The coming of age for hydrogen fuel cells

Dr. Billy Wu - Senior Lecturer - Imperial College London

Professor Nigel Brandon - Dean of Engineering - Imperial College London

The need for net-zero emissions and emergent technologies

In order to address the global warming challenge, nations around the world have made legal commitments to achieve net-zero carbon emissions. In the UK, the goal is to reach this target by 2050 [1] with enablers such as banning new petrol and diesel vehicles [2], decarbonisation of electricity generation, and use of carbon capture and storage technologies. Within the area of transport, electrification of passenger vehicles has seen the most traction due to the rapidly maturing lithium-ion battery technology. Other transport vectors such as large freight vehicles, rail, marine and aerospace also need to decarbonise however there are a number of technical challenges.

The specific energy (Wh/kg) of the technology is one of these key metrics, with lithium-ion batteries currently at 250 Wh/kg. Whilst some comment on the fact that this is far short of the 12 kWh/kg of gasoline, the reality is that a whole system approach is needed for a fair comparison. For instance, this argument does not include the difference in the conversion efficiency, which for the internal combustion engine is far lower than an electric motor (20-40% vs. 90-95%, respectively). Furthermore, new chemistries such as solid-state batteries have the potential of increasing this, with demonstrators already achieving 500 Wh/kg. However, for heavier modes of transport, lithium-ion batteries do not scale well and thus necessitate investigation of alternative low-carbon technologies.

Hydrogen is one such technology which has the potential to fill this gap, with compressed hydrogen (690 bar-25°C) having a promising specific energy of 33 kWh/kg (lower heating value). However, when comparing the volumetric energy density, the challenges become evident; with hydrogen, petrol and lithium-ion batteries demonstrating values of 1,250 Wh/L, 9,500 Wh/L and 700 Wh/L, respectively. Furthermore, the energy density of hydrogen-based technologies also needs to consider the fuel cell stack, balance of plant components and hydrogen storage tanks. Yet, despite these challenges, there have been significant advances in hydrogen fuel cell technology, which when compounded with the advantageous refuelling times and more secure supply chains, means there has been a renewed industrial interest in the technology. 

The hydrogen fuel cell enabler

In order to unlock the potential of hydrogen, fuel cells are needed. These are electrochemical devices that convert a chemical fuel stock directly into electricity. There are a range of different types of fuel cells (see Table 1) but the proton exchange membrane fuel cell has seen the most interest in transport applications.

Table 1: Different types of fuel cell and their typical characteristics. Adapted from [3]

A proton exchange membrane fuel cell is a low temperature type of fuel cell in which hydrogen and oxygen, from the air, are combined to produce electricity with the only by-product being water. Figure 1 shows the cross section of a unit cell, which highlights the key components of this device. This includes the anode and cathode gas diffusion layers, catalyst layers and proton exchange membrane. Platinum is often used as a catalyst in fuel cells in order to increase the efficiency of the electrochemical reactions, and has, in the past, been quoted as the cost bottleneck. However, advances in optimising the catalyst layer has reduced the amount of platinum required to <1 mg/cm2 or <1 gPt/kW. The gas diffusion layers which sit on top of these platinum layers are often made of carbon papers which need simultaneously to provide electronic, gas and water transport pathways. These are reasonably well optimised, however heat, reactant and water management of the fuel cell remain important considerations which require fine control. 

Figure 1: (Left) Diagram showing the cross section of a single fuel cell or MEA. (Right) Diagram showing additional flow fields, current collectors, end plates and tie rods needed to provide mechanic support and reactant delivery to the MEA.

In earlier iterations of fuel cell designs, graphite flow plates were used to deliver the hydrogen and air to the reaction areas as shown in Figure 1 (right). Whilst these offer chemical resistance to corrosion, they were mechanically fragile and resulted in fuel cell stacks with poor volumetric power density. Furthermore, the compression of these layers necessitated heavy end plates which provided mechanical support. More recently, a number of automotive manufacturers have developed carbon-coated steel stamped flow field plates. This advance not only decreases the cell pitch (increasing volumetric energy density) but also significantly reduces cost and increases manufacturability. An example of this metal flow field design and the evolution of its optimisation is shown in Figure 2 for a Toyota Mirai. Here the improvement in power density from 1.4 kW/L (0.8 kW/kg) in 2008 to 3.1 kW/L (2.0 kW/kg) in 2014 can be seen. 

Figure 2: Fuel cell flow field designs for the Toyota Mirai [4].

Keeping the balance

A fuel cell stack requires a number of ancillary components, termed the balance of plant. These include: blowers/compressors to provide air, humidifiers to maintain appropriate hydration, hydrogen recirculation pumps and cooling systems to avoid overheating. An example of such a system is shown in Figure 3 which has seen considerable optimisation in recent years. For example, advances in self-humidifying membranes, which involve hydrophobic coatings to retain water better at the cell level, have reduced some of the air hydration requirements. Other innovations include the use of evaporation cooling systems to reduce the stack pitch and total mass of the system which is currently being promoted by UK stack manufacturer, Intelligent Energy. 

