Novel carbon and sulfur-tolerant anode material FeNi3?PrBa(Fe,Ni)1.9Mo0.1O5+:δ for intermediate temperature solid oxide fuel cells

Article abstract of DOI:10.1039/c9ta07027c

Suppression of carbon deposition and sulfur poisoning without sacrificing electrochemical performance is crucial for operating solid oxide fuel cells (SOFCs), especially at intermediate temperatures (IT). In this work, nickel-doped A-site deficient perovskite oxides (PrBa)_0.95Fe_1.9-xNi_xMo_0.1O_6-δ (PBFMNix, x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized and investigated as potential anodes for IT-SOFCs. With increased amounts of Ni²⁺, the ratios of Fe²⁺/Fe³⁺ and Mo⁵⁺/Mo⁶⁺ in as-prepared PBFMNix continuously decreased under a charge-compensating mechanism, which simultaneously depressed the formation of the impurity phase (BaMoO₄). Interestingly, the substitution of Ni²⁺ benefits the reduction of Fe³⁺ to Fe²⁺ and Mo⁶⁺ to Mo⁵⁺, and small portions of Fe and Ni elements are exsolved from the parent oxides, forming FeNi₃ alloy nano-particles that greatly accelerate the chemical adsorption and surface reaction kinetics of H₂, and thus improve the electrochemical performances of oxide-based anodes. Transformation of the electrical conduction from p-type to n-type after reduction was also observed. A very small polarization resistance of 0.028 Ω cm² at 750 °C was achieved for the cell with the PBFMNi0.3 anode. Importantly, fueled with syngas with 50 ppm H₂S, the maximum power density of a button cell based on the PBFMNi0.3 anode and an Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte-supporting configuration can reach 498 mW cm^-2 at 750 °C, and long-term stability over 100 hours can be demonstrated with negligible performance decay. All these results indicate that PBFMNi0.3 is a promising high-performance anode material with good coking resistance and sulfur tolerance in intermediate temperatures.

Full text of DOI:10.1039/c9ta07027c

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DOI: 10.1039/C9TA07027C
ARTICLE
Novel carbon and sulfur tolerant anode material
FeNi3@PrBa(Fe,Ni)1.9Mo0.1O5+δ for IT-SOFCs
Shuangshuang Xue,a Nai Shi,a Yanhong Wan,a Zheqiang Xu,a Daoming Huan,a Shaowei Zhang,a
Received 00th January 20xx,
Accepted 00th January 20xx
Changrong Xia,a Ranran Peng,*a,b,c Yalin Lu*a,b,c,d
DOI: 10.1039/x0xx00000x
Suppression of carbon deposition and sulfur poisoning without sacrificing electrochemical performance is crucial for
operating solid oxide fuel cells (SOFCs), especially in intermediate temperatures (IT). In this work, nickel doped A-site
deficiency perovskite oxides (PrBa)0.95Fe1.9-xNixMo0.1O6-δ (PBFMNix, x=0, 0.1, 0.2, 0.3, 0.4) are synthesized and investigated as
potential anodes for IT-SOFCs. With the increased contents of Ni2+, the ratios of Fe2+/Fe3+ and Mo5+/Mo6+ in as-prepared
PBFMNix reduce continuously under a charge compensating mechanism, which simultaneously depresses the formation of
impurity phase (BaMoO4). Interestingly, the substitution of Ni2+ benefits the reduction of Fe3+ to Fe2+ and Mo6+ to Mo5+, and
small portions of Fe and Ni elements are exsolved from the parent oxides forming FeNi3 alloy nano-particles, which greatly
accelerates chemical adsorption and surface reaction kinetics of H2, and thus improves the electrochemical performances
of based oxide anodes. Transformation of the electrical conduction from p-type to n-type after reduction is also observed.
A very small polarization resistance of 0.028 Ω·cm2 at 750 °C is achieved for the cell with PBFMNi0.3 anode. Importantly,
fuelled with syngas with 50 ppm H2S, the maximum power density of the button cell based on PBFMNi0.3 anode and SDC
electrolyte supporting configuration can reach 498 mW·cm-2 at 750 °C and a long-term stability over 100 hours can be
demonstrated with negligible performance decay. All these results indicate that PBFMNi0.3 is a promising high-performance
anode material with good coking resistance and sulfur tolerance in intermediate temperatures.
energy conversion efficiency and economic benefit of coal, and
reduce the operational cost of power generation. Unfortunately,
severe catalytic poisoning and coke deposition occur when applying
1. Introduction
In nowadays, the innovative development of energy conversion and
storage systems is becoming an urgent issue to confront the growing
energy demands and global ecocrisis1. Solid oxide fuel cells (SOFCs)
have drawn special attention as a promising clean energy technology
thanks to their high energy conversion efficiency and environment-
friendly operation2. To increase the volume energy density and the
storage and transport safety of fuels, numerous carbon-containing
fuels, including coal-derived syngas3, ethanol4, 5, natural gas6,
these coal-derived syngas in traditional Ni-based anodes. This should
be ascribed to the unfavourable reactions of Ni with sulfur9, 10 and
the strong catalytic activity of Ni toward carbon formation, which
results in serious performance degradation of the anodes and thus
the service life reduction of SOFCs. In particular, when operating at
the goal temperatures (intermediate temperatures, typically 600-
750 °C) of SOFC stacks11, 12, sulfur poisoning and coke formation over
Ni-based anodes are more grievous than those at high
temperatures13, 14. Therefore, exploring suitable anode materials
with good carbon and sulfur tolerance is the key for practical
application of SOFCs.
