Splitting water in remote, separated half cells
Using sunlight collected by solar panels to split water into hydrogen and oxygen is a green, sustainable way to generate hydrogen fuel, involving only sunlight, water, hydrogen, and oxygen. No carbon dioxide is produced, in contrast with the more common method of generating hydrogen through the steam reforming of natural gas. In a conventional water splitting apparatus, the electrochemical dissociation of hydrogen and oxygen takes place in an electrolyte solution at two electrodes located in the same cell, separated only by an ion exchange membrane. Sharing the same cell is convenient for electrolyzers, but it poses numerous logistical problems when it comes to photoelectrochemical solar cells that convert solar power to hydrogen fuel by splitting water. Specifically, the numerous single cells that generate hydrogen and oxygen must be located near a vast field of solar panels, and the hydrogen must be collected and transported over long distances to fueling stations for fuel-cell powered vehicles. Now, as reported in Nature Materials, researchers have succeeded in splitting water in two widely separated cells using a second set of auxiliary electrodes to exchange ions and close the loop of the electrolytic reaction.
Avner Rothschild of the Department of Materials Science and Engineering at The Technion—Israel Institute of Technology, along with colleague Gideon Grader of the Department of Chemical Engineering at The Technion and their team members Avigail Landman, Hen Dotan, and Gennady Shter, had spent years pondering a method to directly, continuously, and sustainably convert sunlight and water to hydrogen and oxygen on an industrial scale. Conventionally, this would involve an array of millions of photoelectrochemical solar cells distributed in a solar field. But the magnitude of such a challenge was staggering. First the water has to be split into hydrogen and oxygen. Second, the two gases must be separated, which involves large quantities of membranes and gas-tight sealing materials to keep the gases apart. Then hydrogen has to be collected from every one of these millions of photoelectrochemical cells and transported into a central distribution facility—a massive piping construction project that could greatly exceed the cost of the rest of the solar plant.
“This research is a complete game-changer in the design of not only electrolysis cells, but more importantly photoelectrochemical cells,” says Rothschild. “This is the first time that we have found a way to separate the two half cells into two separate cells. I can put one at a distance of kilometers from the other, instead of millimeters—that’s the game-changing innovation here.”
By separating the half cells, “you can have a solar field doing half the work of the water splitting by producing oxygen, while the other half of the reaction that produces hydrogen is done at a central location far away from the solar field,” Rothschild says. This eliminates the problems of separation, collection, and transportation of the hydrogen gas from millions of cells to a nearby hydrogen refueling station. Furthermore, the photoelectrochemical solar cells produce only oxygen that can be simply discharged to the atmosphere, while the hydrogen is produced elsewhere. This eliminates the need for expensive membranes and gas-tight sealing of millions of cells. In addition, “the hydrogen can be produced in a compact fashion under pressure,” Grader says.
The scientific key to the group’s success was introducing a second set of auxiliary electrodes to overcome the challenge of exchanging ions between the oxygen-producing electrode and hydrogen-producing one, which is normally handled through the ion exchange membrane. Standard Pt and Ni primary electrodes were used for splitting water into hydrogen and oxygen, respectively. NiOOH/Ni(OH)2 electrodes, which have a long history of use in rechargeable alkaline batteries, were used as the auxiliary electrodes in an alkaline solution in each of the half cells. A NiOOH electrode reversibly converts to Ni(OH)2 in the oxygen cell, while a Ni(OH)2 electrode converts to NiOOH in the hydrogen cell: NiOOH + H20 + e- ↔ Ni(OH)2 + OH-. So, the auxiliary electrodes serve as redox relays that exchange OH- ions with the primary electrodes in each separate cell, providing the ion exchange necessary to complete the reaction. This OH- ion exchange between electrodes normally happens across the membrane in a conventional cell.
These auxiliary electrodes can be cycled thousands of times by reversing the current polarity in the wire connecting them; they can also be swapped out from one cell to another in lieu of this polarity cycling. The solar-to-hydrogen conversion efficiency of this system was measured to be 7.5% over 1 hour, although the researchers anticipate that this could reach 11.7% by optimizing the photovoltaic device and other parameters of the system.
“This is a clever breakthrough—both conceptually and practically—that solves a major safety challenge,” says Scott Warren, an assistant professor in the Department of Chemistry at the University of North Carolina at Chapel Hill, who was not involved with this study. “Because this cell design is simple, scalable, and efficient, it seems likely to become the preferred design for future industrial devices.”
Read the abstract in Nature Materials.