Stimulation of ethylene glycol electrooxidation on electrodeposited Ni–PbO2–GN nanocomposite in alkaline medium

In this work, a novel system composed of non-precious nickel-based metal oxide/reduced graphene oxide nanocomposite (Ni–PbO2–GN) is used for electrooxidation of ethylene glycol (EG) in 1.0 M NaOH solution and compares its activity with that of Ni, Ni–GN, and Ni–PbO2. The facile electrodeposition technique is used to prepare the catalysts on glassy carbon (GC) substrates. The outcomes of electrochemical measurements show a high performance towards EG oxidation is obtained for Ni-nanocomposite electrodes compared to that of Ni mainly due to their higher surface areas. The excellent electrocatalytic properties of the Ni-nanocomposite could be ascribed to the synergistic contributions of PbO2 and graphene (GN) nano-sheets that help the reduction of Ni grains. A smaller charge transfer resistance value of 34.5 Ω cm2 for EG oxidation reaction at + 360 mV is recorded for GC/Ni–PbO2–GN compared to the other prepared electrodes. Moreover, it exhibits higher kinetic parameters of EG such as diffusion coefficient (D = 3.9 × 10–10 cm2 s−1) and charge transfer rate constant (ks = 32.5 mol−1 cm3 s−1). The overall performance and stability of the prepared catalysts towards EG electrooxidation have been estimated to be in the order of GC/Ni–PbO2–GN > GC/Ni–GN > GC/Ni–PbO2 > GC/Ni.


Introduction
Due to the current deficit of conventional energy resources, developing sustainable energy resources is of great urgency. Direct fuel cells are promising renewable and clean sources due to environmental considerations [1]. Ethylene glycol (EG) is a promising alternative fuel used in proton-exchange membrane fuel cells (PEMFCs), it had received great attention because it displays superiorities over methanol as a fuel. It has high energy density, less toxic and less volatile liquid with high water solubility, and low flammability. Moreover, its electron transfer rate during its oxidation is much higher than that of methanol. EG could be produced as the byproduct of blooming biodiesel with a high yield and nowadays, it is produced from the hydrolysis of ethylene oxide [2,3]. Also, it can be synthesized from heterogeneous hydrogenation of cellulose derivatives [4]. The complete oxidation of one molecule of EG to CO 2 results in a gain of 10 electrons, while the complete oxidation of one molecule of methanol produces 6 electrons [5]. In the acidic media, the complete electrochemical oxidation of EG to CO 2 involves 10 electrons per molecule [6], as described in the following reaction:-Relatively high-cost catalysts are used for this reaction in the acidic medium such as Pt or Pt-based surfaces and several substances are formed as a result of the oxidation process, such as glycolaldehyde, glyoxal, glycolic acid, glyoxilic acid, oxalic acid, and others [7,8]. Additionally, the energy efficiency of ethylene glycol oxidation reaction (EGOR) in the acidic media is low, because Pt is easily poisoned by CO ads that is formed during the oxidation of the alcohol at the anode of a fuel cell [6,[9][10][11]. On the contrary, the oxidation reaction of EG is more feasible in alkaline media from a kinetic point of view because of the higher activity of EGOR and the easier electron transfer in the presence of hydroxide ions [12][13][14][15][16][17][18][19]. A variety of catalyst materials can be used in alkaline media that have high catalytic activity and relatively low cost. The complete oxidation of EG in alkaline media can be presented as follows [20]: (1) HO − CH 2 − CH 2 − OH + 2 H 2 O → 2 CO 2 + 10 H + + 10 e − (2) CH 2 OH 2 + 10 OH − → 2 CO 2 + 8 H 2 O + 10 e − Different oxidation products of EG are produced depending on the reaction conditions and the catalytic materials. Oxalate was detected as a final product during EGOR by a non-poisonous pathway as follows [20,21]: Another pathway leads to the formation of glycolates and formate which can be further oxidized to poisonous products as follows [20][21][22]: But, generalizing the use of EG fuel cells on a commercial scale requires highly efficient and cheap anode catalysts for EGOR. For many years, EGOR has been extensively investigated on various metal and modified metal electrodes such as Pt, Pd Au, Co, Ag, Fe, and Ni in alkaline media [19,[23][24][25][26][27][28][29][30][31]. The use of Pt and Pt-based catalyst anodes are of great importance due to their high efficiency towards the EGOR and the reaction takes place at lower potential values, but its high cost and its poisoning by CO adsorbed species are challenges [6, 10-12, 24, 25]. Moreover, Pd and its alloys or composites are used as alternative catalysts to Pt for EGOR because they have high catalytic activity as Pt and are of relatively low cost [17, 26-30, 32, 33].
