Selective ReductionâOxidation Strategy to the Conductivity-Enhancing

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A selective reduction-oxidation strategy to the conductivityenhancing Ag-decorated Co-based 2D hydroxides as efficient electrocatalyst in oxygen evolution reaction Ruohao Dong, Haoran Du, Yixuan Sun, Kuangfu Huang, Wen Li, and Baoyou Geng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03153 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Sustainable Chemistry & Engineering

A selective reduction-oxidation strategy to the conductivity-enhancing Agdecorated Co-based 2D hydroxides as efficient electrocatalyst in oxygen evolution reaction

Ruohao Dong, Haoran Du, Yixuan Sun, Kuangfu Huang, Wen Li and Baoyou Geng*

College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Center for Nano-Science and Technology, NO.189 South Jiuhua Road, Anhui Normal University, Wuhu, 241000, P. R. China. *Email: [email protected]

KEYWORDS: Reduction-Oxidation, Conductivity, Ag-decorated, Co-based 2D hydroxides, OER Abstract The design and synthesis of high-performance catalyst for oxygen evolution reaction (OER) are crucial for electro-catalysis reaction. In this study, the new Ag-decorated Co-based hydroxides nanosheets with good performance as well as gram-scale yield are prepared by a selective reduction-oxidation method in virtue of the different stability of metal in air. The doping of silver obviously raises the electrical conductivity of the catalysts. Due to the synergistic reaction of the high electrical conductivity and the original active sites, the overpotential of the as-prepared Agdecorated Co(OH)2 nanosheets is low to 270 mV at 10 mA/cm2. Compared to the pure Co(OH)2 nansheets, the overpotential has decreased about 80 mV at the same current

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density. Especially, different from the general preparation methods, this process can be carried out at room temperature and does not need any strict condition. And the gram-scale output makes this method potential for industrial production. Introduction Different from the bulk counterparts, two-dimensional (2D) materials have attracted wide attention due to their special properties such as sensitive features to optical, mechanical and electrical factors, which possess excellent morphology tenability.1 The layers among 2D nanosheets are generally combined by van der Waals force. This heterostructures promise a greater combination than traditional 3D structures.2 Particularly, ultrathin nanosheets usually have good catalytic performance that relies on numerous active sites caused by the large surface area.3, 4 Moreover, because of this larger surface area, there is more contact between the ultrathin material and electrolyte, thereby promoting the charge transfer in the reaction, which enhances the conductivity compared to 3D counterparts.5 In addition, the ordered 2D nanosheets are beneficial for the permeation of the electrolyte and provide an effective way for the rapid ion diffusion, which is propitious to the mass diffusion in water splitting.6 As a result of the mentioned advantages, 2D materials have the potential to become a suitable catalyst for water splitting. Among the family of 2D materials, cobalt compounds have been paid close attention to the field of electrocatalysis, such as oxygen evolution reaction (OER),7 hydrogen evolution reaction (HER),8 oxygen reduction reaction (ORR)9 and carbon dioxide reduction reaction (CRR).10 Besides, many cobalt based materials can be used as multifunctional electrocatalysts.11-13 As for electrocatalysis, cobalt based materials have been considered as a candidate to replace noble metal catalysts. The good catalytic effect of them is attributed to the loosely bonded electrons that lead to high


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electrical conductivity.14 Another important reason is the existence of CoII which is advantageous to form CoOOH that is propitious to OER and ORR.15,16 As the most common two-valence cobalt compound, cobalt hydroxide can provide a large amount of CoII. In the Co(OH)2 nanosheet, near surface oxygen vacancies could enhance the electrophilic property of absorbed O, encourage the adsorption of -OH on the active sites and form the adsorbed -OOH species, which plays an important role in the improvement of OER activity.17.18 Apart from the existence of CoII, the interlayer spacing of Co(OH)2 are larger than common transition metal hydroxide, causing the great ion transport characteristics and benefiting to increase the kinetics performance of the OER.19 Therefore, Co(OH)2 has been noticed by researchers in virtue of the promising OER performance and great corrosion resistance in alkaline solutions especially for 2D cobalt hydroxides.20 Although there are lots of CoII easily transforming to CoOOH in 2D Co(OH)2, some tricky problem such as poor conductivity also greatly limits the further application in water splitting.21 Hence, the pure cobalt hydrogen oxide materials usually cannot exhibit the good catalytic properties. To resolve the problems mentioned above, most of studies devote to obtain layered double hydroxides (LDH) consisted of cobalt hydroxide and other transition metal hydroxides with outstanding performance.22 By means of this process, the new cations could replace the original cations in the crystal lattice due to the interaction between different cations,

