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rGO hybrid for oxygen reduction reaction in alkaline medium at low temperature
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Available online at www.sciencedirect.com
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Enhanced electrocatalytic performance of Cu0.02S0.01/rGO hybrid for oxygen reduction reaction in alkaline medium at low temperature Bafrin Shakhseh, Behnam Seyyedi* Institute of Nanotechnology, Urmia University, Urmia, Iran
highlights
graphical abstract
A facile low temperature preparation method without pyrolysis. The unique bio-inspired and stoichiometric structure. Ultra-low content of metal atoms. An effective 4-electron pathway in a wide range of over-potentials. Extremely economical.
article info
abstract
Article history:
Graphene, is a carbon allotrope, which is widely used as a substrate for various catalysts
Received 4 June 2019
due to its interesting physicochemical properties. In the present study, graphene oxide
Received in revised form
sheets were prepared from graphite, then, the graphene oxide surface was modified by a
23 July 2019
low-temperature method using sulfur and copper atoms to obtain pseudo-enzyme Cu/S/
Accepted 4 August 2019
Graphene prosthetic group. The current density passing through Cu/S/Graphene catalyst
Available online 29 August 2019
was four times higher than that passing through graphite. The novel copper-based catalyst had an extraordinary performance for oxygen reduction reaction (ORR) due to the unique
Keywords:
bio-inspired and stoichiometric structure. The results of Raman and Dispersive X-ray
Copper
spectroscopy confirmed the presence of ultra-low content of copper (2%) and sulfur (1%)
Electrocatalyst
atoms on the graphene surface. Thermogravimetric analysis indicated a strong interaction
Graphene
between nanoparticles and graphene layers. The number of electrons transferred for ORR
Oxygen reduction reaction
varied from 3.98 to 4.16 in a wide range of over-potentials indicating an effective 4-electron
Pseudo-enzyme
pathway form O2 to H2O. The Tafel slopes indicated insignificant amount of formed copper
Prosthetic group
* Corresponding author. E-mail addresses: [email protected], [email protected] (B. Seyyedi). https://doi.org/10.1016/j.ijhydene.2019.08.018 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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oxide on the catalyst surface. The catalyst showed excellent electrochemical durability and its half-wave potential (E1/2) was exhibited a negative shift only 8.2 mV after 10000 cycles. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy in different forms is a key factor for the sustainable development of industrial countries. Given the increased rate of industrial development, environmental concerns, increased demand, and decreased fossil energy reserves, the use of generation and storage systems of renewable energy is inevitable [1,2]. Alternative systems should be highly stable and have good efficiency and fewer economic limitations [3]. Continuous conversion of chemical energy into electrical energy using renewable non-pollutant resources, as the nextgeneration renewable energy systems, is of great importance. Oxygen is used as an abundant nontoxic oxidizer for cathodic reactions in electricity generation systems [4]. In general, oxygen is reduced through two pathways under different conditions [5]: 1. A single-step two-electron pathway: O2 þ 2e / H2O2 2. A two-step successive two-electron pathway: O2 þ 2e / H2O2 þ 2e / H2O The energy required for ORR is significant due to its slow kinetics [6]. This indicates the necessity for the use of effective catalysts with high efficiency. Various compounds have been studied as catalysts and most studies have focused on carbonbased catalysts (such as carbon/metal or carbon/metal oxide catalysts) [7,8]. Carbon-supported platinum and its derivatives are among conventional catalysts for ORR. However, development of this class of catalysts is not considerable due to numerous limitations [9]. According to the literature, metalnitrogen-carbon (M-N4-C) is reported as the most important alternative for platinum-based catalysts [10,11]. M-N4-C is prepared by pyrolysis of metal, nitrogen, and carbon precursors at high temperatures. Due to the random nature of pyrolysis, the obtained structure is rarely reproducible. The initial structure and properties of precursors change during pyrolysis process [12]. Therefore, stoichiometric compounds production through low temperature processes have received much attention to produce catalysts without pyrolysis of precursors. Low-temperature processes preserve initial properties of precursors which allow investigation of the relationship between catalyst structure, activity, and its stability for the ORR. Catalytic centers in natural enzymes are placed in this class of catalysts. Enzymes have catalytic activity at low temperatures. The ordered arrangement, property preservation, reproducibility, simple preparation method, performance stability, and high efficiency are among the unique features of pseudo-enzyme centers. Cytochrome 450 enzyme is used in nature for ORR at ambient temperature with high efficiency [13]. Cytochrome 450 uses iron centers (FeeN4) bonded to the protein body by sulfur bridges for oxygen reduction (Fig. 1) [14].
Fig. 1 e Heme prosthetic group of Cytochrome 450 enzyme.
Carbon and its various allotropes including graphene have been used as the substrate of ORR catalysts [15]. Graphene is commonly used due to affordable production methods, unique physiochemical properties, especially zero-overlap semimetal property with high electrical conductivity [16]. Various methods are used to prepare graphene. Graphite oxide, a material made of many layers of graphene oxide (GO), includes oxygen functional groups. Graphite oxide and GO have interesting properties that can be different than those of graphene. By reducing graphite oxide and GO, these oxidized functional groups are removed, to obtain a graphene material. This graphene material is called reduced graphene oxide, often abbreviated to rGO. The carbon texture and its electrical properties are considerably improved by doping graphene sheets with heteroatoms of high electron density such as nitrogen and sulfur [17]. In this study, the pseudo-enzyme stoichiometric structure based on copper atoms and graphene sheets were prepared at low temperatures with high efficiency for ORR (Fig. 2). Based on the appropriate performance of M N4 centers for ORR, copper atoms were used in the form of CueN4 in the 2-dimensional copper phthalocyanine (CuPc) framework. CuPc is selected for the following reasons: - Very good physicochemical stability [18]. - A large molecular surface which fixes the appropriate direction of the stoichiometric reaction. - An environmentally friendly compound lacking toxic groups with an affordable synthesis method [19]. The sulfur-modified graphene was prepared through the low-temperature reaction of graphene oxide (GO) sheets with the Na2S and the CueN4 centers attached to the graphene surface by sulfur bridges. The physical characterizations of samples were carried out by Raman spectroscopy, Thermogravimetric Analysis (TGA), Fourier-Transform Infrared Spectroscopy (FTIR), Energy-Dispersive X-ray Spectroscopy (EDS), Inductively Coupled Plasma Optical Emission Spectrometry
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Fig. 2 e Copper phthalocyanine (left) and copper-based prosthetic group (right).
(ICP-OES), X-ray Powder Diffraction (XRD), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM) methods. The catalytic performance for ORR was
evaluated through cyclic voltammetry (CV) and linear sweep voltammetry (LSV) methods on a rotating disc electrode (RDE) system.
Fig. 3 e Raman spectra of, a) CuPc, b) Graphite, c) S/Graphene composite, and d) catalyst Cu/S/Graphene. The Raman spectra were recorded in the range of 200e4000 cm¡1.
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Synthesis of graphene oxide (GO) sheets The GO sheets were prepared through the improved Hummers’ method [21]. Typically, 1.2 g graphite powder and 1.5 g sodium nitrate were added to 25 mL concentrated sulfuric acid (98%) and stirred for 1 h. The mixture was then placed in an ice-water bath and mixed continuously. Then 4.45 g potassium permanganate was slowly added to the mixture for 3 h. The mixture temperature was raised to 40 C and mixed for 2 h. The reaction was stopped by adding 120 mL of ice water and 4.2 mL H2O2 (30%). The resulting GO powder was rinsed several times with plenty of deionized water and hydrochloric acid (3%). The powder was filtered and dried at room temperature.
