Under the strong impetus of China’s “dual carbon” (carbon peak and carbon neutrality) strategy, renewable and clean energy sources such as wind and solar power are developing rapidly and are expected to play a central role in the nation’s future energy landscape. However, the inherent intermittency and variability of these sources, influenced by uncontrollable natural conditions, present significant challenges to their large-scale integration and stable operation within the power grid. As a result, the development of energy storage systems that offer high safety, long lifespan, low cost, and scalability has become essential for enhancing the efficiency and reliability of clean energy utilization.
Aqueous organic redox flow batteries (AORFBs), with intrinsic safety, decoupled energy and power, and modular scalability, have emerged as a frontier in next-generation energy storage research. Among various organic redox-active species, cyclic nitroxide radicals have attracted considerable attention as catholyte candidates, owing to their high redox potentials, excellent electrochemical reversibility, and environmentally friendly synthesis pathways. However, in practical applications, these molecules are prone to side reactions during cycling, such as autocatalytic oxidation, disproportionation, molecular aggregation, and ring-opening, leading to irreversible degradation and rapid capacity fading—key bottlenecks hindering their commercialization.
To address these challenges, Professor Jiangxuan Song’s team at the School of Materials Science and Engineering (MSE), Xi’an Jiaotong University (XJTU), has conducted a series of innovative studies on stabilizing key electrolyte molecules in AORFBs, achieving significant breakthroughs.
Focusing on the issue of ring-opening and activity loss of high redox-potential pyrroline-based nitroxide radicals during cycling, the team pioneered a host-guest chemical modulation strategy to construct a novel, stable structure with a “molecular armor” effect. By encapsulating pyrroline nitroxide radical within the hydrophobic cavity of water-soluble cyclodextrins, they formed an inclusion complex in which the N-O• moiety is oriented toward the cavity bottom. This spatial configuration effectively shields reactive hydrogen sites on the pyrroline ring from nucleophilic attack (e.g., by Lewis bases), thereby inhibiting detrimental ring-opening side reactions. Electrochemical tests demonstrated that this encapsulated system achieved a remarkably low capacity decay rate of 0.002% per cycle (daily decay <0.233%) over 500 cycles within a 0.05-0.5 M concentration range, in stark contrast to the 0.039% per cycle (5.23% per day) observed for the unprotected molecule. These results validate the molecular armor strategy as a robust and broadly applicable approach to enhancing nitroxide radical stability in AORFBs.
In a further innovation, the team broke through conventional linear design paradigms for piperidine-based nitroxide radicals by developing a new class of branched molecules bearing bis-quaternary ammonium substituents. These branched architectures leverage spatial and electrostatic repulsion to significantly enhance both the redox stability and performance of the electrolyte. The branched dual cationic centers introduce strong intermolecular repulsion and steric hindrance, effectively suppressing nucleophilic degradation pathways.
When paired with viologen-based anolytes, the resulting AORFB system demonstrated exceptional performance, with a per-cycle capacity retention of 99.992% (daily retention >99.85%) and a peak power density of up to 140.3 mW cm⁻². In situ UV-Vis spectroscopy and theoretical simulations confirmed that the branched design increases the energy barrier between redox states and enhances intermolecular repulsion, thereby stabilizing the molecular structure and inhibiting side reactions. In collaboration with Prof. Duanyang Kong of Beijing University of Chemical Technology, the team extended this system from all-organic aqueous batteries to Zn-organic hybrid systems, successfully developing an aqueous flow battery that combines high areal capacity with long-term cycling stability.
Figure 1: Research articles enabled by molecular armor and branched design strategies.
The above research findings were published in the international journals eScience and Angewandte Chemie International Edition, titled:
· “Spatial structure regulation towards armor-clad five-membered pyrroline nitroxides catholyte for long-life aqueous organic redox flow batteries”
· “Branching-Induced Intermolecular Repulsion Effects Drive Stable and Sustainable Flow Batteries on Condensed Nitroxyl Radicals”
Associate Professor Hao Fan of the MSE-XJTU is the first author of the papers, with Professor Jiangxuan Song serving as the corresponding author, and Xi’an Jiaotong University is the sole institution credited.
Figure 2: From laboratory innovation to engineering demonstration—megawatt-scale AORFB projects underway in multiple regions.
Building on these research achievements, the team is actively advancing the integration of research, industry, application, and finance to commercialize the outcomes. Based on the key electrolyte technology, they have established in-depth collaborations with leading enterprises such as Suqian Time Energy Storage Technology Co., Ltd., China Huaneng Group Co., Ltd., China Three Gorges Corporation, and the Energy Storage Technology Company of Pipe China. Together, they are addressing critical technical challenges and pushing forward practical implementation. At present, megawatt-scale demonstration projects for the AORFBs are under joint development in regions including Inner Mongolia and Fujian. A complete technological chain has been preliminarily established—from materials design and system integration to engineering application—providing a replicable and scalable solution for large-scale clean energy storage in China.
This series of studies was supported by the National Natural Science Foundation of China, the National Foreign Experts Project of the Ministry of Human Resources and Social Security, the Shaanxi Provincial Key R&D Program, the Discipline Innovation and Talent Introduction Program 2.0 (“111 Plan 2.0”), the Young Talents Program of Xi’an Jiaotong University, and the Open Fund of the State Key Laboratory for Mechanical Behavior of Materials. Characterization and testing were conducted with the assistance of the Instrument Analysis Center and the Materials College Testing Platform at Xi’an Jiaotong University, while theoretical simulations were performed using the university’s high-performance computing resources.
Paper Links:
1. https://doi.org/10.1016/j.esci.2023.100202 (Cover)
2. https://doi.org/10.1002/anie.202504932
3. https://www.oaepublish.com/articles/energymater.2024.161
Professor Jiangxuan Song’s team homepage: http://jxsong.xjtu.edu.cn/
