LSM cathodes fabricated on YSZ electrolyte hollow fibres by dip-coating

Materials Today Chemistry 16 (2020) 100252

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Electrical conductivities and microstructures of LSM, LSM-YSZ and LSM-YSZ/LSM cathodes fabricated on YSZ electrolyte hollow fibres by dip-coating O.O. Agbede a, b, *, K. Hellgardt a, G.H. Kelsall a a b

Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, UK Department of Chemical Engineering, Ladoke Akintola University of Technology Ogbomoso, P.M.B. 4000, Ogbomoso, Nigeria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 October 2019 Received in revised form 7 January 2020 Accepted 9 February 2020 Available online xxx

Lanthanum strontium manganite e La0.80Sr0.20MnO3 (LSM), LSM-Yttria stabilised zirconia (LSM-YSZ) composite and LSM-YSZ/LSM double-layer cathodes were separately fabricated on Yttria stabilised zirconia (YSZ) electrolyte hollow fibres by dip coating; their electrical conductivities and microstructures were then determined by the direct current four-probe method and scanning electron microscopy (SEM), respectively. Excellent cathode-electrolyte and cathode-cathode adhesion without delamination were achieved by the dip-coating fabrication method. The apparent electrical conductivities of porous LSM, LSM-YSZ and LSM-YSZ/LSM cathodes manufactured on YSZ hollow fibre by dip-coating and sintered at various temperatures in the range 1273e1473 K for 3 h, were 1.8
103e5.5
103 S/m, 0.32e209 S/m and 1.3
103e5.5
103 S/m, respectively, at measurement temperatures of 673e1073 K. The operating temperature dependence of the apparent electrical conductivity of the LSM, LSM-YSZ and LSM-YSZ/LSM cathodes was defined by the Arrhenius equation for electrical conductivity. The activation energies for electrical conductivity were derived as 0.106e0.147 eV, 0.83e0.94 eV, and 0.104e0.146 eV for the LSM, LSM-YSZ and LSM-YSZ/LSM cathodes, respectively. The LSM-YSZ and LSM-YSZ/LSM cathodes were strongly influenced by the YSZ and LSM phases, respectively. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Lanthanum strontium manganite YSZ electrolyte hollow fibre Electrical conductivity LSM-YSZ composite Dip-coating Microstructure

1. Introduction The world needs efficient and environmentally benign energy conversion devices to efficiently convert the fast depleting and non-renewable fossil fuels to electricity with low emissions to the environment [1e5]. The solid oxide fuel cell (SOFC) is an electrochemical energy conversion device, which consists basically of a porous oxygen-reducing cathode, a dense oxide ion conducting solid oxide electrolyte and a porous fuel oxidizing anode [6,7]. It can convert the chemical energy in a fuel, directly and efficiently, into electricity with low emissions to the environment; so, it is actively being investigated as a cleaner energy conversion device able to replace conventional power plants [7e11]. At the cathode of an SOFC, oxygen is reduced to oxide ion, which is conducted through the electrolyte to the anode/electrolyte interface where it reacts with a fuel (e.g. hydrogen, methane or synthesis gas) to

* Corresponding author. E-mail address: [email protected] (O.O. Agbede). https://doi.org/10.1016/j.mtchem.2020.100252 2468-5194/© 2020 Elsevier Ltd. All rights reserved.

produce (e.g. steam and carbon dioxide) and release electrons, which pass through an external circuit to the cathode [6,9,12]. The SOFC operates at a high temperature, usually in the range 800e1000  C. Owing to the high operating temperature, the startup of conventional SOFC is slow and more energy demanding [13]. There is also the need to use cell materials, which are durable at high temperatures; this limits the choice of material and results in high cost [14]. Operation at intermediate temperatures (500e800  C) allows the use of less expensive materials in interconnects and balance-of-plant components, it also greatly simplifies system requirements coupled with more rapid start-up and shut down [11]. Hence, the means of improving cathode performance at a lower temperature and novel cathodes that can operate at lower temperatures are currently being investigated [15]. The performance of SOFC at lower operating temperatures can be enhanced by improving cathode microstructure and using dualphase LSM-YSZ cathodes [13]. The two main designs of SOFCs are the planar and tubular SOFCs [6]; however, there is increased research interest in microtubular SOFCs (mt-SOFCs), which are small millimetre-scale tubular

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O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

Nomenclature A D Ea I kB L R T V

s so

Area (m2) Diameter (m) Activation energy for electrical conductivity (kJ/ mol) Current (A) Boltzmann constant (kJ/mol/K) Length of cathode (m) Cathode resistance (U) Absolute temperature (K) Voltage (V) Apparent electrical conductivity of porous cathode (S/m) Pre-exponential factor (S/m K)

