Table of Content

19 December 2022, Volume 5 Issue S2

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Advances in Graphene-Supported Single-Atom Catalysts for Clean Energy Conversion

Yunkun Dai, Fanrong Kong, Xuehan Tai, Yunlong Zhang, Bing Liu, Jiajun Cai, Xiaofei Gong, Yunfei Xia, Pan Guo, Bo Liu, Jian Zhang, Lin Li, Lei Zhao, Xulei Sui, Zhenbo Wang

2022, 5(S2):  22.  doi:10.1007/s41918-022-00142-w

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Recently, heterogeneous single-atom catalysts (SACs) have attracted enormous attention in electrochemical applications owing to their advantages of high metal utilization, well-defined active sites, tunable selectivity, and excellent activity. To avoid the aggregation of atomically dispersed metal sites, an appropriate support has to be adopted to reduce the surface free energy of catalysts. Graphene with a high surface area, outstanding conductivity, and unique electronic properties has generally been utilized as the substrate for SACs. Moreover, the correlations between metal-support interactions and the electrocatalytic performance at the atomic scale can be studied on graphene-supported single-atom catalyst (G-SAC) nanoplatforms. In this review, we start from an overview of the synthetic methods for G-SACs. Subsequently, several advanced and effective characterization techniques are discussed. Then, we present a comprehensive summary of recent progress in G-SACs for a variety of electrochemical applications. Finally, we present challenges for and an outlook on the development of G-SACs with outstanding catalytic activity, stability, and selectivity.



Focus on the Electroplating Chemistry of Li Ions in Nonaqueous Liquid Electrolytes: Toward Stable Lithium Metal Batteries

Hongmei Liang, Li Wang, Li Sheng, Hong Xu, Youzhi Song, Xiangming He

2022, 5(S2):  23.  doi:10.1007/s41918-022-00158-2

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Lithium metal anodes (LMAs) show unique superiority for secondary batteries because they possess the lowest molar mass and reduction potential among metallic elements. It can diminish the large gap in energy density between secondary batteries and fossil fuels. However, notorious dendrite propagation gives rise to large volume expansion, low reversibility and potential safety hazards, making the commercial application of LMAs a perennial challenge. The booming development in material characterization deepens the understanding of the dendrite formation mechanism, and the great progress made via nanotechnology-based solutions hastens practical procedures. In this paper, we highlight the current understanding of lithium dendrites. We first illustrate different nucleation theories and growth patterns of lithium dendrites. According to the growth patterns, we classify dendrites into three categories to accurately describe their different formation mechanisms. Then, we concentrate on the factors that may lead to dendritic deposits in each electroplating step. The dendritic morphology originates from the inhomogeneity of Li atoms, electrons, mass transport in the bulk electrolyte and the solid electrolyte interphase. Different inducements lead to different growth patterns. Based on this understanding, strategies for controlling lithium plating are divided into five methodologies. Reasonable integration of the strategies is expected to provide new ideas for basic research and practical application of LMAs. Finally, current limitations and advice for future research are proposed, aiming at inspiring engaged contributors and new entrants to explore scalable solutions for early realization of industrialization.



Atom Doping Engineering of Transition Metal Phosphides for Hydrogen Evolution Reactions

Huawei Bai, Ding Chen, Qianli Ma, Rui Qin, Hanwen Xu, Yufeng Zhao, Junxin Chen, Shichun Mu

2022, 5(S2):  24.  doi:10.1007/s41918-022-00161-7

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Transition metal phosphides (TMPs) have attracted attention in electrocatalytic hydrogen production because of their multiple active sites, adjustable structures, complex and variable composition, and distinctive electronic structures. However, the catalytic performance of pure TMPs in the hydrogen evolution reaction (HER) is not ideal. Fortunately, this situation can be changed by atom doping engineering because atom doping can efficaciously adjust the electronic structure, Gibbs free energy (ΔG_H*) and d-band center to enhance the kinetics of catalytic reactions. Thus, atom doping engineering has aroused widespread interest. This review examines, analyzes and summarizes our previous work and that of others on atom doping engineering, including the activity origin of doped TMPs, doping with nonmetals (B, S, N, O, F, etc.), doping with metals (Ni, Co, Fe, Mn, Mo, Al, etc.) and codoping with nonmetals and metal atoms, as well as direct doping and synergetic doping, doping methods and the resulting HER properties. Finally, the key problems and future directions for development of atom doping in TMPs are discussed. This review will aid the design and construction of high-performance nonnoble metal catalysts for the HER and other electrocatalytic processes.



