- Email: [email protected]
SOEC oxygen electrodes
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Increased performance by use of a mixed conducting buffer layer, terbia-doped ceria, for Nd2NiO4þd SOFC/SOEC oxygen electrodes Devaraj Ramasamy a, Narendar Nasani a,b, D. Pukazhselvan a, Duncan P. Fagg a,* a
Nanotechnology Research Group, Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810 193, Aveiro, Portugal b Centre for Materials for Electronics Technology (C-MET), Ministry of Electronics & Information Technology (MeitY), Govt. of India, Panchawati, Pune, 411 008, India
highlights Formation of Ce0.8R0.2O2-d þ 2 mol% Co buffer layers (R ¼ Gd, Tb) on YSZ electrolytes with Nd2NiO4þd electrodes. Dramatic decreases in polarization resistance, Rp, of up to an order of magnitude, in order, Tb << Gd < no buffer layer. Improved performance associated to increased ambipolar conductivity in the mixed conducting buffer layer. Improved performance maintained on anodic and cathodic polarization.
article info
abstract
Article history:
The present study is focused on the extension of electrochemically active sites in oxygen
Received 18 August 2019
electrodes for solid oxide fuel cells (SOFC) or electrolyser cells (SOEC), at the same time as
Received in revised form
preventing degradation, by the introduction of thin mixed ionic electronic conductive
27 September 2019
buffer layer materials between electrode and electrolyte. The performance of a Nd2NiO4
Accepted 3 October 2019
electrode material with YSZ electrolyte was studied with a mixed conducting, Ce0$8Tb0$2O2-
Available online 31 October 2019
d,
buffer layer material and compared to that of a more typical approach using a pre-
dominately ionic conducting buffer layer, Ce0.8Gd0.2O2-d. Each buffer layer and oxygen Keywords:
electrode were coated on YSZ electrolytes by spin-coating. The chemical reactivity of ox-
Solid oxide fuel cells
ygen electrode, buffer layer and electrolyte sintered powders were analysed by the X-ray
Solid oxide electrolyzer cells
diffraction technique. Scanning electron microscopy (SEM) and Energy-dispersive X-ray
Buffer layers
spectroscopy analysis (EDS) revealed that spin coated layers have good adhesion, are
Mixed conductors
continuous and offer very good chemical compatibility. Impedance spectroscopy under a
Ceria
range of applied DC bias was used to analyze the contribution of the buffer layer materials
Polarization resistance
under SOFC and SOEC operational modes. The cell with the buffer layer that offers mixed conduction significantly reduces the total electrode polarization resistance across the whole of the studied temperature range (600-850 C) by around an order of magnitude when compared to an otherwise identical cell without the buffer layer. In comparison, only half of this performance increase can be obtained for a predominantly ionic buffer layer. The critical nature of mixed conductivity in the buffer layer to maximize performance is
* Corresponding author. E-mail address: [email protected] (D.P. Fagg). https://doi.org/10.1016/j.ijhydene.2019.10.008 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
31467
further reinforced by comparison of current results to the literature performance of another mixed conducting buffer layer, Ce0.8Pr0.2O2-d. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Over last few decades, interest in solid oxide fuel cells (SOFC) and solid oxide electrolyser cells (SOEC) has steadily grown due to their implementation in environmental friendly clean energy cycles, where electrical energy can be generated from chemical fuels with high efficiency in SOFCs, or, inversely, converted into chemicals for the purpose of energy storage in SOECs. Typical chemical fuels can be that of hydrogen or that syngas and its derivatives [1,2]. The process of a SOEC corresponds to the reverse reaction of a SOFC and can be performed using very similar electrochemical devices [3e6]. It is opportune, therefore, to design adequate oxygen electrodes that may be compatible for stable operation in both applications. Recently mixed ionic electronic conducting (MIEC) oxides have been shown to be attractive for oxygen electrodes for solid oxide fuel cells (SOFC) and also solid oxide electrolyser cells (SOEC) due to promoting an improvement in electrode properties, related to extension of reaction sites from the traditional triple-phase-boundary region (TPB) to the whole electrode surface [7,8]. Among potential MIEC oxygen electrode materials, Nd2NiO4þd has attracted great interest due to its high oxygen diffusion and surface exchange coefficients [9,10]. Negatively, however, Nd2NiO4þd shows chemical interaction with the most commonly used electrolyte material yttria stabilized zirconia (YSZ) at the high temperatures that are needed for electrode preparation, leading to the creation of an undesirable Nd2Zr2O7 pyrochlore impurity phase of lowered performance [11]. In order to prevent chemical interaction at electrode/electrolyte interface and to avoid depleted electrochemical performance, many researchers have suggested the application of buffer/barrier layers between the electrolyte and the electrode layer. In this respect, to date, purely ionic conducting Y, Sm or Gd doped ceria-based materials have been typically employed as a buffer layers [12e19]. Although these efforts have been shown to be successful to avoid the formation of impurity phases, thereby, preventing performance degradation, these purely ionic buffer layers do not actively participate to improve electrode kinetics via enlargement of the TPB at the electrode/electrolyte interface [20e23]. In contrast, our group recently demonstrated how tailoring of the transport properties of ceria based buffer layers can not only provide the necessary phase stability against chemical interaction at the electrode/electrolyte interface but also can promote radical improvements in the electrochemical performance [24]. It was proven that using a buffer layer material offering high ambipolar conductivity, praseodymium doped cerium oxide (Ce0.8Pr0.2O2-d) with 2 mol% cobalt oxide sintering aid (CPO þ Co), the polarization resistance of a half cell consisting of Nd2NiO4þd oxygen electrode and YSZ electrolyte could be vastly improved over that of the use of a typical CGO buffer layer [24].
Generally, fluorite type cerium oxide systems can be tuned to offer high ionic conductivity by the introduction of accepter doping [25,26]. These materials also can be modified to introduce mixed conductivity in oxidizing conditions by substitution with multivalent cations like Tb and Pr [27e29]. This potential to obtain MIECs in oxidizing conditions is in addition to the intrinsic n-type electronic conductivity of these materials under reducing atmospheres that proceeds via small polaronic hopping due to partial reduction of Ce4þ to Ce3þ. In this respect, terbium-doped ceria has attracted attention due to its MIEC properties in both oxidizing and reducing atmospheres, where the p-type electronic conductivity in oxidizing conditions occurs via small polaron hopping arising from Tb3þ/Tb4þ [30]. Shuk et al. highlighted that the highest levels of ambipolar conductivity in Ce1-xTbxO2-d materials can be obtained in the compositional range x ¼ 0.15e0.25 [31]. More recently, Balaguer et al., reported that the addition of 2 mol% cobalt oxide sintering aid into the terbium-doped cerium oxide system can further boost its total and ambipolar conductivities [30]. Despite this promise, our previous study highlighted that one must be very careful with the processing conditions of this material as the transport properties of Ce0$8Tb0$2O2-d materials with 2 mol% cobalt oxide (CTO þ Co) are significantly influenced by sintering temperature [32]. The work showed that the CTO þ Co sample sintered at the minimum temperature required for densification, 900 C, offered the highest total conductivity and oxygen permeation flux. On the contrary, further increases in sintering temperature lead to degradation in electronic conductivity due to disruption of a Co rich grain boundary, where the cobalt additions are instead manifested as isolated grains [32,33]. Within this perspective, the present work focuses on the preparation of a new, thin CTO þ Co buffer/barrier layer between YSZ electrolyte and Nd2NiO4þd oxygen electrode by the spin coating technique and low temperature sintering. Microstructure and chemical interaction of the half cells fabricated with thin CTO þ Co buffer layer were systematically analysed. The effect of the CTO þ Co buffer layer was investigated in detail through electrochemical characterization under applied DC bias to represent prospective SOFC and SOEC modes of operation. Comparison is also made to an otherwise identical buffer layer of Ce0.8Gd0.2O2-d þ 2 mol% cobalt oxide (CGO þ Co) that has been previously shown to be a predominantly ionic conductor under these conditions [26].
