Electrochemical energy storage to power the 21st century
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Why electrochemical energy storage matters more than ever before The recognition that energy can be stored at charged interfaces dates to the ancients: from borrowing the Greek word for amber (ηλεκτρον) to name the “electric ion,” electron; to the apparent electrochemical cell used over two millennia ago (the “Baghdad battery,” Figure 1a), which comprised an iron rod inserted into an electrolyte within a cylindrical copper vessel (all packaged in a clay pot) that functioned under load as a primary battery (discharge-only) as the Fe corroded; to the metal–air production of verdigris on copper by interspersing copper sheets with linens soaked in wine dregs. Batteries as a technology came of age during the scientific, technological, and intellectual fervor that characterized the Industrial Revolution. From Benjamin Franklin’s borrowing of the military term “battery” to designate Leyden jars (Figure 1b) connected in parallel to jolt 18th century experiments in electricity to life and amuse the curious in the salons of Europe―to the electric current generated by Volta’s pile of series-coupled Galvanic cells―to the chemicals (zinc, silver, lead) in use since the early 19th century to provide portable battery power. What can possibly be new under the sun from such a venerable―and practical―system? Not enough is the common lament. The Baghdad battery captures the necessary component parts of any battery or
electrochemical capacitor: two physically separated electrodes of sufficient electronic conductivity with a medium for ion transport between them (Figure 1a). With technologists and consumers accustomed to the rate of improvement in computer operations known as Moore’s Law―paying half the cost every two years to double the number of operations per second― why should the performance of batteries trundle along with a mere 10% increase in energy density per year,3,4 taking a decade to double performance? The answer―physics versus electrochemistry―is both simple and complex. The metric of computation depends on the mobility of the electronic charge carrier, so performance improvements arise by optimizing this one functionality, typically by shortening the distance an electron or hole travels. To store and release (charge/discharge) energy from a battery, three primary mobilities are involved: electronic transport in the solid state, ionic transport in the liquid and solid state, and molecular (mass) transport. If yearly performance improvement tracks at 1/2n, where n is the number of transport functions, integrated circuit computation has an n of 1 and doubles performance in two years, while n equals at least 3 for batteries for only a 12.5% improvement per year. Even in electrochemical double-layer capacitors in which an excess (or deficit) of electron charge on the electrode is balanced by a plane of counter-signed mobile ions (Figure 2), the frequency response of the two-electrode
Debra R. Rolison, U.S. Naval Research Laboratory, Washington, DC, USA; [email protected] Linda F. Nazar, Department of Chemistry and Departme
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