Preparation of polybenzimidazole/ZIF-8 and polybenzimidazole/UiO-66 composite membranes with enhanced proton conductivity

Metal-organic frameworks (MOFs) are considered emerging materials as they further improve the various properties of polymer membranes used in energy applications, ranging from electrochemical storage and purification of hydrogen to proton exchange membrane fuel cells. Herein, we fabricate composite membranes consisting of polybenzimidazole (PBI) polymer as a matrix and MOFs as filler. Synthesis of ZIF-8 and UiO-66 MOFs are conducted through a typical solvothermal method, and composite membranes are fabricated with different MOF compositions (e.g., 2.5, 5.0, 7.5, and 10.0 wt %). We report a significant improvement in proton conductivity compared with the pristine PBI; for example, more than a three-fold increase in conductivity is observed when the PBI-UiO66 (10.0 wt %) and PBI-ZIF8 (10.0 wt %) membranes are tested at 160 °C. Proton conductivities of the composite membranes vary between 0.225 and 0.316 S cm^?1 at 140 and 160 °C. For the comparison, pure PBI exhibits 0.060 S cm^?1 at 140 °C and 0.083 S cm^?1 at 160 °C. However, we also report a decrease in permeability and mechanical stability with the composite membranes.     

Proton-conducting composite membranes based on polybenzimidazole and sulfonated mesoporous organosilicate

Sulfonated mesoporous organosilicate (s-MPOs) was synthesized by the one-step sol–gel method as a novel inorganic additive derived for use in the fuel cell. TEM observations revealed that the s-MPOs has well-ordered structure and many SO₃H groups on the inner surface of the mesopores. The s-MPOs was added to the proton-conductive polymer matrix, polybenzimidazole (PBI) in the presence of H₃PO₄, and the proton conductivities were measured at 60–100 °C under controlled humidity. The PBI composites filled with only 1 wt% of s-MPOs gave proton conductivity more than 10-times higher than the original PBI/H₃PO₄ membrane. The s-MPOs possessing many SO₃H groups were able to form effective proton conductive pathways via its periodic structure and to improve the conductivity. The greatest conductivity was estimated to be 0.21 S cm⁻¹ at 80 °C and 98 %RH in case of a PBI/s-MPOs20 (incl. approx. 20 mol% of the SO₃H units in MPS) composite.     

Sulfonated MWNT and imidazole functionalized MWNT/polybenzimidazole composite membranes for high-temperature proton exchange membrane fuel cells

Composite membranes used for proton exchange membrane fuel cells comprising of polybenzimidazole (PBI) and carbon nanotubes with certain functional groups were studied, because they could enhance both the mechanical property and fuel cell performance at the same time. In this study, sodium poly(4-styrene sulfonate) functionalized multiwalled carbon nanotubes (MWNT-poly(NaSS))/PBI and imidazole functionalized multiwalled carbon nanotubes (MWNT-imidazole)/PBI composite membranes were prepared. The functionalization of carbon nanotubes involving non-covalent modification and covalent modification were confirmed by FITR, XPS, Raman spectroscopy, and TGA. Compared to unmodified MWNTs and MWNT-poly(NaSS), MWNT-imidazole provided more significant mechanical reinforcement due to its better compatibility with PBI. For MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes at their saturated doping levels, the proton conductivities were up to 5.1 × 10⁻² and 4.3 × 10⁻² S/cm at 160 °C under anhydrous condition respectively, which were slightly higher than pristine PBI (2.8 × 10⁻² S/cm). Also, MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes showed relatively improved fuel cell performance at 170 °C compared to pristine PBI.     

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.     

Developments of new proton conducting membranes based on different polybenzimidazole structures for fuel cells applications

The current goal on PEMFCs research points towards the optimization of devices working at temperatures above 100 °C and at low humidity levels. Acid-doped polybenzimidazoles are particularly appealing because of high proton conductivity without humidification and promising fuel cells performances.In this paper we present the development of new proton conducting membranes based on different polybenzimidazole (PBI) structures. Phosphoric acid-doped membranes, synthesized from benzimidazole-based monomers with increased basicity and molecular weight, are presented and discussed. Test of methanol crossover and diffusion were performed in order to check the membrane suitability for DMFCs.Both the acid doping level and proton conductivity remarkably increase with the membrane molecular weight and basicity, which strictly depend on the amount of NH-groups as well as on their position in the polymer backbone. In particular, a conductivity value exceeding 0.1 S cm⁻¹ at RH = 40% and 80 °C was reached in the case of the pyridine-based PBI.     

