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C as an electrocatalyst for methanol oxidation in KOH solution for fuel cell application
Accepted Manuscript Title: Microwave irradiated Ni−MnOx /C as an electrocatalyst for methanol oxidation in KOH solution for fuel cell application Author: R.M. Abdel Hameed PII: DOI: Reference:
S0169-4332(15)02025-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.201 APSUSC 31148
To appear in:
APSUSC
Received date: Revised date: Accepted date:
12-5-2015 24-8-2015 25-8-2015
Please cite this article as: R.M.A. Hameed, Microwave irradiated Ni
Highligts Ni−MnOx/C had nickel nanoparticles with an average diameter of 4.5 nm. Oxidation current density increased by 1.43 times at Ni−MnOx/C.
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Adding MnOx to Ni/C lowered its phase angle and impedance values.
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Ni−MnOx/C showed ks value of 3.26 x 103 cm3 mol−1 s−1.
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Microwave irradiated Ni−MnOx/C as an electrocatalyst for methanol oxidation
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in KOH solution for fuel cell application
R.M. Abdel Hameed*
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Chemistry department, Faculty of Science, Cairo University, Giza, Egypt.
*
Corresponding author Tel.: +201145565646; fax: +235727556.
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Abstract
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E-mail address: [email protected] (R.M. Abdel Hameed).
Ni−MnOx/C electrocatalyst was synthesized by the reduction of nickel precursor salt
on MnOx/C powder using NaBH4 and the deposition process was motivated with the aid of microwave irradiation. Finer nickel nanoparticles were detected in Ni−MnOx/C using transmission electron microscopy with a lower particle size of 4.5 nm compared to 6 nm in Ni/C. Cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS) were applied to study the electrocatalytic activity of Ni−MnOx/C for methanol oxidation in 0.5 M KOH solution. The presence of 7.5 wt.% MnOx in Ni−MnOx/C enhanced the oxidation current density by 1.43 times. The catalytic rate constant of methanol oxidation at Ni−MnOx/C was calculated as 3.26 x 103 cm3 mol−1 s−1. An appreciable shift in the maximum frequency at the transition from the resistive to capacitive regions to a higher value in Bode plots of Ni−MnOx/C was shown when compared to Ni/C. It was accompanied by lowered phase angle values. The lowered Warburg impedance value (W) of Ni−MnOx/C at 400 mV confirmed the faster methanol diffusion rate at its surface. 1 Page 1 of 23
Keywords
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Nickel; manganese oxide; microwave irradiation; alkaline medium; fuel cells.
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1. Introduction
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Direct methanol fuel cells (DMFCs) are emerging alternative power sources for many applications in transportation and portable electronics. Many advantages are gained such as much higher energy density and very low environmental intrusion without reformer than
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gaseous fuels such as hydrogen and natural gas [1−4]. Pt and Pt-based alloys were widely employed as anode catalysts in DMFCs in acidic media because they have high catalytic activity towards the electrooxidation of small organic molecules [5−7]. Pt/GO electrocatalyst,
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prepared by the polyol process, shows better tolerance to CO during methanol oxidation reaction when compared to Pt/CNTs [8]. The oxidation potential of CO is significantly
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reduced from 750 mV at Pt/C to 520 mV at Pt/polyaniline/WC/C [9]. The introduction of boron or nitrogen containing functional groups into the reduced graphene sheets could
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modulate the particle size and dispersion of supported PtRu nanoparticles and improve their electrocatalytic performance for methanol oxidation reaction [10]. Pt/C/TiO2NTs electrode,
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prepared by facile pyrolysis of chloroplatinic acid and glucose on TiO2NTs support simultaneously, possesses a large electrochemically active surface area and better stability for methanol oxidation than Pt/TiO2NTs electrode [11]. Mesoporous CoNi@Pt nanorods, formed from the water-in-ionic liquid microemulsion, could provide a significantly higher massnormalized current density (1326 mA mg−1) for methanol oxidation in H2SO4 solution [12]. However, CO poisoning, effective methanol crossover, degradation of membranes and corrosion of carbon materials and cell hardware may hinder the incorporation of Pt-based electrocatalysts in DMFCs [13]. The application of alkaline solution in fuel cells tends to improve their performance. It increases the fuel cell efficiency [14, 15]. Many electrode materials are suitable to be examined and the oxygen cathode becomes more efficient. Moreover, the oxidation reaction of organic fuels is not almost sensitive to the surface structure with smaller or negligible poisoning effects. Many non-platinum catalysts could be also investigated in alkaline DMFCs 2 Page 2 of 23
such as Au [16, 17], Pd [18] and Ni [19−22]. PdCu alloys with smaller particle size associated with triethanolamine and the surface confined Pd replacement exhibit an enhanced catalytic performance for methanol oxidation [23]. Superior performance of (Au−Cu) bimetallic catalyst on Nb2O5 and Nb/MCF supports is due to the synergy between gold and
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copper [24]. The mass activity of Co@Pt/MWCNTs towards methanol oxidation in 0.5 M KOH solution is 1.61 and 3.36 times higher than those at Pt–Co/MWCNTs and Pt/MWCNTs, respectively [25]. A great attention was paid to nickel-based electrodes due to their several
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applications in alkaline batteries, fuel cells, capacitors, sensors and catalysts for different electrochemical reactions. El-Shafei [26] has concluded that methanol oxidation reaction at
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modified nickel hydroxide/glassy carbon electrode is catalyzed by NiOOH formation. Alloying nickel with other metals or metal oxides created electroactive catalyst materials for
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alcohol oxidation. A higher activity was achieved at Ni−Co alloy coating on glassy carbon electrode towards methanol oxidation in Ni(III) and Co(IV) oxidation states [27]. The activation energy of methanol oxidation at Ni-promoted Cd/C electrocatalyst is 2.6 times
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lower than that at Ni/C. It suggests that Ni-promoted Cd surface structure is kinetically beneficial for the oxidation reaction in alkaline media [28]. The electrocatalytic performance of Pd40Ni60 alloy catalyst towards methanol oxidation in alkaline media was enhanced when
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compared to that at nanoporous Pd [29]. Electrochemical impedance spectra of titanium-
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supported nano-scale nickel catalyst reveal that the charge transfer process of Ni(OH)2 oxidation to NiOOH is improved after adding methanol to NaOH solution [30]. Modifying
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Pd−Ni alloy with 5 wt.% CeO2 decreases its particle size and promotes the formation of surface Pd−Ni alloy species and amorphous NiO [31]. The apparent electrocatalytic activity of modified La1.5Sr0.5NiO4 electrode is much higher than that of unmodified electrode [32]. The methanol oxidation peak current density at amorphous Ni−B/TiO2 electrode records 72.1% of its original value after 1300 cycles [33]. The onset potential value of ethanol oxidation reaction at Pd/(NiO−MgO@C) electrocatalyst in 1 M KOH solution is 200 mV lower than that at Pd/C [34].
MnO2 was widely examined as an electrode material for supercapacitors [35−42] and lithium batteries [43, 44]. It also showed high catalytic activity for oxygen reduction reaction [45−47] and H2O2 decomposition [48]. Pd/β-MnO2 nanotubes composite displays an excellent electrochemical performance for methanol oxidation in alkaline solution [49]. The methanol oxidation peak current density at Pd−MnO2/MWCNTs electrocatalyst in 0.5 M NaOH solution is 1.49 times higher than that at Pd/MWCNTs [50]. Pt nanoparticles 3 Page 3 of 23
supported on MnO2 nanowire array electrodes showed a lowered overpotential value by 110 mV and an increased methanol oxidation current density by 2.1 folds relative to those at Pt nanowire array electrode [51]. The synergistic interaction of carbon-supported manganese octahedral molecular sieves with 5 wt.% Ru/C facilitates CO oxidation during methanol
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oxidation reaction. Here, Ru sites act as nucleating centers for CO clustering within the octahedral sieve framework [52].
In the present work, Ni−MnOx/C electrocatalyst was prepared by the chemical
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reduction method using NaBH4 as the reducing agent with the aid of microwave irradiation. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron
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spectroscopy (XPS) and energy dispersive X-ray analysis (EDX) were employed to physically characterize the prepared electrocatalysts. The electrocatalytic activity of
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Ni−MnOx/C towards methanol oxidation reaction in KOH solution was studied using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy.
