Improving the Conductivity of Sulfonated Polyimides as Proton Exchange Membranes by Doping of a Protic Ionic Liquid

Abstract

Proton exchange membranes (PEMs) are a key component of a proton exchange membrane fuel cell. Sulfonated polyimides (SPIs) were doped by protic ionic liquid (PIL) to prepare composite PEMs with substantially improved conductivity. SPIs were synthesized from diamine, 2,2-bis[4-(4-amino-phenoxy)phenyl]propane (BAPP), sulfonated diamine, 4,4'-diamino diphenyl ether-2,2'-disulfonic acid (ODADS) and aromatic anhydride. BAPP improved the mechanical and thermal properties of SPIs, while ODADS enhanced conductivity. A PIL, 1-vinylimidazolium trifluoromethane-sulfonate ([VIm][OTf]), was utilized. [VIm][OTf] offered better conductivity, which can be attributed to its vinyl chemical structure attached to an imidazolium ring that contributed to ionomer-PIL interactions. We prepared sulfonated polyimide/ionic liquid (SPI/IL) composite PEMs using 50 wt% [VIm][OTf] with a conductivity of 7.17 mS/cm at 100 °C, and in an anhydrous condition, 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride (DSDA) was used in the synthesis of SPIs, leading to several hundred-times improvement in conductivity compared to pristine SPIs.

Highly Sulfonated Diamine Synthesized Polyimides and Protic Ionic Liquid Composite Membranes Improve PEM Conductivity

“…Three diamines were used: 2,2-bis[4-(4-amino-phenoxy) phenyl] propane (BAPP), 2,7-bis(4-aminophenoxy) naphthalene (BAPN), and 4,4′-(9-fluorenylidene) dianiline (9FDA). The impact of aromatic dianhydride on the conductivity of SPI/PIL PEMs was noted in our recent study [14]. Three aromatic dianhydrides were investigated: 4,4′-oxydiphthalic anhydride (ODPA), naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTDA), and 3,3′,4,4′-diphenyl sulfone tetracarboxylic dianhydride (DSDA).…”

“…The temperature range of 80-130 °C is within the domain of many novel materials. Sulfonated hydrocarbon-based membranes and inorganic composite membranes have been explored, along with the application of ionic liquid (IL) doped into hydrocarbon-based membranes, to promote the performance of PEM above 100 °C [3,[8][9][10][11][12][13][14]. ILs are molten salts that have attracted much attention in recent years because of their exceptional properties, including negligible vapor pressure, inflammability, high thermal and electrochemical stability, and outstanding ionic conductivity even under anhydrous conditions [15,16].…”

“…Conrad and Vankelecom have studied imidazolium methanesulfonate as a proton conductor for high-temperature PEMFCs [9]. The continuing improvements in PIL-based PEMs have led to many attractive results in a growing number of research studies [8,9,[12][13][14][17][18][19][20][21]. Various PILs have also been applied in SPI PEMs.…”

“…We attempted to improve the conductivity of PEMs by utilizing a sulfonated diamine, 4,4′-diamino diphenylether-2,2′-disulfonic acid (ODADS) to synthesize SPIs that were doped with a PIL 1-vinyl-3-H-imidazolium trifluoromethanesulfonate [VIm][OTf] to fabricate composite membranes [13,14]. We prepared SPI/PIL composite PEM using 50 wt % [VIm][OTf] with a high conductivity of 7 mS/cm at 100-120 °C and anhydrous condition depending on the ODADS content and dianhydride [14].…”

“…We attempted to improve the conductivity of PEMs by utilizing a sulfonated diamine, 4,4′-diamino diphenylether-2,2′-disulfonic acid (ODADS) to synthesize SPIs that were doped with a PIL 1-vinyl-3-H-imidazolium trifluoromethanesulfonate [VIm][OTf] to fabricate composite membranes [13,14]. We prepared SPI/PIL composite PEM using 50 wt % [VIm][OTf] with a high conductivity of 7 mS/cm at 100-120 °C and anhydrous condition depending on the ODADS content and dianhydride [14]. To further improve the conductivity of composite PEMs in this study, we first synthesized a novel sulfonated diamine 1,4-bis(4-aminophenoxy-2-sulfonic acid) benzenesulfonic acid [pBABTS], and fabricated high-conductivity composite PEMs using pBABTS synthesized SPIs as a matrix polymer that were doped with the PIL [VIm][OTf] 40%.…”