The electrochemical characteristics of SrTi0.3Fe0.7O3-delta (STF), Sr0.95Ti0.3Fe0.63Ni0.07 O3-delta (STFN), and Sr0.95Ti0.3Fe0.63Ru0.07 O3-delta (STFR) fuel electrodes were compared for a range of H2/H2O and CO/CO2 fuel mixtures. Electrodes prepared in symmetrical solid oxide cells were measured using impedance spectroscopy over a range of temperatures. Impedance spectra were fitted well in all cases using an equivalent circuit that featured three R-Q elements, but the response was generally dominated by a low frequency process associated with adsorption and charge transfer. The electrodes all yielded similar polarization resistances RP values at relatively high temperature (>= 800 degrees C) in H2/H2O, but STFN and STFR were substantially better than STF at lower temperatures. STFR yielded a much lower RP than the other electrodes when operating in CO/CO2. In full cells with these electrodes, STFR-based cells yielded superior power density and smaller polarization resistance compared to both STFN and STF cells, consistent with the symmetric cell results. It is suggested that the superior performance of STFR in CO/CO2 resulted from an enhancement of electrode processes in the presence of exsolved Ru-Fe nanoparticles.

Reversible solid oxide cells (ReSOCs) can provide significant value by playing different roles - electrical generation or electricity storage - depending on the prevailing electrical grid requirements. Relatively few studies have focused on the life testing of solid oxide cells in this mode-switching operation, and much of that work has focused on small "button" cells. The present study describes electrochemical characterization and post-test characterization from an -2600 h life test of a 10 cm x 10 cm fuel-electrode-supported cell operated with periodically reversing current. The cell performance gradually decreased for the first -1000 h and then stabilized with low degradation rate until an abrupt test-ending gas crossover failure that resulted in open circuit voltage drop. Post-test characterization of the (La,Sr)CoO3-delta air-electrode showed evidence of Cr vapor contamination from stainless-steel manifolds and Ag contamination from current collection contacts. Partial delamination of the air-electrode and Gd-doped ceria interlayer was observed, along with fractures in the electrolyte near the fuel inlet that probably caused a drop in open circuit voltage near the end of the test. Ni-Ni connectivity was reduced in both the functional and support layers accompanied by Ni particle coarsening, especially at the fuel outlet.

SrTi 1-x Fe x O 3-δ (STF) has shown promise as a fuel electrode, and as a basis for fuel electrodes with catalytically active nanoparticle exsolution. This study aims to determine trends in performance and stability of STF in highly reducing environments as Fe-content is varied from x = 0.5 to 0.8. Here we report that, while polarization resistance of STF fuel electrodes decreases with higher Fe-content, this is accompanied with lower stability in highly reducing conditions. For too-reducing conditions and too-high x, partial decomposition of the original perovskite phase is observed with the formation of metallic Fe and a Ruddlesden-Popper STF phase.

