Foam Synthesis of Nickel/Nickel (II) Hydroxide Nanoflakes Using Double Templates of Surfactant Liquid Crystal and Hydrogen Bubbles: A High-Performance Catalyst for Methanol Electrooxidation in Alkaline Solution

This work demonstrates the chemical synthesis of two-dimensional nanoflakes of mesoporous nickel/nickel (II) hydroxide (Ni/Ni(OH)2-NFs) using double templates of surfactant self-assembled thin-film and foam of hydrogen bubbles produced by sodium borohydride reducing agent. Physicochemical characterizations show the formation of amorphous mesoporous 2D nanoflakes with a Ni/Ni(OH)2 structure and a high specific surface area (165 m2/g). Electrochemical studies show that the electrocatalytic activity of Ni/Ni(OH)2 nanoflakes towards methanol oxidation in alkaline solution is significantly enhanced in comparison with that of parent bare-Ni(OH)2 deposited from surfactant-free solution. Cyclic voltammetry shows that the methanol oxidation mass activity of Ni/Ni(OH)2-NFs reaches 545 A/cm2 gcat at 0.6 V vs. Ag/AgCl, which is more than five times higher than that of bare-Ni(OH)2. Moreover, Ni/Ni(OH)2-NFs reveal less charge transfer resistance (10.4 Ω), stable oxidation current density (625 A/cm2 gcat at 0.7 V vs. Ag/AgCl), and resistance to the adsorption of reaction intermediates and products during three hours of constant-potential methanol oxidation electrolysis in alkaline solution. The high-performance electrocatalytic activity of Ni/Ni(OH)2 nanoflakes is mainly derived from efficient charge transfer due to the high specific surface area of the 2D mesoporous architecture of the nanoflakes, as well as the mass transport of methanol to Ni2+/Ni3+ active sites throughout the catalyst layer.


Introduction
Methanol is regarded as an attractive fuel to feed direct methanol fuel cells (DMFCs) because of the advantages of high energy density, reduced CO 2 emission, low operating temperature, and ease of storage and transport [1][2][3]. Nevertheless, the DMFC suffers from several challenges that limit its performance in commercial markets due to the high price of platinum catalyst and the slow kinetics of the methanol oxidation reaction (MOR) as a result of catalyst surface poisoning [1,[4][5][6][7]. Consequently, it is necessary to develop low-cost, non-noble, and efficient transition-metal electrocatalysts to improve electrocatalytic activity and durability for MOR [8][9][10].
Recently, heterostructures of Ni cores encapsulated within NiO shells and supported on carbon nanotubes (NiO-Ni/CNT) were shown to boost the electrocatalytic methanol oxidation reaction with an onset oxidation potential of 0.325 V vs. Hg/HgO and normalized mass activity of about 2.1 A/mg at 0.58 V vs. Hg/HgO, and the catalyst was stable for up to 300 cycles during methanol oxidation in alkaline solution [19]. Interestingly, a high-surfacearea porous nickel hydroxide catalyst electrodeposited using a dynamic hydrogen bubble template was reported to exhibit durable catalytic methanol oxidation electroactivity in alkaline solution and achieved a current density of 181 mA/cm 2 [20]. Wang et al. [21] demonstrated that a molecular catalyst of four-six-coordinated nickel hydroxide (NR-Ni(OH) 2 ) nanoribbon resulted in a significantly lower onset potential of 0.55 V vs. RHE for the oxidation of methanol in alkaline solution. The enhanced methanol performance was related to the presence of charge-transfer orbitals for delocalized electrons near the Fermi level that efficiently enhance methanol oxidation to CO 2 via a six-electron transfer reaction in alkaline media.
