•Using Ullmann C-N coupling to synthesize poly(arylene ether ketone)s with dense sulfonic-acid-functionalized pendants.•Multiphenyl substituted difluoride monomer is not required in synthesis procedure, avoiding complex synthetic procedures.•The large difference in polarity of polymer matrix enables the formation of well-defined nanophase-separated structures.•With an IEC value of 1.72 meq.·g−1, the membrane exhibits excellent dimensional stability and high proton conductivity.A series of poly(arylene ether ketone)s (PAEKs) with dense sulfonic-acid-functionalized pendants were successfully synthesized by the Ullmann C–N coupling of a polymer precursor containing pendant iodobenzene rings and 3,6-ditrityl-9H-carbazole, followed by sulfonation using chlorosulfonic acid. A tough transparent membrane, which exhibited suitable dimensional stability and high proton conductivity, was obtained by casting an N,N-dimethylacetamide polymer solution. DSPAEK-CZ-25 (IEC = 2.03 meq. g−1, where IEC = ion-exchange capacity) exhibited the highest proton conductivity of 177 mS cm−1 at 100 °C; this value is significantly greater than that of Nafion 117. In terms of molecular design, the large difference in polarity between the fluorinated polymer backbone and the concentrated pendant sulfonic acid groups possibly induces good phase-separation morphology, which enhances conductivity and dimensional stability. The obtained membranes exhibited excellent thermal stabilities and good mechanical properties. Overall, the DSPAEK-CZ polymers exhibited outstanding comprehensive performance as potential materials for proton exchange membranes (PEMs).Preparation of proton exchange membranes with functional pendants using Ullmann C-N coupling reaction is detailed in this work. Membranes show well-defined phase-separation resulting in sufficient proton conductivity and modest dimensional stability. The membranes exhibited outstanding performance as good candidate proton-exchange membrane materials for fuel cells applications.Download high-res image (260KB)Download full-size image

•A series of novel hexa-sulfonated poly(aryl ether ketone)s (HS-PAEK-x) membranes have been obtained.•The membranes displayed comparable proton conductivity to Nafion 117.•The membranes exhibited adequate resistance to swelling and considerable oxidative stability.•The clear phase separation of the membranes attributes to the structure of concentrated sulfonation on the side chain.A new difluoride monomer containing an aliphatic bridge between 2,6-difluoropheyl and hexaphenylbenzene is designed and synthesized. Then, a series of novel hexa-sulfonated poly(aryl ether ketone)s (HS-PAEK-x) containing an aliphatic spacer between hydrophobic poly(aryl ether ketone)s main chain and hydrophilic sulfonated hexaphenylbenzene are prepared from the new difluoride monomer with 4,4′-(hexafluoroisopropylidene) diphenol and 4,4′-difluorobenzophenone by nucleophilic polycondensation, and subsequent postsulfonation using chlorosulfonic acid. All the polymers give tough, flexible, and transparent membranes by solvent casting. The membranes with comparatively low ion exchange capacity (IEC) values ranging from 0.87 to 1.42 meq g−1, achieve good mechanical properties, suitable proton conductivity (111 mS cm−1) compared with Nafion 117 at 80 °C, and excellent dimensional stability (the highest swelling ratio of length is 9.2% at 80 °C). Furthermore, compared with the previous reported polymers with similar main chain, such as the polymer with pendant sulfoalkyl groups and hollow aromatic hexa-sulfonated polymer, the HS-PAEK-x exhibit higher proton conductivities at similar IEC level, which attributes to the specific molecular structure and improving morphology. Overall, all of the above-mentioned results indicate that the HS-PAEK-x membranes are good candidate materials for proton exchange membranes (PEMs).

A series of octa-sulfonated poly(arylene ether)s (PAEs) was prepared via a low-temperature grafting reaction and subsequent postsulfonation. The rigid backbone with high molecular weight is conducive to higher integrity of the hydrophobic domain. And the grafting ionic clusters are expected to promote the appearance of phase separation. These membranes, with ion exchange capacity (IEC) values ranging from 1.19 to 1.90 meq. g−1, exhibited excellent dimensional stability, mechanical properties and oxidative stability. The membrane with higher IEC (>1.4 meq. g−1) exhibited adequate conductivity (>100 mS cm−1) in water at 80 °C. Furthermore, the membrane with IEC = 1.90 meq. g−1 exhibited comparable conductivity to Nafion 117 under various humidity levels. In addition, SAXS profiles confirmed the well-defined phase-separated morphology of SPAE-x. DMFC single cell performance demonstrates that SPAE-x are good candidates for proton exchange membranes in fuel cell applications.

