CO2 capture and conversion to value-added products promoted by MXene-based materials

Journal Pre-proof CO2 capture and conversion to value-added products promoted by MXene-based materials Yu Chen, Chong Liu, Shien Guo, Tiancheng Mu, Lei Wei, Yanhong Lu PII:

S2468-0257(20)30201-6

DOI:

https://doi.org/10.1016/j.gee.2020.11.008

Reference:

GEE 350

To appear in:

Green Energy and Environment

Received Date: 10 August 2020 Revised Date:

30 October 2020

Accepted Date: 12 November 2020

Please cite this article as: Y. Chen, C. Liu, S. Guo, T. Mu, L. Wei, Y. Lu, CO2 capture and conversion to value-added products promoted by MXene-based materials, Green Energy & Environment, https:// doi.org/10.1016/j.gee.2020.11.008. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020, Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd.

CO2 capture and conversion to value-added products promoted by MXene-based materials Yu Chen*,a, Chong Liu a, Shien Guob, Tiancheng Mu*,c, Lei Wei a, Yanhong Lu*,a a

Department of Chemistry and Material Science, Langfang Normal University, NO.

100 Aimin West Road, Anci District, Langfang 065000, Hebei, P.R. China. E-mail [email protected]; [email protected]; Phone: +86-0316-2188211; Fax:

b

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+86-0316-2112462. College of Chemistry and Chemical Engineering, Jiangxi Inorganic Membrane

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Materials Engineering Research Centre, Jiangxi Normal University, NO. 99 Ziyang

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Avenue, Nanchang 33002, Jiangxi, P.R. China. E-mail: [email protected];

Department of Chemistry, Renmin University of China, NO. 59 Zhongguancun

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c

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Phone: +86- 0791-8812002; Fax: +86-0791-8812060.

Street, Haidian District, Beijing 100872, P.R. China. E-mail: [email protected];

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Phone: +86-010-62514925; Fax: +86-010-62516444.

Tel: +86-316-2188211. E-mail: [email protected] (Yu Chen)

*

Tel: +86-010-62514925. E-mail: [email protected] (Tiancheng Mu)

*

Tel: +86-316-2188211. E-mail: [email protected] (Yanhong Lu)

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*

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Abstract: Carbon dioxide (CO2) capture and conversion is the key route for the mitigation of the greenhouse effect and utilization of carbon sources to obtain value-added products or fuels. Much attention is paid to the development of novel materials with high CO2 adsorption capacity and conversion rate. MXene is the graphene-like two-dimensional metal carbide/nitride/carbonitride owning favorable structure, morphology, high surface–bulk ratio, and physicochemical properties. Here, we review the CO2 capture, sensing, and conversion by MXene and

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MXene-based materials. Furthermore, the underlying mechanism involved the

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capture, sensing, and conversion of CO2 is summarized. This review would open a

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new horizon for CO2 valorization with high efficiency and promising widespread

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applications.

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Keywords: Green chemistry; carbon dioxide; carbon capture, carbon conversion; 2D

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material.

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1. Introduction Carbon dioxide (CO2) is the greenhouse gas with inert and stable feature because of the covalent double bond between carbon atom and oxygen atom. Due to the double bond, CO2 is thermodynamically-stable, which is difficult to be activated for its conversion to value-added products. Moreover, CO2 in the atmosphere is the main carbon building lock for the biology on earth; CO2 is also an important carbon source for producing value-added products and fuels for industrial applications [1-2]. The volume fraction of CO2 in the atmosphere reaches ca. 400 ppm and keeps a tendency

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of increasing concentration [3]. The environmental hazards from greenhouse gas (e.g.,

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CO2) are more evident recently as demonstrated by the melting even disappearance of

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the iceberg. CO2 capture and conversion to chemicals and fuels can both mitigate the

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greenhouse effect and supply renewable energy [3]. However, the CO2 conversion suffers from the high temperature, high pressure, high-energy substrate, or long time

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reaction due to the high thermodynamic stability of CO2 [3-5]. Developing functional

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materials with high CO2 capacity and catalytic effect is a key factor for CO2 high efficient conversion at mild condition.

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Many materials have been proposed as the favorable CO2 sorbents and catalysts,

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including solid, liquid, and gel-like materials [4, 6-8]. Ionic liquids (ILs) are the representative liquid absorbents and catalysts for CO2 capture and conversion [9-15]. For example, Li et al. found that ILs choline imidazolate showed the best catalyzing effect (yield > 99%) on the formylation of amine with CO2/H2 [9]. Zhao et al. achieved the efficient CO2 conversion to quinazoline-2,4-(1H,3H)-diones at room temperature by utilizing 1,8-diazabicyclo[5.4.0]undec-7-ene-based ILs as the catalysts [10]. Luo et al. made a significant improvement on the CO2 absorption capacity by ILs via the strategy of multiple-site interaction [12]. He et al. summarized the enhancement of CO2 capture and conversion to epoxides by polyoxometalate-based ILs [13]. The deep eutectic solvents (DESs) are also the promising media for the CO2 utilization [16-20]. Cui et al. found that CO2 interacted with hydroxyl group (OH) of azolide-based DESs rather than azolide anion of DESs to obtain nearly 1 mole CO2 per mole solvent [16]. Gu et al. used hydrophobic DESs to capture CO2 with high 3

efficiency and high reusability for the purpose of the refraining from the effect of atmospheric water (H2O) [17]. Zhang et al. designed functional DESs for the efficient CO2 capture with high capacity via chemical interaction between amine group and CO2 to form carboxylate [19]. In order to remove CO2 from flue gases or biosyngas, ChCl:urea shows higher absorption capacity of CO2 than CH4, H2, CO and N2 [20]. Apart from ILs and DESs, solid materials are also the favorable media, including zero-dimensional

(0D),

one-dimensional

(1D),

two-dimensional

(2D)

and

three-dimensional (3D) substances [21-23]. Different kinds of materials possess

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specific advantages and disadvantages. For example, ILs are regarded as sustainable

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solvents and catalysts for CO2 utilization. However, some ILs show complex

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synthesizing procedure, high price, detectable volatility and low stability [24-27]. The

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liquid route for the CO2 capture and conversion is simple and efficient; however, the solid absorbents and catalysts for CO2 utilization is very helpful for the separation. It

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is still necessary to develop novel high performance materials for CO2 capture and

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conversion.

MXenes could be metal carbide, metal nitride material or metal carbonitride with

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a 2D-layered structure [28]. Several functional MXenes (including Ti2CO2, Zr2CO2,

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Hf2CO2, Sc2CO2) have been theoretically investigated and identified as promising directly photocatalysts with suitable electronic structures, excellent visible light adsorption, sufficient active points and high carrier motilities. Though some of them have not been synthesized in lab yet, their synthesis can be well anticipated as the corresponding MAX phases exist [29-33]. Discovery of MXenes is co-founded by Barsoum and Gogotsi group in 2011 by overcoming the strong covalent and metallic bond among adjacent layer in hydrofluoric acid solution [28]. MXenes are derived from naturally layered structure called MAX phase, which have been under investigation in Barsoum's group for many years as thermal barrier and hard coating material. With the specific layered structure, MXene shows high conductivity, favorable chemical stability, and multiple catalytic sites. With the favorable structure and properties, MXene has attracted worldwide attention in many fields, such as energy, catalysis, and separation [34-40]. Many reviews focusing on the 4

electrochemical, biomedical, and photochemical applications on MXene have been published [41-44]. Important role of MXene could be seen in the application field of catalysis,

including

evolution/reduction

hydrogen

reaction,

(H2)

nitrogen

evolution reduction

reaction, reaction,

oxygen

(O2)

photocatalytic H2

production and CO2 reduction [45-46]. Review on the application of MXene in the field of batteries is also carried out by Kannan et al. in addition to CO2 conversion and energy storage [47]. However, to the best of our knowledge, there is no review exclusively reporting

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the CO2 conversion and utilization based on MXene-based absorbents/catalysts due to

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the very recent attentions [30, 48-62]. Here, we summarize the CO2 capture and

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conversion by the materials based on MXene and their composites. The review is

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focused on comprehensive and exclusive summary of CO2 capture, CO2 sensing, thermal, electrochemical, and photochemical conversion of CO2 and the related green

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chemistry, which is different from published reviews. There is no published literature

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on the capture of toxic gases such as SO2, H2S and NO2 by MXene-based materials. However, MXene-based materials could act as promising absorbents to capture or

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[67-68].

