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SOEC operation using ammonia as a fuel was investigated. At high temperatures (T>800oC) cell performance closely resembles performance in equivalent gas mixtures of N2 and H2. Between 600 oC < T < 750 oC, however, a mild impeding effect of presence of ammonia on the hydrogen oxidation rate is observed. Further, the fuel cell durability when operating on ammonia was tracked over test periods between 2000 and 6000 hours at different temperatures. In general, degradation rates observed in ammonia were similar to degradation rates observed when operating in a 1:3 gas mixture of N2/H2. Some morphological changes in the fuel electrode support and auxiliary Ni-meshes were observed after test. These indicate Ni-nitride formation and whereas this is detrimental to the Ni-mesh, it did not affect overall cell performance. Experiments with ammonia operation at stack level were also conducted illustrating the potential to realize a stack cooling by internal ammonia cracking.

Limiting the degradation of solid oxide fuel cells is an important challenge for their widespread use and commercialization. The computational expense of long-term simulation of a full stack with conventional models is immense. In this study, we present a multiscale three-dimensional model of a degrading full stack of solid oxide cells, where we integrate degradation phenomena of nickel particle coarsening in the anode electrode, chromium poisoning of the cathode electrode, and oxidation of the interconnect into a multiscale model of the stack. This approach makes this type of simulation computationally feasible, and 38 thousand hours of the stack operation can be simulated in 1 h and 15 min on a high-end workstation. Hereby one can start to explore the optimum operating conditions for a range of parameters. The model is validated with experimental data from an 18-cell Julich Mark-F stack experiment and predicts common trends reported in the literature for evolutions of the stack performance, degradation phenomena, and the related model variables. Moreover, it captures how different regimes in the full stack degrades at different rates and how the various degradation phenomena interact over time. The model is used to investigate the effects of galvanostatic and potentiostatic operation modes, operating conditions, and flow configurations on the long-term performance of the stack. Results demonstrate, as expected, that potentiostatic operation mode, moderate temperature, lower load current, and counter-flow configuration improve the long-term performance of the stack.

Operating the PEM fuel cell in the dead-ended anode mode reduces the overall cost and complexity of the system but causes a voltage loss and carbon corrosion in the cathode catalyst layer due to hydrogen starvation in the anode. Whereas allowing an ultra-low flowrate at the anode outlet offers a very high utilization of hydrogen and achieves a stable voltage transient. Here, a time-dependent pseudo-three-dimensional, two-phase, and non-isothermal model is developed to study the optimum bleeding rate, which maximizes the hydrogen utilization, achieves a stable cell voltage and avoids carbon corrosion, which is commonly observed when the bleed rate is set to zero, i.e. the dead-ended mode. The model is validated against the experimental data by comparing the polarization curves and cell voltage transients during the dead-ended anode operation of small experimental cells with serpentine and straight anode flow channels. Moreover, the effects of operating conditions on cell performance during the anode bleeding operation mode are investigated. Results demonstrate that the hydrogen utilization exceeds 99% in the anode-bleeding mode without hydrogen starvation, and the cell performance improves significantly for higher anode pressure, lower cell temperature, and lower relative humidity at the cathode inlet. Lastly, it is found that serpentine channels in the anode improve the uniformity of the distribution of hydrogen compared to straight and interdigitated channels in the anode-bleeding mode while the cathode flow field consists of serpentine channels.

Chromium poisoning of the air electrode is a primary degradation mechanism for solid oxide cells (SOCs) operating under fuel cell mode. Recent experimental findings show that reversed pulse operation for SOCs operated as electrolyser cells can reverse this degradation and extend the lifetime. Here, we use a multiphysics model of an SOC to investigate the effects of reversed pulse operation for alleviating chromium poisoning of the air electrode. We study the effects of time fraction of the operation under fuel cell and electrolysis modes, cyclic operation starting after a certain duration, and fuel cell and electrolysis current densities on the cell lifetime, total power, and hydrogen production. Our modeling shows that reversed pulse operation enhances cell lifetime and total power for all different cases considered in this study. Moreover, results suggest that the cell lifetime, total power, and hydrogen production can be increased by reversed pulse operation at longer operation times under electrolysis mode, cyclic operation starting from the beginning, and lower electrolysis current densities. All in all, this paper documents and establishes a computational framework that can serve as a platform to assess and quantify the increased profitability of SOCs operating under a co-production operation through reversed pulse operation.

