CoP修飾Ti3C2Tx MXene納米復(fù)合材料作為高效析氫反應(yīng)電催化劑

摘要：高效、經(jīng)濟(jì)和環(huán)保是電化學(xué)水分解制氫電催化劑的關(guān)鍵要素。二維（2D） MXene 材料因其獨(dú)特的物理化學(xué)性質(zhì)而受到廣泛關(guān)注。雖然有許多不同種類的MXene 材料，但只有少數(shù)具有本征析氫反應(yīng)（HER）催化活性。然而，MXene 材料具有很多優(yōu)點(diǎn)，如較大的比表面積、高電導(dǎo)率和豐富的表面官能團(tuán)，因此可以作為與其他物質(zhì)復(fù)合的理想平臺。本研究首先通過密度泛函理論（DFT）預(yù)測了CoP 與Ti3C2Tx MXene （其中Tx =―F 和―OH 官能團(tuán)）具有低的氫吸附自由能（ΔGH*）。接著，我們合成了CoP-Ti3C2Tx MXene 納米復(fù)合材料，并在0.5 mol?L?1 H2SO4 中測試了其電催化HER 性能。該材料在電流密度為10mA?cm?2 時表現(xiàn)出了低的過電位（135 mV）和Tafel 斜率為48 mV?dec?1。理論計算表明，CoP-Ti3C2Tx MXene 納米復(fù)合材料的優(yōu)異電催化性能源于Ti3C2Tx 的高金屬導(dǎo)電性、良好的界面電荷轉(zhuǎn)移、快速的氫吸附/解吸過程以及優(yōu)化的電子結(jié)構(gòu)。

關(guān)鍵詞：Ti3C2Tx MXene；析氫反應(yīng)；CoP；密度泛函理論；界面電荷轉(zhuǎn)移

中圖分類號：O646

Abstract： Electrocatalysts play a pivotal role in theelectrochemical water splitting process to producehydrogen fuel. The advancement of this technologyrelies on the development of efficient， cost-effective，and readily available electrocatalysts. Twodimensional（2D） MXene materials have garneredsignificant attention due to their uniquephysicochemical properties， rendering them promisingcandidates for electrocatalytic applications. Whilethere are numerous types of MXene materialsavailable， only a few possess intrinsic hydrogen evolution reaction （HER） catalytic activity. However， MXene materials canserve as excellent platforms for enhancing catalytic HER activity by combining them with other substances， owing to theirlarge specific surface area， high conductivity， and abundant surface functional groups. In this study， we initially conducteda predictive analysis using density functional theory （DFT） to assess the potential of combining CoP with Ti3C2Tx MXenematerials （where Tx represents ―F and ―OH functional groups） in reducing the adsorption free energy of hydrogen （ΔGH*）.The results indicated that the CoP-Ti3C2Tx nanocomposites exhibited a ΔGH* value approaching 0， suggesting promisingHER performance. Following this theoretical prediction， we synthesized the CoP-Ti3C2Tx MXene nanocomposites.Comprehensive characterization of the synthesized nanocomposites was performed using various techniques， includingscanning electron microscopy （SEM）， transmission electron microscopy （TEM）， X-ray diffraction （XRD）， and X-rayphotoelectron spectroscopy （XPS）. These analyses confirmed the successful decoration of CoP on the MXene nanosheetsand provided insights into the structural and compositional properties of the nanocomposites. Furthermore， we evaluatedthe electrochemical performance of the CoP-Ti3C2Tx nanocomposites through linear sweep voltammetry andchronoamperometry measurements. The results demonstrated superior catalytic activity and stability for the HERcompared to pure Ti3C2Tx and CoP catalysts. Specifically， the as-synthesized CoP-Ti3C2Tx MXene nanocompositesexhibited remarkable electrocatalytic HER kinetics， featuring a low overpotential of 135 mV at a current density of 10mA?cm?2 and a small Tafel slope of 48 mV?dec?1 in a 0.5 mol?L?1 H2SO4 solution， with the electrocatalyst maintaining stability for up to 50 h. Subsequent theoretical calculations were conducted to elucidate the factors contributing to theexceptional electrocatalytic performance of the CoP-Ti3C2Tx MXene nanocomposites. It was determined that the metallicconductivity of Ti3C2Tx MXene materials， well-structured interface charge transfer， and optimized electronic structure ofCoP played significant roles in enhancing catalytic activity. In conclusion， this study underscores the potential of CoPdecoratedTi3C2Tx MXene nanocomposites as promising electrocatalysts for efficient HER in various energy conversionand storage devices. These findings represent a significant contribution to the development of robust and efficient catalystsfor hydrogen generation， a critical component of renewable energy applications and sustainable development.

