Mesoporous Co3O4 as an electrocatalyst for water oxidation

Nano Research 2013, 6(1): 47–54 DOI 10.1007/s12274-012-0280-8 Mesoporous Co3O4 as an electrocatalyst for water oxidation Harun Tüysüz1,†, Yun Jeong Hwang1,‡, Sher Bahadar Khan2, Abdullah Mohamed Asiri2, and Peidong Yang1,2 () 1 Department of Chemistry, University of California, Berkeley, California 94720, USA The Center of Excellence for Advanced Materials Research (CEAMR ), Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia † Present address: Max-Planck Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany ‡ Present address: Clean Energy Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea 2 Received: 3 October 2012 ABSTRACT Revised: 13 November 2012 Mesoporous Co3O4 has been prepared using porous silica as a hard template via a nanocasting route and its electrocatalytic properties were investigated as an oxygen evolution catalyst for the electrolysis of water. The ordered mesostructured Co3O4 shows dramatically increased catalytic activity compared to that of bulk Co3O4. Enhanced catalytic activity was achieved with high porosity and surface area, and the water oxidation overpotential (η) of the ordered mesoporous Co3O4 decreases significantly as the surface area increases. The mesoporous Co3O4 also shows excellent structural stability in alkaline media. After 100 min under 0.8 V (versus Ag/AgCl) applied bias, the sample maintains the ordered mesoporous structure with little deactivation of the catalytic properties. Accepted: 24 November 2012 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2012 KEYWORDS water oxidation, electrocatalyst, ordered mesoporous materials, nanocasting 1 Introduction Since their discovery in the early 1990s [1, 2], ordered mesoporous materials have become one of the most investigated classes of materials for catalysis [3–5] and other applications, such as sorption [6], separation [7, 8], drug delivery [9, 10], sensors [11, 12], photonics [13], plasmonic [14], fuel cells [15, 16] and magnetism [17, 18]. Recently, ordered mesoporous materials have also attracted considerable attention in the field of direct Address correspondence to [email protected] solar energy conversion to hydrogen production by utilizing various semiconductor compositions such as Ta2O5 [19, 20], Ta3N4 [21], Nb2O5 [22], BiVO4 [23], TiO2 [24], Al2O3/TiO2 [25], and C3N4 [26, 27]. In addition, ordered mesoporous silica based materials, such as MCM-41, SBA-15, and KIT-6, have been used as a support for the deposition of nanostructured materials like CdS [28], Cd1–xZnxS [29], MnxOy [30], Co3O4 [31] and anchoring of oxo-bridged heterobinuclear units like TiOMn [32] and TiOFe [33] 48 Nano Res. 2013, 6(1): 47–54 for photo- and electrocatalytic hydrogen production. Currently, world energy consumption is based mainly on non-renewable fossil fuels and needs to be shifted to renewable energy sources because of eventual fossil energy supply depletion and climate change concerns due to carbon dioxide accumulation in the atmosphere. The renewable hydrogen produced by photo- and electrochemical splitting of water is one of the most promising alternative clean energy sources. Water splitting can be catalyzed by several inorganic semiconductors, the first of which, TiO 2 , was discovered in the early 1970s by Fujishima and Honda [34]. Following this report, numerous electrochemical and photochemical water splitting studies on various metal oxides as catalytic materials for hydrogen and oxygen evolution have been conducted [35–46]. Metal or metal oxide co-catalysts are often employed to facilitate solar water splitting reactions. The main function of the co-catalysts is to lower the electrochemical overpotentials related to the multi-electron water oxidation and reduction reactions. For the water oxidation half reaction, noble metal oxides such as ruthenium oxide and iridium oxide are the most active oxidation catalysts [47]. However, these elements are expensive and less abundant, thus making them unsuitable to use on a large scale. Therefore, it is important to develop catalysts composed of more abundant elements. Nowadays, there is great interest in the development of novel co-catalyst materials that do not consist of precious metals. The recent study of Nocera et al. where amorphous Co-oxyphosphates were used as water oxidation catalysts was found to be quite promising [48]. Some transition metal oxides such as oxides of Fe, Mn, Ni, and Co [49, 50] have also been investigated. Recently Esswein et al. reported the size-dependent activity of Co3O4 nanoparticle anodes for alkaline water electrolysis [51]. They observed a maximal activity for nanoparticles having the highest surface area. Their electrochemical study indicates the promise of Co3O4 materials for the development of efficiently nanostructured catalysts for water oxidation with comparable overpotential to that of RuO2. In this study, we report on the electrocatalytic activity of ordered mesoporous Co3O4 for water electrolysis with enhanced catalytic activity because of its large porosity and high surface area. By taking advantage of the well-established nanocasting method, a series of highly ordered crystalline mesoporous Co3O4 materials with various particle size, porosity and surface area were prepared, and the catalytic activities of these materials were investigated for electrochemical water splitting. 