Direct air capture (DAC), as indicated by its name, is a process that captures CO2 from the ambient air. This technology is appealing because, to some extent, it de-couples the emission source and carbon sink (i.e., the relative locations of the emission source and downstream CO2 utilization/storage sites will not be a necessary concern). Therefore, infrastructure and expenditures for long-distance transportation of CO2 can be avoided, resulting in the great potential to reduce the cost of CO2 abatement. Additionally, DAC directly uses air as the upstream source, which is a preferred technology to address mobile emission sources (such as vehicles) and micro emission sources (such as buildings) (Sanz-Pérez et al., 2016).
DAC delivers its carbon reduction capacity by coupling with either CO2 conversion or CO2 storage. Firstly, CO2 obtained from DAC can be used as a raw material to replace fossil resources to produce carbon-based chemicals. In this context, the resulting products will be carbon neutral throughout their life cycles. Practically, this reshapes the existing chemical industry, leading to a disruptive impact on the entire industry and supply chain. Secondly, coupling of DAC with CO2 storage has a significant carbon-negative effect, and its capacity is huge if large-scale deployment and application can be carried out. This offers a solution to the historical emissions and meets the demand of “reducing atmospheric CO2 concentration” in response to climate change. Furthermore, such a carbon-negative effect may also have beneficial impacts on the energy and industrial system, promoting DAC to become an important part in the new model of carbon-neutral social and economic infrastructure (Goglio et al., 2020).
Compared with industrial emission sources, the concentration of CO2 in the air is extremely low, only slightly higher than 400 ppm. This means that the DAC process is very unfavourable in terms of thermodynamics. Therefore, it faces great technological challenges in aspects of absorbents/adsorbents, efficiency, and energy costs. These adverse factors often cause the consumption of a large amount of materials and energy, even achieving a level that may offset the CO2 reduction capacity of the DAC process (Deutz and Bardow, 2021). Overall, the technological maturity of DAC is still low, and its application is still in early infancy. In recent years, related studies have mainly focused on high-performance absorbents/adsorbents, prototype demonstrations, and life cycle assessments.
Absorbents and adsorbents are the most important components of a DAC technology system, and their performance directly dictates the overall cost and efficiency of the process. At present, most DAC studies and demonstrations have focused on solid adsorbents, and only a few experiments have used liquid absorbents with alkali metal hydroxide solution as the main component. The reason is because those solid adsorbents possess obvious advantages in terms of kinetics, stability, and environmental footprints (Shi et al., 2020).
Physical adsorbents (such as activated carbons, molecular sieves, etc.) are rarely used for DAC, mainly due to their weak interaction with the CO2 molecules, which leads to extremely low adsorption capacity and selectivity in low CO2 concentration environments (i.e., air). For example, Kumar et al. compared the CO2 adsorption performance of four physical adsorbents and an immobilized organic amine sample, which is a typical chemical adsorbent. They found that although the performances of these samples only showed slight differences in 15 vol.% CO2, the CO2 adsorption capacities of physical adsorbents are more than one order of magnitude lower than those of chemical adsorbents in DAC conditions (Kumar et al., 2015).
Compared with physical adsorbents, chemical adsorbents are more widely used for DAC. Among them, immobilized organic amines are a kind of important CO2 chemical adsorbent. The earliest prototype of these adsorbents comes from the “molecular basket” material proposed by Song’s group (Fig. 1) (Xu et al., 2002; Ma et al., 2009). Then, Sayari’s group (Franchi et al., 2005; Sayari and Belmabkhout, 2010) and Jones’ group (Hicks et al., 2008; Didas et al., 2015) systematically developed the preparation method of immobilized organic amines, and in-depth studies on their adsorption behaviours and structure-performance relationship for CO2 capture were performed. Due to the strong interaction between these adsorbents and the CO2 molecules, their application in the DAC process has attracted extensive attention. For example, Goeppert et al. reported that fumed silica supported poly (ethyleneimine) (PEI) can effectively adsorb CO2 from the air (Goeppert et al., 2011). In 2011, Choi et al. (2011) used 3-aminopropyl trimethoxysilane and tetraethylorthotitanate modified PEI as the adsorption components, which were loaded on porous SiO2, and they found that this type of material exhibited great DAC performance (adsorption capacity > 2 mmol g–1) and the use of modifiers can significantly improve the stability of PEI, rendering the material with excellent stability (Choi et al., 2011). Keller et al. used carbon nanotubes as a carrier for PEI to prepare a hollow fibre adsorbent. The adsorption capacity of CO2 reached 1.07 mmol g–1 under DAC conditions (Keller et al., 2018). In addition to the commonly used impregnation method, immobilization of amines can also be achieved by surface grafting. Although adsorbents prepared in this way have slightly lower CO2 uptakes under DAC conditions, their overall stability can be significantly improved (Potter et al., 2017; Sabatino et al., 2021).
