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CHEMICAL PROBLEMS 2020 no. 1 (18) ISSN 2221-8688
9
UDC 621.352.6, 535.375.54, 544.6.076.2
IN-SITU RAMAN SPECTROSCOPY STUDIES OF OXYGEN SPILLOVER AT SOLID
OXIDE FUEL CELL ANODES
G.M. Eliseeva1, I.N. Burmistrov1,2, D.A. Agarkov1,2, A.A. Gamova1, I.V. Ionov3, M.N. Levin4, A.A. Solovyev3, I.I. Tartakovskii1,5, V.V. Kharton1, S.I. Bredikhin1,2
institute of Solid State Physics RAS, 142432, Russia, Moscow region, Chernogolovka, Academician Osipyan str., 2 2Moscow Institute of Physics and Technology, 141700, Russia, Moscow region, Dolgoprudniy, Institutskiy per., 9 3Institute of High Current Electronics, Siberian Branch RAS, 634055, Russia, Tomsk, Akademichesky Ave., 2/3 4EFKO Group, Biruch Innovation Center, 309927, Russia, Belgorod region, Malobykovo 5National Research University Higher School of Economics, 101000, Russian, Moscow, Myasnitskaya Str. 20, e-mail: agarkov@issp.ac.ru
Received 28.05.2019
Abstract: The present work is centered on the combined studies of anodic processes in electrode-supported solid oxide fuel cells (SOFCs) usingin-situ Raman spectroscopic and electrochemical measurements. A possibility to perform direct spectroscopic measurements at the inner electrolyte | anode interface in the case of anode-supported SOFC with polycrystalline thin-film zirconia solid electrolyte was experimentally demonstrated. The application of anode-supported SOFC architecture makes also it possible to significantly extend the range of operating temperatures and current densities across the electrochemical cell. The results confirma direct incorporation of oxygen anions transported through zirconia electrolyte into the ceria-based anode sublayer, with subsequent fuel oxidation at the cermet anode.
Keywords: solid oxide fuel cell, in-situ Raman spectroscopy, anode-supported SOFC, optical transparency, inner interface. D01:10.32737/2221-8688-2020-1-9-19
Introduction
Solid oxide fuel cells (SOFCs) transform chemical energy of hydrogen [1] and hydrocarbon fuels [2] into the electrical and heat energy [3].The SOFC efficiency is highercompared to other power generation technologies [4], especially in the case of hybrid systems where SOFCs are combined with gas turbines [5].The SOFC efficiency depends, however, on an electrode optimization level and degradation processes[6]. The in-situ spectroscopic analyses of SOFC components under operating conditions have a key importance for electrode optimization. In particular, one important challenge is to understand microscopic mechanisms of the anodic fuel oxidation in order to optimize anode architecture on the
basis of direct experimental data. Conventional techniques of the structural and elemental analyses (e.g., scanning electron microscopy, SEM, energy-dispersive X-ray spectroscopy, EDX, X-ray diffraction, XRD, X-ray photoelectron spectroscopy, XPS) cannot be used for this goal due to severe working conditions of SOFCs, namely operating temperatures from 550 to 900oC [7], high current densities up to 3-4 A/cm2 [8-9], chemically aggressive atmospheres, and separated gas chambers with fuel and oxidant mixtures.On the contrary, Raman spectroscopy provides possibilities for thein-situ studies of operating SOFCs [10-11]. Moreover,very large databases of Raman spectra exist in the literature for different types of chemical
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CHEMICAL PROBLEMS 2020 no. 1 (18)
reactions and materials, including those used in SOFCs [12-13]. Attractive applications of Raman spectroscopy for the in-situ studies of operating solid oxide fuel cells include so-called "spectro-chronopotentiometry"
(correlation of the electrochemical performance with spectral data) [14], monitoring of Ag-Cr interactions at the SOFC cathode [15], assessment of mechanical stresses [16-17], and surface mapping [18]. At the same time, most literature data were only collected from the outer surfaces of electrodes or solid electrolytes due to the low penetration depth of light.
