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The relationship between impedance and shape on the surface of a SOFC sensor
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 17 (2019) 1921–1930
www.materialstoday.com/proceedings
MRS-Thailand 2017
The relationship between impedance and shape on the surface of a SOFC sensor Suthawee Phaijita*, Montri Sukluenga, Sutham Niyomwasb, Sutida Marthosac, Chainuson Kasagepongsarnd, Mladen Micanovice a*,a
Faculty of Engineering, PSU Energy Systems Research Institute (PERIN), Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand b Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand c Faculty of Science and Industrial Technology, Prince of Songkla University (Surat Thani Campus),Surat Thani 84000, Thailand d Renewable Energy and Environmental Research for Local Community Unit (REERC) Department of Physics Faculty of Science and Technology Surat Thani Rajabhat University 84000, Thailand e Faculty of technical sciences, University of Novi Sad, Serbia
Abstract Solid Oxide Fuel Cells (SOFCs) are a type of power source that can convert hydrocarbons and hydrogen gas to electricity at temperatures of between 600-800°C with a flow rate of 0.2 l/min. A BYCF cathode, GDC10 electrolyte and NiO anode were used for the SOFC system. A Spray Pyrolysis technique was used for the fabrication of the cathode and anode layers which were 5-10 µm in thickness. The Micro-SOFC should be able to detect hydrocarbon gases such as methane, butane, and so forth, which can generate a small electrical signal depending on the quantity of gas. Therefore, the novel design of Micro-SOFCs can be used as a sensor for measuring the quantity of methane. In particular, it can measure the content of methane in biogas after fermentation. This investigation used a novel design of sensor surface with Cat, Square, and Yin-Yang shapes. The impedance showed a relationship between resistivity and ionic transfers in the Micro-SOFC system depending on the surface shapes. The length of the © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Micro-SOFCs; Impedance; BYCF cathode; Biogas; Sensor
* Corresponding author. Tel : +66 95257 4505; fax: +66 74282260 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.
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gaps on the surface between the cathode and anode which were 21mm, 33.5mm, and 20.41 mm respectively affected the impedance value. The Square shape revealed the lowest impedance based on RΩ+Rp values of 3910.00Ω, 2152.30Ω, 1536.40Ω, 1647.30Ω, and 751.52Ω at 600°C, 650°C, 700°C, 750°C, and 800°C, respectively. The Square shape of the Micro-SOFC indicated high signals which varied with the quantity of the methane. Therefore, this investigation can promote the Square shape of Micro-SOFC to be used as a sensor for the detection of methane in biogas.
1. Introduction The development of Solid Oxide Fuel Cells [1, 2] is driven by such factors as their high potential for excellent energy efficiency, fuel flexibility and environmentally friendly properties [3], and this electrical device can convert chemical energy directly into electrical energy. These convert chemical energy in fuel into electricity directly without the involvement of any combustion process [2, 4, 5]. However, the operational SOFCs technology is hindered when exposed to a high temperature in the range of 800-1000°C [1]. Many investigations have been conducted that reduced the operating temperature to an intermediate temperature [4] in the range of 500-700°C, as well as using a thin film of electrolyte with higher ionic conductivity, and the high electrical conductivity of the cathode and anode can reduce the temperature are required [6-9]. Due to development intermediate temperatures for solid oxide fuel cells (IT-SOFCs) materials that can be used at low operating temperatures have been widely investigated. There have been example of pure ceria being operated in low temperatures that were modified in terms of structure to create a significant number of oxygen vacancies to enhance high ionic conductivity of the partial substitution of ce4+ ions with the trivalent cations such as Sm3+, Gd3+, Pt3+, Y3+, La3+ and Nd3+ which creates large a density of oxygen vacancies in ceria lattice results in high ionic conductivity [4, 10, 11]. The small sizes of SOFC systems are currently being researched in terms of their potential application as portable power units for domestic heat and power generation, for example [12, 13]. These systems are also being researched regarding their operation capabilities on a wide variety of fuels such as hydrogen, propane, methane and butane. Kendall and Palin [13] have investigated a Micro-SOFC using a butane/air mix as the fuel, and obtained promising results with the generation of a small amount of electricity. N.M. Sammes [12] have revealed that a Micro-SOFC running on butane is useful under the operation of a micro-tubular SOFC. Direct flame fuel cells have been studied in micro-tubular SOFC operating in under a high temperature without a heating element powered by an electrical current [14, 15]. However, micro fuel cells generated low currents of electricity that were limited in terms of their use in a power source applied to electrical equipment. The micro fuel cells should be able to provide high sensitivity with methane gas. Therefore, micro fuel cells can be a novelty-designed sensor for the measurement of methane that is mixed with the NiO composition at the anode side [16]. Micro SOFCs are usually fabricated as anode supported the system which caused very thin electrolyte film can be deposited on the anode support, thus the ohmic resistance decreased and the cell’s high performance can be increased. Additionally, the ohmic resistance and oxygen ion transfer are indicated by an impedance measurement [17, 18]. Impedance has three semicircles of the electrolyte of SOFC systems in the typical impedance spectra. High-frequency semicircle represented the grain conduction; intermediate-frequency semicircle represented the grain boundary and/or impurity phase contributions to ionic conduction; and lower-frequency semicircle represented ion and electron transfers at the sample surface contacting the electrode. At a high temperature, the semicircle involved with the grain and grain boundary disappears. The reason for this phenomenon is due to the fact that the time constant associated with grain and grain boundary impedances are smaller in amount when compared to electrode processes [19]. In the present study, the composition of BYCF cathode, NiO anode and GDC10 electrolyte created a layer for the micro fuel cell to be a sensor system. This newly-designed sensor used the electrolyte supported and deposited cathode and anode on the electrolyte layer by way of Spray Pyrolysis technique. This investigation created novelty-designed sensor surfaces with Cat, Square, and Yin-Yang shapes. The impedance spectrometer was used to study the relationship between impedance and shape.
