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Carbon Dioxide Recycling Makes Waves

2019; Elsevier BV; Volume: 3; Issue: 8 Linguagem: Inglês

10.1016/j.joule.2019.07.019

2542-4785

Michael B. Ross,

Carbon Dioxide Capture Technologies

Recycling CO2 into fuels and chemicals would lessen dependence on fossil fuels and help to mitigate CO2 emissions. Writing in Proceedings of the National Academy of Sciences, U.S.A., Patterson et al. propose that solar methanol islands, man-made oceanic structures that use renewable energy to harvest CO2 from seawater, perform catalysis, and generate methanol, could provide a new approach to recycling carbon dioxide into liquid fuels. Recycling CO2 into fuels and chemicals would lessen dependence on fossil fuels and help to mitigate CO2 emissions. Writing in Proceedings of the National Academy of Sciences, U.S.A., Patterson et al. propose that solar methanol islands, man-made oceanic structures that use renewable energy to harvest CO2 from seawater, perform catalysis, and generate methanol, could provide a new approach to recycling carbon dioxide into liquid fuels. As early as the 18th century, Antoine Lavoisier and Joseph Priestley identified the existence of a natural carbon cycle, where carbon sinks captured carbon dioxide (CO2) and carbon sources released CO2.1Galvez M.E. Gaillardet J. Historical constraints on the origins of the carbon cycle concept.C. R. Geosci. 2012; 344: 549-567Crossref Scopus (10) Google Scholar Anthropogenic CO2 emissions, driven primarily by fossil fuel combustion, have since distorted the natural carbon cycle, changing the Earth's climate and increasing ecological and economic risks globally. Now, considerable research effort is focused on new technologies to store, mitigate, and even reverse rising CO2 levels in the atmosphere.2Chu S. Cui Y. Liu N. The path towards sustainable energy.Nat. Mater. 2016; 16: 16-22Crossref PubMed Scopus (2473) Google Scholar, 3Ross M.B. De Luna P. Li Y. Dinh C.T. Kim D. Yang P. Sargent E.H. Designing materials for electrochemical carbon dioxide recycling.Nat. Catal. 2019; (Published online July 1, 2019)https://doi.org/10.1038/s41929-019-0306-7Crossref PubMed Scopus (533) Google Scholar, 4Montoya J.H. Seitz L.C. Chakthranont P. Vojvodic A. Jaramillo T.F. Nørskov J.K. Materials for solar fuels and chemicals.Nat. Mater. 2016; 16: 70-81Crossref PubMed Scopus (927) Google Scholar, 5Sanchez D.L. Kammen D.M. A commercialization strategy for carbon-negative energy.Nat. Energy. 2016; 1: 15002Crossref Scopus (88) Google Scholar Additionally, recent work has revealed new insights about natural CO2 sinks; for example, we now know that the Earth's oceans have been the largest carbon sink for anthropogenic emissions, absorbing an astounding 40% of CO2 emissions since the industrial age.6DeVries T. Holzer M. Primeau F. Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning.Nature. 2017; 542: 215-218Crossref PubMed Scopus (178) Google Scholar Now, researchers are asking if the ocean can be used, in part, to help artificially recycle CO2. Writing in Proceedings of the National Academy of Sciences, U.S.A., Patterson et al. describe a new strategy for carbon dioxide recycling: marine-based floating islands that combine renewable energy harvesting, CO2 capture, and heterogeneous catalysis to generate liquid fuel, namely, methanol.7Patterson B.D. Mo F. Borgschulte A. Hillestad M. Joos F. Kristiansen T. Sunde S. van Bokhoven J.A. Renewable CO2 recycling and synthetic fuel production in a marine environment.Proc. Natl. Acad. Sci. USA. 2019; 116: 12212-12219Crossref PubMed Scopus (61) Google Scholar Recycling carbon dioxide into fuels and chemicals remains a grand challenge of science and engineering. This capability could further decouple CO2 emissions from economic growth, in addition to aiding the mitigation of rising global CO2 emissions.2Chu S. Cui Y. Liu N. The path towards sustainable energy.Nat. Mater. 2016; 16: 16-22Crossref PubMed Scopus (2473) Google Scholar, 8Obama B. The irreversible momentum of clean energy.Science. 