The replacement of fossil fuels by a clean and renewable energy source is one of the most urgent and challenging issues our society is facing today, which is why intense research has been devoted to this topic recently. Nature has been using sunlight as the primary energy input to oxidise water and generate carbohydrates (solar fuel) for over a billion years. Inspired, but not constrained, by nature, artificial systems can be designed to capture light and oxidise water and reduce protons or other organic compounds to generate useful chemical fuels. This tutorial review covers the primary topics that need to be understood and mastered in order to come up with practical solutions for the generation of solar fuels. These topics are: the fundamentals of light capturing and conversion, water oxidation catalysis, proton and CO2 reduction catalysis and the combination of all of these for the construction of complete cells for the generation of solar fuels.

A very efficient homogeneous system for visible-light driven hydrogen production in water is reported. This comprises the [Co(CR)Cl2](+) cobalt(III) tetraaza-macrocyclic complex (Cat1) as a noble metal-free catalyst, [Ru(bpy)3]Cl2 as a photosensitizer and ascorbate/ascorbic acid as a sacrificial electron donor and buffer. This system gives up to 1000 turnovers at pH 4.0 versus the catalyst with a relatively low photosensitizer/catalyst ratio (10/1) and a high concentration of catalyst (1 × 10(−4) M), thus producing a significant amount of H2 (12.3 mL for 5 mL of solution). It also exhibits long-term stability (more than 20 hours). The efficiency of Cat1 has been compared under the same experimental conditions to those of three other H2-evolving catalysts, which are known to operate in water, [Co{(DO)(DOH)pn}Br2] (Cat2), [Co(dmbpy)3]Cl2 (Cat3) and [Rh(dmbpy)2Cl2]Cl (Cat4). These comparative studies show that Cat4, although based on a noble metal, is about four times less active, while Cat2 and Cat3 produce more than one hundred times less hydrogen than Cat1. The low-valent CoI form of Cat1 has been successfully electrogenerated in CH3CN. Its high stability can be related to the high catalytic performance of the Cat1 system. We have also shown that in acidic aqueous solution (photocatalytic conditions) reduction at a slightly more negative potential than the Co(II)/Co(I) couple is needed to ensure efficient catalysis; this reduction is performed by the photogenerated [Ru(II)(bpy)2(bpy(˙−))](+) species.