Understanding activity diversity among Ni-based chalcogenide pre-catalysts under oxygen evolution conditions: the role of oxyanions

Abstract

Ni-based chalcogenides have long been regarded as “pre-catalysts”, in which the electrochemically transformed oxyhydroxides are believed to be the real active species for the alkaline oxygen evolution reaction (OER). However, the origin of activity diversity among oxyhydroxides reconstructed from different Ni-based chalcogenides remains ambiguous. Herein, we utilized experiments and theoretical calculations to study the OER activity trend among NiS₂, NiSe₂, and NiTe, with a special focus on the anion modulation mechanism. Specifically, it was found that anions convert into oxyanions chemisorbed on the basal plane of NiOOH, with a thermodynamic priority over chalcogen dopants, upon the electrochemical reconstruction of Ni-based chalcogenides. Compared with selenates and tellurites, sulfates perform better as not only a proton-transfer station to accelerate the deprotonation of OH_ads, but also as a promoter to regulate the 3d orbital of the Ni active sites to thereby optimize the adsorption strength of the intermediates. Therefore, the derived sulfates make the OER facile by both adsorbate evolution and lattice oxygen mechanisms, thereby greatly improving the OER activity of NiOOH than do selenates and tellurites. The more obvious promotion effect of sulfates on NiOOH rationalized our experiment results showing that NiS₂ had an optimal OER activity among the studied Ni-based chalcogenides. Moreover, the electrochemical transformation of anions was found to be more thermodynamically beneficial than that of oxyanions in the solvent for the formation of an oxyhydroxides–oxyanions complex, rationalizing the fact that additive oxyanions have less chance to influence the OER activities of (oxy)hydroxides than in situ-constructed oxyanions. This work shows that the OER activity diversity among Ni-based chalcogenides is related to the in situ-formed oxyanions from anions, which can inspire the rational design of metal–metalloid compound pre-catalysts via engineering appropriate anion groups.