Very recently, Ni 3 S 2 enriched with defects and supported on carbon nanofibers (CNFs) showed significant electroactivity and selectivity in the oxidation of methanol to valuable formate with the co-production of hydrogen at an applied potential of 1.37 to 1.62 V vs. RHE and current density of more than 700 mA/cm 2 [12,22]. The Faradic efficiency of the process reached 99%, and the synergistic effect of Ni-OOH and SO x active sites was responsible for selective oxidation of methanol to formate and cathodic hydrogen-intensive production. In related work, our research group reported nickel/nickel hydroxide nanoflake (Ni/Ni(OH) 2 -NF) catalysts obtained by chemical deposition and exfoliation through the self-assembly of a surfactant hexagonal template mixture. The nanoarchitecture of Ni/Ni(OH) 2 -NFs revealed a high mesoporous surface area of 135 m 2 /g and semi-crystalline Ni-incorporated α-Ni(OH) 2 nanoflakes [23]. The obtained Ni/Ni(OH) 2 nanostructures showed superior activity over the bare-Ni(OH) 2 catalyst for electrooxidation of small organic molecules such as ethanol [23], urea [24], and glucose [25] in alkaline solution.
In this study, we developed a thin-film self-assembly of surfactant liquid crystal template (in a Petri dish) to prepare 2D mesoporous nanoflakes of Ni/Ni(OH) 2 by chemical reduction and exfoliation of the confined nickel deposit by hydrogen foam generated by sodium borohydride reducing agent. The physicochemical properties of the obtained catalyst were evaluated by different techniques. The electrochemical performance, long-term stability, and charge transfer kinetics of the fabricated Ni/Ni(OH) 2 nanoflake structure were examined for the electrocatalytic oxidation of methanol in alkaline conditions using electrochemical techniques, namely, cyclic voltammetry, current-time transients, and electrochemical impedance spectroscopy (EIS), respectively.

Nickel/Nickel Hydroxide Ni/Ni(OH) 2 Nanoflake Synthesis
Scheme 1 shows a schematic diagram for the synthesis of 2D nanoflakes of Ni/Ni(OH) 2 material (Ni/Ni(OH) 2 -NFs) using the approach of thin-film self-assembly of Brij ® 78 surfactant mixed with nickel precursor solution. Firstly, the template mixture of nickel precursor was obtained by physically mixing Brij ® 78 surfactant (2.0 g, melted at 40 • C) with a solution of nickel nitrate (conc. 0.5 M, 10.0 mL) in a Petri dish (10 cm diameter). After that, the template mixture in the Petri dish was transferred to an ultrasonic bath for 20 min to obtain a homogeneous solution and then transferred to an oven at 25 ± 2 • C to evaporate the excess of water to reach a surfactant/water ratio of 40% to obtain the hexagonal template of surfactant liquid crystal. After that, an excess of sodium borohydride solution (NaBH 4 , 2.0 M, 10 mL) in a ratio of 1:4 (nickel nitrate/NaBH 4 ) was sprayed over the nickel template mixture in the Petri dish. Scheme 1 shows a schematic diagram for the synthesis of 2D nanoflakes of Ni/Ni(OH)2 material (Ni/Ni(OH)2-NFs) using the approach of thin-film self-assembly of Brij ® 78 surfactant mixed with nickel precursor solution. Firstly, the template mixture of nickel precursor was obtained by physically mixing Brij ® 78 surfactant (2.0 g, melted at 40 °C) with a solution of nickel nitrate (conc. 0.5 M, 10.0 mL) in a Petri dish (10 cm diameter). After that, the template mixture in the Petri dish was transferred to an ultrasonic bath for 20 min to obtain a homogeneous solution and then transferred to an oven at 25 ± 2 °C to evaporate the excess of water to reach a surfactant/water ratio of 40% to obtain the hexagonal template of surfactant liquid crystal. After that, an excess of sodium borohydride solution (NaBH4, 2.0 M, 10 mL) in a ratio of 1:4 (nickel nitrate/NaBH4) was sprayed over the nickel template mixture in the Petri dish.
Effervescence of hydrogen foam began to form upon the addition of NaBH4, as shown in Scheme 1, and the template mixture gradually changed from green to black due to the commencement of the Ni/Ni(OH)2-NFs catalyst deposition. The Petri dish with black catalyst was maintained overnight to ensure the completion of the deposition reaction. Then, the surfactant was removed by washing the precipitated catalyst with hot distilled water (50 °C) five times under magnetic stirring. Lastly, the catalyst deposit was separated via centrifugation and placed in an oven overnight to dry at 50 °C. The control catalyst (bare-Ni(OH)2) was precipitated in the absence of the Brij ® 78 surfactant by reacting NaBH4 solution (2.0 M) with 1.0 M aqueous nickel nitrate solution.

