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B. = 54 o. 7 Li MAS-NMR Investigation of MnO 2 Infused Carbon Nanofoam Supercapacitors. Christian E. Mejia (HSS), Prof. Steve Greenbaum (PI), Dr. Phil Stallworth (co-PI) Hunter College of the City University of New York , Dept. of Physics and Astronomy . Introduction. Results.
7Li MAS-NMR Investigation of MnO2 Infused Carbon Nanofoam Supercapacitors
Christian E. Mejia (HSS), Prof. Steve Greenbaum (PI), Dr. Phil Stallworth (co-PI)
Hunter College of the City University of New York, Dept. of Physics and Astronomy
Samples and Preparation
The samples were prepared by Dr. Jeff Long of the U.S. Naval Research Lab (NRL). The carbon nanofoam was purchased from MarkeTech and then treated with MnO2, and soaked for 24 hours in 1.0 M LiOH at 85 oC. One sample was annealed in argon at 300 oC for 4 hours and then oxidized at 200 oC in air for 4 hours. The other sample was only annealed in Ar.
The samples were crushed into a fine powder and packed into 1.6mm rotors, which are then placed within the NMR probe.
Supercapacitors, also known as electrolytic double layer capacitors (DLC), are a major advance in energy storage technology. As their name implies, supercapacitors are capacitors with a much higher capacitance than conventional capacitors, by up to three orders of magnitude. This allows for high energy density and high charge-discharge rates. Supercapacitors also show the promise of being able to work in conjunction with a battery in various applications, like the power system of an electric car. With their high power rating, supercapacitors can rapidly provide the energy to accelerate the vehicle, while the battery provides the energy for the range of the vehicle. The supercapacitor’s ability to deliver and store charge quickly also makes it ideal for a regenerative breaking mechanism, making it very efficient.
Magic Angle Spinning Nuclear
Magnetic Resonance (MAS NMR)
NMR is a powerful and useful technique when it comes to identifying and analyzing molecular structure. Each nucleus is affected by its chemical environment, which causes distinct shifts from the resonance frequency of the “bare” nucleus. By spinning the sample, usually at frequencies up to 40 kHz, at an angle of 54o in respect to the magnetic field, broad lines can be resolved into more defined peaks.
The thin layer comprising the electrode/electrolyte interface is the most important layer, as it is responsible for the electrical properties of the supercapacitor.
Discussion & Conclusions
Two sets of spectra corresponding to a sample that was annealed in Ar only and the other oxidized after annealing are shown above. The spectra were acquired at different spinning speeds to resolve the true resonances from the spinning sidebands. Both samples exhibit a main resonance at zero ppm corresponding to surface Li, but also show secondary shifted resonances attributed to intercalated Li. Both species contribute to the charge storage mechanism, and the NMR method described herein is one of the only techniques that can be used to quantify the Li in each site. Several differently prepared samples are under investigation and the results will be correlated with electrical characterization being performed at NRL.
The origin of the “chemical shift” in the NMR spectrum is the local magnetic field arising from orbital and spin angular momentum of nearby electrons. In the carbon nanofoam, there are two definite sites for the Li+ to be found, namely on the surface and within the nanoporous material. The non-intercalated (surface) lithiumis identified by zero shift in the NMR spectrum, and the intercalated lithium presents a range of shifts due to the paramagnetic Mn4+, as shown in the figure.
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)