Preparation of S/Graphene composite Fig. 4 e FT-IR spectra of, a) CuPc, b) Graphite, c) S/Graphene composite, and d) catalyst Cu/S/Graphene. FT-IR spectra of the samples were recorded in the range of 400e4000 cm¡1 with a spectral resolution of 2 cm¡1.
Materials and methods All initial chemicals were in guaranteed reagent (GR) grade from Sigma-Aldrich, Merck and Alfa Aesar and used without further purification unless otherwise noted.
GO was reduced to prepare the composite S/Graphene as follows. First, 0.2 g dry GO powder was dispersed in 100 mL of the dimethylformamide (DMF). The resulting mixture was placed in an ultrasonic bath for 60 min to obtain a uniform suspension. Then 1.3 g of sodium sulfide was dissolved in 20 mL of the DMF. The solution was dropped to the suspension in the ultrasonic bath. For sulfur-functionalization of GO, the mixture was refluxed at 150 C for 8 h. The powder S/ Graphene was obtained with a high degree of GO reduction and low sulfur functionalization (z1%) [22]. The resulting powder was rinsed with deionized water to remove excess sodium sulfide. The powder was then filtered and dried at room temperature.
Synthesis of copper phthalocyanine (CuPc) CuPc was synthesized through the method reported by our group [20]. A temperature-controlled microwave synthesis reactor (Anton Paar-Monowave 200) was used to synthesis of CuPc at atmospheric pressure. The initial materials copper (II) chloride (1 mmol), high-purity urea (4 mmol), and phthalic anhydride (4 mmol) were dissolved in a mixture of saturated solution of sodium chloride (45 mL) and ethanol (5 mL). Then 0.1 mmol of the ammonium heptamolybdate catalyst was added to the mixture. The resulting mixture was exposed to microwave irradiation for 30 min at a controlled temperature (120 C). The obtained CuPc powder was filtered and rinsed with water-ethanol mixture several times. The final product was dried in an air oven at 80 C for 12 h.
Table 1 e Infrared frequencies of important groups of Pc skeleton. Group OeH CeH CeH
CeH CeC CeC
Frequencies (cm1)
Assigned to
3440 3040 1049 1170 1300 1460 1739 1522 1642 1450
Absorbed H2O Benzene
Plane vibration
Aryl Pyrrole Isoindole
Preparation of catalyst Cu/S/Graphene: anchoring of CuPc on the surface of S/Graphene composite About 0.1 g S/Graphene powder and 0.1 g CuPc powder were dispersed in 50 mL of the DMF. The mixture was placed in an ultrasonic bath for 60 min. The suspension was refluxed at 150 C for 8 h. The final product was rinsed several times with deionized water and then filtered and dried at room temperature.
Results and discussion CuPc, GO, S/Graphene composite and the catalyst Cu/S/Graphene were synthesized through a low-temperature method. The CuPc powder was synthesized in a microwave reactor with cheap raw materials through an unknown radical mechanism at low temperature. GO was prepared by improved Hummers’ method in a strong oxidizing medium from graphite powder. The GO sheets were then modified by sulfur atoms via a low-temperature process (Cu/S/Graphene with a high reduction degree). The CuPc molecules were then fixed on the graphene surface by sulfur bridges. The pseudoenzyme structure of Cu/S/Graphene contains 1.99 wt% of copper atoms and 1.05 wt% of sulfur atoms (EDS and XRF results). The Cu/S weight ratio in the catalyst structure is consistent with the molecular weight ratio of copper (63.546 g mol1) and sulfur (32.06 g mol1) atoms confirming the 1:1 stoichiometric synthesis of Cu/S.
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Fig. 5 e EDS spectrum of Cu/S/Graphene.
The Raman spectra specify carbon allotropes such as graphite and graphene. Generally, G and D bands (carbon sheet vibrations with sp2 hybridization) and 2D bands of carbon allotropes are observed in the range of 1000e3000 cm1. Raman shift, shape and the ratio of bands determine the type and changes in the carbon structure. For graphite, 2D/G < 1 and the Raman shift is about 1582 cm1 for the G band. The G band for bilayer and
Table 2 e EDS and ICP-OES structural analysis data. Analysis Method
Cu%
S%
WCu/WS
EDS ICP-OES
1.99 2.1
1.05 1.03
1.90 2.04
monolayer modified graphene is respectively observed at 1584 and 1587 cm1 [23]. G/D ratio and (D þ G) Raman shift determine the degree of defects and irregularity of carbon sheets. The Raman spectra of the synthesized samples were recorded in the range of 200e4000 cm1 on a Raman spectrophotometer (RENISHAW) (S1). The Raman spectrum of a-CuPc is consistent with that reported by Anghelone et al. (Fig. 3a) [24]. The D, G and 2D peaks for graphite are observed at 1351, 1582 and 2695 cm1, respectively (Fig. 3b). Due to the compact and irregular structure of graphite sheets, 2D/G < 1 [25]. The G band in the S/Graphene composite Raman spectrum is observed at 1588 cm1 with G Dz1(Fig. 3c). The 2D peak is completely disappeared due to replacement of oxide groups with sulfur atoms in the GO structure. Thus, a monolayer modified graphene was obtained with high quality and order of layers. Sharp peaks (due to crystalline structure) are observed in the range of 100e500 cm1 =
Structural characterization
Fig. 6 e XRD patterns of, a) Graphite, b) S/Graphene composite prepared @ 150 C, c) catalyst Cu/S/Graphene prepared @ 150 C, and d) CuPc prepared using microwave irradiation @ controlled temperature (120 C). The XRD spectra were recorded in the range of 5e80 at a radiation wavelength of 1.54 A.
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Fig. 7 e TGA analysis of, solid line) CuPc, dot line) Graphite, dash line) S/Graphene composite, and dosh-dot line) catalyst Cu/S/Graphene. The TGA diagrams were carried out in a temperature range of 25e700 C in the air atmosphere. The heating rate and fluid flow were 10 C.min¡1 and 50 ml min¡1, respectively.