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2. Experimental 2.1. Chemicals
Vulcan XC-72R carbon black was purchased from Cabot Corporation in USA with a
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specific surface area (BET) of 240 m2 g−1 and an average particle size of 40 nm. Nafion
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(perfluorosulphonic acid-polytetrafluoroethylene (PTFE) copolymer, 5 wt.% solution) and methanol were obtained from Sigma−Aldrich and Merck Corporation, respectively in
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Germany. KOH and NaBH4 were supplied by Eka Chemicals AB, while MnO2 and NiCl2.6H2O were purchased from Solveco AB. All these chemicals were used as received without further purification. Double distilled water was used for electrodes washing and preparation of all aqueous solutions. 2.2. Preparation of Ni−MnOx/C electrocatalyst The preparation of Ni−MnOx/C electrocatalyst was carried out in two steps: MnOx/C
was firstly synthesized, followed by chemical reduction of nickel ions using sodium borohydride as a reducing agent and assisted by microwave irradiation. To prepare MnOx/C powder, calculated amounts of MnO2 and Vulcan XC-72R carbon black were dispersed in a mixture of double distilled water and isopropyl alcohol (1:1). Here, 7.5 wt.% MnO2 was added to carbon support. This mixture was stirred for 30 min, followed by heating in a household microwave oven (Caira CA-MW 1025, touch pad digital control, 50 MHz, 1400 W) using the pulse mode of 20 s on/60 s off for 6 times with a total working time of 120 s at a heating temperature of 140ºC. The resultant powder was then filtered, washed with double 4 Page 4 of 23
distilled water for 6 times and dried in an air oven at 80°C for 6 h. Nickel nanoparticles were then reduced onto MnOx/C. A suspended solution of MnOx/C powder was sonicated for 30 min. Nickel chloride solution was then added keeping the metal loading of 25 wt.% with stirring for 30 min. Sodium borohydride solution was added dropwisely with constant stirring
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for 30 min. The molar ratio of nickel metal to the reducing agent was adjusted to be 1:70. The reduction step was activated by heating in the microwave oven using the pulse mode of 20 s on/10 s off for 15 times with a total working time of 5 min at a heating temperature of 140ºC.
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The above mixture was then filtered, washed and dried. Ni/C electrocatalyst was prepared
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under the same conditions for comparison. 2.3. Physical characterization of Ni−MnOx/C electrocatalyst
Transmission electron microscopy was employed to characterize the electrocatalyst
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morphology and determine its particle size. JEOL-JEM 2010 transmission electron microscope was working at an accelerating voltage of 160 kV. Gatan program was used to
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estimate the size of electrocatalyst nanoparticles. X-ray diffraction was applied to study the crystalline structure of the prepared electrocatalyst. For this purpose, Rigaku-D/MAX-PC 2500 X-ray diffractometer equipped with Ni filtered Cu Kα as the radiation source was used.
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The tube current was 40 mA with a voltage of 40 kV. The electrocatalyst powder was fixed on a glass slide and then dried in vacuum overnight. X-ray diffraction pattern was scanned for
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2θ values ranging between 10° and 80° at a scan rate of 10° min−1. The electronic structure of
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Ni−MnOx/C was evaluated by X-ray photoelectron spectroscopy. It was carried out on a Perkin-Elmer PHI-5000C ESCA system equipped with a hemispherical electron energy analyzer using a monochromatic Mg Kα radiation (1253.6 eV). The binding energies were calibrated using the signal of carbon as reference [C 1s peak at 284.6 eV]. The chemical composition of Ni−MnOx/C electrocatalyst was determined using energy dispersive X-ray analysis. Its unit "INCA X-sight, OXFORD instruments, England" was used. It was attached to scanning electron microscope "JXA−840A, Electron Prob Microanalyzer, JEOL, Japan". The actual amount of loaded nickel on carbon surface was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Hitachi P-4010. N2 adsorptiondesorption measurements were determined at 77 K using a Micromeritics ASAP 2020 Analyzer. Prior to analysis, the samples were degassed under vacuum at 200ºC for 3 h. Their specific surface area values were calculated using the multipoint Brunauer-Emmett-Teller (BET) method.
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2.4. Electrode fabrication and electrochemical measurements Commercial carbon rod was used as a support for Ni/C and Ni−MnOx/C powders. It was pretreated by polishing its surface with soft emery papers followed by washing with double distilled water and acetone. Electrochemical activation step for carbon surface was
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proposed to clean its surface and prepare it for adding the electrocatalyst powder. It was operated by cyclization in 0.5 M H2SO4 solution in the potential window from −800 to +1600 mV versus Hg/Hg2SO4/1.0 M H2SO4 (MMS) for 50 cycles at 50 mV s−1. The electrocatalyst
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ink was prepared by dispersing 10 mg of Ni/C or Ni−MnOx/C powder into 5 ml isopropyl alcohol with the aid of ultrasonic agitation for 5 min. 0.5 ml of this suspension was spread
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over the carbon surface. At the same time, 1 wt.% Nafion solution was prepared by diluting 5 wt.% solution with ethanol. 0.1 ml of this diluted Nafion solution was added on the carbon
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surface to fix the electrocatalyst layer. It was dried in air and kept in a desiccator till it is used. The mass loading of the electrocatalyst active material on carbon support surface was
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about 2 mg cm−2.
All cyclic voltammetry and chronoamperometry experiments were performed on Gamry potentiostat at room temperature and in aerated electrolytes. A conventional three-
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electrode system was used. It consisted of a modified commercial carbon rod with Ni/C or
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Ni−MnOx/C powder as the working electrode, a platinum wire as the counter electrode, and an Hg/HgO/1.0 M NaOH (MMO) as the reference electrode. All potentials in this work were
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referred to (MMO). A constant dc potential value with ac voltage amplitude of 10 mV and a frequency range of 1x104–0.1 Hz were adjusted to carry out the electrochemical impedance spectroscopy measurements. 3. Results and discussion
TEM images of Ni/C and Ni−MnOx/C electrocatalysts were represented in Figs. (1a)
and (1b), respectively. They were composed of spherical particles of carbon black with a mean diameter of 35 nm. Dense Ni nanoparticles were deposited on carbon surface in Ni/C. On the other hand, finer nickel nanoparticles appeared in Ni−MnOx/C with a lower particle size of 4.5 nm compared to 6 nm in Ni/C. Fig. (2a) showed X-ray diffraction patterns of Ni/C and Ni−MnOx/C electrocatalysts. The broad peak at 2θ value of about 25° was related to the graphite (002) facet of Vulcan XC-72R carbon black. For Ni/C, another three diffraction peaks appeared corresponding to the formation of metallic nickel and nickel hydroxide. According to JCPD (Joint Committee 6 Page 6 of 23
on Powder Diffraction), Ni(111) plane was observed at 2θ value of 46.89°. On the other hand, (100) and (110) facets of Ni(OH)2 appeared at 2θ values of 34.05° and 60.45°, respectively [53−55]. For Ni−MnOx/C, (100) diffraction plane of Ni(OH)2 was positively shifted to 2θ value of 34.41°, while that of (110) plane was almost unchanged [60.46°]. The
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diffraction peaks associated with pure manganese oxide could not be detected. This is due to the presence of a very small amount of metal oxide in the prepared electrocatalyst in an
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amorphous form or incorporated in a solid solution with Ni.
XPS measurements were employed to analyze the surface composition of
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Ni−MnOx/C electrocatalyst. Fig. (2b) represented XPS spectra of Ni 2p. The Ni 2p peaks at binding energies of 852.1 and 855.8 eV were attributed to metallic Ni and Ni(OH)2 species, respectively [56]. This satisfactorily confirmed the result obtained by XRD analysis. The
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intensity of metallic Ni peak was somewhat small in relation to that of Ni(OH)2 species. It indicated that high percentage of nickel at the electrocatalyst surface had been oxidized
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during the preparation step. Quantitative XPS results of Ni−MnOx/C showed that the surface composition percentage of Ni(OH)2 species was about 79.45%. On the other hand, three different oxidation states of manganese have been detected in XPS spectra of Mn 2p region in
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Fig. (2c). They were shown as three pairs of doublets for Mn(III), Mn(IV) and Mn(V) species
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in 2p1/2 and 2p3/2 orbits. The two peaks of Mn(IV) were mainly observed at binding energies of 641.9 and 653.8 eV. On the other hand, Mn 2p3/2 and Mn 2p1/2 of Mn(III) were noticed at
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641.3 and 653.4 eV, respectively. The remaining two peaks at 646.0 and 657.1 eV were assigned to Mn(V) in MnOx. This was in a good agreement with the results obtained by Xie et al. [57] for manganese dioxide that was fabricated by in situ coating method on carbon nanotubes. The percentage values of these Mn oxidation states could not be easily predicted due to the influence of Mn 3d electrons on the XPS intensity of Mn 2p signals [58]. Energy dispersive X-ray spectrum of Ni−MnOx/C in Fig. (2d) confirmed the presence of carbon, nickel, manganese and oxygen in the prepared electrocatalyst. The weight percentage of nickel was about 23.56%. Manganese and oxygen were detected in small percentage values as 3.05% and 1.56%, respectively. The actual loading content of Ni in the prepared electrocatalysts was estimated using ICP-AES. It was 23.68% and 24.18% in Ni/C and Ni−MnOx/C electrocatalysts, respectively which was close to the added metal percentage during the preparation step.
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Fig. (3) represented the nitrogen adsorption-desorption isotherms of Ni/C and Ni−MnOx/C electrocatalysts. They exhibited type IV isotherm with H3 hystersis loop which is a characteristic feature of porous materials. This porous structure could enhance ion transportation and facilitate rapid charge/discharge reactions by maintaining smooth electron pathways. The Brunauer-Emmett-Teller (BET) specific surface area values of Ni/C and
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Ni−MnOx/C electrocatalysts were calculated as 51.72 and 49.57 m2 g−1, respectively.
