Effect of CuO as Sintering Additive in Scandium Cerium and Gadolinium-Doped Zirconia-Based Solid Oxide Electrolysis Cell for Steam Electrolysis

Abstract

The effect of CuO as a sintering additive on the electrolyte of solid oxide electrolysis cells (SOECs) was investigated. 0.5 wt% CuO was added into Sc_0.1Ce_0.05Gd_0.05Zr_0.89O₂ (SCGZ) electrolyte as a sintering additive. An electrolyte-supported cell (Pt/SCGZ/Pt) was fabricated. Phase formation, relative density, and electrical conductivity were investigated. The cells were sintered at 1373 K to 1673 K for 4 h. The CuO significantly affected the sinterability of SCGZ. The SCGZ with 0.5 wt% CuO achieved 95% relative density at 1573 K while the SCGZ without CuO could not be densified even at 1673 K. Phase transformation and impurity after CuO addition were not detected from XRD patterns. Electrochemical performance was evaluated at the operating temperature from 873 K to 1173 K under steam to hydrogen ratio at 70:30. Adding 0.5 wt% CuO insignificantly affected the electrochemical performance of the cell. Activation energy of conduction (E_a) was 72.34 kJ mol⁻¹ and 74.93 kJ mol⁻¹ for SCGZ and SCGZ with CuO, respectively.

1. Introduction

Global climate change is an important issue that is clearly found to affect the environment and humanity. Greenhouse gas emission is the major cause for retaining heat in the atmosphere, leading to climate change. Greenhouse gases are mainly generated from the combustion of fossil fuel such as petroleum, natural gas, and coal. Hydrogen is the one of the promising energy carriers for better environment since only water and energy are produced from hydrogen combustion [1,2]. Moreover, hydrogen can be used as a feedstock for various industrial chemical productions [3,4,5].

Solid oxide electrolysis cells (SOECs) are used to produce high purity hydrogen from steam electrolysis reaction at elevated temperatures. Material selection is one of the challenges faced in the implementation of SOEC. The operation may operate under severe condition in the production scale. Sealant of gas leakage between the stack needs to be modified [6]. Material choice for SOEC electrode is often similar to solid oxide fuel cell [7,8,9]. Therefore, 8 mol% yttria stabilized zirconia (YSZ), which is a conventional electrolyte in solid oxide fuel cell [10,11,12,13], is generally used as the SOEC electrolyte. Attempt has been made to seek for fast ionic conductor instead of conventional YSZ [14,15,16,17,18]. Doped-cerate- or doped-zirconia-based electrolyte has been studied. Scandium oxide (Sc₂O₃) is the one among the dopants for zirconia-based electrolyte (Scandium stabilized zirconia, ScSZ) that improves the ionic conductivity when compared with YSZ electrolyte. Similar ionic radius between Sc³⁺ and Zr⁴⁺ can cause low internal stress and the activation energy of conduction [16]. However, phase transition can occur in ScSZ electrolyte, leading to loss of conductivity. Abbas et al. [19] prepared co-doping of 1 mol% Gd₂O₃, CaO, and CeO₂ with 10 mol% Sc₂O₃ into zirconia structure to reduce phase transition and it was found that 1 mol% CeO₂ and 10 mol% Sc₂O₃ as dopants provide relatively highest conductivity, likely because of the close ionic radius of Ce⁴⁺ and Zr⁴⁺ compared to other dopants. Shin et al. [20] investigated the co-doping of trivalent oxides (Gd₂O₃, Yb₂O₃, and Y₂O₃) with CeO₂ into scandium stabilized zirconia structure. It was reported that co-doping of Gd₂O₃ with CeO₂ provides relatively higher ionic conductivity when compared with other oxides. However, doped-zirconia electrolyte often requires relatively high sintering temperature (~1723 K) for electrolyte densification. High sintering temperature of the electrolyte can lead to degradation issue in other cell compartments, leading to difficulty in fabrication process. For example, in electrode-supported cells, nickel (Ni) component in an electrode can agglomerate at high temperature, leading to decreasing three phase boundary of the electrode. In metal-supported cell, metal part (often containing iron (Fe) or Ni content) cannot withstand high sintering temperature because of a large thermal expansion coefficient of the metal. Therefore, lower sintering temperature for electrolyte is required to enhance the fabrication process of the cell.