Figure 1. “Molecular basket” sorbents [Reprinted from (Ma et al., 2009)].
Porous framework materials are a type of functional materials developed rapidly in recent years. The structure of these materials is highly ordered and can be finely characterized by various spectroscopic methods, thereby providing new dimensions for studying DAC mechanisms and performance tunning (Custelcean, 2021). For example, by the engineering of ligand for metal-organic frameworks (MOFs), Kumar et al. replaced the SiF62− pillar in SIFSIX-3-Ni material with TiF62−, which led to enhanced interaction between the framework and CO2, and thus the DAC performance of the material was effectively improved (Kumar et al., 2017). Bhatt et al. further explored possible strategies by adjusting the pore size, pore shape, and surface functional groups of MOFs, and a CO2 adsorption capacity of 1.30 mmol g–1 could be achieved at the condition of 400 ppm and 25°C, which is on a similar level to immobilized amine-based adsorbents (Bhatt et al., 2016). It should be noted that although the low-pressure CO2 adsorption performance can be significantly improved through the optimization of MOFs structure, in general, its adsorption mechanism still relies on physical interactions, so it is easily affected by competitive adsorption of moisture. In fact, MOFs can also be used as carriers to prepare amine-based DAC adsorbents (Lu et al., 2013; Lee et al., 2014; Liao et al., 2016; Sen et al., 2019), and under the synergistic effect of multiple functional sites, the resulting materials may exhibit excellent DAC performance through several types of new adsorption mechanisms. For example, McDonald et al. used N, N’-dimethylethylenediamine (mmen) to functionalize Mg2(dobpdc) (dobpdc4−=4,4’-dioxidobiphenyl-3,3’-dicarboxylate). It was found that during the adsorption of CO2, there is a process of CO2 insertion into the metal-amine bond, so the obtained CO2 adsorption isotherm has step-shaped characteristics. This unique behaviour makes the material possess a higher working capacity than other chemical adsorbents (McDonald et al., 2015). Through the tunability of MOFs structure and the optimization of external amines, CO2 adsorption capacity, kinetics, and regeneration performance under DAC, conditions can be further improved (Lee et al., 2014; Liao et al., 2016; Kim et al., 2020b; Martell et al., 2020).
In recent years, some new adsorption strategies have also been used for DAC processes. Based on the affinity difference between materials to CO2 and water, researchers have proposed the concept of moisture-swing adsorption. In this process, the material captures CO2 from air in a dry environment, while the competitive adsorption of moisture in a wet environment will liberate CO2 molecules from the adsorbent surface (Shi et al., 2016; Yang et al., 2018). Inagaki et al. found that the aqueous solution of m-xylylene diamine (MXDA) can absorb CO2 in the air and generate water-insoluble MXDA·CO2 crystals. After simple filtration and separation, heating of MXDA·CO2 can release CO2 and complete the regeneration of MXDA. This process avoids the large amount of energy consumption required for the evaporation of solvent water in the traditional liquid absorption method (Inagaki et al., 2017). Based on a similar strategy, Brethomé et al. used a cheap and easily available amino acid aqueous solution to absorb CO2, and then the CO2 loaded solution further reacted with guanidine compounds to form insoluble carbonates, which regenerated the amino acids simultaneously, and then the separated carbonate can be decomposed under relatively mild conditions to release CO2 (Brethomé et al., 2018).
Table 1 summarizes recent research results on DAC adsorbents. Overall, the research and development of DAC absorbents/adsorbents still face great challenges. In this study, a model was designed to estimate the cost of DAC adsorbents and the corresponding relationship between adsorbent performance and its highest allowable cost was studied (Fig. 2). Based on this model, it is found that if the service life of an adsorbent is 1000 cycles, its cost must be less than $1 kg–1. Currently, there are hardly any adsorbents so low in cost. Increasing adsorbent service life can reduce its highest allowable cost; with 100 000 cycles, the allowable cost of MOF (Diamine) materials is the highest, reaching $90 kg–1. However, the price of MOF material is generally higher than $10 000 kg–1, and its stability is far from the requirements (Shi et al., 2020). This work verifies the difficulty of developing DAC adsorbents from a cost perspective, but it should be mentioned that recently, several groups have reported several CO2 adsorbents with excellent comprehensive performance, which provides new opportunities for reducing the cost of DAC technology (Nandi et al., 2015; Yue et al., 2017; Cavalcanti et al., 2018; Mukherjee et al., 2019; Lin et al., 2021).
