Recent advances in structural engineering of MXene electrocatalysts

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Recent advances in structural engineering of MXene electrocatalysts

Article in Journal of Materials Chemistry A · May 2020

DOI: 10.1039/D0TA03271A

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Recent advances in structural engineering of MXene electrocatalysts

Hao Wang and Jong-Min Lee *

Most recently, two-dimensional (2D) transition-metal carbides (MXenes) have been demonstrated to be promising electrocatalysts owing to their unique chemical and electronic properties, e.g., metallic conductivity, high hydrophilicity, and tunable surface terminations. Herein, representative progress achieved in MXenes as hydrogen evolution reaction electrocatalysts is reviewed both experimentally and theoretically. Briefly, termination modification and heteroatom incorporation are applied to optimize the chemical and electronic configurations of active sites for intrinsically enhanced catalytic kinetics while various nanostructures and hybridizations are fabricated to increase the density and accessibility of active sites. Then, the achievements of MXene-based catalysts in other electrocatalysis processes are also summarized, including the oxygen evolution/reduction reaction, carbon dioxide reduction reaction and nitrogen reduction reaction. Finally, current challenges and future research directions for MXene-based electrocatalysis are discussed.

1. Introduction

Currently, fossil fuels (coal, oil, and gas) account for the majority (80%) of the global energy demand. The Interna- tional Energy Agency reported that the global energy demand would increase from 18 TW in 2014 to24 TW in 2040.¹Such rapid growth, coupled with the associated environmental issues, drives the development of clean and sustainable energy storage and conversion to reduce our reliance on fossil fuels.

Electrocatalysis plays a critical role in clean energy conversion by converting atmospheric gases (e.g., H₂O, CO₂, and N₂) into high-value products with renewable energy.² Till now, consid- erable eﬀort has been devoted to the following electrocatalytic reactions: the hydrogen evolution reaction (HER),³ oxygen evolution reaction (OER),⁴ oxygen reduction reaction (ORR),⁵ nitrogen reduction reaction (NRR)⁶and carbon dioxide reduction reaction (CRR).⁷ In particular, the HER involved in water splitting is the most studied, in which the precious metal-based electrocatalysts remain the most eﬃcient. Thus, exploring earth-abundant element-based electrocatalysts with high

Dr Hao Wang is currently a postdoc researcher at Nanyang Technological University under the supervision of Prof. Jong-Min Lee. He received his PhD in New Energy Science and Engineering from Soochow University in 2018. He worked as a visiting researcher in the A. J. Drexel Nanomaterials Institute, Drexel University from 2017 to 2018.

His research interests focus on the development of nano- structured materials for electrocatalysis.

Prof. Jong-Min Lee received his PhD degree from the Department of Chemical Engineering at Columbia University in 2003.

He worked in the Chem. Sci.

Division at Lawrence Berkeley National Laboratory and in the Department of Chemical Engi- neering at the University of California at Berkeley as a post- doctoral fellow in 2006–2008.

He is currently an Associate Professor in the School of Chemical and Biomedical Engineering at Nanyang Technological University. His research interests are in the development of ionic liquids for various applications and in the development of meso- porous materials for electrochemical energy systems.

School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore. E-mail: [email protected]

Cite this:J. Mater. Chem. A, 2020,8, 10604

Received 23rd March 2020 Accepted 4th May 2020 DOI: 10.1039/d0ta03271a rsc.li/materials-a

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eﬃciency and stability for the involved electrochemical reaction is of great importance.^8–12

Most recently, heterogeneous electrocatalysts based on non- noble metal elements have been developed as high- performance electrocatalysts, such as carbides,¹³⁰ phosphides,¹⁴ suldes,¹⁵ oxides,¹⁶ nitrides,¹⁷ and even metal-free nanocarbons.¹⁸ For instances, Li et al.¹⁹ developed a MoS₂- based HER catalyst, in which the basal plane was activated and optimized by the formation of strained sulfur vacancies. Most recently, defect-rich and ultrathin N-doped carbon nanosheets designed by self-foaming pyrolysis served as trifunctional electrocatalysts for the ORR, OER and HER.²⁰While extensive eﬀorts have been made in developing electrocatalysts for these electrochemical reactions during the past few decades, low-cost and high-performance electrocatalysts that can be commercialized remain challenging.

Transition metal carbides/carbonitrides/nitrides (MXenes) are an emerging family of two-dimensional materials, which can be represented by the general chemical formula M_n+1X_nT_x (where M stands for metal atoms, such as Ti, Mo, Nb, and V; X represents carbon or nitrogen;n ¼1, 2, or 3; T_xrefers to the surface terminations of –O, –OH, or –F).²¹ Typical examples include Ti₃C₂T_x, Mo₂CT_x, and Nb₄C₃T_x, in which the M layers cover the X layers in an [MX]_nM arrangement. Since Ti₃C₂T_x MXenes were rst reported in 2011,²² MXenes have become a large and growing family of 2D materials. MXenes are produced by selectively etching the “A” elements from their layered ternary carbide or nitride (MAX phases) precursors with a general formula of Mn+1AXn(A mostly stands for group 13 and 14 elements). Upon etching the A elements using uoride- containing acidic solutions, the M layers are terminated by hydrophilic groups (–O,–OH, or–F). Due to the unique merits, e.g., metallic conductivity (10 000 S cm¹), large surface area, tunable structure and hydrophilic nature, MXenes have exten- sively been investigated and applied in variouselds, including energy storage,^23,24 electromagnetic interference shielding,^25,26 environmental and water purication,^27,28 catalysis,^29,30 sensors,^31,32and so on.³³^–³⁵

