Enhancing Oxygen Reduction Reactions in Solid Oxide Fuel Cells

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Enhancing Oxygen Reduction Reactions in Solid Oxide Fuel Cells with Ultra-Thin Nanofilm Electrode-Electrolyte Interfacial Layers Aniruddha Kulkarni, Sarb Giddey, and Sukhvinder P.S. Badwal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09345 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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The Journal of Physical Chemistry

Enhancing Oxygen Reduction Reactions in Solid Oxide Fuel Cells with Ultra-thin Nanofilm Electrode-Electrolyte Interfacial Layers Aniruddha P. Kulkarni*, Sarb Giddey, S.P.S. Badwal CSIRO Energy Private Bag 10, Clayton South 3169, Victoria, Australia *Corresponding author email: [email protected] Abstract Low and intermediate-temperature solid oxide fuel cells (SOFCs) and solid oxide membrane reactors are gaining considerable attention for applications in energy conversion, chemical synthesis and electrolysis. The sluggish oxygen reduction reaction (ORR) causes significant voltage losses for the air electrodes (cathode) in these systems, particularly at lower temperatures (400–600 °C). Surface engineering of electrolytes with nanograined, thin-film interfaces introduced between the cathode and electrolyte is a promising method of reducing the voltage losses associated with ORR on the cathode. In this work, we deposited a nanocrystalline ceria ultra-thin film (nanofilm) interface layer on yttria-stabilised zirconia (YSZ) and samaria-doped ceria (SDC) electrolytes using a simple and scalable solution-based deposition process. The effect of the interfacial layers on cathode polarisation resistance was studied using impedance spectroscopy and surface imaging techniques. At low temperature (400 °C), the nanofilm interface layer reduced cell polarisation resistance substantially for both YSZ and SDC

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electrolytes. The reduction in the polarisation resistance is primarily attributed to the increased interfacial surface area between the platinum electrode and the electrolyte, as confirmed by almost an order of magnitude increase in the interfacial capacitance with nanofilm interface, and three-dimensional reconstruction of the surface structures using Confocal Microscopy and Atomic Force Microscopy. The testing of anode-supported thin electrolyte SOFCs at 600°C clearly demonstrated the benefits of nanofilm interfacial layer in improving the power output of cell. Key words: nano materials, SOFC, cathode, interface layers, thin film ceria Introduction Fuel cells have long been cherished as a potential source of clean, high-efficiency electrical power. Solid oxide fuel cells (SOFCs), which are based on an oxide-ion-conducting electrolyte, traditionally operate within a high temperature range of around 800–1000 °C to ensure sufficient ionic transport and fast electrode kinetics. However, such a high-temperature operation causes problems regarding material selection and degradation, and also limits the application of SOFCs in stationary power generation.1 With recent developments in electrolyte thin-film deposition technologies and cell fabrication methods, the operating temperature can be significantly reduced; in fact, with advanced microfabrication technologies, SOFCs are even being considered for portable applications at low temperatures (300–500 °C).2-4 In addition to fuel cells, solid-state electrochemical reactors are being considered for applications such as steam electrolysis, oxidative coupling of methane and CO2 conversion to value-added chemicals. However, several technical issues need to be addressed for such technologies to be successfully commercialised. These include high cost of fabrication, low electrolyte conductivity and poor cathode (air electrode) performance, particularly at low temperatures (300–600 °C). 2 ACS Paragon Plus Environment

