Supporting Information

Supporting Information

Electronic optimization by coupling FeCo

nanoclusters and Pt nanoparticles in carbon

nanotubes for efficient hydrogen evolution

Fengcai Lei,

†,§,‡

_{Zhao Tang,}

†,‡

_{Wenli Xu,}

†

_{Jing Yu,}

#

_{Kun Li,}

†

_{and Jimmy C. Yu,}

§,*

†

_{College of Chemistry, Chemical Engineering and Materials Science, Institute of}

Biomedical Sciences, Shandong Normal University, Jinan, 250014, P.R. China.

§

_{Department of Chemistry, The Chinese University of Hong Kong, Shatin, New}

Territories 999077, Hong Kong SAR, China. E-mail:

[email protected]

#

_{School of Physics and Electronics, Institute of Materials and Clean Energy, Shandong}

Normal University, Jinan 250014, P.R. China

Number of pages: 15

Number of tables: 2

Number of figures: 14

Experimental details

Chemicals

Melamine (99%), Nafion (5 wt%), iron(ii) chloride tetrahydrate (reagent grade, 99.99 %) were purchased from Sigma-Aldrich. Cobalt(ii) chloride hexahydrate, urea, hydrazine hydrate aqueous solution (85%), sulfuric acid, chloroplatinic acid hexahydrate, N-doped carbon nanotube and perchloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. Benchmark 20 wt% Pt/C (commercial) catalyst was purchased from Johnson Matthey. All the chemicals were of analytical grade and used as received without further purification, unless otherwise specified.

Synthesis of FeCo@NCNT

In a typical synthesis, 0.12 g (0.6 mmol) FeCl2·4H2O, 0.12 g (0.5

mmol) CoCl2·6H2O (for weight ratio is Fe:Co=1:1), 2.4 mmol melamine, 2.5 mmol urea were

ground and mixed at room temperature, then put in a porcelain boat and heated at 5 ℃/min to 800℃ under nitrogen atmosphere for two hours. The other weight ratios of Fe salts and Co salts (2:1, 1:2, 0:1 and 1:0) were synthesized by the same method except for the weight ratios were 0.16:0.08 g, 0.08:0.16 g, 0:0.24 g, 0.24:0 g. Finally, the as-obtained FeCo@NCNT was washed with 0.5 M H2SO4, deionized water and ethanol at room temperature several times, then put it in a vacuum

drying oven at 40℃ for drying.

Synthesis of FeCo nanoplate

The FeCo nanoplate was synthesized according to previous work.1 _{In a typical experiment, 2.6 mmol FeCl}

2·4H2O and 2.7 mmol CoCl2·6H2O were dissolved

in 50 ml water under argon atmosphere, then a mixture composed of NH2–NH2·H2O (85 wt%, 10

ml) and NaOH (0.06 mol) was added. After a reaction of 0.5 h at 66℃, the supernatant was discarded by centrifugation. The products were washed with water and ethanol and then dried in vacuum at room temperature for further study.

Electrochemical Pt deposition

The Pt loadings on the synthesized FeCo@NCNT, NCNT, FeCo nanoplate were carried out with a three-electrode system using an electrochemical station (CHI660E). Glassy carbon electrode (GCE, geometric area of 0.07065 cm2_{), graphite rod and}

saturated calomel electrode (SCE) served as the working electrode, counter electrode and reference electrode, respectively. To prepare working electrode, 5 mg of each sample and40 µL of 5 wt% Nafion solution were dispersed in 1 mL ethanol solvent with half an hour of sonication to form a homogeneous ink, and then 5 µL of the above catalyst solution was dropped onto the GCE. During the deposition process, 10 mL of a chloroplatinic acid aqueous solution (1 mg mL–1_{) was poured}

into 60 mL of a 0.1 M HClO4 electrolyte to serve the platinum source. The deposition process was

carried out by 3 cyclic voltammetry cycles under a bias potential from +0.3 to -0.3 V (vs the reversible hydrogen electrode (RHE) at a scan rate of 100 mV s–1_{).Then the working electrodes of}

Pt-loaded catalysts have been prepared successfully and used for further electrochemical measurements.

