Nanostructured Materials for Energy Storage and Conversion

Abstract

Efficient, cost effective, and environmentally friendly energy storage and conversion systems are highly desirable to meet ever increasing demands. Nanostructured materials have attracted great interest due to their many superior characteristics in these energy applications. These materials, typically nanoporous or nanostructured, exhibit faster charge transports, better contact, and sometimes new electrochemical reactivity, which leads to their high energy density, high power and/or great catalytic performances. A series of functional nanostructured materials have been fabricated with new synthetic schemes. Nanoporous materials technology and solid state electrochemistry have been attempted to be integrated in this study. New functional nanoporous materials have been sought for electrochemical purposes. By employing a simple dilution strategy, homogeneously sized, ordered mesoporous silica nanorods (SBA-15), spanning about 10 porous channels in width and ranging from 300 to 600 nm in length were prepared. By employing SBA-15 nanorods as a template, ordered mesoporous carbon (OMC) CMK-3 nanorods were prepared. These porous nanorods exhibit enhanced mass transfer kinetics in their applications owing to their short dimensions. To improve the electronic conductivity of OMC and exploit otherwise wasted copolymer surfactant cross-linked in the channels of as-synthesized SBA-15, direct graphitic mesoporous carbon (termed as DGMC) were synthesized from the copolymer surfactant by employing transition metals (Fe, Co, Ni) as a catalyst. DGMC exhibit three orders higher conductivity and better thermal stability than non-graphitic OMC materials. A series of nanostructured composites were fabricated by employing OMC as structure backbones and/or electronic conduits. DGMC/MoO2 as a Li ion battery anode exhibits a reversible capacity more than twice the value that a graphite anode can provide. Due to the confined and nanosized dimensions of the MoO2, the composite exhibits a cycle life with no capacity fading. Polymer modified OMC/sulfur interwoven nanostructures were prepared and applied as a cathode in Li-S batteries. The nanostructure displays all of the benefits of confinement effects at a small length scale. The nanostructure provides not only high electronic conductivity but also great access to Li+ ingress/egress for reactivity with the sulfur. The tortuous pathways within the framework and the surface polymer strongly retard the diffusion of polysulfide anions out from the channels into the electrolyte and minimize the loss of active mass in the cathode, resulting in a stabilized cycle life at reasonable rates. The Li-S batteries can supply up to near 80% of the theoretical capacity of sulfur (1320 mA∙h/g). This represents more than five times the specific capacity of conventional intercalation Li ion batteries. The assembly process for OMC/S is simple and broadly applicable, conceptually providing new opportunities for materials scientists for tailored design that can be extended to many different electrode materials. Size-controlled supported metal and intermetallic nanocrystallites are of substantial interest because of their wide range of electrocatalytic properties. These intermetallics are normally synthesized by high temperature techniques; however, rigorous size control at high temperature is very challenging. A simple and robust chemically controlled process was developed for synthesizing size controlled noble metal, or bimetallic nanocrystallites, embedded within the porous structure of OMC. The method is applicable to a wide range of catalysts, namely bimetallic PtBi but also including Pt, Ru, Rh and Pd. By using surface-modified OMC, nanocrystallites are formed with monodisperse sizes as low as 1.5 nm, that can be tuned up to 2 and 3.5 nm (equivalent to the channel size of OMC) by thermal treatment. The method is also tailored for the deposition of catalysts on conventional fuel-cell carbon supports. OMC-PtBi nanohybrids were investigated as catalysts for formic acid oxidation for the first time. OMC-PtBi catalysts show an absence of CO poisoning. The excellent catalytic properties can be attributed to the successful catalyst preparation and the faithful practice of the “ensemble effect” at the nanoscale level. A new agitation-friction methodology was developed to prepare the nano-OMC/S composite. The method is completely different from any conventional impregnation which requires the voluntary molecular mobility of guest phases. The method relies on frictional forces, and the hydrophobic attraction of the mixing components. This is the first example of a nanoporous solid which can be infiltrated by another solid phase at room temperature. The C/S nanocomposite exhibits not only better Pt ion sorption kinetics than its bulk counterpart, but also a higher pseudo-second-order rate constant than chitosan sorbents.