Over the few past decades, nickel-based materials such as metallic nickel, nickel oxides, hydroxides and oxy-hydroxides, Ni alloys, and composites are extensively used as anodic catalysts for the electrooxidation processes because they exhibited excellent electrocatalytic performance toward many small organic molecules [17,27,[34][35][36][37][38][39][40]. Raney nickel is used as an electrocatalyst for EG electrooxidation, it is oxidized at high current densities at elevated temperatures and glycolate is estimated as an intermediate that could be further oxidized to oxalate [41]. Other authors suggested a selective production of formate followed by further oxidation to carbonate, suggesting that the C-C bond cleavage occurred readily on the Ni surface as follows [42]:-On the other hand, Ni when alloyed with noble metals such as Pt, Pd, and Au showed enhanced electrocatalytic activity toward EGOR [17,28,37,43,44]. The nanostructured electrocatalysts Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C, and Pd/C are very active compared to the smooth Pd electrode towards EGOR. The oxidation products oxalate and carbonate were obtained with Pd-(Ni-Zn)/C or Pd-(Ni-Zn-P)/C [42]. Au-decorated Pt 1 Ni 3 /C electrode (Au/Pt 1 Ni 3 /C) was prepared and used as an anode catalyst for the oxidation of EG in an alkaline solution. It was found a significant enhancing effect of Au adatoms on the activity of Pt 1 Ni 3 /C for EGOR [44]. The low-cost AuPt/Ni/reduced graphene oxide was prepared by electrodeposition of AuPt on Ni nanoparticles. Its electrocatalytic activity towards EGOR is similar to monometallic Pt/reduced graphene oxide [37]. It was found that Ni enhanced the electrocatalytic activity of Au for the electrooxidation of ethylene glycol and glycerol in the alkaline media. These improvements are attributed to the ability of Ni to enhance the OH − adsorbed species on the Au surface, thereby inducing a lower onset potential and helping to remove intermediates from the active site [45]. Also, the addition of Ni to Pd, as an oxophilic element, enhances the adsorption of OH − leading to the oxidation of the chemisorbed intermediates [46]. Pt-Ni nanoparticles supported on (MoS 2 )/reduced graphene oxide (RGO) is synthesized and used for EGOR. The catalytic activity of the prepared composite electrode showed large enhancement with the optimized molar ratio of Pt to Ni being 3:1 [38].
It was reported that metal oxides spontaneously form surface hydroxyl groups that help to complete oxidation of intermediates especially CO species to CO 2 [47]. Non-precious metals/metal oxides, such as Ni and Co, are promising candidates for electrocatalysts towards the EGOR in alkaline media [48,49]. Electrooxidation of EG was studied on nickel ion implanted-modified indium tin oxide electrode (NiNPs/ ITO) in NaOH solution. The prepared catalyst exhibited a satisfactory electrocatalytic activity and good stability in the electrooxidation process [50]. Also, bimetallic Ni and Ti nanoparticle-modified indium tin oxide electrode (Ni-Ti NPs/ITO) was prepared by ion implantation method and its catalytic activity was investigated towards the oxidation of EG. It showed higher electrocatalytic activity and stability for EG oxidation than NiNPs/ITO [39].
Combining carbon materials such as graphene with metal oxides was effective to produce high surface area catalysts of high conductivity and catalytic performance [64]. Moreover, the co-presence of metal/metal oxides can remarkably affect the electrocatalytic activity and stability of the catalyst [66]. Pd/C with nanocrystalline oxides (CeO 2 , Co 3 O 4 , Mn 3 O 4 , and NiO x ) electrocatalysts were used for electrocatalytic oxidation of methanol, ethanol, glycerol, and ethylene glycol in 1 3 alkaline media. Those electrocatalysts were superior to Ptbased electrocatalysts in terms of electrocatalytic activity and poison tolerance. On the other side, metal oxides and non-noble metals would increase the concentration of OH ads species on the electrode surface, thus facilitating complete oxidation of the reaction intermediates [67]. Pd mixed valance Mn-Ti oxide-3DG nanoarchitecture catalyst was prepared and used for the electrochemical oxidation of ethylene glycol. It is capable of enhancing EGOR by extracting active oxygen atoms from the electrolyte [68]. Pd-Co oxide nanoparticles (Pd-PdO-CoO x ) were prepared by in situ oxidation of Pd-Co precursor and used for EG electrooxidation in an alkaline medium. The enhanced electrocatalytic activity of Pd-PdO-CoO x catalyst is due to the strong metal-support interactions between PdO-CoO x and Pd nanoparticles and the synergistic effect between PdO and CoO x [69]. Additionally, Ni-metal oxide nanocomposite catalysts (Ni-P-TiO 2 , Ni-Cr 2 O 3 , Ni-ZnO, Ni-Fe 2 O 3 , Ni-Co 3 O 4 , Ni-MnO 2 , Ni-MgO, Ni-Bi 2 O 3 , and Ni-PbO 2 ) have attracted attention as electrocatalysts for electrooxidation processes due to the easiness of their preparations, high surface areas, and improved catalytic activities and stabilities [34,[70][71][72][73][74]. Recently, PbO 2 is used as a metal oxide electrode in many electrochemical applications owing to some attractive properties. It has low electrical resistivity, considerable chemical stability, and high overpotential for the oxygen evolution reaction. Moreover, it is inexpensive and a strong oxidizing agent, but the main concern when using PbO 2 anodes is the possible release of toxic Pb 2+ ions [75,76].
Yet, so far, continuous efforts are made for the development of highly active and cost-effective catalysts for ethylene glycol oxidation reaction (EGOR). Herein, our efforts will focus on tailoring Ni-based binary and ternary nanocomposite systems of Ni-GN, Ni-PbO 2 , and Ni-PbO 2 -GN catalysts on glassy carbon (GC) substrates by a direct, facile, and fast electrodeposition technique. Also, GC/Ni is prepared under the same conditions for comparison. Elucidation of the catalytic roles of Ni, graphene (GN), and PbO 2 will be investigated. It is anticipated for the binary and ternary nanocomposites to boost the electrocatalytic activity and achieve considerable stability for EGOR in alkaline solution to use for direct ethylene glycol fuel cell (EGFC) applications.