Characterization

X-ray diffraction (XRD) was performed on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation (λ = 1.54178 Å). The scanning electron microscopy (SEM) images were taken on a JEOL JSM-6700F SEM. The transmission electron microscopy (TEM) was carried out on a JEM-2100F field emission electron microscope at an acceleration voltage of 200 kV. The content of metals for the samples were determined by inductively coupled plasma optical emission spectrum (ICP-OES) on a Perkin Elmer Optima 7300DV ICP emission spectroscope. X-ray photoelectron spectroscopy (XPS) analyses were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an excitation source of Mg Kα = 1253.6 eV, and the resolution level was lower than 1 atom%. Raman spectra were performed on a LabRAM HR Evolution Raman spectrometer, the laser excitation was 532 nm, and the exposure time was 2 s. The H2 content was measured by gas

chromatography (Agilent Technologies 7890A Gas Chromatograph)

Electrochemical measurements

All the electrochemical measurements were performed under the same condition as the Pt loading process except for that 0.5 M H₂SO₄ solution was used as electrolyte instead of mixed solution of chloroplatinic acid and HClO4 electrolyte. All potentials were calibrated to a reversible hydrogen

electrode (RHE, Figure S8). For catalysts of FeCo@NCNT and commercial Pt/C, 5 mg of catalyst and 40 μL Nafion solution (Sigma Aldrich, 5 wt%) were dispersed in 1 mL ethanol by sonicating for at least 30 min to form a homogeneous ink. Then 5 μL of the dispersion (containing 25 μg of catalyst) was loaded onto a glassy carbon electrode with 3 mm diameter, leading to a catalyst loading of 0.357 mg cm-2_{. The as-prepared catalyst film was allowed to be dried at room temperature. The}

linear sweep voltammetry (LSV) with a scan rate of 5 mV s-1 _{were conducted in 0.5 M H} 2SO4

solution. The electrochemical impedance spectroscopy (EIS) measurements were operated at an overpotential of 26 mV vs. RHE from 1-105 _{Hz with a 5 mV perturbation amplitude. The ECSA}

was estimated by the electrochemical double layer capacitance (Cdl) of the electrode surface by CV

at a potential range from 0.1-0.2 V vs RHE. Plots shown in Figure 3D were collected by difference of anodic current and cathodic current as a function of scan rate.

TOF calculations

The TOF for Pt-based catalysts was estimated by the following equation:2

TOF =Total hydrogen turnovers cm ―2 Numbers of Pt active sites

In this equation, the total hydrogen turnovers can be calculated from linear sweep voltammetry curve as follows:

Total hydrogen trunovers = |𝑗| mA cm―2 ₁

₍

_{C s}―1 ₁₀―3 _mA―1

₎

_{1 mol e}― _{(96485 C)}―1

(

1 mol

2 mol e―

)

(

6.022 × 1023 molecules H2

1 mol H2

)

The numbers of Pt active sites can be calculated by the Pt content of each catalysts and the Pt atomic weight:

N umbers of Pt sites =

catalysts loading on each electrode

(

g

cm2

)

× Pt wt% 195.1

(

g mol)

DFT calculations

The DFT calculations were calculated by Vienna Ab-inito Simulation Package (VASP). The Perdew–Burke–Ernzerhof functional for the exchange correlation term was used with the projector augmented wave method and a cutoff energy of 500 eV was used to achieve the accurate density of the electronic states. Ionic relaxations were carried out under the conventional energy (10-4 _{eV) and}

force (0.02 eV/Å) convergence criteria. The model of carbon nanotube (CNT) in this work was shown in Fig. S6, which is built by integrating a CNT (6, 6) in a 20×20×9.8 Å supercell. A 1×1×5 Monkhorst-Pace k-point mesh was used in the calculations. As the CNT are prepared from melamine, pyridinic N doped is expected to be the dominant species of N in the sample. A structure of C92N2 is built to mimic N doped CNT. The Pt and FeCo nanocluster were anchored on the surface

and inside the NCNT, respectively. Though the metal element structure built in the calculations are smaller than that exhibited in experiment, the essential effect can still be demonstrated entirely by the simple geometry applied here.3 _{The differential charge density ( ) was also calculated to ∆𝜌}

explore the electron transition (Fig. 4B), ∆𝜌 is defined as ∆𝜌 = 𝜌

(

Pt/FeCo@NCNT

)

―𝜌 .

