Synthesis and Electrochemical Study of Mesoporous Nickel-Cobalt

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Synthesis and Electrochemical Study of Mesoporous Nickel-Cobalt Oxides for Efficient Oxygen Reduction Boopathi Sidhureddy, Scott Prins, Jiali Wen, Antony Raj Thiruppathi, Maduraiveeran Govindhan, and Aicheng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22351 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Applied Materials & Interfaces

Synthesis and Electrochemical Study of Mesoporous Nickel-Cobalt Oxides for Efficient Oxygen Reduction Boopathi Sidhureddy,† Scott Prins,† Jiali Wen,† Antony Raj Thiruppathi,† Maduraiveeran Govindhan,‡ Aicheng Chen†* †

Electrochemical Technology Center, Department of Chemistry, University of Guelph, 50 Stone

Road East, Guelph, ON N1G 2W1, Canada. ‡

Department of Chemistry & Research Institute, SRM Institute of Science and

Technology, Chennai, Tamil Nadu 603 203, India. KEYWORDS. nanomaterials; nickel-cobalt oxide; mesoporous; SECM; oxygen reduction.

ABSTRACT: Development of a cost-effective and efficient electrocatalyst for the sluggish oxygen reduction reaction (ORR) is a crucial challenge for clean energy technologies. In this study, we have synthesized various Ni and Co oxide (NCO) nanomaterials via a facile co-precipitation followed by the calcination method. The morphology of the formed NCO nanomaterials was controlled by varying the percentage of the Ni and Co precursors, leading to the formation of template-free mesoporous spinel phase structure of NixCo3-xO4. It was found that the number of the octahedral site cations and the defect sites with lower oxygen in the spinel oxides can be tunable

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by taking appropriate ratios of the Ni and Co precursors. The optimized NCO nanomaterial exhibits superior electrocatalytic activity compared to the mono-metal oxides of NiO and Co3O4 with over three times higher current density and ~0.250 V lower onset potential toward ORR in a 0.1 M KOH solution. Scanning electrochemical microscopy (SECM) was utilized in mapping the activity of the catalyst and monitoring the ORR products, further confirming that a four-electron transfer pathway was facilitated by the NCO nanomaterial. Moreover, the developed mesoporous NCO nanomaterial exhibits a high methanol tolerance capability and long-term stability when compared to the commercial state-of-the-art Pt/C electrocatalyst. The improvement of the catalytic activity and stability of this advanced NCO nanomaterial toward ORR may be attributed to the facile accessible mesoporous structure, and the abundance of octahedral site cations and defective oxygen sites.

INTRODUCTION Increased energy demands, depletion of fossil fuel, and pressing climate change issues have generated a world-wide effort to develop efficient technology for both energy production and environmental sustainability.1–5 Among sustainable energy technologies, fuel cells have been considered as a potential power source for both electric vehicles and portable electronic devices.612

However, fuel cell technology is still in a developmental stage due to some of the technological

challenges, including economic viability, catalyst activity, membrane stability, fuel crossover, carbon corrosion, and erosion.13 Specifically, the slow oxygen reduction reaction (ORR) at the cathode dramatically hampers the efficiency of various types of fuel cells.6,8,14 Hence, recent efforts have focused on the development of efficient carbon-free ORR electrocatalysts with a low cost and high stability.15-17 In the pioneering of electrode development for the ORR, various


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catalysts have been investigated, for instance, Pt based bimetals/alloys, metallated porphyrin, nonnoble metal, and metal free electrocatalysts.3,16-19 Non-noble metal based electrode materials have recently attracted considerable attention for electrochemical oxygen evolution reaction (OER) and ORR.20-25 Specifically, Co3O4 based nanomaterials have been extensively investigated as an electrocatalyst for both OER and ORR due to its abundance and environmental compatibility.26-28 However, the electrocatalytic activity of Co3O4 is still limited because of its inherent poor conductivity and instability.29,30 To overcome these issues, various transition metals doped with Co3O4 and carbon supported composites have recently been explored, with substantially improved electrochemical properties being observed as compared to Co3O4.124,26,31-37 Unfortunately, the decay of the carbon nanomaterials is inevitable when fuel cells are under alkaline conditions. Among the metal doped Co3O4 based metal oxides, Ni-doped spinel metal oxides have been widely explored for various electrochemical applications, such as electrochemical capacitors, sensors, Li-ion battery, Li-O2 and fuel cells. This is due to their multiple redox chemistry in alkaline solution, high electrochemical activity, and enhanced electronic conductivity than that of pure NiO and Co3O4.10,38-44 Generally speaking, spinel structured mixed metal oxides (MMOs) have closely packed oxygen anions with two trivalent and one divalent cation, which are occupied in the octahedral and tetrahedral sites, respectively.43 The cations at octahedral sites and oxygen vacancies are critical for the enhanced electrochemical properties, which are considered as active sites in the spinel structured metal oxides.24 In addition, size, shape and composition of the Ni and Co based MMOs are crucial parameters in determining electrochemical properties.25,34,41,44-49 Thus, the rational design of NixCo3-xO4 with more active sites is essential for further improvement of the electrocatalytic activity.


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Herein, we synthesized various NCO nanomaterials with controlled morphologies for the efficient electrocatalytic ORR in basic electrolytes. The influence of Ni percentage on the structural, morphological, and electrochemical behaviors of the synthesized NixCo3-xO4 was systematically studied. The physical and electrochemical properties of the synthesized NCO nanomaterials were characterized using field-emission scanning electron microscope (FE-SEM), high-resolution transmission electron microscope (HR-TEM), X-ray diffraction (XRD), BrunauerEmmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), and various electrochemical methods. Rotating disk electrode (RDE) was also utilized to test the activity of the developed NCO nanomaterials in a O2 saturated 0.1 M KOH solution. Further, SECM was used to study the localized activity of the catalysts and to monitor the peroxide ion (HO2-) formation during the ORR on the electrocatalytic surfaces. These physicochemical and electrochemical characterization results articulate the advanced electrocatalytic activity of the developed NCO nanomaterials. EXPERIMENTAL SECTIONS Materials Ni(NO3)2.6H2O (Sigma-Aldrich, 99.9%), Co(NO3)2.6H2O (Sigma-Aldrich, 99.9%), urea (BDH Chemicals), a 10 wt.% Nafion solution (Sigma-Aldrich), methanol (ACP Chemical Inc.) and KOH (Sigma-Aldrich, 99.9%) were purchased and used as-received for the experiments. All solutions were prepared using pure water (18.2 MΩ.cm) produced by the NANOpure Diamond UVWater Purification System. Synthesis of nanostructured metal oxides


