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The distribution of internal resistance in a 5-cell electrochemical capacitor comprising an electrode material with 80 wt.% RuO2·xH2O and 20 wt.% activated carbon and an electrolyte of 38 wt.% H2SO4 acid solution was analyzed. It was found that over 94% of the internal resistance in the capacitor was contributed to various contacts with the current collector of the conducting plastic sheet. Careful analysis of three different contact sources including the current collector to the end metal plate, the current collector to the neighboring current collector, and the current collector to the electrode, it was found that the resistance was dominated by the contact resistance between the current collector and the electrode. A composite current collector, made with conducting plastic sheet coated with metal films on both surfaces, was proposed for reducing all kinds of contact resistances. Using the composite current collector, the internal resistance can be reduced to about only 26% of that using conventional conducting plastic sheet. [Copyright &y& Elsevier]

A rate-dependent nucleation theory is developed to analyze the dependence of the capacity of lithium-sulfur (Li-S) batteries on discharge rates. The theory is based on expressing the nucleation rate of solid products in terms of the surface oversaturation of the precipitating species and rewriting the classical Kolmogorov-Avrami model in the form of a differential system of equations. By coupling this system of equations with the multicomponent transport equations we are able to explain qualitatively well the decrease of the specific capacity with increasing discharge rates and the characteristics of the discharge curves under variable discharge currents in Li-S batteries. Special attention is given to accurately determine the values of model parameters from experimental data, including the solubility constants of the solid products and the mobilities and diffusivities of the ions. The anodes of the batteries used in our analysis were made using Li foils and the cathodes were fabricated using freestanding carbon nanotube foams, on which solid sulfur was uniformly distributed to maximize the dissolution rate. The experimental data combined with the theoretical analysis show that the capacity limitation in our batteries is due to surface passivation by the final reaction products. In order to increase the specific capacity of Li-S batteries under normal discharge rates we conclude that it is better to avoid the solid precipitation of intermediate lithium polysulfides (LiPS) by using electrolytes with a high solubility of LiPS. In addition, it is better to use catalyst particles in the cathode that would promote the nucleation rate at specified locations. The number of catalyst particles should not be too small to avoid electrolyte oversaturation or too large to avoid the rapid passivation of the carbon surface with solid products. [ABSTRACT FROM AUTHOR]

Developing high‐capacity electrodes requires the evaluation of electrochemical behaviors with an increasing current density. Currently, the current density for evaluation of high‐capacity electrodes has reached a new stage where the polarization at the lithium counter electrode has become a technical barrier for the accurate evaluation of battery electrodes, resulting in severe performance and mechanism mischaracterizations. Here, the accurate electrochemical behavior for high‐capacity electrodes via a single‐channel three‐electrode vehicle is decoupled, by which the impact of lithium counter electrode is minimized. The testing high‐capacity graphite electrode is capable of delivering an excellent rate capability with 81.7% capacity retention at 0.3 C, as well as stable cycling performance retaining 97.5% practical reversible capacity after 225 cycles, much higher than the graphite electrode tested with traditional half‐cell testing vehicle but in close agreement with the results obtained from a well‐matched full cell, reflecting accurate electrochemical performance evaluations of high‐capacity electrodes. Moreover, detailed electrochemical mechanisms of impedance and diffusion properties for working electrodes are also successfully decoupled individually. This work uncovers the mismatch between traditional evaluation configuration and increasing testing current density and provides a guideline for accurate electrochemical evaluation for ever‐increasing high‐capacity electrodes, which is of great significance for high‐energy lithium or other alkali‐metal ion batteries. [ABSTRACT FROM AUTHOR]

A stable solid electrolyte interphase (SEI) is highly desired to prevent parasitic reactions during normal operation of lithium‐ion batteries (LIBs). Lithium carbonate (Li2CO3) is one of the most significant components for smooth SEI passivation layers; while the formation mechanism and special distribution of the Li2CO3 layer has not yet been illustrated. Here, an over‐potential tailored Li2CO3 growth mechanism based on the typical hard carbon anode is demonstrated. With an increase in the over‐potential, the size of Li2CO3 decreases gradually as the amount increases. When the over‐potential is large (potential at 0.01 V), a Li2CO3‐rich thin and dense inorganic layer with the average thickness of 4.4 nm in the SEI is constructed. The special SEI the completely wraps the boundaries of the anode enables a larger Li‐ion de‐solvation energy barrier and a lower Li‐ion diffusion energy barrier, which supports low self‐discharge behavior and a fast kinetic rate at the anode. More generally, this Li2CO3 growth mechanism is also applicable to commercialized graphite anodes and similar results are also obtained. Therefore, this work provides a new insight into the Li2CO3 growth mechanism in SEIs, as well as a guideline for the design of stable artificial SEIs. [ABSTRACT FROM AUTHOR]

