No abstract

The impact of the micropore width, external surface area, and meso-/macropore size on the charging performance of electrochemical double-layer capacitor (EDLC) electrodes is systematically investigated. Nonactivated carbon xerogels are used as model electrodes in aqueous and organic electrolytes. Monolithic porous model carbons with different structural parameters are prepared using a resorcinol-formaldehyde-based sol-gel process and subsequent pyrolysis of the organic precursors. Electrochemical properties are characterized by utilizing them as EDLC half-cells operated in aqueous and organic electrolytes, respectively. Experimental data derived for organic electrolytes reveals that the respective ions cannot enter the micropores within the skeleton of the meso-and macroporous carbons. Therefore the total capacitance is limited by the external surface formed by the interface between the meso-/macropores and the microporous carbon particles forming the xerogel skeleton. In contrast, for aqueous electrolytes the total capacitance solely depends on the total surface area, including interfaces at the micropore scale. For both types of electrolytes the charging rate of the electrodes is systematically enhanced when increasing the diameter of the carbon xerogel particles from 10 to 75 nm and the meso-/macropore size from 10 to 121 nm.

The charge transport properties of thin films of sol-gel processed undoped and Al-doped zinc oxide nanoparticles with variable doping level between 0.8 and 10 at.% were investigated. The x-ray diffraction studies revealed a decrease of the average crystallite sizes in highly doped samples. We provide estimates of the conductivity and the resulting charge carrier densities with respect to the doping level. The increase of charge carrier density due to extrinsic doping was compared to the accumulation of charge carriers in field effect transistor structures. This allowed us to assess the scattering effects due to extrinsic doping on the electron mobility. The latter decreases from 4.6 × 10(-3) to 4.5 × 10(-4) cm(2) V(-1) s(-1) with increasing doping density. In contrast, the accumulation leads to an increasing mobility up to 1.5 × 10(-2) cm(2) V(-1) s(-1). The potential barrier heights related to grain boundaries between the crystallites were derived from temperature dependent mobility measurements. The extrinsic doping initially leads to a grain boundary barrier height lowering, followed by an increase due to doping-induced structural defects. We conclude that the conductivity of sol-gel processed nanocrystalline ZnO:Al is governed by an interplay of the enhanced charge carrier density and the doping-induced charge carrier scattering effects, achieving a maximum at 0.8 at.% in our case.

No abstract

We present results of an approach to incorporate redox-active manganese oxide into the 3-dimensional porous structure of carbon xerogels under self-limiting electroless conditions. By varying the structure of the carbon backbone, we found that deposition of manganese oxide preferably takes place on the external surface area of the carbon xerogels. From our detailed analysis we conclude that 3-dimensional carbon xerogels with particle and pore sizes ranging from 10 to 20 nm and low manganese oxide precursor concentration combined with long deposition times are beneficial for fast operating pseudocapacitance electrodes with high capacitance and effective use of the redox active component.