Confined Kirkendall Strain Engineering Promotes Oxygen Evolution on Co ₃ O ₄ for Proton Exchange Membrane Water Electrolyzer

ABSTRACT While Kirkendall oxidation (KO) facilely fabricates transition metal oxides for oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWEs), it concomitantly triggers detrimental lattice expansion. For Co 3 O 4 , an appealing IrO 2 ‐alternative catalyst, KO generates a free lattice expansion above 2.4%, resulting in suboptimal activity and stability. Here, we design a confined KO approach and coat the precursor Co nanoparticles with atomic carbon layers, enabling controllable lattice expansion of Co 3 O 4 between 0.2% and 1.8%. For the first time, a strain‐dependent OER activity of Co 3 O 4 is established in acidic media, with an expansion strain of 1.4% exhibiting the lowest overpotential of 377 mV at 10 mA cm −2 . Synchrotron X‐ray absorption spectroscopies and theoretical calculations unravel the adjustable lattice strain regulates the d‐band center of Co sites and further modifies the oxygenate adsorptions. When integrated into a PEMWE, the 1.4%‐strained Co 3 O 4 catalyst sustainably operates at 250 mA cm −2 over 100 h. The reported confined KO provides a new means for strain engineering of transition metal oxides for energy electrocatalysis.