The role of alkali metal cations in the alkaline hydrogen evolution reaction
By Zhuoying Lin & Aamir Hassan Shah
This article was originally published by UCLA Chemistry & Biochemistry
A collaborative research team led by Professor Xiangfeng Duan’s group and Professor Anastassia N. Alexandrova’s group in the UCLA Department of Chemistry & Biochemistry, and Professor Yu Huang’s group in the UCLA Department of Materials Science and Engineering, has elucidated the elusive role of alkali metal cations in regulating alkaline hydrogen evolution reaction (HER) on the platinum surface, a reaction of critical importance for water electrolysis and green hydrogen production.
Water electrolysis using intermittent renewable electricity (from solar cells and windmills) offers an attractive solution to green hydrogen production that is essential for enabling hydrogen economy and achieving carbon neutrality. The discovery of more efficient catalysts for the HER is important for sustainable and cost-effective hydrogen production from renewable electricity. Although platinum (Pt) is known as a highly efficient catalyst for the HER, the reaction rate also depends on the local chemical environment. The electrolyte composition, including the presence of alkali metal cations (Li+, Na+, and K+) could greatly affect the electrocatalytic reaction rate, although their exact role in HER has been elusive and a topic of considerable debates.
To understand the role of alkali metal cations in the HER, the research team used the home-developed electron transport spectroscopy (ETS) to measure the conductance change of platinum nanowires (PtNWs) during electrocatalysis. ETS is a technique used to probe the surface adsorbates (adsorbed molecules or ions) by measuring the electrical conductance of the surface (Figure 1A). ETS measures the conductance change resulting from surface scattering caused by surface adsorbates and provides a unique way to exclusively probe the surface adsorbates with minimal interference from the electrochemical potential or the bulk electrolyte background. The higher conductance corresponds to the surface coverage of smaller atoms/molecules such as H-atoms, while the lower conductance indicates surface adsorbates with larger size atoms, such as O-atoms (Figure 1B).
Using the same technique, the group also determined the hydronium pKa on the Pt surface in another recently published research.
The ETS results indicate that the conductance level varies based on the type of alkali metal cation. Li+ showed less conductance increase compared to Na+ and K+. This suggests that fewer OHad are being replaced with H2O or Had in the presence of Li+ cations, resulting in a higher number of OHad remaining on the catalyst surface that leads to lower conductance.
“For the first time, we resolved the elusive role of alkali metal cations in the HER kinetics on the Pt surface and experimentally proved that the smaller cations don’t specifically adsorb on the electrode surface,” commented Professor Xiangfeng Duan. “Rather, they indirectly modulate the number of surface hydroxyls that could fundamentally influence the HER kinetics.”
The theoretical calculation of OHad binding strength to the Pt surface matches the trend seen in the experimental measurement of OHad coverage on the Pt surface. The energy calculations also show that OHad is more stable in the Li+ solution and stays on the Pt surface more, which is consistent with the ETS results.
Researchers further showed that hydroxide on the surface affects nearby water molecules through hydrogen bonding (Figure 2). “This makes it easier for water to dissociate, a critical step in the hydrogen evolution, because the bond between oxygen and hydrogen in water becomes longer,” commented co-author Zisheng Zhang, a graduate student in Professor Anastassia Alexandrova’s group. “As a result, increased surface coverage of hydroxyls in the presence of smaller alkali metals helps improve the overall HER reaction rate.”
The research work was also highlighted by the News and Views in Nature Catalysis. “The composition of an electrolyte has a significant effect on electrocatalytic reaction rates and product selectivities. One mechanism by which spectator alkali cations can dictate reaction kinetics is now better understood.” commented Professor Ian T. McCrum from the Department of Chemical and Biomolecular Engineering at Clarkson University.
The research team is further adapting their approach for investigating other important electrochemical reactions that of great importance for renewable energy industry, such as the hydrogen oxidation reaction and the oxygen reduction reaction for fuel cells.
About the Lead UCLA Authors
Professor Xiangfeng Duan joined the chemistry and biochemistry faculty in 2008. His group’s research interests include nanoscale materials, devices and their applications in future electronics, energy technologies and biomedical science.
Aamir Hassan Shah received his undergraduate and master degree in physical chemistry from Quaid-i-Azam University, Islamabad, Pakistan. He is now a fourth year Ph.D. candidate in Prof. Duan’s group. His research at UCLA focused on the fundamental understanding of electrode-electrolyte interface in water splitting reactions using on-chip Electrical transport spectroscopy (ETS).
Zisheng Zhang was born in Wuhan, PRC. He received a B.Sc. in Chemistry from South University of Science and Technology of China in 2019 advised by Prof. Jun Li. At UCLA, he was a UCLA-CSST fellow in 2018, obtained a M.Sc. in Chemistry in 2021, and is currently a Ph.D. candidate advised by Prof. Anastassia N. Alexandrova. In 2022, he worked with Dr. Maria K. Chan as a research intern at Argonne National Lab. His research interests include realistic modeling of catalytic interfaces and inverse design of functional molecules.