COMPREHENSIVE COVERAGE ON DIFFERENT TYPE OF FUEL CELLS FOR MULTIPLE APPLICATIONS

Direct generation of electric power by an electrochemical reaction!

Principle of power generation

＜Electrolysis of water＞

Electrolysis of water is a well-known experiment that is also performed by students in junior high schools. When water is charged with a platinum electrode and electric current is passed, the water (H2O) is decomposed into hydrogen and oxygen.

How a fuel cell generates power

A fuel cell operates as a reverse of the electrolysis of water. A fuel cell makes water from hydrogen and oxygen and extracts the electricity that is generated in the process. At the anode, hydrogen ions (H+) and electrons (e-) are released by a catalytic reaction when hydrogen fuel is supplied. The hydrogen ions (H+) move through the electrolyte to the cathode because the electrolyte is a substance that allows ions to pass but not electrons. Since the electrons are blocked by the electrolyte and cannot move, it is possible to generate electricity by removing them to the outside. At the cathode, oxygen (O2) is separated into two oxygen atoms by a catalytic reaction when oxygen is supplied. Water (H2O) can be created by combining electrons that have moved to this oxygen atom and hydrogen ions that have passed through the electrolyte.

Introduction to Fuel Cell

Conventional power plants convert chemical energy into electrical energy in three steps:

• Production of heat by burning fuel
• Conversion of heat into mechanical energy
• Conversion of mechanical energy into electrical energy

The efficiency of the second step is limited (by the Second Law of Thermodynamics) to the Carnot efficiency, since the conversion of heat into mechanical energy occurs in a closed-cycle heat engine. An efficiency of about 41% can be reached by modern systems. A fuel cell is an electrochemical device that converts the chemical energy in fuels (e.g. hydrogen, methane, butane or even gasoline and diesel) into electrical energy. It exploits the natural tendency of oxygen and hydrogen to react to form water. The direct reaction is prevented by the electrolyte, which separates the two reactants. Therefore two half-reactions occur at the electrodes:

• Anode: Fuel (e.g. H2, CO, CH4) is oxidised
• Cathode: Oxygen is reduced

The ions are transported to the other electrode through the electrolyte. The fuel cell contains no moving parts and only four active elements: cathode, anode, electrolyte and interconnect; it is a simple and robust system. Fuel cells have a number of advantages compared to conventional electricity generation:

• Negligible air pollution (if fossil fuels are used, otherwise none)
• Reduced weight, especially in mobile applications
• 100% theoretical efficiency, 80% efficiency in high temperature turbine hybrid systems, that can use the generated heat High efficiency in low power systems
• Constant efficiency at low load
• Flexible output with fast adjustment
• Low maintenance cost and very few moving parts (or none)
• Quiet or completely silent

Fuel cells have many interesting applications. This short video shows a demonstration fuel cell car. Note that hydrogen and oxygen being used up by the reactions.

The principle

Oxygen and hydrogen, when mixed together in the presence of enough activation energy have a natural tendency to react and form water, because the Gibbs free energy of H2O is smaller than that of the sum of H2 and ?O2 added together (Hence, we don’t smoke our pipes on Zeppelins!). If hydrogen and oxygen were combined directly, we would see combustion:

H2 + ?O2 → H2O

Combustion involves the direct reaction of H2 gas with O2. The hydrogen donates electrons to the oxygen . We say that the oxygen has been reduced and the fuel oxidised. This combustion reaction releases heat energy.

The fuel cell separates hydrogen and oxygen with a gas-impermeable electrolyte through which only ions (e.g. H+, O2-, CO32–) can migrate. Hence two half reactions occur at the two electrodes. The type of reactions at the electrodes is determined by the type of electrolyte.

Grove's fuel cell is one of the simplest examples.

The half-reaction at the anode: H2 → 2H+ + 2e–

The half-reaction at the cathode: O2 + 4e–+ 4H+ → 2H2O

The net reaction is the combustion reaction: H2 + ?O2 → H2O

Activation polarization is caused by the energy intensive activity of the making and breaking of chemical bonds: At the anode, the hydrogen molecules enter the reaction sites so that they are broken into ions and electrons. The resulting ions form bonds with the catalyst atoms and the electrons remain in the vicinity until new hydrogen molecules start bonding with the catalyst, breaking the bond between the earlier ion. The electrons migrate through the bipolar plate if the bonding energy of the ion is low enough and the ions diffuse through the electrolyte. A similar process occurs at the cathode: Oxygen molecules are broken up and react with the electrons from the anode and the protons that diffused through the electrolyte to form water. Water is then ejected as a waste product and the fuel cell runs (can supply a current), as long as fuel and oxygen is provided.

The exact reactions at the electrodes depend upon which species can be transported across the electrolyte. Fuel cells are classified according to the type of electrolyte. The most common electrolytes are permeable for protons and the reactions are as discussed above. The second most common electrolytes, found in solid oxide fuel cells (SOFCs), are permeable for oxide ions and the following half-reactions occur:

The half-reaction at the anode: H2 + O2– → H2O + 2e–

The half-reaction at the cathode: O2 + 4e– → 2O2– The net reaction is the same as before: H2 + ?O2 → H2O

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A third type of electrolyte, used for molten carbonate fuel cells at high temperatures conducts carbide ions (CO32–):

The half-reaction at the anode: H2 + CO32– → H2O + CO2 + 2e–

The half-reaction at the cathode: ?O2 + CO2 + 2e– → CO32–

The net reaction is the combustion reaction: H2 + ?O2 → H2O

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We also commonly see alkaline electrolytes, across which OH– is the transported species. In this case the half-reactions would be:

The half-reaction at the anode: H2 + 2OH– → 2H2O + 2e–

The half-reaction at the cathode: O2 + 4e– + 2H2O → 4OH–

The net reaction is the combustion reaction: H2 + ?O2 → H2O

History of the Fuel Cell technology

The fuel cell concept was first demonstrated by William R. Grove, a British physicist, in 1839. The cell he demonstrated was very simple, probably resembling this:

???Electrolysis setup

By application of a voltage across the two electrodes, hydrogen and oxygen could be extracted (the process is called electrolysis) and captured as shown (William Nicholson first discovered this in 1800). The fuel cell, or “gas battery” as it was first known, is the reverse of this process. In the presence of platinum electrodes, which are necessary as catalysts, the electrolysis will essentially run in reverse and current can be made to flow through a circuit between the two electrodes.

Nobody tried to make use of the concept demonstrated by William R Grove until 1889 when Langer and Mond tried to engineer a practical cell fuelled by coal gas. Further early attempts carried on into the early 1900’s but the development of the internal combustion engine made further research into the technology sadly unnecessary.

Francis Bacon developed the first successful fuel cell in 1932, running on pure O2 and H2 and using an alkaline catalyst and nickel electrodes. It was not until 1959 that Bacon and his colleagues first demonstrated a 5?kW device; the 27 year delay is perhaps an indication of just how difficult it is to make progress in this field of development. Harry Karl Ihrig demonstrated a 20?bhp fuel cell tractor in the same year.

Around about this time, NASA started researching the technology with a view to produce a compact electricity generator for use on spacecraft. Due to their astronomical budget, it was not long before they got the job done. The Gemini program used early PEM fuel cells (PEMFCs) in its later missions, and the Apollo program employed alkaline fuel cells. On a spacecraft the water produced by the reaction was available for the spacemen to drink. NASA continued to use alkaline cells in the space shuttle until the 90’s when PEMFC development meant a switch back to PEMs was considered a possibility, however, the high cost of design, development, test and evaluation prevented the switch, in spite of several technical advantages.

PEM fuel cells being installed in a Gemini 7 spacecraft (Source: Smithsonian Institution, from the Science Service Historical Images Collection, courtesy of General Electric)

The alkaline fuel cell system as used on the space shuttles. Three such modules were installed in each shuttle

Recent developments are thick and fast as the technology begins to come to fruition. Automotive applications are high on the agenda due to the huge consumer market and the need for an environmentally friendly, renewable alternative to the internal combustion engine and fossil fuels.

Honda fuel cell car

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PROTON EXCHANGE MEMBRANE FUEL CELL (PEMFCs)

Low temperature cells

The proton exchange membrane (a.k.a. polymer electrolyte membrane) fuel cell uses a polymeric electrolyte. This proton-conducting polymer forms the heart of each cell and electrodes (usually made of porous carbon with catalytic platinum incorporated into them) are bonded to either side of it to form a one-piece membrane-electrode assembly (MEA). A quick overview of some key advantages that make PEMs such a promising technology for the automotive markets:

• Low temperature operation, and hence
• Quick start up
• No corrosive liquids involved
• Will work in any orientation (or zero g for that matter)
• Thin Membrane-electrode assemblies allow compact cells

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Brief history

The PEM fuel cell was developed in the 1960’s in General Electric’s labs. As with so many technologies, the space program and military funded research fast-forwarded it’s development. PEM membranes were first applied to a US Navy project and projects for the US Signal Corps. PEM cells were used in NASA’s Gemini program, which was to serve as a means of testing technology for the Apollo missions. Batteries were not suitable for a journey to the moon because of the extended flight duration. Early PEM systems were, however, unreliable and plagued with leakages and contamination. The systems installed in Gemini spaceships had an operational lifetime of just 500?hrs, although this was considered suitable. Another issue was the water management systems, which are required to keep the membrane hydrated to the correct extent. Apollo designers opted for the more mature technology of AFCs, as did the Space Shuttle designers in the 70's. Recently however, as part of NASA’s program of continuous upgrade on the Shuttles, PEM systems have replaced the aging AFC technology as the primary power source for the Shuttles’ systems.? GE decided to abandon their research on PEMFCs in the 70’s, probably due to the cost. At that time, the catalysis required 28?mg of Platinum per cm2 of electrode, compared to the current figure of 0.2?mg?cm–2, or less.

Automobiles are arguably one of the most important consumer products on the planet. The finite fuel reserves, which they are chewing through, are not currently a limiting factor, but they will be soon. Much investment has been aimed at developing fuel cell technology for the automotive industry and the electrolyte of choice is the PEM. We’ll look at the problems which automotive companies need to overcome before fuel cell cars hit the street.

However, recent developments in PEMFCs have brought their current densities up to around 1?A?cm–2 and cut the platinum requirement to 1% of what used to be needed. The scope of PEMFCs is, arguably, wider than that of any other power supply technology; with the potential to power a range of devices from mobile phones and laptops to busses, boats and houses.

Construction of the PEM cell (PROTON EXCHANGE MEMBRANE FUEL CELL)

The PEMFC is constructed in layers of bipolar plates, electrodes and membranes:

PEMFC components

Each individual cell produces about 0.7?V EMF when operating in air, as calculated by the expressions outlined in the efficiency section. In order to produce a useful voltage, the electrodes of many cells must be linked in series. In addition to connecting the cells, we must ensure that reactant gases can still reach the electrodes and that the resistance of the electrodes has a minimal effect. If, for example, we were to simply wire up the edge of the anode of one cell to the cathode of another, electrons would have to flow across the face of the electrodes. Each cell only produces ~0.7?V, even a small reduction in this isn’t permissible, so cells aren’t normally wired up this way.

