Ni Doped Co-MOF-74 Synergized with 2D Ti 3 C 2 T x MXene as an Efficient Electrocatalyst for Overall Water-Splitting

: Metal-organic framework (MOF)-based materials with abundant pore structure, large specific surface area, and atomically dispersed metal centers are considered as potential electrocatalysts for oxygen-evolution reaction (OER), while their ligand-saturated metal nodes are inert to electro-catalysis. In this work, heteroatom doping and interface engineering are proposed to improve the OER performance of Co-MOF-74. Using two-dimensional Ti 3 C 2 T x MXene as a conductive support, Ni-doped Co-MOF-74 (CoNi-MOF-74/MXene/NF) was in situ synthesized through a hydrothermal process, which exhibits excellent OER and hydrogen evolution reaction (HER) properties. For OER, the CoNi-MOF-74/MXene/NF achieves a current density of 100 mA/cm 2 at an overpotential of only 256 mV, and a Tafel slope of 40.21 mV/dec. When used for HER catalysis, the current density of 10 mA/cm 2 is reached at only 102 mV for the CoNi-MOF-74/MXene/NF. In addition, the two-electrode electrolyzer with CoNi-MOF-74/MXene/NF as both the cathode and anode only requires 1.49 V to reach the current density of 10 mA/cm 2 . This work provides a new approach for the development of bimetallic MOF-based electrocatalysts.


Introduction
Electrocatalytic water splitting offers an appealing, rapid, and sustainable pathway for producing hydrogen in a clean manner, reducing reliance on conventional fossil fuels and contributing to a healthier environment [1][2][3].However, both half reactions in the water splitting process, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), suffer from sluggish reaction kinetics due to their multi-step proton coupled electron transfer processes [4].Precious metal catalysts, such as Pt/C and IrO 2 , are highly coveted for their ability to function as electrocatalysts for both HER and OER.These catalysts exhibit ultralow overpotential and exceptional long-term durability, making them ideal for industrial purposes.Unfortunately, the rarity and high cost of these materials render electrocatalytic water splitting a costly process [1,5].Therefore, there is a crucial need to develop affordable and durable electrocatalysts based on non-noble metals [6].
Recently, metal-organic frameworks (MOFs) have become known for their high porosity, large surface area, customizable composition, abundant active sites, and well-ordered nanostructures that facilitate effective ionic and electron transfer [7].MOFs are structures that consist of organic ligands and metal ions or clusters coordinated together [8].Because there are multiple metal ions and organic ligands with unique properties, they can coordinate with each other to form a wide range of MOFs [9].For MOFs, it is not only suitable for alkaline environments but also a perfect platform for designing efficient and cost-effective acidic OER catalysts in acidic environments.For example, Xu et al. [10] provide a comprehensive review of recent advances in MOF-based acidic OER catalysts and discuss some catalyst design strategies aimed at improving the acidic OER catalytic performance of MOF-based candidate catalysts.Among them, MOF-74 stands out with a porous channel, featuring a unique one-dimensional hexagonal shape.First synthesized by Yaghi's team in 2005, MOF-74 has the general formula M 2 (DOBDC) (M: Co, Ni, Zn, Mg, Mn, Fe, H 4 DOBDC: 2,5-dihydroxyterephthalic acid) [11].In the MOF-74 crystal structure, each metal ion is coordinated to five oxygen atoms, with an associated unsaturated coordination site [12].These properties have made it renowned for the divalent tunability of its metal nodes, which offers unlimited possibilities for electrocatalyst design in terms of topology, pore size and functionality [13].Among the various MOF-based OER catalysts, the Co-based MOFs have received a great deal of attention due to their high activity and stability [14,15].Theoretical calculations and experimental results indicate that incorporating one or more species can significantly improve electrocatalytic performance due to a synergistic effect.This includes improved charge transfer between dopants and host metal atoms and optimized electronic structures to reduce kinetic energy barriers during catalytic processes [16].Unfortunately, most MOFs are natively suffering from poor electrical conductivity [17].Therefore, the combination of MOFs and conductive nanostructures is expected to further enhance the performance in various electrocatalytic processes [8].
MXenes, an emerging group of 2D materials, are prepared through selective removal of the A layers from MAX phases (M is an early transition metal, A is an A group element, and X is either C and/or N) [18].They display excellent electrical conductivity, surface hydrophilicity, and stability.Additionally, MXenes, with low work function and electronegative surfaces, serve as promising substrates to modify the electrophilicity of active sites in catalysts, thereby regulating catalytic properties in multicomponent catalyst systems [8].
Herein, composite electrocatalysts with self-supported Ni-doped Co-MOF-74 on Ti 3 C 2 T x MXene (designated as CoNi-MOF-74/MXene/NF) were synthesized through a facile method consisting of electrostatic deposition and hydrothermal synthesis.This method provides a high degree of control over the synthesis process, resulting in efficient electrocatalysts.The incorporation of Ni modifies the electronic structure of Co sites in the MOF-74 and promotes the formation of high oxidation-state Co 3+ , ultimately improving the OER performance [6].Moreover, the robust interfacial interaction and electronic coupling between MOF and Ti 3 C 2 T x nanosheets facilitate fast charge transfer kinetics and provide a stable structure, both crucial factors in optimizing catalytic performance [19].The CoNi 0.04 -MOF-74/MXene/NF composite exhibits superior catalytic activity for both HER and OER in alkaline electrolytes, outperforming individual Co-MOF-74 and MXene at current densities of 10 mA•cm −2 and 100 mA•cm −2 , respectively.Specifically, the catalyst achieves a current density of 10 mA•cm −2 and 100 mA•cm −2 at overpotentials of 102 mV and 256 mV for HER and OER, respectively.When employed as both cathode and anode catalysts in the two-electrode electrolyzer, the CoNi 0.04 -MOF-74/MXene/NF heterostructure catalyst achieves a current density of 10 mA•cm −2 at 1.49 V in alkaline solution, surpassing the performance of a traditional Pt/C||IrO 2 couple.Remarkably, the electrolyzer maintains a nearly constant current density over 40 continuous hours, demonstrating its satisfactory long-term durability.These findings not only demonstrate the potential of CoNi 0.04 -MOF-74/MXene/NF for practical applications but also offer insights for the development of other efficient MOF-based composite catalysts.

