Nanostructured Cu-CGO anodes fabricated using a microwave-assisted glycine–nitrate process

Journal of Physics and Chemistry of Solids 98 (2016) 91–99

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Nanostructured Cu-CGO anodes fabricated using a microwave-assisted glycine–nitrate process Shabana P.S. Shaikh a,c, Mahendra R. Somalu a,n, Andanastuti Muchtar a,b a

Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Department of Mechanical and Materials Engineeering, Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia c Department of Physics, University of Pune, Pune 411007, Maharashtra, India b

art ic l e i nf o

a b s t r a c t

Article history: Received 30 May 2016 Accepted 27 June 2016 Available online 27 June 2016

This work reports a study of nanostructured copper-doped gadolinium cermet (Cu-CGO) composite anodes prepared via conventional synthesis (CS) and microwave-synthesis (MS) involving the glycine– nitrate process (GNP). A detailed investigation on the mechanical properties, electrical conductivity and electrochemical performance of prepared Cu0.5(Ce0.9Gd0.1)0.5O2
δ anodes is included. The prepared samples were characterized by techniques, such as XRD, EDX, SEM and electrical characterizations. After reduction in 10% H2 and 90% N2, the DC conductivities of the Cu-CGO anodes prepared via CS-GNP and MS-GNP are found to be 5.43  103 and 1.09  104 S cm
1 at 700 °C, respectively. The electrochemical performances of the spin-coated anode symmetrical cells sintered at 700 °C are evaluated at cell operating temperatures of 600, 700 and 800 °C. The lowest area specific resistance (ASR) values for the CuCGO/CGO/Cu-CGO symmetrical cells prepared via the MS-GNP route at operating temperatures of 600, 700 and 800 °C are found to be 0.34, 0.71 and 1.10 Ω cm2, respectively. The as-prepared (via MS-GNP) CuCGO anode exhibits excellent electrical and electrochemical performance consistent with the uniform nanostructured morphology compared with the anode prepared via CS-GNP. & 2016 Elsevier Ltd. All rights reserved.

Keywords: SOFC Cu-CGO Conductivity Area specific resistance Activation energy

1. Introduction Solid-oxide fuel cells (SOFCs) are environmental friendly electrochemical devices that convert chemical energy directly into electrical energy from an extensive range of fuels (e.g., hydrogen and methane) and emit minimal amounts of harmful gases. Most SOFCs are fabricated using the conventional yttrium-stabilized zirconia (YSZ) electrolyte, which is sandwiched between a porous nickel/YSZ (Ni/YSZ) anode electrode and a porous lanthanum strontium manganite cathode electrode material [1,2]. These SOFCs generally operate at temperatures greater than 800 °C for optimal performance. The anode is one of the main components of SOFCs for oxidation of fuel and conduction of electrons via an external circuit. Numerous techniques such as mechanical mixing [3–5], solidstate reaction [6], sol-gel [7], co-precipitation [8], Pechini methods [9], citrate-nitrate combustion [3,10], and polymer organic complex synthesis [11] have been proposed for production of SOFC anode composite powders. However, these methods n

Corresponding author. E-mail address: [email protected] (M.R. Somalu).

http://dx.doi.org/10.1016/j.jpcs.2016.06.016 0022-3697/& 2016 Elsevier Ltd. All rights reserved.

generally require further calcination processes at relatively high temperatures (i.e., 600–800 °C) to remove water, by-products and other impurities. This high-temperature calcination process induces grain growth and particle agglomeration, which in turn reduce the triple-phase boundary (TPB) length during fabrication of the cermet anodes. Thus, the calcination temperature should be reduced or eliminated to reduce grain growth and particle agglomeration. Alternatively, conventional powder calcinations can be performed using a microwave furnace to control grain growth or particle agglomeration [12,13]. Kawada et al. [10] and Pelosato et al. [14] also showed that this synthesis technique significantly improves the performance and stability of SOFC electrodes by enhancing the mechanical and microstructural properties of the prepared powder. Electrochemical performance can vary due to microstructural characteristics such as grain growth, porosity, particle-to-particle contact, and distribution of Cu2 þ particles in the CGO matrix [10]. In this study, the microwave-assisted glycine–nitrate process (MS-GNP) was used as an advanced synthesis technique to synthesize CuO–CGO anode composites and was compared with the conventional synthesis technique. CuO–CGO was selected because this type of anode has received minimal attention for low-

