Improving the proton conductivity and water uptake of polybenzimidazole-based proton exchange nanocomposite membranes with TiO2 and SiO2 nanoparticles chemically modified surfaces

Poly [2,2′-(m-pyrazolidene)-5,5′-bibenzimidazole] (PPBI) was synthesized from pyrazole-3,5-dicarboxylic acid and 3,3′,4,4′-tetraaminobiphenyle (TAB) through polycondensation reaction in polyphosphoric acid (PPA) as reaction solvent. And polymer-grafted SiO₂ and TiO₂ nanoparticles were prepared through radical polymerization of 1-vinylimidazole and sulfonated vinylbenzene on the surface-vinylated nanoparticles. The polymer-grafted SiO₂ and TiO₂ nanoparticles were utilized as a functional additive to prepare PPBI/polymer-grafted SiO₂ and TiO₂ nanocomposite membranes. Imidazole and sulfonated vinylbenzene groups on the surface of modified nanoparticles forming linkages with PPBI chains, improved the compatibility between PPBI and nanoparticles, and enhanced the mechanical strength of the prepared nanocomposite membranes. The prepared nanocomposite membranes showed higher water uptake and acid doping levels comparing to PPBI. Also, after acid doping with phosphoric acid, nanocomposite membranes exhibited enhanced proton conductivity in comparison to the pristine PPBI and PPBI/un-modified SiO₂ and TiO₂ nanocomposite membranes. The enhancement in proton conductivity of nanocomposite membranes resulted from modified SiO₂ nanoparticles showed higher conductivity than modified TiO₂ nanoparticles. The above results indicated that the PPBI/modified SiO₂ and TiO₂ nanocomposite membranes could be utilized as proton exchange membranes for medium temperature fuel cells.     

Novel epoxy-based cross-linked polybenzimidazole for high temperature proton exchange membrane fuel cells

An approach has been proposed to prepare the reinforced phosphoric acid (PA) doped cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells (HT-PEMFCs), using 1,3-bis(2,3-epoxypropoxy)-2,2-dimethylpropane (NGDE) as the cross-linker. FT-IR measurement and solubility test showed the successful completion of the crosslinking reaction. The resulting cross-linked membranes exhibited improved mechanical strength, making it possible to obtain higher phosphoric acid doping levels and therefore relatively high proton conductivity. Moreover, the oxidative stability of the cross-linked membranes was significantly enhanced. For instance, in Fenton’s reagent (3% H₂O₂ solution, 4 ppm Fe²⁺, 70 °C), the cross-linked PBI-NGDE-20% membrane did not break into pieces and kept its shape for more than 480 h and its remaining weight percent was approximately 65%. In addition, the thermal stability was sufficient enough within the operation temperature of PBI-based fuel cells. The cross-linked PBI-NGDE-X% (X is the weight percent of epoxy resin in the cross-linked membranes) membranes displayed relatively high proton conductivity under anhydrous conditions. For instance, PBI-NGDE-5% membrane with acid uptake of 193% exhibited a proton conductivity of 0.017 S cm⁻¹ at 200 °C. All the results indicated that it may be a suitable candidate for applications in HT-PEMFCs.     

Incorporation of H3PO4 into three-dimensional polyacrylamide-graft-starch hydrogel frameworks for robust high-temperature proton exchange membrane fuel cells

To enhance the anhydrous proton conductivities of proton exchange membranes, we report here the incorporation of H₃PO₄ into three-dimensional (3D) framework of polyacrylamide-graft-starch (PAAm-g-starch) hydrogel materials using extraordinary absorption of hydrogels to H₃PO₄ aqueous solution. Intrinsic microporous structure can close to seal H₃PO₄ molecules in the interconnected 3D frameworks of PAAm-g-starch after suffering from dehydration. The hydrogel membranes are thoroughly characterized by morphology observation, thermal stability, swelling kinetics, proton-conducting performances as well as electrochemical behaviors. The results show that the H₃PO₄ loadings and therefore the proton conductivities of the hydrogel membranes are dramatically enhanced by employing PAAm-g-starch matrix. H₃PO₄ loading of 88.68 wt% and an anhydrous proton conductivity as high as 0.046 S cm⁻¹ at 180 °C are recorded. A fuel cell using a thick membrane shows a peak power density of 517 mW cm⁻² at 180 °C by feeding with H₂/O₂ streams. The high H₃PO₄ loading, reasonable proton conductivity in combination with simple preparation, low cost and scalable matrix demonstrates the potential use of PAAm-g-starch hydrogel membranes in high-temperature proton exchange membrane fuel cells.     

