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DOI: 10.1039/C5RA05706J (Communication) RSC Adv., 2015, 5, 39075-39079

Ultra-tiny Co(OH)2 particles supported on graphene oxide for highly efficient electrocatalytic water oxidation†

Jiong Liuab, Fuping Dua, Haojie Zhanga, Chao Lina, Peng Gaoa, Yuyun Chena, Zhifang Shia, Xiaopeng Li*a, Tiejun Zhao*a and Yuhan Sunac
aCAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201412, PR China. E-mail: lixp@sari.ac.cn; zhaotj@sari.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, PR China
cInstitute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China

Received 31st March 2015 , Accepted 22nd April 2015

First published on 23rd April 2015

Abstract

Here, we report a novel highly efficient cobalt-based catalyst for the oxygen evolution reaction (OER), which consists of ∼2 nm Co(OH)2 clusters supported on graphene oxide (Co(OH)2/GO). Compared with hydrothermally treated Co3O4/GO (HT-Co3O4/GO) reported in the literature, Co(OH)2/GO shows comparable OER performance but with 42% less cobalt loading mass. The turnover frequency (TOF) of Co(OH)2/GO is 2.8 times as high as that of HT-Co3O4/GO. Our result presents a new opportunity for researchers to design efficient OER electrocatalysts.

In renewable energy technologies, the development of durable and highly efficient electrocatalysts that can convert water into oxygen, and vice versa, is of pivotal importance.1–3 The bottleneck of electrochemical water splitting is the oxygen evolution reaction (OER) with sluggish reaction kinetics.4 So far heterogeneous OER electrocatalysts mainly include transition metal oxides such as RuO2, IrO2, PtO2, MnO2, and Co3O4.5 Although the most active catalysts are the scarce noble metal oxides such as RuO2, IrO2 and PtO2, there are constant interests in developing cost-effective OER catalysts based on earth-abundant metals. Co3O4 is slightly less active than the noble metal oxides for electrocatalytic water oxidation in alkaline solution.5 Therefore, different techniques and strategies have been employed to further improve the electrochemical activity of cobalt oxides, such as particle size reduction,6 specific morphological design,7 alloying with other foreign metal elements8,9 and hybrid with conductive carbon materials.10,11

Since the report by Dai et al.,11 hybrid cobalt oxides with conductive graphene and its derivatives especially has been considered as one of the most promising routes to promote the electrochemical activity of cobalt oxides due to several advantages: (I) graphene has a theoretical surface area up to 2630 m2 g−1. It can act as an ideal support to disperse fine catalytic metal oxide particles. Moreover, its unique two-dimensional lamellar structure allows full surface accessibility to the electrolyte. (II) Graphene and its oxides contain abundant oxygenated functional groups, such as epoxides and hydroxyl on the basal plane and carbonyl and carboxyl groups at the edges. These oxygenated groups can act as nucleation sites for metal ions to form ultra-fine metal oxide nanoparticles on graphene materials. (III) Conductive graphene materials can effectively compensate the poor conductivity of metal oxides. (IV) The formed metal–carbon bonds can generate synergistic effects resulting in the remarkable enhancement of the electrochemical activity of the composite electrocatalysts.12–14 Therefore, many studies have been carried out to utilize graphene and its derivatives as the support of Co3O4 nanoparticles as the catalyst for ORR,11 OER,11 and degradation of orange II in water.15

In this report, we started with graphene oxide (GO) and firstly synthesized the reference samples including Co3O4/GO and hydrothermal treated Co3O4/GO (HT-Co3O4/GO) according to the typical recipe reported in the ref. 11. Through tuning the preparation conditions, we successfully obtained a new type of cobalt based GO nanocomposite electrocatalyst. The cobalt based particle size is as small as ∼2 nm. According to the FT-IR and XPS data, the cobalt particle phase was confirmed as Co(OH)2. The Co(OH)2/GO exhibited slightly higher OER performance in comparison with the reference samples while the cobalt loading mass is 36–42% less. The turnover frequency (TOF) of Co(OH)2/GO is 2.8 times as high as that of HT-Co3O4/GO.

