Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (M x P y ) have recently been identified as a promising family of Earth abundant electrocatalysts for the hydrogen-evolution reaction (HER) and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions. In this review, we highlight the progress that has been made in this field and provide insights into the synthesis, characterization, and electrochemical behavior of transition metal phosphides as HER electrocatalysts. We also discuss strategies for the incorporation of metal phosphides into integrated solar-driven water-splitting systems and highlight key considerations involved in the testing and benchmarking of such devices.

Metal phosphides have emerged as promisingEarth-abundant alternatives to platinum for catalyzing the hydrogen evolution reaction (HER) in acidic aqueous solutions. Here, Co 2 P nanoparticles having a hollow, multifaceted, crystalline morphology have been evaluated as HER electrocatalysts at a mass loading of 1 mg cm −2 on Ti foil substrates. The Co 2 P/Ti electrodes required low overpotentials of −95 and −109 mV to produce operationally relevant cathodic current densities of −10 and −20 mA cm −2 , respectively. These values establish Co 2 P nanoparticles as highly active Earth-abundant HER catalyst materials. Importantly, the Co 2 P nanoparticles are morphologically equivalent to previously reported CoP nanoparticle HER catalysts, allowing a direct side-by-side evaluation of their HER activities. Such comparisons of different metal phosphide HER catalysts with the same constituent elements and morphologies are important for identifying the key materials characteristics that lead to high activity.

Nanostructured transition-metal phosphides have recently emerged as Earth-abundant alternatives to platinum for catalyzing the hydrogen-evolution reaction (HER), which is central to several clean energy technologies because it produces molecular hydrogen through the electrochemical reduction of water. Iron-based catalysts are very attractive targets because iron is the most abundant and least expensive transition metal. We report herein that iron phosphide (FeP), synthesized as nanoparticles having a uniform, hollow morphology, exhibits among the highest HER activities reported to date in both acidic and neutral-pH aqueous solutions. As an electrocatalyst operating at a current density of -10 mA cm(-2), FeP nanoparticles deposited at a mass loading of ∼1 mg cm(-2) on Ti substrates exhibited overpotentials of -50 mV in 0.50 M H2SO4 and -102 mV in 1.0 M phosphate buffered saline. The FeP nanoparticles supported sustained hydrogen production with essentially quantitative faradaic yields for extended time periods under galvanostatic control. Under UV illumination in both acidic and neutral-pH solutions, FeP nanoparticles deposited on TiO2 produced H2 at rates and amounts that begin to approach those of Pt/TiO2. FeP therefore is a highly Earth-abundant material for efficiently facilitating the HER both electrocatalytically and photocatalytically.