Catalyst Synthesis and Characterization
Figure 1 schematically illustrates the synthetic process of CoNi-MOF-74/MXene/NF using the two-step method.Firstly, the Ti 3 C 2 T x MXene nanosheets prepared using the MILD method were electrostatically deposited on nickel foam (NF).Scanning electron microscopy (SEM) images show that Ti 3 C 2 T x MXene with fewer layers was successfully prepared (Figure S1, Supplementary Materials).Due to the strong electrostatic interaction between the negatively charged Ti 3 C 2 T x MXene nanosheets and the positively charged NF, Ti 3 C 2 T x MXene can easily adhere to the three-dimensional framework of NF, imparting the black color to the substrate.SEM images reveal the even distribution of Ti 3 C 2 T x MXene nanosheets on the NF skeleton (Figure 2a).After that, CoNi-MOF-74 was grown in situ on MXene/NF through a hydrothermal process.Taking advantage of the distinctive terminal groups of -OH and -F, the CoNi-MOF-74 can be successfully anchored to the surface of MXene.SEM observation shows that the bimetallic MOF-74 have the same identical crystal structure as the single metal MOF-74.In comparison with Co-MOF-74 (Figure 2c), all Ni-doped bimetallic MOF-74 show consistent and regular nanorod morphology (Figures 2b and S2 (Supplementary Materials)).Interestingly, a Co/Ni molar ratio of 25:1 results in a thinner and longer nanorod morphology for CoNi 0.04 -MOF-74/MXene/NF, compared to CoNi 0.1 -MOF-74/MXene/NF and CoNi 0.03 -MOF-74/MXene/NF.This may be attributed to the influence of Ni atoms, which may promote MOF crystal growth along specific lattice planes while limiting growth in other lattice planes [16].Figure 2b illustrates that the prepared CoNi-MOF-74 microrods exhibit a hexagonal structure and a smooth surface, with an average diameter of approximately 10 µm.Elemental mapping reveals a uniform dispersion of Co, Ni, O, and C within the rod-like structure of CoNi-MOF-74 (Figure 2d-h).Furthermore, the Energy-Dispersive X-ray Spectroscopy (EDS) analysis on CoNi 0.04 -MOF-74/MXene/NF confirms the presence of Ti element, indicating the successful integration of MOF with MXene (Figure S3, Supplementary Materials).X-ray diffraction (XRD) was used to characterize crystal structure of the as-synthesized catalysts.As shown in Figure S4 (Supplementary Materials), compared with the Ti 3 AlC 2 phase, the (104) diffraction peak corresponding to the Al layer completely disappears after etching, confirming the successful preparation of Mxene [20].Figure 3a displays the XRD patterns of the prepared CoNi-MOF-74 samples, which match well with the simulated peaks of MOF-74.Upon closer examination (Figure 3a), the diffraction peaks associated with the bimetallic MOFs shift to a smaller angle, indicating the successful introduction of Ni element into the MOF-74 backbone, resulting in a larger interplanar crystal spacing [6,21,22].Figure 3b presents the FT-IR spectra of the as-prepared MOF-74 and H 4 DOBDC for comprehending the coordination between metal ions and H 4 DOBDC.For Co-MOF-74 and bimetallic CoNi-MOF-74, the absorption peak observed above 800 cm −1 corresponds to the organic ligand 2,5-dihydroxyterephthalic acid vibration.In the low wavenumber region, the peak at 585 cm −1 indicates the M-O bond vibration, confirming the successful synthesis of CoNi-MOF-74 [7].S1, Supplementary Materials).Therefore, the incorporation of the Ni dopant can modify the electronic structure of Co species in Co-MOFs, resulting in a slightly higher oxidation state [24,25].Since Co 3+ is often considered as the main active site for OER, the increased content in this high oxidation state may be beneficial for enhancing OER catalytic activity [26][27][28].Furthermore, the Ti 2p high-resolution spectrum (Figure 4b) shows peaks at 458.3 eV/464.5 eV and 455.7 eV/461.3eV, attributed to Ti-O and Ti-C bonds, respectively [29].The C1s spectrum (Figure 4c S6, Supplementary Materials).This shift underscores the significant electronic interactions at the interface of CoNi 0.04 -MOF-74 and MXene, positively impacting the regulation of electrocatalytic activity.In the Raman spectrum of CoNi 0.04 -MOF-74/MXene/NF (Figure S7, Supplementary Materials), the stretching of the M-O (M=Co/Ni) bond was confirmed at 565.4 cm −1 , while the peak at 818.9 cm −1 is assigned to the C-H bending of the benzene ring [33].Additionally, the peaks at 1274.8 and 1409.6 cm −1 correspond to C=O stretching and O-C-O symmetric stretching, respectively [34].

