Water-Based Synthesis of Supported Ultrasmall High-Entropy Alloy Nanoparticles

High-entropy alloys (HEAs) are emerging as a class of materials with immense potential in electrocatalysis. While several synthesis methods have been reported, simple and environmentally benign synthesis of HEA nanoparticles is lacking. In this work, we report a methodology for the synthesis of ultrasmall (1-3 nm) IrPdPtRuRh HEA nanoparticles through a hot-injection approach in water, using l-ascorbic acid as reducing agent. The nanoparticles are formed directly on a carbon support and exhibit a mean crystallite size of just 1.2 ± 0.7 nm. All five elements are well-dispersed throughout the particles, with no elemental separation. The supported particles outperformed a Pt/C benchmark in the alkaline hydrogen evolution reaction, requiring 30 mV less overpotential to achieve 10 mA cm^-2_ECSA. Reaction kinetics experiments reveal a large increase of the metal reduction rates in the presence of the carbon support, highlighting the synergistic effect of the support material and the reducing agent.