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All-vanadium redox flow batteries hold promise for the next-generation grid-level energy storage technology in the future. However, the low electrocatalytic activity of initial graphite felt constrains the development of VRFBs. Furthermore, the positive VO2+/VO2+ reaction involves complex multistep processes and more sluggish kinetics than negative V2+/V3+ reaction. Therefore, enhancing the kinetics of positive reaction is especially important. Heteroatom doping is one of the effective strategies for preparing carbon electrodes with high electrocatalytic activity and good stability. Here, a nitrogen, phosphorus co-doped graphite felt is prepared. Nitrogen introduces more negative charge into the carbon lattice due to the higher electronegativity, and more oxygen-containing functional groups will be introduced into the carbon lattice due to phosphorus-doped graphite felt. N, P co-doping provides more adsorption sites for vanadium ions. As a result, nitrogen, phosphorus co-doped graphite felt shows high electrochemical activity and good stability, and the corresponding VRFB presents a good voltage efficiency of 75% at a current density of 300 mA cm−2, which is 11% higher than the pristine graphite felt. During 100 charge/discharge cycles, the energy efficiency and voltage efficiency remain at 84% and 86% under the current density of 150 mA cm−2.

Although considered as the most viable approach for mobile powers, traditional polymer electrolyte membrane fuel cells require burdensome humidification and water management systems. The bipolar fuel cell (BPFC) or hybrid mem- brane fuel cell won noticeable interest because its potential self-humidification. BPFC is a novel polyelectrolyte membrane fuel cell involving both anion and cation polyelectrolyte and a new kind of acidic-alkaline bipolar reaction interface was pre- sented in the cell. The potential advantages of a BPFC are twofold: (a) the alkaline cathode with inherently faster kinetics that allows use of non-platinum catalysts, thereby significantly reducing the total cost of a fuel cell; (b) the water generation or dissociation reaction takes place at the bipolar interface providing the possibility to devise self-humidification over the entire cell. Despite the BPFC have such attractive potentials, the BPFC developed to date had not yet demonstrated these feature and have operated at very modest power densities. As the development of BPFC is still in their infancy, it still remains un- certain how the bipolar interface works and effects in the cell. In the present study, four types of bipolar membrane electrode configuration containing two or one kind of bipolar interface, either water generation or dissociation interfaces, were fabri- cated to evaluate the effect of bipolar interface on the cell output performance. Results show that the preferred configuration with only one bipolar interface generating water can benefit the cell output. The reason could be the faster water formation reaction kinetics in this kind of bipolar interface and lower interfacial potential loss with only one bipolar interface. Within this preferred membrane electrode configuration, we have realized and demonstrated the BPFC that operate under completely self-humidifying conditions for prolonged periods successfully. As we can see, optimization of the membrane electrode con- figurations and further advances in fabricating bipolar interface would open the way for the development of practical self-regulating portable fuel cells. Keywords bipolar interface; membrane electrode configuration; self-humidifying; polyelectrolyte membrane; fuel cell