SOFCs that possess the relative advantages of rapid start-up, improved power density and better tolerance to thermal cycling, over conventional planar and tubular SOFCs, and can still be potentially fuelled with hydrocarbons [16,17]. The mt-SOFCs may be used in portable applications, including automotive power supplies, battery chargers, mobile electricity generators, auxiliary power units compared to mainly stationary applications that conventional SOFCs are used for [18e20]. Mt-SOFCs can be anodesupported [21e25], cathode supported [26e29] or electrolyte supported [25,30,31]. Jamil et al. [32] recently reviewed the fabrication techniques for mt-SOFC support; they identified extrusion, co-extrusion and the more recent phase inversion-based (single and multilayer) extrusion for single (e.g. electrolyte-supported) and multilayer (e.g. electrolyte/anode dual-layer) hollow fibres, as the commonly employed techniques for the fabrication of mt-SOFC support. The cathode layer of cells, which are not cathode-supporting are usually fabricated by dip-coating [22,33e36] or slurry coating/painting [21,23,24,30,31,37,38]. The dip-coating method may be easily automated for large scale fabrication of mt-SOFCs cathodes [17]. Yttria-stabilised zirconia (ZrO2)0.92(Y2O3)0.08 [YSZ] is the most widely used electrolyte for the SOFC because of its good ion conductivity, stability in both oxidising and reducing environment, abundance, relatively low cost, easiness to fabricate and chemical stability with other cell components [9,40]. The SOFC cathode catalyses the reduction of molecular oxygen, transports charged species to the electrolyte and distributes the electrical current associated with the oxygen reduction reaction. Hence, it must have high catalytic activity for oxygen reduction and a thermal expansion coefficient that matches those of other cell components but must not be reactive towards the other cell components; it must also be a good electronic conductor and porous to allow the diffusion of gases to the triple-phase boundary [7,11,40e42]. The most commonly used cathode for zirconia electrolyte-based solid oxide fuel cells is lanthanum strontium manganite - La1-xSrxMnO3 (LSM) because it has high catalytic activity for oxygen reduction and is chemically stable in oxidizing environments, it also has sufficient electrical conductivity and a close thermal expansion match with YSZ electrolyte at high operating temperature [9,18,42e46]. Researchers investigating zirconia electrolyte mt-SOFCs have used either LSM [27], LSM-YSZ composite [22e24,37] or doublelayer LSM-YSZ/LSM [21,38,47] cathodes. The LSM-YSZ is a composite of LSM and YSZ, which extends the active triple-phase boundary, enhances the porosity and resistance to sintering, improves the thermal match of the cathode with the zirconia

electrolyte and also possesses the required electronic conductivity [9,42,43]. The cathode resistance, and consequently, its electrical conductivity contributes to the ohmic loss in a fuel cell; this resistance is determined by the cathode material, its microstructure and operating temperature. The porous cathode's performance depends on the microstructural characteristics, such as particle size, pore size, particle-particle connection and porosity, in addition to its chemical, structural, and thermodynamic properties [40,48,49]. The electrical conductivity of dense material is altered when it is made porous [48], so the apparent electrical conductivity of a porous solid is a function of the relative density and degree of connection between the particles along the potential gradient [50]. The processing method used in fabricating the electrode affects its microstructural characteristics, and consequently, the performance [40]. Thus the electrical conductivity of a porous cathode considerably depends on the method of fabrication, which largely determines the microstructure [39,40,45,51]. The fabrication of the cathode usually involves sintering, which requires heating the material at a high temperature for a period sufficient for its constituent particles to adhere together. The sintering temperature and time affect the microstructure and electrical conductivity of the cathode, reactivity of LSM with YSZ, and adhesion of the porous electrode to the YSZ electrolyte [23,52,53]. Li et al. [54] have shown that the electrical conductivity of LSM prepared by plasma spraying was ca 50% lower than those made by conventional solid-state sintering method; they reported that depending on composition, the electrical conductivities of the plasma sprayed samples were 5.0
103e20.1
103 S/m compared to 4.0
103e48.5
103 S/m at 1273 K of sintered samples. Jiang, Love and Apateanu [55] have reported electrical conductivities of 3.4
103e5.6
103 S/m at 1073 K for porous La0.72Sr0.18MnO3 electrodes fabricated by screen printing and sintered in air at 1423 K. The electrical conductivity of porous LSM cathode fabricated by screen printing was much lower than that of the bulk LSM material at the same measurement temperature due to the lowering effect of porosity on electrical conductivity. Mattiot et al. [56] also fabricated the La0.83Sr0.17MnO3 electrode by screen printing; they measured electrical conductivities of 2100, 3400, and 6400 S/m for electrodes sintered at 1373, 1473, and 1573 K, respectively and of porosity in the range of 40%e35%. Otoshi et al. [57] fabricated LSM pellets by pressing the powders and sintering at 1573 K for 24 h, and reported a decrease in the apparent electrical conductivity at 1073 K from ca. 14.8
103 S/m for 10% porosity to ca. 4.5
103 S/m for 41% porosity. Marinsek [48] studied the electrical conductivity of porous La0.85Sr0.15MnO3 tablets fabricated by pressing LSM powders and sintering at various temperatures in the range 1273e1603 K for 1 h. They reported electrical conductivities of 0.1
103e6.5
103 S/m at 293e1073 K and porosity of 42.71 to 0.42%. Moriche et al. [58] prepared LSM materials by a mechanochemical method and fabricated pellets, which were sintered at 1573 K for 8 h, and then measured electrical conductivities of 10
103e40
103 S/m in the temperature range of 278e1123 K. Yang, Wei and Roosen [59] investigated the electrical conductivity of LSM-YSZ composite cathode, which was fabricated by diepressing a mixture of A-site deficient La0.65Sr0.3MnO3 and 8 mol% YSZ powders into a bar/disk and sintered at 1673 K for 24 h; they reported that the electrical conductivity increased from 4 to 227 S/ m at operating temperature of 1273 K as the content of LSM in the composite electrode was increased from 10 to 50 vol%, showing that the electrical conductivity of the composite electrode is influenced by its LSM content. Zhang et al. [60] prepared LSM-YSZ composite cathodes by atmospheric plasma spraying and investigated the microstructure and electrical conductivity; they observed that the electrical conductivity of YSZ-50%LSM cathode increased

