Gadolinium doped ceria-impregnated nickel–yttria stabilised zirconia cathode for solid oxide electrolysis cell

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Gadolinium doped ceria-impregnated nickeleyttria stabilised zirconia cathode for solid oxide electrolysis cell Pattaraporn Kim-Lohsoontorn a,b,*, Yu-Mi Kim c, Navadol Laosiripojana d, Joongmyeon Bae b,c,** a

Department of Chemical Engineering, Mahidol University, Nakorn Pathom 73170, Thailand KI for Eco-Energy, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305 701, Republic of Korea c Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305 701, Republic of Korea d The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand b

article info

abstract

Article history:

Steam electrolysis (H2O / H2 þ 0.5O2) was investigated in solid oxide electrolysis cells

Received 10 March 2011

(SOECs). The electrochemical performance of GDC-impregnated NieYSZ and 0.5% wt

Received in revised form

RheGDC-impregnated NieYSZ was compared to a composite NieYSZ and NieGDC elec-

26 April 2011

trode using a three-electrode set-up. The electrocatalytic activity in electrolysis mode of

Accepted 27 April 2011

the NieYSZ electrode was enhanced by GDC impregnation. The RheGDC-impregnated

Available online 16 June 2011

NieYSZ exhibited significantly improved performance, and the electrode exhibited comparable performance between the SOEC and SOFC modes, close to the performance of

Keywords:

the composite NieGDC electrode. The performance and durability of a single cell GDC-

Solid oxide electrolysis cell

impregnated NieYSZ/YSZ/LSMeYSZ with an H2 electrode support were investigated. The

Hydrogen production

cell performance increased with increasing temperature (700  Ce800  C) and exhibited

Steam electrolysis

comparable performance with variation of the steam-to-hydrogen ratio (50/50 to 90/10).

Ceria

The durability in the electrolysis mode of the NieYSZ/YSZ/LSMeYSZ cell was also signif-

Impregnation

icantly improved by the GDC impregnation (200 h, 0.1 A/cm2, 800  C, H2O/H2 ¼ 70/30). Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years, there has been increasing interest in hightemperature steam electrolysis to produce hydrogen (H2) using solid oxide electrolysis cells (SOECs). Most of the hydrogen currently produced is made from hydrocarbon fuels, which results in the liberation of CO2 and consumes valuable hydrocarbon fuel resources. Water electrolysis (H2O / H2 þ 1/ 2O2) can be a promising way to achieve zero-emission and high purity H2 production. Moreover, the high operating temperatures of steam electrolysis thermodynamically reduce the electricity energy requirement in the electrolysis

process and allow the use of waste heat from power stations or other industrial processes. The kinetics of SOECs is also improved at increasing temperatures, which lead to lower internal cell resistance and higher current density at a given cell voltage and steam partial pressure. Due to their similarity to solid oxide fuel cells (SOFCs), advances have been made in the development of high-temperature SOECs based on cell assemblies with the structure nickeleyttria stabilised zirconia (NieYSZ) hydrogen electrode/YSZ electrolyte/lanthanum strontium manganiteeYSZ (LSMeYSZ) oxygen electrode [1e5]. However, a previous study showed that the performance discrepancies of the cell in operation between

* Corresponding author. Department of Chemical Engineering, Mahidol University, Thailand. Tel.: þ66 2 889 2138. ** Corresponding author. KI for Eco-Energy, KAIST, Republic of Korea. Tel.: þ82 42 350 3045. E-mail addresses: [email protected] (P. Kim-Lohsoontorn), [email protected] (J.-M. Bae). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.199

