Quantitative study of anode microstructure related to SOFC stack degradation

As the performances of Solid Oxide Fuel Cells (SOFC) get attractive, long term degradation becomes the main issue for this technology. Therefore it is essential to localise the origin of degradation and to understand its processes in order to find solutions and improve SOFC durability. The electrode microstructure ageing, in particular nickel grain coarsening at the anode side, is known to be a major process to cause performance loss. The increase in nickel particle size will diminish the Triple Phase Boundary (TPB), where fuel oxidation takes place, and decrease the anode electronic conductivity. These two effects degrade the electrochemical performance of the fuel electrode. Degradation is defined as the decrease of potential at constant current density with time in %/1000h or mV/1000h. This study is based on HTceramix® anode supported cells tested in stack conditions from 100 to more than 1000 hours. The anode microstructure has been characterized by Scanning Electron Microscopy (SEM). As the back scattered electron yield coefficients of nickel and yttria stabilized zirconia (YSZ) are very close, the contrast of the different phases (Ni, YSZ and pores) is low. Various techniques are used to enhance the contrast. A new technique is presented here using impregnation and SEM observation based on secondary electron yield coefficients to separate the phases. Image treatment and analysis is done with an in-house Mathematica® code. Image analysis gives information about phase proportion, particle size, particle size distribution, contiguity and finally a new procedure is developed to compute TPB density. A model to describe the coarsening of the nickel particles is also developed. The model assumes an exponential growth of the nickel particles. Using a particle population balance, it estimates the growth of the nickel particles and the concomitant drop in the TPB length. This model is in very good agreement with experimental data, especially for relatively low fuel cell operation times (up to 100-200 hours). This model can be used in the estimation of operational parameters of the anode electrode such as the degradation rate using fundamental parameters of the cermet anode like the anode overpotential and the work of adhesion of the nickel particles on the YSZ substrate. This model gives the portion of stack degradation that corresponds to anode performance decrease due to particle sintering. Finally this study gives the possibility to isolate the degradation coming from the anode sintering and compare to the full SOFC stack degradation

The effect of electrochemical oxygen pumping on the steady-state and oscillatory behavior of CO oxidation on polycrystalline Pt

Summarization: The effect of electrochemically pumping O2− to or from a porous polycrystalline Pt catalyst film used for CO oxidation at atmospheric pressure and temperatures 250–600 °C was studied. The Pt film served both as a catalyst and as an electrode of the solid electrolyte cell CO, O2, Full-size image (<1 K) (Full-size image (<1 K). Under open-circuit conditions the Pt catalyst film operates as a regular CO oxidation catalyst. It was found that electrochemical O2− pumping has a dramatic non-Faradaic effect on the steady-state and oscillatory behavior of CO oxidation on Pt. The steady-state reaction rate typically increases or decreases by a factor of 2 but a 500% increase in reaction rate is observed under severely reducing conditions. The induced changes in reaction rate are typically two orders of magnitude higher than the rate of O2− transfer to or from the catalyst and are always accompanied by the appearance of activation overpotential at the catalyst electrode. Reaction rate oscillations can be induced or stopped at will by adjusting the rate of O2− transfer and consequently the potential of the catalyst-electrode. The frequency of electrochemically induced oscillations is linearly related to the applied O2− current. The observed phenomena are completely reversible and are due to electrochemically induced changes in the oxidation state and catalytic properties of the platinum surface. These changes appear to result from changes in the work function of the metal due to the interaction of O anions with the Pt surface. The very pronounced reaction rate increase upon O2− removal under reducing conditions appears to be caused by CO decomposition followed by fast carbon combustion by gaseous O2.Presented on: Journal of Catalysi

In situ controlled promotion of Pt for CO oxidation via NEMCA using CaF2, as the solid electrolyte

Μη διαθέσιμη περίληψηNot available summarizationPresented on: Journal of Catalysi

Effectiveness factors for reactions between volatile and non-volatile components in partially wetted catalysts

