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Inkjet printing infiltration of Gd:CeO₂ interlayer in commercial anode-supported SOFC

Abstract

Single step inkjet printing infiltration with doped ceria Ce_0.9Me_0.1O_1.95 (Me=Y; Sm) and cobalt oxide precursor inks was performed in order to modify the properties of the doped ceria interlayer in commercial (50x50x1 mm size) anode supported SOFCs. Deep penetration of the inks through the La_0.8Sr_0.2Co_0.5Fe_0.5O_3-δ porous cathode to the Gd_0.1Ce_0.9O₂ interlayer was achieved by optimization of the ink jetting parameters. A low temperature calcination (750^oC) resulted in densification of the porous interlayer as well as decoration of the cathode scaffold with nano-particles (~ 20-50 nm in size). The I-V testing in pure hydrogen showed maximum power density gain of 82% for Sm doped ceria and 97% for Y doped ceria infiltrated cells at 800^oC. The effect was largely assigned to the improvement in the Ohmic resistance. A comparison of the polarization resistances of the reference and the infiltrated cells at 800^oC revealed small changes in the activation and concentration polarizations losses due to the low nano-decoration density of the La_0.8Sr_0.2Co_0.5Fe_0.5O_3-δ scaffold surface. This work demonstrated that single-step inkjet printing infiltration, a non-disruptive scalable low-cost technique, could produce significant and reproducible performance enhancements in commercial SOFCs.

Keywords: Inkjet printing, Infiltration, Solid oxide fuel cells, Doped ceria, Cobalt oxide

1. Introduction

Currently available commercial SOFCs are highly efficient electrochemical systems operating at high temperatures (800-1000 ^oC) and offering fuel flexibility and combined heat and power generation [1-8]. Operation at these temperatures causes undesirable deteriorations and long term instability due to the chemical interaction between the dissimilar components of the SOFC stack [9,10], which ultimately leads to high production and operating costs. Therefore, a reduction of the operating temperature is considered critical for SOFCs widespread commercialisation. However, lowering the operating temperature also leads to performance deterioration caused by increased Ohmic losses due to the reduced ion conductivity and electrode polarisation losses associated with slower electrode reactions kinetic. A common approach towards resolving the issue is utilisation of mixed ion-electron conductor (MIEC), instead of the classical La_1-xSr_xMnO_3-δ (LSM), in order to extend the active zones for the oxygen reduction reaction (ORR) beyond the triple-phase boundary (TPB) [10]. Experimentation with MIEC materials, such as La_1-xSr_xCoO_3-δ (LSC) [12], La_1-xSr_xFeO_3-δ (LSF) [13], or La_1-xSr_xCo_1-yFe_yO_3-δ (LSCF) [14] have been extensively reported. LSCF is often considered a material of choice due to a combination of a number of desirable properties – high electronic conductivity (~340 S cm^-1 at 550 °C) [15], high oxygen ionic conductivity (~1×10^-1 S cm^-1 at 800 °C in air) and high tolerance towards Cr species compared to LSM electrodes [16,17]. However, Co-containing MIEC were found to react with Y-stabilized zirconia (YSZ) electrolytes during cell processing forming deleterious secondary phases like SrZrO₃ and La₂Zr₂O₇ [18]. Additionally, the substantial differences in thermal expansion coefficients (TEC) between the MIEC and the electrolyte lead to thermal cycling instability and short SOFCs lifetimes [19]. An introduction of a diffusion barrier layer (interlayer) of doped ceria between the electrolyte and the cathode is commonly used to alleviate the issue [20,21]. Such interlayer has to be thin and dense in order to avoid an addition of an undesirable Ohmic resistance. Vacuum techniques such as pulsed laser deposition [22] and magnetron sputtering [23] have been employed to deposit efficient dense barrier coatings of doped ceria. Unfortunately, utilization of vacuum techniques in SOFC processing is not readily scalable and thus considered prohibitively expensive. Commercial SOFCs most often have doped ceria interlayers prepared by conventional screen-printing technique. The interlayer sintering temperatures are usually kept below electrolyte sintering temperature in order to prevent formation of undesirable (Zr_1-xCe_x)O_2-y solid solution [24]. This temperature limitation along with the difference in TECs of doped ceria and doped zirconia logically result in a formation of porous interlayers. Consequently, diffusion of Sr species through the porous interlayer is observed, reaching and reacting with YSZ electrolyte [25]. Significant efforts were spent also on wet chemistry methods for improved interlayer deposition e.g. spin coating [26], dip coating [27], spray-pyrolysis [28] and electrostatic spray pyrolysis [29], all producing doped ceria interlayers with various degree of densification. Although offering scalability, the above-mentioned techniques often need multistep processing with repeated heat treatments due to their low deposition rates. The wet chemical methods also lead to a certain degree of non-uniformity and are inefficient in terms of ink waste outside cell area [30].

