Enhancing Proton Conductivity of Sulfonated Poly(ether ether ketone)-Based Membranes by Incorporating Phosphotungstic-Acid-Coupled Graphene Oxide

Short-Side-Chain Membranes with Stabilized Superacid on Graphitic Carbon Nitride for Polymer Electrolyte Fuel Cells under Low-Humidity Conditions

The integration of superacid-like heteropolyacids offers a promising route to develop highly proton-conducting membranes for energy storage and conversion. However, the inherent hydrophilicity of these acids can cause leaching, which undermines the fuel cell performance. In our research, we engineered a proton-conductive membrane with a facile hydrothermal synthesis approach to form stabilized hybrid superacids, namely, phosphotungstic acid (PTA), with graphitic carbon nitride (PCN) and its incorporation in a short-side-chain ionomer, namely, Aquivion. This unique approach via PCN nanohybrids enhances the proton transport within the membrane. These nanohybrids effectively combined the strong acidity of PTA with continuous 2D proton-transport pathways via a short side chain, resulting in a notable proton conductivity of 0.228 S cm-1 at 80 °C under 95% relative humidity. The real impact was evident in the performance of fuel cells using the Aquivion/PCN nanocomposite membrane, which demonstrated a significant improvement of 34% in the peak power density (1.0 W cm-2), and 44% cell performance (0.98 A cm-2) was retained for the nanocomposite membrane at a low relatively humidity (30% RH) at 0.6 V. This advancement represents a major leap in energy conversion and storage technologies at low-humidity conditions.

Utilization of Water-Insoluble Carbon Nitride-Phosphotungstic Acid Hybrids in Composite Proton Exchange Membranes

Phosphotungstic acid (HPW) can retain water in proton exchange membranes to increase proton conductivity; however, its water-soluble nature limits further application. In this work, we combined HPW and graphitic carbon nitride (g-C3N4) via sintering to prepare water-insoluble hybrids (HWN), where HPW was chemically linked to g-C3N4 to fix HPW. Then, HWN fillers were added to a sulfonated polyether ether ketone (SPEEK) matrix to prepare composite membranes. The conductivity of the composite membrane with 10 wt% HWN is up to 0.066 S cm-1 at room temperature, which is 53% higher than that of the SPEEK control membrane (0.043 S cm-1). The composite membrane also showed stable proton conductivity after being immersed in water for 2000 h. Therefore, our study demonstrates that preparing water-insoluble nanofillers containing HPW components through sintering is a promising approach.

Covalent organic frameworks with flexible side chains in hybrid PEMs enable highly efficient proton conductivity

Electrochemical hydrogen compression (EHC) is an emerging energy conversion technology. Proton exchange membranes (PEMs) with high proton conductivity and high mechanical strength are highly required to meet the practical requirements of EHC. Herein, ionic covalent organic frameworks (iCOFs) with tunable side chains were synthesized and introduced into the sulfonated poly (ether ether ketone) (SPEEK) matrix to fabricate hybrid PEMs. In our membranes, the rigid iCOFs afford ordered proton conduction channels, whereas the flexible side chains on iCOFs afford abundant proton conduction sites, adaptive hydrogen bonding networks, and high local density short hydrogen bonds for highly efficient proton transport. Moreover, the hydrogen bond interactions between the side chains on iCOFs and the SPEEK matrix enhance the mechanical stability of membranes. As a result, the hybrid PEM acquires an enhanced proton conductivity of 540.4 mS cm-1 (80 °C, 100%RH), a high mechanical strength of 120.41 MPa, and a superior performance (2.3 MPa at 30 °C, 100%RH) in EHC applications.

A Maximization of the Proton Conductivity of Sulfonated Poly(Ether Ether Ketone) with Grafted Segments Containing Multiple, Flexible Propanesulfonic Acid Groups

To enhance the proton conductivity of sulfonated poly(ether ether ketone) (SPEEK), proton-conducting groups are required to be covalently connected to SPEEK and form proton-conducting channels. Herein, SPEEK fully grafted with segments containing multiple, flexible propanesulfonic acid groups (MS-SPEEK-102) is successfully prepared. Compared with SPEEK, MS-SPEEK-102 exhibits a higher proton conductivity of 8.3 × 10^-2  S cm^-1 at 80 °C with 98% relative humidity, and consequently a greater power density of 0.530 W cm^-2 at 60 °C. These can be ascribed to the increased number of sulfonic acid groups, and ample, uninterrupted proton-conducting channels constructed by the movement of the maximum content, flexible side-chain segments. This approach offers an idea for obtaining a proton exchange membrane with good proton conductivity based on SPEEK.

Enhancing Physicochemical Properties and Single Cell Performance of Sulfonated Poly(arylene ether) (SPAE) Membrane by Incorporation of Phosphotungstic Acid and Graphene Oxide: A Potential Electrolyte for Proton Exchange Membrane Fuel Cells

The development of potential and novel proton exchange membranes (PEMs) is imperative for the further commercialization of PEM fuel cells (PEMFCs). In this work, phosphotungstic acid (PWA) and graphene oxide (GO) were integrated into sulfonated poly(arylene ether) (SPAE) through a solution casting approach to create a potential composite membrane for PEMFC applications. Thermal stability of membranes was observed using thermogravimetric analysis (TGA), and the SPAE/GO/PWA membranes exhibited high thermal stability compared to pristine SPAE membranes, owing to the interaction between SPAEK, GO, and PWA. By using a scanning electron microscope (SEM) and atomic force microscope (AFM), we observed that GO and PWA were evenly distributed throughout the SPAE matrix. The SPAE/GO/PWA composite membrane comprising 0.7 wt% GO and 36 wt% PWA exhibited a maximum proton conductivity of 186.3 mS cm^-1 at 90 °C under 100% relative humidity (RH). As a result, SPAE/GO/PWA composite membrane exhibited 193.3 mW cm^-2 of the maximum power density at 70 °C under 100% RH in PEMFCs.

Anchoring Water Soluble Phosphotungstic Acid by Hybrid Fillers to Construct Three-Dimensional Proton Transport Networks

Phosphotungstic acid (HPW)-filled composite proton exchange membranes possess high proton conductivity under low relative humidity (RH). However, the leaching of HPW limits their wide application. Herein, we propose a novel approach for anchoring water soluble phosphotungstic acid (HPW) by polydopamine (PDA) coated graphene oxide and halloysite nanotubes (DGO and DHNTs) in order to construct hybrid three-dimensional proton transport networks in a sulfonated poly(ether ether ketone) (SPEEK) membrane. The introduction of PDA on the surfaces of the hybrid fillers could provide hydroxyl groups and secondary amine groups to anchor HPW, resulting in the uniform dispersion of HPW in the SPEEK matrix. The SPEEK/DGO/DHNTs/HPW (90/5/5/60) composite membrane exhibited higher water uptake and much better conductivity than the SPEEK membrane at low relative humidity. The best conductivity reached wass 0.062 S cm-1 for the composite membrane, which is quite stable during the water immersion test.