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Nickel-cobalt based mixed binary metal oxides were synthesized using a two-step method. In the first step, aqueous solutions of Ni(NO3)2•6H2O and Co(NO3)2•6H2O were mixed under vigorous stirring at an appropriate molar ratio of Ni/Co precursors (3:1, 1:1 and 1:3), followed by the quick addition of 0.64 M of urea to the reaction mixture while stirring. The mixture was heated at 150 °C in a Teflon-lined autoclave setup for 10 h, and then cooled to room temperature. Following the co-precipitation of NCO, the product was thoroughly washed with pure water, followed by ethanol, and then dried at 60 °C for 12 h. In the second step, the product was calcinated in air at 450 °C for 4 h. Likewise, Co3O4 and NiO were synthesized using either Ni(NO3)2•6H2O or Co(NO3)2•6H2O in the synthetic methods. The obtained NCO with the different ratio of Ni and Co precursors (1:3, 1:1, and 3:1) named as NCO-1, NCO-2, and NCO-3, respectively. Physical characterization Surface morphology and composition of the synthesized mono- and mixed metal oxides were analyzed using FE-SEM and energy dispersive X-ray spectroscopy (EDX) with a Hitachi SU-70. Structural information of metal oxides was obtained from powder XRD measurements (Pananalytical X’pert Pro Diffractometer). HR-TEM images were recorded using a JEOL 2010F TEM with a resolution of 0.23 nm. The oxidation state and chemical composition of the metal oxides were investigated by XPS measurements using a ThermoFisher XPS system, where Al Kɑ (X-ray source) was used. Quantachrome Nova 2200 was used for the BET surface area analysis; prior to analysis, all of the samples were degassed at 250 °C for over 3 h under vacuum. Electrochemical measurements VoltaLab 40 (PGZ301) was employed to conduct the electrochemical measurements at room temperature (20  2 ᵒC). A three-electrode electrochemical cell setup was used for electrochemical


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experiments, in which Pine Instruments-Rotating disk electrode (RDE, glassy carbon (GC) disk area - 0.196 cm2) was employed as a working electrode; Ag/AgCl (saturated KCl) and Pt wire served as the reference and counter electrode, respectively. The measured potential was converted to the standard reversible hydrogen electrode (RHE) potential using the following equation: 𝐸𝑅𝐻𝐸 = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.059(𝑝𝐻) + 0.197

(1)

Prior to modifying the working electrode, it was polished using various grade 1, 0.3 and 0.05 µm alumina powders and sonicated in a water acetone mixture to clean the GC electrode. The polished GC electrode was modified as follows: 4 mg of MMO catalyst was added to a mixture containing water (0.95 mL) and 10 wt.% Nafion (0.05 mL), and subsequently sonicated for 30 min to obtain a homogenous dispersion. In the prepared homogeneous mixture, a 10 µl aliquot was drop-cast on the GC disk and allowed to dry at room temperature for 30 min. The metal oxides loading on GC disk was 0.20 mg cm-2. These modified electrodes were electrochemically activated by running cyclic voltammetry (CV) between 0.964 to 1.564 V vs RHE for 5 cycles in 0.1 M KOH at 0.050 V s-1. Following the activation, the RDE experiments were performed at various rotation rates in an O2 saturated 0.1 M KOH electrolyte at 0.010 V s-1, while the electrode potential was changed from 1.064 to 0.164 V. Electrochemical impedance spectroscopic (EIS) measurements were performed at 0.664 V with ac frequency of 100 kHz - 0.1 Hz and the amplitude of 0.01 V. Scanning electrochemical microscopy (SECM) (HEKA Elektronik) was utilized to map the possible formation of peroxide ion (HO2-) at the NiO, Co3O4, and NCO catalysts in the course of the ORR. Measurements were performed at a constant distance of 2.0 μm above the nanostructured catalyst surface using a Pt microelectrode with a 10 μm diameter purchased from HEKA and outfitted with shear-force piezos in order to gather topological and electrochemical activity simultaneously. The SECM was operated with the Substrate Generator/Tip Collector (SG/TC) mode in which


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electroactive species are produced by the substrate and detected using the Pt microelectrode. During the SECM measurements, a constant potential (0.464 V) was applied to the metal oxide catalysts coated on the GCE to conduct the ORR, while 1.064 V was applied to the Pt microelectrode to probe the possible formation of HO2-. The methanol tolerance test was performed by applying the constant potential at 0.764V. The relative stability of the catalyst was measured using chronoamperometric technique by applying the electrode potential of 0.764 V for 300 min. RESULTS AND DISCUSSIONS A two-step approach was employed to prepare the carbon-free NCO nanomaterials with various ratios of Ni/Co and the mono- metal oxides of NiO and Co3O4 were prepared for comparative purposes. The morphologies of the as-formed MMOs were characterized using FE-SEM. Various ratios of the Ni/Co precursors were utilized in the growth solutions, in which degree of co-addition significantly altered the morphology of the final products of mixed metal oxides when compared to Co3O4 and NiO. Figure 1A shows the morphology of the Co3O4, with the particles assembled like a rice spikelet structure. The average length and diameter of an individual spikelet of Co3O4 were measured to be ~64 and ~30 nm, respectively. These nanostructures were well organized as can be seen in the high magnification image of Figure 1a. The addition of the Ni2+ precursors in the synthesis altered the spikelet structure of Co3O4; in which ratio of Ni/Co-1:3 (denoted as NCO1). For NCO-1, a porous sheet-like morphology (~3 µm length x 1.5 µm width) was obtained as shown in Figure 1B and 1b. In general, surfactants are employed as the size/shape directing agents in the synthesis of nanostructured materials. In the present approach, the morphology of the mixed metal oxides was significantly altered by utilizing the appropriate ratio of Ni2+/Co2+ precursors, as can be seen in Figure 1. Further increasing the concentration of Ni2+ in the synthesis to an


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approximate ratio of 1:1 (Ni/Co) resulted in a morphology similar to that of a dandelion, as shown in Figures 1C and 1c. To demonstrate the importance of the Ni2+/Co2+ precursor ratio, it was observed that the morphology was completely transformed from sheets (NCO-1) to a flower-like structure (NCO-2). At 3:1 ratio of Ni/Co combinations, the morphology of NCO-3 was similar to that of a cotton-ball (Figures 1D and 1d). Finally, for the control experiments, NiO nanomaterial was synthesized using the same experimental procedure and had its morphology compared to the mixed metal oxides. NiO nanomaterial displayed a structure similar to agglomerated platelets, as evident in Figures 1E and 1e. These FE-SEM images showed that the role of Ni2+ content in the shape of formed NCO nanostructures was crucial. Ibupoto et al. synthesized various nanostructured Co3O4 by changing the ratio between urea and Co2+.50 Different geometries of NiCo2O4 were synthesized by controlling the temperature in the hydrothermal process.41 In our synthesis, the amount of urea and temperature were kept constant. The morphology of the mixed metal oxides can be easily controlled with varied ratio of Ni and Co precursors. To determine the Ni and Co entities in mixed metal oxides, EDX measurements were performed in three different spots. The atomic ratio of Ni to Co was calculated to be 1.00:2.06, 1.31:1.00, and 2.80:1.00 for NCO-1, NCO-2 and NCO-3, respectively. In order to confirm the mixed metal oxide features of porous NCO-1, HR-TEM analysis was employed.