A poly(R1R2R3)–N/H3PO4 composite membrane for phosphoric acid polymer electrolyte membrane fuel cells

A poly(R₁R₂R₃)–N⁺/H₃PO₄ composite membrane has been developed for use in a polymer electrolyte fuel cell (PEMFC). The quaternized polysulfone (QNPSU) membrane doped with H₃PO₄ showed high proton conductivity (0.12 S cm⁻¹) at 160 °C and gave good performance in a single fuel cell tests. The peak power density with the QNPSU/H₃PO₄ composite membrane (at 150 °C, with dry H₂/O₂) was greater than 0.7 W cm⁻². The effect of the phosphoric acid doping level on fuel cell performances with the QNPSU membrane was investigated. The data show that the QNPSU/H₃PO₄ composite membrane is promising for higher temperature PEMFC applications. The study demonstrated that the poly(R₁R₂R₃)–N⁺/H₃PO₄ composite system produced an effective method to connect phosphoric acid to a non-conducting polymer structure, to produce a promising membrane for phosphoric acid polymer electrolyte membrane fuel cells.     

Proton conductive membranes based on doped sulfonated polytriazole

This work reports the preparation and characterization of proton conducting sulfonated polytriazole membranes doped with three different agents: 1H-benzimidazole-2-sulfonic acid, benzimidazole and phosphoric acid. The modified membranes were characterized by scanning electron microscopy (SEM), infrared spectra, thermogravimetric analysis (TGA), dynamical mechanical thermal analysis (DMTA) and electrochemical impedance spectroscopy (EIS). The addition of doping agents resulted in a decrease of the glass transition temperature. For membranes doped with 85 wt.% phosphoric acid solution proton conductivity increased up to 2·10⁻³ S cm⁻¹ at 120 °C and at 5% relative humidity. The performance of the phosphoric acid doped membranes was evaluated in a fuel cell set-up at 120 °C and 2.5% relative humidity.     

Ethyl phosphoric acid grafted amino-modified polybenzimidazole with improved long-term stability for high-temperature proton exchange membrane applications

A series of membranes based on amino-modified polybenzimidazole with grafted ethyl phosphonic acid have been successfully prepared. Firstly, the amino-modified polybenzimidazole (PBI-NH₂-X) was synthesized, then the ethyl phosphoric acid (EPA) were grafted on to PBI-NH₂-X to obtained amino-modified polybenzimidazoles with grafted ethyl phosphonic acid groups (PBI-NH₂-EPA-X). EPA is introduced into the matrix to improve the H₃PO₄ doping level and proton conductivity of the membranes. The obtained membranes display good dimensional and thermal stability. Long-term stability of the grafted membranes is much better than the pristine PBI membranes. Moreover, PBI-NH₂-EPA-15 shows a proton conductivity of 0.062 S cm⁻¹ at 170 °C, which is 342% higher than that of PBI.     

