Segmented tetrasulfonated copoly(arylene ether sulfone)s: improving proton transport properties by extending the ionic sequence

Understanding the hydrocarbon - PFSA ionomer conductivity gap in hydrogen fuel cells

Hydrocarbon ionomers (HCs) have the potential to replace perfluorinated sulfonic acids (PFSAs), which are currently used in electrolyser or fuel cell membranes. To be a truly viable alternative, HCs must have conductivity across the operating range and cell lifetime comparable to PFSAs. Conductivity is an important property of membranes because it affects the energy efficiency of a fuel cell or electrolyser. By examining conductivity as a function of water volume fraction, it becomes evident that HC ionomers have consistently lower conductivity at low relative humidity. To better understand this 'conductivity gap', conductivity was converted to proton diffusivity and analysed using General Effective Media (GEM) theory for the first time. This analysis revealed that all ionomers require similar hydration levels for proton dissociation, and proton diffusion coefficients in the dry polymer are responsible for the conductivity gap. It is suggested that the membrane tortuosity ultimately accounts for the dry membrane's proton diffusivity and low RH conductivity. As the membrane hydrates however, all ionomers exhibit similar diffusion coefficients, indicating that conductivity at high humidity is limited by proton concentration.

Branched Poly(arylene ether ketone sulfone)s with Ultradensely Sulfonated Branched Centers for Proton Exchange Membranes: Effect of the Positions of the Sulfonic Acid Groups

Branched sulfonated polymers present considerable potential for application as proton exchange membranes, yet investigation of branched polymers containing sulfonated branched centers remains to be advanced. Herein, we report a series of polymers with ultradensely sulfonated branched centers, namely, B-x-SPAEKS, where x represents the degree of branching. In comparison with the analogous polymers bearing sulfonated branched arms, B-x-SPAEKS showed a reduced water affinity, resulting in less swelling and lower proton conductivity. The water uptake, swelling ratio (in-plane), and proton conductivity of B-10-SPAEKS at 80 °C were 52.2%, 57.7%, and 23.6% lower than their counterparts, respectively. However, further analysis revealed that B-x-SPAEKS featured significantly better proton conduction under the same water content due to the formation of larger hydrophilic clusters (∼10 nm) that promoted efficient proton transportation. B-12.5-SPAEKS exhibited a proton conductivity of 138.8 mS cm^-1 and a swelling ratio (in-plane) of only 11.6% at 80 °C, both of which were superior to Nafion 117. In addition, a decent single-cell performance of B-12.5-SPAEKS was also achieved. Consequently, the decoration of sulfonic acid groups on the branched centers represents a very promising strategy, enabling outstanding proton conductivity and dimensional stability simultaneously even with low water content.

Facile Fabrication and Characterization of Improved Proton Conducting Sulfonated Poly(Arylene Biphenylether Sulfone) Blocks Containing Fluorinated Hydrophobic Units for Proton Exchange Membrane Fuel Cell Applications

Sulfonated poly(arylene biphenylether sulfone)-poly(arylene ether) (SPABES-PAE) block copolymers by controlling the molar ratio of SPABES and PAE oligomers were successfully synthesized, and the performances of SPABES-PAE (1:2, 1:1, and 2:1) membranes were compared with Nafion 212. The prepared membranes including fluorinated hydrophobic units were stable against heat, nucleophile attack, and physio-chemical durability during the tests. Moreover, the polymers exhibited better solubility in a variety of solvents. The chemical structure of SPABES-PAEs was investigated by ¹H nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectroscopy (FT-IR), and gel permeation chromatography (GPC). The membrane of SPABES-PAEs was fabricated by the solution casting method, and the membranes were very flexible and transparent with a thickness of 70⁻90 μm. The morphology of the membranes was observed using atomic force microscope and the ionic domain size was proved by small angle X-ray scattering (SAXS) measurement. The incorporation of polymers including fluorinated units allowed the membranes to provide unprecedented oxidative and dimensional stabilities, as verified from the results of ex situ durability tests and water uptake capacity, respectively. By the collective efforts, we observed an enhanced water retention capacity, reasonable dimensional stability and high proton conductivity, and the peak power density of the SPABES-PAE (2:1) was 333.29 mW·cm-2 at 60 °C under 100% relative humidity (RH).

