Author’s Accepted Manuscript Comparative electrochemical investigation of zinc based nano-composite anode materials for solid oxide fuel cell Fida Hussain, Ghazanfar Abbas, M. Ashfaq Ahmad, Rizwan Raza, Zohaib Ur Rehman, Saleem Mumtaz, M. Akbar, Raja Ali Riaz, Saad Dilshad www.elsevier.com/locate/ceri
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S0272-8842(18)32763-9 https://doi.org/10.1016/j.ceramint.2018.09.288 CERI19687
To appear in: Ceramics International Received date: 1 August 2018 Revised date: 17 September 2018 Accepted date: 28 September 2018 Cite this article as: Fida Hussain, Ghazanfar Abbas, M. Ashfaq Ahmad, Rizwan Raza, Zohaib Ur Rehman, Saleem Mumtaz, M. Akbar, Raja Ali Riaz and Saad Dilshad, Comparative electrochemical investigation of zinc based nanocomposite anode materials for solid oxide fuel cell, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.288 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Comparative electrochemical investigation of zinc based nanocomposite anode materials for solid oxide fuel cell Fida Hussaina, Ghazanfar Abbasb*, M. Ashfaq Ahmadb, Rizwan Razab,c, Zohaib Ur Rehmanb, Saleem Mumtazd, M. Akbarb, Raja Ali Riaza, Saad Dilshada a
Department of Electrical Engineering, COMSATS University, Islamabad-44000, Pakistan Department of Physics, COMSATS University Islamabad, Lahore Campus-54000, Pakistan c Department of Energy Technology, Royal Institute of Technology (KTH), Stockholm 10044, Sweden d Institute of Chemical Sciences, Bahauddin Zakariya University, 60800 Multan, Pakistan b
*
Corresponding Author: Department of Physics, COMSATS University Islamabad, Lahore CampusPakistan,Tel.: +923007506858;
[email protected]
Abstract The structural and electrochemical properties of zinc based nano-composites anode materials with a composition of X0.25Ti0.05Zn0.70 (where X= Cu, Mn, Ag) have been investigated in this present study. . The proposed X0.25Ti0.05Zn0.70 oxide materials have been synthesized through solgel method. The doping effect of Cu, Mn, and Ag on TiZn oxides were analyzed in terms of electronic conduction and power density in hydrogen atmosphere at comparatively low temperature in the range of 650oC. The crystal structure and surface morphology were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis techniques. The XRD patterns of composites depict that the average crystalline sizes lie in the range of 20-100 nm. Four-probe DC conductivity technique was used to measure the conductivity of the materials and maximum electrical conductivity of Ag0.25Ti0.05Zn0.70 oxide was found to be 7.81 S/cm at 650oC. The band gap and absorption spectra were determined by ultra-violet visible (UV-Vis)
and Fourier Transform Infrared spectroscopy (FTIR) techniques respectively. The maximum power density was achieved to be 354 mW/cm2 at 650oC by Ag0.25Ti0.05Zn0.70 oxide anode with SDC (electrolyte) and BSCF (conventional cathode) materials. Keywords: Nano-composite anode, low temperature SOFC, zinc based materials, Non-Symmetrical Cell, silver catalyst
1. Introduction Fuel cell can be considered one of the best promising technologies for power applications and has great consequences in energy conversion sector [1-3]. Fuel cell converts chemical energy of hydrogen (fuel) into electrical energy with low emissions and great efficiency by electrochemical reaction [4-6]. Solid oxide fuel cell (SOFC) has many advantages over other types of fuel cells like enabling quick kinetic process due to its operating temperature range and can be operated under large electric current due to its robust behavior [7, 8]. Due to its solid structure, solid oxide fuel cell does not face any kind of leakage and corrosion [9]. In context of fuel flexibility pros, apart from hydrogen, SOFC can use hydrocarbon, ammonia, syngas, vegetable oil, and even an untreated coal as fuel [10-14]. However, high cost and conventional high operating temperature (1000oC) are the main hurdles to bring into market at commercial state [15-16]. Thermally and chemically compatibility with electrolyte material, , high electronic conductivity to create minimum resistance to escape electrons from the fuel cell, fine particle size, high porosity, and large surface area are the main properties of anode materials [17,18]. Nano structuring technique is the best tool to obtain the above mentioned properties of the anode materials which got great interest of researchers with an aim to lower the manufacturing and
