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We successfully report a liquid−liquid chemical reduction and hydrothermal synthesis of a highly stable columbite-tantalite electrocatalyst with remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance in acidic media. The reduced Fe 0.79 Mn 0.21 Nb 0.16 Ta 0.84 O 6 (CTr) electrocatalyst shows a low overpotential of 84.23 mV at 10 mA cm −2 and 103.7 achieved at 20 mA cm −2 current density in situ for the HER and OER, respectively. The electrocatalyst also exhibited low Tafel slopes of 104.97 mV/dec for the HER and 57.67 mV/ dec for the OER, verifying their rapid catalytic kinetics. The electrolyzer maintained a cell voltage of 1.5 V and potential−time stability close to that of Pt/C and RuO 2 . Complementary first-principles density functional theory calculations identify the Mn sites as most active sites on the Fe 0.75 Mn 0.25 Ta 1.875 Nb 0.125 O 6 (100) surface, predicting a moderate Gibbs free energy of hydrogen adsorption (ΔG H* ≈ 0.08 eV) and a low overpotential of η = 0.47 V. The |ΔGMn H *| = 0.08 eV on the Fe 0.75 Mn 0.25 Ta 1.875 Nb 0.125 O 6 (100) surface is similar to that of the wellknown and highly efficient Pt catalyst (|ΔGPt H *| ≈ 0.09 eV).

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The development of robust and inexpensive electrocatalysts that are capable of catalyzing the overall water splitting reaction is highly essential for large scale production of hydrogen. Herein, we report the successful liquid-liquid extraction and hydrothermal synthesis of a highly stable columbite-tantalite electrocatalysts (Fe0.79Mn0.21Nb0.16Ta0.84O6) with remarkable HER and OER performance in alkaline media. The extracted Fe0.79Mn0.21Nb0.16Ta0.84O6 electrocatalyst shows a low overpotential of 190.2 and 284.8 mV at 10/mA cm-2 in current density in situ for HER and OER, respectively. The electrocatalyst also exhibited low Tafel slopes of 56.36 mV/dec for HER and 112.85 mV/dec for OER, verifying their rapid catalytic kinetics. The electrolyzer maintained the cell voltage of 1.63 V and potential-time stability close to that of Pt/C & RuO2/C. The intrinsic mechanism for the exceptional HER and OER performance was further unravelled through first-principles density functional theory (DFT) calculations, predicting very low Gibbs free energy of hydrogen adsorption (ΔGH* ≈ 0.09 eV) and low overpotential (η = 0.47 eV at the Mn sites) for OER on the Fe0.75Mn0.25Ta1.875Nb0.125O6 catalyst. Our results demonstrate that columbite-tantalite electrocatalysts offer great promise for efficient overall water splitting. Graphical abstract