The utilization of CO2 as a cheap and abundant carbon source to produce useful chemicals or fuels could be regarded as one of the promising ways for sustainable development. The water splitting for CO2 reduction with solar energy is one of the most potential process, however, a high solar-to-fuel efficiency keeps a great challenge. In this work, a Cu-catalyzed highly efficient and robust method for splitting water to reduce CO2 into formic acid with a simple commercially available Cu and Fe powder was presented. Results showed that the formic acid production with Cu catalyst was much higher than that without Cu or with Ni catalyst, and an excellent formic acid production of about 71%, which represented the highest formic acid production from CO2 using Fe reductant to date, was obtained under a mild hydrothermal condition at 300 degrees C for 2 h. The Cu catalyst was relatively stable and could be used repeatedly without causing a significant change in the formic acid production. A reaction mechanism of the conversion of HCO3- into formic acid was also proposed. The water splitting for CO2 reduction with Fe is an exothermic reaction, thus the proposed process may provide a viable method for highly efficient water splitting and CO2 reduction into value-added chemicals by combining the reduction of FexOy into Fe with bio-derived chemicals. (C) 2015 Elsevier B.V. All rights reserved.