PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN00033/1440
Development of In-Stack Reforming for Solid Oxide Fuel Cells
Olga A. Marina
The goal of this project was to evaluate anode materials and designs for the solid oxide fuel cell operating on partially
pre-reformed hydrocarbons and develop Pacific Northwest National Laboratory’s capabilities to fabricate and test planar
solid oxide fuel cells operating on partially pre-reformed hydrocarbon fuels.
Project Description
The primary objective of the project is to develop,
fabricate, and test a thick porous anode that is compatible
with other solid oxide fuel cell components, is coke
resistant, exhibits excellent electrochemical properties,
and has catalytic activity for internal reforming of
methane. The development of such an electrode would
allow solid oxide fuel cell stack operation on such
inexpensive methane-containing fuels as natural gas or
pre-reformed higher hydrocarbons, gasoline and diesel,
provided they were desulfurized prior to use.
Introduction
For the solid oxide fuel cell with an yttria-stabilized
zirconia electrolyte, a nickel-yttria-stabilized zirconia
cermet is currently a favored anode material. State-ofthe-art
Ni-yttria-stabilized zirconia anodes exhibit
polarization resistance as low as 30 and 60 mΩcm 2 at
1000 and 850 o C, respectively, in moist hydrogen
(Primdahl and Mogensen 1999) and are being used
successfully by most developers for solid oxide fuel cells
operating on the clean reformed fuel. However, solid
oxide fuel cells require the anode be tolerant to fuels less
expensive than hydrogen hydrocarbon fuels (natural gas,
gasoline) without external reforming. The major
disadvantage of the nickel cermet electrode arises from
the promotion of competitive catalytic cracking of
hydrocarbons leading to rapid carbon deposition. Steam
reforming of methane at high steam-to-carbon ratio
potentially prevents sooting of the anode, but causes
intolerable temperature gradients throughout the anode
due to the high reactivity of nickel toward this reaction.
Moreover, nickel is unsusceptible to an oxidizing
atmosphere and is readily poisoned by sulfur, which is
always present in the feedstock. Also, sintering of nickel
on prolonged operation is a potential problem for any type
of fuel (van Berkel et al. 1993).
314 FY 2000 Laboratory Directed Research and Development Annual Report
Application of internal reforming of methane in the solid
oxide fuel cell offers several important advantages in
comparison with external reforming: 1) no separate
steam-reforming unit is needed; 2) smaller system size,
lower equipment and operation costs, and lower capital
investment are expected; 3) less cooling is required;
4) less steam is required; 5) more evenly distributed
hydrogen load; and 6) higher methane conversion are
achieved. However, internal reforming also poses some
flaws: 1) intolerable temperature gradients, 2) sooting,
3) need to dilute fuel with steam in order to prevent
carbon depositions, 4) recirculation and incomplete shift
reaction adversely affect power generation, 5) gradual
catalyst deactivation due to metal particle sintering, and
6) additional catalysts may be required.
Results and Accomplishments
A comprehensive literature review was conducted to
search for a new anode material and improvements of the
traditional nickel cermet anode capable of in-stack
reforming. Two promising research directions have been
identified: redox tolerant anode concept, and a new anode
for internal reforming.
A multipurpose test rig was designed and built to enable
anode materials testing in-house (see Figure 1). The test
rig consisted of an electric furnace, a potentiostate, gas
supplying/controlling system, and computer monitoring of
anode and cathode temperatures. Electrolyte-supported
cells can be tested both in hydrogen and methane by
potentiometric and impedance spectroscopy techniques
with the future possibility of simultaneous gas analyzing.
PNNL’s cell fabrication technique was adopted for
manufacturing two- and three-electrode cells for the test
in that rig.