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operating temperature including cost effective benefits [16, 19]. Conventionally, Ni based anodes are used in SOFC, but they create many problems like sulfur poisoning, nickel sintering, carbon sulfur deposition. The said problems can cause the degradation and instability in the material even in fabricated cell. Therefore, the attention has been modified towards Ni free anodes to overcome the addressed problems/drawbacks [19]. Initially the study is motivated to produce novel Ni free anodes which may work at comparatively low temperature at equal dimensions and characteristics as conventionally. Silver (Ag) and copper (Cu) is widely analyzed because of its superb electrical conductivity. It has been studied that titanium oxide (TiO2) has tremendous behavior to increase the structural stability under reduced atmosphere [20, 21]. TiO2 combination with other materials can improve electronic properties and oxidation activity [22]. Lower price of titanium oxide can also contribute to decrease the overall cost of fuel cell [23]. Zinc (Zn) contributes to decrease the polarization losses, stabilize the material, and also used to improve the electrical conduction [19]. The current study is motivated to decrease operating temperature and/or to improve properties and lower the cost of SOFC. In this study Ag0.25Ti0.05Zn0.70, Mn0.25Ti0.05Zn0.70, Cu0.25Ti0.05Zn0.70 composites are synthesized as anode materials for SOFC. These compositions are characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Fourier transform infrared (FTIR), UV-Visible spectroscopy, conductivity and fuel cell performance measurements.
2. Experimental 2.1. Synthesis of Anode Materials The anode materials with sample compositions X0.25Ti0.05Zn0.70 (Where X= Cu, Mn and Ag) were synthesized through sol-gel method. Ag(No)3, Cu(NO3)2.6H2O (Sigma Aldrich), 3
Mn(NO3)2.H2O (Sigma Aldrich), TiO2 (Sigma Aldrich), and Zn (NO3)2.6H2O (Sigma Aldrich) were utilized as starting materials. Nitrate materials were solved in 300ml deionized water, while TiO2 in nitric acid. Then the nitrates and acids solutions were combined for each sample. The citric acid was used as chelating agent and 20wt. % of citric acid was mixed in combined solutions for each sample followed by stirring on a hot plate with magnetic stirrer @ 300rpm at temperature of 80oC for several hours until gel fabrication. Gels of CTZ oxide (CuTiZnO) and MTZ oxide (MnTiZnO) were fine dried, but in drying of ATZ oxide (AgTiZnO) self-combustion took place by giving us a brown powder. After grinding, mixtures were sintered in furnace at 700oC for 4 hours. The samples were collected after furnace cool and grinded one by one to make homogenized composite oxides.
2.2. Conductivity measurements The electrical conductivities of prepared anode materials were tested by making the pellets of each sample. For this purpose, three pellets of CTZO, MTZO, and ATZO were fabricated via dry pressing under a pressure of 4000Pa. The dimensions of pellets were controlled as 13mm diameter and 2mm thickness. The fabricated pellets were then sintered at 670oC for 50 min. Silver paste is coated with soft brush on both sides of the pellets to provide good current contacts. The electrical conductivities of prepared anodes were measured in the temperature range of 300-650oC using 4-probe DC method by Keithley instrument, Taiwan. Following equation is used to calculate conductivity;
L / ( R * A)
(1)
Where σ, L, R, and A show conductivity, thickness, resistance and active area (0.64cm2) of the pellets respectively.
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2.3. Calculation of Activation Energies (Ea) From the given data of electrical conductivities, Arrhenius curves were plotted in order to calculate the activation energies. The following formula was applied for calculations;
A *exp Ea k *T
(2)
Where σ, A, Ea, k, and T represent the conductivity, exponential factor, activation energy, Boltzmann constant and absolute temperature in kelvin respectively.
