Dynamic structure evolution of free-standing S-doped porous Co-Fe microspheres with enhanced oxygen evolution electrocatalysis in alkaline media

Herein, S-doped Co-Fe hybrid films with unique 3D porous microsphere arrays stacked by ultrathin nanoflakes directly grown on nickel foam (Co-Fe-S NFs@MS/NF) are synthesized via a single-step co-electrodeposition strategy. The synergistic effects among Fe, S and Co enable superior oxygen evolution reaction (OER) catalytic activity in basic environments. Impressively, this hybrid material can achieve further enhanced OER catalytic performance with surface self-assembly through in-situ electrochemical oxidation activation in both gentle alkaline (GA: 1.0 M KOH at 298 K) and industrial alkaline (IA: 30 wt.% KOH at 353 K) water electrolysis conditions. The transition processes are revealed in detail regarding the chemical, structural, and catalytic properties of the catalysts served in the GA and IA conditions. The pristine quasi-amorphous Co-Fe-S is found to partially transform into crystallized Co(Fe)OOH in the GA condition, while it remains amorphous nature in the IA condition. The enhanced OER activity is attributed to the formation of active Co(Fe)OOH species with massively proliferated specific surface area, optimized electronic structure, and accelerated kinetics. The activated Co-Fe-S NFs@MS/NF exhibits excellent OER activity, achieving 10 and 500 mA cm −2 at ultralow potentials of 1.453 (iR corrected) and 1.517 V with outstanding durability in the GA and IA conditions, respectively. For full water splitting, the assembled two-electrode system based on the Co-Fe-S NFs@MS/NF anode and a Co-S based cathode affords voltages of only 1.518 and 1.685 V to robustly deliver 10 and 500 mA cm −2 in GA and IA conditions over 200 h, respectively.