Scheme 1.
A schematic diagram shows the synthesis procedure of the Ni/Ni(OH)2 nanoflake catalyst via surfactant liquid crystal thin-film and hydrogen bubble templates.

Characterizations of Ni/Ni(OH)2 Catalysts
The nanostructure and morphology of the as-prepared Ni/Ni(OH)2 nanoflake catalyst were investigated by transmission electron microscope (TEM, JEM 2100F-JEOL, Tokyo, Japan) and scanning electron microscope (SEM, JSM-7600F; JEOL), respectively. The V-Sorb 2800 Porosimetry Analyzer was employed to measure the specific surface area of Ni/Ni(OH)2 catalyst, which was determined using the Brunauer-Emmer-Teller (BET) approach. In addition, the as-synthesized catalyst crystal profile was examined by MiniFlex-600 (Rigaku) X-ray diffraction (XRD) analysis with Cu Kα radiation (λ = 1.5418 A°) at operation conditions of 40 kV and 15 mA.
Cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) characterizations were performed using AutoLab Potentiostatic Galvanostat PGSTAT302N (Metrohm). The electrochemical studies were executed using a Scheme 1. A schematic diagram shows the synthesis procedure of the Ni/Ni(OH) 2 nanoflake catalyst via surfactant liquid crystal thin-film and hydrogen bubble templates.
Effervescence of hydrogen foam began to form upon the addition of NaBH 4 , as shown in Scheme 1, and the template mixture gradually changed from green to black due to the commencement of the Ni/Ni(OH) 2 -NFs catalyst deposition. The Petri dish with black catalyst was maintained overnight to ensure the completion of the deposition reaction. Then, the surfactant was removed by washing the precipitated catalyst with hot distilled water (50 • C) five times under magnetic stirring. Lastly, the catalyst deposit was separated via centrifugation and placed in an oven overnight to dry at 50 • C. The control catalyst (bare-Ni(OH) 2 ) was precipitated in the absence of the Brij ® 78 surfactant by reacting NaBH 4 solution (2.0 M) with 1.0 M aqueous nickel nitrate solution.

Characterizations of Ni/Ni(OH) 2 Catalysts
The nanostructure and morphology of the as-prepared Ni/Ni(OH) 2 nanoflake catalyst were investigated by transmission electron microscope (TEM, JEM 2100F-JEOL, Tokyo, Japan) and scanning electron microscope (SEM, JSM-7600F; JEOL), respectively. The V-Sorb 2800 Porosimetry Analyzer was employed to measure the specific surface area of Ni/Ni(OH) 2 catalyst, which was determined using the Brunauer-Emmer-Teller (BET) approach. In addition, the as-synthesized catalyst crystal profile was examined by MiniFlex-600 (Rigaku) X-ray diffraction (XRD) analysis with Cu K α radiation (λ = 1.5418 A • ) at operation conditions of 40 kV and 15 mA.
Cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) characterizations were performed using AutoLab Potentiostatic Galvanostat PGSTAT302N (Metrohm). The electrochemical studies were executed using a glass cell with a three-electrode configuration with a thin sheet of platinum (0.5 cm × 0.5 cm) and Ag/AgCl as counter and reference electrodes, respectively. The working electrode was made of SIGRACET ® gas diffusion carbon paper (GDL-24BC, 1.0 cm × 1.0 cm) loaded with catalyst ink immersed in methanol and potassium hydroxide test solution. The Ni/Ni(OH) 2 nanoflake ink was prepared by mixing 10 mg of the catalyst, 0.5 mL of deionized water, 0.5 mL of isopropanol, and 10 µL of Nafion in a glass vial and placed in an ultrasonic bath for 30 min. Catalyst ink with predefined loading was placed on one side of the GDL carbon paper using a micropipette and dried in an open atmosphere by hot air to obtain the working electrode. The obtained current was normalized to the catalyst loading and 1.0 cm 2 electrode geometric area.
For comparison, electrochemical measurements of the reference (bare-Ni(OH) 2 ) catalyst were also performed using the same procedure, and throughout this work, all test solutions were prepared using deionized water (DI; resistivity of 18.2 MΩ cm) obtained from a Milli-Q ultrapure water purification system.

