An Amorphous Cobalt Borate Nanosheet-Coated Cobalt Boride

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An amorphous cobalt borate nanosheets coated cobalt boride hybrid for highly efficient alkaline water oxidation reaction Tan Tan, Pengyu Han, Hengjiang Cong, Gongzhen Cheng, and Wei Luo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00258 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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ACS Sustainable Chemistry & Engineering

An amorphous cobalt borate nanosheets coated cobalt boride hybrid for highly efficient alkaline water oxidation reaction Tan Tan,ǂ Pengyu Han,ǂ Hengjiang Cong, Gongzhen Cheng and Wei Luo* College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China, Tel.: +86-27-68752366 *Corresponding author. E-mail addresses: [email protected]. KEYWORDS Core-shell structure, cobalt borate, oxygen evolution reaction, RRDE

ABSTRACT We present a simple in situ sodium borohydride (NaBH4) reduction approach to produce amorphous core-shell like cobalt borate nanosheets coated cobalt boride hybrid (Co-B@Co-Bi). Benefiting from the unique rich amorphous Co-Bi layer, it exhibits extraordinary catalytic activity and good stability toward oxygen evolution reaction (OER) in 1.0 M KOH, with an overpotential of 291 mV to obtain current density of 10 mA cm-2, outperforming most of the documented metalmetalloid based electrocatalysts. This reverse addition strategy might provide a promising pathway for designing various transition metal borate and boride hybrid for more applications.

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INTRODUCTION

Hydrogen is regarded as an ideal alternative to fossil fuels, owing to its high energy density and environmental benignity1-3. Electrochemical water splitting is an efficient and clean way to convert electric power to hydrogen with high purity4-10. However, the oxygen evolution reaction (OER) at anode is considered as the bottleneck for the overall water splitting, due to its sluggish and complicated multistep proton-coupled four-electron transfer process11-14. Currently, precious metal oxides (RuO2/IrO2) are still regarded as the benchmarked OER electrocatalysts, their large-scale applications are severely limited by the high cost and scarcity15. Consequently, developing highly efficient and cost-effective precious metal free electrocatalysts for OER is highly desirable, but still remains a great challenge. Recently, metal borates have received great attention for catalyzing OER or nonlinear optical materials owing to their low price, superior intrinsic activity and good stability16-20. For example, Yao and coworkers reported the synthesis of amorphous cobalt borate together with reduced graphene oxide and carbon black nanocomposites as a highly efficient nonprecious catalyst for OER, and their OER performance with overpotential of 360 mV to achieve to the current density of 10 mA cm-2 in 1.0 M potassium hydroxide21; Wang et al. reported cobalt@cobalt-borate core– shell nanosheets on Ti mesh afford overpotential of 329 mV at 10 mA cm-2 in alkaline condition22; Sun and coworkers reported amorphous Co-borate layer over Co3O4 nanowire arrays on the Ti mesh support also shows good OER activity23; Karadas and coworkers reported Co3(BO3)2@CNT hybrid with overpotential of 487 mV to obtain the current density of 10 mA cm-2 at pH 724. Despite great efforts have been made25-29, their electrocatalytic performances are still far lower than those of benchmarked RuO2/IrO2.


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It has been reported that transition metal borides can be used as promising precatalysts toward OER30-32. A thin layer of metal borate is observed outside the metal borides during the OER process, which has been demonstrated as the truly active species33. Here, inspired by the previous works, we reported the synthesis of a core-shell like catalyst with amorphous cobalt borate (CoBi) nanosheets coated Co-B (Co-B@Co-Bi) through a simple in situ sodium borohydride (NaBH4) reduction approach. As expected, the as-synthesized amorphous Co-B@Co-Bi exhibits remarkable OER performance with overpotential of 291 mV to achieve the current density of 10 mA cm-2 in 1.0 KOH, with excellent durability over 8 hours.