A Bipolar Plate is used to interconnect the anode of one cell to the cathode of the next. It must evenly distribute reactant gases over the surface of the anode, and oxygen/air over the cathode. Bipolar plates may also need to carry a cooling fluid, and in addition, need to keep all these gases and cooling fluids separate. Design considerations:

• The electrical contacts should be as large as possible
• The plate should be thin to minimise resistance
• Gas needs to flow easily across the plate

Often these factors are antagonistic to each other, for instance, large contact area would reduce the width of the gas channels. A very simple bipolar plate might look like this:

A typical bipolar plate (Left) found in a plate-type PEM assembly (Right)

Reactant gases flow at right angles to each other. In a simple plate design as above, the channels extend right to the edge. The reactant gases would probably be supplied to the system via external manifolding in this case.

External manifolding

External manifolding is a very simple solution, and therefore carries out the job cheaply, but the technique has two major disadvantages. 1) The gaskets needed to seal the plates don’t form a tight seal where the channels come to the edge of the plate, leading to localised leaks of the reactant gases. 2)? Additional channels for cooling fluids are very difficult to incorporate into an externally manifolded system, so all the cooling must be done by the air flowing across the cathode.? This means more air than is necessary for the reaction must be pumped through the channels, which in turn means the channels must be wider, that the chance of leaks is increased and that some of the energy produced must be used to power blowers. Whilst simplicity is always a bonus, external manifolding is rarely used in modern systems.

In this image of a Ballard Nexus? fuel cell system, the fan used to blow air through the stack for cooling is visible on the left of the stack.

Most modern bipolar plates make use of internal manifolding. The three examples below show how this might be achieved. In each case, the channels do not run to the edge of the plates so a gasket could be fitted here and a gas-tight seal would be more easily achieved.

Inernal manifolding

• The design on the left is a fairly simple parallel channels design; reactant gases would be blown into one end of the channel through one hole, and removed at the other hole. There are many different designs possible, and designers of bipolar plates are yet to reach an agreement on which type is best. In parallel designs, water or gas may build up along one of the channels causing a temporary blockage. In this case the reactants will happily continue to pass through the other channels and not clear the blockage.

• The second design, a serpentine design, guarantees that if reactants are flowing at all, they’re flowing all along the channel and blockages are easily cleared. The problem in this case is that it takes more effort to push reactants through the long, winding path.

• The third design is more of a compromise between the two and is the type of thing often seen in bipolar plate design. The channels are typically about 1?mm in width and depth. The pressure difference between the start and end of a channel must be engineered to overcome the surface tension of water droplets forming on the channel walls in order to clear blockages. Ballard, for example, achieve this pressure difference with rectangular plates in which the gases run across the long axis in a long parallel design.

The material properties of a bipolar plate, as summed up by Ruge and Büchi (2001), must take into account several important factors:

• Electrical conductivity >10?S cm–1
• Heat conductivity of 20?W m–1 K–1 if cooling fluid is integrated, 100 W m–1 K–1 if heat is removed from the edges.
• Gas permeability < 10–7 mbar L s–1 cm–2
• Resistant to corrosion in an environment of acidic electrolyte, hydrogen, oxygen, heat and humidity.
• Reasonably high stiffness E > 25?MPa
• As ever, it should cost as little as possible.

The plates must also be manufactured so that they are:

• Thin for maximum stack volume
• Light for minimum stack mass
• Able to be produced quickly with a short cycle time

These various and difficult specifications which must be met, along with the fact that modern electrodes require very little catalytic platinum, mean that the bipolar plate is the most expensive part of a modern fuel cell.

?Materials for constructing bipolar plates

PEMFCs without bipolar plates

As discussed, bipolar plates may provide excellent contact between cells, but they are expensive and complex. Some manufacturers, often on the smaller industrial scale, choose different techniques to link their cells. Cells could be connected simply edge to edge, reducing the possibility of leakage. One manufacturer (Intelligent Energy) produces cells with stainless steel bases through which hydrogen channels pass. The cathode current collector is a porous metal and these individual cell units are simply stacked with a piece of corrugated stainless steel between them. It’s a simple solution which may gain popularity.

In conclusion, we should note that although a broad range of bipolar plates techniques exist, none of them fully meet the criteria set above. There is lots of development still to be done in this area before we meet a new industry standard.

PROTON EXCHANGE MEMBRANE FUEL CELL

MEMBRANE

Dupont’s Nafion? ion exchange membrane forms the basis of the proton exchange membrane fuel cell. Each company involved in the development of PEMFCs may have their own variation on Nafion, however, they’re all based on the same sulphonated fluoropolymers and Nafion remains something of an industry standard in membranes, to which all others are compared (although it is not always most suitable). Nafion is a polymer based on PTFE (polytetrafluoroethylene).

PTFE

Nafion is essentially PTFE containing a fraction of pendant sulphonic acid groups. (Nomenclature: “sulphonic acid group” usually refers to the un-dissociated SO3H group, where as “sulphonate” refers to the ionised SO3– group after the proton has dissociated). The ion containing fraction is normally given in terms of equivalent weight (i.e. number of grams of dry polymer per mole of acidic groups). The useful equivalent weight for Nafion ranges from 800?1500?g?mol–1.

Nafion structure (Left) and a fluoropolymer (Right), made by DOW chemical company, also used in PEMFCs.

The length of and the precise nature of the side chains vary between different brands of polymer. Common to all is the PTFE based fluorocarbon “backbone” of the polymer that has several desirable properties:

• PTFE is hydrophobic - this means the hydrophilic sulphonate groups are effectively repelled by the chains and cluster together.

• PTFE is extremely resilient to chemical attack – the environment within the membrane is hostile and very acidic. Hydrocarbon-based polymers would tend to degrade rapidly.

• PTFE is a thermoplastic with high mechanical strength – meaning very thin membranes can be produced, reducing the thickness of each cell and increasing the power density of the stack.

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Transport through the membrane (PROTON EXCHANGE MEMBRANE FUEL CELL)

The animation below demonstrates schematically the mechanism of proton transport in the proton exchange membrane.

https://www.doitpoms.ac.uk/tlplib/fuel-cells/videos/pem1.mp4

In reality, the protons would be strongly associated with water molecules and transported in the form of H3O+ hydronium ions, or even higher order cations. The Zundel (H5O2+ - basically a protonated water dimer) and the Eigen (H7O3+) cations are thought to be particularly important in transfer of protons from one hydronium to another.

Points to note:

• Sulphonic acids are highly acidic (pKa ~ –6 in Nafion) meaning they have a high tendency to dissociate into anions and protons (the effect of the aliens’ blood in the “Alien” films was produced with chlorosulphonic acid). It is of course, these protons that act as the charge carriers through the membrane.

• In order for the polymer to conduct H+ it must be hydrated to the correct degree, in order to promote dissociation of ionic groups and provide a mechanism for proton transport.? Proton conductivity is strongly dependant on the water content of the membrane. The water in the membrane is localised to the hydrophilic groups, where the protons dissociate and are transported in a vehicular manner (by diffusion of hydrated protons) and also structurally (via proton transfer between hydrated clusters).

• Typical PEMs have conductivity in the order of 0.01–0.1?S?cm–1 at 80–90?°C, which is a far lower temperature than other solid-state (usually ceramic) electrolytes.

PROTON EXCHANGE MEMBRANE FUEL CELL

Electrodes and membrane-electrode assembly

?Catalyst

In the first fuel cells, platinum was used in relatively large quantities. This perhaps led to the false belief that most of the cost of a fuel cell is down to the platinum in it. Generally this is not the case. Platinum particles are deposited very finely onto carbon powders so that the platinum is very finely divided with a maximal surface area. With catalysts produced in this way, the raw material platinum cost is just $10 for a 1?kW cell stack.

Catalyst made of carbon powders deposited with platinum particles

Bonding

Before the catalyst layer is applied to the electrolyte, a coating of soluble electrolyte is brushed onto it. This ensures that there is good contact between the platinum and the electrolyte to achieve the important three-phase interaction between gas, catalyst and electrolyte necessary for the reaction to proceed.

The catalyst can be applied to the membrane in one of two ways: Either the catalyst powder can be applied directly to the membrane, by rolling, spraying or printing, and then have the supporting electrode structure (often called the gas-diffusion layer) added afterwards, or the electrodes can be assembled separately and bonded to the membrane in complete form by hot pressing. The catalyst powder is sometimes mixed with PTFE to drive out product water and prevent the electrode becoming water logged. The “gas diffusion layer” is added between the catalyst and the bipolar plate to provide some rigidity to the MEA and to ensure ease of diffusion. This layer is usually composed of carbon cloth or carbon paper 0.2–0.5mm thickness, with more PTFE added to expel water.

The membrane electrode assembly, once completed.

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DIRECT METHANOL FUEL CELL

How does a methanol fuel cell work?

As a clean alternative to fossil-fueled combustion engines, fuel cells are gaining growing attention. Yet much of the focus remains on operation with gaseous hydrogen. But there are also alternative concepts: one of them is the methanol fuel cell.

Design and principle of a methanol fuel cell

Similar to any other type of fuel cell, methanol fuel cells consist of two electrodes (anode and cathode), which are separated from each other through an electrolyte. The electrodes are conductive for electrons, while the electrolyte is only permeable for positively charged hydrogen atoms.

Typically, methanol fuel cell systems build on a polymer electrolyte membrane (PEM). In terms of design, a distinction can be made between two modes of operation: Fuel cells that use methanol directly and those that use methanol indirectly.

Direct methanol fuel cell (DMFC)

Direct methanol fuel cells (DMFC) are the most common form of methanol fuel cell. Compared to other methanol fuel cells, they are characterized by a simple system design and fast start-up times. They are usually low-power systems with an output of < 200 Watts.

Electrical efficiency: 20 – 30%

Operating temperature: 70 – 90°C

Reaction: 2 CH3OH + 3 O2 ->? 4 H2O + 2 CO2

Design and principle (DMFC)

DMFCs supply a methanol-water mixture at the anode side. The mixture is split into hydrogen and carbon dioxide. A catalyst (platinum) divides the hydrogen molecules into positively charged hydrogen atoms (protons).

The hydrogen protons pass through the electrolyte (a proton exchange membrane) to the cathode and react with oxygen to form water. The process takes place at a comparatively low cell temperature of 70 to 90°C, which enables fast start-up times.

Principle and basic design of a direct methanol fuel cell (DMFC) – Schematic illustration

Applications

DMFCs are used both in commercial and private settings. The fields of application include remote monitoring stations, video surveillance systems or smaller electrical systems in traffic control – mostly with a demanded power of less than 150 Watts. In the leisure sector, DMFCs are used for on-board power supply in caravans and boats.

Advantages and disadvantages

Besides the general advantages of fuel cells over conventional combustion engines, direct methanol fuel cells are characterized by a simple system design and fast start-up times. However, DMFCs are relatively sensitive to impurities in the fuel and have a relatively low efficiency.

Advantages

• Fast start-up time
• Simple and compact system design
• Widely used and well-established for low power applications

Disadvantages

• Require high purity methanol
• Comparatively low efficiency (20 – 30%)
• Storage temperature at sub-zero temperatures problematic, as formation of water crystals can damage the membrane
• Methanol “cross-over” on membrane can affect lifetime and system efficiency
• High platinum content required as catalyst

Advantages and disadvantages of direct methanol fuel cells (DMFCs) compared to other methanol fuel cells

Indirect methanol fuel cells (RMFC)

The indirect methanol fuel cell (also reformed methanol fuel cell or RMFC) uses hydrogen as fuel. The hydrogen is extracted from methanol in a pre-process. This system design allows for a higher power output and improved electrical efficiency compared to DMFCs.

