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 which rely on iron- and nickel-based catalysts. Commercial electrolyzers typically employ supported nickel-based catalysts.^[2]

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,^[3] 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^[4] 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:^[5]

2H⁺ + 2e⁻ → H₂

In neutral or alkaline conditions, the reaction follows the formula:^[5]

2H₂O + 2e⁻ → H₂ + 2OH⁻

Both of these mechanisms can be seen in industrial practices at the cathode 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.^[6]

Catalysts for HER

The HER process is more efficient in the presence of catalysts. Commercial alkaline electrolyzers use nickel-based catalysts at the cathode and steel at the anode.^[2] Proton exchange membrane based technology is an alternative to conventional high pressure electrolyzers.^[7] The alkalinity of the electrolyte in these processes enables the use of less expensive catalysts^[3] In PEM electrolyzers, the standard catalyst for HER is platinum supported on carbon, or Pt/C,^[7] 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.^[3] Anion exchange membrane (AEM) water electrolyzers are newly developed electrolyzers. In AEM electrolyzers, the standard catalyst for HER is still non-precious metal-based catalysts, such as nickel or iron.^[8]

Challenges

The high cost and energy input from water electrolysis poses a challenge to the large scale implementation of hydrogen power. The electrolysis of water is only practical where energy is cheap.^[2] 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.^[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.

HER as a competing reaction

HER can also be an unwelcome side reaction that could compete with other reductions such as the electrolyzed nitrogen fixation or electrochemical reduction of carbon dioxide. Neither of these processes commercial, however.^[10] HER does compete in chrome plating.