Fabrication and Characterization of Ni/ScSZ Cermet Anodes for Intermediate Temperature SOFCs
Author(s)
Somalu, Mahendra Rao
Type
Thesis
Abstract
Solid oxide fuel cells (SOFCs) are of increasing interest as low emission, high efficiency,
energy conversion devices for the production of electricity, and in some cases heat, from a
wide range of fuels. In general, a porous cermet of nickel/ytrria-stabilized-zirconia (Ni/YSZ)
is used as an anode with a dense electrolyte of ytrria stabilized zirconia (YSZ), enabling
operation at temperatures above around 750 oC. Nevertheless, operating at high
temperature leads to various problems such as metal corrosion, electrode sintering, and
unwanted interfacial diffusion in the cell. In this regard, cermet anodes such as
nickel/samarium-doped-ceria (Ni/SDC) and nickel/gadolinium-doped-ceria (Ni/CGO) have
been proposed as alternative anodes to be operated with SDC and CGO electrolytes at
intermediate temperatures (< 700 oC), respectively. However, less attention has been
given to nickel/scandia-stabilized-zirconia (Ni/ScSZ) cermet anodes as alternatives for
intermediate temperature SOFCs, despite the fact that the ScSZ electrolyte exhibits a
higher ionic conductivity compared to other zirconia electrolytes, which may offer some
advantages, especially at lower temperature. Furthermore, Ni/ScSZ anodes have shown
improved tolerance towards carbon deposition and sulphur poisoning in addition to
improved durability when compared to Ni/YSZ anodes.
To date there have only been limited studies into the relationship between the materials
used, the processing conditions, and the properties and performance of Ni/ScSZ anodes.
This thesis is therefore aimed at (i) studying the optimum fabrication conditions and
properties of Ni/ScSZ anodes, (ii) investigating the effect of ingredients such as binder
content, solvent type and solid content in the ink formulation on the rheological properties
of NiO/ScSZ inks and their applicability for screen-printing and (iii) relating the rheological
properties of NiO/ScSZ inks to the performance and properties of the resultant anode films. A large part of the work is focussed on the fabrication and rheological properties of
NiO/ScSZ screen-printing inks. The properties are linked to the particle network strength
within the formulated inks and relate to the microstructure, mechanical strength, electrical
performance and electrochemical performance of the resultant anode films.
Overall, Ni/ScSZ anodes having 40 vol% Ni were found to be optimum in terms of both
electronic conductivity and electrode polarization resistance. The anode exhibited
improved tolerance towards carbon deposition compared to Ni/YSZ at intermediate
temperature (700 oC). The effects of binder and solid content on the rheological properties
of NiO/ScSZ screen-printing inks were studied by evaluating the thixotropic properties,
yield stress and viscoelastic properties of the inks. The study indicated improved
thixotropic properties, yield stress and particle network strength within the inks as the
binder and solid content increased. These improved properties can be related to better
particle bridging within the inks. A percentage ink recovery of 40 to 65 % was determined
sufficient for the production of quality films with minimum defects. From the study, inks
having 26 vol% solid with 3 wt% binder or 28-30 vol% solid with 2 wt% binder were
determined as optimum for screen-printing using squeegee load, squeegee length, printing
speed, snap-off and screen type of 6 kg, 5 in, 0.02 m/s, 2 mm and 325, respectively.
Furthermore, films fabricated using these inks revealed improved particle connectivity,
higher electronic conductivity, lower electrode polarization resistance and improved
mechanical hardness. The solvent type was determined to have only a small impact
compared to the binder and solid contents in the inks.
energy conversion devices for the production of electricity, and in some cases heat, from a
wide range of fuels. In general, a porous cermet of nickel/ytrria-stabilized-zirconia (Ni/YSZ)
is used as an anode with a dense electrolyte of ytrria stabilized zirconia (YSZ), enabling
operation at temperatures above around 750 oC. Nevertheless, operating at high
temperature leads to various problems such as metal corrosion, electrode sintering, and
unwanted interfacial diffusion in the cell. In this regard, cermet anodes such as
nickel/samarium-doped-ceria (Ni/SDC) and nickel/gadolinium-doped-ceria (Ni/CGO) have
been proposed as alternative anodes to be operated with SDC and CGO electrolytes at
intermediate temperatures (< 700 oC), respectively. However, less attention has been
given to nickel/scandia-stabilized-zirconia (Ni/ScSZ) cermet anodes as alternatives for
intermediate temperature SOFCs, despite the fact that the ScSZ electrolyte exhibits a
higher ionic conductivity compared to other zirconia electrolytes, which may offer some
advantages, especially at lower temperature. Furthermore, Ni/ScSZ anodes have shown
improved tolerance towards carbon deposition and sulphur poisoning in addition to
improved durability when compared to Ni/YSZ anodes.
To date there have only been limited studies into the relationship between the materials
used, the processing conditions, and the properties and performance of Ni/ScSZ anodes.
This thesis is therefore aimed at (i) studying the optimum fabrication conditions and
properties of Ni/ScSZ anodes, (ii) investigating the effect of ingredients such as binder
content, solvent type and solid content in the ink formulation on the rheological properties
of NiO/ScSZ inks and their applicability for screen-printing and (iii) relating the rheological
properties of NiO/ScSZ inks to the performance and properties of the resultant anode films. A large part of the work is focussed on the fabrication and rheological properties of
NiO/ScSZ screen-printing inks. The properties are linked to the particle network strength
within the formulated inks and relate to the microstructure, mechanical strength, electrical
performance and electrochemical performance of the resultant anode films.
Overall, Ni/ScSZ anodes having 40 vol% Ni were found to be optimum in terms of both
electronic conductivity and electrode polarization resistance. The anode exhibited
improved tolerance towards carbon deposition compared to Ni/YSZ at intermediate
temperature (700 oC). The effects of binder and solid content on the rheological properties
of NiO/ScSZ screen-printing inks were studied by evaluating the thixotropic properties,
yield stress and viscoelastic properties of the inks. The study indicated improved
thixotropic properties, yield stress and particle network strength within the inks as the
binder and solid content increased. These improved properties can be related to better
particle bridging within the inks. A percentage ink recovery of 40 to 65 % was determined
sufficient for the production of quality films with minimum defects. From the study, inks
having 26 vol% solid with 3 wt% binder or 28-30 vol% solid with 2 wt% binder were
determined as optimum for screen-printing using squeegee load, squeegee length, printing
speed, snap-off and screen type of 6 kg, 5 in, 0.02 m/s, 2 mm and 325, respectively.
Furthermore, films fabricated using these inks revealed improved particle connectivity,
higher electronic conductivity, lower electrode polarization resistance and improved
mechanical hardness. The solvent type was determined to have only a small impact
compared to the binder and solid contents in the inks.
Date Issued
2012
Date Awarded
2012-05
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Brandon, Nigel
Sponsor
Malaysia. Kementerian Pengajian Tinggi
Publisher Department
Earth Science and Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)