Phosphoric Acid Doping Levels in PBI-Based Membranes for the Application in High Temperature Fuel Cells: A Comprehensive Evaluation of Different Measurement Techniques

Abstract

Alternative energy studies are crucial since the energy demand in the world increases rapidly. Nowadays, due to limited reserves of fossil fuel and CO2 emission, environmentally green energy is the most promising and desirable energy source for the future. In this context, fuel cells becoming one of the most promising alternative energy conversion devices. Especially polymer electrolyte membrane fuel cells (PEMFCs) are of great research interest.Theoverall efficiency of PEMFCs is strongly influenced by the polymer membrane placed between the anode and cathode. The current state-of-the-art perfluorosulfonic acid (PFSA)-based membranes used in PEMFCs, such as Nafion, rely on the presence of water as the charge carrier for an efficient proton conductivity, and operate usually only up to 80 °C. Therefore, Nafion-based systems require water management, which can be avoided by using polybenzimidazole (PBI)-based membranes instead; operating at higher temperatures (up to 180 °C). Nevertheless, pristine PBI-based membranes need to be doped with acid in order to produce a highly proton conductive system. In this context, phosphoric acid (PA) is the most promising acid as the proton conduction medium. PA doping levels are essential as they govern conductivity, while an excess acid doping can deteriorate mechanical and thermal properties of the membrane. Therefore, the optimization and determination of the doping levels are important for high temperature polymer electrolyte membrane fuel cells. Commonly, titration or weighing of the membranes are used for the determination of the acid doping level, but they suffer from low accuracy and precision.In this work, the acid doping level (ADL) of PBI-based membranes was studied by Raman, impedance, and energy-dispersive X-ray (EDX) spectroscopy, gravimetric and thermogravimetric analysis, and titration. The use of Raman spectroscopy is of great interest due to its non-destructive nature. It can be performed on the sample prior or post application in a HT-PEMFC. This study presents a new measurement protocol for ADLs by using Raman spectroscopy, which is a helpful tool for choosing the optimal ADL for a PBI-based membrane. Thus, the manufacturing process of high temperature fuel cells and their overall efficiency can be optimized. Keywords: Acid Doping Level (ADL), Fuel Cells (FCs), High Temperature (HT), Polymer Electrolyte Membrane (PEM), Phosphoric Acid (PA), Polybenzimidazole (PBI)