Nanocrystalline Co0.85Se Anchored on Graphene Nanosheets as a

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Nanocrystalline Co Se Anchored on Graphene Nanosheets as a Highly Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction Bo Yu, Fei Qi, Yuanfu Chen, Xinqiang Wang, Binjie Zheng, Wanli Zhang, Yanrong Li, and Lai-Chang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09108 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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ACS Applied Materials & Interfaces

Nanocrystalline Co0.85Se Anchored on Graphene Nanosheets as a Highly Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction Bo Yu‡a, Fei Qi‡a, Yuanfu Chen*‡a, Xinqiang Wanga, Binjie Zhenga, Wanli Zhanga, Yanrong Lia, and Lai-Chang Zhang*b a

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of

Electronic Science and Technology of China, Chengdu 610054, PR China b

School of Engineering, Edith Cowan University, 270 Joondalup Dr, Perth, WA 6027,

Australia *Corresponding authors, emails: [email protected], [email protected], [email protected] ‡These authors have contributed equally. KEYWORDS: Co0.85Se nanocrystals, graphene, solvothermal reaction, transition metal chalcogenide, hydrogen evolution reaction, electrocatalytic activity

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ABSTRACT For the first time, porous and conductive Co0.85Se/graphene network (CSGN), constructed by Co0.85Se nanocrystals being tightly connected with each other and homogenously anchored on few-layered graphene nanosheets, has been synthesized by a facile one-pot solvothermal method. Compared to unhybridized Co0.85Se, CSGN exhibits much faster kinetics and better electrocatalytic behavior for hydrogen evolution reaction (HER). The HER mechanism of CSGN is improved to Volmer-Tafel combination, instead of Volmer-Heyrovsky combination, for Co0.85Se. CSGN has a very low Tafel slope of 34.4 mV/dec, which is much lower than that of unhybridized Co0.85Se (41.8 mV/dec) and is the lowest ever reported for Co0.85Se-based electrocatalysts. CSGN delivers a current density of 55 mA/cm2 at 250 mV overpotential, much larger than that of Co0.85Se (33 mA/cm2). Furthermore, CSGN shows superior electrocatalytic stability even after 1500 cycles. The excellent HER performance of CSGN is attributed to the unique porous and conductive network, which can not only guarantee interconnected conductive paths in the whole electrode but also provide abundant catalytic active sites thereby facilitating charge transportation between the electrocatalyst and electrolyte. This work provides insight into rational design and low-cost synthesis of non-precious transition-metal chalcogenide-based electrocatalysts with high efficiency and excellent stability for HER.

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INTRODUCTION

Hydrogen energy as a clean energy has attracted a great deal of interest because its combustion product is water

1, 2

. A clean method to prepare hydrogen energy is water

electrolysis through hydrogen evolution reaction (HER) with the help of electrocatalysts 3. So far, the most effective electrocatalysts are based on noble metals, such as platinum (Pt) and palladium. Yet, the rareness and preciousness of noble metal electrocatalysts have greatly restricted the practical applications in hydrogen evolution 4. Therefore, extensive endeavor has been made to develop earth-abundant, low-cost, and highly efficient electrocatalysts 5-12.

Recently, transition metal chalcogenides (TMCs) 13-15 with low cost, abundant resources, relatively good HER performance have received increasing attentions as HER electrocatalysts 16-20

. As typical TMCs materials, cobalt selenides including CoSe2 and CoSe are regarded as

highly efficient hydrogen evolution electrocatalysts due to their earth-abundance, inexpensiveness, and superior electrocatalytic performances

21-24

. Compared with

stoichiometric phase of cobalt selenides (CoSe2, CoSe) for water splitting use

24-27

, the

nonstoichiometric phase of cobalt selenide (Co0.85Se) has been rarely studied. The nonstoichiometric Co0.85Se has intrinsic half-metallic character (high conductivity) owing to the presence of an overlap between Co 3d and Se 4p spin-up (alpha) electrons, making it uniquely advantaged as an electrocatalyst material 28, 29. To our best knowledge, very limited work has reported on the synthesis and HER activity of Co0.85Se Hou et al.

29

29, 30

. Huang et al.

