Wenjia Ma

Silane-cross-linked polybenzimidazole with improved conductivity for high temperature proton exchange membrane fuel cells

Silane-cross-linked polybenzimidazole (PBI) membranes with high proton conductivity and excellent mechanical properties were successfully prepared by using a silane monomer, γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH560), as a cross-linker. Fourier transform infrared spectroscopy and solubility tests were used to characterize and confirm the cross-linked structure in the membranes. The silane-cross-linked membranes displayed excellent chemical stability and improved mechanical strength. Especially at high temperature (130 °C), where the tensile strength value was in the range of 68.6 to 99.3 MPa, while that of the pristine PBI was 61.7 MPa. Moreover, the proton conductivity was significantly enhanced because the silane-cross-linked structure in the membranes could absorb more phosphoric acid. Considering the tradeoff of mechanical properties and proton conductivity, 3% KH560 in weight was demonstrated to be the optimum content in the membranes, for instance, the SCPBI-3/7.95 PA (the cross-linker content was 3 wt% and the PA doping level was 7.95) had a proton conductivity of 0.081 S cm−1 and that of the SCPBI-3/9.07 PA was 0.114 S cm−1 at 200 °C, while that of pristine PBI was 0.015 S cm−1 at 200 °C.

Cross-linked aromatic cationic polymer electrolytes with enhanced stability for high temperature fuel cell applications

Diamine-cross-linked membranes were prepared from cross-linkable poly(arylene ether ketone) containing pendant cationic quaternary ammonium group (QPAEK) solution by a facile and general thermal curing method using 4,4′-diaminodiphenylmethane with rigid framework and 1,6-diaminohexane with flexible framework as cross-linker, respectively. Self-cross-linked cationic polymer electrolytes membranes were also prepared for comparison. The diamines were advantageously distributed within the polymeric matrix and its amine function groups interacted with the benzyl bromide of QPAEK, resulting in a double anchoring of the molecule. Combining the excellent thermal stability, the addition of a small amount of diamines enhanced both the chemical and mechanical stability and the phosphoric acid doping (PA) ability of membranes. Fuel cell performance based on impregnated cross-linked membranes have been successfully operated at temperatures up to 120 °C and 180 °C with unhumidified hydrogen and air under ambient pressure, the maximum performance of diamine-cross-linked membrane is observed at 180 °C with a current density of 1.06 A cm−2 and the peak power density of 323 mW cm−2. The results also indicate that the diamine-cross-linked membranes using the rigid cross-linker show much improved properties than that using the flexible cross-linker. More properties relating to the feasibility in high temperature proton exchange membrane fuel cell applications were investigated in detail.

Highly stable poly(ethylene glycol)-grafted alkaline anion exchange membranes

A mechanically and chemically stable poly(ethylene glycol) (PEG)-grafted poly(styrene-ethylene-co-butylene-styrene) (SEBS)-based alkaline anion exchange membrane (AAEM) was designed, prepared and characterized. When subjected to tensile strain, the elongation at breaking of these SEBS-based AAEMs was up to 500%, a value 80 times greater than that of an AAEM using polystyrene as the main chain. Remarkably, the ion exchange capacity (IEC), conductivity, dimensions and mechanical properties of this AAEM could remain almost unchanged in 2.5 M KOH at 60 °C for about 3000 h, indicating the excellent alkaline stability of the PEG-grafted SEBS-based AAEMs. As confirmed by TEM, the grafting of PEG could enlarge the size of the ion-conducting channels, significantly enhancing the conductivity of these AAEMs (80 °C, from 29.2 mS cm−1 to 51.9 mS cm−1). Furthermore, the peak power density of an H2/O2 single fuel cell using this SEBS-based AAEM was up to 146 mW cm−2 at 50 °C. Based on these outstanding properties, this membrane has potential application not only for fuel cells, but also for other electrochemical energy conversion and storage devices, such as redox flow and alkaline ion batteries.

Cross-linked membranes with a macromolecular cross-linker for direct methanol fuel cells

With the goal of reducing water swelling and methanol permeability in sulfonated proton exchange membranes (PEM), bromomethylated poly(ether ether ketone) was synthesized and used as a macromolecular cross-linker. The cross-linking reaction was performed at 195 °C for 5 h and resulted in cross-linked membranes with high cross-linked density. Compared to the pristine membrane, the cross-linked membranes displayed greatly reduced water uptake and methanol permeability. Other properties of the cross-linked membranes, including proton conductivity, mechanical properties, and oxidative stability, were also investigated and compared with the pristine membrane. All the results indicated that the novel macromolecular cross-linker and the resulting cross-linked membranes are promising for fuel cell applications.