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detect acetaldehyde [39, 63], acetone [39-40], NO [64-66], benzene [40] and NH3

The outline of this review is shown as below. First, we summarize the capacity, kinetic, and mechanism of CO2 capture by MXene-based materials. Then, we review the CO2 sensing aided by MXene-based materials. Finally, CO2 conversion via thermo-, electro- and photo- methods based on MXene-based materials is summarized. Furthermore, the underlying mechanism involved the capture, sensing, and conversion of CO2 is summarized. This review would open a new horizon for CO2 valorization with high efficiency and promising widespread applications.

2. CO2 capture by MXene-based materials 2.1 Single component MXene Transition metal carbides were used as potential materials for CO2 capture. However, they owned only a modest exposed surface area [69]. Viñes et al. utilized 5

2D MXene to capture CO2 with high efficiency, where MXene possessed the general formula of Mn+1XnTx (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, n = 1, 2, 3, 4) [43, 51]. Mn+1XnTx could be M2XTx, M3X2Tx, M4X3Tx and so on. The highest CO2 abatement capacity could reach as high as 8.25 mole CO2 per kilogram MXene. The CO2 capture capacity of MXene could be ordered as Ti2CTx > V2CTx > Zr2CTx > Nb2CTx > Mo2CTx > Hr2CTx > Ta2CTx > W2CTx. The adsorption capacity was still high even at low CO2 partial pressure and relatively high temperature. The higher CO2 capture capacity by MXene could be ascribed to the higher surface area, more adsorption

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sites, MXene→CO2 charge transfer and higher adsorption energy [51]. It was

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anticipated to achieve higher CO2 conversion yield by MXene because of the high

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CO2 adsorption capacity even at high temperatures and low CO2 partial pressure. The

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work concluded that the CO2 desorption was mainly affected by the adsorption strength and the adsorption conformation.

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Regeneration of CO2 from absorbent was also very important for the industrial

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application of carbon capture. It was favorable if the CO2 desorption occurred at mild temperature and mild pressure. Apart from adsorption strength and adsorption

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conformation, the numbers of interaction sites might be crucial for CO2 capture and

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regeneration. Multiple sites would contribute to higher capacity of CO2 capture and less energy consumed for CO2 desorption [12]. This might be applicable for MXene and MXene-based composite. As the solid absorbents, adding increasing the ratio, size and distribution of pores in MXene might be a favorable route to improve the CO2 capture. Furthermore, many other factors might affect the CO2 capture by MXene. Zhou et al. found that a higher specific area of prepared MXene enhanced the CO2 capture. Specifically, the adsorption capacity by Ti3C2Tx (21 m2 g-1 surface area) was 5.79 mmol g-1, which was comparable to that of the common CO2 absorbents [70]. Particularly, the theoretical CO2 capture capacity by MXene with the highest surface area could reach 44.2 mmol g-1 [70]. It indicated that tuning the surface area of MXene would be a favorable method for the improvement of CO2 capture capacity. One effective method is to design and prepared ultrathin MXene. However, by using 6

the density functional theory, Morales-Garcia suggested that the effect of atomic layers of MXene on the CO2 capture was rather small [71]. Sun et al. ascribed the high CO2 absorption capacity to the surface lone pair electrons of MXene by first principles calculations [72]. Nine MXene (i.e., Ti2CTx, Zr2CTx, Hf2CTx, V2CTx, Nb2CTx, Ta2CTx, Cr2CTx, Mo2CTx and W2CTx) are selected for the test. Results showed that the Gibbs free energies of CO2 adsorption on MXene were all negative [72], which indicated that the CO2 capture by MXene was thermodynamically favorable. Specifically, the value of Gibbs free energies of CO2

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adsorption by MXene was ordered as Ti2CTx< Hf2CTx < Zr2CTx < V2CTx < Nb2CTx <

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Ta2CTx < Cr2CTx < Mo2CTx < W2CTx. Interestingly, this order was adhere to the

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tendency of IVB > VB > VIB, which was correlated by the lone pair electron density

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of M atoms of MXene that transfers to CO2 [72]. The high adsorption of CO2 by MXene could also be corroborated by the elongated bond length of C-O bond (e.g.,

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1.16 Å to 1.36/1.37 Å) and the reduced angle of ∠O-C-O (e.g., 180o to 116o) [72].

Figure 1. Hydrogen bonds between MXene and Pebax in Pebax@MXene composite (a). MXene prepared in different solvents and the corresponding Pebax@MXene 7

composite (b). Interaction mechanism between Pebax and MXene (c). Fractional free volume of Pebax@MXene composite via theoretical calculation (d) [50]. Reproduced with permission from ref. [50]. Copyright (2020) American Chemical Society.

2.2 MXene-based composite Soroush et al. found that Pebax@MXene composite (Pebax is the trade name of polyether block amide), prepared by embedding 2D Ti3C2Tx MXene nanosheet into a

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rubbery polymer, could achieve high separation performance of CO2/N2 and CO2/H2

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[50]. Furthermore, this MXene-based composite for CO2 separation was sustainable,

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stable, energy-efficient, and cost-efficient [50]. The high CO2 capacity could be

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ascribed to the well-formed galleries of MXene nanosheets created by hydrogen-bonding interaction between MXene and rubbery polymer (Figure 1a) [50].

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Also, the uniform dispersion of MXene in composite would also contribute to the high

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efficiency of CO2 separation (Figure 1b). Theoretical calculation showed that hard segment of Pebax was the main interacting site to bind the surface of MXene (Figure

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1c). Fractional free volume of Pebax@MXene composite was also higher than that of

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Pebax@graphene oxide and Pebax, (Figure 1d), which was consistent with the higher CO2 separation by Pebax@MXene [50]. CO2 capture and separation by Pebax@MXene was not efficient because the strength of interaction was weak and the channel for transporting CO2 was deficient. However, the CO2 separation could be improved by the Pebax@MXene composite. Mechanism of CO2 and N2 separation by Pebax@MXene could be proposed as Figure 2. Specifically, CO2 tended to be easier to bind to the MXene surface than N2 due to the higher quadrupole moment of CO2 than N2. The terminated OH in MXene enhanced the CO2 capture capacity, hence a high CO2 selectivity. The size of MXene galleries was about 0.35 nm, which was greater than that of CO2 while smaller than that of N2 [50]. It indicated that CO2 was easy to permeate and diffuse while N2 was obstructed [50]. Also, the introduction of MXene in Pebax@MXene composite would increase the transport pathways for CO2 diffuse. Pebax showed higher extent of phase separation after mixing with MXene, 8

which was favorable for the CO2 permeability. Cost of CO2 capture by Pebax@MXene was about $29/ton CO2, which was much lower than that by Pebax membrane (ca. $40/ton CO2). The decrease in cost for CO2 capture and separation by

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Pebax@MXene was very important for the industry.

Figure 2. Transportation of CO2 and N2 via the Pebax@MXene composite [50].

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Reproduced with permission from ref. [50]. Copyright (2020) American

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Chemical Society.