in the Raman spectrum of sulfur. However, these peaks are not observed in Fig. 3c and d due to low crystallinity of sulfur [26]. G band shift and G/D ratio for the catalyst Cu/S/Graphene did not change compared with S/Graphene composite. The D band was significantly displaced by 16 cm1 due to copper and sulfur ionic bonds. The peak appeared at 1462 cm1 is attributed to CuPc. The triple (D þ G) peaks appeared at 2849, 2895 and 2966 cm1 indicate defects in the layered structure of the catalysts. A regional sandwich structure is likely formed on the surface of graphene sheets [27]. FTIR spectrum monitors the presence or absence of various functional groups in a compound (Fig. 4). The FTIR spectra of the samples were recorded (ThermoNicolet model NEXUS FTIR 670 spectrometer) in the range of 400e4000 cm1 with a spectral resolution of 2 cm1(S2). The characteristic peaks of the phthalocyanine (Pc) skeleton in the range of 500e1650 cm1 confirm the a-CuPc structure [28]. Obviously, all peaks cannot be easily interpreted due to strong overlaps (Table 1). Due to the replacement of copper atoms with hydrogen atoms in the Pc ring, the strong peaks for the NeH bond are not observed at 3290 and 1006 cm1. Due to complexity and overlap of peaks in the range of 1000e1400 cm1 with strong graphene peaks, CeC stretching vibration in Isoindole, and CeCl and CeS vibrations cannot be differentiated [29]. The apparent peak at 724 cm1 confirms CuPc molecules in the catalyst Cu/S/Graphene. A displacement of about þ9 cm1 is observed due to ionic bonds of Cu/S [30]. EDS and ICP-OES are used for structural analysis and determination of chemical composition. Fig. 5 shows the EDS spectrum of the Cu/S/Graphene (S3). The EDS and ICP-OES indicate the presence of copper and sulfur atoms in the catalyst. Quantitative analysis indicates the presence of z2 and z1 wt% of copper and sulfur atoms, Cu respectively (Table 2). %W %WS is in good agreement with the stoichiometric structure of the catalyst and a Cu/S ratio of 1:1. The Cu/S atomic weight ratio is about 1.98 under 1:1 conditions.
X-ray diffraction is widely used for studying the configuration of molecules, phase analysis, and estimating the size and crystalline lattice parameters. The XRD spectra of synthesized samples were recorded (X’ Pert Pro-Panalytical with Cu-Ka) in the range of 5e80 at a radiation wavelength of 1.54 A at room temperature (S4). A sharp peak is observed in the graphite XRD pattern at 2q ¼ 26.60 (Fig. 6a). A sheet spacing of 3.35 A was calculated accordingly [31]. After attaching the sulfur atoms on the surface of carbon sheets at 150 C, a broad peak is observed in the XRD patterns of the S/Graphene composite and catalyst Cu/S/Graphene at about 2q ¼ 25.00 . This corresponds to a sheet spacing of about 3.56 A. The increase in the sheet spacing (0.21 A) relative to graphite is attributed to the presence of sulfur atoms and CuPc molecules. Fig. 6d shows the XRD pattern of a-CuPc [32]. The peaks at 7.63 , 9.03 , 14.18 , 16.45 , 19.23 , 31.5 , 39.46 , and 43.08 are related to the a-CuPc structure in CuPc and catalyst Cu/S/Graphene. TGA diagrams show the thermal behavior and stability of compounds. The diagrams are plotted as the changes in the mass versus temperature. Information on physical and chemical phenomena (such as phase transition, adsorption, desorption, chemical adsorption, and thermal degradation) can be derived from TGA diagrams. The samples were analyzed in a temperature range of 25e700 C in the air atmosphere. The air flow rate was 50 mL min1 and temperature increased at a rate of 10 /min (S5). Fig. 7 shows TGA diagrams for CuPc, graphite, S/Graphene composite, and Cu/S/Graphene. The weight loss of CuPc occurs in three steps. The first step between 25 and 150 C corresponds to moisture removal. Thermal degradation of CuPc skeleton is initiated in the range of 225e300 C (second step) and the sample is completely oxidized in the range of 325e450 C (third step). The weight loss for graphite, the weight sharply decreases with complete oxidation of carbon. The TGA diagram for S/Graphene composite shows weight loss in three stages. In the first stage, water molecules adsorbed on the surface are removed from 25 to 125 C. The sulfur atoms attached to the graphene surface are removed in the second stage in the range of 125e400 C. Weight loss in the range of 400e500 C is related to complete oxidation of carbon atoms (third stage). The oxidation temperature of carbon atoms decreases relative to graphite sheets due to a decrease in the density of carbon sheets in graphene (which is in good agreement with an increase in sheet spacing calculated by XRD) [33]. The catalyst Cu/S/Graphene shows a thermal behavior between CuPc and the S/Graphene composite. Three distinct weight loss steps are as follows: - Moisture removal with a mild slope in the range of 25e150 C. - Degradation of the CuPc skeleton in the range of 250e400 C. - Removal of sulfur atoms and complete oxidation of carbon atoms in the range of 400e450 C. The thermal behavior indicates the significant thermal stability of the catalyst. HR-TEM, TEM, and SEM micrographs are used to find out differences in density, particle size, and structural morphology. Fig. 8(aec) shows the TEM micrographs (TEM
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Fig. 8 e (a and b) TEM micrographs of catalyst Cu/S/Graphene with different resolutions, (c) HR-TEM image of Cu/S/ Graphene, (d) SEM surface image of graphite, and (e) SEM surface image of catalyst Cu/S/Graphene.
Philips EM 208S) of the catalyst. Several micrographs were taken with different resolutions to compare the size and shape of particles. CuPc nanoparticles are seen as catalytic centers in the carbon substrate in the form of dark points with a size of less than 5 nm. The presence of sulfur atoms in the carbon substrate caused considerable changes in the morphology and order of sheets. The bright background indicates the low density of carbon sheets (monolayer graphene structure). The TEM and HR-TEM micrographs are confirmed through Raman, TGA, and XRD data (S6). Fig. 8 shows the SEM micrographs of graphite (8d) and catalyst Cu/S/Graphene (8e). The graphite particles are larger than 1 mm and the surface is
completely irregular. The SEM micrograph for the catalyst Cu/ S/Graphene indicates a particle size of less than 100 nm. The micrograph also shows the greater roughness and order of catalyst surface than graphite (S7).
Electrochemical evaluation of catalyst Cu/S/Graphene for ORR The catalytic performance of samples for ORR was evaluated through cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on a rotating-disk electrode (RDE) system (Autolab PGSTAT302N). Measurements were made using a
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Fig. 9 e Cyclic voltammograms of CuPc (dash line), Graphite (dot line), and catalyst Cu/S/Graphene (solid line) in O2-saturated 0.1 mol/L KOH solution at a scan rate of 50 mV s¡1. The catalyst loading is ≈ 0.4 mg cm¡2.
conventional three electrode cell with a counter electrode (a carbon glass rod of 2 mm diameter), a reference electrode (Ag/ AgCl) and a working electrode (a carbon glass RDE electrode of 5 mm diameter). Catalyst ink was prepared to deposit a catalyst film on the electrode surface. To this end, 2 mg catalyst powder was dispersed in 450 mL deionized water and 50 mL Nafion solution (5%). The resulting suspension was placed in an ultrasonic bath for 30 min. Twenty microliters of the ink was coated on the electrode surface. The catalyst film was formed by solvent evaporation (catalyst loading z 0.4 mg/ cm2). Oxygen-saturated KOH (0.1 mol. L1) solution was used as an electrolyte. CV and LSV diagrams were plotted in the range of 0.02e1.18 V vs. a reversible hydrogen electrode (RHE) with a scan rate of 50 and 10 mV s1, respectively. The electrolyte temperature was adjusted at 25 ± 1 C. The background current density was obtained at nitrogen-saturated KOH solution. CV is used to investigate species and electrochemical reactions. Fig. 9 shows CVs for CuPc, graphite, and the catalyst Cu/S/Graphene (S8). CuPc and graphite lack a significant catalytic activity for ORR (Table 3). The electrical conductivity of CuPc is also insignificant [34]. The catalyst Cu/S/Graphene shows electrochemical activity in two ranges: (i)
electrochemical activity in the range of 0.34e0.72 V which is attributed to Cu(II)eN4/Cu(I)eN4 where copper complexes participate in high-potential redox reactions [35], and (ii) an excellent catalytic activity for ORR at 0.85 V. The broad peak observed in the range of 0.3e0.7 V is assigned to the overlapping reduction peaks of electrochemical activities and high current density in the Cu/S/Graphene bed. Linear sweep voltammetry is a sensitive method for the quantitative determination of electrochemical activities. LSV diagrams were plotted at different rotating rates (200e2200 rpm) for quantitative evaluation of the catalyst Cu/ S/Graphene for the ORR. The onset (EOnset) and half-wave
Table 3 e Comparison of current density and catalytic activity of CuPc, graphite, and Cu/S/Graphene. The catalyst loading is ≈ 0.4 mg cm¡2. Sample CuPc Graphite Catalyst Cu/S/Graphene
|J| (mA.cm2)
Activity for ORR
0.3 1.3 5.5
Insignificant @ 0.85 V e Excellent @ 0.85 V
Fig. 10 e LSVs of catalyst Cu/S/Graphene @ different rotating rates in O2-saturated 0.1 mol/L KOH solution at a scan rate of 10 mV s¡1. The catalyst loading is ≈ 0.4 mg cm¡2.
