What makes H2 making by water electrolysis expensive?
Contents
Process
Kinetics and Thermodynamics
Side reactions and inefficiencies
This note highlights ‘Why and How ‘ high energy consumption in water electrolysis is a challenge
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The main cost issues in producing hydrogen gas from water electrolysis include the high energy consumption required for the electrolysis process, the high cost of electricity needed to power the electrolyzers, and the high initial capital costs associated with purchasing and installing the necessary equipment. Additionally, the cost of maintaining and operating the electrolysis system, as well as the cost of transporting and storing the hydrogen gas, can also be significant factors contributing to the overall cost of producing hydrogen from water electrolysis.
Electrolysis is an energy-intensive process for producing hydrogen. It takes?50 megawatt hours?(MWh) of electricity to produce one ton of hydrogen through electrolysis using PEM or alkaline electrolysis.?If the water needs to be desalinated, an additional 9 MWh of energy is required, bringing the total energy demand to 59 MWh.?
Efficiency
A theoretical electrolysis system that's 100% efficient would require 39.4 kWh of electricity to produce 1 kg of hydrogen.?However, commercial devices are less efficient, typically requiring around 50 kWh per kg
?The process of making H2 by water electrolysis is made up of two half-cell reactions:
Hydrogen evolution reaction (HER):?Water is reduced at the cathode to produce hydrogen gas
Oxygen evolution reaction (OER):?Water is oxidized at the anode to produce oxygen gas
Both thermal and electrical energy are used in water electrolysis because each has its own advantages and applications
Thermal energy
Thermal energy is cheaper than electricity, and high-temperature electrolysis (HTE) is more efficient economically than traditional room-temperature electrolysis.?HTE can be used when high-temperature heat is available as waste heat from other processes
Electrical energy
Electrolysis is technologically mature when applied at low temperatures, but it requires large quantities of electrical energy.?Electricity is the major component in the cost of hydrogen production via electrolysis.?
High-temperature electrolysis (HTE) is a method that uses both heat and electricity to generate hydrogen.?HTE can convert up to 50% of the initial heat energy into chemical energy, which is double the efficiency of low-temperature electrolysis.?
Thermal energy reduces the need for electricity:?At high temperatures, the voltage required for electrolysis is reduced, which means less electricity is needed.?This can make electrolysis cheaper because electricity is more expensive than heat
Thermal energy improves reaction kinetics:?Heat improves the reaction kinetics for splitting water molecules. Thermal energy can come from various sources:?Thermal energy can come from sources like nuclear reactors, solar concentration power plants, and geothermal energy.?
Process
Water electrolysis is a process in which an electric current is passed through water to split it into its constituent elements, hydrogen and oxygen. This process involves the use of an electrolyzer, which is a device that contains two electrodes (anode and cathode) separated by an electrolyte solution. When an electric current is applied to the electrolyzer, the water molecules at the cathode are reduced to form hydrogen gas (H2) and hydroxide ions (OH-). At the anode, water molecules are oxidized to form oxygen gas (O2) and protons (H+). The overall reaction for water electrolysis can be represented as: 2H2O(l) -> 2H2(g) + O2(g) Hydrogen gas collected at the cathode can be used as a clean and efficient energy source for various applications, such as fuel cells and transportation. Oxygen gas produced at the anode is typically released into the atmosphere.
Water
About 9000 litters of water is required to produce one ton H2 gas.
Water purification
The water used for electrolysis needs to be purified first.?Purifying water typically requires 2 tons of impure water to produce 1 ton of purified water.?
Desalination
If the water needs to be desalinated, it can add an additional 9 MWh of energy demand
Kinetics and thermodynamics
In water electrolysis for hydrogen gas production, kinetics and thermodynamics play significant roles in determining the efficiency and feasibility of the process.
Kinetics:
- Kinetics refers to the rate at which a reaction occurs. In the context of water electrolysis, the kinetics of the electrode reactions at the anode and cathode determine how quickly hydrogen and oxygen gas are produced.
- The kinetics of the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode are critical in determining the efficiency of the electrolysis process.
- Factors such as the electrode material, surface area, concentration of electrolyte, temperature, and applied current density can influence the kinetics of the electrode reactions. Optimizing these parameters can improve the efficiency of hydrogen gas production.
Thermodynamics:
- Thermodynamics refers to the study of energy and heat transfer in a system. In the context of water electrolysis, thermodynamics governs whether the overall reaction is spontaneous or not.
