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6 Nano- and Micro scale Approaches to Energy Storage and Corrosion Henry S. White, University of Utah Investigations of electrochemical phenomena at chemical structures that are measured and defined on the nanometer-length scale constitute a rapidly emerg- ing frontier of electrochemical science. Measurement of single electron transfer events, electrochemical syntheses of patterned and well-defined nanostructures, and electrochemical characterization of nanometer-scale surface features have all occurred over the past decade. Technological applications of these scientific advances are on the horizon. An example of this is in the fabrication of new higher energy density batteries, which require new scientific strategies in the assembly and deployment of nanoscale structures to control macroscopic electro- chemical phenomena. The three-dimensional electrochemical cell is a hypothetical device that illustrates how some of the advances in microscale and nanoscale electrochemistry over the past two decades may be applied to its construction (Figure 6.1~. The three-dimensional electrochemical cell is a conventional battery in the sense that it has a cathode and anode, but they are configured in an interpenetrating array with electrodes anywhere from micron dimensions if they are prepared using lithographic techniques down to the nanometer scale. The advantage of the three-dimensional electrochemical cell over conven- tional two-dimensional batteries is that the additional dimension, termed L, can be increased indefinitely. Increasing L results in an increase in energy capacity without any loss of power density in the cell, because electrons travel the same distance between the anode and cathode regardless of the thickness of the cell. This cannot be accomplished with a conventional battery. Potential applications for this technology are Micro-Electro-Mechanical Systems devices and other microelectronic devices. 40

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NANO- AND MICROSCALE APPROACHES TO ENERGY STOOGE AND CORROSION 41 3-D battery 2-D battery Increased capacity with L without losing power density per geometric footprint FIGURE 6.1 The three-dimensional electrochemical cell. ..... 1 i' / L / Actual construction of such a device presents a number of technical chal- lenges. When electrodes are synthesized at 10- to 100-nm diameters, with the anode and cathode separated by similar distances, problems in hard wiring and assembly are to be expected. In addition, there currently is no three-dimensional architecture for an electrochemical cell that would achieve uniform current density. Also, at the nanometer scale, noncontinuum effects, especially mass transport, become a concern. Other issues of concern include ensuring that there is enough territory for phase nucleation to occur and quantized charging when the electrode material approaches nanoscale dimensions. While a simplified assembly may be pictured showing carbon nanotubes hardwired in an electronic circuit (Figure 6.2), it is difficult to envision how this could be accomplished using gold interconnects between carbon nanotubes. This approach is unlikely to be successful at least over the coming decade. A different approach uses solid state chemistry to hard wire a three-dimensional battery-type structure. Solid gel chemistry has been used to produce a void-SiO2 composite aerogel that has been electronically wired with a conformal nanoscopic web of RuO2 to which 50- to 100-nm colloidal Au particles have been electro- chemically attached (Figure 6.3~. This configuration provides the contacts for collecting electricity out of the cell. Ruthenium oxide can act as one of the active electrode materials in the battery, and silicon dioxide (as opposed to a Nation membrane) can act as the separating material. All that is needed to develop a truly functional three-dimensional battery is to put a second electrode material in place and make the electrical contact to the outside world. The result is a very high energy density device with no power losses because the anode and cathode are spaced together so closely. While this device has not yet been built, and the actual device may be several years off in the future, progress toward it has been made.

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42 ENERGY AND TRANSPORTATION FIGURE 6.2 Hardwiring electrochemical nanocells. FIGURE 6.3 Schematic of a void-SiO2 composite aerogel that has been electronically wired with a conformal nanoscopic web of RuO2 to which 50- to 100-nm colloidal Au particles have been electrochemically attached. Ryan et al., Nature, 406: 169. When making electrodes on the order of a few nanometers in diameter, as required for these cells, quantized double-layer charging must be addressed. With these small devices, electrons are not added continuously as a function of poten- tial. Rather, electrons are added in discrete steps at different potentials as a conse- quence of the extremely small capacitance of the particle.

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NANO- AND MICROSCALE APPROACHES TO ENERGY STORAGE AND CORROSION 43 If this in fact is a quantized electrode, the resulting battery will be quantized and a discharge curve will have discrete steps. The result is that this type of device will have totally different operational parameters. Not only will these devices be based on different materials with different physical and chemical properties, but the output and function also would be inherently different. Energy storage batteries are normally thought of as having a voltage deter- mined by the free energy of the reactions at the anode and cathode. If the anode and cathode are very close together such that the electrical double layers begin to overlap, it is possible that the free energy of the cell will no longer be solely determined by the reactions at the anode and cathode but also by the electrostatics of the interaction between the electrodes. Electrostatics is just one of the properties of materials that must be con- sidered when approaching the nanoscale. The oxidation, kinetics, and general electrochemistry of materials have been observed to behave unpredictably at nanoscale dimensions. The fact that these properties are not well understood allows for many research opportunities as devices such as the three-dimensional batteries are made. Because the three-dimensional battery has not yet been made, the question remains whether there is enough electrode area to have a nucleation event such that the anodic or cathodic reaction can occur. THE CHEMISTRY OF CORROSION In addition to energy storage, another broad challenge facing transportation research is corrosion. Many opportunities exist in materials chemistry related to the issue of corrosion, particularly when faced with challenges such as producing an economically viable automobile that achieves 80 miles per gallon fuel effi- ciency. One way to help meet this challenge would be to require a 40 percent reduction in the mass of the vehicle. One means to achieve this is to replace steel in automobiles with aluminum alloys. Challenges like this will require a fundamental understanding of corrosion. Metallurgical issues such as the role of the preexisting distribution of elements in the alloys requires a detailed understanding of microstructure, which in turn is important in order to understand oxidative breakdown and the chemistry that causes the propagation of a stress corrosion crack, for example. Work at Sandia National Laboratory has looked into the fundamentals of corrosion. Macroscopic results of experiments into the pitting of aluminum wire when exposed to sodium chloride solution indicate that the pitting potential is not a thermodynamic value but rather the potential associated with the kinetics of oxide breakdown. As a result, as a device becomes increasingly small, the prob- ability of oxide breakdown will likewise decrease. At nanoscale a device made of this material would be highly unlikely to undergo oxide breakdown, and such a device would be expected to exhibit stability for long periods of time.

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44 ENERGY AND TRANSPORTATION Characterization tools must also be developed if a fundamental understand- ing of corrosion is to be achieved. For example, observation of a titanium surface with an oxide film at the nanometer scale shows oxide grains on the surface. Conductivity atomic force microscopy measurements can be used to indicate defect-free TiO2 by showing no current flow. This is visually indicated by a dark image. Therefore, light spots on the image indicate defects in the oxide. With cur- rent analysis techniques, these defects can be found down to 10-nm resolution. However, fundamental structural information about these defect sites is still lack- ing because the analysis and characterization tools do not yet exist. To advance a fundamental understanding of corrosion, researchers must move beyond empirical or phenomenological descriptions of corrosion mecha- nisms to a more molecular understanding. Characterization tools, including com- putational tools, are needed to achieve this goal. The overarching goal is to develop these corrosion mechanisms at a level of detail and sophistication similar to those found in chemistry and molecular biology.