Influence of the electrocatalyst layer thickness on alkaline DEFC performance

Abstract

Determining the optimum layer thickness, for the anode and cathode, is of utmost importance for minimizing the costs of the alkaline direct ethanol fuel cell (DEFC) without lowering the electrochemical performance. In this study, the influence of layer thickness on the performance of the ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR) in an alkaline medium and resistance was investigated. The prepared gas diffusion electrodes (GDEs) were fully characterized, with scanning electron microscopy to determine the layer thickness and electrochemically in half-cell configuration. Cyclic voltammetry and polarization curve measurements were used to determine the oxidation and reduction processes of the metals, the electrochemical active surface area, and the activity towards the ORR and EOR. It was demonstrated that realistic reaction conditions can be achieved with simple and fast half-cell GDE measurements. Single cell measurements were conducted to evaluate the influence of factors, such as membrane or ethanol crossover. In addition, electrochemical impedance spectra investigation was performed to identify the effect of layer thickness on resistance. This successfully demonstrated that the optimal layer thicknesses and high maximum power density values (120 mW cm^-2) were achieved with the Pt-free catalysts and membranes used.

Fig. 1. Experimental setup for the half-cell measurements (CV: cyclic voltammetry, ORR: oxygen reduction reaction, EOR: ethanol oxidation reaction, WE: working electrode, CE: counter electrode, RE: reference electrode, and GDL: gas diffusion layer).

Fig. 2. SEM images of the cathodes and anodes with varying active catalytic material loadings (a) C1, (b) C2, (c) C3, (d) C4, (e) A1, (f) A2, (g) A3, and (h) A4.

Fig. 3. Electrochemical half-cell characterization of (a) the Ag-Mn_xO_y/C (C1–C4) and (b) the Pd₈₅Ni₁₀Bi₅/C (A1–A4) electrodes (inset: ECSA) with a scan rate of 10 mV s⁻¹ in 5 M KOH at RT.

Fig. 4. Polarization curves for the determination of the ORR activity for the cathodes C1–C4 under (a) condition I and (b) condition III (resistances of the impedance spectra for iR-compensation in the insets); (c) onset potential at −10 mA cm⁻² and (d) current density at 0.75 V vs. RHE for the different conditions.

Fig. 5. Polarization curve determination of the EOR activity for the anodes A1–A4 under (a) condition I and (b) condition III (resistances of the impedance spectra for iR-compensation in the insets); (c) onset potential at 10 mA cm⁻² and (d) current density at 0.5 V vs. RHE for the different conditions.

Fig. 6. Comparison of the IR analysis (bars) and the power density (grey line) during sampling for the evaluation of the ethanol conversion.

Fig. 7. Power density (filled symbols) and polarization curves (unfilled symbols) of the single cell measurements of MEA1–MEA4 under (a) condition I and (b) condition III; (c) open circuit potential and (d) maximum power density for the different conditions.

Fig. 8. Electrochemical impedance spectra measurements (data: circles—ZHIT algorithm is used and fitted spectrum: line) for (a) MEA1, (b) MEA2, (c) MEA3, (d) MEA4 and (e) comparison of the resistances.

Fig. 9. Power density (filled symbols) and polarization curves (unfilled symbols) of the single cell measurements of MEA5–MEA8 under (a) condition I and (b) condition III; (c) open circuit potential and (d) maximum power density for the different conditions.

Fig. 10. Electrochemical impedance spectra measurements (data: circles—ZHIT algorithm is used and fitted spectrum: line) for (a) MEA5, (b) MEA6, (c) MEA7, (d) MEA8 and (e) comparison of the resistances.