Discharge Capacity Decay Research Articles

Design of ZnO coating on diatomite ceramic separator and construction of stable electrolyte/anode interface for high-performance aqueous zinc-ion battery

In aqueous zinc-ion batteries, Zn dendrite growth, hydrogen evolution reaction and by-product generation upon Zn anode seriously affect the battery performance. To address these issues, in this paper, a diatomite ceramic separator with a porous structure is prepared by sintering diatomite. The diatomite ceramic separator possesses excellent electrolyte affinity, high porosity, and remarkable electrolyte retention ability, and the high mechanical strength of the diatomite ceramic separator has a physical inhibition effect on Zn dendrite growth. When assembled into a Zn ‖ NHVO cell, diatomite ceramic separator can effectively inhibit the discharge capacity decay by stabilizing electrolyte/anode interface. To further improve the performance of the diatomite ceramic separator, nanoscale ZnO is coated on the separator surface to construct a more uniform 3D pore structure, which achieves the efficient transport of Zn2+ in the coated separator and improves the ion conductivity of the coated separator (12.0 mS cm-1). ZnO coating expands the electrochemical window of diatomite ceramic separator (2.10 V), homogenizes the deposition of Zn2+ on the surface of Zn anode, reduces the nucleation overpotential (26.5 mV), and further inhibits the growth of Zn dendrites and the production of by-products. The Zn ‖ MnO2 cell assembled with the ZnO coated diatomite ceramic separator exhibits a higher specific discharge capacity of 228.7 mA h g-1 and a higher capacity retention rate at a current density of 0.5 A g-1, and the cell still has a specific discharge capacity of 99.5 mA h g-1 at a current density of 2.0 A g-1, which is superior to the electrochemical performance of the cell with glass fiber separator. The design of ZnO coating on diatomite ceramic matrix opens up a new idea for the practical application of high-performance aqueous zinc-ion batteries.

Evolution of Lithium Metal Anode Along Cycling in Working Lithium–Sulfur Batteries

AbstractThe cycling lifespan of high‐energy‐density lithium–sulfur (Li–S) batteries is dominantly limited by the rapid failure of Li metal anodes. Herein, the evolution of Li metal anode along cycling is systematically investigated in Li–S batteries to clarify the Li plating and stripping behaviors, the generation and accumulation of inactive Li, and the failure mechanism of Li metal anodes. Concretely, plated Li strips preferentially than bulk Li to leave massive stripping Li shells during initial discharge, and the stripping Li shells are refilled by plated Li during the subsequent charge to generate thick Li dendrites. During the following cycles, inactive Li generates and accumulates on top of bulk Li due to solid electrolyte interphase formation and incomplete removal of plated Li. Eventually, the accumulation of inactive Li hinders the diffusion of Li ions through the inactive Li layer to induce cell polarization at the end of the second discharge plateau and rapid decay of discharge capacity. This work elucidates the detailed evolution mechanism of Li metal anode along cycling in working Li–S batteries and highlights the reactivation of inactive Li as an effective strategy to prolong the cycling lifespan of Li–S batteries.

Sodium salt assisted room-temperature synthesis of Prussian blue analogues as high-performance cathodes for sodium-ion batteries

Prussian blue analogues (PBAs) are widely used as cathodes for sodium-ion batteries (SIBs) due to their stable framework structure, simple synthesis method, and low fabrication cost. However, during the synthesis process, a large number of [Fe(CN)6]4− vacancies are easily caused, which leads to the destruction of crystal structure and the rapid decay of discharge capacity. In this work, the effect of sodium salts on lattice defects and electrochemical properties of iron hexacyanoferrates (FeHCF) was studied by adding different sodium salts (Na2SO4, NaCl, C6H5Na3O7, and EDTA-2Na) in the room-temperature coprecipitation method. It was found that the synthesis of FeHCF by adding Na2SO4 based on the chelating agent can not only increase the atomic ratio of Na+ but also effectively reduce [Fe(CN)6]4− vacancy defects and significantly increase the capacity contribution of low-spin Fe redox reaction. Due to the reduction of [Fe(CN)6]4− vacancy defects, the crystal structure is not easy to be destroyed during the cycle process, and the cycle stability of FeHCF is significantly improved. The sample shows good electrochemical performance, with a discharge capacity of 130.8 mAh g−1 in the first cycle and 114.3 mAh g−1 after 100 cycles. This work provides a new way to synthesize PBAs with low [Fe(CN)6]4− vacancy defects.

