b. High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
Currently, developing renewable energy technologies, such as water electrolysis and metal-air batteries, has gained increasing attention due to the impending global energy crisis as well as the increasing environmental concerns caused by the overuse of fossil fuels. Among these crises, the oxygen evolution reaction (OER), as a key half-reaction, is a bottleneck for the commercialization due to the high overpotential resulting from the intrinsic sluggish reaction kinetics. Despite numerous research efforts and great progresses, most non-precious metal based electrocatalysts for OER are still inferior to state-of-the-art precious metals such as IrO
For decades, great progress has been achieved to develop earth-abundant materials toward OER activity, including 3d transition metal oxyhydroxides, layered double hydroxides materials [5-7] (Ni-Fe LDH , Ni-Co LDH , Zn-Co LDH ). Subsequently, a hybrid of transition metals (Fe, Co, Ni, Mn) compounded nonmetallic elements (S , Se , P , N , B ) have been developed as efficient OER catalysts. Very recently, a family of metal (Co and Ni) phosphate and borate have been investigated for OER activity, including amorphous cobalt-phosphate-based materials (Co-Pi, Co-Pi/
Unlike the 3d transition metals with incomplete d-shells such as Fe, Co, Ni, the noble metal silver (Ag) with the unique 4d electronic track arrangement is generally regarded as sufficiently active for electrocatalytic activity. Moreover, owing to the lowest price of the silver among the platinum group materials, it provides further opportunity to improve the activity of non-precious electrocatalysts while keeping cost-effectiveness with the introdution of Ag and the proper control of Ag contents. To the best of our knowledge, Ag has aroused extensive attention as oxygen reduction reaction (ORR)  and carbon dioxide reduction reaction (CO
For synthesis of Co-BTC, 0.2 g C
Solution A: The as-prepared 40 mg of Co-BTC was dissolved in 20 mL ethanol solution and dissolved by magnetic stirring. Solution B: 240 mg Na
25 mg of the as-prepared CoSO
The Co-BTC was prepared according to the previous researches with a minor modication (detailed information is in the supplementary materials) [29, 30]. To identify the morphology and structural information of as-synthesized product, a series of characterization methods were conducted as shown in FIG. 1. The scanning electron microscope images (SEM) (FIG. 1(a)) and transmission electron microscope TEM images (FIG. 1(b)) show that the synthesized Co-BTC particles are all regular, rod-like structures. FIG. 1(c) shows the X-ray diffraction (XRD) pattern of Co-BTC with two characteristic peaks at 17.5
The SEM images and the TEM image of S-2 in FIG. 2 (a)-(c) clearly revealed that a large number of nanosheets grew on the Co-BTC precursor matrix with carbon layers after adding sodium sulfide under high temperature hydrothermal conditions. The corresponding XRD of all the samples was carried out, as shown in FIG. 2(d). It is clearly found that all the diffraction patterns are almost the same, which correspond to the monoclinic structure CoSO
To further investigate the molecular structure of as-prepared samples after soaking in the alkaline condition during the electrochemical test, FTIR spectrum and Raman spectrum were implemented. As shown in FIG 3(a), the characteristic peak at 1128 and 600 cm
Raman spectrum for S-2 after in FIG. 3(b) displays the four characteristic peaks (187, 467, 515, 674 cm
The XPS was conducted to learn the chemical composition and electronic state on the surface of as-prepared samples. The wide spectra of S-2 and S-2 after are shown in the FIG. S2 (supplementary materials) and the chemical compositions of S-2 and S-2-after are listed in Table S1 (supplementary materials). From Table S1 (supplementary materials), we can find that there is small amount of Ag in the prepared material, corresponding to the weaker peak of Ag in FIG. S2 (supplementary materials), which could explain the absence of characteristic peak related to Ag species in above XRD patterns. Moreover, the Ag 3d spectrum is illustrated in FIG. 4(b), the peak at the binding energy of 368.2 and 374.1 eV could be ascribed to the metallic Ag. Besides, there is a peak at about 407 eV due to +5 valence N, meaning the existence of AgNO
In order to further confirm the valence for Co element, the X-ray absorption near edge structure measurements (XANES) are carried out, the results are shown in FIG. 4(d). The Co L edge of S-2 after showed a shift to high photon energy compared with that of S-2, and we can find that it matched well with the energy peak for Co
The OER properties of the formed hybrid catalysts were investigated by a three-electrode system in N
To ensure the rigorousness of the experiment, we performed the OER performance test of the sample S-2 after soaking in 1 mol/L KOH at the same time. As shown in FIG. 5(c), we can see that the LSV polarization curve changes very little, which can illustrate the test results of the sample S-2-a are representative. In addition, we also tested and compared the electrocatalytical performance of samples S-2-a and RuO
To get better understanding of the intrinsic reaction mechanism of the OER process, the Tafel slope is also presented in FIG. 6. FIG. 6(a) shows the Tafel slope plot for all the Ag-CoSO
In summary, we have successfully obtained a sheet of Ag-CoSO
Supplementary materials: Synthesis of Co-BTC, CoSO
This study was supported by the National Natural Science Foundation (No.21271163, No.1232211, No.21571168), the Fundamental Research Funds for the Central Universities (WK2060140021), the CAS/SAFEA International Partnership Program for Creative Research Teams and the Hefei Science Center CAS (2016HSC-IU011). The DFT calculations were completed on the supercomputing system in the Supercomputing Center of USTC.Supporting Information
Synthesis of Co-BTC nanoparticles:
All chemicals are of analytical grade and used without purification. Co-BTC used as MOF precursors was prepared according to the method described in the related literatures with some modications. For synthesis of Co-BTC, 0.2 g C4H6CoO4 ·4H2O and 1.2 g PVP was dissolved into the mixed solution of ethanol (20mL) and deionized water (20 mL), formed solution A, which was put on the magnetic stirrer with a low speed. 0.36 g H3BTC was dissolved into the mixed solution of ethanol (20 mL) and deionized water (20 mL), formed solution B. Then solution B was poured into solution A with a constant speed by using an injector (10 mL). The mixed solution was kept stirring until the precipitation formed. Finally, the product was centrifuged after 24 hours' standing and washed four times by ethanol beforedrying at 60 C in vacuum.
