Vanadium Redox Flow Battery Application Research Articles

Polypyrrole thin film composite membrane prepared via interfacial polymerization with high selectivity for vanadium redox flow battery

In order to improve the selectivity of porous membrane for vanadium redox flow battery (VRB). A polypyrrole (PPy) thin film composite (TFC) membrane was prepared via interfacial polymerization (IP) method and investigated for VRB application. The optimized PPy TFC membrane has shown both low VO2+ permeability and low area resistance compared to that of commercial Nafion 115 membrane. The VRB cell assembled with PPy TFC membrane shows a coulombic efficiency (CE) of 98.1% and energy efficiency of 81.0% at 80 mA cm−2, which is 2.0% and 6.3% higher than that of Nafion 115 membrane, respectively. Furthermore, 93 cycles charge-discharge test proves that the PPy TFC membrane is very stable and the CE decrease rate is only 0.4% for per-cycle on average. Considering the high selectivity, facile preparation procedure and high stability of PPy TFC membrane, it is thus an ideal system for VRB application to overcome the trade-off between vanadium ions permeability and proton conductivity.

The effect of counter-ion substitution on poly(phthalazinone ether ketone) amphoteric ion exchange membranes for vanadium redox flow battery

The study proposed a novel method to prepare poly(phthalazinone ether ketone) amphoteric ion exchange membranes (Q/S-M) with both increased efficiency and chemical stability for vanadium redox flow battery (VRB) applications. Q/S-M membranes were obtained after the successive amination and acidification process of blend base membranes prepared from brominated poly(phthalazinone ether ketone) (BPPEK) and sulfonated poly(phthalazinone ether ketone) substituted by counter-ions (SPPEK-M), where M was defined as counter-ions: Li+, Na+ and K+. The study analyzed the effect of counter-ion size on properties of Q/S-M membranes, which were compared with those for Q/S membrane obtained from BPPEK and SPPEK in acid form. Q/S and Q/S-M membranes maintained the same composition of ionic groups. The Q/S-M membranes induced by bigger counter-ion size showed the increasing of water content and ion diffusion rate, which was higher than that of Q/S membrane. Related to Q/S membrane, Q/S–K membrane possessed almost half area resistance (0.67 Ω cm2 vs 1.43 Ω cm2), and the value was close to that of Nafion115. Q/S-M membranes exhibited a 96.0–98.8% decrease in VO2+ permeability over Nafion115. Q/S–K membrane displayed a 6.5% increase in VE and correspondingly a 6.2% increase in EE over Q/S membrane, maintaining superior EE than Nafion115 (89.7% vs 86.5%). The ex-situ degradation test in VO2+ solutions indicated that despite close EE, Q/S–K membrane showed superior chemical stability over amphoteric ion exchange membranes (AIEMs) in our previous works. There was no obvious efficiency decay of Q/S–K membrane during 100-cycles test, indicating the good duration in operating VRB.

Nitrogen and sulfur co-doped graphene composite electrode with high electrocatalytic activity for vanadium redox flow battery application

Nitrogen and sulfur co-doped graphene has been decorated on graphite felt electrode via a hydrothermal treatment using thiourea as sulfur and nitrogen sources for vanadium redox flow battery application. Our results demonstrate that the modification of sulfur and nitrogen co-doped graphene on graphite felt provides an effective way to improve surface area and introduce functional groups for vanadium redox reaction, resulting in a significant improvement in energy efficiency (85.37% at a current density of 80 mA/cm2), power density (0.4925 W/cm2), discharge capacity and capacity retention. These results reveal that the modification of nitrogen and sulfur co-doped graphene on graphite felts is a promising method for the enhancement of vanadium redox flow battery performance.

Novel sulfonated polyimide membrane blended with flexible poly[bis(4-methylphenoxy) phosphazene] chains for all vanadium redox flow battery

