Carbon Nanotube Electrodes for batteries and fuel cells
Carbon nanotubes have attracted considerable attention for basic and applied research based on their extraordinary electrical, thermal, and mechanical properties. The NRPL is investigating the production of carbon nanotube electrodes for chemical energy conversion applications. Specifically targeted technologies are lithium ion batteries and PEM fuel cells. Both of these devices rely on electrodes that can facilitate charge, ion, and gaseous transport. In addition, specific attributes such as large catalytic surface area, good thermal conductivity, and material strength and flexibility are extremely sought after for these applications. Based on previous work in the NPRL, we are well positioned to capitalize on our fundamental understanding of the material properties to better enhance the performance of these devices.
(a) as-produced MWCNT product harvested from the CVD reactor;
(b) SEM and TEM images representative of the MWCNTs synthesized using xylenes; and
(c) MWCNT paper fabricated using vacuum filtration.
The NPRL has investigated the lithium ion capacity for multi-walled carbon nanotubes (MWCNTs) synthesized by the injection chemical vapor deposition (CVD) process using the cyclopentadienyl iron dicarbonyl dimer catalyst. The high quality of the as-synthesized MWCNTs has enabled free-standing electrodes to be fabricated independent of polymeric binder or copper support. Results of galvanostatic cycling of these electrodes have proved very promising whereby excellent reversibility and coulombic efficiency (>97% after cycle 3) were demonstrated using propylene carbonate based electrolytes, with no evidence for material degradation. A reversible capacity exceeding 225 mAh/g was measured after 20 cycles when using the electrolyte combination of (1:1:1 v/v) ethylene carbonate (EC):propylene carbonate (PC):diethyl carbonate (DEC) at a charge rate of 74 mA/g (equivalent of C/5 for LiC6). With the in-house CVD capabilities, the NPRL has also investigated the effects of modified synthesis parameters (e.g., exchanging xylenes with pyridine as the precursor solvent). These modifications have improved the lithium ion capacity in the resulting MWCNT paper to 340 mAh/g. In addition, this MWCNT paper showed a stable reversible capacity after 10 cycles, exceeding 225 mAh/g when cycled at an equivalent 1C rate. The high production rate and high quality material make MWCNTs a high-capacity alternative for Li-ion battery anodes.
XRD data for the purified SWCNT paper before (blue, bottom) and after lithiation (red, top). The theoretical Bragg reflections (hk) are listed at each peak for a 2-dimensional triangular lattice (see schematic in the center) with a lattice constant of 17 Å. The peaks attributed to the lithiation process in SWCNTs are designated with an asterisk (*).
The NPRL has also investigated the electrochemical cycling performance of high-purity single-walled carbon nanotube (SWCNT) paper electrodes measured vs. Li for a series of electrolyte solvent components. It has been found that the addition of propylene carbonate (PC) into the conventional ethylene carbonate (EC):dimethyl carbonate (DMC) co-solvent mixture enables a reversible Li-ion capacity of 520 mA-h/g for high purity SWCNTs. A free-standing SWCNT electrode (no polymer binder or metal substrate support) with this electrolyte combination had enhanced cycleability, retaining >95% of the initial capacity after 10 cycles. The NPRL is current investigated methods to mitigate the 1st cycle hysteresis which is common in these materials. Results have indicated the electrolyte selection is critical and which emphasizes the importance of fundamental investigations in to the solid-electrolyte interface (SEI) formation on SWCNT materials. Improvements have also been observed in the Li-ion capacity at higher charge rates (e.g., 1C) resulting in a 2× improvement [in capacity per current] over reported values for conventional graphite anode materials. In addition to the many electrochemical results, NPRL’s expertise in materials characterization has been utilized in postmortem analyses of the SWCNT electrodes. Marked changines in SWCNT paper XRD and Raman spectrectra have been observed and understanding these variations are expected to be paramount in optimizing the battery performance.
- Recent Publications:
- Landi, Brian J.; Ganter, Matthew J.; Schauerman, Christopher M.; Cress, Cory D.; Raffaelle, Ryne P., J. Phys. Chem. C, 112(19), 2008.
- Journal of Nanoscience and Nanotechnology Vol.8, 1–5, 2008
- Landi, B.J.; Ganter, M.J.; Schauerman, C.M.; Cress, C.D.; Raffaelle, R. P. “Lithium Ion Capacity of Single Wall Carbon Nanotube Paper Electrodes” J. Phys. Chem. C., 2008, 112, 7509-7515.
- Landi, B.J., DiLeo, R.A., Schauerman, C.M., Cress, C.D., Ganter, M.J., Raffaelle, R.P. “Multi-walled carbon nanotube paper anodes for lithium ion batteries.” Journal of Nanoscience and Nanotechnology Vol.8, 1–5, 2008