1-4

While the issue

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related to electrolyte resistance can be mitigated using thin films (few microns) or ultrathin films (submicron) electrolytes, the higher cathode polarisation resistances leading to a significant increase in cathode overpotential (voltage loss across the cathode/electrolyte interface), at low to intermediate temperatures (300–650 °C) remains a challenge. Strategies such as the use of alternative, multiphase or composite cathodes, as well as nanostructured cathode fabrication by infiltration techniques, are being explored to improve SOFC performance at low temperatures.5 The alternative cathode materials such as La or Ba doped strontium cobalt ferrite (LSCF or BSCF) have shown acceptable performance at intermediate temperatures (500-600°C), however, their stability with yttria stabilised zirconia (YSZ) electrolyte when sintered at higher temperatures is still a major concern.1-4 The modification of cathode-electrolyte interface via electrolyte surface modification is one of the approaches studied extensively in recent years and have shown promising performances.6 The introduction of an electrocatalytic thin film as an active interface between the cathode and electrolyte has considerably reduced polarisation resistance and improved oxygen reduction reaction (ORR) kinetics.7 In particular, nanocrystalline thin-film interfaces improve the performance of SOFC cathodes significantly in the temperature range of 300–600 °C.8-9 Huang et al. reported an order-of-magnitude reduction in cathodic polarisation of platinum (Pt) electrodes due to insertion of nanocrystalline ceria thin films between the Pt electrode and YSZ electrolyte.10 Although the concept of introducing a ceria-based interlayer between the electrolyte and cathode has been studied widely in the past, a dramatic improvement in cathodic performance has only been observed for submicron thin films with a nanocrystalline structure. The improved performance of thin-film cathodes is thought to be due to better surface exchange and faster incorporation of oxygen in nanocrystalline thin films, though the exact mechanism of



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oxygen exchange reactions in such systems is still under investigation.7-10 In most studies, the processes used in thin-film cathodes or interface deposition typically involve the use of single crystal substrate, and/or expensive deposition methods such as magnetron sputtering and pulsed laser deposition. From a commercial perspective, such methods are too costly for scale up and may not be viable for mass production. In this work, we report the effect of a nanocrystalline ceria thin-film interface layer (referred to as ‘nanofilm’ hereafter) on the polarisation resistance of Pt electrodes on polycrystalline, 8 mol% YSZ electrolyte discs. In this work, platinum is chosen as electrode material as it is well studied electrode for the solid electrolytes, and also it has been considered as air electrode of interest for low temperature SOFCs especially for microsolid oxide fuel cells with ultra-thin film electrolytes.2, 3 The nanofilm was prepared by a simple method of spin-coating a polymeric precursor. Cathode polarisation was studied as a function of temperature and current loading. We also investigated the performance of an anode-supported fuel cell with and without the nanofilm. Further, to compare the effect of nanograined ceria with micrograined ceria on cell polarisation, the polarisation resistance of Pt electrodes was measured on polycrystalline, micrograined, 20 mol% samaria-doped ceria (SDC) electrolyte discs with and without nanofilm.

1. Experimental 1.1 Material Synthesis YSZ and SDC electrolyte discs were prepared by die pressing YSZ (Tosoh, TZ8Y) and SDC powders (SDC-20, Fuel Cell Materials, OH, USA) followed by iso-static pressing at 30,000 psi. The pressed electrolyte discs were sintered at 1500 °C for 2 h in air and then ground and polished on both sides with an automatic polisher (Struers) with final polish using 500 grit size abrasive.


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Intentionally, very fine grit size abrasive was not used so that the bare electrolyte surface texture finish should be somewhat similar to the electrolyte surface prepared by tape casting used in commercial SOFCs. SDC nanofilm was deposited by a spin-coating method using a polymeric precursor. The cation source compounds used were high purity grade Ce(NO3)3.6H2O and Sm(NO3)3.6H2O (Sigma Aldrich) in appropriate molar ratios to yield a final composition of Ce0.8Sm0.2O1.9. Glycine (Sigma Aldrich) was used as a fuel and ethylene glycol (Sigma Aldrich) was used as a polymerising agent. The polymerisation was carried out at 80 °C in a constant temperature bath. The details of this method are reported elsewhere.11 The precursor, with a nominal viscosity of 80 centipoise, was spin-coated on polished YSZ or SDC electrolyte discs using a two-stage spin coater (Chemat, China) at speeds of 500 and 4000 rpm for 1 min each. Spin-coated discs were fired at 700 °C for 1 h to form a thin film on the disc. The spin-coating process was carried out twice in repeated cycles. After the disc was coated with the film, a platinum electrode was deposited on top of the film by spray coating with Pt paste (Engelhard 6082)–triethylene glycol mixture. Specimens prepared with ceria nanofilm are referred to as YSZ-N, and bare specimens prepared without the nanofilm, are referred to as YSZ-B. Similarly, SDC samples are referred to as SDC-N and SDC-B with and without the nanofilm. After depositing the Pt electrode, specimens were again fired at 700 °C for 4 h to burn the organic matter.

