A novel hierarchically hybrid structure of MXene and bi‐ligand ZIF‐67 based trifunctional electrocatalyst for zinc‐air battery and water splitting

The development of cost‐effective and durable electrocatalysts possesses a broad spectrum of applications in sustainable energy systems. Herein, a hierarchical composite of Co‐based bi‐ligand zeolite imidazole framework (ZIF‐67) with highly conducting 2D MXene as highly efficient noble metal free electrocatalyst for electrochemical oxygen reduction reaction (ORR), complete water splitting, along with zinc‐air battery (ZAB) has been studied. ZIF‐67 is reported as an efficient electrocatalyst due to its porous structures, high surface area and atomically dispersed active metal centres while low conductivity and structural instability have been addressed by pyrolysis. In this work, structural disintegration due to temperature effect has been handled by using bi‐ligand linkers in ZIF (b‐ZIF‐67) which controls its sharp morphology and uniform mesoporous structure. This b‐ZIF‐67 has been supported on highly conducting 2D MXene material which exposes ample accessible active sites to accelerate the electroactivity of the synthesized catalyst. The resultant b‐CZIF‐67/MXene catalyst exhibits superior onset of 0.91 and 0.93 V in acidic and alkaline medium respectively for ORR. At the current density of 10 mA/cm2 catalyst shows a very low overpotential of 0.170 mV and 1.47 V for HER and OER, respectively. The excellent specific charge storage of 550.6 mAh/g was displayed by the homemade ZAB pouch.


| INTRODUCTION
Hydrogen has been recognized as a 'green' source to substitute fossil fuels due to its properties like high specific heat value and zero carbon emission. 1Fuel cells (FCs), water electrolysers, metal-air batteries and other hydrogen-based technologies are currently the focus of intense research due to their practical applicability in the present scenario where environmental safety is of utmost importance. 2 The ever increasing demand for energy in all fields necessitates continued research in the rational design of catalyst architecture to address this issue. 3atalytic reactions involving oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) undoubtedly play an important role in the development of energy conversion devices such as FCs (involves ORR-HER-OER), metal-air batteries (OER-ORR) 4 and water electrolyser's (HER-OER), 5 and so forth.More often single reaction per catalyst has been studied [6][7][8][9][10] which hardly satisfies the desire for the development in the sustainable energy field. 11Henceforth, integration of the aforementioned reactions within a single device is required.Currently, Platinum (Pt) and its alloys are known to be the most efficient catalyst for HER and ORR 12 while ruthenium oxide (RuO 2 ) and iridium oxide (IrO 2 ) are well-known catalysts for OER. 13 However, the applicability of clean energy technology is impeded by their 'rare earth' group status, which results in high cost, limited availability, and consequently limited usage. 14Therefore, it is required to search for highly efficient and active trifunctional catalysts, to meet immense requirement in the energy sector.
Over the last decade, various electrocatalysts have been developed where the origin of catalytic activity was studied but applicability in terms of usage still remained the challenge.Hence, in this direction, the primary requirement is to set a new avenue for the structural analysis of the catalyst and aim for the uniform dispersion of the embedded active centre for maintaining the catalytic performance. 15Recently, there have been tremendous efforts taken that emphasize the performance of material development.Zeolitic Imidazolate Framework (ZIF), is a new class of highly porous materials which possess unique frame like structure with a uniform dispersion of metallic canters and a high surface-to-volume ratio. 16Due to this, ZIF has emerged as an ideal precursor for the development of a tailormade transition metal-based heteroatom (e.g., nitrogen and carbon) doped electrocatalyst (M-N-C, where M=Fe, Co, Zn) for various energy applications. 7,17,18However, its low electrical conductivity and structural disintegration is the main issue which needs to be addressed.To address these issues, pyrolysis is one of the preferred techniques. 19High temperature-treated ZIF-based electrocatalysts show improved conductivity and produce atomically dispersed M-Nx type active centres. 20The active centres that are located at the electrode-electrolyte interface contribute to the electrocatalytic activity, while those trapped inside the deep carbon network are electrochemically inactive. 162][23][24] Particularly, in the last decade, Co based ZIF-67 has been studied in detail and high electrochemical activity has been reported owing to its Co-N 4 active centres.
