Updating the Notion that Poor Cathode Performance Typically Dominates Overall Solid Oxide Fuel Cell Response
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Journal of The Electrochemical
Society
OPEN ACCESS
Updating the Notion that Poor Cathode Performance Typically
Dominates Overall Solid Oxide Fuel Cell Response
To cite this article: Yubo Zhang and Jason D. Nicholas 2021 J. Electrochem. Soc. 168 034513
View the article online for updates and enhancements.
This content was downloaded from IP address 46.4.80.155 on 14/11/2021 at 17:22
Journal of The Electrochemical Society, 2021 168 034513
Updating the Notion that Poor Cathode Performance Typically
Dominates Overall Solid Oxide Fuel Cell Response
Yubo Zhang* and Jason D. Nicholas*,z
Chemical Engineering and Materials Science Department, Michigan State University, East Lansing, Michigan 48824, United
States of America
Here, Solid Oxide Fuel Cells (SOFCs) utilizing high performance La0.6Sr0.4Co0.8Fe0.2O3−x (LSCF)—Gd0.1Ce0.9O1.95−x (GDC)
nano-composite cathodes (NCCs) on commercially-available [GDC diffusion barrier ∣ (Y2O3)0.08(ZrO2)0.92 (YSZ) electrolyte ∣
Ni-YSZ anode functional layer ∣ Ni-YSZ gas transport layer] supports had the same 550 °C–650 °C current-voltage behavior as
identical SOFCs utilizing commercial La0.6Sr0.4CoO3 (LSC) cathodes, despite differences in open-circuit cathode polarization
resistance (RP). Nickel anode infiltration also produced a ∼25% SOFC peak power density improvement in these cells. These
results, combined with literature data showing that either the cathode RP, anode RP, or ohmic losses within the cell can limit state-
of-the-art SOFC performance (depending on the exact compositions, microstructures, testing conditions etc.), suggest that it is time
to retire the old adage that poor cathode performance typically limits overall SOFC performance.
© 2021 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access
article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/
by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/
1945-7111/abed21]
Manuscript submitted September 2, 2020; revised manuscript received November 7, 2020. Published March 18, 2021.
Supplementary material for this article is available online
Solid Oxide Fuel Cells (SOFCs) are electrochemical-based is generally thought to be the more difficult reaction to activate
chemical to electrical energy conversion devices offering some of [compared to oxygen evolution on the anode] on SOFCs operating at
the highest gravimetric and volumetric power densities,1,2 highest commercially relevant temperatures” have become common in the
demonstrated efficiencies,3–6 and lowest emission intensity7 of any literature.47–49 Further, it is easy to find statements in the literature
hydrocarbon-based electricity generation technology. This, com- like “cathode overpotential is often the main factor limiting SOFC
bined with their ability to operate on a variety of fuels,8 allows them performance”,50 “it has been well accepted that in intermediate-
to reduce the environmental impact of today’s hydrocarbon-based temperature SOFCs, oxygen reduction at the cathode is the main rate
economy while simultaneously providing a path to a CO2-neutral limiting factor to the performance of the whole system”,51 “cathodic
hydrogen- or biofuel-based economy. In addition, SOFCs can be activation loss stemming from sluggish ORR dominates in SOFC”,52
operated in reverse as Solid Oxide Electrolysis Cells (SOECs) to and even in our own work “SOFC cathodes have historically limited
efficiently store energy, produce fuels or chemicals, and/or tie SOFC performance”.53 Together, these statements have contributed
together electrical and hydrocarbon distribution networks for im- to the impression that future SOFC research should primarily focus
proved grid reliability.2,9 on improved cathodes. However, as shown here, other SOFC
Unfortunately, widespread SOFC and SOEC commercialization components (even high-performance, state-of-the-art ones) can be
is complicated by the high operating temperatures typically required just as limiting, if not more limiting, than state-of-the-art SOFC
to achieve high performance. These high operating temperatures cathodes.
