Fe stabilized metallic phase of NiS2 for highly efficient oxygen evolution reaction
This work reports a fundamental study on the relationship of electronic structure, catalytic activity and surface reconstructure process of Fe doped NiS2 (FexNi1-xS2) for oxygen evolution reaction (OER). A combined photoemission and X-ray absorption spectroscopic study reveal Fe doping introduces more occupied Fe 3d6 states at top of the valence band and thereby induce a metallic phase. Meanwhile, Fe doping also significantly increases the OER activity and much better stability with optimum found at Fe0.1Ni0.9S2. More importantly, we performed detailed characterizations to track the evolution of the structure and composition of the catalysts after different cycles of OER testing. Our results further confirmed the catalysts gradually transform into amorphous (oxy)hydroxide which is the actual active species for OER. However, a fast phase transformation in NiS2 accompanies with a decrease of OER activity, because of the formation of a thick insulating NiOOH layer limiting electron transfer. On the other hand, Fe doping retards the process of transformation, because of a shorter Fe-S bond length (2.259 Å) than Ni-S (2.400 Å), explaining a better electrochemical stability of Fe0.1Ni0.9S2. These results suggest the formation of a thin surface layer of NiFe (oxy)hydroxide as active OER catalyst and the remaining Fe0.1Ni0.9S2 as conductive core for fast electron transfer is the base for the high OER activity of FexNi1-xS2. Our work provides important insight and design principle for metal chalcogenides as highly active OER catalysts.