Research Article www.acsami.org
Exceptionally Active and Stable Spinel Nickel Manganese Oxide Electrocatalysts for Urea Oxidation Reaction Sivakumar Periyasamy,†,§ Palaniappan Subramanian,‡,§ Elena Levi,† Doron Aurbach,† Aharon Gedanken,*,† and Alex Schechter*,‡ †
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Department of Chemistry, Bar-Ilan Institute of Nanotechnology and Advanced Materials (BINA), Bar-Ilan University, Ramat-Gan 52900, Israel ‡ Department of Biological Chemistry, Ariel University, Ariel 40700, Israel S Supporting Information *
ABSTRACT: Spinel nickel manganese oxides, widely used materials in the lithium ion battery high voltage cathode, were studied in urea oxidation catalysis. NiMn2O4, Ni1.5Mn1.5O4, and MnNi2O4 were synthesized by a simple template-free hydrothermal route followed by a thermal treatment in air at 800 °C. Rietveld analysis performed on nonstoichiometric nickel manganese oxide-Ni1.5Mn1.5O4 revealed the presence of three mixed phases: two spinel phases with different lattice parameters and NiO unlike the other two spinels NiMn2O4 and MnNi2O4. The electroactivity of nickel manganese oxide materials toward the oxidation of urea in alkaline solution is evaluated using cyclic voltammetric measurements. Ni1.5Mn1.5O4 exhibits excellent redox characteristics and lower charge transfer resistances in comparison with other compositions of nickel manganese oxides and nickel oxide prepared under similar conditions.The Ni1.5Mn1.5O4modified electrode oxidizes urea at 0.29 V versus Ag/AgCl with a corresponding current density of 6.9 mA cm−2. At a low catalyst loading of 50 μg cm−2, the urea oxidation current density of Ni1.5Mn1.5O4 in alkaline solution is 7 times higher than that of nickel oxide and 4 times higher than that of NiMn2O4 and MnNi2O4, respectively. KEYWORDS: nickel manganese oxide, urea oxidation, cyclic voltammetry, electrochemical impedance spectroscopy, hydrothermal synthesis
1. INTRODUCTION Fuel cells can run on a variety of hydrogen-rich chemicals such as methanol, ethanol, and ethylene glycol by either direct feed or external reforming. In the past few years, there has been a growing interest in utilizing urea as an alternative fuel in fuel cells since it is abundantly available, stable, nontoxic, and nonflammable. Urea is also considered as a promising hydrogen carrier for a sustainable supply of energy.1 Urea can be synthesized from ammonia or produced from natural gas or coal in large quantities. Human and animal urine contain about 2−2.5 wt % urea and the calculated urine production from human resources alone is 240 million tons compare to 0.5 million tons of fossil fuels.2 In the past few years, several works have been published on direct conversion of urea energy in fuel cells.2−4 Nevertheless, the power density of urea fuel cells is limited to a few milliwatts per square centimeter, due to slow kinetics of urea oxidation at the anode. The inexpensive nonnoble catalysts employed in the direct urea fuel cell anodes reported recently are nickel based monometallic2,3 and few bimetallic4,5 materials. The theoretical potential of urea oxidation process is −0.46 V versus standard hydrogen electrode (SHE). The lowest overpotential of ca. 0.45 V versus © 2016 American Chemical Society
SHE reported so far for electrochemical oxidation of urea on nickel based electrocatalysis still considerably higher than the theoretical value. Hence, it is desirable to produce new stable nickel-based catalysts that can oxidize urea efficiently at lower/ comparable overpotential and thereby help to improve the overall efficiency of direct urea based fuel cell systems. In this context, we propose to develop a bimetallic catalyst composed of nickel and manganese oxide based on the premise that the incorporation of multivalent manganese in the bimetallic oxide and/or redox characteristics of manganese oxide could potentially downshift the onset potential/reduce the activation energy of nickel oxyhydroxide formation and thereby facilitate urea oxidation at lower overpotentials. Several groups have reported the preparation and properties of nickel manganese oxide materials for a variety of application. For example, Pang et al. prepared porous bipyramid, fusiform, and plate structured NiMn2O4 materials by calcining oxalate precursors in air without using any template or surfactant for Received: February 28, 2016 Accepted: April 28, 2016 Published: April 28, 2016 12176