Key Words： Ti3C2Tx MXene; Hydrogen evolution reaction; CoP; Density functional theory;Interface charge transfer

1 Introduction

Due to the increasing demand of energy， the consumption offossil fuels and the total amount of CO2 emissions are risingrapidly， the energy transformation of “clean， low-carbon， safeand efficient” has become as a general trend 1–3. The large-scaleuse of renewable energy source （such as solar energy， windenergy， hydropower， etc.） is limited by its inherent intermittency，volatility and randomness; while hydrogen could be an idealsecondary energy carrier with high calorific value， its energydensity （140 MJ?kg?1） is more than twice that of solid fuel （50MJ?kg?1） 4–6. Additionally， its combustion product is water，which makes it to be the most environmental friendly energysource 7. Using renewable energy to achieve large-scalehydrogen evolution reaction （HER）， through the bridging effectof hydrogen can not only provide hydrogen source for fuel cells，but also， converting liquid hydrogen fuel in a green way. So that，it is possible to realize a sustainable cycle of smooth transitionfrom fossil energy to renewable energy， spawning a sustainablehydrogen economy 8，9. At present， more than 95% of thehydrogen production comes from the reforming of fossil fuels，and the production process needs to emit a large amount of CO2.In contrast， the condition for HER from water splitting is mild，and no CO2 discharge. However， due to cost constraints， onlyabout 4%–5% of hydrogen comes from the water electrolysisprocess 10–13. Therefore， in order to reduce the cost of waterelectrolysis for HER， efficient and inexpensive HER catalystdesign and preparation is receiving more and more extensiveattention.

2D nanomaterials have unique sheet-like morphologies withlateral dimensions ranging from hundreds of nanometers to tensof micrometers， but only one or a few atomic layers thick.Therefore， they have the characteristics of large specific surfacearea， more exposed atoms， and short electron charge transportdistance， which make the materials have important applicationprospect in the field of HER electrocatalysis 14. Among the earthabundant2D electrocatalysts， MXene materials with the meritsof high conductivity， excellent thermo stability， tunable surfacefunctional groups， and superior support to interaction with other substance， have been paid more attentions for electrocatalyticHER 15–25. Ling et al. 15 have theoretically predicted the HERactivity of fully oxygen terminated surface V2C-MXenematerials by first-principles calculations for the first time. Theyfind that the activity of the V2C-MXene material alone is notideal， but after introducing transition metal promoter atoms onthe surface， the promoter atoms will provide electrons to thesurface O， resulting an optimal hydrogen adsorption free energyclose to 0 eV. Combining theory and experiments， Seh et al. 26have screened 2D Mo2CTx MXene materials among dozens ofM2XTx as HER electrocatalysts， which delivers the overpotentialof 283 mV at the current density of 10 mA?cm?2 in 0.5 mol?L?1H2SO4 solution. Although various compositions of MXene havebeen discovered， there are still very few MXene-basedelectrocatalysts with promising HER activity.

Typically， MXene materials are regarded as the versatile andfavorable electrocatalytic construction platforms due to itsabundant surface functional groups and ultrathin sheet-likemorphology 27–33. Benefiting from the unique 2D structure andexcellent metallic conductivity， the MXene supports canstabilize severe aggregation of nanocomposites， and enhance theinterface electron/charge transfer between the electrode surfaceand electrolyte 34–36. Besides， the surface terminations of MXenematerials also could couple with other electrocatalysts byinterface engineering， resulting tunable electronic structures andelectrochemical activities for HER performance 37. Based on theconsideration of synergistic effect， it is of great significance todevelop MXene-based nanocomposites with strong interfacialinteraction for electrocatalytic HER.

