(190c) Catalyst Design Strategies for the Alkaline Hydrogen Evolution Reaction

The hydrogen oxidation and evolution reactions (HOR/HER) are important reactions in hydrogen fuel cells and water electrolyzers. While devices which operate under alkaline conditions offer potential cost-savings over those operated under acidic conditions, the rate of hydrogen oxidation and evolution are significantly slower in an alkaline electrolyte than in an acid electrolyte, even on some of the most active catalysts known for these reactions. Design strategies are therefore needed to develop more active catalysts. In our recent work, we used a combination of detailed experiments on single crystal electrodes and density functional theory calculations to determine the mechanism of the hydrogen evolution reaction in an alkaline electrolyte [1]. We found that the activity of the hydrogen evolution reaction in an alkaline electrolyte depends not only on how strongly hydrogen binds to the catalyst but also how strongly hydroxide binds, yielding a volcano shaped relationship. On the too-weak binding side of the hydroxide volcano, the activation barrier for electrochemical water dissociation (the rate determining step) correlates with how strongly hydroxide binds to the surface, even if it is not necessarily a product of this reaction. On the too-strong binding side, hydroxide does adsorb, and the rate is limited by that of hydroxide desorption. Combining this with the well-known hydrogen volcano yields a 3-dimensional volcano shaped map (Figure 1) which allows us to predictively design and improve catalyst activity by finding materials with hydrogen and hydroxide binding strengths closer to the optimum. In this talk, I will discuss two strategies to tune hydrogen and hydroxide adsorption strength, namely via surface modification and alloying. Results from both experiment and density functional theory will be presented and particular attention will be given to possible trade-offs identified between surface modification and site-blocking effects.

1) I.T. McCrum, M. T.M. Koper, Nature Energy, 5, 891-899 (2020).