Boosting the Overall Electrochemical Water Splitting Performance of Pentlandites Through Non-Metallic Heteroatom Incorporation&Nbsp;

Abstract

Material doping with foreign elements  is a powerful method to tune their properties. Yet, doping of metal-rich sulfides which revealed interesting properties as bifunctional catalysts for the overall water splitting has been hitherto rarely reported. We report herein on the synthesis of the heterotrimetallic pentlandite-type material Fe₃Co₃Ni₃S₈ (abbreviated as FCNS) in presence of suitable phosphorus- (FCNSP) and nitrogen-(FCNSN) donors and compare the performances of the resulting materials in the overall electrochemical water splitting. Throughout the experiments, a preferential incorporation of N in the FCNS-lattice is observed whereas the addition of phosphorus generally leads to metal-phosphate-FCNS composites. The obtained FCNSP, FCNSN, and FCNSNP materials facilitate the oxygen evolution reaction (OER) at 100 mA cm^-2 in 1.0M KOH with overpotentials of 479 mV, 440 mV, and 427 mV, respectively, outperforming the often-used literature benchmark IrO₂ (564 mV) and commercial Ni metal powder (> 600 mV) under identical conditions. Likewise, FCNSN and FCNSNP reveal an improved performance towards the hydrogen evolution reaction (HER) in 0.5M H₂SO_4,outperforming the pristine FCNS material. All materials revealed high stability, high morphological and structural robustness during OER and HER as evidenced by SEM and PXRD analysis after the electrochemical measurements. Notably, quantum-theoretical calculations at the level of density functional theory (DFT) suggest that N and P doping leads to optimized binding strengths of the surface-bound intermediates to boost the OER activity of the pristine FCNS. Likewise, N doping enhances the hydrogen adsorption activity at Ni and Fe in octahedral sites, leading to a significant improvement in the HER activity.

Keywords: Transition metal-rich sulfides, pentlandites, heteroatom doping, bifunctional inorganic electrodes, water oxidation, hydrogen production