Vanadium Redox Flow Battery Application Research Articles

VRFBs based on benzotriazole grafted polybenzimidazole / polymeric ionic liquid with high ion selectivity

A series of innovative branching OPBI composite membranes (OPBI-BTZ-PIL) were created based on benzotriazole side chain grafted OPBI (OPBI-BTZ) and double-bond ionic liquids for vanadium redox flow battery applications. The benzotriazole contained three nitrogen sites, which helped enhance the proton conductivity and low area resistance. The cation in the polymeric ionic liquid in the matrix and the vanadium ion produces the Donnan effect, which reduces vanadium permeability. Moreover, the hydrogen bond cross-linking network formed by the polymeric ionic liquid in the membrane further helps proton transport. The vanadium permeability of OPBI-BTZ10-PIL10 is as low as 1.51 × 10−9 cm2 min−1, which is one 40th of the Nafion117 (62.54 × 10−9 cm2 min−1). Its ion selectivity is up to 2.45 × 106 S min cm−3. The OPBI-BTZ10-PIL10 membrane performs well at 60 mA cm−2, with CE of 94.12% and EE of 86.2%, both of which are higher than the Nafion117 membrane.

Anion Exchange Membranes Based on Bis-Imidazolium and Imidazolium-Functionalized Poly(phenylene oxide) for Vanadium Redox Flow Battery Applications.

Although the Nafion membrane has a high energy efficiency, long service life, and operational flexibility when applied for vanadium redox flow battery (VRFB) applications, its applications are limited due to its high vanadium permeability. In this study, anion exchange membranes (AEMs) based on poly(phenylene oxide) (PPO) with imidazolium and bis-imidazolium cations were prepared and used in VRFBs. PPO with long-pendant alkyl-side-chain bis-imidazolium cations (BImPPO) exhibits higher conductivity than the imidazolium-functionalized PPO with short chains (ImPPO). ImPPO and BImPPO have a lower vanadium permeability (3.2 × 10-9 and 2.9 × 10-9 cm2 s-1) than Nafion 212 (8.8 × 10-9 cm2 s-1) because the imidazolium cations are susceptible to the Donnan effect. Furthermore, under the current density of 140 mA cm-2, the VRFBs assembled with ImPPO- and BImPPO-based AEMs exhibited a Coulombic efficiency of 98.5% and 99.8%, respectively, both of which were higher than that of the Nafion212 membrane (95.8%). Bis-imidazolium cations with long-pendant alkyl side chains contribute to hydrophilic/hydrophobic phase separation in the membranes, thus improving the conductivity of membranes and the performance of VRFBs. The VRFB assembled with BImPPO exhibited a higher voltage efficiency (83.5%) at 140 mA cm-2 than that of ImPPO (77.2%). These results of the present study suggest that the BImPPO membranes are suitable for VRFB applications.

Preparation of Sulfonated Poly(arylene ether)/SiO2 Composite Membranes with Enhanced Proton Selectivity for Vanadium Redox Flow Batteries

Proton exchange membranes (PEMs) are an important type of vanadium redox flow battery (VRFB) separator that play the key role of separating positive and negative electrolytes while transporting protons. In order to lower the vanadium ion permeability and improve the proton selectivity of PEMs for enhancing the Coulombic efficiency of VRFBs, herein, various amounts of nano-sized SiO2 particles were introduced into a previously optimized sulfonated poly(arylene ether) (SPAE) PEMs through the acid-catalyzed sol-gel reaction of tetraethyl orthosilicate (TEOS). The successful incorporation of SiO2 was confirmed by FT-IR spectra. The scanning electron microscopy (SEM) images revealed that the SiO2 particles were well distributed in the SPAE membrane. The ion exchange capacity, water uptake, and swelling ratio of the PEMs were decreased with the increasing amount of SiO2, while the mechanical properties and thermal stability were improved significantly. The proton conductivity was reduced gradually from 93.4 to 76.9 mS cm-1 at room temperature as the loading amount of SiO2 was increased from 0 to 16 wt.%; however, the VO2+ permeability was decreased dramatically after the incorporation of SiO2 and reached a minimum value of 2.57 × 10-12 m2 s-1 at 12 wt.% of SiO2. As a result, the H+/VO2+ selectivity achieved a maximum value of 51.82 S min cm-3 for the composite PEM containing 12 wt.% of SiO2. This study demonstrates that the properties of PEMs can be largely tuned by the introduction of SiO2 with low cost for VRFB applications.