Sintering additive can promote densification and help reduce sintering temperature of the electrolyte. The additive can melt to form liquid phase during the sintering process. Solid grain of the electrolyte is wet by liquid phase which provide capillary force pulling the grain denser [16]. There are varied metal oxide such as CuO, NiO, Fe₂O₃, and MnO₂ that have been used as sintering additives for zirconia-based electrolyte [21,22,23,24]. However, adding sintering additives which are often metal oxides can induce electronic conductivity in the electrolyte. Myung et al. [25] investigated scanning electron microscope (SEM) images of sintered YSZ by varying amount of CuO additive from 0 to 1.5 wt%. It was found that optimum quantity of additive and sintering temperature was 0.5 wt% of CuO and 1623 K, respectively. Our previous work [26] reported other metal oxides (NiO, Co₂O₃, and ZnO) as additives in barium cerate-based electrolyte. The additives helped increase sinterability of the electrolyte and provided different conductivity for the electrolyte.

In this research, Sc_0.1Ce_0.05Gd_0.05Zr_0.89O₂ (SCGZ) was used as an electrolyte in SOEC. CuO was added into the electrolyte as a sintering additive. The effect of sintering additive on phase formation, cell densification, and electrical performance were investigated.

2. Experimental

2.1. Cell Fabrication

Scandium cerium- and gadolinium-doped zirconia (Sc_0.1Ce_0.05Gd_0.05Zr_0.89O₂, SCGZ) was fabricated into an electrolyte-supported SOEC. Three grams of SCGZ powder (Kceracell, Chungcheongnam-do, Republic of Korea) were pressed at 24 MPa into a pellet with diameter of 25 mm and thickness of 1.4 mm. The pellets were then sintered at 1373 K to 1673 K for 4 h. The electrolyte pellet with sintering additive was fabricated using the same method when 0.5 wt% CuO was added into the SCGZ staring powder. The relative density of the sintered electrolyte pellet was calculated following Equation (1).

$$\mathrm{Relative}\mathrm{density}=\frac{\mathrm{Actual}\mathrm{density}}{\mathrm{Theoretical}\mathrm{density}}\times 100$$

(1)

where actual density is the bulk density of sintered cell. The theoretical density was obtained by lattice parameter from X-ray diffraction (XRD) analysis (Bruker D8 Advance, Billerica, MA, USA).

Platinum (Pt) conductive paste (70 wt% Pt, Nexceris) was coated on the both sides of the electrolyte pellet as electrodes with thickness and area of ~30 µm and ~0.5 cm², respectively. The schematic of cell component was shown as Figure 1. The cell was then fired at 1173 K for 2 h. It should be noted that the choices of electrode used depend on the deposition technique, the operating temperature, and the type of conductivity. Using gold or platinum as working electrode was reported to perform well at high temperature but below 873 K they are relatively blocking oxygen [27].

2.2. Characterization

Phase and crystallite size of sintered electrolyte were determined using XRD with CuKα source (Bruker D8 Advance, Billerica, MA, USA). The microstructure of the samples was investigated using SEM (Hitachi S-3400N JEOL model S-3400, Tokyo, Japan). The average grain sizes were calculated by linear intersection method.

2.3. Electrochemical Performance Measurement

Electrochemical performance was measured in a controlled temperature from 873 K to 1173 K with the feed containing steam and hydrogen at the ratio of 70:30. The fabricated cell was attached with Pt mesh and wire (Kceracell, Chungcheongnam-do, Korea) for electrical connection. The cell was placed on a cell holder with high temperature sealant (Ceramabond 552, Aramco, Houston, TX, USA). The holder was installed inside a vertical furnace (Chavachote, Bangkok, Thailand). High performance liquid chromatography (HPLC) liquid pump (Teledyne SSI, State College, PA, USA) was used to supply deionized water through a heated-pipe for steam generation in the system. Linear sweep voltammetry procedure was applied to generate current/voltage (I/V) curves by controlling the potential from 0.4 V to 1.8 V with a scan rate of 20 mVs⁻¹ (Metrohm Autolab, Utrecht, The Netherlands).

