MXenes: Applications in electrocatalytic, photocatalytic hydrogen evolution reaction and CO2 reduction

Molecular Catalysis 486 (2020) 110850

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Review

MXenes: Applications in electrocatalytic, photocatalytic hydrogen evolution reaction and CO2 reduction

T

Thang Phan Nguyena,b, Dinh Minh Tuan Nguyenc, Dai Lam Trand,e, Hai Khoa Led,e, Dai-Viet N. Vof, Su Shiung Lamg, Rajender S. Varmah,*, Mohammadreza Shokouhimehri,*, Chinh Chien Nguyenj,*, Quyet Van Lej,* a

Laboratory of Advanced Materials Chemistry, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam c Faculty of Chemical Engineering, University of Science and Technology, The University of Da Nang, 54 Nguyen Luong Bang, Da Nang, 550000, Viet Nam d Graduate University of Science and Technology, Vietnam Academy of Science and Technology (VAST), Viet Nam e Institute for Tropical Technology, Vietnam Academy of Science and Technology (VAST), Viet Nam f Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh University, 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City, 755414, Viet Nam g Pyrolysis Technology Research Group, Institute of Tropical Aquaculture and Fisheries (Akuatrop) & Institute of Tropical Biodiversity and Sustainable Development (Bio-D Tropika), Universiti Malaysia Terengganu, 21030, Kuala Terengganu, Terengganu, Malaysia h Regional Center of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, Šlechtitelů 27, 783 71, Olomouc, Czech Republic i Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul, 08826, Republic of Korea j Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam b

A R T I C LE I N FO

A B S T R A C T

Keywords: MXenes Composites HER CO2RR Electrocatalysis Photocatalysis

Two-dimensional MXenes have gained tremendous interest as frontier materials for a wide variety of applications and play a pivotal role in the development of future energy, electronic and optoelectronic devices as they exhibit high catalytic activity in diverse electrocatalytic and photocatalytic devices. The fabrication and application of MXenes as catalysts have become more progressive in recent years and more than 30 different varieties have been experimentally discovered and utilized. In this review, we rationally summarized and discussed the most recent advances in the synthesis and specific applications of MXenes as electrocatalysts and photocatalyst for hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR) including strategies for boosting their catalytic activity for target products. Finally, we highlight the lingering challenges and direction for the future development of MXenes as catalysts for HER and CO2RR.

1. Introduction Energy and environment are currently one of the highest priority issues for mankind that need to be addressed [1]. Fossil fuel-derived energy has been the primary energy source for industry and transportation which is being severely depleted at a rapid rate [2]. Consequently, the search for a renewable and sustainable energy source is of utmost priority. Among the emerging sources, hydrogen energy has been identified as clean, sustainable and renewable [3] for industrial usage in various forms [4]. The hydrogen can be produced via various approaches such as water gas shift [5], formic acid decomposition [6,7], reforming and partial oxidation of hydrocarbons [8], biomass

conversion [9], and water splitting [10]. However, the clean, safe and cheapest method can be attributed to the water-splitting approach in which the hydrogen was generated through electrocatalysis or photocatalysis of water. The burning of fossil fuel contributes to severe environmental problems due to the formation and release of by-product gases such as NOx, SO2, and CO2 [11] and the reduction of these toxic gaseous needs to consider. CO2 is the primary pollutant gas due to the burning of fossil fuels besides being released from many other human activities [12] and contributing to the greenhouse effect and culminating in global warming [13]. Thus, the removal and consumption of CO2 are imperative to protect the global environment. Two general approaches for

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Corresponding authors. E-mail addresses: [email protected] (T.P. Nguyen), [email protected] (R.S. Varma), [email protected] (M. Shokouhimehr), [email protected] (C.C. Nguyen), [email protected] (Q.V. Le). https://doi.org/10.1016/j.mcat.2020.110850 Received 10 February 2020; Received in revised form 18 February 2020; Accepted 19 February 2020 2468-8231/ © 2020 Elsevier B.V. All rights reserved.

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2.2. Properties

reducing CO2 levels in the atmosphere comprise geological sequestration [14], and the chemical reduction [15], the second approach being more promising which creates many of valuable chemical products such as CO, HCOOH, CH4, C2H3, CH3OH, and C2H5OH and deployed as clean fuels [16,17]. To date, the most efficient catalysts for either HER or CO2RR are attributed to noble metals namely Au, Pd, and Pt. Nevertheless, tremendous progress has been made to produce high efficient catalysts for HER and CO2RR based on non-noble metals [18–42]. Among the new emerging catalysts, low dimensional materials such as transition metal dichalcogenide [43–52], and transition metal carbide [53–57] are highly considered due to the excellent catalytic activity, earth-abundant and low-cost production via solution process which can be scaled up for commercialization. Besides, other layered structures have also been rationally developed such as layered silicates [58–60], boron nitrides [61], and layered perovskite oxides [62,63]. Discovered in 2011 [64], MXenes have become one of the most intriguing classes of 2D materials that can be utilized in a wide range of applications such as energy storage [65–67], batteries [68–70], magnetic shielding [71,72], and catalysis [73,74], owing to their extraordinary and unique optical and physical properties [75]. In general, MXenes can be represented by a chemical formula Mn+1XnTz (n = 1–3) (M = Ti, Sc, Nb, Ta, Cr, Mo, Zr, Hf and V; X = C; T = hydroxyl, fluorine, or oxygen). The use of MXenes in electrocatalytic and photocatalytic hydrogen evolution reaction (HER), [76–78] and CO2 reduction reaction (CO2RR) [56,79,80], have been widely undertaken where they exhibit exceptional catalytic activity toward the HER and CO2RR. Consequently, they are potential candidates to replace noble metal catalysts for HER and CO2RR in terms of industrialized use. In this review, we rationally recapitulate and discuss the recent advancement in the fabrication and utilization of MXenes as catalysts for HER and CO2RR; a brief description on fabrication and properties is followed by their comprehensive application as efficient catalysts for HER and CO2RR. The emerging strategies for improving the performance of MXenes towards HER and CO2RR including structural engineering, surface modification, compositional engineering, and doping are taken into account including the persisting impediments and road map for their future development.