The silica-doped sulfonated poly(fluorenyl ether ketone)s membrane using hydroxypropyl methyl cellulose as dispersant for high temperature proton exchange membrane fuel cells

The sulfonated poly(fluorenyl ether ketone)s (SPFEK) membranes doped with SiO₂ and dispersed by hydroxypropyl methyl cellulose (HPMC) were prepared and investigated for polymer electrolyte membrane fuel cells (PEMFCs) used at high temperature and low relative humidity (RH). The above membrane was prepared by solution dispersion of SPFEK and SiO₂ using HPMC as dispersant. The physio-chemical properties of the hybrid membrane were studied by means of scanning electron microscope (SEM), ion-exchange capacity (IEC), proton conductivity, and single cell performance tests. The hybrid membranes dispersed by HPMC were well dispersed when compared with common organic/inorganic hybrid membranes. The hybrid membranes showed superior characteristics as a proton exchange membrane (PEM) for PEMFC application, such as high ionic exchange content (IEC) of 1.51 equiv/g, high temperature operation properties, and the satisfactory ability of anti-H₂ crossover. The single cell performances of the hybrid membranes were examined in a 5 cm² commercial single cell at both 80 °C and 120 °C under different relative humidity (RH) conditions. The hybrid membrane dispersed by HPMC gave the best performance of 260 mW/cm² under conditions of 0.4 V, 120 °C, 50% RH and ambient pressure. The results demonstrated HPMC being an efficient dispersant for the organic/inorganic hybrid membrane used for PEM fuel cell.     

Investigation on sulfuric acid sulfonation of in-situ sol-gel derived Nafion/SiO2 composite membrane

In this paper, an effective method to cover the conductivity loss of Nafion/SiO₂ through sulfonation and its mechanism are studied. Nafion/SiO₂ composite membranes are prepared via an in-situ sol-gel route, and then sulfonated with concentrated sulfuric acid (marked as Nafion/S-SiO₂). The effects of the sulfonation on properties of the Nafion/SiO₂ composite membrane are investigated. The results show that sulfonation can improve the proton conductivity of the Nafion/SiO₂ effectively, though it brings water-uptake loss to the composite membrane to some extent simultaneously. According to the results of FT-IR, UV resonance Raman spectroscopy, ²⁹Si solid-state MAS NMR and XRD, it’s proved that higher conductivity of Nafion/S-SiO₂ should be relevant to hydrogen bonds & chemical bonds between SiO₂ nano particles and sulfuric acid molecules. While, lower water uptake & swelling ratio should be caused by hydroxyl-elimination on the surface of SiO₂ nano-particles during sulfonation. Single cell tests show that the performance of Nafion/S-SiO₂ composite membranes substantially exceeds Nafion/SiO₂ at 110 °C and 59%RH, and in the initial testing stage no performance reduction is observed.     

Characterizations of polybenzimidazole based electrochemical hydrogen pumps with various Pt loadings for H2/CO2 gas separation

Carbon capture and storage (CCS) technologies have been intensively researched and developed to cope with climate change, by reducing atmospheric CO₂ concentration. The electrochemical hydrogen pumps with phosphoric acid doped polybenzimidazole (PBI) membrane are evaluated as a process to concentrate CO₂ and produce pure H₂ from anode outlet gases (H₂/CO₂ mixture) of molten carbonate fuel cells (MCFC). The PBI-based hydrogen pump without humidification (160 °C) can provide higher hydrogen separation performances than the cells with perfluorosulfonic-acid membranes at a relative humidity of 43% (80 °C), suggesting that the pre-treatment steps can be decreased for PBI-based systems. With the H₂/CO₂ mixture feed, the current efficiency for the hydrogen separation is very high, but the cell voltage increase, compared to the pure hydrogen operation, mainly due to the larger polarization resistance at electrodes, as confirmed by electrochemical impedance spectroscopy (EIS). The performance evaluation with various Pt loadings indicates that the hydrogen oxidation reaction at anodes is rate determining, and therefore the Pt loading at cathodes can be decreased from 1.1 mg/cm² to 0.2 mg/cm² without significant performance decay. The EIS analysis also confirms that the polarization resistances are largely dependent on the Pt loading in anodes.     

