CERAMICS
INTERNATIONAL
Ceramics International 40 (2014) 13105–13113
Ni
–CGO cermet anodes from nanocomposite powders: Microstructural and
electrochemical assessment
Daniel A. Macedo
a,n,1, Filipe M.L. Figueiredo
b, Carlos A. Paskocimas
a, Antonio E. Martinelli
a,
Rubens M. Nascimento
a, Fernando M.B. Marques
baMaterials Science and Engineering Postgraduate Program– PPGCEM, UFRN, 59072-970 Natal, Brazil b
CICECO, Deptartment of Materials & Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal Received 7 January 2014; received in revised form 4 April 2014; accepted 5 May 2014
Available online 14 May 2014
Abstract
In this study, composite powders synthesized by a novel one-step sol–gel method were used to obtain Ni–CGO anodes, while anodes of the same composition prepared from commercial powders were used as reference. The anodes performance was studied by impedance spectroscopy and dc polarization in the temperature range of 650–750 1C in flowing humidified 10% H2þ90% N2gas mixtures, using a three-electrode
configuration cell, with clear advantage for the novel one-step route. One-step anodes fired at 1450 1C showed an area specific resistance of 0.15Ω cm2 under open circuit conditions and an anodic overpotential of 91 mV at 7501C for a current density of 322 mA/cm2, which are amongst the best results mentioned in the literature. The enhanced electrochemical performance of one-step anodes is mainly attributed to unique microstructural features, namely small grain size (submicrometer scale even afterfiring at 1450 1C) and homogeneous phase distribution, which is expected to extend the triple-phase boundary length.
& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Composites; C. Electrical properties; E. Fuel cells; Ni–CGO anodes
1. Introduction
Solid oxide fuel cells (SOFCs) are among the most efficient and environment friendly energy conversion devices. How-ever, high performance electrodes and long term micro-structural stability at low and intermediate temperatures (500–750 1C) are still needed. For optimum SOFC perfor-mance, the electrodes should have high electrical conductivity, chemical stability towards the electrolyte and gas environment, thermal expansion coefficient matching that of the electrolyte, high electrocatalytic activity to yield low overpotential losses and suitable porosity (around 30–40%)[1–7].
Electrode performance is determined by composition and microstructure. The nature of the starting powders and the applied manufacturing technique are influential in these aspects [3,8,9,10]. One potential way to enhance the perfor-mance of electrodes is to increase the number of reaction sites where the electrochemical reaction takes place, i.e. the triple phase boundaries (TPBs) [11–14]. In other words, TPBs are sites where charge transfer easily occurs and their length is directly related to the microstructure and particle size of the electrode constituents. The smaller the particle size, the larger the TPB.
Typical anode cermets for SOFCs include one ceramic phase (frequently, YSZ-yttria stabilized zirconia or CGO-gadolinia doped ceria) and one metal (often Ni). In classical processing routes both cermet constituents are mixed as oxides, afterwards used to prepare inks for screen-printing. Homogeneous mixtures require a significant amount of work to match the grain size of both phases, also to avoid potential
www.elsevier.com/locate/ceramint
http://dx.doi.org/10.1016/j.ceramint.2014.05.010
0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
nCorresponding author. Tel.:þ55 83 32167860; fax: þ55 83 32167906.
E-mail address:damaced@gmail.com(D.A. Macedo).
1Present address: Department of Materials Engineering, Federal University
segregation of constituents due to distinct densities and particle size and shape. Therefore, the development of cost-effective synthesis routes to produce homogeneous nanocomposite powders and adequate electrode microstructures, namely anode cermets, is highly desirable [15–18]. Hereby, we follow this approach, after preliminary tests showing that a newly devel-oped one-step synthesis of NiO–CGO provided promising Ni–CGO anodes [19,20].
The electrochemical performance of electrodes is widely investigated by impedance spectroscopy [18,21,22]. Often, only symmetrical cells are used to assess the electrode area specific resistance (ASR) from the so-called electrode impe-dance arc. This information is quite interesting to screen promising electrodes and adjust processing routes (e.g., elect-rode firing conditions). The present study focused on the preparation and electrochemical performance of screen-printed one-step Ni–CGO anodes and the comparison with conven-tional anodes obtained by mechanical mixture of commercial powders. The investigation of the performance of one-step anodes, by impedance spectroscopy and anodic overpotential measurements, as well as their microstructural correlation has not been previously reported.