Experimental Buffer layer material terbium-doped cerium oxide (Ce0$8Tb0$2O2-d) nano powders (CTO) were synthesised hydrothermally, using Ce(NO3)3$6H2O, (Aldrich, 99%) with terbium (III)
31468
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
nitrate pentahydrate (Tb(NO3)3$5H2O, 99.9% pure, Aldrich), as precursors. Stoichiometric amounts of these precursors were independently dissolved in distilled water, mixed, and coprecipitated with ammonium hydroxide at pH ~10. The precipitated gel was heat treated at the temperature of 250 C for 5 h with 5 C/min ramp rates in heating and cooling in a sealed Teflon-lined steel autoclave. The end solution containing CTO nano powder was washed thoroughly using deionised water and subsequently dried in air at the temperature of 70 C for 12 h. The preparation of Ce0.8Gd0.2O2-d (CGO) nano powder is described elsewhere [24]. The resultant nano powders were subjected to ball milling with a slow rotation velocity of 150 rpm for the time period of 5 h to fracture weak agglomerates. The sintering aid at a concentration of 2 mol% was added into the CTO and CGO nano powders, in the form of an aqueous solution of nitrate, Co(NO3)2$6H2O (Aldrich >98%), followed by ultrasonication for 1 h and drying. The resultant powders were dry ground for further use using an agate mortar and pestle. The oxygen electrode material, Nd2NiO4þd, was synthesised using a citrate route, as previously described [24]. Electrochemical cells were prepared by sequentially spin coating a buffer layer and the oxygen electrode on top of 8 mol % yttria stabilized-zirconia electrolyte pellets, which had been pre-densified. A control sample of just the oxygen electrode spin coated on the YSZ electrolyte was also prepared under the same conditions. The initial slurry suspensions were prepared using absolute ethanol as solvent, and polyvinylpyrrolidone (PVP) as binder with the respective ceramic powders by ball milling at a rotational speed of 350 rpm for the 2 h followed by ultrasonication for 1 h. Spin coating conditions used a rotation speed of 3000 rpm for 30 s. The prepared CTO þ Co or CGO þ Co buffer layers were sintered at 900 C for 5 h before the Nd2NiO4þd electrode deposition on top of each buffer layer. The complete Nd2NiO4þd/CTO þ Co/YSZ half cell was subsequently co-sintered at the temperature of 900 C using a dwell time of 10 h. To prepare three probe electrochemical cells, porous Pt paint counter electrodes were brush painted on the reverse face of the electrolyte with an identical area and symmetrical to the working electrode. A Pt reference electrode was applied as a ring around the working electrode. The distance between the reference and working electrode was > 3 times the electrolyte thickness, to provide negligible interference of current fluxes at the reference electrode [34]. All Pt electrodes were sintered at 850 C for a dwell time of 10 min. Room temperature powder XRD patterns were recorded to assess the phase purity of CTO and CGO powders using a Rigaku Geigerflex diffractometer and CuKa radiation. In addition, X-ray diffraction was also used to provide information on the chemical stability of each buffer layer with the respective Nd2NiO4þd electrode and YSZ electrolyte materials by intimately mixing these powders in a 50:50 wt% ratio followed by calcination at 900 C for 10 h conditions that mirror those used in cell preparation. The microstructure and morphology of the cells were analysed by SEM (model Bruker. QuantaxGermany), coupled with chemical analysis by EDS. A.C. impedance spectroscopy was performed using a frequency response analyser, Electrochemie-Autolab (PGSTAT302N) with measurements made under an air
atmosphere in the temperature range from 850 C to 600 C, in the direction of decreasing temperature. Measurements were made in the frequency range 1 MHz - 0.01 Hz, with signal amplitude of 100 mV. Impedance spectra were deconvoluted using ZView software. Polarization was performed under applied DC bias in potentiostatic mode to yield information in both SOFC and SOEC operation modes in the applied potential range 0.3 V to þ0.3 V. Intervals of 0.05 V were used with a dwell time of 30 min at each potential step to achieve stabilisation. Zero applied DC bias conditions were measured before and after the anodic branch measurement as a method of ensuring that permanent changes in behaviour had not occurred. Subsequently, the cathodic branch was measured in a similar way.
Results and discussion Fig. 1 exhibits the XRD patterns of CTO powder with Coadditions that had been hydrothermally prepared. The results show that the prepared powder is single phase, with the absence of impurities at the resolution of the XRD technique. In order to check the chemical stability of the buffer layer, intimate mixtures (50:50 wt%) of the CTO þ Co buffer layer with the YSZ electrolyte and the CTO þ Co buffer with the Nd2NiO4þd electrode were prepared, pressed into pellets and heated at the temperature of 900 C for a dwell time of 10 h. The obtained results show the absence of reaction products with only the desired phase mixtures exhibited. The results of the CGO þ Co analogue can be found elsewhere, showing similar purity and compatibility [24]. This analysis indicates that cobalt containing CTO and CGO can serve as compatible buffer layer/barrier layers to prevent interaction between Nd2NiO4þd electrodes and YSZ electrolytes under these preparation conditions. Micrographs of Nd2NiO4 electrode deposited on YSZ electrolytes using a CTO þ Co buffer layer are shown in Fig. 2. The
Fig. 1 e XRD patterns of Terbium-doped Ceria (CTO þ Co) buffer layer and (50:50 wt%) intimate mixtures of this buffer layer with Nd2NiO4þd electrode and YSZ electrolyte fired at 900 C for 10 h. △ Nd2NiO4, ▽ YSZ, ; CTO þ Co.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
31469
Fig. 2 e Microstructure Cross-Section of CTO þ Co buffer layer deposited on electrolyte/electrode assemblies.
SEM results show a dense, thin and uniform buffer layer, of approximately 0.5 mm thickness, that is well adhered to the substrate, YSZ. In all cases the Nd2NiO4 electrode thickness is shown to be approximately 2 mm and well adhered to the support. Fig. 3 presents EDS compositional mapping of the Nd2NiO4/ CTO þ Co/YSZ assembly, highlighting, clear separation of each layer of the architecture, with no notable cation interdiffusion; a result that underscores successful electrolyte/ buffer layer/electrode fabrication by the spin coating technique under the present preparation conditions. Similar results for the CGO þ Co buffer layer can be found elsewhere [24].
Fig. 4 e Example impedance spectra of samples with CTO þ Co buffer layer, CGO þ Co buffer layer and no buffer layer (BL). Measurement made at zero applied DC bias. Ohmic offset, R1, has been subtracted.
All measured impedance spectra show an offset at high frequency along the real Z0 axis, R1, followed by overlapping distributed responses at lower frequencies. Fig. 4 shows example impedance spectra of the cell with and without the CTO þ Co buffer layer measured at 850 C under zero applied DC bias. In order to further understand the improved electrode kinetics of MIEC CTO þ Co buffer layer, the results were also compared with the more typical CGO þ Co buffer layer
Fig. 3 e SEM-EDS analysis across electrode/electrolyte interfaces of an Nd2NiO4/CTO þ Co/YSZ assembly.
31470
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
prepared and measured at the same conditions [24]. Note in Fig. 4 the offset resistance (R1) has been subtracted to aid clarity. The spectra consist of a minor intermediate frequency response (R2) that is followed by a larger response in the lower frequency range (R3). The inset of Fig. 4 accentuates this feature in the case of the CTO þ Co spectra. All impedance spectra were fitted to the simple equivalent circuit shown in Fig. 4, where resistance (R) and constant phase (CPE) elements are associated in series to account each distributed response. The extracted fitting parameters consist of the resistance, R, the pseudo-capacitance, Q, and an additional parameter n. These terms provide the true capacitance values of each response by the following equation (1), C ¼ Rð1nÞ=n Q 1=n
(1)
All calculated capacitance values are noted to be greater than that of 105 Fcm2, suggesting that the observed responses R2 and R3 correspond to electrode phenomena [35e37]. The (R2) higher frequency response and the (R3) lower frequency response can, thus, be attributed to the total electrode polarization resistance, given by the equation Rp ¼ R2 þ R3. The sample with CTO þ Co buffer layer shows a
Table 1 e Activation energies of 1/R1 and 1/Rp for samples with CTO þ Co and CGO þ Co buffer layer or No buffer layer (BL). Measurements made at zero applied DC bias. Sample CTO þ Co CGO þ Co No BL
Activation energy Ea (eV) R1
Rp
0.88 ± 0.03 0.81 ± 0.03 0.86 ± 0.03
1.85 ± 0.03 1.90 ± 0.02 1.68 ± 0.03
significant reduction in the overall resistance Rp, in comparison to the others samples. For instance, Rp is shown to be lower for the sample with CTO þ Co buffer layer by approximately an order of magnitude, when compared to the sample with no buffer layer (2.64 cm2 down to 0.26 U cm2), under zero applied DC bias. With respect to the performance of the sample containing the CTO þ Co buffer layer in comparison to that of the CGO þ Co buffer layer, an improvement of around 4 times in Rp can be obtained for the former under equivalent conditions. Fig. 5a and b shows the Arrhenius behavior of the extracted ohmic resistance R1 and the total electrode resistance Rp for
Fig. 5 e Arrhenius behavior of polarization resistances a) R1 and b) Rp, for samples containing no buffer layer (BL) or buffer layers CTO þ Co and CGO þ Co.