This work presents the results obtained on model SOFCs with a novel geometry, based onan optically transparent thin-film polycrystalline solid electrolyte membrane made of stabilized cubic zirconia. The membrane transparency in combination
withring-shaped cathode enables to pass the laser excitation beam across solid electrolyte and to collect scattered radiation from the inner "anode | electrolyte" interface, simultaneously measuring the current vs. voltage dependencies and impedance spectra of the electrochemical cell. Similar in-situ approach proposed in our previous works [1923] was based on the use of zirconia single crystals as the membrane of electrolyte-supported cells (ESCs). However, the ESC geometry substantially limits the range of working temperatures and currents due to high ohmic resistance of the thick (~ 250 p,m) zirconia membrane; its thickness also leads to the dominance of cubic zirconia contribution visible in the Raman spectra. In order to solve these problems, thin anode-supported zirconia films were magnetron-sputtered and tested in the present work.
Experimental methods and materials
Cell fabrication and characterization membrane of yttria-stabilized zirconia (YSZ) The geometry of model anode- is illustrated in Fig. 1. supported cells with the thin-film electrolyte
YSZ thin film GDC thin film
/ * / a ■ ■ ■
Anode support
MM
1 2
Figure 1. Architecture(left) and photograph (right) of anode-supported SOFCs developed in this work. Red arrow shows laser beam; black layer at the top of electrochemical cell corresponds to the ring-shaped cathode.
The commercial anode supports made of NiO-YSZ composite were supplied by SOFCMAN (China). Thin anode sublayers of 10 mol.% gadolinia-doped ceria (10GDC) were deposited onto the porous anode supports by magnetron sputtering at the Institute of High Current Electronics SB RAS. In standard SOFCs, such sublayers are necessary to facilitate anodic reactions and to suppress
carbon deposition due to high catalytic activity and mixed ionic-electronic conductivity of reduced ceria. In the case of Raman spectroscopy, these sublayers play an additional role, indicating local overpotential as their spectra lines exhibit a significant dependence on the oxygen nonstoichiometry of GDC; the related phenomena are discussed below. Then gas-tight membrane films of
8YSZ (92 mol% ZrO2 + 8 mol% Y2O3) solid electrolyte with the thickness of 3-5 p,m were magnetron-sputtered onto 10GDC sublayers. The deposition was performed using two 85/15 at.%Zr/Y targets (size of 300x100x6 mm3, distance from the anode support of 10 cm)in Ar-O2 mixture at 300oC and operating total pressure of 0.3 Pa. Subsequent screen-printing
of the ring-shaped cathodes made of lanthanum-strontium manganite (LSM), was similar to that in our previous reports [24-27]. Fig.2 presents two SEM micrographs illustrating high quality of the solid-electrolyte films and specific morphology of the electrode layers (LSM cathode is at the top, NiO-YSZ anode is at the bottom).
Figure 2. SEM images showing cross-sections of the model SOFCs with different magnifications. The cathode layer is at the top.
The optical transparency of polycrystalline 8YSZ films was studied using a Specord M40 spectrometer (Carl Zeiss Jena, Germany) in a wavelength range from 200 to 900 nm. These measurements were performed for the films with thicknesses of 1.6, 3.0 and
4.7 p,m, sputtered onto single-crystal 8YSZ supports (300 p,m). The magnetron sputtering conditions were identical to those used in the course of SOFC fabrication. As an illustration, Fig.3 displays cross-section of the sample with 4.7 p,m thickness, finally annealed at 1100oC.
10 um
Figure 3.Cross-section of 8YSZ sample comprising a 4.7 p,m thick film deposited onto single crystal of the same composition, for the optical transparency studies.
The microstructure of individual Electrochemical measurements were
materials and electrochemical cells was performed using Gamry Reference 3000
characterized by scanning electron microscopy potentiostat/galvanostat/ZRA. (SEM) using a LEO Supra 50VP and a Dual Setup for in-situ Raman studies and
Beam VERSA 3DHighVac (FEI) instruments. electrochemical measurements
The in-situ Raman studies of SOFC anodes were carried out using a combined experimental setup consisting of optical and electrochemical blocks. The main part of this
setup, except for potentiostat and temperature controller connected to the measuring cell, is shown in Fig. 4.
microscope
laser 532 nm
laser cleaning system polarizer shutter
monochromator
CCD-camera
sample holder
f = 95
crossed slot movable prism
Figure 4. Principal scheme of the combined experimental setup for in-situ Raman studies and electrochemical measurements.
