Zacharie Wuillemin

Modeling and Study of the Influence of Sealing on a Solid Oxide Fuel Cell

The properties of sealing materials are important for the performance and reliability of solid oxide fuel cells (SOFCs). Even if the properties of a sealing material can be studied separately, it remains difficult to quantify the effect of an imperfect seal on the repeat-element behavior. In this study, simulation is used to investigate the effects of an imperfect seal behavior on the performance and reliability of SOFCs. Diffusion through the sealing material and inherent local combustion of fuel are added to the computational fluid dynamics (CFD) repeat-element model, which also allows us to compute the flow field, the electrochemical reactions, and the energy equations. The results are in good agreement with experiments. The zones of parasitic combustion and local overheating are well reproduced. Furthermore, the model predicts a risk of reoxidation under polarization that is well observed. The model also shows the necessity to take into account the diffusion transport for the development of compressive seal materials, hence verifying the hypotheses made by other groups. The modeling approach presented here, which includes the imperfections of components, allows us to reproduce experiments with good accuracy and gives a better understanding of degradation processes. With its reasonable computational cost, it is a powerful tool for a design of SOFC based on reliability.

Sulfur as pollutant species on the cathode side of a SOFC system

Sulfur poisoning in a strontium doped lanthanum cobaltite (LSC) cathode current collection layer was revealed in a solid oxide fuel cell (SOFC) tested in repeat-element and stack configuration. Sources of sulfur contamination, other than trace SOx in air, were identified. Strontium sulfate (SrSO4) and strontium chromate (SrCrO4) enriched with sulfur were found at the interface between LSC and air channels. Understanding degradation mechanisms is a major issue in the development of SOFCs. In particular, in repeat-element and stack configuration, an important coupling of different degradation processes exists, with internal sources at the stack level and exogenous sources coming from system components. A specific diagnostic test station was developed to allow locally-resolved measurements of electrochemical performance and degradation in a repeat-element. Large differences in local degradation behavior were observed, affecting different electrochemical processes. Post-mortem analysis, mainly done by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), allowed to identify the pollutant species responsible for degradation. Complete results are published elsewhere [1-2]. In this study, an anode-supported cell, with an active strontium doped lanthanum manganite (LSM) - yttria stabilized zirconia (YSZ) composite cathode and a LSC current collection layer, was tested over 1900 h around 1073 K. Chromium and sulfur were found as major pollutant species on the cathode side; both are dependent on upstream conditions and components. While chromium poisoning is well known for SOFC cathodes, sulfur contamination of the cathode has received more attention recently. Sulfur poisoning is a major limitation for wide use of perovskite catalysts for treatment of auto exhaust gas [3]. Sulfur pollution from trace SOx in air and involving sulfate formation was reported by Xu et al. [4], where perovskite-type ceramics were used for oxygen permeation membranes. Yokokawa et al. identified sulfur as impurity after long-term operation of SOFC stacks in [5], and Xiong et al. reported and studied sulfur poisoning of different cathode materials just recently [6]. In the present experiment, a commercial vulcanized polymer tube for air inlet and an insulating high temperature sealing paste were identified as potential sulfur sources. Figure 1 shows the LSC microstructure before and after exposure to sulfur containing air. Strontium sulfate growth on porous LSC surface leads to an almost dense sulfate layer. Strontium sulfate was found over the whole LSC thickness without affecting the LSM/YSZ cathode. The more pronounced strontium sulfate formation in LSC compared to LSM is explained by the higher activity of strontium oxide (SrO) in LSC [7]. Chromium on the LSC surface was only found at the air-inlet, indicating that Cr was principally evaporated from upstream system components. SEM/EDX analysis allowed to identify sulfur-rich strontium chromate with a composition close to Sr(Cr0.85S0.15)O4. The chromate structure is changing along the airflow direction, from an almost dense structure to isolated particles, while maintaining the same composition. Further analyses are ongoing to confirm sulfate and chromate crystalline structures.

Locally-Resolved Study of Degradation in a SOFC Repeat-Element

The locally-resolved degradation behavior was studied during 1900 hours in an SOFC repeat-element. In-situ measurements of local electrochemical performance were made on 18 locations over a segmented anode-supported cell. The evolution of local current densities, overpotentials and area-specific resistances was studied, showing a reorganization of the electrochemical reaction with time. The extent and the spatial distribution of degradation were established for different electrochemical reactions steps using impedance spectroscopy. The low- frequency cathode contribution was the mostly altered process, followed by the charge transfer reaction on anode side. Post-experiment analyses allowed to identify three major pollutants on the cathode side (chromium, silicon and sulfur), whose spatial distributions corresponded to the observed local degradation. Sources of pollutants were identified in system components as well as within the stack repeat-element.