Perovskite oxides with catalytically active metal nanoparticle exsolution are receiving considerable attention as alternative Solid Oxide Cell fuel-electrode materials. For exsolution, a small amount of a cation is substituted on the B-site of the base oxide and then is reduced out of the host lattice to form nanoparticles that promote electrochemical processes. During exsolution, the host lattice generally becomes more B-site deficient. In some cases, the oxide is made initially A-site deficient to compensate for the loss of B-site cations. Thus, the stability of these oxides under a range of stoichiometries and fuel electrode conditions is of interest. Here we focus on one perovskite host material of interest, SrTi1-xFexO3- δ (STF), which has shown good fuel electrode characteristics that can be enhanced by the substitution of Ru or Ni, resulting in exsolution to form Ru-Fe or Ni-Fe alloy nanoparticles, respectively. (1, 2) Although the amounts of Ru or Ni substituted are small, typically ~ 7% of the B-site cations, the co-exsolution of a comparable amount of Fe is usually observed, resulting in substantial B-site deficiency, potentially de-stabilizing the host perovskite phase. The stability of Fe in STF is also of interest because it helps to determine the Fe content of the alloy nanoparticles. (3) This study seeks to determine the stability of STF in various reducing fuel environments. While STF has been mostly studied as SrTi0.3Fe0.7O3-δ (STF-7), it is not known if this composition provides the best combination of stability and electrochemical performance. Thus, a few additional compositions, SrTi0.5Fe0.5O3- δ (STF-5), SrTi0.4Fe0.6O3- δ (STF-6), and SrTi0.2Fe0.8O3- δ (STF-8), have been studied. In general, the Fe-rich compositions are expected to possess higher electronic and ionic conductivity, leading to good electrochemical performance, whereas the more Ti-rich compositions are expected to provide better stability. Exposure to 97% H2 – 3% H2O at 850°C for 4 h resulted in STF decomposition into BCC α-Fe and a Ruddlesden-Popper (RP) phase, but the decomposition became more limited for the more Ti-rich compositions (Figure 1a). The main Fe peak overlaps with one of the RP peaks, but the presence of α-Fe particles is clearly visible in STEM energy dispersive x-ray spectroscopy chemical mapping, shown for STF-7 in Figure 1b. Figure 1c shows that there is a critical p(O2), 1.3 x 10-20 atm, below which decomposition of perovskite STF-7 occurs. In addition to ex situ XRD as shown in Figure 1, in situ XRD results will be presented and used to confirm and quantify phase changes in combination with thermogravimetric analysis (TGA). Additionally, the conductivity and electrochemical performance of the various STF compositions will be reported and discussed relative to the phase change from perovskite to Ruddlesden Popper and the Fe exsolution. The other main materials variable to be explored is the A-to-B site stoichiometry, which will be studied over the range expected during exsolution. The implications of these results for various exsolution electrode compositions will be discussed. Figure 1: a) Reductions of STF-5 through STF-8 all yield significant decomposition as the initially pristine perovskite acquires several additional peaks. b) Using energy dispersive x-ray spectroscopy, it can be shown that significant Fe deposits appear in STF-7 after reduction at 850°C (effective pO2 1.2e-21 atm for 4 hours). c By increasing pO2 by a factor of 10, XRD patterns for STF-7 show no clear Ruddlesden-Popper or α-Fe peaks, indicating that decomposition requires significantly reducing conditions and that STF may remain stable under more oxidizing conditions. References R. Glaser, T. Zhu, H. Troiani, A. Caneiro, L. Mogni and S. Barnett, Journal of Materials Chemistry A, 6, 5193 (2018). T. Zhu, H. Troiani, L. V. Mogni, M. Santaya, M. Han and S. A. Barnett, Journal of Power Sources, 439 (2019). T. Zhu, H. E. Troiani, L. V. Mogni, M. Han and S. A. Barnett, Joule, 2, 478 (2018). Figure 1

Ni-YSZ electrode support symmetric cells were operated at 0, 0.75, 1.00, and 1.50 A/cm 2 for 1000 h in 50% H 2 -50% H 2 O at 800 ˚C. Electrochemical fracture at the anode-electrolyte interface is observed to occur under high anodic overpotential. Ni migration is observed and quantified over time at the anode of the polarized cells; however, the cathode shows no migration compared to control. Gas diffusion calculations show that steam is significantly enriched and depleted at the anode and cathode respectively, leading to the formation or suppression of volatile Ni(OH) x species, which have been hypothesized as a transport pathway for Ni. However; gas flux calculations show that chemical evaporation alone is unlikely to be fast enough to induce the Ni loss observed.