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2. Experimental 2.1. Materials and methods A composition of a BYCF (Ba0.054Y0.029Co1.8Fe0.062O2.89) formed a mixture with 5wt% BaO, 3wt% Fe2O3, 2wt% Y2O3 and 90 wt% Co3O4 which was obtained by a method of conventional ceramic powder processing. The cathode was prepared using re-agent grade metal oxide powders from Sigma Aldrich, Germany. And a composite GDC10 electrolyte (10mol% gadolinium doped 90mol% Ceria, Ce0.9Gd0.1O1.95) was also prepared using re-agent grade metal oxide powders from Sigma Aldrich, Germany. A composite anode used NiO powder which was sourced from Fuel Cell Materials USA with a surface area of 2.9 m2/g and particle size of 0.5-1.5 µm. The thermal expansion coefficient (TEC) was controlled in order for it to be close to that of GDC10 electrolyte which is 13.08 x10-6 oC-1 at 900oC[20]. The cathode side was a 50/50 mixture of BYCF and GDC10 and the anode side was also a 60/40 mixture of NiO and GDC10. The chemical powder was ground for 1 hour and then mixed with distilled water, including 10% PVA (polyvinyl alcohol), by weight. The mixture was milled for 24 hours in a cylindrical-capped container with alumina balls as filling using a horizontal rotary ball mill and then dried in an oven at 150°C for 5 hours. The powder was ground again for 24 hours in our in-house-made grinding machine and sieved through a 150 mesh. The fine powder, which now exhibited some residual moisture content, was then pressed into 15 mm diameter and 2 mm thickness pellets using a uniaxial press under 8,534 psi of pressure. Finally, the BYCF+GDC10 pellets were sintered at 1,100°C with a heating rate of 30°C/min for 10 hours under the air.
Fig. 1. Spray Pyrolysis system
The experimental set-up to deposit a thin layer of the BYCF cathode and NiO anode on the GDC10 electrolyte substrate used a Spray Pyrolysis machine. The spray gun was operated at an air pressure rate of 0.82737 MPa and the nozzle was set at a height of 20 cm with a 16 cm spraying diameter. The pellet was placed on a stainless steel substrate heated to 450°C. This enabled crack-free layers to be formed on the 60/40 mixture of NiO
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and GDC10 anode substrate. The spraying time (in seconds) and the deposited thickness is shown in Figure 1. A dilute solution of the mixture for Spray Pyrolysis was prepared as follows: slurry with BYCF and GDC10 powder and water used at a ratio of 50:50 vol% and milled for 24 hours.
H2O
H2 , Air, CH4
Alumina tube
Cat
Square
Yin-Yang
Cathode
Cathode
Cathode
Anode
Anode
Pt wire
Gap Anode Cathode
Surface
Pt-past
Anode
Electrolyte
BCABS-Zr Glass Sealant
Micro-SOFC sensor
Fig. 2. The pellet with the platinum probes was placed inside an in-house designed furnace
A uniaxial press die was used to produce a GDC10 pellet 15 mm in diameter and 3 mm in thickness. Spray pyrolysis was used to deposit the BYCF cathode and NiO anode with 20 µm thicknesses; these depositions used a novel design of sensor surface with Cat, Square, and Yin-Yang shapes. Platinum electrodes as probes were primed with a platinum paste and then pressed to ensure good contact could be made as shown in Figure 3. In the real experiment, the pellet with the platinum probes was placed in an in-house designed furnace as shown in Figure 2. Microstructures were observed using a scanning electron microscope [21]. For impedance measurement and Nyquist plots, Impedance Spectroscopy was carried out using an Analog Device EVAL-AD5933/34EBZ as an impedance analyser in the frequency range 1 Hz - 1 MHz at 1Vrms signal amplitude over temperatures of 30°C to 800°C with 50°C steps. A PROVA 200A PV analyser was used to measure the power density. The experiments used 99.99% hydrogen (H2) or 98.00% methane (CH4) gas, fed in with an air-pump.