To obtain Co3O4/GO nanocomposite electrocatalyst, commonly used methods involve the precipitation of cobalt ions on GO under alkaline conditions (ammonia solution), followed by calcination or hydrothermal treatment at high temperature in early literatures.11,16–18 Detailed preparation procedures are described in the Experimental section. Fig. 1(a) and (d) shows cobalt oxide nanoparticles before and after hydrothermal treatment. Nanoparticles are densely distributed on the GO surface as shown in the transmission electron microcopy (TEM) images in Fig. 1(a) and (c). The average nanoparticles size increases from 3.3 nm to 6.9 nm after hydrothermal treatment (see inset pictures of Fig. 1(a) and (c)).


image file: c5ra05706j-f1.tif
Fig. 1 TEM images of (a) Co3O4/GO and (c) HT-Co3O4/GO. (b) and (d) are the corresponding HRTEM images. The inset pictures in (a) and (c) are particle-size distribution diagrams.

The high-resolution TEM (HRTEM) image clearly shows the crystalline nature (seen in Fig. 1(b) and (d)). The interplanar distances with the d-spacings of 0.20 nm, 0.24 nm, 0.29 nm, and 0.47 nm correspond to the (400), (311), (220), and (111) facets of spinel Co3O4, respectively.19

The ultra-tiny cobalt-based nanoparticle/GO was obtained via reducing the precipitation temperature to 0 °C and decreasing the cobalt ion concentration. A large number of small crystal nucleus will form at low temperature once the solution system reaches the precipitation threshold,20 while low cobalt ion concentration can stop the further growth of crystal nucleus. Fig. 2(a) and (b) shows that nanoparticles are uniformly distributed all over the GO surfaces. The HRTEM image shown in Fig. 2(c) reveals the crystalline nature of the nanoparticles. The average diameter of nanoparticles is 2.2 nm (see Fig. 2(d)). However, the actual average diameter is slightly less than 2.2 nm, because it has been observed that the cobalt-based nanoparticles grew slowly after exposing to high energy electron beam in the TEM.


image file: c5ra05706j-f2.tif
Fig. 2 (a), (b) and (c) are TEM image and HRTEM images of Co(OH)2/GO. (d) is particle-size distribution diagram of Co(OH)2/GO.

The phases of the aforementioned samples were firstly investigated by XRD measurements. The XRD patterns of the GO, Co3O4/GO and HT-Co3O4/GO are presented in Fig. S1† and 3. GO shows a diffraction peak at 10.06° corresponding to the (001) plane,21 an additional small and low broad (002) diffraction peak appears at 2θ of 19.90° (see ESI Fig. S1†), which can be indexed to the disorderedly stacked graphene sheets.16 The (001) plane reflection peak of GO also appears in the XRD pattern of Co3O4/GO (Fig. 3(b)), all other diffraction peaks can be ascribed to the well-crystallized Co3O4 with a face-centered cubic structure (fcc, Fd3m (227), a = 0.808 nm, JCPDS no. 42-1467).16 Compared to the XRD pattern of Co3O4/GO, the (001) plane reflection peak of GO disappears for HT-Co3O4/GO (Fig. 3), indicating partial reduction of GO after the hydrothermal treatment. However, the ultra-tiny cobalt-based nanoparticle/GO sample only shows a diffraction peak at 10.06° corresponding to the (001) plane of GO. No appearances of other diffraction peaks confirms that the formed nanoparticles are extremely small less than 5 nm.


image file: c5ra05706j-f3.tif
Fig. 3 XRD spectra of (a) Co(OH)2/GO (red), (b) Co3O4/GO (blue), and (c) HT-Co3O4/GO (olive).

In order to further evaluate the phase and functional groups of nanocomposites, all samples were characterized by FT-IR spectroscopy. As shown in Fig. 4, the ultra-tiny cobalt-based nanoparticle/GO sample demonstrates a unique absorption band at 617 cm−1 can be ascribed to the formation of Co(OH)2.22,23 In terms of both Co3O4/GO and HT-Co3O4/GO samples, two distinct and sharp absorption bands at 661–664 cm−1 and 576–577 cm−1 are owing to the formation of Co3O4.15,21,24–27 The broad bands at 3324–3406 cm−1 belong to stretching vibrations of hydroxyl groups which is related to the adsorbed water and carboxy of graphene oxide.28 A significant red-shift of O–H band to 3324 cm−1 can be attributed to the formation of CoO–H (Fig. 4(b)) which again verifies the phase of Co(OH)2. It is also worthy to note that GO also goes through chemical changes after deposition of Co3O4 and Co(OH)2. The absorption peaks of carboxy group (C[double bond, length as m-dash]O) and hydroxyl group (C–OH) at 1735 cm−1 and 1226 cm−1, are missing after cobalt deposition, indicating that GO was partially reduced in the alkaline precipitation solution.


