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Decoration of NiO hollow spheres composed of stacked nanosheets with CeO2 nanoparticles: Enhancement effect of CeO2 for electrocatalytic methanol oxidation
Journal Pre-proof Decoration of NiO hollow spheres composed of stacked nanosheets with CeO2 nanoparticles: Enhancement effect of CeO2 for electrocatalytic methanol oxidation Weili Li, Zhongxin Song, Xiaohui Deng, Xian-Zhu Fu, Jing-Li Luo PII:
S0013-4686(20)30075-X
DOI:
https://doi.org/10.1016/j.electacta.2020.135684
Reference:
EA 135684
To appear in:
Electrochimica Acta
Received Date: 29 July 2019 Revised Date:
26 November 2019
Accepted Date: 8 January 2020
Please cite this article as: W. Li, Z. Song, X. Deng, X.-Z. Fu, J.-L. Luo, Decoration of NiO hollow spheres composed of stacked nanosheets with CeO2 nanoparticles: Enhancement effect of CeO2 for electrocatalytic methanol oxidation, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2020.135684. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Decoration of NiO hollow spheres composed of stacked nanosheets with CeO2 nanoparticles: enhancement effect of CeO2 for electrocatalytic methanol oxidation Weili Li,a, b Zhongxin Song,a Xiaohui Deng,a Xian-Zhu Fu*a and Jing-Li Luo*a, c
a
College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060,
China. Email: [email protected] b
Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and
Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. c
Department of Chemical and Materials Engineering, University of Alberta, Edmonton,
Alberta T6G 2G6, Canada. Email: [email protected]
Abstract High-performance and low-cost electrocatalysts for electrochemical methanol oxidation reaction (MOR) are essential for the wide application of direct methanol fuel cells (DMFC). Herein, CeO2-NiO composites with CeO2 nanoparticles homogeneously supported on the surface of NiO hollow spheres composed of stacked nanosheets are synthesized via a facile two-step route and tested as noble-metal-free electrocatalysts towards methanol oxidation in alkaline solution. Optimal loading of CeO2 nanoparticles with a uniform size of ~ 8 nm greatly enhances the
electrocatalytic activity and stability comparing with pristine NiO hollow spheres for MOR. It is proposed that a synergistic effect resulting from the proper loading content and high dispersion of CeO2 nanoparticles on the surface of NiO nanosheets boosted the formation of active NiOOH species, which enhanced the electrocatalytic activity and stability of CeO2-NiO catslysts for MOR. CeO2 nanoparticles exhibit good promotion effect as co-catalyst in electrocatalysis, and CeO2-NiO nanocomposites show potential as inexpensive anode catalysts for DMFC applications.
Keywords CeO2-NiO composite; Ultra fine CeO2 nanoparticles; Methanol oxidation reaction (MOR); Synergistic effect;
1. Introduction Direct methanol fuel cell (DMFC) is a promising power source with the advantages of low pollutant emission, higher energy density, mild operating condition, low-cost and abundant supply of methanol, and liquid methanol is also easy to transport, store and distribute[1-3]. The overall performance of DMFC depends on several factors, such as the electrocatalytic activity of anode and cathode materials[4], the ionic conductivity and resistance to methanol cross-over of the polymer proton exchange membrane[5], etc. Of which, electrode material is one of the key factors that impact the DMFC performance. A lot of progresses have been made in developing practical DMFC, however, its performance is still limited by the poor kinetics of methanol oxidation at
the anode side. To present, noble metal catalysts, especially Pt and Pt based composite materials are the most high-efficient anode catalysts[6, 7] for methanol oxidation reaction. Due to the drawbacks of high-cost and easy CO poisoning of Pt-based catalyst, exploring the platinum-group metal (PGM)-free anode catalysts with low cost and advanced electrocatalytic activity is highly desirable[8].
Constant effort has been made to develop low-noble metal, or even non-noble metal electrocatalyst for methanol oxidation[9-11]. Combining transition metal oxides with noble metals is proved to be an effective method to enhance the electrocatalytic activity and increase the noble metal utilization efficiency[12-15]. Nickel (Ni) is an inexpensive, easily available, non-precious metal, Ni-based catalysts have been comprehensively used in many traditional industrial catalysis processes, such as reforming of methane[16], hydrogenation reactions[17, 18], hydrocracking[19], and oxidation reactions[20, 21]. Benefiting from the low cost and high activity, Ni-based materials have also been widely investigated in electrochemical energy storage[22, 23] and electrocatalytic process[24]. Ni based materials are promising candidates as non-platinum catalysts for electrocatalytic oxidation of methanol and ethanol in alkaline media owing to their easily reversible redox states (Ni2+/Ni3+), unpaired and vacant d orbitals[25]. Many kinds of Ni based catalyst for electrocatalytic oxidation of methanol have been developed, such as NiO film[26], NiCo2O4[14], Ni-Cu alloy[27], Ni-B-Co nanoparticles[28], Ni-MCM-41[29], Ni-SBA-15[30], Ni ions dispersed onto poly(o-toluidine)/Triton X-100 film at the surface of multi-walled carbon nanotube
paste electrode[31] etc. Although many research works have been done on the Ni-based catalysts, it is still a long way to develop a catalyst meeting the harsh requirements of practical application.
Ceria (CeO2) is an important rare earth metal oxide whose cations can switch between +3 and +4 oxidation states, so it is able to store, transport and release oxygen[32]. CeO2 is extensively used in traditional catalysis as support or co-catalyst. The promotion effect of CeO2 was also investigated in electrocatalytic reactions[3, 33, 34]. Zhou et al.[35] prepared Pt-CeO2/carbon nanotubes (CNTs), Pt nanoparticles and CeO2 were deposited onto the outer surfaces and inner surfaces of carbon nanotubes. The Pt-CeO2/CNTs catalyst showed superior catalytic performance compared with the Pt/CNTs catalyst. Zhang et al.[36] reported the shape effect of CeO2 on the catalytic performance of Pt/CeO2 for methanol electrooxidation, the highest active Pt catalyst was observed on CeO2 nanorods support. Tao et al.[37] prepared CeO2 nanorods with rich oxygen vacancies and rough surface by plasma irradiation (CeO2-P). The as-obtained Pt/CeO2-P catalyst exhibited high catalytic activity and improved durability for methanol oxidation reaction (MOR) due to the enhanced electrical conductivity and pronounced abilities to catalyze CO oxidation. The promotion effect of CeO2 was also observed on non-noble metal catalysts. Kaur et al.[38] prepared a catalyst by the calcination of a physical mixture of nanocrystalline CeO2 and Nano-ZSM-5. CeO2/Nano-ZSM-5 with 30 wt% of CeO2 exhibited outstanding electrocatalytic activity and stability in electrochemical methanol oxidation compared
with nanocrystalline CeO2 and commercial 20% Pt/C catalyst due to the synergistic effect between CeO2 and ZSM-5.
