The Marriage of Hydrazone‐Linked Covalent Organic Frameworks and MXene Enables Efficient Electrocatalytic Hydrogen Evolution

Electrochemical water splitting is long regarded as a green and feasible pathway to realize the scalable hydrogen production, while the overall hydrogen evolution reaction (HER) efficiency is largely dependent on the electrocatalytic ability of the cathode catalysts. Herein, the in situ growth of hydrazone‐linked covalent organic framework (COF‐42) nanocrystals with a unique nanoflower‐shaped morphology on 2D ultrathin Ti3C2Tx MXene nanosheets (COF/Ti3C2Tx) is achieved through a convenient and robust stereoassembly strategy. Strikingly, the marriage of COF‐42 and Ti3C2Tx nanosheets not only offers multiscale porous channels for the fast transportation of electrolyte and electrons, but also enables the full exposure and activation of numerous catalytically active centers. As a consequence, the optimized COF/Ti3C2Tx nanoarchitecture displays exceptional HER properties in terms of a very low onset potential of 19 mV, a small Tafel slope of 50 mV dec−1 as well as reliable long‐term durability, which are comparable to those of commercial Pt/C catalyst. Density functional theory calculations further disclose that the rational combination of COF‐42 with Ti3C2Tx provides more diversified active positions with appropriate ΔGH values, thus leading to a boosted hydrogen generation rate.


Introduction
Nowadays, the overuse of traditional fossil fuels has led to severe environmental degradation and energy shortage, which has seriously hampered the sustainable development of the social economy. [1]To tackle the above issues, it is urgent to search novel green energy resources and explore advanced energy generation technologies. [2]Among diverse renewable energy sources, hydrogen is environment-friendly and possesses high energy density, which has unparalleled advantages in resource sustainability. [3]Especially, electrochemical water splitting has been considered as one viable and continuous hydrogen generation strategy due to its low hazard emission and high flexibility. [4]To accelerate the H 2 production efficiency, it is necessary to introduce highly active electrocatalyst to the cathode surface, which can overwhelm the relative high overpotential during the hydrogen evolution reaction (HER) process and thus significantly reduce the electric energy consumption. [5]Although platinum (Pt) and its derivatives with extraordinary electrocatalytic performance have been served as common commercial electrodes toward the HER, their finite reserves and expensive price have largely cramped the marketing promotion. [6]Therefore, the exploitation and utilization of non-Pt HER electrocatalysts has evolved as an active field in energy sector in recent years. [7]D transition metal carbides or nitrides, also known as MXene, are provided with impressive ultrathin-sheet nature, rich surface chemistries, and exceptional metallic conductivity. [8]In particular, 2D Ti 3 C 2 T x nanosheets have proved to be rich in plentiful efficient hydrogen binding sites, which have been considered as one of the most promising MXene materials to construct the next-generation HER electrocatalysts. [9]However, due to the van der Waals forces between the adjacent nanosheets, Ti 3 C 2 T x -based catalysts have a natural tendency to longitudinal reagglomeration or restacking, which would shelter the reactive sites and render it difficult to transport the external electrolytes to the interior Ti 3 C 2 T x surface. [10]To overcome this obstacle, the rational intercalation of Ti 3 C 2 T x nanosheets with sophisticated porous structural units has been demonstrated as an efficient strategy to prevent the deactivation and concurrently enhance the electrochemical performance through the synergistic coupling effects, which opens up new possibilities for the design and construction of advanced Ti 3 C 2 T x -based HER electrocatalysts. [11]s an emerging type of metal-free crystalline polymers, covalent organic frameworks (COFs) have recently attracted DOI: 10.1002/sstr.202300279Electrochemical water splitting is long regarded as a green and feasible pathway to realize the scalable hydrogen production, while the overall hydrogen evolution reaction (HER) efficiency is largely dependent on the electrocatalytic ability of the cathode catalysts.Herein, the in situ growth of hydrazone-linked covalent organic framework (COF-42) nanocrystals with a unique nanoflower-shaped morphology on 2D ultrathin Ti 3 C 2 T x MXene nanosheets (COF/Ti 3 C 2 T x ) is achieved through a convenient and robust stereoassembly strategy.Strikingly, the marriage of COF-42 and Ti 3 C 2 T x nanosheets not only offers multiscale porous channels for the fast transportation of electrolyte and electrons, but also enables the full exposure and activation of numerous catalytically active centers.As a consequence, the optimized COF/Ti 3 C 2 T x nanoarchitecture displays exceptional HER properties in terms of a very low onset potential of 19 mV, a small Tafel slope of 50 mV dec À1 as well as reliable long-term durability, which are comparable to those of commercial Pt/C catalyst.Density functional theory calculations further disclose that the rational combination of COF-42 with Ti 3 C 2 T x provides more diversified active positions with appropriate ΔG H values, thus leading to a boosted hydrogen generation rate.