30

and

reported that the Co0.85Se-based composites exhibit attractive hydrogen

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evolution performances for water splitting. However, compared to Pt-based electrocatalysts, the electrocatalytic performance of these reported Co0.85Se is still lower and needs to be further improved. Therefore, it is expected to obtain highly efficient Co0.85Se-based electrocatalyst with abundant active sites and excellent conductivity.

There are several strategies to realize the excellent HER performance of Co0.85Se. Firstly, nanostructurization is an effective method to enhance the HER performances of Co0.85Se-based electrocatalysts. Many reports on nanostructured TMCs, such as nanowires 31, nanosheets

32

, nanoarrays

10

, and nanoparticles

33

, have illustrated that nanostructured

electrocatalysts deliver higher efficiency than their corresponding coarse-grained counterparts because nanostructure can supply much more abundant electrocatalytic active sites. Similarly, the nanocrystallization technique can also provide more electrocatalytic active sites for improving HER performance 34, 35. Furthermore, it is feasible to make a conductive matrix for Co0.85Se, which is favorable for charge transportation. Graphene with high conductivity could be a fantastic option for this purpose, as it has been extensively used to improve the conductivity of electrocatalysts and electrode materials

8, 36-39

In this regard, graphene

modified Co0.85Se composite could deliver fascinating HER performances. Nevertheless, up to now, there is no report on the combination of nanocrystallization and graphene coating to improve the electrocatalytic performance of Co0.85Se.

Herein, for the first time, porous and conductive Co0.85Se/graphene network (CSGN) has been synthesized through a facile one-pot solvothermal method. CSGN is constructed by ultra-small sized Co0.85Se nanocrystals being tightly connected with each other and 4


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homogeneously anchored on few-layered graphene nanosheets. Due to its unique structure, CSGN exhibits superior HER performance with the smallest Tafel slope of 34.4 mV dec-1 among the reported Co0.85Se-based electrocatalysts and excellent long-term stability. The excellent HER performance of CSGN is attributed to the synergistic effect of its unique self-assembled porous and conductive network: the Co0.85Se nanocrystals with ultra-small size are being strongly connected with each other and uniformly anchored on highly conductive graphene nanosheets. Such unique network can not only guarantee high conductivity of the whole electrode, but also provide abundant catalytic active sites to facilitate the charge transportation between the electrocatalyst and the electrolyte.

Experimental section

Synthesis of graphene oxide

Graphene oxide (GO) was synthesized from natural flake graphite by Hummers' method, similar to our previous report

15

. 10 g graphite and 40 g KMnO4 were added into 400 mL

concentrated H2SO4 and stirred for 2 h at 35 °C. Then 800 mL deionized water (DI water) was slowly added into the mixed solution and the temperature of solution was kept at 95 °C. Next, 10 mL H2O2 was added into the previous solution when its temperature decreased to 60 °C. Afterward, 200 mL diluted HCl was added. Finally, the GO solution was filtrated several times with DI water and then dried.

Synthesis of Co0.85Se nanocrystals

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Co0.85Se was fabricated through a facile one-pot solvothermal method. 0.316 g Se and 0.189 g NaBH4 were dissolved into 50 mL N, N-dimethylformamide (DMF). The mixture was stirred until a homogeneously dispersed black-color solution was obtained. Then, 0.809 g CoCl2•6H2O was added into this black-color solution. Next, the as-prepared solution was transferred into a 65 mL Teflon-lined autoclave and heated at 160 °C for 24 h. After cooled naturally, the acquired products were collected and filtrated with ethanol and DI water for several times and dried in vacuum at 60 °C for 12 h. It is noted that during synthesis in this study, NaBH4 is acted as the reductant to reduce Se to Se2- and Se2- further reacted with Co ion to form Co0.85Se, whose role is similar to previous researches to synthesize selenium compounds 40, 41.

Synthesis of CSGN

The CSGN was synthesized by the same solvothermal reaction conditions as those for Co0.85Se nanocrystals. The prepared process is schematically demonstrated in Figure 1. A GO solution of 2.4 mg/mL was added to the mixture of Se and NaBH4, and the total volume of the solution was adjusted to 50 mL. All other procedures were same as the synthesis processes for Co0.85Se nanocrystals.

