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Electrochemical oxidation of chlorine-doped Co(OH) nanosheet arrays on carbon cloth as a bifunctional oxygen electrode Yue Kou, Jie Liu, Yingbo Li, Shengxiang Qu, Chao Ma, Zhishuang Song, Xiaopeng Han, Yida Deng, Wenbin Hu, and Cheng Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17002 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Materials & Interfaces

Electrochemical oxidation of chlorine-doped Co(OH)2 nanosheet arrays on carbon cloth as a bifunctional oxygen electrode Yue Kou,a Jie Liu,a Yingbo Li,a Shengxiang Qu,a Chao Ma,a Zhishuang Song,a Xiaopeng Han,b Yida Deng,b Wenbin Hub and Cheng Zhonga,b* a

Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Edu

cation), School of Materials Science and Engineering, Tianjin University, Tianjin 300 072, China b

Tianjin Key Laboratory of Composite and Functional Materials, School of Materials

Science and Engineering, Tianjin University, Tianjin 300072, China

* Corresponding author. E-mail address: [email protected] (C. Zhong)

KEYWORDS: Co(OH)2, chlorine, bifunctional electrocatalyst, electrochemical oxidation, electrodeposition.

Abstract: The primary challenge of developing clean energy conversion/storage systems is to exploit an efficient bifunctional electrocatalyst both for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with low-cost and good durability. Here, we synthesized chlorine-doped Co(OH)2 in situ grown on carbon cloth (Cl-doped Co(OH)2) as an integrated electrode by a facial

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electrodeposition method. The anodic potential was then applied to the Cl-doped Co(OH)2 in alkaline solution to remove chlorine atoms (EO/Cl-doped Co(OH)2), which can further enhance the electrocatalytic activity without any thermal treatment. EO/Cl-doped Co(OH)2 exhibits a better performance both for ORR and OER in activity and durability, because of the formation of the defective structure with larger electrochemically active surface area after the electrochemical oxidation. This approach provides a new idea for introducing defects and developing active electrocatalyst.

Introduction In recent decades, under the decline of fossil fuels and the increasingly environmental issues, it is urgent to develop renewable, clean and sustainable energy to ensure the rapid and healthy development of the economy.1-3 As to the storage/conversion systems of clean energy in terms of metal-air batteries,4 water-splitting,5 and fuel cells6 has been significantly enhanced. The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) occupy the significance position in the complex redox reactions of these energy conversion/storage systems, especially metal-air batteries.7-9 However, due to the kinetically sluggish of ORR and OER, it is highly necessary to develop an effective bifunctional catalyst to promote the reaction processes and to enhance the catalytic stability.10-12 Up to date, although noble metal-based catalysts have aroused much interest as result of their high catalytic activity, they don’t possess bifunctional catalytic characteristics and have limited


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natural resources and high prices.13-15 Therefore, it is eagerly to exploit earth-abundant and highly efficient bifunctional catalysts. Transition-metal catalysts have recently drawn great attention thanks to their earth-abundant nature, cost-effective and environmentally friendliness.16,17 Among them, a large number of studies have shown that cobalt-based materials have considerable catalytic properties, electrochemical stability and reversibility for both ORR and OER,18,19 including their hydroxide,20 oxides,21 nitrides.13 Co-based catalyst can effectively accelerate the kinetics of oxygen catalytic reaction and promote the process of complex redox reaction in alkaline electrolytes. Since layered α-Co(OH)2 has a large interlayer spacing,22 it has good ion transport characteristics to promote the diffusion of reactants and products in redox reactions.23 Therefore, it has been widely used as bifunctional electrocatalysts in the alkaline electrolyte both for ORR and OER. In recent studies, various morphologies of α-Co(OH)2, such as nanosheet,24 nanocone,20 nanowire,25 nanodisk,26 were synthesized by different methods. The nanosheet structure can effectively improve the size of active surface areas and speed up the charge transport that can significantly accelerate the reaction kinetics. In our earlier work,27 α-Co(OH)2 nanosheet arrays coated carbon cloth were successfully synthesized by one-step electrodeposition method. Unfortunately, although it has a high electrocatalytic activity for OER, its ORR performance is relatively poor. It has been reported that atomic-level defects can enhance the catalytic performance but