Materials
All the chemical materials used in this study are of analytical grade and used as received without any purification. Graphene (GN) is of 99% purity was purchased from Sigma-Aldrich. Ethylene glycol (EG) of 99%, NaOH of 98%, NiSO 4 of 95%, NiCl 2 of 95%, H 3 BO 3 of 99.5% and PbO 2 of 97% were purchased from Fluka. The double-distilled water of resistivity 17 MΩ cm −1 was used for the preparation of solutions.

Preparation of nanocomposite systems
The standard nickel Watts' bath is used for the preparation of the catalyst samples using the galvanostatic deposition technique using Ni rode electrode as an anode and glassy carbon (GC) as a cathode. The chemical composition of the electrolyte used for the deposition is NiSO 4 ; 240 g L −1 , NiCl 2 ; 45 g L −1 , H 3 BO 3 ; 30 g L −1 ,10 g L −1 PbO 2 , 0.02 g L −1 graphene (GN), at pH 5, 55 °C for 30 min deposition time and at a constant current density of 40 mA cm −2 . Boric acid is one of the main compounds in electrolytic baths for nickel deposition. The pK a1 value of boric acid is 9.14, so its predominant form at pH 5 is the un-dissociated molecule [77]. So its role is not a buffering agent but this species has a clear influence on the crystallographic structure, improved morphology, brightness, and adhesion of the deposited Ni [78]. Glassy carbon (GC) substrate electrode of d = 3.0 mm is used as catalyst support. GC/Ni, GC/Ni-PbO 2 , GC/ Ni-GN, and GC/Ni-PbO 2 -GN nanocomposite catalysts are prepared and used as anodes for EG electrooxidation reaction (EGOR). Details of the preparation method are adapted from Ref. [40]. The deposition solution was stirred using a magnetic stirrer at 150 r/min before and during the deposition process to ensure homogenous distribution of the suspended particles (PbO 2 or GN) in the solution and to avoid the assembly of these particles together and to keep them dispersed and avoid their sedimentation in the electrolyte. Zeta potential for PbO 2 and GN is measured using a laser zeta meter (Zetasizer 2000, Malvern Instruments, UK) using a sample powder of 0.01 g PbO 2 and/or GN in 25 ml of Ni Watts' solution of pH 5 and the sample was stirred for 60 min then the zeta potential was measured. The amount of electrodeposited Ni in each electrode is determined by inductively coupled plasma mass spectrometer [ICP(MS)] (Teledyne Leeman Labs ICP model Prodigy 7) after dissolving the deposited layer in 36% nitric acid. Moreover, the real surface areas of the prepared catalysts were estimated using Nova 2000 series depending on Brunauer, Emmett, and Teller's (B.E.T) theory. The specific surface area was measured by N 2 adsorption-desorption isotherm, a type IV isotherm is obtained for the prepared catalysts and from the relation between [1/[n(P/P°) − 1] and the relative pressure (P/P°), we get the slope and intercept that are used for the determination of monolayer (n m ) and the BET surface area is calculated using the following Eq. (7): where N is Avogadro's number, A is the cross-sectional area of the adsorbate and equals 0.162 nm 2 for an absorbed nitrogen molecule, n m is the volume of monolayer (cm 3 ), V° is the molar volume of N 2 at STP which equals 22400 cm 3 and W is the weight of the catalyst sample.

Surface analysis techniques
To examine the surface morphologies and the elemental chemical composition of the prepared nanocomposite catalysts, a scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) with an accelerating voltage of 30 kV (FEI company, The Netherlands) was used. Furthermore, the X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical compositions of the different nanocomposite catalysts, it was performed on K-ALPHA (Thermo Fisher Scientific, USA) with monochromatic X-ray Al K-alpha radiation − 10 to 1350 eV, spot size 400 µm at pressure 10 -9 mbar with full-spectrum pass energy 200 eV and at narrow-spectrum 50 eV. Additionally, the nanocomposite catalyst structures were estimated by X-ray diffractometer (XRD) which operated on a PANalytical X'Pert Pro MPD system with Cu Ka radiation source (λ = 1.54056 Å) at 45 kV and a step rate, size, and range were 0.050/s, 0.020 and 10°-100°, respectively.

Electrochemical measurements
Various electrochemical techniques are used to evaluate the prepared nanocomposite catalysts towards EGOR. Cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy (EIS) are performed in a conventional standard three-electrode Pyrex glass cell composed of a glassy carbon electrode modified with Ni, Ni-GN, Ni-PbO 2 , and Ni-PbO 2 -GN as working electrodes, the reference electrode is the saturated calomel electrode (SCE), E° = 240 mV vs RHE to which all potentials are referred and a Pt sheet as a counter electrode. All measurements were performed at room temperature of 25 ± 0.2 °C in 1.0 M NaOH and in the presence of 1.0 M EG. The amount of Ni catalyst of each electrode in mg is used to calculate the current density as mA mg Ni −1 and the geometric surface area of the electrode is used to calculate the current density in mA cm −2 for all electrochemical performance data. EIS data were performed at a constant voltage mode at frequencies ranging from 100 kHz to 100 mHz with an applied amplitude of 10 mV. The holding time of 15 min was applied at a definite potential to approach a near steady-state before each measurement. The electrochemical system used is IM6 Zahner potentiostat, Germany, joined to Thales software for analyses of the electrochemical data. The equivalent circuit is estimated using SIM program which is built-in the IM6 package to best fit the experimental impedance spectra. The correctness of the elements in the proposed equivalent circuits to best fit the experimental curves has an error of 2.1% of the fitting.

