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Composite manganate oxygen electrode enhanced with iron oxide nanocatalyst for high temperature steam electrolysis in a proton-conducting solid oxide electrolyzer
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Composite manganate oxygen electrode enhanced with iron oxide nanocatalyst for high temperature steam electrolysis in a proton-conducting solid oxide electrolyzer Huaxin Li a,1, Xiaoli Chen a,1, Shigang Chen a, Yucheng Wu a, Kui Xie a,b,* a
School of Materials Science and Engineering, Hefei University of Technology, No. 193 Tunxi Road, Hefei, Anhui 230009, China b Key Lab of Design & Assembly of Functional Nanostructure, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, Fujian 350002, China
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abstract
Article history:
Composite electrode based on La0.8Sr0.2MnO3-d (LSM) can be utilized in a proton-conducting
Received 26 February 2015
solid oxide electrolyzer for steam electrolysis; however, the insufficient electro-catalytic
Received in revised form
activity of LSM still restricts the electrode performance and Faraday current efficiency. In
4 April 2015
this work, catalytic-active iron oxide nanoparticles are loaded on the surface of LSM
Accepted 14 April 2015
composite oxygen electrode to improve electro-catalytic performance as well as extend the
Available online 6 May 2015
three-phase boundaries. SEM and EDS results together confirm the loading of Fe2O3 nanoparticles with the size of approximately 20e40 nm on the surface of LSM composite
Keywords:
oxygen electrode. The effects on electrode performance due to different contents of Fe2O3
Iron oxide
are loaded into LSM composite electrodes are systemically studied using symmetric cells.
Manganate
The electrical property of LSM is investigated and correlated to the electrochemical per-
Proton conductor
formance of the composite oxygen electrode in electrolysis cells. The maximum Faraday
Steam electrolysis
current efficiency is approximately 65% with the Fe2O3-loaded LSM composite electrode for steam electrolysis in a proton-conducting solid oxide electrolyzer at 800 C. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The consumption of limited fossil fuels in the current world causes serious energy crisis and leads to greenhouse effect due to carbon dioxide emission [1e3]. Hydrogen attracts a lot
of interest as it can be produced using renewable resources and can also be utilized as a potential green fuel for many applications, such as heating, electricity and vehicles [4,5]. Solid oxide electrolyzers have shown the great advantages of efficient electrochemical conversion of steam into hydrogen using renewable electrical energy because of the high
* Corresponding author. Fujian Institute of Research on The Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, Fujian 350002, China. Tel./fax: þ86 591 63179173. E-mail address: [email protected] (K. Xie). 1 Equal contribution. http://dx.doi.org/10.1016/j.ijhydene.2015.04.067 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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temperature which contributes to energy for the steam dissociation leading to favorable kinetic and thermodynamic properties [6e8]. Proton-conducting solid oxide electrolyzer has many advantages compared to oxide-ion-conducting electrolyzer for the reason following. First, high temperature proton conductors illustrate higher ionic conductivity compared with that of oxygen-ion conductors in the intermediate temperature range [9,10]. Furthermore, dry and pure hydrogen is obtained at the fuel electrode side and no further gas separation is needed in a proton-conducting electrolyzer. Under external applied potentials, a proton-conducting solid oxide electrolyzer can electrolyze H2O into H2 and O2. At the oxygen electrode, the H2O molecules are electrochemically oxidized and split into O2 while the generated protons transport through the protonconducting electrolyte to the fuel electrode compartment to form H2 in the three-phase boundaries [11e13]. Irvine et al. have reported that a higher Zr-containing composition, BaCe0.5Zr0.3Y0.16Zn0.04O3-d (BCZYZ) as the electrolyte material for a proton-conducting solid oxide electrolyzer has performed excellent performance in electrochemical reduction of carbon dioxide [14,15]. The perovskite lanthanum strontium manganite (LSM) is a traditional oxygen electrode material for the solid oxide electrolyzer (SOE) and fuel cell (SOFC) which presents outstanding electrode performance as well as good stability in long-term test [16]. In recent research reports, Irvine et al. studied the utilization of LSM as oxygen electrode in SOFC for direct conversion of waste-derived carbon successfully [17]. In addition, it has been reported that LSM shows reasonable performance of catalytic property for direct synthesis of methane from CO2eH2O co-electrolysis [18]. However, the Faraday current efficiency of steam electrolysis is low with LSM oxygen electrode due to the limited electro-catalytic activity of LSM ceramic. It is reported that electro-catalyst Co3O4 is impregnated on the electrode surface to enhance the LSM electro-catalytic performance in a proton-conducting solid oxide electrolyzer, but the Faraday current efficiency is less than 50% [19]. It is therefore necessary to improve the electrocatalytic activity of the LSM oxygen electrode to enhance the performance of steam electrolysis. The traditional Ni-based fuel electrode is easily oxidized to NiO leading to a loss of electronic conductivity and even the failure of the electrode [20,21]. Compared with the traditional Ni-based fuel electrode, the perovskite La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM) is an active and redox-stable material for the direct high-temperature steam electrolysis as reported by Irvine et al. [22,23]. Promising electrode polarization resistances of LSCM have been achieved in an-oxide-conducting solid oxide electrolyzer in our previous work [24,25]. Thus, LSCM can also be utilized for fuel electrode in a proton-conducting solid oxide electrolyzer. Iron-based materials are excellent catalysts for electrochemical reaction because it is low-cost, relatively non-toxic, environmentally friendly and abundant. In the past decades, it has been studied that various iron salts were applied as Lewis acids (Fe3þ) in homogeneous catalysis and different catalytically active iron complexes [26]. Furthermore, in heterogeneous catalysis, iron oxides have been frequently employed as catalysts and supports in bulk industrial processes, usually at high temperature and pressure [27]. What is
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more, iron oxide (Fe2O3) plays an important role in the photoanode of cells for photoelectrochemical water oxidation because of balancing the needs for photon absorption and charge separation against the demands of stability particularly for catalytic ability [28]. In addition, it is reported that iron doped into electrode material is able to improve electrode performance as well as catalytic activity in solid oxide fuel cells and electrolyzers [29,30]. So, Fe2O3-loaded LSM composite oxygen electrode is expected to effectively improve the performances in a proton-conducting solid oxide electrolyzer for high temperature steam electrolysis. In this work, catalytic-active Fe2O3 nanoparticles are loaded on LSM-BCZYZ oxygen electrode to enhance the electro-catalytic activity for steam electrolysis in a proton conducting electrolyzer. Electrical property of the LSM is systematically investigated which is closely relevant to electrochemical performance of the composite electrodes in symmetrical or electrolysis cells. It is systemically studied with symmetric cells that different contents of Fe2O3-loaded LSM-BCZYZ composite electrodes affect on the electrode performance. Steam electrolysis performances of LSM-BCZYZ and Fe2O3-loaded LSM-BCZYZ are systematically estimated in a proton-conducting solid oxide electrolyzer at 800 C with 5% H2O/Ar introduce to the oxygen electrode while the fuel electrode is exposed to 5% H2/Ar.
