[1] Univ Calif San Diego, Dept Elect & Comp Engn, La Jolla, CA 92093 USA
[2] Univ Calif San Diego, Energy Res Ctr, La Jolla, CA 92093 USA
[3] Arizona State Univ, Sch Mat, Tempe, AZ 85287 USA
[4] Univ Wisconsin, Dept Chem & Biol Engn, Madison, WI 53706 USA