MXene-Based Photocatalysts and Electrocatalysts for CO2 Conversion to Chemicals

The interest in CO2 conversion to value-added chemicals and fuels has increased in recent years as part of strategic efforts to mitigate and use the excessive CO2 concentration in the atmosphere. Much attention has been given to developing two-dimensional catalytic materials with high-efficiency CO2 adsorption capability and conversion yield. While several candidates are being investigated, MXenes stand out as one of the most promising catalysts and co-catalysts for CO2 reduction, given their excellent surface functionalities, unique layered structures, high surface areas, rich active sites, and high chemical stability. This review aims to highlight research progress and recent developments in the application of MXene-based catalysts for CO2 conversion to value-added chemicals, paying special attention to photoreduction and electroreduction. Furthermore, the underlying photocatalytic and electrocatalytic CO2 conversion mechanisms are discussed. Finally, we provide an outlook for future research in this field, including photoelectrocatalysis and photothermal CO2 reduction.


Introduction
Excessive emissions of carbon dioxide (CO 2 ) from the burning of fossil fuels have led to serious and potentially irreversible impacts on climate and the environment, such as global warming, rising sea levels, ocean acidification, and glacial ablation. As the atmospheric CO 2 concentration is predicted to increase from 404 × 10 -6 in 2017 to 600 × 10 -6 by 2100, an urgent need has arisen to reduce the carbon footprint generated by human activities [1,2]. To mitigate this ever-growing CO 2 concentration, significant efforts have been dedicated to developing different technologies, including CO 2 capture and storage, suppression of CO 2 emission at the source, and direct CO 2 conversion into value-added fuels and chemicals. Among these emerging technologies, the transformation of CO 2 gas into more valuable fuels and chemicals, known as the CO 2 reduction reaction (CO 2 RR), is an attractive approach to alleviating excessive CO 2 emissions while generating a series of valuable C 1 (CO, HCOOH, CH 3 OH, and CH 4 ) and C 2+ (CH 3 COOH, C 2 H 4 , C 2 H 5 OH, and C 3 H 7 OH) products [3][4][5]. Given their energy-efficient, low-cost, sustainable, and mild operating conditions, photocatalytic and electrocatalytic CO 2 RRs have emerged as the most promising methods for converting CO 2 into valueadded products. Photocatalytic CO 2 reduction (PCR) harvests intermittent light from solar energy to drive direct chemical conversion of atmospheric CO 2 into solar fuels with the aid of water [6,7]. Although PCR can be performed under mild operating temperatures and conditions, the low efficiency due to rapid electron-hole pair recombination hinders its practical relevance [8,9]. Moreover, the product 1 3 selectivity of PCR is difficult to control, and thus, the products generally consist of two or more of the aforementioned chemicals and fuels [7,10]. On the other hand, electrocatalytic CO 2 reduction (ECR) offers higher product yields and conversion efficiency, particularly when connected to renewable electricity sources, yet it is a complicated reaction because it involves multiple elementary reactions along with the transfer of coupled proton-electron pairs to produce diverse products [11,12]. Furthermore, the sluggish reaction kinetics due to the stable C=O bond and the competition with the hydrogen evolution reaction (HER) are fundamental challenges faced by ECR [13][14][15][16]. Therefore, exploring an efficient catalyst that can overcome these limitations is of significance to the real-world implementation of PCR and ECR technologies.
Two-dimensional (2D) transition metal carbides and nitrides, also known as MXenes, are a new family of 2D materials, having a general chemical formula of M n+1 X n T x (n = 1-4; M = early transition metal; X = C and/or N; T x = surface-terminated group) [17,18]. They are produced by selective etching of A layers (A represents a IIIA/IVA group element) from the respective MAX phases [19,20]. The chemical composition of MXenes is largely diverse, where more than one M element can be in their layered structure, forming solid-solution and ordered double structures [21]. Even high entropy MXene, which contains more than five metals, can be successfully synthesized [22]. Therefore, hundreds of possible MXene compositions have been predicted to date. Since their introduction in 2011 [23], MXenes have revolutionized many research areas in the 2D material systems, including the sustainability field [24]. The excellent electronic, mechanical, chemical, and physical properties, as well as controllable surface groups and large surface area, make them promising as high-performance catalysts for energy conversion reactions [25][26][27]. Numerous works are available on the functionalization of MXene-based catalysts for photocatalytic and electrocatalytic CO 2 reduction due to their highlighted properties [28,29]. As pristine MXenes cannot generally absorb light energy, they act as co-catalysts in tandem with other photocatalysts because of their impressive electrical conductivity and abundant active sites [30,31]. In this regard, MXenes serve as robust support and 2D platform for photocatalyst growth to promote efficient photoinduced electron-hole pair separation and transfer processes [30]. Their abundant functional groups also enhance the CO 2 molecular adsorption and activation, enabling higher conversion efficiency [32]. Meanwhile, the knowledge and research development of MXenes as catalysts for electrochemical CO 2 reduction is relatively inadequate because it is still a growing field [33]. Most of this work mainly focuses on computation-guided discovery to screen the most optimized composition and structure of MXenes [29,34] as well as to obtain deeper knowledge on CO 2 capture and reduction pathways and the fundamental reaction mechanisms converting CO 2 into chemicals and fuels [35]. Despite the progress made in this emerging field, further theoretical and experimental studies are still needed to gain deep insight into the structure-activity relationships of MXene-based photocatalysts and electrocatalysts for CO 2 reduction.
In this review, we have primarily summarized the recent advances of MXene-based photocatalysts and electrocatalysts for CO 2 reduction applications. This review is divided into three major parts: (1) a mechanistic understanding of photocatalytic and electrocatalytic CO 2 reduction; (2) research progress on photocatalytic CO 2 conversion over MXene-based nanocomposites; and (3) research progress on MXene-based electrocatalysts for CO 2 reduction. Photoelectrocatalysis (PEC) and photothermal CO 2 reduction are also briefly discussed. Finally, the remaining challenges and future directions in developing MXene-based CO 2 reduction catalysts are provided to spur further progress.

