Vanadium Flow Battery Application Research Articles

High Performance Vanadium Redox Flow Battery Electrodes

Abstract A high temperature ammonia treatment was applied to carbon felt electrodes to enhance vanadium redox flow battery (VRFB) performance. Samples were heated to 900 °C in the presence of ammonia gas for up to 4 h. While all heating times resulted in an overall improvement in current density at 80% voltage efficiency, samples treated for 4 h showed the greatest increase in current density (325%) compared to untreated carbon felt. Raman spectroscopy showed a 74% increase in edge sites as a result of the 4 h treatment. Electrochemical surface area increased by 142% and scanning electron microscopy showed the appearance of pores on felt fiber surfaces, indicating that the performance improvement may be due to enhanced surface area in addition to functionalization. Impedance spectroscopy showed decreased charge transfer resistance and increased durability (during cycling) compared to other published electrode treatments. These results indicate that heated ammonia can be used to increase the performance of electrodes for vanadium flow battery applications, with excellent durability.

Sub‐2‐nm Channels within Covalent Triazine Framework Enable Fast Proton‐Selective Transport in Flow Battery Membrane

AbstractIon conductive membranes (ICMs) with robust sub‐2‐nm channels show high proton transport rate in flow battery, but it remains a great challenge to precisely control the ion sieving of the membranes. Herein, as a promising proton‐selective carrier, sulfonated piperazine covalent triazine framework (s‐pCTF) with the channel size of ≈1.5 nm and abundant fast proton hopping sites is introduced into sulfonated poly(ether ether ketone) (SPEEK) to fabricate advanced ICM for vanadium flow battery (VFB) application. The interior protoplasmic channels of s‐pCTF demonstrate significant Donnan exclusion effect, resulting in a high proton/vanadium ion selectivity in theory (6.22 × 105). Meanwhile, the nitrogen‐rich sub‐2‐nm channels yield fast proton highway, and exterior‐grafted sulfonic acid groups further facilitate the proton transfer. By regulating the ion sieving and proton conductivity, the optimal hybrid membrane exhibits synchronously improved battery performance with an enhanced energy efficiency (92.41% to 78.53% at 40–200 mA cm−2) and long‐term stability for 900 cycles over 400 h (EE: 87.2–85% at 120 mA cm−2), outperforming pure SPEEK and Nafion212 membranes. This study validates the applicability of organic porous CTF with sub‐2‐nm channels and desired functionality in ICMs for high‐performance VFB application.

Improvement of proton conductivity and efficiency of SPEEK-based composite membrane influenced by dual-sulfonated flexible comb-like polymers for vanadium flow battery

A sulfonated poly(ether ether ketone)-based composite membrane (S/SMA-SN) is developed by introducing dual-sulfonated flexible comb-like polymer (SMA-SN), varying from 0.1 to 1.5 wt%, which was prepared from the poly(styrene-co-maleic anhydride)-ethylenediamine through a main-/side-chain two-step sulfonation procedure. The S/SMA-SN composite membranes demonstrate ultrahigh proton conductivity and excellent cell performance at ultralow contents of SMA-SN polymers. Synergistic main-/side-chain proton channels formed by SPEEK and SMA-SN offer the highest proton conductivity (49.2 mS cm-1) and the lowest vanadium ion permeability (19.6 × 10-7 cm2 min-1) for S/SMA-SN-0.5, which is superior to SPEEK and Nafion membranes. Moreover, S/SMA-SN-0.5 displays its remarkable energy efficiency (72–81.3%) at 100–200 mA cm-2 current density, further illustrating the tradeoff effect of conductivity and resistance. Even at 150 mA cm-2 current density, stable coulombic efficiency (97%) and energy efficiency (80%) maintained after 500 charge-discharge cycles confirms the stable membrane structure and increased chemical stability, which originated from the main-/side-chain synergistic structure. This opens a promising method to explore the high-performance proton exchange membrane for vanadium flow battery application by the side-chain structuralized comb-like polymer at ultralow concentrations.