The high-temperature holder made of a single-crystal sapphire tube (produced byISSP RAS) is fixed on a computer-controlled mechanical system (Avesta-Project, Russia) for precise placing of the sample; a buttontype SOFC sample is sealed onto one end of this tube. The gas system comprising a set of mass-flow controllers (Bronkhorst, Netherlands) makes it possible to supply mixtures ofH2, N2 and CO into the anode chamber. The optical part of the setup consists of an excitation radiation sub-system and an optical sub-system for the scattered radiation detection. A multimode semiconductor green laser (wavelength of 532 nm; power output of 20 mW), with a 532 nm spectroscopic filter and a polarizer,was selected as a source of the excitation radiation. A mechanical iris-type shutter placed on the laser beam trajectory was
used to collect background spectra for their subtraction from the final Raman spectra. All the lenses used for the setup were supplied by the Lytkarino Factory of Optical Glass (Russia). A movable prism-type mirror was used to direct scattered radiation to a microscope with a computer-controlled CCDcamera (ToupView, China) during tuning of the sample position. A pair of lenses (focus distances of 95 and 190 mm) directs the scattered radiation onto the edge filter and then onto a hole of a diffraction grating spectrometer (MDR-12, LOMO, Russia); finally the spectra were collected using a liquid nitrogen-cooled CCD-camera LN/CCD-1340/400-EHRB/1 produced by Roper Scientific (USA) with the size of 1340x400 pixels.
Results and discussion
The data on optical transparency of the polycrystalline 8YSZ films sputtered onto 300 pm-thick 8YSZ single crystals are summarized in Figure 5.
The black line corresponding to the single-crystal support is almost constant, close
to 75% in thestudied wavelength range. The deposition of polycrystalline thin films leads to a progressive decrease in the transmittance at large wavenumbers. As expected, this effect associated with extended defects in the polycrystalline films becomes more
pronounced when the film thickness increases. Nonetheless, the films remain optically-transparent in the wavelength range used for the Raman studies, marked by black rectangle in Fig. 5. and should not noticeably affect collected Raman spectra. One should also mention that the transmittance of polycrystalline filmsexhibits periodic
dependenceon the wavenumbers due to lightinterference.The average period of these dependences can be used to estimate refractive index,varying from (2.4±0.2) for the film thickness of 4.7 pmup to (2.5±0.2) for 1.6 p,m. Such values well correspond to the refractive indexes of stabilized zirconia [28-30].
Figure 5. Transmittance spectra of magnetron-sputtered 8YSZ layers compared to that of 8YSZ single-crystal support. The film thicknesses are given in the legend.
Figures 6 and 7 illustrate electrochemical SOFC with thin-film 8YSZ membrane performance of the model anode-supported supported by cermet anode.
Figure 6. Voltage and power vs. current density dependencies of the model SOFCs for two different fuel mixture compositions (vol.%): 97% H2 - 3% H2O and 48.5% H2 - 48.5% N2 - 3% H2O. The total fuel mixture flow rate was 600 ml/min.
Figure 7. Impedance spectra of the model anode-supported SOFC for two different fuel mixture compositions (vol.%): 97% H2 -3% H2O and 48.5% H2 - 48.5% N2 - 3% H2O, under open circuit conditions (Iioad = 0) and at the current density of 0.56 A/cm2. The total fuel mixture flow rate was 600 ml/min.
The open circuit voltage (OCV) for humidified hydrogen is higher than 1 V, which gives a direct evidence of satisfactory gas tightness of the solid electrolyte membrane and sealing. The maximum power density is higher than 0.6 W/cm2, acceptable for practical applications. The ohmic resistance of the cell is lower than 0.2 Ohm*cm2. Decreasing hydrogen pressure leads to a modest decrease of low-frequency (10-1-1 Hz) contribution in the impedance spectra, Fig.7; the intermediate-frequency contribution observed at 101-102 Hz tends to slightly increase, probably due to partial surface oxidation of nickel in the cermet anode. Note that the low-frequency arc diminishes at high current densities.
Raman spectracollected from the inner GDC | electrolyte interface of the anode-supported SOFC at room temperature and at 750oC are presented in Figures 8a and 8b, respectively.
