- Email: [email protected]
Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer Shisong Li a, Ruiqiang Yan b, Guoijan Wu a, Kui Xie a,*, Jigui Cheng a,* a
School of Materials Science and Engineering, Hefei University of Technology, No. 193, Tunxi Road, Hefei, Anhui 230009, China b Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
article info
abstract
Article history:
A
Received 15 July 2013
BaCe0.5Zr0.3Y0.16Zn0.04O3d (BCZYZ) is investigated for steam electrolysis in a proton-
Received in revised form
conducting solid oxide electrolyzer. The conductivity of LSM is studied with respect to
14 September 2013
temperature and oxygen partial pressure and correlated to the electrochemical properties
Accepted 15 September 2013
of the composite oxygen electrodes in symmetric cells and solid oxide electrolyzers at
Available online xxx
composite
oxygen
electrode
based
on
Co3O4-loaded
La0.8Sr0.2MnO3
(LSM)-
800 C. The optimal Co3O4 loading in the composite oxygen electrode is systematically investigated in symmetric cells; loading catalytically active Co3O4 significantly enhances
Keywords:
the electrode performance, unlike the bare LSM-BCZYZ electrode. Steam electrolysis was
Proton conductor
then performed using LSM-BCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes,
Solid oxide electrolyzer
respectively. The Co3O4-loaded catalyst significantly improves the electrode process and
Steam electrolysis
enhances the current density below a certain applied voltage. The current efficiencies
Electrode improvement
reach approximately 46% with a 10% H2O/air feed for the oxygen electrode. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen attracts research interest because it may be created using renewable resources before storage and transportation; moreover, it is an environmentally friendly source of energy [1e6]. There are many sources used to produce hydrogen, including fossil fuels, biomass, and electrochemistry. From a long-term perspective, using water (steam) electrolysis (H2O/H2 þ 1/2O2) in a solid oxide electrolyzer (SOE) to produce hydrogen has become more popular because the high temperature contributes energy to the steam dissociation,
leading to favorable kinetic and thermodynamic properties [7e12]. Proton-conducting solid oxide electrolyzers (SOE) are inverted proton-conducting solid oxide fuel cells (SOFC); they convert electrical energy directly into chemical energy [13e16] and can efficiently produce pure hydrogen via high temperature steam electrolysis. Using external electricity, steam is split into oxygen and protons. The protons diffuse across the proton-conducting electrolyte to the fuel electrode, where pure hydrogen forms from protons with in the three-phase boundary. Hydrogen formation only occurs in the fuel
* Corresponding authors. E-mail addresses: [email protected] (K. Xie), [email protected] (J. Cheng). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.082
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
electrode compartment, making it unnecessary to separate the hydrogen from the steam. In a proton-conducting solid oxide electrolyzer, the oxygen electrode is under strongly oxidizing conditions during the steam electrolysis, conditions under which a purely redox active metal cannot operate. Stable oxygen electrode materials with efficient catalytic performances must still be developed. We have recently reported a composite oxygen electrode based on La0.6Sr0.4Co0.2Fe0.8O3d, Ba0.5Sr0.5Co0.8Fe0.2 O3d and (La0.75Sr0.25)0.95Mn0.5Cr0.5O3d for direct steam electrolysis in a proton-conducting solid oxide electrolyzer [15,16]. However, the electrode polarization resistance of the electrolyzer is still considerable, and the steam electrolysis remains limited by the kinetics processes of the electrode. Enhancing the electro-catalytic performance of the electrode is an effective way to improve the electrolytic process. Co3O4 is reportedly an efficient electro-catalyst for oxygen evolution reactions (OER) in metal-air batteries [17e19] and alkaline water electrolyzers [20e22]. In the oxygen electrode of a proton-conducting electrolyzer, the H2O molecules are split into O2 ions and protons under an external electrolysis potential, and the O2 ions are oxidized to O2 gas before being released from the electrode. This process is also a typical oxygen evolution reaction that occurs on the electrode of an electrolyzer. Therefore, a Co3O4 electro-catalyst might equally enhance the electrode electro-catalytic performance of an electrolyzer. In this work, composite electrodes based on LSM-BCZYZ and Co3O4-loaded LSM-BCZYZ are systematically investigated as oxygen electrodes for steam electrolysis in protonconducting solid oxide electrolyzers. The electrical conductivity of La0.8Sr0.2MnO3 is studied with respect to temperature and oxygen partial pressure and correlated to the electrochemical properties of the composite oxygen electrodes in the symmetric cells and solid oxide electrolyzers. The electrochemical performance of the composite electrodes based on LSM-BCZYZ and Co3O4 catalyst-loaded LSM-BCZYZ are systemically studied in proton-conducting solid oxide electrolyzers for steam electrolysis.
2.
Experimental
The La0.8Sr0.2MnO3 powder was synthesized using a solidstate reaction with an appropriate mixture of La2O3, SrCO3 and MnO2 powders. The mixture was ball-milled in acetone before being dried and pressed into pellets at room temperature; subsequently, the material was treated at 1400 C for 10 h in air. The pellets were then ground, repelletized and fired at same temperature for 10 h in air. The LSM powder loaded with 10 wt.% Co3O4 was generated by impregnating the LSM powder with a Co(NO3)2 solution followed by a heat treatment at 500 C for 30 min in air. All of the reagents (Chemical grade) were purchased from the SINOPHARM Chemical Reagent Co., Ltd (China). X-ray diffraction (Rigaku, Japan) was conducted to analyze the phase formation of the LSM and Co3O4-loaded LSM powders. Appropriate amounts of the LSM powders were pressed into a bar, followed by a 10 h heat treatment at 1400 C in air before the conductivity test. The relative sample density reached 76%. Conductivity tests were performed in air using
the dc four-terminal method from room temperature to 800 C; conductivity was recorded versus temperature with an online system in 0.4 C steps. The relationship between conductivity and oxygen partial pressure was tested at 800 C with oxygen partial pressures ranging from 0.2 to 1020 atm, which were adjusted by flowing dry 5% H2/Ar at 20 ml min1. BaCe0.5Zr0.3Y0.16Zn0.04O3d (BCZYZ) was synthesized using a solid-state reaction method. The mixture of BaCO3, CeO2, ZrO2, Y2O3 and ZnO powders was ball-milled at room temperature and subsequently fired at 1300 C in air for 10 h. A 2mm-thick BCZYZ disc for an electrolyte support was prepared by dry-pressing the BCZYZ powders into a green disk with a 20 mm diameter before sintering at 1400 C for 10 h. The composite oxygen electrode slurry was prepared by milling LSM with BCZYZ (60:40 in weight ratio) in alpha-terpineol with the proper amount of cellulose additive. Similarly, the composite fuel electrode slurry was prepared using NiO and BCZYZ (60:40 in weight ratio) with the method described above. The solid oxide symmetric cell and solid oxide electrolyzer with LSM-BCZYZ/BCZYZ/LSM-BCZYZ and LSMBCZYZ/BCZYZ/NiO-BCZYZ configurations were coated with electrode slurry in symmetrical positions on electrolyte supports within a 1 cm2 area followed by an appropriate heat treatment (1400 C for the NiO-BCZYZ electrode; 1100 C for the LSM-BCZYZ electrode) for 3 h in air. Different contents of Co3O4-loaded LSM electrodes were achieved by impregnating the LSM electrodes with an appropriate Co(NO3)2 nitrate solution several times, with a 30 min heat treatment at 500 C in air after each impregnation. The maximal content of each impregnation treatment was 2 wt.%. Silver paste was printed onto both electrode surfaces followed by a 500 C heat treatment (3 C min1) for 30 min in air for current collection. Silver electrical wire (0.2 mm in diameter) was then connected to both current collectors using silver paste followed by firing at 500 C (3 C min1) for 30 min in air. The microstructures of the LSM-BCZYZ and 6 wt.% Co3O4-LSM electrodes of the symmetric cell were observed with a scanning electron microscope (SEM, SU8020, HITACHI, Japan). The LSM-BCZYZ electrode symmetric cell and the Co3O4-loaded LSM-BCZYZ electrode symmetric cells with different Co3O4 contents were tested at different applied current densities in air, as well as under different oxygen partial pressures under OCV conditions, at 800 C using an electrochemical workstation (IM6, Zahner, Germany). All of the gas flow rates were maintained at 30 ml min1 with a mass flow meter (D08-3F, Sevenstar, Beijing, China). The LSM-BCZYZ (40%) and (LSM-BCZYZ (40%))-Co3O4 (6 wt.%) powders were produced by ball-milling LSM and BCZYZ powders followed by the appropriate impregnation treatment. The specific surface area (SSA) of these powders was characterized by BET (Brunauer, Emmett and Teller) measurements (SA 3100, Beckman Coulter). The solid oxide electrolyzers based on the LSM-BCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes were sealed onto homemade testing jigs using ceramic paste (JD767, Jiudian, Dongguan, China) for the electrochemical measurements. The electrolyzers were typically tested at 800 C with 5% H2/Ar at 30 ml min1 fed into the fuel electrode and 10% H2O/Air (saturated steam concentration at 46 C [23]) at 30 ml min1 introduced to the oxygen electrode for
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
3
Fig. 1 e The XRD patterns of the LSM powders (a) and 10 wt.% Co3O4-loaded LSM powders (b) prepared via solid-state reaction and impregnation treatments.