Fig. 3. Surface of Micro SOFCs in the shapes (a) Cat, (b) Square, and (c) Yin-Yang.
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3. Result and Discussion 3.1 Microstructure and layer thickness Micro-SOFCs for sensors in this study used an electrolyte layer for the substrate and supported the anode and cathode layers, as exhibited in Figure 4(a). The cathode side contained a 50/50 mixture of BYCF and GDC10 by %weight and used the Spray Pyrolysis technique to deposit on the GDC10 electrolyte on the surface, as shown in Figure 4(b). On the surface of the cathode, there was indication that there was obviously many pin-holes of 20 µm and these pin-holes represented the porosity of cathode surface. The effective transport properties of the SOFC composite structure depend on the microstructural characteristics of the electrode, such as the particle diameter, the porosity and the composition, compatibility with the electrolyte, long-term stability and intrinsic charge-transfer resistance. Thus, having a high porosity is one such property of the material that determines the cathode’s efficiency, which is inversely proportional to its polarization resistance[21, 22]. The properties of the LSCF and BYCF cathodes must be operated for intermediate temperatures SOFCs due to both high electronics and oxygen conductivity and also high activity towards oxygen reduction at low temperatures around 600°C [8, 23]. However, the abundance of pores on the surface of the cathode may use the Spray Pyrolysis technique that is an absolutely physical cathode composition. LSCF has deposits by the Spray Pyrolysis technique that indicate the pore size of 0.4 µm with a nano-particle size of 0.05 µm, which allows for the oxygen diffusion inside the cathode [24] to take place. Therefore, the Spray Pyrolysis is high in performance in terms of generating the porosity in the cathode layer. The layer of the cathode deposited of 40 µm on the electrolyte, as exhibited in Figure 5(b). Both of the two layers clearly show an adhesive layer In between each other. The thin layer of the cathode can reduce the electrical resistivity, high oxygen levels and high ionic transfer rate [25, 26]. Additionally, the anode side contained a 60/40 mixture of NiO and GDC10 that was also deposited on the electrolyte, as shown in Figure 4(c). The surface of the anode side needed a high level of which is the as same as at the cathode side. From this investigation, it was found that pin-holes occurred of 4 µm; this revealed the porosity, however, the number was less than at the cathode side. However, the majority of anodes included NiO, which is frequently in order to stabilize the morphology based on the porosity and thermal expansion. The anode must be a supported reactant and be able to adsorb fuel gas. Diana M. Amaya has reportedly been able to achieve this with a porosity between 20 and 40% porosity of Cu/YSZ [27]. Therefore, the porosity on the anode side is still very important for the transfer of gas and conductivity in SOFCs. The layer of anode deposited of 80 µm on electrolyte as exhibited in Figure 5(c). Both of two layers obviously reveal an adhesive layer together. The thin layer of the anode can reduce the electrical resistivity and high ionic transfer as well as the absorbed hydrogen gas [28]. The electrolyte side was a GDC10 supported anode and cathode, as shown in Figure 4(d). The surface of the electrolyte side indicated there were exactly no pores (zero) which represented a high density.The surface of the electrolyte with a high dense exhibited a grain size of 12 µm lead that was sintered at a high temperature. However, the electrolyte properties must respond to the conducting of the ions between the electrodes for the separation of the gases (from the chemical reaction) and for the internal electrical conduction blocking, driving the electrons to flow through the external circuit[29]. The GDC10 composite represented the Gd3+ doped ceria shows a higher ionic conductivity because of a smaller enthalpy associated between the doped cation Gd3+ and the oxygen vacancy of the host lattice based on ceria. Intermediate and low temperatures of the SOFCs are the best properties for the composition. GDC is one of the materials that can be operated at an intermediate temperature with the electrolyte needing to be near-full in density[30]. Our study focused on the novel design of the SOFC structure for a methane sensor that used the electrolyte for a substrate and deposited the cathode and anode on the electrolyte layers. Between the cathode and anode, a gap of 900 µm was produced, as exhibited in Figure 5(d). Generally, the SOFC structure is fabricated by a triple phase boundary (TPB) such as at the anode, cathode and electrolyte. The ionic transfer directly flows from the cathode to the electrolyte and anode. The TPB generated the ohmic polarization, representing electrical resistance of the electrode and resistance to the flow of ions through the electrolyte. Also, processes in the operational SOFCs can
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significantly contaminate reactant gases which tends to increase the ohmic polarization[31]. On the other hand, in this investigation, the structure of our sensor shows the cathode and anode on the electrolyte that is partitioned with the gap. Therefore, the ion transfer should be hopped from the cathode through to the electrolyte, as shown in Figure 6.