1.2 Materials characterisation The precursors used for deposition of the nanofilm were fired under similar conditions as that of the film in a crucible, and the resulting powder was used to obtain an X-ray diffraction (XRD) pattern. The images of cross section of the deposited thin film were obtained using Field



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Emission SEM (FESEM; Merlin-Zeiss Ultra-Plus with a Gemini II column, Germany). Confocal microscopy was used to observe surface morphology and texture over the area of up to 130 µm (LEXT OLS400, Olympus, Japan) on the surface of the electrolyte before Pt electrode deposition. Atomic Force Microscope (AFM, Bruker FastScan) was used to obtained high resolution images and particle sizes of nanofilm sample. For AFM, the tip force constant was 42 N/m with a resonant frequency of 300 kHz. Scans were performed at 0.8 - 1 Hz with 512 data points per scan line. All AFM images were processed using NanoScope Analysis 1.5 software (Bruker) along with ImageJ (Wayne Rasband, Research Services Branch, NIMH, Bethesda, USA).

1.3 Electrochemical Characterisation and fuel cell testing The impedance analysis was carried out using three electrode cells with Zahner IM6e (Zahner Inc., Germany) impedance analyser. The details of the measurement method used are described in Reference 12.12 In brief, the Pt working electrode was spray coated on top of the ceria nanofilm interface layer, and the Pt counter electrode was spray coated on the opposite sides of the electrolyte discs. Both electrodes were 10 mm in diameter. The ring reference electrode, which had a strip width of about 2 mm, was brush painted on the same side as the counter electrode. The nominal distance between the counter electrode and reference electrode was about 4 mm. For testing of cells, a test fixture with a spring loading arrangement was used with Au meshes (Fiaxell SOFC Technologies, Switzerland) pressed on Pt electrodes for current collection. All measurements were carried out in air. Fuel cell testing was carried out with commercial anode supported half cells (Fuel Cell Materials, USA). As obtained cells consisted of 0.5 mm thick NiO-YSZ composite anode coated


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with 10 µm electrolyte (YSZ) layer. To complete the cell, Pt was used as the cathode and was coated by spraying Pt paste as described above. Two cells were prepared: one with the nanofilm interface layer deposited on the YSZ electrolyte and one without. To deposit the nanofilm on the cell, the same procedure was followed as for coating the electrolyte discs. For fuel cell testing, Ceramabond-552 (Aremco) was used as a sealant to separate the anode and cathode chambers. Humidified hydrogen (BOC, Industrial grade) was used as a fuel and air as an oxidant. Before cell operation at 600°C, the anode support was pre reduced insitu in H2 for 4 hrs at 650° C. Pt paste was used along with Au mesh as anode and cathode side current collectors. The current collector meshes were spring loaded to ensure firm contact between electrode and the mesh. The voltage-current-power data were obtained using a current sink (MTI, USA) at the scan rate of 2.5 mV/S at cell operating temperature of 600° C.

2. Results and Discussion 2.1 Structural and microscopic characterisation Figure 1 shows the XRD pattern of the powder obtained by burning the precursor used to deposit the nanofilm at 650 °C. The powder was used in place of a deposited thin film, because the XRD pattern of the thin film on the electrolyte could not be separated from that of the bulk electrolyte substrate. This is due to the very small thickness of the nanofilm (discussed later) and much higher intensity of diffraction peaks from the electrolyte substrate suppressing the thin-film peaks. The average crystallite size, calculated using Scherrer’s equation at full width half maxima, was about 7 nm. The results are in agreement with previously reported data for SDC nanoparticles prepared via combustion of the polymeric precursor.13



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Figure 1. XRD Pattern obtained by burning the nanofilm precursor solution at 700°C. Blue lines show the positions for 20 mol % Sm doped Ceria (SDC) peaks from JCPD data base

Figure 2. Scanning electron microscope image (cross-section) of the yttria-stabilised zirconia (YSZ) electrolyte coated with ceria nanofilm interface layer and platinum (Pt) electrode on top of the interface; (a) low magnification and (b) high magnification.