(which are typically greater than ~600 °C) raise costs and often
result in unacceptably high degradation rates brought on by
degradation mechanisms such as diffusion-enabled surface composi- Experimental Methods
tion changes,10 electrode microstructural changes,11,12 and/or un- Sample fabrication.—To help demonstrate this point, Solid
wanted side chemical reactions.13 Oxide Fuel Cells (SOFCs) utilizing high performance La0.6Sr0.4
Historically, efforts to improve SOFC performance and/or reduce Co0.8Fe0.2O3−x (LSCF)—Gd0.1Ce0.9O1.95−x (GDC) nano-composite
SOFC operating temperatures have largely focused on improving cathodes (NCCs) on commercially-available SOFC supports were
cathode performance. This emphasis resulted from the fact that 1) compared to commercially-available La0.6Sr0.4CoO3−x (LSC) cath-
traditional, colloidal thick film nickel-yttria stabilized zirconia (Ni- odes on the same supports. These supports were supplied by the Fuel
YSZ) anodes performed much better than traditional, colloidal thick Cell Materials subsidiary of Nexceris LLC (Lewis Center, OH) and,
film lanthanum strontium manganate (LSM)-YSZ cathodes,36,37 as shown in Figs. 1a–1b, consisted of GDC diffusion barrier layers,
and 2) the fact that many SOFC anodes display lower oxygen (Y2O3)0.08(ZrO2)0.92 (YSZ) electrolytes, Ni-YSZ anode functional
exchange activation energies (∼1.0 eV for Ni-YSZ, ∼1.4 eV for layers, and mechanically-supporting Ni-YSZ anode gas diffusion
Ni-GDC, ∼0.5 eV for Sr(Ti,Fe)O3, etc.)38,39,17 than SOFC cathodes layers. Conventional LSCF-GDC NCCs (i.e. those not subject to
(>1.4 eV for La0.8Sr0.2MnO3, ∼1.8 eV for Ba0.5Sr0.5Co0.8Fe0.2O3−x, GDC pre-infiltration,54,55 atomic layer deposited overcoating,56 or
∼1.8 eV for La0.6Sr0.4FeO3−x, ∼1.6 eV for La0.6Sr0.4Fe0.8Co0.2 precursor solution desiccation53,57) with LSM current collectors and
O3−x, ∼1.3 eV for La0.6Sr0.4Co0.8Fe0.2O3−x, ∼1.3 eV for La0.6Sr0.4 a gold grid surface mesh were produced atop these SOFC supports
CoO3−x, and ∼1.3 eV for Sm0.5Sr0.5CoO3−x).40,41 As a result, via LSCF nitrate solution infiltration into porous, partially-sintered,
even though microstructural tailoring,42 grain size,43,44 testing screen-printed GDC scaffolds following the procedures described
atmospheres,14 electrical polarization,45,16 and other effects have previously in the literature.42,53–57 The LSC cathodes were prepared
been shown to modulate SOFC electrode performance (such that by Nexceris using proprietary fabrication procedures.
high oxygen surface exchange activation energy does not necessarily To provide evidence that the Nexceris anodes contributed
correlate with poor electrode performance), over the past ∼20 years significantly to the overall SOFC resistance, the aforementioned
sentiments similar to those in Ref. 46 stating that “oxygen reduction anode supports were surface-decorated with nano-sized nickel
particles through nickel nitrate solution anode infiltration. The nickel
nitrate precursor solutions were made by mixing high purity
*Electrochemical Society Member. Ni(NO3)2·6H2O (Alfa Aesar, Haverhill, MA) with Milli-Q water.
z
E-mail: jdn@msu.edu The precursor solutions were then pipetted into the Nexceris Ni-YSZ
Journal of The Electrochemical Society, 2021 168 034513
anodes, held in air in 5 min, dried in an 80 °C oven for 5 min, and
fired at 700 °C for an hour to form nano-sized NiO particles.
Multiple solution infiltration processes were performed to reach a
∼4% Ni loading level (a mid-range loading level based upon the
literature28,58). ∼10 μm thick gold grid current collectors made up of
6 mm wide Au paste (C5756, Heraeus, West Conshohocken, PA)
lines and ∼0.11 cm2 of open area between each set of gridlines were
then screen printed onto each anode.
