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Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution
Accepted Manuscript Original article Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution Merfat S. Al-Sharif, Prabhakarn Arunachalam, Twaha Abiti, Mabrook S. Amer, Matar Al-Shalwi, Mohamed A. Ghanem PII: DOI: Reference:
S1878-5352(18)30165-5 https://doi.org/10.1016/j.arabjc.2018.07.017 ARABJC 2359
To appear in:
Arabian Journal of Chemistry
Received Date: Accepted Date:
30 January 2018 23 July 2018
Please cite this article as: M.S. Al-Sharif, P. Arunachalam, T. Abiti, M.S. Amer, M. Al-Shalwi, M.A. Ghanem, Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution, Arabian Journal of Chemistry (2018), doi: https://doi.org/10.1016/j.arabjc.2018.07.017
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Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution Merfat S. Al-Sharif, Prabhakarn Arunachalam,* Twaha Abiti, Mabrook S. Amer, Matar AlShalwi, Mohamed A. Ghanem Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia. *Corresponding author email: [email protected]; Tel.: +96614670405; Fax: +96614675991.
Abstract
A crystalline mesoporous cobalt phosphate (meso-CoPi) electrocatalyst is prepared using liquid crystal template of non-ionic surfactant of Brij®78. The physicochemical investigations of the electrocatalyst executed by surface area analyzer, XRD, transmission electron microscope submits creation of a mesoporous crystalline nanostructured of meso-CoPi with a surface area of 124 m2g-1. This is an 10-fold greatness superior than that for bulk-CoPi particles produced without surfactant template. The meso-CoPi electrocatalyst comprises of metallic cobalt layered with a cobalt-oxo/hydroxo-phosphate layer which facilitates the electro-oxidation of methanol at modest overpotential of < 1.2 V vs RHE in alkaline solution. The methanol oxidation activity of the meso-CoPi catalyst shows more than 20-fold current increase at 1.4 VRHE in comparison to bulk-CoPi counterpart which due to the enhancement of the electroactive specific surface area. Liquid crystal template chemical approach provides a reproducible stage to synthesize mesoporous metal phosphates with improved electrocatalytic activities. Keywords: mesoporous, crystal template method, methanol, electrocatalysts, cobalt phosphate
Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution Merfat S. Al-Sharif, Prabhakarn Arunachalam,* Twaha Abiti, Mabrook S. Amer, Matar AlShalwi, Mohamed A. Ghanem Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia. *Corresponding author email: [email protected]; Tel.: +96614670405; Fax: +96614675991.
Abstract
A crystalline mesoporous cobalt phosphate (meso-CoPi) electrocatalyst is prepared using liquid crystal template of non-ionic surfactant of Brij®78. The physicochemical investigations of the electrocatalyst executed by surface area analyzer, XRD, transmission electron microscope submits creation of a mesoporous crystalline nanostructured of meso-CoPi with a surface area of 124 m2g-1. This is an 10-fold greatness superior than that for bulk-CoPi particles produced without surfactant template. The meso-CoPi electrocatalyst comprises of metallic cobalt layered with a cobalt-oxo/hydroxo-phosphate layer which facilitates the electro-oxidation of methanol at modest overpotential of < 1.2 V vs RHE in alkaline solution. The methanol oxidation activity of the meso-CoPi catalyst shows more than 20-fold current increase at 1.4 VRHE in comparison to bulk-CoPi counterpart which due to the enhancement of the electroactive specific surface area. Liquid crystal template chemical approach provides a reproducible stage to synthesize mesoporous metal phosphates with improved electrocatalytic activities.
Keywords: mesoporous, crystal template method, methanol, electrocatalysts, cobalt phosphate
1. Introduction Direct methanol fuel cells (DMFC) are a subcategory of proton-exchange membrane fuel cells and it delivers benefits over their hydrogen counterparts and much determination has been dedicated towards their improvement.1,2 The DMFC is categorized by its low pollution and noise, high efficiency and less toxicity nature that will allow to engage in transportable applications namely cellular phones, laptop computers, micro electro-mechanical systems devices and military applications.3 In this area, extensive researches has been focused towards the advancement in the efficiency of anode electrocatalysts.4-6 Recently, the most effective electrocatalysts for DMFC are Pt-based alloys but their lower abundance and higher cost of production limits the marketable applications.7-8 Conversely, Pt-based electrode materials can be simply contaminated by trace amount of CO which is retarded the methanol electro-oxidation.9 Significant research efforts have been conducted on the noble free metals or their complexes10-12 as alternatives. Among these, NiO,11 CeO2,13 MoOx,14 Mn3O4,15 Co3O4,16 and CuO17 have been engaged to enhance the electro-catalytic performances of methanol oxidation. Cobalt phosphate (CoPi) is considered to be efficient oxygen evolution reaction (OER) catalyst shown promising performance18 and over the last decades, several metal phosphate based catalysts were fabricated.19-20 Cobalt and its oxide composites are recognized as good electrocatalysts due to presence of Co(II)/Co(III) redox sites in alkaline media.21-23 Very recently, we demonstrate the synthesis of mesoporous cobalt hydroxide (meso-Co-OH) using liquid crystal template with high surface area of high surface area of 457 m2/g and average pore size of 4.0 nm. The meso-Co-OH exhibits enhanced oxygen evolution activity which was ascribed to the considerable improvement in the electroactive surface area originated the creation of mesoporous framework.24
Using a similar approach the mesoporous nickel hydroxide25 and nano-flakes26 were produced which shown significant electrocatalytic performances for methanol and urea oxidation respectively. In addition, preparation of mesoporous structure and introducing nickel phosphate displayed the improved electrocatalytic behavior and are recognized as suitable candidates for catalysis applications.27-28 Among the available transition metal series, cobalt and its mixed oxide/hydroxides and oxyhydroxide composites have been recognized as potential candidate for catalytic material for OER in alkaline media.29-34 It is generally recognized that the electro-catalytic activity of cobalt based catalysts was ascribed due to presence of Co 3+/Co4+ redox sites in alkaline media.35 Among them, Cobalt phosphate (CoPi) is considered to be efficient OER catalyst has precisely drawn much consideration due to their electrocatalytic performance with the formation of Co 4+– O intermediates.20 Under alkaline conditions, the stability and efficiency of CoPi during electrocatalytic process cannot gratify the future energy necessity, which is considered to major challenge for their commercialization. However, these problems can be astounded by increasing the density of Co3+/Co4+ active sites by augmenting the surface area of the CoPi materials. Consequently, mesoporous structured CoPi materials are very cognizable prospect due to the benefits of their extraordinary specific surface area, 3D open assemblies, competent reactants and products mass transfer. Quite recently, Dan Yang et al. report the ordered 2D nanoflakes of amorphous transition-metal phosphates (Co3(PO4)2, FePO4 and Mn3(PO4)2) catalysts using Tetradecylphosphonic acid (TDPA) as the template as well as the phosphate precursor and shown promising performances in lithium ion batteries (BET surface area 100-120 m2g-1 and pore size around 2–3 nm).36 One of the best reported techniques available to the synthesis of materials using liquid crystalline template method and its effective way to control the phase
structure, surface area pore ordering and pore size of the catalysts.37-41 In comparison, meso-CoPi synthesis is easier than that of NiPi due to difference in bond length of NiO6 and PO4 and easy precipitation of phosphorous ions.42 However, it is indispensable to find the greatest scheme to advance simple and compatible tactics to synthesis crystalline CoPi materials with greater specific surface area, which should be promising to mass production of the materials. 43 This work unprecedentedly describes the synthesis of crystalline meso-CoPi with surface area of upto 124 m2g-1 using a liquid crystal template of non-ionic surfactant Brij®78. The obtained meso-CoPi materials were investigated via XRD, N2 desorption, SEM, TEM and electrochemical methodologies. The electrocatalytic performances and its stability of the unsupported meso-CoPi materials for electrooxidation of methanol in alkaline solution were investigated and evaluated against the bulk-CoPi electrocatalyst chemically deposited using the same condition without surfactant. The higher methanol oxidation activity of the meso-CoPi is mostly ascribed to an upsurge in the real specific surface area and the open porous structure in association with bulkCoPi catalyst.
