CO2 conversion to solar fuels and chemicals: Opening the new paths

Abstract

This future article discusses the new prospects and directions of CO2 conversion via the photo-electro-catalytic (PEC) route. The second (2nd) generation solar fuels and chemicals (SFs) are generated directly in PEC systems via electrons/protons reactions without forming molecular H2 as an intermediate, overcoming the thermodynamics limitations and practical issues encountered for electro-fuels produced by multistep thermocatalytic processes (i.e. CO2 conversion with H2 coming from water electrolysis). A distributed and decentralized production of SFs requires very compact, highly integrated, and intensified technologies. Among the existing reactors of advanced design (based on artificial leaves or photosynthesis), the integrated photovoltaic plus electrocatalytic (PV-EC) device is the only system (demonstrated at large scale) to produce SFs with high solar-to-fuel (STF) efficiency. However, while the literature indicates STF efficiency as the main (and only) measure of process performance, we remark here the need to refer to productivity (in terms of current density) and make tests with reliable flow PEC systems (with electrodes of at least 5–10 cm2) to accelerate the scaling-up process. Using approaches that minimize downstream separation costs is also mandatory. Many limitations exist in PEC systems, but most can be overcome by proper electrode and cell engineering, thus going beyond the properties of the electrocatalysts. As examples of current developments, we present the progress of (i) artificial leaf/tree devices for green H2 distributed production and (ii) a PEC device producing the same chemicals at both cathode and anode parts without downstream operations for green solvent distributed production. Based on these developments, future directions, such as producing fertilizers and food components from the air, are outlined. The aim is to provide new ideas and research directions from a personal perspective.