To better separate the cathode performance from the overall
SOFC performance, LSCF-GDC NCC∣GDC∣LSCF-GDC NCC sym-
metric cells with LSM current collectors and a gold grid surface
mesh were produced via LSCF nitrate solution infiltration into
porous, partially-sintered, screen-printed GDC scaffolds following
the procedures described previously in the literature.42,53–57
Sample characterization.—For current-voltage (I–V) measure-
ments, full cells were sealed on top of a homemade 316 stainless-
steel test rig with silver paste (C8728, Heraeus, West Conshohocken,
PA). Silver wires were connected to the gold current collectors with
gold paste (C5756, Heraeus, West Conshohocken, PA). Before
electrochemical tests, the cells were fired at 700 °C for an hour in
100 sccm of flowing air using 5 °C min−1 nominal heating and
cooling rates to sinter the silver paste. After this silver sealing
procedure, NiO in the anodes was reduced at 650 °C for at least
20 min with 100 sccm of humidified hydrogen (97% H2—3% H2O)
and until the open circuit voltage (OCV) of the full cell was stable.
After the NiO was fully reduced, I–V measurements were conducted
at 550 °C, 600 °C and 650 °C with static air as oxidant on the
cathode side and humidified hydrogen with a flow rate of 100 sccm
as the fuel on the anode side. At each temperature, a hold time of at
least 20 min was used to achieve thermal equilibrium.
Electrochemical Impedance Spectroscopy (EIS) measurements
on symmetric cells were conducted using an impedance analyzer
(IM6, Zahner, Kronach, Germany). Symmetric cathode cell tests
were conducted in static air using a Pt plate push-contact setup
utilizing Pt wire leads. For all the cells, impedance data was
collected at 550 °C, 600 °C and 650 °C. At each temperature, a
hold time of at least 20 min was used to achieve thermal equilibrium.
A 0.1 to 100 kHz frequency range was used for symmetric cathode.
Due to use of ∼1.8 meter long leads, a nominal 100 mV AC
amplitude (∼75 mV across the sample) was utilized for all EIS
measurements, after identical (but noisier) results were obtained
with nominal 50 mV (∼25 mV across the sample) AC amplitudes.
The cathode area specific open-circuit polarization resistance (RP)
was determined by multiplying the resistance difference between the
x-axis intercepts on each symmetric cell EIS Nyquist plot by the
0.5 cm2 geometric area of the electrode and dividing by two (since
each symmetric cell had two electrodes).
Cross-sectional Scanning Electron Microscopy (SEM) analyses
on fractured cells were taken by sputtering ∼1 nm of Pt over each
fractured specimen and imaging it using a 15 kV electron beam
voltage and a ∼4 mm working distance in an Auriga SEM (Carl
Zeiss, White Plains. NY). Images for the infiltrated LSCF particles
were taken within cracks at the electrode/electrolyte interface.
Results and Discussion
Figures 1a and 1b show microstructural overviews of the LSCF-
GDC NCC and LSC-containing SOFCs, respectively. In both cases,
the 30 μm thick LSCF-GDC NCCs or 12 μm thick LSC cathodes
were supported by 1) a ∼3 μm-thick GDC interlayer designed to
inhibit formation of the strontium zirconate and/or lanthanum
zirconate that often form when La/Sr-containing cathode materials
contact YSZ,59,60 2) a dense, ∼3 μm-thick YSZ electrolyte, 3) a
high-surface area ∼15 μm-thick Ni-YSZ functional layer where
Figure 1. Scanning electron microscopy images of (a) LSCF-GDC cathodes most of the electrochemical reaction occurred,61 and 4) a mechani-
on Nexceris half cells, (b) LSC cathodes on Nexceris Half Cells, and cally-supported ∼385 μm-thick Ni-YSZ anode gas transport layer.
(c) symmetric cell LSCF-GDC cathodes. Consistent with previous literature reports on identically processed
Journal of The Electrochemical Society, 2021 168 034513
cathodes,42,53–57 the LSCF-GDC cathodes contained partially-sin-
tered GDC particles a few hundred nanometers in diameter which, as
shown in Fig. 1c, were surface decorated with LSCF particles ∼40–
50 nm in average hemispherical diameter. In contrast, as shown in
Fig. 1a, the commercial LSC cathodes contained LSC particles
∼200 nm in diameter.