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ACS Applied Materials & Interfaces supercapacitor application.6 Kang et al. reported porous hierarchical NiMn2O4/C tremella-like nanostructures obtained through a simple solvothermal and calcination method that exhibited a superior specific capacity and an excellent long-term cycling performance even at a high current density in lithium battery cathodes.7 Garcia et al. investigated the catalytic activity of Ni-doped MnOx catalysts by mild hydrothermal reaction between MnVII and MnII in the presence of different carbon powder substrates, at controlled pH and temperature so as to characterize the effects of these substrates toward the Oxygen reduction reaction (ORR) kinetics in alkaline medium.8 Menezes et al. have prepared nickel−manganese oxides with variable Ni:Mn ratios from heterobimetallic single-source precursors that turned out to be efficient water oxidation catalysts. In this report, the authors have used nickel and manganese oxalate precursors in microemulsions containing cetyltrimethylammonium bromide (CTAB) as a surfactant, 1hexanol as cosurfactant and hexane as the lipophilic phase and mixed them with an aqueous solution containing Ni2+, Mn2+, and oxalate ions with tunable ratios.9 To date, there is no report on spinel nickel manganese oxide prepared by templateless hydrothermal synthetic route. More importantly, this report unravels the activity of different spinel nickel manganese oxides toward electrochemical oxidation of urea. Herein, we also report the physicochemical properties and electrocatalytic activity of nickel manganese oxide materials toward oxidation of urea. The hydrothermal synthesis process has led to the formation of nonstoichiometric spinel structure Ni1.5Mn1.5O4. This material has excellent electrochemical properties with highest activity toward electro-oxidation of urea in alkaline solutions.
Table 1. Experimental Conditions for the Synthesis of Nickel Manganese Oxides Oxides metal precursors Ni2+ sample NiMn2O4 Ni1.5Mn1.5O4 MnNi2O4 NiO Mn2O3
Mn2+
molar concn
mass (g)
molar concn
mass (g)
molar ratio (Ni:Mn)
0.05 0.075 0.1 0.15
0.99 1.49 1.99 2.99
0.1 0.075 0.05
1.96 1.47 0.98
0.15
2.94
1:2 1.5:1.5 2:1 1:0 0:1
inductively coupled plasma (ICP)-optical emission spectrometer (OES). The samples were dissolved in hot aqua regia, diluted and the results of three independent measurements were averaged to verify if they were in accordance with the expected atomic ratio. The X-ray diffraction (XRD) studies were performed with a Bruker Inc. (Germany) AXS D8 ADVANCE diffractometer (reflection θ−θ geometry, Cu Kα radiation, receiving slit 0.2 mm, high-resolution energy-dispersive detector). Diffraction data for the Rietveld refinement were collected in the angular range of 10° < 2θ< 140°, step size 0.02°, step time 6 s/step for NiMn2O4 and MnNi2O4, 3 s/step for Ni1.5Mn1.5O4. The data were analyzed by the Rietveld structure refinement program, FULLPROF.10 The structural data for the modeling were taken from previously reported articles for spinel NiMn2O411 and NiO.12 The Thompson−Cox−Hastings pseudo-Voigt function was used for the peak-shape approximation. The background was fitted manually by linear interpolation. The composition of the materials was measured by using a Bruker (TENSOR27) Fourier transform infrared (FTIR) spectrophotometer. The size and morphology of the samples were investigated by transmission electron microscopy (TEM) with energy-dispersive X-ray (EDX) mapping, and the corresponding selected area electron diffraction (SAED) patterns were conducted on a JEOL, JEM-1400 transmission electron microscope at an acceleration voltage of 120 kV and a high-resolution scanning electron microscope (HRSEM, FEI, Magellan 400L, accelerating voltage 15 kV), respectively. Samples for TEM measurements were prepared by making a suspension of the particles in isopropanol, using water-bath sonication. Two small droplets were then applied on a TEM copper grid, coated with a carbon film, and dried in air in a covered Petri dish before sample processing. The specific surface area (SSA) was determined by BET (Brunauer− Emmett−Teller) N2 adsorption−desorption isotherms measured at liquid nitrogen temperature using a NOVA-3200e Quantachrome surface area analyzer. 