Resistance was determined by the slope of current/voltage (I/V) curves and the conductivity of fabricated cell (σ) was then calculated using Equation (2).

σ = L/RA

(2)

where σ is the conductivity (S cm⁻¹); L is the thickness of fabricated cell (cm); R is the cell resistance (Ω); and, A is the area of electrode (cm²).

The activation energy of conduction (E_a) was obtained by using Arrhenius, Equation (3) with conductivity value as mentioned above.

$$\mathsf{\sigma }\mathrm{T}=\mathrm{A}·\exp \left(-\frac{E_a}{\mathrm{RT}}\right)$$

(3)

where σ is the conductivity (S cm⁻¹); T is the absolute temperature (K); A is a constant; E_a is the activation energy of conduction (J mol⁻¹); and R is the gas constant (8.314 J K⁻¹ mol⁻¹). It should be noted that the activation energy of conduction in this study was calculated from I/V curves ranging from open circuit voltage to 1.8 V. The slope of I/V curve is total resistance, which includes electrode resistance. The I/V slope was not constant and was derived using a linear regression with R-squared (R²) ranging from 0.90–0.97. In this study, Pt was applied as both electrodes (Pt/Electrolyte/Pt) and was expected to provide rather low resistance at operating conditions.

3. Results and Discussion

3.1. Densification of the Fabricated Cell

SEM images of SCGZ electrolyte with and without 0.5 wt% CuO sintered at varied temperature from 1423 K to 1673 K are presented in Figure 2 and Figure 3, respectively. The microstructure images reveal that the SCGZ without the sintering additive could not be densified although high sintering temperature was used. Porosity were observed all over the SCGZ without the sintering additive. Grain boundary was observed from 1623 K but the grain size was rather small. The porosity decreased when the sintering temperature was increased. However, the relative density of the SCGZ without the sintering additive was only <90%, although high sintering temperature was increased up to 1673 K. On the other hand, densification and larger grain size were observed in the sample with the sintering additive. The grain growth was observed when increasing sintering temperature (

Table 1. Average grain size of sinterted Sc_0.1Ce_0.05Gd_0.05Zr_0.89O₂ (SCGZ) electrolyte and sintered SCGZ electrolyte with 0.5 wt% CuO when sintering temperature was varied.

Sintering Temperature (K)	Average Grain Size (μm)
	SCGZ	SCGZ with 0.5 wt% CuO
1423	-	1.58
1473	-	2.66
1523	1.04	5.02
1573	1.20	5.11
1623	1.51	3.32
1673	2.28	3.77

). The added CuO could diffuse along the grain boundary and substituted in the vacancy position of the microstructure. Myung et al. [25] investigated CuO (0.3 wt% to 1.5 wt%) as sintering additive in yttria-stabilized zirconia (YSZ) sintering between 1423 K to 1673 K. It was reported that 0.5 wt% of CuO is the optimal amount providing the highest densification. Amount of optimal CuO additive at 0.5 wt% was the same for YSZ and SCGZ, likely because both materials are zirconia-based electrolyte. However, although having same optimal CuO amount, YSZ requires relatively higher sintering temperature (1623 K) when compared with SCGZ (1573 K) at the same sintering additive amount.

