Self-Organized One-Dimensional Cobalt Compound Nanostructures

Oct 14, 2013 - In this work, we reported that a self-organized cobalt oxalate (CoC2O4) with one-dimensional structures was applied to investigate the ...
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Self-Organized One-Dimensional Cobalt Compound Nanostructures from CoC2O4 for Superior Oxygen Evolution Reaction Jin Won Kim,† Jae Kwang Lee,‡ Doungkamon Phihusut,† Youngmi Yi,‡ Hye Jin Lee,§ and Jaeyoung Lee*,†,‡ †

School of Environmental Science and Engineering and ‡Ertl Center for Electrochemistry and Catalysis, RISE, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea § Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 700-701, South Korea S Supporting Information *

ABSTRACT: In this work, we reported that a self-organized cobalt oxalate (CoC2O4) with one-dimensional structures was applied to investigate the electrocatalytic behavior of an oxygen evolution reaction (OER) in alkaline media. The one-dimensional CoC2O4 structure was fabricated by a facile anodization process, i.e., reacting the Co metal surface with oxalate ions. The dense CoC2O4 nanowires were evenly distributed on the Co substrate. By thermal treatment, Co3O4 one-dimensional structures were easily formed from CoC2O4 structures. In addition, we confirmed that fabricated CoC2O4 could also be transformed to Co(OH)2 with one-dimensional structures via a spontaneous chemical reaction in alkaline, which serves better electrocatalytic performance for an OER.

and FeC2O4,13,14 and (iv) easy alteration by the thermal decomposition to metal oxides.15 Despite the excellent advantages above, only few studies on the use of metal oxalates were reported,12−15 and CoC2O4 for OER electrode materials has never been addressed up to date.

1. INTRODUCTION In the new energy generation systems, the oxygen evolution reaction (OER) plays capital roles such as a water electrolyzer, unitized regenerative fuel cell (URFC), and metal air battery cathode.1−4 Among various barriers, the key technology is to develop cost-effective electrodes composed of non-noble electrocatalysts with less overpotential of the OER. Currently, relevant OER catalysts mostly consist of noble metal oxides such as Ru and Ir.1 However, the cost issue has hindered the commercialization of energy systems, and transition metal oxide based electrodes have thus been widely developed.5−7 Of late, considering stable redox cycling, superior physicochemical properties, and relatively low costs, spinel structured cobalt compounds have been attractive as an active catalyst for OERs in alkaline media.8,9 The high surface area and defect structure of electrodes resulted in the rapid transfer of electroactive species.10 Interestingly, bifunctional oxygen electrocatalysts are needed for the development of metal-air batteries and URFCs.1 Recently, hybrid construction of Co3O4 and graphene electrodes showed high electrocatalytic activity for both the OER and oxygen reduction reaction (ORR) due to the synergetic chemical coupling effect.11 In this work, we demonstrate a new observation of selforganized cobalt oxalates (CoC2O4) for improved OER kinetics. From the literature, metal oxalates has several advantages including: (i) a simple method to fabricate onedimensional structures,12 (ii) an easy process to construct multicomponent layers, (iii) representing reversibly electrochemical redox properties for supercapacitors, such as NiC2O4 © 2013 American Chemical Society

2. EXPERIMENTAL SECTION Pure Co foil (>99.95%, Sigma-Aldrich) was used as a starting substance for the fabrication of cobalt complexes. To remove impurities from the Co surface, appropriate ultrasonication was applied to the immersed Co foil in a mixed solution of acetone and ethanol for 5 min, and air was immediately dried in the oven at 50 °C for 30 min. The fabrication of one-dimensional CoC2O4 was immediately initiated on the pretreated Co foil after soaking in 0.5 M H2C2O4 solution at 10 °C with pH value around 1. To observe the steps of formation and morphology changes for one-dimensional CoC2O4, different reaction times (1 min, 5 min, and 1 h) were used in 0.5 M H2C2O4 solution at 25 °C. The one-dimensional CoC2O4 was converted to cobalt oxides by thermal decomposition under O2 conditions at 400 °C for 4 h. The structural and morphological changes were identified by X-ray diffraction (XRD, Rigaku Miniflex II) with a Cu Kα1 radiation source (λ = 1.54056 Å), field-emission scanning electron microscope (FE-SEM, Hitachi S-4700), and transmission electron microscope (TEM, JEOL JEM-2100). Received: July 18, 2013 Revised: October 12, 2013 Published: October 14, 2013 23712