Herein， CoP-Ti3C2Tx MXene nanocomposites weresuccessfully designed and fabricated through hydrothermalreaction and in situ phosphating process， in which CoP grow onthe surface of Ti3C2Tx MXene nanosheets and formed a layeredstructure with large surface area and high electrical conductivity.The theoretical calculation results show that the electricalconductivity of CoP-Ti3C2Tx MXene nanocomposites isenhanced and the electronic structure of Co species areoptimized. In addition， compared with CoP， the d-band center ofCoP in CoP-Ti3C2Tx MXene nanocomposites moves furtherdownward relative to Fermi level and has lower anti-bondingenergy， which leads to weaker hydrogen adsorption capacity ofCoP and makes the Gibbs free energy of hydrogen adsorption ofthe composite closer to 0. Consequently， the nanocompositesgive full play to the synergistic advantages of CoP and Ti3C2TxMXene nanosheets， provide abundant active sites， promote thecontact between catalyst and electrolyte， and improve thecharge/electron transfer and hydrogen rapid release during theelectrocatalytic HER process.

2 Experimental

2.1 Synthesis of Ti3C2Tx MXene

First， 1.6 g of LiF was dissolved in 20 mL of 9 mol?L?1hydrochloric acid at 40 °C with stirring to form a homogeneous solution. 1 g of Ti3AlC2 precursor was added to the abovesolution， and the metal aluminum layer was removed bycontinuous etching for 48 h. After etching， the obtainedsuspension was centrifuged at a speed of 3500 r?min?1 for 1 min，and 2 mol?L?1 HCl was added to the obtained precipitate to washaway the unreacted LiF， and the process was repeated 3 times.Then deionized water was added and continued to centrifuge andwash until the pH of the supernatant was neutral. Finally， theprecipitate was vacuum-dried at 60 °C for 6 h to obtain Ti3C2TxMXene materials.

2.2 Synthesis of CoP-Ti3C2Tx MXene nanocomposites

CoP-Ti3C2Tx MXene nanocomposites were successfullyprepared by hydrothermal method and in situ phosphatingprocess. Specifically， 1.5 mmol of cobalt acetate tetrahydrate（Co（OAc）2?4H2O） and 1.5 mmol of hexamethylenetetramine（HMT） were dissolved in 70 mL of water， then a certain amountof Ti3C2Tx MXene materials were added for ultrasonicdispersion. The suspension was then placed into a 100 mL PTFElinedautoclave and reacted at 180 °C for 12 h. After the reaction，it was naturally cooled to room temperature， the precipitate waswashed three times with deionized water， then dried in a vacuumoven at 60 °C. 100 mg of the above sample and 1.5 g of sodiumhypophosphite （NaH2PO2） were mixed uniformly， placed in aclosed porcelain boat， and reacted at 300 °C for 2 h undernitrogen atmosphere. After the phosphating reaction wascompleted， it was naturally cooled to room temperature， and thesamples were washed with water and ethanol three timesrespectively， and then dried in a vacuum drying oven at 60 °Cfor 12 h to obtain CoP-Ti3C2Tx MXene nanocomposites.

2.3 Characterization

The crystal structure analysis of the samples was carried outby a powder X-ray diffractometer （AXS D8 ADVANCE， BrukerGermany）. The morphology and structure of the samples werecharacterized by field emission scanning electron microscopy（FESEM， Gemini SEM 300， Germany Zeiss）， energy dispersiveX-ray spectroscopy （EDX）， transmission electron microscopy（TEM， JEM-2100）， and selective electron diffraction （SAED）analyzers. The elemental state of the catalyst surface wasanalyzed using X-ray photoelectron spectroscopy （XPS，ESCALAB XI+-600W X）.

2.4 Computational method

We have employed the Vienna Ab Initio Package （VASP） 38，39to perform all the density functional theory （DFT） calculationswithin the generalized gradient approximation （GGA） using thePBE 40 formulation. We have chosen the projected augmentedwave （PAW） potentials 41，42 to describe the ionic cores and takevalence electrons into account using a plane wave basis set witha kinetic energy cutoff of 400 eV. Partial occupancies of theKohn-Sham orbitals were allowed using the Gaussian smearingmethod and a width of 0.05 eV. The electronic energy wasconsidered self-consistent when the energy change was smallerthan 10?5 eV. A geometry optimization was considered convergent when the force change was smaller than 0.2 eV?nm?1.Grimme’s DFT-D3 methodology 43 was used to describe thedispersion interactions.

The equilibrium lattice constants of orthorhombic CoP unitcell were optimized to be a = 0.5017 nm， b = 0.3222 nm， c =0.5499 nm. We then use it to construct a CoP （110） surface model（model 1） with p（1 × 1） periodicity in the x and y directions andone stoichiometric layer in the z direction separated by a vacuumlayer in the depth of 1.5 nm in order to separate the surface slabfrom its periodic duplicates. During structural optimizations， a 5 ×5 × 1 k-point grid in the Brillouin zone was used for k-pointsampling， and the bottom half stoichiometric layer was fixedwhile the top half was allowed to relax.