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Fig. 11 e K-L plots of Cu/S/Graphene for ORR in O2saturated 0.1 mol/L KOH solution. Scan rate: 10 mV s¡1. Current densities were measured at 0.2 Ve0.7 V (vs. RHE).
potentials (E1/2) for ORR were respectively observed at 1.09 and 0.89 V (Fig. 10). The obtained potentials are comparable with those for the conventional platinum reference electrode (S9) [36]. The number of electrons transferred is exactly calculated to identify the ORR mechanism. Koutecky-Levich (KeL) equation was used to calculate the number of electrons transferred at different over-potentials [36]. 1 J1 ¼ J1 L þ JK ¼
qffiffiffiffiffiffiffiffi pffiffiffi 1 1 3 þ ; B ¼ ð0:201Þ:n:F:CO2 : D2O2 :1 6 v 0:5 Bu JK
(1)
The K-L diagrams for the catalyst Cu/S/Graphene were plotted in the range of 0.1e0.7 V using Eq. (1) (Fig. 11). The plots are linear and the number of electrons transferred can be exactly calculated from the slope of lines. The linear plots are classified into two groups (Table 4). The first group are plotted in the over-potential range of 0.35e0.7 V where the number of electrons transferred is very close to 4. Only two-electron series and parallel ORRs occur in this range. The number of electrons transferred was 3.98 indicating over 99% contribution of the 4-electron ORR pathway. The second group are plotted in the range of 0.1e0.35 V where the number of electron transferred is greater than 4 (n ¼ 4.16). Reduction of oxygen and copper atoms occurs simultaneously and
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Fig. 12 e Calculated Tafel slope of catalyst Cu/S/Graphene @ 1800 rpm for ORR in O2-saturated 0.1 mol/L KOH solution. The catalyst loading is ≈ 0.4 mg cm¡2. Scan rate: 10 mV s¡1.
independently in this range so that over 95% of electrons are transferred through a 4-electron ORR pathway. The Tafel slope can be used to evaluate the electrocatalytic activity of the sample (Fig. 12). The Tafel slope is a function of the diffusion-corrected kinetic current density which determines the mechanism and controller of reaction kinetics. The Tafel slope for the platinum catalyst varies from 60 to 120 mV.dec1. The corresponding Tafel slope for non-precious metal catalysts (NPMCs) in alkaline media varies from 30 to 130 mV.dec1 [37]. Large Tafel slopes indicate the lack of formation of metal oxide species on the surface of catalytic centers. The metal/
Table 4 e The number of electron transferred of Cu/S/ Graphene in the over-potential range of 0.1e0.7 V vs. RHE. E (V) 0.1 0.2 0.3 0.4 0.5 0.6 0.7
n 1
V |J | V |J1| V |J1| V |J1| V |J1| V |J1| V |J1|
¼ ¼ ¼ ¼ ¼ ¼ ¼
6754.8 6727.1 6724.9 7039.5 7032.1 7029.7 7040.7
1/2
u -47.781, u1/2-45.528, u1/2-45.445, u1/2-52.321, u1/2-51.224, u1/2-50.465, u1/2-49.880,
2
R R2 R2 R2 R2 R2 R2
¼ ¼ ¼ ¼ ¼ ¼ ¼
0.99) 0.99) 0.99) 0.99) 0.99) 0.99) 0.99)
4.15 4.16 4.17 3.98 3.98 3.98 3.98
Fig. 13 e ORR polarization plots of the catalyst Cu/S/ Graphene before and after 10,000 potential cycles between 0.6 V and 1.1 V (vs. RHE) in O2-saturated KOH solution. Scan rate: 10 mV s¡1.
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metal oxide system directs ORR through a 2-electron pathway. Two Tafel slopes with a slight difference are observed for the catalyst Cu/S/Graphene in the KOH medium. The Tafel slopes of 128.8 mV.dec1 and 112.6 mV.dec1 were calculated for high and low over-potentials, respectively. A large Tafel slope (>100 mV.dec1) indicates the dominance of the 4-electron ORR pathway over the 2-electron pathway. In other words, such a high Tafel slope for catalyst Cu/S/Graphene indicates easy protonation of O2 on the surface of catalyst; however, protonation of O2 is the kinetics-control step of ORR on the active sites. The difference in the Tafel slope (16.2 mV.dec1) at low over-potentials is attributed to the overlap of Cu (II) reduction reactions with ORR. Electrochemical durability is among basic challenges for the use of electrocatalysts. This parameter determines economic evaluations and limitations in the use of this type of catalysts. The electrochemical durability of the catalyst Cu/S/Graphene was examined in the range of 0.6e1.1 V at a scan rate of 50 mV s1 (Fig. 13). The electrochemical durability of catalysts is usually reported after 4000 cycles. The half-wave potential for the catalyst Cu/S/Graphene after 5000 and 10000 cycles was displaced by 6 and 8.2 mV, respectively, which are lower than those reported in the literature [38]. The catalyst showed reasonable electrochemical durability.
Conclusions The graphene surface was modified by a low-temperature method using sulfur and copper atoms to obtain a nonplatinum catalyst based on copper and graphene. The very active catalyst with ultra-low content of copper atoms (z2%) showed an onset potential of 1.09, a half-wave potential of 0.89 V, a high selectivity (n z 3.98), and excellent electrochemical durability (DE1/2 ¼ 8.2 mV.dec1 @ 10000th cycle) for ORR. The outstanding performance of the catalyst for ORR is related to the unique stoichiometric structure while preserving the initial framework and properties of precursors. The lack of pyrolysis at high temperature prevents the formation of a random structure. A large current density was obtained through modifying GO surface by sulfur atoms at a low temperature which can be an interesting feature in a wide range of applications.
