THERMOCHEMICAL ACTIVATION OF THE SURFACE OF CARBON MATERIALS

The use of a gas-diffusion carbon-graphite electrode for depolarizing the anode process in the sulfuric acid cycle of hydrogen production is promising. The sulfuric acid cycle is currently the most promising solution for addressing issues in atomic-hydrogen energy. The integration of large-scale electrochemical production and carbon-free electricity generation into a single system can solve the problem of "valley" and "peak" loads. The use of porous graphite-based anodes enables the process to be conducted in a gas- diffusion mode. A significant challenge is the high overpotential of the electrode process, which can be reduced by activating the surface of carbon materials. Porous graphite of the PG-50 grade was used as the base for the gas-diffusion electrode. To enhance catalytic activity and increase the specific surface area of the electrode, active carbon (AC) was deposited on the surface and in the pores of the graphite electrodes. Experimental results show that two impregnations of graphite with a polysaccharide solution followed by carbonization produce porous electrodes with an active carbon content of 82…85 % of the initial electrode weight. To establish the role of catalytically active carbon forms in the oxidation of SO2, cyclic voltammograms were obtained on glassy carbon (GC 12) in 1 mol·dm⁻ ³ sulfuric acid, both with and without the addition of sulfur(IV) oxide (0.24 mol·dm⁻ ³). GC 12 was chosen as a carbonaceous material with a low degree of surface development, whose actual surface area is close to its geometrically measured area. The experimental data indicate that sulfur(IV) oxide oxidation occurs on the glassy carbon surface. This oxidation process occurs at potentials where weakly bound oxygen forms on the graphite surface. Therefore, it can be assumed that the oxidation of sulfur(IV) oxide on graphite proceeds through weakly bound oxygen on the electrode surface. The activation of the carbon electrode surface to intensify SO2 oxidation was achieved by depositing catalytic additives in the form of active carbon onto the porous graphite surface. Multistage impregnation of the carbon-graphite base with a polysaccharide solution, followed by thermal decomposition and activation in nitric acid solution, regulated the amount of deposited AC. The amount of AC deposited per activation on a PG-50 sample was 9…12 mg·cm⁻ ². The AC content was increased by increasing the number of impregnation cycles with concentrated nitric acid, followed by thermal decomposition after each cycle. At an AC content of 33…39 mg·cm⁻ ², the majority of the electrode surface becomes available for free SO2 oxidation. Further increases in AC content lead to deteriorating electrode performance. With AC activation of the graphite base, the anodic current density reaches 3200…3300 A·m⁻ ², which is sufficient to recommend activated carbon-graphite materials for industrial application in the sulfuric acid cycle of hydrogen production.