Silicon modified ultrafiltration-based proton-conductive membranes with improved performance for H2/Cl2 fuel cell application

Ultrafiltration (UF)-based proton-conductive membranes, which comprised nanosize SiO₂, polyethersulfone and aqueous acid absorbed, as an alternative to traditional ion exchange membranes, were first proposed and successfully prepared for H₂/Cl₂ fuel cell. Various membranes were prepared with different weight fractions of SiO₂ nanoparticles. The effect of silicon content on the performance of membranes was characterized. The ionic conductivity of a membrane doped with 3 M hydrochloric acid increased with the silicon content and reached 0.150 S cm⁻¹ at 15 wt.% SiO₂. A non-optimized H₂/Cl₂ fuel cell assembled with the modified UF membrane (115 μm thick) exhibited better performance than that with Nafion 115 membrane. It demonstrated that 12.67% and 55.03% improved at 10 wt.% and 15 wt.% SiO₂, respectively. The study provides an effective way to fabricate high performance porous membranes for H₂/Cl₂ fuel cell application.     

Improvement of PEMFC performance with Nafion/inorganic nanocomposite membrane electrode assembly prepared by ultrasonic coating technique

Membrane electrode assemblies with Nafion/nanosize titanium silicon dioxide (TiSiO₄) composite membranes were manufactured with a novel ultrasonic-spray technique and tested in proton exchange membrane fuel cell (PEMFC). Nafion/TiO₂ and Nafion/SiO₂ nanocomposite membranes were also fabricated by the same technique and their characteristics and performances in PEMFC were compared with Nafion/TiSiO₄ mixed oxide membrane. The composite membranes have been characterized by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction, water uptake, and proton conductivity. The composite membranes gained good thermal resistance with insertion of inorganic oxides. Uniform and homogeneous distribution of inorganic oxides enhanced crystalline character of these membranes. Gas diffusion electrodes (GDE) were fabricated by Ultrasonic Coating Technique. Catalyst loading was 0.4 mg Pt/cm² for both anode and cathode sides. Fuel cell performances of Nafion/TiSiO₄ composite membrane were better than that of other membranes. The power density obtained at 0.5 V at 75 °C was 0.456 W cm⁻², 0.547 W cm⁻², 0.477 W cm⁻² and 0.803 W cm⁻² for Nafion, Nafion/TiO₂, Nafion/SiO₂, and Nafion/TiSiO₄ composite membranes, respectively.     

High performance polymer electrolytes based on main and side chain pyridine aromatic polyethers for high and medium temperature proton exchange membrane fuel cells

Novel aromatic polyether type copolymers bearing side chain polar pyridine rings as well as combination of main and side chain pyridine units have been evaluated as potential polymer electrolytes for proton exchange membrane fuel cells (PEMFCs). The advanced chemical and physicochemical properties of these new polymers with their high oxidative stability, mechanical integrity and high glass transition temperatures (T_g's up to 270 °C) and decomposition temperatures (T_d's up to 480 °C) make them promising candidates for high and medium temperature proton exchange membranes in fuel cells. These copolymers exhibit adequate proton conductivities up to 0.08 S cm⁻¹ even at moderate phosphoric acid doping levels. An optimized terpolymer chemical structure has been developed, which has been effectively tested as high temperature phosphoric acid imbibed polymer electrolyte. MEA prepared out of the novel terpolymer chemical structure is approaching state of the art fuel cell operating performance (135 mW cm⁻² with electrical efficiency 45%) at high temperatures (150-180 °C) despite the low phosphoric acid content (<200 wt%) and the low platinum loading (ca. 0.7 mg cm⁻²). Durability tests were performed affording stable performance for more than 1000 h.     

Proton conductivity and fuel cell performance of organic–inorganic hybrid membrane based on poly(methyl methacrylate)/silica

Organic–inorganic hybrid membranes based on poly(methyl methacrylate) (PMMA)/silica have been synthesized using a sol-gel technique for use in polymer electrolyte fuel cells (PEFCs). The properties of these membranes were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetric analysis. The results indicate that these membranes are formed through hydrogen bonds between the carbonyl group of PMMA and the uncondensed alcohol functional groups of the inorganic clusters. The proton conductivity of these membranes is on the order of 10⁻¹ S cm⁻¹, and the 60PMMA–30SiO₂–10P₂O₅ membrane displays the highest proton conductivity of 3.85 × 10⁻¹ S cm⁻¹ at 90 °C and 50% RH. The performance of a fuel cell using these membranes was tested. A maximum power density of 370 mW cm⁻² is obtained at 80 °C, and the current density at 0.4 V remains almost unchanged during the 100-h test time under the test conditions. This class of hybrid membranes is an extremely promising material for use in PEFCs.     

