Bifunctional oxygen electrode cobalt–nickel sulfides catalysts originated from intercalated LDH precursors

Bimetallic cobalt–nickel sulfide nanoparticles anchored on S-, N-codoped holey carbon nanosheets (CoNi-S-T@NCFs) with a hydrangea-like morphology, were synthesized via a confinement synthesis route, in which an intercalated LDH precursor was subjected to the interlayer-confined carbonization and host-layer sulfurization. The phase transformation and structure evolution (e.g., atom site occupancy, crystallite size, and cell volume) of the CoNi-S-T@NCFs electrocatalysts, as a function of sulfurization temperatures, were confirmed by X-ray diffraction and Rietveld analyses. The sulfur vacancies effectively enhance the electrocatalytic activity, while the synergistic effect of (Co,Ni)₇S₈ alloy and S, N-codoped carbon matrix facilitates the electron transfer and accelerates reaction kinetics, making CoNi-S-900@NCFs an efficient and stable bifunctional electrocatalyst for oxygen reduction reaction (ORR). The rich high-valence Co (III) and Ni (III) of CoNi-S-900@NCFs facilitates the in-situ transformation of the metal (oxy)hydroxides intermediates with high catalytic activity for oxygen evolution reaction (OER). Thus, with a bifunctional parameter, ΔE, of 0.75 V (E_{j=10, OER} - E_{1/2, ORR}), this electrocatalyst slightly outperforms the state-of-the-art commercial Pt/C + RuO₂/C catalyst (ΔE = 0.76 V) in alkaline medium. This work demonstrates the influence that the sulfurization temperature has on the relationship between the structure and electrocatalytic performance of bimetallic sulfides prepared by the synthesis strategy using the intercalated LDH precursor. This strategy can be extended to prepare other chalcogenides with binary or ternary transition metals.