enormous attention in the subjects of energy and environment owing to their highly ordered periodic crystalline architectures, tunable porous structures, abundant active spots, and high chemical stability. [12]Among diverse categories, hydrazone-linked porous COF-42 has been widely studied as it can be fabricated simply by hydrazine and aldehyde as building blocks through condensation reaction. [13]Currently, COF-42 displays a variety of applications benefiting from the distinctive channels and high structural stability, such as photocatalysis, drug delivery, artificial muscles membranes, chemicals isolation, and purification. [14]lthough the poor electron conductivity of this hydrazone-linked COF has limited its electrochemical applications, there are still abundant textural features that are suitable for the HER electrocatalysis: 1) the regular mesoporous architectures afford the rapid transport of electrolyte with unobstructed large channels; 2) the lamellar complex structure exposes adequate surface active centers, guaranteeing the reactants accessible and the products quickly disconnected; and 3) the appreciable nitrogen elements of the long-range ordered crystalline are able to adjust the surface electronic structure of the integral catalytic system.Recent studies have also demonstrated that the coupling of well-regulated porous COFs with MXene nanosheets could offer massive efficient channels and diverse exposed functional groups, which are benefit for the diffusion of electrolytes and performance enhancement of the heterostructures. [15]Accordingly, COF-42 associated with conductive Ti 3 C 2 T x MXene is expected to make an alliance for the combination of their respective structural advantages, which are very favorable to generate extra catalytic functions.Nevertheless, given the complex chemical environment of Ti 3 C 2 T x surface, the controllable assembling of COF-42 on the Ti 3 C 2 T x nanosheets still remains a great challenge in this emerging area.
Herein, we present a facile and robust stereoassembly method to the bottom-up construction of nanoflower-shaped COF-42 nanocrystals strongly coupled with ultrathin Ti 3 C 2 T x MXene nanosheets (COF/Ti 3 C 2 T x ) via an in situ growth process.
Interestingly, the marriage of hydrazone-linked COFs and Ti 3 C 2 T x nanosheets endows the newborn hybrid nanoarchitectures with a series of unique structural features, such as large specific surface areas, well-separated thin nanolayers, abundant porosity, numerous N species, and excellent electron conductivity, which not only provide rapid transportation channels for electrons and ions, but also create massive accessible catalytically active sites during the electrocatalytic HER process.Ultimately, the newly developed COF/Ti 3 C 2 T x electrocatalysts demonstrate unusual HER properties in terms of low onset potentials, small Tafel slopes, and reliable long-term durability, all of which are close to those of commercial Pt/C and more competitive than those of pristine Ti 3 C 2 T x and COF-42 catalysts.
The overall synthetic route of the COF/Ti 3 C 2 T x nanoarchitectures is illustrated in Figure 1.Initially, 2D ultrathin Ti 3 C 2 T x nanoflakes with an average thickness of %3.5 nm were harvested from bulk Ti 3 AlC 2 material via a wet etching approach in a mixed LiF/HCl solution based on our previous work (Figure S1-S3, Supporting Information). [16]Meanwhile, 1,3,5-triformylbenzene was dissolved in a mixed solution of 1,4-dioxane, mesitylene, and acetic acid to form a clear homogenous dispersion (Figure S4, Supporting Information).Afterward, the as-synthesized Ti 3 C 2 T x lamellae were suspended in the aforementioned solution by ultrasonic treatment, during which the surface oxygen-containing functional groups of Ti 3 C 2 T x nanoflakes would adsorb and partly react with the aldehyde groups.Subsequently, 2,5-diehoxyterephthalamide were introduced to the above black suspension under vigorous stirring, and then the mixture was further transferred into an autoclave and heated at 60 °C for 12 h.In the solvothermal process, the hydrazonelinked COF-42 with a nanoflower morphology was gradually stereoassembled on the surface of Ti 3 C 2 T x nanoflakes, thus giving birth to the COF/Ti 3 C 2 T x nanoarchitectures.It is noticeable that the chemical compositions of the COF/Ti 3 C 2 T x nanoarchitectures are conveniently tunable, and the four COF-42 mass fractions of 20%, 40%, 60%, and 80% were chosen in this work, Figure 1.Schematic of the synthetic procedures for the COF/Ti 3 C 2 T x nanoarchitectures: 1) preparation of 2D Ti 3 C 2 T x nanoflakes from bulk Ti 3 AlC 2 through a LiF/HCl-assisted wet etching approach; 2) surface functionalization of the Ti 3 C 2 T x nanoflakes with aldehyde groups; and 3) stereoassembly of nanoflower-shaped COF-42 onto the Ti 3 C 2 T x nanoflakes.

