7 Li MAS-NMR Investigation of MnO 2 Infused Carbon Nanofoam Supercapacitor Materials

1. Jovi D. Rodriguez (HSS), Christian E. Mejia (UGS), Prof. Steve Greenbaum (PI), Dr. Phil Stallworth (co-PI) Hunter College of the City University of New York , Dept. of Physics and Astronomy Results Introduction Samples and Preparation The samples were prepared by Dr. Jeff Long of the U.S. Naval Research Lab (NRL), Washington D.C. The carbon nanofoam was purchased from MarkeTech Inc. and then treated with MnO2, soaked for 24 hours in 1.0 M LiOH at 25 oC, annealed in Argon at 300 oC for 4 hours and then oxidized at 200 oC in air for 6 hours for the exception of one sample which was not oxidized. The samples were crushed into a fine powder and packed into 1.6mm rotors, which were then placed within the NMR probe and subjected to spectral measurements via excitation of their magnetizations. Recently, with the increase of environmental awareness, scientists as well as major consumer product companies are performing research to find an efficient and inexpensive way to power numerous electronics as well as vehicles. One of the major accomplishments achieved by the research done is the electrochemical double-layer capacitor also known as supercapacitors. These have many advantages and therefore many applications. Some characteristics that make supercapacitorssuperior power sources are long cycle life (>100,000 cycles), their manufacture and function are simple; they charge in a short amount of time, they have a high power density and are rather safe. An important application is supercapacitors are their use in conjunction with batteries for the use in electric hybrid vehicles. The supercapacitor plays a very significant in electric hybrid vehicles due to its ability to charge and deliver energy quickly. 2040 ppm 7Li MAS-NMR Investigation of MnO2 Infused Carbon Nanofoam Supercapacitor Materials Fig.5 Magic Angle Spinning-Nuclear Magnetic Resonance (MAS-NMR) NMR is a phenomenon that occurs when atomic nuclei are exposed to a magnetic field, subjected to radio frequency pulses and as a result emit a signal corresponding to their magnetic properties. The sample containing a macroscopic number of atomic nuclei is spun at a certain frequency, usually around 40 kHz, and at an angle of 54.7° resulting in the acquisition of highly resolved peaks in the NMR spectra making it easier to identify the local atomic environment of lithium in the samples being analyzed. Supercapacitor Schematic Fig. 6 Fig.3 An important property used in NMR spectroscopy that can be seen in the NMR spectra is called chemical shift. In simple terms, chemical shift isthe position of an NMR absorption peak relative to a reference compound such as an aqueous solution of LiCl. This property is key in identifying the local atomic environment of Li in the carbon nanofoam being studied. MAS (54.7o) Fig.1 Fig.2 Fig.1 Fig.7 Fig. 1 depicts the structure of an electrolytic double layer capacitor Fig. 2 shows the carbon-MnO2nanofoam Discussion & Conclusions In an early sample provided by NRL, 9688-41-9, we see four sites, namely those at 0, 517, 1020 and 2040 ppm (Fig. 5). The 0 ppm peak is associated with lithium that resides on the surface of the material (non-intercalated). The 517 ppm peak is associated with lithium in a bulk tetrahedral site. The site present at 2040 ppm is associated with bulk octahedral lithium. The peak at 1020 is in some intermediate configuration between tetrahedral and octahedral lithium. As a result of this feedback provided to NRL, new samples were fabricated and measured by NMR (Fig. 6). It can be seen that the octahedral site was eliminated, and the non-intercalated lithium has also been reduced. The presence of a larger amount of tetrahedral lithium is correlated with superior performance of the supercapacitor. Fig.4 Sponsors: National Aeronautics and Space Administration (NASA) NASA Goddard Space Flight Center (GSFC) NASA Goddard Institute for Space Studies (GISS) NASA New York City Research Initiative (NYCRI)