(509ar) Surface Proton Dynamics on BaZrO3: Implications for High Temperature Solid Electrolytes in Electrocatalysis

Solid-oxide ceramic materials are an important class of proton conducting electrolytes. Due to their high operating temperatures, they enable improved electrode reaction kinetics over systems reliant on aqueous electrolytes at ambient pressures. Furthermore, for electrochemical reactions in which hydrogen evolution reaction (HER) is a competing reaction at the cathode (eg. nitrogen reduction, CO₂ reduction, etc.), these materials offer an alternative to using H₂O, which has been shown to provide hydrogen with easy access to the active sites [1]. Yet very few fundamental studies have looked at these materials for electrochemical reactions.

Here, we use Density Functional Theory (DFT) calculations to rationalize proton conduction behavior in BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ (BZCYYb). A common electrode in these systems is a composite material comprised of La_0.7Sr_0.3TiO₃ (LST) and BZCYYb. We use BaZrO₃ (BZO) to model the BZCYYb electrolyte and SrTiO₃ (STO) to model LST for computational feasibility. We analyze the AO- and BO₂-terminated (001) surfaces of both STO and BZO and consider existence of different transition metal (TM) catalysts on the surface. Kinetic barriers to hydrogen dissociation/recombination are presented on STO, BZO, and catalyst TMs on BZO and combined with the thermodynamic modeling of hydrogen incorporation into the BZO electrolyte bulk. We show that hydrogen recombination can occur directly on the BZO (001) surface without catalyst present, but that it is thermodynamically unfavorable for hydrogen gas to adsorb on the electrolyte. Thus, a catalyst is needed to dissociate hydrogen and inject the protons into the electrolyte, but that these protons are able to recombine and evolve into the gas phase directly from the electrolyte surface. Using these systems for electrochemical reactions that compete with HER will require more detailed electrolyte engineering to quench the parasitic HER.

References:

1. Singh, A. R. et al. Strategies toward Selective Electrochemical Ammonia Synthesis. ACS Catal. 9, 8316–8324 (2019).