Oxygen Evolution Reaction Performance
OER typically occurs via a four-electron process on the anode, but it is a slow process with low energy conversion efficiency, while the energy barrier peak represents a major hurdle for electrocatalytic water splitting [35].The slow reaction kinetics of OER involve the four-electron process of O=O bond formation.Low overpotential, high current density, and a low Tafel slope value constitute the primary criteria for indicating OER activity [36].
To reveal the kinetic behaviors of the catalysts, the smallest Tafel slope of 40.21 mV/dec is achieved by a CoNi 0.04 -MOF-74/MXene heterostructure compared with those of Co-MOF-74/MXene (67.31 mV/dec), CoNi 0.04 -MOF-74/NF (111.47 mV/dec) and MXene (165.09mV/dec), implying the fastest kinetics (Figure 5c).Furthermore, electrochemical impedance spectroscopy (EIS) was utilized to study the kinetics at the electrode/electrolyte interface of the prepared catalysts.Generally, the electrocatalytic kinetics are influenced by the charge transfer resistance (R ct ), with a lower R ct value indicating faster electronic transport.As shown in Figure 5d, the CoNi 0.04 -MOF-74/MXene heterostructure displays a smaller R ct compared to Co-MOF-74/MXene, CoNi 0.04 -MOF-74/NF and MXene (Table S3, Supplementary Materials), which correlates well with the Tafel slope results.R ct and R s denote charge transfer resistance and equivalent series resistance, respectively; W means Warburg impedance, and CPE is a constant phase element, which simulates the doublelayer capacitance.The electrochemically active surface areas (ECSAs) were estimated using the electrochemical double-layer capacitance (C dl ) of the catalysts recorded in the non-Faradaic region (0.644-0.744V vs. RHE).This method was chosen due to its linear relationship with ECSA in 1.0 M KOH (Figure S10, Supplementary Materials).Based on Figure 5e, the C dl value of CoNi 0.04 -MOF-74/MXene (18.13 mF cm −2 ) is higher than the values for Co-MOF-74/MXene (13.73 mF cm −2 ) and CoNi 0.04 -MOF-74/NF (10.43 mF cm −2 ).CoNi 0.04 -MOF-74/MXene displays a greater C dl value, indicating a larger electrochemically active surface area and an increased exposure of active sites.These results demonstrate that the hybridization of MOF-74 with Ti 3 C 2 T x MXene can enhance active site exposure, intrinsic activity, and charge transfer kinetics [45].
Stability is another important factor in the assessment of the commercial value of catalysts.The long-term durability of CoNi 0.04 -MOF-74/MXene was measured using chronopotentiometry at j = 100 mA/cm 2 .Even after 40 h of testing, the potential required only increases by 2%, indicating that CoNi 0.04 -MOF-74/MXene has good stability (Figure 5f).Furthermore, there is almost no discernible change in the LSV curve following 1000 cycles of cyclic voltammetry (Figure S11, Supplementary Materials).And the CoNi 0.04 -MOF-74/MXene electrode also maintains relatively intact structural characteristics after 1000 CV cycles.SEM observation confirms the remained micro-rod structure with some nanoflakes on the surface, suggesting good structural stability of the catalyst (Figure S12, Supplementary Materials).TEM characterization further confirms the retention of the micro-rod structure of CoNi 0.04 -MOF-74/MXene after OER and the presence of nanosheets on the surface (Figure S13a, Supplementary Materials).The lattice fringes observed in Figure S13b (Supplementary Materials), measuring 0.233 nm, 0.142 nm, and 0.154 nm, correspond to the (002) plane of Co(OH) 2 (PDF#30-0443) and CoOOH (PDF#26-0480), as well as the (003) crystal plane of Ni(OH) 2 (PDF#14-0117).In addition, selected area electron diffraction detects (101) crystal planes of Co(OH) 2 , and (002) and (131) crystal planes of CoOOH (Figure S13c, Supplementary Materials).
Moreover, the surface chemical composition of the post-OER catalyst was also analyzed using XPS characterization (Figure S14, Supplementary Materials).It is evident that the Co 3+ /Co 2+ ratio increases from 2.47 to 3.62, and the Ni 3+ /Ni 2+ ratio increases from 0.34 to 0.54 after OER test (Table S4, Supplementary Materials).This signifies that more reactive hydroxyl oxide substances were generated after OER, as supported by the TEM results.In addition, the increase in M-O content in the O 1s spectrum (Figure S14c, Supplementary Materials) is also corroborated by the production of hydroxyl oxides.In previous reports, Co(Ni)OOH has been identified as the true active species of OER [46][47][48].The formed Ni 3+ and Co 3+ on the surface possess more valence 3D electron orbitals, leading to higher electron affinity which enhances the adsorption of OER intermediates [46].Or, following the lattice oxygen mechanism, the formed Co(Ni)OO − on the catalyst surface may facilitate the evolution of oxygen molecule, boosting the intrinsic catalytic activity for OER [49,50].In the Ti 2p spectrum (Figure S14d, Supplementary Materials), the disappearance of the Ti-C bond and the increased presence of Ti-O species are attributed to the oxidation of MXene through a prolonged hydrolysis process.