Composite bipolar plates composed of conductivity fillers and resins are widely employed in stationary proton exchange membrane fuel cell (PEMFC) due to their high corrosion resistance in acidic environments. Phenol-formaldehyde resin is a commonly used adhesive in composite bipolar plates; while numerous benzene rings in phenol-formaldehyde resin result in the poor flexibility of composite bipolar plates. Embedding the flexible segment (–CHOH–CH 2 -O-) in the resin network structure of phenol-formaldehyde resin by copolymerization with epoxy resin is an effective method for improving the defects of phenol-formaldehyde resin. In this contribution, to tailor flexible-segment-rich (–CHOH–CH 2 -O-) resin network structure, the co-polymerization mechanism of phenol-formaldehyde resin and epoxy resin is investigated in detailed by in-situ Fourier Transform Infrared (FTIR), based on which an Alter-T strategy is proposed for flexible-segment-rich (–CHOH–CH 2 -O-) resin network structure during composite bipolar plates fabrication process. The obtained composite bipolar plates consisted of graphite and flexible-segment-rich resin network structure delivers improved flexural strength (86.97 MPa), electrical conductivity (203.22 S cm−1) and areal resistance (3.41 mΩ cm2) in comparison to previously reported composite bipolar plates with same components, which are also over the technical standard of the DOE. It is further demonstrated that enriching the flexible segment for improving the poor flexibility of resin network structure is of great importance to enhance the performance of the composite bipolar plates. A two-step alteration of temperature (Alter-T) strategy was proposed to tailor flexible-segment-rich (–CHOH–CH 2 -O-) in the resin network structure during polymerization, for improving the performance of CBP including mechanical properties and electrical conductivity. This approach provides a new insight of improving CBP performance by regulating resin network structure. [Display omitted] • Epoxy is embedded in the resin network structure of Phenol-Formaldehyde. • The resin network structure flexibility is improved by embedding flexible-segment. • The composite bipolar plates exhibit improved comprehensive performance. [ABSTRACT FROM AUTHOR]

Composite bipolar plates (CBP) composed of conductive carbon and flexible but insulated resin are considered promising skeletons in proton exchange membrane fuel cell (PEMFC) with the characteristics of desirable mechanical and electrical conductivity properties, as well as chemical stability. However, the paradox of mechanical and electrical properties caused by insulating resins limits the overall performance of the CBP. Recently, massive studies have been reported to develop high-performance practical CBP; while there is few detailed summarizations yet. This review covers the recent advances in materials and technologies of high-performance CBP comprehensively. First, the core materials supporting the electrical conductivity and mechanical properties are summarized in detail. Furthermore, the interaction between resins and carbon-based materials is analyzed carefully. Subsequently, the fabrication process and novel structure of CBP are reviewed. Finally, the existing challenges are also discussed, and several future research directions are proposed. Overall, this review can provide guidelines for selecting appropriate strategies for improved CBP in PEMFC. • The materials and technologies of composite bipolar plate are reviewed. • The core materials for electrical and mechanical properties are summarized. • The fabrication process and novel structure of CBP are reviewed. • Suggestions for future development of composite bipolar plate are discussed. [ABSTRACT FROM AUTHOR]

Next‐generation Li‐ion batteries (LIBs) with higher energy density adopt some novel anode materials, which generally have the potential to exhibit higher capacity, superior rate performance as well as better cycling durability than conventional graphite anode, while on the other hand always suffer from larger active lithium loss (ALL) in the first several cycles. During the last two decades, various pre‐lithiation strategies are developed to mitigate the initial ALL by presetting the extra Li sources to effectively improve the first Coulombic efficiency and thus achieve higher energy density as well as better cyclability. In this progress report, the origin of the huge initial ALL of the anode and its effect on the performance of full cells are first illustrated in theory. Then, various pre‐lithiation strategies to resolve these issues are summarized, classified, and compared in detail. Moreover, the research progress of pre‐lithiation strategies for the representative electrochemical systems are carefully reviewed. Finally, the current challenges and future perspectives are particularly analyzed and outlooked. This progress report aims to bring up new insights to reassess the significance of pre‐lithiation strategies and offer a guideline for the research directions tailored for different applications based on the proposed pre‐lithiation strategies summaries and comparisons. [ABSTRACT FROM AUTHOR]

Mesoporous hollow TiO2 microboxes are synthesized by a two-step solvothermal method via CaTiO3 intermediate. Composite consisting of TiO2 microboxes and carbon nanotubes (CNT) is applied as a coating layer on conventional polymer separator for lithium-sulfur batteries. The cell using a modified separator exhibits enhanced cycling performance and rate capability. A discharge capacity of 852.3 mAh g−1 is achieved at 0.2 C after 200 cycles and more than 500 mAh g−1 of discharge capacity could be retained after 1000 cycles at 1 C, much better than the cells employing untreated or sole CNT-coated separator. Even at a high C-rate of 10 C, the cell with CNT-TiO2-coated separator could deliver a high specific discharge capacity of 561 mAh g−1. Such enhanced electrochemical performance is ascribed to the synergistic effect of CNT and TiO2 on suppressing shuttle reactions of lithium polysulfides. [ABSTRACT FROM AUTHOR]

