(Digital Presentation) Non-Metallic Atom (B, C, N, O, P) Doped-Nickel Sulfide for Efficient Electrochemical Oxygen Evolution Reaction: A First-Principles Study
Authors: Zheng Xingqun , Lu Shun 
Published: September 2023

Due to the relatively high energy barrier and the inertia involved in the 4e- process, oxygen evolution reaction (OER), greatly impedes the efficiency of water splitting devices.1 Ni-based sulfides, especially Ni3S2 based material has regarded as promising catalysts to replace noble OER catalysts due to its facile synthesis and good conductivity.2 Nevertheless, the intrinsic activity of Ni3S2 is still inferior to the Ru/Ir oxides catalysts. Heteroatom doping has been widely used to tune catalytic activity of Ni3S2. Similarly, nonmetal heteroatom can also engineer geometric and electronic structure and activate surface sites of catalysts.3 Moreover, as the radius and electronegativity of nonmetal atoms usually differ greatly from that of transition-metal atoms, nonmetal atoms are more likely to bring about changes in the local geometric and electronic structures of active sites of catalysts. This makes nonmetal heteroatoms more attractive in adjusting the electrocatalytic activity of materials.4 Herein, nonmetal doping would be a more promising way to regulate the electronic structure and OER activity of Ni3S2. Until now, there are few investigations on modulating the OER activity of Ni3S2 by doping nonmetal-atoms.

[Figure]

Figure 1. (a) The bond length of X-Ni1 (dX-Ni1) corelates with the electronegativity (χx) and atom radius (Rx) of X (the inset shows the top-view of Ni3S2(100) with labled Ni sites and two doping sites, Xout and Xin). (b)The Bader charge of Ni1, Ni2, Ni3 and Ni4 atoms. (c) The correlation between calculated overpotential (η) vs. difference between the adsorption energy of *OH and *O on different sites of all surfaces. (d) The improved active sites proportion (nsite) on X-Ni3S2(100) and the different between minimum η of each surface and η of Ni3S2(100). (e) OER polarization curves and (f) experimental overpotential (η) at current density of 10 mA cm-2, average turnover frequency and exchange current density(J0) of Ni3S2 NSs and C-Ni3S2 NSs.
Inspired by the above considerations, we systematically investigated the OER activities of five kinds of nonmetal atoms (X, X = B, C, N, O, P) doped Ni3S2 (X-Ni3S2) electrocatalysts and screened out the most promising X-Ni3S2 OER catalysts. The geometric and electronic structure of X-Ni3S2, intermediates adsorption, potential determining step (PDS), and theoretical overpotentials of OER were studied using the density functional theory (DFT) calculations. With 2.5 at% X doping content, the X-Ni3S2 shows good electronic conductivity which can benefit the charge transfer between the surface and the intermediates during electrocatalysis. Among all X, C and N cause more prominent local structure disturbance on Ni3S2(100) surface (Fig.1a). This change of local coordination environment furhter disturbe the surface charge distribution, enableing the charge of adjacent Ni sites are intensely altered X (Fig.1b). Further, the OER free energy diagram denonting that the formation of OOH is the the PDS on pristine Ni3S2(100) and various X-Ni3S2(100). Among all X dopants, C is most effective dopant which can reduce the OER theorotical overpotential of adjacent Ni sites by 0.16 V and 0.23V when X doping on top most layer (Cout-)) and sublayer (Cin-) of Ni3S2(100) respectivelly (Fig.1c). Moreover, C doping can effectively active the surface Ni sites and enable the 62.5% of Ni sites on C-Ni3S2(100) have better OER activity than that of Ni sites on pristine Ni3S2 (Fig.1d).Furthermore, C doped Ni3S2 nanosheets (C-Ni3S2 NSs), and Ni3S2 nanosheets (Ni3S2 NSs) were synthesized through the same method3 to verify its application in OER. Our electrochemical results verify that C-Ni3S2 NSs catalyst has higher OER activity than pristine Ni3S2 NSs, exhibiting lower overpotential of 261 mV at current density of 10 mA cm-2 (Fig.1e) and lower Tafel slope of 96.18 mV dec-1 compared with Ni3S2 NSs (310 mV, 109.35 mV dec-1 respectively). Besides, C- Ni3S2 NSs has higher exchange current density (J0) and average turnover frequency than pristine Ni3S2 NSs(Fig.1f). Our DFT calculation and experimental results jointly highlight the promising C dopant in promoting the OER performance of Ni3S2. Our study offers guidance for screening and fabricating promising OER catalysts through nonmetal doping engineering, which can inspire more exploration of nonmetal doping of other electrocatalysts.
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