New modified Nafion-bisphosphonic acid composite membranes for enhanced proton conductivity and PEMFC performance

Proton exchange membranes remain a crucial material and a key challenge to fuel cell science and technology. In this work, new Nafion membranes are prepared by a casting method using aryl- or azaheteroaromatic bisphosphonate compounds as dopants. The incorporation of the dopant, considered at 1 wt% loading after previous selection, produces enhanced proton conductivity properties in the new membranes, at different temperature and relative humidity conditions, in comparison with values obtained with commercial Nafion. Water uptake and ionic exchange capacity (IEC) are also assessed due to their associated impact on transport properties, resulting in superior values than Nafion when tested in the same experimental conditions. These improvements by doped membranes prompted the evaluation of their potential application in fuel cells, at different temperatures. The new membranes, in membrane-electrode assemblies (MEAs), show an increased fuel cell maximum power output with temperature until 60 °C or 70 °C, followed by a decrease above these temperatures, a Nafion-like behaviour when measured in the same conditions. The membrane doped with [1,4-phenylenebis(hydroxymethanetriyl)]tetrakis(phosphonic acid) (BP2) presents better results than Nafion N-115 membrane at all studied temperatures, with a maximum power output performance of ～383 mW cm^?2 at 70 °C. Open circuit potentials of the fuel cell were always higher than values obtained for Nafion MEAs in all studied conditions, indicating the possibility of advantageous restrain to gas crossover in the new doped membranes.     

Modeling of high temperature proton exchange membrane fuel cells with novel sulfonated polybenzimidazole membranes

In this study, a three-dimensional, steady-state, non-isothermal numerical model of high temperature proton exchange membrane fuel cells (HT-PEMFCs) operating with novel sulfonated polybenzimidazole (SPBI) membranes is developed. The proton conductivity of the phosphoric acid doped SPBI membranes with different degrees of sulfonation is correlated based on experimental data. The predicted conductivity of SPBI membranes and cell performance agree reasonably with published experimental data. It is shown that a better cell performance is obtained for the SPBI membrane with a higher level of phosphoric acid doping. Higher operating temperature or pressure is also beneficial for the cell performance. Electrochemical reaction rates under the ribs of the bipolar plates are larger than the values under the flow channels, indicating the importance and dominance of the charge transport over the mass transport.     

CHS-WSiA doped hexafluoropropylidene-containing polybenzimidazole composite membranes for medium temperature dry fuel cells

Various loadings (0, 5, 10, 20 wt%) of wet-mechanochemically prepared CsHSO₄–H₄SiW₁₂O₄₀ (CHS-WSiA) composite were incorporated into polymer electrolyte membranes (PEM) based on commercial polybenzimidazole (PBI) and synthesized fluorine-containing PBI (F₆PBI). The membranes thus obtained were evaluated by thermal analysis, Fenton's test, acid leaching test, proton conductivity and fuel cell performance. The addition of CHS-WSiA inorganic composite improved acid retention ability and proton conductivity. F₆PBI composite membrane exhibited enhanced chemical stability against hydroxyl radical attack, while improving acid doping characteristics and acid retention ability significantly. With 8 mol phosphoric acid doping level (PADL) (173 wt% phosphoric acid uptake), F₆PBI with 10 wt% CHS-WSiA, namely F₆PBI(10), exhibited the highest conductivity of 2.14 mScm⁻¹ at 150 °C under dry conditions. Under similar conditions as for fuel cell performance, F₆PBI(10) achieved a maximum power density of 498 mWcm⁻² with a stable voltage of 0.614 V under a constant current 0.2 Acm⁻² durability test over 24 h.     

High temperature proton exchange porous membranes based on polybenzimidazole/ lignosulfonate blends: Preparation,morphology and physical and proton conductivity properties

Porous polybenzimidazole (PBI) based blend membranes were prepared by adding different amounts of lignosulfonate (LS) in the presence of LiCl salt. The morphology characteristics of the PBI/LS blends were investigated by FT-IR, atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses. The relation between the membrane morphology and membrane proton conductivity was studied. Results showed that LS content has a significant influence on the membrane morphology. High amount of LS in the blend created micro-pores within the membrane where increase in the LS content up to 20 wt% resulted in membranes containing pores with a mean diameter of about 0.8 μm. The resulting PBI/LS (0–20 wt%) membranes indicated high PA doping levels, ranging from 3 to 16 mol of PA per mole of PBI repeat units, which contributed to their unprecedented high proton conductivities of 4–96 mS cm⁻¹, respectively, at 25 °C. The effect of temperature on the proton conductivity of blends was also investigated. The results showed that by rising the temperature, the proton conductivity increases in PBI/LS blends. In the blend containing 20 wt% LS, proton conductivity increased from 98 mS cm⁻¹ at 25 °C to 187 mS.cm⁻¹at 160 °C which can be considered as an excellent candidate for use in both high and low temperature proton exchange membrane fuel cells.     