The ongoing green energy transition is increasing the need for dynamic and efficient Power-to-X (PtX) systems to convert surplus wind and solar power to high-value products. The solid oxide electrolysis cell (SOEC) technology offers the highest energy conversion efficiency. However, high degradation and thermal variations that cause thermomechanical stress hinders up-scaling of the SOEC technology. Here we present a novel operation method that alleviates temperature variations and minimize degradation caused by impurities and nickel migration. By rapidly switching between electrolysis mode and brief periods in fuel cell mode, a flat thermal profile is obtained. Our results thus establish a new, simple way to achieve increased SOEC stack and module size and extended lifetime. The new operation method enables dynamic operation of large SOEC modules for renewable energy powered PtX systems which could drastically decrease costs associated with production of high-value green fuels and chemicals from wind and solar power.

Ammonia is one of the promising fuels that can be used as a hydrogen source for solid oxide fuel cells (SOFC) due to its high energy density, high hydrogen content, no carbon emission at the consumption site, easy storage and transportation, and potential for low cost owing to a well-developed infrastructure. Ammonia-fueled SOFCs have been studied through several experimental works at both cell and stack levels, while a limited number of numerical models at cell levels have been devoted to this subject. Both experimental and numerical studies show that the ammonia decomposes chemically in the fuel support layer at the common operating temperatures of the SOFCs due to the catalytic role of the nickel particles for the ammonia decomposition. Here, we present the first Multiphysics model of an ammonia-fueled SOFC stack, which couples and solves the transport equations of mass, momentum, species, charges, and heat. The model includes the whole stack with manifolds, active domain, headers, and sealings. For the layered domains of the stack, a homogenization approach is used, which replaces them with an equivalent domain and solves for the effective variables [1-3]. Several research groups and stack manufacturers have been using this homogenization approach to model SOFC stacks due to the advantage of reasonable computational expenses and so runtimes. This makes systematic studies of operating conditions, design changes, and even degradation at the stack-scale computationally feasible [1]. An ammonia decomposition reaction rate model has been developed and validated thoroughly in [4]. The model shows that the ammonia decomposition occurs over the thickness of the fuel support layer, depending on the amount of ammonia in the fuel channel and the operating temperature. With the homogenized model, the variations over the thickness of the electrode are effectively described by a well-accepted 0D model developed, see e.g. [5]. Here, we present such a 0D model that has been validated with a 2D through-plane model of the fuel side of a single cell. We show that the 0D model can describe the effective penetration depth of the ammonia in the fuel support layer through the balance of the fluxes to/from the support layer. The developed relation for the effective ammonia penetration depth in the support layer approximates well the one obtained from the detailed 2D model. A similar idea can be used to find the effective penetration depth of methane in the support layer for the case of a methane-fired SOFC and facilitate its stack-scale modeling with the homogenized model. Ammonia is added to the stack-scale model with the 0D model, which correctly describes the decomposition rate per unit area. The model reproduces well-reported trends from experiments with ammonia-fueled SOFC stacks. Internal cracking of the ammonia is advantageous from a cost/efficiency perspective as the heat consumption of the cracking process facilitates stack cooling; however, depending on the conditions of operation, the ammonia decomposition could be too fast, making a sharp temperature drop at the inlet. This leads to increased thermal stresses in the region and increases the risk of mechanical failure. The model is used to investigate the effects of the operating conditions, e.g. temperature and flow rates of the fuel and air, and flow configuration on the temperature gradients at the inlet to identify safe operating conditions. References [1] Rizvandi, O. B., Miao, X. Y., & Frandsen, H. L. (2021). Multiscale modeling of degradation of full solid oxide fuel cell stacks. International Journal of Hydrogen Energy, 46, 27709-27730. [2] Miao, X. Y., Rizvandi, O. B., Navasa, M., & Frandsen, H. L. (2021). Modelling of local mechanical failures in solid oxide cell stacks. Applied Energy, 293, 116901. [3] Navasa, M., Miao, X. Y., & Frandsen, H. L. (2019). A fully-homogenized multiphysics model for a reversible solid oxide cell stack. International Journal of Hydrogen Energy, 44(41), 23330-23347. [4] Kishimoto, M., Furukawa, N., Kume, T., Iwai, H., & Yoshida, H. (2017). Formulation of ammonia decomposition rate in Ni-YSZ anode of solid oxide fuel cells. International Journal of Hydrogen Energy, 42(4), 2370-2380. [5] Leonide, A., Apel, Y., & Ivers-Tiffee, E. (2009). SOFC modeling and parameter identification by means of impedance spectroscopy. ECS Transactions, 19(20), 81. Figure 1