2. Experimental
Chemicals and materials supply Acetone, Cobalt acetate (CoC₄H₆O₄.4H2O, 99.5%) and non-ionic surfactant of Brij®78 (C18H37(OCH2CH2)20OH, Aldrich, purity >97 %), were obtained from Sigma-Aldrich. Phosphoric acid (85% w/w), sodium hydroxide and potassium hydroxide (purity > 97%,) were purchase from Alfa Aesar. Phosphoric acid (purity, supply) was used to form phosphate buffer solution at pH = 7.0. Aceton (purity %, BDH AnalaR) was used for dissolving the surfactant and for the removal of templates.
Synthesis of mesoporous cobalt phosphate (meso-CoPi) The meso-CoPi was prepared using the liquid crystal template of non-ionic Brij®78 as surfactant, by melting of 4.9 g of Brij®78 at 45 °C, then 4.0 ml of 1.0 M cobalt acetate (0.236 mole Co(II)) solution was added and the resultant mixture was mixed to attain a homogenous gel. The concentration of the Brij®78 was around 55 wt. % (surfactant: water) and then obtained mixture was cooled down naturally. Subsequently, the resultant mixture was subjected to aging process for an hour and then again heated to 45 °C in glass vial. The aging process was replicated three times to attain a constant mixing which exhibited a homogeneous pink color. The creation of hexagonal template mixture was established by observing the optical texture by polarizing optical microscope (MeijiTechno, Japan). The mesoporous cobalt phosphate (meso-CoPi) catalyst was synthesized by adding 3.0 ml of 1.0 M (0.285 moles) phosphate buffer solution (PBS) at pH 7.0 to the Brij®78 surfactant mixture followed by mixing until the homogenous mixture was obtained. The mixture was left in the sealed vial for overnight to complete the mesoCoPi catalyst precipitation. The surfactant template was further removed by washing with acetone followed by three times by deionized water and centrifuging, then drying in oven at 80 °C to obtain the meso-CoPi catalyst powder. For control experiment, a bulk cobalt phosphate (bulk-CoPi) catalyst was synthesized by precipitation through adding cobalt acetate solution and PBS but without surfactant. The catalytically active catalysts obtained in this work were finally used for characterization and application in methanol oxidation. 2.2 Characterizations N2-sorption isotherms of the materials have been executed by the Brunauer-Emmer-Teller (BET) method on a NOVA 2200e analyzer and prior to the measurements sample was degassed at 373
K for over 3 h. The morphological structures of meso-CoPi catalysts were investigated by XRD (Rigaku Miniflex 600), EDAX (JED-2200), SEM (JSM-6380) and TEM (JEOL-6330). All the electrochemical analysis was carried out in 3-electrode system using a Potentiostat/Galvanostat (Autolab PGSTAT30). The glassy carbon electrode (GCE, 3.0 cm) was engaged as working electrode. Saturated calomel electrode (SCE) and Pt-foil (1.0 cm2) were engaged as reference and counter electrode correspondingly. The catalyst ink was made by sonicating of 5.0 mg of the meso-CoPi based catalyst in a mixture of 1 ml isopropanol/distilled water and 10 μl Nafion solution for 15 min. All the measured potential data was converted to RHE scale unless otherwise state. 3. Results and Discussion To improve the electrocatalytic performance of phosphate based electrocatalysts for methanol oxidation, a mesostructure of meso-CoPi was advanced by template synthesis method and examined for electrochemical methanol oxidation in alkaline media. Fig. 1 displays the XRD patterns of meso-CoPi catalyst having a crystalline structure with sharp diffraction peaks and high intensity. The main peaks for meso-CoPi catalysts at 2θ of 13.34°, 22.28°, 28.16°, 30.31°, 33.46°, 33.95° and 37.53° and can be indexed to diffraction planes (020), (130), (031), (211), (141), (231) and (301) respectively and showed good consistent with the literature.44 The characteristic peaks displayed in Fig. 1 can be well correspond to hydrated cobalt phosphate with composition of monoclinic Co3(PO4)2.8H2O (JCPDS-033-0432) which was produced due to precipitation of Co 2+ ions by the phosphate anions. No other kinds of peaks or impurities were noticed from the XRD pattern. The grain size of this catalyst was valued to be about 33.9 nm by Scherrer equation and on the basis of the strongest peak at 2θ which suggest the crystalline nature of the XRD patterns suggesting that the meso-CoPi catalyst. Further, Inset of Fig. 1
evidences the low-angle X-ray signals for the meso-CoPi catalysts. The low-angle X-ray patterns of meso-CoPi contain a well-resolved peak with a q value of 0.65 nm−1 in addition to broad peak
10
20
0.5
1.0 -1 q, nm
1.5
2.0
30
40
(130)
(132)
(-141)
JCPDS # 33-0432 (231) (301) (051)
(031)
(130) (130)
(200)
(110)
(211)
0.0
(350)
Intensity(a.u)
Intensity (a.u)
(020)
at 1.14 nm−1, which indicate the formation of a hexagonal mesoporous structure.45
50
60
2
Fig. 1. Wide angle X-ray diffraction patterns of meso-CoPi catalysts (inset-small angle X-ray diffraction patterns)
Fig. 2a displays the N2 sorption isotherm and Fig. 2b shows the equivalent Barrett-JoynerHalenda (BJH) pore size distribution plot of meso-CoPi and bulk-CoPi. As evidenced from the Fig 2a, the isotherm for meso-CoPi belong to type IV adsorption-desorption isotherm with sharp hysteresis of H2 type at high relative pressure which is an indication of the effective creation of mesoporous structure by the template synthesis method.46 On the other hand, bulk-CoPi electrocatalyst displays type II isotherm which is characteristics for macroporous materials with an estimated BET surface area, total pore volume and average pore size 12 m2/g, 0.048 cm3/g,
and 1.52 nm correspondingly. On the other hand the meso-CoPi catalyst have a BET specific surface area, total pore volume and average pore size of 124 m2/g, 0.432 cm3/g and 3.9 nm correspondingly. The specific BET surface area of meso-CoPi is ten-fold increase than that for bulk-CoPi catalyst demonstrating the formation of mesoporous structure throughout the cobalt phosphate precipitate. The BET specific surface area of present meso-CoPi (124 m2/g) prepared using Brij®78 surfactant are relatively higher than the surface area obtained with amorphous meso-Co3(PO4)2 (104 m2/g)36 prepared using tetradecylphosphonic acid as template method and indicates that the present method is more advantageous to prepare non-precious mesoporous metal phosphates with higher surface area. Moreover, the BJH pore size distribution for mesoCoPi catalysts shown in Fig. 2b suggests the presence of small pore size distribution of bimodal
300
25
(b)
(a) 20
(ii)
-1
dS/dD (nm g nm )
3
Volume adsorbed N2, cm /g
(ii) 2 -1
200
100
(i) 10
5
(i) 0 0.0
15
0
0.2
0.4
0.6
0.8
1.0
P/P
4
8
12
Pore diameter/nm
mesoporous structures with an average pore size around 3.9 nm which is consistent with mesoporous pore size obtained using similar surfactant.25 Fig. 2 (a) Nitrogen adsorption–desorption isotherms for (i) bulk-CoPi and (ii) meso-CoPi; (b) equivalent pore size distribution plot.