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A

a

B

b

C

c

D

d

E

e

Figure 1. FE-SEM images of the NCO nanostructures: Co3O4 (A), NCO-1 (B), NCO-2 (C), NCO3 (D) and NiO (E). The corresponding high magnification images of the nanostructured oxide materials (a-e) presented parallel to A-E. Figure 2A shows the HR-TEM image of the small crystalline domain of NCO-1. Two distinct lattice spaces were measured to be 0.28 and 0.24 nm, which are associated with the miller indices of (220) and (311) planes, respectively. This shows the existence of nickel cobaltite phase in the


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synthesized nanostructure materials. Figure 2B shows the porous sheet-like structure of NCO-1. Figures 2 C & D illustrate the elemental mapping of Co and Ni, showing the homogeneous distribution of the Ni and Co in the entire mesoporous structure of NCO-1:3. In order to further understand the mesoporous nature of NCO-1, BET surface area analysis was performed (Figure 2E and 2F). The surface area (SABET) of the sample was measured to be 22.01 m2 g-1, and the average pore diameter was found to be 10.53 nm. The SABET was lower than that of some reported mesoporous materials, which might be attributed to its low pore volume [29]. Further, the volume (0.066 cm3 g-1) and diameter (3.28 nm) of a mesopore were estimated using the Barrett-JoynerHalenda (BJH) analysis, further confirming the mesoporous nature of NCO-1.

Figure 2. HR-TEM images of the mesoporous NCO-1. The high-resolution image of single entities of NCO-1 (A), single sheets like bulk structure NCO-1 (B), and the elemental mapping of NCO-1 (C&D). Nitrogen adsorption/desorption isotherm (E) and pore size distributions (F) of the NCO-1 at 77 K.


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Figure 3 shows the XRD pattern of Co3O4, NiO and mixed NCO nanomaterials, indicating the polycrystalline nature of the formed nanostructured metal oxides. In the case of Co3O4, the major peaks were identified at 2 values of 19.02, 31.32, 36.89, 44.908, 59.47, and 65.34, which are indexed based on the miller indices of (111), (220), (311), (400), (511) and (440), respectively. The XRD pattern of synthesized Co3O4 matched the standard reference pattern of cubic spinel Co3O4 (JCPDS- 09-0418).33 The crystallite size and lattice parameters were calculated from the obtained XRD pattern. The spikelet structure Co3O4 possessed the average crystallite size of 36.5 nm and lattice constant of 0.807 nm. By doping Ni into the Co3O4, the crystallite size (67.8 nm) and lattice constant (0.810 nm) increased. The XRD pattern of NCO-1 matches well with the standard reference pattern of nickel cobaltite (JCPDS-73-1702).40 The major peaks of NCO-1 were indexed to the miller indices of (111), (220), (311), (400), (511) and (440) of a cubic spinel NixCo3xO4.

In the spinel structured mixed metal oxide NCO-1, Ni(II) and Ni(III) may present in the

tetrahedral and octahedral sites of spinel oxides. Hence, its lattice constant was observed to increase to 0.810 nm compared to pure Co3O4. Further increasing the Ni content in the preparatory solution for the formation of NCO-2, the lattice constant increased to 0.811 nm. In addition, these XRD pattern clearly elaborated the existence of cubic spinel mixed metal oxides in NCO-2. In contrast to NCO-1 and NCO-2, for NCO-3, the peak intensity decreased and broadening was observed at 36.89 (2). This might be due to the solid solution of Ni3O4•Co3O4.51 Hence, the solid solution had a lattice constant value of 0.808 nm, which is close to the lattice constant of pure Co3O4. The calculated lattice constant and crystallite size of the synthesized metal oxides were summarized in Table S1. Furthermore, the XRD pattern of the NiO was recorded to compare with NCO and Co3O4 (Figure 3 (pink)). The XRD pattern matched well with the cubic crystal phase


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NiO (JCPDS card 78- 0429).51 The platelet structured NiO nanomaterials have an average crystallite size of 30.18 nm and its lattice constant is 0.42 nm. JCPDS-09-0418 (311) (111)

(220)

(400)

(511) (440)

Co3O4

NCO-1

JCPDS-73-1702

Intensity(a.u.)

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NCO-2

NCO-3 (200) (111) (220)

NiO

JCPDS-78- 0429

20

60

40

80

2 Figure 3. XRD pattern of the Co3O4, NCO-1, NCO-2, NCO-3 and NiO. XPS measurements were carried out in order to understand the chemical state and near surface chemical composition of the synthesized metal oxides. High-resolution Co (2p), Ni (2p) and O (1s) core spectra were recorded for Co3O4, NCO and NiO. The high-resolution spectra of metal oxides were deconvoluted using XPS peakfit software and the results are presented in Figure 4. In the deconvoluted spectra of Co(2p), the obtained Gaussian peaks and their corresponding binding energy values are attributed to Co(III) (779.1 eV - 2p3/2 & 794.87 eV - 2p1/2) and Co(II) (781.31 eV - 2p3/2 & 796.71 eV-2p1/2). Except for the aforementioned peaks, the remaining shoulder peaks were associated to satellite (sat) shakeup peaks.52 The ratio of Co(III)/Co(II) was estimated to be 1.27 for Co3O4 nanomaterial. In the case of NCO-1, the lower binding energy shift of ~0.1 eV was


