1 Introduction

Elevated greenhouse gas emissions, resulting from heavy dependence on fossil fuels, have adversely impacted the global climate. Therefore, a great interest has developed among scientists to implement greener technologies to shift the global energy situation towards a more sustainable model, utilizing energy sources with less carbon footprint [1].

Methanol is a feedstock chemical with relatively high energy density that can be used as a fuel in direct methanol fuel cells (DMFCs), with far enhanced efficiency as compared to internal combustion engines [2]. The vast majority of such DMFCs utilize Pt as the catalyst on both the anode (methanol oxidation reaction) and the cathode (oxygen reduction reaction) electrodes. Due to the high cost and limited abundance of Pt, catalysts based on non-precious group elements (NPG) are of high interest [3]. The use of NPG catalysts in DMFCs is hindered by the catalyst poisoning and the slow methanol electro-oxidation kinetics at the anodes utilizing NPGs catalysts [2]. Therefore, developing an NPG-based electrocatalyst for methanol oxidation in DMFCs that demonstrates (i) low overpotential, (ii) rapid kinetics, and (iii) maintained activity for prolonged reaction time is of prime interest [3]

The methanol electro-oxidation reaction is categorized based on the supporting electrolyte pH (acidic and alkaline). Alkaline medium prevails over acidic medium in offering faster oxidation kinetics [4, 5] due to facilitated alkoxide ion production at high pH, as pointed out by Kwon and co-workers in their study of the direct effect of base concentration on methanol electro-oxidation kinetics [6]. Despite the fact that methanol electro-oxidation has a thermodynamic potential of 0.03 V (vs. SHE), this value is not realizable even with the best catalysts due to the number of electrons involved in this step [7]. Precious metals such as Pt [8,9,10], Au [11,12,13], and Ag [14] demonstrate high catalytic activity for electro-oxidation of short-chained alcohols at potentials below 1.0 V, however, their elevated cost and their relatively low abundance preclude their use in commercial DMFCs. Alternative anodic modifiers utilizing NPG metals, especially those based on Co2+, Ni2+, and Cu2+, are currently of great interest as redox mediators for the methanol electro-oxidation reactions in alkaline media [15]. For such NPG catalysts, it is argued that the catalytic activity is ascribed to the generation of oxidative M3+ species (M3+ = Co3+, Ni3+, or Cu3+) along the reaction pathway, as key intermediates to facilitate their catalytic activity towards methanol oxidation. For example, A. Gopalakrishnan and his colleagues have prepared MoS2 nanostructure supported on Ni foam (MoS2/NF-5) via a hydrothermal method, which demonstrated activity towards methanol oxidation up to 73 mA/cm2 at 0.7 V vs RHE in 0.1 M KOH and 0.5 M methanol at a scan rate of 30 mV/s [16].

Electrochemically active catalysts based on metal–organic frameworks (MOFs) have specifically gained significant interest in recent years as potential NPG porous catalysts with superior performance. MOFs are considered novel microporous hybrid inorganic–organic solids with promising activities in numerous demanding applications [17]. The utilization of MOFs in DMFCs as heterogeneous catalysts is attributed to their defined pore size [18] and ability to adsorb methanol species on their surface. Their high specific surface areas can as well accommodate the generated intermediate species through physical adsorption, at which the intermediates will subsequently react in two-step reactions to reach CO2 as the final product [7]. Several hypotheses have been presented in literature to explain the methanol oxidation catalytic activity at the MOF surfaces. Fleischmann and co-workers suggested MeOH oxidation proceeds through formation of M3+OOH species (M = Co, Ni, or Cu) from metal hydroxides [19]. The M3+OOH species serve as redox mediators for methanol electro-oxidation in alkaline media. Fleischmann proposed the adsorption of methanol substrate on M3+ species to provoke radical intermediates, which will further interact with M3+ species and produce M2+ species along with other oxidation products. In a separate study by Wang and co-workers, Ni complexes demonstrated involvement of the Ni2+/Ni3+ redox pair throughout the methanol oxidation process[20]. Nonetheless, pristine MOFs as anodic modifiers in DMFCs are rarely reported accounting for their modest electrical conductivity regulated by the poor overlap between valence orbitals and the electronic states on metal ions and ligands [21,22,23]. As a result, pristine MOFs demonstrated reduced current densities when utilized in DMFCs.

