NiO x , long studied for its use in nickel-based secondary batteries, has been the subject of much recent interest due to its efficacy as an oxygen evolution catalyst. Despite extensive study over more than a century, however, many outstanding questions remain surrounding both the structure and the activity of NiO x . Further compounding this ambiguity is the recent finding that much of the previous experimental work on NiO x may have been influenced by incidental doping. Here, we report a computational study of the two simplest members of the NiO x family: β-Ni­(OH)2 and β-NiOOH. Using DFT+U calculations, we first identify a β-NiOOH structure with a staggered arrangement of intercalated protons that is more consistent with experimental crystal structures of β-NiOOH than previously proposed geometries. Next, by conducting a thorough study of various initial spin configurations of this β-NiOOH structure, we found that a low-spin d7 Ni3+ configuration is always favored, which suggests a Jahn–Teller distortion, rather than disproportionation, explains the different Ni–O bond distances found in experiment. G 0 W 0 calculations performed on β-Ni­(OH)2 and β-NiOOH reveal electronic structures consistent with previous experimental results. Lastly, calculations of various low-index surface energies of both β-Ni­(OH)2 and β-NiOOH demonstrate that the (001) surface is the most thermodynamically stable surface, in keeping with numerous experimental results but in contrast to recent computational models.

NiO x has long been studied both as a battery cathode material and electrocatalyst for the oxygen evolution reaction (OER). Numerous investigations have demonstrated that Fe-doped nickel oxyhydroxide (NiOOH) is one of the most active OER catalysts in alkaline media. Despite extensive research, however, many unanswered questions pertaining to the OER mechanism on this material remain. Here, using density functional theory + U calculations, we compare several surfaces of β-NiOOH studied for the OER and determine that unlike some earlier models selected, the (001) surface is the most stable surface under electrochemical conditions. We then examine several magnetic states of this material and predict that, unlike bulk β-NiOOH, (001)-β-NiOOH manifests a slight preference to be ferromagnetic. We then use the resulting structural model to compare in detail four commonly proposed OER mechanisms. In addition to excluding a proposed mechanism involving hydrogen peroxide formation, we identify multiple binuclear mechanisms with slightly lower overpotentials than the commonly studied associative mechanism. All exhibit overpotentials that coincide well with measured values. However, the similarity in calculated overpotentials highlights the fact that several mechanisms are likely to be operative under electrochemical conditions on β-NiOOH. This finding suggests that much of the complexity of studying the OER on NiOOH is due to multiple competing mechanisms occurring under given conditions, which should be accounted for in subsequent analyses.

Computational searches for catalysts of the hydrogen evolution reaction commonly use the hydrogen binding energy (HBE) as a predictor of catalytic activity. Accurate evaluation of the HBE, however, can involve large periodic supercell slab models that render high-throughput screening relatively expensive. In contrast, calculations of other relevant surface properties, such as the surface energy, work function, and potential of zero charge (PZC), require only small surface unit cells and are hence less expensive to compute. Correlations between catalytic activity and these surface properties warrant exploration because of this reduced computational cost. Here, we use density functional theory in conjunction with three different exchange-correlation functionalsthe local density approximation (LDA), the Perdew−Burke− Ernzerhof (PBE) generalized gradient approximation, and the PBEsol functional (a reparameterization of the PBE functional)to calculate the lattice constants, surface energy, cohesive energy, and work function of six common catalysts: three metals (Au, Pd, and Pt) and three transitionmetal carbides (TMCs; WC, W 2 C, and Mo 2 C). The three exchange-correlation functionals produce identical trends, and PBEsol yields results between those calculated using LDA and PBE and most often closer to experiment. We therefore use PBEsol to obtain the surface energy, work function, and PZC of nine novel hybrid catalysts, each containing a metal monolayer on a TMC substrate. Importantly, a volcano-shaped correlation between the experimental exchange current density and the theoretical surface energies emerges. We also investigate solvation effects on the surface energy and work function using a polarizable continuum model within the framework of joint density functional theory. For these particular materials, the surface energies in vacuum are nearly unchanged upon exposure to an aqueous solution, prior to any reaction with water. The volcano-shaped correlation observed between the exchange current densities and the surface energies is not observed for the work function or PZC. Our work thus reveals opportunities for more rapid computational screening of reduced Pt-loading catalysts using the surface energy as a computationally efficient catalytic descriptor.

Iron-doped nickel oxyhydroxide has been identified as one of the most active alkaline oxygen evolution reaction (OER) catalysts, exhibiting an overpotential lower than values observed for state-of-the-art precious metal catalysts. Several computational investigations have found widely varying effects of doping on the theoretical overpotential of the OER on NiOx. Comparisons of these results are made difficult by the numerous differences in the structural and computational parameters used in these studies. In this work, within a consistent framework, we calculate the theoretical overpotentials for reactions occurring on the most stable, basal plane of undoped and doped β-NiOOH. We compare the activities of Fe(iii), Co(iii), and Mn(iii) doping using density functional theory with Hubbard-like U corrections on the transition-metal d orbitals. We compare the effect of surface and subsurface doping in order to establish whether the dopants act as new active sites for the reaction or whether they induce more widespread changes in the material. The results of our study find only a small reduction in the overpotential (∼0.1 and ≤0.05 V when doped in the surface and subsurface layers, respectively) for the three dopants, if doped in the dominant basal plane. This is much less than the reductions of 0.3 V experimentally observed for the most active Fe-doped systems. Furthermore, the magnitudes of reductions in overpotentials for the three dopants are similar. This work therefore disqualifies the possibility of enhancing the activity of the dominant exposed basal plane of β-NiOOH through substitutional doping.

A method of forming crystalline tungsten carbides was reported by exposing the heated tungsten filament to 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) in a hot-wire chemical vapor deposition process. Methyl radicals produced from the decomposition of TMDSCB on the filament serve as the carbon source. The formation of tungsten carbides was investigated by X-ray diffraction, cross-sectional scanning electron microscopy, and in-situ filament resistance measurements. A pure W2C phase was formed at a high temperature of 2400 °C after 1–2 h exposure time with a growth rate of 4.4 μm min–1. The growth of the W2C layer is found to be a diffusion-controlled process. Our study at longer deposition time of 3–4 h shows that once the metal filament is fully carburized to form W2C, the carbon-rich WC phase starts to form on the outside layer upon further exposure to TMDSCB. A WC layer with no contamination from the W2C phase was found to be formed at 2400 °C and 4 h deposition time.