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Fig. (4a) displayed the cyclic voltammogram of Ni/C in 0.5 M KOH solution at 10 −1
mV s . It was recorded after scanning this electrode for 10 cycles. A redox couple was
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noticed at potential values of +550 and +340 mV in the forward and backward directions, respectively. It corresponds to Ni(II)/Ni(III) species transformation according to the
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following reaction [19, 59−61]:
Ni(OH)2 + OH− ↔ NiOOH + H2O + e−
(1)
For Ni−MnOx/C electrocatalyst, this redox couple was formed at more negative potential
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values of +536 and +319 mV with much higher current densities. It indicated the influence of MnOx in enhancing the electron transfer process during Ni(OH)2/NiOOH transformation at
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Ni−MnOx/C due to the ability of manganese oxide to easily adsorb OH species at its surface.
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The presence of MnOx also improved the rate of oxygen evolution reaction as evidenced by its increased current density when compared to that at Ni/C. Raj et al. [62] have studied the
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electrocatalytic activity of electrolytic manganese dioxide, dispersed with silver and lanthanum through a thermal route in selected non-stoichiometric compositions, towards oxygen evolution reaction in KOH solution. It was found that α-MnO2 nanowires possessed an enhanced electrocatalytic performance when compared to nanotubes and nanoparticles [63].
When 0.4 M methanol was added to the supporting electrolyte in Fig. (4b), the
oxidation current density at Ni−MnOx/C increased with the start of NiOOH formation to get an oxidation peak at a potential value of +830 mV. The reduction of area under peak of NiOOH/Ni(OH)2 transformation was a clue for the consumption of an appreciable percentage of NiOOH during methanol oxidation in the forward scan. This observation confirmed that NiOOH is an electrocatalyst for the oxidation reaction. This result is in a good agreement with the oxidation mechanism assumed by Fleischmann et al. [64, 65] as follows: NiOOH + methanol → Ni(OH)2 + product
(2)
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The cyclic voltammogram of methanol oxidation reaction at Ni−MnOx/C was compared with that at Ni/C in Fig. (4c). It was observed that the presence of 7.5 wt.% manganese oxide in Ni−MnOx/C resulted in 1.43 folds increment in the oxidation current density in relation to that at Ni/C. However, it showed a positive potential shift by 63 mV.
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Wu et al. [66] have prepared carbon-supported Ni(OH)2−MnOx/C composite by reducing the amorphous MnO2/C suspension in the presence of Ni2+ with NaBH4. They observed that
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Mn(OH)2/MnOOH transformation peak at MnOx/C showed a fast potential shift in the positive direction in alkaline solution. This may account for the positive potential shift of
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methanol oxidation peak at Ni−MnOx/C. The reduction peak of NiOOH species was almost absent at Ni/C, while it was clearly detected at Ni−MnOx/C electrocatalyst. Therefore, lower amount of NiOOH species was required for methanol oxidation reaction with enhanced
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electrocatalytic activity when manganese oxide was incorporated in Ni/C. The addition of metal oxides can significantly increase the catalytic activity at the same Ni loading.
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Amorphous Ni−B/TiO2 electrode, prepared by electroless plating, had a much higher methanol oxidation peak current density than Ni−B/Ti electrode. This was attributed to the large specific surface area of TiO2 nanotube arrays as observed by field emission scanning
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electron microscopy [33]. The modification of Pd−Ni alloy catalyst with 5 wt.% CeO2,
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through the extensively utilized deposition–precipitation route, improved its adsorption capacity towards ethanol [31]. Pd−MnO2/reduced graphene oxide had a more negative onset
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potential value for methanol oxidation than that at Pd/reduced graphene oxide by 7 mV [67]. If/Ib ratio of methanol oxidation reaction at Pd/β-MnO2 nanotubes was much higher than that at Pd/C [5.87 vs 1.49] [49].
Double-step chronoamperometry was employed to estimate the catalytic rate constant
for the reaction between methanol and Ni−MnOx/C electrocatalyst. The chronoamperograms of Ni−MnOx/C in 0.5 M KOH solution in presence of methanol over a concentration range of 0−0.50 M were displayed in Fig. (5a). The applied potential step values were 750 and 350 mV, respectively. As methanol concentration increases, the oxidation current density would increase at the first potential step. The catalytic rate constant k can be calculated according to the following equation [68]: IC/IL = [γ0.5 (π0.5 erf (γ0.5)) + exp (‒γ/γ0.5)]
(3)
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Where: IC and IL are the current density values of the electrocatalyst in the presence and in absence of methanol, respectively and γ = kC0t is the argument of the error function. The catalytic rate constant is k in cm3 mol‒1 s‒1, C0 is the bulk concentration of methanol in mol cm−3, erf is the error function and t is the elapsed time in s. In such cases where γ > 1.5,
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erf (γ0.5) is almost equal to unity and the above equation is reduced to the following law: IC/IL = γ0.5 π0.5 = π0.5 (kC0t)0.5
(4)
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The current ratio (IC/IL) was plotted as a function of the square root of time in different methanol concentrations (0.025−0.30 M) at Ni−MnOx/C electrocatalyst in Fig. (5b).
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Straight lines were obtained. The catalytic rate constant of methanol oxidation reaction at Ni−MnOx/C electrocatalyst was calculated in 0.5 M KOH solution containing 0.2 M
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methanol based on the slope value of its corresponding linear relationship. It had a value of 3.26 x 103 cm3 mol−1 s−1, which was 1.81 times higher than that at Ni/C under the same operating conditions [69]. Therefore, the presence of manganese oxide enhances the rate of
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the oxidation reaction at Ni−MnOx/C electrocatalyst.
Plotting the net current of methanol oxidation reaction at Ni−MnOx/C electrocatalyst
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against the minus square root of time in various methanol concentrations [0−0.20 M] in 0.5
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M KOH solution resulted in linear relationships in Fig. (5c). Based on this observation, the oxidation reaction is a diffusion-controlled process. The diffusion coefficient of methanol
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could be estimated by applying Cottrell′s equation as follows [70]: I = nFAD0.5Cπ‒0.5t‒0.5
(5)
Where: I is the net current in Ampere, D is the diffusion coefficient in cm2 s‒1 and C
is the bulk concentration of methanol in mol cm‒3. The diffusion coefficient value of methanol at Ni−MnOx/C electrocatalyst in (0.2 M methanol + 0.5 M KOH) solution was about 4 times higher than that at Ni/C in accordance with Ref. [69]. It indicated that methanol could be easily diffused at Ni−MnOx/C electrocatalyst when compared to that at Ni/C. The chronoamperograms of methanol oxidation reaction at Ni/C and Ni−MnOx/C electrocatalysts in (0.4 M methanol + 0.5 M KOH) solution were recorded in Fig. (6) at a potential step value of 750 mV for 1800 s. In current density-time curves, the oxidation current density was initially dropped in the first 100 s, followed by a slower decay until it
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reached a steady state value. The oxidation current density at Ni/C after 1800 s was 8.62 mA cm−2, which was 1.29 times lower than that at Ni−MnOx/C electrocatalyst. Electrochemical impedance spectroscopy technique was also applied to examine the electrocatalytic behaviour of the prepared electrocatalysts after long-time operation. Fig. (7)
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showed the phase shift diagrams of methanol oxidation reaction at Ni/C electrocatalyst at different potential values ranging from 400 to 650 mV. It was observed that the phase angle
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was almost unchanged in the frequency range from 10 kHz to 6.5 Hz with varying the electrode potential. It suggested a resistive behaviour including charge transfer process at the
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outermost electrode surface. At 400 mV, the phase angle was slightly changed at lower frequency values below 2.5 Hz. Therefore, the electrode reaction was dominated by the electric double layer capacitance with low electrocatalytic activity in this frequency range.
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An arc was then evident at lower frequency values. The transition from the resistive to capacitive behaviour could be distinguished by a maximum frequency value. This maximum
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was slightly shifted to higher values as the electrode potential increased up to 550 mV to imply an increased reaction rate in the resistive region. Above 550 mV, a slight shift towards lower maximum frequency value was shown with increased phase angle values. Ni−MnOx/C
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electrocatalyst also showed the same characteristics in its phase shift diagrams in Fig. (8) at
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potential values of 400, 550 and 650 mV. However, the maximum frequency at the transition from the resistive to capacitive regions was appreciably shifted to a higher value when
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manganese oxide was added to Ni/C. Moreover, lower phase angle values were generally recorded at Ni−MnOx/C electrocatalyst. Figs. (9a, b) showed the corresponding Nyquist plots of methanol oxidation reaction
at Ni/C at different potential values. It was observed that Nyquist diagram at 400 mV consisted of an inclined line in the low frequency region and a semicircle in the high frequency region. This linear part referred to a diffusion-limited process, while the semicircle portion was related to the electron transfer-limited process [68]. This linear part was replaced by another defined semicircle at a potential value of 450 mV. It was then overlapped with the semicircle at high frequency section at higher potential values up to 650 mV. The diameter of this semicircle is a measure of the charge transfer resistance of methanol oxidation reaction [71]. It decreased with increasing the potential value up to 550 mV, indicating a faster electron transfer process. However, it increased again at higher potential values to reflect the increased resistance of the charge transfer step. This may be in a good agreement with the proposed mechanism of methanol oxidation reaction at nickel-based electrocatalysts. 11 Page 11 of 23
Ni(OH)2 starts to be oxidized to NiOOH at 400 mV. Since NiOOH is an electrocatalyst for methanol oxidation reaction, the linear part in Nyquist diagram at 400 mV could account for the start of methanol adsorption and diffusion through the electrocatalyst surface. As the potential value increased up to 550 mV, more NiOOH species would be formed to enhance
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the rate of methanol adsorption and oxidation at Ni/C surface. However, thick oxyhydroxide species may play an adverse role to somewhat hinder the oxidation reaction at higher potential values. The Nyquist diagrams of Ni−MnOx/C electrocatalyst were displayed in Figs.
