Electrocatalytic Performance of Nickel Hydroxide-Decorated Microporous Nanozeolite Beta-Modified Carbon Paste Electrode for Formaldehyde Oxidation

In this paper, aluminosilicate nanozeolite beta has been prepared and described using X-ray diffraction (XRD), nitrogen sorption isotherm, Fourier transform infrared (FT-IR), transmission electron micrograph (TEM), and field emission scanning electronic microscopy (FESEM) techniques; TEM image demonstrated semispherical particles with dimensions under 50 nm. The BET surface area, total pore volume, and pore diameter of it were attained to be 321 m2 g−1, 0.053 cm3 g−1, and 1.22 nm, respectively. The modified carbon paste electrode by aluminosilicate nanozeolite beta and nickel hydroxide (Ni(OH)2-Beta/CPE) was applied for formaldehyde (HCHO) electrocatalytic oxidation. The obtained results specify that Ni(OH)2-Beta/CPE demonstrates worthy electrocatalytic activity for oxidation of HCHO due to mesoporous construction and the great surface area of nanozeolite. The electron-transfer coefficient, catalytic rate constant, and diffusion coefficient are found to be 0.69, 2.08 × 106 cm3 mol−1 s−1, and 4.4 × 10−7 cm2 s−1, respectively. The Ni(OH)2-Beta/CPE exhibited low background current, simplicity of surface renewal, good reproducibility, and stability and also displayed high stability up to 300 cycles and 3000 s without an important loss in the current density. This modified electrode has better poisoning tolerance capability than bare CPE for HCHO electrocatalytic oxidation and is a higher device for the long term accomplishment.


Introduction
With the rapid progress of the economy and society, the energy crisis of old sources which produced environmental contamination has converted progressively serious. In this progressively environmentally sensible time, we have revolved our responsiveness to green renewable energy. However, effective adaptation and storage of chemical energy have developed one of the major challenges [1,2]. The electrocatalytic oxidation of methanol (CH 3 OH) and formaldehyde (HCHO) is a reaction topic to intense exploration because of its potential presentation in fuel cell technology [3]. HCHO is a damaging material and not exact appropriate for the fuel cells, but the knowledge of its electrooxidation is significant for the full considerate of CH 3 OH electrooxidation in direct methanol fuel cells (DMFCs), because it is formed by fractional oxidation of CH 3 OH [4]. The HCHO can be applied as a process for elimination and alteration of it to fewer poisonous materials from dilute waste streams [5,6].
Zeolites have identical network size and exceptional molecular form selectivity and robust acidity and worthy hydrothermal and thermal stability. The BEA class of zeolite includes an intergrowth of 2 or additional polymorphs involving a 3-dimensional system of 12 membered ring networks [31]. Zeolite beta framework topology has benefits, for example, great accessible micropore volume, large pore channel, and the existence of energetic locations that progress metal zeolite interaction by growing the negative charge of the zeolite framework with cationes, e.g., Ni 2+ [32]. Carbon paste electrodes (CPEs) are preferred because of low background current, low prices, and extensive potential window. Uses of improved CPEs with mesoporous materials such as zeolites have involved greatly consideration [33]. Fabrication of zeolite-modified electrodes (ZMEs) is simple, fast, and inexpensive that were firstly considered and encouraged by Murray et al. [34].
Electrode surface modification is one of the significant progresses in the latest years. Ni(OH) 2 is an energetic constituent in the electrochemistry because of its sharp redox presentation, high theoretical capacity, and low cost [24]. As an example, Hoque et al. used the MnO 2 -Ni(OH) 2 catalyst decorated at the interface of the zeolite NaY and MWCNT for the electrochemical methanol oxidation reaction (MOR) [35]. Also, they utilized Cu(OH) 2 -Ni(OH) 2 engulfed by zeolite NaY and MWCNT for effective MOR [36]. Rh/RhO x -Ni(OH) 2 -Y/rGO was also synthesized using zeolite NaY, and reduced graphene oxide (rGO) and was applied for MOR and ethanol oxidation reaction (EOR) [37]. According to the literature review, nanozeolite beta-modified CPE was not applied for electrocatalytic oxidation of HCHO. Here, we exerted effort to discover the influence of Ni(OH) 2 distributed on the nanozeolite beta modified CPE (Beta/CPE) as a novel, easy, and sensitive substance for HCHO oxidation. Structure characterization of the synthesized nanozeolite beta is considered by using X-ray diffraction (XRD), field emission scanning electronic microscopy (FESEM), transmission electron micrograph (TEM), Fourier transform infrared (FT-IR), and nitrogen sorption isotherms. The electrochemical performance of Ni(OH) 2 -Beta/CPE is deliberated by cyclic voltammetry (CV), chronoamperometry, chronocoulometry, and linear sweep voltammetry techniques.

