Synthesis of Clustered Mn3O4 Nanoparticles through a Polymer Surfactant Mediated Route on Few-Layer Exfoliated Graphene Platelet Surface and Its Application for Electrochemical Energy Storage: Lithium-Ion Battery Anode

Abstract

Composites synthesized through the deposition of Mn3O4 on graphene, carbon nanotube and other carbon based materials have attracted much attention recently as potential electrode materials for different electrochemical applications such as pseudocapacitor; Lithium-ion battery; and catalysis. The primary reason Mn3O4 is grown on these substrates in spite of having high charge storage capacity as pseudocapacitor or Lithium-ion battery electrodes on its own is to enhance its electrical conductivity and/or to impart flexibility to the electrode, which is difficult for a fully metallic electrode. Higher electrical conductivity prolongs the cycle life of an electrode. In addition, the substrate contributes capacity and thus, enhances the overall energy density of an electrode. Mn3O4 acts as a spacer and keeps graphene nanosheets separated when used as the substrate for capacitor electrode fabrication. This helps retain the high surface area of graphene nanosheets in the electrode which contributes additional capacitance. Mn3O4 supported on graphene and other carbon substrates have recently been investigated as catalyst for methanol electro-oxidation in alkaline media; CO oxidation; and Oxygen reduction reaction (ORR). High surface area substrate uniformly distributes metal particles and prevents their agglomeration and dissolution during catalytic process. In addition, high electrical conductivity of graphene and carbon substrates enhances the electronic conductivity of Mn3O4 which is of importance for superior catalytic activity. Composite of Mn3O4 combined with carbon based substrate has also found non-electrochemical application such as the removal of Pb and Cu ions from aqueous solution because of their adsorptive behaviour. A myriad of procedures have been adopted for the synthesis of Mn3O4 on graphene or other carbonaceous substrates. All of these methods involve one or more of the following factors that complicate the process, such as: long synthesis time; high synthesis temperature; use of hazardous/toxic chemicals; multistep process and the requirement for sophisticated device or highly controlled environment. In fact, the complicacies associated with the synthesis of Mn3O4 have already been acknowledged and investigations have been directed at finding relatively simpler route such as the use of microwave technique. In this research, we report the synthesis of clusters of nearly octahedral shaped Mn3O4 nanoparticles on few-layer exfoliated graphene platelet (15µm diameter, thickness 6-8 nanometer, 120-150 m2/g surface area) surface through a simple, wet-chemical, polyethyleneimine (PEI) mediated route. Few-layer exfoliated graphene platelets are ultrathin particles of graphite that can also be thought of as short stacks of graphene sheets prepared through proprietary manufacturing processes. Several grades and sizes with thicknesses ranging from 1-20 nanometers and width ranging from 1-50µm are manufactured by XG Sciences, Lansing, MI, USA. Unique manufacturing processes for these platelet materials are non-oxidizing; thus, yielding a pristine graphitic surface of sp2 carbon molecules. This makes these platelet particles especially suitable for applications requiring high electrical and thermal conductivities. The surface areas of the resulting platelets predominantly consist of the open surface areas of the graphene basal planes. The components involved in this synthesis method are manganese salts (KMnO4 and MnSO4.H2O); water; PEI; and few-layer exfoliated graphene platelet as the substrate. The synthesis is carried out at a temperature of 80°C only and in open air. Highly crystallized Mn3O4 particles, as observed by X-Ray Diffraction (XRD), can be synthesized on few-layer exfoliated graphene platelet surface. It has also been observed that PEI acts as a reducing agent and as a capping agent on a continuous network of ribbon-like Birnessite-MnO2 (IV) to produce a nearly octahedral shaped nanoparticles of Mn3O4 (II, III). It has already been mentioned that composites of Mn3O4 on graphene or other carbonaceous substrates find a myriad of applications. Thus, our research findings to synthesize few-layer exfoliated graphene platelet-Mn3O4 composite through a simple method should be of interest to a broad group of researchers. In this research, we have investigated the performance of this composite system as a Lithium-ion battery anode only. Our preliminary investigations reveal that the Mn3O4 composite synthesized through this method has just as much potential as the ones prepared through other alternative methods.