there the effect of atomic defects on the electrocatalytic properties of bifunctional electrocatalysts has not been study so far.28,29 Here, we introduced chlorine atoms to form atomic defects and vacancies to improve the electrocatalytic performance of Co(OH)2. By comparing the evolution reaction potentials of chlorine and H2O, it is obvious that the chlorine atoms in the electrocatalysts will be oxidized under the positive voltage, thus destroying the internal structure of the catalyst and introducing vacancies and defects. In the present work, Cl-doped Co(OH)2 nanosheet were in situ grown on the flexible carbon cloth via a facile electrodeposition method by the addition of chlorine ions in the electrolyte. Then the final catalysts were obtained by electrochemical oxidation of the precursor. This approach provides two major advantages. First, the electrocatalyst is in situ electrodeposited on the surface of the carbon cloth without adding any binder and surfactant, which is highly beneficial for the characterization of intrinsic properties. Second, all treatments are carried out at room temperature without any heat treatment, and the whole process is very simple and quick (20 min). It has been found that the introduction of chlorine atoms helps to create defective structure, which greatly improves the electrochemical active surface area and enhance the adsorption and diffusion ability of the catalyst to molecular oxygen.30 The ORR/OER performance is further improved and also has good stability. This work also provides a new way of activating electrocatalysts. Experimental section Materials Synthesis


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Synthesis of Cl-doped Co(OH)2 Cl-doped Co(OH)2 nanosheet was obtained by a one-step electrodeposition method in a three-electrode standard mode under the temperature of a room. With the three-electrode system, the working electrode was carbon cloth (WOS1002, Taiwan), the counter electrode was a platinum plate (~1 × 1 cm) and its reference electrode was saturated calomel electrode (SCE), respectively. Carbon cloth (~1 × 1.5 cm) was carefully washed with acetone, ethanol, and deionized water (DI water) in turn by ultrasonication before electrodeposition. After cleaning, carbon cloth was dried at 50 °C in ambient atmosphere. The deposition electrolyte contained 0.05 M Co(NO3)2 and 0.01 M NaCl. Then, the cathodic electrodeposition was performed on IviumStat workstation at –1.0 V (vs. SCE) for 600 s. The specimen with green precursor was obtained from DI water within several times before dried at room temperature. Finally, a carbon cloth with green color deposits was obtained (mass loading: ~1.0 mg cm–2). Synthesis of Co(OH)2 In addition to adding NaCl to the electrolyte, other synthesis methods were the same as the above. In this case, carbon cloth with green color products was finally obtained (mass loading: ~1.0 mg cm–2). Oxidization pretreatment EO/Cl-doped Co(OH)2 and EO/ Co(OH)2 were obtained by oxidation with an electrochemical method before further tests. The oxidization pretreatment was carried out for 20 cycles at the range from 1.26 to 1.86 V under cyclic voltammetric scans



(CV) method. For Cl-doped Co(OH)2 sample, chlorine atoms would be effortlessly oxidized during the electrochemical oxidation treatment. Materials Characterization The phase structures were observed using an X-ray diffractometer (XRD, Bruker D8 Advanced, German, using Cu Kα radiation) over a range from 10° to 70°for 15 min. The morphology of surface was observed using scanning electron microscope (SEM, Hitachi S-4800, Japan) furnished with an energy-dispersive X-ray (EDX) spectrometer (Genesis XM2, USA). The thickness of the nanosheet was measured by atomic force microscope (AFM, Agilent 5500, USA). The Brunauer-Emmett-Teller (BET) measurement aimed at analyzing the size of specific surface area by N2 adsorption isotherms at 77 K using AutosorbiQ instrument (Quantachrome, USA). The microstructure was investigated by transmission electron microscope (TEM, JEOL JEM-2100F, Japan). The surface chemistry of the electrocatalysts was evaluated through X-ray photoelectron spectroscopy (XPS, Escalab 250XI, ThermoFisher Scientific, USA) with the 100 eV pass energy for survey and 20 eV for high-resolution scan. Electrochemical measurements In order to conduct an electrochemical test for catalysts at room temperature, the standard three-electrode was applied on an IviumStat working system. The as-prepared catalysts deposited on carbon cloth were directly employed as the working electrode, the platinum plate and SCE were employed as the counter electrode and reference electrode, separately. Electrochemical tests were conducted in