electrolysis. Currentevoltage (IeV) and short-term performance under externally applied loads were tested at 800 C. The AC impedances of the solid oxide electrolyzers were tested with different applied voltages for electrode activation using an electrochemical workstation (IM6, Zahner, Germany). The gas flow rate was controlled with a mass flow meter (D08-3F, Sevenstar, Beijing, China), and the product, hydrogen, was detected using an online gas chromatograph (GC9750II, Fuli, China).
3.
Results and discussion
Fig. 1(a) and (b) display the XRD patterns for the LSM powders with and without 10 wt.% Co3O4 prepared via solid-state reactions and impregnation treatments, indicating that both La0.8Sr0.2MnO3 and Co3O4 samples remain in pure phases (LSM: PDF#86-1233; Co3O4: PDF#78-1969). The LSM is a p-type electronic conductor that displays typical semiconducting behavior with a positive temperature coefficient up to 800 C in air, as illustrated in Fig. 2(a). The conductivity reaches approximately 32 S cm1 at 800 C. The p-type behavior is also observed in Fig. 2(b). The conductivity decreases when the oxygen partial pressure decreases because there are significantly fewer charge carriers (h) in reducing atmospheres. The decreased oxygen partial pressure causes oxygen loss, simultaneously decreasing the hole concentration [15].
The BET results indicate that the specific surface area (SSA) of the LSM-BCZYZ (40 wt.%) powder is 1.938 m2/g. In contrast, the SSA of the (LSM-BCZYZ (40 wt.%))-Co3O4 (6 wt.%) powder is 6.574 m2/g. The SSA increases approximately 239% after the Co(NO3)2 solution (6 wt.% Co3O4) is added to the LSM-BCZYZ powder before the 30 min, 500 C heat treatment. The Co(NO3)2 solution decomposes to form Co3O4 nanoparticles that are loaded on the electrode surface after the heat treatment, enhancing the oxygen evolution reaction and ameliorating the three-phase boundary of the electrode. Fig. 3(a) and (b) reveal the microstructure of the electrodes based on LSMBCZYZ and LSM-BCZYZ-6 wt.%Co3O4. Fig. 3(a) presents the electrode microstructure of LSM-BCZYZ electrode, revealing the porosity of the electrode. Nano-floccus Co3O4 filled some pores in the electrode, as shown in Fig. 3(b). Fig. 4(a1)e(a3) displays the AC impedance of the solid oxide symmetric cell with a LSM-BCZYZ/BCZYZ/LSM-BCZYZ configuration that was tested at 800 C while passing different current densities through two electrodes in air. The series resistance (Rs) is approximately 4.37 U cm2 at OCV and is reasonable for the 2-mm-thick BCZYZ electrolyte at 800 C in air. In Fig. 4, the series resistance gradually decreases at a large current density; this trend might be related to the p-type conducting properties of BCZYZ electrolyte because it is a mixed conductor in air [15,16,24]. As shown in Fig. 6(a), the electrode polarization resistance (Rp) of the cell is approximately 0.1 U cm2, and it decreases further when the current
Fig. 2 e The conductivity of La0.8Sr0.2MnO3 versus (a) temperature in air and (b) oxygen partial pressure at 800 C. Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
Fig. 3 e The electrode microstructure of symmetric cells based on LSM-BCZYZ (a) and 6 wt.% Co3O4-(LSM-BCZYZ) (b) electrodes before testing.
densities increase, demonstrating that the activity of LSMBCZYZ electrode is efficient under different current densities in air. The symmetric cell based on the 6 wt.% Co3O4-loaded LSM-BCZYZ composite with a 6 wt.% Co3O4-LSM-BCZYZ/
BCZYZ/6wt.% Co3O4-LSM-BCZYZ configuration exhibits the same phenomenon shown in Figs. 4(b1)e(b3) and 6(a). The Rs is approximately 4.23 U cm2, and the Rp is approximately 0.03 U cm2 under the OCV conditions; it decreases further at
Fig. 4 e AC impedance of the symmetric solid oxide cells LSM-BCZYZ/BCZYZ/LSM-BCZYZ (a1ea3) and 6 wt.% Co3O4-(LSMBCZYZ)/BCZYZ/6 wt.%Co3O4-(LSM-BCZYZ) (b1eb3) tested at 800 C in air under different current densities. Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
5
Fig. 5 e AC impedance of the symmetric solid oxide cells LSM-BCZYZ/BCZYZ/LSM-BCZYZ (a1)e(a3) and 6 wt.% Co3O4-(LSMBCZYZ)/BCZYZ/6 wt.% Co3O4 -(LSM-BCZYZ) (b1)e(b3) tested at 800 C at different oxygen partial pressure under OCV conditions.
Fig. 6 e The electrode polarization resistance (Rp) of symmetric cells with different Co3O4-loaded LSM-BCZYZ composite electrode contents versus (a) passing current density and (b) oxygen partial pressure. Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
Fig. 7 e IeV curve of the solid oxide electrolyzers based on LSM-BCZYZ (a) and 6 wt.% Co3O4-LSM-BCZYZ (b) composite oxygen electrode for steam electrolysis with 10% H2O/Air fed to the oxygen electrode and 5% H2/Ar introduced to the fuel electrode; the dV/dI (cell total resistance) of solid oxide electrolyzers based on LSM-BCZYZ (c) and 6 wt.% Co3O4-LSM-BCZYZ (d) composite oxygen electrode.