image file: c5ra05706j-f4.tif
Fig. 4 FT-IR spectra of (a) GO (black), (b) Co(OH)2/GO (red), (c) Co3O4/GO (blue), and (d) HT-Co3O4/GO (olive).

X-ray photoelectron spectroscopy (XPS) measurements were carried out to analyse the phase, chemical components and valence of nanocomposite electrocatalysts in further detail. As seen in Fig. 5(a), the wide scan spectra of the nanocomposites demonstrates photoelectron lines at binding energies (BE) of ∼285, 530 and 780 eV, corresponding to C 1s, O 1s and Co 2p, respectively. As indicated in Fig. 5(b), two strong peaks are centered at BE of ∼780 and ∼796 eV for all samples, which are in agreement with the BE of Co 2p3/2 and Co 2p1/2, respectively.26 In the XPS spectra of Co3O4/GO and HT-Co3O4/GO (Fig. 5(b)), the Co 2p3/2–Co 2p1/2 splitting of approximately 15 eV and the shake-up satellite structure of the Co 2p3/2 at ∼9 eV higher than the main peaks are associated with the Co3O4.26,29 Co(OH)2/GO shows peaks at 780.2, 785.8, 796.3, and 802.3 eV which match the Co 2p peaks of Co(OH)2 reported in early literatures30–32 (Fig. 5(b)) and in turn confirm the FT-IR data.


image file: c5ra05706j-f5.tif
Fig. 5 (a) Wide scan XPS spectra and (b) high resolution Co 2p spectra of Co3O4/GO, Co(OH)2/GO, respectively. XPS curve fit of (c) C 1s and (d) O 1s spectra of GO, Co3O4/GO and Co(OH)2/GO, respectively.

The state of GO in the nanocomposites is analysed by the C 1s spectra in detail shown in Fig. 5(c). The spectrum of the C 1s spectra can be deconvoluted into five components including C[double bond, length as m-dash]C sp2 (284.0 eV), C–C sp3 (284.8 eV), C–OH and/or C–O–C (286.4 eV), C[double bond, length as m-dash]O (287.8 eV) and O–C[double bond, length as m-dash]O (289.2 eV).33–36 In the C 1s spectra, the intensity of C–OH and C–O–C peak is higher than that of the C[double bond, length as m-dash]C sp2 peak, indicating that GO is highly oxidized and contains lots of hydroxyl and epoxy groups in comparison with the carbonyl and carboxylate groups.

After deposition of Co(OH)2 or Co3O4, the atomic ratio of oxygenated functional groups decreased, particularly for C–OH and C–O–C (see Fig. 5(c)). These results clearly suggest that GO undergoes partial reduction due to partial removal of epoxide and hydroxyl groups, which were deoxygenated under alkaline conditions during preparation of nanocomposites. The total atomic concentrations of C[double bond, length as m-dash]C and C–C for Co(OH)2/GO and Co3O4/GO are 85.6% and 84.1%, respectively. Therefore, we can conclude that even though the preparation temperature for Co(OH)2/GO is 80 °C lower than that for Co3O4/GO. The reduction level of GO for both samples is similar.