Fig. 4. Scanning electron microscopy (SEM) micrograph of the surface of a Micro-SOFC (a) the Micro-SOFC’s sensor (b) surface of the cathode (c) surface of the anode and (d) surface of the electrolyte.
Fig. 5. SEM micrographs of the cross-section of (a) the Micro-SOFC’s sensor (b) the cathode (c) the anode.
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Fig. 6. The construction of a Micro-SOFC set-up for determining the O2- ion flow.
3.2 Electrochemical Impedance Spectroscopy (EIS) Impedance spectroscopy is a powerful tool used to study the electrical properties of solid electrolytes. The bulk and grain boundary resistance and total ionic conductivity of solid oxide materials use the impedance spectroscopy for investigation based on the SOFC, and the impedance analysis of all the specimens was operated under an intermediate temperature in the range from 600-800°C. Nyquist plot is the complex impedance trend of electrolyte, cathode and anode materials characterized by three successive semicircles. Each semicircle corresponds to higher a frequency related to the grain (bulk) contribution, and the semicircle corresponds to intermediate frequency related to grain boundary contribution and the semicircle that corresponds to a lower frequency is related to electrode contribution [32-35]. Generally, the impedance plot can be an advantage when there is strong impedance on the frequency, as is the case with a capacitance. The log|z| vs, log ω curve can yield values of polarization Rp and ohmic RΩ resistance[36]. The electrical performance of a SOFC is involved with the impedance plot that RΩ represents mainly the ohmic resistance from lead wires and electrolyte GDC10 [37]. The complex impedance plots of all specimens show semicircles with a combination of three contributions such as grain, grain boundary and electrode represented by Rpvalue [37, 38] as shown in Fig.7. The Nyquist plot of the Cat, Square and Yin -Yang sensors at 650°C and 700°C indicated that Yin-Yang is higher than Rp and RΩ is higher than Cat and Square. The gap’s length of the physical Yin-Yang sensor is shorter than others and may only generate low ion conductivity that affects the high RΩ and Rp[38] revealed in Fig.8.
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Z" (Imaginary), Ω
Decreasing frequency max Z"
RΩ
Zʹ (Real), Ω
RΩ+ Rp
Fig.7. Conventional Nyquist plot.
Fig. 8. Nyquist plot of the Cat, Square and Yin -Yang sensors at 650°C and 700°C.
Comparison of Nyquist plot of (a)Cat, (b)Square and (c)Yin Yang at varied temperatures, as shown in Fig 9. Increasing temperatures can affect the decrease of impedance, while Cat, Square and Yin Yang also reduce in terms of impedance when the temperature is increased. The Square shape is lower in impedance at 800oC and the Nyquist plot also represents a more true and clear semicircle compared with the other shapes. The increasing temperature in the impedance test system creates a reduction in impedance and the semicircles which depends on the shift forwards towards the higher frequency side. Grain boundary resistance (Rgb) and grain resistance (Rg) contributions were significantly affected at higher temperatures, therefore, the total resistances (Rt = Rg+ Rgb) of the
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semicircular arcs were intercepted in the electrical properties of the materials[34]. Therefore, all of the impedances of Cat, Square and Yin Yang are resolved Rg and Rgb when there was an increase of temperature. The reduction of impedance involves decreasing Rg and Rb.
Fig. 9. Comparison of Nyquist plot of (a) Cat, (b) Square, (c) Yin-Yang at varied temperatures
4. Conclusions This investigation designed a novel sensor based on a solid oxide fuel cell system using different shapes of a cathode, anode and electrolyte. The novel sensor used a GDC10 electrolyte for the substrate, BYCF cathode, NiO anode, and deposit on the GDC10 electrolyte. The shape of the cathode and anode were in the form of Cat, Square, and Yin Yang which were deposited by the use of a Spray Pyrolysis technique that is able to generate high porosity, with a 18.28 µm thickness of the anode and a 146.73 µm thickness of the cathode. The Square shape of the novel sensor indicated that there is low impedance at high temperatures compared with Cat and Yin Yang shapes. The Square shape had a longer gap length that can have a significant contribution to the transfer rate of ions. Therefore, the Square shape is deemed to be the best and most suitable for use in the application of the sensor. Acknowledgements This research was supported by Innovation’s fund (RDO590769S), Center of Excellence in Materials Engineering, CEME and Travel Funding for conferences (graduate student of Faculty of Engineer), Prince of Songkla University. We thank our colleagues from the Faculty of Engineering; PSU Energy Systems Research Institute (PERIN) who provided their insight and expertise that greatly assisted this research.