Effects of mesoporous fillers on properties of polybenzimidazole composite membranes for high-temperature polymer fuel cells

In this study, we elucidated the effects of the addition of various mesoporous silicates (0–20 wt%) to the membranes used for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) on cell performance. Two types of polybenzimidazole (PBI)-based hybrid membranes were prepared by homogeneously dispersing a predetermined amount of MCM-41 or SBA-15 within the PBI matrix. Compared to the pure PBI membrane, those with MCM-41 and SBA-15 exhibited significantly enhanced phosphoric acid doping and better mechanical properties, leading to improved HT-PEMFC performance and reduced acid migration. However, the membranes with 20 wt% silicate showed inferior performance compared to those with 10 wt% silicate. In addition, the membranes with SBA-15 exhibited noticeable aggregation, lower phosphoric acid doping, and greater phosphoric acid migration during the leaching test than did the membranes with MCM-41. Finally, during the short-term durability test, the PBI/MCM-41 (10 wt%) membrane showed the best performance (maximum power density of 310  mW cm^?2).     

Comprehensive performance enhancement of polybenzimidazole based high temperature proton exchange membranes by doping with a novel intercalated proton conductor

On the study of high temperature proton exchange membrane (HTPEM), the trade-off between proton conductivity and physico-chemical property (such as mechanical strength, dimensional stability and methanol resistance) remained a main obstacle for comprehensive performance enhancement. To address this issue, novel HTPEM was prepared by doping phosphotungstic acid intercalated ferric sulfophenyl phosphate (FeSPP-PWA) into polybenzimidazole (PBI) via hot pressed method. Intense hydrogen bonding network was built between PBI and FeSPP-PWA, rendering construction of proton channels and reinforcement of physico-chemical property. As a novel proton conductor, FeSPP-PWA facilitated formation of efficient proton transfer pathway. The layered morphology and inorganic intrinsity of FeSPP-PWA also improved the mechanical and dimensional stability while reducing the methanol permeability of the PBI/FeSPP-PWA membranes. The composite membrane exhibited good thermal stability up to 200 °C. The proton conductivity of PBI/FeSPP-PWA (30 wt%) reached 110 mS cm^?1 at 170 °C and 100% RH, and was 69.3 mS cm^?1 at 180 °C and 50% RH. The PBI/FeSPP-PWA also showed low methanol permeability and high membrane selectivity for application in direct methanol fuel cells.     

Polymer electrolyte fuel cells based on phosphoric acid doped polybenzimidazole (PBI) membranes

In order to make fuel cells with high power density the structure and morphology for the three-dimensional gas diffusion electrodes (GDEs) are very important. A preparation technique for GDEs for phosphoric acid doped polybenzimidazole (PBI) is presented. Teflon treatment of the backing material was found to be beneficial for the performance of the electrodes, and explained by higher total porosity. In general the open circuit voltage (OCV) with PBI-based cells is 0.9 V. The observed low OCV was explained by slow kinetic for the oxygen reduction and cross over of the reactants. The performance of the fuel cells is found to increase with increasing temperature; this was explained by faster reaction kinetic and higher membrane conductivity. A typical power output was 0.3–0.4 W cm⁻² at 0.6 V and 175 °C.     