Reference Sorbents Mechanisms Adsorption capacity
(Kumar et al., 2015) TEPA-SBA-15 chemisorption 158 20 400 ppm Zeolite 13X physisorption 5.8 20 HKUST-1 physisorption 2.1 20 Mg-MOF-74/ physisorption 40 20 SIFSIX-3-Ni physisorption 58 20 (Goeppert et al., 2011) FS-PEI-33 chemisorption 75 25 400 ppm (Choi et al., 2011) PEI-silica chemisorption 103 25 400 ppm A-PEI/silica chemisorption 99 25 T-PEI/silica chemisorption 96 25 (Keller et al., 2018) CNT-PEI chemisorption 47 25 350 ppm (Bhatt et al., 2016) NbOFFIVE-1-Ni chemisorption 57 25 400 ppm (McDonald et al., 2015) mmen-Mg2(dobpdc) chemisorption 154 25 1000 ppm (Kim et al., 2020b) Mg2(dobpdc) (3-4-3) chemisorption 150 90 4000 ppm
Table 1. Summary of research results on DAC sorbents.
Figure 2. Allowable price of DAC adsorbents [Reprinted from (Shi et al., 2020)].
Devices and demonstrations are also major focuses in the DAC field. This is because the concentration of CO2 in the air is low, so the gas processing capacity of the DAC process is huge, and there is little experience in an engineering perspective to date. Over the past decade, developed countries in Europe and America have attached great importance to the development and application of DAC, and several small-scale prototypes and demonstrations have been reported, which has laid an important foundation for the future development of DAC technology.
Climeworks, a company based in Switzerland, is the first company in the world that provides customers with CO2 captured from the air. In 2017, the company built the world’s first commercial DAC device in Switzerland, which adopted modular design and used 18 adsorption units. An overall capture capacity of several hundred tons of CO2 per year was achieved. In 2021, Climeworks built its latest Orca plant in Iceland. The Orca plant has a capture capacity of 4000 t yr–1 (Fig. 3) and is the largest DAC demonstration in the world to date. The captured CO2 is injected 700 m underground for mineralization and storage (Climeworks, 2021).
DAC technology from Global Thermostat uses temperature-swing adsorption and adopts immobilized organic amine adsorbents. Global Thermostat established the first pilot plant in 2010 and the first commercial DAC plant in Alabama in 2018. It is reported that the energy consumption can be reduced to less than 6 GJ (t CO2)–1, which is around the minimum for existing DAC demonstrations (Zhu et al., 2021).
Carbon Engineering established its first DAC pilot plant by using KOH and Ca(OH)2 solutions as absorbents in 2015, and in 2017, the company succeeded in the conversion of air-captured CO2 to liquid fuels. In 2019, the company began to design and build a million-ton DAC demonstration project. Its carbon capture cost is estimated to be $94–$232 (t CO2)–1, with an energy consumption intensity of about 8.81 GJ (t CO2)–1 (Keith et al., 2018; Engineering, 2021).
At present, there are still some controversies around the economic and environmental benefits of DAC. Therefore, its full life cycle assessment is also a research hotspot. By applying LCA to the actual operation data, Deutz et al. found that the DAC demonstration currently carried out by Climeworks has already achieved a carbon-negative effect. The carbon capture efficiencies of its two DAC plants reached 85.4% and 93.1%, respectively. However, the environmental benefits of the DAC process are closely related to energy sources. In an energy structure dominated by renewables, it is expected that the large-scale deployment of DAC technology (reaching 1% of global carbon emissions) will not be limited by materials and energy, and related environmental issues, if any, are relatively limited (Deutz and Bardow, 2021). According to calculations on carbon balance, Jonge et al. carried out LCA studies on a NaOH absorption DAC system, with a special focus on the life cycle carbon efficiency. Their results also showed that the energy source is very important to the emission reduction benefits of DAC. In general, the utilization of renewable energy is the simplest and most significant way to improve the carbon efficiency of DAC at this stage (de Jonge et al., 2019).
Terlouw and co-workers (2021b) proposed that it is necessary to incorporate CO2 storage into the DAC’s LCA process in order to fully reflect the emission reduction benefits and other environmental impacts of DAC. Based on this idea, the authors compared and studied the environmental impact of direct air capture and storage (DACCS) under different power/heat sources and a variety of technology combinations. It was found that although a negative carbon effect can be achieved in several scenarios, it is necessary to select the best energy supply mode according to the energy infrastructure of different countries and regions. This is of pivotal importance to the emission reduction capability and the environmental impact of DAC (Terlouw et al., 2021a).
It should be mentioned that DAC technology is still very immature, so there are significant challenges in technology evaluation. Terlouw et al. (2021b) systematically summarized the current LCA works for carbon-negative technologies and pointed out that existing methods may induce confusion between avoided emission and negative emission, resulting in misleading conclusions. In order to better support the development of DAC technology in the future, it is necessary to strengthen the comprehensiveness of the evaluation and the transparency of the methods.