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(9a', b'). They showed the same characteristic shape for those of Ni/C. The improved performance of Ni−MnOx/C electrocatalyst for methanol oxidation reaction was apparent in
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two points. The diameter of similar semicircles was lower than that of Ni/C to confirm the decreased charge transfer resistance values at Ni−MnOx/C electrocatalyst surface. The
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impedance values were also much lowered after the addition of manganese oxide to Ni/C. An equivalent electric circuit in Fig. (10a) was proposed to fit the electrochemical impedance results. It was composed of two (R−CPE) networks in a serial connection with the electrolyte
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resistance (Rs). The CPE (constant phase element) represents the double-layer capacitance distributed between the ohmic and faradic processes [72, 73]. It was used in this model, rather than a capacitor, to compensate for the inhomogenity of the system caused by the porous
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nature of the catalytic layer and the interface roughness [74]. R1 and R2 values stand for the
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charge transfer resistances at the outermost surface of the electrocatalyst and within its porous structure, respectively. The insertion of Warburg impedance element in the circuit of
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Fig. (10b) was essential to fit the linear part of EIS curve at a potential value of 400 mV. Zview software was applied to analyze the electrochemical impedance curves based on the proposed electric circuits. Tables (1) and (2) listed the values of different elements of these circuits for EIS data of Ni/C and Ni−MnOx/C, respectively. The resistance values generally decreased at both electrocatalysts with increasing the potential value up to 550 mV, afterwards they increased. Ni−MnOx/C showed lower resistance values when compared to Ni/C. Moreover, W value of EIS curve of Ni−MnOx/C at 400 mV was 7.54 times lower than that of Ni/C. It indicated faster methanol diffusion rate at Ni−MnOx/C electrocatalyst surface. The stability of Ni/C electrocatalyst for long-time operation during methanol oxidation reaction was studied using electrochemical impedance spectroscopy. This test was carried out by subjecting Ni/C for potentiostatic step at a potential value of 750 mV for long time intervals in (0.4 M methanol + 0.5 M KOH) solution. Phase shift and Nyquist plots were then displayed in the frequency range from 10 kHz to 0.1 Hz at a potential value of 550 mV. 12 Page 12 of 23
This phase shift diagram was represented in Fig. (11a). It was found that prolonged operation at the peak potential value of methanol oxidation reaction resulted in an increased phase angle of the maximum position of the arc when compared to the performance of a fresh Ni/C electrocatalyst. It referred to the increased charge transfer resistance after long-time
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operation. The increased diameter value of the semicircle in the corresponding Nyquist plots of Ni/C with the operation time in Fig. (11b) confirmed this result. It was accompanied by a gradual increase in the impedance value. Similar diagrams of Ni−MnOx/C electrocatalyst
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were observed in Figs. (11a', b'). They showed lower phase angle and impedance values when compared to those of Ni/C. The equivalent electric circuit in Fig. (10a) was employed to
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simulate the electrochemical impedance curves of Ni/C and Ni−MnOx/C electrocatalysts after the stability test. The values of electrochemical elements were shown in Tables (3) and (4),
an
respectively. The variation of R1 and R2 values of both electrocatalysts with the operation time was plotted in Fig. (12). The polarization of prepared electrocatalysts at the peak potential value of methanol oxidation for long time starting from 2 h resulted in an increased
M
impedance value. After 4 h, R1 and R2 values increased by 1.13 and 1.23 times, respectively at Ni−MnOx/C relative to those at fresh electrocatalyst [0 h]. On the other hand, Ni/C had increased R1 and R2 values by 1.44 and 1.47 times. This result supported the improved
te
d
stability of Ni/C after incorporating manganese oxide. The synergistic action of manganese oxide in Ni−MnOx/C electrocatalyst could be
Ac ce p
arisen from the oxophilic nature of transition metal oxides. Manganese oxide can create large hydrophilic regions on carbon surface that can facilitate the diffusion of nickel ions and effectively prevent the agglomeration of nickel nanoparticles [75]. It also increases the formation rate of OHads species [76−78] which in turn would participate in Ni(OH)2/NiOOH transformation. This could enhance the charge transfer at Ni−MnOx/C surface, resulting in lowered phase angle and impedance values. The small-sized and well-dispersed nickel nanoparticles on MnOx/C as shown in TEM images can also offer higher surface area that hosts more accessible active sites, thus achieving high electrocatalytic activity towards methanol oxidation reaction. 4. Conclusion Microwave irradiation technique was employed to reduce nickel ions on MnOx/C using NaBH4. Incorporating manganese oxide in Ni/C was found to improve its electrocatalytic performance for methanol oxidation in KOH solution. The catalytic rate 13 Page 13 of 23
constant and diffusion coefficient values of methanol during the oxidation process at Ni−MnOx/C were about 1.81 and 4 times higher than that at Ni/C. Ni−MnOx/C showed faster electron transfer rate at E ≤ 550 mV. However, thick NiOOH species at higher potential values would retard the charge transfer process. The impedance value of Ni−MnOx/C
ip t
increased after potentiostatic polarization at methanol oxidation peak potential for long time. However, its poisoning degree was lower than that at Ni/C. This study elects Ni−MnOx/C as
cr
a good candidate for anode manufacture in direct methanol fuel cells.
us
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Table (1): Electrochemical impedance parameters of methanol oxidation reaction at Ni/C
400
114.60
25.83
450
110.70
24.86
500
109.10
24.66
n1
R2 / Ω cm2
Y02 x 10−3/ Ω −1 sn cm−2
n2
3.217
0.91
40.47
1.720
1.00
0.88
39.83
3.232
0.80
4.769
0.87
38.33
4.642
0.81
Y01 x 10−3 / Ω −1 sn cm−2
M
R1 / Ω cm2
d
Rs / Ω
4.180
Ac ce p
te
Potential value / mV
an
electrocatalyst at different potential values.
550
109.90
20.57
4.912
0.92
28.69
6.314
0.80
600
108.40
33.38
4.339
0.83
42.28
4.984
0.98
650
108.10
35.14
4.068
0.84
44.44
4.220
1.00
Table (2): Electrochemical impedance parameters of methanol oxidation reaction at Ni−MnOx/C electrocatalyst at different potential values. Potential value /
Rs / Ω
R1 / Ω cm2
Y01 x 10−3 / Ω −1 sn cm−2
n1
R2 / Ω cm2
Y02 x 10−3/ Ω −1 sn cm−2
n
21 Page 21 of 23
mV
110.00
14.46
7.723
0.85
15.75
6.298
0.82
450
107.90
12.96
12.978
0.88
14.53
7.635
0.88
500
107.70
11.42
23.885
0.80
550
108.60
9.79
34.315
0.86
600
109.30
14.62
20.780
650
108.30
15.37
13.933
ip t
400
10.41
37.978
0.85
12.19
6.823
1.00
13.09
5.231
0.85
M
0.81
0.88
cr
us
an
0.88
18.471
11.57
d
Table (3): Electrochemical impedance parameters of Ni/C electrocatalyst at 550 mV after
te
operation for long time during methanol oxidation reaction. Y01 x 10−3 / Ω −1 sn cm−2
0
109.90
20.57
4.912
0.92
28.69
1
103.80
19.58
8.041
0.83
25.04
2
102.90
21.40
6.608
0.87
33.61
3
102.50
24.73
5.525
0.89
37.30
4
101.30
29.65
3.336
0.94
42.16
Rs / Ω
R1 / Ω cm2
Ac ce p
Operation time / h
n1
R2 / Ω cm2
Y Ω
22 Page 22 of 23
Table (4): Electrochemical impedance parameters of Ni−MnOx/C electrocatalyst at 550 mV
Rs / Ω
R1 / Ω cm2
Y01 x 10−3 / Ω −1 sn cm−2
n1
R2 / Ω cm2
Y02 x 10 Ω −1 sn cm
0
108.60
9.79
34.315
0.86
10.41
37.978
1
98.17
9.00
41.879
10.10
54.379
2
93.25
10.20
38.137
0.83
11.00
28.185
3
92.48
10.60
32.006
0.88
11.81
17.914
4
92.55
11.09
29.936
0.84
12.79
15.207
cr
ip t
Operation time /h
an
after operation for long time during methanol oxidation reaction.
