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    A Beginners’ Guide to Modelling of Electric Double Layer under Equilibrium, Nonequilibrium and AC Conditions
    Lu-Lu Zhang, Chen-Kun Li, Jun Huang
    Journal of Electrochemistry    2022, 28 (2): 2108471-.   DOI: 10.13208/j.electrochem.210847
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    In electrochemistry, perhaps also in other time-honored scientific disciplines, knowledge labelled classical usually attracts less attention from beginners, especially those pressured or tempted to quickly jam into research fronts that are labelled, not always aptly, modern. In fact, it is a normal reaction to the burden of history and the stress of today. Against this context, accessible tutorials on classical knowledge are useful, should some realize that taking a step back could be the best way forward. This is the driving force of this article themed at physicochemical modelling of the electric (electrochemical) double layer (EDL). We begin the exposition with a rudimentary introduction to key concepts of the EDL, followed by a brief introduction to its history. We then elucidate how to model the EDL under equilibrium, using firstly the orthodox Gouy-Chapman-Stern model, then the symmetric Bikerman model, and finally the asymmetric Bikerman model. Afterwards, we exemplify how to derive a set of equations governing the EDL dynamics under nonequilibrium conditions using a unifying grand-potential approach. In the end, we expound on the definition and mathematical foundation of electrochemical impedance spectroscopy (EIS), and present a detailed derivation of an EIS model for a simple EDL. We try to avoid the omission of supposedly ‘trivial’ information in the derivation of models, hoping that it can ease the access to the wonderful garden of physical electrochemistry.

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    Fundamentals of Electrochemical Impedance Spectroscopy for Macrohomogeneous Porous Electrodes
    Xiang Li, Qiu-An Huang, Wei-Heng Li, Yu-Xuan Bai, Jia Wang, Yang Liu, Yu-Feng Zhao, Juan Wang, Jiu-Jun Zhang
    Journal of Electrochemistry    2021, 27 (5): 467-497.   DOI: 10.13208/j.electrochem.201126
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    Electrochemical impedance spectroscopy (EIS) can be used to diagnose charge transfer reactions and mass transport in porous electrodes. The charge transfer reactions include interfacial charge accumulation and charge conduction as well as electrochemical reaction. In this paper, the complex phasor method is developed under the macrohomogeneous assumption to build an impedance model of porous electrodes for clarifying several vague expressions in the traditional approaches. The following researches are carried out: (1) Identifying characteristic parameters for the porous electrodes, including electrode electronic conductivity σ1, electrolyte ionic conductivity σ2, interface charge transfer conductivity gct, unit area interface capacitance C, solid phase diffusion coefficient D, rate constant k, electrode thickness d, characteristic hole depth Lp and unit volume surface area Sc ; (2) elucidating characteristic output parameters for the impedance spectroscopic response, including field diffusion constant K, characteristic frequencies ω0, ω1, ω2, ω3, and ωmax for interface conduction reaction, finite field diffusion, redox reaction, pore diffusion and minimum characteristic pore size, respectively. In addition, the transition frequencies fk1 and fk2 from conduction to diffusion area and from diffusion to saturation area are also defined and studied respectively; (3) defining the parameters X and Z, herein, X = σ1,Z = dSc, Lp , C, gct , D, k,which are responsible for the evolution trend of the characteristic parameters for impedance spectroscopic response, the competition effects of X and Z parameters coupled in charge transfer reaction are analyzed; (4) Further analyzing the competition effects of X and Z parameters coupled in the charge transfer reaction, the diverging frequencies fXZ and fXZ are phenomenologically defined. The locations of fXZ and fXZ can indicate the depth and breadth of the charge transfer reaction affected by the parameters X and Z. The non-existence of fXZ and fXZ indicates that the parameter X or Z can affect the charge transfer reaction over the whole frequency range. With the help of characteristic frequency and diverging frequency, the effects of electrode kinetic and microstructure parameters on the charge transfer reaction in porous electrodes are studied; on the other hand, the shape change and trend evolution of the impedance responses for porous electrodes are analyzed. The research results in this paper should be able to provide theoretical basis for system simulation and system identification of impedance spectroscopy, technical support for competitive analysis of charge transfer reaction in porous electrodes, and diagnostic tool for optimal design of electrochemical energy storage system.