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observed and the ratio of Co(III)/Co(II) increased to 1.3, suggesting that Ni(II)/(III) was present in the spinel structure, in the form of NixCo3-xO4. In order to confirm the Ni(II)/(III) presence, XPS spectra of Ni entities were analyzed. In the Ni(2p) spectra (Figure 3B), four major peaks were associated to Ni(II) (853.82 eV - 2p3/2 & 871.44 eV - 2p1/2) and Ni(III) (855.53 eV - 2p3/2 & 873.457 eV-2p1/2).43 As a counterpart to Co(2p), the peak values of Ni (2p) shifted towards higher values and increased peak intensity with increasing Ni content in the preparatory solution. These results may be attributed to the partial charge transfer between Co and Ni. In the formed bimetallic oxides, a high amount of Ni(III) was noticed when compared with pure NiO (Table S2). These Ni(III) ions might be occupied at the octahedral sites of spinel structured metal oxides.38 Moreover, O1s XPS spectra of metal oxides were also de-convoluted to find the defect densities of NCO (Supporting Information - Figure S1). The O1s XPS spectrum showed two main peaks at 529.17 (O(1)) and 530.90 eV (O(2)); they could be attributed to the metal-oxygen bond and defect sites with low oxygen, respectively.53 The ratio of O(2)/O(1) was calculated and listed in Table S2. Furthermore, the ratio of Ni/Co was calculated to be 1.00:2.09, 1.19:1.00, and 3.55:1.00 for NCO-1, NCO-2 and NCO-3, respectively, which is in thorough agreement with the EDX results. From the XP spectra, it was calculated that the ratio of octahedral site and tetrahedral site cations and defective oxygen sites (Table S2), which are critical for the electrocatalytic activity of metal oxides as we discussed in the introduction. In the present study, NCO-1 possesses higher amount of active sites; therefore, it may exhibit better electrochemical performance than other combinations of synthesized metal oxides.


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Co

3+

2p3/2 Co

2p1/2

2+

Co

sat

3+

Co

Co3O4

2+

sat

NCO-1

Intensity(a.u.)

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NCO-2

NCO-3

2+

Ni

780

800

2p3/2

840

2p1/2 3+ Ni 3+ 2+ Ni NiO sat Ni sat

860

880

Binding energy/eV Figure 4. XPS spectra of the Co3O4, NCO-1, NCO-2, NCO-3 and NiO. Figure 5 presents the CV curves of the Co3O4 (navy blue), NCO-1 (red), NCO-2 (green), NCO3 (dark red) and NiO (pink) electrodes recorded in 0.1 M KOH at 0.050 V s-1. The CV of the Co3O4 shows the typical crystalline Co3O4 redox behavior in alkaline solution, as displayed in Figure 4 (navy blue). In the anodic sweep, two peaks appeared at 1.314 and 1.544 V, which are concomitant to the oxidation of Co(II)/Co(III) and Co(III)/Co(IV), respectively. During the reduction, a broad peak appeared at ~1.464V. The CV curve of the Co3O4 is consistent with the reported CV of polycrystalline Co3O4.28,54 For comparison, the CV of NiO showed the characteristic oxidation


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peak at 1.444 V and the reduction peak at ~1.344 V for the conversion of Ni(II)/Ni(III) and Ni(III)/Ni(II), respectively. In the Ni doped mixed metal oxides (NCO-1), two broad peaks were noticed during the anodic sweep at ~1.344 and ~1.494V, they might be associated to oxidation of Ni(II), Co(II), and Co(III), but it is difficult to distinguish corresponding redox behavior due to the similar redox potential of Ni and Co.55 During the cathodic sweep, the reduction of corresponding metal ions overlapped and displayed a broad reduction peak at ~1.279 V. In addition, a high amount of charge under the curve indicates the improved conductivity and increased surface roughness in the mixed oxide NCO-1 when compared to Co3O4 and NiO.43 Further increasing the Ni content, the redox behavior is similar, but charge under the oxidation and reduction peak decreased. This might be due to structural and morphological changes of NCO-2 and NCO-3. Obviously, NCO-1 showed the high electrochemical redox features due to optimal loading of Ni in the spinel structure as well as having a mesoporous nature. The electrochemical double-layer capacitance (Cdl), which is directly proportional to the electrochemical surface area (ECSA), was determined by collecting CV curves at various sweep rates (0.005 to 0.100 V s-1) in the potential range of 0.914 - 1.04 V, where no red-ox processes occurred as displayed in the supporting information (Figure S2 A-E). The Cdl values calculated from the slope of the plots (Figure S2 F) for Co3O4, NCO-1, NCO-2, NCO-3, and NiO were 0.10, 1.25, 1.07, 0.14 and 0.24 mF cm-2, respectively, revealing that the Cdl of NCO-1 was much higher than that of Co3O4, NCO-2, NCO3, and NiO. Based on the standard capacitance value of metal oxide (40 µF cm-2), the electrochemical surface areas (ECSAs) were estimated to be 2.38, 31.25, 26.75, 3.60, 5.91 cm2 for Co3O4, NCO-1, NCO-2, NCO-3, and NiO, respectively.


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Figure 5. CV curves of the Co3O4, NCO-1, NCO-2, NCO-3 and NiO recorded in 0.1 M KOH solution at 0.050 V s-1. Cyclic voltammetry was used to characterize the activity of the developed metal oxide towards ORR by scanning from 1.064 to 0.164 V in a 0.1 M KOH solution. Figure 6A depicts the CV curves of the NCO-1 nanomaterial-based electrode recorded in the Ar-saturated (blue) and O2saturated (red) alkaline solution at 0.050 V s-1. As expected, almost no cathodic peak was observed in the Ar-saturated solution. However, a well-defined cathodic peak was observed in the O2saturated electrolyte solution, which was due to the reduction of dissolved oxygen. The NCO-1 nanomaterial exhibited an excellent electrocatalytic activity towards ORR in 0.1 M KOH with an


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onset potential of -0.9 V and the reduction peak obtained maximum at 0.764 V. The achieved negative onset potential was the lowest towards ORR in alkaline solution when compared to other metal oxide based electrocatalysts reported in the literature, as listed in Table S2. Followed by the CV study, RDE experiments were conducted in order to compare the ORR performance. Figure 6B shows the ORR performance of the synthesized metal oxides, including Co3O4 (navy blue), NCO-1 (red), NCO-2 (green), NCO-3 (dark brown) and NiO (pink) electrodes at 1600 rpm. Based on the half wave potential, the order of ORR activity was determined for the fabricated electrodes as follows: NCO-1 > NCO-2 > NCO-3 > Co3O4 > NiO. Among the five catalysts, NCO-1 demonstrated enhanced electrocatalytic performance towards ORR, with its activity being about three times greater than that of the mono-metal oxides of Co3O4 and NiO. The NCO nanomaterial demonstrated a less negative ORR onset potential, which is ~0.250 V lesser than that of the monometal oxides. To study the charge transfer properties of the metal oxides, EIS analysis was carried out for Co3O4, NCO-1 and NiO. The electrochemical impedance spectra were presented in the supporting information (Figure S3), and the electrical circuit was employed to fit the EIS data was displayed as an inset of Figure S3. The fitting results were listed in Table S3, showing that the charge transfer resistance of NCO-1 was much smaller than that of Co3O4 and NiO, which is consistent with the ORR results. Further, the mass activity of the metal oxides was calculated at 0.814 V and plotted in Figure 6C. The mass activity of the NCO-1 nanomaterial electrode was over four times higher than that of the mono-metal oxides. As shown in Figure 6B and 6C, NCO1 nanomaterial exhibited a promising ORR when compared to other electrodes investigated in the present study. Moreover, the NCO-1 nanomaterial electrode is favorable towards ORR when compared with recent ORR catalysts based on metal oxides with and without carbon supports reported in the literature as shown in Table S4. The exemplary performance of the carbon


17


unsupported mesoporous NCO-1 nanomaterial is due to an abundance of octahedral site cations the

mesoporous

j/mA cm-2

-0.5 -1.0

4

-1

-2 Co3O4 NCO-1 NCO-2 NCO-3 NiO

-3 NCo-1 (With Ar) NCo-1 (With O2)

-1.5 0.2

0.4

0.6

E/V vs. RHE

0.8

1.0

-4

structure.