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KOH solution with O2-saturated, which was bubbled with high purity O2 (purity of 99.995%, Air Product) before the test for 30 min at least, and the gas purge was preserved in the electrolyte during the experiment. ORR activities were tested in 0.1 M KOH, and other electrochemical experiments were conducted in 1.0 M KOH. In all experiments, the potentials were calibrated with respect to the reversible hydrogen electrode

(RHE)

according

to

the

formula

of

EሺRHEሻ = EሺSCEሻ + 0.0591 × pH + 0.24 . ORR and OER curves collected from linear sweep voltammetry (LSV) were performed with 5 mV s–1 over a range of 1.1 to 0.2 V and 1.2 to 2.0 V. Electrochemical impedance spectra (EIS) were tested from 100 mHz to 10 kHz at 1.6 V. In order to measure electrochemical active surface area (ECSA), the electrochemical double layer capacitance (Cdl) was displayed through CV test with different scan rate (50, 100, 150, 200, 250, 300, 400 mV s–1, respectively) at open circuit voltage. The long-term electrochemical stability of catalysts was conducted with a three-electrode system on a CHI 660D working system. The stability of OER was examined for 12 hours in 1 M KOH by chronopotentiometry measurement applied with 10 mA cm–2 and the stability of ORR was evaluated in 0.1 M KOH for 12 hours by chronoamperometry measurement applied with a voltage of 0.66 V. Results and discussion Cl-doped Co(OH)2 in situ grown on carbon cloth followed by electrochemical oxidation is illustrated in Figure 1. Initially, uniform Cl-doped Co(OH)2 nanosheets were deposited on carbon cloth (denoted as Cl-doped Co(OH)2) by a facial



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electrodeposition method in a solution which contains Co(NO3)2 and NaCl. Co(OH)2 with hydrotalcite-like structure have a large interlayer space. With the addition of chlorine ion, chlorine ions occupy Co tetrahedral sites and exist in the interlayer space.31,32 The Co(OH)2 nanosheets were synthesized without adding NaCl to the electrolyte. During the electrodeposition (Figure S1), NO3– ions are reacted with H2O to form OH– ions which subsequently react with Co2+ ions in the electrolyte to form Co(OH)2 on the carbon cloth.21 This series of reactions can be described as:33 –

NO3 + H2 O + 2e– → NO2 + 2OH–

–

(1)

Co2+ + 2OH– → Co(OH)2

(2)

Then, the as-prepared sample with green color was transferred into alkaline solution (1 M KOH) for electrochemical oxidation treatment (denoted as EO/Co(OH)2 or EO/Cl-doped Co(OH)2). After electrochemical oxidation process (Figure S2), Co(OH)2 was changed into CoOOH based on the following reaction:34 Co(OH)2 + OH– → CoOOH + e–

(3)

Finally, EO/Cl-doped Co(OH)2 and EO/Co(OH)2 with black color can be obtained (Figure S3).


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Figure 1. Schematic graph of the synthesis procedure of Co(OH)2 and Cl-doped Co(OH)2 before and after oxidation on carbon cloth. The crystal phase of these samples was identified by XRD measurement (Figure 2a). Both for Co(OH)2 and Cl-doped Co(OH)2, the major diffraction peaks of (003), (101), (012), (015), and (110) are observed, corresponding to the standard peaks of hydrotalcite-like α-Co(OH)2 (JCPDS 02-0925). No other diffraction peaks are found in the XRD, that means the formation of high-purity Co(OH)2 and the addition of chlorine doesn’t alter the phase formation. After electrochemical oxidation, the phase was transformed into CoOOH with the diffraction peaks of (003), (012) and (015) (JCPDS 07-0169). XRD pattern indicates that after electrochemical oxidation process, Co(OH)2 has been completely converted to CoOOH with the better crystallinity. The surface morphologies of EO/Co(OH)2, EO/Cl-doped Co(OH)2 and their precursors on carbon cloth were featured by SEM (Figure 2). As shown in Figure 2b and 2c, a dense 3D network of Co(OH)2 nanosheet can be observed, which are



