Tin pyrophosphate (SnP2O7) membranes need a sintering process to achieve a required mechanical strength as the electrolyte of intermediate temperature fuel cells (IT-FCs) operating at temperatures of 200-300 degrees C at large scale. However, sintering causes a severe drop of proton conductivity due to the decomposition of SnP2O7 and release of residual fused phosphoric acid and/or phosphorous oxides. Here we demonstrate a route to restore the proton conductivity by introducing phosphoric acid inside a sintered SnP2O7 membrane to react with the degraded SnP2O7 for restoration. After restoration, the decomposition product SnO2 is converted back to SnP2O7 and fused phosphoric acid and/or phosphorous oxides (17.7-20.0%) is regenerated. The proton conductivity is tremendously enhanced from 9.7 x 10(-4) S cm(-1) to 0.061 S cm(-1) at 225 degrees C. A planar IT-FC (active area = 5 cm(2)) with a restored membrane (thickness = 0.85 mm, diameter = 40 mm) generates a peak power density of 78 mW cm(-2) without using intermediate catalyst layers at 225 degrees C. It can steadily run for 45 h at 100 mA cm2 with a degradation rate of 0.7 mV h(-1) at 225 degrees C. The fuel and oxidant are, respectively, H-2 (50 sccm) and air (100 sccm) humidified at 30 degrees C. (C) 2015 Elsevier B.V. All rights reserved.