Systematic Modeling for Triple Phase Boundary of Ni/ZrO2 SOFC Anode from First Principles

Introduction

In solid electrolyte-based electrochemical devices such as fuel cells and gas sensors, the devices include hetero interfaces between solid electrolyte and metal, which play an important role for the device working mechanism. The interface between solid-electrolyte and solid-metal can be recognized as a two phase boundary, and the two phase boundary exposed to vacuum (i.e., gas phase) is called as triple (or three) phase boundary (TPB). TPB in electrochemical devices is a key boundary for electrochemical reactions because the boundary is an active site for charge transfer reactions, and thus for the production of electricity. In addition, a rate limiting step in electrochemical devices is often identified in the electrochemical reactions at TPB, although a specific investigation is required in each system. In this study, we carried out first principles electronic structure calculations for the precise understanding of electrochemical processes on Ni-YSZ TPB, and show how TPB models should be constructed in the atomistic level, and what is the suitable index for the characterization of the electrochemical activity of TPB. 

Method

For the investigation of electrochemical reactions in the atomistic level, first principles electronic structure method is a powerful theoretical tool, but the calculation procedures are not so simple when the target system includes complicated interface TPBs. The TPB atomistic model must include the following ingredients: 1) metal region (i.e., slab), 2) solid electrolyte region (slab), 3) the metal-solid electrolyte interface, and 4) the interface exposed to vacuum [1,2]. When we construct a TPB model holding these conditions, the number of atoms included in the computational cell becomes large, and reaction analysis based on the large models is sometimes found to be inappropriate depending on the computational costs. Therefore, theoretical calculations for reaction analysis on TPB were sometimes carried out using small metal-cluster deposited on solid electrolyte slab. However, the electronic structures of metal-slab and metal-cluster are very different because of the finite size effect of the metal cluster model. At the first part of this study, we will present how the metal cluster models on solid electrolyte inappropriately affect computational results at TPB, and how an acceptable TPB model should be constructed based on metal-slab layers (see an example for TPB structure in Figure). 

The second step is the selection of the stoichiometry and the determination of impurity positions. For example in yttrium stabilized zirconia (YSZ) as solid electrolyte, the numbers of dopant Y and oxygen vacancy are determined depending on the stoichiometry and dopant concentration. Thus, we usually fix the system stoichiometry and dopant concentration at first (e.g., stoichiometric or oxygen-rich or oxygen poor in YSZ), and in turn determine the position of dopants from the calculated total energies. The reaction analysis using the atomistic models are thereby carried out as the final step by using first principles calculations with the nudged elastic band method. Since the standard calculations are done along the procedure, the computational results are usually characterized in terms of the system stoichiometry. However, in electrochemical reactions in TPB, the system stoichiometry is sometimes not a good index for the characterization of TPB activity. At the second part of this study, we will present the calculated reaction energies at TPB are highly scattered when those are classified with the system stoichiometry, but clearly characterized when those are classified with a new index, TPB local stoichiometry. We will present the detail of the new concept for the TPB local stoichiometry.

[1] Tomofumi Tada, Shusuke Kasamatsu, and Satoshi Watanabe, First principles study on electronic structures of Ni/H/ZrO₂ triple phase boundary, ECS Transactions 16(51) , 265 (2009).

[2] Tomofumi Tada and Satoshi Watanabe, Chemically softened boundary of metal/vacuum/solid-electrolyte from first principles, J. Phys. Chem. C 113 , 17780 (2009).