Figure 2a shows that nearly identical performance was observed
for Nexceris SOFC half cells with either micro-porous Nexceris LSC
cathodes or Michigan State LSCF-GDC nano-composite cathodes.
This identical full cell performance would be difficult to explain if
the cathode were the most resistive part of these SOFCs, since
previous reports have shown that LSC cathodes with 200 nm LSC
particles have 400 °C–700 °C RP values more than four times larger
than those of the LSCF-GDC NCC produced here.56,62 Further,
Fig. 2b shows that the performance of SOFC half cells utilizing
nano-composite LSCF-GDC cathodes can be dramatically improved
by infiltrating nickel into the anode. This result, which is consistent
with previous literature reports showing that nickel infiltration can
improve the performance of micro-composite Ni-YSZ anodes63 is
another indication that commercially-available, bi-layer Ni-YSZ
anodes (i.e. those utilizing anode functional layers to promote
electrolyte-adjacent electrochemical reactions, and coarse porous
Ni-YSZ support layers to promote rapid gas transport) can dominate
the SOFC electrode response when placed in series with high
performance SOFC cathodes. This conclusion is also backed up by
the open-circuit area-specific cathode, electrolyte, and anode resis-
tances for the LSCF-GDC on Nexceris SOFCs with and without
nickel infiltration shown in Fig. S1 and Table SI (available online at
stacks.iop.org/JES/168/034513/mmedia) of the Supplemental
Materials.
Figure 3 shows open-circuit, symmetric cell polarization resis-
tance measurements for the LSCF-GDC NCCs produced here.
Similar to previous reports, multiple impedance arcs can be Figure 2. SOFC current-voltage (left axis) and power (right axis) curves for
observed: a “low frequency” arc on the right which previous commercial half cells (i.e. cells with a GDC barrier layer, a YSZ electrolyte,
controlled atmosphere experiments have shown is related to gas a Ni-YSZ anode functional layer (AFL), and a Ni-YSZ anode support) with
transport through the cathode, and additional, higher-frequency arcs a) either LSC or LSCF-GDC cathodes or b) with LSCF-GDC cathodes with
that are related to oxygen transport through the cathode and and without anode nickel infiltration. Note that these curves are representa-
interfacial charge-transfer.56 The RP values extracted from the tive of the behavior of three “identically produced” cells, which had
x-axis Nyquist plot intercepts were 0.25, 0.12 and 0.07 Ωcm2 at 550 °C–650 °C peak power density standard deviations less than 10% of
550 °C, 660 °C, and 650 °C, respectively, which are similar to those the average value at each tested temperature.
reported previously for LSCF-GDC NCCs.53,56
Figure 4 shows an open-circuit, symmetric cell literature com-
parison indicating that the RP’s obtained from Fig. 3 and from other
literature cathodes can be lower than the RP’s from many symmetric
cell anodes operated on hydrogen.17,56,28,62,64–67,25,32 This, com-
bined with the fact that many SOFC anodes perform even worse on
hydrocarbon fuels than they do on hydrogen,68–70 suggest that it is
time to update the notion that poor cathode performance typically
limits overall SOFC performance.
Further, Table I shows that while identification and separation of
the anode and cathode resistances in many of the world’s best-
performing Solid Oxide Fuel Cells (i.e. those with peak power
densities >1 W cm−2) are absent from the literature, the analyses
that do exist indicate that under open-circuit-conditions, either
SOFC anode polarization losses, cathode polarization losses, or
ohmic losses can be the greatest source of resistance. Table I also
shows that, more-often-than-not, the total open-circuit electrode
polarization resistance is greater than the ohmic resistance resulting
from the electrolyte, current collectors, electrical leads, etc.