O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

from 217 to 360 S/m along the direction parallel to the coating surface but increased from 9.7 to 20 S/m along the perpendicular direction to the coating surface when the test temperature increased from 773 to 1173 K. Campana et al. [22] fabricated and electrochemically characterised an anode-supported microtubular SOFC, consisting of Ni-YSZ anode prepared by cold isostatic pressing, YSZ electrolyte made by spray coating and LSM-YSZ cathode fabricated by dip-coating. They reported that cathodes sintered at temperatures below 1273 K did not have a good electrolytecathode adhesion while those sintered at 1523 K lost ca. 30% manganese due to evaporation of manganese oxide during the sintering process. Luo et al. [61] pressed LSM powder into bars, which were sintered in air at 1623 K for 5 h, and measured the electrical conductivity in the temperature range of 623e1073 K in air and humidified hydrogen. They reported an increase in conductivity with increasing temperature with maximum conductivities of 12.38
103 and 201 S/m at 1073 K in air and hydrogen, respectively. Abdelaziem et al. [62] investigated the electrical resistivity of La0.8Sr0.2MnO3 thin films fabricated by pulsed laser deposition and annealed at 673e1273 K. They showed that the LSM film resistivity decreased with increasing temperature in the range of 298e473 K due to the semiconductor behaviour of LSM. The lowest resistivity with an activation energy of 0.18 eV was seen at 1073 K, but the LSM film cracked at 1273 K. The apparent electrical conductivities of porous LSM, LSM-YSZ composite and LSM-YSZ/LSM doublelayer cathodes fabricated by dip-coating on YSZ electrolyte hollow fibre have not been reported. Similar to other previously reported techniques used for the fabrication of LSM cathodes [48,54e62], the dip-coating technique and sintering temperature would affect the microstructure, and consequently, the apparent electrical conductivities of cathodes fabricated by this technique. Hence, this study aimed to: - determine the electrical conductivities, at various operating temperatures, of LSM, LSM-YSZ composite and LSM-YSZ/LSM double-layer cathodes fabricated on YSZ electrolyte hollow fibres by dip-coating, and sintered at different temperatures. - describe the influence of sintering temperature on cathode microstructure, and consequently, on the apparent electrical conductivities of the cathodes. - define the temperature dependence of the apparent electrical conductivity of the LSM, LSM-YSZ, and LSM-YSZ/LSM cathodes fabricated by dip-coating on YSZ hollow fibres.

2. Materials and methods 2.1. Materials The open-ended YSZ electrolyte hollow fibres used in this study, which had outer diameters of ca. 1.3 mm were obtained from the Department of Chemical Engineering, Imperial College London. The fabrication of these hollow fibres has been described by Wei and Li [33]. The LSM and LSM-YSZ electrodes were fabricated with LSM (La0.80Sr0.20MnO3) and LSM-YSZ (50 wt% La0.80Sr0.20MnO3 and 50 wt% (Y2O3)0.08(ZrO2)0.92) pastes purchased from NexTech Materials (fuelcellmaterials.com) Ohio, USA. A vehicle ink, which is a terpineol-based solvent, from the same company, was used to dilute the pastes during their application. The air used for sintering purposes was generated by a Chrompack AO100 air generator. Silver wire of 0.25 mm diameter and 99.99% purity was purchased from Sigma Aldrich, and silver paste was obtained from Alfa Aesar, UK.