2 2.1 Experimental Material preparation Cubic ordered mesoporous KIT-6 was prepared according to the Ref. [52]. Briefly, 13.5 g of surfactant (Pluronic 123, EO20PO70EO20) was dissolved in a mixture of 487.5 g of distilled water and 26.1 g of concentrated HCl (37%). 13.5 g n-butanol was added to the homogenous solution at 35 °C. After stirring the solution for 1 h, 29 g of tetraethyl orthosilicate (TEOS) was quickly added to the solution and followed by stirring at the same temperature for 24 h. The mixture was then heated at different temperatures (35, 100 or 130 °C) for another 24 h under static conditions (for an aging temperature of 130 °C, the mixture was transferred into an autoclave). The solid product was filtered without washing, dried at 90 °C overnight, and then calcined at 550 °C for 6 h. KIT-6, with different textural parameters, was used as the hard template to fabricate cubic ordered mesoporous Co3O4. In a typical synthesis, 0.5 g of KIT-6 was dispersed in 5 mL of 0.8 M Co(NO3)2.6H2O in ethanol and stirred for 1 h at room temperature. Subsequently, the ethanol was evaporated at 50 °C. The sample was calcined at 200 °C for 6 h. The composite was re-impregnated again, followed by calcination at 450 °C for 6 h (with an intermediate plateau at 200 °C for 4 h). The silica template was then removed using 2 M NaOH aqueous solution. 2.2 Working electrode preparation and electrochemical measurements A gold electrode (5 mm diameter, Pine Research Instrumentation AFE1E050AU) loaded with Co3O4 was chosen as a working electrode. The surface of the gold disk electrode (GE) was polished with sandpaper and diamond suspension with different particle sizes (6, 1, and 0.25 μm) to obtain a clean surface. After | www.editorialmanager.com/nare/default.asp Nano Res. 2013, 6(1): 47–54 49 polishing, 5 μL of a 0.02 M ethanol solution of Co3O4 (0.13 mg/cm2), which was sonicated for at least 30 min, was dropped on a GE and dried at room temperature. Subsequently, 5 μL of a 0.25 wt.% Nafion solution was dropped on the GE as the binding agent. Finally, the electrode was dried in a vacuum oven at 120 °C for 2 h. Catalytic current measurements were performed in aqueous KOH electrolytes with a three electrode configuration (EG&G Princeton Applied Research Potentiostat, VersaSTAT Ⅱ), consisting of a GE loaded with Co3O4 as the working electrode, a Pt gauze as the counter electrode, and a Ag/AgCl reference electrode. All three electrodes were fixed in a glass cell to retain the distance between electrodes, and Ar gas was bubbled through to remove the dissolved oxygen during the measurements. All the current versus potential measurements were carried out at a 50 mV/s sweep rate. Chronoamperometric measurements were carried out in 0.1 M KOH solution by applying a bias of 0.8 V (vs. Ag/AgCl) for 10 min and zero current for an additional 10 min sequentially. 3 fabricated from KIT-6 aged at low temperature, has a more open uncoupled sub-framework structure while Co3O4-100, and Co3O4-130 have a dense coupled framework as expected due to the different interconnectivity between the channels of the KIT-6 hard template [53]. TEM indicated that the domain sizes of the samples are in the range of several hundred nm and the crystallite size of the Co3O4-35, Co3O4-100, and Co3O4-130 are around 6, 8, and 10 nm, respectively. The surface areas (SA) of the samples were determined by using the N2-sorption technique. As shown in Table 1, Co3O4-35 had a surface area of 156 m2/g, while those of Co3O4-100 and Co3O4-130 were 113 and 72 m2/g, respectively. By increasing the aging temperature of the silica hard template, the surface area of the nanocast Results and discussion The morphology, structure, porosity and textural parameters of nanocast Co3O4 can be controlled by using an appropriate silica hard template. In this study, we prepared three different ordered mesoporous Co3O4 structures. A detailed structure analysis of the Co3O4 replica has been discussed elsewhere [53]. Transmission