Environmentally Friendly and Hostile Water and Aqueous Solutions

The IPCC's 5th assessment report has heightened the urgency of steep reductions in atmospheric CO2 concentrations. The transportation sector contributes 22% of global CO2 emissions from fuel combustion. Solar-to-fuel technologies thus offer tremendous potential to contribute significantly to such reductions. Engineering offers a multitude of pathways to produce liquid hydrocarbon fuels starting from CO2 and H2O. In these pathways, downstream processes such as fuel synthesis are generally well understood and often commercially available. In contrast, upstream processes such as the dissociation of CO2 and H2O are usually less mature (exception: low temperature water electrolysis). Here, we first analyze recent research on various CO2 and H2O dissociation technologies and then assess their future potential to be used at scale. The dissociation technologies range from thermolysis, to thermochemical cycling, low and high temperature electrolysis, photo-electro-chemical splitting, and artificial photosynthesis. In addition to the state-of-the-art of these technologies, we compare their inherent disadvantages and advantages as well as key future R&D requirements. For completion, other steps of the pathways, namely CO2 capture, providing water, and fuel synthesis, are addressed briefly as well. We conclude with a discussion of the relative future potential of the various pathways and an R&D outlook.