(724b) Membrane-Free Water Splitting for Hydrogen Generation

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 over the past decades. Specifically, hydrogen production by water electrolysis (2H₂O → 2H₂ + O₂) is a promising P2G technology. Photoelectrochemical (PEC) water splitting is a promising path to solar hydrogen production. However, separation of the H₂ and O₂ gas products as well as hydrogen collection and transport in large solar fields becomes an overwhelming technical challenge. State of the art water splitting technologies make use single cell units, separated into anode and cathode compartments by membranes. This implies that millions of PEC cell units in the solar field would have to be hermetically sealed and fitted with membranes, gas tubes and tube adaptors in order to separate and collect the hydrogen gas, resulting in a very complicated and expensive construction. The hydrogen would then have to be transported to the end user, either by pipelines or by high-pressure H₂ vessels. These obstacles, in addition to efficiency and stability challenges, render PEC hydrogen production economically questionable.

In this work, which was published recently in Nature Materials [1], we aim to solve these problems by totally separating the H₂-generation electrochemical cell from the O₂-generation PEC solar cell. This is achieved by introducing an additional set of electrodes, called the auxiliary electrodes. These are Ni(OH)₂/NiOOH electrodes, commonly used in rechargeable alkaline batteries, which can be cycled 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. Using suitable photoanodes, the PEC cell can generate O₂, which can then be discharged to the atmosphere, alleviating the need for sealing and piping. Since the separate cells are connected to each other by metal wires only, the H₂ can be generated at any location, for example, directly at a Hydrogen Refueling Station (HRS).