Hypersulfonated polyelectrolytes: preparation, stability and conductivity

A new sulfonation strategy enables the preparation of durable aromatic polymers with octasulfonated biphenyl units. This leads to polyelectrolytes with extremely high degrees of sulfonation, reaching high proton conductivities at low water contents.

Microphase separated sepiolite-based nanocomposite blends of fully sulfonated poly(ether ketone)/non-sulfonated poly(ether sulfone) as proton exchange membranes from dual electrospun mats

In this study, nanocomposite blends of fully sulfonated poly(ether ketone) (PEK) and non-sulfonated poly(ether sulfone) (PES) were prepared from a dual electrospinning process.

Ion distribution in quaternary-ammonium-functionalized aromatic polymers: effects on the ionic clustering and conductivity of anion-exchange membranes

A series of copoly(arylene ether sulfone)s that have precisely two, three, or four quaternary ammonium (QA) groups clustered directly on single phenylene rings along the backbone are studied as anion-exchange membranes. The copolymers are synthesized by condensation polymerizations that involve either di-, tri-, or tetramethylhydroquinone followed by virtually complete benzylic bromination using N-bromosuccinimide and quaternization with trimethylamine. This synthetic strategy allows excellent control and systematic variation of the local density and distribution of QA groups along the backbone. Small-angle X-ray scattering of these copolymers shows extensive ionic clustering, promoted by an increasing density of QA on the single phenylene rings. At an ion-exchange capacity (IEC) of 2.1 meq g(-1), the water uptake decreases with the increasing local density of QA groups. Moreover, at moderate IECs at 20 °C, the Br(-) conductivity of the densely functionalized copolymers is higher than a corresponding randomly functionalized polymer, despite the significantly higher water uptake of the latter. Thus, the location of multiple cations on single aromatic rings in the polymers facilitates the formation of a distinct percolating hydrophilic phase domain with a high ionic concentration to promote efficient anion transport, despite probable limitations by reduced ion dissociation. These findings imply a viable strategy to improve the performance of alkaline membrane fuel cells.

Towards high conductivity in anion-exchange membranes for alkaline fuel cells

Quaternized poly(2,6-dimethylphenylene oxide) materials (PPOs) containing clicked 1,2,3-triazoles were first prepared through Cu(I) -catalyzed "click chemistry" to improve the anion transport in anion-exchange membranes (AEMs). Clicked 1,2,3-triazoles incorporated into AEMs provided more sites to form efficient and continuous hydrogen-bond networks between the water/hydroxide and the triazole for anion transport. Higher water uptake was observed for these triazole membranes. Thus, the membranes showed an impressive enhancement of the hydroxide diffusion coefficient and, therefore, the anion conductivities. The recorded hydroxide conductivity was 27.8-62 mS cm(-1) at 20 °C in water, which was several times higher than that of a typical PPO-based AEM (TMA-20) derived from trimethylamine (5 mS cm(-1) ). Even at reduced relative humidity, the clicked membrane showed superior conductivity to a trimethylamine-based membrane. Moreover, similar alkaline stabilities at 80 °C in 1 M NaOH were observed for the clicked and non-clicked membranes. The performance of a H2 /O2 single cell assembled with a clicked AEM was much improved compared to that of a non-clicked TMA-20 membrane. The peak power density achieved for an alkaline fuel cell with the synthesized membrane 1a(20) was 188.7 mW cm(-2) at 50 °C. These results indicated that clicked AEM could be a viable strategy for improving the performance of alkaline fuel cells.