SnP 2 O 7 -based proton conductors were characterized by Fourier transform infrared spectroscopy ͑FTIR͒, temperature-programmed desorption ͑TPD͒, X-ray diffraction ͑XRD͒, and electrochemical techniques. Undoped SnP 2 O 7 showed overall conductivities greater than 10 −2 S cm −1 in the temperature range of 75-300°C. The proton transport numbers of this material at 250°C under various conditions were estimated, based on the ratio of the electromotive force of the galvanic cells to the theoretical values, to be 0.97-0.99 in humidified H 2 and 0.89-0.92 under fuel cell conditions. Partial substitution of In 3+ for Sn 4+ led to an increase in the proton conductivity ͑from 5.56 ϫ 10 −2 to 1.95 ϫ 10 −1 S cm −1 at 250°C, for example͒. FTIR and TPD measurements revealed that the effects of doping on the proton conductivity could be attributed to an increase in the proton concentration in the bulk Sn 1−x In x P 2 O 7 . The deficiency of P 2 O 7 ions in the Sn 1−x In x P 2 O 7 bulk decreased the proton conductivity by several orders of magnitude, which was explained as due to a decrease in the proton mobility rather than the proton concentration. The mechanism of proton incorporation and conduction is examined and discussed in detail.

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Al3+ -doped SnnormalP2normalO7 proton conductors were prepared by controlling the initial composition of the reactants [ SnnormalO2 , Al(OH)3 , and normalH3PnormalO4 ]. normalSn1−xnormalAlxnormalPynormalOz with y<2 displayed conductivities approximately two orders of magnitude lower than normalSn1−xnormalAlxnormalP2normalO7 , while those of normalSn1−xnormalAlxnormalPynormalOz with y2 exhibited conductivities at a maximum of 1.99 times higher. However, because the conductivity values of normalSn1−xnormalAlxnormalPynormalOz with y2 were not stable, the optimal value of y in normalSn1−xnormalAlxnormalPynormalOz was determined to be 2. Partial substitution of Al3+ for Sn4+ in normalSn1−xnormalAlxnormalP2normalO7 led to an increase in the conductivity up until x=0.05 . As a result, the conductivity reached 0.045Scm−1 at 100°C , 0.15Scm−1 at 200°C , and 0.19Scm−1 at 300°C when the x and y values were 0.05 and 2, respectively. A hydrogen concentration cell with this material demonstrated that the ionic transport number was ∼1 , and a fuel cell using this material demonstrated that the dc conductivity was comparable to the ac conductivity.

Expanding the range of supercapacitor operation to temperatures above 100°C is important because this would enable capacitors to operate under the severe conditions required for next-generation energy storage devices. In this study, we address this challenge by the fabrication of a solid-state supercapacitor with a proton-conducting Sn0.95Al0.05H0.05P2O7 (SAPO)-polytetrafluoroethylene (PTFE) composite electrolyte and a highly condensed H3PO4 electrode ionomer. At a temperature of 200°C, the SAPO-PTFE electrolyte exhibits a high proton conductivity of 0.02 S cm−1 and a wide withstanding voltage range of ±2 V. The H3PO4 ionomer also has good wettability with micropore-rich activated carbon, which realizes a capacitance of 210 F g−1 at 200°C. The resulting supercapacitor exhibits an energy density of 32 Wh kg−1 at 3 A g−1 and stable cyclability after 7000 cycles from room temperature to 150°C.

Performance of a fuel cell using Sn 0.9 In 0.1 P 2 O 7 as the electrolyte was evaluated in the temperature range of 150-300°C under unhumidified conditions. The IR drop and electrode overpotential of the cell were measured separately by the current interruption method. The dc conductivity values of the electrolyte between 150 and 300°C, estimated from the IR drop, were comparable to the ac conductivity values ͑1.48 ϫ 10 −1 -1.95 ϫ 10 −1 S cm −1 ͒ of the electrolyte. The cell performance was improved by forming an intermediate layer consisting of Sn 0.9 In 0.1 P 2 O 7 and Pt/C catalyst powders at the interface between the electrolyte and cathode, which significantly reduced the cathode polarization. As a result, the peak power density reached 264 mW cm −2 at 250°C using the 0.35-mm-thick electrolyte. The present fuel cell also showed high stability at low relative humidities ͑p H 2 O Ϸ 0.0075 atm͒ and 10% CO concentration.