23, 24

which can promote electron transfer to reduce overpotential. A great quantity of research about cobalt based catalysts has been concentrated on the doping between different first transition metals, which show preeminent performance including Zn-Co, Mn-Co, Fe-Co, Ni-Co, etc.25-30 And all of the studies are meaningful for the development of these materials. Unfortunately, the mutual doping between transition



metals is hard to further enhance the catalytic activity. Hence in the up to date research, OER catalysts decorated by noble metals have attracted wide attention. For example, single-atom Au loaded LDH materials have been successfully synthesized. These favorable catalyst-gold interfacial interactions greatly reduce the overpotential of Au loaded LDH materials compared with raw LDH.31 At the same time, iridiumcobalt composites have been successfully prepared. The introduction of iridium increases the performance and durability of the catalysts by enhancing electrical connectivity.32 Through the addition of noble metals, the catalytic properties of the materials have been obviously improved. However, the high cost of noble metal limits the practical application. Meanwhile, single atomic materials must be faced with the difficulties in preparation and characterization. It is noteworthy that, distinguishing from other metal doping, the addition of silver can also change the conductivity of transition metal materials more directly and thus enhance the catalytic performance of the sample,33 which have a significantly lower overpotential than the raw materials. In addition, compared to gold, iridium, ruthenium and other precious metals, the cost of silver is relatively low so that the materials with large amounts of silver can be obtained. Besides, it is not necessary to make silver as a single-atom material, which reduces the difficulty in preparation and characterization of the materials. In this study, the Ag-decorated 2D Co(OH)2 nanosheets with the excellent OER performance of 270 mV at 10 mA.cm-2 have been successfully obtained by an ambient reduction-oxidation strategy from nitrates. In contrast, the Co(OH)2 nanosheets prepared from the similar method show the overpotential of 350 mV at the current density of 10 mA.cm-2 due to the poor electrical conductivity,34,35 which is difficult to change by doping other transition metals in our work, such as Zn-, Fe-, and Ni-doping Co(OH)2 nanosheets. In order to the further optimization of electroconductivity of the


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Co(OH)2 nanosheets, decorating a little silver on the layers of Co(OH)2 is a feasible method. As we expected, the addition of silver largely reduces the overpotential of hydroxide because of the improved conductivity. It is highlighted that there are many outstanding advantages to obtain the high-performance catalyst by this method with the potential of commercialization such as at the simple procedure, the mild reaction condition and the high yield. Moreover, the ratio of silver plays the key role in the performance of the catalyst, which could be easily controlled by adding different amounts of silver. In the follow-up experiments, we also tried to prepare ternary catalytic materials containing silver, cobalt and another transition metal element. After the decoration of Ag, the performance of these composites, such as Agdecorated Co-Fe LDH and Ag-decorated Co-Ni LDH, was also superior to the original LDH. In general, the good combination of silver and cobalt leads to the excellent catalytic properties and broaden the application prospect of this composite. Experiment Section Materials Cobalt nitrate hexahydrate (Co(NO3)2.6H2O, Macklin), iron nitrate nonahydrate (Fe(NO3)3·9H2O, Aladdin), silver nitrate (AgNO3, Macklin) , nickel nitrate hexahydrate (Ni(NO3)2 .6H2O, Aladdin), zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Aladdin),

manganese

nitrate

solution

(Mn(NO3)2,

Aladdin),

sodium

borohydride(NaBH4, Macklin), polyvinyl- pyrrolidone (PVP, Mw = 40 000, Aladdin), potassium hydroxide (KOH, 99.999%, Aladdin), dimethyl formamide (DMF, Aladdin), Vucan XC-72 carbon, and Nafion solution (5 wt%, Shanghai Geshi Energy Technology Co., Ltd). All chemicals are AR and are not subjected to special treatment prior to use. Synthesis of Ag-decorated Co(OH)2 Nanosheets



First, 1.45 g (5 mmol) of Co(NO3)2·6H2O accompanied by 0.085 g (0.5 mmol) of AgNO3 were dissolved in 10.0 mL deionized water to form a homogeneous solution. Second, 0.5 g of PVP was added to the mentioned solution. Then 50.0 mL deionized water was supplied to the solution with the mole ratio of Co and Ag is 10:1 (solution A). After sonicating for 10 min, the beaker loading solution A was put on a magnetic stirrer for stirring. At the same time, 10 mL solution B containing 0.5 g NaBH4 should be prepared in other beaker. Next, solution B was slowly dropped to solution A in ice water bath and the mixture was kept stirring 10 h. When the reaction was over, the obtained brown floccules were washed and centrifuged three times by water and ethanol, respectively. Finally, the sample was transferred to a 60 °C oven dried for 12 h. As a contrast, the above methods were used to prepare cobalt hydroxide and silver respectively, and then the two products were ground together to obtain the product of mechanical mixing.