Synthesis of Nanozeolite Beta
The production process of nanozeolite beta has been designated in another place [31]. 27 mL of TEAOH was gradually added to 1.2 g NaOH in 4.0 mL DW. The combination was agitated for 10 min and step up to a TEOS solution (22.3 mL) in 9.5 mL DW at 25 °C and was permitted to blend for 30 min. After it, 2.2 g of Al 2 (SO 4 ) 3 ·16H 2 O dissolved in 8.1 mL of DW was added to the mixture and agitated for 1 h. In the next step, 1.4 g CTAB was dissolved in 40 mL ethanol and added slowly to the mixture and the gel was stirred for 2 h. The consequential viscous gel was heated at 90 °C with stirring to comprehensive dryness. The dry originator lumps were obscenely rumpled and moved into a Teflon-lined autoclave. DW (ca. 0.2 mL per 1 g (dry gel)) was added into the bottom of the autoclave. The crystallization of it was performed at 175 °C for 24 h. After this treatment, the powdered results were recuperated by a runs of centrifugation at 12,000 rpm and washed with DW until pH < 8 and then dried at 25 °C for 24 h. The templates were eliminated from the nanozeolite beta by calcination for 8 h at 550 °C.

Apparatus and Characterization
X-ray diffraction pattern was obtained by X-ray diffractometer (XRD, MPD 3000 Instrument, Italy) with Be Filtered Cu K α radiation (1.5418 Å) operating at 35.4 kV and 28.0 mA. Filed emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) spectroscopy were detailed by a Mira-3 XMU instrument (Czech Republic). The Perkin Elmer Fourier transform infrared spectrometer was utilized for register of IR spectrum. Nitrogen sorption isotherms were acquired by a volumetric adsorption equipment Belsorp-max instruments, Japan. Also, a transmission electron microscope model Philips (model CM120) was used for recording of nanoparticle morphology and size. The electrochemical tests were carried out by a potentiostat/galvanostat (SAMA500 electroanalyzer system, Isfahan, Iran) with a voltammetry cell in a three electrodes configuration. The Ag|AgCl|KCl(3 M) and platinum wire (from Azar Electrode Co., Iran) were utilized as reference and auxiliary electrodes. The fabricated home-made CPEs were utilized as the working electrode in the all electrochemical tests.

Fabrication of Electrodes
For fabrication of Beta/CPE (25 wt.% with respect to the graphite powder), 0.05 g nanozeolite beta and 0.2 g graphite powder were carefully mixed by diethyl ether. After evaporation of diethyl ether, two drops of paraffin oil (35 wt.%) were added and combined with a mortar by hand mixing for 30 min awaiting a regularly wetted paste was gained. This paste was crammed into the finale of a glass tube (with 4.5 mm inner diameter and 10 cm length), with a copper wire as electrical connection. A fresh surface was gotten by pushing an extra of the paste out of the tube and brush up with a weighing paper. Revision of the Beta/CPE was implemented in the two steps. Initially, the Beta/CPE was immersed in the solution of 0.1 M NiCl 2 for 15 min, and the arranged Ni-Beta/CPE was washed by DW to eliminate adsorbed materials in the electrode surface. Then, the Ni-Beta/CPE was conditioned in 0.1 M NaOH by potential cycling in the range of 0.0 and 1.0 V for about 15 cycles at a sweep rate of 0.05 V s −1 for providing Ni(OH) 2 -Beta/CPE. To obtain the operations of beta nanozeolite and Ni(OH) 2 in the electrocatalytic oxidation of HCHO, the Ni(OH) 2 /CPE and Beta/CPE were prepared in similar method lacking addition the nanozeolite beta and without soaking modified electrode in the NiCl 2 solution, respectively.