Results and Discussion
The nanostructure and morphology of the as-synthesized COF/ Ti 3 C 2 T x nanoarchitectures were initially observed by means of field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM).As shown in Figure 2a-c and S2, Supporting Information, the ultrathin and flexible Ti 3 C 2 T x nanoflakes are freestanding as the growing platform without the occurrence of aggregation or overlapping.Additionally, the hydrazone-linked COF-42 with porous nature is found to spread as flower-like nanoclusters growing on the lamellar Ti 3 C 2 T x substrate (Figure S5, Supporting Information), which can effectively separate the MXene lamellae from each other and introduce abundant pore channels on their surface.Under close inspection, it is clearly seen that the nanoflower-shaped COF-42 crystals consist of plentiful winding and interweaved nanorods with an average lateral size of %45 nm (Figure 2d-f ).Such a unique configuration is expected to tremendously increase the reactive surface areas as well as fully expose the underlying catalytic sites.The detailed crystal textures of the COF/Ti 3 C 2 T x nanoarchitectures were further disclosed by high-resolution TEM (HR-TEM) technique.It is noteworthy that the lattice fringes with interplanar spacings of 2.6 and 3.2 Å correspond to the (0110) crystal planes of Ti 3 C 2 T x nanoflakes and (003) crystal planes of COF-42, respectively, evidencing their successful coupling (Figure 2g).Meanwhile, the well-defined heterointerfaces between COF-42 and Ti 3 C 2 T x are also clearly visible (Figure S6, Supporting Information), which are favorable to generate synergistic coupling effects.Moreover, the high-angle annular dark-field scanning TEM (HAADF-STEM) and element mapping analysis reveals that the COF/Ti 3 C 2 T x nanoarchitecture is composed of C, Ti, O, and N elements (Figure 2h-l), and all of them are homogeneously dispersed in the whole heterostructure.
COF-42 was obtained through the condensation reaction, and then the similar procedure was adopted to acquire COF/Ti 3 C 2 T x with diverse ratios for performance optimization.The corresponding solid-state 13 C NMR and liquid-phase 1 H NMR spectra of COF-42 are shown in Figure 3a, S7 and Table S1, Supporting Information, which are consistent with the theoretical chemical shifts data.Especially, the 13 C NMR spectrum demonstrates that the characteristic peak at %149 ppm can be arising from the imine carbon atom (ÀC = NÀ), identifying the successful synthesis of hydrazone-linked COF-42.The small-angle X-ray diffraction (XRD) analysis within the angle ranging from 2θ = 1°t o 30°was then performed to confirm the crystallographic structure of the COF/Ti 3 C 2 T x nanoarchitecture.As can be seen from Figure 3b, the XRD pattern of COF/Ti 3 C 2 T x exhibits the similar characteristic peaks as the pristine COF-42 at 2θ = 3.4°, 6.8°, 9.0°, and 26.1°, corresponding to its (100), ( 200), ( 210), and (003) crystal faces, respectively.At the meantime, the typical diffraction peak for Ti 3 C 2 T x nanosheets at 2θ = 7.6°is also detected, which is apparently different from that for bulk Ti 3 AlC 2 (Figure S8, Supporting Information).Noteworthily, the broad and weak intensity of these diffraction peaks for the COF/Ti 3 C 2 T x hybrid should be attributed to the intercalation of COF-42 nanoflowers into the Ti 3 C 2 T x lamellae along with the decreased crystallinity.
