Solar-Electrochemical Devices for Water Splitting and Chlorine Generation

Solar technologies hold the potential to ultimately counteract the effects of climate change. Nevertheless, the intermittency of sunlight poses technical challenges regarding the storage of the surplus production and the supply of the deficit. The generation of chemical fuels is of great interest to store energy for prolonged periods. Hydrogen has been a key candidate to address the issue. The possibility to generate hydrogen exploiting sunlight has been widely investigated in recent years. Fully decoupled devices seem the most promising pathway. The exploitation of earth-abundant elements is indispensable for the future of solar hydrogen, as they are cost-effective and they are inclined to meet the requirements of generation at a global scale. We have designed and tested a solar hydrogen generator solely employing earth-abundant materials; its efficiency is the highest reported to date for such class of devices. The solar-to-hydrogen efficiency we have recorded is > 14%, improving by more than 40% the prior state of the art. We have employed tailor-made silicon hetero-junction solar cells, coupled to an alkaline electrolyzer. The main hurdle that prevents the large deployment of solar hydrogen technologies is their cost. Those financial barriers can be overcome by coupling hydrogen evolution with a more cost-viable oxidation reaction. Generation of halides is a promising pathway to substitute oxygen evolution. The chlor-alkali process is a major electrochemical operation in the industry, and generates hydrogen, chlorine and sodium hydroxide. In my work, I have designed and tested a novel solar chlor-alkali reactor, which integrates a novel solar concentrator developed by a local startup and multi-junction solar cells. I have recorded solar-to-chemical conversion efficiencies > 25% under real outdoor illumination, doubling prior values reported in literature. Chlorine and chlorinated products can be deployed for disinfection of drinking water. Liquid solutions of sodium hypochlorite can be preferred over chlorine gas for such applications. I have demonstrated and compared the performances of solar-powered devices for hypochlorite generation, employing multi-junction technologies or silicon hetero-junction solar cells. I have characterized the efficiencies under real outdoor conditions, revealing solar-to-chemical conversion efficiencies of 10 – 14% for silicon-based cells, and confirming > 25% values for multi-junction cells. I have then developed a computational model that predicts efficiencies, throughputs and production cost in three locations of choice. The results suggest that solar-hypochlorite can outcompete traditional hypochlorite. Deployment of solar hypochlorite generators in developing countries can address the lack of access to drinking water those communities suffer. I have evaluated the performances of a simpler and more robust batch-type hypochlorite generator. The results highlight that the throughput can be enhanced by structuring the electrode plates. I have assessed the economics of hypochlorite production via solar-powered batch reactors; the cost is higher, but is still competitive with respect to the supply of bulk hypochlorite. My work holds the potential to inspire and spur further innovation in the field of solar-electrochemical operation, in which the platforms I have demonstrated and assessed can represent a starting point towards a new generation of devices.