Fig. 6 e Temperature dependence of the Capacitance a) and Relaxation frequency b), for samples of no buffer layer (BL) or buffer layers CTO þ Co and CGO þ Co. Filled symbols ¼ intermediate frequency response (R2), open symbols ¼ lower frequency response (R3).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
31471
Fig. 7 e Schematic of the oxygen reduction and incorporation reaction via potential electrode surface path (a), bulk path (b) and additional buffer layer parallel path (c). (i) Electrode (ii) mixed conducting layer (iii) electrolyte.
the samples with CTO þ Co and CGO þ Co buffer layers compared to that with no buffer layer. The respective active energies of these terms are documented in Table 1. The high frequency offset resistance (R1) shows an activation energy that is approximately 0.8 eV for samples with and without CTO þ Co buffer layer, corresponding well to that expected for ionic conductivity of the electrolyte material, YSZ [38], supporting the conclusion that the R1 term can be attributed to ohmic resistance, in agreement with previous literature [24]. With respect to electrode polarization resistance, Fig. 5b clearly underscores that the large reduction in Rp for the MIEC CTO þ Co buffer layer sample is maintained throughout the studied temperature range over the other samples tested. To understand this improved performance one should note that all samples have identical Nd2NiO4 electrode materials and similar electrode microstructures, with the only differences being in the presence and nature of the buffer layers. In this respect, previous literature shows the CTO þ Co material to have higher electronic (and ambipolar) conductivity than CGO þ Co over the measured temperature range, with these differences becoming larger as temperature decreases [32,33]. In agreement, Fig. 5b highlights a greater improvement in electrode performance for the CTO þ Co buffer layer sample over that of CGO þ Co with decreasing temperature. This observation reinforces the hypothesis outlined by the current authors in their previous work [24], that was based on the beneficial nature of a mixed conducting buffer layer due to providing additional paths for electrochemical reaction. Fig. 6 shows the capacitance values of the intermediate and low frequency responses, C2 and C3, and their corresponding relaxation frequency values for impedance spectra measured at zero applied DC bias for both samples. Fig. 6 clearly demonstrates that the capacitance values corresponding to the electrode polarization phenomena R2 and R3 are two orders of magnitude higher for the sample with the MIEC, CTO þ Co, buffer layer in comparison to that of the buffer layer free
sample or typical CGO þ Co buffer layer, Fig. 6a. This dramatic increase in specific capacitance of the polarization phenomena for the MIEC buffer layer containing sample is subsequently also mirrored in the corresponding relaxation frequencies of these polarization responses Fig. 6b. Significantly lower relaxation frequencies are noted in the cell containing the MIEC buffer layer CTO þ Co in comparison to the buffer layer free sample or ionically dominated CGO þ Co buffer layer. These observations closely correspond to those reported in our previous study where a different MIEC buffer layer, that of praseodymium doped cerium oxide with 2 mol% cobalt oxide sintering aid (CPO þ Co), was used [24].
Fig. 8 e Impedance spectra of sample with CTO þ Co buffer layer. Measurement made under anodic bias in air at 800 C.
31472
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
Fig. 9 e Total polarization resistance as a function of applied DC bias (anodic and cathodic modes of operation) and temperature. Measurements made in air, for cells with no buffer layer (BL) or CTO þ Co and CGO þ Co buffer layers. Fig. 9f contains information on a CPO þ Co buffer layer from Ref. [24]. Simulations provided in our previous publication highlighted that an improved electrode performance and a characteristically large increase in the associated capacitance of all electrode phenomena can be explained by the presence of an additional parallel path for electrochemical reaction that is introduced by the incorporation of a mixed conducting buffer layer [24]. This scenario is described by Fig. 7 that provides a schematic of potential competing paths for electrochemical reaction in which the mixed conducting buffer layer also contributes as a viable mechanistic route. From these results, we can understand that introducing a thin MIEC buffer layer, in this case CTO þ Co, between oxygen electrode and electrolyte can potentially lead to better electrode performance by providing an additional path at which the electrochemical reaction can occur. Taking into account
the above characteristic phenomena, ultimately one can tune the electrode kinetics improvement by tailoring the transport properties of thin intermediate buffer layer to maximize its electronic and ambipolar conductivity [24]., as demonstrated in the present case. In order to assess if this improvement in performance is maintained under both SOFC and SOEC operating conditions, the samples were polarised under anodic and cathodic modes of operation. Fig. 8 shows examples of typical impedance spectra obtained under anodic polarization for the Nd2NiO4 electrode with CTO þ Co buffer layer at 800 C in air atmosphere. Again, for clarity, the represented spectra are plotted after subtraction of the ohmic resistance (R1). The impedance spectra can be seen to change in magnitude upon polarization, while retaining their general shape comprising of two
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
overlapping semicircular responses. The impedance spectra for all samples and under cathodic polarization exhibit similar behaviour. For this reason, all impedance spectra were fitted to a similar equivalent circuit to that use under zero applied DC bias conditions, as shown in Fig. 4. The corresponding total polarization resistance, Rp, is shown in Fig. 9 as a function of applied DC potential in the range between þ0.3 V and 0.3 V with 0.05 V intervals, at different temperatures. In general, a decreased polarization resistance is observed with increasing applied potential for all modes of operation. This phenomenon was previously reported to be characteristic when alterations in the local chemical potential of oxygen at the electrode/electrolyte interface, induced by the applied potential, are not efficiently compensated from the gas phase due to the presence of limiting surface exchange [22,39]. Exceptions to this trend are noted at high temperatures and low applied potentials, and are classically explained to be due to increased oxygen surface exchange at higher temperatures that attenuates the presence of local gradients in oxygen chemical potential [22,39]. The most important result of Fig. 9 is that the sample with the CTO þ Co buffer layer is shown to continue to offer substantially lower polarization resistances compared to the other two samples under both cathodic and anodic polarization and at all measured temperatures. This result highlights the important role of the MIEC buffer layer, CTO þ Co, to increase oxygen electrode performance in both SOFC and SOEC modes of operation, a characteristic, thus, highly relevant for these two devices. Interesting, secondary information can also be obtained from Fig. 9, such as the fact that improvements in polarization resistance for the CGO þ Co buffer layer sample over that of the sample with no buffer layer can only be obtained at high temperatures. This result mirrors the result obtained at open circuit, Fig. 5b, and can be explained by previous literature that indicated the CGO þ Co material to only start to present a significant p-type electronic component to total conductivity at very high temperatures, while this contribution is rapidly depleted as temperature decreases [33]. In contrast, the new CTO þ Co buffer layer offers a considerable electronic contribution to total conductivity over the entire temperature range [33] and, thus, can promote improved electrode performance in all conditions. The critical nature of mixed conductivity to maximize performance is further reinforced by comparison of the current results to the performance of the CPO þ Co (Ce0.8Pr0.2O2-dþCo) buffer layer of our previous publication [24], Fig. 9f. This comparison demonstrates that both the current CTO þ Co buffer layer and the previously measured CPO þ Co buffer layer provide large performance improvements over that of the use of the predominately ionic buffer layer, CGO þ Co, as result of their much higher levels of mixed conductivity [32].
Conclusions The electrochemical performance of the oxygen electrode Nd2NiO4 has been investigated using a new terbium-doped ceria buffer layer for SOFC and SOEC applications that offers mixed ionic and electronic conduction. A thin film CTO buffer
31473
layer with cobalt additions was successfully prepared on the top of a pre-prepared YSZ electrolyte by a cost effective spin coating method and utilizing a low co-sintering temperature (900 C). Microscopy results show well separated and adhered layers that are free of observable cation interdiffusion. The YSZ/CTO þ Co/Nd2NiO4 half cell was characterized by EIS measurements under anodic and cathodic polarization and results were compared with samples of identical configuration but containing the typical buffer layer material CGO þ Co and no buffer layer. The sample with MIEC buffer layer CTO þ Co show vastly improved electrode behavior over that obtainable in an equivalent cell without a buffer layer or with a predominantly ionic CGO þ Co buffer layer, reinforcing that noted for the previously published mixed conducting buffer layer CPO þ Co. Such dramatic improvements in polarization resistances are, thus, suggested to be the existence of additional paths for electrochemical reaction due to the presence of a high level of mixed conductivity in the buffer layer.