2.4. Fuel Cell Performance Measurements Three consecutive layers of anode/electrolyte/cathode for fuel cells have been fabricated through dry pressing technique. Samarium doped ceria (SDC) and BSCF were used as electrolyte and cathode materials respectively with prepared XTZO anodes [24,3]. The prepared XTZO anodes (X = Ag, Cu, and Mn) were further modified by mixing as 20 wt. % SDC electrolyte and 80wt. % anode(s). The said weight ratios were mixed with mortar and pestle for each anode and used as composite anode materials in order to enhance the ionic conduction mechanism. The illustrated three layer fuel cell scheme was mentioned as 80wt. % XTZO-20wt. % SDC / SDC / BSCF for composite anode/electrolyte/cathode respectively. The fuel cell dimensions were controlled as 13mm diameter and 1mm thickness of each cell/pellet. However the cell layers were kept 40mm, 35mm, and 25mm as anode, electrolyte and cathode respectively. The pressed cells/pellets were then sintered at 650oC for 50 min. In order to improve the electrical contacts, outside surfaces of cells were coated with silver paste. Fuel cell performance of each cell was obtained by using fuel cell testing unit S12, China at 650oC. The H2 (purity 99.999%) at anode side as a fuel and O2 at cathode side as oxidant were
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used to complete the reaction process. Flow rate of hydrogen was 110 ml/min at a pressure of 1 atm.
2.5. Characterizations PAN-Alytical X'Pert Pro MPD, Netherlands was used for X-ray diffraction to acknowledge the crystal and phase analysis with Cu Kα radiation (having scanning rate 0.005, 30 mA current and 35kV voltage at room temperature). Philips XL-30, Netherlands was used for SEM analysis to observe porosity, particle size, and surface morphology of the prepared anode samples. Perkin Elmer Lambda 750, USA, was used for band gap measurements in the wavelength range 300800 nm. FTIR spectroscopy with a spectral range of 4000-500 cm-1 was observed by using Perkin Elmer spectrum RX I, USA.
3. Results and Discussions 3.1. Structural Analysis The XRD patterns have been displayed in figure 1 (a-c). The crystallographic structure of anode materials X0.25Ti0.05Zn0.70 oxide (where X = Cu, Mn, Ag) sintered at 700oC for 4 hours. Usually material ratios are altered to find out the best results in a series of samples; however, in the present study one component is changed having the same ratios (X0.25Ti0.05Zn0.70 oxide (where X = Cu, Mn, and Ag). The crystallite sizes were calculated from the XRD patterns and found to be nano-structure in the range of 20-100nm. The crystallite sizes of each sample have been listed in Table 1. The possible uncertainty in the calculation of crystallite sizes of the prepared materials has been evaluated with the help of standard deviation formula; the used formula is given as:
6
( xi x)2 S .D. N 1 i N
(3)
Where xi is the value of crystallite size and i = 1, 2, 3, ….., ̅ is the average value of crystallite size and N is the number of measurements. However, the values of evaluated errors are represented in small brackets in the Table 1. The XRD patterns of Cu0.25Ti0.05Zn0.70 oxide shown in figure 1(a) were examined by Match! Software and found to be two phases. The ZnO phase has hexagonal structure (COD Card No. 96-900-4179) and the other CuTiZn has a cubic structure (COD Card No. 96-152-4535). Figure 1(b) depicts the XRD patterns of Mn0.25Ti0.05Zn0.70 oxide and shows two-phase structure. The ZnO phase has hexagonal structure (COD Card No. 96-101-1259) and MnO phase has tetragonal structure (COD Card No. 96-810-3497). However, the XRD patterns do not show any peak of TiO2 contents. It has been observed that the peaks of small amount (ratio 0.05mole) of TiO2 may be doped in Zn-oxide. This dopant phenomenon can be observed with a peak at angle of 2θ = 43o. It has been reported that the doping of TiO2 into ZnO improve the quality of the material [25]. In literature it is also cited that less amount of Ti-oxide may enhance electrical properties of the composite material [26-27]. The XRD patterns of Ag0.25Ti0.05Zn0.70 oxide has been displayed in figure1(c). The results show that the patterns of AgTiZn oxide have also two-phase structure. Once again ZnO has hexagonal structure (COD Card No. 96-101-1259) while AgTi has tetragonal structure (COD Card No. 96-150-9562). Some noise has been detected (intensity lower than 100) which may be due to instrument alignment issues or water vapors [16]. In the XRD patterns indexing of X0.25Ti0.05Zn0.70 oxide, it has been noticed that the ZnO has hexagonal structure and found to be common in all samples whereas X content in composition almost varies its structure. 7