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    Synthesis of Lithium-Rich Manganese-Based Layered Cathode Materials and Study on Its Structural Evolution of First Cycle Overcharge
    Chen-Xu Luo, Chen-Guang Shi, Zhi-Yuan Yu, Ling Huang, Shi-Gang Sun
    Journal of Electrochemistry    2022, 28 (1): 2006131-.   DOI: 10.13208/j.electrochem.200613
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    Lithium-rich manganese-based cathode materials have become one of promising cathode materials due to their low cost and large discharge specific capacity exceeding 250 mAh·g-1. However, their problems such as low coulombic efficiency of first cycle and apparent voltage decay influence commercialization process. The high charging voltage will cause instability of structure and increase the hidden danger of the battery. Therefore, structural evolution of first cycle at higher voltage needs to be further studied. In this work, the precursor was synthesized by the co-precipitation method, and the lithium-rich manganese-based layered cathode materials were prepared by lithium-mixed and high-temperature sintering, and the effects of coulombic efficiency and cycle performance were studied at different charge cut-off voltages. Results have shown that high charging voltage would increase the capacity, but reduce the coulombic efficiency greatly in the first cycle, leading to the decayed specific capacity of long cycle. Cyclic voltammetric investigation proves that when the charge cut-off voltage was 5.0 V, part of the bulk lattice oxygen underwent a reversible oxidation reaction, which lead to the increase of capacity. TEM, XRD and SEM characterization results show that the electrode not only went deep into the bulk phase structural changes, including a large number of stacking faults and spinel phases MnOx and NiOx, and other irreversible phase changes, but also reacted with the electrolyte. Mapping and XPS results show that when the charging voltage became higher, more bulk lattice oxygen participated during redox reaction, which causes stronger oxidizing peroxygen and superoxide ions to undergo side reactions with the electrolyte and accelerates the structural collapse of the electrode, ultimately, becomes not conducive to long cycle performance of the battery accompanied by the dissolution of the transition metal.

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    In- Situ/ Operando 57Fe Mössbauer Spectroscopic Technique and Its Applications in NiFe-based Electrocatalysts for Oxygen Evolution Reaction
    Jafar Hussain Shah, Qi-Xian Xie, Zhi-Chong Kuang, Ri-Le Ge, Wen-Hui Zhou, Duo-Rong Liu, Alexandre I. Rykov, Xu-Ning Li, Jing-Shan Luo, Jun-Hu Wang
    Journal of Electrochemistry    2022, 28 (3): 2108541-.   DOI: 10.13208/j.electrochem.210854
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    The development of highly efficient and cost-effective electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a significant barrier to establish effective utilization of renewable energy storage systems and water splitting to produce clean fuel. The current status of the research in developing OER catalysts shows that NiFe-based oxygen evolution catalysts (OECs) have been proven as excellent and remarkable candidates for this purpose. But it is critically important to understand the factors that influence their activity and underlying mechanism for the development of state-of-the-art OER catalysts. Therefore, the development of in-situ/operando characterizations is urgently required to detect key intermediates along with active sites and phases responsible for OER. 57Fe Mössbauer spectroscopy is one of the appropriate and suitable techniques for determining the phase structure of catalysts under their electrochemical working conditions, identifying the active sites, clarifying the catalytic mechanisms, and determining the relationship between catalytic activity and the coordination structure of catalysts. In this tutorial review, we have discussed the current status of research on NiFe-based catalysts with particular attention to introduce in detail the knowhow about the development and utilization of in-situ/operando57Fe Mössbauer-electrochemical spectroscopy for the study of OER mechanism. A brief overview using NiFe-(oxy)hydroxide catalysts, derived from ordered porous metal-organic framework (MOF) material NiFe-PBAs (Prussian blue analogues), as a typical model study case for the OER electrocatalyst and self-designed in-situ/operando57Fe Mössbauer-electrochemical instrument, has been provided for the better understanding of readers. Moreover, using in-situ/operando57Fe Mössbauer spectroscopy, the crucial role of Fe species during OER reaction has been explained very well.

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    Progress of Pt-Based Catalysts in Proton-Exchange Membrane Fuel Cells: A Review
    Long Huang, Hai-Chao Xu, Bi Jing, Qiu-Xia Li, Wei Yi, Shi-Gang Sun
    Journal of Electrochemistry    2022, 28 (1): 2108061-.   DOI: 10.13208/j.electrochem.210806
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    Fuel cells are energy conversion devices that convert chemical energy directly into electricity. It has the advantages of high energy density, high utilization efficiency of fuel, clean and noiseless during working. Among all kinds of fuel cells, proton exchange membrane fuel cells (PEMFCs) are most popular since PEMFCs function at near ambient temperature, while their power densities are higher than those of other fuel cells. Currently, Pt-based nanomaterials are still the unreplaceable catalysts in commercialized PEMFCs. The lack of low-cost and high-performance cathode catalysts is still one of key factors that hampers the commercialization of PEMFCs. In this review, the structurally controlled syntheses of catalysts and their influences on the performances of oxygen reduction reaction (ORR) and membrane electrode assembly (MEA) are summarized. The performance of membrane electrode assembly (MEA) can also be adjusted by regulating the structure of catalyst layer. Special attention has been paid with a focus on the achievement of enhanced utilization of noble metal, and thus, lowering the loading of noble metals in MEA.