Acknowledgements The authors acknowledge the Foundation for Science and Technology Portugal(FCT) project grants PTDC CTM-ENE/6319/ 2014, POCI-01-0145-FEDER-032241, UID/EMS/00481/2019-FCT, CENTRO-01-0145-FEDER-022083 - QREN, FEDER and COMPETE Portugal and the European Union for the financial support.
references
[1] Zhang L, Hu S, Zhu X, Yang W. Electrochemical reduction of CO2 in solid oxide electrolysis cells. J Energy Chem 2017;26:593e601. [2] Vialetto G, Noro M, Colbertaldo P, Rokni M. Enhancement of energy generation efficiency in industrial facilities by SOFCSOEC systems with additional hydrogen production. Int J Hydrogen Energy 2019;44:9608e20. [3] Eguchi K, Hatagishi T, Arai H. Power generation and steam electrolysis characteristics of an electrochemical cell with a zirconia- or ceria-based electrolyte. Solid State Ion 1996;2738:1245e9. [4] Schiller G, Ansar A, Lang M, Patz O. High temperature water electrolysis using metal supported Solid Oxide Electrolyser Cells (SOEC). J Appl Electrochem 2009:293e301. [5] Iora P, Chiesa P. High efficiency process for the production of pure oxygen based on solid oxide fuel cell e solid oxide electrolyzer technology. J Power Sources 2009;190:408e16. [6] Zhang W, Zheng Y, Yu B, Wang J, Cheng J. Electrochemical characterization and mechanism analysis of high temperature Co-electrolysis of CO2 and H2O in a solid oxide electrolysis cell. Int J Hydrogen Energy 2017;42:29911e20. [7] Lein H, Wiik K, Grande T. Thermal and chemical expansion of mixed conducting La0.5Sr0.5Fe1xCoxO3d materials. Solid State Ion 2006;177:1795e8. [8] Sunarso J, Baumann S, Serra JM, Meulenberg WA, Liu S, Lin YS, Diniz da Costa JC. Mixed Ionic e Electronic Conducting (MIEC) ceramic-based membranes for oxygen separation. J Membr Sci 2008;320:13e41. [9] Wan J, Goodenough J, Zhu J. Nd2xLaxNiO4þd, a mixed ionic/ electronic conductor with interstitial oxygen, as a cathode material. Solid State Ion 2007;178:281e6.
31474
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
[10] Chauveau F, Mougin J, Bassat JM, Mauvy F, Grenier JC. A new anode material for solid oxide electrolyser : the neodymium nickelate Nd2NiO4þd. J Power Sources 2010;195:744e9. ndez A, Vega-Castillo J, Mogni L, [11] Montenegro-Herna Caneiro A. Thermal stability of Ln2NiO4þd (Ln: La, Pr, Nd) and their chemical compatibility with YSZ and CGO solid electrolytes. Int J Hydrogen Energy 2011;36:15704e14. [12] Liu M, Shi D, Suo H, Ye S, Zhao Y, Zhu Y, Li Q, Wang L, Jihyun A, Zhou M. A simple MOD method to grow a single buffer layer of Ce0.8Gd0.2O1.9 (CGO) for coated conductors. Physica C 2009;469:230e3. [13] Zhang W, Yu B, Xu J. Investigation of single SOEC with BSCF anode and SDC barrier layer. Int J Hydrogen Energy 2012;37:837e42. [14] Mesguich D, Aymonier C, Bassat J, Mauvy F, You E, Watkins JJ. Low-temperature deposition of undoped ceria thin films in scCO2 as improved interlayers for IT-SOFC. Chem Mater 2011;23:5323e30. [15] Min Park Y, Kim H. Porous Gd-doped ceria barrier layer on solid oxide fuel cell with Sm0.5Sr0.5CoO3d Cathodes. Ceram Int 2013;39:2037e43. [16] Xie Z, Zhao H, Du Z, Chen T, Chen N. Electrical, chemical, and electrochemical properties of double perovskite oxides Sr2Mg1xNixMoO6d as anode materials for solid oxide fuel cells. J Phys Chem C 2014;118:18853e60. [17] Nguyen T, Kobayashi K, Honda T, Iimura Y, Kato K, Neghisi A, et al. Preparation and evaluation of doped ceria interlayer on supported stabilized zirconia electrolyte SOFCs by wet ceramic processes. Solid State Ion 2004;174:163e74. [18] Milcarek R, Wang K, Falkenstein-Smith R, Ahn J. Performance variation with SDC buffer layer thickness. Int J Hydrogen Energy 2016;41:9500e6. n-Ruiz A, Laguna-Bercero M, [19] Wain-Martin A, Mora ~ aga A, Slater P, Arriortua M. SOFC Campana R, Larran cathodic layers using wet powder spraying technique with self synthesized nanopowders. Int J Hydrogen Energy 2019;44:7555e63. [20] Bohac P, Orliukas A, Gauckler L. Lowering of the cathode overpotential of SOFC by electrolyte doping. Electroceramics 1994;IV(II):771e4. rez-Coll D, Aguadero A, Escudero MJ, Nu´n ~ ez P, Daza L. [21] Pe Optimization of the interface polarization of the La2NiO4based cathode working with the Ce1exSmxO2ed electrolyte system. J Power Sources 2008;178:151e62. rez-Coll D, Aguadero A, Escudero MJ, Daza L. Effect of DC [22] Pe current polarization on the electrochemical behaviour of La2NiO4þd and La3Ni2O7þd-based systems. J Power Sources 2009;192:2e13. [23] Fagg DP, Kharton VV, Frade JR. Transport in ceria electrolytes modified with sintering aids: effects on oxygen reduction kinetics. J Solid State Electrochem 2004;8:618e25. rez-Coll D, [24] Ramasamy D, Nasani N, Ana-Brandao D, Pe Fagg DP. Enhancing electrochemical performance by control of transport properties in buffer layers e solid oxide fuel/
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
electrolyser cells. Phys Chem Chem Phys: PCCP 2015;17:11527e39. Christie GM, Van Berkel FPF. Microstructure e ionic conductivity relationships in ceria-gadolinia electrolytes. Solid State Ion 1996:17e27. Fagg DP, Kharton VV, Frade JR. P-type electronic transport in Ce0.8Gd0.2O2d:the effect of transition metal oxide sintering aids. J Electroceram 2003;9:199e207. de Vries KJ, Meng G-Y. Phase formation and thermal stability of fcc (fluorite) Ce1xTbxO2d solid solutions. Mater Res Bull 1998;33:357e67. Ye F, Mori T, Ou DR, Takahashi M, Zou J, Drennan J. Compositional dependence of electrical conductivity of Ce1xTbxO2d (0x1). Renew Energy 2008;33:331e5. Fagg DP, Garcı´a-martin S, Kharton VV, Frade JR. Transport properties of fluorite-type Ce0.8Pr0.2O2-d: optimization via the use of cobalt oxide sintering aid. Chem Mater 2009;21:381e91. Balaguer M, Solı´s C, Serra M. Study of the transport properties of the mixed ionic electronic conductor Ce1xTbxO2-d þ Co (x ¼ 0 . 1 , 0 . 2) and evaluation as oxygen. Transport Membrane 2011;23:2333e43. Shuk P, Greenblatt M, Croft M. Hydrothermal synthesis and properties of mixed conducting Ce1-xTxO2-d solid solutions. Chem Mater 1999;11:473e9. mez-Herrero A, Kharton VV, Ramasamy D, Shaula AL, Go Fagg DP. Oxygen permeability of mixed-conducting Ce0.8Tb0.2O2d membranes: effects of ceramic microstructure and sintering temperature. J Membr Sci 2015;475:414e24. Ramasamy D, Shaula AL, Nasani N, Kovalevsky AV, Fagg DP. Comparative study of fluorite-type ceria-based Ce1-xLnxO2d (Ln¼Tb, Gd, and Pr) mixed ionic electronic conductors densified at low temperatures. J Mater Sci 2016;51:10293e300. Adler SB, Henderson BT, Wilson MA, Taylor DM, Richards RE. Reference electrode placement and seals in electrochemical oxygen generators. Solid State Ion 2000;134:35e42. Bauerle JE. Study of solid electrolyte polarization by a complex admittance method. J Phys Chem Solids 1969;30:2657e70. Nasani N, Ramasamy D, Antunes I, Perez J, Fagg DP. Electrochemical behaviour of Ni-BZO and Ni-BZY cermet anodes for Protonic Ceramic Fuel Cells (PCFCs) e a comparative study. Electrochim Acta 2015;154:387e96. Loureiro FJA, Yang T, Stroppa DG, Fagg DP. Pr2O2SO4eLa0.6Sr0.4Co0.2Fe0.8O3-d: a new category of composite cathode for intermediate temperature-solid oxide fuel cells. J Mater Chem 2015;3:12636e41. Gibson IR, Dransfield GP, Irvine JTS. Sinterability of commercial 8 mol% yttria-stabilized zirconia powders and the effect of sintered density on the ionic conductivity. J Mater Sci 1998;33:4297e305. Svenssona AM, Sunde S, Nisancioglu K. A mathematical model of the porous SOFC cathode. Solid State Ion 1996;86e88:1211e6.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Increased performance by use of a mixed conducting buffer layer, terbia-doped ceria, for Nd2NiO4þd SOFC/SOEC oxygen electrodes Devaraj Ramasamy a, Narendar Nasani a,b, D. Pukazhselvan a, Duncan P. Fagg a,* a
Nanotechnology Research Group, Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810 193, Aveiro, Portugal b Centre for Materials for Electronics Technology (C-MET), Ministry of Electronics & Information Technology (MeitY), Govt. of India, Panchawati, Pune, 411 008, India
highlights Formation of Ce0.8R0.2O2-d þ 2 mol% Co buffer layers (R ¼ Gd, Tb) on YSZ electrolytes with Nd2NiO4þd electrodes. Dramatic decreases in polarization resistance, Rp, of up to an order of magnitude, in order, Tb << Gd < no buffer layer. Improved performance associated to increased ambipolar conductivity in the mixed conducting buffer layer. Improved performance maintained on anodic and cathodic polarization.