Catalyst Characterizations
As shown in Scheme 1, the mesoporous nickel/nickel (II) hydroxide nanoflakes (Ni/Ni(OH) 2 -NFs) were synthesized using the modified approach with a surfactant thinfilm template and sodium borohydride (NaBH 4 ) reducing agent at room temperature (25 • C). During this synthesis procedure, nickel ions confined within the interstitial aqueous domain of the surfactant template were chemically precipitated by NaBH 4 to instantaneously produce hydrogen gas, which exfoliated and fragmented the confined deposited catalyst layers in the form of nanoflakes. Figure 1 shows the morphology of the catalyst surface and fine nanostructure of the Ni/Ni(OH) 2 nanoflakes, as characterized by scanning (SEM) and transmission (TEM) electron microscopes. The SEM images in Figure 1a reveal that the assembled Ni/Ni(OH) 2 catalyst has a spherical microparticle morphology, and the microparticles themselves are formed from an array of overlapping nanometer-sized flakes. Moreover, the TEM micro-images in Figure 1b,c show that the 2D nanoflakes are very thin (thickness of about 10.0 nm) and strongly crumpled with nonuniform assembly and an open mesoporous channel. The elemental analysis, performed by energy-dispersive X-ray spectroscopy (EDX), as shown in Figure 1d, confirms that the nickel and oxygen contents (wt.%) are 47.33% and 52.67%, respectively, which correspond to a Ni/O mole ratio of 1:3 and are consistent with the hydrated Ni(OH) 2 composition.
The N 2 adsorption-desorption isotherm curves of Ni/Ni(OH) 2 -NF catalyst and corresponding bare-Ni(OH) 2 are shown in Figure 2b. According to the IUPAC classification of adsorption-desorption isotherms [27], the isotherms of Ni/Ni(OH) 2 -NFs and bare-Ni(OH) 2 both show a type IV (H 3 ) shape with a N 2 gas capillary condensation hysteresis loop in a P/P • range of 0.45-1.0.
The N 2 adsorption-desorption isotherm confirms that the Ni/Ni(OH) 2 -NFs architecture possesses a mesoporous structure with disordered slit and lamellar shapes, which is consistent with the Ni/Ni(OH) 2 -NFs nanoflake morphology [28]. The BET-specific surface area of Ni/Ni(OH) 2 -NFs obtained from the N 2 adsorption-desorption isotherms was estimated to be 165 ± 5.0 m 2 /g, which is significantly higher (about seven times) than that of the bare-Ni(OH) 2 counterpart (23.5 m 2 /g). The results indicate the presence of a typical mesoporous material throughout the nanoflake architecture, which is consistent with that mentioned in the literature for mesoporous nickel hydroxide architecture [24,28]. The N2 adsorption-desorption isotherm curves of Ni/Ni(OH)2-NF catalyst and corresponding bare-Ni(OH)2 are shown in Figure 2b. According to the IUPAC classification of adsorption-desorption isotherms [27], the isotherms of Ni/Ni(OH)2-NFs and bare-Ni(OH)2 both show a type IV (H3) shape with a N2 gas capillary condensation hysteresis loop in a P/P° range of 0.45-1.0.   The N2 adsorption-desorption isotherm confirms that the Ni/Ni(OH)2-NFs architecture possesses a mesoporous structure with disordered slit and lamellar shapes, which is consistent with the Ni/Ni(OH)2-NFs nanoflake morphology [28]. The BET-specific surface area of Ni/Ni(OH)2-NFs obtained from the N2 adsorption-desorption isotherms was estimated to be 165 ± 5.0 m 2 /g, which is significantly higher (about seven times) than that of the bare-Ni(OH)2 counterpart (23.5 m 2 /g). The results indicate the presence of a typical mesoporous material throughout the nanoflake architecture, which is consistent with that mentioned in the literature for mesoporous nickel hydroxide architecture [24,28].  During the first cycle, the anodic/cathodic redox peaks observed at potentials of 0.465/0.180 V vs. Ag/AgCl for Ni/Ni(OH) 2 -NFs and 0.480/0.165 V for bare-Ni(OH) 2 can be assigned to the Ni(II)/Ni(III) redox couple as a result of the reaction (Ni(OH) 2 + OH − → NiOOH + H 2 O + e − ) [29,30]. As cycling continues, very small changes in redox peak potential and current are observed, and the redox peak separation (∆E = E anodic − E cathodic ) reaches 0.320 and 0.285 V for bare-Ni(OH) 2 and Ni/Ni(OH) 2 -NFs, respectively, indicating that the Ni(II)/Ni(III) redox reaction is more stable and more reversible with the Ni/Ni(OH) 2 -NF catalyst. As shown above, the X-ray diffraction result suggests the Ni/Ni(OH) 2 -NF catalyst has a non-stoichiometric Ni and α-Ni(OH) 2 structure, and because of the reaction with air/moisture, an intercalated buffer layer (1-2) of NiO can form (Ni/NiO/α-Ni(OH) 2 ), which is electrochemically converted to nickel oxyhydroxide (NiOOH) according to the reported mechanism by Medway et al. [31].