Hydrogen Evolution Reaction Performance
To achieve high catalyst efficiency for HER in alkaline conditions, two crucial factors must be addressed.One is to improve conductivity.Another crucial aspect is the optimization of the ∆E H2O and ∆G H for water and hydrogen adsorption, respectively [51,52].
The HER performance of the as-synthesized catalysts was tested in an alkaline electrolytic cell with a 1 M KOH solution using a three-electrode system.All potentials were converted relative to the reversible hydrogen electrode (RHE) and 95% IR compensation was carried out.In Figure 6a, it can be observed that CoNi 0.04 -MOF-74/MXene exhibits superior HER activity with a low η 10 of only 102 mV, which is significantly lower than Co-MOF-74/MXene (175 mV), CoNi 0.04 -MOF-74/NF (203 mV) and MXene/NF (217 mV) (Figure 6b).The Tafel slope shown in Figure 6c is a significant index for the kinetics of the HER reaction.It is important to note that the mechanisms behind the OER and the HER reactions differ significantly.For the HER in basic media, three primary routes have been proposed, which are as follows:  The reaction routes have a close connection to the Tafel slope.The Tafel slope ranges from approximately 30 to 40 mV dec −1 when the rate-determining step (RDS) for HER is either the Heyrovsky or the Tafel step.If the Tafel slope is changed to ~120 mV dec −1 , then the RDS becomes the Volmer step [53].Herein, the Tafel slope of CoNi 0.04 -MOF-74/MXene (123.5 mV dec −1 ) is significantly lower than that of Co-MOF-74/MXene (135.62 mV dec −1 ), CoNi 0.04 -MOF-74/NF (146.8 mV dec −1 ) and MXene/NF (150.3 mV dec −1 ), indicating faster HER kinetics.Also, the Tafel slope value suggests that the Volmer step is the RDS for our catalysts.The great HER performance of CoNi 0.04 -MOF-74/MXene was bolstered by the synergetic collaboration of CoNi 0.04 -MOF-74 and MXene.In particular, MXene speeds up the Volmer step in HER through augmented catalyst interfacial water adsorption/activation and charged chemical synergistic interactions with MOF.The fantastic hydrophilicity and conductivity of MXene facilitate such interactions, which ultimately affect its HER performance [54,55].
The Nyquist curves display a semicircular shape, which indicates complete control of their electrode reaction processes through the charge transfer process (Figure 6d).In Figure 6d, the R ct of CoNi 0.04 -MOF-74/MXene is lower compared to Co-MOF-74/MXene, CoNi 0.04 -MOF-74/NF and MXene/NF, suggesting an accelerated charge transfer process for CoNi 0.04 -MOF-74/MXene (Table S5, Supplementary Materials).This finding is consistent with the Tafel analysis.Then, the double layer capacitance (C dl ) was measured by conducting CV scans at various scanning speeds within a narrow range of the nonfaradic region (0.1-0.2 V) (Figure S15, Supplementary Materials). Figure 6e shows that the CoNi 0.04 -MOF-74/MXene displays the highest C dl , suggesting that the hybridization of CoNi 0.04 -MOF-74 with MXene can enhance the ECSA of the material and offer a greater HER reaction site.In addition, the study also measured the average activity of the intrinsically active sites using the turn-over frequency (TOF).As shown in Figure S16 (Supplementary Materials), the TOF of CoNi 0.04 -MOF-74/MXene is 0.00224 s −1 at an overpotential of 100 mV, better than Co-MOF-74/MXene (0.00212 s −1 ) and CoNi 0.04 -MOF-74/NF (0.00076 s −1 ), suggesting the Ni doping enhances the intrinsic activity for HER.The durability of CoNi 0.04 -MOF-74/MXene was tested using a chronopotentiometer at the current density of 100 mA/cm 2 .Clearly, CoNi 0.04 -MOF-74/MXene exhibits reasonable stability, with around a 7% increase in the potential even after 40 h of testing (Figure 6f).Furthermore, the LSV curve does not undergo significant changes after 1000 CV cycles (Figure S17, Supplementary Materials).The micro-rod structure of the CoNi 0.04 -MOF-74/MXene electrodes remains well-preserved even after the HER test (Figure S18, Supplementary Materials).The Raman analysis conducted on post-HER catalyst exhibits peaks at approximately 485 and 503 cm −1 , indicating the production of hydroxide and hydroxyl oxide [56][57][58].

