Effect of Step Density and Orientation on the Apparent pH Dependence of Hydrogen and Hydroxide Adsorption on Stepped Platinum Surfaces

Abstract

The effect of the alkali-metal cation (Li+, Na+, K+, and Cs+) on the non-Nernstian pH shift of the Pt(554) and Pt(533) step-associated voltammetric peak is elucidated over a wide pH window (1–13), through computation and experiment. In conjunction with our previously reported study on Pt(553), the non-Nernstian pH shift of the step-induced peak is found to be independent of the step density and the step orientation. In our prior work, we explained the sharp peak as due to the exchange between adsorbed hydrogen and hydroxyl along the step and the non-Nernstian shift as a result of the adsorption of an alkali-metal cation and its subsequent weakening of hydroxyl adsorption. Our density functional theory results support this same mechanism on Pt(533) and capture the effect of alkali-metal cation identity and alkali cation coverage well, where increasing electrolyte pH and cation concentration leads to increased cation coverage and a greater weakening effect on hydroxide adsorption. This work paints a consistent picture for the mechanism of these effects, expanding our fundamental understanding of the electrode/electrolyte interface and practical ability to control hydrogen and hydroxyl adsorption thermodynamics via the electrolyte composition, important for improving fuel cell and electrolyzer performance.

Understanding the pH Dependence of Underpotential Deposited Hydrogen on Platinum

“…As imilar trend also holds true for the H upd peak corresponding to the Pt(100) facet (Supporting Information, Figure S3 A), showing that the peak shift with the electrolyte pH is general for stepped surfaces.W enote that no discernable shift of the H upd feature on the Pt(111) facet with pH was observed in previous studies, [22,25,27,28] and these diverging trends on different facets of Pt have been attributed to the adsorption of metal cations on step sites. [4,5,17,22] If cation adsorption affects the H upd peak, changes in the concentration of the cation are expected to change its surface coverage and cause ashift of the H upd peak position. To test this hypothesis,t he same set of data are plotted against the concentration of metal cations (Figure 1B).…”

“…[1][2][3] Molecular level insights into how these two reactions proceed at the electrode surface are key to the understanding and catalyst design of many electrochemical processes.F urther,t hey are also model reactions to investigate interactions of species in the electric double layer with surface bound species,a nd in turn their impact on the activity of surface mediated reactions.Despite their seeming "simplicity" and the extensive previous research effort, much debate still exists on the HOR/HER catalyzed by Pt surfaces,f or example,t he nature of surface bound species in the relevant potential range, [4][5][6] the identity of the rate limiting step, [7][8][9] and the impact of electrolyte on the reaction. [4,5,10] In particular,the dependence of the peak potential of the oxidation of the under potential deposited hydrogen (H upd )on platinum group metals (PGMs) in cyclic voltammograms (CVs) on the reversible hydrogen electrode (RHE) scale has attracted much recent attention owing to the interest in developing PGM-free hydrogen cells with hydroxide exchange membranes. [3][4][5][10][11][12][13] Thermodynamically,the adoption of the RHE scale should account for the effect of proton concentration, and thus the H upd features in CV are expected to be identical regardless of the electrolyte pH.…”

“…[4,5,10] In particular,the dependence of the peak potential of the oxidation of the under potential deposited hydrogen (H upd )on platinum group metals (PGMs) in cyclic voltammograms (CVs) on the reversible hydrogen electrode (RHE) scale has attracted much recent attention owing to the interest in developing PGM-free hydrogen cells with hydroxide exchange membranes. [3][4][5][10][11][12][13] Thermodynamically,the adoption of the RHE scale should account for the effect of proton concentration, and thus the H upd features in CV are expected to be identical regardless of the electrolyte pH. However, existing works,including several recent publications from our laboratories,d emonstrated as ystematic shift of the H upd peaks on PGMs to higher potentials with the increase of pH in the electrolyte.…”

“…[21,22] Thes hift in the (apparent) hydrogen binding energy has long been recognized as ak ey descriptor in predicting the performance of HOR/HER catalysts. [1-3, 10, 14, 18, 19] H upd peaks have also been proposed to be the result of the exchange of Hand OH on the surface, [4,5,17,[23][24][25][26] with the peak shifts with the pH of the electrolyte attributed to the change in the PtÀOH bond strength due to the specific adsorption of cations. [4,5,17] Both hypotheses recognize that the diverging trends of the electrode potential on the SHE and the RHE scales with changing pH are key to understanding the impact of the electrolyte pH on the H upd peak and HOR/HER activities,but the proposed molecular level pathways are quite different.…”

Enhancement of Oxygen Evolution Activity of Nickel Oxyhydroxide by Electrolyte Alkali Cations

“…Theresults displayed in Figure 3would imply that bigger cations (in this case Cs + )provide abetter stabilization to the Ni À OO À À M + intermediate.T his species acts as the precursor to O 2 evolution, explaining the higher activity.T he exact nature of this cation stabilization should be the subject of further computational study (see,e .g., Refs. [34,43]). The model expressed in Equation (2) is subtly different form the model recently put forward by Grimaud et al, [28] who studied the influence of large organic tetra-alkyl ammonium cations,o nt he OER activity of NiOOH and Ni(Fe)OOH catalysts.T hey suggested that the strong interaction with organic cations actually blocks the active site and lowers the OER activity,s pecifically of the Ni(Fe)OOH catalyst.…”