Recent development of polymer electrolyte membranes for fuel cells.

Mimicking proton conduction mechanism of Nafion to construct novel proton-conducting materials with low cost and high proton conductivity is of wide interest. Herein, we have designed and synthesized a cationic covalent organic framework with high thermal and chemical stability by combining a cationic monomer, ethidium bromide (EB) (3,8-diamino-5-ethyl-6-phenylphenanthridinium bromide), with 1,3,5-triformylphloroglucinol (TFP) in Schiff base reactions. This is the first time that the stable cationic crystalline frameworks allowed for the fabrication of a series of charged COFs (EB-COF:X, X = F, Cl, Br, I) through ion exchange processes. Exchange of the extra framework ions can finely modulate the COFs’ porosity and pore sizes at nanoscale. More importantly, by introducing PW12O403– into this porous cationic framework, we can greatly enhance the proton conductivity of ionic COF-based material. To the best of our knowledge, EB-COF:PW12 shows the best proton conductivity at room temperature among ever report...

Cationic Covalent Organic Frameworks: A Simple Platform of Anionic Exchange for Porosity Tuning and Proton Conduction

Crystalline porous materials are currently a hot research topic in the field of proton-conducting materials. Crystalline porous materials include metal–organic frameworks (MOFs), coordination polymers (CPs), polyoxometalates (POMs) and covalent organic frameworks (COFs). The designable structures and high surface areas of these materials provide great opportunities to orderly accommodate proton carriers and to systemically modify the concentration and mobility of proton carriers in available spaces. Based on the understanding of the relationship between the structure and proton conductivity, controllable synthesis of porous materials with high proton conductivity will gradually be achieved. This review summarizes the emerging studies of these materials and their unique proton conductivities.

Proton-conducting crystalline porous materials

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. Proton exchange membranes with short-pathway through-plane proton conductivity are attractive for proton exchange membrane fuel cells. Here the authors align proton conducting channels orthogonal to the plane of composite proton exchange membranes using a magnetic field for improved fuel cell performance.

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Magnetic field alignment of stable proton-conducting channels in an electrolyte membrane.

Because of their strong acidity and water affinity, sulfated zirconia nanoparticles were evaluated as inorganic additives in the formation of composite Nafion-based membranes. Two types of sulfated zirconia were obtained according to the preparation experimental conditions. Sulfated zirconia-doped Nafion membranes were prepared by a casting procedure. The properties of the composite membranes were compared with those of an unfilled Nafion membrane obtained by the same preparation method. The water uptake, measured at room temperature in a wide relative humidity range, was higher for the composite membranes, this confirming the hydrophilic nature of the selected additives. The membrane doped by zirconia particles having the highest sulfate group concentration showed the highest water diffusion coefficient in the whole range of temperature and relative humidity investigated because of the presence of SO42− providing extra acid sites for water diffusion. The proton diffusivity calculated from impedance spect...

Composite Nafion/Sulfated Zirconia Membranes: Effect of the Filler Surface Properties on Proton Transport Characteristics†

An inorganic filler prepared by impregnation of phosphotungstic heteropolyacid on zirconia (HPW/Zr) was developed to be inserted into a perfluorosulphonic polymer matrix for a polymer electrolyte fuel cell (PEFC) operating at a medium temperature (80–120 °C) and low relative humidity (RH). Two different phosphotungstic acid (PWA) loadings (30 and 45% w/w) were anchored on a nanopowdered ZrO2. Such compounds were characterised by different techniques: differential scanning calorimetry (DSC), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and porosity and surface area by Brunauer–Emmett–Teller (BET), to verify the introduction and anchorage of PWA on ZrO2. Two composite Nafion membranes were prepared and characterised in terms of chemical–physical characteristics and electrochemical tests. Thermogravimetric analysis (TGA) provided evidence that HPW/Zr had been incorporated into composite membranes and it was not eluted. A good proton conductivity of about 6 × 10–3 S cm–1 at 120 °C and 25% RH was recorded.



Accelerated in situ ageing tests highlighted a good electrochemical stability (more than 150 cycles at 90 °C with dry gases) of the composite membranes with a slow decay and a reasonable integrity of the analysed membrane-electrodes assembly (MEA). Finally, a post-mortem SEM–EDX analysis on MEAs confirmed the presence of HPW/Zr in the membrane after the in situ testing.

R. Barrera

Papers

Phosphotungstic Acid Supported on a Nanopowdered ZrO2 as a Filler in Nafion‐Based Membranes for Polymer Electrolyte Fuel Cells