Novel octopus shaped organic-inorganic composite membranes for PEMFCs

© 2016 Hydrogen Energy Publications LLC.Phosphoric acid doped polybenzimidazoles are among the most interesting proton exchange membrane materials for high temperature proton exchange membrane fuel cell applications. As a major challenge the proton conducting decline due to free phosphoric acid leaching during the long term fuel cell operation is addressed by fixing overmuch phosphoric acid in the polymer matrix. Novel organic-inorganic composite membranes are prepared via in situ synthesis of poly(2,5-benzimidazole) (ABPBI) and OctaAmmonium POSS (AM-POSS) hybrid composites (ABPBI/AM-POSS) following phosphoric acid doping and membrane casting procedures. Compared with the pristine ABPBI membrane, the introduction of AM-POSS into ABPBI polymer membrane caused water and phosphoric acid absorbilities increasing dramatically, resulting in the significant increase of proton conductivities at whether hydrous or anhydrous condition. ABPBI/3AM composite membranes with phosphoric acid uptake above 250% showed best proton conductivities from room temperature to 160 °C, indicating these composite membranes could be excellent candidates as a polymer electrolyte membrane for low and intermediate temperature applications

Fabricating rapid proton conduction pathways with sepiolite nanorod-based ionogel/Nafion composites via electrospinning

The major limitation of conventional sulfonated polymer proton-exchange membranes (PEMs) is their strong reliance on water molecules for proton conduction, causing a significant reduction in proton conductivity under low-humidity conditions. In this study, a one-dimensional ionogel (IL@Sep) confined within sepiolite (Sep) nanorods was prepared using ionic liquid (1-butyl-3-methylimidazolium trifluoromethanesulfonate) and supercritical CO2. Subsequently, IL@Sep was blended with a Nafion solution, and electrospinning was used to fabricate the composite fiber PEM suitable for low-humidity environments. The results revealed that the electrospun (ES)-Nafion/IL@Sep composite fiber membrane exhibited significantly enhanced mechanical properties, water absorption, and proton conductivity. At an IL@Sep content of 2 wt%, the Nafion/2IL@Sep membrane exhibited a proton conductivity of 231 mS cm−1 at 80 °C/98 % relative humidity (RH) and 113 mS cm−1 at 80 °C/40 % RH. Moreover, the single-cell assembled with this composite membrane exhibited good gas tightness and achieved a peak power density of 779 mW cm−2 at 60 °C/80 % RH, which was ∼1.45 times that of the Nafion 212 membrane single-cell. This study indicates that electrospinning-assisted ionogel-modified ES-Nafion/IL@Sep composite fiber membranes have potential suitability for use in proton-exchange membrane fuel cells under varying humidity conditions.</p

Amination modification of graphene oxide for the in-situ synthesis of sulfonated polyimide-based composite proton exchange membranes

To mitigate the severe degradation of mechanical properties and stability of highly sulfonated proton exchange membranes (PEMs) caused by the sulfonic acid groups, amino-functionalized graphene oxide (AGO) was embedded into sulfonated polyimide (SPI) for the in-situ synthesis of a composite proton exchange membrane. The introduction of AGO nanoparticles facilitated the enhancement of the crystallinity of the composite membrane, with the well-constructed interface and dispersion. The resulting AGO-SPI composite membrane exhibited high mechanical strength and stability. The fracture strength of AGO-SPI composite membrane was approximately 61 MPa, which was 1.85 times higher than that of pure SPI. Meanwhile, the weight loss of AGO-SPI composite membrane in Fenton's reagent was only 2.11 %. Additionally, at 90 °C/98 % RH, the proton conductivity of AGO-SPI composite membrane reached 50.1 mS cm−1, which was 3.53 times higher than that of pristine SPI. The results suggest the promising application prospects of the proton exchange composite membrane.</p