Characterization and surface analysis
The scanning electron microscopy (SEM) images Fig. 1a-d presents the surface morphology of electrodeposited Ni, Ni-GN, Ni-PbO 2 , and Ni-PbO 2 -GN, respectively. The surface morphology of electrodeposited Ni (Fig. 1a) has a regular pyramidal structure well distributed on the substrate with some aggregations. While the SEM image of Ni-GN ( Fig. 1b) is characterized by more fine granular morphologies than that of Ni homogeneously distributed on the entire surface. On the other hand, the SEM images of Ni-PbO 2 and Ni-PbO 2 -GN ( Fig. 1c, d) indicate fine surfaces with a significant reduction in the Ni grain size with a relatively uniform distribution on the entire substrate surfaces. Reduction in Ni grains is detected by the naked eye from the SEM images and the grain size of Ni was found in the order of Ni-PbO 2 -GN ˂ Ni-PbO 2 ˂ Ni-GN ˂ Ni. The inclusion of GN and/or PbO 2 in the Ni matrix results in the decrease of Ni grain size, this could be due to the adsorption of PbO 2 and /or GN particles on the Ni grain boundary which limits the Ni crystal growth according to the Guglielmi model of deposition [79] and leads to more fine surface and high dispersity of Ni particles on the entire surface. The measured zeta potential values of − 22 and − 2.5 V for PbO 2 and GN particles, respectively greatly support this vision.
The EDX analysis (Table 1) represents the energy dispersive X-ray (EDX) data of the four catalysts prepared at 40 mA cm −2 deposition current density. It shows the W% of C, O, and Ni for the electrodeposited Ni on glassy carbon electrode and it is also represented in Fig. 2a  Additionally, the microstructures of the prepared Ni, Ni-PbO 2 , and Ni-PbO 2 -GN catalysts were inspected by X-ray diffraction (Fig. 4). The diffraction peaks for Ni electrode ( Fig. 4a) situated at the 2Ɵ values of 44.30°, 51.55°, 76.05° and 82.55° are corresponding to the crystal planes (1 1 1), (2 0 0), (2 2 0), and (3 1 1), respectively for Ni according to the standard crystallographic pattern of Ni (JCPDS 40-0850), which indicate the deposition of nickel particles. In addition, a small peak is detected at 2Ɵ of 43.5° corresponds to NiO x (0 0 2), and a diffraction peak presented  XPS is a powerful analysis tool is employed to characterize the surface chemical compositions and states of catalysts. Figure 5a indicates XPS survey spectra of Ni-PbO 2 and Ni-PbO 2 -GN, respectively, on glassy carbon substrates. XPS spectra of Ni-PbO 2 -GN catalyst once again manifested the formation of the nanocomposites. The presence   [84][85][86][87].
The high-resolution XPS spectra of C1s of Ni-PbO 2 electrode (Fig. 5c′) indicate two peaks at 284.57 and 288.07 eV are due to C-C and C=O, respectively. Additional peaks are detected for C1s of Ni-PbO 2 -GN electrode (Fig. 5c),

Electrochemical analysis
To evaluate the electrochemical properties of the prepared catalysts, the amount of nickel catalyst in each electrode is determined by ICP MS and it is used to calculate the current density as mA mg Ni −1 . The determined amount of electrodeposited Ni in GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN is 2, 1.88, 1.8, and 1.65 mg, respectively on a glassy carbon surface of diameter 3 mm. Moreover, the current density is calculated as mA cm −2 . The cyclic voltammogram curves were firstly recorded in 1.0 M NaOH solution, then in the presence of 1.0 M EG at a sweep rate of 50 mV s −1 . From Fig. 6a and a′ it is obvious that two redox peaks are observed for all the prepared catalysts in 1.0 M NaOH, the forward peak is related to the Ni(II)/ Ni(III) redox couple, and the backward one is attributed to Ni(III)/Ni(II) redox species. The oxidation peaks current (I p ) is taken as criteria to evaluate the catalytic activity of different prepared catalysts. It is worth noticing that different heights of the peaks' current density and slightly different onset potentials are observed for the formation of Ni(III) species of the prepared electrode catalysts. It is well known that Ni(III) species is crucial for electrooxidation processes. solution and it is electrooxidized to NiOOH species then NiOOH chemically oxidizes EG, and Ni(OH) 2 species are regenerated [91,92]. The order of increasing the oxidation peaks current and accordingly the catalytic activity is GC/ Ni-PbO 2 -GN > GC/Ni-GN > GC/Ni-PbO 2 > GC/Ni. It is worth noticing that the electrochemical activity of the Nibased nanocomposite electrodes is much higher than that of the Ni electrode and the highest peaks′ current is recorded for GC/Ni-PbO 2 -GN electrode compared to the other prepared electrodes, which indicates higher catalytic activity. The onset potential for the formation of the Ni(III) species at GC/Ni, GC/Ni-GN and GC/Ni-PbO 2 is at about 0.3 V (SCE) and slightly shifted to a lower potential value for the GC/Ni-PbO 2 -GN nanocomposite as shown in Fig. 6a and a′. On the other hand, Ni and Ni-based nanocomposite electrodes show separation peak-to-peak potential is about 100, 90, 100, and 250 mV for GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN, respectively. This deviation from the theoretical value of zero confirms the irreversibility of the electrooxidation process.
