An Fe-doped Co11(HPO3)8(OH)6 nanosheets array for high-performance water electrolysis

Abstract

The water electrolysis for commercial hydrogen production is dependent on efficient and economical electrocatalysts toward both half reactions of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In this work, an Fe-doped Co11(HPO3)8(OH)6 nanosheets array was fabricated on Ni foam via a one-step liquid-phase approach in the solvents of deionized water and isopropyl alcohol as such a bifunctional electrocatalyst. The different proton acceptors of hydroxyl and phosphite in Co11(HPO3)8(OH)6 nanosheets expedited the proton and electron transfer. The Fe dopant in Co11(HPO3)8(OH)6 adjusted the electronic structure of Co11(HPO3)8(OH)6, which significantly increased the hydrophilicity, conductivity, and active surface area. The density functional theory (DFT) calculations showed the Gibbs free energy of OER intermediates and H* adsorption energy in HER process were lowered, while the density of states (DOS) in the d-band center was increased by Fe doping, suggesting that Fe-doped Co11(HPO3)8(OH)6 is more active towards OER and HER processes. As a result, the optimized Fe11.7%-Co11(HPO3)8(OH)6 shows outstanding activity, durability and robust structural stability toward both OER and HER in 1 M KOH electrolyte. For OER, it outputs 20 mA cm−2 merely at an overpotential of 206 mV. Benefited from monolithically integrated nanosheets array structure on Ni foam, it can stably drive 500 mA cm−2 at a fairly low overpotential of 268 mV. For HER, low overpotentials of 102 mV for 10 mA cm−2 and 263 mV for 500 mA cm−2 are acquired on Fe11.7%-Co11(HPO3)8(OH)6. This offers a water-alkali electrolyzer by two Fe11.7%-Co11(HPO3)8(OH)6 electrodes implements a low cell voltage of 1.494 V for 10 mA cm−2, and 1.772 V for 500 mA cm−2 toward overall water electrolysis with striking stability. This work opens a promising avenue in exploring highly active and stable catalysts by transition metal phosphites with suitable element doping toward scale-up water electrolysis.