The equilibrium lattice constants of hexagonal O-Ti-C-Ti-CTi-O MXene monolayer unit cell in a vacuum layer of 1.5 nmwas optimized to be a = 0.3018 nm. We then use it to constructa MXene （001） surface model （model 2） with p（1 × √3）periodicity. Half of the O atoms were replaced by F atoms.During structural optimizations， a 5 × 5 × 1 k-point grid in theBrillouin zone was used for k-point sampling， and all atoms wereallowed to relax.

The free energy of a gas phase molecule or an adsorbate onthe surface was calculated by the equation G = E + ZPE ? TS，where E is the total energy， ZPE is the zero-point energy， T is thetemperature in kelvin （298.15 K is set here）， and S is the entropy.The reported standard hydrogen electrode （SHE） model 44 wasadopted in the calculations of Gibbs free energy changes （ΔG）of all reaction steps， which was used to evaluate the reactionbarrier. The chemical potential of a proton-electron pair， μ（H+） +μ（e?）， is equal to the half of the chemical potential of one gaseoushydrogen molecule， 1/2μ（H2）， at U = 0 V vs. SHE at pH = 0.

2.5 Electrochemical measurements

4 mg of the catalyst was dispersed in 1 mL of 0.1% Nafionsolution and then sonicated for 60 min to form a homogeneousink. Then， 20 μL of the ink solution was dropped onto a glassycarbon electrode with a diameter of 5 mm， and the catalystloading was about 0.408 mg?cm?2. All electrochemical measurements were performed at room temperature on theCHI660E electrochemical workstation using a conventionalthree-electrode system and RDE type rotating disk electrodes.The electrolyte was 0.5 mol?L?1 H2SO4 solution. Beforemeasurement， the electrolyte was purged with nitrogen for 30min to remove dissolved oxygen from the solution. Thereference electrode used in the electrochemical experiments wasa saturated silver/silver chloride electrode （Ag/AgCl）， thecounter electrode was a graphite rod， and the working electrodewas a glassy carbon electrode loaded with a catalyst. Linearsweep voltammetry （LSV） polarization curve was measured atscanning rate of 5 mV?s?1. All the potentials mentioned in thispaper were converted to reversible hydrogen electrode （RHE）potentials by ERHE = EAg/AgCl + 0.059 × pH + 0.198. Allelectrochemical measurements conducted with 95% iRcompensation.

3 Results and discussion

The DFT calculations have been applied to predict thesuperior performance of the CoP-Ti3C2Tx MXenenanocomposites. The hydrogen adsorption sites andconfigurations on CoP， Ti3C2Tx MXene， and CoP-Ti3C2TxMXene nanocomposites have been calculated in Fig. S1 andTable S1 （Supporting Information） by the first principlescalculations. Apparently， the optimized structures of CoP，Ti3C2Tx MXene and CoP-Ti3C2Tx MXene nanocomposites havebeen constructed as shown in Fig. 1a–c. As a reasonabledescriptor， the adsorption free energy of hydrogen （ΔGH*） iswidely used to evaluate the performance of electrocatalytichydrogen evolution 45. In general， when the |ΔGH*| value is zero，there is a rapid proton/electron transfer process and hydrogenrelease process in the electrocatalytic hydrogen evolutionreaction. As can be seen from Fig. 1d， the |ΔGH*| value of CoPTi3C2TxMXene nanocomposites is 0.03 eV， which is much lowerthan that of CoP （0.12 eV） and Ti3C2Tx MXene （0.18 eV），suggesting the positive synergistic effect between CoP andTi3C2Tx MXene materials for favorable HER catalytic activity.