Electrochemical Characterization of Ni/Ni(OH) 2 -NF Catalyst
The Ni(II)/Ni(III) redox peak current density is significantly affected by the Ni/Ni(OH) 2 -NF loading, as illustrated in Figure 3b. The cyclic voltammograms displayed in Figure 3b show the effect of various catalyst loadings (10, 50, and 100 µg) alongside a comparison with bare-Ni(OH) 2 (100 µg, blue line) in a 2.0 M KOH solution at a scan rate of 50 mV s −1 . It can be seen that at similar catalyst loadings, the Ni(II)/Ni(III) redox peak current of the Ni(OH) 2 -NF electrode is significantly higher than that of bare-Ni(OH) 2 , and the current density of the redox peak steadily increases when the Ni(OH) 2 -NF loading increases. The results indicate that the electrochemical active surface area (ECSA) of Ni(OH) 2 -NFs is superior due to the unique nanoflake architecture with enhanced porosity, which increases the rate of the redox process and facilitates the mass transport of OH− ions to the catalyst surface [32].
The electroactive surface area (ECSA) of Ni(OH) 2 -NFs can be estimated using the equation ECSA = Q/(mq), where Q is the charge under the reduction peak of the Ni(OH) 2 /NiOOH redox reaction, q is the charge required for Ni(OH) 2 monolayer formation (equivalent to 257 µC/cm), and m is the catalyst mass, [33]. Herein, the ECSA of Ni/Ni(OH) 2 -NFs and bare-Ni(OH) 2 catalyst is estimated at 354.3 and 23.5 m 2 /g, respectively. The ECSA of Ni/Ni(OH) 2 -NFs is 15 times higher than that of bare-Ni(OH) 2 , which is a significant difference and attributed to the presence of nanoflakes with a highly mesoporous ultrathin architecture, in agreement with the BET results above. Figure 3c shows the cyclic voltammograms of 100 µg/cm 2 Ni(OH) 2 nanoflake catalyst and illustrates the effect of increasing KOH electrolyte concentrations on the electrochemical performance of the catalyst. A significant shift in the onset potential of the oxidation peak is observed as the KOH concentration increases from 0.1 to 2.0 M, which is attributed to the Nernst effect, in which the potential is inversely proportional to the hydroxyl ion concentration.
The nature of the redox process at the Ni(OH) 2 -NF electrode surface was investigated by observing the effect of increasing the scan rate from 5 mV s −1 to 50 mV s −1 , as shown in Figure 3d. As soon as the scan rate increases, a slightly positive shift in the anodic peak potential is observed, while the cathodic peak moves in the negative direction. In addition, a linear relation is obtained between the redox peak currents and the square root of the scan rate (inset in Figure 3d), which is evidence that the formation of the β-NiOOH reaction is controlled by ion diffusion; this result agrees well with previously reported results [23,25].