The transmittance spin-coated MXene films were reported to be 97 % per nanometer thickness which is approaching that of single-layer graphene sheets (97.7 %), 53which is tunable depending on the etchant used in their synthesis. For example, the MXenes that are etched with NH4HF2 exhibited transparency higher than that of MXenes etched with HF due to the difference in terminal functional groups [86]. The optical bandgap is more important to be considered when using MXenes as catalysts and is dictated by many different parameters including structures, composition and terminal functional groups [87,88]. Most of MXenes exhibit indirect bandgap except Sc2C(OH)2 [67,89]. Depending on the composition, the bandgap for MXenes can range from 0.05 to 2.87 eV [88], which can be modulated via the engineering of surface termination group [64,90]. For example, the bandgap of Sc2C was reported to be 1.8, 1.03 and 0.45 eV with O, F, and OH as functional groups, respectively [91]. This property allows us to tailor the structure of MXenes featuring desirable properties toward high catalytic activity for HER or CO2RR. For use as catalysts, the conductivity of MXenes is another key parameter that needs to be investigated; MXenes can be categorized into metallic, semimetallic and semiconductors [88]. As an example, low defect Ti3C2Tz exhibited electrical conductivity of 9880 S cm−1 [60], which is superior to that of graphene ∼6000 S cm−1 [92], which is superior compared to that of graphene ∼6000 S cm-1 [93]. On the contrary, the highly defected Ti3C2Tz showed a much lower conductivity of 1000 S cm−1 [94].

2.3. Synthesis A common route for the synthesis of MXenes is shown in Fig. 2 [95] which specifically entails 2 steps, etching and exfoliating. Originally, the fabrication of MXens was introduced by Naguib et al. in 2011 where the Al was extracted from Ti3AlC2 to yield OH or F-terminated Ti3C2, according to the equations below: Ti3AlC2 + 3 H F = AlF3 +3/2H2 + Ti3C2 Ti3C2 + H2O = Ti2C2(OH)2 + H2 Ti3C2 + HF = Ti3C2F2 + H2

2. Structure, properties, and synthesis

The transformation of Ti3AlC2 to Ti3C2 is illustrated in Fig. 3A–C wherein the removal of Al upon etching can be vividly observed in the scanning electron microscopy (SEM) image as shown in Fig. 3d. The corresponding TEM and simulated TEM images are shown in Fig. 3E-G. In the final step, the MXenes are exfoliated by sonication to obtain ultrathin MXenes nanosheets [84]. Depending on the structure of the parent’s MAX phase, the requisite synthesis conditions such as reaction temperature, amount of acid and reaction time can be significantly varied. For instance, 10 % HF was enough for etching away Al from Ti2AlC to yield Ti2CTx in 10 h [95], while 50 % HF was needed to produced Nb2CTx from Nb2AlC for 90 h at room temperature [96]. The diverse prerequisite conditions to produce MXenes from various MAX precursors have been postulated to originate from the bond energies between M and A atoms [97,98]. The reaction time for removing A layer from the MAX phase can be drastically decreased via the downsizing of MAX particles or increasing the reaction temperature [99]. However, it is noteworthy that the overheating may cause the recrystallization of the Mxenes [100] or dissolution of carbide [95]. For instance, the etching of Ti3AlC2 in HF at 180 °C did not result in MXenes (Ti3C2) but rock-salt structured titanium carbide [100]. Besides HF, other etchants such as ammonium fluoride (NH4HF2) [86], and HCl + LiF solution have also been successfully used to produce MXenes from MAX phase [101]. In addition, the fluorine-free etchant was also reported [102]at elevated reaction temperature (270 °C) and high concentration NaOH solution (27 M) in a hydrothermal method to remove Al to afford production yield up to 92 %. Other than solution

2.1. Structures MXenes are a new emerging 2D materials comprising transition metal carbides and nitrides that are exfoliated from MAX phase precursors (Mn+1AXn) (M = Mo, Ti, Zr, Cr; A = Al, Ga, Ge, Si; and X = C, N) [81]; structure and composition of MXenes being determined by the MAX phase types. Typically, the MAX phases are classified into 3 different types including 211, 312 and 413 in accordance with n range from 1 to 3 as shown in Fig. 1 [82]. The first MXenes reported by Naguib et al. had a structure Ti3C2 [64], and more than 30 different MXenes have been experimentally fabricated to date using assorted approaches [83,84]. Because the synthesis of MXenes related to etching with strong acids such as hydrofluoric acid, their surface are generally grafted with various terminal functional groups such as OH, O, and F [85]. Notably, the distribution of these functional groups strongly depends on the synthesis conditions deployed such as etchants, pH, and temperature [85]. Because of the presence of terminal functional groups, the structure of MXenes is written as Mn+1XTx or (M: early transition metal, X: carbide or nitride, T is the terminal functional groups). The crystal structure of MXenes, in general, is hexagonal closed-packed (HCP). However, the M atoms in M2X displayed HCP sequence (ABABAB), while M3X2 and M4X3 exhibit face center cubic (FCC) sequence (ABCABC) [66,81].

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Fig. 1. Structural features of MAX phase parent and their corresponding MXenes.

3. MXenes for hydrogen generation

etching, a molten salt (KF, NaF, and LiF) approach at high temperature (550 °C) has been investigated [103]wherein few-layer MXenes with Otermination can be archived. It should be noted that the exfoliation after etching is of paramount importance to obtain single or few layers from bulk MXenes. In fact, the exfoliation strongly depends on the etchant that use to remove the A atoms from MAX phases. For example, single or few layers MXenes can be easily obtained through sonication or shaking if the etchant is HCl + LiF solution due to the presence of Liion [101]. However, if the MXenes is produced using only HF as an etchant, the small organic molecule or ion such as is the prerequisite to delaminate the MXenes [104].

