Abstract
Platinum electrodes are commonly used in devices such as fuel cells, neurostimulators, and sensors. Device performance can be improved by increasing the electrochemically active surface area of the platinum, which increases the charge storage capacitance, oxygen reduction reaction (ORR) rates, and catalytic activity. Therefore, there is significant motivation to fabricate platinum 3D structures, but it is challenging to accomplish successfully. In this study, graphenated carbon nanotubes (gCNTs), a type of carbon nanotube with leaf-like graphene foliates, were decorated with platinum nanoparticles to fabricate a 3D structure with increased Pt surface area. The carbon nanotubes act as a conductive scaffold and the foliates provide increased surface area and highly reactive edge sites.
The development of gCNTs was accomplished by systematically adjusting growth parameters in a microwave plasma enhanced chemical vapor deposition system. The primary growth parameters were growth temperature, growth time, process gas composition (methane flow rate vs ammonia flow rate), and microwave power. gCNTs were characterized to obtain the best relevant electrochemical properties, i.e., impedance, voltage window, stability, and charge storage capacity. Compared to platinum, typical gCNT electrodes had lower impedance at 100 Hz at 230 Ω vs 375 Ω for Pt. The electrochemical properties of the gCNTs were compared to scanning electron microscope images (Fig. 1) and it was found that a medium foliate density (3.0 CH4: 1 NH3) had the best electrochemical properties.
Next, the gCNTs electrodes can be further enhanced by decoration with nanoparticles of platinum via atomic layer deposition (ALD). Atomic layer deposition alternates two reagents so that the first adsorbs on the surface in a monolayer and the second reacts with the monolayer. The most reactive sides tend to nucleate first, so that at low deposition density, particles preferentially deposit at the edges of the graphene foliates. A major advantage of ALD is that it produces much more monodispersed nanoparticles, which yields more uniform properties. The combined electrodes will have the larger electrochemical surface area of gCNTs and the catalytic and charge storage properties of platinum electrodes. Electrochemical properties of Pt-gCNTs will be compared to gCNTs and planar Pt electrodes.
Figure 1