Raman and Fourier transform infrared (FT-IR) spectroscopy measurements were then conducted to reveal the chemical structures of the COF/Ti 3 C 2 T x nanoarchitecture.As depicted in  ). [17]Meanwhile, the two strong peaks at 1265 and 1608 cm À1 are corresponding to the C═N and C═O bond signals of the COF-42 nanocrystal.Figure 3d shows the FT-IR spectra of the COF/Ti 3 C 2 T x , COF-42, and Ti 3 C 2 T x samples.In the case of COF/Ti 3 C 2 T x , the typical peaks at 600, 1641, and 3480 cm À1 are ascribed to the Ti-C vibration, C═O and -OH of surface functional group, respectively.The absorption peaks at %1610 cm À1 can be attributed to the bending mode of N─H bond, while the intensive peak at 1210 cm À1 refers to the stretching vibration of C═N bonds in COF-42, which further testified their presence in the COF/Ti 3 C 2 T x nanoarchitecture. [18]e next adopted the nitrogen adsorption-desorption isotherm measurements to explore the porosity of the COF/Ti 3 C 2 T x nanoachitecture, together with that of COF-42 and Ti 3 C 2 T x for comparison.As presented in Figure 3e,f, the isotherm plots of the COF/Ti 3 C 2 T x hybrid approximates the type IV curve with a Brunauer-Emmett-Teller (BET)-specific surface area of 565 m 2 g À1 , which is smaller than that of COF-42 (1224 m 2 g À1 ) but obviously larger than that of primary Ti 3 C 2 T x (20 m 2 g À1 ).
Besides, the corresponding pore size distribution of the COF/ Ti 3 C 2 T x structure locates in the range of 0.3-50 nm, indicating that the COF/Ti 3 C 2 T x nanoachitecture demonstrates hierarchical porous networks with abundant micro-and mesopores, which are able to facilitate the exposure of catalytically active sites and simultaneously provide adequate unimpeded channels for the rapid transportation of the electrolyte.
X-ray photoelectron spectroscopy (XPS) analysis was further carried out to acquire more information about the chemical compositions and valence states of the COF-42/Ti 3 C 2 T x nanoachitecture.The full surface survey scan shown in Figure 4a verifies four main elemental signals without any impurity, including C 1s, O 1s, N 1s, and Ti 2p, in good agreement with the energy dispersive X-ray (EDX) result (Figure S9, Supporting Information).In addition, the high-resolution C 1s spectrum consists of four different energy peaks at 281.4, 284.6, 285.9, and 287.8 eV (Figure 4b), which can be assigned to the C─Ti─T x , C─C, C─N/C─O, and C═O bonds, respectively.Comparing to the pristine Ti 3 C 2 T x , the increased intensity of C 1s signal in the COF-42/Ti 3 C 2 T x spectrum signifies the successful combination of COF-42 with Ti 3 C 2 T x .The peak fitting of Ti 2p spectrum yields six distinctive peaks between 454.0 and 466.0 eV (Figure 4c), representing the combination modes of the C─Ti─T x , TiO 2 and Ti─O─X species.The presence of TiO 2 component in the composite should be attributed to the inevitable surface oxidation of the Ti 3 C 2 T x nanosheets in the air.In Figure 4d, the O 1s spectra with five deconvoluted peaks at 528.9, 530.2, 531.0, 532.2, and 532.9 eV are linked to the adsorbed Ti─O, C─Ti─O x , C─Ti─(OH) x , C═O, and C─O groups, respectively.Furthermore, the deconvoluted N 1s spectrum in Figure 4e displays three characteristic peaks, which are associated to N─O (399.2 eV), -N═C-(399.7 eV), and C─N (400.3 eV).Notably, the presence of C─N functional groups validated by the above C 1s and N 1s spectra convincingly demonstrate the formation of hydrazone structure on the surface of Ti 3 C 2 T x lamellae.