2.3. Electrode Fabrication and Electrocatalytic Studies. Electrodes with catalysts prepared from slurries of suspended particles were applied to the electrode surface. Each suspension contained 5 mg of the prepared catalyst, 2.4 mg of the Vulcan XC-72 Carbon, 280 μL of Nafion solution (ion power, 5 wt % in isopropanol) in a mixture of 30% isopropanol (Frutarom Ltd., 99.5%) and 70% water (v/v). A 5 μL aliquot of this suspension was pipetted onto a 5 mm diameter glassy carbon disk electrode (geometric area 0.197 cm2, Pine Instruments) that was used as a working electrode. After drying in air for 20 min, the metal oxide loading on the disk electrode was 50 μg·cm−2. A platinum wire (99.99%) and an Ag/AgCl electrode (Metrohm) were used as the counter and reference electrode, respectively. Electrochemical impedance spectroscopic (EIS) measurements were recorded under the open circuit voltage (OCV) with alternating current (ac) amplitude of 5 mV over the frequency range of 100 kHz to 1 mHz. For conducting urea electrolysis on nonstoichiometric Ni1.5Mn1.5O4, the electrodes were fabricated by drop-coating the slurry composed of 70 wt % catalyst and 30 wt % Vulcan XC-72 carbon black along with 250 μL of Nafion onto the teflonized carbon cloth. In this case, the mass loading of active materials was about 5 mg cm−2. The electrolytes for tests were 1 M KOH (pH = 14) in the absence and presence of 0.33 M urea. Electrochemical experiments (cyclic voltammetry and
2. EXPERIMENTAL SECTION 2.1. Synthesis of Metal Oxides. All reagents were of analytical grade and used as received. Manganese(II) acetate tetrahydrate (Mn(CH3COO)2·4H2O, 99+%), nickel(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98%), sodium hydroxide (NaOH, ≥97.0%) were purchased from Sigma-Aldrich. Absolute ethanol (dehydrated) (C2H5OH) was purchased from Bio-Lab Ltd. Israel. In a typical synthesis, nickel and manganese acetate salts were dissolved in 80 mL of double distilled water (DDW) in a 100 mL beaker under magnetic stirring. 5 M NaOH was added dropwise to this solution to adjust the pH to10 at room temperature. After stirring for about 15 min, the resultant solution was transferred into a Teflon lined stainless steel autoclave with a capacity of 125 mL, sealed, and heated at 150 °C for a period of 12 h in a programmable electric oven without shaking or stirring to carry out the hydrothermal reaction. When the reaction was completed, the autoclave was naturally cooled to room temperature, the synthesized products were centrifuged and washed with DDW and anhydrous ethanol several times to remove plausible residual impurities and dried at 100 °C for 3 h in a programmable electric oven. The as-prepared product was milled uniformly in an agate mortar. Finally, the sample was annealed at 800 °C for 2 h at the heating rate of 10 °C min−1 in the ambient atmosphere to obtain the metal oxide powders. After cooling, the sample was grounded again into a fine powder for electrochemical measurements. Please refer to Table 1 for the experimental conditions used for the preparation of each of the mixed metal oxide samples. The formation mechanism of mixed metal oxide is based on the coprecipitation method. The metal acetates form hydroxides in basic solution and these metal hydroxides undergo thermal treatment in air to form mixed metal spinel oxides. The stoichiometry of the final mixed metal oxide materials is dependent on the composition of precursor metal hydroxides. 2.2. Characterization Techniques. The chemical composition of the nickel manganese oxide samples was confirmed by a Varian 710-ES 12177
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ACS Applied Materials & Interfaces electrochemical impedance spectroscopy) were conducted at room temperature using a CHI 760C bipotentiostat (CH Instrument, Bee Cave, Texas). Pure N2 (99.999%) gas was passed to purge dissolved oxygen in the electrolytes before and during measurements.