electron microscopy (TEM) images of the nanocast Co3O4 samples are presented in Fig. 1. The ordered structure of all samples can be clearly seen from the images. Co3O4-35 (35 indicates the aging temperature of the silica hard template), which was Figure 1 TEM images of Co3O4-35 (a), Co3O4-100 (c), Co3O4130 (d), and Co3O4-35 after electrochemical measurement (b). Table 1 Overpotential (at 5 and 10 mA/cm2) and current density (at 1 V vs. Ag/AgCl) of commercial (bulk) and various cubic ordered mesoporous Co3O4 materials at pH 13 (0.1 M KOH) BET SA (m2/g) Crystallite size (nm) Overpotential (mV) at 5 mA/cm2 Overpotential (mV) at 10 mA/cm2 Current density (mA/cm2) at overpotential 735 mV GE — — — — 1.6 Co3O4 Bulk 2 µm range — — 2.8 Co3O4-130 72 10 592 — 8.7 Co3O4-100 113 8 486 636 13.6 Co3O4-35 156 6 453 525 27.2 www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 50 Nano Res. 2013, 6(1): 47–54 Co3O4 decreases because of the increasing pore size of the silica template, generating a replica with a larger crystallite size. Co3O4 is an active electrocatalyst for oxygen evolution in alkaline conditions. We have investigated electrocatalytic properties of nanocast Co3O4 series in water splitting. The linear sweeps of a bare gold electrode, commercial Co3O4, and ordered mesoporous Co3O4-35, Co3O4-100, and Co3O4-130 loaded gold electrodes in 0.1 M KOH solution (pH 13) are given in Fig. 2. Here, the potential for the anode EOH–/O2 = 0.463 V vs. the normal hydrogen electrode (NHE) at pH 13— which corresponds to 0.265 V vs. the Ag/AgCl reference electrode—was used for all overpotential measurements. The overpotentials of bulk and nanocast Co3O4 samples at a specified current density (5 mA/cm2 or 10 mA/cm2) and the current densities observed at 1 V (vs. Ag/AgCl) are presented in Table 1. The commercial Co3O4 shows the lowest current density while Co3O4-35 reveals the highest current density at a given potential. Commercial Co3O4 reaches a current density of 2.8 mA/cm2 while with ordered mesoporous Co3O4-130, Co3O4-100, and Co3O4-35, current densities of 8.7, 13.6, and 27.2 mA/cm2 can be achieved at 1 V (vs. Ag/AgCl, or an overpotential of 735 mV). A current density of 5 mA/cm2 was obtained at 857, 751, and 718 mV (vs. Ag/AgCl) for Co3O4-130, Co3O4-100, and Co3O4-35, respectively, corresponding to overpotentials of 592, 486, and 453 mV, respectively. Figure 2 Linear voltammograms of a bare gold electrode, and commercial bulk and nanocast Co3O4 samples. To compare electrocatalytic activity of the mesoporous structure with that of nanoparticles, we also prepared Co3O4 nanoparticles through hydrothermal synthesis according to the Ref. [54]. TEM investigation indicated that (Fig. S1(a) in the Electronic Supplementary Material (ESM)) the nanoparticles were not very uniform in size with an average particle size of 5 nm. Linear voltammograms of Co3O4 with ordered mesoporous structure and nanoparticle morphologies in 0.1 M KOH solution are shown in Fig. S1(b) (in the ESM). It can be seen that the two types of nanostructures exhibit comparable activities. One key advantage of using ordered mesoporous structures over nanoparticles is the easy dispersion and formation of a stable film on working electrode. Although nanocasting is more time consuming in comparison with hydrothermal synthesis, the flexible synthetic process allows the synthesis of composite nanostructures with precisely controlled composition, crystallite size, and interconnectivity for electrochemical applications. Assuming that the Faraday efficiency is 100% and that every Co atom is an active site (lower bound of activity), the turnover frequency (TOF) for oxygen evolution can be calculated from the anodic current density. At 400 mV of overpotential, commercial Co3O4 particles exhibit a TOF of 0.142 × 10–3 s–1 on a GE electrode, while the ordered mesoporous Co3O4-130, Co3O4-100, and Co3O4-35 achieve TOFs of 1.58 × 10–3 s–1, 3.16 × 10–3 s–1, and 4.55 × 10–3 s–1, respectively, under the same conditions. The TOFs of mesoporous Co3O4-130, Co3O4-100, and Co3O4-35 are respectively 11, 22, and 32 times larger than that of commercial Co 3O 4. Comparison of the TOFs and overpotentials of various Co3O4 anodes prepared in this study illustrates the importance of a large surface for high catalytic activity. Electrolyte type and the concentration used are important parameters for electrochemical reactions. Typically, a concentrated alkali solution is used as the electrolyte for oxygen evolution in the cases of basic metal oxides like Co3O4. A series of KOH solutions with different pH was prepared to investigate the effect of different concentrations of alkali solutions on the electrocatalytic