Xiaoan Li

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.

Electrochemical Energy Production Using Fuel Cell Technologies

Fuel cells are highly efficient and environmentally friendly energy conversion devices that are receiving increasing attention and are steadily moving toward commercialization. Fuel cells deliver electricity and heat, based on the spontaneous electrochemical oxidation of fuels at the anode and the reduction of oxygen at the cathode, without combustion. In many ways, fuel cells are similar to batteries, although they do not require recharging and operate as long as fuel continues to be provided. There are four leading types of fuels reviewed in this chapter, proton exchange membrane fuel cells (PEMFCs) operating on clean hydrogen, direct alcohol (primarily methanol) fuel cells (DAFCs), solid oxide fuel cells (SOFCs), and molten carbonate fuel cells (MCFCs). PEMFCs and DAFCs normally operate at below 100 °C and are targeted primarily for transportation and mobile applications, while SOFCs and MCFCs, which run at temperatures above 600 °C, can run on a wide variety of fuels and are intended mostly for stationary combined heat and power applications. This review is focused primarily on a description of each of these technologies, with an emphasis on the materials used in the electrodes, the electrolyte that separates them, and the current collectors.