Cobalt-Borate Nanoarray: An Efficient and Durable Electrocatalyst for

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Cobalt-borate nanoarray: an efficient and durable electrocatalyst for water oxidation under benign conditions Ruixiang Ge, Hongbin Du, Kai Tao, Qiuju Zhang, and Liang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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ACS Applied Materials & Interfaces

Cobalt-borate nanoarray: an efficient and durable electrocatalyst for water oxidation under benign conditions Ruixiang Ge,†,‡ Hongbin Du,† Kai Tao,† Qiuju Zhang,† and Liang Chen*,†,‡ †

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, Zhejiang, P.R. China. ‡

University of Chinese Academy of Sciences, Beijing, P.R. China 100049

*Corresponding author. Tel: +86 57486685160. *E-mail: [email protected]

Abstract:

The development of efficient earth-abundant electrocatalysts for oxygen

evolution reaction (OER) under benign conditions is still urgent and challenging. Herein, we report the electrochemical generation of novel Co-Bi nanoarray on carbon cloth (Co-Bi NA/CC) from CoS2 nanoarray precursor. As a three-dimensional anode, such Co-Bi NA/CC exhibits excellent electrocatalytic performance for OER with the overpotential requirement of 411 mV to drive 10 mA cm-2. Notably, this electrode also demonstrates outstanding long-term electrochemical durability for 20 hours.

Keywords: cobalt-borate, nanoarray, water oxidation, electrocatalysis, benign conditions

1

ACS Paragon Plus Environment


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■Introduction With the rapid depletion of conventional energy resources, global energy crisis and environmental issues have prompted researchers to search for renewable and clean energy carriers. Hydrogen is a promising alternative for future energy supply owing to its eco-friendly character and high energy density1,2 Electrochemical water splitting driven by renewable resources offers us a sustainable and attractive method for large-scale hydrogen fuel production. Unfortunately, the sluggish anodic oxygen evolution reaction (OER), which involves multiproton-coupled electron-transfer steps, still limits the efficiency of overall water splitting.3,4 RuO2 and IrO2 are the best OER electrocatalysts,5-7 but the low abundance and high price of these metal oxides impede their widespread application. Consequently, searching for high-performance alternatives composed of earth-abundant elements has attracted tremendous research efforts during the past decades.8-16 Commercial water electrolyzers operate in harsh environments to reduce ohmic drops,17 which, however, suffer from serious corrosion problems and high running costs. Moreover, the practical application of solar fuel generator is constrained by the limited stability of its complex components in strongly acidic or alkaline electrolytes.18 Increasing research effort has therefore been directed to develop non-precious metal OER electrocatalysts operating under benign conditions. In 2008, Nocera and colleagues reported that electrodeposited cobalt-phosphate (Co-Pi) film is active for OER at pH 7.19 After that, widespread attention has been focused on the development of such Co-Pi catalyst for water oxidation in neutral media.20-24 Nickel-borate (Ni-Bi) is efficient for water oxidation in borate electrolyte,25-29 but the use of cobalt-borate (Co-Bi) for OER has been rarely reported.30 On the other hand,


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rationally designed nanostructures are deemed to be beneficial for the electrocatalytic efficiency of catalysts,31 and more importantly, nanoarray catalyst has advantages of exposing more active sites and facilitating diffusion of electrolyte and gas, leading to enhanced electrochemical performance.32-34 As such, developing Co-Bi nanoarray as a 3D water oxidation anode is highly attractive but still remains unexplored and challenging. Herein, for the first time, we report the in situ electrochemical generation of Co-Bi nanoarray on carbon cloth (Co-Bi NA/CC) from CoS2 nanoarray on CC (CoS2 NA/CC) 0.1 M potassium borate (K-Bi, pH: 9.2). As an integrated 3D electrode for water oxidation, such Co-Bi NA/CC shows superior catalytic activity with the overpotential requirement of only 411 mV to drive 10 mA cm-2 in 0.1 M K-Bi. Moreover, it is also excellent in durability with its catalytic activity being maintained for 20 hours.

