Blending polybenzimidazole with an anion exchange polymer increases the efficiency of vanadium redox flow batteries

The Critical Analysis of Membranes toward Sustainable and Efficient Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFB) are a promising technology for large-scale storage of electrical energy, combining safety, high capacity, ease of scalability, and prolonged durability; features which have triggered their early commercial implementation. Furthering the deployment of VRFB technologies requires addressing challenges associated to a pivotal component: the membrane. Examples include vanadium crossover, insufficient conductivity, escalated costs, and sustainability concerns related to the widespread adoption of perfluoroalkyl-based membranes, e.g., perfluorosulfonic acid (PFSA). Herein, recent advances in high-performance and sustainable membranes for VRFB, offering insights into prospective research directions to overcome these challenges, are reviewed. The analysis reveals the disparities and trade-offs between performance advances enabled by PFSA membranes and composites, and the lack of sustainability in their final applications. The potential of PFSA-free membranes and present strategies to enhance their performance are discussed. This study delves into vital membrane parameters to enhance battery performance, suggesting protocols and design strategies to achieve high-performance and sustainable VRFB membranes.

Membrane Assemblies with Soft Protective Layers: Dense and Gel-Type Polybenzimidazole Membranes and Their Use in Vanadium Redox Flow Batteries

Polybenzimidazole (PBI) membranes show excellent chemical stability and low vanadium crossover in vanadium redox flow batteries (VRFBs), but their high resistance is challenging. This work introduces a concept, membrane assemblies of a highly selective 2 µm thin PBI membrane between two 60 µm thick highly conductive PBI gel membranes, which act as soft protective layers against external mechanical forces and astray carbon fibers from the electrode. The soft layers are produced by casting phosphoric acid solutions of commercial PBI powder into membranes and exchanging the absorbed acid into sulfuric acid. A conductivity of 565 mS cm^-1 is achieved. A stability test indicates that gel mPBI and dense PBI-OO have higher stability than dense mPBI and dense py-PBI, and gel/PBI-OO/gel is successfully tested for 1070 cycles (ca. 1000 h) at 100 mA cm^-2 in the VRFB. The initial energy efficiency (EE) for the first 50 cycles is 90.5 ± 0.2%, and after a power outage stabilized at 86.3 ± 0.5% for the following 500 cycles. The initial EE is one of the highest published so far, and the materials cost for a membrane assembly is 12.35 U.S. dollars at a production volume of 5000 m² , which makes these membranes very attractive for commercialization.

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.

Synthesis of Anion Exchange Membranes Containing PVDF/PES and Either PEI or Fumion®

In this work, the preparation of dense blended membranes, from blends of poly(vinylidene fluoride) (PVDF), poly(ether sulfone) (PES) and polyethyleneimine (PEI) or Fumion^®, with possible applications in alkaline fuel cell (AEMFC) is reported. The blended PEI/Fumion^® membranes were prepared under a controlled air atmosphere by a solvent evaporation method, and were characterized regarding water uptake, swelling ratio, thermogravimetric analysis (TGA), infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), ion exchange capacity (IEC), OH^- conductivity and novel hydroxide ion exchange rate (HIER), which is related to the mass transport capacity of the OH^- ions through the membrane. The effect of the chemical composition on its morphological and anion exchange properties was evaluated. It was expected that the usage of a commercial ionomer Fumion^®, in the blended membranes would result in better features in the electrical/ionic conductivity behaviour. However, two of the membranes containing PEI exhibited a higher HIER and OH^- conductivity than Fumion^® membranes, and were excellent option for potential applications in AEMFC, considering their performance and the cost of Fumion^®-based membranes.

Surface Modification of Polybenzimidazole (PBI) with Microwave Generated Vacuum Ultraviolet (VUV) Photo-Oxidation

Background:

Polybenzimidazole (PBI) is used in high temperature proton exchange membrane fuel cells (HT-PEMFCs) and redox flow batteries, where proton transfer occurs with the nitrogen-containing groups in PBI, and in aerospace applications exposed to oxygen and radiation.

Objective:

The objective is to investigate VUV photo-oxidation of PBI for the first time in order to incorporate polar functional groups on the surface to potentially enhance proton conductivity in HT-PEMFCs.

Methods:

A low-pressure microwave discharge of Ar generated 104.8 and 106.7 nm vacuum UV (VUV) radiation to treat PBI with VUV photo-oxidation. Analysis was done with X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), water contact angle (WCA) and Thermal Gravimetric Analysis (TGA) to detect changes in chemistry, surface roughness, hydrophilicity, and adhesion, respectively.