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    Brain Electrochemistry
    Cong Xu, Ying Jiang, Ping Yu, Lan-Qun Mao
    Journal of Electrochemistry    2022, 28 (3): 2108551-.   DOI: 10.13208/j.electrochem.210855
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    Brain, as the source of neural activities such as perceptions and emotions, consists of the dynamic and complex networks of neurons that implement brain functions through electrical and chemical interactions. Therefore, analyzing and monitoring neurochemicals in living brain can greatly contribute to uncovering the molecular mechanism in both physiological and pathological processes, and to taking a further step in developing precise medical diagnosis and treatment against brain diseases. Through collaborations across disciplines, a handful of analytical tools have been proven to be befitting in neurochemical measurement, spanning the level of vesicles, cells, and living brains. Among these, electrochemical methods endowed with high sensitivity and spatiotemporal resolution provide a promising way to precisely describe the dynamics of target neurochemicals during various neural activities. In this review, we expand the discussion on strategies to address two key issues of in vivo electrochemical sensing, namely, selectivity and biocompatibility, taking our latest studies as typical examples. We systematically elaborate for the first time the rationale behind engineering electrode/brain interface, as well as the unique advantages of potentiometric sensing methods. In particular, we highlight our recent progress on employing the as-prepared in vivo electrochemical sensors to unravel the molecular mechanism of ascorbate in physiological and pathological processes, aiming to draw a blueprint for the future development of in vivo electrochemical sensing of brain neurochemicals.

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    Mathematical Expression and Quantitative Analysis of Impedance Spectrum on the Interface of Glassy Carbon Electrode
    Lei Cheng, Pu-Xuan Yan, You-Jun Fan, Hua-Hong Zou, Hong Liang
    Journal of Electrochemistry    2021, 27 (5): 518-528.   DOI: 10.13208/j.electrochem.200821
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    Glassy carbon electrode (GCE) is a common basic electrode for various electrochemical sensors, and the detection properties are determined by its interfacial characteristics. In this paper, we established an equivalent circuit including electrolyte resistance (Rel), charge transport resistance (Rct), diffusion impedance (Rdi, Cdi), electrochemical (oxidation/reduction) reaction impedance (RR, CR), surface adsorption impedance (Rads , Cads), double-layer capacitance (CDL), and derived the mathematical expression for the equivalent circuit. The Rel and CDL are contributed by inactive ions in electrolyte to produce non-faradaic impedance, while the Rct and RR are contributed by the active ions of redox reaction in electrolyte to produce faradaic impedance. The Rct directly corresponds to the electrode potential (E) of the reaction, which represents the difficulty of electrode reaction. When the potential E is the only state variable in the impedance spectrum of electrode reaction, that is, there is only one time constant in the impedance spectrum, the Rct can represent the whole Faraday impedance of the system. However, when the electrode reaction is also affected by other variables such as diffusion, surface film or surface adsorption ion coverage, the Faraday impedance of the system also includes the impedance produced by the diffusion impedance and the changes of the surface film (RR, CR) and the coverage of the surface absorbed ions caused by electrochemical reaction (Rads, Cads). The impedance spectrum of the electrode system in different states were simulated by changing the five parameters of the mathematical expression. The contribution of different factors to the impedance spectrum of GCE was revealed. Finally, the impedance spectra of bare/modified GCE in potassium ferricyanide solution were analyzed by the mathematical model. The fitting results are in good agreement with the experimental data. Based on the parameters obtained by fitting, the changes of the electrode surface characteristic before and after modifications were quantitatively compared and analyzed. The charge transport resistance increases from 5827.8 Ω to 25104.3 Ω, and the diffusion conductance of Fe3+/Fe2+ ions on the electrode surface also increases by an order of magnitude. However, there is no significant difference with the double-layer capacitance and the frequency dispersion coefficient. The surface of the modified electrode remains electrically neutral. The aggregation state and oxidation-reduction mechanism of Fe3+/Fe2+ on the electrode surface are the same as those on the bare GCE surface.