■Experimental Methods Chemicals. Cobalt nitrate hexahydrate, ammonium fluoride and urea were provided by Tianjin Fuchen Chemical Reagent Factory. Sulfur powder and potassium borate were provided by Chengdu Kelong Chemical Reagent Factory. The RuCl3·3H2O and Nafion (5 wt%) were purchased from Sigma-Aldrich. Purified water was obtained from a Millipore system. CC is commercial available and was pretreated in concentrated HNO3 (65%) at 393 K for 2 hours before use. Preparation of CoS2 NA/CC. Cobalt nitrate hexahydrate (2 mmol), ammonium fluoride (5 mmol), urea (10 mmol) and ultrapure water (36 mL) were mixed together under stirring. The resulting solution and a piece of cleaned CC (2 cm × 3 cm) were transferred to a 50 mL Teflon-lined stainless-steel autoclave and kept at 393 K for 6 hours. Then the autoclave



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cooled

down

naturally

and

Co(CO3)0.5OH·0.11H2O

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nanowire

array

on

CC

(Co(CO3)0.5OH·0.11H2O NA/CC) was prepared. After washing and drying, the resulting Co(CO3)0.5OH·0.11H2O NA/CC and sulfur powder (2 g) were placed at two sides of a ceramic boat with Co(CO3)0.5OH·0.11H2O/CC at the downstream side. The sample was then heated at 673 K for 1 hour with a heating speed of 5 K min-1 under Ar flow. After cooling down to room temperature, the CoS2 NA/CC was obtained. Preparation of Co-Bi NA/CC. To prepare Co-Bi NA/CC, electrochemical transformation was conducted by prolonged electrolysis in 0.1 M K-Bi using a typical three-electrode system at 1 V (vs. saturated calomel electrode, SCE ) for 2 hours. The CoS2 NA/CC is used as working electrode. Graphite plate and SCE serve as counter electrode and reference electrode, respectively. Electrodeposited Co-Bi on CC was prepared according to a previously reported method30 for comparison. Preparation of RuO2. The RuO2 was prepared according to previous report.35 Briefly, 2.61 g RuCl3·3H2O and 30 mL NaOH (1.0 M) were added into 100 mL distilled water under stirring and kept at 373 K for 45 minutes. The precipitates were collected then washed thoroughly, followed by drying at 353 K. After annealing at 573 K for 3 hours under air atmosphere, the RuO2 powder was obtained. Characterizations.

X-ray

diffraction

(XRD)

patterns

were

collected

by

a

RigakuD/MAX 2550. The scanning electron microscopy (SEM) analysis was conducted on a a XL30 ESEM FEG microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images are taken by a HITACHI H-8100 electron microscopy. The X-ray photoelectron spectroscopy (XPS) data was recorded on an


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ESCALABMK II surface analysis system with Mg exciting source. Electrochemical measurements. Electrochemical data was collected using a CHI 660E electrochemical analyzer (CH Instruments, China). A typical three-electrode system was used. The Co-Bi NA/CC, graphite plate and SCE is working, reference and counter electrode, respectively. The current densities were calculated with respect to the geometrical area of the electrodes (0.25 cm2). The reference electrode was calibrated to the reversible hydrogen electrode (RHE) scale in all measurements using following equation: E (RHE) = E (SCE) + (0.242 + 0.059 pH) V. Scan rate for all the linear sweep voltammetry (LSV) tests is 2 mV s-1. All experiments were carried out at 298 K. Turnover frequency (TOF). To compare the activity of Co-Bi with that of other earth-abundant catalysts, we make a rough estimation of TOF for each active site using the following equation36: TOF = J A / 4 F m, where J is current density (A cm-2) at defined overpotential during the LSV measurement in 0.1 M K-Bi; A is the geometric area of the electrode; 4 indicates the mole of electrons consumed for evolving one mole of O2 from water; F is the Faradic constant (96485 C mol-1); m is the number of active sites (mol), which can be calculated from the following equation37: slope = n2F2m/4RT, where slope is obtained from the linear relationship between the oxidation peak current and scan rate; n is the number of electrons transferred (n is 1, assuming an one-electron process for oxidation of Co centers in Co-Bi); F is the Faradic constant; R is the ideal gas constant and T is absolute temperature.

■Results and discussion CoS2 NA/CC was derived from Co(CO3)0.5OH·0.11H2O NA/CC via sulfurization (see



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Experimental Section for preparation detail). Figure 1a shows the XRD pattern of CoS2 NA/CC. Five peaks at 32.4°, 36.3°, 39.9°, 46.4° and 55.0° are indexed to the (200), (210), (211), (220) and (311) planes of CoS2, respectively (JCPDS No. 41-1471),32,38-40 and other peaks originate from CC. SEM images of CoS2 NA/CC (Figure 1b) show that CoS2 inherits the nanowire morphology of Co(CO3)0.5OH·0.11H2O (Figure S1). Following potentiostatic electrolysis at anodic potential in 0.1 M K-Bi, diffraction peaks of CoS2 disappeared completely, and only peaks corresponding to CC are observed (Figure 1a), indicating the transformation from CoS2 to an amorphous product. SEM images of Co-Bi NA/CC (Figure 1c) demonstrate the complete coverage of CC by Co-Bi nanowires with well preserved 1D morphology and an enlarged diameter of 180-850 nm (60-360 nm for CoS2 nanowires, Figure 1b). TEM and HRTEM analysis (Figures 1d-1g) also strongly support the formation of amorphous Co-Bi nanowire from crystalline CoS2. Figure 2 displays the XPS spectra in the Co 2p, B 1s, and O 1s regions. The Co 2p3/2 and Co 2p1/2 peaks at 780.3 and 795.7 eV (Figure 2a) are in a range typical of Co2+ or Co3+ bound to oxygen.19 The binding energies of B 1s and O 1s appear at 191.5 and 531.3 eV, corresponding to the core levels of central boron and oxygen atom in borate species, respectively.28

