Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (6): 731-738

#### The article information

Meng-si Li, Zhi-yu Lina, Qian-wang Chen

Metal-Organic Frameworks Derived Ag-CoSO4 Nanohybrids as Efficient Electrocatalyst for Oxygen Evolution Reaction

Chinese Journal of Chemical Physics, 2019, 32(6): 731-738

http://dx.doi.org/10.1063/1674-0068/cjcp1805104

### Article history

Accepted on: June 3, 2018
Metal-Organic Frameworks Derived Ag-CoSO4 Nanohybrids as Efficient Electrocatalyst for Oxygen Evolution Reaction
Meng-si Lia , Zhi-yu Linaa , Qian-wang Chena,b
Dated: Received on May 13, 2018; Accepted on June 3, 2018
a. Hefei National Laboratory for Physical Sciences at the Microscale, Department of Materials Science & Engineering and Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China;
b. High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China
Abstract: Cobalt-based nanomaterials have been intensively explored as one of the most promising noble-metal-free oxygen evolution reaction (OER) electrocatalysts. However, most of their performances are still inferior to state-of-the-art precious metals especially for Ru and Ir. Herein, we apply a continuous ion exchange method and further hydrothermal treatment to synthesize the flake-like Ag-CoSO$_4$ nanohybrids beginning from Co-BTC (BTC: benzene-1, 3, 5-tricarboxylic acid) metal-organic frameworks precursor. The catalyst exhibits superior OER performance under the alkaline electrolyte solution (a low overpotential of 282 mV at 10 mA/cm$^{2}$ in 1 mol/L KOH), which is even better than RuO$_2$ due to the improved conductivity and rapid electrons transfer process via introducing small amount of Ag. The existence of Ag in the hybrids is beneficial for increasing the Co(Ⅳ) concentration, thus promoting the $^*$OOH intermediate formation process. Besides, due to the very low requirement of Ag content (lower than 1 atom%), the cost of the catalyst is also limited. This work provides a new insight for designing of inexpensive OER catalysts with high performance and low cost.
Key words: Metal-organic frameworks    Ag-CoSO4    Oxygen evolution reaction    Electrons transfer
Ⅰ. INTRODUCTION

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$_2$ and RuO$_2$ based materials. Efficient as it is, the high cost and poor durability largely impede its widespread applications [1-4]. Accordingly, it is of paramount importance to seek the cost-effective OER catalysts with superior stability.

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 [8], Ni-Co LDH [9], Zn-Co LDH [10]). Subsequently, a hybrid of transition metals (Fe, Co, Ni, Mn) compounded nonmetallic elements (S [11], Se [12], P [13], N [14], B [15]) 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/$\alpha$-Fe$_2$O$_3$ composites) [16, 17], hollow-structured CoB$_2$O$_4$ nanowire array [18], etc., which display superior activity towards OER. In this case, Hu and co-workers designed an efficient electrocatalyst with a core-shell structure composed of a nickel boride (Ni$_3$B) nanoparticles core encapsulated with nickel(Ⅱ) borate (Ni-Bi), displaying an outstanding performance by controlling the crystallinity of the Ni-Bi shells, which is originated from the enhanced intrinsic activity of the catalytic sites [19]. Besides, previous studies have proven that the topotactic transformation of Co(OH)$_2$ leads to a synergistic interaction for enhancing the OER electrocatalytic activity [20]. However, the OER activity of most reported Co-based electrocatalysts is still far away from the commercial RuO$_2$ due to its limited intrinsic activity.