large current densities. The electrode performance is significantly improved when the Co3O4 catalyst is added; the Co3O4 ameliorates the three-phase boundary of the electrode, improves the reaction overpotential and enhances the oxygen evolution reaction. The AC impedance of the LSM-BCZYZ and 6 wt.% Co3O4-LSM-BCZYZ composite oxygen electrodes in symmetric cells are also tested under different oxygen partial pressures at 800 C. As shown in Fig. 5(a1)e(a3) and (b1)e(b3), the Rs of both cells gradually decrease with the oxygen partial pressure up to 20% (0.2 atm), demonstrating that a small amount of p-type conduction might occur in BCZYZ electrolyte [24]. Similarly, the Rp of the cells also improves when increasing the oxygen partial pressure; this effect is closely related to p-type conducting properties of LSM ceramics, as discussed above. To understand the changes in electrode performance, symmetric cells with different Co3O4 contents loaded into LSM-BCZYZ electrodes are systemically studied. Fig. 6(a) illustrates the relationship between the electrode polarization resistance (Rp) of LSM-BCZYZ composite electrode symmetric cells loaded with different Co3O4 contents and under different passing current densities at 800 C in static air. The Rp decreases with increased passing current densities in a cell, and the Rp is lowered as the Co3O4 content loaded increases at a fixed passing current. When a considerable amount of Co3O4 is loaded, the electrode polarization resistance (Rp) decreases significantly and remains stable when the additive content exceeds 6 wt.% because using the Co3O4 as a catalyst possibly improved the three-phase boundary of the electrode,
enhancing the overpotential and electrode electrochemical process [25e30]. Similar regularity is observed in Fig. 6(b) in the pattern of electrode polarization resistance (Rp) in the LSM-BCZYZ composite electrode symmetric cells loaded with different Co3O4 contents versus oxygen partial pressure at 800 C under OCV conditions. The Rp gradually decreases when the Co3O4 content increases at a fixed oxygen partial pressure. The Rp also decreases when the oxygen partial pressure increases in one cell; this pattern is closely related to the p-type conductivity of LSM ceramic, as discussed above. To assess the seal on a single solid oxide electrolyzer based on a LSM-BCZYZ composite oxygen electrode, 10% H2O/Air is introduced to the oxygen electrode while the fuel electrode is exposed to 5% H2/Ar. The open circuit voltage (OCV) reaches 0.92 V at 800 C, indicating a reasonable separation between the anodic and cathodic gases. At this point, the electrochemical cell is a solid oxide fuel cell, and the OCV mainly arises from the electrochemical potential (2H2þO2 / 2H2O). Fig. 7(a) displays the current density against the applied voltage (IeV curve) at 800 C with 10% H2O/Air supplied to the oxygen electrode while the fuel electrode is exposed to 5% H2/ Ar. The IeV curve is not linear, revealing the change in cell resistance across the entire voltage region. The maximum current density reaches 157 mA cm2 at 2 V. To study the change in electrolyzer resistance under different applied voltages, the dV/dI curve (cell total resistance) is plotted versus voltage, as shown in Fig. 7(c). The resistance gradually decreases when the voltage increases to 1.2 V, remaining stable at approximately 6.5 U cm2 at higher voltages. In situ
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
7
Fig. 8 e AC impedance of the solid oxide electrolyzers based on LSM-BCZYZ (a1, a2) and 6 wt.% Co3O4-LSM-BCZYZ (b1, b2) composite oxygen electrodes for steam electrolysis with 10% H2O/Air fed into the oxygen electrode and 5% H2/Ar introduced to the fuel electrode; (c) the Rp of electrolyzers versus applied voltages.
AC impedance spectroscopy was also utilized under different applied voltages to investigate the changes in Rs and Rp; Rs and Rp correspond to the series resistance and polarization resistance of the electrodes, respectively. As shown in Fig. 8(a1) and (a2), the series resistance has its maximum value at the OCV before decreasing relative to the applied voltage from 0.92 to 2.0 V due to the contribution of the p-type conduction of the electrolyte [16,24]. The Rp sharply decreases with increasing voltage, most likely due to the reoxidation of the LSM-BCZYZ electrode. The electrode polarization resistance and total resistance are 0.38 U cm2 and 6.92 U cm2 at 2.0 V, respectively. However, higher voltages improve the electrode reactions, further enhancing the electrode process and decreasing the electrode polarization resistance at higher voltages. The electrolyzer based on a 6 wt.% Co3O4-loaded LSMBCZYZ composite oxygen electrode is tested in a similar manner as the electrolyzer with the LSM-BCZYZ oxygen
electrode. The OCV is 0.93 V at 800 C with 10% H2O/Air being fed to oxygen electrode while the fuel electrode receives 5% H2/Ar. The current density versus the applied voltage (IeV curve) is displayed in Fig. 7(b); this curve is also nonlinear, and the maximum current density is approximately 184 mA cm2 at 2 V. As shown in Fig. 7(d), the dV/dI (cell total resistance) is approximately 4 U cm2 at a 2.0 V applied voltage. In Fig. 8(b1) and (b2), the in situ AC impedance spectroscopy data for the 6 wt.% Co3O4-loaded LSM-BCZYZ composite oxygen electrode electrolyzer reveals that the Rp decreases significantly when the applied voltage increases because the reoxidation of LSMBCZYZ electrode enhances the conductivity of the LSM ceramic. The electrode polarization resistance and the total resistance are 0.24 U cm2 and 6.26 U cm2 at 2.0 V, respectively. Fig. 8(c) compares the polarization resistance of the electrolyzers based on the LSM-BCZYZ and the 6 wt.% Co3O4-loaded LSM-BCZYZ composite oxygen electrodes under different applied voltages. Loading Co3O4 onto the LSM-BCZYZ
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
Fig. 9 e Short-term performance of the solid oxide electrolyzers based on LSM-BCZYZ (a) and 6 wt.% Co3O4-LSM-BCZYZ (b) composite oxygen electrodes for steam electrolysis at 800 C with 10% H2O/Air fed into the oxygen electrode and 5% H2/Ar introduced to the fuel electrode; the H2 production and current efficiency of the electrolyzers based on LSM-BCZYZ (c) and 6 wt.% Co3O4-LSM-BCZYZ (d) composite oxygen electrodes.
electrode greatly improves the electrode, enhancing the electrode electrochemical process and decreasing the Rp of the electrolyzer. Fig. 9 reveals the short-term performance of the electrolyzers based on LSM-BCZYZ and 6 wt.% Co3O4-loaded LSMBCZYZ composite electrodes for electrolyzing steam at 800 C under different applied voltages. Fig. 9(a) and (b) displays the current density versus time and applied voltage of the electrolyzers based on LSM-BCZYZ and 6 wt.% Co3O4-loaded LSMBCZYZ electrodes tested at 800 C when 10% H2O/Air is fed into the oxygen electrode and 5% H2/Ar is fed into the fuel electrode, respectively. For the electrolyzer based on the LSM electrode, the current densities are approximately 31, 53, 84, 113 and 152 mA cm2 at 1.2, 1.4, 1.6, 1.8 and 2.0 V, respectively. Fig. 9(c) illustrates the H2 production: 0.102, 0.166, 0.23, 0.279 and 0.348 ml min1 cm2 at 1.2, 1.4, 1.6, 1.8 and 2.0 V, respectively. The corresponding current efficiencies are 46.6%, 45.3%, 38.9%, 33.8% and 32.6%. These values exceed the 22% current efficiency achieved by the La0.75Sr0.25Cr0.5Mn0.5O3d-based oxygen electrode for steam electrolysis in a thin-membrane, proton-conducting, solid oxide electrolyzer [16] and are comparable with La0.6Sr0.4Co0.2Fe0.8O3d and Ba0.5Sr0.5Co0.8Fe0.2 O3d electrode electrolyzers during steam electrolysis in proton-conducting solid oxide electrolyzers [15]. However, the efficiency decreases when the voltage improves. The current efficiency lost under higher voltages and currents are most likely caused by the transportation of other ions or holes, as well as the localized steam starvation at high voltages that limits the electrochemical oxidation of steam in the oxygen electrode [15,31]. For the electrolyzer based on the 6 wt.% Co3O4-loaded LSM-BCZYZ electrode, the current densities are approximately 41, 68, 98, 132 and 173 mA cm2 at 1.2, 1.4, 1.6,
1.8 and 2.0 V, respectively. The H2 production is 0.131, 0.205, 0.266, 0.326 and 0.369 ml min1 cm2 at 1.2, 1.4, 1.6, 1.8 and 2.0 V, respectively. The corresponding current efficiencies are 45.4%, 43.3%, 38.4%, 34.8% and 30.8% and are comparable to the cell based on the LSM-BCZYZ electrode. The lower current efficiency in the electrolyzer is dictated by the ion conduction from the proton-conducting electrolyte. The proton conductivity in the BCZYZ electrolyte relies on the partial filling of vacant oxygen sites by water, yielding mobile protons bound to the lattice oxide ions. The necessity of these oxide vacancies complicates the transport properties. Under oxidative conditions, atmospheric oxygen enters the lattice, creating hole-type electronic conduction and lowering the current efficiency of the electrolyzer [24]. Consequently, the electrolyzer based on the proton-conducting electrolyte has a lower current efficiency than that based on the oxide-ion conducting electrolyte. There are two main electrochemical processes that occur from OCV to 2 V during steam electrolysis: the reoxidation of the LSM-BCZYZ electrode and steam electrolysis. The oxidation of LSM is the major process below 1.2 V, while the electrochemical oxidation of H2O dominates at high voltages. Fig. 7(c) and (d) shows that the dV/dI (cell resistance) remains constant above 1.2 V, indicating there is sufficient oxidation of the oxygen electrode and a stable steam electrolysis process.
