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

• 3-TMA PROXYL as a strong candidate for aqueous redox flow battery technologies • First report of a five-membered ring nitroxide posolyte for AORFBs • Unprecedented combination of high redox potential, solubility, and stability 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.