Combining electrochemical and photocatalytic degradation of organic pollutants for the simultaneous wastewater remediation and hydrogen production

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 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 [2]. In this frame, the electrochemical treatment of wastewater could be considered a valuable solution for green H2 production: it has been demonstrated that organic pollutants in wastewaters containing high level of chemical energy are excellent electrons donors and suitable candidates for producing H2 [3]. In parallel, heterogeneous photocatalysis has emerged as an interesting strategy to treat wastewaters, since it operates under mild conditions, and degrades many pollutants without chemical oxidant addition. Unfortunately, it is recognized as a slow process, assuring high efficiency only when it works in low polluted waters. According to these premises, coupling electroxidation and photocatalysis in cascade could be considered a suitable solution in solving the issues related to the environmental pollution, producing green energy at the same time. Herein, we present our preliminary results aimed at coupling electrolysis and photocatalysis, reducing the environmental pollution and alternatively producing green hydrogen. Simulated drugs-containing wastewaters can 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. These latter can be eventually treated in a subsequent step in which the photocatalysis can be exploited in the presence of proper photocatalytic materials with the final aim to obtain clean water. By the combination of the described technologies, polluted wastewaters, containing recalcitrant species, are efficiently purified and green H2 is simultaneously produced. Concerning the electrolytic step, innovative, cheap and durable noble metal-free electrodes have been tested as well as for the photocatalytic treatment titanium dioxide-free photocatalysts (e.g., graphitic carbon nitride, bismuth oxybromide, strontium titanate) have been investigated. [1] A. Arregi, et al. Energy conv. Manag. 165 (2018) 696. [2] R. Bernasconi, et al. J. Mater. Sci 57 (2022) 9370. [3] Z. Wu, et al. Electrochimica Acta 254 (2017) 140-147.