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What are Electrochemical Materials and Interfaces? Advances in nearly all industrial products are tied, at least in part, to improvements in materials. The microelectronics industry is the quintessential example where major technological advances move concurrently with advances in material processing and properties.[1] In recent years, a spate of papers in Science, Nature, and other premier journals have made it clear that revolutionary advances in the electrochemical technology sector are also inseparably wedded to materials innovation.[2-10] For example, low temperature fuel cells that directly utilize methanol are hampered by the performance of electrocatalyst and membrane materials;[2] one cannot expect to improve lithium ion batteries,[3] and then move beyond lithium,[4] without the development of alternative materials; advances in biological sensors depend on engineering highly selective electroactive interfaces;[5] control of electrodeposition has advanced from art to science by understanding nucleation and growth at electrified interfaces;[6,7] and, novel electrochemical microfabrication methods are creating unique microsystems.[8-10] Moreover, the economic, health, and safety impact of corrosion processes are tremendous. Aside from microelectronic devices, it is difficult to think of another technological realm that is more materials-dependent than electrochemical systems. Given that electronic and electrochemical technologies are both paced by materials innovations and both have high economic and societal impact, it seems odd that the term “electronic materials” is so common while the term “electrochemical materials” is largely unknown. What exactly are electrochemical materials? Electrochemical materials are ionic and/or electronic conductors whose properties dictate the performance of electrochemical systems, processes, or devices. Given this definition, electrochemical materials play a central role in a remarkably diverse group of sciences and technologies, ranging from the specific examples cited above to membranes (biological and industrial), solid state ionics, and electrosynthesis. Not only are the applications diverse, but the scientific underpinning of how electronic and ionic conductors interact has interesting subtleties. Figure 1 illustrates a series of material transformations that occur for a variety of electrochemical materials as they perform their intended function. Figure 1 (top) illustrates the intercalation of lithium into a graphitic carbon electrode during the recharging of a lithium ion battery. To maintain charge neutrality, an equal number of electrons (supplied by an external power supply) and lithium cations (from an adjacent polymer electrolyte) must enter the structure.[11] At the negative potential present during the recharging process, the electrons move through the electronically conducting graphite sheets and react with the cations between the graphitic layers to form zero valent lithium metal. In a similar vein, the middle of Figure 1 shows when an electrochemical ion exchanger is selectively loaded with cations, electrons move through the conductive lattice while ions intercalate into the interstitial regions.[12] In this case, however, the potential of the matrix is considerably positive compared to the recharging of a lithium battery, so the charge-balancing electrons prefer to reside within the lattice framework rather than on the intercalated cation. For a glucose sensor (bottom, Figure 1), the electrochemical material is a redox enzyme immobilized near an electrode surface via a polymer network capable of conducting current.[5] When glucose interacts with glucose oxidase (GOX), a redox reaction occurs and the charge is propagated, via redox centers on the polymer, to the electrode where it can be measured as a current. Thus, a glucose sensor of this type represents yet another way that coupled electronic and ionic properties of a material can be harnessed for useful purposes. Despite significant differences in each of the materials in Figure 1, they all function because of the simultaneous conduction of electrons (via the solid state) and the transport of ions (through various condensed phases). Anything that affects the ionic or electronic conductivity of the material (such as defects, doping, grafting of donors/acceptors on polymers, ion size and affinity for the matrix, hydration state, etc.) affects the performance of the resulting device. As Fig. 1 illustrates, electrochemical materials are underpinned by the physics of electronic and ionic conductivity within condensed phases and charge transfer at the boundary between phases. Moreover, the processing, structure, and properties of electrochemical materials must be understood at the nano-, micro-, and continuum-scales as well. There are many reasons to believe that the science and application of electrochemical materials will continue to grow in importance. Electrochemical energy conversion is not limited by the thermodynamic efficiency of a heat engine and, thus, fits well with the worldwide trend toward higher efficiency, cleaner, more flexible energy. Much of the biological world works via redox chemistry and electrolytic transport processes, and thus, can likely be harnessed as electrochemical materials with useful sensing, separation, therapeutic, and