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    Nitrogen-Sulfur Co-Doped Porous Carbon Preparation and Its Application in Lithium-Sulfur Batteries
    Gui-Xiang Zhao, Wail Hafiz Zaki Ahmed, Fu-Liang Zhu
    Journal of Electrochemistry    2021, 27 (6): 614-623.   DOI: 10.13208/j.electrochem.201210
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    In recent years, lithium-sulfur (Li-S) batteries have been considered as a promising candidate for the next generation of energy storage system due to their ultrahigh theoretical capacity (1675 mAh·g-1) and energy density (2600 Wh·kg-1). However, the practical application of Li-S batteries is seriously limited by their insulating nature of sulfur, the shuttle effect of polysulfides (LiPSs), and volume expansion during charging and discharging. To overcome those disadvantages, one of the commonly methods is to infiltrate sulfur into porous conductive carbon framework, such as porous carbon, hollow carbon spheres, graphene, carbon nanotubes and some composites of the above structures to achieve the purpose of physically limiting the shuttle effect of polysulfides, thereby improving the performance of Li-S batteries. However, due to the nonpolarity of traditional carbon materials, the interaction with polar polysulfides is very weak, which cannot effectively inhibit the shuttle effect of polysulfides. Previous studies have shown that introducing heteroatom (N, S, P, B, etc.) doping into carbon matrix is a feasible method to adjust the nonpolarity of carbon materials. It is reported that the introduction of N atoms is conducive to improving the electrochemical activity. The Li-N bond formed by the interaction between N and Li+ can anchor polysulfides, effectively inhibit the dissolution of polysulfides and improve the utilization rate of sulfur. The introduction of nitrogen and sulfur heteroatoms can increase polar sites and active centers, thus, enhancing the adsorption capacity of carbon materials for polysulfides and capturing polysulfides. Therefore, ionic liquids are selected as nitrogen and sulfur sources to improve the polarity of carbon materials. In this paper, nitrogen and sulfur co-doped porous carbon (NSPC) was synthesized by using glucose as carbon source, KCl and ZnCl2 as templates, KOH as activator and ionic liquid as heteroatom source. XPS and adsorption experiments show that nitrogen and sulfur heteroatoms had been successfully introduced into NSPC, which improved the adsorption capacity of carbon materials for polysulfides, effectively alleviated the shuttle effect of polysulfides. The higher specific surface area (1290.67 m2·g-1) could help to improve the sulfur loading. After loading 70.1wt.% sulfur into NSPC (S@NSPC) and tested as a cathode material of Li-S battery, the initial discharge capacity was 1229.2 mAh·g-1 at 167.5 mA·g-1, higher than the 861.6 mAh·g-1 of S@PC, and the capacity remained at 328.1 mAh·g-1 after 500 cycles. When the current density returned to 167.5 mA·g-1, the reversible capacity almost went back to its initial value, which was 80% of its initial value. The good performance was mainly ascribed to both the porous structure and N, S co-dopants, which provided physical blocks and chemical affinity, respectively, for the efficient immobilization of intermediate lithium polysulfides. The results would provide an effective example in the surface chemistry and sulfur host materials design for high performance Li-S batteries.

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    Research Progress and Performance Improvement Strategies of Hard Carbon Anode Materials for Sodium-Ion Batteries
    Xiuping Yin, Yufeng Zhao, Jiujun Zhang
    Journal of Electrochemistry    DOI: 10.13208/j.electrochem.2204301
    Accepted: 13 June 2022

    Acetate Solutions with 3.9 V Electrochemical Stability Window as an Electrolyte for Low-Cost and High-Performance Aqueous Sodium-Ion Batteries
    Dao-Yun Lan, Xiao-Feng Qu, Yu-Ting Tang, Li-Ying Liu, Jun Liu
    Journal of Electrochemistry    2022, 28 (1): 2102231-.   DOI: 10.13208/j.electrochem.210223
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    Low-cost and high-safety aqueous sodium-ion batteries have received widespread attention in the field of large-scale energy storage, but the narrow electrochemical stability window (1.23 V) of water limits the energy density of aqueous sodium-ion batteries. The “water-in-salt” strategy which uses the interaction between cations and water molecules in the solution can inhibit water decomposition and broaden the electrochemical stability window of water. In this work, two types of low-cost salts, namely, ammonium acetate (NH4CH3COOH) and sodium acetate (NaCH3COOH), were used to configure a mixed aqueous electrolyte for aqueous sodium-ion batteries. The solution consisted of 25 mol·L -1 NH4CH3COOH and 5 mol·L-1 NaCH3COOH, used as an aqueous electrolyte, exhibited a wide electrochemical stability window of 3.9 V and high ionic conductivity of 28.2 mS·cm-1. The composite of layered manganese dioxide and multi-wall carbon nanotubes (MnO2/CNTs) was used as a positive electrode material, while the carbon-coated NaTi2(PO4)3 with NASICON structure was used as a negative electrode material. Both of these electrode materials had excellent electrochemical performances in the aqueous electrolyte. A full cell achieved an average working voltage of about 1.3 V and a discharge capacity of 74.1 mAh·g-1 at a current density of 0.1 A·g-1. This aqueous sodium-ion battery displayed excellent cycling stability with negligible capacity losses (0.062% per cycle) for 500 cycles. The safe and environmentally friendly aqueous acetate electrolyte, with a wide electrochemical stability window, showed the potential to be matched with positive materials having higher potential and negative materials having lower potential for further improving the voltage of aqueous sodium-ion batteries and promoting the development of aqueous batteries for large-scale energy storage technology.