Despite the high surface area of MOFs, it suffers from low electrical conductivity, which is essential for electrocatalytic applications, therefore one strategy is to utilize a conductive support in order to enhance the catalyst conductivity [24,25,26]. Alternatively, MOFs were rather utilized as precursors or hard templates for the synthesis of homogenously dispersed metal/metal oxides on porous carbon under pyrolysis at different temperatures to enhance their electrical conductivity. For example, Rezaee and co-workers reported NiCo/NiO-CoO/nanoporous carbon as a redox mediator for methanol oxidation through direct pyrolysis of bimetallic NiCo-MOF [27]. Such composite exhibited a maximum current density of 185 mA/cm2 at 0.65 V vs. Ag/AgCl. Similarly, Qian et al. pyrolyzed Ni2+/Co2+ MOF using an organic linker L-aspartic acid in air at 300 °C to achieve NF/Co3O4/NiCo2O4 with nano-spindle arrays morphology [28]. The calcination process of Ni2+/Co2+ ensured fast diffusion rates of electrolyte ions to the accessible active sites, thereby enhancing its electrocatalytic activity towards methanol oxidation. Moreover, Sheikhi and co-workers fabricated a zirconium oxide-based porous carbonaceous platform as a support for Ni nanoparticles through MOF calcination at 800 °C [29]. Despite the exceptional electrocatalytic performance, nevertheless, such synthetic strategy is energy-consuming, which could limit its large-scale application. In this regard, for successful commercially applicable DMFCs, further exploitation is required to approach an economic synthesis procedures of MOFs catalysts at a large scale.

To approach the inherent issue of MOFs’ low electrical conductivity, conductive carbon supports such as, graphene or its derivatives, carbon nanotubes and activated carbon are incorporated to construct MOF composites with adequate electrical conductivity for electrocatalytic applications [30]. For example, a composite of Co-benzenedicarboxylate MOF with 5 wt% graphene oxide (Co-BDC MOF@5wt% GO) exhibited a current density of 291 mA/cm2 at 1.77 V vs. RHE in 1 M KOH and 3 M MeOH [31]. Similarly, a current density of 130 mA/cm2 at 1.59 V vs. RHE was attained using Co-benzenetricarboxylate MOF with 1 wt% reduced graphene oxide (Co-BTC MOF@1wt% rGO) in 1 M NaOH and 2 M MeOH [32]. Despite the enhanced conductivity of rGO, increasing its amount in the composite in fact reduced the attained current density due to sheets restacking and blocking of MOF active sites, and hindering methanol oxidation at surface that is now occupied by the hydroxyl groups that is found in the alkaline media [30]. Another examples of Ni-benzenetricarboxylate MOF and reduced graphene oxide composite (Ni-BTC MOF@4wt% rGO) demonstrated a current density of 200.22 mA/cm2 at 1.61 V vs. RHE in 1 M NaOH and 2 M MeOH [26]. The observed improvement in electrocatalytic performance of MOF-graphene oxide composites towards methanol oxidation is attributed to the synergy between conductivity of graphene oxide sheets and the high surface area and catalytic activity of MOF.

Herein, we utilized Ni-UiO-66-NH2@MWCNTs through simple incipient wetness impregnation method. The MOF@MWCNTs provides a platform for efficient deposition of Ni(OH)2@MWCNTs, with notable activity towards methanol oxidation in an alkaline medium at low onset potential through MOF degradation and dispersion of nickel hydroxide nanoparticles on 1D MWCNTs support, Scheme 1.

Scheme 1
scheme 1

In-situ deposition of MOF@MWCNTs as a precursor to Ni(OH)2 active catalyst for methanol oxidation

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2 Experimental and methods

All reagents of commercial grade were used without any further purification. Electrochemical measurments were done in three electrode system using reference electrode (RE-61AP) Hg/HgO filled with 1 M NaOH (from ALS Instruments) and data was recorde using EC-Lab software, graphite rod as counter electrode and glassy carbon electrode as working electrode. FTIR was measured using ThermoScientificiS10 spectrophotometer. ZrCl4 anhydrous > 98%, 2-aminoterephthalic acid 99%, HCl 37% were purchased from Acros Organics. N,N’dimethylformamide (DMF) analytical reagent grade 99.99%, Methanol 99% HPLC grade and acetonitrile (ACN) HPLC grade were purchased from Fisher Scientific-UK Chemicals. Ni(NO3)2.6H2O 98% was purchased from Alfa Aesar from USA, KOH 90% from PioChem Egypt, Carboxylated multi-walled carbon nanotubes (MWCNT-COOH) > 95 wt%/30–50 nm was purchased from Cheap Tubes (SKU:050306) were used as received without further purification.

