High-Entropy Metallene Aerogels: A New Balancer for *H Production and Consumption in Nitrate Reduction Reaction.

High-entropy metallene aerogels (HEMAs), synergizing high-entropy metallenes and metallic aerogels, face challenges in achieving single-phase structures due to multi-metallic nucleation/growth disparities, hindering two-dimensional anisotropic growth and three-dimensional assembly of multi-component nanocrystals. Herein, the universal preparation of HEMAs was achieved by a seed-mediated synthetic route for electrochemical nitrate reduction reaction (NO₃RR). PdCuSnCoNi HEMAs exhibit maximum Faradaic efficiency and yield rate of NH₃ up to 99.5% and 4117.8 µg h^-1 mg_cat. ^-1, surpassing those of Pd metallene aerogels (MAs). In situ attenuated total reflection infrared absorption spectroscopy, online differential electrochemical mass spectrometry and density functional theory calculations reveal kinetic match for *NO₃ to *NO₂ and *NO₂ to *NH₃, with the energy barrier for *NO₂ formation (potential-determining step) being lower than that for *H to H₂, balancing production and consumption of *H and facilitating NH₃ generation on PdCuSnCoNi HEMAs. This study paves the way for efficient NO₃RR catalysts and guides rational design for diverse electrocatalytic systems.