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ScienceDirect Materials Today: Proceedings 17 (2019) 1921–1930
www.materialstoday.com/proceedings
MRS-Thailand 2017
The relationship between impedance and shape on the surface of a SOFC sensor Suthawee Phaijita*, Montri Sukluenga, Sutham Niyomwasb, Sutida Marthosac, Chainuson Kasagepongsarnd, Mladen Micanovice a*,a
Faculty of Engineering, PSU Energy Systems Research Institute (PERIN), Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand b Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand c Faculty of Science and Industrial Technology, Prince of Songkla University (Surat Thani Campus),Surat Thani 84000, Thailand d Renewable Energy and Environmental Research for Local Community Unit (REERC) Department of Physics Faculty of Science and Technology Surat Thani Rajabhat University 84000, Thailand e Faculty of technical sciences, University of Novi Sad, Serbia
Abstract Solid Oxide Fuel Cells (SOFCs) are a type of power source that can convert hydrocarbons and hydrogen gas to electricity at temperatures of between 600-800°C with a flow rate of 0.2 l/min. A BYCF cathode, GDC10 electrolyte and NiO anode were used for the SOFC system. A Spray Pyrolysis technique was used for the fabrication of the cathode and anode layers which were 5-10 µm in thickness. The Micro-SOFC should be able to detect hydrocarbon gases such as methane, butane, and so forth, which can generate a small electrical signal depending on the quantity of gas. Therefore, the novel design of Micro-SOFCs can be used as a sensor for measuring the quantity of methane. In particular, it can measure the content of methane in biogas after fermentation. This investigation used a novel design of sensor surface with Cat, Square, and Yin-Yang shapes. The impedance showed a relationship between resistivity and ionic transfers in the Micro-SOFC system depending on the surface shapes. The length of the © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Micro-SOFCs; Impedance; BYCF cathode; Biogas; Sensor
* Corresponding author. Tel : +66 95257 4505; fax: +66 74282260 E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.
1922
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gaps on the surface between the cathode and anode which were 21mm, 33.5mm, and 20.41 mm respectively affected the impedance value. The Square shape revealed the lowest impedance based on RΩ+Rp values of 3910.00Ω, 2152.30Ω, 1536.40Ω, 1647.30Ω, and 751.52Ω at 600°C, 650°C, 700°C, 750°C, and 800°C, respectively. The Square shape of the Micro-SOFC indicated high signals which varied with the quantity of the methane. Therefore, this investigation can promote the Square shape of Micro-SOFC to be used as a sensor for the detection of methane in biogas.
1. Introduction The development of Solid Oxide Fuel Cells [1, 2] is driven by such factors as their high potential for excellent energy efficiency, fuel flexibility and environmentally friendly properties [3], and this electrical device can convert chemical energy directly into electrical energy. These convert chemical energy in fuel into electricity directly without the involvement of any combustion process [2, 4, 5]. However, the operational SOFCs technology is hindered when exposed to a high temperature in the range of 800-1000°C [1]. Many investigations have been conducted that reduced the operating temperature to an intermediate temperature [4] in the range of 500-700°C, as well as using a thin film of electrolyte with higher ionic conductivity, and the high electrical conductivity of the cathode and anode can reduce the temperature are required [6-9]. Due to development intermediate temperatures for solid oxide fuel cells (IT-SOFCs) materials that can be used at low operating temperatures have been widely investigated. There have been example of pure ceria being operated in low temperatures that were modified in terms of structure to create a significant number of oxygen vacancies to enhance high ionic conductivity of the partial substitution of ce4+ ions with the trivalent cations such as Sm3+, Gd3+, Pt3+, Y3+, La3+ and Nd3+ which creates large a density of oxygen vacancies in ceria lattice results in high ionic conductivity [4, 10, 11]. The small sizes of SOFC systems are currently being researched in terms of their potential application as portable power units for domestic heat and power generation, for example [12, 13]. These systems are also being researched regarding their operation capabilities on a wide variety of fuels such as hydrogen, propane, methane and butane. Kendall and Palin [13] have investigated a Micro-SOFC using a butane/air mix as the fuel, and obtained promising results with the generation of a small amount of electricity. N.M. Sammes [12] have revealed that a Micro-SOFC running on butane is useful under the operation of a micro-tubular SOFC. Direct flame fuel cells have been studied in micro-tubular SOFC operating in under a high temperature without a heating element powered by an electrical current [14, 15]. However, micro fuel cells generated low currents of electricity that were limited in terms of their use in a power source applied to electrical equipment. The micro fuel cells should be able to provide high sensitivity with methane gas. Therefore, micro fuel cells can be a novelty-designed sensor for the measurement of methane that is mixed with the NiO composition at the anode side [16]. Micro SOFCs are usually fabricated as anode supported the system which caused very thin electrolyte film can be deposited on the anode support, thus the ohmic resistance decreased and the cell’s high performance can be increased. Additionally, the ohmic resistance and oxygen ion transfer are indicated by an impedance measurement [17, 18]. Impedance has three semicircles of the electrolyte of SOFC systems in the typical impedance spectra. High-frequency semicircle represented the grain conduction; intermediate-frequency semicircle represented the grain boundary and/or impurity phase contributions to ionic conduction; and lower-frequency semicircle represented ion and electron transfers at the sample surface contacting the electrode. At a high temperature, the semicircle involved with the grain and grain boundary disappears. The reason for this phenomenon is due to the fact that the time constant associated with grain and grain boundary impedances are smaller in amount when compared to electrode processes [19]. In the present study, the composition of BYCF cathode, NiO anode and GDC10 electrolyte created a layer for the micro fuel cell to be a sensor system. This newly-designed sensor used the electrolyte supported and deposited cathode and anode on the electrolyte layer by way of Spray Pyrolysis technique. This investigation created novelty-designed sensor surfaces with Cat, Square, and Yin-Yang shapes. The impedance spectrometer was used to study the relationship between impedance and shape.
