Hydrogen evolution reaction

Hydrogen evolution reaction (HER) is a chemical reaction that yields H₂.^[1] 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.^[2] HER, however, can also be an unwelcome side reaction that competes with other reductions such as nitrogen fixation, or electrochemical reduction of carbon dioxide^[3] 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,^[4] 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^[5]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:^[6] ${\displaystyle {\ce {2H^+ + 2e^- -> H2}}}$ 

In neutral or alkaline conditions, the reaction follows the formula:^[6] ${\displaystyle {\ce {4H2O + 4e^- -> 2H2 + 4OH^-}}}$ 

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.^[7]

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.^[8] The alkalinity of the electrolyte in these processes enables the use of less expensive catalysts^[4] In PEM electrolyzers, the standard catalyst for HER is platinum supported on carbon, or Pt/C,^[8] 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.^[4]

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^[9] 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.^[10] 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

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. "Frequently Asked Questions (FAQs) - U.S. Energy Information Administration (EIA)". www.eia.gov. Retrieved 2023-11-21.
10. 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.