Electrochemical Model of Solid Oxide Fuel Cell for Simulation at the Stack Scale I. Calibration Procedure on Experimental Data

Lifetime prediction and improvement of solid oxide fuel cell (SOFC) devices require a reliable electrochemical model that supports the implementation of degradation phenomena. This study comprises two parts. This Part I describes the calibration of an electrochemical model based on physical principles for simulation at the stack scale. Part II presents the further implementation of degradation models. A distinction is made between the two most common cathode materials, lanthanum strontium manganite and lanthanum strontium cobalt ferrite. The experimental data used for the parameter estimations was gathered by two segmented setups. The calibrations enabled to reproduce adequately the measurements over a wide range of operating conditions. The optimal values of the physical parameters were inside the ranges reported in literature. Unambiguous discrimination could not be achieved between variations (i) in the choice of electrode rate-determining steps, (ii) data on the properties of the materials found in literature and (iii) empirical relations for the steam-methane reforming reaction. However, these model variations do not affect significantly the predicted magnitudes and distributions of the field variables assumed to govern the degradation processes at the SRU scale, compared with the uncertainties on the degradation phenomena to be implemented in Part II

Impact of Materials and Design on Solid Oxide Fuel Cell Stack Operation

Planar SOFC stack technology based on a unique concept (SOFConnex™) uses structured gas distribution layers between unprofiled metal sheet interconnects and thin Ni-YSZ anode supported electrolyte cells. The layers are flexible both in material and designand allow to implement new configurations relatively simply; manifolding can be internal, external, or combined. Together with thin stack components, independent of the supplier, the SOFConnex™ stacking approach allows compact planar assembly with low cost potential and adequate power density. Different cell and flow designs have been realized. With a basic flow configuration, short stacks (50 cm2 cell active area) were assembled and tested, power density at 800°C reaching 0.5 W/cm2 at 0.7 V average cell voltage (1.5 kWe /L, 0.36 cm2 area specific resistance), for 65% fuel utilization and 35% lower heating value electrical efficiency. Short stacks were thermally cycled and operated with both hydrogen and syngas. Degradation was essentially Ohmic(confirmed from impedance spectroscopy on stacks) and at first mainly due to the cathode-electrolyte interfacial reaction, performance loss was subsequently strongly reduced after cathode replacement. Using multiple voltage probes with additional interconnects allowed to separately monitor current collection losses during polarization. With an improved design in terms of sealing, postcombustion control and flow field, stacks up to 1 kWe have been operated.

Glass-Forming Exogenous Silicon Contamination in Solid Oxide Fuel Cell Cathodes

Spatially resolved analyses, by energy-dispersive X-ray spectroscopy (EDS) scanning electron microscopy (SEM), allowed the quantification of exogenous Si contamination in a solid oxide fuel cell (SOFC) cathode after operation. The Si quantification, taking into account the endogenous Si impurity level, correlated well with the expectation from the condensation of Si(OH)4 vapor, originating from upstream alloy components and saturated in the hot inlet air. At higher resolution, EDS-transmission electron microscopy (TEM) pointed out the deposition of Si vapor in the form of amorphous SiO2, blocking oxygen incorporation into the electrolyte phase within a composite SOFC cathode.

Robust Real-Time Optimization of a Solid Oxide Fuel Cell Stack

On-line control and optimization can improve the efficiency of fuel cell systems whilst simultaneously ensuring that the operation remains within a safe region. Also, fuel cells are subject to frequent variations in their power demand. This paper investigates the real-time optimization (RTO) of a solid oxide fuel cell (SOFC) stack. An optimization problem maximizing the efficiency subject to operating constraints is defined. Due to inevitable model inaccuracies, the open-loop implementation of optimal inputs evaluated off-line may be suboptimal, or worse, infeasible. Infeasibility can be avoided by controlling the constrained quantities. However, the constraints that determine optimal operation might switch with varying power demand, thus requiring a change in the regulator structure.

Multi-Scale Assessment of Cr Contamination Levels in SOFC Cathode Environment

This study aims to quantify in a holistic approach chromium (Cr) contamination in solid oxide fuel cell (SOFC) stack testing using adapted tools: i) a hot gas sampling method to analyze volatile Cr species in the air flux at the cathode inlet location; ii) a rapid quantification method for Cr as condensed matter in cathode material of post-test samples. The hot air sampling method reveals itself as a reproducible and time-resolved quantification technique for Cr trace amounts in a gas flow; this technique is seen as a promising evaluation tool for Cr contamination issues in SOFC systems. The quantification method reveals severe Cr-poisoning in a cell. The combined findings indicate that Cr contamination generated by system components located upstream the cell must be suppressed by hindering the access of Cr pollutants to the cathode compartment.

Modeling and Designing of a Radial Anode Off-Gas Recirculation Fan for Solid Oxide Fuel Cell Systems

To improve the industry benchmark of solid oxide fuel cell (SOFC) systems, we consider anode off-gas recirculation (AOR) using a small-scale fan. Evolutionary algorithms compare different system design alternatives with hot or cold recirculation. The system performance is evaluated through multi-objective optimization (MOO) criteria, i.e., maximization of electrical efficiency and cogeneration efficiency. The aerodynamic efficiency and rotordynamic stability of the high-speed recirculation fan is investigated in detail. The results obtained suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable, including for small power scale (10 kWe).

Spotting Solid Oxide Fuel Cell Degradation Effects by Electron Microscopy

In the present international energy context, the landscape is being redesigned. Decentralised combined heat and power, fuel diversity and increased use of renewable sources will gain in importance. Solid Oxide Fuel Cells (SOFCs) represent an attractive conversion technology and are considered as part of the answer to decentralised generation at small to medium scale with the highest possible electrical efficiency (60% even for standalone kW-scale systems).