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High-temperature electrolysis of synthetic seawater using solid oxide electrolyzer cells
Journal of Power Sources 342 (2017) 79e87
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
High-temperature electrolysis of synthetic seawater using solid oxide electrolyzer cells Chee Kuan Lim a, b, c, *, Qinglin Liu a, b, Juan Zhou a, b, Qiang Sun a, d, Siew Hwa Chan a, b, c, ** a
Singapore-Peking University Research Centre, Campus for Research Excellence & Technological Enterprise (CREATE), Singapore 138602, Singapore Energy Research Institute at NTU (ERIAN), Nanyang Technological University, Singapore 639798, Singapore c School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore d College of Engineering, Peking University, Beijing 100871, China b
h i g h l i g h t s Contaminants have not been found in the steam of seawater. Performance of cells running on pure water and seawater are the same. Contamination of sea salt has no in-situ and ex-situ effects on the cell. Sea salt is volatile at the typical operating temperature of the cell.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2016 Received in revised form 4 December 2016 Accepted 5 December 2016
A Ni-YSZ/YSZ/LSCF-GDC solid oxide electrolyzer cell (SOEC) is used to investigate the effects of seawater electrolysis for hydrogen production through electrolyzing steam produced from simulated seawater bath. Steam electrolysis using an SOEC with its fuel electrode contaminated by sea salt is also investigated. Steam produced from seawater is found to be free of contaminants, which are present in the seawater. Similar electrochemical performance is observed from the polarization curves and impedance spectra when using steam produced from pure water and seawater. Their short-term degradation rates are similar, which are registered at 15% 1000 h1 for both cases. For the case of direct sea salt contamination in an SOEC's fuel electrode, both the uncontaminated and contaminated cells exhibit rather similar performance as observed from the polarization curves and impedance spectra. The difference in ASR values from the polarization curves and impedance spectra between the uncontaminated and contaminated cell are all within a 10% range. Rather similar short-term degradation rates of 15% 1000 h1 and 16% 1000 h1 are recorded for the uncontaminated and contaminated cells, respectively. Post-mortem analysis shows that the sea salt impregnated into the cell has been vaporized at a typical SOEC operating temperature of 800 C over the period of operation. © 2016 Elsevier B.V. All rights reserved.
Keywords: High-temperature electrolyzer Seawater electrolysis Solid oxide cell (SOC)
1. Introduction In recent years, rising energy costs and changing of global
* Corresponding author. Singapore-Peking University Research Centre, Campus for Research Excellence & Technological Enterprise (CREATE), Singapore 138602, Singapore. ** Corresponding author. Singapore-Peking University Research Centre, Campus for Research Excellence & Technological Enterprise (CREATE), Singapore 138602, Singapore. E-mail addresses: [email protected] (C.K. Lim), [email protected] (S.H. Chan). http://dx.doi.org/10.1016/j.jpowsour.2016.12.019 0378-7753/© 2016 Elsevier B.V. All rights reserved.
climate due to the combustion of fossil fuels are of major concerns [1]. In order to combat the depletion of fossil fuels and to reduce carbon emissions, the use of fossil fuels must be replaced by renewable resources. One of the fundamental problems associated with the use of renewable sources is that a balance between electrical energy supply and demand is required. Therefore, storing excess energy is essential during times when the supply exceeds the demand. Several energy storage technologies such as compressed air energy storage (CAES), flywheel energy storage (FES), pumped hydro energy storage (PHES) and batteries are potential technologies to store energy [2]. However, some of the disadvantages associated with these technologies are the high capital costs
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required for large scale energy storage, transportability issues, high dependence on appropriate geographical regions as well as energy losses such as mechanical losses in flywheels and loss of charge overtime due to self-discharge in batteries. Hence, there has been an increasing interest in chemical energy storage particularly hydrogen in recent years [3]. This is mainly due to hydrogen being a clean alternative energy carrier as well as its capability of being converted into hydrocarbon fuels [4]. Hydrogen must be generated from hydrogen-containing substances since it is not readily available as a fuel on Earth [5]. An alternative green method to produce pure hydrogen is through the electrolysis of water using renewable energy or excess power from thermal power plants. The development of low-temperature water electrolysis technologies such as alkaline and proton exchange membrane based water electrolyzers has been ongoing for many years [6e8]. Solid oxide fuel cells (SOFCs) have gained popularity decades ago. Many researchers, such as Sun et al. and Wang et al., have studied the fabrication techniques, electrode development as well as operating parameters of SOFCs extensively [9e11]. The reverse of SOFCs, which are the solid oxide electrolyzer cells (SOECs), has gained much attention lately due to its advantages over low-temperature electrolysis for its excellent efficiency in hydrogen production (28e39 kwh1 H2) [12]. During the early 1980s, steam electrolysis in SOECs for production of hydrogen started to undergo development as an interesting alternative to low-temperature water electrolysis [13e15], where popular research areas are related to its materials, design and operating parameters. For example, researchers like Moritz et al. have studied the effect of pressure on solid oxide cells [16,17]. From a thermodynamic point of view, high-temperature electrolysis of steam using SOECs is a promising technology to produce hydrogen as it requires less electrical energy per unit hydrogen produced as compared to low-temperature water electrolysis due to the combination of more favorable thermodynamics and kinetics at high temperatures [18,19]. An increase in operating temperature leads to a reduction in electrical energy demand for steam electrolysis and hence a reduction in the production cost of hydrogen. As of today, most of the water electrolysis technologies are related to freshwater electrolysis. However, only less than 1% of all the freshwater on Earth are accessible to us at an affordable cost, whereas 97.5% of water on Earth is seawater which is more abundant than freshwater [20]. Electrical energy generated from renewable energy technologies such as offshore wind and wave farms can be used to directly electrolyze steam produced from seawater, which has a higher availability around these regions as compared to freshwater. Research and developments related to electrode materials for low temperature seawater electrolysis to resolve problems such as chlorine production at the anode and the gradual build-up of insoluble precipitates at the cathode surface are gaining popularity [21e23]. Nevertheless, high temperature steam electrolysis of seawater using SOECs remains a new research which is worthwhile to be explored. Most studies related to SOECs are carried out using pure water as a source of moisture content. The degradation mechanisms in SOECs based on pure water had been studied extensively over the years. The typical causes of degradation in the fuel electrode of an SOEC are the relocalization of nickel due to formation of Ni(OH)2 [24], electrode poisoning due to the formation of nickel sulfides and Si(OH)4 [25,26], changes in electrode microstructure such as coarsening of nickel grains due to the sintering effect [27] and local oxidation of nickel in the Ni-YSZ electrode at higher moisture content (>50% pH2O) [28]. There are several ways to obtain the desired moisture content in the feedstock gas; one of the ways is by bubbling feedstock gas through the seawater bath using a gas tube and another way is by
direct spray injection of seawater into the SOEC using an atomizing spray nozzle. In an actual case of seawater electrolysis using an SOEC, spray injecting seawater into a heated chamber to provide the required moisture content in the feedstock gas is one of the most practical ways. By directly using seawater, there is a high possibility of contamination by sea salt in the fuel electrode. The potential damages of sea salt on the fuel electrode include poisoning of nickel and blocking of triple phase boundaries near the electrode-electrolyte interface which reduces catalytic activity. Hence, the electrochemical performance and degradation mechanism in SOECs utilizing seawater remains unknown. In this paper, two cases related to seawater electrolysis using SOECs have been investigated. 1. The first case involves the investigation of the purity of steam produced from seawater to check for any impurities in the steam originated from seawater during vaporization process. 2. The second case involves the investigation of the simulated case for contamination of sea salt in the Ni-YSZ fuel electrode of a button SOEC on its electrochemical performance and degradation behavior. A button cell consisting of Ni-YSZ as fuel electrode, YSZ as electrolyte and LSCF-GDC as air electrode was tested for these two cases to show the feasibility of SOECs for direct seawater electrolysis application. Post-mortem analysis, which includes scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), have been performed on the cells before and after operation. 2. Experimental 2.1. NiO-YSZ/YSZ/LSCF-GDC cell fabrication The fuel electrode (cathode) supported button cells used in this study were prepared by homogeneously mixing 60 wt % nickel oxide powder and 40 wt % yttria-stabilized zirconia powder (8 mol % Y2O3-stabilized ZrO2) with a small amount of carbon black (poreformer) in ethanol using ball milling overnight. After drying on a hot plate to eliminate the solvent, a small amount of binder was added into the electrode substrate using a mortar and pestle. The mixture was then allowed to dry and NiO-YSZ powder was compacted under a uniaxial pressure to form circular disks of 24 mm in diameter. The electrode disks were then baked at 900 C to increase the mechanical strength for handling and to remove the poreformer. YSZ electrolyte suspension was applied sequentially onto the baked fuel electrode discs using a spray coating method. The coated fuel electrode disks were subsequently sintered at 1400 C for 3 h to obtain dense YSZ electrolyte films supported on the substrates. A 50-50 wt% of La0.8Sr0.2Co0.2Fe0.8O3d (LSCF) e Ce0.8Gd0.2O2d (GDC) composite oxygen electrode (anode) was prepared by mixing LSCF-GDC50 powder with a proper amount of polyethylene glycol. The mixture was then applied onto the center of the electrolyte film by screen printing technique, followed by sintering at 900 C for 2 h to form a porous electrode layer with an active area of 0.5 cm2. A porous Pt layer was coated on both electrodes to serve as the current collectors to minimize the contact resistance between the electrodes with the Pt mesh. Subsequently, sintering was carried out at 900 C. EDX was performed on the cross-section and electrode-electrolyte interfaces to ensure that no Pt has infiltrated into the electrochemically active sites of the electrode. The thickness of the NiO-YSZ cathode, YSZ electrolyte and LSCF-GDC anode are roughly 1 mm, 10 mm and 35 mm respectively. The cross-sectional view of the completed button cell which is sealed to an alumina tube is shown in Fig. 1.