To evaluate the electrocatalytic performance towards the methanol oxidation reaction (MOR) in an alkaline medium, the CVs of Ni(OH) 2 -NF and bare-Ni(OH) 2 catalysts were recorded at 50 mV s −1 in 2.0 M KOH solution containing 0.5 M methanol, and the results are shown in Figure 4a. As illustrated in Figure 4a, the mesoporous Ni(OH) 2 -NF catalyst shows a substantial anodic current enhancement reaching 54.5 mA/cm 2 at 0.6 V vs. Ag/AgCl, which is five times higher than that observed for the bare-Ni(OH) 2 catalyst, confirming the superior electrocatalytic activity of the Ni(OH) 2 -NF catalyst towards methanol oxidation in alkaline solution. Additionally, the mesoporous Ni(OH) 2 -NFs display a methanol oxidation onset potential of 0.330 V vs. Ag/AgCl, which is consistent with the potential of Ni 2+ /Ni 3+ oxidation of the Ni(OH) 2 -NF catalyst in the absence of methanol. Figure 4c shows the CVs of different loadings of Ni(OH) 2  results are shown in Figure 4a. As illustrated in Figure 4a, the mesoporous Ni(OH)2-NF catalyst shows a substantial anodic current enhancement reaching 54.5 mA/cm 2 at 0.6 V vs. Ag/AgCl, which is five times higher than that observed for the bare-Ni(OH)2 catalyst, confirming the superior electrocatalytic activity of the Ni(OH)2-NF catalyst towards methanol oxidation in alkaline solution. Additionally, the mesoporous Ni(OH)2-NFs display a methanol oxidation onset potential of 0.330 V vs. Ag/AgCl, which is consistent with the potential of Ni 2+ /Ni 3+ oxidation of the Ni(OH)2-NF catalyst in the absence of methanol. Figure   The results reveal that methanol oxidation currents at the mesoporous Ni(OH)2-NF electrode gradually increase with increasing catalyst loading due to the enhancement of Ni 2+ /Ni 3+ active sites, which are required for methanol adsorption and oxidation in 2.0 M KOH. To identify the reaction mechanism of methanol oxidation at the Ni(OH)2-NF cata-  The results reveal that methanol oxidation currents at the mesoporous Ni(OH) 2 -NF electrode gradually increase with increasing catalyst loading due to the enhancement of Ni 2+ /Ni 3+ active sites, which are required for methanol adsorption and oxidation in 2.0 M KOH. To identify the reaction mechanism of methanol oxidation at the Ni(OH) 2 -NF catalyst, CVs were performed at various scan rates from 5.0 to 50 mVs −1 in 2.0 M KOH containing 0.5 M methanol, and the results are shown in Figure 4d. Clearly, the CVs show that the methanol oxidation current increases with the scan rate. As shown in Figure 4d (inset), a linear plot between peak current density (i peak ) and the square root of the scan rate is obtained, and the line intercepts with the current axis at about 30.5 mA/cm 2 (R 2 = 0.9892). This indicates that a mixed kinetic-diffusion-controlled methanol oxidation process occurs at the Ni(OH) 2 -NFs catalyst in alkaline conditions.
The mechanism of methanol oxidation at the Ni(OH) 2 -NFs electrode can be adapted according to the EC-type mechanism described by Fleischman et al. [33], in which the redox couple of Ni 2+ /Ni 3+ species is generated during the anodic electrooxidation of Ni(OH) 2 in alkaline solution according to Equation (1).
n NiOOH + CH 3 OH → n Ni(OH) 2 + Products (2) The observed decrease in the cathodic peak current, which is around 0.25 V vs. Ag/AgCl in the presence of methanol (Figure 4a), is associated with the consumption of NiOOH species in the chemical reaction with methanol and the regeneration of Ni(OH) 2 according to Equation (2). Interestingly, this cathodic peak current decreases as the methanol concentration increases (Figure 4b) due to the depletion of Ni 2+ /Ni 3+ active sites, while its current increases when the catalyst loading increases (Figure 4c). This confirms that the methanol oxidation reaction follows the EC-type mechanism [34].
Nyquist plots were generated to investigate the electrode/electrolyte interface charge resistance of electrochemical methanol oxidation. Figure 5a shows EIS Nyquist plots and the equivalent circuit recorded in 2.0 M KOH containing 0.5 M methanol at 100 µg/cm 2 Ni(OH) 2 -NFs, as compared to bare-Ni(OH) 2 electrodes, at an applied potential of 0.6 V vs. Ag/AgCl and frequency of 100 kHz to 0.1 Hz. Figure 5b illustrates the Nyquist plots of the Ni(OH) 2 -NF catalyst at different applied potentials from 0.35 to 0.60 V vs. Ag/AgCl. The Nyquist profile displays a very small and depressed arc in the high-frequency zone, while a large semicircle in the low-frequency zone can be observed at all studied potentials. As shown in Figure 5, the small semicircle in the high-frequency region can be attributed to the charge transfer characteristic of methanol oxidation at the electroactive Ni(OH) 2 /NiOOH sites, while the large semicircle in the low-frequency domain is associated with the impedance of the intermediates' adsorption process.