A series of sulfonated methoxyphenyl-containing poly(arylene ether ketone)s (SMP-PAEKs) were synthesized via polycondensation from 2-(3-methoxy)phenylhydroquinone and other commercial monomers, followed by a postsulfonation approach under mild reaction conditions. Controlled substituted sites and the degree of sulfonation were realized by using various quantities of 2-(3-methoxy)phenylhydroquinone, given that sulfonated polymers (SMP-PAEK) can be soluble in common organic solvents such as DMSO, NMP, and DMAc. The tough and transparent polymer membrane was prepared by a solution casting method, exhibiting excellent mechanical properties and high proton conductivities. The tensile stress at maximum load and elongation at break of these membranes are 30.0–32.4 MPa and 120–171% in a dry state respectively. The proton conductivities of these membranes are higher than that of Nafion 117 in water. In particular, SMP-PAEK-80 with an IEC of 1.62 mequiv. g−1 exhibited the highest proton conductivity of 294 mS cm−1 at 80 °C in water. In addition, SMP-PAEK-90 with an IEC of 1.83 mequiv. g−1 exhibited suitable proton conductivities in different relative humidities (RHs) (30–98%) at 80 °C, which was higher than Nafion 117. The clear micro-phase separation morphology was observed by SAXS, which was powerful evidence to explain their high conductive behaviors. In addition, the current density of the SMP-PAEK-80 membrane was measured to be 235 mA cm−2 at 0.36 voltage (V) under fully hydrated conditions at room temperature with 1.5 bar back pressure during a MEA cell performance test.

•Poly(arylene ether)s containing flexible disulfonic acid pendant was synthesized.•The I-50 exhibited comparable conductivity to Nafion 117 and lower swelling ratio.•Disulfonic acid side chain I-x was better than SP-90 with similar IEC value.New difluoride monomer containing a biphenyl pendant group is designed and synthesized for preparing side chain style polyelectrolyte. As a result, a novel poly(arylene ether)s containing flexible disulfonic acid pendant are obtained by a two-step synthetic procedure. First, poly(arylene ether)s with electron-rich side groups as polymer precursor (P-x) are prepared from designed difluoride monomer, 4,4′-difluorobenzophenone (DFDPK), and 4,4′-(hexafluoroisopropylidene)diphenol (6FBPA) by nucleophilic polycondensation. Then objective sulfonated polymers (I-x) are obtained from P-x and chlorosulfonic acid by sulfonation reaction. Tough, flexible, and transparent membranes having high mechanical strength are obtained by solution casting of I-x polymer. The membrane I-x exhibits high proton conductivity and good dimensional stability, which attributes their special side chain style structure. For example, I-50 membrane (IEC = 1.32 meq. g−1) shows low swelling of 8.1% and comparable conductivity with Nafion 117 at 80 °C. Suitable morphology is observed in I-x membranes by SAXS test, which may explain that I-x possess perfect overall properties.

A new difluoride monomer containing electron-rich tetraphenylmethane was designed and synthesized. Based on this monomer along with 4,4′-(hexafluoroisopropylidene)diphenol (6FBPA) and 4,4′-difluorobenzophenone (DFB), a series of tetra-sulfonated poly(aryl ether ketone)s (TS-PAEK-x) were prepared by nucleophilic polycondensation, followed by a sulfonation reaction using chlorosulfonic acid. Tough, flexible, and transparent membranes were obtained by solvent casing. These membranes with ion exchange capacity (IEC) values ranging from 0.85 to 1.43 meq. g−1 exhibited good mechanical properties, excellent dimensional stability, and suitable proton conductivity. The largest swelling ratio (in-plane direction) with a value of 10.3% was observed from TS-PAEK-25 (IEC = 1.43 meq. g−1) membranes at 100 °C, which was much lower than that of Nafion 117 under the same conditions. The highest proton conductivity of 151 mS cm−1 was obtained from the TS-PAEK-25 membrane at 100 °C in the fully hydrated state. Compared to Nafion 117, TS-PAEK-25 with comparable water content exhibited a superior effective proton mobility (μ) value (up to 1.19 × 10−3 cm2 s−1 V−1). Under reduced humidity conditions, TS-PAEK-25 showed desired conductivity considering its lower IEC level. The results indicate that the TS-PAEK-x membranes are promising candidates for application as proton exchange membranes.

The preparation and characterization of new polymeric ionomers based on a fully aromatic poly(arylene ether) backbone with locally pentasulfonated pendent groups for applications as proton exchange membranes is reported. The high molecular weight sulfonated polymers were obtained by the polycondensation of new (2,6-difluorophenyl) (4-(1,2,3,4,5-pentaphenylbenzene)phenyl)methanone, 4,4′-difluorodiphenyl methanone, and 4,4′-dihydroxydiphenylsulfone, followed by sulfonation using sulfuric acid in high yields. The polymers produced tough, flexible, and transparent membranes by solvent casting. Membranes with ion exchange capacities between 0.8 and 1.7 mEq g-1 showed high proton conductivities and low methanol permeabilities. Compared to Nafion 117, these sulfonated membranes exhibited better microphase separation morphologies. The fully humidified membranes also exhibited considerably good mechanical properties, with tensile strengths from 35 to 45 MPa and elongations at break from 23 to 49%. This investigation demonstrates a controllable high density sulfonated side group of a poly(arylene ether sulfone) membrane with tunable and balanced properties, which is promising for proton exchange membrane fuel cell technology.

