Electrochemical Energy Storage to Power the 21st Century
a1 U.S. Naval Research Laboratory, Washington, DC 20375, USA; firstname.lastname@example.org
a2 Département de Chimie, Université du Québec à Montréal, Canada H3C 3P8; email@example.com
a3 University of Nantes, France; firstname.lastname@example.org
a4 Shinshu University, Ueda, Nagano 386-8567, Japan; email@example.com
a5 U.S. Naval Research Laboratory, Washington, DC 20375, USA; firstname.lastname@example.org
a6 University of Nantes, France; email@example.com
Ongoing technological advances in such disparate areas as consumer electronics, transportation, and energy generation and distribution are often hindered by the capabilities of current energy storage/conversion systems, thereby driving the search for high-performance power sources that are also economically viable, safe to operate, and have limited environmental impact. Electrochemical capacitors (ECs) are a class of energy-storage devices that fill the gap between the high specific energy of batteries and the high specific power of conventional electrostatic capacitors. The most widely available commercial EC, based on a symmetric configuration of two high-surface-area carbon electrodes and a nonaqueous electrolyte, delivers specific energies of up to ∼6 Whkg–1 with sub-second response times. Specific energy can be enhanced by moving to asymmetric configurations and selecting electrode materials (e.g., transition metal oxides) that store charge via rapid and reversible faradaic reactions. Asymmetric EC designs also circumvent the main limitation of aqueous electrolytes by extending their operating voltage window beyond the thermodynamic 1.2 V limit to operating voltages approaching ∼2 V, resulting in high-performance ECs that will satisfy the challenging power and energy demands of emerging technologies and in a more economically and environmentally friendly form than conventional symmetric ECs and batteries.