It was accepted that metal oxides can supply rich surface OH- to improve the catalytic efficiency. And, it is also recognized that CeO2 is a good cocatalyst to enhance electrocatalytic performance. Herein, we combined CeO2 and NiO to prepare a composite oxide as Pt-free catalyst for electrochemical oxidation of methanol in alkaline medium anticipating to improve the electrocatalytic activity of NiO. Ni(OH)2 hollow spheres composed of Ni(OH)2 nanosheets were fabricated through hydrothermal synthesis. After high temperature pyrolysis, Ni(OH)2 hollow spheres were converted into the NiO, maintaining their initial hollow spheres morphology. Subsequently, CeO2-NiO composites with CeO2 nanoparticles homogeneously dispersed on the surface of NiO spheres were prepared through a solvothermal reaction and then calcination approach. The as-prepared catalysts were used as anode catalysts for MOR, it was found that the electrocatalytic activity of CeO2-NiO was significantly enhanced comparing with that of NiO spheres without CeO2 decoration.
2. Experimental 2.1 Materials and reagents All chemicals were analytical reagent (A.R.) grade and used as received without further purification. Cerium nitrate hexahydrate (Ce(NO3)3·6H2O), Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium sulfate decahydrate (Na2SO4·10H2O), glycine
(NH2CH2COOH) and sodium hydroxide (NaOH) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., China. Hexamethylenetetramine (C6H12N4) was purchased from Sigma-Aldrich Co., Ltd., USA. Potassium hydroxide (KOH) was purchased from Shanghai Macklin Biochemical Co., Ltd., China. Methanol (CH3OH) was purchased from Xilong Scientific Co., Ltd., China. Deionized water from a Millipore Milli-Q system (resistivity 18 MΩ cm) was used throughout the studies. Electrochemical measurements were performed in 1.0 M KOH solution.
2.2 Synthesis of NiO hollow spheres Ni(OH)2 hollow microspheres were first synthesized through hydrothermal synthesis method described elsewhere[39]. In a typical procedure, 1.454g of Ni(NO3)2·6H2O, 2.0 g of glycine and 2.0 g of Na2SO4·10H2O were dissolved in 25 mL of deionized water, and stirred for 0.5 h at ambient temperature. Subsequently, 10 mL of NaOH solution (5.0 M) was added into the above mixture dropwise under magnetic stirring, the solution turned clearly blue, and was stirred for another 0.5 h. Then, the homogeneous blue solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave for hydrothermal reaction at 180 ℃ for 24 h. After naturally cooling down to room temperature, the precipitate was centrifuged, washed with deionized water and ethanol alternately for several times, and dried in an oven at 60 ℃ for 12 h. The obtained light green powder was Ni(OH)2, after calcined in air or argon atmosphere at 600 ℃ for 2 h, the final product became light gray and was designated as NiO-Air and
NiO-Ar respectively.
2.3 Synthesis of CeO2-NiO composites For the synthesis of CeO2-NiO composites, a solvothermal method was employed in this work. Typically, 150 mg of as-prepared Ni(OH)2 was dispersed in 15 mL of ethanol, and stirred for 0.5 h to form a homogeneous slurry. Then, 0.24 mmol hexamethylenetetramine (HMT) and 0.08 mmol Ce(NO3)3·6H2O (Ce molar amount of 5 %) was added in turn, and stirred for another 0.5 h at room temperature. The mixture was then transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 180 ℃ for 6 h before it was cooled to room temperature. The precipitate was collected via centrifugation, washing with deionized water and ethanol alternately for several times, and dried at 60 ℃ for 12 h. Finally, the obtained gray powder was calcined in air at 600 ℃ for 2h. The obtained products were named as 5CeO2-NiO. CeO2-NiO composites with Ce molar amount of 1.5%, 2.5%, 10% and 15% was also prepared with the same method, while changing the Ce(NO3)3℃6H2O amount to 0.024
mmol,
0.04
mmol,
0.16
mmol
and
0.32
mmol
respectively.
Ce(NO3)3℃6H2O/HMT=1:3 is maintained throughout the synthetic processes. The corresponding CeO2-NiO products were named as 1.5CeO2-NiO, 2.5CeO2-NiO, 10CeO2-NiO, and 15CeO2-NiO, respectively.
2.4 Characterizations X-ray diffraction (XRD) analyses were performed on a Rigaku SmartLab (Japan)
diffractometer, using monochromatic Cu Kα radiation (λ = 0.1542 nm, 45 kV, 200 mA, room temperature). XRD patterns were recorded in the 2θ range of 10-90° with a scan speed of 10°/min. Nitrogen adsorption/desorption measurements were performed on a nitrogen adsorption apparatus (Micromeritics ASAP 2020, USA). The specific surface area of the as-prepared materials was measured based on Brunauer-Emmett-Teller (BET) theory. X-ray photoelectron spectroscopy (XPS) investigations were performed in a Thermo fisher Scientific K-Alpha+ XPS system with a monochromatic Al Kα source. All binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Scanning electron microscopy (SEM) investigations were carried out using a Hitachi SU-70 microscope with energy dispersive X-ray spectroscopy (EDS) to investigate both the chemical composition and the surface morphology of the prepared materials. Transmission electron microscopy (TEM) was carried out on a JEOL 2100F operated at 200 kV.
2.5 Electrochemical measurements Electrochemical experiments were performed on a CHI 760E electrochemical workstation (China). A conventional three-electrode cell was used for cyclic voltammetry (CV) tests. A square platinum foil (area of 1 cm2) was used as the counter electrode, and an Ag/AgCl electrode (saturated KCl solution) as the reference electrode. A modified glassy carbon electrode (GCE, 6 mm in diameter) covered with a thin layer of catalyst was used as the working electrode. Before modification, the GCE was first polished with alumina slurry and then ultrasonicated in ethanol and
deionized water for 30 seconds. The catalyst ink was made as follows: firstly, 10 mg of catalyst and 10 mg of acetylene black was mixed and grinded together in a mortar; secondly, 5 mg of the mixture, 1 mL of ethanol and 30 µL of Nafion solution (5 wt.% Nafion) was mixed and sonicated for at least 0.5 h to produce a homogeneous suspension; thirdly, 10 µL of the suspension was dropped onto the GCE surface by pipette, and dried in air naturally. The CV tests were conducted in 1.0 M KOH and 1.0 M KOH +1.0 M methanol solution successively at 50 mV s-1 in the potential range of 0 to 1 V (vs. Ag/AgCl). The electrocatalytic active surface areas (ECSA) of the catalysts were measured with the double layer capacitance (Cdl) method by recording CVs in a non-faradaic potential range (0 to 0.1V vs. Ag/AgCl) under different scan rates. Nitrogen was bubbled into the solution for 30 min prior to each test to remove the dissolved gases.