C

0

0.5 0.0

spinel

0.2

0.4

0.6

0.8

3

2

1

0

1.0

E/V vs. RHE

N iO

B

in

N C O -3

1.0

sites

N C O -2

oxygen

C o 3O 4 N C O -1

A

defective

Mass activity(A/g)

and

j/mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

Figure 6. CV curves of the NCO-1 recorded in Ar-saturated or O2-saturated 0.1 KOH at a scan rate of 0.050 V s-1 (A). The comparison of LSV curves obtained with RDE in O2 saturated 0.1 KOH during the rotation of 1600 rpm at 0.010 V s-1 (B). The plot of mass activity at 0.814 V against various Ni and Co oxides-based electrodes (C). Furthermore, RDE experiments were conducted in various rotation speeds in order to get kinetic information of the electrocatalysts. Figure 7A shows the representative RDE curves of the NCO1 electrode with varied rotation rates in O2 saturated 0.1 M KOH solution at 0.010 V s−1. KouteckyLevich (KL) plot of the NCO-1 electrode was obtained by plotting 1/j vs. 1/ω1/2 at different potentials and is displayed in the Figure 7B. The KL plot showed a linear response in the selected potential range, which exemplifies the first-order kinetics with respect to dissolved oxygen concentration, which is in good agreement with the reported results.11,56 The number of electrons involved in the reduction of each molecule of oxygen could be determined using the KouteckyLevich Equation (2).57

1 1 1 1 1 = + = + 𝑗 𝑗𝑘 𝑗𝑙 𝑗𝑘 𝐵𝜔1/2

(2)


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Where, jk-1 kinetic current density (A cm-2), jl -limiting current density (A cm-2), and ɷ is the rotation rate (rpm). B is the Levich constant and is given by, 𝐵 = 0.2𝑛𝐹𝐷2/3 𝜗 −1/6 𝐶0

(3)

Where, n = number of electrons, F=Faraday constant (96487 C mol-1), Co=concentration of the dissolved oxygen in the electrolyte (1.2 x 10-6 mol cm-3), ν = kinematic viscosity of water (0.01 cm2 s-1), and D = diffusion coefficient of oxygen (1.9x10-5 cm2 s-1).58 From the KL equation, the average number of electrons transferred for ORR was determined to be ~3.96 for NCO-1 at the potential range from 0.614 to 0.414 V. Similarly, all synthesized metal oxides were also examined for ORR and the number electrons transferred during ORR were compared as shown in Figure 7C. The number of electrons transferred during ORR was calculated to be 3.79, 3.10, 2.60, and 2.35 on NCO-2, NCO-3, Co3O4 and NiO, respectively, revealing that NCO-1 nanomaterial exhibited the highest ORR performance compared to all the others investigated in the present study. In the ORR, formation of HO2- was identified as an intermediate product attributed to the two-step electron transfer process. In order to examine the HO2- formation on the electrocatalyst, SECM was utilized to map the electrocatalytic activity of metal oxide coated GCE surface. Figures 7D, E and F display the SECM tip current profile for the HO2- produced on the NiO, Co3O4, and NCO-1 coated GCE, respectively. The tip current profile reveals that NiO and Co3O4 produce a greater amount of HO2- than NCO-1, which further confirms the high electrocatalytic activity of NCO-1 towards the four-electron ORR.


19

100 rpm 400 rpm 900 rpm 1600 rpm 2500 rpm

-4 0.2

0.4

0.6

0.8

0.4

0.2

1.0

0.04

0.06

1/(,

E/V vs RHE

D

0.02

0.08

3 2 1 0

0.10

rpm)1/2

E

4

N iO

-3

0.6

C

C o 3O 4 N C O -1

-2

0.8

0.614 V 0.564 V 0.514 V 0.464 V 0.414 V

N C O -3

-2

-1

1.0

N C O -2

B

0

1/(j, mA cm )

A j/mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

no. of electrons


F

Figure 7. The representative RDE curves of NCO-1 recorded at NCO-1 in O2-saturated 0.1 KOH with varied rotation speed and at a sweep rate of 0.010 V s-1 (A). KL plot for NCO-1 (B). Number of electrons transfer during the ORR on various metal oxides (C). SECM tip current profile for the HO2- produced on the NiO (D), Co3O4 (E), NCO-1 (F). In order to be an efficient cathodic electrocatalyst of DMFCs, methanol tolerance ORR activity is another important criterion, because methanol crossover from anode to cathode decreases the electrocatalytic activity and overall efficiency of the DMFC.13 Hence, it is a crucial to evaluate the performance of the cathodic electrocatalyst in presence of methanol for the tolerance level and selectivity of the catalyst towards ORR. To check methanol tolerance of the developed catalyst, chronoamperometric (CA) experiments were performed by applying the constant potential at 0.764 V in O2-saturated 0.1 M KOH at 1600 rpm. Figure 8A depicts the relative current density of ORR on the NCO-1 nanomaterial and 20 wt.% metal loading of commercial Pt/C electrodes prior to and following the addition of 0.5 M methanol. Following the addition of methanol, it can be clearly


20

Page 21 of 32

seen that the instant drop in the ORR current density with almost no activity on Pt/C (lavender) due to methanol oxidation in the applied potential. However, the NCO-1 nanomaterial electrode showed a stable ORR response and the current density was unchanged in the CA response. This study revealed that NCO-1 nanomaterial was much more methanol tolerant towards ORR compared to the commercial 20 wt.% Pt/C catalyst.