RESULTS AND DISCUSSION

The amorphous Co-B@Co-Bi was obtained through a simple in situ chemical reduction approach by adding cobalt (II) chloride (CoCl2) solution into the solution of sodium borohydride (NaBH4) (seeing the experimental section in supporting information). For comparison, cobalt boride (CoB) was also prepared by adding the solution of NaBH4 into CoCl2 solution though the traditional approach. As displayed in Figures 1a-1d, similar 3D morphologies of Co-B@Co-Bi and Co-B formed by poly-dispersed nanoparticles were observed from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The core-shell morphology of Co-B@CoBi can be observed clearly from the high-resolution TEM image shown in Figure 1e, suggesting the thick amorphous layer of Co-Bi coating on the surface of Co-B (vide infra), which was further confirmed by high annular dark-field scanning TEM (HADDF-STEM) image and corresponding elemental mapping (Figure S1). However, aggregated Co-B varies in the range of about 50 nm are observed from the TEM of Co-B as shown in Figure 1f, which is similar to the previous report34.


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Furthermore, the selected area electron diffraction (SAED) images further indicate the amorphous structures of Co-B@Co-Bi and Co-B (Figure S2). In addition,

Figure 1. (a) SEM images of Co-B@Co-Bi, (b) SEM images of Co-B; (c), (e) TEM images of Co-B@Co-Bi, (d), (f) TEM images of Co-B.

as shown in Figure 2a, no obvious diffraction peaks in the powder X-ray diffraction (XRD) was observed, which further suggest the amorphous states of the as-synthesized Co-B@Co-Bi and CoB. Moreover, the as-obtained Co-B@Co-Bi was further annealed at 800 °C (donated as Co-B@CoBi-800), and the resulted XRD shown in Figure S3 demonstrate identical pattern of Co3(BO3)2 (PDF#75-1808). One possible formation mechanism is briefly discussed as follows. The overall reaction formula of CoCl2 and NaBH4 can be expressed below (using Co2B instead of Co-B to simplify the equation):


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2𝐶𝑜2+ (𝑎𝑞) + 4𝐵𝐻4− (𝑎𝑞) + 9𝐻2 𝑂(𝑙) → 𝐶𝑜2 𝐵(𝑠) + 12.5𝐻2 (𝑔) + 3𝐵(𝑂𝐻)3 (𝑎𝑞) (1) The obvious differences between those two approaches can be attributed to the amount of the reduction agents NaBH4 across the reaction. When cobalt source was added into the NaBH4 solution, the cobalt boride is formed immediately. It has been reported that the as-synthesized Co2B can be served as efficient catalyst to catalyze hydrolysis of NaBH4 by the equation (2): 𝐶𝑜2 𝐵

𝐵𝐻4− (𝑎𝑞) + 4𝐻2 𝑂(𝑙) →

4𝐻2 (𝑔) + 𝐵(𝑂𝐻)− 4 (𝑎𝑞) (2)

Given that our cobalt source was added dropwisely, the process (2) might be the major reaction. As the sodium borohydride consuming, instead of the formation of Co-B, the newly added Co2+ might follow the equation (3): 𝐶𝑜2+ (𝑎𝑞) + 2𝐵(𝑂𝐻)− 4 (𝑎𝑞) → 𝐶𝑜(𝐵𝑂2 )2 (𝑠) + 4𝐻2 𝑂(𝑙) (3) Thus, a thick layer of cobalt borate (Co-Bi) was formed on the surface of Co-B. By using this universal strategy, Ni-B and Ni-B@Ni-Bi were also successfully synthesized. Their corresponding SEM and TEM images are presented in Figure S4 and S5, respectively. The X-ray photoelectron spectroscopy (XPS) was further used to characterize the chemical states of Co-B@Co-Bi and Co-B. The XPS results suggest the Co, B and O are coexisted with the atomic ratio of Co and B is nearly 3:1 for Co-B and 1:1 for Co-B@Co-Bi, which is in consistent with inductively coupled plasma atomic emission spectroscopy (ICP-AES) results (Table. S2). In the Co 2p 3/2 (Figure 2b), three peaks located at 778.0 eV, 782.1 eV, and 780.4 eV can be assigned to the Co(0), Co(II) and Co(III), respectively. However, there are only two character peaks located at 782.3 eV and 780.8 eV can be observed in Co-B@Co-Bi, corresponding to the Co(II) and Co(III), respectively. It can be seen clearly that the peaks of Co(II) and Co(III) in Co-B@Co-Bi display positive shift of about 0.2 eV and 0.4 eV compared to those in Co-B, respectively. In the