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temperature SOFC applications. CuO–CGO anode materials exhibit high electronic conductivity, which is an essential requirement for an anode in SOFC applications. The Ni-CGO anode exhibits degradation of electrochemical performance due to the deposition of carbon. In contrast to the Ni-based anodes, Cu-based anodes do not support coke formation, and Cu is less expensive than Ni [15]. Furthermore, although the melting point of Cu is 1024 °C, CuO– CGO anodes require a low sintering temperature. MS-GNP is generally more advantageous than the conventional synthesis with the glycine–nitrate process (CS-GNP) [16]. Microwave-assisted techniques have been applied for synthesis of various advanced nanostructured ceramics [17]. In microwave synthesis, heating of molecular materials is accelerated by microwave irradiation in contrast to the conventional heating process [18]. However, the ability of the materials to convert the electromagnetic energy absorbed from microwave radiation into heat energy depends on the dielectric properties (dielectric loss factor and dielectric constant) of the materials [19–21]. The dielectric constant (ε′) is related to the ability of a material to absorb microwaves, and the dielectric loss (ε′′) factor represents the ability of a material to convert microwave energy into heat energy. The susceptibility of a material to microwave energy is generally high if the material shows a high dissipation factor (ε′′/ε′) [19–21]. In MS-GNP, microwaves generate heat inside the bulk reactant materials with a minimum temperature gradient by absorbing electromagnetic waves in the microwave oven. The heat generated is ascribed to the fast kinetic motion of dipoles in the reactant molecules [22]. Ceramics possess excellent dielectric properties because they generally oppose the translational motion of electrons by inducing loss and attenuation of the electric field. The loss of the electric field generates heat energy in a highly compact manner within the bulk material in a microwave oven, which volumetrically enhances the heating of ceramic anode materials [22,23]. However, the temperatures of the samples inside a microwave oven cannot be monitored using a thermocouple, unlike in a conventional furnace, but the power can be adjusted in the form of Watts supplied. Powdered metal composite reactants are predicted to be good absorbers of microwaves for effective uniform heating to produce powders with ideal nano-sized particles and notably low grain growth [16,24]. Thus, the MS-GNP technique has attracted much attention for synthesis of nanostructured anode materials with a uniform crystalline microstructure for SOFC application. These materials are advantageous for SOFCs because of their unique properties, which include short processing time, fast heating rate, minimal sintering time, low processing temperature, high fuel conversion efficiency, and reduced power requirement compared with the conventional techniques. Consequently, the structural, mechanical, and electrical properties of nanostructured anode materials prepared via MS-GNP are more significantly improved. In addition, the proposed technique eliminates the brittle intermetallic particles in the synthesized powders caused by rapid heating [24]. In the current study, nanostructured CuO–CGO anode powders were prepared using the MS-GNP and CS-GNP techniques, and the properties of the synthesized powders were evaluated, such as crystalline size. The mechanical properties, electrical conductivity, and electrochemical performance of the anodes fabricated using powders synthesized via both techniques were determined and compared. In addition, the microstructures of the fabricated anodes were investigated and correlated with the properties of the anodes.

