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Developing high energy storage lithium ion batteries (LIBs) using manganese oxides as anodes is an attractive challenge due to their high theoretical capacity and abundant resources. However, the manganese oxides anodes still suffer from the low initial Coulombic efficiency and poor rate performance. Herein, we demonstrate that nano-sized morphological engineering is a facile and effective strategy to improve the electrochemical performance of the manganese dioxide (MnO 2 ) for LIBs. The tailored MnO 2 nanoparticles (NPs) exhibit high reversible capacity (1095 mAh g −1 at 100 mA g −1 ), high initial Coulombic efficiency (94.5%) and good rate capability (464 mAh g −1 at 2000 mA g −1 ). The enhanced electrochemical performance of MnO 2 NPs can be attributed to the presences of numerous electrochemically active sites and interspaces among the NPs. [ABSTRACT FROM AUTHOR]

Abstract: Water management in PEM fuel cells has received extensive attention due to its key role in fuel cell performance. The unavoidable water, from humidified gas streams and electrochemical reaction, leads to gas–liquid two-phase flow in the flow channels of fuel cells. The presence of two-phase flow increases the complexity in water management in PEM fuel cells, which remains a challenging hurdle in the commercialization of this technology. Unique water emergence from the gas diffusion layer, which is different from conventional gas–liquid two-phase flow where water is introduced from the inlet together with the gas, leads to different gas–liquid flow behaviors, including pressure drop, flow pattern, and liquid holdup along flow field channels. These parameters are critical in flow field design and fuel cell operation and therefore two-phase flow has received increasing attention in recent years. This review emphasizes gas–liquid two-phase flow in minichannels or microchannels related to PEM fuel cell applications. In situ and ex situ experimental setups have been utilized to visualize and quantify two-phase flow phenomena in terms of flow regime maps, flow maldistribution, and pressure drop measurements. Work should continue to make the results more relevant for operating PEM fuel cells. Numerical simulations have progressed greatly, but conditions relevant to the length scales and time scales experienced by an operating fuel cell have not been realized. Several mitigation strategies exist to deal with two-phase flow, but often at the expense of overall cell performance due to parasitic power losses. Thus, experimentation and simulation must continue to progress in order to develop a full understanding of two-phase flow phenomena so that meaningful mitigation strategies can be implemented. [Copyright &y& Elsevier]

Abstract: In the present work, instantaneous gas flow rates in each of two parallel channels of gas–liquid two-phase flow systems were investigated through measurements of the pressure drop across the entrance region. Liquid flow rates in two branches were pre-determined through liquid injection independently into each channel. Experiments were conducted in two different manners, i.e., the gas flow rate was varied in both ascending and descending paths. Flow hysteresis was observed in both gas flow rate distributions and the overall pressure drop of two-phase flow systems. Effects of liquid flow rates on gas flow distributions were examined experimentally. The presence of flow hysteresis was found to be associated with different flow patterns at different combinations of gas and liquid flow rates and flow instability conditions. A new and simple method was developed to predict gas flow distributions based on flow regime-specific pressure drop models for different experimental approaches and flow patterns. In particular, two different two-phase pressure drop models were used for slug flow and annular flow, separately. Good agreement was achieved between theoretical predictions and our experimental data. The developed new method can be potentially applied to predict gas flow distributions in parallel channels for fuel cells. [Copyright &y& Elsevier]

Abstract: In the present study, gas–liquid two-phase flow in a parallel square minichannel system oriented horizontally and at an incline is studied under operating conditions relevant to fuel cell operations. Flow mal-distribution in parallel channels occurs at low gas and liquid flow rates. In general, high superficial gas velocities are required to ensure even flow distribution, and the minimum gas flow rates required to achieve even distribution depend on the liquid flow rates, channel orientation and experimental procedures. As the inclination angle is increased, a higher gas flow rate is required to ensure even gas–liquid flow distribution while flow channels inclined downward seems to help in improving the even flow distribution. The presence of flow hysteresis phenomena indicate that multiple flow distributions exist at the same given flow conditions when the gas flow rates are varied in ascending and descending manners. Flow mal-distribution and flow hysteresis are directly linked with flow stability. More specifically, the actual gas and liquid distribution in parallel channels is determined by the stability of mathematical solutions of mass and momentum balance equations and also the flow history. For the first time, the present work investigates flow distributions in fuel cell flow fields by accounting for two-phase flow conditions. In addition, a novel approach is introduced to ensure flow distributions and their stability through contour construction of isobars where unstable flow region can be identified, which can be used in the design of parallel channel flow fields, especially for fuel cells. [Copyright &y& Elsevier]