The preparation of other cobalt-based composites is similar to this method. The unique difference is the type and ratio of the added metal nitrates including AgNO3, Fe(NO3)3, Ni(NO3)2 and Zn(NO3)2. Material Characterizations The morphologies and the sizes of the samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Hitachi, HT-7700). And the high-resolution TEM (HRTEM, FEI, Tecnai G2 F30) was used to obtain the information of lattice fringes and energy dispersive spectroscopy (EDS) mapping of materials. X-ray powder diffraction (XRD, Bruker AXS, D8 Advance) characterizations were carried out by the Cu Kα radiation. And Xray photoelectron spectroscopy (XPS, Thermo Fisher, ESCALAB 250XI) was selected to analysis the valence state with the Al Kα monochromatized radiation. The


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ratio of cobalt and silver in the final product was characterized by inductively coupled plasma test (ICP, Agilent, THS-106). The slurries used for resistivity measurement were prepared according to the weight ratio of active materials/carbon black/ carboxymethyl cellulose/styrene-butadiene rubber=8:1:0.5:0.5, and the mass loading of all active materials is 1.3 mg·cm-2. Then the as-prepared films were dried in vacuum at 60 oC for 12 h. After the compaction at 5 MPa, the films were cut into 4×10 cm. The resistivity of the films was measured on the film resistance tester. Electrocatalytic Measurements Electrocatalytic measurements were actualized by an electrochemical workstation (Chenhua, CHI660C) with the three electrode system in 1.0 M KOH solution (pH ≈ 13.85) at room temperature. The three electrode system is composed of platinum wire electrode as the counter electrode, Ag/AgCl electrode as referring electrode, and glass carbon electrode (0.3 cm in diameter) as the working electrode. Before the measurements, each working electrode is polished with 0.05 micron alumina powder for ten minutes, and the oxygen needs to be passed into the KOH solution half an hour in advance to obtain an oxygen saturated solution. And during the measurements, oxygen needs to be continuously accessed to keep the equilibrium potential value. The work electrode is prepared as follows: taking 2 mg sample powders and 1 mg carbon black (Vucan XC-72) dissolved in a mixed solution of 665 µL deionized water and 335 µL DMF. The resulting solution was than sonicated in an ultrasound machine for 10 min. After sonicating, 10 µL Nafion solution (5 wt %) was added. Subsequently, the resulting mixture was once again placed in an ultrasonic machine for 20 minutes to obtain a final homogeneous ink. After the ultrasonication, 7 µL of the as-prepared ink were loaded onto the glass carbon electron and ensure the



catalysts loading is 0.2 mg·cm-2. Finally, the glass carbon electrons loaded with the ink were dried at room temperature for at least 6 h. The polarization curves were obtained by sweeping the potential from 1.3 to 1.7 V (vs. RHE) at room temperature with the sweep rate of 10 mV·s-1. From the polarization curves we can know the overpotenial of the catalysts when the current is 0.0007A. The electrochemical impedance spectroscopy (EIS) test was carried out in the same condition at room temperature. And the stability test were performed using chronopotentiometry with polarization current density of 10 mA·cm-1. Results and Discussion

Scheme 1. The illustration of the ambient reduction-oxidation strategy to prepare Agdecorated Co-based 2D hydroxides. The reaction process has been shown in scheme 1. With the adding of NaBH4, the pink solution rapidly turns to dark and produces numerous bubbles, which means the reduction from cation to metal. During the subsequent stir, the Co, Fe, Ni and Zn could be oxidized to form hydroxides because of the existence of H2O and O2 in air. In contrast with those active metals, Ag steadily exists in the solution, which is the whole reaction mechanism to prepare Ag-decorated Co(OH)2.