Improving solid oxide cell power density will enable cheaper commercial-scale SOFCs and SOECs. In Ni-YSZ electrode-supported cells, gas diffusion through the electrode support layer can be a major limitation at high H 2 or H 2 O utilization and high temperature. Conventional methods of improving diffusion through the electrode support include increasing the support porosity and reducing the support thickness, but these can reduce the cell’s structural integrity. Freeze casting 1 and 3D printing 2 have also been explored to enhance diffusion. Here we explore cells in which the Ni-YSZ supports have macroscopic channels, produced by laser ablation, that reduce the average gas diffusion length. To test the difference in mass transport through patterned and pristine electrode supports, symmetric electrode-supported Ni-YSZ cells are patterned on one side, which enables comparison of the electrochemical performance of two otherwise identical electrodes (Figure 1a). Cells with varying pattern geometry, pore geometries, Ni/YSZ ratios, and Ni-YSZ particle sizes were created to fully understand how the support layer microstructure and macrostructure affects the cell performance. Patterned and pristine cells were tested together at three temperatures (600C, 700C, and 800C) in a 97%H 2 :3%H 2 O environment, chosen to produce a clear gas diffusion limitation in H 2 O electrolysis. j-V sweeps and electrochemical impedance spectroscopy (EIS) were carried out on each cell at each test condition, and the microstructure of patterned and control supports was characterized through SEM imaging. Upon removing the ohmic portion and fitting the j-V data to Equation 1 (see Figure 1), patterned electrodes consistently demonstrated higher limiting current density and lower mass transport losses than the control (Figure 1c-d). Three-point-bend mechanical testing revealed that the mean flexural fracture strengths of pristine cells and patterned cells are 36.0 ± 11.4 MPa and 33.0 ± 8.9 MPa respectively. Combining the equations for limiting current density with diffusion in a system with prismatic channels, we determined the ratio of the limiting current density for a patterned and pristine electrode using Equation 2 (see Figure). For the cell data shown in Figures 1c-1d, we expect an average 33% increase (with a standard deviation of 3.5%) in limiting current density based on microscopy measurements of channel and support layer thicknesses. In 1c, we see that the limiting current density of the patterned electrode is 32% higher than the pristine electrode, which matches our expectation. Furthermore, the Nyquist plot in Figure 1d demonstrates that patterned symmetric cells have similar impedance to pristine cells in the high-frequency regime with less impedance in the low-frequency regime, which is consistent with our expectations that the patterned cells have reduced mass transport losses with similar ohmic and activation losses. These j-V and EIS results suggest that macroscopic support patterning is a promising method for improving performance without compromising structural integrity. Figure 1: a) Ni-YSZ symmetric cell cross-section, laser-patterned with 600 µ m by 600 µ m channels and 200 µ m channel spacing. b) Comparison of the flexural fracture strength for pristine and patterned cells, under 3-point-bend testing. c) IR-free current density-voltage profiles of a patterned Ni-YSZ symmetric cell with a predicted current density improvement of 33%, at 700C in a 97%H 2 :3%H 2 O environment. Two distinct j-V responses are observed, based on whether the pristine electrode or the patterned electrode is under a cathodic bias. d) Unbiased potentiostatic EIS (1 MHz - 100 mHz) of a pristine cell versus two patterned cells at 700 C in 97%H 2 :3%H 2 O. The cells all have similar Ohmic resistances, but the impedance of the patterned cells is significantly lower in the low-frequency regime, due to the reduction in mass-transport losses. Equation 1: a is a constant, α is the transfer coefficient, j is current density, and j lim,H2O is the limiting current density for H 2 O diffusion. Equation 2: j lim /j lim,0 is the limiting current density ratio, A is the fraction of the cell surface that is patterned, and t c is the ratio of the channel depth to the patterned electrode thickness. References: (1) Gaudillere, C.; Serra, J. M. Freeze-Casting: Fabrication of Highly Porous and Hierarchical Ceramic Supports for Energy Applications. Bol. Soc. Esp. Cerámica Vidr. 2016 , 55 (2), 45–54. https://doi.org/10.1016/j.bsecv.2016.02.002. (2) Geisendorfer, N. R.; Barnett, S. A. Fuel Cell and Electrolysis Operation of Solid Oxide Cells Containing 3D-Printed Electrode Supports in H2/H2o and CO/CO2 Gas Mixtures. ECS Meet. Abstr. 2020 , MA2020-01 (36), 1463. https://doi.org/10.1149/MA2020-01361463mtgabs. Figure 1