Characterization

The morphology and size of samples were characterized by scanning electron microscopy (SEM, JSM-7000F, JEOL) and transmission electron microscopy (TEM, Tecnai F20) equipped with a Gatan imaging filter (GIF). The crystal structure of samples was analyzed through X-ray diffraction (XRD, Rigaku D/MAX-rA 6


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diffract-ometer). The Raman spectrum was acquired by the Raman microscope (532 nm, Horiba). The chemical composition measurements were performed by X-ray photoelectron spectroscopy (XPS, Kratos XSAM800) using Al Kα radiation. Surface area analysis was conducted using the Brunauer-Emmett–Teller (BET) theory (ASASP2020).

Measurements of Electrocatalytic Properties

All electrocatalytic measurements were performed on an electrochemical station (CHI660D) in 0.5 M H2SO4 with a three-electrode system. Detailed electrochemical characterizations are similar to previous report

18

. To prepare the work electrode, 4 mg

CSGN was homogeneously dispersed into 250 µL ethanol and 750 µL DI water mixed solution with 50 µL Nafion (5% w/w in water and 1-propanol). Then 10 µL of the above slurry was dropped on polished glassy carbon electrode (GCE) with a diameter of 3 mm. For comparison, Co0.85Se and commercial 20 wt% Pt/C powder were also both dispersed on the polished GCE in the same way. Linear sweep voltammetry (LSV) was performed with 5 mV s-1 scan rate and electrodes were cycled at least 40 cycles prior any measurements. AC impedance measurements were performed at -0.17 V vs RHE in the frequency range 0.1 to 100 KHz with an AC voltage of 5 mV. In order to estimate the effective active surface area (ECSA) of Co0.85Se and CSGN, the cyclic voltammograms measurements at various scan rates (20 ~ 200 mV/s) in the region of 0.2 ~ 0.3 V (vs RHE) were carried out. The current density differences (∆J = Ja - Jc) at 0.25 V (vs RHE) are then plotted against the scan rate from cyclic voltammograms. The electrochemical double-layer capacitance (Cdl) values, which is equivalent to the linear slope of the curve of ∆J/2 vs scan rate, are used to represent 7



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and compare the ECSA of the bare Co0.85Se and the CSGN.

Results and discussion

Figure 2a shows the XRD patterns of the synthesized CSGN and Co0.85Se. The XRD peaks are observed at 33.3°, 44.7°, 50.6°, 60.4°, 61.9°, and 69.9° for both CSGN and Co0.85Se. These identified peaks are respectively assigned to the crystal planes of (101), (102), (110), (103), (112), and (202) of standard hexagonal structure of Co0.85Se (JCPDS 52-1008). It is noted that the diffraction peak of the (002) crystal plane for graphene could not be clearly observed at 26.5° for the CSGN, which is similar to previous studies

42

. This is attributed to the low content of reduced graphene oxide

(RGO) and the no stack of graphene in the CSGN. Figure 2b displays the Raman spectrum of CSGN. The peak identified at 176 cm-1 corresponds to the Co-Se stretching mode of hexagonal Co0.85Se. The other three Raman peaks at 1345, 1585, and 2675 cm-1 can be attributed to the D, G, and 2D bands of graphene, respectively, which indicate the presence of RGO in the hybrid.

The chemical composition of the CSGN was characterized from XPS, as shown in Figure 3a. The four elements including C, Co, Se, and O are obviously identified in the hybrid. The core 1s level peak of C element can be deconvoluted into four peaks, as illustrated in Figure 3b. The main peak at 284.78 eV is assigned to the sp2 hybridized graphite carbon atom (C‒C). The other three peaks at 285.7 eV, 287.6 eV, and 289.4 eV can be attributed to the C‒O, C=O, and O‒C=O chemical bonds,

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respectively. These values are consistent with those for RGO reported in previous studies

8, 36, 43

, which again prove the presence of RGO in the hybrid. As shown in

Figure 3c, the both Co 2p1/2 and Co 2p3/2 peaks have been deconvoluted into three peaks. The peaks at S1 (780.9 eV) and S1’ (797.2 eV) are originated from the Co2+ cations. The peaks at S2 (783.2 eV) and S2’ (799.2 eV) are originated from the Co3+ cations of the surface oxide phase. The S3 and S3’ peaks at 786.2 eV and 803.4 eV correspond to the shakeup satellite peaks, which is similar to the previous report

44-47

.