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) [21] and carbon dioxide reduction reaction (CO$_2$RR) [22] materials. Very recently, Zeng et al. explored that via introducing a trace amount of Ag$^+$ cations in the Ag-CoSe$_2$ nanobelts, the electrical conductivity is improved in the material, significantly enhancing the overall OER performance. Nevertheless, its performance is still inferior to commercial RuO$_2$ [23]. On the other hand, it has been shown that the supported Au nanostructures could effectively enhance electrocatalytic activity owing to the local catalyst-gold interfacial interactions [24-27]. Zhang et al. have demonstrated that the individual Au monolayers supported on NiFe LDH ($^{ \rm{s}}$Au/NiFe LDH) exhibited an excellent OER performance over the state-of-the-art commercial precious metal based catalysts. The Au monolayers on NiFe oxyhydroxide could induce the charge redistribution of active Fe atoms as well as its surrounding atoms accumulation of charges and consequently modify the adsorption energies of $^*$O and $^*$OOH for electrocatalytic active sites, thereby accelerating the electrocatalytic OER. They also discovered that the interfacial CO$_3 $$^{2-} and H _2 O interfacing with LDH could contribute to the OER process [28]. Enlightened by these considerations, the flake-like Ag-CoSO _4 nanohybrids were successfully achieved by a continuous ion exchange strategy with further hydrothermal treatment using Co-BTC (BTC: benzene-1, 3, 5-tricarboxylic acid) as precursor. The formed nanohybrid exhibits superior electrocatalytic activities in alkaline media. As the OER electrocatalysts, our Ag-CoSO _4 catalyst exhibited a low overpotential of 282 mV at 10 mA/cm ^2 , which was even superior to the noble commercial RuO _2 catalyst. The existence of trace amount of Ag in CoSO _4 could produce more Co (Ⅳ) active sites, changing electronic charge redistribution density of the Co and improving the electrical conductivity, as a result, enhancing the overall OER performance. Ⅱ. EXPERIMENTS A. Synthesis of Co-BTC nanoparticles For synthesis of Co-BTC, 0.2 g C _4 H _6 CoO _4\cdot 4H _2 O and 1.2 g PVP was dissolved into the mixed solution of ethanol (20 mL) and deionized water (20 mL), solution A was formed, which was put on the magnetic stirrer with a low speed. 0.36 g H _3 BTC was dissolved into the mixed solution of ethanol (20 mL) and deionized water (20 mL), solution B was formed. 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 h standing and washed four times by ethanol before drying at 60 ℃ in vacuum. B. Synthesis of CoSO _4 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 Na _2 S \cdot 9H _2 O 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 autoclave, heated to 130 ℃ in the oven, and kept for 6 h. The reaction solution was centrifuged and washed alternately with deionized water and ethanol solution for 5 times. Then, the product was dried in a vacuum oven. The obtained solid was designated as S-0. C. Synthesis of Ag-CoSO _4 nanohybrids 25 mg of the as-prepared CoSO _4 nanoparticles were dissolved in 25 ml deionized water to form uniformly dispersed solution after stirring. And then 0.5, 1.0, 2.0, and 4.0 mL AgNO _3 (100 mL containing 22.4 mg of AgNO _3 ) 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. Ⅲ. RESULTS AND DISCUSSION A. CHARACTERIZATION OF Co-BTC AND Ag-CoSO _4 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 ^\circ and 18.7 ^\circ , which is consistent with the cubic structure reported before. As revealed by Fourier transform infrared (FTIR) spectroscopy in FIG. 1(d), the absorption bands at 1523 and 1423 cm ^{-1} are the result of asymmetric stretching vibration and symmetric stretching vibration caused by the interaction between carboxyl groups and Co ^{2+} in BTC, which made the peak around 1720 cm ^{-1} of carbon dioxide double bond disappear. The relatively broad absorption bands of 3453 and 1617 cm ^{-1} indicate the presence of water in the metal-organic framework. In addition, the absorption band at the region of 1300 - 600 cm ^{-1} correspond to out-of-plane deformation vibration of BTC, and this result is consistent with the report in Ref.[29].  FIG. 1 (a) Field emission scanning electron microscopy of Co-BTC. (b) Transmission electron microscopy images of Co-BTC. (c) X-ray diffraction pattern of Co-BTC. (d) FT-IR spectrum of Co-BTC. 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 _4\cdot 6H _2 O phase (JCPDS card No.16-0304), indicating that the existence of Ag did not change the material structure. Significantly, the XRD patterns of the nanocomposites Ag/CoSO _4\cdot 6H _2 O did not show the peak of Ag owing to the very low content of Ag, which could be identified by the X-ray photoelectron spectroscopy (XPS) results in Table S1 (supplementary materials). In addition, we measured the OER properties of our materials in 1 mol/L KOH electrolyte. In order to guarantee the rigorousness of the experiment, the S-2 sample was soaked in 1 mol/L KOH for 6 h, and then the obtained product was marked as S-2 after. The XRD pattern of S-2 after is shown in FIG. 2(d), we can find that there exists the phase transition in the alkaline medium, which corresponds to the hexagonal structure Co(OH) _2 (JCPDS card No.74-1057). In order to verify whether this phase is stable, we have also made the sample of S-2 to be soaked in 1 mol/L KOH for 0.5 h and 3 h, respectively, then the corresponding XRD was also conducted shown in FIG. S1 (supplementary matrials), which is consistent with the results before, implying this Co(OH) _2 phase is stable.  FIG. 2 (a, b) Field emission SEM of S-2. (c) TEM images of S-2. XRD patterns of (d) S-0, S-1, S-2, S-3, S-4, (e) S-2 after, respectively. 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 ^{-1} in the sample of S-2 could be derived from the vibration of SO _4$$ ^{2-}$ groups [31], which is well fit with the CoSO$_4$ phase detected in the XRD patterns. Distinguished from the pristine sample of S-2, there is a poignant characteristic absorption peak at 3626 cm$^{-1}$, which is assigned to the O$-$H stretching vibration of free hydroxyl, implying that there is the phase transition in the alkaline medium and consequently there are hydroxyl functional groups in the sample after alkaline treatment, which are in good agreement with the XRD results discussed above. Beyond that, the broad absorption band located at 3436 and 1634 cm$^{-1}$ could be originated from the O$-$H stretching vibration and bending vibration of absorbed water in the as-prepared catalyst, respectively [32]. Besides, the stretching vibration of C$-$O bond (1014 cm$^{-1}$) could be also identified in FTIR spectra. Furthermore, the absorption bands at 1383 and 720 cm$^{-1}$ are ascribed to symmetrical stretching vibration and asymmetric stretching vibration of CO$_3 $$^{2-} group, which is resulted from the absorption of carbon dioxide and H _2 O from atmosphere by alkaline medium. According to the previous researches, the CO _3$$ ^{2-}$ group has stronger affinity for OH$^{-}$ and superior ability to neutralize change so as to boost the OER activity of samples [28, 31].