2. Experimental procedures 2.1. Synthesis of CuO-CGO anode powder using MS-GNP and CS-GNP The Cu0.5(Ce0.9Gd0.1)0.5O2
δ (CuO–CGO) anode material was synthesized via the MS-GNP and CS-GNP techniques. Glycine was used as the fuel for combustion, and nitrate functioned as an oxidant during the synthesis. Stoichiometric amounts of the starting precursors of Cu(NO3)3  3H2O (Sigma-Aldrich, 499%), Ce(NO3)3  6H2O (Sigma-Aldrich, 4 99%), and Gd(NO3)3  6H2O (Sigma-Aldrich, 4 99%) were weighed based on the required stoichiometric proportions using an electronic balance (70.1 mg, Mettler 163AE), and the compounds were dissolved in 100 mL deionized water. The dissolved solution was stirred for 2 h on a hot plate to obtain a homogeneous solution. After 3 h of continuous stirring, glycine (Sigma-Aldrich, 99.0%, glycine-to-nitrate molar ratio of 1:1.5) was added to the nitrate solution. In the CS-GNP route, this solution was stirred for 4 h at 400 °C. The solution turned into a viscous gel that spontaneously charred to form a foamy ash. The as-prepared powder was subsequently calcined at 700 °C for 5 h in a conventional laboratory-made muffle furnace to remove any unburned residuals and impurities [16]. In the MS-GNP technique [11,12,16], the nitrate solution with glycine was stirred for 2 h at 100 °C to obtain a viscous gel. This viscous gel was placed in a microwave oven (IFB-SC2) for heating by setting the power at 100 W (P-HI¼ 100 W) for 15 min. As soon as the viscous gel was placed in the microwave oven, the gel simmered and burned completely within 2–3 min. The final product took the form of foamy gray-colored ash. The frothy solid powder was mixed and mashed into fine powder, followed by calcination at 700 °C for 2 h in a high-temperature microwave furnace (Phoenix CEM, Microwave Technology, USA). 2.2. Preparation and characterization of CuO–CGO pellets CuO–CGO anode pellets were prepared using the powders synthesized in Section 2.1. Anode pellets with a diameter of 9 mm and thickness of 1 mm to 2 mm were prepared by uniaxially pressing the CuO–CGO powder using a circular stainless steel die-punch (Specac, UK) and a hydraulic press with an applied pressure of 4 t/ cm2. The pellets prepared using the CS-GNP synthesized powders were sintered at 900 °C for 4 h in a laboratory-made conventional muffle furnace. In contrast, the pellets prepared using the MS-GNP synthesized powders were sintered at 900 °C for 2 h in a microwave furnace (Phoenix CEM, Microwave Technology). The crystallinity of the fabricated CuO–CGO anodes was examined via X-ray diffraction (XRD, PANalytical X’Pert Pro, Phillips, Holland) using Cu Kα radiation (α ¼0.15418 nm). The scanning range was varied from 20° to 80° with a step size increment of 0.02° and a counting rate of 5 s per scanning step, and the data were refined using X’pert High Score plus software. The density of the sintered pellet was calculated using the Archimedes principle and a Mettler Toledo balance with a density kit. The morphology of the fabricated pellets was characterized via secondary scanning electron microscopy (SEM, JEOL JSM-6380A). The elemental analyses of the pellets were characterized using an energy dispersive X-ray (EDX) analyzer (FEI Quanta 00 field) coupled with an emission electron microscope (FEI-North America NanoPort, Hillsboro, Oregon, USA). The hardness of the sample was measured using the Vickers indentation technique with an HMV-2 microhardness tester (Shimadzu, Japan) [16]. 2.3. Measurement of the DC electrical conductivity of CuO–CGO and Cu-CGO anodes The sintered CuO–CGO pellets were sputtered with platinum