Novel microporous sulfonated polyimide membranes with high energy efficiency under low ion exchange capacity for all vanadium flow battery

High proton conductivity and low vanadium permeation created great challenges in designing of highly efficient membranes for all vanadium redox flow battery. To overcome this trade-off phenomenon, a series of novel microporous sulfonated polyimides (SPI) with gradient sulfonic acid group concentrations (6FTMA-X) were prepared by a simple one-step polymerization. They demonstrated high molecular weight (Mn of 23 to 59 KDa), modest microporosity (SBET = 437.2 - 39.4 m2 g−1) and suitable interchain space (between 3.7 and 5.2 Å), which provided excellent ion sieving for proton and vanadium ions (2.4 and 6.0 Å). Consequently, although the ion exchange capacities (0.38 - 1.47 mmol g−1) of 6FTMA-Xs were low compared with the reported SPIs, they still achieved high energy efficiency (80.4%) under high current density (100 mA cm−2), which was comparable to Nafion 117 (N117, 81.7%) at the same current density. This was because of the acceptable proton conductivity and ultra-low vanadium permeation (0.59 - 3.56 × 10−7 cm2 min−1). We suppose the micropore inside the sulfonated polyimides behaviors as ion channel for proton transport and a vanadium barrier. All these results demonstrate that the 6FTMA-X membranes show great potential for vanadium redox flow battery applications. This polymer design strategy provided new insight for highly efficient membranes for VRFB applications.

Low Vanadium Permeability Membranes Based on Flexible Hydrophilic Side Chain Grafted Polybenzimidazole/Polymeric Ionic Liquid for VRFBs

Based on amino polybenzimidazoles with flexible hydrophilic side chains (AmPBI-MOE) and polymeric ionic liquid (PIL), a series of composite membranes (AmPBI-MOE-PIL-X) were fabricated for vanadium redox flow battery applications. Here, 1-Bromo-2-(2-methoxyethoxy)ethane was grafted onto amino polybenzimidazole (AmPBI) by the method of halogenated hydrocarbons, and PIL was synthesized from ionic liquids by in situ radical polymerization to build a hydrogen-bonded cross-linked network within the film. The hydrophilic side chain improves the proton conductivity. With the increase in ionic liquids, the vanadium transmittance and the proton conductivity increase. The AmPBI-MOE-PIL-5 membrane not only exhibits a vanadium ions permeability of 0.88 × 10−9 cm2 min−1, which is much lower than Nafion117 (6.07 × 10−8 cm2 min−1), but also shows a very excellent blocking ability for vanadium ion. The AmPBI-MOE-PIL-5 membrane shows excellent performances at 60 mA cm−2, with VE of 87.93% and EE of 82.87%, both higher than that of Nafion117 membrane in VRFB.

Synthesis, fabrication and characterization of 2-naphthyloxy group-substituted bis(2-pyridylimino)isoindoline and its derivatives as a positive electrode for vanadium redox flow battery applications.

In recent years, tridentate nitrogen donor ligands have played a vital role in inorganic chemistry. The ease of synthesis, readily modifiable structure and high stability of 1,3-bis(2-pyridylimino)isoindole (BPIs) compounds make them suitable candidates for many potential applications. In this study, a 1,3-bis(2-pyridylimino)isoindoline derivative bearing a naphthoxy unit and its palladium complex (PdBPI) were synthesized and characterized by single crystal X-ray diffraction, NMR, FT-IR, UV-Vis, and mass spectroscopic methods. The BPI- or PdBPI-modified pencil graphite electrodes were clarified via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), EDX, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The efficiency of these substances in a vanadium redox flow battery (VRB) system was investigated for the first time. The behaviors of the BPI-modified carbon felt electrode (BPI-CF) and PdBPI-modified carbon felt electrode (PdBPI-CF) were investigated in the redox flow battery (RFB) applications. These modified electrodes were obtained by the electrodeposition method. The respective charge potentials of BPI-CF and PdBPI-CF reached 1.63 V and 1.88 V, respectively. The discharge capacity maxima obtained were ∼301 mA h (1204 mA h L-1) and ∼303 mA h (1212 mA h L-1) for BPI-CF and PdBPI-CF at the VRB system under a charge current density of 4.0 mA cm-2 and discharge current density of 0.4 mA cm-2, respectively.