Based on the results of theoretical calculation， the CoPTi3C2TxMXene nanocomposites were constructed throughetching， hydrothermal and subsequent phosphating processes（see “Experimental” for detailed steps）， which was expected tobe a promising HER electrocatalyst. The morphology andcomposition of CoP-Ti3C2Tx MXene nanocomposites werecharacterized by SEM （Fig. S2） and TEM （Fig. 2a）. After peelingoff the Ti3AlC2 precursor， the 2D Ti3C2Tx MXene nanosheetswith a clean surface was obtained. The insets in Fig. 2a are theHRTEM and selected area electron diffraction （SAED） patternof Ti3C2Tx MXene， which exhibit the distinct lattice fringe of0.457 nm for （004） crystal plane 46. As can be seen in Fig. 2b，CoP nanoparticles are exhibiting homogeneous spherical shapewith a diameter of about 20 nm. Similarly， the inset in Fig. 2bshow the HRTEM image and corresponding SAED pattern，which suggest that the lattice fringe is 0.221 nm， correspondingto the （210） crystal plane of the CoP 47. The SEM images of CoPTi3C2TxMXene nanocomposites have been presented in Fig. 2cand Fig. S3. It can be seen that CoP nanoparticles tightly adhereto the surrounding Ti3C2Tx MXene surface and form stablenanocomposites. This finding not only highlights thecompatibility between the two materials but also hints at thepossibility of synergistic effects that may arise due to theirinteraction. The nanosheet structure of Ti3C2Tx MXene offers alarge surface area， making it an ideal candidate for applicationsthat require high surface reactivity. Fig. 2d–f show the presenceof Co， P， and Ti elements， Ti element in the center of thenanostructure， while， Co and P elements in the periphery of thenanostructure， which proves the successful growth of CoP on thesurface of Ti3C2Tx MXene nanosheets. The surface of thenanocomposites is rough， which is beneficial to theelectrocatalyst and electrolyte contact as well as the rapid releaseof gas.

The crystal and phase structures of these samples wereanalyzed by XRD measurements， and the results are shown inFig. 3a. For Ti3C2Tx MXene materials， the phase ofdiffraction peak at 2θ = 39° almost disappeared relative toTi3AlC2 MAX phase， indicating that the Al layer is successfullyremoved. After conducting a thorough comparison， we haveobserved that the XRD diffraction peaks of Ti3C2Tx MXenealign remarkably well with the simulated pattern 48. This findingindicates that our Ti3C2Tx MXene materials have beensuccessfully prepared and are of high quality. According to theXRD spectrum of CoP， a detailed analysis reveals that the maincharacteristic diffraction peaks are located at 31.6°， 36.3°， 48.1°，and 56.0°. These peaks correspond remarkably well with thediffraction planes （011）， （111）， （211）， and （020） of orthorhombicCoP （JCPDS No. 29-0497） respectively. In the XRD of thenanocomposites， the diffraction peaks of CoP are obvious， butpart of the diffraction peaks of MXene materials are weakenedor even disappeared， which means the CoP nanoparticles grownon the surface of Ti3C2Tx MXene materials can effectivelyinhibit the repacking of MXene nanosheets. The detailedchemical composition and electronic structure of CoP-Ti3C2TxMXene nanocomposites had been investigated by XPS. Thesurvey XPS spectrum in Fig. 3b shows that CoP-Ti3C2Tx MXene nanocomposites contain Co， O， Ti， C， F， and P elements. Asshown in Fig. 3c and Table S2， there are two distinct peaks canbe identified from the Co 2p core level spectra. The peak at780.5–781.5 eV is attributed to Co 2p3/2 and the other bindingenergy located at 796.7–797.6 eV is assigned to Co 2p1/2.Obviously， compared with CoP， the peaks of Co 2p in CoPTi3C2TxMXene nanocomposites move towards the direction oflow binding energy， which means the electron transfer from CoPto Ti3C2Tx MXene materials. The modulated electronic structureof Co is beneficial for the HER activity. In addition， the highresolutionXPS spectra of P 2p can be deconvoluted into threepeaks （Fig. 3d and Table S3）， the peaks at 128.1 and 128.8 eVare assigned to P 2p3/2 and P 2p1/2， respectively， and the peaks at132.4 eV are assigned to peak of P oxide species. Compared withthe P 2p peak in CoP， the area of the P 2p peak in CoP-Ti3C2TxMXene increases， exhibiting the strong chemical bindingbetween CoP and Ti3C2Tx MXene materials. In the case of Ti 2pspectra of these samples， there are four peaks can bedeconvoluted （Fig. 3e and Table S4）. Specifically， the bindingenergies centered at 458.3 and 463.1 eV are attributed to the 2p3/2and 2p1/2 characteristic peaks of Ti―C bonds， while the peakpositions located at 459.3 and 464.6 eV are corresponded to the2p3/2 and 2p1/2 characteristic peaks of Ti―O bonds 48，49.Compared with Ti3C2Tx MXene materials， the slight shift of Ti2p peaks of CoP-Ti3C2Tx MXene nanocomposites to higherbinding energy demonstrates that the interaction betweenTi3C2Tx MXene materials and CoP. Meanwhile， the two majorpeaks located at the binding energy of 283.4 and 284.9 eV areassigned to Ti―C and C―C bond， respectively （Fig. 3f andTable S5）. After decorating CoP， the XPS peaks of thenanocomposites deconvolate four distinct peaks of Ti―C，C―C， C＝O， and C―O around at the binding energies of 283.2，284.1， 287.4， and 290.9 eV.