Figure 3: Fuel cell system balance of plant [5]

Scaling up fuel cell systems further into vehicle applications highlights some of the further challenges/opportunities. The Sankey diagrams in Figure 4, compares a pure fuel cell vehicle and a fuel cell-battery hybrid vehicle, and shows the sources of system inefficiencies. In recent years, improvements have been made in all areas, from reduction of electrochemical losses through better catalyst designs to reduction in parasitic power consumption through improved compressors and reduction in heat generation and thus cooling power needs. The regenerative power recovery potential in vehicles through braking events can be appreciable and given that conventional fuel cells only generate electricity means a hybrid design is advantageous. This both increases the overall efficiency and also reduces the overall stack size and thus system cost. 

Figure 4: Sankey diagram showing the energy flows in a pure fuel cell system for a bus [6](left) and a fuel cell -battery hybrid system [7](right).

Hydrogen and fuel cell applications beyond transport

Outside of transport, there is significant role for hydrogen and fuel cells to play. For instance, in domestic and industrial applications, the energy requirements for space and water heating, in addition to electrical demands, can be significant. Combined heat and power units, whereby the waste heat from the fuel cell is captured and used to supplement/replace the heating requirements, can significantly reduce the overall energy requirements and the carbon intensity of the electricity generation. When coupled together, the total electrical and thermal efficiency can reach as high as 90%, compared to 40-50% for pure electricity generation.

In the majority of cases, commercially deployed units fall into the micro fuel cell-combined heat and power category, with an electrical power rating of < 1 kW. These units predominantly use proton exchange membrane and solid oxide fuel cells, with the main markets currently being Japan and Korea for domestic applications. Given the development of proton exchange membrane fuel cells in automotive applications, this is the more technologically mature technology. However, due to the fuel flexibility and increased efficiency advantages of solid oxide fuel cells, there have been significant developments in this area.

Traditionally, solid oxide fuel cells operate at temperatures of 800°C because of the high ionic resistance (and thus low efficiency) of the materials used in their construct at lower temperatures. The electrolyte is one of the key components which govern the operating temperature. Here, yttria-stablised zirconia is one of the most commonly used materials however, the high operating temperature requirements, leads to poor thermal cycling characteristics. In more recent years, breakthroughs in the development of materials such as gadolinium doped ceria allowed for lower temperature operation (500 °C). Ceres Power, a UK solid oxide fuel cell developer, pioneered the use of gadolinium doped ceria, which due to its lower operating temperature, allowed for the use of perforated steel plates in its construction which reduces the cost of manufacturing and increases thermal cycling resistance. These low temperature solid oxide fuel cells from Ceres have since found application in combined heat and power units as well as in hybrid buses as range extenders.

Beyond combined heat and power applications, hydrogen is also a potential technology solution for long duration and large scale energy storage applications, in what is termed power-to-gas. Here excess electricity, ideally from renewables, is used in an electrolyser to produce hydrogen. This hydrogen can either be stored and used later through the reconversion back to electricity in a fuel cell, or it can be put onto the natural gas network, used directly or converted into products such as syngas, methane or liquefied petroleum gas. Notable examples of power-to-gas projects include the activities of UK based proton exchange membrane electrolyser producer, ITM Power, who are constructing MW-scale units to help the UK reach its net zero carbon emission targets. 

Prospects for the golden era of hydrogen

Given the advances and potential of hydrogen and fuel cells, many nations have been keen to support their deployment, with one notable announcement including China’s $1.2 billion fund for hydrogen production and consumption projects [8]. However, whilst this is a significant ramp up in hydrogen production, many of these projects derive their hydrogen from fossil fuel based sources, and ‘green’ hydrogen synthesis routes need support to be cost comparable. Yet, given the lack of an incumbent large fuel cell manufacturer which dominates the market in the same way as do tier 1 battery manufacturers such as LG Chem, Panasonic and Samsung, there is an urgency to fill that leading position. With that in mind, the hydrogen taskforce, which is a cross-industry coalition of leading organisation focused on providing evidence based support for the UK government, is calling for a ￡1 billion commitment in the UK, for hydrogen-based technologies. Whilst the number of fuel cell vehicles such as the Toyota Mirai (Figure 5) is increasing, challenges remain and government support will be important, however there is a huge opportunity for the new era of hydrogen to play a key part in our low carbon future.

Figure 5: Image of a Toyota Mirai with decals representing the location of key components

Seeing through the hype

With the increased interest in hydrogen based technologies it is also important that accurate sources of information can be found. For the avid reader a wealth of information can be found at the following sources:

• The H2FC Supergen hydrogen and fuel cell research hub - https://www.h2fcsupergen.com/
• The Future of Hydrogen – Seizing today’s opportunities – IEA - https://www.iea.org/reports/the-future-of-hydrogen
• Path to hydrogen competitiveness – A cost perspective – Hydrogen council - https://hydrogencouncil.com/wp-content/uploads/2020/01/Path-to-Hydrogen-Competitiveness_Full-Study-1.pdf
• Options for producing low-carbon hydrogen at scale – The Royal Society - https://royalsociety.org/~/media/policy/projects/hydrogen-production/energy-briefing-green-hydrogen.pdf

References

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[8]      China’s Hebei approves $1.2 bln hydrogen production and consumption projects | Nasdaq, (n.d.). https://www.nasdaq.com/articles/chinas-hebei-approves-%241.2-bln-hydrogen-production-and-consumption-projects-2020-04-03 (accessed April 5, 2020).