Self-supporting sulfonated covalent organic framework as a highly selective continuous membrane for vanadium flow battery

The application of sulfonated covalent organic frameworks (SCOFs) in vanadium redox flow battery (VRFB) is limited by the nano-scale non-sieving pores and low additive amount in mixed matrix membranes. Herein, a self-supporting continuous SCOF membrane interlaced with Nafion chains (SCOF/Nf) is proposed to possess high H+/Vn+ selectivity via the wedge-tenon like reinforce structure. The continuous self-supporting SCOF layer provides crystalline ordered dense sulfonic acid groups for proton hopping and contributes to the considerably high proton conductivity (143.9 mS cm−1, 25 °C) at a very low swelling ratio (3.1 %). The flexible Nafion polymer chains can be inserted into the pores of SCOF by hydrogen bonds and ionic interactions, reducing the pore size from 15.0 Å to 5–10 Å for efficiently sieving H+/Vn+ ions. With a SCOF weight proportion of 0.6, the SCOF/Nf-0.6 membrane achieves to a high energy efficiency and extremely low discharge capacity decay rate (85.5 % and 0.13 % per cycle at 100 mA cm−2, respectively), which are much superior to the most state-of-the-art Nafion and COFs based ion conductive membranes. With the interlaced Nafion layer, the SCOF/Nf-0.6 membrane keeps stable and intact in the 500-cycle test (>900 h) in VRFB. The application of SCOF in VRFB from mixed matrix membranes to self-supporting continuous membrane provides an efficient way to maximize the function of the crystalline ordered two-dimensional materials.

Ammonium Bifluoride‐Etched MXene Modified Electrode for the All−Vanadium Redox Flow Battery

AbstractThe development of electrodes with high performance and long‐term stability is crucial for commercial application of vanadium redox flow batteries (VRFBs). This study compared the performance of VRFB with thermal‐treated and MXene‐modified carbon paper. To prepare the MXene, a modified‐etching process with ammonium−bifluoride (NH4HF2) led to a mild and efficient conversion of the MAX‐phase to MXene compared to etching process with hydrofluoric‐acid (HF). Electron microscopy and X‐ray diffraction studies revealed that the etching process with NH4HF2 led to MXene nanostructures with a large interlayer spacing. The results show that at a current density of 60 mA cm−2, the energy efficiency increased by 25.5 % when using a NH4HF2 ‐etched MXene‐modified negative electrode, by 12.5 % with a thermal‐treated MXene‐modified electrode, and by 4 % with an HF‐etched MXene‐modified electrode, in comparison to the pristine electrode. The maximum power density of the battery was increased by more than 40 %. In long‐term cycling experiments the MXene modified electrode exhibited excellent stability over 1000 cycles of charge‐discharge, with 0.05 % discharge capacity decay per cycle, amongst the lowest values reported to date and four times lower than for thermally‐treated electrode. The superior performance was linked to the improved electrical conductivity and wettability, higher interlayer spacing, and lower charge transfer resistance for the V2+/V3+ redox reaction.

A Comparative Study of Nafion 212 and Sulfonated Poly(Ether Ether Ketone) Membranes with Different Degrees of Sulfonation on the Performance of Iron-Chromium Redox Flow Battery.

