Phosphoric
acid-doped polybenzimidazole (PA-PBI) used in high-temperature
proton exchange membranes (HT-PEMs) frequently suffers from a serious
loss of mechanical strength because of the “plasticizing effect”
of the dopant acid. Conventional cross-linking approaches generally
enhance membrane stability. However, acid doping levels (ADLs) and
consequently proton conductivity inevitably decrease. This is due
to the formation of more compact molecular structures and a reduced
amount of functional imidazole units, caused by their consumption
in introducing the cross-linker. To resolve the common problems of
current PA-PBI-based HT-PEMs, herein, a highly acidophilic imidazole-rich
cross-linked network with superior “antiplasticizing”
ability is constructed based on a novel multifunctional cross-linker.
This unique bischloro/bibenzimidazole (“A2B2-type”)
molecular structure has extremely high reactivity, including “self-reaction”
among the cross-linkers and “inter-reaction” between
the cross-linker and PBI molecules. The resulting imidazole-rich cross-linked
membranes exhibit the desired combination of high ADLs, high conductivity,
outstanding dimensional–mechanical stability, and excellent
fuel cell performance. In comparison to a corresponding linear PBI
membrane, one membrane with a high content of the cross-linker of
30% has a 100 wt % increased acid uptake, a doubling in proton conductivity
at 200 °C, and a maximum power density of 533 mW·cm–2 at 160 °C without humidification.
The practical applications
of phosphoric acid-doped polybenzimidazole
(PA–PBI) as high-temperature proton exchange membranes (HT-PEMs)
are mainly limited by their poor dimensional-mechanical stability
at high acid doping levels (ADLs) and the leaching of PA from membranes
during fuel cell operation. In this work, to overcome these issues,
we fabricated novel cross-linked PBI networks with additional imidazole
groups by employing a newly synthesized bibenzimidazole-containing
dichloro compound as cross-linker and an arylether-type Ph-PBI as
matrix. Ph-PBI featured by good solubility under high molecular weight
offers satisfactory film-forming ability and mechanical strength using
for the matrix. Importantly, the additional imidazole moieties in
BIM-2Cl endow the cross-linked PBI membranes improved dimensional-mechanical
stability with simultaneously enhanced ADLs and proton conductivity.
Furthermore, superior acid retention capability is obtained by incorporating
porous polyhydroxy SiO2 nanoparticles into these cross-linked
networks. As a result, the SiO2/cross-linked PBI composite
membranes are suitable to manufacture membrane electrode assemblies
(MEAs), and an excellent H2/O2 cell performance
with a peak power density of 497 mW cm–2 at 160
°C under anhydrous conditions can be achieved.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.