Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group: A progress report

Abstract

The melting behaviour and transport properties of straight chain alkanes mono- and difunctionalized with phosphonic acid groups have been investigated as a function of their length. The increase of melting temperature and decrease of proton conductivity with increasing chain length is suggested to be the consequence of an increasing ordering of the alkane segments which constrains the free aggregation of the phosphonic acid groups. However, the proton mobility is reduced to a greater extent than the proton diffusion coefficient indicating an increasing cooperativity of proton transport with increasing length of the alkane segment. The results clearly indicate that the "spacer concept", which had been proven successful in the optimization of the proton conductivity of heterocycle based systems, fails in the case of phosphonic acid functionalized polymers. Instead, a very high concentration of phosphonic acid functional groups forming "bulky" hydrogen bonded aggregates is suggested to be essential for obtaining very high proton conductivity. Aggregation is also suggested to reduce condensation reactions generally observed in phosphonic acid containing systems. On the basis of this understanding, the proton conductivities of poly(vinyl phosphonic acid) and poly(meta-phenylene phosphonic acid) are discussed. Though both polymers exhibit a substantial concentration of phosphonic acid groups, aggregation seems to be constrained to such an extent that intrinsic proton conductivity is limited to values below sigma = 10(-3) S cm(-1) at T = 150 degrees C. The results suggest that different immobilization concepts have to be developed in order to minimize the conductivity reduction compared to the very high intrinsic proton conductivity of neat phosphonic acid under quasi dry conditions. In the presence of high water activities, however, (as usually present in PEM fuel cells) the very high ion exchange capacities (IEC) possible for phosphonic acid functionalized ionomers (IEC >10 meq g(-1)) may allow for high proton conductivities in the intermediate temperature range (T approximately 120 -160 degrees C).

High temperature proton exchange membranes based on polybenzimidazoles for fuel cells

“…From proton conducting mechanism points of view, phosphoric and phosphonic acids are interesting because they are more amphoteric, having both proton donor (acidic) and proton acceptor (basic) groups to form dynamic hydrogen bond networks, in which protons can readily transfer by hydrogen bond breaking and forming processes, as recently rationalized by Kreuer and co-workers [18][19][20]. Other important features of phosphoric or phosphonic acids are their excellent thermal stability and low vapour pressure at elevated temperatures.…”

Protic ionic liquids: an alternative proton-conducting electrolyte for high temperature proton exchange membrane fuel cells

“…3 A breakthrough was achieved when polybenzimidazole (PBI) was first proposed for preparing aciddoped membranes. [4][5][6] Besides, solid acids (e.g., CsH 2 PO 4 ), are known to undergo a structural change which leads to a highly conductive superprotonic phase at temperatures above 140 uC.…”

In-Situ Dynamic Characterization of Energy Storage and Conversion Systems

“…The advantages of PBI over other polymers, including low cost [54], high glass transition temperature [55], excellent textile fiber properties [56], and great thermal stability [57], promised it to be an excellent polymer for membrane fabrications. One of the most significant advantages of a PA doped PBI membrane over a PFSA based membrane is that its conductivity no longer relies on the water content due to its unique proton conduction mechanism [50,[58][59][60], but strongly depends on the PA doping level [50,[61][62][63] and the operating temperature [50,63,64].…”