Motivated by the fascinating textural features, the COF/ Ti 3 C 2 T x nanoachitectures with varying COF-42 contents were deposited onto the glassy carbon electrode and measured as cathode catalysts toward the HER to assess their electrocatalytic performances in acidic medium.As shown in Figure 5a,b, the comparison of linear sweep voltammetry (LSV) polarization curves discloses that the onset potential of the COF (40%)/ Ti 3 C 2 T x electrocatalyst is only 19 mV (vs RHE), followed by the COF (20%)/Ti 3 C 2 T x (33 mV), COF (60%)/Ti 3 C 2 T x (45 mV), COF (80%)/Ti 3 C 2 T x (145 mV), COF-42 (205 mV), and Ti 3 C 2 T x (276 mV).Meanwhile, among these investigated catalysts, COF (40%)/Ti 3 C 2 T x is found to deliver the lowest overpotentials of 72 and 206 mV to achieve 10 and 100 mA cm À2 , respectively, which are even close to those of the commercial Pt/C catalyst (45 and 141 mV).Additionally, the Tafel plots of various electrodes describe their kinetic pathway for the HER with the dependence of potential on the current density.As can be seen from Figure 5c and Table S2, Supporting Information, the Tafel slope of the COF (40%)/Ti 3 C 2 T x electrode is calculated to be only 50 mV dec À1 , which is significantly smaller than that of other COF/Ti 3 C 2 T x (72-132 mV dec À1 ), bare COF-42 (162 mV dec À1 ), and Ti 3 C 2 T x (195 mV dec À1 ) electrodes, as well as slightly inferior to that of Pt/C electrode.Besides, the electrocatalytic HER ability of the optimized COF (40%)/Ti 3 C 2 T x catalyst is also more competitive than that of recent state-of-the-art MXene-based catalysts, such as N-doped RGO/Ti 3 C 2 T x , [19] MnCo 2 O 4 /Ti 3 C 2 T x , [20] CoSe/Ti 3 C 2 T x , [21] LDH/RGO/Ti 3 C 2 T x , [22] quantum dots/Ti 3 C 2 T x , [23] MOF/Ti 3 C 2 T x , [24] and so on (Figure 5d and Table S3, Supporting Information).
The newly developed COF/Ti 3 C 2 T x as well as the reference COF-42 and Ti 3 C 2 T x electrodes were then subjected to the cyclic voltammetry (CV) tests at different scan rates from 20 to 120 mV s À1 to estimate their electrochemical active surface area (ECSA) values.As depicted in Figure 5e,f and S10, Supporting Information, no obvious Faradaic currents were observed in the potential region of 0.25-0.45V, and a rectangular profile for the COF/Ti 3 C 2 T x electrodes could be obtained during the scan rounds, attesting their ideally capacitive behaviors.Among these COF/Ti 3 C 2 T x electrodes, the largest mean electrochemical double layer capacitance (C dl ) value of 30.0 mF cm À2 is achieved on the COF (40%)/Ti 3 C 2 T x electrode, which is also apparently larger than that of COF-42 (2.7 mF cm À2 ) and Ti 3 C 2 T x (1.5 mF cm À2 ).Therefore, the COF (40%)/Ti 3 C 2 T x electrode exhibits a larger ECSA value relative to the other electrodes, testifying that COF(40%)/Ti 3 C 2 T x is electrochemically more accessible (Figure S11, Supporting Information).This significant improvement is attributable not only to the accessible catalytically active sites of highly dispersed COF-42, but also to the rich functional groups on Ti 3 C 2 T x nanolayers that could regulate the electronic structures and chemical activities of adjacent atoms.It is worth noting that the feeding ratio between Ti 3 C 2 T x and COF-42 exhibits a remarkably high correlation with the electrocatalytic performance.In general, the excessive introduction of COF-42 would partially destroy the flexibility of MXene nanoflakes and decrease the electrical conductivity of the COF/Ti 3 C 2 T x hybrid, while insufficient COF-42 may deteriorate the porous configuration and inevitably give rise to the deficiency of active surface areas.Within this context, an appropriate growth density of COF-42 nanoflowers on 2D Ti 3 C 2 T x matrix is expected to balance various conflicting factors and thereby gives full play to the catalytic functions.
Considering that the long-term durability of electrocatalysts is another important indicator for further practical HER application, the chronoamperometric technique was employed to examine the electrocatalytic stability of the COF (40%)/Ti 3 C 2 T x electrode.As presented in Figure 6a and S12, Supporting Information, the as-recorded chronoamperometric curves manifest a serrate shape under a fixed overpotential, which is linked to the alternating accumulation and release of the H 2 bubbles on the electrode surface.It is impressive that the large cathode current (%700 mA cm À2 ) on the COF (40%)/Ti 3 C 2 T x electrode is able to maintain steady for a long time of 24 h.Further SEM, XRD, and XPS analysis indicates that the micromorphology, crystal structure, and elemental composition of the COF (40%)/Ti 3 C 2 T x catalyst were well protected after the stability test (Figure S13 and S14, Supporting Information), and simultaneously the corresponding LSV polarization curve almost coincides with the initial line (Figure S15, Supporting Information), its outstanding structural and catalytic stability.Moreover, the continuous LSV tests were also carried out to evaluate the cycling stability of the COF (40%)/Ti 3 C 2 T x electrode.As clearly seen from Figure 6b, the LSV curve of the COF (40%)/ Ti 3 C 2 T x electrode after 2000 cycles is almost identical to the initial data with negligible current change, proving that our COF (40%)/Ti 3 C 2 T x electrocatalyst is capable to afford sustainable catalytic activity during the continuous HER process.Besides, a drainage system presented in Figure S16, Supporting Information, was adopted to collect the gases and evaluate the overall water splitting behaviors for practical application.Typically, the acquisition of 13.7 mL H 2 can be realized within approximately 1200 s (corresponding to a H 2 production rate of about 1.83 mmol h À1 ), with a Faradaic efficiency (FE) of almost 97.2%.
