Electrochemical energy conversion between electricity and chemicals through electrocatalysis is a promising strategy for the development of clean and sustainable energy sources. This is because efficient electrocatalysts can greatly reduce energy loss during the conversion process. However, poor catalytic performances and a shortage in catalyst material resources have greatly restricted the widespread applications of electrocatalysts in these energy conversion processes. To address this issue, earth-abundant two-dimensional (2D) materials with large specific surface areas and easily tunable electronic structures have emerged in recent years as promising high-performance electrocatalysts in various reactions, and because of this, this review will comprehensively discuss the engineering of these novel 2D material-based electrocatalysts and their associated heterostructures. In this review, the fundamental principles of electrocatalysis and important electrocatalytic reactions are introduced. Following this, the unique advantages of 2D material-based electrocatalysts are discussed and catalytic performance enhancement strategies are presented, including the tuning of electronic structures through various methods such as heteroatom doping, defect engineering, strain engineering, phase conversion and ion intercalation, as well as the construction of heterostructures based on 2D materials to capitalize on individual advantages. Finally, key challenges and opportunities for the future development of these electrocatalysts in practical energy conversion applications are presented.

Advanced energy storage systems hold critical significance in satisfying the ever-increasing global demand for energy. And as a viable and effective alternative to lithium-ion batteries that dominate the current energy market, Zn-based batteries [i.e. Zn-ion batteries (ZIBs) and Zn-air batteries (ZABs)] have attracted extensive research efforts. Zn metal possesses many advantages because of its high theoretical capacity, its inexpensiveness and its good safety characteristic, and in recent years, tremendous efforts have been carried out to accelerate the development of ZIBs and ZABs with various electrode materials and electrocatalysts being proposed and investigated. In addition, with advances in characterization techniques, the underlying reaction mechanisms of these materials are also being elucidated. Therefore, this review will provide a comprehensive summary of the latest progress in various electrode materials adopted in the current ZIBs and ZABs along with corresponding mechanisms. Specifically, Mn- and V-containing cathode materials for ZIBs and associated reaction mechanisms will be thoroughly discussed, and emerging cathodes such as Prussian blue analogues, NASICON-type nanostructures and organic compounds will be presented. In terms of ZABs, this review will discuss three major types of electrocatalysts, including noble metals, heteroatom-doped carbons and transition metal oxides/sulphides/phosphides/nitrides. In addition, as a critical factor in the performance of Zn-based batteries, challenges encountered by the current Zn anodes and strategies developed to tackle these issues will be discussed as well. Finally, a short summary including the current progress and future perspectives of ZIBs and ZABs will be provided.

Computational modeling has played a key role in advancing the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs). In recent years there has been a significant focus on PEMFC catalyst layers because of their determining impact on cost and and durability. Further progress in the design of better performance, cheaper and more durable catalyst layers is required to pave the way for large scale deployment of PEMFCs. The catalyst layer poses many challenges from a modeling standpoint: it consists of a complex, multi-phase, nanostructured porous material that is difficult to characterize; and it hosts an array of coupled transport phenomena including flow of gases, liquid water, heat and charged occurring in conjunction with electrochemical reactions. This review paper examines several aspects of state-of-the-art modeling and simulation of PEMFC catalyst layers, with a view of synthesizing the theoretical foundations of various approaches, identifying gaps and outlining critical needs for further research. The review starts with a rigorous revisiting of the mathematical framework based on the volume averaging method. Various macroscopic models reported in the literature that describe the salient transport phenomena are then introduced, and their links with the volume averaged method are elucidated. Other classes of modeling and simulation methods with different levels of resolution of the catalyst layer structure, e.g. the pore scale model which treats materials as continuum, and various meso- and microscopic methods, which take into consideration the dynamics at the sub-grid level, are reviewed. Strategies for multiscale simulations that can bridge the gap between macroscopic and microscopic models are discussed. An important aspect pertaining to transport properties of catalyst layers is the modeling and simulation of the fabrication processes which is also reviewed. Last but not least, the review examines modeling of liquid water transport in the catalyst layer and its implications on the overall transport properties. The review concludes with an outlook on future research directions.

Rechargeable batteries dominate the energy storage market of portable electronics, electric vehicles and stationary grids, and corresponding performance advancements are closely related to the fundamental understanding of electrochemical reaction mechanisms and their correlation with structural and chemical evolutions of battery components. Through advancements in aberration-corrected transmission electron microscopy (TEM) techniques for significantly enhanced spatial resolution, in situ TEM techniques in which a nanobattery assembly is integrated into the system can allow for the direct real-time probing of structural and chemical evolutions of battery components under dynamic operating conditions. Here, open-cell in situ TEM configurations can provide the atomic resolution imaging of the intrinsic response of materials to ion insertion or extraction, whereas the development of sealed liquid cells can provide new avenues for the observation of electrochemical processes and electrode-electrolyte interface reactions that are relevant to real battery systems. And because of these recent developments in in situ TEM techniques, this review will present recent key progress in the utilization of in situ TEM to reveal new sciences in rechargeable batteries, including complex reaction mechanisms, structural and chemical evolutions of battery materials and their correlation with battery performances. In addition, scientific insights revealed by in situ TEM studies will be discussed to provide guiding principles for the design of better electrode materials for rechargeable batteries. And challenges and new opportunities will also be discussed.

Flow batteries have received increasing attention because of their ability to accelerate the utilization of renewable energy by resolving issues of discontinuity, instability and uncontrollability. Currently, widely studied flow batteries include traditional vanadium and zinc-based flow batteries as well as novel flow battery systems. And although vanadium and zinc-based flow batteries are close to commercialization, relatively low power and energy densities restrict the further commercial and industrial application. To improve power and energy densities, researchers have started to investigate novel flow battery systems, including aqueous and non-aqueous systems. Here, novel non-aqueous flow batteries possess low conductivity and low safety, limiting further application. Therefore, the most promising systems remain vanadium and zinc-based flow batteries as well as novel aqueous flow batteries. Overall, the research of flow batteries should focus on improvements in power and energy density along with cost reductions. In addition, because the design and development of flow battery stacks are vital for industrialization, the structural design and optimization of key materials and stacks of flow batteries are also important. Based on all of this, this review will present in detail the current progress and developmental perspectives of flow batteries with a focus on vanadium flow batteries, zinc-based flow batteries and novel flow battery systems to provide an effective and extensive understanding of the current research and future development of flow batteries.