Ni(OH)2-NF catalyst at different applied potentials from 0.35 to 0.60 V vs. Ag/AgCl. The Nyquist profile displays a very small and depressed arc in the high-frequency zone, while a large semicircle in the low-frequency zone can be observed at all studied potentials. As shown in Figure 5, the small semicircle in the high-frequency region can be attributed to the charge transfer characteristic of methanol oxidation at the electroactive Ni(OH)2/NiOOH sites, while the large semicircle in the low-frequency domain is associated with the impedance of the intermediates' adsorption process. The Nyquist plots in Figure 5a of the bare-Ni(OH)2 and Ni/Ni(OH)2-NF electrodes can be adequately modeled with the equivalent circuit shown in Figure 5a (inset). The fitted equivalent circuit parameters are defined as the electrolyte resistance (Rs), the charge transfer resistance (R1), and the capacitance of the double layer at the catalyst/electrolyte interface (C1), while R2 is related to the rate of intermediate adsorption resistance, and Q1 is the equivalent adsorption capacitance of methanol oxidation at the Ni(OH)2/NiOOH interface. Table 1 clearly shows that Rs, representing the electrolyte resistance, remains almost constant with an average value of 7.50 Ω. At an applied potential of 0.50 V or higher, the methanol charge transfer resistance R1 is less than 0.480 Ω for the mesoporous Ni/Ni(OH)2 nanoflakes, which is less than that of the bare-Ni(OH)2 electrode (1.46 Ω), indicating that superior methanol oxidation kinetics take place at the Ni/Ni(OH)2-NFs electrode. However, the charge transfer R1 significantly increases as the oxidation potential changes from 0.50 to 0.35 V vs. Ag/AgCl. This confirms that the kinetics of methanol electrooxidation at the Ni/Ni(OH)2 nanoflakes are enhanced at a higher potential, while, at a The Nyquist plots in Figure 5a of the bare-Ni(OH) 2 and Ni/Ni(OH) 2 -NF electrodes can be adequately modeled with the equivalent circuit shown in Figure 5a (inset). The fitted equivalent circuit parameters are defined as the electrolyte resistance (R s ), the charge transfer resistance (R 1 ), and the capacitance of the double layer at the catalyst/electrolyte interface (C 1 ), while R 2 is related to the rate of intermediate adsorption resistance, and Q 1 is the equivalent adsorption capacitance of methanol oxidation at the Ni(OH) 2 /NiOOH interface. Table 1 clearly shows that R s , representing the electrolyte resistance, remains almost constant with an average value of 7.50 Ω. At an applied potential of 0.50 V or higher, the methanol charge transfer resistance R 1 is less than 0.480 Ω for the mesoporous Ni/Ni(OH) 2 nanoflakes, which is less than that of the bare-Ni(OH) 2 electrode (1.46 Ω), indicating that superior methanol oxidation kinetics take place at the Ni/Ni(OH) 2 -NFs electrode. However, the charge transfer R 1 significantly increases as the oxidation potential changes from 0.50 to 0.35 V vs. Ag/AgCl. This confirms that the kinetics of methanol electrooxidation at the Ni/Ni(OH) 2 nanoflakes are enhanced at a higher potential, while, at a lower potential, the oxidation of the Ni/Ni(OH) 2 to NiOOH species is predominant, as shown by the cyclic voltammetry results.  (26.8 Ω) and decrease as the applied potential becomes more positive. This decrease in R 2 is related to the higher tolerance of mesoporous Ni/Ni(OH) 2 -NFs to the adsorption of methanol oxidation intermediates due to the availability of Ni 2+ /Ni 3+ active sites.