To improve SPI stability, poly[bis(4-methylphenoxy) phosphazene] (PMPP) was designed and synthesized using polyphosphazene's unique and strong flexibility. A novel sulfonated polyimide (SPI) composite membrane embedded with flexible PMPP chains was prepared for all vanadium redox flow battery applications. SPI/PMPP composite membrane characterizations, such as proton conductivity, vanadium ion permeability, stability, and single cell performance, were thoroughly investigated. PMPP's unique properties not only allow the membrane to exhibit excellent anti-hydrolysis and anti-oxidation stability, but also maintain high proton conductivity and low vanadium ion permeability (as low as 4.81 × 10−7 cm2 min−1). It can also form a rigidity-flexibility network with SPI to improve membrane mechanical properties. It can be deduced that introducing PMPP has achieved many things in one stroke. Results show that the hydrolysis and oxidation stabilities of SPI/PMPP membrane were significantly improved. The SPI/2% PMPP membrane exhibits higher columbic efficiency (CE: 97.52%), voltage efficiency (VE: 84.88%), and energy efficiency (EE: 82.21%) than that of the N212 membrane (CE: 94.97%, VE: 82.04%, EE: 76.99%) at a high current density of 100 mA cm−2. In addition, the charging capacity loss of a battery with a SPI/2% PMPP membrane has also significantly decreased by about 206.70 mAh after 110 cycles at 100 mA cm−2 with the same electrolyte volume compared to a pristine SPI. Meanwhile, the high temperature resistance of SPI/PMPP membrane also shows the possibility of being applied with fuel cells.

Chemically stable anion exchange membranes based on C2-Protected imidazolium cations for vanadium flow battery

Imidazolium-based anion exchange membranes (AEMs) are attractive as the separator for vanadium redox flow battery (VFB) application. However, the lack of fundamental understanding of the correlations between imidazolium chemical structure and their physical properties as well as cell performance constraints the design of advanced AEMs in VFBs. In this work, by designing the “clickable” imidazolium compounds from C2-methyl or C2-phenyl substituted imidazolium to C2-phenyl substituted benzimidazolium, a series of PSf-based AEMs having pendant C2-protected imidazolium derivatives were prepared by efficient CuAAC reaction to explore how the nature of C2 substitution affected the electrochemical property of AEMs and the resulting VFB performance. Interestingly, PSf-MIm with C2-methyl protected imidazolium group showed lowest area resistance (0.3 Ω cm2, IEC = 1.66 meq./g) in 3 M H2SO4 aqueous solution but unfavorable vanadium permeability due to its highest swelling ratio. The introduction of bulky C2-phenyl substituted benzimidazolium led to the distinct microphase separation in PSf-PhBIm membrane, and thus reduced water uptake, high vanadium selectivity, and comparable conductivity were observed. As a result, the single VFB with PSf-MIm-1.2 membrane exhibited better electrochemical performance with a coulombic efficiency (CE) of 97.0%, and an energy efficiency (EE) of 82.4% at a current density of 120 mA/cm2, higher than those of Nafion N115 membrane (EE = 75.5%) and unsubstituted imidazolium-based AEMs (EE = 80.6%). More importantly, both ex situ stability testing in 1.5 M (VO2)2SO4/3 M H2SO4 solution for 90 days and in situ cycling performance at a current density of 120 mA/cm2 demonstrated that the chemical structure of C2-substituted imidazolium based AEMs remained intact after stability tests as confirmed by NMR analysis, while significant degradation was found for unsubstituted PSf-Im membrane via possible nucleophilic addition mechanism. Therefore, the VFB with PSf-MIm membrane showed the best long-term durability with only 34.1% loss in EE for 3638 h of operating (4800 cycles). This work not only fills the knowledge gap on the structure-property relationship of imidazolium-based AEMs for VFB application, but also gives us new directions to design stable AEMs for durable VFBs.

Fluorinated poly(fluorenyl ether)s with linear multi-cationic side chains for vanadium redox flow batteries

The cationic group distribution along the polymeric backbones of anion exchange membranes (AEMs) has significant influence on their microscopic morphology and anion conductivity. To develop high-performance AEMs for vanadium redox flow batteries (VRFBs), a series of poly (fluorenyl ether) samples bearing di- and tri-quaternary ammonium side chains with similar ion exchange capacities (IECs) were synthesized by grafting cationic alkyl chains with tertiary amine-containing poly(fluorenyl ether) precursors. The experimental results indicate that the introduction of the multi-cationic side chains facilitates the formation of micro-phase-separated morphologies and enhances anion conductivity. Moreover, the number of spacer atoms between the quaternary ammonium groups on the side chains affects the water uptake of the membranes, thus complicating the relationship between the density of cationic group distribution and anion conductivity. The poly(fluorenyl ether)s with dicationic side chains and six spacing atoms (DQA-PFE-C6) showed the highest anion conductivity. A VRFB assembled with DQA-PFE-C6 exhibited a maximum power density of 239.80 mW cm−2 at 250 mA cm−2, which is significantly higher than a VRFB assembled with Nafion 212. Therefore, side chain engineering is an effective chemical approach to enhance the properties of AEMs for VRFB applications.