Significant efforts on developing carbon and sulfur tolerant anodes
have been focused on mixed conducting perovskite-based materials,
such as La0.75Sr0.25Cr0.5Mn0.5O3-δ (LSCM)15, La1-xSrxCr1-xFexO3-δ
(LSCrF)16, La0.2Sr0.8TiO3 (LST)17, and double perovskite oxides such as
Sr2Fe1.5Mo0.5O6-δ (SFM)18, PrBaMn2O5+δ (PBM)19,20. However, these
alternative perovskite-based anodes typically exhibit insufficient
electro-catalytic activity for fuel oxidation. Recently, adding a second
metal which will be in-situ exsolved from oxides as nano-metal
particles has been intensively studied to optimize perovskite anode
catalysts in SOFCs21-25. Much energy have been invested to decorate
Fe-based perovskite oxides using Ni or Co substitute, such as
LaSrFeNiO6-δ (LSFN)26, Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (PSCFN)27. It was
found that the exsolved second metal (Ni, Co…) can form a bimetallic
alloy with the host metal (Fe), and thus synergistically promote the
biogas7,and wood-derived gases have been applied in SOFCs as
8
substitutes for traditional H2 fuel. Moreover, the application of
carbon-containing fuels may bring forth other commercial
advantages. For example, direct utilization of coal-derived syngas
(containing H2, CO, N2, H2O, H2S…) can greatly simplify the
purification equipment (compared with those for H2), increase the
a. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials
Science and Engineering, University of Science and Technology of China, Hefei,
230026 Anhui, China.
b. Synergetic Innovation Center of Quantum Information & Quantum Physics,
University of Science and Technology of China, Hefei, Anhui 230026, China.
c. Hefei National Laboratory of Physical Science at the Micro-scale, University of
Science and Technology of China, Hefei, 230026 Anhui, China.
d. National Synchrotron Radiation Laboratory, University of Science and Technology
of China, Hefei 230026, P. R. China.
e. *. Corresponding author.
f. Electronic Supplementary Information (ESI) available: [Rietveld refinement
parameters, the change in PO2 in ECR experiments, conductivity results, ECR
theory and results, electrochemical stability results and SEM results]. See
DOI: 10.1039/x0xx00000x
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catalysts’ catalytic activity and remarkably enhance the resistance to of the SDC electrolyte via the same method. The samples were co-
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carbon accumulation and sulfur poisoning26. Unfortunately, these fired at 1000 °C in air for 2 hours to completeDtOheI: f1a0b.1r0ic3a9t/iCo9nTpA0ro7c0e2s7sC.
studies are mainly confined in operating at high temperatures (800 - Thickness of each electrode was about 20 μm (Figure. S10). As
900 °C) for lacking of sufficient catalytic activities toward carbon- comparison, NiO-SDC (6:4) |SDC|LSCF-SDC (6:4) cells were also
containing fuels in intermediate temperatures, which is more prone prepared via the same process.
to occur severe carbon deposition and sulfur poisoning.
In this work, we prepared nickel doped A-site deficiency perovskite
2.3 Characterization of powders
oxides (PrBa)0.95Fe1.9-xNixMo0.1O6-δ (PBFMNix, x=0-0.4) with the aim
of improving the electrochemical activity of anode materials and
enhancing its resistance to carbon deposition and sulfur poisoning at
intermediate temperatures (550-750 °C). Mo elements with high and
changeable valences were introduced into B-site to stabilize Fe-
based perovskite lattices28 and to contribute to their electronic
conductivities in reducing atmosphere. Thus, the crystal structures,
valence states of B-site elements and conductivities of the series
samples were explored before and after reduction. Surface
adsorption kinetics of these samples in reducing atmosphere were
also explored as functions of Ni contents and temperatures.
Electrochemical performances and stabilities of single cells using
these anodes were investigated with fuels like wet hydrogen, wet
syngas (H2: CO=2:1), syngas with 50 ppm H2S and wet propane.