Adding 0.5 wt% CuO enhanced the densification of SCGZ. The relative densities of the fabricated cells are presented in Figure 4. The SCGZ without sintering additive provided rather low relative density (<90%) at all sintering temperatures, corresponding to porosity observed in the SEM images. The SCGZ with 0.5 wt% CuO could be densified at lower sintering temperature. Increasing sintering temperature from 1423 to 1673 K could help increase the relative density of the fabricated cell. The cell was densified at 95% relative density at 1573 K respect to other sintering temperature. It was reported that CuO exhibit relatively low melting point (around 1599 K) and can enhance sintering by pore filling during liquid phase sintering [28]. Increasing temperature above 1623 K was found to decrease the relative density of the fabricated cell, likely relating to the liquid phase sintering. Liou et al. [29] studied the effect of CuO as sintering additives on CaTiO₃ perovskite ceramics. Liquid phase sintering at grain boundary is found at sintered sample at 1723 K for 6 h and increases significantly when increasing sintering temperature to 1743 K. However, liquid phase did not occur when increasing sintering temperature up to 1773 K for 6 h, leading to less densification. Moreover, when sintering soak time was increased to 8 h at 1723 K, liquid phase sintering significantly increases when compared with 6 h. A proper sintering temperature and soak time are important factors affecting the densification of the sample.

3.2. Phase Identification

XRD patterns of the SCGZ pellet sintered at 1673 K and the SCGZ with 0.5 wt% CuO pellet sintered at 1523 K are shown in Figure 5. The XRD patterns included main peaks at (111), (200), (220), (311), and (222) planes (COD Database ID: 1529100). Impurity phase was not detected. The XRD patterns of two samples were identical in term of the peak positions. Shifting in peaks position of the XRD patterns was not detected. This result could confirm that CuO did not form into a solid solution with SCGZ but well-mixed with SCGZ as a composite form. This results corresponded to the previous work [30]. It should be noted that CuO peaks were not detected, likely due to small amount of CuO in the sample. The average crystallite sizes were 184 nm and 202 nm for SCGZ sintered at 1673 K and SCGZ with sintering additive sintered at 1523 K, respectively, corresponding to the work that reported higher densification provided a larger grain size leading to a larger crystallite size [31].

3.3. Activation Energy of Conduction

The electrochemical performance of the SOEC was evaluated. Linear sweep voltammetry was conducted from 873 K to 1173 K under a constant steam to hydrogen ratio (70:30). The I/V curves of the cells having SCGZ electrolyte without and with CuO addition are shown in Figure 6 and Figure 7, respectively. The conductivity and activation energy of conduction are presented in Figure 8. It can be seen that the conductivity increased with increasing operating temperature. The activation energy of conduction (E_a) was 72.34 kJmol⁻¹ and 74.93 kJmol⁻¹ for SCGZ and SCGZ with CuO, respectively. In this study, adding 0.5 wt% CuO did not significantly affect the conductivity of the electrolyte. The use of CuO as sintering additive in various ceramic electrolytes has been reported differently. It was reported that adding CuO can lead to ionic and electrical properties modification [25,32,33,34,35,36]. Zhang et al. [36] found that addition of 1 wt% CuO improved the sinterability of Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. The SDC could be densified even at lower sintering temperature than 1273 K but the ionic conductivity was also decreased as a result of microstructure alteration. On the other hand, 0.5 mol% of CuO as sintering additive could provide high ionic conductivity and insignificant change in the activation energy of conduction for gadolinium-doped ceria (GDC) electrolyte [37]. In this study, 0.5 wt% CuO was used as a potential sintering additive for SCGZ electrolyte in SOEC, decreasing the sintering temperature without any significant change in the activation energy of conduction.

4. Conclusions

0.5 wt% CuO was employed as a sintering additive for SCGZ electrolyte in SOEC. Phase formation, microstructure, relative density, and electrochemical performance of electrolyte-supported SOEC were investigated. Adding 0.5 wt% CuO helped increasing the sinterability of the electrolyte which could achieve 95% relative density with a large grain size at 1573 K. The average grain size was measured at 5.11 μm when sintering temperature was 1573 K for SCGZ with CuO. Phase transformation and impurity was not detected in the electrolyte after adding CuO. Neither peak shifting nor impurity peak were detected in the XRD patterns. Without CuO addition, the SCGZ could not be densified although sintering temperature was increased up to 1673 K. Adding CuO into SCGZ insignificantly affected the electrochemical performance of the cell. The activation energy of conduction (E_a) of the SCGZ with and without CuO was 74.93 kJ mol⁻¹ and 72.34 kJ mol⁻¹, respectively.