Spin configuration modulation of Co3O4 by Ru doping for boosting the overall water splitting and hydrazine oxidation reactions

Abstract

Co-based spinel oxides with unique crystal structures have been identified as a class of promising candidates for alkaline water oxidation catalysis in oxide structural families. Unfortunately, their relatively poor conductivity exerts significant negative impacts on efficient charge transfer, thereby inhibiting the enhancement of their intrinsic activity for electrochemical water splitting. Optimizing the electronic structure of the catalytically active sites of Co-based spinel oxide electrocatalysts is of great significance for improving the electrocatalytic performance. Herein, a facile heterometal doping strategy is reported to synthesize an oxygen-vacancy-rich Ru-doped Co₃O₄ nanosheet array on a cobalt foam substrate (denoted as Ru-Co₃O₄|V_O/CF) as a trifunctional catalyst for significantly boosting the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and hydrazine oxidation reaction (HzOR) electrocatalytic activities, which involves the electrodeposition process, cation exchange approach, and subsequent thermal oxidation treatment. Benefiting from the 3D hierarchical structural merits and more exposed active sites caused by local Jahn–Teller distortions, the optimized catalyst exhibits excellent catalytic performance towards the HER and OER, affording a current density of 100 mA cm⁻² at low overpotentials of 48.6 and 270 mV in 1 M KOH, respectively, along with remarkable stability and durability. Interestingly, the integrated Ru-Co₃O₄|V_O/CF water electrolyzer only requires a low cell voltage of 1.47 V to reach 10 mA cm⁻², outperforming the benchmark couple of the Pt/C(−)||RuO₂(+) electrode and most reported counterparts. More impressively, a hydrazine-assisted water electrolysis electrolyzer is subsequently assembled by using Ru-Co₃O₄|V_O/CF electrodes, delivering a current density of 100 mA cm⁻² with an ultralow operating voltage of 0.70 V, which is 1.1 V lower than that of its hydrazine free counterpart. Density functional theory results reveal that the synergistic effect of Ru doping and oxygen vacancies could effectively modify the spin configuration (e_g orbital occupancy), tune the d-band center of Co ions and optimize the Gibbs free energy of hydrogen adsorption and the adsorption energy of OER intermediates, consequently resulting in the enhancement of the intrinsic catalytic activity of Co₃O₄. This work provides important referential value for fabricating highly efficient and stable catalysts towards sustainable energy conversion and storage.