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Fig. 1. Experimental set-up for the electrochemical performance testing of a button SOEC fed with simulated seawater.
2.2. Equipment set-up Fig. 1 shows the experimental set-up for simulated seawater electrolysis in this study. The button cell was sealed with a glass sealant to separate the gas environment between the cathode and the anode. A temperature of 800 C was chosen for the reduction of NiO-YSZ as well as the operating temperature of the cell during electrochemical testing. The water bath was heated up to provide a high moisture content of 70% pH2O balanced with H2. Dry H2 was fed into the water bath at a flow rate of 50 mlmin1, whereas the oxygen electrode side was exposed to ambient air. The cell was left for about 3 h after reaching its operating temperature and fed with H2 to ensure complete reduction of nickel oxide before the electrochemical performance of the cell was measured. The pipe connecting the water bath to the SOEC was heated up to more than 100 C to prevent any condensation of steam in the pipe. Platinum gauze was used for current collection on both sides of the cell. Each electrode side was connected to two platinum wires, which serve as respective voltage and current probes to eliminate the contribution of platinum wire to the total resistance. 2.3. Preparation of simulated seawater The simulated seawater was prepared by dissolving sea salt in pure water to obtain the average salinity of actual seawater which is about 3.5%. The main constituents of sea salt and their respective melting points can be found in Table 1. Some other trace amounts of elements which include bromine, carbon, amino acids, etc., are considered to be insignificant. The seawater bath was heated up to provide a moisture content of 70/30 pH2O/pH2 in the feedstock gas under a high flow rate of H2. The steam in the feedstock gas (humidified H2) was condensed and collected for purity testing. The presence of chloride and sulfate
Table 1 Main components of sea salt and their melting points. Type of salt
Percentage (%)
Melting point ( C)
NaCl MgCl2 MgSO4 CaSO4 KCl NaBr
77.83 9.66 6.10 4.05 2.11 0.24
801 714 1124 1460 770 747
anions in the steam condensate was tested using an ion chromatograph mass spectrometry (Metrohm Model 930 Compact IC Flex). 2.4. Electrolysis of steam produced from seawater For comparison purpose, one button cell was tested in this case, first using steam produced from pure water and followed by steam produced from seawater. The water bath (Fig. 1) consisting of pure water and followed by seawater was heated up to provide a feedstock gas containing 70/30 pH2O/pH2. The cell was left in the test station under open circuit voltage for 24 h for stabilization after the switch from pure water to seawater before any measurements were taken. For both cases, their electrochemical performance was evaluated by measuring the IeV polarization curves and impedance spectra of the cell. A durability test was also conducted for a total of 120 h. 2.5. Direct contamination of sea salt in cell In this case, a contaminated cell was being tested under the same conditions as the one in the case of electrolysis of steam produced from pure water and seawater. Only pure water bath
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was used to obtain the required moisture content. The results were then compared with the uncontaminated cell using steam produced from pure water in the previous case. For the preparation of the contaminated cell, sea salt was dissolved in pure water up to the point of saturation at room temperature. The initial weight of a button cell was recorded and a small amount of saturated seawater solution was applied onto the fuel electrode and was allowed to penetrate into the porous NiO-YSZ fuel electrode. This will ensure that sea salt is present in the whole volume as well as at the triple-phase boundaries of the electrodeelectrolyte interface. The cell was then allowed to dry in air before its final weight was taken. This process is repeated until 0.03 g of sea salt was deposited into the porous fuel electrode. SEM and EDX were used to verify that sea salt is present at the cross-section and electrode-electrolyte interface. 2.6. Electrochemical performance measurement The button cell's electrochemical performance was characterized by polarization curves (IeV) and electrochemical impedance spectroscopy (EIS). Polarization curves were generated using linear sweep current techniques, where a potentiostat/galvanostat instrument (Solartron Model 1470E) was used to control the voltage between 0.8 and þ 0.8 V of bias against OCV at a sweep rate of 1 mV1. EIS measurements were performed using a sinusoidal signal amplitude of 10 mV across a frequency range of 100 kHz to 0.05 Hz using a frequency response analyzer (Solartron Model 1255B). All the impedance spectra were fitted to curves using ZView2 by constructing an electrical circuit consisting of an inductor in series with a resistor and two other resistors in parallel with a constant phase element (CPE). All the fitted values had reasonable errors of within 10% from the original values. 2.7. Ex-situ characterization SEM and EDX analysis were then carried out on the surface, cross-section and electrode-electrolyte interface of the fuel electrode at the end of every durability test for both cases. IC-MS was carried out on the condensate of steam produced from simulated seawater to check its purity due to the possibility of sea salt being transported by the steam. Thermogravimetric analysis (TGA Model Q500) was performed on the sea salt used in this study to understand its thermal properties such as the disintegration/vaporization temperature.
3. Results and discussion 3.1. Electrolysis of steam produced from seawater Fig. 2 shows the impedance spectra measured under open circuit condition with a feedstock gas composition of 3/97 pH2O/pH2 and 70/30 pH2O/pH2. It can be seen that the polarization resistance of the cell, which is contributed by the electrode processes, decreases significantly with an increase in moisture content. However, the ohmic resistance, which is mainly contributed by the electrolyte resistance, has remained unaffected. These observation is well known and is consistent with similar results obtained in the literature [29]. Two arcs can be clearly distinguished in the frequency domain, suggesting that the overall electrochemical reaction is dominated by at least two electrode processes in series. It is understood that the high frequency arc is associated with the interfacial charge transfer at the electrode-electrolyte interface, while the lower frequency arc is associated with the gas-solid interactions (adsorption, diffusion, etc.) [28,30]. The moisture dependence of the lower frequency arc is due to the remarkable enhancement of dissociative adsorption and diffusion process of hydrogen on the nickel surface with increasing moisture content up to a certain level [31,32]. The impedance spectra in Fig. 2 show that an increase in moisture content would lead to a size reduction of both high and low frequency arcs. It would then be more favorable to operate an SOEC under a high amount of moisture content due to its reduced polarization resistance. However, size reduction of the low frequency arc appears to be more significant than that of the high frequency arc. This suggests that there is an overlapping of both arcs and it will not be easy to assign the different electrode processes to their respective arcs. Fig. 3(a) shows the initial polarization curves measured for the electrolyzer cell utilizing steam produced from pure water and simulated seawater. The activation region, which is prevalent at low current densities (~0 to ±0.4 Acm2) cannot be clearly observed whereas the ohmic region, which is prevalent at intermediate current densities (~±0.4 Acm2 to ±1.4 Acm2) can be clearly observed. This is because the electrodes are sufficiently activated at a temperature of 800 C. A smooth transition across the open circuit voltage from electrolyzer to fuel cell mode can be observed, indicating that the cell is reversible for the charge transfer process when the cell was switched between different modes. In the electrolyzer mode, the ASR values calculated from the linear portion of the IeV curves are 0.23 Ucm2 and 0.24 Ucm2 for the utilization of pure water and simulated seawater, respectively. The ASR values calculated form the IeV curves for both cases corresponds to their respective ohmic and polarization resistances at their respective current loads. The voltages measured at a current density of 1.0 Acm2 are 1.17 V and 1.18 V for the utilization of pure water and simulated seawater, respectively. The difference in voltage can be considered to be negligible. Assuming a Faradaic efficiency of 100%, which is reasonable with the oxygen partial pressure range under study, the hydrogen production rate (in gs1cm2) can be calculated based on the equation below:
m_ ¼
Fig. 2. Impedance spectra under OCV for two water contents at 800 C.
IM nF A
where I is the current in A, M is the molar mass of hydrogen (2 gmol1), n is the number of electrons transferred, F is the Faraday's constant (96485 Cmol1) and A is the active electrode area. This corresponds to a hydrogen production rate of 0.0207 gs1cm2 at a current density of 1.0 Acm2. The volumetric flow rate of hydrogen and oxygen production can then be calculated based on some modifications to the equation provided above [33].
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Fig. 3. (a) IeV curves at 800 C utilizing steam produced from pure water and seawater at initial state; (b) Impedance spectra under a current density of 1.1 Acm2 at 800 C.
