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Tandem and Hybrid Processes for Carbon Dioxide Utilization

2021; Elsevier BV; Volume: 5; Issue: 1 Linguagem: Inglês

10.1016/j.joule.2020.12.004

2542-4785

Sean Overa, Tony G. Feric, Ah‐Hyung Alissa Park, Feng Jiao,

Carbon dioxide utilization in catalysis

Graphical AbstractView Large Image Figure ViewerDownload Hi-res image Download (PPT)Graphical AbstractView Large Image Figure ViewerDownload Hi-res image Download (PPT)Graphical AbstractView Large Image Figure ViewerDownload Hi-res image Download (PPT) As fossil fuels continue to dominate the energy portfolio, the atmospheric carbon dioxide (CO2) concentration has exceeded 400 ppm, posing a major threat to our environment. Conventional chemical processes utilize fossil carbon sources to produce chemicals and fuels, which inevitably emit a tremendous amount of the greenhouse gas CO2. To realize sustainable chemical production and to minimize environmental impacts, CO2 captured from industrial sources and ambient air can serve as an alternative carbon source. A variety of CO2 conversion technologies, including thermochemical, biological, and electrochemical approaches, are currently under development. Thermochemical hydrogenation processes are capable of converting CO2 into single carbon (C1) products such as methane, methanol, and carbon monoxide. These technologies have high technology readiness levels (TRL) compared to the electrochemical and biological approaches (Figure 1A) but use hydrogen primarily derived from methane through steam reforming, a process that emits CO2.1Jarvis S.M. Samsatli S. Technologies and infrastructures underpinning future CO2 value chains: A comprehensive review and comparative analysis.Renew. Sustain. Energy Rev. 2018; 85: 46-68Crossref Scopus (94) Google Scholar Biological approaches, such as artificial photosynthesis and algae growth, suffer from high operational costs but are capable of producing long-chain C2–C6 products with high selectivity.2Roh K. Bardow A. Bongartz D. Burre J. Chung W. Deutz S. Han D. Heßelmann B. Kohlhaas Y. König A. et al.Early-stage evaluation of emerging CO2 utilization technologies at low technology readiness levels.Green Chem. 2020; 22: 3842-3859Crossref Google Scholar Electrochemical approaches, such as CO2 electrolysis, have the unique advantage of being able to operate using solely renewable electricity. With renewable electricity becoming steadily more available and affordable, CO2 electrolysis can effectively produce C1 and C2+ products directly from CO2 at rapid and cost-effective rates. Recent techno-economic analyses have found that the CO2 electrolysis products formic acid, acetic acid, and ethylene are all very close to being economically viable, given the current trajectory of renewable electricity prices.3Jouny M. Lv J.J. Cheng T. Ko B.H. Zhu J.J. Goddard 3rd, W.A. Jiao F. Formation of carbon-nitrogen bonds in carbon monoxide electrolysis.Nat. Chem. 2019; 11: 846-851Crossref PubMed Scopus (50) Google Scholar As a result, significant research should be invested in CO2 electrolysis technologies to improve performance and deploy them for chemical manufacturing. The performance of CO2 electrolyzers has been improved significantly with the operational current density (reaction rate) approaching 1 A cm-2.4Jeng E. Jiao F. Investigation of CO2 single-pass conversion in a flow electrolyzer.React. Chem. Eng. 2020; 5: 1768-1775Crossref Google Scholar However, CO2 electrolyzers still suffer from several practical issues which stem from the reliance of CO2 electrolyzers on alkaline electrolyte to achieve high reaction rates. The alkaline electrolyte together with locally generated hydroxide anions inevitably reacts with fed CO2 to form carbonates at the electrode-electrolyte interface, which not only causes flooding issues for the electrodes but also results in low single-pass conversion of CO2 toward desired products. Furthermore, if anion exchange membranes (AEM) are used, the carbonate (and bicarbonate) anions will cross the membrane and reach the anode chamber, requiring additional recycling and separation steps to be recovered as well as producing unwanted interactions with the anode. This also has the added issue of limiting CO2 conversion to < 50% toward desired products.4Jeng