Photoelectrochemical Water Treatment Coupled with Generation of In Situ Oxidant and Hydrogen

Abstract

Photoelectrochemistry is the field of study of physical chemistry involving the interaction of light with electrochemical system. Photocatalysts have energy band gaps, and generate an electron-hole pairs when the energy of the photon has higher than the band gap energy of the semiconductor. The electron-hole pairs are used to redox reaction on the photocatalyst. This characteristic of photocatalyst has been successfully used to convert solar energy to chemical or electrical energy. However, the photocatalytic system suffer from not only fast recombination of electron-hole pairs but also needing additional separation step after reaction owing to powder type of catalyst. To deal with this problem, this study demonstrated the photoelectochemical (PEC) system in which an external potential bias with light irradiation effectively separates electron-hole pairs into an anode and cathode. In addition, the PEC system provides an ideal method for achieving various photochemical conversions including the organic pollutants degradation, via the generation of reactive species and the reduction of proton or water to molecular hydrogen (H2) or hydrogen peroxide (H2O2). In the first study, electrochromic TiO2 nanotube arrays were prepared by electrochemical self-doping with dark blue coloration (denoted as Blue-TNTs) for the dual-funtional photoelectrochemical water treatment coupled with H2 generation. The surface charge of TiO2 nanotube arrays (TNTs) was partial reduced Ti4+ to Ti3+ by cathodic polarization, which revealed a marked enhancement in donor density and electrical conductivity of TNTs. The degradation rate of organic compound (i.e. 4-chlorophenol) was greatly enhanced on Blue-TNTs particularly in PEC condition rather than PC and EC conditions, where the H2 generation rate was more than doubled from that of bare TNTs. The superior PEC properties of Blue-TNTs was excellent for bromine activation, as well. Bromide ion (Br-) can be oxidized to reactive bromine species (RBS) which are known as powerful oxidant. The second study demonstrated a PEC system with Blue-TNTs as a photoanode for in-situ RBS generation by the activation of Br-, which achieved directly removal of ammonium ion (NH4+) to nitrogen gas (N2). In addition, this system suppressed bromate (BrO3-) formation, a byproduct of bromination, and produced H2 at the cathode for energy saving. However, TiO2 has a drawback in that it can absorb only UV light due to the wide band gap (~3.1 eV). To overcome this drawback, the catalyst which can absorb visible light was used to enhance the visible light activity. Tungsten trioxide (WO3) has been used as a popular photoanode material for visible light conversion processes. The visible light absorbing WO3 with a band gap of approximately ~2.8 eV and valence band (VB) edge located at ~3.0 V (vs NHE) was proper to generate reactive chlorine species (RCS). Not only RBS but also RCS are also widely used for wastewater treatment owing to strong oxidation power and relatively long lifetime. The third study investigated a PEC system of RCS generation from chloride ions using a WO3 thin film electrode in visible light. In this study, the degradation of organic compounds coupled with H2 generation was compared among EC, PC, and PEC systems. The degradation efficiency was remarkably enhanced by in-situ RCS generated in PEC system, whereas the activities were negligible in that of EC and PC conditions. Because the activities of the WO3 thin film were limited by fast electron-hole pair recombination in the PC condition, and the potential bias of +0.5 V did not induce any significant reactions in the EC condition. In addition, the visible light-irradiated PEC system generated no chlorate, unlike UV-irradiated system. However, the stability of WO3 electrode was limited only in acidic condition. Therefore, the last study investigated a PEC system with combining of WO3 thin film and crystalline structure of tantalum oxynitride (TaON) layer as a passivation layer where WO3 can be stable even in neutral pH. Moreover, the developing a semiconductor heterojunction with suitable band edge positioning can geneate a potential gradient at the junction interface, which retard electron-hole pairs recombination through interfacial charge transfer. In this study, the in-situ generated RCS reacted with cyanide (CN-) and were converted to final products CO2 and N2 via intermediates cyanate (OCN-) and NH4+. In addition, H2O2 generated from the cathode reacted with CN- to further increase the removal efficiency of CN-. The use of solar energy for water treatment with simultaneous energy recovery (e.g., H2 or H2O2 generation) is an attractive, environment-friendly and economically viable technology.