Amorphous Co2B Grown on CoSe2 Nanosheets as a Hybrid

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Amorphous Co2B grown on CoSe2 nanosheets as a hybrid catalyst for efficient overall water splitting in alkaline medium Yaxiao Guo, Zhaoyang Yao, Changshuai Shang, and Erkang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10605 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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ACS Applied Materials & Interfaces

Amorphous Co2B grown on CoSe2 nanosheets as a hybrid catalyst for efficient overall water splitting in alkaline medium Yaxiao Guo,†, ‡ Zhaoyang Yao,§ Changshuai Shang†, ‡ and Erkang Wang*, † †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China. ‡

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

§

Department of Chemistry, University of Washington, Seattle, 98105, USA.

KEYWORDS: Co2B nanosheets, CoSe2 nanosheets, oxygen evolution reaction, hydrogen evolution reaction, water splitting

ABSTRACT: In this work, we synthesized a novel hybrid catalyst (Co2B/CoSe2) by growing amorphous Co2B on the surface of CoSe2 nanosheets. Benefiting from the prominent coupled effects between Co2B and CoSe2 nanosheets, the efficient OER catalyst Co2B/CoSe2 exhibits a very low overpotential of 320 mV@ 10 mA cm−2 with a Tafel slope of 56.0 mV dec−1 in alkaline medium. An overpotential of 300 mV can be also achieved by Co2B/CoSe2 at the same condition for HER. Notably, at the applied potential of 1.73 V, the electrocatalyst Co2B/CoSe2

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demonstrates a 10 mA cm−2 current density for overall water splitting and displays the outstanding long-term stability. The Faradaic efficiencies of Co2B/CoSe2 for both hydrogen and oxygen evolution are close to 100%.

1. INTRODUCTION With the gradually diminishing fossil fuels and rapidly increasing environmental contamination, the exploration of alternative energy storage and conversion system becomes more and more urgently. So far, numerous kinds of clean energy are being explored, such as solar power,1,2 CO2 reduction,3,4 hydrogen evolution5-10 and so on. As an efficient and environment friendly technology for hydrogen manufacture, electrochemical water splitting, especially in alkaline condition, has some unique merits such as abundant raw materials, good outputs and low costs.11-15 Nevertheless, the activity of electrocatalytic hydrogen evolution reaction (HER) usually decreases obviously in alkaline environment compared with that of acidic electrolytes.13,16,17 In addition, the hysteretic kinetics of multistep proton-coupled electron transfer usually have a negative influence on oxygen evolution reaction (OER), which limits the efficiency of overall water splitting greatly.11,18 Up to now, RuO2 and IrO219-23 and Pt-based alloys have been proved the highly efficient catalysts for OER and are more advantageous to the catalytic HER.20 However, the intrinsic drawback of scarcity for noble metals hampers their further development in practical application.22 Therefore, seeking cheap, earth-abundant, durable and efficient catalysts for simultaneously generating O2 and H2 is still a great challenge.24,25 It has been reported that Co-based composites perform better electrocatalytic activity toward HER and OER.25-28 Recently, Yu’ group prepared a series of Co-based composites for electrocatalysis. For example, at the same current density of 10 mA cm−2, Ni/NiO/CoSe2 and MoS2/CoSe2 exhibit efficient electrocatalytic activity toward HER with the 88 mV and 68 mV


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overpotential respectively,29,30 CeO2/CoSe2 displays an excellent performance for OER with overpotential of 288 mV @ 10 mA cm−2 in 0.1 M KOH.31 CoO/CoSe2 also exhibits a good performance for OER with overpotential of 510 mV @ 10 mA cm−2 in 0.5 M phosphate buffer solution.32 Meanwhile, CoSe2/DETA nanobelts acting as an substrate has exhibited good conductivity and decent electrocatalytic activity. Mn-doped CoSe2 ultrathin nanosheets developed by Xie’s group have possessed excellent HER properties33 and the further reduction of the thickness of CoSe2 sheets to atomic scale will improve the OER electrocatalytic activity significantly.34 Boride has also attracted widespread attention as a promising class of electrocatalyst. Wu’s group fabricated a hybrid of cobalt-based borate (Co-Bi) ultrathin nanosheets and graphene by using a chemical synthesis method at room temperature. The Co-Bi NS/G hybrid exhibits high OER activity in alkaline and neutral medium owing to their abundant active sites on surface, improved electron transfer, and enhanced synergetic effects.11 The Co2B reported by Masa’s group has shown its powerful electrocatalytic ability of HER and OER in alkaline electrolytes.19 In this context, amorphous Co2B was grown on the surface of CoSe2/DETA nanosheets (DETA= diethylenetriamine) at room-temperature to form a composite catalyst (Co2B/CoSe2), which exhibits highly efficient OER electrocatalytic activity and decent HER performance in alkaline medium. The surface of CoSe2/DETA nanosheets contains abundant amino group as the nucleation sites for the nucleate and subsequent growth of Co2B. Scheme 1 outlined the fabrication procedure and the detailed experimental data were listed in the supporting information.