Advancements in polyelectrolyte membrane designs for vanadium redox flow battery (VRFB)

The polyelectrolyte membrane (PEM) is deputed as a pivotal component of vanadium redox flow battery (VRFB) devices and allows long-cycling life for practical applications. However, the bottlenecks in stability and high performance PEM designs has limited the utilization of this technology to laboratory wisdom. Herein, this review focuses on the critical topic of recent advancements in PEM designs and state-of-the art methodologies to acquire alternate stable membrane structures for its application in VRFB. The importance of these advanced membrane materials underlies determining the economic feasibility, safety and desire performance outputs. Moreover, there is a drastic shift in exploring the anion exchange polyelectrolyte materials over cation exchange polyelectrolyte material with performance equivalence of Nafion®. And this has elaborated a newer prospect in polymer synthesis and functionalization. Embarking towards advanced materials, a much comprehensive literature on recent structures and various designing strategies is critically required. This literature comprehensively and systematically summarises the basics of VRFB and developments of high performance membranes. In addition, we cover different fabrication strategies, ionic designs, composite material reinforced PEMs and alternate fluorinated backbone based PEMs attributed with better performance than Nafion®. Furthermore, degradation mechanisms and oxidative stability enhancing strategies is also included. The modification of microstructures in these materials is critical to acquire desired electrochemical properties and hence influence of variable functional architecture is briefly reviewed. The objective of this review is to summarise the recent membrane designs for efficient VRFB performances.

A novel permselective branched sulfonated polyimide membrane containing crown ether with remarkable proton conductance and selectivity for application in vanadium redox flow battery

As a key component of vanadium redox flow battery (VRFB), the ideal membrane with high proton conductance together with high vanadium ions resistance is urgently required. In this work, a permselective monomer di(aminobenzo)-18-crown-6 with proper aperture of 0.26–0.32 nm is synthesized to prepare branched sulfonated polyimide containing crown ether (ce-bSPI-x) membranes. In virtue of the excellent balance between vanadium ions resistance and proton conductance, the ce-bSPI-60 membrane possesses the optimum physico-chemical properties among all ce-bSPI-x membranes. Subsequently, in comparison with commercial Nafion 212 membrane, the ce-bSPI-60 membrane demonstrates higher coulomb efficiencies (CEs) (96.50%–99.39%) and energy efficiencies (EEs) (69.36%–85.45%). Furthermore, the ce-bSPI-60 membrane can achieve 80% of EE at 140 mA cm−2, which is in a superior position compared with other SPI-based membranes reported in recent years. 1000-time cycles at 140 mA cm−2 without obvious structural and morphological changes proves the excellent durability of ce-bSPI-60 membrane. To sum up, the as-prepared ce-bSPI-60 membrane exhibits a great potential to match the demand of application in VRFB.

Porous Membranes of Polysulfone and Graphene Oxide Nanohybrids for Vanadium Redox Flow Battery.

Porous nanohybrid membranes of polysulfone (PSF) with graphene oxide (GO) nanosheets (PSF/GO membrane) were developed to serve as proton exchange membranes in a vanadium redox flow battery (VRFB). Various ratios of PSF/GO and thickness were investigated to evaluate the optimal voltage efficiency (VE), coulombic efficiency (CE), and energy efficiency (EE) of the VRFB. The pore size, distribution, and hydrophilicity of PSF/GO membranes were studied using scanning electron microscopy (SEM) images and contact angles. Functional groups of GO were evaluated using Raman spectroscopy. The mechanical properties and thermal stability of PSF/GO membranes were analyzed using a tensile tester and thermogravimetric analysis (TGA), respectively. The results show that the mechanical properties of the PSF porous membrane with GO nanosheets were significantly improved, indicating that the addition of graphene oxide nanosheets consolidated the internal structure of the PSF membrane. Cyclic voltammetry revealed an obviously different curve after the addition of GO nanosheets. The CE of the VRFB in the PSF/GO membrane was significantly higher than that in the pristine PSF membrane, increasing from 80% to 95% at 0.6 wt.% GO addition. Moreover, PSF/GO membranes displayed great chemical stability during long-term operation; thus, they can evolve as potential porous membranes for application in VRFBs for green energy storage.

3D Carbon Nanonetwork Coated Composite Electrode with Multi-Heteroatom Doping for High-Rate Vanadium Redox Flow Batteries.

