Water Oxidation
An Efficient CeO2/CoSe2 Nanobelt Composite for Electrochemical Water Oxidation Ya-Rong Zheng, Min-Rui Gao, Qiang Gao, Hui-Hui Li, Jie Xu, Zhen-Yu Wu, and Shu-Hong Yu* Oxygen evolution reaction (OER) plays an important role in many energy storage technologies, such as rechargeable metal-air batteries and solar H2 fuel production from water splitting.[1] However, large-scale electrochemical water splitting is greatly hindered by the sluggish OER kinetics at the anode where O-H bond breaking and attendant O-O bond formation are necessary.[2] Currently, precious metal oxides (such as RuO2 and IrO2) are considered as the state-of-theart catalysts for OER. However, the limited resources and high cost of Ru and Ir pose tremendous limitations to widespread use. Therefore, seeking for high efficiency, stable, and inexpensive alternative catalysts which provide the availability for large-scale application is crucial for viable water electrolytic system. Recently, cheap cobalt-based materials have been intensively investigated owing to their promising applications in electrochemical catalysts. A great many cobalt-based materials, such as metal oxides,[3] phosphates,[4] perovskite,[5] chalcogenides,[6] and hydro(oxy)oxides[2] have been developed and exhibited remarkable OER activities. The OER activity of catalyst is to a large extent determined by the bond strength of the reaction intermediates (HO-, HOO-) to the catalyst surface,[7] which could be presumably regulated by incorporating with other functional materials. The strong chemical and electrical coupling between functional materials and substrates, leading to improved electrocatalytic performances.[8] Recently, our group used a new lamellar mesostructured CoSe2/diethylenetriamine (DETA) nanobelts (NBs) as substrate and modified them rationally by various metal or metal oxide NPs (e.g., Pt, Mn3O4, Fe3O4, and Ni/NiO) to construct hybrid catalysts.[6,9] These newly developed hybrids exhibit much better OER, oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER)
Y.-R. Zheng, M.-R. Gao, Q. Gao, H.-H. Li, J. Xu, Z.-Y. Wu, Prof. S.-H. Yu Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Collaborative Innovation Center of Suzhou Nano Science and Technology Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026, PR China E-mail: [email protected] DOI: 10.1002/smll.201401423 small 2014, DOI: 10.1002/smll.201401423
performance than the original CoSe2/DETA NBs owing to the chemical and electrical coupling effects. Ceria (CeO2) is an effective oxygen storage material, which is related to the ease in forming and repairing oxygen vacancies at CeO2 surface, that is, the remarkable redox property.[10] The relatively small CeO2 NPs could induce the increase of lattice constant, thus create more oxygen vacancies (corresponding to Ce3+ in CeO2) to maintain the charge neutrality.[11] The vacancies could bind adsorbates much stronger than normal oxide sites. Therefore, the introduction of CeO2 onto CoSe2/DETA NBs could be beneficial to form hydroperoxy species (OOHad) on the surface of the hybrid due to the unique surface structure with high mobility of oxygen vacancies, and also the chemical synergistic effect between CoSe2 substrate and CeO2 would make contribution to improve the OER catalytic activity. Herein, we report that CeO2 NPs with little OER activity by itself, however, when anchored onto CoSe2/DETA NBs can exhibit exhilaratingly high OER performance in alkaline medium. This new hybrid catalyst exhibits superior OER catalytic activity to the state-of-the-art RuO2 catalyst and its stability is also remarkable, thus representing a very promising noble-metal-free OER electrocatalyst. Ultrathin CoSe2/DETA NBs were firstly prepared by following our previous work (see Supporting Information for details on the synthesis).[12] Then, CeO2 NPs were anchored on the surface of CoSe2/DETA NBs by a facile polyol reduction method at 278 °C for 1 h. Figure 1a-c show scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images with different magnifications of CeO2/CoSe2 hybrid. The decorated CoSe2 NBs do not change their original flexible and ultrathin belt-like structure, but they may bend and form porous structures by connecting each other, which favors the transformation of gas and electrolyte thus have advantage of improving its OER performance. The selected-area electron diffraction (SAED) pattern (inset in Figure 1b) shows the distinct diffraction spots indexed to (400), (521) planes of the CoSe2 matrix (marked by white arrows), and also diffraction ring of the CeO2 loadings can be clearly detected (marked by yellow arrows) (Supporting Information Figure S1). The CeO2 NPs are fairly well grown on the matrix of CoSe2 NBs with an average size of ∼3.5 nm (Figure 1c and inset). High resolution TEM (HRTEM) investigation in Figure 1d shows the CeO2 NPs on the CoSe2 matrix, which reveals the resolved lattice fringes of CeO2 (200) planes with a spacing of 2.76 Å, as well
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Figure 1. (a) SEM image of CeO2/CoSe2 nanobelt composite. (b, c) TEM images with different magnifications of CeO2/CoSe2 nanobelt composite prepared at 278 °C for 1 h. The inset in (b) shows the corresponding SAED pattern, and the inset in (c) shows the corresponding particle size histogram. (d, e) HRTEM images of the CeO2/CoSe2 nanobelt composite. Scale bar: 5 nm. (f) XRD patterns and (g) EDX spectrum of the nanocomposite.