3

2.2. Experimental techniques, apparatus and procedure 2.2.1. Fabrication of cathodes on YSZ hollow fibres by dip-coating The original LSM-YSZ and LSM pastes were diluted with ink vehicle to reduce their viscosities and make them suitable for use; 58.8 wt% LSM-YSZ paste was mixed with 41.2 wt% ink vehicle while 76.9 wt% LSM paste was mixed with 23.1 wt% ink vehicle. After diluting the paste, five sets of YSZ hollow fibres were dipcoated with only LSM paste and allowed to dry at room temperature; they were then sintered separately in air at different temperatures of 1273, 1323, 1373, 1423 and 1473 K for 3 h, to fabricate only LSM electrode directly on the YSZ hollow fibre electrolyte. Another five sets of YSZ hollow fibres were coated with only LSMYSZ composite paste and then sintered separately in air at different temperatures of 1273, 1323, 1373, 1423 and 1473 K for 3 h, to fabricate only LSM-YSZ composite electrode directly on YSZ hollow fibre electrolyte. A third five sets of YSZ microtubes were dip-coated with the LSM/YSZ paste and allowed to dry at room temperature. After complete drying of the first layer, each set of microtubes was dip coated with LSM paste, dried and then sintered separately in air at different temperatures of 1273, 1323, 1373, 1423 and 1473 K for 3 h, to produce an LSM-YSZ/LSM double-layer cathode on YSZ electrolyte hollow fibres prepared by co-sintering the composite LSM-YSZ and pure LSM layers. Lastly, five sets of YSZ tubes were dip coated with the LSM-YSZ paste, dried and sintered in air at different temperatures of 1273, 1323, 1373, 1423 and 1473 K for 3 h. After sintering the first layer, the tubes were dip coated with the LSM paste to apply a second layer of LSM cathode, dried and sintered in the air again, to produce an LSM-YSZ/LSM double-layered cathode on YSZ hollow fibre electrolyte prepared by sintering the two cathode layers one after the other (double sintering). This was to check whether cosintering the two layers of the double-layer cathodes produces similar effects as double sintering, on the microstructure and electrical properties of the LSM-YSZ/LSM double-layer cathode fabricated by dip coating. The sintering temperature was limited to 1473 K because LSM reacts with zirconia at temperatures above 1473 K and forms a resistive layer of lanthanum zirconate (La2ZrO7) or strontium zirconate (SrZrO3) at higher strontium levels that could lower the conductivity of the electrode [9,14,63,64]. 2.2.2. Measurement of cathode resistance The resistance of 30 mm length of each sintered cathode was measured in the temperature range 650e1073 K using the direct current four-point probe resistance measurement technique and equipment described schematically in Fig. 1. The cathodes were connected to voltage and current probes with silver wires, and some silver paste was also applied to ensure good contact between the cathode and the wires wound around it. A fixed potential, which was varied from 0.2 to 1 V was applied across a known resistor placed between the working and reference electrodes of an AUTOLAB PGSTAT30 potentiostat to generate current. The potential drops due to the resistance of the measured length of the cathode were measured using a Keithley precision multimeter. A potential of 0.02e0.1 V was applied during the measurement of the resistance of the LSM-YSZ cathode. A resistor of 10 kU was used for the measurement of the resistance of the LSMYSZ composite cathodes, while a resistor of 999.8 U was used for the LSM and LSM-YSZ/LSM double-layer cathodes. The measured cathode resistance (Rc) was computed from the current applied and voltage drop:

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O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

were afterwards determined from the binary images. A relationship was then defined between sintering temperature and porosity.

POTENTIOSTAT Counter Electrode

Reference Electrode

Working Electrode

Furnace Icircuit

cathode

Rknown

Furnace Vmeasured

Mulmeter Fig. 1. Schematic of the experimental set-up for the measurement of cathode resistance by the direct current four-point technique.

Rc ¼

Vmeasured Icircuit

(1)

The electrical conductivity (sc) of the electrode was then obtained from the average value of measured resistances:

sc ¼

Lc Rc Ac

(2)

where Lc was the length of the electrode. Ac, the area of current flow, was defined as:

Ac ¼

p 4

D2od;coated  D2od;uncoated



(3)

where Dod, uncoated was the outer diameter of the un-coated electrolyte tube and Dod, coated the outer diameter of the electrolyte after it had been coated with LSM, LSM-YSZ composite, or LSM-YSZ/ LSM double-layer cathode. These diameters were determined from scanning electron microscope (SEM) micrographs of the coated electrolyte tubes. 2.2.3. Scanning electron microscope imaging of cathode The microtubes were broken into smaller lengths after the resistance had been measured and a Leo Gemini 1525 highresolution field emission gun scanning electron microscope (SEM) was then used to take a high-resolution image of the cross-section of each tube to determine the diameter of the coated microtube and characterise the microstructure of the fabricated cathode. A low vacuum sputter coating of samples with an ultrathin layer of chromium preceded the SEM imaging, to prevent static electric fields accumulating at the material during imaging. An acceleration voltage of 5 kV was used for all SEM imaging. 2.3. ImageJ analysis of SEM micrographs A public domain image analysis software, ImageJ (www.nih. com), was used to determine the average porosities of the LSM cathodes by analysis of several SEM micrographs. The grayscale SEM micrographs were first thresholded using the Otsu thresholding algorithm [65] in ImageJ for the binarisation of pore and solid phases of the cathode, to obtain binary images. The porosities