Metal-organic frameworks (MOFs) are one of the emerging and rapidly growing focuses in material science. These materials are constructed from complexation of metal ions or metal ion clusters with bridging organic linkers, which exhibit regular crystalline lattices with high surface area and well-defined pore structures (Furukawa et al., 2013; Yuan et al., 2018). In MOFs, both the metal nodes and the organic linkers can be readily adjusted. This offers an effective approach for the customization and delicate adjusting of the materials’ pore structure and functionalities. Therefore, MOFs can be widely applied in separation and catalysis science. In recent years, it was found that due to the dynamic properties of the framework component, MOFs that are responsive to external stimuli (pressure, temperature, ultraviolet light, etc.) can be synthesized (Matsuda, 2014). Based on this, construction of a reversible phase transformable CO2 adsorption material can be realized to achieve an ultra-low cost CO2 capture process (Schneemann et al., 2014) (Table 2). It should be noted that the energy consumption of the existing carbon capture process is relatively high, often accounting for 70% of the entire CCUS technology chain.
Reference Sample Trigger of flexibility Properties alteration upon flexibility (Park et al., 2012) PCN-123 UV irradiation At 1 bar pressure, CO2 adsorption decreased from 21 to 10 cm3 g–1 after UV irradiation. (Lyndon et al., 2013) Zn(AzDC)
UV irradiation Dynamic exclusion of CO2 during UV on-.and-off switching cycles. (Dong et al., 2021) NTU-65 Temperature C2H4 adsorption capacity (1 bar pressure) decreased from 90 to ~0 cm3 g–1 when increase temperature from 195 to 263 K. (Marks et al., 2020) CPL-2 and CPL-5 Rotation of the pillar ligands Due to lattice expansion, CO2 adsorption increased rapidly at pressure lower than 20 bar. (Taylor et al., 2018) Co(bdp) Formation of CO2 clathrate At appropriate pressure, CH4 can be largely excluded from the pores, leading to high CO2/CH4 adsorption selectivity.
Table 2. Properties before and after breathing-MOF.
One of the general strategies to construct flexible MOFs is to use organic linkers with light-responsive function groups as subunits for the framework. Modrow et al. constructed the first porous MOF (CAU-5) with photo-switchable linker molecules (3-azo-phenyl-4,4’-bipyridine). Under the irradiation of UV light (365 nm), the azo-functionality switches from its thermodynamically stable trans-isomer to cis-isomer (Modrow et al., 2011). Similarly, CO2 adsorption behavior can be reversibly altered upon photochemical or thermal treatment in an MOF (PCN-123) with an azobenzene functional group, which can switch its conformation (Park et al., 2012). Sensharma et al. reported the synthesis and characterization of a photoactive MOF (TCM-15), which revealed a dynamic response upon UV irradiation, leading to instant desorption of pre-adsorbed CO2. Based on FT-IR experiments and DFT calculations, the author verified that such release of CO2 could be attributed to the structural flexibility of the materials (Sensharma et al., 2019). Lyndon et al. synthesized an MOF loaded with azo-type light-responsive groups. The material showed rapid response towards CO2 adsorption during the light on-and-off switching cycles. This work directly verified the possibility of developing ultra-low energy consumption carbon capture technology based on flexible framework materials (Fig. 4) (Lyndon et al., 2013).
Figure 4. Light-responsive capture of CO2 [Reprinted from (Lyndon et al., 2013)].
Interaction between MOFs and guest molecules is an important way to stimulate flexibility. As early as 2003, Kitaura et al. introduced displacement freedom during the self-assembly process of rigid motifs, and they proved experimentally that the obtained material has a “gate-opening pressure” for different gases, which caused the MOF material to transition from a crystalline “closed” form into an “open” form. The authors suggest that the reason for this flexibility is the displacement of π-π stacked moieties in the material. At the same time, due to the different interactions between different gas molecules and materials, the gate-opening pressure also varied in different atmospheres. For CO2, the opening and closing pressure are only 0.4 and 0.2 bar, respectively (Kitaura et al., 2003). Stavitski et al. prepared amino-modified NH2-MIL-53(Al) and found that the modified material has excellent CO2 adsorption properties compared with the unmodified counterpart. Very interestingly, it was found that instead of binding CO2 chemically, the introduced amino groups actually played a role in adjusting the framework flexibility. In this way, the CO2 adsorption capacity could be enhanced while the physical adsorption nature was not altered, and therefore, a significant increase in energy consumption for desorption was avoided (Stavitski et al., 2011). Using Cu as the metal center, 2,3-pyazinedicarboxylic acid (pzdc) as the ligand, and 4,4’-bipyridine (bpy) and 1,2-di-(4-pyridil)-ethylene (bpe) as the pillar, CPL-2 (Cu2(pzdc)2(bpy)) and CPL-5 (Cu2(pzdc)2(bpe)) were prepared, respectively. Both samples showed excellent CO2 adsorption capacity and selectivity, and based on in situ synchrotron X-ray diffraction experiments, the authors showed that the chemical interaction between CO2 and the materials triggered rotation of the pillar ligands. This distortion further introduced expansion of the framework lattice, which is the key factor for the high CO2 adsorption performance (Marks et al., 2020).