 FIG. 3 (a) FTIR spectra of S-2 and S-2 after. (b) Raman spectra of S-2 after. (c) Raman spectra of S-2 and S-2 after from 1000 cm$^{-1}$ to 3000 cm$^{-1}$.

Raman spectrum for S-2 after in FIG. 3(b) displays the four characteristic peaks (187, 467, 515, 674 cm$^{-1}$), which is well fit with the Co(OH)$_2$ phase. Concretely, the peaks at 467 and 515 cm$^{-1}$ are originated from the dipole oscillation of the symmetrical stretching vibration of O$-$Co$-$O structure. In addition, the initial peak at 190 cm$^{-1}$ belongs to the second formant of O$-$Co$-$O structure. As illustrated in FIG. 3(c), the Raman shift at 1045 cm$^{-1}$ indicated the bending vibration of hydroxyl, which could be originated from the O$-$H structure of H$_2$O. Besides, G band (1586 cm$^{-1}$) and 2D band confirmed the presence of a few carbon layers [33].

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$_3$ in the sample, which also suggested that we have successfully introduced the Ag into the material with the AgNO$_3$ [34]. In addition, as shown in FIG. S2 (supplementary materials), after the sample was soaked into 1 mol/L KOH, the peak of S almost disappeared, which is consistent with the FTIR and XRD results. To get further information for the valence state of Co element, the deconvolution results of the Co 2p spectrum is presented in FIG. 4(a). As can be seen from FIG. 4(a), the signals at binding energy of 781.4 and 798.1 eV are assigned to Co$^{2+}$ species, interestingly, the two peaks at about 779 and 794 eV in FIG. 4(a) should be assigned to metallic state of Co. This is because that the Na$_2$S solution will be oxidated into the sodium thiosulfate, thus leading to the reduction of Co(Ⅱ) nanoclusters to the metallic Co under high temperature. However, the deconvolution of Co 2p spectrum for S-2 after soaking in the 1 mol/L KOH for 6 h (FIG. 4(c)) demonstrated the existence of Co$^{3+}$ at 782.3 eV, which indicated that the Co(OH)$_2$ obtained after alkaline treatment contains Co$^{2+}$ and Co$^{3+}$ species.