An ultrahigh conductivity and efficiency of SPEEK-based hybrid proton exchange membrane containing amphoteric GO-VIPS nanofillers for vanadium flow battery

Sulfonated poly(ether ether ketone) (SPEEK)-based hybrid membranes (S/GO-VIPS) have been fabricated by the solution blending method, which amphoteric GO-VIPS nanofillers prepared by grafting 3-(1-vinyl-3-imidazole) propane sulfonate (VIPS) onto graphene oxide (GO) flake. With various GO-VIPS contents changing from 1 to 10 wt% in the S/GO-VIPS membranes, the interfacial interaction between SPEEK and GO-VIPS contributes to enhanced membrane structures and performance. Against pristine SPEEK (3.9 × 103 S min cm−3) and Nafion 117 (4.0 × 103 S min cm−3), S/GO-VIPS-5 gives the highest ion selectivity at 24.4 × 103 S min cm−3, demonstrating the amphoteric GO-VIPS nanosheets afford a superior vanadium ion resistance and proton conductivity. At 100–200 mA cm−2 current density, S/GO-VIPS-5 presents excellent coulombic efficiency (97.4–99%), energy efficiency (78.9–66.3%), and self-discharge time (78 h), which is better than that of SPEEK and Nafion 117, respectively. Further cycling at 100 mA cm−2 for 350 times, its energy efficiency maintains at 80%, confirming its excellent structural stability and chemical stability. Combining with the physical barrier and the amphoteric acid-base functional groups of the GO-VIPS nanosheets, the increased proton conductivity and ion selectivity, and the decreased permeability of SPEEK-based membranes are well-regulated, demonstrating a promise in developing high-performance proton exchange membrane for vanadium flow battery applications.

Pyridine-extended proton sponge enabling high-performance membrane for flow batteries

The development of high-performance and low-cost membrane is of significance towards vanadium flow battery (VFB). The membrane usually suffer from a trade-off challenge between conductivity and selectivity especially in long-term operation of flow battery. Herein, using a proton sponge drived by the proton absorption capacity of extended pyridine groups, a highly ion-selective membrane is fabricated. During acid-assisted doping behaviors, transport nanochannels with highly proton-selective conductivity are constructed based on well-defined nanophase separation structure. The protonated nanochannels display effectively acidic absorption during hydration process for enabling fast proton conduction (a low area resistance of 0.33 Ω cm2) and repelling vanadium ion (a low vanadium ion permeability of 3.04 × 10−10 cm2 s−1). Consequently, prepared membrane with proton sponge exhibits remarkable performance of high energy efficiency of 85.5%, together with excellent chemical durability over 1000 charge-discharge cycles (more than 1100 h) at 120 mA cm−2 in VFB application. These notable properties make prepared membrane as a potential alternative used in large-scale energy storage systems.

Swelling-Induced Quaternized Anthrone-Containing Poly(aryl ether ketone) Membranes with Low Area Resistance and High Ion Selectivity for Vanadium Flow Batteries.

A vanadium flow battery (VFB) is one of the most promising electrochemical energy storage technologies. However, membranes for VFBs still suffer from high cost or low conductivity and poor stability. Here, we report new quaternized anthrone-containing poly(aryl ether ketone) (QAnPEK) membranes for VFBs. QAnPEK membranes with moderate ion exchange capacity (1.26 mmol g-1) were swelling-induced in H3PO4 (50 wt %) to form wider ion transport pathways that significantly enhanced membrane conductivity (e.g., 0.49 Ω cm2 for the QAnPEK-virgin membrane and 0.12 Ω cm2 for the swelling-induced QAnPEK-90 membrane). The bulky rigid anthrone-containing backbone provided high swelling resistance and enabled QAnPEK membranes to have high ion selectivity. As a result, QAnPEK membranes displayed low area resistance, high ion selectivity, and robust mechanical strength. The QAnPEK-90 membrane yielded excellent energy efficiencies (92.4% at 80 mA cm-2, 85.1% at 200 mA cm-2, and 80.3% at 280 mA cm-2). Moreover, QAnPEK membranes exhibited outstanding in situ and ex situ stability, for example, the VFB with the QAnPEK-40 membrane demonstrated highly stable battery performance for 3000 cycles at 160 mA cm-2. QAnPEK membranes are attractive candidates for VFB application.