article info
abstract
Article history:
The present study is focused on the extension of electrochemically active sites in oxygen
Received 18 August 2019
electrodes for solid oxide fuel cells (SOFC) or electrolyser cells (SOEC), at the same time as
Received in revised form
preventing degradation, by the introduction of thin mixed ionic electronic conductive
27 September 2019
buffer layer materials between electrode and electrolyte. The performance of a Nd2NiO4
Accepted 3 October 2019
electrode material with YSZ electrolyte was studied with a mixed conducting, Ce0$8Tb0$2O2-
Available online 31 October 2019
d,
buffer layer material and compared to that of a more typical approach using a pre-
dominately ionic conducting buffer layer, Ce0.8Gd0.2O2-d. Each buffer layer and oxygen Keywords:
electrode were coated on YSZ electrolytes by spin-coating. The chemical reactivity of ox-
Solid oxide fuel cells
ygen electrode, buffer layer and electrolyte sintered powders were analysed by the X-ray
Solid oxide electrolyzer cells
diffraction technique. Scanning electron microscopy (SEM) and Energy-dispersive X-ray
Buffer layers
spectroscopy analysis (EDS) revealed that spin coated layers have good adhesion, are
Mixed conductors
continuous and offer very good chemical compatibility. Impedance spectroscopy under a
Ceria
range of applied DC bias was used to analyze the contribution of the buffer layer materials
Polarization resistance
under SOFC and SOEC operational modes. The cell with the buffer layer that offers mixed conduction significantly reduces the total electrode polarization resistance across the whole of the studied temperature range (600-850 C) by around an order of magnitude when compared to an otherwise identical cell without the buffer layer. In comparison, only half of this performance increase can be obtained for a predominantly ionic buffer layer. The critical nature of mixed conductivity in the buffer layer to maximize performance is
* Corresponding author. E-mail address: [email protected] (D.P. Fagg). https://doi.org/10.1016/j.ijhydene.2019.10.008 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
31467
further reinforced by comparison of current results to the literature performance of another mixed conducting buffer layer, Ce0.8Pr0.2O2-d. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Over last few decades, interest in solid oxide fuel cells (SOFC) and solid oxide electrolyser cells (SOEC) has steadily grown due to their implementation in environmental friendly clean energy cycles, where electrical energy can be generated from chemical fuels with high efficiency in SOFCs, or, inversely, converted into chemicals for the purpose of energy storage in SOECs. Typical chemical fuels can be that of hydrogen or that syngas and its derivatives [1,2]. The process of a SOEC corresponds to the reverse reaction of a SOFC and can be performed using very similar electrochemical devices [3e6]. It is opportune, therefore, to design adequate oxygen electrodes that may be compatible for stable operation in both applications. Recently mixed ionic electronic conducting (MIEC) oxides have been shown to be attractive for oxygen electrodes for solid oxide fuel cells (SOFC) and also solid oxide electrolyser cells (SOEC) due to promoting an improvement in electrode properties, related to extension of reaction sites from the traditional triple-phase-boundary region (TPB) to the whole electrode surface [7,8]. Among potential MIEC oxygen electrode materials, Nd2NiO4þd has attracted great interest due to its high oxygen diffusion and surface exchange coefficients [9,10]. Negatively, however, Nd2NiO4þd shows chemical interaction with the most commonly used electrolyte material yttria stabilized zirconia (YSZ) at the high temperatures that are needed for electrode preparation, leading to the creation of an undesirable Nd2Zr2O7 pyrochlore impurity phase of lowered performance [11]. In order to prevent chemical interaction at electrode/electrolyte interface and to avoid depleted electrochemical performance, many researchers have suggested the application of buffer/barrier layers between the electrolyte and the electrode layer. In this respect, to date, purely ionic conducting Y, Sm or Gd doped ceria-based materials have been typically employed as a buffer layers [12e19]. Although these efforts have been shown to be successful to avoid the formation of impurity phases, thereby, preventing performance degradation, these purely ionic buffer layers do not actively participate to improve electrode kinetics via enlargement of the TPB at the electrode/electrolyte interface [20e23]. In contrast, our group recently demonstrated how tailoring of the transport properties of ceria based buffer layers can not only provide the necessary phase stability against chemical interaction at the electrode/electrolyte interface but also can promote radical improvements in the electrochemical performance [24]. It was proven that using a buffer layer material offering high ambipolar conductivity, praseodymium doped cerium oxide (Ce0.8Pr0.2O2-d) with 2 mol% cobalt oxide sintering aid (CPO þ Co), the polarization resistance of a half cell consisting of Nd2NiO4þd oxygen electrode and YSZ electrolyte could be vastly improved over that of the use of a typical CGO buffer layer [24].
Generally, fluorite type cerium oxide systems can be tuned to offer high ionic conductivity by the introduction of accepter doping [25,26]. These materials also can be modified to introduce mixed conductivity in oxidizing conditions by substitution with multivalent cations like Tb and Pr [27e29]. This potential to obtain MIECs in oxidizing conditions is in addition to the intrinsic n-type electronic conductivity of these materials under reducing atmospheres that proceeds via small polaronic hopping due to partial reduction of Ce4þ to Ce3þ. In this respect, terbium-doped ceria has attracted attention due to its MIEC properties in both oxidizing and reducing atmospheres, where the p-type electronic conductivity in oxidizing conditions occurs via small polaron hopping arising from Tb3þ/Tb4þ [30]. Shuk et al. highlighted that the highest levels of ambipolar conductivity in Ce1-xTbxO2-d materials can be obtained in the compositional range x ¼ 0.15e0.25 [31]. More recently, Balaguer et al., reported that the addition of 2 mol% cobalt oxide sintering aid into the terbium-doped cerium oxide system can further boost its total and ambipolar conductivities [30]. Despite this promise, our previous study highlighted that one must be very careful with the processing conditions of this material as the transport properties of Ce0$8Tb0$2O2-d materials with 2 mol% cobalt oxide (CTO þ Co) are significantly influenced by sintering temperature [32]. The work showed that the CTO þ Co sample sintered at the minimum temperature required for densification, 900 C, offered the highest total conductivity and oxygen permeation flux. On the contrary, further increases in sintering temperature lead to degradation in electronic conductivity due to disruption of a Co rich grain boundary, where the cobalt additions are instead manifested as isolated grains [32,33]. Within this perspective, the present work focuses on the preparation of a new, thin CTO þ Co buffer/barrier layer between YSZ electrolyte and Nd2NiO4þd oxygen electrode by the spin coating technique and low temperature sintering. Microstructure and chemical interaction of the half cells fabricated with thin CTO þ Co buffer layer were systematically analysed. The effect of the CTO þ Co buffer layer was investigated in detail through electrochemical characterization under applied DC bias to represent prospective SOFC and SOEC modes of operation. Comparison is also made to an otherwise identical buffer layer of Ce0.8Gd0.2O2-d þ 2 mol% cobalt oxide (CGO þ Co) that has been previously shown to be a predominantly ionic conductor under these conditions [26].