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    Cyclic Voltammetric Simulations on Batteries with Porous Electrodes
    Xue-Fan Cai, Sheng Sun
    Journal of Electrochemistry    2021, 27 (6): 646-657.   DOI: 10.13208/j.electrochem.210210
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    Lithium-ion batteries (LIBs) are among the most widely used energy storage devices. Whole-cell modeling and simulations of LIBs can optimize the design of batteries with lower costs and higher speeds. The Pseudo-Two-Dimensional (P2D) electrochemical model is among the most famous whole-cell models and widely applied in LIB simulations. P2D model consists of a series of kinetic equations to model Li+/Li diffusion in working/counter electrodes and electrolytes, which are filled in the porous electrodes and separator, and reactions at the interface of electrolyte and active particles. The traditional applications of P2D model, however, are limited to the cases where the current is the control variable and the voltage is the dependent variable. The present work tries to apply boundary conditions with the electrode potential as the control variable to simulate cyclic voltammetric (CV) experiments on the whole battery, based on a detailed analysis on different potentials, including Galvani potential, Volta potential, electrode potential and battery terminal voltage, as well as their relationships. In many CV experiments, only two electrodes, the working and the counter electrodes, are used. The experimental results are usually explained by using theoretical results directly taken from textbooks. The theories of CV are, however, based on three-electrode systems with a reference electrode to provide a reference voltage. The differences of CV curves between the two- and the three-electrode systems have never been studied by using P2D models. The present work performs numerical simulations of CV on both two- and three-electrode systems by using finite element methods, brought with the software of COMSOL Multiphysics, to study the influences of scanning rate, effective radius of active particles, lithium ion diffusivity and stoichiometric maximum concentration in electrode on CV curves. The three-electrode system is simulated by applying a potential detector at the separator region of a battery. The applied potentials are changed in time based on the magnitude of the detected potential. Results show that, for CV curves on both two- and three-electrodes systems, the peak current determined by the complex electrode dynamics process increases with the increases of scanning rate, lithium ion diffusivity in electrode and stoichiometric maximum lithium ion concentration, but with the decrease of the radius of electrode active particles. The peak currents obtained from CV curves are larger in three-electrode systems than in two-electrode systems under the same applied parameters. CV curves of three-electrode systems are more symmetric for the anodic and cathodic currents than those in two-electrode systems.

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    Synthesis of Nickel Phosphide/Nitrogen Phosphorus Co-Doped Carbon and Its Application in Lithium Ion Batteries
    Jian Hu, Yan-Shuang Meng, Qian-Ru Hu
    Journal of Electrochemistry    2021, 27 (5): 540-548.   DOI: 10.13208/j.electrochem.200713
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    In recent years, the nickel-based phosphide has drawn great attention because of its low intercalation/deintercalation platform and lower polarization compared to sulfides and oxides as anodes for next-generation high-energy lithium-ion batteries. The Ni2P anode can deliver high theoretical specific capacity of 542 mAh·g-1, but it subject to a conversion reaction mechanism, which make them unsuitable for commercial applications. The agglomeration of Ni2P nanoparticles during material fabrication and the structural deterioration of electrode associated with large volume change during charge-discharge process lead to poor cycle stability and low utilization of active materials. Meanwhile, the low intrinsic conductivity of Ni2P is also sluggish electrochemical reaction kinetics. Herein, we design a facile and viable approach to synthesize Ni2P/NPC composites with a stable structure to address these issues. This new approach entails synthesis of Ni2P/NPC by a N and P co-doped carbon framework with ionic liquids assistance during synthesis. This stable composite structure can serve as anode material of lithium ion batteries with good electrochemical performance. The Ni2P/NPC composites were prepared by one-step method using ionic liquids as carbon and nitrogen sources, while sodium hypophosphite and nickel acetate as phosphorus and nickel sources, respectively. The results of SEM and TEM show that Ni2P nanoparticles were uniformly distributed on the N and P co-doped carbon framework. When the Ni2P/NPC composite was used as an anode material of lithium ion batteries, the discharge specific capacities were 377.7, 294.1, 265.4, 211.7 and 187.5 mAh·g-1 at 0.1, 0.5, 1, 3 and 5 A·g-1, respectively. When the current density returned to 0.1 A·g-1, the discharge specific capacity reached 368.1 mAh·g-1. The Ni2P/NPC structure could be kept stable at high rate, showing excellent rate performance. The fabricated Ni2P/NPC anode delivered the discharge specific capacity of 301.8 mAh·g-1 with the capacity retention of 80.7% after 200 cycles at 0.5 A·g-1. Finally, CV curves confirmed that the lithium storage of Ni2P/NPC colud be controlled by diffusion process and capacitance behavior.