Poly(2,5-benzimidazole)/sulfonated sepiolite composite membranes with low phosphoric acid doping levels for PEMFC applications in a wide temperature range

o broaden the operating temperature range of phosphoric acid (PA) doped polybenzimidazole membrane-based proton exchange membrane fuel cells (PEMFCs) toward low temperatures, a novel series of poly(2,5-benzimidazole) (ABPBI)/sulfonated sepiolite (S-Sep) composite membranes (ABPBI/S-Sep) with low PA doping levels (DLs) were prepared via in-situ synthesis. The desirably enhanced mechanical, thermal, and oxidative stabilities of ABPBI/S-Sep composite membranes were achieved by constructing ABPBI chains arranged along the sepiolite (Sep) fibers and acid-base crosslinks formed between S-Sep fibrous particles and ABPBI chains. Benefiting from the richness of high temperature stable bound water and the excellent water absorbability of Sep particles that enable the formation of additive proton conducting paths, the composite membranes retained bounded PA and achieved much higher proton conductivities under both anhydrous and hydrous conditions compared to PA-doped ABPBI membranes. Proton conductivity values above 0.01 S/cm at 40-90 °C/20-98% RH conditions and 90-180 °C/anhydrous conditions as well as peak power density of 0.13 and 0.23 W/cm2 at 80 and 180 °C with 0% RH, respectively from the ABPBI/2S-Sep composite membrane are more holistic compared to Nafion at low temperatures and polybenzimidazole-based membranes at high temperatures, respectively. The excellent properties of ABPBI/S-Sep composite membranes suggest them as prospective candidates for PEMFCs applications in a wide temperature range

Polyethyleneimine-filled sepiolite nanorods-embedded poly(2,5-benzimidazole) composite membranes for wide-temperature PEMFCs

Proton-exchange membrane fuel cells (PEMFCs) that operate from room temperature to high temperatures (e.g., 200 °C) are desired for fuel cells used in vehicles and combined heat and power systems. In this work, polyethyleneimine-filled sepiolite nanorods (PEI@SNR)-embedded poly(2,5-benzimidazole) composites (ABPBI/PEI@SNR) were synthesized in-situ to enhance their proton conductivity and minimize phosphoric acid (PA) leaching. They were then applied in PEMFCs between room temperature and 200 °C. The physicochemical and electrochemical properties of the composite membranes were characterized. The composite membranes showed enhanced thermal, oxidative, and dimensional stability and achieved proton conductivities above 0.01 S/cm from 40 to 200 °C at a relative humidity of 0–100%. This performance was attributed to abundant hydrogen bonds between PA, ABPBI, and PEI, and the strong retention of bound water within sepiolite nanorods (SNRs). The maximum power density of the cell based on the PA-doped ABPBI/5PEI@SNR composite membrane reached 0.16 W/cm2 at 80 °C and 0.27 W/cm2 at 180 °C and an anhydrous environment, which were respectively 2.2 and 1.5 times higher than those of the PA-doped ABPBI membrane. The cell performance was much better than previously reported zeolite-embedded polybenzimidazole membrane-based PEMFCs, indicating that the composite membranes have good application prospects in PEMFCs operating over a wide temperature range.</p

Advances in ionogels for proton-exchange membranes

To ensure the long-term performance of proton-exchange membrane fuel cells (PEMFCs), proton-exchange membranes (PEMs) have stringent requirements at high temperatures and humidities, as they may lose proton carriers. This issue poses a serious challenge to maintaining their proton conductivity and mechanical performance throughout their service life. Ionogels are ionic liquids (ILs) hybridized with another component (such as organic, inorganic, or organic-inorganic hybrid skeleton). This design is used to maintain the desirable properties of ILs (negligible vapor pressure, thermal stability, and non-flammability), as well as a high ionic conductivity and wide electrochemical stability window with low outflow. Ionogels have opened new routes for designing solid-electrolyte membranes, especially PEMs. This paper reviews recent research progress of ionogels in proton-exchange membranes, focusing on their electrochemical properties and proton transport mechanisms.</p

Halloysite ionogels enabling poly(2,5-benzimidazole)-based proton-exchange membranes for wide-temperature-range applications

