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[Display omitted] • NGO nanoparticles were embedded into SPI polymers through in-situ synthesis. • In-situ synthesis was more favorable for NGO dispersion and well-constructed interface compared to physical blending. • The fracture strength of NGO-SPI composite membrane was approximately 61 MPa. 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. [ABSTRACT FROM AUTHOR]

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 CO 2. 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. [Display omitted] • Sep-encapsulated ILs (IL@Sep) were prepared using supercritical CO 2. • Nafion/2IL@Sep membrane-based single-cell achieved a peak power density of 779 mW cm−2 at 60 °C/80 %. • The composite membrane's improved performance was due to the axial alignment of IL@Sep within the Nafion nanofibers. [ABSTRACT FROM AUTHOR]

[Display omitted] • IL@HNTs were prepared utilizing supercritical fluid technology. • Proton conductivity of SPEEK/6IL@HNTs was above 20 mS/cm at −10–90 °C. • SPEEK/6IL@HNTs single-cell achieved 732 mW/cm2 at 60 °C. • IL@HNTs alignment in polymer fibers boosted composite membrane performance. The most fatal drawback of traditional sulfonated polymer proton-exchange membranes (PEMs) is their heavy dependence on water molecules to conduct protons, which causes a sharp decrease in their proton conductivity in low-humidity environments. Here, composite membranes were fabricated by electrostatically spinning halloysite nanotube-encapsulated ionic-liquids (IL@HNTs) incorporated into sulfonated poly(ether ether ketone) (SPEEK) (SPEEK/IL@HNTs). Using electrospinning, one-dimensional IL@HNTs ionogels with good water absorption were axially and uniformly aligned along the fiber filament direction. This formed long-range uninterrupted proton conduction pathways that greatly improved the proton conductivity of the composite membrane under various humidity environments. The axially-aligned structure facilitated crystallization, which improved the mechanical properties and thermal stability of the composite membrane. The SPEEK/6IL@HNTs membrane exhibited a through-plane proton conductivity of 139.2 mS cm−1 at 90 °C/98 % RH, representing a significant increase compared with the intrinsic SPEEK membrane. The SPEEK/6IL@HNTs membrane demonstrated a maximum power density of 732 mW cm−2, which exceeded that of the Nafion115 membrane (328 mW cm−2) by a factor of 2.2. It exhibited a single-cell performance equivalent to that of the Nafion212 membrane (723 mW cm−2), demonstrating that this provides a feasible approach to operating fuel cells in low-humidity environments without additional humidification. [ABSTRACT FROM AUTHOR]

[Display omitted] • The sHNT improved the interfacial compatibility between the inorganic particles and the polymer matrix due to the acid–base interactions. • High proton conductivity was due to the fast proton conduction paths of the sHNT and the well-constructed interface. • The peak power density of the PA-doped 3 % sHNT/ABPBI composite membrane reached 0.212 W cm−2 at 160 °C under anhydrous conditions. 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 –SO 3 H 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. [ABSTRACT FROM AUTHOR]

Abstract To 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. Highlights • Sepiolite nanoparticles with high specific surface area and porosity are synthesized. • Proton conductivity of ABPBI/2S-Sep is above 0.01 S/cm at 40–180 °C with RH ≥ 20%. • High proton conductivity is due to the multi-conduction paths of composites. [ABSTRACT FROM AUTHOR]

To prevent phosphoric acid (PA) from leaching and harming traditional polybenzimidazole-based proton exchange membranes, polyethyleneimine-confined halloysite nanotubes (PEI@HNTs) were embedded in poly (2,5-benzimidazole) membranes to construct rich ion pairs that stably conducted protons. With the aid of supercritical carbon dioxide, PEI oligomers with basic groups were confined in HNTs. Then, through in-situ synthesis, highly-dispersed PEI@HNT enhanced the mechanical stability of composite membranes, whose basicity improved the absorption and retention capacity of PA. The PEI@HNTs/ABPBI composite membrane exhibited excellent mechanical strength and electrical performance over a temperature range of 40–180 °C. The through-plane proton conductivity of the 5%PEI@HNTs/ABPBI composite membrane was 0.061 S cm−1 at 98% relative humidity and 90 °C and 0.049 S cm−1 under anhydrous conditions at 180 °C. This performance was attributed to strong hydrogen bonding between PEI and PA, as well as the excellent retention of bound water within the HNTs. The peak power density of PA-doped 5%PEI@HNTs/ABPBI composite membrane reached 0.235 W cm−2 and 0.418 W cm−2 at 120 °C and 180 °C under anhydrous conditions, respectively, which were 1.68 and 1.96 times higher than those obtained with the pristine PA-ABPBI membrane. These results highlight the potential of this composite membrane for applications requiring a wide temperature range. [Display omitted] • PEI-confined halloysite nanotubes were prepared by supercritical fluid-assisted filling. • Peak power density of PA-doped 5%PEI@HNTs/ABPBI reached 0.418 W cm−2 at 180 °C/0% RH. • High proton conductivity was due to fast proton conduction paths of PEI@HNTs. [ABSTRACT FROM AUTHOR]

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 CO 2 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. [Display omitted] • Halloysite ionogels were prepared by supercritical fluid-assisted filling. • Proton conductivity of ABPBI/5IL@HNTs is above 10 mS/cm at 20–180 °C with RH ≥ 20%. • High proton conductivity is due to the abundant proton conduction paths of the composite. [ABSTRACT FROM AUTHOR]