Summarization: A steady state model is formulated to describe the reactions between one gaseous and two non-volatile components on partially wetted catalysts in a trickle-bed reactor. Unlike previous studies, the model does not assume a gaseous or liquid limiting reactant. The computed results show the effect of intraparticle and interphase mass transfer resistances as well as of wetting efficiency and number and location of wetted zones on catalyst performance. Maximum effectiveness factors are generally obtained with intermediate values of the wetting efficiency. The model is applied to a typical HDS process of heavy gas oil in trickle-bed reactors.Presented on: Chemical Engineering Scienc

Methane to ethylene with 85 percent yield in a gas recycle electrocatalytic reactor-separator

Summarization: Methane was oxidatively coupled to ethylene with an ethylene yield up to 85 percent and a total C2 hydrocarbon yield up to 88 percent in a gas recycle high-temperature (800°C) electrocatalytic or catalytic reactor where the recycled gas passes continuously through a molecular sieve trap in the recycle loop. Oxygen is supplied either electrocatalytically by means of the solid electrolyte support of the silver-based catalyst or in the gas phase. The C2 products are obtained by subsequent heating of the molecular sieve trap. The selectivity to ethylene is up to 88 percent for methane conversion up to 97 percent.Παρουσιάστηκε στο: Scienc

Potentional-programmed reduction: a new technique for investigating the thermodynamics and kinetics of chemisorption on catalysts supported on solid electrolytes

Summarization: A new technique is presented for investigating the kinetics and thermodynamics of chemisorption of oxygen and, potentially, of other adsorbates on conductive catalyst films deposited on solid electrolytes, such as yttria-stabilized zirconia, an O2− conductor. In this technique, termed potential-programmed reduction (PPR), the catalyst surface is first exposed to the chemisorbing gas and then the catalyst potential is swept linearly by, typically −1 V, causing the reduction of chemisorbed oxygen to O2− at distinct and well-resolved catalyst potential values Ep. In this way current peaks centered at Ep are generated. The area of each peak provides direct quantitative information about the coverage of adsorbed oxygen and the corresponding Ep value about its Gibbs free energy. The PPR technique has certain similarities with temperature-programmed reduction but is isothermal and provides direct thermodynamic as well as kinetic information. It is, however, limited to conductive catalyst films which can be supported on solid electrolytes. In the present work the PPR technique is used to investigate oxygen chemisorption on Ag and Pt. In the case of Ag two types of oxygen are resolved at temperatures 300 to 450°C (atomically adsorbed and subsurface), while on Pt there is usually one peak corresponding to atomic oxygen with a second peak developing after prolonged exposure to positive potentials.Presented on: Journal of Catalysi

Oxidative coupling of methane to ethylene with 85% yield in a gas recycle electrocatalytic or catalytic reactor separator

Summarization: A novel gas-recycle electrocatalytic or catalytic reactor-separator has been developed which gives very high ethylene yields during the oxidative coupling of methane. The recycled gas passes continuously through a molecular sieve trap in the recycle loop, which traps and thus protects an easily controllable percentage (up to 100%) of C2H4 and C2H6 produced during each gas cycle. The C2 products are obtained by subsequent heating of the molecular sieve trap. Ethylene yields up to 85% (88% C2H4 selectivity at 97% methane conversion) were achieved during batch operation while ethylene yields up to 50% (65% C2H4 selectivity at 76% methane conversion) were obtained during continuous-flow, steady-state operation.Presented on: Studies in Surface Science and Catalysi

Ethylene production from methane in a gas recycle electrocatalytic reactor separator

Summarization: It was found that methane can be oxidatively coupled to ethylene with an ethylene yield up to 85% and a total C2 hydrocarbon yield up to 88%, in a novel gas-recycle electrocatalytic reactor-separator where the recycled gas passes continuously through a molecular sieve trap in the recycle loop. The molecular sieve traps and thus protects a controllable percentage of ethylene and ethane produced during each gas cycle. These products are obtained by subsequent heating of the trap. In this way we have obtained, using the batch operating mode of the recycle reactor, ethylene yields up to 85%, i.e. 88% selectivity to ethylene at 97% CH4 conversion. Oxygen is supplied electrochemically to the Ag-Sm2O3 or Ag anode via a solid electrolyte, i.e. Y2O3-stabilized-ZrO2, which is a O2- conductor. The cathode is exposed to ambient air and the electrocatalytic reactor operates at 750–830 °C.Presented on: Ionic