Direct electrosynthesis of sodium hydroxide and hydrochloric acid from brine streams

Seawater is an abundant resource across the world, and its purification and by-product recovery methods are crucial for economical, environmentally safe and sustainable utilization. Desalinating seawater generally produces brine as a by-product that cannot be purified economically with current technologies and which is instead released to the environment. In this Perspective, we discuss direct electrosynthesis of sodium hydroxide (NaOH) and hydrochloric acid (HCl) from sea-water desalination brine as an emerging alternative solution. In this direct electrosynthesis (DE) process, the water splitting reaction is used to produce H+ and OH–, which combine with the brine stream to produce NaOH and HCl. After introducing the scope of the process, we describe developments in earth-abundant catalysts for water splitting and the competing chlorine evolution reaction (CER), as well as challenges in inefficiency and productivity associated with these processes. Finally, we discuss the economic impact and feasibility of direct electrosynthesis. Desalination processes generally leave brine as a by-product, which is then discharged back into the environment. This Perspective looks at recent procedures for using this brine instead as resource for the production of hydrochloric acid and sodium hydroxide through direct electrosynthesis.