High Density Active-Site MnO 2 Nanofibers for Energy Storage and Conversion Applications
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ABSTRACT This investigation involves the synthesis of MnO 2 nanofibrous materials, via an aqueous chemical synthesis route. A critical step in the synthesis is the extended period of the refluxing of the reactive constituents, which enables the gradual transformation of the initial amorphous nanoparticles into a random-weave nanofibrous structure, in the form of a bird's nest superstructure. The bird's nest has a diameter of approximately 10 L.tm, which is an assemblage of many individual nanofibers with a diameter of about 15 rm, and a length up to I gim. Partial transformation of the nanostructured MnO 2 realizes a novel bimodal morphology, which combines a high density of chemically active sites with an enhanced percolation rate. Characterization of these nanofibers include SEM, TEM, surface area, and chemical analysis. High resolution TEM observations reveals that the as-synthesized MnO 2 nanofibers contain lattice defects, including molecular pores, meso- and micro-pores. INTRODUCTION In recent years, much interest has been focused on developing synthetic manganese oxide (MnO 2) [1-7]. This material is composed of MnO 6 octahedrons linked at their vertices and edges to form single or double chains [8-101. These chains share comers with other chains, leading to structures with channels of empty sites. The size of these channels is related to the number of Mn-O chains on each side [11-12]. Naturally occurring MnO 2 coexists with many impurity oxide phases, including SIO 2, Fe 20 3, A120 3, and P20 5 [13-18]. These impurities limit possible applications and complicates the chemical and structural analyses of these materials. Approaches for the preparation of pure hollandite phases include ion-exchange reactions [19], hydrothermal synthesis [5,20], electrolytic synthesis [7], and chemical synthesis methods [5, 21-22]. Chemical methods for the preparation of phase pure MnO 2, in a variety of crystalline forms, were developed by McKenzie in the early 1970s [21 ]. Since then, several studies have been performed on the synthesis of layered MnO 2 by reacting manganese salts, such as MnC12 or MnSO 4 , with strong oxidizers, such as KMnO 4 or ozone/oxygen mixtures [6, 23-26]. Most of this work has addressed synthetic methods and possible applications. Only a limited amount of research has been concerned with morphological aspects and lattice defect structures. Here, we report a unique high percolation rate nanofibrous MnO 2 material for a wide range of applications, including rechargeable batteries and fuel cells. This nanostructured MnO 2 is synthesized via a patent pending wet synthesis process route that is scalable to large volume manufacturing and anticipated to be low in cost. The synthesis combines a refluxing technique with highly intense ultrasonic radiation. The morphology, microstructure, possible reaction mechanism, and possible application of these novel nanofibrous MnO 2 are also discussed. 409 Mat. Res. Soc. Symp. Proc. Vol. 496 01998 Materials Research Society
EXPERIMENTAL Nanostructured MnO 2 is synthe
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