Acknowledgments We thank the staff of Institute of Nanotechnology (Assistant Professor. Dr. Habibi and Dr. Sehatnia) for their assistance with the electrochemical discussion and Associate Professor. Dr. Keshipour at Urmia university for the discussion on RAMAN and FT-IR results. The authors gratefully acknowledge the support of the Iran National Science Foundation: INSF.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.018.
references
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Available online at www.sciencedirect.com
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Enhanced electrocatalytic performance of Cu0.02S0.01/rGO hybrid for oxygen reduction reaction in alkaline medium at low temperature Bafrin Shakhseh, Behnam Seyyedi* Institute of Nanotechnology, Urmia University, Urmia, Iran
highlights
graphical abstract
A facile low temperature preparation method without pyrolysis. The unique bio-inspired and stoichiometric structure. Ultra-low content of metal atoms. An effective 4-electron pathway in a wide range of over-potentials. Extremely economical.
article info
abstract
Article history:
Graphene, is a carbon allotrope, which is widely used as a substrate for various catalysts
Received 4 June 2019
due to its interesting physicochemical properties. In the present study, graphene oxide
Received in revised form
sheets were prepared from graphite, then, the graphene oxide surface was modified by a
23 July 2019
low-temperature method using sulfur and copper atoms to obtain pseudo-enzyme Cu/S/
Accepted 4 August 2019
Graphene prosthetic group. The current density passing through Cu/S/Graphene catalyst
Available online 29 August 2019
was four times higher than that passing through graphite. The novel copper-based catalyst had an extraordinary performance for oxygen reduction reaction (ORR) due to the unique
Keywords:
bio-inspired and stoichiometric structure. The results of Raman and Dispersive X-ray
Copper
spectroscopy confirmed the presence of ultra-low content of copper (2%) and sulfur (1%)
Electrocatalyst
atoms on the graphene surface. Thermogravimetric analysis indicated a strong interaction
Graphene
between nanoparticles and graphene layers. The number of electrons transferred for ORR
Oxygen reduction reaction
varied from 3.98 to 4.16 in a wide range of over-potentials indicating an effective 4-electron
Pseudo-enzyme
pathway form O2 to H2O. The Tafel slopes indicated insignificant amount of formed copper
Prosthetic group
* Corresponding author. E-mail addresses: [email protected], [email protected] (B. Seyyedi). https://doi.org/10.1016/j.ijhydene.2019.08.018 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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oxide on the catalyst surface. The catalyst showed excellent electrochemical durability and its half-wave potential (E1/2) was exhibited a negative shift only 8.2 mV after 10000 cycles. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy in different forms is a key factor for the sustainable development of industrial countries. Given the increased rate of industrial development, environmental concerns, increased demand, and decreased fossil energy reserves, the use of generation and storage systems of renewable energy is inevitable [1,2]. Alternative systems should be highly stable and have good efficiency and fewer economic limitations [3]. Continuous conversion of chemical energy into electrical energy using renewable non-pollutant resources, as the nextgeneration renewable energy systems, is of great importance. Oxygen is used as an abundant nontoxic oxidizer for cathodic reactions in electricity generation systems [4]. In general, oxygen is reduced through two pathways under different conditions [5]: 1. A single-step two-electron pathway: O2 þ 2e / H2O2 2. A two-step successive two-electron pathway: O2 þ 2e / H2O2 þ 2e / H2O The energy required for ORR is significant due to its slow kinetics [6]. This indicates the necessity for the use of effective catalysts with high efficiency. Various compounds have been studied as catalysts and most studies have focused on carbonbased catalysts (such as carbon/metal or carbon/metal oxide catalysts) [7,8]. Carbon-supported platinum and its derivatives are among conventional catalysts for ORR. However, development of this class of catalysts is not considerable due to numerous limitations [9]. According to the literature, metalnitrogen-carbon (M-N4-C) is reported as the most important alternative for platinum-based catalysts [10,11]. M-N4-C is prepared by pyrolysis of metal, nitrogen, and carbon precursors at high temperatures. Due to the random nature of pyrolysis, the obtained structure is rarely reproducible. The initial structure and properties of precursors change during pyrolysis process [12]. Therefore, stoichiometric compounds production through low temperature processes have received much attention to produce catalysts without pyrolysis of precursors. Low-temperature processes preserve initial properties of precursors which allow investigation of the relationship between catalyst structure, activity, and its stability for the ORR. Catalytic centers in natural enzymes are placed in this class of catalysts. Enzymes have catalytic activity at low temperatures. The ordered arrangement, property preservation, reproducibility, simple preparation method, performance stability, and high efficiency are among the unique features of pseudo-enzyme centers. Cytochrome 450 enzyme is used in nature for ORR at ambient temperature with high efficiency [13]. Cytochrome 450 uses iron centers (FeeN4) bonded to the protein body by sulfur bridges for oxygen reduction (Fig. 1) [14].
Fig. 1 e Heme prosthetic group of Cytochrome 450 enzyme.
Carbon and its various allotropes including graphene have been used as the substrate of ORR catalysts [15]. Graphene is commonly used due to affordable production methods, unique physiochemical properties, especially zero-overlap semimetal property with high electrical conductivity [16]. Various methods are used to prepare graphene. Graphite oxide, a material made of many layers of graphene oxide (GO), includes oxygen functional groups. Graphite oxide and GO have interesting properties that can be different than those of graphene. By reducing graphite oxide and GO, these oxidized functional groups are removed, to obtain a graphene material. This graphene material is called reduced graphene oxide, often abbreviated to rGO. The carbon texture and its electrical properties are considerably improved by doping graphene sheets with heteroatoms of high electron density such as nitrogen and sulfur [17]. In this study, the pseudo-enzyme stoichiometric structure based on copper atoms and graphene sheets were prepared at low temperatures with high efficiency for ORR (Fig. 2). Based on the appropriate performance of M N4 centers for ORR, copper atoms were used in the form of CueN4 in the 2-dimensional copper phthalocyanine (CuPc) framework. CuPc is selected for the following reasons: - Very good physicochemical stability [18]. - A large molecular surface which fixes the appropriate direction of the stoichiometric reaction. - An environmentally friendly compound lacking toxic groups with an affordable synthesis method [19]. The sulfur-modified graphene was prepared through the low-temperature reaction of graphene oxide (GO) sheets with the Na2S and the CueN4 centers attached to the graphene surface by sulfur bridges. The physical characterizations of samples were carried out by Raman spectroscopy, Thermogravimetric Analysis (TGA), Fourier-Transform Infrared Spectroscopy (FTIR), Energy-Dispersive X-ray Spectroscopy (EDS), Inductively Coupled Plasma Optical Emission Spectrometry
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Fig. 2 e Copper phthalocyanine (left) and copper-based prosthetic group (right).
(ICP-OES), X-ray Powder Diffraction (XRD), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM) methods. The catalytic performance for ORR was
evaluated through cyclic voltammetry (CV) and linear sweep voltammetry (LSV) methods on a rotating disc electrode (RDE) system.
Fig. 3 e Raman spectra of, a) CuPc, b) Graphite, c) S/Graphene composite, and d) catalyst Cu/S/Graphene. The Raman spectra were recorded in the range of 200e4000 cm¡1.
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Synthesis of graphene oxide (GO) sheets The GO sheets were prepared through the improved Hummers’ method [21]. Typically, 1.2 g graphite powder and 1.5 g sodium nitrate were added to 25 mL concentrated sulfuric acid (98%) and stirred for 1 h. The mixture was then placed in an ice-water bath and mixed continuously. Then 4.45 g potassium permanganate was slowly added to the mixture for 3 h. The mixture temperature was raised to 40 C and mixed for 2 h. The reaction was stopped by adding 120 mL of ice water and 4.2 mL H2O2 (30%). The resulting GO powder was rinsed several times with plenty of deionized water and hydrochloric acid (3%). The powder was filtered and dried at room temperature.
