MWCNT-modified MXene as cost-effective efficient bifunctional catalyst for overall water splitting

Utilization of cost-effective, bifunctional, and efficient electrocatalysts for complete water splitting is desirable for sustainable clean hydrogen energy. In last decade, MXenes, a family of emerging two-dimensional (2D) materials with unique physiochemical properties, enticed scientists because of their use in different applications. However, insufficient electron transport, lower intrinsic chemical activity and limited active site densities are the factors inhibiting their use in electrocatalytic cells for hydrogen production. Here, we have presented material design to address this issue and introduced carbon nanotubes (CNTs) on V2CTx MXene sheets for conductive network channels that enhance the ion diffusion for enhanced electrochemical activity. The SEM reveals the uniform dispersion of the MWCNTs over the MXene surface that resulted in the formation of conductive network channels and enhances reaction kinetics. The as-synthesized electrocatalyst was subjected to linear sweep voltammetry (LSV) measurements for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The hybrid catalyst M2 exhibited an enhanced HER activity with a lower over-potential of 27 mV which is comparable to commercially available Pt-based catalysts (32 mV). Similarly, an enhanced OER activity was observed with a lower over-potential of 469 mV as compared to pristine V2CTx MXene. The electrocatalyst was subjected to a durability test through chronoamperometry and was observed to be stable for 16 hours. Hence, this study opens a new avenue for future cost-effective efficient catalysts for overall water splitting as a solution to produce clean hydrogen.


Introduction
Global warming, increased carbon emission causing environmental hazards and shortage of renewable energy resources result in an ever growing demand for green energy production and conversion. Hydrogen fuel is considered to be the most green energy carrier to address the energy crises and environmental issues. [1][2][3] Currently, hydrogen fuel is produced by the steam process via burning of fossil fuels. However, during this process, CO 2 is produced causing environmental pollution. [4][5][6][7][8][9] Electrochemical water splitting is considered as an advanced clean energy technology for hydrogen production. 10,11 Hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode are the two key reactions proceeding in an electrocatalytic cell for the generation of hydrogen (H 2 ) and oxygen (O 2 ), respectively. Precious metals (Pt, Ir, Ru) or metal oxides (Ru 2 O, IrO) are considered as efficient catalysts for electrochemical water splitting but their high cost and unavailability limit their commercialization and industrial use. [12][13][14] Moreover, it is very difficult to achieve high HER and OER performance simultaneously using a single precious metal hence, there is a need to develop a cost effective, non-precious metal, bifunctional, durable catalyst for overall water splitting. In the past, transition metal chalcogenides (TMDC), 15,16 oxides, [17][18][19][20][21] phosphides, [22][23][24][25][26][27] nitrides, 28-31 oxy hydroxides, 32,33 carbides 13,34 and metal free hybrids 35,36 have been thoroughly investigated for low-cost and effective electrocatalytic activity.
In recent years, a new family of 2D early transition metal carbides and nitrides known as MXenes have attracted much attention for their unique physical, optical, electronic, optoelectronic, thermal and magnetic properties. 37 MXenes have general formula of M n X n+1 T x where M is early transition metal, X is carbon or nitrogen and T x are the surface terminations like -OH, -F, -Cl or -O. This rich surface chemistry and high surface area enabled them as potential candidates for energy storage and conversion systems. MXenes are synthesized via wet chemical etching of A layer with hydrouoric acid or salt soln. containing uorine from the MAX phases. Where A is either Al or Si an element of group III A or IV A. M is early transition metal and X is carbon or nitrogen. 38 MXenes have been widely explored for their use in lithium-ion-battery (LIBS) 39 and supercapacitors. [40][41][42] Due to the active surface functionality and chemical inertness in electrochemical potential, MXene is predicted to be a heterogeneous catalyst. 43,44 However, very limited literature is available for the catalytic behavior of MXene. Only a few MXenes (Ti 3 C 2 T x , Mo 2 TiC 2 T x , Nb 2 CT x , V 4 C 3 T x , Mo 2 CT x ) out of the largest known 2D family have been explored for catalytic activities. [45][46][47][48][49][50][51] Contrary to the theoretical calculation, 43 experimental studies reveal that Ti 3 C 2 T x and V 4 C 3 T x have poor HER performance with an over-potential of 600 mV (ref. 46 and 49) than Mo-based MXene (Mo 2 CT x ). 46 Therefore, many research groups are attempting to improve the catalytic performance of MXene by modication of terminal groups so that it can be used as bifunctional catalyst for overall (for both HER and OER activities) water splitting. Due to high H-binding strength (high Gibbs free energy) of oxygen, vulnerable vanadium based MXene shows a poor HER performance. 47,52 So, the use of V 2 CT x as an efficient catalyst is still a challenge.
Previous research shows that the introduction of the CNTs can effectively improve the conductivity of the Ti 3 C 2 T x . 57,62-67 Their unique hollow geometry offers a high specic area that makes them suitable support for heterogeneous catalysis. One more important feature of carbon nanotube is relatively high oxidation stability due to their chemically inert nature. 67 In this study, a facile method was adopted for the modication of V 2 CT x MXene by introducing MWCNT for its effective use as an electrochemical catalyst. The MWCNT forms a uniform networking over the surface of the MXene sheets preventing oxidation as well as are intercalated between them that enhances the electrical conductivity and provides more active sites for the ion diffusion hence, resulting in an increased electrocatalytic activity. The modied MXene not only exhibits excellent HER activity comparable to precious metal industrial catalysts but also presented an enhanced OER hence, showed bifunctional catalyst for the overall water splitting.