Jin et al. also fabricated Pebax@MXene composite to improve the capacity of CO2 capture, the permeance of CO2 (ca. 21.6 GPU) and the selectivity of CO2/N2 separation (ca. 72.5) [49]. The superior CO2 capacity was mainly explained by the 2D channels of the composite [49]. The CO2 capture capacity and selectivity might be improved by tuning the categories of the polymer. Soroush et al. found that polyurethane@MXene composite also showed comparable efficiency of CO2 separation [50]. It implied that this strategy would provide a promising route for the improvement of CO2 capture and separation by MXene-based composite. 2.3 Atomic defects of MXene

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Double transition metal carbide MXene is very interesting and meaningful. Khaledialidusti et al. noticed that the intrinsic defects of MO2TiC2Tx MXene favor the CO2 adsorption because of the specific surface terminated with F, O or OH [48]. Results showed that the synthesized structural defects were determined by the categories of terminators as indicated by the theoretical formation energy of defect. For the defect formation, the outer layer (i.e., molybdenum layer) was easier than the inner layer (i.e., titanium layer). Also, the formation of an O-typed terminator consumed higher energy than that of F- and OH-typed terminators [48]. CO2 was

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strongly attached to the defective sites of MXene spontaneously via an exothermic

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process. However, perfect sites of MXene interacted with CO2 weakly and

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nonspontaneously via an endothermic process [48]. It was predicted that increasing

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the ratio of atomic defects on the MXene would improve the CO2 capture capacity. 2.4 Conclusion of CO2 capture by MXene-based materials

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Improving the CO2 adsorption by MXene could enhance the CO2 sensing and

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conversion by MXene-based materials. Commonly, CO2 capture capacity was affected by external factors, such as temperature, pressure, impurities. The structural

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factors for the improvement of CO2 capture included adsorption strength, adsorption

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conformation, number of interaction sites, and the ratio/size of pores in MXene-based hybrids. Absolutely, CO2 capture capacity could also be enhanced by the functionalized of MXene and the introduction of CO2-philic components in the MXene-based composite. More attention should be paid on the capture of CO2 with high capacity and high selectivity at mild temperature and low CO2 concentration, particularly in the case of CO2-containing mixed gases. Other gas might be occupy the interaction sites on the MXene-based absorbents/catalysts aimed for CO2 capture and conversion, which was one of the practical and troublesome kinds of stuff for the industrial application of CO2 capture and conversion to value-added products sustainably.

3. CO2 sensing by MXene-based materials 3.1. CO2 sensing 10

Sensor could be used to detect temperature, pressure, color, force, humidity, taste, magnetism and so on. CO2 sensing is important for the detection of CO2 concentration in industrial, environmental, and home. CO2 sensor relies on the signal generated from the information detected, and transformation of the signal to electrical signal or other output. A good CO2 sensor possesses the features of being fast, cheap, automatical, stable, interference-free, efficient and green. An underlying premise for the CO2 sensor is the interaction between the materials in sensor and CO2. Also, CO2 sensing requires the contact of CO2 with the sensor. Commonly, stronger interaction would

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make the CO2 sensor more sensitive. The current material for CO2 sensor is based on

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dye [73], nanoparticle [74], ILs [75], phononic crystal [76], moist paper [77],

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copolymer [78] and composite [79]. The voltage requirement, reusability and

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recyclability for CO2 sensor are also favorable. Performance of CO2 sensor could be improved by designing novel materials. Compared to the internet of things (IoT)

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devices for CO2 sensing, MXene-based CO2 sensor owns the advantage of high

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sensitivity theoretically due to the high conductivity and high tunability of MXene-based materials.

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Chen et al. utilized V2CTx@poly(2-(dimethylamino)ethylmethacrylate) hybrid

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materials to achieve a smart response of CO2, where polymer was loaded into the interlayer of V2CTx [62]. V2CTx MXene was obtained by immersing V2AlC into aqueous HF at mild temperature. V2CTx MXene was then attached by poly(2-(dimethylamino)ethylmethacrylate) via the terminated OH and fluorine groups on the surface through self-initiated photografting and photopolymerization [62]. The conductivity of V2CTx@poly(2-(dimethylamino)ethylmethacrylate) could be tuned by purging CO2 or desorbing CO2. Mechanism showed that the amine group in V2CTx@poly(2-(dimethylamino)ethylmethacrylate) interacted with CO2 to form ammonium bicarbonate, which could be regenerated by bubbling N2. Correspondingly, the conductivity was increased about 30 μS cm-1 after absorbing CO2 and turned back to the original value while desorbing CO2 [62]. The key for the favourable CO2 sensing is to develop MXene-based materials with high conductivity, polymer with CO2-philic groups, and strong interaction 11

between MXene and polymer to form stable hybrid and to achieve a synergistic effect. It is anticipated to explore better hybrids containing responsive polymer and functionalized MXene for better detection of CO2, particularly at super low/high concentration, mixed atmosphere, fast response, or temperature tolerance. Theoretically, the development of single component functionalized MXene with high performance of CO2 detection capacity is also feasible and promising, which could be cheaper, simpler, and more time-saving than by MXene-based hybrids. However, there are only limited reports investigating the MXene-based CO2 sensor.

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3.2. Conclusion of CO2 sensing by MXene-based materials

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The reports related to the CO2 sensing by MXene-based materials are rare. The

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design of alkaline MXene is one of possible strategies for improving the efficiency of

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CO2 sensing. Possible routes include the introduction of alkaline terminated group, the alkaline mixtures, or the MXene-based materials with high surface area and

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appropriate pore distribution. The effect of thickness, pore size, surface area,

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alkalinity, elements of MXene-based materials on the CO2 sensor should be elaborated for the purpose of obtaining the rule of designing novel MXene-based

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materials. To obtain the flexible and green CO2 sensor, the inclusion of green solvents

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(e.g., DESs and ILs) in MXene-based materials might also be one of the promising solutions.[80]

4. CO2 conversion by MXene-based materials 4.1 Thermoconversion of CO2 4.1.1 Single-atom@MXene. Catalyst was the key for the efficient conversion of CO2 powered by heat. Single-atom catalysts were highly efficient and widely applied for thermal conversion of CO2 to value-added products. However, they are not stable and tend to aggregate to lose their catalytic activity [81]. Stabilization of single-atom catalysts required support, such as heteroatom-doped carbon, metal oxides, and metal-organic framework, which commonly needed high temperature or dangerous H2 atmosphere [81]. Chen et al. achieved the stabilization of single Pt atom on Ti-deficit MXene (i.e., Pt1/Ti3-XC2Ty) at room temperature without dangerous H2 atmosphere 12

[61]. Specifically, single Pt atom stabilized at the deficit places in MXene via the

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formation of Pt-C bond (Figure 3) [61].

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Figure 3. Synthesis of Pt1@MXene composite by the self-reduction stabilizing

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American Chemical Society.

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process [61]. Reproduced with permission from ref. [61]. Copyright (2019)

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Pt1@Ti3-XC2Ty could catalyze 1 atm CO2 to react with amines and silane to

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produce formamides with almost 100% conversion rate and 100% selectivity (Figure 4a). There was nearly no formanide produced catalyzed only by Ti3-XC2Ty or only by

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the precursor of Pt1 [61]. Moreover, Pt nanoparticles supported on MXene showed only 18% conversion rate, which is much lower than the nearly 100% conversion rate catalyzed by Pt1@Ti3-XC2Ty (Figure 4b). Moreover, CO2 conversion catalyzed by MXene or by the precursor of Pt single atom was negligible (Figure 4b). The conversion rate would decrease by substituting anilines with electron-withdrawing groups, while increase by the functionalization of anilines with electron-donating groups. Furthermore, aliphatic and secondary amines led to a higher yield of products [61]. It should be noted that the temperature for the CO2 conversion was relatively high (140 oC) and the time needed for the reaction was still long (10 h). More importantly, volatile organic compounds (e.g., DMF) were used during the process of CO2 conversion, which is not environmentally friendly. It is anticipated to design other single-atom@MXene hybrids to achieve the efficient conversion of CO2 at room 13

temperature and atmospheric pressure at a high rate without the usage of organic solvents. It seemed that the recyclability of Pt1@Ti3-XC2Ty needed to be improved because there was about 20% loss of CO2 conversion rate after five times reusability

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although the selectivity kept very high (Figure 4c).

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Figure 4. Chemical reaction of CO2 thermal conversion with aniline and Et3SiH (a). CO2 thermal conversion catalyzed by different catalysts (b). Regeneration of

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MXene-based composite catalysts for CO2 thermal conversion (c) [61].

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Reproduced with permission from ref. [61]. Copyright (2019) American Chemical Society.