Reconstructing proton channels via Zr-MOFs realizes highly ion-selective and proton-conductive SPEEK-based hybrid membrane for vanadium flow battery

There is an urgent need to break through the trade-off between proton conductivity and ion selectivity of proton exchange membrane (PEM) in vanadium flow battery (VFB). Proton channels in PEM are the key to controlling the ion sieving and proton conductivity in VFB. Herein, two types of proton channels are reconstructed in the hybrid membrane via introducing modified Zr-MOFs (IM-UIO-66-AS) into SPEEK matrix. Internal proton channels in IM-UIO-66-AS and interfacial proton channels between grafted imidazole groups on Zr-MOFs and SPEEK greatly improve the conductivity of the IM-UIO-66-AS/SPEEK hybrid membrane. More importantly, both reconstructed proton channels block the vanadium-ion permeation to realize enhanced ion selectivity according to the size sieving and Donnan exclusion effects, respectively. Moreover, the hybrid membrane exhibits good mechanical property and dimensional stability. Benefiting from such rational design, a VFB loading with the optimized membrane exhibits enhanced voltage efficiency of 79.9% and outstanding energy efficiency of 79.6% at 200 mA cm−2, and keeps a notable cycle stability for 300 cycles in the long-term cycling test. Therefore, this study provides inspiration for preparing next-generation PEMs with high ion selectivity and proton conductivity for VFB application.

(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

Pyridinium functionalized poly(phthalazinone ether ketone) with pendant phenyl groups porous membranes for vanadium flow battery application by vapor induced phase separation

A novel porous membrane based on pyridinium-functionalized poly(phthalazinone ether ketone) with pendant phenyl groups (PyPPEK-P) is synthesized from poly(phthalazinone ether ketone) with benzyl chloride moieties by the vapor induced phase separation (VIPS) process. The membrane is used for vanadium redox flow batteries (VRFB). Pyridine is used as an amination reagent to improve the chemical stability of the membrane, and its successful introduction is experimentally confirmed. The structure and properties of the porous membrane are adjusted by VIPS to enhance the membrane performance when used in batteries. It is found that a lower temperature and a longer exposure time are beneficial to obtaining a large pore size in cross-section, while relative humidity exert a complex effect on the membrane structure. The PyPPEK-P membrane prepared by mixed NMP and DMAc solvents as casting solution exhibit the best battery performance. Compared with Nafion 115, the PyPPEK-P membrane display higher coulombic efficiency (97.2% vs. 96.8%), voltage efficiency (83.1% vs. 76.9%), and energy efficiency (81.0% vs. 76.8%) at a 140 mA cm−2 current density. After 1000 charge-discharge cycles, the PyPPEK-P membrane exhibit a low capacity-decay, while the battery performance do not decline. These observations indicate that PyPPEK-P porous membranes have good application prospects in VRFB.

Robust Adamantane-Based Membranes with Enhanced Conductivity for Vanadium Flow Battery Application.

Membranes with high conductivity, high selectivity, and high stability are urgently needed for high-power-density vanadium flow batteries (VFBs). Enhancing membrane conductivity presents many challenges, often resulting in sacrificing membrane selectivity and mechanical strength. To overcome this, new robust adamantane-based membranes with enhanced conductivity are constructed for VFB. Low-content basic piperazine (IEC = 0.78 mmol g−1) and hydrophilic hydroxyl groups are introduced into highly rigid, hydrophobic adamantane containing poly(aryl ether ketone) backbone (PAPEK) and then selectively swelled to induce microphase separation and form ion transport pathways. The highly rigid and hydrophobic PAPEK exhibits high swelling resistance and provides the membranes with slight swelling, high selectivity, and high mechanical strength. The selective swelling temperature has a significant influence on the areal resistance of the resulting membrane, e.g., the PAPEK-130 membrane, when selectively swelled at 130 °C, has low areal resistance (0.22 Ω∙cm2), which is approximately two-fifths that of the PAEKK-60 membrane (treated at 60 °C, 0.57 Ω∙cm2). Consequently, the resulting PAPEK membranes exhibit low swelling, high selectivity, and low areal resistance, with the VFB constructed with a PAPEK-90 membrane exhibiting excellent energy efficiency (91.7%, at 80 mA∙cm−2, and 80.0% at 240 mA∙cm−2) and stable cycling performance for 2000 cycles.