Considering that MXenes with an atomically thin structure could maximally expose the surcial active sites, coupled with the excellent metallic conductivity and hydrophilicity, MXenes show great promise as electrocatalysts. Though vast advance- ments have been made in the designing and applications of MXene-based materials, it was not until the very recent few years that their potential in electrocatalysis drew the attention of researchers. In 2016, Sehet al.³⁶for therst time reported Mo2CTxMXene as an HER electrocatalyst, paving the way for developing MXene-based electrocatalysts for various clean

energy reactions. As countless investigations on MXenes for electrocatalysis are rapidly growing, here we provide a brief summary of the recent progress in MXene-based electrocatalysts, with a special emphasis on the intrinsic and extrinsic optimizationviastructural engineering. First, combining theory and experiments we summarize the achievements of MXene- based HER electrocatalysts and discuss the engineering of the morphological and electronic structure of MXenes and MXene- based composites, including the electronic regulation by termination modication and heteroatom doping for intrinsically optimized active sites, the construction of nanostructures for more accessible active sites and the hybridization with other active materials towards synergistically promoted HER activity.

Then, we briey overview the applications of MXenes in other important electrochemical reactions including the OER, oxygen reduction reaction (ORR), CRR, and nitrogen reduction reaction (NRR). Finally, we oﬀer a perspective on the future development of MXene-based electrocatalysts.

2. MXene-based electrocatalysts for the HER

Hydrogen is considered an ideal alternative to fossil fuels owing to its high gravimetric energy density and zero pollution emis- sion.³⁷At present, H₂is mainly produced by steam reforming, which still consumes fossil fuels. In recent decades, water splitting (H2O (liquid) / H2 (gas) + ¹₂O2 (gas)) driven by renewable energy has stimulated intensive interest as an emerging H2-production technology.^38,39 As one of the half reactions in water splitting, the HER is a multi-step reaction consisting of hydrogen adsorption, hydrogen reduction and H2

desorption processes, as described in Table 1.⁴⁰Typically, the

“Volmer”step refers to the adsorption of hydrogen from either H⁺(in acidic solution) or dissociated H2O molecules (in alkaline or neutral solutions). Then molecular hydrogen is formed by the combination of adsorbed hydrogen (*H) via a Heyrovsky or Tafel step. The dominant reaction step can be indicated by the Tafel slope derived from experimental data, as shown in Table 1. To date, platinum-based materials are the most eﬃcient HER electrocatalysts with nearly zero overpotential. However, their scarcity and high cost severely hinder the scalable application.

Thus, the exploration of non-noble metal HER electrocatalysts (carbides, suldes, phosphides, nitrides, etc.) is of great importance for electrocatalytic H2production.^41–48

Most recently, MXenes as HER electrocatalysts have widely been investigated with experiments and theory. Pandeyet al.⁴⁹ systematically investigated the HER performance of 72 diﬀerent

Table 1 HER reaction steps in various media

Acid Alkaline/neutral Tafel slop

Overall *+ 2H⁺+2e/H₂ *+ 2H₂O + 2e/H₂+ 2OH —

Volmer *+ H⁺+ e/*H *+ H₂O + e/*H + OH 120 mV dec¹

Heyrovsky *+ H⁺+ e+*H/*+ H₂ *+ H₂O + e+*H/H₂+ OH+* 40 mV dec¹

Tafel 2*H/2*+ H₂ 2*H/2*+ H₂ 30 mV dec¹

Review Journal of Materials Chemistry A

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MXenes (Mn+1XnTx,n ¼ 1, 2, 3)viadensity functional theory (DFT) calculations. The thermodynamic stability of bare and functionalized MXenes was analyzed with the heat of formation (DH), indicating that bare and F-terminated MXenes are unstable for the HER. Accordingly, theDHand free energy of hydrogen adsorption (DGH) of various O-terminated MXenes were mainly calculated and those MXenes with a negativeDH and |DGH|#0.5 eV are summarized in Fig. 1a. As circled in the

gure, some MXenes like Mo2CO₂, V₃C₂O₂, and Cr₄C₃O₂ have

|DGH|#0.1 eV, suggesting suitability toward the HER.⁵⁰It is obvious that diverse kinds of MXenes exhibit different HER activity. The components affect not only theDGH, but also the conductivity, which inuence the HER performance. For instance, Mo2CO2has a metallic nature, while Ti2CO2is semi- conducting. On the other hand, Seh et al. experimentally studied the HER performance of pristine MXenes (Ti2CTxand Mo2CTx). As shown in Fig. 1b, Mo2CTxas an HER electrocatalyst could deliver a current density of 10 mA cm² at an overpotential of 283 mV, which is much lower than that of Ti₂CT_x (609 mV). Coupled theoretical calculations demonstrated that the basal planes of Mo₂CT_xare catalytically active towards the HER, which is different from another widely studied layered material, molybdenum disulde (MoS2).⁵¹ While the MXenes have potential as HER electrocatalysts as demonstrated by experiments and theoretical simulations, their performance remains far from satisfactory so they cannot replace Pt-based materials. In the following sections, various strategies for optimizing the HER activity of MXenes will be typically discussed, including tuning the terminations, heteroatom doping, nanostructuring and hybridization.