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the electrolytic and galvanic modes could be varied, depending on the electrode materials [6,7]. In previous work, the electrochemical performance of the NieYSZ H2 electrode for steam electrolysis was significantly lower than that for H2 oxidation, while comparable activity for operating between the SOEC and SOFC mode was achieved with the Niegadolinium doped ceria (GDC) electrode [7]. Thus, the ceria-based electrode is preferable for electrolysis. Alternatives to the widely used NieYSZ/YSZ/ LSMeYSZ cell are also under investigation [6e12]. The work reported here therefore seeks to utilise ceria in the H2 electrode for SOEC. Because NieGDC exhibited a relatively high performance and comparable performance between the SOEC and SOFC modes [7], it is interesting to use a composite NieGDC H2 electrode with the GDC electrolyte for SOEC. However, an optimised and time-consuming process to fabricate the cell is required [13]. Because ceria-based materials exhibit mixed ionic and electronic conductivity, the open circuit voltage (OCV) of the cell with ceria as an electrolyte is generally lower than the corresponding Nernst voltage. Moreover, the high applied voltage in the SOEC system can deteriorate the ionic transfer number due to electronic conduction by the reduction of the ceria electrolyte from Ce4þ to Ce3þ [14]. The double layer electrolyte of YSZ/GDC can be used, where YSZ acts as a buffer layer for electronic conduction [13,15]. However, ceria-based electrolyte also exhibit relatively low mechanical strength compared to YSZ electrolytes [16]. Therefore, a NieGDC H2 electrode-supported cell can be a disadvantage. Metal-supported SOFCs can be advantageous for ceria-based electrolytes to increase the mechanical strength of the cell, but, in the case of SOECs, the oxidation of the metal substrate in environments containing high steam content can be a major concern [17]. In this study, the performance and durability of a H2 electrode-supported SOEC with a conventional NieYSZ H2 electrode impregnated with GDC solution were investigated. High-temperature steam electrolysis of the GDC-impregnated NieYSZ/YSZ/LSMeYSZ cell was studied over a range partial pressures of H2O/H2 (50%e90% H2O:H2) and applied voltages (0.2e1.5 V). The durability of the cell during electrolysis (0.1 A/cm2, 800  C, H2O/H2 ¼ 70/30, 200 h) was monitored, and the microstructure of the cell after the electrolysis was analysed. A comparison of the electrochemical performance among GDC-impregnated NieYSZ, RheGDC-impregnated NieYSZ, composite NieYSZ, and composite NieGDC electrodes is also reported. The three-electrode set-up was used to individually access the electrochemical performance of the H2 electrode, and the performance discrepancy between the electrolysis and galvanic modes was investigated.

followed by sintering at 1500  C for 4 h to produce electrolyte pellets with a diameter of w25.0 mm and a thickness of w1.5 mm. The NieYSZ electrode slurry was blended with compositions of 15 wt% of binder (Butvar B-98, Sigma Aldrich), 1 wt% of dispersant (polyvinyl pyrrolidone, Sigma Aldrich), 1 wt% of plasticiser (polyethylene glycol, Sigma Aldrich), and 60:40 wt% NieYSZ powder balance (Ni, J.T. Baker, USA). The NieYSZ electrodes were then screen-printed on YSZ electrolyte and fired at 1200  C for 1 h, giving an electrode layer with an area of 0.785 cm2 and a thickness of w30 mm. A reference electrode should be stable and placed in a stable gaseous environment. Otherwise, changes in the electrode and gas partial pressure can cause alterations in the reference potential. Our previous study showed that O2 electrodes affected the discrepancies in the performance of the cell operated in the SOFC and SOEC modes [6,7]. Therefore, platinum, Pt (Gwi-joo Metal, Korea), was chosen as a reference and counter electrode in this study to maintain the electrode performance stability in both modes of operation. A Pt counter electrode with an area of 0.785 cm2, surrounded by a ring-type reference electrode (Pt), was screen-printed and fired at 900  C for 1 h. The reference electrode had an internal diameter of 19 mm and an external diameter of 21 mm. The working electrode and counter electrode were identical circular discs. For the cell with a GDC-impregnated NieYSZ, the NieYSZ electrode of the sintered cell was then impregnated with 10 mol% gadolinium doped ceria (GDC, Praxair, USA). The GDC solution was 1.2 mol L1 (20% wt GDC power, 78% wt solvent of butanol-mixed xylene solution, 1% wt polyvinyl pyrrolidone, 0.5% wt polyethylene glycol, and 0.5% wt butvar). Two kinds of samples were used in this study: an electrolyte-supported cell with a w30 mm H2 electrode and an H2 electrode-supported cell with a thickness of w1.2 mm. The impregnation of the GDC solution was performed by considering the volume of the NieYSZ substrates and using a GDC loading of 0.26 mg cm2 for the electrolyte-supported cell and 10.4 mg cm2 for the H2supported cell. The GDC-impregnated NieYSZ cell was sintered in-situ at an operating temperature of 800  C for 2 h in ambient air before steam/H2 was introduced to the electrode chamber. The cell with RheGDC-impregnated NieYSZ was fabricated using the same process but 0.5% wt Rh e 99.8% wt GDC power (Praxair, USA) was used for the RheGDC impregnating solution. To fabricate a composite NieGDC electrode cell, the process described above was followed, and the NieGDC electrode slurry was blended with the same compositions used in the NieYSZ slurry except for the 60:40 wt% NieGDC powder balance used instead.