The meticulous chemical and surface states of the meso-CoPi were further evidenced by the XPS analysis as shown in Fig. 3. The XPS survey of the meso-CoPi catalyst shown in Fig. 3a discloses the main photoelectron peaks agreeing to Co 2p, P 2p and O 1s with a very small peak for C1s, which confirms the presence of cobalt, phosphorus and oxygen with little background infection of carbon. The core XPS spectra in Fig. 3b of Co 2p composed of two spin-orbit components of 2p3/2 (Co2+; 781.0 eV) and 2p1/2 (Co3+; 796.9 eV) peaks are observed, which is accordance with the literatures.47-48 The P2p region of meso-CoPi
(Fig. 3c) displays two-
distinctive peaks with 133.8 eV and 134.5 eV binding energies (ratio of 2:1) equivalent to the 2p3/2 and 2p1/2 core levels can be ascribed to phosphate group in the meso-CoPi.49 The O1s signal (Fig. 3d) is centered at 531.7 eV and the 1.2:1 Co/P ratio in the meso-CoPi, estimated by integrating the Co 2p and P 2p XPS spectra, which resembles to the stoichiometric ratios of CoPi. The EDAX analyses (Fig. S1) indicate a Co/P atomic ratio of 1.53:1, which coincides with the XPS results. As the Co2p and O1s core levels binding energies50 of hydroxides and oxide form of cobalt are in the similar range as those of CoPi, we cautiously designate the surface of mesoCoPi as a blend of cobalt (II) phosphate with a cobalt oxo/hydroxo species Co xOy(OH)z, perhaps in the Co(II) state, as witnessed for janus cobalt-oxo/hydroxo-phosphate at the surface of metallic cobalt.51
800 600 400 Binding energy (eV)
(d)
810
P 2p
529.6
532.3
534 532 530 Binding energy (eV)
528
800 790 Binding energy(eV)
138
780
P 2p
(c)
O 1s
3+
2+
2+
0
531.2
536
Co 2p1/2 Satellite peak
Relative intensity (a.u)
Relative Intensity (a.u)
Co 3p
P 2s
200
Co 2p3/2
Relative intensity (a.u)
O1s
Co 2p
C1s
1000
Co 2p
(b)
Wide
Co LMM
Relative intensity (a.u)
O KLL Co 2s
(a)
P 2p3/2 P 2p1/2 P-O
136
134 132 Binding energy (eV)
130
128
Fig. 3 XPS spectra analysis of the chemical state of meso-CoPi (a) XPS survey, (b) Co 2p, (c) P 2p and (d) O1s core level. The surface morphological and mesoporous structure of hexagonal meso-CoPi catalyst are inspected by scanning and transmission microscope characterizations. The SEM in Fig. 4a shows the meso-CoPi catalyst morphology is rather formed from aggregated irregular particles with below micrometer size. The TEM image in Fig. 4b clearly show the fine structure of hexagonal meso-CoPi catalyst comprise a highly porous network structure that extended over a micrometersize domain. The open pores have an estimated diameter of roughly 4 nm which shown consistence with the pore size investigated by the BJH method (Fig. 2b). However, the porous
structure is rather disordered, presumably due to the disruption induced by the chemical precipitation of cobalt phosphate in the hydrophilic interstitial space the soft Brij®78 template.
(a)
(b)
Fig.
4
(a)
SEM
image and (b) TEM images of meso-CoPi catalyst synthesized by liquid crystal template using Brij®78 surfactant. The electrochemical behavior of meso-CoPi catalyst with respect to redox reactions in alkaline solution were assessed and evaluated with bulk-CoPi fabricated without surfactant. Fig. 5 displays the cyclic voltammograms at 50 mVs-1 in 1.0 M KOH for both meso-CoPi and bulkCoPi loaded on GC electrode and in the potential varied from 0.9 V to 1.7 V vs. RHE. The voltammograms for meso-CoPi show well distinguished two redox peaks (A1/C1 = Co(II)/Co(III)) and (A2/C2 = Co(III)/Co(IV) positioned at about 1.12/1.05 and 1.48/1.42 V vs. RHE respectively, which are typical analogous to those characteristic cobalt redox peaks previously reported in literature.51-52 Typically , in alkaline media the Co(III)/Co(IV) transition is precede before the onset of OER and follow the distinguished redox couple of Co(II)/Co(III) species53-54 according to equation 1. Co3(PO4)2 + OH- ↔ Co3(PO4)2 (OH-) + e- (1) Interestingly and as shown in table 1, for both redox centers, it can be observed that the redox peaks current of meso-CoPi catalyst voltammogram is suggestively more than thirty times higher than the current for bulk-CoPi signifying a substantial advanced electrochemical active surface area and higher capacitance current the existence of mesoporous network structure as (Table 1).
Table 1. Electrochemical parameters obtained for redox peak A1/C1 of bulk-CoPi, meso-CoPi and meso-Co(OH)225 catalysts at loading 2.1 mg/cm2 in 1.0 M KOH at scan rate 50mV s-1. Catalyst
EA1/ mV
EC1/ mV
ΔE/ mV
IA1/ mA cm-2
IC1/ mA cm-2
bulk-CoPi
1144
1108
36
1.15
0.47
meso-CoPi
1190
1050
140
35.70
28.64
meso-Co(OH)2
1160
1104
56
2.34
1.34
100
Current/mAcm
-2
80
(iii)
60
A1
40
A2
(ii)
20
(i) 0
C2
C1
-20
-40 1.0
1.2
1.4
1.6
1.8
Potential vs. RHE/V
Fig. 5 Cyclic voltammetry at 50 mVs-1 in 1.0 M KOH solution for 2.1 mg/cm2 of (i) glassy carbon, (ii) bulk-CoPi and (ii) meso-CoPi. Cyclic voltammogram data for the redox nature of meso-CoPi catalyst at altered scan rates are portrayed in Fig. 6a. The linear connection among anodic peak current vs. the square root of the scan rate are presented in Fig. 6b. The anodic current upsurge with altering scan rate varied from 5 to 100 mVs-1. The linear development of the anodic peak current with square root of the scan
rate designates that the Co2+/Co3+ redox process of meso-CoPi catalyst electrooxidation in alkaline solution is diffusion limited process and consistent with reported literatures55 reflecting faster rate of electrochemical reaction.
80
(a) 60
(b)
40
20
0
-20
-1
-40
-2
5 mVs 10 20 30 50 70 100
Current, mAcm
Current/mAcm
-2
40
20
Oxidation Reduction
0
-20
-40 1.0
1.2
1.4
1.6
1.8
2
4
6
8 -1
Potential vs. RHE/V
[Scanrate (mVs )]
10
1/2
Fig. 6 Cyclic voltammetry for 2.1 mg/cm2 loaded meso-CoPi catalyst in 1.0 M KOH (a) various scan rate from 5 mVs-1 to 100 mVs-1, (b) linear connection between redox peak current and square root of scan rate. Fig. 7a depicts the linear sweep voltammograms for meso-CoPi with different loadings in 1.0 M KOH. Fig. 7b noticeably displays as the meso-CoPi catalyst loading upsurges, the peak A1 current density is linearly increases with a line slope of 19.52 mA/mg cm-2 due to an enhancement in electrochemical active surface area. Further, the peak A1 potential considerably shifted about 100 mV to more positive over the considered loading presumable due to film thickness limitations. In addition, additional increase in the meso-CoPi loading effects in less current which can be credited to destructive OER and effects in the physical removal of electrocatalyst from the electrode surface.
1.26
45
(b)
(vi)
150
(a)
40 1.24
(iii)
-2
35 1.22
30 25
1.20
20
Peak current, mA cm
Current/mAcm
-2
(iv) 100
Peak potential, vs. RHE/V
(v)
1.18
50
15
(ii) 1.16
10
(i)
5 1.14
0 1.0
1.2
1.4
Potential vs. RHE/V
1.6
1.8
0.0
0.5
1.0
1.5
2.0
2.5
0 3.0
2
Loading, mg/cm
Fig. 7. a) Linear sweep voltammetry at 50 mVs-1 in 1.0 M KOH solution at numerous loading of meso-CoPi catalyst, (i) 0.0, (ii) 0.14, (iii) 0.56, (iv) 0.84, (v) 1.4 and (vi) 2.1 mg/cm2. (b) Plots for the effect of meso-CoPi catalyst loadings on OER onset potential and current at 1.76 V vs. RHE. Fig. 8 shows the cyclic voltammetry of meso-CoPi catalyst in various concentration of methanol solution. The voltammograms display that the methanol oxidation current steadily upsurges at the onset potential of peak A1 and its corresponding peak potential moves to positive potential as the methanol concentration increases. The inset in Fig. 8b displays a linear increase between methanol oxidation current and the methanol concentration. The results of voltammogram indicate that the electrocatalytic activity of methanol oxidation depends intensely on methanol concentration. This might be owing to the fact that more methanol molecules would
be adsorbed by Co3+ species resulting in an efficient conversion from Co 2+ to Co3+ species. On addition of methanol, the reduction peak current of Co(II)/Co(III) (A1/C1) redox at about 1.07 V vs. RHE during the reverse scan is totally reduced in comparison to oxidation peak current for the forward scan as well as the disappearance of the redox peaks (A2/C1) around 1.45 corresponding to Co(III)/Co(IV). This could be ascribed to the consumption of Co(III) and (Co(IV) if existed) by methanol oxidation consequently a decrease in the reduction current at C1 peak. Literature suggests that the methanol is oxidized into intermediates/products (CoOOH) by Co4+ species and then Co4+ species is reduced into Co3+ species.56 In the presence of methanol, meso-CoPi catalyst provides an intensive anodic peak current at 1.41 VRHE and about 225 mAcm2
which is ascribed to the oxidation of methanol. This demonstrates that our meso-CoPi has
relatively higher methanol electro-oxidation current (about 7 times higher) than that in absence
300
(vi)
(a)
(b) 240
(v)
200
(iii)
(i) 100
-2
200
(ii)
Current/mAcm
Current/mAcm
-2
(iv)
160
120
0
80
1.0
1.2
1.4
1.6
1.8
Potential vs. RHE/V
0.0
0.2
0.4
0.6 c[CH3OH]/M
0.8
1.0
of methanol. The high catalytic current could be attributed to the open porous channel of mesoCoPi catalyst and the easy access of active Co sites by methanol molecules and acts as an efficient electrocatalyst for methanol oxidation.
