The Influence of Acidity, Water Content and Temperature of Ionic Liquids on the Double Layer Properties of Pt/Pil Interface
Authors: Suo Yanpeng , Wippermann Klaus , Rodenbücher Christian , Korte Carsten 
Published: February 2020

The use of green energy for transport applications is becoming ever more important, with fuel cells showing the potential to be a highly competitive candidate in the automobile industry. Traditional low temperature polymer electrolyte fuel cells (LT-PEFCs) can only be operated at temperatures below 80 °C. High temperature polymer electrolyte fuel cells (HT-PEFCs), based on phosphoric acid (H3PO4) doped polybenzimidazole membranes (PBI), can be operated in a temperature range of 160-180 °C. Because of the higher operating temperature, HT-PEFCs have many advantages, such as a much easier water and heat management, a more compact cooling system and a higher CO tolerance in the feed gas.[1] However, the major drawback of H3PO4 as an electrolyte is the poisoning effect on the redox catalyst and low O2 solubility. This leads to a significant decrease in the cathodic oxygen reduction reaction (ORR) kinetics.

An approach to overcome these disadvantages is to substitute the electrolyte H3PO4 with a proton conducting ionic liquids (PILs). PILs exhibit a high ionic conductivity, a wide electrochemical window and a high chemical and thermal stability.[2] Therefore, they are promising candidates as alternative non-aqueous electrolytes in intermediate temperature fuel cells for operation temperatures at 120 °C.

In this contribution, a study on the double layer properties of the interface between platinum and PILs, investigated by means of electrochemical impedance spectroscopy (EIS), is presented. Three PILs – [2-SEMA][TfO], [1-EIm][TfO] and [DEMA][TfO] – with different cation acidities are compared. Water is produced unavoidably during fuel cell operation at the anode side. Even at temperatures above 100 °C residual water will be present in the electrolyte. Thus, for all three ionic liquids, the layer structures at the interface of the electrodes are studied as a function of the water concentration and temperature by means of atomic force microscopy (AFM).

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 obtained, depending on the potential (U = 0 – 1.5 V vs RHE), water concentration (c
H
2
O = 0.6 – 7.2 wt%) and temperature (T = 30 – 90 °C). The differential capacitance of the fast process can mainly be attributed to charge redistributions at the interface and the ion transport in the double layer. The slow capacitive process is probably caused by charge redistributions in the innermost ion layer, which requires a higher activation energy for the reorientation of the ions, as the ions in this layer are strongly bound to the electrode.

In addition, the PILs are studied as a function of c
H
2
O and T by means of AFM to obtain a more direct image of the structure at the electrode interface. It is known that ionic liquids usually form a structure of alternating cation and anion layers, significantly different compared to (diluted) aqueous electrolytes. In the case of [DEMA][TfO] up to four distance dependent force maxima can be recognized, which indicate the ordered interfacial nanostructure. The structure at the interface is affected by the molecular structure and size of the cations (and anions) of the PIL, as well as by the water, absorbed to the surface and incorporated in the cation and anion layer, which results in a perturbance of the structure.

In conclusion, the combined electrochemical double layer measurements and microscopic studies provide a deeper insight into the double layer structure at the interface between the Pt electrode and ionic liquids depending on the PIL acidity, water concentration and temperature.

[1] K. Wippermann, J. Wackerl, W. Lehnert, B. Huber, C. Korte, J. Electrochem., 2016, 162, F25-F37.

[2] K. Wippermann, J. Giffin, S.Kuhri, W. Lehnert, C. Korte, Phys. Chem. Chem. Phys., 2017, 19, 24706-24723.