Sulfonated poly (vinylidene fluoride‐co‐hexafluoropropylene) nanocomposite membranes with high selectivity, stability, and vanadium‐ion barrier for vanadium redox flow batteries

Polyethyleneimine‐functionalized reduced graphene oxide (PEI‐RGO)‐embedded sulfonated poly (vinylidene fluoride‐co‐hexafluoropropylene) (SPVDF‐co‐HFP) acid‐base composite membranes are fabricated by solution casting method for vanadium redox flow battery (VRFB) applications. The denser and crumbled morphology revealed by FESEM images and EDX results indicates the successful synthesis of PEI‐RGO nanosheets. The interfacial acid‐base pairs formed between PEI‐RGO and SPVDF‐co‐HFP matrix of composite membranes result in improvement of ion exchange capacity, water uptake, proton conductivity, as well as effective control in swelling ratio and vanadium‐ion permeability. In addition, the dispersing ability of PEI‐RGO in the SPVDF‐co‐HFP matrix improved the thermal and mechanical stability of the composite membranes. Homogeneous dispersion of PEI‐RGO nanosheets at the SPVDF‐co‐HFP surface is revealed by the FESEM images of the composite membranes, and their presence is confirmed by EDX results. The SPVDF‐co‐HFP membrane with 0.75 wt% PEI‐RGO exhibited the highest proton conductivity of 5.69 × 10−3Scm−1at 25°C, membrane selectivity of 25.52 × 10−4Scm−3min, and lowest vanadium‐ion permeability of 2.23 × 10−8cm2min−1. Overall results suggested that the SPVDF‐co‐HFP‐0.75 nanocomposite membranes are found to be a suitable alternative for commercially costly Nafion in VRFB applications.

Improved chemical stability and proton selectivity of semi‐interpenetrating polymer network amphoteric membrane for vanadium redox flow battery application

AbstractIn this work, semiinterpenetrating polymer network (semi‐IPN), consisting of sulfonated poly (arylene ether sulfone) (SPAES) and crosslinked vinyl imidazole grafted polysulfone (VMPSU), is prepared and characterized. FTIR, EDS, and solubility test indicate the successful preparation of amphoteric membranes. The semi‐IPN amphoteric membranes exhibit better stability than pure SPAES membrane, as demonstrated by thermogravimetric analysis and ex situ immersion testing results. More importantly, it is shown that the amphoteric membrane can effectively hinder vanadium ion crossover through the membrane, which is attributed to the semi‐IPN structure and Donnan exclusion. As expected, the amphoteric membrane containing 20% VMPSU exhibits the highest proton selectivity (6.86 × 104 S min cm−3), comparing to pristine SPAES (1.90 × 104 S min cm−3) as well as Nafion117 (1.31 × 104 S min cm−3).

Mesoscopic modeling and characterization of the porous electrodes for vanadium redox flow batteries

• Pore-scale simulations on 3D models of porous electrode reconstructed from X-ray micro-computed tomography were performed. • Effective transport properties of the reconstructed electrode models were computed. • A 2D macroscopic model for VRFB using the effective transport properties obtained from pore-scale simulations was developed. Porous electrodes are commonly used in electrochemical devices such as fuel cells and vanadium redox flow batteries (VRFBs). The performance of these electrodes depends on their transport properties including diffusivity, permeability and electric/thermal conductivities, and further on their surface properties if two-phase flow occurs at high current conditions. This paper reports a pore-scale investigation on the transport processes involved in a carbon felt for VRFB applications. The microstructure of a carbon felt over a range of compression ratios is first reconstructed from micro-computed tomography. Pore-scale model simulation, which solves the coupled transport of electrolyte in porous materials, is employed to compute the effective transport properties of the reconstructed model. The permeability and diffusivity of the carbon felt are found to decrease with increasing compression ratio. Transport of liquid water within the reconstructed carbon felt is studied based on the multiple-relaxation-time lattice Boltzmann method and the simulation result indicates that compression on the electrode causes a large drop in porosity and flow resistance increases accordingly. Furthermore, the reconstructed model is converted to a finite-element model and then solid mechanics simulations are performed to gain insight to the stress distribution of the microstructure under compression. These effective transport properties are used in a two-dimensional macroscopic model to demonstrate the mesoscopic-macroscopic approach. Compression on a porous electrode is found to contribute notably on the increase of vanadium concentrations and pressure drops, which can be beneficial to the performance of VRFB. The methodology developed from this research is readily applicable to other porous electrodes such as the gas diffusion layers for fuel cells.