2.1 Synthesis of Ni-UiO-66-NH2@MWCNTs

In this work, Ni-UiO-66-NH2@MWCNTs, as an electrochemical catalyst for methanol oxidation, was prepared according to our previously reported work [33]. Briefly,in two clean separate vials, 42.5 mg (0.18 mmol) of ZrCl4 was suspensed in 5 mL DMF and 1 mL HCl (37%) (Solution A), while 45.5 mg (0.428 mmol) of 2-aminoterphethalic acid and 50 mg of MWCNTs-COOH were suspensed in 10 ml DMF (Solution B). Both solutions were left to sonicate for 20 min. Solution A was added to solution B while stirring at 400 rpm with a magnetic bar. The vial was capped and left under stirring at 80 °C overnight. The as-prepared MOF@MWCNTs was filterd, washed and left to exchange in ACN at 85 °C for 2 h, then the solid was filtered and dried overnight at 80 °C. For Ni-metallation, 50 mg of the UiO-66-NH2@MWCNTs was suspensed in 5 ml acetonitrile, followed by addition of 15.2 mg Ni(NO3)2.6H2O to the solution and sonication for 20 min. The vial was transferred to thermalblock and dried while stirring at 85 °C. The temperature was then raised to 120 °C to ensure complete evaporation of acetontitrile. The synthetic process is represented and summarized in Scheme 2.

Scheme 2
scheme 2

A pictorial representation of the synthetic process

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2.2 Electrochemical measurements

About 8 mg of Ni-UiO-66-NH2@MWCNTs was dispersed in 1 ml of isopropanol followed by the addition of 7 µl of Nafion (10 wt% in isopropanol). The ink was sonicated then 20 µl were drop-casted on a glassy carbon electrode and left to dry at room temperature. The cyclic voltametry (CV), impedance (EIS) and chronoamperometry measurements were performed in 1 M KOH solution as supporting electrolyte at room temperature from 0 to 0.6 V vs Hg/HgO at scan rates ranging from 5 to 100 mV/s. EIS measurements were performed by using a voltage amplitude of 10 mV and frequencies from 0.1 Hz to 105 Hz.

3 Results and discussion

To confirm the successful deposition of UiO-66-NH2 MOF nanoparticles (NPs) atop the MWCNTs, Fourier-transform infrared (FTIR) spectroscopy was utilized (Fig. 1a), where the MOF characteristic peaks were evident in the FTIR spectrum of the UiO-66-NH2@MWCNTs and Ni-UiO-66-NH2@MWCNTs composite. The peaks at 1575 cm−1 and 1432 cm−1 correspond to the stretching vibration frequencies of C=O and C=C of the 2-aminoterephthalic acid ligand, respectively, which are absent in the MWCNTs-COOH spectrum thereby confirming the successful deposition of MOF NPs atop the MWCNTs-COOH [34]. For further investigation, X-ray diffraction (XRD) was also performed to ensure the successful preparation of UiO-66-NH2@MWCNTs. As shown in Fig. 1b, the XRD pattern of UiO-66-NH2@MWCNTs and Ni-UiO-66-NH2@MWCNTs revealed the characteristic peaks of the pristine MOF. An additional peak at 2θ = 25.7 was evident (marked by * in Fig. 1b), which is characteristic of (002) planes of graphitic carbon of MWCNTs [35].

Fig. 1
figure 1

(a) FTIR spectra of UiO-66-NH2, UiO-66-NH2@MWCNTs, Ni-UiO-66-NH2@MWCNTs, and MWCNTs-COOH and (b) XRD diffraction patterns of UiO-66-NH2, UiO-66-NH2@MWCNTs, Ni-UiO-66-NH2@MWCNTs, and simulated UiO-66. The asterisk (*) demonstrates the additional peak representing the (002) planes of MWCNTs

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Thermogravimetric analysis (TGA), Fig. 2a, was performed to investigate the thermal stability of the composite. Compared to pristine MOF, the thermal stability of the composite was enhanced, which indicates the strong interaction between MWCNTs-COOH and MOF NPs [36]. To confirm the microporosity of the UiO-66-NH2 MOF NPs, N2 sorption isotherm was measured (Fig. 2b), which exhibited Type-I characteristics indicating the presence of micropores. The Brunauer–Emmett–Teller (BET) surface area was calculated to be 1276 m2/g, which is consistent to previous literature.