However, since SOFC electrode polarization resistances often
decrease exponentially under current (in a manner which can be fit
to the Butler-Volmer equation, even if the theoretical foundation for Figure 3. Electrochemical Impedance Spectroscopy (EIS) Nyquist Plots for
doing so is questionable),45,46,71,72 while ohmic resistances do not, symmetric LSCFGDC∣GDC∣LSCF-GDC cells at various temperatures. Note,
the dominant source of resistance can switch from the electrode here the ohmic portion of each EIS spectrum has been removed due to slight
differences in the electrolyte thicknesses, and the x-axis numerical values
polarization resistance to the ohmic resistance with the application of have been divided by two to reflect the polarization response of a single
current, as demonstrated by the SOFC performance reported for LSCF-GDC NCC. The shown EIS spectra are representative of those
Ref. 26 in Table I. Literature impedance modeling performed under obtained from the three “identically-processed” LSCFGDC∣GDC∣LSCF-
current has also shown that the anode RP can be higher than the GDC cells which had 500 °C–700 °C RP standard deviations less than
cathode RP under SOFC operation.73 Together, this literature data 10% of the average at each tested temperature.Table I. Characteristics of several SOFCs with peak power densities ⩾ 1 W cm−2 produced from 2007–2020.
SOFC Anode ∣Electrolyte∣ Institution(s), Last Author
Temp. Peak Power Density Geometry Cathode RO@ OCV RP,T @ OCV RP,A @ OCV RP,C @ OCV Ro @ 0.7 V RP,T@ 0.7 V Country on paper References
(°C) (W cm−2) (Ωcm2) (Ωcm2) (Ωcm2) (Ωcm2) (Ωcm2) 2
(Ωcm )
800 2.2 ASCD Ni-YSZ∣YSZ∣ LN 0.08 0.11 — — — — U. Utah, USA A. Virkar 14
800 2.1 ASCD Ni-YSZ
∣YSZ + GDC∣ 0.06 0.12 0.075 0.045 Northwestern, USA S. Barnett 15
LSCF-GDC
800 1.5 ASCD Ni-YSZ∣YSZ∣ 0.25 0.70 — — 0.21 0.04 Ecole Europeenne de M. Rolland 16
Chimie, France
LSCF
800 1.0 ASCD STFN∣LSGM + 0.22 0.13 0.105 0.025 Northwestern, USA S. Barnett 17
LDC∣LSCF +
Journal of The Electrochemical Society, 2021 168 034513
GDC
750 1.5 ASCD + Ni-YSZ∣YSZ∣ 0.07 0.52 0.39 0.13 Northwestern USA S. Barnett 18
Infil
LSM-YSZ + STFC
750 1.4 ASCD + Ni-YSZ
Infl.
∣YSZ∣ 0.15 0.30 — — — — Huazhong U., China/ Li Jian 19
NTU Singapore
Pd-LSM-YSZ
700 3.3 ASCD + Ni-Fe-SDC
TF
∣LSGM + SDC∣ — — — — — — Kyushu U., Japan J. Yan 20
SSC
700 1.1 ASCD + Ni-YSZ
TF
∣YSZ + SDC∣ 0.08 0.09 — — — — PNNL/U. Houston, A. Ignatiev 21
USA
LSF
700 1.1 ASCD + Ni-YSZ
TF
∣YSZ + SDC∣ 0.15 0.03 — — — — U. Sci. & Tech., China W. Liu 22
SSC-SDC
650 2.9 ASCD Ni-GDC
∣GDC∣ — — — 0.01 † — — UNIST,S. Korea/ G. Kim/M. 23
Georgia Tech., USA Liu
PBSCF
650 2.0 ASCD + Ni-GDC
TF
∣ESB + GDC∣ 0.05 0.03 — — — — U. Maryland, USA E. Wachsman 24
BRO-ESB
650 2.0 ASCD Ni-GDC
∣GDC∣ — — — 0.16 † — — UNIST,S. Korea G. Kim 25
LSC-GDC
650 1.8 ASCD + Ni-YSZ
TFTable I. (Continued).
SOFC Anode ∣Electrolyte∣ Institution(s), Last Author
Temp. Peak Power Density Geometry Cathode RO@ OCV RP,T @ OCV RP,A @ OCV RP,C @ OCV Ro @ 0.7 V RP,T@ 0.7 V Country on paper References
(°C) (W cm−2) (Ωcm2) (Ωcm2) (Ωcm2) (Ωcm2) (Ωcm2) 2
(Ωcm )
∣YSZ + GDC∣ 0.03 0.36 — — — — KIST,S. Korea J. Son 26
LSCF-GDC
650 1.3 ASCD + Ni-GDC
Infl.
