Introducing planar hydrophobic groups into an alkyl-sulfonated rigid polyimide and how this affects morphology and proton conductivity

A High-Performance Polyimide Composite Membrane with Flexible Polyphosphazene Derivatives for Vanadium Redox Flow Battery

A sulfonated polyimide, S-F-abSPI, with alkyl sulfonic acid side chains, and a polyphosphonitrile derivative, poly[4-methoxyphenoxy (4-fluorophenoxy) phosphazene] (PFMPP), were designed and synthesized. Composite modification of the S-F-abSPI membrane was carried out using PFMPP, resulting in the preparation of composite membranes with different composite ratios, which were then subjected to performance testing and characterization. Experimental results revealed a significant enhancement in the proton conductivity of the S-F-abSPI membrane, reaching 0.116 S cm-1, slightly higher than that of the N212 membrane. The S-F-abSPI/1% PFMPP composite membrane exhibited the optimal comprehensive performance, with a surface resistance as low as 0.54 Ω cm2, comparable to that of the N212 membrane. At a high current density of 200 mA cm-2 during charge-discharge, the composite membrane achieved a voltage efficiency (VE) of 83.12% and an energy efficiency (EE) of 81.95%. Cycling tests over 200 cycles demonstrated the composite membrane's excellent long-term cycling stability. The alkyl sulfonic acid side chains enhanced the proton conductivity of the membrane, while electrostatic potential distribution calculations indicated strong interactions between PFMPP and the base membrane, enhancing the membrane's mechanical strength, reducing vanadium ion permeability, and improving chemical stability and vanadium ion selectivity. This composite membrane holds promise for high-performance VRFB applications.

Unravel-engineer-design: a three-pronged approach to advance ionomer performance at interfaces in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs), which use hydrogen as fuel, present an eco-friendly alternative to internal combustion engines (ICEs) for powering low-to-heavy-duty vehicles and various devices. Despite their promise, PEMFCs must meet strict cost, performance, and durability standards to reach their full potential. A key challenge lies in optimizing the electrode, where a thin ionomer layer is responsible for proton conduction and binding catalyst particles to the electrode. Enhancing ion transport within these sub-μm thick films is critical to improving the oxygen reduction reaction (ORR) at the cathodes of PEMFCs. For the past 15 years, our research has targeted this limitation through a comprehensive "Unravel - Engineer - Design" approach. We first unraveled the behavior of ionomers, gaining deeper insights into both the average and distributed proton conduction properties within sub-μm thick films and at interfaces that mimic catalyst binder layers. Next, we engineered ionomer-substrate interfaces to gain control over interfacial makeup and boost proton conductivity, essential for PEMFC efficiency. Finally, we designed novel nature-derived or nature-inspired, fluorine-free ionomers to tackle the ion transport limitations seen in state-of-the-art ionomers under thin-film confinement. Some of these ionomers even pave the way to address cost and sustainability challenges in PEMFC materials. This feature article highlights our contributions and their importance in advancing PEMFCs and other sustainable energy conversion and storage technologies.

Lyotropic Liquid Crystalline Property and Organized Structure in High Proton-Conductive Sulfonated Semialicyclic Oligoimide Thin Films

Fully aromatic sulfonated polyimides with a rigid backbone can form lamellar structures under humidified conditions, thereby facilitating the transmission of protons in ionomers. Herein, we synthesized a new sulfonated semialicyclic oligoimide composed of 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA) and 3,3'-bis-(sulfopropoxy)-4,4'-diaminobiphenyl to investigate the influence of molecular organized structure and proton conductivity with lower molecular weight. The weight-average molecular weight (M _w) determined by gel permeation chromatography was 9300. Humidity-controlled grazing incidence X-ray scattering revealed that one scattering was observed in the out-of-plane direction and showed that the scattering position shifted to a lower angle as the humidity increased. A loosely packed lamellar structure was formed by lyotropic liquid crystalline properties. Although the ch-pack aggregation of the present oligomer was reduced by substitution to the semialicyclic CPDA from the aromatic backbone, the formation of a distinct organized structure in the oligomeric form was observed because of the linear conformational backbone. This report is the first-time observation of the lamellar structure in such a low-molecular-weight oligoimide thin film. The thin film exhibited a high conductivity of 0.2 (±0.01) S cm^-1 under 298 K and 95% relative humidity, which is the highest value compared to the other reported sulfonated polyimide thin films with comparable molecular weight.

Synthesis and characterization of a novel sulfonated poly (aryl ether ketone sulfone) semi-crosslinked membrane with high proton selectivity through click reaction for direct methanol fuel cells

A novel sulfonated polyvinyl alcohol containing alkynyl groups (SPVA-C≡C) and a new sulfonated poly (aryl ether ketone sulfone) (SPAEKS) are synthesized. Semi-crosslinked membrane (semi-crosslink-SPAEKS-x) was prepared by click reaction of mercapto-alkynes between 1, 5-pentanedithiol and SPVA-C≡C. The chemical structures of SPAEKS, SPVA-C≡C and semi-crosslink-SPAEKS-x are confirmed by 1H-NMR and FTIR spectra. The semi-crosslink-SPAEKS-x membranes show good mechanical properties, excellent dimensional stability and oxidative stability. The proton conductivity of SPAEKS and semi-crosslink-SPAEKS-x membranes is in the range of 25.6–52.5 mS/cm. The methanol permeability of semi-crosslink-SPAEKS-x membranes is in the range of 1.4–1.7 × 10−7 cm−2 s−1, which is much lower than that of Nafion 117 membrane (18.3 × 10−7 cm−2 s−1). Especially, the proton selectivity of semi-crosslink-SPAEKS-15 membrane (24.3 × 104 S s cm−3) is above seven times higher than that of Nafion 117 membrane (3.4 × 104 S s cm−3).

The preparation of novel sulfonated poly(aryl ether ketone sulfone)/TiO2 composite membranes with low methanol permeability for direct methanol fuel cells

A sulfonated poly(aryl ether ketone sulfone) (SPAEKS) with locally dense sulfonic acid groups is synthesized and different amounts of TiO2 is doped into SPAEKS matrix to prepare composite membranes (SPAEKS/TiO2-x). SEM shows that TiO2 in the composite membranes has good dispersibility when TiO2 content is not higher than 3%. The composite membranes show good mechanical properties, dimensional stability and oxidative stability. The proton conductivity of composite membranes is near to that of Nafion 117 membrane and methanol permeability of composite membranes is much lower than that of Nafion 117 membrane. Therefore, the proton selectivity of composite membranes is higher than that of Nafion 117 membrane. In particular, proton selectivity of SPAEKS/TiO2-3% (12.8 × 104 S s cm−3) is four times higher than that of Nafion 117 membrane (3.2 × 104 S s cm−3).