Fig. 8 a) Cyclic voltammogram for 2.1 mgcm-2 loaded meso-CoPi catalyst at 50 mVs−1 obtained at various ethanol concentration, (i) 0.0, (ii) 0.1, (iii) 0.3, (iv) 0.5, (v) 0.7 and (vi) 1.0 M. b) linear relationship between current density vs. methanol concentration.
This methanol electrooxidation performance at meso-CoPi surface is concordant with the Fleischmann EC’-type mechanism.57 Electro-reactive CoOOH kinds are produced from the oxidation of meso-CoPi electrocatalyst as displayed by Equation 1. Then methanol reacts with CoOOH via the electrochemical reaction in Equation 2 and rejuvenate Co(OH)2. Finally, the obtained product under ambient circumstances is presently not known but multiple electron transfers and whole depletion of methanol act likely, in particular at elevated temperatures. nCoOOH+CH3OH → nCo(OH)2 +Product
[2]
By considering the above clarifications and the catalytic role of CoOOH plays a vital role in catalyzing the methanol electrooxidation process. To further assess the stability and long-term activity of the meso-CoPi catalyst towards methanol electro-oxidation. As shown in Fig. 9a, the oxidation peaks current slightly decreases with the number of potential cycles after 1 st scan. The peak current of the 100th scan is about 92.3% of that of 1 st scan which indicates that meso-CoPi exhibits good long-term electrocatalytic stability and storage properties during methanol oxidation cycling under alkaline conditions. Tafel regions with slope are changed from 76 to 80 mV/ dec for 1st and 100th cycle respectively.
350
(a)
(b)
180 150 -2
250 200 150 100 50
1.16
Potential vs. RHE/V
Current/mAcm
Current/mAcm
-2
300
1201.15 1.14
901.13 1.11
60
1.0
st
1 th 100
0
(v) 1st 100th
1.12
1.1 1.2 1.3 log(i/mAcm-2)
1.4
(iv)
1.5
(iii)
30
-50
(ii) (i)
0 1.0
1.2 1.4 Potential vs. RHE/V
1.6
1.8 -2
0
1000
2000 Time(sec)
3000 −1
Fig. 9 a) Cyclic voltammogram for 2.1 mgcm loaded meso-CoPi catalyst at 50 mVs
4000
obtained at
1st and 100th cycle. b) Chronoamperometry at for meso-CoPi and bulk-CoPi catalysts in 1.0 M KOH in presence of 1.0 M methanol, (i) bulk-CoPi in presence of 1.0 M methanol at 1.41 V, meso-CoPi in (ii) absence of methanol at 1.41 V, in presence of methanol at 1.26 V (iii), 1.56 V (iv) and (v) at 1.41 V vs. RHE. Fig. 9b shows the long-term chronoamperometry measurements for meso-CoPi and bulk-CoPi catalysts in 1.0 M KOH with and without 1.0 M methanol. At the beginning, the current drops quickly for all the chronoamperometric measurements owing to double layer charging current and then gradually declines till reaching a pseudo-steady state. In the presence of methanol, the steady state current at 1.4 and 1.6 V vs RHE is considerably higher compared to without methanol which is concordant with earlier studies. The steady state methanol oxidation current of meso-CoPi catalyst at both oxidation potentials stays constant for more than 1 hour signifying the good long-standing stability and respectable tolerance against adsorption of methanol oxidation intermediates and products. From chronoamperometry results, meso-CoPi exhibited 20-fold increase at 1.4 V vs RHE related to the bulk-CoPi and have a large pore space for better reactant molecules. Table 2 listed the specific and mass catalytic activities in parallel to other metal oxide catalysts reported in the literatures. The specific and mass catalytic performances of the
fabricated meso-CoPi electrocatalyst with related to the electrooxidation of methanol were 225 mAcm−2 and 1512 mAmg−1, correspondingly; these are 3-fold increase related to the previously reported materials. Consequently, the fabricated meso-CoPi revealed considerably improved electrocatalytic performances with related to electrooxidation of methanol. Table 2 Specific and mass activities of present meso-CoPi and previously reported electrocatalysts for methanol/ethanol oxidation in alkaline media.
Sr. No
Catalyst
Method
1.
meso- CoPi
Liquid crystal template
3.
meso-CoPi
4.
BET surface area (m2/g)
Mass activity, mA mg-1/ specific activity mA cm-2
Ref.
1512/225
This work
846/126
58
Electrolyte (M)
Potential range (V)
124
KOH:1 CH3OH:1
0.0~0.7 SCE
V/
Microwave
20
KOH:0.1 CH3OH:1
-0.2~0.7 SCE
V/
meso NiCoO
Microwave
160
KOH:1 CH3OH:1
0~0.7 V / SCE
750/160
59
5.
Meso Ni(OH)2
Liquid crystal template
90
KOH:1 C2H5OH:1
0~0.7 V / SCE
370/72
11
6.
Ni/NiO
Microwave
68
KOH:1 CH3OH:1
0~0.7 V / SCE
275/40
60
7.
NiO/CNF
Electrospinning
31.85
KOH:6 CH3OH:1
0~0.9 V/ SCE
1800/300
12
8.
Pt/C
Chemical reduction
-
KOH:1 CH3OH:1
-0.8~0.3/ Hg/HgO
-/40
61
9.
Co-Cu-CNF
Electrospinng
-
KOH:1 CH3OH:2
0.0~0.8 V / Ag/AgCl
-/16
62
10.
RGONiCo2O4
Hydrothermal method
42
KOH:0.1 CH3OH:0.5
0~0.9 V Ag/AgCl
-/45
63
Two-step solution method
64.1
KOH:0.1 CH3OH:0.5
-1~0 V/ SCE
400/-
64
11.
Pt/Ni(OH)2/ Graphene
4. Conclusions
/
In this work, crystalline meso-CoPi catalyst with higher active surface area (124 m2/g) was successfully prepared by sol gel template method using Brij®78surfactant. The meso-CoPi exhibited 20-fold increase (228 mA/cm2) at 1.4 V vs RHE related to the bulk-CoPi and have a large pore space for better reactant molecules. Our findings showed that methanol oxidation on meso-CoPi catalyst in alkaline solution was catalyzed by the Co 3+/Co4+ redox couples and the catalyst exhibited an impressively high electrocatalytic activity for methanol oxidation. Considering the benefit of the catalysts stability, relatively high surface area and the large pore size, it can be established that the synthesized meso-CoPi catalyst with remarkable electrochemical activities can assist as an auspicious anode electrode material for DMFC and methanol sensors. Acknowledgements This project was funded by The National Plan for Science, Technology and Innovation, King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award number 83-36. References 1. Abdel Rahim, M.A., Abdel Hameed, R.M., Khalil, M.W., 2004. Nickel as a catalyst for the electro-oxidation of methanol in alkaline medium. J. Power Sources 134, 160-169. 2. Liu, P., Yin, G.P., Du, C.Y., 2008. Composite anode catalyst layer for direct methanol fuel cell. Electrochem. Commun. 10, 1471-1473. 3. Kim, D.B., Chun, H.J., Lee, Y.K., Kwon, H.H., Lee, H.I., 2010. Preparation of Pt/NiO-C electrocatalyst and heat-treatment effect on its electrocatalytic performance for methanol oxidation. Inter. J. Hydrogen Energy 35, 313-320. 4. Das, A.K., Sahoo, S., Arunachalam, P., Zhang, S, 2016. Facile synthesis of Fe 3O4 nanorod decorated reduced graphene oxide (RGO) for supercapacitor application. RSC Advances 6, 107057-107064.
