Impacts of electrode potentials and solvents on the electroreduction of CO2: a comparison of theoretical approachesElectronic supplementary information (ESI) available: Bader charges for selected adsorbates as a function of potential, all geometries optimized at zero-charge in a vacuum and a shell-script to post-process VASP computations according to the correction proposed by Filhol and Neurock. See DOI: 10.1039/c5cp00946d

Since CO 2 is a readily available feedstock throughout the world, the utilization of CO 2 as a C1 building block for the synthesis of valuable chemicals is a highly attractive concept. However, due to its very nature of energy depleted "carbon sink", CO 2 has a very low reactivity. Electrocatalysis offers the most attractive means to activate CO 2 through reduction: the electron is the "cleanest" reducing agent whose energy can be tuned to the thermodynamic optimum. Under protic conditions, the reduction of CO 2 over many metal electrodes results in formic acid. Thus, to open the road to its utilization as a C1 building block, the presence of water should be avoided to allow a more diverse chemistry, in particular for C-C bond formation with alkenes. Under those conditions, the intrinsic reactivity of CO 2 can generate carbonates and oxalates by C-O and C-C bond formation, respectively. On Ni(111), almost exclusively carbonates and carbon monoxide are evidenced experimentally. Despite recent progress in modelling electrocatalytic reactions, determining the actual mechanism and selectivities between competing reaction pathways is still not straight forward. As a simple but important example of the intrinsic reactivity of CO 2 under aprotic conditions, we highlight the shortcomings of the popular linear free energy relationship for electrode potentials (LFER-EP). Going beyond this zeroth order approximation by charging the surface and thus explicitly including the electrochemical potential into the electronic structure computations allows us to access more detailed insights, shedding light on coverage effects and on the influence of counterions. Investigating in detail the intrinsic reactivity of CO 2 under aprotic conditions highlights the benefit of explicitly including the electrochemical potential into electronic structure computations together with an implicit solvent.