With the advantages of benign mechanical property, electrochemical stability, and low cost, graphite fibers (GFs) have been widely used as electrodes for vanadium redox flow batteries (VRFBs). However, GFs usually possess inferior electrochemical activity and ion diffusion kinetics for electrode reaction, vastly limiting their application in VRFBs. Here, a 3D carbon nanonetwork coated GFs with multi-heteroatom doping was constructed for application in VRFBs via low temperature polymerization between linear polymer monomer and phytic acid, and subsequent carbonization (900 °C) on the GFs (GF@PCNs-900). Benefiting from the 3D structural features and multi-heteroatom doping (O, N and P), the composite electrode displayed sufficient diffusion of vanadium ions, rapid electron conduction, and highly enhanced electrochemical activity of reactive site on the electrodes. As a result, the GF@PCNs-900 delivered a high discharge capacity of 21 Ah L-1 and energy efficiency of above 70% with extraordinary stability during 200 cycles at 200 mA cm-2. Even at a huge current density of 400 mA cm-2, the GF@PCNs-900 still maintained a discharge capacity of 5.0 Ah L-1, indicating an excellent rate of performance for VRFBs. Such design strategy opens up a clear view for further development of energy storage field.

Clean preparation of mixed trivalent and quadrivalent vanadium electrolyte for vanadium redox flow batteries by catalytic reduction with hydrogen

The vanadium redox flow battery (VRFB) is a promising technology for large-scale stationary energy storage systems. However, the high preparation cost of mixed valent vanadium electrolyte hinders the large-scale commercial application of VRFB. In this work, a simple, green and low-cost method is proposed to prepare the mixed valent vanadium electrolyte for VRFB. The clean hydrogen is chosen as the reducing agent to obtain trivalent vanadium ions from quadrivalent vanadium ions. The Pt/C material is used as the catalyst to accelerate the reduction rate at atmospheric pressure. The impurity-free mixed valent vanadium electrolyte, with a mean valence number of 3.5, prepared by the catalytic reduction process exhibits excellent battery performance with CE of 93% and EE of 85%. Furthermore, a catalytic reactor using Pt/C decorated graphite felt is designed and used to continuously produce the mixed valent vanadium electrolyte. According to the result of simple cost analysis, the proposed catalytic hydrogen reduction process can reduce theoretically the manufacturing cost by approximately 22.6% compared with the present industrial electrolytic process. As a consequence, the simple, clean and low-cost manufacture method of the mixed valent vanadium electrolyte exhibits remarkable competitiveness and promising potential for the practical production application of VRFB.

High-Performance and Low-Cost Membranes Based on Poly(vinylpyrrolidone) and Cardo-Poly(etherketone) Blends for Vanadium Redox Flow Battery Applications

Energy storage systems have aroused public interest because of the blooming development of intermittent renewable energy sources. Vanadium redox flow batteries (VRFBs) are the typical candidates owing to their flexible operation and good cycle durability. However, due to the usage of perfluorinated separator membranes, VRFBs suffer from both high cost and serious vanadium ions cross penetration. Herein, we fabricate a series of low-budget and high-performance blend membranes from polyvinylpyrrolidone (PVP) and cardo-poly(etherketone) (PEKC) for VFRB. A PEKC network gives the membrane excellent mechanical rigidity, while PVP endows the blend membranes with superior sulfonic acid uptake owing to the present N-heterocycle and carbonyl group in PVP, resulting in low area resistance. Meanwhile, blend membranes also display low vanadium ion permeability resulting from the electrostatic repulsion effect of protonated PVP polymer chains towards vanadium ions. Consequently, the 50%PVP-PEKC membrane has a high ionic selectivity of 1.03 × 106 S min cm−3, while that of Nafion 115 is nearly 17 times lower (6.03 × 104 S min cm−3). The VRFB equipped with 50%PVP-PEKC membrane has high coulombic efficiencies (99.3–99.7%), voltage efficiencies (84.6–67.0%) and energy efficiencies (83.9–66.8%) at current densities of 80–180 mA cm−2, and possesses excellent cycle constancy, indicating that low-cost x%PVP-PEKC blend membranes have a great application potentiality for VRFBs.

