Produção de compostos carbonílicos e hidrogênio em um eletrolisador de glicerol com eletrodos de grafite

Abstract

Glycerol is a co-product of biodiesel production that has attracted interest due to its potential use, being widely available and of low value. One strategy for adding value to this alcohol is to use it as a matrix for electrolysis. The use of metal-free catalysts requires energy to complete the coupled reaction - glycerol electro-oxidation (GEOR) at the anode and the H₂ evolution reaction (HER) at the cathode. This energy can be supplied by alternative energies, such as solar panels. In this work, the use of graphite electrodes free of noble metals was explored for the electrolysis of glycerol, with concomitant production of H₂. The anodic and cathodic reactions of graphite were investigated, showing that it is possible to couple GEOR to HER at an open circuit potential of 1.7 V. We then prototyped an H-type electrolyzer with septa for sampling liquids and gases. The operation of this electrolyzer shows that the current density, carbonyl products and H₂ depend on the concentration of glycerol used. The electrolyzer produces formate, lactate, glycerate and glycolate at the anode, with coupled production of H₂ at the cathode. The glycerol electro-oxidation and oxygen evolution reactions (OER) take place in parallel, competing for the anode surface. But although the kinetics of OER is faster than that of GEOR on graphite, GEOR has a lower onset potential. In addition, the electro-oxidation of alcohol has a lower current at high concentrations. High concentrations of glycerol (1.0-1.5 mol/L) lead to less oxidized compounds, such as glycolate, while low concentrations (0.05-0.5 mol/L) induce more oxidized compounds, such as formate. In addition, increasing the cell voltage leads to a mixture rich in format, circulating more electrons in the external circuit and producing more H₂. It is possible to adjust the composition of the product mixture at the anode and the amount of H₂ at the cathode by changing the power supplied. This work shows that low-cost, simple and widely available bulk graphite can be exploited as a base or model electrode for glycerol electrolysis with parallel H₂ production. Ultimately, the work proves that graphite-based current collectors used in commercial electrolysers can take part in the reactions. This work proves that graphite is not a completely inert material.