3. RESULTS AND DISCUSSION 3.1. The Chemical Composition of Nickel Manganese Oxides and X-ray Diffraction Analysis. The chemical composition and quantification of the bulk Ni:Mn ratio in the prepared metal oxide powders were obtained by ICP-OES. The values shown in Table 2 exhibit the formation of nickel manganese oxides in different stoichiometry, namely, NiMn2O4, Ni1.5Mn1.5O4, and MnNi2O4. Table 2. Chemical Composition of Nickel Manganese Oxide Materials sample
Ni:Mn (theoretical atomic ratio)
Ni:Mn atomic ratio (ICP-OES)
NiMn2O4 Ni1.5Mn1.5O4 MnNi2O4
1:2 1.5:1.5 2:1
0.99:2.01 1.49:1.51 2.01:0.99
The Rietveld profiles for three mixed Mn−Ni oxides are presented in Figure 1. The results of the quantitative phase analysis can be seen in Table 3. A single spinel phase formed only for the Mn-rich composition, NiMn2O4. Increase in the Ni content in the oxide mixture (composition Ni1.5Mn1.5O4) results in the crystallization of two spinel phases with different lattice parameters and a small amount of NiO. For the Ni-rich composition, the Rietveld plot was fitted as a mixture of spinel, MnNi2O4, and NiO. It should be noted that different number of phases in the refinement model leads to drastic changes in the quality of fitting. In particular, a replacement of one-phase model for composition MnNi2O4 by the two-phase model decreases χ2 from 92.8 to 3.12 (the refinement of the MnNi2O4 sample with one-phase model is presented in Figure S1). Similar, for composition Mn1.5Ni1.5O4χ2 = 49.7, 24.13, and 5.32 for one, two and three-phase models, respectively (for details, see Figure S2). The cation distribution obtained by the Rietveld analysis (Table 4) indicated that MnNi2O4 is a normal spinel (MgAl2O4 structural type): Mn and Ni atoms occupy tetrahedral and octahedral sites, respectively. The increase in the Mn content results in the Ni substitution by Mn and formation of inverse spinel, in accordance with literature data.11 Table 3 presents also the crystalline size obtained by the profile fitting. As discussed below, the results agree with those obtained by SEM. The XRD patterns of Mn2O3 and NiO (Figure 2) are in agreement with the database. The main peaks detected in Mn2O3 and NiO patterns are at 32.97° (222), 55.17° (440) and 37.27° (111), 43.29° (200), 62.89°(220), respectively. Of importance is the phase composition obtained by the Rietveld analysis for two mixed Mn−Ni oxides (MnNi2O4 and Ni1.5Mn1.5O4) disagrees with the results of the chemical analysis. This may be caused by a presence of the Mn-rich amorphous phase in the mixtures. In addition, it is difficult to determine the exact cation distribution and phase composition in the samples under study based on the X-ray diffraction scans due to similar scattering of Ni and Mn atoms. Nevertheless, the instability of the material with the Ni1.5Mn1.5O4 composition, namely, its separation into three different phases, may explain
Figure 1. Rietveld plots. Calculated patterns are shown by solid curves; red dots show the observed intensities. Differences between the observed and calculated intensities are presented by blue curves. Short vertical bars indicate the position of Bragg reflections for the respective phases in the list. From the top to the bottom: (a) MnNi2O4(63%), NiO (37%); (b) MnNi2O4(63%), Ni1.2Mn1.8O4 (27%), NiO (10%); (c) NiMn2O4.