Enhanced proton selectivity and stability of branched sulfonated polyimide membrane by hydrogen bonds construction strategy for vanadium flow battery

Vanadium redox flow battery (VFB) is promising for use as an energy storage/conversion device, however membrane imposes a limitation in enhancing the performance of VFB. Herein, a novel diamine monomer bis(2-trifluoromethyl-4-aminobenzene) amine containing imino groups is synthesized by nucleophilic substitution and reduction reactions. Then, we develop four branched sulfonated polyimide (I-bSPI) membranes with 40%–70% theoretical sulfonation degrees for application in VFB. The trade-off between proton conduction and vanadium ion blocking of I-bSPI membrane is broken, and the stability of I-bSPI membrane is also effectively enhanced owing to hydrogen bonds network and Donnan repulsion effect. Surprisingly, the proton selectivity of I-bSPI-50 membrane is 4.6 times that of commercial Nafion 212 membrane. In addition, I-bSPI-50 membrane exhibits superior coulomb and energy efficiencies to Nafion 212 membrane at 100–300 mA cm−2. And I-bSPI-50 membrane has steady efficiencies and high capacity holding abilities during 600-time cycles. This work illustrates a promising pathway to obtain cost-effective membrane by hydrogen bonds construction strategy for VFB application.

N-Sulfonated Poly(arylene-oxindole) for Vanadium Redox Flow Battery Applications

N-Sulfonated Poly(arylene-oxindole) for Vanadium Redox Flow Battery Applications

Polybenzimidazole Membrane for Vanadium Flow Battery Applications

The vanadium redox flow battery (VRFB) is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy.1 Perfluorinated-based ion exchange membranes such as Nafion are the most widely used membranes in VRFB due to their high proton conductivity. However, the extremely high cost and low ion selectivity of Nafion have limited their further application in VRFB. Among the aromatic hydrocarbon-based membranes that combine both high ion selectivity and high proton conductivity, polybenzimidazole- based membranes have served as one of the most promising alternatives to Nafion due to their excellent chemical and mechanical stability and low cost.2 In the present work m-PBI membranes of varying thickness and modified PBI were prepared, and their properties and VRFB performances were compared. The voltage efficiency (VE) increased with decreasing membrane thickness because of the decreasing ohmic resistance. The excellent current efficiency (CE) of VRFBs using meta-PBI based membranes is attributed to the negligible vanadium crossover observed in ex-situ permeability tests.The stability of the membranes after long term cycling is one of the most challenging problems in large-scale VRB applications. The durability of PBI membrane in VRFB cell testing and the capacity decay was measured by operating the cells for 100 cycles under a high current density of 200 mA cm−2. Fig 1: Flow battery efficiencies using electrodes: 2.5 mm SGL, CR = 20%, membrane: PBI 7 µm at current density of 200 mA/cm2, the electrolyte solution V(IV):V(III) (50:50) in 2.0 M H2SO4 and 0.05 M H3PO4 . The chemical stability of PBI membranes was analyzed after immersion of the PBI samples in various highly oxidizing and reducing sulfuric acid based vanadium and cerium solutions. The membranes stayed unchanged for more than 4 months. This result suggests that the PBI membranes demonstrate excellent ex-situ chemical stability in the harsh acid, oxidizing and reducing conditions.

Enhancing energy efficiency and long-term durability of vanadium redox flow battery with catalytically graphitized carbon fiber felts as electrodes by boron doping

A boron doping on a commercial carbon fiber felt (CFF) is used to assemble a high-performance vanadium redox flow battery (VRFB). The CFF is immersed in the boric acid solution and subjected to a heat-treatment up to 2700 °C. The crystallinity of the boron-doped samples increases more than threefold when compared to only heat-treated CFF, indicating that boron induces CFF to highly graphitized CFF. As a result, electrochemical impedance spectroscopy and cyclic voltammetry reveal that boron-doped CFFs increase electrical conductivities. In addition, when the boron-doped CFFs are applied to VRFB electrodes, their electrochemical performances are improved compared to CFF and only heat-treated CFF. The CFFB5 that is a sample containing 0.6 at.% of boron contents exhibits the best electrochemical performance in the single-cell test compared to the other samples. In particular, the CFFB5 sample provides excellent energy efficiency (90% at 50 mA cm–2) and long-term durability (98.1% over 150 cycles). Consequently, our approach introduces effective modulation of commercially available CFF with a significant graphitization via a feasible boron doping method and we conclude that the resulting boron-doped CFF induces improved electrochemical performance for VRFB applications when the appropriate amount of a doping agent is provided.