E. Jiao F. Investigation of CO2 single-pass conversion in a flow electrolyzer.React. Chem. Eng. 2020; 5: 1768-1775Crossref Google Scholar Bipolar membrane (BPM) CO2 electrolyzers could address this issue, as they do not suffer from the same crossover issues as AEM electrolyzers.5Li Y.C. Zhou D. Yan Z. Gonçalves R.H. Salvatore D.A. Berlinguette C.P. Mallouk T.E. Electrolysis of CO2 to Syngas in Bipolar Membrane-Based Electrochemical Cells.ACS Energy Lett. 2016; 1: 1149-1153Crossref Scopus (143) Google Scholar However, BPM-based electrolyzers suffer from high cell voltages and are still in the early stage of their development (TRL 2-3). High-temperature CO2 electrolyzers, using solid oxide (ceramic) based electrolytes, are another potential route for direct CO2 electroreduction. These systems are mature (TRL 8) and have been demonstrated with thousands of hours of stability.6Küngas R. Review-Electrochemical CO2 Reduction for CO Production: Comparison of Low- and High-Temperature Electrolysis Technologies.J. Electrochem. Soc. 2020; 167: 044508Crossref Scopus (75) Google Scholar Since solid oxide electrolyzers do not rely on alkaline electrolytes to convert CO2 to CO, carbonate formation is not an issue. However, these systems are only capable of directly converting CO2 to CO. Solid oxide electrolyzers also suffer from coke formation at high CO2 conversion, necessitating recycle loops and separation steps for the production of concentrated CO product streams. Both low-temperature and high-temperature routes for CO2 electrolysis suffer from significant limitations. The primary challenges of the two systems are that low-temperature CO2 electrolysis is limited to < 50% conversion of CO2 to products, while high-temperature electrolysis is limited to solely producing CO. To overcome these issues, tandem and hybrid processes (Figure 1B) including tandem electrocatalytic, tandem electrocatalytic-biological, tandem electrocatalytic-thermocatalytic, and reactive capture (coupling carbon capture with electrocatalytic conversion) processes are all potential options. Here, we present a brief discussion of each route to provide insights and stimulate more interest in the integration and hybridization of CO2 electrolysis for practical applications as well as possible future research directions. The CO2 electrolysis process can be conducted in two consecutive steps: CO2 reduction to CO in a non-alkaline environment and then CO reduction to C2+ products in an alkaline electrolyte (Figure 2A). CO2-to-CO in non-alkaline conditions benefits from reduced formation of carbonate species as well as > 90% selectivity toward CO.4Jeng E. Jiao F. Investigation of CO2 single-pass conversion in a flow electrolyzer.React. Chem. Eng. 2020; 5: 1768-1775Crossref Google Scholar This CO will then be fed to a CO electrolyzer, which has recently been demonstrated to be capable of reaching > 90% selectivity toward C2+ products as well as > 1 A/cm2 operating current densities.8Jouny M. Luc W. Jiao F. High-rate electroreduction of carbon monoxide to multi-carbon products.Nat. Catal. 2018; 1: 748-755Crossref Scopus (186) Google Scholar Moreover, the CO electrolyzer also showed a unique ability to produce acetate selectively using a Cu nanosheet catalyst (Figure 2B).9Luc W. Fu X. Shi J. Lv J.J. Jouny M. Ko B.H. Xu Y. Tu Q. Hu X. Wu J. et al.Two-dimensional copper nanosheets for electrochemical reduction of carbon monoxide to acetate.Nat. Catal. 2019; 2: 423-430Crossref Scopus (142) Google Scholar This is a significant improvement in selectivity toward C2+ products compared to direct CO2 electrolysis, especially when carbonate formation is taken into account. Currently, CO2 electrolyzer and CO electrolyzer have TRLs of 4 and 3, respectively. The feasibility of two-step tandem CO2 electrolysis has been demonstrated experimentally by Romero Cuellar and coworkers, who reported a total CO2 reduction current density of 300 mA/cm2 with a cumulative Faradaic efficiency of 62% toward C2+ products.10Romero Cuellar N.S. Scherer C. Kaçkar B. Eisenreich W. Huber C. Wiesner-Fleischer K. Fleischer M. Hinrichsen O. Two-step electrochemical reduction of CO2 towards multi-carbon products at high current densities.J. CO2 Util. 2020; 36: 263-275Crossref Scopus (11) Google Scholar While this is an excellent proof