Experimental Buffer layer material terbium-doped cerium oxide (Ce0$8Tb0$2O2-d) nano powders (CTO) were synthesised hydrothermally, using Ce(NO3)3$6H2O, (Aldrich, 99%) with terbium (III)
31468
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
nitrate pentahydrate (Tb(NO3)3$5H2O, 99.9% pure, Aldrich), as precursors. Stoichiometric amounts of these precursors were independently dissolved in distilled water, mixed, and coprecipitated with ammonium hydroxide at pH ~10. The precipitated gel was heat treated at the temperature of 250 C for 5 h with 5 C/min ramp rates in heating and cooling in a sealed Teflon-lined steel autoclave. The end solution containing CTO nano powder was washed thoroughly using deionised water and subsequently dried in air at the temperature of 70 C for 12 h. The preparation of Ce0.8Gd0.2O2-d (CGO) nano powder is described elsewhere [24]. The resultant nano powders were subjected to ball milling with a slow rotation velocity of 150 rpm for the time period of 5 h to fracture weak agglomerates. The sintering aid at a concentration of 2 mol% was added into the CTO and CGO nano powders, in the form of an aqueous solution of nitrate, Co(NO3)2$6H2O (Aldrich >98%), followed by ultrasonication for 1 h and drying. The resultant powders were dry ground for further use using an agate mortar and pestle. The oxygen electrode material, Nd2NiO4þd, was synthesised using a citrate route, as previously described [24]. Electrochemical cells were prepared by sequentially spin coating a buffer layer and the oxygen electrode on top of 8 mol % yttria stabilized-zirconia electrolyte pellets, which had been pre-densified. A control sample of just the oxygen electrode spin coated on the YSZ electrolyte was also prepared under the same conditions. The initial slurry suspensions were prepared using absolute ethanol as solvent, and polyvinylpyrrolidone (PVP) as binder with the respective ceramic powders by ball milling at a rotational speed of 350 rpm for the 2 h followed by ultrasonication for 1 h. Spin coating conditions used a rotation speed of 3000 rpm for 30 s. The prepared CTO þ Co or CGO þ Co buffer layers were sintered at 900 C for 5 h before the Nd2NiO4þd electrode deposition on top of each buffer layer. The complete Nd2NiO4þd/CTO þ Co/YSZ half cell was subsequently co-sintered at the temperature of 900 C using a dwell time of 10 h. To prepare three probe electrochemical cells, porous Pt paint counter electrodes were brush painted on the reverse face of the electrolyte with an identical area and symmetrical to the working electrode. A Pt reference electrode was applied as a ring around the working electrode. The distance between the reference and working electrode was > 3 times the electrolyte thickness, to provide negligible interference of current fluxes at the reference electrode [34]. All Pt electrodes were sintered at 850 C for a dwell time of 10 min. Room temperature powder XRD patterns were recorded to assess the phase purity of CTO and CGO powders using a Rigaku Geigerflex diffractometer and CuKa radiation. In addition, X-ray diffraction was also used to provide information on the chemical stability of each buffer layer with the respective Nd2NiO4þd electrode and YSZ electrolyte materials by intimately mixing these powders in a 50:50 wt% ratio followed by calcination at 900 C for 10 h conditions that mirror those used in cell preparation. The microstructure and morphology of the cells were analysed by SEM (model Bruker. QuantaxGermany), coupled with chemical analysis by EDS. A.C. impedance spectroscopy was performed using a frequency response analyser, Electrochemie-Autolab (PGSTAT302N) with measurements made under an air
atmosphere in the temperature range from 850 C to 600 C, in the direction of decreasing temperature. Measurements were made in the frequency range 1 MHz - 0.01 Hz, with signal amplitude of 100 mV. Impedance spectra were deconvoluted using ZView software. Polarization was performed under applied DC bias in potentiostatic mode to yield information in both SOFC and SOEC operation modes in the applied potential range 0.3 V to þ0.3 V. Intervals of 0.05 V were used with a dwell time of 30 min at each potential step to achieve stabilisation. Zero applied DC bias conditions were measured before and after the anodic branch measurement as a method of ensuring that permanent changes in behaviour had not occurred. Subsequently, the cathodic branch was measured in a similar way.
Results and discussion Fig. 1 exhibits the XRD patterns of CTO powder with Coadditions that had been hydrothermally prepared. The results show that the prepared powder is single phase, with the absence of impurities at the resolution of the XRD technique. In order to check the chemical stability of the buffer layer, intimate mixtures (50:50 wt%) of the CTO þ Co buffer layer with the YSZ electrolyte and the CTO þ Co buffer with the Nd2NiO4þd electrode were prepared, pressed into pellets and heated at the temperature of 900 C for a dwell time of 10 h. The obtained results show the absence of reaction products with only the desired phase mixtures exhibited. The results of the CGO þ Co analogue can be found elsewhere, showing similar purity and compatibility [24]. This analysis indicates that cobalt containing CTO and CGO can serve as compatible buffer layer/barrier layers to prevent interaction between Nd2NiO4þd electrodes and YSZ electrolytes under these preparation conditions. Micrographs of Nd2NiO4 electrode deposited on YSZ electrolytes using a CTO þ Co buffer layer are shown in Fig. 2. The
Fig. 1 e XRD patterns of Terbium-doped Ceria (CTO þ Co) buffer layer and (50:50 wt%) intimate mixtures of this buffer layer with Nd2NiO4þd electrode and YSZ electrolyte fired at 900 C for 10 h. △ Nd2NiO4, ▽ YSZ, ; CTO þ Co.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
31469
Fig. 2 e Microstructure Cross-Section of CTO þ Co buffer layer deposited on electrolyte/electrode assemblies.
SEM results show a dense, thin and uniform buffer layer, of approximately 0.5 mm thickness, that is well adhered to the substrate, YSZ. In all cases the Nd2NiO4 electrode thickness is shown to be approximately 2 mm and well adhered to the support. Fig. 3 presents EDS compositional mapping of the Nd2NiO4/ CTO þ Co/YSZ assembly, highlighting, clear separation of each layer of the architecture, with no notable cation interdiffusion; a result that underscores successful electrolyte/ buffer layer/electrode fabrication by the spin coating technique under the present preparation conditions. Similar results for the CGO þ Co buffer layer can be found elsewhere [24].
Fig. 4 e Example impedance spectra of samples with CTO þ Co buffer layer, CGO þ Co buffer layer and no buffer layer (BL). Measurement made at zero applied DC bias. Ohmic offset, R1, has been subtracted.
All measured impedance spectra show an offset at high frequency along the real Z0 axis, R1, followed by overlapping distributed responses at lower frequencies. Fig. 4 shows example impedance spectra of the cell with and without the CTO þ Co buffer layer measured at 850 C under zero applied DC bias. In order to further understand the improved electrode kinetics of MIEC CTO þ Co buffer layer, the results were also compared with the more typical CGO þ Co buffer layer
Fig. 3 e SEM-EDS analysis across electrode/electrolyte interfaces of an Nd2NiO4/CTO þ Co/YSZ assembly.
31470
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
prepared and measured at the same conditions [24]. Note in Fig. 4 the offset resistance (R1) has been subtracted to aid clarity. The spectra consist of a minor intermediate frequency response (R2) that is followed by a larger response in the lower frequency range (R3). The inset of Fig. 4 accentuates this feature in the case of the CTO þ Co spectra. All impedance spectra were fitted to the simple equivalent circuit shown in Fig. 4, where resistance (R) and constant phase (CPE) elements are associated in series to account each distributed response. The extracted fitting parameters consist of the resistance, R, the pseudo-capacitance, Q, and an additional parameter n. These terms provide the true capacitance values of each response by the following equation (1), C ¼ Rð1nÞ=n Q 1=n
(1)
All calculated capacitance values are noted to be greater than that of 105 Fcm2, suggesting that the observed responses R2 and R3 correspond to electrode phenomena [35e37]. The (R2) higher frequency response and the (R3) lower frequency response can, thus, be attributed to the total electrode polarization resistance, given by the equation Rp ¼ R2 þ R3. The sample with CTO þ Co buffer layer shows a
Table 1 e Activation energies of 1/R1 and 1/Rp for samples with CTO þ Co and CGO þ Co buffer layer or No buffer layer (BL). Measurements made at zero applied DC bias. Sample CTO þ Co CGO þ Co No BL
Activation energy Ea (eV) R1
Rp
0.88 ± 0.03 0.81 ± 0.03 0.86 ± 0.03
1.85 ± 0.03 1.90 ± 0.02 1.68 ± 0.03
significant reduction in the overall resistance Rp, in comparison to the others samples. For instance, Rp is shown to be lower for the sample with CTO þ Co buffer layer by approximately an order of magnitude, when compared to the sample with no buffer layer (2.64 cm2 down to 0.26 U cm2), under zero applied DC bias. With respect to the performance of the sample containing the CTO þ Co buffer layer in comparison to that of the CGO þ Co buffer layer, an improvement of around 4 times in Rp can be obtained for the former under equivalent conditions. Fig. 5a and b shows the Arrhenius behavior of the extracted ohmic resistance R1 and the total electrode resistance Rp for
Fig. 5 e Arrhenius behavior of polarization resistances a) R1 and b) Rp, for samples containing no buffer layer (BL) or buffer layers CTO þ Co and CGO þ Co.
