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.
Proton-conducting composite membranes were fabricated by blending Sn 0.95 Al 0.05 P 2 O 7 having an excess of phosphates with polybenzimidazole ͑PBI͒ and polytetrafluoroethylene ͑PTFE͒. The addition of PBI to Sn 0.95 Al 0.05 P 2 O 7 -P x O y powder stabilized the conductivity of the composite, providing higher conductivities than those of stoichiometric Sn 0.95 Al 0.05 P 2 O 7 . The addition of PTFE to Sn 0.95 Al 0.05 P 2 O 7 -P x O y -PBI powder reduced the conductivity but increased the tensile strength. The resulting composite membrane exhibited a conductivity of 0.04 S cm −1 at 200°C and a tensile strength of 2.30 MPa. Moreover, a fuel cell made with this composite membrane yielded high power densities exceeding 200 mW cm −2 above 100°C and good durability under unhumidified conditions. Proton conductors capable of operating at 100-250°C are currently of great interest because of their advantages over Nafion-type fluoropolymers that are conventionally used at temperatures of 80°C or less in proton exchange membrane fuel cells. 1,2 Operating a fuel cell at intermediate temperatures provides the anode catalyst with a high tolerance to CO, eliminating the need for a CO-removal unit ͑water-gas-shift and CO-preferential-oxidation reactors͒. In addition, the kinetics of the electrode kinetics are enhanced at intermediate temperatures, permitting low Pt loadings. Other advantages include good drainage at the cathode and effective heat dissipation.Because anhydrous proton conductors do not require the presence of water as the charge carrier, they can be operated at intermediate temperatures ͑at least in principle͒. In addition, anhydrous proton conductors potentially can make proton conductivity independent of humidity so that a complicated humidity control system is not required. Thus, considerable effort has been devoted to developing anhydrous proton conductors. Although a number of anhydrous proton conductors have already been developed, their proton conductivities have been reported to be in the range of 10 −2 -10 −3 S cm −1 . [3][4][5][6][7][8][9][10] We previously reported that 10 mol % In 3+ -or 5 mol % Al 3+ -doped SnP 2 O 7 exhibited high proton conductivities of above 10 −1 S cm −1 between 100 and 300°C in water-free conditions. 11,12 These materials were also investigated for use as electrolytes in intermediate-temperature fuel cells. These fuel cells exhibited stable performance in low-humidity conditions and high CO concentrations; 13 they also permitted the use of alternative anodes to Pt 14 and had good fuel flexibility. 15 In a series of studies, excess P was observed as a highly hygroscopic P x O y layer on the exterior of the crystal, when the H 3 PO 4 /MO x ͑M = Sn and Al͒ molar ratio of the raw materials was much higher than its stoichiometric value. 12 The resulting conductivity reached 0.3 S cm −1 at 275°C, which is approximately 2 times higher than that of the stoichiometric Al 3+ -doped SnP 2 O 7 ͑hereafter Sn 0.95 Al 0.05 P 2 O 7 ͒ at the same temperature. This increase in conductivity was d...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.