Results :

XPS showed: an increase in the O concentration up to a saturation level of 15 ± 1 at %; a decrease of the C concentration by about the same amount; and little change in the N concentration. With increasing treatment time, there were significant decreases in the concentrations of C-C sp2, C-C sp3 and C=N groups, and increases in the concentration of C=O, O-C=O, O-(C=O)-O, CN, and N-C=O containing moieties. The water contact angle decreased from 83° for pristine PBI down to 43°, making the surface more hydrophilic, primarily due to the oxidation, since AFM detected no significant changes in surface roughness. TGA analysis showed an improvement of water adhesion to the treated surface.

Conclusion:

Microwave generated VUV photo-oxidation is an effective technique for oxidizing the surface of PBI and increasing hydrophilicity.

A Chemistry and Microstructure Perspective on Ion-Conducting Membranes for Redox Flow Batteries

Redox flow batteries (RFBs) are among the most promising grid-scale energy storage technologies. However, the development of RFBs with high round-trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion-conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.

Performance and stability comparison of Aemion™ and Aemion+™ membranes for vanadium redox flow batteries

Anion exchange membranes (AEMs) have shown a significant rise in performance and durability within recent years for applications such as electrolysis and fuel cells. However, in vanadium redox-flow batteries, their use is of particular interest to lower costs and self-discharge rates compared to conventional perfluorinated sulfonic acid-based ionomers such as Nafion. In this work we evaluate the properties of two commercial AEMs, Aemion™ and Aemion+™, based on ex situ characterizations, an accelerated stress test degradation study (>1000 hours storage in highly oxidizing VO₂⁺ electrolyte at 35 °C) and electrochemical battery cycle tests. All membranes feature low ionic resistances of below 320 mΩ cm², enabling battery cycling at 100 mA cm⁻². Aemion shows considerable VO²⁺ formation within a VO₂⁺ stress test, whereas Aemion+ remains almost unaffected in the 1058 h stress test. Evaluating self-discharge data, cycling performance and durability data, Aemion+™ (50 μm thickness) features the best properties for vanadium redox-flow battery operation.

Cycling behaviour of Aemion™ (50 μm), Aemion+™ (50 μm), Aemion+™ (15 μm) and Nafion® 212 (50 μm) at 100 mA cm⁻². (a) Coulombic efficiency, (b) energy efficiency and (c) membrane resistance.

Sulfonated component-incorporated quaternized poly(phthalazinone ether ketone) membranes with improved ion selectivity, stability and water transport resistance in a vanadium redox flow battery

Novel poly(phthalazinone ether ketone)-based amphoteric ion exchange membranes with improved ion selectivity, stability and water transport resistance were prepared for vanadium redox flow battery (VRB) applications. The preparation method ensured the absence of electrostatic interaction. A small amount of sulfonated poly(phthalazinone ether ketone) (SPPEK) with different ion exchange capacity (IEC) values was mixed with brominated poly(phthalazinone ether ketone) (BPPEK) to prepare base membranes with the solution casting method, and they were aminated in trimethylamine to obtain the resulting membranes (Q/S-x, x represents the IEC value of SPPEK). Compared with the AEM counterpart (QBPPEK) prepared from the amination of the BPPEK membrane, Q/S-1.37 showed lower swelling ratio and area resistance (R). The R value of Q/S-1.37 (0.58 Ω cm²) was close to that of Nafion115. The VO²⁺ and V³⁺ permeability values of Q/S-x were 96.7–97.6% and 98.5–99.2% less than those of Nafion115, respectively, demonstrating the excellent ion selectivity of Q/S-x. Compared with Nafion115 and QBPPEK, Q/S-1.37 displayed 90.0% and 92.1% decrease in the static water transport volume and 93.2% and 66.7% decrease in the cycling transport rate, respectively, revealing good water transport resistance. Compared with Nafion115, Q/S-1.37 exhibited an increase of 1.0–5.7% in the coulombic efficiency (CE) and an increase of 2.5–8.7% in the energy efficiency (EE) at 20–200 mA cm⁻². Q/S-x showed better chemical stability in VO₂⁺ solutions than QBPPEK. VRB with Q/S-1.37 could be steadily operated for 400 h without sudden capacity and efficiency drop, while VRB with QBPPEK could hold for only around 250 h. Q/S-1.37 retained higher CE, EE and capacity retention than Nafion115, displaying good long-term stability. Thus, the Q/S-x are promising for use in commercial VRBs.

Novel AIEMs were prepared through successive blending and amination processes, and they exhibited good ion selectivity, stability and water transport resistance.