- The standard Gibbs free energy change (?G°) of the water splitting reaction determines the feasibility of the electrolysis process. For water electrolysis to occur spontaneously, the ?G° value must be negative.
- The cell voltage required for water electrolysis can be calculated using the Nernst equation, which considers the standard electrode potentials of the anode and cathode reactions. The cell voltage should be higher than the theoretical voltage required for the electrolysis reaction to overcome overpotential losses and ensure efficient hydrogen gas production.
Role of Nernst equation and Gibbs free energy
The Nernst equation and Gibbs free energy play crucial roles in understanding and predicting the behavior of chemical reactions, including water electrolysis for hydrogen gas production.
Nernst Equation:
- The Nernst equation relates the standard electrode potential of a half-reaction to the actual electrode potential under non-standard conditions. It allows for the calculation of the cell voltage (Ecell) of an electrochemical cell based on the concentrations of the reactants and products involved.
- In the context of water electrolysis, the Nernst equation helps determine the cell voltage required to drive the overall reaction forward. This voltage must be higher than the standard potential difference for the water splitting reaction to proceed.
- The Nernst equation is given by: Ecell = E°cell - (RT/nF) * ln(Q), where E°cell is the standard cell potential, R is the gas constant, T is the temperature, n is the number of moles of electrons transferred in the reaction, F is Faraday's constant, and Q is the reaction quotient.
Gibbs Free Energy (?G°):
- Gibbs free energy is a measure of the thermodynamic feasibility of a chemical reaction. It determines whether a reaction is spontaneous (ΔG < 0) or non-spontaneous (ΔG > 0) under standard conditions.
- In the context of water electrolysis, the standard Gibbs free energy change (?G°) for the overall reaction (2H2O → 2H2 + O2) must be negative for the process to be thermodynamically favorable.
- The relationship between Gibbs free energy and the equilibrium constant (K) of a reaction is described by the equation: ?G° = -RT * ln(K), where R is the gas constant, T is the temperature, and ln denotes the natural logarithm. This equation provides insights into the direction and extent of a chemical reaction at equilibrium.
- By understanding the Gibbs free energy change of the water splitting reaction and comparing it to the Nernst equation-derived cell voltage, researchers can assess the thermodynamic feasibility and efficiency of water electrolysis for hydrogen gas production.
Nernst equation and Gibbs free energy compliment each other in electrolysis
Nernst equation is not a concept that is used often in our routine work. This has become an important concept with lots of focus on H2 production by electrolysis.
How the Nernst equation is important can be explained by that water electrolysis is an energy-intensive process. One reason is the cell potential of water electrolysis is negative which makes it not a thermodynamically favorable process.
The Nernst equation and Gibbs free energy are related concepts that complement each other and provide different perspectives on electrochemical reactions, including those occurring in electrolysis.
The Nernst equation provides a way to calculate the cell potential (Ecell) of an electrochemical cell under non-standard conditions, taking into account the concentrations of the reactants and products involved in the cell reaction. It helps us understand how the cell potential changes with varying concentrations and temperature.
The equation is as follows:
Ecell = E°cell - (RT/nF) * ln(Q)
Where:
- Ecell is the cell potential under non-standard conditions
- E°cell is the standard cell potential
- R is the gas constant (8.314 J/mol·K)
- T is the temperature in Kelvin- n is the number of electrons transferred in the reaction
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- F is the Faraday constant (96485 C/mol)
- Q is the reaction quotient
On the other hand, the Gibbs free energy change (ΔG) provides information about the thermodynamic feasibility and spontaneity of a reaction. It indicates the maximum amount of work that can be obtained from a reaction under constant temperature and pressure conditions.
The relationship between Gibbs free energy and cell potential is given by the equation
:ΔG = -nFE
Where:- ΔG is the Gibbs free energy change for the reaction
- n is the number of electrons transferred in the reaction
- F is the Faraday constant
- E is the cell potential
The Nernst equation allows us to calculate the cell potential under non-standard conditions, accounting for changes in concentrations, temperature, and other factors.
Meanwhile, the Gibbs free energy equation helps us understand the thermodynamic feasibility of the reaction and the relationship between the Gibbs free energy change and cell potential.
Together, these two equations provide valuable insights into the energetics and feasibility of electrochemical reactions, including those involved in electrolysis. They help us optimize reaction conditions, predict the behavior of electrochemical cells, and design efficient and sustainable electrochemical processes for various applications.