2017; 355: 126-129Crossref PubMed Scopus (635) Google Scholar At present, modern society relies on coal, natural gas, and oil to synthesize the fuels and chemicals that are essential for heating, materials, and transportation. If these feedstocks could instead be generated from CO2, particularly using renewable energy sources, net carbon dioxide emissions from these processes could be minimized, or even negative. Tremendous progress has been made over the past decade toward developing effective CO2 recycling processes.3Ross M.B. De Luna P. Li Y. Dinh C.T. Kim D. Yang P. Sargent E.H. Designing materials for electrochemical carbon dioxide recycling.Nat. Catal. 2019; (Published online July 1, 2019)https://doi.org/10.1038/s41929-019-0306-7Crossref PubMed Scopus (533) Google Scholar, 4Montoya J.H. Seitz L.C. Chakthranont P. Vojvodic A. Jaramillo T.F. Nørskov J.K. Materials for solar fuels and chemicals.Nat. Mater. 2016; 16: 70-81Crossref PubMed Scopus (927) Google Scholar Much of the research focus has been on photocatalytic (light driven) and electrocatalytic (electricity driven) processes. The former is promising for decentralized approaches to generating fuels, and the latter could provide a readily scalable approach to commodity chemical synthesis. While recent technoeconomic analyses suggest they are nearing technological relevance, both approaches require both fundamental and applied advances to reach impactful scales and performance.9De Luna P. Hahn C. Higgins D. Jaffer S.A. Jaramillo T.F. Sargent E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?.Science. 2019; 364: eaav3506Crossref PubMed Scopus (805) Google Scholar More specifically, present-day renewable-driven CO2 recycling technology does not yet achieve sufficient production rates, >1 A/cm2, stability, >1,000 s of h, efficiency, 60% electrical-to-chemical conversion, or scalability.2Chu S. Cui Y. Liu N. The path towards sustainable energy.Nat. Mater. 2016; 16: 16-22Crossref PubMed Scopus (2473) Google Scholar, 3Ross M.B. De Luna P. Li Y. Dinh C.T. Kim D. Yang P. Sargent E.H. Designing materials for electrochemical carbon dioxide recycling.Nat. Catal. 2019; (Published online July 1, 2019)https://doi.org/10.1038/s41929-019-0306-7Crossref PubMed Scopus (533) Google Scholar, 4Montoya J.H. Seitz L.C. Chakthranont P. Vojvodic A. Jaramillo T.F. Nørskov J.K. Materials for solar fuels and chemicals.Nat. Mater. 2016; 16: 70-81Crossref PubMed Scopus (927) Google Scholar, 5Sanchez D.L. Kammen D.M. A commercialization strategy for carbon-negative energy.Nat. Energy. 2016; 1: 15002Crossref Scopus (88) Google Scholar, 9De Luna P. Hahn C. Higgins D. Jaffer S.A. Jaramillo T.F. Sargent E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?.Science. 2019; 364: eaav3506Crossref PubMed Scopus (805) Google Scholar Thus, bridge technologies that take advantage of existing chemical approaches and implement them in new, energy efficient, and ideally renewably driven ways are of great importance. In their description, Patterson et al. combine existing conventional chemical processes and energy harvesting methods in a new approach to CO2 recycling. They walk through a series of considerations for the implementation of solar methanol islands (Figure 1A), including: (1) using seawater to electrochemically generate H2, (2) extracting CO2 from seawater using a series of membrane cells, (3) synthesizing methanol using a looped gas-phase heterogeneous catalysis process, (4) using photovoltaic modules on top of the island to power this process, and (5) describing marine dynamics, island placement, and reactor optimization. Thereafter, the authors provide a technoeconomic and geographic analysis and suggest that a single solar methanol island could output 15,300 tons/year of methanol. Practical and technical challenges need to be addressed before realizing a solar methanol island at a meaningful scale. That said, many of the individual CO2 conversion steps rely on previously demonstrated chemical and physical processes (Figure 1C). Electrolysis through the hydrogen evolution reaction would generate H2 from desalinated seawater, while electrodialysis would be used to extract CO2 from seawater through acidification and transport through a series of bipolar or cation-exchange membranes. The latter step is particularly noteworthy because it takes advantage of the factor 125 greater amount of CO2 in seawater (0.099 kg CO2/m3) compared to ambient air (0.00079 kg/m3), avoiding the challenge of direct atmospheric CO2 capture.7Patterson B.D. Mo F. Borgschulte A. Hillestad M. Joos F. Kristiansen T. Sunde S. van Bokhoven J.A. Renewable CO2 recycling