Anion exchange polyelectrolytes for membranes and ionomers

Blended Anion Exchange Membranes for Vanadium Redox Flow Batteries

In this study, blended anion exchange membranes were prepared using polyphenylene oxide containing quaternary ammonium groups and polyvinylidene fluoride. A polyvinylidene fluoride with high hydrophobicity was blended in to lower the vanadium ion permeability, which increased when the hydrophilicity increased. At the same time, the dimensional stability also improved due to the excellent physical properties of polyvinylidene fluoride. Subsequently, permeation of the vanadium ions was prevented due to the positive charge of the anion exchange membrane, and thus the permeability was relatively lower than that of a commercial proton exchange membrane. Due to the above properties, the self-discharge of the blended anion exchange membrane (30.1 h for QA–PPO/PVDF(2/8)) was also lower than that of the commercial proton exchange membrane (27.9 h for Nafion), and it was confirmed that it was an applicable candidate for vanadium redox flow batteries.

Insight into the Alkaline Stability of N-Heterocyclic Ammonium Groups for Anion-Exchange Polyelectrolytes

The alkaline stability of N-heterocyclic ammonium (NHA) groups is a critical topic in anion-exchange membranes (AEMs) and AEM fuel cells (AEMFCs). Here, we report a systematic study on the alkaline stability of 24 representative NHA groups at different hydration numbers (λ) at 80 °C. The results elucidate that γ-substituted NHAs containing electron-donating groups display superior alkaline stability, while electron-withdrawing substituents are detrimental to durable NHAs. Density-functional-theory calculations and experimental results suggest that nucleophilic substitution is the dominant degradation pathway in NHAs, while Hofmann elimination is the primary degradation pathway for NHA-based AEMs. Different degradation pathways determine the alkaline stability of NHAs or NHA-based AEMs. AEMFC durability (from 1 A cm^-2 to 3 A cm^-2 ) suggests that NHA-based AEMs are mainly subjected to Hofmann elimination under 1 A cm^-2 current density for 1000 h, providing insights into the relationship between current density, λ value, and durability of NHA-based AEMs.

Side-Chain-Type Anion Exchange Membranes Based on Poly(arylene ether sulfone)s Containing High-Density Quaternary Ammonium Groups

To obtain anion exchange membranes with both high ionic conductivity and good dimensional stability, a series of side-chain-type poly(arylene ether sulfone)s (PAES-QDTPM-x) were designed and synthesized. Quaternary ammonium (QA) groups were densely aggregated and grafted onto the main chain via flexible hydrophobic spacers. Well-defined microphase separation was confirmed by small-angle X-ray scattering. PAES-QDTPM-0.30 exhibited reasonably high conductivity (39.4 mS cm^-1 at 20 °C and 76.1 mS cm^-1 at 80 °C) and excellent dimensional stability at 80 °C (11.9% in length, 11.2% in thickness) due to the concentration of ion clusters and the side-chain-type structure. All membranes maintained over 82% of the conductivity after alkali treatment for 14 days. In the H₂/O₂ fuel cell performance test, the maximum power density of PAES-QDTPM-0.30 at 60 °C was 225.8 mW cm^-2.

Poly(fluorenyl aryl piperidinium) membranes and ionomers for anion exchange membrane fuel cells

Low-cost anion exchange membrane fuel cells have been investigated as a promising alternative to proton exchange membrane fuel cells for the last decade. The major barriers to the viability of anion exchange membrane fuel cells are their unsatisfactory key components-anion exchange ionomers and membranes. Here, we present a series of durable poly(fluorenyl aryl piperidinium) ionomers and membranes where the membranes possess high OH- conductivity of 208 mS cm-1 at 80 °C, low H2 permeability, excellent mechanical properties (84.5 MPa TS), and 2000 h ex-situ durability in 1 M NaOH at 80 °C, while the ionomers have high water vapor permeability and low phenyl adsorption. Based on our rational design of poly(fluorenyl aryl piperidinium) membranes and ionomers, we demonstrate alkaline fuel cell performances of 2.34 W cm-2 in H2-O2 and 1.25 W cm-2 in H2-air (CO2-free) at 80 °C. The present cells can be operated stably under a 0.2 A cm-2 current density for ~200 h.

Alcohol-Treated Porous PTFE Substrate for the Penetration of PTFE-Incompatible Hydrocarbon-Based Ionomer Solutions

For a mechanically tough proton exchange membrane, a composite membrane incorporated with a porous polymer substrate is of great interest to suppress the ionomer swelling and to improve the dimensional stability and mechanical strength of the ionomers. For the composite membranes, good impregnation of substrate-incompatible ionomer solution into the substrate pores still remains one of the challenges to be solved. Here, we demonstrated a facile process (surface treatment with solvents compatible with both substrate and the ionomer solution) for the fabrication of the composite membranes using polytetrafluoroethylene (PTFE) as a porous substrate and poly(arylene ether sulfone) (SPAES) as a hydrocarbon-based (HC) ionomer. Appropriate solvents for the surface treatment were sought through the contact angle measurement, and it was found that alcohol solvents effectively tuned the surface property of PTFE pores to facilitate the penetration of the SPAES/N-methyl-2-pyrrolidone (NMP) solution into ∼300 nm pores of the substrate. Using this simple alcohol treatment, the SPAES/NMP contact angle was reduced in half, and we could fabricate the mechanically tough PTFE/HC composite membranes, which were apparently translucent and microscopically almost void-free composite membranes.

Poly(Alkyl-Terphenyl Piperidinium) Ionomers and Membranes with an Outstanding Alkaline-Membrane Fuel-Cell Performance of 2.58 W cm-2

Aryl-ether-free anion-exchange ionomers (AEIs) and membranes (AEMs) have become an important benchmark to address the insufficient durability and power-density issues associated with AEM fuel cells (AEMFCs). Here, we present aliphatic chain-containing poly(diphenyl-terphenyl piperidinium) (PDTP) copolymers to reduce the phenyl content and adsorption of AEIs and to increase the mechanical properties of AEMs. Specifically, PDTP AEMs possess excellent mechanical properties (storage modulus>1800 MPa, tensile strength>70 MPa), H2 fuel-barrier properties (<10 Barrer), good ion conductivity, and ex-situ stability. Meanwhile, PDTP AEIs with low phenyl content and high-water permeability display excellent peak power densities (PPDs). The present AEMFCs reach outstanding PPDs of 2.58 W cm-2 (>7.6 A cm-2 current density) and 1.38 W cm-2 at 80 °C in H2 /O2 and H2 /air, respectively, along with a specific power (PPD/catalyst loading) over 8 W mg-1 , which is the highest record for Pt-based AEMFCs so far.