Preparation of graphitic carbon foam current collectors (MTU: T. Rogers; Clemson University: O. Mefford)
To date, work at MTU has focused on making cathodes by depositing nickel oxyhydroxide active mass into the pores of carbon foams marketed by Poco Graphite, Inc. (Decatur, Texas). We now desire to make a carbon foam current collector in-house and tailor its properties. Using equipment and expertise in the School of Material Science and Engineering at Clemson University, we are learning how to manufacture foams from a suitable carbon source and control the foams' electrical conductivity, strength, and pore size distribution.
Some of the more promising foams produced at Clemson will be sent to MTU and made into cathodes. The Clemson foams will be tested at MTU to determine the extent to which nickel oxyhydroxide active mass can be electrochemically deposited in the pores. Repeated charge-discharge cycles, using a flooded laboratory cell connected to a potentiostat, will also assess the electrodes' performance (e.g., discharge capacities and the charge cycles available prior to 20% permanent capacity loss) versus those made from Poco Graphite foams.
We want to do a side-by-side comparison, using SEM and XPS, density, and optical microscopy to see what surface and elemental differences there are between the bare and impregnated foams. The observed differences will be correlated to the fabrication and deposition conditions.
In the next quarter, we will focus on making a graphitized carbon foam with morphology and properties similar to Poco Graphite foam. Both the electrical conductivity (degree of graphitization) and the porosity of the foam are important for current collector applications.
After making an analog to a Poco Graphite foam, future Clemson work will attempt to optimize the foam in two ways: (1) increase the macropore space to hold more active mass volume, and (2) increase the accessible surface area so that a thin layer of active material can be deposited throughout the foam. The latter may allow us to make an ultracapacitor that stores charge through electric double layer and Faradaic (redox) mechanisms.
The carbon foams of varying pore sizes will be formed from PAN precursors. To form the precursor materials we will thermally induce phase separation (TIPS) of the polymer from the solvent phase. The resulting foam can then be pyrolized to produce a carbon foam. This will be accomplished by heating the PAN foam in an oxygen free environment to 1200ºC.
In future works, we will also explore modifications of this procedure to include silica nanoparticles which can be etched out with hydrofluoric acid or high pH solutions. This step will increase the porosity and the surface area of the materials. Our goal will be to produce a series of foams with varying pores sizes and morphology for energy storage devices.
MTU is in contact with a company, Keystone Materials LLC, which specializes in electroless plating of metal throughout carbon foams. A very thin nickel coating may make better contact with the deposited active mass and allow direct soldering of connections to the electrode for deposition and cycling experiments. In the future, promising Clemson foams may be sent to Keystone Materials for nickel plating.