(

Pt/NCNT

)

―𝜌(FeCo)

The Gibbs free energy (∆GH∗) has been calculated as follows: ∆G = ∆𝐸 + ∆𝐸𝑍𝑃𝐸― 𝑇∆𝑆

where ∆𝐸 is the binding energy of H calculated by DFT, ∆𝐸𝑍𝑃𝐸 represents the different zero point energy between the adsorbed H and the gas state, and ∆𝑆 is the difference of entropy change of adsorbed H and the gas hydrogen. The value of 𝑇∆𝑆 was 0.20 eV according to the work by Norskov.4 _{∆𝐸𝑍𝑃𝐸 was calculated by the formula of 𝐸𝑍𝑃𝐸( ∗ 𝐻) ―1/2𝐸𝑍𝑃𝐸(𝐻2). From ref. 4, the}

value of 𝐸𝑍𝑃𝐸(𝐻2) was about 0.27 eV. Finally, the ∆𝐸𝑍𝑃𝐸 was calculated to be 0.17 eV for NCNT and 0.04 eV for Pt(111).

Supplementary figures

10 20 30 40 50 60 70 80 JCPDS No. 49-1567 In te n si ty ( a. u .) 2 (degree) Pt/FeCo@NCNT

Figure S1. X-ray diffraction (XRD) of Pt/FeCo@NCNT. One can clearly observe that the appearance of two peaks (at 26º and 44º), which can be assigned to graphite and FeCo alloy.

Figure S2. HRTEM of Pt/FeCo@NCNT, the lattice fringes in orange circles indicate graphite carbon and FeCo nanoclusters marked by yellow circles.

Figure S3. XPS spectra of Pt/FeCo@NCNT. The core level spectra of (A) C 1s, (B) Pt 4f, (C) Fe 2p, (D) Co 2p, (E) N 1s.

For further to analyze the chemical composition of as-prepared PtFeCo@NCNT, X-ray photoelectron spectroscopy (XPS) was carried out. As shown in Fig. S3A, the high-resolution spectra of C can be deconvoluted into six peaks. The main peak centered at 284.6 eV is attributed to graphitic carbon, Peaks at 285.4, 286.4 and 288.8 eV are assigned to C–O, C≡N bonds and C– (N)3 bonds (originated from g-C3N4) respectively.5 The remained two characteristic peaks of 291.6

and 292.2 eV is originated from C–F and C–F2 derived from Nafion which is used in process of

electrochemical deposition of Pt. The N 1s core level spectrum shown in Fig. S3E can be deconvoluted into two peaks. The peaks located at 399.6 eV can be assigned to pyridinic-N and the 401.3 eV peak is attributed to graphitic-N.6

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 -60 -40 -20 0 20 C u rr en t d en si ty ( m A c m -2 ) Potential (V

vs

. RHE) FeCo@NCNT-1 FeCo@NCNT-2 FeCo@NCNT-3 FeCo@NCNT-4 FeCo@NCNT-5

Figure S4. LSV curves based on FeCo@NCNT of different FeCo weight ratios.

Figure S5. TEM images of (A) Pt/FeCo nanoplate, (B) Pt/NCNT.

Figure S6. Exchange current density of different catalysts. (A) Calculated exchange current density derived from the Tafel plots by an extrapolation method. (B) The exchange current values of Pt/FeCo@NCNT, Pt/C, Pt/NCNT, Pt/FeCo nanoplate, FeCo@NCNT are 2.57, 1.47, 0.63, 0.31, 0.13

-0.2 -0.1 0.0 0.01 0.1 1 10 100 T O F ( s -1) Potential (V vs. RHE) Pt/FeCo@NCNT Pt/NCNT Pt/FeCo nanoplate Pt/C

Figure S7. Turn over frequencies of Pt/FeCo@NCNT, Pt/NCNT, Pt/FeCo nanoplate and commercial Pt/C.

