The combination of conducting atomic force microscopy (AFM) and electrical measurements (AC impedance spectroscopy and cyclic voltammetry) offers many advantages for measuring fuel cell electrode kinetics. The use of a conducting AFM probe as a nanoelectrode enables isolation and characterization of a single electrode-electrolyte interface without the need for a reference electrode. Furthermore, this technique permits studies of the spatial dependence of mechanistic phenomena while providing controllable electrode-electrolyte contact areas. The feasibility of using a nanoscale probe as a fuel cell electrode has been examined for the polymer electrolyte membrane system at room temperature (1-2). Higher temperature capabilities would make this technique useful for a wide variety of material systems, including low-to-intermediate temperature solid electrolytes.

Here, we demonstrate the viability of conducting AFM under controlled environments and at temperatures relevant to proton conducting solid acid compounds. Solid acid compounds have been demonstrated as viable proton conducting electrolytes for fuel cells (3-4), with peak power densities of ~ 400 mW/cm² at ~ 240 ºC (5). Such fuel cells provide several advantages over polymer membrane fuel cells, including improved kinetics due to higher operating temperatures, impermeability of the membrane to fuels, and elimination of the need for complex water management systems. However, activation overpotential losses, particularly at the cathode, limit the performance (6), and electrode kinetics are not yet well understood.

We select cesium hydrogen sulfate, CsHSO₄, as a representative solid acid electrolyte for the study of oxygen electroreduction. Experiments are performed with a platinum-coated probe in contact with CsHSO₄. The Pt-based counter electrode, which covers the entire opposing area of the electrolyte, is effectively reversible and contributes negligible overpotential to the measurements. We discuss the experimental challenges associated with high impedance systems and mitigation strategies to extract meaningful information. We also present impedance spectra and cyclic voltammograms for Pt|CsHSO₄.

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5. T. Uda and S. M. Haile, Electrochem Solid State Lett 8(5), A245-A246 (2005)

6. S. M. Haile, C. R. I. Chisholm, K. Sasaki, D. A. Boysen, T. Uda, Faraday Discussions 134, 17-39 (2007)