Anhydrous Proton-Conducting Polymers

Abstract

▪ Abstract Anhydrous proton-conducting polymers usually consist of a more or less inert polymer matrix that is swollen with an appropriate proton solvent (in most cases, phosphoric acid). An outline of the different materials is provided, with a focus on PBI/H3PO4 blends that are currently most suitable for fuel cell applications. Also discussed are alternative concepts for fully polymeric materials, which establish proton conductivity as an intrinsic property using amphoteric heterocycles such as imidazole as a proton solvent. The development of some of the first polymers is described, and the fundamental relations between their material properties and conductivity are discussed. Closely related to this relatively new concept are mechanistic investigations focusing on intermolecular proton transfer and diffusion of (protonated) solvent molecules, the contributions of both transport processes to conductivity, and the dependence of these ratios on composition, charge carrier density, etc. Although the development of fully polymeric proton conductors is inseparably related to mechanistic considerations, relatively little attention has been paid to these concepts in the field of conventional membranes (hydrated ionomers, H3PO4-based materials). Consequently, their general relevance is emphasized, and according investigations are summarized to provide a more comprehensive picture of proton transport processes within proton exchange membranes.

Proton conducting composite membranes based on poly(1-vinyl-1,2,4-triazole) and nitrilotri (methyl triphosphonic acid)

“…In the last decade, research on PEM has focused on the developing polymer electrolyte anhydrous in order to maintain the existence of proton conductivity at high temperature. The operation of the fuel cell at high temperature can provide benefits such as improved CO tolerance of platinum catalyst, increasing the mass transport, improved reaction kinetics, and simplifying humidity [5,6]. A common technique used in the synthesis of anhydrous proton conducting material, namely the doping of proton solvent into the polymer matrix so that proton transport occurs almost entirely through the diffusion structure.…”

Synthesis and proton conductivity of anhydrous dendritic electrolytes

“…The conductivity difference between PEG-HA0.5Im and PEG-HA1Im is more pronounced at lower temperatures and approaches each other at higher temperatures. In this system conductivity increase was found to depend on the glass transition temperature rather than imidazole volume fraction [5,17]. This is proved by plotting σ dc versus T-Tg and presented in Figure 7.…”

“…Recently, several water-free proton conductors were produced either through doping of basic polymers with strong acid, i.e., H 3 PO 4 [3][4][5] or acidic polymers with heterocycles such as imidazole or benzimidazole [6][7][8]. Both systems have already been shown to possess high proton conductivity in the dry state.…”

Transport and Electro‐Optical Properties in Polymeric Self‐Assembled Systems

“…There has been an intense search for alternative amphoteric hydrogen-bonded compounds capable of rendering high proton conductivity. Phosphoric acid, pyrazole, imidazole and benzimidazole have been studied [143,149,[160][161][162]. Phosphoric acid is relatively highly conducting [163].…”