Marcel Vogler

Modelling Study of Surface Reactions, Diffusion, and Spillover at a Ni/YSZ Patterned Anode

This paper presents a modelling study of the electrochemical hydrogen oxidation reaction at nickel/yttria-stabilized zirconia (Ni/YSZ) patterned anodes. An elementary kinetic reaction-diffusion model accounts for coupled heterogeneous chemistry and transport on the Ni and YSZ surfaces. Charge transfer is modeled as a spillover of adsorbates between the Ni and YSZ surfaces at the three-phase boundary (TPB). No a priori assumptions on rate-determining processes are made. Thermodynamic, kinetic, and transport parameters are compiled from various literature sources serving as a base for quantitative simulations. Seven different spillover reaction pathways of the hydrogen oxidation reaction are compared to experimental patterned anode data obtained previously by Bieberle et al. (J. Electrochem. Soc., 148, A646 2001) under a range of operating conditions. Only one reaction pathway, based on two hydrogen spillover reactions, is able to describe consistently the complete experimental data set. A sensitivity analysis for this case allows identification of rate-determining processes. Surface concentrations close to the TPB are predicted to differ from the concentration derived from thermodynamical equilibrium by up to 2 orders of magnitude. The simulation results and the validity of the model are critically discussed. Directions for future theoretical and experimental studies for elucidating the mechanistic details of Ni/YSZ anodes are given.

Model anodes and anode models for understanding the mechanism of hydrogen oxidation in solid oxide fuel cells

This article presents a literature review and new results on experimental and theoretical investigations of the electrochemistry of solid oxide fuel cell (SOFC) model anodes, focusing on the nickel/yttria-stabilized zirconia (Ni/YSZ) materials system with operation under H(2)/H(2)O atmospheres. Micropatterned model anodes were used for electrochemical characterization under well-defined operating conditions. Structural and chemical integrity was confirmed by ex situ pre-test and post-test microstructural and chemical analysis. Elementary kinetic models of reaction and transport processes were used to assess reaction pathways and rate-determining steps. The comparison of experimental and simulated electrochemical behaviors of pattern anodes shows quantitative agreement over a wide range of operating conditions (p(H(2)) = 8×10(2) - 9×10(4) Pa, p(H(2)O) = 2×10(1) - 6×10(4) Pa, T = 400-800 °C). Previously published experimental data on model anodes show a strong scatter in electrochemical performance. Furthermore, model anodes exhibit a pronounced dynamics on multiple time scales which is not reproduced in state-of-the-art models and which is also not observed in technical cermet anodes. Potential origin of these effects as well as consequences for further steps in model anode and anode model studies are discussed.

Direct-Flame Solid-Oxide Fuel Cell (DFFC): A Thermally Self-Sustained, Air Self- Breathing, Hydrocarbon-Operated SOFC System in a Simple, No-Chamber Setup

We present a combined experimental and modeling study of a direct-flame type solid oxide fuel cell (DFFC). The operation principle of this system is based on the combination of a flame with an SOFC in a simple, no-chamber setup. Experiments were performed using 13-mm-diameter planar SOFCs with Ni-based anode, samaria-doped ceria electrolyte and cobaltite cathode. At the anode, a 7-mm-diameter flat-flame burner provided methane/air rich premixed flames. The cell performance reaches power densities of up to 200 mW/cm2. A detailed analysis of the electrical efficiency is carried out. Observed system efficiencies are below 0.5%. Equilibrium calculations of the flame exhaust gas were performed. From the simulations, both H2 and CO were identified as species that are available as fuel for the SOFC.

The role of interstitial hydrogen species in Ni/YSZ patterned anodes: A 2D modeling study

The mechanistic details of the hydrogen oxidation at Ni/YSZ anodes are subject of controversial discussion. In the light of potential accumulation of secondary phases at the three-phase boundary, a mechanism involving interstitial hydrogen species in the bulk phases and charge-transfer at the Ni/YSZ two-phase boundary has been proposed. We present a quantitative analysis of this mechanism based on a two-dimensional elementary kinetic model of electrochemistry and bulk diffusion. The use of literature diffusion coefficients yields diffusion-limited current densities that are below those observed in experiments. Assuming increased diffusivity allows to quantitatively reproduce published experimental pattern anode data, including their dependence on gas composition and temperature. This shows the feasibility of interstitial mechanisms when impurities are present and explains their decreased performance. For clean three-phase boundaries, surface spillover pathways are preferred.