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Physicochemical properties of proton-conducting SmNiO3 epitaxial films
Journal of Materiomics 5 (2019) 247e251
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Journal of Materiomics journal homepage: www.journals.elsevier.com/journal-of-materiomics/
Physicochemical properties of proton-conducting SmNiO3 epitaxial films Xing Xu a, Chen Liu a, Jing Ma a, Allan J. Jacobson b, Cewen Nan a, *, Chonglin Chen a, c, ** a
School of Material Science and Engineering, Tsinghua University, Beijing, 100086, China Department of Chemistry and Texas Center for Superconductivity, University of Houston, Houston, TX, 77204, USA c Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, 78249, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 October 2018 Received in revised form 7 January 2019 Accepted 30 January 2019 Available online 9 March 2019
Proton conducting SmNiO3 (SNO) thin films were grown on (001) LaAlO3 substrates for systematically investigating the proton transport properties. X-ray Diffraction and Atomic Force Microscopy studies reveal that the as-grown SNO thin films have good single crystallinity and smooth surface morphology. The electrical conductivity measurements in air indicate a peak at 473 K in the temperature dependence of the resistance of the SNO films, probably due to oxygen loss on heating. A Metal-Insulator-Transition occurs at 373 K for the films after annealing at 873 K in air. In a hydrogen atmosphere (3% H2/97% N2), an anomalous peak in the resistance is found at 685 K on the first heating cycle. Electrochemical Impedance Spectroscopy studies as a function of temperature indicate that the SNO films have a high ionic conductivity (0.030 S/cm at 773 K) in a hydrogen atmosphere. The activation energy for proton conductivity was determined to be 0.23 eV at 473e773 K and 0.37 eV at 773e973 K respectively. These findings demonstrate that SNO thin films have good proton conductivity and are good candidate electrolytes for low temperature proton-conducting Solid Oxide Fuel Cells. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: SmNiO3 Thin film Transport properties Ionic conductivity Impedance measurements
A Solid Oxide Fuel Cell (SOFC) is an electrochemical device that directly converts chemical fuels into electricity efficiently and in an environmental friendly way [1]. Like a traditional battery, an SOFC consists of an anode, a cathode, and an electrolyte [2]. In the past decade, the Mixed Ionic Electronic Conductors (MIECs), especially the double perovskite cobaltate, have reduced the cathode operating temperature [3,4]. The electrolyte should be good ionic conducting but an electronic insulator. Traditionally, there are two types of electrolytes for SOFCs: oxygen-ion-conducting and protonconducting. The oxygen-ion-conducting SOFCs are preferred because they can operate using a variety of fuels, such as hydrogen, carbon monoxide, ethanol, methane, and other hydrocarbons. The high operating temperature (>1073 K), however, dramatically reduces the operation lifetime and increases the cost of device packaging. In contrast, proton-conducting SOFCs can be operated at a relatively low operating temperature with high power density [5].
* Corresponding author. ** Corresponding author. Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, 78249, USA. E-mail addresses: [email protected] (C. Nan), [email protected] (C. Chen). Peer review under responsibility of The Chinese Ceramic Society.
For instance, proton-conducting SOFCs with Y-doped BaZrO3 electrolytes can be operated below 973 K [6]. Normally, ionic conducting electrolytes are designed by doping aliovalent elements into an electrically insulating system, i.e. Ydoped BaZrO3 (Y-BZO) [7] and Yttria-Stabilized-Zirconia (YSZ). Such aliovalent ion doping, however, can introduce defects and strain, resulting in electronic leakage and lower ionic conductivity [8]. Thus the ionic conductivity of traditional oxide electrolytes is limited by the dilemma of aliovalent ion doping. A new strategy is needed to achieve high ionic conductivity. Recently, SmNiO3 (SNO) was reported to be a new protonconducting electrolyte at low temperature [9]. SNO is a distorted orthorhombic perovskite with the GdFeO3-type structure with the Pbnm space group. The lattice parameters are a ¼ 5.366, b ¼ 5.452, c ¼ 7.567 Å [10,11], which correspond to pseudo cubic unit cell (a’√2 a’√2 2a0 ) with a’ ¼ 3.796 Å [12]. SNO has been widely studied as electronic material due to its sharp Metal-InsulatorTransition (MIT) at 400 K [13]. It was reported that the strain, oxygen vacancies, and other defects can strongly alter its electronic and magnetic transport properties [14]. Protons and lithium ions can be applied to gate the resistance states and tune the optical properties of SNO [3,15], suggesting that the SNO could be a good proton-conducting electrolyte. Unfortunately, the synthesis of bulk
https://doi.org/10.1016/j.jmat.2019.01.011 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
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single crystal SNO requires high temperature and high oxygen pressure (i.e. 1273 K, 200 bar [12]; 1173 K, 50 bar [10]) due to the instability of the Ni3þ valence state [12,16]. In hydrogenated SNO (H-SNO), the insertion of an electron introduces a large Coulomb repulsion energy U, which results in an insulating state [9]. Based on the electronic insulating state of H - SNO, Zhou et al. developed proton-conducting micro-SOFCs with operating temperatures as low as 673 K and characterized the transport properties of films deposited by magnetron sputtering on Si3N4 and epitaxially on LaAlO3 substrates [9]. The conductivity was shown to be due to protons by measurement of the open-circuit voltage of a microSOFC (OCV) at 773 K. A stable OCV with a value close to the Nernst potential indicated a proton transference number close to 1. Further measurements of the electrochemical properties and ionic transport behavior of H-SNO electrolytes in reducing/ oxidizing environments are needed on epitaxial thin films with different degrees of crystal perfection prepared by different routes. The grain boundary and defects in