Investigating the Application of Graphitic Carbon Nitrides as Additives in Proton Exchange Membranes for Fuel Cells

Abstract

Proton exchange membranes (PEMs) that conduct protons between electrodes, whilst minimizing the transport of reactant molecules are vital components of energy storage and conversion devices, such as fuel cells (FC), electrolysers and redox flow batteries. Despite over half a century of synthetic development, perfluorosulfonic acid membranes, such as Nafion, remain the industry standard. Performance improvements have thus relied on use of additives to improve water retention and proton conductivity. However, understanding the way in which these additives arrange favourably into the phase separated morphology of the polymer structure and positively influence water uptake and proton conductivity has eluded researchers.Polytriazine imide structured graphitic carbon nitride (PTI) with a graphitic structure of repeating C12N12H3 structural voids is an ideal candidate additive due to its specific size, functional groups, and hydrophilicity.[1] Using neutron scattering we directly revealed the importance of triazine and bridging N-H groups for facilitated water transport through the intralayer voids and speculate their importance in hydrated proton transport.[2] Herein, ultrasonic spray printing (USP) was used to incorporate PTI nanosheets into the phase separated morphology of Nafion. Fabrication was optimised via PTI wt.% loading and USP fabrication parameters to obtain a high-performance PEM with proton conductivity higher than commercial Nafion at low and high hydration. USP allowed an increased proportion of PTI to be added over conventional solution casting method without deleterious effect of restacking and agglomeration. Neutron reflectivity (NR) and quasi-elastic neutron scattering were used to probe the structure and dynamics of polymer framework, water and ions. Comparison with pure Nafion and other composite materials provided insight into the way in which graphitic additives arrange inside the polymer aggregates and enhance water transport. This work highlights PTI as an effective 2D material for structural tuning of phase separated polymers for contiguous proton and water transport within the semi-crystalline framework, as well as highlighting an experimental approach to reveal nanoscale structure-property relationships.[1] T. M. Suter, T. S. Miller, J. K. Cockcroft, A. E. Aliev, M. C. Wilding, A. Sella, F. Corà, C. A. Howard, P. F. McMillan, Chem. Sci., 2019, 10, 2519-2528[2] F. Foglia, A. J. Clancy, J. Berry-Gair, K. Lisowska, M. C. Wilding, T. M. Suter, T. S. Miller, K. Smith, F. Demmel, M. Appel, V. G. Sakai, C. A. Howard, M. Tyagi, F. Coràand, and P. F. McMillan, Sci. Adv., 2020, 6, eabb6011