Furthermore, the enhanced HER performance of the COF (40%)/Ti 3 C 2 T x electrocatalyst should also be dependent on its improved electrical conductivity.As a proof of concept, we adopted the electrochemical impedance spectroscopy (EIS) method to study the diverse capabilities of electron transportation for the COF (40%)/Ti 3 C 2 T x , COF-42, and Ti 3 C 2 T x electrodes.As revealed in Figure 6c,d, all the obtained EIS spectra depict a half arc in the high-frequency region, which is commonly used to estimate the charge-transfer resistance of the electrocatalyst.According to the standard equivalent circuit in Figure 6e, the charge-transfer resistance of COF (40%)/ Ti 3 C 2 T x is identified to be 9.6 Ω, much lower than that bare COF-42 (298.5 Ω) and close to that of pristine Ti 3 C 2 T x (8.1 Ω), unraveling that the cooperation of COF-42 and Ti 3 C 2 T x could largely boost the electron transport rate and thus achieve more rapid electrocatalytic kinetics for the hydrogen production.
In order to have an in-depth theoretical understanding in the electrocatalytic mechanisms, spin-unrestricted density functional theory (DFT) calculations were conducted to disclose the H adsorption behaviors on the COF-42 and Ti 3 C 2 O 2 models, where the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional was used as the exchange-correlation functional.A 3 Â 3 Ti 3 C 2 O 2 supercell was applied for simulating the adsorption of H atom (Figure 7a).Due to the large lattice parameter of 29.9768 Å for the unit cell of COF-42, the 1 Â 1 unit cell was used to build the COF-42 for simulating the adsorption of H atom (Figure 7b-d).
As is well known, the free energy of adsorbed H atom (ΔG H ) can serve as a key indicator to appraise the intrinsic electrocatalytic capacity for diverse active sites of the HER electrocatalysts. [25]igure 7 presents four possible positions for the adsorption of H atoms on the COF-42 and Ti 3 C 2 O 2 models, which consists of the top of O site within the Ti 3 C 2 O 2 nanolayer (Figure 7a), the top of N site (Figure 7b), O 1 site (Figure 7c), and O 2 site (Figure 7d) within the COF-42 structure.On the basis of the standard computational principle, [26] ΔG H is simplified to ΔG H = ΔE H þ 0.28, where ΔE H is the H adsorption energy and is defined as Here, E 2DþH represents the total energy of the 2D material adsorbed with H atom, while E 2D and E H2 represent the total energies of isolated 2D  material and H 2 molecule, respectively.Based on the above, the ΔG H values of the adsorbed H atom on O site, N site, O 1 site, and O 2 site were determined to be À0.57, 0.51, 0.83, and 2.66 eV, respectively.It is universally acknowledged that the ideal HER ability is linked to a ΔG H value of 0 eV, while the more negative or positive ΔG H values indicate the strong or weak interactions between H atom and catalytic sites that are detrimental to the hydrogen generation, Accordingly, the H atoms are found to be easier to adsorb on the O atom of the Ti 3 C 2 O 2 model as well as the N site and O 1 site of the COF-42 model because of their suitable ΔG H values.Therefore, the marriage of hydrazone-linked COF-42 and MXene is very beneficial to expose more varieties of efficient active sites, which could drastically accelerate the overall hydrogen production efficiency.

Conclusion
In summary, a facile and robust stereoassembly approach is developed to achieve the marriage of nanoflower-shaped hydrazone-linked COF-42 nanocrystals and ultrathin Ti 3 C 2 T x MXene nanosheets via an in situ growth process.In virtue of the distinctive textural features, such as large specific surface area, well-separated nanolayers, hierarchical porous canals, plentiful N species, and low charge-transfer resistance, the as-derived COF/Ti 3 C 2 T x hybrid nanoachitecture with the preferred COF loading of 40 wt% demonstrates exceptional HER performance with a low onset potential of 19 mV, a small Tafel slope of 50 mV dec À1 , and good cycling stability, which is significantly superior to that of bare COF-42 and Ti 3 C 2 T x catalysts and even close to that of commercial Pt/C catalyst.It is believed that the current design concept may bring new opportunities to the bottom-up construction of versatile delicate nanoarchitectures based on porous organic frameworks and MXene nanosheets for a wide range of applications in the fields of energy and environment.