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S1878-5352(18)30165-5 https://doi.org/10.1016/j.arabjc.2018.07.017 ARABJC 2359
To appear in:
Arabian Journal of Chemistry
Received Date: Accepted Date:
30 January 2018 23 July 2018
Please cite this article as: M.S. Al-Sharif, P. Arunachalam, T. Abiti, M.S. Amer, M. Al-Shalwi, M.A. Ghanem, Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution, Arabian Journal of Chemistry (2018), doi: https://doi.org/10.1016/j.arabjc.2018.07.017
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Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution Merfat S. Al-Sharif, Prabhakarn Arunachalam,* Twaha Abiti, Mabrook S. Amer, Matar AlShalwi, Mohamed A. Ghanem Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia. *Corresponding author email: [email protected]; Tel.: +96614670405; Fax: +96614675991.
Abstract
A crystalline mesoporous cobalt phosphate (meso-CoPi) electrocatalyst is prepared using liquid crystal template of non-ionic surfactant of Brij®78. The physicochemical investigations of the electrocatalyst executed by surface area analyzer, XRD, transmission electron microscope submits creation of a mesoporous crystalline nanostructured of meso-CoPi with a surface area of 124 m2g-1. This is an 10-fold greatness superior than that for bulk-CoPi particles produced without surfactant template. The meso-CoPi electrocatalyst comprises of metallic cobalt layered with a cobalt-oxo/hydroxo-phosphate layer which facilitates the electro-oxidation of methanol at modest overpotential of < 1.2 V vs RHE in alkaline solution. The methanol oxidation activity of the meso-CoPi catalyst shows more than 20-fold current increase at 1.4 VRHE in comparison to bulk-CoPi counterpart which due to the enhancement of the electroactive specific surface area. Liquid crystal template chemical approach provides a reproducible stage to synthesize mesoporous metal phosphates with improved electrocatalytic activities. Keywords: mesoporous, crystal template method, methanol, electrocatalysts, cobalt phosphate
Mesoporous cobalt phosphate electrocatalyst prepared using liquid crystal template for methanol oxidation reaction in alkaline solution Merfat S. Al-Sharif, Prabhakarn Arunachalam,* Twaha Abiti, Mabrook S. Amer, Matar AlShalwi, Mohamed A. Ghanem Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia. *Corresponding author email: [email protected]; Tel.: +96614670405; Fax: +96614675991.
Abstract
A crystalline mesoporous cobalt phosphate (meso-CoPi) electrocatalyst is prepared using liquid crystal template of non-ionic surfactant of Brij®78. The physicochemical investigations of the electrocatalyst executed by surface area analyzer, XRD, transmission electron microscope submits creation of a mesoporous crystalline nanostructured of meso-CoPi with a surface area of 124 m2g-1. This is an 10-fold greatness superior than that for bulk-CoPi particles produced without surfactant template. The meso-CoPi electrocatalyst comprises of metallic cobalt layered with a cobalt-oxo/hydroxo-phosphate layer which facilitates the electro-oxidation of methanol at modest overpotential of < 1.2 V vs RHE in alkaline solution. The methanol oxidation activity of the meso-CoPi catalyst shows more than 20-fold current increase at 1.4 VRHE in comparison to bulk-CoPi counterpart which due to the enhancement of the electroactive specific surface area. Liquid crystal template chemical approach provides a reproducible stage to synthesize mesoporous metal phosphates with improved electrocatalytic activities.
Keywords: mesoporous, crystal template method, methanol, electrocatalysts, cobalt phosphate
1. Introduction Direct methanol fuel cells (DMFC) are a subcategory of proton-exchange membrane fuel cells and it delivers benefits over their hydrogen counterparts and much determination has been dedicated towards their improvement.1,2 The DMFC is categorized by its low pollution and noise, high efficiency and less toxicity nature that will allow to engage in transportable applications namely cellular phones, laptop computers, micro electro-mechanical systems devices and military applications.3 In this area, extensive researches has been focused towards the advancement in the efficiency of anode electrocatalysts.4-6 Recently, the most effective electrocatalysts for DMFC are Pt-based alloys but their lower abundance and higher cost of production limits the marketable applications.7-8 Conversely, Pt-based electrode materials can be simply contaminated by trace amount of CO which is retarded the methanol electro-oxidation.9 Significant research efforts have been conducted on the noble free metals or their complexes10-12 as alternatives. Among these, NiO,11 CeO2,13 MoOx,14 Mn3O4,15 Co3O4,16 and CuO17 have been engaged to enhance the electro-catalytic performances of methanol oxidation. Cobalt phosphate (CoPi) is considered to be efficient oxygen evolution reaction (OER) catalyst shown promising performance18 and over the last decades, several metal phosphate based catalysts were fabricated.19-20 Cobalt and its oxide composites are recognized as good electrocatalysts due to presence of Co(II)/Co(III) redox sites in alkaline media.21-23 Very recently, we demonstrate the synthesis of mesoporous cobalt hydroxide (meso-Co-OH) using liquid crystal template with high surface area of high surface area of 457 m2/g and average pore size of 4.0 nm. The meso-Co-OH exhibits enhanced oxygen evolution activity which was ascribed to the considerable improvement in the electroactive surface area originated the creation of mesoporous framework.24
Using a similar approach the mesoporous nickel hydroxide25 and nano-flakes26 were produced which shown significant electrocatalytic performances for methanol and urea oxidation respectively. In addition, preparation of mesoporous structure and introducing nickel phosphate displayed the improved electrocatalytic behavior and are recognized as suitable candidates for catalysis applications.27-28 Among the available transition metal series, cobalt and its mixed oxide/hydroxides and oxyhydroxide composites have been recognized as potential candidate for catalytic material for OER in alkaline media.29-34 It is generally recognized that the electro-catalytic activity of cobalt based catalysts was ascribed due to presence of Co 3+/Co4+ redox sites in alkaline media.35 Among them, Cobalt phosphate (CoPi) is considered to be efficient OER catalyst has precisely drawn much consideration due to their electrocatalytic performance with the formation of Co 4+– O intermediates.20 Under alkaline conditions, the stability and efficiency of CoPi during electrocatalytic process cannot gratify the future energy necessity, which is considered to major challenge for their commercialization. However, these problems can be astounded by increasing the density of Co3+/Co4+ active sites by augmenting the surface area of the CoPi materials. Consequently, mesoporous structured CoPi materials are very cognizable prospect due to the benefits of their extraordinary specific surface area, 3D open assemblies, competent reactants and products mass transfer. Quite recently, Dan Yang et al. report the ordered 2D nanoflakes of amorphous transition-metal phosphates (Co3(PO4)2, FePO4 and Mn3(PO4)2) catalysts using Tetradecylphosphonic acid (TDPA) as the template as well as the phosphate precursor and shown promising performances in lithium ion batteries (BET surface area 100-120 m2g-1 and pore size around 2–3 nm).36 One of the best reported techniques available to the synthesis of materials using liquid crystalline template method and its effective way to control the phase
structure, surface area pore ordering and pore size of the catalysts.37-41 In comparison, meso-CoPi synthesis is easier than that of NiPi due to difference in bond length of NiO6 and PO4 and easy precipitation of phosphorous ions.42 However, it is indispensable to find the greatest scheme to advance simple and compatible tactics to synthesis crystalline CoPi materials with greater specific surface area, which should be promising to mass production of the materials. 43 This work unprecedentedly describes the synthesis of crystalline meso-CoPi with surface area of upto 124 m2g-1 using a liquid crystal template of non-ionic surfactant Brij®78. The obtained meso-CoPi materials were investigated via XRD, N2 desorption, SEM, TEM and electrochemical methodologies. The electrocatalytic performances and its stability of the unsupported meso-CoPi materials for electrooxidation of methanol in alkaline solution were investigated and evaluated against the bulk-CoPi electrocatalyst chemically deposited using the same condition without surfactant. The higher methanol oxidation activity of the meso-CoPi is mostly ascribed to an upsurge in the real specific surface area and the open porous structure in association with bulkCoPi catalyst.