Fuel Cell Reactors for the Clean Cogeneration of Electrical Energy and Value-Added Chemicals

Fengzhan Si, Subiao Liu, Yue Liang, Xian-Zhu Fu, Jiujun Zhang, Jing-Li Luo

2022, 5(S2):  25.  doi:10.1007/s41918-022-00168-0

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Fuel cell reactors can be tailored to simultaneously cogenerate value-added chemicals and electrical energy while releasing negligible CO₂ emissions or other pollution; moreover, some of these reactors can even "breathe in" poisonous gas as feedstock. Such clean cogeneration favorably offsets the fast depletion of fossil fuel resources and eases growing environmental concerns. These unique reactors inherit advantages from fuel cells:a high energy conversion efficiency and high selectivity. Compared with similar energy conversion devices with sandwich structures, fuel cell reactors have successfully "hit three birds with one stone" by generating power, producing chemicals, and maintaining eco-friendliness. In this review, we provide a systematic summary on the state of the art regarding fuel cell reactors and key components, as well as the typical cogeneration reactions accomplished in these reactors. Most strategies fall short in reaching a win-win situation that meets production demand while concurrently addressing environmental issues. The use of fuel cells (FCs) as reactors to simultaneously produce value-added chemicals and electrical power without environmental pollution has emerged as a promising direction. The FC reactor has been well recognized due to its "one stone hitting three birds" merit, namely, efficient chemical production, electrical power generation, and environmental friendliness. Fuel cell reactors for cogeneration provide multidisciplinary perspectives on clean chemical production, effective energy utilization, and even pollutant treatment, with far-reaching implications for the wider scientific community and society. The scope of this review focuses on unique reactors that can convert low-value reactants and/or industrial wastes to value-added chemicals while simultaneously cogenerating electrical power in an environmentally friendly manner.



Understanding and Control of Activation Process of Lithium-Rich Cathode Materials

Tongen Lin, Trent Seaby, Yuxiang Hu, Shanshan Ding, Ying Liu, Bin Luo, Lianzhou Wang

2022, 5(S2):  27.  doi:10.1007/s41918-022-00172-4

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Lithium-rich materials (LRMs) are among the most promising cathode materials toward next-generation Li-ion batteries due to their extraordinary specific capacity of over 250 mAh g⁻¹ and high energy density of over 1 000 Wh kg⁻¹. The superior capacity of LRMs originates from the activation process of the key active component Li₂MnO₃. This process can trigger reversible oxygen redox, providing extra charge for more Li-ion extraction. However, such an activation process is kinetically slow with complex phase transformations. To address these issues, tremendous effort has been made to explore the mechanism and origin of activation, yet there are still many controversies. Despite considerable strategies that have been proposed to improve the performance of LRMs, in-depth understanding of the relationship between the LRMs' preparation and their activation process is limited. To inspire further research on LRMs, this article firstly systematically reviews the progress in mechanism studies and performance improving attempts. Then, guidelines for activation controlling strategies, including composition adjustment, elemental substitution and chemical treatment, are provided for the future design of Li-rich cathode materials. Based on these investigations, recommendations on Li-rich materials with precisely controlled Mn/Ni/Co composition, multi-elemental substitution and oxygen vacancy engineering are proposed for designing high-performance Li-rich cathode materials with fast and stable activation processes.



