NiP₂: A Story of Two Divergent Polymorphic Multifunctional Materials

Abstract

In this paper, we explore the drastic differences in transport properties and catalytic activities for two structural polymorphs of NiP 2 : cubic (Pa3; No. 205) and monoclinic NiP 2 (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 NiP 2 is a semimetal, while the monoclinic polymorph is an ntype direct band gap semiconductor. Both compounds exhibit low thermal conductivities, with cubic NiP 2 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 NiP 2 excels in both half-cell HER and OER measurements. In alkaline half-cell OER, cubic NiP 2 outperforms the RuO 2 standard. More importantly, HER tests in a PEM electrolysis single cell showed high promise for cubic NiP 2 , which requires only 13% higher overpotentials when compared to state-of-the-art Pt/IrRuO x -based assemblies, far surpassing any reported properties of metal pnictide or chalcogenide full cells.

Crystallographic facet selective HER catalysis: exemplified in FeP and NiP2 single crystals

“…45 The more difficult to remove ca. 5 nm layer on top of the m-NiP 2 single-crystal material is compositionally similar to the oxidation layers on as-prepared polycrystalline 17 and nanostructured Ni phosphide catalysts, 45,46 consistent with the previous interpretation of the Ni 2p 3/2 component at BE of 857.5 AE 0.2 eV as primarily Ni phosphate. Finally, the difficulty in removing the phosphate overlayer from m-NiP 2 , highlights the apparently signicant effect of the specic transition metal and bulk stoichiometry on the properties of the oxidized overlayers.…”

“…8c), the phosphide Ni 2p 3/2 and P 2p 3/2 components are, respectively, at BEs of 853.3 and 129.5 eV, in excellent agreement with the values characteristic of the monoclinic m-NiP 2 crystal structure. 17 The qualitatively different spectra (in terms of line shapes and BEs) of the as-prepared m-NiP 2 sample (Fig. 8a) are also consistent with the change of the sputtering rate aer removal of the topmost ca.…”

“…We have recently shown by studying the properties of two polymorphs of NiP 2 that the structure of bulk phases has an impact on the catalytic properties of the corresponding phosphide. 17 One possible explanation is that the monoclinic polymorph of NiP 2 is a semiconductor, while cubic one has metallic properties, which might be sufficient to explain the observed difference in reactivity instead of their bulk crystal structures.…”

“…We have recently shown that the catalytic performance of two polymorphs of the same metal phosphide in powder form, NiP 2 , was drastically different in both HER and OER. 17 Here, we address this issue by studying the catalytic activities of the different facets of the same material, either FeP or m-NiP 2 . Most studies conducted on TMPs for HER applications are focused on the reduction of the overpotential required to drive the catalyst.…”

The Role of Non‐Metallic and Metalloid Elements on the Electrocatalytic Activity of Cobalt and Nickel Catalysts for the Oxygen Evolution Reaction

“…A recent study that compared the properties of two different NiP 2 polymorphs, cubic NiP 2 ( Pa 3 space group), and monoclinic NiP 2 ( C 2/ c space group) revealed drastic differences in their electronic and electrochemical properties. This study established that cubic NiP 2 (c‐NiP 2 ) is metallic whereas monoclinic NiP 2 (m‐NiP 2 ) is a semiconductor and the latter exhibited considerably higher OER activity than the former (Figure ) . The difference in the OER activity of these two NiP 2 polymorphs was attributed to the electronic band structure based on density of states calculations of the two NiP 2 polymorphs.…”