Fundamental Mechanisms of PCR and ECR
As CO 2 reduction involves multiple electron and proton transfer processes, understanding the chemical reaction pathway is essential in developing efficient photo(electro) catalysts to select the preferred products. Photoreduction and electroreduction of CO 2 can occur under ambient conditions; thus, they should compete with the traditional CO 2 hydrogenation. PCR generally involves the three stages shown in Fig. 1, which will occur if the illumination energy is higher than the photocatalyst bandgap energy (E g ) [36]. First, the electron-hole pair is generated through photon absorption, wherein the electron is excited to the conduction band, and the hole circulates around the upper level in Fig. 1 Schematic illustration of CO 2 photoreduction. Reproduced with permission from Ref. [37]. Copyright 2020, John Wiley and Sons, Inc the valence band in Step (1) [37]. Step (2) involves charge transport and spatial separation toward the photocatalyst surface, which results in rapid charge carrier recombination on the photocatalyst surface due to the effect of high Coulomb force [38]. In Step (3), a redox reaction occurs between photogenerated carriers and the molecules on the photocatalyst surface. Holes will mainly participate in the oxidation process of water molecules, while the adsorbed CO 2 is reduced by photoexcited electrons to produce fuels and chemicals. The charge carrier recombination in Step (2) should ideally be suppressed; otherwise, this redox process will not occur. Several governing factors affect the efficiency of PCR, such as the quantity of uptake CO 2 , the concentration of the absorbed CO 2 on the surface of photocatalyst materials, and the activated catalytic sites [39].
In ECR, the CO 2 RR takes place at the cathode, while water oxidation occurs at the anode. The competing HER also occurs at the cathode with a similar potential [2,40]. The thermodynamic potentials for the major CO 2 RR halfreactions and HER in an aqueous electrolyte under standard conditions (1.0 atm, 25 °C, and pH = 7.0) are shown in Fig. 2a. Notably, the reduction process of CO 2 involves different electron transfers, ranging from two electrons for CO and formic acid (HCOOH) to 18 electrons for propanol. These differences seem problematic because the reduction potentials of these processes are adjacent to each other, which makes selectivity toward a single product low [1]. Much research has been devoted to understanding fundamental CO 2 activation and reduction mechanisms in different electrocatalysts to explore the critical parameters influencing the efficiency and selectivity of ECR products. In general, the first stage of ECR is CO 2 adsorption and activation, turning the linear CO 2 to interact with electrons on the catalyst surface to generate the carbon dioxide radical anion (CO 2 · -), occurring at a negative thermodynamic potential (− 1.49 V vs. RHE) [41]. This result is mainly due to the high dissociative energy of the C=O bond (~ 750 kJ/ mol), so CO 2 is thermodynamically stable. Thus, the CO 2 RR has sluggish reaction kinetics and poor energy conversion efficiency. In the next step, hydrogenation of CO 2 generates *COOH or *OCHO, followed by the formation of *CO or *HCOOH intermediates, respectively, through protoncoupled electron transfer (* represents the binding site of intermediates on the catalyst surface) [42]. The formed *CO can be further desorbed from the surface or undergo a reduction reaction to produce C 1 products (hydrocarbons, alcohols, and acids) [43]. Schouten et al. [3] showed the different pathways of *CO dimerization to generate *OC-CO before the hydrogenation process starts. This step will lead to C 2+ product formation, where the adsorbed *CO species dimerize before the reduction process to become C 2 H 4 or undergo a reduction reaction to form C 1 products, followed by dimerization and a reduction process to create C 2+ products [44]. For instance, ethanol production from CO 2 involves 12-electron transfer and goes through the following reactions: CO 2 → *COOH → *CO → *COCO → *COCOH → *COCHOH → *COCH 2 OH → *CHOCH 2 OH → *CH 2 OCH 2 OH → CH 3 CH 2 OH [45]. The CO 2 reaction pathway differs between electrocatalysts; for example, transition metals such as Cu form hydrocarbons, and oxygenates Sn, Pb, and Bi form HCOOH as the primary product, while Ag, Au, and Zn mainly form CO [13,46]. CO 2 reduction pathways also depend on electrolytes, the applied potential, pH, and so on [47]. The CO 2 reduction pathways in different metal catalysts and reaction media are shown in Fig. 2b. In this context, the selection of electrocatalysts and suitable Fig. 2 a Thermodynamic potentials for the CO 2 RR and HER in the aqueous electrolyte under standard conditions. The left is the thermodynamic equilibrium, and the colors highlight the overpotential for the specific products. Reproduced with permission from Ref. [46].
Copyright 2020, John Wiley & Sons, Inc. b Pathways for the CO 2 RR over metal catalyst surfaces. Reproduced with permission from Ref. [47]. Copyright 2020, Elsevier B.V conditions for CO 2 reduction is crucial for regulating the formation of key intermediates and subsequently producing the targeted C 1 or C 2+ chemicals.