2.1. Termination modication

The surface terminations (–O,–OH,–F) play a critical role in the electronic properties of MXenes.^52–54Huet al.⁵⁵demonstrated that the terminations stabilize the Ti3C2 matrix by saturating the nonbonding valence electrons of surface Ti using the low-

energy orbitals of the termination. Schultz et al.⁵⁶ identied the adsorption sites of terminations on the MXene surface and conrmed the tuning of the work function with the modication of terminations. Typically, when heating in a vacuum, the work function increases from 3.9 to 4.8 eV due to desorption of water, contamination and hydroxyl species, and decreases again at higher temperatures to 4.1 eV due to uorine desorption.

Hartet al.⁵⁷experimentally demonstrated the increase of electronic conductivity of MXenesviasurface de-functionalization.

Most recently, the surface terminations were experimentally demonstrated to affect the HER activity of the MXenes. Han- dokoet al.⁵⁸combined experimental and theoretical approaches to investigate the effect of basal plane terminations (Tx) on the HER activity of various MXenes. Taking Ti3C2Txas an example, the effect of F coverage of the basal plane on the HER activity was studied by preparing Ti3C2Txwith different etchants (HF and LiF–HCl). As shown in Fig. 2a, the HER activity of Ti3C2Tx

increases with the decrease of F coverage on the basal plane as follows: LiF–HCl (F : Ti¼0.09) > 10% HF (F : Ti¼0.19) > 50%

HF (F : Ti ¼ 0.46). As these MXenes possess similar electrochemical surface areas (within a factor of 2.6), it is safe to say that the HER activity of Ti₃C₂T_xcan be greatly attenuated by the presence of F terminations. Meanwhile, as displayed in Fig. 2b, DFT calculations reveal that theDGHincreases monotonically with the F coverage. Obviously, the experimental activity trend generally matches the DFT calculation trend. Furthermore, other MXenes including Ti2CTx and Mo-based MXenes were also investigated and similar trends were observed (Fig. 2c).

Specically, Mo2CTxwith very low basal planeuorine coverage achieved a current density of 10 mA cm²at an overpotential of 189 mV. At the same time, the theoretical results indicate that the O terminations on the basal planes of Mo2CTxare catalytically active toward the HER. Accordingly, Jianget al.⁵⁹developed an eﬃcient HER electrocatalyst of Ti3C₂O_x, which is synthesized by dispersing pristine Ti₃C₂T_x in KOH solution followed by heating under an Ar atmosphere (Fig. 2d).^60,61 As shown in

Fig. 1 (a) Heat of formation (DH)versusfree energy of hydrogen adsorption (DGH) plot for O-terminated MXenes that have |DGH|#0.5 eV.

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Fig. 2e, the in situ Fourier-transform infrared spectroscopy measurements reveal termination modication from Ti3C2(- OH)x to Ti3C2Ox during the calcination process. The HER polarization curves (Fig. 2f) show that Ti3C2Oxexhibits a much lower overpotential of190 mV at 10 mA cm²than Ti3C2(OH)x

(217 mV) and Ti3C2Tx-450 (266 mV). It is experimentally evident that the O terminations on the basal plane of Ti3C2 serve as active sites for the HER.

In short, current studies have demonstrated well the eﬀects of terminations on the HER activity of Mxenes both experimentally and theoretically. The F terminations generally play a negative role in the HER activity, which shows sluggish hydrogen adsorption kinetics. In contrast, O terminations are determined to highly contribute to the HER performance. This progress could pave the way to optimization of MXene-based HER electrocatalysts by tuning their surface termination groups.

2.2. Heteroatom doping

As mentioned in the previous sections, the HER activity of pristine and even termination optimized MXenes remains unsatisfactory compared to reported transition metal-based electrocatalysts.^44,47,62This may be attributed to their relatively low intrinsic activity toward the HER due to the low adsorption

energy and limited number of active sites. As one of the feasible strategies, chemical doping with heteroatoms has been widely developed to optimize HER electrocatalysts viatailoring electronic properties, manipulating surface chemistry and modi- fying the elemental composition. To date, various 2D materials, including graphene,⁶³ MoS2,⁶⁴ and graphdiyne,⁶⁵ exhibit enhanced HER activities aer doping with heteroatoms. In this section, we will summarize the progress of heteroatom doping in MXenes, which can be divided into two categories: metal- atom doping and non-metal-atom doping.

2.2.1 Metal-atom doping.Most recently, metal-atom doped catalysts are widely known as single atom catalysts, which could utilize every metal atom active site.⁶⁶ Diﬀerent from metal nanoparticles/2D material composites, single metal atoms are stabilized by the lattice or the coordination environment of 2D materials through strong covalent bonds, serving as active sites for the HER. Notably, the strong electronic interaction between single atoms and the 2D support synergistically enhances the intrinsic activity of the active sites. Inspired by other 2D single atom catalysts, MXenes were also investigated to conne single metal atoms to improve the HER activity. Liet al.⁶⁷utilized high- throughput computational methods to investigate the optimization of M2XO2-type MXenesviathe doping of various transition metal atoms. Since very limited nitride MXenes can be experimentally synthesized, here we focus on M₂CO₂MXenes.

