Ultralow-Ru-Activated Amorphous NiCoFe Medium-Entropy Boride for Overall Water Splitting in a Proton-Exchange-Membrane Electrolyzer

초록

Platinum-free electrocatalysts that combine high activity for the hydrogen evolution reaction (HER) with the stability required for proton-exchange-membrane (PEM) water electrolysis remain a central challenge for scalable green-hydrogen production. Herein, we report an amorphous Ru-incorporated NiCoFe medium-entropy boride (Ru-NiCoFe-B, denoted Ru-MEB; configurational entropy Delta S-config approximate to 1.33 R for the cation sublattice, within the medium-entropy range), synthesized by a one-step, room-temperature aqueous co-reduction strategy. The optimized 2 wt% Ru-MEB drives the HER at an overpotential of only 75 mV at 10 mA cm(-2) in 0.5 M H2SO4 (vs. 110 mV for commercial Pt/C measured under identical loading conditions) and exhibits an OER overpotential of 253 mV at 10 mA cm(-2) in 1 M KOH (vs. 274 mV for the Ru-free MEB). Drawing on density functional theory (DFT) studies of B-Ru and Ru/oxophile systems, we propose that synergistic incorporation of ultralow Ru (2 wt%) and electron-deficient B into the NiCoFe matrix modulates the local electronic structure, weakens the Ru-H bond, and provides oxophilic B sites that adsorb OH*, accelerating the rate-determining steps. Assembled into a symmetric Ru-MEB || Ru-MEB membrane electrode assembly (MEA) with a Nafion 117 proton-exchange membrane at 25 degrees C, the device drives overall water splitting at 1.95 V at 100 mA cm(-2) with > 99% Faradaic efficiency and > 100 h of stable operation. On a Ru-mass basis, Ru-MEB delivers a mass activity of similar to 14.5 A mg Ru-1 at eta = 100 mV (HER), over an order of magnitude higher than commercial Pt/C, highlighting the cost-effectiveness of the trace-Ru design. This work establishes Ru-engineered medium-entropy borides as a viable platinum-free platform for practical PEM water electrolysis.

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