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Urea-assisted Room Temperature Stabilized Metastable #NiMoO4: Experimental and Theoretical Insights into its Unique Bifunctional Activity towards Oxygen Evolution and Supercapacitor Satyajit Ratha, Aneeya K. Samantara, Krishna Kanta Singha, Abhijeet Sadashiv Gangan, Brahmananda Chakraborty, Bikash Kumar Jena, and Chandra Sekhar Rout ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 1, 2017
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ACS Applied Materials & Interfaces
Urea-assisted Room Temperature Stabilized Metastable βNiMoO4: Experimental and Theoretical Insights into its Unique Bi-functional Activity towards Oxygen Evolution and Supercapacitor Satyajit Rathaa,‡, Aneeya K. Samantarab,c,‡, Krishna Kanta Singhaa, Abhijeet Sadashiv Gangand, Brahmananda Chakrabortyd,Bikash Kumar Jenab,c,*, Chandra Sekhar Routa,** a
School of Basic Sciences, Indian Institute of Technology, Bhubaneswar, Odisha, India-751013
b
CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India-751013
c
Academy of Scientific & Innovative Research, New Delhi, India-110001
d
High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre,
Trombay, Mumbai-400085, India KEYWORDS: nickel molybdate, urea, supercapacitor, oxygen evolution, computational study
ABSTRACT: Room temperature stabilization of metastable β-NiMoO4 is achieved through urea-assisted hydrothermal synthesis technique. Structural and morphological studies provided significant insights for the metastable phase. Furthermore, detailed electrochemical investigations showcased its activity toward energy storage and conversion, yielding intriguing results. Comparison with the stable polymorph, α-NiMoO4 has also been borne out to support the enhanced electrochemical activities of the as-obtained β-NiMoO4. A specific capacitance of ~4188 F g-1 (at a current density of 5 A g-1) has been observed showing its exceptional faradic capacitance. We qualitatively and extensively demonstrate through the analysis of density of states(DOS) obtained from First principles calculations that enhanced DOS near top of the valence band and empty 4d orbital of Mo near Fermi level make β-NiMoO4 better energy storage and conversion material compared to α-NiMoO4. Likewise, from the oxygen evolution reaction experiment, it is found that the state of art current density of 10 mA cm-2 is achieved at overpotential of 300 mV which is much lower than that of IrO2/C. First principles calculations also confirm a lower overpotentialof 350 mV for β-NiMoO4.
1. INTRODUCTION Since the last few decades, concept of energy storage and conversion has grown by leaps and bounds. Several alternatives, such as membrane electrode assemblies (MEA),1 solar cells,2 Li-ion batteries,3 and supercapacitors etc.4 have been ventured successfully and are found to be effective in providing a potential platform for future energy storage/conversion. Supercapacitor devices, being one of these influential alternatives have been acclaimed for their simple operational principle and safety. Though at a nascent stage, supercapacitors hold an advantage over Li-ion batteries in terms of power density. Besides, supercapacitor electrodes do not require any specific designing/modification, facilitating higher degree of portability. Supercapacitors based on pure carbon or
carbon-derivatives are highly efficient, show ultrafast charge-discharge, and can run for ages without wearing out. However, the lack of enough energy density limits their application to electric vehicles and high power electronics only. To escalate the specific capacitance (energy density), a great degree of focus has been shifted upon redox reaction based charge storage.5,6 Much like batteries, redox reaction based charge storage has the capacity to provide enormous amount of specific capacity eventually promoting the energy density to a desired level. In this context, inorganic metal oxides (oxides of transition metals in particular) of various kinds have seized much attention in recent times because of their unparalleled performances and potential.7,8 Due to their abundancy in
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earth’s crust, they can be derived by implementing various synthetic tools and techniques which are both cost effective and environmentally friendly. There are plenty of reports on such materials, however only those having more than one cationic component are preferred due to their high electrical conductivities and robustness arising out of metal-metal correlation.9 So far, many metal oxides having transition metals as core cationic component such as NiCo2O4,10 CoMoO4-NiMoO4,11 Fe3O4,7 NiMoO4,12 ZnCo2O4,13 MnCo2O4,14 ZnV2O4,15 LiZnVO4,16 and ZnFe2O4 etc.,17 have been well established owing to their unique physical and chemical properties. In addition to the enhanced redox (faradic) behavior, the promising catalytic properties of metal oxides intensify their candidature as potential, cost-effective catalysts to overcome the sluggish anodic kinetics in electrochemical oxygen evolution reaction (OER).18 OER is crucial, as