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thin film using a DC sputtering unit (HINDHIVAC, Bangalore) to achieve good ohmic contact during the DC conductivity measurement of the samples. The as-prepared samples were spring loaded in a ceramic high-temperature sample holder (Amel, Italy) and heated to 700 °C. The resistance of the samples was measured via the four-probe technique as a function of temperature ranging from 700 °C to 400 °C using a computer-controlled current source (Keithley 6221) and a nano-voltmeter (2182A). The similar procedures and temperature range were employed to measure the conductivity of reduced Cu-CGO pellets. However, prior to the measurement, the samples were reduced for 2 h in a mixture of 10% H2 and 90% N2 at 700 °C. Further details of this measurement can be found in [16]. 2.4. Preparation of CuO–CGO symmetrical cells Symmetrical cells of CuO–CGO/CGO/CuO–CGO were prepared via spin coating of CuO–CGO slurry on both flat surfaces of Ce0.9Gd0.1O2-δ (CGO) pellets. The CuO–CGO slurry was prepared by mixing 1 g ground Cu0.5(Ce0.9Gd0.1)0.5O2-δ powder (prepared via MS-GNP) with 3 wt% (poly vinyl buteral) binder, sodium-free corn oil, and ethyl methyl ketone. The as-prepared mixture was thoroughly mixed and homogenized using a Pulverisette-6 ball mill (Fritsch Germany) for 1 h at 300 rpm. The ball-milling media consisted of 35 tungsten carbide balls with a diameter of 10 mm and an 80 mL tungsten carbide bowl. The CGO nanopowders prepared via CS-GNP and MS-GNP were similarly pressed to obtain pellets with a thickness of 12 mm. These electrolyte pellets were sintered at 1400 °C for 2 h in a microwave furnace (Phoenix CEM, Microwave Technology) to obtain dense electrolytes. The prepared CuO–CGO slurry was spin coated on both flat surfaces of the sintered CGO pellets at 3000 rpm for 1 min using a spin coater (Millmann, India). The symmetrical cells were initially baked in a microwave oven at 80 W to burn out the organic binders, followed by sintering at 700 °C for 3 h in the microwave and the conventional furnace. The conventional- and microwave-sintered cells were designated as cell-600, cell-700, and cell-800 with respect to the operating temperature for electrochemical impedance spectroscopy. 2.5. Electrochemical performance of Cu-CGO/CGO/Cu-CGO symmetrical cells AC impedance measurements were performed using an electrochemical interface and impedance analyzer (Solartron SI 1287) coupled with a computer-controlled frequency response analyzer (Solartron1255B FRA) over the frequency range from 0.01 Hz to 1 MHz under an applied voltage amplitude of 50 mV. These measurements were conducted using fabricated symmetrical cells of Cu-CGO/CGO/Cu-CGO in the temperature range from 600 °C to 800 °C under a mixture of 10% H2 and 90% N2 gas atmosphere. Prior to impedance measurement, the spring-loaded CuO–CGO/ CGO/CuO–CGO cell was heated to 700 °C, followed by reduction for 2 h under a mixture of 10% H2 and 90% N2 in order to obtain reduced Cu-CGO/CGO/Cu-CGO symmetrical cells. The measured impedance data at three different operating temperatures (600, 700, and 800 °C) were analyzed using Scribner advanced software for electrochemical research and development, as explained by Blennow et al. [25].

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prepared via the CS-GNP and MS-GNP techniques were crushed and finely ground in an agate mortar and characterized by X-ray diffractometer. The XRD patterns of CuO–CGO anodes prepared using CS-GNP and MS-GNP are shown in Fig. 1, respectively. The obtained XRD patterns were analyzed with the aid of X’Pert High Score computer software and indexed with the hkl plane. The XRD peaks were analyzed, smoothed, and backgrounded, and unwanted Cu-Kα peaks were removed. The XRD patterns clearly show peaks corresponding to the (1 1 1) hkl plane with a cubic fluorite structure of mixed oxides of CuO and CGO in the fabricated anodes. The peaks also confirm that the prepared CuO–CGO anodes exhibit a cubic fluorite structure at room temperature. All of the diffracted lines are in good agreement with the Joint Committee for Powder Diffraction Standard data of CuO (File no. 00002-1041) and CGO (File no. 01-075-0161). The XRD patterns also indicate that the MS-GNP synthesized samples have higher nanocrystallinity than those prepared using the CS-GNP synthesized powder with greater peak broadening. In addition, XRD analysis confirms that the CuO–CGO anodes possess a uniform distribution. The Debye-Scherer relation was used to calculate the crystallite sizes of CuO–CGO anodes from the obtained XRD patterns using the following equation [26].

t=

0. 9 λ βcosθ B

(1)

where t is the average crystallite size, λ is the wavelength of the Cu Kα radiation, β is the calibrated full width at half maximum of the peaks for the CuO-CGO anodes (in radians), and θB is the Bragg angle. Additionally, β is obtained using the following equation [26].

β 2 = βm2 − βs2

(2)

where βm and βs are the measured and standard FWHM values of the diffracted lines, respectively. The lattice parameters and cell volume corresponding to the CGO and CuO phases were calculated from the XRD pattern using Unit Cell computer program [26], and the values are found to be 0.4613 and 0.4773 nm with cell volumes of 9.7 and 10.9 nm for the cubic fluorite structures of CuO and CGO, respectively. X’Pert High Score Plus software based on the Debye-Scherer formula was used to determine the mean particle size of the fabricated CuO–CGO samples [26]. Data from the CuO–CGO anodes prepared via CSGNP and MS-GNP are shown in Table 1. The results are consistent

3. Results and discussion 3.1. XRD characterization Sintered

samples

of

CuO0.5(Ce0.9Gd0.1)0.5O2
δ

(CuO–CGO)

Fig. 1. XRD patterns of sintered CuO–CGO anode prepared by (a) CS-GNP and (b) MS-GNP.