2. Experimental procedure
NiO–Ce0.9Gd0.1O1.95 (NiO–CGO, 50 wt% NiO) composite
powders were prepared following two different experimental procedures: one-step synthesis and mechanical mixture of commercially available powders. The one-step synthesis approach is a novel sol–gel based method in which NiO and CGO resins are obtained from polymeric precursors, mixed together, and then thermally treated to directly produce the NiO–CGO precursor powder [19,20]. The starting materials used were cerium nitrate hexahydrate (Sigma-Aldrich) 99.99%, gadolinium nitrate hexahydrate (Sigma-Aldrich) 99.9% and nickel nitrate hexahydrate (Sigma-Aldrich) Z97%. Here, the as-prepared nanocomposite was calcined at 7001C for 0.5 h after thermogravimetric analysis (Setaram SetSys 16/18 instru-ment) of the powder precursor. Afterwards, the calcined powder was ball milled with zirconia balls at 50 rpm for 1 h to reduce the agglomerate size.
A conventional composite powder was also prepared from a mixture of commercial NiO (J. T. Baker, Phillipsburg, NJ) and Ce0.9Gd0.1O1.95 (Praxair Specialty Ceramics, Seattle) powders,
with a weight ratio of 1:1. Given the larger grain size of NiO particles with respect to the CGO powder, the NiO powder was first milled in a planetary ball mill at 300 rpm for 4 h and then added to CGO and ethanol for a second milling step at 50 rpm during 1 h. The particle size distribution of sieved powders was measured by a laser particle size analyzer (Coulter LS, USA).
Both NiOþCGO composite powders were mixed with a commercial organic vehicle (Quimiceram, Portugal), and the resulting slurries were screen-printed onto dense Ce0.9Gd0.1
O1.95 (CGO) pellets prepared from the previously mentioned
commercial powder (Praxair), after being shaped by uniaxial and cold isostatically pressing (300 MPa), and sintered to almost full densification at 1550 1C for 4 h.
Screen-printed NiO–CGO layers were fired between 1350 and 14501C for 4 h in air in order to assess the effect of the firing temperature on the microstructure of NiO–CGO pre-cursor anodes and Ni–CGO cermets. The area of the electrodes was 0.2 cm2and their thickness was in the range of 20–40 mm after sintering. Before testing the anodes electrochemical performance, NiO was reduced in situ to metallic nickel at 7501C for 3 h in a wet hydrogen gas mixture (10% H2þ90%
N2humidified at room temperature) flowing at 100 cm3/min.
The relations between the anodic area specific resistance (ASR) and overpotential (η) with current density were studied using three-electrode cells, with porous Pt counter and reference electrodes (CE and RE, respectively) sintered at 10001C for 1 h. The cell geometry was chosen according to previous work [23]. The porous Ni–CGO working electrode (WE) and CE were symmetrically deposited onto both sides of the CGO pellets. The distance between the WE and RE was 6 mm. The electrochemical performance was evaluated using an Autolab PGSTAT302 fitted with a frequency response analyzer (FRA) (Autolab, EcoChemie, Netherlands). Impe-dance measurements were carried out with a test signal amplitude of 10 mV, over the frequency range from 100 kHz to 0.1 Hz, under an hydrogenflow (100 mL cm3) saturated with water vapor at 251C (the same used for NiO reduction). The operating temperature varied between 650 and 7501C, with a stabilization time between measurements of 1 h. An YSZ oxygen sensor was placed close to the samples to evaluate the partial pressure of oxygen (pO2), which was
found to be around 1021 atm at 7501C.