and synthetic fuel production in a marine environment.Proc. Natl. Acad. Sci. USA. 2019; 116: 12212-12219Crossref PubMed Scopus (61) Google Scholar For catalyzing reaction (1),3H2+CO2 → CH3OH + H2O,(1) a previously demonstrated process using Cu/ZnO/Al2O3 as a catalyst is suggested in a looped (multi-cycle) plug flow reactor to achieve sufficient CO2 conversion. The energetic considerations are broad in scope—the authors suggest that photovoltaics on a 100-m-diameter island could produce 0.35MWaverage to power these onboard processes. Alternatively, they also suggest that offshore wind could also serve as a power source. Further analysis of how a diverse energy mixture could contribute would be beneficial. It would also be useful to compare the energetics of their described chemical process with direct electrochemical catalysis in seawater. In principle, electrochemical catalysis could be used to directly synthesize a liquid fuel3Ross M.B. De Luna P. Li Y. Dinh C.T. Kim D. Yang P. Sargent E.H. Designing materials for electrochemical carbon dioxide recycling.Nat. Catal. 2019; (Published online July 1, 2019)https://doi.org/10.1038/s41929-019-0306-7Crossref PubMed Scopus (533) Google Scholar, 4Montoya J.H. Seitz L.C. Chakthranont P. Vojvodic A. Jaramillo T.F. Nørskov J.K. Materials for solar fuels and chemicals.Nat. Mater. 2016; 16: 70-81Crossref PubMed Scopus (927) Google Scholar or used to generate synthesis gas, H2 and CO,10Ross M.B. Li Y. De Luna P. Kim D. Sargent E.H. Yang P. Electrocatalytic rate alignment enhances syngas generation.Joule. 2019; 3: 1-8Abstract Full Text Full Text PDF Scopus (48) Google Scholar in the correct ratio for subsequent thermochemical methanol production. Many of the barriers to solar methanol island realization are at the systems level. What materials, both catalytic and photovoltaic, as well as structural, are resilient and corrosion-resistant enough to survive the marine environment while maintaining adequate performance? Aside from the ecological concern of marine chemical spills, would the local alkalization of seawater, due to proton consumption and CO2 extraction, disrupt marine life? Would this approach be economically competitive with a land-based system that relied on direct-atmospheric CO2 capture, water splitting, and gas-phase thermochemical methanol production? In the most optimistic scenario described by the authors—solar methanol islands that cover 1.5% of the total ocean area at optimal locations (Figure 1B)—it is suggested that 12 gigatons of carbon emissions per year could be avoided, which would exceed the total global emissions from fossil fuels. Claims such as this will require more rigorous modeling and more context within the fossil fuel and CO2 emissions domains. This includes comparison with state-of-the-art carbon capture and storage (CCS) technologies in terms of both economic feasibility and scalability.5Sanchez D.L. Kammen D.M. A commercialization strategy for carbon-negative energy.Nat. Energy. 2016; 1: 15002Crossref Scopus (88) Google Scholar There is also considerable uncertainty in the technoeconomic analysis, which relies on variable assumptions regarding future oil and methanol prices, capital costs, and overhead for constructing solar methanol islands.9De Luna P. Hahn C. Higgins D. Jaffer S.A. Jaramillo T.F. Sargent E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?.Science. 2019; 364: eaav3506Crossref PubMed Scopus (805) Google Scholar This work proposing the realization of solar methanol islands illustrates the potential for combining catalysis, energy harvesting, and engineering to recycle CO2 in new and innovative ways. Ambitious conceptual proposals such as this one will challenge researchers, industry, and governments alike to consider novel solutions to climate challenges. They will also challenge the field with new questions. For example, how would production on a solar methanol island be taxed, regulated, and domiciled? Grappling with concepts and questions like these will drive us toward technological solutions to climate change. Anthropogenic climate change and CO2 emissions mitigation is a global challenge, which will require global solutions. Faring to the sea—approximately 70% of the Earth's surface—for new approaches is representative of the ambitious innovations needed to realize CO2 recycling at scale.