A study of Ni based fuel reforming anodes for solid oxide fuel cells.
The anode material in a conventional design of solid oxide fuel cell (SOFC) operating
above 1123 K is typically made from NiO/Zirconia. NiO/Zirconia anodes are known
to perform well in hydrogen but exhibit difficulties when natural gas is used as a fuel.
Natural gas is much cheaper than hydrogen and widely available but causes carbon
deposition and deactivation of the NiO/Zirconia SOFC anode.
One objective of this work was to prepare and characterize NiO/Zirconia anodes both
as powders and as applied to extruded zirconia tubes. The problem of carbon
deposition when NiO/Zirconia anodes operate in methane, the main component of
natural gas, was investigated. Another aim was to address the problem of coking with
an effort to moderate carbon deposition by using additives to the NiO/Zirconia anode.
Temperature programmed reduction (TPR) was used to study the reaction
characteristicso f NiO/Zirconia anodes.T he carbon depositedo n thesea nodesa fter
methane decomposition and reforming was characterized using temperature
programmed oxidation (TPO). The anodes were placed in a reactor (stainless-steel,
alumina or zirconia) tube in a test assembly developed for an extruded tubular SOFC.
The reactor inlet was connected to a flexible gas handling system and the exhaust to a
continuously sampling mass spectrometer. This system also allowed simultaneous
study of electrical and catalytic measurementsThe various conditions for methane reforming have been shown to influence the
character of carbon deposited and the quantity deposited. Conditions such as anode
calcination temperature, anode reduction regime, reforming temperature and
reforming time have all been shown to influence the reactions occuring on the anode
including carbon deposition, subsequently characterized by TPO. NiO/Zirconia
powders have also been shown to behave differently from NiO/Zirconia anodes
adhered to zirconia tubes.
An alkali metal additive has been shown to moderate carbon deposition and improve
cell performance. Small additions of Li, typically 1 mol %, to the Ni/Zirconia anode
cause a decrease in carbon deposition after reforming at temperatures of 1123 K and
1173 K. The activation energy of surface carbon removed by oxygen is lowered by
approximately 50 kJ mot' for the 1 mol % Li doped Ni/Zirconia anode compared to
the undoped powder. Anodes doped with Li displayed greater cell performances. The
improvements seen with these additives show that their use could offer a viable
alternative to conventional anodes in current SOFC systems.
Tubular solid oxide fuel cells have been tested in a custom built rig whereby electronic
and catalytic measurements can be sampled simultaneously. This was used to monitor
the influence of drawing current on the reactions occurring on the anode. The
presence of alkali Li on the doped Ni anode surface has been shown to interfere with
surface reactions under electrochemical load/steam reforming