it could effectively be used as a rich source of oxygen for hydrogen fuel cells,19 metal-air batteries etc.20 employing a clean energy conversion principle. Therefore, screening of suitable electrocatalyst for OER is in fact a matter of immense importance and challenge in the context of integrated solar water-splitting devices.21 The key is to search for an electrocatalyst having a minimal overpotential and strategically investigate its overall kinetics and long term stability. Besides the ternary metal oxides mentioned in the previous section, compounds like CoMoO4, MnMoO4, and NiMoO4 etc., collectively termed as molybdates are known for their excellent catalytic properties of industrial repute.22,23 They are complex in both nature and structure which keeps them in a distinguished category of mixedmetal oxides. Amongst them, NiMoO4 shows better prospect for both supercapacitor electrode material and OER catalyst due to the dynamicity and superior electrochemical activity shown by its cations.24 Apart from that, NiMoO4 has got a huge number of polymorphs and two of them, i.e. α-NiMoO4 and β-NiMoO4 are noteworthy due to their peculiar lattice arrangement and mechanochemical transition effect.22 The basic structural difference between these two polymorphs is the coordination state of Mo cation. In α-NiMoO4, the molybdenum ions coordinate with the oxygen atoms to form a pseudo octahedral symmetry,25 whereas in the case of β-NiMoO4, the molybdenum ions are coordinated in a distorted tetrahedral manner.25 Though detailed studies have not yet been performed as to ascertain which one of these has superior electrochemical activity, it is however believed that the cationic defects/instability might have some significant impact on the intrinsic electrochemical activity of the β-phase.26–28 Extensive study on synthesis and application of α-phase in various fields has already been reported.29,30 Though the β-phase has also been discussed in several reports, its possible application in energy storage/conversion is rather scarce.29–31 This is because of the arduousness associated with the controlled synthesis and optimization
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of the β variant.32,33 The α-state is stable up to a temperature of ~700 °C, after which it gradually attains the β-phase. When the temperature falls below ~200 °C, retrogression to α-phase occurs, making it difficult to study some vital physicochemical properties of β-phase at standard temperature.32 Moreover, a facile and costeffective method for its controlled synthesis is yet to be explored in order to make it industrially more affordable. As such, a facile hydrothermal synthetic tool to produce room temperature stabilized β-phase NiMoO4 (β-NMO) has been detailed in this report. Electrochemical profiles were studied to check its supercapacitor performance and catalytic activity towards OER. Density Functional Theory (DFT) simulations have been performed to explain enhanced supercapacitor performance and computed overpotential for β-NiMoO4 which readily supports experimentally obtained superioir catalytic activity towards OER. Promising results were obtained which are discussed in the sections to follow. To the best of our knowledge, no reports on the bi-functional activity of βNMO towards energy storage (Supercapacitor) and conversion (OER) have been published yet. 2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All the chemicals were of analytical grade and used as received. Both Ni(NO3)2.6H2O and urea were purchased from HiMedia Laboratories Pvt. Ltd., India. Na2MoO4.2H2O was purchased from Merck Specialties Pvt. Ltd., India. K2IrCl6, sodium hydrogencitrate sesquihydrate (C6H9NaO8), carbon powder (graphitized carbon black), and acetylene black were purchased from Sigma Aldrich. 2.1.1. Synthesis of NiMoO4 (NMO). Water dissolved precursors of both nickel and molybdenum were treated hydrothermally in the presence of urea as a complex agent yielding the final product. In a typical synthesis process, 30 mM of Ni(NO3)2.6H2O and 30 mM of Na2MoO4.2H2O were dissolved in 30 ml of de-ionized (DI) water under constant stirring. Further, 10 ml of aqueous urea solution (at different molarities) was added to the mixture solution to make the final volume 40 ml. After further stirring for about an hour, the final solution was transferred to a Teflon lined stainless steel autoclave of 50 ml capacity and put under hydrothermal treatment at 200 °C for 12 hrs. Four samples were prepared at 50, 100, 200, and 400 mM of urea following the above method. The molarities of all the precursors discussed above were calculated with respect to the volume of the final mixture solution, i.e. 40 ml. Besides the above urea treated samples, another sample was also prepared without taking urea, for comparison. In each of the above reaction conditions, the final product of hydrothermal treatment was a light green colored precipitate and was collected via filtration, dried at 70 °C for 12 hrs prior to annealing at 450 °C for 8 hrs. Then the samples were characterized both structurally and morphologically.