Novel Anion-Exchange Blend Membranes Comprised of a Commercially Available & Water-Soluble Ionomer for All-Vanadium Redox Flow Batteries

The All-Vanadium Redox Flow Battery is a promising technology for solving the problem of cost-efficient and safe large-scale storage of renewable energies [1]. One key component of the batteries, which alters the performance and long-term stability of those systems, is the membrane. For this purpose, Nafion based membranes are often used. However, these membranes are high in cost and show a significant capacity fading and reduction of coulombic efficiency mainly due to crossover of vanadium species [2]. One way around this is to use commercially available, low-cost materials with positively charged ammonium groups, which can effectively repel positively charged vanadium species due to the Donnan exclusion effect [3, 4].In this work we present novel anion-exchange blend membranes comprised of a commercially available and water-soluble ionomer and a PBI blend matrix, which can be used in All-Vanadium Redox Flow batteries. For this approach the water-soluble ionomer was modified to achieve solubility in organic solvents and miscibility with the matrix polymer. The resulting membranes show good mechanical properties even at high IECs and can be modified in a wide range of properties by adjusting the amounts of ionomer and matrix polymer and/or by the addition of crosslinking agents.The above-mentioned membranes were characterised ex-situ by using EIS, NMR spectroscopy, IR spectroscopy, DMA, TGA, tensile tests and permeability tests. In addition, first promising in-situ characterisations were performed in a VRFB single-cell test station.Literature[1] Fan, X; Liu, B; Liu, J; Ding, J; Han, X; Deng, Y; Lv, X; Xie, Y; Chen, B; Hu, W; Zhong, C.: Battery Technologies for Grid-Level Large-Scale Electrical Energy Storage. Trans. Tianjin Univ. 2020, 92 – 103.[2] Düerkop, D; Widdecke, H; Schilde, C; Kunz, U; Schmiemann, A.: Polymer Membranes for All-Vanadium Redox Flow Batteries: A Review. Membranes 2021.[3] Xi, J; Wu, Z; Teng, X; Zhao, Y; Chen, L; Qiu, X.: Self-assembled polyelectrolyte multilayer modified Nafion membrane with suppressed vanadium ion crossover for vanadium redox flow batteries. J. Mater. Chem. 2008, 1232.[4] Mohammadi, T; Kazacos, M.: Modification of anion-exchange membranes for vanadium redox flow battery applications. Journal of Power Sources 1996, 179 – 186.

(Digital Presentation) High-Temperature Ammonia Treatment for Carbon Felt Electrodes in All-Vanadium Redox Flow Batteries

Ammonia gas treatments of varying temperature were performed on carbon felt electrodes in this study. Their physical and electrochemical properties were investigated.Carbon felt electrodes are often used in vanadium redox flow batteries (VRFBs). Without processing, carbon felt has poor wettability which frustrates electrochemical activity. This material must be modified to improve performance, which has been historically accomplished with thermal treatments ostensibly to promote oxygen functional groups, although this has been the subject of debate as morphological impacts have also been observed. [1-3] Many attempts have been made to improve performance of carbon felt in VRFBs, but some of the more successful modifications have resulted in an increase in the specific surface area. [4-6] Previous research at the University of Tennessee Knoxville suggests edge sites play a role in improving performance. Preliminary experiments on carbon felt performed by this lab with ammonia at high temperatures have shown promising results. Additional experiments were performed at 900°C by varying length of treatment time, finding four hours to be optimal. [3] New experiments have been done to investigate the kinetic effects of temperature.For these experiments, commercially available carbon felts (SIGRACELL® GFD3 by SGL Carbon, Meitingen, Germany) were modified by exposure to flowing ammonia gas through a furnace at (500°C, 700°C, 900°C, and 1100°C) varying temperatures for four hours. The samples were tested in a single-cell flow battery using cyclic voltammetry (CV), polarization curves, and electrochemical impedance spectroscopy (EIS). This treatment results in a significant increase in electrochemical surface area and performance.The physical properties were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and Raman spectroscopy. This was done to study the mechanism and kinetic effects of ammonia on the carbon felt material as temperature increases during the treatment itself. Samples treated at the highest temperatures exhibited a notable loss of mass, which is observed in the SEM images in Figure 1. This resulted in a significant increase in edge sites and active surface area, along with a substantial increase in cell performance, which was achieved with the ammonia-modified carbon felts described in this work, with over three times the current density at 80% voltage efficiency compared to untreated felt. Sun, B. and M. Skyllas-Kazacos, Modification of graphite electrode materials for vanadium redox flow battery application—I. Thermal treatment.Electrochimica Acta, 1992. 37(7): p. 1253-1260.Pezeshki, A.M., "Impedance-Resolved Performance and Durability in Redox Flow Batteries. " PhD diss., University of Tennessee, 2016.Gass, K., High Performance Vanadium Redox Flow Battery Electrodes. [manuscript under submission], 2021.Lu, W., et al., High-performance porous uncharged membranes for vanadium flow battery applications created by tuning cohesive and swelling forces.Energy & Environmental Science, 2016. 9(7): p. 2319-2325.Zhou, X.L., et al., A high-performance dual-scale porous electrode for vanadium redox flow batteries.Journal of Power Sources, 2016. 325: p. 329-336.Wei, L., et al., Highly catalytic hollow Ti3C2Tx MXene spheres decorated graphite felt electrode for vanadium redox flow batteries.Energy Storage Materials, 2020. 25: p. 885-892. Figure 1. SEM images at 30,000X magnification of carbon felt electrodes: a) untreated, b) 4 hours NH3 treated at 500°C, c) 4 hours NH3 treated at 700°C, d) 4 hours NH3 treated at 1100°C. Scale was identical for all micrographs Figure 1