the higher catalytic activity of this composition as compared to others (see below). 3.2. X-ray Photoelectron Spectroscopy (XPS). The valence states and surface atomic concentration of Ni, Mn and O were evaluated by XPS analysis. The XPS survey spectra of Ni1.5Mn1.5O4, NiMn2O4, and MnNi2O4 displayed in Figure 3 12178
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ACS Applied Materials & Interfaces Table 3. Results of the Rietveld Analysis for the Ni−Mn−O system
fitting quality composition MnNi2O4 Ni1.5Mn1.5O4
NiMn2O4
phase
space group
(Mn)tet(Ni)2octO4 NiO (Mn)tet(Ni,Mn)2octO4 (Mn)tet(Ni)2 octO4 NiO (Mn)tet(Ni,Mn)2octO4
Fd−3m Fm−3m Fd−3m Fd−3m Fm−3m Fd−3m
lattice parameters, Å a a a a a a
= = = = = =
8.338 4.176 8.344 8.361 4.175 8.391
atom
16c
Ni1/ Mn1
8b
32e
Mn2/ Ni2
O
x in NixMn3‑xO4
X
Y
Z
occ
2
0
0
0
1/0
1.5 1 2
0 0 3/8
0 0 3/8
0 0 3/8
0.6/0.4/1/0 0.5/0.5 1/0
1.5 1 2 1.5 1
3/8 3/8 0.239 0.236 0.236
3/8 3/8 0.239 0.236 0.236
3/8 3/8 0.239 0.236 0.236
1/0 1/0 1 1 1
crystallite size, nm
Rb, %
χ2
63 37 39 50 11 100
100 85 130 157 69 114
3.74 2.32 2.64 2.77 2.65 4.54
3.12 3.12 5.32 5.32 5.32 2.44
indicate that the mixed-metal oxide surface is composed of nickel, manganese, and oxygen. The calculated signals in Figure 3A−C give the surface atomic ratio of elements, in agreement with the atomic ratio calculated from ICP-OES measurements.NiMn2O4, MnNi2O4, and NiO exhibited Ni 2p3/2peaks at a binding energy of 854.79 eV, 854.4 and 853.98 eV, respectively, which can be attributed to Ni2+13while the Ni 2p3/2 peaks of Ni1.5Mn1.5O4 shifted to a marginally higher energy of 855.09 eV. This signal can be assigned as the mixture of Ni2+/3+.9,13 The high-resolution XPS spectrum of the Mn 2p in nickel manganese oxide sample shows a major peak centered at ∼641 eV and a satellite peak at 653 eV, a characteristic of Mn3+ or Mn4+ ions14 (Figure 4). The positive binding energy shift observed in the Ni 2p and Mn 2p signals may indicate the charge transfer between nickel and manganese and this effect appears pronounced in Ni1.5Mn1.5O4. The O 1s spectrum for single oxides (NiO) showed a signal at 529.58 eV and all mixed-metal oxides exhibited a sharp signal at ∼530 eV. This signal is attributed to bridging oxides and to the hydroxide species.15 3.3. Fourier Transform Infrared Spectroscopic (FTIR) Studies. The FTIR spectra results served as an evidence to confirm the formation of mixed-metal oxides and single oxides. The overlaid FTIR spectra of nickel manganese oxides (NiMn2O4, Ni1.5Mn1.5O4, and MnNi2O4), and single oxides (nickel oxide (NiO), and manganese oxide (Mn2O3)) from 375 to 1500 cm−1 is shown in Figure 5. From 375 to 1000 cm−1, the IR bands of inorganic products are usually assigned to the vibration of ions in the crystal lattices. The peaks observed in this range for all the samples are due to metal−oxygen bands. This indicated the presence of different mixed and single oxide particles. The absorption of highest bands normally observed in the range of 600−550 cm−1 corresponds to intrinsic stretching vibration of atoms at the tetrahedral site (Mtetra−O), while the lowest bands observed in the range 450−385 cm−1 correspond to stretching vibration of atoms at the octahedral site (Mocta− O). An intense band at 388 cm−1 corresponds to the Ni−O stretching mode of the NiO product. Four absorption peaks are revealed for the Mn2O3 sample. The observed peaks at around 561 and 663 cm−1 exhibit vibrational distortion of the Mn−O octahedral site and Mn−O stretching mode at the tetrahedral site, respectively. The peaks at around 478 and 395 cm−1 are ascribed to the vibration of the Mn3+ species at the octahedral site. An obvious observation of a small red shift in IR spectrum in the mixed oxides compares with single oxides, which reflect the vibrational change due to the formation of different compositions of nickel manganese oxides. These results are in good agreement with the reported literature.9,16 3.4. Surface Morphology, Elemental Composition, Surface Area, and Crystal Structure Analysis. Highresolution scanning electron microscopic (HRSEM) images of nickel manganese oxides, nickel and manganese oxide are
Table 4. Structural Data Obtained by Rietveld Analysis for Spinel NixMn3‑xO4 site
phase content, %
Figure 2. Powder XRD patterns of (a) NiO and (b) Mn2O3.
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Figure 3. XPS survey spectrum of (A) Ni1.5Mn1.5O4, (B) NiMn2O4, (C) MnNi2O4, and (D) NiO.
