Metal-free carbon-based materials for electrocatalytic and photo-electrocatalytic CO2 reduction

Nowadays, reducing carbon dioxide emission in the atmosphere is one of the most important environmental issues that must be overcome. At the same time, low-cost and environmentally friendly technologies are necessary to produce renewable fuels able to replace the conventional fossil ones. Electrochemical cells (driven by solar energy) and photo-electrochemical cells (PECs) are among the main efficient technologies to get these challenging goals. Taking into account the PEC working mechanism, two different electrodes, based on photo-electrocatalytic and electrocatalytic materials able to drive reactions both under illumination or in dark conditions, are involved. In this review, recent results on carbon-based materials for electrocatalytic and photo-electrocatalytic carbon dioxide reduction are discussed. The properties and synthesis conditions applied to the preparation of conducting polymer and graphitic carbon nitride (g-C3N4) are described and discussed for their application in the photoactive electrodes. As for the electrodes to be applied in the electrocatalytic CO2 activation and conversion, light heteroelement-doped carbon nanomaterials have been taken into account as highly valuable metal-free candidate to run the process efficiently and selectively. For the latter process, also the influence of the electrolyte and the selectivity towards different reaction products will be discussed. All these data taken together indicate that a lot of work still has to be done to achieve high efficiency with metal-free organic-based electro- and photo-electrocatalysts applied to the carbon dioxide conversion. Anyhow, many seminal outcomes collected in the literature up to now clearly indicate the real possibility to replace highly costly metal-based materials with simply organic ones. © 2019, Accademia Nazionale dei Lincei.