Covalent/ionic co-crosslinking constructing ultra-densely functionalized ether-free poly(biphenylene piperidinium) amphoteric membranes for vanadium redox flow batteries

A novel covalently/ionically co-crosslinked poly(biphenylene piperidinium) amphoteric membrane was designed for improved stability and efficiencies in vanadium redox flow batteries (VRFB). The amphoteric-side-chain structure provides an ultrahigh functionalization degree (4.93 mmol·g − 1) and unobstructed ion channels, which endowed the membrane with an excellent ion conduction capacity comparable to Nafion 212 (area resistance: 0.23–0.29 Ω·cm2). Surpassing traditional sulfonated membranes and single ionic crosslinked membranes, even at high level functionalization, the covalently/ionically co-crosslinking structures guaranteed the membrane a low swelling ratio (10.1%) and VO2+ permeability of 0.79 × 10−8 cm2·s − 1. Moreover, due to the lack of sensitive aryl ether bonds, these membranes showed very good chemical stability, with a slight mass loss of 3.43% (nearly 5.5 times lower than SPEEK) after soaking in vanadium oxygen species for 30 days. On this basis, the membrane exhibited excellent battery performance. The Coulombic, voltage, and energy efficiencies of a battery using the above membrane reached 98.12, 87.27, and 85.98% at 120 mA·cm−2 current density and no obvious decline occurred after 400 cycles. The self-discharge time of battery up to 136 h. These observations indicated that this membrane appeared very promising for use in VRFB applications.

Quaternized poly(arylene ether benzonitrile) membranes for vanadium redox flow batteries

A series of quaternized poly (arylene ether benzonitrile)s (QAxCN; x = percent quaternization) were synthesized for the vanadium redox flow battery (VRFB) application. The presence of highly polar nitrile groups incorporated in the quaternized polymer chain provides desirable polymer properties (low water uptake, high dimensional stability and low VO2+ permeability) without compromising ion conductivity. VRFB single cells using the aforementioned membranes showed excellent performance. The cell using a 15 μm-thick QA50CN membrane exhibited superior coulombic efficiency (98.2%) and comparable energy efficiency (86.5%) at 40 mA cm-2 compared to the single cell using Nafion-212 with a coulombic efficiency of 95.5% and an energy efficiency of 86.2%. The cell cycling stability and membrane oxidative stability data confirmed that the QAxCN membranes have the potential to replace Nafion membranes for energy storage in VRFB applications.

Preparation of sulfonated polyimide/polyvinyl alcohol composite membrane for vanadium redox flow battery applications

A novel series of sulfonated polyimide (SPI)/polyvinyl alcohol (PVA) composite membranes applied in vanadium redox flow battery (VRFB) is prepared and characterized in this work. With the hydrophilicity and good barrier of PVA, the composite membranes exhibit better hydrolysis stability and vanadium ion resistance than normal SPIs with five-membered rings. In VRFB single-cell tests, because of the highest proton selectivity, the SPI5/PVA5 composite membrane shows lower self-discharge rate, higher coulombic efficiency, and energy efficiency (EE) (79.5% vs 77.8% at 30 mA cm−2) than that of Nafion117. Besides, the composite membranes present stable performances up to 100 cycles without significant decline in EE.

Synergistic effects of niobium oxide–niobium carbide–reduced graphene oxide modified electrode for vanadium redox flow battery

A powerful, low-cost niobium oxide–niobium carbide–graphene oxide (NbO2/NbC/rGO) hybrid electrocatalyst is prepared by refluxing, hydrothermal method, followed by annealing process at the optimum temperature of 1050 °C for all vanadium redox flow battery (VRFB) application. The NbO2/NbC/rGO electrocatalyst shows the best electrochemical activity with the highest peak current densities and the lowest peak differences toward VO2+/VO2+ and V2+/V3+ redox reactions from the cyclic voltammetry compared to other samples. The rate performance of VRFB cell using NbO2/NbC/rGO modified graphite felts (NbO2/NbC/rGO/GF) shows the voltage efficiencies of 89.3%, 81.7%, 71.0%, and 59.3% by the charge-discharge current densities of 40 mA cm−2, 80 mA cm−2, 120 mA cm−2, and 160 mA cm−2, respectively, which these performances are superior to those using the pretreated GFs and the untreated GF. The charge-discharge stability test of the VRFB cell using NbO2/NbC/rGO/GF demonstrates no significant decay after 100 cycles by the current density of 80 mA cm−2. This observation is due to the synergistic effects related to the presence of excess oxygen vacancies on NbO2/NbC, the high content of oxygen functional groups, and the high electrical conductivity characteristics of the graphene sheet.

