High surface area N-doped mesoporous carbon capsules with iron traces exhibit outstanding electrocatalytic activity for the oxygen reduction reaction (ORR) in both alkaline and acidic media. In alkaline conditions, they exhibit a more positive onset (0.94 V vs. RHE) and half-wave potentials (0.83 V vs. RHE) than commercial Pt/C, while in acidic media the onset potential is comparable to that of commercial Pt/C with a peroxide yield lower than 10 %. The Fe-N-doped carbon catalyst combines the high catalytic activity with remarkable performance stability (3500 cycles between 0.6 and 1.0 V vs. RHE), which stems from the fact that iron is coordinated to nitrogen. Additionally, the newly developed electrocatalyst is unaffected by the methanol cross-over effect in both acid and basic media, contrary to commercial Pt/C. The excellent catalytic behavior of the Fe-N-doped carbon, even in the more relevant acid medium, is attributable to the combination of chemical functions (N-pyridinic, N-quaternary and Fe-N coordination sites) and structural properties (large surface area, open mesoporous structure and short diffusion paths), which guarantees a large number of highly active and fully accessible catalytic sites and rapid masstransfer kinetics. Thereby, this catalyst represents an important step forward towards replacing Pt catalysts with cheaper alternatives. In this regard, an alkaline anion exchange membrane fuel cell was assembled with the Fe-Ndoped mesoporous carbon capsules as the cathode catalyst providing current and power densities matching those of a commercial Pt/C, which glimpses the practical applicability of the Fe-N-carbon catalyst.
A hybrid membrane of superacid sulfated Zr-MOF (SZM) and Nafion shows much superior performance to Nafion, particularly for fuel cell operating under low humidity. The Brønsted acidic sites in SZM networks retain an ample amount of water which facilitated proton conduction under low humidity. The water retention properties of Nafion-SZM hybrid membranes with 1 wt % loading of SZM increased at 35% relative humidity and outperformed commercial unfilled Nafion membrane. The proton conductivity increases by 23% for Nafion-SZM hybrid compared to unfilled Nafion membrane. The Nafion-SZM membrane also shows higher performance stability at 35% relative humidity than Nafion, as confirmed by close monitoring of the change of open circuit voltage for 24 h.
The hot pressing process for fabricating membrane electrode assemblies (MEAs) has been widely adopted, yet little is known of its effects on the microstructural properties of the different components of the MEA. In particular, the interaction of the electrolyte, electrode and gas diffusion layer (GDL) due to lamination is difficult to probe as conventional imaging techniques cannot access the internal structure of the MEA. Here, a novel approach is used, which combines characterisation of hot-pressed membrane electrode assemblies using X-ray computed tomography, thermogravimetric analysis, differential scanning calorimetry and atomic force microscopy, with electrochemical performance measurements from polarisation curves and high-frequency impedance spectroscopy. Membrane electrode assemblies hot pressed at 100 o C, 130 o C and 170 o C reveal significant differences in microstructure, which has a consequence for the performance. When hot pressed at 100 o C, which is lower than the glass transition temperature of Nafion (123 o C), the catalyst only partially bonds with the Nafion membrane, leading to increased Ohmic resistance. At 170 o C, the Nafion membrane intrudes into the electrode, forming pinholes, degrading the catalyst layer and filling pores in the GDL. Finally, at 130 o C, the interfacial contact is optimum, with similar roughness factor between the catalyst and Nafion membrane surface, indicating effective lamination of layers.
Graphitic
carbon nitrides are investigated for developing highly
durable Pt electrocatalyst supports for polymer electrolyte fuel cells
(PEFCs). Three different graphitic carbon nitride materials were synthesized
with the aim to address the effect of crystallinity, porosity, and
composition on the catalyst support properties: polymeric carbon nitride
(gCNM), poly(triazine) imide carbon nitride (PTI/Li+Cl–), and boron-doped graphitic carbon nitride (B-gCNM).
Following accelerated corrosion testing, all graphitic carbon nitride
materials are found to be more electrochemically stable compared to
conventional carbon black (Vulcan XC-72R) with B-gCNM support showing
the best stability. For the supported catalysts, Pt/PTI-Li+Cl– catalyst exhibits better durability with only
19% electrochemical surface area (ECSA) loss versus 36% for Pt/Vulcan
after 2000 scans. Superior methanol oxidation activity is observed
for all graphitic carbon nitride supported Pt catalysts on the basis
of the catalyst ECSA.
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