the polycrystalline sample will strongly alter the ionic diffusion process [17]. Recently, we have grown SNO thin films by pulsed laser deposition epitaxially on LaAlO3 (001) substrates with high crystallinity for further investigation of the electrochemical properties. Our results show that the SNO thin film in a hydrogen ambient has excellent proton conductivity of 0.030 S/cm at 773 K with very small activation energy of 0.23 eV at 473e773 K and 0.37 eV at 773e973 K, respectively. The activation energy range is similar to the previous report (~0.3 eV) but the conductivity is larger at 773 K [9]. These findings further demonstrate that SNO with good proton conductivity can be a good candidate electrolyte for low/intermediate temperature protonconducting Solid Oxide Fuel Cell device applications. Thin films of GdFeO3-type SNO were grown on LaAlO3 (001) single crystal substrates by the Pulsed Laser Deposition (PLD) technique. The KrF excimer laser was operated to ablate the target with 1.5e2 J/cm2 energy density at a 5 Hz repetition rate. The SNO films were deposited in 200 mTorr oxygen gas at 1073 K. The as-grown films were annealed in situ in 200 Torr oxygen gas and kept at 1073 K for 15 min before cooling down to room temperature at a rate of 5 K/min. X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM) studies reveal that the films are c-axis oriented with good single crystallinity and smooth surface morphology. Electrochemical properties were studied by Electrochemical Impedance Spectroscopy (Solartron 1260) and the DC resistance was measured (Keithley 2400) in air and in hydrogen gas (3% hydrogen/97% nitrogen). The films were connected to two platinum probes by silver paste and cured at room temperature. The samples were installed in a home-made high temperature measurement system. A Keithley 2400 source meter was used to monitor the DC resistance change of the SNO films in both air and hydrogen gas to understand the electronic/ionic conductivities. In air, the sample was heated to 873 K at a rate of 5 K/min and then cooled down at the same rate. In hydrogen gas, the resistance measurements were made on heating the SNO films to 973 K at 5 K/min and holding at 973 K for 1 h to fully hydrogenate the sample; that film was then cooled down at 5 K/min. The impedance spectroscopy study was carried out in the frequency range of 1 MHz to 0.1 Hz with 100 mV amplitude to determine the surface chemical dynamics. The impedance spectroscopy was conducted from 473 K to 973 K with a ramping rate of 5 K/min to each measuring temperature point. Data were taken after 5 min to stabilize the temperature after which the temperature deviated less than 1 K during the measurement process. Fig. 1 shows the XRD patterns and an AFM image of the asgrown SNO (001) film on the LaAlO3 (001) substrate. Fig. 1(a) is the q-2q scan showing that only (00l)LAO and (00l)SNO peaks (indexed as pseudo-cubic) appear in the XRD pattern, indicating that the SNO thin film has c-axis orientation. Fig. 1(b) is an
Fig. 1. (a) The XRD pattern of SNO on LAO and (b) an expanded view showing the (002)SNO and (002)LAO peaks. The inset of (b) shows the X-ray reflectivity; (c) 4 scans of LAO(111) (red line) and SNO(113) (blue line), the inset shows the rocking curve of the SNO(113) peak with a FWHM of 0.34 ; (d) is an AFM image of an SNO thin film surface.
expanded view of the pattern near the (002)LAO peak and the inset in Fig. 1 (b) shows the X-ray reflectivity. Thus, the thin film thickness (d) can be estimated to be 80 nm from the diffraction satellites (d ¼ l/2Dq, where l ¼ 0.154 nm). The phi-scan (Fig. 1 (c)) shows that four-fold symmetric (113)SNO and (111)LAO peaks are well matched, suggesting that SNO (001) thin film is cubic-on-cubic grown on the LAO(001) surface. The Full Width at Half Maximum (FWHM) of SNO (113) rocking curve peak (the inset of Fig. 1 (c)) is ~0.34 , suggesting that the as-grown film has a good single crystallinity. Furthermore, the small surface roughness of less than 1 nm in the AFM image (Fig. 1(d)) indicates that the SNO film has a smooth surface. To systematically investigate the transport properties of SNO thin films, samples with two platinum probes glued by silver paste were installed in a homemade high temperature measurement system. The temperature dependence of resistance for the as-grown SNO films was monitored in various cycles of heating and cooling in air. On heating, the resistance of the SNO film shows a typical semiconductor behavior with an anomalous peak appearing near 473 K (Fig. 2 (a)). This peak can be attributed to the loss of the surface oxygen from the thin film surface reducing Ni3þ and increasing the band gap due to the electron-electron Coulomb repulsion interaction [18]. The reaction can be described as equation (1), where O2 ad corresponds to the surface adsorption oxygen ion. 3þ 2þ O2 ad þ 2Ni /1=2O2 þ 2Ni
(1)
When the temperature reaches 573 K, the resistance decreases to several hundred Ohms and stabilizes in a metallic state. An example of the resistance of SNO on cooling is shown in Fig. 2 (b) inset. In the high temperature range (from 473 K to 873 K), the resistance of SNO films increases with the temperature and show metallic behavior. A small deviation in the resistance curve is observed near 673 K which may arise from the instability of oxygen vacancies. Oxygen vacancies in the SNO crystal lattice can significantly alter the electron hopping between the oxygen octahedra. On cooling cycles, the resistance showed a Metal-Insulator Transition at a temperature (TMI) of 373 K (in Fig. 2(b)). SNO is a chargetransfer complex insulator at low temperature [19]. Epitaxial SNO thin films on LAO (001) substrates (cubic a ¼ 0.3821 nm) are subject to compressive strain (0.1%) which makes the TMI (393 K) lower
X. Xu et al. / Journal of Materiomics 5 (2019) 247e251
Fig. 2. The temperature dependence of the resistance of SNO in air; (a) the resistance on cycle 1 (heating) and the 1st derivative, (b) the resistance of cycle 4 (cooling), the FFT smoothed jdR/dTj vs. temperature shows the Metal-Insulator Transition Temperature at 373 K. The inset of (b) shows the resistance over the whole temperature range on cooling.