Figure S8. Calibration of reference electrode. The reversible hydrogen electrode (RHE) calibration was performed in H2 saturated 0.5 M H2SO4 with a Pt foil as the work electrode and SCE as

Figure S9. The HER polarization curves of Pt/FeCo@NCNT tested repeatedly for different batch samples. 0 1000 2000 3000 0.0 0.5 1.0 1.5 2.0 H2 (1 0 -4 m o l) Time (s)

Figure S10. The amount of H2 theoretically calculated (black line) and experimentally measured

(purple balls) at -0.26 V vs SCE for 3600 s. From the results one can see the experimental H2

production is close to the theoretical value, indicating nearly 100% faradaic efficiency can be achieved.

Figure S11. TEM of Pt/FeCo@NCNT after 3000 cycles of CV, the Pt are shown in dotted line circles, which indicated no aggregation after long-time catalysis.

Figure S13. Top view of the calculated structures of (A) Pt/NCNT, (B) Pt/FeCo@NCNT, (C) Pt(111) and adsorbed H atoms on surface of each structures (D) H-Pt/NCNT, (E) H-Pt/FeCo@NCNT, (F) H-Pt(111).

Figure S14. Side view of the calculated structure of H-Pt/FeCo@NCNT, the same as the structure in Figure 4B.

Table S1 Content analysis of Pt/FeCo@NCNT by ICP-OES Catalyst Fe (wt%) Co (wt%) Pt (wt%) Pt/FeCo@NCNT 3.8 2.7 0.30 Pt/FeCo@NCNT after 3000 CV 3.0 2.5 0.27

Table S2 Comparison of the HER performance between Pt-based catalysts. Pt Loading

Catalysts Overpotential at 10

mA cm-2 _(mV) µg

cm-2 wt%

Reference

Pt/FeCo@NCNT 14 1.1 0.30 This work

Pt@DNA 26 15 -- Reference7 Mo2C@NC@Pt 27 -- 7.49 Reference 8 Pt−MoO2/MWC NTs 60 -- 0.5 Reference 9 Pt1/N-C 19 -- 2.5 Reference10 HCS-O-Pt 14.4 1.7 0.05 Reference11 Pt/MoS2/CC 18 -- 1.26 Reference12 AL-Pt/Pd3Pb 13.8 1.6 -- Reference13 Pt/np-Co0.85Se 58 -- 1.03 Reference14 PtSA-NT-NF 30 -- 1.76 Reference15 Pt1/OLC 38 0.27 Reference2

Reference

(1) Yang, Y.; Xu, C. L.; Xia, Y. X.; Wang, T.; Li, F. S. Synthesis and microwave absorption properties of FeCo nanoplates. J. Alloy Compd. 2010, 493, 549-552, DOI: 10.1016/j.jallcom.2009.12.153.

(2) Liu, D.; Li, X.; Chen, S.; Yan, H.; Wang, C.; Wu, C.; Haleem, Y. A.; Duan, S.; Lu, J.; Ge, B.; Ajayan, P. M.; Luo, Y.; Jiang, J.; Song, L. Atomically dispersed platinum supported on curved carbon supports for efficient electrocatalytic hydrogen evolution. Nat. Energy 2019, 4, 512-518, DOI: 10.1038/s41560-019-0402-6.

(3) Deng, J.; Ren, P.; Deng, D.; Yu, L.; Yang, F.; Bao, X. Highly active and durable non-precious-metal catalysts encapsulated in carbon nanotubes for hydrogen evolution reaction. Energy Environ. Sci. 2014,

7, 1919-1923, DOI: 10.1039/c4ee00370e.

(4) Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Norskov, J. K. Trends in the exchange current for hydrogen evolution. J. Electrochem. Soc. 2005, 152, J23-J26, DOI: 10.1149/1.1856988.

(5) Khabashesku, V. N.; Zimmerman, J. L.; Margrave, J. L. Powder synthesis and characterization of amorphous carbon nitride. Chem. Mater. 2000, 12, 3264-3270, DOI: 10.1021/cm000328r.

