Porous silicon ͑PS͒ with a micro-nano-hybrid structure has been successfully fabricated with an electrochemical etching process. The micropores consist of one-dimensional tunnels, which vary from ca. 1 to 1.5 m in pore diameter and extend up to 15 m in depth. The walls of these micropores are covered with a nanoporous structure that consists of small spherical particles, the feature size of which is of the order of tens of nanometers or smaller. The as-prepared PS structures show cathodic/anodic peaks for lithiation/delithiation during cyclic voltammetry with minimal destruction of either the micropore or nanopore of its wall even after 50 cycles. Furthermore, the peak current, the cumulative charge increase, and the electrochemical impedance for electrode reactions consistently decreases with the surface area of the tunnel wall, indicating that processes at the tunnel wall govern the overall lithiation/delithiation reactions. A hybrid porous structure consisting of microtunnels with nanostructured surface layers appears to provide a viable and practical way to utilize silicon for anode materials in rechargeable microbatteries.One of the largest obstacles to the breakthrough of high-capacity lithium-ion batteries is the limited reversible capacity of the anode material, i.e., graphite, which is used commercially in most of the rechargeable lithium batteries. Much effort has recently been made to find a reasonable substitute to meet the rapid advancement of telecommunication devices toward digital multifunction systems. [1][2][3] Silicon has been widely studied as a promising candidate for the next-generation anode material, due to its high theoretical specific capacity ͑4200 mAh g −1 ͒ and its applicability for an on-chip microbattery. 4,5 A variety of structures and Si-based composites have been examined in order to reduce the lithiation-induced stress and suppress the structural destruction of silicon, which is believed to be the main cause for the loss of sustainability and the lack of capacity retention during charge/discharge cycling. 6-12 Nevertheless, finding an optimal structure/composition of silicon and/or siliconbased materials is still a great challenge to most of the researchers in this field.In a previous communication, 13 we have reported preliminary results on the use of porous silicon ͑PS͒ with a one-dimensional microtunnel structure as the anode material for rechargeable microbatteries. The results showed that lithiation/delithiation reactions readily take place in PS with minimal structural change. Nevertheless, PS has many scientific and practical issues still unexplored for battery applications. These include the effect of the area/structure of the tunnel wall on electrochemical activity, difference in the activity between the surface and the tunnel wall in the PS structure, and sustainability of the charge/discharge reactions. Here we report recent results on PS with a hybrid micro-nano-porous structure, prepared by a select electrochemical etching process. In particular, the structural chan...
The electrochemical characteristics of Ta 2 O 5-IrO 2 electrodes prepared from different chemical compositions and coating methods were observed by using cyclic voltammetry, potentiostatic polarization, galvanostatic polarization and scanning electron microscopy. The efficiency for chloride oxidation and oxygen evolution processes was not only influenced by the chemical composition but also by the surface morphology of the oxide electrode which was susceptible to the ratio of the two components and the coating method. Ta 2 O 5 (50)-IrO 2 (50) electrodes revealed the highest catalytic activity for the chloride ion oxidation and oxygen evolution reaction because they had the largest effective surface area. The durability of the oxide electrodes in the accelerated life tests was improved as the thickness of the oxide layer increased and the ratio of [IrO 2 ] to [Ta 2 O 5 ] approached 80/20.
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