Theoretical calculations indicate that 2D MXene materials fulfill essential conditional requirements for the hydrogen evolution reaction (HER). Indeed, OH* and O*-terminated MXenes (e.g., Ti2C, V2C, and Ti3C2) are the basis for metallic property, which encourages the charge transfer and transportation. Moreover, oxygen atoms-modified MXenes surface offers active sites because the weak H*-MXene interaction assists in the evolved hydrogen. The volcano curve in Fig. 4 represents the capability of various MXenes for H2 production. High exchange current (i°) associated with the H* adsorption energy close to zero significantly

Fig. 2. Schematic illustration of the MXenes fabrication process. 3

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Fig. 3. Illustration of the exfoliation process for Ti3AlC2. A) Ti3AlC2 structure; B) Al atoms substituted by OH upon reaction with HF; C) Exfoliation of the Ti3C2 nanosheets after sonication in methanol; D) SEM image of Ti3C2 after etching by HF; E-F) TEM images of Ti3C2; G) simulated structures of Ti3C2. Reproduced with permission from ref [64].

impact on the electrocatalytic activity of MXene catalysts Thus, oxygen groups-terminated basal planes of Mo2CTx are beneficial for the HER process while oxygen-functionalized Ti3C2Ox nanosheets boost HER performance because of the essential role, O terminations play as the active sites in acid media [106,107]. Moreover, the DFT calculations unveiled that the layer thickness of 2D MXenes prompting the activity as the hydrogen adsorption energy relies on the number of metal layers in the MXenes structure [108]. Recently, Li et al. ascertained the potential electrocatalytic hydrogen production of MXene through fluorine-terminated Ti2CTx ultrathin nanosheets. Thus, rich F-terminated Ti2CTx ultrathin nanosheets reduce the value of Gibbs free energy of hydrogen adsorption (ΔGH), which is beneficial for the proton adsorption kinetic encouraging the hydrogen production. Also, the presence of F- termination groups force the charge transfer owing to its capability to lower the charge-transfer resistance while nanosheets offer more catalytic sites than conventional materials. These features have resulted in an excellent electrocatalytic achievement for HER. The overpotential of the Ti2CTx nanosheets for HER is considerably reduced compared to layer, pristine and alkalized Ti2CTx, as shown in Fig. 5A-B [109]. In 2016, Seh et al. found that the HER capability of Mo2CTx outperforms Ti2CTx signifying a prominent catalyst. In this study, Mo2CTxbased MXene was synthesized from Mo2Ga2C parent ternary carbide via the etching method with HF (Fig. 5C). Interestingly, Mo2CTx exhibits the notable enhancement in the HER activity and stability compared to Ti2CTx (Fig. 5D) as the overpotential of the fabricated Mo2CTx is remarkably decreased in comparison to the sample Ti2CTx. Thus, the asprepared Mo2CTx exhibited the overpotential of 283 mV to reach the current density of 10 mA.cm−2, which is considerably lower than that of Ti2CTx (609 mV), suggesting its outperformance. Moreover, the theoretical calculations exhibiting a small ΔGH of 0.048 eV is consistent with its higher HER activity [76]. The outcomes derived from this study can inspire the development of a new class of MXenes beyond the

Fig. 4. The volcano plot depicting the relationship between the exchange current and average Gibb free energy of hydrogen adsorption. Reproduced with permission from ref [78].

promotes the H2 generation performance. Hence, the 2D MXenes materials (e.g., Ti2CO2, W2CO2, TiVCO2, and Nb2CO2) located at the peak of the volcano plot are the highly active catalysts [67,78,105]. Additionally, bimetallic MXenes carbide, M’M”CO2, are also the potential candidate as they produce appropriate H2 adsorption free energy signifying the highly active catalysts for HER [105]. For those reasons, MXene-relied systems have become a magnet in the field of electrocatalysts and the design of solar-driven photocatalysts. 3.1. MXene for electrocatalytic hydrogen evolution reaction 3.1.1. First-generation MXenes It has been demonstrated that the termination groups strongly 4

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Fig. 5. A-B) Polarization curves of Ti2CTx nanosheets compared to other samples. Reproduced with permission from ref [109]. C) Schematic illustration for the synthesis of Mo2CTx and D) Anodic-going iR-corrected LSVs of Mo2CTx and Ti2CTx. Reproduced with permission from ref [76].

nitridation with NaNH2 followed by (ii) delamination by sonication in water. The nitridation process thus significantly alters the structure of Ti2CTx as shown in Fig. 6B. An apparent downward shift can be observed for the samples undergone the nitridation treatment indicating the enlargement of the c-lattice parameter. The SEM images (Fig. 6C-D) show the morphologies of pristine- Ti2CTx and sonicated N-Ti2CTx samples which confirm the successful exfoliation. It turns out that the HER ability of nitridated samples outperforms the pristine-Ti2CTx and TiN samples owing to the smaller overpotential, better kinetic and charge transfer resistance (Fig. 6E-G). In this case, the presence of nitrogen on the MXene surface through Ti-Nx motifs significantly facilitates the electron transfer pathway and offers proper active sites for the HER process [113]. Recently, Le et al. reported nitrogen-doped Ti3C2Tx (N-Ti3C2Tx) catalyst synthesized by annealing of Ti3C2Tx in the ammonia environment. The appropriate nitrogen doping enhances the electrical conductivity and reactivity properties, which boost HER performance at low overpotential and high durability. The DFT calculations indicate that a suitable level of N doping results in the Gibbs free energy for hydrogen adsorption (ΔGH*) close to zero suggesting the advantages of doping methods [116]. In addition to N-doped MXene, P-doped MXenes also have attracted consideration; Qu et al. introduced P and O into the Mo2CTx employing a phosphorization procedure wherein the HER activity for P and O co-doped Mo2CTx exceeds the pristine Mo2CTx. It turns out that both P and O-incorporated in each Mo-C-Mo layer enlarged the interlayer distance providing more P and O active sites, which are the primary reason for the significant decrease in overpotential compared to the Mo2CTx sample. Moreover, the dopants also decrease the free energy of hydrogen adsorption and conductivity that enhances the feasibility of HER kinetics and electron transportation [117]. It can be pointed out that such structural modifications and novel fabrication methods can change the nature of catalyst in the context of charge transfer and free energy for hydrogen adsorption that boosts the hydrogen generation performance. Therefore, this direction of study is