High performance positively charged membranes with selective swelling-induced ion transport channels for vanadium flow battery application

The selective swelling technique is adopted for the first time to construct positively charged membranes with selective swelling-induced ion transport channels for vanadium redox flow battery (VRFB). These membranes are prepared based on pyridine and adamantane groups containing poly(aryl ether ketone ketone) (PyAPEKK) polymer. Polymer chains with pyridine groups are selective swelled to form ion transport channels in the polymer membrane using the base-acid interaction between the pyridine groups and mineral acids (H3PO4 and H2SO4). The bulky, rigid, hydrophobic adamantane groups in the polymer increase the rigidity and hydrophobicity of the polymer backbone that reduce the swelling of the membrane and enhance the membrane selectivity. Consequently, a VRFB utilizing this PyAPEKK membrane exhibit impressive battery performance (CE = 98.9%, EE = 92.3% at 80 mA cm−2; CE = 99.3%, EE = 80.56% at 240 mAcm−2). The PyAPEKK membranes exhibited stable VRFB performance for more than 1500 cycles and exhibited excellent chemical stability in ex-situ stability tests. This work demonstrated a convenient, facile, and environmental-friendly way to fabricate advanced positively charged membranes for VRFB.

Poly(arylene ether sulfone) Membrane Crosslinked with Bi‐Guanidinium for Vanadium Flow Battery Applications

AbstractA membrane with low vanadium ion permeability and excellent electrochemical stability is crucial for a vanadium flow battery (VFB). Herein, a highly stable anion exchange membrane based on a bi‐guanidinium crosslinking quaternized poly(arylene ether sulfone) (PSF) is reported. The positively charged guanidinium groups can not only form continuous and smooth ion transport channels but also prevent the mutual penetration of vanadium electrolyte at positive and negative sides, respectively, based on the Donnan exclusion effect. Moreover, the crosslinking structure endows the membrane high mechanical strength to meet the requirements of the harsh environment of VFBs. As a result, the anion exchange membrane fabricated by the poly(arylene ether sulfone) crosslinked with bi‐guanidinium (BG‐PSF) shows an impressive VFB performance with a coulombic efficiency of 99.8% and an energy efficiency of above 85.8% at a current density of 80 mA cm−2, which is much higher than that of Nafion115. Furthermore, this battery can continuously work for over 200 cycles at a current density of 80 mA cm−2, indicating good stability of the membrane. Therefore, the rationally designed BG‐PSFs are promising for VFB applications.

Advanced anion exchange membranes with selective swelling-induced ion transport channels for vanadium flow battery application

Anion exchange membranes with attractive advantages are highly promising for vanadium flow battery (VRFB) applications, but high area resistance limits their uses in high-power-density VRFB. Increasing the ion exchange capacity (IEC) is long considered as the primary choice to enhance the membrane conductivity, which unfortunately results in highly swelling and much sacrifice of the membrane selectivity and stability. In this work, new quaternized phenyl and adamantane containing poly(aryl ether ketone ketone) (QPAPEKK) membranes with selective swelling-induced ion transport channels are prepared. QPAPEKK membranes with much low IEC (0.62 mmol g−1) are selective swelled with H3PO4 to form ion transport channels. Consequently, the area resistance of resulting QPAPEKK-110 membrane (0.24 Ω cm2) is nearly one-fifth of QPAEKK-virgin membrane (1.21 Ω cm2). Moreover, VRFB utilizing QPAPEKK-90 membrane exhibit impressive battery performance (energy efficiency 91.4% at 80 mAcm−2; energy efficiency 80.0% at 220 mAcm−2). The QPAPEKK membranes combined high battery performance and high chemical stability are promising for high-power density VRFB application.