Fig. 6 e Temperature dependence of the Capacitance a) and Relaxation frequency b), for samples of no buffer layer (BL) or buffer layers CTO þ Co and CGO þ Co. Filled symbols ¼ intermediate frequency response (R2), open symbols ¼ lower frequency response (R3).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
31471
Fig. 7 e Schematic of the oxygen reduction and incorporation reaction via potential electrode surface path (a), bulk path (b) and additional buffer layer parallel path (c). (i) Electrode (ii) mixed conducting layer (iii) electrolyte.
the samples with CTO þ Co and CGO þ Co buffer layers compared to that with no buffer layer. The respective active energies of these terms are documented in Table 1. The high frequency offset resistance (R1) shows an activation energy that is approximately 0.8 eV for samples with and without CTO þ Co buffer layer, corresponding well to that expected for ionic conductivity of the electrolyte material, YSZ [38], supporting the conclusion that the R1 term can be attributed to ohmic resistance, in agreement with previous literature [24]. With respect to electrode polarization resistance, Fig. 5b clearly underscores that the large reduction in Rp for the MIEC CTO þ Co buffer layer sample is maintained throughout the studied temperature range over the other samples tested. To understand this improved performance one should note that all samples have identical Nd2NiO4 electrode materials and similar electrode microstructures, with the only differences being in the presence and nature of the buffer layers. In this respect, previous literature shows the CTO þ Co material to have higher electronic (and ambipolar) conductivity than CGO þ Co over the measured temperature range, with these differences becoming larger as temperature decreases [32,33]. In agreement, Fig. 5b highlights a greater improvement in electrode performance for the CTO þ Co buffer layer sample over that of CGO þ Co with decreasing temperature. This observation reinforces the hypothesis outlined by the current authors in their previous work [24], that was based on the beneficial nature of a mixed conducting buffer layer due to providing additional paths for electrochemical reaction. Fig. 6 shows the capacitance values of the intermediate and low frequency responses, C2 and C3, and their corresponding relaxation frequency values for impedance spectra measured at zero applied DC bias for both samples. Fig. 6 clearly demonstrates that the capacitance values corresponding to the electrode polarization phenomena R2 and R3 are two orders of magnitude higher for the sample with the MIEC, CTO þ Co, buffer layer in comparison to that of the buffer layer free
sample or typical CGO þ Co buffer layer, Fig. 6a. This dramatic increase in specific capacitance of the polarization phenomena for the MIEC buffer layer containing sample is subsequently also mirrored in the corresponding relaxation frequencies of these polarization responses Fig. 6b. Significantly lower relaxation frequencies are noted in the cell containing the MIEC buffer layer CTO þ Co in comparison to the buffer layer free sample or ionically dominated CGO þ Co buffer layer. These observations closely correspond to those reported in our previous study where a different MIEC buffer layer, that of praseodymium doped cerium oxide with 2 mol% cobalt oxide sintering aid (CPO þ Co), was used [24].
Fig. 8 e Impedance spectra of sample with CTO þ Co buffer layer. Measurement made under anodic bias in air at 800 C.
31472
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
Fig. 9 e Total polarization resistance as a function of applied DC bias (anodic and cathodic modes of operation) and temperature. Measurements made in air, for cells with no buffer layer (BL) or CTO þ Co and CGO þ Co buffer layers. Fig. 9f contains information on a CPO þ Co buffer layer from Ref. [24]. Simulations provided in our previous publication highlighted that an improved electrode performance and a characteristically large increase in the associated capacitance of all electrode phenomena can be explained by the presence of an additional parallel path for electrochemical reaction that is introduced by the incorporation of a mixed conducting buffer layer [24]. This scenario is described by Fig. 7 that provides a schematic of potential competing paths for electrochemical reaction in which the mixed conducting buffer layer also contributes as a viable mechanistic route. From these results, we can understand that introducing a thin MIEC buffer layer, in this case CTO þ Co, between oxygen electrode and electrolyte can potentially lead to better electrode performance by providing an additional path at which the electrochemical reaction can occur. Taking into account
the above characteristic phenomena, ultimately one can tune the electrode kinetics improvement by tailoring the transport properties of thin intermediate buffer layer to maximize its electronic and ambipolar conductivity [24]., as demonstrated in the present case. In order to assess if this improvement in performance is maintained under both SOFC and SOEC operating conditions, the samples were polarised under anodic and cathodic modes of operation. Fig. 8 shows examples of typical impedance spectra obtained under anodic polarization for the Nd2NiO4 electrode with CTO þ Co buffer layer at 800 C in air atmosphere. Again, for clarity, the represented spectra are plotted after subtraction of the ohmic resistance (R1). The impedance spectra can be seen to change in magnitude upon polarization, while retaining their general shape comprising of two
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
overlapping semicircular responses. The impedance spectra for all samples and under cathodic polarization exhibit similar behaviour. For this reason, all impedance spectra were fitted to a similar equivalent circuit to that use under zero applied DC bias conditions, as shown in Fig. 4. The corresponding total polarization resistance, Rp, is shown in Fig. 9 as a function of applied DC potential in the range between þ0.3 V and 0.3 V with 0.05 V intervals, at different temperatures. In general, a decreased polarization resistance is observed with increasing applied potential for all modes of operation. This phenomenon was previously reported to be characteristic when alterations in the local chemical potential of oxygen at the electrode/electrolyte interface, induced by the applied potential, are not efficiently compensated from the gas phase due to the presence of limiting surface exchange [22,39]. Exceptions to this trend are noted at high temperatures and low applied potentials, and are classically explained to be due to increased oxygen surface exchange at higher temperatures that attenuates the presence of local gradients in oxygen chemical potential [22,39]. The most important result of Fig. 9 is that the sample with the CTO þ Co buffer layer is shown to continue to offer substantially lower polarization resistances compared to the other two samples under both cathodic and anodic polarization and at all measured temperatures. This result highlights the important role of the MIEC buffer layer, CTO þ Co, to increase oxygen electrode performance in both SOFC and SOEC modes of operation, a characteristic, thus, highly relevant for these two devices. Interesting, secondary information can also be obtained from Fig. 9, such as the fact that improvements in polarization resistance for the CGO þ Co buffer layer sample over that of the sample with no buffer layer can only be obtained at high temperatures. This result mirrors the result obtained at open circuit, Fig. 5b, and can be explained by previous literature that indicated the CGO þ Co material to only start to present a significant p-type electronic component to total conductivity at very high temperatures, while this contribution is rapidly depleted as temperature decreases [33]. In contrast, the new CTO þ Co buffer layer offers a considerable electronic contribution to total conductivity over the entire temperature range [33] and, thus, can promote improved electrode performance in all conditions. The critical nature of mixed conductivity to maximize performance is further reinforced by comparison of the current results to the performance of the CPO þ Co (Ce0.8Pr0.2O2-dþCo) buffer layer of our previous publication [24], Fig. 9f. This comparison demonstrates that both the current CTO þ Co buffer layer and the previously measured CPO þ Co buffer layer provide large performance improvements over that of the use of the predominately ionic buffer layer, CGO þ Co, as result of their much higher levels of mixed conductivity [32].
Conclusions The electrochemical performance of the oxygen electrode Nd2NiO4 has been investigated using a new terbium-doped ceria buffer layer for SOFC and SOEC applications that offers mixed ionic and electronic conduction. A thin film CTO buffer
31473
layer with cobalt additions was successfully prepared on the top of a pre-prepared YSZ electrolyte by a cost effective spin coating method and utilizing a low co-sintering temperature (900 C). Microscopy results show well separated and adhered layers that are free of observable cation interdiffusion. The YSZ/CTO þ Co/Nd2NiO4 half cell was characterized by EIS measurements under anodic and cathodic polarization and results were compared with samples of identical configuration but containing the typical buffer layer material CGO þ Co and no buffer layer. The sample with MIEC buffer layer CTO þ Co show vastly improved electrode behavior over that obtainable in an equivalent cell without a buffer layer or with a predominantly ionic CGO þ Co buffer layer, reinforcing that noted for the previously published mixed conducting buffer layer CPO þ Co. Such dramatic improvements in polarization resistances are, thus, suggested to be the existence of additional paths for electrochemical reaction due to the presence of a high level of mixed conductivity in the buffer layer.
