Compact solid oxide fuel cells and catalytic reformers based on microtubular membranes

Article abstract of DOI:10.1016/j.cattod.2018.11.009

AC heated oxygen-permeable microtubular membranes with the composition Ba_0.5Sr_0.5Co_0.78W_0.02Fe_0.2O_3-δ were used to provide catalytic reforming of methane into C₂- hydrocarbons. The methane conversion degree about 60% and acetylene yield about 27% at 1200 °C was achieved. Microtubular solid oxide fuel cells based on gadolinium-doped ceria with perovskite-like cathode material composed of Ba_0.5Sr_0.5Co_0.75Mo_0.05Fe_0.2O_3-δ were prepared. The MT-SOFC demonstrates maximum power densities of 50, 100, 200 mW/cm² at 550, 600, 650 °C, respectively with humidified H₂ as fuel and ambient air as oxidant.

Full text of DOI:10.1016/j.cattod.2018.11.009

Accepted Manuscript
Title: Compact solid oxide fuel cells and catalytic reformers
based on microtubular membranes
Authors: Mikhail P. Popov, Daniel V. Maslennikov, Igor I.
Gainutdinov, Igor P. Gulyaev, Andrey N. Zagoruiko,
Alexander P. Nemudry
PII:
S0920-5861(18)31524-4
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.11.009
CATTOD 11743
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
20 September 2018
17 October 2018
2 November 2018
Please cite this article as: Popov MP, Maslennikov DV, Gainutdinov II,
Gulyaev IP, Zagoruiko AN, Nemudry AP, Compact solid oxide fuel cells and
catalytic reformers based on microtubular membranes, Catalysis Today (2018),
https://doi.org/10.1016/j.cattod.2018.11.009
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Compact solid oxide fuel cells and catalytic reformers based on microtubular
membranes
Mikhail P. Popova, Daniel V. Maslennikova,b, Igor I. Gainutdinova, Igor P. Gulyaevc, Andrey N.
Zagoruikod, Alexander P. Nemudrya
aInstitute of Solid State Chemistry and Mechanochemistry, SB RAS; 630128 Kutateladze 18,
Novosibirsk, Russia
bNovosibirsk State University, 630090 Pirogova 2, Novosibirsk, Russia
cInstitute of Theoretical and Applied Mechanics, SB RAS; 630090 Institutskaya 4/1, Novosibirsk,
Russia
dBoreskov Institute of Catalysis, SB RAS; 630090 pr. Lavrentieva 5, Novosibirsk, Russia
*Corresponding author: popov@solid.nsc.ru
Fax: +7 383 3322847; Tel: +7 383 2332410
Graphical Abstract

Highlights
 Compact catalytic reformers of methane have been successfully fabricated and tested.
 Proposed BSCFW2 perovskite is promising compound for a methane conversion.
 A crack-free dense electrolyte layer (20 µm) was prepared by co-firing method.
 Microtubular solid oxide fuel cell have been successfully fabricated and tested.
Abstract
AC heated oxygen-permeable microtubular membranes with the composition
Ba0.5Sr0.5Co0.78W0.02Fe0.2O3-δ were used to provide catalytic reforming of methane into C2-
hydrocarbons. The methane conversion degree about 60% and acetylene yield about 27% at
1200°C was achieved. Microtubular solid oxide fuel cells based on gadolinium-doped ceria with
perovskite-like cathode material composed of Ba0.5Sr0.5Co0.75Mo0.05Fe0.2O3-δ were prepared. The
MT-SOFC demonstrates maximum power densities of 50, 100, 200 mW/cm2 at 550, 600, 650 °C,
respectively with humidified H2 as fuel and ambient air as oxidant.
Keywords:
Microtubular Solid Oxide Fuel Cell (MT-SOFC); phase inversion; dip-coating; catalytic
reforming; oxygen-permeable membranes.
1. Introduction
Recently, active efforts has been made to develop catalytic membrane reactors (CMR) in
which separation of atmospheric oxygen with the help of oxygen-permeable (OP) membranes and
catalytic conversion of hydrocarbons are combined in one reactor, which allows to significantly
decrease in the prime cost of syngas [1]. In addition, OP membranes can also be used to produce
pure oxygen for efficient fuel combustion in power engineering [2-4] and as cathodes for solid
oxide fuel cells (SOFC) [5,6].