Ultra-low vanadium ion permeable electrolyte membrane for vanadium redox flow battery by pore filling of PTFE substrate

The proton conductive membranes were prepared by impregnating sulfonated poly(arylene ether ketone) (SP) into the porous polytetrafluoroethylene (PTFE) substrates for vanadium redox flow battery application. To impregnate SP, the surface of porous PTFE was chemically hydrophilized with catechol and polyethyleneimine (PEI). The SP filled PTFE membranes (trPTFE/SP) showed significantly enhanced thermal, dimensional, and mechanical stability, compared with the pristine SP membranes. While the water uptake of trPTFE/SP membranes reduced by only 3–5%, the swelling ratio reduced to about a half and tensile strength increased by 5 times. Although this pore filling process slightly decreased the proton conductivity of the pure SP membranes by 10%, it hugely decreased the vanadium ion permeability about 5 times. Their low vanadium permeability, 1.37 ​× ​10−7 ​cm2 ​min−1–4.21 ​× ​10−7 ​cm2 ​min−1, was even 15 times at most lower than that of Nafion117, 20.28 ​× ​10−7 ​cm2 ​min−1. This low vanadium permeability led to high coulombic efficiency over 96% and energy efficiency around 84% during the 100 charge-discharge cycling tests, which are better than Nafion117 membrane.

Custom-made sulfonated poly (vinylidene fluoride-co-hexafluoropropylene) nanocomposite membranes for vanadium redox flow battery applications

Sulfonated poly (vinylidene fluoride-co-hexafluoropropylene) (SPVDF-co-HFP) based nanocomposite proton exchange membranes (PEM) are fabricated by simple solution casting method using polydopamine coated exfoliated molybdenum disulfide (PDA-MoS2) nanosheets as an alternative for Nafion® in vanadium redox flow batteries (VRFBs). PDA-MoS2 is synthesized by the etching of exfoliated MoS2 nanosheets with dopamine molecule by self-polymerization method. Various characteristic results clearly demonstrated that the incorporated PDA-MoS2 nanosheets homogeneously distributed into the SPVDF-co-HFP matrix and the presence of NH/NH2 group electrostatically interacts with SPVDF-co-HFP to form a strong acid-base pair and thus enhances the proton transport via Grotthuss type mechanism. Besides, the improvement in surface hydrophilicity provides the vehicle type conduction also. As a result, SPVDF-co-HFP/PM nanocomposite membranes showed higher proton conductivity in comparison with the pristine membrane. Especially SPVDF-co-HFP/PM-1 membrane demonstrated the excellent proton conductivity of 5.24 × 10−3 Scm−1 at 25 °C, lower vanadium-ion permeability of 1.05 × 10−8 cm2min−1 and highest membrane selectivity of 49.9 × 104 Scm−3min. On the other hand, vanadium-ion stability of the membrane increased by adding the PD-MoS2 content is attributed to their strong electrostatic attraction towards the polymer matrix. Overall results suggested that the SPVDF-co-HFP/PM-1 nanocomposite membrane is found to be a better alternative for commercially costly Nafion in VRFB applications.

Porous polybenzimidazole membranes with high ion selectivity for the vanadium redox flow battery