For an iron-chromium redox flow battery (ICRFB), sulfonated poly(ether ether ketone) (SPEEK) membranes with five various degrees of sulfonation (DSs) are studied. To select the SPEEK membrane with the ideal DS for ICRFB applications, the physicochemical characteristics and single-cell performance are taken into consideration. Following all the investigations, it has been determined that the SPEEK membrane, which has a DS of 57% and a thin thickness of 25 μm, is the best option for replacing commercial Nafion 212 in ICRFB. Firstly, it exhibits a better cell performance according to energy efficiency (EE) and coulombic efficiency (CE) at the current density range between 40 mA cm-2 and 80 mA cm-2. Additionally, it has a more stable EE (79.25-81.64%) and lower discharge capacity decay rate (50%) than the Nafion 212 (EE: 76.74-81.45%, discharge capacity decay: 76%) after 50 charge-discharge cycles, which proves its better oxidation stability as well. In addition, the longer self-discharge time during the open-circuit voltage test further demonstrates that this SPEEK membrane could be employed for large-scale ICRFB applications.

Enhanced catalytic activity of Co-CoO via VC0.75 heterostructure enables fast redox kinetics of polysulfides in Lithium-Sulfur batteries

The shuttle effect and sluggish redox kinetics of lithium polysulfides lead to a short lifespan and poor rate capability of lithium-sulfur (Li-S) batteries. Introducing electrocatalysts into sulfur host materials has been deemed a promising strategy to overcome these issues in Li-S batteries. Herein, an efficient catalytic sulfur host material (i.e., VC0.75/Co-CoO) has been fabricated by simply pyrolyzing the electrospun vanadium and cobalt salt-based polymer fibers. Electrochemical measurements demonstrated that the as-formed heterostructure of Co-CoO with VC0.75 evidently strengthens the redox kinetics of polysulfides due to the enhanced adsorption ability to polysulfides and adjusts the electronic structure of heterostructure via the introduction of VC0.75. Further theoretical computation discloses that Co-CoO mainly plays the role in stretching the SS bond in polysulfides to facilitate the reduction of polysulfide, extending the Li-S bond in Li2S and reducing energy barriers of Li2S decomposition to promote the oxidation process of Li2S. Accordingly, the collaboration of Co-CoO and VC0.75 synergetically facilitates the redox kinetics of polysulfides, prolonging the lifespan of Li-S batteries. Impressively, VC0.75/Co-CoO composites endow Li-S batteries with a discharge capacity decay rate as low as 0.02 % per cycle at the current density of 0.2 C. The specific capacity reaches 626 mAh/g at the high current density of 1.0 C even after 450 cycles. This work sheds light on a facile methodology for improving the catalytic activity of sulfur host materials in Li-S batteries.

Hydrogen evolution mitigation in iron-chromium redox flow batteries via electrochemical purification of the electrolyte

The redox flow battery (RFB) is a promising electrochemical energy storage solution that has seen limited deployment due, in part, to the high capital costs of current offerings. While the search for lower-cost chemistries has led to exciting expansions in available material sets, recent advances in RFB science and engineering may revivify older chemistries with suitable property profiles. One such system is the iron-chromium (Fe–Cr) RFB, which utilizes a low-cost, high-abundance chemistry, but whose efficient and long-term operation is challenged by the poor Cr redox reaction kinetics and high hydrogen evolution reaction (HER) rates . Of late, renewed efforts have focused on HER mitigation through materials innovation including electrocatalysts and electrolyte additives. Here, we show electrochemical purification, where soluble contaminants are deposited onto a sacrificial electrode prior to cell operation, can lead to a ca. 5× reduction in capacity fade rates. Leveraging data harvested from prior literature, we identify an association between coulombic efficiency and discharge capacity decay rate, finding that electrochemical purification can enable cell performance equivalent to that with new and potentially-expensive materials. We anticipate this method of mitigating HER may reduce capacity maintenance needs and, in combination with other advances, further long-duration Fe–Cr RFBs.

