Perovskite PbCoO synthesized at 12 GPa was found to have an unusual charge distribution of PbPbCoCoO with charge orderings in both the A and B sites of perovskite ABO. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to PbPbCoCoO quadruple perovskite structure. It is shown that the average valence distribution of PbCoO between PbCrO and PbNiO can be stabilized by tuning the energy levels of Pb 6s and transition metal 3d orbitals.

Generation of hydrogen by splitting water with the electrocatalytic approach could become a more sustainable way following the discovery of new materials, such as the 2D transition-metal carbides. Developing eco-friendly, low-cost, stable, and highly active nonprecious hydrogen evolution reaction (HER) catalysts is one of key factors for hydrogen energy economy. Two-dimensional metal carbide and nitride (MXenes) materials have shown characteristics of promising HER catalysts. Herein, we explored the conductive and thermal stability and electrocatalyst performance of four 2D ordered double MXenes M2 ′M″C2, Cr2TiC2, Cr2VC2, Mo2TiC2, and Mo2VC2, and their corresponding oxygen (O*)- or hydroxyl (OH*)-terminated MXenes by using density functional calculations. Results indicated that all the above MXenes are conductive, which are favored to charge transfer during HER. Four MXenes are fully terminated by O* under standard conditions [pH = 0, p(H2) = 1 bar, U = 0 V]. The Gibbs free energy for the adsorption of atomic hydrogen (ΔG H*) on the O*-terminated M2 ′M″C2 (e.g., Cr2TiC2O2) is close to 0 eV (the ideal value) at suitable H coverage. The formability of oxygen vacancy in the fully O*-terminated M2 ′M″C2, that is, M2 ′M″C2O2 was studied, and a linear relationship between the formation energy of oxygen vacancy (E f) and ΔG H* was obtained. The electronic structure analysis indicates that the more electrons gained by the terminated O* from M2 ′M″C2, the higher is the occupation of the p orbitals of the terminated O* and thus the weaker is the binding strength between the terminated O* and the adsorbed H. Our results indicated that O*-terminated M2 ′M″C2 are promising HER electrocatalysts for generating hydrogen by water splitting.

Nanostructure-supported single-atom bifunctional catalysts can reduce the usage of catalyst and improve the catalytic activity compared with unifunctional catalysts. Developing effective bifunctional electrocatalysts for water splitting is one of the central issues to the area of renewable energy. Herein, we report a single transition-metal atom anchoring on the Cr2CO2 MXene surface as bifunctional eletrocatalyst for water splitting through density functional theory calculations. Results show that Ni anchored on Cr2CO2 (Ni/Cr2CO2) MXene exhibits satisfactory catalytic activity producing low overpotentials of 0.16 and 0.46 V for HER and OER, respectively. Large amounts of electrons were transferred from Ni to the surface O* of Cr2CO2, promoting the binding strength between Ni and Cr2CO2 (binding energy is −5.16 eV). The ultrahigh Ni oxide formation pressure (O2 pressure is higher, at 3 × 1019Pa) ensures the stability of Ni/Cr2CO2 during electrocatalytic water splitting. Moreover, ab initio molecular dynamics simulations and climbing nudged elastic band calculations suggest that Ni atom can be stably immobilized on Cr2CO2 substrate to prevent its aggregation to form Ni3 and Ni4 clusters. In addition, the possible synthesis route is predicted for a Cr2CO2-supported Ni single-atom catalyst (SAC) system showing that Ni/Cr2CO2 can be experimentally synthesized. This work shows that Cr2CO2-supported Ni SAC can be a potential catalyst for water splitting and therefore provides an opportunity for energy conversion.

First principles calculations on an Al, Ti, Mn, and Ni doped MgH2 (110) surface were carried out to study the influence of dopants on the dehydrogenation properties of MgH2. It was shown that Al prefers to substitute for an Mg atom, whereas Ti, Mn, and Ni prefer to occupy interstitial sites. The dopants used different mechanisms to improve the dehydrogenation properties of MgH2. Al weakens the interactions between the Mg and the H atoms in its vicinity and so slightly improved the dehydrogenation properties of the Al doped system. The H atoms near the dopants of the transition metal doped systems were dramatically distorted. Ti has a high potential to generate a TiH2 phase by attracting two H atoms, which frees one H atom from its host Mg atom. The dehydrogenation properties of the Mn doped system were improved by the formation of a Mn−H cluster with a similar structure to Mg3MnH7 but weaker interactions between its atoms. If the MgH2 (110) surface is doped with Ni, the Ni will attract four H atoms to form a regular tetrahedral NiH4 group almost identical in structure to that in Mg2NiH4. The improvement of the dehydrogenation properties of Ni-doped MgH2 is expected as the bonding between the Mg and the H atoms is weakened, and there is a high possibility that the Mg2NiH4 phase will be formed, which is thermodynamically less stable than the MgH2 in this system.