Recent advances of two-dimensional CoFe layered-double-hydroxides for electrocatalytic water oxidation

Construction of iron-doped nickel cobalt phosphide nanoparticles via solvothermal phosphidization and their application in alkaline oxygen evolution

Multi-metallic phosphides offer the possibility to combine the strategies of surface reconstruction, electronic interaction and mechanistic pathway tuning to achieve high electrocatalytic oxygen evolution activity. Here, iron-doped nickel cobalt phosphide nanoparticles (FexCoyNi2-x-yP) with the crystalline NiCoP phase are for the first time synthesized by the solvothermal phosphidization method via the reaction between metal-organic frameworks and white phosphorus. When used to electrochemically catalyze oxygen evolution reaction (OER), the Fe0.4Co0.8Ni0.8P supported by nickel foam requires only 248 mV overpotential to achieve 10 mA cm-2 current densities, and is robust towards the long-term OER in 1 M KOH. The higher number of electrochemically active sites can account for the good OER activity, along with the improved intrinsic activity which is caused by the electron interaction that optimizes the adsorption energy of hydroxyl intermediates, and that increases the acidity of high-valent metal centers. The OER mechanistic pathway involves both adsorbate and lattice oxygen. Surface conversion is observed after OER in alkaline solution, and metal phosphide layer transforms to metal oxides and (oxy)hydroxides.

Bimetallic metal-organic framework-derived bamboo-like N-doped carbon nanotube-encapsulated Ni-doped MoC nanoparticles for water oxidation

Molybdenum carbide materials with unique electronic structures have received special attention as water-splitting catalysts, but their structural stability in the alkaline water electrolysis process is not satisfactory. This study reports an in situ pyrolysis method for preparing NiMo-based metal-organic framework (MOF)-derived chain-mail oxygen evolution reaction (OER) electrocatalysts and bamboo-like N-doped carbon nanotube (NCNT)-encapsulated Ni-doped MoC nanoparticles (NiMoC-NCNTs). The NCNTs can provide chain mail shells to protect the inner highly reactive Ni-doped MoC cores from electrochemical corrosion by the alkaline electrolyte and regulate their catalytic properties through charge redistribution. Benefiting from high N-doping with abundant pyridinic moieties and abundant active sites of the periodic bamboo-like nodes, the as-prepared NiMoC-NCNTs display an outstanding activity for the OER with an overpotential of 310 mV at 10 mA cm-2 and a superior long-term stability of 50 h. Density functional theory calculations reveal that the excellent electrocatalytic activity of NiMoC-NCNTs comes from the electron transfer from NiMoC nanoparticles to NCNTs, resulting in a decrease in the local work function at the carbon surface and optimized free efficiencies of OER intermediates on C sites. This work provides an effective approach to improve the structural stability of fragile catalysts by equipping them with carbon-based chain.

Substrate self-derived porous rod-like NiS/Ni9S8/NF heterostructures as efficient bifunctional electrocatalysts for overall water splitting

A self-derivation strategy using conductive substrates is used to prepare one-piece highly efficient bifunctional electrodes, where the chosen substrate acts as both an active catalyst precursor and a conductive carrier. Here, a bifunctional catalyst, porous NiS/Ni9S8/NF-2 nanorods, was synthesized by low-temperature vulcanization after an oxalic acid etching process. To reach a current density of 10 mA cm-2, NiS/Ni9S8/NF-2 requires only a tiny overpotential of 115 mV for the HER and 176 mV for the OER, and demonstrates sustained activity for 100 hours with almost any degradation. The substrate self-derived NiS/Ni9S8/NF-2 catalyst for overall water splitting requires only a small voltage of 1.52 V to deliver 10 mA cm-2 with excellent stability. Experimental results show that the heterostructured electrocatalysts impart good catalytic properties of NiS/Ni9S8/NF-2 by modulating the electronic structure and promoting the reconstruction process from sulfides to hydroxides. This work demonstrates the success of the substrate self-derivation strategy to achieve high catalytic activity and provide a new autogenous growth technique for electrode fabrication.