Ac ce p
te
d
M
us
0.87
23 Page 23 of 23
S0169-4332(15)02025-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.08.201 APSUSC 31148
To appear in:
APSUSC
Received date: Revised date: Accepted date:
12-5-2015 24-8-2015 25-8-2015
Please cite this article as: R.M.A. Hameed, Microwave irradiated Ni
Highligts Ni−MnOx/C had nickel nanoparticles with an average diameter of 4.5 nm. Oxidation current density increased by 1.43 times at Ni−MnOx/C.
cr
Adding MnOx to Ni/C lowered its phase angle and impedance values.
ip t
Ni−MnOx/C showed ks value of 3.26 x 103 cm3 mol−1 s−1.
us
Microwave irradiated Ni−MnOx/C as an electrocatalyst for methanol oxidation
an
in KOH solution for fuel cell application
R.M. Abdel Hameed*
M
Chemistry department, Faculty of Science, Cairo University, Giza, Egypt.
*
Corresponding author Tel.: +201145565646; fax: +235727556.
Ac ce p
Abstract
te
d
E-mail address: [email protected] (R.M. Abdel Hameed).
Ni−MnOx/C electrocatalyst was synthesized by the reduction of nickel precursor salt
on MnOx/C powder using NaBH4 and the deposition process was motivated with the aid of microwave irradiation. Finer nickel nanoparticles were detected in Ni−MnOx/C using transmission electron microscopy with a lower particle size of 4.5 nm compared to 6 nm in Ni/C. Cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS) were applied to study the electrocatalytic activity of Ni−MnOx/C for methanol oxidation in 0.5 M KOH solution. The presence of 7.5 wt.% MnOx in Ni−MnOx/C enhanced the oxidation current density by 1.43 times. The catalytic rate constant of methanol oxidation at Ni−MnOx/C was calculated as 3.26 x 103 cm3 mol−1 s−1. An appreciable shift in the maximum frequency at the transition from the resistive to capacitive regions to a higher value in Bode plots of Ni−MnOx/C was shown when compared to Ni/C. It was accompanied by lowered phase angle values. The lowered Warburg impedance value (W) of Ni−MnOx/C at 400 mV confirmed the faster methanol diffusion rate at its surface. 1 Page 1 of 23
Keywords
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Nickel; manganese oxide; microwave irradiation; alkaline medium; fuel cells.
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1. Introduction
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Direct methanol fuel cells (DMFCs) are emerging alternative power sources for many applications in transportation and portable electronics. Many advantages are gained such as much higher energy density and very low environmental intrusion without reformer than
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gaseous fuels such as hydrogen and natural gas [1−4]. Pt and Pt-based alloys were widely employed as anode catalysts in DMFCs in acidic media because they have high catalytic activity towards the electrooxidation of small organic molecules [5−7]. Pt/GO electrocatalyst,
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prepared by the polyol process, shows better tolerance to CO during methanol oxidation reaction when compared to Pt/CNTs [8]. The oxidation potential of CO is significantly
d
reduced from 750 mV at Pt/C to 520 mV at Pt/polyaniline/WC/C [9]. The introduction of boron or nitrogen containing functional groups into the reduced graphene sheets could
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modulate the particle size and dispersion of supported PtRu nanoparticles and improve their electrocatalytic performance for methanol oxidation reaction [10]. Pt/C/TiO2NTs electrode,
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prepared by facile pyrolysis of chloroplatinic acid and glucose on TiO2NTs support simultaneously, possesses a large electrochemically active surface area and better stability for methanol oxidation than Pt/TiO2NTs electrode [11]. Mesoporous CoNi@Pt nanorods, formed from the water-in-ionic liquid microemulsion, could provide a significantly higher massnormalized current density (1326 mA mg−1) for methanol oxidation in H2SO4 solution [12]. However, CO poisoning, effective methanol crossover, degradation of membranes and corrosion of carbon materials and cell hardware may hinder the incorporation of Pt-based electrocatalysts in DMFCs [13]. The application of alkaline solution in fuel cells tends to improve their performance. It increases the fuel cell efficiency [14, 15]. Many electrode materials are suitable to be examined and the oxygen cathode becomes more efficient. Moreover, the oxidation reaction of organic fuels is not almost sensitive to the surface structure with smaller or negligible poisoning effects. Many non-platinum catalysts could be also investigated in alkaline DMFCs 2 Page 2 of 23
such as Au [16, 17], Pd [18] and Ni [19−22]. PdCu alloys with smaller particle size associated with triethanolamine and the surface confined Pd replacement exhibit an enhanced catalytic performance for methanol oxidation [23]. Superior performance of (Au−Cu) bimetallic catalyst on Nb2O5 and Nb/MCF supports is due to the synergy between gold and
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copper [24]. The mass activity of Co@Pt/MWCNTs towards methanol oxidation in 0.5 M KOH solution is 1.61 and 3.36 times higher than those at Pt–Co/MWCNTs and Pt/MWCNTs, respectively [25]. A great attention was paid to nickel-based electrodes due to their several
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applications in alkaline batteries, fuel cells, capacitors, sensors and catalysts for different electrochemical reactions. El-Shafei [26] has concluded that methanol oxidation reaction at
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modified nickel hydroxide/glassy carbon electrode is catalyzed by NiOOH formation. Alloying nickel with other metals or metal oxides created electroactive catalyst materials for
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alcohol oxidation. A higher activity was achieved at Ni−Co alloy coating on glassy carbon electrode towards methanol oxidation in Ni(III) and Co(IV) oxidation states [27]. The activation energy of methanol oxidation at Ni-promoted Cd/C electrocatalyst is 2.6 times
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lower than that at Ni/C. It suggests that Ni-promoted Cd surface structure is kinetically beneficial for the oxidation reaction in alkaline media [28]. The electrocatalytic performance of Pd40Ni60 alloy catalyst towards methanol oxidation in alkaline media was enhanced when
d
compared to that at nanoporous Pd [29]. Electrochemical impedance spectra of titanium-
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supported nano-scale nickel catalyst reveal that the charge transfer process of Ni(OH)2 oxidation to NiOOH is improved after adding methanol to NaOH solution [30]. Modifying
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Pd−Ni alloy with 5 wt.% CeO2 decreases its particle size and promotes the formation of surface Pd−Ni alloy species and amorphous NiO [31]. The apparent electrocatalytic activity of modified La1.5Sr0.5NiO4 electrode is much higher than that of unmodified electrode [32]. The methanol oxidation peak current density at amorphous Ni−B/TiO2 electrode records 72.1% of its original value after 1300 cycles [33]. The onset potential value of ethanol oxidation reaction at Pd/(NiO−MgO@C) electrocatalyst in 1 M KOH solution is 200 mV lower than that at Pd/C [34].
MnO2 was widely examined as an electrode material for supercapacitors [35−42] and lithium batteries [43, 44]. It also showed high catalytic activity for oxygen reduction reaction [45−47] and H2O2 decomposition [48]. Pd/β-MnO2 nanotubes composite displays an excellent electrochemical performance for methanol oxidation in alkaline solution [49]. The methanol oxidation peak current density at Pd−MnO2/MWCNTs electrocatalyst in 0.5 M NaOH solution is 1.49 times higher than that at Pd/MWCNTs [50]. Pt nanoparticles 3 Page 3 of 23
supported on MnO2 nanowire array electrodes showed a lowered overpotential value by 110 mV and an increased methanol oxidation current density by 2.1 folds relative to those at Pt nanowire array electrode [51]. The synergistic interaction of carbon-supported manganese octahedral molecular sieves with 5 wt.% Ru/C facilitates CO oxidation during methanol
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oxidation reaction. Here, Ru sites act as nucleating centers for CO clustering within the octahedral sieve framework [52].
In the present work, Ni−MnOx/C electrocatalyst was prepared by the chemical
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reduction method using NaBH4 as the reducing agent with the aid of microwave irradiation. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron
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spectroscopy (XPS) and energy dispersive X-ray analysis (EDX) were employed to physically characterize the prepared electrocatalysts. The electrocatalytic activity of
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Ni−MnOx/C towards methanol oxidation reaction in KOH solution was studied using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy.