Solid oxide cell long-term durability experiments are resource-intensive and have limited ability to capture the interdependence of microstructural evolution and electrochemical performance. Studies of microstructural degradation mechanisms are usually limited to before and after life-test images. Here we describe a life testing method that simultaneously operates multiple symmetric cells under different conditions, simultaneously providing information on electrolysis and fuel cell operation, while sampling the microstructure during operation. The method utilizes laser-cutting to exactly define different cell areas, allowing testing under different current densities with a single current source, and facilitating removal of segments of the cells during life tests, allowing for microstructural evaluation at intermediate times. The method is demonstrated in Ni-YSZ / YSZ / Ni-YSZ fuel-electrode-supported cells at low H 2 O/H 2 ratios. Characterization using SEM-based imaging techniques shows pronounced microstructural damage that increases rapidly with increasing current density and time, mirroring observed electrochemical degradation. The present results agree with prior reports for SOC operation under such conditions but reveal new features of the degradation process via the unique capability of time-resolved imaging.

Lifetime performance stability is a key issue for the commercialization of solid oxide cells. Ni migration in Ni-YSZ electrode supported cells is an important degradation mechanism. Here we present the results of life tests on symmetric Ni-YSZ electrode-supported cells. These symmetric cells have similar processing and microstructure as conventional Ni-YSZ-supported cells, but the symmetric structure provides information on the Ni-YSZ electrode operating as both an anode and a cathode in the same test. Life tests of up to 1000 h were carried out at 800 ˚C in 50-50 H 2 -H 2 O at current densities of 0, 0.75, 1.00, and 1.50 A/cm 2 . Total applied voltage to each cell was tracked over the lifetime, and small portions of the cells were removed during the life tests to study the time-dependent changes in microstructure. Figure 1 summarizes the microstructural results from a Ni-YSZ cell operated at 1.0 A/cm 2 , observed at 100, 500, and 1000 h. 2D and 3D microstructure characterization was used to show the volume fractions of pore and active (electrically connected) Ni versus position. Somewhat surprisingly, the cathode microstructure remains relatively unchanged compared to a non-polarized cell, with no evidence of Ni migration or isolation. In contrast, Ni migration and isolation was observed to increase with time at the anode in polarized cells, with the thickness of the Ni deactivated region growing from ~5 mm at 500 h to ~15 mm at 1000 h for 1.0 A/cm 2 and ~2 μm at 500 h and ~8 μm at 1000 h for 0.75 A/cm 2 . In addition, total cleavage at the anode-electrolyte interface occurs at 1.5 A/cm 2 by 100 h, and at 1.0 A/cm 2 by 1000 hr. The increase of porosity in the altered zones clearly shows where Ni is depleted. There is no evidence of Ni enrichment adjacent to the depleted region, as might be expected if Ni was moving via surface diffusion. Although there have been numerous recent reports of Ni migration/isolation in Ni-YSZ cathodes during electrolysis cell operation 1 , Ni migration has also been reported in Ni-YSZ fuel cell anodes 2,3 . Here we suggest that such results can be explained by a relatively high steam content in the Ni-YSZ anode functional layer. Conversely, the lack of Ni migration in the electrolysis cathode can be explained by a relatively low steam content in the cathode functional layer. To quantitatively assess the gas compositions, one-dimensional modeling of the electrochemical and gas diffusion processes of these cells was performed using a finite difference method (FDM) with modified Butler-Volmer kinetics and the dusty-gas model respectively. Using diffusivity values calculated via electrochemical impedance spectroscopy and microstructural measurements, the modeling reveals that electrochemically active Ni sees significantly different gas composition than the inlet, creating steam-rich anodes (P H2O = 0.73, 0.8, and 0.95 atm for 0.75, 1.0 and 1.5 A/cm 2 respectively) and steam-depleted cathodes (P H2O = 0.27, 0.2, and 0.05 atm for 0.75, 1.0 and 1.5 A/cm 2 respectively). This is in accord with results and models suggesting that Ni migration is important mainly under high steam conditions, probably due to vapor transport. Note that these effects are exacerbated by the relatively low porosity and small pore size in the present electrodes that lead to asymmetric Knudsen diffusion, wherein H 2 O diffuses at ~1/3 the rate of H 2 . These results suggest that the nature of the porosity in Ni-YSZ supports can lead to significant variations in the extent and directionality of Ni migration. Figure 1: (a, b, c) Polished cross sectional backscatter electron (BSE) images and (d,e,f) low voltage secondary electron (LV-SE) images of an electrode-supported symmetric Ni-YSZ cell at 100, 500, and 1000 h of galvanostatic operation at 1.00 A/cm 2 in 50-50 H 2 -H 2 O and T = 800 ˚C. BSE images reveal a depletion of total Ni at the anode-electrolyte interface (AEI) by a net increase in porosity, and eventual cleavage at that line. LV-SE images reveal that near total deactivation of Ni occurs near the AEI as well. (g,h,i) Quantitative image analysis reveals the extent of porosity increase (via Ni depletion) and Ni deactivation at 100, 500, and 1000 hours. The regimes for both events exactly overlap, with the depletion/deactivation extending 0, 5, and 15 μm from the AEI, respectively. M. B. Mogensen et al., Fuel Cells , 21 , 415–429 (2021). J. Geng et al., J. Power Sources , 495 , 229792 (2021). Z. Jiao and N. Shikazono, J. Power Sources , 396 , 119–123 (2018). Figure 1

Solid oxide cell systems are often designed for operation with a pressurized stack. Although the cell performance is expected to improve with pressurization, the details of how pressure affects the performance of various technologically-relevant electrodes are typically not known. Here we investigate the electrochemical characteristics of Ni-YSZ and GDC-infiltrated Ni-YSZ fuel electrodes in Ni-YSZ-supported cells as a function of total pressure P from 1 to 5 atm in H2/H2O fuel mixtures with humidification of 25%, 50%, and 75% and temperatures of 600˚C and 700˚C. Using electrochemical impedance spectroscopy, the two limiting electrode processes are identified: charge transfer reactions and gas diffusion. The charge transfer resistance is significantly reduced for Ni-YSZ:GDC compared to Ni-YSZ for all conditions, with total polarization resistance RP reduced by 30 - 40%. Fitting the data to a power-law dependence, RP ∝ P−n, yields a power law exponent of n = 0.28 for Ni-YSZ and 0.36 for Ni-YSZ:GDC (at 600˚C) and n = 0.32 for Ni-YSZ and 0.39 (at 700˚C). That is, GDC infiltration improved electrode performance more at higher pressure. Increasing the total pressure from 1 to 5 atm results in a 42% and 47% reduction in RP for infiltrated electrodes at 600˚C and 700˚C; these values are averaged for all humidities. Increasing humidity from 25 to 50% at 1 atm resulted in a ~26% reduction in total RP. The Ni-YSZ:GDC electrode at 5 atm had a RP value 63% - 65% lower than that of the Ni-YSZ electrode at 1 atm, a very substantial combined effect. The impact of pressurization on overall cell area-specific resistance is assessed based on the present data combined with prior measurements of oxygen electrode pressurization effects.

Ni–(Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 (YSZ) and Ce 0.8 Gd 0.2 O 2 - δ (GDC) infiltratedNi-YSZ fuel electrodes are investigated using impedance spectroscopy as a function of total pressure P from 1 to 5 atm in 25%, 50%, and 75% humidified H 2 mixtures at a temperature of 600˚C. The charge transfer resistance decreases significantly with infiltration for all conditions, and the total polarization resistance R P is reduced by ~20%. Fitting the P dependence to a power-law, R P ∝ P −n , yields an exponent of ~0.21 for both un-infiltrated and infiltrated tests. Increasing the total pressure from 1 to 5 atm results in an average 29% reduction in R P for both electrodes. Increasing the humidification from 25 to 75% generally results in a reduction in R P . The Ni-YSZ:GDC electrode at 5 atm had a R P value ~44% lower than that of the Ni-YSZ electrode at 1 atm, a substantial combined effect.

In Ni-YSZ electrode-supported cells, gas diffusion through the electrode support layer can be a significant limitation at high H 2 or H 2 O utilization and high temperature. Conventionally, higher-porosity electrode supports are used to improve diffusion, but this diminishes the cell’s structural integrity. Alternative fabrication methods like freeze-casting and 3D-printing allow for the creation of hierarchical structures with cutouts in the cell surface that improve gas diffusion, but these methods require redesigning processing procedures to obtain the desired materials properties. This work explores the use of laser ablation to pattern cutouts into the electrode support after sintering, enabling a faster mass transport without redesigning the entire fabrication process. Current-voltage measurements of symmetric Ni-YSZ electrode-supported cells with one patterned and one un-patterned electrode demonstrate that laser-patterning improves limiting current density and effective diffusivity by as much as 30%. Mechanical testing of patterned and un-patterned cells demonstrates that patterned cells suffer relatively small reductions in fracture strength.

Solid oxide cells (SOCs) can have a significant impact on climate change over the next decade and beyond, in applications such as balancing renewable grid electricity via electrolytic fuel production, and producing electricity from bio-fuels combined with CO2 product sequestration. However, long-term performance degradation remains a key variable that may limit further implementation of SOCs. This talk focuses on the Ni-YSZ fuel electrode that is widely used but is known to be an important contributor to SOC degradation. Various processes that cause Ni-YSZ degradation are discussed. Results on 3D tomography measurements of accelerated Ni coarsening are described and a quantitative model is developed to predict long-term degradation. Although the results indicate that coarsening effects can be minimized in well-designed Ni-YSZ microstructures, degradation can still occur, especially during high-current-density electrolysis operation. For operation under low H2O/H2 conditions, high electrolysis current density can yield reduction of zirconia to form Ni-Zr compounds, along with substantial microstructural damage. For operation under high H2O/H2conditions, high electrolysis current density can yield Ni migration away from the electrolyte. A phase-field simulation is described that predicts this Ni migration, using actual Ni-YSZ microstructures measured using 3D tomography as the starting point, and compared with experimental observations. The model assumes that Ni transport is driven by a spatial gradient in surface tensions, i.e., a decrease in the Ni/YSZ contact angle with increasing distance from the electrolyte. Recent results on electrolysis and reversibly operated SOCs, making use of Ceria-infiltrated Ni-YSZ to improve stability, are described.

Solid oxide cells (SOC) are exciting both for their capability of fuel-flexible electricity generation and their promise in the production of green H2; however, their degradation and long-term performance remain important questions. Operating individual button cells to characterize these changes is prone to cell-to-cell variations, whereas large-area cell/stack life testing requires considerable resources. In both cases, full cell characterization usually must wait until the completion of the life test, which may delay getting results for months or years. While progress has been made in observing microstructural changes over time (termed here time-resolved characterization) using techniques like non-destructive transmission x-ray microscope tomography, such experiments are difficult to implement, especially the effect of current is unexplored due to geometrical and gas environmental limitations of the system. Presented here is a method to rapidly obtain time-resolved microstructural information of the same cell in tandem with electrochemical measurements. While SOC degradation arises due to a range of processes, this work focuses on the microstructural evolution of Ni-YSZ fuel electrodes that occurs during electrolysis operation, which is known to be a key issue. Two key experimental features allowed time-resolved characterization of multiple cells simultaneously: (1) the use of Ni-YSZ symmetric cells allows multiple cells of different size in a single test by avoiding the need for gas seals, and (2) the use of laser milling to allow controlled removal of a portion of each cell during the test. Here we illustrate the method for the case of low steam content and high current density, designed to exacerbate and accelerate electrolysis degradation due to the highly reducing conditions achieved. Tape-cast planar electrode-supported Ni-YSZ symmetric cells were laser cut into precise geometries with well-defined projected areas and easily removable sections. Four cells with projected areas of 1, 1, 0.75, and 0.5 cm2 were connected in series within the same furnace at 800 C and a gas environment of 97% H2 and 3% H2O. A current of 0.75 A was run through three of the cells, resulting in current densities of 0.75, 1.0, and 1.5 A/cm2, while one was maintained with no current. The life test lasted for 500 h, and the microstructure was observed at 0, 200, and 500 hours. Figure 1 shows the microstructures observed at the highest current density by polished cross-sectional SEM and FIB-polished sections under conditions that provided Ni/YSZ contrast. Microstructural degradation had occurred by 200 h and became immense at 500 h, due to high current density in these reducing conditions. By 200 hours, grain boundaries become enriched with Ni, likely due to the formation of Ni-Zr intermetallics in the ultra-low pO2 induced by local overpotential as reported by Chen[1] and Szasz[2]. Additionally, the electrode-electrolyte interface becomes nanoporous and a Ni enriched fracture is present ~2 μm into the electrolyte. By 500 hours, the Ni rich nanoporosity has progressed more than 10 μm into the electrolyte and created islands of large grains surrounded by Ni-rich deposits. Electrochemical impedance spectroscopy measurements show a clear initial decrease in polarization resistance followed by an overall impedance increase by 200 hours. These correspond to the initial electrochemical boost from the production of a nanoporous structure, followed by the deactivation of areas of the cell due to the large fractures through the electrolyte. [1] M. Chen et al., “Microstructural Degradation of Ni/YSZ Electrodes in Solid Oxide Electrolysis Cells under High Current,” J. Electrochem. Soc., vol. 160, no. 8, pp. F883–F891, 2013, doi: 10.1149/2.098308jes. [2] J. Szász et al., “High-Resolution Studies on Nanoscaled Ni/YSZ Anodes,” Chem. Mater., vol. 29, no. 12, pp. 5113–5123, 2017, doi: 10.1021/acs.chemmater.7b00360. Figure 1: Microstructural and electrochemical changes of a symmetric Ni-YSZ SOC undergoing 1.5 A/cm2 in reducing conditions at 0, 200, and 500 hours. Cathode is on the left in all images; anode is on the right. a, b, c) Backscatter electron imaging reveals destruction of the dense YSZ electrolyte over the lifetime of the experiment. Lines of brighter phases on the anode side of fractures are Ni deposits. d, e, f) Focused Ion Beam cross sections reveal nanoporosity forming in the first 10 μm from the cathode-electrolyte interface at 200 and 500 hours. Charging is evident as a result of curtaining from the nanoporous features in the electrolyte. g) Electrochemical impedance spectroscopy shows the initial decrease in resistance from 0 to 48 hours followed by increasing resistance due to microstructure evolution. Figure 1

In this talk, I introduce topics, that I have been working on, in solid oxide electrolysis cells involving complicated interfacial structures and dynamics of interfaces. Then I will focus on my recent work on nickel (Ni) particle migration in electrodes consisting of Ni, yttria-stabilized zirconia (YSZ), and pores during the operation of oxygen-ion conducting solid oxide electrolysis cells (o-SOECs) and Faraday efficiency in proton-conducting solid oxide electrolysis cells (p-SOECs) under electrolysis operations. SOECs can have a significant impact on climate change over the next decade and beyond, in applications such as balancing renewable grid electricity via electrolytic fuel production. However, long-term performance degradation remains a key issue that may limit further implementation of O-SOECs, and the dependency of operation conditions on Faraday efficiency in P-SOECs has been under debate. In particular, in Ni/YSZ/pore electrode of O-SOEC, a phase-field model is proposed that employs the Ni-YSZ 3D microstructure as the initial condition and large-scale numerical simulation is implemented that predicts the directional Ni migration. The results are thus directly comparable to experimental observations. Quantitative predictions of the evolution of the Ni/YSZ/pore system's microstructures due to Ni particles' migration are studied through theoretical analysis and data analysis. In P-SOECs, an electrochemical model is proposed to study the dependency of Faraday efficiency on operation conditions for P-SOECs with yttrium-doped barium zirconates (BZY) and co-doping barium zirconate-cerate oxides with ytterbium and yttrium (BCZYYb) as electrolytes respectively. Our numerical predictions are verified by experimental results obtained in INL. An optimal structure of electrolyte is proposed to boost the Faraday efficiency in P-SOECs.