Results and Discussion
Characterization of the Nanozeolite Beta Figure 1A exhibits XRD pattern of prepared nanozeolite beta. The crystallization yields complemented the specific peaks of it with worthy crystallinity [31,38]. The crystalline dimension of nanozeolite beta was also measured to be 33.6 nm via Debye-Scherrer Equation [39]. According to the FT-IR spectrum of the nanozeolite beta in Fig. 1b, doublet bands at 464 and 567 cm −1 are ascertained to the bending vibrations of Si-O or Al-O bands in β-cages, which support the existence of a zeolitic building with great crystallinity that is distinguishing of beta zeolite [40,41]. The absorption band at 795 cm −1 is recognized to internal tetrahedral symmetrical stretching of Si-O-Si [42]. The band positioned at 1080 cm −1 is ascribed to the external linkage asymmetrical stretching mode of Si-O-Si and can overlay with the Al-O-Si stretching mode [40,43]. The band located at about 1630 cm −1 is ascribed to the bending vibration of H 2 O adsorbed on OH − groups [44,45].
The BET surface area, pore diameter, and pore volume of the prepared beta nanozeolite were estimated from N 2 sorption isotherm and pore size distribution investigations. Figure 1c displays N 2 adsorption/desorption isotherms of beta nanozeolite at 77 K that revealed H 4 type hysteresis at P/P 0 0.8-1.0 [46]. This isotherm was type-III agreeing to Brunauer arrangement and type-I according to IUPAC organization, which is a distinctive structures of microporous compounds and support the presence of great porosities in synthesized sample [47]. Figure 1d illustrates pore size distribution curve for nanozeolite beta was calculated as of the adsorption branches of isotherms. The textural possessions of the synthesized nanozeolite beta are specified that this substantial has great BET surface area (321 m2 g−1), mean pore diameter of 1.22 nm, and total pore volume (0.053 cm 3 g −1 ). These results identified that beta nanozeolite is active for adsorption of Ni ion for electrocatalytic investigation. Figure 2a demonstrates FESEM of nanozeolite beta, which specifies the foundation of semispherical forms of beta nanozeolite by average particle size under 50 nm. These consequences indicated that particle dimension of beta nanozeolite is in settlement by gotten consequence from XRD technique. Also, Fig. 2b-e exhibits elemental mapping of nanozeolite beta that specifies sodium, oxygen, silicon, and aluminum elements are existed in the structure of synthesized nanozeolite beta. Also, transmission electron micrographs (TEM) and high resolution transmission electron micrographs (HRTEM) of the synthesized beta nanozeolite are demonstrated in Fig. 3a, b, respectively. Figure 3c shows the displayed selected area of electron diffraction (SAED) of Fig. 3b which shows the formation of nanoparticles. The HRTEM of it displays the agglomeration of beta nanozeolite with particle dimension in the range of 20-25 nm (see Fig. 3d).

FESEM and Electrochemical Examination of Fabricated Electrodes
FESEM was done to analyze the surface arrangement of the all prepared electrodes and are illustrated in Fig. 4. As can be seen in Fig. 4a, the layer of irregular flakes of graphite powder was presented and insulated from each other on the surface of CPE. After adding beta nanozeolite to the graphite powder for production of Beta/CPE, it can be appreciated that it was totally dispersed onto electrode surface, deducing that the beta nanozeolite was irregularly integrated into the graphite powder (Fig. 4c). Similarly, Fig. 4b, d displays nickel hydroxide was wholly dispersed on the CPE and Beta/ CPE surface [48].
EDX spectroscopy was assisted to obtain useful data in the existence of elements and their alignment in the structure of electrodes. The EDX spectrum and elemental mapping of Ni(OH) 2 -Beta/CPE are presented in Fig. SM1, and the weight percent of elements in all above electrodes is accessible in Table 1. The attained data from Fig. SM1 and Table 1 represented that C, O, Na, Al, Si, and Ni existed on the Ni(OH) 2 -Beta/CPE structure. Also, Fig. SM1 exposed homogeneous distribution of the above elements in the Ni(OH) 2 -Beta/CPE approving the identical distribution of the Ni(OH) 2 and beta nanozeolite in Ni(OH) 2 -Beta/CPE structure.
