ith the growing importance and falling prices of renewable electricity, the issue of electricity storage to deal with the intermittent nature of renewable energy sources is becoming urgent. Storing renewable electricity in chemical bonds ('electrofuels') is particularly attractive, as it allows for high-energy-density storage and potentially high flexibility. While hydrogen is the most likely and realistic candidate for electricity storage in electrofuels, research on the electrochemical conversion of carbon dioxide and water into carbon-based fuels has intrigued electrochemists for decades, and is currently undergoing a notable renaissance [1][2][3][4] . In contrast to hydrogen production by water electrolysis, carbon dioxide electrolysis is still far from a mature technology. Significant hurdles regarding energy efficiency, reaction selectivity and overall conversion rate need to be overcome if electrochemical carbon dioxide reduction is to become a viable option for storing renewable electricity.Many electrocatalysts have been reported for the production of different compounds from the electrocatalytic carbon dioxide reduction reaction (CO 2 RR). Table 1 gives an overview of some of the most active and selective metal or metal-derived electrocatalysts towards specific products in aqueous media. The two-electron transfer products, CO and HCOOH, can be produced with low overpotential and high Faradaic efficiency on suitable electrocatalysts, but substantially higher overpotentials and lower selectivities are observed for multi-electron transfer products such as methane, ethylene and alcohols 2 . For a recent discussion about the economic perspectives of CO 2 RR, the reader is referred to a previous analysis 5 .The aim of this Review is not to be exhaustive, but rather to selectively (and subjectively) discuss some recent advances and pertinent challenges in this field, focusing on themes that have recently witnessed important progress 2,3,6,7 . An overview of some of the themes covered in this Review is shown in Fig. 1. We also discuss two important methodologies used to increase fundamental understanding of CO 2 RR: in situ spectroscopic techniques and computational techniques.

Table S1: Total electronic energies for low and high spin states and relative energies of the high spin states (vs low spin states) for calculated negatively charged intermediates. * Relative values = E high spinE low spin ** The close-shell singlet does not converged probably because the lowest singlet configuration is open-shell. *** The numbers in the brackets are the spin multiplicity.

We provide a complete and computationally detailed picture of the mechanism of the initial stages of the electrocatalytic reduction of CO2 in water catalysed by cobalt porphyrin complexes.

Polypyridine ruthenium sulfoxide complexes are intriguing compounds which can display both photochromic and electrochromic properties. These properties are based on the Ru-S → Ru-O linkage isomerization capability of the sulfoxide group. The photoisomerization mechanism is of particular importance in order to understand the photophysical properties of such molecules. Density functional theory calculations demonstrate that the main photoisomerization mechanism is nonadiabatic for the system under study in agreement with the experimental observations. Indeed, funnels for efficient radiationless decay back to the ground state are shown to be easily accessible compared to transition states on the adiabatic triplet potential energy surface. However, we highlight for the first time that triplet metal-centered states play a central role in the photoisomerization mechanism of these compounds.