2.2.

2.

Experimental

2.1.

Three-electrode cell fabrication

SOEC electrolyte-supported cells with a reference electrode were prepared to evaluate the performance of a GDC-impregnated NieYSZ compared to RheGDC-impregnated NieYSZ, composited NieGDC, and composited NieYSZ electrodes. YSZ powder (Tosho, Japan) was pressed at 1 metric ton for 30 s

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Hydrogen electrode-supported cell fabrication

A SOEC H2 electrode-supported cell was fabricated. NiO powder (J.T. Baker, USA) and 8 mol% yttria stabilised zirconia, YSZ (Tosho, Japan), were mixed with the weight ratio of 60:40 and ball-milled for 24 h using ethanol as the medium. Starch (12 wt%) was added as the pore former to create sufficient porosity in the cathode. The mixed powder was pressed into pellets (1 metric ton, 30 s), followed by sintering at 1200  C for 2 h, giving pellets with a diameter of w26.0 mm and thickness of w1.2 mm. The YSZ slurry was blended with compositions of

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15 wt% of binder, 2 wt% of dispersant, 10 wt% of plasticiser and a YSZ powder balance (8 mol% yttria in YSZ). The YSZ slurry was ball-milled for 36 h. The dip coating technique was used to deposit the YSZ electrolyte layer, followed by sintering at 1500  C for 4 h to give an electrolyte layer thickness of w25 mm. The LSMeYSZ ink was prepared using a commercial powder (Hanchem, Korea) dispersed in a mixture of aterpineol (Sigma Aldrich, USA) and cellulose (Sigma Aldrich, USA). The ratio between LSM and YSZ in the composite electrode was 50:50 wt%. The oxygen electrode ink was then screen-printed on the YSZ electrolyte and fired at 1100  C to fabricate oxygen electrode layers with an area of 0.785 cm2 and a thickness of w30 mm. The NieYSZ electrode was then impregnated with GDC to the desired level. Fig. 1(a)e(d) show the SEM images before the in-situ sintering of the GDC loading on NieYSZ electrodes for 0 mg cm2, 5.2 mg cm2, 10.4 mg cm2, and 20.4 mg cm2, respectively.

2.3.

Electrochemical performance measurement

Single cell polarisation curves were generated using linear sweep current techniques. A potentiostat (Solartron, SI 1287) was used to control the voltage range with a scan rate of 20 mV/s. The study was carried out over a range of SOEC operating conditions by varying the partial pressure of H2O/H2 in the process gas (50%e90% H2O:H2), the applied voltage (0.2e1.5 V) and the electrolysis current. The electrical connection was made to the cell electrodes via platinum wires and paste (wire 99.99% Pt, 0.25 mm diameter, Gwi-joo Metal, South Korea) and was placed with compression. The cell ridge was sealed using glass sealant (Untra-Temp 516, Aramco, USA) to separate the gas environment of the two electrodes. Fig. 2 shows the experimental setup for the delivery of the gas to the cell in the furnace. The test

system allowed variable gas compositions of steam, H2, and N2 to be introduced to the H2 electrode as well as air to the O2 electrode. N2 was used as a carrier gas to control the steam ratio in the gas compositions. Deionised water, supplied using an HPLC liquid pump (Chrom Tech, USA), was evaporated in a heated sand bath and mixed with the N2 line. The steam/N2 stream was combined with the H2 directed to the furnace using a heating line for steam electrolysis. The exact gas composition for each operating mode is shown in Table 1.

2.4.

Microstructure analysis

The microstructure images were taken using a scanning electron microscope (SEM) with an electron acceleration voltage of 15 kV and a vacuum of 1.5  105 torr (HITACHI FESEMS-4300, Japan). The images of NieYSZ with varied amounts of GDC loading were taken before in-situ sintering in operating conditions and after in-situ sintering for 2 h, following the temperature ramping profile for sealing material. The SEM images of the cell after the durability tests were also taken.

3.