as (210) plane of supported CoSe2 with a spacing of 2.69 Å. The highly ordered lamellar CoSe2 NBs is well preserved after the deposition of CeO2 NPs (Figure 1e). The interlayer distance decreases from 1.08 to 0.68 nm, owing to the removal of DETA molecules during the high-temperature reaction (278 °C). Correspondingly, the X-ray diffraction (XRD) diffraction peak of (002) shift to higher angle compared with pure CoSe2 (Figure 1f), agreeing with the HRTEM image in Figure 1e. The XRD patterns (Figure 1e) reveal the phases of CeO2/CoSe2 composite, which correspond to CeO2 (JCPDS 34–0394) and CoSe2 (JCPDS 09–0234). Additionally, energydispersive X-ray spectroscopy (EDS) confirms the formation of CeO2/CoSe2 composite, which was composed of the elements Ce, Co, Se, and O, the presence of Cu and C peaks were emanated from the carbon coated TEM grid (Figure 1g). Scanning TEM energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping was employed to obtain elemental distribution of Ce, O, Co, and Se in the
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hetero-nanostructure (Figure 2). The red regions in the image shown in Figure 2e are the Ce-containing portions of the nanocomposite, whereas the yellow and green regions are Co and Se-containing segments, respectively. X-photoelectron spectroscopy (XPS) analysis further confirmed the formation of CeO2/CoSe2 nanobelt composite (Supporting Information Figure S2a). Anchoring of CeO2 NPs onto the CoSe2/DETA NB surface could be easily achieved by a heterogeneous nucleation process, as depicted in Scheme 1a. The confined growth of CeO2 on the CoSe2/DETA NB surface was owing to the copious amino groups on the CoSe2/DETA surface, which can served as nucleation sites to couple Ce precursors and then lead to the deposition of CeO2 on CoSe2 matrix under the thermal reduction condition.[12b] The FT-IR spectra also show that almost no DETA molecules remain on the obtained CeO2/CoSe2 nanobelt composite, which should be due to the CeO2 coverage and the high-temperature reaction process
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small 2014, DOI: 10.1002/smll.201401423
An Efficient CeO2/CoSe2 Nanocomposite for Electrochemical Water Oxidation
Figure 2. (a-e) STEM-EDS Elemental maps of CeO2/CoSe2 nanobelt composite overlap, Co (yellow), Se (green), O (blue), and Ce (red), respectively. Scale bar: 200 nm.
(Supporting Information Figure S2b). In contrast, we only obtained transparent brown solution without any product in the absence of CoSe2 despite the synthesis strategy is exact the same (Scheme 1b). Such difference proves the crucial role of CoSe2/DETA NBs to mediate the growth of other functional nanomaterials. The loading amount of CeO2 NPs could be readily controlled by changing the cerium precursor concentration (Supporting Information Figure S3). Obviously, increasing the concentration of Ce(acac)3 and reaction time lead to higher coverage density of CeO2 NPs and induce more aggregations (Supporting Information Figure S4), however the overloaded CeO2 NPs would cover most CoSe2 surface, which is not beneficial to the OER catalytic activity of the CeO2/CoSe2 nanobelt composite. We assessed the OER properties of CeO2/CoSe2 nanobelt composite in O2-satureated 0.1 M KOH solution in a standard
three-electrode system. The film of CeO2/CoSe2 nanobelt composite catalyst was uniformly cast onto glass carbon electrode (GCE) for recording OER polarization curves. Similar measurements for bare GC, pure CoSe2/DETA NBs, CeO2 NPs, and the commercial RuO2 (Sigma-Aldrich) catalysts were also performed for comparison. Figure 3a shows the linear sweeps in an anodic direction, the CeO2/CoSe2 nanobelt composite exhibits earlier onset potential (∼1.394 V vs. RHE) and greater OER current density as compared with other referential catalysts (Supporting Information Table S1). By contrast, bare GC does not affect OER activity. It is meaningful to compare overpotential (η) requirements for achieving the current density of 10 mA cm−2, which is a metric relevant to solar fuel synthesis.[13] Remarkably, the CeO2/CoSe2 nanobelt composite can afford the current density at a small η of ∼0.288 V, much smaller than pure CoSe2
Scheme 1. (a) Schematic illustration of the synthesis of the CeO2/CoSe2 nanobelt composite by a facile polyol reduction method at 278 °C for 1 h. (b) Polyol synthesis without CoSe2/DETA NBs in the same procedure, resulting in nothing to obtain. small 2014, DOI: 10.1002/smll.201401423
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Figure 3. Electrochemical performance of CeO2/CoSe2 nanobelt composite catalyst. (a) iR-corrected polarization curves for OER on bare GCE and modified GCEs comprising the CeO2/CoSe2 nanobelt composite, RuO2, CoSe2/DETA NBs and CeO2, respectively. (b) iR-corrected Tafel plot (overpotential versus log current) derived from (a). (c) iR-corrected OER polarization curves for the CeO2/CoSe2 nanobelt composite before and after different cycles of accelerated stability test. (d) Chronopotentiometry curves of CeO2/CoSe2 nanobelt composite on GCE at a constant current density of 10 mA cm−2 for 36,000 s. All the measurements were performed in O2-purged 0.1 M KOH (pH ∼13). Catalyst loading: ∼0.2 mg cm−2. Sweep rate: 5 mV s−1.