Recent Advances of Modified Ni (Co, Fe)-Based LDH 2D Materials for Water Splitting

Water splitting technology is an efficient approach to produce hydrogen (H₂) as an energy carrier, which can address the problems of environmental deterioration and energy shortage well, as well as establishment of a clean and sustainable hydrogen economy powered by renewable energy sources due to the green reaction of H₂ with O₂. The efficiency of H₂ production by water splitting technology is intimately related with the reactions on the electrode. Nowadays, the efficient electrocatalysts in water splitting reactions are the precious metal-based materials, i.e., Pt/C, RuO₂, and IrO₂. Ni (Co, Fe)-based layered double hydroxides (LDH) two-dimensional (2D) materials are the typical non-precious metal-based materials in water splitting with their advantages including low cost, excellent electrocatalytic performance, and simple preparation methods. They exhibit great potential for the substitution of precious metal-based materials. This review summarizes the recent progress of Ni (Co, Fe)-based LDH 2D materials for water splitting, and mainly focuses on discussing and analyzing the different strategies for modifying LDH materials towards high electrocatalytic performance. We also discuss recent achievements, including their electronic structure, electrocatalytic performance, catalytic center, preparation process, and catalytic mechanism. Furthermore, the characterization progress in revealing the electronic structure and catalytic mechanism of LDH is highlighted in this review. Finally, we put forward some future perspectives relating to design and explore advanced LDH catalysts in water splitting.

Pearson’s Principle-Inspired Robust 2D Amorphous Ni-Fe-Co Ternary Hydroxides on Carbon Textile for High-Performance Electrocatalytic Water Splitting

Layered double hydroxide (LDH) is widely used in electrocatalytic water splitting due to its good structural tunability, high intrinsic activity, and mild synthesis conditions, especially for flexible fiber-based catalysts. However, the poor stability of the interface between LDH and flexible carbon textile prepared by hydrothermal and electrodeposition methods greatly affects its active area and cyclic stability during deformation. Here, we report a salt-template-assisted method for the growth of two-dimensional (2D) amorphous ternary LDH based on dip-rolling technology. The robust and high-dimensional structure constructed by salt-template and fiber could achieve a carbon textile-based water splitting catalyst with high loading, strong catalytic activity, and good stability. The prepared 2D NiFeCo-LDH/CF electrode showed overpotentials of 220 mV and 151 mV in oxygen evolution and hydrogen evolution reactions, respectively, and simultaneously had no significant performance decrease after 100 consecutive bendings. This work provides a new strategy for efficiently designing robust, high-performance LDH on flexible fibers, which may have great potential in commercial applications.

Facile Synthesis of Sulfate-Intercalated CoFe LDH Nanosheets Derived from Two-Dimensional ZIF-9(III) for Promoted Oxygen Evolution Reaction

Layered double hydroxide (LDH) has emerged as a promising electrocatalyst; however, the synthetic method usually requires high temperature and high pressure, and sulfate-intercalated LDH is rarely reported. Herein, the sulfate-intercalated CoFe LDH nanosheets were successfully fabricated at ambient temperature via a facile strategy, using two-dimensional ZIF-9(III) as a template and FeSO4 as both etchant and iron source. When the as-prepared sulfate-intercalated CoFe LDH acts as an electrocatalyst, it presents superior electrocatalytic performance for the oxygen evolution reaction (OER), requiring low overpotential (η@10 mA cm−2 = 218 mV) with a small Tafel slope of 59.9 mV dec−1 in 1.0 M KOH, which compares favorably with commercial RuO2 and most reported transition-metal electrocatalysts. The high catalytic activity of CoFe LDH might be ascribed to the large interlayer space distance originating from special SO42− ions and the strong synergistic effects between Fe and Co. This work provides a novel and feasible approach to designing highly efficient electrocatalysts based on advanced LDH materials for OER.