Anion-exchange membranes (AEM) containing saturated-heterocyclic benzyl-quaternary ammonium (QA) groups synthesised by radiation-grafting onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films are reported. The relative properties of these AEMs are compared with the benchmark radiation-grafted ETFE-g-poly(vinylbenzyltrimethylammonium) AEM. Two AEMs containing heterocyclic-QA head groups were down-selected with higher relative stabilities in aqueous KOH (1 mol dm-3) at 80°C (compared to the benchmark): these 100 μm thick (fully hydrated) ETFE-g-poly(vinylbenzyl-Nmethylpiperidinium)- and ETFE-g-poly(vinylbenzyl-N-methylpyrrolidinium)-based AEMs had as-synthesised ion-exchange capacities (IEC) of 1.64 and 1.66 mmol g-1, respectively, which reduced to 1.36 mmol dm-3 (ca. 17 – 18% loss of IEC) after alkali ageing (the benchmark AEM showed 30% loss of IEC under the same conditions). These down-selected AEMs exhibited as-synthesised Cl- ion conductivities of 49 and 52 mS cm-1, respectively, at 90°C in a 95% relative humidity atmosphere, while the OH- forms exhibited conductivities of 138 and 159 mS cm-1, respectively, at 80°C in a 95% relative humidity atmosphere. The ETFE-g-poly(vinylbenzyl-N-methylpyrrolidinium)-based AEM produced the highest performances when tested as catalyst coated membranes in H2/O2 alkaline polymer electrolyte fuel cells at 60°C with PtRu/C anodes, Pt/C cathodes, and a polysulfone ionomer: the 100 μm thick variant (synthesised from 50 μm thick ETFE) yielded peak power densities of 800 and 630 mW cm-2 (with and without 0.1 MPa back pressurisation, respectively), while a 52 μm thick variant (synthesised from 25 μm thick ETFE) yielded 980 and 800 mW cm-2 under the same conditions. From these results, we make the recommendation that developers of AEMs, especially pendent benzyl-QA types, should consider the benzyl-Nmethylpyrrolidinium head-group as an improvement to the current de facto benchmark benzyltrimethylammonium headgroup
Alkaline polymer electrolyte fuel cells are a class of fuel cells that enable the use of non-precious metal catalysts, particularly for the oxygen reduction reaction at the cathode. While there have been alternative materials exhibiting Pt-comparable activity in alkaline solutions, to the best of our knowledge none have outperformed Pt in fuel-cell tests. Here we report a Mn-Co spinel cathode that can deliver greater power, at high current densities, than a Pt cathode. The power density of the cell employing the Mn-Co cathode reaches 1.1 W cm −2 at 2.5 A cm −2 at 60 o C. Moreover, this catalyst outperforms Pt at low humidity. In-depth characterization reveals that the remarkable performance originates from synergistic effects where the Mn sites bind O 2 and the Co sites activate H 2 O, so as to facilitate the proton-coupled electron transfer processes. Such an electrocatalytic synergy is pivotal to the high-rate oxygen reduction, particularly under water depletion/low humidity conditions.
Anion exchange membranes (AEMs) are a promising class of materials that enable non-noble metals to be used as catalysts in fuel cells. Compared to their acidic counterparts, typically Nafion and other perfluorosulfonate-based membranes, the low OH– conductivity in AEMs remains a concern as these materials are developed for practical applications. Cross-linked macromolecular structures are a popular way to optimize the trade-off between the ionic conductivity and the water swelling of AEMs with high ion exchange capacities (IECs). However, common cross-linked AEMs (e.g., x(QH)QPPO) that have high degrees of cross-linking with low molecular weight between cross-links are usually mechanically brittle. Moreover, the cross-links in AEMs can hinder the transport of OH–, leading to unsatisfactory conductivities. Here we report a series of elastic and highly conductive poly(2,6-dimethylphenylene oxide) (PPO)-based AEMs (x(QH)3QPPO) containing flexible, long-chain, multication cross-links. The strength and flexibility of the x(QH)3QPPO samples are significantly improved as compared to the conventional x(QH)QPPO membranes and multication un-cross-linked materials reported previously. The high conductivities in these new materials (x(QH)3QPPO-40, IEC = 3.59 mmol/g, σOH– = 110.2 mS/cm at 80 °C) are attributed to the distinct microphase separation observed in the x(QH)3QPPO membranes by SAXS and TEM analyses. Furthermore, the x(QH)3QPPO samples exhibit good dimensional (swelling ratio of x(QH)3QPPO-40 is 25.0% at 80 °C) and chemical (22% and 25% decrease in IEC and OH– conductivity in 1 M NaOH at 80 °C for 30 days, respectively) stabilities, making this cross-linking motif suitable for potential membrane applications in fuel cells and other electrochemical devices.