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2. Experimental 2.1. Chemicals
Vulcan XC-72R carbon black was purchased from Cabot Corporation in USA with a
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specific surface area (BET) of 240 m2 g−1 and an average particle size of 40 nm. Nafion
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(perfluorosulphonic acid-polytetrafluoroethylene (PTFE) copolymer, 5 wt.% solution) and methanol were obtained from Sigma−Aldrich and Merck Corporation, respectively in
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Germany. KOH and NaBH4 were supplied by Eka Chemicals AB, while MnO2 and NiCl2.6H2O were purchased from Solveco AB. All these chemicals were used as received without further purification. Double distilled water was used for electrodes washing and preparation of all aqueous solutions. 2.2. Preparation of Ni−MnOx/C electrocatalyst The preparation of Ni−MnOx/C electrocatalyst was carried out in two steps: MnOx/C
was firstly synthesized, followed by chemical reduction of nickel ions using sodium borohydride as a reducing agent and assisted by microwave irradiation. To prepare MnOx/C powder, calculated amounts of MnO2 and Vulcan XC-72R carbon black were dispersed in a mixture of double distilled water and isopropyl alcohol (1:1). Here, 7.5 wt.% MnO2 was added to carbon support. This mixture was stirred for 30 min, followed by heating in a household microwave oven (Caira CA-MW 1025, touch pad digital control, 50 MHz, 1400 W) using the pulse mode of 20 s on/60 s off for 6 times with a total working time of 120 s at a heating temperature of 140ºC. The resultant powder was then filtered, washed with double 4 Page 4 of 23
distilled water for 6 times and dried in an air oven at 80°C for 6 h. Nickel nanoparticles were then reduced onto MnOx/C. A suspended solution of MnOx/C powder was sonicated for 30 min. Nickel chloride solution was then added keeping the metal loading of 25 wt.% with stirring for 30 min. Sodium borohydride solution was added dropwisely with constant stirring
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for 30 min. The molar ratio of nickel metal to the reducing agent was adjusted to be 1:70. The reduction step was activated by heating in the microwave oven using the pulse mode of 20 s on/10 s off for 15 times with a total working time of 5 min at a heating temperature of 140ºC.
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The above mixture was then filtered, washed and dried. Ni/C electrocatalyst was prepared
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under the same conditions for comparison. 2.3. Physical characterization of Ni−MnOx/C electrocatalyst
Transmission electron microscopy was employed to characterize the electrocatalyst
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morphology and determine its particle size. JEOL-JEM 2010 transmission electron microscope was working at an accelerating voltage of 160 kV. Gatan program was used to
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estimate the size of electrocatalyst nanoparticles. X-ray diffraction was applied to study the crystalline structure of the prepared electrocatalyst. For this purpose, Rigaku-D/MAX-PC 2500 X-ray diffractometer equipped with Ni filtered Cu Kα as the radiation source was used.
d
The tube current was 40 mA with a voltage of 40 kV. The electrocatalyst powder was fixed on a glass slide and then dried in vacuum overnight. X-ray diffraction pattern was scanned for
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2θ values ranging between 10° and 80° at a scan rate of 10° min−1. The electronic structure of
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Ni−MnOx/C was evaluated by X-ray photoelectron spectroscopy. It was carried out on a Perkin-Elmer PHI-5000C ESCA system equipped with a hemispherical electron energy analyzer using a monochromatic Mg Kα radiation (1253.6 eV). The binding energies were calibrated using the signal of carbon as reference [C 1s peak at 284.6 eV]. The chemical composition of Ni−MnOx/C electrocatalyst was determined using energy dispersive X-ray analysis. Its unit "INCA X-sight, OXFORD instruments, England" was used. It was attached to scanning electron microscope "JXA−840A, Electron Prob Microanalyzer, JEOL, Japan". The actual amount of loaded nickel on carbon surface was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on a Hitachi P-4010. N2 adsorptiondesorption measurements were determined at 77 K using a Micromeritics ASAP 2020 Analyzer. Prior to analysis, the samples were degassed under vacuum at 200ºC for 3 h. Their specific surface area values were calculated using the multipoint Brunauer-Emmett-Teller (BET) method.
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2.4. Electrode fabrication and electrochemical measurements Commercial carbon rod was used as a support for Ni/C and Ni−MnOx/C powders. It was pretreated by polishing its surface with soft emery papers followed by washing with double distilled water and acetone. Electrochemical activation step for carbon surface was
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proposed to clean its surface and prepare it for adding the electrocatalyst powder. It was operated by cyclization in 0.5 M H2SO4 solution in the potential window from −800 to +1600 mV versus Hg/Hg2SO4/1.0 M H2SO4 (MMS) for 50 cycles at 50 mV s−1. The electrocatalyst
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ink was prepared by dispersing 10 mg of Ni/C or Ni−MnOx/C powder into 5 ml isopropyl alcohol with the aid of ultrasonic agitation for 5 min. 0.5 ml of this suspension was spread
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over the carbon surface. At the same time, 1 wt.% Nafion solution was prepared by diluting 5 wt.% solution with ethanol. 0.1 ml of this diluted Nafion solution was added on the carbon
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surface to fix the electrocatalyst layer. It was dried in air and kept in a desiccator till it is used. The mass loading of the electrocatalyst active material on carbon support surface was
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about 2 mg cm−2.
All cyclic voltammetry and chronoamperometry experiments were performed on Gamry potentiostat at room temperature and in aerated electrolytes. A conventional three-
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electrode system was used. It consisted of a modified commercial carbon rod with Ni/C or
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Ni−MnOx/C powder as the working electrode, a platinum wire as the counter electrode, and an Hg/HgO/1.0 M NaOH (MMO) as the reference electrode. All potentials in this work were
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referred to (MMO). A constant dc potential value with ac voltage amplitude of 10 mV and a frequency range of 1x104–0.1 Hz were adjusted to carry out the electrochemical impedance spectroscopy measurements. 3. Results and discussion
TEM images of Ni/C and Ni−MnOx/C electrocatalysts were represented in Figs. (1a)
and (1b), respectively. They were composed of spherical particles of carbon black with a mean diameter of 35 nm. Dense Ni nanoparticles were deposited on carbon surface in Ni/C. On the other hand, finer nickel nanoparticles appeared in Ni−MnOx/C with a lower particle size of 4.5 nm compared to 6 nm in Ni/C. Fig. (2a) showed X-ray diffraction patterns of Ni/C and Ni−MnOx/C electrocatalysts. The broad peak at 2θ value of about 25° was related to the graphite (002) facet of Vulcan XC-72R carbon black. For Ni/C, another three diffraction peaks appeared corresponding to the formation of metallic nickel and nickel hydroxide. According to JCPD (Joint Committee 6 Page 6 of 23
on Powder Diffraction), Ni(111) plane was observed at 2θ value of 46.89°. On the other hand, (100) and (110) facets of Ni(OH)2 appeared at 2θ values of 34.05° and 60.45°, respectively [53−55]. For Ni−MnOx/C, (100) diffraction plane of Ni(OH)2 was positively shifted to 2θ value of 34.41°, while that of (110) plane was almost unchanged [60.46°]. The
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diffraction peaks associated with pure manganese oxide could not be detected. This is due to the presence of a very small amount of metal oxide in the prepared electrocatalyst in an
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amorphous form or incorporated in a solid solution with Ni.
XPS measurements were employed to analyze the surface composition of
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Ni−MnOx/C electrocatalyst. Fig. (2b) represented XPS spectra of Ni 2p. The Ni 2p peaks at binding energies of 852.1 and 855.8 eV were attributed to metallic Ni and Ni(OH)2 species, respectively [56]. This satisfactorily confirmed the result obtained by XRD analysis. The
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intensity of metallic Ni peak was somewhat small in relation to that of Ni(OH)2 species. It indicated that high percentage of nickel at the electrocatalyst surface had been oxidized
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during the preparation step. Quantitative XPS results of Ni−MnOx/C showed that the surface composition percentage of Ni(OH)2 species was about 79.45%. On the other hand, three different oxidation states of manganese have been detected in XPS spectra of Mn 2p region in
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Fig. (2c). They were shown as three pairs of doublets for Mn(III), Mn(IV) and Mn(V) species
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in 2p1/2 and 2p3/2 orbits. The two peaks of Mn(IV) were mainly observed at binding energies of 641.9 and 653.8 eV. On the other hand, Mn 2p3/2 and Mn 2p1/2 of Mn(III) were noticed at
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641.3 and 653.4 eV, respectively. The remaining two peaks at 646.0 and 657.1 eV were assigned to Mn(V) in MnOx. This was in a good agreement with the results obtained by Xie et al. [57] for manganese dioxide that was fabricated by in situ coating method on carbon nanotubes. The percentage values of these Mn oxidation states could not be easily predicted due to the influence of Mn 3d electrons on the XPS intensity of Mn 2p signals [58]. Energy dispersive X-ray spectrum of Ni−MnOx/C in Fig. (2d) confirmed the presence of carbon, nickel, manganese and oxygen in the prepared electrocatalyst. The weight percentage of nickel was about 23.56%. Manganese and oxygen were detected in small percentage values as 3.05% and 1.56%, respectively. The actual loading content of Ni in the prepared electrocatalysts was estimated using ICP-AES. It was 23.68% and 24.18% in Ni/C and Ni−MnOx/C electrocatalysts, respectively which was close to the added metal percentage during the preparation step.
7 Page 7 of 23
Fig. (3) represented the nitrogen adsorption-desorption isotherms of Ni/C and Ni−MnOx/C electrocatalysts. They exhibited type IV isotherm with H3 hystersis loop which is a characteristic feature of porous materials. This porous structure could enhance ion transportation and facilitate rapid charge/discharge reactions by maintaining smooth electron pathways. The Brunauer-Emmett-Teller (BET) specific surface area values of Ni/C and
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Ni−MnOx/C electrocatalysts were calculated as 51.72 and 49.57 m2 g−1, respectively.