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Scheme 1. The procedure of Co2B/CoSe2 fabrication.

2. EXPERIMENTAL SECTION Sample Preparation. The preparation method of CoSe2/DETA nanosheets is same as that of our previous work.35 Co2B/CoSe2 hybrid was prepared by the following method. Firstly, in 10 ml DI were dispersed 0.01g freshly made CoSe2/DETA nanosheets and the resulting mixture was sonicated for 10 min. After a homogeneous solution was achieved by stirring at room temperature, 8 mL different concentrations of Co(NO3)2·6H2O deionized water solution (0.2, 0.4, 0.5, 0.6, 0.8 mM) was dissolved into and then stirred for 30 minutes. And then, 2.5 ml NaBH4 solution (0.2, 0.4, 0.5, 0.6, 0.8 mol L−1) was added and further stirred at room-temperature for 1 h. The resulting products have been collected by centrifuging and the possible ions were removed by several times washing with the mixture of water and ethanol. Finally, product was frozen by liquid nitrogen and lyophilized for 24 h. Co2B was fabricated by using the same procedure without CoSe2/DETA nanosheets. Other experimental details were shown in the Supporting Information.

3. RESULTS AND DISCUSSION Firstly, we employed the XRD method to obtain the structural information of different samples. No obvious diffraction peaks were found in the Co2B XRD pattern, suggesting the formation of an amorphous structure of Co2B nanosheets (Figure 1a). Similar to the previous reports, the reflections of CoSe2/DETA can be indexed to a standard pattern (JCPDS #09-


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234).35,36 A little shift of reflection peaks of CoSe2/DETA can be observed with respect to the standard pattern, which is attributed to the interaction between CoSe2 and DETA. As expected, the characteristic peaks of CoSe2 in Co2B/CoSe2 hybrid are weaker compared with that of CoSe2/DETA. This is caused by the coverage of Co2B on the surface of CoSe2/DETA. Furthermore, there are no characteristic peaks of Co2B in the Raman spectrum of Co2B, verifying its amorphous character. (Figure S1) In addition, four sharp peaks at 188, 463, 504, and 672 cm−1 correspond to the characteristic peaks of CoSe2 phase.29,35

Figure 1. (a) XRD patterns, (b-d) TEM images of three catalysts: CoSe2/DETA, Co2B and Co2B/CoSe2. (e and f) HRTEM images of CoSe2/DETA and Co2B/CoSe2. (g) STEM and EDX elemental mapping images of Co2B/CoSe2. The SEM (Figure S2a) and TEM (Figure 1b) images demonstrated that CoSe2/DETA nanosheets as an excellent substrate form a thin flake-like morphology, which is in favor of the regular growth of nanomaterials. Instead of the aggregated flake hierarchical structure of Co2B product (Figure 1c and S2b), much smaller Co2B nanosheets are uniformly grown on the surface of CoSe2/DETA nanosheets (Figure 1d and S2c). This morphology can be seen clearly from the high-magnification TEM image of Co2B/CoSe2 (Figure S3a). The rich amino groups on the


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surface of CoSe2/DETA nanosheets, which could afford abundant nucleation sites for the nucleate and subsequent growth of Co2B, may contribute to the formation of this desirable morphology. Figure 1e demonstrates the HRTEM image of CoSe2/DETA nanosheets. The lattice planes cricled use red circle parts in Figure 1e have a d spacing of 2.70 Å, which can be indexed to CoSe2 (210) crystal plane.34-36 The same lattice planes can also be found on the surface of Co2B/CoSe2. (Figure 1f and S3b) Meanwhile, a typical hierarchical structure can be observed from the HRTEM image of Co2B/CoSe2 nanosheets, revealing the smaller and ultrathin nature of the Co2B nanosheets. (Figure 1f) The EDX analysis verified the existence of Co, Se, B and O elements (Figure S4) and the SETM and EDX elemental mapping images confirmed the well-proportioned dispersion of these elements across the whole Co2B/CoSe2 nanosheets. Furthermore, the inductively coupled plasma optical emission spectrometry (ICP-OES) results displayed the atomic percentage (at.%) of different elements in CoSe2/DETA nanosheets, Co2B nanosheets and Co2B/CoSe2 hybrid. The Co:B atomic ratio is 1.92:1, indicating the generation of Co2B (Table S1). In order to further study the composition and electron states of surface elements, we resorted to the XPS. In good agree with our EDX analysis, the XPS survey spectrum of Co2B/CoSe2 shown in Figure 2a also reveals the presence of Co, Se, B and O elements. As observed in Figure 2b, two emission peaks in the high-resolution XPS spectrum of Co 2p are in line with Co 2p3/2 and Co 2p1/2 respectivly, indicating the existence of Co2+.34-36 Two Se 3d emission peaks at 54.4 and 59.9 eV can be ascribed to the Se 3d5/2 and Se 3d3/2.

