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Application of Rare-Earth Doped Ceria and Natural Minerals for Solid Oxide Fuel Cells

Time: Fri 2019-09-27 10.00

Location: K1, Teknikringen 56, Stockholm (English)

Subject area: Energy Technology Materials Science and Engineering Chemical Engineering

Doctoral student: Yanyan Liu , Energiteknik, Solid Oxide Fuel Cells

Opponent: Professor Shanwen Tao, School of Engineering, University of Warwick, UK

Supervisor: Andrew R. Martin, Kraft- och värmeteknologi, Energiteknik; Wujun Wang, Kraft- och värmeteknologi

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Although solid oxide fuel cell (SOFC) technology exhibits considerable advantages as compared to other energy conversion devices, e.g. high efficiency, low emission and fuel flexibility, its high operating temperature leads to rapid component degradation and has thus hampered commercialization. In recent years, intensive research interests have been devoted to lowering the operating temperature from the elevated temperature region (800-1,000 ℃) to intermediate or low-temperature range (<800 ℃). To achieve this goal, material selection plays a dominant role, involving improving the conductivity of existing electrolytes and developing new exploitable materials. This dissertation is focused on enhancing the ionic conductivity of rare-earth oxides (principally doped ceria) and exploring new candidate materials (e.g. natural minerals) for low temperature (LT) SOFCs.

In this work, the scientific contributions can be divided into four aspects:

i)                To develop desirable superionic conductors, Sm3+/Pr3+/Nd3+ triple-doped ceria is designed to realize the desired doping for Sm3+ in bulk and Pr3+/Nd3+ at surface domains via a two-step wet chemical co-precipitation method. It exhibits high ionic conductivity, 0.125 S cm-1 at 600 ℃. The SOFC device using this material as electrolyte displays a high output power density of 710 mW cm-2 at 550 ℃.

ii)              To further clarify the individual effect of Pr3+ in the doped ceria, a single-element (Pr3+) doped ceria is studied, exhibiting a mixed electronic/ionic conduction property, capable of being employed as the core component of electrolyte-layer free solid oxide fuel cells (EFFCs).

iii)             To investigate various rare-earth doped-ceria materials in double- and triple-element doping solutions for LT-SOFCs, Sm3+/Ca2+ co-doped ceria and La3+/Pr3+/Nd3+ triple-doped ceria are synthesized and then further incorporated with semiconductors, e.g. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) or Ni0.8Co0.15Al0.05Li-oxide (NCAL), to serve as a semiconducting-ionic conducting membrane in EFFCs.

iv)             To exploit the feasibility of natural mineral cuprospinel (CuFe2O4) as an alternative material for LT-SOFCs, three different types of fuel cell devices are fabricated and tested. The device using CuFe2O4 as cathode exhibits a maximum power density of 180 mW cm-2 with an open circuit voltage of 1.07 V at 550 °C, while the device using a homogeneous mixture membrane of CuFe2O4, Li2O-ZnO-Sm0.2Ce0.8O2 (LZSDC), and LiNi0.8Co0.15Al0.05O2 (NCAL) demonstrates an improved power output, 587 mW cm-2 under the same measurement conditions.  

Based on this work, a new triple-doping strategy is exploited to improve the ionic conductivity of doped ceria materials by surface- and bulk-doping methodology. Furthermore, the material developments of single-phase mixed electronic/ionic conducting doped ceria and doped ceria/semiconductor composites are realized and verify the feasibility of EFFC technology. Investigations on CuFe2O4 indicate the utility of natural minerals in developing cost-effective materials for LT-SOFCs.