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2. Experimental 2.1. Materials and methods A composition of a BYCF (Ba0.054Y0.029Co1.8Fe0.062O2.89) formed a mixture with 5wt% BaO, 3wt% Fe2O3, 2wt% Y2O3 and 90 wt% Co3O4 which was obtained by a method of conventional ceramic powder processing. The cathode was prepared using re-agent grade metal oxide powders from Sigma Aldrich, Germany. And a composite GDC10 electrolyte (10mol% gadolinium doped 90mol% Ceria, Ce0.9Gd0.1O1.95) was also prepared using re-agent grade metal oxide powders from Sigma Aldrich, Germany. A composite anode used NiO powder which was sourced from Fuel Cell Materials USA with a surface area of 2.9 m2/g and particle size of 0.5-1.5 µm. The thermal expansion coefficient (TEC) was controlled in order for it to be close to that of GDC10 electrolyte which is 13.08 x10-6 oC-1 at 900oC[20]. The cathode side was a 50/50 mixture of BYCF and GDC10 and the anode side was also a 60/40 mixture of NiO and GDC10. The chemical powder was ground for 1 hour and then mixed with distilled water, including 10% PVA (polyvinyl alcohol), by weight. The mixture was milled for 24 hours in a cylindrical-capped container with alumina balls as filling using a horizontal rotary ball mill and then dried in an oven at 150°C for 5 hours. The powder was ground again for 24 hours in our in-house-made grinding machine and sieved through a 150 mesh. The fine powder, which now exhibited some residual moisture content, was then pressed into 15 mm diameter and 2 mm thickness pellets using a uniaxial press under 8,534 psi of pressure. Finally, the BYCF+GDC10 pellets were sintered at 1,100°C with a heating rate of 30°C/min for 10 hours under the air.
Fig. 1. Spray Pyrolysis system
The experimental set-up to deposit a thin layer of the BYCF cathode and NiO anode on the GDC10 electrolyte substrate used a Spray Pyrolysis machine. The spray gun was operated at an air pressure rate of 0.82737 MPa and the nozzle was set at a height of 20 cm with a 16 cm spraying diameter. The pellet was placed on a stainless steel substrate heated to 450°C. This enabled crack-free layers to be formed on the 60/40 mixture of NiO
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and GDC10 anode substrate. The spraying time (in seconds) and the deposited thickness is shown in Figure 1. A dilute solution of the mixture for Spray Pyrolysis was prepared as follows: slurry with BYCF and GDC10 powder and water used at a ratio of 50:50 vol% and milled for 24 hours.
H2O
H2 , Air, CH4
Alumina tube
Cat
Square
Yin-Yang
Cathode
Cathode
Cathode
Anode
Anode
Pt wire
Gap Anode Cathode
Surface
Pt-past
Anode
Electrolyte
BCABS-Zr Glass Sealant
Micro-SOFC sensor
Fig. 2. The pellet with the platinum probes was placed inside an in-house designed furnace
A uniaxial press die was used to produce a GDC10 pellet 15 mm in diameter and 3 mm in thickness. Spray pyrolysis was used to deposit the BYCF cathode and NiO anode with 20 µm thicknesses; these depositions used a novel design of sensor surface with Cat, Square, and Yin-Yang shapes. Platinum electrodes as probes were primed with a platinum paste and then pressed to ensure good contact could be made as shown in Figure 3. In the real experiment, the pellet with the platinum probes was placed in an in-house designed furnace as shown in Figure 2. Microstructures were observed using a scanning electron microscope [21]. For impedance measurement and Nyquist plots, Impedance Spectroscopy was carried out using an Analog Device EVAL-AD5933/34EBZ as an impedance analyser in the frequency range 1 Hz - 1 MHz at 1Vrms signal amplitude over temperatures of 30°C to 800°C with 50°C steps. A PROVA 200A PV analyser was used to measure the power density. The experiments used 99.99% hydrogen (H2) or 98.00% methane (CH4) gas, fed in with an air-pump.
