Dr. Ekaterina Pomerantseva gets the same reaction each time she tells people about her most recent work: we didn’t realize Drexel University is doing water desalination research.
Rest assured. Drexel University is doing water desalination research.
At least three College of Engineering labs—including Pomerantseva’s Materials Electrochemistry Group—are engaged in advancing the technology of removing salt ions from brackish or ocean water. Pomerantseva’s team is creating new materials and developing new electrode manufacturing approaches for a more efficient process.
As Pomerantseva characterizes it, desalination—or deionization—is one of the great challenges of science as researchers seek to improve the harvest of fresh water at a time when the poet’s famous line, “Water, water everywhere/nor any drop to drink,” is becoming alarmingly relevant.
Of the Earth’s total water resources, just 1.2% of it is fresh, liquid, and accessible. About 3.4 billion people—40% of the human population—already experience some degree of water scarcity. These numbers are likely to increase dramatically in the coming decades. The looming crisis fuels the need to develop a scalable, cost-effective, environmentally sustainable way to produce fresh water.
Recipe for Success
Pomerantseva, the Anne Stevens Assistant Professor in Materials Science and Engineering (MSE), and colleagues have published two recent papers detailing the promise behind their next-generation desalination research. Pairing the process of electrochemistry with an unconventional deionization “recipe” activated through hybrid capacitive deionization (HCDI), researchers have yielded a higher ion-removal performance than traditional CDI.
In their recipe, CoE researchers replace a more typical carbon electrode with the electrode containing manganese oxides. The crystal structure of manganese oxides has tunnels or layers available for incorporating ions from salty water; thus ions are stored in volume rather than just on the surface, as is the case with carbon electrodes, providing a higher ion removal efficiency.
Questions remain on the rate of permeability of ions through the manganese oxide structures—diffusion is slower than with carbon-based materials. But Pomerantseva said manganese oxides have a “superior” ion-removal performance. Her investigation manipulates the size of the tunnels to maximize the number of ions that are removed from water. As research proceeds, she also gains an increased understanding of the fundamental processes governing CDI.
The two articles appeared in Nano Energy and ACS Applied Materials & Interfaces. Together, they underscore the performance of manganese oxides as a lower-cost, abundant, environmentally friendly deionization agent.
“Creating new low-cost materials that would allow us to increase the salt uptake from saline water is critical to enable more efficient and affordable electrochemical water desalination technology, which could be used locally whenever it is needed," said Pomerantseva. “I worked with tunnel manganese oxides for energy storage purposes, but I also know they’re stable in water. So, just for the fun of it, I tried to use these materials for water desalination. They showed quite a high performance. This is very promising work.”
Picture the Planet
Three major processes are currently used to remove salt from ocean water: electrodialysis, reverse osmosis, and thermal distillation. Some cruise ships, for example, use thermal distillation to produce freshwater from the very element they’re cruising on. When tanks of ocean water in the bowels of the ship are heated to extremely high temperatures, water evaporates. In that process, fresh water is separated out and the brine product is released back into the ocean. But heating the tanks requires enormous amounts of energy. Dialysis and reverse osmosis in turn are based on the use of expensive porous membranes that allow some ions to pass through them while preventing others, thus separating out the constituent ions and yielding fresh water.
Each of these desalination processes has significant drawbacks, some related to cost and some to energy input.
Pomerantseva’s research provides compelling evidence for using an alternative process—CDI, based on low voltages for activation—and for using cheap and efficient materials that are both surface- and volume-reactive to significantly improve ion uptake. She replaces one layer of the carbon materials with a layer of manganese oxides, which results in a higher performance: unlike carbon-based materials, manganese oxides provide both surface and volume storage.
To understand the difference, picture the planet Earth. It “stores” people on its surface, not within its core or volume. The appeal of living underground notwithstanding, consider how many more billions of people could populate the planet if able to inhabit the core, or volume, as well as the surface.
That’s the chief advantage manganese oxides bring to the desalination process.
Salty water is passed through a desalination device, or cell, which features two layers, called electrodes, of materials. As the water passes between these materials layers, voltage is applied across the electrodes, which causes ions in the salt water to move towards oppositely charged electrodes. Salt water contains sodium, potassium, and magnesium ions that are attracted to manganese oxide electrodes. That attraction drives the uptake: positively charged ions are taken up by negatively charged electrodes, and negatively charged ions are taken up by positively charged electrodes.
Once the ions are completely taken up, the consequent liquid is fresh and can be collected for use. When the voltage polarity is changed, ions get released from the electrodes and brine product can be collected and disposed of.
“Carbon is a fantastic material – it’s conductive, it’s light, it’s electrochemically active. But it has one big limitation for this desalination process: it can store charge only on the surface. So if you have a particle of carbon, you don’t involve the volume. If you think about these ions being absorbed on the surface, it’s a limited number of ions that can fit on the surface,” said Pomerantseva.
“So, now what’s happening is we use manganese oxides – one of my favorite materials – which is both surface and volume active,” she added. “Now, we have intercalation of ions going into that volume.”
Diffusion and Other Matters
When ions are inserted into the volume of the manganese oxide materials, however, diffusion – the process by which ions are intercalated into the crystal lattice structure – is limited. Superficially, the process moves more quickly; but when storage in volume is involved, the process is “more and more” slowed down. That’s another of the behaviors Pomerantseva’s group wants to understand.
“For me, as a materials scientist, it’s a very exciting challenge,” said Pomerantseva. “How do I improve diffusion? How do I involve more of the volume of my materials so there is more volume being occupied by these ions?
“The experiment we’re planning right now is making a solution that would have different salts in it, and pass that solution through our desalination device, or cell, to see what we have in the end,” said Pomerantseva. “Would our electrode preferentially absorb any specific type of ion like sodium ion or potassium ion, or would they absorb all ions in a similar way? In reality, we just don’t know what’s happening.”
As electrochemical desalination research advances and is scaled for larger and larger applications, scientists will also need to look at the brine disposal issue. If desalination is deployed worldwide, the amount of waste could invite environmental problems that will need a new level of intervention.
Ultimately, a thorough understanding of the operational conditions and their effects on performance would guide development efforts as the research attempts to answer one of the world’s most pressing needs. New intercalation compounds that can replace carbon electrodes and efficiently and quickly remove ions from water are essential. Pomerantseva’s group is moving towards developing new material and electrode manufacturing solutions to meet those needs.
Pomerantseva’s research is supported by funding from the National Science Foundation (grant CMMI-1635233) to develop materials synthesis methods and to support water desalination experiments.