4.2 Electroconversion of CO2 4.2.1 Single component MXene. To the best of our knowledge, almost all the reports on the electroreduction of CO2 were limited to the single component MXene. DFT theoretical calculation results by Sun et al. showed that Cr3C2Tx and Mo3C2 TxMXene delivered the best efficiency for the electrochemical CO2 conversion to CH4 among all the M3C2Tx MXene investigated [82]. The functionalization of MXene surface with O or OH would reduce the energy barrier for CO2 electroconversion to CH4. The release of H2O required less energy from OH-covered MXene than from O-covered MXene (Figure 5), which implied that MXene tended to be dehydroxylation [82]. More important, Cr3C2Tx and Mo3C2Tx MXene were predicted 14

to be more CO2-philic than H2O-philic, indicating a higher possibility for CO2 electroreduction than H2 evolution [82]. This advantage of MXene would make CO2 capture and electroconversion in the situation of moisture environment [82]. Previous reports by Liu et al. also suggested that the presence of ILs would also enhance the

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CO2 reduction while suppress H2 evolution [83].

Figure 5. The mechanism of deoxidation and dehydroxylation of O/OH group on

spontaneous

and

nonspontaneous

reaction,

respectively

[82].

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indicates

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O-covered Mo3C2Tx (a) and OH-covered Mo3C2Tx (b). Blue and red color

Reproduced with permission from ref. [82]. Copyright (2017) American Chemical Society.

Using a theoretical DFT study, Seh et al. investigated the M2XO2-typed MXene for CO2 electroconversion [84]. The theoretical calculation showed that W2CO2 and Ti2CO2 were the two best electrocatalysts for the conversion of CO2 to CH4, which owned 0.52 and 0.69 V overpotential theoretically [84]. Moreover, most of MXene catalyzed CO2 via the *HCOOH route rather than *CO route. Intermediates of the CO2 electroreduction could be stabilized by the O-terminated groups on MXene, allowing the CO2 electroreduction to proceed at mild condition [84]. Mechanism showed that charge around H would be transferred to the area between C and H (Figure 6a); however, it would transfer from O-H bond to C-H bond for *(H)COOH 15

and *(H)CHO (Figures 6b and 6c) [84]. Energy barriers for *(H)COOH → *CO (Figure 6d) and *CO→ *(H)CHO (Figure 6e) were 0.4 eV and 0.21 eV, respectively [84]. It suggested that MXene was the favorable electrocatalyst for CO2

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electroreduction.

Figure 6. Deformation charge density for CO2 (a), *(H)COOH (b) and *(H)CHO (c). Potential-limiting step existed in (d) while there is no transition state in (e) [85]. Reproduced with permission from ref. [85]. Copyright (2019) American Chemical Society.

Zhang et al. utilized DFT-based first-principle schemes for the simulation of CO2 electroreduction by 17 kinds of OH-terminated MXene [85]. Results showed that CH4 was the main product and ScC(OH)2 was the most favorable electrocatalysts owning 16

the least negative limiting potential of -0.53 V [85]. Function of OH on MXene could stabilize the adsorbed species to proceed via an alternative reaction route [85]. Furthermore, by DFT calculation, Xiao et al. concluded that CO2 could be activated by M3C2Tx MXene and hence reduced to colorful products (e.g., CO, CH3OH, HCHO, HCOOH, CH4) via the pathway of bicarbonate species [86]. Bicarbonate species were formed by the combination of CO2 and surface hydrogen, while HCHO was estimated as the main intermediate [86]. Due to the higher suppression of H2 evolution, the efficiency of CO2 electroreduction was high [86].

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Recently, Zhang et al. found that M2XO2 type MXene with O-terminated group

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and defects could improve the electrochemical reduction of CO2 by the first-principles

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modeling simulation [87]. Results showed that *COOH and *CHO were the –C

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coordinated fragments intermediates, while *HCOOH and *H2COH were –H coordinated intermediates with the form of molecule [87]. During the process of CO2

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electroreduction, the defect on the MXene could make the bound of *COOH and

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*CHO stronger while *HCOOH and *H2COH were influenced slightly [87]. The Fermi level of MXene also altered significantly after introduction of vacancy in

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MXene, which could be used to estimate the electrochemical catalytic efficiency of

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MXene. Among all the defected MXene investigated, Hf2NO2 with Hf vacancy showed the most promising application [87]. Although theoretical prediction was simple and cheap, it was important to design the targeted MXene to achieve the targeted products. It was because that the real structure of MXene would be different from that of the synthesized MXene. The electrolyte was also the key factor for the CO2 electroreduction [88-90]; however, it was hard to simulate the effect of electrolyte on the CO2 electroconversion in the theoretical calculation. It was still interesting to combine experiments and theoretical calculations to obtain the high efficiency of CO2 electroreduction and mechanism interpretation. By combining experiments and theoretical calculations, Seh et al. used Ti2CTx and MO2CTx as the electrocatalysts of CO2 reduction and found that HCOOH was the major product [91]. Ti2CTx electrocatalyst could obtain over 56% Faradaic efficiency 17

and MO2CTx electrocatalyst could obtain 2.5 mA cm-2 partial current density. Moreover, Seh revealed the functionalization of F on MXene resulted in a lower

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overpotential of CO2 electroreduction [91].

Figure 7. Conventional (a), expanded (b) and high-resolution (c) TEM of

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MXene-based composite. The corresponding STEM image (d) and the elemental

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mapping profiles (e-h) of Ti, Pb, Br and Cs [60]. Reproduced with permission

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from ref. [60]. Copyright (2019) American Chemical Society.

4.2.2 MXene-based composite. Apart from the single component MXene, MXene-based nanocomposite was also the promising electrocatalyst for CO2 electroreduction. Sadasivuni et al. found that ZnO-Fe@MXene nanocomposite showed a higher current density (18.7 mA cm-2) of CO2 electroreduction than that of ZnO, MXene, ZnO-Fe and ZnO-MXene [92]. Instead, there was no current peak under the N2 gas atmosphere for ZnO-Fe@MXene. In comparison with other electrocatalysts (e.g., Cu@Sn, B-doped graphene), ZnO-Fe@MXene also showed more favorable current density due to the high surface area, high conductivity and many active sites. Furthermore, ZnO-Fe@MXene was more stable than that of ZnO-MXene, MXene, ZnO and ZnO-Fe, as demonstrated by the high retention rate (88%) of oxidation current after 1000 cycles for ZnO-Fe@MXene [92]. The ternary 18

components in ZnO-Fe@MXene for CO2 electroreduction played individual functions. ZnO was helpful for the CO2 adsorption. Fe favored the charge transfer and suppressed the H2 evolution. MXene played the role of electron transport and other functions as discussed above. The synergistic effects of the three components (Fe, ZnO and MXene) resulted in the highly efficient CO2 electroreduction. Common electrolytes of CO2 electroreduction were inorganic solutions or ILs; however, seawater was cheaper, more renewable and more conductive. Liu et al. found that the CO2 electroreduction in the media of seawater was highly efficient by

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MXene-based materials. The Faradaic efficiency and the CO partial current density

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could reach 92% and -16.2 mA cm-2, respectively, which was comparable to that of

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noble-metal electrocatalysts [93]. The key for the high efficiency of CO2

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electroconversion was the introduction of N-doping and the metal vacancies into Ti3C2Tx MXene, which reduced the energy barrier for the intermediates (e.g., *COOH,

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*CO) significantly. The efficiency of CO2 electroreduction by MXene with doping

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and vacancies was much higher than that of pristine MXene [93]. The more negative free-energy change of doped-MXene with *H (-0.19 eV) than that of MXene with *H

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(-0.13 eV) suggested that H2 evolution was more suppressed on doped-MXene than

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on MXene. Moreover, the free-energy change of doped-MXene with CO2 (-0.01 eV, negative) was much lower than that of MXene with CO2 (0.02 eV, positive), indicating a more favorable capture of CO2 by doped-MXene. The positive value of 0.02 eV implied that pristine MXene could not capture CO2 spontaneously at that condition [93]. 4.3 Photoconversion of CO2 4.3.1 Perovskite@MXene. MXene nanosheet was the favorable material to improve the photoreduction of CO2. Liu et al. found that perovskite@MXene (e.g., CsPbBr3@Ti3C2Tx) composite could reduce CO2 under simulated solar light irradiation into CO and CH4 with the yield of 26.32 and 7.25 μmol g−1 h−1, respectively [60]. This yield was higher than that of CsPbBr3 and CsPbBr3-based heterostructures [60]. It could be ascribed to the efficient charge separation, favorable charge transfer from CsPbBr3 to Ti3C2Tx. Also, CsPbBr3 was cubic and uniformly 19

dispersed on the surface of MXene (Figures 7a~7d). Elemental mapping profiles also suggested that the perovskite@MXene composite was the successful loading of CsPbBr3 on MXene (Figures 7e~7h). It would also contribute to the high efficiency of

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CO2 photoreduction.