than that observed in the bulk state (400 K) [14,20]. The compressive strain in SNO films will compress the Ni-O bond more than the Sm-O bond to increase the tolerance factor and decrease TMI [16,21]. In the heating cycles, the oxygen ions escapes from SNO lattice and the oxygen vacancies convert Ni3þ into Ni2þ(with larger ion radius), resulting in the lattice expansion along the c-axis. The lattice parameters along the a and b axes are constrained by the substrate increasing the overlap of Ni-O bond. Therefore, TMI further decreases to 373 K (100 C) (Fig. 2(b)). To study the proton conductivity of epitaxial SmNiO3 thin films, the temperature dependence of the resistance for the pristine SNO sample is determined in a hydrogen atmosphere (3%H2/97%N2). The resistance on heating and cooling and the corresponding derivatives as a function of temperature are shown in Fig. 3(a) and (b). On heating, the resistance increases gently within 1e2 kU and then increases to a maximum of 6.2 107 U in the Temperature Zone I (TZ-I, 373e685 K), as seen in Fig. 3 (a). This increase in resistance can be attributed to the oxygen desorption from the surface (equation (1)) and from the thin film. The oxygen desorption will reduce the nickel ion from Ni3þ to Ni2þ and block electron hopping by the large electron-electron Coulomb repulsion energy [22]. When temperature increases in TZ-II (685e738 K), the resistance decreases to 1 106 U from the maximum (685 K) since the proton conductivity increases (equation (2a) and (2b)). At 738 K, the conductivity of the SNO films is comparable to the proton conductivity given in previous reports [9].
H2 /2Hþ
(2a)
Hþ þ O2 /OH
(2b)
When the temperature increases to TZ-III (738e973 K), the resistance increases and then decreases. This can be interpreted that the oxygen vacancies are generated and activated (around 773 K) during the hydrogen reaction, which induces the increase of the proton diffusion scattering by the oxygen vacancies. When the
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Fig. 3. The temperature dependence of the resistance of the as-grown SNO thin film in a hydrogen atmosphere (3%H2/97%N2); (a) on the 1st heating cycle (b) on the first cooling cycle and the corresponding resistance derivatives with temperature. The inset is (b) is the zoom-in of the derivative peak.
temperature increases the resistance decreases as the proton conductivity becomes dominating and stabilizes in this temperature range. The sample is held at 973 K for 1 h and then coole down. In Fig. 3 (b), the resistance increases monotonically when temperature decreases. The resistance shows a sharp discontinuity at 495 K which can only been observed on cooling. A much broader feature is observed at higher temperature on heating. The same behavior is observed in cycles 2 and 3 given in the in the supplementary data. The detailed mechanism is still under investigation. To further understand the nature of proton conductivity of SNO films, Electrochemical Impedance Spectroscopy (EIS) studies were performed on the hydrogen stabilized SNO (H-SNO) films at various temperatures. The spectra are in the form of a single semicircle. Fig. 4 (a) shows the impedance spectrum at 971 K; the inset is the equivalent circuit used to fit the data using Zview software. The fitting simply uses on RC circuit which represents the bulk H-SNO thin film. R2 is a series resistance (leads, contacts) [7]. The detailed fitting results are shown in Table S1. The value of CPE is in the small range of 1 109 F and changes slightly with the temperature. The experiment results are well described by this equivalent circuit with small errors (<1%) in the resistance R1. The fitting results are represented in the Arrhenius plot of the conductivity in Fig. 4 (b). The conductivity shows typically thermal activation behavior in the whole temperature range. However, the conductivity plot can be divided into two different ranges. The activation energy Ea1 is 0.23 eV in the lower temperature range (473e773 K) and Ea2 is 0.37 eV in the higher temperature range (773e973 K). The two different activation energies can be considered as the high temperature oxygen vacancy activation and the low temperature proton diffusion. At high temperature region, diffusion of the oxygen vacancies occurs in perovskite structure [23,24] which scatter the proton diffusion. Thus, the activation energy for the proton diffusion is significantly increased. At lower temperature, more inactive oxygen vacancies makes the hopping of protons in the lattice easier. The conductivity is 0.030 S/cm at 773 K which is much larger than 0.0079 S/cm in the previous report [9]. The proton conductivity of H-SNO is very stable at 773 K in hydrogen gas (Figure S 2). In conclusion, SmNiO3 thin films with the GdFeO3-type
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Fig. 4. An Electrochemical Impedance spectrum of H-SNO and the fitting results at 971 K in 3%H2 gas, the inset is the equivalent circuit. (b) an Arrhenius plot of the conductivity from the fitted data and the activation energy. Data from Ref. R [9]. for a SNO film on LAO prepared by magnetron sputtering are shown for comparison.