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Taking the preparation of pure Co(OH)2 for example, the sample obtained in the first 30 seconds after the addition of NaBH4 is obviously magnetic, suggesting the appearance of Co nanoparticles as shown in Figure 1a. The dark green sample after complete reaction does not exhibit magnetism, which proves the ambient reductionoxidation process as shown in Figure 1b. With the time goes on, the color of the liquid slowly turns brown (Figure S1), which illustrates the formation of Ag-decorated Co(OH)2. Moreover, the Fe-doped, Ni-doped and Zn-doped Co(OH)2 have also been obtained through the same steps as shown in Figure S2. The products originating from different metal nitrates have different colors, but the whole black intermediates appearing in the process means the similar route that is a transformation from metal nanoparticles into the corresponding hydroxides.

Figure 1. Magnetic properties of samples taken at different times. a) The sample at the beginning of the reaction. b) The sample at the end of the reaction. From SEM characterization, the as-prepared Co(OH)2 shows flower-like morphology with the structural units of nanosheets (Figure 2), which show the thickness of nanosheets range from 5 to 10 nm. It is noticed that the TEM images of Ag-decorated Co(OH)2 nanosheets exhibits some small particles dispersing on the nanosheets (Figure 3a, b and S3). These particles can be proved to silver particles by the subsequent characterizations. From the HRTEM imaging, the clear lattice fringes



with interplanar spacings of 0.24 nm were displayed in Figure 3c, which relate to the (111) crystal planes of Ag. Figure 3d is the SAED image of 10 % Ag-decorated Co(OH)2 that exhibits the typical feature of polycrystal. It is interesting that the Ag nanoparticles have not been found in the surface of nanosheets but in TEM images. In addition, some amorphous materials could be observed in the edge of Ag shown in Figure 3c. These two aspects prove that these nanoparticles are coated in Co(OH)2 nanosheets. There is hardly influence on the 2D morphology deriving from the increasing ratio of Ag except for the amount of Ag particles. The hydroxides obtained from different transition metal elements also keep the flower-like structure. The differences of the mentioned pure/composite hydroxides are only reflected in the thickness and size of the nanosheets (Figure S4 to S6). For example, the thickness of Fe(OH)3 was thicker than other transition metal hydroxide in this work (Figure S4). In Co-Ni LDH, the sample with higher nickel content has smaller size than that with more cobalt content (Figure S5). According to the SEM and TEM images, the 2D nanosheets could promote the charge transfer and expose more active sites due to the larger surface area, which may be beneficial for the great catalytic performance.


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Figure 2. a, c) low magnification SEM images of 10 % Ag-decorated Co(OH)2 nanosheets from different areas and b, d) high magnification images corresponding to the rectangular frames.

Figure 3. a, b) TEM images of 10 % Ag-decorated Co(OH)2 nanosheets. c) HRTEM images of 10 % Ag-decorated Co(OH)2 nanosheets and d) selective area electron diffraction (SAED) of 10 % Ag-decorated Co(OH)2. The pure cobalt hydroxide and the ordinary transition metal doped sample are amorphous products with nanosheets morphology due to the precipitation method, which lead to the poor signal-to-noise ratio (Figure 4a), including pure Co(OH)2 and other transition metals doped Co(OH)2. Fortunately, Ag prepared by this method



shows good crystallinity according to XRD pattern, which could be ascribed to PDF 04-0783 (Figure 4b). In order to definitely proof the phase, XPS was selected to characterize these samples. As shown in the XPS results, the binding energy of Co 2p1/2 is 798 eV and Co 2p3/2 is 782.1 eV corresponding Co2+ cations in Co(OH)2 (Figure 4c). And the peak at 367.9 eV and 374.27 eV belong to Ag 3d5/3 and Ag 3d3/2 separately, which correspond to metal silver (Figure 4d). From the image of mapping, the blue part represents silver (Figure 5b), which could certify that the small particles in TEM images. Other elements, cobalt and oxygen, are also shown in Figure 5c and 5d, respectively. According to ICP, the reactant ratio of cobalt and silver is very close to that of the final product (Table S1), which means that the contrast experiments made by different feeding ratios are believable.

Figure 4. The XRD patterns of pure a) Co(OH)2 and b) Ag-decorated Co(OH)2. The XPS spectrums of c) Co 2p and d) Ag 3d in the Ag-decorated Co(OH)2.