LSCF based cathodes were found to suffer from substantial long-term degradation [31-34] at the SOFC operating temperatures. The effect is often ascribed to Sr enrichment of the surface, where insulating SrO, (Sr(OH)₂ and SrCO₃ species form and subsequently suppress oxygen surface exchange kinetics [35]. Rupp et al [36] established that application of only ~4% of a SrO monolayer on the surface of a LSC thin film electrode resulted in severe deactivation of the cathode. At the same time, a minor decoration with Co-oxide (~2% of a monolayer) enhanced the oxygen exchange rate by ~13%. Wang et al [37] confirmed that B-site metal atoms (Co or Fe) are more reactive than A-site atoms (La or Sr) suggesting that ORR in cobaltite-based cathodes is strongly dependant on the surface morphology of the co-existing A- and B–site metal atoms.

Nano engineering of perovskite cathodes by infiltration have been actively researched in recent years. Improved performances and stability of the LSCF based cathodes were demonstrated via decoration of the porous cathode scaffolds with nanoparticles (noble and transition metal oxides, doped and un-doped ceria, MIEC compounds). Nano decoration of screen printed La_0.8Sr_0.2Co_0.5Fe_0.5O_3-δ (LSC8255) with Gd_0.2Ce_0.8O₂ [38] and tape-casted La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF6428) cathodes with Sm_0.2Ce_0.8O_1.95 (SDC) [39] was reported reduce the polarization resistance between two and four times at 750^oC. Infiltration of both doped ceria and Co_xO_y inks was shown to deliver additional benefits. Imanishi et al [40] reported that dual infiltration of Co(NO₃)₂ and Ce(NO₃)₃ into a LSM/YSZ cathode effectively suppressed the aggregation of the Co₃O₄ nanoparticles without any significant degradation of the catalytic activity of the co-infiltrated electrode at 800 °C for 100 hours. In general, the improvement in infiltrated cathodes was observed to be more pronounced at the lower end of the intermediate temperature region (500-800^oC) with most of the work done on symmetrical cells with doped-ceria electrolytes [39-41]. Co_xO_y was found to act as sintering aid improving the densification of Gd doped ceria [42-44] as well as to accelerate adsorption-dissociation-surface exchange reactions of oxygen as well as suppress Sr precipitation on the surface of LSCF cathodes [45].

This study focuses on utilisation of a non-disruptive scalable technique, namely inkjet-printing infiltration, for improving the performance of commercial anode-supported SOFCs (50 x 50 x 1 mm). Yttrium (YDC) and samarium (SDC) doped ceria inks were jetted in nano- litter volumes drops impinging with high velocity on the surface of the LSCF cathodes. The momentum of the drops forced a substantial amount of ink to reach the interface between the electrolyte and the cathode, depositing it into the pores of the interlayer. A second infiltration with Co_xO_y ink was applied in order to promote the sintering behaviour of the infiltrated doped ceria. The influence of the nanoparticles formed after the infiltration on the electrochemical performance was studied, taking into account the changes in both the Ohmic resistance of the Ce_0.9Gd_0.1O_1.95 (GDC) interlayer and the polarisation resistance of the LSCF cathode. A single step infiltration, leading to low overall infiltrate loading levels, was implemented in order to avoid blocking of the porous gas channels and introduction of concentration polarisation losses.

2. Methodology

Depending on the desired loading the infiltration can be done in several steps with low temperature (<800^oC) calcination in-between. In order to achieve higher loading levels, deeper penetration and more uniform distribution some research groups perform vacuum treatments after each infiltration step. The infiltrate inks are often tailored with surfactants and gelling agents in order to attain control over the phase and morphology of the infiltrated product – particle size, distribution, coverage etc. The infiltration strategies can be classified by two basic types of scaffold being infiltrated – (i) composite cathode (e.g. LSCF/GDC) or (ii) mono-phase scaffold (e.g. LSCF). The second type of scaffold generally requires higher loading levels of ink in order to provide connectivity of the infiltrated phase while the composite scaffolds are not restricted by this requirement as low amount of infiltrate can induce percolative short-range nano-decoration paths.

