Water electrolysis is considered as a key energy source to produce hydrogen for achieving a clean and sustainable society. Improving the sluggish kinetics of the oxygen evolution reaction (OER) is focused to overcome the limit efficiency of water electrolysis. Transition metals, especially Ni, are one of the most promising materials for the alkaline OER because of good catalytic activities, stabilities and cost efficiency. Ni based materials, such as Ni oxyhydroxides, hydroxides, phosphides and sulfides, are intensively developed for use as OER catalysts. Recently, the sulfurization of transition metals is introduced as an effective method to improve the intrinsic electrocatalytic activities. The role of S has been summarized in 2 ways. S accelerates the regeneration of transition metals in alkaline media that S is dissolved and exchanged with O species. Mullins et al. reported Nickel oxide derived from Ni-S. It exhibits amorphous and large active surface morphology which is favorable OER. The other role is that remaining S modulates the electronic structures of Ni catalysts reducing the energy barrier for the OER. Wu et al. developed ultrathin Fe-doped Ni₃S₂ for overall water splitting. Based on density functional theory calculations, the Fe-doped Ni₃S₂ surface exhibits an optimized energy for water adsorption and dissociation, which enhances the intrinsic activity for the OER. However, in-situ/operando experimental studies regarding the effects of residual S are rare.

Herein, we report the role of the remaining S in Ni catalysts that exhibit longer Ni-O bonding in the NiOOH phase produced in-situ. C-supported Fe- and Co-doped nickel sulfide catalysts (NiFeCo-S/C) is prepared using a simple impregnation method with heating under an H₂S atmosphere. We tested the electrochemical properties of the NiFeCo-S/C in alkaline media of 1 M KOH. It achieved current density of 10 mA cm^-2 with only overpotential of 267.2 mV. Also, it maintained the OER activities for 24 h at current densities of 10 and 50 mA cm^-2, respectively. Moreover, the prepared catalyst was analyzed the change of electronic structure and bonding group using in-situ/operando X-ray absorption spectroscopy (XAFS) and surface-enhanced Raman spectroscopy (SERS) under OER condition. Based on these results, we could elucidate the effects of the remaining S during the OER.