Recent advances in phase, size, and morphology-oriented nanostructured nickel phosphide for overall water splitting

Abstract

Currently, world is dealing with two major hurdles towards a prosperous economy, i.e., energy crisis and environmental pollution, which challenge the development and growth of the living society. In this regard, semiconductor-oriented heterogeneous photocatalysis, electrocatalysis, and photoelectrocatalysis via fuel cell technology are green and sustainable technologies that have been receiving wide attention by the scientific community for tackling these two issues. In addition, in the present scenario, the synthesis of cost-effective and robust catalysts with high stability, corrosion resistance in any medium, and size and morphology oriented in the micro and nanoscale is a hot topic of research. Therefore, transition metal phosphides (TMPs) and their composite-based catalysts are extensively used in these fields, replacing previously used noble metals such as Pt, Au, and Ag due to scarcity, instability, and high price. Predominantly, nickel phosphide-based materials among all TMPs have proven to be excellent catalysts regarding these fields of application. The metal–phosphorus bonding, basically the Ni–P bonding present in the catalyst, and its resemblance with the [NiFe] hydrogenase makes it more stable and resistive. Several phases of nickel phosphides are available with different lattice structures. Moreover, according to their metal and phosphorus proportion, they can be divided in two categories as metal-rich nickel phosphides and phosphorus-rich nickel phosphides. With respect to this, their activities are also reflected by their elemental contents; the phosphorus-rich nickel phosphides are excellent in electrocatalytic hydrogen evolution reactions (HER), whereas metal-rich nickel phosphides are excellent in electrocatalytic oxygen evolution (OER). However, some metal-rich nickel phosphides with high P/Ni ratio, i.e., Ni₅P₄, show better activity towards HER because of the high P content. The Ni₂P phase also plays an active role in both HER and OER, though it belongs to the metal-rich phase. This shows good catalytic behavior towards HER due to presence of well-defined exposed facets and proper proportion of Ni and P atoms. Furthermore, different types of composites are still being developed by modifying their morphology and size through different synthetic strategies with the expectation of more enhanced activity. This review gives an overview of the design and development of nanostructured nickel phosphide-based materials and their recent significant trends in electrocatalytic, photoelectrocatalytic, and photocatalytic water splitting reactions. Moreover, we present the state-of-art various nickel phosphide materials in last five years, which highlight challenges and future prospects involving some critical issues in these emerging fields that await further exploration, and opportunities in the present and future research.