# Hydrogen evolution reaction

Hydrogen evolution reaction (HER) is a chemical reaction that yields H$_2$. The conversion of protons to H$_2$ requires reducing equivalents and usually a catalyst. In nature, HER is catalyzed by hydrogenase enzymes. Commercial electrolyzers typically employ supported platinum as the catalyst at the anode of the electrolyzer. HER is useful for producing hydrogen gas, providing a clean-burning fuel. HER, however, can also be an unwelcome side reaction that competes with other reductions such as nitrogen fixation, or electrochemical reduction of carbon dioxide 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, 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 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:
    2 H$^+$ + 2 e$^−$ → H$_2$
In neutral or alkaline conditions, the reaction follows the formula:
    4 H$_2$O + 4 e$^−$ → 2 H$_2$ + 4 HO$^−$
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.

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

### 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 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. 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.