Clean Production of Chlorine (Cl2) and Hypochlorous Acid (HOCl) from Photocatalytic and Photoelectrochemical Seawater Splitting

Abstract

Seawater splitting has emerged as a promising alternative to overall water splitting because it eliminates the kinetically sluggish oxygen evolution reaction (OER) that is the bottleneck in water splitting, and the low economic value of O2. Moreover, in seawater splitting H2 evolution coupled with oxidation of chloride (Cl-) to value-added chlorine (Cl2) and/or hypochlorous acid (HOCl) can simultaneously benefit energy and environmental sectors. Cl2 and HOCl are widely used in bleaching, disinfection, sanitisation and sterilisation in a medical sector, safe drinking water and swimming pools owing to its strong oxidizing and antibacterial properties. The mainstream industrial production employs the chlor-alkali electrolysis of sodium chloride (NaCl) which requires significant energy input and releases enormous CO2. To achieve sustainable production of Cl2 and HOCl that reduces energy consumption and environmental impacts, photocatalytic (PC) and photoelectrochemical (PEC) technology have been attempted as a green alternative. Importantly, PC and PEC enable on site production of Cl2/HOCl in remote areas which can circumvent the instability (decomposition), storage and transport issues. This article reviews the recent progress of PC and PEC production of Cl2/HOCl, catalytic materials, photocatalyst designs and their photocatalytic performance. The applications of in situ HOCl production in anti-bacterial treatment, ammonia removal, selective oxidation and conversion of organic compounds, and CO2 conversion are discussed. We also address the challenges in this area and highlight the prospects for future directions. All in all, we demonstrate that PC and PEC production of Cl2/HOCl serve as green and sustainable alternatives to the chlor-alkali process. This research area is still at infancy, and we hope this review article will garner the attention of researchers to contribute to this area leading to a step closer toward practical applications.