Recently, two-dimensional (2D) materials especially transition metal carbides, nitrides and carbonitrides, attracted a lot of attention due to their high surface area, low ionic diffusion resistance and fast charge transfer rate. 25The 2D carbide family especially MXene materials are synthesized by the etching process of M n+1 AX n (n = 1, 2 or 3, M-transition metal, A-13 or 14 group element for example, aluminium, carbon etc., and X-Carbon or/and Nitrogen). 26MXene has been widely studied due to its significant properties like metal end structure, chargecarrying ability and high conductivity. 27In addition to this, its controlled flexibility and infusible surface along with hydrophilic terminal functional groups make it an appropriate candidate for energy applications. 6,8,9,26,28,29owever, aggregation and stacking of sheets due to van der Waals force and hydrogen bonding reduces its intrinsic properties like surface area and overall electrochemical efficiency.To address this issue and consider the high electrochemical activity of ZIF-67, it is anticipated that a composite of ZIF-67 and MXene could show a significant synergistic effect and boost the electrochemical activity. 30ecently, Pang and colleagues reported the usage of MXene/MOF composite for the supercapacitor application.The composite shows enhanced stability and conductivity as the metal ions cross-linked with the surface groups of MXene nanosheets, reduced the interlayer electrostatic repulsion and formed the nucleation site for the attachment of ZIF. 31 Furthermore, the addition of extra transition metal-based ZIF creates strong coupling between the d-orbit and adsorbate valance state, enhancing the adsorption of gases such as oxygen and hydrogen on the active sites and thereafter enhancing overall activity of the catalyst. 32,33Therefore, the composite of MXene and d-electron rich transition metal could shift d-band toward lower fermi energy which leads to enhancing the overall catalytic activity. 15,34,35n the present work, a unique bi-ligand strategy was employed for the first time to synthesize a composite of bi-ligand Co based carbonized ZIF-67 supported on conducting 2D sheets of MXene.This hybrid composite shows exceptionally high electrocatalytic activity as trifunctional electrocatalyst for ORR, HER and OER.The controlled and sharp architectural morphology of the b-ZIF-67 was obtained and retained post pyrolysis due to the usage of 2-aminobenzimidazole (2-abIM) and 2-methyl imidazole (2-IM) as organic linkers during the synthesis.The partial substitution of one ligand with another not only introduces uniform accessible active sites but also maintains the structural porosity of the catalyst postpyrolysis by balancing ensembles and frameworks.Herein, MXene acts as a highly conducting support for stabilizing and uniformly distributing active centres incorporated in ZIF-67.This work opens a new area for the design and optimization of ZIF-67/MXene based electrode materials for multifunctional electrochemical applications.MXene was prepared by using the method reported in the literature. 27Typically, selective aluminium layer (Al) removal was done by etching Ti 3 AlC 2 powder in a mild aqueous acid solution containing LiF and HCl.Then the solution was kept at 45°C for 40 h following constant stirring.The sediment was carefully washed with DI water till pH = 7, dried at 60°C and named MXene.

| Synthesis of pyrolyzed ZIF-67 supported MXene
In a typical synthesis, two solutions were prepared namely A and B. Solution A contains Co (NO 3 ) 2 .6H 2 O (2.9 g) dissolved in 100 mL of methanol, while solution B contains 3.25 g of 2-methyl imidazole (2MI) and primarily dissolved 2-aminobenzimidazole (abIM) in 100 mL of methanol.Both the solutions were kept for continuous stirring (1 h).A dark purple solution of bi-ligand ZIF-67 was obtained after the addition of solution A to B, called b-ZIF-67.This was followed by the addition of 100 mg of synthesised Ti 3 C 2 T x, MXene.Then the entire solution was kept overnight for vigorous stirring.To form a well-bound structure with the amount of precipitate, the solution was kept undisturbed for 10 h.Finally, the obtained precipitate was washed with methanol to remove excess and unreacted particles, followed by overnight drying.
To obtain the final material, synthesized purple powder was kept in a quartz tube and pyrolyzed in a tubular furnace by maintaining an inert atmosphere (under N 2 flow) at 800°C with a ramp rate of 5°C for 2 h.This material was called b-CZIF-67/MXene.

| Material characterization
The crystal structure of the as-prepared catalyst was studied using the Rikagu ultima IV machine with the Cu Kα as the monochromatic source of radiation of a wavelength of 0.154 nm.The data were collected with a 2θ value ranging from 10°to 70°with a scan rate of 2 °per minute.The morphology and elemental composition were investigated using Field emission Scanning electron microscopy (FE-SEM) by JFEI company-made Nova Nano SEM-450, equipped with Energy Dispersive Spectroscopy (EDS) and elemental mapping.The scanning was performed at 10 kV and 10 mm working distance.The detailed elemental analysis was studied by XPS from Thermo-Fisher Scientific Company Nexsa base.To quantify the Brunauer-Emmett-Teller (BET) surface area and pore size of material, N 2 adsorption-desorption isotherm was measured by using Metrohm BELSORP miniX instrument at 77 K with N 2 as adsorptive gas.Before N 2 sorption measurement, samples were degassed at 220°C for 6 h to remove the moisture in the samples.The pore size and surface area of the catalyst were obtained by using the BET and Quenched Solid Density Functional Theory (QSDFT) method.The conductivity of the as-obtained catalyst was calculated by using the fourprobe method.