Fig. 5(d) shows the O 1s spectra of GO, Co(OH)2/GO and Co3O4/GO HT-Co3O4/GO. The deconvoluted O 1s spectra of the original GO consists of three peaks: (I) the O element in carboxylate and/or carbonyl (O–C[double bond, length as m-dash]O; C[double bond, length as m-dash]O: 532.2 eV), and (II) the O element in the epoxy and/or hydroxyl (C–O–C; C–OH: 533.2 eV), (III) the O element that binds to the trace sulphate group (O ex SO4: 531.2 eV).37 After deposition of Co3O4 and Co(OH)2, the O 1s peak shifted to lower BE and broadened. For Co3O4/GO, the major reason of such shift is partial reduction of GO, the emerging peak of lattice oxygen in Co3O4 (Co–O: 529.4 eV) and the formation of Co–O–C bonds (∼531.0 eV).26,31 For Co(OH)2/GO, the weak lattice oxygen peak at 529.4 eV again confirms the phase of Co(OH)2, and the major peak at 531.0 eV can be ascribed to the OH group in Co(OH)2 and the formation of Co–O–C bonds (∼531.0 eV).

We tested the OER catalytic activity of Co(OH)2/GO, Co3O4/GO, HT-Co3O4/GO, commercial 10 wt% Pt/C, and IrO2 (see Fig. 6(a)). The Co(OH)2/GO sample shows slightly higher performance than Co3O4/GO and HT-Co3O4/GO. In comparison with commercial noble metal electrocatalysts including 10 wt% Pt/C and IrO2, all cobalt based GO nanocomposite electrocatalysts demonstrate overwhelming advantages. In order to evaluate the actual cobalt loading mass in GO, we conducted thermogravimetric (TG) analysis of all nanocomposite samples in the air atmosphere (see Fig. 6(b)). The final residual ratio is 35% for Co(OH)2/GO, 70% for Co3O4/GO and 75% for HT-Co3O4/GO. Based on the TG data, we calculated the real cobalt oxide/hydroxide loading mass ratio in nanocomposites (calculation details seen in the ESI†). The loading ratio is 30 wt% for Co(OH)2/GO, much less than 66 wt% for Co3O4/GO and 72 wt% for HT-Co3O4/GO. We further evaluated the intrinsic activity of Co(OH)2/GO, Co3O4/GO, and HT-Co3O4/GO, via calculating the turnover frequency (TOF) at 1.5 V (vs. Ag/AgCl) (see Fig. 6(c)). The TOF values of Co3O4/GO and HT-Co3O4/GO are 0.99 and 0.94 s−1, respectively. On contrast, the TOF value for Co(OH)2/GO is 2.66 s−1, which is 2.7 times higher than that of Co3O4/GO. Considering the reduction level of GO for both samples are similar, we ascribed such high electrochemical activity of Co(OH)2/GO to the extremely small size of Co(OH)2 nanoparticles. The ultra-high surface-to-volume ratio of Co(OH)2 nanoparticles allows the exposing huge number of active sites catalyzing the OER. Moreover, we also measured the OER stability of Co(OH)2/GO. As shown in Fig. 6(d), it shows no degradation in the strong alkaline condition under the constant applied potential of 1.5 V (vs. Ag/AgCl).


image file: c5ra05706j-f6.tif
Fig. 6 (a) OER polarization curves of Co(OH)2/GO, Co3O4/GO, HT-Co3O4/GO, commercial 10 wt% Pt/C and IrO2 (Sigma-Aldrich). The linear scanning rate is 50 mV s−1. (b) TG curves of GO, Co(OH)2/GO, Co3O4/GO and HT-Co3O4/GO in air atmosphere. (c) Histogram of TOF (detailed calculations of TOF are presented in the ESI†). (d) OER stability test of Co(OH)2/GO and 10 wt% Pt/C under constant applied potential of 1.5 V (vs. Ag/AgCl).

In summary, we have successively prepared ∼2 nm Co(OH)2 clusters supported on GO. Such Co(OH)2/GO electrocatalyst demonstrates slightly higher OER performance than recent intensively studied Co3O4/GO and HT-Co3O4/GO, while the loading mass of catalytic cobalt for Co(OH)2/GO is 36% less than Co3O4/GO and 42% less than HT-Co3O4/GO. The turnover frequency (TOF) of Co(OH)2/GO is 2.8 times as high as that of HT-Co3O4/GO. Moreover, it also shows perfect OER stability in the strong alkaline electrolyte. We believe our approach can be generalized to other carbon materials and applied in various areas such as supercapacitor and lithium ion battery while minimizing the metal loading amount.

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c5ra05706j
This journal is © The Royal Society of Chemistry 2015
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