Neutron scattering technique provides new data on adsorption of ions in microporous materials

Feb 28, 2013
This schematic shows the experimental setup for in-situ studies of ion adsorption on the surface of microporous carbon electrodes. Credit: Gleb Yushin

(Phys.org)—The adsorption of ions in microporous materials governs the operation of technologies as diverse as water desalination, energy storage, sensing and mechanical actuation. Until now, however, researchers attempting to improve the performance of these technologies haven't been able to directly and unambiguously identify how factors such as pore size, pore surface chemistry and electrolyte properties affect the concentration of ions in these materials as a function of the applied potential.

To provide the needed information, researchers at the Georgia Institute of Technology and the Oak Ridge National Laboratory have demonstrated that a technique known as small angle (SANS) can be used to study the effects of ions moving into nanoscale pores. Believed to be the first application of the SANS technique for studying ion surface adsorption in-situ, details of the research were reported recently in the journal .

Using conductive nanoporous carbon, the researchers conducted proof-of-concept experiments to measure changes in the adsorption of in pores of different sizes within the same material due to variations in solvent properties and applied electrical potential. Systematic studies performed with such a technique could ultimately help identify the optimal , and electrolyte solvent properties necessary for either maximizing or minimizing the adsorption of ions under varying conditions.

"We need to understand this system better so we can predict the kind of surface chemistry required and the kinds of solvents needed to control the levels of ion penetration and adsorption in pores of different sizes," said Gleb Yushin, an associate professor in the Georgia Tech School of . "Understanding these processes better could lead to the development of improved , water purification and desalination systems. This new experimental methodology may also give us paths to better understand ion transport in biological systems and contribute to the development of improved drugs and artificial organs."

The research was supported partially by the U.S. Army Research Office, the Georgia Institute of Technology and the Oak Ridge National Laboratory (ORNL).

"The advantage of neutron scattering is that it can be used to study real systems," said Yushin. "You can study most electrode materials and electrolyte combinations as long as they have a high sensitivity for neutron scattering."

Yushin and his collaborators – Georgia Tech graduate research assistant Sofiane Boukhalfa, and Oak Ridge scientists Yuri Melnichenko and Lilin He – conducted the research using ORNL's High Flux Isotope Reactor, which produces a beam of high-energy neutrons. Their experimental setup allowed them to immerse activated carbon fabric samples – each sample containing pores of different sizes – in different electrolyte materials while varying the applied electrical potential.

By measuring how the neutron beam was scattered when it passed through the carbon fabric and electrolytes, the researchers could determine how the solvent, pore size and electrical potential affected the average ion concentration in the carbon material samples.

Georgia Tech associate professor Gleb Yushin (left) and graduate research assistant Sofiane Boukhalfa assemble a test cell used to study the adsorption of ions. Credit: Gary Meek

"You can learn whether the ions get adsorbed into small pores or large pores by simply comparing the changes in the neutron scattering," Yushin explained. "This experimental technique allows us to independently change the surface chemistry to see how that affects the ion concentrations, and we can use different solvents to observe how the interaction between electrolyte and pore walls affects the ion adsorption in pores of different sizes. We can further identify exactly where the ion adsorption takes place even when no potential is applied to an electrode."

Earlier work in this area had not provided clear results.

"There have been multiple prior studies on the pore size effect, but different research groups worldwide have obtained contradictory results depending on the material selection and the model used to determine the specific surface area and pore size distribution in carbon electrodes," Yushin said. "Neutron scattering should help us clarify existing controversies. We have already observed that depending on the solvent-pore wall interactions, either enhanced or reduced ion electro-adsorption may take place in sub-nanometer pores."

In their experiments, the researchers used two different electrolytes: water containing sulfuric acid and deuterium oxide – also known as heavy water – which also contained sulfuric acid. The two were chosen for the proof-of-concept experiments, though a wide range of other hydrogen-containing electrolytes could also be used.

Now that the technique has been shown to work, Yushin would like to expand the experimentation to develop better fundamental understanding about the complex interactions of solvent, ions and pore walls under applied potential. That could allow development of a model that could guide the design of future systems that depend on ion transport and adsorption.

"Once you gain the fundamental knowledge from SANS experiments, predictive theoretical models could be developed that would guide the synthesis of the optimal structures for these applications," he said. "Once you clearly understand the structure-property relationships, you can use materials science approaches to design and synthesize the optimal material with the desired properties."

Information developed through the research could lead to improvements in supercapacitors and hybrid battery-capacitor devices for rapidly growing applications in hybrid electrical vehicles, energy efficient industrial equipment, smart grid-distributed energy storage, hybrid-electric and electrical ships, high-power energy storage for wind power and uninterruptible power supplies.

Explore further: Could 'Jedi Putter' be the force golfers need?

More information: Boukhalfa, S., et al., "Small-Angle Neutron Scattering for In Situ Probing of Ion Adsorption Inside Micropores." Angew. Chem. Int. Ed (2013). www.dx.doi.org/10.1002/anie.21209141

Related Stories

Recommended for you

Could 'Jedi Putter' be the force golfers need?

Apr 18, 2014

Putting is arguably the most important skill in golf; in fact, it's been described as a game within a game. Now a team of Rice engineering students has devised a training putter that offers golfers audio, ...

Better thermal-imaging lens from waste sulfur

Apr 17, 2014

Sulfur left over from refining fossil fuels can be transformed into cheap, lightweight, plastic lenses for infrared devices, including night-vision goggles, a University of Arizona-led international team ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

StarGazer2011
1 / 5 (1) Mar 01, 2013
now thats some real nice sciencing

More news stories

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...