Electrical efficiency: 35 – 50%

Operating temperature: depending on membrane 70 – 90°C (LT-PEM) or 160 – 200 °C (HT-PEM)

Reaction: CH3OH + H2O -> 3 H2 + CO2 ?(Reformer) ??2 H2 + O2 ->? 2 H2O (Fuel cell)

Design and principle

RMFCs can differ with regard to their membrane. Some systems use conventional low-temperature polymer electrolyte membranes (LT-PEM), while others use high-temperature polymer electrolyte membranes (HT-PEM). More on the different types here. However, the basic working principle is similar:

In the first step, methanol is converted into a hydrogen-containing gas (reformate gas). This so-called steam reformation process takes place at temperatures between 200 – 220°C. More about the process can be found below in the text. Methanol serves as a liquid hydrogen carrier.

In contrast to a DMFC, no watery methanol is fed to the anode, but gaseous hydrogen. However, since LT-PEMs – a fuel cell type commonly used in the automotive industry – require high-purity hydrogen, the reformate gas must be purified before being used for power generation. This may be accomplished by means of a palladium membrane.?

In contrast to LT-PEMs, HT-PEMs are tolerant to impurities in the reformate gas due to their elevated operating temperature. The step of conditioning is therefore not required for this system design.

Principle and basic design of an indirect methanol fuel cell (RMFC) with reformer and HT-PEM (LT-PEM requires an additional step of purification between fuel cell and reformer) – Schematic illustration

Applications

RMFCs are predominantly used for stationary power generation, as well as in the automotive sector. Fields of application include remote telecommunication sites or backup power supply for critical infrastructure. Systems and modules vary from 150 watts to several hundred kilowatts, depending on the field of application.

Advantages and disadvantages (DMFC)

Compared to a DMFC, the system design of the RMFC is more complex. However, the requirements regarding the methanol purity are lower. In addition, a higher efficiency can be achieved.

Advantages

• High efficiency (35- 50%)
• Cold storage since no liquid is in direct contact with the membrane
• Lower platinum content
• Use of industrial methanol (for example according to IMPCA)

Disadvantages

• Slower startup time due to warm-up phase
• More complex system design

Advantages and disadvantages of indirect methanol fuel cells (RMFCs) compared to DMFCs

All methanol fuel cells require a methanol-water mixture. However, since water (vapor) is formed on the cathode side of the fuel cell during operation, this can be condensed out and mixed with pure methanol. For this reason, some manufacturers allow the use of pure methanol as a fuel.

Advantages of methanol for fuel cells (DMFCs)

Containing four hydrogen, one carbon, and one oxygen atom, methanol (H3COH) is the simplest organic alcohol. High energy density and easy production from renewable energy sources or biomass make methanol an interesting fuel for future energy systems.

Advantages of methanol for fuel cells

Transport and storage

Methanol is liquid at room temperature and can be transported and stored in tanks, canisters or barrels. In addition, it has a low freezing point of – 97°C, making methanol fuel cells suitable for use in cold ambient temperatures.

Low cost and globally available

Methanol is an important feedstock for the chemical industry and is used, for instance, as a solvent. Due to its versatility, methanol is traded globally. In Europe, the net price for methanol – supplied in an IBC – is around €0.50 per liter.

High energy density

Methanol has a high proportion of chemically bonded hydrogen and is characterized by its high energy density. To put this into perspective: 10 liters of methanol contain approximately 1 kilogram of hydrogen. A 25 liter canister consequently contains the same amount as 2 pressurized gas cylinders with gaseous hydrogen at 200 bar.

Production of hydrogen from methanol

Today, hydrogen is mainly produced from hydrocarbons. This is a group of compounds consisting only of carbon (C) and hydrogen (H). These include fossil fuels such as natural gas, coal and petroleum, but also alcohols such as methanol and ethanol (C2H4O).

The most common process is that of steam reforming. In this process, the chemically bound hydrogen is extracted from the source materials at high temperature. The result is a reformate gas rich in hydrogen.

In contrast to other hydrocarbons, methanol can be converted into a hydrogen-containing reformate gas at a relatively low temperature of 200 – 220 °C. For power generation, the reformate gas can be used directly in HT-PEM fuel cells.

Purification of hydrogen allows for usage of hydrogen in industrial processes or in other types of fuel cells. Today, this can be accomplished by Pressure Swing Adsorption (PSA), Palladium membranes, or Electrochemical Hydrogen Seperation (EHS).

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ALKALINE FUEL CELL

Alkaline fuel cells (AFCs) was one of the first extensively researched fuel cell types and was used by NASA for the Gemini, Apollo, and Space Shuttle missions. The first alkali electrolyte fuel cell was built by Francis Thomas Bacon (1904–1992) in 1939. He used potassium hydroxide for the electrolyte and porous “gas-diffusion” electrodes instead of the acid electrolytes and solid electrodes used in previous fuel cell prototypes. Like today’s fuel cells, Bacon also used pressurized gases to keep the electrolyte from “flooding” the electrodes. During World War II, he thought the alkali electrolyte fuel cell would provide a good source of power for the Royal Navy submarines instead of the dangerous storage batteries used at the time. For the next 20 years, he created large-scale demonstrations with his alkali cells using potassium hydroxide as the electrolyte. One of the first demonstrations was a 1959 Allis–Chalmers farm tractor that pulled a weight of 3000 pounds powered by a stack of 1008 alkaline cells. Allis–Chalmers continued fuel cell research for many years, and also demonstrated a fuel cell–powered golf cart, submersible vehicle, and forklift. In the late 1950s and 1960s, Union Carbide also experimented with alkali cells. Karl Kordesch and his colleagues designed alkali cells with carbon gas–diffusion electrodes based upon the work of G. W. Heise and E. A. Schumacher in the 1930s. They demonstrated a fuel cell–powered mobile radar set for the U.S. Army, as well as a fuel cell–powered motorbike. Pratt & Whitney licensed the Bacon patents in the early 1960s and won the National Aeronautics and Space Administration (NASA) contract to power the Apollo spacecraft with alkali cells.

Chemical Reactions:

The operating temperature of alkaline fuel cells range between room temperature to approximately 250 ?C and can achieve power-generating efficiencies of up to 70 percent. A diagram of the alkaline fuel cell is shown in Figure 1. The chemical reactions that occur in an AFC are as follows:

Anode: 2H2 (g) + 4(OH)- (aq) → 4H2O (l) + 4e-

Cathode: O2 (g) + 2H2O (l)+ 4e- → 4(OH)-(aq)

Overall: 2H2 (g) + O2 (g) → 2H2O (l)

Figure 1. Chemical Reactions in an Alkaline Fuel Cell.

In AFCs, the oxygen reacts at the cathode to produce either hydroxide (OH-) or a carbonate ion (CO32-), depending upon the electrolyte composition. The ion travels through the electrolyte to react with hydrogen at the cathode. AFCs use lower cost materials compared with other fuel cells.

The catalyst layer can use either platinum or nonprecious metal catalysts such as nickel and the electrolyte has historically consisted of liquid KOH or a KOH filled matrix. A disadvantage of AFCs is that pure hydrogen and oxygen have to be fed into the fuel cell because it cannot tolerate the small amount of carbon dioxide from the atmosphere.

Electrolyte Layer

The electrolyte layer is essential for a fuel cell to work properly. In low temperature fuel cells, when the fuel in the fuel cell travels to the catalyst layer, the fuel molecule gets broken into protons (H+) and electrons. The electrons travel to the external circuit to power the load, and the hydrogen proton (ions) travel through the electrolyte until it reaches the cathode to combine with oxygen to form water. In AFMs, the reactions are similar, but they occur in the opposite direction, with the O2 producing the hydroxide and then reacting with hydrogen at the anode.

Traditionally, the electrolyte for alkaline fuel cells is an aqueous solution of alkaline potassium hydroxide soaked in a matrix with normalities ranging from 6 to 9. The solution must be as pure as possible to prevent the impurities from contaminating the catalyst. The electrolyte can be in a mobile or immobile form. The mobile electrolyte is pumped through the cells and removes water and waste heat from the fuel cell. These cells typically have large 2 to 3 mm flow channels to allow rapid flow. The large thickness increases the ohmic polarization which is a major design consideration for AFCs.

In AFCs that use immobile electrolyte, the KOH/H2O solution is held in an asbestos matrix. The electrolyte layer can be as thin as 0.05 mm; therefore, ohmic polarization is not as much of an issue in this type of AFC. The electrolyte typically used is 30-percent potassium hydroxide, which yields the optimal ionic conductivity when AFCs are operated at 60 to 80 C. Increasing the KOH concentration helps AFC performance, but it is not practical and feasible to use high concentrations of KOH in water due to the nonuniformity of KOH concentrations in operating cells. The oxygen reduction reaction may reduce the water concentration near the cathode and then the electrolyte solution may solidify, thus preventing reactant transport.

As with other fuel cell types, the electrolyte must meet the following requirements:

? High ionic conductivity ? Present an adequate barrier to the reactants ? Be chemically and mechanically stable ? Low electronic conductivity ? Ease of manufacturability/availability ? Preferably low-cost

Finding an electrolyte material that meets all of these requirements is tough. The toughest requirements are high ionic conductivity, and a material that is stable in both an oxidizing and reducing environment. There are many research groups around the world that are investigating solutions to overcome these limitations. One solution is to use solid electrolyte layers such as anion exchange membranes (AEMs) instead of liquid or matrix electrolytes.

AFC Electrodes

The AFC electrodes can be hydrophobic or hydrophilic. The hydrophobic electrodes are carbon-based with PTFE, while the hydrophilic electrodes are usually made of metallic materials such as nickel and nickel-based alloys. The electrodes usually have several layers with different porosities for the liquid electrolyte, fuel, and oxidant. AFCs can use both precious and nonprecious metal catalysts. The precious metal catalysts used are platinum or platinum alloys that are deposited on carbon supports or manufactured on nickel-based metallic electrodes. The catalyst loading is typically 0.25 mg Pt/cm2 and up. The most commonly used nonprecious metal catalysts are Raney nickel for the anodes at a loading of 120 mg Ni/cm2, and silver-based powders for the cathodes with a loading between 1.5 to 2 mg Ag/cm2.

Sintered nickel powder. When the alkaline fuel cell was originally designed, it was made to use precious metal catalysts and employ low-cost materials. The electrodes were made of porous powdered nickel that was sintered to make it a rigid structure. To ensure good three-phase contact between the reactant gas, the liquid electrolyte, and the solid electrode, the nickel was made of two layers of different sizes of nickel powders. This structure gave good results, but it was hard to optimize the fuel cell catalyst layer at the time due to limited coating technology and instrumentation to investigate the properties of materials. This structure is still sometimes used today with and without additional catalysts.

Raney metals. Raney metals can be used to achieve very active and porous forms of a metal. These are prepared by mixing the active metal (nickel) with an inactive metal such as aluminum. The mixing is accomplished so that the two metals are not mixed to form an alloy, but the regions and properties of both metals are maintained. A strong alkali is then applied to the mixture to dissolve the aluminum. This leaves a porous material with a very high surface area. This process allows the pore size to be changed by altering the degree of mixing between the two metals.