Solid oxide cells can deliver highly efficient energy conversions between electricity and fuels/chemicals. A central challenge of upscaling solid oxide cells is the probability of failure of the brittle ceramic components. The failures of the ceramic components may lead to significant degradation or eventual failure of a stack. To predict mechanical failures in a stack, a full stack model is needed, together with a local assessment of stresses at the vicinity of failing regions, e.g. the contact points between the cells and interconnects. A conventional three-dimensional model requires a very fine discretization of the mesh to capture stress intensities. Computational resources needed for such a model are therefore immense, and it is highly unlikely to compute at stack scale, as well describe the evolution over time. In this work, the homogenization modelling framework for solid oxide cell stacks is extended to identify local mechanical failures. Thus, the fracturing within a local failing point is examined by using a localization approach, where stresses in the stack model are linked to the local stresses and the energy release rate at the crack tip of the relevant interface. This is done in a general manner, such that the local stresses and the energy release rate can be evaluated at every point in the stack at every instant of time without loss of computational efficiency. A 100-cell stack can be modelled in three dimensions with all coupled multiphysics in steady state within 3 min on a current workstation computer.

The high energy density of hydrogen is very advantageous for automotive applications, however the high cost and low durability of proton exchange membrane (PEM) fuel cells limit their commercial use. The anode-bleeding operation mode offers a very high hydrogen utilization and cost reduction by eliminating the components necessary for the recovery of hydrogen in the flow-through mode and avoids the carbon corrosion reaction, which causes degradation of the catalyst layer when the bleeding rate is set to zero in the dead-ended mode. A two-phase, non-isothermal, transient and pseudo-three-dimensional model is developed here to study the cell performance and carbon corrosion during dead-ended and anode bleeding operation modes. The model is validated against the experimental data from the literature and used to investigate the effects of the geometric and operation parameters on the voltage transient during dead-ended anode (DEA) operation mode. Results demonstrate that a lower load-current density, higher anode pressure, lower relative humidity at the cathode inlet, higher stoichiometric flow in the cathode, higher cell temperature, and shorter, deeper and wider channels can improve the cell performance under the DEA operation. The bleeding rate is optimized to sustain a stable transient cell voltage without carbon corrosion in the cathode catalyst layer while the hydrogen utilization is more than 99%. (C) 2019 Elsevier Ltd. All rights reserved.

Microfluidic fuel cells (MFCs) are effective energy conversion devices in microscales due to their simplicity and high energy density as they are based on laminar co-flow of reactants in microchannels without a separator. However, fuel utilization at practical flow rates remains as one of the important challenges. In this study, a two-dimensional (2D) model is developed based on Brinkman equations for flow, Fick's law for mass transport and Butler-Volmer equations for reaction kinetics to study MFCs and address the effects of geometric and operation parameters on the performance and fuel utilization. Commercial finite-element software, COMSOL, is used to solve coupled equations and to analyze the performance of MFCs for different Peclet numbers, concentrations of reactants and geometric variables. According to simulation results, the 2D model compares very well with the three-dimensional model based on Navier-Stokes equations and with the experimental data reported in the literature. Moreover, the model is used for the analysis of the proposed branched-channel design of microfluidic cells that improves the fuel utilization and power output of the MFC.

