Ion motion and electrochemistry in nanostructures
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Introduction Ionic motion in bulk materials and at interfaces has played an enormous role in the development of many technologies.1 The motion of ions in an electrolyte is essential for conventional batteries;2 in solids, it is critical for the kinetics of lithiation and delithiation in modern lithium-ion batteries2–5 (see schematic in Figure 1). Under discharge conditions, lithium ions diffuse through an electrolyte/separator from a Li-containing anode (shown schematically as Li-intercalated graphite). The electrolyte is pristine (in the case of solid electrolytes) or contains a macroscale ion-transporting separator that prevents direct electrical contact between anode and cathode. Lithium ions and electrons that have passed through the external load are reunited at the cathode, where lithium is intercalated into, or reacted with, the cathode material. In fuel cells, a solid material is often used as the electrolyte, acting as a transport medium for (negatively charged) oxygen anions or (positively charged) protons.6,7 As shown in Figure 1, molecular hydrogen is cracked catalytically at the anode, and protons diffuse through a macroscale proton exchange membrane (PEM) while electrons are transported through an external electrical load. The protons and electrons are united with oxygen species produced catalytically at the cathode to form water.
In recent years, with our increasing ability to engineer materials at the nanometer scale, the motion of ions and the modification of surfaces via electrochemical reactions have become of increasing importance, even as their effects have sometimes been challenging to interpret. While these effects can complicate many situations, they also present new opportunities for materials engineering.8 Indeed, both ionic motion and electrochemical reactions have a greater impact at the nanoscale because of the same underlying physical reasons that have made nanoscale materials engineering so attractive: nanoscale materials generally have large surface-to-volume ratios. Chemical reactivity and defect motion at surfaces are both enhanced—or at least modified—from their bulk values. Changes in surface termination and charge can have profound effects on the electronic distribution in the comparatively small bulk, depending on electronic and dielectric screening. Modest voltages or differences in chemical potential across nanoscale distances can produce large electric fields and diffusive forces. In short, decreasing the characteristic scale of a system to the “nano” regime enhances ionic and electrochemical effects because driving gradients can readily become large, and the relevant length scale required for “interesting” ionic motion becomes small. Ions can move in response to these diffusive
Douglas Natelson, Rice University, Houston, TX 77005, USA; [email protected] Massimiliano Di Ventra, University of California San Diego, La Jolla, CA 92093–0319, USA; [email protected] DOI: 10.1557/mrs.2011.266
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MRS BULLETIN • VOLUME 36 • NOVEMBER 2011 • www.mrs.org/bulletin
© 2011 Mater
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