CVs of the bare CPE and Beta/CPE were registered in 5 mM K 4 Fe(CN) 6 + 5 mM K 3 Fe(CN) 6 and 0.1 M KCl solution at sweep rate 20 mV s −1 (display Fig. 5a). The investigational consequences demonstrate reproducible anodic and cathodic peaks ascribed to the Fe(CN) 6 3− /Fe(CN) 6 4− redox couple on the CPE and Beta/CPE surfaces. As can be seen, the anodic and cathodic peak current densities of Beta/CPE are greater than those at bare CPE. The variance in potential of anodic and cathodic peaks (ΔE p ) for ferricyanide was 477 mV at Beta/CPE and 400 mV at CPE. The electrochemically active surface areas of the Beta/CPE and bare CPE were expected via the slope of I pa vs. υ 0.5 plots for a well-known concentration of K 4 Fe(CN) 6 , according to the Randles-Sevcik Equation [49]. (see Fig. 5b). The electrochemically active areas were calculated to be 1.16 and 0.14 cm 2 for Beta/CPE and bare CPE, respectively, and then, it was at Beta/CPE 8.3-fold greater than that in bare CPE. These consequences specified that revision of CPE with nanozeolite beta causes to improve the effective surface area of the modified CPE.  Fig. 6a, c, no redox peak can be detected on the CPE and Beta/CPE surfaces which ascends from high overpotential for HCHO electrooxidation at these electrodes and the existence of a kinetically sluggish oxidation procedure [26]. Also, the background current for Beta/CPE is greater than that on bare CPE because of greater surface area of nanozeolite beta in the structure of Beta/CPE [47,50].

Electrocatalytic oxidation of HCHO
The presence of Ni 2+ at the fabricated electrodes was considered by CV comparison of Ni(OH) 2 -Beta/CPE and Ni(OH) 2 /CPE with and without 20 mM HCHO (see Fig. 6c, d). In the absence of HCHO, a couple of wellidentified redox peaks with great oxidation peak current density (j pa = 1.97 mA cm −2 ) and ΔE p of 180 mV can be distinguished on Ni(OH) 2 -Beta/CPE (see Fig. 6d). But, the redox peaks with weak anodic signal (j pa = 0.15 mA cm −2 ) were appeared on bare CPE with ΔE p of 120 mV (see Fig. 6c). Since the active microscopic area of Beta/CPE is about 8.3-fold bigger than that at bare CPE, accumulation of Ni 2+ and achieved oxidation currents on Beta/CPE are larger than that on the bare CPE. It can be deduced that the existence of nanozeolite beta in the structure of fabricated CPE had an excessive influence on the improvement of the oxidation current densities for Ni(OH) 2 to Ni(OOH) conversion (see Fig. 6e). It is proposed that at the interface of Ni(II)-Beta/CPE and electrolyte in 0.1 M NaOH, Ni 2+ reacts with OH − to generate Ni(OH) 2 -Beta/CPE that acts as an active location in the redox procedure. In the lack of HCHO in Fig. 6d, the anodic peak was related to the oxidation of Ni(OH) 2 -Beta/CPE to Ni(OOH)-Beta/CPE, and the cathodic peak was ascribed to the inverse transformation [51][52][53][54].
To distinguish the influence of nanozeolite beta, the CVs of CPE, Beta/CPE, Ni(OH) 2 /CPE, and Ni(OH) 2 -Beta/CPE were displayed in the existence of 20 mM HCHO in Fig. 6f. According to the figure, no anodic currents were observed for electrooxidation of HCHO on the CPE and Beta/CPE surfaces. A small anodic peak was detected around 0.52 V on Ni(OH) 2 /CPE that is specialized to HCHO oxidation in the existence of little Ni(OH) 2 on the bare CPE. This statement specifies little affinity of graphite to the adsorption of Ni 2+ . Diffusion of Ni 2+ ions in nanozeolite beta is greatly more rapidly because of Ni 2+ coordination to the nanozeolite framework, the larger cages, and channels and fast movement of Ni 2+ ions from the cages [26,45]. Also, the broad cathodic peak in some plots in Fig. 6 might be ascribed to the phase conversion of β-NiOOH to γ-NiOOH because of sluggish, irreversible overcharging for the period of cycling, and the matching reduction to α-Ni(OH) 2 [55]. Meanwhile, catalytic current of HCHO electrooxidation onto Ni(OH) 2 -Beta/CPE is larger than that obtained on the Ni(OH) 2 /CPE. The greater presentation of Ni(OH) 2 -Beta/CPE for electrooxidation of HCHO can be accredited to the existence of mesoporous nanozeolite beta in the composition of Beta/CPE that improve electroactive surface area of modified CPE.
