Compositional and Structural Impact on the Hydrogen Evolution Reaction Activity across Noble-Metal-Based Compositionally Complex Solid Solutions Thin Film Libraries

Compositionally complex solid solutions (CCSS), often called high-entropy alloys (HEA) have emerged as a promising platform for electrocatalysts for the hydrogen evolution reaction (HER). However, due to their complexity, the interplay among surface composition, structure, and catalytic activity remains insufficiently understood. Herein, noble-metal-based CCSS thin-film materials libraries, prepared by combinatorial magnetron sputtering, are employed as a platform to elucidate how HER activities of CCSS electrocatalysts are linked to their volume composition through surface segregation and crystal structure. High-throughput investigations of three CCSS material libraries, Ir-Pd-Pt-Rh-Ru, Ag-Au-Pd-Pt-Ru, and Ag-Au-Cu-Pd-Pt, were conducted to assess their structural, compositional, and electrochemical properties, using high-throughput X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) coupled with Gaussian process regression, and long-range scanning electrochemical cell microscopy (SECCM). Correlative data analysis revealed that the composition-dependent surface segregation, driven by surface energy differences, modulates the distribution of active sites on the surface. We identified Ir₁₀Pd₁₅Pt₄₆Rh₁₂Ru₁₇ as the most active volume composition for HER which corresponds to a surface composition of Ir₆Pd₁₇Pt₅₀Rh₁₁Ru₁₆. Moreover, the HER trends were governed not only by the intrinsic activity of the constituent elements but also by unexpected synergetic effects between them. The findings offer direct insights into composition-structure-activity relationships in compositionally complex electrocatalysts.