Room temperature X-ray diffraction (RT-XRD) patterns of the series
samples were recorded on a Rigaku D/Max 2100 powder X-ray
λ
diffractometer with Cu-Kɑ radiation ( =1.5418 Å) at a scan rate of 3
deg min-1 in the range of 20-80 °. Rietveld refinements were
conducted with the program FullProf to determine the space group
and the lattice parameters. High-resolution transmission electro-
microscopy (HRTEM), scanning transmission electron microscopy
(STEM) and corresponding energy dispersive spectroscopy (EDS)
analyses were performed on a JEOL JEM-ARM200F TEM/STEM with
a spherical aberration corrector. H2-temperature programmed
reduction (TPR) experiments were conducted on 0.05 g PBFMNix
(x=0-0.3) using a Thermo Finnigan TPDRO 1100 apparatus equipped
with a thermal conductivity detector. The flow rate of 5% H2/Ar for
TPR analysis was 20 ml·min-1, and the temperature ramp rate was 10
°C·min-1. X-ray photoelectron spectroscopy (XPS) analysis was
performed using a Kratos Analytical AXIS 165 with a monochromatic
Al Kɑ source to investigate the oxidation states of powder samples. A
nonlinear Shirley-type background method was used to analyse the
core peaks. In the fitting, the peak positions and areas were
optimized via the Lorentzian–Gaussian method. Morphology of the
powders was characterized using a SEM (JSM-6700F).
2. Experimental sections
2.1 Powders synthesis
(PrBa)0.95Fe1.9-xNixMo0.1O6-δ (PBFMNix, x=0-0.4) powders were
synthesized using a citric acid - ethylene diaminetetraacetic acid
(EDTA) complexing method. Stoichiometric amounts of
Pr(NO3)·6H2O, Ba(NO3)2, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O, and
(NH4)6Mo7O24·4H2O were dissolved in deionized water. Citric acid
and EDTA were added to the solution as the complexing agents and
fuels with a molar ratio of citric acid: EDTA: metal cations = 1.5: 1: 1.
The pH value of the solution was adjusted to 7 with ammonia. After
being stirred for 2 hours, the solution was heated in a microwave
oven until self-ignition. The combustion ashes were heat-treated at
1100 °C for 6 hours in air to form the desired PBFMNix powders.
PBFMNix powders were reduced in wet H2 at 800 °C for 10 hours to
in-situ exsolute Ni-based nano-particles. Powders of Sm0.2Ce0.8O1.9
(SDC) electrolyte were prepared with carbonate co-precipitation
method, described previously29,30, using Ce(NO3)3·6H2O (99%) and
Sm(NO3)3·6H2O (99.95%) as the precursors. La0.6Sr0.4Co0.2Fe0.8O3-δ
(LSCF) powders were synthesized via a solution combustion method
as described elsewhere31. All the raw chemicals were purchased from
Sinopharm Chemical Reagent Co. Ltd.
2.4 Characterization of cells
Electrochemical performances of cells were characterized in a home-
developed testing system using multiple fuels such as hydrogen,
propane, syngas (H2: CO=2:1) and syngas with 50 ppm H2S. The flow
rates of the gases were set as ~ 50 ml·min−1. Ambient air was used as
the oxidant. Before electrochemical testing in propane and syngas
containing fuels, wet 5% H2/Ar was injected into anode area at 800
°C for 2 hours to reduce PBFMNix anodes. A.C. impedance spectra
and electrochemical performances of the cells were investigated
from 750 down to 550 °C using an electrochemistry workstation
(Garmary framework) in the typical frequency range of 0.01 Hz ~ 1
MHz with the AC voltage amplitude of 10 mV. Polarization curves
were obtained by performing linear sweep voltammetry from open
circuit potential to 0.2 V with a scan rate of 50 mV·s-1. Fractured
microstructures of the cells were characterized using a scanning
electron microscopy (SEM, JSM-6700F).
2.2 Cell fabrication
2.5 Electrical performance evaluation
Single cells with the structure of PBFMNix–SDC|SDC|LSCF–SDC were
prepared taking electrolyte supporting configurations. SDC
electrolyte substrates with a thickness of about 200 μm were
prepared by dry-pressing 0.15 g SDC powders uniaxially at 200 MPa,
followed by sintering at 1450 °C for 5 hours in air. PBFMNix and SDC
powder mixtures (weight ratio of 7:3) were blended with ɑ-terpilenol
in weight ratio of 1:1.5 to form anode ink, which was then applied
onto one surface of SDC electrolyte by a screen-printing method.