The electrical energy efficiency of steam electrolysis can be expressed as:
Electrical energy efficiency ðhelec Þ ¼
Rate of H2 production LHV of H2 100% Electrical energy input
helec ¼
M DH 1:291 Volts 100% ¼ nFV V
where DH is the enthalpy change of hydrogen (124.6 MJkg1 at 800 C) and V is the applied voltage. Hence, the electrical efficiencies, which are the amount of electrical energy input per unit of hydrogen produced, at a current density of 1.0 Acm2 can be calculated as 110.3% and 109.4% for the electrolysis of steam produced from pure water and simulated seawater, respectively. The electrical efficiencies of both cases are more than 100% because their voltages at a current density of 1.0 Acm2 are below the thermoneutral voltage of steam electrolysis at 800 C, which is 1.291 V. Hence, part of the energy required by the SOEC comes from the heat supplied by the furnace, which leads to a reduction in electrical energy required for the electrolysis process. Fig. 3(b) shows the impedance spectra measured under a current density of 1.1 Acm2 (electrolyzer mode) for the cell utilizing steam produced from pure water and simulated seawater. A similar impedance spectra trend was observed for both cases. Table 2 shows the values for the ohmic, polarization (high and low frequency arcs) and total resistances measured under a current density of 1.1 Acm2 for steam produced from pure water and simulated seawater. Similar ohmic resistance can be observed from the impedance spectra. The high and low frequency contributions to the polarization resistance are also very similar to each other leading to a difference in total polarization resistance of only 0.003 Ucm2 or 2.8%. It can be seen that the contribution of the low frequency electrode process is about two times of the high frequency electrode process in the total polarization resistance. The difference in total area specific resistance (ASR), which is a combination of the ohmic and polarization resistances between both cases is only 0.001 Ucm2 or 0.4%. This difference in total ASR is
negligible as compared to their individual values of the total ASR. Fig. 4(a) shows the short-term durability test for the electrolyzer cell utilizing steam produced from pure water and simulated seawater for up to 120 h under a constant electrolysis current of 0.8 Acm2. It should be noted that a single cell was used in the durability test for pure water and simulated seawater electrolysis, where the steam origin was changed from pure water to simulated seawater after 50 h. Both pure water and simulated seawater electrolysis show similar degradation rate of about 15% 1000 h1 in terms of increment in the cell's operating voltage under constant current. The similar degradation rates show that the electrolysis of steam produced from simulated seawater has no noticeable effects on the cell's degradation rate. At a constant current density of 0.8 Acm2, the volumetric flow rate of hydrogen and oxygen production can be calculated as 6.07 sccm cm2 and 3.04 sccm cm2 respectively. The fluctuation in current is due to the slight variations in steam partial pressure in the feedstock gas due to heating effect of the water bath and sample holder. In this study, the degradation of the cell may be caused by several typical factors such as the contamination of the fuel electrode due to silica impurities from the glass sealant and the changes in electrode microstructure such as coarsening of nickel grains due to the sintering effect [26,27]. It was reported in the literature that the larger degradation for the first 300 h may be caused by initial passivation of the cell due to the evaporation of Si(OH)4 from the glass sealant [26]. It should be noted that the observations of insignificant degradation and performance loss in this study are related to the cell materials only. Fig. 4(c) shows the ICP testing results for the condensate of steam produced from simulated seawater. Only three different anions, which are fluoride, chloride and nitrate, were detected with low concentrations in the condensate. The concentration of chloride ions, which has a high concentration in seawater, is only less than 1 ppm in the condensate. This shows that the impurities from seawater are not likely to be transported by the steam even if it is produced at a high gas flow rate through the heated seawater. Fig. 5 shows the SEM images for various parts of the single cell utilizing steam produced from simulated seawater after the durability test. No salt particles were detected in the cell after 48 h of simulated seawater electrolysis. The surface of the Ni-YSZ electrode
Table 2 Breakdown of different resistances measured under a current density of 1.1 Acm2 at initial state. Steam origin (70/30 pH2O/pH2)
Pure water Seawater
RU (Ucm2)
0.14 0.142
Rp (Ucm2)
Total ASR (Ucm2)
High frequency
Low frequency
Total
0.033 0.032
0.076 0.074
0.109 0.106
0.249 0.248
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Fig. 4. (a) Durability test using pure water and simulated seawater under a constant electrolysis current of 0.8 Acm2 at 800 C; (b) ICP for the condensate of steam produced from simulated seawater.
Fig. 5. SEM images of (a) surface of Ni-YSZ electrode (b) Ni-YSZ electrode cross-section, and (c) electrode-electrolyte interface.
has a porous structure as seen in Fig. 5(a). EDX shows that there was no presence of sodium or chloride elements on the surface of the electrode. In Fig. 5(c), it can be seen that the button cell possesses two porous electrodes and a dense YSZ electrolyte of about 10 mm. Also, it can be seen that both the fuel and air electrodes are well adhered to the electrolyte without delamination. EDX analysis shows no presence of sodium or chloride elements in the Ni-YSZ electrode cross-section as well as at the Ni-YSZ/YSZ/LSCF-GDC interfaces. All these results have proven that sea salt was not being transported to the cell fed with steam produced from simulated seawater; at least these impurities were not detectable by EDX. Impurities such as Si, Na and Al, which originate from the glass sealant, were not detected probably because longer exposure duration may be needed for substantial amounts of these
impurities to be accumulated. 3.2. Direct contamination of sea salt in cell Fig. 6(a) shows the polarization curves for the uncontaminated and contaminated cell. Similar to the case of pure water and simulated seawater electrolysis, the activation region which occurs at low current densities cannot be clearly observed whereas the ohmic region which occurs at intermediate current densities can be clearly observed in both curves. A similar polarization trend can be observed for both cells, where the mass transport limitation is not present in the contaminated cell up to a current density of 1.5 Acm2. This shows that the sea salt may not affect the diffusion/adsorption of gas species into the reaction sites at the
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Fig. 6. (a) Comparison of polarization behavior between the uncontaminated and contaminated cell; (b) Impedance spectra for the both cells; (c) Durability test for the contaminated cell under a constant electrolysis current of 0.8 Acm2 at 800 C.
electrode-electrolyte interface. A smooth transition across the open circuit voltage from electrolyzer to fuel cell mode can also be observed. When the button cells were operated in electrolyzer mode, the ASR values calculated from the linear portion of the IeV curves were 0.27 Ucm2 and 0.29 Ucm2 for the uncontaminated and contaminated cell, with only a 7.4% difference in value. The contaminated cell, with its fuel electrode contaminated with sea salt, shows only a slightly lower performance as compared to the uncontaminated cell. The voltages measured at a current density of 1.0 Acm2 were 1.16 V and 1.19 V for the uncontaminated and contaminated cell, which corresponds to electrical energy efficiencies of 111.3% and 108.5%, respectively, with a hydrogen production rate of 0.0207 gs1cm2. The difference of only 2.8% in electrical efficiency between both cells at this particular current density can be assumed to be negligible, when compared to the difference in electrical efficiencies between low and high temperature electrolyzers. Fig. 6(b) shows the impedance spectra measured under OCV and a current density of 1.1 Acm2 (electrolyzer mode) for the uncontaminated and contaminated cell. It can be observed that rather similar impedance spectra trend and values were obtained for both cells. The difference in ohmic resistances between the two cells,
which are mainly contributed by the electrolyte and contact resistances, was similar under OCV and a current density of 1.1 Acm2 with a value of 0.02 Ucm2 (~14% difference). This large difference in ohmic resistance may arise from the slight difference in electrolyte thickness between the two cells as well as their contact area with the probe. The difference in polarization resistances between the two cells, which are contributed by the electrode polarization during electrochemical activity, was 0.007 Ucm2 (~8.6% difference) and 0.009 Ucm2 (~8.3% difference), respectively, under OCV and a current density of 1.1 Acm2. The maximum difference in total area specific resistance (ASR), which is a combination of the ohmic and polarization resistances, between both cells was 0.024 Ucm2 (~9.6% difference) under electrolyzer mode, which is mainly contributed by the difference in ohmic resistances. Unlike the case of electrolysis of steam produced from seawater, this case involves the testing of two separate cells at different times, where it is expected that there will be slight differences in the ohmic, polarization and total resistances between the uncontaminated and contaminated cell. Hence, the contamination of sea salt in the Ni-YSZ fuel electrode can be said to have no significant effects on the electrolysis performance of an SOEC since the difference in their polarization resistances and total ASR were all within 10% with respect to the uncontaminated cell.
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Fig. 7. SEM images of (a, d) surface, (b, e) cross-section and (c, f) electrode-electrolyte interfaces before (top row) and after (bottom row) testing for Cell B.
Fig. 6(c) shows the short-term durability test of the contaminated cell under a constant electrolysis current of 0.8 Acm2. Steam produced from pure water was used in the feedstock gas in this case. The degradation rate of the uncontaminated cell is about 15% 1000 h1 (Fig. 4) whereas the degradation rate of the contaminated cell is about 16% 1000 h1. The relatively similar degradation rates between both cells shows that the presence of sea salt in the fuel electrode may not pose any effects on the cell's degradation rate. After 50 h of durability test, the operating voltages of the uncontaminated and contaminated cells were about 1.066 V and 1.084 V with a 1.7% difference with respect to the uncontaminated cell. Since a constant current density of 0.8 Acm2
is being appiled, the volumetric flow rate of hydrogen and oxygen production will be constant throughout the durability test. It should be noted that the uncontaminated and contaminated cells were sealed with the same type of glass sealant; hence both cells would be subjected to the same contaminant from the glass sealant, if any. Fig. 7 shows the SEM images for various parts of the contaminated cell before and after the durability test. Salt particles were observed in the fuel electrode of the cell before testing was conducted, where the surface, cross-section and interface of the Ni-YSZ electrode were covered with sea salt as seen in Fig. 7(aec). EDX analysis has confirmed the presence of sea salt with the detection of sodium and chloride elements in various parts of the fuel electrode.
