Fuel cells
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 efcient 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 specifc 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.
Full-text：https://link.springer.com/article/10.1007/s41918-019-00045-3/fulltext.html
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 signifcant 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 difcult to characterize; and it hosts an array of coupled transport phenomena including fow 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 diferent 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.
Full-text：https://link.springer.com/article/10.1007/s41918-019-00043-5/fulltext.html
Electrochemical energy storage systems such as fuel cells and metal-air batteries can be used as clean power sources for electric vehicles. In these systems, one necessary reaction at the cathode is the catalysis of oxygen reduction reaction (ORR), which is the rate-determining factor afecting overall system performance. Therefore, to increase the rate of ORR for enhanced system performances, efcient electrocatalysts are essential. And although ORR electrocatalysts have been intensively explored and developed, signifcant breakthroughs have yet been achieved in terms of catalytic activity, stability, cost and associated electrochemical system performance. Based on this, this review will comprehensively present the recent progresses of ORR electrocatalysts, including precious metal catalysts, non-precious metal catalysts, single-atom catalysts and metal-free catalysts. In addition, major technical challenges are analyzed and possible future research directions to overcome these challenges are proposed to facilitate further research and development toward practical application.
Full-text：https://link.springer.com/article/10.1007/s41918-019-00052-4
Single-atom catalysis is a powerful and attractive technique with exceptional performance, drastic cost reduction and notable catalytic activity and selectivity. In single-atom catalysis, supported single-atom catalysts contain isolated individual atoms dispersed on, and/or coordinated with, surface atoms of appropriate supports, which not only maximize the atomic efciency of metals, but also provide an alternative strategy to tune the activity and selectivity of catalytic reactions. This review will highlight the attributes of single-atom catalysis and summarize the most recent advancements in single-atom catalysts with a focus on the design of highly active and stable single atoms. In addition, new research directions and future trends will also be discussed.
Full-text：https://link.springer.com/article/10.1007/s41918-019-00050-6
Graphene-based nanomaterials are promising bifunctional electrocatalysts for overall water splitting (OWS) to produce hydrogen and oxygen as sustainable fuel sources because graphene-based bifunctional electrocatalysts can provide distinct features such as large surface areas, more active sites and facile synthesis of multiple co-doped nanomaterials. Based on this, this review will present recent advancements in the development of various bifunctional graphene-based electrocatalysts for OWS reactions and discuss advancements in the tuning of electronic surface-active sites for the electrolytic splitting of water. In addition, this review will evaluate perspectives and challenges to provide a deep understanding of this emerging field.
Full-text：https://link.springer.com/article/10.1007/s41918-020-00066-3
Flow batteries offer solutions to a number of the growing concerns regarding world energy, such as increasing the viability of renewable energy sources via load balancing. However, issues regarding the redox couples employed, including high costs, poor solubilities/energy densities, and durability of battery materials are still hampering widespread adoption of this technology. Flow batteries with a positive half-cell consisting of a halogen based redox couple (Cl-/Cl2, Br-/Br2, I-/I2) offer several advantages over other alternatives, such as being relatively inexpensive, highly soluble, and exhibiting faster kinetics than many other electroactive redox couples. This paper aims to provide a comprehensive comparative review of the thermodynamic and kinetic properties of relevant halogen and polyhalide redox couples, and recent advances in electrode and membrane materials for various halogen-based flow batteries and regenerative hydrogen fuel cells using halogens instead of oxygen.
Full-text： https://link.springer.com/article/10.1007/s41918-020-00069-0
Proton exchange membrane fuel cells (PEMFCs) as power systems have been widely studied in various application fields because of advantages such as cleanness and high efficiency with great progress having been made in the past decades both technologically and fundamentally. Despite the many promising developments however, technical challenges remain in terms of performance and lifespans. This is because PEMFCs are complex systems composed of various components and factors such as material property, engineering design and operating conditions can interact with each other to affect lifespans and performance. To fully understand the coupling effects of different factors on the overall performance and durability of PEMFCs, this review will comprehensively present existing research based on four aspects, including fuel cell stacks, subsystems, system integration and control strategy optimizations. First, this review will outline fuel cell stacks with their multi-physics modeling and engineering design to provide an understanding of the operating mechanisms inside PEMFC reactors. Following this, the progress of research into the structure and function of each subsystem is summarized and integration schemes for different applications are briefly presented. Finally, various control strategies for individual PEMFC subsystems to optimize energy management and dynamic performance are discussed.
