Fangxia Feng

Effect of carbon support nanostructure on the oxygen reduction activity of Pt/C catalysts

This work is focussed on establishing the effect of the nanostructure of a series of ordered mesoporous carbon (OMC) support materials, after Pt loading, on the oxygen reduction reaction (ORR) performance, for application in proton exchange membrane fuel cells (PEMFCs). Hexagonal mesoporous silica (HMS) templates were prepared using alkylamine surfactants with varying carbon chain lengths, producing wormhole pore diameters of 1.5–3.1 nm and silica wall thicknesses of ca. 2.3 nm. The HMS pores were then filled either with sucrose or an aromatic carbon precursor (anthracene or naphthalene), followed by carbonization and removal of the HMS in NaOH, leaving behind an interconnected carbon structure (“nano-strings”), 1.5–3.1 nm in diameter. These OMCs all have a similar, bimodal, pore size distribution, with the smaller pores (1.8 nm) attributed to removal of the HMS walls and the larger pores (∼3.5 nm) arising from incomplete filling of the HMS pores with carbon precursor. The OMCs were loaded with 20 wt% Pt, resulting in very similar Pt particle sizes (ca. 5 nm), as confirmed by XRD, TEM, and electrochemical surface area measurements. The ORR activity was found to decrease as the carbon nano-string diameter decreased, proposed to be due to a higher electronic resistance, while the degree of OMC graphitization, determined by XRD analysis, had only a minor impact on the ORR activity.

First time investigation of Pt nanocatalysts deposited inside carbon mesopores of controlled length and diameter

This work represents the first-reported synthesis and evaluation of Pt-loaded colloid-imprinted carbon (CIC) supports consisting of a porous shell surrounding a solid core, for use as catalysts for cathodes in proton exchange membrane fuel cells (PEMFCs). Increasing the temperature of imprinting during the synthesis of CIC supports, from 250 to 400 °C, gave a five times increase in porous shell thickness (average pore depth), as confirmed by gas sorption and transmission electron microscopy studies. The CIC supports, all with a 26 nm pore diameter, were loaded with 20 wt% Pt and characterized with 3D transmission electron microscopy and electron tomography, showing that the Pt nanoparticles are uniformly deposited throughout the CIC pores. Using 3-electrode electrochemistry in 0.5 M H2SO4, it was found that the rate (per active Pt surface area) of the oxygen reduction reaction is independent of the pore length, with no transport limitations encountered. This demonstrates that full utilization of both the Pt and the CIC internal surface area was achieved under these experimental conditions, which promises benefits in terms of enhanced Pt utilization, thus lowering the cost and improving the durability of PEMFCs. Furthermore, this work has opened up an entirely new direction for fuel cell catalyst layer design by allowing the controlled modification of both carbon support pore diameter and pore length, also of relevance for battery and capacitor applications.

Wettability of Nafion and Nafion/Vulcan Carbon Composite Films

The wettability of the Pt/carbon/Nafion catalyst layer in proton exchange membrane fuel cells is critical to their performance and durability, especially the cathode, as water is needed for the transport of protons to the active sites and is also involved in deleterious Pt nanoparticle dissolution and carbon corrosion. Therefore, the focus of this work has been on the first-time use of the water droplet impacting method to determine the wettability of 100% Nafion films, as a benchmark, and then of Vulcan carbon (VC)/Nafion composite films, both deposited by spin-coating in the Pt-free state. Pure Nafion films, shown by SEM analysis to have a nanochanneled structure, are initially hydrophobic but become hydrophilic as the water droplet spreads, likely due to reorientation of the sulfonic acid groups toward water. The wettability of VC/Nafion composite films depends significantly on the VC/Nafion mass ratios, even though Nafion is believed to be preferentially oriented (sulfonate groups toward VC) in all cases. At low VC contents, a significant water droplet contact angle hysteresis is seen, similar to pure Nafion films, while at higher VC contents (>30%), the films become hydrophobic, also exhibiting superhydrophobicity, with surface roughness playing a significant role. At >80% VC, the surfaces become wettable again as there is insufficient Nafion loading present to fully cover the carbon surface, allowing the calculation of the Nafion:carbon ratio required for a full coverage of carbon by Nafion.

Surface Characteristics of Microporous and Mesoporous Carbons Functionalized with Pentafluorophenyl Groups

The in situ diazonium reduction reaction is a reliable and well-known approach for the surface modification of carbon materials for use in a range of applications, including in energy conversion, as chromatography supports, in sensors, etc. Here, this approach was used for the first time with mesoporous colloid-imprinted carbons (CICs), materials that contain ordered monodisperse pores (10-100 nm in diameter) and are inherently highly hydrophilic, using a common microporous carbon (Vulcan carbon (VC)), which is relatively more hydrophobic, for a comparison. The ultimate goal of this work was to modify the CIC wettability without altering its nanostructure and also to lower its susceptibility to oxidation, as required in fuel cell and battery electrodes, by the attachment of pentafluorophenyl (-PhF5) groups onto their surfaces. This was shown to be successful for the CIC, with the -PhF5 groups uniformly coating the inner pore walls at a surface coverage of ca. 90% and allowing full solution access to the mesopores, while the -PhF5 groups deposited only on the outer VC surface, likely blocking its micropores. Contact angle kinetics measurements showed enhanced hydrophobicity, as anticipated, for both the -PhF5 modified CIC and VC materials, even revealing superhydrophobicity at times for the CIC materials. In contrast, water vapor sorption and cyclic voltammetry suggested that the micropores remained hydrophilic, arising from the deposition of smaller N- and O-containing surface groups, caused by a side reaction during the in situ diazonium functionalization process.