3. Results and discussion Fig. 2 shows SEM micrographs of (a) a typical electrode (b) LSM electrode (c) LSM-YSZ electrode (d) LSM-YSZ and LSM electrodes (cosintering) (e) LSM-YSZ and LSM electrodes (double sintering), fabricated on microtubular YSZ electrolyte hollow fibre by dip coating. Fig. 2a shows that porous cathodes (LSM-YSZ, LSM or LSMYSZ/LSM) were successfully fabricated on YSZ electrolyte hollow fibres by dip coating in LSM-YSZ or/and LSM pastes. There was good adhesion between the LSM cathode and YSZ electrolyte and also between the LSM-YSZ composite cathode and YSZ electrolyte, as shown in Fig. 2 a and b, respectively; the cathode did not delaminate from the surface of the electrolyte after sintering and resistance measurement experiments. Similarly, there was excellent adhesion of the porous LSM-YSZ layer to the dense YSZ electrolyte and porous LSM layer of the double-layer cathode in both cases of cathodes cosintered and those sintered one after the other, as shown in Fig. 2c and d, respectively. Good adhesion at electrode | electrode and electrolyte | electrode interfaces is essential for charge transfer and durability of the electrode throughout the microtubular SOFC life. It is also required to avoid contact resistance that would lower cell performance. 3.1. LSM cathode Fig. 3 shows the plots of apparent electrical conductivity (s) versus absolute operating temperature (T) and ln (sТ) versus reciprocal of absolute operating temperature, for LSM cathodes sintered at 1273, 1323, 1373, 1423 and 1473 K. The apparent electrical conductivity of the LSM cathode increased with increasing operating (measurement) temperature in the range 673e1073 K and with increasing sintering temperature in the range 1273e1473 K considered. The apparent electrical conductivity of the LSM cathode increased with increasing operating temperature because the electrical conductivity of a semiconductor increases with increasing operating temperature [48]. The apparent electrical conductivities of porous La0.80Sr0.20MnO3 cathodes manufactured on YSZ electrolyte hollow fibres by dip coating and sintered at various temperatures in the range 1273e1473 K for 3 h, in this study, were 1.8
103e5.5
103 S/m at measurement temperatures 673e1073 K. They are higher than those of 208 and 350 S/m at 473 and 1273 K, respectively, for porous La0.65Sr0.3MnO3 fabricated by die-pressing LSM powders and sintering at 1673 K for 1 h, reported by Yang, Wei and Roosen [59]. They are similar to those of porous La0.85Sr0.15MnO3 tablets, manufactured by pressing LSM powders and sintering at various temperatures in the range 1273e1603 K for 1 h, reported by Marinsek [48], which were 0.1
103e6.5
103 S/m at 293e1073 K. They are also similar to conductivities of 3.4
103e5.6
103 S/m at 1073 K reported by Jiang, Love and Apateanu [55] for porous La0.72Sr0.18MnO3 fabricated by screen printing and sintered in air at 1423 K. Standard errors were determined, and error bars plotted for all data of electrical conductivities presented in this study. However, the error bars are not conspicuous because the standard errors were very minute; the highest standard errors were 9, 1.35, and 10 S/m, for the apparent electrical conductivities of LSM, LSM-YSZ and LSM-YSZ/LSM cathodes, respectively. This implies a high precision of electrical conductivity measurement by the direct current four-probe technique used in this study. The electrical conductivity - operating temperature data, for all the cathodes (LSM, LSM-YSZ and LSM-YSZ/LSM) investigated in this

O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

5

Fig. 2. SEM micrograph of (a) a typical cathode (b) LSM cathode (c) LSM-YSZ cathode (d) LSM-YSZ and LSM cathodes (cosintering) (e) LSM-YSZ and LSM cathodes (double sintering), fabricated on microtubular YSZ electrolyte hollow fibre by dip coating.

(a)

(b) 6

15.8

1273 K 1323 K 1373 K 1423 K 1473 K

15.6 15.4

ln (σT / S m¯¹ K)

σ / 103 S m-1

5 4 3 2 1

15.2

R² = 0.9997

15

R² = 0.9989

14.8

R² = 0.9998

14.6 14.2

650

750

850

950

1050

14

Temperature / K

13.8

R² = 0.9948

1273 K 1323 K 1373 K 1423 K 1473 K

14.4

0.9

1

R² = 0.9685

1.1

1.2

1.3

1.4

1.5

10³ K / T Fig. 3. Plot of (a) apparent electrical conductivity (s) versus absolute operating temperature (T) (b) ln (sТ) versus reciprocal of absolute operating temperature for LSM cathodes sintered at 1273, 1323, 1373, 1423 and 1473 K.

study, were successfully fitted to the Arrhenius equation for electrical conductivity:

  Ea kB T

(4)

Ea kB T

(5)

sT ¼ so exp

InðsTÞ ¼ Inso 

where so was the pre-exponential factor, Ea the activation energy for electrical conductivity, T the absolute temperature, and KB the Boltzmann constant. The activation energy for electrical conductivity was subsequently obtained from the slope of the plot of ln (sТ) versus the reciprocal of the absolute temperature. Fig. 3b shows the plots of ln (sТ) versus reciprocal of absolute operating temperature, for LSM cathodes sintered at 1273, 1323,

1373 1423 and 1473 K. The plot of ln (sТ) versus reciprocal of absolute operating temperature for the LSM cathode is linear, a good fit of the data to Eq. (4) was achieved, which implies that the relationship between the operating temperature and apparent electrical conductivity may be defined by the Arrhenius equation for the electrical conductivity. The activation energies for the electrical conductivity of the LSM cathodes were derived as 0.106e0.147 eV. This is the hopping energy for electrical conduction in Strontium doped LaMnO3 semiconductor by the small polaron hopping conduction mechanism [66]. The activation energies obtained in this study are similar to that of 0.09 eV reported for 20 mol % Sr-doped LaMnO3 bars fabricated by dry pressing powder and sintering in air at 1450  C for 2 days [66]. They are also close to the value of 0.125 eV obtained for plasma-sprayed La0.8Sr0.2MnO3 [67] and 0.099 eV reported for die-pressed La0.65Sr0.3MnO3 cathode sintered at 1673 K for 1 h [59].