2. Experimental
Chemicals and materials supply Acetone, Cobalt acetate (CoC₄H₆O₄.4H2O, 99.5%) and non-ionic surfactant of Brij®78 (C18H37(OCH2CH2)20OH, Aldrich, purity >97 %), were obtained from Sigma-Aldrich. Phosphoric acid (85% w/w), sodium hydroxide and potassium hydroxide (purity > 97%,) were purchase from Alfa Aesar. Phosphoric acid (purity, supply) was used to form phosphate buffer solution at pH = 7.0. Aceton (purity %, BDH AnalaR) was used for dissolving the surfactant and for the removal of templates.
Synthesis of mesoporous cobalt phosphate (meso-CoPi) The meso-CoPi was prepared using the liquid crystal template of non-ionic Brij®78 as surfactant, by melting of 4.9 g of Brij®78 at 45 °C, then 4.0 ml of 1.0 M cobalt acetate (0.236 mole Co(II)) solution was added and the resultant mixture was mixed to attain a homogenous gel. The concentration of the Brij®78 was around 55 wt. % (surfactant: water) and then obtained mixture was cooled down naturally. Subsequently, the resultant mixture was subjected to aging process for an hour and then again heated to 45 °C in glass vial. The aging process was replicated three times to attain a constant mixing which exhibited a homogeneous pink color. The creation of hexagonal template mixture was established by observing the optical texture by polarizing optical microscope (MeijiTechno, Japan). The mesoporous cobalt phosphate (meso-CoPi) catalyst was synthesized by adding 3.0 ml of 1.0 M (0.285 moles) phosphate buffer solution (PBS) at pH 7.0 to the Brij®78 surfactant mixture followed by mixing until the homogenous mixture was obtained. The mixture was left in the sealed vial for overnight to complete the mesoCoPi catalyst precipitation. The surfactant template was further removed by washing with acetone followed by three times by deionized water and centrifuging, then drying in oven at 80 °C to obtain the meso-CoPi catalyst powder. For control experiment, a bulk cobalt phosphate (bulk-CoPi) catalyst was synthesized by precipitation through adding cobalt acetate solution and PBS but without surfactant. The catalytically active catalysts obtained in this work were finally used for characterization and application in methanol oxidation. 2.2 Characterizations N2-sorption isotherms of the materials have been executed by the Brunauer-Emmer-Teller (BET) method on a NOVA 2200e analyzer and prior to the measurements sample was degassed at 373
K for over 3 h. The morphological structures of meso-CoPi catalysts were investigated by XRD (Rigaku Miniflex 600), EDAX (JED-2200), SEM (JSM-6380) and TEM (JEOL-6330). All the electrochemical analysis was carried out in 3-electrode system using a Potentiostat/Galvanostat (Autolab PGSTAT30). The glassy carbon electrode (GCE, 3.0 cm) was engaged as working electrode. Saturated calomel electrode (SCE) and Pt-foil (1.0 cm2) were engaged as reference and counter electrode correspondingly. The catalyst ink was made by sonicating of 5.0 mg of the meso-CoPi based catalyst in a mixture of 1 ml isopropanol/distilled water and 10 μl Nafion solution for 15 min. All the measured potential data was converted to RHE scale unless otherwise state. 3. Results and Discussion To improve the electrocatalytic performance of phosphate based electrocatalysts for methanol oxidation, a mesostructure of meso-CoPi was advanced by template synthesis method and examined for electrochemical methanol oxidation in alkaline media. Fig. 1 displays the XRD patterns of meso-CoPi catalyst having a crystalline structure with sharp diffraction peaks and high intensity. The main peaks for meso-CoPi catalysts at 2θ of 13.34°, 22.28°, 28.16°, 30.31°, 33.46°, 33.95° and 37.53° and can be indexed to diffraction planes (020), (130), (031), (211), (141), (231) and (301) respectively and showed good consistent with the literature.44 The characteristic peaks displayed in Fig. 1 can be well correspond to hydrated cobalt phosphate with composition of monoclinic Co3(PO4)2.8H2O (JCPDS-033-0432) which was produced due to precipitation of Co 2+ ions by the phosphate anions. No other kinds of peaks or impurities were noticed from the XRD pattern. The grain size of this catalyst was valued to be about 33.9 nm by Scherrer equation and on the basis of the strongest peak at 2θ which suggest the crystalline nature of the XRD patterns suggesting that the meso-CoPi catalyst. Further, Inset of Fig. 1
evidences the low-angle X-ray signals for the meso-CoPi catalysts. The low-angle X-ray patterns of meso-CoPi contain a well-resolved peak with a q value of 0.65 nm−1 in addition to broad peak
10
20
0.5
1.0 -1 q, nm
1.5
2.0
30
40
(130)
(132)
(-141)
JCPDS # 33-0432 (231) (301) (051)
(031)
(130) (130)
(200)
(110)
(211)
0.0
(350)
Intensity(a.u)
Intensity (a.u)
(020)
at 1.14 nm−1, which indicate the formation of a hexagonal mesoporous structure.45
50
60
2
Fig. 1. Wide angle X-ray diffraction patterns of meso-CoPi catalysts (inset-small angle X-ray diffraction patterns)
Fig. 2a displays the N2 sorption isotherm and Fig. 2b shows the equivalent Barrett-JoynerHalenda (BJH) pore size distribution plot of meso-CoPi and bulk-CoPi. As evidenced from the Fig 2a, the isotherm for meso-CoPi belong to type IV adsorption-desorption isotherm with sharp hysteresis of H2 type at high relative pressure which is an indication of the effective creation of mesoporous structure by the template synthesis method.46 On the other hand, bulk-CoPi electrocatalyst displays type II isotherm which is characteristics for macroporous materials with an estimated BET surface area, total pore volume and average pore size 12 m2/g, 0.048 cm3/g,
and 1.52 nm correspondingly. On the other hand the meso-CoPi catalyst have a BET specific surface area, total pore volume and average pore size of 124 m2/g, 0.432 cm3/g and 3.9 nm correspondingly. The specific BET surface area of meso-CoPi is ten-fold increase than that for bulk-CoPi catalyst demonstrating the formation of mesoporous structure throughout the cobalt phosphate precipitate. The BET specific surface area of present meso-CoPi (124 m2/g) prepared using Brij®78 surfactant are relatively higher than the surface area obtained with amorphous meso-Co3(PO4)2 (104 m2/g)36 prepared using tetradecylphosphonic acid as template method and indicates that the present method is more advantageous to prepare non-precious mesoporous metal phosphates with higher surface area. Moreover, the BJH pore size distribution for mesoCoPi catalysts shown in Fig. 2b suggests the presence of small pore size distribution of bimodal
300
25
(b)
(a) 20
(ii)
-1
dS/dD (nm g nm )
3
Volume adsorbed N2, cm /g
(ii) 2 -1
200
100
(i) 10
5
(i) 0 0.0
15
0
0.2
0.4
0.6
0.8
1.0
P/P
4
8
12
Pore diameter/nm
mesoporous structures with an average pore size around 3.9 nm which is consistent with mesoporous pore size obtained using similar surfactant.25 Fig. 2 (a) Nitrogen adsorption–desorption isotherms for (i) bulk-CoPi and (ii) meso-CoPi; (b) equivalent pore size distribution plot.