After drying at 80 °C for 60 min, the cathode ink containing LSCF and
SDC powders (weight ratio of 6:4) was applied onto the other surface
Electrical conductivity relaxation (ECR) measurements were
performed using a standard 4-probe technique to study the oxygen
transport kinetics of samples in reducing atmospheres. For this
measurement, PBFMNix powders were mixed with 5% PVA (polyvinyl
alcohol) by ball-milling for 2 hours. The powder mixtures were
uniaxially pressed at 300 MPa and then sintered in air at 1350 °C for
10 hours to obtain rectangular bar samples with dimensions about
32.0
4.5
0.56 mm3. The surface SEM images of the sintered
×
×
samples were presented in Figure. S8 and the densities of the
sintered samples measured by Archimedes method were all above
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96% of their theoretical ones. Electrical connections to the samples spectroscopy (XPS) investigation suggests that Ni elements show
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were established by four silver wires, and silver conducting resin was divalent valence state Ni2+ (∼854.7/856.5)3D1O,32I: 1(0Fi.1g0u3r9e/.C29aTA) 0i7n02t7hCe
used to enhance the contact between the specimen and the Ag series samples. And therefore, the substitutions of Fe3+ by Ni2+
wires. Before ECR experiments, the bar samples were firstly annealed enables more Mo elements to enter the lattice of perovskites
in 5% H2/Ar at 800 °C for 10 hours to ensure the steady states of (PBFMNix) under a charge compensating mechanism, and thus
samples. Conductivities of specimen were measured as a function of inhibits the formation of impurity phase (BaMoO4). Moreover, as
time following a step-wise decrease of oxygen partial pressure shown in Figure. 2a, b and c, both Fe and Mo elements demonstrate
performed by switching the gas stream from humidified 5% H2/Ar to mixed oxidation states, including Fe2+ (~ 709.8 eV) and Fe3+ (~ 710.8,
humidified 10% H2/Ar at a total gas flow rate of 200 ml min-1 and the 711.6, 713.4, and ~ 719.2 eV), and Mo5+ (~ 231.6 and ~ 234.7 eV) and
resulting changes in oxygen partial pressures (PO2) at different Mo6+ (~ 232.3 and ~ 235.4 eV)31, respectively. Importantly, the ratios
temperatures were shown in Table. S2.
for Fe2+/Fe3+ and Mo5+/Mo6+ in as-prepared PBFMNix (table.1)
reduce continuously with the increasing Ni-doping contents,
testifying the charge compensating mechanism. It should be also
noted that PBFMNix and SDC have good chemical compatibility
testified by the XRD patterns for their mixtures calcined at 1000 °C
for 2 hours in air (Figure. S1).
3. Results and discussion
3.1 In-situ formation of nano-metal particles
Figure. 1a shows the RT-XRD spectra of the as-prepared PBFMNix
(x=0, 0.1, 0.2, 0.3, 0.4) powders calcined at 1100 °C for 6 hours in air.
Peak at around 26.5 °, indexed as BaMoO4 (JCPDS#48-0335) is clearly
observed in the spectra of as-prepared (PrBa)0.95Fe1.9Mo0.1O6-δ (x=0),
indicating that pure perovskite structure failed to be obtained in such
composition. This should be ascribed to the high oxidation state of
Mo6+ under oxidizing atmosphere, similar to that in famous
Sr2FeMoO6-δ32. Substitution of Ni for Fe in PBFMNix has the potential
to stabilize the pure-phase perovskite structure in air. With the
increasing contents of Ni substation, peak for BaMoO4 becomes
weaker and gets invisible when the doping contents x reaches above
0.2, exhibiting a pure perovskite structure. X-ray photoelectron
For the reduced samples treated at 800 °C in wet H2 for 10 hours,
FeNi3 alloy (JCPDS#38-0419, Pm-3m, a = 3.545 Å) instead of pure Ni
(JCPDS#04-0850, Fm-3m, a = 3.524 Å) was successfully exsolved from
parent materials PBFMNix (x=0.2, 0.3, 0.4) , as shown in Figure. 1b
and c, thus forming FeNi3@PBFMNix. The reason why the alloy was
not detected in reduced PBFMNi0.1 may be attributed to the low
contents of resolved FeNi3 particles. It is worth noticing that
secondary impurity phase of PrOx appeared when the doping
amounts of Ni reached 0.4, indicating that perovskite structure has
been partially destroyed due to the severe Ni-Fe exsolution. This may
result in unstable performance when it is used as SOFC anode.
Interestingly, BaMoO4 impure phase existed in PBFMNix (x = 0, 0.1,
Figure. 1 Room-temperature XRD patterns of (a) as-prepared PBFMNix (x =0-0.4) powders in air and (b) the
reduced PBFMNix (x =0-0.4) powders processed in wet H2 at 800 °C in the range of 20-80 °. (c) The magnified
spectra of (b) in the range of 40-50 °. And Refined XRD profiles for (d) as-prepared and (e) reduced PBFMNi0.3.
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DOI: 10.1039/C9TA07027C
Figure. 2 XPS spectra of as-prepared and reduced PBFMNix (x=0, 0.1, 0.2, 0.3): (a, d) Ni 2p (b, e) Fe 2p (c, f) Mo 3d
0.2) powders diminished after reduction, which may be ascribed to refinement results are summarized in table S1. The lattice constants
the lower ratio of Mo6+ under reducing atmosphere as demonstrated of as-prepared PBFMNi0.3 powders are a=b=c=3.905 Å. And after
in the XPS fitting results (Table.1). The XPS spectra (Figure. 2d) of the reduction, the oxide lattice expanded to a=b=c=3.938 Å due to the
reduced samples further indicate that both metallic nickel Ni0 formation of more oxygen vacancy along with the increased ionic
(∼852.6/858.6) and divalent valence state Ni2+ (∼854.7/856.5)32,33 radius of Fe and Mo ions due to their reduced average valence.
