Electrolysis coupled with photocatalytic degradation: a challenging process in cascade for hydrogen production and wastewater remediation

Development of a clean energy system not emitting carbon dioxide is today an urgent task for the creation of a sustainable energy society. Hydrogen (H2) is a promising energy storage medium, whose market is expected to increase in the near future in a 5–10% per year due to its consumption in refineries for treating heavy oil fractions and use as energy vector in the transportation sector [1]. Unfortunately, ca. 96% its production is based on non-renewable sources and 4% comes from water splitting. In this latter, under normal conditions, a considerable electrochemical overpotential is needed to trigger the hydrogen evolution reaction (HER) on the electrode surface. Moreover, highly efficient working electrode requires the use of an electrocatalyst to minimize the energy barrier associated with HER. For these reasons, this method is costly [2]. Among all the possibilities to obtain green H2, NH3 splitting could be an alternative. However, NH3 remains a bulk chemical of great importance in industrial chemistry and, furthermore low amount of H2 can be obtained following this route. In the last decades, it has been demonstrated that organic pollutants in wastewaters containing high level of chemical energy are excellent electron donor and suitable candidates for producing H2 [3]. This promising approach could also help in solving the issues related to the environmental pollution. Herein, we present our preliminary results related to the development of a strategy aimed at coupling electrolysis and photocatalysis approaches for providing alternative green hydrogen and at the same time reducing the environmental pollution (Figure 1). Simulated wastewaters coming from industrial processes or agriculture (containing a mixture of polluting species) will be partially depolluted by electrolysis. In this way, at the cathode H2 is produced, whereas at the anode the organic compounds are partially oxidized, leading to residual wastewaters characterized by lower COD levels. These latter will be eventually treated in a cascade step in which the photocatalysis approach will be exploited in the presence of proper photocatalytic materials with the final aim to obtain clean water. By the combination of the two technologies, green hydrogen is produced under energetically advantageous conditions, as well as, highly polluted wastewaters, containing recalcitrant species, are efficiently purified. Concerning the electrolytic step, innovative, cheap and durable electrodes were tested as well as for the photocatalytic treatment titanium dioxide-free photocatalysts (e.g., zinc oxide, bismuth oxyhalides, Z-scheme heterojunctions, etc.) were evaluated.