3.2. SEM Analysis The SEM results of X0.25Ti0.05Zn0.70 oxides are shown in figure 2(a-c). The micrographs of each sample have been observed deeply and found to be porous in structure. The porous structure is a fundamental characteristic of electrode/anode materials for solid oxide fuel cell. During the cell reaction, porous structure gives advantage of transferring ions/electrons that come from any electrode side. The particle sizes of each proposed synthesized materials X0.25Ti0.05Zn0.70 oxide were observed by line drawing method from obtained micrographs. The results of observations were found to be in the range of 20-100 nm. The obtained results of particle sizes from SEM analysis execute well agreement of the crystallite sizes that than calculated by Scherer’s formula from XRD patterns. Small particle size of X0.25Ti0.05Zn0.70 oxides can play vital role to increase performance and conductivity of the cell. Energy dispersive spectroscopy (EDS) spectrum of Ag0.25Ti0.05Zn0.70 oxide is shown in figure 2(d), where the desired stoichiometry is confirmed by all the elements.
3.3. UV-Visible and FTIR Analysis The UV-Visible spectrum of the Ag0.25Ti0.05Zn0.70 oxide (only one sample on behalf of high conductivity and power density) was obtained in the range of 300-800 nm and is shown in figure 3. The band gap can be calculated by following equation;
h
n
A(h Eg )
(3)
where α, h, Eg, A, n are absorbance coefficient, Plank’s constant, band gap, constant, and type of band gap. In UV spectrum, strong absorption was observed in the range of 300-500 nm regions. The materials information regarding structural and phase transformations can be found through FTIR 8
spectroscopy. FTIR spectrum of Ag0.25Ti0.05Zn0.70 has been shown in figure 4 in the range of 4000−750 cm-1. In the obtained spectrum, several peaks were observed at 3749, 3649, 2173, 2002, 1634, 1516, 1384, and 891 cm-1. The large absorption bands comparatively to other peaks were found at 3749 cm-1 and at 1516 cm-1. The peak originated at 891 cm-1 is most probably due to the groups of NO3− [28-29], while the small peak at 1384 cm-1 is related to Ti−O modes [3031]. The other peak shown at 1516 cm-1 is corresponded to Ag nano-particles [32] and adjacent peak at 1634 cm-1 is due to stretching of Zn−O [33].
3.4. Conductivity Analysis Appropriate electrical conductivity is a major parameter to get better performance results of electrode materials [34]. Four-probe DC measurement method was implemented to get results of electrical conductivity for X0.25Ti0.05Zn0.70 oxide (X = Cu, Mn, and Ag) nano-composite in the temperature range of 300-650oC at air atmosphere individually. The results of measurements were presented in figure 5. It has been found that the electrical conductivity of each sample increases with the increase in temperature. However, the maximum electrical conductivities were achieved at temperature 650oC with an order of Ag0.25Ti0.05Zn0.70 (7.81 S/cm1) ˃ Mn0.25Ti0.05Zn0.70 (7.19 S/cm1) ˃ Cu0.25Ti0.05Zn0.70 (6.01 S/ cm1). Among the three samples, the Ag0.25Ti0.05Zn0.70 possesses a maximum conductivity of 7.81 S/cm1 due to higher international annealed copper standard (IACS) value than Cu and Mn. However Cu is widely used world-wide for electrical purposes because it is cheap as compare to pure silver [35]. The Arrhenius plots were drawn of each conductivity data in order to elucidate the activation energy. The activation energies of Ag0.25Ti0.05Zn0.70, Mn0.25Ti0.05Zn0.70, and Cu0.25Ti0.05Zn0.70 were found to be 0.65, 0.50, and 0.21eV, respectively and prescribed in legend of figure 5. The lowest value of activation energies ensures the shortest time required to start the chemical reaction. Usually 9
electrical conductivity is considered 10 times higher than that of ionic conductivity of electrolyte used in the cell [36-37]. In this present study, all the samples have approximately 60-80 times higher electrical conductivity at 650oC than that of SDC electrolyte which contains ionic conductivity 0.1S/cm [19]. Thus among X0.25Ti0.05Zn0.70 oxide as anode; Ag0.25Ti0.05Zn0.70 is considered a good material under intermediate temperature. From figure it is also noted that conductivity is directly proportional to temperature and this reveals that composite oxides act like semiconductor [38-39].