conventional Ti-based MXene catalysts. In another investigation, Intikhab et al. found that the stoichiometry and surface structure of Mobased MXene strongly impact on its activity. The Mo1.33CTx MXene containing ordered surface divacancies exhibits a less HER activity than that of Mo2CTx. This change could be originated from the crucial modifications in both electronic properties and the coordination geometries, which are associated with the disappearance of hcp sites with six-fold coordinated C atoms in the Mo1.33CTx structure [110]. It is noteworthy that the free energy of hydrogen adsorption for O-terminated hcp sites is favorable for the HER process [76,110]. Therefore, the loss of hcp sites with six-fold coordinated C atoms induces the remarkable decrease in the HER performance. The study is a step closer towards the development of high-quality 2D Mo-based MXene whereas redoubled efforts in future years may aim at providing further computational and experimental results for Mo-derived MXenes to obtain the superior electrocatalysts for HER. Very recently, Cr- derived MXenes have attracted considerable attention for the HER as substantiated by DFT calculations for the capabilities of Cr2CO2 to generate electrocatalytic hydrogen; Cr2C and Cr2CO2 possess the valuable conductive ability for electron transfer. The free energy of hydrogen adsorption could be tuned by either hydrogen coverage or transition metal modification. Furthermore, carbon vacancies-derived Cr2CO2 MXene can enhance HER achievement [111,112]. Such preliminary results offer a roadmap for the experimental preparation of 2D Cr-based MXene for electrocatalytic water splitting.

3.1.2. Structural modifications of MXenes The introduction of heteroatom into the MXenes structure is a propitious tactic to enhance the activity performance as the dopant can expand the lattice parameter of the host materials promoting the electrochemical performance and energy storage capabilities [114,115]. In recent work, Yoon et al. reported the robust catalytic activity for HER over nitrided-Ti2CTx (N-Ti2CTx). Fig. 6A exhibits the schematic illustration of the two-step synthesis of N-Ti2CTx (i) 5

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Fig. 6. Schematic illustration of the nitridation to prepare the N-Ti2CTx nanosheets by heat-treatment with NaNH2 and the effect of degrees of nitridation on the chemical structures and morphological properties. A) Synthesis procedures for the N-Ti2CTx nanosheets and its dispersion (right image); B) XRD patterns of the ratiodependent structural transition in NTi2CTx; SEM images of C) the pristine-Ti2CTx MXene phase (Ti2CTx) before heat treatments with NaNH2 and D) after heat treatment with NaNH2; E) Linear sweep voltammetry (LSV curves) without iR correction of electrocatalysts with a scan rate of 10 mV s−1; F) corresponding Tafel slopes; G) EIS Nyquist plot of different electrocatalysts at an overpotential of – 200 mV vs. NHE. Reproduced with permission from ref [113].

abundant surface Mo vacancies (VMo) and Pt single atoms (SA). The Mo vacancies function as the anchoring sites for Pt single atoms, which then are stabilized by the surrounding C through the formation of Pt-C bond, as shown in Fig. 7A. The high-angle annular dark-field STEM (HAADF–STEM) images (Fig. 7B) confirm the presence of single Pt atoms on the host Mo2TiC2Tx and the immobilization of Pt sites at the Mo positions. Moreover, XANES measurements verified the formation of Pt-C bonds in the structure of rich Mo vacancies- Mo2TiC2Tx nanosheets (Fig. 7C). The ensuing Mo2TiC2Tx-PtSA shows an exceptional activity for the HER as shown in Fig. 7D-E. The overpotential of 33 mV required to reach a current density of 10 mA.cm−2 could be observed. This value was found to be better than that of the commercial Pt/C (> 65 mV), indicating the extraordinary application of this nanocomposite, which can replace commercial Pt/C. The Tafel slope of the sample Mo2TiC2Tx-PtSA is approximating that of the Pt/C catalyst implying the features of single Pt atoms inducing the fast kinetics. Furthermore, the Mo2TiC2Tx-PtSA catalyst also shows high stability for 100 working hours. Thus, the single Pt atoms display superior properties and driving the excellent activity. The ΔGH* of the single Pt site is very close to zero (-0.08 eV), which surpasses the commercial Pt catalyst (-0.10 eV), causing the faster generation of H2 molecules. The presence of single Pt atoms on Mo2TiC2O2-PtSA is, therefore, a paramount factor

desirable and should solicit increased attention. 3.1.3. MXene-based nanocomposite It has been realized that Pt/C material is the most promising material for HER performance [118]. However, scarcity and high cost of Pt are the primary hindrances confronting its use for scalable applications. Therefore, the fabrication of alternative catalysts, which can reduce/ or replace the loaded Pt amount and be competitive to Pt/C, has been the main objective of researchers in this field. MXene-based nanocomposites have appeared as a formidable candidate in this race because they can circumvent these issues (Table 1) and significantly promote HER performance compared to the pristine catalyst. The single metal atoms-integrated MXenes have become the hot research trend in recent years. As-proof-of-concept, single metal atoms can significantly boost the catalytic performance because of the lowcoordinated metal atoms-induced high surface free energy, quantum size effect and strong metal-support interaction prompt chemical interactions, charge transfer between metal single atoms and supports [130,131]. Recently, Zhang et al., for the first time, introduced a Mo2TiC2Tx-immobilized Pt single atom (SA) configuration to remarkably boost the hydrogen generation. The system was fabricated via an electrochemical exfoliation, which simultaneously produces 6

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Table 1 MXene-based nanocomposite for electrocatalytic HER. Materials

Electrolyte

Mass loading (mg. cm−2)

Overpotential (η10. mV)

Tafel slope (mV.dec−1)