Inducing favorable dual-substitution reactivity by doping Mo6+ and F− to enhance electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode materials

Lithium-ion batteries are a hot spot of modern energy research due to the advantage of capacity and cost. As a new type of green energy, lithium-ion batteries have a far-reaching influence on the sustainable development of the environment, and the wide application of it can effectively reduce the exploitation of fossil fuels. However, the high specific capacity is also accompanied by some irreversible problems, such as poor cyclic performance and the rapid decay of discharge capacity with high current density. In view of these problems, we employ a dual-modification strategy to co-dope LiNi0.6Co0.2Mn0.2O2 by Mo6+ and F−. The addition of Mo6+ widens the interval layers and F− was utilized to lessen the release of lattice oxygen with a purpose of enhancing cyclical stability. After 50 cycles, the dual-substitution of LiNi0.6Co0.2Mn0.2O2 displayed distinguished capacity retention of 89.6% with 137.7 mAh g−1 at 0.5C. It's a feasible strategy to enhance the electrochemical performance of cathode materials.

Construction of hierarchical proton sieving-conductive channels in sulfated UIO-66 grafted polybenzimidazole ion conductive membrane for vanadium redox flow battery

Sieving of H+/Vn + ions is essential to vanadium redox flow battery (VRFB). Herein, the interconnected hierarchical proton sieving-conductive channels are constructed through the graft of sulfated UIO-66 (UIO–66OSO3) along the polybenimidazole (PBI) backbone with the highly reactive and acidic sulfate ester functional group. Highly porous UIO-66OSO3 provides numerous angstrom scale sieving channels (about 5 Å) to transport proton and repel vanadium ion, and the sulfated groups ended flexible side chains are densely bonded to UIO-66 and aggregate into broad nanoscale ionic clusters (∼8 nm) for fast proton conduction between UIO-66OSO3 nanoparticles. The as-prepared membrane exhibits low vanadium permeability (7.88 × 10−9 cm2s−1) and area resistance (0.23 Ω cm−2, lower than that of Nafion 212). High energy efficiency (86.1%) and low discharge capacity decay (0.15% per cycle) are achieved even after 100 charge-discharge cycles at 100 mA cm−2, and the battery keeps running for 2000 cycles without morphological and chemical changes. The VRFB proprieties are far superior to Nafion 212 membrane VRFB proprieties (energy efficiency 74.9%, decay rate 0.71% per cycle), and also surpasses most recently reported porous, dense and hybrid ion conductive membranes.

Facile Synthesis of Heterostructured MoS2–MoO3 Nanosheets with Active Electrocatalytic Sites for High-Performance Lithium–Sulfur Batteries

In order to overcome the shuttling effect of soluble polysulfides in lithium-sulfur (Li-S) batteries, we have designed and synthesized a creative MoS2-MoO3/carbon shell (MoS2-MoO3/CS) composite by a H2O2-enabled oxidizing process under mild conditions, which is further used for separator modification. The MoS2-MoO3 heterostructures can conform to the CS morphology, forming two-dimensional nanosheets, and thus shorten the transport path of lithium ion and electrons. Based on our theoretical calculations and experiments, the heterostructures show strong surface affinity toward polysulfides and good catalytic activity to accelerate polysulfide conversion. Benefiting from the above merits, the Li-S battery with a MoS2-MoO3/CS modified separator exhibits good electrochemical performance: it delivers a high discharge capacity of 1531 mAh g-1 at 0.2 C; the initial capacity can be maintained by 92% after 600 cycles at 1 C, and the discharge capacity decay rate is only 0.0135% per cycle. Moreover, the MoS2-MoO3/CS battery still achieves good cycling stability with 78% capacity retention after 100 cycles at 0.2 C with a high sulfur loading of 5.9 mg cm-2. This work offers a facile design to construct the MoS2-MoO3 heterostructures for high-performance Li-S batteries, and may also improve one's understanding on the heterostructure contribution during polysulfide adsorption and conversion.