Fig. 2 (a) Polarization curves with various F coverages in a 0.5 M H2SO4electrolyte. (b) DFT calculated HER overpotential as a function of F coverage on the basal plane (F : Ti ratio) for Ti3C2Tx, with experimental HER overpotentials at10 mA cm². (c) DFT calculated HER overpotential as a function of F basal plane coverage. Reproduced with permission.⁵⁸Copyright 2018, American Chemical Society. (d) Schematic illustration of the synthesis of Ti3C2OxMXene. (e) Magniﬁedin situtemperature-dependent FTIR spectra of the as-synthesized E-Ti3C2(OH)xin an Ar atmosphere. (f) Polarization curves of various MXenes. Reproduced with permission.⁵⁹Copyright 2019, WILEY-VCH.

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Fig. 3a shows the structural model of transition metal (TM) doped M2CO2 MXenes (TM-M2CO2) and the potential H adsorption sites. As shown in Fig. 3b, the total number of active sites for various TM-M₂CO₂ are summarized. Obviously, Ir–

Zr₂CO₂, Ag–V2CO₂ and Zn–Nb2CO₂ have all three kinds of O sites active toward the HER. It is worth noting that not all TM dopants are benecial for HER activity, such as Cu. Fig. 3c gives theDGHvalues of several TM-M₂CO₂with all possible modi- cation cases. Except for Hf2CO2, the other results suggest that suitable TM doping can activate the M2CO2 sites. Strikingly, Mo–V2CO2and Au–Sc2CO2are highly HER active with multiple O sites with |DGH| < 0.1. This work theoretically provides deep insight into the HER mechanism of TM-doped MXenes and thus paves the way for developing highly eﬃcient MXene-based HER electrocatalysts.

Experimentally, single metal doped MXenes have increas- ingly been developed as HER electrocatalysts. Zhanget al.²⁹reported single Pt atoms immobilized on double transition metal MXene (Mo₂TiC₂T_x) as an eﬃcient HER electrocatalyst. The MXene with abundant Mo vacancies (Mo₂TiC₂T_x–VMo) was prepared by electrochemical exfoliation, during which single Pt atoms are simultaneously immobilized on V_Moand stabilized by the covalent Pt–C bonds. The high-angle annular dark-eld scanning transmission electron microscopy (HAADF-STEM) image of Mo2TiC2Tx–PtSA(Fig. 4a) shows that the Pt atoms are

exactly immobilized at the Mo position on Mo2TiC2Tx, which is conrmed by the STEM simulation based on the DFT-optimized structure. Generally, X-ray absorption spectroscopy is a widely recognized strategy to characterize single atom materials.⁶⁸X- ray absorption near-edge structure (XANES) can provide infor- mation on the chemical state and geometric structure of single atoms on supports while the extended X-ray absorption ne structure (EXAFS) can give the average interatomic distance and coordination number between the absorbed single atoms and the nearby backscattered atoms. As shown in Fig. 4b, the Fourier transform spectra of the Pt L3-edge EXAFS oscillations shows that the Pt–Pt contribution at2.7˚A for Pt foil is absent for Mo2TiC2Tx–PtSA, with only a shell at 1.9A referring to Pt–C˚ bonds. These results strongly conrm the formation of single Pt atoms on MXene rather than Pt nanoparticles or nanoclusters.

The HER polarization curves (Fig. 4c) reveal that the HER activity of Mo2TiC2Tx–PtSAis comparable to that of commercial Pt/C on a geometric scale and will be more superior to the latter when normalized to the Pt loading. Typically, Mo₂TiC₂T_x–PtSA

exhibited an overpotential of 30 mV at 10 mA cm²and a mass activity of 8.3 A mg¹ at an overpotential of 77 mV. The DFT computations (Fig. 4d) show that theDGHof Mo₂TiC₂O₂–PtSAis determined to be0.08 eV, smaller than that of Pt (0.1 eV) and Mo2TiC2O2 (0.19 eV). This indicates that the strength of hydrogen adsorption on the positively charged Pt atoms on Fig. 3 (a) Schematic diagrams of the transition metal (TM) modified M2CO2structure, depicted with two kinds of oxygen atom sites on the outermost layer: face centered cubic (fcc) and hexagonal closest packing (hcp). There are three different neighboring adsorption sites (S0, S1, and S2) for H atoms existing at the fcc or hcp site. (b) Color-coded active site number of TM-modified MXenes: the more the active sites, the darker the color. White color means no active sites. (c) TheDGHof TM-modified M2CO2MXenes. Reproduced with permission.⁶⁷Copyright 2018, Royal Society of Chemistry.

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Mo2TiC2O2–PtSA is less than that on Pt/C, resulting in faster formation and release of H2.