electrosynthetic properties. Garnering support for public investment in civil infrastructure is getting more difficult every year, so corrosion science that slows the degradation of infrastructure is increasingly important. Finally, electrodeposition is one of the most facile tools available to a nanotechnologist and, thus, will find increasingly creative applications with a wider range of materials. To foster this sort of growth and diversity in electrochemical materials may require a rethinking of the organizing concepts that serve as the foundation for electrochemical materials science and engineering. At present, the tools needed to broadly understanding electrochemical materials are found within a melange of interfacial electrochemistry, electrochemical engineering, corrosion science, and solid-state ionics. Each application area (batteries, fuel cells, sensors, electrosynthesis, corrosion, electrodeposition, etc.) has adopted a fairly narrow materials focus. Ideally, electrochemical materials science and engineering should be broadly concerned with the generation and application of knowledge that relates the structure and composition of electrochemical materials to processing and to their properties and performance in electrochemical systems. To appreciate the benefits that may accrue from a unified electrochemical materials science and engineering perspective, it is valuable to look at the electronic materials research enterprise. One potential benefit is a stunning synergy between fundamental materials research and its translation into technology. For electronic materials, this synergy is often embodied in Moore’s Law,[13] but it is also evident in advances like blue diode lasers and polymeric semiconductor devices. Thus, a fundamental science-based understanding of structure-property relationships, and of methods for processing materials, can result in the rapid advancement of both science and technology. A second potential benefit is an increase in the recognition that the electrochemical materials community commands. Innovations in electrochemical materials have led to portable power for cellular phones, high-speed copper-plated integrated circuits, smart windows, electrodialysis, cars that resist rust, and biosensors, to name a few examples. Individually, these innovations contribute to modern life, but in aggregate, they are driven by an electrochemical materials revolution that is changing the World for the better (cleaner, more efficient, more productive). This ongoing revolution justifies the attention and [1] R. D. Miller, Science 286, 421 (1999). [2] E. Reddington, A. Sapienza, B. Gurau, R. Viswanathan, S. Sarangapani, E. S. Smotkin, and T. E. Mallouk, Science 280, 1735 (1998) [3] D.R. MacFarlane, J. Huang, and M. Forsyth, Nature 402, 792 (1999). [4] S. Licht, B. Wang, and S. Ghosh, Science 285, 1039 (1999) [5] J. G. Wagner, D. W. Schmidtke, C. P. Quinn, T. F. Fleming, B. Bernacky, and A.Heller, PNAS 95, 6379 (1998) [6] K. Sieradzki, S. R. Brankovic, and N. Dimitrov., Science 284, 138 (1999) [7] J.A.Switzer, M. G. Shumsky, and E. W. Bohannan, Science 284, 293 (1999) [8] J.-C. Bradley, et al., Nature 389, 268 (1997) [9] R. J. Jackman, S. T. Brittain, A. Adams, M. G. Prentiss, and G. M. Whitesides, Science 280, 2089 (1998). [10] R. Schuster, V. Kirchner, P. Allongue, and G. Ertl, Science 289, 98 (2000). [11] M. Broussely, P. Biensan, B. Simon, Electrochimica Acta 45, 3 (1999). [12] D.T. Schwartz and S.M. Haight, Surf. Coll. A 174, 209 (2000). [13] P. A. Packan, Science 285, 2079 (1999). [X]S.D. Leith and D.T. Schwartz, J. Microeng. Microsys. 9 97(1999)
Figure Caption
Figure 1. The behavior of electrochemical materials is a mix of transport issues (electronic, ionic, and neutral species) combined with the energetics of phase transformation. The top figure illustrates the intercalation of lithium (Li) into the graphitic electrode of a Li-ion battery during recharge. Electrons are delocalized in the sp2 bonded graphite layers and ions are mobile between the graphite sheets. At a sufficiently negative potential, electrons prefer to reside on the intercalated Li+ cation (light circle), reducing it to its metallic form (dark circles). Owing to the complex energetics of Li phase formation, a variety of self-organized Li structures form as the carbon electrode is filled. In the middle figure, a separation occurs when a mixture of cations (one with low affinity r+ , one with high affinity O+ ) are allowed to intercalate into nickel hexacyanoferrate (O=Ni+2, ∆=Fe+3, s=Fe+2, and —— denotes CN– ligands that connect the transition metals). Electrons transported through the host matrix reside on iron centers within the matrix itself (rather than the intercalated ion), causing a reduction of Fe+3 to Fe+2. The intercalation affinity of the matrix arises from size exclusion and other effects. The bottom figure illustrates the detection of glucose using redox polymer wired glucose oxidase (GOX). When poised at an oxidizing potential, glucose is enzymatically oxidized to gluconolactone and two protons by modulation of the FAD/FADH2 cofactor reaction center in GOX. Regeneration of the GOX occurs by transferring the charge to redox centers on the immobilizing polymer network, which then propagates the charge to the substrate via electron hopping.
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Daniel T. Schwartz, Director |