uniformly and vertically formed on the carbon cloth (Figure S4). After electrochemical oxidation, Co(OH)2 is changed into CoOOH but still maintains the morphology of the precursor, indicating the stability of the nanostructure. For Cl-doped Co(OH)2 in Figure 2d, the surface morphology of interconnected nanosheet indicates that the intercalated chlorine ions don’t change the morphology of products. However, the surface morphology of Cl-doped Co(OH)2 has some changes after the electrochemical oxidation process. The crisscrossed nanosheets become a little crumpled with some small cracks after the oxidation of chlorine atoms (Figure 2e). Elemental mapping images show that chlorine is hardly found in the elemental mapping of Co(OH)2 (Figure 2f) but can be observed clearly and dispersed uniformly with Co and O in Cl-doped Co(OH)2 (Figure 2g). This suggests that chlorine ions were successfully introduced and distributed uniformly in Co(OH)2. The average thickness of the Cl-doped Co(OH)2 before and after electrochemical oxidation is about 5.1 nm and 3.8 nm, as measured by AFM (Figure 3). The surface area of Cl-doped Co(OH)2 before and after the oxidation process is compared by BET method, as demonstrated by N2 adsorption/desorption measurements (Figure S5). The specific surface area of EO/Cl-doped Co(OH)2 (76.10 m2 g–1) is larger than the area of Cl-doped Co(OH)2 (48.58 m2 g–1) because of the formation of defective structure.


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Figure 2. (a) XRD pattern of Cl-doped Co(OH)2 and Co(OH)2 before and after electrochemical oxidation. SEM images of (b) Co(OH)2, (c) EO/Co(OH)2, (d) Cl-doped Co(OH)2 and (e) EO/Cl-doped Co(OH)2. Elemental mapping of (f) Co(OH)2 and (g) Cl-doped Co(OH)2 on a single carbon fiber.



Figure 3. AFM images and the corresponding thickness measurement data of Cl-doped Co(OH)2 before (a) (c) and (b) (d) after electrochemical oxidation. The detailed morphology of Cl-doped Co(OH)2 nanostructure before and after oxidation was further observed by TEM. Ultrathin nanosheet structure of Cl-doped Co(OH)2 is clearly seen with no obvious cracks or defects (Figure 4a). The nanosheet surface is very uniform and smooth. High-resolution TEM (HRTEM) illustrates the lattice spacings about 0.237 and 0.268 nm, in accordance with the (015) and (012) crystal planes of α-Co(OH)2 (Figure 4c). EO/Cl-doped Co(OH)2 nanosheet become less flat and change into fragments with some defects (Figure 4b), indicating the formation of a defective structure after electrochemical oxidation. As shown in Figure 4d, HRTEM image further illustrates the phase change of EO/Cl-doped Co(OH)2 before and after oxidation, with the lattice spacing about 0.231 and 0.438 nm, consistent with (012) and (003) planes of CoOOH. These results agree well with the XRD measurements (Figure 2a). The EDX elemental mapping (Figure 4e) and EDX spectra (Figure S6) of Cl-doped Co(OH)2 show that chlorine atoms are successfully doped into Co(OH)2 and evenly distributed on a single nanosheet. After electrochemical oxidation, chlorine atoms are removed from the original Cl-doped Co(OH)2 nanosheet (Figure 4f).


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Figure 4. TEM images of Cl-doped Co(OH)2 (a) before and (b) after electrochemical oxidation. HRTEM images of Cl-doped Co(OH)2 (c)before and (d) after electrochemical oxidation. EDX mapping of Co, O, Cl elements in Cl-doped Co(OH)2 (e) before and (f) after electrochemical oxidation.

Before and after oxidation, Cl-doped Co(OH)2 was further studied by XPS to evaluate the detailed information of chemical valence states (Figure 5). As