3. Results and discussion The crystal structures of as-prepared catalysts were investigated by XRD. Fig. 1a shows the XRD patterns of NiO-Air and NiO-Ar, both correspond to highly crystalline structure with high phase purity, which means that crystalline NiO was produced after calcination of Ni(OH)2 at 600 ℃ no matter in air or in argon atmosphere. Fig. 1b presents the XRD patterns of CeO2-NiO composites with different molar ratio of CeO2. The strong diffraction peaks at ca. 37.190°, 43.240°, 62.830°, 75.350°, 79.380° are consistent with the diffraction peaks of NiO (JCPDS card No. 89-7131). While other peaks at ca. 28.540°, 33.080°, 47.490°, 56.354°,
59.082°, 69.423°, 76.659° and 88.610° correspond to the cubic fluorite-type structure of crystalline CeO2, according to the standard crystallographic spectrum of CeO2 (JCPDS card No. 34-0394). The CeO2 peaks show up for 2.5CeO2-NiO, 10CeO2-NiO, and 15CeO2-NiO, but could not be seen for 1.5CeO2-NiO due to the low content of CeO2. As shown in Fig. 1b, two separate phases of NiO and CeO2 could be seen from the XRD pattern, this means that NiO and CeO2 coexist in CeO2-NiO composites.
The morphology and structure of as-prepared NiO and CeO2-NiO composites were analyzed by SEM and TEM techniques. The low-magnification SEM image of NiO-Air is shown in Fig. 2a, which indicates the sample of NiO-Air with homogeneous hollow sphere structures with diameter of 2~3 µm. The surface area is about 19.7 m2/g by BET analysis. Fig. 2b and 2c display the cross section of NiO-Air spheres, it could be clearly seen that NiO spheres exhibit hollow structure, and they are stacked of porous nanosheets. The shell of the hollow spheres is about 0.6~0.9 µm in thickness. Fig. 2d shows the amplified surface of NiO-Air sphere. For NiO nanosheets which are the building blocks of NiO hollow spheres, the thickness is around 50 nm. The SEM images of Ni(OH)2 before calcination are provided in Fig. S1, it is notable that Ni(OH)2 also exhibit the clear morphology of hollow spheres, but the surface of Ni(OH)2 nanosheets is flat and smooth. Fig. S2 shows the SEM images of NiO-Ar, which still presents the morphology of hollow spheres stacked of nanosheets just as Ni(OH)2 and NiO. But it could also be clearly seen that calcination in Ar produced different NiO nanosheets, which shows a coarse surface instead of the
porous surface of NiO-Air and the smooth surface of Ni(OH)2. The XRD analyses (Fig. 1a) confirmed that crystalline NiO was obtained after calcination at 600 ℃ regardless of the atmosphere, and SEM analyses demonstrate that the hollow sphere morphology of Ni(OH)2 is maintained after calcination at 600 ℃, but different calcination atmosphere generated NiO nanosheets with different surface.
CeO2-NiO composites with CeO2 nanoparticles decorated on the surface of NiO spheres were synthesized through solvothermal method with the as-prepared Ni(OH)2 hollow spheres as precursor. Fig. 3 (a-d) shows the SEM images of 5CeO2-NiO calcined in air at 600 ℃. NiO still kept the stacking structure of hollow spheres as NiO-Air. CeO2 nanoparticles (as demonstrated by XRD result) less than 10 nm are homogeneously deposited on the surface of NiO nanosheets. The SEM images of 1.5CeO2-NiO, 2.5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO, are shown in Fig. S4, it is the same case with 5CeO2-NiO. For 1.5CeO2-NiO, and 2.5CeO2-NiO, less CeO2 particles could be seen. For 10CeO2-NiO, and 15CeO2-NiO, NiO nanosheets are covered with more density of CeO2 nanoparticles. Figure 3e shows electron image of the 5CeO2-NiO sphere that is chosen to collect the EDS spectrum, Figure 3f, 3g and 3h show the corresponding O, Ni and Ce elements with a uniform distribution in the 5CeO2-NiO sample, which confirms the homogeneous decoration of CeO2 nanoparticles on NiO nanosheets surface. The energy dispersive X-ray spectra of different CeO2-NiO composites are shown in Fig. S5 in the Supporting Information, which clearly shows the occurrence of O, Ce and Ni. TEM was used to further
investigate CeO2 particle size and morphology on the surface of NiO nanosheets. It is shown in Fig. 4b and 4c that uniform CeO2 particles are dispersed homogeneously on the surface of NiO, and the particle size is about 8~9 nm. Fig. 4d-4f shows the representative TEM images of irregular CeO2 particles, The measured lattice distances of 0.243 nm in Fig. 4d could be indexed to NiO(111) facet, and the lattice distance of 0.313 nm in Fig. 4f could be indexed to CeO2 (111) facet, which confirms the successful preparation of crystalline CeO2 nanoparticles on the NiO nanosheet sphere and is consistent with the XRD patterns result.
XPS was performed to elucidate the chemical status of the as-prepared samples. Fig. S3 shows the XPS spectrum of Ni 2p (BE = 840−890 eV) in NiO-Air and NiO-Ar. Four peaks, designated as B−A and BII−AII, were observed. Peak A and AII corresponds to Ni 2p3/2 of NiO, peak B and BII corresponds to Ni 2p1/2 of NiO. The position of peak B is at 854.19 eV and 854.09 eV for NiO-Ar and NiO-Air, respectively. The binding energy exhibits 0.1 eV positive shift for NiO-Ar compared with NiO-Air. Fig. 5a shows the XPS spectrum of Ni2p (BE = 840−890 eV) in NiO-Air, 5CeO2-NiO and 10CeO2-NiO which are calcined in air atmosphere. The binding energy of peak B is 854.09 eV, 853.9 eV and 853.9 eV respectively. For 5CeO2-NiO and 10CeO2-NiO, the binding energy of Ni 2p1/2 is both 853.9 eV. Compared with NiO-Air, the binding energy of Ni 2p1/2 in CeO2-NiO decreased by 0.19 eV, which indicates the electronic interaction between CeO2 and NiO, and the valence state of Ni is changed. Fig. 5b shows the XPS spectrum of Ce 3d in
5CeO2-NiO and 10CeO2-NiO, the binding energy of peak B is 881.61 eV and 881.91 eV respectively. Compared with the binding energy of Ce3+ (880 eV) and Ce4+ (882 eV), Ce in CeO2-NiO composites is in mixed oxidation states of 3+/4+. For 5CeO2-NiO, the binding energy is 0.3 eV less than that of 10CeO2-NiO, indicating more Ce3+ and more surface oxygen vacancies are present in 5CeO2-NiO.