In Figure 3d, the peaks at 55 eV and 59.45 eV can be assigned to the binding energies of Se 3d and Co 3p, respectviely.

In order to characterize the pore size and distribution, the BrunauerEmmett-Teller (BET) measurements have been performed. The N2 sorption isotherm plots and pore size distribution curves of the Co0.85Se and CSGN have been shown in Figure S1. The calculated specific surface area of CSGN (21.12 m²/g) is much larger than that of Co0.85Se (10.61 m²/g), and the total volume of pores for CSGN (0.126218 cm³/g) is also much larger than that of Co0.85Se (0.073946 cm³/g). It suggests that the CSGN with larger specific surface area and porosity has better electrocatalytic behavior than that of Co0.85Se.

The morphologies of the Co0.85Se and CSGN were performed by SEM. As seen from Figures 4a and b, the Co0.85Se exhibits nanocrystals with quite small size which can expose abundant active sites. Co0.85Se nanocrystals stack together and the interspace, which is favourable to electrolyte penetration, is retained. As shown in 9



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Figure 4c and d, when the RGO is introduced, the Co0.85Se nanocrystals grow on graphene nanosheets and inhibit the graphene nanosheets from stacking. Meanwhile, the RGO constitutes a conductive network in the CSGN, which facilitates the electronic transportation.

TEM and high-resolution TEM (HRTEM) images of Co0.85Se and CSGN were performed. As shown in Figure 5a, the Co0.85Se nanocrystals are quite fine (20~30 nm in size) and with a rough and branchy surface. Each nanocrystal is connected tightly with its neighboring nanocrystals. From the HRTEM image shown in Figure 5b, the interplanar distances are measured to be 0.20 nm and 0.26 nm, corrsponding to the d-spacing values for the (102) and (101) crystal planes of Co0.85Se, respectively, which agree with the XRD data shown in Figure 2a. From the TEM image in Figure 5c, the Co0.85Se nanocrystals are connected with each other and distributed on the RGO nanosheets. Furthermore, the structural characteristics of CSGN with visible lattice fringes are obviously observed in Figure 5d. The interplanar distances of 0.26 nm corresponds to the d-spacing for the (101) lattice plane of Co0.85Se. The interlayer distance of 0.37 nm corresponds to the d-spacing for the (002) lattice plane of graphene.

Figure 6 shows the catalytic activity of Co0.85Se and CSGN as electrocatalytic materials for HER. As presented in Figure 6a, the polarization curves of GCE, Pt/C, Co0.85Se and CSGN are acquired at 5 mV/s and the current density is normalized by geometric surface area of GCE. The GCE exhibits no catalytic activity for HER. The 10


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Co0.85Se and CSGN present 10 mA/cm2 at -210 and -196 mV vs RHE, respectively. Furthermore, the CSGN delivers 55 mA/cm2 at 250 mV overpotential, which is higher than 33 mA/cm2 at 250 mV overpotential for the Co0.85Se. These results indicate that the CSGN exhibits surpassed electrocatalytic HER performance than Co0.85Se. To shed an insight into the electrocatalytic activity of CSGN and Co0.85Se, their Tafel plots were investigated and were fitted to the Tafel equation: η = b log(j) + a, where η is the overpotential, j is the current density, and b is the Tafel slope. As shown in Figure 6b, the Tafel slopes of Co0.85Se, CSGN, and Pt/C electrocatalysts are 41.8, 34.4, 30.6 mV/dec, respectively. The CSGN delivers a smaller Tafel slope than that of Co0.85Se, indicating that the CSGN has higher catalytic activity for HER. To the best of our knowledge, the Tafel slope of 34.4 mV/dec is the smallest value for Co0.85Se-based electrocatalysts, which is even smaller than those for most cobalt or nickel chalcogenides electrocatalysts (shown in Figure 6c and Table S1) 21, 23, 25, 27, 30, 48-52, indicating the superior HER activity for CSGN. The small Tafel slope of the CSGN is advantageous for practical applications, since it leads to a quicker increment of HER rate with rising overpotential 32.

