Atomically dispersed metal sites in COF-based nanomaterials for electrochemical energy conversion

Atomically dispersed metal sites(ADMSs)play key roles in electrochemical energy conversion.The covalent organic frameworks(COFs)enable the precise control of the chemical compositions and structures at the molecular level,making them ideal substrates for supporting ADMSs.In this review,we systematically summarize the recent progress on the design and synthesis of ADMSs in COFs,including embedding molecular catalysts into COFs,immobilizing ADMSs on heteroatom-containing COFs,and preparing COF-derived carbon materials through pyrolysis.The electrocatalytic performance of the resulting catalysts is presented for various electrochemical reactions,involving oxygen reduction reaction(ORR),carbon dioxide reduction reaction(CO_(2)RR),oxygen evolution reaction(OER),hydrogen evolution reaction(HER),and nitrogen reduction reaction(NRR).The modulation strategies of AMDSs in COFs for enhanced activity,selectivity,and stability are highlighted,together with a perspective of the current challenges and the future opportunities in this field.

MOF衍生的分层多孔三维N-CoP_(x)/Ni_(2)P大电流密度下加速析氢

氢气(H_(2))具有能量密度高、环境友好等优点,是一种很有前景的清洁能源载体.目前,电催化水裂解大规模制氢被认为是一种理想可行的方法.析氢反应(HER)涉及多个步骤,首先形成吸附的氢(Volmer步骤),然后是脱附步骤(Heyrovsky步骤)或两个相邻的吸附氢形成H_(2)(Tafel步骤).与酸性介质相比,碱性介质中的HER可与现有的析氧反应(OER)催化剂偶合,降低电解水的设备成本,因此研究碱性条件下HER更具应用价值.但是,HER在碱性介质中不可避免地需要打破较强的共价键H–O–H,动力学缓慢,导致需要高过电位驱动反应.因此,开发适用于广泛的pH范围,特别是碱性介质高催化活性的催化剂,成为当务之急.金属铂是最高效的HER催化剂,但昂贵的价格严重阻碍了其在电解水中的大规模商业化应用.因此,开发过电位低和稳定性持久的非贵金属催化剂,特别是可以在大电流密度(>500 mA cm^(-2)的质子交换膜和碱性电解槽)下稳定工作的催化剂,对实际工业应用至关重要.过渡金属磷化物(TMPs),尤其是CoP和Ni_(2)P在HER中表现出了较好的催化活性,引起广泛关注.但是,有限的电子结构、低电导率和大电流密度测试过程中的团聚仍然是限制其实际应用的瓶颈.近年来,具有金属可调性、多孔型结构、高比表面积和多交叉开放通道的金属有机骨架(MOFs)已被证明是制备TMPs的理想前驱体.但是,在高温煅烧过程中无法避免MOF结构坍塌,导致开放通道和电导率降低,限制了电子/离子的传输以及在高电流密度下的电催化活性.本文通过TMPs和Co-MOF之间的简单拓扑化学转化制备了一种自支撑结构的N掺杂二元TMPs电催化剂(N-CoP_(x)/Ni_(2)P),以Co-MOF作为模板和前驱体,一部分泡沫镍原位磷化成Ni_(2)P,形成异质结构的双金属磷化物.扫描电镜和透射电镜结果表明,该催化剂呈三维多孔结构,有利于充分暴露活性位点.通过X射线光电子能谱分析了催化剂表面化学状态,发现形成了Co–N键,说明N掺杂成功.通过电化学测试结果表明,N-CoP_(x)/Ni_(2)P在全pH范围内表现出较好的HER活性,尤其在碱性介质中,当电流密度为650 mA cm^(-2)时,仅需要152 mV过电位.催化剂转化率为3.2 s^(-1),法拉第效率接近100%,该催化剂在200 mA cm^(-2)电流密度下连续工作24 h无明显衰减.密度泛函理论计算表明,N-CoP_(x)/Ni_(2)P催化活性的增强归因于氮掺杂及双金属磷化物的协同作用提高了催化剂的本征活性位点,从而优化了氢吸附能和水结合能.综上,本文为廉价电催化剂的工业化应用提供了一种有前景的策略.

2D MXene-Based Materials for Electrocatalysis

MXenes,as an emerging 2D material,are expected to exert a great influence on future energy storage and conversion technologies.In this review,we systematically summarize recent advances in MXene-based materials in electrocatalysis,particularly in the hydrogen evolution,oxygen evolution,oxygen reduction,nitrogen reduction,and CO2 reduction reactions.Crucial factors influencing the properties of these materials,such as functional groups,conductivity,and interface,are discussed,and challenges to the future development of MXene-based electrocatalysts are presented.

