NOVELTY - A redox fuel cell comprises anode and cathode separated by an ion selective polymer electrolyte membrane; device for supplying a fuel to the anode region of the cell; device for supplying an oxidant to the cathode region of the cell; device for providing an electrical circuit between the anode and the cathode; non-volatile catholyte solution flowing in fluid communication with the cathode, where the catholyte solution comprises a polyoxometallate redox couple.    USE - In redox fuel cell (claimed) which is used in microfuel cells for electronic and portable electronic components, and also in larger fuel cells for the automotive industry.    ADVANTAGE - The fuel cell at an increased temperature or pressure results in an improved performance; provides optimum temperature for operation which was 80 degrees C, as performance was thought to decrease at higher temperatures, due to its decrease of oxygen solubility to zero as the boiling point of the catholyte is approached; increases the temperature or pressure causes speciation of the polyoxometallate species, which increases the regeneration rate of the polyoxometallate. This causes improvements in steady state performance as the catholyte which is less reduced under the same load. This also increases the redox potential of the polyoxometallate, which results from a dynamic equilibrium between oxidation (occurring via regeneration) and reduction (occurring at the fuel cell electrode). The fuel cell operates straightforwardly with a redox couple catalyzing in operation of the fuel cell the reduction of oxidant in the cathode chamber.    DETAILED DESCRIPTION - A redox fuel cell comprises anode and cathode separated by an ion selective polymer electrolyte membrane; device for supplying a fuel to the anode region of the cell; device for supplying an oxidant to the cathode region of the cell; device for providing an electrical circuit between the anode and the cathode; non-volatile catholyte solution flowing in fluid communication with the cathode, where the catholyte solution comprises a polyoxometallate redox couple being at least partially reduced at the cathode in operation of the cell, and at least partially regenerated by reaction with the oxidant in a regeneration zone after such reduction at the cathode, where the catholyte solution further comprises at least one vanadium species that result from the speciation of the polyoxometallate at an elevated temperature and/or pressure.