Ionic conductivity in electrochemical application is a function of the number of charge carriers and their mobility. In the electrodes containing ionomer both numbers are high, whereas in deionized water, the ion concentration is ~7 orders of magnitude lower than that in a polymer electrolyte, such as Nafion. In ionomer-free electrodes for fuel cell applications, such as Pt-black or extended-surface Pt electrodes, water assists in ion conduction and expected ionic conductivity should be close to that of deionized (DI) water. However, ionic conductivity of ionomer-free electrodes was measured to be 2-3 orders of magnitude higher than DI water (1, 2). These findings suggest that local confinement and electric double layers (EDLs) affect ion concentration or/and ion mobility, especially in porous high surface area electrodes. Over the years several experimental and modeling studies propose two ion transport mechanisms: 1) Ions transport through surface adsorption and subsequent neutral species surface diffusion and 2) water-mediated ion conduction due to EDLs (3). To understand these mechanisms, model electrodes with controlled geometry and active area have to be designed.

In this study, Atomic Layer Deposition (ALD) was used to fabricate model thin-film electrodes to study ion-transport mechanisms under varied levels of relative humidity. The ALD gas-phase fabrication process provides a uniform and conformal Pt layer with precise thickness on a high aspect-ratio substrates. Here, we fabricate Pt electrodes with a novel thermal ALD process. The activity of the deposited electrodes is characterized with RDE tests and within actual fuel cell hardware. Ionic conductivity studies are conducted in a membrane electrode assembly (MEA) fuel cell set-up under various relative humidities and operating potentials to elucidate the range of proton transport mechanisms. Furthermore, various interlayers were added to the MEA to switch on and off the ion transport processes to separate H_ad surface diffusion from charged proton surface migration mechanisms.

Figure 1. SEM images of the Pt nanoelectrode array a) after 500 cycles showing a thickness of 8 µm b) honeycomb structured in-plane view after 500 cycles c) cross sectional side view of 5 µm after 300 cycles d) In-plane view of the nanoelectrode array after 300 cycles e) In-plane view of the nanoelectrode array fabricated using 100 nm pore sized AAO and 200 cycles.

References

1. K. C. Hess, W. K. Epting and S. Litster, Analytical Chemistry, 83, 9492 (2011).
2. P. K. Sinha, W. Gu, A. Kongkanand and E. Thompson, Journal of Electrochemical Society, 158, B831 (2011).
3. I. V. Zenyuk and S. Litster, Electrochimica Acta, 146, 194 (2014).