CuFeS2 anchored in ethylenediamine-modified reduced graphene oxide as an anode material for sodium ion batteries

Tetragonal CuFeS2 anchored in ethylenediamine-modified reduced graphene oxide (CuFeS2/EN-rGO) was synthesized via a simple solvothermal method, and evaluated for the first time as an anode material for sodium ion batteries via constant current charge/discharge, rate charge/discharge, cyclic voltammetry, electrochemical impedance spectroscopy. Ex-situ XRD revealed sodium storage mechanism of CuFeS2, indicating that its capacity is mainly provided by the conversion reaction. Compared with CuFeS2, reversible capacity and cycle performance of CuFeS2/EN-rGO was significantly improved by high electrical conductivity of EN-rGO. At 1.0 A g−1, the discharge capacity of CuFeS2/EN-rGO maintained 247.8 mAh g−1 at 250th cycle, corresponding to 0.14% discharge capacity decay rate per cycle compared with its second discharge capacity. Even at 3.2 A g−1, it can reached a discharge capacity of 296.2 mAh g−1. The results show that CuFeS2/EN-rGO has certain development prospects as an anode material for sodium ion batteries.

Superior acidic sulfate ester group based high conductive membrane for vanadium redox flow battery

Proton conductive group is essential to provide ion transport site and tune micro-phase separation in proton exchange membranes for vanadium redox flow battery (VRFB), while nearly all the fabrication methods are based on the benchmark sulfonic acid group. Herein, the sulfate ester group (-OSO3H) with superior acidity is proposed as a novel proton conductive group to achieve high proton transport at lower IECs, therefore disentangles the trade-off between vanadium crossover and conductive ion diffusion. The electron withdrawing ester oxygen atom endows sulfate ester group with 27 kJ mol−1 lower proton binding energy than the sulfonic acid group, leading to higher proton dissociation ability and better micro-phase separation. The 1,3-propanediol cyclosulfate grafted polybenzimidazole with IEC of 1.85 (OPBI-OSO3H-1.85) reaches comparable area resistance (0.19 Ω cm−2) to the sulfonic acid grafted polybenzimidazole with much higher IEC of 2.95 (OPBI-SO3H-2.95), but 19.1% decrease in vanadium permeability. The cell assembled with OPBI-OSO3H-1.85 achieves high energy efficiency of about 86.7% and low decay of discharge capacity (0.41% per cycle), superior to that of OPBI-SO3H-2.95 (0.51% per cycle) and Nafion 212 (0.71% per cycle). The sulfate ester group is proved to be chemically stable in H2SO4/vanadium environment.

Advanced Nafion hybrid membranes with fast proton transport channels toward high-performance vanadium redox flow battery

The development of high-performance ion exchange membranes to break the trade-off between ion selectivity and conductivity is always a great challenge for the vanadium redox flow battery (VRFB). Herein, a hybrid membrane was successfully prepared via the solution casting method. The UiO-66-SO3H was embedded in Nafion matrix, acting as an effective barrier for vanadium ions to realize enhanced ion selectivity. At the same time, besides the intrinsic proton cluster channels in Nafion, two types of additional proton transport channels formed in the membranes, which further improved the conductivity of the membranes. Consequently, the VRFB with an optimized hybrid membrane exhibited enhanced voltage efficiency (VEs: 91.5-78.4%) and outstanding energy efficiency (EEs: 86.0-76.1%) compared with the one with recasting Nafion membrane (rNf) (VEs: 86.9-69.0%, EEs: 80.3-66.0%) at 80–220 mA cm-2. The efficiency kept stable for 1000 cycles in the long-term cycling tests, and the discharge capacity decay rate of the VRFB with the hybrid membrane is only 0.13% per cycle, which is in obvious contrast to the one with rNf (0.73% per cycle). In addition, the electrolyte utilization rate of the VRFB assembled with the hybrid membrane was much higher than that of the one with rNf.

