Final answer:
A flow battery is rechargeable due to chemical reactions that continuously convert chemical potential energy to electrical energy with the help of reactants supplied externally.
Step-by-step explanation:
A flow battery, also known as a fuel cell, is a type of rechargeable battery where rechargeability is provided by chemical reactions within the battery. These batteries harness the energy of spontaneous redox reactions, which involve a fuel and an oxidizer. Unlike conventional batteries that store energy in solids or pastes, flow batteries require a continuous input of the redox reactants from an external reservoir, enabling the reaction's electron transfer via an external circuit. The electricity generated in a flow battery is a result of the chemical potential energy being converted to electrical energy, which powers the flow of electrons through a circuit.