(21g) Comparison of the Direct and Bifunctional Mechanisms through Steady-State Microkinetic Modeling for Hydrogen Electrocatalysis in Alkaline Media

Experimentally observed slow kinetics of the hydrogen oxidation and hydrogen evolution reactions (HER and HOR) in alkaline compared to acidic media remain a fundamental conundrum in modern electrocatalysis. Despite numerous efforts, the reaction pathway followed by the hydrogen reaction in basic media is still unknown. Thorough understanding of the reaction mechanism is necessary for the rational design and synthesis of electrocatalysts required for efficient energy conversion and storage. Herein, we undertake an experimental and computational investigation to determine the role of adsorbed hydroxide and bifunctional mechanisms in alkaline HER/HOR kinetics.

A possible explanation for the decreased HER/HOR reaction kinetics in basic media, proposed by the Janik and Koper [1,2] research groups, is that in base, hydroxide ions present in solution tend to adsorb onto the catalyst surface, thereby competing with hydrogen and reducing the number of available adsorption sites needed for HER/HOR to progress. Conversely, the Markovic, Zitoun, and Yan [3,4,5,6] groups have suggested that such hydroxide adsorption actually facilitates water dissociation/recombination in alkaline media, and that HER/HOR activity can be significantly improved through the addition of oxophilic sites onto the catalyst surface (such as Ru or Ni(OH)₂ clusters on Pt). The latter observation suggests the existence of a bifunctional mechanism [3,4], where adsorbed hydroxide on oxophilic sites reacts with adsorbed hydrogen on non-oxophilic sites to promote HER/HOR. In our own past work, we have concluded that, on a single-site catalyst, adsorbed hydroxide serves as a competitive spectator in the alkaline Volmer step, and that the bifunctional mechanism is extremely unlikely to contribute significantly. Nonetheless, no major investigations have focused specifically on dual-site catalysts, and the fundamental role of adsorbed hydroxide on oxophilic sites is yet to be deciphered.

In this work, we examine through the means of microkinetic modeling the hypothesis that adsorbed hydroxide, whether on a single-site or a dual-site catalyst, is an active participant in the alkaline HER/HOR. We consider both Volmer-Tafel and Volmer-Heyrovsky for a single-site catalyst, and compare the resulting kinetic expression to a binary-site catalyst operating with either a recombination-Tafel or recombination-Heyrovsky mechanism. Assuming quasi-equilibrium for non-rate-determining steps (RDS) yields eight unique reaction mechanisms. Our results relate the steady-state current as a function of voltage to the hydrogen and hydroxide binding strengths for the direct (hydroxide as spectator, single-site) and indirect (hydroxide-mediated, binary site) mechanisms in alkaline media. The direct mechanism for binary sites is not considered as it provides no opportunity for adsorbed hydrogen and hydroxide species to interact.

Our computations show that, for the bifunctional mechanism on dual-site catalysts, varying the OH_ad binding strength has little to no effect on the HER/HOR kinetics when either the Tafel or the Heyrovsky steps are rate-determining, but has a strong influence on HOR when the recombination step is rate limiting. Stronger hydroxide binding results in an increase in HOR current, consistent with the suggested mechanism of Subbaraman et. al. [4]. For the direct mechanism, we find that the hydroxide binding strength does not affect HER/HOR kinetics when the Volmer step is rate-limiting, but has a significant effect on HOR when either the Tafel or Volmer steps are rate limiting. Due to site competition between hydrogen and hydroxide, however, an increase in OH_ad binding strength leads to a decrease in HOR current. These observations show that stronger hydroxide binding can only have unfavorable effects on HER/HOR kinetics for single-site catalysts, but enhances HOR activity for binary site catalysts if the recombination step is the RDS.

Our results suggest that hydroxide can only improve HER/HOR kinetics if it is adsorbed onto oxophilic sites on a catalyst surface; otherwise, adsorbed hydroxide plays a detrimental or a passive role at best. This study provides a theoretical platform for resolving a fundamental paradox in electrocatalysis by determining the role of adsorbed hydroxide onto oxophilic sites on hydrogen electrocatalysis in alkaline media. In future work, comparisons with experiments using catalysts of dual nature will aid in determining the validity of each reaction mechanism modeled herein.

[1] McCrum et. al. J. Phys. Chem. C, 2015, 457.

[2] Van Der Niet et al. Catal. Today 2013, 105.

[3] Strmcnik et al, Nat. Chem. 2013, 1.

[4] Subbaraman et al, Nat. Mater. 2012, 550-557.

[5] Alesker et al J. Power Sources 304 (2016) 332–339.

[6] Alia et al, J. Am. Chem. Soc. 135 (2013), 13473–13478.