We report on the role of as-grown and plasma treated multi-walled carbon nanotubes (MWCNTs) as cathode components in solid acid fuel cells (SAFCs). SAFCs combine a true solid state, proton conducting electrolyte, CsH₂PO₄ with good conductivity at mild temperatures slightly above 228 °C, with fuel flexibility and a high resistance to catalyst poisoning.^1,2 The widespread application of solid acid fuel cells has been partly hindered by the necessity of the precious metal catalyst platinum in the electrodes.^3,4
A general new approach in electrode catalysis bearing immense potential for electrochemical technologies is the prospect of precious metal free electrodes based on carbon nanomaterials.^5,6
MWCNTs were synthesised by chemical vapour deposition using Ni nanoparticles as the growth catalyst on carbon paper current collectors. Oxygen plasma treatment was applied on selected samples with the dual effect of removing remaining amorphous carbon and increasing the concentration of oxygen functional groups on the surface. Symmetric electrochemical cells (MWCNTs|CsH₂PO₄|MWCNTs) were fabricated by uniaxial cold-pressing for impedance spectroscopy in a humidified oxygen environment at 240 °C. A good long term stability over a 19 day period and electrode impedances as low as 22 Ω cm²were measured, Figure 1. The oxygen plasma treated samples showed significantly lower impedances compared to as-synthesized. However, the application of a 500 mV DC bias voltage caused a dramatic drop in the electrode impedance when using as-synthesised MWCNTs. The impedance data suggests the electrochemical activity towards oxygen reduction and water splitting reaction after an initial reversible activation step. However, carbon corrosion is shown to have a non-negligible but small contribution to the measured currents. Scanning electron microscopy, Raman and XPS spectroscopy before and after the electrochemical measurements show increased oxygen concentration, increased graphitisation and slight morphological changes in all electrodes.

Caption Figure 1: (a) Scanning electron micrograph of representative MWCNTs on carbon paper fiber. (b) Electrode impedance of symmetric electrochemical cells (MWCNTs|CsH₂PO₄|MWCNTs) measured in humidied oxygen at 240 °C as a function of oxygen concentration of MWCNTs.

References
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