Chemically regenerative redox fuel cells are promising alternatives to conventional PEMFCs as stationary energy conversion devices since they avoid platinum-based catalyst at the cathode for the oxygen reduction reaction thanks to the chemical oxidation of the mediators in an external tank[1]. The requirements for the mediator are a high standard redox potential (0.6 V vs. SHE), high diffusion coefficient and fast electron transfer kinetic to reach high power density. Furthermore, the mediator must be stable in acidic medium (pH<1) and at the operating temperature of the system (40 to 80°C). The durability of the system will depend on the physico-chemical properties as well as on the chemical reactivity with oxygen in order to regenerate the reduced redox mediator. Loss of performance over time can be attributed to the crossover of the solubilized species from the cathode to the anode side and to the chemical instability over redox reaction. Crossover phenomena are commonly observed in aqueous organic redox flow battery using (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) as reactive species[2].In this study, several mediators based on TEMPO with different PEG chain length are synthesized and characterized. The goal of the functionalization is to mitigate the crossover rate of the mediator through the membrane. Fuel cell tests were performed giving access to performance and power decrease over time. For a deeper understanding of the origin of performance losses, an ex situ electrochemical setup was developed to assess the impact of functionalization toward crossover.[1] Singh R, Shah AA, Potter A, Clarkson B, Creeth A, Downs C, et al. Performance and analysis of a novel polymer electrolyte membrane fuel cell using a solution based redox mediator. J Power Sources 2012;201:159–63. https://doi.org/10.1016/j.jpowsour.2011.10.078.[2] Small LJ, Pratt HD, Anderson TM. Crossover in Membranes for Aqueous Soluble Organic Redox Flow Batteries. J Electrochem Soc 2019;166:A2536–42. https://doi.org/10.1149/2.0681912jes. Figure 1