dx.doi.org/10.1021/jp407156d | J. Phys. Chem. C 2013, 117, 23712−23715

The Journal of Physical Chemistry C

Article

An electrocatalytic performance of the oxygen evolution reaction was analyzed by linear sweep voltammetry (using potentiostat/galvanostat, Bio-Logic SAS; VSP) in 0.1 M KOH solution under a potential range of 0−0.9 V, at 10 mV/s of scan rate. Obtained samples, directly formed CoC2O4 on Co foil in 0.5 M H2C2O4 for 2 h and Co3O4 after thermal decomposition at 400 °C for 4 h under O2, were used as working electrodes with 0.196 cm2 geometric area (0.5 cm diameter). To compare its OER behavior, pure Co and Ir foil were subjected to linear sweep voltammetry, and platinized Pt wire and Hg/HgO electrode were used as the counter and reference electrode, respectively. All electrochemical experiments were conducted after purging nitrogen for 30 min.

3. RESULTS AND DISCUSSION Changes in chemical composition for three types of synthesized nanostructures, pure Co metal, the one-dimensional CoC2O4 electrode, and cobalt oxide electrodes through the thermal treatment, were further analyzed using the X-ray diffraction method. XRD patterns are exhibited in Figure 1: (a) pure Co

Figure 2. Schematic illustration of SEM and TEM images of the selfassembled one-dimensional CoC2O4 fabrication step mechanism in 0.5 M H2C2O4 aqueous solution for (a) 0 s, (b) 10 s, (c) 2 h, and (d) the one-dimensional Co3O4 and CoO structures transformed by thermal decomposition at 400 °C for 4 h under O2 conditions.

locally happens on the surface with two moles of the H+ ion. The formed Co2+ ions react with divalent oxalic ions, C2O42−, creating CoC2O4 (reaction 1). Because the C2O42− ions adopt both the D2h planar conformation and D2d symmetry, where the O−C−C−O dihedrals reach 90°, metal oxalate (MC2O4) structures generally keep one-dimensional structure.16 Therefore, metal oxalates prepared in this work are generally presented on the two-dimensional sheet or one-dimensional rod-shaped structures. Finally, the thermal decomposition of CoC2O4 easily transformed oxalate species into cobalt oxides (CoO and Co3O4) and CO2 (reactions 2 and 3), maintaining its one-dimensional structures.17

Figure 1. Comparison of XRD peak patterns of (a) Co foil, (b) onedimensional CoC2O4 on Co foil, synthesized in 0.5 M H2C2O4 at room temperature for 2 h, (c) bare Co foil heat-treated at 400 °C for 4 h, (d) one-dimensional Co3O4 and CoO on Co foil, converted by thermal decomposition at 400 °C for 2 h under O2 conditions, and (e) one-dimensional CoC2O4 after immerging into 0.1 M KOH solution at room temperature for 10 min (○, Co [JCPDS 15-0806]; ▲, CoC2O4 [JCPDS 47-0797]; ⧫, Co3O4 [JCPDS 78-1969]; ●, CoO [JCPDS 481719]; ▼, Co(OH)2 [JCPDS 74-1057]).

metal mainly appears at 41.54°, 44.48°, and 47.42°; (b) the one-dimensional CoC2O4 electrodes representing its feature peak at 18.56°, 22.8°, 29.66°, and 37.68° and CoC2O4 peak patterns were further changed to CoO and Co3O4, after the heat treatment process, as shown in Figure 1 (d). To recognize these cobalt oxide origins, pure Co foil was annealed at 400 °C under O2 conditions for 4 h, and their XRD peak patterns are shown in Figure 1(c). After heat treatment of pure Co foil, the CoO peak at 36.74°, 42.4°, and 61.48° was slightly formed by direct thermal oxidation on the surface of bare Co foil, but it did not have a significant effect on the peak of CoO in Figure 1(d). Following the SEM and XRD results, the formation mechanism of CoC2O4 and Co3O4 on Co foil can be supported by the suggested mechanism scheme with SEM morphology images in Figure 2. First, H2C2O4 in water is doubly ionized to H+ and C2O42− in aqueous solution, and then cobalt corrosion

