Nano-structured membranes in fuel cells
With this in mind, the EU-funded project ZEOCELL investigated the properties of several multifunctional nano-structured materials for high-temperature operation. ZEOCELL intended to incorporate ionic conductivity equal to or over 100 milli Siemens (mS)/cm into FC membranes along with good chemical, mechanical and thermal stability. Other important properties included durability for at least 1 000 hours of operation at temperatures between 130 and 200 ºC, low fuel cross-over and manufacturing costs below EUR 400/m2. For this purpose, ZEOCELL developed and comprehensively studied seven electrolyte membrane compositions using one or more of the following materials: porous poly-benzimidazole (PBI), protic ionic liquids (PILs) and microporous zeolites/zeotypes. Their morphological, physicochemical, mechanical and electrochemical properties were evaluated in depth. Polymer membrane architecture is crucial for proton transport. Hence, PBI films with random and straight pores were developed for use as proton conductor supports. Suitable functionalisation protocols for microporous materials were established using grafting and filming techniques. Other aspects affecting performance were also studied through phosphoric acid doping, PIL embedding, and the addition of inorganic fillers such as microporous zeolites and titanosilicate nanocrystals. Polymeric ionic liquid films on randomly porous PBI supports had the best conduction performance (over 275 mS/cm) after 1 000 hours of operation. Hybrid membranes based on acid-doped porous PBI and microporous materials with good conductivity were successfully developed. Their fuel cross-over properties were also adequate. ZEOCELL successfully demonstrated proof-of-concept and superior performance in comparison to commercial membrane electrode assemblies. However, further work is required to increase durability and reduce fuel cross-over at temperatures above 120 ºC through electrode optimisation and improvements in FC components. Cost assessment and business analysis shows promise for applications in micro-combined heat and power systems and back-up power for telecom applications. Mass manufacture of high-temperature PEMFCs would be competitive in the global marketplace once durability issues are successfully addressed. ZEOCELL materials could also find applications in gas- and liquid-phase separation, lithium-ion batteries, adsorption and catalysis, micro FCs and lab-on-chip devices.
