Carbon dioxide (CO) can be reduced to carbon monoxide (CO) and other more reduced compounds, such as methane, using the appropriate photocatalysts. One early example was the use of Tris(bipyridine)ruthenium(II) chloride (Ru(bipy)Cl) and cobalt chloride (CoCl) for CO reduction to CO. In recent years many new catalysts have been found to reduce CO into CO, after which the CO could be used to make hydrocarbons using for example the Fischer-Tropsch process. The most promising system for the solar-powered reduction of CO is the combination of a photovoltaic cell with an electrochemical cell (PV+EC). Using solar-driven processes, CO can also be converted to other products such as formate and alcohols. 
For the photovoltaic cell the highly efficient GaInP/GaAs/Ge solar cell has been used, but many other series-connected and/or tandem (multi-junction) PV architectures can be employed to deliver the required voltage and current density to drive the CO reduction reactions and provide reasonable product outflow. The solar cells/panels can be placed in direct contact with the electrolyzer(s), which can bring advantages in terms of system compactness and thermal management of both technologies, or separately for instance by placing the PV outdoors exposed to sunlight and the EC systems protected indoors.
The currently best performing electrochemical cell is the gas diffusion electrode (GED) flow cell. In which the CO reacts on Ag nanoparticles to produce CO. Solar to CO efficiencies of up to 19% have been reached, with minimal loss in activity after 20h.
CO can also be produced without a catalyst using microwave plasma driven dissociation of CO. This process is relatively efficient, with an electricity to CO efficiency of up to 50%, but with low conversion around 10%. These low conversions are not ideal, because CO and CO are hard to separate at large scale in a efficient manner. The big upside of this process is that it can be turned off and on quite rapidly and does not use scarce materials. The (weakly ionised) plasma is produced using microwaves, these microwaves can accelerate the free electrons in the plasma. These electrons interact with the CO which vibrationally excite the CO, this leads to dissociation of the CO to CO. The excitation and dissociation happens fast enough that only a little bit of the energy is converted to heat, which keeps the efficiency high. The dissociation also produces an oxygen radical, which reacts with CO to CO and O.
Also in this case, the use of microorganisms has been explored. Using genetic engineering and synthetic biology techniques, parts of or whole biofuel-producing metabolic pathways can be introduced in photosynthetic organisms. One example is the production of 1-butanol in Synechococcus elongatus using enzymes from Clostridium acetobutylicum, Escherichia coli and Treponema denticola. One example of a large-scale research facility exploring this type of biofuel production is the AlgaePARC in the Wageningen University and Research Centre, Netherlands.