Designing of root-soil-like polyethylene oxide-based composite electrolyte for dendrite-free and long-cycling all-solid-state lithium metal batteries

Lithium metal with excellent theoretical specific capacity and low reduction potential is thought as the promising anode material for energy storage device. However, the issues of weak security and poor cycle life resulted from the lithium dendrites growth have greatly limited their quick development. In this study, the electrospun polyvinylidene fluoride (PVDF) nanofiber membrane with multi-level structure was introduced into the polyethylene oxide (PEO) polymer as a nano-polymer filler to construct an all-solid-state root-soil-like composite electrolyte. The mutual overlaps of the coarse fiber and the fine fiber in the membrane provide a strong skeleton support for the electrolyte, and the intermolecular hydrogen bond between the PVDF and PEO can further enhance the interfacial interaction between the membrane and polymer, which in turn make the root-soil-like composite electrolytes have excellent mechanical strength to inhibiting the growth of lithium dendrites. Moreover, the existence of the multi-level structure in the membrane can significantly reduce the crystallinity of the polymer and provide more transmission channels for Li+ ions, so that Li+ ions can be uniformly and rapidly deposited during the plating/stripping process, and the interface compatibility between the lithium anode and electrolyte can be effectively enhanced. The voltage value of the Li symmetric battery can be stabilized at 70 mV for 1000 h under 0.3 mA cm−2. And the discharge capacity decay rate of the Li|LiFePO4 battery after 600 cycles at 1 C is only 0.04% per cycle. The study will provide a promising electrolyte candidate for high energy all-solid-state lithium metal batteries.

An optimal electrolyte addition strategy for improving performance of a vanadium redox flow battery

In this paper, the influences of multistep electrolyte addition strategy on discharge capacity decay of an all vanadium redox flow battery during long cycles were investigated by utilizing a 2-D, transient mathematical model involving diffusion, convection, and migration mechanisms across the membrane as well as the contact resistance in the battery. Results show that with various multistep electrolyte addition strategies, the discharge capacity decay of the battery can be diminished. An optimal multistep electrolyte addition strategy is presented, which is corresponding to adding 1.04 mol L−1 V3+ electrolyte to a negative tank while adding 1.04 mol L−1 VO2+ electrolyte to a positive tank. Results show that capacity decay of the battery can be debased by 10.8%, which is due to increased vanadium ions in the negative side and the decreased state-of-charge (SOC) imbalance between two half-cells. This study will propose a practical method for mitigating the discharge capacity decay of the battery during operation.

Mitigating capacity decay and improving charge-discharge performance of a vanadium redox flow battery with asymmetric operating conditions

A two-dimensional transient model with considering vanadium ion crossover was presented to examine the influence of asymmetric electrolyte concentrations and operation pressures strategies on the characteristics of capacity decay, vanadium ions crossover and charge-discharge performance of a vanadium redox flow battery during battery cycling. It was indicated that for asymmetric electrolyte operating concentrations, with increasing the initial concentration of positive electrolyte while keeping initial concentration of negative electrolyte unchanged, the discharge capacity decay behavior during battery cycling can be effectively mitigated due to the reason that the imbalance of vanadium ions crossover is alleviated by the increased diffusion flux of VO2+/VO2+ couple from positive to negative side. Also, the overall charge-discharge performance of the battery is greatly improved due to the reduced potential losses of both electrode reactions. Moreover, it was shown that the discharge capacity decay can also be suppressed by increasing the outlet pressure of positive electrode, which is attributed to the reason that the imbalance of vanadium ions crossover can be eliminated with the increased vanadium osmotic convection driven by pressure gradient. However, asymmetric operation pressures showed little impact on batter charge-discharge voltages.

