Available online at www.sciencedirect.com Current Opinion in ScienceDirect Electrochemistry Review Article Use of CO2 for electrosynthesis Silvia Mena, Jose Peral and Gonzalo Guirado Abstract average atmospheric CO2 concentration of 421 ppmv A review of the main ideas related to the capture and valori- [1], the highest in the last 800,000 years [2e4]. zation of CO2 by using electrochemical processes is presen- ted. The advantages of using an electrode-controlled potential Atmospheric CO2 levels can be reduced by replacing the that provides the possibility of reducing CO2 to non-fossil current massive consumption of fossil fuels with the use carbon-based fuels, or of using this atmospheric pollutant in of alternative sources of energy, but also by capturing organic synthesis processes that rely on electrocarboxylation and removing the molecule from the atmosphere while, steps are outlined. The implications of a reduction in atmo- at the same time, finding ways for its chemical valori- spheric CO2 emissions in the global climate change emer- zation [5]. Several means of atmospheric CO2 removal, gency, while at the same time finding alternative and clean such as reforestation, soil carbon sequestration in crop- ways of producing valuable fuels and chemicals, are empha- lands and grasslands, carbon capture and storage [CCS], sized. Particular attention is paid to the powerful role that ionic enhanced weathering (alkalinity enhancement), ocean liquids can play as ideal solvents and electrolytes in such fertilization, are currently being proposed and/or tested electrochemical reactions. A general description of the ad- [6,7], although a clear worldwide use of any of them is vantages that these liquids have for the solubilization and the not yet underway. support of the electrochemical reduction of CO2 is presented. Post-combustion CCS technologies of flue gases in Addresses traditional gas or coal-fired power plants or in industrial Departament de Química, Universitat Autònoma de Barcelona, Bella- facilities such as cement kilns, or iron and steel pro- terra Barcelona, 08193 Spain duction can be visualized as chain technologies created Corresponding author: Guirado, Gonzalo (gonzalo.guirado@uab.cat) to: i) separate CO2 from industrial gas sources, ii) transport it to a storage location and iii) secure long- term isolation from the atmosphere [8e10]. Current Opinion in Electrochemistry 2023, 42:101392 This review comes from a themed issue on Organic and Molecular Currently available CO2 capture strategies Electrochemistry (2023) Chemical absorption by aqueous amine-based solvents Edited by Cyrille Costentin appears to be the most promising short-term solution for For a complete overview see the Issue and the Editorial CO2 capture [11]. Large-scale processes using an Available online 12 September 2023 aqueous solution of monoethanolamine (MEA) and https://doi.org/10.1016/j.coelec.2023.101392 diethanolamine (DEA) with overall efficiencies above 90% already exist. Once captured, the CO2 is liberated 2451-9103/© 2023 The Authors. Published by Elsevier B.V. This is an o p e n a c c e s s a r t i c l e u n d e r t h e C C B Y- N C l i c e n s e ( h t t p : / / in higher concentrations by steam stripping. This kind creativecommons.org/licenses/by-nc/4.0/). of process is called amine scrubbing [12] (Figure 1). Unfortunately, implementation of these CO2 capture Keywords: CO2 capture, CO2 valorization, Ionic liquids, Electrocarboxylation, systems has been limited due to the costs associated Electrochemical reduction of CO2. with the high energy consumption during amine regen- eration, as well as to solvent degradation, and corro- sion [13]. Introduction As the increasing concentration of greenhouse gases in Ionic liquids (ILs) have been proposed as viable alter- the atmosphere is triggering a global climate change, a natives to amines as CO2 absorbers [14,15]. Indeed, clear need exists to control the level of atmospheric ionic liquids have extremely low volatility and many of carbon dioxide, by far the most important anthropogenic them can solubilize large amounts of carbon dioxide. pollutant with greenhouse effects. CO2 accumulated in Ramdin et al. [14] have provided a very extensive review the atmosphere at an average rate of 1.4 ppmv per year of the advantages and problems of using ionic liquids for between 1959 and 2006 and roughly 2.0 ppmv per year the purpose of capturing carbon dioxide. They are also between 2006 and 2020, with a current (March 2023) electrical conductors, and ideal absorbers/solvents for www.sciencedirect.com Current Opinion in Electrochemistry 2023, 42:101392 2 Organic and Molecular Electrochemistry (2023) Figure 1 CO2 capture from flue gas by an aqueous amine system. the electrochemical capture and transformation of CO2 produced from CO2. Hydrogenation