NBs and commercial RuO2 catalyst, comparable to the performance of the best reported noble-metal-free OER catalylsts (Supporting Information Table S1). To test whether pure CeO2 can contribute OER activities, we prepared CeO2 NPs for comparison, and found that they are OER inactive (Supporting Information Figure S5). We proposed that the excellent OER activity of CeO2/CoSe2 composite was derived from CoSe2/DETA NBs, and CeO2 served as the synergist. In contrast, the overloaded OER-inactive CeO2 NPs would shelter a large part of CoSe2 surface, thus cause the decrease of OER activity (Supporting Information Figure S6), agreeing well with our previous work.[6] In addition, the physical mixture of CeO2 and CoSe2 did not effectively improve the OER activity (Supporting Information Figure S5d). The OER kinetics of these catalysts was estimated by corresponding Tafel plots (log j – η), as shown in Figure 3b. The Tafel slope of CeO2/CoSe2 nanobelt composite is only 44 mV decade−1, which is much smaller than the value of 66 mV decade−1 for CoSe2 NBs and 69 mV decade−1 for commercial RuO2 catalyst. The Tafel slope is smaller or comparable to those of the previous reported Co-based OER catalysts in the literature (Supporting Information Table S1), proving the superior OER kinetics of CeO2/CoSe2 composite. In addition, CeO2/CoSe2 composite only need a η of 0.288 V to reach the current density of 10 mA cm−2, which is much smaller than those of other Co-based OER catalysts. The low Tafel slope, small overpotential, and high current densities of
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CeO2/CoSe2 composite demonstrate that the superior OER catalytic activity after decorating CoSe2 NBs with CeO2 NPs. To assess the OER stability of CeO2/CoSe2 catalyst, we performed continuous potential cycling between 0.30 and 0.80 V (vs. Ag/AgCl) for the CeO2/CoSe2 composite in O2-saturated 0.1 M KOH. As shown in Figure 3c, after 1,000 cycles, the CeO2/CoSe2 catalyst needs a mere 14 mV increase in η to achieve the current density of 10 mA cm−2. Additionally, chronoamperometry measurement (j ∼ t) was employed to further assay the long-term stability of CeO2/CoSe2 catalyst (Figure 3d), at a constant current density of 10 mA cm−2, the CeO2/CoSe2 catalyst showed a bit increase in η by ∼20 mV after maintained in 0.1 M KOH for 36,000 s. Furthermore, CeO2/CoSe2-modified-CFP also showed steadily vigorous effervescence even after 36,000 s (Supporting Information, Movie S1.), demonstrating its remarkable stability under harsh OER conditions. It is interesting to understand the intrinsic reason of the improved OER performance of CoSe2/DETA NBs after anchoring CeO2 NPs. The greatly enhanced activity indicates the interactions between CoSe2 and CeO2 in the hybrid materials, which were demonstrated by XPS and Raman scattering measurements (Figure 4). Note that pure CeO2 NPs did not affect OER activity (Figure 3a), it thus stands to reason that the extraordinary OER activity of CeO2/ CoSe2 hybrid was originated from CoSe2 NBs, where CeO2 served as a synergist. It was reported that CoIV cations could
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An Efficient CeO2/CoSe2 Nanocomposite for Electrochemical Water Oxidation
Figure 4. (a) High-resolution Co 2p XPS spectra for pure CoSe2/DETA NBs NBs and CeO2/CoSe2 nanobelt composite. (b) Raman spectra of pure CoSe2/DETA NBs and as-constructed CeO2/CoSe2 nanobelt composite. The inset shows a high resolution Raman spectra. (c) Schematic image depicts the electron donation from CeO2 to CoSe2.