The
electrocatalytic
performance
of
NiO
and
CeO2-NiO
composites
in
electrochemical oxidation of methanol is investigated by scanning the cyclic voltammetry (CV) curves for each sample in alkaline solution. Fig. 6a and 6b shows the CVs of samples of NiO and 5CeO2-NiO in 1.0 M KOH solution and 1.0 M KOH solution containing 1.0 M methanol, respectively. It can be clearly seen that no peak is observed in the absence of methanol for both NiO and 5CeO2-NiO. The obvious current density increase after 0.6 V is due to the occurrence of oxygen evolution reaction. However, a forward peak and a stronger reverse current density peak appear when methanol is added to the 1.0 M KOH solution. These suggest that the oxidation of methanol happened at the electrode surface with both NiO and 5CeO2-NiO. Moreover, the much higher peak current density demonstrated by 5CeO2-NiO catalyst, indicates the much better catalytic activity of 5CeO2-NiO than that of pristine NiO catalyst. To further investigate the impact of CeO2 content on the catalytic activity, CeO2-NiO composites with different CeO2 molar content (1.5%, 2.5%, 10% and 15%) were prepared and the corresponding activity toward MOR are explored. Fig. 6c shows a comparison of the 10th CV scanning curves of CeO2, NiO, 1.5CeO2-NiO,
2.5CeO2-NiO, 5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO samples. It could be seen that there is no methanol oxidation peak appear for the CeO2 sample, indicating that CeO2 is unable to oxidize methanol. The peak current density follows the order of 2.5CeO2-NiO > 5CeO2-NiO > 1.5CeO2-NiO ≈ 10CeO2-NiO > 15CeO2-NiO > NiO, all CeO2-NiO composites show stronger peak current density than NiO. This means the electrocatalytic activity of NiO for methanol oxidation is enhanced when CeO2 nanoparticles are deposited onto the surface of NiO. 2.5CeO2-NiO obviously shows the highest peak current density (159.62 mA/cm2), almost twice over NiO (84.67 mA/cm2), but as more CeO2 is formed, the current density gradually decreases. The electrochemically active surface area (ESCA) is also a important characteristic parameter related with the electrocatalytic activity. The ESCAs of every catalysts were estimated by recording CVs in a non-faradaic potential range (Figure S6). The ECSAs of different catalyst were listed in Table S1, it could be seen that 2.5CeO2-NiO has the largest ECSA of 100.25 cm2/g, which is consistent with its activity for MOR. XPS result reveals the electronic interaction between CeO2 and NiO, the superior electrocatalytic activity of CeO2-NiO composites can be attributed to the synergistic effect between CeO2 nanoparticles and NiO nanosheets. As the amount of CeO2 increase, more surface of NiO nanosheets are covered, this is the reason of the dropping of electrocatalytic activity. Therefore, a proper amount and high dispersion of CeO2 on the surface of NiO nanosheets is important for high electrocatalytic activity of methanol oxidation reaction. The stability of CeO2-NiO composites for electrocatalytic oxidation of methanol was
examined by recording repetitive CVs for 500 cycles in 1.0 M KOH containing 1.0 M methanol at a scan rate of 50 mV/s. As shown in Figure 6d, 2.5CeO2-NiO shows the highest electrocatalytic activity during the initial 250 scanning cycles, but as the CV scanning repeats, the current densities of 1.5CeO2-NiO, 2.5CeO2-NiO and 5CeO2-NiO grow closer to each other, and become almost the same between the 250th and 350th cycles. 2.5CeO2-NiO and 5CeO2-NiO also show better stability than 1.5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO. So, it could be concluded that 2.5-5% of CeO2 is a proper molar content for CeO2-NiO to show the best electrocatalytic activity and stability.
Small reduction peaks are observed between 0.2 to 0.4 V during the repetitive CV sweeps. Fig. 7a shows the CV result of 5CeO2-NiO in the range of 0.1-0.45 V, negligible peak is observed for the first CV cycle, as CV sweeps went on, the peak current density grows stronger, and becomes steady between 200 and 300 cycles, the 200th and 300th peak almost coincide with each other. Then the peak current density continuously drops for 400th and 500th cycle, this is consistent with the change of methanol oxidation peaks. According to literature[40], one reversible redox process can occur during the potential sweep for the NiO electrode in alkaline solution: NiO + OH- → NiOOH + eNiO is oxidized by OH- to form NiOOH during the forward sweep. The oxidation peak is not observed, because it was covered up by the strong methanol oxidation peak. The reduction peak is reflection of the reduction of NiOOH to NiO. It is
reported[41, 42] that NiOOH is the active state for methanol oxidation, as potential sweep is repeated, the reduction peak becomes stronger, this means that more NiOOH is activated during the forward sweep, and this is also in accordance with the increasing of methanol oxidation activity (Fig. 6d). Fig. 7b shows a comparison of the 300th CV cycle in the range of 0.1-0.45 V, Compared with Fig. 6d, the one with better methanol oxidation activity show stronger reduction peak. The presence of CeO2 enhances the Ni2+/Ni3+ redox process, and the formation of Ni3+ species (NiOOH) is essential for methanol oxidation process.
4. Conclusions CeO2-NiO composites with hollow structures are constructed by a two-step method. Ni(OH)2 hollow spheres stacked by nanosheets are first prepared by hydrothermal synthesis, then CeO2 nanoparticles are homogeneously deposited onto the surface of NiO nanosheets through solvothermal reaction and calcination process. Compared to NiO hollow spheres, the CeO2-NiO composites exhibit better electrocatalytic activity for methanol oxidation in alkaline solution. The superior activity of CeO2-NiO is attributable to the synergistic effect between nanocrystalline CeO2 and NiO nanosheets which is proved by XPS analysis. The addition of CeO2 enhances the reversible conversion of NiO and NiOOH during the potential sweeps which provides the required active sites for methanol oxidation.
Acknowledgements
This research was supported by China Postdoctoral Science Foundation (No. 2018M633136). The authors would also like to acknowledge the technical support provided by Instrumental Analysis Center of Shenzhen University (Xili Campus).
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Fig. 1 XRD patterns of NiO (a) and different CeO2-NiO composites (b).
Fig. 2 SEM images of NiO-Air.
Fig. 3 SEM images of 5CeO2-NiO composites (a-d) and EDS mapping images of 5CeO2-NiO composites sphere (f-h) taken from (e).
Fig. 4 TEM images of 5CeO2-NiO composites.
Fig. 5 XPS spectrum of (a) Ni 2p states and (b) Ce 3d states.
Fig. 6 CV results of NiO (a) and 5CeO2-NiO (b) in 1 M KOH solution and 1 M KOH solution containing 1 M methanol. (c) Cyclic voltammetry (CV) curves of of CeO2, NiO and CeO2-NiO composites in 1 M KOH containing 1 M methanol; (d) Reverse peak current density vs. cycling time of NiO, 1.5CeO2-NiO, 2.5CeO2-NiO, 5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO in 1 M KOH containing 1 M methanol. The scanning rate is 50 mV/s for all above CV tests.
Fig. 7 (a) Comparison of different CV cycles of 5CeO2-NiO in the range of 0.1-0.45V; (b) Comparison of the 300th CV cycle of NiO, 1.5CeO2-NiO, 2.5CeO2-NiO, 5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO in the range of 0.1-0.45V.
