Perspectives of electrochemical and photocatalytic technologies for the water-energy nexus potential of water splitting of brines

• Water splitting as promising technology to brine valorization by green H 2 production. • Energy recovery and freshwater generation as water-energy nexus of water splitting. • Photo- and electrocatalysis show feasible opportunities for sustainable water splitting. • Reactor design and brine composition impact photo- and electrocatalysis of brines. The economic, environmental, technological and social development of society are linked with two crucial resources: energy and water. The increasing energy costs and the scarcity of fresh water have caused concern across the globe due to limited access to these resources. Consequently, academia and industry are combining efforts to enhance technological processes, optimize resources, and valorize waste management by improving the water-energy nexus. In this context, brine waters from ocean, brackish groundwater and industrial desalination have been identified as potential waste from which value-added products can be sourced. In this perspective paper, firstly, an overview of the main current treatment methods for brines and their chemical composition is presented. Most processes solely focus on the recovery of water, being over 70 %, with energy consumption from 2 to 100 kWh/m 3 . The high variability is based upon disposal costs of concentrated brines – with the highest associated with zero liquid discharge (ZLD) plus salt disposal. The salinity, concentration of ions, and chemical oxygen demand (COD) of brines differ depending on their respective sources. Second, the water-energy potential nexus of the water splitting of brines was contextualized. The perspective proposed herein is based on the integration of the production of H 2 through water splitting using renewable energy and the subsequent H 2 oxidation in a fuel cell to produce energy (recirculated within the process) and water (for drinking or industrial use). Finally, the prospects of electrochemical and photocatalytic technologies for water splitting of brines are outlined. Reactor designs and the influence of brine composition are considered the main aspects to be compared, identifying important advantages and challenges for a sustainable water-energy nexus in the treatment of brines.