facilitate the formation of hydroperoxy (OOH) groups (key intermediates in the OER) and then further oxidized to O2.[8a] The presence of CoIV cations (Co 2p3/2 at 778.0 eV, Figure 4a) in cubic phase CoSe2 guarantees the decent OER activity of pure CoSe2.[6] As shown in Figure 4a, the electron binding energy of Co 2p for CeO2/CoSe2 composite showed a ∼0.6 eV decrease compared with pure CoSe2 NBs, which is presumably caused by the electron transfer from CeO2 to CoSe2.[6] When the loading amount of CeO2 increased, the more negative shifting can be observed (Supporting Information Figure S7). The chemical synergistic effect between CeO2 NPs and CoSe2 NB substrates was also revealed by Raman spectra. The detailed change of the CoSe2 NBs structure after incorporating with CeO2 was evaluated, as shown in Figure 4b. Both the Raman spectra of CoSe2/DETA NBs and CeO2/ CoSe2 composite show four characteristic peaks which are contributed to the cubic CoSe2.[14] Notably, the Raman peak at ca. 657 cm−1 of CoSe2/DETA NBs (inset in Figure 4b) shifts to higher wavelength after loading CeO2 NPs, highly indicating that the electron transfer between CeO2 NPs and CoSe2 NBs substrate.[15] Moreover, this modification of electronic structure will make CeO2 more acidic (Lewis acid) and thus facilitate the activation of H2O molecules (Lewis base). Additionally, after the introduction of CeO2, the conductivity of the composite was increased (Supporting Information Figure S8), which could contribute to a faster charge transfer during the catalytic reaction and thus an improved catalytic efficiency. small 2014, DOI: 10.1002/smll.201401423
Generally, the elementary steps of the OER are accepted to involve the adsorbtion of O and OH species on the catalysts surface (denoted as Oad and OHad), whereas, the further oxidation of the intermediates to produced O2 could via two different pathways, as shown in Figure 5. The one-step pathway involves Oad species direct recombinant to produce O2, however, the thermodynamic barrier of the reaction is always larger[16] (Figure 5a). So, it is widely accepted that the adsorbed Oad would react with OH− to form hydroperoxy species (OOHad), which is then further oxidized to H2O and O2 by a two-step way (Figure 5b).[3g] The OOHad specie is considered as a key intermediate in the OER, and the presence of CoIV cations (from CoSe2-substrate) could enhance the electrophilicity of Oad, thus to facilitate the formation of OOHad via nucleophilic attack and also promote
Figure 5. Schematic illustration of the oxidation of water molecules on the surface of catalysts in two different pathways. (a) One-step way: The adsorbed Oad direct recombinant to produce oxygen. (b) Two-step way: The adsorbed Oad react with OH− to form a reaction intermediate (OOHad), which is then further oxidized to H2O and O2.
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the deprotonation of OOHad to produce O2 via electro-withdrawing inductive effect.[8a] We note that the CeO2/CoSe2 composite also performs much more excellent OER activity than the previous reported Mn3O4/CoSe2 nanocomposite,[6] which indicates the presence of CeO2 makes a significant contribution to enhance the OER activity, besides the chemical synergistic effect between CoSe2 substrate and CeO2. The relatively small anchored CeO2 NPs (∼3.5 nm) could induce the increase of lattice constant, and create more oxygen vacancies to maintain the charge neutrality.[11] Accordingly, the vacancies could bind adsorbates much stronger than normal oxide sites.[17] Therefore, the introduction of CeO2 could be more beneficial to format OOHad on the surface of the hybrid, due to the unique surface structure with high mobility of oxygen vacancies in nanoscale, thus promote the water oxidation to produce O2 on the CeO2/ CoSe2 composite. All these advantages in the hybrid together are responsible for the surprisingly excellent OER catalytic activity of this new CeO2/CoSe2 composite catalyst. In summary, we have successfully grown CeO2 NPs in situ onto CoSe2/DETA NBs to obtain a novel CeO2/CoSe2 nanobelt composite by a simple polyol reduction route. This easily obtained and earth-abundant catalyst exhibited excellent OER catalytic activity with a small η of ∼0.288 V at the current density of 10 mA cm−2, small Tafel slope of 44 mV decade−1, large anodic currents and good durability in alkaline solution. To our knowledge, this nanocomposite catalyst is currently one of the best non-precious electrocatalysts for OER in alkaline medium. We attributed the improved OER activity to the chemical synergistic effect between CoSe2 substrate and loaded CeO2 NPs, as well as the unique surface structure of CeO2 with high mobility of oxygen vacancies in nanoscale. The presented synthesis approach points a promising way for designing advanced OER catalysts by rational coupling of multi-functional foreign materials, which is in great request for energy conversion technologies.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. It includes full synthetic procedures, details of electrochemical characterization, RHE calibration, and FT-IR, XPS, TEM, XRD, additional electrochemical data, and the current-voltage (I–V) characteristics of the CoSe2/DETA NBs and CeO2/CoSe2 nanocomposite. The movie shows the O2 evolution on a CeO2/CoSe2 nanocomposite modified carbon fiber paper (CFP) electrode in the electrochemical cell at an applied potential of 0.8 V vs. Ag/AgCl for 36 000 s (electrolyte: 0.1 M KOH; CeO2/CoSe2 loading: 1 mg cm−2).
Acknowledgements Y.-R. Zheng and M.-R. Gao contributed equally to this work. S.-H. Yu. supervised the project and conceived the experiments.
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Y.-R. Zheng and M.-R. Gao planned and performed the experiments, colletcted and analyzed the data. Q. Gao, H.-H. Li, J. Xu, and Z.-Y. Wu assisted with the experiments and characterizations. All authors discussed the results and commented on the manuscript. This work is by the National Basic Research Program of China (Grants 2010CB934700, 2013CB933900, 2014CB931800), the National Natural Science Foundation of China (Grants 91022032, 91227103, 21061160492, J1030412), and the Chinese Academy of Sciences (Grant KJZD-EW-M01–1).