Weili Li: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing Original Draft, Visualization, Funding acquisition Zhongxin Song: Writing - Review & Editing Xiaohui Deng: Writing - Review & Editing Xian-Zhu Fu: Conceptualization, Resources, Writing - Review & Editing, Supervision, Project administration Jing-Li Luo: Resources, Supervision, Funding acquisition
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(20)30075-X
DOI:
https://doi.org/10.1016/j.electacta.2020.135684
Reference:
EA 135684
To appear in:
Electrochimica Acta
Received Date: 29 July 2019 Revised Date:
26 November 2019
Accepted Date: 8 January 2020
Please cite this article as: W. Li, Z. Song, X. Deng, X.-Z. Fu, J.-L. Luo, Decoration of NiO hollow spheres composed of stacked nanosheets with CeO2 nanoparticles: Enhancement effect of CeO2 for electrocatalytic methanol oxidation, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2020.135684. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Decoration of NiO hollow spheres composed of stacked nanosheets with CeO2 nanoparticles: enhancement effect of CeO2 for electrocatalytic methanol oxidation Weili Li,a, b Zhongxin Song,a Xiaohui Deng,a Xian-Zhu Fu*a and Jing-Li Luo*a, c
a
College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060,
China. Email: [email protected] b
Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and
Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. c
Department of Chemical and Materials Engineering, University of Alberta, Edmonton,
Alberta T6G 2G6, Canada. Email: [email protected]
Abstract High-performance and low-cost electrocatalysts for electrochemical methanol oxidation reaction (MOR) are essential for the wide application of direct methanol fuel cells (DMFC). Herein, CeO2-NiO composites with CeO2 nanoparticles homogeneously supported on the surface of NiO hollow spheres composed of stacked nanosheets are synthesized via a facile two-step route and tested as noble-metal-free electrocatalysts towards methanol oxidation in alkaline solution. Optimal loading of CeO2 nanoparticles with a uniform size of ~ 8 nm greatly enhances the
electrocatalytic activity and stability comparing with pristine NiO hollow spheres for MOR. It is proposed that a synergistic effect resulting from the proper loading content and high dispersion of CeO2 nanoparticles on the surface of NiO nanosheets boosted the formation of active NiOOH species, which enhanced the electrocatalytic activity and stability of CeO2-NiO catslysts for MOR. CeO2 nanoparticles exhibit good promotion effect as co-catalyst in electrocatalysis, and CeO2-NiO nanocomposites show potential as inexpensive anode catalysts for DMFC applications.
Keywords CeO2-NiO composite; Ultra fine CeO2 nanoparticles; Methanol oxidation reaction (MOR); Synergistic effect;
1. Introduction Direct methanol fuel cell (DMFC) is a promising power source with the advantages of low pollutant emission, higher energy density, mild operating condition, low-cost and abundant supply of methanol, and liquid methanol is also easy to transport, store and distribute[1-3]. The overall performance of DMFC depends on several factors, such as the electrocatalytic activity of anode and cathode materials[4], the ionic conductivity and resistance to methanol cross-over of the polymer proton exchange membrane[5], etc. Of which, electrode material is one of the key factors that impact the DMFC performance. A lot of progresses have been made in developing practical DMFC, however, its performance is still limited by the poor kinetics of methanol oxidation at
the anode side. To present, noble metal catalysts, especially Pt and Pt based composite materials are the most high-efficient anode catalysts[6, 7] for methanol oxidation reaction. Due to the drawbacks of high-cost and easy CO poisoning of Pt-based catalyst, exploring the platinum-group metal (PGM)-free anode catalysts with low cost and advanced electrocatalytic activity is highly desirable[8].
Constant effort has been made to develop low-noble metal, or even non-noble metal electrocatalyst for methanol oxidation[9-11]. Combining transition metal oxides with noble metals is proved to be an effective method to enhance the electrocatalytic activity and increase the noble metal utilization efficiency[12-15]. Nickel (Ni) is an inexpensive, easily available, non-precious metal, Ni-based catalysts have been comprehensively used in many traditional industrial catalysis processes, such as reforming of methane[16], hydrogenation reactions[17, 18], hydrocracking[19], and oxidation reactions[20, 21]. Benefiting from the low cost and high activity, Ni-based materials have also been widely investigated in electrochemical energy storage[22, 23] and electrocatalytic process[24]. Ni based materials are promising candidates as non-platinum catalysts for electrocatalytic oxidation of methanol and ethanol in alkaline media owing to their easily reversible redox states (Ni2+/Ni3+), unpaired and vacant d orbitals[25]. Many kinds of Ni based catalyst for electrocatalytic oxidation of methanol have been developed, such as NiO film[26], NiCo2O4[14], Ni-Cu alloy[27], Ni-B-Co nanoparticles[28], Ni-MCM-41[29], Ni-SBA-15[30], Ni ions dispersed onto poly(o-toluidine)/Triton X-100 film at the surface of multi-walled carbon nanotube
paste electrode[31] etc. Although many research works have been done on the Ni-based catalysts, it is still a long way to develop a catalyst meeting the harsh requirements of practical application.
Ceria (CeO2) is an important rare earth metal oxide whose cations can switch between +3 and +4 oxidation states, so it is able to store, transport and release oxygen[32]. CeO2 is extensively used in traditional catalysis as support or co-catalyst. The promotion effect of CeO2 was also investigated in electrocatalytic reactions[3, 33, 34]. Zhou et al.[35] prepared Pt-CeO2/carbon nanotubes (CNTs), Pt nanoparticles and CeO2 were deposited onto the outer surfaces and inner surfaces of carbon nanotubes. The Pt-CeO2/CNTs catalyst showed superior catalytic performance compared with the Pt/CNTs catalyst. Zhang et al.[36] reported the shape effect of CeO2 on the catalytic performance of Pt/CeO2 for methanol electrooxidation, the highest active Pt catalyst was observed on CeO2 nanorods support. Tao et al.[37] prepared CeO2 nanorods with rich oxygen vacancies and rough surface by plasma irradiation (CeO2-P). The as-obtained Pt/CeO2-P catalyst exhibited high catalytic activity and improved durability for methanol oxidation reaction (MOR) due to the enhanced electrical conductivity and pronounced abilities to catalyze CO oxidation. The promotion effect of CeO2 was also observed on non-noble metal catalysts. Kaur et al.[38] prepared a catalyst by the calcination of a physical mixture of nanocrystalline CeO2 and Nano-ZSM-5. CeO2/Nano-ZSM-5 with 30 wt% of CeO2 exhibited outstanding electrocatalytic activity and stability in electrochemical methanol oxidation compared
with nanocrystalline CeO2 and commercial 20% Pt/C catalyst due to the synergistic effect between CeO2 and ZSM-5.