The enhancement in anodic peak current was detected on Ni(OH) 2 -Beta/CPE at 0.61 V and the cathodic peak current at about 0.28 V was reduced in the existence of HCHO (see Fig. 6d). This consequence identified that employed nanozeolite beta as s modifier in this procedure contributes straightly to the HCHO electrooxidation. Captivating into explanation, all these explanations were in agreement with the previous works [25,53,56,57], and we might mention a mechanism for electrocatalytic oxidation of HCHO on the Ni(OH) 2 -Beta/CPE surface. In aqueous solution, the HCHO is totally hydrated and changed to the methylene    [5,25,53,56,60]. The electrocatalytic oxidation mechanism for HCHO on Ni(OH) 2 -Beta/CPE might be defined by the next equations: It was commonly accepted that NiOOH in the surface of modified electrode at 0.1 M NaOH catalyzes the oxidation of HCHO through one electron procedure for generating format anions as an ending produce 5, 60.. The sum of the above equations could be stated as follows: Prepared electrodes at different ratio of 15, 25, and 40% wt./wt. vs. graphite were considered by the CV technique. It was detected that the ratio of 25% nanozeolite beta in the fabricated electrode has maximum anodic current for electrooxidation of HCHO. In 15% wt./wt. of nanozeolite beta in CPE, the low accessible pores of the nanozeolite for Ni 2+ loading and in 40% wt./wt. more resistance of modified electrode because of nonconductive characteristic of nanozeolite beta are chief aspects which origin to reduce anodic current densities [26].

Effect of Sweep Rate with and without HCHO
Influence of sweep rate was considered on the redox performance of Ni(OH) 2 -Beta/CPE without HCHO in 0.1 M NaOH solution and is shown in Fig. SM2(a). A couple of well-formed redox peaks by a ΔE p of 170 mV is identified at a sweep rate of 10 mV s −1 . The ΔE p growths with improving sweep rate that specifies the attendance of a inadequacy in charge transfer kinetics [61]. At ΔE p > 0.2/n V from a theory described by Laviron [62], we can estimate the electron transfer coefficient (α) by calculating the alteration of the peak potential (E p ) vs. log υ and apparent charge transfer rate constant (k s ). Fig. SM2(b) displays the curve of E p vs. log υ in 5-600 mV s −1 for cathodic and anodic peaks. It can be appreciated that E p is proportionate to log υ at υ > 100 mV s −1 definite by Laviron [62]. The quantity of α is obtained to be 0.52 signifying speed limiting stages for cathodic and anodic can be nearly identical to step [63]. The average quantity of k s over this range of sweep rate is obtained to be 0.043 s −1 . In relation to the slope of two lines in Fig. SM2(c), the surface coverage of the Ni(OH) 2 / Ni(OOH on Ni(OH) 2 -Beta/CPE can be expected to be 2.3 × 10 −8 mol cm −2 [64]. The quantity of loaded Ni on the modified electrode surface is achieved to be 0.215 μg. At great sweep rates in Fig. SM2(d), the peak current densities are proportionate to υ o.5 , indicating the free diffusion controlled process [65,66].
CVs of the Ni(OH) 2 -Beta/CPE in the existence of 22 mM HCHO at different sweep rates (5-600 mV s −1 ) in 0.1 M NaOH are displayed in Fig. 7(a). As the sweep rate increases, the anodic peak current for HCHO oxidation improves. The remaining cathodic peak ascertained for the reduction of the residual NiOOH to Ni(OH) 2 increases by sweep rate. This may be because of the statement that the persisted NiOOH increases with growing the sweep rate because of lower consumption of the NiOOH at greater sweep rates 5.. A curve of j pa vs. υ did not display a linear plot (Fig. 7b); meanwhile, the curve of j pa against υ 0.5 was establish to be linear (see Fig. 7c). This item suggested that this procedure is diffusion controlled procedure instead of surface controlled process [67].