Highly Active Ag Nanoparticle Electrocatalysts toward V2+/V3+ Redox Reaction

A high power density vanadium flow battery (VFB) is essential for reducing the cost of a VFB system. The power density of a VFB is currently limited by the negative half-cell since the negative V2+/V3+ reaction has sluggish reaction kinetics and high activation overpotential. In this work, a silver (Ag) nanoparticle-decorated graphite felt (GF) was designed and prepared by the electrodeposit method for VFB applications. By regulating the deposition current density and loading amount of metallic Ag, the Ag nanoparticles with a size of 30–80 nm are evenly deposited on the carbon fibers. Combining high electrical conductivity and high electrocatalytic activity of Ag nanoparticles, the activation energy for the V2+/V3+ redox reaction is reduced dramatically. Thus, the reaction kinetics of the V2+/V3+ redox couple is accelerated, and a very low charge-transfer resistance of only 1.743 Ω is obtained for GF-Ag-3, which is much lower than that of pristine GF (525 Ω). Accordingly, the voltage efficiency and energy efficiency with GF-Ag-3 can reach 85 and 82% in a VFB single cell, respectively, at a current density of 200 mA cm–2. This paper shows an effective and simple way to prepare a highly effective electrocatalyst toward the V2+/V3+ redox reaction.

Advanced Membranes with Selective Swelling-Induced Ion Transport Channels Constructed from Hydroxyl and Piperazine for Vanadium Flow Battery Application

Advanced Membranes with Selective Swelling-Induced Ion Transport Channels Constructed from Hydroxyl and Piperazine for Vanadium Flow Battery Application

Modifying Carbon Felt Electrodes Using High-Temperature Ammonia Treatment to Improve the Performance of All-Vanadium Redox Flow Batteries

High temperature ammonia treatments 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 intervention, carbon felt has poor wettability which frustrates electrochemical activity. This material must be modified to make it more effective, which has been historically accomplished with thermal treatments to promote oxygen functional groups. [1] 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. [2-4] Preliminary experiments on carbon felt performed by this lab with ammonia at high temperatures have shown promising results. [5]For our experiments, commercially available carbon felts (SIGRACELL® GFD3 by SGL Carbon, Meitingen, Germany) were modified by exposure to flowing ammonia gas through a furnace at 900°C for varying lengths of time. 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. A pitting effect was observed in SEM images, preceded by surface blistering at shorter lengths of exposure.Greatly increased active surface area, along with a substantial increase in cell performance, was achieved with the ammonia-modified carbon felts described in this work. 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.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.Pezeshki, A.M., "Impedance-Resolved Performance and Durability in Redox Flow Batteries. " PhD diss., University of Tennessee, 2016. Figure 1

Facile fabrication of sulfonated poly(aryl ether sulfone)/polybenzoxazine crosslinked membrane for vanadium flow battery application

A novel series of sulfonated poly(aryl ether sulfone) (SPAES)/polybenzoxazine (PBa) crosslinked membranes applied in vanadium redox flow battery (VRFB) are prepared and characterized. The introduction of PBa with vinyl groups by benzoxazine (Ba) ring-opening polymerization has a significant impact on the physical and chemical properties of the crosslinked membranes. Compared to pristine SPAES membrane, the crosslinked membranes exhibit a better thermal, dimension and chemical stability owing to the three-dimensional network structure and the hydrophobic segments of PBa. In addition, the narrow ion channel caused by low swelling ratio and protonated nitrogen atoms of SPBa can hamper the vanadium permeation immensely. As expected, the SPBa-33 membrane shows an extremely low vanadium permeability of 1.49 × 10−7 cm2 min−1. Moreover, the proton selectivity of SPBa-33 membrane (8.19 × 104 S min cm−3) is nearly four times higher than that of Nafion117 (2.32 × 104 S min cm−3).