Besides the electrochemical exfoliation method, additional strategies have further been developed to fabricate single metal atom doped MXenes. Ramalingamet al.⁶⁹developed N and S coordinated Ru single atoms on Ti3C2Tx (RuSA–N–S-Ti3C2Tx) through a pyrolysis strategy, which exhibits superior HER activity in a wide pH range. The HAADF-STEM image (Fig. 4e) of the Ru_SA–N–S–Ti3C₂T_xdisplays the homogeneous dispersion of Ru atoms (bright dots circled). The Fourier transform-EXAFS spectrum of Ru_SA–N–S–Ti3C₂T_x (Fig. 4f) exhibits a super- imposed peak at 1.67˚A corresponding to both Ru–N(O) and Ru–

S scattering pairs. Coupled with the absence of Ru–Ru and Ru–

O scattering pairs, Ru atoms are conrmed to isolate on Ti₃C₂T_x. In addition, single metal atom doped MXenes can be realized from doped MAX precursors. Kuznetsovet al.⁷⁰reported a cobalt-doped Mo2CTx MXene (Mo2CTx: Co) from a Co- substituted Mo2Ga2C MAX. As shown in Fig. 4g, the HAADF- STEM image shows the ultrathin hexagonal sheet morphology and the energy dispersive X-ray (EDX) spectroscopy map shows the uniform distribution of Mo, C, and O, without detection of Co due to the ultralow content (0.04 wt%). The Co K-edge EXAFS data (Fig. 4h) can be excellentlytted with the proposed structural model, in which Co atoms locate at the Mo position of Mo₂CT_x. As a result, Mo₂CT_x: Co exhibited a lower overpotential of 180 mV at 10 mA cm²than the pristine Mo₂CT_x (230 mV), evidencing the enhancement of HER performance by Co doping. DFT calculations attribute the enhancement of the HER activity to the improved thermodynamics of hydrogen

binding on the MXene surface upon cobalt substitution into Mo2CTx.

2.2.2 Non-metal-atom doping.In addition to metal atom doping, nonmetals like N, S, P,etc, are also widely doped into 2D materials to tune the electronic structure thereby facilitating HER activity. For example, Xiaoet al.⁷¹revealed the dual function of N dopants in MoS2as an HER electrocatalyst: (1) acti- vating the S edge sites and (2) promoting the conductivity of the basal plane. Inspired by this, nonmetal doped MXenes were experimentally and theoretically explored as HER electrocatalysts. Dinget al.⁷²performed DFT calculations to study the eﬀect of heteroatom doping (N, B, P, and S) on the HER activity of Ti₂CO₂MXene. As shown in Fig. 5a, theDGHof N–Ti2CO₂and B–Ti2CO₂is 0.087 and0.097 eV, respectively, similar to that of Pt. Notably, the theoretical overpotentials of N–Ti2CO₂and B–

Ti2CO2are also comparable to that of Pt, indicating that the doping of N and B will substantially enhance the HER activity of Ti2CO2. TheDGHagainst H coverage (Fig. 5b) shows a generally upward trend with the H coverage increase and less H coverage (1/9 and 2/9 ML) can improve the HER performance of the X–

Ti2CO2. This computational work opens a new method for optimizing the HER activity of MXenesvianonmetal doping. On the other hand, feasible strategies have been developed to synthesize N or P doped MXenes. As depicted in Fig. 5c, N- doped Ti₃C₂T_x could be obtained via ammoniation of Ti₃C₂T_x.⁷³Yoonet al.successfully prepared N-doped Ti₂CT_xby nitridation of Ti₂CT_x in sodium amide (NaNH₂).⁷⁴ Moreover, they synthesized P doped V₂CT_x through phosphorization of V₂CT_x with triphenyl phosphine (Fig. 5d).⁷⁵ The DFT Fig. 4 (a) HAADF-STEM image of Mo2TiC2Tx–PtSAand its corresponding simulated image with the structural model. (b) Thek²-weighted Fourier transform of EXAFS spectra derived from EXAFS of Pt foil, PtO2and Mo2TiC2Tx–PtSA. (c) Polarization curves of various electrocatalysts in 0.5 m H2SO4. (d) Calculated free energy proﬁles of the HER at the equilibrium potential for Mo2TiC2O2, Mo2TiC2O2–PtSA, and Pt. Reproduced with permission.²⁹Copyright 2018 Nature Publishing Group. (e) Magniﬁed HAADF-STEM image of RuSA–N–S–Ti3C2Tx. (f) The FT-EXAFS spectra of Ru foil, RuO2and RuSA–N–S–Ti3C2Tx. Reproduced with permission.⁶⁹Copyright 2019, WILEY-VCH. (g) HAADF-STEM image and elemental mapping image for delaminated Mo2CTx: Co sheets. (h) Phase-uncorrected Fourier-transform spectra of the Co K-edge EXAFS function for Mo2CTx: Co (inset: two projections of the coordination environment of cobalt in Mo2CTx: Co). Reproduced with permission.⁷⁰Copyright 2019, American Chemical Society.

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calculations suggest that the C–P bonds induced by P doping are the most stable chemical composition. This supports the experimental results that the formation of P–C bonds can eﬃ- ciently weaken the H-bonding strength, improving H adsorption towards fast HER kinetics.⁷⁵ In addition, the N doped Ti2CTx and N doped Ti3C2Tx both exhibit improved HER performance compared to the pristine MXenes.

Summarizing the studies reviewed above, it is clear that doping MXenes with heteroatoms can tune the electronic structures and improve the H adsorption barriers, thereby enhancing the HER activity. Various strategies have been developed to realize metal (Pt, Ru, and Co) and nonmetal (N and P) atom doping. It is worth noting that more than thirty MXenes experimentally synthesized can be used as catalyst supports for heteroatom doping, providing great potential for the development of high-performance HER electrocatalysts.