of CO2 offers the described in this review [15]. possibility of recycling CO2, storing H2, producing a liquid fuel, and storing excess green electric energy Metal organic frameworks (MOFs) are also a most (e.g., solar, wind, biomass) by transforming it into non- promising class of adsorbents, especially for gas separa- fossil carbon-based fuel sources that help reduce the tion applications. MOFs have extraordinary surface use of fossil fuels [21e23]. areas, ultra-high porosity, and flexibility to tune their porous structure and their surface functionalities due to In addition to synthetic fuels, CO2 can be used as a the potential modification of their organic ligands. Many feedstock to produce a large array of fine chemicals, the MOFs have carbon dioxide capture capabilities ranging most important applications being the production of from 4 to 26 mmol g1 [16e18], although they also urea, inorganic carbonates, polyurethane, acrylic acid present several drawbacks like low thermal stabilities, and acrylates, polycarbonates and alkylene carbon- the use of complex and costly ligands, and the diffi- ates [24,25]. culties of large-scale production. The present review provides a description of the state of Of all the different CO2 capture approaches, the bio- the art of a group of new CO2 capture processes based on logically based one seems to be especially attractive the use of electrochemical reactions that reduce CO2 to since CO2 can be converted by photosynthesis into other carbon-containing molecules with potential ben- organic matter by using sunlight as a source of energy. In efits. The great features of electrochemistry as a method particular, algae can convert CO2 into organic com- of synthesis of materials, being a clean, powerful and pounds more efficiently than other terrestrial plants, easy-to-control way of carrying out chemical processes, being a promising source of biofuels or food. This are highlighted, and its potential for a large-scale alternative, while valuable, still requires further research effective removal of atmospheric CO2 is emphasized. and development [19,20]. Electrochemical valorization of CO2 CO2 as feedstock for production of fuels and Electrochemical reduction of CO2 in aqueous and chemicals non-aqueous conventional electrolytes CO2 conversion into fuels is considered the best alter- Electrochemical reduction or activation of CO2 can native in CO2 utilization. Methane, methanol, syngas, produce a number of valuable products, such as oxalic and alkanes are some of the compounds that can be acid, CO, formic acid, methane, methanol, ethane, Current Opinion in Electrochemistry 2023, 42:101392 www.sciencedirect.com Use of CO2 for electrosynthesis Mena et al. 3 ethylene, ethanol, as well as other hydrocarbons The second strategy mentioned above would be the use and oxygenates. of non-aqueous solvents as electrolytes. Organic solvents usually dissolve gases much better than water, and in The electrochemical reduction of CO2 usually requires inert electrodes the CO2 reduction products are high overpotential (h > 1), due to the energy re- normally oxalate and CO when the solvent is dry, quirements of the single electron reduction of CO2 into although the presence of water promotes the appear- the CO2-$ anion radical, which depend on electrolytic ance of formate [37]. The reduction of CO2 begins with media and electrode. With gold electrodes, the value of the formation of the carbon dioxide radical anion, which electrochemical carbon dioxide reduction occurs can dimerize leading to a C2þ product, oxalate, avoiding at 1.9 V (vs standard hydrogen electrode, SHE), proton-assisted multiple-electron-transfer processes determined in 0% of water content electrolyte media (Figure 2, equation 1) [26,38,39]. Depending on the [26]. The electrochemical CO2 reactivity and selectivity experimental conditions as well as the cathode material obtained is strongly dependent on the type of electrodic a primary competitive reaction is through a nucleophilic material and solvent employed, so highly active and coupling radical anion CO2$- with CO2 (ads), leading to selective electrocatalyst are necessary to reduce the CO and CO32 (Figure 2, equation 3-4). Hence, in the activation energy of the reaction and to enable an case of Fe, Cr, Mo, Ni and Ti electrodes substantial energetically efficient process. amounts of oxalic acid and CO are obtained. For Ni, Pd, Pt, Cu, Ag, Au, Cd and Sn cathodes the main product In aqueous environments, it is reported that the target obtained is CO; whereas C2 product is yielded for Pb, products of the CO2 electrochemical reduction pro- Tl, and Hg electrodic materials [40]. cesses can be tuning depending on the nature of the electrode, being mainly formate/HCOOH derivatives on On the other hand, another approach related to the Cd, In, Sn