Carbon Ceramic Electrodes Modified with Alpha-Nickel Hydroxide Applied to the Electro-Oxidation of Methanol in Alkaline Medium

Abstract

Direct methanol fuel cells (DMFCs) are attractive power sources for portable electronic devices owing to their advantages of low operating temperature, easy transportation and fuel storage, high-energy efficiency, low exhaustion and fast start-up. The alkalines DMFCs have drawn particular attention in this area for the advantage of high alcohol oxidation kinetics and of the potential use of cheaper non-Pt catalysts. Nickel hydroxide has been extensively studies as electro-catalysts for methanol oxidation. In this present work alpha nickel hydroxide was synthesized and incorporated into carbon ceramic electrodes. The nickel hydroxide was obtained by mixing nickel acetate and glycerin with heating at 50 ° C under constant agitation. The obtained solution was mixed with a solution of NaOH dissolved in butanol, the mixture obtained was stirred for 6 hours and then allowed to stand for 15 days. Carbon ceramic (CC) was obtained by the Sol-Gel process, methyltrimethoxysilane (MTMS) was mixed with ethanol and the catalyst used was NaOH after partial hydrolysis of MTMS graphite was added to the mixture, the material obtained was allowed to stand for 48 h. The ceramic-carbon (CC) was mixed with nickel hydroxide in the proportion of 5, 10, 30, 50 and 70% (w/w) Ni(OH)2/CC. The mixture was homogenized and packaged in glass tubes of 0.03 cm2 and 1 cm in height, and nickel-chromium wire was inserted as electrical contact. The material obtained was designated as ECC/α-Ni(OH)2 and was used as an electrode for electro-oxidation of methanol in alkaline medium. The x-ray diffraction confirmed the presence of lamellar graphite. The images of SEM/EDS confirmed the presence of nickel hydroxide dispersed irregularly in the sample. Initial studies indicated that the electrodes with the higher concentrations of nickel hydroxide produce higher current intensities at higher anode potential. In studies of the electro - oxidation of methanol was chosen electrode produced with 10 % of Ni(OH)2, because it has the lowest potential anodic peak. The ECC has no catalytic activity for the electro-oxidation of methanol, but the ECC/α-Ni(OH)2 promotes the electro-oxidation of methanol potential of 0.53 V vs. Ag/AgCl, yielding a current density of about 100 mA/cm2. The highest oxidation current was obtained with 1.90 mol/L of methanol. The ECC has no signs of poisoning, because there is no decrease in the intensity of the anodic current after 1000 cycles. The electrode showed promising thermal behavior, with no loss of electrochemical activity until the temperature of 200oC. All results indicate that the ECC/α-Ni(OH)2 is a promising material for electro-oxidation of methanol in alkaline medium, to obtain high current densities.