structure were epitaxially grown on (001) LAO single crystal substrates by PLD. The XRD and AFM image indicates that the films have good single crystallinity and high quality epitaxial nature. A Metal-Insulator Transition was found near 373 K. The temperature dependence of Electrochemical Impedance data shows a high ionic conductivity of 0.030 S/cm at 773 K and the ionic conductivity shows a small activation energy of 0.23 eV at 473e573 K and 0.37 eV at 773e973 K. The resistance shows good stability in hydrogen for a long period. These findings demonstrate that the SNO has good proton conductivity, suggesting that it can be a good candidate electrolyte for the development of a low temperature proton-conducting Solid Oxide Fuel Cell. Conflicts of interest
[2] Jacobson AJ. Materials for solid oxide fuel cells. Chem Mater 2010;22:660e74. [3] Taskin AA, Lavrov AN, Ando Y. Achieving fast oxygen diffusion in perovskites by cation ordering. Appl Phys Lett 2005;86:091910. [4] Wang H, Enriquez E, Collins G, Ma C, Liu M, Zhang Y, et al. Anomalous redox properties and ultrafast chemical sensing behavior of double perovskite CaBaCo2O5þd thin films. J Materiom 2015;1:113e7. [5] Zuo C, Zha S, Liu M, Hatano M, Uchiyama M. Ba(Zr0.1Ce0.7Y0.2)O3ed as an electrolyte for low-temperature solid-oxide fuel cells. Adv Mater 2006;18: 3318e20. [6] Kreuer KD. Proton-conducting oxides. Annu Rev Mater Res 2003;33:333e59. [7] Pergolesi D, Fabbri E, D'Epifanio A, Di Bartolomeo E, Tebano A, Sanna S, et al. High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition. Nat Mater 2010;9:846e52. [8] Filal M. Ionic conductivity of yttrium-doped zirconia and the “composite effect”. Solid State Ionics 1995;80:27e35. [9] Zhou Y, Guan X, Zhou H, Ramadoss K, Adam S, Liu H, et al. Strongly correlated perovskite fuel cells. Nature 2016;534:231e4. [10] Nikulin IV, Novojilov MA, Kaul AR, Mudretsova SN, Kondrashov SV. Oxygen nonstoichiometry of NdNiO3d and SmNiO3d. Mater Res Bull 2004;39: 775e91. ~ oz JL, Rodríguez-Carvajal J, Lacorre P, Torrance JB. Neutron[11] García-Mun diffraction study ofRNiO3(R¼La,Pr,Nd,Sm): electronically induced structural changes across the metal-insulator transition. Phys Rev B 1992;46:4414e25. [12] Perez-Cacho J, Blasco J, Garcia J, Castro M, Stankiewicz J. Study of the phase transitions in SmNiO3. J Phys Condens Matter 1999;11:405e15. [13] Torrance J, Lacorre P, Nazzal A, Ansaldo E, Niedermayer C. Systematic study of insulator-metal transitions in perovskites RNiO3 (R¼Pr,Nd,Sm,Eu) due to closing of charge-transfer gap. Phys Rev B 1992;45:8209e12. [14] Catalano S, Gibert M, Bisogni V, Peil OE, He F, Sutarto R, et al. Electronic transitions in strained SmNiO3 thin films. Apl Mater 2014;2:116110. [15] Li Z, Zhou Y, Qi H, Pan Q, Zhang Z, Shi NN, et al. Correlated perovskites as a new platform for super-broadband-tunable photonics. Adv Mater 2016;28: 9117e25. [16] Catalan G. Progress in perovskite nickelate research. Phase Transitions 2008;81:729e49. [17] Luo J. Interfacial engineering of solid electrolytes. J Materiom 2015;1:22e32. [18] Zaaneen J. Band gaps and electronic structure of transition-metal compounds. Phys Rev Lett 1985;55:418. [19] Granados X, Fontcuberta J, Obradors X, Torrance JB. Metastable metallic state and hysteresis below the metal-insulator transition inPrNiO3. Phys Rev B 1992;46:15683e8. [20] Torriss B, Margot J, Chaker M. Metal-insulator transition of strained SmNiO3 thin films: structural, electrical and optical properties. Sci Rep 2017;7:40915. [21] Medarde ML. Structural, magnetic and electronic properties of perovskites (R ¼ rare earth). J Phys Condens Matter 1997;9:1679e707. [22] Shi J, Zhou Y, Ramanathan S. Colossal resistance switching and band gap modulation in a perovskite nickelate by electron doping. Nat Commun 2014;5:4860. nez C, Caneiro A. Oxygen orderedisorder phase tran[23] Mogni L, Prado F, Jime sition in layered GdBaCo2O5þd perovskite: thermodynamic and transport properties. Solid State Ionics 2013;240:19e28. [24] Streule S, Podlesnyak A, Mesot J, Medarde M, Conder K, Pomjakushina E, et al. Effect of oxygen ordering on the structural and magnetic properties of the layered perovskites PrBaCo2O5þd. J Phys Condens Matter 2005;17:3317e24. Dr. Xu Xing is a postdoc fellow in Tsinghua University. He received his PhD in Physics from University of Texas at San Antonio (UTSA) in Aug 2015.
Authors declare that there are no conflicts of interest. Funding Financial support by National Science Foundation of China Contract No. 51672149, 51332001 and 51402164, the National Basic Research Program of China (No. 2016YFA0300103). AJJ acknowledges support from the R. A. Welch Foundation, Grant #E0024). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.01.011. References [1] Li F, Zeng R, Jiang L, Wei T, Lin X, Xu Y, et al. Enhanced electrochemical activity in Ca3Co2O6 cathode for solid-oxide fuel cells by Cu substitution. J Materiom 2015;1:60e7.
Miss Chen Liu was a master student at the School of Materials Science and Engineering, Tsinghua University. She received her master degree in June 2018.
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Dr. Jing Ma is an assistant professor in School of Materials Science and Engineering, Tsinghua University. She received her B.S. and Ph.D degree in materials science and engineering from Tsinghua University in 2006 and 2011 respectively, and completed post-doc period in Stockholm University from 2012 to 2013. Her research interests include high performance multiferroic magnetoelectric composites and device prototypes, and functional oxide films.
Dr. Cewen Nan is Academician of the Chinese Academy of Sciences, Cheung-Kong Professor, Tsinghua University, China. He received his PhD degree from Wuhan University of Technology in 1992.
Dr. Allan Jacobson is Robert A. Welch Chair of Science Professor in University of Houston. He is Director of the Texas Center for Superconductivity at UH. He is a fellow of National Academy of Inventors, US Editor of Solid State Ionics, Associate Editor of Materials Research Bulletin and on Editorial Advisory Board of Progress in Solid State Chemistry, Journal of Solid State Chemistry and Solid State Sciences. He received his PhD in from Oxford in 1969.
Dr. Chonglin Chen is currently a professor of physics in the Department of Physics and Astronomy, University of Texas at San Antonio, a joint professor at the Texas Center for Superconductivity at the University of Houston (TcSUH), the “Thousand Talent Program - B00 professor at Tsinghua University, and the fellow of the American Ceramics Society. He received his PhD degree in solid state science (Materials) from the Pennsylvania State University in 1994.