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Figure 2 (a) and (b) are SEM cross-sectional images of a YSZ pellet coated with nanofilm and Pt electrode after heat treatment at 650 °C for 4 h. SEM images taken at different sites of the fractured specimen indicate a nominal nanofilm thickness of about 45–50 nm. It should be noted that the thickness of the nanofilm may vary across the sample as the substrate (electrolyte) disc surface itself is somewhat non-uniform and has been discussed later in this manuscript. The thickness of Pt cathode was about 6 µm. Another key feature of the SEM image is the fine microstructure of the Pt electrode. This is attributed to the lower temperature of the heat treatment (650 °C) along with shorter hold time at 650 °C. The particle size of heat-treated Pt electrode is about 6–8 nm, which is consistent with the literature report.14 Figure 3 is 3D confocal microscopic images of the surface, taken before (a) and after (b) coating of nanofilm on polished YSZ electrolyte. Images of uncoated substrate (YSZ-B) clearly show more ledges, crests and troughs (Figure 3 (a)), as compared to the surface after coating (Figure 3 (b)). Some scratches, perhaps the polishing marks are still visible even on coated surface (YSZ-N), however, the topographical differences between YSZ-B and YSZ-N surfaces are clearly evident. It should be noted that the colour contrast in both image is a function of both, surface morphology and some sample tilt at the time of imaging. It appears that with the spin coating method used in this work, the very fine ceria nano-particles penetrated inside the surface irregularities of the electrolyte making it smooth and “levelled” to some extent. It should further be noted that the bare electrolyte surface texture used here is similar to that of electrolytes prepared by tape casting used in commercial SOFCs.15



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Figure 3. Confocal microscopy images of the yttria-stabilised zirconia electrolyte coated (a) with and (b) without ceria nanofilm interface layer. Z-scale units are in µm. Figure 4 (a) (b) (c) (d) and (e) are AFM images obtained at the centre of ceria coated (b, d & e) and uncoated (a and c) electrolyte disc which clearly shows filling or levelling of the electrolyte surface in alignment with con-focal data. The AFM image of YSZ-B (Figure 5 a) surface shows some troughs almost 450 nm deep and crests at 450 nm in height. Thus the thickness of film could vary from 50 nm to higher values, however SEM image taken at a number of spots indicated that the thickness on grain surfaces was around 50 nm.


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Figure 4. Atomic force microscope images of the yttria-stabilised zirconia (YSZ) electrolyte (a) without nanofilm, (b) with nanofilm (YSZ-N); (c) high-resolution view of YSZ-B, (d) highresolution view of YSZ-N and (e) two-dimensional view of YSZ-N surface 11 ACS Paragon Plus Environment


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Further, the comparison of AFM images shows that the specimen with nanofilm interface would offer more points of contacts per unit electrode area as compared to the uncoated electrolyte surface (Figure 4 c and 4 d). The effect of this enhanced contact area, between deposited Pt electrode and nanofilm surface on the electrolyte, is further discussed in the electrochemical characterisation section. From the surface image (Figure 4e) the average grain size of ceria was estimated to be around 6 to 7 nm, which is in agreement with that determined from XRD data and also with that in a previous report where ceria thin films were deposited using a similar method.16 In comparison, the grain size of the underlying YSZ electrolytes, prepared using the same electrolyte composition and sintering conditions, was about 8-10 µm from the previously reported data.17 The surface area, determined semi-qualitatively from AFM image, was higher by a factor of about 2.1 for YSZ-N (Figure 4 d) compared to the geometric surface area and this factor was about 1.1 for YSZ-B. It should be noted that the nanoscale features are not fully resolved due to the large scan size used during AFM imaging and also there is a limitation on the image processing software resolution. Thus the true surface area of YSZ-N may be much higher than that determined by AFM image analysis. The coatings obtained with solution-based processes are affected by the underlying substrate conditions and morphology.18 Thus although the thickness of ceria nanofilm on YSZ was measured to be ~50nm, some thickness variation is possible across the entire electrolyte surface based on general morphology of YSZ surface.