Fe-containing N-doped carbons (Fe/N/C) are a promising Pt-alternative catalyst for the oxygen reduction reaction (ORR) and are believed to be more stable in alkaline media than in acids and thus particularly suitable to be applied as the cathode catalyst for alkaline polymer electrolyte fuel cells (APEFCs). However, there has hitherto been no successful report on high-performance APEFC based on the Fe/N/C cathode, the reason for which is still not quite clear. Here we report a highperformance Fe/N/C catalyst and its application in APEFC. The catalyst precursor is adenosine, an environmentally benign Nrich biomolecule, which is polymerized via a solvothermal process and then carbonized through pyrolysis. The resulting Fe/N/C nanotubes are thoroughly characterized by a variety of microscopy and spectroscopy (SEM, TEM, XRD, XPS, Raman, Mossbauer, and STEM-EELS), which reveal a high surface N/C ratio (8 at%) and atomic Fe sites well dispersed at the wall of the nanotubes. The catalytic sites are identified to be Fe−N 4 . The volume-specific catalytic activity of the Fe/N/C catalyst toward the ORR is as good as that of the commercial 20 wt % Pt/C catalyst in alkaline solutions, and better in durability. The electronic conductivity of Fe/N/C turns out to be trivial in rotating-disk electrode experiments but key for fuel cell tests. The APEFC with Fe/N/C cathode (2 mg/cm 2 in catalyst loading) exhibits a peak power density greater than 450 mW/cm 2 , the thus-far highest record in the literature for APEFC using a nonprecious metal cathode. Our findings not only deepen the understanding of the structure−activity relationship of the Fe/N/C catalyst but also mark a step toward its real application in APEFC.
In order to improve the alkaline stability of polyarylether-based anion exchange membranes (AEMs), the strategy of attaching a hydrophobic side-chain to the backbone is proposed. A series of AEMs which include three original quaternary ammonia AEMs (QAPSF, QAPPSU, and QAPPO) and three hydrophobic side-chain attached AEMs (sQAPSF, sQAPPSU, and sQAPPO) are synthesized. Although a gradual enhancement of chemical stabilities for QAPSF, QAPPSU, and QAPPO is observed by reducing backbone polar groups, degradation of the original QA-type AEMs is still obvious. After testing in 1 M N2-saturated NaOH solution at 60 °C for 30 days, the weight losses of the original QA-type AEMs exceed 29%; the IEC losses of the AEMs exceed 24%, and the IC losses of the AEMs exceed 25% (IEC, ion exchange capacity). While for s-type AEMs, due to the hydrophobic side-chains endowing the membranes with hydrophilic/hydrophobic microphase separation morphologies, the backbones are protected from OH– attacking; significant improvements of the AEMs’ chemical stabilities are achieved. After the 30 day test, the side-chain attached AEMs exhibit much higher stabilities than those of original AEMs. The weight loss percentages for sQAPSF (IEC = 1.02 mmol g–1), sQAPPSU (IEC = 1.05 mmol g–1), and sQAPPO (IEC = 2.52 mmol g–1) are 8%, 8%, and 10%, respectively. The IECs of sQAPSF, sQAPPSU, and sQAPPO are reduced by 12%, 10%, and 15%, respectively. The ionic conductivities of the membranes are reduced by 15%, 12%, and 8%, respectively. Besides, compared to the original AEMs, the side-chain attached AEMs show enhanced ionic conductivity and suppressed swelling behavior. The results explicitly show that the approach of introducing a hydrophobic side-chain is an effective way to elevate the chemical stabilities of polyarylether-based AEMs.
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