Book	FZJ-2024-00831

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Investigation of the influence of cation acidity in proton-conducting ionic liquids for the application as electrolyte in a future intermediate-temperature fuel cell

Suo, Y.RWTH* ; Korte, C.FZJ*RWTH*

2023
RWTHpublications online

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Please use a persistent id in citations: doi:10.18154/RWTH-2023-08372

Abstract: The operation of a Polymer electrolyte membrane fuel cells in an intermediate temperature range between 100–140 °C (IT-PEMFCs) would allow a significant simplified setup for system (no water management, more efficient cooling), and thus a cost cut. The use of proton-conducting ionic liquids (PILs) as electrolytes for future IT-PEMFCs is a promising approach, because of their unique properties. As water is produced during fuel cell operation, the bulk properties of PILs and the properties of the catalyst/PIL interface, including the ORR kinetics, are significantly affected. It is therefore crucial to investigate thoroughly the interface structure and properties of binary PIL/H2O systems. In this thesis, three PILs with different cation acidities were selected to investigate the influence of the cation acidities on the physicochemical and electrochemical properties of PILs: 2-sulfoethylmethylammonium triflate [2-Sema][TfO] ([2-Sema]+: pKa = 0.94), 1-Ethylimidazolium triflate [EIm][TfO] ([EIm]+: pKa = 7.70) and Diethylmethylammonium triflate [Dema][TfO] ([Dema]+: pKa = 10.55). PILs with higher pKa exhibit better thermal stability due to a complete proton transfer to the cation. All three PILs are thermally stable at 120 °C. The conductivity of PILs depends strongly on their cation acidities and the interaction between cations and anions. All three PILs have 3–40 times higher values of the product of oxygen diffusivity and solubility (DO2 ∙ cO2) compared to the common electrolyte H3PO4 for high temperature (HT-)PEMFCs, which is beneficial especially for the mass transport in the ORR. The analysis of polarization curves and cyclic voltammograms show that the ORR on platinum electrodes takes place via an associative mechanism with molecular adsorbed oxygen, where the first electron and proton transfer to the oxygen molecules is rate determining. It turned out, that only [2-Sema][TfO] with the highly acidic [2-Sema]+ cation provides sufficient current densities in the potential range relevant for fuel cell application. Hence, a high acidity of the PIL cation is crucial. Moreover, only the [2-Sema]+ cation participates in the proton transfer to the oxygen molecules. On the other hand, the significantly higher DO2 ∙ cO2 value and ORR limiting current of the low acidic PILs suggests combining favorable kinetic and bulk properties by mixing PILs with highly and low acidic cations. For a future improvement of the ORR, a detailed knowledge of the structure and properties of the catalyst/PIL interface, such as the double layer capacitance, is mandatory. The impedance analysis revealed high and low frequency capacitances, C1 and C2, assigned to (fast) ion transport and (slow) pseudo-capacitive processes. The PILs exhibit significant differences in the capacitance C1, which are attributed to a higher compacity of the ions in the double layer in case of [2-Sema][TfO] compared to the low acidic PILs. The increase of peaks of C1 and C2 in the HUPD (hydrogen underpotential deposition) and Pt oxidation region correlate well with the corresponding effects in cyclic voltammograms. By addition of water, C1 tends to increase. This is explained by a change in the dielectric properties and the structure of the double layer, including a lower stiffness of the ion layers, a higher permittivity and a lower thickness.

Keyword(s): Hochschulschrift ; fuel cells ; protic ionic liquids ; oxygen reduction reaction (ORR) ; electrode kinetics ; double layer capacitance ; Brennstoffzellen ; protische ionische Flüssigkeiten ; Sauerstoffreduktionsreaktion ; Elektrodenkinetik ; Doppelschichtskapazität

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Contributing Institute(s):

1. Elektrochemische Verfahrenstechnik (IEK-14)

Research Program(s):

1. 1231 - Electrochemistry for Hydrogen (POF4-123) (POF4-123)

Appears in the scientific report 2023

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