Recently, microtubular (MT) membranes manufactured by phase inversion method have
been considered as the most promising among OP membranes. They provide not only high
mechanical strength but also stability to thermocycling and electric load that ensures rapid
initiation, and thin (20–50 µm) gas-tight layer, which allows achieve high oxygen fluxes [7-11].
In this paper, we will discuss about two most promising applications of MT membranes (1)
MT catalytic reformers based on perovskite-like Ba0.5Sr0.5Co0.78W0.02Fe0.2O3-δ (BSCFW2) oxide
and (2) MT SOFC with perovskite-like cathode material composed of
Ba0.5Sr0.5Co0.75Mo0.05Fe0.2O3-δ (BSCFM5) oxide. The above compounds were selected due to the
following qualities. We demonstrated previously that partial substitution of cobalt in BSCF by
high-valence cations (like tungsten [7,11] and molybdenum [12]) improves the functional
properties of the material: causes an increase of its oxygen permeability, suppresses the
polymorphous transition from cubic to hexagonal perovskite, increases the stability of the material
in CO2, which ensures the stable operation of the membrane in long-term tests.
Experimental
To obtain BSCFW2 and BSCFM5 materials [R2.1], ethanol was added into the
stoichiometric mixture of the oxides (Co, Fe and W/Mo) and nitrates (Ba and Sr) in the amount of
1 ml of С2Н5ОН / 1 g of the mixture; then the mixture was treated in a planetary ball mill AGO-2
for 30 s. The resulting mixture was annealed at 900°C for 7 hours to remove gaseous products,
then, after grinding in a mortar, the mixture was ground in the ball mill at the frequency of 1.5
r.p.s. for 48 hours. MT membranes (BSCFW2 and NiO/CGO : 60/40 [R2.3]) were obtained by
means of phase inversion technique. Dried dispersed powder was mixed with the solvent (N-
Methyl-2-pyrrolidone [R2.2]) and polymer binder (Polyethersulfone [R2.2]) at a ratio of 12:4:1
[6, 8]. The slurry was homogenized in a jar roller mill for 24 hours; the homogeneous mixture was
subjected to several cycles of degassing. Then the resulting slurry was extruded through spinneret
(Fig. 1a) to make the green bodies of OP membranes. The air-gap was in the range 5–10 mm.

Figure 1. Scheme of spinneret and dip-coater.
The resulting green bodies of MT membranes were annealed at 600°C [R2.4] in air for 2
hours, then heated at 1200°C [R2.4] in air for 6 hours. After such procedure BSCFW2 MT
membranes were ready for measurements. Opposite, NiO/CGO tubes were dip-coated (Fig. 1b) in
a slurry which is composed of CGO electrolyte powder; binder (Polyvinyl butyral [R2.2]),
plasticizer (Dibutyl phthalate [R2.2]); dispersant (Dysperbyk-111 «BYK» [R2.2]) and solvents
(Butyl glycol [R2.2]). The coatings were left to dry in air, then sintered at 1400°C for 6 h in air.
After, the anode tubes with electrolyte were dip-coated in cathode slurry, which consists of
BSCFM5/CGO:60/40 [R2.3] powder and organic components (similar with those of the electrolyte
slurry). After dip-coated the cathode slurry, the tubes were dried in air and sintered at 1200°C for
2 h in air to complete a cell. The state of the surface was determined by the scanning electron
microscope Hitachi TM–1000.
To carry out catalytic reforming of methane measurements, we used a reactor, described in
detail in [11] (Fig. 2).

Figure 2. Scheme of reactor 2 for catalytic reforming measurements on AC heated MT
membranes.
Pure CH4 was used in experiment [R1.12] (F=100 ml/min). Ambient air was used as an
oxygen source (F=150 ml/min). The reactor in which the BSCFW2 MT membrane (2) is fixed at
the center has channels for gas input (1a, с) and output (1b, d). Channels 1a and 1b are connected
with the MT membrane through polymer tubes; the connection sites are sealed by heat-shrink
tubes. With the help of the deposited current-conducting (Ag) coating (3), a reliable contact
between the MT membrane and the source of AC electric current (5) was provided. The
temperature of the membrane was determined with the accuracy of 0.1o with the help of an IMPAC
IGA 300 infrared pyrometer (4) through the window cut in the case. The window is sealed with a
heat-resistant Kapton tape, which is able to transmit IR rays. AC current regulator (5) connected
with the pyrometer allowed us to carry out controllable heating of the sample (heating rate was 60
°C/min, exposure time at each temperature was about ~ 15 min [R1.1]). Gas flows were controlled
with a gas mixing unit UFPGS-4 (SoLO, Novosibirsk) (6). The concentrations of outgoing gases
were determined by the quadrupole mass spectrometer QMS 200. The size of the membrane:
inner/outer diameter = 0.55/1.75 mm [R2.6]; total length = 50 mm; active length = 35 mm; active
area = 0.60 cm2 [R2.6].
The scheme of single MT-SOFC measurements is shown in Fig. 3:

