Method of operating a regenerative fuel cell for enhancement of characteristics

A regenerative fuel cell comprises two half-cells separated by a cation-exchange membrane. There is a halogen/halide electrolyte in one half of the cell, a sulphide/polysulphide electrolyte in the other half of the cell and cations in both halves of the cell, which act as charge carriers therebetween. The state of charge of the sulphide/polysulphide electrolyte is defined as the ratio of the total number of sulfur atoms which make up all sulphur species present in the sulphide/polysulphide electrolyte to the total number of units of negative charge carried by all sulphur species present in the sulphide/polysulphide electrolyte, one unit of negative charge being equivalent to the charge on an electron. The state of charge of the sulphide/polysulphide electrolyte is in the range of from 1. 8 to 2. 5 for at least a part of the charge/discharge cycle over a plurality of charge/discharge cycles. .