(141d) Comparison of Hydroxide-Mediated and Direct Mechanisms for Alkaline Hydrogen Electrocatalysis

It has long been recognized that the reaction rates of the hydrogen oxidation and hydrogen evolution reactions (HOR and HER) are slower in basic than acidic electrolytes, even though the surface intermediate of adsorbed hydrogen is independent of solution pH. Understanding the root of this observation is critical to designing catalysts for a multitude of electrochemical reactions with relevance to energy conversion and storage. In this work, we undertake a fundamental investigation to determine if adsorbed hydroxide plays an active or spectator role in HER/HOR kinetics.

One proposed explanation for the pH dependence of HER/HOR kinetics, suggested by the research groups of both Gasteiger and Yan, is that in base, hydroxide ions stabilize the Pt-H bond for stronger binding and slower catalysis [1,2]. Central to this hypothesis is an experimentally measured shift with pH in the peak potential for hydrogen underpotential deposition (H-UPD) on the (100) and (110) steps on the Pt surface. However, the Janik and Koper groups have found that the shift in H-UPD potential is caused not by changes to hydrogen adsorption, but instead by competitive adsorption of hydroxide in the presence of alkalai cations [3,4]. Markovic et al have proposed that such hydroxide adsorption is actually necessary to facilitate water recombination/dissociation in base, and that oxophilic sites such as Ni(OH)2 generate a bifunctional mechanism for efficient alkaline hydrogen electrocatalysis [5].

In this work, we investigate specifically the hypothesis that adsorbed hydroxide is an active participant in the alkaline HER and HOR by combining electroanalytical techniques with microkinetic modeling. We develop expressions for time-dependent current and voltage as a function of hydrogen and hydroxide binding strength for the indirect (hydroxide-mediated) and direct (hydroxide-as-spectator) Volmer steps in base. These expressions are compared to experimental cyclic voltammograms of H-UPD on polycrystalline platinum. The free energy of hydroxide adsorption is controlled experimentally by the electrolyte cation via changes in cation solvation strength [6]. Varying the sweep rate results in greater peak separation that can be used to extract rate constants for the adsorption reaction via traditional electroanalysis [7].

Our computations show that at both slow and fast scan rates, either the direct or indirect mechanism can describe the peak potential splitting and peak current. However, hydroxide binding strength affects the mechanisms differently. In the indirect mechanism, stronger hydroxide binding reduces overpotential and, therefore, peak splitting. In the direct mechanism, stronger hydroxide binding decreases available surface sites, leading to lower exchange current densities and greater overpotential. Comparison with experiment shows that on the (110) and (100) facets of polycrystalline platinum, moving from 0.1M KOH to 0.1M LiOH decreases the peak potential location, but increases peak potential splitting. This trend shows that stronger hydroxide binding has detrimental effects on H-UPD kinetics, and that the direct mechanism is therefore more likely to dominate observed behavior.

Our results strongly suggest that adsorbed hydroxide serves as a competitive spectator in the alkaline Volmer step, and that the bifunctional HOR/HER mechanism plays only a minor role at best. This study contributes to resolving a long-standing paradox in electrocatalysis and surface science by determining that oxophilicity is not an accurate descriptor for alkaline hydrogen electrocatalysts. Other parameters, such as water orientation and non-covalent interactions, must play a greater role in overall activity. Future work will discuss these and other factors in the relation between H-UPD kinetics and HER/HOR catalyst activity .

(1) Durst et al. Energy Environ. Sci. 2014, 2255.

(2) Sheng et al. Nat. Commun. 2015, 5848.

(3) McCrum et. al. J. Phys. Chem. C, 2015, 457.

(4) Van Der Niet et al. Catal. Today 2013, 105.

(5) Strmcnik et al, Nat. Chem. 2013, 1.

(6) Danilovic, N. et al. Electrocatalysis 3, 2012, 221.

(7) Angerstein-Kozlowska et al. J. Electroanal. Chem. Interfacial Electrochem. 1979, 1.