Experimental All chemicals and powders which were acquired from SINOPHARM Chemical Reagent CO., Ltd (China) were of analytical grade unless otherwise specified. The La0.8Sr0.2MnO3-d (LSM) powders were synthesized using a combustion method with stoichiometric amounts of La2O3, SrCO3, C4H6MnO4$4H2O and glycine which were mixed and dissolved in nitric acid, then heated until they were combusted, fired at 800 C (3 C min1) for 3 h in air. The LSM powders were loaded with 10 wt% Fe(NO3)3 solution followed by a heat treatment at 550 C for 30 min in air. With a similar method, the La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM) powders were synthesized with stoichiometric amounts of Cr(NO3)3$9H2O, La2O3, SrCO3, C4H6MnO4$4H2O and glycine which were mixed and dissolved in nitric acid, then heated until it was combusted, fired at 1000 C (3 C min1) for 3 h in air [31,32]. The BaCe0.5Zr0.3Y0.16Zn0.04O3-d (BCZYZ) powders were synthesized by modified glycine-assisted combustion method as reported by Xie at al [33]. Stoichiometric amounts of BaCO3, Ce(NO3)3$6H2O, Zr(NO3)4$5H2O, Y2O3, ZnO and glycine were mixed and dissolved in nitric acid, then heated, fired at 1200 C (3 C min1) for 5 h in air to obtain BCZYZ. X-ray diffraction (XRD, Cu Ka, 2q ¼ 3 min1, D/MAX2500V, Rigaku Corporation, Japan) was conducted to determine the phase formation of the LSM, 10 wt% Fe2O3, LSCM and BCZYZ powders with 2q ranging from 10 to 80 and XRD Rietveld refinement was performed using GSAS software. High-resolution transmission electron microscopy analysis (HR-TEM) (JEM-2100F, JEOL, Japan) with selected area diffraction was performed to observe the lattice structure of LSM, LSCM and BCZYZ. X-ray Photoelectron Spectroscopy (XPS) (ESCALAB25, Thermo, USA) was performed using monochromatized Al Ka at hg ¼ 70
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1486.6 eV to analyze the chemical state of LSM and 6 wt% Fe2O3-LSM. About 1.5 g LSM powders were pressed into a bar followed by sintering at 1000 C (3 C min1) for 10 h in air for conductivity test. The conductivity test was performed using the DC four-terminal method with temperature ranging from 25 C to 800 C in air. Four Ag wires (0.4 mm in diameter) were tied on the bar with the conductive adhesive (DAD87, Shanghai Research Institute for Synthetic Resins, Shanghai, China) followed by heat treatment at 550 C (3 C min1) for 30 min in air. Then the bar was mounted onto a four terminal jig coupled with an oxygen sensor device. The results of the conductivity test were recorded versus temperature with an online system at a step 0.5 C. The conductivity was obtained at 800 C with the oxygen partial pressure ranging from 102 to 1018 atm. The change of oxygen partial pressure was achieved by flowing 5% H2/Ar at the flow rate of 1 mL min1 with a mass flow meter (D08-3F, Sevenstar, China). The oxygen partial pressure and conductivity were recorded using an online sensor (Type 1231, ZrO2-based oxygen sensor, Noveltech, Australia) and an online multi-meter (Keithley 2000, Digital Multimeter, Keithley Instrument Inc., USA). BCZYZ electrolyte green disks were synthesized using a solidestate reaction method. The mixture of BaCO3, CeO2, ZrO2, Y2O3 powders were ball-milled at room temperature and fired at 1400 C (3 C min1) in air for 10 h and then an appropriate amount of ZnO powders were mixed in the prefired powders after grinding evenly. A 2-mm-thick BCZYZ disc with a diameter of 20 mm was prepared by dry-pressing the mixed BCZYZ powders before firing at 1400 C (2 C min1) in air for 10 h. The composite electrode LSMBCZYZ slurry was prepared by directly milling the BCZYZ powders (combustion method) and LSM powders at a 35:65 weight ratio with a amount of alpha-terpineol in a mortar. The LSCM-BCZYZ slurry was prepared as the same way above. In addition, a proper amount of cellulose was added to the slurry. The symmetric electrolyzers were fabricated by printing the LSM-BCZYZ and LSCM-BCZYZ electrode slurry onto the two surfaces of the BCZYZ electrolyte support with an area of 1 cm2 followed by a heat treatment at 1000 C (3 C min1) for 3 h in air, respectively. Different contents of Fe2O3 loaded in LSM electrodes were achieved by impregnating the LSM electrodes with an a appropriate Fe(NO)3 nitrate solution several times, with a 30 min heat treatment at 550 C (3 C min1) in air after each impregnation. The maximal content of each impregnation treatment was 2 wt%. The microstructures of the LSM-BCZYZ and 6 wt% Fe2O3LSM-BCZYZ electrodes were observed with Scanning Electron Microscopy (SEM, SU8020, Japan). The current collection layers were made by printing silver paste (SS-8060, Xinluyi, Shanghai, China) on both surfaces of the electrode which were then heated at 550 C (3 C min1) for 30 min in air. The external circuit was made with silver wires which were fastened to both current collectors using conductive adhesive followed by firing at 550 C (3 C min1) for 30 min in air. The AC impedance of the symmetric cells were tested at a 2electrode mode at 800 C using an electrochemical station (IM6, Zahner, Germany) in a frequency ranging from 4 M to 0.1 Hz. The oxygen partial pressure was controlled by mixing different ratios of air and N2 with the aid of mass flow
meters, and hydrogen partial pressure was controlled by mixing different ratios of H2 and N2 as the same way. The microstructures of symmetric cell of 10 wt% Fe2O3-LSMBCZYZ/BCZYZ/10 wt% Fe2O3-LSM-BCZYZ and LSCM-BCZYZ/ BCZYZ/LSCM-BCZYZ were observed with SEM after tests. The solid oxide electrolyzers with configurations of (fuel electrode) LSCM-BCZYZ/BCZYZ/LSM-BCZYZ (oxygen electrode) and (fuel electrode) LSCM-BCZYZ/BCZYZ/4 wt% Fe2O3BCZYZ (oxygen electrode) were sealed to a home-made testing jig using ceramic paste (JD-767, Jiudian, Dongguan, China) for systematic electrochemical measurements, respectively. Electrochemical measurements including AC impedance and currentevoltage (IeV) curves of the electrolysis cells were performed (two-electrode type) with 5% H2/Ar fed into the fuel electrode at 30 mL min1 and 5% H2O/Ar (saturated steam concentration at 33 C [34]) fed into the oxygen electrode at 30 mL min1 for steam electrolysis. The steam electrolysis was performed under the external potentials at 800 C. The output gas from the fuel electrode was analyzed using an online gas chromatograph (GC9790II, Fuli, Zhejiang, China) aiming to detect the concentration of hydrogen.
Results and discussions The XRD Rietveld refinement pattern of LSM, 10 wt% Fe2O3LSM, LSCM and BCZYZ powders are shown in the Fig. 1. Fig. 1(a) and (b) show the XRD pattern for the LSM powders with and without 10 wt% Fe2O3 prepared via a combustion method and impregnation treatments, indicating that both LSM and Fe2O3 samples remain in pure phases [LSM: PDF#530058; Fe2O3: PDF#39-0238]. Fig. 1(a) indicates that the space group of LSM is R-3c with a perovskite structure (a ¼ 5.515525 Ǻ; b ¼ 5.515525 Ǻ; c ¼ 13.357562 Ǻ; a ¼ 90 ; b ¼ 90 ; g ¼ 120 ). The refinement of LSM sample gives c2, wRp and Rp values of 1.064, 8.42% and 7.29%, respectively, which is consistent with those reported in literature [35]. Fig. 1(b) shows that the cell parameters of 10 wt% Fe2O3-LSM powders are similar to these of LSM powders. Fig. 1(c) reveals that the space group of LSCM is R-3c (a ¼ 5.499654 Ǻ; b ¼ 5.499654 Ǻ; c ¼ 13.327922 Ǻ; a ¼ 90 ; b ¼ 90 ; g ¼ 120 ) with a perovskite structure. The refinement of LSCM sample gives c2, wRp and Rp values of 1.337, 30.02% and 10.90%, respectively, which is consistent with the reported data in a previous work [36]. Fig. 1(d) shows the XRD Rietveld refinement of the BCZYZ sample indicating a cubic phase with a ¼ b ¼ c ¼ 4.319011 and a ¼ b ¼ g ¼ 90 . The refinement of LSM sample gives c2 ¼ 2.681, wRp ¼ 15.72%, and Rp ¼ 8.35%. Fig. 2 shows the HR-TEM images of LSM, LSCM and BCZYZ. The LSM, LSCM and BCZYZ which have revealed lattice spacing of 0.31 nm (110), 0.27 nm (002) and 0.31 nm (002) consistent with the separation spacing determined by the XRD analysis. In order to study the electrical properties of the LSM, conductivity tests were performed in both air and 5% H2/Ar versus temperature and oxygen partial pressure at 800 C, respectively. Fig. 3(a) indicates that the LSM is a p-type electronic conductor that displays typical semiconducting behavior with a positive temperature coefficient in air from 25 to 800 C and finally the conductivity of LSM reaches
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Fig. 1 e XRD Retvield refinement patterns of (a) LSM, (b) 10 wt% Fe2O3-LSM, (c) LSCM and (d) BCZYZ powders.
Fig. 2 e The TEM images of (a) LSM, (b) LSCM and (c) BCZYZ.