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Figure 8. Photocurrent of MXene-based composite irradiated by LED (a) and X-ray

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(b). Energetic diagram of MXene-based composite (c) and the yield of products (d) [60]. Reproduced with permission from ref. [60]. Copyright (2019) American

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Chemical Society.

Furthermore, the photocurrent from the irradiation of perovskite@MXene by visible light and X-Ray could be repeated for several times with only negligible loss (Figures 8a and 8b). Also, the photogenerated electron was easy to transfer from perovskite to MXene due to the energy alignment between perovskite and MXene, and the conduction band offset of 1.5 eV (Figure 8c). CO and CH4 were the main products while other product could not be detected (Figure 8d). Note that organic solvent (i.e., ethyl acetate) was used in the photocatalytic system of CO2 reduction for the purpose of dissolving a high concentration of CO2. We believed that improving the CO2 solubility in the solvent could enhance the yield of CO and CH4, even change

20

the categories of products. Moreover, CO was produced by the photodecomposition of ethyl acetate, indicating that ethyl acetate was not photochemically stable [60]. Procedure of synthesizing perovskite@MXene was simple and facile. However, the etching of Ti3AlC2 still required toxic HCl-HF solution. The yield of MXene with monolayered flakes would be extremely low due to the gradient centrifugation, where multilayered MXene in the bottom was settled and monolayered MXene was collected. Moreover, volatile and toxic organic compounds (toluene, DMF, hexane), were needed as the liquid media to obtain perovskite@MXene composite, because the

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utilization of water would degrade perovskite. Mechanism of perovskite@MXene

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synthesis was related to the formation of nucleation sites from oleylamine and oleic

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acid on the terminated groups of MXene by absorbing precursors. During the process,

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cesium and precursors were both consumed [60].

4.3.2 1D Cu2O@0D MXene quantum dots (QDs). MXene QDs enhanced the

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photoreduction of CO2 reduction significantly. Chen et al. synthesized a novel

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heterostructure by combining Ti3C2Tx MXene QDs and 1D Cu2O nanowire via a simple and efficient method [59]. Specifically, the aluminum in Ti3AlC2 was first

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etched away by HF and DMSO, where was presumed to be replaced by terminated

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moieties, such as OH, =O and F. MXene microsheets could be obtained after ultrasonication, followed by the passivation via polyethylenimine to produce QDs. The essence of synthesizing Cu2O@MXene composite was the electrostatic interaction between the positively-charged surface of Cu2O nanowires by poly(sodium 4-styrenesulfonate) and negatively-charged surface of MXene by polyethylenimine. Moreover, poly(sodium 4-styrenesulfonate) and polyethylenimine were easy to be removed by calcination in protective atmosphere. However, the porous surface on the Cu2O nanowire was covered after the calcination although morphology of the composite was slightly affected [59]. This hybrid material improved the efficiency of CO2 photoconversion into CH3OH because of the stabilization of the Cu2O nanowire, the improvement of charge transfer/carrier density/light adsorption, and the suppression of charge recombination [59]. Photogenerated electrons were first transferred from the valence band (1.497 V) 21

to the conductive band (-0.703 V vs normal hydrogen electrode) of Cu2O nanowire under the simulated solar light. Then, the photogenerated electrons transferred from conductive band of Cu2O nanowire (-0.703 V) to MXene due to the less negative Fermi level of MXene (-0.523 V), which was able to reduce CO2 into CH3OH (-0.38 V) rather than CO (-0.53 V). Also, MXene could capture CO2 with a higher tendency than H2O, leading to the preferable CO2 photoreduction than H2 evolution. The yield of CH3OH from CO2 catalyzed by Cu2O@MXene nanowire was 8.25 times of that by Cu2O nanowire, indicating the importance of MXene for the photoreduction of CO2

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[59]. It should be aware that MXene might be unstable during the photocatalytic

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process although it would not inhibit the CO2 photoconversion, as suggested by Chen

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et al. [59].

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4.3.3 TiO2@MXene. Surface alkalinization of MXene could be deemed as the booster for the CO2 photoreduction. Wang et al. claimed that Ti3C2Tx MXene was a

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noble-metal-free cocatalyst with commercial TiO2 for efficient CO2 photoconversion [58]. Surface alkalinization MXene with F or OH supported on TiO2 could boost the

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production of CO (11.74 μmol g−1 h−1) and CH4 (16.61 μmol g−1 h−1) from CO2

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photoreduction, which was 3 times and 227 times higher than that of TiO2. Moreover,

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the synthesis procedures of TiO2@MXene were facile, i.e., simply by mixing MXene with TiO2. The alkalinization of MXene was achieved by mixing with KOH. In this way, the F atom on the surface of MXene could be replaced by OH, which could be demonstrated by energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Nevertheless, synthesis via simple mixing might not be favorable for the long-term reusability due to the weak interaction [58]. Wang et al. attributed this high efficiency to the high electrical conductivity, superior charge separation, high CO2 capture, and multiple catalytic sites of the functionalized surface of MXene. Moreover, the photogenerated electrons transferred from the conductive band of TiO2 to functionalized MXene because of the lower Femi energy level of functionalized MXene than that of TiO2 [58]. It was understandable because the alkalinization of MXene was easier to bind the acid CO2 via base-acid interaction, hence higher adsorption and activation of CO2. The 22

efficiency of CO2 photoconversion by this system could be further improved by the functionalization of MXene with other groups (e.g., NH2) and control of size/layer numbers of MXene. TiO2@MXene hybrid (no surface alkalinization of MXene) could be used as a favorable photocatalyst for CO2 reduction [54]. However, CH4 yield from CO2 photoreduction catalyzed by TiO2@MXene was ca 3.7 times that of commercial TiO2 [54], which seemed lower than that of TiO2@functionalized MXene (277 times) [58]. Although TiO2@MXene hybrid showed lower efficiency, the interaction between

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TiO2 and MXene was stronger because TiO2 grafted on the surface of MXene in situ

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by calcination via chemical reaction [54]. The high temperature also contributed the

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formation of anatase TiO2. It would increase the specific surface area and extended

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the cycles for reusability for CO2 conversion. Moreover, both the content and the crystallinity of TiO2 increased at a higher temperature [54]. The product distribution

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was different too. CH4 was mainly obtained while there was no CO produced [54].

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There was also minor CH3OH and C2H5OH obtained, while it was less than that of CH4. It was because that the reduction potential of CO2→CH4, CO2→CH3OH,

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CO2→C2H5OH was -0.24 V, -0.38 V, -0.33 V, respectively, among which reduction

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potential of CO2→CH4 was less negative indicating the most favorable tendency for CH4 production. The common points for the CO2 photoreduction by TiO2@MXene and TiO2@functionalized MXene were the high conductivity, efficient charge transfer/separation, and low charge recombination after the introduction of MXene. 4.3.4 CeO2@MXene. Liu et al. synthesized the CeO2@Ti3C2-MXene composite simply by hydrothermal method at 180 oC for 24 h, where CeO2 showed cubic shape. CeO2@MXene showed higher efficiency (ca. 1.5 times) of CO2 reduction to CO than pristine CeO2 under solar light irradiation [55]. It might be due to the faster charge transfer, higher photocurrent, and lower interfacial resistance for CeO2@MXene than for CeO2. Moreover, the improvement of CO2 photoreduction by CeO2@MXene might be attributed to the built-in electric field between CeO2 and MXene, which enhanced the electron transfer from MXene to CeO2 and led to the Schottky junction between MXene and CeO2 [55]. CeO2@MXene with 5% MXene content was found to 23

be the composite with the highest performance for CO2 photoreduction among all the CeO2@MXene hybrids investigated [55]. The best CO yield catalyzed by CeO2@MXene could reach 40.2 μmol m−2 h−1 [55]; however, products of CO2 photoreduction by TiO2@MXene included both CO and CH4 [58]. It indicated that the categories of metal oxide in the MXene-based composite influence the categories of products. It was anticipated that other kinds of metal-oxide semiconductors hybridized with MXene would tune the product distribution and the yield from the CO2 photoreduction.