 FIG. 4 Structural analysis of Ag-CoSO$_4$ surface. (a) XPS spectrum of Co 2p in the sample of S-2. (b) XPS spectrum of the Ag 3d in the sample of S-2. (c) XPS spectrum of Co 2p in the sample of S-2 after. (d) Co L edge of S-2, S-2 after, Co$_2$O$_3$ by the XANES.

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$^{3+}$ in the reference sample Co$_2$O$_3$, suggesting the rise of the valence for Co element and the presence of Co$^{3+}$ in S-2 after[35]. On the basis of all the discussion above, it indicated that there is the phase transition in our material under the alkaline electrolyte solution and the final phase Co(OH)$_2$ is composed of Co$^{2+}$ and Co$^{3+}$ species.

B. ELECTROCHEMICAL CHARACTERIZATION FOR OER CATALYSIS

The OER properties of the formed hybrid catalysts were investigated by a three-electrode system in N$_2$-saturated 1 mol/L KOH. Particularly, according to the above discussion, the phase transition of our sample did occur after testing under electrolyte solution, so we additionally marked the tested samples as S-0-a, S-1-a, S-2-a, S-3-a and S-4-a in order to distinguish them from the samples before the OER test (S-1, S-2, S-3, S-4), respectively. FIG. 5(a) shows the polarization curves of Ag-CoSO$_4$ with the varied addition of AgNO$_3$ in the alkaline media. Notably, the present S-2-a material exhibited attractive electrocatalytic activity with an overpotential $(\eta)$ of 282 mV to deliver a 10 mA/cm$^2$ current density for the OER, and the trend in the overpotentials was found to be S-2-a$<$S-3-a$<$S-4-a$<$S-1-a, suggesting that the activity was increased with the increasing amount of Ag. Interestingly, the activity would decrease with the further increase of Ag amount, which could be the result of the phenomenon of particle aggregation under high temperature. In addition, for comparison, we also measured the activity of CoSO$_4$ catalyst under the same conditions (S-0-a) (FIG. 5(b)). Clearly, we can find that the S-2-a showed the better OER activity, indicating that the existence of Ag could promote the catalytic process.

 FIG. 5 Electrocatalytic OER performance test in N$_2$-saturated 1 mol/L KOH solution. (a) OER polarization curves of Ag-CoSO$_4$ samples with the varied addition of AgNO$_3$. (b) LSV polarization curves of S-2-a and S-0-a, respectively. (c) LSV polarization curves of S-2-a and S-2 after, respectively. (d) OER polarization curves of S-2-a and RuO$_2$ before and after continuous 1000 potential cycles sweeping at 50 mV/s in alkaline media, respectively.

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$_2$, as well as the durability. As shown in FIG. 5(d), when the current density is 10 mA/cm$^2$, the overpotential of sample S-2-a (282 mV) is significantly lower than overpotential of RuO$_2$ (338 mV). In addition, after 1000 cycles, the OER performance of sample S-2-a was found to be reduced but still better than that of RuO$_2$. Furthermore, the catalytic behavior of the sample S-2-a was also superior to the most reported cobalt-based catalysts, rendering it the most promising OER electrocatalyst in alkaline solution (Table S2 in supplementary materials).