and Pb; CO on Au, Ag, Pd, and Zn; and al- conversion of CO2 to high value-added molecules, in cohols and more valuable hydrocarbon on Cu cathodes aprotic organic solvents, considers its incorporation in [27e30]. The relationship between the binding en- larger chemical structures by using electrochemistry. In ergies of key intermediates and the free energy of the that respect, electrochemical carboxylation is one of the catalyst surface determines the main product yielded. most useful methods for CO2 fixation to organic mole- The use of fully or partially oxidized metals can also cules, because it is a mild and easy-to-handle pro- improve selectivity and activity under more favorable cess [41]. conditions, such as low overpotential [27,30]. Finally, the use of electrocatalyst nanostructures affects the Electrochemical carboxylation of CO2 in organic CO2 reduction reaction activity and product selectivity aprotic electrolytes by modifying the morphology, size, shape, and grain The general mechanism of electrochemical carboxylation boundary [31e33]. In this sense, the design of those involves the reduction of a substrate, and/or CO2, leading nanostructured cathodes can also incorporate different to the formation of the corresponding radical anion, and metals for improving its performance [29]. the reaction between the radical anion and the substrate (or the CO2) to render the carboxylate anion [41]. Another drawback in progress of carbon dioxide elec- Figure 3a shows different types of substrates that feature troreduction is the low solubility of carbon dioxide in carbonecarbon double bonds and that can be employed aqueous electrolytes (c.a. 33 mM at normal room tem- in the synthesis of the corresponding carboxylic acid perature and pressure) [34]. There are two types of through an electrocarboxylation process [42]. strategies have been developed to enhance the effi- ciency of the process; (i) the utilization of gas diffusion By using a one-compartment cell equipped with a Pt electrodes (GDEs), as already used in fuel-cell tech- cathode and an Mg anode in DMF, Senboku et al. have nology, and (ii) the use of non-aqueous electrolytes. successfully accomplished the electrochemical carbox- ylation of several flavones at the C2-position, producing GDEs are porous electrodes with a catalyst layer in flavanone-2-carboxylic acids with moderate to good contact with the electrolyte. Due to the high CO2 mass yields [43]. transport and reduced diffusion lengths within the catalyst layer, GDEs can achieve current densities Following the achievements obtained in the electro- higher than those of traditional electrodes [35]. Recent carboxylation of styrene derivatives [44], Jiang et al. developments in GDE technology have increased have studied the electrochemical carboxylation of carbon dioxide reduction efficiencies. Ma et al. incor- phenylacetylene with CO2 [45]. An unsaturated aryl- porated multi-walled carbon nanotubes (MWCNT) in maleic anhydride was the main product under anhy- the Ag catalyst layer of gas diffusion electrodes and re- drous conditions, while saturated 2-arylsuccinid acid ported a current density up to 350 mA cm2, with 95% could also be obtained as a major product in the pres- faradaic efficiencies towards the formation of CO [36]. ence of H2O. www.sciencedirect.com Current Opinion in Electrochemistry 2023, 42:101392 4 Organic and Molecular Electrochemistry (2023) Figure 2 Proposed mechanism of CO2 electroreduction in organic electrolytes. Organic halide electrocarboxylation has also been capture or to tune the CO2 reactivity [52] after a SeS or widely studied, using silver in the electrochemical CeS electrochemical bond cleavage reaction, respec- reduction of arylethyl chlorides [46], or showing that tively. Note that more sophisticated carboxylated the direct CO2 reduction at Ag and Cu working elec- products, such as pharmaceutical derivatives [53e56] or trodes occurs at more negative potentials than the molecular switches [57] can be obtained following reduction potentials of a series of bromobenzenes similar routes. Finally, it is worthy to remark that there (ArBr), allowing the selective reduction of ArBr in are also other kind of molecules which can be used as CO2-saturated DMF [47e49]. It was also shown that substrates for electrocarboxylations reactions, such as hydrodebromination yielding ArH is always in pseudo-halides, imines, ketones and aldehydes. In this competition with the desirable carboxylation reac- sense, Zhang et al. summarized recent advances in tion (Figure 3b). electrochemical carboxylation based on those substrates for the synthesis of fine chemicals [58]. For halobenzonitriles, after a four-electron reduction process and two electrocarboxylic processes, it is Nevertheless, the volatility and toxicity [59], the low possible to carry