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Received: May 22, 2014 Revised: July 10, 2014 Published online:
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An Efficient CeO2/CoSe2 Nanobelt Composite for Electrochemical Water Oxidation Ya-Rong Zheng, Min-Rui Gao, Qiang Gao, Hui-Hui Li, Jie Xu, Zhen-Yu Wu, and Shu-Hong Yu* Oxygen evolution reaction (OER) plays an important role in many energy storage technologies, such as rechargeable metal-air batteries and solar H2 fuel production from water splitting.[1] However, large-scale electrochemical water splitting is greatly hindered by the sluggish OER kinetics at the anode where O-H bond breaking and attendant O-O bond formation are necessary.[2] Currently, precious metal oxides (such as RuO2 and IrO2) are considered as the state-of-theart catalysts for OER. However, the limited resources and high cost of Ru and Ir pose tremendous limitations to widespread use. Therefore, seeking for high efficiency, stable, and inexpensive alternative catalysts which provide the availability for large-scale application is crucial for viable water electrolytic system. Recently, cheap cobalt-based materials have been intensively investigated owing to their promising applications in electrochemical catalysts. A great many cobalt-based materials, such as metal oxides,[3] phosphates,[4] perovskite,[5] chalcogenides,[6] and hydro(oxy)oxides[2] have been developed and exhibited remarkable OER activities. The OER activity of catalyst is to a large extent determined by the bond strength of the reaction intermediates (HO-, HOO-) to the catalyst surface,[7] which could be presumably regulated by incorporating with other functional materials. The strong chemical and electrical coupling between functional materials and substrates, leading to improved electrocatalytic performances.[8] Recently, our group used a new lamellar mesostructured CoSe2/diethylenetriamine (DETA) nanobelts (NBs) as substrate and modified them rationally by various metal or metal oxide NPs (e.g., Pt, Mn3O4, Fe3O4, and Ni/NiO) to construct hybrid catalysts.[6,9] These newly developed hybrids exhibit much better OER, oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER)
Y.-R. Zheng, M.-R. Gao, Q. Gao, H.-H. Li, J. Xu, Z.-Y. Wu, Prof. S.-H. Yu Division of Nanomaterials and Chemistry Hefei National Laboratory for Physical Sciences at Microscale Collaborative Innovation Center of Suzhou Nano Science and Technology Department of Chemistry University of Science and Technology of China Hefei, Anhui 230026, PR China E-mail: [email protected] DOI: 10.1002/smll.201401423 small 2014, DOI: 10.1002/smll.201401423
performance than the original CoSe2/DETA NBs owing to the chemical and electrical coupling effects. Ceria (CeO2) is an effective oxygen storage material, which is related to the ease in forming and repairing oxygen vacancies at CeO2 surface, that is, the remarkable redox property.[10] The relatively small CeO2 NPs could induce the increase of lattice constant, thus create more oxygen vacancies (corresponding to Ce3+ in CeO2) to maintain the charge neutrality.[11] The vacancies could bind adsorbates much stronger than normal oxide sites. Therefore, the introduction of CeO2 onto CoSe2/DETA NBs could be beneficial to form hydroperoxy species (OOHad) on the surface of the hybrid due to the unique surface structure with high mobility of oxygen vacancies, and also the chemical synergistic effect between CoSe2 substrate and CeO2 would make contribution to improve the OER catalytic activity. Herein, we report that CeO2 NPs with little OER activity by itself, however, when anchored onto CoSe2/DETA NBs can exhibit exhilaratingly high OER performance in alkaline medium. This new hybrid catalyst exhibits superior OER catalytic activity to the state-of-the-art RuO2 catalyst and its stability is also remarkable, thus representing a very promising noble-metal-free OER electrocatalyst. Ultrathin CoSe2/DETA NBs were firstly prepared by following our previous work (see Supporting Information for details on the synthesis).[12] Then, CeO2 NPs were anchored on the surface of CoSe2/DETA NBs by a facile polyol reduction method at 278 °C for 1 h. Figure 1a-c show scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images with different magnifications of CeO2/CoSe2 hybrid. The decorated CoSe2 NBs do not change their original flexible and ultrathin belt-like structure, but they may bend and form porous structures by connecting each other, which favors the transformation of gas and electrolyte thus have advantage of improving its OER performance. The selected-area electron diffraction (SAED) pattern (inset in Figure 1b) shows the distinct diffraction spots indexed to (400), (521) planes of the CoSe2 matrix (marked by white arrows), and also diffraction ring of the CeO2 loadings can be clearly detected (marked by yellow arrows) (Supporting Information Figure S1). The CeO2 NPs are fairly well grown on the matrix of CoSe2 NBs with an average size of ∼3.5 nm (Figure 1c and inset). High resolution TEM (HRTEM) investigation in Figure 1d shows the CeO2 NPs on the CoSe2 matrix, which reveals the resolved lattice fringes of CeO2 (200) planes with a spacing of 2.76 Å, as well
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Figure 1. (a) SEM image of CeO2/CoSe2 nanobelt composite. (b, c) TEM images with different magnifications of CeO2/CoSe2 nanobelt composite prepared at 278 °C for 1 h. The inset in (b) shows the corresponding SAED pattern, and the inset in (c) shows the corresponding particle size histogram. (d, e) HRTEM images of the CeO2/CoSe2 nanobelt composite. Scale bar: 5 nm. (f) XRD patterns and (g) EDX spectrum of the nanocomposite.