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Physicochemical properties of proton-conducting SmNiO3 epitaxial films Xing Xu a, Chen Liu a, Jing Ma a, Allan J. Jacobson b, Cewen Nan a, *, Chonglin Chen a, c, ** a
School of Material Science and Engineering, Tsinghua University, Beijing, 100086, China Department of Chemistry and Texas Center for Superconductivity, University of Houston, Houston, TX, 77204, USA c Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, 78249, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 October 2018 Received in revised form 7 January 2019 Accepted 30 January 2019 Available online 9 March 2019
Proton conducting SmNiO3 (SNO) thin films were grown on (001) LaAlO3 substrates for systematically investigating the proton transport properties. X-ray Diffraction and Atomic Force Microscopy studies reveal that the as-grown SNO thin films have good single crystallinity and smooth surface morphology. The electrical conductivity measurements in air indicate a peak at 473 K in the temperature dependence of the resistance of the SNO films, probably due to oxygen loss on heating. A Metal-Insulator-Transition occurs at 373 K for the films after annealing at 873 K in air. In a hydrogen atmosphere (3% H2/97% N2), an anomalous peak in the resistance is found at 685 K on the first heating cycle. Electrochemical Impedance Spectroscopy studies as a function of temperature indicate that the SNO films have a high ionic conductivity (0.030 S/cm at 773 K) in a hydrogen atmosphere. The activation energy for proton conductivity was determined to be 0.23 eV at 473e773 K and 0.37 eV at 773e973 K respectively. These findings demonstrate that SNO thin films have good proton conductivity and are good candidate electrolytes for low temperature proton-conducting Solid Oxide Fuel Cells. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: SmNiO3 Thin film Transport properties Ionic conductivity Impedance measurements
A Solid Oxide Fuel Cell (SOFC) is an electrochemical device that directly converts chemical fuels into electricity efficiently and in an environmental friendly way [1]. Like a traditional battery, an SOFC consists of an anode, a cathode, and an electrolyte [2]. In the past decade, the Mixed Ionic Electronic Conductors (MIECs), especially the double perovskite cobaltate, have reduced the cathode operating temperature [3,4]. The electrolyte should be good ionic conducting but an electronic insulator. Traditionally, there are two types of electrolytes for SOFCs: oxygen-ion-conducting and protonconducting. The oxygen-ion-conducting SOFCs are preferred because they can operate using a variety of fuels, such as hydrogen, carbon monoxide, ethanol, methane, and other hydrocarbons. The high operating temperature (>1073 K), however, dramatically reduces the operation lifetime and increases the cost of device packaging. In contrast, proton-conducting SOFCs can be operated at a relatively low operating temperature with high power density [5].
* Corresponding author. ** Corresponding author. Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, 78249, USA. E-mail addresses: [email protected] (C. Nan), [email protected] (C. Chen). Peer review under responsibility of The Chinese Ceramic Society.
For instance, proton-conducting SOFCs with Y-doped BaZrO3 electrolytes can be operated below 973 K [6]. Normally, ionic conducting electrolytes are designed by doping aliovalent elements into an electrically insulating system, i.e. Ydoped BaZrO3 (Y-BZO) [7] and Yttria-Stabilized-Zirconia (YSZ). Such aliovalent ion doping, however, can introduce defects and strain, resulting in electronic leakage and lower ionic conductivity [8]. Thus the ionic conductivity of traditional oxide electrolytes is limited by the dilemma of aliovalent ion doping. A new strategy is needed to achieve high ionic conductivity. Recently, SmNiO3 (SNO) was reported to be a new protonconducting electrolyte at low temperature [9]. SNO is a distorted orthorhombic perovskite with the GdFeO3-type structure with the Pbnm space group. The lattice parameters are a ¼ 5.366, b ¼ 5.452, c ¼ 7.567 Å [10,11], which correspond to pseudo cubic unit cell (a’√2 a’√2 2a0 ) with a’ ¼ 3.796 Å [12]. SNO has been widely studied as electronic material due to its sharp Metal-InsulatorTransition (MIT) at 400 K [13]. It was reported that the strain, oxygen vacancies, and other defects can strongly alter its electronic and magnetic transport properties [14]. Protons and lithium ions can be applied to gate the resistance states and tune the optical properties of SNO [3,15], suggesting that the SNO could be a good proton-conducting electrolyte. Unfortunately, the synthesis of bulk
https://doi.org/10.1016/j.jmat.2019.01.011 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
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single crystal SNO requires high temperature and high oxygen pressure (i.e. 1273 K, 200 bar [12]; 1173 K, 50 bar [10]) due to the instability of the Ni3þ valence state [12,16]. In hydrogenated SNO (H-SNO), the insertion of an electron introduces a large Coulomb repulsion energy U, which results in an insulating state [9]. Based on the electronic insulating state of H - SNO, Zhou et al. developed proton-conducting micro-SOFCs with operating temperatures as low as 673 K and characterized the transport properties of films deposited by magnetron sputtering on Si3N4 and epitaxially on LaAlO3 substrates [9]. The conductivity was shown to be due to protons by measurement of the open-circuit voltage of a microSOFC (OCV) at 773 K. A stable OCV with a value close to the Nernst potential indicated a proton transference number close to 1. Further measurements of the electrochemical properties and ionic transport behavior of H-SNO electrolytes in reducing/ oxidizing environments are needed on epitaxial thin films with different degrees of crystal perfection prepared by different routes. The grain boundary and defects in the polycrystalline sample will strongly alter the ionic diffusion process [17]. Recently, we have grown SNO thin films by pulsed