Evaluation of Phosphoric Acid Leaching in PBI Membranes for High-Temperature PEM Fuel Cells By Ex-Situ and Operando Characterization Methods

Abstract

High-Temperature PEM Fuel Cells (HT-PEMFCs) based on PA-doped-PBI membranes have become a promising sustainable power generation system due to their high proton conductivity, high tolerance to hydrogen impurities, thermal stability and high utilization of waste heat [1]. Unlike low-temperature fuel cells based on perfluorosulfonic acid polymers (Nafion), HT-PEMFCs do not need water to act as a proton conductor, instead a hydrogen-bonded matrix is established between PA and PBI, negating the need for humidification in the system. As a result, the proton conductivity of the PA-doped PBI membrane is related to its PA loading. When PA is added to the PBI membrane, each PBI repeat unit can only bond with two PA molecules, leading to higher doping levels due to PA distributed in the PBI matrix in a free state. This extra PA leaches from the membrane to the electrodes during cell operation [2] resulting in several problems, the reduction of the proton conductivity of the membrane, blocking of the pathway for the gases to and from catalytically active sites and drive the migration of Pt-based catalyst, resulting in an increase in both mass and charge transfer resistances [3].To prevent acid leaching out of the membrane and the subsequent negative effects on the fuel cell performance, Single Layer Graphene (SLG) has been incorporated as barrier material between the PEM and cathode/anode. SLG acts as sieve material to block all atoms and molecules from passing through with the exception of protons improving the performance and the long-term of the HT-PEMFC [4]. However, although the electrochemical characterization proves this enhancement, the mechanism of the phosphoric acid migration and the effect of the SLG on the leaching is still unknown.In this work, different techniques, X-rat CT [4, 5]y, Raman microscopy [5] and µ-XRF, have been used to evaluate the PA leaching under ex-situ and in-operando conditions to study the mechanism of the migration of the acid and to prove that the electrochemical improvement is due to SLG has a positive effect in the HT-PEMFC.