are detected in the reduced samples. As calculated from the fitting Figure. 3 display the morphology of the formed FeNi3@PBFMNix
results, most of the nickel elements are still in the lattice and only a powders. With no Ni substituting (x=0), glossy particle surface is
fraction are reduced to metallic nickel. This seems to suggest that Ni shown; while in case of Ni substituted samples (x>0), many small
cannot be totally exsolved from the parent oxide, similar to that in sphere-like particles (15-25 nm) are uniformly distributed on the
Sr2FeMo0.65Ni0.35O3-δ31。For Fe elements, peaks corresponding to Fe0 oxide surface, indicating the formation of FeNi3 nano-particles (even
in addition to mixed oxidation state of Fe2+/ Fe3+ are clearly observed, for x=0.1). Notably, although the amounts of FeNi3 particles increase
as shown in Figure. 2e. The coexistence of Ni and Fe metal spectra with the Ni contents, the size of particles seems changeless. Scanning
confirms the formation of FeNi3 alloy.
transmission electron microscopy (STEM) along with energy X-ray
To obtain an accurate lattice parameter, Rietveld refinements of XRD spectroscopy (EDS) analysis was conducted on the
spectra for as-prepared and reduced PBFMNi0.3 were conducted FeNi3@PBFMNi0.3 powders to further confirm the composition of
based on a cubic structure (space group: Pm-3m, No. 221). The the formed small particles. As clearly shown in the STEM and
refined XRD profiles are shown in Figure. 1c and d, and the elemental EDS mapping images (Figure. 4a), both Ni and Fe elements
are present in these small particles, confirming the exsolution of Ni-
Fe alloy nano-particles on the surface of the PBFMNi0.3 particles.
This is consistent with XRD and XPS analysis. High resolution
transmission electron-microscopy (HRTEM) pictures of the
FeNi3@PBFMNi0.3 powder are shown in Figure. 4b. The lattice space
in the sphere-like nano-particle is determined as 0.20 nm,
corresponding to the (111) plane of the cubic FeNi3 alloy, and for the
oxide part, the lattice space is measured as 0.28 nm, in
correspondence with the (110) plane of reduced PBFMNi0.3
perovskite structure. All these results clearly demonstrate FeNi3 alloy
was exsolved from PBFMNix perovskite oxides. It is worth noticing
that most Ni elements are still observed in the oxides, as shown in
Table.1 Ratios of Fe 2p and Mo 3d peaks of PBFMNix
obtained by X-ray photoelectron spectroscopy
Fe2+/Fe3+
Mo5+/Mo6+
As-
prepared
0.34
0.31
0.30
Reduce
d
0.53
0.48
0.50
0.51
As-
prepared
0.78
0.60
0.56
Reduce
d
0.75
1.21
1.07
0.89
x=0
x=0.1
x=0.2
x=0.3
0.28
0.39
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DOI: 10.1039/C9TA07027C
Figure. 3 SEM images of reduced PBFMNix (x = 0-0.3) at
800 °C in wet H2 for 10 hours. Nano-sized FeNi3 particles
(15-25 nm) exsolved from the backbone dispersed
uniformly on the reduced oxides surface.
Figure. 5 TPR profiles of PBFMNix (x=0-0.3) powders
reduction, and that Fe0 and Ni0 peaks appear in XPS spectra of
reduced PBFMNix (x=0.1-0.3). Moreover, with the increase of Ni
substitution (x), the reduction peaks for Fe3+ and Mo6+ gradually shift
to a lower temperature and the peaks’ areas for FeNi3 alloy
formation are obviously improved. This is consistent with the larger
peak’ areas for Ni0 and Fe0 presented in the XPS spectra of reduced
samples with more Ni-doping contents (Figure. 2d). These results all
indicate that Ni-doping could facilitate the reduction of Fe3+ to Fe2+
and Mo6+ to Mo5+, and prompt more FeNi3 metallic nano-particles
generated.
Figure. 4a. This is consistent with the above XPS results. Notably, the
exsolved FeNi3 are embedded to a considerable depth in the parent
oxide (Figure. S2), which may depress the carbon formation due to
strong interaction between the exsolved socketed particles and the
parent oxides34.
To figure out the exsolution properties of FeNi3, TPR profiles of
PBFMNix in a 5% H2/Ar gas mixture were conducted, in which the
hydrogen consumption was measured as a function of the operating
temperatures. As depicted in Figure. 5, PBFMNi0 presents one main
reduction peak at ~ 500 °C related to the reduction of Fe3+ to Fe2+ 35,36
without the reduction peak of Mo6+ in the perovskite bulk. While for
PBFMNix (x=0.1-0.3), in addition to the reduction peaks for Fe3+, two
additional peaks locate in ~ 630 and ~ 680 °C can be clearly observed,
which should be ascribed to the reduction of Mo6+ to Mo5+ 33 and the
formation of FeNi336,37, respectively. This is consistent with XPS
results that the ratio for Mo5+ hardly changes in PBFMNi0 while
obviously increases in PBFMNix (x=0.1-0.3) before and after
3.2 Conductivity
As shown in Figure.6a, the conductivity plots of all PBFMNix samples
exhibite an up-down transition at 500-600 °C, indicating
semiconductor behavior at low temperatures (200-500 °C), and
metallic behaviour at higher tempretures (550-800 °C). Notably, the
electrical conductivities increase with the Ni contents in PBFMNix,
Figure.4 (a) the STEM pictures and the EDS mappings of reduced PBFMNi0.3 powders in HAADF-STEM mode; (b) HRTEM
images of a typical reduced PBFMNi0.3 particle and the crystal lattice analyses associated with fast Fourier transformation of
FeNi3 alloy (c region) and PBFMNi0.3(d region).