One can observe several overlapped peaks associated with zirconium and cerium
oxides. The most intensive of them is attributed to ceria; zirconia signals are much lower compared to the electrolyte-supported cells [21,23]. The NiO peaks could also be separated from the room-temperature spectrum of oxidized anode, but their intensity is too low. Furthermore, due to relatively high operating temperature of SOFCs, the intensity of thermal radiation becomes excessive. In order to improve the measurement statistics, cyclic accumulation of the signal was used and the subtraction of the heated body radiation spectrum was performed for each cycle.One example of the resultant spectrum is shown in Figure 8b. Such spectra were then normalized to the temperature-frequency factor (Fig. 8c) and used for deconvolution by Gaussian lines (Fig.8d). The latter procedure was necessary in order to suppress the errors associated with partial overlapping with the zirconia peaks. The deconvolution makes it possible to separate the Ce-O-Ce peak (~460 cm-1) sensitive to the local oxygen stoichiometry.
a
|t 25°C
1 \ — c©o2
Zr02 11 \ Zr°-
/A!
A/
c
200 300 400 500 600 700
200 300 400 500 600 700 Raman shift, cm 1
b
H—i—i—i—i—i—i—i—i—i—i—•-
750°C, 97% H, + 3% H O, 0 mA
200 300 400 500 600 700
750°C, 97% H2 + 3% H20, 0 mA
-Experimental data
— -Fit Peak 1
■ Fit Peak 2 "
Cumulative Fit Peak
d
300 400 500
Raman shift, cm1
Figure 8. Raman spectra from the inner "anode | electrolyte" interface at room temperature (a) and at 750oC (b). The inset in (a) displayscharacteristic peak with the shift of 460 cm-1 in the spectra of doped ceria, which is extremely sensitive to oxygen stoichiometry variations [31-32]. The high-temperature Raman spectrum was collected for 100 measuring cycles (1 second each). The parts (c) and (d) show the high-temperature spectrum normalized to the temperature-frequency factor and deconvolution of this spectrum by Gaussian lines, respectively.
The dependences of the extracted ~460 cm-1 peak intensity on fuel mixture composition and current load are displayed in Fig.9.
Raman shift, cm'1 Raman shift, cm'1
Figure 9. Intensity of the Ce-O-Cespectral line vs. fuel gas mixture composition (a) and current (b).
As expected, the intensity of the Ce-O-Ce peak potential of oxygen at the anode, determining is strongly dependent on the electrochemical oxygen nonstoichiometry of doped ceria at a
fixed temperature. Namely, this signal becomes higher when the content of oxygen in the crystal lattice increases as photons scatter more frequently on the phonons corresponding to symmetric oscillations of Cecations between oxygen anions. As a result, the peak intensity decreases with increasing hydrogen partial pressure and increases with increasing anodic current density. This behavior is similar
to that observed for the electrolyte-supported SOFCs with ceria-containing anodes [23]. It indicates a direct incorporation of oxygen anions transported across zirconia membraneinto the GDC lattice with subsequent fuel oxidation, confirming the so-called oxygen spillover mechanism of the anodic process.
Conclusions
The combined studies of electrode processes in the anode-supported solid oxide fuel cells using in-situ Raman spectroscopic and electrochemical measurements, were performed. The possibility of direct spectroscopic measurements at the inner electrolyte | anode interface in the case of polycrystalline thin-film zirconia solid electrolyte was experimentally demonstrated by the optical transparency tests and Raman
investigations. The use of anode-supported SOFC architecture makes also it possible to significantly extend the range of operating temperatures and current densities compared to the electrolyte-supported cells. The results confirm that oxygen anions are directly transported fromYSZ electrolyte membrane into the ceria-based anode sublayer, providing an evidence in favor of the oxygen spillover mechanism.
Acknowledgements
This study was financially supported by Russian Scientific Foundation, grant 17-7930071 "Scientifically grounded optimization of power and mass-dimensional characteristics of planar SOFC stacks and development of fuel processor for highly-efficient transport and stationary power plants ". Study was supported by the Ministry of Science and Higher Education of Russian Federation, unique identifier of contract RFMEFI60819X0279 "Development of stacks of anode-supported solid oxide fuel cells for highly-efficient generator of hydrogen energy" in the part of development of model anode-supported SOFC fabrication technology.