Characteristics of proton exchange membrane fuel cells cold start with silica in cathode catalyst layers

In this study, a novel strategy is reported to improve the cold start performance of proton exchange membrane (PEM) fuel cells at subzero temperatures. Hydrophilic nano-oxide such as SiO₂ is added into the catalyst layer (CL) of the cathode to increase its water storing capacity. To investigate the effect of nanosized SiO₂ addition, the catalyst coated membranes (CCMs) with 5 wt.% and without nanosized SiO₂ are fabricated. Although at normal operation conditions the cell performance with nanosized SiO₂ was not so good as that without SiO₂, cold start experiments at −8 °C showed that the former could start and run even at 100 mA cm⁻² for about 25 min and latter failed very shortly. Even at −10 °C, the addition of SiO₂ dramatically increased the running time before the cell voltage dropped to zero. These results further experimentally proved the cold start process was strongly related with the cathode water storage capacity. Also, the performance degradation during 8 cold start cycles was evaluated through polarization curves, cyclic voltammetry (CV) and electrochemical impedance spetra (EIS). Compared with the cell without SiO₂ addition, the cell with 5 wt.% SiO₂ indicated no obvious degradation on cell performance, electrochemical active surface area and charge transfer resistance after experiencing cold start cycles at −8 °C.     

Proton-conducting methacrylate–silica sol–gel membranes containing tungstophosphoric acid

The increase of the operation temperature in proton-exchange membrane fuel cell (PEMFC) above 100 °C is a great concern for the application of this type of cells in electric vehicles. Hybrid organic–inorganic membranes with nano-sized interfaces can combine the main properties to meet this objective. Methacrylate–silica covalent hybrid membranes have been synthesised by polymerization of 2-hydroxyethyl methacrylate and 3-methacryloxypropyl trimethoxysilane, and hydrolysis–polycondensation of alkoxide radicals. Tungstophosphoric acid hydrate was incorporated to endow the membranes with proton conductivity. The composition and synthesis conditions to promote organic polymerisation and sol–gel condensation avoiding phase separation have been optimised. The structural analysis shows homogeneous membranes without phase separation. The water retention properties provided by SiO₂ and tungstophosphoric acid leads to high proton conductivity (maximum values around 3 × 10⁻² S cm⁻¹) at 100–150 °C. A 0.5 M % of Tungstophosphoric acid (PWA) is enough to build well-connected paths for proton conduction.     

High CO tolerance of new SiO2 doped phosphoric acid/polybenzimidazole polymer electrolyte membrane fuel cells at high temperatures of 200–250 °C

The high CO tolerance or resistance is critical for the practical application of proton exchange membrane fuel cells (PEMFCs) coupled with on board reformers for transportation applications due to the presence of high level of CO in the reformats. Increasing the operating temperature is most effective to enhance the CO tolerance of PEMFCs and therefore is of high technological significance. Here, we report a new PEMFC based on SiO₂ nanoparticles doped phosphoric acid/polybenzimidazole (PA/PBI/SiO₂) composite membranes for operation at temperatures higher than 200 °C. The phosphoric acid within the polymer matrix is stabilized by PA/phosphosilicate nanoclusters formed via prior polarization treatment of the membrane cells at 250 °C at a cell voltage of 0.6 V for 24 h, achieving a high proton conductivity and excellent stability at temperatures beyond that of conventional PA/PBI membranes. The proton conductivity of PA/PBI/SiO₂ composite membranes is in the range of 0.029–0.041 S cm^?1 and is stable at 250 °C. The PA/PBI/SiO₂ composite membrane cell displays an exceptional CO tolerance with a negligible loss in performance at CO contents as high as 11.7% at 240 °C. The cell delivers a peak power density of 283 mW cm^?2 and is stable at 240 °C for 100 h under a cell voltage of 0.6 V in 6.3% CO-contained H₂ fuel under anhydrous conditions.     

Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells

Graphite oxide/polybenzimidazole synthesized by 3, 3′-diaminobenzidine and 5-tert-butyl isophthalic acid (GO/BuIPBI) and isocyanate modified graphite oxide/BuIPBI (iGO/BuIPBI) composite membranes were prepared for high temperature polymer proton exchange membrane fuel cells (PEMFCs). All membranes were loaded with different content of phosphoric acid to provide proton conductivity. The GO/BuIPBI and iGO/BuIPBI membranes were characterized by SEM which showed that the filler GO or iGO were well dispersed in the polymer matrix and had a strong interaction with BuIPBI, which can improve the chemical stability of BuIPBI membrane and support a higher acid content. The proton conductivities of the GO/BuIPBI and iGO/BuIPBI with high acid loading were 0.016 and 0.027 S/cm, respectively, at 140 °C and without humidity.     

Membrane electrode assemblies doped with H3PO4 for high temperature proton exchange membrane fuel cells

H₃PO₄ content plays a critical role in high temperature proton exchange membrane fuel cells (HT-PEMFC), as it is responsible for the majority of the conductivity of the key components under high temperature operation. The conductivities of commercial AB-PBI membranes doped by immersing in 85 wt.% H₃PO₄ for different times and temperatures are investigated. The effect of H₃PO₄ loading in electrodes, including the AB-PBI polymer and a Pt/C catalyst, is also studied. The as-prepared electrodes and membranes are combined to fabricate a membrane electrode assembly for HT-PEMFCs. The results reveal that AB-PBI membranes doped with 85 wt.% H₃PO₄ at 90 °C for 9 h display a maximum conductivity of 33 mS cm⁻¹. This membrane was selected and combined with electrodes including 15 wt.% AB-PBI and 0.75 mg cm⁻² Pt with different H₃PO₄ loadings. A maximum current density of 260 mA cm⁻² was achieved in the as-prepared MEA (with 5 mg cm⁻² H₃PO₄ in electrodes) operating at 0.6 V and 160 °C, using oxygen and hydrogen.     

Superprotonic KH(PO3H)–SiO2 composite electrolyte for intermediate temperature fuel cells

Novel thin film composite electrolyte membranes, prepared by dispersion of nano-sized SiO₂ particles in the solid acid compound KH(PO₃H), can be operated under both oxidizing and reducing conditions. Long-term stable proton conductivity is observed at 140 °C, i.e., slightly above the superprotonic phase transition temperature of KH(PO₃H), under conditions of relatively low humidification (pH₂O ≈ 0.02 atm).     

Preparation and characterization of Sm0.2Ce0.8O1.9/Na2CO3 nanocomposite electrolyte for low-temperature solid oxide fuel cells

Sm_0.2Ce_0.8O_1.9 (SDC)/Na₂CO₃ nanocomposite synthesized by the co-precipitation process has been investigated for the potential electrolyte application in low-temperature solid oxide fuel cells (SOFCs). The conduction mechanism of the SDC/Na₂CO₃ nanocomposite has been studied. The performance of 20 mW cm⁻² at 490 °C for fuel cell using Na₂CO₃ as electrolyte has been obtained and the proton conduction mechanism has been proposed. This communication demonstrates the feasibility of direct utilization of methanol in low-temperature SOFCs with the SDC/Na₂CO₃ nanocomposite electrolyte. A fairly high peak power density of 512 mW cm⁻² at 550 °C for fuel cell fueled by methanol has been achieved. Thermodynamical equilibrium composition for the mixture of steam/methanol has been calculated, and no presence of C is predicted over the entire temperature range. The long-term stability test of open circuit voltage (OCV) indicates the SDC/Na₂CO₃ nanocomposite electrolyte can keep stable and no visual carbon deposition has been observed over the anode surface.