Impedance Spectroscopy of Direct Ethanol Fuel Cells with Anodes Containing Pt Group Metals
Authors: Polczynski Piotr , Miklolajczak Aleksandra , Lewera Adam , Jurczakowski Rafal Robert 
Published: February 2020

Alcohols might be a promising alternative to gaseous hydrogen in fuel cells technologies. In this study ethanol electrooxidation and oxygen electroreduction were studied by using classical electrochemical methods and electrochemical impedance spectroscopy (EIS). Studies have been carried out for both Pt and Pt-Ru catalysts for the cathode and anode separately in 0.5 M sulfuric acid solutions and for the assembled fuel cells with Nafion membranes. EIS investigations for anodes were carried out in the potentiodynamic conditions because otherwise a stationary state necessary for impedance measurements does not occur [1]. Impedance of the ethanol electrooxidation was recorded in voltammetric experiments with a sinusoidal perturbation of single frequency and small amplitude additionally applied at the electrode. After several voltammetric runs impedance spectra were built for selected electrode potentials. Thus obtained time and potential resolved impedance spectra were characterised by good reproducibility and their validity was confirmed in Kramers–Kronig transformations. The impedance spectra were qualitatively similar to those reported earlier and obtained by using Dynamic EIS (DEIS) experiments [1]. Complex mechanism of ethanol oxidation manifest itself as a negative resistance of the charge transfer, which is related to decrease in the rate of ethanol electrooxidation with increasing anodic potential. This effect is also accompanied by a significant drop in the capacity of the double layer and can be ascribed to the accumulation of carbon monoxide at the electrode surface.

Impedance measurements were also performed for assembled fuel cells under different loads with current perturbation equal to 5% of the overall current density. Since the stationary state was assured by a constant flow of the reactants, in this configuration impedance was recorded in a broader frequency range and for much lower frequencies than in ex situ experiments. Equivalent circuit were proposed to model experimentally recoded spectra. The resistance of the charge transfer and the capacity of the double layer at the cathode were virtually independent from the oxygen partial pressure within studied range. This effect, observed particularly for current densities greater than 4 mA cm-2, suggest nearly constant potential of the cathode. Ethanol concentration largely affects impedance of the anode. For higher ethanol concentration a decrease in the double layer capacity accompanied by an increase in the absolute values of the negative resistance were observed. These effects reflect a strong poisoning of the electrode surface. Results points out that the current efficiency is strongly limited by the electrooxidation process at the anode. The influence of other experimental conditions such as temperature and fuel to oxygen stoichiometry on the impedance of direct ethanol fuel cells will be also discussed.

[1] E.Desilets, A. Lasia, Electrochimica Acta 78 (2012) 286-293