Fig. 3 e The conductivity of LSM (a) sample in air; the dependence of conductivity of LSM (b) on oxygen partial pressure at 800 C.
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27.99 S cm1 in air at 800 C. The p-type behavior is also observed in Fig. 2(b). The conductivity of LSM decreases with decreasing oxygen partial pressure attributed to that the lower oxygen partial pressure leads to the generation of oxygen vacancy resulting in the decrease of concentration of charge carrier in stronger reducing atmosphere. The conductivity of LSM decreases to 8.09 S cm1 in 5% H2/Ar at the pO2 of about 1018 atm at 800 C. In order to describe the different chemical states of elements in the samples, XPS is performed to test the LSM and 10 wt% Fe2O3-LSM samples. All XPS spectroscopies are fitted with a Shirley-type background subtraction method. The background-functions for different spectroscopies of elements are fitted by 80% Gaussian and 20% Lorenz. The XPS spectroscopies of LSM and 10 wt% Fe2O3-LSM samples are presented in Fig. 4(a1ea3) and (b1eb3), respectively. The Sr3d core level XPS spectra of LSM are shown in Fig. 4(a1). There is a doublet with binding energies at 132.70 and 134.40 eV assigned
as Sr3d5/2 and Sr3d3/2 peaks, respectively. Their binding energies are very close to the similar compounds [37], which can be the Sr2þ ions in LSM. Fig. 4(a2) shows the Mn2p core level XPS spectra of LSM. The Mn3þ 2p1/2 and Mn4þ 2p3/2 peaks are observed at 653.40 and 641.90 eV, respectively. According to the binding energy of Mn2p, the main chemical states of Mn are þ3 and þ4 in the LSM. The O1s core level XPS spectra of LSM are shown in Fig. 4(a3), binding energies are observed at 533, 531 and 529.3 eV, respectively. On the basis of the binding energy, the main line is assigned to O2 ion of the metal oxide, while the broad shoulder peak is probably caused by the water and hydroxide absorbed on the surface [38]. The Sr3d and Mn2p core level XPS spectrum of 10 wt% Fe2O3-LSM are presented in Fig. 4(b1) and (b2), respectively. In contrast to LSM, Fe2O3 loaded LSM present no obvious change of the chemical state. More importantly, Fe3þ peaks are observed at 724 eV for 2p1/2 and 710.8 eV for 2p3/2 as shown in the Fig. 4(b3), indicating that the main chemical state of Fe is mainly þ3 from the in the
Fig. 4 e XPS results for (a1) Sr, (a2) Mn and (a3) O in the LSM; (b1) Sr, (b2) Mn and (b3) Fe in the LSM þ 10 wt% Fe2O3.
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form of Fe2O3, after Fe(NO3)3 solution was loaded on the LSM followed by a heat treatment at 550 C for 30 min in air. In order to firmly demonstrate Fe2O3 nanoparticles were loaded on the electrodes surface, the SEM graphs and EDS maps of the LSM composite electrodes surface were obtained before and after loading of 6 wt% Fe2O3. As shown in Fig. 5(c) and (d), all the elements are homogeneous dispersed in the porous electrode, indicating the uniform microstructure of the composite electrode. There exist Fe2O3 nanoparticles on the 6 wt% Fe2O3-loaded electrodes surface compared with the bare electrodes, as shown in Fig. 5(a) and (b). The presence of Fe2O3 nanoparticles are firmly confirmed by SEM and EDS in addition to XRD and XPS as discussed above. It is reported that impregnation technology can prepare nanostructured high performance electrodes due to extend three-phase boundaries (TPB) as well as improve electrode microstructures [39e43]. Furthermore, the backbone structure has little effect on nanoparticles surface area, but significantly affects TPB length, suggesting a strategy to identify electrode reaction mechanisms. Decreasing impregnation particle size contributes its surface area, enhances the peak TPB length, and decreases the optimal impregnation loading, indicating small impregnated particles essentially benefit electrode performance, as reported by Ni and Xia [44]. More importantly, the nanosized electrocatalytic Fe2O3 particles are found in the attractive range of 20e40 nm on the electrode surface, which
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are expected to effective extend the three-phase boundaries length as well as improve electrode microstructures, thereby enhance the electrode performance. The Fe2O3 nano-catalyst can significantly enhance the electrode performance and Faraday current efficiency of stream electrolysis, which is further investigated through electrode polarization and electrolysis performances. To understand the effects of Fe2O3 nanocatalyst on electrode performance, symmetric cells with different contents of Fe2O3-loaded LSM composite electrodes were systemically studied by AC impedance of the symmetric cells tested at 800 C at different oxygen partial pressure (pO2 ¼ 0, 1, 2, 5, 10 and 21% atm). The series resistance (Rs) and the polarization resistance (Rp), depicted by the first intercept and the difference between the first and second intercepts, were calculated by Zview 2.1c software as reported in our previous work [45e47]. In Fig. 6(a1)e(a3), (b1)e(b3), (c1)e(c3), (d1)e(d3) and (e1)e(e3), the series resistances (Rs) gradually increase with decreasing oxygen partial pressure, which is probably due to the p-type conducting behavior of BCZYZ electrolyte as it is a mixed conductor in air [48]. Similarly, the Rp of the cells also gradually increase on account of the p-type conducting property of LSM ceramics when the oxygen partial pressure decreases, as discussed above. As shown in Fig. 6(e1)e(e3), Rp of the cells gradually decrease with increase contents of Fe2O3. The Rp of cells gradually decrease from about 0.155 to
Fig. 5 e SEM images of (a) LSM-BCZYZ electrode and (b) 6 %wt Fe2O3-LSM-BCZYZ; SEM and EDS images of (c) LSM-BCZYZ electrode and (d) 6 %wt Fe2O3-LSM-BCZYZ.
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Fig. 6 e The AC impendence of the symmetric cell based on x wt% (x ¼ 0(a1, a2, a3), 2(b1, b2, b3), 4(c1, c2, c3), 6(d1, d2, d3), 10(e1, e2, e3) Fe2O3-LSM-BCZYZ in different oxygen partial pressure and (f1, f2, f3) polarization resistance (Rp) with x (x ¼ 0, 2, 4, 6, 10) wt% Fe2O3 loading change of relation in different oxygen partial pressure at 800 C.