Overall Water Splitting Performance
Encouraged by the excellent performance of OER and HER, we used CoNi 0.04 -MOF-74/MXene as both the anode and cathode in a two-electrode system cell to evaluate its ability as a catalyst for overall water splitting.Figure 7a depicts a schematic diagram of the assembly for the alkaline electrolyzer.The results indicate that CoNi 0.04 -MOF-74/MXene requires only 1.49 V to achieve a current density of 10 mA cm −2 , which is significantly better than Pt/C-IrO 2 pair (1.69 V) (Figure 7b).This performance is superior to or comparable to other electrocatalysts reported in the literature (Table S6, Supplementary Materials), such as Ti 2 NT x @MOF-CoP(1.61V) [38], CdFe-BDC (1.64 V) [59], CoNiBDC/CC (1.625 V) [60], Ni-ZIF/Ni-B@NF (1.54 V) [61], Fe 2 Ni-MIL-88B/NFF(1.56V) [62], NiFe-MOF-74 (1.58 V) [63], NiFe-MOF (1.57V) [64] and FeMn 6 Ce 0.5 -MOF-74/NF (1.65 V) [65]. Figure 7c shows that the CoNi 0.04 -MOF-74/MXene electrode maintains a consistent current density of approximately 10 mA cm −2 at 1.49 V during a continuous 40 h test.While the Pt/C-IrO 2 pair shows a significant decrease in current density after operation for 40 h.This excellent stability may be attributed to the synergistic effect of MXene and CoNi-MOF-74.The results indicate that the CoNi 0.04 -MOF-74/MXene material has potential applications as a versatile and efficient electrocatalyst for both the OER and the HER.
The practical application ability of CoNi 0.04 -MOF-74/MXene in water electrolysis electrochemical systems was evaluated by exploring its electron utilization efficiency.The Faraday efficiency (FE) was measured in 1.0 M KOH. Figure 7d shows that the amount of collected H 2 is approximately double that of collected O 2 , consistent with the theoretical ratio of H 2 and O 2 being close to 2:1.This suggests that water has been fully decomposed into oxygen and hydrogen.Upon comparison of the actual molar amount of gas with the theoretical value, the Faraday efficiency was found to be approximately 97.0%.This Faraday efficiency close to 100% indicates that the CoNi 0.04 -MOF-74/MXene catalyst has great potential as an advanced bifunctional catalyst for electrochemical hydrogen production.