Energy-dispersive

X-ray

(EDX)

spectrum

roughly

verifies

the

0.14:0.17:0.68:0.009 atomic ratio between Co, B, O and S (Figure S2), suggesting that S is nearly completely removed while B is incorporated. Elemental mapping images show the uniform distribution of Co, B, and O elements in Co-Bi NA/CC (Figure S3). All these analyses confirm that CoS2 nanoarray was successfully converted into Co-Bi nanoarray. Electrocatalytic water oxidation activity of Co-Bi NA/CC (catalyst loading: 1.8 mg cm-2)


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was examined in 0.1 M K-Bi using a typical three-electrode setup. Blank CC and RuO2 coated on CC (RuO2/CC, RuO2 loading: 1.8 mg cm-2) were also examined for comparison. In order to reflect the intrinsic behavior of catalysts,41 Figure 3a shows LSV curves after iR correction (Figure S4 shows LSV curve without iR correction). As observed, blank CC is not active for OER within the examined potential window. Co-Bi NA/CC shows excellent OER activity to deliver 10 mA cm-2 at a low overpotential of 411 mV, which is 94 mV larger than that of RuO2/CC. In addition, an overpotential of 553 mV is needed for Co-Bi NA/CC to drive 50 mA cm-2, whereas RuO2/CC requires 592 mV to achieve the same current density. These overpontentials compare favorably to most reported values for noble metal-free OER catalysts under benign conditions (Table S1). Besides, distinguishable redox waves were

observed before water oxidation catalysis, indicating that Co species in Co-Bi undergoes redox reaction. To monitor the change of oxidation state and understand redox kinetics of Co-Bi NA/CC, electron paramagnetic resonance (EPR) spectroscopy and in-situ X-ray absorption near edge spectroscopy (XANES) analyses are needed in future work.23,42-43 Figure 3b shows the corresponding Tafel plots of RuO2/CC and Co-Bi NA/CC, which are fit with the Tafel equation (η = b log j + a, where j is the current density and b is the Tafel slope), yielding a Tafel slope of 146 mV dec-1 for Co-Bi NA/CC and 131 mV dec-1 for RuO2/CC, respectively. The LSV curves of Co-Bi NA/CC toward different concentrations of K-Bi (Figure 3c) show that increasing concentrations of K-Bi from 0.1 to 0.5 M lead to increased catalytic current and Co-Bi NA/CC only demand an overpotential of 285 mV to drive 10 mA cm-2 in 0.5 M K-Bi. These experimental results demonstrate that Co-Bi NA/CC can electrocatalyze water oxidation efficiently under benign conditions.



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Electrocatalytic activity of Co-Bi NA/CC at different pH values was also investigated. As shown in Figure S5, Co-Bi NA/CC needs overpotentials of 470, 410, 403, 410, and 426 mV to drive 10 mA cm-2 in 0.1 M K-Bi with pH values of 8.5, 9.2, 10.2, 11.2 and 12.0, respectively. The pH dependence of catalytic activity was further probed using chronopotentiometry in 0.1 M K-Bi containing 2 M KNO3. As shown in Figure S7, a slope of -23 mV/pH unit is calculated from the linear relationship between the steady-state electrode potential and pH, indicating an inverse first order dependence of reaction rate on boric species.20, 27 Durability is of great importance for the practicability of catalyst electrode. We thus examined the stability of Co-Bi NA/CC by continuous cyclic voltammetry (CV) scanning between 1.38 and 1.78 V vs. RHE in 0.1 M K-Bi at a scan rate of 100 mV s-1. After 1000 cycles, only slight decrease in current density is observed from LSV curves (Figure 3d). Chronopotentiometry test further demonstrates the long-term electrochemical stability with maintained activity for 20 hours. Figure S8 displays a multi-step chronopotentiometric curve for Co-Bi NA/CC. The initial current density is set as 10 mA cm-2 and an increment of 6 mA cm-2 is applied per 300 s. When current increment is applied, the potential raises abruptly and then remains constant until next current increment, indicating the excellent mass transportation, conductivity, and mechanical robustness of this catalyst electrode.44,45 Considering the excellent OER catalytic performance of Co-Bi NA/CC, we probed the possibility to use Co-Bi NA/CC as anode for water-splitting device. A two-electrode system based on CoS2 NA/CC cathode and Co-Bi NA/CC anode (CoS2 NA/CC‖Co-Bi NA/CC) was assembled. In 0.1 M K-Bi, CoS2 NA/CC‖Co-Bi NA/CC demands a cell voltage of 2.08