Solid Oxide Fuel Cells (SOFC) converts chemical energy of a fuel (hydrogen or reformed hydrocarbons) to electricity directly with high efficiency. High-temperature operation of the SOFC makes it a fuel-flexible device; however, the high temperatures induce some degradation mechanisms. Degradation of the SOFC is a barrier against its widespread use and commercialization. Therefore, a thorough analysis is necessary to investigate mitigation strategies for reducing the degradation rate and so improving performance. Degradation experiments are time consuming and costly. Modeling of the SOFC stack is therefore a good alternative for studying degradation mechanisms and mitigation techniques. However, computational cost of numerical simulations of an SOFC is high because of the multiphysics modeling approach that is needed to take care of the interactions of physics such as flow distribution, mass transport, charge and energy transfer. The high computational cost of the numerical models hinders 3D modeling of the SOFC even at a single-cell scale. Therefore, 1D and 2D models with all physics included have been developed, or 3D models with simplifying assumptions, such as isothermal approach, have been developed. To the best of the authors’ knowledge, all the degradation models of the SOFC are developed at a single-cell scale, even most of them concentrate on a part of the electrodes and electrolyte. Here, a homogenized model of a full stack in 3D is used to investigate the degradation with a reasonable computational time. The homogenized model considers flow in manifolds, and in the active domain it considers all relevant physics, i.e. currents, gas flows, heat transport, mass balances and mechanical stresses. It does so by use of a homogenized description of all physics, such that it correctly represent the physics without describing the geometrical detail but rather represent the response of all layers effectively. It was shown that the model predicts the overall responses of the stack in a realistic manner, while it is about two orders of magnitude faster than conventional stack models where the geometry inside the stack is explicitly represented. The model includes transport equations of mass, momentum, species, charges, and energy. In this study, the following degradation mechanisms are added to the model: nickel particles coarsening (agglomeration) which decreases triple-phase boundary (TPB) length and electric conductivity of anode electrode, chromium poisoning at the cathode side which reduces TPB length of cathode electrode, and corrosion of interconnect which increases ASR (area specific resistance) through the resistance of oxide scale against electric charge transfer. The transient model takes less than a day to simulate tens of thousands of hours of degradation on a workstation with Intel Xeon 3.7 GHz 6-core processor. The peak RAM (random access memory) is about 10 GB for the simulations. The volume averages of the aforementioned degradation in the active domain over time are qualitatively consistent with common trends reported in the literature. Moreover, the model resolves distributions of the degradation mechanisms over the active domain. Therefore, it can be used to modify the stack design to minimize degradation rates. Moreover, cells that experience high degradation rates could be exchanged with the ones at safe regions of the stack to improve its lifetime. Figure 1

Three-dimensional models of the proton exchange membrane fuel cell are necessary to study important issues such as water and thermal management and flow field design, but not practical due to computational cost of simulation of a full cell. Here, we present a pseudo three-dimensional model to mitigate the computational cost of three-dimensional models. The model includes in-plane transport equations in channels and gas diffusion layers explicitly, and through-plane transport is based on resistance relationships through the thin membrane and catalyst layers. Polarization curves and distributions of species for a small section of a fuel cell that consists of straight channels, gas diffusion layers, catalyst layers and the membrane are compared with the results from a full three-dimensional model of the same cell for the variations in the channel height, gas diffusion layer thickness, widths of the channels and ribs, operation temperature, and relative humidity of the reactant gases at the inlets. Overall, results from the pseudo three-dimensional model compare very well with the ones from the full three-dimensional model and experimental data. (C) 2019 Elsevier Ltd. All rights reserved.

High utilization of hydrogen is desired in operation of PEM fuel cells. Typically, additional devices for hydrogen recovery at the anode exit are necessary. Alternatively, dead-ended anode (DEA) operation may be considered, however this mode causes severe voltage transients and loss of catalyst support in hydrogen-depleted regions of the active area. Here. ultra-low stoichiometric (ULS) flow conditions that deliver very high hydrogen utilization is considered, however uniform flow distribution is necessary in this case. In order to obtain the flow distribution in the channels and in the inlet and exhaust headers in the anode, a three-dimensional CFD model is developed based on the finite-element method to solve Stokes equations subject to no-slip boundary conditions on the walls, specified pressure at the inlet and specified flow rate at the exit, which is set to a very low value comparable to the typical rate of nitrogen accumulation in the anode side in order to simulate the ULS flow. Uniformity of the flow distribution between the channels is quantified by means of two performance metrics: (i) the root-mean-square (rms) of the channel average velocities; (ii) the ratio of maximum and minimum values of the channel averages. Effects of geometric parameters, such as the widths of the channels and ribs and the position and lengths of the baffles in the inlet and exhaust headers are studied. The final design has less than 5% rms, and less than 1.2 for maximum to minimum average channel velocity ratio.