2.2 Electrochemical Characterisation Figure 5 shows the EIS spectra obtained at open circuit voltage (OCV) and under 1 V polarisation (shown as inset) for the YSZ electrolyte discs without (YSZ-B) and with nanofilm


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(YSZ-N) at 400 °C. The intercepts of the high frequency arc were attributed to grain and grain boundary resistances of YSZ electrolyte which are known to appear distinctively in Nyquist plot at lower temperatures (450°C or less) for YSZ. These did not change with applied potential as expected. However, for both YSZ-N and YSZ-B specimens, the magnitude of only the lowfrequency arc decreased substantially with applied potential. Hence, this arc was ascribed to cathode polarisation resistance (RP), noting that a three-electrode arrangement was used and the data here was recorded between working (cathode) and reference electrodes. A comparison of EIS spectra at 400 °C under OCV and under applied direct current (DC) voltage conditions clearly showed that the cathode polarisation resistance is lower by a factor of around 4–5 for the specimen with the nanofilm interfacial layer between the Pt electrode and YSZ electrolyte.

Figure 5. Electrochemical Impedance Spectra (EIS) obtained at open circuit voltage (OCV) and under 1 V polarisation (inset) for the YSZ electrolyte discs with (YSZ-N) and without nanofilm (YSZ-B) at 400 °C 13 ACS Paragon Plus Environment


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A closer inspection of the shape of the EIS spectra for YSZ-B and YSZ-N showed a nearsemicircular arc for YSZ-N, while for YSZ-B, the arc was more depressed or flattened. Figure 6 compares the total cathode polarisation resistance (RP) as a function temperature for Pt/YSZ-N and Pt/YSZ-B cells at OCV. At all temperatures, the cathode polarisation resistance at the Pt/YSZ-N interface is significantly lower than at the Pt/YSZ-B interface. However, the magnitude of the difference between RP decreases somewhat as the temperature rises. It therefore appears that the nano structured interface is more beneficial in reducing polarisation resistance at lower temperatures.

Figure 6. Arrhenius plot of polarisation resistances obtained from electrochemical impedance spectra obtained at open circuit voltage for yttria-stabilised zirconia (YSZ) electrolyte discs with (YSZ-N) and without (YSZ-B) nanofilm interface


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The polarisation resistance of Pt/YSZ-B and Pt/YSZ-N interfaces under different cathodic current loadings are compared in Figure 7. Both specimens showed a significant decrease in polarisation resistance with increasing current density. This is consistent with the polarisation characteristics of SOFC cathodes, the overpotential losses for which decrease on application of increasing current load. In this work, to avoid current induced zirconia reduction (blackening), the applied voltage was kept below 1.7 V and thus data only up to 100 mA/cm2 current densities was acquired.19 The cathodic polarisation resistance of YSZ-N was considerably lower at OCV than YSZ-B; however, the difference became less conspicuous at higher current densities (Figure 7). The ohmic resistances determined from the high-frequency intercept of the electrode arc on the real axis at 600 °C were about 5.1 ohms for both YSZ-B and YSZ-N. This close match indicates that the nanofilm interface has no significant contribution to the ohmic resistance, and there was no detrimental phase formation between the ceria nanofilm and YSZ electrolyte.

Figure 7. Plot of polarisation resistances versus current density for yttria-stabilised zirconia (YSZ) electrolyte discs with (YSZ-N) and without (YSZ-B) nanofilm interface at 600 °C 15 ACS Paragon Plus Environment


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2.3 Impedance Modelling and influence of the nanofilm interfacial layer on cathodic processes The polarisation resistance in SOFC cathodes arises from overall processes of electrochemical oxygen reduction reaction (ORR) which comprises several chemical and electrochemical steps.20 The elementary steps can be: gas phase diffusion to near the electrode/electrolyte interface, oxygen adsorption and dissociation on the electrode as well as the electrolyte surface, diffusion of adsorbed oxygen to the triple phase boundary (TPB), the charge transfer step involving formation of oxygen anion near TPB, and oxygen incorporation into the vacancy in the electrolyte. Although the exact mechanism is debated, possible reaction pathways are depicted in Figure 8.