Synthesis and Delamination of Ti 3 C 2 T x MXene
Ti 3 C 2 T x MXene was prepared through the selective etching of the Al layer method using Ti 3 AlC 2 powder as the substrate.Firstly, LiF (1.98 g) was added to 20 mL of concentrated HCl and stirred.Then, Ti 3 AlC 2 (2 g) powder was slowly added into the above solution and stirred at room temperature for 24 h.The obtained suspension was washed with deionized water and centrifuged several times until the pH of the supernatant reached 6-7.Subsequently, the multilayers of Ti 3 C 2 T x were converted into nanosheets by stripping in an ultrasonic machine.Finally, the collected supernatant was dispersed in water to form the desired solution (4 mg•mL −1 ).

Synthesis of MXene/NF
First, the NF (2 × 3 cm 2 ) was immersed in HCl solution (3 M) for 2 h to remove impurities and oxides, followed by ultrasonic cleaning with acetone, ethanol, and deionized water for 15 min each and drying at 60 • C in an oven.A piece of treated NF (2 × 3 cm 2 ) was immersed in Ti 3 C 2 T x MXene (4 mg mL −1 ) solution for 1 h and dried in vacuum for 12 h, resulting in the MXene/NF.3.2.3.Synthesis of CoNi 0.04 -MOF-74/MXene/NF Typically, 2.4 mmol Co(NO) 3 •6H 2 O, 0.1 mmol Ni(NO) 3 •6H 2 O, and 0.171 g of 2,5- dihydroxybenzene dicarboxylic acid (H 4 DOBDC) were dissolved in a mixed solution comprising 10 mL of distilled water, 10 mL of ethanol, and 10 mL of N, N-dimethylformamide (DMF).Subsequently, the resulting solution was transferred to a 50 mL autoclave and the MXene/NF (NF, 2 × 3 cm 2 ) prepared above was added and kept at 120 • C for 24 h.After cooling to room temperature, the sample was washed with water and ethanol several times, and dried in an oven at 60 • C. The sample of CoNi 0.04 -MOF-74/MXene/NF with a deep-red color on the surface was obtained.For comparison, various catalysts were obtained by maintaining a constant total salt amount of 2.5 mmol and adjusting Co:Ni ratios to be 10:1 or 30:1.Co-MOF-74/MXene/NF catalysts were prepared through the described synthesis without the addition of Ni(NO) 3 •6H 2 O.

Synthesis of CoNi 0.04 -MOF-74/NF
The CoNi 0.04 -MOF-74/NF can be obtained by adding cleaned NF, following the same steps as previously described for synthesis.3.2.5.Synthesis of Pt/C @NF or IrO 2 @NF A total of 4 mg of IrO 2 or Pt/C powder samples were dispersed in a mixture of 50 uL of 5 wt% Nafion solution and 950 uL of anhydrous ethanol.The resulting solution was sonicated for 1 h.Subsequently, the prepared solutions were uniformly added to a clean NF, 50 uL at a time, 20 times.The NF was then dried at room temperature.

