Electronic Supplementary Information

Local Spin-State Tuning of Cobalt-Iron Selenide Nanoframes for Boosted Oxygen

Evolution

Jun-Ye Zhang, a,+ Ya Yan, b,+ Bingbao Mei, c,+ Ruijuan Qi, d Ting He, a Zhitong Wang, a Wensheng Fang, a

Shahid Zaman, a Yaqiong Su, e* Shujiang Ding, e* and Bao Yu Xia a*

a Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Key

Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering,

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037

Luoyu Road, Wuhan 430074, PR China

Corresponding author: byxia@hust.edu.cn (B. Y. Xia)

b School of Materials Science and Engineering, University of Shanghai for Science and Technology, 516

Jungong Road, Shanghai 200093, PR China

c Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of

Sciences, Shanghai 201204, PR China

d Key Laboratory of Polar Materials and Devices (MOE), Department of Electronics, East China Normal

University, 500 Dongchuan Road, Shanghai 200241, PR China

e School of Chemistry, Xi'an Key Laboratory of Sustainable Energy Materials Chemistry, MOE Key

Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory of

Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an 710049, PR China

Corresponding author: yqsu1989@xjtu.edu.cn (Y. Su); dingsj@mail.xjtu.edu.cn (S.Ding)

+ These authors contribute equally to this work.

Electronic Supplementary Material (ESI) for Energy & Environmental Science.This journal is © The Royal Society of Chemistry 2020

mailto:dingsj@mail.xjtu.edu.cn

Experimental section

1. Material synthesis.

Cobalt nitrate hexahydrate, sodium citrate, potassium ferricyanide, selenium powder and RuO2 were

purchased from Aladdin company. Potassium hydroxide from J. T. Baker and Nafion 5% in lower aliphatic

alcohols and water was purchased from Sigma-Aldrich. All chemical reagents were directly used without

further purification.

2. Precursor preparation.

In a typical procedure, 2.4 mmol of cobalt nitrate hexahydrate and 5.4 mmol of sodium citrate were dissolved

in 80 mL of DI water to form solution A. 1.6 mmol of potassium ferricyanide was dissolved in 80 mL of DI

water to form solution B. Next, the blend solutions A and B under magnetic stirring for 5 min. The obtained

homogeneous solution was aged for 24 h at room temperature. After the centrifugation and washing several

times by water and ethanol, the Fe0.4Co0.6 precursor was collected and dried at 60℃ overnight. For the

Fe0.6Co0.4 precursor, potassium ferricyanide and cobalt nitrate hexahydrate were replaced by potassium

hexacyanocobaltate and ferrous chloride hexahydrate respectively, and other procedure was the same. For

Fe0.2Co0.8 precursor, 0.5 mmol cobalt nitrate hexahydrate, 2 mmol ferric nitrate hexahydrate, 25 mmol urea,

and 10 mmol ammonium fluoride were dissolved in 40 ml of DI water, and subsequently hydrothermally

treated at 120 ℃ for 16 hours, and other procedure was the same. For Co-Co precursor, only the potassium

ferricyanide was replaced by potassium hexacyanocobaltate, and other procedure was the same.

3. Fe0.4Co0.6Se2 nanoframe preparation.

20 mg precursor and 300 mg selenium powder were both set in a porcelain boat. The selenium precursor was

put at upstream position of a tube furnace. The heating rate was 2 °C min−1 with 100 sccm Ar gas until the

annealing temperature reached 350 °C. After 2 hours annealing treatments, the samples were collected and

washed by water/ethanol.

4. Material Characterization.

Scanning electron microscopy (SEM) images were obtained from JEOL JSM-7100F. Transmission electron

microscopy (TEM) test was carried on TecnaiG2 20 (Philips) with 200 kV accelerating voltage and scanning

EDS spectrum. XRD measurement was performed on Empyrean (PANalytical B.V. with Cu-Kα radiation).

The Raman spectra were obtained from LabRAM HR800. Inductively coupled plasma optical emission

spectrometry (ICP-OES) was tested on Agilent ICPOES730. The X-ray photoelectron spectroscopy (XPS)

data was implemented on a Kratos AXIS Ultra DLD-600W XPS system with a monochromatic Al Kα

(1486.6 eV) X-ray source. The Electron paramagnetic resonance (EPR) data was acquired on the Bruker

EMXmicro. The XAFS measurements at Co K-edge in both transmission mode was performed at the BL14W1

in Shanghai Synchrotron Radiation Facility (SSRF). The electron beam energy was 3.5 GeV and the stored

current was 230 mA (top-up). A 38-pole wiggler with the maximum magnetic field of 1.2 T inserted in the

straight section of the storage ring was used. XAFS data were collected using a fixed-exit double-crystal Si

(111) monochromator. The energy was calibrated using Co foil. The photon flux at the sample position was

2.6×1012 photons per second. The raw data analysis was performed using IFEFFIT software package

according to the standard data analysis procedures. The spectra were calibrated, averaged, pre-edge

background subtracted, and post-edge normalized using Athena program in IFEFFIT software package. The

Fourier transformation of the k3-weighted EXAFS oscillations, k3·χ(k), from k space to R space was

performed to obtain a radial distribution function. Data fitting was completed by Artemis program in IFEFFIT.

5. Electrochemical test.

All electrochemical experiments were performed on AUTOLAB 302N electrochemistry workstation at 25

oC. In three-electrode system, glassy carbon (RDE, 5 mm diameter), graphite rod and calibrated Hg/HgO,

was used as work electrode, counter electrode and reference electrode, respectively. The following equation

was used for conversion versus RHE: E (RHE) = E (Hg/HgO) +0.098 V +0.059×pH. Rotating disk electrode

was operated at 1600 rpm. The scan rate of linear sweep voltammetry (LSV) was 5 mV s-1. The LSV curves

were iR compensated. The mass loading of catalyst is 0.5 mg cm-2. Electrochemical impedance spectroscopy

(EIS) was collected at 1.53 V versus RHE from 0.01 Hz to 100 KHz. Chronopotentiometry was operated at

a constant current density of 10 mA cm-2. For The scan rates of electrochemically active surface area (ECSA)

measurements were 20, 40, 60, 80, 100 mV s-1. Turnover frequency (TOF) calculation is based on the

equation: , where the j is the current density at overpotential of 0.3 V, the A is the TOF =

𝑗 * A

4 * m * F

electrode area, the F is the faraday constant equal to 96485 C/mol, the m is the Co site mole number.

5. Computational details.

Spin-polarized calculations within the density-functional theory framework were carried out as implemented

in the Vienna ab initio simulation package (VASP).1 The ion-electron interactions were represented by the

projector-augmented wave (PAW) method2 and the electron exchange-correlation by the generalized gradient

approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.3 The Kohn-

Sham valence states were expanded in a plane-wave basis set with a cut-off energy of 400 eV. The DFT+U

approach was used, in which U is a Hubbard-like term describing the on-site Coulombic interactions4. For Co

and Fe, a value of U = 4.0 eV was adopted. For Fe-doped CoSe2 bulk, we substitute Fe atoms for 50% Co

atoms of CoSe2 bulk, named Co0.5Fe0.5Se2. The optimzied lattice parameter of CoSe2 and Co0.5Fe0.5Se2 bulk

is 5.84 and 5.81 Å. For the Brillouin zone integration, a 11×11×11 Monkhorst-Pack mesh was used. The

geometry optimization was performed when the convergence criterion on forces became smaller than 0.02

eV/Å and the energy difference was

Figure S1. SEM images of Co-Co PBAs precursors (a and b) and CoSe2 nanoframes (c and d).

Figure S2. N2 sorption isotherms (a) and pore size distribution curves (b) of Fe0.4Co0.6Se2 and precursors.

Figure S3. EDS profile of Fe0.4Co0.6Se2 sample (Selected area: Figure 1g).

Figure S4. XRD pattern of precursor sample.

Figure S5. XRD pattern of CoSe2 sample.

Figure S6. (a) XPS full survey, and (b) Se 3d spectra of CoSe2, Co0.2Fe0.8Se2, Co0.4Fe0.6Se2 and Fe0.6Co0.4Se2

samples.

Figure S7. (a) XPS Co 2p and (b) Fe 2p spectra of precursor sample.

Figure S8. EXAFS fitting curves of Fe0.4Co0.6Se2 sample in R space.

Figure S9. LSV curves of Fe0.4Co0.6Se2 loaded on the nickel foam, carbon cloth and carbon paper.

Figure S10. Overpotentials at 10 mA cm-2 for different Fe doping electrocatalysts.

Figure S11. LSV curves of Fe0.4Co0.6Se2 using different scan rates to verify the mass transfer rate.

Figure S12. CV curves of (a) CoSe2, (b) Co0.2Fe0.8Se2, (c) Co0.4Fe0.6Se2 and (d) Co0.6Fe0.4Se2 at different

scanning rates.

Figure S13. Capacitance-normalized LSV curves to reflect the intrinsic OER activity (1 mF cm-2).

Figure S14. The specific Co mass activity at a 300 mV overpotential.

Figure S15. OER turnover frequency (TOF) of Co site at a 300 mV overpotential.

Figure S16. The cell LSV curves of Pt/C-Fe0.4Co0.6Se2 and Pt/C-RuO2 couples.

Figure S17. XRD pattern of Fe0.4Co0.6Se2 after OER test.

Figure S18. HRTEM images of Fe0.4Co0.6Se2 after OER test.

Figure S19. EDS profile of Fe0.4Co0.6Se2 after OER test (Selected area: Figure 4d).

Figure S20. Raman spectra of CoSe2, Co0.2Fe0.8Se2, Co0.4Fe0.6Se2 and Fe0.6Co0.4Se2 samples acquired before

(a) and after (b) OER test.

Table S1. Fitted EXAFS results for CoSe2 and Fe0.4Co0.6Se2.

Sample Scattering Pair CN R(Å) σ2 (10-3Å2)

E (eV)

R-factor

CoSe2 Co-Se 6 2.44 -- -- --

Co-Se 5.5 2.46 5.3 10.0Fe0.4Co0.6Se2

Co-Fe 3.4 2.62 3.0 -101.21%

Table S2. Activity summary of recent cobalt-based catalysts.

Catalysts Electrolyte Overpotential

(η10) (mV)

Tafel slope (mV dec-1)

Reference

Fe0.4Co0.6Se2 nanoframes

1.0 M KOH 270 36 This work

Mn-Co oxyphosphide 1.0 M KOH 320 52 Angew. Chem. Int. Ed. 2017, 56, 2386

Co/VN 1.0 M KOH 320 55 Nano Energy 2017, 34, 1A-CoS4.6O0.6-PNCs 1.0 M KOH 290 67 Angew. Chem., Int. Ed.

2017, 56,4858Ni0.6Co1.4(OH)2 1.0 M KOH 300 80 Adv. Funct. Mater. 2018,

28, 1706008A-CoS4.6O0.6-PNCs 1.0 M KOH 290 67 Angew. Chem. Int. Ed.

2017, 56, 4858N-CoFe LDHs 1.0 M KOH 281 40.03 Adv. Funct. Mater. 2018,

28, 1703363CoOx 1.0 M KOH 306 67 Nano Energy 2018, 43, 110

CoOx 0.1 M KOH 370 76 Adv. Mater. 2019, 1807468Zn0.35Co0.65O 1.0 M KOH 322 42 Adv. Energy Mater. 2019,

9, 1900328Co-Fe−N−C 1.0 M KOH 321 40 J. Am. Chem. Soc. 2019,

141, 14190Fe3C-Co/NC 1.0 M KOH 340 - Adv. Funct. Mater. 2019,

29, 1901949PHI-Co 1.0 M KOH 324 44 Adv. Mater. 2020, 32,

1903942Fe-Co3O4 1.0 M KOH 262 43 Adv. Mater. 2020, 32,

2002235NiO/Co3O4 1.0 M KOH 262 58 ACS Catal. 2020, 10,

12376Ba4Sr4(Co0.8Fe0.2)4O15 0.1 M KOH 340 47 Adv. Mater. 2020, 32,

1905025.LaCo1-xNixO3-δ 1.0 M KOH 330 62 Angew. Chem., Int. Ed.

2020, 59, 19691

Table S3. EDS results of elements content for Fe0.4Co0.6Se2 before and after OER test.

Fe (At %) Co (At %) Se (At %)

Fe0.4Co0.6Se2 before test 13.4 19.2 67.4

Fe0.4Co0.6Se2 after test 36.0 54.0 10.1

Table S4. Surface Se and O contents from XPS characterizations before and after OER test.

Se (At %) O (At %)

The Fe0.4Co0.6Se2 before test 11.93 16.17

The Fe0.4Co0.6Se2 after test 7.87 52.43