Bifunctional Ion-Conducting Polymer Electrolyte for the Vanadium Redox Flow Battery with High Selectivity

Abstract

The need for electrochemical energy storage increases along with the growing share of fluctuating renewables for power generation. The all-vanadium redox flow battery (VRB) has experienced increasing attention in this context. Its electrolyte membrane is a key component, yet currently perfluorinated materials (e.g., Nafion) are mainly used in VRBs, which have neither been designed for this particular application, nor are they cost-effective. The cross-over of redox active vanadium species impairs cell efficiency, in particular in case of thin Nafion membranes with low ohmic resistance. Here, we present a potentially generic concept of a bifunctional ion-conducting membrane prepared by radiation grafting with sulfonic acid proton exchange sites and amidoxime moieties. The crossover of vanadium-ions is reduced four-fold in the presence of amidoxime groups without markedly impairing the conductivity of the membrane. A VRB cell containing such membrane shows increased energy efficiency and negligible capacity fading over 122 charge / discharge cycles compared to cells with Nafion membrane, which showed a loss of discharge capacity of around 35% after 35 cycles and considerable electrolyte imbalance. The growing need for energy and urge to reduce CO 2 emissions require clean and renewable energy sources, such as wind and solar power.1-3 Since these primary energies are inherently intermittent, 4 the development of adequate energy storage scenarios is essential. 5,6Among the various energy storage concepts, batteries have been shown to be especially promising. 7,8 Batteries for grid-scale energy storage require good cycling stability, rapid response to changes, high roundtrip efficiency and reasonable lifecycle costs.Redox flow batteries (RFBs) can fulfill these requirements 9 to target applications in the MW to GW power scale with typical charge/ discharge times of 4 to 6 h. 10 Key advantages over conventional rechargeable grid-scale batteries, such as Na-S batteries, are the independent scalability of energy and power, inherent safety, deep discharge capability and low self-discharge. 7,11,12 A redox flow battery consists of two external tanks filled with liquid electrolyte and a stack of cells that contain porous electrodes separated by an ion exchange membrane (IEM). The IEM avoids cross-mixing of redoxactive species and allows the transport of background electrolyte ions. The all-vanadium redox flow battery (VRB) is the most advanced type.13 VRBs use the same redox-active element on the negative and positive electrode, preventing irreversible cross-contamination of the electrolytes.14 The membrane is a key cost driver in a VRB system and can account for the major share of the stack cost. 15 An ideal membrane for an RFB should have low crossover of redox-active species and water, low cost, high conductivity, and high chemical stability in the respective electrolyte solution. 16Depending on the nature of the exchange groups, IEMs are generally classified into cation and anion exchange membranes (CEMs, AEMs). Variou...

Amphoteric Ion‐Exchange Membranes with Significantly Improved Vanadium Barrier Properties for All‐Vanadium Redox Flow Batteries

“…In contrast to our previously reported amidoximated and sulfonated membranes, which showed amphoteric character only after being exposed to an acidic environment leading to protonation of the amidoxime groups (induced amphotericity), the functionalized VP‐based membranes reported here exhibit an intrinsic amphoteric nature owing to the presence of alkylated and, thus, positively charged vinylpyridinium units.…”

“…Whereas for a cell containing a CEM net volumetric transfer toward the positive half‐cell was observed, for cells with AEM and non‐ionic separators net water transport was in the opposite direction . To the best of our knowledge, the volumetric transfer in cells with AIEMs was reported for the first time in our previous work using amphoteric membranes with amidoxime and sulfonic acid groups …”

“…The volumetric imbalance in the cell containing amphoteric‐grafted membranes with vanadium‐blocking amidoxime and sulfonic acid groups was studied in our previous work . As a result of significantly suppressed vanadium crossover in the cells with amidoximated membranes, much less pronounced capacity fading and volumetric imbalance of around 11 % was determined.…”

“…In addition, despite of reduced vanadium permeability, AEMs suffer from lower conductivities compared to CEMs . Furthermore, amphoteric ion‐exchange membranes (AIEMs), which combine advantages of AEMs and CEMs, have been shown to be promising for VRB applications …”

“…Improved vanadium barrier properties of the new AIEMs are targeted by introduction of positively charged alkylated vinylpyridinium units and a small degree of crosslinking, whereas the presence of protogenic sulfonic acid groups is expected to reduce their ohmic resistance. The performance, capacity decay, and electrolyte transfer phenomena in cells with amphoteric IEMs are investigated and compared with results obtained from cells containing Nafion 117 CEMs and AIEMs carrying amidoxime and sulfonic acid from our previous study …”