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B 1s region, as shown in Figure 2c, a sharp peak located at 188.0 eV corresponding to the boron in cobalt boride (Co-B) and a characteristic oxidized boron peak (Co-B-O) located at 191.8 eV

Figure 2. (a) XRD patterns of Co-B@Co-Bi (i) and Co-B (ii), high resolution XPS spectra of (b) Co 2p, (c) B 1s, (d) O 1s of (i) Co-B@Co-Bi and (ii) Co-B.

can be observed in Co-B. However, in Co-B@Co-Bi, no peaks corresponding to the Co-B region can be observed. In addition, beyond (Co-B-O) peak, another symbolic peak belongs to B-O bond at 192.5 eV is observed. As shown in the O 1s region (Figure 2d), three peaks located at 530.5, 531.5 and 532.6 eV were observed, which can be assigned to the lattice oxygen, substituted


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hydroxyl species and surface oxygen, respectively. However, only substituted hydroxyl species and surface oxygen peaks are observed in Co-B@Co-Bi.

Figure 3. (a) LSV curves for IrO2, Co-B@Co-Bi, Ni-B@Ni-Bi, Co-B and Ni-B with a scan rate of 1 mV s-1 for OER in 1 M KOH; (b) Tafel plots for IrO2, Co-B@Co-Bi, Ni-B@Ni-Bi, Co-B and Ni-B; (c) LSV curves recorded for Co-B@Co-Bi and Co-B before and after 5000 CV cycles in 1 M KOH and (d) chronopotentiometry curves for Co-B@Co-Bi, Co-B, IrO2 at 10 mA cm-2 in 1 M KOH.


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The electrocatalytic activities of Co-B, Ni-B, Co-B@Co-Bi and Ni-B@Ni-Bi catalysts, as well as commercial IrO2 were tested in the 1.0 M KOH electrolyte using a three-electrode system (pH = 13.6). Those catalysts were dropped onto the glass carbon electrode (GCE) to evaluate their OER activity and the catalyst loading was 0.306 mg cm-2. Polarization curves were recorded from linear sweep voltammetry (LSV) test with a scan rate of 1.0 mV s-1. In order to eliminate the effect of ohmic resistance, we conducted an iR correction to all raw data in the further discussions. As shown in Fig. 3a, Co-B@Co-Bi exhibits the highest catalytic activity with an overpotential of 291 mV to drive a current density of 10 mA cm-2 (η10 = 291 mV), lower than that of Co-B (η10= 348 mV), and comparable to that of IrO2 (η10 = 296 mV). As expected, the Ni-B@Ni-Bi also exhibits much higher catalytic performance than Ni-B (η10= 310 mV for Ni-B@Ni-Bi, and η10= 365 mV for Ni-B). In addition, the OER activity of annealed Co-B@Co-Bi-800 was also tested and it showed much lower OER activity than the amorphous Co-B@Co-Bi, further highlighting the key role of amorphous state of Co-Bi in facilitating the OER process (Fig. S6). The catalytic activity of Co-B@Co-Bi is among the top of reported metal borates/borides, and higher than most of the reported metal-metalloid based electrocatalysts as indicated in Table S3. As shown in Fig. 3b, the Tafel slope of Co-B@Co-Bi is calculated to be 105 mV dec-1, followed by Co-B (111.4 mV dec1

), IrO2 (120.73 mV dec-1), Ni-B@Ni-Bi (150.1 mV dec-1), and Ni-B (108.5 mV dec-1). In addition,

we conducted the electrochemical impedance spectroscopy (EIS) to evaluate the charge transfer resistance. As shown in Figure S7, the Nyquist plot of Co-B@Co-Bi shows the smallest semicircle diameter, suggesting the fastest charge transfer because of the lowest resistance of charge transfer (Rct). This result indicates faster OER reaction kinetics of Co-B@Co-Bi, probably for the excellent electron transfer capacity. In addition, both 5000 CV cycling test and chronopotentiometry results show the superior stability of the Co-B@Co-Bi in alkaline media. As