4.
Conclusions
In this work, composite oxygen electrodes based on LSMBCZYZ and Co3O4-loaded LSM-BCZYZ were investigated for steam electrolysis in proton-conducting solid oxide electrolyzers. The LSM sample has p-type conducting properties; this
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
material is investigated and correlated with the electrochemical properties of the symmetric cells and electrolyzers. The optimal Co3O4 loading in the composite oxygen electrode is systematically investigated in symmetric cells: adding catalytically active Co3O4 significantly enhances the electrode performance compared to the bare LSM-BCZYZ electrode. Steam electrolysis was conducted on electrolyzers using LSMBCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes; adding the Co3O4 catalyst significantly improves the electrode processes and enhances the current density under a certain applied voltage. The significantly improved electrode performance after adding the catalytically active Co3O4 may be because the Co3O4 ameliorates the three-phase boundary of electrode, improving the reaction overpotential and enhancing the oxygen evolution reaction.
Acknowledgment This work is supported by the Natural Science Foundation of China No. 21303037, China Postdoctoral Science Foundation No. 2013M53150 and the Fundamental Research Funds for the Central Universities No. 2012HGZY0001.
references
[1] Brisse A, Schefold J, Zahid M. High temperature water electrolysis in solid oxide cells. Int J Hydrogen Energy 2008;33:5375e82. [2] Hauch A, Ebbesen S, Jensen S, Mogensen M. Highly efficient high temperature electrolysis. J Mater Chem 2008;18:2331e40. [3] Schiller G, Ansar A, Lang M, Patz O. High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC). J Appl Electrochem 2009;39:293e301. [4] Jin C, Yang C, Zhao F, Cui D, Chen F. La0.75Sr0.25Cr0.5Mn0.5O3 as hydrogen electrode for solid oxide electrolysis cells. Int J Hydrogen Energy 2011;36:3340e6. [5] Yang X, Irvine JTS. (La0.75Sr0.25)0.95Mn0.5Cr0.5O3 as the cathode of solid oxide electrolysis cells for high temperature hydrogen production from steam. J Mater Chem 2008;18:2349e54. [6] Ni M, Leung MKH, Leung DYC. Technological development of hydrogen production by solid oxide electrolyzer cell (SOEC). Int J Hydrogen Energy 2008;33:2337e54. [7] Xie K, Zhang YQ, Meng GY, Irvine JTS. Direct synthesis of methane from CO2/H2O in an oxygen-ion conducting solid oxide electrolyser. Energy Environ Sci 2011;4:2218e22. [8] Xie K, Zhang YQ, Meng GY, Irvine JTS. Electrochemical reduction of CO2 in a proton conducting solid oxide electrolyser. J Mater Chem 2011;21:195e8. [9] Doenitz W, Schmidberger R, Steinheil E, Streicher R. Hydrogen production by high temperature electrolysis of water vapour. Int J Hydrogen Energy 1980;5:55e63. [10] Balthasar W. Hydrogen production and technology: today, tomorrow and beyond. Int J Hydrogen Energy 1984;9:649e68. [11] Dutta S. Technology assessment of advanced electrolytic hydrogen production. Int J Hydrogen Energy 1990;15:379e86.
9
[12] Li SS, Li YX, Gan Y, Xie K, Meng GY. Electrolysis of H2O and CO2 in an oxygen-ion conducting solid oxide electrolyzer with a La0.2Sr0.8TiO3þd composite cathode. J Power Sources 2012;218:244e9. [13] Iwahara H, Uchida H, Yamasaki I. High-temperature steam electrolysis using SrCeO3-based proton conductive solid electrolyte. Int J Hydrogen Energy 1987;12:73e7. [14] Schober T. Applications of oxidic high-temperature proton conductors. Solid State Ionics 2003;162:277e81. [15] Li SS, Xie K. Composite oxygen electrode based on LSCF and BSCF for steam electrolysis in a proton-conducting solid oxide electrolyzer. J Electrochem Soc 2013;160:F224e33. [16] Gan Y, Zhang J, Li YX, Li SS, Xie K, Irvine JTS. Composite oxygen electrode based on LSCM for steam electrolysis in a proton conducting solid oxide electrolyzer. J Electrochem Soc 2012;159:F763e7. [17] Kong F. Synthesis of rod and beadlike Co3O4 and bifunctional properties as air/oxygen electrode materials. Electrochim Acta 2012;68:198e201. [18] Lee J, Kim S, Cao R, Choi N, Liu M, Lee K, et al. Metal-air batteries with high energy density: Li-air versus Zn-air. Adv Energy Mater 2011;1:34e50. [19] Nikolova V, Iliev P, Petrov K, Vitanov T, Zhecheva E, Stoyanova R, et al. Electrocatalysts for bifunctional oxygen/ air electrodes. J Power Sources 2008;185:727e33. [20] Singh R, Mishra D, Anindita Sinha A, Singh A. Novel electrocatalysts for generating oxygen from alkaline water electrolysis. Electrochem Commun 2007;9:1369e73. [21] Lu B, Cao D, Wang P, Wang G, Gao Y. Oxygen evolution reaction on Ni-substituted Co3O4 nanowire array electrodes. Int J Hydrogen Energy 2011;36:72e8. [22] Li Y, Hasin P, Wu Y. NixCo3xO4 nanowire arrays for electrocatalytic oxygen evolution. Adv Mater 2010;22:1926e9. [23] Yue XL, Irvine JTS. (La, Sr)(Cr, Mn)O3/GDC cathode for high temperature steam electrolysis and steam-carbon dioxide co-electrolysis. Solid State Ionics 2012;225:131e5. [24] Tao SW, Irvine JTS. A stable. Easily sintered protonconducting oxide electrolyte for moderate-temperature fuel cells and electrolyzers. Adv Mater 2006;18:1581e4. [25] Lee WH, Kim H. Oxidized iridium nanodendrites as catalysts for oxygen evolution reactions. Catal Commun 2011;12:408e11. [26] Garsuch A, Panchenko A, Querner C, Karpov A, Oesten R. FeAgMo2O8-A novel oxygen evolution catalyst material for alkaline metal-air batteries. Electrochem Commun 2010;12:1642e5. [27] Ma L, Sui S, Zhai Y. Preparation and characterization of Ir/TiC catalyst for oxygen evolution. J Power Sources 2008;177:470e7. [28] Wang W, Zhao Q, Dong J, Li J. A novel silver oxides oxygen evolving catalyst for water splitting. Int J Hydrogen Energy 2011;36:7374e80. [29] Hamdani M, Singh R, Chartier P. Co3O4 and co-based spinel oxides bifunctional oxygen electrodes. Int J Electrochem Sci 2010;5:556e77. [30] Rios E, Reyes H, Ortiz J, Gautier JL. Double channel electrode flow cell application to the study of HO-2 production on MnxCo3xO4 (0 x 1) spinel films. Electrochim Acta 2005;50:2705e11. [31] Xu SS, Li SS, Yao WT, Dong DH, Xie K. Direct electrolysis of CO2 using an oxygen-ion conducting solid oxide electrolyzer based on La0.75Sr0.25Cr0.5Mn0.5O3d electrode. J Power Sources 2013;230:115e21.
