Chlorine is a chemical commodity widely used; it is estimated to be employed in half of the goods used and consumed on a daily basis. One of the main sector is water treatment, where chlorine and other chlorinated compounds are employed as powerful disinfectant agents to remove waterborne pathogens from reservoirs or effluents. Chlorine is most commonly produced via electrochemical routes (the so-called chlor-alkali process), in a membrane-based reactor. Concentrated sodium chloride brines (20-26% in weight) are utilized as anolytes; chloride ions undergo oxidation to generate molecular chlorine, which is later separated. The common catholyte of choice is sodium hydroxide (30% in weight); the counter reaction – water reduction – allows to concentrate the sodium hydroxide brines and generate hydrogen. The process accounts for three useful products as opposed to water-splitting which produces hydrogen and oxygen, but the latter is often considered a waste. The chlor-alkali process produces 60 million tons of chlorine each year, accounting for 1% of the overall global energy consumption. Currently, the process involves an electrochemical apparatus fed by almost exclusively grid electricity.

Here we present a stand-alone, solar-powered chlor-alkali device potentially able: (i) to be deployed in remote locations, where accessing the grid is unfeasible or unpractical, and where waterborne diseases are generally a great threat; (ii) to decrease the external energy demand of centralized chlor-alkali facilities by using cheap photovoltaic electricity during daytime. Our prototype comprised three key elements: (i) an innovative planar solar concentrator working at high efficiencies (>80%) over a broad angular acceptance (±40°) illuminating (ii) multi-junction, gallium-arsenide solar cells, illuminated by the solar concentrator; (iii) an electrochemical reactor fabricated via additive manufacturing. The electrolyzer is constituted by a nickel-based cathode and a dimensionally-stable-anode (DSA), separated by a cation exchange membrane.

The device showed a continuously stable operation when exposed to natural sunlight illumination, with performances that closely match the predictions based on the nearest weather station. Under the tested experimental conditions, 25.1% sun-to-chlorine efficiencies (SCE) were recorded over 2 hours at mid day. A full summer 12 hour-day was reproduced indoor in terms of illumination direction and intensity for each hour of the day; results show the capability of employing the innovative tracking strategy without strict angular limitations over an entire typical day. Our analysis demonstrated that the technology could be scaled to meet the chlorine demands of a real case scenario, e.g. a small size hospital accommodating forty patients. Preliminary techno-economical evaluations revealed that the levelized cost of solar chlorine could be competitive with the current market.

Despite the relatively small input area of the device tested (5 cm²), we demonstrated that it holds the potential to be scaled up and practically implemented. We believe our technology could trigger the deployment of solar-chlorine generators to communities suffering access to poor quality water reservoirs; moreover, our approach could spur further penetration of renewable energy in industrial processes.