| RESULTS AND DISCUSSION
Scheme 1 illustrates a detailed schematic representation of the composite b-CZIF67/MXene.First, a layered MXene was obtained by acid leaching (LiF/HCl) of the MAX phase, which selectively removes the Al layer.MXene acts as a conducting support for b-CZIF67 where it adheres to the surface due to the presence of the surface terminal groups.A high-temperature pyrolysis treatment (at 800°C) was used to construct a highly conducting and porous electrocatalyst.To evaluate the conductivity of the prepared catalyst a probe technique was employed (SI).The conductivity of MXene was calculated by using Supporting Information: Equation S1 and found to be 10.1 S cm −1 , which is higher than b-CZIF67/MXene that is, 1.5 S cm −1 .However, the absence of electrochemically active sites limits MXene application despite its higher conductivity.][38] Figure 1 illustrates the morphological assessment of the as-synthesized materials.Albeit, active centres distribution and conductivity are the key features that increase catalyst activity, well-defined morphology is another crucial factor that has an impact on the catalyst functionality.The precise control of the catalyst's morphology is necessary for the uniform distribution of active sites. 39Figure 1A shows the tightly stacked layered morphology of the pristine MAX material.A complete transformation from a closely packed structure to the exfoliated layered MXene observed in Figure 1B showing layered structure as a result of the delamination of the Al layer due to the acid treatment.Further, Energy dispersive spectra (EDS) mapping, confirms the presence of Ti, F and C (Figure 1C).The existence of F moieties might correspond to the surface terminatal group.The bi-ligand b-ZIF-67 structure obtained prepyrolysis treatment shows the same morphology as that of single linker ZIF-67 (Supporting Information: Figure S2). Figure 1D shows a perfect cuboid-shaped b-ZIF-67 particle that maintains the same morphology and structure, as a single linker ZIF-67 (Supporting Information: Figure S2A).As illustrated in Figure 1E, b-ZIF-67 maintains its parent morphology post pyrolysis at 800°C temperature in an N 2 atmosphere.This suggests that the introduction of one ligand right after another prevents the structure from breaking/shrinking at higher temperatures, exposing a high surface area for the catalytic activity, which is in contrast with single linker ZIF-67 (Supporting Information: Figure S2B).The cuboid b-ZIF-67 (as shown in Figure 1F) supported between MXene layers (inset Figure 1F) was obtained as a result of the in-situ synthesis of MXene sheets and b-ZIF-67 followed by the pyrolysis treatment.This clearly shows the uniform distribution of b-ZIF-67 in between the MXene sheets which also acts as a spacer and hinders its stacking.The EDS mapping corresponding to b-CZIF-67/ MXene (Figure 1G) reveals the traces of Co, N, C, Ti and O further confirming the successful incorporation of ZIF-67 between MXene layers.This confirms the excellent structural support of MXene to obtain the uniformly doped ZIF-67 without any aggregations. 40igure 2A-C illustrates the XRD patterns of b-ZIF-67, MXene and b-CZIF-67/MXene.The diffraction pattern of MXene (Figure 2A) depicts the presence of characteristic peaks in comparison with pristine Ti 3 AlC 2 as per the JCPDS 52-0875 data file. 10We infer from comparative analysis of XRD patterns of MAX phase and MXene, that major structural and crystallinity loss was observed as a result of LiF/HCL treatment. 41The slight shift of peak at S C H E M E 1 Schematic representation of the steps followed to synthesize the b-CZIF67/MXene.9.8°toward a lower angle of 9.3°was observed predominantly for (002) peaks of MXene owing to the increased d-spacing as calculated from Supporting Information: Equation S2 (d MAX -9.1 Å to d MXene -9.45 Å), hence c parameter enhancement (from 1.82 to 1.9 nm, obtained from Supporting Information: Equation S3). 10,42his suggests that MXene maintains its parent hexagonal symmetry except for a small alteration in d-spacing.However, an extensive increase in c value for b-CZIF-67/ MXene to 1.96 nm suggests strong intercalation of ZIF between MXene sheets. 43This was facilitated due to the removal of the Al layer which exfoliated the MXene sheets and the introduction of terminal groups (-OH, -F, =O). 26Moreover, the desertion of the sharp peak (104) at 39°further supports this information and hence confirms the formation of MXene.The XRD pattern of b-ZIF-67 (Figure 2B) exemplifies that despite linker composition crystallinity of the material remains intact as a single ligand ZIF-67 (s-ZIF-67).The XRD pattern (Figure 2C) obtained postpyrolysis treatment of the hybrid catalyst (b-CZIF-67/MXene) shows a drastic change in the crystallinity of the ZIF structure.The peak obtained at 26°with miller indices (002) confirms the formation of graphitic carbon due to pyrolysis treatment.The peaks obtained at 45°, 51°a nd 75°with facet (111), ( 200) and (220) correspond to the formation of metallic Co (JCPDS 15-0806). 18The incorporation of 2-abIM ligand not only produces activated carbon and N site but also the presence of NH x group from this ligand (attached on the surface before pyrolysis), which prevents the oxidation of MXene into TiO 2 , and forms TiN species. 44The inherited peaks of TiN observed (marked as ♣) in XRD pattern suggests successful incorporation of ZIF on MXene sheets (JCPDS 38-1420). 28igure 2D,E illustrate, BET adsorption-desorption and Barrett-Joyner-Halenda (BJH) pore size analysis of MXene, b-ZIF-67 and b-CZIF-67/MXene materials.This study was performed to measure the surface area and porosity of as-synthesized materials.The highest value of the surface area was obtained for b-ZIF-67 (1467 m 2 /g), possibly due to the synergistic effect between the two ligands.The b-ZIF-67 structure shows a Type I isotherm with a predominant microporous nature of pore size 1.5 nm.While MXene shows the lowest surface area of 21 m 2 /g with large pores size of 20 nm.Subsequently, the hybrid catalyst (b-CZIF-67/MXene) exhibits optimized mesoporous Type IV isotherm with a surface area of 541 m 2 /g, as an effect of high pyrolysis temperature followed by acid treatment.It was observed that the intercalation of ZIF-67 within MXene sheets alters the porosity of the hybrid catalyst. 