Preparation of S/Graphene composite Fig. 4 e FT-IR spectra of, a) CuPc, b) Graphite, c) S/Graphene composite, and d) catalyst Cu/S/Graphene. FT-IR spectra of the samples were recorded in the range of 400e4000 cm¡1 with a spectral resolution of 2 cm¡1.
Materials and methods All initial chemicals were in guaranteed reagent (GR) grade from Sigma-Aldrich, Merck and Alfa Aesar and used without further purification unless otherwise noted.
GO was reduced to prepare the composite S/Graphene as follows. First, 0.2 g dry GO powder was dispersed in 100 mL of the dimethylformamide (DMF). The resulting mixture was placed in an ultrasonic bath for 60 min to obtain a uniform suspension. Then 1.3 g of sodium sulfide was dissolved in 20 mL of the DMF. The solution was dropped to the suspension in the ultrasonic bath. For sulfur-functionalization of GO, the mixture was refluxed at 150 C for 8 h. The powder S/ Graphene was obtained with a high degree of GO reduction and low sulfur functionalization (z1%) [22]. The resulting powder was rinsed with deionized water to remove excess sodium sulfide. The powder was then filtered and dried at room temperature.
Synthesis of copper phthalocyanine (CuPc) CuPc was synthesized through the method reported by our group [20]. A temperature-controlled microwave synthesis reactor (Anton Paar-Monowave 200) was used to synthesis of CuPc at atmospheric pressure. The initial materials copper (II) chloride (1 mmol), high-purity urea (4 mmol), and phthalic anhydride (4 mmol) were dissolved in a mixture of saturated solution of sodium chloride (45 mL) and ethanol (5 mL). Then 0.1 mmol of the ammonium heptamolybdate catalyst was added to the mixture. The resulting mixture was exposed to microwave irradiation for 30 min at a controlled temperature (120 C). The obtained CuPc powder was filtered and rinsed with water-ethanol mixture several times. The final product was dried in an air oven at 80 C for 12 h.
Table 1 e Infrared frequencies of important groups of Pc skeleton. Group OeH CeH CeH
CeH CeC CeC
Frequencies (cm1)
Assigned to
3440 3040 1049 1170 1300 1460 1739 1522 1642 1450
Absorbed H2O Benzene
Plane vibration
Aryl Pyrrole Isoindole
Preparation of catalyst Cu/S/Graphene: anchoring of CuPc on the surface of S/Graphene composite About 0.1 g S/Graphene powder and 0.1 g CuPc powder were dispersed in 50 mL of the DMF. The mixture was placed in an ultrasonic bath for 60 min. The suspension was refluxed at 150 C for 8 h. The final product was rinsed several times with deionized water and then filtered and dried at room temperature.
Results and discussion CuPc, GO, S/Graphene composite and the catalyst Cu/S/Graphene were synthesized through a low-temperature method. The CuPc powder was synthesized in a microwave reactor with cheap raw materials through an unknown radical mechanism at low temperature. GO was prepared by improved Hummers’ method in a strong oxidizing medium from graphite powder. The GO sheets were then modified by sulfur atoms via a low-temperature process (Cu/S/Graphene with a high reduction degree). The CuPc molecules were then fixed on the graphene surface by sulfur bridges. The pseudoenzyme structure of Cu/S/Graphene contains 1.99 wt% of copper atoms and 1.05 wt% of sulfur atoms (EDS and XRF results). The Cu/S weight ratio in the catalyst structure is consistent with the molecular weight ratio of copper (63.546 g mol1) and sulfur (32.06 g mol1) atoms confirming the 1:1 stoichiometric synthesis of Cu/S.
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Fig. 5 e EDS spectrum of Cu/S/Graphene.
The Raman spectra specify carbon allotropes such as graphite and graphene. Generally, G and D bands (carbon sheet vibrations with sp2 hybridization) and 2D bands of carbon allotropes are observed in the range of 1000e3000 cm1. Raman shift, shape and the ratio of bands determine the type and changes in the carbon structure. For graphite, 2D/G < 1 and the Raman shift is about 1582 cm1 for the G band. The G band for bilayer and
Table 2 e EDS and ICP-OES structural analysis data. Analysis Method
Cu%
S%
WCu/WS
EDS ICP-OES
1.99 2.1
1.05 1.03
1.90 2.04
monolayer modified graphene is respectively observed at 1584 and 1587 cm1 [23]. G/D ratio and (D þ G) Raman shift determine the degree of defects and irregularity of carbon sheets. The Raman spectra of the synthesized samples were recorded in the range of 200e4000 cm1 on a Raman spectrophotometer (RENISHAW) (S1). The Raman spectrum of a-CuPc is consistent with that reported by Anghelone et al. (Fig. 3a) [24]. The D, G and 2D peaks for graphite are observed at 1351, 1582 and 2695 cm1, respectively (Fig. 3b). Due to the compact and irregular structure of graphite sheets, 2D/G < 1 [25]. The G band in the S/Graphene composite Raman spectrum is observed at 1588 cm1 with G Dz1(Fig. 3c). The 2D peak is completely disappeared due to replacement of oxide groups with sulfur atoms in the GO structure. Thus, a monolayer modified graphene was obtained with high quality and order of layers. Sharp peaks (due to crystalline structure) are observed in the range of 100e500 cm1 =
Structural characterization
Fig. 6 e XRD patterns of, a) Graphite, b) S/Graphene composite prepared @ 150 C, c) catalyst Cu/S/Graphene prepared @ 150 C, and d) CuPc prepared using microwave irradiation @ controlled temperature (120 C). The XRD spectra were recorded in the range of 5e80 at a radiation wavelength of 1.54 A.
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Fig. 7 e TGA analysis of, solid line) CuPc, dot line) Graphite, dash line) S/Graphene composite, and dosh-dot line) catalyst Cu/S/Graphene. The TGA diagrams were carried out in a temperature range of 25e700 C in the air atmosphere. The heating rate and fluid flow were 10 C.min¡1 and 50 ml min¡1, respectively.
