Solar energy storage through the homogeneous electrocatalytic reduction of carbon dioxide : photoelectrochemical and photovoltaic approaches

The sun is the most abundant resource of renewable energy available to the Earth. More energy strikes the surface of the earth in one hour than all primary energy consumption by humanity in an entire year. However, solar energy is intermittent, and if it is to become a major contributor to the electricity supply, an inexpensive and reliable form of massive energy storage will be necessary. The ability to convert solar electricity into a liquid fuel is an attractive solution to the energy storage problem. A challenging goal will be to use only H₂O and CO₂ as feedstocks for making synthetic hydrocarbon fuels. Electroreduction of CO₂ to liquid fuels necessitates the use of efficient electrocatalysts to increase efficiency and rate for the essential development of practical industrial processes. Two approaches towards the storage of energy in chemical bonds are investigated. The photoelectrocatalytic production of CO using CO₂ as a feedstock demonstrates the capture of solar energy and subsequent electrochemical conversion into a useful chemical commodity. CO₂ is reduced at illuminated p- Silicon (p-Si) cathodes using the electrocatalyst fac-Re(2, 2'-bipyridyl)CO₃Cl at a 440 mV less anodic potential when compared to a glassy carbon electrode. Cyclic voltammograms of the electrocatalyst with CO₂ show an increase in current at the second reduction wave. In the second approach, a fully integrated system for a directly coupled solar photovoltaic driven CO₂ electrolyzer was built and characterized. The design and theoretical voltage requirements show a minimum practical voltage of 3.4 V even though the thermodynamic minimum is only 1.33 V. The balancing of a non-linear power supply to a non- linear load reveals a self-stabilizing nature. An overall solar conversion efficiency ([eta]TOT) of 2.1% is achieved by using the electrocatalyst Re(4,4'-di-tert-butyl-2,2'- bipyridine)(CO)₃Cl. Theoretical calculations predict an upper efficiency limit of 21% for a single junction solar cell coupled to an electrolyzer. This stand-alone performance shows great promise and demonstrates the need for further development of more efficient CO₂ electrocatalyst