Composite bipolar plates (CBPs) are promising candidates in proton exchange membrane fuel cell (PEMFC) for achieving mechanical strength and electrical conductivity as well as chemical stability in acidic environments simultaneously. While widely-explored CBPs composed with Phenol-Formaldehyde (PF) resin as adhesive (donated as PF-CBP) deliver poor flexibility due to the numerous hard π-conjugation (benzene ring) in PF network structure, which is becoming the main bottleneck index and largely hindering the practical applications of CBP. Herein, we propose a co-polymerization method by introducing Epoxy (EP) into PF resin as adhesive for CBP (donated as PF-EP-CBP), by which abundant flexible segments (-CHOH-CH 2 -O-) from EP are demonstrated to be well embedded into the structure of PF resin for enhanced flexibility. The obtained PF-EP-CBP exhibits significantly improved flexural strength of 46.20 MPa, 1.57 times higher than pristine PF-CBP (29.58 MPa), which is also higher than the 2025 DOE targets (40 MPa). Moreover, the detailed areal specific resistance, thermal conductivity, H 2 permeability and corrosion behaviors are conducted systematically to support the single cell of PEMFC. The results provide a novel insight and convenient method on the adhesive tailoring by incorporating flexible linkers into resin network structure, which is of significance to drive the practical development of the CBPs in PEMFC fields. [Display omitted] • Epoxy is embedded in the resin network structure of Phenol-Formaldehyde. • The flexibility of resin network structure could be improved by embedding Epoxy. • The composite bipolar plates exhibit improved flexural strength of 46.20 MPa. [ABSTRACT FROM AUTHOR]

Lithium-sulfur (Li–S) batteries suffer from poor utilization of active material and short cycle life due to the complicated multi-step reaction mechanisms. Herein, three conditional cycling methods, i.e. asymmetrical cycling, constant voltage (CV) discharge cycling, and partial cycling are designed in order to increase the cyclability of Li–S batteries. It is found that the solid deposition process that takes place during the lower plateau of discharge is the major limiting step for achieving high discharge capacity and cycle retention, and the cathode surface coverage can be deferred by applying an optimal discharge/charge rate and CV discharge cycling. The asymmetrical cycling renders a specific capacity of ca. 700 mAh g−1 after 200 cycles, 30% higher than that under symmetrical cycling, while applying a CV discharge cycling enables a full retention of target specific capacity of ca. 800 mAh g−1 over 50 cycles. The partial cycling with a low number of phase transformation steps and reduced surface coverage at the end of discharge/charge also enhances cyclability. This work paves the way for understanding and improving the cycling performance of Li–S batteries without increasing the cost of electrode design or changing the configuration of the cell. [ABSTRACT FROM AUTHOR]

Hybrid lithium-ion capacitors (HyLICs), combining a battery-type negative electrode and a composite positive electrode, can display a more comprehensive performance than conventional lithium-ion capacitors (LICs), e.g. high energy density. However, owing to the coexisting of non-Faradaic and Faradaic energy storage mechanism in the composite positive electrode, the insufficient cycle stability of device becomes an urgent problem. In this paper, we discussed the influence of electrode matching on the cycling stability of HyLICs with a L30/HC@SLMP structure. The three L30/HC mass ratios were 0.8, 1.0 and 1.2, respectively. Three full cells with above ratios exhibit great difference in cycle stability when charging/discharging at a current density of 1 A g−1. Various methods, including galvanostatic charging/discharging, three-electrodes test and scanning electron microscopy, were used to analyze the performance of difference cells. It turns out that the poor rate performance of HC electrode is the main limited factor in HyLICs' cycling stability. Compared with the composite positive electrode, HC electrode is more sensitive to the current densities. Under high current density, solid electrolyte interface layers on the HC electrode surface accumulated fast, which leads to the increase in the impedance of HC electrode. From the Ragone plot, the best matching strategy of L30/HC electrode was achieved when the mass ratio of L30/HC was 1.0. Finally, the optimal HyLICs with L30/HC mass ratio of 1.0 were assembled and performed an excellent cycle stability of more than 80% retention after 62,000 cycles in 40C, as well as better power and energy density compared with LICs. Unlabelled Image • The hybrid lithium ion capacitors consist with LFP-AC electrode and pre-lithiated HC electrode. • The influence of electrode matching on capacity decaying is well-studied. • HC electrode is the main limited factor for capacity decaying. • The optimized device delivers better energy density and durable cycling life (80% retention after 62,000 cycles at 40C). [ABSTRACT FROM AUTHOR]

Abstract Lithium sulfur (Li-S) batteries are promising candidates to replace lithium ion batteries as the next-generation energy storage devices. However, the operation of conventional Li-S batteries at low electrolyte/sulfur ratios is rather challenging with many obstacles to be addressed. In this work we use solid-state Li 2 S 4 as the cathode active material to construct a new type of Li-Li 2 S 4 battery. With the aid of electrochemical impedance spectrum, we find that the use of Li 2 S 4(S) in the cathode alleviates the issue of surface coverage encountered under the lean electrolyte condition. We also find that the discharge capacity is not determined by the solubility of Li 2 S 4 on the lower plateau of discharge. Consequently, the Li-Li 2 S 4 cell with a high sulfur content of 84% and a low E/S ratio of 4.4 ml g−1 delivered a much higher discharge capacity compared to a conventional Li-S cell. In addition, improved cycle retention was achieved when controlling the Li-Li 2 S 4 cell cycling between Li 2 S 4 and Li 2 S. We believe that the reported results open a new direction of cathode development for high energy density Li-S batteries. Highlights • Lithium polysulfide electrochemical reactions are restricted by its solubility. • Li 2 S 4 as cathode is not limited by the electrolyte. • Theoretical specific energy of Li 2 S 4 batteries is given. [ABSTRACT FROM AUTHOR]

