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

Abstract

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.

Trends for Methane Oxidation at Solid Oxide Fuel Cell Conditions

“…44 Also an increased overpotential will, in our approach, essentially drive oxygen to the surface. So beyond the influence of diffusion processes, and that the kinetic models need to include several concurrent RDSs, the effect of oxygen-oxygen repulsion at higher coverages also needs to be addressed.…”

“…[40][41][42][43] It is generally believed that the surface processes of hydrogen and methane oxidation for the most commonly used catalyst, the Ni-YSZ cermet, are fast. 44 Recent studies suggest that chargetransfer and hydrogen spillover reactions are rate-limiting steps for this material. 41,45 Pressure conditions may also influence which reaction steps become rate-limiting.…”

On Modeling of Heat and Mass Transfer and Other Transport Phenomena in Fuel Cells

“…Despite the big amount of experimental electrochemical data available, the mechanistic details of even the simple hydrogen oxidation reaction are still unclear, as claimed by Vogler, et al (2009). It is often assumed that the electrochemical reactions take place close to the TPB.…”

“…However there is some controversy regarding the actual pathway and nature of the elementary steps. More discussion can be found in, e.g., Vogler, et al (2009). It is believed that an electrochemical oxidation reaction at the anode produces electrons that flow through the intercollector (bipolar plate, for PEMFC) orconnector (for SOFC) to the external circuit, while the oxide ions (in SOFCs) or protons (in PEMFCs) pass through the electrolyte to the opposing electrode.…”

Bayesian inference of chemical kinetic models from proposed reactions

“…The standard approach to building models in such a case is to postulate reaction models and to compare them based on their ability to reproduce indirect system-level experimental data. Data-driven model comparison thus involves defining a metric of fit, e.g., the sum of squared residuals, and selecting a model that minimizes this metric, e.g., least-squares or regularized least-squares fitting (Hanna et al, 2014;Vogler et al, 2009;Yurkiv et al, 2011). The principal limitation of the least-squares fitting approach is that it only identifies a single "best" model and yields point estimates of the underlying parameter values, without providing a meaningful description of the uncertainty among competing models and in their parameters.…”