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer Shisong Li a, Ruiqiang Yan b, Guoijan Wu a, Kui Xie a,*, Jigui Cheng a,* a
School of Materials Science and Engineering, Hefei University of Technology, No. 193, Tunxi Road, Hefei, Anhui 230009, China b Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
article info
abstract
Article history:
A
Received 15 July 2013
BaCe0.5Zr0.3Y0.16Zn0.04O3d (BCZYZ) is investigated for steam electrolysis in a proton-
Received in revised form
conducting solid oxide electrolyzer. The conductivity of LSM is studied with respect to
14 September 2013
temperature and oxygen partial pressure and correlated to the electrochemical properties
Accepted 15 September 2013
of the composite oxygen electrodes in symmetric cells and solid oxide electrolyzers at
Available online xxx
composite
oxygen
electrode
based
on
Co3O4-loaded
La0.8Sr0.2MnO3
(LSM)-
800 C. The optimal Co3O4 loading in the composite oxygen electrode is systematically investigated in symmetric cells; loading catalytically active Co3O4 significantly enhances
Keywords:
the electrode performance, unlike the bare LSM-BCZYZ electrode. Steam electrolysis was
Proton conductor
then performed using LSM-BCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes,
Solid oxide electrolyzer
respectively. The Co3O4-loaded catalyst significantly improves the electrode process and
Steam electrolysis
enhances the current density below a certain applied voltage. The current efficiencies
Electrode improvement
reach approximately 46% with a 10% H2O/air feed for the oxygen electrode. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen attracts research interest because it may be created using renewable resources before storage and transportation; moreover, it is an environmentally friendly source of energy [1e6]. There are many sources used to produce hydrogen, including fossil fuels, biomass, and electrochemistry. From a long-term perspective, using water (steam) electrolysis (H2O/H2 þ 1/2O2) in a solid oxide electrolyzer (SOE) to produce hydrogen has become more popular because the high temperature contributes energy to the steam dissociation,
leading to favorable kinetic and thermodynamic properties [7e12]. Proton-conducting solid oxide electrolyzers (SOE) are inverted proton-conducting solid oxide fuel cells (SOFC); they convert electrical energy directly into chemical energy [13e16] and can efficiently produce pure hydrogen via high temperature steam electrolysis. Using external electricity, steam is split into oxygen and protons. The protons diffuse across the proton-conducting electrolyte to the fuel electrode, where pure hydrogen forms from protons with in the three-phase boundary. Hydrogen formation only occurs in the fuel
* Corresponding authors. E-mail addresses: [email protected] (K. Xie), [email protected] (J. Cheng). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.082
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
electrode compartment, making it unnecessary to separate the hydrogen from the steam. In a proton-conducting solid oxide electrolyzer, the oxygen electrode is under strongly oxidizing conditions during the steam electrolysis, conditions under which a purely redox active metal cannot operate. Stable oxygen electrode materials with efficient catalytic performances must still be developed. We have recently reported a composite oxygen electrode based on La0.6Sr0.4Co0.2Fe0.8O3d, Ba0.5Sr0.5Co0.8Fe0.2 O3d and (La0.75Sr0.25)0.95Mn0.5Cr0.5O3d for direct steam electrolysis in a proton-conducting solid oxide electrolyzer [15,16]. However, the electrode polarization resistance of the electrolyzer is still considerable, and the steam electrolysis remains limited by the kinetics processes of the electrode. Enhancing the electro-catalytic performance of the electrode is an effective way to improve the electrolytic process. Co3O4 is reportedly an efficient electro-catalyst for oxygen evolution reactions (OER) in metal-air batteries [17e19] and alkaline water electrolyzers [20e22]. In the oxygen electrode of a proton-conducting electrolyzer, the H2O molecules are split into O2 ions and protons under an external electrolysis potential, and the O2 ions are oxidized to O2 gas before being released from the electrode. This process is also a typical oxygen evolution reaction that occurs on the electrode of an electrolyzer. Therefore, a Co3O4 electro-catalyst might equally enhance the electrode electro-catalytic performance of an electrolyzer. In this work, composite electrodes based on LSM-BCZYZ and Co3O4-loaded LSM-BCZYZ are systematically investigated as oxygen electrodes for steam electrolysis in protonconducting solid oxide electrolyzers. The electrical conductivity of La0.8Sr0.2MnO3 is studied with respect to temperature and oxygen partial pressure and correlated to the electrochemical properties of the composite oxygen electrodes in the symmetric cells and solid oxide electrolyzers. The electrochemical performance of the composite electrodes based on LSM-BCZYZ and Co3O4 catalyst-loaded LSM-BCZYZ are systemically studied in proton-conducting solid oxide electrolyzers for steam electrolysis.
2.
Experimental
The La0.8Sr0.2MnO3 powder was synthesized using a solidstate reaction with an appropriate mixture of La2O3, SrCO3 and MnO2 powders. The mixture was ball-milled in acetone before being dried and pressed into pellets at room temperature; subsequently, the material was treated at 1400 C for 10 h in air. The pellets were then ground, repelletized and fired at same temperature for 10 h in air. The LSM powder loaded with 10 wt.% Co3O4 was generated by impregnating the LSM powder with a Co(NO3)2 solution followed by a heat treatment at 500 C for 30 min in air. All of the reagents (Chemical grade) were purchased from the SINOPHARM Chemical Reagent Co., Ltd (China). X-ray diffraction (Rigaku, Japan) was conducted to analyze the phase formation of the LSM and Co3O4-loaded LSM powders. Appropriate amounts of the LSM powders were pressed into a bar, followed by a 10 h heat treatment at 1400 C in air before the conductivity test. The relative sample density reached 76%. Conductivity tests were performed in air using
the dc four-terminal method from room temperature to 800 C; conductivity was recorded versus temperature with an online system in 0.4 C steps. The relationship between conductivity and oxygen partial pressure was tested at 800 C with oxygen partial pressures ranging from 0.2 to 1020 atm, which were adjusted by flowing dry 5% H2/Ar at 20 ml min1. BaCe0.5Zr0.3Y0.16Zn0.04O3d (BCZYZ) was synthesized using a solid-state reaction method. The mixture of BaCO3, CeO2, ZrO2, Y2O3 and ZnO powders was ball-milled at room temperature and subsequently fired at 1300 C in air for 10 h. A 2mm-thick BCZYZ disc for an electrolyte support was prepared by dry-pressing the BCZYZ powders into a green disk with a 20 mm diameter before sintering at 1400 C for 10 h. The composite oxygen electrode slurry was prepared by milling LSM with BCZYZ (60:40 in weight ratio) in alpha-terpineol with the proper amount of cellulose additive. Similarly, the composite fuel electrode slurry was prepared using NiO and BCZYZ (60:40 in weight ratio) with the method described above. The solid oxide symmetric cell and solid oxide electrolyzer with LSM-BCZYZ/BCZYZ/LSM-BCZYZ and LSMBCZYZ/BCZYZ/NiO-BCZYZ configurations were coated with electrode slurry in symmetrical positions on electrolyte supports within a 1 cm2 area followed by an appropriate heat treatment (1400 C for the NiO-BCZYZ electrode; 1100 C for the LSM-BCZYZ electrode) for 3 h in air. Different contents of Co3O4-loaded LSM electrodes were achieved by impregnating the LSM electrodes with an appropriate Co(NO3)2 nitrate solution several times, with a 30 min heat treatment at 500 C in air after each impregnation. The maximal content of each impregnation treatment was 2 wt.%. Silver paste was printed onto both electrode surfaces followed by a 500 C heat treatment (3 C min1) for 30 min in air for current collection. Silver electrical wire (0.2 mm in diameter) was then connected to both current collectors using silver paste followed by firing at 500 C (3 C min1) for 30 min in air. The microstructures of the LSM-BCZYZ and 6 wt.% Co3O4-LSM electrodes of the symmetric cell were observed with a scanning electron microscope (SEM, SU8020, HITACHI, Japan). The LSM-BCZYZ electrode symmetric cell and the Co3O4-loaded LSM-BCZYZ electrode symmetric cells with different Co3O4 contents were tested at different applied current densities in air, as well as under different oxygen partial pressures under OCV conditions, at 800 C using an electrochemical workstation (IM6, Zahner, Germany). All of the gas flow rates were maintained at 30 ml min1 with a mass flow meter (D08-3F, Sevenstar, Beijing, China). The LSM-BCZYZ (40%) and (LSM-BCZYZ (40%))-Co3O4 (6 wt.%) powders were produced by ball-milling LSM and BCZYZ powders followed by the appropriate impregnation treatment. The specific surface area (SSA) of these powders was characterized by BET (Brunauer, Emmett and Teller) measurements (SA 3100, Beckman Coulter). The solid oxide electrolyzers based on the LSM-BCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes were sealed onto homemade testing jigs using ceramic paste (JD767, Jiudian, Dongguan, China) for the electrochemical measurements. The electrolyzers were typically tested at 800 C with 5% H2/Ar at 30 ml min1 fed into the fuel electrode and 10% H2O/Air (saturated steam concentration at 46 C [23]) at 30 ml min1 introduced to the oxygen electrode for
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
3
Fig. 1 e The XRD patterns of the LSM powders (a) and 10 wt.% Co3O4-loaded LSM powders (b) prepared via solid-state reaction and impregnation treatments.