To understand the process of converting H+ to H2 in acidic media, the following three reaction steps have been proposed 32, 33, 47, Volmer reaction: catalyst + H3O+ + e- → catalyst-Hads + H2O

(1)

followed by either Heyrovsky reaction: catalyst-Hads + H2O+ + e- → H2 (g) + H2O or 11


(2)


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Tafel reaction: catalyst-Hads + catalyst-Hads → 2 catalyst + H2 (g)

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(3)

The Tafel slope determined by the rate-limiting step of HER is an inherent property of electrocatalysts. Generally, when the rate-limiting step of HER is the Volmer reaction, the Tafel slope should be 120 mV/dec. Additionally, the Heyrovsky or Tafel reaction is the rate-limiting step of HER corresponding to a Tafel slope of 40 or 30 mV/dec. It is noted that the values for Tafel slope of unhybridized Co0.85Se, and CSGN are 41.8, 34.4 mV/dec, respectively. Obviously, Volmer-Heyrovsky combination mechanism is operative for unhybridized Co0.85Se in HER process, while Volmer-Tafel combination mechanism is operative for CSGN. It suggests that CSGN hybrid as an electrocatalyst exhibits much faster HER kinetics and better electrocatalytic behavior compared to Co0.85Se.

Stability is another significant criterion by which an electrocatalyst to be evaluated. To investigate the stability in 0.5 M H2SO4 solution, the polarization curves of CSGN was performed by taking the continuous cyclic voltammetry (CV) test. As shown in Figure 7a, after 1500 CV cycles at 100 mV/s with the range of -0.35 to 0.25 V vs RHE, we can clearly find the similar curve to the initial one, indicating the superior stability of CSGN in a long-term electrochemical process. Moreover, the chronoamperometry (j-t) measurement was carried out to further probe the durability of CSGN in an acidic environment. Figure 7b shows the time dependence of current density under static potential of -0.2 V vs RHE. The current density exhibits only slight degradation even after a long period of 20 h, which might be caused by the consumption of H+ or the remaining of H2 bubbles on the surface of the electrode 12


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thereby hindering the reaction

32

. The outstanding electrocatalytic activity as well as

stability of the CSGN catalyst may make it a promising HER catalyst for practical applications.

In order to understand why the CSGN exhibits excellent HER performance, the effective electrochemically active surface area (ECSA) is estimated by electrochemical double-layer capacitance (Cdl) of the Co0.85Se and CSGN, which were determined from the cyclic voltammograms measured at various scan rates (20 ~ 200 mV/s) in the region of 0.2 ~ 0.3 V (vs RHE). Figure 8a and b shows the cyclic voltammograms of the Co0.85Se and CSGN. The electrochemical double-layer capacitance (Cdl) value is equivalent to the slope shown in Figure 8c. The Cdl values of Co0.85Se and CSGN are 2.91 and 6.42mF/cm2, respectively. It is noteworthy that the Cdl of the CSGN is 120.6% larger than that of Co0.85Se, which suggests that CSGN has many more exposed active sites and better electrocatalytic performance. Figure 8d shows the electrochemical impedance spectroscopy (EIS) performed at -0.17 V vs RHE. The EIS can be fitted to the equivalent circuit (the Figure 8d inset), where Rs is the series resistance and Rct is the charge-transfer resistance. Fitted results show the Rs and Rct of Co0.85Se are 13.6 and 228 Ω respectively and the Rs and Rct of CSGN are 9.3 and 177 Ω respectively. The smaller Rs value indicates that the CSGN has better electronic conductivity than Co0.85Se. The smaller Rct value suggests that CSGN has a quicker charge transfer at the interface between electrocatalyst and electrolyte in comparison to Co0.85Se. The smaller Rct of CSGN can be attributed to its unique structure: the Co0.85Se and graphene form a three-dimensional porous and conductive network, 13



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which can guarantee interconnected conductive paths in the whole electrode; this network will facilitate the highly efficient electrical communication between the catalytic sites and the graphene nanosheets, leading to a faster charge transfer and smaller Rs and Rct.