Directly scalable preparation of sandwiched MoS2/graphene nanocomposites via ball-milling with excellent electrochemical energy storage performance

The few-layered transitional sulphide/oxide nanosheets and graphene composites attract increasingly interest as electrode materials although facing the significant preparation challenge such as low quantities and uncontrollable homogeneity. Using MoS2 and graphene oxide (GO) bulk materials as the raw materials, we demonstrate a novel solvent-free and one-step ball-milling approach to prepare massive sandwiched MoS2/reduced graphene oxide (rGO) nanocomposites. The reduction of GO to rGO, the exfoliation of bulk layered materials and assembly of the obtained MoS2 and rGO nanosheets are achieved all together in the ball-milling process. The orderly assembled MoS2/rGO composite as electrode material used in supercapacitor delivers a high specific capacitance of 306 F g−1 at a current density of 0.5 A g−1 and approximately two times higher than that of the pure MoS2. Besides, it exhibits nearly no discharge capacity decay used both in lithium-ion batteries (0.2 A g−1 after 100 cycles) and supercapacitor (4 A g−1 after 5000 cycles). This work has developed an encouraging approach for preparation of this series of graphene-based hybrid electrode materials with ideal performance for potential application in energy storage devices.

Delineating Contributions from Vanadium Crossover and Electrode Degradation to Capacity Decay in Vanadium Redox Flow Batteries

All-vanadium redox flow batteries (VRFBs) are promising candidates for large-scale energy storage thanks to their scalability, relatively high-performance and safety 1. However, wide-spread integration of VRFBs requires mitigation of rapid capacity decay over long-term cycling. There are three major contributors to discharge capacity decay during cycling in VRFBs: 1) undesired transport of vanadium ions and water through the ion-exchange membrane 2) degradation of cell components (predominantly electrodes, and membrane) and 3) gas-generation from side reactions. With careful selection of upper and lower voltage limits for cycling, gas-generation can be mitigated to a great degree. However, capacity decay due to undesired vanadium ion and water crossover along with component degradation is inevitable. It is important to note that the capacity decay over long-term cycling is cumulative and therefore, capacity decay due to vanadium crossover and degradation are combined in cycling experiments making it challenging to distinguish the contribution from these two sources independently. In this work, we will present our latest experimental data focused on delineating contributions from vanadium crossover and electrode degradation. The data include two ion-exchange membranes and two electrodes with different degradation behavior. To this end, we have designed and built a unique facility utilizing multiple electrochemical and flow cells equipped with UV/Vis spectroscopy to measure the vanadium ion crossover in real-time 2-6. To assess the capacity decay due to combined degradation effect of all components, we have developed another diagnostic in which we use a symmetric cell set-up for negative (V(II)/V(III)) and positive (V(IV)/V(V)) sides separately. The degradation diagnostics utilize electrochemical impedance spectroscopy to quantify the contributions of cell overpotential during cycling stemming from charge-transfer, ohmic and mass transport. The results of this study should provide more in-depth insight to optimize VRFBs with enhanced performance and reduced ion-crossover and degradation.

Influence of Membrane Equivalent Weight and Reinforcement on Ionic Species Crossover in All-Vanadium Redox Flow Batteries.

One of the major sources of lost capacity in all-vanadium redox flow batteries (VRFBs) is the undesired transport (usually called crossover) of water and vanadium ions through the ion-exchange membrane. In this work, an experimental assessment of the impact of ion-exchange membrane properties on vanadium ion crossover and capacity decay of VRFBs has been performed. Two types of cationic membranes (non-reinforced and reinforced) with three equivalent weights of 800, 950 and 1100 g·mol−1 were investigated via a series of in situ performance and capacity decay tests along with ex situ vanadium crossover measurement and membrane characterization. For non-reinforced membranes, increasing the equivalent weight (EW) from 950 to 1100 g·mol−1 decreases the V(IV) permeability by ~30%, but increases the area-specific resistance (ASR) by ~16%. This increase in ASR and decrease in V(IV) permeability was accompanied by increased through-plane membrane swelling. Comparing the non-reinforced with reinforced membranes, membrane reinforcement increases ASR, but V(IV) permeability decreases. It was also shown that there exists a monotonic correlation between the discharge capacity decay over long-term cycling and V(IV) permeability values. Thus, V(IV) permeability is considered a representative diagnostic for assessing the overall performance of a particular ion-exchange membrane with respect to capacity fade in a VRFB.