With the development of renewable energy and electrified transportation, electrochemical energy storage will be more important in the future than it has ever been in the past. Although lithium-ion batteries (LIBs) are traditionally considered to be the most likeliest candidate thanks to their relatively long cycle life and high energy efficiency, their limited energy density as well as cost are still causing a bottleneck for their long-term application. Alternatively, metal-air batteries have been proposed as a very promising large-scale electricity storage technology with the replacement of the intercalation reaction mechanism by the catalytic redox reaction of a light weight metal-oxygen couple. Generally, based on the electrolyte, these metal-air batteries can be divided into aqueous and nonaqueous systems, corresponding to two typical batteries of Zn-air and Li-air, respectively. The prominent feature of both batteries are their extremely high theoretical energy density, especially for nonaqueous Li-air batteries, which far exceeds the best that can be achieved with LIBs. In this review, we focus on the major obstacle of sluggish kinetics of the cathode in both batteries, and summarize the fundamentals and recent advances related to the oxygen catalyst materials. According to the electrolyte, the aqueous and nonaqueous electrocatalytic mechanisms of the oxygen reduction and evolution reactions are discussed. Subsequently, seven groups of oxygen catalysts, which have played catalytic roles in both systems, are selectively reviewed, including transition metal oxides (single-metal oxides and mixed-metal oxides), functional carbon materials (nanostructured carbons and doped carbons), metal oxide-nanocarbon hybrid materials, metal-nitrogen complexes (non-pyrolyzed and pyrolyzed), transition metal nitrides, conductive polymers, and precious metals (alloys). Nonaqueous systems have the advantages of energy density and rechargeability over aqueous systems and have gradually become the research focus of metal-air batteries. However, there are considerable challenges beyond catalysts from aqueous to nonaqueous electrolytes, which are also discussed in this review. Finally, several future research directions are proposed based on the results achieved in this field, with emphasis on nonaqueous Li-air batteries.

Heterogenous electrocatalysts based on transition metal sulfides (TMS) are being actively explored in renewable energy research because nanostructured forms support high intrinsic activities for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review article, the authors describe how researchers are working to improve the This article is protected by copyright. All rights reserved. 2 performance of TMS-based materials by manipulating its internal and external nanoarchitectures. A general introduction to the water splitting reaction is initially provided to explain the most important parameters in accessing the catalytic performance of nanomaterials catalysts. Later, the general synthetic methods used to prepare TMS-based materials is explained in order to delve into the various strategies being used to achieve higher electrocatalytic performance in HER. Complementary strategies can be used to increase the OER performance of TMS, resulting in bifunctional water-splitting electrocatalysts for both HER and OER. Finally, the current challenges and future opportunities of TMS materials in the context of water splitting are summarized. The authors aim to provide insights gathered in the process of studying TMS, and describe valuable guidelines for engineering other kinds of nanomaterial catalysts for energy conversion and storage technologies.

Lithium‐oxygen (Li‐O2) batteries are one of the most promising candidates for high‐energy‐density storage systems. However, the low utilization of porous carbon and the inefficient transport of reactants in the cathode limit terribly the practical capacity and, in particular, the rate capability of state‐of‐the‐art Li‐O2 batteries. Here, free‐standing, hierarchically porous carbon (FHPC) derived from graphene oxide (GO) gel in nickel foam without any additional binder is synthesized by a facile and effective in situ sol‐gel method, wherein the GO not only acts as a special carbon source, but also provides the framework of a 3D gel; more importantly, the proper acidity via its intrinsic COOH groups guarantees the formation of the whole structure. Interestingly, when employed as a cathode for Li‐O2 batteries, the capacity reaches 11 060 mA h g−1 at a current density of 0.2 mA cm−2 (280 mA g−1); and, unexpectedly, a high capacity of 2020 mA h g−1 can be obtained even the current density increases ten times, up to 2 mA cm−2 (2.8 A g−1), which is the best rate performance for Li‐O2 batteries reported to date. This excellent performance is attributed to the synergistic effect of the loose packing of the carbon, the hierarchical porous structure, and the high electronic conductivity of the Ni foam.