Impedance spectra were fitted to equivalent circuits using the ZView software from Scribner Associates in order to estimate the electrolyte and polarization resistances. The anode overpotential (η) was measured in potentiostatic mode and calculated using:
η ¼ U–IR ð1Þ
where U is the potential difference between WE and RE, I is the current between WE and CE, and IR is the ohmic contribution to the potential drop between WE and RE. The anode polarization resistance was obtained from thefits to the impedance spectra as corresponding to the amplitude of the semicircles, and was corrected for the electrode area to obtain the so-called area specific resistance (ASR).
The surface and cross-section of sintered NiO–CGO pre-cursor anodes and Ni–CGO cermets were inspected using scanning electron microscopy (Hitachi SU-70FEG-SEM). Compositional mappings were gathered using energy disper-sive X-ray spectroscopy (EDS). The porosity of the anode precursor layers (NiO–CGO) was estimated by SEM image analysis using ImageJ software.
3. Results and discussion
3.1. Microstructural characterization
The thermogravimetric analysis (TGA) of the as-prepared one-step NiO–CGO nanocomposite powder (Fig. 1) demonstrates that
major weight loss (60%) occurred below 6001C, due to burning of residual organic compounds present in the synthesis based on polymeric precursors. Since only marginal weight losses (o1%) were observed in the temperature range above 6001C, the as-prepared one-step powder was subjected to calcination at 7001C for 0.5 h in order to crystallize both NiO and CGO phases as a fine composite powder. Given the following reported results, this calcination temperature can be understood as a maximum borderline sincefiner and better powders are likely to be obtained at lower calcination temperatures.
Fig. 2depicts the particle size distribution and morphology for both calcined one-step and conventional (mechanical mixed) powders. Although suggesting a wider particle size distribution, compared to the narrower one observed for the conventional composite powder (Fig. 2a), the one-step synth-esis also showed a high fraction of particles smaller than 200 nm. However, the SEM image of the one-step powder, shown inFig. 2b, not only exhibits agglomerates with size of 1 mm, in good agreement with particle size distribution analysis, but also confirms the presence of nanoparticles smaller than 50 nm. In contrast, the conventional powder exhibits a morphology consisting of nanosized CGO and larger NiO particles (Fig. 2c). These observations indicate that the particle size distributions just mentioned might be mis-leading. In fact, agglomerates obtained with the one-step route are indeed composite powders including nanoparticles of both phases, whereas the conventional anodes are composed of coarser particles, mostly NiO, as confirmed by EDS analysis of screen-printed and fired electrode layers.
The microstructures of NiO–CGO screen-printed layers obtained from the two different composite powders and subsequentlyfired between 1350 and 1450 1C were examined by SEM to identify a suitable firing temperature. The micro-structures for the two series of anode precursors are different, as shown in Fig. 3. As it can be seen fromFig. 3a(1), after firing at 1350 1C the size of the NiO particles in the conventional powder reached approximately 2mm. Regardless of the firing temperature used, large NiO grains are clearly
Fig. 1. TGA of the as-prepared one-step NiO–CGO nanocomposite powder.
observed in conventionalfilms, indicating a poor effectiveness of both milling steps used in the preparation of these cermets. On the contrary, the microstructures of one-step layers (Fig. 3a and b(2)) are characterized by very fine and uniformly distributed NiO and CGO grains, and homogeneous thickness. Even after firing at high temperature (Fig. 3b(2)), one-step films contain both NiO and CGO grains smaller than 1 mm, which show that the one-step synthesis process here adopted is a suitable method to attain reduced particle size even when high temperatures are used to improve bonding to the electrolyte layer.
Partial distribution of Ni and Ce in the conventional and one-step electrodes fired at 1350 1C was studied by energy dispersive X-ray spectroscopy (EDS). As it can be seen from EDS maps shown in Fig. 4, the electrode made with the one-step composite powder has not only smaller particles but also a more uniform distribution of NiO particles if compared with the layer obtained by a conventional powder. This confirms once more that one-step synthesis is an effective chemical route to fabricate anode precursor layers with well-connected submicron-sized grains of NiO and CGO, which is expected to extend the TPB length in reduced anodes.