as (210) plane of supported CoSe2 with a spacing of 2.69 Å. The highly ordered lamellar CoSe2 NBs is well preserved after the deposition of CeO2 NPs (Figure 1e). The interlayer distance decreases from 1.08 to 0.68 nm, owing to the removal of DETA molecules during the high-temperature reaction (278 °C). Correspondingly, the X-ray diffraction (XRD) diffraction peak of (002) shift to higher angle compared with pure CoSe2 (Figure 1f), agreeing with the HRTEM image in Figure 1e. The XRD patterns (Figure 1e) reveal the phases of CeO2/CoSe2 composite, which correspond to CeO2 (JCPDS 34–0394) and CoSe2 (JCPDS 09–0234). Additionally, energydispersive X-ray spectroscopy (EDS) confirms the formation of CeO2/CoSe2 composite, which was composed of the elements Ce, Co, Se, and O, the presence of Cu and C peaks were emanated from the carbon coated TEM grid (Figure 1g). Scanning TEM energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping was employed to obtain elemental distribution of Ce, O, Co, and Se in the
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hetero-nanostructure (Figure 2). The red regions in the image shown in Figure 2e are the Ce-containing portions of the nanocomposite, whereas the yellow and green regions are Co and Se-containing segments, respectively. X-photoelectron spectroscopy (XPS) analysis further confirmed the formation of CeO2/CoSe2 nanobelt composite (Supporting Information Figure S2a). Anchoring of CeO2 NPs onto the CoSe2/DETA NB surface could be easily achieved by a heterogeneous nucleation process, as depicted in Scheme 1a. The confined growth of CeO2 on the CoSe2/DETA NB surface was owing to the copious amino groups on the CoSe2/DETA surface, which can served as nucleation sites to couple Ce precursors and then lead to the deposition of CeO2 on CoSe2 matrix under the thermal reduction condition.[12b] The FT-IR spectra also show that almost no DETA molecules remain on the obtained CeO2/CoSe2 nanobelt composite, which should be due to the CeO2 coverage and the high-temperature reaction process
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An Efficient CeO2/CoSe2 Nanocomposite for Electrochemical Water Oxidation
Figure 2. (a-e) STEM-EDS Elemental maps of CeO2/CoSe2 nanobelt composite overlap, Co (yellow), Se (green), O (blue), and Ce (red), respectively. Scale bar: 200 nm.
(Supporting Information Figure S2b). In contrast, we only obtained transparent brown solution without any product in the absence of CoSe2 despite the synthesis strategy is exact the same (Scheme 1b). Such difference proves the crucial role of CoSe2/DETA NBs to mediate the growth of other functional nanomaterials. The loading amount of CeO2 NPs could be readily controlled by changing the cerium precursor concentration (Supporting Information Figure S3). Obviously, increasing the concentration of Ce(acac)3 and reaction time lead to higher coverage density of CeO2 NPs and induce more aggregations (Supporting Information Figure S4), however the overloaded CeO2 NPs would cover most CoSe2 surface, which is not beneficial to the OER catalytic activity of the CeO2/CoSe2 nanobelt composite. We assessed the OER properties of CeO2/CoSe2 nanobelt composite in O2-satureated 0.1 M KOH solution in a standard
three-electrode system. The film of CeO2/CoSe2 nanobelt composite catalyst was uniformly cast onto glass carbon electrode (GCE) for recording OER polarization curves. Similar measurements for bare GC, pure CoSe2/DETA NBs, CeO2 NPs, and the commercial RuO2 (Sigma-Aldrich) catalysts were also performed for comparison. Figure 3a shows the linear sweeps in an anodic direction, the CeO2/CoSe2 nanobelt composite exhibits earlier onset potential (∼1.394 V vs. RHE) and greater OER current density as compared with other referential catalysts (Supporting Information Table S1). By contrast, bare GC does not affect OER activity. It is meaningful to compare overpotential (η) requirements for achieving the current density of 10 mA cm−2, which is a metric relevant to solar fuel synthesis.[13] Remarkably, the CeO2/CoSe2 nanobelt composite can afford the current density at a small η of ∼0.288 V, much smaller than pure CoSe2
Scheme 1. (a) Schematic illustration of the synthesis of the CeO2/CoSe2 nanobelt composite by a facile polyol reduction method at 278 °C for 1 h. (b) Polyol synthesis without CoSe2/DETA NBs in the same procedure, resulting in nothing to obtain. small 2014, DOI: 10.1002/smll.201401423
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Figure 3. Electrochemical performance of CeO2/CoSe2 nanobelt composite catalyst. (a) iR-corrected polarization curves for OER on bare GCE and modified GCEs comprising the CeO2/CoSe2 nanobelt composite, RuO2, CoSe2/DETA NBs and CeO2, respectively. (b) iR-corrected Tafel plot (overpotential versus log current) derived from (a). (c) iR-corrected OER polarization curves for the CeO2/CoSe2 nanobelt composite before and after different cycles of accelerated stability test. (d) Chronopotentiometry curves of CeO2/CoSe2 nanobelt composite on GCE at a constant current density of 10 mA cm−2 for 36,000 s. All the measurements were performed in O2-purged 0.1 M KOH (pH ∼13). Catalyst loading: ∼0.2 mg cm−2. Sweep rate: 5 mV s−1.