demonstrated in Figure 5a, the XPS survey spectrum proves the introduction and removal of chlorine once again. The C 1s peak comes from the carbon cloth substrate. In Figure 5b–c, the spectra of Co 2p, O 1s, and Cl 2p are illustrated separately. Two kinds of Co states including Co2+ and Co3+ can be observed in Figure 5b. Before oxidation, the signals of Co 2p are equipped with two spin-orbit doublets, which are assigned to Co 2p3/2 at 781.1 eV and Co 2p1/2 at 797.2 eV corresponding to Co2+.35,36 The distance between Co 2p1/2 and Co 2p3/2 was 16.1 eV, further proving the existence of Co2+ in Co(OH)2.37 Meanwhile, there are two peaks exist in 785.6 and 802.9 eV, which can be regarded as the shakeup satellites (denoted as Sat. in Figure 5b).35 After electrochemical oxidation, a significant difference was noted. After electrochemical oxidization, the peaks of Co 2p at 779.6 and 794.6 eV are related to Co 2p3/2 and Co 2p1/2, suggesting the presence of Co3+ without any signal of Co2+.38,39 This further confirms that Co(OH)2 is changed into CoOOH after electrochemical oxidization. In the O 1s spectra (Figure 5c), three kinds of peaks can be observed. Before electrochemical oxidation, three peaks are displayed in O 1s spectra of 533.1, 531.4 and 529.4 eV, which represent to structural water,40,41 bonds of O–H,36 and the typical metal-oxygen bonds of Co–O,42,43 respectively. The main oxygen peak is adsorbed oxygen (denoted as Oads), which is corresponding to the O 1s spin-orbit of Co(OH)2.36 The distinct difference of Cl-doped Co(OH)2 before and after oxidation lies in the peak of lattice oxygen (denoted as Olat). The Olat peak comes from the covalent bond of Co–O after the formation of CoOOH. In the high-resolution spectra


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of chlorine (Figure 5d), Cl 2p3/2 at 198.4 eV and Cl 2p1/2 at 200.0 eV can be detected44 and no chlorine signals can be found after oxidation.

Figure 5. (a) XPS survey spectra (b) Co 2p, (c) O 1s, (d) Cl 2p high-resolution spectra of Cl-doped Co(OH)2 before and after electrochemical oxidation, separately. To characterize the effect of phase change and defective structure on the catalysts bifunctional performance, the ORR activity of as-prepared specimens was first tested in 0.1 M KOH with O2-saturated under a three-electrode system. Before ORR test, EO/Co(OH)2 and EO/Cl-doped Co(OH)2 were obtained by electrochemically treating their precursors in 1 M KOH using CV method, ranging from 1.26 to 1.86 V (Figure S2). A distinct oxidation peak can only be detected on the first cycle at 1.56 V, which is

connected

with

the

irreversible

change

of


Co(OH)2

to

CoOOH


( Co(OH)2 + OH– → CoOOH + e– ). Since cobalt hydroxide will be oxidized to CoOOH at 1.56 V, the CV curves are swept from the voltage of 1.11 to 0.41 V before the test to ensure surface stability (Figure S8). The ORR polarization curves of as-prepared samples were tested by LSV method with 5 mV s–1 scan rate (Figure 6a). Contrasted with Co(OH)2 and Cl-doped Co(OH)2, carbon cloth displays poor ORR activity, but there is not much difference after the introduction of chlorine. This presents that the intercalated chlorine doesn’t affect the electrocatalytic activities of ORR. After electrochemical oxidation, a great polarization at the initial of the ORR curves has been detected, which is attributed to the reversible convert between CoOOH and CoO2 (CoOOH + OH– ↔ CoO2 + H2O + e– ).45 This is also indicated by CV curves in 1 M KOH (Figure S7). To Co(OH)2, after the electrochemical oxidation, the reduction current density is much higher indicates the ORR performance is improved as a result of the formation of CoOOH. However, the onset potential of EO/Cl-doped Co(OH)2 located at 0.74 V is more positive than EO/Co(OH)2. The reduction current density of EO/Cl-doped Co(OH)2 at –2.24 mA cm–2 is better than EO/Co(OH)2 (–1.69 mA cm–2) and nearly three times larger than the current of Co(OH)2 (–0.76 mA cm–2). All the results indicate that EO/Cl-doped Co(OH)2 exhibits higher ORR activity. For purpose of further illustrating the effect of the defective structure of EO/Cl-doped Co(OH)2, the Cdl is used to measure the ECSA by the capacitive current (Figure 6b). The value of Cdl was measured with different scan rates over a range of ±50 mV across the open circuit potential (OCP) by the CV method (Figure S9). The Cdl of Co(OH)2, Cl-doped Co(OH)2, EO/Co(OH)2 and


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EO/Cl-doped Co(OH)2 are calculated to be 0.66, 0.88, 2.46 and 3.47 mF cm–2. This suggests that the largest effective active areas of EO/Cl-doped Co(OH)2 due to the electrochemical oxidation and dissolution of chlorine atoms, which can provide more active sites and defective structures that further improve the ORR activity. The ORR stabilities of Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation for long-term were tested in 0.1 M KOH with O2 purging continuously under chronoamperometry measurement (Figure 6c). EO/Cl-doped Co(OH)2 displays higher current density and there is no obvious loss of activity at a voltage of 0.66 V for both two catalysts after 12 hours. The surface morphology of EO/Cl-doped Co(OH)2 after long-term stability test was characterized by SEM (Figure 6d). The sample still remains nanosheet morphology with no significant change, indicating CoOOH can stably exist and has good durability.