It was accepted that metal oxides can supply rich surface OH- to improve the catalytic efficiency. And, it is also recognized that CeO2 is a good cocatalyst to enhance electrocatalytic performance. Herein, we combined CeO2 and NiO to prepare a composite oxide as Pt-free catalyst for electrochemical oxidation of methanol in alkaline medium anticipating to improve the electrocatalytic activity of NiO. Ni(OH)2 hollow spheres composed of Ni(OH)2 nanosheets were fabricated through hydrothermal synthesis. After high temperature pyrolysis, Ni(OH)2 hollow spheres were converted into the NiO, maintaining their initial hollow spheres morphology. Subsequently, CeO2-NiO composites with CeO2 nanoparticles homogeneously dispersed on the surface of NiO spheres were prepared through a solvothermal reaction and then calcination approach. The as-prepared catalysts were used as anode catalysts for MOR, it was found that the electrocatalytic activity of CeO2-NiO was significantly enhanced comparing with that of NiO spheres without CeO2 decoration.
2. Experimental 2.1 Materials and reagents All chemicals were analytical reagent (A.R.) grade and used as received without further purification. Cerium nitrate hexahydrate (Ce(NO3)3·6H2O), Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium sulfate decahydrate (Na2SO4·10H2O), glycine
(NH2CH2COOH) and sodium hydroxide (NaOH) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., China. Hexamethylenetetramine (C6H12N4) was purchased from Sigma-Aldrich Co., Ltd., USA. Potassium hydroxide (KOH) was purchased from Shanghai Macklin Biochemical Co., Ltd., China. Methanol (CH3OH) was purchased from Xilong Scientific Co., Ltd., China. Deionized water from a Millipore Milli-Q system (resistivity 18 MΩ cm) was used throughout the studies. Electrochemical measurements were performed in 1.0 M KOH solution.
2.2 Synthesis of NiO hollow spheres Ni(OH)2 hollow microspheres were first synthesized through hydrothermal synthesis method described elsewhere[39]. In a typical procedure, 1.454g of Ni(NO3)2·6H2O, 2.0 g of glycine and 2.0 g of Na2SO4·10H2O were dissolved in 25 mL of deionized water, and stirred for 0.5 h at ambient temperature. Subsequently, 10 mL of NaOH solution (5.0 M) was added into the above mixture dropwise under magnetic stirring, the solution turned clearly blue, and was stirred for another 0.5 h. Then, the homogeneous blue solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave for hydrothermal reaction at 180 ℃ for 24 h. After naturally cooling down to room temperature, the precipitate was centrifuged, washed with deionized water and ethanol alternately for several times, and dried in an oven at 60 ℃ for 12 h. The obtained light green powder was Ni(OH)2, after calcined in air or argon atmosphere at 600 ℃ for 2 h, the final product became light gray and was designated as NiO-Air and
NiO-Ar respectively.
2.3 Synthesis of CeO2-NiO composites For the synthesis of CeO2-NiO composites, a solvothermal method was employed in this work. Typically, 150 mg of as-prepared Ni(OH)2 was dispersed in 15 mL of ethanol, and stirred for 0.5 h to form a homogeneous slurry. Then, 0.24 mmol hexamethylenetetramine (HMT) and 0.08 mmol Ce(NO3)3·6H2O (Ce molar amount of 5 %) was added in turn, and stirred for another 0.5 h at room temperature. The mixture was then transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 180 ℃ for 6 h before it was cooled to room temperature. The precipitate was collected via centrifugation, washing with deionized water and ethanol alternately for several times, and dried at 60 ℃ for 12 h. Finally, the obtained gray powder was calcined in air at 600 ℃ for 2h. The obtained products were named as 5CeO2-NiO. CeO2-NiO composites with Ce molar amount of 1.5%, 2.5%, 10% and 15% was also prepared with the same method, while changing the Ce(NO3)3℃6H2O amount to 0.024
mmol,
0.04
mmol,
0.16
mmol
and
0.32
mmol
respectively.
Ce(NO3)3℃6H2O/HMT=1:3 is maintained throughout the synthetic processes. The corresponding CeO2-NiO products were named as 1.5CeO2-NiO, 2.5CeO2-NiO, 10CeO2-NiO, and 15CeO2-NiO, respectively.
2.4 Characterizations X-ray diffraction (XRD) analyses were performed on a Rigaku SmartLab (Japan)
diffractometer, using monochromatic Cu Kα radiation (λ = 0.1542 nm, 45 kV, 200 mA, room temperature). XRD patterns were recorded in the 2θ range of 10-90° with a scan speed of 10°/min. Nitrogen adsorption/desorption measurements were performed on a nitrogen adsorption apparatus (Micromeritics ASAP 2020, USA). The specific surface area of the as-prepared materials was measured based on Brunauer-Emmett-Teller (BET) theory. X-ray photoelectron spectroscopy (XPS) investigations were performed in a Thermo fisher Scientific K-Alpha+ XPS system with a monochromatic Al Kα source. All binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Scanning electron microscopy (SEM) investigations were carried out using a Hitachi SU-70 microscope with energy dispersive X-ray spectroscopy (EDS) to investigate both the chemical composition and the surface morphology of the prepared materials. Transmission electron microscopy (TEM) was carried out on a JEOL 2100F operated at 200 kV.
2.5 Electrochemical measurements Electrochemical experiments were performed on a CHI 760E electrochemical workstation (China). A conventional three-electrode cell was used for cyclic voltammetry (CV) tests. A square platinum foil (area of 1 cm2) was used as the counter electrode, and an Ag/AgCl electrode (saturated KCl solution) as the reference electrode. A modified glassy carbon electrode (GCE, 6 mm in diameter) covered with a thin layer of catalyst was used as the working electrode. Before modification, the GCE was first polished with alumina slurry and then ultrasonicated in ethanol and
deionized water for 30 seconds. The catalyst ink was made as follows: firstly, 10 mg of catalyst and 10 mg of acetylene black was mixed and grinded together in a mortar; secondly, 5 mg of the mixture, 1 mL of ethanol and 30 µL of Nafion solution (5 wt.% Nafion) was mixed and sonicated for at least 0.5 h to produce a homogeneous suspension; thirdly, 10 µL of the suspension was dropped onto the GCE surface by pipette, and dried in air naturally. The CV tests were conducted in 1.0 M KOH and 1.0 M KOH +1.0 M methanol solution successively at 50 mV s-1 in the potential range of 0 to 1 V (vs. Ag/AgCl). The electrocatalytic active surface areas (ECSA) of the catalysts were measured with the double layer capacitance (Cdl) method by recording CVs in a non-faradaic potential range (0 to 0.1V vs. Ag/AgCl) under different scan rates. Nitrogen was bubbled into the solution for 30 min prior to each test to remove the dissolved gases.
