Surface Activation of High‐Entropy Alloy Nanoparticles Using ns‐Pulsed UV Laser Postprocessing for Enhanced Water Splitting and Fuel Cell Performance

Mild pulsed laser postprocessing of catalyst dispersions via pulsed laser defect engineering in liquids has emerged as an effective tool to enhance catalytic activity. This approach enables the introduction of cations or point defects and the modification of surface properties with single‐pulse precision in flow‐through reactors, while requiring only low laser energies. To date, it has been applied primarily to oxides, but not to metallic or alloyed catalysts. Here, we investigate the impact of nanosecond‐pulsed ultraviolet (ns‐UV) laser irradiation on Pd 3 0 Ru 3 0 Pt 1 0 Ir 1 0 Rh 2 0 high‐entropy alloy (HEA) nanoparticles, a theoretically proposed composition for highly active electrocatalytic oxygen reduction reaction (ORR). After UV laser processing, the HEA nanoparticles exhibit an 80 mV lower overpotential for the acidic oxygen evolution reaction (OER) and a tenfold increase in acidic ORR activity compared to the untreated sample. These enhancements correlate with a laser‐induced increase in surface charge density, while particle size and composition remain unchanged. Control experiments with iridium nanoparticles confirm an enrichment of negatively charged surface groups as the underlying factor. Remarkably, substantial performance gains are achieved at very low laser fluences of 1 mJ cm −2 in water. Optical modeling rationalizes this unusually high efficiency, highlighting the scalability of this method for kg‐scale catalyst activation, with direct relevance to green hydrogen production (OER) and fuel cell applications (ORR).