Phosphoric acid is used in most high temperature polymer electrolyte membrane fuel cells (HT-PEFC) especially because it exhibits high proton conductivity together with high availability. The nature of the proton conductivity in phosphoric acid and the interaction between the polymer matrix and phosphoric acid were investigated recently by MD simulations [1]. Experimental conductivity data are available extensively for temperatures up to 100°C and different water contents, while it gets scares for higher temperatures [2, 3]. The reason for this is the complex nature of the phosphoric acid, tending to polymerize from its monomeric form at low temperature and surplus of water towards chain or even ring structures at temperatures above 100°C with depletion of water [4]. However, the different species have different chemical but also physical properties. This circumstance is either neglected or not considered for most examinations. In contrast to most published methods starting with P2O5 and defined water content, we start from aqueous orthophosphoric acid (85w%) and impose a fixed water vapour pressure using a climate chamber. The development of the equilibrium state with the gas phase is investigated using time sequenced impedance measurements. Temporal snapshot samples are taken and examined using Karl-Fischer titration method to determine the content of unbound water and Raman spectroscopy to determine changes in molecular interactions. The transitions between the different species and adducts of phosphoric acid can be precisely determined.

Liquid – gas phase equilibria of phosphoric acid at high temperature electrolyte polymer fuel cell condition

CONTI, FOSCA;
2014

Abstract

Phosphoric acid is used in most high temperature polymer electrolyte membrane fuel cells (HT-PEFC) especially because it exhibits high proton conductivity together with high availability. The nature of the proton conductivity in phosphoric acid and the interaction between the polymer matrix and phosphoric acid were investigated recently by MD simulations [1]. Experimental conductivity data are available extensively for temperatures up to 100°C and different water contents, while it gets scares for higher temperatures [2, 3]. The reason for this is the complex nature of the phosphoric acid, tending to polymerize from its monomeric form at low temperature and surplus of water towards chain or even ring structures at temperatures above 100°C with depletion of water [4]. However, the different species have different chemical but also physical properties. This circumstance is either neglected or not considered for most examinations. In contrast to most published methods starting with P2O5 and defined water content, we start from aqueous orthophosphoric acid (85w%) and impose a fixed water vapour pressure using a climate chamber. The development of the equilibrium state with the gas phase is investigated using time sequenced impedance measurements. Temporal snapshot samples are taken and examined using Karl-Fischer titration method to determine the content of unbound water and Raman spectroscopy to determine changes in molecular interactions. The transitions between the different species and adducts of phosphoric acid can be precisely determined.
2014
65th Annual Meeting of the International Society of Electrochemistry
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/3157728
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