Taylor and co-workers prepared a Co (bdp) (bdp2− = 1,4 benzenedipyrazolate) MOF material with a flexible framework, which showed excellent adsorption selectivity for the separation of CO2 and CH4. Based on in situ X-ray Powder Diffraction (XRD) measurements, the authors suggest that the high selectivity originates from a “reversible guest templating” effect. That is, in the adsorption process, the framework expanded to form CO2 clathrates, and the expended framework collapsed back to the non-templated phase in the subsequent desorption process. Based on such a mechanism, CH4 was completely removed during adsorption of a 1:1 CO2-CH4 mixture (Fig. 5) (Taylor et al., 2018).
Figure 5. Separation of CO2 and CH4 by flexible framework material [Reprinted from (Taylor et al., 2018)].
Although flexible MOFs have shown many advantages in CO2 adsorption, how to regulate their flexibility to match practical conditions is still a challenging question.
Based on the excellent structural flexibility and mechanical resistance of MIL-53 series materials, Chanut et al. found that when an external force is applied to the materials, they gradually change from an open-pore form to a contracted form. More importantly, the pore size of the materials can be accurately controlled by the strength of the applied force. Based on this idea and considering specific separation objects, efficient separation of CO2/N2 and CO2/CH4 could be achieved under optimized external force conditions. At the same time, when the external force is unloaded, the material structure switched back to the open form, which led to gas desorption and material regeneration (Fig. 6). Clearly, such a strategy is superior to the conventional pressure-swing or temperature-swing processes (Chanut et al., 2020).
Figure 6. Flexibility regulation by external force [Reprinted from (Chanut et al., 2020)].
Using NH2-MIL-53(Al) as a parent material, Bitzer et al. introduced Sc, V, Cr, and Fe as a second metal node to prepare NH2-MIL-53 (Al, M). Based on systematic characterizations, the authors found that the flexibility of the framework can be effectively adjusted by the presence of the second metal, which further altered the CO2 capture performance of the resulting samples. In general, the construction of polymetallic MOFs can easily and effectively regulate the framework flexibility in a wide range, which has great potential in future applications (Bitzer et al., 2020).
Ghoufi’s calculation revealed that the flexibility of MIL-53 could be affected by the existence of an electric field in addition to the stimulation of the guest molecules, heat, and external forces, with a volume change of up to 40%. At the same time, varying the electric field strength led to the regulation of the unit cell volume. This provided a ready path for adjusting gas adsorption, which resulted in the efficient separation of CO2 and CH4 (Ghoufi et al., 2017).
Dong et al. prepared a flexible MOF (NTU-65), and its flexibility is sensitive to temperature. Based on this characteristic, and together with the different gate-opening pressure of different gases, the authors successfully found a suitable operation window for the one-step separation of multi-component gases containing ethylene, ethane, and CO2. High purity ethylene of polymerization grade was obtained by this method, which may find important potential in the petrochemical industries (Dong et al., 2021).
Electro-catalytic CO2 reduction (ECR) represents the processes that convert CO2 via an electro-chemical way. In sharp contrast to the conventional thermal-catalytic process where high temperature and pressure are normally needed for CO2 activation and conversion, ECR processes can be readily carried out in much milder conditions (atmospheric pressure and room temperature). Meanwhile, as the required energy can be largely provided in the form of electrons, the ECR processes can be directly integrated with low-grade renewables, and H2O, rather than other energized molecules, can be used as a hydrogen source. As such, ECR enables the transformation of renewable energies to chemical energies in form of fuels and/or chemicals including CO, hydrocarbons (methane, ethylene), hydrocarbon oxygenates (formic acid, methanol, ethanol, etc.), or a mixture of them, and after their consumption, the released CO2 can be recycled to close the carbon loop (Yang et al., 2016). All these merits give ECR great promise for the realization of carbon neutrality. In Table 3, we summarized several representative results from recent publications on ECR.
Reference Sample Potential
(V vs. RHE)
Products and FE (Kim et al., 2020a) Ag nanoparticles −0.82 400 CO, 92.6% (Zhang et al., 2020) NiPc-OMe −0.64 300 CO, 99.5% (Xu et al., 2016) N-doped CNT −0.9 5.8 CO, 90% (Dinh et al., 2018) Cu electrocatalyst −0.55 100 C2H4, 70% (Zhong et al., 2020) Cu-Al alloy electrocatalyst −1.5 400 C2H4, 80% (Xiong et al., 2021) Ag@Cu2O −1.2 178 ± 5 CH4, 74% ± 2% (Yadav et al., 2022) N-doped GQDs −0.85 170 CH4, 63% (Bai et al., 2017) Pd-Sn alloy electrocatalyst −0.43 V − HCOOH, >99% (Yan et al., 2021) s-SnLi −1.2 1000 HCOOH, 92% (Xu et al., 2020) Cu catalyst −0.7 1.8 C2H5OH, 91% (Wang et al., 2020) N-C/Cu −0.68 300 C2H5OH, 52% ± 1% (Song et al., 2017) N-C −0.56 − C2H5OH, 77%
Table 3. Summary of research results on ECR catalysts.