A straight line is attained (R 2 = 0.9974) which is in agreement with the Randles-Sevcik equation for totally irreversible diffusion-controlled procedure and specified by the following Equation [64].: where I p pointed to the anodic peak current (A), A is the surface area of the modified electrode (cm 2 ), D is the diffusion coefficient of HCHO (cm 2 s −1 ), n is the total number of electrons (e.g., 1), C is the bulk concentration of HCHO (mol cm −3 ), and υ is the sweep rate (V s −1 ). Also, α is the charge transfer coefficient, n α is the number of electrons in the rate determining stage, F is Faraday's constant (96,485 C mol −1 ), R is the gas constant (8.314 J mol −1 K −1 ), and T is the absolute temperature (298 K). The amount of αn α can be measured using the following Equation [68]: where E p is the peak potential and E p/2 is the potential at half peak current. The information in Fig. 7a was utilized to estimate the parameter αn α at various sweep rates. The mean value was obtained to be 0.344. It can be stated that this amount is fewer than that found in the blank that may argument to some kinetic limitations in the existence of the HCHO because of probable poisoning of the modified electrode surface with the oxidation produces. From Eq. (6), the slope of the straight line in the inset of Fig. 7c and the amount of αn α (i.e., 0.344) the D value was calculated to be 2.97 × 10 −7 cm 2 s −1 .
A linear segment is detected on the curve of log j pa vs. log υ by slope of 0.2976 (see Fig. 7d) which is nearly the theoretically estimated value of 0.5 for a justly diffusioncontrolled process. Small alteration by theoretical amount rises possibly from kinetic limitation in the total reaction 49.. A polynomial decrease in the curve of normalized current density (j pa /υ 0.5 ) versus υ in Fig. 7e described as an EC′ procedure, and it was highlighted that an irreversible continuation chemical stage is elaborate in the total procedure [69,70]. Influence of sweep rate on the proportion of anodic to cathodic current with and without HCHO is exhibited in Fig. 7f. This figure demonstrates that the I pa /I pc in the existence of HCHO was reduced meaningfully by growing sweep rate. Consequently, drop in the period for HCHO oxidation at greater sweep rates abstained the superficial electron transfer between NiOOH and HCHO molecule [60].  Fig. SM3(b) displays the Tafel plots (log I vs. E) strained from the data resulting by the growing portion of the current voltage curves. This raising section of voltammogram, identified as the Tafel region, is influenced by electron transfer kinetics between the HCHO and Ni(OH) 2 -Beta/CPE. The Tafel slope is identical to n(1 − α)F/2.303RT, where n is the number of electron in the rate measuring stage (i.e., n = 1) and α, F, R, and T have their common meaning. According to the above declared quasireversible electron transfer kinetics and Tafel slope for four curves, the quantity of α was achieved to be 0.69 presuming one electron developed in the rate measuring stage [71,72].  Figure 8a, b exhibits CVs of Ni(OH) 2 -Beta/CPE at different concentrations of HCHO. As can be detected, the anodic peak current improved by growing HCHO concentration. It can be identified that upon growing HCHO concentration, the onset potential of the Ni(OH) 2 oxidation motivated to positive values. This is because of the adsorption of intermediates on the residual active locations that obstruct the additional HCHO oxidation, and then, more overpotential is needed for HCHO oxidation. Plot of the current density versus HCHO is represented in Fig. 8c that contained 2 linear sections in the concentration ranges of 0 to 20 mM and 20 to 140 mM. At a higher concentration range (20-140 mM), adsorption of the oxidation produced at Ni(OH) 2 -Beta/CPE may be a reason for the discontinuation of further oxidation, and then, the slope of calibration curve was decreased [49,73]. The curve of normalized current voltammogram at Ni(OH) 2 -Beta/CPE is exhibited in Fig. 8(d). The value of inserted Ni 2+ is obtained from the surface coverage.

