Complementary 3d-4 <i>d</i> /5d Pairing in Medium-Entropy Sulfides for Bifunctional Water Electrolysis

The incorporation of multiple elements has become a prevalent strategy for developing advanced electrocatalysts. However, current approaches often prioritize enhancing configurational entropy or empirically adding elements without a rational framework for achieving genuine functional complementarity. Herein, we propose the concept of complementary 3d-4<i>d</i>/5d element pairing, demonstrated through the synthesis of a nanoporous medium-entropy multimetal sulfide, (CoNiMoW)S₂. This design establishes a cooperative architecture in which 3d metals (Co, Ni) serve as dynamic sites for the in situ generation of catalytically active (oxy)hydroxides, while 4<i>d</i>/5d metals (Mo, W) play distinct roles: Mo acts as an electronic modulator that optimizes adsorption energetics, and W functions as a structural stabilizer that enhances durability. The resulting catalyst exhibits exceptional bifunctional performance, requiring only 36 mV for the hydrogen evolution reaction and 120 mV for the oxygen evolution reaction at 10 mA cm^-2, along with outstanding operational stability exceeding 900 h. Operando Raman spectroscopy and density functional theory analyses confirm that the complementary 3d-4<i>d</i>/5d pairing creates a cooperative catalytic system, where 3d metals drive the surface reconstruction into active phases while 4<i>d</i>/5d metals concurrently modulate the electronic structure and enhance durability.