Selective ECR to CO is a promising route toward industrial implementation because the produced CO can be used as a chemical feedstock to produce numerous higher-value chemicals or fuels, such as liquid fuel, low carbon olefin, and low carbon alcohol. However, direct reduction of CO2 to *COO− involves a one-electron transfer process with a very negative redox potential (−1.9 V versus reversible hydrogen electrode (RHE)), which significantly inhibits the activation of CO2 (Hunt et al., 2003). Therefore, suitable electrocatalysts should be developed to stabilize the *COO− intermediate, thereby reducing the activation overpotential.
Metal catalysts including Au, Ag, Zn, and Cd with both weak hydrogen and oxygen adsorption (Uesaka et al., 2018) have been widely investigated for ECR to CO. Among them, Ag exhibits extraordinary ECR to CO performance. For example, Yang’s group reported that when an Ag nanoparticle/ordered-ligand interlayer catalyst is applied in a gas-diffusion environment, a high FE of 92.6% for CO formation at a high current density of 400 mA cm–2 could be achieved. It was verified that the interlayer structure facilitated the synergistic effect between multiple components and is responsible for such excellent performance (Kim et al., 2020a). Single-atom catalysts (SACs) with individual metal atoms dispersed on solid substrates can maximize atom utilization efficiency and thereby enhance catalytic performance. In comparison with bulk Ni metal that catalyses hydrogen evolution exclusively under ECR conditions, the Ni single-atom counterpart can selectively electrochemically reduce CO2 to CO. Zhang et al. designed a methoxy group of functionalized nickel phthalocyanine (NiPc-OMe) molecules supported on carbon nanotubes, which catalyses the ECR to CO process with >99.5% selectivity. The electron-donating OMe groups could enhance the Ni-N bond strength in the Ni-N4 sites and accelerate CO desorption, thereby improving the catalyst stability (Zhang et al., 2020).
Metal-free carbon-based materials were also found to be effective for ECR to CO. However, the pristine, defect-free carbon materials are less effective, and incorporating heteroatoms such as nitrogen into the carbon matrix is necessary to improve activity. Su’s group reported an N-doped CNT synthesized through the pyrolysis of mixtures of poly (diallyl dimethylammonium chloride) and oxidized CNTs. By adjusting types and contents of the used nitrogen dopants, a maximum FE of 90% for CO formation and stable operation over 60 h with total current density and FE of 5.8 mA cm–2 and 85% can be achieved, respectively. During the reaction, the N-containing functionalities played an important role in stabilizing the *COO− intermediate, and this is the key for the sample to possess high performance (Xu et al., 2016).
With the combination of hydrogen from water, CO2 can be electrocatalytically converted to hydrocarbons, mainly containing methane (CH4) and ethylene (C2H4), via eight electron-proton and twelve electron-proton reaction processes, respectively. Cu-based nanoparticles can produce hydrocarbons at moderate overpotentials. This is attributed to the appropriate strength of CO chemisorption on Cu, and protonation or dimerization of CO is considered to be a key step towards the formation of single- (CH4) or multi-carbon (C2H4) hydrocarbons (Peterson et al., 2010) (Kortlever et al., 2015). During the above mechanism, the *CO coverage on the catalysts can significantly influence the ECR activity and selectivity. A high *CO coverage can readily trigger C-C coupling that enhances C2H4 formation, while less *CO on the catalyst surface might not be competitive enough over hydrogen evolution reaction (Huang et al., 2017).
In the past several years, significant progress has been made in ECR to C2H4 conversion, including catalysts, electrolytes, and electrodes. A typical example was reported by Sargent’s group where a Cu electrocatalyst at an abrupt reaction interface in an alkaline electrolyte (7 M KOH) reduces CO2 to C2H4 with 70% FE at a potential of −0.55 V vs. RHE. The remarkable performance is correlated to hydroxide ions on or near the Cu surface, which lowered the activation energy barriers of ECR and C-C coupling (Dinh et al., 2018). A graphite/carbon NPs/Cu/PTFE electrode was further constructed to prevent flooding problems and stabilize the Cu catalyst surface, thereby resulting in enhanced stability over the prolonged operation for 150 h. Afterwards, they developed a Cu-Al alloy electrocatalyst to further improve the FE of C2H4 to 80% at a current density of 400 mA cm−2 in 1 M KOH electrolyte. They suggested that the Cu-Al alloys provide multiple sites and surface orientations with near-optimal CO binding for both efficient and selective ECR (Zhong et al., 2020).