NBs and commercial RuO2 catalyst, comparable to the performance of the best reported noble-metal-free OER catalylsts (Supporting Information Table S1). To test whether pure CeO2 can contribute OER activities, we prepared CeO2 NPs for comparison, and found that they are OER inactive (Supporting Information Figure S5). We proposed that the excellent OER activity of CeO2/CoSe2 composite was derived from CoSe2/DETA NBs, and CeO2 served as the synergist. In contrast, the overloaded OER-inactive CeO2 NPs would shelter a large part of CoSe2 surface, thus cause the decrease of OER activity (Supporting Information Figure S6), agreeing well with our previous work.[6] In addition, the physical mixture of CeO2 and CoSe2 did not effectively improve the OER activity (Supporting Information Figure S5d). The OER kinetics of these catalysts was estimated by corresponding Tafel plots (log j – η), as shown in Figure 3b. The Tafel slope of CeO2/CoSe2 nanobelt composite is only 44 mV decade−1, which is much smaller than the value of 66 mV decade−1 for CoSe2 NBs and 69 mV decade−1 for commercial RuO2 catalyst. The Tafel slope is smaller or comparable to those of the previous reported Co-based OER catalysts in the literature (Supporting Information Table S1), proving the superior OER kinetics of CeO2/CoSe2 composite. In addition, CeO2/CoSe2 composite only need a η of 0.288 V to reach the current density of 10 mA cm−2, which is much smaller than those of other Co-based OER catalysts. The low Tafel slope, small overpotential, and high current densities of
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CeO2/CoSe2 composite demonstrate that the superior OER catalytic activity after decorating CoSe2 NBs with CeO2 NPs. To assess the OER stability of CeO2/CoSe2 catalyst, we performed continuous potential cycling between 0.30 and 0.80 V (vs. Ag/AgCl) for the CeO2/CoSe2 composite in O2-saturated 0.1 M KOH. As shown in Figure 3c, after 1,000 cycles, the CeO2/CoSe2 catalyst needs a mere 14 mV increase in η to achieve the current density of 10 mA cm−2. Additionally, chronoamperometry measurement (j ∼ t) was employed to further assay the long-term stability of CeO2/CoSe2 catalyst (Figure 3d), at a constant current density of 10 mA cm−2, the CeO2/CoSe2 catalyst showed a bit increase in η by ∼20 mV after maintained in 0.1 M KOH for 36,000 s. Furthermore, CeO2/CoSe2-modified-CFP also showed steadily vigorous effervescence even after 36,000 s (Supporting Information, Movie S1.), demonstrating its remarkable stability under harsh OER conditions. It is interesting to understand the intrinsic reason of the improved OER performance of CoSe2/DETA NBs after anchoring CeO2 NPs. The greatly enhanced activity indicates the interactions between CoSe2 and CeO2 in the hybrid materials, which were demonstrated by XPS and Raman scattering measurements (Figure 4). Note that pure CeO2 NPs did not affect OER activity (Figure 3a), it thus stands to reason that the extraordinary OER activity of CeO2/ CoSe2 hybrid was originated from CoSe2 NBs, where CeO2 served as a synergist. It was reported that CoIV cations could
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An Efficient CeO2/CoSe2 Nanocomposite for Electrochemical Water Oxidation
Figure 4. (a) High-resolution Co 2p XPS spectra for pure CoSe2/DETA NBs NBs and CeO2/CoSe2 nanobelt composite. (b) Raman spectra of pure CoSe2/DETA NBs and as-constructed CeO2/CoSe2 nanobelt composite. The inset shows a high resolution Raman spectra. (c) Schematic image depicts the electron donation from CeO2 to CoSe2.