18The enhanced mass transfer results in the consistency of performance thereafter improving the stability of the catalyst. 44o study the surface chemical composition of MXene and b-CZIF-67/MXene, the samples were investigated by using XPS analysis.The XPS survey spectra (Figure 2F) confirm the existence of C, Ti, O, N and Co.Table 1, illustrates the atomic percentage of obtained elements with their respective binding energies.The Ti 2p spectra of b-CZIF-67/MXene results show a slight shift toward higher binding energy, as a result of the coupling effect between ZIF and MXene.To understand the resultant variation in the subspecies of obtained spectra these were further analysed by using XPSCasa software.The C 1s spectra of MXene were deconvoluted into five significant peaks (Figure 2G).The peaks at 281.85, and 282.55 eV  45 The high percentage of C-N peaks illustrates the conducting nature of the catalyst which is beneficial for ORR as it absorbs more O 2 molecules. 28The existence of π-π* suggests a strong conjugated structure between MXene and ZIF which is beneficial for electrocatalytic activity due to their structural transformation.The core-level Ti-2p spectra of MXene split into another five peaks corresponding to 457.02/464.1 eV (Ti-C) and 457.1/461.67/465.3 eV (Ti-O) binding energies (Figure 2I).It has been reported that particularly Ti-O bond is favourable for O 2 adsorption in the ORR process. 44The newly found Ti-N (458.71/464.05eV) and Ti-C-Co x (462.7 eV) in the composite material (Figure 2J) further elucidates the formation of interfacial heterojunction between marched cuboid ZIF onto/in between MXene, which improves the stability of the catalyst. 1This newly formed bond (Ti-C-Co x ) suggests that Co-ZIF gets adhered on MXene which limits the migration and aggregation of active species thus maintaining uniform distribution, and that is found to modulate the electronic structure and hence the enhanced activity of the catalyst. 46The in-situ synthesis, followed by pyrolysis of the b-CZIF-67/ MXene catalyst leads to the incorporation of N into the newly formed electronic structure with an atomic content of 4.70% for b-CZIF-67/MXene.Figure 2K illustrates the obtained high-resolution N 1 s spectra of b-CZIF-67/MXene further deconvoluted into five peaks with their respective binding energies namely, pyridinic N (Pyr-N), pyrrolic N (Pyro-N), Metal-N (Co-N), graphitic N (gra-N) and oxidized N (N-O) at 398.81, 399.67, 400.66, 401.79 and 404.05 eV, respectively.Each bond represents special significance toward the electrochemical activity of the catalyst.The high percentage of pyre-N increases the electron density near heteroatom, which could be the possible reason for enhanced ORR performance. 47ccording to the reports, pyro-N improves the Lewis acid-base interaction and improves the binding energy and affinity of polar titanium further improving the electron transfer mechanism. 48The presence of the N-O bond helps in improving the charge transfer kinetics of the electrode materials. 6As shown in Figure 2L the sharp peaks located at 778.03 and 793.3 eV correspond to Co 2p 1/2 and Co 2p 3/2 , 49 when further deconvoluted, it illustrates the presence of metallic Co and active moieties (Co-Nx).Moreover, it is demonstrated as N doping into MXene which adds extra hydrophilic character and results in improving the wettability of the electrode, this further helps to enhance the performance by introducing the electron-donor properties. 6,50

| Electrocatalytic ORR activity in acidic and alkaline mediums
The ORR performance of the as-synthesized catalyst was first evaluated in O 2 -saturated 0.5 M H 2 SO 4 by using a typical rotating disc electrode (RDE) in a three-electrode system.For comparison commercial Pt/C, pristine MXene and b-ZIF-67 were also tested in identical conditions.As observed from LSV curves resulted in Figure 3A and Supporting Information: Figure S3, IT is clear that b-CZIF-67/MXene exhibits enhanced catalyst activity with positive onset (E onset ) = 0.91 V and high half-wave potential (E 1/2 ) of 0.81 V at the current density (j) of 5.56 mA/cm 2 compared to Pt/C (E onset = 0.917, E 1/2 = 0.798 V and j = 5.4 mA/cm 2 ).Notably, the obtained results in this work are significantly higher than those recently reported MXene-based catalysts as summarized in Supporting Information: Table S3.This performance is attributed to the strong electronic interaction between MXene and ZIF-67, which avoids aggregation of active sites and offers extra active centres owing to the bi-ligand mechanism.To obtain the available active surface area (ECSA) of the catalyst, first, the double layer capacitance (C dl ) was calculated as explained in Supporting Information: Equation S4 as it shows the direct linear relationship with ECSA (Supporting Information: Equation S5).To evaluate C dl , CV curves were recorded in the faradic region within the potential range of 0.25-0.45V RHE, as shown in Figure 3B and Supporting Information: Figure S4.The obtained results show a higher C dl value (Figure 3C) of 79.5 mF/cm 2 for b-CZIF-67/MXene catalyst, while 3.56 and 6.1 mF/cm 2 for b-ZIF67 and Mxene, respectively.To calculate the total number of active centres participating in catalytic activity, first, the surface charge was obtained by integrating the CV curve in the complete potential range (Supporting Information: Equation S7).This directs the evaluation of active sites that contribute to forming all the active centres.The obtained value for active site density (ASD) was 9.42 × 10 23 g −1 which is in accordance with ECSA (2271.4cm 2 ), and the roughness factor (11574).The enhanced intrinsic behaviour of the catalyst is further certified by calculating turnover frequency (TOF) by using Supporting Information: Equation S8, as a figure of merit irrespective of the active area. 14,51Notably, the obtained TOF of b-CZIF-67/MXene is 0.12 S −1 , which is significantly higher than the reported ORR catalyst (Supporting Information: Table S3).
