(Invited) Decoupled Water Splitting for Green Hydrogen Production: Reshaping Water Electrolysis

Abstract

Green hydrogen produced by splitting water molecules into hydrogen and oxygen using electricity from renewable energy sources is essential to reduce CO2 emissions, especially in hard-to-abate industrial sectors such as steelmaking and ammonia production. It is a critical component in the energy transition to fight climate change. The present technologies for green hydrogen production are alkaline and polymer electrolyte membrane water electrolysis. One thing in common to these technologies is the coupling between the hydrogen and oxygen evolution reactions (HER and OER, respectively), where both reactions occur concurrently in the same electrolytic cell. Therefore, a membrane or diaphragm divides the cell into two compartments to avoid hazardous H2/O2 mixing. This membrane architecture complicates the electrolyzers construction by adding gaskets and compression sealing to prevent H2/O2 crossover. This adds substantial costs and limits operation at high pressures and low-purity water. In addition, substantial energy losses, mostly due to the difficult OER, increase the cost of energy in this energy intensive technology. These drawbacks give rise to high cost of green hydrogen.To overcome these challenges, we developed alternative processes that decouple the generation of hydrogen and oxygen into two consecutive stages (time separation),1 or two cells (space separation),2 avoiding the need for membrane and sealing. In addition, we divided the OER, a difficult electrochemical reaction that requires four electrons and protons to generate an oxygen molecule on a single atomic reaction site, into two sub-reactions that occur on four sites, thereby enabling facile reactions and reducing the energy losses in water electrolysis. An ultrahigh efficiency of nearly 99% was demonstrated at lab scale,2 and we expect reaching above 90% at system scale. To bring this breakthrough process to reality we established H2Pro, an Israeli startup company that aims to produce green hydrogen at $1/kg by the end of this decade, based on our invention.3 Landman et al., Photoelectrochemical water splitting in separate oxygen and hydrogen cells, Nature Materials 16, 646–651 (2017).Dotan et al., Decoupled hydrogen and oxygen evolution by a two-step electrochemical – chemical cycle for efficient overall water splitting, Nature Energy 4, 786–795 (2019).Rothschild et al., Methods and system for hydrogen production by water electrolysis, United States Patent No. 2020/0040467 A1 (2020).