Closed Bipolar Electrodes for Spatial Separation of H₂ and O₂ Evolution during Water Electrolysis and the Development...

Electrolytic water splitting could potentially provide clean H₂ for a future "hydrogen economy". However, as H₂ and O₂ are produced in close proximity to each other in water electrolyzers, mixing of the gases can occur during electrolysis, with potentially dangerous consequences. Herein, we describe an electrochemical water-splitting cell, in which mixing of the electrogenerated gases is impossible. In our cell, separate H₂- and O₂-evolving cells are connected electrically by a bipolar electrode in contact with an inexpensive dissolved redox couple (K₃Fe(CN)₆/K₄Fe(CN)₆). Electrolytic water splitting occurs in tandem with oxidation/reduction of the K₃Fe(CN)₆/K₄Fe(CN) redox couples in the separate compartments, affording completely spatially separated H₂ and O₂ evolution. We demonstrate operation of our prototype cell using conventional Pt electrodes for each gas-evolving reaction, as well as using earth-abundant Ni₂P electrocatalysts for H₂ evolution. Furthermore, we show that our cell can be run in reverse and operate as a H₂ fuel cell, releasing the energy stored in the electrogenerated H₂ and O₂. We also describe how the absence of an ionically conducting electrolyte bridging the H₂- and O₂-electrode compartments makes it possible to develop H₂ fuel cells in which the anode and cathode are at different pH values, thereby increasing the voltage above that of conventional fuel cells. The use of our cell design in electrolyzers could result in dramatically improved safety during operation and the generation of higher-purity H₂ than available from conventional electrolysis systems. Our cell could also be readily modified for the electrosynthesis of other chemicals, where mixing of the electrochemical products is undesirable.