35,37-40

(Figure

2c) According to Figure 2d, the B 1s characteristic peak at 192.3 eV, which agrees very well


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with the B3+ state.8 Furthermore, a dominant peak at the binding energy of 531.3eV should be assigned to the O 1s signals (Figure S5).11,41

Figure 2. (a) XPS survey spectrum of Co2B/CoSe2. (b) Co 2p, (c) Se 3d and (d) B 1s XPS spectra in Co2B/CoSe2 composites. We firstly evaluated the OER activity of the Co2B/CoSe2 catalyst in 1 M KOH alkaline solution. The effect of the loading amount of Co2B on CoSe2 for OER performance is studied (Figure S6). Originally, with increasing Co2B concentration, the performance of the hybrid catalyst increases. It is interesting to note that as the continuing increase of the Co2B loading amount, the OER performance of the catalysts decline. The results show that Co2B/CoSe2 exhibits the best OER activity when the precursor of Co is 0.5 mM. The overpotential of CoSe2/DETA nanosheets is 392 mV @ 10 mA cm−2 in alkaline solution, decreasing 8 mV compared with previously reported.34 (Figure 3a) In contrast, the overpotential of Co2B decreases 19 mV compared with that of CoSe2/DETA nanosheets. By further incorporating Co2B with CoSe2/DETA nanosheets, Co2B/CoSe2 exhibits the best catalytic activity for OER with a much lower overpotential of 320 mV @ 10 mA cm−2, and a small Tafel slope (56.0 mV dec−1),


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suggesting the more favorable OER kinetics. (Figure 3b) The impressive Tafel slope, which is smaller than that of most non-precious-metal OER catalysts (listed in Table S2), is comparable to the value of commercial RuO2 catalyst, implying the similar surface reaction routes. The homogeneously distributed amino groups induce Co2B to nucleate, disperse, and reduce size, thus exposing more active sites for OER. This can be further confirmed by the electrochemical double layer capacitor (Cdl) measurement which can be used to estimate the effective surface area of electrode. As shown in Figure S7, the Cdl of Co2B/CoSe2 (36.2 mF cm-2) is higher than those of Co2B (28.0 mF cm-2) and CoSe2/DETA (25.4 mF cm-2). The enhanced Cdl indicates that more effective active sites are exposed after combining Co2B with CoSe2/DETA, which is in favor of electrocatalytic activity. Moreover, Co2B/CoSe2 displays an amazing exchange current density (j0) of 33.5 µA cm−2 by extrapolation of Tafel plots, demonstrating its high OER catalytic activity. All of samples have a low charge transfer resistances (Rct), corresponding to a large reaction rate (Figure S7e). In addtion, the current density only decreases 4.5% after 30 h controlled potential electrolysis at static potential of 1.55 V, revealing the excellent stability of Co2B/CoSe2 (Figure S7f). It is noting that Co2B/CoSe2 also exhibits decent catalytic ability for HER in alkaline solution. Incorporating with different amount of Co2B on CoSe2, Co2B/CoSe2 catalysts also exhibit different HER activities in 1M KOH (Figure S8). Similar to the OER, Co2B/CoSe2 also exhibits the best HER activity when the precursor of Co is 0.5 mM. As listed in Table 1, Co2B also exhibits superior HER electrocatalytic activity than CoSe2/DETA, indicating the intrinsic good electrocatalytic activity of boride in alkaline condition.11,19 As shown in Figure 3c, Co2B/CoSe2 demonstrates superior HER electrocatalytic activity to that of Co2B and CoSe2/DETA. The overpotential is 300 mV @ 10 mA cm−2, and the Tafel slope is as small as


