How Energy Efficient Is Electrolysis?
Electrolysis is the process of using electricity to split water into hydrogen and oxygen. It involves passing an electric current through water to separate hydrogen gas (H2) from oxygen gas (O2). This process was first discovered in 1800 by William Nicholson and Anthony Carlisle. Today, electrolysis has several important industrial and commercial applications.
Electrolysis is used for electroplating, producing certain metals like aluminum and sodium, manufacturing chemicals like chlorine and sodium hydroxide, and generating hydrogen gas. Recently, interest in electrolysis for producing “green hydrogen” from renewable electricity has grown due to hydrogen’s potential as an emissions-free fuel. Overall, electrolysis allows elements to be separated in their pure form through electrical energy.
In this article, we will examine the energy efficiency of electrolysis. This refers to how much of the electrical energy put into the electrolytic cell is converted into stored chemical energy in the hydrogen and oxygen gases produced. We will look at how the process works, key efficiency factors, ways to improve efficiency, and the economic and environmental implications. Understanding electrolysis efficiency is important for evaluating its viability in applications like renewable hydrogen production.
How Electrolysis Works
Electrolysis is an electrochemical process that uses an electrical current to drive an otherwise nonspontaneous chemical reaction. It involves passing an electric current through an ionic substance that is either molten or dissolved in an aqueous solution. This causes a chemical reaction and the separation of materials (1).
In the electrolysis of water, the electric current provides the energy needed to split water molecules into hydrogen and oxygen. Water naturally dissociates into hydrogen (H+) and hydroxide (OH-) ions. When an electric voltage is applied via two electrodes immersed in the water, the positively charged hydrogen ions migrate to the negatively charged cathode, while the negatively charged hydroxide ions migrate to the positively charged anode. At the cathode, each hydrogen ion gains an electron from the electrode to form a hydrogen atom. The hydrogen atoms then bond together to form hydrogen gas. At the anode, water molecules react with the hydroxide ions, losing electrons to form molecular oxygen gas and hydrogen ions (2).
The half reactions occurring during the electrolysis of water are:
Anode (oxidation): 4OH- → O2 + 2H2O + 4e-
Cathode (reduction): 4H+ + 4e- → 2H2
Overall reaction: 2H2O → 2H2 + O2
So by providing sufficient electrical energy, the electrolysis process splits water into its constituent elements, hydrogen and oxygen (3). The amount of hydrogen and oxygen gases liberated is in a 2:1 ratio.
(1) https://adhetron.com/what-is-electrolysis/
(2) https://www.youtube.com/watch?v=8KNWMxQDIow
Energy Input Required
Electrolysis requires an electrical energy input to split water into hydrogen and oxygen. The reaction occurs when an electric current passes between two electrodes submerged in water. This provides the electrical potential energy needed to separate the chemical bonds in water molecules. The amount of electrical energy consumed is directly related to the voltage applied between the electrodes.
According to the Physics Stack Exchange, the theoretical minimum potential needed for water electrolysis is 1.23V. However, in practice a higher voltage is required – typically between 1.8 to 2.0V – to overcome various inefficiencies in the system.[1] The voltage applied determines the electrical energy input per mole of hydrogen produced. With higher applied voltage, more electrical energy is consumed by the electrolysis process.
The relationship can be calculated using Faraday’s laws of electrolysis. The energy input (in kJ/mol) is equal to the applied voltage (in V) multiplied by Faraday’s constant (96485 C/mol). So at the theoretical 1.23V, the minimum electrical energy input is 152 kJ/mol of hydrogen. In practice, with higher voltages around 1.8-2.0V, the actual energy input is closer to 180-200 kJ/mol.[2]
So in summary, the amount of electrical energy consumed in electrolysis depends directly on the cell voltage required to drive the chemical reaction. Minimizing this voltage is key to improving energy efficiency.
[1] https://physics.stackexchange.com/questions/24624/water-electrolysis-calculations
[2] https://www.quora.com/When-comparing-the-energy-required-for-electrolysis-splitting-of-H2O-then-combusting-to-result-in-water-and-energy-I-found-237-Input-was-the-minimum-to-split-H2O-and-286-being-output-KJ-This-is-per-mole-Is-this-true
Theoretical Minimum Energy
The theoretical minimum voltage required for the electrolysis of water is 1.23V as calculated from thermodynamic data [1]. This is the ideal reversible voltage calculated from the Gibbs free energy of the reaction. Using Faraday’s laws of electrolysis, this minimum voltage translates to a theoretical minimum energy input of 237 kJ per mole of water split into hydrogen and oxygen [2].
However, in real-world electrolysis, the actual voltage applied needs to be higher than the theoretical 1.23V. This is because of various efficiency losses that occur during the process. Overpotentials at both electrodes, resistance losses in the electrolyte, and back reactions mean extra energy needs to be supplied. Typical electrolysis systems operate at 1.8-2.0V, significantly above the theoretical minimum [3]. As a result, real-world electrolyzers consume more electrical energy than the thermodynamic minimum.
[1] https://en.wikipedia.org/wiki/Electrolysis_of_water
[2] https://www.nrel.gov/docs/fy10osti/47302.pdf
[3] https://www.quora.com/How-many-watts-are-required-to-perform-an-electrolysis-on-a-gallon-of-water-How-much-oxygen-would-be-produced-by-it
Efficiency Losses
Electrolyzers have lower efficiencies than the theoretical maximum due to various sources of losses. A major source of efficiency loss is overvoltage at the electrodes, particularly at the anode where oxygen evolution occurs. According to a study by the U.S. Department of Energy, the majority of electrolysis efficiency losses arise from the oxygen evolution overpotential, which is needed to drive the sluggish oxygen evolution reaction [1].