in the Raman spectrum of sulfur. However, these peaks are not observed in Fig. 3c and d due to low crystallinity of sulfur [26]. G band shift and G/D ratio for the catalyst Cu/S/Graphene did not change compared with S/Graphene composite. The D band was significantly displaced by 16 cm1 due to copper and sulfur ionic bonds. The peak appeared at 1462 cm1 is attributed to CuPc. The triple (D þ G) peaks appeared at 2849, 2895 and 2966 cm1 indicate defects in the layered structure of the catalysts. A regional sandwich structure is likely formed on the surface of graphene sheets [27]. FTIR spectrum monitors the presence or absence of various functional groups in a compound (Fig. 4). The FTIR spectra of the samples were recorded (ThermoNicolet model NEXUS FTIR 670 spectrometer) in the range of 400e4000 cm1 with a spectral resolution of 2 cm1(S2). The characteristic peaks of the phthalocyanine (Pc) skeleton in the range of 500e1650 cm1 confirm the a-CuPc structure [28]. Obviously, all peaks cannot be easily interpreted due to strong overlaps (Table 1). Due to the replacement of copper atoms with hydrogen atoms in the Pc ring, the strong peaks for the NeH bond are not observed at 3290 and 1006 cm1. Due to complexity and overlap of peaks in the range of 1000e1400 cm1 with strong graphene peaks, CeC stretching vibration in Isoindole, and CeCl and CeS vibrations cannot be differentiated [29]. The apparent peak at 724 cm1 confirms CuPc molecules in the catalyst Cu/S/Graphene. A displacement of about þ9 cm1 is observed due to ionic bonds of Cu/S [30]. EDS and ICP-OES are used for structural analysis and determination of chemical composition. Fig. 5 shows the EDS spectrum of the Cu/S/Graphene (S3). The EDS and ICP-OES indicate the presence of copper and sulfur atoms in the catalyst. Quantitative analysis indicates the presence of z2 and z1 wt% of copper and sulfur atoms, Cu respectively (Table 2). %W %WS is in good agreement with the stoichiometric structure of the catalyst and a Cu/S ratio of 1:1. The Cu/S atomic weight ratio is about 1.98 under 1:1 conditions.
X-ray diffraction is widely used for studying the configuration of molecules, phase analysis, and estimating the size and crystalline lattice parameters. The XRD spectra of synthesized samples were recorded (X’ Pert Pro-Panalytical with Cu-Ka) in the range of 5e80 at a radiation wavelength of 1.54 A at room temperature (S4). A sharp peak is observed in the graphite XRD pattern at 2q ¼ 26.60 (Fig. 6a). A sheet spacing of 3.35 A was calculated accordingly [31]. After attaching the sulfur atoms on the surface of carbon sheets at 150 C, a broad peak is observed in the XRD patterns of the S/Graphene composite and catalyst Cu/S/Graphene at about 2q ¼ 25.00 . This corresponds to a sheet spacing of about 3.56 A. The increase in the sheet spacing (0.21 A) relative to graphite is attributed to the presence of sulfur atoms and CuPc molecules. Fig. 6d shows the XRD pattern of a-CuPc [32]. The peaks at 7.63 , 9.03 , 14.18 , 16.45 , 19.23 , 31.5 , 39.46 , and 43.08 are related to the a-CuPc structure in CuPc and catalyst Cu/S/Graphene. TGA diagrams show the thermal behavior and stability of compounds. The diagrams are plotted as the changes in the mass versus temperature. Information on physical and chemical phenomena (such as phase transition, adsorption, desorption, chemical adsorption, and thermal degradation) can be derived from TGA diagrams. The samples were analyzed in a temperature range of 25e700 C in the air atmosphere. The air flow rate was 50 mL min1 and temperature increased at a rate of 10 /min (S5). Fig. 7 shows TGA diagrams for CuPc, graphite, S/Graphene composite, and Cu/S/Graphene. The weight loss of CuPc occurs in three steps. The first step between 25 and 150 C corresponds to moisture removal. Thermal degradation of CuPc skeleton is initiated in the range of 225e300 C (second step) and the sample is completely oxidized in the range of 325e450 C (third step). The weight loss for graphite, the weight sharply decreases with complete oxidation of carbon. The TGA diagram for S/Graphene composite shows weight loss in three stages. In the first stage, water molecules adsorbed on the surface are removed from 25 to 125 C. The sulfur atoms attached to the graphene surface are removed in the second stage in the range of 125e400 C. Weight loss in the range of 400e500 C is related to complete oxidation of carbon atoms (third stage). The oxidation temperature of carbon atoms decreases relative to graphite sheets due to a decrease in the density of carbon sheets in graphene (which is in good agreement with an increase in sheet spacing calculated by XRD) [33]. The catalyst Cu/S/Graphene shows a thermal behavior between CuPc and the S/Graphene composite. Three distinct weight loss steps are as follows: - Moisture removal with a mild slope in the range of 25e150 C. - Degradation of the CuPc skeleton in the range of 250e400 C. - Removal of sulfur atoms and complete oxidation of carbon atoms in the range of 400e450 C. The thermal behavior indicates the significant thermal stability of the catalyst. HR-TEM, TEM, and SEM micrographs are used to find out differences in density, particle size, and structural morphology. Fig. 8(aec) shows the TEM micrographs (TEM
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Fig. 8 e (a and b) TEM micrographs of catalyst Cu/S/Graphene with different resolutions, (c) HR-TEM image of Cu/S/ Graphene, (d) SEM surface image of graphite, and (e) SEM surface image of catalyst Cu/S/Graphene.
Philips EM 208S) of the catalyst. Several micrographs were taken with different resolutions to compare the size and shape of particles. CuPc nanoparticles are seen as catalytic centers in the carbon substrate in the form of dark points with a size of less than 5 nm. The presence of sulfur atoms in the carbon substrate caused considerable changes in the morphology and order of sheets. The bright background indicates the low density of carbon sheets (monolayer graphene structure). The TEM and HR-TEM micrographs are confirmed through Raman, TGA, and XRD data (S6). Fig. 8 shows the SEM micrographs of graphite (8d) and catalyst Cu/S/Graphene (8e). The graphite particles are larger than 1 mm and the surface is
completely irregular. The SEM micrograph for the catalyst Cu/ S/Graphene indicates a particle size of less than 100 nm. The micrograph also shows the greater roughness and order of catalyst surface than graphite (S7).
Electrochemical evaluation of catalyst Cu/S/Graphene for ORR The catalytic performance of samples for ORR was evaluated through cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on a rotating-disk electrode (RDE) system (Autolab PGSTAT302N). Measurements were made using a
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Fig. 9 e Cyclic voltammograms of CuPc (dash line), Graphite (dot line), and catalyst Cu/S/Graphene (solid line) in O2-saturated 0.1 mol/L KOH solution at a scan rate of 50 mV s¡1. The catalyst loading is ≈ 0.4 mg cm¡2.
conventional three electrode cell with a counter electrode (a carbon glass rod of 2 mm diameter), a reference electrode (Ag/ AgCl) and a working electrode (a carbon glass RDE electrode of 5 mm diameter). Catalyst ink was prepared to deposit a catalyst film on the electrode surface. To this end, 2 mg catalyst powder was dispersed in 450 mL deionized water and 50 mL Nafion solution (5%). The resulting suspension was placed in an ultrasonic bath for 30 min. Twenty microliters of the ink was coated on the electrode surface. The catalyst film was formed by solvent evaporation (catalyst loading z 0.4 mg/ cm2). Oxygen-saturated KOH (0.1 mol. L1) solution was used as an electrolyte. CV and LSV diagrams were plotted in the range of 0.02e1.18 V vs. a reversible hydrogen electrode (RHE) with a scan rate of 50 and 10 mV s1, respectively. The electrolyte temperature was adjusted at 25 ± 1 C. The background current density was obtained at nitrogen-saturated KOH solution. CV is used to investigate species and electrochemical reactions. Fig. 9 shows CVs for CuPc, graphite, and the catalyst Cu/S/Graphene (S8). CuPc and graphite lack a significant catalytic activity for ORR (Table 3). The electrical conductivity of CuPc is also insignificant [34]. The catalyst Cu/S/Graphene shows electrochemical activity in two ranges: (i)
electrochemical activity in the range of 0.34e0.72 V which is attributed to Cu(II)eN4/Cu(I)eN4 where copper complexes participate in high-potential redox reactions [35], and (ii) an excellent catalytic activity for ORR at 0.85 V. The broad peak observed in the range of 0.3e0.7 V is assigned to the overlapping reduction peaks of electrochemical activities and high current density in the Cu/S/Graphene bed. Linear sweep voltammetry is a sensitive method for the quantitative determination of electrochemical activities. LSV diagrams were plotted at different rotating rates (200e2200 rpm) for quantitative evaluation of the catalyst Cu/ S/Graphene for the ORR. The onset (EOnset) and half-wave
Table 3 e Comparison of current density and catalytic activity of CuPc, graphite, and Cu/S/Graphene. The catalyst loading is ≈ 0.4 mg cm¡2. Sample CuPc Graphite Catalyst Cu/S/Graphene
|J| (mA.cm2)
Activity for ORR
0.3 1.3 5.5
Insignificant @ 0.85 V e Excellent @ 0.85 V
Fig. 10 e LSVs of catalyst Cu/S/Graphene @ different rotating rates in O2-saturated 0.1 mol/L KOH solution at a scan rate of 10 mV s¡1. The catalyst loading is ≈ 0.4 mg cm¡2.