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Table 1 Crystallite size, sintered density, mechanical strength and porosity of CuO–CGO anodes prepared using powders synthesized by CS-GNP and MS-GNP. Method of Synthesis

Crystallite size, Cs (nm)

Sintered density d (g/cm3)

Mechanical hardness (HV)

Porosity (%)

CS-GNP MS-GNP

38 29

2.531 1.862

30.41 33.85

33.21 41.20

*CS-GNP represents conventional synthesis glycine–nitrate process and MS-GNP represents microwave synthesis glycine–nitrate process.

with previously reported data [11]. The CuO–CGO composite anodes prepared using MS-GNP synthesized powder display higher mechanical hardness with a smaller crystallite size compared with those prepared using the CS-GNP synthesized powder. However, the sintered density of anodes prepared using the MS-GNP synthesized powder is lower than those prepared using CS-GNP

synthesized powder because of the slightly higher porosity of the anode prepared using the former technique. 3.2. Microstructural characterization Fig. 2 shows the microstructures of the sintered CuO–CGO anodes and reduced Cu-CGO anodes prepared using powders synthesized via CS-GNP and MS-GNP, respectively. Substantial structural differences can be observed in the scanning electron microphotographs of the sintered CuO–CGO and reduced Cu-CGO anodes. The figures demonstrate that the particles are aggregated with adequate porosity and homogeneous dispersion. Therefore, the anode forms a spongy nanostructure. However, the microstructure of the sintered and reduced anodes indicated that the grain size of the anode prepared using the CS-GNP synthesized powder is larger with higher agglomeration compared with the one prepared using the MS-GNP synthesized powder. This result is

Fig. 2. SEM images of (1) sintered CuO–CGO anodes and (2) reduced Cu-CGO anodes prepared using powders synthesized by (a) CS-GNP and (b) MS-GNP.

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Fig. 3. (1) Cross sectional view and (2) surface view of symmetric cells of CuO–CGO/CGO/CuO–CGO for (a) cell-600, (b) cell-700 and (c) cell-800.

in agreement with the larger crystallite size of the CuO–CGO powder prepared using CS-GNP shown in Table 1. It is important to note that microwave heating offers an ultra-fast method for ceramic preparation at an ultra-high heating rate. The grain size of ceramics obtained via this method is smaller than that sintered by the conventional method. Hence, the MS-GNP material results in a powder with a finer microstructure than that of CS-GNP. The microstructure of the CuO–CGO/CuO–CGO symmetrical cells prepared by MS-GNP was studied due the fact that the microwaveassisted synthesis technique exhibited enhanced structural and electrochemical performance compared with that of the conventional synthesis. Fig. 3 shows the cross-sectional and surface view images of the symmetrical cells prepared by MS-GNP for cell-600, cell-700 and cell-800. From the cross-sectional view, it is clear that the thicknesses of cell-600, cell-700 and cell-800 are 1.63, 3.89 and 5.10 mm, respectively, and the thickness of the electrolyte CGO is on the order of 2 mm. The SEM images exhibit well-connected nanosized grains in the fabricated anode layer. The cross-sectional and surface views of the symmetrical cells show that the nanostructured particles of cell-700 and cell-800 are more agglomerated with less porosity compared with those of cell-600 due to the

increased operating temperature of the cell. The improved particle connectivity is important for improving the TPB in the fabricated anode, which consequently improves the electrochemical performance of the anode (see Section 3.3). The uniform and fine microstructures can be attributed to the nanosized distribution of the CuO and CGO particles. 3.3. Elemental analysis Fig. 4 and Table 2 show the EDX data and summary of the elemental distribution of CuO–CGO anode, respectively. The SEM image in Fig. 6 offers the information on the surface structures of freshly prepared materials and the elemental composition of the CuO–CGO anode. The EDX spectra of the CuO–CGO anode prepared using the powder synthesized by MS-GNP technique reveal the elemental evidence of isolated Cu and Gd particles in the cerium phase with good crystallite size and a fine-grained particle structure (Fig. 4). In a previous study [27], microphotographs showed highly compact particles with a uniform dispersion of CuO and CGO constituents. The EDX analysis demonstrates that the performance

