MICRORODS SYNTHESIZED OF MoO3 WITH CORN STRAW AS BIOLOGICAL TEMPLATES AND ITS ELECTROCHEMICAL PERFORMANCE IN AQUEOUS ALUMINUM-ION BATTERY

In this paper, MoO3 microrods were prepared using corn straw as biological template via roasting process The components and crystal characterization of the material were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and the electrochemistry property and mechanism were studied. The results showed that the MoO3 material synthesized by template method is orthorhombic structures. The MoO3 particles were submicron and micron rods with uniform distribution and a smooth surface. MoO3 microrods had an average diameter that ranged from 1 to 2 μm. The result indicated that the MoO3 as the new negative of aluminum battery delivers a higher discharge capacity of 190 mA·h·g at a scanning rate of 1 mV·s, which showing good capacity and cycling performance.


INTRODUCTION
With growing concern about the environment, climate change and a sustainable energy supply, studies have been focused on the development of green energy storage systems with high volumetric energy density, low price and improved safety. In order to solve discontinuous and instable problems of renewable electricity (such as solar arrays and wind farms), energy storage systems can be used. The advanced new batteries are needed to have not only high energy density, but also good safety and affordable electric storage systems that enable better use of the intermittent renewable energy sources [1][2][3]. Because of superior performance in various electrochemical aspects, Li-ion batteries are often viewed as an energy storage device. However, limited resource and high price of lithium salts restrict its application in this area.
Orthorhombic molybdenum trioxide (α-MoO3) has been extensively investigated as a key material for fundamental research and technological applications in photocatalyst, solar cells and electrochemical storage. The most important structural characteristic of α-MoO3 is its structural anisotropy, which can be considered as a layered structure parallel to (010). Each layer is composed of two sub-layers, each of which is formed by corner-sharing octahedral along [001] and [100]; the two sub-layers stack together by sharing the edges of the octahedra along [001].
An alternate stack of these layered sheets along [010] would lead to the formation of α-MoO3, where a van der Waals interaction would be the major binding force between the piled sheets [4,5]. However, there are few literatures about the report of its electrochemical performance in aqueous rechargeable aluminum-ion battery.
Biological templates have attracted considerable attention for the syntheses of inorganic materials in the recent several years [6], because they are generally performed under mild condition, it is energy-conserving, green, and has little requirement for equipment. Moreover, most natural templates and building blocks can be harvested in large amounts at low costs, thus biomorphic assembly is cheap compared with conventional assembly methods to form nanostructures.
Corn is one of major crops cultivated in China. There are about 0.2 billion tons of corn stalk produced annually. As agricultural waste in the world, the corn stalk has been widely used because of its many advantages, including easy preparation and low cost economical [7].
Aqueous aluminum ion batteries have become a hot spot for research because of the good safety performance, simple production and non-toxic electrolyte, high ionic conductivity and environmental friendliness [8][9][10][11]. Molybdenum trioxide (MoO3), as a potential negative electrode material has a lot of advantages, such as high electrochemical activity, low cost, and environmentally friendly nature [12,13]. However, there are few literatures about the report of its electrochemical performance in aqueous rechargeable aluminum-ion battery. In this study, MoO3 was prepared by the solid-phase method and hydrothermal method and its electrochemical behavior in 1.0 mol·L −1 AlCl3 aqueous solution is discussed.

