2-Sulfoethylammonium Trifluoromethanesulfonate as an Ionic Liquid for High Temperature PEM Fuel Cells

Abstract

Tafel Oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFC).-High temperature polymer electrolyte fuel cells (HT-PEFCs) are currently based on phosphoric acid-doped polybenzimidazole-type membranes (PBI) and are operated in a temperature range of 120-180• C. HT-PEFCs have several advantages over low temperature PEFCs, such as: easier water and heat management, the possibility of recovering high-grade waste heat, a more compact cooling system and a higher CO tolerance by the anode catalyst.However, a major drawback of HT-PEFCs is the sluggish oxygen reduction reaction (ORR) kinetics in the presence of phosphoric acid. This is primarily caused by three effects: (i) The inhibition effect of phosphoric acid on the ORR because of the poisoning of the Pt catalyst due to the adsorption of the H 3 PO 4 species onto active catalyst sites. (ii) The low solubility of oxygen in phosphoric acid. (iii) The slow diffusion of oxygen through the film of phosphoric acid which covers the platinum catalyst.The adsorption of phosphate by platinum has been studied for more than 30 years 1-15 by means of cyclic voltammetry (CV), [2][3][4][5]9,[11][12][13][14][16][17][18][19][20][21] rotating disc electrode measurements (RDE), [2][3][4][5]8,9,11,13,14 electrochemical impedance spectroscopy (EIS), 13,16,22 transient measurements, 9,23,24 Fourier transform infrared spectroscopy (FTIR), 20,25-27 electrochemical quartz crystal microbalance (QCM), 18 radio tracer experiments, 28 X-ray photoelectron spectroscopy (XPS) 17 and X-ray absorption spectroscopy (XAS). 14,15 The in-operando XAS measurements recently published by Kaserer et al. show the adsorption behavior of H 3 PO 4 species on Pt/C fuel cell cathode catalysts in the temperature range of 50-170• C under the operating conditions of an HT-PEFC. 15In diluted aqueous solutions and at moderate temperatures, sulphuric acid and sulfonic acids like methanesulfonic acid (MSA) or trifluoromethanesulfonic acid (triflic acid, TFMSA) have been found to be less poisonous than phosphoric acid because sulfonate groups are less strongly adsorbed on Pt. 3,29,30 For example, Zelenay et al.,29 in a comparative study of phosphoric acid and TFMSA, showed that * Electrochemical Society Active Member. z E-mail: k.wippermann@fz-juelich.de 0.02 M TFMSA is indeed less strongly adsorbed by comparison to 0.02 M phosphoric acid at room temperature. However, the free space available for oxygen reduction is found to be similar at elevated temperatures and for concentrated solutions. 29 Under these conditions, the advantage of TFMSA lies in its higher oxygen solubility and diffusivity rather than its poisoning effect. 29 Although this result cannot be generalized, it still indicates the importance of oxygen solubility and the diffusion coefficient of oxygen in the electrolyte being used. Proton conducting ionic liquids (PIL).-With respect to alternative electrolytes for HT-PEFCs, proton-conducting ionic liquids (PILs) appear to be suitable, as they do not rely on solvents like water. Moreover...

Atomic Force Spectroscopy on Ionic Liquids

“…When the ionic liquid is in contact with a solid, the interface structure is also affected by the topography, atomic arrangement, reactivity, local charge, and polarizability of the solid surface [11]. This interface structure becomes additionally distorted when electrical potential is applied to the solid, as is the case in electrochemical cells such as PEMFCs that employ ionic liquids as an electrolyte [48]. In consequence, a complex layered structure is formed in the ionic liquid that determines the kinetics of ion transport and reactions at the interface.…”

PBI‐type Polymers and Acidic Proton Conducting Ionic Liquids – Conductivity and Molecular Interactions

“…Despite the high operation temperature of 160°C, the presence of H 3 PO 4 causes a slow cathodic oxygen reduction reaction kinetics (ORR). This is primarily caused by an inhibition effect due to a poisoning by adsorption of H 3 PO 4 species onto active catalyst sites [4]. Also discussed is a low solubility of O 2 and a slow diffusion of O 2 through the H 3 PO 4 film which covers the platinum catalyst [5].…”

In Situ Determination of the Water Content of Ionic Liquids

“…Electrochemical applications, which demand practically anhydrous electrolytes include, for instance, lithium batteries, 1,2 electrochemical supercapacitors, 3,4 solar cells, 5,6 field-effect double-layer transistors 7,8 and electrochemical synthesis. 9,10 For other applications, the presence of water is either unavoidable (e.g., fuel cells [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] ) or even desirable (humidity sensors 28 or drying agents [29][30][31][32] ).…”