The progress on ECR to CH4, which has the highest heating value of 55.5 MJ kg−1 among all the ECR products, is far behind that of C2H4. There is still a lack of applicable catalysts with satisfactory CH4 selectivity. Only very recently, Xiong et al. fabricated an Ag@Cu2O core-shell structure. By fixing the Ag core and adjusting the Cu2O envelope size, the *CO coverage and *H adsorption at the Cu surface can be modulated to steer the ECR pathway towards CH4. The optimal catalyst delivered a high CH4 FE of 74% ± 2% and a partial current density of 178 ± 5 mA cm−2 at −1.2 V vs. RHE (Xiong et al., 2021). Yadav and co-workers reported an amine functionalized N-doped GQDs for efficient ECR to CH4. It revealed that the CH4 yield (partial current density) increased linearly with amino group (NH2) content. Consequently, a maximum CH4 FE of 63% could be obtained over the catalyst with the maximum NH2 content of 9.07 atomic concentration (at. %) (Yadav et al., 2022).
Investigations on ECR to oxygenates mainly focus on formic acid (or formate) and ethanol via two electron-proton and twelve electron-proton reaction processes, respectively. Formic acid is an important product from ECR that has been widely explored as a hydrogen carrier. The common strategy for industrial production of formic acid is carbonylation of methanol, which, however, requires intensive energy input. It is thus highly desired to directly convert CO2 to formic acid via the ECR process, which was predicted to have a production potential of 475 kt yr–1 by 2030 globally. (Contentful, 2021).
Several metals, including Sn, In, and Bi, have been investigated as electro-catalysts for selective ECR to formic acid or formate. Among these metals, Sn commands the most attention for its low toxicity and cost. Bai and some of the co-authors of this paper developed a Pd-Sn alloy electrocatalyst for exclusive formic acid formation (FE>99%) in a 0.5 M KHCO3 solution. The presence of Pd modified the electronic configuration and oxygen affinity of Sn, which stabilized the HCOO* intermediate and the subsequent formic acid formation (Bai et al., 2017). Recently, Zheng’s group developed a surface-Li-doped Sn (s-SnLi) catalyst, which exhibited a high FE of 92% and a partial current density of 1.0 A cm–2 for producing formate. The introduction of Li dopants into the Sn lattice enabled the localization of negative charges and lattice strains to their neighbouring Sn atoms; thereby, both activity and selectivity of ECR to formate were enhanced (Fig. 11) (Yan et al., 2021).
Figure 11. Scheme illustrations of (a) electrochemical lithiation preparation of the s-SnLi catalyst, and (b) its function mechanism for ECR to formate [Reprinted from (Yan et al., 2021)].
Ethanol, a kind of clean and renewable liquid fuel with a heating value of –1366.8 kJ mol–1, is a preferred product from ECR. Owing to the higher energy density and ease of storage and transportation compared to gas products, ethanol has also been considered as one of the optimal candidate fuels that substitute or supplement fossils in many applications (Shih et al., 2018). Moreover, ethanol is also an important and widely used common chemical feedstock for organic chemicals and medical disinfectants. Based on the considerable market demand, direct conversion of CO2 to ethanol using only water and driven by renewable energy is highly desired.
Cu is the only reported metal so far that can electrochemically catalyse ECR to ethanol. However, the selectivity is extremely low due to its moderate binding energy with most reaction intermediates. To promote the selectivity towards ethanol, manipulation of the binding strength of reaction intermediates on Cu is a commonly used strategy. Xu et al. reported a carbon-supported Cu catalyst via an amalgamated Cu–Li method, by which most Cu is atomically dispersed on the carbon surface. The high initial dispersion of single Cu atoms favors the selective ECR to ethanol with FE reaching ~91% at –0.7 V vs. RHE and outstanding durability of 16 h. However, the current density (around 1.8 mA cm–2) is still below the industrial level (Xu et al., 2020). Based on recent investigations, the improvement of current density is at the expense of ethanol FE. For instance, Sargent’s group coated a nitrogen-doped carbon (N-C) layer on a Cu surface to build a confined reaction volume, which promoted C-C coupling and suppressed the breaking of the C-O bond in HOCCH*, thereby promoting ethanol selectivity in ECR. Under a current density of 300 mA cm–2, an ethanol FE of (52 ± 1) % is achieved on 34% N-C/Cu (Wang et al., 2020). Metal-free nitrogen-doped carbon materials have also been recently reported to be capable of ethanol production from ECR, which delivered comparable catalytic activities to Cu-based catalysts and even better durability. Song and co-workers developed a metal-free cylindrical mesoporous nitrogen-doped carbon as a robust catalyst for selective ECR to ethanol. The synergy of nitrogen heteroatoms and highly uniform cylindrical channel structures dramatically boosted C−C bond formation in ECR. Therefore, the catalyst enabled efficient production of ethanol with a high FE of 77% at –0.56 V vs. RHE in 0.1 M KHCO3 (Song et al., 2017). Inspired by the potential of adjusting the nanostructure of the catalyst to acquire multi-carbon compounds, the group further developed a hierarchical porous N-doped carbon with micropores embedded in the channel walls of N-doped ordered mesoporous carbon. By controlling the micropore content, the ethanol formation rate is improved by one order of magnitude compared to that of the counterpart without medium micropores. These reports have provided new insights for designing highly efficient electrocatalysts for ECR to ethanol in the future (Song et al., 2020).