laser deposition epitaxially on LaAlO3 (001) substrates with high crystallinity for further investigation of the electrochemical properties. Our results show that the SNO thin film in a hydrogen ambient has excellent proton conductivity of 0.030 S/cm at 773 K with very small activation energy of 0.23 eV at 473e773 K and 0.37 eV at 773e973 K, respectively. The activation energy range is similar to the previous report (~0.3 eV) but the conductivity is larger at 773 K [9]. These findings further demonstrate that SNO with good proton conductivity can be a good candidate electrolyte for low/intermediate temperature protonconducting Solid Oxide Fuel Cell device applications. Thin films of GdFeO3-type SNO were grown on LaAlO3 (001) single crystal substrates by the Pulsed Laser Deposition (PLD) technique. The KrF excimer laser was operated to ablate the target with 1.5e2 J/cm2 energy density at a 5 Hz repetition rate. The SNO films were deposited in 200 mTorr oxygen gas at 1073 K. The as-grown films were annealed in situ in 200 Torr oxygen gas and kept at 1073 K for 15 min before cooling down to room temperature at a rate of 5 K/min. X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM) studies reveal that the films are c-axis oriented with good single crystallinity and smooth surface morphology. Electrochemical properties were studied by Electrochemical Impedance Spectroscopy (Solartron 1260) and the DC resistance was measured (Keithley 2400) in air and in hydrogen gas (3% hydrogen/97% nitrogen). The films were connected to two platinum probes by silver paste and cured at room temperature. The samples were installed in a home-made high temperature measurement system. A Keithley 2400 source meter was used to monitor the DC resistance change of the SNO films in both air and hydrogen gas to understand the electronic/ionic conductivities. In air, the sample was heated to 873 K at a rate of 5 K/min and then cooled down at the same rate. In hydrogen gas, the resistance measurements were made on heating the SNO films to 973 K at 5 K/min and holding at 973 K for 1 h to fully hydrogenate the sample; that film was then cooled down at 5 K/min. The impedance spectroscopy study was carried out in the frequency range of 1 MHz to 0.1 Hz with 100 mV amplitude to determine the surface chemical dynamics. The impedance spectroscopy was conducted from 473 K to 973 K with a ramping rate of 5 K/min to each measuring temperature point. Data were taken after 5 min to stabilize the temperature after which the temperature deviated less than 1 K during the measurement process. Fig. 1 shows the XRD patterns and an AFM image of the asgrown SNO (001) film on the LaAlO3 (001) substrate. Fig. 1(a) is the q-2q scan showing that only (00l)LAO and (00l)SNO peaks (indexed as pseudo-cubic) appear in the XRD pattern, indicating that the SNO thin film has c-axis orientation. Fig. 1(b) is an
Fig. 1. (a) The XRD pattern of SNO on LAO and (b) an expanded view showing the (002)SNO and (002)LAO peaks. The inset of (b) shows the X-ray reflectivity; (c) 4 scans of LAO(111) (red line) and SNO(113) (blue line), the inset shows the rocking curve of the SNO(113) peak with a FWHM of 0.34 ; (d) is an AFM image of an SNO thin film surface.
expanded view of the pattern near the (002)LAO peak and the inset in Fig. 1 (b) shows the X-ray reflectivity. Thus, the thin film thickness (d) can be estimated to be 80 nm from the diffraction satellites (d ¼ l/2Dq, where l ¼ 0.154 nm). The phi-scan (Fig. 1 (c)) shows that four-fold symmetric (113)SNO and (111)LAO peaks are well matched, suggesting that SNO (001) thin film is cubic-on-cubic grown on the LAO(001) surface. The Full Width at Half Maximum (FWHM) of SNO (113) rocking curve peak (the inset of Fig. 1 (c)) is ~0.34 , suggesting that the as-grown film has a good single crystallinity. Furthermore, the small surface roughness of less than 1 nm in the AFM image (Fig. 1(d)) indicates that the SNO film has a smooth surface. To systematically investigate the transport properties of SNO thin films, samples with two platinum probes glued by silver paste were installed in a homemade high temperature measurement system. The temperature dependence of resistance for the as-grown SNO films was monitored in various cycles of heating and cooling in air. On heating, the resistance of the SNO film shows a typical semiconductor behavior with an anomalous peak appearing near 473 K (Fig. 2 (a)). This peak can be attributed to the loss of the surface oxygen from the thin film surface reducing Ni3þ and increasing the band gap due to the electron-electron Coulomb repulsion interaction [18]. The reaction can be described as equation (1), where O2 ad corresponds to the surface adsorption oxygen ion. 3þ 2þ O2 ad þ 2Ni /1=2O2 þ 2Ni
(1)
When the temperature reaches 573 K, the resistance decreases to several hundred Ohms and stabilizes in a metallic state. An example of the resistance of SNO on cooling is shown in Fig. 2 (b) inset. In the high temperature range (from 473 K to 873 K), the resistance of SNO films increases with the temperature and show metallic behavior. A small deviation in the resistance curve is observed near 673 K which may arise from the instability of oxygen vacancies. Oxygen vacancies in the SNO crystal lattice can significantly alter the electron hopping between the oxygen octahedra. On cooling cycles, the resistance showed a Metal-Insulator Transition at a temperature (TMI) of 373 K (in Fig. 2(b)). SNO is a chargetransfer complex insulator at low temperature [19]. Epitaxial SNO thin films on LAO (001) substrates (cubic a ¼ 0.3821 nm) are subject to compressive strain (0.1%) which makes the TMI (393 K) lower
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Fig. 2. The temperature dependence of the resistance of SNO in air; (a) the resistance on cycle 1 (heating) and the 1st derivative, (b) the resistance of cycle 4 (cooling), the FFT smoothed jdR/dTj vs. temperature shows the Metal-Insulator Transition Temperature at 373 K. The inset of (b) shows the resistance over the whole temperature range on cooling.