MXene-Based Catalysts for PCR
MXenes have various intriguing properties, such as tunable functional groups, hydrophilicity, large interlayer spacing, good chemical stability, and outstanding electrical/thermal conductivity [9,48]. Thus, MXenes are promising noblemetal-free co-catalysts and catalyst supports for efficient PCR [49,50]. MXenes can boost photocatalytic activity in several ways [38]; they help in charge carrier transfer, serve as robust support, restrict the photocatalyst size, favor reactant adsorption, and act as an electron or hole reservoir [51,52]. Thus, they have been combined with several photocatalyst materials, such as CdS, Bi 2 WO 6 , perovskites, and g-C 3 N 4 , to enhance the efficiency of PCR [53].
MXenes as co-catalysts have excellent conductivity to facilitate photocatalytic activity [54] through accelerated charge separation and suppression of carrier recombination [55]. In this case, the photocatalysts absorb visible light, and photogenerated electrons are excited to the conduction band, whereas the holes remain in the valence band. Furthermore, the excited charge carriers are moved to the interface of MXenes mainly due to the higher potential of MXenes. Electrons can transfer to MXenes without recombination and react on the MXene surface to generate CO by reducing CO 2 . The charge transfer process from the photocatalyst to MXenes enhances the electron-hole pair separation and suppresses charge recombination in photocatalysts, thus enhancing the photoactivity [55]. The rich functional terminations of MXene surfaces also make them potential candidates for combining with other semiconductor materials having a close contact interface and hence generating charge transfer, which enables them to increase the charge carrier separation efficiency [38,49,51]. Moreover, the shape and dimension tunability of MXenes also provides a significant advantage; it not only makes them suitable for combining with other materials in the nanocomposite form but also enhances their overall catalytic properties [54].

MXene-Based Heterostructures
Several functional MXenes (including Ti 2 CO 2 , Zr 2 CO 2 , Hf 2 CO 2 , and Sc 2 CO 2 ) are theoretically promising as cocatalysts, having suitable electronic structures, sufficient active sites, and high carrier mobilities [28]. Thus far, Ti 3 C 2 T x MXene has largely been explored as a photocatalyst support because of its large work function and the relatively low activation energy of CO 2 photoreduction [53].
In particular, many reports have shown MXenes combined with 2D g-C 3 N 4 as efficient photocatalyst materials. g-C 3 N 4 has excellent physical and chemical stability, low cost, nontoxicity, and a suitable energy band position for photocatalyst CO 2 reduction [70]. However, g-C 3 N 4 has low CO 2 adsorption and fast recombination of photoinduced electron-hole pairs, limiting its photocatalytic performance; thus, MXenes solve this problem with their high conductivity. In combining MXenes with perovskites, photocatalyst enhancement can also be obtained because MXenes have good electron transfer properties that can facilitate the charge separation and transport in perovskite [59]. In most cases, perovskite materials, such as Cs 2 AgBiBr 6 , suffer from a large exciton binding energy, which diminishes the charge separation and limits photocatalytic reactions [59]. Nevertheless, by exploiting the outstanding optical properties of perovskite, combining MXene with perovskite could induce large light absorption for the photocatalytic reaction [59]. MXenes can also improve the photocatalytic activity of metal oxides, such as CeO 2 , wherein the noble-metal-free co-catalyst of MXenes could couple with CeO 2 to generate a built-in electric field and induce a Schottky junction, as illustrated in Fig. 3 [49]. This strategy is effective in accelerating electron transfer to separate photogenerated carriers. This strategy may also apply to TiO 2 because TiO 2 has low light usage ability and rapid electron-hole recombination. Thus, MXenes loaded in TiO 2 as co-catalyst could boost the photocatalyst reaction [9]. In the case of Bi-based compounds as photocatalysts, a co-catalyst is also important in increasing reactivity. The co-catalyst of MXenes could significantly enhance the PCR performance of the [Bi 2 O 2 ] 2+ layer containing the Bi 2 O 2 SiO 3 photocatalyst [50]. Moreover, the distinctive 2D structure of MXenes supports the strong construction of 2D-2D MXene/Bi-based semiconductor-layered heterojunctions having a strong interface contact [50]. Similar to previous cases, although LDH is a typical 2D-layered photocatalyst, it suffers from poor charge mobility, aggregated layers, and rapid charge recombination, leading to low photocatalytic activity [60]. Thus, because of great conductivity and an interfacial Schottky junction, LDH as an n-type semiconductor could be combined with MXenes, leading to enhancement of photocatalyst activity [60].

MXene-Based Z-and S-Scheme Photocatalysts
Modification of the shape and morphology of MXenes and tandem materials could lead to more efficient photocatalysts by maximizing surface areas as well as the number of adsorption sites [51,71]. MXene-based nanocomposite photocatalysts can be modified into core-shell heterostructures, having an intimate interaction between MXenes and the semiconductor materials [60]. A tandem semiconductor material, such as Co-Co LDH nanosheets, can also be assembled vertically on layered MXenes to accelerate the separation of photogenerated charge carriers [61]. As MXenes can be modified easily into 0D, 1D, and 3D forms instead of their original 2D form [20,25], the design of MXene-based nanocomposites for photocatalyst applications relates to not only a conventional layered combination but also various forms and designs. For instance, 2D/2D/0D TiO 2 /C 3 N 4 /Ti 3 C 2 MXene nanocomposites with an S-scheme photocatalyst have excellent CO 2 reduction activity [53]. In this case, 0D Ti 3 C 2, as well as the trapped electrons from g-C 3 N 4, could induce photogenerated carrier separation [53]. Conversely, most semiconductor photocatalysts that can be formed in various low-dimensional forms, such as perovskite formamidinium lead bromide (FAPbBr 3 ) quantum dots, can also be combined with 2D layered MXenes to form a hybrid 0D/2D architecture [57]. Some rare design of hydrangealike morphology of InVO 4 /Ti 3 C 2 T x has also been reported, exhibiting excellent photocatalyst activity due to enhanced specific surface areas and a multiple photon scattering cross section [67]. The heterostructure scheme is also important in determining the efficiency of MXene nanocomposite-based photocatalysts. For instance, Z-scheme heterojunctions can avoid charge carrier recombination, wherein the co-catalyst Ti 3 C 2 can serve as an electron sink to achieve a quick shift of photoinduced electrons and supply many active sites for photocatalytic reactions [63]. In contrast, based on semiconductors with narrow bandgaps, constructing S-scheme heterojunctions can extend the light absorption range and achieve the requirements of a higher conduction band and lower valence band levels [38]. Here the S-scheme heterojunction not only helps in charge transfer but also has a strong redox potential, which is important in photocatalyst activity. The Z-and S-scheme CO 2 photoreduction mechanisms of MXene-semiconductor nanocomposites are illustrated in Fig. 4.