Figure 8. Schematic of proposed reaction pathways in literature for oxygen reduction reactions for platinum electrode (adapted from reference 21)21 The reaction path and step/s that dominate or becomes rate limiting, will depend on the type of electrode materials used; its microstructure and porosity; and its ability to dissolve and transport oxygen to the electrode/electrolyte interface (surface or bulk diffusion) for charge transfer across 16 ACS Paragon Plus Environment

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the interface. The surface diffusion of oxygen ions onto the electrode surface is most likely the dominating step in the case of metallic electrodes such as Pt, because of the low oxygen solubility and diffusivity through the bulk of the Pt.21 In addition to the bulk and surface pathways, oxygen adsorption may occur directly on the electrolyte surface near TPB followed by diffusion to TPB area on the electrolyte surface. The deconvolution of EIS spectrum for such a complicated processes is a challenging task, however, there is a general agreement that the oxygen incorporation process involving the charge transfer step at triple phase boundary (electrode/electrolyte interface) has a smaller time constant, and therefore appears at higher frequency end on the EIS spectrum immediately before electrolyte processes (oxygen ion conduction). The diffusion related processes, including gas phase diffusion and surface/bulk diffusion, appear at relatively lower frequency ranges.22 In the present work, the EIS data obtained were modelled using a simple circuit consisting of a series resistor and two resistorconstant phase elements (R-CPE) sub-circuits, shown as inset in Figure 9 (a). A series resistor (R1) represents the ohmic losses, which include electrolyte resistance as well as all other ohmic losses due to Pt wires, Au mesh, electrode ohmic resistance and contact resistance. Two R-CPE elements represent low frequency (diffusion processes related to oxygen species) and high frequency (charge transfer) processes at or near TPB. Although cathodic processes can be modelled using more complex circuits or more R-CPE elements, we found that a simple circuit gave the best fit (less than 2% error in fitting) for all the EIS data acquired in this work. As most of the data were obtained either at OCV or at low current densities, no gaseous diffusion-limited behaviour was obvious from EIS spectra, and hence no Warburg-type element was used in the model. Figures 9 (a) and (b) compare the values of polarisation resistance components associated with low frequency and high frequency cathode processes for YSZ-B and YSZ-N samples. It



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appears that both high and low frequency processes were significantly affected by the presence of ceria nanofilm interface.

Figure 9. Arrhenius plots comparing (a) high-frequency resistance (R2) and (b) low-frequency resistance (R3) for yttria-stabilised zirconia (YSZ) electrolyte discs without (YSZ-B) and with (YSZ-N) nanofilm interface. (Inset: equivalent circuit used for fitting) 18 ACS Paragon Plus Environment

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As both specimens have similar Pt electrodes and electrolytes with nanofilm interface being the only difference, it clearly obvious that in the present systems, the major electrochemical processes are taking place in the close vicinity of TPBs near the electrode/electrolyte interface. Both high and low frequency processes show a strong dependence on temperature. The apparent activation energy for the low frequency cathodic process was calculated to be 64 kJ/mol for YSZ-B sample and 51 kJ/mol for YSZ-N. The calculated apparent activation energies for high frequency processes are 73 kJ/mol for YSZ-B sample and about 66 kJ/mol for YSZ-N. The lower values obtained for both low and high-frequency resistive components for the specimen with nanofilm interface indicate that nanofilm influencing the slower surface adsorption/diffusionrelated processes, as well as the faster charge transfer processes.23 To further understand the reason for decrease in the cathodic polarisation resistance for specimens with ceria nanofilm, the capacitance values were calculated from fitting EIS data using the formula: = ()/ /

Eq. 1

Where C is capacitance in F/cm2 and R is polarisation resistance in Ω-cm2. Pseudo-capacitance Q and n were found by fitting the data and n value was 1>n>0.8. These capacitance values are plotted in Figure 10 (a) and (b), which clearly show that the capacitance associated with both high and low frequency processes is about an order of magnitude higher in the case of YSZ- N than that for YSZ-B. This is perhaps the first time an effect of nanocrystalline ultrathin film interface on the interfacial capacitance is clearly demonstrated.