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Figure. 6 (a) conductivities of PBFMNix (x = 0-0.3) in air at 200-800 °C; (b) Arrhenius plots of conductivity at 550-800 °C in
O2 and air for PBFMNi0.3; (c) conductivities of after-reduced PBFMNix (x = 0-0.3) in wet H2 at 550-800 °C; (d) Arrhenius
plots of conductivity at 550-800 °C in wet 5% H2/Ar, 10% H2/Ar and 100% H2 for PBFMNi0.3, respectively.
suggesting that the doping of Ni could facilitate the conductivities of
PBFMNix in air. This may partially result from the less impurity phase
formed in Ni substituting samples. Arrhenius plots of conductivity for
PBFMNi0.3 measured at 550-800 °C in pure O2 and air are shown in
Figure. 6b. The higher conductivities under pure O2 than that in air
suggests that the sample is a p-type conductor. The electrical
conductivities of FeNi3@PBFMNix (x = 0-0.3) formed by annealing
PBFMNix in humidified 5% H2/Ar at 800 °C for 10 hours were also
investigated in the temperature range of 550-800 °C under
humidified H2. As shown in Figure. 6c, all the electrical conductivities
increase with the increase of testing temperatures, demonstrating a
semiconductor
behavior.
Interestingly,
largest
electrical
conductivities of 14.38 S·cm2 at 800 °C was achieved at x=0.1, unlike
that measured in air. Arrhenius plots of electrical conductivities for
FeNi3@PBFMNi0.3 in different reducing atmosphere are shown in
Figure. 6d. The higher conductivity in pure H2 than that in 5% H2/Ar
indicating the sample exhibits n-type semiconductor characteristic,
different from that of as-prepared PBFMNi0.3. Such transformation
of the electrical conduction mechanism from p-type to n-type after
reduction also occurred in other two samples PBFMNix (x=0.1, 0.2)
(Figure. S3), which may root in the following equations:
퐇ퟐ + 퐎퐎× ⟶ 퐕퐎∙∙ + ퟐ퐞′ + 퐇 퐎
(g)
(1)
and
퐎
ퟐ
퐎 ×
퐕 ∙∙
Where H2 extracted
to form H2O with an oxygen vacancy
퐎
two electron generated. Therefore, the amount of extrinsic electron
in samples after reduction increases and gradually depletes the
number of electron hole (h•), which explains why the materials
transformed from p-type conductor to n-type conductor. The
generated electrons can reduce the valence of B-site elements via
following equations:
Figure. 7 (a) Electrical conductivity relaxation curves of
reduced PBFMNix (x=0.1, 0.2, 0.3) at 700 °C; (b) Arrhenius
plots of the surface exchange coefficient of the three
samples at 550-750 °C.
FeF×e + MoM×o + 2e′ ⟶ FeF′ e + MoM′ o
(2)
′′′
Ni′Fe + 2e′ ⟶ V + Ni
(3)
Fe
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(4) expressed by equation (1) in Kröger-Vink notation39. According to the
′′′
Fe′Fe + 2e′ ⟶ V + Fe
Fe
View Article Online
DOI: 10.1039/C9TA07027C
Electrical conductivity is often derived from the exclusive movement linear interface kinetics assumption40,41, the total surface reaction
of electrons through B-O-B or B-O-B’ in perovskites (e.g. Fe3+ and rate, r (mol.s-1), can be expressed as:
Mo5+ pair38). As shown in TPR and XPS results, the substitution of Ni2+
r =SKchem[c (t) - c (∞)]
(5)
can facilitate the reduction of Mo6+ and Fe3+, and thus improves the
electronic conduction. While with the increase of Ni substitution,
more alloy FeNi3 is formed, which indicates more electrons
consumed (equation (3) and (4)), which in turn leads to the
depressed electronic conduction. According to the XPS fitting results
above (Table. 1), the ratios for Fe3+ and Mo5+ electronic configuration
in the series reducing samples reach its maximum value when the Ni
contents reaches 0.1, which corresponds well with the maximum
electronic conductivity of x=0.1 among these reduced samples.
Meanwhile, the conductivity activation energies of these samples
under H2 are much higher than that in air. The high activation
energies in the reduced system imply electrons are more localized
and electronic conduction occupy smaller proportion in total
conductivity.