out the friendly synthesis of phthalate electric conduction of aprotic solvents and the addition derivatives that can potentially be used for making of large quantities of supporting electrolytes represent plastic polymers [50]. The use of benzyldisulfide [51] an obstacle for their use as solvents in many electro- and benzyl phenyl thioethers also allows to reversibly chemical reactions. Current Opinion in Electrochemistry 2023, 42:101392 www.sciencedirect.com Use of CO2 for electrosynthesis Mena et al. 5 Figure 3 a) Various examples of olefins for the electrocarboxylation process b) Electrochemical synthesis of benzoic acid derivatives in DMF using different cathodes through electrocarboxylation processes. Electrochemistry of CO2 in ionic liquids: capture and them an attractive choice of solvent in many important electrocarboxylation processes chemical processes [15,60]. In order to avoid the use of volatile and toxic solvents, and simplify separation, electrocarboxylation in CO2- Room-temperature ionic liquids (RTILs) have received saturated room-temperature ionic liquids have been a lot of attention as potential “green” and “custom- developed. Ionic liquids (ILs), as electrically con- izable” solvents. RTILs have better properties than ducting “organic” solvents which have moderate con- aprotic solvents: a) very large liquid range (e.g. BMIM ductivities and high CO2 absorption capacity, represent TFSI: 89e450  C as compared to EtOH: 114 to a good compromise for that kind of processes. 78  C), b) good solvency for a wide range of inorganic, organic and polymeric materials, c) Brønsted, Lewis and The modular nature of ILs means that they can be Franklin acidity, hydrogen-bonding ability, pep in- submitted to many structural modifications, making teractions, etc. d) neglectable vapor pressure, e) water www.sciencedirect.com Current Opinion in Electrochemistry 2023, 42:101392 6 Organic and Molecular Electrochemistry (2023) impurities do not restrict their industrial applications, f) solubility of CO2 in ionic liquids is relatively high, best thermally stable up to 200  C., and g) relatively cheap absorption capacities at low post-combustion partial and easy to prepare. Moreover, RTILs have intrinsic pressures are lower than 5 mol%. Therefore, IL-based ionic conductivity at room temperature and a wide functional deep eutectic solvents (DESs) and other electrochemical window (electrochemical stability in combinations with functional CO2-philic groups have the range of 4.0e5.7 V [61]. been created, to increase the molar absorption capacity of these materials [63,64]. Nevertheless, there are two common drawbacks to ILs: (a) mass transport rates (reported as diffusion co- Zhao et al. used an ionic liquid as electrolyte for the efficients) are smaller by a factor of between 10 and 100 electrochemical reduction of high-pressure CO2, in all IL media (compared to molecular solvents), due to obtaining syngas as the electrolysis product [65]. the viscous nature of the media; and (b) the heteroge- Different reviews of electrochemical transformation of neous electron-transfer rate constants can be smaller by carbon dioxide in ionic liquids within the framework of as much as two orders of magnitude for processes capture and sequestration are already available [15,66], controlled by outer-sphere dynamics [62]. showing that the trend has been to move from less abundant substances, such as noble metals, to more Ionic liquids are appealing solvents (compared to alka- commonly available metals and materials. Rosen et al. nolamines) for carbon dioxide capture in flue-gas from [67], described that some ionic liquids can significantly coal-fired power plants because of their low volatility, lower the overpotential of CO2 reduction, and proposed and potential to be recycled. However, even though that some ILs may stabilize the CO2 anion that is formed Figure 4 a) Electrocarboxylation reaction of benzyl chloride in acetonitrile and BMIM BF4 under almost the same experimental conditions b) electrochemical synthesis for ibuprofen and naproxen extracted to references [54,71,72]. Current Opinion in Electrochemistry 2023, 42:101392 www.sciencedirect.com Use of CO2 for electrosynthesis Mena et al. 7 in the first step of the electrochemical reaction, through could potentially displace the petrochemical processes formation of a complex, thus decreasing the activation in a future. barrier and the overpotential. Compton et al. discovered that the ionic liquid 1-butyl-3-ethylimidazolium acetate Conclusions showed an abnormally high solubility of carbon dioxide The capture and valorization of CO2 in protic-, organic- due to the formation of molecular complexes [68], or IL-based electrochemical processes, either by direct making it clear that in order to facilitate the electro- CO2 reduction to valuable