The Influence of Non-Charged Sidechains on the Performance of Meta-Polybenzimidazole Membranes in Vanadium Redox Flow Batteries

The global goal to combat climate change is growing year by year, as for example indicated by the latest climate conference by the United Nations, COP26. Hereby, one of the main goals concerns the transition from fossil fuels to more renewable energy means. In doing so, the importance of highly efficient energy storage technologies, such as supercapacitors, Li-ion batteries and/or redox flow batteries that both store and release energy on a short-, mid- and long-term scale, grows alongside it1. Among storage technologies, redox flow batteries (RFBs) are considered as promising candidates for mid-term storage (daily discharge for 2-12 h) due to their unique characteristic of a decoupled energy storage and power output2. To enable the incorporation of these systems into society further and thus smoothen the transition towards renewable energy technologies, highly efficient and low cost membranes in terms of efficiency and durability are needed, that will simultaneously improve the efficiency and reduce the cost of energy storage.One of the membrane types for vanadium redox flow battery (VRFB) applications that gained the interest of industry and the scientific community are of the polybenzimidazole (PBI) type. Although these materials possess excellent chemical stability and vanadium barrier properties, they suffer from a poor proton conductivity compared to commercial standards such as Nafion® 212. Several strategies were proposed to tackle this issue, ranging from the preparation of composite membranes with a thin PBI skin layer3, alkaline pretreatments that improve the swelling characteristics of the PBI film4 and chemical modifications to attach ionic conducting groups on the polymer backbone, thus directly improving the conductivity5. This latter approach showed that a highly conductive PBI membrane could be obtained by the addition of sidechains containing charged moieties such as sulfonic acid5. However, this improved conductivity comes at a price, specifically, a decrease in its vanadium barrier properties due to a significant crossover of vanadium ions, thus losing one of the main benefits of PBI type membranes5.The work presented herein describes the preparation of multiple PBI derivatives containing small and readily available non-charged sidechains, such as alkylated or benzylated PBI. Herein, it was envisaged that the addition of non-charged sidechains to the PBI backbone could increase the free volume of the membrane and thus enable the incorporation of electrolyte into the film without significantly affecting its repelling effect for vanadium ions. With this in mind, the influence of these non-charged sidechains on the ex-situ properties under VRFB conditions was studied in terms of dimensional swelling, acid uptake, vanadium barrier properties and mechanical strength. Hereby, key differences in the behavior of the membranes were observed between each individual type of functionalization, such as swelling, uptake and composition in acidic media. Subsequently, the correlation between the ex-situ properties of the PBI derivatives and in-situ VRFB cycling performance was studied to further understand their significance. Through this process, valuable knowledge was obtained that showed a clear correlation between the ex-situ properties and the observed in-situ area specific resistance, energy efficiency and capacity fading of the various materials, with the best material outperforming Nafion® 212 in terms of energy efficiency by 1% at a current density of 200 mA∙cm2, a current density regime that is normally dominated by cation exchange membranes due to their low resistance. Thereby proving that even a small modification of the PBI backbone can have a significant influence on the performance in a VRFB cell. Yuan, Z.; Duan, Y.; Zhang, H.; Li, X.; Zhang, H.; Vankelecom, I., Advanced porous membranes with ultra-high selectivity and stability for vanadium flow batteries. Energy & Environmental Science 2016, 9 (2), 441-447. Chae, I. S.; Luo, T.; Moon, G. H.; Ogieglo, W.; Kang, Y. S.; Wessling, M., Ultra-High Proton/Vanadium Selectivity for Hydrophobic Polymer Membranes with Intrinsic Nanopores for Redox Flow Battery. Advanced Energy Materials 2016, 6 (16), 1600517. Gubler, L.; Vonlanthen, D.; Schneider, A.; Oldenburg, F. J., Composite Membranes Containing a Porous Separator and a Polybenzimidazole Thin Film for Vanadium Redox Flow Batteries. Journal of The Electrochemical Society 2020, 167 (10), 100502. Noh, C.; Serhiichuk, D.; Malikah, N.; Kwon, Y.; Henkensmeier, D., Optimizing the performance of meta-polybenzimidazole membranes in vanadium redox flow batteries by adding an alkaline pre-swelling step. Chemical Engineering Journal 2020, 126574. Yan, X.; Dong, Z.; Di, M.; Hu, L.; Zhang, C.; Pan, Y.; Zhang, N.; Jiang, X.; Wu, X.; Wang, J.; He, G., A highly proton-conductive and vanadium-rejected long-side-chain sulfonated polybenzimidazole membrane for redox flow battery. Journal of Membrane Science 2020, 596, 117616.