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However, the content of MXene in CeO2@MXene composite was very low (5%),

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leading to the non-detection of MXene diffraction peaks [55]. The low content of

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MXene in the composite implied a low cost. Moreover, hydrothermal method was

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simple for the synthesis of composite. It should be noted that the presence of water in the composite would increase the difficulty for the water removal due to the surface of

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MXene was hydrophilic. The structure of MXene-based materials might also changes

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after drying.

4.3.5 C3N4@MXene. Due to the lower solubility of CO2 in the MXene-based

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composite, alkalization of MXene-based composite was an efficient route for the

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improvement of CO2 capture and activation, resulting in the favorable CO2 photoreduction. Wang et al. found that CO production (11.21 μmol g−1) from CO2 photoreduction by C3N4@alkalized MXene (5%) was ca. 6 times of that by C3N4 (1.88 μmol g−1) [56]. Moreover, 5% alkalized MXene in C3N4@alkalized MXene was the optimal concentration for CO2 photoreduction to obtain CO; instead, C3N4@alkalized MXene with 1% and 10% MXene only owned 10.09 and 8.85 μmol g−1, respectively [56]. Except for CO, some other gaseous products were detected, such as CH4, C2H4, CH3CHO, although the liquid product was not detectable [56]. The interesting finding was the detection of C2H4 from CO2 photoreduction, which could be improved by tuning the components and composition of MXene-based composited. More value-added products (e.g., C3H6, C3H8) might be promising targets. The high photocatalytic performance of C3N4@alkalized MXene was ascribed to the high electric conductivity, high CO2 adsorption capacity, and favorable 24

electron-hole separation [56]. It should be noted that the nanosheets’ shape of C3N4@alkalized MXene might be destroyed into curved porous sheets in some sense due to the alkalization of MXene by KOH. It should be noted that the synthesis of C3N4@alkalized MXene was simply by mixing C3N4 and alkalized Ti3C2. However, too much toxic HF (80 mL HF for 4 g Ti3AlC2) was used for synthesizing MXene. Afterwards, much water would be used to wash the MXene to obtain the product with the PH about 6, which implied that a large volume of acid wastewater would be released to the environment. Furthermore, the

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ratio of KOH to MXene (200 mL 2 mol L-1 KOH to 1 g MXene) was very high for the

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purpose of obtaining alkalized MXene [56], which would lead to the release of

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alkalized wastewater. It could be mixing the acid wastewater and alkalized

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wastewater above to minimize the effect on the environment. The ratio of the above two kinds of wastewater could be tuned to be neutralization. C3N4@alkalized MXene

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was synthesized via simple mechanical stirring [56]. Although the procedure was

MXene composite.

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simple, the weak interaction might cause the loss of C3N4 from the C3N4@alkalized

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Lv et al. obtained ultrathin 2D/2D C3N4@MXene by utilizing urea as the remover

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of Ti3C2Tx to nanosheets and as the precursor of C3N4 [57]. Similarly, CO2 photoreduction by C3N4 alone could not produce abundant products due to the easy charge recombination [57]. No product was observed by MXene because MXene possessed metallic property. Instead, C3N4@MXene composite enhanced the CO and CH4 yield significantly under visible-light illumination in the absence of sacrificial agent [57]. The high yield was due to the high charge transfer, charge separation, conductivity, and CO2 adsorption/activation [57]. The maximal yield of CO and CH4 yield could reach 5.19 and 0.044 μmol g−1 h−1. In this case, there was no detection of C2H4 and CH3CHO [57] although the photocatalysts were similar to that of Wang et al [56]. It might be ascribed to the difference in the shape, size, degree of contact, and alkalization of the composite. Too low and too much MXene in C3N4@MXene was not favorable for the CO2 photoreduction, which was similar to that of other reports [55, 57]. Here, another half-reaction was the oxidation of H2O into O2, which was 25

clear and cheap. This half-reaction could be used to produce more colorful and valuable products, such as biomass valorization. 4.3.6 TiO2@C3N4@MXene S-scheme. TiO2@C3N4@MXene was found to be the favorable heterojunction for CO2 photocatalysis. Macyk et al. found that van der Waals interaction accounted for the 2D/2D core-shell structure, while 0D MXene QDs were attached on the surface of C3N4 via electrostatic interaction [53]. The synthesis MXene QDs would require at least five kinds of solvents, i.e., H2O, HF, DMSO, glycerol and ammonia. Functions of H2O, HF and DMSO were described as

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above. Glycerol played the role of structure-directing agent for the synthesis of

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TiO2@C3N4 core–shell nanosheets by in situ calcining urea on the surface of TiO2

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nanosheets at high temperature. The function of ammonia was to passivate

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functionalized MXene QDs by reacting with epoxy groups to produce amine groups [53]. Due to the enrichment of terminated groups in MXene QDs, the

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TiO2@C3N4@MXene composite could be easy obtained via electrostatic interaction

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after long-term stirring and strong sonication [53]. It should be noted that only the filtered suspension with 200 nm membrane was MXene QDs while the precipitate

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was called MXene bulk [53]. The yield of MXene QDs was very low, which would

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increase the cost of application. It is anticipated to recycle of MXene bulk to synthesize MXene QDs to improve the yield of MXene QDs. 2D/2D/0D TiO2@C3N4@MXene composite with S-scheme showed higher photoactivity of CO2 reduction into CO and CH4 than TiO2, C3N4, TiO2@C3N4, and C3N4@Ti3C2. It indicated that the S-scheme existed in TiO2@C3N4@MXene composite was necessary for the high performance of CO2 photoreduction. The maximal yield of CO and CH4 catalyzed by TiO2@C3N4@MXene could reach 4.39 and 1.20 μmol g−1 h−1, respectively, which was 3 and 8 times higher than that of TiO2. The high efficiency was ascribed to the enhanced photoinduced electrons transfer, charge separation, and active catalytic sites from MXene. However, it seemed that the absolute value of CO and CH4 yield was not very high when compared with the similar photocatalysts, such as C3N4@MXene [56-57]. The categories of products by TiO2@C3N4@MXene [53] were the same as that by C3N4@MXene (i.e., CO, CH4) 26

[57], which instead was different from that by C3N4@alkalized MXene (i.e., CO, CH4, C2H4, CH3CHO) [56]. 4.3.7 Bi2WO6@MXene. Ultrathin 2D/2D Bi2WO6@MXene was successfully synthesized by Yu et al. via the electrostatic interaction between positive Bi3+ and negative Ti3C2 MXene [52]. The electrostatic interaction was the basis of forming Bi2WO6 nanosheets on the surface of MXene to obtain the Bi2WO6@MXene composite with intimate contact. Compared to simply mechanical mixing, method, this procedure would endow a more favorable recyclability and a higher stability of

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Bi2WO6@MXene. Moreover, the intimate contact of the ultrathin 2D/2D structure of

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Bi2WO6@MXene resulted in the short charge transport and large contact area, which

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was favorable for the charge transfer [52].