Fig. 8. TGA analysis on sea salt.
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However, salt particles can no longer be observed in the fuel electrode surface, cross-section and interface after the durability test as shown in Fig. 7(def). EDX analysis further confirmed that there was no presence of sodium or chloride elements on the surface, crosssection and interface of the electrode. This study suggests that sea salt may have been vaporized from the surface of the fuel electrode over the period of operation at 800 C. Fig. 8 shows the thermogravimetric analysis of the sea salt used in this study. From the illustration, it can be deduced from the interception of the gradients of the curves that the decomposition temperature, the temperature where the sea salt starts to undergo significant mass loss, is about 803 C which is close to the operating temperature of 800 C used in this study. This means that the sea salt may be volatile and have vaporized over the cell's operating period at ~800 C with no reaction occurring with the electrode material, which is the reason for the disappearance of sea salt from the fuel electrode after the operation. Hence, it is highly unlikely that sea salt would cause any serious contamination or poisoning effect on the fuel electrode of a SOEC when operating at a temperature of 800 C or above. 4. Conclusion In this study, the in-situ and ex-situ characteristics of an SOEC utilizing steam produced from pure water and simulated seawater, as well as the contamination of the SOEC's fuel electrode by the sea salt were investigated. It has been found that contaminants are not present in steam produced from the seawater. The electrolysis of steam produced from both pure water and seawater gave almost the same initial performance in terms of current-voltage curves as well as impedance spectra. The degradation rate for seawater electrolysis is also found to be similar to the degradation rate for pure water electrolysis. Post-mortem analysis showed that no sodium chloride is present on the surface, cross-section, and the electrode-electrolyte interface of the Ni-YSZ fuel electrode. For the direct contamination of sea salt in the SOEC's fuel electrode, it has been found that the contamination has posed no significant effects on the electrochemical performance of the cell. The uncontaminated and contaminated cell demonstrated almost similar initial performance as measured by polarization curves and impedance spectra. The degradation rate of the contaminated cell was also found to be very close to the degradation rate of the uncontaminated cell. Post-mortem analysis has confirmed that sea salt did not react with the electrode material and has been vaporized over the operating period of the cell at the typical operating temperature of 800 C. Acknowledgement This research programme/project is funded by the National Research Foundation, Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. References [1] M. Hoel, S. Kverndokk, Depletion of fossil fuels and the impacts of global warming, Resour. Energy Econ. 18 (1996) 115e136. [2] T.M.I. Mahlia, T.J. Saktisahdan, A. Jannifar, M.H. Hasan, H.S.C. Matseelar, A review of available methods and development on energy storage; technology update, Renew. Sustain. Energy Rev. 33 (2014) 532e545. [3] K. Mazloomi, C. Gomes, Hydrogen as an energy carrier: prospects and challenges, Renew. Sustain. Energy Rev. 16 (2012) 3024e3033. [4] M.A. Laguna-Bercero, Recent advances in high temperature electrolysis using
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solid oxide fuel cells: a review, J. Power Sources 203 (2012) 4e16. [5] S. Yoon, I. Kang, J. Bae, Suppression of ethylene-induced carbon deposition in diesel autothermal reforming, Int. J. Hydrogen Energy 34 (2009) 1844e1851. [6] A. Manabe, M. Kashiwase, T. Hashimoto, T. Hayashida, A. Kato, K. Hirao, et al., Basic study of alkaline water electrolysis, Electrochim. Acta 100 (2013) 249e256. [7] S.M. Mousavi Ehteshami, S.H. Chan, Optimal design and operation of polymer electrolyte membrane reactors for pure hydrogen production, Chem. Eng. Sci. 98 (2013) 282e290. [8] W. Smith, The role of fuel cells in energy storage, J. Power Sources 86 (2000) 74e83. [9] Y. Liu, X. Hao, Z. Wang, J. Wang, J. Qiao, Y. Yan, et al., A newly-developed effective direct current assisted sintering technique for electrolyte film densification of anode-supported solid oxide fuel cells, J. Power Sources 215 (2012) 296e300. [10] J. Kong, K. Sun, D. Zhou, N. Zhang, J. Mu, J. Qiao, NieYSZ gradient anodes for anode-supported SOFCs, J. Power Sources 166 (2007) 337e342. [11] T.S. Li, W.G. Wang, H. Miao, T. Chen, C. Xu, Effect of reduction temperature on the electrochemical properties of a Ni/YSZ anode-supported solid oxide fuel cell, J. Alloys Compd. 495 (2010) 138e143. [12] IEA, Energy Technology Essentials (ETE 05), April 2007. [13] A.O. Isenberg, Energy conversion via solid oxide electrolyte electrochemical cells at high temperatures, Solid State Ionics 3e4 (1981) 431e437. [14] Z. Pan, Q. Liu, L. Zhang, X. Zhang, S.H. Chan, Effect of Sr surface segregation of La0.6Sr0.4Co0.2Fe0.8O3 d electrode on its electrochemical performance in SOC, J. Electrochem. Soc. 162 (2015). F1316eF23. [15] Z. Pan, Q. Liu, L. Zhang, X. Zhang, S.H. Chan, Study of activation effect of anodic current on La0.6Sr0.4Co0.2Fe0.8O3d air electrode in solid oxide electrolyzer cell, Electrochim. Acta 209 (2016) 56e64. [16] M. Henke, C. Willich, J. Kallo, K.A. Friedrich, Theoretical study on pressurized operation of solid oxide electrolysis cells, Int. J. Hydrogen Energy 39 (2014) 12434e12439. [17] M. Henke, C. Willich, C. Westner, F. Leucht, R. Leibinger, J. Kallo, et al., Effect of pressure variation on power density and efficiency of solid oxide fuel cells, Electrochim. Acta 66 (2012) 158e163. [18] J.D. Holladay, J. Hu, D.L. King, Y. Wang, An overview of hydrogen production technologies, Catal. Today 139 (2009) 244e260. [19] J.P. Stempien, Q. Sun, S.H. Chan, Performance of power generation extension system based on solid-oxide electrolyzer cells under various design conditions, Energy 55 (2013) 647e657. [20] P.H. Gleick, The World's Water 2000-2001: the Biennial Report on Freshwater Resources, Island Press, 2000. [21] N.A. Abdel Ghany, N. Kumagai, S. Meguro, K. Asami, K. Hashimoto, Oxygen evolution anodes composed of anodically deposited MneMoeFe oxides for seawater electrolysis, Electrochim. Acta 48 (2002) 21e28. [22] E. Baniasadi, I. Dincer, G.F. Naterer, Electrochemical analysis of seawater electrolysis with molybdenum-oxo catalysts, Int. J. Hydrogen Energy 38 (2013) 2589e2595. [23] D.W. Kirk, A.E. Ledas, Precipitate formation during sea water electrolysis, Int. J. Hydrogen Energy 7 (1982) 925e932. [24] Z. Jiao, N. Takagi, N. Shikazono, N. Kasagi, Study on local morphological changes of nickel in solid oxide fuel cell anode using porous Ni pellet electrode, J. Power Sources 196 (2011) 1019e1029. [25] T.S. Li, W.G. Wang, Sulfur-poisoned Ni-based solid oxide fuel cell anode characterization by varying water content, J. Power Sources 196 (2011) 2066e2069. [26] A. Hauch, S.D. Ebbesen, S.H. Jensen, M. Mogensen, Solid oxide electrolysis cells: microstructure and degradation of the Ni/Yttria-Stabilized zirconia electrode, J. Electrochem. Soc. 155 (2008). B1184eB93. [27] S.P. Jiang, S.P.S. Badwal, An electrode kinetics study of H2 oxidation on Ni/ Y2O3eZrO2 cermet electrode of the solid oxide fuel cell, Solid State Ionics 123 (1999) 209e224. [28] H.P. Dasari, S.-Y. Park, J. Kim, J.-H. Lee, B.-K. Kim, H.-J. Je, et al., Electrochemical characterization of Nieyttria stabilized zirconia electrode for hydrogen production in solid oxide electrolysis cells, J. Power Sources 240 (2013) 721e728. [29] S.-D. Kim, D.-W. Seo, A.K. Dorai, S.-K. Woo, The effect of gas compositions on the performance and durability of solid oxide electrolysis cells, Int. J. Hydrogen Energy 38 (2013) 6569e6576. , Solid oxide electrolysis cell analysis by [30] A. Nechache, M. Cassir, A. Ringuede means of electrochemical impedance spectroscopy: a review, J. Power Sources 258 (2014) 164e181. [31] W.G. Bessler, J. Warnatz, D.G. Goodwin, The influence of equilibrium potential on the hydrogen oxidation kinetics of SOFC anodes, Solid State Ionics 177 (2007) 3371e3383. [32] M. Vogler, A. Bieberle-Hütter, L. Gauckler, J. Warnatz, W.G. Bessler, Modelling study of surface reactions, diffusion, and spillover at a Ni/YSZ patterned anode, J. Electrochem. Soc. 156 (2009). B663eB72. [33] C. Yang, A. Coffin, F. Chen, High temperature solid oxide electrolysis cell employing porous structured (La0.75Sr0.25)0.95MnO3 with enhanced oxygen electrode performance, Int. J. Hydrogen Energy 35 (2010) 3221e3226.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
High-temperature electrolysis of synthetic seawater using solid oxide electrolyzer cells Chee Kuan Lim a, b, c, *, Qinglin Liu a, b, Juan Zhou a, b, Qiang Sun a, d, Siew Hwa Chan a, b, c, ** a
Singapore-Peking University Research Centre, Campus for Research Excellence & Technological Enterprise (CREATE), Singapore 138602, Singapore Energy Research Institute at NTU (ERIAN), Nanyang Technological University, Singapore 639798, Singapore c School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore d College of Engineering, Peking University, Beijing 100871, China b
h i g h l i g h t s Contaminants have not been found in the steam of seawater. Performance of cells running on pure water and seawater are the same. Contamination of sea salt has no in-situ and ex-situ effects on the cell. Sea salt is volatile at the typical operating temperature of the cell.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2016 Received in revised form 4 December 2016 Accepted 5 December 2016
A Ni-YSZ/YSZ/LSCF-GDC solid oxide electrolyzer cell (SOEC) is used to investigate the effects of seawater electrolysis for hydrogen production through electrolyzing steam produced from simulated seawater bath. Steam electrolysis using an SOEC with its fuel electrode contaminated by sea salt is also investigated. Steam produced from seawater is found to be free of contaminants, which are present in the seawater. Similar electrochemical performance is observed from the polarization curves and impedance spectra when using steam produced from pure water and seawater. Their short-term degradation rates are similar, which are registered at 15% 1000 h1 for both cases. For the case of direct sea salt contamination in an SOEC's fuel electrode, both the uncontaminated and contaminated cells exhibit rather similar performance as observed from the polarization curves and impedance spectra. The difference in ASR values from the polarization curves and impedance spectra between the uncontaminated and contaminated cell are all within a 10% range. Rather similar short-term degradation rates of 15% 1000 h1 and 16% 1000 h1 are recorded for the uncontaminated and contaminated cells, respectively. Post-mortem analysis shows that the sea salt impregnated into the cell has been vaporized at a typical SOEC operating temperature of 800 C over the period of operation. © 2016 Elsevier B.V. All rights reserved.