The completion of the catalytic reaction was confirmed by its four-electron pathway in ORR performance, evaluated using the Kentucky-Levich (K-L) plot (Supporting Information: Equation S10) as shown in Figure 3D.The obtained plot illustrates good linearity (Supporting Information: Figure S5), confirming four electron pathways with a number of electron (n) value of n catalyst = 4 and n Pt/C = 3.9 respectively.The results imply that the mesoporous structure of the catalyst obtained as a result of bi-ligand linkers allows facile mass transfer thereafter boosting the reaction mechanism.Figure 3E illustrates a lower Tafel value of 73 mV/dec corresponding to b-CZIF-67/MXene than pristine MXene and Pt/C (99 and 78 mV/dec) further confirming enhanced reaction kinetics.The higher activity of b-CZIF-67/MXene is assigned to the synergistic effect between MXene and bi-ligand ZIF which allows a feasible mesoporous mass transfer channel.The high content of N pyri (attributed to bi-ligand precursors), Ti-O (attributed to MXene) and Co-N (as a result of metallic species from ZIF) all together boost the ORR performance of the catalyst.The chronoamperometric test was performed to examine the stability of the catalyst (Figure 3F).The results illustrate the superior stability of b-CZIF-67/MXene (for 50,000 s at 0.6 V) compared to Pt/C.This is mainly attributed to the noncorrosive and infusible nature of MXene support as compared to C support for Pt, which easily gets corroded in an acidic atmosphere.For Pt/C the current density was observed to decrease with time whereas b-CZIF-67/MXene shows less change in current density.To further support the structural stability of b-CZIF67/MXene, XRD and FESEM analysis were studied post stability measurement, as shown in Supporting Information: Figures S6  and S7.The obtained results demonstrate no loss in the crystallinity and in the morphology of the catalyst, this is mainly attributed to the high structural stability obtained due to the application of bi-ligand and strong conducting support that prevents catalyst agglomeration and maintains the sharp morphology.The atomic percentage obtained before and after the stability test illustrates a slight change in the active species that is, N, Co and Ti attributed to the long term stability of the catalyst.The inset in Figure 3F shows the methanol tolerance study of the catalyst in comparison with Pt/C.A drastic decrease in current density was observed in Pt/C after the methanol addition, while b-CZIF-67/MXene showed a negligible change.
Given the notable ORR performance in an acidic medium, the catalyst was further tested for its ORR activity in an alkaline medium (0.1 M KOH).As shown in Figure 3G, synthesized catalysts follow a similar trend that is, b-CZIF-67/MXene exhibits higher activity (E onset = 0.93 V, E 1/2 = 0.84 with j = 5.90 mA/cm 2 ) compared to Pt/C (E onset = 0.91 V, E 1/2 = 0.8 with j = 5 mA/cm 2 and pristine materials.For further perception, the LSV of the prepared catalyst was obtained at a different set of rotation speeds (400-1600 rpm), which shows that the current density increases with an increase in rotation speed as shown in Supporting Information: Figure S8.The corresponding C dl value for b-CZIF-67/MXene obtained from the faradic region CV curve (Figure 3H and Supporting Information: Figure S9) is 40.6 mF/cm 2 (Figure 3I), which further confirms the high ECSA value of 1015.5 cm 2 and roughness factor of 5171.9.The ECSA values obtained for b-ZIF67 and MXene are 5 and 8 mF/cm 2 respectively.Additionally, the n value obtained from the K-L plot (Figure 3J and Supporting Information: Figure S10) confirms the four electron pathways.This proves that like Pt/C (n = 3.96), b-CZIF-67/MXene also shows a four-electron pathway with an n value equal to 3.91, more inclined to the 4e -n pathway.The Tafel value for electrocatalyst was obtained in the following order for alkaline ORR, b-CZIF-67/MXene (68 mV/dec) <Pt/C (71 mV/dec) <MXene (106 mV/dec) <pristine ZIF (112 mV/dec), suggesting faster kinetics for b-CZIF-67/ MXene.Owing to its porous nature and faster kinetics, the chronoamperometric study was employed to study the stability of the catalyst.Figure 3I demonstrates the alkaline stability of the catalyst performed at 0.6 V for 50,000 s.The inset in Figure 3L shows the methanol tolerance study of the catalyst in comparison with Pt/C.A drastic decrease in current density was observed in Pt/C after the methanol addition, while b-CZIF-67/MXene showed a negligible change.Similar to the evaluation of active sites in an acidic medium, the active sites corresponding to b-CZIF-67/MXene in the alkaline medium are 1.11 × 10 24 g −1 and TOF of 0.107 S −1 .The overall results reveal excellent ORR activity of b-CZIF-67/MXene compared to other MXenebased materials reported earlier (Supporting Information: Table S3).The higher performance was elucidated by the ample active centres, uniformly distributed metal sites, high surface area and porous nature which all together attributed as a result of the electronic junction between MXene and bi-ligand ZIF.