The electrocatalytic process of CSGN is schematically illustrated in Figure 9. Electrons can transfer through the graphene matrix to Co0.85Se nanocrystals catalyst uniformly anchored on highly conductive RGO surface. Hydrogen ions at the exposed active sites of Co0.85Se nanocrystals catalyst are reduced by the transferred electrons, followed by release of hydrogen gas. The fast HER kinetics and excellent HER performance and long-term stability of CSGN can be simply interpreted as follows. The CSGN hybrid has a unique porous and conductive network constructed by ultra-small sized Co0.85Se nanocrystals being tightly connected with each other and uniformly anchored on highly conductive graphene nanosheets, which guarantees its superior performance: (i) the nanostructured Co0.85Se grains with size of ~10 nm and with rough and branchy surface are strongly connected with each other and retain interspace, which can be favourable to porous networks, providing abundant active sites and facilitating the electrolyte penetration and the charge transportation; (ii) the Co0.85Se nanocrystals are homogenously anchored on the few-layered graphene nanosheets, which guarantees a conductive network and facilitates rapid charge transportation; (iii) the Cdl calculated from CV curves demonstrates that the CSGN has much larger Cdl than that of Co0.85Se, which further confirms that CSGN has many more electrocatalytic active edge sites; (iv) the EIS data indicate that CSGN has much 14


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lower series resistance and charge transfer resistance, which suggests that after introducing graphene, the conductivity of the whole electrode is effectively enhanced and the charge transportation ability between the electrocatalysts and electrolyte is improved.

Conclusions

In summary, for the first time, the porous and conductive Co0.85Se/RGO hybrid is prepared by a facile one-pot solvothermal method. The Co0.85Se/RGO hybrid as an electrocatalyst delivers excellent electrocatalytic hydrogen evolution reaction (HER) performances with a very small Tafel slope of 34.4 mV/dec and superior electrocatalytic stability. The excellent HER activity and durability, earth abundance, together with facile and low-cost synthesis method, make Co0.85Se/RGO hybrid a promising candidate alternative to precious metal catalysts for HER.

AUTHOR INFORMATION

Corresponding Author

[email protected]; [email protected]

ACKNOWLEDGMENT

The research was supported by the National High Technology Research and Development

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Program of China (Grant No. 2015AA034202), the National Natural Science Foundation of China (Grant No. 51372033), and the 111 Project (Grant No. B13042).

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Figures

Figure 1. Schematic illustration of the synthesis process of Co0.85Se/graphene network (CSGN).

Figure 2. (a) XRD patterns of Co0.85Se/graphene network (CSGN) and Co0.85Se. (b) Raman spectrum of CSGN.

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Figure 3. Survey spectrum (a) and high-resolution XPS spectra of C 1s (b), Co 2p (c), and Se 3d (d) of CSGN.

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(a)

(b)

3 μm

1 μm

(c)

(d)

5 nm

1 μm

3 μm

Figure 4. SEM images of as-prepared Co0.85Se (a-b) and Co0.85Se/graphene network (c-d).

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Figure 5. (a) TEM image and (b) HRTEM image of Co0.85Se. (c) TEM image and (d) HRTEM image of Co0.85Se/graphene network.

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Figure 6. Electrocatalytic performance of CSGN and Co0.85Se: (a) polarization curves and (b) corresponding Tafel plots. (c) Comparison of Tafel slopes for GSNG versus cobalt chalcogenides.

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Figure 7. (a) Durability test for CSGN after 1500 CV cycles. (b) Time dependence of current density under static potential of −0.2 V (vs RHE) for CSGN.

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Figure 8. Cyclic voltammograms of Co0.85Se (a) and Co0.85Se/graphene network (CSGN) (b). (c) Estimated Cdl and relative electrochemically active surface area for the Co0.85Se and CSGN. (d) Nyquist plots (100 kHz–10 mHz) of Co0.85Se and CSGN at -0.17 V vs RHE. The data were fitted to the equivalent circuit shown in the inset.

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Figure 9. Schematic illustration of the Co0.85Se/graphene network and its application in the hydrogen evolution reaction.

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Table of Contents (TOC)

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Nanocrystalline Co0.85Se Anchored on Graphene Nanosheets as a

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