3-TMA PROXYL: A high-potential, highly soluble nitroxide for enhanced stability and performance in aqueous organic redox flow batteries

In response to the growing demand for sustainable and efficient stationary energy storage solutions, this study introduces and evaluates a novel posolyte, 3-TMA PROXYL, tailored for aqueous organic redox flow battery (AORFB) applications. Easily synthesized at the gram scale, 3-TMA PROXYL presents an unprecedented combination of high redox potential (1.06 V vs SHE) and remarkable solubility (> 3 M in 1 M NaCl), resulting in an increased theoretical capacity of 80 Ah L⁻¹. Paired with methyl viologen as a negolyte, the 3-TMA PROXYL battery achieves an 8 % improvement in cell voltage (1.45 V) surpassing the benchmark 4-TMA TEMPO in energy and power density and competing with it regarding stability. Detailed 1 H NMR and UV-visible spectroscopy analyses indicate that 3-TMA PROXYL undergoes minimal irreversible chemical degradation, with observed capacity fade attributed primarily to self-discharge. These results establish 3-TMA PROXYL as a reliable alternative to state-of-the-art nitroxides, paving the way for five-membered ring nitroxides in high-performance, sustainable redox flow applications.