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: lijian@hust.edu.cn (Dr. J. Li), jingli.luo@ualberta.ca (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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