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O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

Fig. 4 shows the SEM micrographs of LSM cathodes sintered at different temperatures in the range 1273e1473 K considered. The micrographs in the first, second and third rows were obtained at 20,000, 40,000 and 60,000 magnifications, respectively, for electrodes sintered at 1273, 1323, 1373, 1423 and 1473 K as denoted in each column of micrographs. There was a homogeneous distribution of particles and pores in the LSM cathodes, the particle size and density of the electrodes increased while the porosity decreased with increasing sintering temperature. The homogeneity in particle and pore distribution also decreased with increasing sintering temperature. Similar findings have been reported by Jørgensen et al. [52] and Piao et al. [51], who fabricated LSM cathodes by the methods of airborne spraying and screen-printing, respectively. The SEM micrographs revealed that the densification of the cathodes resulted in the observed increase in apparent electrical conductivity with increasing sintering temperature shown in Fig. 3a; the denser the material, the higher the apparent electrical conductivity achieved at each operating temperature. Hence, the apparent electrical conductivity of the porous LSM cathode is not only a function of the bulk density of the LSM material but also a function of the microstructure of the sintered porous electrode. Fig. 5a shows a typical binary image of an SEM micrograph, which had been thresholded using the Otsu algorithm in ImageJ software, from which the porosity of the LSM cathode was subsequently determined. Fig. 5b shows the plot of porosity of the LSM cathodes versus sintering temperature; the porosity is a function of sintering temperature, it decreased with increasing sintering temperature in the investigated range 1373e1473 K. This further confirms that the densification (decrease in porosity) of the electrode with increasing sintering temperature resulted in the apparent increase in electrical conductivity of the porous LSM with increasing sintering temperature as shown in Fig. 3a. Consequently, it can be inferred that the apparent electrical conductivities of porous La0.80Sr0.20MnO3 cathodes, manufactured on YSZ electrolyte hollow fibres by dip coating, of porosity 51e31%, were 1.8
103e5.5
103 S/m at operating

temperatures 673e1073 K. They are similar to 0.1
103e6.5
103 S/m at 293e1073 K for La0.85Sr0.15MnO3 of porosity of 42.71 to 0.42%, reported by Marinsek [48]. Thus, the porous cathode microstructure must be carefully controlled to ensure sufficient porosity for gas diffusion while not compromising the electrical conductivity of the electrode. 3.2. LSM-YSZ composite cathode Fig. 6 shows the plot of apparent electrical conductivity (s) versus absolute operating temperature (T) and plot of ln (sТ) versus reciprocal of absolute operating temperature for LSM-YSZ cathodes sintered at 1273, 1323, 1373, 1423 and 1473 K. The apparent electrical conductivity of the LSM-YSZ composite electrode increased with increasing operating temperature in the range 673e1073 K and sintering temperature in the range 1273e1473 K. The conductivities of the LSM-YSZ of 0.32e209 S/m were much lower than those of the LSM cathodes because of the highly resistive YSZ particles dispersed in the composite cathode. Zhang et al. [60] reported electrical conductivities of 217e360 S/m for YSZ-50 wt%LSM along the parallel direction to the coating surface and 9.7e20 S/m along the perpendicular direction, at 773e1173 K. Comparably low electrical conductivities of 4e227 S/m at 1273 K for porous LSMYSZ (consisting of 10e50 vol% LSM) fabricated by die-pressing a mixture of La0.65Sr0.3MnO3 and 8 mol% YSZ powders into a bar and sintering at 1673 K for 24 h have also been reported [59]. The LSMYSZ layer improves the thermal match of the cathode with YSZ electrolyte and extends the active triple-phase boundary, however, as shown in Fig. 6a, it does not possess sufficient electrical conductivity, and consequently, an LSM layer is usually added to improve the electrical conductivity of the cathode. Fig. 6b shows that the plot of ln (sТ) versus absolute operating temperature for the LSM-YSZ composite cathode sintered at 1273, 1323, 1373, 1423 and 1473 K, is linear. This implies that the operating temperature dependence of the apparent electrical conductivity of the porous LSM-YSZ composite cathode fabricated on YSZ

Fig. 4. SEM micrographs of LSM cathodes fabricated on YSZ electrolyte hollow fibres by dip coating and sintered at different temperatures in the range 1273e1473 K.

O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

(a)

7

(b) 55

Porosity / %

50 45 40 R² = 0.9931 35 30 25 1250

1300

1350

1400

1450

1500

Sintering Temperature / K Fig. 5. (a) Thresholded SEM image of LSM cathode (b) Plot of porosity of LSM cathode versus sintering temperature.