Keywords: High-temperature electrolyzer Seawater electrolysis Solid oxide cell (SOC)
1. Introduction In recent years, rising energy costs and changing of global
* Corresponding author. Singapore-Peking University Research Centre, Campus for Research Excellence & Technological Enterprise (CREATE), Singapore 138602, Singapore. ** Corresponding author. Singapore-Peking University Research Centre, Campus for Research Excellence & Technological Enterprise (CREATE), Singapore 138602, Singapore. E-mail addresses: [email protected] (C.K. Lim), [email protected] (S.H. Chan). http://dx.doi.org/10.1016/j.jpowsour.2016.12.019 0378-7753/© 2016 Elsevier B.V. All rights reserved.
climate due to the combustion of fossil fuels are of major concerns [1]. In order to combat the depletion of fossil fuels and to reduce carbon emissions, the use of fossil fuels must be replaced by renewable resources. One of the fundamental problems associated with the use of renewable sources is that a balance between electrical energy supply and demand is required. Therefore, storing excess energy is essential during times when the supply exceeds the demand. Several energy storage technologies such as compressed air energy storage (CAES), flywheel energy storage (FES), pumped hydro energy storage (PHES) and batteries are potential technologies to store energy [2]. However, some of the disadvantages associated with these technologies are the high capital costs
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required for large scale energy storage, transportability issues, high dependence on appropriate geographical regions as well as energy losses such as mechanical losses in flywheels and loss of charge overtime due to self-discharge in batteries. Hence, there has been an increasing interest in chemical energy storage particularly hydrogen in recent years [3]. This is mainly due to hydrogen being a clean alternative energy carrier as well as its capability of being converted into hydrocarbon fuels [4]. Hydrogen must be generated from hydrogen-containing substances since it is not readily available as a fuel on Earth [5]. An alternative green method to produce pure hydrogen is through the electrolysis of water using renewable energy or excess power from thermal power plants. The development of low-temperature water electrolysis technologies such as alkaline and proton exchange membrane based water electrolyzers has been ongoing for many years [6e8]. Solid oxide fuel cells (SOFCs) have gained popularity decades ago. Many researchers, such as Sun et al. and Wang et al., have studied the fabrication techniques, electrode development as well as operating parameters of SOFCs extensively [9e11]. The reverse of SOFCs, which are the solid oxide electrolyzer cells (SOECs), has gained much attention lately due to its advantages over low-temperature electrolysis for its excellent efficiency in hydrogen production (28e39 kwh1 H2) [12]. During the early 1980s, steam electrolysis in SOECs for production of hydrogen started to undergo development as an interesting alternative to low-temperature water electrolysis [13e15], where popular research areas are related to its materials, design and operating parameters. For example, researchers like Moritz et al. have studied the effect of pressure on solid oxide cells [16,17]. From a thermodynamic point of view, high-temperature electrolysis of steam using SOECs is a promising technology to produce hydrogen as it requires less electrical energy per unit hydrogen produced as compared to low-temperature water electrolysis due to the combination of more favorable thermodynamics and kinetics at high temperatures [18,19]. An increase in operating temperature leads to a reduction in electrical energy demand for steam electrolysis and hence a reduction in the production cost of hydrogen. As of today, most of the water electrolysis technologies are related to freshwater electrolysis. However, only less than 1% of all the freshwater on Earth are accessible to us at an affordable cost, whereas 97.5% of water on Earth is seawater which is more abundant than freshwater [20]. Electrical energy generated from renewable energy technologies such as offshore wind and wave farms can be used to directly electrolyze steam produced from seawater, which has a higher availability around these regions as compared to freshwater. Research and developments related to electrode materials for low temperature seawater electrolysis to resolve problems such as chlorine production at the anode and the gradual build-up of insoluble precipitates at the cathode surface are gaining popularity [21e23]. Nevertheless, high temperature steam electrolysis of seawater using SOECs remains a new research which is worthwhile to be explored. Most studies related to SOECs are carried out using pure water as a source of moisture content. The degradation mechanisms in SOECs based on pure water had been studied extensively over the years. The typical causes of degradation in the fuel electrode of an SOEC are the relocalization of nickel due to formation of Ni(OH)2 [24], electrode poisoning due to the formation of nickel sulfides and Si(OH)4 [25,26], changes in electrode microstructure such as coarsening of nickel grains due to the sintering effect [27] and local oxidation of nickel in the Ni-YSZ electrode at higher moisture content (>50% pH2O) [28]. There are several ways to obtain the desired moisture content in the feedstock gas; one of the ways is by bubbling feedstock gas through the seawater bath using a gas tube and another way is by
direct spray injection of seawater into the SOEC using an atomizing spray nozzle. In an actual case of seawater electrolysis using an SOEC, spray injecting seawater into a heated chamber to provide the required moisture content in the feedstock gas is one of the most practical ways. By directly using seawater, there is a high possibility of contamination by sea salt in the fuel electrode. The potential damages of sea salt on the fuel electrode include poisoning of nickel and blocking of triple phase boundaries near the electrode-electrolyte interface which reduces catalytic activity. Hence, the electrochemical performance and degradation mechanism in SOECs utilizing seawater remains unknown. In this paper, two cases related to seawater electrolysis using SOECs have been investigated. 1. The first case involves the investigation of the purity of steam produced from seawater to check for any impurities in the steam originated from seawater during vaporization process. 2. The second case involves the investigation of the simulated case for contamination of sea salt in the Ni-YSZ fuel electrode of a button SOEC on its electrochemical performance and degradation behavior. A button cell consisting of Ni-YSZ as fuel electrode, YSZ as electrolyte and LSCF-GDC as air electrode was tested for these two cases to show the feasibility of SOECs for direct seawater electrolysis application. Post-mortem analysis, which includes scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), have been performed on the cells before and after operation. 2. Experimental 2.1. NiO-YSZ/YSZ/LSCF-GDC cell fabrication The fuel electrode (cathode) supported button cells used in this study were prepared by homogeneously mixing 60 wt % nickel oxide powder and 40 wt % yttria-stabilized zirconia powder (8 mol % Y2O3-stabilized ZrO2) with a small amount of carbon black (poreformer) in ethanol using ball milling overnight. After drying on a hot plate to eliminate the solvent, a small amount of binder was added into the electrode substrate using a mortar and pestle. The mixture was then allowed to dry and NiO-YSZ powder was compacted under a uniaxial pressure to form circular disks of 24 mm in diameter. The electrode disks were then baked at 900 C to increase the mechanical strength for handling and to remove the poreformer. YSZ electrolyte suspension was applied sequentially onto the baked fuel electrode discs using a spray coating method. The coated fuel electrode disks were subsequently sintered at 1400 C for 3 h to obtain dense YSZ electrolyte films supported on the substrates. A 50-50 wt% of La0.8Sr0.2Co0.2Fe0.8O3d (LSCF) e Ce0.8Gd0.2O2d (GDC) composite oxygen electrode (anode) was prepared by mixing LSCF-GDC50 powder with a proper amount of polyethylene glycol. The mixture was then applied onto the center of the electrolyte film by screen printing technique, followed by sintering at 900 C for 2 h to form a porous electrode layer with an active area of 0.5 cm2. A porous Pt layer was coated on both electrodes to serve as the current collectors to minimize the contact resistance between the electrodes with the Pt mesh. Subsequently, sintering was carried out at 900 C. EDX was performed on the cross-section and electrode-electrolyte interfaces to ensure that no Pt has infiltrated into the electrochemically active sites of the electrode. The thickness of the NiO-YSZ cathode, YSZ electrolyte and LSCF-GDC anode are roughly 1 mm, 10 mm and 35 mm respectively. The cross-sectional view of the completed button cell which is sealed to an alumina tube is shown in Fig. 1.