electrolysis. Currentevoltage (IeV) and short-term performance under externally applied loads were tested at 800 C. The AC impedances of the solid oxide electrolyzers were tested with different applied voltages for electrode activation using an electrochemical workstation (IM6, Zahner, Germany). The gas flow rate was controlled with a mass flow meter (D08-3F, Sevenstar, Beijing, China), and the product, hydrogen, was detected using an online gas chromatograph (GC9750II, Fuli, China).
3.
Results and discussion
Fig. 1(a) and (b) display the XRD patterns for the LSM powders with and without 10 wt.% Co3O4 prepared via solid-state reactions and impregnation treatments, indicating that both La0.8Sr0.2MnO3 and Co3O4 samples remain in pure phases (LSM: PDF#86-1233; Co3O4: PDF#78-1969). The LSM is a p-type electronic conductor that displays typical semiconducting behavior with a positive temperature coefficient up to 800 C in air, as illustrated in Fig. 2(a). The conductivity reaches approximately 32 S cm1 at 800 C. The p-type behavior is also observed in Fig. 2(b). The conductivity decreases when the oxygen partial pressure decreases because there are significantly fewer charge carriers (h) in reducing atmospheres. The decreased oxygen partial pressure causes oxygen loss, simultaneously decreasing the hole concentration [15].
The BET results indicate that the specific surface area (SSA) of the LSM-BCZYZ (40 wt.%) powder is 1.938 m2/g. In contrast, the SSA of the (LSM-BCZYZ (40 wt.%))-Co3O4 (6 wt.%) powder is 6.574 m2/g. The SSA increases approximately 239% after the Co(NO3)2 solution (6 wt.% Co3O4) is added to the LSM-BCZYZ powder before the 30 min, 500 C heat treatment. The Co(NO3)2 solution decomposes to form Co3O4 nanoparticles that are loaded on the electrode surface after the heat treatment, enhancing the oxygen evolution reaction and ameliorating the three-phase boundary of the electrode. Fig. 3(a) and (b) reveal the microstructure of the electrodes based on LSMBCZYZ and LSM-BCZYZ-6 wt.%Co3O4. Fig. 3(a) presents the electrode microstructure of LSM-BCZYZ electrode, revealing the porosity of the electrode. Nano-floccus Co3O4 filled some pores in the electrode, as shown in Fig. 3(b). Fig. 4(a1)e(a3) displays the AC impedance of the solid oxide symmetric cell with a LSM-BCZYZ/BCZYZ/LSM-BCZYZ configuration that was tested at 800 C while passing different current densities through two electrodes in air. The series resistance (Rs) is approximately 4.37 U cm2 at OCV and is reasonable for the 2-mm-thick BCZYZ electrolyte at 800 C in air. In Fig. 4, the series resistance gradually decreases at a large current density; this trend might be related to the p-type conducting properties of BCZYZ electrolyte because it is a mixed conductor in air [15,16,24]. As shown in Fig. 6(a), the electrode polarization resistance (Rp) of the cell is approximately 0.1 U cm2, and it decreases further when the current
Fig. 2 e The conductivity of La0.8Sr0.2MnO3 versus (a) temperature in air and (b) oxygen partial pressure at 800 C. Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
Fig. 3 e The electrode microstructure of symmetric cells based on LSM-BCZYZ (a) and 6 wt.% Co3O4-(LSM-BCZYZ) (b) electrodes before testing.
densities increase, demonstrating that the activity of LSMBCZYZ electrode is efficient under different current densities in air. The symmetric cell based on the 6 wt.% Co3O4-loaded LSM-BCZYZ composite with a 6 wt.% Co3O4-LSM-BCZYZ/
BCZYZ/6wt.% Co3O4-LSM-BCZYZ configuration exhibits the same phenomenon shown in Figs. 4(b1)e(b3) and 6(a). The Rs is approximately 4.23 U cm2, and the Rp is approximately 0.03 U cm2 under the OCV conditions; it decreases further at
Fig. 4 e AC impedance of the symmetric solid oxide cells LSM-BCZYZ/BCZYZ/LSM-BCZYZ (a1ea3) and 6 wt.% Co3O4-(LSMBCZYZ)/BCZYZ/6 wt.%Co3O4-(LSM-BCZYZ) (b1eb3) tested at 800 C in air under different current densities. Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
5
Fig. 5 e AC impedance of the symmetric solid oxide cells LSM-BCZYZ/BCZYZ/LSM-BCZYZ (a1)e(a3) and 6 wt.% Co3O4-(LSMBCZYZ)/BCZYZ/6 wt.% Co3O4 -(LSM-BCZYZ) (b1)e(b3) tested at 800 C at different oxygen partial pressure under OCV conditions.
Fig. 6 e The electrode polarization resistance (Rp) of symmetric cells with different Co3O4-loaded LSM-BCZYZ composite electrode contents versus (a) passing current density and (b) oxygen partial pressure. Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
Fig. 7 e IeV curve of the solid oxide electrolyzers based on LSM-BCZYZ (a) and 6 wt.% Co3O4-LSM-BCZYZ (b) composite oxygen electrode for steam electrolysis with 10% H2O/Air fed to the oxygen electrode and 5% H2/Ar introduced to the fuel electrode; the dV/dI (cell total resistance) of solid oxide electrolyzers based on LSM-BCZYZ (c) and 6 wt.% Co3O4-LSM-BCZYZ (d) composite oxygen electrode.
