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