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Fig. 1. Experimental set-up for the electrochemical performance testing of a button SOEC fed with simulated seawater.
2.2. Equipment set-up Fig. 1 shows the experimental set-up for simulated seawater electrolysis in this study. The button cell was sealed with a glass sealant to separate the gas environment between the cathode and the anode. A temperature of 800 C was chosen for the reduction of NiO-YSZ as well as the operating temperature of the cell during electrochemical testing. The water bath was heated up to provide a high moisture content of 70% pH2O balanced with H2. Dry H2 was fed into the water bath at a flow rate of 50 mlmin1, whereas the oxygen electrode side was exposed to ambient air. The cell was left for about 3 h after reaching its operating temperature and fed with H2 to ensure complete reduction of nickel oxide before the electrochemical performance of the cell was measured. The pipe connecting the water bath to the SOEC was heated up to more than 100 C to prevent any condensation of steam in the pipe. Platinum gauze was used for current collection on both sides of the cell. Each electrode side was connected to two platinum wires, which serve as respective voltage and current probes to eliminate the contribution of platinum wire to the total resistance. 2.3. Preparation of simulated seawater The simulated seawater was prepared by dissolving sea salt in pure water to obtain the average salinity of actual seawater which is about 3.5%. The main constituents of sea salt and their respective melting points can be found in Table 1. Some other trace amounts of elements which include bromine, carbon, amino acids, etc., are considered to be insignificant. The seawater bath was heated up to provide a moisture content of 70/30 pH2O/pH2 in the feedstock gas under a high flow rate of H2. The steam in the feedstock gas (humidified H2) was condensed and collected for purity testing. The presence of chloride and sulfate
Table 1 Main components of sea salt and their melting points. Type of salt
Percentage (%)
Melting point ( C)
NaCl MgCl2 MgSO4 CaSO4 KCl NaBr
77.83 9.66 6.10 4.05 2.11 0.24
801 714 1124 1460 770 747
anions in the steam condensate was tested using an ion chromatograph mass spectrometry (Metrohm Model 930 Compact IC Flex). 2.4. Electrolysis of steam produced from seawater For comparison purpose, one button cell was tested in this case, first using steam produced from pure water and followed by steam produced from seawater. The water bath (Fig. 1) consisting of pure water and followed by seawater was heated up to provide a feedstock gas containing 70/30 pH2O/pH2. The cell was left in the test station under open circuit voltage for 24 h for stabilization after the switch from pure water to seawater before any measurements were taken. For both cases, their electrochemical performance was evaluated by measuring the IeV polarization curves and impedance spectra of the cell. A durability test was also conducted for a total of 120 h. 2.5. Direct contamination of sea salt in cell In this case, a contaminated cell was being tested under the same conditions as the one in the case of electrolysis of steam produced from pure water and seawater. Only pure water bath
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was used to obtain the required moisture content. The results were then compared with the uncontaminated cell using steam produced from pure water in the previous case. For the preparation of the contaminated cell, sea salt was dissolved in pure water up to the point of saturation at room temperature. The initial weight of a button cell was recorded and a small amount of saturated seawater solution was applied onto the fuel electrode and was allowed to penetrate into the porous NiO-YSZ fuel electrode. This will ensure that sea salt is present in the whole volume as well as at the triple-phase boundaries of the electrodeelectrolyte interface. The cell was then allowed to dry in air before its final weight was taken. This process is repeated until 0.03 g of sea salt was deposited into the porous fuel electrode. SEM and EDX were used to verify that sea salt is present at the cross-section and electrode-electrolyte interface. 2.6. Electrochemical performance measurement The button cell's electrochemical performance was characterized by polarization curves (IeV) and electrochemical impedance spectroscopy (EIS). Polarization curves were generated using linear sweep current techniques, where a potentiostat/galvanostat instrument (Solartron Model 1470E) was used to control the voltage between 0.8 and þ 0.8 V of bias against OCV at a sweep rate of 1 mV1. EIS measurements were performed using a sinusoidal signal amplitude of 10 mV across a frequency range of 100 kHz to 0.05 Hz using a frequency response analyzer (Solartron Model 1255B). All the impedance spectra were fitted to curves using ZView2 by constructing an electrical circuit consisting of an inductor in series with a resistor and two other resistors in parallel with a constant phase element (CPE). All the fitted values had reasonable errors of within 10% from the original values. 2.7. Ex-situ characterization SEM and EDX analysis were then carried out on the surface, cross-section and electrode-electrolyte interface of the fuel electrode at the end of every durability test for both cases. IC-MS was carried out on the condensate of steam produced from simulated seawater to check its purity due to the possibility of sea salt being transported by the steam. Thermogravimetric analysis (TGA Model Q500) was performed on the sea salt used in this study to understand its thermal properties such as the disintegration/vaporization temperature.
3. Results and discussion 3.1. Electrolysis of steam produced from seawater Fig. 2 shows the impedance spectra measured under open circuit condition with a feedstock gas composition of 3/97 pH2O/pH2 and 70/30 pH2O/pH2. It can be seen that the polarization resistance of the cell, which is contributed by the electrode processes, decreases significantly with an increase in moisture content. However, the ohmic resistance, which is mainly contributed by the electrolyte resistance, has remained unaffected. These observation is well known and is consistent with similar results obtained in the literature [29]. Two arcs can be clearly distinguished in the frequency domain, suggesting that the overall electrochemical reaction is dominated by at least two electrode processes in series. It is understood that the high frequency arc is associated with the interfacial charge transfer at the electrode-electrolyte interface, while the lower frequency arc is associated with the gas-solid interactions (adsorption, diffusion, etc.) [28,30]. The moisture dependence of the lower frequency arc is due to the remarkable enhancement of dissociative adsorption and diffusion process of hydrogen on the nickel surface with increasing moisture content up to a certain level [31,32]. The impedance spectra in Fig. 2 show that an increase in moisture content would lead to a size reduction of both high and low frequency arcs. It would then be more favorable to operate an SOEC under a high amount of moisture content due to its reduced polarization resistance. However, size reduction of the low frequency arc appears to be more significant than that of the high frequency arc. This suggests that there is an overlapping of both arcs and it will not be easy to assign the different electrode processes to their respective arcs. Fig. 3(a) shows the initial polarization curves measured for the electrolyzer cell utilizing steam produced from pure water and simulated seawater. The activation region, which is prevalent at low current densities (~0 to ±0.4 Acm2) cannot be clearly observed whereas the ohmic region, which is prevalent at intermediate current densities (~±0.4 Acm2 to ±1.4 Acm2) can be clearly observed. This is because the electrodes are sufficiently activated at a temperature of 800 C. A smooth transition across the open circuit voltage from electrolyzer to fuel cell mode can be observed, indicating that the cell is reversible for the charge transfer process when the cell was switched between different modes. In the electrolyzer mode, the ASR values calculated from the linear portion of the IeV curves are 0.23 Ucm2 and 0.24 Ucm2 for the utilization of pure water and simulated seawater, respectively. The ASR values calculated form the IeV curves for both cases corresponds to their respective ohmic and polarization resistances at their respective current loads. The voltages measured at a current density of 1.0 Acm2 are 1.17 V and 1.18 V for the utilization of pure water and simulated seawater, respectively. The difference in voltage can be considered to be negligible. Assuming a Faradaic efficiency of 100%, which is reasonable with the oxygen partial pressure range under study, the hydrogen production rate (in gs1cm2) can be calculated based on the equation below:
m_ ¼
Fig. 2. Impedance spectra under OCV for two water contents at 800 C.
IM nF A
where I is the current in A, M is the molar mass of hydrogen (2 gmol1), n is the number of electrons transferred, F is the Faraday's constant (96485 Cmol1) and A is the active electrode area. This corresponds to a hydrogen production rate of 0.0207 gs1cm2 at a current density of 1.0 Acm2. The volumetric flow rate of hydrogen and oxygen production can then be calculated based on some modifications to the equation provided above [33].
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Fig. 3. (a) IeV curves at 800 C utilizing steam produced from pure water and seawater at initial state; (b) Impedance spectra under a current density of 1.1 Acm2 at 800 C.
