An Emergent Integrated Technology: Solar-Assisted Redox Flow Batteries

Abstract

Over the last several years, the use of energy from renewable energy sources is becoming urgent in order to decrease fossil fuel demand and large CO2 emissions. Among all renewable energy sources, the sun is probably the most exploitable and limitless resource. However, the intermittent nature of the sunlight makes imperative the development of efficient energy storage devices to store the electricity generated from solar irradiation. In this context, solar flow batteries (SFBs) have been proposed as an attractive technology for sunlight harvesting and storage into electrochemical fuels. The photoelectrodes integrated in the photoelectrochemical cell (PEC) promote the photocharging of the redox couples in the liquid electrolytes, storing the photogenerated electrons or holes as chemical energy, which can be then discharged in electricity. It is expected that the solar battery is able to be charged under sunlight without needing an external supply. The integrated SFBs show some advantages with respect to the non-integrated devices, such as i) safe performance of the photoelectrode because of the recirculation of the electrolyte and, ii) cost-effective production due to their compact design and iii) allow co-generation, i.e. heat up the electrolytes for sanitary/thermal comfort applications [1][2]. Nevertheless, the development of unbiased and high-efficient SFBs requires finding the optimal voltage match between the photoelectrode and chemical redox pair. In this communication, it is shown recent advances in SFBs by exploring low-cost, environmentally friendly, Earth-abundant, and non-toxic materials for the decentralized solar energy collection, storage, and utilization. In particular, photoelectrodes based on nitrides and oxides have been studied in combination with aqueous and non-aqueous electrolytes. Special attention has been paid to the shape and morphology of the semiconductors and how it affects to the efficiency of the PEC cells. In parallel, the stability and the efficiency of the semiconductors have been also characterized in different electrolytes, aqueous-based and using organic solvents. It is shown how the nature of the electrolyte directly influences the performance of the semiconductors regardless the photovoltage and the photocurrents, which are crucial parameters for improving the performance of the overall SFBs. In addition, the research efforts are not only in the field of materials, but also on the engineering challenges of device architectures. It is proposed to develop a membrane-free SFB in which the charge transfer process takes place at the liquid-liquid interface [3]. The research shown in this work has been carried out in the framework of a European project and an international collaboration with the University of Porto, Portugal.