Development of high ion-selective polymer membranes is of great importance for vanadium redox flow batteries (VRFBs). In this work, porous polybenzimidazole (PBI) membranes are successfully prepared for improved efficiencies and stability in VRFB applications. The porous structure is constructed via a hard templating method using monodispersed SiO2 solid spheres and 3 M NaOH as template and etching solution, respectively. The influence of the adding content of SiO2 on the microstructure of the membrane is illustrated. The properties of porous membranes, including acid doping, swellings, area resistance, mechanical properties, vanadium ion permeation, and single cell and cycling performance, are investigated systematically. Results show that asymmetric porous PBI membranes exhibit increased sulfuric acid (SA) uptake (>35 wt%), remarkably low area resistance (<0.58 Ω cm2), ultralow vanadium ion permeability (<10-9 cm2 min-1) and superior mechanical strength (>20 MPa) simultaneously. At high current densities (80–200 mA cm-2), the cell with the porous PBI-40%SiO2 membrane possesses superior coulombic efficiencies (CE: 99.5–100%) and energy efficiencies (EE: 87.9–71.5%), together with excellent cycling stability at 120 mA cm-2 in the vanadium flow battery. Accordingly, this work not only provides a kind of advanced membranes for the VRFB, but also offers a facile and effective method to design, optimize and fabricate membranes incorporating the features according to needs.

Recovery of spent VOSO4 using an organic ligand for vanadium redox flow battery applications

To recover the spent vanadium compound, Rhodamine-B-based Schiff’s base ligand (L1) was synthesized via ultrasonication process and was evaluated with vanadyl sulfate (VOSO4), which has shown considerable selectivity towards V(IV). The change of the solution color from colorless to pink is attributed to L1 after the reaction with vanadium ion owing to the successful formation of the vanadium complex and the opening of the spirolactam ring in the L1 structure. In FT–IR spectra, the vanadyl peaks are co-existed with the L1 structure, which confirmed the complex formation of the L1 with vanadium. Similarly, the binding energy of V(IV) was identified at 516.2 eV for V2p3/2 in XPS spectra. The new strategy for VOSO4 recovery was established through solvent extraction and acid leaching. After recovery process, the absence of vanadium peak in the XPS confirmed the complete removal of V(IV) from the complex. The recovered VOSO4 solution used as an electrolyte in vanadium redox flow battery (VRFB) systems, where the unit cell performance is comparable with the conventional electrolyte solution. The advantage of study is reuse of VOSO4 as a resource for energy storage applications.

A facile strategy for disentangling the conductivity and selectivity dilemma enables advanced composite membrane for vanadium flow batteries

To resolve the long-standing conductivity-selectivity dilemma in ion exchange membranes (IEMs), a facile strategy for the preparation of the composite membrane based on perfluorosulfonic acid (PFSA) membrane with thin polybenzimidazole (PBI) selective layer is presented for vanadium redox flow batteries (VFBs). By simple impregnation method, the dense thin PBI selective layer with tunable thickness was coated on the highly conductive commercial PFSA proton exchange membrane (DMR). The thin PBI selective layer not only provided the membrane with high selectivity and proton conductivity, but the strong acid-base interaction between basic PBI and acidic PFSA membrane was also responsible for the formation of a stable layer on the PFSA supporting. With this design, the optimized composite membrane T-DMR-100s exhibited a low area resistance of 0.14 Ω cm2 in 3 M H2SO4 as well as low vanadium permeability of 3.6 × 10−8 cm2/min, which surpass the benchmark Nafion® 212 membrane under the same testing conditions (area resistance of 0.15 Ω cm2 and vanadium permeability of 1.2 × 10−7 cm2/min). Consequently, both high coulombic efficiency (CE) of 98.9% and energy efficiency (EE) of 86.1% were achieved for the VFB with T-DMR-100s composite membrane at a current density of 140 mA/cm2, much higher than the cell with the Nafion® 212 membrane (CE of 92.0% and EE of 80.2%). The chemical stability of the composite membrane was confirmed by stable operating for 960 cycles (700 h) at a current density of 140 mA/cm2 and ex situ oxidation stability tests. Moreover, the failure mechanism of the composite membrane in the operating VFBs was also investigated in the extended cycling testing, and the post-cell sample analysis of the aged membranes by XPS and permeability measurement indicated that the degradation of the NPBI selective layer may be the main reason for the continuous decay in EE after 960 cycles. This work provided a new insight into the design of the composite membranes with both high conductivity and selectivity for VFB application.

