Gideon Grader1 Avigail Landman1 Hen Dotan1 Gennady Shter1 Avner Rothschild1

1, Technion, Haifa, , Israel

Power-to-Gas (P2G) technologies that convert renewable energies such as solar and wind to chemical fuels have been the focus of many scientific endeavors recently. Specifically, H2 production by water electrolysis (2H2O à 2H2 + O2) is a promising P2G technology. The emerging market of Fuel Cell Vehicles (FCV) and fast-growing Hydrogen Refueling Stations (HRS) increase the demand for clean, pure, high-pressure hydrogen at a competitive price. The state-of-the-art electrolyzers operating today include Alkaline Electrolyzers (AEL) and Proton Exchange Membrane Electrolyzers (PEMEL). Alkaline electrolysis represents a commercial technology that produces high purity H2 using non-precious metal catalysts. However, it has several shortcomings that limit its potential integration as a P2G technology, including limited partial load and high-pressure capabilities, and a low tolerance to electrolyte impurities. These limitations stem from the required diaphragm and single-cell configuration. PEM electrolysis, on the other hand, can operate at higher differential pressures, but also faces dangerous gas crossover at partial load, as well as low tolerance to impurities and membrane degradation. It also suffers from the high cost of catalysts and membranes [1], [2]. In addition, both technologies work in corrosive high/low pH environments, requiring expensive construction materials, safety measures and maintenance. In a recent publication in Nature Materials [3] we suggested a new Photoelectrochemical (PEC) water splitting technology wherein hydrogen and oxygen are produced in two completely separate cells, connected to each other externally only by electrical wires. This is achieved by introducing an additional set of Ni(OH)2/NiOOH electrodes, called the auxiliary electrodes (AEs). Nickel hydroxide is commonly used in rechargeable alkaline batteries, and can be electrochemically cycled (charged/discharged) many times with minimal energy loss. By placing a "charged" (NiOOH) auxiliary electrode in the oxygen cell, and electrically connecting it to a "discharged" (Ni(OH)2) auxiliary electrode in the hydrogen cell, electrolysis can be performed in two separate cells. During electrolysis, one auxiliary electrode charges while the other discharges. Thereafter, the process can be repeated by cycling the auxiliary electrodes between the charged/discharged states. The presentation will describe the operation of our membraneless electrolysis system, its benefits and potential.
[1] A. Buttler and H. Spliethoff, “Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids : A review,” Renew. Sustain. Energy Rev., no. September, 2017
[2] D. V Esposito, “Membraneless Electrolyzers for Low-Cost Hydrogen Production in a Renewable Energy Future,” Joule, pp. 1–8, 2017
[3] A. Landman et al., “Photoelectrochemical water splitting in separate oxygen and hydrogen cells,” Nat. Mater., vol. 16, pp. 646–651, 2017