Development of an Additional Selective Layer to Mitigate Crossover in Vanadium Redox Flow Batteries: Influence of Composition on Efficiency and Capacity Decay

Vanadium redox flow battery (VRFB) is a promising technology for energy storage because of its independent energy to power ratio and long cycle life. However, VRFB commercialization is still hindered by some technological issues, among which the capacity loss induced by the undesired transport of ions across the ion-exchange membrane. Depending on the nominal operating condition, the choice of a suitable membrane for VRFB application results from the trade-off between low vanadium ions permeability and high proton conductivity. Usually, in order to reduce undesired ion fluxes between the two half-cell electrolytes, the membrane thickness is relatively high, implying increased ohmic losses (i.e., reduced energy efficiency) and system capital cost. In fact, state of the art membranes can represent up to 50% of stack capital cost [1]. Nafion® is widely used due to its high conductivity, but it is not ideally selective towards vanadium ions, leading to the adoption of thicker membrane to limit capacity loss. Alternative cation exchange membranes like SPEEK or SPI are promising because of their reduced permeability, but the low conductivity limits system power density. Instead, anion exchange membranes are still limited by the poor chemical stability and low conductivity [2].In a recent work [3], the authors demonstrated the proof of concept of an additional selective layer to mitigate vanadium crossover. The selective layer, termed as barrier, is a porous component in which pores size, tortuous path, thickness and composition are designed to improve ion/proton selectivity. The proof of concept was manufactured with reactive spray deposition technology (RSDT), which is a flame-based synthesis process unique to Dr. Radenka Maric’s research group. For the fabrication of the proof of concept, carbon-rich particles ∼4-10 nm in diameter were formed in the RSDT flame and were deposited directly onto Nafion® 212 (50 μm thick) simultaneously with a mixture of 1100EW Nafion® and Vulcan® XC-72R (∼40 nm diameter) that was sprayed from air-assisted secondary nozzles. The presence of the barrier layer significantly reduced battery self-discharge, as reported in [3].In this work, different compositions and morphologies of the barrier layers were analysed in order to improve ion/proton selectivity. In particular, the influence of ionomer to carbon ratio (I/C), the amount of carbon-rich particles and the introduction of silica were investigated. Moreover, the effect of Nafion® 211 (25 μm thick) as a support for barrier deposition was also evaluated. The barrier layers were characterized in a 25 cm2 cell [4] equipped with reference electrodes at both positive and negative electrode in order to monitor the corresponding electrolyte potential and get an insight into battery state of charge (SoC). In addition to electrochemical testing, the structure of the barrier layer was characterized using TEM and SEM.The most suitable I/C ratio was found to be included between 1 and 2, while the presence of carbon-rich particles significantly contributed to crossover reduction, with a minor impact on proton conductivity. Also the introduction of silica nanoparticles from primary nozzle was effective for vanadium ion selectivity, in particular when the amount of carbon from the secondary nozzle is reduced. In fact, the most promising barrier layer resulted the one composed by only silica and ionomer. This layer was also deposited on Nafion® 211 (Figure 1), exhibiting an excellent trade-off between ion selectivity and proton conductivity. This barrier was proved to be stable over 1,000 cycles, presenting a stable coulombic efficiency of 99.5%, with an average capacity decay of 0.08%/cycle at 100 mA cm-2. Figure 1 – SEM image of only Silica barrier deposited on Nafion® 211.