Experimental Section
Preparation of the COF/Ti 3 C 2 T x Nanoarchitectures: Initially, 2D thin Ti 3 C 2 T x nanoflakes with less than five layers were yielded by the selective etching of Al atomic layers in bulk Ti 3 AlC 2 (11 Technology Co., Ltd.) with the assistance of mixed LiF/HCl solution according to our primary work. [16]uring the entire etching process, the surface of Ti 3 C 2 T x was protected by argon flow to prevent excessive oxidation of the lamellas.The suspension of the obtained Ti 3 C 2 T x nanoflakes exhibits a typical Tyndall phenomenon (Figure S3, Supporting Information).The synthetic procedures of the COF/Ti 3 C 2 T x nanoarchitecture with a COF-42 loading amount of 40 wt% are exemplified as follows: 6 mg of 1,3,5-triformylbenzene was dissolved in a mixed solution containing 1.5 mL of 1,4-dioxane and 1.0 mL of mesitylene along with 0.2 mL of acetic acid (11 M).Thereafter, 30 mg of as-synthesized Ti 3 C 2 T x nanoflakes were added to the above solution under ultrasonic treatment for 30 min.Afterward, 17 mg of 2,5-diehoxyterephthalamide was introduced to the mixture with vigorous magnetic stirring for another 30 min.Then, the prepared black suspension was transferred to an autoclave and kept at 60 °C over 12 h.After the solvothermal process, the as-generated product, labeled as COF(40%)/Ti 3 C 2 T x , was collected by centrifugation and washed with deionized water for several times, and finally freeze-dried at À50 °C for 24 h.Similarly, other COF/Ti 3 C 2 T x nanoarchitectures with diverse COF-42 contents were obtained by adjusting the feeding ratios of Ti 3 C 2 T x to organic precursors.Besides, bare COF-42 material was also prepared by the same method with the absence of Ti 3 C 2 T x support.
Characterization: The microstructure and surface topography of the as-synthesized COF/Ti 3 C 2 T x nanoarchitectures were carefully examined by using FE-SEM (JEOL 6500F) and TEM (JEOL JEM-2100F).The crystal and chemical structures of the COF/Ti 3 C 2 T x as well as reference samples were studied by XRD (Bruker D8 Advanced) and XPS (Physical Electronics Quantera).The molecular connectivity and integrity of the molecular building blocks of guest-free COF-42 were assessed by FT-IR (Bruker Vertex 70), Raman (LabRam HR Evolution), and 13 C cross-polarization with magic-angle spinning (CP-MAS) nuclear magnetic resonance (NMR, Bruker Avance 500 MHz) spectroscopies.The specific surface areas of the samples were determined by N 2 adsorption/desorption tests using a Micromeritics ASAP 2020 Plus system.
Electrochemical Measurements: The electrocatalytic HER tests were performed on the CHI760E electrochemical workstation with the use of a standard three-electrode system.In this three-electrode system, the working electrode is a glass carbon electrode (GCE, 3 mm), the counter electrode is a graphite rod, and the reference electrode is saturated calomel electrode (SCE).The preparation process of the working electrode is as follows: to begin with, the GCE was ground with alumina powder, rinsed with deionized water, and dried at room temperature.Then, 4 mg of catalyst powder was dissolved in a mixture containing 950 μL of water, 950 μL of ethanol, and 100 μL of 5% Nafion 117, and then sonicated for 20 min to form a uniform black suspension.Afterward, 5 μL of the above solution was dropped onto the surface of glassy carbon electrodes, which were dried at room temperature before corresponding electrochemical tests.The electrocatalytic HER properties, including the onset potential, current density, cycling stability, and electron conductivity, were systematically studied by LSV, CV, chronoamperometry, and EIS tests in a N 2 -purged 0.5 M H 2 SO 4 aqueous solution.

Figure 2 .