3. Results and discussion The crystal structures of as-prepared catalysts were investigated by XRD. Fig. 1a shows the XRD patterns of NiO-Air and NiO-Ar, both correspond to highly crystalline structure with high phase purity, which means that crystalline NiO was produced after calcination of Ni(OH)2 at 600 ℃ no matter in air or in argon atmosphere. Fig. 1b presents the XRD patterns of CeO2-NiO composites with different molar ratio of CeO2. The strong diffraction peaks at ca. 37.190°, 43.240°, 62.830°, 75.350°, 79.380° are consistent with the diffraction peaks of NiO (JCPDS card No. 89-7131). While other peaks at ca. 28.540°, 33.080°, 47.490°, 56.354°,
59.082°, 69.423°, 76.659° and 88.610° correspond to the cubic fluorite-type structure of crystalline CeO2, according to the standard crystallographic spectrum of CeO2 (JCPDS card No. 34-0394). The CeO2 peaks show up for 2.5CeO2-NiO, 10CeO2-NiO, and 15CeO2-NiO, but could not be seen for 1.5CeO2-NiO due to the low content of CeO2. As shown in Fig. 1b, two separate phases of NiO and CeO2 could be seen from the XRD pattern, this means that NiO and CeO2 coexist in CeO2-NiO composites.
The morphology and structure of as-prepared NiO and CeO2-NiO composites were analyzed by SEM and TEM techniques. The low-magnification SEM image of NiO-Air is shown in Fig. 2a, which indicates the sample of NiO-Air with homogeneous hollow sphere structures with diameter of 2~3 µm. The surface area is about 19.7 m2/g by BET analysis. Fig. 2b and 2c display the cross section of NiO-Air spheres, it could be clearly seen that NiO spheres exhibit hollow structure, and they are stacked of porous nanosheets. The shell of the hollow spheres is about 0.6~0.9 µm in thickness. Fig. 2d shows the amplified surface of NiO-Air sphere. For NiO nanosheets which are the building blocks of NiO hollow spheres, the thickness is around 50 nm. The SEM images of Ni(OH)2 before calcination are provided in Fig. S1, it is notable that Ni(OH)2 also exhibit the clear morphology of hollow spheres, but the surface of Ni(OH)2 nanosheets is flat and smooth. Fig. S2 shows the SEM images of NiO-Ar, which still presents the morphology of hollow spheres stacked of nanosheets just as Ni(OH)2 and NiO. But it could also be clearly seen that calcination in Ar produced different NiO nanosheets, which shows a coarse surface instead of the
porous surface of NiO-Air and the smooth surface of Ni(OH)2. The XRD analyses (Fig. 1a) confirmed that crystalline NiO was obtained after calcination at 600 ℃ regardless of the atmosphere, and SEM analyses demonstrate that the hollow sphere morphology of Ni(OH)2 is maintained after calcination at 600 ℃, but different calcination atmosphere generated NiO nanosheets with different surface.
CeO2-NiO composites with CeO2 nanoparticles decorated on the surface of NiO spheres were synthesized through solvothermal method with the as-prepared Ni(OH)2 hollow spheres as precursor. Fig. 3 (a-d) shows the SEM images of 5CeO2-NiO calcined in air at 600 ℃. NiO still kept the stacking structure of hollow spheres as NiO-Air. CeO2 nanoparticles (as demonstrated by XRD result) less than 10 nm are homogeneously deposited on the surface of NiO nanosheets. The SEM images of 1.5CeO2-NiO, 2.5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO, are shown in Fig. S4, it is the same case with 5CeO2-NiO. For 1.5CeO2-NiO, and 2.5CeO2-NiO, less CeO2 particles could be seen. For 10CeO2-NiO, and 15CeO2-NiO, NiO nanosheets are covered with more density of CeO2 nanoparticles. Figure 3e shows electron image of the 5CeO2-NiO sphere that is chosen to collect the EDS spectrum, Figure 3f, 3g and 3h show the corresponding O, Ni and Ce elements with a uniform distribution in the 5CeO2-NiO sample, which confirms the homogeneous decoration of CeO2 nanoparticles on NiO nanosheets surface. The energy dispersive X-ray spectra of different CeO2-NiO composites are shown in Fig. S5 in the Supporting Information, which clearly shows the occurrence of O, Ce and Ni. TEM was used to further
investigate CeO2 particle size and morphology on the surface of NiO nanosheets. It is shown in Fig. 4b and 4c that uniform CeO2 particles are dispersed homogeneously on the surface of NiO, and the particle size is about 8~9 nm. Fig. 4d-4f shows the representative TEM images of irregular CeO2 particles, The measured lattice distances of 0.243 nm in Fig. 4d could be indexed to NiO(111) facet, and the lattice distance of 0.313 nm in Fig. 4f could be indexed to CeO2 (111) facet, which confirms the successful preparation of crystalline CeO2 nanoparticles on the NiO nanosheet sphere and is consistent with the XRD patterns result.
XPS was performed to elucidate the chemical status of the as-prepared samples. Fig. S3 shows the XPS spectrum of Ni 2p (BE = 840−890 eV) in NiO-Air and NiO-Ar. Four peaks, designated as B−A and BII−AII, were observed. Peak A and AII corresponds to Ni 2p3/2 of NiO, peak B and BII corresponds to Ni 2p1/2 of NiO. The position of peak B is at 854.19 eV and 854.09 eV for NiO-Ar and NiO-Air, respectively. The binding energy exhibits 0.1 eV positive shift for NiO-Ar compared with NiO-Air. Fig. 5a shows the XPS spectrum of Ni2p (BE = 840−890 eV) in NiO-Air, 5CeO2-NiO and 10CeO2-NiO which are calcined in air atmosphere. The binding energy of peak B is 854.09 eV, 853.9 eV and 853.9 eV respectively. For 5CeO2-NiO and 10CeO2-NiO, the binding energy of Ni 2p1/2 is both 853.9 eV. Compared with NiO-Air, the binding energy of Ni 2p1/2 in CeO2-NiO decreased by 0.19 eV, which indicates the electronic interaction between CeO2 and NiO, and the valence state of Ni is changed. Fig. 5b shows the XPS spectrum of Ce 3d in
5CeO2-NiO and 10CeO2-NiO, the binding energy of peak B is 881.61 eV and 881.91 eV respectively. Compared with the binding energy of Ce3+ (880 eV) and Ce4+ (882 eV), Ce in CeO2-NiO composites is in mixed oxidation states of 3+/4+. For 5CeO2-NiO, the binding energy is 0.3 eV less than that of 10CeO2-NiO, indicating more Ce3+ and more surface oxygen vacancies are present in 5CeO2-NiO.