MXene-Based Ternary Nanocomposites
In addition, MXene-based nanocomposite photocatalysts can also involve more than two material combinations, for instance, TiO 2 /C 3 N 4 /Ti 3 C 2 [53], meso-TiO 2 @ZnIn 2 S 4 / Ti 3 C 2 [69], TiO 2 /g-C 3 N 4 /Ti 3 C 2 [72], CdS/Ti 3 C 2 /g-C 3 N 4 [73], g-C 3 N 4 /Bt/Ti 3 C 2 [74], and BiOIO 3 /g-C 3 N 4 /Ti 3 C 2 [63]. In these nanocomposites, each material makes a contribution, such as TiO 2 as photocatalyst [69], ZnIn 2 S 4 as co-catalyst [69], C 3 N 4 as a provider of trapped electrons [53], BiOIO 3 [63], and Bt as a mediator in Schottky junctions to provide new electron transfer channels [74]. In the case of triple combination material nanocomposites, MXenes could serve as excellent additional support. For example, although TiO 2 /C 3 N 4 composites show enhanced photocatalytic activity compared to their pristine form (TiO 2 and C 3 N 4 alone), defect formation is a major problem because of limited contact between TiO 2 and C 3 N 4 , which can degenerate the charge carrier migration, and bulky or thick C 3 N 4 having light shielding and a long charge transfer distance, lowering the efficiency of photocatalyst activity [53]. This situation requires a sufficient Fig. 3 Enhancement mechanism of the photocatalytic CO 2 reduction activity of CeO 2 /Ti 3 C 2 -MXene with a built-in electric field-induced Schottky junction. Reproduced with permission from Ref. [49]. Copyright 2019, Elsevier and unified contact as well as a distinct morphological characteristic supporting the photocatalyst reaction mechanisms [53]. Inhibiting the recombination of photoexcited electrons and holes driven by a strong Coulombic interaction is also a challenge to be solved, where MXenes could be the answer [53]. Moreover, doping has also been explored to obtain high-efficiency MXene-based photocatalysts. The doping could be incorporated in semiconductor sites as well as in MXenes; for example, boron doping in g-C 3 N 4 sites could increase visible light absorption and achieve a lifted valence band for excellent photocatalytic activity in g-C 3 N 4 /MXene-based nanocomposites [75]. Gold atoms could also be used as doping for the recovery of oxygen vacancies toward photocatalytic aerobic oxidation in MXene-based photocatalysts [76]. In addition, Qu et al. [77] found that, in general, the important key to boosting the efficiency of CO 2 conversion using Ti 3 C 2 T x MXenes is introducing N-doping and metal vacancies to significantly reduce the energy barrier for the intermediates (e.g., *COOH, and *CO). We have summarized recent experimental works on photocatalytic CO 2 reduction over MXene-based catalysts and co-catalysts in Table 1.

MXene-Based Catalysts for ECR
Similar to general catalytic reactions, in electrocatalytic reactions, electrocatalysts provide favorable reaction sites that ease the substrate adsorption process and promote alternative reaction pathways, thereby increasing the reaction rate and minimizing energy use, i.e., lowering the overpotential. While a large surface area and an extensive metal-terminated surface make MXenes promising materials for CO 2 adsorption [82], their inherent metallic character is considered beneficial for electrochemical reaction processes; therefore, numerous studies using experimental and theoretical methods have explored applying MXenes for the ECR. Theoretical methods have considerable power for simulating various catalytic reaction processes for predicting plausible reaction mechanisms of electrochemical CO 2 reduction.

Theory-Guided CO 2 Reduction Mechanism
Li et al. [34] initiated the first computational study of MXenes as electrocatalysts for the ECR by investigating the CO 2 capturing mechanism and its electrocatalytic reduction process on M 3 C 2 -type MXenes. The CO 2 molecule is likely Fig. 4 Diagram of possible photocatalytic Z-scheme and S-scheme mechanisms of MXene-semiconductor nanocomposites. Reproduced with permission Ref [38]. and [63] Copyright 2022, Elsevier to be chemisorbed on the terminated metal atom surface for all metal elements due to spontaneous binding energies (∆G < 0), as shown in Fig. 5a, instead of forming a physisorption interaction. This attribute indicates that M 3 C 2 -type MXenes are generally strong CO 2 capture materials, despite CO 2 capture being thermodynamically unfavorable. The capture of CO 2 on MXenes originates from the electron donation of carbides to terminated metals, which increases the metal electronegative charge to interact with the carbon of CO 2 . The strength of the CO 2 chemisorption mode is likely affected by the oxygen affinity of the metal and the distance between each terminated metal. For instance, CO 2 chemisorption on Mo 3 C 2 is less spontaneous compared to Cr 3 C 2 because of the lower oxygen affinity of Mo and the longer metal-metal distance on the Mo 3 C 2 terminal layer, which do not permit the O-Mo interaction.
After the CO 2 capturing process, CO 2 activation on the MXenes is crucial for allowing the electrocatalytic reduction reaction to proceed. In representative Mo 3 C 2 MXene, the strong interaction between CO 2 and Mo 3 C 2 changes the angular structure of CO 2 molecules, spontaneously activating the CO 2 molecules. Although the first hydrogenation of activated CO 2 may occur on the C atom and one of the terminal O atoms of CO 2 , the formation of *OCHO is more thermodynamically preferred than that of HOCO*, which means that further hydrogenation to the C atom will occur, and CO and HCOOH formation will be minimal. This pathway is different from other classical electrocatalyst materials, such as Cu-based catalysts, where the first hydrogenation mostly occurs on the oxygen of CO 2 . The first hydrogenation of the C atom causes the breaking of C-M interactions while chemisorption of *OCHO intermediates is formed through the two O atoms of CO 2 with the M surface termination of MXenes. In ECR, the first hydrogenation step usually requires energy input to surmount the barrier activation energy or the net nonspontaneous reaction step.
The as a fifth-reduced radical species also proceeds spontaneously over most of the catalysts examined and is thermodynamically preferred to CH 2 OH •· radical formation. Although the next hydrogenation step of the CH 3 O • radical requires the release of its radical for the access of the H + /e − pair and is a path that faces a large energy barrier, the reactive nature of the transition metal carbides provides an alternative path involving the sixth H + /e − pair gain on the CH 3 O · radical taking place on the CH 3 moiety, leading to the release of CH 4 and an O atom inserted on the material. In the final step, the highly reactive, chemisorbed O atom on the metal surface undergoes the highly spontaneous hydrogenation reaction, yielding a chemisorbed OH • radical species and releasing the largest amount of energy of all the elementary reactions. Moreover, in contrast to the other electrocatalysts, where the first hydrogenation step is rate-limiting, the release of such chemisorbed OH · radical species as a relatively strongly chemisorbed H 2 O molecule on M 3 C 2 -type MXene is the rate-limiting step. The whole CO 2 adsorption, activation, and reduction pathway processes on bare M 3 C 2 -type MXene are displayed in Fig. 5b.

Computational Screening of Promising MXenes
Following Li et al. [34], researchers attempted to find the best candidates among the MXene family. Unfortunately, a wide variety of MXenes require laborious experimental work to investigate the suitable MXene-based materials for the electrocatalytic CO 2 RR, while MXene preparation itself is currently challenging [17,24,32,83,84]. Within this context, computation-guided discovery is one of the best strategies to minimize experimental work by screening suitable materials and optimizing factors affecting their catalytic performance. Among the MXene family, M 2 CT x synthesis is more extensive, with some materials even having no surface group terminal [24,33,85]. The reaction mechanisms of M 2 C-type and M 3 C 2 -type MXenes were generally predicted to be similar, and Mo-and Cr-based MXenes were the most promising, with relatively similar rate-limiting step energies [33]. Moreover, Mo 2 C MXene was a distinctly better electrocatalyst than the earlier reported bulk Mo 2 C [82]. Although the Fig. 5 a Proposed path for the interaction of CO 2 with M 3 C 2 MXene surfaces. b Side view of the minimum energy path followed for the CO 2 conversion mechanism into *CH 4 and **H 2 O catalyzed by Mo 3 C 2 . Reproduced with permission from Ref. [34]. Copyright 2017, American Chemical Society CO 2 adsorption process was likely affected by the electron or charge transfer from MXene to adsorbate, the hydrogenation process mostly originates from the various binding strengths between CO 2 molecules and individual MXenes. Too strong binding between the MXene surface and the captured CO 2 likely hinders the first hydrogenation process, as its Gibbs free energy was higher for group IV (△G > 0) than that for group V (△G ≈ 0) and group VI (△G < 0) [33]. According to the reaction mechanism, the hydrogenation is mostly spontaneous on the C atom but nonspontaneous on the O atom because of the strong M-O bond.

Surface Termination Effect
In theoretical studies of ECR, an ideal bare MXene surface is the standard model despite most of the successfully synthesized MXenes having -OH, -O, and -F functional groups, depending on etchant materials [24,86]. Consistent with this model, the study on the effect of surface terminals in MXenes is of great interest, where the effect of -O and -OH terminal species on ECR were reported for M 2 CT x and M 3 C 2 T x [35,87]. In general, while the presence of -O and -OH species on the MXenes surface does not tend to enhance ECR, some of these catalysts exhibited even better performance relative to their corresponding bare M 2 C and M 3 C 2 -type MXenes, namely Mo 3 C 2 O 2 , Mo 3 C 2 (OH) 2 , W 2 CO 2 , Ti 2 CO 2 , Sc 2 C(OH) 2 and Y 2 C(OH) 2 . The -O and -OH species facilitate the physisorption of CO 2 molecules in M 2 CO 2 , M 3 C 2 O 2 , M 2 C(OH) 2 , and M 3 C 2 (OH) 2 , which directs to the various favorable reaction pathways. For the first hydrogenation step, HOCO* formation is thermodynamically preferred. The different steps begin from the second hydrogenation, wherein Mo 3 C 2 O 2 , Mo 3 C 2 (OH) 2 , Ti 2 CO 2 , and W 2 CO 2 prefer to form HCOOH*, and Sc 2 C(OH) 2 and Y 2 C(OH) 2 generate CO* with the release of H 2 O instead. The third hydrogenation step of the -O surface also differs from that of -OH species for Mo 3 C 2 O 2 and Mo 3 C 2 (OH) 2 , in which the catalyst prefers to generate CHO* and H 2 COOH, respectively. The remaining hydrogenation steps show mostly spontaneous processes until the release of the CH 4 molecule.