Co2 + + C2O4 2 − → CoC2O4

(1)

3CoC2O4 + 2O2 → Co3O4 + 6CO2

(2)

CoC2O4 + 1/2O2 → CoO + 2CO2

(3)

The step change of Co substance as a function of reaction time is investigated using SEM (see Figure 2). Figure 2b shows the plate-shape surface after 10 s of reaction time with 0.5 M H2C2O4, originating from bare Co metal (Figure 2a). After 5 min, we could observe the one-dimensional rod structure with uniform dimensions, and then very regular CoC2O4 nanowires were observed after 2 h of the reaction time, evenly distributed on the substrate (Figure 2c). It is obvious that the length and thickness of individual nanostructures were strongly dependent on the reaction time. It grows to 200 nm thickness and several tens of micrometers in length. Finally, CoC2O4 nanowires were converted to CoO and Co3O4, keeping one-dimensional structure via the thermal decomposition process under O2 atmosphere at 400 °C for 4 h (Figure 2d). 23713

dx.doi.org/10.1021/jp407156d | J. Phys. Chem. C 2013, 117, 23712−23715

The Journal of Physical Chemistry C

Article

oxide electrodes, generally positioned in the range from 40 to 60 mV/dec of the Tafel slope in the previous studies, and this value reasonably agrees with other studies for the OER of metal hydroxide and oxide species.18,19 This observation strongly supports the advantage of using one-dimensional CoC2O4 as a catalyst for OERs.20,21 It is believed that the excellent activity for OER is related to the surface change of CoC2O4 in alkaline media, and the CoC2O4 could be immediately changed to the Co(II)(OH)2 structure before the electrochemical reaction (reaction 4). The onedimensional structure with the appeared flake-like structure13 was maintained on the surface. These flake-like structures were also observed after spontaneous reaction of the one-dimensional CoC2O4 electrode with 0.1 M KOH as shown in the inset of Figure S1 (Supporting Information). The XRD peak patterns clearly confirmed the change of CoC 2 O 4 to Co(II)(OH)2 after immersing it in 0.1 M KOH for 10 min, as shown in Figure 1e. The possible mechanistic origin of OER at CoC2O4 electrodes might be described by the following four steps20

Figure 3 presents the electrocatalytic activities for OERs in alkaline solution of pure Co, Ir foil, one-dimensional CoC2O4

CoC2O4 + 2OH− → Co(II)(OH)2 + 2CO2 + 2e−

(4)

Co(II)(OH)2 + OH− → Co(III)OOH + H 2O + e−

(5)

Co(III)OOH + OH− → Co(IV)O2 + e− + H 2O

(6)

Co(IV)O2 → Co(IV) + O2

(7)

The oxidation of Co(II)(OH)2 takes place with extra OH− species in the alkaline solution, resulting in the formation of Co(III)OOH (reaction 5). This corresponds to the big oxidation peak at 0.36 V in Figure 3a (i). The Co(III) ion in the produced Co(III)OOH is transformed to the Co(IV) ion with the oxidation of Co(III)OOH (reaction 6). Following the reaction 6, Co(III)OOH reacts with OH− species, producing Co(IV)O2 and H2O, and releases a single electron at the potential of 0.54 V. The oxygen evolution reaction occurs as soon as reaction 6 finishes, in which the absorbed O2 to Co(IV) species is desorbed from the Co(IV), as described in reaction 7. The formation step of Co(II)(OH)2 from CoC2O4 and the oxidation step of Co(II)(OH)2 to Co(III)OOH are very important in terms of the electrochemical activity for the OER. While CoC2O4 has low electrical conductivity, there are many studies of Co(OH)2 or CoOOH electrodes for OER. In the previous studies, the incorporation of quasi-reversible oxidation of M(OH)2/MOOH to metal substrates enhances conductivity, chargeability, and low overpotential for the OER. 21−25 Thus, synthesized one-dimensional CoC 2 O 4 structures from the facile fabrication process lead to a highly effective and superior active Co(II)(OH)2 and Co(III)OOH species. Also, the one-dimensional CoC2O4 showed highly reversible CV curves after the second cycles, and the one-dimensional morphology still remained as shown in Figure S1 (Supporting Information). In the cathodic curves, two distinguished peaks at 0.48 and 0.04 V appeared, in which a reduction reaction of Co(IV) and Co(III) occurred. To investigate long-term stability of the one-dimensional CoC2O4 electrodes, the chronoamperometry performed at 0.7 V revealed that the electrochemical activity for the OER remained after 5 h as displayed in Figure S2 (Supporting Information).