The long-term stability of a catalyst is one of the important requirements of methanol fuel cell application. Figure 6a shows long-term oxidation for 100 multicycles using 100 µg/cm 2 Ni/Ni(OH) 2 -NF at 50 mVs −1 in 0.5 M methanol in 2.0 M KOH. As shown in Figure 6a, after 100 cycles over a potential range of −0.200 to 0.700 V vs. Ag/AgCl, the methanol oxidation current density remains relatively stable, indicating that the mesoporous Ni/Ni(OH) 2 -NF catalyst is extremely stable during the methanol oxidation reaction. Figure 6b shows the chronoamperometric (current/time) transient during the long-term oxidation of 0.5 M methanol in 2.0 M KOH at different applied potentials (0.400, 0.500, and 0.700 V vs. Ag/AgCl) using different loadings of Ni/Ni(OH) 2 -NFs in comparison to the bare-Ni(OH) 2 catalyst. The chronoamperometric measurements confirm that the methanol oxidation current density obtained at the Ni/Ni(OH) 2 nanoflake electrode remains very stable and significantly increases upon increasing either the applied potential or the catalyst loading over an extended oxidation process of 200 min. Moreover, the current density at 0.700 V vs. Ag/AgCl shows that the methanol oxidation mass activity of the Ni/Ni(OH) 2 -NF catalyst (current density/catalyst mass) reaches about 625 A/cm 2 g cat , which is more than seven times higher than the parent bare-Ni(OH) 2 catalyst, which is consistent with the cyclic voltammetry results.
Nanomaterials 2022, 12, x FOR PEER REVIEW 13 of 16 [22], and a nanoribbon structure (NR-Ni(OH)2) (180 A/cm 2 gcat) [21]. In addition, with the advantage of a simple synthesis approach in a one-pot template mixture and under room conditions, the Ni/Ni(OH)2-NF catalyst exhibits a comparable methanol oxidation mass activity to that of the nickel metal-organic framework supported on reduced graphene oxide (NiO-MOF/rGO, 540 A/cm 2 gcat) [34].   The outstanding stability and enhanced methanol oxidation activity exhibited by the Ni/Ni(OH) 2 -NF catalyst can be attributed to the presence of a mesoporous network of overlapping Ni/Ni(OH) 2 nanoflakes with high surface area and Ni 2+ /Ni 3+ active sites required for methanol adsorption and oxidation [13,17,23,25]. In addition, the methanol oxidation current stability indicates the tolerance and resistance of Ni/Ni(OH) 2 -NFs to passivation by the adsorption of the intermediates and products of the methanol oxidation reaction [7,35].
The electrochemical activity of our mesoporous Ni/Ni(OH) 2 nanoflake catalyst was evaluated and compared with previously reported catalysts. Table 2 displays a comparison of characteristic methanol oxidation parameters, namely, onset potential, stability, and mass activity (current/cm 2 g cat ), between our mesoporous Ni/Ni(OH) 2 nanoflake catalyst and cutting-edge nickel-based electrocatalysts reported in the literature. For instance, our Ni/Ni(OH) 2 nanoflake catalyst showed significantly higher methanol oxidation mass activity (545 A/cm 2 g cat at 0.6 V vs. Ag/Cl) and stability than most of the cutting-edge nickel-based catalysts, such as NiCo 2 O 4 /SS (50 A/cm 2 g cat ) [35], NiO-NS@NW/NF (∼55 A/cm 2 g cat ) [36], NiCo 2 O 4 -rGO (∼63 A/cm 2 g cat ) [37], Ni 3 S 2 -CNFs/CC (∼120 A/cm 2 g cat ) [22], and a nanoribbon structure (NR-Ni(OH) 2 ) (180 A/cm 2 g cat ) [21]. In addition, with the advantage of a simple synthesis approach in a one-pot template mixture and under room conditions, the Ni/Ni(OH) 2 -NF catalyst exhibits a comparable methanol oxidation mass activity to that of the nickel metal-organic framework supported on reduced graphene oxide (NiO-MOF/rGO, 540 A/cm 2 g cat ) [34]. Table 2. Comparison of current densities and onset potentials between the presented catalyst and previously reported nickel-based electrocatalyst materials for alkaline methanol electrooxidation reaction.

Catalysts
Onset Potential, V Stability, min