Fabrication of crosslinked polybenzimidazole membranes by trifunctional crosslinkers for high temperature proton exchange membrane fuel cells

Two trifunctional bromomethyls containing crosslinkers, 1,3,5-tris(bromomethyl)benzene (B3Br) and 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (Be3Br), are employed to covalently crosslink polybenzimidazole (PBI) membranes for the high temperature proton exchange membrane fuel cell. The presence of three bromomethyl groups in each crosslinker molecule is expected to create more free volume for acid doping while enhancing the adhesive strength of the PBI chains. In addition, the influence of the two crosslinker structures on the property of the crosslinked membranes is compared and analyzed. All the crosslinked PBI membranes exhibit longer morphology durability over the pristine PBI membrane toward the radical oxidation. Moreover, the crosslinked PBI membranes with the crosslinker Be3Br containing three ethyl groups display superior acid doping level, high conductivity and excellent mechanical strength simultaneously, over those with the crosslinker B3Br and the pristine PBI membrane. Single cell measurements based on the acid doped membrane with a crosslinking degree of 7.5% by Be3Br demonstrate the technical feasibility of the prepared membranes for high temperature proton exchange membrane fuel cells.     

Influence of catalyst layer polybenzimidazole molecular weight on the polybenzimidazole-based proton exchange membrane fuel cell performance

The catalyst layer (CL) of a polybenzimidazole (PBI) membrane electrode assembly (MEA) consists of Pt–C (Pt on a carbon support), PBI, and H₃PO₄. Two series of catalyst ink solutions each containing Pt–C, N,N′-dimethyl acetamide, and PBIs comprising four different molecular weights (MWs) (i.e., M_w = 1.1 × 10⁴, 4.4 × 10⁴, 9.0 × 10⁴, and 17.4 × 10⁴ g mol⁻¹) are used to fabricate CLs. One catalyst ink solution series is mixed with LiCl, while the other solution series lacks LiCl. We demonstrate that the CL prepared using a lower MW PBI has a higher electrochemical surface area, lower charge transfer resistance, and higher fuel cell performance. The addition of LiCl enhances the dispersion of the high MW PBIs in the catalyst ink solution and acts as a foaming agent in CL, thus improving fuel cell performance. However, LiCl exerts small influence on the fuel cell performance of the MEAs fabricated using low MW PBIs.     

PVDF-g-PSSA and Al2O3 composite proton exchange membranes

Poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF-g-PSSA) membranes doped with different amount of Al₂O₃ (PVDF/Al₂O₃-g-PSSA) were prepared based on the solution-grafting technique. The microstructure of the membranes was characterized by IR-spectra and scanning electron microscope (SEM). The thermal stability was measured by thermal gravity analysis (TGA). The degree of grafting, water-uptake, proton conductivity and methanol permeability were measured. The results show that the PVDF-g-PSSA membrane doped with 10% Al₂O₃ has a lower methanol permeability of 6.6 × 10⁻⁸ cm² s⁻¹, which is almost one-fortieth of that of Nafion-117, and this membrane has moderate proton conductivity of 4.5 × 10⁻² S cm⁻¹. Tests on cells show that a DMFC with the PVDF/10%Al₂O₃-g-PSSA has a better performance than Nafion-117. Although Al₂O₃ has some influence on the stability of the membrane, it can still be used in direct methanol fuel cells in the moderate temperature.     

Polybenzimidazole and benzyl-methyl-phosphoric acid grafted polybenzimidazole blend crosslinked membrane for proton exchange membrane fuel cells

We synthesize polybenzimidazole (PBI; M_w = 1.65 × 10⁵ g mol⁻¹) and benzyl-methyl-phosphoric acid grafted PBI (PBI-BP; 24 mol% degree of grafting). We demonstrate that blending 20 to −40 wt.% PBI-BP in the PBI membrane enhances the H₃PO₄ doping level, proton conductivity, and mechanical strength. However, the membrane is highly dissolved in an 85 wt.% H₃PO₄ aqueous solution as the PBI-BP content in the blend membrane is larger than 50 wt.%. To prevent PBI-BP from being dissolved out of the blend membrane by the H₃PO₄ aqueous solution, we fabricated a PBI/PBI-BP/epoxy (8/2/1.23 by wt.) crosslinked membrane. The crosslinked membrane demonstrated good fuel cell performance and excellent stability after a 23 on/off (12 h on at 160 °C with a current density of 200 mA cm⁻² and 12 h off at room temperature) fuel cell cycle test with an unhumidified H₂/O₂.     