Cell Components

The electrodes contain high loadings of noble metals: 80 percent Pt – 20 percent Pd anodes are loaded at 10 mg/cm2 on Ag-plated Ni screen; 90 percent Au – 10 percent Pt cathodes are loaded at 20 mg/cm2 on Ag plated Ni screen. Both are bonded with PTFE to achieve high performance at the lower temperature of 85 to 95 °C. A wide variety of materials (e.g., potassium titanate, ceria, asbestos, zirconium phosphate gel) have been used in the micro-porous separators for AFCs. The electrolyte is 35 percent KOH and is replenished via a reservoir on the anode side. Gold-plated magnesium is used for the bipolar plates. A typical configuration, Apollo uses carbonbased plastic-bonded gas diffusion electrodes with a current collector (nickel) inside. Due to the ease of preparation, the electrodes in present stacks use noble metals loaded to less than 0.5mg/cm2 . The 0.3 cm thick cells are stacked in a monopolar order and are commonly connected in series via edge connectors. Neither membranes nor bipolar plates are needed. The stacks operate at 75 °C, using a 9N KOH electrolyte. The gases are fed at ambient pressure; either pure hydrogen or cracked ammonia is used. Lifetime testing has not been finished, but is >1,000 hours at intermittent operation (a few hours per day). Several types of catalysts are used or are being considered for the electrodes: ? noble metals (expensive but simple, and acceptable for low volume stack preparation) ? “classic” non-noble metals (silver for the cathode and Raney nickel for the anode) ? spinels and perovskites (often referred to as alternative catalysts, these are being developed because they cost less than the noble metal catalysts.

Conclusion

AFCs offer potential benefits over PEMFCs due to non-noble metal catalysts and the ability to use cheaper and more versatile hydrocarbon-based membranes. PEMFCs face an issue of H2 and O2 crossover because of electro-osmosis and diffusion; however, for AEMFCs, the transport of hydroxide occurs from cathode to anode while water moves from anode to cathode, therefore, the crossover problems are solved in AEMFCs. The alkaline environment of AEMFCs allow the use of non-precious metal catalysts such as iron, cobalt, silver, and graphene, which significantly reduces the cost of the fuel cell system. Water management issues can also potentially be solved by tuning the properties of the polymer to allow for water diffusion from the anode to the cathode.

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PHOSPHORIC ACID FUEL CELL

Phosphoric acid fuel cells (PAFC) currently represent one of the fuel cell technologies that have been demonstrated in many countries around the world and for many applications. PAFCs can be purchased, complete with a warranty, maintenance and spare parts service. The first PAFC power plants were installed in the 1970s, and now more than 500 units have been installed all around the world.

The phosphoric acid fuel cell (PAFC) is the most widely used and best documented type of fuel cell. Since the 1970s, more than 500 PAFC power plants have been installed and tested around the world. With every new product release, the number of units sold became larger as well as the power rating per unit. The largest fuel cell ever built to date is an 11?MW PAFC power plant for the Tokyo Electric Power Co. (TEPCO) in Japan, which was operated for more than 230?000?h between 1991 and 1997 . The most important PAFC developers are UTC Fuel Cells , Toshiba and Fuji Electric. All the installations have been used for stationary applications, with the exception of the Georgetown University Fuel Cell Transit Program, in which a 100?kW UTC fuel cell was deployed . PAFCs have shown a remarkable reliability. UTC PAFC systems are characterized by a Mean Time Before Failure (MTBF) that ranges between 2500?h for the PC25 to 6750?h for the 400?kW advanced PAFC . Due to the high reliability, that is well above those of traditional systems, PAFC represents, for some applications, the answer to electricity quality and availability needs.

The PAFC is the first type of FC that has been commercialized. It is commercially available in hotels, houses, hospitals, and some power stations in a range from 50?kW to 11?MW.

Efficiency of PAFC is ~?35%–45%, which is higher than PEMFC, but lower than MCFC and SOFC. When it works with CHP, heat and power are applied simultaneously, so the efficiency grows dramatically and reaches to about 80%.

PAFCs have been found useful in stationary power distribution, defense, and military applications. Heat and power cogeneration is another major advantage. In spite of these advantages, PAFC technology is constrained by various factors. Use of precious metal electrocatalyst makes it costly. The system weight is high owing to the use of heavier materials for bipolar plates. The power density of the PAFC Systems is therefore low.

Introduction

Of the hydrogen-oxygen fuel cell systems the most mature is the phosphoric acid fuel cell (PAFC). It operates at 150-190°C and pressure ranging from ambient to 5 atm. PAFC systems use primarily Pt as catalyst both for hydrogen and oxygen electrodes. The operating temperature range of PAFC allows it to take up hydrogen directly from hydrogen sources like reformer gases. Less than one percent of CO present in the reformer gases are not adsorbed on Pt sites owing to high operating temperature. The other components used in PAFC are mainly made of graphite and carbon. All these factors make PAFC a versatile member of the hydrogen-oxygen fuel cell family.

Concentrated phosphoric acid (90-100% based on ortho phosphoric acid) is used as electrolyte in this fuel cell, that operates at 150 to 190°C. Some of the pressurized systems are reported to work upto 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor), and CO poisoning of the Pt electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids; consequently the PAFC is capable of operating at the high end of the acid temperature range (100 to 220°C). In addition, the use of concentrated acid minimizes the water vapor pressure and hence water management in the cell is not as difficult as it is for polymer electrolyte fuel cell (PEMFC). The matrix, that is universally used to retain the acid, is silicon carbide. The electrocatalyst typically used in both the anode and cathode is Pt loaded on carbon.

Phosphoric Acid Fuel Cell (PAFC): System Definition and Principle of Operation.

A phosphoric acid fuel cell (PAFC) is composed of two porous gas diffusion electrodes, namely, the anode and cathode (Fig. 1) juxtaposed against a porous electrolyte matrix. The gas diffusion electrodes are porous substrates that face the gaseous feed. The substrate is a porous carbon paper or cloth. On the other side of this substrate, which faces the electrolyte (phosphoric acid), platinized fine carbon powder electrocatalyst is roll coated with polytetrafluroethylene (PTFE) as a binder. PTFE also acts as a hydrophobic agent to prevent flooding of pores so that reactant gas can diffuse to the reaction site easily.

At anode, hydrogen ionizes to H+ and migrates towards cathode to combine with oxygen, forming water. The product water then diffuses out to the oxygen stream and comes out of the system as steam. An emf is generated between the two electrodes through conversion of reaction free energy to electricity and on connecting an external load, electrical power can be extracted. The reactions at anode and cathode are as follows.

Fuel cells, which use phosphoric acid solution as the electrolyte, are called phosphoric acid fuel cells (PAFCs). As Eq. 1 indicates, the phosphoric acid in aqueous solution dissociates into phosphate ions and hydrogen ions; the hydrogen ions (H+ ) act as the charge carrier.

H3PO4 → H+ + H2PO4 (1)

Phosphoric acid is chemically stable, and is easy to handle. It also has an extremely low vapor pressure even at an operating temperature of 200 °C (473 K). This implies that phosphoric acid in the electrolyte layer cannot be easily discharged from the fuel cell together with the cell exhaust gas, although even such minute discharge, results in the degradation of cell performance in the long term.

A conceptual working principle is described in Figure 1.

At the fuel electrode, pure hydrogen or reformate fuel gases the principal component being hydrogen is supplied, and air is supplied at the air electrode; the resulting electrochemical reaction yields an electric power output. At the fuel electrode, hydrogen reacts at the electrode surface to become hydrogen ions and electrons, and the hydrogen ions migrate toward the air electrode within the electrolyte.

Fuel electrode: H2 → 2H+ + 2e- (2)

At the air electrode, the hydrogen ions, which have migrated from the fuel electrode; electrons, which have passed through the external circuit, and oxygen supplied from outside, combine to produce water in the following reaction:

Air electrode: (1/2)O2 + 2H+ + 2e – → H2O (3)

Hence the net fuel cell reaction produces water as follows:

H2 + (1/2) O2 → H2O (4)

Figure 1. Principle of Operation of Phosphoric Acid Fuel Cells (PAFCs)

2. Cell Structure

The PAFC itself consists of a pair of porous electrodes (the fuel electrode and air

electrode) formed from mainly carbon material, between which is placed an electrolyte layer consisting of a matrix impregnated with highly concentrated phosphoric acid solution. The catalytic layer of the electrodes where reactions take place consists of the carbon material, minute metal catalyst particles, and water repellant material, in a construction such that the reaction gas is supplied and the electrolyte retained effectively.

The voltage obtained from a single fuel cell is from 0.6 to 0.8 V or so; in actual power plants several hundred cells are stacked and connected in series, forming a sub unit called a “cell stack.” Heat is generated due to energy losses in the course of the electrochemical reaction of hydrogen with oxygen, and so cooling plates are inserted at regular intervals between fuel cells, and cooling water is passed through them to maintain a cell operating temperature of about 200 °C (473 K).

3. Features of Phosphoric Acid Fuel Cells

The PAFC do not suffer the carbon dioxide-induced electrolyte degeneration seen in alkaline fuel cells, and so can use reformed gas derived from fossil fuels, though expensive platinum catalyst is necessary in order to promote the electrode reactions.

Thus it can make use of city gas (natural gas-based) and other existing fuel

infrastructure. However, when CO exists at high concentrations, as in coal-gasified gas, the platinum catalyst used in electrodes is poisoned, leading to performance degradation, so that use of such fuels is impractical without effective means of eliminating CO. This gives an additional constraint.

The operating temperature is about 200 °C (473 K). Consequently if the cell is designed such that it does not make direct contact with the phosphoric acid, copper, iron and other metals can be used. Also, in order to endow the electrode catalyst layer with water-repelling properties, a fluoride resin (PTFE) or other highly heat-resistant organic material may also be used. In order to remove the heat generated by the electrode reactions, the fuel cell is itself water-cooled as mentioned above.

Waste heat at a temperature range below 200 °C is available; which cannot just be used for space heating and water heating, but can also be extracted in part as steam and used as the heat source of refrigeration equipment for cooling. The electric power generation efficiency of PAFCs under atmospheric pressure operation is approximately 40% (LHV basis), which is superior, or at least competitive with existing gas turbine and gas engines. Properties of low Nox and low noise make them suitable for cogeneration systems for urban environmentally friendly power sources.

Unlike the high temperature fuel cell systems such as MCFCs and SOFCs, a combined cycle system with gas turbine or steam turbine generators to maximize the system efficiency is generally difficult for PAFCs, since the quality of plant exhaust heat is inadequate for such purposes.

In pressurized PAFC systems, thought reformer exhaust gas at elevated pressure and temperature can be passed through an expander to drive an air compressor or an electric power generator, the total power generation efficiency stays in a range of 44–46% (LHV basis).