Clearly, CCUS will play an important role in reaching the carbon neutrality target, not just at its current stage, but more so after future development. In particular, deep decarbonization requires a considerable revolution of the current energy and industrial infrastructure. Therefore, cutting-edge/disruptive CCUS technologies are becoming increasingly important and influential.
In this paper, we reviewed four technologies that are regarded as frontiers of CCUS. In Table 4, their relevance to carbon neutrality over other low-carbon technologies, advantages over regular CCUS technologies, and difficulties for future development are summarized.
Technology Relevance to carbon
neutrality over other
Advantages over regular CCUS technologies Technological difficulties DAC • Negative emission effects
• Solution to historical emissions
• Flexibility in localization
• Minimize transportation demand
• Solution to dispersed and small-scale emission sources
• High-performance adsorbents/absorbents
• Coupling with renewable energies
• Process engineering
Flexibilt MOFs • Enable low-cost CCUS for carbon neutrality • Lowering energy consumption for carbon capture • High-performance materials
• Process engineering
ICCC • A new way of carbon recycling
• Re-shape industrial system
• Lowering CCUS cost
• Source-sink matching
• Efficient coupling of absorption and conversion process
• Extension of products
• Matching with application scenarios
ECR • Avoiding fossil fuels
• Storing low-grade renewable energies
• Milder conditions
• Easy to scale-up
• High-performance catalysts
• Coupling with low-grade renewable energy
Table 4. Summary of selected CCUS technologies.
DAC is one of the rare technologies that is able to offer negative emission, and very uniquely, DAC also provides a solution to historical emissions. Both characteristics are necessary to suppress climate change. When comparing with other CCUS technologies, localization of DAC is highly flexible. Therefore, the necessity for CO2 transportation can be minimized by deployment of DAC in close vicinity of CO2 emitters and/or downstream utilization/storage sites. Based on such a feature, DAC is an ideal approach for distributed, mobile, and small-scale emitters. Currently, DAC is an immature technology, and the process cost is relatively high. Efficient absorbents/adsorbents, coupling with renewable energies, process engineering, and scaling-up are among the most urgent issues to be solved.
Flexible MOFs are a type of smart materials that can potentially alter the landscape of many applications. When they are used as CO2 adsorbents, their reversible breathing behavior toward external stimuli may change the fundamental thermodynamic driving force for adsorbent regeneration and thus enable less energy-intensive strategies over traditional temperature-swing and pressure-swing processes. Lowering the cost of CO2 capture by flexible MOFs is of particular importance to facilitate the large-scale application of CCUS, which in turn guarantees carbon neutrality to be achieved. However, the performance of the reported flexible MOFs cannot meet the requirement of practical application. Further research, including design of a suitable process, is needed.
ICCC technology represents a new way of carbon recycling, which may promote the formation of low-carbon models and processes of the industrial system. This is of promise as decarbonization of industrial departments is considerably difficult. Compared with existing CCUS technologies, ICCC has obvious advantages in terms of cost reduction, source-sink matching, and so forth. Nevertheless, ICCC is still in the laboratory verification stage. How CO2 capture and its conversion can be efficiently combined, how value-added products can be obtained, and how the process can be adjusted to match practical application scenarios are still questionable at this stage.
Recently, ECR has been one of the most eye-catching areas in catalysis. Due to its natural connection with renewable electricity, the process manages to close the carbon loop of fuels and chemicals by storing the low-grade renewable energy. Therefore, large-scale deployment of ECR can effectively avoid consumption of fossil fuels and thus contribute carbon reduction in an indirect way. Compared with other CO2 conversion technologies, the ECR process can be carried out under very mild conditions. Additionally, ECR can be readily modularized, which greatly facilitates its scaling up. The primary technical difficulties of ECR include the design and preparation of the catalyst with high activity and selectivity and engineering challenges related to coupling with low-grade renewable energy.
|Reference||Sorbents||Mechanisms||Adsorption capacity |
|(Kumar et al., 2015)||TEPA-SBA-15||chemisorption||158||20||400 ppm|
|(Goeppert et al., 2011)||FS-PEI-33||chemisorption||75||25||400 ppm|
|(Choi et al., 2011)||PEI-silica||chemisorption||103||25||400 ppm|
|(Keller et al., 2018)||CNT-PEI||chemisorption||47||25||350 ppm|
|(Bhatt et al., 2016)||NbOFFIVE-1-Ni||chemisorption||57||25||400 ppm|
|(McDonald et al., 2015)||mmen-Mg2(dobpdc)||chemisorption||154||25||1000 ppm|
|(Kim et al., 2020b)||Mg2(dobpdc) (3-4-3)||chemisorption||150||90||4000 ppm|