Abstract: Two-phase flow in horizontal parallel channels has been experimentally investigated under fuel cell related operating conditions. Pronounced hysteresis is observed in the pressure drop versus flow characteristic curve when starting from either flooded or dry conditions. When gas is introduced into channels initially filled with water (flooded initial condition), both gas and liquid tend to flow predominantly in one channel at low gas or liquid flow velocities. As the gas flow velocity increases, even distribution of gas and liquid flow in both channels is observed, accompanied with a sudden decrease in the pressure drop. On the other hand, even gas and liquid flow distribution between both channels is found at comparatively lower gas flow velocities when starting with dry-gas flow conditions with liquid introduced into channels filled with gas (stratified flow regime). The flow regimes of this system are visualized in plots of the pressure drop against gas and liquid flow velocities. However, this phenomenon tends to vanish at high gas and liquid flow velocities, suggesting that high gas and liquid flow velocities are required to ensure even flow distribution in parallel channels. The hysteresis points appear at the same level of the pressure drop, reflecting intrinsic characteristics of the parallel channels used in this study. These results have important implications for PEM fuel cell operational strategies. In order to avoid reactant mal-distribution in parallel flow channels in the flow field in the two-phase flow regime, fuel cells should be operated at sufficiently high gas flow velocities. [Copyright &y& Elsevier]

Abstract: Mechanically flexible mats consisting of electrospun carbon nanofibers (ECNFs) were prepared by first electrospinning aqueous mixtures containing a natural product of alkali lignin together with polyvinyl alcohol (PVA) into composite nanofiber mats followed by stabilization in air and carbonization in an inert environment. Morphological and structural properties, as well as specific surface area, total pore volume, average pore size, and pore size distribution, of the lignin-based ECNF mats were characterized; and their electrochemical performances (i.e., capacitive behaviors) were evaluated by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The lignin-based ECNF mats exhibited outstanding performance as free-standing and/or binder-free electrodes of supercapacitors. For example, the ECNFs made from the composite nanofibers with mass ratio of lignin/PVA being 70/30 (i.e., ECNFs (70/30)) had the average diameter of ∼100 nm and the Brunauer–Emmett–Teller (BET) specific surface area of ∼583 m2 g−1. The gravimetric capacitance of ECNFs (70/30) electrode in 6 M KOH aqueous electrolyte exhibited 64 F g−1 at current density of 400 mA g−1 and 50 F g−1 at 2000 mA g−1. The ECNFs (70/30) electrode also exhibited excellent cycling durability/stability, and the gravimetric capacitance merely reduced by ∼10% after 6000 cycles of charge/discharge. [Copyright &y& Elsevier]

Abstract: Flexible electrospun titanium carbide (TiC) nano-felts were converted into carbide-derived carbon (CDC) by dry chlorination at temperatures between 200 and 1000°C and used as binder-free supercapacitor electrodes. In the carbide nano-felt, TiC nano-crystals (20–30nm) were embedded in a matrix of disordered carbon. After chlorination, the porous CDC nano-fibers/felts maintain their size, shape, and flexibility. With the increase of synthesis/chlorination temperature, the degree of carbon ordering increased. Electrochemical characterizations in 1M H2SO4 and 1.5M tetraethylammonium tetrafluoroborate in acetonitrile were carried out on binder-free electrodes with galvanostatic cycling, cyclic voltammetry, and electrochemical impedance spectroscopy. The highest gravimetric capacitance was identified for the CDC nano-felt synthesized at the highest temperature of 1000°C, reaching 135Fg−1 in aqueous and 120Fg−1 in organic electrolytes. In contrast to powder or monolithic supercapacitor electrodes made of conventional activated, templated, or carbide-derived carbons, this material demonstrated excellent high-power handling ability; and ∼50% of the low-rate capacitance was maintained at a very high scan rate of 5Vs−1. [Copyright &y& Elsevier]

Abstract The 4-D imaging and quantification of the desaturation of an initially flooded gas diffusion layer (GDL) with a serpentine gas flow channel by synchrotron radiography is presented. An imaging area with a diameter of 10 mm allows for the study of how the natural anisotropy of a GDL affects the desaturation profile. The GDL is progressively spatially segmented from the overall domain down to sections of the individual channels and ribs. Temporal saturation profiles and desaturation rates are presented for each step of this progressive segmentation. Although the desaturation of the overall domain initially appears to be a constant and steady process with a desaturation rate of 0.0030 μL cm−2 s−1, segmented results display heterogeneous behavior with over two-fold differences between areas of the flow field. Segmented saturation surface plots are presented that spatially show how this heterogeneity occurs within the entire domain. Results indicate that initial conditions, anisotropic material properties, and flow field geometry all play a key role in determining local desaturation behavior. The larger domain considered here provides valuable data for future modeling studies that combine continuum studies of flow in the gas flow channels with GDL pore network models that consider two-phase flow behavior. Highlights • Spatial and temporal GDL saturation is analyzed from Synchrotron X-ray CT data. • As the GDL is segmented spatially, variations in the desaturation rate are noted. • Overall desaturation rate of 0.0030 ± 0.0013 μL cm−2 s−1 is spatially investigated. • Spatially and temporally resolved desaturation rates are key to model validation. [ABSTRACT FROM AUTHOR]

