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[Display omitted] • The porous rhodium nanosheets with tunable compressive strain were successfully obtained by directly annealing metastable trigonal rhodium oxide. • The record HzOR performance. • A clear strain effect and catalytic mechanism for alkaline HzOR in single-metal-element systems. Electrocatalytic water decomposition is an essential pathway for large-scale preparation of high-purity hydrogen, while the conventional oxygen evolution reaction (OER) with a kinetically sluggish four-electron process becomes a huge challenge for its further application. Hydrazine oxidation reaction (HzOR) is considered an effective strategy to replace the sluggish OER process, attributed to its lower thermodynamic oxidation potential. Here, porous rhodium nanosheets (Rh-NSs) with tunable lattice strain are successfully obtained by directly annealing metastable trigonal rhodium oxide (Tri-RhO 2) in a hydrogen atmosphere. More importantly, ligand and synergetic effects can be effectively avoided in a single-metal-element system, providing an ideal model electrocatalyst for understanding the correlation between reactivity and lattice strain. The electrochemical results show that the optimal Rh-NSs-300 (the Tri-RhO 2 precursor is annealed at 300 °C) may deliver ultralow working potential (vs. RHE) of −103 mV at the current density of 10 mA cm−2 with a Tafel slope of 15.4 mV dec-1 for HzOR. In addition, the working potential (vs. RHE) of Rh-NSs-300 only drops 35 mV after the 72 h test by chronopotentiometry method at the constant current density of 10 mA cm−2. Density functional theory (DFT) calculations indicate that HzOR can more easily occur on Rh-NSs with tunable compressive strain than pure Rh. [ABSTRACT FROM AUTHOR]