than that observed in the bulk state (400 K) [14,20]. The compressive strain in SNO films will compress the Ni-O bond more than the Sm-O bond to increase the tolerance factor and decrease TMI [16,21]. In the heating cycles, the oxygen ions escapes from SNO lattice and the oxygen vacancies convert Ni3þ into Ni2þ(with larger ion radius), resulting in the lattice expansion along the c-axis. The lattice parameters along the a and b axes are constrained by the substrate increasing the overlap of Ni-O bond. Therefore, TMI further decreases to 373 K (100 C) (Fig. 2(b)). To study the proton conductivity of epitaxial SmNiO3 thin films, the temperature dependence of the resistance for the pristine SNO sample is determined in a hydrogen atmosphere (3%H2/97%N2). The resistance on heating and cooling and the corresponding derivatives as a function of temperature are shown in Fig. 3(a) and (b). On heating, the resistance increases gently within 1e2 kU and then increases to a maximum of 6.2 107 U in the Temperature Zone I (TZ-I, 373e685 K), as seen in Fig. 3 (a). This increase in resistance can be attributed to the oxygen desorption from the surface (equation (1)) and from the thin film. The oxygen desorption will reduce the nickel ion from Ni3þ to Ni2þ and block electron hopping by the large electron-electron Coulomb repulsion energy [22]. When temperature increases in TZ-II (685e738 K), the resistance decreases to 1 106 U from the maximum (685 K) since the proton conductivity increases (equation (2a) and (2b)). At 738 K, the conductivity of the SNO films is comparable to the proton conductivity given in previous reports [9].
H2 /2Hþ
(2a)
Hþ þ O2 /OH
(2b)
When the temperature increases to TZ-III (738e973 K), the resistance increases and then decreases. This can be interpreted that the oxygen vacancies are generated and activated (around 773 K) during the hydrogen reaction, which induces the increase of the proton diffusion scattering by the oxygen vacancies. When the
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Fig. 3. The temperature dependence of the resistance of the as-grown SNO thin film in a hydrogen atmosphere (3%H2/97%N2); (a) on the 1st heating cycle (b) on the first cooling cycle and the corresponding resistance derivatives with temperature. The inset is (b) is the zoom-in of the derivative peak.
temperature increases the resistance decreases as the proton conductivity becomes dominating and stabilizes in this temperature range. The sample is held at 973 K for 1 h and then coole down. In Fig. 3 (b), the resistance increases monotonically when temperature decreases. The resistance shows a sharp discontinuity at 495 K which can only been observed on cooling. A much broader feature is observed at higher temperature on heating. The same behavior is observed in cycles 2 and 3 given in the in the supplementary data. The detailed mechanism is still under investigation. To further understand the nature of proton conductivity of SNO films, Electrochemical Impedance Spectroscopy (EIS) studies were performed on the hydrogen stabilized SNO (H-SNO) films at various temperatures. The spectra are in the form of a single semicircle. Fig. 4 (a) shows the impedance spectrum at 971 K; the inset is the equivalent circuit used to fit the data using Zview software. The fitting simply uses on RC circuit which represents the bulk H-SNO thin film. R2 is a series resistance (leads, contacts) [7]. The detailed fitting results are shown in Table S1. The value of CPE is in the small range of 1 109 F and changes slightly with the temperature. The experiment results are well described by this equivalent circuit with small errors (<1%) in the resistance R1. The fitting results are represented in the Arrhenius plot of the conductivity in Fig. 4 (b). The conductivity shows typically thermal activation behavior in the whole temperature range. However, the conductivity plot can be divided into two different ranges. The activation energy Ea1 is 0.23 eV in the lower temperature range (473e773 K) and Ea2 is 0.37 eV in the higher temperature range (773e973 K). The two different activation energies can be considered as the high temperature oxygen vacancy activation and the low temperature proton diffusion. At high temperature region, diffusion of the oxygen vacancies occurs in perovskite structure [23,24] which scatter the proton diffusion. Thus, the activation energy for the proton diffusion is significantly increased. At lower temperature, more inactive oxygen vacancies makes the hopping of protons in the lattice easier. The conductivity is 0.030 S/cm at 773 K which is much larger than 0.0079 S/cm in the previous report [9]. The proton conductivity of H-SNO is very stable at 773 K in hydrogen gas (Figure S 2). In conclusion, SmNiO3 thin films with the GdFeO3-type
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Fig. 4. An Electrochemical Impedance spectrum of H-SNO and the fitting results at 971 K in 3%H2 gas, the inset is the equivalent circuit. (b) an Arrhenius plot of the conductivity from the fitted data and the activation energy. Data from Ref. R [9]. for a SNO film on LAO prepared by magnetron sputtering are shown for comparison.
