The most knotty problem of (electro)catalysis that arises ubiquitously is the notion that the more active catalysts for oxygen evolution reaction (OER) will simultaneously suffer from low resistance toward corrosion. In developing cost-effective stable materials for the oxygen evolution reaction with high activity, precisely tuning the structure of transition-metal materials in atomic level to obtain ideal chemical properties is urgent while difficult to achieve¹. In this study, we propose a “dual ligand synergistic modulation” tactics to completely solve the painful compromise between the two conflicted indicators: stability and activity. By substituting S ligand into the adjustable and stable structure of layered hydroxides, which was selected as the start noumenon, we obtain a new class of NiCo₂(S_x·OH_2-x)_y with single phase and homogenous composition. The elaborated designed dual-ligand catalyst ideally inherits the excellent activity of the conventional transition metal sulfides and the superior stability of the conventional transition metal hydroxides and even exceeds the predecessors. Impressively, the optimized NiCo₂(SOH)_x performs a small overpotential of 0.32 V at 10 mA cm^-2, and shows no decay after 30h accelerated ageing even at a large current density of 100 mA cm^-2, both of which have entered the top of all the transition-metal hydroxides and sulfides reported to date and exceeded the commercial RuO₂ and Ir/C benchmarks. Theoretical calculations further disclose that the OH ligands on the surface of NiCo₂(SOH)_x could attracts electrons from the antibonding orbital of M-S bonds to M-O bonds, resulting in a strengthened binding energy between metal and S and thus enhancing the stability. Meanwhile, the synergy between S ligands and OH groups appropriately tunes the electronic structure of NiCo₂(SOH)_x and thus renders it with optimal binding energies for OER intermediates (*OH, *O, and *OOH), which is benefit for facilitate the O₂ evolution proceeding. Besides, the varied magnetism induced by the modulation of catalysts’ electronic structure significantly influences the desorption action of paramagnetic O₂ from the catalyst surface. The special non-magnetic NiCo₂(SOH)_x suffers the lowest O₂ desorption resistance and proceeds the smoothest reaction kinetics pathway among NiCo₂(S_xOH_2-x)_y catalysts, thus owning the topmost OER activity.

Figure 1. (a) The ultimate unity of OER performance of NiCo₂(S_xOH_2-x)_y. (b) The reaction steps and corresponding free energy diagram of the OER process. (c) OER polarization curves of Ni, RuO₂, NiCo₂S₄, NiCo₂(OH)_x, and NiCo₂(SOH)_x and (d) Chronoamperometric curves of Ni, NiCo₂(SOH)_x and NiCo₂S₄ at a current density of 100mA/cm².

Key words: Oxygen evolution, electrocatalysis, dual-ligand modulation

Reference:

1. Lishan Peng, Jun Wang, Yao Nie, Kun Xiong, Yao Wang, Ling Zhang, Ke Chen, Wei Ding, Li Li, and Zidong Wei*. ACS Catal., 2017, 7, 8184-8191.