Abstract
The rapid increasing of the population in combination with the emergence of new energy-consuming technologies has risen worldwide total energy consumption towards unprecedent values. Furthermore, fossil fuel reserves are running out very quickly and the polluting greenhouse gases emitted during their utilization need to be reduced. In this scenario, a few alternative energy sources have been proposed and, among these, proton exchange membrane (PEM) fuel cells are promising. Recently, polybenzimidazole-based polymers, featuring high chemical and thermal stability, in combination with fillers that can regulate the proton mobility, have attracted tremendous attention for their roles as PEMs in fuel cells. Recent advances in composite membranes based on polybenzimidazole (PBI) for high temperature PEM fuel cell applications are summarized and highlighted in this review. In addition, the challenges, future trends, and prospects of composite membranes based on PBI for solid electrolytes are also discussed.
Highlights
Proton conductivity has received intense attention owing to its application in chemical sensors, electrochemical devices, and power generation [1,2]
Hooshyari et al [144] studied the behavior of PBI-BaZrO3 (PBZ) nanocomposite membranes for HT-proton exchange membrane fuelas cell (PEMFC); their results showed that the water uptake, acid doping level, and proton conductivity of the PBZ were higher than that of the virgin PBI membrane owing to the presence of BaZrO3 perovskite nanoparticles (Figure 15)
Studies carried out on the PBI-based membranes prepared by covalent crosslinking with triglycidylisocyanurate (TGIC) and doped with highly sulfonated polyaniline (SPAN) showed the good thermal, dimensional, mechanical, and oxidative stability of these membranes applied in membrane-electrode assemblies (MEAs) of direct methanol fuel cells (DMFCs) [231]
Summary
Proton conductivity has received intense attention owing to its application in chemical sensors, electrochemical devices, and power generation [1,2]. PBI is a basic polymer, which dissociates PA releasing protons, as sketched by the following reaction: H3 PO4 + PBI → H2 PO4 − + PBI− H+ , where the equilibrium constant is about K = 1.17 × 103 and H+ ions can migrate through the polymer backbone by means of hydrogen bonds, in this case, assisted by phosphate anions by means of the Grotthuss mechanism, as we previously mentioned In some cases, such as PA-doped composite membranes of PBI containing ionic liquids, the dependence of the conductivity versus temperature presents a different behavior than the typical. Similar activation energies were obtained for PBI–PA 1 M, PBI–PA 0.1 M, and PBI–phytic acid membranes, but PBI–PA 14 M displayed a lower value as the PA concentration was much higher and, the proton transport was more favored According to these results, the Grotthuss mechanism dominates the proton transport in acid doped PBI membranes. The presence of PGO with an exfoliated structure may have disrupted the crystalline structure of PyPBI, which results in more available and stronger sites for PA trapping and provided additional diffusion pathways for proton hopping across the membrane via the Grotthuss mechanism
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