(a)

(b)

250

150 100

1273 K

1323 K

1373 K

1423 K

1473 K

13

1273 K 1323 K 1373 K 1423 K 1473 K

12

ln (σТ / S m-1 K)

σ / S m-1

200

14

50

R² = 0.9998

11

R² = 0.999

10

R² = 0.9999

9 R² = 0.9988

8 7 6

R² = 0.9997

5

0 650 700 750 800 850 900 950 1000 1050 1100

Temperature / K

4

0.9

1

1.1

1.2

1.3

1.4

1.5

103 K / T

Fig. 6. Plot of (a) apparent electrical conductivity (s) versus absolute operating temperature (T) (b) ln (sТ) versus reciprocal of absolute operating temperature, for LSM-YSZ electrodes sintered at 1273, 1323, 1373, 1423 and 1473 K.

electrolyte hollow fibre by dip coating may be defined by the Arrhenius equation for electrical conductivity. The activation energies for electrical conductivities of the LSM-YSZ cathodes, which were higher than those obtained for the LSM cathodes, ranged from 0.83 to 0.94 eV. These activation energies are similar to those of 0.84e1.18 eV reported for YSZ [59,68], which implies that the YSZ phase of the LSM-YSZ composite cathode greatly influenced the electrical conductivities measured in this study. A percolation threshold/limit must be achieved by LSM particles dispersed in YSZ at which the LSM particles would have sufficient interconnectivity for them to influence the electrical conductivity of the composite LSM-YSZ cathode [59,69]. The method of preparation of the LSM-YSZ material and the technique used for the cathode fabrication also affect the interconnectivity of LSM particles and the influence of LSM on the electrical conductivity of the composite cathode [59,60,69]. It is believed that the method of preparation of the LSM-YSZ slurry and dip-coating technique used for the fabrication of LSM-YSZ composite cathodes in this study affected the LSM particle distribution and connectivity in the fabricated cathode such that the LSM did not have sufficient interconnectivity to strongly influence the electrical conductivity of the LSM-YSZ cathode. In agreement with this study, Zhang et al. [60] observed that the electrical conductivities of plasma-sprayed YSZ-50 wt%LSM, YSZ-40 wt%LSM and YSZ-20 wt%LSM composite cathodes were strongly influenced by the YSZ phase. They reported activation energies of 0.80e0.83 eV for electrical conductivities measured along the parallel direction of the coating and 1.05e1.13 eV along the perpendicular direction.

Fig. 7 shows the SEM micrographs of LSM-YSZ cathodes sintered at different temperatures in the range 1273e1473 K considered. The micrographs in the first, second and third rows were obtained at 20,000, 40,000 and 60,000 magnifications, respectively, for cathodes sintered at 1273, 1323, 1373, 1423 and 1473 K, as indicated in each column of micrographs. The cathode pores and particles were homogeneously distributed; however, the homogeneity in particle and pore distribution decreased with increasing sintering temperature. Likewise, the particle size and density of the porous LSM-YSZ cathode increased while the porosity decreased with increasing sintering temperature, resulting in the observed increase in apparent electrical conductivity with increasing sintering temperature. This effect of sintering temperature on the microstructure of LSM-YSZ is similar to that reported by Jørgensen et al. [52] for LSMYSZ cathodes fabricated by air spraying and sintered at temperatures in the range 1423e1573 K. 3.3. LSM-YSZ/LSM double-layer cathode Fig. 8 shows the plots of apparent electrical conductivity of double-layer LSM-YSZ/LSM cathode versus absolute operating temperature, for the cathodes sintered once and those sintered twice at 1273, 1323, 1373, 1423 and 1473 K. For the cathodes sintered once (Fig. 8a), as well as those sintered twice (Fig. 8b), the apparent electrical conductivity increased with increasing operating temperature in the range 673e1073 K and sintering temperature in the range 1273e1473 K considered. The magnitudes (1.7
103e5.5
103 S/m) of the apparent electrical conductivity of

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Fig. 7. SEM micrographs of LSM-YSZ electrodes fabricated on YSZ hollow fibre by dip coating and sintered separately at 1273, 1323, 1373, 1423 and 1473 K.

cathodes sintered once are similar to those (1.3
103e4.8
103 S/ m) of the cathodes sintered twice. The apparent electrical conductivities of the double-layer cathodes are similar to those (1.8
103e5.5
103 S/m) of the LSM cathode, suggesting that the LSM layer greatly influenced the electrical conductivity of the LSMYSZ/LSM cathodes. Fig. 9 shows the plot of ln (sТ) versus the reciprocal of absolute operating temperature for double-layer LSM-YSZ/LSM cathodes sintered once and those sintered twice at 1273, 1323, 1373, 1423 and 1473 K. The plots are linear indicating that the operating temperature dependence of the apparent electrical conductivity of the double-layer LSM-YSZ/LSM may also be defined by the Arrhenius equation for electrical conductivity. The activation energies for electrical conductivity of LSM-YSZ/ LSM cathodes sintered once ranged from 0.104 to 0.146 eV, while those sintered twice were 0.109e0.122 eV, the activation energies derived in both categories of LSM-YSZ/LSM cathodes were of the same order of magnitude as those (0.106e0.147 eV) of the LSM cathodes. This confirms that the electrical

(a)

conductivity of the LSM-YSZ/LSM double-layer cathode is strongly influenced by the pure LSM layer. Fig. 10 shows the SEM micrographs of the LSM-YSZ and LSM layers of the LSM-YSZ/LSM double-layer cathodes fabricated on YSZ electrolyte hollow fibres by dip coating and sintered at 1273, 1323, 1373, 1423 and 1473 K. Similar to the observations for LSM and LSM-YSZ cathodes when considered separately (Figs. 4 and 7), there was homogeneous distribution of particles and pores in both layers of the porous LSM-YSZ/LSM cathodes, the particle size and density of the electrodes (and consequently, the measured apparent electrical conductivity) increased as the porosity decreased with increasing sintering temperature. The similarities in the microstructure and magnitude of the apparent electrical conductivities of the double-layer LSM-YSZ/LSM cathodes sintered once (Fig. 10a) and those sintered twice (Fig. 10b), show that the sintering approach (cosintering or double sintering) does not have a significant effect on the microstructure and consequently on the apparent electrical conductivity of the porous double-layer cathode.