of concept, the system is far from optimized when compared to standalone CO2 and CO electrolysis technologies. Low CO2 conversion in the first reactor led to a scrubber being necessary to remove excess CO2 before the CO electrolyzer, vastly reducing the conversion of fed CO2 to C2+ products. A CO2/CO membrane separator could provide a more sustainable option for product stream purification, as CO2 would not have to be recovered from the scrubber after capturing, thus allowing for steady-state operation. Even with a CO2/CO separator, a significant improvement in single-pass conversion of CO2 is still necessary. Most CO2 research now operates at extremely low single-pass conversion of CO2, which is not industry viable. Future research efforts should focus on maximizing CO2 conversion to CO to minimize the energy requirement for separating unreacted CO2 from the product stream. A potential route for this could be a renewed focus on CO2 delivery to the catalyst surface. Improvements in flow cell design and catalyst layer design (i.e., porosity) could significantly improve CO2 conversion by allowing a higher percentage of fed CO2 to be reacted. Even with a 100%-efficient CO2 electrolyzer, CO2 conversion to desired products will always be limited to 50% in a low-temperature CO2 electrolyzer.4Jeng E. Jiao F. Investigation of CO2 single-pass conversion in a flow electrolyzer.React. Chem. Eng. 2020; 5: 1768-1775Crossref Google Scholar This issue could be addressed by utilizing CO produced from more mature renewable technologies such as solid oxide electrolysis (TRL 7–8) or the reverse water-gas shift reaction (TRL 6). This would also decrease the risk associated with CO electrolysis, as the CO feed would be from a well-established process. However, these processes will require more complicated integration due to the large differences in operating conditions between the high-pressure and -temperature thermal reactors and the low-temperature and -pressure CO electrolyzer. Improvements on current CO electrolysis technologies, such as increased selectivity toward a single product and long-term stability, will be necessary before it can be considered to be coupled with these mature reactors. Research efforts should be focused on first improving CO electrolysis performance and then on the coupling of the CO electrolyzer with these mature technologies. A major challenge of CO2 electrolysis is the production of chemicals containing more than two carbon atoms per molecule in a selective manner. In comparison, biological carbon utilization technologies can selectively produce valuable C4–C6 carboxylic acids and their corresponding alcohols as biofuel precursors. However, biological processes often suffer from slow production rates and the complexity of co-culturing multiple organisms. To improve upon these rates, a hybrid approach that combines easily scalable CO2 electrolysis and biological upgrading could be utilized. Recently, the feasibility of the electrocatalytic-biological hybrid process was reported, where CO2 and H2O were electrocatalytically converted into synthesis gas (i.e., a mixture of CO and H2) followed by a biological upgrading process to produce butanol and hexanol using a bacteria solution containing C. autoethanogenum and C. kluyveri.11Haas T. Krause R. Weber R. Demler M. Schmid G. Technical photosynthesis involving CO2 electrolysis and fermentation.Nat. Catal. 2018; 1: 32-39Crossref Scopus (238) Google Scholar The production rate of butanol and hexanol by these microorganisms is significantly boosted in the presence of CO, as compared to CO2 and H2 alone. Future efforts in this area will be required to address the solubility issue of synthesis gas in the aqueous media of the biological reactor. An alternative CO2 reduction product, formate, has also been investigated, which overcomes these solubility limitations. However, only a select few bacteria are capable of using formate as their carbon source, emphasizing the need to find a compatible and soluble CO2 electrolysis product that can be further upgraded through biological means. The CO2 electrolysis process can also be coupled with a thermocatalytic process to produce chemicals that cannot be obtained by CO2 electrolysis alone. For instance, the synthesis gas produced through the CO2 electrocatalytic