Results and discussion

3.1. Comparison of electrolytic and galvanic modes of operation using the three-electrode set-up The overpotential of H2 electrode was evaluated in both SOEC and SOFC modes while the steam content delivered to the H2 electrode was maintained constant at H2O/H2 ¼ 70/30 (54% H2O þ 23%H2 þ 23%N2) at 800  C. To observe any discrepancies in the H2 electrode performance between SOEC and SOFC mode, a three-electrode set-up was used, and the results were

Fig. 1 e The SEM images before in-situ sintering of the GDC loading on NieYSZ electrodes for (a) 0 mg cmL2; (b) 5.2 mg cmL2; (c) 10.4 mg cmL2; and, (d) 20.4 mg cmL2, respectively.

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Fig. 2 e Schematic drawing of the test system.

analysed using a Tafel plot. The overpotential as a function of current density of GDC-impregnated NieYSZ and RheGDCimpregnated NieYSZ were compared to those of the composite NieYSZ and NieGDC electrodes, as shown in Fig. 3. As reported previously [7], the activity of the NieYSZ electrode for H2 oxidation was significantly higher than that for steam electrolysis, which agreed with the results reported by Eguchi et al. [14] and Marina et al. [11]. In contrast to the NieYSZ electrode, the relatively higher performance and comparable activity between the SOEC and SOFC modes were achieved with the composite NieGDC electrode, which can be

Table 1 e Gas composition for electrolysis operating mode. Operating mode H2O electrolysis

Gas composition H2O/H2 ¼ 50/50, 38%H2O þ 38%H2 þ 24%N2 H2O/H2 ¼ 70/30, 54%H2O þ 23%H2 þ 23%N2 H2O/H2 ¼ 90/10, 69%H2O þ 8%H2 þ 23%N2

significantly advantageous to an SOEC/SOFC reversible system. Ideally, if SOEC and SOFC can operate with the same device with high and comparable efficiency, there can be significant overall cost benefits. Much effort has been spent to investigate reversible solid oxide cells (RSOCs) using symmetrical cells [8,18]. The cell can take advantage of excess electrical energy demand during the off-peak hours to produce H2 and then utilise it later during high electrical energy demand. In this study [7], the Ni content in both electrodes (NieYSZ and NieGDC) was 60% wt, indicating the benefit of the GDC component for electrolysis operation, likely due to the suppression of Ni surface oxidation in environments containing a large amount of steam. The surface oxidation of Ni under such an environment, which leads to the formation of a less active layer, has been proposed as another possible cause of performance degradation of the electrode under electrolysis conditions [14,18]. When the NieYSZ electrode was impregnated with GDC solution, the electrode performance was significantly improved in the electrolysis mode and the GDC-impregnated NieYSZ electrode exhibited closer to symmetrical behaviour between

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Fig. 3 e Current/overpotential of GDC-impregnated NieYSZ, RheGDC-impregnated NieYSZ, composite NieYSZ, and composite NieGDC electrode during electrolysis and galvanic mode (800  C, H2O/H2 [ 70/30).

the SOEC and SOFC modes, as shown in Fig. 3. However, the performance in the SOFC mode was lower when compared to that of the bare NieYSZ electrode. It is unclear for the lower performance in SOFC mode; however, it is likely that the GDC impregnation was not well-distributed on the NieYSZ base and parts of electrode porosity were blocked. This can be the cause of a relatively larger overpotential at high current density in the GDC-impregnated NieYSZ electrode compared to bare NieYSZ electrode while suppression of Ni surface oxidation in high steam environment is more dominant effect in SOEC mode. Fig. 4(a) shows the SEM image of the GDC starting powder for the GDC impregnating solution, and it can be seen the GDC particles are small (<1 mm) but highly agglomerated. In Fig. 4(b), the SEM image of the GDCimpregnated NieYSZ sample after in-situ fired at 800  C for 2 h in ambient air shows that the impregnated GDC was not welldistributed on the NieYSZ base. This result suggests that the impregnation process in this study must be optimised. It is expected that a smaller GDC particle size and well-distributed impregnation could lead to even more improved performance of the electrode. However, with the 0.5% wt RheGDCimpregnation on the NieYSZ electrode, the performance could be significantly improved close to that of the composite NieGDC electrode likely due to the catalytic activity of the Rh. The electrode performance also exhibited near symmetrical behaviour between the SOEC and SOFC modes.

