A nanostructured NiO electrode was synthesized through an initial deposition of layered basic nickel acetate having a yarn-ball-shaped morphology and subsequent heat-treatment at 500 degrees C in oxygen. A sandwich-type open cell using a NiO/ C343 photocathode yielded a photocurrent density of 1.61 mA cm(-2), an open-circuit photovoltage of 113 mV, a fill factor of 0.31, and a solar energy conversion efficiency of 0.057%, reaching the highest level ever reported. A tandem solar cell was also produced together with a TiO2/ N719 photoanode, which resulted in an increase of the open-circuit photovoltage to 0.80 V. (C) 2008 The Electrochemical Society.