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Figure 5. The images of elemental mapping of Ag, Co, O in 10 % Ag-decorated Co(OH)2 nanosheets. In order to compare the effects of different elements on the properties of cobalt hydroxide nanosheets, all materials were tested in the same preparation method in 1M KOH solution, and the temperature was maintained at room temperature. The total loading of catalysts on the GC electrode is 200 µg. cm-2. And the polarization curves were tested by linear sweep voltammetry (LSV) with the scan rate of 5 mV.s-1. It can be found that the catalytic effect of Ag decorated materials is much better than the pure Co(OH)2 nanosheets (Figure 6a). Moreover, different proportion of silver also has a significant impact on the performance. In a certain range, the overpotential could be decreased with the addition of Ag. However, the extra silver will reduce the catalytic performance of the material, which arise the excessive silver with no OER catalytic performance occupying the active sites of Co(OH)2. Finding a suitable proportion of silver is crucial to study the influence of different ratio of Ag in the Co(OH)2 nanosheets. From the data, when the amount of Ag is 10 %, the overpotential is 270 mV. When it up to 20 %, the overpotential increases to 320 mV. And when the amount of Ag addition is 15 %, the overpotential of the sample is



closing to that of the 20 % Ag-decorated Co(OH)2 nanosheets (Figure 6a), which obviously illustrates that some specific content of Ag can reach the optimum catalytic performance of Co(OH)2 nanosheets. Of course, we also find that all the Ag-decorated samples in this study have the lower overpotential than pure Co(OH)2 nanosheets (overpotential is 350 mV) and lower than that of most common transition metal doped Co(OH)2 nanosheets. At the same time, in order to more easily compare the catalytic performance of the material under the certain potential, we summarized the current density of all the Ag-decorated Co(OH)2 nanosheets when the potential is 350 mV (Figure 6d). The 10 % Ag-decorated Co(OH)2 has the highest current density of 37.6 mA.cm-2, which is obviously higher than 20 % Ag-decorated Co(OH)2 (18.25 mA.cm2

) , 15 % Ag-decorated Co(OH)2 (18.51 mA.cm-2) and 5% Ag-decorated Co(OH)2 (12

mA.cm-2), and it is more than 3 times as high as the pure Co(OH)2 nanosheets. Considering the potential value of constant current and the current value at constant potential, 10 % Ag-decorated Co(OH)2 obviously has the better catalytic performance than other materials in this experiment. Moreover, according to the overpotential and current density, the catalytic performance of the Ag-decorated Co(OH)2 nanosheets is better than the most recent research results. At the same time, 10 % Ag-decorated Co(OH)2 exhibits an obviously lower Tafel slope of 67 mV/dec than other Agdecorated Co(OH)2 nanosheets (Figure 6c), which shows the remarkable OER kinetic increment attributed to the decoration and synergistic effect of the suitable amount of Ag. It is worth mentioned that, all the samples doped with Ag show the evidently lower Tafel slope than pure Co(OH)2 nanosheets (109 mV/dec).


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Figure 6 a) The LSV curves, b) EIS curves, c) Tafel slope and d) current densities at the overpotential of 0.35V of different ratio of Ag-decorated Co(OH)2 nanosheets. In order to further characterize the kinetic properties of the Ag-decorated Co(OH)2 nanosheets, we have carried out the EIS test on this set of samples (Figure 6b). The diameter of the semicircle in the figure represents the impedance of the sample. As shown in the picture, the impedance of the sample reduces with the increase of silver content. The results also clearly show that in a certain range, the increase of silver content has a significant promoting effect on the reduction of the material impedance, which infers the improved charge transfer speed with the increased Ag. In addition, in order to investigate the actual resistance of materials, the different samples coated on the Cu foil with the same mass and thickness are tested by the film resistance tester as shown in Table S2. It is clear that the resistance of materials tend to decrease with the adding of Ag, which highly exhibits the function of Ag to enhance the conductivity of electrodes. However, the excessive content of silver also leads to the increase of impedance, overpotential and Tafel slope because the Ag itself does not have OER catalytic properties. In other words, excessive Ag plays the negative impact on the performance of the materials. When the excess Ag is added, the proportion of Co(OH)2/Ag in the material is decreased, resulting in the loss of the acting component.



