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...

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Polybenzimidazole (PBI) membranes doped with phosphoric acid are used as proton-conducting membrane in high-temperature polymer electrolyte fuel cells. There is no general model for the thermodynamics of the absorption process over the whole accessible doping range for all published data. The interactions between H 3 PO 4 and polymer chains, the polycondensation equilibria of H 3 PO 4 and the implications on proton conductivity are still largely unknown. In this study, we demonstrate that the uptake of a protic electrolyte by a polymer with basic moieties, i.e. a PBI-type polymer, can be described satisfactorily with a BET-like absorption isotherm. The absorption equilibria of the uptake process are analysed using literature data on the H 3 PO 4 , H 2 SO 4 and HClO 4 uptake of non-crosslinked m-PBI and AB-PBI as well as using our own investigations on the H 3 PO 4 uptake of a commercial crosslinked PBI derivative, Fumapem AM-55. In addition to the thermodynamic data of the absorption process, Raman data on m-PBI taken from the literature and our own Raman investigations on Fumapem AM-55 are taken into account. It is possible to correlate domains in the absorption isotherms with specific features in the Raman spectra. Two stages for the uptake of a protic electrolyte can be distinguished: (i) the protonation of the polymer chains and thus coulombic interactions with the electrolyte anions, (ii) the formation of H bonds directly with the chains and with electrolyte molecules that are still absorbed.

Synthesis and sintering properties of the (La 0.8 Ca 0.2−x Sr x )CrO 3 samples doped by two alkaline earth metals in comparison to the doped only by one alkaline earth metal were evaluated by phase analysis, sintering properties, thermal expansion behaviors, and electrical conductivity. The sintered (La 0.8 Ca 0.2−x Sr x )CrO 3 (x = 0, 0.05, and 0.1) and (La 0.8 Ca 0.2−x Sr x )CrO 3 (x = 0.2) were found to have orthorhombic and rhombohedral symmetries, respectively. Relative density of the (La 0.8 Sr 0.2 )CrO 3 sample sintered at 1500 • C for 5 h was lower than that of the (La 0.8 Ca 0.2−x Sr x )CrO 3 (x = 0, 0.05, and 0.1) sample. TECs of the (La 0.8 Ca 0.2−x Sr x )CrO 3 (x = 0, 0.05, 0.1, and 0.2) in air were 11 × 10 −6 / • C, 11.2 × 10 −6 / • C, 11.2 × 10 −6 / • C, and 11.3 × 10 −6 / • C, respectively. The electric conductivity of the (La 0.8 Ca 0.2−x Sr x )CrO 3 sample was determined.