Construction of Defect-Rich Ni-Fe-Doped K0.23 MnO2 Cubic Nanoflowers via Etching Prussian Blue Analogue for Efficient Overall Water Splitting.

Designing elaborate nanostructures and engineering defects have been promising approaches to fabricate cost-efficient electrocatalysts toward overall water splitting. In this work, a controllable Prussian-blue-analogue-sacrificed strategy followed by an annealing process to harvest defect-rich Ni-Fe-doped K0.23 MnO2 cubic nanoflowers (Ni-Fe-K0.23 MnO2 CNFs-300) as highly active bifunctional catalysts for oxygen and hydrogen evolution reactions (OER and HER) is reported. Benefiting from many merits, including unique morphology, abundant defects, and doping effect, Ni-Fe-K0.23 MnO2 CNFs-300 shows the best electrocatalytic performances among currently reported Mn oxide-based electrocatalysts. This catalyst affords low overpotentials of 270 (320) mV at 10 (100) mA cm-2 for OER with a small Tafel slope of 42.3 mV dec-1 , while requiring overpotentials of 116 and 243 mV to attain 10 and 100 mA cm-2 for HER respectively. Moreover, Ni-Fe-K0.23 MnO2 CNFs-300 applied to overall water splitting exhibits a low cell voltage of 1.62 V at 10 mA cm-2 and excellent durability, even superior to the Pt/C||IrO2 cell at large current density. Density functional theory calculations further confirm that doping Ni and Fe into the crystal lattice of δ-MnO2 can not only reinforce the conductivity but also reduces the adsorption free-energy barriers on the active sites during OER and HER.