2.3. Nanostructuring

The termination modication and heteroatom doping of MXenes reviewed above are aimed at optimizing the electronic structure to intrinsically improve the HER activity. On the other hand, the accessibility of active sites during electrocatalysis is also critical for HER performance. It is well recognized that 2D materials are prone to restacking during the fabrication of electrodes, which will severely limit the utilization of active sites. To improve the exposure and accessibility of active sites on MXenes, a one-dimensional nanostructure is considered benecial because the high active surface area provides abundant sites while the axial dimension facilitates the charge transfer and hinders the restacking of active components.

Yanget al.⁷⁶theoretically exploited the HER activity of MXene nanoribbons, which showed that the edges of MXene nanoribbons Fig. 5 (a) The volcano plot of the calculated overpotential (hr¼ DGH/e)vs.DGHon X-doped Ti2CO2(X¼B, N, P, S) and pristine Ti2CO2. (b) The calculatedDGHwith diﬀerent H coverages for X–Ti2CO2. Reproduced with permission.⁷²Copyright 2020, Elsevier. (c) Illustration of the synthesis of N-doped Ti3C2Tx MXene. Reproduced with permission.⁷³Copyright 2019, American Chemical Society. (d) Schematic illustration of the phosphorization for synthesizing the P-doped V2CTxand the possible chemical compositions which can be determined by calculating the surface formation energy. Reproduced with permission.⁷⁵Copyright 2019, WILEY-VCH.

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served as active sites for the HER much more eﬃciently than the O- terminated surface of MXenes. Yuan et al.⁷⁷ designed Ti₃C₂T_x nanobers with a high specic surface area through the hydro- lyzation of bulk Ti₃AlC₂ MAX ceramics and a subsequent HF etching process, as illustrated in Fig. 6a. The OHions highly aﬀect the Ti–C bonds near defect sites and serve as scissors shearing the 2D Ti3AlC2into nanobers. Aer etching the Al, the nanober morphology of Ti3C2Txis well retained with a diameter of50 nm (Fig. 6b). The specic surface area of Ti3C2Txnanobers is determined to be 58.5 m²g¹,6.9 times larger than that of pristine Ti3C2Tx(8.5 m²g¹). As a result, as displayed in Fig. 6c, Ti3C2Txnanobers exhibit an overpotential of 169 mV at 10 mA cm², far superior to that of Ti3C2Txakes (385 mV). Such a facile strategy can be further extended to fabricate a series of MXene nanobers with abundant active sites. Besides, MXenes are also fabricated into other hierarchical structures,e.g., nanorolls⁷⁸and 3D architectures,⁷⁹ and then applied in HER electrocatalysts coupled with other active materials. These studies will be discussed in the next section.

2.4. Hybridization

Besides serving as HER electrocatalysts individually, MXenes are found to couple with other active materials for enhanced HER performance, including chalcogenides,^78,80^–⁸⁸ phosphides,^79,89 layered hydroxides (LDHs),⁹⁰ carbides,^91,92 metal nanoparticles/alloys^93–99 and even metal-free black

phosphorus.¹⁰⁰ Wu et al.⁸⁰ reported hierarchical MoS₂/Ti₃C₂- T_x@C composites, in which the plated carbon layer stabilized the MXenes against spontaneous oxidation. As shown in Fig. 7a, few-layered MoS₂ nanoplates with an interlayer spacing of 0.64 nm are loosely packed on the Ti₃C₂T_x surface while an ultrathin carbon layer is observed around the MoS2/Ti3C2Tx

architecture. The unique structure greatly decreases the pathway for mass diﬀusion and facilitates the charge transfer during the HER process. As shown in Fig. 7b, the MoS2/Ti3C2- Tx@C composite exhibits an onset potential of20 mV and an overpotential of 135 mV, much better than those of benchmark MoS2/rGO@C (260 mV and 352 mV) and MoS2/oxidized MXene (210 mV and 332 mV). Apart from MoS2,^85,87 many other transition metal chalcogenides, e.g., NiSe₂,⁸² NiS₂,⁸³ MoSe₂,^84,86and VS₂,⁸⁸are also coupled with MXenes to improve the HER activities. For example, Wang et al.⁸⁸ developed VS₂@V₂CT_x via in situ sulfurization of V₂CT_x MXenes using a hydrothermal method, which exhibited universal HER activity toward seawater splitting in the full pH range.

Recently, metal nanoparticles and binary alloys supported on MXenes have been demonstrated as highly efficient HER electrocatalysts. Li et al.⁹⁶ prepared Pt3Ti nanoparticles on Ti3C2Txthroughin situco-reduction, in which Pt alloyed with Ti from Ti3C2Tx to form Pt3Ti. As displayed in Fig. 7c, bright particles of Pt3Ti are uniformly distributed on the Ti3C2Txwith a diameter of 6.63.5 nm. The inset atomic resolution HAADF- Fig. 6 (a) Schematic diagram for the synthesis of Ti3C2Txnanofibers. (b) SEM image of Ti3C2Txnanofibers (inset: TEM image of a Ti3C2Tx

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STEM image conrms the ordered intermetallic compound structure of Pt3Ti nanoparticles. Fig. 7d reveals that Pt3Ti/

Ti₃C₂T_x exhibits better HER activity than commercial Pt/C.