Experimental section
Synthesis of V 2 CT x MXene and MWCNT@V 2 CT x MXene hybrid Fig. 1 represents the schematic for the synthesis of MXene treating MAX and MWCNT decorated MXene. The V 2 CT x MXene was synthesized from the V 2 AlC MAX phase by wet chemical etching process. Typically, 1 g of MAX powder was added to the 15 ml of 50% Hydrouoric Acid (HF) and kept on stirring at 200 rpm in a Teon lined bottle for 90 hours at room-temperature. Then the mixture was centrifuged at 3500 rpm/5 min and washed several times with DI water until the PH reaches up to 6 followed by vacuum ltration to obtain multilayer (ML) V 2 CT x .
Co-precipitation route was opted for the synthesis of MWCNT/MXene hybrid. Cetyl Trimethyl Ammonium Bromide (CTAB) graed MWCNT was prepared by dissolving 2 mg of CTAB in 20 ml DI water via magnetic stirring to achieve a uniform aqueous solution. Aer that, MWCNTs were dispersed in CTAB solution via probe sonication to a concentration of 0.5 mg ml À1 . Briey, 20 ml of CTAB graed MWCNT solution was added dropwise to 50 ml (0.4 mg ml À1 ) of V 2 CT x solution. The resultant mixture was probe sonicated for 10 minutes followed by vacuum ltration and vacuum drying at 50 C overnight to get a hybrid material. In a similar fashion, four ratios 1 : 1, 1 : 2, 0.5 : 2, and 0.25 : 2 of MWCNT : MXene samples were prepared labelled as M1, M2, M3, and M4.

Material characterization
The crystal structure and phase identication were carried out using powder X-ray diffraction (DRON-8) diffractometer equipped with Cu K-a (l ¼ 0.154 nm) source in the 2q range of 3-70 . VEGA3/TESCAN 51-ADD007 scanning electron microscope (SEM) was used for the study of surface morphology. Transmission electron microscopy (TEM) analysis was carried out to further investigate the morphology. The high-resolution images were acquired on Titan 60-300 from Thermo Fischer Scientic equipped with an imaging Cs-corrector and working at 300 kV.
Electrochemical measurements. Gamry 1010B potentiostat workstation was used for the electrochemical testing in a threeelectrode conguration using Pt wire as counter electrode and  Ag/AgCl as a reference electrode in 3.5 M KCl. The working electrode was fabricated on Ni foam by dispersing 10 mg of assynthesized MWCNT@V 2 CT x hybrid catalyst in 500 ml DI water, 450 ml isopropyl alcohol and 50 ml Naon (wt 5%) by ultrasonication for 35 min. Then 100 ml ink was spread on cleaned NF (1 cm Â 1 cm) surface with a loading of 2-3 mg cm À2 . The prepared electrodes were overnight vacuum-dried at roomtemperature. The dried electrodes were pressed under 5-10 MPa pressure for 10 seconds. Linear Sweep Voltammetry (LSV) towards HER and OER activity was performed at constant scan rate of 10 mV s À1 in 1 molar KOH. All the measured potentials against Ag/AgCl were converted to RHE based on the  formula of Nernst equation: E RHE ¼ E Ag/AgCl + 0.059pH + 0.1976. The CV voltammograms were recorded in the voltage range of À0.4 to 0.7 V under a scan rate range of 2 mV s À1 to 200 mV s À1 . The EIS was acquired at open circuit potential (OCP) in a frequency range of 20 kHz to 10 MHz using a sinusoidal signal of 10 mV. The long term durability was tested through chronoamperometry at 0.6 V.