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The pore structure and the high specific surface of Bi2WO6@MXene enhanced the CO2 capture and activation. These particular properties of Bi2WO6@MXene

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improved the CO2 photoreduction into CH4 and CH3OH under simulated solar

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irradiation, which was 4.6 times higher than that of pristine Bi2WO6 [52]. Mechanism showed that conductive band of Bi2WO6 was more negative than that of Fermi level

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of MXene, improving the transfer of photogenerated electrons from Bi2WO6 to

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MXene. Furthermore, the ultrathin structure made the electron-transferring process faster. Then, the photogenerated electrons on the surface of MXene were used for the CO2 reduction. It should be noted that CH4 and CH3OH were obtained, which was related to the categories of Bi2WO6@MXene photocatalysts. The types and yield of products were anticipated to be improved by tuning the categories of salt, the concentration, the morphology, and the size of salt@MXene composite. 4.3.8 Co-Co@MXene. Ultrathin Co-Co layered double hydroxide@MXene nanocomposite was used as the photocatalysts for the conversion of CO2 into CO. The CO generation rate could reach as high as 12500 μmol g−1 h−1 under the visible-light irradiation (> 400 nm), which was much higher than that of the pristine Co-Co photocatalyst [94]. The high efficiency could be ascribed to the enhanced separation and the fast transmission of photogenerated electrons, the high conductivity and the hierarchical architecture. Also, this system showed excellent stability and high 27

apparent quantum efficiency. However, Ru-based photosensitizer was used for the visible-light photocatalysis of CO2. The usage of noble metal (e.g., Ru) in photosensitizer would increase the cost although the efficiency was enhanced. Moreover, Deng et al. found that the expensive Ru-based photosensitizer would be exhausted as demonstrated by the decrease of rate of CO as the time [94]. Therefore, new photosensitizer should be added for each cycle experiment in the visible-light photocatalysis. After the photocatalysis, waste photosensitizer containing the heavy metal ion (i.e., Ru) would be released to cause water pollution, do harm to the

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ecosystem and increase the cost. This kind of Co-Co@MXene photocatalyst coupled

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with Ru-based photosensitizer was still in its fancy; however, it provided a novel

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protocol of MXene-based photocatalysis system for CO2 photoreduction. The cost and

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the pollution could be minimized if methods were developed to regenerate the Ru-based photosensitizer.

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Photocatalysis mechanism showed that Ru-based photosensitizer was firstly

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transformed into the excited state under the visible-light irradiation, followed by the quench via sacrificial reagent to generate reduced photosensitizer [94]. Then, the

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electron on the reduced photosensitizer was transferred to the MXene-based

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composite [94]. Finally, the active site (i.e., Co) received the transferred electron for the CO2 reduction. Meanwhile, the proton in the system was converted to H2 by the reduction of the above electron [94]. Mott-Schottky test showed that the flat band potential of Co-Co@MXene composite (-1.21 V vs. Ag/AgCl, i.e., -1.01 V vs. NHE) was lower than that of redox potential value of Ru-based photosensitizer (-1.09 V vs. NHE), indicating that the electron could be transferred from the latter to the former spontaneously for the reduction of CO2 into CO. Energy level diagram suggested that conduction band minimum (i.e., 3.33 eV) of Co-Co@MXene composite was between the LUMO (3.19 eV) and HOMO (i.e., 5.68 eV) energy of Ru-based photosensitizer. Thus, electron in LUMO could be favorably transmitted into conduction band of MXene-based composite, where the CO2 reduction occurs. 4.3.9 Theoretical mechanism. The mechanism of photoreduction of CO2 on MXene without other cocatalyst was very important. CO2 photocatalysis merely by 28

MXene would be simple, cheap, and convenient if the yield and selectivity of products were also favorable.

Zhou et al. used first-principle computations to study

the CO2 photoconversion catalyzed by the MXene with oxygen vacancy [30]. Among all the MXene investigated (TiCO2, V2CO2, and Ti3C2O2), TiCO2 showed the highest ability for the CO2 photoreduction. A two-step mechanism was proposed as below: (1) the oxygen vacancy on MXene captured CO2 with a favorable energy barrier (ca. 0.53 eV). (2) HCOOH was produced from the oxygen vacancy with the energy barrier of 0.59 eV while keeping the structure of oxygen vacancy stable. It could be seen that

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the CO2-adsorbing process was the rate-limiting step (0.53 eV), which was much

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easier than the CO2 reduction on TiO2 with the energy barrier of 0.87 eV [30].

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However, the overall energy barrier for producing HCHO, CH3OH and CH4 were

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0.75, 1.32 and 1.92 eV, respectively, which were all bigger than that of HCOOH. It implied that HCOOH was the preferable product from CO2 photoreduction on

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MXene.

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4.4. Conclusion of CO2 conversion by MXene-based materials MXene-based ternary composite seemed to possess special structure and

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properties. However, the yield from CO2 photoreduction seemed to be not very

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favorable [53]. The MXene-based ternary composite would be cheaper, which is simpler than other type of corresponding ternary composite while keeping the similar even higher yield of products from CO2 photoreduction. C3N4@MXene might be the most promising reported photocatalyst for CO2 photoconversion due to the absence of metal and the low cost in C3N4. After doping of C3N4, functionalization of MXene, size control, morphology design of C3N4@MXene, the yield and categories of products would be significantly improved. Zhou et al. seemed to provide the best though via a theoretical calculation that CO2 photocatalysis could be processed efficiently by MXene alone, just with some minor change (e.g., MXene with vacancy) [30]. However, it remained to test whether it would be efficient in practical experiments. CO2 photochemical conversion based on the MXene-based materials would attract more attention. 29

Moreover, the solvents used for CO2 conversion by MXene-based materials owned limited CO2 solubility. Also, the introduction of CO2-philic green solvents (e.g., ILs and DESs) could capture and activate CO2 for the purpose of improving thermochemical, electrochemical or photochemical conversion of CO2. For example, Liu et al. found that the CO producing rate from CO2 photochemical reduction in the ILs [P4444][p-2-O] was much higher than that in NaHCO3 aqueous solution [95]. ILs could also reduce the energy barrier for the electrochemical reduction of CO2 to obtain a lower overpotential and higher Faradaic efficiency. Interestingly, the usage of

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seawater was proposed as the media for the CO2 conversion [93], which implied that

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alkaline wastewater from industry or form daily life might also possible solvents for

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the efficient CO2 conversion by using MXene-based material as the catalysts.

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Moreover, apart from the source of media for CO2 conversion, the source of CO2 (e.g., diluted CO2, CO2 from the air) was also the route for the improvement of CO2

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conversion [95-101]. In this way, CO2 capture and conversion could be achieved

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anywhere and anytime by using the uniform equipment. Chen et al. found that nanoconfined ion hydration could capture CO2 from air with high capacity driven by

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the cheap H2O [96]. Prakash et al. converted the atmospheric CO2 (about 400 ppm)

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into energy and fuel (e.g., CH3OH) with high efficiency (turnover number > 2000) by using the homogeneous ruthenium catalyst and the polyamine absorbent [97]. Another limitation of MXene was that MXene could not absorb intensive visible light when used for photochemical conversion of CO2. Although many reports revealed that MXene showed the favorable efficiency for CO2 photoreduction under visible-light irradiation, there was nearly no light absorption when the wavelength was higher than 450 nm. It indicated that only limited range of light was utilized, which could be improved at this point by modification of the MXene. However, even after modification, (e.g., heterojunction [52, 57], S-scheme [53], and alkalization [56, 58]), the absorption of visible light was still a very small proportion of the all visible-light range. It seemed that Schottky-junction [55], QDs [59], photosensitizers [94] could enhance the visible light absorption of MXene, which was favorable for the visible light reduction of CO2. 30

Based on the reports, the major products from CO2 photoconversion catalyzed by the MXene-based composite were limited mainly to CO and CH4. Some minor products were CH3OH, C2H4, CH3CHO, and so on. It was difficult to obtain C2 products with high efficiency and high selectivity by MXene-based composite up to now. More efforts should be used to design novel MXene-based photocatalysts for more value-added products. The possible strategies could concentrate on the decoration of MXene, the selection of liquid media, and the size/morphology of MXene.

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Except for photochemical catalysis and thermal catalysis, there was no report on

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the combination of photochemical and thermal strategy (i.e., photothermal) to convert

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CO2 [102]. In theory, under visible light or solar light irradiation, MXene or

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MXene-based composite could increase the temperature of the system, which could be used as the energy for the CO2 conversion. It is anticipated to do such an

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investigation for the better utilization of light energy and CO2. Actually, flower-like

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Co2C, similar structure of MXene, was found to display efficient photothermal ability for the reaction of CO2 with epoxides to generate cyclic carbonates under mild

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temperature [103]. We believed that MXene and MXene-based materials should also

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own the photothermal ability for CO2 conversion. Similarly, the photoelectrochemical CO2 reduction to value-added products [104-106] was also desirable by MXene or MXene-based materials in the near future research.

