NiP₂: A Story of Two Divergent Polymorphic Multifunctional Materials

In this paper, we explore the drastic differences in transport properties and catalytic activities for two structural polymorphs of NiP2: cubic (Pa3; No. 205) and monoclinic NiP2 (C2/c; No. 15). The former one has been long assumed to be the high-pressure metastable phase, as it had been originally synthesized through high-pressure methods. Synthetic and in situ synchrotron X-ray diffraction studies unambiguously show that the cubic polymorph can be synthesized at ambient pressure but irreversibly transforms into the monoclinic structure above 876 K. Band structure calculations and transport measurements show that cubic NiP2 is a semimetal, while the monoclinic polymorph is an n-type direct band gap semiconductor. Both compounds exhibit low thermal conductivities, with cubic NiP2 exhibiting a value of 1.7 W•m–1 K–1 at 300 K. The bulk structure of the phosphides may affect the surface-related properties. Unlike the monoclinic polymorph, cubic NiP2 excels in both half-cell HER and OER measurements. In alkaline half-cell OER, cubic NiP2 outperforms the RuO2 standard. More importantly, HER tests in a PEM electrolysis single cell showed high promise for cubic NiP2, which requires only 13% higher overpotentials when compared to state-of-the-art Pt/IrRuOx-based assemblies, far surpassing any reported properties of metal pnictide or chalcogenide full cells.