The
electrocatalytic
performance
of
NiO
and
CeO2-NiO
composites
in
electrochemical oxidation of methanol is investigated by scanning the cyclic voltammetry (CV) curves for each sample in alkaline solution. Fig. 6a and 6b shows the CVs of samples of NiO and 5CeO2-NiO in 1.0 M KOH solution and 1.0 M KOH solution containing 1.0 M methanol, respectively. It can be clearly seen that no peak is observed in the absence of methanol for both NiO and 5CeO2-NiO. The obvious current density increase after 0.6 V is due to the occurrence of oxygen evolution reaction. However, a forward peak and a stronger reverse current density peak appear when methanol is added to the 1.0 M KOH solution. These suggest that the oxidation of methanol happened at the electrode surface with both NiO and 5CeO2-NiO. Moreover, the much higher peak current density demonstrated by 5CeO2-NiO catalyst, indicates the much better catalytic activity of 5CeO2-NiO than that of pristine NiO catalyst. To further investigate the impact of CeO2 content on the catalytic activity, CeO2-NiO composites with different CeO2 molar content (1.5%, 2.5%, 10% and 15%) were prepared and the corresponding activity toward MOR are explored. Fig. 6c shows a comparison of the 10th CV scanning curves of CeO2, NiO, 1.5CeO2-NiO,
2.5CeO2-NiO, 5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO samples. It could be seen that there is no methanol oxidation peak appear for the CeO2 sample, indicating that CeO2 is unable to oxidize methanol. The peak current density follows the order of 2.5CeO2-NiO > 5CeO2-NiO > 1.5CeO2-NiO ≈ 10CeO2-NiO > 15CeO2-NiO > NiO, all CeO2-NiO composites show stronger peak current density than NiO. This means the electrocatalytic activity of NiO for methanol oxidation is enhanced when CeO2 nanoparticles are deposited onto the surface of NiO. 2.5CeO2-NiO obviously shows the highest peak current density (159.62 mA/cm2), almost twice over NiO (84.67 mA/cm2), but as more CeO2 is formed, the current density gradually decreases. The electrochemically active surface area (ESCA) is also a important characteristic parameter related with the electrocatalytic activity. The ESCAs of every catalysts were estimated by recording CVs in a non-faradaic potential range (Figure S6). The ECSAs of different catalyst were listed in Table S1, it could be seen that 2.5CeO2-NiO has the largest ECSA of 100.25 cm2/g, which is consistent with its activity for MOR. XPS result reveals the electronic interaction between CeO2 and NiO, the superior electrocatalytic activity of CeO2-NiO composites can be attributed to the synergistic effect between CeO2 nanoparticles and NiO nanosheets. As the amount of CeO2 increase, more surface of NiO nanosheets are covered, this is the reason of the dropping of electrocatalytic activity. Therefore, a proper amount and high dispersion of CeO2 on the surface of NiO nanosheets is important for high electrocatalytic activity of methanol oxidation reaction. The stability of CeO2-NiO composites for electrocatalytic oxidation of methanol was
examined by recording repetitive CVs for 500 cycles in 1.0 M KOH containing 1.0 M methanol at a scan rate of 50 mV/s. As shown in Figure 6d, 2.5CeO2-NiO shows the highest electrocatalytic activity during the initial 250 scanning cycles, but as the CV scanning repeats, the current densities of 1.5CeO2-NiO, 2.5CeO2-NiO and 5CeO2-NiO grow closer to each other, and become almost the same between the 250th and 350th cycles. 2.5CeO2-NiO and 5CeO2-NiO also show better stability than 1.5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO. So, it could be concluded that 2.5-5% of CeO2 is a proper molar content for CeO2-NiO to show the best electrocatalytic activity and stability.
Small reduction peaks are observed between 0.2 to 0.4 V during the repetitive CV sweeps. Fig. 7a shows the CV result of 5CeO2-NiO in the range of 0.1-0.45 V, negligible peak is observed for the first CV cycle, as CV sweeps went on, the peak current density grows stronger, and becomes steady between 200 and 300 cycles, the 200th and 300th peak almost coincide with each other. Then the peak current density continuously drops for 400th and 500th cycle, this is consistent with the change of methanol oxidation peaks. According to literature[40], one reversible redox process can occur during the potential sweep for the NiO electrode in alkaline solution: NiO + OH- → NiOOH + eNiO is oxidized by OH- to form NiOOH during the forward sweep. The oxidation peak is not observed, because it was covered up by the strong methanol oxidation peak. The reduction peak is reflection of the reduction of NiOOH to NiO. It is
reported[41, 42] that NiOOH is the active state for methanol oxidation, as potential sweep is repeated, the reduction peak becomes stronger, this means that more NiOOH is activated during the forward sweep, and this is also in accordance with the increasing of methanol oxidation activity (Fig. 6d). Fig. 7b shows a comparison of the 300th CV cycle in the range of 0.1-0.45 V, Compared with Fig. 6d, the one with better methanol oxidation activity show stronger reduction peak. The presence of CeO2 enhances the Ni2+/Ni3+ redox process, and the formation of Ni3+ species (NiOOH) is essential for methanol oxidation process.
4. Conclusions CeO2-NiO composites with hollow structures are constructed by a two-step method. Ni(OH)2 hollow spheres stacked by nanosheets are first prepared by hydrothermal synthesis, then CeO2 nanoparticles are homogeneously deposited onto the surface of NiO nanosheets through solvothermal reaction and calcination process. Compared to NiO hollow spheres, the CeO2-NiO composites exhibit better electrocatalytic activity for methanol oxidation in alkaline solution. The superior activity of CeO2-NiO is attributable to the synergistic effect between nanocrystalline CeO2 and NiO nanosheets which is proved by XPS analysis. The addition of CeO2 enhances the reversible conversion of NiO and NiOOH during the potential sweeps which provides the required active sites for methanol oxidation.
Acknowledgements
This research was supported by China Postdoctoral Science Foundation (No. 2018M633136). The authors would also like to acknowledge the technical support provided by Instrumental Analysis Center of Shenzhen University (Xili Campus).
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Fig. 1 XRD patterns of NiO (a) and different CeO2-NiO composites (b).
Fig. 2 SEM images of NiO-Air.
Fig. 3 SEM images of 5CeO2-NiO composites (a-d) and EDS mapping images of 5CeO2-NiO composites sphere (f-h) taken from (e).
Fig. 4 TEM images of 5CeO2-NiO composites.
Fig. 5 XPS spectrum of (a) Ni 2p states and (b) Ce 3d states.
Fig. 6 CV results of NiO (a) and 5CeO2-NiO (b) in 1 M KOH solution and 1 M KOH solution containing 1 M methanol. (c) Cyclic voltammetry (CV) curves of of CeO2, NiO and CeO2-NiO composites in 1 M KOH containing 1 M methanol; (d) Reverse peak current density vs. cycling time of NiO, 1.5CeO2-NiO, 2.5CeO2-NiO, 5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO in 1 M KOH containing 1 M methanol. The scanning rate is 50 mV/s for all above CV tests.
Fig. 7 (a) Comparison of different CV cycles of 5CeO2-NiO in the range of 0.1-0.45V; (b) Comparison of the 300th CV cycle of NiO, 1.5CeO2-NiO, 2.5CeO2-NiO, 5CeO2-NiO, 10CeO2-NiO and 15CeO2-NiO in the range of 0.1-0.45V.
Weili Li: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing Original Draft, Visualization, Funding acquisition Zhongxin Song: Writing - Review & Editing Xiaohui Deng: Writing - Review & Editing Xian-Zhu Fu: Conceptualization, Resources, Writing - Review & Editing, Supervision, Project administration Jing-Li Luo: Resources, Supervision, Funding acquisition
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: