Sr-secondary phase formation is a potentially significant degradation mode with direct impact upon solid-oxide cell (SOC) commercial viability. A first-principles based thermodynamic study was performed for La1-xSrxMnO3 +/-delta (LSM) perovskites to assess their stability against formation of different Sr-secondary phases, including SrO, SrCrO4, SrSO4, SrCO3, and Sr(OH)2, for SOC applications. The Sr-secondary phase formation reaction free energies were determined via a thermodynamic model by combining ab initio lattice dynamics calculations for the solid phases and ab initio thermodynamic data for the gas phases. The current approach expands the previously reported thermodynamic modeling studies by integrating first-principles based point defect equilibria into the thermodynamic analysis. The modeling results obtained using this new approach indicate an increased tendency to form the SrO oxide upon decreasing the oxygen partial pressure. Additionally, the enhancing factors to form the Sr-related secondary phase from the associated SrO activity in LSM are further quantified by considering the equilibrium of SrO reacting with the contaminant gas species as a function of temperature and pressure.

Microstructures control the properties of materials and their performance in applications, including solid oxide cell (SOC) electrodes. Direct calculation of microstructural parameters from 2D or 3D images requires phase segmentation of experimental greyscale data. Subtle differences in segmentation outcomes can lead to significant differences in calculated microstructure parameters, especially for higher-order ones such as triple phase boundary (TPB) density. Unfortunately, it is nearly impossible to reproduce published segmentation results and their uncertainties are difficult to assess. We present a modification to typical watershed segmentation that addresses these issues. Markers are first placed based on the gradient distribution, using a single threshold parameter, and then grown by watershed to generate a marker segmented image. Phase-labeling is readily carried out from a post-watershed marker averaged greyscale distribution, using two threshold parameters. Sequentially performing these steps and simplifying the methods to determine the optimal values of the three threshold parameters enables the process to be automated and readily reproduced. With this new method, common segmentation errors can be avoided and sensitivity of computed microstructural properties around the optimal threshold values can be assessed. The method is applied to several large-scale 3D SOC electrode datasets, and the outputs are compared to published values.

One of the major degradation mechanisms in the long-term operation of solid oxide electrolysis cells (SOECs) is the delamination of oxygen electrodes (OEs), driven by high inner oxygen pressure near the OE-electrolyte interface. However, the effects of transport properties and electrode thickness on the inner oxygen pressure are not well understood. This study employs a microstructure-based electrochemical model that accounts for electron and oxygen ion conduction, along with Butler-Volmer-type chemical reactions at triple-phase-boundaries (TPBs), to investigate the oxygen pressure in lanthanum strontium manganate (LSM)-based SOECs. The model is applied to both two-dimensional (2D) prototype microstructures and three-dimensional (3D) realistic microstructures, with the oxygen pressure evaluated as a function of transport properties and electrode thickness under both potentiostatic and galvanostatic operations. The simulation results suggest that electrode delamination can be mitigated by enhancing the ionic conductivities and thickness of the OE under potentiostatic operation, and by enhancing the ionic conductivities in the electrolyte and OE, as well as optimizing the thickness of OE under galvanostatic operation. The simulation results are compared to an analytical solution, with the discrepancies attributed to the Butler-Volmer-type kinetics included in the microstructure-based model.

As part of the US Department of Energy’s Reversible Solid Oxide Fuel Cell (R-SOFC) and H2NEW programs, the National Energy Technology Laboratory (NETL) solid oxide cell (SOC) research group focuses on characterizing SOC degradation and improving the lifetime performance of SOC electrodes. A major tool used by NETL is its multiphysics performance degradation modeling framework, which considers microstructure, electrochemistry and mass/charge transport when simulating the lifetime energy/hydrogen output of an SOEC/SOFC sample. By simulating thousands of potential electrode microstructures varied across over 10 independent microstructural parameters, NETL analyzes which electrode features have the greatest impact on lifetime performance. Simulations and experiments are also used to explore how much lifetime performance can be increased by creating more advanced electrodes using additive manufacturing and electrode infiltration or by adjusting the operating conditions. The talk will review the current capabilities of the degradation modeling framework and will use case studies to provide lessons on SOC electrode design and operation.

Reversible solid oxide cells(rSOCs) are an enabling energy storage/production technology for a dynamic grid environment. Reversible operation requires strong working knowledge of fuel cell (SOFC) and electrolyzer (SOEC). Performance degradation of SOECs has been observed; however, the details of physical processes related to the performance degradation remain unknown. Multiphysics simulations were performed to investigate the performance degradation of solid oxide electrolysis cells under various working conditions.

High performance computing simulations of Cr-poisoning are used to develop systematic trends for overpotential driven degradation modes in microstructurally resolved fuel cell cathodes. Oxygen reduction and chromium oxide deposition at triple phase boundaries (tpbs), and species transport, are numerically computed within 19 microstructures (103 µm3 domains) over a range of operating conditions. Three primary input parameters drive degradation: tpb density of the microstructure ρtpb, normalized charge-transfer current density for Cr-poisoning i∗, and galvanostatic current density j. Simulations converged over large fractions of the potential operating time, ranging from thousands to over a hundred thousand hours. For all simulations, normalized overpotential η∗ versus time t curves can be modeled using a progressive, asymptotic function η∗=ma(ta−t) with two fitting parameters: the initial slope m and the asymptote a. A third output parameter tl was defined as the time when a specific fraction of overpotential range was lost. Several methods to determine each output parameter are presented. More importantly, using the large amount of data generated from over 95 time-dependent simulation series (over 29,000 individual simulations), simple correlations are made between the input and output parameters that provide predictive capability of operational lifetime tl (and m and a), without needing further high-performance computing.