Commonly reported infiltration procedures have been predominantly performed in a laboratory environment using sample immersion or micropipettes. Such processes are wasteful, very slow and not scalable as well as non-uniform in terms of ink distribution. Recently, several attempts have been reported on scaling up the procedure in order to achieve compatibility with conventional ceramic SOFC production routes. Lee et al [46] reported using foam roller, which was then applied onto the porous anode functional layer. Kiebach et al [47] infiltrated stacks of anode supported SOFCs by ‘‘flushing’’ an aqueous solution containing metal nitrates and surfactants through the manifold compartments. Both methods appeared to be scalable but with limited control over the infiltration process and could lead to non-uniformities and large amounts of wasted ink. Mitchell-Williams et al [48] recently demonstrated the feasibility of GDC nano-infiltration into thick, commercial SOFC anodes with a simple, low-cost and industrially scalable inkjet printing procedure.

Inkjet printing infiltration (IJI) is characterized by the uniform delivery of precisely positioned small droplets (in the range of pico-L to nano-L volumes) at high rates (kHz). It is inherently cost-effective and environmentally friendly due to minimisation of the ink amount used and the lack of wastage. IJI ensures excellent lateral and in depth uniformity of ink delivery into the porous scaffolds. Commercial inkjet printing systems are widely available, from experimental platforms working with customised inks, up to mass manufacturing systems that can print rapidly and competitively at an industrial scale. The production of SOFC anodes and electrolyte coatings with a Domino print head was reported previously by Tomov et al [49] and Wang et al [50]. Tomov et al [45] used inkjet printing infiltration successfully to produce GDC nano-decorated LSCF/GDC composite cathodes with improved performance and durability.

3. Experimental

3.1 Anode supported SOFC preparation

Commercially available AS-SOFCs (CEREL, Poland) with LSCF cathodes were infiltrated by IJI with doped ceria and cobalt oxide inks precursors. The anode supports were made using high-pressure injection-molding. A functional layer (8YSZ+NiO – 1:1) and electrolyte layer (8YSZ) were applied using screen-printing. After drying, the coatings were sintered at 1400^oC for 3 h. Then, GDC interlayer was deposited by screen-printing and sintered at 1350^oC for 1h. La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ cathode (LSCF – thickness ~25 µm, porosity ~25 vol. %) was screen printed and sintered at 1100^oC for 1h. In the as-prepared 50 x 50 x 1 mm cells a 5 μm-thick YSZ electrolyte was supported by ~ 500 μm-thick Ni-YSZ anode (including the anode functional layer). La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ cathode with an effective area of 16 cm² was separated from the electrolyte by thin GDC interlayer (~2-3 μm thick). A current conducting LSM coating was applied on the top of the LSCF before testing. A detailed description of the anode supported SOFC manufacturing is published elsewhere [51].

3.2 Symmetric LSCF cathode cells preparation

GDC pellets were formed from 10 mol% gadolinium doped cerium oxide powder (99.9%, Sigma Aldrich) uniaxially pressed and sintered at 1400 °C in air. The sintered pellet had a diameter of 10.9 mm (area = 0.93 cm²) and thickness 0.6 mm. Suspension LSCF ink was synthesized from commercial La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF, Fuel Cell Materials), α-Terpineol (Sigma-Aldrich) and hydroxypropyl cellulose (Sigma-Aldrich) powders by wet milling with 3YSZ milling balls in a planetary mill. Terpineol (Sigma-Aldrich) was used as a carrier as well as natural dispersant. Hydroxypropyl cellulose was used as a polymeric dispersant for steric stabilisation The rheology of the ink was adjusted with Methanol (reagent grade, Sigma-Aldrich) due to its high volatility, which enabled fast drying of the droplets. LSCF cathodes were deposited by inkjet printing on the both sides of GDC pellets and sintered in air at 1100 °C with heating and cooling rates of 5 K min^-1. The final cathode thickness was ~25 μm.