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2.1.2. Synthesis of IrO2/C. The IrO2/C was synthesized following the procedure reported elsewhere with certain modifications.34 In a typical procedure, 50 ml aqueous solution of 6.3×10-4 M sodium hydrogen citrate sesquihydrate (0.167 g) was prepared and to it 0.1 g of K2IrCl6 was added. The as obtained red-brown coloured solution was taken and the pH was adjusted to 7.5 by adding 0.25 M NaOH solution. The solution was then heated at 95 °C in an oil bath under stirring condition. After 30 minutes, the solution was cooled to room temperature and again the pH was adjusted to 7.5 by using the aforesaid procedure. The pH adjustment and heating at 95 °C was repeated until a stabilized pH value of 7.5 is obtained. After that, 0.184 g of carbon powder added to the solution and dispersed properly by ultrasonication. The mixed solution was refluxed at 95 °C for next 2 hour with bubbling of O2 gas (purity: 99.999%). The solution was cooled down to room temperature, filtered and vacuum dried at 70 °C. To remove the organic contaminants, the as dried IrO2/C sample was annealed at 300 °C for 30 minutes. The annealed sample was washed repeatedly with double distilled water, dried at 60 °C and stored in vacuum desiccator for future use. 2.2. Characterization Techniques. Morphological analyses of the samples were carried out with the help of field emission scanning electron microscopy (Merlin Compact with Gemini-I electron column, Zeiss Pvt. Ltd., Germany). Elemental compositions of the samples were checked using elemental mapping and energy dispersive X-ray spectroscopy (INCA, Oxford Instruments, UK). Structural characterisation was done by X-ray diffraction method (Bruker D8 Advanced diffractometer having CuKα radiation, λ = 0.154184 nm). Transmission electron microscopy was performed with the help of TEM JEOL (JEM3010). Raman spectroscopic studies were carried out with the help of a micro Raman spectrometer (Renishaw inVia with an excitation source of 532 nm). All XPS measurements were performed in VG Microtech, England (Multi-Lab, ESCA-3000. Sr.No - 8546/1, Multi-Lab) under ultra-high vacuum condition. Electrochemical characterisation of the samples was done by carrying out cyclic voltammetry (CV) and constant current charge-discharge (CD) measurements with the help of a standard Potentiostat/Galvanostat (PG262A, Technoscience Instruments, Bangalore, India). The electrochemical oxygen evolution reaction was performed by a Bio-Logic instrument (EC-Lab, V: 10.37) connected to a rotor (Pine instruments, USA). 2.3. Brunauer–Emmett–Teller (BET) Analyses. The specific surface area for the samples have been obtained via BET surface area analysis with the help of Quantachrome Instruments Autosorb-iq automatic gas sorption analyzer equipped with oil-free vacuum pumps (ultimate vacuum