Synthesis of CoSO4 nanoparticles:
Solution A: The as-prepared 40 mg of Co-BTC was dissolved in 20 ml ethanol solution and dissolved by magnetic stirring. Solution B: 240 mg Na2S·9H2O was dissolved in 10 ml deionized water solution to form a colorless transparent solution after continuous stirring. The solution B is slowly poured into solution A, and then it is sonicated for 30 min to make the mixture disperse uniformly. After that, the solution was transferred into a 35 ml reactor, heated to 130℃ in the oven, and kept for 6 hours. The reaction solution was centrifuged and washed alternately with deionized water and ethanol solution for 5 times finally dried in a vacuum. The obtained solid was designated as S-0.
Synthesis of Ag-CoSO4 nanohybrids:
25 mg of the as-prepared CoSO4 nanoparticles were dissolved in 25 ml deionized water to form uniformly dispersed solution after stirring. And then 0.5 ml, 1.0 ml, 2.0 ml and 4.0 ml AgNO3 (100 ml containing 22.4 mg of AgNO3) solution was dropwise added to above solution with vigorous stirring for 40 min at room temperature. The mixed solution was transferred to a 30 ml autoclave, heated to 160 ℃ and kept for 6 h. The obtained product was centrifuged and washed with deionized water and ethanol solution for 3 times and dried in a vacuum oven. The obtained samples were designated as S-1, S-2, S-3, and S-4, respectively.
Transmission electron microscopy (TEM) measurements were performed on a Hitachi H-7650 and JEOL JEM-2100F field emission transmission electron microscope and a HRTEM (JEOL-2011) was operated at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were acquired on a JEOL JSM-6700 M scanning electron microscope, The powder XRD data were obtained on a Japan RigakuD/MAX-γA X-ray diffractometer using Cu-Kα radiation (λ=1.54178Å) with 2θ range of 20-80°. Fourier transform infrared (FT-IR) spectra were determined by a Magna-IR 750 spectrometer. Raman spectra were recorded with a LabRAM HR Raman spectrometer ranging from 500 to 3000 cm-1. XPS was conducted on an ESCALAB 250 X-ray photoelectron spectrometer instrument. XPS was performed on an ESCALAB 250 X-ray photoelectron spectrometer using Al Ka radiation. Co XANES were obtained at soft x-ray magnetic circular dichroism station in national synchrotron radiation laboratory in USTC, Hefei.
All of the electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (CHI660D) in 1M KOH electrolyte. To prepare the working electrode, typically, 4 mg of catalyst and 30μL Nafion solution (Sigma Aldrich, 5 wt %) were dispersed in 1 mL ethanol solution, and then sonicated it for 40min to form a homogeneous dispersion. Then 5 uL of the dispersion was dropped onto a glassy carbon electrode with 3 mm diameter (loading 0.285 mg/cm2). While a Ag/AgCl (in 3 M KCl solution) electrode and a platinum foil were served as the reference electrode and counter electrodes, respectively. All of the potentials were calibrated to a reversible hydrogen electrode (RHE). The linear sweep voltammetry (LSV) were applied at a scan rate of 5 mV s-1 from 0 V to 0.7 V (vs. RHE) into 1 M KOH electrolyte with a stable flow of N2 gas maintained over the electrolyte during the OER experiment. In order to investigate the stability of the samples, the cyclic voltammetry (CV) were applied at a sweep rate of 50 mV/s in 1 M KOH solution in the potential from 0 V to 0.7 V (vs. RHE) for 1000 cycles. In addition, we used the commercial RuO2 catalyst was used as a referencet to evaluate the catalytic activity of the our catalyst.
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b. 中国科学院合肥物质科学研究院强磁场中心，合肥 230031