A novel bisphenol monomer containing concentrated electron rich phenyls is synthesized, which provides a locally and densely postsulfonation sites. Based on this monomer, a series of new sulfonated poly (ether sulfone)s (SPESs) are successfully obtained by nucleophilic substitution reaction, followed by postsulfonation using concentrated sulfuric acid. All the polymer membranes are readily prepared by solvent casting and exhibit excellent thermal stability and mechanical properties. As ion exchange capacity (IEC) ranging from 0.98 to 1.66 mequiv g−1, the polymers afford considerable proton conductivity and water absorption. SPES-3 with IEC = 1.66 mequiv g−1 shows equal proton conductivity (0.131 S cm−1) to Nafion 117 at 100 °C under fully hydrated state. At low IEC level (IEC = 0.92 mequiv g−1), SPES-1 also exhibits higher conductivity (>10−2 S cm−1) than some earlier reported sulfonated random polymers. Their excellent performance is attributed to the internal structure of the polymers, which formed distinct phase separation between hydrophilic and hydrophobic moiety observed by SAXS. A combination of high proton conductivities, moderate water uptake, suitable mechanical properties and low swelling ratios for some of the obtained SPES indicates that they are good candidate materials for proton exchange membranes (PEMs).Graphical abstractHighlights► A series of new concentrated sulfonated poly (ether sulfone)s were synthesized. ► These polymers own lower activation energy (Ea) of conductivity compared to Nafion. ► The membrane with IEC = 1.66 mequiv g−1 exhibits higher conductivity than Nafion. ► These membranes show considerable water management and proton conductivity.

A series of tetra-sulfonated poly(arylene ether)s are prepared from a new tetra-sulfonated difluoride monomer and commercial 4,4′-difluorobenzophenone and 4,4′-(hexafluoroisopropylidene)diphenol via polycondensation process. With the content of tetra-sulfonated monomer raising from 15% to 35% in difluoride monomer, sulfonated polymers with ion exchange capacity (IEC) ranging from 0.92 to 1.66 mequiv g−1 are obtained. The high thermal stable polymer owns the glass transition temperature higher than 190 °C and onset decomposition temperature higher than 300 °C. The polymers exhibit good solubility in dimethylacetamide (DMAc) and the tough, flexible and transparent films are obtained by solution casting method. These membranes exhibit suitable proton conductivity, low methanol permeability and excellent dimensional stability. The membrane with IECE = 1.81 mequiv g−1 shows considerable proton conductivity (84 mS cm−1) and swelling ration (only 8.6%) under fully hydrated state at 100 °C. Their excellent performance is attributed to distinct phase separation between hydrophilic and hydrophobic morphology, which is observed by TEM. This work demonstrates that the strategy of combining locally high densities hydrophilic segment with fluorine-containing hydrophobic segment in a polymer chain can balance on proton conduction and dimensional stability efficiently. Furthermore, these membranes own much lower methanol permeability and higher selectivity than Nafion.Graphical abstractHighlights► We synthesized novel tetra-sulfonated poly(arylene ether ketone)s as PEM materials. ► The membranes exhibited lower water uptake and swelling ratio than Nafion 117. ► Combination of considerable proton conductivity and low methanol permeability. ► The membranes are promising candidate PEM materials for direct methanol fuel cell.

A series of octa-sulfonated poly(arylene ether)s (PAEs) was prepared via a low-temperature grafting reaction and subsequent postsulfonation. The rigid backbone with high molecular weight is conducive to higher integrity of the hydrophobic domain. And the grafting ionic clusters are expected to promote the appearance of phase separation. These membranes, with ion exchange capacity (IEC) values ranging from 1.19 to 1.90 meq. g−1, exhibited excellent dimensional stability, mechanical properties and oxidative stability. The membrane with higher IEC (>1.4 meq. g−1) exhibited adequate conductivity (>100 mS cm−1) in water at 80 °C. Furthermore, the membrane with IEC = 1.90 meq. g−1 exhibited comparable conductivity to Nafion 117 under various humidity levels. In addition, SAXS profiles confirmed the well-defined phase-separated morphology of SPAE-x. DMFC single cell performance demonstrates that SPAE-x are good candidates for proton exchange membranes in fuel cell applications.

The preparation and characterization of new polymeric ionomers based on a fully aromatic poly(arylene ether) backbone with locally pentasulfonated pendent groups for applications as proton exchange membranes is reported. The high molecular weight sulfonated polymers were obtained by the polycondensation of new (2,6-difluorophenyl) (4-(1,2,3,4,5-pentaphenylbenzene)phenyl)methanone, 4,4′-difluorodiphenyl methanone, and 4,4′-dihydroxydiphenylsulfone, followed by sulfonation using sulfuric acid in high yields. The polymers produced tough, flexible, and transparent membranes by solvent casting. Membranes with ion exchange capacities between 0.8 and 1.7 mEq g-1 showed high proton conductivities and low methanol permeabilities. Compared to Nafion 117, these sulfonated membranes exhibited better microphase separation morphologies. The fully humidified membranes also exhibited considerably good mechanical properties, with tensile strengths from 35 to 45 MPa and elongations at break from 23 to 49%. This investigation demonstrates a controllable high density sulfonated side group of a poly(arylene ether sulfone) membrane with tunable and balanced properties, which is promising for proton exchange membrane fuel cell technology.