PAFC technology is the most mature fuel cell technology. About 100 MW of PAFC systems have been installed and operated worldwide over the past 20 years with installations in the size range from 50 kW to 11 MW. Some of the field test systems supplied by UTC and Fuji Electric have demonstrated the “magical” 40 000 h operational lifetime mark. Up to the mid-1990s there were a number of developers in the USA, Europe and Japan, but currently only UTC Power, Fuji Electric and more recently HydroGen LLC (acquired Westinghouse's air cooled technology) are still producing PAFC. The major hurdle to large-scale commercialisation is still the cost of the units of 4000–5000 USD/kW that needs to be reduced by a factor of three. Interest in PAFCs has been revived as PEMFC have not been able to match the lifetimes demonstrated by PAFC, and UTC, HydroGen and Fuji Electric believe that a target market price of USD 1500 is achievable through further development and through economies of scale.

Typical applications lie in hospitals, where the waste heat can be used in laundry and other areas and where consistent and reliable power is required; in computer equipment power provision, where the absence of power surges and spikes from the fuel cell enables systems to be kept running; and in army facilities and leisure centers that have a suitable heat and power requirement.

Advantages of Phosphoric Acid Fuel Cell

1. High energy efficiency – Phosphoric Acid Fuel Cells (PAFCs) are highly energy efficient. They convert fuel into electricity in a direct and effective way, reducing energy waste.
2. Long operational life – They also have a long operational life. This means they can keep working for a long time without needing replacement or repair.
3. Low emissions – PAFCs produce low emissions, making them environmentally friendly. They don’t release harmful gases or pollutants that can harm our planet.
4. Operates at high temperatures – These cells can operate at high temperatures. This makes them suitable for situations where heat resistance is crucial. At an operating range of 150 to 200?°C, the expelled water can be converted to steam for air and water heating (combined heat and power). This potentially allows efficiency increases of up to 70%.
5. Can use variety of fuels – PAFCs can use a variety of fuels. This versatility allows them to utilize different energy sources, making them adaptable to various circumstances.

Disadvantages of Phosphoric Acid Fuel Cell

1. High production and maintenance cost – Phosphoric Acid Fuel Cells (PAFCs) can be expensive to produce and maintain, which can limit their widespread use.
2. Limited lifespan – These fuel cells also have a restricted lifespan, which means they’ll need to be replaced after a certain period, adding to the overall cost.
3. Slow startup time – PAFCs are known for their slow startup time, which can be a drawback in situations where immediate power is needed.
4. Requires pure hydrogen fuel – They require pure hydrogen fuel, which can be difficult and costly to obtain, limiting their practicality.
5. Potential for acid corrosion – There’s also a risk of acid corrosion due to the phosphoric acid electrolyte, which can damage the cell and shorten its life.
6. At lower temperatures phosphoric acid is a poor ionic conductor, and CO poisoning of the platinum electro-catalyst in the anode becomes severe.
7. disadvantages include rather low power density and chemically aggressive electrolyte.

Applications

PAFC have been used for stationary power generators with output in the 100?kW to 400?kW range and are also finding application in large vehicles such as buses.

Major manufacturers of PAFC technology include Doosan Fuel Cell America Inc.(formerly ClearEdge power & UTC Power and Fuji Electric.

India's DRDO has developed PAFC based air-independent propulsion air-independent propulsion for integration into their Kalvari-class submarines.

Typical applications lie in hospitals, where the waste heat can be used in laundry and other areas and where consistent and reliable power is required; in computer equipment power provision, where the absence of power surges and spikes from the fuel cell enables systems to be kept running; and in army facilities and leisure centers that have a suitable heat and power requirement.

PAFC FUEL CELL APPLICATIONS

1. Electrical Power Generation

a. Quiet electrical power generation for recreation vehicles, mobile homes, etc.

b. Lighthouse applications (propane powered).

c. Short-term underwater power plants.

d. Emergency and standby electric power (hospital, industry).

e. Offshore oil/gas drilling rig.

f. Offshore oil production platform.

g. Remote power for entertainment industry.

2. Non-Conventional Applications (Remote/Third World)

a. Rural water pumps.

b. Third world power generation (indigenous fuels).

c. Arctic village power generation.

d. Power generation for food processing - Third World.

e. Power for remote mineral processing plants.

3. Agriculture

a. Tractors.

b. Farm machines (self-powered).

c. Logging machines.

d. Grain drying.

e. Lumber mills.

f. Water desalination plant power.

4. Mining

Underground Rock Mines

a. Load haul dumps (LHDs).

b. Digging equipment.

c. Locomotives.

Surface Mining

d. Small hauling equipment.

e. Large hauling equipment.

f. Small digging equipment.

g. Large digging equipment.

Underground Coal Mining

h. Auxiliary vehicles inside mines.

i. Continuous mining equipment.

j. Large-wall equipment.

k. LHDs.

5. Transportation

a. Passenger cars.

b. Highway trucks and buses.

c. Distribution van.

d. Mail car.

e. Long haul locomotives.

f. 'Switching locomotives.

g. Taxi.

h. Total energy system for pleasure boats.

i. Delivery trucks.

j. City buses.

k. Rail maintenance equipment.

1. Above ground rail rapid transit.

6. Construction

a. Construction vehicles.

b. Portable welders.

c. On-site power.

d. Portable air compressors.

e. Concrete pumps.

7. Industrial Applications

a. Indoor forklift trucks.

b. Mobile refrigeration equipment.

c. Natural gas pipeline compressors.

d. Pipeline auxiliary power.

e. Submersibles or crawlers for offshore oil and gas industry.

f. Undersea mining equipment.

g. Oil and gas field power.

h. Oil pipeline remote pump stations,

i. Coal slurry pipeline pumping.

8. Marine Applications (Commercial)

a. LNG/LPG tanker.

b. VLCCs.

c. Petroleum product tankers.

d. Methanol tankers.

e. Other merchant vessels.

f. Cruise ships.

g. Coastal and inland waterways diesel vessels,

h. Submersible tankers.

CONCLUSION

Developed in the mid-1960s and field-tested since the 1970s, the PAFCs are one of the most mature types of fuel cells and the first type to be commercially used. Phosphoric acid fuel cells (PAFCs) use phosphoric acid as an electrolyte and an anode and cathode made of a finely dispersed platinum catalyst on a carbon and silicon carbide structure. They have been typically used for stationary power generation in buildings, hotels, hospitals, and utilities in USA, Europe and Asia. The units have been technically successful and very reliable, with 40% plus efficiency levels and tens of thousands of operating hours. Water management in these fuel cells is easier than in PEMs, and they are more tolerant of impurities in hydrogen. However, the emission of phosphoric acid vapor is problematic and good ventilation is mandatory. PAFCs are less powerful than other fuel cells for the same weight and volume and require much more platinum than other fuel cells, which raises their cost.

A fuel cell is a device which reacts a fuel with an oxidant electrochemically, directly generating DC power. Several classes of fuel cells are being developed for application using oxygen from the air as the oxidant and a hydrogen-rich gas as the fuel. Among these is the phosphoric acid fuel cell. It takes its name, as do most other fuel cell systems, from its electrolyte: phosphoric acid.

The phosphoric acid fuel cell has several advantages over most conventionally used fuel-based systems for generating electricity or shaft power. Perhaps most important, it offers high fuel efficiency.

Even allowing for the inefficiency involved in processing a light hydrocarbon or alcohol fuel into a hydrogen-rich gas, it can offer efficiencies of power generation between 40 and 48%.

The phosphoric acid fuel cell is a very clean power source. It can be operated with essentially negligible emissions of carbon monoxide, oxides of nitrogen, unburned hydrocarbons, and sulfur dioxide, with no discharge of polluted water. In addition to being much cleaner than any currently commercial internal combustion engine it is also much quieter. Furthermore, being based on a continuously conducted electrochemical process, with moving parts limited to a few pumps and valves, it might be expected that technologically mature fuel cell systems would display higher reliability than internal combustion engines with all their moving parts.

Given these attractions, it is not surprising that the phosphoric acid fuel cell is being actively evaluated for several classes of applications. Most notable are large, grid connected electric utility applications, residential/commercial total energy system applications, and small military power sources. The U. S. Department of Energy and the National Aeronautics and Space Administration concluded that it would be advantageous to commission a multi-disciplinary study to identify and begin evaluation of additional classes of applications, with the hope of finding highly promising but heretofore overlooked uses for the phosphoric acid fuel cell. The study was meant to consider:

? Major market segments. An example would be the railroad locomotive application .

? Market entry application. Examples of these potentially high, value applications would include the mine locomotive and Arctic village systems.

? Specialty applications. An example of such an attractive but small market application would be the robotic submersible.

? Space applications

? Industrial cogeneration in the developed world;

? Opportunistic use of industrial waste hydrogen;

? Applications smaller than 10 kilowatts.

Merits & Shortcomings of Phosphoric Acid Cells

Five conclusions are developed on the comparative advantages and disadvantages of PAFC vis-a-vis conventional alternatives:

? In situations where it is feasible to provide the necessary power via a steam cycle, based on a solid fuel fired boiler, this is likely to be the preferred system. Under these circumstances, the modest energy efficiency advantage of the

fuel cell can seldom overcome the capability of the steam system to use a much less expensive fuel.

? In most other applications of concern in this study, the principal obstacle to fuel cell use is capital cost. PAFC capital costs are higher than those of Conventional systems, and must be reduced substantially for the fuel cell to be

competitive in most applications. Careful investigation can, however, identify applications which might be economically served by fuel cell systems whose capital costs do substantially exceed those of the conventionally used systems.

? Current phosphoric acid fuel cell systems probably cannot compete with internal combustion engines in applications where weight and volume are of substantial importance. This includes most light duty vehicles, whose overall fuel

efficiency will be reduced significantly by the fuel cell's added weight.

The fuel cell's key advantage over internal combustion engines is fuel efficiency. This can yield substantial savings in applications where high load factors are achieved for large numbers of operating hours per year.

? Industrial practice is to substitute batteries for internal combustion engines in applications where the fuel cell's cleanliness was expected to offer a large advantage over these engines. Essentially all of these applications are vehicular. The fuel cell can offer substantial advantages over the battery in weight, utility of use, and possibly even in capital costs in situations where the vehicle in question

must be capable of performance of lengthy missions without battery pack charge or recharge.

Specific Applications

The principal conclusion concerning use of PAFC in remote applications is that economics will probably be attractive in Arctic applications before they compete in other environments. Outside the Arctic PAFC must compete with photovoltaic power as well as diesel generators. This competitor, which eliminates the need for costly remote-delivered fuel altogether, probably has an overwhelming long-term advantage in this market, particularly as its capital costs decline.

Concerning vehicular applications, fuel cells appear to be well suited for use in traction vehicles in which fuel cell weight can be tolerated. Examples of such traction vehicles which have attractively high load factors and large numbers of annual operating hours include railroad locomotives and some underground mine locomotives (such as those used in tunnelling and some hardrock mines). Farm and construction traction vehicles are not good applications since they exhibit limited annual operating hours.

For use in vehicles which operate in environments where ventilation is limited, the fuel cell is likely to be in competition with lead acid batteries. Here, the fuel cell can enjoy substantial advantages in applications which daily consume energy equivalent to that required for full power operation for 8 to 12 hours. While most

indoor industrial utility vehicles and underground mining vehicles receive far less use than this, some few high duty vehicles can be identified in which PAFC looks quite attractive. The best example is probably the underground mine locomotive and some forklift truck applications.