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$_4$ samples with the varied addition of AgNO$_3$. It suggested that the slope rate of Tafels decreases after adding AgNO$_3$. Besides, the Tafel slope of the S-2-a sample is the lowest, which also indicates that such a trace amount of Ag could contribute to the overall OER activity. Furthermore, it is visibly found from FIG. 6(b) that the fitted Tafel slope of S-2-a was 75 mV/dec, which was even smaller than that of commercial RuO$_2$ (89.5 mV/dec), implying the rapid electron transfer in OER process. Previous studies have shown that there is a four-electron transfer process during the OER in the alkaline electrolyte, and at the same time the cobalt-based catalyst would possess the oxidation process simultaneously: Co(Ⅱ)$\to$Co(Ⅲ)$\to$Co(Ⅳ), significantly, in which the real active sites are Co(Ⅳ) [36-39]. According to the above experimental results, the CoSO$_4$ in our sample would change into the Co(OH)$_2$ phase after testing under electrolyte solution, which is consisted of Co$^{2+}$ and Co$^{3+}$ in alkaline medium, making it easier to generate Co(Ⅳ) active sites and consequently promoting the OER performance. Besides, based on Zhang's previous study [28], the interfacial CO$_3$$^{2-}$ identified by the FTIR spectrum in our material could also contribute to the OER. On the other hand, although the precise OER mechanism of metal hydroxides is not particularly clear, but it is possible to effectively improve its OER performance by controlling the adsorption energy of the catalysts on oxygen species [28]. When introducing a small amount of Ag, it will not only improve the conductivity of the catalyst, but also promote the electron transfer during the reaction due to the special electronic structure, thereby promoting the formation of Co(Ⅳ) in situ and changing their charge redistribution, which is directly related to the adsorption of intermediates such as $^*$O and $^*$OOH during OER process, thus enabling fast OER kinetics.

 FIG. 6 Tafel plots of Ag-CoSO$_4$ with the varied addition of AgNO$_3$. (b) Tafel plots of S-2-a and RuO$_2$ catalysts.
Ⅳ. CONCLUSION

In summary, we have successfully obtained a sheet of Ag-CoSO$_4$ nanohybrids as an efficient electrocatalyst towards OER by a continuous ion exchange method using Co-BTC as a precursor. During the electrochemical measurements, the CoSO$_4$ in our sample would change into the Co(OH)$_2$ phase under basic electrolyte solution, which exhibits high intrinsic activity. Meanwhile, the existence of trace amount of Ag in CoSO$_4$ could produce more Co(Ⅳ) active sites, changing electronic charge redistribution density of the active sites and improving the electrical conductivity, as a result, enhancing the overall OER performance. The as-prepared S-2 catalyst exhibits excellent OER activity, when the current density is 10 mA/cm$^2$, the overpotential of the sample is only 282 mV in alkaline medium, which is comparable to or even better than the RuO$_2$ catalyst. The adsorption process for oxygen species at active sites during OER is optimized via introducing a trace amount of Ag in our material, enabling fast OER kinetics, which may open up new insight to design other efficient catalysts.

Supplementary materials: Synthesis of Co-BTC, CoSO$_4$ nanoparticles and Ag-CoSO$_4$ nanohybrids, details of materials characterization and electrochemical measurements, XRD patterns of S-2 soaked in 1 mol/L KOH at different hours, XPS spectra and corresponding chemical compositions of S-2 and S-2 after and comparison of the OER activity between S-2 with some reported cobalt-based OER catalysts in basic condition are available.

Ⅴ. ACKNOWLEDGEMENTS

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

Experimental Section

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.

Material characterization:

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.

Electrochemical Measurements:

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.

Supplementary Figures and Tables

 FIG. S1 (a), (b) the XRD pattern of S-2 soaked in 1M KOH for 0.5h and 3h, respectively.
 FIG. S2 (a), (b) the XPS spectra of S-2, S-2 after, respectively.
Table S1 Chemical compositions of samples after etching process prepared at different annealing temperatures by XPS measurement
Table S2 Comparison of the OER activity between S-2 with some cobalt-based OER catalysts in basic condition from literatures.

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a. 中国科学技术大学材料科学与工程系，合肥微尺度物质科学国家研究中心，合肥 230026;
b. 中国科学院合肥物质科学研究院强磁场中心，合肥 230031