Alkyl Spacer Grafted ABPBI Membranes with Enhanced Acid-Absorption Capabilities for Use in Vanadium Redox Flow Batteries

Abstract

Achieving high proton conductivity, low vanadium ion permeability, and high chemical stability using a single material remains a key challenge for hydrocarbon-based membranes for use in vanadium redox flow batteries (VRFBs). Herein, we report amorphous poly(2,5benzimidazole) (ABPBI) membranes with alkyl spacers which can meet these requirements. Spacer-grafted poly(2,5-benzimidazole)s (ABPBIs) with different grafting ratios were synthesized via an N-substitution reaction and denoted as ABPBI-0, -10, -25, -40, and -50, respectively. The structure to battery performance relationship was investigated through spectroscopic and electrochemical analysis. As the grafting ratio increased, the microstructure of the ABPBI derivative membranes was transformed from a semicrystalline to an amorphous structure because of reduced chain packing, increasing H 2 SO 4 -absorption capability and consequently reducing area-specific resistances. Also, they exhibited lower vanadium permeabilities compared with Nafion 115 owing to Donnan exclusion effect. As a combined effect of these characteristics, the membranes outperformed Nafion 115 membrane with notably high coulomb efficiencies and energy efficiencies. Furthermore, a grafted ABPBI membrane showed stable battery cycling performance more than 500 cycles at 200 mAcm −2 , and 1000 h continuous operation at 1C-rate, demonstrating excellent chemical stability against highly oxidizing VO 2 + solution.

Design of polybenzimidazolium membranes for use in vanadium redox flow batteries

“…These strategies can be divided into three main categories: (i) the reduction of membrane thickness, resulting in the need of porous materials providing mechanical support to the PBI skin layer, 19–22 (ii) the increase of porosity and/or free volume of the polybenzimidazole membrane, 23–26 thereby increasing the swelling and uptake characteristics of the membrane, and lastly (iii) the chemical modification of the benzimidazole core, providing an improved ionic conductivity and chain separation of the polymer. 27–31 In the latter, the typical approach is to introduce ionically charged pendant chains to the polymer backbone that form the basis of charged channels within the membrane. Doing so, Yan et al introduced a sulfonic acid-terminated sidechain onto a diphenylether–PBI backbone, thereby creating proton conducting moieties and enabling the formation of a 55 μm thick PBI based membrane that outperforms Nafion™ NR212 in both energy efficiency and capacity retention.…”

Alkali-stable polybenzimidazole anion exchange membranes tethered with N,N-dimethylpiperidinium cations for dilute aqueous KOH fed water electrolyzers

“…To this end, we here report on PBI membranes functionalized with piperidinium side groups for dilute solution-fed AEMWEs. The introduction of cationic side chains to form AEMs is expected to improve ionic clustering, as well as increase the free volume in the PBI material, 16–20 thus possibly enabling an increased absorption of electrolyte solution, especially at dilute concentrations. In the present study, two different cationic haloalkyl compounds were synthesized and tethered to PBI.…”

Tuning Polybenzimidazole‐Derived Crosslinked Interpenetrating Network Membranes for Vanadium Redox Flow Batteries

“…[6] Additionally it should be chemically and mechanically stable and have low cost. [7] The redox-active vanadium species and hydrated protons are both positively charged, which means that the selectivity of the IEM needs to be controlled by means of size sieving. [8] Perfluorosulfonic acid (PFSA) membranes currently used for proton transport suffer from high cost and crossover of the positively charged vanadium ions.…”

“…The development of PBI membrane for VRFB applications has focused on exploration of different backbone chemistries, [7,11,13] pretreatment methods, [4,14] polymer blends, [15][16][17][18] and post-functionalization strategies [19][20][21] to balance ionic conductivity and undesired crossover. As reviewed by Chen et al [22] the area-specific resistance, permeability and energy efficiency are parameters that can be tuned by modifying the chemical structure and composition of the membrane.…”

“…Most of the modified PBI membranes reported have an area specific resistance from 0.2 Ω cm 2 to 0.5 Ω cm 2 with some notable exceptions. Jang et al [7] grafted alkyl groups levels on AB-PBI by N-coupling at different grafting. Because the alkyl groups act as spacers, the morphology of the polymer is more amorphous, which increases the electrolyte uptake.…”