activity of the nanocast Co3O4-35. Plots of current density vs. potential for various pH solutions are presented in Fig. 3. Relatively low | www.editorialmanager.com/nare/default.asp Nano Res. 2013, 6(1): 47–54 51 Table 2 Overpotential (at 10 and 20 mA/cm2) and current density (at 1 V vs. Ag/AgCl) of cubic ordered mesoporous Co3O4-35 at different pH Overpotential (mV) at 10 mA/cm2 Overpotential (mV) at 20 mA/cm2 Current density (mA/cm2) at 1 V (vs. Ag/AgCl) pH 11 — — 0.3 pH 12 — — 2.6 pH 13 525 646 27.2 pH 13.5 483 541 62.3 pH 14 476 514 126.9 Figure 3 Linear voltammograms of Co3O4-35 in solutions with various KOH concentrations. current densities were observed at pH 11 and 12, (0.3 and 2.6 mA/cm2 at 1 V vs. Ag/AgCl). However, the effect of the alkali solution concentration becomes more apparent at pH 13, where a current density of 27.2 mA/cm2 at 1 V (vs. Ag/AgCl) was obtained. Further increasing the solution pH to 13.5 or 14 gives a higher current density of 62.3 and 126.9 mA/cm2 at 1 V (vs. Ag/AgCl), respectively. To compare the catalytic activities at different pH, the thermodynamic potentials (and hence catalyst overpotentials) were calculated at particular current densities. As seen from Table 2, overpotentials of 646, 541, and 514 mV were measured at 20 mA/cm2 current density for pH 13, 13.5, and 14, respectively. These results suggest that the oxidative catalytic activity of the mesoporous Co3O4 increases – with increasing concentration of OH ions in the electrolyte. Besides catalytic activity, material stability is also a critical issue in order to develop cost effective catalysts for water splitting. Although RuO2 is the most active material for oxygen evolution, it is generally unstable at high overpotential. RuO 2 easily dissolves as ruthenate in basic solutions or dissolves as volatile RuO4 if the potentials for the oxidation of surface sites to Ru(Ⅵ) or Ru(Ⅷ) are reached [47]. Co3O4 has gained more attention due to its strong stability and realistic affordability. Figure 4 shows a chronoamperometric measurement of the Co3O4-35 sample biased at 0.8 V vs. Ag/AgCl at pH 13 for 100 min. At that potential, the sample reaches Figure 4 Chronoamperometric measurement of the Co3O4-35 sample biased at 0.8 V vs. Ag/AgCl at pH 13 for 100 min. a current density of 8 mA/cm2 and does not show any deactivation over 100 min, despite the high surface area and nanocrystalline structure. The TEM image of this sample after 100 min under bias still shows the presence of an ordered mesoporous structure with 6 nm average crystallite size (Fig. 1(b)). This finding provides solid evidence for the structural stability of nanocast Co3O4-35, making it a promising electrocatalyst for oxygen evolution. 4 Conclusion We have presented the design of a series of ordered mesoporous Co3O4 materials by using the nanocasting route and the catalytic activities of these materials were investigated in electrochemical water splitting. It was found that the electrocatalytic activities of the samples significantly depend on the structure, surface www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research 52 Nano Res. 2013, 6(1): 47–54 area, and pH of electrolyte solution. The highest catalytic activity was achieved in a more concentrated alkali solution with Co3O4 that has an open subframework structure and high surface area. As the surface area increases, the overpotential decreases significantly. Cubic ordered mesoporous Co3O4 has excellent structural stability during the electrolysis of water; this material holds promise as a cheap anode for overall water splitting. A further study combining the use of this nanocast Co3O4 with high surface area semiconductor materials for photo–electrochemical water splitting is in progress. Acknowledgements This work was funded by King Abdulaziz University (KAU), under grant No. (HiCi/30-3-1432). The authors, therefore, acknowledge technical and financial support of KAU. H. Tüysüz thanks the German Research Foundation (DFG) for his research fellowship. Electronic Supplementary Material: Supplementary material (TEM image of Co3O4 nanoparticles and linear voltammograms of Co 3 O 4 with ordered mesoporous and nanoparticle morphologies) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-012-0280-8. References [1] [2] [3] [4] [5] Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. The preparation of alkyltrimethylammonium-kanemite complexes and their conversion to microporous materials. Bull. Chem. Soc. Jpn. 1990, 63, 988–992. Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 359, 710–712. Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. Rev. 1997, 97, 2373–2419. 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