Densifying Crystalline-Amorphous Ni₃S₂/NiOOH Interfacial Sites To Boost Electrocatalytic O₂ Production

The development of an efficient and low-cost electrocatalyst for oxygen evolution reaction (OER) is the key to improving the overall efficiency of water electrolysis. Here, we report the design of a three-dimensional (3-D) heterostructured Ni₉S₈/Ni₃S₂ precatalyst composed of unstable Ni₉S₈ and inert Ni₃S₂ components, which undergoes in situ electrochemical activation to generate an amorphous-NiOOH/Ni₃S₂ heterostructured catalyst. In situ Raman spectroscopy combined with ex situ characterizations, such as X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy, reveals that during the activation, Ni₉S₈ loses the sulfur element to form nickel oxides and eventually transforms to amorphous NiOOH at O₂-evolving potentials, while the Ni₃S₂ component is rather inert that its majority in the bulk remains, thus forming a 3-D congee-like NiOOH/Ni₃S₂ heterostructure with the Ni₃S₂ crystalline particles randomly dispersed among amorphous NiOOH species. Unlike the sparse heterostructure that consists of a layer of NiOOH on top of Ni₃S₂, our unique congee-like NiOOH/Ni₃S₂ heterostructure provides plentiful reactive amorphous-crystalline interfacial sites. Moreover, the partial electron transfer between the NiOOH and remaining Ni₃S₂, benefiting from their dense interfacial sites, contributes to a higher valence state of the Ni³⁺ active centers in NiOOH, hence optimizing the adsorption of OER intermediates. Density functional theory calculations further disclose that the electronic structure regulation not only optimizes the Gibbs free energy of intermediate adsorption but also tunes the OH* absorption behavior to be exothermic, elucidating the spontaneous occurrence of OH* absorption and hence improves the OER. Therefore, a low overpotential of only 197 mV at an O₂-evolving current density of 10 mA/cm², a small Tafel slope of 38.8 mV/dec, and good stability are achieved on the amorphous-NiOOH/crystalline-Ni₃S₂ heterostructured catalyst.