facilitate the formation of hydroperoxy (OOH) groups (key intermediates in the OER) and then further oxidized to O2.[8a] The presence of CoIV cations (Co 2p3/2 at 778.0 eV, Figure 4a) in cubic phase CoSe2 guarantees the decent OER activity of pure CoSe2.[6] As shown in Figure 4a, the electron binding energy of Co 2p for CeO2/CoSe2 composite showed a ∼0.6 eV decrease compared with pure CoSe2 NBs, which is presumably caused by the electron transfer from CeO2 to CoSe2.[6] When the loading amount of CeO2 increased, the more negative shifting can be observed (Supporting Information Figure S7). The chemical synergistic effect between CeO2 NPs and CoSe2 NB substrates was also revealed by Raman spectra. The detailed change of the CoSe2 NBs structure after incorporating with CeO2 was evaluated, as shown in Figure 4b. Both the Raman spectra of CoSe2/DETA NBs and CeO2/ CoSe2 composite show four characteristic peaks which are contributed to the cubic CoSe2.[14] Notably, the Raman peak at ca. 657 cm−1 of CoSe2/DETA NBs (inset in Figure 4b) shifts to higher wavelength after loading CeO2 NPs, highly indicating that the electron transfer between CeO2 NPs and CoSe2 NBs substrate.[15] Moreover, this modification of electronic structure will make CeO2 more acidic (Lewis acid) and thus facilitate the activation of H2O molecules (Lewis base). Additionally, after the introduction of CeO2, the conductivity of the composite was increased (Supporting Information Figure S8), which could contribute to a faster charge transfer during the catalytic reaction and thus an improved catalytic efficiency. small 2014, DOI: 10.1002/smll.201401423
Generally, the elementary steps of the OER are accepted to involve the adsorbtion of O and OH species on the catalysts surface (denoted as Oad and OHad), whereas, the further oxidation of the intermediates to produced O2 could via two different pathways, as shown in Figure 5. The one-step pathway involves Oad species direct recombinant to produce O2, however, the thermodynamic barrier of the reaction is always larger[16] (Figure 5a). So, it is widely accepted that the adsorbed Oad would react with OH− to form hydroperoxy species (OOHad), which is then further oxidized to H2O and O2 by a two-step way (Figure 5b).[3g] The OOHad specie is considered as a key intermediate in the OER, and the presence of CoIV cations (from CoSe2-substrate) could enhance the electrophilicity of Oad, thus to facilitate the formation of OOHad via nucleophilic attack and also promote
Figure 5. Schematic illustration of the oxidation of water molecules on the surface of catalysts in two different pathways. (a) One-step way: The adsorbed Oad direct recombinant to produce oxygen. (b) Two-step way: The adsorbed Oad react with OH− to form a reaction intermediate (OOHad), which is then further oxidized to H2O and O2.
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the deprotonation of OOHad to produce O2 via electro-withdrawing inductive effect.[8a] We note that the CeO2/CoSe2 composite also performs much more excellent OER activity than the previous reported Mn3O4/CoSe2 nanocomposite,[6] which indicates the presence of CeO2 makes a significant contribution to enhance the OER activity, besides the chemical synergistic effect between CoSe2 substrate and CeO2. The relatively small anchored CeO2 NPs (∼3.5 nm) could induce the increase of lattice constant, and create more oxygen vacancies to maintain the charge neutrality.[11] Accordingly, the vacancies could bind adsorbates much stronger than normal oxide sites.[17] Therefore, the introduction of CeO2 could be more beneficial to format OOHad on the surface of the hybrid, due to the unique surface structure with high mobility of oxygen vacancies in nanoscale, thus promote the water oxidation to produce O2 on the CeO2/ CoSe2 composite. All these advantages in the hybrid together are responsible for the surprisingly excellent OER catalytic activity of this new CeO2/CoSe2 composite catalyst. In summary, we have successfully grown CeO2 NPs in situ onto CoSe2/DETA NBs to obtain a novel CeO2/CoSe2 nanobelt composite by a simple polyol reduction route. This easily obtained and earth-abundant catalyst exhibited excellent OER catalytic activity with a small η of ∼0.288 V at the current density of 10 mA cm−2, small Tafel slope of 44 mV decade−1, large anodic currents and good durability in alkaline solution. To our knowledge, this nanocomposite catalyst is currently one of the best non-precious electrocatalysts for OER in alkaline medium. We attributed the improved OER activity to the chemical synergistic effect between CoSe2 substrate and loaded CeO2 NPs, as well as the unique surface structure of CeO2 with high mobility of oxygen vacancies in nanoscale. The presented synthesis approach points a promising way for designing advanced OER catalysts by rational coupling of multi-functional foreign materials, which is in great request for energy conversion technologies.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. It includes full synthetic procedures, details of electrochemical characterization, RHE calibration, and FT-IR, XPS, TEM, XRD, additional electrochemical data, and the current-voltage (I–V) characteristics of the CoSe2/DETA NBs and CeO2/CoSe2 nanocomposite. The movie shows the O2 evolution on a CeO2/CoSe2 nanocomposite modified carbon fiber paper (CFP) electrode in the electrochemical cell at an applied potential of 0.8 V vs. Ag/AgCl for 36 000 s (electrolyte: 0.1 M KOH; CeO2/CoSe2 loading: 1 mg cm−2).
Acknowledgements Y.-R. Zheng and M.-R. Gao contributed equally to this work. S.-H. Yu. supervised the project and conceived the experiments.
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Y.-R. Zheng and M.-R. Gao planned and performed the experiments, colletcted and analyzed the data. Q. Gao, H.-H. Li, J. Xu, and Z.-Y. Wu assisted with the experiments and characterizations. All authors discussed the results and commented on the manuscript. This work is by the National Basic Research Program of China (Grants 2010CB934700, 2013CB933900, 2014CB931800), the National Natural Science Foundation of China (Grants 91022032, 91227103, 21061160492, J1030412), and the Chinese Academy of Sciences (Grant KJZD-EW-M01–1).
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Received: May 22, 2014 Revised: July 10, 2014 Published online:
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