3.2 Infiltrate inks synthesis

Ce_0.9Me_0.1O_1.95 (Me=Y or Sm) precursor solutions were prepared by diluting stoichiometric quantities of Cerium nitrate hexahydrate (99.999%, Alfa Aesar) and Yttrium or Samarium nitrate hexahydrate (99.9% Alfa Aesar) in absolute ethanol. Urea (>99.5%, Fisher Scientific) was added as a complexing agent in 1:1.5 molar ratio (Metals:Urea). For the Co₃O₄ ink the precursor ink was prepared in a similar manner using Cobalt nitrate hexahydrate (99%, VWR) and Urea. The powders were dissolved with stirring and heating at 40 °C. The solutions were cooled to room temperature before being passed through 3 μm glass fibre filter before storage.

3.4 Jetting optimization

The jetting behaviour of a print-head nozzle can be described with the help of dimensionless Reynolds (Re), Weber (We) and Ohnesorge (Oh) numbers –

Re=ρu0r0μ

;

We=ρu02r0γ

;

Oh=We0.5Re

where u_o, r_o, ρ, μ and γ denote the drop velocity, drop radius, ink specific mass density, ink viscosity and surface tension coefficient, respectively [52,53]. The Oh number was identified as a suitable grouping of the ink physical constants characterizing drop generation. The conditions leading to stable jetting without formation of satellite drops were suggested as 10 > Z > 1 (where Z = 1/Oh). Avoiding formation of delayed satellite drops is essential for achieving the desired jetting and infiltration control and requires knowledge of the drop velocity and the drop volumes specific for any particular printing system. The use of an integrated drop visualisation system allowed us to examine drops volumes and velocities and to assess the optimum printing parameters window. Such optimisation for the electromagnetic print heads was done by varying the opening times and nozzle pressures.

The system for visualisation consisted of a collinear LED strobe and camera (Stingray F-125B, Allied Vision Technologies) fitted with zoom lens (ML-Z07545, Moritex). The LED backlit the drops in flight. The nozzle and camera shutter were precisely triggered with increasing delay times between nozzle and camera to image the entire jetting process. The images were then analysed using in-house software that quantified the drop volumes and velocities. This enabled optimisation of the jetting parameters in order to produce desired drop formation and ensure nearly identical molar loading of each ink uniformly distributed on the surface of the cathodes. The drops jetted towards the surface of the cathode had defined kinetic energy prior to their contact with the porous medium. Hence, while the axial momentum of the impact was transformed to radial spreading, the pressure of the impact facilitated the ink penetration into the substrate. According to the numerical model developed by Reis et al [54] spreading and penetration of the drop into a porous medium was found to be governed by a number of parameters – Reynolds number (Re), Weber number (We), porosity (ε), and contact angle (θ). Thus, the penetration depth and spreading was a complex function of fixed parameters (ε), parameters with variability restricted by the rheological window of stable jetting for the particular print head (ρ, μ, γ and θ) and sufficiently variable parameters (u_o, r_o). Hence, in the present study, an optimization of u_o, r_o was made aiming at formation of small drops with high impingement velocity while simultaneously preserving stable jetting without formation of satellites or splashing. Additionally, the uniformity of the ink distribution was controlled by the choice of the printer lateral step size and the overlap between the drop replicas on the surface.

3.3 Inkjet printing infiltration

The cathodes of the AS-SOFC (NiO-8YSZ/8YSZ/GDC/LSCF) were infiltrated with a commercial 16 nozzle Domino Macrojet print head. The infiltrations were performed using optimized jetting parameters for each ink such as to have similar drop volumes and drop velocities as shown in Table 1. The doped ceria inks were deposited on to the cathode surface at room temperature with drop volumes of approximately ~57-58 nL and velocities of ~1.5-2.2 m s^-1. Single step infiltration was performed across the entire cathode surface in a square array pattern with a spacing of 1 mm between drops. Infiltration passes were repeated until the ink was no longer absorbed into the porous scaffolds. The cells were heated to 500 °C in air with heating and cooling rates of 5 °C min^-1. The room temperature ink deposition was then repeated for the Co₃O₄ precursor ink (58 nL, 2.2 m s^-1). The final stage calcination was done at 750 °C in air for 30 min (heating and cooling rates of 5 °C min^-1). The cells were weighed (SI-234 Analytic Balance, Denver Instrument) before and after the infiltration process to determine the mass loading of the inks. In as-described single step infiltration (without intermediate high temperature treatments or vacuum assistance), the loading levels for both doped ceria and Co₃O₄ infiltrates were found to be ~5 wt% with respect to the LSCF electrode.