Fig. 2
figure 2

(a) TGA of UiO-66-NH2 and UiO-66-NH2@MWCNTs, and (b) N2 sorption isotherms for UiO-66-NH2

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Figure 3 shows the scanning electron microscopy (SEM) images of (a) MWCNTs-COOH (b) UiO-66-NH2@MWCNTs and (c) Ni-UiO-66-NH2@MWCNTs, which confirm the successful deposition of MOF NPs on the surface of MWCNTs-COOH with uniform coverage.

Fig. 3
figure 3

SEM images at the same magnification of (a) MWCNTs-COOH, (b) UiO-66-NH2@MWCNTs and (c) Ni-UiO-66-NH2@MWCNTs

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Energy-dispersive X-ray spectroscopy (EDX) measurements (Fig. 4) were also conducted on the Ni-loaded composite, which revealed the homogeneous distribution of Ni within the composite, as evidenced by the Ni map aligning closely with the elemental maps of C, O, and Zr. Moreover, the analysis indicated that the composite contains 2.7 wt% of Ni.

Fig. 4
figure 4

EDX maps for Ni-UiO-66-NH2@MWCNTs showing the homogenous distribution of the elements (labeled images) throughout the sample

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Metalation of UiO-66-NH2@MWCNTs with Ni(II) ions was conducted through incipient wetness impregnation, where simply soaking the composite in an acetonitrile solution of Ni(NO3)2 for 4 h was sufficient to induce metalation by Ni(II) ions. Cyclic voltammetry (CV) of Ni-UiO-66-NH2@MWCNTs in alkaline medium (1 M KOH) demonstrated well-defined anodic and cathodic peaks of Ni2+/Ni3+ redox pair at 0.494 V and 0.352 V vs Hg/HgO respectively, Fig. 5, confirming the inclusion of Ni2+ ions within the composite. The current density of this redox couple increased with consecutive CV scans, Fig. 3 inset, indicating the continuous adsorption of OH– ions to activate Ni-UiO-66-NH2@MWCNTs and form the electrocatalytically active species NiOOH according to Eq. (1) [29].

Fig. 5
figure 5

Activation of Ni-UiO-66-NH2@MWCNTs in 1 M KOH at scan rate 100 mV/s

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Ni(OH)2 + OH – NiOOH + H2O + e – (1).

Upon measuring CVs at different scan rates from 5–100 mV/s, the current density increased with increasing the scan rate indicating a diffusion-controlled mechanism.for Ni(OH)2/NiOOH on the electrode surface, Fig. 6a. Figure 6b shows the linear dependency of the anodic current density on square root of the scan rate, which confirms the aforementioned explanation.

Fig. 6
figure 6

(a) CVs of Ni-UiO-66-NH2@MWCNTs in 1 M KOH at different scan rates and (b) linear relationship between anodic current density at 0.49 V and square root of scan rate

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In order to assess the electrocatalytic acitivty of Ni-UiO-66-NH2@MWCNTs towards methanol (MeOH) oxidation, CVs were measured at different alkaline methanolic concentrations, Fig. 7a. The observed current density increases upon increasing the MeOH concentration up to a maximum of 143 mA/cm2 at 0.6 V (vs Hg/HgO) at 1 M MeOH concentration, beyond which the current density starts to decline (Fig. 7a). This can be ascribed to the increasing availability of.

Fig. 7
figure 7

(a) CVs of Ni-UiO-66-NH2@MWCNTs in 1 M KOH and different methanol concentrations at 50 mV/s scan rate and (b) CVs of UiO-66-NH2@MWCNTs and Ni-UiO-66-NH2@MWCNTs in 1 M KOH and 1 M MeOH at 50 mV/s scan rate

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MeOH to be adsorbed on the abundant active sites. However, beyond 1 M MeOH concentration, there exists no more additional unoccupied active sites as well as the reaction intermediates and unreacted methanol tend to accumulate, thereby decreasing the current density [37]. In order to ensure that the presence of Ni ions is responsible for methanol oxidation catalytic activity, CV was carried out for UiO-66-NH2@MWCNTs under similar conditions. As shown in Fig. 7b, the current density is negligible and there are no discernible redox peaks contrary to Ni-UiO-66-NH2@MWCNTs.