The mechanical mixture of commercial powders was not fully optimized in this work, but the same is true in the case of the adopted one-step route, since the calcination temperature of precursor powders was one of the parametersfixed at extreme
conditions, able to be modified in order to obtain even finer powders. Thus, we cannot say that the conventional route is unable to produce suitable anodes (corresponds to the state of the art preparation method to obtain performing anodes for SOFCs). However, we can indeed say at this stage that the one-step route is clearly highly versatile and effective in the production of intimate mixtures offinely dispersed phases. 3.2. Electrochemical characterization
The electrochemical performance of one-step and conven-tional cermet anodes was evaluated by impedance spectro-scopy in the temperature range of 650–750 1C in humidified 10% H2þ90% N2gas mixtures. Typical impedance spectra of
the Ni–CGO anodes fired at different temperatures, acquired at 7501C under open circuit voltage (OCV), are shown inFig. 5. In these spectra, the high frequency intercept with the real axis represents the ohmic resistance of the electrolyte (with marginal contributions from electrode and lead wires), while the overall amplitude of the arcs corresponds to the anode polarization resistance. These spectra werefitted assuming an equivalent circuit R1(R2CPE2)HF(R3CPE3)LF, where R1 is the
electrolyte ohmic resistance in series with two distinct elec-trode contributions consisting of resistances (R2 and R3) in
parallel with constant phase elements (CPE2 and CPE3).
Usually, the low frequency arc can be attributed to diffusional
Fig. 3. SEM cross-section images of the NiO–CGO/CGO interfaces fired at (a) 1350 1C and (b) 1450 1C, for conventional (1) and one-step (2) anode precursor layers.
processes (gas through the porous material or surface diffusion of electroactive species to the TPB sites). The high frequency arc is frequently associated with charge transfer processes
[24,25]. Similar equivalent circuits have been previously described for Ni-based anodes [26–28]. We believe that this type of description fits quite well with the present results, although no attempt was made to fully elucidate the exact electrode mechanism and identify the phenomena which could be ascribed to these impedance contributions.
The overall ASR, which characterizes the electrochemical performance of each electrode, can be obtained from the sum of the electrode resistance components (R2HFþR3LF) multiplied
by the electrode surface area. It is clear from Fig. 5 that the electrochemical response is dominated by the low frequency impedance. Despite the clearly visible high frequency (HF) limitation of the FRA, the entire spectra were suitably fitted. Obviously, in some cases more complex equivalent circuits could be considered for improved fitting, but the limited information available in this study would increase the spec-ulation about their exact physical meaning.
The overall electrode resistance was studied by changing the anode precursor powder as well as the operating and firing temperatures. As it can be observed fromFig. 5, there was a significant reduction in the electrode impedance on both one-step and conventional anodes fired at 1450 1C, indicating a clear enhancement in their electrochemical performance if compared to the corresponding anodes fired at 1400 1C. The ASR values shown inTable 1obtained from these impedance spectra decreased with increasing temperature, indicating that the overall electrode polarization resistance is, as expected, thermally activated. According toTable 1, the ASR values for
one-step and conventional anodes decrease with increasing firing temperatures from 0.27 to 0.15 Ω cm2
and from 0.76 to 0.30Ω cm2, respectively, at the highest tested operating tem-perature (7501C).
For the purpose of comparison, a screen-printed Ni–CGO anode from a composite powder with 70 wt% NiO showed an ASR of around 1.2Ω cm2at 8501C under conditions similar to those used in this study [12]. Despite the higher operating temperature and Ni content used (60 vol% metal after NiO reduction), this ASR value is eight times higher than that found for our best anode. Babaei et al. [26] also reported an high ASR of 0.59Ω cm2for a Ni–CGO anode in 97% H2/3% H2O
at 8501C. Even with added 0.11 mg cm2 palladium nano-particles, ASR of these anodes reached 0.43Ω cm2at 7501C (almost three times the best value reported herein). Lastly, although a very low ASR value of 0.09Ω cm2 has been reported for a Ni–CGO anode tested as low as 550 1C [29], such electrode was not prepared by a two-step process (powder then electrode), but using an electrostatic-assisted ultrasonic spray pyrolysis method, which produces porous layers with fine and active cauliflower-like structures. This emphasizes the role of processing on electrode performance and enhances the promising results here obtained with the one-step route.