0.047 U cm2 with the content of Fe2O3 increasing from 0 to 4 wt % in pO2 ¼ 21% atm, which is probably due to that Fe2O3 nanocatalyst can significantly enhance the electro-catalytic activity of the composite electrode leading to the remarkable
electrode polarization improvement. More importantly, there are not obvious improvement of the Rp of the cells with the contents of Fe2O3 increasing from 4 to 10 wt% in the range from pO2 ¼ 21% atm to pO2 ¼ 1% atm. What is worse, the Rp of
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the cells increases when the contents of Fe2O3 are in a range from 4 to 10 wt% in pO2 ¼ 0% atm, indicating that overweight contents of Fe2O3-loaded LSM composite electrodes cannot improve the electrode performance. Furthermore, 4 wt% Fe2O3-loaded LSM composite electrodes show the most outstanding electrode performance. The Rp of LSM electrodes impregnated with gadolinia-doped ceria and samaria-doped ceria reach the lowest value of 0.21 and 0.23 U cm2 at 700 C at the optimal loadings, as reported by Jiang and Xia [49e51]. In addition, it is reported that the Rp of the ethanol solution precursor plasma spraying LSM are 1.04 and 0.15 U cm2 at 850 C and 1000 C, respectively [52]. Compared with other previous work, the Rp of the cell based on the optimal loadings Fe2O3 nanocatalyst enhanced LSM composite electrodes is 0.047 U cm2 in pO2 ¼ 21% at 800 C, which exhibit excellent performance. Fig. 7 displays the AC impendence spectra of the symmetric solid oxide electrolyzers with the electrodes based on the LSCM-BCZYZ at different hydrogen partial pressure (pH2 ¼ 5, 10, 20, 50, 80 and 100%) at 800 C. The Rp of the cell gradually decreases from approximately 5 to 1.5 U cm2 with the increasing hydrogen partial pressure from pH2 ¼ 0e100% atm, as shown in the Fig. 7(a1) and (a2). Implying that the stronger reducing atmosphere can be beneficial to the electro-catalytic activity of the composite electrodes as well as LSCM-BCZYZ composite fuel electrodes are outstanding for the proton conducting solid oxide electrolyzers. LSCM acts as an excellent electrode material have been widely reported. Jiang et al reported that LSCM-YSZ composite anodes show reasonable performance for the methane oxidation reaction in wet CH4 and the best electrode performance is achieved for the composite with LSCM contents of 50e60 wt% which show Rp of 2-3 U cm2 in 97% CH4/ 3% H2O at 850 C [53]. Moreover, it is reported that the Rp of SOFCs based on LSCM anode is 1.52 U cm2 in pure H2 at 850 C and is 9.50 U cm2 in pure CH4 at 850 C [54]. In comparison with other previous work, LSCM-BCZYZ composite electrodes display reasonably good performance. Fig. 8 presents the microstructure of the BCZYZ electrolyte-supported symmetric cells based on LSM and 10 wt% Fe2O3-loaded LSM electrodes, respectively. It can be found that the LSM and 10 wt% Fe2O3loaded LSM electrodes are porous, and both of two porous electrodes adhere to the dense BCZYZ electrolyte very well with thickness of about 15 mm. The porous electrode
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microstructure is expected to optimize the TPB where electrode, electrolyte and gas phase meet, thereby improving the electrode performance, which have been demonstrated by theoretical and experimental researches [55e57]. The steam electrolysis was performed in two kinds of solid oxide electrolyzers with (fuel electrode) LSCM-BCZYZ/BCYZY/ LSM-BCZYZ (oxygen electrode) and (fuel electrode) LSCMBCZYZ/BCZYZ/4 wt% Fe2O3-LSM-BCZYZ (oxygen electrode) with 5% H2O/Ar introduced to the oxygen electrode while the fuel electrode is exposed to 5% H2/Ar, under a series of applied voltages ranging from 0 to 1.6 V at 800 C, respectively. Fig. 9(a) shows the current density against the applied voltage (IeV curve) at 800 C with oxygen electrode fed of 5% H2O/Ar 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 0.063 A cm2 at 1.6 V for the cell based on the LSM oxygen electrode at 800 C. In contrast, the cell based on 4 wt% Fe2O3loaded LSM oxygen electrode presents improved performance and the current densities finally reach approximately 0.073 A cm2 under the same condition. To study the change in electrolyzer resistance under different applied voltages, the dV/dI curve (cell total resistance) is shown versus voltage in Fig. 9(b). The resistance gradually decreases when the voltage increases to 1.0 V, remains stable at approximately 9.4 U cm2 for the cell based on LSM oxygen electrode in comparison with that of the cell based on 4 wt% Fe2O3-loaded LSM oxygen electrode, approximately 7.4 U cm2 from 1.5 V to 1.6 V. The in situ AC impedance tests of the solid oxide electrolyzers were carried out under different applied potentials to investigate the change of Rs and Rp. In Fig. 10, the Rs is approximately 5.5 U cm2, which is consistent with the ionic resistance of a 2-mm-thick BCZYZ disk at 800 C and generally stable in the whole range of applied voltages; however, the Rp of the electrolyzers based on LSM and 4 wt% Fe2O3-loaded LSM oxygen electrode are considerably improved with the increase of applied voltages in steam electrolysis. It is likely due to the re-oxidation of the oxygen electrode. Higher voltages improve the electrode reactions, further enhancing the electrode progress and decreasing the electrode polarization resistance. The Rp is considerably large at low voltage, and decreases with the external potential increasing from 1.0 to 1.6 V, from approximately 23 U cm2 at 1.0 V to 2 U cm2 at 1.6 V for the solid oxide electrolyzer based on LSM, for the solid oxide
Fig. 7 e The AC impendence of the symmetric cell based on (a1, a2) LSCM-BCZYZ/BCZYZ/LSCM-BCZYZ in different hydrogen partial pressure at 800 C.
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Fig. 8 e The microstructures of symmetric cell of 10 wt% Fe2O3-LSM-BCZYZ/BCZYZ/10 wt% Fe2O3-LSM-BCZYZ and LSCMBCZYZ/BCZYZ/LSCM-BCZYZ after tests.
Fig. 9 e Current-voltage curves of the electrolysis cells (a) (fuel electrode) LSCM-BCZYZ/BCZYZ/LSM-BCZYZ (oxygen electrode) and (fuel electrode) LSCM-BCZYZ/BCZYZ/4 wt% Fe2O3-LSM-BCZYZ (oxygen electrode) for steam electrolysis at 800 C, respectively; (b) the dV/dI curve of the electrolyzers based on LSM-BCZYZ and 4 wt% Fe2O3-LSM-BCZYZ anode, respectively.
Fig. 10 e The AC impendence of the electrolysis cells (a) (fuel electrode) LSCM-BCZYZ/BCZYZ/LSM-BCZYZ (oxygen electrode) and (b) (fuel electrode) LSCM-BCZYZ/BCZYZ/4 wt% Fe2O3-LSM-BCZYZ (oxygen electrode) for steam electrolysis at 800 C, respectively.
electrolyzer based on 4 wt% Fe2O3-loaded LSM, the Rp is from about 22 U cm2 at 1.0 V to 1.9 U cm2 at 1.6 V. To further investigate the performance for the steam electrolysis, the short-term performances were recorded versus time with applied voltages of 1.2, 1.4 and 1.6 V Fig. 11(a1) and (b1) indicate that the current densities increase with the applied potentials from 1.2 to 1.6 V, and each current density are consistent with the experimental data shown in Fig. 9. As shown in Fig. 11(a2) and (b2), with the increase of external
applied potentials, the production of H2 is from 0.06 to 0.12 mL min1 cm2 for the solid oxide electrolyzer based on LSM oxygen electrode, from 0.08 to 0.16 mL min1 cm2 for the solid oxide electrolyzer based on Fe2O3-loaded LSM oxygen electrode. More importantly, Faraday current efficiency of solid oxide electrolyzer based on Fe2O3-loaded LSM oxygen electrode is approximately 65% which is 15% higher than that of the LSM-based solid oxide electrolyzer at 1.2 V. Both of the trends further demonstrate that the Fe2O3 nanocatalyst is
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Fig. 11 e Short-term performances of the electrolysis cells based on (a1) LSM and (b1) 4 wt% Fe2O3-LSM oxygen electrode for steam electrolysis at 800 C, respectively; H2 production and Faraday current efficiencies for the cells based on (b1) LSM and (b2) 4 wt% Fe2O3-LSM oxygen electrode, respectively.
capable of enhancing electrical properties, and significantly increases the production of H2 with an accompanied increased electro-catalytic activity. In our previous work, the Faraday current efficiencies of proton-conducting solid oxide electrolyzers based on BSCF, LSCF and impregnated LSM oxygen electrodes all are less than 50% [7,19]. In contrast, Fe2O3 nanocatalyst impregnated LSM oxygen electrodes can effectively improve the steam electrolysis current efficiency in the proton-conducting solid oxide electrolyzers. As shown in Fig. 12, the short term performance of cell based on 4 wt% Fe2O3-LSM oxygen electrode is generally stable under 1.4 V and the current density is approximately 0.03 A cm2, indicating the Fe2O3 nanocatalyst enhanced LSM is a stable
electrode material for steam electrolysis in the protonconducting solid oxide electrolyzers.
Conclusions In our work, catalytic-active Fe2O3 nanoparticles are loaded on LSM composite oxygen electrode to enhance the catalytic activity for steam electrolysis in a proton-conducting solid oxide electrolyzer. The effects of different contents of Fe2O3loaded LSM composite electrodes on electrode performance are systemically studied in symmetric cells, demonstrating that overweight contents of Fe2O3 are loaded on the LSM composite electrodes cannot improve the electrode performance and electrodes based on 4 wt% Fe2O3-loaded show the most outstanding electrode performance. Stable shortterm performance with current efficiency as high as 65% is achieved by solid oxide electrolyzer based on 4 wt% Fe2O3loaded LSM composite oxygen electrode, which is improved approximately 15% compared with that without Fe2O3loaded. The present results indicate that the Fe2O3-loaded LSM is a highly potential oxygen electrode material for steam electrolysis in a proton-conducting solid oxide electrolyzer.
Acknowledgments Fig. 12 e Short-term performance of the electrolysis cell based on 4 wt% Fe2O3-LSM oxygen electrode for steam electrolysis under 1.4 V at 800 C.