Figure 4. High resolution Mn2p spectrum of spinel nickel manganese oxide samples. Figure 5. FTIR spectra of (A) NiMn2O4, (B) Ni1.5Mn1.5O4, (C) MnNi2O4, (D) NiO, and (E) Mn2O3.
represented in Figure 6. The morphological features of mixed/ individual oxides are somewhat similar. The particle size in these samples is distributed in the range between 50 and 250 nm (Figure 6). The global maximum in the particle size distribution curve of nickel manganese oxides shows 50% of the particles are 150 nm in size. To further analyze the morphological features, a typical transmission electron micrograph and the diffraction pattern of the as-prepared nickel manganese oxide powders are presented in Figure 7. Hexagonal structures with the size in the range of 100−150 nm are seen in the micrograph of spinel nickel manganese oxides (Figure 7A− C) whereas NiO and Mn2O3 are composed of distorted
hexagonal shape particle and some particles with undefined structures, respectively. BET measurements were done on all of the reported nickel manganese oxide materials and the specific surface area is 7.4, 7.6, and 8.3 m2 g−1 for NiMn2O4, Ni1.5Mn1.5O4, and MnNi2O4, respectively. The crystallite sizes obtained from XRD data ranged from 85 to 150 nm. From these values, the particles seen in microscopic images are made up of 1 (in the case of 150 nm) or 2 (in the case of 85 nm) single/mixed metal crystallite aggregates. EDS elemental mappings of nickel manganese oxides, nickel and manganese 12180
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Figure 6. HRSEM images and particles size distribution curves of (A) NiMn2O4, (B) Ni1.5Mn1.5O4, (C) MnNi2O4, (D) NiO, and (E) Mn2O3.
OES data shown in Table 2 and surface atomic concentration estimated by XPS. This proves the formation of nickel manganese oxides in three different stoichiometries as discussed earlier. Furthermore, most of the diffraction rings that are appearing in the selected-area electron diffraction (SAED)
oxide materials are shown in Figure 7 (second column). The elemental distribution mapping has identified Ni, Mn and O in nickel manganese oxides, Ni and O in NiO, Mn and O in Mn2O3. The atomic ratio of nickel manganese oxides evaluated from the EDS (data not shown) is in agreement with the ICP12181
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Figure 7. TEM images, SAED patterns, and elemental mapping images of (A) NiMn2O4, (B) Ni1.5Mn1.5O4, (C) MnNi2O4, (D) NiO, and (E) Mn2O3.
pattern shown in Figure 7 (third column) is identified as reflections from respective nickel manganese oxides, nickel and manganese oxide phases. The position and intensity of these reflections unambiguously confirmed the crystalline spinel structures of nickel manganese oxides samples. The results are consistent with the standard JCPDS file cards and good agreement with the X-ray diffraction results. 3.5. Electrochemical and Electrocatalytic Studies. Electrochemical behavior of single oxide and mixed metal oxides was studied using the cyclic voltammetry (CV) technique. Figure 8A depicts the CV traces recorded on NiO and Mn2O3 samples in 1 M KOH electrolyte. NiO shows a
redox couple, assigned to the redox process involving the formation of NiOOH and Ni(OH)2 and the voltammogram is featureless in the case of Mn2O3. Figure 8B displays the voltammograms of Ni1.5Mn1.5O4, NiMn2O4, and MnNi2O4 in 1 M KOH electrolyte at a scan rate of 10 mV s−1. A reversible redox couple is present in Ni1.5Mn1.5O4 and NiMn2O4 at approximately the same half-wave potential (E1/2 = 0.32 and 0.28 V) ascribed to the formation of NiOOH and Ni(OH)2(βNi(OH)2/β-NiOOH) in the catalyst surface.17 In the case of MnNi2O4 in addition to the aforementioned redox couple, there is a second anodic peak at 0.4 V vs Ag/AgCl. The appearance of this oxidative process is attributed to quasi12182
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Figure 8. Overlay of cyclic voltammograms recorded in 1 M KOH electrolyte at 10 mV s−1 on (A) NiO (a) and Mn2O3 (b); (B) Ni1.5Mn1.5O4, (a) NiMn2O4 (b), and MnNi2O4 (c).