large current densities. The electrode performance is significantly improved when the Co3O4 catalyst is added; the Co3O4 ameliorates the three-phase boundary of the electrode, improves the reaction overpotential and enhances the oxygen evolution reaction. The AC impedance of the LSM-BCZYZ and 6 wt.% Co3O4-LSM-BCZYZ composite oxygen electrodes in symmetric cells are also tested under different oxygen partial pressures at 800 C. As shown in Fig. 5(a1)e(a3) and (b1)e(b3), the Rs of both cells gradually decrease with the oxygen partial pressure up to 20% (0.2 atm), demonstrating that a small amount of p-type conduction might occur in BCZYZ electrolyte [24]. Similarly, the Rp of the cells also improves when increasing the oxygen partial pressure; this effect is closely related to p-type conducting properties of LSM ceramics, as discussed above. To understand the changes in electrode performance, symmetric cells with different Co3O4 contents loaded into LSM-BCZYZ electrodes are systemically studied. Fig. 6(a) illustrates the relationship between the electrode polarization resistance (Rp) of LSM-BCZYZ composite electrode symmetric cells loaded with different Co3O4 contents and under different passing current densities at 800 C in static air. The Rp decreases with increased passing current densities in a cell, and the Rp is lowered as the Co3O4 content loaded increases at a fixed passing current. When a considerable amount of Co3O4 is loaded, the electrode polarization resistance (Rp) decreases significantly and remains stable when the additive content exceeds 6 wt.% because using the Co3O4 as a catalyst possibly improved the three-phase boundary of the electrode,
enhancing the overpotential and electrode electrochemical process [25e30]. Similar regularity is observed in Fig. 6(b) in the pattern of electrode polarization resistance (Rp) in the LSM-BCZYZ composite electrode symmetric cells loaded with different Co3O4 contents versus oxygen partial pressure at 800 C under OCV conditions. The Rp gradually decreases when the Co3O4 content increases at a fixed oxygen partial pressure. The Rp also decreases when the oxygen partial pressure increases in one cell; this pattern is closely related to the p-type conductivity of LSM ceramic, as discussed above. To assess the seal on a single solid oxide electrolyzer based on a LSM-BCZYZ composite oxygen electrode, 10% H2O/Air is introduced to the oxygen electrode while the fuel electrode is exposed to 5% H2/Ar. The open circuit voltage (OCV) reaches 0.92 V at 800 C, indicating a reasonable separation between the anodic and cathodic gases. At this point, the electrochemical cell is a solid oxide fuel cell, and the OCV mainly arises from the electrochemical potential (2H2þO2 / 2H2O). Fig. 7(a) displays the current density against the applied voltage (IeV curve) at 800 C with 10% H2O/Air supplied to the oxygen electrode while the fuel electrode is exposed to 5% H2/ Ar. The IeV curve is not linear, revealing the change in cell resistance across the entire voltage region. The maximum current density reaches 157 mA cm2 at 2 V. To study the change in electrolyzer resistance under different applied voltages, the dV/dI curve (cell total resistance) is plotted versus voltage, as shown in Fig. 7(c). The resistance gradually decreases when the voltage increases to 1.2 V, remaining stable at approximately 6.5 U cm2 at higher voltages. In situ
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
7
Fig. 8 e AC impedance of the solid oxide electrolyzers based on LSM-BCZYZ (a1, a2) and 6 wt.% Co3O4-LSM-BCZYZ (b1, b2) composite oxygen electrodes for steam electrolysis with 10% H2O/Air fed into the oxygen electrode and 5% H2/Ar introduced to the fuel electrode; (c) the Rp of electrolyzers versus applied voltages.
AC impedance spectroscopy was also utilized under different applied voltages to investigate the changes in Rs and Rp; Rs and Rp correspond to the series resistance and polarization resistance of the electrodes, respectively. As shown in Fig. 8(a1) and (a2), the series resistance has its maximum value at the OCV before decreasing relative to the applied voltage from 0.92 to 2.0 V due to the contribution of the p-type conduction of the electrolyte [16,24]. The Rp sharply decreases with increasing voltage, most likely due to the reoxidation of the LSM-BCZYZ electrode. The electrode polarization resistance and total resistance are 0.38 U cm2 and 6.92 U cm2 at 2.0 V, respectively. However, higher voltages improve the electrode reactions, further enhancing the electrode process and decreasing the electrode polarization resistance at higher voltages. The electrolyzer based on a 6 wt.% Co3O4-loaded LSMBCZYZ composite oxygen electrode is tested in a similar manner as the electrolyzer with the LSM-BCZYZ oxygen
electrode. The OCV is 0.93 V at 800 C with 10% H2O/Air being fed to oxygen electrode while the fuel electrode receives 5% H2/Ar. The current density versus the applied voltage (IeV curve) is displayed in Fig. 7(b); this curve is also nonlinear, and the maximum current density is approximately 184 mA cm2 at 2 V. As shown in Fig. 7(d), the dV/dI (cell total resistance) is approximately 4 U cm2 at a 2.0 V applied voltage. In Fig. 8(b1) and (b2), the in situ AC impedance spectroscopy data for the 6 wt.% Co3O4-loaded LSM-BCZYZ composite oxygen electrode electrolyzer reveals that the Rp decreases significantly when the applied voltage increases because the reoxidation of LSMBCZYZ electrode enhances the conductivity of the LSM ceramic. The electrode polarization resistance and the total resistance are 0.24 U cm2 and 6.26 U cm2 at 2.0 V, respectively. Fig. 8(c) compares the polarization resistance of the electrolyzers based on the LSM-BCZYZ and the 6 wt.% Co3O4-loaded LSM-BCZYZ composite oxygen electrodes under different applied voltages. Loading Co3O4 onto the LSM-BCZYZ
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
Fig. 9 e Short-term performance of the solid oxide electrolyzers based on LSM-BCZYZ (a) and 6 wt.% Co3O4-LSM-BCZYZ (b) composite oxygen electrodes for steam electrolysis at 800 C with 10% H2O/Air fed into the oxygen electrode and 5% H2/Ar introduced to the fuel electrode; the H2 production and current efficiency of the electrolyzers based on LSM-BCZYZ (c) and 6 wt.% Co3O4-LSM-BCZYZ (d) composite oxygen electrodes.
electrode greatly improves the electrode, enhancing the electrode electrochemical process and decreasing the Rp of the electrolyzer. Fig. 9 reveals the short-term performance of the electrolyzers based on LSM-BCZYZ and 6 wt.% Co3O4-loaded LSMBCZYZ composite electrodes for electrolyzing steam at 800 C under different applied voltages. Fig. 9(a) and (b) displays the current density versus time and applied voltage of the electrolyzers based on LSM-BCZYZ and 6 wt.% Co3O4-loaded LSMBCZYZ electrodes tested at 800 C when 10% H2O/Air is fed into the oxygen electrode and 5% H2/Ar is fed into the fuel electrode, respectively. For the electrolyzer based on the LSM electrode, the current densities are approximately 31, 53, 84, 113 and 152 mA cm2 at 1.2, 1.4, 1.6, 1.8 and 2.0 V, respectively. Fig. 9(c) illustrates the H2 production: 0.102, 0.166, 0.23, 0.279 and 0.348 ml min1 cm2 at 1.2, 1.4, 1.6, 1.8 and 2.0 V, respectively. The corresponding current efficiencies are 46.6%, 45.3%, 38.9%, 33.8% and 32.6%. These values exceed the 22% current efficiency achieved by the La0.75Sr0.25Cr0.5Mn0.5O3d-based oxygen electrode for steam electrolysis in a thin-membrane, proton-conducting, solid oxide electrolyzer [16] and are comparable with La0.6Sr0.4Co0.2Fe0.8O3d and Ba0.5Sr0.5Co0.8Fe0.2 O3d electrode electrolyzers during steam electrolysis in proton-conducting solid oxide electrolyzers [15]. However, the efficiency decreases when the voltage improves. The current efficiency lost under higher voltages and currents are most likely caused by the transportation of other ions or holes, as well as the localized steam starvation at high voltages that limits the electrochemical oxidation of steam in the oxygen electrode [15,31]. For the electrolyzer based on the 6 wt.% Co3O4-loaded LSM-BCZYZ electrode, the current densities are approximately 41, 68, 98, 132 and 173 mA cm2 at 1.2, 1.4, 1.6,
1.8 and 2.0 V, respectively. The H2 production is 0.131, 0.205, 0.266, 0.326 and 0.369 ml min1 cm2 at 1.2, 1.4, 1.6, 1.8 and 2.0 V, respectively. The corresponding current efficiencies are 45.4%, 43.3%, 38.4%, 34.8% and 30.8% and are comparable to the cell based on the LSM-BCZYZ electrode. The lower current efficiency in the electrolyzer is dictated by the ion conduction from the proton-conducting electrolyte. The proton conductivity in the BCZYZ electrolyte relies on the partial filling of vacant oxygen sites by water, yielding mobile protons bound to the lattice oxide ions. The necessity of these oxide vacancies complicates the transport properties. Under oxidative conditions, atmospheric oxygen enters the lattice, creating hole-type electronic conduction and lowering the current efficiency of the electrolyzer [24]. Consequently, the electrolyzer based on the proton-conducting electrolyte has a lower current efficiency than that based on the oxide-ion conducting electrolyte. There are two main electrochemical processes that occur from OCV to 2 V during steam electrolysis: the reoxidation of the LSM-BCZYZ electrode and steam electrolysis. The oxidation of LSM is the major process below 1.2 V, while the electrochemical oxidation of H2O dominates at high voltages. Fig. 7(c) and (d) shows that the dV/dI (cell resistance) remains constant above 1.2 V, indicating there is sufficient oxidation of the oxygen electrode and a stable steam electrolysis process.