A fundamental analysis of multicomponent gas transport models was performed in application to the oxygen electrodes of solid oxide cells. It is common practice to neglect the effect of pressure gradients within oxygen electrodes, even though a net molar flux at the electrolyte surface implies that a pressure gradient must exist. The influence of both Darcy velocity and Knudsen flux are considered in the context of ordinary (Fickian) diffusion, the dusty gas model, and the binary friction model. Comparisons between the models and different sets of assumptions are made via parametric studies on operating load, oxygen partial pressure, microstructural properties, and electrode thickness. Results show that the pressure gradient will have a significant impact on the oxygen concentration distribution and therefore the concentration overpotential. In electrolysis mode, pressure increases up to 1 atm are predicted, indicating that pressure at the electrode/electrolyte interface could contribute to electrode delamination. Additionally, it is found that Darcy's law is insufficient for calculating the pressure distribution without accounting for the flux due to Knudsen diffusion. Additionally, it is found that for the range of properties typical of oxygen electrodes, there is a negligibly small difference between the dusty gas model and binary friction model from a practical standpoint. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Ni coarsening and migration is the most important degradation in the hydrogen electrode of solid oxide cells. This presentation reviews our works on Ni coarsening and migration in NETL over the past years. We used simulation techniques including phase-field modeling and density function theory to investigate possible mechanisms of Ni coarsening and migration, including self-diffusion of Ni, diffusion of gaseous Ni(OH)2, surface diffusion of Ni(OH)x and Ni-YSZ wettability change, and examined the effect of operating conditions on these mechanisms. So far, none of the mechanisms can fully explain the experiments. Remaining questions and possible paths forward are summarized and discussed.

Oxygen partial pressure is an important thermodynamic state variable that affects both the performance and degradation of solid oxide electrolysis cells. In this work, a three-dimensional model developed from Virkar’s one-dimensional model has been applied to reconstructed and synthetic microstructures of solid oxide cells with hydrogen electrodes made from Ni and yttria stabilized zirconia (YSZ), and with oxygen electrodes made of gadolinium doped ceria (GDC) and lanthanum strontium cobalt ferrite (LSCF). The effect of microstructures, including the thickness of the YSZ and GDC layer, and the compositions of the hydrogen and oxygen electrodes on the distribution of oxygen partial pressure was investigated. The results show that the maximum oxygen partial pressure occurs on the interface between the GDC and YSZ layers. Under the constant current mode, a thicker GDC layer may decrease or increase the maximum oxygen partial pressure in the cell, depending on the rate of oxygen ion exchange between GDC and YSZ. A thicker YSZ layer lowers the maximum oxygen partial pressure in the hydrogen electrode but increases the maximum oxygen partial pressure in the cell. In addition, the Ni:YSZ/LSCF:GDC ratio and porosity also affect the distribution of oxygen partial pressure. These findings provide insights on mitigating degradation in solid oxide electrolysis cells by tuning the cell microstructures.

Distinct from the proton defect, the hydride defect species may be present in certain perovskite materials in reducing environments such as in fuel electrodes of solid oxide cells (SOCs) or on the reducing side of ceramic membranes. A generalized defect thermodynamic model was developed for the triple-conducting perovskites (La,Ba)Fe1-xMxO3-delta (M = Y and Zr) to allow inclusion of the hydride defect formation reaction in addition to the other three main defect reactions, namely, the oxygen vacancy formation, hydration, and charge disproportionation reactions. This comprehensive defect model also allows the incorporation of polynomial functional forms of oxygen nonstoichiometry delta to describe the defect reaction energies and entropies and to enable refinements of the defect reaction equilibrium constants in the defect thermodynamic analysis. As a first step, the developed model is applied to the Ba0.95La0.05FeO3-delta material as an illustrative system to obtain its Brouwer diagrams with both the proton and hydride defects in relevant SOC conditions, particularly for more reducing environments. The results provide direct guidance on the influence of electronic and ionic defect concentrations upon thermodynamic properties and ultimately on the performance of Ba0.95La0.05FeO3-delta and potentially other (La,Ba)Fe1-xMxO3-delta perovskite materials involved in SOC applications.

To investigate the difference between ionic and electronic conductors as infiltrates in solid oxide fuel cells (SOFCs), high -throughput and high-performance finite element simulations were carried out on 51 different cathode microstructures. Five cathode backbones, reconstructed from a commercial SOFC, were infiltrated computationally with varying number densities of nanoscale electronically or ionically conducting particles. Local electrochemical quantities were computed within the volumetric meshes that represent the complex 3D microstructural morphologies that include the infiltrated particles. As infiltrates, ionic conductors improve the performance more than electronic conductors. By differentiating transport and reaction pathways originating from backbone phases and infiltrates, we show that new ionic transport pathways opened by the ionically conducting infiltrates are the origin of this difference. These new transport paths redistribute current throughout the cathode, thereby increasing (decreasing) the available local activation (Ohmic) overpotential at triple phase boundaries and rendering them more active than for the case of electronic conductors as infiltrates. These results give us insight to engineering improved electrodes for SOFCs via infiltration with surface active nano -particles.