The rational design of catalysts is crucial to make power-to-X technologies viable. Here the authors introduce the delafossite PdCoO2 as a highly active hydrogen evolution reaction catalyst due to the growth of a tensile-strained Pd-rich capping layer under reductive conditions. Image credit: Christop Hohmann.

The rational design of hydrogen evolution reaction electrocatalysts that can compete with platinum is an outstanding challenge in the process of designing viable power-to-gas technologies. Here, we introduce delafossites as a family of hydrogen evolution reaction electrocatalysts in acidic media. We show that, in PdCoO2, the inherently strained Pd metal sublattice acts as a pseudomorphic template for the growth of a tensile-strained Pd-rich capping layer under reductive conditions. The surface modification ranges up to 400 nm and continuously improves the electrocatalytic activity by simultaneously increasing the exchange current density and by reducing the Tafel slope down to 38 mV dec(-1), leading to overpotentials eta(10) < 15 mV. The improved activity is attributed to the operando stabilization of a beta-PdHx phase with enhanced surface catalytic properties with respect to pure or nanostructured palladium. These findings illustrate how operando-induced electrodissolution can be used as a top-down design concept through the strain-stabilized formation of catalytically active phases.