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    A High-Performance Continuous-Flow MEA Reactor for Electroreduction CO 2 to Formate
    Pei-Xuan Liu, Lu-Wei Peng, Rui-Nan He, Lu-Lu Li, Jin-Li Qiao
    Journal of Electrochemistry    2022, 28 (1): 2104231-.   DOI: 10.13208/j.electrochem.210423
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    The electrochemical carbon dioxide reduction reaction (CO2RR) is a promising approach to produce liquid fuels and industrial chemicals by utilizing intermittent renewable electricity for mitigating environmental problems. However, the traditional H-type reactor seriously limits the electrochemical performance of CO2RR due to the low CO2 solubility in electrolyte, and high ohmic resistance caused by the large distance between two electrodes, which is unbeneficial for industrial application. Herein, we demonstrated a high-performance continuous flow membranes electrode assembly (MEA) reactor based on a self-growing Cu/Sn bimetallic electrocatalyst in 0.5 mol·L-1 KHCO3 for converting CO2 to formate. Compared with an H-type cell, the MEA reactor not only shows the excellent current density (66.41 mA·cm-2 at -1.11 VRHE), but also maintains high Faraday efficiency of formate (89.56%) with the steady work around 20 h. Notably, we also designed the new CO2RR system to effectively separate the gaseous/liquid production. Surprisingly, the production rate of formate reached 163 μmol·h-1·cm-2 at -0.91 VRHE with the cell voltage of 3.17 V. This study provides a promising path to overcome mass transport limitations of the electrochemical CO2RR and to separate liquid from gas products.

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    In Situ Characterization of Electrode Structure and Catalytic Processes in the Electrocatalytic Oxygen Reduction Reaction
    Ya-Chen Feng, Xiang Wang, Yu-Qi Wang, Hui-Juan Yan, Dong Wang
    Journal of Electrochemistry    2022, 28 (3): 2108531-.   DOI: 10.13208/j.electrochem.210853
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    As an electrochemical energy conversion system, fuel cell has the advantages of high energy conversion efficiency and high cleanliness. Oxygen reduction reaction (ORR), as an important cathode reaction in fuel cells, has received extensive attention. At present, the electrocatalysts are still one of the key materials restricting the further commercialization of fuel cells. The fundamental understanding on the catalytic mechanism of ORR is conducive to the development of electrocatalysts with the enhanced activity and high selectivity. This review aims to summarize the in situ characterization techniques used to study ORR. From this perspective, we first briefly introduce the advantages of various in situ techniques in ORR research, including electrochemical scanning tunneling microscopy, infrared spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. Then, the applications of various in situ characterization techniques in characterizing of the catalyst morphological evolution and electronic structure as well as the identification of reactants and intermediates in the catalytic process are summarized. Finally, the future development of in situ technology is outlooked.

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    Magnetic Resonance in Metal-Ion Batteries: From NMR (Nuclear Magnetic Resonance) to EPR (Electron Paramagnetic Resonance)
    Bing-Wen Hu, Chao Li, Fu-Shan Geng, Ming Shen
    Journal of Electrochemistry    2022, 28 (2): 2108421-.   DOI: 10.13208/j.electrochem.210842
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    Metal-ion batteries have changed our quotidian lives. The research on the electrode materials for metal-ion battery is the key to improve the performance of the battery. Therefore, understanding the structure-performance relationship of the electrode materials can help to improve the energy density and power density of the materials. Magnetic resonance, including nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR), has been continuously improved during the past three decades, and has gradually become one of the important technologies to study the structure-performance relationship of electrode materials. This paper summarizes the progress of magnetic resonance research from our group on several interesting electrode materials and demonstrates the important role of NMR and EPR in the study of electrode materials. This article will help to grasp the important value of magnetic resonance technology for battery research, which will promote the further development of advance magnetic resonance technology.

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    Advances of Phosphide Promoter Assisted Pt Based Catalyst for Electrooxidation of Methanol
    Meng Li, Li-Gang Feng
    Journal of Electrochemistry    2022, 28 (1): 2106211-.   DOI: 10.13208/j.electrochem.210621
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    Transition metal phosphide (TMP), as an ideal catalytic promoter in methanol fuel oxidation, has received increased attention because of its multifunctional active sites, tunable structure and composition, as well as unique physical and chemical properties and efficient multi-composition synergistic effect. Some advances have been made for this catalyst system recently. In the current review, the research progresses of transition metal phosphides (TMPs) in the assisted electrooxidation of methanol including the catalysts fabrication and their performance evaluation for methanol oxidation are reviewed. The promotion effect of TMPs has been firstly presented and the catalyst systems based on the different metal centers of TMPs are then mainly discussed. It is concluded that the TMPs can greatly promote methanol oxidation through the electronic effect and the oxyphilic property based on the bifunctional catalytic mechanism. The problems and challenges in methanol fuel oxidation by using TMPs are also described at the end with the attention being paid to the precise catalyst design. The catalytic mechanism probing and application of the fuel cells device are proposed. The current effort might be helpful to the community for novel catalyst system design and fabrication.