Where Kchem (cm·s-1) is the surface reaction rate constant, often
referred as the chemical surface oxygen exchange coefficient that
can be obtained by the fitting curves of normalized conductivity; S
(cm2) is the surface area, c (t) (mol·cm-3) presents the oxygen
concentration at time t, and c (∞) (mol·cm-3) presents the oxygen
concentration in the final equilibrium time. The final equilibrium time
reduces with the increase of Ni-doping contents (x) in PBFMNix
(Figure. 7a), indicating the accelerated H2 oxidation process (lattice
oxygen reduction process). The time dependences of obtained
normalized conductivities in ECR experiments for FeNi3@PBFMNix
(x=0.1-0.3) are plotted in Figure. S4 and the corresponding fitting
results of Kchem are plotted in Figure. 7b. Obviously, PBFMNi0.3 shows
×
the largest Kchem values among the investigated samples, of 3.25
10-4 and 1.50 10 cm·s-1 at 700 and 600 °C, respectively. Such
enhanced Kchem can be attributed to more nano-catalysts FeNi3
exsolved from the oxides but with changeless particle size, which
enlarged triple phase boundary (TPB) length and accelerated the
transmission of the charged species41.
-4
×
3.3 Surface adsorption kinetics
ECR technique was used to evaluate the the surface oxygen exchange
properties of FeNi3@PBFMNix in a humidified H2/Ar atmosphere at
550-750 °C (see the Experimental section). By abruptly switching the
gas from humidified 5% H2/Ar to humidified 10% H2/Ar, the oxygen
3.4 Electrochemical properties
partial pressure (PO2) decreases sharply from 1.07 10-20 atm to 2.69
×
To assess the electrochemical performance of PBFMNix (x=0.1-0.3)
anodes, I-V curves of the single cells using PBFMNix-SDC anodes (in-
situ reducing to FeNi3@PBFMNix-SDC) are characterized at 700 °C
with humid (3 vol % H2O) H2 and ambient air as fuel and oxidant,
10-21 atm (750 °C, Table. S2). To meet the change of environmental
×
oxygen partial pressure, lattice oxygen of PBFMNix will partially
escape from the lattice and react with hydrogen, similar to the redox
reaction taking place in the anode of SOFCs. This reaction can be
Figure. 8 (a) I-V and I-P curves and (b) EIS of SDC electrolyte-supported single cells PBFMNix-SDC|SDC|LSCF-
SDC (x=0.1, 0.2, 0.3) operating in humid H2 at 700 °C; (c) I-V and I-P curves and (d) EIS of SDC electrolyte-
supported single cell PBFMNi0.3-SDC|SDC|LSCF-SDC operating in humid H2.
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respectively. Electrochemical impedance spectra (EIS) of these fuel
cells are measured accordingly. The ohmic resistances of the
electrolyte are subtracted from the impedance spectra to have a
comparison of Ni-doping impact on the electrode polarization. As
shown in Figure. 8a, b and Figure. S5, the peak power densities of the
cells increase and the polarization resistances (Rp) decrease with Ni
contents increasing in anodes, indicating that the increase of Ni
contents can largely accelerate anode reactions and may be partially
attribute to the increased amounts of FeNi3 alloy nano-particles in
reduced-PBFMNi0.3 for its strong catalytic activities toward fuel
oxidation reactions. This is in good accordance with the above ECR
results (Figure. 7). SDC electrolyte-supported single cells using
PBFMNi0.3-SDC anode have achieved excellent electrochemical
performance, as shown in Figure. 8c and d, with the peak power
densities of 588, 437 and 297 mW·cm-2 at 750, 700 and 650 °C,
respectively. It is comparable to famous Sr2FeMo0.65Ni0.35O3-δ anode
reported previously (590 mW·cm-2 at 750 °C). The corresponding Rp
values are 0.028, 0.061, 0.149 Ω·cm2 at 750, 700, 650 °C, respectively,
much lower than other single cells with nano-catalysts decorated Fe-
based oxide anodes like La0.8Sr1.2Fe0.9Co0.1O4±δ (0.550 Ω·cm2 at
750 °C)42, La0.7Sr0.3Fe0.9Ni0.1O3-δ (0.403 Ω·cm2 at 750 °C)43, Sr2Fe1.5-
xMo0.5NixO6-δ (0.31 Ω·cm2 at 750 °C)36, Sr0.95Ti0.3Fe0.63Ni0.07O3-δ (0.10
Ω·cm2 at 800 °C)44 as listed in table. 2. The good electrochemical
performance of PBFMNi0.3 anode verifies its effectiveness in
catalysing H2 oxidizing. Moreover, the cells using PBFMNi0.3 anode
have reached a long term stable performance for 100 h in hydrogen
with a constant current density of 200 mA·cm-2 applied (Figure. S6).
