Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Andreas Heyden

Abstract

A key challenge in the development of solid oxide fuel cell (SOFC) technology is related to finding a suitable replacement for Ni-based cermet anodes. Although conventional Ni-based electrodes exhibit excellent catalytic activity and current collection, they suffer from several limitations such as instability upon redox cycling, nickel sintering, and sulfur and carbon poisoning when exposed to practical hydrocarbon fuels. Therefore, alternative anode materials need to be developed for SOFCs. Among the novel anode electrodes, perovskite based materials are of great interest because they have been shown to satisfy most intrinsic SOFC anode requirements such as high thermodynamic stability in anodic environments and strong resistance to carbon deposition and sulfur poisoning.

In this dissertation, we described how "first principles" modeling can be used to rationally develop a doping strategy to obtain mixed ionic/electronic conductivity in SrTiO3 perovskites under anodic SOFC conditions. First, constrained ab initio thermodynamic calculations were employed to evaluate the thermodynamic stability of the doped SrTiO3 phases at synthesized and anodic SOFC conditions. Then, we computed and analyzed the density of states (DOS) of p- and n-doped SrTiO3 to determine the number of charge carriers per unit cell in each phase. In agreement with experimental observations, the computational results reveal that mixed p- and n-doping is an efficient strategy to obtain mixed ionic/electronic conductivity in perovskite oxides such as SrTiO3. Moreover, we have proven that this strategy is valid independent of p- and n-doping site (A- or B-site) in the perovskite structure. We used La and Nb as n-type dopants and Na and Ga as p-type dopants to replace the A-site and B-site cations in the SrTiO3 perovskite structure, respectively. All p- and n-doped SrTiO3 perovskite oxides exhibit mixed ionic and electronic conductivity in a reducing environment as long as the concentration of p-dopants is significantly below, e.g., half, the concentration of the n-dopant.

Next, we explain how multiscale simulations can help understand the rate / performance limiting steps in SOFCs based on Sr2Fe1.5Mo0.5O6-δ (SFM) anodes running on H2. First, we performed constrained ab initio thermodynamic simulations to identify the surface phase of SFM (001) under anodic SOFC conditions. Then, we studied the reaction mechanism of the electrochemical H2 oxidation from first principles and developed a microkinetic model that identified the second H transfer step to be rate determining under operating voltage and temperature. As a result, adding a transition metal to the SFM surface such as Ni that facilitates H transfer should improve the overall cell performance. Indeed, experimental observations confirm this predicted SOFC cell behavior.

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