Exceptionally Active and Stable Spinel Nickel Manganese Oxide

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02491 • Publication Date (Web): 28 Apr 2016 Downloaded from http://pubs.acs.org on April 29, 2016

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ACS Applied Materials & Interfaces

Exceptionally active and stable spinel nickel manganese oxide electrocatalysts for urea oxidation reaction Sivakumar Periyasamy#a, Palaniappan Subramanian#b, Elena Levia, Doron Aurbacha, Aharon Gedanken a*, Alex Schechterb,* a

Department of Chemistry, Bar-Ilan Institute of Nanotechnology and Advanced Materials (BINA), Bar-Ilan University, Ramat-Gan 52900, Israel b Department of Biological Chemistry, Ariel University, Ariel 40700, Israel

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 non-stoichiometric nickel manganese oxideNi1.5Mn1.5O4 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 electro activity of nickel manganese oxide materials towards 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 oxidesand nickel oxide prepared under similar conditions.The Ni1.5Mn1.5O4modified electrodeoxidizes urea at 0.29 V versus Ag/AgClwith 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 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. #Equally contributed first authors.*Corresponding authors: A.Schechter - Tel: +972-3-9371470; fax: +972-3-9076586, E-mail address: [email protected]; A.Gedanken - Tel: +972-3-5318315; fax: +972-3-7384053; E-mail address: [email protected];

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1. Introduction Fuel cells can run on a variety of hydrogen-rich chemicals such as methanol, ethanol, and ethylene glycol either by direct feed or external reforming. In the last few years there is a growing interest in utilising urea as an alternative fuel in fuel cells since it is abundantly available, stable, non-toxic and non-flammable. 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 last 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 non-noble 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 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.


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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 the air without using any template or surfactant for 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 micro-emulsions containing cetyltrimethylammonium bromide (CTAB) as a surfactant, 1-hexanol as co-surfactant and hexane as the lipophilic phase and mixed 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 template-less hydrothermal synthetic route. More importantly, this report unravels the electro-activity of different spinel nickel manganese oxide towards electrochemical oxidation of urea. Herein, we also report the physicochemical properties and electrocatalytic activity of nickel manganese oxide materials towards oxidation of urea. The hydrothermal synthesis process has led to the formation of non-stoichiometric spinel structure Ni1.5Mn1.5O4. This material has excellent electrochemical properties with highest activity towards electro-oxidation of urea in alkaline solutions.



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2. Experimental 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 having a capacity of 125 mL, sealed, and followed by heating 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 process 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 using a programmable electric furnace 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 metallic spinel oxides. The


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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 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 K 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°

Exceptionally Active and Stable Spinel Nickel Manganese Oxide

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