Upon the addition of 1.0 M EG, a sudden increase in the anodic current is observed at the prepared Ni and Ni-based nanocomposite electrodes (Fig. 6b-e, b′-e′). The oxidation of EG begins once NiOOH species is formed [93,94], and the oxidation current increases in the anodic direction reaching its maximum at the peak. Another re-oxidation peak appears in the backward sweep very close to the peak that appeared in the forward sweep which could be due to the oxidation of EG or the intermediate products of its oxidation for GC/Ni, GC/Ni-GN, and GC/Ni-PbO 2 electrodes. For GC/Ni-PbO 2 -GN, no obvious EG electrooxidation peak is observed but just an increase of the oxidation current in the anodic direction, and it is merged with the oxygen evolution reaction. Also, no obvious reverse oxidation peak is observed in the cathodic direction probably due to nearly complete oxidation of EG in the forward sweep or nearly no poisonous intermediate species are adsorbed on the electrode surface. This is because the evolved oxygen could help in the oxidation of the poisonous intermediates.
On the other side, a decrease in the cathodic current in the backward sweep of EG is recorded confirming that most of NiOOH species formed in the forward sweep are consumed in the oxidation process. The oxidation current densities of EG on GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN are 68, 87.3, 130, 172 mA cm −2 , respectively at + 0.80 V (SCE). Additionally, the oxidation current densities in mA mg Ni −1 are 2.4, 3.3, 5.1, and 7.3, for GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN, respectively (see Table 2). It is clear that the catalytic activities of Ni-based nanocomposite electrodes are much higher than that of the GC/Ni electrode. The catalytic activity of GC/Ni-PbO 2 -GN is about threefold higher than that of GC/ Ni, while the catalytic activity of GC/Ni-GN is about twice higher than that of GC/Ni, and that of GC/Ni-PbO 2 is about 1.3 higher than that of GC/Ni. Generally, the catalytic activity of any catalyst relates to many factors such as the surface area, synergism between the different catalytic species, identity of the catalytic species, amount and morphology of the prepared catalysts. In this work the surface areas of the prepared Ni and Ni-nanocomposites were estimated based on B.E.T. theory using N 2 adsorption-desorption isotherm. Isotherms of Ni, Ni-PbO 2 , Ni-GN and Ni-PbO 2 -GN catalysts are presented in (Fig. S1,  supplementary data), a type IV isotherm with H3 hysteresis loop based on the IUPAC classification [95] is obtained for all the prepared catalysts which is a characteristic isotherm for mesoporous materials. The mesoporous nature of the catalysts can help access the electrolyte inside the pores and accordingly facilitate the reaction to occur on a higher surface area. Comparative values obtained from surface area and porosity analysis are shown in Table 3. It includes BET surface area in m 2 /g, the average particle radius in nm, the average pore radius in nm and the average pore volume in cc/g for Ni, Ni-PbO 2 , Ni-GN, and Ni-PbO 2 -GN, respectively. BET specific surface area values of 58, 96, 105, and 122 m 2 g −1 are estimated for GC/Ni, GC/Ni-PbO 2 , GC/ Ni-GN, and GC/Ni-PbO 2 -GN, respectively. The high surface area of Ni-nanocomposite catalysts could due to the better dispersion of Ni particles, in addition to the smaller particle size of Ni. Reduction in Ni grains size for the Ninanocomposites is also detected by SEM (see Fig. 1). The average particle radius of Ni is 4.4, 3.9, 3.3, and 3.2 nm for GC/Ni, GC/Ni-PbO 2, GC/Ni-GN and GC/Ni-PbO 2 -GN, respectively ( Table 3). The inclusion of GN and/or PbO 2 in the Ni matrix results in the decrease of Ni particle radius, this could be due to the adsorption of PbO 2 and /or GN particles on the Ni grain boundary which limits the Ni crystal growth as mentioned above and leads to more fine surface. The higher surface areas of Ni-based nanocomposite electrodes could increase the number of active sites available for EG molecules to bind and oxidize [40]. So, it plays a very important role in boosting the catalytic activity. Moreover, the smaller size nanoparticles in the Ni-based nanocomposite electrodes could increase the number of active sites and the higher pore volume (0.26, 0.31 and 0.34 cc/g for Ni-PbO 2 , Ni-GN, and Ni-PbO 2 -GN, respectively) can access and facilitate the diffusion of EG reactant molecules to the electrode surface to be oxidized [72].
The catalytic activity of GC/Ni-PbO 2 could be due to the higher surface area and the cooperation between Ni and Pb oxides in the oxidation process, PbO 2 stimulates the adsorption of EG and the Ni oxide causes the mediation of the oxidation process. Similar work was done by Shi et al. on Ni-Mo/graphene nanocatalysts towards urea electrooxidation reaction [96], in which a favorable synergy between the different oxides is proposed which leads to a higher absorbability of the fuel molecules on the Ni-nanocomposite electrode surface. A dual role is played by the mixed oxides in the Ni-nanocomposite electrodes; one enhances the adsorption of the fuel molecules and the other causes the mediation of the oxidation process. Another attractive feature of PbO 2 is that it is a strong oxidizing agent [75,76] that could help in the Ni 2+ /Ni 3+ transformation process which leads to a higher oxidation current of EG. Moreover,  the highly oxophilic property of metal oxides likes PbO 2 provides conditions for the occurrence of a bifunctional mechanism that boosts the catalyst performance by improving the adsorption of hydroxide ions on the catalyst surface [97]. Additionally, the oxygen of PbO 2 could help in oxidizing the intermediates formed during EG oxidation thus helping complete removal of intermediates and reduce the catalyst poisoning [40].
