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| Fig. 1: PEM Electrolysis. (Image Source: D. LaJoie) |
Decarbonizing the planet is a major objective set to mitigate the impacts of climate change. Accordingly, there has been a growing interest in recent years towards the large-scale implementation of renewable energy-based power plants, along with other industrial and transportation applications, driven by the increased focus on the production of green hydrogen via electrolysis. [1] The majority (98%) of hydrogen is currently produced from carbon-intensive energy sources, primarily through steam methane reforming (SMR) (76%) and coal gasification (22%), also known as gray and black hydrogen, respectively. However, hydrogen is only environmentally sustainable when generated through water electrolysis using photovoltaic (PV), wind, water, or other renewable energy sources and is referred to as green hydrogen. Currently, a mere 2% of global hydrogen production is produced in this manner. [2]
Electrolysis is the process by which water is split into hydrogen and oxygen via electricity applied to an electrolyzer. The most desirable form of this technology is known as proton exchange membrane (PEM) water electrolysis. It is praised for its high-purity hydrogen production and seamless alignment with the intermittency of renewable energies. More specifically, it has garnered significant interest over the past decades from its high current density, enhanced energy efficiency, compact mass-volume profile, and ease of handling. [3] In the process of PEM water electrolysis, water undergoes electrochemical splitting into hydrogen and oxygen, occurring at distinct electrodes, the cathode for hydrogen and the anode for oxygen. The PEM water electrolysis system involves the pumping of water to the anode, where it is divided into oxygen, protons, and electrons. These protons traverse through a proton-conducting membrane to reach the cathode. Simultaneously, electrons exit the anode through the external power circuit, generating the driving force (cell voltage) for the reaction. Upon reaching the cathode, protons and electrons recombine, resulting in the production of hydrogen. [4] Other examples of this technology include alkaline water electrolysis (AWE), solid oxide electrolysis (SOE), and microbial electrolysis. [3]
Despite the numerous benefits and promising outlook for PEM electrolyzers, their practical industrial implementation faces challenges due to the excessively high cost and severe degradation of electrocatalysts in acidic environments. This is particularly notable for OER catalysts exposed to both acidic and strongly oxidative conditions. Additionally, the overall operating efficiency of PEM electrolyzers, due to the required extra voltage (overpotential) needed to jump-start the reactions, is typically around 67-82%. [3] These required overpotentials come as a direct result of the interfacial resistance between the electrode and membrane in a given electrolyzer, with greater resistance leading to larger overpotentials and lower efficiencies. [5] To overcome these issues, significant research endeavors have focused on exploring noble-metal-free catalysts, enhancing the intrinsic catalytic performance and durability of catalysts in acidic electrolytes, decreasing electrolyzer interfacial resistance, and optimizing overall efficiency. Of note, one study was able to yield a cell electrolysis efficiency of 94.4% and a very low cell voltage of 1.567 V to reach the current density of 1 A cm-2, showing promise of what is to come in the future. [3]
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| Fig. 2: Green Hydrogen Production. (Image Source: D. LaJoie) |
The whole concept behind prioritizing hydrogen production is to reduce greenhouse gas emissions in the energy, transport, and chemical industries. SMR is the most frequently applied process to produce hydrogen today. Because natural gas steam reforming is entirely based on fossil resources, it produces substantial process-related CO2 emissions (530 Mt/a) and makes a very significant contribution to climate change (11.956 CO2-eq/kg H2). [6,7] Coal gasification, on the other hand, is the production method with the highest carbon footprint, (19.37-22.87 CO2-eq/kg H2) nearly double that of SMR. [8,9] Other studies found even higher emissions for coal gasification (28.6 CO2-eq/kg H2). [9] However, when CO2 capture and storage technology (CCS) was implemented, the carbon footprint of coal gasification was shown to decrease by 52.34-74.59%. [8] The lowest value of global warming potential comes from the wind-based electrolysis (0.0325 CO2-eq/kg H2), followed by solar based electrolysis (0.37 CO2-eq/kg H2). [7] Analyzing the data, it is obvious that green hydrogen is significantly more environmentally friendly and has a much smaller carbon footprint than other hydrogen evolution methods.
Looking at hydrogen production from an economic perspective is a whole different story. Depending on the hydrogen production method and kind of energy used, final hydrogen costs could vary significantly. It has been found that the production of grey hydrogen is generally associated with the lowest costs, coming in between €0.8-2.1 ($0.86-$2.27) per kg of hydrogen. In regions with low natural gas prices, however, grey hydrogen can be produced as cheap as €0.8 ($0.86) per kg of hydrogen. Although the investment costs associated with coal gasification are higher than that of SMR, fuel input for coal gasification is cheaper than the natural gas used in SMR, with an overall production cost range of approximately €1.2-2.0 ($1.30-$2.16) per kg of hydrogen. [9] Another economic analysis found the cost of coal gasification to be about $0.90-$1.46 per kg of hydrogen. With the addition of CCS to the coal gasification process, the total cost significantly increased to around $1.44-$2.11 per kg of hydrogen. [8] Lastly, coinciding with the aforementioned challenges and inefficiencies associated with PEM electrolysis, green hydrogen was found to be by far the most expensive method of hydrogen production, ranging mostly between €2.2-8.2 ($2.38-$8.85) per kg of hydrogen. [9]
Water electrolysis, when paired with renewable energy sources, is an alternative method of producing hydrogen that offers multiple environmental advantages over other conventional methods. It boasts a significantly reduced amount of greenhouse gas emission and total carbon footprint. However, significantly increased attention and efforts need to be directed towards improving the efficiency of producing green hydrogen via electrolysis. Henceforth, improving the cost effectiveness of making hydrogen in this manner is vital for the decarbonization and overall sustainability of the world.
© Dominic LaJoie. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. S. Kumar amd H. Lim, "An Overview of Water Electrolysis Technologies for Green Hydrogen Production," Energy Rep. 8, 13793 (2022).
[2] T. Terlouw et al., "Large-Scale Hydrogen Production via Water Electrolysis: A Techno-Economic and Environmental Assessment," Energy Environ. Sci. 15, 3583 (2022).
[3] T. Wang, X. Cao, and L. Jiao, "PEM Water Electrolysis for Hydrogen Production: Fundamentals, Advances, and Prospects," Carbon Neutrality 1, 21 (2022).
[4] S. S. Kumar and V. Himabindu, "Hydrogen Production by PEM Water Electrolysis a Review," Mater. Sci. Energy Technol. 2, 442 (2019).
[5] Han, Bo, et al. Electrochemical Performance Modeling of a Proton Exchange Membrane Electrolyzer Cell for Hydrogen Energy. International Journal of Hydrogen Energy, vol. 40, no. 22, June 2015, pp. 700616. https://doi.org/10.1016/j.ijhydene.2015.03.164.
[6] M. Hermesmann and T. Müller, "Green, Turquoise, Blue, or Grey? Environmentally Friendly Hydrogen Production in Transforming Energy Systems," Prog. Energy Combust. Sci. 90, 100996 (2022).
[7] F. Suleman, I. Dincer and M. Agelin-Chaab, "Environmental Impact Assessment and Comparison of Some Hydrogen Production Options," Int. J. Hydrog. Energy 40, 6976 (2015).
[8] J. Li et al., "The Carbon Footprint and Cost of Coal-Based Hydrogen Production With and Without Carbon Capture and Storage Technology in China," J. Clean. Prod. 362, 132514 (2022).
[9] A. Ajanović, M. Sayer, and R. Haas, "The Economics and the Environmental Benignity of Different Colors of Hydrogen," Int. J. Hydrog. Energy 47, 24136 (2022).