Aqueous Redox Flow Batteries: Small Organic Molecules for the Positive Electrolyte Species

Computational framework for discovery of degradation mechanisms of organic flow battery electrolytes

The stability of organic redox-active molecules is a key challenge for the long-term viability of organic redox flow batteries (ORFBs). Electrolyte degradation leads to capacity fade, reducing the efficiency and lifespan of ORFBs. To systematically investigate degradation mechanisms, we present a computational framework that automates the exploration of degradation pathways. The approach integrates local reactivity descriptors to generate reactive complexes and employs a single-ended process search to discover elementary reaction steps, including transition states and intermediates. The resulting reaction network is iteratively refined with heuristics and human-guided validation. The framework is applied to study the degradation mechanisms of quinone- and quinoxaline-based electrolytes under acidic and basic aqueous conditions. The predicted reaction pathways and degradation products align with experimental observations, highlighting key degradation modes such as Michael addition, disproportionation, dimerization, and electrochemical transformation. The framework provides a valuable tool for in silico screening of stable electrolyte candidates and guiding the molecular design of next-generation ORFBs.

Constructing Hydrophilic Polymer Membranes with Microporosity for Aqueous Redox Flow Batteries

Ion selective membranes (ISMs) are key components of aqueous redox flow batteries (ARFBs), and their property in selective ion transport largely determines the energy storage efficiency of ARFBs. Traditional ISMs are based on microphase-separated structures and have been advanced for many years, but most of them show poor performance as membrane separators in ARFBs due to their conductivity-selectivity. In recent years, using confined micropores instead of dense hydrophilic regions as ion channels has been demonstrated to effectively break this tradeoff. We here summarize the synthetic strategies for constructing hydrophilic polymer membranes with microporosity and highlight the performance of some typical microporous ISMs in ARFBs. We also propose fundamental issues that remain to be addressed for the further development of ISMs.

A Multielectron and High-Potential Spirobifluorene-Based Posolyte for Aqueous Redox Flow Batteries

The rising energy demand driven by human activity has posed pressing challenges in embracing renewable energy, necessitating advances in energy storage technologies to maximize their utilization efficiency. Recent studies in aqueous organic redox flow batteries have focused primarily on the development of organic negative electrolytes, while the progress in organic positive electrolytes remains constrained by limitations in their redox potentials and effective electron concentrations. Herein, we report a spatially twisted chlorinated spirobifluorene ammonium salts (CSFAs), created through an unexpected green chlorination-protection pathway during the initial cycling in the flow battery cell, utilizing chloride ions from counterions in aqueous solution. The chlorinated, nonplanar spiral structure of CSFAs possesses a one-step four-electron transfer electrochemical property and offers exceptional resistance to nucleophilic attacks, exhibiting an unprecedented redox potential as high as 1.05 V (vs. SHE). A full redox flow battery based on CSFA-Cl (chloride ions as the counter ions) with 1.4 M electron concentration achieved an average coulombic efficiency exceeding 99.4 % and a capacity utilization reaching 95 % of the four-electron capacity for a stable cycling over 250 cycles (~22 days). The present work exemplifies the use of side reactions to develop new redox species, which can be extended to create more structurally versatile energy storage materials.

Membrane-Electrolyte System Approach to Understanding Ionic Conductivity and Crossover in Alkaline Flow Cells

Membrane transport properties are crucial for electrochemical devices, and these properties are influenced by the composition and concentration of the electrolyte in contact with the membrane. We apply this general membrane-electrolyte system approach to alkaline flow batteries, studying the conductivity and ferricyanide crossover of Nafion and E-620. We report undetectable crossover for as-received Nafion and E-620 after both sodium and potassium exchange but high ferricyanide permeability of 10-7 to 10-8 cm2 s-1 for Nafion subjected to pretreatment prevalent in the flow battery literature. We show how the electrolyte mass fraction in hydrated membranes regulates the influence of ion concentration on membrane conductivity, identifying that increasing electrolyte concentration may not increase membrane conductivity even when it increases electrolyte conductivity. To illustrate this behavior, we introduce a new metric, the membrane penalty, as the ratio of the conductivity of the electrolyte to that of the membrane equilibrated with the electrolyte. We discuss the trade-off between flow battery volumetric capacity and areal power density that arises from these findings. Finally, we apply insights from this approach to provide recommendations for use of membranes in alkaline flow cells and electrochemical reactors in general.