One key challenge arises when the region of suitable equilibrium binding constants is superimposed on that of oxygen sensitivity 11, 12. 1e, plotted against their reduction potential, exhibits a linear trend. The equilibrium binding of the activated form of these molecules with CO 2, shown in Fig. A subset of redox-active molecules used in the direct cycle are shown in Fig. Several reviews summarize the various chemistries employed in these cycles 5, 6, 7, 8, 9, 10, although it is useful to briefly address one of the key challenges associated with the direct cycle. Panels adapted with permission from: a, c, ref. Horizontal dashed lines indicate the minimum necessary equilibrium binding for direct air capture (blue) and flue gas (green) conditions 11. ![]() e, Equilibrium binding between the activated redox sorbents in d and CO 2 plotted against reduction potential (quinones, brown squares bipyridines, red diamonds disulfide, orange circle transition metal complexes, purple triangles). d, Typical molecules employed in the direct and semi-direct cycles. Inset graphs in a– c show the fraction of redox species active as a function of applied potential E, with the standard potential, E 0, marked by a dashed gray line. B and B + represent the dormant and activated states of the blocker, respectively. c, Indirect cycle: an electrochemically inert absorbent with a redox-active blocker species. AH represents the activated protonated state. b, Semi-direct cycle operating in protic media. A and A − represent the dormant and activated states, respectively. Substantial effort in the literature has focused on developing appropriate chemistries to avoid side reactions 5, 6, 7, 8, 9, and little work has been directed to the engineering of such systems to address the scale of the target separations in relation to emissions associated with fossil-fuel power generation natural gas treatment cement, ammonia and steel production distributed direct air capture and, recently, marine inorganic carbon extraction.Ī, Direct cycle for a generic redox-active absorbent operating in aprotic media. This Perspective focuses on the challenges faced in the development and scaled implementation of the latter class of EMCC systems using redox-active molecules. Another class of EMCC introduces an additional redox-active species (in protic or aprotic media) to modulate the total CO 2 sorption capacity. 3 and 4, respectively) to drive pH changes or react with CO 2 to swing CO 2 capacity. EMCC can be realized in aqueous media by means of water dissociation in electrolytic cation exchange (ECC) or bipolar membrane electrodialysis (BPMED), or by the chemical looping of water oxidation and reduction products (O 2 or H 2 refs. Electrochemically mediated carbon capture (EMCC), on the other hand, with sorption capacity modulated by an applied potential, offers potentially lower energetics, direct integration with high capacity factor, low-carbon-intensity energy sources, and modular scaling and ease of implementation. Traditional CO 2 capture systems modulate temperature or pressure to swing CO 2 sorption capacity 2. With atmospheric carbon dioxide (CO 2) concentrations increasing, there is an obvious need for inexpensive, scalable and low-energy CO 2 separation technologies across a range of operating conditions to mitigate the effects of climate change 1.
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