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Fig. 6. Conductivity of Cu–CGO anodes after reduction prepared using powder synthesized by CS-GNP and MS-GNP. Fig. 4. Energy dispersive X-ray (EDX) spectra of CuO–CGO anode. Table 2 Elemental composition of CuO–CGO anode composite from EDX analysis. Element wt% Ce Gd Cu Total

55.9 9.5 34.6 100

of the CuO–CGO cermet anode is significantly enhanced such that these anodes could be distinguished by their microstructures, which strongly depend on CuO particles. This result confirms the successful preparation of CuO–CGO cermets as alternative anodes with appropriate microstructures [11]. 3.4. Electrical performance of CuO–CGO and Cu-CGO anodes Four-probe DC conductivity testing was performed to measure the DC conductivity of the CuO–CGO and Cu–CGO anodes prepared

using powders synthesized via both the CS-GNP and MS-GNP techniques. Figs. 5 and 6 show the conductivity of CuO–CGO and reduced Cu–CGO anodes, respectively. The reduction of Cu2 þ to Cu þ , Ce4 þ to Ce3 þ and Gd4 þ to Gd3 þ improve the electronic conductivity of reduced Cu-CGO anode. The figures clearly indicate that the DC conductivities of the CuO–CGO and Cu–CGO anodes are higher for anodes prepared using the MS-GNP synthesized powder compared with those prepared using the CS-GNP technique. This improved conductivity result is consistent with previously published data [28] and is expected due to the finer grain size with reduced particle agglomeration in the powder synthesized by the MS-GNP technique, as illustrated in Fig. 2. Table 3 summarizes the differences in the DC conductivities and activation energies of CuO–CGO and reduced Cu-CGO anodes prepared using powders synthesized via the MS-GNP and CS-GNP techniques. The CuO–CGO and reduced Cu-CGO anodes prepared using the MS-GNP synthesized powder exhibited the maximum DC conductivities and relatively minimal activation energies compared with those prepared using the CS-GNP synthesized powder due to the smaller grain size with reduced particle agglomeration in the powder synthesized by the MS-GNP technique, as illustrated in Fig. 2. These results are consistent for all anode samples before and after reduction. In addition, similar results have also been reported in a few studies [14,25,27,29]. Importantly, all of the samples followed the Arrhenius law and exhibited a substantial increase in the conductivity with changes in the preparation route before and after the reduction, as shown in Figs. 5 and 6, consistent with the structural morphology, as described previously in the illustrated SEM images. These results are consistent with the data in our previous report [11]. Thus, microwave synthesis enhances the electrical performance of the Cucermet anode materials for IT-SOFCs. Table 3 DC conductivity and activation energy of the CuO–CGO and Cu–CGO anodes prepared through CS-GNP and MS-GNP.

Fig. 5. Conductivity of CuO–CGO anodes before reduction prepared using powders synthesized by CS-GNP and MS-GNP.

Anode preparation method

Conductivity of CuO–CGO at 700 °C, rBR (Scm
1)

Conductivity reduced Cu– CGO at 700 °C, rAR (Scm
1)

Activation energy of CuO–CGO, Ea,BR (eV)

Activation energy of reduced Cu– CGO, Ea,AR (eV)

CS-GNP MS-GNP

4.4  103 5.18  103

5.43  103 1.09  104

0.96 0.78

0.61 0.36

*BR represents before reduction and AR represents after reduction.

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Fig. 7. Impedance spectra of symmetrical cells of Cu-CGO/CGO/Cu-CGO for (a) cell-600, (b) cell-700 and (c) cell-800 prepared by (1) CS-GNP and (2) MS-GNP.

3.5. Electrochemical performance of reduced Cu-CGO anodes Electrochemical impedance spectroscopy (EIS) is extensively used to study the mechanism of the electrochemical reaction at the electrode and electrolyte interface in the form of charge transfer, bulk resistance of the electrode, and capacitances. The catalytic activity, triple phase boundary (TPB) density, charge transfer reaction, surface diffusion, and adsorption/dissociation of ions at the anode/electrolyte interface are general parameters that determine the effective electrochemical performance of the CuCGO/CGO/Cu-CGO symmetrical cells at operating temperatures of 600, 700 and 800 °C [25]. From the electrochemical impedance spectra for the anode symmetrical cells (i.e., cell-600, cell-700 and cell-800 prepared by the CS-GNP and MS-GNP techniques), it is observed that all Nyquist plots for Cu-CGO/GDC/Cu-CGO cells, as depicted in Fig. 7, reveal two incomplete semicircles. The first

semicircle, corresponding to the high frequency region, is responsible for the charge transfer process (RE1) while the second semicircle, corresponding to the low frequency region, is attributed to the gas diffusion and transport process (RE2). The total electrode polarization resistance (Rp) can be expressed using Eq. (3), which is obtained by fitting the impedance spectra using the Z-View software introduced by Scribner Association Inc.

