Proton exchange membranes with short-pathway through-plane orientated proton conductivity are highly desirable for use in proton exchange membrane fuel cells. Magnetic field is utilized to create oriented structure in proton exchange membranes. Previously, this has only been carried out by proton nonconductive metal oxide-based fillers. Here, under a strong magnetic field, a proton-conducting paramagnetic complex based on ferrocyanide-coordinated polymer and phosphotungstic acid is used to prepare composite membranes with highly conductive through-plane-aligned proton channels. Gratifyingly, this strategy simultaneously overcomes the high water-solubility of phosphotungstic acid in composite membranes, thereby preventing its leaching and the subsequent loss of membrane conductivity. The ferrocyanide groups in the coordinated polymer, via redox cycle, can continuously consume free radicals, thus helping to improve the long-term in situ membrane durability. The composite membranes exhibit outstanding proton conductivity, fuel cell performance and durability, compared with other types of hydrocarbon membranes and industry standard Nafion^® 212.

Achieving high power output from proton exchange membrane fuel cells (PEMFCs) requires efficient proton transport in proton exchange membranes (PEMs). Since proton conductivity is closely related to membrane moisture content, operation at low relative humidity (RH) and elevated temperature has become a critical bottleneck for the practical application of PEMFCs due to severe PEM dehydration. While several strategies have sought to mitigate this, including external thermal and water management, coating of nano-cracked hydrophobic layers and optimization of membrane intrinsic water retention, only partial improvements have been realized. Here, using a membrane formulation of ferrocyano-coordinated poly(4-vinylpyridine) (CP4VP), phosphotungstic acid (PWA) and polysulfone (PSf), novel highly water-retentive PEMs are fabricated via a strong magnetic field. During magnetic-assisted membrane casting, CP4VP and PWA form a microporous Prussian blue analogue (PBA) framework with the new type of Fe–CN–W bonding, which is paramagnetic and is thus simultaneously aligned in the through-plane (TP) direction of the membrane. The neutral PSf membrane component affords mechanical strength to the embedded TP-aligned conducting channels. This new type of microporous PBA framework is highly hydrophilic and proton conductive, with micropores of ∼5.4 Å diameter, which act as nano-sponges to absorb only more retentive non-freezable water, effective for proton conduction. These nano-sponges display efficient water absorption and retention at low RH and elevated temperatures, together with a much faster hydration process than the dehydration process. Furthermore, the TP-aligned PBA channels also enable faster water transport to promote PEM proton conduction beyond any previously reported water-retentive membrane. Consequently, the novel nano-sponge-like PEMs exhibit remarkable performance in both ex situ and in situ evaluations, especially at low RH and elevated temperature, largely prevailing over the commercial benchmark Nafion® 212.

Sulfonated graphene oxide (SGO) is already utilized as an effective filler in proton exchange membranes (PEMs) for performance enhancement in direct methanol fuel cells (DMFCs). Previously, SGO has been mostly prepared via functionalization based on the intrinsic groups of graphene oxide (GO), which may cause inhomogeneity/insufficient sulfonation and possibly limit the improvement of membrane performance. To overcome this widely overlooked problem, herein, a new type of GO sulfonation product, SDGO, is fabricated by carrying out the sulfonation on polydopamine (PDA) deposited GO, which results in improved uniformity/sufficient sulfonation on SDGO compared to PDA-free SGO. Therefore, the sulfonated poly(ether ether ketone) (SPEEK) composite PEMs filled with SDGO show better matrix–filler compatibility, larger hydrophilic domains and less voids than those using PDA-free SGO, which lead to much improved proton conductivity, methanol barrier and mechanical properties. SPEEK–SDGO based DMFCs largely outperform Nafion 212 based DMFCs, achieving an output of 177.93 mW cm⁻² (80 °C, 1 M methanol/O₂), one of the most competitive DMFC power densities reported for DMFCs based on SGO composite membranes. Moreover, the radical scavenging ability of PDA boosts the stability of SPEEK–SDGO composite membranes in chemical accelerated stress tests. Mechanical accelerated stress tests and constant voltage experiments further confirm the superior durability of SPEEK–SDGO composite membranes during DMFC operations.