Sep 30 2020
According to a group of scientists from Pennsylvania State University (Penn State), the power of the sun, sea, and wind may soon integrate to yield clean-burning hydrogen fuel.
Le Shi, a postdoctoral researcher in environmental engineering and first author of the paper, and Bruce Logan, Kappe Professor of Environmental Engineering and Evan Pugh University Professor, examine their newly-designed seawater electrolyzer. Image Credit: Tyler Henderson.
The researchers combined water purification technology into a new proof-of-concept design for a seawater electrolyzer, which employs an electric current to separate the oxygen and hydrogen present in water molecules.
The new technique meant for “seawater splitting” could render it easier to convert solar and wind energy into a portable and storable fuel, according to Bruce Logan, Kappe Professor of Environmental Engineering, and Evan Pugh, University Professor.
Hydrogen is a great fuel, but you have to make it. The only sustainable way to do that is to use renewable energy and produce it from water. You also need to use water that people do not want to use for other things, and that would be seawater. So, the holy grail of producing hydrogen would be to combine the seawater and the wind and solar energy found in coastal and offshore environments.
Bruce Logan, Kappe Professor of Environmental Engineering and Evan Pugh University Professor, The Pennsylvania State University
Although seawater is abundantly available, it is not generally used for water splitting. The water has to be desalinated before it is allowed to enter the electrolyzer—a costly additional step—or else, the chloride ions in seawater will change into poisonous chlorine gas, which would damage the equipment and leak into the environment.
Hence, to avert this, the team placed a thin, semipermeable membrane, which was initially created for purifying water in the reverse osmosis (RO) treatment process. This RO membrane substituted the ion-exchange membrane normally used in electrolyzers.
The idea behind RO is that you put a really high pressure on the water and push it through the membrane and keep the chloride ions behind.
Bruce Logan, Kappe Professor of Environmental Engineering and Evan Pugh University Professor, The Pennsylvania State University
Seawater in an electrolyzer would no longer be forced through a RO membrane but confined by it. A membrane is utilized to help isolate the reactions that take place close to two submerged electrodes— a negatively charged cathode and a positively charged anode —linked by an external energy source.
When the power is switched on, water molecules begin to split at the anode, discharging tiny hydrogen ions known as protons and forming oxygen gas. The protons subsequently travel via the membrane and integrate with electrons at the cathode to produce hydrogen gas.
With the RO membrane in place, seawater is contained on the cathode side, and the chloride ions are too big to travel via the membrane and reach the anode, preventing the generation of chlorine gas.
However, in water splitting, other salts are deliberately dissolved in the water to help render it conductive, noted Logan. The ion-exchange membrane, which sifts ions by their electrical charge, permits salt ions to travel through. But the RO membrane does not.
“RO membranes inhibit salt motion, but the only way you generate current in a circuit is because charged ions in the water move between two electrodes,” added Logan.
Since the movement from the larger ions is limited by the RO membrane, the team wanted to see if there were sufficient tiny protons traveling via the pores to maintain a high electrical current.
Basically, we had to show that what looked like a dirt road could be an interstate. We had to prove that we could get a high amount of current through two electrodes when there was a membrane between them that would not allow salt ions to move back and forth.
Bruce Logan, Kappe Professor of Environmental Engineering and Evan Pugh University Professor, The Pennsylvania State University
Through a range of experiments recently reported in the Energy & Environmental Science journal, the scientists evaluated a pair of commercially available RO membranes and a pair of cation-exchange membranes, a kind of ion-exchange membrane that enables all positively charged ions to move in the system.
Each product was analyzed for membrane resistance to the movement of ions, the production of oxygen and hydrogen gas, the quantity of energy required to finish a reaction, the interaction with chloride ions, and the deterioration of the membrane.
Logan revealed that while one RO membrane became a “dirt road,” the other did better in comparison to the cation-exchange membranes. The team is still exploring why there was such a variation between these two RO membranes.
“The idea can work,” he added. “We do not know exactly why these two membranes have been functioning so differently, but that is something we are going to figure out.”
The team recently received a $300,000 grant from the National Science Foundation (NSF) to continue exploring seawater electrolysis. Logan is confident that their study will have a critical role to play in decreasing the global carbon dioxide emissions.
“The world is looking for renewable hydrogen. For example, Saudi Arabia has planned to build a $5 billion hydrogen facility that is going to use seawater. Right now, they have to desalinate the water. Maybe they can use this method instead,” Logan concluded.
Others who contributed to this project are Penn State researchers Le Shi, a postdoctoral researcher in environmental engineering, Ruggero Rossi, a postdoctoral researcher in environmental engineering, Derek Hall, assistant professor of energy engineering, Michael Hickner, professor of materials science and engineering and chemical engineering, and Christopher Gorski, associate professor of civil and environmental engineering.
The study was funded by the Stan and Flora Kappe Endowment in the Penn State Department of Civil and Environmental Engineering, the NSF, the United States Agency for International Development, and the National Academy of Sciences, as well as extra funding from Penn State.
Journal Reference:
Shi, L., et al. (2020) Using reverse osmosis membranes to control ion transport during water electrolysis. Energy & Environmental Science. doi.org/10.1039/D0EE02173C.