Abstract Mesoporous hollow TiO 2 microboxes synthesized by a two-step solvothermal method via CaTiO 3 intermediate were applied as host materials for sulfur cathodes of lithium-sulfur batteries. 3TiO 2 /7S composite containing 70 wt% sulfur exhibited the best electrochemical performance among all composites. A high discharge capacity of 924.8 mAh g−1 was achieved for the 1st cycle and 67.4% of it could be retained after 200 cycles at 0.2 C. Even at a higher C-rate of 1 C, more than 600 mAh g−1 of discharge capacity could be delivered after 500 cycles. The efficient polysulfide adsorption of TiO 2 microboxes demonstrated by visualized adsorption test and UV–vis measurement as well as the mesoporous hollow feature was responsible for the large discharge capacity, excellent capacity retention and decent rate capability. Graphical abstract Image 1 Highlights • Hollow mesoporous TiO 2 microboxes were synthesized. • TiO 2 microboxes exhibited excellent polysulfides adsorption ability. • 3TiO 2 /7S was found to be the best proportion for electrochemical performance. • 626 mAh g−1 of discharge capacity could be delivered after 500 cycles at 1 C. • TiO 2 microboxes were potential candidates for host of sulfur cathode. [ABSTRACT FROM AUTHOR]

Abstract A three-dimensional catalyst layer (CL) with gradient distribution of key components based on double-layered buckypaper (BPCL) as cathode for proton exchange membrane fuel cells (PEMFCs) was fabricated. This novel structured BPCL was prepared by using multi-walled carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) as main framework structure and Pt/C catalyst as active sites. The gradient distribution of platinum (Pt) nanoparticles, proton conductor (Nafion ionomer) and electrode porosity along the thickness of CL were established to reduce Pt loading and facilitate the transportation of protons, reactants and products. Two critical parameters for fabricating membrane electrode assembly (MEA), Nafion content which influences the proton transfer process in CL and hot-pressing temperature which affects the combination situation of CL with electrolyte membrane, were further optimized to promote proton transfer efficiency and decrease the contact resistance between CL and membrane. By using this unique structure, a maximum power density of 536.8 mW cm−2 was achieved at cell temperature of 80 °C, inlet gas pressure of 100 kPa and relative humidity of 80% with an ultra-low Pt loading of 0.09 mg Pt cm−2 in cathode. The generated cathodic Pt utilization is 0.168 g Pt kW−1, reducing by 20.1% compared with the MEA based on traditional catalyst coated on membrane (CCM) process (0.21 g Pt kW−1). Graphical abstract Image 1 Highlights • Gradient distribution of Pt, Nafion and electrode porosity in catalyst layer. • The optimal Nafion content of Buckypaper catalyst layer has been investigated. • Hot-pressing temperature for fabricating MEA has been optimized. • Pt utilization improved by 20.1% compared to that of traditional MEA. [ABSTRACT FROM AUTHOR]

• A homemade microsensor measures CCL dynamic temperature. • An improved transient thermal model is developed to analyze the process. • Both experiment and simulation indicate 0–7 °C temperature variation. • Temperature variation is dramatically affected by cathode operation. • Underlying mechanisms for temperature variation are analyzed. Temperature variation inside the cathode catalyst layer (CCL) is significant, which could greatly damage fuel cell performance and durability. However, limited by CCL microstructure and complex physicochemical processes, the knowledge regarding CCL dynamic temperature is indeed insufficient. Thus, this work integrates a pre-prepared thin-film microsensor into the fuel cell to measure CCL surface temperature under dynamic load. At the same time, an improved transient thermal model considering local mass transport and liquid effect on thermal contact resistance (TCR) is established for the first time to analyze the internal process. Results show that CCL temperature is greatly affected by local resistance and interface accumulated water. And, as confirmed by both experiment and simulation, the CCL surface exhibits 0–7 °C temperature variation, whose profile changes sharply at the beginning and then evolutes slowly, eventually taking tens or hundreds of seconds to reach a new steady state due to thermal inertia. Moreover, the temperature is further increased by ∼1 °C as cathode humidity increases or stoichiometric ratio decreases, but only slight changes with the anode condition. The significant role of cathode operation is attributed to the greater mass transfer resistance and more sensitive intrinsic oxygen reduction reaction to reactant change. This study is expected to provide a deeper understanding of CCL dynamic temperature, thus improving fuel cell thermal management. [ABSTRACT FROM AUTHOR]

Polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) functionalized carbon nanotubes (CNTs) supported Pt electrocatalyst was synthesized as a substitute for commonly used Pt/C and Pt/CNTs (modified by harsh acid-oxidation treatment) catalysts. In addition, this catalyst was fabricated as the cathode catalyst layer (CL) with a unique double-layered structure for proton exchange membrane fuel cells (PEMFCs). Thermogravimetric analysis shows an enhanced thermal stability of Pt/PDDA-CNTs. The Pt/PDDA-CNTs catalyst with an average Pt particle size of ∼3.1 nm exhibits the best electrocatalytic activity and a significantly enhanced electrochemical stability. Scanning electron microscope, energy dispersive spectrometer and mercury intrusion porosimetry results demonstrate the gradient distribution of Pt content and pore size along the thickness of buckypaper catalyst layer (BPCL). The accelerated degradation test results of BPCLs indicate that this gradient structure can ensure a high Pt utilization in the BPCLs (up to 90%) and further improve the catalyst durability. In addition, the membrane electrode assembly (MEA) fabricated with cathode BPCL-PDDA shows the best single cell performance and long-term stability, and a reduction of Pt loading can be achieved. The feasibility of BPCL for improving the Pt utilization is also demonstrated by the cathode cyclic voltammetry in MEA. [ABSTRACT FROM AUTHOR]