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Fig. 11 e K-L plots of Cu/S/Graphene for ORR in O2saturated 0.1 mol/L KOH solution. Scan rate: 10 mV s¡1. Current densities were measured at 0.2 Ve0.7 V (vs. RHE).
potentials (E1/2) for ORR were respectively observed at 1.09 and 0.89 V (Fig. 10). The obtained potentials are comparable with those for the conventional platinum reference electrode (S9) [36]. The number of electrons transferred is exactly calculated to identify the ORR mechanism. Koutecky-Levich (KeL) equation was used to calculate the number of electrons transferred at different over-potentials [36]. 1 J1 ¼ J1 L þ JK ¼
qffiffiffiffiffiffiffiffi pffiffiffi 1 1 3 þ ; B ¼ ð0:201Þ:n:F:CO2 : D2O2 :1 6 v 0:5 Bu JK
(1)
The K-L diagrams for the catalyst Cu/S/Graphene were plotted in the range of 0.1e0.7 V using Eq. (1) (Fig. 11). The plots are linear and the number of electrons transferred can be exactly calculated from the slope of lines. The linear plots are classified into two groups (Table 4). The first group are plotted in the over-potential range of 0.35e0.7 V where the number of electrons transferred is very close to 4. Only two-electron series and parallel ORRs occur in this range. The number of electrons transferred was 3.98 indicating over 99% contribution of the 4-electron ORR pathway. The second group are plotted in the range of 0.1e0.35 V where the number of electron transferred is greater than 4 (n ¼ 4.16). Reduction of oxygen and copper atoms occurs simultaneously and
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Fig. 12 e Calculated Tafel slope of catalyst Cu/S/Graphene @ 1800 rpm for ORR in O2-saturated 0.1 mol/L KOH solution. The catalyst loading is ≈ 0.4 mg cm¡2. Scan rate: 10 mV s¡1.
independently in this range so that over 95% of electrons are transferred through a 4-electron ORR pathway. The Tafel slope can be used to evaluate the electrocatalytic activity of the sample (Fig. 12). The Tafel slope is a function of the diffusion-corrected kinetic current density which determines the mechanism and controller of reaction kinetics. The Tafel slope for the platinum catalyst varies from 60 to 120 mV.dec1. The corresponding Tafel slope for non-precious metal catalysts (NPMCs) in alkaline media varies from 30 to 130 mV.dec1 [37]. Large Tafel slopes indicate the lack of formation of metal oxide species on the surface of catalytic centers. The metal/
Table 4 e The number of electron transferred of Cu/S/ Graphene in the over-potential range of 0.1e0.7 V vs. RHE. E (V) 0.1 0.2 0.3 0.4 0.5 0.6 0.7
n 1
V |J | V |J1| V |J1| V |J1| V |J1| V |J1| V |J1|
¼ ¼ ¼ ¼ ¼ ¼ ¼
6754.8 6727.1 6724.9 7039.5 7032.1 7029.7 7040.7
1/2
u -47.781, u1/2-45.528, u1/2-45.445, u1/2-52.321, u1/2-51.224, u1/2-50.465, u1/2-49.880,
2
R R2 R2 R2 R2 R2 R2
¼ ¼ ¼ ¼ ¼ ¼ ¼
0.99) 0.99) 0.99) 0.99) 0.99) 0.99) 0.99)
4.15 4.16 4.17 3.98 3.98 3.98 3.98
Fig. 13 e ORR polarization plots of the catalyst Cu/S/ Graphene before and after 10,000 potential cycles between 0.6 V and 1.1 V (vs. RHE) in O2-saturated KOH solution. Scan rate: 10 mV s¡1.
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metal oxide system directs ORR through a 2-electron pathway. Two Tafel slopes with a slight difference are observed for the catalyst Cu/S/Graphene in the KOH medium. The Tafel slopes of 128.8 mV.dec1 and 112.6 mV.dec1 were calculated for high and low over-potentials, respectively. A large Tafel slope (>100 mV.dec1) indicates the dominance of the 4-electron ORR pathway over the 2-electron pathway. In other words, such a high Tafel slope for catalyst Cu/S/Graphene indicates easy protonation of O2 on the surface of catalyst; however, protonation of O2 is the kinetics-control step of ORR on the active sites. The difference in the Tafel slope (16.2 mV.dec1) at low over-potentials is attributed to the overlap of Cu (II) reduction reactions with ORR. Electrochemical durability is among basic challenges for the use of electrocatalysts. This parameter determines economic evaluations and limitations in the use of this type of catalysts. The electrochemical durability of the catalyst Cu/S/Graphene was examined in the range of 0.6e1.1 V at a scan rate of 50 mV s1 (Fig. 13). The electrochemical durability of catalysts is usually reported after 4000 cycles. The half-wave potential for the catalyst Cu/S/Graphene after 5000 and 10000 cycles was displaced by 6 and 8.2 mV, respectively, which are lower than those reported in the literature [38]. The catalyst showed reasonable electrochemical durability.
Conclusions The graphene surface was modified by a low-temperature method using sulfur and copper atoms to obtain a nonplatinum catalyst based on copper and graphene. The very active catalyst with ultra-low content of copper atoms (z2%) showed an onset potential of 1.09, a half-wave potential of 0.89 V, a high selectivity (n z 3.98), and excellent electrochemical durability (DE1/2 ¼ 8.2 mV.dec1 @ 10000th cycle) for ORR. The outstanding performance of the catalyst for ORR is related to the unique stoichiometric structure while preserving the initial framework and properties of precursors. The lack of pyrolysis at high temperature prevents the formation of a random structure. A large current density was obtained through modifying GO surface by sulfur atoms at a low temperature which can be an interesting feature in a wide range of applications.
Acknowledgments We thank the staff of Institute of Nanotechnology (Assistant Professor. Dr. Habibi and Dr. Sehatnia) for their assistance with the electrochemical discussion and Associate Professor. Dr. Keshipour at Urmia university for the discussion on RAMAN and FT-IR results. The authors gratefully acknowledge the support of the Iran National Science Foundation: INSF.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.08.018.
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