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76.0 mV dec−1. The HER electrocatalytic activity of Co2B/CoSe2 in alkaline solution is better than many other similar catalysts reported so far (listed in Table S3). In addition, Co2B/CoSe2 also exhibits a remarkable exchange current density (j0) of 23.8 µA cm−2, showing its good HER catalytic activity. The largest electrochemical double layer capacitor (Cdl) of Co2B/CoSe2 (30.9 mF cm-2) indicates that more effective active sites are exposed after Co2B is grown on the surface of CoSe2/DETA (Figure S9). The low charge transfer resistances (Rct) for all composites suggest the faster HER kinetics (Figure S9e). In addition, after operating HER at static overpotential of 300 mV for 30 h, and the current density only decreases 1.9%, exhibiting the excellent stability of Co2B/CoSe2 (Figure S9f).

Figure 3. (a) Linear sweep voltammograms and (b) Tafel plots for OER in 1 M KOH. (c) Linear sweep voltammograms and (d) Tafel plots for HER in 1 M KOH. (e) Linear sweep voltammograms of water electrolysis employing Co2B/CoSe2 catalyst. (f) The stability test of electrolyzer at static potential of 1.55 V for 30h. (f) The optical photograph of the cell during electrolysis.


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Table 1. Electrochemical parameters of different materials

OER

HER

Materials

ηj=10 [a]

b

[b]

j0 [c]

Cdl [d]

CoSe2/DETA Co2B Co2B/CoSe2 CoSe2/DETA Co2B Co2B/CoSe2

392 373 320 386 350 300

83.6 80.2 56.0 107.8 78.5 76.0

17.9 24.8 33.5 9.6 16.5 23.8

25.4 28.0 36.2 15.4 23.1 30.9

[a] Overpotential (mV) at current density of 10 mA cm-2. [b] Tafel slope (mV dec-1). [c] Exchange current density (µA cm-2). [d] Double-layer capacitance (mF cm-2).

As shown in Figure 3e, Co2B/CoSe2 was acted as a good electrocatalyst for simultaneously generating O2 and H2 in 1 M KOH. As the inset in Figure 3f shown, two pieces of carbon fiber cloth were deposited with catalysts to act as both anode and cathode. Obvious gas evolution was observed at both electrodes from the photographic image and the video of the electrolysis cell for overall water splitting during electrolysis. Co2B/CoSe2 exhibits the best performance for electrocatalyst with the current density of 10 mA cm−2 at the applied potential of 1.73 V. Additionally, the Faradaic efficiency of hydrogen and oxygen evolution are close to 100%, and as shown in Figure S10, the volume ratio of H2 and O2 is close to 2. Moreover, continuous watersplitting test at 1.73 V was conducted to probe the durability of Co2B/CoSe2. (Figure 3f) It is notable that the current density only decreases 1.87% after operating for 30 h. The excellent stability of Co2B/CoSe2 for water splitting can be attributed to the good CoSe2/DETA nanosheet substrate for nucleation of Co2B and their positive interaction to promote the mechanical stability of the hybrid. These all demonstrate the practicability of Co2B/CoSe2 as an excellent electrocatalyst for overall water splitting.

4. CONCLUSIONS To sum up, we synthesized a novel electrocatalyst for overall watersplitting in alkaline solution. Amorphous Co2B was grown on the surface of CoSe2/DETA nanosheets at roomtemperature to construct the composite catalyst (Co2B/CoSe2), which exhibits both excellent


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electrocatalytic activity for OER with a much lower overpotential of 320 mV and good HER performance with 300 mV overpotential at current density of 10 mA cm−2 in 1 M KOH. More importantly, the current density of 10 mA cm−2 is achieved at the applied potential of 1.73 V by applying Co2B/CoSe2 for overall water splitting and this catalyst system simultaneously demonstrates the outstanding long-term stability. In this efficient system, CoSe2/DETA nanosheets afford a conductive substrate, not only demonstrate superior electrocatalytic activity, but also provide copious sites for the nucleation, dispersion and size reduction of Co2B, which affords sufficient electrocatalytic active sites. Meanwhile, the synergetic effects of Co2B and CoSe2/DETA promote the mechanical stability of the hybrid. This facile method can simulate further exploration of other electrocatalysts for overall watersplitting in the future. ASSOCIATED CONTENT Supporting Information Available: Description of the material. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. ACKNOWLEDGMENTS We are grateful for the financial support from the NSFC (No.21427811) and Most China YFA 0203200. REFERENCES


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Table of Contents


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Amorphous Co2B Grown on CoSe2 Nanosheets as a Hybrid

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