Lithium-ion capacitors (LICs) were investigated for high power, moderate energy density applications for operation in extreme environments with prolonged cycle-life performance. The LICs were assembled as three-layered pouch cells in an asymmetric configuration employing Faradaic pre-lithiated hard carbon anodes and non-Faradaic ion adsorption-desorption activated carbon (AC) cathodes. The capacity retention was measured under high stress conditions, while the design factor explored was electrolyte formulation using a set of carbonates and electrolyte additives, with a focus on their stability. The LIC cells were evaluated using critical performance tests under the following high stress conditions: long-term voltage floating-cycling stability at room temperature (2.2–3.8 V), high temperature storage at 3.8 V, and charge voltages up to 4.4 V. The rate performance of different electrolytes and additives was measured after the initial LIC cell formation for a 1C–10C rate. The presence of vinylene carbonate (VC) and tris (trimethylsilyl) phosphate (TMSP) were found to be essential to the improved electrochemical performance of the LIC cells under all testing conditions. [ABSTRACT FROM AUTHOR]

Although the cathode of lithium-sulfur (Li-S) batteries has a theoretical specific capacity of 1,672 mAh g −1 , its practical capacity is much smaller than this value and depends on the electrolyte/sulfur ratio. The operation of Li-S batteries under lean electrolyte conditions can be challenging, especially in the case when the solubility of lithium polysulfide (LiPS) sets an upper bound for polysulfide dissolution. In this work, specially designed cathode structures and electrolyte configurations were built in order to analyze the effects of LiPS solubility on cell capacity. Two reaction pathways involving the reduction of LiPS in liquid and solid phase are proposed and analyzed. We show that at discharge rates above 0.4 mA cm −2 the reaction in the liquid phase dominates the discharge process. Once the electrolyte becomes saturated, the solid phase LiPS cannot be further reduced and does not contribute to the capacity of the cells. This phenomenon prevents Li-S batteries from achieving their high theoretical specific capacity. Finally, the specific energy of the Li-S cell is reevaluated and discussed considering the limitation imposed by LiPS solubility. [ABSTRACT FROM AUTHOR]

Lithium sulfur (Li S) batteries are one of the most promising energy storage devices owing to their high energy and power density. However, the shuttle effect as a key barrier hinders its practical application by resulting in low coulombic efficiency and poor cycling performance. Herein, a novel design of in situ formed polysulfide adsorptive-blocking layer (PAL) on the cathode surface was developed to tame the polysulfide shuttling and promote the cycling stability for Li S batteries. The PAL is consisted of La 2 S 3 , which is capable to chemically adsorb polysulfide via the strong interaction of La S bond and S S bond, and build an effective barrier against sulfur escaping. Moreover, the La 2 S 3 is capable to suppress the crystallization of Li 2 S and promote the ion transfer, which contributes to the reduced internal resistance of batteries. Furthermore, the by-product LiNO 3 simultaneously forms a stable anode solid and electrolyte interface to further inhibit the polysulfide shuttle. By this simple and convenient method, the resultant Li S batteries achieved exceptional cycling stability with an ultralow decay rate of 0.055% since the 10th cycle. [ABSTRACT FROM AUTHOR]

The catalyst layer is a key component in the proton exchange membrane fuel cell (PEMFC) for it is where the conversion of fuel into electricity takes place. Traditionally, electrocatalyst is uniformly distributed in the catalyst layers of the membrane electrode assembly (MEA) and the high Pt consumption in catalyst layers blocks the widely use of PEMFC. Here we proposed a Pt content and pore structure gradient distributed, two-layer catalyst layer for PEMFC to improve the MEA performance. Energy-dispersive X-ray (EDX) spectroscopy results show Pt nanoparticles gradient distributed on the vertical direction of catalyst layer. The pore size in the Pt poor layer is larger than that in the Pt rich layer, and this structure can improve the Pt utilization and enhance the mass transfer in the catalyst layer. The single cell test result shows this new MEA has a better performance (11%) than the traditional MEA. [ABSTRACT FROM AUTHOR]

• A two-dimensional, two-phase, PEMFC transient thermal model is established. • Effects of operating&structure parameters on CCL thermal transient are studied. • CCL temperature overshoot and fluctuation are analyzed under dynamic loading. • Both electrochemical and two-phase flow transients cause temperature overshoot. • Optimal loading rate, operating temperature, and width ratio are suggested. The temperature variation of the cathode catalyst layer (CCL) inside the proton exchange membrane fuel cell (PEMFC) caused by dynamic loading will damage the material and consequently shorten cell lifetime. To address this problem, the thermal transient of CCL needs to be clarified first before further action is taken. In this work, the effects of current load, operating temperature, and channel to rib width ratio on the thermal transient of CCL are systemically investigated by a two-dimensional, two-phase transient PEMFC thermal model. At the same time, the action mechanisms of electrochemical and two-phase flow transients on the thermal transient are detailly described. We observe that the overshoot of CCL temperature will happen as the current steps from nearly zero to a certain level. Interestingly, under the action of local water and thermal evolution, the overshoot amplitude first increases and then decreases with the rise of the step current density. Moreover, with the increase in loading time and operating temperature, temperature overshoot significantly decreases. The former is attributed to the quasi-steady state operation of two-phase flow at a long loading time, while the latter is caused by the reduction of membrane water absorption with atmosphere drying. It is also found that the larger channel to rib width ratio will induce more uniform temperature fluctuation in CCL regions under rib and channel due to the more uniform electrochemical reaction caused by shorter O 2 transport path length. The above model simulation results can guide cell operation and design for better thermal management under dynamic loading. [ABSTRACT FROM AUTHOR]