Table 2 shows the result of the deconvolution of the electrode impedance into high and low frequency tions. The relative importance of this low-frequency contribu-tion (values in parentheses) is also listed. Regardless of the type of anode and firing temperature, the electrochemical process taking place at low frequencies represents at least 55% of the overall electrode resistance. These results suggest that the oxidation of H2 is dominated by diffusion as a low
frequency process [30], which dominates over the entire temperature range, reaching 83% of the overall resistance.
Fig. 6displays the Arrhenius plots of the ASR values for the different anodes as a function of the firing temperature. All samples show a linear trend of ln(ASR) versus 1/T, confirming the usual thermally activated performance of anodes under H2
[31]. As emphasized in this plot, the conventional anodefired at 14001C exhibited the lowest ASR values. However, after further optimization of the anodes performance, increasing the firing temperature to 1450 1C, the one-step route showed better performance. The activation energy of the overall anodic resistance was in the range of 1.58–1.93 eV, slightly larger than for nanocrystalline Ni–CGO thin layer anodes prepared by spray pyrolysis and pulsed laser deposition [31]. It is worthwhile mentioning that these results were obtained under
zero dc current and facing no chemical potential gradient. Therefore, the anode performance must be different in a real cell (facing reducing and oxidizing atmospheres) under a dc current, situation described in the next paragraphs.
In addition to the area specific resistance, the anodic over-potential is another important parameter which must be considered while designing high performance Ni-based cermet anodes [18,24,25,28,32,33]. Fig. 7 shows the anodic over-potential as a function of current density for one-step and conventional anodes for different operating and firing tem-peratures. The one-step anode prepared at the highest sintering temperature showed the lowest values of overpotential over the entire range of current density. This is coherent with the lowest ASR values measured under zero dc current. The anodic overpotential for the Ni–CGO cermet anode prepared by one-step synthesis andfired at 1450 1C was only 91 mV at 750 1C for a current density of 322 mA/cm2. The reference conven-tional anodes sintered at 1400 and 14501C showed over-potential values far higher under identical experimental conditions. This is probably due to the considerable enhance-ment of the TPB length in one-step Ni–CGO cermet anodes.
Fig. 5. Impedance spectra of the one-step and conventional Ni–CGO cermet anodesfired at 1400 and 1450 1C. Spectra were acquired at 750 1C and under open circuit conditions.
Table 1
Area specific resistance of different anodes, under open circuit conditions, as a function of operating and firing temperatures. Operating temperature (1C) Area specific resistance (ASR) in Ω cm2
One-step/14001C One-step/14501C Conventional/14001C Conventional/14501C
750 0.27 0.15 0.76 0.30
700 0.65 0.41 2.10 0.73
650 2.88 1.22 6.28 2.10
Table 2
High (HF) and low-frequency (LF) resistances obtained from the deconvo-lution of impedance spectra and normalized to the electrode area. The values in parentheses represent the fraction of the low-frequency contribu-tion with respect to the overall electrode resistance.
Ni–CGO anode HF (Ω cm2) LF (Ω cm2) One-step/14501C 7501C 0.05 0.10 (67%) 7001C 0.13 0.28 (68%) 6501C 0.46 0.76 (62%) One-step/14001C 7501C 0.12 0.15 (55%) 7001C 0.17 0.48 (73%) 6501C 0.64 2.24 (78%) Conventional/14501C 7501C 0.13 0.17 (56%) 7001C 0.24 0.49 (67%) 6501C 0.71 1.39 (66%) Conventional/14001C 7501C 0.16 0.60 (79%) 7001C 0.35 1.75 (83%) 6501C 1.55 4.73 (75%)
By comparing the best overpotential values reported herein with the literature data, the importance of anode microstruc-tural design can be confirmed. The superior performance of one-step Ni–CGO cermets in terms of anodic overpotential is shown in Table 3. The overpotential value of 80 mV was measured for a Ni–SDC anode for a current density of 200 mA/cm2 at 8001C [18]. Even though these authors also
reported an enhanced anode performance (33.1 mV at 200 mA/ cm2and 8001C) by decreasing the powder calcining tempera-ture from 1200 to 10001C, given the operating temperature (maximum of 7501C) and gas composition (lower H2content)
used in the current work, once more the superior performance of one-step anodes was confirmed.