The LSV polarization curves of these electrocatalysts are presented in Fig. 5a. It is noted that CoP-Ti3C2Tx MXeneelectrocatalysts only need overpotential of 135 mV to achievethe current density of 10 mA cm?2. In contrast， the potentials ofTi3C2Tx materials and CoP reaching the same current density areas high as 449 and 199 mV， respectively. The promising HERperformance with low overpotential of CoP-Ti3C2Tx MXeneelectrocatalysts result from the synergistic effect betweenTi3C2Tx MXene and CoP. Pt/C electrode has a minimumoverpotential of 69 mV at a current density of 10 mA cm–2.Although it has the best HER performance， the scarcity and highcost of precious metals seriously restrict its large-scale practicalapplication. In addition， the corresponding Tafel plots of theseelectrocatalysts are also calculated to investigate the underlyingcatalytic kinetics （Fig. 5b）. The calculated Tafel slopes ofTi3C2Tx MXene， CoP， and CoP-Ti3C2Tx MXene nanocompositesare 350， 107， and 48 mV?dec?1， respectively. As predicted， thesmallest value for CoP-Ti3C2Tx MXene electrode indicates thesuperior HER kinetics. Furthermore， the dominated ratedeterminingstep of electrocatalytic HER is the Volmer–Heyrovsky mechanism 50. The TOF values of Ti3C2Tx MXene，CoP， and CoP-Ti3C2Tx MXene nanocomposites have beenestimated to understand the activity of each site in theseelectrocatalysts. The potential dependent TOF curves arepresented in Fig. 5c. Obviously， the TOF values of CoP-Ti3C2TxMXene nanocomposites are higher than those of the Ti3C2TxMXene and CoP catalyst at different potentials， indicating theimproved intrinsic catalytic activity of single active site for CoPTi3C2TxMXene nanocomposites 51. EIS measurement has alsobeen carried out to assess the electrochemical behaviors of theseelectrodes， and the results are shown in Fig. 5d. It is noted thatthe CoP-Ti3C2Tx MXene electrode exhibits the lowest chargetransfer resistance， demonstrating the appropriate charge/ionmobility at the interface between the CoP-Ti3C2Tx MXenesurface and electrolyte， which is consistent with the Tafel plot.The fast charge transfer ability can attribute to the enhancedconductivity and electron coupling between Ti3C2Tx MXene andCoP. The long-term stability of electrocatalysts is an importantfactor for practical application. The durability of CoP-Ti3C2TxMXene nanocomposites is evaluated by chronoamperometry atthe potential of 0.135 V （vs. RHE）. It is worth noting that thecurrent density of this electrode can be maintained for 50 hwithout significant attenuation （Fig. 5e）. Moverover， the doublelayercapacitances （Cdl） of Ti3C2Tx MXene， CoP， CoP-Ti3C2TxMXene nanocomposites have been estimated to calculated theelectrochemical active surface area， which is the indication ofthe number of catalytic sites. The values of Cdl are calculated byCV measurements in the potential of 0.2–0.4 V （vs. RHE） withthe scan rates of 20-200 mV?s?1 （Fig. S4）. As shown in Fig. S3d，the Cdl value of CoP-Ti3C2Tx MXene nanocomposites isconfirmed at 7.41 mF?cm?2， which is 1.4 times of CoP and 1.9times of Ti3C2Tx MXene materials. The large double layercapacitance of CoP-Ti3C2Tx MXene nanocomposites meansincreasing available exposed active sites due to the synergisticeffect between Ti3C2Tx MXene and CoP， which is beneficial forboosting the electrocatalytic HER 52，53.