| Electrocatalytic activity toward HER
The HER kinetics of the as-synthesized materials were examined in N 2 saturated 0.5 M H 2 SO 4 by using three electrodes set-up.To avoid Pt dissociation, a graphite rod was used as a counter electrode instead of platinum.MXene) shows an onset potential of 59 mV which is closest to benchmark Pt/C (21 mV) but far lower than b-ZIF-67 and MXene, indicating superior electrocatalytic activity.To comprehend the reaction mechanism, the Tafel slope was obtained by the linear fitting of the overpotential (η) vs log ( j) plot in the low η region (Figure 4B).Tafel slope The electrochemical HER and OER performance of as synthesized catalyst (A) HER polarisation curve in N 2 saturated 0.5 M H 2 SO 4 at10 mV/sec (B) Tafel slop as a function of η (C) chrono potentiometric stability test performed at η 10mA/cm 2 = 0.170 V (D) CV recorded in nonfaradic region (E) double layer capacitance plotted against current density Vs scan rate (F) TOF of the MXene based catalyst reported so far (G) OER polarisation curve of pristine MXene, ZIF, optimized catalyst, state-of-the-art Pt/C and RuO 2 recorded in O 2 saturated 0.1 M KOH at 10 mV/sec (H) Tafel plot associated with liner fitting (I) chronoamperometric stability test of catalyst recorded for 20,000 s at working potential of 1.43 V (η 10mA/cm 2), inset shows stability corresponding to 5000 CV cycles with respect to RuO 2 (J) CV recorded in nonfaradic region at scan rate from 20 to 100 mV/sec (K) double layer capacitance obtained with respect to anodic and cathodic current density verses scan rates (L) obtained onset values in comparison with MXene based electrocatalyst.HER, hydrogen evolution reaction.
obtained for the synthesized catalysts are 148, 159, 69 and 61 mV/dec for MXene, b-ZIF-67, commercial (Pt/C) and b-CZIF-67/MXene respectively.This indicates the Volmer-Heyrovsky mechanism of hydrogen evolution (H*+ H + + e -n → H 2 ).Whereas, pristine catalyst shows a sluggish Volmer mechanism (H + + e -n → H*) which is a ratedetermining step.Apparently, the hydrophilic nature of MXene resulted in the Volmer-Heyrovsky mechanism.It also improves the overall hydrophilicity of b-CZIF-67/ MXene, which aids in the smooth adoption and dissociation process of adsorbate that is, hydronium ions. 30This overall mechanism of the catalyst controlled by mesoporosity, hydrophilicity and high surface area leads to enhanced mass transport and makes it appropriate for electrocatalytic HER activity.Figure 4C illustrates the chrono-potentiometric stability study obtained for b-CZIF-67/MXene, the inset shows a feasible adsorptiondesorption process.The stability test was performed for 20 K, where the potential was found to be maintained at 0.170 V. To further understand the intrinsic electrochemical activity of the active sites, ECSA was analysed using electrochemical double-layer capacitance.The CV was recorded at various scan rates in the nonfaradic region for b-ZIF67, MXene and b-CZIF-67/MXene (as shown in Figure 4D and Supporting Information: Figure S11) to calculate C dl (Figure 4E) value.And the values obtained are 3, 6.02 and 10.1 mF/cm 2 , respectively.This is further used to calculate ECSA and hence the total number of active sites contributing to the catalytic activity.To further investigate the catalytic activity, TOF was calculated (Supporting Information: Equations S13-S16) as a function of electrochemically active sites which are 8.30 × 10 23 g −1 , in accordance with a higher ECSA value of 288.57cm 2 .Interestingly, at η = 0.170 V hybrid catalyst b-CZIF-67/ MXene shows a TOF value equal to 0.171 S −1 which is significantly higher than most of the reported catalysts as summarized in Supporting Information: Table S4.For more comparison purposes a graph of TOF was plotted against different MXene-reported materials (Figure 4F), illustrating higher TOF obtained in this work.