Chronoamperometric and Chronocoulometric Studies
Some chronoamperometric and chronocoulometric readings were accomplished to estimate the electrocatalytic management of the Ni(OH) 2 -Beta/CPE for HCHO. Figure 9a displays chronoamperograms of redox process registered by the location potential of Ni(OH) 2 -Beta/CPE at 0.65 and 0.25 V vs. Ag|AgCl|KCl(3 M) with and without various HCHO concentrations. It was detected that attained currents from these tests were in worthy arrangement by the achieved date from CV experiments and the current densities progress as the HCHO concentration increases. This consequence confirms our deduction in the case of the catalytic character of NiOOH for HCHO oxidation on the Ni(OH) 2 -Beta/CPE surface [74].
For an electroactive compound by a diffusion process, the practical current for the electrochemical reaction underneath mass transportation limited situations can be stated with the Cottrell Equation [75]: where n is the number of electron (i.e., 1), A is the area of the electrode (0.159 cm 2 ), and F, C, and D have their common meaning. Practical curves of I versus t -0.5 for all HCHO concentrations were plotted, and the best fits for different HCHO concentrations were obtained (Fig. 9b). The slopes of the consequential lines were plotted against three concentrations of HCHO and are displayed in Fig. 9c. From the obtained slope and Eq. (8), the average quantity of D was gotten to be 4.4 × 10 −7 cm 2 s −1 . This amount is comparable with the value estimated from Eq. (6).
The chronoamperometry technique can be employed for the assessment of the catalytic rate constant (k cat ) of an analyte on the active positions of the fabricated Ni(OH) 2 -Beta/ CPE rendering to the next Equation [76]: where, I cat and I L are the currents with and without HCHO, respectively. The C 0 is the bulk concentration of HCHO (mol cm −3 ), and t is the elapsed time (second). Using the slopes of I cat /I L against t 0.5 (Fig. 9d) for all concentrations, the average value of k cat was estimated to be 2.08 × 10 6 cm 3 mol −1 s −1 . Judgment of the expected k cat in this work by others in the earliest papers is done and exhibited in Table 2. According to this table, the Ni(OH) 2 -Beta/CPE can act as a comparable catalyst in HCHO electrocatalytic oxidation. Also, j pa and E pa were compared for HCHO electrocatalytic oxidation at the surface of modified electrode by several previous papers. According to this table, j pa with Ni(OH) 2 -Beta/CPE is greater than that on the some of the earlier works.
Nano-NiPh/GCE nanoporous nickel phosphate/glassy carbon electrode, Ni(OH) 2 (30-140 mM) with potential stages of 0.65 and 0.25 V. The chronocoulometric plot of Ni(OH) 2 -Beta/CPE in the 0.1 M NaOH displayed a practically symmetrical form. It can be deduced that nearly same charges were consumed for the oxidation and reduction of Ni(OH) 2 /NiOOH in the surface of modified electrode. In the existence of HCHO, the charge amount related to the advancing chronocoulometry is larger than that detected on backward chronocoulometry. The charge quantity attendant with the backward chronocoulometry is generally decreased by growth in HCHO concentration demonstrating that the electrocatalytic oxidation procedures are irreversible [64,70]. This consequence discovered a like performance to those defined by CV technique.
Also, chronocoulometry method was also applied to estimate the D of HCHO in the modified CPE surface. The charge answer under diffusion-controlled is designated using the next Equation [64,70]: For estimate of the D, Fig. 10b shows displayed curves of Q vs. t 0.5 for all sHCHO concentrations at Ni(OH) 2 -Beta/CPE surface. In the next step, the slopes of the resultant straight lines were planned versus HCHO concentration (see Fig. 10c). From the slope of the subsequent plots and applying Eq. (10), the average quantity of D was obtained to be 2.13 × 10 −6 cm 2 s −1 in the HCHO concentration scale of 30-140 mM.