Defective MXenes
Defects on MXenes should also affect the electrocatalytic activity, considering the change in localized electron density on M-terminals [11,88]. A metal vacancy (V M ) or X-vacancy (V X ) of M 2 XO 2 affects the interaction between the surface and the fragment-type radical (·COOH, ·CHO, etc.) intermediates in which the V M strengthens and the V X weakens the interaction [88]. Hence, it brings the possibility of tuning the rate-limiting step of ECR by controlling the vacancy site. V Zr -Zr 2 NO 2 , V Hf -Hf 2 NO 2 , V W -W 2 NO 2 , and V N -Ta 2 NO 2 were listed as the most promising defective M 2 XO 2 -type MXenes with very low theoretical overpotential. Substituting the inner M-layer of Mo 3 C 2 with transition metals also enhanced the CO 2 adsorption ability and selectivity [89]. Mo 2 TiC 2 , Mo 2 ZrC 2 , and Mo 2 HfC 2 were the highest among them by having the most electron transfers from MXenes to CO 2 . Accordingly, compared to groups V and VI, group IV of transition metal (TM)-substituted M 3 C 2 also exhibited remarkable activity in the electrocatalytic process as the only catalysts that spontaneously proceed with the second hydrogenation, as displayed in Fig. 6a, b. Therefore, Mo 3 C 2 (M 2 (TM)C 2 ) TM-substituted by Ti, Zr, Hf, and Cr exhibits a relatively low limiting potential of 0.350, 0.367, 0.353, and 0.369 V, respectively, compared to the pristine Mo 3 C 2 , due to strong localization of lone pair  [89]. Copyright 2022, Elsevier electrons that stabilize and activate the intermediates [11,89]. These TM-substituted MXenes also exhibited high stability and excellent CH 4 product selectivity over CO, HCOOH, and CH 3 OH. Meanwhile, the single-atom dopants (Cr, Mn, Fe, and Co) on M-terminal Mo 2 C caused the electron density to be concentrated at the dopant, although this phenomenon was not observed in Ni-, Ru-, or Rh-doped Mo 2 C because of the similar configuration with the pristine Mo 2 C [90]. M-doped Mo 2 C MXenes (M = Cr, Mn, Fe, and Co) have a high ECR activity and selectivity that promote CH 4 and CH 3 OH generation, although CH 4 is energetically unfavorable. In particular, Co-doped Mo 2 C and Fe-doped Mo 2 C exhibited the most remarkable activity, with a lower overpotential of ~ 0.5 V (the rate-limiting step energy barrier is ca. 0.35 eV) compared to the pristine Mo 2 C (0.9 V). Moreover, a smaller *OH binding energy of Fe-doped Mo 2 C could break the inherent linear scaling relationship to trigger high activity for ECR to the C 2 products of C 2 H 4 and CH 3 CH 2 OH [90].

MXene-Based Nanocomposites
The potential use of a single-atom catalyst (SAC)-incorporated MXenes for ECR is promising, particular by anchoring the single-atom TMs. On this topic, CO 2 was predicted to be adsorbed on all TM SAC surfaces with a C*O*O configuration, wherein each atom in CO 2 is coordinated with the SACs. For the reduction process, however, CH 4 selectivity is an issue in the TM SAC-loaded MXene, as the HER could be a strong competitor for the CO 2 RR because of the high possibility of H* formation in the first hydrogenation process, in addition to possible protonation on a C or O atom of CO 2 . Baskaran and Jung [91] predicted that Ru-Mo 2 CS 2 has CH 4 product selectivity with the lowest rate-limiting step energy barrier of 0.24 eV. All in all, the SAC-anchored MXene catalysts with nonspontaneous CO 2 reduction require a large amount of energy for the CO 2 adsorption and hydrogenation steps, and the product selectivity varies from CO to HCOOH because the barrier energy for CH 4 production is high [91]. The complex structure of MXene hybrids becomes a major challenge in performing a DFT calculation.
The theoretical results suggest that the characteristics of bare and -O or -OH terminated MXenes seemed to make ECR preferable to the HER even in acidic conditions because proton adsorption on the MXene surfaces is thermodynamically unfavorable because of the CO 2 -philic surfaces [28,34,35,88,92]. Likewise, the defective MXenes, such as single-metal dopants and metal substitution in the inner layer of M 3 C 2 MXenes, strengthen their CO 2 capture abilities and enhance ECR selectivity against the HER according to energy profiles of all possible H* intermediate configurations [89,90]. Despite the vacancies (V X and V M ) offering promising enhancement in ECR, their strong effect on the fragment-type intermediate adsorption caused some of them to have higher selectivity to the HER than ECR [88]. In this case, the metal elements seemed to play an essential role in the selectivity of CO 2 adsorption, in which hafnium (Hf) and tantalum (Ta) exhibited remarkable ECR selectivity among them. Interestingly, the anchoring of a single-atom catalyst on the Mo 2 CS 2 -type MXenes could generally enhance the ECR selectivity against the HER. However, most noble metals have poor ECR selectivity because of their lower CO 2 -philicity, while a high ECR selectivity was shown by Fe, Co, Ni, and Ru [91].

