Carbon dioxide—a themed issue

Frederik C. Krebs

Frederik Christian Krebs received his PhD from the Technical University of Denmark (2000) and has since then worked in the field of energy conversion involving light. He is currently professor at DTU within areas of research that include development of new materials and methods for processing of thin functional films and multilayer structures with application within the areas of selective gas separation membranes, carbon dioxide extraction and conversion, resource extraction, polymer solar cells, polymer fuel cells and polymer light emitting devices.

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Carbon dioxide is a colorless and odorless gas that is present in our atmosphere. Plants consume and use it to grow with the aid of sunlight and we exhale it in our breath as exhaust from our living organisms consuming organic food. This in principle constitutes a balanced system as long as the rate of production matches the rate of generation. Mankind has however found new ways of producing carbon dioxide at excessive rates, thus tilting the balance. Atmospheric carbon dioxide levels have risen to the point where they influence our ecosystem with respect to temperature, acidification of oceans and much, much more.

Carbon dioxide as a chemical is particular in its reactivity and while it is the end product of almost all combustion processes involving organic matter it is not easily reduced. It is relatively inert while reactive towards certain metals, metal oxides, and readily dissolves in water forming carbonic acid. In spite of these peculiarities it should be a simple matter to extract carbon dioxide directly from the atmosphere and revert the atmospheric carbon dioxide level to that of the pre-industrial era. This challenge sometimes termed “the teraton challenge” is difficult for several reasons.

Firstly it is a question of scale and secondly one of distribution. In addition to those reasons are of course the peculiarities of carbon dioxide as a molecule and the caveat that it is impossible to eliminate carbon dioxide from the atmosphere without introducing more with our current methods unless the energy comes from the outside (i.e., the sun). In terms of scale, carbon dioxide is emitted on a multigigaton scale annually whereas our current capacity for handling carbon dioxide is more than an order of magnitude lower. In addition to the annual emission there is of course all the carbon dioxide that we have emitted during many past decades. Thus to revert the current state we need a capacity for converting carbon dioxide that is significantly larger than our emission.

This is where the second reason comes in. Our way of emitting carbon dioxide is highly distributed through automobiles, trains, boats, households, powerplants, iron smelters, concrete factories, etc. The latter emit enormous quantities and while it is possible to extract carbon dioxide from the flue gasses in such locations all the other sources would be impossible to implement with carbon dioxide “filters”. To handle it one would have to either make sure that the carbon dioxide is produced in a neutral way or by changing the energy source.

The current state of affairs is that we are aware of the challenge and of its scale. There are also a lot of good ideas that are being tested and the challenge is being addressed from many new angles.

This themed issue presents a selection of activities all concerned with carbon dioxide from the laboratory scale to a systematic overview. Starting from the large end, Markewitz et al. present a review on innovations within the fields of carbon capture and utilization of carbon dioxide backed by a paper presented by Middleton et al. on CO₂ capture and storage at four different scales (from the very small to the very large). The issue also contains a review on looping processes for carbon dioxide extraction by Fan et al. backed by a paper on a calcium looping process by Arias et al. and a series of contributions on the extraction of carbon dioxide in a perspective on using polymeric membranes for gas separation by Du et al. to a paper on a bipolar membrane from carbon dioxide extraction from seawater by Eisaman et al. to the sorbent based methods of carbon dioxide extraction in a communication using N-doped carbon by Xing et al. backed by a paper by Qi et al. where silica is used as a sorbent. The issue is rounded off by conversion of carbon dioxide using light in a review by Mori et al. backed by a paper on catalytic interconversion between carbon dioxide/hydrogen and formic acid at ambient temperature and pressure by Maenaka et al. The issue finally presents a review by Jessop et al. on the use of carbon dioxide to trigger solvent properties.

I would personally like to thank all the authors for their excellent contributions and would especially like to thank the editorial team at EES for the exceptionally fast response time, professionalism and positive spirit. It has been a pleasure to guest-edit this themed issue on carbon dioxide.

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