This work is supported by the Natural Science Foundation of China No. 21303037.
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Composite manganate oxygen electrode enhanced with iron oxide nanocatalyst for high temperature steam electrolysis in a proton-conducting solid oxide electrolyzer Huaxin Li a,1, Xiaoli Chen a,1, Shigang Chen a, Yucheng Wu a, Kui Xie a,b,* a
School of Materials Science and Engineering, Hefei University of Technology, No. 193 Tunxi Road, Hefei, Anhui 230009, China b Key Lab of Design & Assembly of Functional Nanostructure, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, Fujian 350002, China
article info
abstract
Article history:
Composite electrode based on La0.8Sr0.2MnO3-d (LSM) can be utilized in a proton-conducting
Received 26 February 2015
solid oxide electrolyzer for steam electrolysis; however, the insufficient electro-catalytic
Received in revised form
activity of LSM still restricts the electrode performance and Faraday current efficiency. In
4 April 2015
this work, catalytic-active iron oxide nanoparticles are loaded on the surface of LSM
Accepted 14 April 2015
composite oxygen electrode to improve electro-catalytic performance as well as extend the
Available online 6 May 2015
three-phase boundaries. SEM and EDS results together confirm the loading of Fe2O3 nanoparticles with the size of approximately 20e40 nm on the surface of LSM composite
Keywords:
oxygen electrode. The effects on electrode performance due to different contents of Fe2O3
Iron oxide
are loaded into LSM composite electrodes are systemically studied using symmetric cells.
Manganate
The electrical property of LSM is investigated and correlated to the electrochemical per-
Proton conductor
formance of the composite oxygen electrode in electrolysis cells. The maximum Faraday
Steam electrolysis
current efficiency is approximately 65% with the Fe2O3-loaded LSM composite electrode for steam electrolysis in a proton-conducting solid oxide electrolyzer at 800 C. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The consumption of limited fossil fuels in the current world causes serious energy crisis and leads to greenhouse effect due to carbon dioxide emission [1e3]. Hydrogen attracts a lot
of interest as it can be produced using renewable resources and can also be utilized as a potential green fuel for many applications, such as heating, electricity and vehicles [4,5]. Solid oxide electrolyzers have shown the great advantages of efficient electrochemical conversion of steam into hydrogen using renewable electrical energy because of the high
* Corresponding author. Fujian Institute of Research on The Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, Fujian 350002, China. Tel./fax: þ86 591 63179173. E-mail address: [email protected] (K. Xie). 1 Equal contribution. http://dx.doi.org/10.1016/j.ijhydene.2015.04.067 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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temperature which contributes to energy for the steam dissociation leading to favorable kinetic and thermodynamic properties [6e8]. Proton-conducting solid oxide electrolyzer has many advantages compared to oxide-ion-conducting electrolyzer for the reason following. First, high temperature proton conductors illustrate higher ionic conductivity compared with that of oxygen-ion conductors in the intermediate temperature range [9,10]. Furthermore, dry and pure hydrogen is obtained at the fuel electrode side and no further gas separation is needed in a proton-conducting electrolyzer. Under external applied potentials, a proton-conducting solid oxide electrolyzer can electrolyze H2O into H2 and O2. At the oxygen electrode, the H2O molecules are electrochemically oxidized and split into O2 while the generated protons transport through the protonconducting electrolyte to the fuel electrode compartment to form H2 in the three-phase boundaries [11e13]. Irvine et al. have reported that a higher Zr-containing composition, BaCe0.5Zr0.3Y0.16Zn0.04O3-d (BCZYZ) as the electrolyte material for a proton-conducting solid oxide electrolyzer has performed excellent performance in electrochemical reduction of carbon dioxide [14,15]. The perovskite lanthanum strontium manganite (LSM) is a traditional oxygen electrode material for the solid oxide electrolyzer (SOE) and fuel cell (SOFC) which presents outstanding electrode performance as well as good stability in long-term test [16]. In recent research reports, Irvine et al. studied the utilization of LSM as oxygen electrode in SOFC for direct conversion of waste-derived carbon successfully [17]. In addition, it has been reported that LSM shows reasonable performance of catalytic property for direct synthesis of methane from CO2eH2O co-electrolysis [18]. However, the Faraday current efficiency of steam electrolysis is low with LSM oxygen electrode due to the limited electro-catalytic activity of LSM ceramic. It is reported that electro-catalyst Co3O4 is impregnated on the electrode surface to enhance the LSM electro-catalytic performance in a proton-conducting solid oxide electrolyzer, but the Faraday current efficiency is less than 50% [19]. It is therefore necessary to improve the electrocatalytic activity of the LSM oxygen electrode to enhance the performance of steam electrolysis. The traditional Ni-based fuel electrode is easily oxidized to NiO leading to a loss of electronic conductivity and even the failure of the electrode [20,21]. Compared with the traditional Ni-based fuel electrode, the perovskite La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM) is an active and redox-stable material for the direct high-temperature steam electrolysis as reported by Irvine et al. [22,23]. Promising electrode polarization resistances of LSCM have been achieved in an-oxide-conducting solid oxide electrolyzer in our previous work [24,25]. Thus, LSCM can also be utilized for fuel electrode in a proton-conducting solid oxide electrolyzer. Iron-based materials are excellent catalysts for electrochemical reaction because it is low-cost, relatively non-toxic, environmentally friendly and abundant. In the past decades, it has been studied that various iron salts were applied as Lewis acids (Fe3þ) in homogeneous catalysis and different catalytically active iron complexes [26]. Furthermore, in heterogeneous catalysis, iron oxides have been frequently employed as catalysts and supports in bulk industrial processes, usually at high temperature and pressure [27]. What is
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more, iron oxide (Fe2O3) plays an important role in the photoanode of cells for photoelectrochemical water oxidation because of balancing the needs for photon absorption and charge separation against the demands of stability particularly for catalytic ability [28]. In addition, it is reported that iron doped into electrode material is able to improve electrode performance as well as catalytic activity in solid oxide fuel cells and electrolyzers [29,30]. So, Fe2O3-loaded LSM composite oxygen electrode is expected to effectively improve the performances in a proton-conducting solid oxide electrolyzer for high temperature steam electrolysis. In this work, catalytic-active Fe2O3 nanoparticles are loaded on LSM-BCZYZ oxygen electrode to enhance the electro-catalytic activity for steam electrolysis in a proton conducting electrolyzer. Electrical property of the LSM is systematically investigated which is closely relevant to electrochemical performance of the composite electrodes in symmetrical or electrolysis cells. It is systemically studied with symmetric cells that different contents of Fe2O3-loaded LSM-BCZYZ composite electrodes affect on the electrode performance. Steam electrolysis performances of LSM-BCZYZ and Fe2O3-loaded LSM-BCZYZ are systematically estimated in a proton-conducting solid oxide electrolyzer at 800 C with 5% H2O/Ar introduce to the oxygen electrode while the fuel electrode is exposed to 5% H2/Ar.
