Study of Nanocellulose as a New Proton Conducting Material for Fuel Cells and Electrolysers

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Cellulose nanofibers (CNF) and cellulose nanocrystal (CNC) membranes were used as benchmark materials (lower and upper limit of crystallinity). Previously prepared sulfonated nanocellulose (S-CNF) was further investigated and characterized.The CNF membrane had high oxidative stability and retained most of its mass during Fenton test (97.5%), comparable to Nafion (99.3 %). S-CNF was less stable (64.3% mass-retention). The IEC of CNF, CNC and S-CNF are 0.016, 0.003 and 0.090 mmol g-1. A PEFC fabricated with a 30 micron thick S-CNF membrane showed an approx. 2 orders of magnitude lower in-situ hydrogen crossover than Nafion, meeting DOE requirements. In order to investigate the impact of crystallinity on conductivity, blends of CNF and CNC were prepared and electrochemical characterized via EIS. The proton conductivity increased with increasing CNC/CNF ratio, approaching the conductivity of pure CNC. Blocking layer measurements confirmed same proton conduction mechanism as in Nafion. In- and through-plane measurements unveiled high conductivity anisotropy in CNF and CNC. TEMPO-oxidized CNF was prepared. The proton conductivity increased by factor 10 (max. 1.9 mS/cm at 120°C, increased activation energy) and the membrane maintained high mechanical strength and bending capability.For benchmark reasons, water electrolysis MEAs were prepared using Nafion and Aquivion, and their performance and durability was investigated from 80 to 120°C. Aquivion based MEAs showed better performance and high durability at 120°C, whereas the Nafion-based MEA failed after only 15h operation.