Acknowledgements The authors acknowledge the Foundation for Science and Technology Portugal(FCT) project grants PTDC CTM-ENE/6319/ 2014, POCI-01-0145-FEDER-032241, UID/EMS/00481/2019-FCT, CENTRO-01-0145-FEDER-022083 - QREN, FEDER and COMPETE Portugal and the European Union for the financial support.
references
[1] Zhang L, Hu S, Zhu X, Yang W. Electrochemical reduction of CO2 in solid oxide electrolysis cells. J Energy Chem 2017;26:593e601. [2] Vialetto G, Noro M, Colbertaldo P, Rokni M. Enhancement of energy generation efficiency in industrial facilities by SOFCSOEC systems with additional hydrogen production. Int J Hydrogen Energy 2019;44:9608e20. [3] Eguchi K, Hatagishi T, Arai H. Power generation and steam electrolysis characteristics of an electrochemical cell with a zirconia- or ceria-based electrolyte. Solid State Ion 1996;2738:1245e9. [4] Schiller G, Ansar A, Lang M, Patz O. High temperature water electrolysis using metal supported Solid Oxide Electrolyser Cells (SOEC). J Appl Electrochem 2009:293e301. [5] Iora P, Chiesa P. High efficiency process for the production of pure oxygen based on solid oxide fuel cell e solid oxide electrolyzer technology. J Power Sources 2009;190:408e16. [6] Zhang W, Zheng Y, Yu B, Wang J, Cheng J. Electrochemical characterization and mechanism analysis of high temperature Co-electrolysis of CO2 and H2O in a solid oxide electrolysis cell. Int J Hydrogen Energy 2017;42:29911e20. [7] Lein H, Wiik K, Grande T. Thermal and chemical expansion of mixed conducting La0.5Sr0.5Fe1xCoxO3d materials. Solid State Ion 2006;177:1795e8. [8] Sunarso J, Baumann S, Serra JM, Meulenberg WA, Liu S, Lin YS, Diniz da Costa JC. Mixed Ionic e Electronic Conducting (MIEC) ceramic-based membranes for oxygen separation. J Membr Sci 2008;320:13e41. [9] Wan J, Goodenough J, Zhu J. Nd2xLaxNiO4þd, a mixed ionic/ electronic conductor with interstitial oxygen, as a cathode material. Solid State Ion 2007;178:281e6.
31474
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 1 4 6 6 e3 1 4 7 4
[10] Chauveau F, Mougin J, Bassat JM, Mauvy F, Grenier JC. A new anode material for solid oxide electrolyser : the neodymium nickelate Nd2NiO4þd. J Power Sources 2010;195:744e9. ndez A, Vega-Castillo J, Mogni L, [11] Montenegro-Herna Caneiro A. Thermal stability of Ln2NiO4þd (Ln: La, Pr, Nd) and their chemical compatibility with YSZ and CGO solid electrolytes. Int J Hydrogen Energy 2011;36:15704e14. [12] Liu M, Shi D, Suo H, Ye S, Zhao Y, Zhu Y, Li Q, Wang L, Jihyun A, Zhou M. A simple MOD method to grow a single buffer layer of Ce0.8Gd0.2O1.9 (CGO) for coated conductors. Physica C 2009;469:230e3. [13] Zhang W, Yu B, Xu J. Investigation of single SOEC with BSCF anode and SDC barrier layer. Int J Hydrogen Energy 2012;37:837e42. [14] Mesguich D, Aymonier C, Bassat J, Mauvy F, You E, Watkins JJ. Low-temperature deposition of undoped ceria thin films in scCO2 as improved interlayers for IT-SOFC. Chem Mater 2011;23:5323e30. [15] Min Park Y, Kim H. Porous Gd-doped ceria barrier layer on solid oxide fuel cell with Sm0.5Sr0.5CoO3d Cathodes. Ceram Int 2013;39:2037e43. [16] Xie Z, Zhao H, Du Z, Chen T, Chen N. Electrical, chemical, and electrochemical properties of double perovskite oxides Sr2Mg1xNixMoO6d as anode materials for solid oxide fuel cells. J Phys Chem C 2014;118:18853e60. [17] Nguyen T, Kobayashi K, Honda T, Iimura Y, Kato K, Neghisi A, et al. Preparation and evaluation of doped ceria interlayer on supported stabilized zirconia electrolyte SOFCs by wet ceramic processes. Solid State Ion 2004;174:163e74. [18] Milcarek R, Wang K, Falkenstein-Smith R, Ahn J. Performance variation with SDC buffer layer thickness. Int J Hydrogen Energy 2016;41:9500e6. n-Ruiz A, Laguna-Bercero M, [19] Wain-Martin A, Mora ~ aga A, Slater P, Arriortua M. SOFC Campana R, Larran cathodic layers using wet powder spraying technique with self synthesized nanopowders. Int J Hydrogen Energy 2019;44:7555e63. [20] Bohac P, Orliukas A, Gauckler L. Lowering of the cathode overpotential of SOFC by electrolyte doping. Electroceramics 1994;IV(II):771e4. rez-Coll D, Aguadero A, Escudero MJ, Nu´n ~ ez P, Daza L. [21] Pe Optimization of the interface polarization of the La2NiO4based cathode working with the Ce1exSmxO2ed electrolyte system. J Power Sources 2008;178:151e62. rez-Coll D, Aguadero A, Escudero MJ, Daza L. Effect of DC [22] Pe current polarization on the electrochemical behaviour of La2NiO4þd and La3Ni2O7þd-based systems. J Power Sources 2009;192:2e13. [23] Fagg DP, Kharton VV, Frade JR. Transport in ceria electrolytes modified with sintering aids: effects on oxygen reduction kinetics. J Solid State Electrochem 2004;8:618e25. rez-Coll D, [24] Ramasamy D, Nasani N, Ana-Brandao D, Pe Fagg DP. Enhancing electrochemical performance by control of transport properties in buffer layers e solid oxide fuel/
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
electrolyser cells. Phys Chem Chem Phys: PCCP 2015;17:11527e39. Christie GM, Van Berkel FPF. Microstructure e ionic conductivity relationships in ceria-gadolinia electrolytes. Solid State Ion 1996:17e27. Fagg DP, Kharton VV, Frade JR. P-type electronic transport in Ce0.8Gd0.2O2d:the effect of transition metal oxide sintering aids. J Electroceram 2003;9:199e207. de Vries KJ, Meng G-Y. Phase formation and thermal stability of fcc (fluorite) Ce1xTbxO2d solid solutions. Mater Res Bull 1998;33:357e67. Ye F, Mori T, Ou DR, Takahashi M, Zou J, Drennan J. Compositional dependence of electrical conductivity of Ce1xTbxO2d (0x1). Renew Energy 2008;33:331e5. Fagg DP, Garcı´a-martin S, Kharton VV, Frade JR. Transport properties of fluorite-type Ce0.8Pr0.2O2-d: optimization via the use of cobalt oxide sintering aid. Chem Mater 2009;21:381e91. Balaguer M, Solı´s C, Serra M. Study of the transport properties of the mixed ionic electronic conductor Ce1xTbxO2-d þ Co (x ¼ 0 . 1 , 0 . 2) and evaluation as oxygen. Transport Membrane 2011;23:2333e43. Shuk P, Greenblatt M, Croft M. Hydrothermal synthesis and properties of mixed conducting Ce1-xTxO2-d solid solutions. Chem Mater 1999;11:473e9. mez-Herrero A, Kharton VV, Ramasamy D, Shaula AL, Go Fagg DP. Oxygen permeability of mixed-conducting Ce0.8Tb0.2O2d membranes: effects of ceramic microstructure and sintering temperature. J Membr Sci 2015;475:414e24. Ramasamy D, Shaula AL, Nasani N, Kovalevsky AV, Fagg DP. Comparative study of fluorite-type ceria-based Ce1-xLnxO2d (Ln¼Tb, Gd, and Pr) mixed ionic electronic conductors densified at low temperatures. J Mater Sci 2016;51:10293e300. Adler SB, Henderson BT, Wilson MA, Taylor DM, Richards RE. Reference electrode placement and seals in electrochemical oxygen generators. Solid State Ion 2000;134:35e42. Bauerle JE. Study of solid electrolyte polarization by a complex admittance method. J Phys Chem Solids 1969;30:2657e70. Nasani N, Ramasamy D, Antunes I, Perez J, Fagg DP. Electrochemical behaviour of Ni-BZO and Ni-BZY cermet anodes for Protonic Ceramic Fuel Cells (PCFCs) e a comparative study. Electrochim Acta 2015;154:387e96. Loureiro FJA, Yang T, Stroppa DG, Fagg DP. Pr2O2SO4eLa0.6Sr0.4Co0.2Fe0.8O3-d: a new category of composite cathode for intermediate temperature-solid oxide fuel cells. J Mater Chem 2015;3:12636e41. Gibson IR, Dransfield GP, Irvine JTS. Sinterability of commercial 8 mol% yttria-stabilized zirconia powders and the effect of sintered density on the ionic conductivity. J Mater Sci 1998;33:4297e305. Svenssona AM, Sunde S, Nisancioglu K. A mathematical model of the porous SOFC cathode. Solid State Ion 1996;86e88:1211e6.