The meticulous chemical and surface states of the meso-CoPi were further evidenced by the XPS analysis as shown in Fig. 3. The XPS survey of the meso-CoPi catalyst shown in Fig. 3a discloses the main photoelectron peaks agreeing to Co 2p, P 2p and O 1s with a very small peak for C1s, which confirms the presence of cobalt, phosphorus and oxygen with little background infection of carbon. The core XPS spectra in Fig. 3b of Co 2p composed of two spin-orbit components of 2p3/2 (Co2+; 781.0 eV) and 2p1/2 (Co3+; 796.9 eV) peaks are observed, which is accordance with the literatures.47-48 The P2p region of meso-CoPi
(Fig. 3c) displays two-
distinctive peaks with 133.8 eV and 134.5 eV binding energies (ratio of 2:1) equivalent to the 2p3/2 and 2p1/2 core levels can be ascribed to phosphate group in the meso-CoPi.49 The O1s signal (Fig. 3d) is centered at 531.7 eV and the 1.2:1 Co/P ratio in the meso-CoPi, estimated by integrating the Co 2p and P 2p XPS spectra, which resembles to the stoichiometric ratios of CoPi. The EDAX analyses (Fig. S1) indicate a Co/P atomic ratio of 1.53:1, which coincides with the XPS results. As the Co2p and O1s core levels binding energies50 of hydroxides and oxide form of cobalt are in the similar range as those of CoPi, we cautiously designate the surface of mesoCoPi as a blend of cobalt (II) phosphate with a cobalt oxo/hydroxo species Co xOy(OH)z, perhaps in the Co(II) state, as witnessed for janus cobalt-oxo/hydroxo-phosphate at the surface of metallic cobalt.51
800 600 400 Binding energy (eV)
(d)
810
P 2p
529.6
532.3
534 532 530 Binding energy (eV)
528
800 790 Binding energy(eV)
138
780
P 2p
(c)
O 1s
3+
2+
2+
0
531.2
536
Co 2p1/2 Satellite peak
Relative intensity (a.u)
Relative Intensity (a.u)
Co 3p
P 2s
200
Co 2p3/2
Relative intensity (a.u)
O1s
Co 2p
C1s
1000
Co 2p
(b)
Wide
Co LMM
Relative intensity (a.u)
O KLL Co 2s
(a)
P 2p3/2 P 2p1/2 P-O
136
134 132 Binding energy (eV)
130
128
Fig. 3 XPS spectra analysis of the chemical state of meso-CoPi (a) XPS survey, (b) Co 2p, (c) P 2p and (d) O1s core level. The surface morphological and mesoporous structure of hexagonal meso-CoPi catalyst are inspected by scanning and transmission microscope characterizations. The SEM in Fig. 4a shows the meso-CoPi catalyst morphology is rather formed from aggregated irregular particles with below micrometer size. The TEM image in Fig. 4b clearly show the fine structure of hexagonal meso-CoPi catalyst comprise a highly porous network structure that extended over a micrometersize domain. The open pores have an estimated diameter of roughly 4 nm which shown consistence with the pore size investigated by the BJH method (Fig. 2b). However, the porous
structure is rather disordered, presumably due to the disruption induced by the chemical precipitation of cobalt phosphate in the hydrophilic interstitial space the soft Brij®78 template.
(a)
(b)
Fig.
4
(a)
SEM
image and (b) TEM images of meso-CoPi catalyst synthesized by liquid crystal template using Brij®78 surfactant. The electrochemical behavior of meso-CoPi catalyst with respect to redox reactions in alkaline solution were assessed and evaluated with bulk-CoPi fabricated without surfactant. Fig. 5 displays the cyclic voltammograms at 50 mVs-1 in 1.0 M KOH for both meso-CoPi and bulkCoPi loaded on GC electrode and in the potential varied from 0.9 V to 1.7 V vs. RHE. The voltammograms for meso-CoPi show well distinguished two redox peaks (A1/C1 = Co(II)/Co(III)) and (A2/C2 = Co(III)/Co(IV) positioned at about 1.12/1.05 and 1.48/1.42 V vs. RHE respectively, which are typical analogous to those characteristic cobalt redox peaks previously reported in literature.51-52 Typically , in alkaline media the Co(III)/Co(IV) transition is precede before the onset of OER and follow the distinguished redox couple of Co(II)/Co(III) species53-54 according to equation 1. Co3(PO4)2 + OH- ↔ Co3(PO4)2 (OH-) + e- (1) Interestingly and as shown in table 1, for both redox centers, it can be observed that the redox peaks current of meso-CoPi catalyst voltammogram is suggestively more than thirty times higher than the current for bulk-CoPi signifying a substantial advanced electrochemical active surface area and higher capacitance current the existence of mesoporous network structure as (Table 1).
Table 1. Electrochemical parameters obtained for redox peak A1/C1 of bulk-CoPi, meso-CoPi and meso-Co(OH)225 catalysts at loading 2.1 mg/cm2 in 1.0 M KOH at scan rate 50mV s-1. Catalyst
EA1/ mV
EC1/ mV
ΔE/ mV
IA1/ mA cm-2
IC1/ mA cm-2
bulk-CoPi
1144
1108
36
1.15
0.47
meso-CoPi
1190
1050
140
35.70
28.64
meso-Co(OH)2
1160
1104
56
2.34
1.34
100
Current/mAcm
-2
80
(iii)
60
A1
40
A2
(ii)
20
(i) 0
C2
C1
-20
-40 1.0
1.2
1.4
1.6
1.8
Potential vs. RHE/V
Fig. 5 Cyclic voltammetry at 50 mVs-1 in 1.0 M KOH solution for 2.1 mg/cm2 of (i) glassy carbon, (ii) bulk-CoPi and (ii) meso-CoPi. Cyclic voltammogram data for the redox nature of meso-CoPi catalyst at altered scan rates are portrayed in Fig. 6a. The linear connection among anodic peak current vs. the square root of the scan rate are presented in Fig. 6b. The anodic current upsurge with altering scan rate varied from 5 to 100 mVs-1. The linear development of the anodic peak current with square root of the scan
rate designates that the Co2+/Co3+ redox process of meso-CoPi catalyst electrooxidation in alkaline solution is diffusion limited process and consistent with reported literatures55 reflecting faster rate of electrochemical reaction.
80
(a) 60
(b)
40
20
0
-20
-1
-40
-2
5 mVs 10 20 30 50 70 100
Current, mAcm
Current/mAcm
-2
40
20
Oxidation Reduction
0
-20
-40 1.0
1.2
1.4
1.6
1.8
2
4
6
8 -1
Potential vs. RHE/V
[Scanrate (mVs )]
10
1/2
Fig. 6 Cyclic voltammetry for 2.1 mg/cm2 loaded meso-CoPi catalyst in 1.0 M KOH (a) various scan rate from 5 mVs-1 to 100 mVs-1, (b) linear connection between redox peak current and square root of scan rate. Fig. 7a depicts the linear sweep voltammograms for meso-CoPi with different loadings in 1.0 M KOH. Fig. 7b noticeably displays as the meso-CoPi catalyst loading upsurges, the peak A1 current density is linearly increases with a line slope of 19.52 mA/mg cm-2 due to an enhancement in electrochemical active surface area. Further, the peak A1 potential considerably shifted about 100 mV to more positive over the considered loading presumable due to film thickness limitations. In addition, additional increase in the meso-CoPi loading effects in less current which can be credited to destructive OER and effects in the physical removal of electrocatalyst from the electrode surface.
1.26
45
(b)
(vi)
150
(a)
40 1.24
(iii)
-2
35 1.22
30 25
1.20
20
Peak current, mA cm
Current/mAcm
-2
(iv) 100
Peak potential, vs. RHE/V
(v)
1.18
50
15
(ii) 1.16
10
(i)
5 1.14
0 1.0
1.2
1.4
Potential vs. RHE/V
1.6
1.8
0.0
0.5
1.0
1.5
2.0
2.5
0 3.0
2
Loading, mg/cm
Fig. 7. a) Linear sweep voltammetry at 50 mVs-1 in 1.0 M KOH solution at numerous loading of meso-CoPi catalyst, (i) 0.0, (ii) 0.14, (iii) 0.56, (iv) 0.84, (v) 1.4 and (vi) 2.1 mg/cm2. (b) Plots for the effect of meso-CoPi catalyst loadings on OER onset potential and current at 1.76 V vs. RHE. Fig. 8 shows the cyclic voltammetry of meso-CoPi catalyst in various concentration of methanol solution. The voltammograms display that the methanol oxidation current steadily upsurges at the onset potential of peak A1 and its corresponding peak potential moves to positive potential as the methanol concentration increases. The inset in Fig. 8b displays a linear increase between methanol oxidation current and the methanol concentration. The results of voltammogram indicate that the electrocatalytic activity of methanol oxidation depends intensely on methanol concentration. This might be owing to the fact that more methanol molecules would
be adsorbed by Co3+ species resulting in an efficient conversion from Co 2+ to Co3+ species. On addition of methanol, the reduction peak current of Co(II)/Co(III) (A1/C1) redox at about 1.07 V vs. RHE during the reverse scan is totally reduced in comparison to oxidation peak current for the forward scan as well as the disappearance of the redox peaks (A2/C1) around 1.45 corresponding to Co(III)/Co(IV). This could be ascribed to the consumption of Co(III) and (Co(IV) if existed) by methanol oxidation consequently a decrease in the reduction current at C1 peak. Literature suggests that the methanol is oxidized into intermediates/products (CoOOH) by Co4+ species and then Co4+ species is reduced into Co3+ species.56 In the presence of methanol, meso-CoPi catalyst provides an intensive anodic peak current at 1.41 VRHE and about 225 mAcm2
which is ascribed to the oxidation of methanol. This demonstrates that our meso-CoPi has
relatively higher methanol electro-oxidation current (about 7 times higher) than that in absence
300
(vi)
(a)
(b) 240
(v)
200
(iii)
(i) 100
-2
200
(ii)
Current/mAcm
Current/mAcm
-2
(iv)
160
120
0
80
1.0
1.2
1.4
1.6
1.8
Potential vs. RHE/V
0.0
0.2
0.4
0.6 c[CH3OH]/M
0.8
1.0
of methanol. The high catalytic current could be attributed to the open porous channel of mesoCoPi catalyst and the easy access of active Co sites by methanol molecules and acts as an efficient electrocatalyst for methanol oxidation.