• Two-dimensional model considering interface and compression effect is developed. • Thermal responses under gas channel and land are significantly different. • Interface resistances greatly affect thermal response and performance. • Phase change can cause temperature overshoot/undershoot as load changes. • An optimal assembly pressure is found with consideration heat transfer. The improper thermal response is one of the main causes of proton exchange membrane fuel cell degradation. In this study, a two-dimensional, non-isothermal, two-phase transient fuel cell model has been established combined with newly measured experimental data. This model fully incorporates the land and gas channel (GC) of the bipolar plate, comprehensively accounting for the electrical contact resistance (ECR) and the thermal contact resistance (TCR) at the carbon paper/land interface, as well as the changes of carbon paper structure and electrical/thermal resistance induced by assembly pressure. By simulation, the impacts of interface resistance, working conditions, and assembly pressure on the thermal responses under land and GC are systematically investigated. The existence of TCR is found to increase the two regions' temperature significantly, while ECR only slightly raises it by generating a surface heat source. Furthermore, uneven distribution, ununiform fluctuation, and rapid overshoot/undershoot of temperature are observed in the two areas during overload and change load operations. It is mainly caused by the different distributions and responses of reaction and phase change at various locations. In the end, the average temperature is found to fall significantly and reach a new steady fastly with the increase of assembly pressure due to the reduction of bulk and interface thermal resistances of carbon paper. However, performance degradation and hot spot temperature rise are further observed if the pressure is too high. To balance performance and heat transfer, we should choose an optimal assembly pressure. [ABSTRACT FROM AUTHOR]

Abstract: Electrochemical capacitor electrodes were fabricated by depositing an ultra-thin layer of vanadium oxide on a high conducting, large specific surface area (SSA) materials (substrates) using a supercritical fluid adsorption–calcination method. The high SSA materials included binder free single walled carbon nanotube–activated carbon (SWCNT–AC) composites and the traditional electrode of activated carbon–carbon black–polymer binder (AC–CB–binder). The uptake of the organometallic precursor for the oxide (vanadium (III) acetylacetonate) on the substrates were investigated and related to their SSA. Precursor uptakes up to 54.7 wt% of the initial carbon substrate was achieved. Calcination conditions for converting the precursor to oxide and electrochemical properties of the electrodes were thoroughly investigated. The V2O5, which showed extremely high specific pseudo-capacitance (>1000 F g−1 at 100 mV s−1), greatly enhanced the overall electrode performances. Conversely, the V2O5 pseudo-capacitance was better utilized in the SWCNT–AC substrate, particularly at high working speeds, because of the significantly higher electrical conductivity. For example, the SWCNT–AC–V2O5 composite (weight ratio 40:60) had its volumetric capacitance doubled, comparing to the ∼50% increase in the AC–CB–binder sample at 100 mV s−1. [Copyright &y& Elsevier]

Abstract: Many studies of the discharge performance of traditional lithium oxygen (Li-O2) cells with binder also include electrochemical impedance spectroscopy (EIS) data and equivalent circuit models (ECM) to provide additional insight on the electrochemical kinetics driving cell operation. However, no EIS and ECM data have been reported for binder-free Li-O2 cells. In this work, EIS and ECM data are analyzed and first reported for a binder-free Li-O2 cell. The EIS results are consistent with the overall discharge capacity results for a cell that is experiencing increased polarization due to cycling, and the fitted equivalent circuit accurately models the same behavior. Surface morphology is also presented for the cell and exhibits product formation on the cathode that is consistent with increased polarization. [Copyright &y& Elsevier]

Vanadium pentoxide (V2O5) deposited on porous multiwalled carbon nanotube (MWCNT) buckypaper using supercritical fluid CO2(scCO2) deposition shows excellent performance for electrochemical capacitors. However, the low weight loading of V2O5 is one of the main problems. In this paper, design of experiments and response surface methods were employed to explore strategies for improving the active material loading by increasing the organo-vanadium precursor adsorption. A second-order response surface model was fitted to the designed experiments to predict the loading of the vanadium precursors onto carbon nanotube buckypaper as a function of time, temperature and pressure of CO2, buckypaper functionalization, precursor type, initial precursor mass and stir speed. Operation conditions were identified by employing a model that led to a precursor loading of 19.33%, an increase of 72.28% over the initial screening design. CNTs-V2O5 composite electrodes fabricated from deposited samples using the optimized conditions demonstrated outstanding electrochemical performance (947.1 F g-1 of V2O5 at a high scan rate 100 mV s-1). The model also predicted operation conditions under which light precursor aggregation took place. The V2O5 from aggregated precursor still possessed considerable specific capacitance (311 F g-1 of V2O5 at a scan rate 100 mV s-1), and the significantly higher V2O5 loading (∼81%) contributed to an increase in overall electrode capacitance. [ABSTRACT FROM AUTHOR]