3.2. Performance and durability of H2 electrolytesupported GDC-impregnated NieYSZ/YSZ/LSMeYSZ electrolysis cell The performance and durability tests of the H2 electrode-supported GDC-impregnated NieYSZ/YSZ/LSMeYSZ cell were performed. The variation of the SOEC performance with operating temperature was studied. The H2O/H2 was maintained at a constant 70/30 while the temperature was varied. The I/V response at different temperatures ranging from 700  C to

Fig. 4 e The SEM images of (a) GDC starting power; and, (b) GDC-impregnated NieYSZ electrode after fired at 800  C 2 h.

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800  C is shown in Fig. 5. The SOEC performance improved with increasing temperature as expected, similar to the increasing SOFC performance with the operating temperature. The effect of the steam content on the cell performance was investigated in both the SOFC and SOEC modes. Fig. 6 shows the I/V response of the cell when the steam-to-H2 ratio was varied (H2O/H2 ¼ 50/50, 70/30, and 90/10), and the operating temperatures were maintained at 800  C. The OCV decreased with increasing steam content in accordance with the Nernst potential. Under electrolysis conditions, similar performance was observed for the cell exposed to varied H2O/H2 from 50/50 to 90/10 in contrast to the results of a previous study on a NieGDC composite electrode [7]. The overpotential of the NieGDC composite electrode during steam electrolysis slightly increased when the H2O/H2 was changed from 50/50 to 70/30 and significantly increased with H2O/H2 ¼ 90/10, likely due to the result of the sensitivity of the GDC conductivity to pO2. This sensitivity can be a drawback for the use of GDC in a composite electrode, but there was no significant negative effect when a relatively small amount of GDC was used as an impregnating component. It should be noted that the cell performance in this study was rather low, compared to other studies using bare NieYSZ electrodes [1e5]. The GDC-impregnated NieYSZ electrode exhibited low electrochemical performance because the performance of the bare NieYSZ in this study was primarily low. It is likely due to low and nonwell distributed porosity of the electrode as mention earlier (Section 3.1). The fabrication and impregnation process in this study is still needed to be optimised; however, the results from this study demonstrated a potential of the GDCimpregnated NieYSZ as a cathode for SOECs. Durability studies on the GDC-impregnated NieYSZ/YSZ/ LSMeYSZ cell were performed over 200 h at constant operating conditions (an electrolytic current density of 0.1 A/cm2, an operating temperature of 800  C, H2O/H2 ¼ 70/30). Compared to a conventional NieYSZ/YSZ/LSMeYSZ button cell under the same operating conditions, the performance durability was significantly improved with the GDC impregnation of the H2 electrode, as shown in Fig. 7. This result implied that the GDC impregnation of a well-sintered NieYSZ electrode could be a simple but effective way to increase the performance and

Fig. 5 e I/V characteristics of H2 electrode-supported GDCimpregnated NieYSZ/YSZ/LSMeYSZ cell during the steam electrolysis (H2O/H2 [ 70/30 to H2 electrode) and galvanic modes for temperatures from 700  C to 800  C.

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Fig. 6 e I/V characteristics of H2 electrode-supported GDCimpregnated NieYSZ/YSZ/LSMeYSZ during the steam electrolysis and galvanic modes at 800  C when steam-tohydrogen ratio to H2 electrode was varied (H2O/H2 [ 50/50; 70/30; and 90/10).

durability of solid oxide electrolysis cells. A large noise band in the performance response was observed on both samples, and the exact cause of it is uncertain. It may be due to the uneven introduction of steam to the H2 electrode or poor control of water addition in the experiment. However, it should be noted that in the experimental set-up, the gas line connecting from the heated sand bath to the test rig were heated by a heating band. Therefore, the temperature of the gas inlet to the test rig was controlled at 150  C. The gas inlet temperature was recorded by thermocouple placed near the inlet of the test rig. The microstructure analysis was performed after the GDCimpregnated NieYSZ cell was operated in the electrolysis mode for 200 h (Fig. 8). Cross-sectional SEM in Fig. 8(a) and (b) indicated that the H2 electrode exhibited signs of agglomeration near the surface of the electrode (Fig. 8(a)), but no discernable agglomeration was observed on the inner structure (Fig. 8(a) and (b)). This result suggests that well-designed functional layers for H2 electrodes could be developed to help minimise the agglomeration of the electrode. It also suggests that agglomeration near the H2 electrode surface may have less negative impact on the cell performance durability. The delamination of O2 electrodes in the electrolysis mode has been reported [6,19,20]. The delamination of the O2 electrode made of barium strontium cobalt ferrite (BSCF) after only 20 h of electrolysis was reported in our previous work [6]. Mawdsley et al. reported the delamination of the LSMeYSZ electrode after a prolonged operation of 2000 h [19]. It was suggested that a high rate of oxygen release into any defects at the perovskiteezirconia interfaces within the electrolyte layer could cause localised pressure-induced cracking at the interfaces [19]. However, in this study for a shorter period of time, the layers of the GDC-impregnated NieYSZ H2 electrode, YSZ electrolyte, and LSMeYSZ O2 electrode remained fused together in the electrolysis mode for 200 h, as shown in Fig. 8(b). Chen et al. reported that an LSM electrode was delaminated under high polarisation current, and GDC impregnation of the LSM electrode not only increased the electrocatalytic oxygen oxidation reaction but also inhibited the delamination of the electrode [20]. Both H2- and O2-electrodes are the main factors for performance

