Synthesis and characterisation of proton conducting composite materials resilient to dehydration at elevated temperatures

Abstract

The optimal operating temperature range for a low temperature polymer exchange membrane fuel cell begins at 80 °C. At this temperature water will begin to leave the system as steam and any membrane that relies on water to transport protons will dehydrate and thus, the proton conductivity of the membrane will start to degrade. Here the development of a new proton conducting material that is resilient to dehydration is presented. The exploitation of the oxide surface of the ca. 20 Å pore walls of a mesoporous transition metal oxide (mX2O5) as a means of anchoring sulfonate groups and suppressing moisture loss to encourage proton conduction pathways in the naphthalene sulfonate formaldehyde (NSF) impregnated analogues is conceived and studied. Composites of mX2O5 doped with H2SO4 and subsequently impregnated with NSF or having NSF oligomerised in situ of the mX2O5 pores were synthesised, characterised and had their proton conductivity recorded over a range of temperatures. Initially six mesoporous Ti oxide composites of NSF were made. The most promising sample displayed a proton conductivity of 1.837 mS cm-1 at 100 °C. This surpasses that of a pellet of Nafion 117 (1.143 mS cm-1) whilst having a greater conductivity than pure hydrated NSF (0.122 mS cm-1), confirming a synergistic interaction between the NSF and the oxide mesostructure. Next a series of mX2O5-NSF composites were synthesised with C6, C12, and C18 templates. Here the most promising sample displayed a conductivity of 21.96 mS cm-1 at 100 °C, surpassing the literature value for a Nafion 117 film (ca. 8 mS cm-1). Finally, studies were performed on a series of mNb₂O₅ composites with NSF resin polymerised within the pores. The most promising sample displayed a conductivity of 21.77 mS cm-1 at 80 °C. Subsequent thermal durability tests demonstrated that this composite maintains superior conductivity to Nafion 117 at 80 °C for the length of the study (24 h). These observations were subsequently rationalised by in depth solid-state NMR studies.

Date of Award	4 May 2017
Original language	English
Supervisor	David Antonelli (Supervisor) & Jon Maddy (Supervisor)