Simultaneous excellent proton conductivity and high mechanical properties from ambient temperatures to 200 °C are the most pressing challenges to the long-term applications of polybenzimidazole-based proton-exchange membranes for methanol steam reformer-proton-exchange membrane fuel cell units. Their performance is subject to the loss and plasticizing effects of free phosphoric acid (PA) during their long-term operation. Herein, novel proton carriers, termed halloysite ionogels (IL@HNTs), were prepared by filling the ionic liquid (IL) into the inorganic framework of halloysite nanotubes (HNTs) with the assistance of supercritical CO2 to replace free PA in polybenzimidazole membranes. IL@HNTs-embedded poly(2,5-benzimidazole) (ABPBI) composite membranes (ABPBI/IL@HNTs) were obtained by in situ synthesis and then doped with low levels of PA. Experimental characterization results showed that the ILs were confined within the lumen of the HNTs. Benefiting from the introduction of IL@HNTs, the composite membranes showed excellent proton conductivity (>10 mS/cm) from ambient temperature to 180 °C and a greatly enhanced mechanical strength (>75 MPa), water uptake, and PA absorbability. The ABPBI/5IL@HNTs composite membrane achieved peak power outputs of 219 and 380 mW/cm2 under anhydrous conditions at 80 and 160 °C, respectively, which were respectively 1.9 and 2.1 times greater than those of PA-doped ABPBI membrane. Satisfactory single-cell performance was obtained at a low PA doping level and without free PA. The results suggest that this approach of introducing novel ionogels to construct wide-temperature proton-exchange membranes can overcome the limitations of traditional low-temperature and high-temperature membranes, thus broadening the application temperature range of existing PEMFCs.</p

In situ synthesis of star copolymers consisting of a polyhedral oligomeric silsesquioxane core and poly(2,5-benzimidazole) arms for high-temperature proton exchange membrane fuel cells

Star copolymers with good film-forming and mechanical properties were in situ synthesized for fabricating proton exchange membranes. The monomers of 3,4-diaminobenzoic acid were first grafted onto glycidyl-polyhedral oligomeric silsesquioxane (G-POSS) cores and then propagated to the poly(2,5-benzimidazole) (ABPBI) chains. The introduction of the star copolymer improves the movement of the ABPBI polymer chains, resulting in a lower internal viscosity and larger free volume that favor increased membrane flatness and absorbilities of water and phosphoric acid molecules, respectively. It was found that the star copolymers with 1.0 wt% of incorporated POSS (ABPBI-1.0POSS) had the best balance of the acid retentivity and film-forming property as well as mechanical properties that are desirable for proton exchange membranes without PA loss operating at high temperatures. The enhanced cell performance characteristics obtained using the ABPBI-1.0POSS-based membranes indicate that star copolymers are promising materials for use in high-temperature proton exchange membrane fuel cells

Sulfonation modification of halloysite nanotubes for the in-situ synthesis of polybenzimidazole-based composite proton exchange membranes in wide-temperature range applications

Phosphoric acid-doped polybenzimidazole (PA–PBI) membranes face challenges, such as the easy loss of free PA and the reduced mechanical strength caused by the “plasticization effect” of PA, limiting their application in a wide temperature range. In this study, sulfonated halloysite (sHNT) was used to modify poly(2,5-benzimidazole) (ABPBI) for the in-situ synthesis of a composite proton exchange membrane. The introduction of halloysites in the composite membrane enabled the capturing of PA and water, while its nanoporous structure provided additional paths for proton conduction. Sulfonation modification of halloysite improved the interfacial compatibility between the inorganic particles and the polymer matrix, with the –SO3H groups providing extra proton hopping sites. Due to the well-constructed interface, the resulting sHNT/ABPBI composite membrane exhibited high mechanical strength and excellent proton conductivity across a wide temperature range. The 3 % sHNT/ABPBI composite membrane exhibited a breaking strength of approximately 130 MPa, which was 1.6 times that of pure ABPBI. Moreover, the proton conductivity of the composite exceeded 0.01 S cm−1 at temperatures ranging from 40 to 180 °C. At 160 °C, the peak power density of the PA-doped 3 % sHNT/ABPBI composite membrane was 0.212 W cm−2, which was 1.33 times higher than that of the pure ABPBI membrane. These results show that the composite membrane has potential applications in a wide temperature range.</p