Supporting information
Exploring Ni(Mn1/3Cr2/3)2O4 Spinel-Based Electrode for Solid Oxide Cell
Nanqi Duan†, ‡, Minrui Gao‡, Bin Hua‡, Meng Li‡, Bo Chi†, Jian Li†, *, Jing-Li Luo‡, *
† Center for Fuel Cell Innovation, School of Materials Science and Engineering, Huazhong
University of Science and Technology, Wuhan, Hubei, 430074, China
‡ Department of Chemical and Materials Engineering, University of Alberta, Edmonton,
Alberta T6G 1H9, Canada
* Corresponding author.
E-mail: [email protected] (Dr. J. Li), [email protected] (Dr. J.-L. Luo)
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2020
1. DFT calculations
In order to calculate the Gibbs free energy change of spinel reduction reaction shown in eq.1
and eq.2,
(Mn16Ni8)(Cr32Ni16)O96 + 32H2 = 32H2O + 24Ni + 16MnCr2O4 (1)
24Ni + 16MnCr2O4 + 16O2 = (Mn16Ni8)(Cr24Ni16)O96 (2)
density functional theory (DFT) calculations were carried out by Vienna Ab initio Simulation
Package (VASP). Projector-augmented plane-wave method and Perdew-Burke-Ernzer (PBE)
function were employed to treat the electron-ion interaction and exchange-interaction effect
respectively. Cutoff energy was set at 400 eV and spin polarization was switched on for
magnetic elements including Cr, Mn and Ni. Brillouin Zone k points were sampled by
Monkhorst-Pack (8×8×8) for optimization of Ni and MnCr2O4 unit cell. Based on the optimized
cell, (3×1×1) MnCr2O4, (2×1×1) MnCr2O4, (3×2×1) Ni supercells were built up. To determine
the substitution positions of Ni on A sites (Mn) and B sites (Cr) of spinel, multiple geometries
were considered and the one with lower energy was chosen. It was revealed that Ni prefers to
stay together when substituting Mn sites but to distribute discretely when substituting Cr sites.
Gaussian smearing was set at 0.1 eV for spinel and metal (Ni), while for O2, H2 and H2O, Fermi
smearing was set at 0.01 eV. Relaxation of degree of ions was not terminated until a maximum
force component of 0.05 eV/Angstrom reached. K points were changed to Monkhorst-Pack
(1×3×3) for Ni substituted MnCr2O4 supercell, Monkhorst-Pack (2×4×4) for MnCr2O4
supercell and Monkhorst-Pack (2×3×6) for Ni supercell. Gibbs free energy of species can be
expressed by eq.3,
G = E + ZPE + ∫CpdT - TS (3)
where E, ZPE, ∫CpdT and -TS represent the DFT calculated electronic energy, zero-point energy,
enthalpy correction and entropy correction individually. PV contributions was neglected. 3N
freedom degrees were treated as frustrated harmonic vibrations to calculate ZPE and enthalpy
correction, entropy contribution was calculated by proposed standard method and transferred
to thermodynamic data at room temperature. ZPE, ∫CpdT and -TS are all functions of vibration
frequencies of ions. Considering the fact the value of corrections items (ZPE, ∫CpdT, -TS) from
crystalline solids are usually very limited and vibrations of ions in crystalline solids are usually
cancelled between nearest layers[10], only the values of ZPE, ∫CpdT and -TS from O2, H2 and
H2O were counted. The Gibbs free energy change of eq.1 and eq.2 were then calculated by
∆G = 32*G[H2O] + 24*G[Ni] + 16*G[MnCr2O4] - 32*G[H2] - G[(Mn16Ni8)(Cr24Ni16)O96
(4)
∆G = G[(Mn16Ni8)(Cr24Ni16)O96] - 16*G[MnCr2O4] - 16*G[O2] - 24*G[Ni] (5)
References
[1]. G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558.
[2]. G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251
[3]. G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169
[4]. B. Hammer, L.B. Hansen, J.K. Nørskov, Phys. Rev. B 59 (1999) 7413
[5]. H. Monkhorst, J. Pack, Phys. Rev. B 13 (1976) 5188
2. Supplemented figures
Figure S1 Rietveld refined XRD results: (a) synthesized NMC, (b) 1st reduced NMC, (c) 1st
re-oxidized NMC, (d) 10th reduced NMC, (e) 10th re-oxidized NMC, (f) NMC reduced in 5
mol.% CO-CO2, and (g) wet hydrogen reduced NMC re-oxidized in CO2.
Figure S2 The SEM pictures of NMC powders after different processing history: synthesized
NMC (a, b), first reduced NMC (c), first re-oxidized NMC (d), 10th reduced NMC (e), and
10th re-oxidized NMC (f). Partial nickel particles are marked by dot yellow circles.
Figure S3 XPS surveys of NMC powders processed at different atmosphere: C1s (a) and O1s
(b).
The O1s spectrums (Figure S3b) of NMC suffered through air condition appeared a lattice
oxygen (O2-) peak and an adsorption oxygen peak centered at 530.1 eV and 532.1 eV
respectively. The lattice oxygen peak shifted a little and centered at 530.3 eV because a larger
lattice parameter of the spinel phase in the reduced NMC decreases the electrostatic attraction
between the metal ionic and the extranuclear electrons of oxygen ionic, which benefits the
electrostatic attraction between the nuclear of oxygen ionic and its extranuclear electrons.
Figure S4 The XRD pattern of fresh prepared symmetrical cells.
Figure S5 NMC-GDC electrode at different processing history: (a) as prepared symmetrical
cell, (b) fresh NMC-GDC electrode, (c, d) NMC-GDC electrode after 8 cycles of redox, and
(e, f) NMC-GDC anode after working in wet hydrogen for 96 h.
Compared microstructure of NMC-GDC electrode at origin (Figure S5b) and after 8 cycles of
redox (Figure S5c & S5d), the infiltrated particles did have certain coarsen phenomenon. On
the other hand, the polarization resistance kept relative stable at later redox cycles, and it
indicated the coarsen was not an endless process, since the nano-sized nickel particles would
dissolve into the spinel lattice completely to form single phase NMC and new nickel particles
would generate again in wet hydrogen. What’s more, NMC nano-sized particles planted by
infiltration processes instead of forming mechanical mixed composite electrode could avoid
and/or impress the influence of volume change of NMC during redox on the stability of
electrode structure. In wet hydrogen, the arcs corresponded to low frequencies zone shows an
apparent difference with the original spectra, this might be due to the lower redox
temperature limited the exsolution of nickel particles compared with original spectra (Figure
3b) reduced at 800 °C.
Figure S6 (a) EIS at open circuit of symmetrical cell in 30 mol.% CO-CO2 mix gases and (b)
the V-j curves and V-P curves of symmetrical cellfueled with 30 mol.% CO-CO2 in fuel
electrode at 700-850 °C.
Figure S7 The EIS of NMC symmetrical cell at SOEC mode under different voltage (vs. open
circuit voltage): (a) 700 °C, (b) 750 °C, (c) 800 °C, and (d) 850 °C. The open circuit voltage
was 0.087, 0.088, 0.091, and 0.096 V at 700, 750, 800, and 850 °C respectively.
Figure S8 Changing of calculated open circuit potential between cathode and anode (CO(g) +
0.5O2(g) = CO2(g)) against the CO content in the cathode. Assuming an oxygen partial pressure
of 0.21 in anode side in this calculation.
The data used to calculate the open circuit potential.
T (°C) T (K) ΔG (kJ mol-1) Equilibrium constant (K0)
700 973 -197.901 4.201E+010
750 1023 -193.549 7.621E+009
800 1073 -189.206 1.623E+009
850 1123 -184.873 3.969E+008
𝐸′ = −𝑅𝑇
𝑧𝐹ln
𝐾′
𝐾0
𝐾′ =𝑝𝐶𝑂2
𝑝𝐶𝑂 ∗ (𝑝𝑂2)0.5
Here, R is ideal gas constant (8.314 J K−1 mol−1), F is the Faradaic constant (96485 C mol−1),
z is the number of electrons transferred in the reaction (2).
Figure S9 Assuming a faradaic efficiency of 100% and an oxygen partial pressure of 0.21 in
anode, the calculated open circuit voltage between cathode and anode (CO(g) + 0.5O2(g) =
CO2(g)) by adopting the tested V-j curve at 800 °C (blue curve).
If the faradaic efficiency was 100%, CO would generate in cathode and the open circuit
potential between cathode and anode was even higher than the applied voltage, which could
not be true. The cross point of calculated potential curve and applied voltage curve was 0.742
V, indicating the faradaic efficiency was much lower than 100% at an applied voltage lower
than 0.742 V.
Detailed calculation method:
𝑝𝐶𝑂 =𝐹𝐶𝑂
𝐹𝐶𝑂2, 𝑖𝑛𝑙𝑒𝑡
𝐹𝐶𝑂 =𝑗 ∗ ɳ ∗ 𝑆 ∗ V𝑖𝑑𝑒𝑎𝑙
𝑧 ∗ 𝑁𝐴
Here, 𝐹𝐶𝑂2, 𝑖𝑛𝑙𝑒𝑡 is the inlet flow rate of CO2 to the cathode ( 20 ml min-1); 𝑗 is the current
density varies with the applied voltage (V-j curve); ɳ is the faradaic efficiency of CO,
assuming 100% in this calculation; 𝑆 is the active area of electrode (0.2 cm2); V𝑖𝑑𝑒𝑎𝑙 is the
ideal gas volume at room temperature (24.0 L mol-1 at 20 °C); 𝑧 is the number of electrons
transferred to generate one CO molecular; 𝑁𝐴 is the Avogadro constant (6.022 × 1023 mol-1).
𝐾′ =𝑝𝐶𝑂2
𝑝𝐶𝑂 ∗ (𝑝𝑂2)0.5
𝐸′ = −𝑅𝑇
𝑧𝐹ln
𝐾′
𝐾0
Fig. S10 In the 3-unit cells model, A-site Ni atoms are favored to locate at the neighbor (1 1
0) faces (a), the B-site Ni atoms are favored to locate at different (1 1 0) faces (b), the
assumed Ni atoms distribution (c)
Spinel structure oxides are formulated as AB2O4, where A and B are usually tetrahedrally and
octahedrally coordinated cations with a valence of +2 and +3 respectively. In normal spinel
structure, such as MgAl2O4, A-site cations fill the 1/8 of the tetrahedral holes and B-site
cations fill half of the octahedral holes; in some situation, A-site cations and B-site cations
would replace their positions and thus form the reverse spinel structure. Since 1/3 atoms in
both A site and B site were taken by Nickel, 3-unit cells including 16 Mn atoms, 32 Cr atoms,
24Ni atoms, and 96 O atoms were considered in the DFT calculation. Ni atoms are suspected
to uniformly distribute in both A sites and B sites from a large scale, which would lead to
humorous workloads. According to the calculation, in the 3-unit cells model, A-site Ni atoms
are favored to locate at the neighbor (1 1 0) faces, while the B-site Ni atoms are favored to
locate at different (1 1 0) faces, as shown by the Figure S10a & S10b. Basing on these results,
the Ni atoms distribution is assumed as Figure S10c. Different distribution conditions of nickel atoms only
have small effects on the value of G, the influenced value is smaller than 2 KJ mol-1.
Table S1 A-site and B-site elements and its ionic radius of NMC spinel before and after
reduction.
Table S2 Rietveld refined XRD results.
Stages of NMC material
Spinel phase Nickel
Space Group a = b = c (nm) Space group a = b = c (nm)
NMC Fd-3m 0.835
NMC 1st reduction Fd-3m 0.842 Fm-3m 0.352
NMC 1st re-oxidation Fd-3m 0.836
NMC 10th reduction Fd-3m 0.841 Fm-3m 0.352
NMC 10th re-oxidation Fd-3m 0.836
NMC reduced in 5 vol. % CO Fd-3m 0.842 Fm-3m 0.353
Reduced NMC re-oxidized in
CO2
Fd-3m 0.837
Synthesized NMC NMC after reduction
Site Element Coordination
Ionic radius
(Å)
Element Coordination
Ionic radius
(Å)
A Site
Ni 4 o.69
Mn 4 0.8
Mn 4 0.8
B Site
Ni 6 0.69 Ni (residual) 6 0.69
Cr 6 0.755 Cr 6 0.755
Table S3 Detailed results of Ni-YSZ anode supported cells at 650-800 °C
Table S4 Detailed results of the symmetrical cells fueled with wet hydrogen in anode side at
650-850 °C
Temperature (°C) Peak power density (W·cm-2)
Resistance (Ω·cm-2)
Ohmic Polarization
650 0.106 1.916 1.801
700 0.157 1.023 1.416
750 0.269 0.679 0.587
800 0.416 0.510 0.292
850 0.573 0.374 0.202
Temperature (°C) Peak power density (W·cm-2)
Resistance (Ω·cm-2)
ohmic polarization
650 0.391 0.181 0.142
700 0.752 0.112 0.105
750 0.981 0.093 0.096
800 1.293 0.077 0.098
Table S5 Comparison of peak powder density of multi reported materials.
Material
Composite or
not
Support
Peak power
density (mW cm-2)
Ref.
NMC
Composite
with GDC
YSZ support
(200 μm)
416,
at 800 °C This
work Anode support
(YSZ)
1293,
at 800 °C
MnCo2O4 No
Anode support
(YSZ)
160,
at 800 °C
[6]
Cu0.5MnCo1.5O4 No
Anode support
(YSZ)
506,
at 800 °C
[7]
Mn1.5Co1.5O4
Composite
with GDC
Anode support
(YSZ)
912,
at 800 °C
[8]
Cu1.4Mn1.6O4 No
Anode support
(ScSZ)
809,
at 800 °C
[8]
CuCo2O4 No
Anode support
(SSZ)
970,
at 800 °C
[9]
CuCo2O4 GDC
Anode support
(SSZ)
1074,
at 800 °C
[10]
Co1.5Mn1.5O4 SDC
Anode support
(YSZ)
859,
at 800 °C
[11]
CuFe2O4 No
LSGM support
(300 μm), Ni-
SDC as anode
326,
at 800 °C
[12]
CoFe2O4 No
LSGM support
(300 μm), Ni-
SDC as anode)
293,
at 800 °C
[12]
NiFe2O4 No
LSGM support
(300 μm), Ni-
SDC as anode)
277,
at 800 °C
[12]
Pd infiltrated
LSM-YSZ
Anode support
(YSZ)
1054,
at 800 °C
[13]
LSCF GDC-YSZ
Anode support
(YSZ)
1240,
at 800 °C
[14]
Table S6 Detailed results of symmetrical cells at 700-850 °C fueled with 30 mol.% CO-CO2
Temperature (°C)
Peak power density
(W·cm-2)
Resistance (Ω·cm-2)
Ohmic Polarization
700 0.072 1.951 2.775
750 0.148 1.043 1.207
800 0.250 0.662 0.642
850 0.376 0.469 0.335
Table S7 A summarization of previously reported results related with CO2 electrolysis.
Material
Anode
material /
structure
Support
Current density
(mW cm-2) @
applied voltage
Atmospher
e
Ref.
NMC-GDC
Symmetri
cal
YSZ support
(200 μm)
2.320 @ 2 V
Pure CO2
at 850 °C
This
wor
k 1.739 @ 2V
Pure CO2
800 °C
La0.3Sr0.7Fe0.7Ti0.3O3
Symmetri
cal
YSZ support
(400 μm)
0.521 @ 2V
Pure CO2
800 °C
[15]
(La0.75Sr0.25)0.9(Cr0.5Mn0.
5)0.9Ni0.1O3-δ
Symmetri
cal
YSZ support
(1 mm)
0.38 @ 2 V
Pure CO2
800 °C
[16]
La0.75Sr0.25Mn0.5Cr0.5O3−
δ
Composited with SDC
Symmetri
cal
YSZ support
(2 mm)
0.18 @ 2 V
Pure CO2
800 °C
[17]
La0.75Sr0.25Mn0.5Cr0.5O3−
δ
Composited with SDC
and Fe added
Symmetri
cal
YSZ support
(2 mm)
0.38 @ 2 V
Pure CO2
800 °C
[18]
La0.6Sr0.4Fe0.8Ni0.2O3-δ
Composited with GDC
Symmetri
cal
YSZ support
(400 μm)
1.52 @ 2V
Pure CO2
850 °C
[19]
La0.75Sr0.25Cr0.5Mn0.5O3−
δ
Symmetri
cal
Yb0.6Sc0.4SZ
support (160
μm)
0.7 @ 1.7 V
Pure CO2
at 850 °C
[20]
La0.65Sr0.3Ce0.05Cr0.5Fe0.
5O3−δ
Composited with GDC
LSCF-
GDC
YSZ support
(300 μm)
1.283 @ 2.071
V
30 mol.%
CO-CO2
at 850 °C
[21]
La0.3Sr0.7Ti0.3Fe0.7O3-δ
Infiltrated on ScSZ
LSM-
ScSZ
ScSZ support
(70 μm)
3.40 @ 2 V
30 mol.%
CO-CO2
at 800 °C
[22]
La0.5Sr0.5Fe0.95V0.05O3-δ
Composited with YSZ
LSM-
YSZ
YSZ support
(0.5 mm)
~ 0.8 @ 1.8 V
Pure CO2
at 800 °C
[23]
Sr1.9Fe1.5Mo0.4Ni0.1O6−δ
Composited with SDC
LSM-
YSZ
YSZ support
(180 μm)
2.16 @ 1.5 V
Pure CO2
800 °C
[24]
La0.6Sr0.4Fe0.8Ni0.2O3-δ
Composited with GDC
LSCF-
GDC
YSZ support
(220 μm)
1.78 @ 1.6 V
30 mol.%
CO-CO2
at 850 °C
[25]
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