Intrinsically Proton-Conducting Benzimidazole Units Tethered to Different Polymer Chain Architectures

Sammanfattning: Proton transport is a fundamental process that can be found in many systems in nature, as well as in technical devices, such as fuel cells, electrochromic devices and various types of sensors. Fuel cell technology is considered as environmentally friendly because fuel cells are not limited by the Carnot cycle, and efficiently converts chemical energy into electrical energy which can be used for powering cars, laptops, buildings etc. One of the most promising fuel cell for portable devices is the polymer electrolyte membrane fuel cell (PEMFC). However, commercialization of PEMFC's is limited by several factors, including the insufficient properties of the current polymer membrane at temperatures above 100 º C, which are basically a consequence of the water that is necessary in the polymer membrane to facilitate the proton conduction. By operating the cell at temperatures in the range 150-200 ºC, many advantages could be reached at a system level. Consequently, the concept of fully polymeric proton conductors was investigated where the proton carriers were tethered to polymer backbones in order to avoid any leakage of the proton carrying component at elevated temperatures. Benzimidazole was chosen as the proton carrying component due to its inherent proton conducting properties and its high chemical and thermal stability. A series of model materials consisting of polymers with different chain structures and architectures and having benzimidazole units tethered via side chains were synthesized and investigated as proton conductors. A modular scheme to conveniently incorporate benzimidazole via thiol-ene coupling was developed. In general, the glass transition temperature was drastically increased when benzimidazole were tethered to the polymers with short side chains, indicating suppression of the segmental mobility of the polymers. This effect was attributed to the formation of hydrogen bonds between the benzimidazole units. When the length of the side chains was increased, the benzimidazole seemed to become decoupled from the polymer mobility of the backbone. It was concluded that the benzimidazole concentration and the segmental mobility of the polymers were the most important parameters to obtain high proton conductivities regardless of the polymer architecture. The conductivity was facilitated by high benzimidazole concentrations and high segmental mobility. However, since high benzimidazole concentrations suppressed the segmental mobility by the formation of strong hydrogen bonds, the benzimidazole content had to be balanced so that a reasonably high segmental mobility was retained. These findings suggested that polymers with a high concentration of the proton carrying species and a high segmental mobility have to be prepared in order to reach high conductivities. This may be accomplished by using long side chains when tethering the benzimidazole.