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    Functional Sulfate Electrolytes Enable the Enhanced Cycling Stability of NaTi 2(PO 4) 3/C Anode Material for Aqueous Sodium-Ion Batteries
    Shu-Jin Li, Zhi-Kang Cao, Wen-Kai Wang, Xiao-Han Zhang, Xing-De Xiang
    Journal of Electrochemistry    2021, 27 (6): 605-613.   DOI: 10.13208/j.electrochem.210125
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    Aqueous sodium-ion batteries show promising application in fields of large-scale storage of intermittent renewable energies owing to the earth-abundant sodium resources and incombustible aqueous electrolytes. Primary factors determining whether they can be commercially utilized are low cost and long lifetime. Among current electrode materials, NASICON-type NaTi2(PO4)3 arouses wide interests as an anode material for aqueous sodium-ion batteries as it offers a high specific capacity, fast Na-transport ability and reasonable working potential, however, suffering from insufficient cycling performance caused by severe dissolution of active materials in traditional aqueous electrolytes. In this work, a functional sulfate electrolyte (2 mol·L-1 Na2SO4 + 0.3 mol·L-1 MgSO4) was designed by coupling concentrated Na2SO4 salt and functional MgSO4 additive to enhance the cycling stability of NaTi2(PO4)3/C material. Experimental results from cyclic voltammetry and galvanostatic measurements suggest that the electrolyte can improve electrochemical reversibility and cycling performance of NaTi2(PO4)3/C material relative to traditional electrolyte (1 mol·L-1 Na2SO4). In specific, the material harvested a reversible capacity of 93.4 mAh·g-1 and impressive capacity retention of 96.5% at the specific current of 100 mA·g-1 in the functional sulfate electrolyte, but exhibited a reversible capacity of 88.6 mAh·g-1 and much lower capacity retention of 72.1% in the traditional electrolyte. In order to explore intrinsic causes of the performance improvement, structural properties of the material before and after cycling were comparatively investigated by using X-ray diffraction and X-ray photon spectroscopy. It is found that the material showed excellent structural stability and formation of protective Mg-containing interfacial layer during cycling in the functional sulfate electrolyte. Both the raised electrolyte-salt concentration and functional MgSO4 additive should be responsible for the enhanced structural stability. The high electrolyte-salt concentration could decrease electrochemical activity and widen electrochemical stability window of electrolyte solvents, while the MgSO4 additive could timely capture the hydroxyl group resulting from water-solvent decomposition to prevent the alkalization of aqueous electrolytes and spontaneously form protective Mg(OH)2 interfaces. As a result, the electrolyte could suppress the dissolution of active NaTi2(PO4)3, thus, resulting in the enhanced structural stability and cycling performance. With an aim to further exhibit the feasibility for practical application, full aqueous sodium-ion batteries were assembled by coupling Na2Ni[Fe(CN)6] cathode, functional sulfate electrolyte and NaTi2(PO4)3/C anode. Charge/discharge tests show that the battery could deliver a working voltage of 1.3 V and a reversible capacity of 84.2 mAh·g-1 (calculated as the mass of active anode material) at the current of 100 mA·g-1, achieving a specific energy of about 110 Wh·kg-1. After being continuously charging and discharging for 500 cycles at the current of 500 mA·g-1, it achieved high capacity retention of 80%. The results in this work suggest that designing functional additive-containing sulfate electrolytes is an effective strategy to fabricate low-cost, long-lifetime aqueous sodium-ion batteries.

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    Structural Degradation of Ni-Rich Layered Oxide Cathode for Li-Ion Batteries
    Jia-Yi Wang, Sheng-Nan Guo, Xin Wang, Lin Gu, Dong Su
    Journal of Electrochemistry    2022, 28 (2): 2108431-.   DOI: 10.13208/j.electrochem.210843
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    Nickel(Ni)-rich layered oxide has been regarded as one of the most important cathode materials for the lithium-ion batteries because of its low cost and high energy density. However, the concerns in safety and durability of this compound are still challenging for its further development. On this account, the in-depth understanding in the structural factors determining its capacity attenuation is essential. In this review, we summarize the recent advances on the degradation mechanisms of Ni-rich layered oxide cathode. Progresses in the structure evolution of Ni-rich oxide are carefully combed in terms of inner evolution, surface evolution, and the property under thermal condition, while the state-of-the-art modification strategies are also introduced. Finally, we provide our perspective on the future directions for investigating the degradation of Ni-rich oxide cathode.