∣GDC∣ 0.09 0.12 0.09 † 0.03 † — — U. Maryland, USA E. Wachsman 27
LSC-LSCF-GDC
650 1.2 ASCD + Ni-LSGM
Infl.
∣LSGM∣ 0.13 0.30 0.03 † 0.27 † — — Northwestern, USA S. Barnett 28
Journal of The Electrochemical Society, 2021 168 034513
LSCF-LSGM
600 1.8 TF Ni-YSZ
∣YSZ + GDC∣ 0.10 0.16 0.04 0.12 — — UCSD, USA N. Minh 29
LSCF-YSZ
600 1.5 TF Ni-YSZ
∣YSZ + GDC∣ 0.05 0.35 — — 0.05 0.10 KIST,S. Korea J. Son 30
LSC
600 1.2 ASCD + Ni-LSGM
Infl.
∣LSGM∣ 0.06 0.32 0.04 0.28 — — Northwestern, USA S. Barnett 31
LSCF-GDC
600 1.0 ASCD Ni-SDC∣SDC∣BSCF 0.15 0.02 — X — — Caltech, USA S. Haile 32
550 2.0 ASCD Ni-GDC
∣GDC∣ 0.05 0.05 — X — — Yonsei U.,S. Korea Y. Shul 33
BSCF-GDC
500 1.3 TF Pt∣YSZ∣Pt 0.05* 5.9* — — — — Stanford, USA F. Prinz 34
500 1.0 ASCD Ni-GDC
∣GDC∣ 0.07* 0.08* — X — — Colorado School of R. O’hayre 35
Mines/Clemson/
Coorstec, USA
BCFZY
Ro, RP,T, RP,A, and RP,C denote the ohmic resistance, the total electrode polarization resistance, the anode polarization resistance, and the cathode polarization resistance, respectively. OCV denotes open circuit
voltage. All the peak power density and resistance at 0.7 V data in this table is for anodes operating on 3% humidified hydrogen and cathodes operating on air, unless denoted by a * for cells tested under dry
hydrogen. Null dashes indicate that these values were unreported. Data derived, at least in part, from symmetric cell measurements are denoted by a †. In the Anode∣Electrolyte∣Cathode column, hyphens denote
composite mixtures and plus signs denote multilayers. X’s denote symmetric cell cathode RP,C at OCV values that were greater than the corresponding full cell, EIS-measured RP,T at OCV values, and hence were
not believed to represent full SOFC cathode performance. ASCD = anode supported colloidally deposited, BCFZY = barium cobalt ferrite doped with yttria and zirconia, BRO = bismuth ruthenate, ESB =
Erbium stabilized bismuth oxide, GDC = gadolinium doped ceria, Infl. = Infiltration, LDC = Lanthanum Doped Ceria, LN = Lanthanum nickelate, LSC = Lanthanum strontium cobaltite, LSCF = Lanthanum
strontium cobalt ferrite, LSGM = Lanthanum strontium gallium manganate, LSM = Lanthanum Strontium Manganate, NBCCO = Neodymium barium calcium cobaltite, PBSCF = Praseodymium barium
strontium cobalt ferrite, SDC = Samarium doped ceria, STFC = Strontium titanium cobalt ferrite, STFN = Strontium titanium nickel ferrite, TF = thin film, YSZ = yttria doped zirconia. Unfortunately, no studies
could be found on SOFCs with peak power densities >1 W cm−2 where the anode and cathode polarizations had been separated under SOFC operation.Journal of The Electrochemical Society, 2021 168 034513
suggests that all these sources of resistance should receive research
and development attention moving forward.
Acknowledgments
This work was supported by the Nissan Technical Center North
America. The authors would like to thank Mohammed Hussain
Abdul Jabbar at the Nissan Technical Center North America for
SOFC current-voltage measurements independently confirming
the Michigan State University measurements shown in Fig. 2a.
SEM analyses were conducted at the Michigan State University
Composite Materials and Structures Center which is supported by
the NSF Major Research Instrumentation Program and Michigan
State University.
ORCID
Yubo Zhang https://orcid.org/0000-0001-8840-9422
Jason D. Nicholas https://orcid.org/0000-0001-7986-209X
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