Experimental All chemicals and powders which were acquired from SINOPHARM Chemical Reagent CO., Ltd (China) were of analytical grade unless otherwise specified. The La0.8Sr0.2MnO3-d (LSM) powders were synthesized using a combustion method with stoichiometric amounts of La2O3, SrCO3, C4H6MnO4$4H2O and glycine which were mixed and dissolved in nitric acid, then heated until they were combusted, fired at 800 C (3 C min1) for 3 h in air. The LSM powders were loaded with 10 wt% Fe(NO3)3 solution followed by a heat treatment at 550 C for 30 min in air. With a similar method, the La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM) powders were synthesized with stoichiometric amounts of Cr(NO3)3$9H2O, La2O3, SrCO3, C4H6MnO4$4H2O and glycine which were mixed and dissolved in nitric acid, then heated until it was combusted, fired at 1000 C (3 C min1) for 3 h in air [31,32]. The BaCe0.5Zr0.3Y0.16Zn0.04O3-d (BCZYZ) powders were synthesized by modified glycine-assisted combustion method as reported by Xie at al [33]. Stoichiometric amounts of BaCO3, Ce(NO3)3$6H2O, Zr(NO3)4$5H2O, Y2O3, ZnO and glycine were mixed and dissolved in nitric acid, then heated, fired at 1200 C (3 C min1) for 5 h in air to obtain BCZYZ. X-ray diffraction (XRD, Cu Ka, 2q ¼ 3 min1, D/MAX2500V, Rigaku Corporation, Japan) was conducted to determine the phase formation of the LSM, 10 wt% Fe2O3, LSCM and BCZYZ powders with 2q ranging from 10 to 80 and XRD Rietveld refinement was performed using GSAS software. High-resolution transmission electron microscopy analysis (HR-TEM) (JEM-2100F, JEOL, Japan) with selected area diffraction was performed to observe the lattice structure of LSM, LSCM and BCZYZ. X-ray Photoelectron Spectroscopy (XPS) (ESCALAB25, Thermo, USA) was performed using monochromatized Al Ka at hg ¼ 70
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1486.6 eV to analyze the chemical state of LSM and 6 wt% Fe2O3-LSM. About 1.5 g LSM powders were pressed into a bar followed by sintering at 1000 C (3 C min1) for 10 h in air for conductivity test. The conductivity test was performed using the DC four-terminal method with temperature ranging from 25 C to 800 C in air. Four Ag wires (0.4 mm in diameter) were tied on the bar with the conductive adhesive (DAD87, Shanghai Research Institute for Synthetic Resins, Shanghai, China) followed by heat treatment at 550 C (3 C min1) for 30 min in air. Then the bar was mounted onto a four terminal jig coupled with an oxygen sensor device. The results of the conductivity test were recorded versus temperature with an online system at a step 0.5 C. The conductivity was obtained at 800 C with the oxygen partial pressure ranging from 102 to 1018 atm. The change of oxygen partial pressure was achieved by flowing 5% H2/Ar at the flow rate of 1 mL min1 with a mass flow meter (D08-3F, Sevenstar, China). The oxygen partial pressure and conductivity were recorded using an online sensor (Type 1231, ZrO2-based oxygen sensor, Noveltech, Australia) and an online multi-meter (Keithley 2000, Digital Multimeter, Keithley Instrument Inc., USA). BCZYZ electrolyte green disks were synthesized using a solidestate reaction method. The mixture of BaCO3, CeO2, ZrO2, Y2O3 powders were ball-milled at room temperature and fired at 1400 C (3 C min1) in air for 10 h and then an appropriate amount of ZnO powders were mixed in the prefired powders after grinding evenly. A 2-mm-thick BCZYZ disc with a diameter of 20 mm was prepared by dry-pressing the mixed BCZYZ powders before firing at 1400 C (2 C min1) in air for 10 h. The composite electrode LSMBCZYZ slurry was prepared by directly milling the BCZYZ powders (combustion method) and LSM powders at a 35:65 weight ratio with a amount of alpha-terpineol in a mortar. The LSCM-BCZYZ slurry was prepared as the same way above. In addition, a proper amount of cellulose was added to the slurry. The symmetric electrolyzers were fabricated by printing the LSM-BCZYZ and LSCM-BCZYZ electrode slurry onto the two surfaces of the BCZYZ electrolyte support with an area of 1 cm2 followed by a heat treatment at 1000 C (3 C min1) for 3 h in air, respectively. Different contents of Fe2O3 loaded in LSM electrodes were achieved by impregnating the LSM electrodes with an a appropriate Fe(NO)3 nitrate solution several times, with a 30 min heat treatment at 550 C (3 C min1) in air after each impregnation. The maximal content of each impregnation treatment was 2 wt%. The microstructures of the LSM-BCZYZ and 6 wt% Fe2O3LSM-BCZYZ electrodes were observed with Scanning Electron Microscopy (SEM, SU8020, Japan). The current collection layers were made by printing silver paste (SS-8060, Xinluyi, Shanghai, China) on both surfaces of the electrode which were then heated at 550 C (3 C min1) for 30 min in air. The external circuit was made with silver wires which were fastened to both current collectors using conductive adhesive followed by firing at 550 C (3 C min1) for 30 min in air. The AC impedance of the symmetric cells were tested at a 2electrode mode at 800 C using an electrochemical station (IM6, Zahner, Germany) in a frequency ranging from 4 M to 0.1 Hz. The oxygen partial pressure was controlled by mixing different ratios of air and N2 with the aid of mass flow
meters, and hydrogen partial pressure was controlled by mixing different ratios of H2 and N2 as the same way. The microstructures of symmetric cell of 10 wt% Fe2O3-LSMBCZYZ/BCZYZ/10 wt% Fe2O3-LSM-BCZYZ and LSCM-BCZYZ/ BCZYZ/LSCM-BCZYZ were observed with SEM after tests. The solid oxide electrolyzers with configurations of (fuel electrode) LSCM-BCZYZ/BCZYZ/LSM-BCZYZ (oxygen electrode) and (fuel electrode) LSCM-BCZYZ/BCZYZ/4 wt% Fe2O3BCZYZ (oxygen electrode) were sealed to a home-made testing jig using ceramic paste (JD-767, Jiudian, Dongguan, China) for systematic electrochemical measurements, respectively. Electrochemical measurements including AC impedance and currentevoltage (IeV) curves of the electrolysis cells were performed (two-electrode type) with 5% H2/Ar fed into the fuel electrode at 30 mL min1 and 5% H2O/Ar (saturated steam concentration at 33 C [34]) fed into the oxygen electrode at 30 mL min1 for steam electrolysis. The steam electrolysis was performed under the external potentials at 800 C. The output gas from the fuel electrode was analyzed using an online gas chromatograph (GC9790II, Fuli, Zhejiang, China) aiming to detect the concentration of hydrogen.
Results and discussions The XRD Rietveld refinement pattern of LSM, 10 wt% Fe2O3LSM, LSCM and BCZYZ powders are shown in the Fig. 1. Fig. 1(a) and (b) show the XRD pattern for the LSM powders with and without 10 wt% Fe2O3 prepared via a combustion method and impregnation treatments, indicating that both LSM and Fe2O3 samples remain in pure phases [LSM: PDF#530058; Fe2O3: PDF#39-0238]. Fig. 1(a) indicates that the space group of LSM is R-3c with a perovskite structure (a ¼ 5.515525 Ǻ; b ¼ 5.515525 Ǻ; c ¼ 13.357562 Ǻ; a ¼ 90 ; b ¼ 90 ; g ¼ 120 ). The refinement of LSM sample gives c2, wRp and Rp values of 1.064, 8.42% and 7.29%, respectively, which is consistent with those reported in literature [35]. Fig. 1(b) shows that the cell parameters of 10 wt% Fe2O3-LSM powders are similar to these of LSM powders. Fig. 1(c) reveals that the space group of LSCM is R-3c (a ¼ 5.499654 Ǻ; b ¼ 5.499654 Ǻ; c ¼ 13.327922 Ǻ; a ¼ 90 ; b ¼ 90 ; g ¼ 120 ) with a perovskite structure. The refinement of LSCM sample gives c2, wRp and Rp values of 1.337, 30.02% and 10.90%, respectively, which is consistent with the reported data in a previous work [36]. Fig. 1(d) shows the XRD Rietveld refinement of the BCZYZ sample indicating a cubic phase with a ¼ b ¼ c ¼ 4.319011 and a ¼ b ¼ g ¼ 90 . The refinement of LSM sample gives c2 ¼ 2.681, wRp ¼ 15.72%, and Rp ¼ 8.35%. Fig. 2 shows the HR-TEM images of LSM, LSCM and BCZYZ. The LSM, LSCM and BCZYZ which have revealed lattice spacing of 0.31 nm (110), 0.27 nm (002) and 0.31 nm (002) consistent with the separation spacing determined by the XRD analysis. In order to study the electrical properties of the LSM, conductivity tests were performed in both air and 5% H2/Ar versus temperature and oxygen partial pressure at 800 C, respectively. Fig. 3(a) indicates that the LSM is a p-type electronic conductor that displays typical semiconducting behavior with a positive temperature coefficient in air from 25 to 800 C and finally the conductivity of LSM reaches
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Fig. 1 e XRD Retvield refinement patterns of (a) LSM, (b) 10 wt% Fe2O3-LSM, (c) LSCM and (d) BCZYZ powders.
Fig. 2 e The TEM images of (a) LSM, (b) LSCM and (c) BCZYZ.
Fig. 3 e The conductivity of LSM (a) sample in air; the dependence of conductivity of LSM (b) on oxygen partial pressure at 800 C.