Materials Characterization
The morphologies were characterized with field-emission scanning electron microscopy (FE-SEM, Quanta FEG450, Thermo, Waltham, MA, USA) and transmission electron microscopy (TEM, TECNAI F30, Thermo, Waltham, MA, USA).The elemental composition and distribution were investigated with energy dispersive X-ray spectroscopy (EDX, JEOL-2010, Tokyo, Japan) attached to the FE-SEM.The X-ray diffraction (XRD) spectrum was measured using a Bruker D8-Advance X-ray diffractometer (Bruker, Bremen, Germany) with Cu Ka radiation to analyze the crystal structure of the samples.The Raman spectrum of the as-prepared samples was conducted with a LabRAM HR 800 system (Horiba Jobin Yvon, Longjumeau, France) at 532 nm laser.FT-IR spectra were recorded in the range of 4000-450 cm −1 with a JASCO FT/IR-460 spectrophotometer.X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Thermo, Waltham, MA, USA) was carried out to analyze the chemical composition and valence states of the constituent elements in the samples.

Electrochemical Measurement
All electrochemical tests were performed on the Gamry electrochemical workstation (INTERFACE 1010E, Warminster, PA, USA) with a standard three-electrode system, while using the as-obtained catalysts on NF as the working electrode, Pt sheets as the counter electrode, and an Hg/HgO as the reference electrode.A total of 1 M KOH was employed as the electrolyte solution.For HER, the linear sweep voltammograms (LSVs) were measured in a potential range of −0.576 to 0.024 V vs. RHE at a scan rate of 5 mV s −1 , while for OER, the LSVs were obtained from 1.024 to 2.024 V vs. RHE at a scan rate of 2 mV s −1 .Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 0.1 Hz to 100 kHz with a 5 mV AC dither for HER and OER, respectively.Cyclic voltammograms (CVs) were tested at different scan rates, which are employed to estimate the double-layer capacitances (C dl ) of the catalysts.Chronopotentiometry curves were obtained with a constant current density at 100 mA cm −2 .Overall water splitting performance was evaluated in a two-electrode system by using CoNi 0.04 -MOF-74/NF as the catalysts for HER and OER in 1 M KOH.The polarization curves were recorded at a scan rate of 2 mV s −1 .The Faraday efficiency (FE) theoretical precipitation is calculated as follows: n(O 2 ) = Q/nF (Q = it), where n(O 2 ) is the theoretical molar amount of O 2 precipitated, Q is the transferred charge, n is the number of electrons transferred in the OER/HER reaction (n = 4/2), F is the Faraday's constant (F = 96,485 C/mol), i is the value of the electric current applied to the water electrolysis, and t is the time of water electrolysis.In this paper, the drainage method was used to derive the amount of oxygen precipitated at a fixed time point, the current applied was 50 mA, and the reaction time of water electrolysis was 1 h.All potentials reported in this paper were converted from vs. Hg/HgO to vs. RHE by adding a value of 0.098 + 0.059×pH and were corrected for ohmic losses.

Conclusions
A heterostructure electrocatalyst with Ni-doped Co-MOF-74 grown in situ on TiC 2 T x MXene was synthesized using a facile hydrothermal process.Benefiting from the Ni doping and synergistic effect between CoNi-MOF-74 and Ti 3 C 2 T x , the obtained CoNi-MOF-74/MXene/NF exhibits an impressive electrocatalytic performance, achieving 100 mA/cm 2 at the overpotential of 256 mV for OER and 10 mA/cm 2 at the overpotential of 102 mV, respectively.When employed as both the cathode and anode catalyst, the assembled electrolyzer may drive the current density of 10 mA/cm² at a low cell voltage of 1.49 V, meanwhile maintaining excellent durability over 40 h.This work opens a new avenue for improving the activity of transition metal MOF-based catalysts and elucidates the tunability of the catalytic performance through modulating electronic interactions.

Figure 7 .
Figure 7. (a) Schematic illustration for the overall water splitting with the CoNi 0.04 -MOF/MXene/NF couple.(b) The overall water splitting performance of the CoNi 0.04 -MOF/MXene/NF and Pt/C-IrO 2 couple.(c) The catalytic stability of the CoNi 0.04 -MOF /MXene/NF and Pt/C-IrO 2 couple for overall water splitting in 1.0 M KOH.(d) Under a current of 50 mA, the calculated and measured H 2 and O 2 production vs. time.