Polybenzimidazole membrane with dual proton transport channels for vanadium flow battery applications

Polybenzimidazole membranes with dual proton transport channels (B-PBI) are designed and fabricated for vanadium flow battery (VFB) applications. The dual channels are established by introducing pyridine groups (channel-1) in the PBI chain (B-PBI), which contains imidazole rings (channel-2). The proton transport channels can endow the B-PBI membrane with dramatically enhanced proton conductivity, while the repulsion between the positively charged pyridine and imidazole nitrogens after doping with protons and the vanadium ions can afford the membrane with excellent ion selectivity. Hence, the prepared B-PBI membrane delivers an excellent performance in terms of high proton conductivity, ion selectivity and chemical stability. A VFB with a B-PBI membrane exhibits a coulombic efficiency (CE) of 99.16% and a voltage efficiency (VE) of 88.86% at a current density of 80 mA cm−2, which is much higher than does a battery with a pristine PBI membrane (CE of 98.54%, VE of 84.10%) and with a Nafion 115 membrane (CE of 95.13%, VE of 87.05%). The chemical stability of the B-PBI is confirmed by a stable performance of 600 cycles at 160 mA cm−2 and the ex-situ oxidation stability test. These results suggest that the B-PBI membrane with dual proton transport channels can be served as promising candidate for vanadium flow battery.

Cyclodextrin templated nanoporous anion exchange membrane for vanadium flow battery application

A β-cyclodextrin (β-CD) based template method is proposed for fabricating high performance nanoporous anion exchange membranes for vanadium flow batteries (VFBs). The β-CD template can be uniformly dispersed in hydroxyl modified polysulfone precursor membrane with a low degree of quaternization; removal of the template will result in the formation of AEMs with well controlled pore structure. This new template method is advantageous due to the hydrogen bonding between the β-CD and polymer so that aggregation of β-CD in the membrane can be minimized. The well dispersed β-CD is beneficial for fabricating porous AEM with small pores, and further for selective transport of protons and anions in the battery. The fabricated porous AEM has low IEC, high conductivity and low vanadium ions permeability. The VFB employing such a membrane showed a coulombic efficiency of 99% and an energy efficiency of 80% at 120 mA cm-2. This work demonstrates the synergistic effect of nanopore and imidazolium cation for high performance VFB membrane fabrication.

(Invited) RKP Vanadium Flow Battery and its Application

Vanadium flow battery(VFB) is deemed to be one of the most promising technologies for large-scale industrial energy storage, due to its attractive features like flexible capacity, active thermal management, long cycle life and high safety. Dalian Rongke Power Co., Ltd. (RKP) has been devoted to the fundmental study and advanced research of Vanadium flow battery applications. It has become one of the world-leading enterprises in VFB development, manufacturing and delivery industry. The products have evolved from earlier indoor structure to the latest containerized system designs. More than a dozen VRFB systems have been deployed and end products have been launched into operations across the world over the past decades. The 500kW/2MWh VFB product was developed for the large scale energy application, which is consisted of 16 cell stacks. The high integration level, low shunt current loss, low auxiliary power supply consumption and high reliability were achieved. The novel cell stack is designed by optimizing the key materials and cell stack structure, which can achieve a better performance and higher reliability at a relatively low cost. The energy efficiency(EE) could reach 80% at the rated power of 33kW, and the current density could be improved to 140~150 mA/cm2.The cycle life test indicated that no obvious performance decay was found even after 10000 cycles. The advanced two-stage power conversion system (PCS) was selected in order to realiaze the independent control of each sub-systems, which would improve the overall reliability ultimately. The AC/AC energy efficiency could reach 70% at the rated power of 500kW. The 500kW/2MWh VFB product has been installed in power plants for wind and solar energy storage and has obtained better application effect. The world largest battery system, a 200MW/800MWh VFB peaking power plant conducted by RKP, is currently in progress in Dalian, China. The purpose is to improve the Northeast grid resiliency and facilitate smooth integration of renewables. A 500kW/2MWh VFB product is the minimum storage unit of the project. Each unit product could be controled independently. The foreseeable success of the project would exert great influence on the development of the flow battery in the future. In this presentation, new technolgy and product developments of vanadium flow battery at RKP will be introduced and further challenges will be addressed as well.