In this study, with the motivation of elucidating the effect of H₂S and HCl on solid oxide fuel cell anodes, nickel and ceria pattern anodes are prepared on yttrium-stabilized zirconia electrolyte, and the effect of H₂S and HCl on their performance is tested using electrochemical impedance spectroscopy. However, it has been found that while H₂S adversely impacts both nickel and ceria, the poisoning caused is reversible for nickel and only partially reversible for ceria. Poisoning kinetics are similar and fast for both materials, while recovery kinetics are slower for ceria than nickel. High sulfur coverage is the rate-limiting factor inferred from the elementary kinetic modeling. Unlike H₂S, the presence of HCl appeared to be favorable for electrochemical oxidation as the polarization resistance of both pattern electrode cells decreased upon feeding HCl contaminated hydrogen gas. Similar behavior has not been reported previously, and the conclusion regarding underlying mechanisms requires further investigation.

@en

Conventional energy technologies and fossil fuels are causing irreversible damage to the environment. A transition from conventional to sustainable technologies is inevitable to address the environmental concerns. Solid oxide fuel cells (SOFCs) can play a key role in this transition because of their high efficiency and fuel flexibility – SOFCs can operate with fossil fuels as well as with renewable fuels. However, several challenges concerning cost reduction, operability, and long-term durability remain in SOFC development. A good physio-chemical and electrochemical understanding of the fuel-electrode is crucial to overcome the operability and durability limiting factors, as well as to design the new, improved, and low-cost electrodes.@en

Using the Nernst-Planck-Poisson model and a detailed reaction mechanism, we studied the hydrogen electrochemical oxidation on a ceria anode. Resistances caused by surface kinetics, and bulk transport of oxide-ion vacancies and electrons are computed individually to identify the dominant resistive process. The effect of operating conditions like temperature and gas-phase composition on the polarization resistance is evaluated and compared with the experimental data obtained by Electrochemical Impedance Spectroscopy (EIS). The rate-determining step is found to be the charge-transfer reaction in which hydrogen adsorbs at the surface oxide ions and forms hydroxyls along with the charge-transfer to adjacent cerium ions. Based on the rate-determining step, the exchange-current density is also calculated and validated with the experimental data.@en

Solid oxide fuel cell exhibits its advantages such as fuel variability. Capable of running on renewable fuels, SOFCs are regarded as potential solution to carbon-neutral energy conversion path. SOFCs operate on fuel supplied to the anode and oxidant supplied to the cathode. The anode should facilitate the oxidation of fuel and transport of electrons from the reaction site to the current collector. It should also enable the diffusion of gaseous fuel to the reaction sites and reaction products away from the reaction sites. Meeting abovementioned requirements, Ni/YSZ cermets are competent SOFC anodes, where Ni is an electronic conductor, YSZ is an ionic conductor, and pores in between Ni and YSZ facilitate the fuel supply. Fuel oxidation occurs in the vicinity of triple phase boundaries (TPBs), where Ni particles, YSZ particles and pore interact. Thus, TPB length quantifies the electrochemically active sites within the anode. The fuel oxidation involves physical processes and multi-elementary reaction steps in reality, among which exists a rate-limiting step. The overall reaction rate can be represented by the rate-limiting step whose kinetics are proposed as a function of the TPB length. In many past computational studies of the cermet anode, a global electrochemical reaction expressed by Butler-Volmer equation is put forward, simplifying electrode reaction to a single step process and probably inadequately to present insights into intrinsic processes occurring within the anode. In this work, TPB-based kinetics, derived from the pattern anode experiment, are implemented in a CFD model to evaluate the performance of Ni/YSZ cermet-based cells. For model validation, simulated polarization curves are compared with the experimental ones. TPB length as a fitting parameter is determined to ensure the agreement between simulated polarization curves and experimental ones. The fitted TPB length is found to be several orders of magnitudes lower than physical TPB length of typical Ni-YSZ cermet anode, which might imply that only a small fraction of the physical TPB length actually participates in the electrochemical reactions. CFD behavior like species distribution and electrochemical behavior like overpotential breakdown are investigated. The gradient of species molar fraction in anode turns to be larger than that in fuel channel, agreeing with the fact that fluid flow is much slower in anode where species transport in porous electrode is dominated by diffusion rather than convection. The cathode activation overpotential makes the most significant contribution to the overall overpotential. The anode activation overpotential at high current density region in the case of low molar fraction of inlet hydrogen yields rapid increase, which could be explained as a consequence of rapid decrease of exchange current density in the vicinity of anode/electrolyte interface. Parametric study of operating temperature is conducted later to see its effect on the cell performance. The higher the operating temperature is, the slower drop of cell voltage with respect to cell current is observed. At a fixed cell voltage, an elevated temperature will cause intensified fuel consumption/vapor generation within anode, leading to a larger gradient of species molar fraction and accordingly a larger concentration overpotential.

Fuel flexibility of solid oxide fuel cells enables the use of low cost and practical fuels like syngas. Understanding of the oxidation kinetics with syngas is essential for proper selection of anode material and its design optimization. Using nickel and ceria pattern anodes, we study the electrochemical oxidation of syngas in both dry and wet environments. In dry environment, the polarization resistance of CO oxidation drops drastically with the addition of small amounts of hydrogen to CO gas stream. In wet environment (4 % moisture), the polarization resistance of CO is only slightly higher than syngas and hydrogen. Observation in the first case is related to the hydrogen preferential oxidation whereas latter is a combined effect of water gas shift reaction and preferential oxidation of hydrogen. Kinetic modeling is also carried out to understand hydrogen and CO co-oxidation. Simulation suggests that CO, besides hydrogen, may also electrochemically oxidize depending upon its concentration in the syngas. At higher concentration, CO electrochemical oxidation may be non-negligible especially in case of ceria anodes.@en

A summary of research activities and on-going multiple projects at Delft University of Technology aimed at the development of Solid Oxide Fuel Cells (SOFCs) integrated waste to energy systems are presented in this paper. A wide range of studies ranging from pattern cell impedance analysis to integrated system development is also presented. Cleaning the gas to match constraints of SOFCs requires complex approaches. A brief discussion on the achievable system efficiencies with SOFC integrated waste to energy systems is presented and novel concepts such as gasifier-Reversible SOFC integrated power production cum energy storage systems are discussed. Additionally, an overview of a wide range of national, European and international projects that helped or currently helping us to carry out the above mentioned research activities are presented.@en