(b)

6

1273 K

1323 K

1373 K

1423 K

6

1473 K

σ / 103 S m-1

5

σ / 103 S m-1

1273 K

1323 K

1373 K

1423 K

1473 K

5

4 3 2

4 3 2 1

1 650

700

750

800

850

900

950

Temperature / K

1000 1050 1100

0 650

700

750

800

850

900

950

1000

1050

1100

Temperature / K Fig. 8. Temperature dependence of the apparent electrical conductivity of LSM-YSZ/LSM double-layer cathode fabricated by dip coating (a) single sintering (b) double sintering.

O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

( a)

( b) 15.8

1273 K

15.6

1323 K

1373 K

1423 K

15.6

1473 K

15.4 15.2

15.2

R² = 0.9981

15

R² = 0.9994

14.8

ln(σТ / S m-1 K)

15.4

ln (σТ / S m-1 K)

9

R² = 0.9979

14.6 14.4

R² = 0.9978

14.2

R² = 0.9938

14.8

R² = 0.9835

14.6 14.4

R² = 0.9992

14.2 13.8 13.6

0.9

1

1.1

1.2

103 K / T

1.3

1.4

1.5

13.4

R² = 0.9993

1273 k 1323 K 1373 K 1423 K 1473 K

14

14 13.8

R² = 0.997

15

0.9

R² = 0.9985

1

1.1

1.2

1.3

1.4

1.5

103 K / T

Fig. 9. Plot of ln (sТ) versus the reciprocal of absolute temperature for LSM-YSZ/LSM double-layer cathode fabricated by dip coating (a) single sintering (b) double sintering.

Fig. 10. SEM micrographs of LSM-YSZ and LSM layers of the LSM-YSZ/LSM double-layer cathodes fabricated on YSZ electrolyte hollow fibres by dip coating (a) single sintering (b) double sintering.

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O.O. Agbede et al. / Materials Today Chemistry 16 (2020) 100252

4. Conclusions a) LSM (La0.80Sr0.20MnO3), LSM-YSZ (50 wt% La0.80Sr0.20MnO3 and 50 wt% (Y2O3)0.08(ZrO2)0.92) composite and LSM-YSZ/ LSM double-layer cathodes were successfully fabricated on YSZ electrolyte hollow fibres by dip coating, with excellent cathode-electrolyte and cathode-cathode adhesion, without delamination of the cathode from the electrolyte. b) The apparent electrical conductivities of porous LSM, LSMYSZ and LSM-YSZ/LSM cathodes fabricated on YSZ hollow fibre by dip coating and sintered at various temperatures in the range 1273e1473 K for 3 h, were 1.8
103e5.5
103 S/m, 0.32e209 S/m and 1.3
103e5.5
103 S/m, respectively, at measurement temperature of 673e1073 K. c) The densification of porous LSM, LSM-YSZ and LSM-YSZ/LSM cathodes with increasing sintering temperature resulted in an increase in apparent electrical conductivity with increasing sintering temperature. d) The operating temperature dependence of the apparent electrical conductivity of the LSM, LSM-YSZ and LSM-YSZ/ LSM cathodes was defined by the Arrhenius equation sT ¼ so expðEa =kB TÞ for electrical conductivity. e) The activation energies for electrical conductivity of LSM, LSM-YSZ and LSM-YSZ/LSM cathodes were 0.106e0.147 eV, 0.83e0.94 eV, and 0.104e0.146 eV, respectively. f) The YSZ phase strongly influenced the electrical conductivity of the LSM-YSZ composite cathode, while the LSM layer greatly influenced the electrical conductivity of the LSM-YSZ/ LSM cathode.

Credit author statement Oluseye Omotoso Agbede: Conceptualization, Methodology, Investigation, Visualization, Formal Analysis, Writing- Original draft preparation, Writing- Reviewing and Editing, Project Administration. Klaus Hellgardt: Resources, Fund Acquisition, Conceptualization, Methodology, Supervision. Geoffrey H. Kelsall: Resources, Fund Acquisition, Conceptualization, Methodology, Supervision. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. Declaration of competing interest None. Acknowledgement The authors thank the Petroleum Technology Development Fund (PTDF), Nigeria for a studentship awarded to Oluseye Agbede. References [1] IPCC special report on carbon dioxide capture and storage, in: B. Metz, O. Davidson, H.C. de Coninck, M. Loos, L.A. Meyer (Eds.), Prepared by Working Group III of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2005, p. 442. [2] International Energy Agency, Worldwide Trends in Energy Use and Efficiency Key Insights from IEA Indicator Analysis, 2008. http://www.iea.org/ publications/freepublications/publication/Indicators_2008.pdf. (Accessed 15 June 2017).

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