Figure 9. Overlay of cyclic voltammograms recorded in 1 M KOH electrolyte at 10 mV s−1 on (A) bare glassy carbon electrode in the absence (a) and presence of 0.33 M urea (b); (B) Vulcan carbon (XC-72) modified glassy carbon electrode in the absence (a) and presence of 0.33 M urea (b); 0.33 M aqueous solution of urea on (C) NiO (a) and Mn2O3 (b), (D) Ni1.5Mn1.5O4 (a), NiMn2O4 (b), MnNi2O4 (c), 20% Ni/C (d), and 20% Pt/C (e).
reversible reaction of α-Ni(OH)2/γ-NiOOH reported to be forming at higher positive potentials.18 It is also evident from voltammetric traces of nickel manganese oxide materials in 1 M KOH (Figure 8B) that the double-layer charging currents, indicative of surface area and representative of nonfaradaic process seen at low potentials are almost identical and are certainly in the same range. This observation is in complete agreement with the BET values. A complete oxidation of urea on nickel to CO2 and N2 proceeds via the formation of NiOOH surface as shown below.
6Ni(OH)2 (s) + 6OH− → 6NiOOH(s) + 6H 2O(l) + 6e− 6NiOOH(s) + CO(NH 2)2 (aq) + H 2O(l) → 6Ni(OH)2 (s) + N2(g) + CO2 (g)
The cyclic voltammograms of a bare glassy carbon electrode and Vulcan carbon (XC-72) modified glassy carbon electrode, respectively, in the absence (Figure 9A.a and B.a) and presence 12183
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Figure 10. (A) Nyquist plots in 1 M KOH electrolyte with 0.33 M aqueous solution of urea collected using NiO (a) Ni1.5Mn1.5O4 (b) electrode. Inset: Enlarged plot in the lower frequency region. (B) Current−time transients recorded on Ni1.5Mn1.5O4 coated teflonized carbon cloth polarized at 0.5 V vs Ag/AgCl in presence (red trace) and absence (black trace) of 0.33 M urea.
lower impedance and thus markedly faster kinetics toward urea electro-oxidation than NiO (Figure 10A). These results clearly reveal that Ni1.5Mn1.5O4 facilitates the charge transfer between urea and NiOOH sites better and promotes electrooxidation more than the other nickel manganese oxides studied. The chronoamperometric experiments were performed to evaluate the stability of the Ni1.5Mn1.5O4catalyst. Figure 10B represents the current transients recorded on Ni1.5Mn1.5O4 coated carbon cloth polarized at a potential of 0.5 V (vs Ag/ AgCl) during 1000 consecutive seconds. The plot clearly shows a constant and higher current density in the presence of 0.33 M urea during 1000s indicating that Ni1.5Mn1.5O4 is active and stable during urea electrolysis. Botte et al. have proposed an indirect EC mechanism for electro-oxidation of urea on Ni(OH)2 catalyst in alkaline medium. In brief, Ni(OH)2 is electrochemically oxidized to NiOOH, mediator for the urea oxidation reaction. During the urea oxidation process, the catalytically active NiOOH is chemically reduced to Ni(OH)2 by urea, concurrently, urea is chemically oxidized to its products (nitrogen, carbon dioxide). The inactive Ni(OH)2 is electrochemically oxidized again to NiOOH at high oxidation potential thus regenerating catalytically active NiOOH for further oxidation of urea molecules. The mechanism of urea oxidation occurring on nickel manganese materials is not different from those shown in the set of chemical equations provided above.