Stability

Ref

Co-doped MoS2/Mo2CTx MoS2/Ti3C2 MoS2/Ti3C2Tx PtxNi NWs/Ti3C2

1 M KOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.1 M KOH 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 1.0 M KOH 0.5 M H2SO4 0.5 M H2SO4 1.0 M KOH 0.5 M H2SO4

0.35 0.35 0.27 NA NA 1 0.1 1 4.9 0.38 0.4

112 ∼280 152 18.55 55.6 (η5) 30 132 76 72 55 135 179 > 65

82 68 70 13.37 39.5 30 70 90 45 70 45 85 32

> 18 hr 36 hr 12 hr < 2 hr < 1 hr > 100 hr 280 h > 10 hr > 12h 20 hr > 20 hr

[119] [120] [121] [122]

PtSA/Mo2TiC2Tx NiFe-LDHs/Ti3C2/NF RuSA-N-S-Ti3C2Tx NF/Ni3S2/Ti3C2Tx Pt/Ti3C2Tx MoS2/Ti3C2‐MXene@C NiS2/V2CTx Pt/C

1

> 100 hr

[123] [124] [125] [126] [127] [128] [129] [123]

SA: Single-atom; Pt/C catalyst was added for comparison; NA: not available; NF: Nikel foam.

Fig. 7. A) the formation of rich Mo vacancies flowing Pt single atoms in the structure of Mo2TiC2O2; B) HAADF-STEM of Mo2TiC2Tx-PtSA and its corresponding structural illustration); C) XANES spectra at the Pt L3-edge; D, E) polarization curves and corresponding Tafel slope; and F) stability of Mo2TiC2Tx-PtSA. Reproduced with permission from ref [123].

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Fig. 8. A) Schematic illustration of the synthesis of RuAS-N-S-Ti3C2Tx; B-D)TEM and HAADF-STEM images of RuAS-N-S-Ti3C2Tx; E-F) polarization curves and corresponding Tafel plots, and G) Calculated Gibbs free energy of hydrogen adsorption. Reprinted by permission from ref [125].

hydrogen adsorption over RuAS-N-S-Ti3C2Tx is 0.08 eV, which is very close to zero, suggesting the distinct nature of this material that is beneficial for H2 production [125].

addressing the challenges and boosting HER performance [123]. Furthermore, this material exhibited excellent stability under the working condition as shown in Fig. 7F. The extraordinary results derived from this study thus provide an essential tactic for the preparation of other single metal atoms/ MXenes heterojunction. Very recently, Ramalingam et al. reported the synthesis of ruthenium single-atom (RuSA) on nitrogen (N) and sulfur (S) species- modified Ti3C2Tx MXene (denoted as RuAS-N-S-Ti3C2Tx (Fig. 8A). It is noteworthy that N and S species on the Ti3C2Tx surface stabilize RuSA via the formation of Ru-N and Ru-S bonds eliminating the agglomeration. Indeed, the TEM and HAADF-STEM images (Fig. 8B-D) confirms the existence of isolated Ru dots (< 1 nm), implying the successful formation of RuSA on the Ti3C2Tx MXene few layers. It has been found that the as-prepared Ru single atoms are stable owing to the Ru-N and Ru-S bonds. Such imperative chemical bonds restrict the agglomeration and increase the metal-support interaction. Consequently, RuAS-N-S-Ti3C2Tx exhibited excellent HER ability, as shown in Fig. 8E-F. The overpotential was found to be 76 mV to attain a current density of 10 mA.cm−2. Also, the RuAS-N-S-Ti3C2Tx catalyst exhibited a Tafel slope of 90 mV.dec-1 indicating the rapid reaction kinetics. Such outstanding performance is derived from the combination of RuSA and active Ti3C2Tx MXene. The computation study was also employed to explain the enhanced activity (Fig. 8G). The Gibbs free energy of

3.2. Cocatalysts-based MXenes for photocatalytic hydrogen production The usage of MXenes, which function as cocatalysts for attracting the photogenerated electrons, enables a novel pathway to prepare efficient solar-driven hydrogen production catalysts; as-photogenerated electrons transfer to MXene cocatalysts to participate in the reduction reactions. In this setting, MXenes display a dual-role as suppressing the electro-hole recombination and offering active sites for the redox reactions. Also, the MXenes-incorporated photocatalysts are the alternative solution towards free-noble- metal photocatalysts. In 2017, Ran et al. reported the first fascinating study of robust Ti3C2 MXene cocatalyst integrated on the CdS photo-absorber to boost the visible-light-driven photocatalytic hydrogen evolution [132]. The density functional theory calculations, as shown in Fig. 9A, implies that the Gibbs free energy value, |ΔGH*|, of the intermediate state over Oterminated Ti3C2, is very close to zero indicating the strong capability for the HER activity compared to Pt, MoS2 and WS2 catalysts (Fig. 9B). Coupling this MXene with CdS accordingly resulted in a high-powered photocatalyst simultaneously promoting light absorption, charge 8

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Fig. 9. A,B) Calculation of free energy for HER at various and 1/2 H* coverage, respectively; C) Time-resolved PL spectra; D) Photocatalytic hydrogen production; E) Charge separation and transportation within the system and F) proposed mechanism. Reprinted by permission from ref [132].

Ti3C2 MXene as cocatalysts to inspire future studies (Table 2). Very recently, Li et al. constructed graphitic carbon nitride g-C3N4based g-C3N4/Ti3C2 MXene quantum dots (QDS) photocatalyst via the self-assembly of PEI-incorporated Ti3C2 QDs and g-C3N4 nanosheets. It is noteworthy that the positively charged PEI polymer interacts with the negatively charged Ti3C2 QDs and converts the surface to the negative charge. As a result, the Ti3C2 QDs are immobilized on g-C3N4 nanosheets via electrostatic force between Ti3C2-PEI (+) and g-C3N4 (-) inducing the intimate contact within two components, as shown in Fig. 10A – B. The as-fabricated g-C3N4/Ti3C2 QDs prolong the lifetime of charge carriers, implying the enhancement of electron-hole separation. Notably, this composite system exhibited a dominated photocatalytic performance for hydrogen production as compared to Ti3C2 MXene sheet/g-C3N4 and Pt/g-C3N4. Thus, the produced hydrogen was