Moreover, MXene supported PtNi nanowires,⁹⁵ Pt nanoparticles^94,97,98and NiCo nanoalloys⁹³are well demonstrated to be excellent HER electrocatalysts. As shown in Fig. 7e, highly crystalline CoNi nanodots with an average diameter of 5 nm are homogeneously dispersed on the Ti_2.5Nb_0.5C₂T_x nanosheets.

They exhibit an overpotential of 43.4 mV at 10 mA cm², comparable to that of commercial 10% Pt/C (Fig. 7f).

Strikingly, MXenes can greatly promote the HER activity of NiFe LDH in an alkaline medium.⁹⁰As shown in Fig. 7g, ultrathin NiFe LDHs are densely grown on Ti3C2Tx, which exposes more edge and defect sites and shortens the charge transfer distance during the HER process. Such a composite delivers better activity than commercial Pt/C at high current density (Fig. 7h). Furthermore, Wuet al.for therst time synthesized

cobalt molybdenum carbide nanoparticles on Ti3C2Tx(Fig. 7i), in which the dopamine protected MXene from self-oxidation during annealing. The composite shows comparable HER activity to the commercial Pt/C (Fig. 7j) and delivers high performance in seawater splitting. In addition to these metal compounds mentioned above, black phosphorus quantum dots (BP QDs) are decorated on MXenes (Fig. 7k).¹⁰⁰The synergy of BP QDs and MXene delivers largely enhanced HER activity compared to the sole counterparts (Fig. 7l).

As reviewed in the last section, MXenes can be fabricated into 1D nanober structures to improve the accessibility of active sites. Accordingly, MXenes with designed nanostructure can be benecial for further promoting the HER performance of MXene-based composites. For example, as illustrated in Fig. 8a, Liuet al.⁷⁸developed a quick-freezing method to obtain a MoS2/ Ti3C2Tx composite with a unique nanoroll-like hierarchical structure. As shown in Fig. 8b, the composite consists of Fig. 7 (a) TEM image of MoS2/Ti3C2Tx@C nanohybrids (inset: HRTEM image of MoS2edges). (b) Polarization curves of various electrocatalysts in 0.5 M H2SO4. Reproduced with permission.⁸⁰Copyright 2017, WILEY-VCH. (c) HAADF-STEM image of Pt3Ti NPs on a Ti3C2TxMXene support (the inset shows the atomic resolution HAADF-STEM image of Pt3Ti with a sphere model of the (110) facet). (d) Polarization curves of various electrocatalysts in H2saturated 0.1 M HClO4. Reproduced with permission.⁹⁶Copyright 2019, American Chemical Society. (e) TEM image of the Ni0.9Co0.1@NTM nanohybrid. The inset shows the HRTEM image of Ni0.9Co0.1. (f) Polarization curves of various electrocatalysts in 1 M KOH.

Reproduced with permission.⁹³Copyright 2019, WILEY-VCH. (g) TEM image of NiFe LDH/Ti3C2Txwith the inset showing the HRTEM image of the NiFe LDH edge. (h) Polarization curves of various electrocatalysts in 1 M KOH. Reproduced with permission.⁹⁰Copyright 2019, Elsevier. (i) TEM image of Co0.31Mo1.69C/MXene/NC (inset: HRTEM image of a Co0.31Mo1.69C nanoparticle). (j) Polarization curves of various electrocatalysts in 1 M KOH. Reproduced with permission.⁹¹Copyright 2019, WILEY-VCH. (k) TEM image of the BP QD/MXene nanohybrids. The inset shows the HRTEM image of BP QDs. (l) Polarization curves of various electrocatalysts in 1 M KOH. Reproduced with permission.¹⁰⁰Copyright 2019, Royal Society of Chemistry.

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Fig. 8 (a) Schematic illustration of the preparation of the MoS2/Ti3C2Txnanoroll hybrid. (b) SEM and (c) high-resolution TEM images of the MoS2/ Ti3C2Tx. (d) Polarization curves of various electrocatalysts in 0.5 M H2SO4. Reproduced with permission.⁷⁸Copyright 2019, Elsevier. (e) Capillary- forced assembly of MXene to 3D architecture and related hybrid systems. (f) TEM and (g) magniﬁed TEM images of the CoP@3D Ti3C2-MXene architecture. (h) Polarization curves of various electrocatalysts in 1 M KOH. Reproduced with permission.⁷⁹Copyright 2018, American Chemical Society.

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nanorolls with a diameter of 200 nm and length of several microns. The HRTEM image (Fig. 8c) reveals that few-layered MoS2 is vertically aligned on the MXene surface with highly exposed edge sites. As a result, the MoS₂/Ti₃C₂T_x nanoroll exhibits a low onset potential of30 mV and an overpotential of 152 mV at 10 mA cm²(Fig. 8d), which is superior to those of 2D MoS₂/Ti₃C₂T_xcomposites.

