High concentrations of proton carriers (5.0 x 10(21) cm(-3)) were injected into tungsten phosphate glass (8WO(3)-35NaO(1/2)-8NbO(5/2)-5LaO(3/2)-44PO(5/2)) by electrochemical substitution of sodium ions with protons at 345 degrees C. The electromotive force of a hydrogen concentration cell that used the substituted glass as a solid electrolyte indicated a mixed conduction of protons and electrons in the glass with a mean proton transport number of 0.8 at 300 degrees C The partial conductivity of,the protons was 8.0 x 10(-4) Scm(-1). The fuel cell generated electricity at a maximum power density of 1.3 mWcm(-2), even though its open circuit voltage was only 0.77 V because of the electronic contribution to the conductivity. Methods to increase the proton conductivity for improving fuel cell performance are discussed. (C) 2013 Elsevier B.V. All rights reserved.