The improved electrochemical performance of one-step anodes is believed to be mainly due to the fine and homo-geneous phase distribution, and pore structure. The grain size distribution was indeed preserved during reduction of the anodes, as confirmed by SEM inspection (Fig. 8). The electrode derived from the one-step powder (Fig. 8a) clearly shows well connected submicron-sized grains of Ni and CGO. For the conventional anode (Fig. 8b), Ni grains are coated by CGO, limiting the Ni connectivity, thus, decreasing anode perfor-mance. The distinct microstructural features resulting from different NiO–CGO anode precursor composite powders result in different TPB lengths, affecting thefinal anode performance. The microstructural evidence combined with impedance spectroscopy and anodic overpotential results confirmed that microstructural design is essential to improve the performance of Ni based anodes. The potential for microstructural improve-ment in any of the adopted routes is enormous as improve-mentioned throughout this text. However, the one-step nanocomposite powders prepared here showed enormous and promising potential, considering the above results.
Fig. 6. Arrhenius plots of the ASR values for different anodesfired at 1400 and 14501C.
4. Conclusions
Ni–CGO cermet anodes were prepared at different firing temperatures from NiO–CGO composite powders obtained by distinct synthesis routes. Under identical experimental condi-tions the one-step anodes exhibited better microstructure and electrochemical performance than conventional anodes pre-pared by simple mixture of oxide precursors. The hereby named one-step synthesis route favored the formation of well-connected submicrometric grains of Ni and CGO even when bonded within large size agglomerates, showing char-acteristics quite competitive with respect to the state of the art anodes.
Acknowledgments
The authors express their appreciation for thefinancial support granted by CAPES (PRÓ-ENGENHARIAS and PDEE program BEX 6775/10-1), CICECO (PEst-C/CTM/LA0011/2011), and FCT/COMPETE/FEDER (Portugal). The electrochemical charac-terization was performed with the assistance of Dr. Aleksey Yaremchenko (CICECO, University of Aveiro, Portugal).
References
[1]Y.C. Hsiao, J.R. Selman, The degradation of SOFC electrodes, Solid State Ion. 98 (1997) 33–38.
Table 3
Literature survey on anodic performance and on own data on one-step and conventional anodes.
Cermet anode Overpotential and current density Electrode preparation method Fuel Reference One-step Ni–CGO 91 mV at 7501C and 322 mA/cm2 Powder obtained by one-step synthesis and
calcined at 7001C/0.5 h. Screen-printing and sintering at 14501C/4 h.
Wet 10% H2þ90% N2 This work
Conventional Ni–CGO 95 mV at 750 1C and 215 mA/cm2 NiO/CGO commercial powders. Screen-printing and sintering at 14501C/4 h.
Wet 10% H2þ90% N2 This work
Ni–CGO 350 mV at 8001C and 200 mA/cm2 Cellulose-precursor synthesis and calcined at 9001C/2 h. Screen-printing and sintering at 13001C/2 h.
Wet 10% H2þ90% N2 [33]
Surface modified Ni–CGO
110 mV at 8001C and 200 mA/cm2 Cellulose-precursor synthesis and calcined at 9001C/2 h. Screen-printing and sintering at 13001C/2 h.
Wet 10% H2þ90% N2 [33]
Ni–SDC 130 mV at 8001C and 200 mA/cm2 As-prepared NiO-SDC powder via
urea-combustion synthesis. Spin-coating and sintering at 13001C/4 h.
40% H2þ60% N2 [18]
Ni–SDC 80 mV at 8001C and 200 mA/cm2 NiO-SDC powder via urea-combustion synthesis
calcined at 12001C. Spin-coating and sintering at 13001C/4 h.
40% H2þ60% N2 [18]
Surface modified Ni–CGO
80 mV at 8001C and 150 mA/cm2 Cellulose-precursor synthesis and calcined at 9001C/2 h. Screen-printing and sintering at 13001C/2 h.