Fig. 3. Surface of Micro SOFCs in the shapes (a) Cat, (b) Square, and (c) Yin-Yang.
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3. Result and Discussion 3.1 Microstructure and layer thickness Micro-SOFCs for sensors in this study used an electrolyte layer for the substrate and supported the anode and cathode layers, as exhibited in Figure 4(a). The cathode side contained a 50/50 mixture of BYCF and GDC10 by %weight and used the Spray Pyrolysis technique to deposit on the GDC10 electrolyte on the surface, as shown in Figure 4(b). On the surface of the cathode, there was indication that there was obviously many pin-holes of 20 µm and these pin-holes represented the porosity of cathode surface. The effective transport properties of the SOFC composite structure depend on the microstructural characteristics of the electrode, such as the particle diameter, the porosity and the composition, compatibility with the electrolyte, long-term stability and intrinsic charge-transfer resistance. Thus, having a high porosity is one such property of the material that determines the cathode’s efficiency, which is inversely proportional to its polarization resistance[21, 22]. The properties of the LSCF and BYCF cathodes must be operated for intermediate temperatures SOFCs due to both high electronics and oxygen conductivity and also high activity towards oxygen reduction at low temperatures around 600°C [8, 23]. However, the abundance of pores on the surface of the cathode may use the Spray Pyrolysis technique that is an absolutely physical cathode composition. LSCF has deposits by the Spray Pyrolysis technique that indicate the pore size of 0.4 µm with a nano-particle size of 0.05 µm, which allows for the oxygen diffusion inside the cathode [24] to take place. Therefore, the Spray Pyrolysis is high in performance in terms of generating the porosity in the cathode layer. The layer of the cathode deposited of 40 µm on the electrolyte, as exhibited in Figure 5(b). Both of the two layers clearly show an adhesive layer In between each other. The thin layer of the cathode can reduce the electrical resistivity, high oxygen levels and high ionic transfer rate [25, 26]. Additionally, the anode side contained a 60/40 mixture of NiO and GDC10 that was also deposited on the electrolyte, as shown in Figure 4(c). The surface of the anode side needed a high level of which is the as same as at the cathode side. From this investigation, it was found that pin-holes occurred of 4 µm; this revealed the porosity, however, the number was less than at the cathode side. However, the majority of anodes included NiO, which is frequently in order to stabilize the morphology based on the porosity and thermal expansion. The anode must be a supported reactant and be able to adsorb fuel gas. Diana M. Amaya has reportedly been able to achieve this with a porosity between 20 and 40% porosity of Cu/YSZ [27]. Therefore, the porosity on the anode side is still very important for the transfer of gas and conductivity in SOFCs. The layer of anode deposited of 80 µm on electrolyte as exhibited in Figure 5(c). Both of two layers obviously reveal an adhesive layer together. The thin layer of the anode can reduce the electrical resistivity and high ionic transfer as well as the absorbed hydrogen gas [28]. The electrolyte side was a GDC10 supported anode and cathode, as shown in Figure 4(d). The surface of the electrolyte side indicated there were exactly no pores (zero) which represented a high density.The surface of the electrolyte with a high dense exhibited a grain size of 12 µm lead that was sintered at a high temperature. However, the electrolyte properties must respond to the conducting of the ions between the electrodes for the separation of the gases (from the chemical reaction) and for the internal electrical conduction blocking, driving the electrons to flow through the external circuit[29]. The GDC10 composite represented the Gd3+ doped ceria shows a higher ionic conductivity because of a smaller enthalpy associated between the doped cation Gd3+ and the oxygen vacancy of the host lattice based on ceria. Intermediate and low temperatures of the SOFCs are the best properties for the composition. GDC is one of the materials that can be operated at an intermediate temperature with the electrolyte needing to be near-full in density[30]. Our study focused on the novel design of the SOFC structure for a methane sensor that used the electrolyte for a substrate and deposited the cathode and anode on the electrolyte layers. Between the cathode and anode, a gap of 900 µm was produced, as exhibited in Figure 5(d). Generally, the SOFC structure is fabricated by a triple phase boundary (TPB) such as at the anode, cathode and electrolyte. The ionic transfer directly flows from the cathode to the electrolyte and anode. The TPB generated the ohmic polarization, representing electrical resistance of the electrode and resistance to the flow of ions through the electrolyte. Also, processes in the operational SOFCs can
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significantly contaminate reactant gases which tends to increase the ohmic polarization[31]. On the other hand, in this investigation, the structure of our sensor shows the cathode and anode on the electrolyte that is partitioned with the gap. Therefore, the ion transfer should be hopped from the cathode through to the electrolyte, as shown in Figure 6.