Full-text：https://link.springer.com/article/10.1007/s41918-020-00068-1
Hydrogen is an ideal energy carrier in future applications due to clean byproducts and high efficiency. However, many challenges remain in the application of hydrogen, including hydrogen production, delivery, storage and conversion. In terms of hydrogen storage, two compression modes (mechanical and non-mechanical compressors) are generally used to increase volume density in which mechanical compressors with several classifications including reciprocating piston compressors, hydrogen diaphragm compressors and ionic liquid compressors produce significant noise and vibration and are expensive and inefficient. Alternatively, non-mechanical compressors are faced with issues involving large-volume requirements, slow reaction kinetics and the need for special thermal control systems, all of which limit large-scale development. As a result, modular, safe, inexpensive and efficient methods for hydrogen storage are urgently needed. And because electrochemical hydrogen compressors (EHCs) are modular, highly efficient and possess hydrogen purification functions with no moving parts, they are becoming increasingly prominent. Based on all of this and for the first time, this review will provide an overview of various hydrogen compression technologies and discuss corresponding structures, principles, advantages and limitations. This review will also comprehensively present the recent progress and existing issues of EHCs and future hydrogen compression techniques as well as corresponding containment membranes, catalysts, gas diffusion layers and flow fields. Furthermore, engineering perspectives are discussed to further enhance the performance of EHCs in terms of the thermal management, water management and the testing protocol of EHC stacks. Overall, the deeper understanding of potential relationships between performance and component design in EHCs as presented in this review can guide the future development of anticipated EHCs.
Full-text：https://link.springer.com/article/10.1007/s41918-020-00077-0
Solid oxide cells (SOCs) are highly efficient and environmentally benign devices that can be used to store renewable electrical energy in the form of fuels such as hydrogen in the solid oxide electrolysis cell mode and regenerate electrical power using stored fuels in the solid oxide fuel cell mode. Despite this, insufficient long-term durability over 5-10 years in terms of lifespan remains a critical issue in the development of reliable SOC technologies in which the surface segregation of cations, particularly strontium (Sr) on oxygen electrodes, plays a critical role in the surface chemistry of oxygen electrodes and is integral to the overall performance and durability of SOCs. Due to this, this review will provide a critical overview of the surface segregation phenomenon, including influential factors, driving forces, reactivity with volatile impurities such as chromium, boron, sulphur and carbon dioxide, interactions at electrode/electrolyte interfaces and influences on the electrochemical performance and stability of SOCs with an emphasis on Sr segregation in widely investigated (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3-δ. In addition, this review will present strategies for the mitigation of Sr surface segregation.
Full-text：https://link.springer.com/article/10.1007/s41918-020-00078-z
High-temperature proton exchange membrane fuel cells based on phosphoric acid-doped polybenzimidazole membranes are a technology characterized by simplified construction and operation along with possible integration with, e.g., methanol reformers. Significant progress has been achieved in terms of key materials, components and systems. This review is devoted to updating new insights into the fundamental understanding and technological deployment of this technology. Polymers are synthetically modified with basic functionalities, and membranes are improved through cross-linking and inorganic-organic hybridization. New insights into phosphoric acid along with its interactions with basic polymers, metal catalysts and carbon-based supports are recapped. Recognition of parasitic acid migration raises acid retention issues at high current densities. Acid loss via evaporation is estimated with respect to the acid inventory of membrane electrode assembly. Acid adsorption on platinum surfaces can be alleviated for platinum alloys and non-precious metal catalysts. Binders have been considered a key to the establishment of the triple-phase boundary, while recent development of binderless electrodes opens new avenues toward low Pt loadings. Often ignored microporous layers and water impacts are also discussed. Of special concern are durability issues including acid loss, platinum sintering and carbon corrosion, the latter being critical during start/stop cycling with mitigation measures proposed. Long-term durability has been demonstrated with a voltage degradation rate of less than 1 μV h-1 under steady-state tests at 160℃, while challenges remain at higher temperatures, current densities or reactant stoichiometries, particularly during dynamic operation with thermal, load or start/stop cycling.
Full-text：https://link.springer.com/article/10.1007/s41918-020-00080-5
Typical catalyst inks in proton exchange membrane fuel cells (PEMFCs) are composed of a catalyst, its support, an ionomer and a solvent and are used with solution processing approaches to manufacture conventional catalyst layers (CLs). Because of this, catalyst ink formulation and deposition processes are closely related to CL structure and performance. However, catalyst inks with ideal rheology and optimized electrochemical performances remain lacking in the large-scale application of PEMFCs. To address this, this review will summarize current progress in the formulation, characterization, modeling and deposition of catalyst inks. In addition, this review will highlight recent advancements in catalyst ink materials and discuss corresponding complex interactions. This review will also present various catalyst ink dispersion methods with insights into their stability and introduce the application of small-angle scattering and cryogenic transmission electron microscopy (cryo-TEM) technologies in the characterization of catalyst ink microstructures. Finally, recent studies in the kinetic modeling and deposition of catalyst inks will be analyzed.
Full-text：https://link.springer.com/article/10.1007/s41918-020-00083-2