Figure 3. (a) Linear sweep voltammogram curves with 10 mV/s scan rate in 0.1 M KOH solution. Inset: molecular image of CoC2O4 and Co(OH)2 (blue sphere: Co, white sphere: O, black sphere: C, red sphere: H). (b) Tafel slope of oxygen evolution region. (i) Onedimensional CoC2O4 on the Co foil, fabricated in 0.5 M H2C2O4 solution for 2 h, (ii) Ir foil, (iii) one-dimensional Co3O4 synthesized by thermal decomposition at 400 °C for 4 h under O2 conditions, and (iv) Co foil.

formed on Co foil, and thermally prepared cobalt oxides from CoC2O4. The one-dimensional CoC2O4 electrode showed comparably high enough activity to Ir foil for OER. In Figure 3a (i), the one-dimensional CoC2O4 electrode shows a similar high slope value at 0.8 V of 44.33 mA·cm−2/V to the Ir foil of 47.81 mA·cm−2/V in Figure 3a (ii) and higher than the cobalt oxides of 22.33 mA·cm−2/V (Figure 3a (iii)) and Co foil of 30.56 mA·cm−2/V (Figure 3a (iv)). Moreover, the onset potential of the OER on CoC2O4 was shifted toward a more negative direction around 0.1 V than cobalt oxides and Co foil. The slope of one-dimensional CoC2O4 electrodes showed a dramatic increase from 0.54 V. To express how effectively charge transfer occurs within the double layer, the Tafel slope was defined as shown as Figure 3(b). The Tafel slope value 27.6 mV/dec of one-dimensional CoC2O4 electrodes was almost similar to 25 mV/dec of Ir foil and much lower than 46.4 mV/dec of cobalt oxide electrodes and 32.1 mV/dec of Co foil. These results mean that CoC2O4 required lower overpotential for electrochemical OER, compared with cobalt 23714

dx.doi.org/10.1021/jp407156d | J. Phys. Chem. C 2013, 117, 23712−23715

The Journal of Physical Chemistry C

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4. CONCLUSION A novel but simple and facile approach is demonstrated for the creation of self-assembled one-dimensional CoC2O4 with superior activity for OERs. The Co3O4 structure can also be obtained by thermal decomposition with the preservation of one-dimensional structures. The synthesized CoC2O4 shows a remarkable activity for OER in alkaline solution, rather than that of thermally prepared cobalt oxide electrodes. It is possible that CoC2O4 can facilitate OER by forming its favorable species such as Co(II)(OH)2 and Co(III)OOH with one-dimensional structures. Our suggested simple and facile process could be an excellent alternative to the fabrication of the one-dimensional structured metal electrocatalysts for OER, which may lead to facilitate the development of energy storage and conversion systems.

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ASSOCIATED CONTENT

S Supporting Information *

Cyclic voltammetry (CV) curves, SEM image of the surface after a CV test, and chronoamperometry results for 5 h of the one-dimensional CoC2O4 electrode. These results explain reversible redox reaction and long-term stability of the onedimensional CoC2O4 electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-62-715-2440. Fax: +8262-715-2434. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Core Technology Development Program for NextGeneration Energy Storage of the Research Institute for Solar and Sustainable Energies (RISE) at GIST supported this work.

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REFERENCES

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dx.doi.org/10.1021/jp407156d | J. Phys. Chem. C 2013, 117, 23712−23715