Analysis and design of flow fields for proton exchange membrane fuel cell (PEMFC) require coupled solution of the flow fields, gas transport and electrochemical reaction kinetics in the anode and the cathode. Computational cost prohibits the widespread use of three-dimensional models of the anode and cathode flow fields, gas diffusion layers (GDL), catalyst layers (CL) and the membrane for fluid flow and mass transport. On the other-hand, detailed cross-sectional two-dimensional models cannot resolve the effects of the anode and cathode flow field designs. Here, a two-dimensional in-plane model is developed for the resolution of the effects of anode and cathode flow channels and GDLs, catalyst layers are treated as thin-layers of reaction interfaces and the membrane is considered as a thinlayer that resist the transfer of species and the ionic current. Brinkman equations are used to model the in-plane flow distribution in the channels and the GDLs to account for the momentum transport in the channels and the porous GDLs. Fick’s law equations are used to model transport of gas species in the channels and GDLs by advection and diffusion mechanisms, and electrochemical reactions in the CL interfaces are modeled by Butler-Volmer equations. Complete features of the flow in the channels and inlet and outlet manifolds are included in the model using resistance relationships in the through-plane direction. The model is applied to a small cell having an active area of 1.3 cm2 and consisting of 8 parallel channels in the anode and a double serpentine in the cathode. Effects of the anode and cathode stoichiometric ratios on the cell performance and hydrogen utilization are investigated. Results demonstrate that for a sufficiently high cathode stoichiometric ratio enough, anode stoichiometric ratio can be lowered to unity to obtain very high hydrogen utilization and output power.

A multi-physical 3D model of the stack is a useful tool to increase the lifetime of an SOC stack, as this can be used to study the impact of modifying operating parameters, design and/or predict the requirements to the stack component materials. The multi-physics model can estimate the variation of physical quantities (e.g. temperature, overpotentials, thermal stresses), which could be used to assess local degradation or mechanical failures. Multi-physical 3D models describing all relevant physical phenomena (current, gas flows, heat transport, mass balances and mechanical stresses) with the geometric details are computationally expensive to run, i.e. days on a cluster is required for a single steadystate simulation. However, by using multi-scale models or so-called homogenized models, the computational resources are tremendously decreased, such that a steady-state model can be solved within minutes on a workstation. A wide range of models have been developed to describe the degradation and failure of cells and other components in the stack for both electrolysis and fuel cell modes of operation. These include Ni migration and agglomeration, Chromium poisoning, corrosion and failure in the oxygen electrode / interconnect interface. Most of these relate simple physical models or empirical models to the observed degradation in actual cells and stacks by fitting a few unknown parameters. These models are rather useful to describe the local degradation in SOC stack, but the local evolution will influence the entire stack, and a full 3D stack model is needed to predict the spatial variation of the degradation. In this paper, we present a 3D model of an SOC stack, where the local degradation and mechanical failures are described everywhere in the stack over the stack lifetime. The model is thus able to describe all relevant physics (currents, gas flows, heat transport, mass balances and mechanical stresses) coupling in space and time. Simulations of tens thousands of hours of operation can be computed within an hour or two on a high-end workstation, a task that would have taken months of simulation time with conventional models. This makes it possible to assess the lifetime, effect of design modifications and material requirements for the SOC stack technology.

The ongoing green energy transition increases the need for dynamic and efficient Power-to-X (PtX) systems to convert surplus wind and solar power to high-value products. The solid oxide electrolysis cell (SOEC) technology offers the highest energy conversion efficiency. However, operation at part-load, i.e. below thermo-neutral potential, creates thermal variations causing thermomechanical stress. Combined with limited lifetime this impedes commercialization of the technology. Here we present a novel operating method that alleviates temperature variations in the SOECs and decreases degradation.A flat thermal profile across the SOEC is obtained by switching several times per second between electrolysis mode and brief pulses in fuel cell mode; the thermal mass of the SOEC prevents temperature fluctuations during the short switching period. The operation method is referred to as AC:DC operation.In this presentation we present experimental results. The observed reduction in the degradation rate is believed to be caused by desorption of impurities that otherwise adsorb on the surface of the electrodes. Finally, we discuss how AC:DC operation can enable dynamic operation of large SOEC systems.