fuels and chemicals, or by chemical reduction process, carbon dioxide capture electrocarboxylation of organic molecules, is a promising should be a physical absorption (as seen for CO2 mixing field of research that can help to tackle the control and in most ionic liquids), rather than chemical absorption, reduction of the increasing levels of atmospheric CO2 which potentiated the extent of capture at the expense while generating alternative green fuels and chemicals of down-stream transformation and separation. and bringing solutions to the pressing global emergency of climate change. The use of aqueous electrolytes Isse et al. showed that benzyl chloride could be allows to move from the obtention of SYNGAS carboxylated, driving its electrochemical reduction in (CO þ H2) to hydrocarbons by using either noble metals CO2-saturated acetonitrile with yields c.a 90% using Ag (such as Au, Ag or Pd) or Cu electrodes, respectively. as working electrode [47]. Lu et al. showed that it was Whereas, the use of organic electrolytes allows to tune possible improve that electrocarboxylation, in terms of from the conversion of CO2 into oxalate, formate or toxicity and sustainability, by using an RTIL (BMIM carbonate derivatives into electrocarboxylated high BF4) as solvent-electrolyte media [69], which could be added-value products. Finally, ionic liquids are consid- recycled and reused four times (Figure 4a). Senboku ered one of the most advantageous electrolytes for such et al. demonstrated that electrochemical carboxylation applications, and although their high cost has been of organohalides was successfully achieved with considered a disadvantage, the enormous potential for reasonable yields in ionic liquids [70]. Description of structural customization due to the huge number of sustainable and highly efficient chemical routes for possible combinations between different cations and synthetizing useful compounds using CO2 as a C1 anions, together with a wide liquid temperature range, symptom through electrocarboxylation reactions for the high thermal stability, and extremely low volatility, could synthesis of NSAIDs in ILs has been also recently allow the design of green, low-cost, low-viscosity sol- described. Controlled potential electrolysis of 1-chloro- vents with high CO2 absorption capacity that could pave (4-isobutylphenyl)ethane and 2-(1-chloroethyl)-6- the way for an effective integration of CO2 capture with methoxynaphthalene in CO2-saturated ionic liquids CO2 conversion, while also contributing to a removal of solutions obtains ibuprofen [71] and naproxen [72]. In atmospheric CO2. those latest cases a comparison between the use of organic aprotic solvents containing 0.1 M of supporting electrolyte and ionic liquids as electrolytes can be done Declaration of competing interest [54]. For the electrochemical synthesis of ibuprofen, in The authors declare that they have no known competing both electrolytes moderate yields were obtained using financial interests or personal relationships that could carbon graphite cathodes, although a greater conversion have appeared to influence the work reported in rate was seen for the piperidonium ionic liquid. In the this article. case of the electrochemical obtention of naproxen, the use of ionic liquids allows to improve the yield from c.a. Data availability 85% to 90% and conversion rates from 74% to 90% Data will be made available on request. compare to conventional electrolytes, as well as the suitability of the process (Figure 4b) [54]. Acknowledgments The authors thank the Ministerio de Ciencia e Innovación of Spain for financial support through projects PID2019-106171RB-I00, TED2021- This methodology offers a “green” way for the synthesis 130797B-I00 and Funding from Generalitat de Catalunya (2021 SGR of different carboxylic acids that could potentially 00052). S.M. acknowledges support from the Margarita Salas UAB post- displace their petrochemical sources in a future. Finally, doctoral grants funded by the Spanish Ministry of Universities with Eu- ropean Union-funds NextGenerationEU. it is remarkable that the electrocarboylation approach is a low-cost alternative for countries where electricity prices are below 4 cents per kWh and for processes with References Papers of particular interest, published within the period of review, at least 60% of electrochemical conversion efficiencies have been highlighted as: [73]. Hence, apart from the technical challenges of * of special interest using electrochemistry for manufacturing marketable * * of outstanding interest products as well as reducing electricity price between 2 and 4 times by using renewable energy sources, this 1. https://www.co2.earth/earths-co2-main-page. methodology offers a “green” way for the synthesis of 2. 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