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    Development Status of Copper Electroplating Filling Technology in Through Glass Via for 3D Interconnections
    Zhi-Jing Ji, Hui-Qin Ling, Pei-Lin Wu, Rui-Yi Yu, Da-Quan Yu, Ming Li
    Journal of Electrochemistry    2022, 28 (6): 2104461-.   DOI: 10.13208/j.electrochem.210446
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    With the slow development of Moore's Law, the high density and miniaturization of microelectronic devices put forward higher requirements for advanced packaging technology. As a key technology in 2.5D/3D packaging, interposer technology has been extensively studied. According to different interposer materials, it is mainly divided into organic interposer, silicon interposer and glass interposer. Compared with the through silicon via (TSV) interconnection, the through glass via (TGV) interposer has received extensive attention in the 2.5D/3D advanced packaging field for its advantages of excellent high-frequency electrical characteristics, simple process, low cost, and adjustable coefficient of thermal expansion (CTE). However, the thermal conductivity of glass (about 1 W·m-1·K-1) is much lower than that of silicon (about 150 W·m-1·K-1), thus, the glass interposer has serious heat dissipation problems. In order to obtain a high-quality TGV interposer, not only an efficient and low-cost via preparation process, but also a defect-free filling process is required. The challenges faced by glass interposer is mainly concentrated in these two aspects. This review firstly introduces the preparation process of TGV, such as ultra-sonic drilling (USD), ultra-sonic high speed drilling (USHD), wet etching, deep reactive ion etching (DRIE), photosensitive glass, laser etching, laser induced deep etching (LIDE), etc. Then it summarizes the defect-free filling of TGV, and outlines several filling mechanisms and some current filling processes of TGV, such as bottom-up filling mechanisms, butterfly filling mechanisms and conformal filling mechanisms. Among the filling mechanisms of the above three filling methods, the filling method of bottom-up is the most studied one, and many scholars have given relevant explanations. Currently, the main ones that are commonly used are the diffusion-consumption mechanism, curvature enhanced adsorbate coverage mechanism (CEAC), convection dependent adsorption mechanism (CDA), and S-shaped negative differential resistance theory. In the process of TGV filling, the type and concentration of base bath, additives and electroplating process will affect the filling status of TGV. At present, the constant current plating mode is most commonly used in the process of TGV filling. Then the research progress of TGV electroplating additives is introduced, including the action mechanism of typical additives and the current research status of some new additives. Through glass via technology can be filled with the synergistic action of accelerators, suppressors and levelers. Finally, the practical application of TGV is briefly reviewed, for example, glass interposer is used in 3D integrated passive device (IPD), embedded glass fan-out technology (eGFO), integrated antenna packaging, micro-electro-mechanical system (MEMS), multi-chip module packaging, as well as the applications in the field of optical integration technology.

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    Structure Analysis of PEMFC Cathode Catalyst Layer
    Rui-Qing Wang, Sheng Sui
    Journal of Electrochemistry    2021, 27 (6): 595-604.   DOI: 10.13208/j.electrochem.201208
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    The sluggish oxygen reduction reaction (ORR) on the cathode of the proton exchange membrane fuel cell (PEMFC) has always been one of the key factors limiting its commercialization. The optimization of the cathode catalytic layer structure plays an important role in improving fuel cell performance and reducing production costs. In this paper, two different catalysts (platinum nanoparticles (Pt-NPs) and platinum nanowires (Pt-NWs)) were prepared by using catalyst coated substrate (CCS) method. By constructing a double-layer catalytic layer structure, we analyzed the effect of different catalytic layer structures by performing a single cell test. The results showed that the dense platinum particle structure in the Pt-rich layer near the proton exchange membrane could promote the ORR rate, while the Pt-poor layer near the gas diffusion layer had higher porosity and average pore size, which is beneficial to the reaction gas transmission and diffusion. When the platinum loading ratio of the rich to poor platinum layer was 1:2, the best single cell performance was achieved. The current density at 0.6 V reached 1.05A·cm-2, and the maximum power density was 0.69 W·cm-2. Compared with the single-layer structure, the peak power density was increased by 21%. When growing Pt-NWs on the Pt-NPs base layer, the presence of Pt particles promoted the reduction of platinum precursors and provided deposition sites for newly formed Pt atoms, and the grown Pt-NWs had a more uniform distribution as well as a denser pile structure. The current density of the optimized Pt-NWs catalytic layer structure at 0.6 V increased by 21%. The MEA fabricated by double-catalytic layer method had a higher catalyst utilization rate and a guiding significance for the optimization of the cathode catalytic layer structure. The high activity shown by the platinum nanowires provides a new idea for the preparation of efficient catalysts.

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