Proton Conduction in Sn[sub 0.95]Al[sub 0.05]P[sub 2]O[sub 7]–PBI–PTFE Composite Membrane

Abstract

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...

Niobium phosphates as an intermediate temperature proton conducting electrolyte for fuel cells

“…However, limited information is available even for the most extensively studied SnP 2 O 7 -based systems. Heo et al 10 reported a 50% decrease in conductivity for Sn 0.95 Al 0.05 P 2 O 7 -P 0.6 O 2.1 after being kept in a dry atmosphere for 10 h, whereas Xu et al 33 reported a higher stability for the SnP 2 O 7 -H 3 PO 4 system. These results are also included in Fig.…”

“…Great efforts are being made to develop new proton conducting materials as electrolytes for fuel cells operational at intermediate temperatures from 200 to 400 C. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] This temperature range is of strategic importance, since it bridges the gap between the currently available high and low temperature fuel cells. A number of biofuels can be directly oxidized or reformed to hydrogen via an internal reformer in this temperature range.…”

“…[10][11][12][13] Research has also been done to develop proton conducting materials including ammonium polyphosphates, caesium hydrogen phosphates as well as other phosphate composites, as recently reviewed by Paschos et al.…”

Catalyst, Membrane, Free Electrolyte Challenges, and Pathways to Resolutions in High Temperature Polymer Electrolyte Membrane Fuel Cells

“…This is done to avoid the high acid loadings required to achieve reasonable conductivity which can cause mechanical stability issues with the PBI membranes [66][67][68]. An avenue to avoid this issue currently being researched involves filling the membrane with solid proton conductors such as solid acids, ionic liquids, and graphite [50,[69][70][71][72][73][74][75][76][77][78] or hygroscopic materials [79][80][81][82].…”

A Flexible CsH5(PO4)2-Doped Composite Electrolyte Membrane for Intermediate-Temperature Fuel Cells

“…Area-specific resistance of pellet-form electrolytes is very high, hindering their application in ITFCs [15]. Hibino et al reported several works on the topic of inorganic/organic composite electrolytes using metal phosphates as the proton conductor for ITFCs [7,[19][20][21][22]. By adding organic binders, they prepared Sn0.9In0.1P2O7 composite membranes with a thickness of less than 150 μm, having a relatively low ASR of 0.24 Ω cm 2 [7].…”