separation, transfer, and surface catalytic reactions. These features prolong the lifetime of photogenerated electrons and notably enhance the produced hydrogen, as shown in Fig. 9C-D. The evolved hydrogen over Ti3C2/CdS composite (denoted as CT2) was higher than those of various cocatalysts (Fig. 9D). The superior activity over the Ti3C2/CdS catalyst was found to be one of the best noble metal-free cocatalyst/ metal sulfide photocatalysts. Fig. 9E-F explains the reason behind the enhanced activity. The Schottky barrier formed at the Ti3C2-CdS interface due to the difference of the Fermi level of O-terminated Ti3C2 and CdS facilitates the electron transfer from CdS to the cocatalyst. Furthermore, the metallic nature and outstanding capability of the Ti3C2 MXene for HER encourage the hydrogen generation [132]. It can be safely concluded that O-terminated Ti3C2 possesses outstanding properties of an excellent cocatalyst and opens up a newer trend for

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Table 2 Photocatalytic activity for hydrogen production of various MXenes- based photocatalysts. Composite (SC/MXenes)

Sacrificial agent

Produced Hydrogen (μmol.g-1.h-1)

Improvement Times (bare catalysts)

Ref

Ti3C2Tx/TiO2 Ti3C2@TiO2@MoS2 Monolayer Ti3C2Tx/TiO2 TiO2-Ti3C2-CoSx Ti3C2/g-C3N4 CdS-MoS2-Ti3C2Tx Zn2In2S5/Ti3C2(O,OH)x g-C3N4/Ti3C2/Pt Ti2C/g-C3N4 Nb2O5/C/Nb2C g-C3N4/O-terminated Ti3C2 MoxS@TiO2@Ti3C2 Carbon-doped TiO2/Ti3C2Tx Ti3C2Tx/CdS

Methanol TEOA Methanol Methanol TEOA Na2S, Na2SO3 Na2S, Na2SO3 TEOA TEOA Methanol TEOA TEOA TEOA Lactic acid

17.8 ∼6425.3 2,650.0 950 72.3 9,679 12,983.8 5100 47.5 (μmol. h−1) 7.81 88 10505.8 33.04 825

∼ 4 (rutile TiO2) 87.1 (TiO2) ∼9 (TiO2) 5.8 (TiO2) ∼10 (g-C3N4) ∼ 3 (CdS) ∼2 (Zn2In2S5) 5 (g-C3N4/Pt) 14.4 (g-C3N4) 4 (Nb2O5) ∼15 (non- annealed g-C3N4/Ti3C2) 193 (TiO2 nanosheets) 9.7 (P25) ∼1.8(CdS)

[133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [146]

TEOA, triethanolamine.

4. MXenes for CO2RR

found to be 3 and 10 times higher than those of Pt/g-C3N and Ti3C2 MXene sheet/g-C3N4, respectively (Fig. 10C). The outperformance of gC3N4/Ti3C2 QDs photocatalyst is achieved by the efficient interfacial electron transfer and the nature of Ti3C2 QDs that are beneficial to the redox reactions (Fig. 10D) [147]. This study thus presents a novel pathway to exploit Ti3C2 MXene-based materials for photocatalytic hydrogen production. Such fabrication means can be employed for various host photocatalysts. Moreover, the usage of quantum dot species not only reduces cocatalyst loading but also can address the light absorption issues induced by the shielding effect of MXene. It can be stated that the MXene-containing photocatalysts remarkably promote the evolved hydrogen compared to the pristine materials. Such enhancements are caused by the high efficiency in charge separation, transportation, redox reactions originated from the superior properties of MXenes. More efforts need to be devoted to investigating MXenen QDs compared to their sheet counterparts. Therefore, further understanding of MXenens QDs and their impact on activity in both, the experimental and computation perspectives, is highly desired.

Extraordinary capabilities of MXene (e.g., excellent electron conductivity, high hydrophilicity, and electronegativity) derived from the metallic and surface terminal groups are also found to be an excellent platform for the catalytic reduction of CO2. Indeed, DFT calculations suggest that O-terminated MXenes (e.g., Ti3CO2 and W2CO2) exhibit low overpotential and good selectivity toward the electrocatalytic conversion of CO2 to HCOOH. Such a reaction pathway is derived from the O-terminating groups that assist in stabilizing the intermediates through –H coordination [55]. The following sections provide the cutting-edge development of MXenes employed for photocatalytic reduction of CO2.

4.1. Titanium-based MXenes Ti-MXenes (Ti3C2Tz) has offered a potential solution for the photocatalytic reduction of CO2. Thus, Ti3C2Tz facilitates the electron-hole separation when coupled to a semiconductor-based photocatalyst

Fig. 10. A) Schematic illustration of the fabrication pathway; B) HR-TEM; C) photocatalytic activity hydrogen production of various samples and D) schematic illustration of electron transfer and reduction reaction. Reproduced with permission from ref [147]. 10

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Fig. 11. A,B) SEM and TEM images of the samples TT550; C) nitrogen adsorption-desorption isotherm; D) photocurrent; E) chemical products and F) CH4 formation of various samples; TT-X where C represents the calcination temperature. Reproduced with permission from ref [148].