process could be fed into the Fischer-Tropsch process for synthetic fuel productions. This process can be done most efficiently by coupling Fischer-Tropsch with a high-temperature solid oxide CO2 electrolyzer and solid oxide water electrolyzer.12van Bavel S. Verma S. Negro E. Bracht M. Integrating CO2 Electrolysis into the Gas-to-Liquids-Power-to-Liquids Process.ACS Energy Lett. 2020; 5: 2597-2601Crossref Scopus (14) Google Scholar With similar operating conditions and high maturity, this system could be rapidly deployed to sustainably convert CO2 into liquid fuels. Instead of coupling two carbon conversion steps in a tandem and/or hybrid process, a reactive carbon capture approach can be employed to combine a CO2 capture step with a CO2 conversion step. In this concept (outlined in Figure 3), once the CO2 capture materials bind to CO2, they do not release CO2 through an energy-intensive desorption process, but rather act as an electrolyte additive to carry the captured CO2 molecules to the catalytic sites for the subsequent electrochemical CO2 reduction. After the conversion step, the CO2 capture materials are regenerated for the next CO2 capture cycle. By doing so, the overall energy demand for CO2 capture and conversion could be significantly reduced. For conventional amine-based solvents that capture CO2 through the exothermic formation of carbamate species (ΔH = −80 kJ/mol), the resulting carbamates are not electrochemically active. Owing to the high CO2 capture capacities and charged nature, novel materials such as nanoparticle organic hybrid materials and ionic liquids have been suggested as attractive candidates for reactive carbon capture. They can be designed via CO2 binding energy optimization. Recent studies have shown that these novel electrolyte materials can improve CO2 conversion rates as well as product distributions.13Verma S. Lu X. Ma S. Masel R.I. Kenis P.J.A. The effect of electrolyte composition on the electroreduction of CO2 to CO on Ag based gas diffusion electrodes.Phys. Chem. Chem. Phys. 2016; 18: 7075-7084Crossref PubMed Google Scholar For example, Park and coworkers showed that the reactivity of nanoparticle organic hybrid materials bound to CO2 is distinct from free CO2, illustrating the potential co-catalytic role of nanoparticle organic hybrid materials during electrochemical CO2 conversion.14Yu W. Wang T. Park A.A. Fang M. Review of liquid nano-absorbents for enhanced CO2 capture.Nanoscale. 2019; 11: 17137-17156Crossref Google Scholar Further research is required in this direction to gain a better fundamental understanding of the properties of these electrolyte materials and to develop integrated reactive systems that are able to capture CO2 and in situ convert it to desired products in an energy-efficient way. Tandem and hybrid processes for CO2 utilization are promising future directions that have great potential in creating alternative routes in sustainable chemical manufacturing and renewable energy storage sectors. However, the present readiness level of the electrochemical components, besides solid oxide electrolysis, is relatively low. As a result, significant work needs to be done to address critical challenges such as long-term stability, product selectivity, and cost-competitiveness over existing industrial processes. Combining CO2 electrolysis, a low TRL process, and a more mature process (e.g., thermochemical CO2 hydrogenation and high-temperature CO2 electrolyzer) could substantially accelerate the research and development process to reach a high TRL. Additionally, the tandem and hybrid strategy also creates new opportunities and flexibility to target more valuable products that cannot be produced through a single-step process. S.O. and F.J. thank the National Science Foundation for financial support (Award No. CBET- 1803200 ). The Columbia team is supported as part of the Breakthrough Electrolytes for Energy Storage (BEES), an Energy Frontier Research Center funded by the U.S. Department of Energy , Office of Science, Basic Energy Sciences under Award No. DE-SC0019409 (exploration of secondary fluid on the physical properties of novel electrolyte) and Shell’s New Energy Research and Technology (NERT) Program (CO2 capture and conversion using NOHMs). A.-H.A. Park is an Advisory Board member of Joule.