R p = RE1 + RE2

(3)

The electrochemical performance of Cu-CGO/CGO/Cu-CGO symmetrical cells prepared by CS-GNP and MS-GNP significantly depends on the synthesis technique and its structural morphology. The better electrochemical performance of the anode prepared using powders synthesized by MS-GNP is primarily ascribed to a smaller grain size with better particle size control compared with that synthesized by the CS-GNP technique, as confirmed from

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Table 4 Area specific resistance of Cu-CGO/CGO/Cu-CGO symmetrical cells prepared by CSGNP and MS-GNP. ASR

CS-GNP Ω cm2

MS-GNP Ω cm2

Cell-600 Cell-700 Cell-800

1.86 2.32 4.91

0.34 0.71 1.10

A 2

The authors gratefully acknowledge the National University of Malaysia and the Ministry of Education of Malaysia for support via Grants GUP-2015-038 and FRGS/1/13/SG06/UKM/01/1.

References

Figs. 2 and 3, respectively. Hence, the MS-GNP-prepared material has an improved particle network with enhanced TPBs in the fabricated anode. Therefore, the MS-GNP technique can be viewed as one of the most promising and time-saving techniques for the synthesis of SOFC anode materials. The area specific resistance (ASR) can be calculated for each cell using the following equation:

ASR = RP ×

Acknowledgment

(4)

where Rp and A are the total area resistance and surface area of the electrode, respectively. Cell-600 showed the highest electrochemical performance and the lowest area specific resistance compared with cell-700 and cell-800 as summarized in Table 4. This is expected due to partial reduction of ceria from C4 þ to Ce3 þ under reducing environment (hydrogen) which gives rise to electronic conduction in the CGO electrolyte [30], and subsequently increases the total area specific resistance of the symmetrical cells. This partial reduction is expected to increase with increase in operating temperature. For example, the reported OCV values of cells fabricated using CGO electrolyte at 600 and 650 °C were 0.67 and 0.62 V, respectively [31]. The reduced OCV value with increasing temperature can be related to increased electronic leakage in the CGO electrolyte under reducing environment. Thus, Cu-CGO anodes are suitable to be used effectively with CGO electrolyte at low operating temperature ( o800 °C).

4. Conclusions MS-GNP is an effective method for obtaining mixed ionic and electronically conducting anode materials. In the case of MS-GNP, the volumetric heating ability of microwaves allows for more rapid and uniform heating, decreased processing time, and often enhanced material properties compared with the conventional synthesis. Thus, this process is low cost and offers an outstanding synthesis method that saves energy and time. The highest DC conductivities for Cu-CGO anodes after reduction are 5.43  103 and 1.09  104 S cm
1 at 700 °C with crystallite sizes of 38 and 29 nm for samples prepared by using powders synthesized by CSGNP and MS-GNP, respectively. The ASR values of Cu-CGO/CGO/ Cu-CGO symmetrical cells prepared using powders synthesized by CS-GNP and MS-GNP at 600 °C are 1.86 and 0.34 Ω cm2, respectively. The lower ASR value for cell prepared using MS-GNP powders is due to a smaller grain size with better particle size control compared with that prepared using CS-GNP powder. The major advantages of MS-GNP in the current study are reduced synthesis time, improved microstructure of fabricated anodes and increased mechanical properties of prepared anodes compared with those prepared using powders synthesized by CS-GNP. In conclusion, the reduced Cu-CGO anodes prepared using MS-GNP powder exhibited excellent electrical and electrochemical performance in addition to uniform microstructure. Thus, MS-GNP is one of the best techniques for preparation of CGO-based composite anode materials for intermediate temperature SOFC applications.

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