Abstract
In this work we study the role of alkali metal cation concentration and electrolyte pH in altering the kinetics of the hydrogen evolution reaction (HER) at gold (Au) electrodes. We show that at moderately alkaline pH (pH 11), increasing the cation concentration significantly enhances the HER activity on Au electrodes (with a reaction order ≈0.5). Based on these results we suggest that cations play a central role in stabilizing the transition state of the rate‐determining Volmer step by favorably interacting with the dissociating water molecule (*H–OHδ−–cat+). Moreover, we show that increasing electrolyte pH (pH 10 to pH 13) tunes the local field strength, which in turn indirectly enhances the activity of HER by tuning the near‐surface cation concentration. Interestingly, a too high near‐surface cation concentration (at high pH and high cation concentration) leads to a lowering of the HER activity, which we ascribe to a blockage of the surface by near‐surface cations.
Highlights
Research on the electrochemical hydrogen evolution reaction (HER) is at the heart of realizing a sustainable and economically feasible hydrogen-based economy
From the experimental evidence presented in this paper we conclude that there exists an intricate interrelation between the cation and the pH effects on the HER kinetics in the alkaline media
We have shown here that on Au electrodes, the rate for the sluggish Volmer step is increased when the near-surface cation concentration is increased
Summary
Research on the electrochemical hydrogen evolution reaction (HER) is at the heart of realizing a sustainable and economically feasible hydrogen-based economy. A shift to a lower wavenumbers with the increasing pH could indicate that the hydrogen bond strength decreases with the increasing pH, though one must be careful in electrosorption systems to correlate changes in metal-adsorbate frequencies to corresponding changes in binding energies because there is no theoretical basis for such a correlation.[17] Interestingly, Mao and co-workers have previously observed a similar pH dependence for the Pt-H vibrational band.[16c] Notably, the observed Stark tuning effect for the Au-H band (see Supporting Information Figure S12) is similar to the previously reported Stark tuning effect for the Pt-H band.[16c] While these spectroscopy measurements do not allow us to draw detailed quantitative conclusions, together with the impedance data, they do lead to two important qualitative conclusions that are in line with the conclusions of the previous section: (i) there is an intrinsic pH dependence of the (double layer structure of the) gold-aqueous electrolyte interface, presumably resulting in associated changes in the (weak) specific OHÀ adsorption in double layer and the strength of the hydrogen adsorption in the HER window, and (ii) cation and pH effects are convoluted in the sense that higher pH invoke higher near-surface cation concentrations. We believe that an investigation into the nature of conclusion (i) requires a detailed study of its own
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