As is well demonstrated, a 3D interconnected structure can ideally address the issues of the accessibility of active sites and charge transfer. Xiu et al.⁷⁹ developed a capillary-forced assembling strategy for converting MXene to a hierarchical 3D architecture with geometry-based high resistance to

aggregation (Fig. 8e). With metal ion incorporation, various metal compound nanoparticles embedded on 3D MXene can be prepared, as demonstrated by CoP@3D Ti3C2Tx(Fig. 8f and g).

Due to the synergistic merits of high conductivity, an enlarged active surface area and an accelerated electron and mass pathway, as shown in Fig. 8h, the CoP@3D Ti₃C₂T_xshows a low overpotential of 168 mV at 10 mA cm², much lower than that of CoP@Ti₃C₂T_x(243 mV) and CoP (303 mV). The work suggests an eﬀective way to address the fundamental diﬃculty in processing and application of the MXene family beyond electrocatalysis.

Till now, MXenes and MXene-based composites as HER electrocatalysts have been widely explored using various

Table 2 Summary of HER activities of MXene-based and some representative non-noble metal electrocatalysts

Catalysts Electrolyte

Mass loading

(mg cm²) honseta(mV) h10b(mV) b^c(mV dec¹) Ref.

Mo₂CT_x 0.5 M H₂SO₄ 0.1 NA^d 283 NA 36

Mo₂CT_x(low F coverage) 0.5 M H₂SO₄ 1 NA 189 75 58

MoS₂/Ti₃C₂T_x@C 0.5 M H₂SO₄ 0.4 20 135 45 80

Ti₂CT_x(rich F termination) 0.5 M H₂SO₄ 0.285 75 170 100 54

N–Ti₂CT_x 0.5 M H₂SO₄ 0.571 NA 215 67 74

Ti₃C₂T_xnanobers 0.5 M H₂SO₄ 0.3 NA 169 97 77

Mo₂TiC₂T_x–Pt_SA 0.5 M H₂SO₄ 1 0 30 30 29

BP QDs/Ti₃C₂T_x 1 M KOH 0.39 NA 190 83 100

NiCo@Nb–Ti₃C₂T_x 1 M KOH 0.38 NA 43.4 116 93

Pt NP–Ti₃C₂T_x 0.5 M H₂SO₄ NA 75.9 226 59.5 94

NiSe₂/Ti₃C₂T_x 0.5 M H₂SO₄ 0.73 NA 200 37.7 82

E-Ti3C2Ox 0.5 M H2SO4 0.196 NA 190 60.7 59

Pt3.21Ni@Ti3C2 0.5 M H2SO4 NA 6.81 18.55 13.3 95

0.1 M KOH NA 6.39 55.6^e 39.5

NiS₂/V₂CT_x 1 M KOH 0.286 50 179 85 83

Mo₂CT_x: Co 1 N H₂SO₄ 1 NA 180 NA 70

N–Ti₃C₂T_x@600 0.5 M H₂SO₄ 0.276 NA 198 92 73

MoSe₂/Ti₃C₂T_x 1 M KOH 2 NA 95 91 84

Pt/Ti₃C₂T_x 0.1 M HClO₄ 1 0 32.7 32.3 96

Co–MoS₂/Mo₂CT_x 1 M KOH 0.354 NA 112 82 85

MoS₂/Ti₃C₂T_xnanoroll 0.5 M H₂SO₄ NA 30 152 70 78

Ru_SA–N–S–Ti₃C₂T_x 0.5 M H₂SO₄ 1 NA 76 90 69

Co_xMo_2xC/MXene/NC 1 M KOH 0.2 NA 75 32 91

0.1 M PBS NA 126 47

0.5 M H₂SO₄ NA 81 24

Mo₂C/Ti₃C₂T_x@NC 0.5 M H₂SO₄ 0.285 6 53 40 92

0.1 M PBS NA 114 80.3

1 M KOH NA 75 59.2

P–V₂CT_x 0.5 M H₂SO₄ 0.179 81 220 74 75

NiFe-LDH/Ti₃C₂T_x 1 M KOH 0.1 NA 132 70 90

TBA–Ti3C2Tx-Pt 0.5 M H2SO4 0.382 NA 55 65 97

MoS2/Mo2CTx 1 M KOH 0.71 NA 176^f 207 87

VS₂/V₂CT_x 1 M KOH NA NA 164^f 47.6 88

0.5 M H₂SO₄ NA NA 138 37.9

Pd/Nb₂C–HF 0.5 M H₂SO₄ 4 NA 34 43 99

Pt–TBA–Ti₃C₂T_x 0.5 M H₂SO₄ NA NA 67.8 69.8 98

Commercial 20% Pt/C 0.5 M H₂SO₄ 0.285 0 27 31 92

0.1 M PBS 0 78 57.9

1 M KOH 0 43 43.4

N–MoS₂/CN 0.5 M H₂SO₄ 0.285 30 114 46.8 101

Ni₃V₂O₈on N-doped GO 1 M KOH 1.84 NA 43 54.8 102

MoC–Mo₂C/PNCDs 1 M KOH 0.4 45 121 60 62

2D W₂N₃ 0.5 M H₂SO₄ 0.254 30.9 98.2 59 103

N–P–Ni 1 M KOH 0.51 NA 25.8 34 104

aOnset overpotential.^bOverpotential at 10 mA cm².^cTafel slope.^dNot available.^eOverpotential at 5 mA cm².^fOverpotential at 20 mA cm².