B100

A

100

80 0.5 M CH3OH

50

NCO-1 20%-Pt/C

0 0

50

100

150

Activity loss (%)

Relative current density (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

NCO-1 20%-Pt/C

60 40 20 0

0

Time/s

100

200

300

Time/min

Figure 8. Methanol tolerance test recorded on NCO-1 and 20 wt.%-Pt/C, Eapp: 0.764 V in 0.5 M methanol + 0.1 M KOH (A). The stability of NCO-1 and 20 wt.%-Pt/C electrodes for ORR (B). Eapp: 0.764 V; Electrolyte: O2 saturated 0.1 KOH. The issue of electrode stability arises due to particle aggregation, carbon corrosion, and the catalyst dissolution. Therefore, it is critical to develop a stable and active electrocatalyst for DMFCs. In order to investigate the stability of the electrocatalysts, additional CA was performed, applying 0.764 V in an O2-saturated 0.1 M KOH electrolyte for 300 min; the ORR activity loss in percentage (%) was plotted in Figure 8B. Over the 300-minute period, ~6% loss of the catalytic activity was observed for the NCO-1 nanomaterial, whereas over 25% losses occurred with the Pt/C catalyst, revealing that the NCO-1 nanomaterial is much more stable towards ORR when compared to the state-of-the-art Pt/C electrocatalyst.


21


Page 22 of 32

CONCLUSIONS Various nanostructured nickel-cobalt oxides with different composition have been successfully synthesized using a hydrothermal method followed by annealing. The physicochemical and electrochemical properties of the formed mixed metal oxides were systemically studied using various surface characterization techniques and electrochemical methods. It was observed that the morphology of the resultant NCO nanomaterials is controllable by using the appropriate ratio of Ni/Co in the preparatory solutions. Among the synthesized metal oxides, NCO-1 possessed a mesoporous sheet structure with abundant active sites. Further, the synthesized NCO nanomaterials were investigated towards ORR in alkaline solution and its activity, was compared with the commercially prevalent 20 wt.% Pt/C electrocatalyst. The nanostructured NCO material exhibits excellent electrocatalytic activity towards ORR in 0.1 M KOH with less negative ORR onset potential (0.9 V) and high mass activity (4.5 A/g). SECM surface mapping analysis provided further insight into the electrocatalysis, where the optimized NCO facilitated a single-step fourelectron transfer pathway for the ORR with minimal production of HO2-. In addition to enhanced electrocatalytic activity, the methanol tolerance and long-term durability of the NCO nanomaterial was effectively tested, revealing the higher performance of the NCO nanomaterial than that of the commercial 20 wt.% Pt/C. These results demonstrate that the developed NCO nanomaterial with high abundant of octahedral site cations and defective oxygen sites in the mesoporous spinel structure are favorable for advancing ORR in alkaline media. The unique carbon-free mesoporous nanostructure and its high methanol-tolerant activity suggest that NCO-1 would be a promising alternative durable non-noble cathode electrocatalyst for the ORR in energy applications.

ASSOCIATED CONTENT


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Supporting Information. XPS of nickel-cobalt oxides; Tables of measured parameters from the characterization results and comparison of catalyst performance. AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was sponsored by the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery Grant, RGPIN-2015-06248). A.C. acknowledges the NSERC and the Canada Foundation for Innovation for the Canada Research Chair Award in Electrochemistry and Nanoscience. REFERENCES (1)

Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.;

Wang, C.; Su, D.; Shao. D.; Wu, G. Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation. J. Am. Chem. Soc. 2017, 139, 14143– 14149. (2)

Chang, A.; Zhang, C.; Yu, Y.; Yu, Y.; Zhang, B. Plasma-Assisted Synthesis of NiSe2

Ultrathin Porous Nanosheets with Selenium Vacancies for Supercapacitor. ACS Appl. Mater. Interfaces 2018, 10, 41861–41865.


23


(3)

Page 24 of 32

Chen, A.; Ostrom, C. Palladium-Based Nanomaterials: Synthesis and Electrochemical

Applications. Chem. Rev. 2015, 115, 11999–12044. (4)

Zhong, H. X.; Wang, J.; Zhang, Y. W.; Xu, W. L.; Xing, W.; Xu, D.; Zhang, Y. F.; Zhang,

X. B. ZIF-8 Derived Graphene-Based Nitrogen-Doped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts. Angew. Chemie 2014, 53, 14235–14239. (5)

Zhong, H.; Wang, J.; Meng, F.; Zhang, X. In Situ Activating Ubiquitous Rust towards

Low-Cost, Efficient, Free-Standing, and Recoverable Oxygen Evolution Electrodes. Angew. Chemie. 2016, 55, 9937–9941. (6)

Wang, H.; Yin, S.; Li, Y.; Yu, H.; Li, C.; Deng, K.; Xu, Y.; Li, X.; Xue, H.; Wang, L. One-

Step Fabrication of Tri-Metallic PdCuAu Nanothorn Assemblies as an Efficient Catalyst for Oxygen Reduction Reaction. J. Mater. Chem. A 2018, 6, 3642–3648. (7)

Shi, Q.; Zhu, C.; Bi, C.; Xia, H.; Engelhard, M. H.; Du, D.; Lin, Y. Intermetallic Pd3Pb

Nanowire Networks Boost Ethanol Oxidation and Oxygen Reduction Reactions with Significantly Improved Methanol Tolerance. J. Mater. Chem. A 2017, 5, 23952–23959. (8)

Shukla, A. K.; Raman, R. K.; Scott, K. Advances in Mixed-Reactant Fuel Cells. Fuel Cells

2005, 5, 436–447. (9)

Li, X.; Faghri, A. Review and Advances of Direct Methanol Fuel Cells (DMFCs) Part I:

Design, Fabrication, and Testing with High Concentration Methanol Solutions. J. Power Sources 2013, 226, 223–240.


24

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10)

Zhang, G.; Xia, B. Y.; Wang, X.; Lou, X. W. Strongly Coupled NiCo2O4-rGO Hybrid

Nanosheets as a Methanol-Tolerant Electrocatalyst for the Oxygen Reduction Reaction. Adv. Mater. 2014, 26, 2408–2412. (11)

Liu, Q.; Zhang, J. Graphene Supported Co-G-C3N4 as a Novel Metal-Macrocyclic

Electrocatalyst for the Oxygen Reduction Reaction in Fuel Cells. Langmuir 2013, 29, 3821–3828. (12)

Jeyabharathi, C.; Venkateshkumar, P.; Mathiyarasu, J.; Phani, K. L. N. Platinum-Tin

Bimetallic Nanoparticles for Methanol Tolerant Oxygen-Reduction Activity. Electrochim. Acta 2008, 54, 448–454. (13)

Ahmed, M.; Dincer, I. A. Review on Methanol Crossover in Direct Methanol Fuel Cells:

Challenges and Achievements. Int. J. Energy Res. 2011, 35, 1213-1228. (14)

Ma, M.; You, S.; Wang, W.; Liu, G.; Qi, D.; Chen, X.; Qu, J.; Ren, N. Biomass–Derived

Porous Fe3C/WC/GC Nanocomposite for Efficient Electrocatalysis of Oxygen Reduction. ACS Appl. Mater. Interfaces 2016, 8, 32307-32316. (15)

Sebastián, D.; Serov, A.; Artyushkova, K.; Gordon, J.; Atanassov, P.; Aricò, A. S.; Baglio,

V. High Performance and Cost-Effective Direct Methanol Fuel Cells: Fe-N-C Methanol-Tolerant Oxygen Reduction Reaction Catalysts. ChemSusChem 2016, 9, 1986–1995. (16)

Li, Q.; Wang, T.; Havas, D.; Zhang, H.; Xu, P.; Han, J.; Cho, J.; Wu, G. High-Performance

Direct Methanol Fuel Cells with Precious-Metal-Free Cathode. Adv. Sci. 2016, 3, 1600140. (17)

Yang, Y.; Fei, H.; Ruan, G.; Li, L.; Wang, G.; Kim, N. D.; Tour, J. M. Carbon-Free

Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. ACS Appl. Mater. Interfaces 2015, 7, 20607–20611.