Table1. Solution infiltration inks viscosities and jetting parameters

3.4 Characterisation

Anode supported SOFCs were tested on single cell test rig (I-V and Electrochemical Impedance Spectroscopy-EIS) at temperature of 800^oC, with pure H₂ flow of 1.0 NL/min acting as fuel and standard air flow of 2.0 NL/min as cathode feed. The microstructure was characterised using high resolution SEM-EDX (Nova NanoSEM) with an acceleration voltage of 15 kV. The cells were fractured and the cross-section sputter coated with palladium in order to prevent charging effects. Polished cross-sections of the cells were used for elemental mapping. Surface chemistries of the bulk and the interfacial areas of the cathodes were characterized using high-resolution X-ray photoelectron spectroscopy (XPS) of edge polished samples. The spectra were recorded on a Thermo Scientific K-Alpha⁺ X-ray photoelectron spectrometer operating at 5×10⁻⁹ mbar base pressure. This system incorporated a monochromated microfocused Al Kα X-ray source (hν = 1486.6 eV) and a 180° double focusing hemispherical analyser with a 2D detector.

4. Results and discussions

Figure 1. Dependence of the Center of Mass (CoM) of jetted drops vs flight delay time superimposed with the relevant visualization drop images for the YDC ink.

Drop visualization optimization enabled ink jetting parameters to be tailored in such a way that each triggering event resulted in a single drop of ink, without formation of satellite drops. Figure 1 shows dependence of the Centre of Mass (CoM) of YDC-EtOH ink drops on the flight delay time. Images of drops superimposed to the relevant delay times show an optimized jetting behaviour. Using a pressure of 200 mbar and an opening time of 240 µs we observed jetting where the initial drop forms an elongated tail after it detaches from nozzle, but soon the tail part catches up with the main drop and forms a single drop with ~57 nL volume and ~1.5 m s^-1 velocity. At optimized jetting parameters, no satellite drop formation was observed leading to uniform lateral distribution of the infiltrated ink. Based on the drop visualization results stable jetting was found to relate to Z = ~ 7 to 8 for all implemented inks.

The differences in the electrochemical performances of the reference and the infiltrated cells were expected to be associated with changes in:

(i) Ohmic (R_s) resistance related to densification of the GDC interlayer;

(ii) Polarization (R_p) resistance associated with modification of the catalytic properties of the bulk LSCF scaffold as well as introduction of catalytic properties in the interlayer by Co₃O₄ nanoparticles.

Figure 2(a) presents the I-V curves and impedance spectra measured on the reference and the infiltrated cells at 800 °C. The open circuit voltages (OCVs) of all cells were higher than 1.05 V at 800^oC. The cells infiltrated with doped ceria inks show substantially improved performances in comparison to the reference cell. The maximum power density increased from 350 mW cm⁻² for the reference cell to 640 mW cm^-2 for SDC and 690 mW cm⁻² for YDC infiltrated cell. This corresponds to a maximum power density gain of 82% and 97% respectively as a result of the infiltration procedure. The EIS data recorded at 0V and 0.8 V for both reference and infiltrated cells are shown in Figure 2(b). The spectra for all measured symmetrical cells were similar in shape, showing overlapping arcs associated with the polarization losses in both electrodes (anode and cathode). The polarization (R_p ) and Ohmic (R_s ) resistances were estimated from the low and high frequency intercepts of the Nyquist plots with the real axis. As far as only the cathode was modified by infiltration, one can assume that the difference observed in the spectra can be assigned to the effects of the infiltration. At open circuit potential (0A) the overall R_p was clearly reduced for the infiltrated cells while under polarization (8A) the shape of the arcs of the reference and infiltrated samples became similar. The data suggested that at 800 °C the performance improvement was largely due to the reduction in the R_s. This effect was more expressed for the YDC infiltrated cathode. This finding corresponds to the expected higher ionic conductivity of YDC (0.1015 S cm^-1, at 700^oC) compared to SDC (0.0200 S cm^-1, at 700^oC) as reported by Steele et al [55]. As seen in Figure 2(b) and the inset therein, the changes in the polarization resistance are small and further decreasing under polarization (at 8A). This is due to the enhanced electrode surface kinetics at 800^oC [56,57]. A slight decrease in R_p was observed between 1.5 and 100 Hz, most likely due to promotion in the oxygen adsorption/dissociation in oxygen reduction reaction (ORR) by the infiltrated nano particles.