structure were epitaxially grown on (001) LAO single crystal substrates by PLD. The XRD and AFM image indicates that the films have good single crystallinity and high quality epitaxial nature. A Metal-Insulator Transition was found near 373 K. The temperature dependence of Electrochemical Impedance data shows a high ionic conductivity of 0.030 S/cm at 773 K and the ionic conductivity shows a small activation energy of 0.23 eV at 473e573 K and 0.37 eV at 773e973 K. The resistance shows good stability in hydrogen for a long period. These findings demonstrate that the SNO has good proton conductivity, suggesting that it can be a good candidate electrolyte for the development of a low temperature proton-conducting Solid Oxide Fuel Cell. Conflicts of interest
[2] Jacobson AJ. Materials for solid oxide fuel cells. Chem Mater 2010;22:660e74. [3] Taskin AA, Lavrov AN, Ando Y. Achieving fast oxygen diffusion in perovskites by cation ordering. Appl Phys Lett 2005;86:091910. [4] Wang H, Enriquez E, Collins G, Ma C, Liu M, Zhang Y, et al. Anomalous redox properties and ultrafast chemical sensing behavior of double perovskite CaBaCo2O5þd thin films. J Materiom 2015;1:113e7. [5] Zuo C, Zha S, Liu M, Hatano M, Uchiyama M. Ba(Zr0.1Ce0.7Y0.2)O3ed as an electrolyte for low-temperature solid-oxide fuel cells. Adv Mater 2006;18: 3318e20. [6] Kreuer KD. Proton-conducting oxides. Annu Rev Mater Res 2003;33:333e59. [7] Pergolesi D, Fabbri E, D'Epifanio A, Di Bartolomeo E, Tebano A, Sanna S, et al. High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition. Nat Mater 2010;9:846e52. [8] Filal M. Ionic conductivity of yttrium-doped zirconia and the “composite effect”. Solid State Ionics 1995;80:27e35. [9] Zhou Y, Guan X, Zhou H, Ramadoss K, Adam S, Liu H, et al. Strongly correlated perovskite fuel cells. Nature 2016;534:231e4. [10] Nikulin IV, Novojilov MA, Kaul AR, Mudretsova SN, Kondrashov SV. Oxygen nonstoichiometry of NdNiO3d and SmNiO3d. Mater Res Bull 2004;39: 775e91. ~ oz JL, Rodríguez-Carvajal J, Lacorre P, Torrance JB. Neutron[11] García-Mun diffraction study ofRNiO3(R¼La,Pr,Nd,Sm): electronically induced structural changes across the metal-insulator transition. Phys Rev B 1992;46:4414e25. [12] Perez-Cacho J, Blasco J, Garcia J, Castro M, Stankiewicz J. Study of the phase transitions in SmNiO3. J Phys Condens Matter 1999;11:405e15. [13] Torrance J, Lacorre P, Nazzal A, Ansaldo E, Niedermayer C. Systematic study of insulator-metal transitions in perovskites RNiO3 (R¼Pr,Nd,Sm,Eu) due to closing of charge-transfer gap. Phys Rev B 1992;45:8209e12. [14] Catalano S, Gibert M, Bisogni V, Peil OE, He F, Sutarto R, et al. Electronic transitions in strained SmNiO3 thin films. Apl Mater 2014;2:116110. [15] Li Z, Zhou Y, Qi H, Pan Q, Zhang Z, Shi NN, et al. Correlated perovskites as a new platform for super-broadband-tunable photonics. Adv Mater 2016;28: 9117e25. [16] Catalan G. Progress in perovskite nickelate research. Phase Transitions 2008;81:729e49. [17] Luo J. Interfacial engineering of solid electrolytes. J Materiom 2015;1:22e32. [18] Zaaneen J. Band gaps and electronic structure of transition-metal compounds. Phys Rev Lett 1985;55:418. [19] Granados X, Fontcuberta J, Obradors X, Torrance JB. Metastable metallic state and hysteresis below the metal-insulator transition inPrNiO3. Phys Rev B 1992;46:15683e8. [20] Torriss B, Margot J, Chaker M. Metal-insulator transition of strained SmNiO3 thin films: structural, electrical and optical properties. Sci Rep 2017;7:40915. [21] Medarde ML. Structural, magnetic and electronic properties of perovskites (R ¼ rare earth). J Phys Condens Matter 1997;9:1679e707. [22] Shi J, Zhou Y, Ramanathan S. Colossal resistance switching and band gap modulation in a perovskite nickelate by electron doping. Nat Commun 2014;5:4860. nez C, Caneiro A. Oxygen orderedisorder phase tran[23] Mogni L, Prado F, Jime sition in layered GdBaCo2O5þd perovskite: thermodynamic and transport properties. Solid State Ionics 2013;240:19e28. [24] Streule S, Podlesnyak A, Mesot J, Medarde M, Conder K, Pomjakushina E, et al. Effect of oxygen ordering on the structural and magnetic properties of the layered perovskites PrBaCo2O5þd. J Phys Condens Matter 2005;17:3317e24. Dr. Xu Xing is a postdoc fellow in Tsinghua University. He received his PhD in Physics from University of Texas at San Antonio (UTSA) in Aug 2015.
Authors declare that there are no conflicts of interest. Funding Financial support by National Science Foundation of China Contract No. 51672149, 51332001 and 51402164, the National Basic Research Program of China (No. 2016YFA0300103). AJJ acknowledges support from the R. A. Welch Foundation, Grant #E0024). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jmat.2019.01.011. References [1] Li F, Zeng R, Jiang L, Wei T, Lin X, Xu Y, et al. Enhanced electrochemical activity in Ca3Co2O6 cathode for solid-oxide fuel cells by Cu substitution. J Materiom 2015;1:60e7.
Miss Chen Liu was a master student at the School of Materials Science and Engineering, Tsinghua University. She received her master degree in June 2018.
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Dr. Jing Ma is an assistant professor in School of Materials Science and Engineering, Tsinghua University. She received her B.S. and Ph.D degree in materials science and engineering from Tsinghua University in 2006 and 2011 respectively, and completed post-doc period in Stockholm University from 2012 to 2013. Her research interests include high performance multiferroic magnetoelectric composites and device prototypes, and functional oxide films.
Dr. Cewen Nan is Academician of the Chinese Academy of Sciences, Cheung-Kong Professor, Tsinghua University, China. He received his PhD degree from Wuhan University of Technology in 1992.
Dr. Allan Jacobson is Robert A. Welch Chair of Science Professor in University of Houston. He is Director of the Texas Center for Superconductivity at UH. He is a fellow of National Academy of Inventors, US Editor of Solid State Ionics, Associate Editor of Materials Research Bulletin and on Editorial Advisory Board of Progress in Solid State Chemistry, Journal of Solid State Chemistry and Solid State Sciences. He received his PhD in from Oxford in 1969.
Dr. Chonglin Chen is currently a professor of physics in the Department of Physics and Astronomy, University of Texas at San Antonio, a joint professor at the Texas Center for Superconductivity at the University of Houston (TcSUH), the “Thousand Talent Program - B00 professor at Tsinghua University, and the fellow of the American Ceramics Society. He received his PhD degree in solid state science (Materials) from the Pennsylvania State University in 1994.