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V to drive 10 mA cm-2 (Figure S9). The turnover frequency (TOF) at a constant overpotential was calculated to further evaluate the intrinsic activity of Co-Bi NA/CC.46,47 Active sites were quantified by electrochemistry on the assumption that all the Co centers in Co-Bi NA/CC perform a one-electron oxidation process. The concentration of surface catalytic sites can be extracted from the linear relationship between the oxidation peak current and scan rate during CV tests (Figure 4).37 At overpotentials of 400 and 600 mV, the TOFs are calculated as 0.0353 and 0.3011 s-1 for Co-Bi NA/CC, respectively. These TOFs are higher than that of previously reported Fe-Bi48 (0.17 s-1, η = 600 mV) and Co-Bi49 (0.0015 s-1, η = 400 mV). The high TOF value of Co-Bi NA/CC may be ascribed to its 3D configuration, which is favourable for facile ion diffusion and thus efficient use of active sites. In addition, the double layer capacitance (Cdl) of Co-Bi NA/CC and electrodeposited Co-Bi on CC are calculated and used to represent the electrochemical active surface area (ECSA). As shown in Figure S11, the higher Cdl of Co-Bi NA/CC (1.8 mF cm-2) than that of electrodeposited Co-Bi (0.5 mF cm-2) indicates the higher ECSA of Co-Bi NA/CC. To estimate Faradic efficiency (FE), the practically generated oxygen was quantified by measuring the pressure change during electrolysis. Comparison between the quantity of practically evolved oxygen and theoretically calculated oxygen amount demonstrates 100 % FE for oxygen evolution (Figure S12).

■Conclusions In summary, Co-Bi nanoarray has been developed successfully from CoS2 nanoarray via electrochemical oxidation tuning in 0.1 M K-Bi. Such Co-Bi NA/CC works efficiently as a water oxidation electrocatalyst electrode at near-neutral pH. The excellent catalytic



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performance could be rationalized as follows. (1) Effective electrical connection between the catalyst and current collector facilitates the electron transport throughout the electrode. (2) The 3D configuration of Co-Bi NA/CC ensures efficient utilization of active sites. This study not only presents a noble metal-free catalyst electrode for efficient water oxidation, but also provides us with a direction to fabricate 3D catalyst electrode by electrochemical transformation.

■ASSOCIATED CONTENT Supporting Information Detailed characterizations of Co(CO3)0.5OH·0.11H2O/CC and Co-Bi NA/CC. Electrochemical properties of Co-Bi NA/CC.

■AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

■Acknowledgements We gratefully acknowledge the financial support of the National Key Basic Research Program of China (Grant No. 2013CB934800), the National Science Foundation of China (Grant Nos. 51472254 and 51572272), Zhejiang NSF (Grant No. LR14E020004) and the program for Ningbo Municipal Science and Technology Innovative Research Team (Grant No. 2015B11002 and 2016B10005).

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(30) Surendranath,

Y.;

Dincă,

M.;

Nocera,

D.

G.

Electrolyte-Dependent

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List of figure captions Figure 1. (a) XRD patterns of blank CC, CoS2 NA/CC and Co-Bi NA/CC. SEM images for (b) CoS2 NA/CC and (c) Co-Bi NA/CC. TEM and HRTEM images of (d, e) CoS2 and (f, g) Co-Bi nanowire. Figure 2. XPS spectra for Co-Bi NA/CC in the (a) Co 2p, (b) B 1s and (c) O 1s regions. Figure 3. (a) LSV curves of RuO2/CC, Co-Bi NA/CC and bare CC for OER after iR correction. (b) Corresponding Tafel plots for RuO2/CC and Co-Bi NA/CC. (c) LSV curves for Co-Bi NA/CC in 0.1, 0.3, and 0.5 M K-Bi for OER. (d) LSV curves for Co-Bi NA/CC before and after 1000 CV cycles. Insert: chronopotentiometric curve of Co-Bi NA/CC at a constant current density of 10 mA cm−2 for 20 hours without iR correction. All experiments were tested in 0.1 M K-Bi unless specifically stated. Figure 4. (a) Cyclic voltammograms of Co-Bi NA/CC at different scan rates in 0.1 M K-Bi. (b) Oxidation peak current versus scan rate plot for Co-Bi NA/CC.


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Cobalt-Borate Nanoarray: An Efficient and Durable Electrocatalyst for

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