Figure 2. Morphological and nanostructural analysis of the COF/Ti 3 C 2 T x nanoarchitecture.Representative a-c) FE-SEM, d-f ) TEM, and g) HR-TEM images of COF/Ti 3 C 2 T x .h) HAADF-STEM image and the corresponding elemental mapping analysis of i) C, j) Ti, k) O, and l) N elements.

Figure
Figure3c, the characteristic Raman scattering peak at about 153 cm À1 in the COF/Ti 3 C 2 T x spectrum is consistent with the A 1g symmetry plane out of the Ti atoms, while the additional strong Raman peaks between 300 and 700 cm À1 are attributed to the A 1g signals of C-Ti (386 cm À1 ) and Ti-O vibration (610 cm À1 ).[17]Meanwhile, the two strong peaks at 1265 and 1608 cm À1 are corresponding to the C═N and C═O bond signals of the COF-42 nanocrystal.Figure3dshows the FT-IR spectra of the COF/Ti 3 C 2 T x , COF-42, and Ti 3 C 2 T x samples.In the case of COF/Ti 3 C 2 T x , the typical peaks at 600, 1641, and 3480 cm À1 are ascribed to the Ti-C vibration, C═O and -OH of surface functional group, respectively.The absorption peaks at %1610 cm À1 can be attributed to the bending mode of N─H bond, while the intensive peak at 1210 cm À1 refers to the stretching vibration of C═N bonds in COF-42, which further testified their presence in the COF/Ti 3 C 2 T x nanoarchitecture.[18]We next adopted the nitrogen adsorption-desorption isotherm measurements to explore the porosity of the COF/Ti 3 C 2 T x nanoachitecture, together with that of COF-42 and Ti 3 C 2 T x for comparison.As presented in Figure3e,f, the isotherm plots of the COF/Ti 3 C 2 T x hybrid approximates the type IV curve with a Brunauer-Emmett-Teller (BET)-specific surface area of 565 m 2 g À1 , which is smaller than that of COF-42 (1224 m 2 g À1 ) but obviously larger than that of primary Ti 3 C 2 T x (20 m 2 g À1 ).Besides, the corresponding pore size distribution of the COF/ Ti 3 C 2 T x structure locates in the range of 0.3-50 nm, indicating that the COF/Ti 3 C 2 T x nanoachitecture demonstrates hierarchical porous networks with abundant micro-and mesopores, which are able to facilitate the exposure of catalytically active sites and simultaneously provide adequate unimpeded channels for the rapid transportation of the electrolyte.

Figure 4 .
Figure 4. a) XPS survey spectrum of COF (40%)/Ti 3 C 2 T x verifies the presence of C, O, N, and Ti components in the nanoarchitecture.High-resolution b) C 1s, c) O 1s, d) N 1s, and e) Ti 2p spectra disclose the detailed chemical valences for these elements.

Figure 5 .
Figure 5. HER performance of the COF/Ti 3 C 2 T x nanoarchitecture.a) LSV polarization curves, b) histograms of required overpotentials, and c) Tafel plots of COF/Ti 3 C 2 T x with diverse COF concentrations, COF-42, Ti 3 C 2 T x , and Pt/C electrodes in 0.5 M H 2 SO 4 solution.d) Comparison of overpotentials at 10 mA cm À2 for COF (40%)/Ti 3 C 2 T x and recent state-of-the-art non-Pt catalysts.e) The CV curves of the COF (40%)/Ti 3 C 2 T x electrode recorded at potentials between 0.25 and 0.45 V (vs RHE) at scan rates from 20 to 120 mV s À1 .f ) The capacitive currents for the COF/Ti 3 C 2 T x , COF-42, and Ti 3 C 2 T x electrodes at 0.35 V plotted as a function of scan rates.

Figure 6 .
Figure 6.a) The current-time and b) cycling test prove a reliable long-term stability of the COF (40%)/Ti 3 C 2 T x electrode.c,d) Nyquist plots of EIS for the COF (40%)/Ti 3 C 2 T x , Ti 3 C 2 T x , and COF-42 electrodes at the corresponding open circuit potential with an amplitude of 10 mV.e) Nyquist plots of EIS and the fitting curve for the COF (40%)/Ti 3 C 2 T x electrode.

Figure 7 .
Figure 7. Hydrogen atom adsorbed on a) O site of the Ti 3 C 2 O 2 model, b) N site, c) O 1 site, and d) O 2 site of the COF-42 model.The brown, gray, red, blue and white balls represent Ti, C, O, N, and saturated H atoms, respectively.The cyan ball represents adsorbed H atom.