For the robotic submersible PAFC offers substantial opportunities for improving utilization. This may prove of great importance to the full development of the robotic submersible, which will swim untethered for many hours, performing simple tasks. For the foreseeable future, however, the phosphoric acid fuel cell must be considered to offer an opportunity to the submersible designer,

rather than the submersible be considered an opportunity for PAFC.

The size of the PAFC market which would be created by full development of the submersible is insignificant in terms of producing capital cost reduction via learning curve effects.

Phosphoric Acid Fuel Cells have the following characteristics:

• Operate at ~40% efficiency
• Operate at up to ~85% efficiency with co-generation
• Operate at ambient pressure
• Operate at low temperature (~400 F)

These four characteristics make PAFC fuel cells one of the most exciting types of fuel cells out there for stationary co-generation power applications. The last characteristic specifically, the low temperature, is a very intriguing one. Because PAFCs operate at a low temperature, around 400 F, they essentially operate at lower cost because the stack does not need replacement as often as compared to higher temperature fuel cells like molten carbonate and solid oxide fuel cells. Carbon oxides will “poison” most types of fuel cells by dirtying the electrodes and diminishing the fuel cell’s functional efficiency over time. An advantage of PAFCs is that they can tolerate a concentration of carbon oxide “poisoning” of about 1.5%.

The stack is the heart of any fuel cell regardless of type. If you follow RMP, you know our mantra: always follow the money. If PAFCs can operate at a lower costs, that means they will make customers happier by providing more value. Listen to Jeff Chung, the President & CEO of Doosan Fuel Cell explain why his company is focused on PAFCs has extensive knowledge in all types of fuel cells. They are focusing on the PAFC because they think they can make this type of fuel cell deliver increased value creation for their customers because of lower operating costs. Always follow the money, it is what drives decisions in the real world and is the shortest route to the truth.

In 2013, CBS installed a Doosan PureCell Model 400 phosphoric acid fuel cell at their Los Angeles studio at 4024 Radford Avenue.?? This time-lapse video shows CBS removing their old power generators and installing a sustainable, reliable, clean, and low emissions PAFC for generating electricity and heat at upwards of 80% efficiency because of co-generation. Many companies operating large facilities in urban areas are switching to PAFCs because no other form of power generation can compete with a fuel cell’s power density; not solar, not wind, nor any other form of power generation. This is good news for America’s & the world’s most storm vulnerable cities.?? Hurricane Sandy in October of 2012 was a category 3 storm that hit New York City & New Jersey and?cut power to many people in and around major population centers.?? Fuel cells and a distributed grid can help stop large outages caused by future storms similar to Sandy because fuel cell systems like the ones discussed here have no transmission lines; electrical power and heat are?generated on site. The cost advantages and resiliency in life & death situations make fuel cells like PAFCs a very smart choice for America’s hospitals when the power has just gotta stay on 24/7. ?Even if fuel cells polluted our air & water like our current centralized power plants pollute, they would still make more sense economically than centralized power plants. Fuel cells, however, do not pollute and are therefore a win-win solution for energy production. Fuel cells provide clean energy on top of all their other fundamental economic advantages.

Frequently asked Questions:

What are the properties of phosphoric acid fuel cell?

PAFC power plants supply usable thermal energy at an efficiency of 37–41% HHV. A portion of the thermal energy can be supplied at temperatures of ～120 °C to ～150 °C; however, the majority of the thermal energy is supplied at ～65 °C. The PAFC has a power density of 1.7–1.9 KW m?2 of active cell area.

What are the major input parameters of any fuel cell?

The fuel cell operating temperature is considered a crucial parameter in a fuel cell operating system. The operating temperature influences the membrane conductivity, current density, synthesis of input gas streams, and water vapor pressure.

What are the important points on fuel cell?

Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes—a negative electrode (or anode) and a positive electrode (or cathode)—sandwiched around an electrolyte.

What is the efficiency of phosphoric acid fuel cell?

Phosphoric Acid Fuel Cell Efficiency of PAFC is ~ 35%–45%, which is higher than PEMFC, but lower than MCFC and SOFC.

What is a phosphoric acid fuel cell?

Phosphoric acid fuel cells (PAFC) operate at temperatures around 150 to 200 C (about 300 to 400 degrees F). As the name suggests, PAFCs use phosphoric acid as the electrolyte. Positively charged hydrogen ions migrate through the electrolyte from the anode to the cathode.

What are the three types of phosphoric acid?

phosphoric acid May be one of three types: orthophosphoric acid (H 3PO 4), metaphosphoric acid (HPO 3), or pyrophosphoric acid (H 4P 2O 7).

What are 3 interesting facts about phosphoric acid?

It serves as an acidic, fruitlike flavouring in food products. Pure phosphoric acid is a crystalline solid (melting point 42.35° C, or 108.2° F); in less concentrated form it is a colourless syrupy liquid. The crude acid is prepared from phosphate rock, while acid of higher purity is made from white phosphorus.

What are the uses and importance of phosphoric acid?

Uses. Phosphoric acid is a component of fertilizers (80% of total use), detergents, and many household cleaning products. Dilute solutions have a pleasing acid taste; thus, it's also used as a food additive, lending acidic properties to soft drinks and other prepared foods, and in water treatment products.

What is the advantage of phosphoric acid?

Phosphoric acid also occurs naturally in many fruits and their juices. Apart from use of phosphoric acid itself, the greatest consumption of phosphoric acid is in the manufacture of phosphate salts. Taking advantage of its ability to lower blood pH, phosphoric acid has been used therapeutically to treat lead poisoning.

Which material is suitable for phosphoric acid?

The recommended alloy for use with phosphoric acid are Incoloy 825, Inconel 625 and Hastelloy C276. There are limited applications of Nickel 200 in phosphoric acid. In pure and unaerated acid, the corrosion rates of Nickel 200 are nominal at all concentrations at normal temperatures.

Is phosphoric acid positive or negative catalyst?

negative catalyst

Phosphoric acid acts as a negative catalyst to decrease the rate of decomposition of hydrogen peroxide.

What are the different types of phosphoric acid?

Cyclic phosphoric acids

• Orthophosphoric acid. H 3PO 4
• Pyrophosphoric acid. H 4P 2O 7
• Tripolyphosphoric acid. H 5P 3O 10
• Tetrapolyphosphoric acid. H 6P 4O 13
• Trimetaphosphoric acid. H 3P 3O 9
• Phosphoric anhydride. P 4O 1What are the parameters of fuel cell design?

There are four design parameters to be optimized: electrolyte, anode, cathode thickness, and anode porosity.

What are the parameters that raise the efficiency of a fuel?

Combustion efficiency is affected by fuel type, bed temperature, gas velocity, and excess air levels. Combustion efficiency increases with fuel volatile matter content and bed temperature. Combustion efficiency decreases with increasing superficial gas velocity.

What is the pressure in a fuel cell?

Generally, if the operating pressure of PEM fuel cells is increased above 4 bar, the effect of the voltage increase is getting smaller due to mass transport issues. Therefore, the optimal PEM-fuel cell operating pressure lies typically between 3 and 4 bar

What are the factors affecting fuel cell performance?

The load current, temperature, relative humidity, membrane thickness, membrane-active area, electrode active area, corrosion, purity, pressure, and concentration of hydrogen fuel, maintenance of water inside the cell, pressure in the electrode particularly on both side of the membrane etc. are the factors.

What are the limitations of fuel cells?

Expensive to manufacture due the high cost of catalysts (platinum) Lack of infrastructure to support the distribution of hydrogen. A lot of the currently available fuel cell technology is in the prototype stage and not yet validated.

What is polarization in fuel cell?

Polarization is caused by chemical and physical factors associated with various elements in the fuel cell, such as temperature, pressure, gas composition, and fuel properties and reactant utilization. These factors limit the reaction processes when the current flows through.

What is the efficiency of a fuel cell?

A conventional combustion-based power plant typically generates electricity at efficiencies of 33 to 35%, while fuel cell systems can generate electricity at ef- ficiencies up to 60% (and even higher with cogeneration).

What are the two most common elements used in fuel cells?

Fuel cells have two main supplies. In order to make electricity, fuel cells will use an external source of fuel and oxygen. The source of fuel could be an element such as hydrogen. Fuel is electrochemically oxidised.

What is the current density of a fuel cell?

The limiting current density of the fuel cell is increased proportional to increase in hydrogen flow rate. The limiting current density for 0.07 lpm is 960 mA/cm2 which is comparably higher than the limiting current density of 915 mA/cm2 corresponding to 0.04 lpm.

How does a phosphoric acid fuel cell work?

Phosphoric acid fuel cell (PAFC) anodes accelerate the hydrogen oxidation reaction rate in phosphoric acid. The anode materials must be stable at high operating temperature in phosphoric acid. During operation, hydrogen starvation may cause reverse polarization and electrochemical corrosion of the anode material.

What is the basic principle of fuel cell?

Fuel cells require a continuous input of fuel and an oxidizing agent (generally oxygen) in order to sustain the reactions that generate the electricity. Therefore, these cells can constantly generate electricity until the supply of fuel and oxygen is cut off.

What are 3 advantages of fuel cells?

Hydrogen fuel cell technology offers the advantages of a clean and reliable alternative energy source to customers in a growing number of applications – electric vehicles, including forklifts, delivery vans, drones, and cars – primary and backup power for a variety of commercial, industrial, and residential buildings;

What are the main components of fuel cell and their function?

A fuel cell is composed of: Negative electrode or anode. Positive electrode or cathode. Intercalated electrolyte between each electrode of a material that facilitates the passage of ions (positively or negatively charged atoms), but not electrons (which are conducted to generate electricity).

What is a fuel cell used for?

• As a power generator in emergency situations when the main power supply fails at companies, hospitals, residential areas, etc. In these cases, hydrogen is used as a storage system to generate electricity and heat.
• In the field of modes of transportation, where the use of hydrogen through fuel cells for vehicles has been proposed as an alternative for electric cars in the future.

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Molten carbonate fuel cells (MCFCs)

High temperature cells

Molten Carbonate Fuel Cells (MCFCs) are another type of high temperature fuel cell. A molten mixture of salts: lithium, sodium, potassium carbonate is used as the electrolyte. These salts melt and conduct carbonate ions (CO32–) from the anode to the cathode when heated to about 600°C. Hydrocarbons have to be used as part of the fuel since the charge carriers in the electrolyte are carbonate ions. Hydrogen is also needed at the anode. It is gained by internal reforming of hydrocarbon based fuels. The electrodes should be resistant to poisoning by carbon. The high exhaust temperature makes cogeneration of electricity with turbines possible; hence the efficiency (60% without and 80% with hybrid technology) is relatively high compared to other fuel cell systems. MCFCs are mainly used for stationary power generation in the 50?kW to 5?MW range. Since it uses a liquid and high temperature electrolyte, it is rather unsuitable for mobile applications. The main problem with MCFC is the slow dissolution of the cathode in the electrolyte. Most of the research is therefore in the area of more durable materials and cathodes.