On the other hand, the higher catalytic activity of GC/ Ni-GN compared with that of GC/Ni is due to the higher surface area provided by graphene (see Table 3) in addition to the high conductivity of the electrode and the better dispersion of Ni nanoparticles preventing agglomeration and giving smaller size of uniform Ni nanoparticles as shown from the abovementioned SEM (Fig. 1b) and BET data Table 3. It is worth noticing that the catalytic activity of GC/ Ni-PbO 2 -GN is the highest among the prepared nanocomposite electrodes. Combining GN with PbO 2 and Ni makes excellent synergism that boosts the catalytic activity due to the previous reasons.
Comparing this work with other similar works on EG oxidation at different electrode catalysts, we will find that this work surpasses them in terms of high efficiency and high oxidation current density of EG as revealed by Tables 2  and 4.
For further discussion of the electrocatalytic behavior of the prepared nanocomposites, a series of electrochemical measurements on the prepared GC/Ni, GC/Ni-PbO 2 , GC/ Ni-GN, and GC/Ni-PbO 2 -GN nanocomposite electrodes were operated in 1.0 M NaOH at various sweep rates from 5 to 1000 mVs −1 (see supplementary data S2). Some electrochemical parameters are extracted from the measurements related to each electrode catalyst such as the surface coverage (Γ*), apparent charge transfer rate constant (k s ), electron transfer coefficient (α), and the formal potential E°. Where E° is calculated using both anodic and cathodic peaks potentials at low sweep rates through Eq. (14).
The estimated values of formal potentials for the four electrodes are almost the same and their values are around 0.3 to 0.31 V that related to the redox species NiOOH/ Ni(OH) 2 . It is worth noticing that for all the prepared electrodes the peak current density of Ni(II)/Ni(III) increases with increasing the sweep rate and the anodic peak potential shows a slight shift towards a more positive direction (towards higher overpotential) and the cathodic peak potential shifts towards the negative potential. The shift is more (12) Ni 2+ HOCH 2 COO − ads + 2 NiOOH → Ni 2+ (CHOCOO − ) ads + 2 Ni(OH) 2 (13) Ni 2+ (CHOCOO − ) ads + 2 NiOOH pronounced in the case of GC/Ni-PbO 2 -GN (see supplementary data S2(d)). This could be due to the IR drop that occurs at high currents. It was found that the peak's currents are proportional to the sweep rates especially at relatively lower sweep rates in the range of 5-100 mVs −1 (see supplementary data S3(a)) indicating the electrochemical catalytic activity of the surface redox species Ni(II)/Ni(III). At a high potential sweep (S3(b)), the peak current (I p ) is proportional to the square root of the sweep rate confirming that the process is diffusion controlled. In addition, to better understand the effects of surface coverage on the electrocatalytic activity, the surface coverage Г * (mol cm −2 ) of the redox species Ni(II)/Ni(III) in 1.0 M NaOH is calculated from (S3(a)) and using Eq. (15): where I p is the peak current density in ampere, υ is the potential sweep rate in volt s −1 , A is the geometric surface area of the working electrode, R is the ideal gas constant which equals 8.314 J K −1 mol −1 , n is the number of exchanged electrons which equals 1 for Ni(II)/Ni(III). The values of Г* are: 0.26 × 10 -7 , 0.77 × 10 -7 , 1.22 × 10 -7 and 2.08 × 10 -7 mol cm −2 for GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN and GC/Ni-PbO 2 -GN, respectively. So a higher catalytic activity surface is related to a higher surface coverage, so the surface coverage and accordingly the catalytic activity increase in the order of GC/Ni-PbO 2 -GN > GC/ Ni-GN > GC/Ni-PbO 2 > GC/Ni. Wherefore, the presence of GN and PbO 2 improves the distribution of Ni over the host electrode and help shaping the Ni size. Also, the favorable synergy between the different species improved the stability of the electrode through repeated cyclization (see Table 2). The charge transfer coefficient is a proportion of the interfacial potential at the electrode-electrolyte interface that aids in lowering the electrochemical reaction's free energy barrier. To explore the transfer coefficients (α) for the anodic part of the voltammograms for the Ni(II)/Ni(III) redox species process of different electrodes, the relation between the sweep rate, ln sweep rate vs the peak potential (E p ) (S3(c, d)), respectively are used in addition to Eqs. 16 and 17. Where Ep a and Ep c are the anodic and cathodic peak potential values in V, respectively, E° is the standard electrode potential, T is the absolute temperature in Kelvin, F is (15) I p = (n 2 F 2 ∕4RT) AΓ * Faraday's constant (96500 C mol −1 ), n is the electron transfer number for Ni(II)/Ni(III), υ is the sweep rate in V s −1 and k s is the electron transfer rate constant in mol −1 L s −1 . Estimated transfer coefficient values of 0.92, 0.73, 0.49, and 0.17 were found for Ni(II)/Ni(III) of GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN, respectively.
The deviation of transfer coefficient (α) value from the standard value of 0.5 could be due to the irreversibility of the oxidation process (see Fig. 6). CVs at different sweep rates for all the prepared electrodes were studied in 1.0 M NaOH in the presence of 1.0 M EG (see supplementary data (S4)). It is clear from the figures that by increasing the sweep rates the oxidation current of EG increases. To better understand the kinetics of the EG oxidation process, (S5 supplementary data) shows the effect of changing the sweep rate on the oxidation peak current density in 1.0 M EG + 1.0 M NaOH on the Ni and Ni-based nanocomposite catalysts. Linear relationships were obtained between the anodic peak current density (Ip a ) and the square root of the sweep rate (S5(a, a′)), which confirms that the EG oxidation process is diffusioncontrolled. The non-linear relations between the sweep rates normalized current and the sweep rate shown in (S5(b, b′)) support our previous vision that the EG oxidation mechanism follows EC (electrochemical-chemical) mechanism.
