Influence of Ru Content on Defect Formation and Electrocatalytic Activity of Au-Pd-Pt-Ru Compositionally Complex Solid Solution Thin Films for the Hydrogen Evolution Reaction.

Compositionally complex solid solutions (CCSSs) consist of a randomly mixed single phase with the potential to enhance electrocatalytic activity through their polyelemental surface atom arrangements. Local structure, chemistry, and lattice strain variation can affect electrocatalytic activity. We investigate the effect of Ru content on defect formation and electrochemistry in Au-Pd-Pt-Ru CCSS thin films. A thin-film material library covering a wide composition range was fabricated by room-temperature combinatorial co-sputtering. High-throughput characterization, including electron microscopy, X-ray diffraction, and electrochemical screening, were used to correlate composition and microstructural features with catalytic activity. Three representative compositions - Au68Pd13Pt15Ru4, Au27Pd23Pt24Ru26, and Au9Pd21Pt18Ru52 - were examined. The samples exhibit face-centered cubic crystal structures, with lattice contraction occurring and a transition from a high density of nanotwins to high-density stacking faults with increasing Ru content. These planar defects could influence electrocatalytic activity when they terminate at surfaces. Moreover, the hydrogen evolution reaction activity improves with higher Ru content, suggesting that Ru can tune the surface energy for hydrogen adsorption through interactions with neighboring atoms. Atom probe tomography reveals local compositional fluctuations at grain boundaries depending on the Ru content. The findings provide a new insight into surface atom arrangement design in the CCSS electrocatalysts with enhanced performance.