Abstract:
There is an urgent need to explore affordable and renewable energy resources because
of the decline in reserves of fossil fuels. Biomass is an abundantly available resource
in nature and can be used to generate energy in a sustainable manner. Fuel cells deliver
a combination of advantages and make use of renewable energy sources. Solid oxide
fuel cells (SOFCs), specifically, overcome the petroleum scarcity issue by using
biofuel. The aim of this thesis is the development of nanocomposite electrolytes and
anode composite catalysts for low-temperature SOFCs fuelled with biogas for clean
energy applications. In the present work, Sr/Sm-doped ceria (Sr-SDC) nanocomposite
electrolytes with a core shell structure are synthesized with different compositions for
low temperature SOFCs. A co-doping technique is successfully used to achieve a
significant enhancement in the ionic conductivity of 0.50 S/cm at 600 ˚C for the
nanocomposite electrolyte Sr0.1Sm0.1Ce0.8O2-δ-carbonate. The carbonate phase (shell
layer) acts as a barrier and protects the SDC (core) from the partial reduction by the
fuel. This carbonate shell introduces an interface between these two phases, which is
the key to realizing the interfacial super-ionic conduction pathways. This work also
describes the development of ceria electrolytes that are doped and co-doped with
lanthanum (La) and zirconium (Zr) and show excellent thermal stability. The ionic
conductivity of La0.2Ce0.8O2-δ (LDC), Zr0.2Ce0.8O2-δ (ZDC) and Zr0.2La0.2Ce0.6O2-δ
(ZLDC) has been measured in the temperature ranges of 450 °C to 650 °C and LDC
achieved a high ionic conductivity of 0.81 × 10-2 S/cm. Thermal expansion coefficients
(TECs) of these electrolytes have also been found to have good concurrence and
compatibility with commonly used electrolytes and electrodes. The main objective of
this work is the development of stable and active anode catalysts that run over biogas
as well as hydrogen for low temperature SOFCs. The anode composite Ni0.6Zn0.4-
Gd0.2Ce0.8O2-δ (NiZn-GDC) has been developed that exhibits semiconductor conductive
behaviour, and a maximum conductivity of 1.37 S/cm has been achieved at 600 ˚C.
This composite anode is found to have excellent thermally stability as well as being
carbon resistant to coking during testing with biogas. A maximum power density of
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820 and 548 mW/cm2 has been reported with hydrogen and biogas fuels, respectively,
at 600 ˚C. This thesis also describes Ni-based and Ni-free anode catalysts NiLiCu-oxide
with LDC for SOFCs fuelled with biogas. The anode composite NLC622-LDC has
reported a maximum DC conductivity of 3.47 S/cm with Pmax of 650 and 390 mW/cm2
for hydrogen and biogas, respectively, at 600 ˚C. A Ni free anode catalyst
Zn0.2Li0.2Cu0.6O2-δ (ZnLiCu-oxide) is also developed as a potential candidate for
biogas-based SOFCs that bypasses the difficulty of carbon deposition and has a
maximum conductivity of 4.0 S/cm at 600 ˚C. An open circuit voltage (OCV) of 0.96
V is achieved with maximum power density of 600 mW/cm2 with biogas (50%
methane) at 650 ˚C. In the present work, a theoretical model of FC system has been
designed using MATLAB software, and it makes use of biomass (animal waste,
redwood, rice husk and sugar cane). In the last part of the thesis, a partial research work
has been conducted to cast the tapes of NiO-GDC (NiO-Gd0.1Ce0.9O1.95) as anode and
GDC (Gd0.1Ce0.9O1.95) as electrolyte via aqueous tape casting method. The aqueous tape
casting is an emerging and cost-effective technique for the commercialization of
SOFCs but faces challenges with ceria tapes due to its poor mechanical strength and
co-sintering of half-cells.