Impedance spectra obtained on high‐performance false(<0.1 normalΩ cm2false) Ni/yttria stabilized zirconia (YSZ) cermet anodes at 1000°C in H2/H2O/normalinert gas mixtures are considered. An impedance arc with a characteristic frequency of about 10 to 100 Hz is related to finite diffusion limitation by considering the dependence on water partial pressure, apparent thermal activation, and effect of the inert gas component. By experiments in different test geometries the diffusion zone is demonstrated to be a stagnant gas layer outside the anode structure. An estimated stagnant gas layer thickness of about 1 mm is arrived at. The diffusion impedance is demonstrated to be avoidable by using a setup geometry with two coupled electrodes in direct electrical contact through opposing sides of a contacting mesh. © 1999 The Electrochemical Society. All rights reserved.

The appearance of an extra arc in impedance spectra obtained on high performance solid oxide fuel cell (SOFC) anodes i recognized when experiments are conducted in a test setup where the working and reference electrodes are placed in separate atmospheres. A simple continuously stirred tank reactor (CSTR) model is used to illustrate how anodes measured with the reference electrode in an atmosphere separate from the working electrode are subject to an impedance contribution from gas conversion. The gas conversion impedance is split into a resistive and a capacitive part, and the dependences of these parameters on gas composition, temperature, gas flow rate, and rig geometry are quantified. The fuel gas flow rate per unit of anode area is decisive for the resistivity, whereas the capacitance is proportional to the CSTR volume of gas over the anode. The model predictions are compared to actual measurements on Ni/yttria stabilized zirconia cermet anodes for SOFC. The contribution of the gas conversion overpotential to dc current-voltage characteristics is deduced for H2/H20 and shown to have a slope of RT/2F in a Tafel plot.

The oxidation of hydrogen on Ni/yttria-stabilized zirconia (Ni/YSZ) is studied by impedance spectroscopy. The active thickness obtained is 20 pm or less. Conditions such as temperature, anodic overvoltage, electrode potential, H2 and H20 partial pressure are varied. Three distinct arcs are identified in impedance spectra, representing at least three rate-limiting processes. One equivalent circuit of the type LR(RQ)(RQ)(RQ), where Q = Yjju4Th, is used to describe all recorded impedance in the temperature range 850 to 1000°C. The n-values are held constant, allowing a direct comparison of R and Y0 values for different structures and conditions. The high-frequency arc (1 to 50 kHz) is sensitive to the cermet structure (particle size) and relatively insensitive to atmospheric composition and overvoltage. The related imperfect capacitance is suggested to be interpreted as a double-layer capacitance in the Ni/YSZ interface. The medium-(10 Hz to 1 kHz) and low-frequency arc (0.1 to 10 Hz) are sensitive to atmospheric composition and overvoltage. Both reaction resistances change their dependency on H2 partial pressure around 0.5 atm. The perfect capacitance related to the low-frequency arc is in the order of 0.5 to 2.5 F/cm2, indicating an absorbed charged species rather than surface adsorption. InfrocluctionThe most widely studied anode for oxidation of hydrogen in solid oxide fuel cells (SOFCs) is the Ni/YSZ cermet anode. Alternative metallic anodes such as Pt, Ru, and Pd have been investigated in conjunction with YSZ,13 and mixed conducting oxides such as CeO2, (CeO2)0 3(SmO, 5)02, and PrO2.

The relations between morphology and electrochemical performance of Ni/yttria-stabilized zirconia (YSZ) anodes are investigated. Four types of anodes are prepared on YSZ electrolyte three-electrode pellets. A fine cermet of 0.5-1 m particles, a coarse cermet of 2-3 m particles, a porous Ni-paste anode, and a Ni-felt anode. The anodes are characterized by impedance spectroscopy at open-circuit potential, and the electrode relevant part (polarization resistance, R P ) of the spectra is identified and investigated. The active thickness of the fine cermet anode is demonstrated to be about 10 m and is believed to relate to the conductivity of the YSZ network. In the temperature range 850-1000ЊC, R P exhibits an apparent activation energy which increases with coarseness of the anode. No significant dependence on p H2 (0.01-0.97 atm) at 1000ЊC is observed. A dependence on p H2O of about R P ϰ p H2O Ϫ1/2 is found. Physical transport limitations are suggested as possible causes for the observed anode polarization.

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