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27.99 S cm1 in air at 800 C. The p-type behavior is also observed in Fig. 2(b). The conductivity of LSM decreases with decreasing oxygen partial pressure attributed to that the lower oxygen partial pressure leads to the generation of oxygen vacancy resulting in the decrease of concentration of charge carrier in stronger reducing atmosphere. The conductivity of LSM decreases to 8.09 S cm1 in 5% H2/Ar at the pO2 of about 1018 atm at 800 C. In order to describe the different chemical states of elements in the samples, XPS is performed to test the LSM and 10 wt% Fe2O3-LSM samples. All XPS spectroscopies are fitted with a Shirley-type background subtraction method. The background-functions for different spectroscopies of elements are fitted by 80% Gaussian and 20% Lorenz. The XPS spectroscopies of LSM and 10 wt% Fe2O3-LSM samples are presented in Fig. 4(a1ea3) and (b1eb3), respectively. The Sr3d core level XPS spectra of LSM are shown in Fig. 4(a1). There is a doublet with binding energies at 132.70 and 134.40 eV assigned
as Sr3d5/2 and Sr3d3/2 peaks, respectively. Their binding energies are very close to the similar compounds [37], which can be the Sr2þ ions in LSM. Fig. 4(a2) shows the Mn2p core level XPS spectra of LSM. The Mn3þ 2p1/2 and Mn4þ 2p3/2 peaks are observed at 653.40 and 641.90 eV, respectively. According to the binding energy of Mn2p, the main chemical states of Mn are þ3 and þ4 in the LSM. The O1s core level XPS spectra of LSM are shown in Fig. 4(a3), binding energies are observed at 533, 531 and 529.3 eV, respectively. On the basis of the binding energy, the main line is assigned to O2 ion of the metal oxide, while the broad shoulder peak is probably caused by the water and hydroxide absorbed on the surface [38]. The Sr3d and Mn2p core level XPS spectrum of 10 wt% Fe2O3-LSM are presented in Fig. 4(b1) and (b2), respectively. In contrast to LSM, Fe2O3 loaded LSM present no obvious change of the chemical state. More importantly, Fe3þ peaks are observed at 724 eV for 2p1/2 and 710.8 eV for 2p3/2 as shown in the Fig. 4(b3), indicating that the main chemical state of Fe is mainly þ3 from the in the
Fig. 4 e XPS results for (a1) Sr, (a2) Mn and (a3) O in the LSM; (b1) Sr, (b2) Mn and (b3) Fe in the LSM þ 10 wt% Fe2O3.
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form of Fe2O3, after Fe(NO3)3 solution was loaded on the LSM followed by a heat treatment at 550 C for 30 min in air. In order to firmly demonstrate Fe2O3 nanoparticles were loaded on the electrodes surface, the SEM graphs and EDS maps of the LSM composite electrodes surface were obtained before and after loading of 6 wt% Fe2O3. As shown in Fig. 5(c) and (d), all the elements are homogeneous dispersed in the porous electrode, indicating the uniform microstructure of the composite electrode. There exist Fe2O3 nanoparticles on the 6 wt% Fe2O3-loaded electrodes surface compared with the bare electrodes, as shown in Fig. 5(a) and (b). The presence of Fe2O3 nanoparticles are firmly confirmed by SEM and EDS in addition to XRD and XPS as discussed above. It is reported that impregnation technology can prepare nanostructured high performance electrodes due to extend three-phase boundaries (TPB) as well as improve electrode microstructures [39e43]. Furthermore, the backbone structure has little effect on nanoparticles surface area, but significantly affects TPB length, suggesting a strategy to identify electrode reaction mechanisms. Decreasing impregnation particle size contributes its surface area, enhances the peak TPB length, and decreases the optimal impregnation loading, indicating small impregnated particles essentially benefit electrode performance, as reported by Ni and Xia [44]. More importantly, the nanosized electrocatalytic Fe2O3 particles are found in the attractive range of 20e40 nm on the electrode surface, which
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are expected to effective extend the three-phase boundaries length as well as improve electrode microstructures, thereby enhance the electrode performance. The Fe2O3 nano-catalyst can significantly enhance the electrode performance and Faraday current efficiency of stream electrolysis, which is further investigated through electrode polarization and electrolysis performances. To understand the effects of Fe2O3 nanocatalyst on electrode performance, symmetric cells with different contents of Fe2O3-loaded LSM composite electrodes were systemically studied by AC impedance of the symmetric cells tested at 800 C at different oxygen partial pressure (pO2 ¼ 0, 1, 2, 5, 10 and 21% atm). The series resistance (Rs) and the polarization resistance (Rp), depicted by the first intercept and the difference between the first and second intercepts, were calculated by Zview 2.1c software as reported in our previous work [45e47]. In Fig. 6(a1)e(a3), (b1)e(b3), (c1)e(c3), (d1)e(d3) and (e1)e(e3), the series resistances (Rs) gradually increase with decreasing oxygen partial pressure, which is probably due to the p-type conducting behavior of BCZYZ electrolyte as it is a mixed conductor in air [48]. Similarly, the Rp of the cells also gradually increase on account of the p-type conducting property of LSM ceramics when the oxygen partial pressure decreases, as discussed above. As shown in Fig. 6(e1)e(e3), Rp of the cells gradually decrease with increase contents of Fe2O3. The Rp of cells gradually decrease from about 0.155 to
Fig. 5 e SEM images of (a) LSM-BCZYZ electrode and (b) 6 %wt Fe2O3-LSM-BCZYZ; SEM and EDS images of (c) LSM-BCZYZ electrode and (d) 6 %wt Fe2O3-LSM-BCZYZ.
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Fig. 6 e The AC impendence of the symmetric cell based on x wt% (x ¼ 0(a1, a2, a3), 2(b1, b2, b3), 4(c1, c2, c3), 6(d1, d2, d3), 10(e1, e2, e3) Fe2O3-LSM-BCZYZ in different oxygen partial pressure and (f1, f2, f3) polarization resistance (Rp) with x (x ¼ 0, 2, 4, 6, 10) wt% Fe2O3 loading change of relation in different oxygen partial pressure at 800 C.