Wet 10% H2þ90% N2 [24]
CGO impregnated Ni anode
150 mV at 8001C and 300 mA/cm2 NiO commercial powder. Slurry painting and sintering at 14001C/2 h. Ion impregnation with Gd0.2Ce0.8(NO3)xnitrate solution andfiring at
8501C/1 h.
Humidified H2 [25]
[2]M. Mogensen, K.V. Jensen, M.J. Jørgensen, S. Primdahl, Progress in understanding SOFC electrodes, Solid State Ion. 150 (2002) 123–129. [3]S.P. Jiang, S.H. Chan, A review of anode materials development in solid
oxide fuel cells, J. Mater. Sci. 39 (2004) 4405–4439.
[4]J. Cheng, L. Deng, B. Zhang, P. Shi, G. Meng, Properties and microstructure of NiO/SDC materials for SOFC anode applications, Rare Met. 26 (2007) 110–117.
[5]R.P. Fiuza, M.A. Silva, J.S. Boaventura, Development of Fe–Ni/YSZ– GDC electrocatalysts for application as SOFC anodes: XRD and TPR characterization and evaluation in the ethanol steam reforming reaction, Int. J. Hydrog. Energy 35 (2010) 11216–11228.
[6]N. Oishi, Y. Yoo, Low polarization resistance of Ni–Cu–Ce oxide anode synthesized by coprecipitation with tetramethylammonium carbonate for intermediate temperature SOFC, Mater. Lett. 64 (2010) 876–878. [7]E.M. Ryan, W. Xu, X. Sun, M.A. Khaleel, A damage model for
degradation in the electrodes of solid oxide fuel cells: modeling the effects of sulfur and antimony in the anode, J. Power Sources 210 (2012) 233–242.
[8]N.Q. Minh, Ceramic fuel cells, J. Am. Ceram. Soc. 76 (1993) 563–588. [9]N.Q. Minh, T. Takahashi, Science and Technology of Ceramic Fuel Cell,
Elsevier, New York, 1995.
[10]A. Atkinson, S. Barnett, R.J. Gorte, J.T.S. Irvine, A.J. McEvoy, M. Mogensen, S.C. Singhal, J. Vohs, Advanced anodes for high-temperature fuel cells, Nat. Mater. 3 (2004) 17–27.
[11]T. Fukui, K. Murata, S. Ohara, H. Abe, M. Naito, K. Nogi, Morphology control of Ni–YSZ cermet anode for lower temperature operation of SOFCs, J. Power Sources 125 (2004) 17–21.
[12]B. Rösch, H. Tu, A.O. Störmer, A.C. Müller, U. Stimming, Electro-chemical characterization of Ni–Ce0.9Gd0.1O2δfor SOFC anodes, Solid State Ion. 175 (2004) 113–117.
[13]H.J. Cho, G.M. Choi, Effect of milling methods on performance of Ni–Y2O3-stabilized ZrO2 anode for solid oxide fuel cell, J. Power Sources 176 (2008) 96–101.
[14]R.J. Gorte, J.M. Vohs, Nanostructured anodes for solid oxide fuel cells, Curr. Opin. Colloid Interface Sci. 14 (2009) 236–244.
[15]C. Ding, H. Lin, K. Sato, T. Kawada, J. Mizusaki, T. Hashida, Improvement of electrochemical performance of anode-supported SOFCs by NiO–Ce0.9Gd0.1O1.95nanocomposite powders, Solid State Ion. 181 (2010) 1238–1243.
[16]C. Ding, T. Hashida, Synthesis and evaluation of NiO–Ce0.8Sm0.2O1.9 nanocomposite powders for low-temperature solid oxide fuel cells, Int. J. Hydrog. Energy 36 (2011) 5567–5573.
[17]L. Holzer, B. Münch, B. Iwanschitz, M. Cantoni, T. Hocker, T. Graule, Quantitative relationships between composition, particle size, triple phase boundary length and surface area in nickel–cermet anodes for solid oxide fuel cells, J. Power Sources 196 (2011) 7076–7089.