Overall, the presented catalyst demonstrated superior activity towards methanol oxidation in comparison with other previously reported catalysts as shown in Fig. 8 despite the noted differences in the reported reaction conditions utilized in the previous publications, which are summarized in Table 1. Furthermore, the stability of Ni-UiO-66-NH2@MWCNTs was investigated by chronoamperometry (CA) in 1 M MeOH/KOH solution at a fixed.potential of 0.6 V vs Hg/HgO for 10 h (Fig. 9a). The high initial current density is most likely due to fast oxidation of adsorbed MeOH on the active sites [38]. The following drop in current density is therefore due to partial depletion of adsorbed MeOH as well as accumulation of adsorbed intermediates like COOH, CHO and CO on the catalyst surface, which may lead to catalyst saturation [39]. Nevertheless, Ni-UiO-66-NH2@MWCNTs retained 84.1% of the current density after 10 h (Fig. 9b). The slight decrease in current density can be explained by the reduction of methanol concentration in solution with time after its oxidation on the electrode surface.

Fig. 8
figure 8

Current density recorder utilizing the Ni-UiO-66-NH2@MWCNTs, in 1 M Methanol/KOH at 50 mV/s scan rate, as compared to previous literature comparing the values of highest recorded current density

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Table 1 Electrochemical catalytic activity of reported MOF composites and MOF-derived catalysts towards methanol oxidation reaction
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Fig. 9
figure 9

(a) CA of Ni-UiO-66-NH2@MWCNTs in 1 M KOH and 1 M MeOH at 0.6 V vs Hg/HgO after 10 h. (b) showing the stability after 10 h

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In an attempt to gain deeper insight into the proposed mechanism, electrochemical impedence spectroscopy (EIS) was performed for Ni-UiO-66-NH2@MWCNTs in 1 M KOH before and after addition of 1 M MeOH (Fig. 10). The experimental points were well-fitted to proper equivalent circuits that contain Rsol, CPEdl, Rct, CPEads, Rads, and W as solution resistance, constant phase element for double layer capacitance, charge transfer resistance, constant phase element for adsorption of reaction intermediates on the electrode surface, adsorption of reaction intermediates on the electrode surface resistance, and Warburg impedance, respectively as given in Table 2. As shown in Fig. 10, the Nyquist plots showed significantly different appearances before and after the addition of MeOH to the alkaline electrolyte, which confirmed the methanol oxidation mechanism on the anode follows that proposed by Harrington and Conway [49, 50]. The large semicicle at high frequency in the EIS for the Ni-UiO-66-NH2@MWCNTs in 1 M KOH could be attributed to double layer capacitance (CPEdl = 4289 μT) and high charge transfer resistance (Rct = 176 Ω) on the electrode surface where no reaction is taking place. After addition of 1 M MeOH, the EIS revealed two smaller semicircles, where the first semicircle can be attributed to higher double layer capacitance (CPEdl = 1244 μT), as well as lower charge transfer resistance (Rct = 14.94 Ω). The additional second semicircle can be correlated to the adsorption of methanol oxidation reaction intermediates onto the electrode surface with lower resistance (Rads = 6.517 Ω and CPEads = 79.62 μT), thus confirming the methanol oxidation process on the catalyst surface [51]. Moreover, a fast mass transfer is evident by the presence of Warburg element, which indicates the fast oxidation process [29].

Fig. 10
figure 10

Nyquist plots for Ni-UiO-66-NH2@MWCNTs at 0.6 V in 1 M KOH before and after addition of 1 M MeOH

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Table 2 Fitting parameters of the different elements of the equivalent circuits used in modeling the EIS data for Ni-UiO-66-NH2@MWCNTs in 1 M KOH before and after addition of 1 M MeOH
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4 Conclusion

In summary, a simple one-pot synthesis method to prepare UiO-66-NH2@MWCNTs followed by Ni2+ ion metalation through facile incipient wetness impregnation is presented, to generate a highly active electrocatalyst for alkaline methanol oxidation reaction. Electrochemical activation of the prepared composite resulted in NiOOH atop MWCNTs, which revealed exceptional electrocatalytic activity towards methanol oxidation in alkaline solution with appreciable stability. The Ni-UiO-66-NH2@MWCNTs catalyst retained 93.5% and 84.1% of the current density after 1 h and 10 h of continuous oxidation, respectively, and demonstrated superior catalytic activity and stability compared to other reported catalysts based on MOF composites/-derived catalysts. The reported approach opens the door for novel electrode materials for direct methanol fuel cell applications.