Abstract: Equivalent circuit model (EMC) of a high-power Li-ion battery that accounts for both temperature and state of charge (SOC) effects known to influence battery performance is presented. Electrochemical impedance measurements of a commercial high power Li-ion battery obtained in the temperature range 20 to 50°C at various SOC values was used to develop a simple EMC which was used in combination with a non-linear least squares fitting procedure that used thirteen parameters for the analysis of the Li-ion cell. The experimental results show that the solution and charge transfer resistances decreased with increase in cell operating temperature and decreasing SOC. On the other hand, the Warburg admittance increased with increasing temperature and decreasing SOC. The developed model correlations that are capable of being used in process control algorithms are presented for the observed impedance behavior with respect to temperature and SOC effects. The predicted model parameters for the impedance elements R s , R ct and Y 013 show low variance of 5% when compared to the experimental data and therefore indicates a good statistical agreement of correlation model to the actual experimental values. [Copyright &y& Elsevier]

Abstract: A polymer gel electrolyte has been prepared based on polyvinyl alcohol, lithium salt and organic solvent N-methyl-2-pyrrolidone (NMP). This gel has the potential for use in secondary batteries and electrochemical capacitors. The conductivity of this gel electrolyte can be as high as 2×10−3–5.8×10−4 S/cm at room temperature. Functioning as separator and electrolyte, this gel system was used to evaluate the specific capacitance of an electrochemical capacitor with carbon nanotube films as the active electrodes. [Copyright &y& Elsevier]

Abstract: RuO2 thin films were grown on (001) LaAlO3 utilizing the pulsed laser deposition technique. Atomic force microscopy was used to check the topography of films at different growth stages. The in situ resistivity measurement was used to monitor the resistance change during and post film growth with changes of resistivity associated with the change of film growth mode. Transmission electron microscopy was used to reveal film quality and crystalline information. The layer-plus-island Stranski–Krastanov growth mode is proposed according to above results. The ambient O2 filled during growth is found to be the main oxygen source for the formed RuO2. [Copyright &y& Elsevier]

Abstract: Nd–Fe–B films were grown on silicon substrate subjected to perpendicular magnetic field using pulsed laser deposition. The effect of substrate temperature (250–650 °C) on the magnetic properties of the film was studied. Magnetic anisotropy was observed in amorphous deposited film. Minor Nd2Fe14B crystalline phase was found when films were deposited at 500 °C and 650 °C. No obvious magnetic crystalline anisotropy shown after the correction of shape anisotropy. [Copyright &y& Elsevier]

The ac and dc performance and leakage current of Evans tantalum oxide-ruthenium oxide hybrid capacitors were characterized at room temperature. The RC time constants were in the range from 2 to 0.45 ms and dependent on the maximum operational voltage of the capacitor. The gravimetric and volumetric energy densities of hybrid capacitors were in the range of 0.074-0.233 J/g and 0.388-1.384 J/cm³, respectively. The gravimetric and volumetric power densities of hybrid capacitors were in the range of 19-259 W/g and 100-1540 W/cm³, respectively. A comparison of the performance between hybrid capacitors to conventional aluminum and tantalum electrolytic capacitors is made in this paper. [ABSTRACT FROM AUTHOR]

Heat is one of the main causes of performance degradation and failure of proton exchange membrane fuel cells (PEMFCs). The 'hot spot' and uneven temperature distribution formed when fuel cells operate can result in the severe degradation of membrane and catalyst layers. To better understand these phenomena, this review presents a systematic and comprehensive summary and discussion on the heat transfer within a PEMFC. Notably, (i) the overall introduction of the basic concept and thermal effect; (ii) the analysis of heat transfer mechanisms, including heat generation and energy transport, as well as the effects of multi-scale; (iii) the summary and discussion of the thermal conductivity and contact resistance of porous media; and (iv) the latest developments in numerical calculation and temperature measurement are crucial. In addition, several outlooks are proposed for further advanced research. We consider that this paper will aid readers in deepening their understanding of this field. • Effects of high temperature and uneven temperature distribution are summarised. • Heat transfer mechanisms are analyzed. • Thermal conductivity and contact resistance of porous media are reviewed. • Thermal models are introduced and compared. • Temperature measurement techniques are summarised. [ABSTRACT FROM AUTHOR]

One dimension, two-phase transient proton exchange membrane fuel cell (PEMFC) thermal models, accounting for most relevant heat sources and energy transport terms, are presented. These thermal models fully consider the thermal contact resistance between gas diffusion backing (GDB) and bipolar plate, and the dependence of thermal conductivity (TC) and specific heat capacity (SHC) on liquid water, enabling the study of static and dynamic heat transfer behaviors under current loads and water conditions. In this paper, we first discussed current loads on temperature distribution, maximum temperature, transient response, and other thermal characters within the fuel cell. A noticeable temperature jump inside the cathode catalyst layer (CCL) is observed as the current step changes, induced by the different transients of heat generation and energy transport. Subsequently, the effects of water conditions are investigated. It is found that the liquid water dramatically affects the distribution and variation of temperature due to its high TC and SHC. Lastly, sensitivity analysis reveals that the TC of dry GDB contributes much to CCL's high temperature, which indicates that improving GDB TC is very important to avoid CCL overheating and explains the lower temperature gradient inside PEMFC under relatively wet conditions. • Thermal models considering most heat transfer mechanisms are developed. • The current load drives the variations of thermal characters within a fuel cell. • Temperature jump inside catalyst layer is observed as current step changes. • Liquid water dramatically affects the distribution and variation of temperature. • Dry gas diffusion backing contributes much to the local high temperature. [ABSTRACT FROM AUTHOR]