Figure 6. (a) ORR polarization curves of carbon cloth, Co(OH)2, EO/Co(OH)2, Cl-doped Co(OH)2 and EO/Cl-doped Co(OH)2 were tested in 0.1 M KOH with O2-saturated with 5 mV s−1. (b) Double-layer capacitance measurements for measuring the electrochemically active surface area of prepared catalysts. (c) Chronoamperometry measurement of ORR at 0.66 V of Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation were tested in 0.1 M O2-saturated KOH for 12 hours. (d) SEM images of Cl-doped Co(OH)2 after ORR stability test. Inset reveals the morphology of Cl-doped Co(OH)2 before durability test. Electrocatalytic OER activities of Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation were tested in 1 M KOH with O2-saturated, as displayed in Figure 7a. The catalysts were also pretreated by the CV method to reach a stable state (Figure S10). EO/Cl-doped Co(OH)2 exhibits an overpotential of ~330 mV at 10 mA cm–2, which is lower than the overpotential of EO/Co(OH)2 (~380 mV). Besides, evolution current density of EO/Cl-doped Co(OH)2 can achieve 228 mA cm–2 at 2.0 V which is better than EO/Co(OH)2 (125.2 mA cm–2). Tafel plots are used to measure the OER kinetics of these catalysts (Figure 7b). The fitted Tafel plots value of EO/Cl-doped Co(OH)2 is 98 mV dec–1, which is lesser than the Tafer plots of EO/Co(OH)2 (130 mV dec–1). This suggests the more efficient kinetics of OER on the defective surface of EO/Cl-doped Co(OH)2. As a common method to measure ion and charge transport kinetics, electrochemical impedance spectroscopy (EIS) was displayed in 1 M KOH at 1.66 V (Figure 7c). Charge-transfer resistance (Rct) of EO/Co(OH)2 and EO/Cl-doped Co(OH)2 was 0.5 and 0.9 Ω, which is significantly


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smaller than carbon cloth (14 Ω). Compared to the EO/Co(OH)2, EO/Cl-doped Co(OH)2 has a higher charge-transfer kinetic, indicating highly efficient electron transfer between the electrocatalyst and substrate. Long-term stability of OER activities was evaluated by chronopotentiometry measurement in 1 M KOH with O2 purging continuously applied with 10 mA cm–2 (Figure 7d). There is no observable fluctuation after testing for 12 hours. The OER curve was obtained again by LSV after long-term stability testing (Figure 7e), which basically overlaps with the initial curve. Moreover, the surface morphology of EO/Cl-doped Co(OH)2 still maintains the vertical and uniform nanosheet appearance (Figure 7f). This indicates the high durability of EO/Cl-doped Co(OH)2 during the OER catalysis. Due to the electrochemical oxidation and dissolution of chlorine, EO/Cl-doped Co(OH)2 can form defective structure which provide large active surface area and better adsorption of H2O.46 The above results demonstrate that EO/Cl-doped Co(OH)2 exhibits considerable electrocatalytic activity and good durability both in ORR and OER, which can obtained by a simple and quick synthesis method at room temperature.



Figure 7. (a) OER polarization curves of carbon cloth, Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation were conducted at 5 mV s−1 in 1.0 M KOH with O2-saturated. (b) Corresponding Tafel slopes. (c) EIS measured on EO/Co(OH)2 and EO/Cl-doped Co(OH)2 at 1.66 V. (d) Chronopotentiometry measurement of OER at 10 mA cm–2 for Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation for 12 hours in 1.0 M KOH with O2-saturated. (e) LSV curves for OER of EO/Cl-doped Co(OH)2 before and after long-term stability test. (f) SEM images of


