Bo Guan

Date of Graduation

2016

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Co-Chair

Xueyan Song

Committee Member

Hui Zhang

Abstract

In the development of intermediate temperature solid oxide fuel cell (IT-SOFC), mixed ionic-electronic conductors (MIEC) have drawn big interests due to their both ionic and electronic species transport which can enlarge the 3-dimension of the cathode network. This thesis presents an investigation of MIEC of Ruddlesden-popper (RP) phases like K2NiF4 type La2NiO4+delta (LNO)-based oxides which have interesting transport, catalytic properties and suitable thermal expansion coefficients. The motivation of this present work is to further understand the fundamental of the effect of Sr doing on the oxygen reduction reaction (ORR) kinetics of LNO cathode.;Porous symmetrical cells of La2-xSrxNiO4+delta (0≤x≤0.4) were fabricated and characterized by electrochemical impedance spectroscopy (EIS) in different PO2 from temperature range of 600∼800°C. The spectra were analyzed based on the impedance model introduced by Adler et al. The rate determining steps (RDS) for ORR were proposed and the responsible reasons were discussed. The overall polarization resistances of doped samples increase with Sr level. Surface oxygen exchange and bulk ionic diffusion co-control the ORR kinetics. With high Sr content (x=0.3, 0.4), oxygen ion transfer resistance between nickelate/electrolyte is observed.;However for porous symmetrical cells it is hard to associate the resistance from EIS directly to each ORR elementary processes because of the difficulty in describing the microstructure of the porous electrode. The dense electrode configuration was adopted in this thesis. By using the dense electrode, the surface area, the thickness of electrode, the interface between electrode and electrolyte and lastly the 3PB are theoretically well-defined. Through this method, there is a good chance to distinguish the contribution of surface exchange from other processes. Dense and thin electrode layers in thickness of ∼40 mum are fabricated by using a novel spray modified pressing method. Negligible bulk diffusion resistance is confirmed by parallel experiment and EIS analysis, resulting in exclusive focus on the surface process. It is ambiguously proved that Sr doping impairs the surface kinetics of lanthanum nickelates. The interstitial oxygen is suggested to be the key role when the oxygen incorporation is rat determining. For the first time, a physical model is proposed to illustrate how those interstitial species work to regulate the exchange rate of the incorporation reaction.;To achieve better surface exchange ability on LNO, Mn is chosen as the doping element substituted for Ni with different levels to improve the surface kinetics because Mn is much active both for adsorption process and for incorporation process due to the high state of Mn leading to the high amount of the interstitial oxygen. Mn is found to substantially promote the surface kinetics, showing highest surface exchange coefficient (k) of 1.57x10-6cm/s at 700°C on composition of La1.8Sr0.2Ni0.9 Mn0.1O4+delta. Such value is ∼80% larger than that of the undoped sample, and is one of the highest k among the currently available R-P phase intermediate temperature (IT) cathode.

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