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IN-SITU ИССЛЕДОВАНИЯ ТРАНСПОРТА КИСЛОРОДА В АНОДАХ ТВЕРДООКСИДНЫХ ТОПЛИВНЫХ ЭЛЕМЕНТОВ МЕТОДОМ СПЕКТРОСКОПИИ КОМБИНАЦИОННОГО РАССЕЯНИЯ СВЕТА
Г.М. Елисеева1, И.Н. Бурмистров1'2, Д.А. Агарков1'2, А.А. Гамова1, И.В. Ионовв3, М.Н. Левин4, А.А. Соловьев?, И.И. Тартаковский1'5, В.В. Хартон1, С.И. Бредихин1'2
1 Федеральное государственное бюджетное учреждение науки Институт физики твердого тела
Российской академии наук (ИФТТ РАН), 142432, Россия, Московская обл., г. Черноголовка, ул. Академика Осипьяна, д. 2 2Федеральное государственное автономное образовательное учреждение высшего образования «Московский физико-технический институт (национальный исследовательский университет)» (МФТИ), 141700, Россия, Московская обл., г. Долгопрудный, Институтский пер., д. 9 3Федеральное государственное бюджетное учреждение науки Институт сильноточной электроники Сибирского отделения Российской академии наук (ИСЭ СО РАН), 634055, Россия, г. Томск, Академический пр-кт, д. 2/3
4ГК ЭФКО, Инновационный центр Бирюч, 309927, Россия, Белогородская обл., с. Малобыково 5Федеральное государственное автономное образовательное учреждение высшего образования «Национальный исследовательский университет «Высшая школа экономики» (НИУ ВШЭ), 101000, Россия, г. Москва, ул. Мясницкая, д. 20 e-mail: agarkov@issp. ac. ru
Данная работа сфокусирована на комбинированных исследованиях анодных процессов в твердооксидных топливных элементах (ТОТЭ) с поддерживающим электродом методом in-situ спектроскопии комбинационного рассеяния света и при помощи электрохимических измерений. Экспериментально показана возможность проведения прямых спектроскопических исследований с внутреннего интерфейса электролит | анод в случае ТОТЭ с поддерживающей анодной
подложкой и твердым электролитом на базе тонкопленочного поликристаллического стабилизированного диоксида циркония. Применение архитектуры ТОТЭ с поддерживающим анодом также дало возможность существенно расширить диапазон рабочих температур и плотности токовых нагрузок через электрохимическую ячейку. Полученные результаты подтверждают прямое встраивание анионов кислорода, транспортируемых через циркониевый электролит в анодный подслой на базе диоксида церия, с последующим окислением топлива на керметном аноде.
Ключевые слова: твердооксидный топливный элемент, т^Шспектроскопия комбинационного рассеяния света, ТОТЭ с поддерживающим анодом, оптическая прозрачность, внутренний интерфейс.
RAMANSPEKTROSKOPiYASIMETODUÎLd BORKOKSÎDLÎ YANACAQ ELEMENTLdRIN ANODLARINDA OKSiGENiN TRANSPORTUNUN IN-SiTU TBDQLqL
Q.M. Yeliseyeva1, i.N. Burmistrov12, D.A. Aqarkov1'2, A.A. Qamova1, i.V. ionov3, M.N. Levin4, A.A. Solovyov3 J.L Tartakovski1'5, V. V. Xarton1, S.i. Bredixin1'2
1Bark cisimlarin fizikasi institutu 142432, Rusiya, Çernoqolovka qah., akad. Osipyan kûç., 2
2Moskva fizika-texniki institutu 141700, Rusiya, Dolqoprudniy qah., institut döngasi, ev. 9 3Daqiq elektronika institutu 634055, Rusiya, Tomsk qah., Akademiçeskiy pr., 2/3
4Biryuç innovasiya markazi 309927, Rusiya, Belqorod vilayati, Malobikovo k. 5 "Ali iqtisad maktabi" Milli tadqiqat universiteti 101000, Rusiya, Myasnitskaya kûç., 20, e-mail: agarkov@,issp.ac.ru
Taqdim olunan maqala in-situ Raman spektroskopiyasi metodu ila barkoksidli yanacaq elementlarinda kômakçi elektrod vasitasi ila anod proseslarin tadqiqina hasr olunub. Tacrûbalar göstarir ki, barkoksidli yanacaq elementlarinda elektrolit-anod daxili interfeysdan birbaça spektrockopik tadqiqat aparmaq mûmkûndûr.
Açar sözfor: barkoksidli yanacaq elementi, in-situ Raman spektroskopiyasi, kômakçi anod, optik çaffafliq, daxili interfeys