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Cl-doped Co(OH)2 after OER stability test. The inset displays the surface morphology of Cl-doped Co(OH)2 before durability test. Conclusion It can be concluded that we have synthesized Cl-doped Co(OH)2 supported on carbon cloth as an integrated electrode, without the addition of any binder, which can better reflect the intrinsic electrocatalytic performance of the catalyst. After electrochemical oxidation, Cl-doped Co(OH)2 is irreversibly changed into CoOOH and the structure is destroyed with some defects introduced by the removal of chlorine atoms. Compared to EO/Co(OH)2, EO/Cl-doped Co(OH)2 nanosheet with defective structure display much better electrocatalytic activity because of the larger electrochemical surface area and thus can be utilized for an effective bifunctional oxygen catalyst. This study also provides a new approach for in situ activating catalysts. ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Additional cathodic polarization curves, oxidation curves, SEM, BET data, CV curves, etc.

AUTHOR INFORMATION



Corresponding Author E-mail: [email protected] (C.Z.).

ORCID

Cheng Zhong: 0000-0003-1852-5860

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Science Fund for Excellent Young Scholar (No. 51722403), the National Youth Talent Support Program, National Natural Science Foundation of China (No. 51771134 and 51571151), National Natural Science Foundation of China and Guangdong Province (No. U1601216), and Tianjin Natural Science Foundation (No.16JCYBJC17600).

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Table of content 245x177mm (300 x 300 DPI)



Figure 1. Schematic graph of the synthesis procedure of Co(OH)2 and Cl-doped Co(OH)2 before and after oxidation on carbon cloth. 103x58mm (300 x 300 DPI)


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Figure 2. (a) XRD pattern of Cl-doped Co(OH)2 and Co(OH)2 before and after electrochemical oxidation. SEM images of (b) Co(OH)2, (c) EO/Co(OH)2, (d) Cl-doped Co(OH)2 and (e) EO/Cl-doped Co(OH)2. Elemental mapping of (f) Co(OH)2 and (g) Cl-doped Co(OH)2 on a single carbon fiber. 287x124mm (300 x 300 DPI)



Figure 3. AFM images and the corresponding thickness measurement data of Cl-doped Co(OH)2 before (a) (c) and (b) (d) after electrochemical oxidation. 287x203mm (300 x 300 DPI)


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Figure 4. TEM images of Cl-doped Co(OH)2 (a) before and (b) after electrochemical oxidation. HRTEM images of Cl-doped Co(OH)2 (c)before and (d) after electrochemical oxidation. EDX mapping of Co, O, Cl elements in Cl-doped Co(OH)2 (e) before and (f) after electrochemical oxidation. 245x282mm (300 x 300 DPI)



Figure 5. (a) XPS survey spectra (b) Co 2p, (c) O 1s, (d) Cl 2p high-resolution spectra of Cl-doped Co(OH)2 before and after electrochemical oxidation, separately. 243x187mm (300 x 300 DPI)


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Figure 6. (a) ORR polarization curves of carbon cloth, Co(OH)2, EO/Co(OH)2, Cl-doped Co(OH)2 and EO/Cldoped Co(OH)2 were tested in 0.1 M KOH with O2-saturated with 5 mV s−1. (b) Double-layer capacitance measurements for measuring the electrochemically active surface area of prepared catalysts. (c) Chronopotentiometry measurement of ORR at 0.66 V of Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation were tested in 0.1 M O2-saturated KOH for 12 hours. (d) SEM images of Cl-doped Co(OH)2 after ORR stability test. Inset reveals the morphology of Cl-doped Co(OH)2 before durability test. 250x182mm (300 x 300 DPI)



Figure 7. (a) OER polarization curves of carbon cloth, Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation were conducted at 5 mV s−1 in 1.0 M KOH with O2-saturated. (b) Corresponding Tafel slopes. (c) EIS measured on EO/Co(OH)2 and EO/Cl-doped Co(OH)2 at 1.66 V. (d) Chronoamperometry measurement of OER at 10 mA cm–2 for Co(OH)2 and Cl-doped Co(OH)2 after electrochemical oxidation for 12 hours in 1.0 M KOH with O2-saturated. (e) LSV curves for OER of EO/Cl-doped Co(OH)2 before and after long-term stability test. (f) SEM images of Cl-doped Co(OH)2 after OER stability test. The inset displays the surface morphology of Cl-doped Co(OH)2 before durability test. 250x275mm (300 x 300 DPI)


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Electrochemical Oxidation of Chlorine-Doped Co ... - ACS Publications

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