Solar fuels: thermodynamics, candidates, tactics, and figures of merit

Inorganic chemistry has been and continues to be a central discipline in the field of renewable energy and solar fuels. A fundamental approach to storing solar energy is artificial photosynthesis, whereby uphill chemical reactions are driven by light, e.g. the water gas shift reaction, halogen acid splitting, or water splitting. This article endeavors to define a common context for these research topics, particularly by analyzing the thermodynamic boundaries of photosynthesis. Specifically, the generalized efficiency restrictions on both light absorption and energy storage are expounded, the analogous limitations for several individual photosynthetic strategies are stated, several synthetic catalyst architectures are highlighted, the advantages of molecular and macroscopic approaches are discussed, and key figures of merit are presented.