4.
Conclusions
In this work, composite oxygen electrodes based on LSMBCZYZ and Co3O4-loaded LSM-BCZYZ were investigated for steam electrolysis in proton-conducting solid oxide electrolyzers. The LSM sample has p-type conducting properties; this
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
material is investigated and correlated with the electrochemical properties of the symmetric cells and electrolyzers. The optimal Co3O4 loading in the composite oxygen electrode is systematically investigated in symmetric cells: adding catalytically active Co3O4 significantly enhances the electrode performance compared to the bare LSM-BCZYZ electrode. Steam electrolysis was conducted on electrolyzers using LSMBCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes; adding the Co3O4 catalyst significantly improves the electrode processes and enhances the current density under a certain applied voltage. The significantly improved electrode performance after adding the catalytically active Co3O4 may be because the Co3O4 ameliorates the three-phase boundary of electrode, improving the reaction overpotential and enhancing the oxygen evolution reaction.
Acknowledgment This work is supported by the Natural Science Foundation of China No. 21303037, China Postdoctoral Science Foundation No. 2013M53150 and the Fundamental Research Funds for the Central Universities No. 2012HGZY0001.
references
[1] Brisse A, Schefold J, Zahid M. High temperature water electrolysis in solid oxide cells. Int J Hydrogen Energy 2008;33:5375e82. [2] Hauch A, Ebbesen S, Jensen S, Mogensen M. Highly efficient high temperature electrolysis. J Mater Chem 2008;18:2331e40. [3] Schiller G, Ansar A, Lang M, Patz O. High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC). J Appl Electrochem 2009;39:293e301. [4] Jin C, Yang C, Zhao F, Cui D, Chen F. La0.75Sr0.25Cr0.5Mn0.5O3 as hydrogen electrode for solid oxide electrolysis cells. Int J Hydrogen Energy 2011;36:3340e6. [5] Yang X, Irvine JTS. (La0.75Sr0.25)0.95Mn0.5Cr0.5O3 as the cathode of solid oxide electrolysis cells for high temperature hydrogen production from steam. J Mater Chem 2008;18:2349e54. [6] Ni M, Leung MKH, Leung DYC. Technological development of hydrogen production by solid oxide electrolyzer cell (SOEC). Int J Hydrogen Energy 2008;33:2337e54. [7] Xie K, Zhang YQ, Meng GY, Irvine JTS. Direct synthesis of methane from CO2/H2O in an oxygen-ion conducting solid oxide electrolyser. Energy Environ Sci 2011;4:2218e22. [8] Xie K, Zhang YQ, Meng GY, Irvine JTS. Electrochemical reduction of CO2 in a proton conducting solid oxide electrolyser. J Mater Chem 2011;21:195e8. [9] Doenitz W, Schmidberger R, Steinheil E, Streicher R. Hydrogen production by high temperature electrolysis of water vapour. Int J Hydrogen Energy 1980;5:55e63. [10] Balthasar W. Hydrogen production and technology: today, tomorrow and beyond. Int J Hydrogen Energy 1984;9:649e68. [11] Dutta S. Technology assessment of advanced electrolytic hydrogen production. Int J Hydrogen Energy 1990;15:379e86.
9
[12] Li SS, Li YX, Gan Y, Xie K, Meng GY. Electrolysis of H2O and CO2 in an oxygen-ion conducting solid oxide electrolyzer with a La0.2Sr0.8TiO3þd composite cathode. J Power Sources 2012;218:244e9. [13] Iwahara H, Uchida H, Yamasaki I. High-temperature steam electrolysis using SrCeO3-based proton conductive solid electrolyte. Int J Hydrogen Energy 1987;12:73e7. [14] Schober T. Applications of oxidic high-temperature proton conductors. Solid State Ionics 2003;162:277e81. [15] Li SS, Xie K. Composite oxygen electrode based on LSCF and BSCF for steam electrolysis in a proton-conducting solid oxide electrolyzer. J Electrochem Soc 2013;160:F224e33. [16] Gan Y, Zhang J, Li YX, Li SS, Xie K, Irvine JTS. Composite oxygen electrode based on LSCM for steam electrolysis in a proton conducting solid oxide electrolyzer. J Electrochem Soc 2012;159:F763e7. [17] Kong F. Synthesis of rod and beadlike Co3O4 and bifunctional properties as air/oxygen electrode materials. Electrochim Acta 2012;68:198e201. [18] Lee J, Kim S, Cao R, Choi N, Liu M, Lee K, et al. Metal-air batteries with high energy density: Li-air versus Zn-air. Adv Energy Mater 2011;1:34e50. [19] Nikolova V, Iliev P, Petrov K, Vitanov T, Zhecheva E, Stoyanova R, et al. Electrocatalysts for bifunctional oxygen/ air electrodes. J Power Sources 2008;185:727e33. [20] Singh R, Mishra D, Anindita Sinha A, Singh A. Novel electrocatalysts for generating oxygen from alkaline water electrolysis. Electrochem Commun 2007;9:1369e73. [21] Lu B, Cao D, Wang P, Wang G, Gao Y. Oxygen evolution reaction on Ni-substituted Co3O4 nanowire array electrodes. Int J Hydrogen Energy 2011;36:72e8. [22] Li Y, Hasin P, Wu Y. NixCo3xO4 nanowire arrays for electrocatalytic oxygen evolution. Adv Mater 2010;22:1926e9. [23] Yue XL, Irvine JTS. (La, Sr)(Cr, Mn)O3/GDC cathode for high temperature steam electrolysis and steam-carbon dioxide co-electrolysis. Solid State Ionics 2012;225:131e5. [24] Tao SW, Irvine JTS. A stable. Easily sintered protonconducting oxide electrolyte for moderate-temperature fuel cells and electrolyzers. Adv Mater 2006;18:1581e4. [25] Lee WH, Kim H. Oxidized iridium nanodendrites as catalysts for oxygen evolution reactions. Catal Commun 2011;12:408e11. [26] Garsuch A, Panchenko A, Querner C, Karpov A, Oesten R. FeAgMo2O8-A novel oxygen evolution catalyst material for alkaline metal-air batteries. Electrochem Commun 2010;12:1642e5. [27] Ma L, Sui S, Zhai Y. Preparation and characterization of Ir/TiC catalyst for oxygen evolution. J Power Sources 2008;177:470e7. [28] Wang W, Zhao Q, Dong J, Li J. A novel silver oxides oxygen evolving catalyst for water splitting. Int J Hydrogen Energy 2011;36:7374e80. [29] Hamdani M, Singh R, Chartier P. Co3O4 and co-based spinel oxides bifunctional oxygen electrodes. Int J Electrochem Sci 2010;5:556e77. [30] Rios E, Reyes H, Ortiz J, Gautier JL. Double channel electrode flow cell application to the study of HO-2 production on MnxCo3xO4 (0 x 1) spinel films. Electrochim Acta 2005;50:2705e11. [31] Xu SS, Li SS, Yao WT, Dong DH, Xie K. Direct electrolysis of CO2 using an oxygen-ion conducting solid oxide electrolyzer based on La0.75Sr0.25Cr0.5Mn0.5O3d electrode. J Power Sources 2013;230:115e21.
Please cite this article in press as: Li S, et al., Composite oxygen electrode LSM-BCZYZ impregnated with Co3O4 nanoparticles for steam electrolysis in a proton-conducting solid oxide electrolyzer, International Journal of Hydrogen Energy (2013), http:// dx.doi.org/10.1016/j.ijhydene.2013.09.082