A parameter study of 20 solid oxide electrolysis cells was carried out to systematically investigate long-term degradation each over 1,000 h under variation of temperature, humidity and current density. The influence of operating temperature was investigated between 750 and 850 °C, the humidity ranged from 40 % to 80 % H 2 O, and the current density varied between open circuit voltage (OCV) and 1.5 A•cm-2. The progress of degradation was monitored insitu by electrochemical impedance spectroscopy. Five different contributions to the spectra were identified by calculating the distribution of relaxation times and separated via a complex non-linear square fitting routine. The present work focuses on the degradation of the fuel electrode. From SEM analysis, Ni depletion and an increased pore fraction close to the electrode/electrolyte interface was derived, which is correlated with an increased ohmic resistance of the cells. This unidirectional transport of Ni away from the fuel electrode/electrolyte interface leads to an effective electrolyte extension and is the main source of degradation. Ni depletion is shown to be driven by current density and its extent is shown to be dependent on the complex interplay between the operating parameters current density, anode overpotential, humidity and temperature. It is particularly pronounced for pH 2 O larger than 0.8 atm and temperatures above 800 °C. Furthermore, the fuel electrode electrochemistry also exhibits degradation in the high-frequency region around 10 4 Hz.

One of the most powerful tools in solid oxide cell (SOC) characterization is electrochemical impedance spectroscopy, which can unfold important insights into SOC performance characteristics and degradation behavior. To obtain a better understanding of the electrochemical behavior of Ni/CGO fuel electrodes, this work presents a comprehensive investigation of state-of-the-art Ni/CGO10based electrolyte-supported cells. Commercial Ni/CGO10|CGO10|3YSZ|CGO10|Ni/CGO10 symmetrical cells were characterized between 550-975°C at pH 2 = 0.8 bar and pH 2 O = 0.2 bar, and for different H 2 /H 2 O gas mixtures at 550°C. (i) Small electrode area, (ii) thin electrodes and (iii) large gas flow rates were used to minimize mass transport contributions. Based on distribution of relaxation times (DRT) analysis an equivalent circuit model was derived. Electrode process contributions on Ni/CGO were determined by means of a complex non-linear least square fit of the equivalent circuit model to the experimental data. One low frequency process at 0.1-1 Hz and one middle frequency process at 10-100 Hz were identified and correlated to a surface and a bulk process, respectively. Values for the apparent activation energy barriers and reaction orders with respect to steam and hydrogen content were determined.

The focus of this study is the measurement and understanding of the sulfur poisoning phenomena of Ni/gadolinium‐doped ceria (CGO) based solid oxide fuel cells (SOFC). Cells with Ni/CGO10 and NiCu5/CGO40 anodes were characterized by using impedance spectroscopy at different temperatures and H2/H2O fuel ratios. The short‐term sulfur poisoning behavior was investigated systematically at temperatures of 800–950 °C, current densities of 0–0.75 A cm−2, and H2S concentrations of 1–20 ppm. A sulfur poisoning mitigation effect was observed at high current loads and temperatures. The poisoning behavior was reversible for short exposure times. It was observed that the sulfur‐affected processes exhibited significantly different relaxation times that depend on the Gd content in the CGO phase. Moreover, it was demonstrated that the capacitance of Ni/CGO10 anodes is strongly dependent on the temperature and gas‐phase composition, which reflects a changing Ce3+/Ce4+ ratio.

An elementary kinetic model is developed and applied to explore the influence of sulfur poisoning on the behavior of solid oxide fuel cell (SOFC) anodes. A detailed multi-step reaction mechanism of sulfur formation and oxidation at Ni/YSZ anodes together with channel gas-flow, porous-media transport and elementary charge-transfer chemistry is established for SOFCs operating on H 2 /H 2 O mixtures with trace amounts of hydrogen sulfide (H 2 S). A thermodynamic and kinetic data set is compiled from various literature sources. The derived chemical model, validated against sulfur chemisorption isobars taken from literature, is used to analyze performance drops of SOFCs working under typical fuel cell operating conditions. Electrochemical results show that at relatively low H 2 S concentrations SOFC button-cell performance can be interpreted using chemical sulfur formation. However, when the concentration is sufficiently high, the inclusion of second stage degradation and triple-phase boundary reconstruction is necessary to describe the performance decrease. Additionally, it is shown that the sulfur surface coverage increases with increasing current density. In order to shed more light on advanced fundamental understanding of cell poisoning, sensitive analyses toward total anode resistance and sulfur coverage for different operating conditions were performed.Solid oxide fuel cells (SOFCs) are a promising technology for supplying electrical energy for future demands. Due to many advantages such as high efficiency, low emissions, low noise, reliability, and fuel flexibility, SOFCs are expected to take over a major role in future stationary energy conversion technologies. 1 It has been demonstrated that SOFCs are a well-suited electrical power source for a variety of applications, ranging from mobile technology (e.g. auxiliary power units) to stationary power plants. 1 Due to high operating temperatures, SOFCs can easily work with natural gas and a variety of hydrocarbons (e.g. methane, propane, dodecane). 2 An alternative and very promising source of hydrocarbons is biomass gasification. The obtained biogas (mainly methane and carbon oxides) is a renewable fuel source which can contribute to the reduction of fossil fuel usage and emission of greenhouse gases. However, depending on the content of biomass, the resulting biogas may contain some undesirable species that upon contact with the SOFC anode could lead to electrode degradation. One of such compounds is hydrogen sulfide which via catalytic dissociation at the anode surface is converted into atomic hydrogen and sulfur. Consequently, SOFC failure in a relatively short period of time can happen depending upon the sulfur amount in the supply gas.Although being widely investigated, the microscopic details of the elementary chemical reaction mechanism of SOFC sulfur poisoning occurring at the Ni/YSZ anode are not fully understood. 3 Yet, the knowledge of elementary kinetics of SOFC electrode degradation is important because the understanding at the fundamental level yie...

The aim of the present study is the measurement and understanding of sulfur poisoning phenomena in nickel/gadolinium-doped ceria (CGO) based solid oxide fuel cells (SOFC) operating on reformate fuels. The sulfur poisoning behavior of commercial, high-performance electrolyte-supported cells (ESC) with Ni/Ce0.9Gd0.1O2−δ (CGO10) anodes operated with different fuels was thoroughly investigated by means of current–voltage characteristics and electrochemical impedance spectroscopy and compared with Ni/Yttria-stabilized zirconia (YSZ) anodes. Various methane and carbon monoxide containing fuels were used in order to elucidate the underlying reaction mechanism. The analysis of the cell resistance increase in H2/H2O/CO/CO2 fuel gas mixtures revealed that the poisoning behavior is mainly governed by an inhibited hydrogen oxidation reaction at low current densities. At higher current densities, the resistance increase becomes increasingly large, indicating a particularly severe poisoning effect on the carbon monoxide conversion reactions. However, the ability of Ni/CGO anodes to convert carbon monoxide even at H2S concentrations up to 20 ppm was demonstrated, while this was not possible for Ni/YSZ. The sulfur poisoning behavior of Ni/CGO in reformate fuels was fully reversible for short exposure times. From methane steam re-forming experiments, it is deduced that the Ni surface is blocked and, thus, the water-gas shift reaction is fully deactivated as well. However, electrochemical CO oxidation on the CGO surface was shown to be still active. The present results clearly demonstrate that the high sulfur tolerance of Ni/CGO not only is limited to H2/H2O fuel systems but also extends to CO-containing gases.