Recent Experimental Validations
Armed with valuable suggestions from the theoretical studies, numerous experimental works have been conducted very recently, as listed in Table 2. All the experimental works reported using Mo-and Ti-based MXenes, as they have been predicted to have high activity and selectivity toward ECR [33,34,93]. Handoko et al. [94] reported the first direct experimental demonstration of MXenes as electrocatalysts for ECR. As seen in Fig. 7a, b, CO 2 reduction on Ti 2 CT x and Mo 2 CT x MXenes preferably goes through the *HCOOH pathway to produce formic acid as the main product. The F/ Ti ratio in Ti 2 CT x had a profound effect on electrocatalytic performance, where a low F/Ti ratio exhibited low overpotential and high Faradaic efficiency (FE) [94]. A peak FE of 56% toward formic acid was achieved on Ti 2 CT x , while a peak partial current density of 2.5 mA/cm 2 was attained on Mo 2 CT x . In a separate study, ECR was also observed on Mo 2 CT x and Ti 3 C 2 T x MXenes that produced mainly CO [95]. The difference in the products may be due to the different ionic liquids that were used.
In addition, Cu-loaded MXenes were also tested for ECR as a dopant, single-atom catalyst, or bulk particle. For instance, Ti 3 C 2 T x and Cu-doped Ti 3 C 2 T x can generate HCOOH as a major product and CH 3 OH as a side product [96]. Introducing a tiny amount of Cu (1.04 wt%) to Ti 3 C 2 T x is enough to enhance the electrocatalytic performance with a high reaction rate, lower potential, and higher FE. This significant enhancement in the ECR performance is ascribed to the combination of the inherent catalytic merits of Cu dopants. Meanwhile, another study reported that incorporating SAC-Cu into Ti 3 C 2 Cl x enhanced its catalytic activity with a high obtained current density and FE, which is also better than using a single-atom Cu decorated carbon membrane and a single-atom Cu catalyst on graphene [97]. The experimental work in ECR was also evaluated for the ZnO-Fe/Ti 3 C 2 nanocomposite, which exhibited good performance with a high current density (18.75 mA/cm 2 ). Although combining ZnO and Ti 3 C 2 should benefit the CO 2 adsorption process and electron transport, Fe helps suppress the HER and direct the HCOO* intermediate formation pathway [98].
In the experimental works, the ECR selectivity over the HER remains a significant, unsolved issue. Therefore, aprotic solvents, such as ionic liquids, were used as the electrolyte for the pristine MXenes to suppress HER activity [94,95]. Eid et al. [96] found that the ECR selectivity in an aqueous solution was enhanced by atomically doping Cu on Ti 3 C 2 T x , suggesting that Cu can preclude the HER, although the mechanism remains undefined. Similar results were also reported by Zhao et al. [97]: the SAC-Cu/MXene has a high ECR selectivity at a voltage of − 1.4 V vs. RHE based on the FE of the products. This result suggests that appropriate chemical modification of the MXenes may bring new opportunities to enhance the ECR selectivity over the HER.

MXene-Based Catalysts for PEC and Photothermal CO 2 Reduction
The PEC system can be an alternative that benefits from photocatalytic and electrocatalytic systems to generate more efficient CO 2 conversion [99]. For instance, energetic photoelectrons induced by photons may help to alleviate the high overpotential of the electrochemical process. At the same time, the external voltages enable the maintenance of photogenerated electron-hole pairs through efficient carrier separation and increase the mobility of carriers in the catalyst, which results in high electron utilization [99]. A schematic representation of PEC and photothermal catalytic CO 2 reduction is depicted in Fig. 8. Recently, MXene-based nanocomposite Ti 3 C 2 /g-C 3 N 4 was reported to be an efficient catalyst for CO 2 reduction in the PEC system. Ti 3 C 2 /g-C 3 N 4 ,  [100]. Ti 3+ benefits recombination suppression of the photogenerated electrons and holes in heterojunctions, increasing the efficiency of PEC, whereas the pyri-N species can generate adsorption of CO 2 molecules. The Ti 3 C 2 /g-C 3 N 4 catalysts are excellent for coupling CO 2 reduction with water splitting to produce chemical fuels [100]. Another MXene used for PEC is TiO 2 / Ti 3 CN [99], which was also used as a photocathode in the PEC system. In TiO 2 /Ti 3 CN, the Ti 3+ and oxygen vacancy play an important role by inducing the generation and transportation of photoelectron-hole pairs [99]. Moreover, MXenes can also be used as catalysts in a photothermal catalytic reaction to realize efficient solar energy use [101,102]. During solar irradiation, the photothermal materials absorb photons from the sunlight, inducing photoexcitation and altering the charge carriers that subsequently generate the so-called light-induced electric field, thus eventually converting the solar energy into heat [103]. MXenes can exhibit an electromagnetic wave absorption capacity and localized surface plasmon resonance (LSPR) effect due to their excellent metallic feature, which makes them a potential material for inducing photothermal conversion [101,102]. MXenes exhibit an internal light-to-heat conversion efficiency of nearly ∼100% [104]. Further development can be conducted by making an exfoliated MXene into a selffloating thin membrane, serving as a heat barrier to generate light-to-water evaporation with an efficiency of up to 84%, which is comparable to a state-of-art photothermal evaporation system [104]. Wu et al. [101] have recently shown the ability of Nb 2 C and Ti 3 C 2 MXenes as excellent supports for Ni nanoparticles to achieve efficient photothermal CO 2 catalysis. A record CO 2 conversion rate of 8.50 mol/(g Ni ·h) was reached using Nb 2 C under 36-sun illumination without external heating.