To evaluate the stability of the CoP-Ti3C2Tx MXene samples，we employed SEM and TEM techniques. The results， as depictedin Fig. S5a–c， clearly indicate that even after undergoing longtermstability tests， the CoP-Ti3C2Tx MXene nanocompositeshave managed to maintain their initial coating structure withoutsuffering significant damage. While it is unfortunate that a smallfraction of the particles detached from the surface. Given therigorous nature of the stability tests， it is natural for someparticles to become dislodged. However， it is crucial toemphasize that the overall coating structure remained largely unaffected， indicating the reliability and durability of thematerial. As shown in Fig. S5d， HRTEM image showed that alattic e spacing of 0.19 nm corresponds to the （211） crystal faceof CoP， indicating that CoP did not undergo structural changesafter stability testing. These results demonstrate the robustnessof the coating structure， which has proven to be highly resistantto deterioration. The fact that the majority of the particlesremained intact after the stability tests is a testament to theeffectiveness of the coating in preserving the integrity of thesamples.

In order to explore the reasons for the HER performances ofthese electrocatalysts， the total density of states （TDOS） forTi3C2Tx MXene， CoP， CoP-Ti3C2Tx MXene nanocompositeshave been depicted in Fig. S6. It can be seen from TDOS， thestate density at the Fermi level of the CoP-Ti3C2Tx MXenenanocomposites increases， indicating that its electricalconductivity is enhanced， which is conducive to theelectrochemical reaction. In order to study the influence ofTi3C2Tx MXene for the electronic structure of CoP， the chargedensity difference （CDD） of CoP-Ti3C2Tx MXenenanocomposites has been calculated. As shown in Fig. 6a， theyellow area around the P atom shows charge accumulation andthe blue area near the O atom shows charge depletion. Thisindicates that the electrons of the Ti3C2Tx MXene are transferredto the P of the CoP through the interface between CoP andTi3C2Tx MXene materials. Additionally， the electron localizationfunction （ELF） mapping is also presented to investigate thedegree of electron localization. As shown in Fig. 6b， the ELFdiagram shows the obvious interaction between CoP and Ti3C2TxMXene surface. According to the Bader charges analysis （TableS6）， prior to the formation of the heterojunction， the CoP （110）portion contains 168 electrons. On the other hand， MXene （001）exhibits a different electron configuration. However， upon theformation of the heterojunction， the combined system exhibits atotal of 166.86 electrons. This observation suggests that the CoP（110） portion has lost approximately 1.14 electrons. This is also consistent with the XPS results of the Co element. In addition，the partial densities of states （DOS） for 3d orbital of Co in CoPand C oP-Ti3C2Tx MXene nanocomposites have been plotted inFig. 6c. Compared with CoP （?1.552 eV）， the d-band center ofCoP in MXene-CoP moves further downward （?1.741 eV） withrespect to Fermi level （0 eV） and has lower anti-bonding energy，which results in weaker hydrogen adsorption capacity andstronger hydrogen desorption energy of CoP.

4 Conclusions

The CoP-Ti3C2Tx MXene nanocomposites have beenrationally predicted and successfully prepared as robustelectrocatalysts for HER. A facile lift-off etch process followedby hydrothermal method is used to decorate CoP nanoparticleson Ti3C2Tx MXene nanosheets. For HER， CoP-Ti3C2Tx MXenenanocomposites exhibit the low overpotential of 135 mV at thecurrent density of 10 mA?cm?2 in 0.5 mol?L?1 H2SO4 solution.Due to the high metal conductivity and large surface area ofTi3C2Tx MXene materials， the CoP-Ti3C2Tx MXenenanocomposites owns well-structured nanoparticle-sheetinterface synergistic effect and numerous exposed active sites.This work has laid a solid foundation for expanding theapplication of MXene materials in energy conversion.

Author Contributions： Conceptualization， W. S. and Y.W.; Methodology， S. B.; H. W. and J. Z.; Writing – originaldraft， K. X.; Visualization， Arramel and K. X.; Writing – Reviewamp; Editing， J. J; Project administration， K. X. and J. J.;Supervision， J. J.

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國家自然科學(xué)基金（62004143， 22174033）， 湖北省重點(diǎn)研發(fā)計劃（2022BAA084）， 光電化學(xué)材料與器件教育部重點(diǎn)實驗室（江漢大學(xué)）開放基金（JDGD-202227）， 武漢市知識創(chuàng)新專項項目-曙光計劃（2022010801020355）資助