MRS Bulletin

Electrochemical Energy Storage to Power the 21st Century

Electrochemical Energy Storage to Power the 21st Century

Asymmetric electrochemical capacitors—Stretching the limits of aqueous electrolytes

Jeffrey W. Longa1, Daniel Bélangera2, Thierry Broussea3, Wataru Sugimotoa4, Megan B. Sassina5 and Olivier Crosniera6

a1 U.S. Naval Research Laboratory, Washington, DC 20375, USA; [email protected]

a2 Département de Chimie, Université du Québec à Montréal, Canada H3C 3P8; [email protected]

a3 University of Nantes, France; [email protected]

a4 Shinshu University, Ueda, Nagano 386-8567, Japan; [email protected]

a5 U.S. Naval Research Laboratory, Washington, DC 20375, USA; [email protected]

a6 University of Nantes, France; [email protected]


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.

Key Words:

  • Energy storage;
  • oxide;
  • nanostructure;
  • surface chemistry