Development of lightweight carbon electrodes using graphitic carbon foams for battery and fuel cell applications (MTU: T. Rogers, B. Cornilsen, M. Chye, W. Yeo)
In the past year under previous DOE funding, cycle-life tests of the MTU nickel-carbon foam cathode (patent pending) have been conducted in flooded cells. (The monolithic graphitic foams are impregnated electrochemically with nickel oxyhydroxide active mass.) These have demonstrated competitive capacity retention and high cycle life at a variety of discharge rates required by consumer electronics and power tools. Replacement of the nickel metal current collector in Ni-MH (nickel-metal hydride) batteries with a lighter, cheaper, and readily available material, carbon foam, is expected to cut the weight of a rechargeable Ni-MH battery by up to 25 percent.
The objective of this task is to understand the cycling behavior of the nickel electrode active mass as we fine-tune the structural parameters of this material so that the electrochemical parameters (such as discharge capacity and cycle life) of the nickel carbon foam cathode can be optimized. Based on the known structural model for active mass, i.e. the close packed, point defect containing structure, we will vary the deposition and cycling parameters (cobalt content, electrochemical deposition conditions, loading of active mass in the carbon foam pores, and charge/discharge conditions) to achieve this optimization.
Activities in the past quarter are summarized below:
- Fourteen (14) electrodes have undergone active mass deposition and formation, and are now being cycled. Two are benchmark electrodes consisting of nickel foil with a deposited active mass layer. The rest are carbon-nickel electrodes prepared from Poco Graphite commercial foams using aqueous nickel nitrate solutions and cycled with a variety of clamp designs and counter electrodes. All electrodes are cycling at a 1C rate, alternating with a few cycles at a 0.2C rate after each set of 100 cycles at a 1C rate. The best electrode has achieved over 1,000 charge-discharge cycles. We are recording the discharge capacities versus time, and noting the decrease in capacity as the number of cycles becomes large. Cell "failure" is defined in this work as a permanent 20% loss in discharge capacity.
- We have performed cyclic voltammetry (CV) on Poco Graphite foam samples purchased in 2005 and 2007 to see whether aging (i.e., exposure to moisture and oxygen) has affected the foam's effectiveness as a current collector. A similar CV plot for both samples indicate that aging effects are not prominent.
In the next quarter, we will do the following:
- Work has begun to design a complete battery cell using a Swagelok tubing connector as a battery casing. This cell will test our nickel-carbon cathode in a true battery system having both an anode and a cathode. To date, we have been charging and discharging cathodes in flooded cells over many cycles using a variety of counter electrodes. The Swagelok battery will have a nickel-carbon foam cathode and a slice of bare carbon foam (probably fairly thick) as a non-limiting anode.
- We are developing an analytical method to measure the concentrations of Ni(II) and Co(II) in solutions leached from impregnated electrodes. This is an independent measurement of the amount of active material deposited. Ion chromatography (IC) may not the best option because the concentrations are too high and the technique involves a great deal of solution preparation for each test. Currently, we are favoring an EDTA titration method for nickel ion analysis.