Influence of Acidity, Water and Temperature on the Double Layer Properties of Protic Ionic Liquids for Future Fuel Cell Applications

Abstract

Polymer electrolyte membrane fuel cells (PEMFC) are a viable alternative to combustion engines and rechargeable batteries for automotive applications. However, the operating temperature of PEMFCs using sulfonated fluoropolymers, e.g. NAFION®, is limited below 80 °C (ambient pressure), because the proton conduction relies on the presence of water. A PEMFC operating above 100 °C would allow a much more simplified system setup for water and heat management. This requires novel non-aqueous protic electrolytes. Proton conducting ionic liquids (PIL) are promising candidates. [1,2]. However, the fuel cell relevant electrode reactions—oxygen reduction and hydrogen oxidation reaction (ORR/HOR)—are not as well understood as in aqueous electrolytes.In this study, we employed electrochemical impedance spectroscopy (EIS), cyclovoltammetry (CV), chronoamperomery (CA) and steady state current measurements to elucidate the double layer properties of the platinum electrode/PIL interface, the kinetics and possible mechanism of the ORR. Three PILs with different cation acidities with an Brønsted-acidic cation [HA+][X−] are compared, [Dema][TfO], [1-EIm][TfO] and [2-Sema][TfO].Comparing the PILs with different cation acidity strongly suggest that the first reduction step including the proton transfer to the (catalytic) active sites on the electrode is mainly determining the ORR rate. The presence of residual water, unavoidable also at fuel cell operation >100 °C, is another important parameter. H2O modifies the ordered structure of the electrochemical double layer. Its protolysis equilibrium with an acidic PIL cation results in the formation of H3O+ that serves as a proton donor in the rate determining step and thus influences the ORR kinetics. Highly acidic PIL cations serve as a proton donor as well, particularly at low H2O concentrations, whereas the role of H3O+ as proton donor in the ORR becomes more prominent at higher water concentrations [3]. In low acidic PILs, H3O+ is the predominant proton donor and the ORR rate is significantly smaller resulting in considerably higher overpotentials. Thus, the onset potential of the ORR in a PIL based fuel cell will depend on both the concentration of residual water and the PIL cation acidity.Plots of the potential-dependent data from EIS measurements in the complex capacitance plane (CCP) show that at least two differential double layer capacitances are present, depending on the cell potential U (vs. RHE), water concentration c(H2O) and temperature T. The double layer properties of the highly acidic [2-Sema][TfO] are significantly different compared to the less acidic PILs [1-EIm][TfO] and [Dema][TfO]. The potential dependent capacitance curves were discussed by taking a mean field model, the presence of water and short range correlations of ions into account. [4, 5] The combined electrochemical kinetics and double layer measurements provide a deeper insight into the double layer structure at the Pt electrode/PIL interface to reveal the rate limiting parameters of the ORR and its mechanism.[1] K. Wippermann, J. Giffin, S. Kuhri, W. Lehnert and C. Korte, Phys. Chem. Chem. Phys. 19, 24706 (2017)[2] K. Wippermann, Y. Suo and C. Korte, J. Phys. Chem. C 125(8), 8 (2021)[3] H. Hou, H. M. Schütz, J. Giffin, K. Wippermann, X. Gao, A. Mariani, S. Passerini and C. Korte, ACS Appl. Mater. Interfaces 13, 8370 (2021)[4] Z. A. H. Goodwin, G. Feng and A. A. Kornyshev, Electrochim. Acta 225, 190 (2017)[5] J. Friedl, I. I. E. Markovits, M. Herpich, G. Feng, A. A. Kornyshev and U. Stimming, ChemElectroChem 4, 216 (2017)Figure: ORR rate constant of PILs [HA+][X−] with different cation acidity vs. content of residual water (50 mol% H2O corresponds to 5–6 wt% H2O) Figure 1