Electrochemical impedance spectroscopy (EIS) is a powerful technique for material characterization and diagnosis of the solid oxide fuel cells (SOFC) as it enables separation of different phenomena such as bulk diffusion and surface reaction that occur simultaneously in the SOFC. In this work, we simulate the electrochemical impedance in an experimentally determined, three-dimensional (3D) microstructure of a mixed ion-electron conducting (MIEC) SOFC cathode. We determine the impedance response by solving the mass conservation equation in the cathode under the conditions of an AC load across the cathode’s thickness and surface reaction at the pore/solid interface. Our simulation results reveal a need for modifying the Adler-Lane-Steele model, which is widely used for fitting the impedance behavior of a MIEC cathode, to account for the difference in the oscillation amplitudes of the oxygen vacancy concentration at the pore/solid interface and within the solid bulk. Moreover, our results demonstrate that the effective tortuosity is dependent on the frequency of the applied AC load as well as the material properties, and thus the prevalent practice of treating tortuosity as a constant for a given cathode should be revised. Finally, we propose a method of determining the aforementioned dependence of tortuosity on material properties and frequency by using the EIS data.

Solid oxide fuel cells (SOFCs) with mixed ion-electron conducting (MIEC) cathodes are a leading technology for achieving electrochemical energy conversion with high efficiency. To maximize the surface area available for the electrochemical reaction, MIEC cathodes are designed to have complex and porous microstructure. However, such a design results in a high degree of tortuosity for ionic transport, which limits the performance of the SOFCs. Thus, it is crucial to accurately determine the tortuosity value of a cathode and optimize it for the best performance. In this work, we propose the application of electrochemical impedance spectroscopy (EIS) for determining the effective tortuosity of a MIEC cathode with a complex microstructure. For our work, first, we experimentally obtained the microstructure of the cathode using FIB-SEM. Then, we calculate the EIS spectrum for the cathode by solving the mass conservation equation for oxygen vacancies in the cathode under the influence of an AC load across the cathode’s thickness and surface reaction at the pore/solid interface. Our simulation results provide two key insights; first, the presence of surface reaction at the pore/solid interface causes the amplitude of the vacancy concentration at the interface to be lower than that in the bulk of solid. To account for this difference, we propose a modification to the Adler-Lane-Steele (ALS) model, a macrohomogeneous model that is widely used to fit the impedance behavior of MIEC cathodes. Second, the tortuosity of the cathode, which we calculate using the modified ALS model, is dependent on the AC-load frequency and the material properties (reaction rate constant and bulk diffusivity). Thus, there is a need to revise the prevalent practice of treating the tortuosity as a quantity that depends solely on the microstructure. In our presentation, we will provide a more detailed and quantitative description of the aforementioned insights.

Studies of Ni-yttria-stabilized zirconia (YSZ) fuel electrode degradation mechanisms in solid oxide electrolysis cells (SOECs) are complicated by the different possible Ni-YSZ microstructures and compositions, and the variations in the H2/H2O ratio encountered in an electrolysis stack. Here we describe a life testing scheme aimed at providing survey results on degradation as a function of the H2O-H2 composition, with life tests carried out at five different steam contents from 90% to 10%. A Ni-YSZ-supported symmetric cell geometry is employed both with and without infiltrated nanoscale gadolinia-doped ceria (GDC). Impedance spectroscopy is utilized to observe changes in electrochemical characteristics during the life test, and a transmission-line-based equivalent circuit is used to model the data. Post-test electrode microstructures were observed. The results suggest that the GDC infiltrant reduces the electrode polarization resistance and provides more stable electrode polarization over a range of conditions.

GDC nanoparticles reduce the reaction resistance associated with three-phase boundaries and improve oxygen transport in the Ni–YSZ electrode, as measured by electrochemical impedance spectroscopy under actual solid oxide cell operating conditions.