0.047 U cm2 with the content of Fe2O3 increasing from 0 to 4 wt % in pO2 ¼ 21% atm, which is probably due to that Fe2O3 nanocatalyst can significantly enhance the electro-catalytic activity of the composite electrode leading to the remarkable
electrode polarization improvement. More importantly, there are not obvious improvement of the Rp of the cells with the contents of Fe2O3 increasing from 4 to 10 wt% in the range from pO2 ¼ 21% atm to pO2 ¼ 1% atm. What is worse, the Rp of
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the cells increases when the contents of Fe2O3 are in a range from 4 to 10 wt% in pO2 ¼ 0% atm, indicating that overweight contents of Fe2O3-loaded LSM composite electrodes cannot improve the electrode performance. Furthermore, 4 wt% Fe2O3-loaded LSM composite electrodes show the most outstanding electrode performance. The Rp of LSM electrodes impregnated with gadolinia-doped ceria and samaria-doped ceria reach the lowest value of 0.21 and 0.23 U cm2 at 700 C at the optimal loadings, as reported by Jiang and Xia [49e51]. In addition, it is reported that the Rp of the ethanol solution precursor plasma spraying LSM are 1.04 and 0.15 U cm2 at 850 C and 1000 C, respectively [52]. Compared with other previous work, the Rp of the cell based on the optimal loadings Fe2O3 nanocatalyst enhanced LSM composite electrodes is 0.047 U cm2 in pO2 ¼ 21% at 800 C, which exhibit excellent performance. Fig. 7 displays the AC impendence spectra of the symmetric solid oxide electrolyzers with the electrodes based on the LSCM-BCZYZ at different hydrogen partial pressure (pH2 ¼ 5, 10, 20, 50, 80 and 100%) at 800 C. The Rp of the cell gradually decreases from approximately 5 to 1.5 U cm2 with the increasing hydrogen partial pressure from pH2 ¼ 0e100% atm, as shown in the Fig. 7(a1) and (a2). Implying that the stronger reducing atmosphere can be beneficial to the electro-catalytic activity of the composite electrodes as well as LSCM-BCZYZ composite fuel electrodes are outstanding for the proton conducting solid oxide electrolyzers. LSCM acts as an excellent electrode material have been widely reported. Jiang et al reported that LSCM-YSZ composite anodes show reasonable performance for the methane oxidation reaction in wet CH4 and the best electrode performance is achieved for the composite with LSCM contents of 50e60 wt% which show Rp of 2-3 U cm2 in 97% CH4/ 3% H2O at 850 C [53]. Moreover, it is reported that the Rp of SOFCs based on LSCM anode is 1.52 U cm2 in pure H2 at 850 C and is 9.50 U cm2 in pure CH4 at 850 C [54]. In comparison with other previous work, LSCM-BCZYZ composite electrodes display reasonably good performance. Fig. 8 presents the microstructure of the BCZYZ electrolyte-supported symmetric cells based on LSM and 10 wt% Fe2O3-loaded LSM electrodes, respectively. It can be found that the LSM and 10 wt% Fe2O3loaded LSM electrodes are porous, and both of two porous electrodes adhere to the dense BCZYZ electrolyte very well with thickness of about 15 mm. The porous electrode
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microstructure is expected to optimize the TPB where electrode, electrolyte and gas phase meet, thereby improving the electrode performance, which have been demonstrated by theoretical and experimental researches [55e57]. The steam electrolysis was performed in two kinds of solid oxide electrolyzers with (fuel electrode) LSCM-BCZYZ/BCYZY/ LSM-BCZYZ (oxygen electrode) and (fuel electrode) LSCMBCZYZ/BCZYZ/4 wt% Fe2O3-LSM-BCZYZ (oxygen electrode) with 5% H2O/Ar introduced to the oxygen electrode while the fuel electrode is exposed to 5% H2/Ar, under a series of applied voltages ranging from 0 to 1.6 V at 800 C, respectively. Fig. 9(a) shows the current density against the applied voltage (IeV curve) at 800 C with oxygen electrode fed of 5% H2O/Ar 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 0.063 A cm2 at 1.6 V for the cell based on the LSM oxygen electrode at 800 C. In contrast, the cell based on 4 wt% Fe2O3loaded LSM oxygen electrode presents improved performance and the current densities finally reach approximately 0.073 A cm2 under the same condition. To study the change in electrolyzer resistance under different applied voltages, the dV/dI curve (cell total resistance) is shown versus voltage in Fig. 9(b). The resistance gradually decreases when the voltage increases to 1.0 V, remains stable at approximately 9.4 U cm2 for the cell based on LSM oxygen electrode in comparison with that of the cell based on 4 wt% Fe2O3-loaded LSM oxygen electrode, approximately 7.4 U cm2 from 1.5 V to 1.6 V. The in situ AC impedance tests of the solid oxide electrolyzers were carried out under different applied potentials to investigate the change of Rs and Rp. In Fig. 10, the Rs is approximately 5.5 U cm2, which is consistent with the ionic resistance of a 2-mm-thick BCZYZ disk at 800 C and generally stable in the whole range of applied voltages; however, the Rp of the electrolyzers based on LSM and 4 wt% Fe2O3-loaded LSM oxygen electrode are considerably improved with the increase of applied voltages in steam electrolysis. It is likely due to the re-oxidation of the oxygen electrode. Higher voltages improve the electrode reactions, further enhancing the electrode progress and decreasing the electrode polarization resistance. The Rp is considerably large at low voltage, and decreases with the external potential increasing from 1.0 to 1.6 V, from approximately 23 U cm2 at 1.0 V to 2 U cm2 at 1.6 V for the solid oxide electrolyzer based on LSM, for the solid oxide
Fig. 7 e The AC impendence of the symmetric cell based on (a1, a2) LSCM-BCZYZ/BCZYZ/LSCM-BCZYZ in different hydrogen partial pressure at 800 C.
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Fig. 8 e The microstructures of symmetric cell of 10 wt% Fe2O3-LSM-BCZYZ/BCZYZ/10 wt% Fe2O3-LSM-BCZYZ and LSCMBCZYZ/BCZYZ/LSCM-BCZYZ after tests.
Fig. 9 e Current-voltage curves of the electrolysis cells (a) (fuel electrode) LSCM-BCZYZ/BCZYZ/LSM-BCZYZ (oxygen electrode) and (fuel electrode) LSCM-BCZYZ/BCZYZ/4 wt% Fe2O3-LSM-BCZYZ (oxygen electrode) for steam electrolysis at 800 C, respectively; (b) the dV/dI curve of the electrolyzers based on LSM-BCZYZ and 4 wt% Fe2O3-LSM-BCZYZ anode, respectively.
Fig. 10 e The AC impendence of the electrolysis cells (a) (fuel electrode) LSCM-BCZYZ/BCZYZ/LSM-BCZYZ (oxygen electrode) and (b) (fuel electrode) LSCM-BCZYZ/BCZYZ/4 wt% Fe2O3-LSM-BCZYZ (oxygen electrode) for steam electrolysis at 800 C, respectively.
electrolyzer based on 4 wt% Fe2O3-loaded LSM, the Rp is from about 22 U cm2 at 1.0 V to 1.9 U cm2 at 1.6 V. To further investigate the performance for the steam electrolysis, the short-term performances were recorded versus time with applied voltages of 1.2, 1.4 and 1.6 V Fig. 11(a1) and (b1) indicate that the current densities increase with the applied potentials from 1.2 to 1.6 V, and each current density are consistent with the experimental data shown in Fig. 9. As shown in Fig. 11(a2) and (b2), with the increase of external
applied potentials, the production of H2 is from 0.06 to 0.12 mL min1 cm2 for the solid oxide electrolyzer based on LSM oxygen electrode, from 0.08 to 0.16 mL min1 cm2 for the solid oxide electrolyzer based on Fe2O3-loaded LSM oxygen electrode. More importantly, Faraday current efficiency of solid oxide electrolyzer based on Fe2O3-loaded LSM oxygen electrode is approximately 65% which is 15% higher than that of the LSM-based solid oxide electrolyzer at 1.2 V. Both of the trends further demonstrate that the Fe2O3 nanocatalyst is
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Fig. 11 e Short-term performances of the electrolysis cells based on (a1) LSM and (b1) 4 wt% Fe2O3-LSM oxygen electrode for steam electrolysis at 800 C, respectively; H2 production and Faraday current efficiencies for the cells based on (b1) LSM and (b2) 4 wt% Fe2O3-LSM oxygen electrode, respectively.
capable of enhancing electrical properties, and significantly increases the production of H2 with an accompanied increased electro-catalytic activity. In our previous work, the Faraday current efficiencies of proton-conducting solid oxide electrolyzers based on BSCF, LSCF and impregnated LSM oxygen electrodes all are less than 50% [7,19]. In contrast, Fe2O3 nanocatalyst impregnated LSM oxygen electrodes can effectively improve the steam electrolysis current efficiency in the proton-conducting solid oxide electrolyzers. As shown in Fig. 12, the short term performance of cell based on 4 wt% Fe2O3-LSM oxygen electrode is generally stable under 1.4 V and the current density is approximately 0.03 A cm2, indicating the Fe2O3 nanocatalyst enhanced LSM is a stable
electrode material for steam electrolysis in the protonconducting solid oxide electrolyzers.
Conclusions In our work, catalytic-active Fe2O3 nanoparticles are loaded on LSM composite oxygen electrode to enhance the catalytic activity for steam electrolysis in a proton-conducting solid oxide electrolyzer. The effects of different contents of Fe2O3loaded LSM composite electrodes on electrode performance are systemically studied in symmetric cells, demonstrating that overweight contents of Fe2O3 are loaded on the LSM composite electrodes cannot improve the electrode performance and electrodes based on 4 wt% Fe2O3-loaded show the most outstanding electrode performance. Stable shortterm performance with current efficiency as high as 65% is achieved by solid oxide electrolyzer based on 4 wt% Fe2O3loaded LSM composite oxygen electrode, which is improved approximately 15% compared with that without Fe2O3loaded. The present results indicate that the Fe2O3-loaded LSM is a highly potential oxygen electrode material for steam electrolysis in a proton-conducting solid oxide electrolyzer.
Acknowledgments Fig. 12 e Short-term performance of the electrolysis cell based on 4 wt% Fe2O3-LSM oxygen electrode for steam electrolysis under 1.4 V at 800 C.
This work is supported by the Natural Science Foundation of China No. 21303037.
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