A metallically conductive TiB 2 is developed to construct multifunctional separator toward improved lithium-sulfur (Li–S) battery performance. Coupled with two-dimensional graphene, the obtained TiB 2 @G separator fulfills a synergistic combination of strong physical and chemical sulfur immobilization as well as a high-efficiency sulfur reutilization, thus establishing a robust barrier against the polysulfide shuttling and enabling a fast and durable sulfur electrochemistry. As a result, cells based on TiB 2 @G separator exhibit a high sulfur utilization with 1350 mA h g−1 capacity at 0.05C, superb rate capability up to 5.0C, and excellent cyclability with a high capacity retention over 85% after 300 cycles at 0.5C. Moreover, significantly mitigated self-discharge and considerably high areal capacity of 5.8 mA h cm−2 can be also achieved, indicating the great promise of this separator design for the development of high-performance Li–S batteries. • A metallically conductive TiB 2 is developed to modify the separator. • The TiB 2 @G separator fulfills strong sulfur immobilization. • The TiB 2 @G separator enables a high-efficiency sulfur reutilization. • A lithium sulfur battery with improved performance is achieved. [ABSTRACT FROM AUTHOR]

Newly Pt loaded catalysts were synthesized by lyophilization method with Nafion® modified carbon nanotube as support (noted as N-CNT). The newly introduced Nafion® could tightly anchor and disperse Pt nanoparticles and efficiently increase the number of ion channels at the same time. Various material characterization methods were adopted to illustrate the properties of as-synthesized catalysts. X-ray diffraction (XRD) analysis and Transmission electron microscopy (TEM) of catalyst reveal that Pt nanoparticles with the diameter of 2–3 nm are well crystallized and uniformly deposited on N-CNT surface. In cyclic voltammetry (CV) and oxygen reduction reaction (ORR) measurements, Pt loaded N-CNT showed much higher catalytic activity as compared to the counterpart with untreated CNT as support. The optimum performance was achieved when the weight ratio of CNT to Nafion® is 9:1. Moreover, Pt loaded N-CNT showed a slower decomposition rate in electrochemically accelerated durability test (ADT). All these advantages made it a potential electro-catalyst for fuel-cell applications. [ABSTRACT FROM AUTHOR]

Lithium ion capacitor is one of the most promising energy storage technologies with a higher power density and a longer cycling lifespan than Li-ion battery, but also a higher energy density than supercapacitor. However, the unbalanced electrochemical performance between the cathode and anode will inevitably result in unsatisfied power density and reduced cycling lifespan, which largely hinders its commercial applications. In order to eliminate the unbalanced kinetic behavior and cycling stability between the cathode and anode, in this study, we propose a practical and universal electrode matching approach for constructing high performance LIC systems. The effect of effective anode potential range on the performance of a typical LIC system was investigated. An optimal anode potential range is determined and then realized through pre-lithiation to pair with cathode, by which a much improved LIC with high energy density, large power density as well as long cycling lifespan is achieved. Based on the above findings, this approach brings some new insights on ideal anode material candidates for future LIC systems. • A practical and universal matching approach is proposed. • The approach for common lithium ion capacitor system is systemically demonstrated. • The approach brings new insight for anode candidates of lithium ion capacitors. [ABSTRACT FROM AUTHOR]

A low cost multi-target holder (MTH) designed for pulsed laser deposition (PLD) systems fit for vacuum chamber ports of all size was designed and fabricated using only one rotary-linear motion feedthrough. This system can be used with vacuum chamber ports of small size because of the elimination of a second feedthrough. It provides current PLD chambers without enough large ports the ability to grow multi-layer films, and can decrease the required PLD chamber size. Its design of separate low cost MTH head and stud modules can be utilized to further prevent cross contamination from different multi-layer film fabrications. The MTH described was successfully utilized to grow SrRuO3–SrTiO3–SrRuO3 tri-layer films on LaAlO3 substrate. [ABSTRACT FROM AUTHOR]

Li ion battery (LIB) and electrochemical capacitor (EC) are considered as the most widely used energy storage systems (ESSs) because they can produce a high energy density or a high power density, but it is a huge challenge to achieve both the demands of a high energy density as well as a high power density on their own. A new hybrid Li ion capacitor (HyLIC), which combines the advantages of LIB and Li ion capacitor (LIC), is proposed. This device can successfully realize a potential match between LIB and LIC and can avoid the excessive depletion of electrolyte during the charge process. The galvanostatic charge-discharge cycling tests reveal that at low current, the HyLIC exhibits a high energy density, while at high current, it demonstrates a high power density. Ragone plot confirms that this device can make a synergetic balance between energy and power and achieve a highest energy density in the power density range of 80 to 300 W kg−1. The cycle life test proves that HyLIC exhibits a good cycle life and an excellent coulombic efficiency. The present study shows that HyLIC, which is capable of achieving a high energy density, a long cycle life and an excellent power density, has the potential to achieve the winning combination of a high energy and power density. [ABSTRACT FROM AUTHOR]