[18]Y. Okawa, Y. Hirata, Sinterability, microstructures and electrical proper-ties of Ni/Sm-doped ceria cermet processed with nanometer-sized particles, J. Eur. Ceram. Soc. 25 (2005) 473–480.
[19] B. Cela, D.A. Macedo, G.L. Souza, A.E. Martinelli, R.M. Nascimento, C. A. Paskocimas, NiO–CGO in-situ nanocomposite attainment: one step synthesis, J. Power Sources 196 (2011) 2539–2544.
[20] D.A. Macedo, G.L. Souza, B. Cela, C.A. Paskocimas, A.E. Martinelli, F. M. Figueiredo, F.M.B. Marques, R.M. Nascimento, A versatile route for the preparation of Ni–CGO cermets from nanocomposite powders, Ceram. Int. 39 (2013) 4321–4328.
[21] F.P.F.V. Berkel, F.H.V. Heuveln, J.P.P. Huijsmans, Characterization of solid oxide fuel cell electrodes by impedance spectroscopy and I–V characteristics, Solid State Ion. 72 (1994) 240–247.
[22] T. Ishihara, T. Shibayama, H. Nishiguchi, Y. Takita, Nickel–Gd-doped CeO2cermet anode for intermediate temperature operating solid oxide fuel cells using LaGaO3-based perovskite electrolyte, Solid State Ion. 132 (2000) 209–216.
[23] F.M. Figueiredo, J. Frade, F.M.B. Marques, Role of the reference position on overpotential measurements, Bol. Soc. Esp. Cerám. Vidr. 38 (2009) 639–642.
[24] S.P. Jiang, S.P.S. Badwal, Hydrogen oxidation at the nickel and platinum electrodes on yttria-tetragonal zirconia electrolyte, J. Electrochem. Soc. 144 (1997) 3777–3784.
[25] N.P. Brandon, D.J.L. Brett, Engineering porous materials for fuel cell applications, Philos. Trans. R. Soc. A 364 (2006) 147–159.
[26] A. Babaei, S.P. Jiang, J. Li, Electrocatalytic promotion of palladium nanoparticles on hydrogen oxidation on Ni/GDC anodes of SOFCs via spillover, J. Electrochem. Soc. 156 (2009) B1022–B1029.
[27] P. Kim, D.J.L. Brett, N.P. Brandon, The effect of water content on the electrochemical impedance response and microstructure of Ni–CGO anodes for solid oxide fuel cells, J. Power Sources 189 (2009) 1060–1065.
[28] S.P. Jiang, W. Wang, Y.D. Zhen, Performance and electrode behaviour of nano-YSZ impregnated nickel anodes used in solid oxide fuel cells, J. Power Sources 147 (2005) 1–7.
[29] J.C. Chen, B.H. Hwang, Microstructure and properties of the Ni–CGO composite anodes prepared by the electrostatic-assisted ultrasonic spray pyrolysis method, J. Am. Ceram. Soc. 91 (2008) 97–102.
[30] M. Gödickemeier, Mixed ionic electronic conductors for solid oxide fuel cells (Dissertation ETH), vol. 11348, Swiss Federal Institute of Technol-ogy, Zurich, 1996.
[31] U.P. Muecke, K. Akiba, A. Infortuna, T. Salkus, N.V. Stus, L. J. Gauckler, Electrochemical performance of nanocrystalline nickel/ gadolinia-doped ceria thinfilm anodes for solid oxide fuel cells, Solid State Ion. 178 (2008) 1762–1768.
[32] S.P. Jiang, S. Zhang, Y.D. Zhen, A.P. Koh, Performance of GDC-impregnated Ni anodes of SOFCs, Electrochem. Solid-State Lett. 7 (2004) A282–A285.
[33] E.V. Tsipis, V.V. Kharton, I.A. Bashmokov, E.N. Naumovich, J.R. Frade, Cellulose-precursor synthesis of nanocrystalline Ce0.8Gd0.2O2δ for SOFC anodes., J. Solid State Electrochem. 8 (2004) 674–680.