Finally, multiple fuels, such as wet (10 vol % H2O) propane, wet (3 vol
% H2O) syngas and syngas with 50 ppm H2S, are injected to the anode
side, respectively, to characterize the coking resistance and sulfur
tolerance of the PBFMNi0.3 anode. The I-V and I-P curves of cells
under these various fuels measured at 750 °C are plotted in Figure.
9a. When wet propane is used as the fuel, a cell displays a peak
power density of 332 mW·cm-2. Continuous and stable operation for
View Article Online
DOI: 10.1039/C9TA07027C
Figure. 9 (a) I-V and I-P curves of SDC electrolyte-
supported single cells PBFMNi0.3-SDC|SDC|LSCF-SDC
operating in wet H2, wet propane, wet syngas and
syngas-50 ppm H2S, respectively, at 750 °C. Operating
stability of the cells fuelled with (b) wet propane and
(c) wet syngas with 50 ppm H2S measured at 750 °C
Table. 2 Cell performance using metal nano-catalysts modified Fe-based perovskite anodes through in-situ reduction
in humidified H2 fuel.
μ
Electrolyte (
m)
LSGM
Temperature
(°C)
Peak power density
(mW·cm-2)
Rp
Anode composition
La0.8Sr1.2Fe0.9Co0.1O4-δ
Reference
42
(Ω·cm2)
750
190
0.550
(1000)
La0.7Sr0.3Fe0.9Ni0.1O3-δ
Sr2FeMo0.65Ni0.35O6-δ
LSGM (300)
LSGM (300)
LSGM
750
750
750
800
750
400
590
380
950
364
0.403
-
43
31
36
44
45
Sr2Fe1.4Mo0.5Ni0.1O6-δ
Sr0.95Ti0.3Fe0.63Ni0.07O3-δ
Pr0.6Sr0.4Fe0.7Ni0.2Mo0.1O3-δ
0.31
0.10
0.14
LSGM (300)
LSGM (300)
La0.6Ca0.4Fe0.8Ni0.2O3-δ
infiltrated SDC
SDC (300)
SDC (200)
750
750
444
588
0.024
0.028
46
This
work
PBFMNi0.3-SDC
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more than 50 hours (Figure. 9b) is achieved without observable Science Center CAS (2016HSC-IU004), and the Fundamental
View Article Online
degradation in performance, demonstrating its good carbon Research Funds for the Central UniversitiDesO(I:W10K.1304309/0C090T0A00740)27C
tolerance. When measured in humid syngas and syngas with 50 ppm
H2S, the cells have achieved high performance of 520 and 498
mW·cm-2 (Figure. 9a), respectively, and the polarization resistances
of the cells are almost the same of 0.035 Ω·cm2 (Figure. S7), slightly
larger than that in humid H2 (0.028 Ω·cm2). Such low polarization
resistances greatly suggest the effectiveness of the anode toward
syngas oxidation and especially the high sulphur tolerance. Yet, a
slightly higher ohmic resistance in wet syngas-H2S is observed than
that in wet syngas (Figure. S7), which may be attributed to the trace
amount of sulfide (Ce2S3) on SDC particles as shown in the Figure. S9.
Notably, the cell reaches a long-term stability for 100 h at 750 °C in
wet syngas with 50 ppm H2S as shown in Figure. 9 c without obvious
degradation. In contrast, a continuous decrease rates of 6 mV h−1 in
cell voltage is observed when NiO-SDC anode is exposed to the same
fuel, indicating PBFMNi0.3 anode exhibits much better coking
resistance and sulfur tolerance than conventional NiO-SDC anode in
intermediate temperatures. After durability test in syngas with 50
ppm H2S, the microstructure of the porous PBFMNi0.3-SDC was
examined and no carbon or carbon fibres can be observed (Figure.
S10).
Notes and references
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Secondly, PBFMNi0.3 anode shown the biggest surface exchange
-4
coefficient of 3.25 10 cm·s-1 and the best electrochemical
×
properties at 700 °C compared with PBFMNi0.1 and PBFMNi0.2,
indicating that Ni-doping is beneficial to increase the catalytic activity
of anode materials. Furthermore, electrolyte supported single cells
with PBFMNi0.3 anode achieved very small polarization resistances
of 0.028 Ω·cm2 at 750 °C and the cells’ peak power densities have
reached 588, 520, 498 mW·cm-2 in wet H2, wet syngas and syngas
with 50 ppm H2S at 750 °C respectively. Moreover, the cells have kept
long-term stability for 50 hours in wet propane and for 100 hours in
wet syngas with 50 ppm H2S at 750 °C, which suggests the excellent
coking resistance and sulfur tolerance of the PBFMNi0.3-SDC fuel
electrode in IT-SOFCs.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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Research
and
Development
Program
of
China
(2017YFA0402800), the Natural Science Foundation of China
(51872276), the External Cooperation Program of BIC, the
Chinese Academy of Science (211134KYSB20130017), Hefei
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DOI: 10.1039/C9TA07027C
PBFMNi0.3 perovskite with in-situ exsolved FeNi3 nano-catalysts is a promising carbon and sulfur tolerant
anode for IT-SOFCs.