(a)

(b)  (c)

Figure 2. (a) I-V curves and power densities of infiltrated and reference cells measured at 800 °C; (b) corresponding EIS spectra recorded under current conditions of 0 and 0.8 A at 800 °C; (c) Nyquist and Bode plots for LSCF/GDC/LSCF symmetrical cells with different infiltrations.

Separate contributions from Co₃O₄/doped-ceria nano decorated LSCF cathode and Co₃O₄/doped-ceria nanoparticles located within the interlayer could not be easily discriminated. Thus, in order to distinguish the effect of nano-decorated LSCF bulk cathode, EIS testing of identically infiltrated LSCF/GDC/LSCF symmetrical cells was perform. Figure 2(c) compares the Nyquist and Bode plots of symmetrical cells – reference and infiltrated – with YDC, Co_xO_y and YDC+Co_xO_y, measured at 750^oC. The infiltration of “native” Co_xO_y did not lead to noticeable performance alteration due to its incorporation in the surface structure of the LSCF. In contrary, the decoration with YDC led to an increase in the polarization resistance assigned to a partial masking of the active area available for ORR by non-catalytically active phase (YDC) obstructing the dissociative adsorption/surface exchange mechanisms. Dual infiltration (YDC + Co_xO_y) led to further increase of Rp at very low frequencies assigned to the introduction of concentration polarization losses caused by reduced porosity of the cathode.

The seemingly contradicting data obtained for the symmetrical and the commercial SOFCs can be explained considering the distribution of the infiltrated nanoparticles. The of SEM cross-section images of the YDC infiltrated commercial cell (see Figure 3) confirmed the densification of the GDC interlayer by the infiltrated nanoparticles which led to the significant reduction in the interlayer Ohmic resistance contribution. Figure 3(a) depicts the microstructure of the infiltrated cell cross-section of the cathode/interlayer/electrolyte interface (bar size – 20µm). The functional layer (~2-3 µm thick) was composed of GDC grains (~1.0–1.5 µm in diameter) with ~50% porosity. The concentration of infiltrated phases in the interface region was clearly visible revealing that the interlayer was filled with the mix of YDC and Co_xO_y nanoparticles without achieving full densification (see Figure 3(b)). The image from the area in the middle of the LSCF bulk (see Figure 3(c)) showed decorations with significantly lower density consisting of non-percolating nano particles. This indicated that the large proportion of the infiltrate ink penetrated through the porous scaffold reaching the GDC interlayer. The EDX maps of the interface areas in the reference and the YDC infiltrated cells are presented in Figure 4 (a) and (b) respectively. One can detect migration of Sr to the surface of the YSZ electrolyte in both cases. More localized presence of Sr was seen in the case of the infiltrated cell.

Figure 3. SEM cross-section images – (a) low resolution image of the anode supported SOFC where the cathode was infiltrated with YDC, (b) high resolution image of the GDC interlayer interface area (c) high resolution image of LSCF bulk area. (note that the bar size of image (a) is 20 µm and for images (b) and (c) is 1 µm).

(a)

(b)

Figure 4. EDX mapping of the interlayer areas for (a) the reference and (b) the YDC infiltrated cells.