Abstract Transition metal oxides can introduce high pseudocapacitance to an electric double layer capacitor for storing more electrical charges. Their role can be more than that since they possess high dielectric constant. Here, we propose a self-sustainable bi-functional configuration by eliminating traditional separator of a metal oxide film supercapacitor that is capable of providing good energy storage performance. We take advantage of super-aligned electrospun carbon nanofibers (SA-ECNFs) as an interconnected scaffold, coupling with electrochemical deposition of α-MnO 2 layers at different currents to introduce pseudocapacitance while providing dielectric layer functioning as a separator to assemble a state-of-the-art supercapacitor. The good electrochemical performance of galvanic charge-discharge specific capacitance at 141 F g−1 and energy density at 12.5 Wh kg−1 offers the promising applications in the energy storage field. Graphical abstract Image 1 Highlights • Manganese dioxide films play bifunctions for separation and pseudocapacitance. • The dielectric property of MnO 2 is critical for the membrane-free configuration. • A mechanistic model is proposed for understanding the bi-functions. [ABSTRACT FROM AUTHOR]

Abstract: Two-phase flow pressure drop hysteresis was studied in a non-operational PEM fuel cell to understand the effect of stoichiometry, GDL characteristics, operating range, and initial conditions (dry vs. flooded) for flow conditions typical of an operating fuel cell. This hysteresis is noted when the air and water flow rates are increased and then decreased along the same path, exhibiting different pressure drops. When starting from dry conditions, the descending pressure drop tended to be higher than the ascending pressure drop at lower simulated current densities. The hysteresis effect was noted for stoichiometries of 1–4 and was eliminated at a stoichiometry of 5. It was found that the hysteresis was greater when water breakthrough occurred at higher simulated current densities, which is a function of GDL properties. The operating range had to reach a critical simulated current density (800mAcm−2 in this case) between the ascending and descending approach to create a pressure drop hysteresis zone. The descending step size does not change the size of the hysteresis effect, but a larger step size leads to lower fluctuations in the pressure drop signal. An initially flooded condition also showed hysteresis, but the ascending approach tended to have a higher pressure drop than the descending approach. [Copyright &y& Elsevier]

Hybrid Li-ion capacitors (LICs) are a new kind of electrochemical energy storage device with high energy and power densities. In this work, phosphorus-doped TiO 2 /phosphorus-doped carbon nanofibers (PTO/PC NFs) are fabricated through a simple electrospinning method with disodium hydrogen phosphate as a phosphorus source. Differing from the PTO prepared by hydrothermal method, sol-gel method or calcining treatment at phosphorous atmosphere. The P-doping evenly distributed in PTO/PC NFs and PTO nanoparticles are embedded in PC, which improves the conductivity, structure stability and Li+ ions diffusion of TiO 2. As a result, the PTO/PC NFs anode shows high capacities of 329 and 150 mA h g−1 (0.1 and 2 A g−1), long cycle life (201 mA h g−1 after 1000 cycles at 1 A g−1, 69 mA h g−1 after 1000 cycles at 5 A g−1), and superior pseudocapacitance (87%, 2 mV s−1). Furthermore, the LICs assembled with PTO/PC NFs anode and AC cathode show outstanding capacity of 35 F g−1 at a current density of 0.1 A g−1, high energy density of 72 Wh kg−1 at the power density of 250 W kg−1, and cycle stability (54%, 10 000 cycles, 1 A g−1). This suggests that PTO/PC NFs is a potential anode material for high-performance LICs. Image 1 • P-doped TiO 2 /P-doped C nanofibers (PTO/PC NFs) are fabricated by electrospinning. • The PTO/PC NFs exhibited good conductivity, stability and Li+ ions diffusion. • The PTO/PC anode showed high capacity, long cycle life and high pseudocapacitance. • LIC assembled with PTO/PC NFs anode showed a high energy density and power density. [ABSTRACT FROM AUTHOR]