25


(18)

Page 26 of 32

Chen, A.; Holt-Hindle, P. Platinum-Based Nanostructured Materials: Synthesis, Properties,

and Applications. Chem. Rev. 2010, 110, 3767–3804. (19)

Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-

Horn, Y. Design Principles for Oxygen-Reduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal–air Batteries. Nat. Chem. 2011, 3, 546–550. (20)

Meng, F.; Zhong, H.; Bao, D.; Yan, J.; Zhang, X. In Situ Coupling of Strung Co4N and

Intertwined N-C Fibers toward Free-Standing Bifunctional Cathode for Robust, Efficient, and Flexible Zn-Air Batteries. J. Am. Chem. Soc. 2016, 138, 10226–10231. (21)

Meng, F. L.; Wang, Z. L.; Zhong, H. X.; Wang, J.; Yan, J. M.; Zhang, X. B. Reactive

Multifunctional

Template-Induced

Preparation

of

Fe-N-Doped

Mesoporous

Carbon

Microspheres Towards Highly Efficient Electrocatalysts for Oxygen Reduction. Adv. Mater. 2016, 28, 7948–7955. (22)

Wang, J.; Li, K.; Zhong, H. X.; Xu, D.; Wang, Z. L.; Jiang, Z.; Wu, Z. J.; Zhang, X. B.

Synergistic Effect between Metal-Nitrogen-Carbon Sheets and NiO Nanoparticles for Enhanced Electrochemical Water-Oxidation Performance. Angew. Chemie - Int. Ed. 2015, 54, 10530– 10534. (23)

Lambert, T. N.; Vigil, J. A.; White, S. E.; Davis, D. J.; Limmer, S. J.; Burton, P. D.; Coker,

E. N.; Beechem, T. E.; Brumbach, M. T. Electrodeposited NixCo3−xO 4 Nanostructured Films as Bifunctional Oxygen Electrocatalysts. Chem. Commun. 2015, 51, 9511–9514. (24)

Wang, Z.-L.; Xu, D.; Xu, J.-J.; Zhang, X.-B. Oxygen Electrocatalysts in Metal–air

Batteries: From Aqueous to Nonaqueous Electrolytes. Chem. Soc. Rev. 2014, 43, 7746–7786.


26

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25)

Gao, X.; Zhang, H.; Li, Q.; Yu, X.; Hong, Z.; Zhang, X.; Liang, C.; Lin, Z. Hierarchical

NiCo2O4 Hollow Microcuboids as Bifunctional Electrocatalysts for Overall Water-Splitting. Angew. Chemie Int. Ed. 2016, 55, 6290–6294. (26)

Wang, D.; Chen, X.; Evans, D. G.; Yang, W. Well-Dispersed Co3O4/Co2MnO4

Nanocomposites as a Synergistic Bifunctional Catalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nanoscale 2013, 5, 5312–5315. (27)

Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co 3O4 Nanocrystals

on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780–786. (28)

Sa, Y. J.; Kwon, K.; Cheon, J. Y.; Kleitz, F.; Joo, S. H. Ordered Mesoporous Co3O4 Spinels

as Stable, Bifunctional, Noble Metal-Free Oxygen Electrocatalysts. J. Mater. Chem. A 2013, 1, 9992-10001. (29)

Ding, R.; Qi, L.; Jia, M.; Wang, H. Simple Hydrothermal Synthesis of Mesoporous Spinel

NiCo2O4 Nanoparticles and Their Catalytic Behavior in CH3OH Electro-Oxidation and H2O2 Electro-Reduction. Catal. Sci. Technol. 2013, 3, 3207–3215. (30)

Hu, C. C.; Lee, Y.; U, T. W. The Physicochemical/ Electrochemical Properties of Binary

Ni–Co Oxides. Mater. Chem. Phys. 1997, 48, 246–254. (31)

Wu, F.; Bai, J.; Feng, J.; Xiong, S. Porous Mixed Metal Oxides: Design, Formation

Mechanism, and Application in Lithium-Ion Batteries. Nanoscale 2015, 7, 17211–17230.


27


(32)

Page 28 of 32

Chen, S.; Xue, M.; Li, Y.; Pan, Y.; Zhu, L.; Qiu, S. Rational Design and Synthesis of

NixCo3-xO4 Nanoparticles Derived from Multivariate MOF-74 for Supercapacitors. J. Mater. Chem. A 2015, 4, 1166–1169. (33)

Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X. Hierarchical ZnXCo3–XO4

Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution. Chem. Mater. 2014, 26, 1889–1895. (34)

Hu, H.; Guan, B.; Xia, B.; Lou, X. W. Designed Formation of Co3O4/NiCo2O4 Double-

Shelled Nanocages with Enhanced Pseudocapacitive and Electrocatalytic Properties. J. Am. Chem. Soc. 2015, 137, 5590–5595. (35)

McCullough, S. M.; Flynn, C. J.; Mercado, C. C.; Nozik, A. J.; Cahoon, J. F.

Compositionally-Tunable Mechanochemical Synthesis of ZnXCo3−xO4 Nanoparticles for Mesoporous P-Type Photocathodes. J. Mater. Chem. A 2015, 3, 21990–21994. (36)

Wu, X.; Scott, K. CuxCo3−xO4 (0 ≤ x < 1) Nanoparticles for Oxygen Evolution in High

Performance Alkaline Exchange Membrane Water Electrolysers. J. Mater. Chem. 2011, 21, 12344-12351. (37)

Yan, W.; Cao, X.; Ke, K.; Tian, J.; Jin, C.; Yang, R. One-Pot Synthesis of Monodispersed

Porous CoFe2O4 Nanospheres on Graphene as an Efficient Electrocatalyst for Oxygen Reduction and Evolution Reactions. RSC Adv. 2016, 6, 307–313. (38)

Yan, X.; Li, K.; Lyu, L.; Song, F.; He, J.; Niu, D.; Liu, L.; Hu, X.; Chen, X. From Water

Oxidation to Reduction: Transformation from NiXCo3–XO4 Nanowires to NiCo/NiCoOX Heterostructures. ACS Appl. Mater. Interfaces 2016, 8, 3208–3214.