Why water electrolysis is thermodynamically unfavorable
For the electrolysis of water can be written as: 2H2O (l) → 2H2 (g) + O2 (g) This equation shows that for every 2 moles of water that are electrolyzed, 1 mole of oxygen gas and 2 moles of hydrogen gas are produced.
In order for this reaction to occur, 2 moles of electrons are required to split the water molecules into their constituent elements.
Overall, the electrolysis of water can be represented as:
2H2O (l) → 2H2 (g) + O2 (g) + 4e-
This equation shows that 4 moles of electrons are required for the electrolysis of 2 moles of water.
The potential required for electrolysis of water is about 1.23V.
Actual is about 1.49V for various inefficiencies.
Since each mole of water requires two moles of electrons, and given that the Faraday constant F represents the charge of a mole of electrons (96485 C/mol), it follows that the minimum voltage necessary for electrolysis is about 1.23
The negative cell potential of water electrolysis makes it thermodynamically unfavorable, which impacts the cost of water electrolysis.
How and why?
Additional voltage required
The low concentration of ions and interfaces that electrons must cross require an extra voltage of about 0.6V at each electrode.
Minimum voltage
The minimum cell voltage required to start water electrolysis is 1.229 V, which results in a loss of at least 21% efficiency
What is cell potential?
Cell Potential is the difference in electrical potential between an electrochemical cell’s two electrodes. The differential in electron affinities between the two electrodes and the electrolytes’ reactivity are what cause the cell potential. Electrons will move from the metal with lower electron affinity to the metal with higher electron affinity when two different metals are in contact, creating an electrical potential difference.
What is positive and negative cell potential?
The cell potential is a measure of the ability of the cell to do work and is related to the Gibbs free energy change (ΔG) of the electrochemical reaction.
Positive and negative cell potential A positive cell potential indicates that the reaction is spontaneous and can generate electrical energy, while a negative cell potential indicates that the reaction is non-spontaneous and requires an external source of energy to occur.
Why does water have negative cell potential?
Water has a negative cell potential because of the presence of solutes in the water:
What is the solute potential of water?
When a solute is dissolved in water, the amount of free water molecules decreases, which reduces the kinetic energy of the molecules. This causes the solute potential to decrease and become negative.
Typical energy losses
The process of producing hydrogen is always associated with energy losses.?Electrolyzers need higher voltage than the standard potential to separate water into hydrogen and oxygen. It also involves energy losses due to side reactions, overpotential, and the bubble effect.?
Side reactions
Bubble effect
Lower voltage efficiency
Side reactions
In many cells, competing side reactions occur, resulting in less-than-ideal faradaic efficiency. Electrolyzers need higher voltage than the standard potential of hydrogen oxidation to separate water into hydrogen and oxygen.
Typical side reactions
Preferential reduction of anions
For example, in the presence of stainless steel, the preferential reduction of NO3– anions can prevent hydrogen from evolving.
Stainless steel is used as an anode in alkaline water electrolysis.Stainless steel can catalyze the reduction of nitrate anions (NO3–) to other N-containing species, such as NO2– and NH2OH.?The reduction of nitrate is autocatalytic, meaning that its reduction product (nitrous acid) can promote its reduction reaction.?
In water electrolysis, the side reactions of nitrate reduction include?the formation of nitrite and ammonium:
Bubble effect
Bubbles on the electrode surface and in the electrolyte cause high ohmic voltage drop and large reaction overpotential. The bubble effect in electrolysis is?the impact that bubbles have on the efficiency of the electrolysis process:
Bubble growth
Bubbles grow as dissolved hydrogen diffuses into the gas phase.?The growth of bubbles can be accelerated by high current densities, which deplete reactants rapidly.?
Bubble detachment
Bubbles eventually detach from the electrode and move away, which mixes species locally and decreases concentration gradients.
Bubble blocking
Bubbles can block ion conduction pathways, which increases the electrolyte's effective resistance.?
Bubble slugs
Bubbles can coalesce and form slugs between the electrodes and gas separators.?These slugs act as insulators, which impedes ion movement and increases resistance
Bubble coverage
Bubbles can cover the electrode surface, which acts as an electrical insulator and disrupts current distribution.?
Other effects of bubbles include:?Changing the local electrolyte concentration and pH, causing excessive overpotential, causing unstable currents, Blocking active sites on electrode surfaces, and Physically degrading catalysts?
Lower voltage efficiency
Membraneless electrolysis cells have lower voltage efficiency at high operating current densities.?This is due to the larger gap required between the two electrodes.