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Fig. 7 e Potential monitoring of hydrogen electrode-supported SOECs during the steam electrolysis (0.1 A/cm2, 800  C, H2O/H2 [ 70/30, 200 h) for cell A: NieYSZ/YSZ/LSMeYSZ; and, cell B: GDC-impregnated NieYSZ/YSZ/LSMeYSZ.

durability in SOEC. The O2 electrode delamination may depend on the O2 electrode materials and operating conditions, and dominates during a prolong operation. The O2 electrode is also needed to be improved. It suggests that

an O2 electrode that is able to accommodate a large oxygen over-stoichiometry is required for SOEC operation. The in-plane view of SEM image revealed signs of agglomeration on the H2 electrode (Fig. 8(c)), corresponding to

Fig. 8 e The SEM images of the cell (H2 electrode-supported GDC-impregnated NieYSZ/YSZ/LSMeYSZ) after the electrolysis mode for 200 h (0.1 A/cm2, 800  C, H2O/H2 [ 70/30, 200 h): (a) cross-sectional view of GDC-impregnated NieYSZ at the electrode surface side; (b) cross-sectional view of GDC-impregnated NieYSZ/YSZ/LSMeYSZ; (c) In-plane view of GDCimpregnated NieYSZ; and (d) GDC-impregnated NieYSZ.

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the cross-sectional image in Fig. 8(a). Fig. 8(c) and (d) show that GDC particles remained attached on Ni surface after the electrolysis mode.

4.

Conclusions

The electrochemical performance of GDC-impregnated NieYSZ and RheGDC-impregnated NieYSZ electrodes were investigated in both electrolysis and galvanic modes and compared to composite NieYSZ and NieGDC electrodes. The electrochemical performance of the composite NieYSZ electrode was significantly lower in the SOEC mode than that in the SOFC mode, while relatively higher performance and comparable performance between the SOEC/SOFC operations was obtained from the composite NieGDC. When the NieYSZ electrode was impregnated with the GDC solution, the electrochemical performance in the electrolysis mode increased significantly. While microstructure analysis indicated that the impregnation process is required an optimisation, the 0.5% wt RheGDC-impregnated NieYSZ exhibited near symmetrical performance between the SOEC/SOFC operations and improved performance close to that of the composite NieGDC. The cell composed of the H2 electrode-supported GDCimpregnated NieYSZ/YSZ/LSMeYSZ was investigated. The cell performance increased with increasing temperature and showed comparable performance when the steam contents were varied from H2O/H2 ¼ 50/50 to 90/10, in contrast to the behaviour of the composite NieGDC electrode, which was reported to exhibited decreasing performance with increasing steam content in the environment of the H2 electrode. The durability for 200 h of the GDC-impregnated NieYSZ/YSZ/ LSMeYSZ was significantly higher than that of bare NieYSZ/ YSZ/LSMeYSZ under the same operating conditions (0.1 A/ cm2, 800  C, H2O/H2 ¼ 70/30). The microstructural analysis of the cell after the electrolysis mode revealed an agglomeration of the H2 electrode near its surface, suggesting that welldesigned functional layers for the H2 electrode could be developed. The results demonstrate that GDC impregnation of well-sintered NieYSZ electrodes could be a simple and effective way to increase the performance and durability of solid oxide electrolysis cells.

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Acknowledgement This work was the outcome of the project with the Korea Electric Power Research Institute (KEPRI), Republic of Korea. The work was also supported by the New & Renewable Energy of the Korea Institute of Energy Technology, Republic of Korea. The authors also thank the Thailand Research Fund (TRF) for supporting P. Kim-Lohsoontorn.

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