Nanostructured catalysts production through electrochemical methods: synthesis, characterization and possible applications

In this work a study of the electrochemical synthesis of TiO2 based nanostructured electrodes for photocatalytic and electrocatalytic applications is presented. The study on the electrochemical synthesis of TiO2 nanotubes (NTs) through anodization has been aimed to develop a novel “single-step” anodization method in symmetric electrochemical cells (i.e. both electrodes are Ti sheets), instead of traditional “double-step” processes, which include the sequence of two anodizations separated by the dissolution of the first formed TiO2 and are carried out in asymmetric cells with Pt electrocatalysts as counter electrodes. Besides the anodization, in relation to the specific applications discussed in this study, further electrochemical synthesis routes, employing the anodized TiO2 electrodes, were investigated. In photocatalytic applications, the electrodeposition of Cu2O nanoparticles over the surface of TiO2 NTs has been adopted as strategy to overcome the limited light absorption of the bare TiO2 photocatalyst (active only under UV irradiation). Here, in agreement with most of the studies among the available literature, under visible irradiation the TiO2/Cu2O based electrodes showed improved performances than the bare TiO2 (inactive under only visible light). These improved performances have been frequently claimed based on the only analysis of the photocatalytic performances under visible light, while in the present study it was found that, when the light source employed was UV + visible rather than only visible, the photo-catalytic performances of the composite electrodes were lower as compared to the bare TiO2 electrode. Furthermore, the bare TiO2 activity registered under UV + visible was higher than the composite electrode activity registered under only visible irradiation. To overcome this unexpected behaviour, at this stage the study has been aimed to the development of photo/electrochemical post-treatments, which allowed to improve the performances of the Cu2O based electrodes also under UV + visible light. Black TiO2 electrocatalysts were finally synthesized in the framework of the activities related to the e.THROUGH EU project (H2020-MSCA-RISE-2017-778045), which is aimed to the recovery of critical raw materials and sustainable remediation. Briefly, this goal would be achieved through the development of electrokinetic technologies for soil remediation, as in the case of the electrodialytic mine tailings remediation performed in this work. Electrodialytic remediation is generally carried out in symmetric cells which employ commercial Ti/MMO (Mixed Metal Oxides; Ir/Ru-based) electrodes, for both anode and cathode. Main limit for the real employment of these technologies are the energy costs related to the stirring and the power supply. Here, considering that the main reactions involved during the process are the water splitting reactions, one of the strategies to minimize the overall process cost is based on the recovery of the produced hydrogen. Considering that Ir-/Ru- based compounds are benchmark catalysts for the anodic reaction involved rather than the cathodic hydrogen evolution reaction, at this stage, the study has been aimed to the development of a non-commercial hydrogen evolution reaction (HER) electrocatalyst, namely the Black TiO2 based electrodes. These electrodes have been synthesized through the electrochemical reduction of the Ti/TiO2 electrodes resulting from the anodization, employing Ti sheets as counter electrodes.