Historical summary

Both the solid oxide and the molten carbonate fuel cells are high temperature devices. ?Their development followed similar lines until the late 1950's. First, E. Baur and H. Preis experimented with solid oxide electrolytes in Switzerland. The technical problems they encountered were again tackled by the Russian scientist O.K. Davtyan without success though. In the late 1950's, Dutch scientists G.H.J. Broers and J.A.A. Ketelaar focused on molten carbonate salts as electrolyte. By 1960, they reported the first MCFC prototype. In the mid-1960's, the US Army’s Mobility Equipment Research and Development Center (MERDC) tested several MCFCs made by Texas Instruments ranging from 100 to 1000 Watts. Ishikawjima Heavy Industries showed in Japan in the early 1990s that a 1000 Watt MCFC power generator can operate for 10000 hours continuously. Other large power plants with outputs of up to 3 megawatts are already planned.

M-C Power's molten carbonate fuel cell power plant in San Diego, California, 1997. Smithsonian Institution, from the Science Service Historical Images Collection, courtesy of National Energy Technology Laboratory.

The MCFC has been under development for 15 years as a stationary electric power plant. Although when most problems with the Solid Oxide Fuel Cell are solved, work on the MCFC might be stopped.

Molten Carbonate Fuel Cell Electrolyte

Electrolyte

In most cases, the electrolyte of the MCFC is made of a lithium-potassium carbonate salt heated to 600-1000°C. At this temperature, the salt is in liquid phase and can conduct ions between the two electrodes. The typical mixture ratio of the electrolyte is 62?m/o Li2CO3 and 38?m/o K2CO3 (62/38 Li/K). This particular mixture of carbonate salts melts at 550°C and when it is mixed with lithium aluminate (LiAlO2 is a ceramic matrix retaining the molten salts, it can be used both as an ion-conducting electrolyte and gasketing for the fuel cell stack. Negative carbonate ions (CO32–) are responsible for conduction. As discussed above, the long term performance is an issue for MCFCs.

The following properties of the electrolyte have to be taken into account:

• Volatility of different alkali metal hydroxides generated in the moist cathode atmosphere
• Solubility of the cathode (NiO)
• Segregation of the electrolytes
• Both oxygen and carbon dioxide solubility in the electrolyte
• Oxygen reduction kinetics

Loss of electrolyte mainly occurs at the cathode via hydrolysis:

MeCO3 + H2O?? 2MeOH + CO2

Especially the electrolyte that has been used unaltered for a long time (since Ketelaar and Broers), Li2CO3/K2CO3 (38/62) eutectic, has a relatively high volatility. This causes the fuel cell to dry out.

The partial pressure due to MeOH varies with the square root of water vapour to carbon dioxide vapour pressures:

??(??????????)=????(??)????2??/??????2

Ki(T) is the equilibrium coefficient of the carbonate ion in the melt according to the equilibrium equation:

CO32– + H2O?? CO2 + 2OH–

The anode off-gas on the other hand can be mixed with air after combustion and reused in the cathode chamber with a sufficiently high air excess. Here oxygen and carbon dioxide are consumed in a ratio of 1:2 (molar) by the cathode process.

The change in composition of the electrolyte due to segregation and the volatility of certain species may result in a change in melting temperature. The electrolyte can solidify, causing the fuel cell to malfunction and sometimes allowing gases to break through. Carbonate ions from the electrolyte are used up in the reactions at the anode, so that we have to compensate for the loss by injecting carbon dioxide at the cathode.

Segregation occurs as the potassium concentration increases near the cathode. This leads to an increased cathode solubility and hence a decline of cell performance.

Recent studies show, that using Na instead of K can decrease the amount of segregation. In a molten binary salt with a common anion, segregation occurs due to the difference in mobilities of the cations. The heavier and bigger potassium and sodium cation is faster lithium in mixtures that have a higher potassium concentration that K2CO3=0.32 (Chemla effect), whereas below this isotachic concentration, lithium is faster.

The partial pressure of carbon dioxide has a much smaller effect on the current voltage curves than the partial pressure of oxygen. Mass transfer limitations are not observed for CO2 but for oxygen at low partial pressures. CO2 transport also hinders the cathode operation much less than oxygen transport since this gas has a much better solubility in carbonate melts than O.

Only the Li2CO3/Na2CO3 eutectic with much lower sodium vapour pressures could assure long term performance of fuel cells. We can conclude that the Li/Na electrolyte is more reliable and safer than Li/K. Since the ionic conductivity of Li/Na carbonate melts is higher than that of Li/K carbonate melts, Li/Na is preferred material as the electrolyte. But since the electrolyte is in the liquid phase, the fuel cell needs a more complex design, compared to other technologies using a solid electrolyte.

MOLTEN CARBONATE FUEL CELL ELECTROCHEMISTRY

Electrochemistry

External fuel processors are not needed for MCFCs since the fuels can be reformed internally at the high operating temperatures. Internal reforming includes converting methane and steam into a hydrogen rich gas.

CH4 + H2O?? CO + 3H2

At the anode, hydrogen reacts with the carbonate ions to produce water, electrons and carbon dioxide.

H2 + CO32– ??H2O + CO2 + 2e–

The electrons are conducted away by an external circuit to do useful work to the cathode. Oxygen from the air and carbon dioxide from the anode react at the cathode with electrons to form water and carbonate ions.

O2 + CO2 +2e– ??CO32–

The carbonate ions migrate through the electrolyte to the anode, and complete the electrical circuit. CO32– is used up at the anode; CO2 is needed at the cathode.

MOLTEN CARBONATE FUEL CELL ELECTRODES

?A significant advantage of the MCFC is that non-noble metals can be used as electrodes. At the high operating temperature, a Nickel anode and the Nickel oxide cathode is able to promote the electrochemical reaction. This means lower production costs compared to low temperature fuel cell, where the catalyst electrode is usually made of platinum. The Ni electrodes are less prone to CO poisoning, hence coal based fuel can be used, especially since internal reforming can take place.

Solubility of electrode in electrolyte

The main problem with the electrodes is their solubility in the electrolyte by Ostwald ripening, which is a dissolution/reprecipitation process. It decreases the internal surface of the porous nickel oxide cathode, causing it to deteriorate. The solubility of nickel oxide (cathode material) is dependant on the cathode potential and temperature. The solubility of Ni and NiO in Li/Na was found to be lower than in Li/K melts.

Although Li/Na melts have been found to have superior performance compared to Li/K melts, the lower oxygen solubility reduces the cathode performance on lean gas with a low oxygen partial pressure (below 0.1?bars).

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SOLID OXIDE FUEL CELLS (SOFCs)

High temperature cells

The high consumption of fossil fuels is increasing the concentration of CO2 in the atmosphere, which is considered to be the major cause of global warming and climate change and ultimately the environmental degradation. The utilization of these resource-limited fossil fuels into the clean and highly efficient form of energy has acquired significant attention in the development of advanced energy technologies with negligible environmental impact. In this regard, solid oxide fuel cells (SOFCs) offer the tremendous potential of delivering high electrical efficiency and substantial environmental benefits in terms of fuel flexibility, cleanest and highly efficient power generation . A SOFC produces electricity by an electrochemical combination of the fuel and oxidant through an ion-conducting solid ceramic which acts as an electrolyte and operates at high temperatures (~600-1000°C) . The high-temperature operation permits internal reforming, promotes rapid electro-catalysis, and results in high-quality heat as a by-product, which can be further utilized. The efficiency of the SOFC can reach up to 70% and a further 20% as a heat recovery . Despite the clear advantages, the commercialization of SOFC technology has not entirely succeeded due to this being a high-temperature operation, cost (especially the sealing and current collection materials), and materials degradation issues. Recently, attention has focused on reducing the operating temperature of the SOFCs and developing high-performing and stable devices.

Working principle of the SOFC

A SOFC unit comprises three basic components: porous fuel electrode (anode), porous air/oxygen electrode (cathode), and a dense solid oxide electrolyte sandwiched between these two electrodes as shown in Figure 1 . A metallic interconnect is required for the electron conduction and to connect the unit cells into stacks . Hydrocarbons or hydrogen-rich fuels are supplied to the anodic side of the SOFC, where they are oxidized and release electrons, a process known as fuel oxidation reaction (FOR) . The electrons produced during the oxidation of fuel are conducted by the current collector placed on the anode to an external circuit. The cathode is supplied with air or pure oxygen (O2) and converts O2 into O2-, a process known as oxygen reduction reaction (ORR). Since hydrogen (H2) gas is commonly used as a fuel on the anode side and the air is fed as an oxidant on the cathode side,; therefore, the associated chemical reactions at the anode and cathode are as follows:

Anode:H2→2H++2e? ?(1)

Cathode:1/2O2+2e?→O2? ?(2)

At the anode, steam forms, according to the following reaction:

2H++O2?→H2O2 (3)

The partial pressure of oxygen (PO2) or oxide ions concentration gradient or oxygen chemical potential across the electrolyte offers the driving force for above mentioned electrochemical processes. The theoretical reversible voltage (Eth) of the SOFC can be given by Nernst equation :

Eth=RT/nF ln PO2,cathode/Po2anode (4)

where R is the general gas constant, T is the temperature, n represents the number of electrons and F is the Faraday’s constant.

The actual SOFC voltage is always less than the theoretical Nernst value due to the polarization or electrochemical losses. It includes the charge transfer or activation polarization, Ohmic resistance, and concentration polarization. Activation polarization is countered during the charge transfer between electronic and ionic conductors. Nonetheless, when the current density is considerably low and can be approximated as :

ηact=(RT/nFi0)i (5)

Here, i0?? is the exchange current density and (RT/nFi0) refers to intrinsic charge transfer resistance limited to the specific interface of electrode and electrolyte . Ohmic polarization arises due to the resistance in the conduction of oxygen ions in a solid electrolyte and lastly, concentration polarization is because of the mass transport limitation in the cell. In other words, when reacting species are consuming at a fast rate or product evacuation is slow. These losses, however, cannot be completely mitigated but can be minimized with proper materials selection and optimized design and/or fabrication techniques.

Figure 1. Schematic representation of the working principle of the SOFC

Materials for the SOFC

The present section summarizes the different materials and general requirements for the major SOFC components such as the anode, cathode and electrolyte.

Anode materials (SOLID OXIDE FUEL CELLS)

The electrochemical oxidation of the fuel takes place at the anode, preferentially at an area of the surface where electrode, electrolyte, and pore/gas coexist simultaneously. This site is referred to as the triple-phase boundary (TPB). A material, to act as a potential anode must possess an electrical conductivity of more than 100 Scm?1 at the aforementioned SOFC operating temperatures . Therefore, nickel (Ni) cermet such as Ni-yttria stabilized zirconia (Ni-YSZ) is the promising anode material with numerous advantages: enhanced catalytic activity for H2 oxidation, high electronic conductivity (102–104 Scm?1 at 1000°C), and reasonable ionic conductivity. In a porous Ni-YSZ anode, the metal phase provides a catalytic activity and electronic conduction while the ceramic phase serves as the backbone and matches the thermal expansion coefficient with the electrolyte, thus extending the reaction zone . However, Ni cermet anode suffers from structural instability during the long-term operation. This is owing to the agglomeration of the Ni particles which reduces the TPB length responsible for the electrochemical reaction. Moreover, this anode material exhibits redox instability and coking problem when altering the fueling environment to hydrocarbons. Redox instability is caused by an interruption in fuel supply to the SOFC anode which leads to the oxidation of Ni to NiO. The oxidation of Ni can also occur if fuel utilization or partial pressure of H2O increases on the anode side and causes a volume expansion of 69.9% . This phenomenon results in stress development, an increase in the polarization resistance of the SOFC anode, and eventually mechanical failure. On the other hand, during the SOFC operation with hydrocarbon fuel, carbon particles settle on the Ni particles of Ni-YSZ anode and hinder the active electrochemical sites and cause performance degradation. This degradation process is termed coking.