Other factors that contribute to efficiency losses include system design and operating temperature. Higher temperatures allow for improved reaction kinetics and conductivity, reducing overpotentials. However, this requires additional energy input for heating and advanced materials that withstand high temperatures [2]. System components like power electronics, gas separators and compressors also dissipate energy.
Typical commercial water electrolyzers operate in the range of 50-80% efficiency. In comparison, the theoretical maximum efficiency is 83% based on the higher heating value of hydrogen. Reaching higher efficiencies requires research and development into advanced electrodes, membranes and system designs.
Improving Efficiency
There are several ways to improve the efficiency of electrolysis. One method is reducing the overvoltage, which is the extra voltage above the theoretical 1.23V required to split water. This overvoltage represents an efficiency loss, so reducing it through improved electrolyzer design and new catalysts can boost efficiency [1]. For example, using nickel-iron hydroxides as cathode catalysts can lower overvoltage compared to platinum [2].
Overall system optimization is another approach. Factors like electrolyte temperature, flow rates, cell configuration, and current density affect efficiency. Optimizing these parameters can minimize losses and achieve efficiencies over 90% [1]. New cell designs like zero-gap electrolyzers also aim to reduce resistances and boost efficiency.
Lastly, new materials like solid oxide electrolysis cells operate at high temperatures (500-850°C) to improve kinetics and reach theoretical efficiencies above 95%. However, high temperature operation requires more robust materials and can offset efficiency gains [1].
Applications and Efficiency
The efficiency of electrolysis can vary significantly depending on the application. Two major industrial applications are hydrogen production and the chloralkali process for chlorine and sodium hydroxide production.
For hydrogen production, conventional alkaline electrolysis has efficiencies around 65-82% according to https://newatlas.com/energy/hysata-efficient-hydrogen-electrolysis/. Proton exchange membrane (PEM) electrolysis can achieve slightly higher efficiencies of 70-85% according to https://assets.siemens-energy.com/siemens/assets/api/uuid:a33a8c39-b694-4d91-a0b5-4d8c9464e96c/efficiency-white-paper.pdf.
For the chloralkali process, the efficiency is typically around 65-70% according to https://en.wikipedia.org/wiki/Electrolysis_of_water. The chloralkali process requires less electrical energy input than hydrogen production, but also achieves a lower efficiency.
In summary, electrolysis efficiency can range from 65-85% depending on the application, with hydrogen production generally requiring more energy input but able to achieve higher maximum efficiencies compared to the chloralkali process.
Economic Considerations
Electrolysis efficiency has a major impact on the operating costs and economic viability of hydrogen production. The electricity input accounts for around 80% of the total operational expenses for electrolysis plants (https://www.iea.org/reports/electrolysers). Therefore, improving efficiency lowers the electricity consumption and production costs per unit of hydrogen.
According to one analysis, increasing the electrolyzer system efficiency from 65% to 75% reduces the modeled hydrogen production cost by 15% (https://www.sciencedirect.com/science/article/pii/S0360319918305741). The study found that at scale, hydrogen from electrolysis could become cost-competitive with $2/kg H2 when combining 75% efficiency, low electricity prices below 2¢/kWh, and high utilization rates above 90%.
More efficient electrolyzers also improve capital costs. Since less capacity is needed to produce the same hydrogen output, the electrolyzer stack and balance of plant can be smaller. This reduces upfront investments in the electrolyzer system. However, efficiency is not the only factor determining competitiveness. The hydrogen market value, electricity prices, equipment costs, and financing rates all play major roles.
Environmental Impact
The environmental impact of electrolysis is closely tied to its energy efficiency. Electrolysis requires electricity to split water molecules into hydrogen and oxygen. This electricity often comes from fossil fuel power plants which generate greenhouse gas emissions. More efficient electrolysis reduces the amount of electricity required, thereby lowering emissions associated with hydrogen production (U.S. Department of Energy, n.d.).
For example, proton exchange membrane (PEM) electrolysis can achieve efficiencies over 70%, compared to only 60-70% for alkaline electrolysis. This reduced energy demand translates into lower carbon dioxide emissions per unit of hydrogen produced (Ullmann et al., 2022). Additionally, if the electricity comes from renewable sources like solar or wind, electrolysis can produce hydrogen with zero greenhouse gas emissions.
More efficient electrolysis technologies are enabling the transition to green hydrogen and decarbonization of industrial processes that rely on hydrogen. By maximizing efficiency, electrolysis can play a key role in reducing emissions and mitigating climate change.
Conclusion
To summarize, electrolysis technology efficiency has steadily improved over time as researchers optimize electrolyzer design and materials. However, at present real-world systems operate at 65-82% efficiency, with losses occurring during electricity transmission, the electrolysis reaction itself, and hydrogen processing. The theoretical minimum energy required by thermodynamics is 39 kWh/kg, but the most efficient lab systems today can reach ~50 kWh/kg. Significant opportunities remain to enhance efficiency through improved membranes, catalysts, heat recovery, and hybrid PV-electrolyzer systems. Though electrolysis is currently more expensive than steam methane reforming of natural gas, costs are declining and green hydrogen is projected to achieve parity by 2030. With continued innovation and scale-up, electrolysis efficiency can progressively advance to make green hydrogen more competitive for decarbonizing transportation, industry, electricity generation, and heating.
