Hydrogen evolution reaction

Hydrogen evolution reaction (HER) is a chemical reaction that yields H₂.<ref>Zheng, Yao; Jiao, Yan; Vasileff, Anthony; Qiao, Shi-Zhang (2018). "The Hydrogen Evolution Reaction in Alkaline Solution: From Theory, Single Crystal Models, to Practical Electrocatalysts". Angewandte Chemie International Edition. 57 (26): 7568–7579. doi:10.1002/anie.201710556. PMID 29194903.</ref> The conversion of protons to H₂ requires reducing equivalents and usually a catalyst. In nature, HER is catalyzed by hydrogenase enzymes. Commercial electrolyzers typically employ platinum supported as the catalyst at the anode of the electrolyzer. HER is useful for producing hydrogen gas, providing a clean-burning fuel.<ref>Gray, Harry B. (2009). "Powering the planet with solar fuel". Nature Chemistry. 1 (1): 7. Bibcode:2009NatCh...1....7G. doi:10.1038/nchem.141. PMID 21378780.</ref> HER, however, can also be an unwelcome side reaction that competes with other reductions such as nitrogen fixation, or electrochemical reduction of carbon dioxide<ref>Sui, Yiming; Ji, Xiulei (2021). "Anticatalytic Strategies to Suppress Water Electrolysis in Aqueous Batteries". Chemical Reviews. 121 (11): 6654–6695. doi:10.1021/acs.chemrev.1c00191. PMID 33900728. S2CID 233409171.</ref> or chrome plating.

HER in electrolysis

HER is a key reaction which occurs in the electrolysis of water for the production of hydrogen for both industrial energy applications,<ref name=":0">Wang, Shan; Lu, Aolin; Zhong, Chuan-Jian (December 2021). "Hydrogen production from water electrolysis: role of catalysts". Nano Convergence. 8 (1): 4. Bibcode:2021NanoC...8....4W. doi:10.1186/s40580-021-00254-x. ISSN 2196-5404. PMC 7878665. PMID 33575919.</ref> as well as small-scale laboratory research. Due to the abundance of water on Earth, hydrogen production poses a potentially scalable process for fuel generation. This is an alternative to steam methane reforming<ref>Sun, Pingping; Young, Ben; Elgowainy, Amgad; Lu, Zifeng; Wang, Michael; Morelli, Ben; Hawkins, Troy (2019-06-18). "Criteria Air Pollutants and Greenhouse Gas Emissions from Hydrogen Production in U.S. Steam Methane Reforming Facilities". Environmental Science & Technology. 53 (12): 7103–7113. Bibcode:2019EnST...53.7103S. doi:10.1021/acs.est.8b06197. ISSN 0013-936X. OSTI 1546962. PMID 31039312. S2CID 141483589.</ref>for hydrogen production, which has significant greenhouse gas emissions, and as such scientists are looking to improve and scale up electrolysis processes that have fewer emissions.

Electrolysis Mechanism

In acidic conditions, the hydrogen evolution reaction follows the formula:<ref name=":1">Shih, Arthur J.; Monteiro, Mariana C. O.; Dattila, Federico; Pavesi, Davide; Philips, Matthew; da Silva, Alisson H. M.; Vos, Rafaël E.; Ojha, Kasinath; Park, Sunghak; van der Heijden, Onno; Marcandalli, Giulia; Goyal, Akansha; Villalba, Matias; Chen, Xiaoting; Gunasooriya, G. T. Kasun Kalhara (2022-10-27). "Water electrolysis". Nature Reviews Methods Primers. 2 (1): 1–19. doi:10.1038/s43586-022-00164-0. hdl:1887/3512135. ISSN 2662-8449. S2CID 253155456.</ref> <chem>2H^+ + 2e^- -> H2</chem>

In neutral or alkaline conditions, the reaction follows the formula:<ref name=":1" /> <chem>4H2O + 4e^- -> 2H2 + 4OH^-</chem>

Both of these mechanisms can be seen in industrial practices at the anode side of the electrolyzer where hydrogen evolution occurs. In acidic conditions, it is referred to as proton exchange membrane electrolysis or PEM, while in alkaline conditions it is referred to simply as alkaline electrolysis. Historically, alkaline electrolysis has been the dominant method of the two, though PEM has recently began to grow due to the higher current density that can be achieved in PEM electrolysis.<ref>Carmo, Marcelo; Fritz, David L.; Mergel, Jürgen; Stolten, Detlef (2013-04-22). "A comprehensive review on PEM water electrolysis". International Journal of Hydrogen Energy. 38 (12): 4901–4934. doi:10.1016/j.ijhydene.2013.01.151. ISSN 0360-3199.</ref>

Catalysts for HER

The HER process is driven forward by electricity and requires a large energy input without a highly efficient catalyst, which is a chemical which lowers the activation energy of a reaction without being consumed. In alkaline electrolyzers, Nickel and Iron based catalysts for HER are typically used at the anode.<ref name=":2">Guo, Yujing; Li, Gendi; Zhou, Junbo; Liu, Yong (2019-12-01). "Comparison between hydrogen production by alkaline water electrolysis and hydrogen production by PEM electrolysis". IOP Conference Series: Earth and Environmental Science. 371 (4): 042022. Bibcode:2019E&ES..371d2022G. doi:10.1088/1755-1315/371/4/042022. ISSN 1755-1307.</ref> The alkalinity of the electrolyte in these processes enables the use of less expensive catalysts<ref name=":0" /> In PEM electrolyzers, the standard catalyst for HER is platinum supported on carbon, or Pt/C,<ref name=":2" /> is used at the anode. The performance of a catalyst can be characterized by the level of adsorption of hydrogen into binding sites of the metal surface, as well as the overpotential of the reaction as current density increases.<ref name=":0" />

Challenges

The high cost and energy input from water electrolysis poses a challenge to the large scale implementation of hydrogen power. While alkaline electroysis is commonly used, its limited current density capacity requires large electrical input, which poses both a cost and environmental concern due to the high carbon content of electricity in the many countries, including the United States<ref>"Frequently Asked Questions (FAQs) - U.S. Energy Information Administration (EIA)". www.eia.gov. Retrieved 2023-11-21.</ref> The electrocatalysts used for electrolysis of PEM electrolyzers currently account for about 5% of the total process cost, however, as this process is scaled up, it is predicted that catalysts costs will rise due to scarcity and become a huge factor in the cost of producing hydrogen.<ref>Liu, Lifeng (2021-12-01). "Platinum group metal free nano-catalysts for proton exchange membrane water electrolysis". Current Opinion in Chemical Engineering. 34: 100743. doi:10.1016/j.coche.2021.100743. ISSN 2211-3398.</ref> As such, low-cost, high-efficiency, and scalable alternative materials for the HER catalysts in PEM electrolyzers are a point of research interest for scientists.

References

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