because of its high electronic conductivity and low Fermi level. Moreover, the computational calculation proved that the Ti3C2O2 MXene containing oxygen vacancies is favorable for the photocatalytic CO2 reduction with high selectivity [56]. Recently, TiO2/Ti3C2 composite was fabricated via a feasible calcination method of as-prepared Ti3C2. The in situ growth of TiO2 obtained by the transformation of Ti3C2 during the calcination step generate an intimate contact between Ti3C2 and TiO2 in a rice crust like configuration possessing large surface area (Fig. 11A–B). Moreover, the TiO2/Ti3C2 composite exhibited a remarkable improvement in the electron separation and was found to be energetic for the multiple electron reactions like CO2 reduction. Thus, the smooth increase and stable decline of the photocurrent corresponding the light on and off, respectively, suggest the improved separation of photogenerated electron-hole pairs and the electron reservoir capability of Ti3C2, as displayed in Fig. 11C. Therefore, Ti3C2 MXene, in this case, functions as a cocatalyst enhancing the charge separation and offering proper active sites for the reduction reaction. The photocatalytic activity for CO2 reduction of the TiO2/Ti3C2 samples witnessed the generation of various organic products in which CH4 is the dominant product (Fig. 11D) indicating the excellent activity and selectivity of the TiO2/Ti3C2 composite. Moreover, the produced CH4

was found to be considerably higher than that of P25 suggesting the extraordinary properties of these photocatalysts [148]. The analogous conclusion has also been reached by Ye et al. in which Ti3C2 MXene-integrated P25 prompts the photocatalytic reduction of CO2. It is noteworthy that superior conductivity and OH-terminated surface are beneficial for the adsorption and activation of CO2 molecules. Moreover, such outstanding properties are found to be essential for the processes commanding the participation of multiple electrons. As a result, the selectivity towards the C1 product (e.g., CH4) is significantly improved [149]. In another study, the configuration of 2D/2D Ti3C2/Bi2WO6 heterojunction was reported in an attempt to fabricate an efficient material possessing strong sunlight absorption, forceful electron separation and reductive capability for the CO2 conversion. The in situ growth of Bi2WO6 on the Ti3C2 via a hydrothermal treatment method results in the strong interfacial contact within two materials, as shown in Fig. 12A–C. Indeed, the electrostatic attraction between positive Bi3+ cations and the negative surface of Ti3C2 is the primary reason driving the uniform formation of Bi2WO6 on the Ti3C2 surface. As-prepared Ti3C2/Bi2WO6 exhibits unique properties that are valuable for the photocatalytic CO2 reduction. Firstly, the composite fabricated under 11

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Fig. 12. A-C TEM and elemental mapping of a selected area of Ti3C2/ Bi2WO6 (the sample TB2); D) CO2 adsorption curves; D) Time resolve photoluminescence; D) Photocatalytic performance of various samples. TBX, where X represents the mass percentage of Ti3C2. Reproduced with permission from ref [80].

where the catalyst fabrication comprises two main steps, the synthesis of polyethyleneimine (PEI)- incorporated Ti3C2 QDs, poly(sodium 4styrene sulfonate) (PSS)- modified Cu2O NWs/Cu and self-assembly to form the desired materials, respectively. Notably, that the electrostatic force between the positive charged PEI-Ti3C2 and negatively charged PSS-Cu2O NWs/Cu is the crucial prerequisite inducing the uniform presence of Ti3C2 QDs on Cu2O NWs/Cu initiating the success of the preparation method, as exhibited in Fig. 13A. The existence of Ti3C2 QDs on Cu2O NWs/Cu, as shown in Fig. 13B-C, has been observed by HR-TEM images, suggesting the successful fabrication [153]. It has been demonstrated that the Ti3C2 QDS considerably contributes to the migration behavior of the photoexcited electro-hole pairs in the host photocatalyst. The Ti3C2 QDs/Cu2O nanowires/Cu (NWs) composite exhibited a remarkable outcome in terms of light absorption, charge separation and transportation depicting the enhanced light absorption and smaller bandgap compared to the Ti3C2 sheets/Cu2O NWs/Cu, and Ti3C2 QDs/Cu2O NWs/Cu heterostructures, respectively ascribable to the presence of Ti3C2 QDs. Furthermore, Ti3C2 QDs prompt the migration of photogenerated electrons from Cu2O NWs/Cu to Ti3C2 QDs, which intensify the current density, as shown in Fig. 13D. The higher current density indicates that Ti3C2 QDs accelerate the charge separation. Therefore, it can be stated that Ti3C2 QDs function as a cocatalyst attracting as-generated electrons and providing active sites for the CO2 reduction reaction. The photocatalytic performance (Fig. 13E) reveals a high yield for the formation of methanol (CH3OH).

the optimum condition, the sample TB2 exhibits a CO2 adsorption ability owing to the better CO2 affinity derived from the large specific surface area and pore volume. Such an advantage can promote the kinetics of the reaction as a high concentration of CO2 on the catalyst surface. Additionally, the intimate contact results in an efficient interfacial charge transfer channel prolonging the lifetime of charge carriers. Fig. 12D reveals the time-resolved photoluminescence (TRPL) of Ti3C2/ Bi2WO6 composite compared to the pristine Bi2WO6 (denoted as TB0). The decrease of short, long and average fluorescence lifetime of the sample TB2 suggests a high-end charge transfer pathway from Bi2WO6 to Ti3C2 leading to overcome the recombination issue. As a result, 2D/ 2D Ti3C2/Bi2WO6 heterojunction showed high photocatalytic CO2 conversion, as displayed in Fig. 12E. The evolved products (e.g., CH3OH and CH4) over this composite were found to be a significant improvement compared to the other samples implying the prominent of the investigated catalyst for CO2 reduction. It is postulated that the unique 2D/2D construction, efficient electron attraction capability and proper surface properties of Ti MXene are the primary reasons facilitating the interfacial electron transfer and reduction of CO2 [80]. The use of Ti3C2 quantum dots (QDs) derived from 2D materials, again, has emerged as a novel fascinating approach to exploit the unique properties of Ti3C2 MXene for CO2 application. Thus, the quantum dots of Ti3C2 introduce distinct properties, which are impossible to obtain in the 2D material counterparts [150–152]. In such a study, Zeng et al. reported Ti3C2 QDs/Cu2O nanowires/Cu (NWs) heterostructure 12

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Fig. 13. A) Schematic illustration of the preparation method; B) TEM image of Ti3C2 QDs/Cu2O NWs/Cu heterostructure with a magnified region shown in the inset. C) HRTEM image of Ti3C2 QDs/Cu2O NWs/Cu; D) Photoelectrochemical performance of Cu2O NWs/Cu, Ti3C2 sheets/Cu2O NWs/Cu, and Ti3C2 QDs/Cu2O NWs/Cu; E) Yields of methanol as a function of time; F) Nyquist plots from electrochemical impedance spectroscopy (EIS); G) Mott–Schottky plots. Reproduced with permission [153].

terms of theoretical and computational aspects. Thus, coupling the Ti3C2 cocatalyst to various semiconductive photocatalysts commands intensive research. Furthermore, the simulation of such a structure will assist the in-depth understanding of the manner of Ti3C2 towards the adsorption, activation and charge transfer mechanism.

