In Operando X-ray Spectroscopic and DFT Studies Revealing Improved H2 Evolution by the Synergistic Ni–Co Electron Effect in the Alkaline Condition

The different electrolyte conditions, e.g., pH value, for driving efficient HER and OER are one of the major issues hindering the aim for electrocatalytic water splitting in a high efficiency. In this regard, seeking durable and active HER electrocatalysts to align the alkaline conditions of the OER is a promising solution. However, the success in this strategy will depend on a fundamental understanding about the HER mechanism at the atomic scale. In this work, we have provided thorough understanding for the electrochemical HER mechanisms in KOH over Ni- and Co-based hollow pyrite microspheres by in operando X-ray spectroscopies and DFT calculations, including NiS2, CoS2, and Ni0.5Co0.5S2. We discovered that the Ni sites in hollow NiS2 microspheres were very stable and inert, while the Co sites in hollow CoS2 microspheres underwent reduction and generated Co metallic crystal domains under HER. The generation of Co metallic sites would further deactivate H2 evolution due to the large hydrogen desorption free energy (−1.73 eV). In contrast, the neighboring Ni and Co sites in hollow Ni0.5Co0.5S2 microspheres exhibited the electronic interaction to elevate the reactivity of Ni and facilitate the stability of Co without structure or surface degradation. The energy barrier in H2O adsorption/dissociation was only 0.73 eV, followed by 0.06 eV for hydrogen desorption over the Ni0.5Co0.5S2 surface, revealing Ni0.5Co0.5S2 as a HER electrocatalyst with higher durability and activity than NiS2 and CoS2 in the alkaline medium due to the synergy of neighboring Ni and Co sites. We believe that the findings in our work offer a guidance toward future catalyst design.

Table S3.Crystallographic data of hollow NiS2 microspheres at different potentials in HER.…….17                  Fitting was done across the k range of 3.3 to 9.9 Å -1 and the R range of 1.0 to 2.4 Å for all samples.
Where N is coordination number, R is distance between absorber and backscatter atoms, σ 2 is Debye-Waller factor value, Rf is R-factor characterizing the goodness of fitting Fitting was done across the k range of 2.7 to 8.4 Å -1 and the R range of 1.0 to 2.5 Å for all samples.
Where N is coordination number, R is distance between absorber and backscatter atoms, σ 2 is Debye-Waller factor value, Rf is R-factor characterizing the goodness of fitting.

Figure S11 .
Figure S11.The plots of (a) micro  strain, (b) Rwp, (c) Uiso Co, (d) Uiso S, and (e) R(F 2 ) for hollow So 2 was fixed to 0.76 as determined from Co foil.Error bounds (accuracies) characterizing the structural parameters obtained by EXAFS data analysis are estimated to be as follows: N, ±20%; R, ±1%; and σ 2 , ±20%.Co foil fitting model is from ICSD Code # 44989, and Co-S fitting model is from CoS2 ICSD Code # 53068.

Figure S12 .
Figure S12.Top and side views of various transition metal pyrite surfaces.The grey, blue and yellow

Figure S13 .
Figure S13.Optimized structures of initial, transition, and final states for breaking the H-OH bond

Figure S14 .
Figure S14.Optimized structures of H adsorption on various surfaces.

Table S5 .
Crystallographic data of hollow CoS2 microspheres at different potentials in HER.……19 TableS6.Structural parameters obtained from Ni K-edge EXAFS fitting analysis for samples.…20TableS7.Structural parameters obtained from Co K-edge EXAFS fitting analysis for samples.…..21

Table S3 .
Crystallographic data of hollow NiS2 microspheres at different potentials in HER.

Table S4 .
Crystallographic data of hollow Ni0.5Co0.5S2microspheres at different potentials in HER.

Table S5 .
Crystallographic data of hollow CoS2 microspheres at different potentials in HER.

Table S6 .
Structural parameters obtained from Ni K-edge EXAFS fitting analysis for samples.

Table S7 .
. So 2 was fixed to 0.77 as determined from Ni foil.Error bounds (accuracies) characterizing the structural parameters obtained by EXAFS data analysis are estimated to be as follows: N, ±20%; R, ±1%; and σ 2 , ±20%.Ni foil fitting model is from ICSD Code # 37502, and Ni-S fitting model is from NiS2 ICSD Code # 68167.Structural parameters obtained from Co K-edge EXAFS fitting analysis for samples.