EXPERIMENTAL
Sample preparation. The MoO3 was synthesized via solid-phase method and biological templates method. MoO3(Ⅰ) was prepared by solid state reaction. Ammonium molybdate (H24Mo7N6O24·4H2O) was placed in a tubular furnace with the heating temperature from 20 to 350 °C at 5 °C/min in air and incubated at 350 ℃ for a further two hours.
The MoO3(Ⅱ) was synthesized by biological templates method. The corn stalk was washed thoroughly with water to remove the adhering soil and dust. Then, in order to improve adsorption ability of corn straw, they were cut and immersed in 5% ammonia solution at room temperature for 8 h to eliminate hemicelluloses, lignin and some other components. The corn stalk was washed to the neutral with distilled water and further dried in an oven at 60 ℃ until they were completely dehydrated before are used. Then the dehydrated corn stalk was simply dipped into 0.5 mol/L ammonium molybdate solution for 24 h. The sample was washed with distilled three times and dried in an oven at 60 ℃. After repeating this procedure for several cycles, the specimens were pyrolyzed at 550 ℃ for 4 h in air atmosphere with slow heating rate of 1 ℃/min up to 300 ℃ and a higher rate of 2 ℃/min up to 550 ℃. After naturally cooling to room temperature, a white product, MoO3 was obtained.
Characterization. X-ray diffraction(XRD) was performed on a Bruker D8A25 X-Ray diffractometer, the X-ray beam was nickel-filtered Cu K (λ = 0.15406 nm) radiation operated at 40 kV and 30 mA; and the data were collected from 10 ° to 80 °(2θ) at a scanning rate of 5 °/min.
The morphology of the samples was observed by JEOL JSM-5600LV SEM. EDX analysis was observed by Oxford Instruments.
Electrochemical measurements. Electrochemical characterization of the MoO3 sample was carried out using three-electrode cells, in which a Graphite rod and an Ag-AgCl electrode served as a counter and reference electrodes. The working electrodes were prepared by pressing a 1 cm 2 thin film onto graphite paper. The sample electrode pellet was prepared by pressing the 8:1:1 (in wt.) mixture of active materials, acetylene black and polyvinylidene fluoride (PVDF). The electrolyte was 1.0 mol/L AlCl3 solution purged with nitrogen before use. Cyclic voltammetry (CV) measurements were performed on a Ivium C16430 Electrochemical Analyst multichannel workstation manufactured by the Ivium Technologies with the cutoff voltage of -0.6/0.2 V at room temperature.

RESULTS AND DISCUSSION
XRD measurement was first used to study the phase and lattice modification of the biotemplated MoO3(Ⅱ) and the MoO3(Ⅰ) sample prepared by solid state reaction. The corresponding X-ray power diffraction patterns obtained at room temperature are presented in Figure 1. The diffraction peaks of the XRD pattern for both samples can be readily indexed to be orthorhombic structures with space group Pbnm (62)  For the biotemplated MoO3 (Ⅱ) microrods, the stronger intensities of (020), (110), and (060) peaks than those for the bulk MoO3 (Ⅰ) sample prepared by solid state reaction indicates the anisotropic growth of the nanostructure as well as the preferred orientation of the microrods on the substrate. Importantly, in comparison to the MoO3 (Ⅰ) , there is a shift of the (020) peak toward a lower diffraction angle for the biotemplated MoO3 (Ⅱ) microrods. This is direct evidence of an expanded b-plane interlayer distance, possibly due to the introduction of corn stalk bio-template [14,15]. According to Bragg's equation, the b-plane interlayer distance of the biotemplated MoO3 (Ⅱ) microrods increased from 1.387 nm to 1.425 nm with decreasing 2θ from 12.7619 to 12.4228. The morphology and microstructure of the products were observed by using SEM. Figure 2 shows microstructure of the specimen. The results showed that the sample MoO3 (Ⅰ) was formed as a massive structure with different particles agglomerated together, and most of the particles are approximately 1-4 μm. The sample MoO3 (Ⅱ) showed a long segments with widths of 1-2 m and lengths of 3-10 μm, and a rectangle like cross section was clearly visible. The MoO3 (Ⅱ) can duplicate the biological structure of corn straw profect, having special structure, uniform morphology and good dispersion. The elemental compositions of the MoO3 (Ⅱ) were determined by energy dispersive X-ray spectroscopy (EDX). The result reflects the ratio of molybdenum to oxygen is about 1:3 in Figure 3.   Figure 4, the main CV feature appears as three pairs of symmetric redox peaks at 0.01/-0.14 V, -0.06/-0.19V, and -0.39/-0.53 V. These results of CV suggest that the redox peaks result from the Al 3+ ions [16]. MoO3 can be used as the negative electrode material for aqueous aluminum ion batteries. Cyclic voltammetry tests showed that the MoO3 (Ⅱ) sample had a more obvious oxidation/reduction peak, the peak type was sharper and the peak current was higher. The discharge capacity of MoO3 (Ⅰ) and MoO3 (Ⅱ) samples were 74 mAh·g -1 and 190 mAh·g -1 after 5 cycles, respectively. Compared with MoO3 (Ⅰ), MoO3 (Ⅱ) clearly showed higher discharge capacities and better cyclic ability.

CONCLUSION
MoO3 materials were prepared by using solid-phase method and biological template method exhibited distinct and reversible electrochemical intercalation behavior. Preliminary electrochemical data demonstrated that MoO3 (Ⅱ) showed a higher peak current, a larger discharge capacity and a better recyclability compared with MoO3 (Ⅰ). The discharge capacity of MoO3 (Ⅱ) is 190 mAh·g -1 in the 5th cycle at a current density of 1 mV/s. An optimization of both the material structure and the electrolyte compatibility is necessary to increase the specific capacity and cycle performance.