Ionic liquids as electrolytes in PEFCs for operating temperatures of around 120 °C

The polymer electrolyte fuel cell (PEFC) with a sulfonated fluoropolymer (PFSA) membrane has set standards for mobile applications over the last few decades. PEFCs with PFSA membranes such as NAFION® or AQUIVION® have reached a mature level of performance and durability. The current state of the art is sufficient for initial market penetration in the field of electromobility. To further expand the field of potential applications and accelerate commercialization, it is necessary to continue intensive research into catalyst and electrolyte materials to enable new cell concepts. PFSA membranes are dependent on sufficient water swelling and corresponding PEFCs require wetting of the supplied hydrogen and air as well as water recycling from the exhaust gas. This limits the (atmospheric) operation to <80 °C. Otherwise, dehydration occurs and the conductivity strongly decreases.

PEFCs operated at 120 °C would provide much more effective cooling at high electrical loads and allow for waste heat recovery. This requires an electrolyte, the proton conduction of which is not based on the presence of the amphoter water. A non-aqueous proton-conducting electrolyte also makes it possible to dispense with active water management. Together with more effective cooling, this enables a simplified design of the overall system and thus much more effective easier system integration of PEFCs, especially in the field of electromobility (passenger cars). A higher operating temperature also reduces the sensitivity to impurities in the hydrogen or air. Such medium-temperature PEFCs, which use hydrogen from renewable energy, can meet the requirements in the transport sector better than NT-PEFCs. For operating temperatures above 120 °C, new membrane materials and proton-conducting electrolytes are required that do not lose their conductivity even at low water activities. Strongly acidic, proton-conducting ionic liquids (PILs) based on sulfonic acids are a promising approach.

In high-temperature polymer membrane fuel cells (HT-PEFCs), a polymer membrane loaded with phosphoric acid is currently used as the electrolyte. Phosphoric acid shows a very high proton conductivity in the temperature range above 150 °C, even at very low water vapor partial pressures (i.e. practically anhydrous), which is due to a very pronounced autoprotolysis and a cooperative proton transfer mechanism (PBI/H₃PO₄ ). However, the phosphoric acid or hydrogen phosphate ions block parts of the surface of the platinum catalyst and lead to performance losses, particularly at the cathode.

We study ionic liquids prepared from a simple aminoalkylsulfonic acid (e.g. from taurine and its derivatives) as a protonable base and a superacid such as trifluoromethanesulfonic acid. Such ionic liquids based on sulfoalkylammonium cations show high acidity. The experimental investigations initially focus on ionic liquids with 2-sulfoethylammonium cations (see Fig. 1). The protonation of the aminoethansulfonic acids can be detected by Raman spectroscopy. Protonation weakens the bonds in the cation compared to the unprotonated form (particularly the C-S bond), which leads to a shift of characteristic bands towards smaller wavenumbers. The 2-sulfoethylammonium and 2-sulfoethylmethylammonium cation are preferably present in a gauche conformation (i.e. with an internal H-bridge (ring); see Fig. 1).

Electrochemical and thermal characterization

Important criteria for the practical application of these electrolytes are properties such as thermal and electrochemical stability, electrical conductivity, and the kinetics of the electrode processes relevant for the fuel cell (ORR and HOR).

Thermal analysis using methods such as TGA and DSC showed thermal stability up to 130 °C for the PILs studied so far [1]. Electrochemical measurements showed that these PILs are electrochemically stable throughout the investigated potential range [1]. The electrical conductivity of the PILs increases exponentially with the water content. So far, values of up to 120 mS/cm have been achieved.

The electrochemical kinetics at the cathode (i.e. the reduction of oxygen) is studied in detail using microelectrode and RDE methods. One cathode performance criterion is the magnitude of the current density as a function of the electrode potential. As shown in Fig. 2, the logarithmically plotted current density is actually three times higher in the presence of an ionic liquid – here [2-SeMA][TfO] – than in phosphoric acid. The kinetics and mechanism of oxygen reduction were studied in detail by simulating cyclic voltammograms on various acidic PILs [2]. It was found that oxygen reduction occurs via molecularly adsorbed oxygen and that the initial electron and proton transfer is rate-determining. An important result of these studies is that only PILs with a strongly acidic cation as a proton donor contribute to oxygen reduction, while PILs with weakly acidic cations are practically inactive in the potential range relevant for the fuel cell. The properties of the PIL ions therefore have a major influence on the oxygen reduction reaction [3,4], which occurs at the interface between the platinum catalyst and the respective ionic liquid. Detailed knowledge of the interfacial properties, such as the double layer capacitance, is therefore essential. For the interface Pt/[2-SeMA][TfO], the double layer capacitance was investigated as a function of electrode potential, temperature, and water content using impedance spectroscopy and cyclic voltammetry [5,6]. Further studies of the structure of the double layer are being conducted using atomic force microscopy and infrared spectroscopy (interfaces). Nuclear magnetic resonance (NMR) measurements are also carried out to investigate the mechanism of proton transport [7].

To be able to test the PILs later in the membrane electrode assembly (MEA) of a PEFC under operating conditions, the polymer membrane, for example polybenzimidazole (PBI), must first be loaded (doped) with a PIL. Initial doping experiments revealed a mass uptake of up to 100 weight percent. Raman spectroscopy was used to observe the doping process, verifying the protonation of the polymer by the PIL.

References

[1] Wippermann, K.; Wackerl, J.; Lehnert, W.; Huber, B.; Korte, C. 2-Sulfoethylammonium Trifluoromethanesulfonate as an Ionic Liquid for High Temperature PEM Fuel Cells, J. Electrochem. Soc. 2016, 163-2, F25–F37, http://dx.doi.org/10.1149/2.0141602jes.

[2] Wippermann, K.; Suo, Y.; Korte, C. Oxygen Reduction Reaction Kinetics on Pt in Mixtures of Proton-Conducting Ionic Liquids and Water: The Influence of Cation Acidity, J. Phys. Chem. C 2021, 125, 4423−4435, https://doi.org/10.1021/acs.jpcc.1c05151.

[3] Hou, H.; Schütz, H. M.; Giffin, J.; Wippermann, K.; Gao, X.; Mariani, A.; Passerini, S.; Korte, C. Acidic Ionic Liquids Enabling Intermediate Temperature Operation Fuel Cells. ACS Appl. Mater. Interfaces 2021, 13, 8370−8382, https://doi.org/10.1021/acsami.0c20679.

[4] Wippermann, K.; Korte, C. Effects of protic ionic liquids on the oxygen reduction reaction – a key issue in the development of intermediate-temperature polymer-electrolyte fuel cells. Current Opinion in Electrochemistry 2022, 32, 100894, https://doi.org/10.1016/j.coelec.2021.100894.

[5] Wippermann, K.; Giffin, J.; Kuhri, S.; Lehnert, W.; Korte, C. Influence of water content in a proton-conducting ionic liquid on the double layer properties of the Pt/PIL interface. Phys. Chem. Chem. Phys. 2017, 19, 24706, https://doi.org/10.1039/C7CP04003B.

[6] Wippermann, K.; Giffin, J.; Korte, C. In Situ Determination of the Water Content of Ionic Liquids. J. Electrochem. Soc. 2018, 165(5), H263–H270, http://dx.doi.org/10.1149/2.0991805jes.

[7] Lin, J.; Wang, L.; Zinkevich, T.; Indris, S.; Suo, Y.; Korte, C. Influence of residual water and cation acidity on the ionic transport mechanism in proton-conducting ionic liquids. Phys. Chem. Chem. Phys. 2020, 22, 1145–1153, https://doi.org/10.1039/C9CP04723A.