Fig. 4. Scanning electron microscopy (SEM) micrograph of the surface of a Micro-SOFC (a) the Micro-SOFC’s sensor (b) surface of the cathode (c) surface of the anode and (d) surface of the electrolyte.
Fig. 5. SEM micrographs of the cross-section of (a) the Micro-SOFC’s sensor (b) the cathode (c) the anode.
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Fig. 6. The construction of a Micro-SOFC set-up for determining the O2- ion flow.
3.2 Electrochemical Impedance Spectroscopy (EIS) Impedance spectroscopy is a powerful tool used to study the electrical properties of solid electrolytes. The bulk and grain boundary resistance and total ionic conductivity of solid oxide materials use the impedance spectroscopy for investigation based on the SOFC, and the impedance analysis of all the specimens was operated under an intermediate temperature in the range from 600-800°C. Nyquist plot is the complex impedance trend of electrolyte, cathode and anode materials characterized by three successive semicircles. Each semicircle corresponds to higher a frequency related to the grain (bulk) contribution, and the semicircle corresponds to intermediate frequency related to grain boundary contribution and the semicircle that corresponds to a lower frequency is related to electrode contribution [32-35]. Generally, the impedance plot can be an advantage when there is strong impedance on the frequency, as is the case with a capacitance. The log|z| vs, log ω curve can yield values of polarization Rp and ohmic RΩ resistance[36]. The electrical performance of a SOFC is involved with the impedance plot that RΩ represents mainly the ohmic resistance from lead wires and electrolyte GDC10 [37]. The complex impedance plots of all specimens show semicircles with a combination of three contributions such as grain, grain boundary and electrode represented by Rpvalue [37, 38] as shown in Fig.7. The Nyquist plot of the Cat, Square and Yin -Yang sensors at 650°C and 700°C indicated that Yin-Yang is higher than Rp and RΩ is higher than Cat and Square. The gap’s length of the physical Yin-Yang sensor is shorter than others and may only generate low ion conductivity that affects the high RΩ and Rp[38] revealed in Fig.8.
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Z" (Imaginary), Ω
Decreasing frequency max Z"
RΩ
Zʹ (Real), Ω
RΩ+ Rp
Fig.7. Conventional Nyquist plot.
Fig. 8. Nyquist plot of the Cat, Square and Yin -Yang sensors at 650°C and 700°C.
Comparison of Nyquist plot of (a)Cat, (b)Square and (c)Yin Yang at varied temperatures, as shown in Fig 9. Increasing temperatures can affect the decrease of impedance, while Cat, Square and Yin Yang also reduce in terms of impedance when the temperature is increased. The Square shape is lower in impedance at 800oC and the Nyquist plot also represents a more true and clear semicircle compared with the other shapes. The increasing temperature in the impedance test system creates a reduction in impedance and the semicircles which depends on the shift forwards towards the higher frequency side. Grain boundary resistance (Rgb) and grain resistance (Rg) contributions were significantly affected at higher temperatures, therefore, the total resistances (Rt = Rg+ Rgb) of the
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semicircular arcs were intercepted in the electrical properties of the materials[34]. Therefore, all of the impedances of Cat, Square and Yin Yang are resolved Rg and Rgb when there was an increase of temperature. The reduction of impedance involves decreasing Rg and Rb.
Fig. 9. Comparison of Nyquist plot of (a) Cat, (b) Square, (c) Yin-Yang at varied temperatures
4. Conclusions This investigation designed a novel sensor based on a solid oxide fuel cell system using different shapes of a cathode, anode and electrolyte. The novel sensor used a GDC10 electrolyte for the substrate, BYCF cathode, NiO anode, and deposit on the GDC10 electrolyte. The shape of the cathode and anode were in the form of Cat, Square, and Yin Yang which were deposited by the use of a Spray Pyrolysis technique that is able to generate high porosity, with a 18.28 µm thickness of the anode and a 146.73 µm thickness of the cathode. The Square shape of the novel sensor indicated that there is low impedance at high temperatures compared with Cat and Yin Yang shapes. The Square shape had a longer gap length that can have a significant contribution to the transfer rate of ions. Therefore, the Square shape is deemed to be the best and most suitable for use in the application of the sensor. Acknowledgements This research was supported by Innovation’s fund (RDO590769S), Center of Excellence in Materials Engineering, CEME and Travel Funding for conferences (graduate student of Faculty of Engineer), Prince of Songkla University. We thank our colleagues from the Faculty of Engineering; PSU Energy Systems Research Institute (PERIN) who provided their insight and expertise that greatly assisted this research.
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