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(a)

(b)

Figure 10. Arrhenius plots comparing of (a) high-frequency capacitance (C1) and (b) lowfrequency capacitance (C2) of yttria-stabilised zirconia (YSZ) electrolyte discs with (YSZ-N) and without (YSZ-B) nanofilm interface


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In general, an increase in the electrochemical double layer capacitance suggests a higher electrochemically active area. The higher the interfacial area between electrode and electrolyte, the lower the polarisation resistance of the electrode: as is obvious from the inverse relation between area (A) and resistance (R). The capacitance data aligns well with the inferences from AFM images (Figure 4), which also indicate significant increases in surface area at the nanoscale and potentially more contact points available between the nanofilm interface and Pt electrode. The increased surface area and/or active reaction sites due to nanofilm interface could also represent more oxygen adsorption and charge transfer sites, which would lead to increased capacitance associated with low and high-frequency processes. The capacitance values in the 104

to 10-6 F/cm2 region for both YSZ-B and YSZ-N provide further indication that the high-

frequency arc is associated with the charge transfer reaction.24,25 In general, the capacitance values do not show a strong dependence on temperature, except for the low frequency capacitance (C2) for YSZ-B sample which appears to increase with temperature. To understand if the observed reduced cathodic polarisation resistance was indeed due to the nanostructure of the ceria interface, or whether the catalytic properties of ceria material had played a role, we compared the polarisation resistances of Pt electrode on micrograined ceria electrolyte with and without nanofilm. Ceria-based materials have higher electrocatalytic activity for oxygen reduction, as well as a higher ionic conductivity than YSZ. The Pt electrodes on ceria electrolytes show lower polarisation resistance than YSZ electrolyte discs under OCV and loading conditions.26 The grain size of as-sintered ceria electrolyte from the same source, composition and sintering condition was estimated to be in the range of 2–5 µm.27 Figure 11 compares the EIS spectra of SDC–B and SDC-N Samples at 400°C. It is clearly evident that the



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polarisation resistance at OCV is significantly reduced with the introduction of nanofilm as in the case of YSZ electrolyte.

Figure 11. Electrochemical impedance spectra obtained at open circuit voltage for samariadoped ceria (SDC) electrolyte discs with (SDC-N) and without (SDC-B) nanofilm interface at 400 °C


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Figure 12. Arrhenius plot of total polarisation Rp (R2 + R3 as per inset in Figure 9) resistances at open circuit voltage for samaria-doped ceria (SDC) electrolyte discs with (SDC-N) and without (SDC-B) nanofilm interface The total polarisation resistances of SDC-B and SDC-N samples at OCV different temperatures is compared in Figure 12 ; the trend is consistent with the findings for YSZ electrolyte based systems. The EIS data for SDC-N and SDC-B samples could be fitted to the same circuit model as for YSZ-N and YSZ-B samples (Figure 9 (a)) indicating similar behaviour and contribution from both charge transfer and surface dissociation/diffusion processes. Nanofilm has pronounced effect on the cathodic polarisation resistance of Pt electrode with SDC electrolyte as well. SDC electrolyte possesses same chemical composition as that of nanofilm and thus the reduction in polarisation resistance cannot be attributed to electro-catalytic activity of ceria alone. It is possible that both increased interfacial area and electro-catalytic activity of ceria work synergistically to enhance overall oxygen reduction reaction at cathode. 23 ACS Paragon Plus Environment


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In the present work, the effect of increased interfacial area (and contact points between Pt electrodes and YSZ or SDC electrolytes) at the surface of the electrolyte, due to incorporation of nanostructured intermediate layer, is clearly evident. However, certain other factors cannot be ignored, and these are discussed below. For example, there may be some contribution from the enhanced defect structure and increased surface vacancy concentration on the nanofilm interface. Nanocrystalline thin films are known to have higher grain boundary density and defect concentrations as well as a higher lattice strain as compared to micro-grained YSZ-B, or SDC-B especially when nanofilm thickness is at the submicron scale.28, influence electrochemical and ionic transport properties.30,

31

29

This can dramatically

Enhanced oxygen vacancy

concentrations on the nanofilm surface may lead to more number of active sites for oxygen adsorption, which reduces the resistance associated with adsorption and consequent surface diffusion process on electrolyte near TPBs. Further the incorporation of adsorbed oxygen into lattice is much faster in nano-crystalline thin films along with a higher oxygen surface exchange coefficient ( ) as compared to micro-grained YSZ.32 Recently, Develos – Bagarinao demonstrated that Ce3+ concentration is significantly higher in the nano-grained (20 nm) ceria film surfaces as compared to micro-grained (~ 1µm) ceria, leading to enhanced oxygen vacancy concentration and up to 10 times higher surface exchange coefficient ( ) as determined by tracer diffusion method.33 In the present work, based on the polarisation resistance values, the for YSZ–N samples can be calculated to be at least five times higher than YSZ-B at 400 °C considering the relation:

=

Eq.2


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where k is the Boltzmann constant, e is the charge on electron, T is the process temperature, R is the area specific polarisation resistance and is the total concentration of lattice oxygen.34 Overall, the role of nanofilm in the reduction of polarisation resistance is evident from the impedance analysis of the data. Based on our observations, the main contributor appears to be the increased interfacial area due to incorporation of the nanofilm. However, other factors that may have influenced the process include the better electrocatalytic activity of ceria, and the unique, intrinsic properties of nanostructured thin films (e.g. large number of surface defects).

2.4 Fuel Cell testing To demonstrate the benefit of incorporating a nanofilm interface between the electrode and electrolyte, we constructed two fuel cells: one with nanofilm and one without. Figure 13 compares the I–V–P curves of the two fuel cells. The power density obtained from the cell with nanofilm interface was about 163 mW/cm2 at 600 °C, compared with about 121 mW/cm2 for the cell without nanofilm.



200

1.0

160

0.8 120 0.6 80 0.4 40

0.2 0.0

Power density, mW/cm2

1.2

Cell voltage, V

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0 0

100

200 300 Current density, mA/cm2

400

Figure 13: I–V–P curves for anode-supported thin film cell with (solid lines) and without nanofilm interface (dashed lines) at 600 °C with humidified H2 fuel The cells were tested without optimising the gas flow field or flow rates, and only at slightly positive gas pressure; if these parameters were optimised, further improvement in power output would be possible. Nonetheless, as the anode and electrolyte were identical, the improvement in performance is clearly attributable to greater cathode performance. As mentioned in the Introduction section, the Pt was chosen as a model electrode to clearly understand the effect of nanofilm interface on electrode polarisation resistance. Instead of Pt electrode, the use of Ag or high-performance ceramic electrodes, such as barium strontium cobalt ferrite (BSCF), may further improve cell performance.


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Conclusion An SDC nanofilm interface layer was successfully coated onto die-pressed electrolyte discs using a simple method of solution-based spin coating. The morphology of the nanofilm was studied using AFM, which indicated ‘smoothing’ of the electrolyte surface with nanograins, which increased the contact area between electrode and electrolyte. The structural characterisation of the thin-film surface aligns well with electrochemical data. The cathode polarisation resistance of specimens with nanofilms was significantly lower than that of specimens without nanofilm, particularly at lower temperatures. This is attributed to increased interfacial area, as clearly demonstrated by order-of-magnitude higher capacitance for specimens with a nanofilm interface layer. The factors such as the better electrocatalytic activity of ceria and the intrinsic properties of nanostructured thin films (e.g. higher number of surface defects) may also have influenced the process. Fuel cell testing further confirmed significant improvement in the performance due to the nanofilm interface. The method used in this work for nanofilm deposition (spin coating) is more cost effective and faster than expensive methods such as sputtering or pulsed laser deposition. In principle, the method is scalable to areas used for typical planar SOFC stacks, and possibly in portable micro-SOFCs.



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AUTHOR INFORMATION Corresponding Author *Aniruddha P. Kulkarni Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT The authors thank Dr HyungKuk Ju for reviewing this paper and Dr Mark Easton and Mr Mark Greaves of CSIRO Characterisation Group for their assistance with microscopy.

ABBREVIATIONS SDC, Samaria Doped Ceria; YSZ, Yttria Stabilized Zirconia; ORR, Oxygen Reduction Reaction; OCV, Open Circuit Potential.


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Doped ceria ultra-thin film interface between electrolyte and electrode of Solid Oxide Fuel Cell increases interfacial area resulting in drastic reduction in the cell polarisation resistance. 254x190mm (96 x 96 DPI)

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Enhancing Oxygen Reduction Reactions in Solid Oxide Fuel Cells

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