Enhanced catalytic conversion of polysulfides using high-percentage 1T-phase metallic WS_(2) nanosheets for Li–S batteries

High-energy-density lithium-sulfur batteries has attracted substantial attention as competitive candidates for large-scale energy storage technologies.Still,the adverse“shuttle effect”and sluggish sulfur conversion reaction kinetics immensely obstruct their commercial viability.Herein,a two-dimensional metallic 1T phase WS_(2)(1T-WS_(2))nanosheets modified functional separator is developed to improve the electrochemical performance.Meanwhile,the semiconducting bulk-WS_(2) crystals,and 2H phase WS_(2)(2H-WS_(2))nanosheets with more basal-plane Svacancy defects are also prepared to probe the contributions of the crystal structure(phase),S-vacancy defects,and edges to the Li–S batteries performance experimentally and theoretically.In merits of the synergistic effect of high ion and electron conductivity,enhanced binding ability to lithium polysulfides(LiPSs),and sufficient electrocatalytic active sites,the 1T-WS_(2) shows highly efficient electrocatalysis of LiPSs conversion and further improves Li–S battery performance.As expected,thus-fabricated cells with 1T-WS_(2) nanosheets present superior cycle stability that maintain capacity decline of 0.039%per cycle after 1000 cycles at 1.0 C.The strategy presented here offers a viable approach to reveal the critical factors for LiPSs catalytic conversion,which is beneficial to developing advanced Li–S batteries with enhanced properties.

Understanding of the electrochemical behaviors of aqueous zinc-manganese batteries:Reaction processes and failure mechanisms

As one of the most common cathode materials for aqueous zinc-ion batteries(AZIBs),manganese oxides have the advantages of abundant reserves,low cost,and low toxicity.However,the electrochemical mechanism at the cathode of aqueous zinc-manganese batteries(AZMBs) is complicated due to different electrode materials,electrolytes and working conditions.These complicated mechanisms severely limit the research progress of AZMBs system and the design of cells with better performance.Hence,the mechanism of AZMBs currently recognized by most researchers according to the classification of the main ions involved in the faradaic reaction is introduced in the review.Then a series of reasons that affect the electrochemical behavior of the battery are summarized.Finally,the failure mechanisms of AZMBs over prolonged cycling are discussed,and the current insufficient research areas of the system are explained,along with the direction of further research being prospected.

过一硫酸盐体系中铜分布模式控制活性物种的产生和污染物降解路径

The distribution pattern of metals as active centers on a substrate can influence the peroxymonosulfate(PMS)activation and contaminants degradation.Herein,atomic layer deposition is applied to prepare Cu single atom(SA-Cu),cluster(C-Cu),and film(F-Cu)decorated MXene catalysts by regulating the number of deposition cycles.In comparison with SA-Cu-MXene(adsorption energy(E_(ads))=-4.236 eV)and F-Cu-MXene(E_(ads)=-3.548 eV),PMS is shown to adsorb preferably on the C-Cu-MXene surface for activation(E_(ads)=-5.435 eV),realizing higher utilization efficiency.More SO_(4)^(·-)are generated in C-Cu-MXene/PMS system with steady-state concentration and 1–3 orders of magnitude higher than those in the SA-Cu-MXene and F-Cu-MXene activated PMS systems.Particularly,the contribution of SO_(4)^(·-)oxidation to sulfamethoxazole(SMX)degradation followed the order,C-Cu-MXene(97.3%)>SA-Cu-MXene(90.4%)>FCu-MXene(71.9%),realizing the larger SMX degradation rate in the C-Cu-MXene/PMS system with the degradation rate constants(k)at 0.0485 min^(-1).Additionally,SMX degradation routes in C-Cu-MXene/PMS system are found with fewer toxic intermediates.Through this work,we highlighted the importance of guided design of heterogeneous catalysts in the PMS system.Appropriate metal distribution patterns need to be selected according to the actual water treatment demand.Metal sites could be then fully utilized to produce specific active species to improve the utilization efficiency of the oxidants.

One-step fabrication of two-dimensional hierarchical Mn_(2)O_(3)@graphene composite as high-performance anode materials for lithium ion batteries

Two-dimensional(2 D) hierarchical Mn_(2)O_(3)@graphene composite is synthesized by a one-step solid-phase reaction. The nanosheets of Mn_(2)O_(3) are vertically grown on few-layered graphene, constructing a unique2 D hierarchical structure. As an anode material for lithium-ion batteries(LIBs), this hierarchical composite displays excellent electrochemical performances, showing an extraordinary reversible discharge capacity of 2125.9 mA hg^(–1). Moreover, a record high reversible capacity of 1746.8 mA hg^(–1) is maintained after 100 cycles at a current density of 100 mA g^(–1), which retains 82.2 % of the initial capacity. Such an outstanding performance could be attributed to its novel structure and the synergistic effects between the Mn_(2)O_(3) and graphene.