Results and discussions
Analysis of crystallographic structure of V 2 CT x MXene and MWCNTs/V 2 CT x MXene hybrid are shown in Fig. 2a and b, respectively. The most intense diffraction peak at 2q ¼ 41.3 is reduced aer the HF treatment conrming the successful removal of Al layers. In addition, the shiing and broadening of (002) peak of MAX at 2q ¼ 13.4 to 2q ¼ 8.9 indicates the high caxis orientation and an increased interlayer spacing (JCPDS no. 29-0101). 53 It is obvious that the two signicant peaks of MWCNTs at 2q ¼ 25.3 and 2q ¼ 42.6 (ref. 54 and 55) and the signicant diffraction peaks of MXene remain intact during the hybrid fabrication, indicating its successful fabrication. The (002) plane of V 2 CT x MXene further shis towards a lower angle of 2q ¼ 8.24 with introduction of the carbon nanotubes. To investigate whether the MWCNT were successfully graed on the V 2 CT x MXene, the scanning electron microscopy (SEM) was carried out. Fig. 3a shows the SEM of MAX phase revealing the multilayer crystalline structure. While in Fig. 3b, the layers are separated forming an accordion-like structure that given an evidence of selective etching of Al aer the acidic treatment. 56 The MWCNTs forms a uniform network over the MXene sheets as can be observed from Fig. 3d; the inset represents a closer look of networking. 57 The high-resolution transmission electron microscopy (HRTEM) is in good agreement of SEM micrographs revealing the separated basal planes of MXene (Fig. 3e). The sheets are more separated out conrming the intercalation of MWCNTs in hybrid structure (Fig. 3f). The selective area electron diffraction (SAED) (inset of Fig. 3e and f) reveals that the basal planes maintained the primitive hexagonal structure of parent MAX phase aer the acidic treatment as well the hybrid formation.

Electrochemical measurements
The electrocatalytic performance was estimated via HER and OER studies in 1 molar KOH using 3-electrode system. Where the assynthesized pristine V 2 CT x and MWCNT@V 2 CT x catalysts were used as working electrode, platinum wire and Ag/AgCl were used as counter and reference electrode, respectively. To minimize the interference of the capacitive current i R , corrected linear sweep voltammetry was acquired at a low scan rate of 10 mV s À1 . Fig. 4a represents the polarization curves for OER catalytic activity of V 2 CT x MXene and MWCNT@V 2 CT x hybrid. The hybrid material possesses lower onset potential and overpotential as compared to the pristine MXene at a current density of 10 mA cm À2 (h 10 ). From Fig. 4c, it can be observed that the over-potentials for M1, M2, M3 and M4 are 560 mV, 469 mV, 562 mV and 570 mV, respectively are lower than the pristine MXene (652 mV @ h 10 ). The higher OER activity of the hybrid is attributed to the enhanced surface area of carbon nanotubes along with the formation of uniform conductive channels for the fast ion transportation through electrode/ electrolyte interface, resulting in an improved electrochemical performance. With the increasing concentration of MWCNTs, the OER activity decreases which could be attributed to the overloading and blocking of active sites. Furthermore, the Tafel plot, log(jjj) versus over-potential (h) was employed to verify the electrocatalytic kinetics of OER. The small Tafel slope (77 mV dec À1 ) of MWCNT@V 2 CT x hybrid sample M2 is lower than the pristine MXene (116 mV dec À1 ) representing the synergetic effects between V 2 CT x MXene and MWCNT (Fig. 4b). 68 The 103 mV dec À1 , 92 mV dec À1 , and 75 mV dec À1 are the Tafel slope values for M1, M3 and M4, respectively.

OER activity
The durability of the electrode is a key parameter to evaluate the performance of electrocatalyst. The catalyst was subjected to continuous cyclic voltammetry for stability test and the polarization curves were plotted aer 1000 CV cycles. There was no signicant change in the initial and nal over-potentials aer 1000 cycles conrming high durability of hybrid catalyst. Fig. 5a represents the LSV polarization curves for HER activity of pristine V 2 CT x MXene and MWCNT@V 2 CT x . It can be observed from the graphs that V 2 CT x shows an average HER activity with an over-potential of 77 mV while the hybrid posses 27 mV that is comparable to commercial Pt catalysis i.e., 32 mV.