Recent Advances and Perspectives of Electrochemical CO₂ Reduction Toward C₂₊ Products on Cu-Based Catalysts

Xiaodeng Wang, Qi Hu, Guodong Li, Hengpan Yang, Chuanxin He

2022, 5(S2):  28.  doi:10.1007/s41918-022-00171-5

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Renewable-electricity-powered electrochemical CO₂ reduction reactions (CO₂RR) to highly value-added multi-carbon (C₂₊) fuels or chemicals have been widely recognized as a promising approach for achieving carbon recycling and thus bringing about sustainable environmental and economic benefits. Cu-based catalysts have been demonstrated as the only candidate metal CO₂RR electrocatalysts that catalyze the C-C coupling. Unfortunately, huge challenges still exist in the highly selective CO₂RR to C₂₊ products due to the higher activation barrier of C-C coupling and complex multi-electron reaction. Key fundamental issues regarding both active species and product formation pathways have not been elucidated by now, but recent developments of advanced strategies and characterization tools allow one to comprehensively understand the Cu-based CO₂RR mechanism. Herein, we review recent advance and perspective of Cu-based CO₂RR catalysts, especially in terms of active phases and product formation pathways. Then, strategies in catalysts design for CO₂RR toward C₂₊ products are also presented. Importantly, we systematically summarized the advanced tools for investigating the CO₂RR mechanism, including in situ/operando spectroscopy techniques, isotope labeling, and theoretical calculations, aiming at unifying the knowledge of active species and product formation pathways. Finally, future challenges and constructive perspectives are discussed, facilitating the accelerated advancement of CO₂RR mechanism research.



Reevaluating Flexible Lithium-Ion Batteries from the Insights of Mechanics and Electrochemistry

Qi Meng, Shuaifeng Lou, Baicheng Shen, Xin Wan, Xiangjun Xiao, Yulin Ma, Hua Huo, Geping Yin

2022, 5(S2):  30.  doi:10.1007/s41918-022-00150-w

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The emerging direction toward the ever-growing market of wearable electronics has contributed to the progress made in energy storage systems that are flexible while maintaining their electrochemical performance. Endowing lithium-ion batteries with high flexibility is currently considered to be one of the most essential choices in future. Here, we first propose the basic deformation mode according to the manifestation of flexibility and constructively reevaluate the concept of flexible lithium-ion batteries. Furthermore, the failure mechanism of flexible lithium-ion batteries is investigated with regard to their mechanical failure and electrochemical failure, and the related strategies of battery design and manufacturing are analyzed. More importantly, an in-depth analysis is conducted on the approaches to overcome mechanical failure through stress dispersion, stress absorption, prestress concentration, stress transfer, and other flexible reinforcement methods. Additionally, the advantages of suppressing electrochemical failure are discussed by enhancing the surface roughness, pore formation, surface coating, chemical bonding, in situ encapsulation, etc. Regarding self-healing technology, the general approaches taken for self-healing batteries to achieve flexibility are explained through the classification of macroscopic self-healing (to avert mechanical failure) and microscopic self-healing (to respond to electrochemical failure). Finally, after considering the current state of flexible lithium-ion batteries, future challenges are presented.



Self-Supported Graphene Nanosheet-Based Composites as Binder-Free Electrodes for Advanced Electrochemical Energy Conversion and Storage

Bowen Ren, Hao Cui, Chengxin Wang

2022, 5(S2):  32.  doi:10.1007/s41918-022-00138-6

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Graphene is composed of single-layered sp² graphite and has been widely used in electrochemical energy conversion and storage due to its appealing physical and chemical properties. In recent years, a new kind of the self-supported graphene nanosheet-based composite (GNBC) has attracted significant attention. Compared with conventional powdered materials, a binder-free electrode architecture has several strengths, including a large surface area, enhanced reaction kinetics, and great structural stability, and these strengths allow users to realize the full potential of graphene. Based on these findings, this review presents preparation strategies and properties of self-supported GNBCs. Additionally, it highlights recent significant developments with integrated binder-free electrodes for several practical applications, such as lithium-ion batteries, lithium-metal batteries, supercapacitors, water splitting and metal-air batteries. In addition, the remaining challenges and future perspectives in this emerging field are also discussed.