Fig. 8 a) Cyclic voltammogram for 2.1 mgcm-2 loaded meso-CoPi catalyst at 50 mVs−1 obtained at various ethanol concentration, (i) 0.0, (ii) 0.1, (iii) 0.3, (iv) 0.5, (v) 0.7 and (vi) 1.0 M. b) linear relationship between current density vs. methanol concentration.
This methanol electrooxidation performance at meso-CoPi surface is concordant with the Fleischmann EC’-type mechanism.57 Electro-reactive CoOOH kinds are produced from the oxidation of meso-CoPi electrocatalyst as displayed by Equation 1. Then methanol reacts with CoOOH via the electrochemical reaction in Equation 2 and rejuvenate Co(OH)2. Finally, the obtained product under ambient circumstances is presently not known but multiple electron transfers and whole depletion of methanol act likely, in particular at elevated temperatures. nCoOOH+CH3OH → nCo(OH)2 +Product
[2]
By considering the above clarifications and the catalytic role of CoOOH plays a vital role in catalyzing the methanol electrooxidation process. To further assess the stability and long-term activity of the meso-CoPi catalyst towards methanol electro-oxidation. As shown in Fig. 9a, the oxidation peaks current slightly decreases with the number of potential cycles after 1 st scan. The peak current of the 100th scan is about 92.3% of that of 1 st scan which indicates that meso-CoPi exhibits good long-term electrocatalytic stability and storage properties during methanol oxidation cycling under alkaline conditions. Tafel regions with slope are changed from 76 to 80 mV/ dec for 1st and 100th cycle respectively.
350
(a)
(b)
180 150 -2
250 200 150 100 50
1.16
Potential vs. RHE/V
Current/mAcm
Current/mAcm
-2
300
1201.15 1.14
901.13 1.11
60
1.0
st
1 th 100
0
(v) 1st 100th
1.12
1.1 1.2 1.3 log(i/mAcm-2)
1.4
(iv)
1.5
(iii)
30
-50
(ii) (i)
0 1.0
1.2 1.4 Potential vs. RHE/V
1.6
1.8 -2
0
1000
2000 Time(sec)
3000 −1
Fig. 9 a) Cyclic voltammogram for 2.1 mgcm loaded meso-CoPi catalyst at 50 mVs
4000
obtained at
1st and 100th cycle. b) Chronoamperometry at for meso-CoPi and bulk-CoPi catalysts in 1.0 M KOH in presence of 1.0 M methanol, (i) bulk-CoPi in presence of 1.0 M methanol at 1.41 V, meso-CoPi in (ii) absence of methanol at 1.41 V, in presence of methanol at 1.26 V (iii), 1.56 V (iv) and (v) at 1.41 V vs. RHE. Fig. 9b shows the long-term chronoamperometry measurements for meso-CoPi and bulk-CoPi catalysts in 1.0 M KOH with and without 1.0 M methanol. At the beginning, the current drops quickly for all the chronoamperometric measurements owing to double layer charging current and then gradually declines till reaching a pseudo-steady state. In the presence of methanol, the steady state current at 1.4 and 1.6 V vs RHE is considerably higher compared to without methanol which is concordant with earlier studies. The steady state methanol oxidation current of meso-CoPi catalyst at both oxidation potentials stays constant for more than 1 hour signifying the good long-standing stability and respectable tolerance against adsorption of methanol oxidation intermediates and products. From chronoamperometry results, meso-CoPi exhibited 20-fold increase at 1.4 V vs RHE related to the bulk-CoPi and have a large pore space for better reactant molecules. Table 2 listed the specific and mass catalytic activities in parallel to other metal oxide catalysts reported in the literatures. The specific and mass catalytic performances of the
fabricated meso-CoPi electrocatalyst with related to the electrooxidation of methanol were 225 mAcm−2 and 1512 mAmg−1, correspondingly; these are 3-fold increase related to the previously reported materials. Consequently, the fabricated meso-CoPi revealed considerably improved electrocatalytic performances with related to electrooxidation of methanol. Table 2 Specific and mass activities of present meso-CoPi and previously reported electrocatalysts for methanol/ethanol oxidation in alkaline media.
Sr. No
Catalyst
Method
1.
meso- CoPi
Liquid crystal template
3.
meso-CoPi
4.
BET surface area (m2/g)
Mass activity, mA mg-1/ specific activity mA cm-2
Ref.
1512/225
This work
846/126
58
Electrolyte (M)
Potential range (V)
124
KOH:1 CH3OH:1
0.0~0.7 SCE
V/
Microwave
20
KOH:0.1 CH3OH:1
-0.2~0.7 SCE
V/
meso NiCoO
Microwave
160
KOH:1 CH3OH:1
0~0.7 V / SCE
750/160
59
5.
Meso Ni(OH)2
Liquid crystal template
90
KOH:1 C2H5OH:1
0~0.7 V / SCE
370/72
11
6.
Ni/NiO
Microwave
68
KOH:1 CH3OH:1
0~0.7 V / SCE
275/40
60
7.
NiO/CNF
Electrospinning
31.85
KOH:6 CH3OH:1
0~0.9 V/ SCE
1800/300
12
8.
Pt/C
Chemical reduction
-
KOH:1 CH3OH:1
-0.8~0.3/ Hg/HgO
-/40
61
9.
Co-Cu-CNF
Electrospinng
-
KOH:1 CH3OH:2
0.0~0.8 V / Ag/AgCl
-/16
62
10.
RGONiCo2O4
Hydrothermal method
42
KOH:0.1 CH3OH:0.5
0~0.9 V Ag/AgCl
-/45
63
Two-step solution method
64.1
KOH:0.1 CH3OH:0.5
-1~0 V/ SCE
400/-
64
11.
Pt/Ni(OH)2/ Graphene
4. Conclusions
/
In this work, crystalline meso-CoPi catalyst with higher active surface area (124 m2/g) was successfully prepared by sol gel template method using Brij®78surfactant. The meso-CoPi exhibited 20-fold increase (228 mA/cm2) at 1.4 V vs RHE related to the bulk-CoPi and have a large pore space for better reactant molecules. Our findings showed that methanol oxidation on meso-CoPi catalyst in alkaline solution was catalyzed by the Co 3+/Co4+ redox couples and the catalyst exhibited an impressively high electrocatalytic activity for methanol oxidation. Considering the benefit of the catalysts stability, relatively high surface area and the large pore size, it can be established that the synthesized meso-CoPi catalyst with remarkable electrochemical activities can assist as an auspicious anode electrode material for DMFC and methanol sensors. Acknowledgements This project was funded by The National Plan for Science, Technology and Innovation, King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award number 83-36. References 1. Abdel Rahim, M.A., Abdel Hameed, R.M., Khalil, M.W., 2004. Nickel as a catalyst for the electro-oxidation of methanol in alkaline medium. J. Power Sources 134, 160-169. 2. Liu, P., Yin, G.P., Du, C.Y., 2008. Composite anode catalyst layer for direct methanol fuel cell. Electrochem. Commun. 10, 1471-1473. 3. Kim, D.B., Chun, H.J., Lee, Y.K., Kwon, H.H., Lee, H.I., 2010. Preparation of Pt/NiO-C electrocatalyst and heat-treatment effect on its electrocatalytic performance for methanol oxidation. Inter. J. Hydrogen Energy 35, 313-320. 4. Das, A.K., Sahoo, S., Arunachalam, P., Zhang, S, 2016. Facile synthesis of Fe 3O4 nanorod decorated reduced graphene oxide (RGO) for supercapacitor application. RSC Advances 6, 107057-107064.
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