3.5. Performance Analysis The SDC and BSCF as an electrolyte and cathode materials, respectively were used with X0.25Ti0.05Zn0.70 oxide anode in order to complete three layers fuel cell. In the presence of H2, fuel cells testing were performed in order to check open circuit voltage (OCV) and its corresponding current at various resistances using rheostat. The I-V data was collected at the temperature of 650oC for all three samples using Ag0.25Ti0.05Zn0.70 / Mn0.25Ti0.05Zn0.70 / Cu0.25Ti0.05Zn0.70 as anode schematic earlier in section 2.3. The power density was also calculated from the I-V data and current densities vs power densities curves were drawn and the results of measurements were displayed in figure 6. The active area of the cell has been taken into account 0.64cm2. It has been noted that the maximum open circuit voltage (OCV) and power density were achieved 1.047 V and 354mW/cm2, respectively of the sample Ag0.25Ti0.05Zn0.70 oxide at 650oC using hydrogen as fuel and air as oxidant. The maximum power density achievement is due to the contribution of silver contents that has higher conductivity values as compare to Cu and Mn contents. The results were taken by implementing fuel cell testing unit S12, China. In figure 7, a four axis chart has been drawn to advertise the complete information regarding crystallite sizes, electrical conductivities, power densities, and activation energies obtained from 10
each sample of X0.25Ti0.05Zn0.70 oxide anode materials. The numerical data also has been displayed in Table 1 in respect of crystallite sizes, OCVs, current and power densities at operating temperature of 650oC. The composite oxide with higher values of electrical conductivity has performed maximum power density.
4. Conclusion The X0.25Ti0.05Zn0.70oxide anode materials have been successfully synthesized via sol-gel technique for fuel cell application. The sintering ability of 700oC for 4 hours ensures the crystalline structure of the X0.25Ti0.05Zn0.70 oxide materials. The effect of Cu, Mn, and Ag oxides were analyzed on zinc based TiZn oxide nano-composite with respect to electronic conduction, current and power densities. XRD and SEM both analyses indicate that their particle sizes were lies in between 20-100nm. The obtained crystallite sizes of all proposed materials were found strongly in the recommendation of nano-scale. The maximum conductivity for Ag0.25Ti0.05Zn0.70 oxide was achieved to be 7.81 S/cm1 at 650oC and the corresponding activation energy is 0.65 eV. The anode material with silver oxide catalytic Ag0.25Ti0.05Zn0.70 oxide has performed better result having maximum power density of 354 mW/cm2 and 1.047V OCV at 650oC. On behalf of the obtained electronic conductivity, OCV, and power density including nano-structuring technique, the proposed material can be considered one of the best alternative cheap and nickel free anode for intermediate temperature solid oxide fuel cell.
Acknowledgments The financial support under the Indigenous 5000 scholarship scheme Vide PIN No. 112-245252EG1-388 from Higher Education Commission (HEC) of Pakistan is acknowledged. COMSATS University Islamabad, Lahore Campus is also acknowledged for providing lab facilities.
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