Cu²⁺ as a dynamic director for Ce-incorporated (CoFeNiCuCe)₉S₈ nanoballs for multifunctional electrocatalysis

High-entropy metal sulfides (HEMSs) have emerged as a new class of electrocatalysts, but their synthesis often faces challenges due to their inherent complexity arising from multi-metal interactions, especially with elements having large differences in atomic/ionic sizes, such as the redox-active rare-earth elements. Here, we report a low-temperature (200°C) hydrothermal strategy to fabricate Ce-incorporated (CoFeNiCuCe)₉S₈ nanoballs by leveraging Cu²⁺ as a dynamic director for this phase evolution. Time-resolved studies reveal a multistage growth pathway involving cation exchange, lattice strain-driven reconstruction, and coalescence of various low and medium entropy intermediates (CoFeNi)₉S₈, CuS/(CoFeNi)₉S₈, (CoFeNiCu)₉S₈, Ce₂S₃, Ce₂S₃/(CoFeNi)₉S₈, (CoFeNiCuCe)/S nanoplates) into monodisperse (CoFeNiCuCe)₉S₈ HEMS nanoballs. By systematically varying Cu:Ce ratios, we obtain five distinct configurations, and Ce-rich HEMS-4 (Cu:Ce = 1:4) exhibits superior multifunctional electrocatalytic performance, outperforming a series of lower- (Co₉S₈, (NiFe)₉S₈, and (CoNiFe)₉S₈), medium-((CoNiFeCu)₉S₈), and high-entropy ((CoNiFeCuCe)₉S₈) analogues in the oxygen evolution reaction (OER; η₁₀ = 175 mV, η₁₀₀ = 260 mV), urea oxidation reaction (UOR; 1.277 V and 1.336 V at 10 and 100 mA.cm⁻²), hydrogen evolution reaction (HER; η₁₀ = 85 mV), and nitrite reduction (NO₂RR; 0.112 V at 100 mA.cm⁻²). Post-catalytic and in-situ Raman analyses, in conjunction with density functional theory (DFT), show that metal (oxy)hydroxides form during the reaction, while d-f orbital interactions protect the active sites from over-oxidation. This work establishes a paradigm for integrating rare-earth elements into HEMSs via controlled solution-phase synthesis, advancing the design of high-entropy electrocatalysts.