Branched polybenzimidazole / polymeric ionic liquid cross-linked membranes with high proton conductivity and mechanical properties for HT-PEM applications

Branched polymers have unique three-dimensional dendritic structures, so they have received a lot of attention in the application of high temperature proton exchange membranes. In this work, we synthesize BOPBI-X (X = 3%, 6%, 9%) membranes with different branched ratios based on the synthesis of OPBI, due to the introduction of a rigid triazine structure with a larger free volume, the membranes could absorb more phosphoric acid (PA) while maintaining sufficient mechanical strength. Among them, the BOPBI-6% membrane obtains splendid comprehensive performance, when the PA doping level (ADL) is 9.55, it achieves a proton conductivity of 99.2 mS cm^?1 at 180 °C, which is 1.8 times higher than OPBI (54.8 mS cm^?1), and it performs well in long-term oxidation stability test after 144 h, it still has a mass retention rate of 90%. For the sake of further boost the performance of the membrane, cross-linkable polymeric ionic liquid (PIL) is introduced to the system. Among them, BOPBI-PIL-30% membrane has sufficient mechanical properties (5.50 ± 0.8 MPa), and the proton conductivity (146.9 mS cm^?1) at 180 °C is also excellent, so BOPBI-PIL-30% membrane is expected to be a promising candidate as HT-PEMs.     

Proton conduction in Nafion composite membranes filled with mesoporous silica

Studies of proton-conductive polymer membranes are vital for the future development of high-performance polymer electrolyte membrane fuel cells (PEM-FC). In particular, a method for inhibiting the volatility of water in the polymer matrix at high temperatures is a crucial issue, directly related to the operation of PEM-FC system. In this study, we focus on polymer composite membranes, which consist of commercial Nafion and mesoporous silica (MPSi) as novel inorganic additives, and investigate an improvement in the total proton conductivities and the good electrochemical stability at high temperatures. MPSi, which can be synthesized with pore sizes from 1 to 10 nm, has a wide range of potential applications because of its extraordinary properties, such as extremely large surface area, flawless surface condition and well-regulated porous structure. We found that the Nafion composites filled with MPSi have approximately 1.5 times higher proton conductivities (more than 0.1 S cm⁻¹ at 80 °C and 95%RH) than pure Nafion and can display good temperature performance relative to pure Nafion and the particle SiO₂ composite. Moreover, the conductivity of Nafion/sulfonated MPSi was the highest (0.094 S cm⁻¹) at 40 °C and 95%RH. These are probably due to the large surface area of MPSi, which can increase the water adsorption in Nafion matrix.     

Experimental and computational study of proton and methanol permeabilities through composite membranes

To design direct methanol fuel cells, proton permeability and methanol crossover have to be evaluated. A study of the transport of methanol and protons through composite membranes of poly(ethylene glycol) (PEG) and polysulfone (PSf) was performed and permeabilities of these components were determined. PSF was treated with dilute sulfuric acid to enhance hydrophilicity. PEG was found to be a good material for the active layer, because it contains OH hydrophilic groups which combine with hydrated protons. A composite membrane made of 15 wt.% PSf and 40–50 wt.% PEG showed a lower methanol crossover (1.0E−06 cm² s⁻¹) than the commercial reference NAFION^® 117. Maximal proton conductivity is also lower than NAFION^® 117. A mathematical deterministic model, considering transport by diffusion through the composite membrane and equilibrium at the membrane–reservoir interfaces, was derived. However, the PEG layer did not present any pores and diffusion in the dense membrane was estimated using a transport probability. On the other hand, the porous PSf layer required an effective diffusivity that is a function of physical properties such as porosity and tortuosity. The contribution made by each mass transfer phenomenon to the total permeation was calculated by an association of mass transfer resistances.     

4,4′-Oxydianiline (ODA) containing sulfonated polyimide/protic ionic liquid composite membranes for anhydrous proton conduction

The aim of this study was to utilize ionic liquids (ILs) in preparation of modified sulfonated polyimide (SPI) composite membranes to substantially increase the conductivity of proton exchange membranes (PEMs). Protic ILs used included 1-vinylimidazolium trifluoromethanesulfonate [ImVH][OTf] and 1-methyl-imidazolium trifluoromethanesulfonate [ImMH][OTf]. SPIs are 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 an aromatic anhydride, ODPA (4,4′-oxy diphthalic anhydride). ODADS improves conductivity, while BAPP enhances the mechanical and thermal properties of SPIs. We have prepared SPI/IL composite PEM using 50 wt.% [ImVH][OTf] with a high conductivity of 3–6 mS/cm at 120 °C and anhydrous condition depending on the ODADS content. [ImVH][OTf] offered better conductivity, which can be attributed to its chemical structure that has a vinyl group attached to an imidazolium ring to provide lower melting temperature and lattice energy, thereby increasing conductivity.