Stability, Repeatability, and Reproducibility of the Ni(OH) 2 -Beta/CPE
For a fresh method, long-term stability is a substantial parameter. It was considered with captivating the answer of Ni(OH) 2 -Beta/CPE in three months. The Ni(OH) 2 -Beta/ CPE was reserved in open air at temperature of 298 K while not in custom. The gained answers were considered with analysis of variance (ANOVA) test. The F exp (i.e., 3.4) matched to its critical quantity at 95% confidence level (F 0.05,5,5 = 4.95). It can be indicated that deviances among the modified electrode answers are because of random errors and the electrode memorized its complete action through this time. It was important to progress the surface by a slight rubbing of the modified CPE on a soft paper, at any time the electrode answer decreased. Also, the Ni(OH) 2 -Beta/ CPE was introduced to fifty cycles with the sweep rate of 50 mV s −1 in 0.1 M NaOH with and without 20 mM HCHO (see Fig. 10d, e). This electrode reserved 97.8 of its primary current answer after 50 repetitive cycles without HCHO, and no important variation was revealed in the potential of anodic peaks. The long-term cycle stability of the Ni(OH) 2 -Beta/CPE has been investigated for HCHO oxidation. The result displays only less than 4.8% decrease in electrooxidation current density was occurred around 300 cycles (see Fig. 10e). This decrease is caused by the poisoning of the electrode surface by oxidative intermediates, especially CO, and other intermediate carbon species and various impurities formed and accumulated either from the electrolyte or from the surrounding atmosphere [81,82]. These consequences specify mechanical and chemical stability and reproducible answer as well as long-term stability of the Ni(OH) 2 -Beta/ CPE for HCHO electrocatalytic oxidation [61].
The reproducibility of the Ni(OH) 2 -Beta/CPE was evaluated through the comparison of the currents of 4 various modified electrodes comprising 25 wt.% of nanozeolite beta by CV technique. The oxidation current density of these electrodes for electrooxidation of 20 mM HCHO was considered individually, and the RSD was 3.45%. A reproducible current answer with a RSD of 3.12% was detected for four consecutive assays of 20 mM HCHO. For additional consideration of the electroactivity and long-term stability of the catalysts, cronoamperograms at potential of 0.65 V were registered for the Ni(OH) 2 -Beta/CPE and Ni(OH) 2 /CPE in 0.1 M NaOH and 20 mM HCHO for 3000 s (see Fig. 10f). According to this figure, a reduce in current density by time is obtained in the order of Ni(OH) 2 -Beta/CPE > Ni(OH) 2 /CPE, that is in worthy covenant with the CV information. These outcomes indicated that the Ni(OH) 2 -Beta/CPE has superior poisoning tolerance capacity for HCHO oxidation and is a better electrode for long term process. As can be understood, the decrease in current at first times is reasonably great for modified electrode. However, the current reaches a moderately stable quantity when the time is above 60 s demonstrating that Ni(OH) 2 -Beta/CPE displays an acceptable stability for HCHO oxidation [45]. Also, nanozeolite beta was stayed stable in the building of modified electrode at 0.1 M NaOH [83].

Conclusions
In this paper, nanozeolite beta was synthesized using a simple hydrothermal process and was characterized by different methods containing XRD, FTIR, FESEM, TEM, and N 2 sorption techniques. The role of synthesized sample in the HCHO electrooxidation was estimated by preparation of modified CPEs including nanozeolite beta and Ni(OH) 2 by cyclic voltammetry, chronocoulometry, chronoamperometry, and linear sweep voltammetry methods. It was obtained that nanozeolite beta was as porous material by good BET surface area and large number of active locations performs was as a host for adsorption of Ni 2+ and NiOOH produced throughout the oxidation of Ni(OH) 2 in the 0.1 M NaOH solution. The consequences demonstrate that Ni(OH) 2 -Beta/CPE can increase the HCHO oxidation by a catalytic procedure via a reduce in overpotential and overawed the low kinetic of reaction in confronting with Ni(OH) 2 /CPE and some of the previous works. Some significant kinetic and transport parameters were calculated and compared by those in literatures. As an example, the diffusion coefficient and catalytic rate constant of HCHO are achieved to be 2.13 × 10 −6 cm 2 s −1 and 2.08 × 10 6 cm 3 mol −1 s −1 by chronocoulometry and chronoamperometry, respectively. Also, the stability and presentation of the Ni(OH) 2 -Beta/CPE were investigated at the experimental conditions. The CV test specified that the current density of HCHO electrooxidation improves strangely on