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Ni phosphide has been widely recognized as a promising candidate for HER electrocatalysts because of its unique properties, such as a high HER activity, good conductivity, and excellent physicochemical stability. However, nickel phosphides still have some drawbacks for practical applications. First, the intrinsic HER activity of nickel phosphides is still too low to match the activity of noble Pt-based catalysts. Second, the mass loading of nickel phosphide is also too high. Therefore, structural modulation at the lattice level is important to enhance HER activity of nickel phosphide electrocatalysts.
Various Ni metal precursors can be prepared by hydrothermal methods and then converted into nickel phosphide electrocatalysts by a low-temperature phosphidation reaction using hypophosphite as a P source. A variety of structures and morphologies have been reported and their HER performances in acidic or alkaline electrolytes are summarized in Table 3.
Direct phosphidation of Ni foam or foil with red phosphorous can be used to produce nanoparticles of nickel phosphides. The synthesis is a simple procedure, but it has some drawbacks. For example, the unphosphidated Ni foam or foil has a tendency to dissolve and the nickel phosphide structures are fragile. In addition, the nickel phosphide structures are difficult to control.
In-situ oxidative reduction of nickel/nickel phosphide on carbon cloth substrates can be used to prepare bifunctional electrodes with remarkable HER and OER activities. In-situ oxidative reduction is performed by an electrochemical deposition method, which is convenient and scalable. In the presence of a 0.5 M H2SO4 solution, the as-prepared Ni/nickel phosphide/carbon cloth electrodes exhibit cathodic currents of 10 mA/cm2 with an overpotential of 164 mV and a Tafel slope of 76 mV/dec.