NOVELTY - A redox fuel cell (C1) comprises: an anode and a cathode separated by an ion selective polymer electrolyte membrane; unit for supplying a fuel to the anode region of the cell; unit for supplying an oxidant to the cathode region of the cell; unit for providing an electrical circuit between the anode and the cathode; and a catholyte solution containing at least one non-volatile catholyte component flowing in fluid communication with the cathode and containing a modified ferrocene species containing at least one electron withdrawing substituent on at least one cyclopentadienyl ring.    USE - As redox fuel cell (claimed) useful in various applications e.g. as power sources for portable products such as portable electronics products; for transport vehicles such as automobiles, both main and auxiliary; auxiliary power for caravans and other recreational vehicles and boats; stationary uses such as uninterruptible power for hospitals computers and combined heat and power for homes and businesses; and for portable applications such as automotive and portable electronics technology.    ADVANTAGE - The redox fuel cell is cheap and inexpensive to assemble; uses inexpensive and environmentally friendly materials; is small in their construction; operates at low a temperature; and doesn't produce unwanted by-products, pollutants and noxious materials. The use of a bi-membrane in the redox fuel cell maximizes the potential of the cell, by maintaining the potential due to a pH drop between the anode and catholyte solution.    DETAILED DESCRIPTION - A redox fuel cell (C1) comprises: an anode and a cathode separated by an ion selective polymer electrolyte membrane; unit for supplying a fuel to the anode region of the cell; unit for supplying an oxidant to the cathode region of the cell; unit for providing an electrical circuit between the anode and the cathode; a catholyte solution containing at least one non-volatile catholyte component flowing in fluid communication with the cathode and containing a modified ferrocene species containing at least one electron withdrawing substituent on at least one cyclopentadienyl ring, where the substituent being separated from the ring by a spacer group, the modified ferrocene species being at least partially reduced at the cathode in operation of the cell, and at least partially regenerated by reaction with the oxidant after such reduction at the cathode by direct reaction with the oxidant, or by indirect reaction with it using a redox catalyst catalyzing the regeneration of the ferrocene mediator. An INDEPENDENT CLAIM is included for a catholyte solution for use in a redox fuel cell (C1).