Figure 3. Scheme of single MT cell measurements.
A spiral of silver wire was placed in the inner side of tube (anode side) in such a way that
it formed a good contact with a cermet support. The cell was heated with a furnace, while the entire
cell was in the non-gradient zone of the furnace [R2.7]. Ambient air was used as an oxygen source
(F=250 ml/min). Pure H2 (humidified by bubbling water at 25°C) was used as a fuel (F=100
ml/min). Current collecting was obtained by Ag wire (d=0.3 mm). The size of the cell: inner/outer
diameter = 1.0/2.0 mm; total length = 50 mm; cathode length = 15 mm; active area = 0.94 cm2.
Gas flows were controlled with a gas mixing unit UFPGS-4 (SoLO, Novosibirsk).
Results and discussion
According to the XRD data analysis, the BSCFW2 membrane material is a single phase
̅
and has a cubic Pm3m perovskite structure (Fig. 4, before). The cell parameter (a = 3.989±0.001Å)
agrees with the data for BSCFW2 obtained previously [11]. The micrographs of MT membranes
obtained by phase inversion method are shown in Fig. 5a,b. Developed bulk finger-like porosity
(Fig. 5b) and gas-tight layer (Fig. 5a) of the membranes are clearly seen. A crack-free dense
electrolyte layer with a thickness of about 20 µm was prepared by co-firing method (Fig. 6). The
thickness of cathode layer was ~ 10 µm [R1.7]. The thickness of the NiO/CGO tube was ~ 200
µm and the porosity of the anode determined to be ~ 36% (before reduction) and ~53% (after
reduction) using helium pycnometer.

Figure 4. XRD pattern of BSCFW2 membrane material before (red line) and after (blue line)
conversion of methane experiments. [R2.8]
Figure 5. SEM data of MT membranes composed of BSCFW2 (a) and NiO/CGO (b).

Figure 6. SEM data of NiO/CGO anode tube with co-fired CGO electrolyte: before (on the left
side) and after (on the right side) reduction. [R2.11]
Figure 7. SEM data (on the left side) and photo (on the right side) of NiO/CGO anode tube with
co-fired CGO electrolyte and cathode layer. [R2.7, 2.11]
Catalytic conversion of methane
The performance of catalytic converter of methane was studied and shown in Fig. 7.
Temperature range of the experiment was 750–1200°C. The experiment was carried out for three
hours, with no significant changes in the process indicators (Fig. 4, after).

Figure 8. Dependencies of conversion of methane (black line) and yield of C2-hydrocarbons (red
line) on temperature. [R1.4], [R2.12]
It can be seen from the Fig. 8 that the proposed compound BSCFW2 provides a yield of
C2-hydrocarbons about ~ 27% and conversion of methane about ~ 60%. The obtained data indicate
the promise of the proposed composition. Note that oxygen enters the reactor, diffusing through
the walls of the catalyst tube. Thus, the formation of explosive mixtures is prevented. Moreover,
the AC heating of the MT membrane generates high temperature gradients in the gas phase. This
allows to not only reduce the requirements for structural materials and avoid additional heater, but
to freeze metastable products and increase the selectivity of the oxidative conversion of methane.
Microtubular Solid Oxide Fuel Cell
The performance of the MT cell (Fig. 3) was studied and shown in Fig. 9. Temperature
range of the experiment was 550–650°C. As can be seen from Fig. 9, the peak power density of
50, 125 and 200 mW/cm2 were obtained respectively at 550, 600 and 650°C. Even though the peak
power densities (N) are relatively high (in comparison with a similar cell which have peak power
density 300 mW/cm2 in [13]), the OCV value obtained in this work are not high enough. To solve
this problem, according to the size and performance of the single tubular cell, a stack with 25 tube
cells (5×5) in 1 cm3 can possibly be fabricated, which can be expected to generate 3.3 W/cm3
650°C, respectively. Therefore, developing of stack fabrication method will be a key for realizing
high volumetric power SOFC system, which is currently under investigation and will be reported
separately.

[R1.4]
Figure 9. V-I characterization data of the MT SOFC with BSCFM5/CGO [R2.3] cathode
material.
Conclusion
Compact tubes based on oxide with mixed conductivity have been successfully fabricated
and tested in terms of catalytic conversion of methane. It is shown, that BSCFW2 oxide is
promising catalysis material and provides up to 60% conversion of methane and 27% yield of C2-
hydtocarbons. At the same time, the stable operation time of the catalyst is 3 hours or more. An
additional advantage of the proposed catalyst is the possibility of separating the streams of oxygen-
containing gas and methane, which reduces the explosiveness of the process.
Microtubular solid oxide fuel cells with BSCFM5 cathode material have been successfully
fabricated and tested in the intermediate temperature region. The cell consists of NiO/CGO as an
anode (support tube), CGO as an electrolyte and BSCFM5/CGO as a cathode. The single cell
generated 200 mW/cm2 at 650°C operating temperature with H2 fuel, which indicated a possibility
of building cell stack with volumetric power density of 3.3 W/cm3 by stacking 25 tubes in 1 cm3.
Acknowledgments
This work was supported by the Russian Academy of Sciences under the Program of SB
RAS «Integration and Development", project No. 27.1 (0301-2018-0011).

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