| Electrocatalytic activity toward OER
The synthesized electrode materials were also studied for OER electrocatalytic activity in 0.1 M KOH solution.The three-electrode system was utilized for this, where a graphite rod was used as a counter electrode.For better understanding, pristine MXene, ZIF, Pt/C and RuO 2 were also studied by maintaining ideal conditions.As shown in Figure 4G, B-CZIF-67/MXene shows higher OER activity with an onset potential of 1.416 V (Vs RHE) and the overpotential at 10 mA/cm 2 was obtained to evaluate more real value and precise overpotential (η = 1.44 V 10 mA/cm 2 ). 52 The onset potential obtained was 1.56 V for RuO 2 , 1.6 V for Pt/C, 1.68 and 1.63 V for MXene and pristine ZIF, respectively.The Tafel value is associated with the rate-determining step of the catalyst.The obtained Tafel value for b-CZIF-67/MXene is 74 mV/ dec, which is low compared to other electrocatalysts studied that is, for Pt/C and RuO 2 the values are 82 and 77 mV/dec respectively (Figure 4H), suggesting rapid OER kinetics.Further, a chronoamperometric stability test was performed for 20000 s which demonstrates negligible reduction in onset value, unlike state-of-theart RuO 2 (Figure 4I), further confirming the high stability of b-CZIF-67/MXene.Additionally, the cyclic stability was also performed for 5000 CV cycles and the polarization curve before and after cycles for b-CZIF-67/ MXene and RuO 2 were compared, as shown in the inset of Figure 4I.
The ECSA value was further obtained by recording CV in the nonfaradic region at different scan rates, as illustrated in (Figure 4J and Supporting Information: Figure S12).The obtained high value of C dl (12.05 mF/cm 2 ) for b-CZIF67/MXene (Figure 4K) in comparison with pristine b-ZIF67 (1.02 mF/cm 2 ) and MXene (4 mF/cm 2 ), confirms the high performance of the hybrid material.The intrinsic OER activity of b-CZIF-67/MXene was further tested by calculating TOF as a function of active sites.The total number of electrochemically active sites was calculated to be 3.90 × 10 23 .A TOF value of 0.023 S −1 was obtained for b-CZIF-67/ MXene which is significantly higher compared to the previously reported catalyst, as illustrated in Figure 4L and as summarized in Supporting Information: Table S5.This finding is strongly ascribed to the electronic interaction of MXene with bi-ligand doped ZIF, 53 which introduces a high number of active sites and improves its porosity.

| ZAB performance and overall water splitting of b-CZIF-67/MXene
Considering the excellent activity and stability of b-CZIF-67/MXene toward ORR, HER and OER, the catalyst was assembled for water splitting and zinc-air battery application.The cathode catalyst was prepared by a conventional drop casting method with a total loading of 2 mg/cm 2 on carbon paper as the substrate.Zinc foil was used as an anode with 6 M KOH and 0.2 M zinc acetate as electrolyte.The electrode-electrolyte assembly was packed in a pouch cell using polythene followed by hot pressing.Figure 5A shows the polarisation curve and power density for b-CZIF-67/MXene and performance compared with Pt/C as cathode using other components.Compared to ZAB assembled with Pt/C, the air battery with b-CZIF-67/MXene as an electrode displays higher power density and specific capacitance.In comparison to Pt/C, which has a power density of 79.51 W/kg and an energy density of 400.2 Wh/kg, the b-CZIF-67/MXene cathode exhibits a power density of 117.5 W/kg with an energy density of 700 Wh/kg.In addition, when cycled for 600 min at 5 mA/cm 2 (Figure 5B), hardly any potential drop was observed for the b-CZIF-67/MXene based battery suggesting its better electrocatalytic activity and long-term stability as a cathode electrocatalyst.To calculate the specific charge storage performance, a galvanostatic discharge test was performed at 5 mA/cm 2 (Figure 5C) which demonstrates the superior value for b-CZIF-67/MXene (550.6 mAh/g) than Pt/C which is 498.7 mAh/g.The inset of Figure 5C demonstrates the charging curve of the catalyst for the ZAB battery in comparison with Pt/C.Clearly, with these excellent values and with high open circuit potential (1.45 V for a single pouch), when two pouch cells were connected in series, it easily glows the LED (Figure 5D,E).These results clearly indicate the practical application of flexible and rechargeable ZAB.
To study overall water splitting, a two-electrode electrolyser was assembled by using a b-CZIF-67/MXenebased cathode and anode electrode via the drop-casting method.For comparison purposes, commercial RuO 2 ||Pt/C was also utilized for this study.As illustrated in Figure 5F, B-CZIF-67/MXene|| b-CZIF-67/MXene requires a bias voltage of only 1.49 V to obtain a current density of 10 mA/cm 2 , unlike Pt/c||RuO 2 for which potential of 1.6 V is required.This illustrates the superior activity of the obtained catalyst for overall water splitting.The electrochemical cell with b-CZIF-67/MXene|| b-CZIF-67/MXene illustrates the vital bubbling of hydrogen and oxygen at the electrode surface as shown in the inset of Figure 5F.These vigorously released bubbles substantiate rapid mass transfer rate, demonstrating the promising feature of the catalyst towards water electrolysis.This was found to be in agreement with the faradic efficiency (Supporting Information: Equations S11,S12) of b-CZIF67/MXene.To calculate the Faradic efficiency of the catalyst, the evolved gases were collected by using a lab-made set-up based on the water displacement method.As shown in Supporting Information: Figure S13, the obtained Faradic efficiency which is the ratio of the volume of experimental to theoretical evolved gases was found to be ~80%, indicating a large part of energy was utilized for the generation of H during the water splitting.Additionally, the catalyst exhibits excellent stability for a prolonged chronoamperometric response of 10 h at an applied potential of 1.49 V showing great stability of b-CZIF-67/MXene with a negligible shift in biased voltage (Figure 5G).