Highly alkaline stable N1-alkyl substituted 2-methylimidazolium functionalized alkaline anion exchange membranes

Steric hindrance and hyperconjugative effects, introduced at the N1 position of 2-methylimidazolium, greatly enhance the alkaline stability of the 2-methylimidazolium functional group. 2-Methylimidazolium small molecule compounds with N1-substituents (butyl, hexyl or octyl) are stable in 1 M KOH at 80 °C for more than 3000 h. Accordingly, the membranes based on N1-butyl, hexyl or octyl-substituted 2-methylimidzolium exhibited much more alkaline stability than membranes based on other substituted 2-methylimidazolium compounds, reflected by the almost unchanged IEC, conductivity and dimensions of the membranes after being exposed to 1 M KOH at 60 °C for hundreds of hours. This work reports the preparation of highly alkaline stable 2-methylimidazolium-based membranes by modifying the N1 position of 2-methylimidazolium.

Macromolecular covalently cross-linked quaternary ammonium poly(ether ether ketone) with polybenzimidazole for anhydrous high temperature proton exchange membranes

Poly(ether ether ketone) bearing benzyl bromide groups (Br–PEEK) was synthesized and a series of cross-linked membranes (Br–PEEK–x%PBI) based on Br–PEEK with polybenzimidazole (PBI) as a macromolecular cross-linker was prepared to improve the dimensional stability and tensile strength without reducing proton conductivity. X-ray photoelectron spectroscopy (XPS) confirmed the success of the cross-linking reaction. After being ammoniated, the quaternary ammonium PEEK membranes were immersed in phosphoric acid and anhydrous phosphoric acid doped membranes were obtained. The phosphoric acid doped membranes without PBI as the cross-linker had excess volume swelling and could not remain integrated. The other cross-linked membranes had good dimensional stabilities. Because PBI could absorb phosphoric acid, the proton conductivities of cross-linked membranes first increased and then decreased with the content of PBI increasing. The highest proton conductivity was 0.081 S cm−1 at 200 °C for the PA–PEEK–20%PBI membrane. The dimensional stabilities, oxidative stabilities and tensile strength of PA–PEEK–x%PBI membranes improved. The PA–PEEK–30%PBI membrane could last for 7.5 h in 3 wt.% H2O2, 4 ppm Fe2+ Fenton solution at 80 °C before breaking into pieces. Energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA) were used for detailed research.

Crosslinked hybrid membranes based on sulfonated poly(ether ether ketone)/γ-methacryloxypropyltrimethoxysilane/phosphotungstic acid by an in situ sol–gel process for direct methanol fuel cells

Proton exchange membranes with high dimensional stabilities and low water uptakes were constructed by incorporating phosphotungstic acid (PWA) into a cross-linked network composed of a crosslinkable sulfonated poly(ether ether ketone) containing dipropenyl groups (SDPEEK) and γ-methacryloxypropyltrimethoxysilane (KH570). The chemical structures of the hybrid membranes were confirmed by FT-IR spectroscopy and scanning electron microscopy (SEM). The results indicated that PWA particles were well dispersed in these membranes. The influences of the dispersed PWA on the properties of membranes such as thermal stability, water uptake, swelling ratio, proton conductivity, methanol permeability and mechanical property were researched. The addition of KH570-5/PWA in the hybrid membranes contributed to the improvement of the dimensional stabilities. And the hybrid membranes with 10–40wt% PWA showed higher proton conductivities than Nafion 117 at 80 °C, while the methanol permeabilities of these membranes were much lower than that of Nafion 117. The membranes also exhibited excellent mechanical properties. These results imply that the SDPEEK/KH570-5/PWA-x membranes are promising materials in the direct methanol fuel cells (DMFC) applications.

Preparation of Anion Exchange Membrane Based on Imidazolium Functionalized Poly(arylene ether ketone)

The authors presented a novel synthetic route for the imidazolium functionalized poly(arylene ether ketone)s, derived from an engineering plastics polymer, a poly(arylene ether ketone) with 3,3′,5,5′-tetramethyl-4,4′-dihydroxybiphenyl moiety(PAEK-TM). The preparation of anion exchange membranes comprised converting benzylic methyl groups to bromomethyl groups by a radical reaction, followed by the functionalization of bromomethylated PAEK with alkyl imidazoles, i.e., methyl, butyl or vinyl imidazole. The structure of imidazolium functionalized PAEK was proved by 1H NMR spectra. A class of flexible and tough membranes was then achieved by subsequent film-forming and anion exchange processes. The water uptake and hydroxide conductivities of membranes are comparable or superior to those of quaternary ammonium(QA) anion exchange membranes. This work demonstrated a new route for non-QA anion exchange membrane design, avoiding the chloromethylation reagent and precisely controlling the degree and location of imidazolium groups.