28

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39)

Yu, X.; Sun, Z.; Yan, Z.; Xiang, B.; Liu, X.; Du, P. Direct Growth of Porous Crystalline

NiCo2O4 Nanowire Arrays on a Conductive Electrode for High-Performance Electrocatalytic Water Oxidation. J. Mater. Chem. A 2014, 2, 20823–20831. (40)

Zhai, Y.; Mao, H.; Liu, P.; Ren, X.; Xu, L.; Qian, Y. Q. Facile Fabrication of Hierarchical

Porous Rose-like NiCo2O4 nanoflake/MnCo2O4 Nanoparticle Composite with Enhanced Electrochemical Performance for Energy Storage. J. Mater. Chem. A 2015, 3, 16142–16149. (41)

Xiang, N.; Ni, Y.; Ma, X. Shape-Controlled Synthesis of NiCo2O4 Microstructures and

Their Application in Supercapacitors. Chem. - An Asian J. 2015, 10, 1972–1978. (42)

Zheng, F.; Zhu, D.; Chen, Q. Facile Fabrication of Porous NiXCo3– XO4 Nanosheets with

Enhanced Electrochemical Performance As Anode Materials for Li-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 9256–9264. (43)

Li, Y.; Hasin, P.; Wu, Y. NixCo3-XO4 Nanowire Arrays for Electrocatalytic Oxygen

Evolution. Adv. Mater. 2010, 22, 1926–1929. (44)

Xiao, Y.; Hu, C.; Qu, L.; Hu, C.; Cao, M. Three-Dimensional Macroporous NiCo2O4

Sheets as a Non-Noble Catalyst for Efficient Oxygen Reduction Reactions. Chem. - A Eur. J. 2013, 19, 14271–14278. (45)

Francesca Liotta, L.; Wu, H.; Giuseppe Pantaleo,

ab; Maria Venezia, A. Co3O4

Nanocrystals and Co3O4–MOX Binary Oxides for CO, CH 4 and VOC Oxidation at Low Temperatures: A Review. Catal. Sci. Technol 2013, 3, 3073–3378.


29


(46)

Page 30 of 32

Cui, B.; Lin, H.; Li, J. B.; Li, X.; Yang, J.; Tao, J. Core-Ring Structured NiCo2O4

Nanoplatelets: Synthesis, Characterization, and Electrocatalytic Applications. Adv. Funct. Mater. 2008, 18, 1441–1447. (47)

Shi, H.; Zhao, G. Water Oxidation on Spinel NiCo2O4 Nanoneedles Anode:

Microstructures, Specific Surface Character, and the Enhanced Electrocatalytic Performance. J. Phys. Chem. C 2014, 118, 25939–25946. (48)

Manivasakan, P.; Ramasamy, P.; Kim, J. Use of Urchin-like NixCo3-xO4 Hierarchical

Nanostructures Based on Non-Precious Metals as Bifunctional Electrocatalysts for AnionExchange Membrane Alkaline Alcohol Fuel Cells. Nanoscale 2014, 6, 9665–9672. (49)

Zhao, J.; Li, Z.; Zhang, M.; Meng, A.; Li, Q. Direct Growth of Ultrathin NiCo2O4/NiO

Nanosheets on SiC Nanowires as a Free-Standing Advanced Electrode for High-Performance Asymmetric Supercapacitors. ACS Sustain. Chem. Eng. 2016, 4, 3598–3608. (50)

Ibupoto, Z. H.; Elhag, S.; AlSalhi, M. S.; Nur, O.; Willander, M. Effect of Urea on the

Morphology of CO3O4 Nanostructures and Their Application for Potentiometric Glucose Biosensor. Electroanalysis 2014, 26, 1773–1781. (51)

Kuboon, S.; Hu, Y. H. Study of NiO−CoO and Co3O4 −Ni3O4 Solid Solutions in

Multiphase Ni−Co−O Systems. Ind. Eng. Chem. Res. 2011, 50, 2015–2020. (52)

Bao, J.; Zhang, X.; Fan, B.; Zhang, J.; Zhou, M.; Yang, W.; Hu, X.; Wang, H.; Pan, B.;

Xie, Y. Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation. Angew. Chemie - Int. Ed. 2015, 54, 7399–7404.


30

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(53)

Roginskaya, Y. E.; Morozova, O. V; Lubnin, E. N.; Ulitina, Y. E.; Lopukhova, G. V;

Trasatti, S. Characterization of Bulk and Surface Composition of CoXNi1-XOY Mixed Oxides for Electrocatalysis. Langmuir 1997, 13, 4621–4627. (54)

Koza, J. A.; He, Z.; Miller, A. S.; Switzer, J. a. Electrodeposition of Crystalline Co3O4 —

A Catalyst for the Oxygen Evolution Reaction. Chem. Mater. 2012, 24, 3567–3573. (55)

Lei, Y.; Wang, Y.; Yang, W.; Yuan, H.; Xiao, D. Self-Assembled Hollow Urchin-like

NiCo2O4 Microspheres for Aqueous Asymmetric Supercapacitors. RSC Adv. 2015, 5, 7575–7583. (56)

Boopathi, S.; Raju, C. V.; Jeyabharathi, C.; Kumar, S. S. Synthesis of poly(N-Vinyl-2-

Pyrrolidone) Adsorbed-AuPt/C Bimetallic Nanoparticles and Their Unusual Electrocatalytic Activity towards Methanol Tolerant Oxygen Reduction Reaction. J. Solid State Electrochem. 2016, 20, 579–587. (57)

Garsany, Y.; Epshteyn, A.; Purdy, A. P.; More, K. L.; Swider-Lyons, K. E. High-Activity,

Durable Oxygen Reduction Electrocatalyst: Nanoscale Composite of Platinum-Tantalum Oxyphosphate on Vulcan Carbon. J. Phys. Chem. Lett. 2010, 1, 1977–1981. (58)

Dhavale, V. M.; Gaikwad, S. S.; Kurungot, S. Activated Nitrogen Doped Graphene Shell

towards Electrochemical Oxygen Reduction Reaction by Its Encapsulation on Au Nanoparticle (Au@N-Gr) in Water-in-Oil “nanoreactors.” J. Mater. Chem. A 2014, 2, 1383–1390.


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Synthesis and Electrochemical Study of Mesoporous Nickel-Cobalt

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