(6) Zhang, T. Y.; Han, X.; Yang, H. B.; Han, A. J.; Hu, E. Y.; Li, Y. P.; Yang, X. Q.; Wang, L.; Liu, J. F.; Liu, B. Atomically Dispersed Nickel(I) on an Alloy-Encapsulated Nitrogen-Doped Carbon Nanotube Array for High-Performance Electrochemical CO2 Reduction Reaction. Angew. Chem. Int. Ed. 2020, 59,

12055-12061, DOI: 10.1002/anie.202002984.

(7) Anantharaj, S.; Karthik, P. E.; Subramanian, B.; Kundu, S. Pt Nanoparticle Anchored Molecular Self-Assemblies of DNA: An Extremely Stable and Efficient HER Electrocatalyst with Ultralow Pt Content. ACS Catal. 2016, 6, 4660-4672, DOI: 10.1021/acscatal.6b00965.

(8) Chi, J. Q.; Xie, J. Y.; Zhang, W. W.; Dong, B.; Qin, J. F.; Zhang, X. Y.; Lin, J. H.; Chai, Y. M.; Liu, C. G. N-Doped Sandwich-Structured Mo2C@C@Pt Interface with Ultralow Pt Loading for pH-Universal

Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2019, 11, 4047-4056, DOI: 10.1021/acsami.8b20209.

(9) Xie, X.; Jiang, Y. F.; Yuan, C. Z.; Jiang, N.; Zhao, S. J.; Jia, L.; Xu, A. W. Ultralow Pt Loaded Molybdenum Dioxide/Carbon Nanotubes for Highly Efficient and Durable Hydrogen Evolution Reaction. J. Phys. Chem. C 2017, 121, 24979-24986, DOI: 10.1021/acs.jpcc.7b08283.

(10) Fang, S.; Zhu, X. R.; Liu, X. K.; Gu, J.; Liu, W.; Wang, D. H.; Zhang, W.; Lin, Y.; Lu, J. L.; Wei, S. Q.; Li, Y. F.; Yao, T. Uncovering near-free platinum single-atom dynamics during electrochemical hydrogen evolution reaction. Nat. Commun. 2020, 11, 1029, DOI: 10.1038/s41467-020-14848-2. (11) Shang, X.; Liu, Z. Z.; Lu, S. S.; Dong, B.; Chi, J. Q.; Qin, J. F.; Liu, X.; Chai, Y. M.; Liu, C. G. Pt-C Interfaces Based on Electronegativity-Functionalized Hollow Carbon Spheres for Highly Efficient Hydrogen Evolution. ACS Appl. Mater. Interfaces 2018, 10, 43561-43569, DOI: 10.1021/acsami.8b10845.

(12) Luo, Y. P.; Huang, D. K.; Li, M.; Xiao, X.; Shi, W. N.; Wang, M. K.; Su, J.; Shen, Y. MoS2 nanosheet decorated with trace loads of Pt as highly active electrocatalyst for hydrogen evolution reaction.

Electrochim. Acta 2016, 219, 187-193, DOI: 10.1016/j.electacta.2016.09.151.

(13) Yao, Y. C.; Gu, X. K.; He, D. S.; Li, Z. J.; Liu, W.; Xu, Q.; Yao, T.; Lin, Y.; Wang, H. J.; Zhao, C. M.; Wang, X. Q.; Yin, P. Q.; Li, H.; Hong, X.; Wei, S. Q.; Li, W. X.; Li, Y. D.; Wu, Y. E. Engineering the Electronic Structure of Submonolayer Pt on Intermetallic Pd3Pb via Charge Transfer Boosts the

(14) Jiang, K.; Liu, B. Y.; Luo, M.; Ning, S. C.; Peng, M.; Zhao, Y.; Lu, Y. R.; Chan, T. S.; de Groot, F. M. F.; Tan, Y. W. Single platinum atoms embedded in nanoporous cobalt selenide as electrocatalyst for accelerating hydrogen evolution reaction. Nat. Commun. 2019, 10, 1743, DOI: 10.1038/s41467-019-09765-y.

(15) Zhang, L. H.; Han, L. L.; Liu, H. X.; Liu, X. J.; Luo, J. Potential-Cycling Synthesis of Single Platinum Atoms for Efficient Hydrogen Evolution in Neutral Media. Angew. Chem. Int. Ed. 2017, 56, 13694-13698, DOI: 10.1002/anie.201706921.