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illustrated in the Fig. 3d, after the continuously CV scanning for 5000 cycles with a range of 1.21.7 V (vs. RHE), negative shift of η10 from the LSV curve is observed, which is also in correspondence with the chronopotentiometry results. The stability test of the Co-B and IrO2 were also conducted. As shown in Fig. 3d, they are both inferior to that of Co-B@Co-Bi. In addition, the double-layer capacitances (Cdl) were also measured by CVs with different scanning rates (see details in Supporting Information) around the open circuit potential (OCP) in Ar-saturated 1.0 M KOH. As shown in Figure S8, the Co-B@Co-Bi possesses much higher active area. In addition, as shown in Figure S9, the intrinsic activity of Co-B@Co-Bi is also higher than that of Co-B when normalized by electrochemical active surface areas (ECSA). Characterizations of Co-B@Co-Bi after OER stability test indicates the well-maintained morphology from SEM and TEM (Figure S10, S11). Furthermore, as displayed in Figure S12, the high-resolution B spectrum is almost kept unchanged. For the high-resolution Co spectrum (Figure S13), Co(III) was formed after the OER reaction which was similar to the previous reports35-37.As for the high-resolution O spectrum, absorbed oxygen was revealed indicating that activity O spices had been formed and attached onto the surface of the catalyst (Figure S14)38,39. The rotating ring-disk electrode (RRDE) was further applied to measure the content of byproduct (peroxide inter-mediates) that conducted at the surface of the Co-B@Co-Bi during the OER process. Figure S15 indicates the rather low ring current (red line) compared to the disk current (black line) during the OER process, suggesting negligible hydrogen peroxide formation. In addition, the ring current is decreased when the disk current uprising, suggesting high potential prevented the formation of peroxide intermediates. This experiment revealed that the majority contribution of the disk current is the four-electron transfer O2 formation path during the OER process instead of the two-electron transfer peroxide formation path. In addition, we examined the


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Faradaic efficiency (FE) of Co-B@Co-Bi to prove the rapid increase of the disk current is derived from the OER process instead of other side reactions (see experimental section for more details). When we fixed the disk current at 100 mA, the ring current is measured to be approximately 36.8 mA, corresponding to FE of 98.4 % (Figure 4a), suggesting that the Co-B@Co-Bi is suitable for a

Figure 4. (a) Disk and ring current of Co-B@Co-Bi catalyst on an RRDE (1600 rpm) with a ring potential of 0.67 V in 1 M KOH solution; (b) Corresponding Faradaic efficiency of Co-B@CoBi catalyst at varying disk potential in the alkaline media. fast 4-electron reaction catalyst generating oxygen at lower potential. Moreover, when we applied a larger disk current (750 mA), a nearly 280 mA ring current can be recorded with a FE calculated to be 100.8 %, indicating the detected oxidation current is derived from the OER process. Meanwhile, Fig. 4b shows the relationship between FE and given potential, which shows great O2 conversion across the large region. CONCLUSION In summary, the cobalt borate coated cobalt boride (Co-B@Co-Bi) has been successfully fabricated through a simple sodium borohydride reduction approach. Thanks to the amorphous state, higher electrochemical surface area (ECSA), superb charge transfer resistance, the resulted


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Co-B@Co-Bi exhibits excellent OER performance under alkaline media, with an overpotential of 291 mV to achieve the current density of 10 mA cm-2 and long-term durability. This work might provide new way for developing advanced metal borates based electrocatalysts for OER and beyond. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acssuschemeng.xxxxxxx. Experimental section, HRTEM and HAADF-STEM images and corresponding EDX elemental mapping, SAED images, XRD pattern, SEM images, TEM and HRTEM images, LSV curve, Nyquist plot, CV curves, ECSA normalized LSV curves, SEM and TEM images, XPS spectra, RRDE results and Table S1-S3 (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ǂ These authors contributed equally.

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (21571145, 21633008), and Program for the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.

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SYNOPSIS An amorphous core-shell like cobalt borate nanosheets coated cobalt boride hybrid (Co-B@CoBi) has been synthesized through a simple in situ NaBH4 reduction approach and further used as electrocatalyst to catalyze oxygen evolution reaction with remarkable performance and long-term stability. TOC


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An Amorphous Cobalt Borate Nanosheet-Coated Cobalt Boride

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