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Fig. (4a) displayed the cyclic voltammogram of Ni/C in 0.5 M KOH solution at 10 −1
mV s . It was recorded after scanning this electrode for 10 cycles. A redox couple was
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noticed at potential values of +550 and +340 mV in the forward and backward directions, respectively. It corresponds to Ni(II)/Ni(III) species transformation according to the
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following reaction [19, 59−61]:
Ni(OH)2 + OH− ↔ NiOOH + H2O + e−
(1)
For Ni−MnOx/C electrocatalyst, this redox couple was formed at more negative potential
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values of +536 and +319 mV with much higher current densities. It indicated the influence of MnOx in enhancing the electron transfer process during Ni(OH)2/NiOOH transformation at
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Ni−MnOx/C due to the ability of manganese oxide to easily adsorb OH species at its surface.
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The presence of MnOx also improved the rate of oxygen evolution reaction as evidenced by its increased current density when compared to that at Ni/C. Raj et al. [62] have studied the
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electrocatalytic activity of electrolytic manganese dioxide, dispersed with silver and lanthanum through a thermal route in selected non-stoichiometric compositions, towards oxygen evolution reaction in KOH solution. It was found that α-MnO2 nanowires possessed an enhanced electrocatalytic performance when compared to nanotubes and nanoparticles [63].
When 0.4 M methanol was added to the supporting electrolyte in Fig. (4b), the
oxidation current density at Ni−MnOx/C increased with the start of NiOOH formation to get an oxidation peak at a potential value of +830 mV. The reduction of area under peak of NiOOH/Ni(OH)2 transformation was a clue for the consumption of an appreciable percentage of NiOOH during methanol oxidation in the forward scan. This observation confirmed that NiOOH is an electrocatalyst for the oxidation reaction. This result is in a good agreement with the oxidation mechanism assumed by Fleischmann et al. [64, 65] as follows: NiOOH + methanol → Ni(OH)2 + product
(2)
8 Page 8 of 23
The cyclic voltammogram of methanol oxidation reaction at Ni−MnOx/C was compared with that at Ni/C in Fig. (4c). It was observed that the presence of 7.5 wt.% manganese oxide in Ni−MnOx/C resulted in 1.43 folds increment in the oxidation current density in relation to that at Ni/C. However, it showed a positive potential shift by 63 mV.
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Wu et al. [66] have prepared carbon-supported Ni(OH)2−MnOx/C composite by reducing the amorphous MnO2/C suspension in the presence of Ni2+ with NaBH4. They observed that
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Mn(OH)2/MnOOH transformation peak at MnOx/C showed a fast potential shift in the positive direction in alkaline solution. This may account for the positive potential shift of
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methanol oxidation peak at Ni−MnOx/C. The reduction peak of NiOOH species was almost absent at Ni/C, while it was clearly detected at Ni−MnOx/C electrocatalyst. Therefore, lower amount of NiOOH species was required for methanol oxidation reaction with enhanced
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electrocatalytic activity when manganese oxide was incorporated in Ni/C. The addition of metal oxides can significantly increase the catalytic activity at the same Ni loading.
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Amorphous Ni−B/TiO2 electrode, prepared by electroless plating, had a much higher methanol oxidation peak current density than Ni−B/Ti electrode. This was attributed to the large specific surface area of TiO2 nanotube arrays as observed by field emission scanning
d
electron microscopy [33]. The modification of Pd−Ni alloy catalyst with 5 wt.% CeO2,
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through the extensively utilized deposition–precipitation route, improved its adsorption capacity towards ethanol [31]. Pd−MnO2/reduced graphene oxide had a more negative onset
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potential value for methanol oxidation than that at Pd/reduced graphene oxide by 7 mV [67]. If/Ib ratio of methanol oxidation reaction at Pd/β-MnO2 nanotubes was much higher than that at Pd/C [5.87 vs 1.49] [49].
Double-step chronoamperometry was employed to estimate the catalytic rate constant
for the reaction between methanol and Ni−MnOx/C electrocatalyst. The chronoamperograms of Ni−MnOx/C in 0.5 M KOH solution in presence of methanol over a concentration range of 0−0.50 M were displayed in Fig. (5a). The applied potential step values were 750 and 350 mV, respectively. As methanol concentration increases, the oxidation current density would increase at the first potential step. The catalytic rate constant k can be calculated according to the following equation [68]: IC/IL = [γ0.5 (π0.5 erf (γ0.5)) + exp (‒γ/γ0.5)]
(3)
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Where: IC and IL are the current density values of the electrocatalyst in the presence and in absence of methanol, respectively and γ = kC0t is the argument of the error function. The catalytic rate constant is k in cm3 mol‒1 s‒1, C0 is the bulk concentration of methanol in mol cm−3, erf is the error function and t is the elapsed time in s. In such cases where γ > 1.5,
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erf (γ0.5) is almost equal to unity and the above equation is reduced to the following law: IC/IL = γ0.5 π0.5 = π0.5 (kC0t)0.5
(4)
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The current ratio (IC/IL) was plotted as a function of the square root of time in different methanol concentrations (0.025−0.30 M) at Ni−MnOx/C electrocatalyst in Fig. (5b).
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Straight lines were obtained. The catalytic rate constant of methanol oxidation reaction at Ni−MnOx/C electrocatalyst was calculated in 0.5 M KOH solution containing 0.2 M
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methanol based on the slope value of its corresponding linear relationship. It had a value of 3.26 x 103 cm3 mol−1 s−1, which was 1.81 times higher than that at Ni/C under the same operating conditions [69]. Therefore, the presence of manganese oxide enhances the rate of
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the oxidation reaction at Ni−MnOx/C electrocatalyst.
Plotting the net current of methanol oxidation reaction at Ni−MnOx/C electrocatalyst
d
against the minus square root of time in various methanol concentrations [0−0.20 M] in 0.5
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M KOH solution resulted in linear relationships in Fig. (5c). Based on this observation, the oxidation reaction is a diffusion-controlled process. The diffusion coefficient of methanol
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could be estimated by applying Cottrell′s equation as follows [70]: I = nFAD0.5Cπ‒0.5t‒0.5
(5)
Where: I is the net current in Ampere, D is the diffusion coefficient in cm2 s‒1 and C
is the bulk concentration of methanol in mol cm‒3. The diffusion coefficient value of methanol at Ni−MnOx/C electrocatalyst in (0.2 M methanol + 0.5 M KOH) solution was about 4 times higher than that at Ni/C in accordance with Ref. [69]. It indicated that methanol could be easily diffused at Ni−MnOx/C electrocatalyst when compared to that at Ni/C. The chronoamperograms of methanol oxidation reaction at Ni/C and Ni−MnOx/C electrocatalysts in (0.4 M methanol + 0.5 M KOH) solution were recorded in Fig. (6) at a potential step value of 750 mV for 1800 s. In current density-time curves, the oxidation current density was initially dropped in the first 100 s, followed by a slower decay until it
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reached a steady state value. The oxidation current density at Ni/C after 1800 s was 8.62 mA cm−2, which was 1.29 times lower than that at Ni−MnOx/C electrocatalyst. Electrochemical impedance spectroscopy technique was also applied to examine the electrocatalytic behaviour of the prepared electrocatalysts after long-time operation. Fig. (7)
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showed the phase shift diagrams of methanol oxidation reaction at Ni/C electrocatalyst at different potential values ranging from 400 to 650 mV. It was observed that the phase angle
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was almost unchanged in the frequency range from 10 kHz to 6.5 Hz with varying the electrode potential. It suggested a resistive behaviour including charge transfer process at the
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outermost electrode surface. At 400 mV, the phase angle was slightly changed at lower frequency values below 2.5 Hz. Therefore, the electrode reaction was dominated by the electric double layer capacitance with low electrocatalytic activity in this frequency range.
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An arc was then evident at lower frequency values. The transition from the resistive to capacitive behaviour could be distinguished by a maximum frequency value. This maximum
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was slightly shifted to higher values as the electrode potential increased up to 550 mV to imply an increased reaction rate in the resistive region. Above 550 mV, a slight shift towards lower maximum frequency value was shown with increased phase angle values. Ni−MnOx/C
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electrocatalyst also showed the same characteristics in its phase shift diagrams in Fig. (8) at
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potential values of 400, 550 and 650 mV. However, the maximum frequency at the transition from the resistive to capacitive regions was appreciably shifted to a higher value when
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manganese oxide was added to Ni/C. Moreover, lower phase angle values were generally recorded at Ni−MnOx/C electrocatalyst. Figs. (9a, b) showed the corresponding Nyquist plots of methanol oxidation reaction
at Ni/C at different potential values. It was observed that Nyquist diagram at 400 mV consisted of an inclined line in the low frequency region and a semicircle in the high frequency region. This linear part referred to a diffusion-limited process, while the semicircle portion was related to the electron transfer-limited process [68]. This linear part was replaced by another defined semicircle at a potential value of 450 mV. It was then overlapped with the semicircle at high frequency section at higher potential values up to 650 mV. The diameter of this semicircle is a measure of the charge transfer resistance of methanol oxidation reaction [71]. It decreased with increasing the potential value up to 550 mV, indicating a faster electron transfer process. However, it increased again at higher potential values to reflect the increased resistance of the charge transfer step. This may be in a good agreement with the proposed mechanism of methanol oxidation reaction at nickel-based electrocatalysts. 11 Page 11 of 23
Ni(OH)2 starts to be oxidized to NiOOH at 400 mV. Since NiOOH is an electrocatalyst for methanol oxidation reaction, the linear part in Nyquist diagram at 400 mV could account for the start of methanol adsorption and diffusion through the electrocatalyst surface. As the potential value increased up to 550 mV, more NiOOH species would be formed to enhance
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the rate of methanol adsorption and oxidation at Ni/C surface. However, thick oxyhydroxide species may play an adverse role to somewhat hinder the oxidation reaction at higher potential values. The Nyquist diagrams of Ni−MnOx/C electrocatalyst were displayed in Figs.