Thus, the evolved CH3OH over the Ti3C2 QDs/Cu2O NWs/Cu is 8.25 and 2.15 times of those Cu2O NWs/Cu and Ti3C2 sheets/ Cu2O NWs/Cu after 6 h reaction. Interestingly, this catalyst also exhibits high selectivity for CH3OH. Therefore, the enhanced CO2 conversion efficiency and high selectivity can be assigned to the essential role of Ti3C2 QDs that promote light absorption, charge separation, transportation, and carrier density, which are confirmed by electrochemical impedance spectroscopy (ESI) and Mott–Schottky measurements in Fig. 13 F-G. The aforementioned examples demonstrate the promising nature of Ti-MXene for highly selective photocatalytic CO2 conversion which bodes well for the near future investigations of these materials, both in

4.2. Other MXenes In 2017, Li et al. disclosed a crucial finding on Cr3C2 and Mo3C2 MXene for selective conversion of CO2 to CH4 through well-resolved density functional theory calculations. The results suggest that the O13

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Fig. 14. Minimum energy pathway for the conversion of CO2 into CH4 and H2O; A) Mo3C2(OH)2; B) Mo3C2O2; Reproduced with permission from ref [79].

and −OH terminated MXenes are beneficial for selective conversion of CO2 to CH4 because they significantly reduce the overall reaction free energy compared to the bare materials. Indeed, Fig. 14A-B represents the minimum energy pathway for the reduction of CO2 to CH4 employed −OH and –O terminated Mo3C2, respectively. As shown in Fig. 14A, the energy for the first hydrogenation step (−CO2→ HOCO∙) is -0.92 eV, which is smaller than that of bare Mo3C2 (-0.64 eV), indicating the essential role of OH groups in the hydrogenation of CO2. Moreover, the subsequent steps are embraced by the energy release

suggesting the higher activity of Mo3C2(OH)2 than bare Mo3C2. Similarly, it can be observed for Mo2C2O2 in which input energies requires the formation of radicals (e.g., *COOH, *CH2O, and *CH3OH) while the rest steps are associated with the exothermic processes. Therefore, it can be articulated that −OH and –O incorporated Mo3C2 are bright candidates for CO2 reduction application [79]. Very recently, Sc2C(OH)2 and Y2C(OH)2 MXenes containing −OH termination groups were explored as a rose-colored platform the CO2 reduction reaction (CO2 RR) via first-principles simulations. It was 14

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found that Sc2C(OH)2 and Y2C(OH)2 MXenes possess less negative limiting potential than Cu. Furthermore, -H atom in −OH serve an essential function to form the stable species with intermediates and decrease the overpotential. Consequently, CO2RR efficiency over these MXenes is greatly enhanced [154]. The computational calculations prove the favorable perspective for the –O and −OH groups- modified MXenes as such surface terminal groups significantly impacts the reaction pathways. However, the experimental investigations of MXenes for photo(electro)catalysis CO2 reduction have been rather limited and therefore numerous future efforts are warranted to provide experimental results, which can verify the outstanding properties described by the computational calculations.

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5. Conclusion and outlook It is obvious that the research on MXene has bloomed since the first work appeared in 2011. The science community has witnessed the expeditious reporting of MXene-based catalysts in the context of structure, synthesis and application investigations. This review aims to provide contemporary progress for the development of MXene catalysts for energy production and the reduction of pollutants. Regarding the electro- and photocatalytic hydrogen production, the progress of MXenes has played a pivotal role in seeking the proper catalysts possessing high catalytic activity, stability and that are affordable for largescale applications. In the context of electrocatalytic HER, the development of novel MXenes is highly desirable to fulfill the general picture. Moreover, structural modification of MXenes to improve the activity is also an attractive pathway. The review has also emphasized the metal singleatom immobilized MXenes composite that display fascinating properties consequential from the electrochemical and highly active properties of MXenes and single atoms, respectively; Such configurations are idealistic candidates toward high efficiencies and durability. In addition to electrocatalytic HER, MXene-based catalysts are also the emerging materials as the cocatalysts for photocatalytic hydrogen production and CO2 reduction. It is obvious that more effort need to be devoted to this research field in future. Hitherto, Ti-based Mxenes have been the leading actor in this field and subsequent deployment of novel MXene for novel applications is highly desirable. Besides this, the strategies mentioned in the HER section can be employed for photocatalytic applications. Undoubtedly, MXenes-based catalysts will be the hot commodity attracting more attention and leading the pathway to address the lingering problems in environmental and energy domains. Declaration of Competing Interest Authors declare no conflict of interest Acknowledgment This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.05-2016.34. References [1] J. Uribe-Toril, J.L. Ruiz-Real, J. Milán-García, J. de Pablo Valenciano, Energy, Economy, and Environment: A Worldwide Research Update, Energies 12 (2019) 1120. [2] M. Höök, X. Tang, Depletion of fossil fuels and anthropogenic climate change—a review, Energy Policy 52 (2013) 797–809. [3] J.O. Abe, A.P.I. Popoola, E. Ajenifuja, O.M. Popoola, Hydrogen energy, economy and storage: review and recommendation, Int. J. Hydrogen Energy 44 (2019) 15072–15086. [4] I. Dincer, C. Acar, Review and evaluation of hydrogen production methods for better sustainability, Int. J. Hydrogen Energy 40 (2015) 11094–11111. [5] T.L. LeValley, A.R. Richard, M. Fan, The progress in water gas shift and steam reforming hydrogen production technologies – a review, Int. J. Hydrogen Energy

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