Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Mesoporous nanostructures of NiCo-LDH/ZnCo2O4 as an efficient electrocatalyst for oxygen evolution reaction

Increasing energy demands for pollution-free and renewable energy technologies have stimulated intense research on the development of inexpensive, highly efficient, and stable non-noble metal electrocatalysts for oxygen evolution reaction (OER). In this study, a superior OER performance was achieved using a tri-metallic (Zn, Co, Ni) high-performance electrocatalyst. We successfully fabricated a peony-flower-like hierarchical ZnCo₂O₄ through an additive-free hydrothermal reaction followed by heat treatment. Then NiCo-LDH (layered double hydroxides) nano-flakes was electrodeposited on the ZnCo₂O₄/GCE surface to prepare NiCo-LDH/ZnCo₂O₄/GCE which was used as electrode for OER. The structure and morphology of the catalysts were characterized by several techniques including Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, elemental mapping and Brunauer-Emmett-Teller method. The NiCo-LDH/ZnCo₂O₄ catalyst provided high catalytic activity toward OER under alkaline condition (1.0 M KOH) with a low overpotential of 260 mV to drive the benchmark current density of 10 mA cm^-2 and Tafel slope of 62 mV dec^-1, as well as long-term stability and high turnover frequency of 0.0641 s^-1 at overpotential of 340 mV. The NiCo-LDH/ZnCo₂O₄ catalyst was found to perform significantly better than NiCo-LDH, ZnCo₂O₄, NiCo-LDH/Co₃O₄, Co₃O₄, and commercial RuO₂ catalysts. The outstanding OER performance of NiCo-LDH/ZnCo₂O₄ catalyst, which may be attributed to the large specific surface area, accelerated mass and electron transport, and synergistic effect of multiple hybrid materials, makes it a promising catalyst for OER.

Self-supported nickel-cobalt nanowires as highly efficient and stable electrocatalysts for overall water splitting

The development and design of highly active and stable electrocatalysts based on cheap and Earth-abundant materials is critically important to enable water splitting as a desirable renewable energy source. Herein, we fulfill the significant electrochemical water splitting enhancement in both electrocatalytic activity and durability by constructing self-supported nickel-cobalt nanowire catalysts with abundant oxygen vacancies. Specifically, the rich oxygen vacancies can largely promote the oxygen evolution reaction (OER) activity of optimal Ni₁Co₁O₂ NWs with a relatively low overpotential of 248 mV to drive a current density of 10 mA cm^-2. More significantly, after the phosphorization of Ni₁Co₁O₂ NWs, the resultant Ni₁Co₁P NWs can also display excellent electrocatalytic hydrogen evolution reaction (HER) performances with an overpotential of only 101 mV to achieve a current density of 10 mA cm^-2. Furthermore, benefiting from the unique 1D nanowire structure, the synergistic effect, and the optimal Gibbs free energy for hydrogen evolution evolved from the phosphorization, the Ni₁Co₁O₂ NWs//Ni₁Co₁P NWs couple is thus highly active and stable for overall water electrolysis with a low voltage of 1.58 V at 10 mA cm^-2, showing extraordinary promise for practical overall water splitting electrolysis.