Elementary kinetics of the oxygen reduction reaction on LSM-YSZ composite cathodes

- Oxygen reduction on LSM-YSZ is described by elementary kinetics.
- Three mechanisms are tested against several sets of experimental data.
- The mechanism proposing the chemisorption of O2 on LSM is the most tenable.
- The thermodynamically consistent kinetics is intrinsic.
- A model-based sensitivity analysis elucidates the rate-limiting steps.

A multi-physics based transient, continuum model of a solid oxide half-cell comprising of a porous LSM-YSZ composite cathode sintered to a dense YSZ electrolyte was developed to investigate the oxygen reduction reaction kinetics. The model coupled species, electron and ion transport through the porous cathode to surface and electro-chemistry. The electrochemical reduction of O2 was modeled using three candidate elementary kinetic mechanisms. Each mechanism included parallel surface and bulk pathways for O2 reduction and was driven by three different electric phase potentials. The mechanisms were compared against three sets of electrochemical impedance spectra and polarization curves measured by Barbucci et al. (2009), Cronin et al. (2012) and Nielsen and Hjelm (2014) over a wide range of operating temperatures (873-1173 K), inlet O2 concentrations (5-100%) and overpotentials (−1 V to +1 V). Two of the three mechanisms were able to quantitatively reproduce the three sets of experiments by only tweaking the microstructural parameters for each individual set. Yet on analyzing their kinetic and thermodynamic parameters, the mechanism postulating the chemisorption of gas-phase O2 on LSM to form the superoxo-like adsorbate O2- was determined to be the most realistic. A model-based sensitivity analysis revealed that ionic transport in the YSZ phase, O2 dissociation in conjunction with surface to bulk charge transfer in the LSM phase and charge transfer at the three phase boundary were the rate limiting steps throughout the operating space. Additionally, the bulk pathway was found to be insignificant.

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