Conclusions and Outlook
Photocatalytic and electrocatalytic CO 2 reduction has been nurtured into a promising technology for the sustainable conversion of CO 2 into value-added chemicals and fuels to cut the rising CO 2 atmospheric concentrations and mitigate their environmental impact. This technology can be economically competitive with the decrease in renewable electricity prices. That being said, the intrinsic competing HER is one of many remaining challenges to the low conversion rate of CO 2 in terms of quantum efficiency (QE) for photoreduction and Faradaic efficiency (FE) for electroreduction. MXenes have recently shown excellent CO 2 activation and conversion to overcome the sluggish kinetics and low efficiency of CO 2 reduction, although multiple challenges must first be solved to realize this technology practically.
In PCR, pristine MXenes cannot absorb solar energy because of their metallic characteristics. Although some modifications have produced MXenes with semiconductor properties, their bandgaps are too small, with inappropriate bandgap alignment to allow the CO 2 RR to take place. The formation of nanocomposites with other photocatalysts is a major strategy where MXenes can act as co-catalysts. Nonetheless, researchers are having difficulty enhancing the rate of PCR because the thermodynamic stability of CO 2 causes the first step of CO 2 activation to require high overpotential. Additionally, the achieved QE is far from the industry standard. Finding the best photocatalysts is not an easy task; Fig. 8 a Photoelectrochemical cell with a photocathode as a working electrode (WE) for CO 2 reduction, a counter electrode (CE) for water oxidation, and a reference electrode (RE) immersed in a CO 2 -containing electrolyte. Reproduced with permission from Ref. [6]. Copyright 2016, Royal Society of Chemistry. b Schematic illustration of photothermal catalysis over a semiconductor with external heat input (CB, conduction band; VB, valence band; E g , bandgap): (I) light absorption, (II) charge separation, and (III) surface reaction. c Schematic illustration of the processes resulting from the LSPR effect over plasmonic metal structures (E f , Fermi level): (1) local enhancement of the electric field, (2) hot-electron injection, and (3) photothermal effect. Reproduced with permission from Ref. [103]. Copyright 2021, Elsevier B.V the conduction band edge of the semiconductor photocatalyst should be over the standard reduction potential of CO 2 , and the valence band edge potential must allow the photooxidation process to occur.
In ECR, the essential indicator of FE should be prioritized for practical applications. However, the increase in FE and current density in multi-carbon products (C 2+ ) is still far from the industry target. The design of an efficient electrocatalyst remains a primary challenge because the various CO 2 -related intermediates create numerous permutations to protonation sites, complicating C 2+ production. From multiple experimental studies, the performance of MXenes-based electrocatalysts in the ECR process is affected by the binding energies of key intermediates on catalyst surfaces and the diffusion behavior of reactants near electrodes. Linear scaling relations between the binding energies of intermediates need to be circumvented to achieve significantly lower overpotential. Therefore, these factors can be prioritized in the CO 2 RR process to achieve high FE and valuable products.
In PCR and ECR, using single metals or defect engineering has been attempted to enhance CO 2 adsorption and reduction over MXene-based catalysts. This approach faces stability challenges under reaction conditions that worsen the performance over time, warranting further research. As the product selectivity from CO 2 conversion remains low, researchers need to focus on enhancing selectivity and/ or product separation. Porous materials with tunable pore sizes, such as metal/covalent organic frameworks, should play an essential role in product separation and purification. Therefore, designing a catalytic reactor incorporating porous materials is an interesting topic to further advance this emerging field.
In addition to PCR and ECR, PEC and photothermal catalytic CO 2 reduction can also be complementary strategies. By combining the photocatalytic and electrocatalytic systems in the PEC process, we may obtain more efficient CO 2 reduction. For instance, energetic photoelectrons induced by photons from sunlight may overcome the high overpotential of the electrochemical process. Conversely, the external voltages during the electrochemical process may help suppress photogenerated electron-hole pair recombination. However, several factors need to be solved to obtain an efficient PEC system, such as the requirement of a good match of bandgap with the incident radiation solar spectrum and well-maintained redox processes at both electrodes [105]. As for the photothermal catalytic system, the ability of MXenes to efficiently absorb the near-infrared spectrum from sunlight (that is further converted into heat) is a vital feature in achieving high-efficiency CO 2 reduction. MXenes are suitable for photothermal catalysis because of their excellent metallic behavior; however, not every MXene type can be used because some have relatively low conductivity. Ti 3 C 2 T x showed good photo-to-thermal storage efficiencies up to 94.5% under sunlight irradiation because of its excellent metallic behavior, generating a tremendous LSPR effect [106]. Nb 2 C also exhibited excellent photothermal catalytic ability, yielding a high conversion rate. We believe that an expanding trial on the other types of MXenes and composite-based MXenes to obtain efficient photothermal catalysis will be carried out shortly.
Apart from the attempts to enhance CO 2 RR efficiency, other features of MXenes need to be improved. For instance, the toxicity issue of MXene synthesis should be addressed since they are generally prepared using toxic chemical reagents (such as HF-containing etchants) and complicated fabrication processes. Although MXene-based catalysts can potentially solve the environmental issues of CO 2 and convert it into valuable chemicals, the waste from their synthesis process should not contribute to more problematic environmental toxicity. Further research can be directed to developing greener, safer, and more sustainable syntheses of MXenes, such as electrochemical, halogen-based, or molten salt etching. Toxic chemicals related to HF-containing etchants should be avoided, or at least carefully handled, so as not to leak into groundwater resources. Conventional fabrication of MXenes, to the best of our knowledge, still involves solid-state reactions at high temperatures to prepare MAX phases, which are precursor materials of MXenes. Alternative fabrication methods of MAX phases, such as microwave-assisted, physical synthesis methods, and sol-gel chemistry, are promising for reducing the use of toxic chemical reagents and lowering the energy requirement during the synthesis process. These technologies may have the potential to obtain MXenes that are more environmentally benign for practical applications.

Conflict of interest
The authors declare that there is no conflict of interest.
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