Other anode materials such as Cu-Sm doped ceria (SDC) composite are given considerable attention,; however,it was concluded that Cu has less catalytic activity toward the hydrocarbon fuels. Furthermore, perovskite-based materials like La-doped SrTiO3 (LST) have been investigated in detail but exhibited relatively low electronic conductivity as compared to Ni cermet anodes. The research and development of the new anode materials are continuing.

Electrolyte materials (SOLID OXIDE FUEL CELLS)

An electrolyte is the key component of the SOFC which conducts the ions from the cathode to the anode and accomplishes the overall electrochemical reaction. The conduction of oxide ions takes place by an oxygen vacancy mechanism, which is usually a thermally activated process. The main requirements for the electrolyte are: sufficiently high ion conductivity (~0.1 Scm?1), low electronic transfer number (<10?3), chemically and thermodynamically stable over a SOFC operation temperature range, high mechanical strength, and compatible TEC with other component layers .

Yttria-stabilized zirconia (YSZ) is recognized as the most favorable electrolyte material for the SOFC due to the characteristics mentioned earlier. Among various electrolytes, 8?mol% YSZ (8YSZ) exhibits the highest oxide ion conductivity of 0.1 Scm?1 at 1000°C. Moreover, numerous other doped zirconia systems, scandia stabilized zirconia (SSZ) exhibits around 1.5 times higher ionic conductivity than the 8YSZ and has received considerable attention . Other materials, such as bismuth oxide (Bi2O3) exhibit significantly high ionic conductivity but suffers from structural instability upon cooling below 600°C. Ceria-based oxide ion conductors, including Gd-doped ceria (GDC) or SDC, are also potential candidates for electrolyte materials but they become partially reduced on the anode side of SOFC and hence cannot be used alone . Another possibility is to use Sr or Mg-doped LaGaO3 (LGSM) perovskites, which are currently the most promising materials . However, their long-term stability needs to be assessed, thus, YSZ or SSZ is the current choice of electrolyte material for the SOFC.

Cathode materials (SOLID OXIDE FUEL CELLS)

The most important requirements for the cathode are the electro-catalytic activity for O2 reduction and compatibility toward the electrolyte material, including TEC match and chemical inertness. Platinum was used as one of the earliest materials for oxygen electrodes in SOFCs. However, the high cost strongly limited its range of applications. A porous perovskite-based electronic conductor, lanthanum strontium manganite ((La, Sr) MnO3; LSM) was the most commonly used cathode material because of the superior chemical stability. However, LSM exhibits poor performance at reduced temperatures as it is primarily an electronic conductor with nearly zero ionic conductivity. Consequently, they only allow oxygen reduction reactions near TPBs. However, cobaltite perovskite-type oxide materials such as La0.6Sr0.4Co0.8Fe0.2O3?δ (LSCF) or Ba0.5Sr0.5Co0.2Fe0.8O3?δ (BSCF) are mixed ionic and electronic conductors (MIECs) and exhibit a significantly higher electrocatalytic activity for the O2 reduction in comparison to LSM-based cathode . The chemical formula of perovskite oxide is ABO3, where A and B represent two different types of cations. Characteristic A-site cations include alkaline earth ions and rare earth metal ions whereas B-site cations are transition metal ions . Numerous perovskite-like derivatives, such as Ruddlesden–Popper (RP) phases and double perovskite oxides are also extensively studied as potential cathode materials for SOFCs because of their structural features promoting fast electron transfer.

It is well-known that (La, Sr)(Co, M)O3 (i.e. M =?Fe, Mn, eetc. cobaltite-based perovskite electrode materials tend to react with YSZ electrolyte and results in the formation of insulating phases such as La2Zr2O7 (LZO) and/or SrZrO3 (SZO) . Therefore, GDC or SDC interlayers are practically being used in the SOFCs to avoid the undesirable interaction between the cathode and electrolyte .

Configurations of the SOFC (SOLID OXIDE FUEL CELLS)

Figure 2 shows various structural configurations of the SOFCs including anode, cathode, and electrolyte supported. However, SOFCs designs are mainly focused on a thin electrolyte film coated on a porous electrode or metallic substrate due to the preference of high-power density at a lower operating temperature. However, between electrode supported, anode supported type SOFCs are given high consideration due to the lower polarization and superior electrochemical performance. Electrolyte supported SOFCs exhibit low power density due to the higher Ohmic resistance of the stabilized zirconia electrolyte, and therefore are not widely used.

Figure 2. Various configurations of the SOFCs

Types of the SOFC (SOLID OXIDE FUEL CELLS)

From a geometrical point of view, the SOFC substrate can be mainly classified into tubular type, planar type, and single bbody-typegeometry. However, tubular and planar type SOFCs are widely studied. Figure 3(a) displays the schematic diagram of the very well-known tubular-type SOFC. In this type of SOFC, the substrate or support is generally made up of the porous electrode produced via the extrusion process. The electrolyte, reaction barrier layer, and the other electrode material are then coated onto the extruded support, simultaneously . Tubular type SOFCs have several distinct advantages: convenient stacking, resistance to thermal stress, and higher mechanical strength. However, tubular SOFCs have the disadvantages of less power density in comparison to planar type SOFCs and relatively high production costs.

Figure 3(b) displays the schematic design of the planar type SOFC. A planar type SOFC resembles the sandwich-type geometry where the electrolyte is sandwiched between two porous electrodes. The planar type SOFCs produce higher current density compared to tubular SOFCs and are relatively easy to fabricate. However, demerits include unstable sealing, resulting in fuel and oxidant leakage, poor thermal cycling stability, and relatively high production cost. Therefore, a geometrically modified SOFC design is in need which should exhibit a high-power density, thermal robustness, fast start-up and shutdown, easy high-temperature sealing, and interconnect fabrication.

Figure 3. Schematic diagrams of (a) tubular and (b) planar type geometries of the SOFCs.

Geometrically modified flat-tubular SOFC

Figure 4 shows the schematic design of the geometrically modified flat-tubular (FT) SOFC which incorporates the advantages of both planar and tubular type SOFC. The FT-SOFC is comprised of extruded anode-support having multiple cylindrical/semi-cyclical channels or ribs for the fuel flow as displayed in Figure 4(c). The anode support appears to be a flattened tube in a cross-sectional view. The ribs inside the anode support are electron conductive and provide a shorter current path in the current collection, in addition to the circumferential current flow. This attribute leads to reduced ohmic resistance and higher power density. The electrolyte, GDC interlayer, and cathode are coated onto the FT anode support. The dense interconnect is coated onto the opposite side of anode support for the current collection and electrical connection of unit cells in a stack as represented in Figure 4(b). Further, advantages of the FT-SOFC include easy gas-tight sealing, thermal robustness, and ease of fabrication in comparison to planar type SOFCs . Table 1 represents the quantitative comparison of advantages and disadvantages among planar, tubular, and FT-SOFCs

Table 1. The planar, tubular, and flat-tubular SOFCs, advantages and disadvantages

Table 1. The planar, tubular, and flat-tubular SOFCs, advantages and disadvantages

Property Planar Tubular Flat tubular

Power density High Low High

Start-up and shutdown Slow Fast Fast

Interconnect fabrication Easy Difficult Easy

Thermal robustness Low High High

High temperature sealing Difficult Easy Easy

The FT-SOFC design is developed by Siemens Westinghouse, USA by modifying the previous tubular-type SOFC design. Further development and modification are being carried out by Kyocera, Japan, and KIER, South Korea. The evolution from the tubular design to the flat-tubular was first shown by Hassmann to improve the low power density of the tubular type SOFCs, by shortening the current passage and thus, decreasing the Ohmic resistance.

Interconnection

For the interconnection, an inert and impervious material is needed. It should withstand both oxidising and reducing environments. Lanthanum chromite seems to have the necessary properties in systems operating at 1000?°C. Depending on doping, this material matches the thermal expansion coefficient of LSM. For lower temperatures, metallic based alloys can be used. Again, plasma spraying is the most economic method of applying the interconnect layer on the electrode. Although lanthanum provides cell life times of up to 70,000?h, it is not perfectly inert: it expands in the presence of hydrogen, causing cracking, especially at large planar stacks.

Section of SOFC stack with interconnect. From top to bottom: steel, Ni-mesh, cell, contact paste, interconnect steel

Large lanthanum chromite interconnects are made from fine powder, which is prepared as a mixture of the desired components: lanthanum, strontium and chromium nitrate. This mixture is reacted with glycene at high temperatures. This can be compacted to form plates or extruded to make tubes. It is difficult to sinter the powder to full density.

?System and outlook

The operating temperature of an SOFC is relatively high.

A typical SOFC power plant is fuelled with natural gas because of the lack of a hydrogen infrastructure. A plant must have three main components:

1. The preheater raises the temperature of the fuel and air to near the operating temperature. At the same time, the preaheater reforms the gas by steam reforming to hydrogen. Steam reforming constitutes of two steps:

Methane Reforming:?? CH4 + H2O → CO + 3H2 Water Gas Shift:??? ???? CO + H2O → CO2 + H2 Overall Reaction:??????? CH4 + H2O → CO2 + 4H2

2. The cell stack electrochemically oxidises the hydrogen stream, drawing oxide ions through the electrolyte from the air stream.

Electrochemical reaction: H2 + ?O2 → H2O

The schematic diagram above depicts a complete, 250 kW fuel cell system ?

3. The lower cycle utilises the exhaust energy. The exhaust gases are so hot that gas turbines can be driven to generate additional electrical energy and thus increasing the efficiency of the fuel cell system up to 80%.

SFC-200, a 125 kW SOFC cogeneration system (Source: Siemens Westinghouse)

Fuel

One of the great advantages of the SOFC is that it can use a big range of fuels, depending on the cathode composition. Due to the high operating temperature, internal reforming can take place at the anode, when steam is added to the fuel. The reaction of methane is as follows:

CH4 + H2O → CO + 3H2

Both hydrogen and carbon monoxide can react with the oxide ions. A shift reaction also occurs at the anode since the reaction of CO is slow, producing more hydrogen.

CO + H2O → CO2 + H2

The disadvantage of using hydrocarbon fuels is the possible formation of coke on the anode:

2CO → CO2 + C CH4→ 2H2 + C

As mentioned above, impurities, such as sulphur are also damaging to the SOFC. Only desulphurised natural gas can be used as fuel. Other additives (more than 100 different molecules are present in commercial gasoline) can have damaging effects on the nickel anode.

The activity of the nickel anode decreases due to sintering and coke formation when carbon containing fuels are used. The ceramic parts can easily break if vibrational forces are present. This is one reason, why SOFCs are best suited for stationary applications rather than mobile applications.

The ultimate goal is to build a decentralised network of medium sized power generating SOFCs that can supply a small community with electricity with a much higher reliability and minor consequences in case of failure compared to the current system of few but very large power plants.

The durability of the SOFC is mainly determined by the processes occurring during thermal cycles, oxidation-reduction cycles and the sulphur contamination (even at high temperatures, sulphur is absorbed by the anode).

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