Real-Space 3D Simulation of Cyclic Voltammetry in Carbon Felt Electrodes for Application in Vanadium Redox-Flow Batteries Based on a Combination of Micro CT Data, Digital Simulation and Convolutive Modelling

Cyclic voltammetry (CV) is the prevalently used technique for investigating the kinetic performance of carbon felt electrodes for application as porous electrocatalysts in vanadium redox-flow batteries. However, the theoretical treatment of internal diffusion-reaction phenomena of such electrodes, allowing for quantitative data evaluation, is still unexplored and mostly relies on diffusion domain approximations assuming the felt as an array of either planar, spherical or cylindrical microelectrodes in a randomly distributed and finite diffusion domain. With this study, we present a novel three-step strategy for real-space CV simulation of felt electrodes. By using micro x-ray tomography (CT) data of the felts, we obtain a locally resolved 3D template of the diffusion space making any diffusion domain approximations obsolete. CV simulation involving this real diffusion domain template is performed by combining the implicit Crank-Nicolson technique in 3D with convolutive modelling. At first, by utilizing the Crank-Nicolson method, we obtain the chronoamperometric current of the 3D porous network for Cottrell-like Dirichlet boundary conditions. Subsequently, the mass transfer functions, usually generated with the aid of Laplace integral transformation techniques, are calculated from these chronoamperometric currents. Then, CV simulation is performed on the basis of these mass transfer functions involving classical convolution integrals. This strategy offers three significant advantages. First, we avoid the difficulty of implementing non-Dirichlet boundaries in the matrices of the Crank-Nicolson scheme. Second, the difficulty of performing an inverse Laplace transformation is avoided since no direct Laplace transformation step is involved. Third, once the mass transfer functions are obtained the simulation can be done with significant savings in time since the 3D network is converted into a one-dimensional problem that can be solved via classical convolution integrals. Our simulation is supported by experimental data acquired for the VO2+-oxidation reaction underlining the validity of our approach. Figure 1

Design and Development of an Innovative Barrier Layer in Vanadium Redox Flow Batteries

Vanadium Redox Flow Battery (VRFB) is an electrochemical energy storage technology that exploits red-ox reactions of vanadium ions dissolved in acidic solutions to convert electrical energy into and from chemical energy. VRFB is an interesting and promising technology because of its high efficiency, independent energy to power ratio, high flexibility and long cycle life [1]. However, VRFB commercialization is still hindered by some technological issues, among which is the capacity loss induced by the undesired transport of vanadium ions across the ion-exchange membrane. Depending on the nominal operating condition, the choice of suitable membrane for VRFB application results from the trade-off between low vanadium permeability and high proton conductivity.Nafion® is widely used in VRFB: it presents high conductivity and good mechanical and chemical stability, but it is not ideally selective towards vanadium ions. This implies the adoption of thicker membrane to limit capacity loss, leading to increased ohmic loss and system costs [2]. Alternative cation exchange membranes (CEMs) are SPEEK or SPI that are promising because of their reduced permeability, but the low conductivity and stability hinder their competitiveness with respect to Nafion®. Anion exchange membranes (AEMs) are interesting alternatives, but the poor chemical stability and low conductivity limit their application in VRFB. Amphoteric ion exchange membranes (AIEM) combine the functional groups of both CEM and AEM, obtaining both low permeability and good conductivity. However, the complex preparation results in high costs [3].In this work, an additional selective layer, described in the patent WO 2019/197917, was deposited on a commercial cation exchange membrane. The selective layer, termed as barrier, is a porous component in which pores size, tortuous path, thickness and the composition are designed to improve layer selectivity towards vanadium ions. The barrier was manufactured with reactive spray deposition technology (RSDT), which is a flame-based synthesis process unique to Dr. Radenka Maric’s research group. Fabrication of the barrier layer by RSDT is a one-step process in which carbon-rich particles are synthesized in the flame and a mixture of commercial carbon black particles and ionomer is sprayed directly onto the membrane. In this work different barrier morphologies have been designed, manufactured and tested.The barrier layer was tested in a 25 cm2 cell during cycles at constant capacity in both charge and discharge [4]. After each charge and discharge step the cell was kept in OCV for 90 seconds in order to acquire cell OCV and get an insight into battery state of charge (SoC). Figure 1 reports the evolution of SoC after each step of charge: it is worth noting that the VRFB with barrier presents a significant reduction of SoC decay (i.e., capacity loss) compared to a reference VRFB employing Nafion® 115. Moreover, the VRFB with barrier exhibits a slightly higher efficiency and a good stability in time.In addition to electrochemical testing, the structure of the barrier layer was characterized using TEM, SEM and nitrogen adsorption. TEM and nitrogen adsorption provide information on pore structure and pore-size distribution, which can be related to the performance of the barrier layer during cell testing, while SEM shows that the coating is uniform in thickness and consistent from sample to sample.