(Figure 9A.b and B.b) of 0.33 M urea is shown for comparative purposes. These featureless voltammograms confirmed that these electrodes are inactive toward the electro-oxidation of urea. Figure 9C and D represents the electro-oxidation of urea on NiO (Figure 9C.a), Mn2O3 (Figure 9C.b), Ni1.5Mn1.5O4 (Figure 9D.a), NiMn2O4 (Figure 9D.b), MnNi2O4 (Figure 9D.c), 20% Ni/C(Figure 9D.d), and 20% Pt/C (Figure 9D.e). Except of Mn2O3, the nickel manganese oxide materials with three different metal compositions show electrochemical activity toward the oxidation of urea. The onset potential of urea oxidation on Ni1.5Mn1.5O4 is 0.29 V vs Ag/AgCl, 0.34 V vs Ag/AgCl for both NiMn2O4 and MnNi2O4. The urea oxidation starts at 0.33 V in 20% Ni/C with a peak current density of 5.5 mA cm−2, while no electroactivity is measured in case of 20% Pt/C. Ni1.5Mn1.5O4 showed the highest urea oxidation current density (6.9 mA cm−2), compared to 1.81 and 1.79 mA cm−2, respectively, for NiMn2O4 and MnNi2O4. The enhancement factor of urea electro-oxidation current is approximately 7 in the case of Ni1.5Mn1.5O4 and 2, 1.98 in NiMn2O4 and MnNi2O4, respectively, in comparison with NiO (0.91 mA cm−2). The current density per milligram of Ni1.5Mn1.5O4 is higher than that of other reported bimetallic electrocatalysts Ni− MoO418and Ni−Co bimetallic hydroxide5,19 along with a relatively lower onset potential. Ni1.5Mn1.5O4 has similar morphological features, particle size, and crystal structure as that of NiMn2O4 and MnNi2O4. However, nonstochiometric Ni1.5Mn1.5O4 exhibits relatively higher electroactivity toward urea oxidation reaction possibly due to the presence of three different phases. The plausible explanation for higher activity observed in Ni1.5Mn1.5O4 may also be given based on two other aspects, namely, oxidation state and redox potential of manganese oxide. Assuming the formal oxidation state of nickel to be “+2/+3”, the calculated oxidation state of manganese in NiMn2O4, MnNi2O4, and Ni1.5Mn1.5O4 is “+1.5/+1.75”, “+4/+2”, and “+1.66/+1.16”, respectively. Apparently, the manganese ions are in a more reduced state in Ni1.5Mn1.5O4 when nickel ions are in the ‘+3′ oxidation state. Further, the influence of oxidoreduction reactions of manganese oxide involving MnIV/MnIII might affect the intrinsic electrocatalytic activity of these materials. The conversion of Mn(OH)2 to Mn2O3 or MnOOH in the hydrated state is reported to facilitate the formation of NiOOH in Ni1.5Mn1.5O4.20 In addition, electrochemical impedance spectroscopy analysis shows that Ni1.5Mn1.5O4 has much
4. CONCLUSIONS Nickel manganese oxide based catalysts (NiMn 2 O 4 , Ni1.5Mn1.5O4, and MnNi2O4) with various Ni:Mn atomic ratios were prepared by a simple hydrothermal process. Such catalysts were investigated for their electrocatalysis, and their activities favorably compared to NiO and Mn2O3. There is a significant difference noticed in the redox behavior of spinel nickel manganese oxides due to the existence of different phases confirmed by Rieteveld analysis of XRD data. Taking into account the insignificant difference in the BET values and double-layer charging currents, the difference in catalytic activity is primarily attributed to the different phases present in the samples. Out of the studied nickel manganese oxide compositions, Ni1.5Mn1.5O4 possesses superior electrocatalytic activity, stability, and tolerance toward urea electro-oxidation, indicating promising applications in direct urea fuel cells anodes 12184
DOI: 10.1021/acsami.6b02491 ACS Appl. Mater. Interfaces 2016, 8, 12176−12185
Research Article
ACS Applied Materials & Interfaces
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and for fabricating catalytic anodes for treatment of urea/urine containing wastewater.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02491. Figures S1 and S2 showing Rieteveld plots of nickel manganese oxides using different models (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Tel: +972-3-9371470. Fax: +972-3-9076586. E-mail: salex@ ariel.ac.il. *Tel: +972-3-5318315. Fax: +972-3-7384053. E-mail:
[email protected]. Author Contributions §
S.P. and P.S. are equally contributing first authors.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Sivakumar Periyasamy thanks the Council for Higher Education, State of Israel for the PBC scholarship for outstanding postdoctoral researchers from China and India. Palaniappan Subramanian acknowledges the support of INREP and Ariel University postdoctoral fellowship program for financial assistance. Alex Schechter would like to thank the Israel Science Foundation (ISF) for funding the research work through the Israel National Research Center for Electrochemical Propulsion (INREP) and I-CORE Program (number 2797/11).
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REFERENCES
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DOI: 10.1021/acsami.6b02491 ACS Appl. Mater. Interfaces 2016, 8, 12176−12185
