Highly Ion Selective Proton Exchange Membrane Based on Sulfonated Polybenzimidazoles for Iron–Chromium Redox Flow Battery

Abstract

The iron–chromium redox flow battery (ICRFB) has great potential for large-scale energy storage, due to its low capital cost of redox-active materials. However, the trade-off between conductivity and selectivity in the membranes limits its applications. Herein, a series of sulfonated polybenzimidazoles with exactly controlled sulfonation degree (SD) (S-PBI-x, x refers to SD) are designed and synthesized via direct copolymerization from the sulfonated monomer. Combined with the electrostatic repulsion of the formed imidazoliums, the S-PBIx membranes facilitate the proton transport and repel the redox-active ion crossover efficiently. Especially, when compared with Nafion 212 membrane, the S-PBI-100 membrane displays a comparable conductivity and more than an order of magnitude lower Fe3+ and Cr3+ permeabilities. Thus, a higher columbic efficiency (CE) of 98.2% and energy efficiency (EE) of 83.17% are achieved at 80 mA cm–2 for the corresponding ICRFB. Most importantly, no chemical degradation is observed for the S-PBI-100 membrane after in situ and ex situ stability tests.

Recent advances in non-perfluorinated sulfonic acid proton exchange membranes in the energy field

“…The outstanding electrochemical performance and stability of the composite membrane position it as a compelling alternative to the expensive Nafion membranes, demonstrating the potential for the development of more economical membrane materials. Wang et al 159 developed the S-PBI-100 membrane for iron–chromium redox flow batteries (ICRFBs). This membrane exhibited a minimum area resistance of 0.69 Ω cm −2 and excellent CE of 98.2% and EE of 83.17% at a current density of 80 mA cm −2 .…”

Review—Flow Batteries from 1879 to 2022 and Beyond

“…177 Although the interest in Cr-Fe RFB waned in the late 1980's (see Fig. H2), as vanadium RFBs were gaining popularity (see below), the 11 times lower cost of energy [178][179][180][181] at 250 $/kWh and 0.20 $/(kWh×cycle) 182 of chromium-iron compared to vanadium chemistry was a sufficient reason to pursue revival of Cr-Fe RFB around 2010. 179,[183][184][185][186][187] While some promising developments have been reported for Cr-Fe RFBs during this period, such as amelioration of the parasitic HER by pre-electrolyzing impurities, 188 by adding electrocatalysts, 189,190 as well as an increase in the open-circuit voltage by adding complexing ligands, 191 apparently, these did not improve the technical and commercial viability of Cr-Fe RFBs.…”

“…H2), as vanadium RFBs were gaining popularity (see below), the 11 times lower cost of energy [178][179][180][181] at 250 $/kWh and 0.20 $/(kWh×cycle) 182 of chromium-iron compared to vanadium chemistry was a sufficient reason to pursue revival of Cr-Fe RFB around 2010. 179,[183][184][185][186][187] While some promising developments have been reported for Cr-Fe RFBs during this period, such as amelioration of the parasitic HER by pre-electrolyzing impurities, 188 by adding electrocatalysts, 189,190 as well as an increase in the open-circuit voltage by adding complexing ligands, 191 apparently, these did not improve the technical and commercial viability of Cr-Fe RFBs. For example, a Silicon Valley startup EnerVault received $26.3M in funding between 2010 and 2012 alone, 192 (and over $35M in total between 2008-2013) 193 and had a ribbon-cutting ceremony for a 250 kW × 1 MWh energy storage facility at an almond farm in central California on 2014-05-22, before getting liquidated in 2015.…”