Performance analysis of polymer electrolyte membranes for direct methanol fuel cells

Highlights

• This review discusses recent advances in polymer electrolyte membranes for DMFCs.

• The PFSA and SAP membranes show remarkable performances at temperatures up to 90 °C.

• The composite membranes appear more suitable for DMFC applications above 100 °C.

Abstract

The status of research and development of polymer electrolyte membranes (PEMs) for direct methanol fuel cells (DMFCs) is described. Perfluorosulfonic acid membranes, e.g. Nafion, are widely used in fuel cell technology; but, despite their success, they show some drawbacks such as high cost, limited operating temperature range and high methanol crossover. These limit their widespread commercial application in DMFCs. Such disadvantages are inspiring worldwide research activities for developing new PEM materials based on non-perfluorinated polymers as alternative to Nafion for DMFCs. A review of membrane properties is carried out on the basis of thermal stability, methanol crossover and proton conductivity. The analysis of DMFC performance covers perfluorosulfonic acid membranes (PFSA), sulfonated aromatic polymers (SAPs) and composite membranes. PFSA membranes are suitable materials in terms of power density, SAPs are more advantageous regarding the low methanol permeability and cost, whereas composite membranes are more appropriate for operation above 100 °C. DMFC power density values reported in literature show that, although there are remarkable research efforts on this subject, the achieved results are not yet satisfying. Further work is especially necessary on non-perfluorinated polymers to improve performance and durability for an effective application in practical DMFC devices.

Introduction

Low temperature fuel cells are emerging technologies for the electrochemical conversion of the chemical energy of a fuel directly into electric energy with low environmental impact and high-energy efficiency. Nevertheless, before this technology can reach very large scale diffusion, some problems related to poor electrochemical performance, high cost of fuel cell systems, long term stability, etc. must be solved. In a fuel cell system, high costs derive from the use of noble metal catalysts, perfluorosulfonate polymer electrolyte membranes, bipolar plates and auxiliary components. Up to now, Nafion-type polymer electrolyte membranes (Nafion is a perfluorosulfonic acid polymer produced by DuPont) are the most widely used polymer electrolytes in H₂/air and direct methanol fuel cells. This wide use is due to the high proton conductivity, excellent mechanical and thermal properties, and high chemical and electrochemical stability of Nafion. Other similar perfluorinated polymers commercialized by Dow Chemicals (XUS^®), Asahi Kasei (Aciplex-S^®), Solvay (Aquivion^®), Fumatech (Fumion^®), Asahi Glass Engineering (FlemionR^®) are under consideration worldwide. However, although perfluorinated membranes show excellent properties, they have also some disadvantages such as high cost, high methanol crossover, fast dehydration with a decrease of proton conductivity as the temperature increases above 100 °C, and loss of fluorine species in the exhaust gas due to OH radicals attack. Because of these constraints, there is a strong interest in the R&D of new and cheaper membranes with similar or higher conductivity and lower fuel cross-over for application in the range of temperature from 100 to 150 °C. Most of the new types of PEMs under investigation for DMFC or PEMFC applications are currently synthesized by using various approaches, such as the synthesis of new ionic random and block copolymers, graft copolymerization of ionic polymers on hydrophobic membranes, blending of ionic and non-ionic polymers, synthesis of interpenetrating networks of ionic and non-ionic polymers and composite membranes incorporating a large variety of fillers (silica, zeolites, titania, zirconia, etc.).