5. Sustainability Several improvements are needed for the sustainable utilization of MXene-based materials for CO2 capture and conversion. For example, MXene-based materials commonly require the toxic HF for the synthesis. Also, the morphology, size, thickness, interlayer spacing of MXene-based materials could change after the utilization particularly for long term recyclability. Due to the complex synthesis in the very strict conditions, MXene-based materials are very expensive although the raw materials are cheap. Solutions to these problems are to develop new strategy for the green synthesis of MXene-based materials with low cost and high stability. 31

Particularly, environment pollution from MXene-based materials should be considered seriously. Toxic gases, volatile organic compounds and dissolved metal ion would release to the air, H2O or soil. Compared to the CO2 capture and conversion by green solvents (e.g., ILs and DESs), MXene-based materials show much potential space

for

further

improvement,

especially

in

the

photoconversion

and

electroconversion of CO2. Green solvents could only act as the thermo catalysts and the media of electro/photocatalysis of CO2. However, MXene-based materials are promising electro/photocatalysts. Combined with green solvents, MXene-based

of

materials would show wide industrial application in CO2 capture and conversion.

ro

Recently, some investigations reported that MXene could be synthesized by avoiding

-p

the poisonous HF. For example, Gogotsi and Barsoum found that fluoride salts and

re

HCl could act as the alternatives to toxic and corrosive HF to synthesize MXene [107]. Barsoum et al. found that organic solvents could also be used as the media for the

lP

synthesis of MXene with high efficiency in presence of ammonium dihydrogen

Jo

6. Outlook

ur

synthesis of MXene.

na

fluoride and NH4HF2 [108]. It is anticipated to develop greener media for the

CO2 capture, sensing and conversion by MXene was reviewed in this work. The investigation of the CO2 capture and sensing was rare, while the CO2 conversion by MXene was hot. The electrochemical conversion of CO2 by MXene seemed to attract much attention recently. However, thermo-conversion of CO2 by MXene was seldom studied. Most of the focus was on the photoreduction of CO2 by MXene and MXene-based composite, which was mainly due to high conductivity, high surface area, light harvesting ability, the enhancement of charge transfer and charge separation of photogenerated electrons by MXene [109-111]. However, these properties of MXene do not contribute significantly to the thermal catalysis of CO2. Therefore, the discussion and reports on the thermal conversion of CO2 is less than the photothermal conversion of CO2 in this review. 32

Green processes are required for the capture, sensing and conversion of CO2 by MXene-based materials. However, one of the most disadvantages during the process of applying MXene and MXene-based composite for CO2 photoreduction was the usage of highly toxic and dangerous HF for MXene synthesis. HF would cause harm to the environment and the human although HF is the efficient solvents for the synthesis of MXene. During the calcination process, there would also be some toxic gases released, which was not environmentally friendly and non-sustainable. The synthesis of MXene might be greener by carefully designing or utilizing green

of

solvents, such as ILs and DESs. One might argue that the green solvents cannot

ro

replace the function of HF to synthesize MXene. However, we believe that ILs and

-p

DESs could be designed to be task-specific and functional to dissolve metal elements

re

in raw materials of MXene with high efficiency and selectivity by using some specific methods. Actually, Syroeshkin et al. have demonstrated that ILs 1-methylimidazolium

lP

tetrafluoroborate ([HMIM][BF4]) and 1-methylimidazolium hexafluorophosphate

na

([HMIM][PF6]) could act as the electrochemical etching agents for formation of porous silicon, which traditionally could only be efficiently achieved in toxic HF

ur

solution [112].

Jo

Cost could be reduced by designing stable MXene-based materials. High stability is the premise for the long-term utilization of absorbents, solvents and catalysts [24, 113-114]. Although MXene was claimed to own excellent stabilities [51], some reports suggested that it should be aware that MXene might be unstable during the photocatalytic process although it would not inhibit the CO2 photoconversion in that case, as suggested by Chen [59]. Moreover, the solvents for CO2 photoreduction might be unstable. It could be exemplified by the CO production by the photodecomposition of ethyl acetate, as suggested by Chen [60]. Little attention was paid to the stability of MXene-based photocatalysts and solvents for CO2 photoreduction. Therefore, the stability of MXene should be paid more attention. Particularly, in the conditions of heat, electricity and light, the decomposition behavior and mechanism of MXene is not clear up to now. For example, the terminated groups (e,g., OH, F) on the surface of MXene might become the leaving 33

groups if reaction temperature is high or if there exists other reactive substances in the systems. The high surface area, the favorable thickness and the desired morphology of MXene might lose if MXene is placed at a specific temperature in long-term range, which would be severely changed if MXene decomposes. These properties of MXene are the key conditions for the high efficiency of CO2 capture, sensing and conversion, which are only investigated for several times in the experimental scale. Previous reports showed that long-term stability is much severer than that of the short-term stability [24-25, 114-116]. This conclusion is also applicative for the stability of

of

MXene.

ro

During the application of MXene-based materials for CO2 capture, sensing and

-p

conversion, the H2O in the air might also have some effect. The terminated groups on

re

MXene are polar, which are the favorable media to bind H2O from air. The presence of H2O would influence the CO2 capture, sensing and conversion. It means that the

lP

effect of H2O on the CO2 capture, sensing and conversion by MXene-based materials

na

should be paid more attention. On the other hand, the hygroscopity of MXene-based could be used for providing protons for CO2 conversion to produce CH4 or other

ur

hydrogen-containing chemicals.

Jo

Although there are many drawbacks related to the process, the application of MXene-based materials on CO2 capture, sensing and conversion would develop rapidly. Once the cost of MXene is low enough and the procedures of MXene synthesis is green, we believe that CO2 capture, sensing and conversion could be industrialized by designing novel MXene-based materials and being decorated by other strategies. Moreover, the relations between the synthesis methods and photocatalytic efficiency are very closely. The size, surface area, and other properties of MXene-based materials could be tuned by the synthesizing procedure; therefore, the efficiency of CO2 capture, sensing and conversion could also be improved. Particularly, the photosynthesis (i.e., converting CO2 and H2O into energy and fuels) catalyzed by MXene-based materials might be possible due to the high CO2– absorbing capacity, favorable light-absorbing ability, multiple active sites and potentially high hygroscopicity. 34

Recent investigations have shown that boron (B) can substitute C/N to occupy octahedral interstitial sites in MXenes (also referred as MBenes), which exhibit excellent electronic conductivity and catalytic activity [117-119], suggesting that MBenes may have great potentials as co-catalysts in CO2 conversion. It is anticipated to experimentally explore new types of 2D MXenes to improve the efficiency of CO2 capture, sensing and conversion.

Corresponding author Yu

Chen,

E-mail:

[email protected];

Phone:

+86-316-2188211;

Fax:

ro

*

of

Author information

-p

+86-316-2112462.

re

* Tiancheng Mu, E-mail: [email protected]; Phone: +86-010-62514925; Fax: +86-010-62516444.

lP

* Yanhong Lu, E-mail: [email protected]; Phone: +86-316-2188211; Fax:

na

+86-316-2112462. Notes

ur

The authors declare no competing financial interests.

Jo

Acknowledgment

The authors thank the Natural Science Foundation of Hebei Province (B2019408018, E2020048004), the Fundamental Research Funds for the Universities in Hebei Province (JYQ201902, JYT201901), Program for the Top Young Talents of Higher Learning Institutions of Hebei Province (BJ2020047), College Students' Innovation and Entrepreneurship Training Program Project Fund of Langfang Normal University (202010100001, S202010100011), National Natural Science Foundation of China (21773307), Hebei Higher Education Teaching Reform Research and Practice Project (2019GJJG357) and Research Project of Langfang Teachers University (LSLB201701) for financial support.

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Capture, sensing and conversion of CO2 to value-added products by MXene-based materials are reviewed. Improvement of MXene synthesis and efficiency in a green method is also discussed for the purpose of achieving the green absorbent, sensor and catalyst.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: