The extraordinarily high capacities delivered by lithium‐rich oxide cathodes, compared with conventional layered oxide electrodes, are a result of contributions from both cationic and anionic redox ...processes. This phenomenon has invoked a lot of research exploring new kinds of lithium‐rich oxides with multiple‐electron redox processes. Though proposed many years ago, anionic redox is now regarded to be crucial in further developing high‐capacity electrodes. A basic overview of the previous work on anionic redox is given, and issues related to electronic and geometric structures are discussed, including the principles of activation, reversibility, and the energy barrier of anionic redox. Anionic redox also leads to capacity loss and structural degradation, as well as voltage hysteresis, which shows the importance of controlling anionic redox reactions. Finally, the techniques used for characterizing anionic redox processes are reviewed to aid the rational choice of techniques in future studies. Important perspectives are highlighted, which should instruct future work concerning anionic redox processes.
Anionic redox plays an important role in obtaining high capacities in lithium‐rich layered oxide cathodes. Previous studies on anionic redox and related electronic‐ and geometric‐structure issues, as well as problems induced by anionic‐redox and suitable characterization techniques, are reviewed, in order to direct future work on high‐capacity electrodes that utilize anionic redox processes and widen the scope of these materials.
Metal sites play an essential role in both electrocatalytic and photocatalytic energy conversion. The highly ordered arrangements of the organic linkers and metal nodes as well as the well‐defined ...pore structures of metal‐organic frameworks (MOFs) make them ideal substrates to support atomically dispersed metal sites (ADMSs) located in their metal nodes, linkers, and pores. Porous carbon materials doped with ADMSs can be derived from these ADMS‐incorporating MOF precursors through controlled treatments. These ADMSs incorporated in pristine MOFs and MOF‐derived carbon materials possess unique advantages over molecular or bulk metal‐based catalysts and bridge the gap between homogeneous and heterogeneous catalysts for energy‐conversion applications. This Review presents recent progress in the design and incorporation of ADMSs in MOFs and MOF‐derived materials for energy‐conversion applications.
A site to behold: Atomically dispersed metal sites in MOFs and MOF‐derived materials offer great potential for the design and modification of advanced catalysts for applications in photocatalytic and electrocatalytic energy conversion. Recent breakthroughs and future perspectives are presented in this Review.
Pt‐based electrocatalysts for the oxygen reduction reaction (ORR) are the topic of extensive and intensive research since a few decades. Nevertheless, the scarcity of these Pt‐based electrocatalysts, ...their high cost and unsatisfactory durability are the primary hindrances to their further commercialization. In recent years, non‐Pt electrocatalysts have garnered considerable interest as alternatives to Pt‐based catalysts for the ORR. This review highlights the synthesis, catalytic activity and key factors, namely also active sites, of various nanostructured non‐Pt catalysts that can be grouped into five categories: monometallic catalysts; bimetallic or multimetallic catalysts; transition metal oxides/chalcogenides/nitrides/oxynitrides/carbides; heteroatom‐doped carbon catalysts with and without transition metals and metal–organic frameworks (MOFs)‐derived catalysts that have emerged as a class of promising catalysts recently. For metallic catalysts, the ORR activity and durability can be tuned by capitalizing on such effects as surface ligands and lattice strain; for heteroatom‐doped carbon catalysts, some factors like mass transport and electric conductivity are nontrivial for enhancement of the ORR activity. Recent research advances are presented and an outlook for future research is provided.
Recent progress on oxygen reduction reaction (ORR) electrocatalysts used in proton‐exchange‐membrane fuel cells (PEMFCs) is presented, including monometallic catalysts, bimetallic or multimetallic catalysts, transition metal oxides, chalcogenides, nitrides, oxynitrides and carbides, heteroatom‐doped carbon materials with and without transition metals, and metal–organic frameworks (MOFs)‐derived catalysts. Key factors are demonstrated to improve ORR catalytic activities.
Abstract
Li-rich layered oxide cathode materials show high capacities in lithium-ion batteries owing to the contribution of the oxygen redox reaction. However, structural accommodation of this ...reaction usually results in O–O dimerization, leading to oxygen release and poor electrochemical performance. In this study, we propose a new structural response mechanism inhibiting O–O dimerization for the oxygen redox reaction by tuning the local symmetry around the oxygen ions. Compared with regular Li
2
RuO
3
, the structural response of the as-prepared local-symmetry-tuned Li
2
RuO
3
to the oxygen redox reaction involves the telescopic O–Ru–O configuration rather than O–O dimerization, which inhibits oxygen release, enabling significantly enhanced cycling stability and negligible voltage decay. This discovery of the new structural response mechanism for the oxygen redox reaction will provide a new scope for the strategy of enhancing the anionic redox stability, paving unexplored pathways toward further development of high capacity Li-rich layered oxides.
Abstract
Nano-ordered intermetallic compounds have generated great interest in fuel cell applications. However, the synthesis of non-preciousearly transition metal intermetallic nanoparticles remains ...a formidable challenge owing to the extremely oxyphilic nature and very negative reduction potentials. Here, we have successfully synthesized non-precious Co
3
Ta intermetallic nanoparticles, with uniform size of 5 nm. Atomic structural characterizations and X-ray absorption fine structure measurements confirm the atomically ordered intermetallic structure. As electrocatalysts for the hydrazine oxidation reaction, Co
3
Ta nanoparticles exhibit an onset potential of −0.086 V (vs. reversible hydrogen electrode) and two times higher specific activity relative to commercial Pt/C (+0.06 V), demonstrating the top-level performance among reported electrocatalysts. The Co-Ta bridge sites are identified as the location of the most active sites thanks to density functional theory calculations. The activation energy of the hydrogen dissociation step decreases significantly upon N
2
H
4
adsorption on the Co-Ta bridge active sites, contributing to the significantly enhanced activity.
Li‐rich manganese based oxides (LRMOs) are considered an attractive high‐capacity cathode for advanced Li‐ion batteries; however, their poor cyclability and gradual voltage fading have hindered their ...practical applications. Herein, an efficient and facile strategy is proposed to stabilize the lattice structure of LRMOs by surface modification of polyacrylic acid (PAA). The PAA‐coated LRMO electrode exhibits only 104 mV of the voltage fading after 100 cycles and 88% capacity retention over 500 cycles. The structural stability is attributed to the carboxyl groups in PAA chains reacting with oxygen species on the surface of LRMO to form a uniform and tightly coated film, which significantly suppresses the dissolution of transition metal elements from the cathode materials into the electrolyte. Importantly, a H+/Li+ exchange reaction takes place between the LRMO and PAA, generating a proton‐doped surface layer. Density functional theory calculations and experimental evidence demonstrates that the H+ ions in the surface lattice efficiently inhibit the migration of transition metal ions, leading to a stabilized lattice structure. This surface modification approach may provide a new route to building a stable Li‐rich oxide cathode with high capacity retention and low voltage fading for practical Li‐ion battery applications.
Voltage fading of a Li‐rich oxide cathode is efficiently suppressed by using a polyacrylic acid (PAA) binder to build a well‐protected and partially‐protonated surface. The PAA‐coated Li‐rich manganese‐based oxides electrode exhibits only 104 mV of voltage fading after 100 cycles and 88% capacity retention over 500 cycles.
► The synthesis of cubic MnO/C was more facile compared with those reported attempts. ► The as-prepared MnO/C shows good electrochemical properties. ► The reason for improvement of electrochemical ...performance was studied.
Cubic MnO with particle sizes of ∼200nm and ∼600nm was synthesized by decomposition of MnCO3. The corresponding MnO/C composite was obtained by thermal treatment of mixture of MnCO3 and sucrose. The structure and morphology of the products were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Electrochemical experiments showed that the as-prepared MnO/C exhibited promising electrochemical properties, and could potentially be used as anode material in lithium-ion batteries. MnO/C delivered a reversible capacity of about 470mAh/g after cycling 50 times, when testing at 75mA/g. The reversible capacity, when tested at 150, 375, 755mA/g, reached 440, 320, 235mAh/g, respectively. The good electrochemical performance was ascribed to the smaller particle size and the efficient carbon coating on MnO.
The search for superior‐energy‐density electrode materials for rechargeable batteries is prompted by the continuously growing demand for new electric vehicles and large energy‐storage grids. The ...structural properties of electrode materials affect their electrochemical performance because their functionality is correlated to their structure at the atomic scale. Although challenging, a deeper and comprehensive understanding of the basic structural operating units of electrode materials may contribute to the advancement of new energy‐storage technologies and many other technologies. Therefore, we must strategically control both the structure and kinetics of electrode materials to achieve optimal electrochemical performance. In this contribution, advancements in synchrotron radiation techniques, specifically in situ/operando experiments on electrode materials for rechargeable batteries, are presented and discussed. Indeed, the latest synchrotron radiation methods offer deeper insights into pristine and chemically modified electrode materials, opening new opportunities to optimize these materials and exploit new technologies. In particular, the most recent results from in situ/operando synchrotron radiation measurements, which play a critical role in the fundamental understanding of the kinetics processes that occur in rechargeable batteries, are discussed.
This review systematically summarizes the main challenges in the fields of Li‐ and Na‐ion batteries, the progress achieved using synchrotron radiation methods in the investigation of electrode materials, and the future strategy for in situ and operando observations of these complex electrochemical reactions.
•CoS2/RGO was in situ synthesized by employing graphene oxides as oxidizer and Na2S2O3 as reductor.•The CoS2 particles of 150nm were uniformly dispersed on the RGO nanosheets.•The enhanced properties ...can be attributed to the small particle size and the RGO networks.
This study reports a novel strategy of preparing CoS2/reduced graphene oxides (RGO) nanocomposites by employing graphene oxides (GO) as an oxidizing agent and Na2S2O3 as a reducing agent. CoS2 can be in situ synthesized with GO being reduced. X-ray diffraction (XRD), Raman spectrometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical test are used to characterize the nanocomposite. The CoS2 particles with the size of 150nm are dispersed in the networks made from thin RGO nanosheets. The CoS2/RGO nanocomposite as an anode material for lithium-ion batteries can deliver excellent reversible capacity retention (640mAhg−1) after cycling 50 times when tested at 100mAg−1 and rate performance. The enhanced electrochemical properties can be attributed to the nanoscale particles sizes of CoS2 in addition to the effects of RGO networks in preventing the agglomeration of CoS2 and absorbing lithium polysulfides during the charge-discharge processes.
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