The thermal treatment at 373 K results in a rapid increase in surface area which is characteristic for most gel systems obtained by a sol–gel method [26]. For high-temperature treatment we observe a gradual decrease in surface area, up to about 47–50 m2/g (after treatment at 773 K). It should be noted that the specific surface area values are significantly higher than for the other porous tin (IV) oxides [27]. We observe the correlation of electrochemical activity and changes in surface area (Figure 5). The decrease in activity is preceded by a decrease in specific surface area and it is associated with the sintering process of small metal particles [28] and with the sintering of nanoparticles - SnO2