Xiaomin Xu

Recent Progress in Metal‐Organic Frameworks for Applications in Electrocatalytic and Photocatalytic Water Splitting

The development of clean and renewable energy materials as alternatives to fossil fuels is foreseen as a potential solution to the crucial problems of environmental pollution and energy shortages. Hydrogen is an ideal energy material for the future, and water splitting using solar/electrical energy is one way to generate hydrogen. Metal‐organic frameworks (MOFs) are a class of porous materials with unique properties that have received rapidly growing attention in recent years for applications in water splitting due to their remarkable design flexibility, ultra‐large surface‐to‐volume ratios and tunable pore channels. This review focuses on recent progress in the application of MOFs in electrocatalytic and photocatalytic water splitting for hydrogen generation, including both oxygen and hydrogen evolution. It starts with the fundamentals of electrocatalytic and photocatalytic water splitting and the related factors to determine the catalytic activity. The recent progress in the exploitation of MOFs for water splitting is then summarized, and strategies for designing MOF‐based catalysts for electrocatalytic and photocatalytic water splitting are presented. Finally, major challenges in the field of water splitting are highlighted, and some perspectives of MOF‐based catalysts for water splitting are proposed.

Co-doping strategy for developing perovskite oxides as highly efficient electrocatalysts for oxygen evolution reaction

A synergistic co‐doping strategy is proposed to identify a series of BaCo0.9–xFexSn0.1O3–δ perovskites with tunable electrocatalytic activity for the oxygen evolution reaction (OER). Simply through tailoring the relative concentrations of less OER‐active tin and iron dopants, a cubic perovskite structure (BaCo0.7Fe0.2Sn0.1O3–δ) is stabilized, showing intrinsic OER activity >1 order of magnitude larger than IrO2 and a Tafel slope of 69 mV dec−1.

Activity and stability of Ruddlesden-Popper-Type Lan+1NinO3n+1 (n=1, 2, 3, and ) electrocatalysts for oxygen reduction and evolution reactions in alkaline media

Increasing energy demands have stimulated intense research activity on cleaner energy conversion such as regenerative fuel cells and reversible metal-air batteries. It is highly challenging but desirable to develop low-cost bifunctional catalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), the lack of which is currently one of the major limiting components towards commercialization of these technologies. Here, we have conducted a systematic study on the OER and ORR performances of the Ruddlesden-Popper family of La(n+1)Ni(n) O(3n+1) (n=1, 2, 3, and ∞) in an alkaline medium for the first time. It is apparent that the Ni-O bond lengths and the hyperstoichiometric oxides in the rock-salt layers correlate with the ORR activities, whereas the OER activities appear to be influenced by the OH(-) content on the surface of the compounds. In our case, the electronic configuration fails to predict the electrocatalytic activity of these compounds. This work provides guidelines to develop new electrocatalysts with improved performances.

Modified template synthesis and electrochemical performance of a Co3O4/mesoporous cathode for lithium–oxygen batteries

Rechargeable lithium–oxygen batteries (LOBs) with much higher energy density than conventional lithium-ion batteries are supposed to be the next generation of electrochemical energy storage devices. The oxygen electrode is the key component that determines the capacity and cycling performance of this type of battery. In this study, a Co3O4/mesoporous carbon composite (Co3O4/C) with a carbon content of 42 wt%, a rich mesoporous pore content and a homogeneous distribution of Co3O4 nanoparticles over the carbon surface was prepared using a facile silica template method with sucrose as the carbon source, and H3BO3 as an agent for expanding the space between the silica and carbon to impregnate and accommodate the Co3O4 precursor. This composite was used directly as the oxygen electrode in LOBs without additional conductive carbon additives. Galvano charge–discharge tests showed that the capacity of the composite electrode based on the mass of the mesoporous carbon reached approximately 4500 mA h gcarbon−1 at a current density of 123 mA gcarbon−1. The cell was further successfully run for over 200 discharge–charge cycles at a fixed current density of 246 mA gcarbon−1 and a trapped capacity of 740 mA h gcarbon−1, which indicated the superior electrochemical performance of the electrode. Different materials have been comparatively tested as oxygen electrodes, including Super P (SP), SP + nano Co3O4, carbon derived from KIT-6 (KIT-6-C), and the as-prepared Co3O4/C, among which the current Co3O4/C composite showed the best performance. The proposed Co3O4/C material has significant potential as an electrode material for rechargeable Li–O2 batteries.

Pt/C–LiCoO2 composites with ultralow Pt loadings as synergistic bifunctional electrocatalysts for oxygen reduction and evolution reactions

Oxygen reduction and evolution reactions (ORR and OER) are of prime importance for many energy conversion and storage devices, such as regenerative fuel cells and rechargeable metal–air batteries. However, the sluggish kinetics of the ORR and OER strongly limit the efficiency and performance of these electrochemical systems and jeopardize the route of commercialization. Therefore, the design and development of bifunctional electrocatalysts with high activity for both the ORR and OER is challenging but urgent and crucial. Here, we took advantage of Pt/C and LiCoO2 with outstanding ORR activity and high intrinsic OER activity, respectively, to develop a composite material with ultralow Pt loading as a bifunctional catalyst for the ORR and OER in alkaline media. This catalyst was fabricated via simple ultrasonic mixing, exhibiting superb electrocatalytic activity and good stability. Its ORR activity is comparable to that of the commercial Pt/C catalyst and its OER activity is better than that of single LiCoO2, owing to the synergetic effect between Pt and LiCoO2, which has been demonstrated through the X-ray photoelectron spectroscopy (XPS) characterisation technique. Remarkably, surprisingly high ORR mass activity (2.04 A mgPt−1 at 0.8 V vs. RHE) and enhanced bifunctionality (ΔE = 0.91 V) were obtained for the Pt–LiCoO2 composite catalyst with a mass ratio of 1 : 49 for Pt/LiCoO2. Our work opens up a new track to exploit highly efficient catalysts with reduced consumption of Pt, meanwhile maintaining the optimal catalytic activity and durability.