| CONCLUSION
Herein we report MXene supported bi-ligand ZIF-67 as a high-performance trifunctional electrocatalyst.This unique approach was used to form a sharp, uniform mesoporous electrode structure that prevents structural decomposition and agglomeration of metallic active species.A remarkable trifunctional activity was achieved as a result of the synergistic effect between conducting MXene support and accessible active centres incorporated in the carbonized ZIF67 framework.The hybrid catalyst with high conductivity and optimized structure endorses an electro-transfer rate along with reduced mass transfer resistance.ZIF67 supported MXene hybrid composite shows the unique morphological and structural advantage of enhanced electrochemical activity for ORR, HER and OER.The ORR activity shows predominantly preferable 4 electron reduction behaviour with a very low overpotential for ORR with the onset of 0.91 and 0.93 V in acidic and alkaline medium compared to Pt.A high TOF of 0.12 S −1 is obtained for b-CZIF-67/MXene for ORR in an acidic environment and 0.107 in an alkaline.The Volmer-Heyrovsky mechanism is followed for HER activity with low overpotential (η = 0.170 V 10 mA/cm 2 ).And low onset potential in the case of OER with an overpotential of (η = 1.43 V 10 mA/cm 2 ) is obtained compared to the commercial catalysts (1.56 V for RuO 2 ).This electrocatalyst shows TOF of 0.162 and 0.171 for HER and OER, respectively.In conclusion, our study paves a new way to design a novel ZIF-MXene-based nonprecious metal electrocatalyst as trifunctional ORR-HER and OER for next-generation energy storage and conversion devices.

F
I G U R E 2 XRD pattern of as-synthesized catalyst (A) MAX phase and MXene (Ti 3 C 2 T x ) (B) bare b-ZIF-67 (C) b-CZIF-67/MXene (D) BET adsorption-desorption measurement performed in N 2 at 77 K (E) BJH pore size distribution analysis (F) XPS spectra obtained for MXene and b-CZIF-67/MXene.Deconvoluted XPS peaks corresponding to (G) C 1 s spectra of MXene (H) C 1 s spectra of catalyst (I) Ti 2p spectra of MXene (J) Ti 2p spectra of catalyst (K) N 1 s spectra of b-CZIF-67/MXene (L) Co 2p spectra of b-CZIF-67/MXene.correspond to the sign C-Ti, while the peaks obtained at 284.95 eV are ascribed to C-C which is mainly formed during MXene synthesis.The remaining two peaks obtained at 286.8 and 288.98 eV correspond to C-O and C=O respectively.As illustrated in Figure 2H the C 1 s spectra of the b-CZIF-67/MXene show two extra peaks located at 286.03 and 290.77eV (C-N and π-π*), confirming the decomposition of the carbon group and incorporation of N moieties at high temperature by forming an unsaturated bond with ZIF group.

F I G U R E 3
The ORR performance of b-CZIF-67/MXene in the acidic and alkaline medium in comparison with state-of-the-art Pt/C (A) ORR LSV polarisation curve recorded in O 2 saturated 0.5 M H 2 SO 4 at 1 mV/sec (B) CV in double layer region at scan rate from 20 to 100 mV/sec (C) double layer capacitance obtained with respect to anodic and cathodic current density versus scan rates (D) K-L plot (E) Tafel plot associated with linear fitting (F) chronoamperometric stability test recorded for 50,000 s at 0.6 V Vs RHE with respect to Pt/C, the inset shows methanol tolerance of catalyst studied for 3 M methanol.(G-H) represents the ORR study recorded in 0.1 M KOH medium, where G) LSV curve at 10 mV/sec H) CV in double layer region at scan rate from 20 to 100 mV/sec (I) Double layer capacitance study (J) K-L plot K) Tafel plot L) Chronoamperometric stability study, inset figure represents methanol tolerance of catalyst studied for 3M methanol.

2 and O 2 F
I G U R E 5 Performance of ZAB and overall water splitting (A) polarisation curve of the optimized catalyst with respect to power density (B) charging and discharging profile of rechargeable ZAB corresponding to final catalyst recorded at 5 mA/cm 2 (C) specific capacity in comparison with Pt/C, inset shows charging curve of the catalyst in comparison with Pt/C (D) digital photograph showing obtained open circuit potential for single ZAB and (E) LED illumed by ZAB made up of hybrid catalyst (F) polarisation curve corresponding to overall water splitting, inset shows respective digital demonstration for the same (G) stability performance recorded before and after 10 h for commercial and as-synthesized catalyst.ZAB, zinc-air battery.
Atomic content obtained from XPS analysis.
T A B L E 1