The outcomes demonstrate a high surface area, high surface coverage(Г*), low transfer coefficient (α), and high catalytic activity for the Ni-based nanocomposite catalysts towards EG oxidation compared with that of GC/Ni, and the ternary GC/Ni-PbO 2 -GN nanocomposite represents the highest catalytic activity among the studied electrodes mainly due to its higher surface area (see Tables 2 and 3).
The prepared electrodes display significant stability with repeated cycling in 1.0 M NaOH + 1.0 M EG at a (16) sweep rate of 50 mV s −1 and the results are presented in Table 2. The catalytic activity for EG electrooxidation on the studied Ni and Ni-based nanocomposite electrodes after 50 cycles reached about 80, 87, 91, and 98% on GC/ Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN, respectively. The partial loss of the electrode catalytic activity could be due to the adsorption of reaction intermediates on its active sites. Notably, Ni-based nanocomposite catalysts show excellent stability over repeated cycling than that of GC/Ni as the residual activity still has a high level after 50 cycles. Additionally, the activity and stability of the prepared catalysts for EG oxidation have been determined using the chronoamperometry (It) curve at a constant potential of + 360 mV for 600 s in 1.0 M NaOH + 1.0 M EG (Fig. 7a, a′). It can be seen that the oxidation current densities decrease at the initial stage for the four electrodes, but the Ni-based nanocomposite GC/Ni-PbO 2 -GN delivers high initial and final steady-state currents and better stability than that of the other electrodes, the ternary GC/Ni-PbO 2 -GN represents the most stable one. It is well known that the higher the amount of oxygen-containing species on the surface of the electrode is the better the removal of poisonous species and, consequently, the better the system performance [101]. For GC/Ni-PbO 2 -GN, the presence of PbO 2 promotes the oxidation of the adsorbed intermediates from the electrode surface because it increases the adsorption ability of the hydroxyl ion onto the catalyst surface and help in the oxidation of the adsorbed carbonaceous intermediate species and thereby liberate the active sites of the electrode surface [73,74,[102][103][104].
The diffusion coefficients (D) of EG on the prepared electrodes during the oxidation process are calculated using the Cottrell equation (Eq. 18) [105] assuming that the process is controlled by diffusion: where I is the oxidation current (A), n is the number of electrons exchanged in EG electrooxidation (n = 10), C is the concentration of EG (mol cm −3 ), A is the geometrical electrode area which equals 1.0 cm 2 , F is Faraday's constant which equals 96500 C mol −1 , D is the diffusion. coefficient (cm 2 s −1 ) and t is the time in second. Figure 7b which represents the relation between the net current and the inverse of the square roots of time after removing the background current, using the slopes of the straight lines obtained, the diffusion coefficients of EG are calculated and they are 0.22 × 10 -10 , 0.64 × 10 -10 , 1.5 × 10 -10 , and 3.9 × 10 -10 cm s −1 on GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN, respectively (see Table 2). A large improvement in the diffusion coefficient for EG was found on the Ni-based nanocomposite electrodes and the highest (18) I = nFAD 0.5 C −0.5 t −0.5 one is recorded for the ternary GC/Ni-PbO 2 -GN. From the slopes of these lines (Fig. 7b), it can be revealed that the active surface areas (cm 2 ) of GC/Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN are about 15.5, 36.7. 94.2, respectively, times that of GC/Ni. The catalytic rate constants of the Ni and Ni-based nanocomposite catalysts can be calculated using Eq. 19 according to Pariente et al. [106]: where I cat and I L are the steady-state oxidation currents of each catalyst in the presence (I cat ) and absence (I L ) of EG, γ is the argument of the error function, k s is the heterogeneous catalytic rate constant, C * is the bulk concentration of EG and t is the elapsed time (s). From the slope of the relation I cat /I L vs. t 0.5 (Fig. 7c) and according to Eq. 19, the value of k s can be determined to be 0.61, 2.46, 8.1, and 32.5 mol −1 cm 3 s −1 for GC/Ni, GC/Ni-PbO 2 , GC/Ni-GN, and GC/ Ni-PbO 2 -GN, respectively. GC/Ni-PbO 2 -GN catalyst presented the highest catalytic activity and the lowest rate of poisoning by intermediates because OH ads species could be readily generated on the oxide surface. Indeed, the generation of OH ads species at lower potentials can convert CO-like poisoning species on Ni to CO 2 or other products that dissolve in water, releasing the metal active sites [107].