In addition, the extra Ag hinders the contact between Co(OH)2 and reactant and covers the active sites, which cause the relative poor performance than the sample with appropriate proportion of Ag. As mentioned before, the structure of Ag@Co(OH)2 shown in SEM and TEM images could be obtained through this selective reduction-oxidation strategy. Compared to the mechanical mixture of Ag and Co(OH)2, the 10 % Ag decorated Co(OH)2 delivers the obvious improvement in performance as shown in Figure S7. The overpotential of the mixing electrode is high to 300 mV and the stability is extremely poor than Ag@Co(OH)2 because of the direct contact between Ag and other materials, which indicates that this special structure could relieve the oxidation process of Ag. Figure S8 shows when the stability test is carried out for more than 8 hours, the catalytic performance of 10 % Ag decorated Co(OH)2 fades only slightly. The SEM images of material after chronopotentiometry are displayed in Figure S9. This electrode could completely maintain the structure of 2D nanosheets after 5 h reflecting the good structural stability during the catalytic process. In order to illustrate the effect of electrocatalytic process on the phase of materials, the XPS spectrums of 10 % Ag decorated Co(OH)2 after the stability test are shown in Figure S10. On the one hand, Co2p spectrum consist of Co(II) and Co(III) ascribed to Co(OH)2 and CoOOH, respectively. It is noticed that the electric resistance of materials could be decreased due to the intermediate spin in the octahedral and square pyramidal symmetry of Co(III) compared with the pure Co(OH)2. 36, 40 At the same time, Co(III) enhances the adsorption energy of H2O because of the lower coordination number and stimulates the transformation from –OOH to O2.21,37 Moreover, Ag nanoparticles could further reduce the resistance of material, which shows the clear performance improvement. On the other hand, there is a part of Ag


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oxided into Ag2O exhibited in the XPS spectrum, which matches well with the stability test. Fortunately, this coating structure acts as a protective layer to avoid the direct oxidation of Ag to some extent, as mentioned before. A large number of literatures reported that the doping between different transition metals can significantly improve the catalytic performance of the samples. The composites prepared by this reduction-oxidation strategy also display the enhanced performance. From the LSV curves, the doping of Fe and Ni can improve the catalytic performance of Co(OH)2, but the promotion is limited (Figure S11, S12), which may be caused that Fe and Ni are easy to generate FeOOH and NiOOH in this system

(Figure S13). Because FeOOH is poor conductivity and unstable in alkaline solution and NiOOH is a poor catalyst for OER.38-40 Besides, some common transition metals such as Zn cannot improve the catalytic performance of Co(OH)2 (Figure S14). The overpotential of the original cobalt hydroxide nanosheets is 350 mV, and the best overpotential of Fe-doped Co(OH)2 is 334 mV when the ratio of Fe:Co is 4:6. Then the performances of Ag-decorated Fe-Co LDH and Ag-decorated Fe-Ni LDH are displayed in Figure S15 and S16 that show the overpotential of 307 mV and 310 mV respectively, which is lower than the corresponding ordinary materials. This means that the addition of silver is universal for improving the catalytic performance of transition metal catalyst in OER. Conclusion In summary, we have designed an efficient reduction-oxidation method to prepare 2D cobalt based OER catalysts. On the basis of retaining the advantage of Co(OH)2 with large amount of CoII, the material has been successfully decorated by the proper amount of silver to enhance the electrical conductivity. At the same time, the content of silver can be effectively controlled to get the optimal load. With the optimal Ag-



decoration, the overpotential of Co(OH)2 drops from 350 to 270 mV at the current density of 10 mA.cm-2. Besides, this method has many typical advantages such as facile, controllable, high-yielding and time-saving. Importantly, the sample exhibits excellent catalytic performance compared with most reported Co-based OER catalysts. Therefore, this study opens a new route to design OER catalysts with high performance by adding a small amount of conductive materials, which also gives a significant hint for fabrication of other electrocatalysts. Associated Content Supporting Information. See supporting information for the digital photographs of the samples, SEM and TEM images, LSV curves of Co-Fe, Co-Ni and Co-Zn LDHs with different ratios of transition metal elements, ICP data of Ag-decorated Co(OH)2, chronopotentiometry curve of 10 % Ag-decorated Co(OH)2 nanosheets and LSV curves of Ag-decorated Co-Fe, Co-Ni LDHs, stability tests, XPS tests. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author * E-mail: [email protected] Acknowledgments This work was supported by the National Natural Science Foundation of China (21471006), the Recruitment Program for Leading Talent Team of Anhui Province, the Program for Innovative Research Team of Anhui Education Committee, and the Research Foundation for Science and Technology Leaders and Candidates of Anhui Province.


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For Table of Contents Use Only

This work has synthesized an effective OER catalyst, which can be used for the sustainable utilization of energy.



Syn nopsis: This work has sy ynthesized aan effective OER catalysst, which cann be used fo or the ssustainable utilization u of o energy.


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