HER activity
The lower Tafel slope of 44 mV dec À1 (Fig. 5b) shows an outstanding HER kinetics obeying Volmer-Heyrovsky mechanism.
Volmer step: where, M is the active metal site, during the Volmer process in an alkaline solution M-H ads intermediate is formed when hydrogen is released from the catalyst surface followed by the desorption process resulted in hydrogen gas evolution (Heyrovsky process). This outstanding catalytic performance is the result of synergetic effects between V 2 CT x and multiwall carbon nanotubes along with the conductive support provided by Ni foam. The porous Ni foam provides the smaller pathways for the movement of ions. 58,59 The MWCNT forms a uniform conductive network that facilitates the ion diffusion and provide large surface area that results in the high catalytic activity. The durability is the key for the industrial scale considerations. The catalyst was subjected to 1000 cycles, and it is observed that the catalyst is stable and there is a very minor change in the  catalytic activity (Fig. 5d). An over-potential of 28 mV was achieved which shows high durability of MWCNT@V 2 CT x as shown in Fig. 5c.

Stability test
From Fig. 6a, it can be observed from the Nyquist plots that the hybrid material has the lowest charge transfer resistance of 245.4 U as compared to pristine MXene (1.502 Â 10 3 ). This result conrms that layered V 2 CT x MXene with high conductivity and with active sites facilitates the fast charge transfer resulting in an efficient catalytic activity because of its strong uniform interfacial linkage with MWCNTS thus, the charge transfer behavior is immensely enhanced. 60,61 The cyclic voltammograms of the hybrid at scan rates of 10 mV s À1 , 100 mV s À1 and 200 mV s À1 are shown in inset of Fig. 6b. It can be seen from the shape of the curve that the total capacitance is the sum of EDLC capacitance of MWCNTs and pseudo-capacitance of V 2 CT x MXene. As MWCNT provides conductive channels for the fast ion diffusion and their increased surface are enhances the active site availability hence an increased electrochemical activity. The redox peaks are visible in the potential range of 2-3.1 V. The slight shiing of the peaks can be observed due to faradaic activity while the shape of the curve is maintained even at higher scan rates showing the symmetry and conrming the reversibility of the electrode material. The current density increases with an increase in the scan rate due to small diffusion resistance. An increase in the redox peak at higher scan rate is attributed to surface conned faradaic process that contributes to the stability of the electrode. The electron impedance spectroscopy EIS was carried out within the frequency range of 0.1 Hz to 20 kHz at a constant AC potential of 10 mV. Since, the stability of electrochemical device is a key parameter for the commercialization of the catalyst, for this purpose, the chronoamperometry was carried out at 0.6 V for 16 h which shows a very small change in current for MWCNT@V 2 CT x conrming high stability of the catalyst. The EIS data was subject to Randle's model tting with additional Warburg element for tting purposes. Table 1 shows a comprehensive analysis for the different components of equivalent circuit conrming the low charge transfer resistance of MWCNT modied V 2 CT x i.e. 245.4 U as compared to pristine MXene with 1.502 Â 10 3 U thus responsible for fast kinetics of the reaction.
A comparison of this research study to MXene-based electrocatalysts is given in Table 2.

Summary
In summary, a novel MWCNT-MXene heterostructure was constructed for efficient bifunctional electrochemical catalyst for an overall water splitting. For comparison, pristine MXene catalyst was also synthesized and characterized. MWCNT@V 2 -CT x showed a remarkable HER activity and an OER performance superior to pristine MXene. This enhanced performance may be attributed to the high conductivity of MXene sheets holding many active sites and uniform networking of MWCNTS over the surface, enhancing charge transfer ability and aggregation prevention. This work reveals that MWCNT@V 2 CT x has a potential to replace commercial noble metal electrocatalysts. Moreover, this study opens the doors for other construction of MWCNT hybrids of MXene based materials for efficient bifunctional electrocatalysis for an overall water splitting.

Author contributions
Syedah Afsheen Zahra performed experimentation and paper writing; and Syed Rizwan conceived the research concept, helped in paper writing, and supervised the complete project.

Conflicts of interest
There are no conicts to declare.