XPS analysis was applied for characterization of bulk and interface LSCF areas for the commercial reference and the infiltrated cells. Figure 5 illustrates the background corrected core level XPS spectra of the Ce 3d (a,b), Sr 3d (c) and Co 2p (d) bands for the YDC-Co_xO_y infiltrated and the reference cells. The intensity was normalised to the area of the La 3d_5/2 band for the LSCF bulk spectra (Fig. 5a, 5c, 5d) and normalised to the area of the Zr 3d_5/2 of the YSZ for the interface spectra (Fig. 5b). The Ce 3d spectra (Fig. 5a and 5b) showed mainly cerium doped oxide in the +4 oxidation state, with the characteristic three doublets contribution namely (v,u), (v’’,u’’), (v’’’,u’’’) at positions in good agreement with literature [58]. As expected, no practical amount of doped ceria was detected in the reference LSCF bulk cathode (with observable Co, Fe and La auger peaks at the Ce 3d position) while a small amount was registered in the LSCF bulk of the YDC infiltrated LSCF (La 3d_5/2 : Ce3d = 80:20). In contrast, at the interface with the GDC buffer in the infiltrated electrode a significant concentration of doped ceria was registered – La 3d_5/2 : Ce3d = 11:89. Additionally, the measured Sr 3d core levels (see Fig. 5c) suggest that the amount of Sr segregated on the cathode surface (detected as Sr(OH)₂ and SrCO₃ species) has been reduced by the infiltration – Sr 3d_5/2 lattice : surface = 43 : 57 for the reference cell and 59 : 41 for the YDC infiltrated cell. A similar effect of Sr segregation suppression was observed previously by inkjet printing infiltration of GDC into composite LSCF/GDC cathodes in symmetric SOFC [45]. A comparison of the Co 2p core level spectra between the reference and the YDC infiltrated cells is presented in Fig. 5(d). Both cells exhibit spectra of mixed Co(II) oxide and Co(III) oxide structure with the main Co 2p_3/2 peak located at BE of 779.4 eV and visible shake-up satellite structures around 786 eV and 789 eV indicative of a mixed Co(II) and Co(III) oxide [35]. The excess of Co in the infiltrated sample is clearly observable.

Figure 5. XPS spectra of Ce 3d (a,b), Sr 3d (c) and Co 2p (d) core levels of the LSCF bulk and interface areas for the reference and YDC-Co_xO_y infiltrated cells.

As-presented analyses correlated with the hypothesis that majority of the infiltrated ink reached the interface area and the resulting densification of the GDC buffer layer led to the observed drop in the Ohmic resistance. This demonstrated the efficiency of a single-step IJP infiltration due to the deep penetration of small amount of ink through the porous cathode scaffold. The synergetic effect of doped ceria and Co_xO_y co-infiltration led to enhanced densification of the GDC interlayer as well as catalytic contribution of the interface region to the ORR reaction. A reasonable performance was previously reported by Samson et al [59] for cathodes prepared by infiltration of Co₃O₄ into porous CGO scaffolds (0.27 Ω cm² at 600^oC). The authors speculated that due to the small particle sizes, despite the low ionic conductivity of the bulk Co₃O₄,the mix would work as MIEC taking into account that electronic conductivities of ∼40 S cm⁻¹ at room temperatures were previously reported for Co₃O₄ thin films [60]. The study by Druce et al [61] showed a substantial enhancement of the surface exchange coefficient (k*) of GDC when coated with Co₃O₄. Treatment with Co nitrate was shown to produce an enhancement of k* by more than an order of magnitude at 700 ^oC (k*_{untreated GDC} = 1.1×10^-8; k*_{coated GDC} = 1.5×10^-7).

Although research studies on cathode nano engineering by infiltration have been widely published in recent years, to our knowledge this is the first report on post processing modification of the doped ceria interlayer in commercial AS-SOFCs. The simplicity of the combined doped ceria plus Co_xO_y IJP infiltration provides encouraging answers on how to scale up the infiltration procedures via a non-disruptive cost saving technique.

5. Conclusions

LSCF cathodes of commercial anode supported SOFCs were successfully infiltrated with doped ceria and Co_xO_y inks via commercial inkjet printing. The infiltration was done in a single step procedure without vacuum treatment or intermediate high temperature calcinations. Relatively low infiltration loading (~5 wt%) led to substantially enhanced electrochemical performance. The infiltrated GDC interlayers were densified which led to a reduction of the overall Ohmic resistance. Less pronounced improvement in the polarization resistance was also registered and assigned to the catalytic contribution of Co₃O₄/doped ceria nanocomposite embedded in the GDC interlayer. Inkjet printing infiltration was demonstrated to be feasible non-destructive and cost effective technology for infiltration nano-engineering of the SOFCs. The combination of high printing speeds, accurate drop delivery, ink conservation, and availability of industrial multi-nozzle systems presents an opportunity for scaling up IJP infiltration to a commercial level SOFC technology.

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