Electrochemically assisted self assembly of mesoporous silica on conducting substrates as templates for electroactive metal oxides

Abstract

Three dimensional, conductive, macroporous nickel foams and mesoporous carbon aerogels have been utilized as skeletal supports for an electrochemically assisted deposition of mesoporous silica particles and films. Substrate-supported mesoporous silica particles were synthesized directly onto the conducting supports using a combined sol-gel/electrochemical method, termed Electrochemically Assisted Self Assembly (EASA). The EASA method resulted in mesoporous silica/nickel foam composites exhibiting high specific surface areas (20-120 m^2 g^-1) and narrow silica mesopore size distributions (2.6-3.8 nm), as determined by nitrogen physisorption characterization. Increasing the EASA deposition time resulted in the increase of mesoporous silica particle diameters (from less than one micron for deposition times of 80 seconds to greater than one micron for deposition times of 3600 seconds), as well as an increase in the number of particles deposited and substrate surface coverage, as determined by scanning electron microscopy (SEM) and cyclic voltammetry, respectively. Transmission electron microscopy (TEM) analysis revealed that silica particles deposited by EASA possess a worm-like disordered morphology, due to poor surfactant ordering during the EASA process. Mesoporous silica particles on carbon aerogels resulted in an increase in electrochemical capacitance, from 7 F g^-1 for a bare carbon aerogel substrate to 20 F ^-1 for a mesoporous silica/carbon aerogel composite. Mesoporous silica/nickel foam composites were utilized as substrate-supported templates in a metal oxide nanocasting procedure. Aqueous and ethanolic cobalt nitrate infiltration and thermal decomposition produced Co3O4/mesoporous silica/nickel foam composites. Removal of the mesoporous silica particle template resulted in hierarchically porous Co_3 O_4 /nickel foam composite electrodes, as determined by X-ray diffractometry (XRD) and nitrogen physisorption. Co_3 O_4 /nickel foam composite electrodes fabricated from aqueous cobalt nitrate nanocasting exhibited relatively high surface areas (37-44 m^2 per gram of electrode), small micropore volumes, and broad mesopore size distributions. Co_3 O_4 /nickel foam composite electrodes fabricated from ethanolic cobalt nitrate nanocasting resulted in Co_3 O_4 /nickel foam electrodes with lower surface areas of 12-29 m^2 per gram of electrode. The electrochemical pseudocapacitance of Co_3 O_4 /nickel foam composite electrodes were investigated by galvanostatic constant current chronopotentiometry, with specific capacitances of 298-845 F per gram of deposited Co_3 O_4 at low current densities.