The electrical energy efficiency of steam electrolysis can be expressed as:
Electrical energy efficiency ðhelec Þ ¼
Rate of H2 production LHV of H2 100% Electrical energy input
helec ¼
M DH 1:291 Volts 100% ¼ nFV V
where DH is the enthalpy change of hydrogen (124.6 MJkg1 at 800 C) and V is the applied voltage. Hence, the electrical efficiencies, which are the amount of electrical energy input per unit of hydrogen produced, at a current density of 1.0 Acm2 can be calculated as 110.3% and 109.4% for the electrolysis of steam produced from pure water and simulated seawater, respectively. The electrical efficiencies of both cases are more than 100% because their voltages at a current density of 1.0 Acm2 are below the thermoneutral voltage of steam electrolysis at 800 C, which is 1.291 V. Hence, part of the energy required by the SOEC comes from the heat supplied by the furnace, which leads to a reduction in electrical energy required for the electrolysis process. Fig. 3(b) shows the impedance spectra measured under a current density of 1.1 Acm2 (electrolyzer mode) for the cell utilizing steam produced from pure water and simulated seawater. A similar impedance spectra trend was observed for both cases. Table 2 shows the values for the ohmic, polarization (high and low frequency arcs) and total resistances measured under a current density of 1.1 Acm2 for steam produced from pure water and simulated seawater. Similar ohmic resistance can be observed from the impedance spectra. The high and low frequency contributions to the polarization resistance are also very similar to each other leading to a difference in total polarization resistance of only 0.003 Ucm2 or 2.8%. It can be seen that the contribution of the low frequency electrode process is about two times of the high frequency electrode process in the total polarization resistance. The difference in total area specific resistance (ASR), which is a combination of the ohmic and polarization resistances between both cases is only 0.001 Ucm2 or 0.4%. This difference in total ASR is
negligible as compared to their individual values of the total ASR. Fig. 4(a) shows the short-term durability test for the electrolyzer cell utilizing steam produced from pure water and simulated seawater for up to 120 h under a constant electrolysis current of 0.8 Acm2. It should be noted that a single cell was used in the durability test for pure water and simulated seawater electrolysis, where the steam origin was changed from pure water to simulated seawater after 50 h. Both pure water and simulated seawater electrolysis show similar degradation rate of about 15% 1000 h1 in terms of increment in the cell's operating voltage under constant current. The similar degradation rates show that the electrolysis of steam produced from simulated seawater has no noticeable effects on the cell's degradation rate. At a constant current density of 0.8 Acm2, the volumetric flow rate of hydrogen and oxygen production can be calculated as 6.07 sccm cm2 and 3.04 sccm cm2 respectively. The fluctuation in current is due to the slight variations in steam partial pressure in the feedstock gas due to heating effect of the water bath and sample holder. In this study, the degradation of the cell may be caused by several typical factors such as the contamination of the fuel electrode due to silica impurities from the glass sealant and the changes in electrode microstructure such as coarsening of nickel grains due to the sintering effect [26,27]. It was reported in the literature that the larger degradation for the first 300 h may be caused by initial passivation of the cell due to the evaporation of Si(OH)4 from the glass sealant [26]. It should be noted that the observations of insignificant degradation and performance loss in this study are related to the cell materials only. Fig. 4(c) shows the ICP testing results for the condensate of steam produced from simulated seawater. Only three different anions, which are fluoride, chloride and nitrate, were detected with low concentrations in the condensate. The concentration of chloride ions, which has a high concentration in seawater, is only less than 1 ppm in the condensate. This shows that the impurities from seawater are not likely to be transported by the steam even if it is produced at a high gas flow rate through the heated seawater. Fig. 5 shows the SEM images for various parts of the single cell utilizing steam produced from simulated seawater after the durability test. No salt particles were detected in the cell after 48 h of simulated seawater electrolysis. The surface of the Ni-YSZ electrode
Table 2 Breakdown of different resistances measured under a current density of 1.1 Acm2 at initial state. Steam origin (70/30 pH2O/pH2)
Pure water Seawater
RU (Ucm2)
0.14 0.142
Rp (Ucm2)
Total ASR (Ucm2)
High frequency
Low frequency
Total
0.033 0.032
0.076 0.074
0.109 0.106
0.249 0.248
84
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Fig. 4. (a) Durability test using pure water and simulated seawater under a constant electrolysis current of 0.8 Acm2 at 800 C; (b) ICP for the condensate of steam produced from simulated seawater.
Fig. 5. SEM images of (a) surface of Ni-YSZ electrode (b) Ni-YSZ electrode cross-section, and (c) electrode-electrolyte interface.
has a porous structure as seen in Fig. 5(a). EDX shows that there was no presence of sodium or chloride elements on the surface of the electrode. In Fig. 5(c), it can be seen that the button cell possesses two porous electrodes and a dense YSZ electrolyte of about 10 mm. Also, it can be seen that both the fuel and air electrodes are well adhered to the electrolyte without delamination. EDX analysis shows no presence of sodium or chloride elements in the Ni-YSZ electrode cross-section as well as at the Ni-YSZ/YSZ/LSCF-GDC interfaces. All these results have proven that sea salt was not being transported to the cell fed with steam produced from simulated seawater; at least these impurities were not detectable by EDX. Impurities such as Si, Na and Al, which originate from the glass sealant, were not detected probably because longer exposure duration may be needed for substantial amounts of these
impurities to be accumulated. 3.2. Direct contamination of sea salt in cell Fig. 6(a) shows the polarization curves for the uncontaminated and contaminated cell. Similar to the case of pure water and simulated seawater electrolysis, the activation region which occurs at low current densities cannot be clearly observed whereas the ohmic region which occurs at intermediate current densities can be clearly observed in both curves. A similar polarization trend can be observed for both cells, where the mass transport limitation is not present in the contaminated cell up to a current density of 1.5 Acm2. This shows that the sea salt may not affect the diffusion/adsorption of gas species into the reaction sites at the
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Fig. 6. (a) Comparison of polarization behavior between the uncontaminated and contaminated cell; (b) Impedance spectra for the both cells; (c) Durability test for the contaminated cell under a constant electrolysis current of 0.8 Acm2 at 800 C.
electrode-electrolyte interface. A smooth transition across the open circuit voltage from electrolyzer to fuel cell mode can also be observed. When the button cells were operated in electrolyzer mode, the ASR values calculated from the linear portion of the IeV curves were 0.27 Ucm2 and 0.29 Ucm2 for the uncontaminated and contaminated cell, with only a 7.4% difference in value. The contaminated cell, with its fuel electrode contaminated with sea salt, shows only a slightly lower performance as compared to the uncontaminated cell. The voltages measured at a current density of 1.0 Acm2 were 1.16 V and 1.19 V for the uncontaminated and contaminated cell, which corresponds to electrical energy efficiencies of 111.3% and 108.5%, respectively, with a hydrogen production rate of 0.0207 gs1cm2. The difference of only 2.8% in electrical efficiency between both cells at this particular current density can be assumed to be negligible, when compared to the difference in electrical efficiencies between low and high temperature electrolyzers. Fig. 6(b) shows the impedance spectra measured under OCV and a current density of 1.1 Acm2 (electrolyzer mode) for the uncontaminated and contaminated cell. It can be observed that rather similar impedance spectra trend and values were obtained for both cells. The difference in ohmic resistances between the two cells,
which are mainly contributed by the electrolyte and contact resistances, was similar under OCV and a current density of 1.1 Acm2 with a value of 0.02 Ucm2 (~14% difference). This large difference in ohmic resistance may arise from the slight difference in electrolyte thickness between the two cells as well as their contact area with the probe. The difference in polarization resistances between the two cells, which are contributed by the electrode polarization during electrochemical activity, was 0.007 Ucm2 (~8.6% difference) and 0.009 Ucm2 (~8.3% difference), respectively, under OCV and a current density of 1.1 Acm2. The maximum difference in total area specific resistance (ASR), which is a combination of the ohmic and polarization resistances, between both cells was 0.024 Ucm2 (~9.6% difference) under electrolyzer mode, which is mainly contributed by the difference in ohmic resistances. Unlike the case of electrolysis of steam produced from seawater, this case involves the testing of two separate cells at different times, where it is expected that there will be slight differences in the ohmic, polarization and total resistances between the uncontaminated and contaminated cell. Hence, the contamination of sea salt in the Ni-YSZ fuel electrode can be said to have no significant effects on the electrolysis performance of an SOEC since the difference in their polarization resistances and total ASR were all within 10% with respect to the uncontaminated cell.
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Fig. 7. SEM images of (a, d) surface, (b, e) cross-section and (c, f) electrode-electrolyte interfaces before (top row) and after (bottom row) testing for Cell B.
Fig. 6(c) shows the short-term durability test of the contaminated cell under a constant electrolysis current of 0.8 Acm2. Steam produced from pure water was used in the feedstock gas in this case. The degradation rate of the uncontaminated cell is about 15% 1000 h1 (Fig. 4) whereas the degradation rate of the contaminated cell is about 16% 1000 h1. The relatively similar degradation rates between both cells shows that the presence of sea salt in the fuel electrode may not pose any effects on the cell's degradation rate. After 50 h of durability test, the operating voltages of the uncontaminated and contaminated cells were about 1.066 V and 1.084 V with a 1.7% difference with respect to the uncontaminated cell. Since a constant current density of 0.8 Acm2
is being appiled, the volumetric flow rate of hydrogen and oxygen production will be constant throughout the durability test. It should be noted that the uncontaminated and contaminated cells were sealed with the same type of glass sealant; hence both cells would be subjected to the same contaminant from the glass sealant, if any. Fig. 7 shows the SEM images for various parts of the contaminated cell before and after the durability test. Salt particles were observed in the fuel electrode of the cell before testing was conducted, where the surface, cross-section and interface of the Ni-YSZ electrode were covered with sea salt as seen in Fig. 7(aec). EDX analysis has confirmed the presence of sea salt with the detection of sodium and chloride elements in various parts of the fuel electrode.
