March 20, 2023

The contribution of water molecules to the hydrogen evolution reaction

Coupling renewable energy sources for hydrogen production with water electrolysis is an effective way to develop new energy use and solve the energy crisis. It is important to fully understand the mechanism of the cathodic hydrogen evolution reaction (HER). Compared to the fast kinetics under acidic conditions, the kinetics under alkaline conditions are two orders of magnitude slower, indicating that the reaction mechanism becomes more complicated than that under acidic conditions. Therefore, a descriptor-based in-depth understanding of the HER mechanism under alkaline conditions is of practical importance for improving HER activity.

Researchers at Chongqing University stated that “water molecules act as solvents in acid HER catalysis, while water molecules play a dual role in alkaline HER catalysis, that is, reactant and solvent.” Based on their research and knowledge in the field of HER, the researchers focused on the contribution of water molecules to HER, summarized the development of the HER activity descriptor over the past decade, and provided their understanding of how water molecules affect the kinetics and dynamics of HER. For example, the commonly used activity descriptor, the hydrogen bond energy, can indicate the alkaline HER activity after being corrected by the water adsorption energy. The water dissociation involved the hydroxide bond and OH and cation adsorption were identified as descriptors for HER. In addition, interfacial water is highlighted as a critical factor affecting pH-dependent HER activity and can be purposefully enriched and reoriented to dramatically improve HER kinetics.

The review was recently published in SCIENCE CHINA Chemistry. The work is led by New Energy Material Chemistry and Chemical Engineering group leader Zidong Wei. Chao Cheng and Mingming Deng are the first authors. The team has spent years researching the underlying mechanism of electrocatalysis, designing new electrocatalysts with high efficiency, thereby improving the conversion efficiency between matter and energy in the new energy field, including water electrolysis, fuel cells, etc. al.

Water molecule as reactant in HER

The researchers said: “Water molecules are the few molecules that can enter the depth of the electrode/electrolyte interface due to a strong interaction between the charged electrode and the water dipole.” And “In neutral and alkaline solutions, water acts as a reactant to provide the source of hydrogen evolution. Continuous HER will be suppressed if water is weakly adsorbed on the catalyst surface and not supplied in time.”

The role of interfacial water in HER catalysis

“The configuration of interfacial water and the dynamic process are of great importance in determining the HER performance of catalysts,” said Zidong Wei. And he further emphasized that “Based on our previous studies, we concluded that the electric field at the electrode/solution interface can regulate the enrichment, reorientation, and dissociation activation of interfacial water, thus promoting the coupling of the elementary steps of HER and improving its reaction dynamics in the highly polarizable region and this coupling mechanism is also universal.’

See the article:

The contribution of water molecules to the hydrogen evolution reaction

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