cr
(9a', b'). They showed the same characteristic shape for those of Ni/C. The improved performance of Ni−MnOx/C electrocatalyst for methanol oxidation reaction was apparent in
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two points. The diameter of similar semicircles was lower than that of Ni/C to confirm the decreased charge transfer resistance values at Ni−MnOx/C electrocatalyst surface. The
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impedance values were also much lowered after the addition of manganese oxide to Ni/C. An equivalent electric circuit in Fig. (10a) was proposed to fit the electrochemical impedance results. It was composed of two (R−CPE) networks in a serial connection with the electrolyte
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resistance (Rs). The CPE (constant phase element) represents the double-layer capacitance distributed between the ohmic and faradic processes [72, 73]. It was used in this model, rather than a capacitor, to compensate for the inhomogenity of the system caused by the porous
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nature of the catalytic layer and the interface roughness [74]. R1 and R2 values stand for the
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charge transfer resistances at the outermost surface of the electrocatalyst and within its porous structure, respectively. The insertion of Warburg impedance element in the circuit of
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Fig. (10b) was essential to fit the linear part of EIS curve at a potential value of 400 mV. Zview software was applied to analyze the electrochemical impedance curves based on the proposed electric circuits. Tables (1) and (2) listed the values of different elements of these circuits for EIS data of Ni/C and Ni−MnOx/C, respectively. The resistance values generally decreased at both electrocatalysts with increasing the potential value up to 550 mV, afterwards they increased. Ni−MnOx/C showed lower resistance values when compared to Ni/C. Moreover, W value of EIS curve of Ni−MnOx/C at 400 mV was 7.54 times lower than that of Ni/C. It indicated faster methanol diffusion rate at Ni−MnOx/C electrocatalyst surface. The stability of Ni/C electrocatalyst for long-time operation during methanol oxidation reaction was studied using electrochemical impedance spectroscopy. This test was carried out by subjecting Ni/C for potentiostatic step at a potential value of 750 mV for long time intervals in (0.4 M methanol + 0.5 M KOH) solution. Phase shift and Nyquist plots were then displayed in the frequency range from 10 kHz to 0.1 Hz at a potential value of 550 mV. 12 Page 12 of 23
This phase shift diagram was represented in Fig. (11a). It was found that prolonged operation at the peak potential value of methanol oxidation reaction resulted in an increased phase angle of the maximum position of the arc when compared to the performance of a fresh Ni/C electrocatalyst. It referred to the increased charge transfer resistance after long-time
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operation. The increased diameter value of the semicircle in the corresponding Nyquist plots of Ni/C with the operation time in Fig. (11b) confirmed this result. It was accompanied by a gradual increase in the impedance value. Similar diagrams of Ni−MnOx/C electrocatalyst
cr
were observed in Figs. (11a', b'). They showed lower phase angle and impedance values when compared to those of Ni/C. The equivalent electric circuit in Fig. (10a) was employed to
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simulate the electrochemical impedance curves of Ni/C and Ni−MnOx/C electrocatalysts after the stability test. The values of electrochemical elements were shown in Tables (3) and (4),
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respectively. The variation of R1 and R2 values of both electrocatalysts with the operation time was plotted in Fig. (12). The polarization of prepared electrocatalysts at the peak potential value of methanol oxidation for long time starting from 2 h resulted in an increased
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impedance value. After 4 h, R1 and R2 values increased by 1.13 and 1.23 times, respectively at Ni−MnOx/C relative to those at fresh electrocatalyst [0 h]. On the other hand, Ni/C had increased R1 and R2 values by 1.44 and 1.47 times. This result supported the improved
te
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stability of Ni/C after incorporating manganese oxide. The synergistic action of manganese oxide in Ni−MnOx/C electrocatalyst could be
Ac ce p
arisen from the oxophilic nature of transition metal oxides. Manganese oxide can create large hydrophilic regions on carbon surface that can facilitate the diffusion of nickel ions and effectively prevent the agglomeration of nickel nanoparticles [75]. It also increases the formation rate of OHads species [76−78] which in turn would participate in Ni(OH)2/NiOOH transformation. This could enhance the charge transfer at Ni−MnOx/C surface, resulting in lowered phase angle and impedance values. The small-sized and well-dispersed nickel nanoparticles on MnOx/C as shown in TEM images can also offer higher surface area that hosts more accessible active sites, thus achieving high electrocatalytic activity towards methanol oxidation reaction. 4. Conclusion Microwave irradiation technique was employed to reduce nickel ions on MnOx/C using NaBH4. Incorporating manganese oxide in Ni/C was found to improve its electrocatalytic performance for methanol oxidation in KOH solution. The catalytic rate 13 Page 13 of 23
constant and diffusion coefficient values of methanol during the oxidation process at Ni−MnOx/C were about 1.81 and 4 times higher than that at Ni/C. Ni−MnOx/C showed faster electron transfer rate at E ≤ 550 mV. However, thick NiOOH species at higher potential values would retard the charge transfer process. The impedance value of Ni−MnOx/C
ip t
increased after potentiostatic polarization at methanol oxidation peak potential for long time. However, its poisoning degree was lower than that at Ni/C. This study elects Ni−MnOx/C as
cr
a good candidate for anode manufacture in direct methanol fuel cells.
us
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Table (1): Electrochemical impedance parameters of methanol oxidation reaction at Ni/C
400
114.60
25.83
450
110.70
24.86
500
109.10
24.66
n1
R2 / Ω cm2
Y02 x 10−3/ Ω −1 sn cm−2
n2
3.217
0.91
40.47
1.720
1.00
0.88
39.83
3.232
0.80
4.769
0.87
38.33
4.642
0.81
Y01 x 10−3 / Ω −1 sn cm−2
M
R1 / Ω cm2
d
Rs / Ω
4.180
Ac ce p
te
Potential value / mV
an
electrocatalyst at different potential values.
550
109.90
20.57
4.912
0.92
28.69
6.314
0.80
600
108.40
33.38
4.339
0.83
42.28
4.984
0.98
650
108.10
35.14
4.068
0.84
44.44
4.220
1.00
Table (2): Electrochemical impedance parameters of methanol oxidation reaction at Ni−MnOx/C electrocatalyst at different potential values. Potential value /
Rs / Ω
R1 / Ω cm2
Y01 x 10−3 / Ω −1 sn cm−2
n1
R2 / Ω cm2
Y02 x 10−3/ Ω −1 sn cm−2
n
21 Page 21 of 23
mV
110.00
14.46
7.723
0.85
15.75
6.298
0.82
450
107.90
12.96
12.978
0.88
14.53
7.635
0.88
500
107.70
11.42
23.885
0.80
550
108.60
9.79
34.315
0.86
600
109.30
14.62
20.780
650
108.30
15.37
13.933
ip t
400
10.41
37.978
0.85
12.19
6.823
1.00
13.09
5.231
0.85
M
0.81
0.88
cr
us
an
0.88
18.471
11.57
d
Table (3): Electrochemical impedance parameters of Ni/C electrocatalyst at 550 mV after
te
operation for long time during methanol oxidation reaction. Y01 x 10−3 / Ω −1 sn cm−2
0
109.90
20.57
4.912
0.92
28.69
1
103.80
19.58
8.041
0.83
25.04
2
102.90
21.40
6.608
0.87
33.61
3
102.50
24.73
5.525
0.89
37.30
4
101.30
29.65
3.336
0.94
42.16
Rs / Ω
R1 / Ω cm2
Ac ce p
Operation time / h
n1
R2 / Ω cm2
Y Ω
22 Page 22 of 23
Table (4): Electrochemical impedance parameters of Ni−MnOx/C electrocatalyst at 550 mV
Rs / Ω
R1 / Ω cm2
Y01 x 10−3 / Ω −1 sn cm−2
n1
R2 / Ω cm2
Y02 x 10 Ω −1 sn cm
0
108.60
9.79
34.315
0.86
10.41
37.978
1
98.17
9.00
41.879
10.10
54.379
2
93.25
10.20
38.137
0.83
11.00
28.185
3
92.48
10.60
32.006
0.88
11.81
17.914
4
92.55
11.09
29.936
0.84
12.79
15.207
cr
ip t
Operation time /h
an
after operation for long time during methanol oxidation reaction.
Ac ce p
te
d
M
us
0.87
23 Page 23 of 23