Fig. 8. TGA analysis on sea salt.
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However, salt particles can no longer be observed in the fuel electrode surface, cross-section and interface after the durability test as shown in Fig. 7(def). EDX analysis further confirmed that there was no presence of sodium or chloride elements on the surface, crosssection and interface of the electrode. This study suggests that sea salt may have been vaporized from the surface of the fuel electrode over the period of operation at 800 C. Fig. 8 shows the thermogravimetric analysis of the sea salt used in this study. From the illustration, it can be deduced from the interception of the gradients of the curves that the decomposition temperature, the temperature where the sea salt starts to undergo significant mass loss, is about 803 C which is close to the operating temperature of 800 C used in this study. This means that the sea salt may be volatile and have vaporized over the cell's operating period at ~800 C with no reaction occurring with the electrode material, which is the reason for the disappearance of sea salt from the fuel electrode after the operation. Hence, it is highly unlikely that sea salt would cause any serious contamination or poisoning effect on the fuel electrode of a SOEC when operating at a temperature of 800 C or above. 4. Conclusion In this study, the in-situ and ex-situ characteristics of an SOEC utilizing steam produced from pure water and simulated seawater, as well as the contamination of the SOEC's fuel electrode by the sea salt were investigated. It has been found that contaminants are not present in steam produced from the seawater. The electrolysis of steam produced from both pure water and seawater gave almost the same initial performance in terms of current-voltage curves as well as impedance spectra. The degradation rate for seawater electrolysis is also found to be similar to the degradation rate for pure water electrolysis. Post-mortem analysis showed that no sodium chloride is present on the surface, cross-section, and the electrode-electrolyte interface of the Ni-YSZ fuel electrode. For the direct contamination of sea salt in the SOEC's fuel electrode, it has been found that the contamination has posed no significant effects on the electrochemical performance of the cell. The uncontaminated and contaminated cell demonstrated almost similar initial performance as measured by polarization curves and impedance spectra. The degradation rate of the contaminated cell was also found to be very close to the degradation rate of the uncontaminated cell. Post-mortem analysis has confirmed that sea salt did not react with the electrode material and has been vaporized over the operating period of the cell at the typical operating temperature of 800 C. Acknowledgement This research programme/project is funded by the National Research Foundation, Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. References [1] M. Hoel, S. Kverndokk, Depletion of fossil fuels and the impacts of global warming, Resour. Energy Econ. 18 (1996) 115e136. [2] T.M.I. Mahlia, T.J. Saktisahdan, A. Jannifar, M.H. Hasan, H.S.C. Matseelar, A review of available methods and development on energy storage; technology update, Renew. Sustain. Energy Rev. 33 (2014) 532e545. [3] K. Mazloomi, C. Gomes, Hydrogen as an energy carrier: prospects and challenges, Renew. Sustain. Energy Rev. 16 (2012) 3024e3033. [4] M.A. Laguna-Bercero, Recent advances in high temperature electrolysis using
87
solid oxide fuel cells: a review, J. Power Sources 203 (2012) 4e16. [5] S. Yoon, I. Kang, J. Bae, Suppression of ethylene-induced carbon deposition in diesel autothermal reforming, Int. J. Hydrogen Energy 34 (2009) 1844e1851. [6] A. Manabe, M. Kashiwase, T. Hashimoto, T. Hayashida, A. Kato, K. Hirao, et al., Basic study of alkaline water electrolysis, Electrochim. Acta 100 (2013) 249e256. [7] S.M. Mousavi Ehteshami, S.H. Chan, Optimal design and operation of polymer electrolyte membrane reactors for pure hydrogen production, Chem. Eng. Sci. 98 (2013) 282e290. [8] W. Smith, The role of fuel cells in energy storage, J. Power Sources 86 (2000) 74e83. [9] Y. Liu, X. Hao, Z. Wang, J. Wang, J. Qiao, Y. Yan, et al., A newly-developed effective direct current assisted sintering technique for electrolyte film densification of anode-supported solid oxide fuel cells, J. Power Sources 215 (2012) 296e300. [10] J. Kong, K. Sun, D. Zhou, N. Zhang, J. Mu, J. Qiao, NieYSZ gradient anodes for anode-supported SOFCs, J. Power Sources 166 (2007) 337e342. [11] T.S. Li, W.G. Wang, H. Miao, T. Chen, C. Xu, Effect of reduction temperature on the electrochemical properties of a Ni/YSZ anode-supported solid oxide fuel cell, J. Alloys Compd. 495 (2010) 138e143. [12] IEA, Energy Technology Essentials (ETE 05), April 2007. [13] A.O. Isenberg, Energy conversion via solid oxide electrolyte electrochemical cells at high temperatures, Solid State Ionics 3e4 (1981) 431e437. [14] Z. Pan, Q. Liu, L. Zhang, X. Zhang, S.H. Chan, Effect of Sr surface segregation of La0.6Sr0.4Co0.2Fe0.8O3 d electrode on its electrochemical performance in SOC, J. Electrochem. Soc. 162 (2015). F1316eF23. [15] Z. Pan, Q. Liu, L. Zhang, X. Zhang, S.H. Chan, Study of activation effect of anodic current on La0.6Sr0.4Co0.2Fe0.8O3d air electrode in solid oxide electrolyzer cell, Electrochim. Acta 209 (2016) 56e64. [16] M. Henke, C. Willich, J. Kallo, K.A. Friedrich, Theoretical study on pressurized operation of solid oxide electrolysis cells, Int. J. Hydrogen Energy 39 (2014) 12434e12439. [17] M. Henke, C. Willich, C. Westner, F. Leucht, R. Leibinger, J. Kallo, et al., Effect of pressure variation on power density and efficiency of solid oxide fuel cells, Electrochim. Acta 66 (2012) 158e163. [18] J.D. Holladay, J. Hu, D.L. King, Y. Wang, An overview of hydrogen production technologies, Catal. Today 139 (2009) 244e260. [19] J.P. Stempien, Q. Sun, S.H. Chan, Performance of power generation extension system based on solid-oxide electrolyzer cells under various design conditions, Energy 55 (2013) 647e657. [20] P.H. Gleick, The World's Water 2000-2001: the Biennial Report on Freshwater Resources, Island Press, 2000. [21] N.A. Abdel Ghany, N. Kumagai, S. Meguro, K. Asami, K. Hashimoto, Oxygen evolution anodes composed of anodically deposited MneMoeFe oxides for seawater electrolysis, Electrochim. Acta 48 (2002) 21e28. [22] E. Baniasadi, I. Dincer, G.F. Naterer, Electrochemical analysis of seawater electrolysis with molybdenum-oxo catalysts, Int. J. Hydrogen Energy 38 (2013) 2589e2595. [23] D.W. Kirk, A.E. Ledas, Precipitate formation during sea water electrolysis, Int. J. Hydrogen Energy 7 (1982) 925e932. [24] Z. Jiao, N. Takagi, N. Shikazono, N. Kasagi, Study on local morphological changes of nickel in solid oxide fuel cell anode using porous Ni pellet electrode, J. Power Sources 196 (2011) 1019e1029. [25] T.S. Li, W.G. Wang, Sulfur-poisoned Ni-based solid oxide fuel cell anode characterization by varying water content, J. Power Sources 196 (2011) 2066e2069. [26] A. Hauch, S.D. Ebbesen, S.H. Jensen, M. Mogensen, Solid oxide electrolysis cells: microstructure and degradation of the Ni/Yttria-Stabilized zirconia electrode, J. Electrochem. Soc. 155 (2008). B1184eB93. [27] S.P. Jiang, S.P.S. Badwal, An electrode kinetics study of H2 oxidation on Ni/ Y2O3eZrO2 cermet electrode of the solid oxide fuel cell, Solid State Ionics 123 (1999) 209e224. [28] H.P. Dasari, S.-Y. Park, J. Kim, J.-H. Lee, B.-K. Kim, H.-J. Je, et al., Electrochemical characterization of Nieyttria stabilized zirconia electrode for hydrogen production in solid oxide electrolysis cells, J. Power Sources 240 (2013) 721e728. [29] S.-D. Kim, D.-W. Seo, A.K. Dorai, S.-K. Woo, The effect of gas compositions on the performance and durability of solid oxide electrolysis cells, Int. J. Hydrogen Energy 38 (2013) 6569e6576. , Solid oxide electrolysis cell analysis by [30] A. Nechache, M. Cassir, A. Ringuede means of electrochemical impedance spectroscopy: a review, J. Power Sources 258 (2014) 164e181. [31] W.G. Bessler, J. Warnatz, D.G. Goodwin, The influence of equilibrium potential on the hydrogen oxidation kinetics of SOFC anodes, Solid State Ionics 177 (2007) 3371e3383. [32] M. Vogler, A. Bieberle-Hütter, L. Gauckler, J. Warnatz, W.G. Bessler, Modelling study of surface reactions, diffusion, and spillover at a Ni/YSZ patterned anode, J. Electrochem. Soc. 156 (2009). B663eB72. [33] C. Yang, A. Coffin, F. Chen, High temperature solid oxide electrolysis cell employing porous structured (La0.75Sr0.25)0.95MnO3 with enhanced oxygen electrode performance, Int. J. Hydrogen Energy 35 (2010) 3221e3226.