The charge transfer kinetics process was investigated by electrochemical impedance spectroscopy (EIS) for the prepared Ni and Ni-nanocomposite catalysts in this work. Nyquist plots (Fig. 8) were recorded for all the studied electrodes at a potential value of + 360 mV in 1.0 M NaOH and in the presence of 1.0 M EG. The diameter of the semicircle is a critical parameter to measure the charge transfer resistance of the electrocatalysts, and the smaller diameter of the impedance arc means the smaller charge transfer resistance for the electrooxidation reaction. Figure 8a and b shows the Nyquist plots of the electrochemical behavior of electrodes in 1.0 M NaOH and in the presence of 1.0 M EG, respectively collected at + 360 mV. A semi-circular arc appears in all EIS figures, and the radius of these arcs represents the electrode response resistance of the electrode reaction. Obviously, the radii of impedance arcs of all catalysts are in the order of GC/Ni-PbO 2 -GN < GC/Ni-GN < GC/ Ni-PbO 2 < GC/Ni, signifying the faster kinetics of EGOR on Ni-based nanocomposite catalysts, especially GC/ Ni-PbO 2 -GN which represents the highest catalytic property towards EGOR. The equivalent electric circuit which fits the electrochemical behavior of the prepared electrodes toward EGOR is presented in Fig. 8c. The circuit is used to fit the experimental results with the theoretical ones. The estimated equivalent circuit is represented as a simple Randle equivalent circuit, where R ct is the charge transfer resistance, R s represents the solution resistance and CPE is the constant phase element, respectively. Because (19) I cat ∕I L = 0.5 0.5 = 0.5 (kC * t) 0.5

3
CPE corresponds to the double-layer capacitance, it was necessary to substitute the double-layer capacitance in the equivalent fitting circuits with a constant phase element to get successfully fitting impedance diagrams. This replacement is due to the microscopic roughness of the electrode surface, which makes an inhomogeneous distribution in the double-layer capacitance and the solution resistance [108]. The R ct for the electrode is the only circuit element that has a simple physical meaning which indicates how fast the rate of the charge transfer during electrocatalytic oxidation varies with the type of catalyst used or the medium used (EG). Good simulated results were obtained using the abovementioned model. The impedance parameters after fitting are listed in Table 5. An empirical exponent (α = 0 to 1) is applied to account for the deviation from the ideal capacitive behavior due to the surface inhomogeneities and adsorption effects [71]. Ideal capacitor has α = 1, while α = 0. was as follows: GC/Ni ˃ GC/Ni-PbO 2 ˃ GC/Ni-GN > GC/ Ni-PbO 2 -GN which demonstrates that GC/Ni-PbO 2 -GN nanocomposite catalyst presents the lowest charge transfer resistance (R ct ) with good electrical conductivity and better charge transfer kinetics of EGOR. This means that the inclusion of GN and PbO 2 within the Ni matrix leads to a faster charge transfer, lower electrochemical impedance, high conductivity, and easier oxidation reaction. This could be interpreted on the basis that EG is oxidized by Ni and some intermediates are formed, then PbO 2 is supplying oxygen atoms to complete oxidation of such intermediates to final products, while GN provides a high surface area and increases the conductivity of the electrode. The values of double-layer capacitances (C dl ) increase in EG from GC/Ni electrode (C dl = 0.254 mF cm −2 ), GC/Ni-PbO 2 (C dl = 0.313 mF cm −2 ), GC/Ni-GN (C dl = 0.491 mF cm −2 ) and finally to GC/Ni-PbO 2 -GN (C dl = 0.931 mF cm −2 ). This behavior is due to EG oxidation taking place on GC/Ni electrode, but it is more feasible on the Ni-based nanocomposite electrodes. The lowest R ct value (34.5 Ω cm 2 ) for EG oxidation reaction was recorded at GC/Ni-PbO 2 -GN electrocatalyst, and it was 308, 121, and 110 Ω cm 2 for GC/Ni, GC/Ni-PbO 2 and GC/ Ni-GN, respectively. Graphene and metal oxides incorporation in the Ni matrix decreases the charge transfer resistance (R ct ) and facilitates the charge transfer process [72] during EG oxidation reaction on GC/Ni-PbO 2 -GN electrode.
In this context, we report how the catalytic activity and stability of the Ni-based nanocomposite electrodes are greatly enhanced as compared to that of the GC/Ni catalyst. In general, the greatly enhanced performance could be attributed to the several combined properties of the Ni-based nanocomposite catalysts. The nano-size unique structure is of great significance in enlarging the surfaceactive area and benefitting the diffusion and accessibility for EG molecules to the active sites of catalysts. The synergetic effect between Ni and PbO 2 greatly enhanced the catalytic activity and stability of the electrode [109,110]. The introduction of graphene can provide excellent support for the distribution and dispersity of Ni nanoparticles on its surface and increases the conductivity which greatly enhances the electrocatalytic performance of the Ni-based nanocomposites.

Conclusions
In summary, graphene and PbO 2 have been used to prepare Ni-based nanocomposite electrodes via a facile, fast, and environmentally friendly electrodeposition method. Benefitting from the presence of PbO 2 and/or GN, high surface areas of the Ni-based nanocomposite catalysts are obtained. Synergistic effects between the Ni, PbO 2 , and GN promote the electrocatalytic activity towards EGOR. The outcomes indicate high surface coverage for Ni-based nanocomposites and values of (0.26, 0.77, 1.22 and 2.08) × 10 -7 mol cm −2 are estimated for GC/Ni, GC/ Ni-PbO 2 , GC/Ni-GN, and GC/Ni-PbO 2 -GN, respectively and the diffusion coefficient values of (3.9, 1.5, 0.64 and 0.22) × 10 -10 cm 2 s −1 are reported for GC/Ni-PbO 2 -GN, GC/Ni-GN, GC/Ni-PbO 2 , and GC/Ni, respectively. A small charge transfers resistance value of 34.5 Ω cm 2 for EG oxidation reaction is recorded at ternary GC/ Ni-PbO 2 -GN nanocomposite, but its values are 308, 121, and 110 Ω cm 2 for GC/Ni, GC/Ni-PbO 2 , and GC/Ni-GN, respectively which means a fast electron transfer rate and lower charge transfer resistance are recorded for the Nibased nanocomposite electrodes and GC/Ni-PbO 2 -GN nanocomposite catalyst exhibits extremely high electrocatalytic performances. The binary and ternary Ni-based nanocomposites are promising alternative catalysts that could greatly boost the substantial development of direct EG fuel cells.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Conflict of interest The authors declare no competing interests.
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