From waterproofing a patio to impeding corrosion of a battleship, from depositing gold on jewelry to building energy-dense devices for fast-starting electric cars, interfaces are ever present. Supercapacitors are devices that store charge within the electrical double layer that is formed at the electrode-electrolyte interface. The energy is readily accessible for short high-powered tasks such as accelerating an electric vehicle. Jennifer Black and co-workers in the Fluid Interface Reactions, Structures and Transport Center (FIRST) studied how molecules and ions interact with the electrode surfaces in supercapacitors. Marrying molecular dynamics simulations with atomic force microscopy (AFM) force spectroscopy, the team chemically and structurally characterized the electrical double layer to improve the understanding of the charge storage mechanism. Their findings can lead to tuning surface interactions to improve the energy density of supercapacitors and their performance in electric vehicles.
During the charging and discharging of a supercapacitor, positively and negatively charged ions separate. They then condense at the surface of the oppositely charged electrodes to form a structure that stores electric charge. Because capacity is restricted to the surface interface (less than 5 nanometers thick), electrochemical capacitors have significantly less energy density than batteries. That is, supercapacitors may allow you to accelerate on a highway, but it is the lithium ion within the electric vehicle's battery that allows you to drive to and from the grocery store. One way to increase the energy density of supercapacitors is to increase the operational voltage, as doubling the voltage quadruples the energy density. Previous generations of supercapacitors used water, but at high voltages, water splits into oxygen and hydrogen gas. Room temperature ionic liquid electrolytes are prime candidates to increase voltage because they are far more stable than water. However, the liquids must support a stable electrical double layer structure for them to be useful in supercapacitors.
The electrical double layer is typically only 5 nanometers thick, so obtaining a detailed structure was difficult. With this in mind, Black and her co-workers turned to AFM to map out the ionic layering at the carbon electrode surface.
"The sensitivity of the AFM probe to incredibly weak forces allows us to determine the structure of the ionic liquid within the electrical double layer with molecular-level resolution," explained Black. "We directly observed restructuring of ions in response to an applied potential, and it was revealed that ions within 1 nanometer of the electrode surface are responsible for charge storage, providing insight into the mechanism of charge storage for this relatively new class of electrolytes."
Computer modeling of the electrical double layer predicted that when the carbon electrode is charged or discharged, the ionic liquid forms a layered structure about 3 nanometers thick at the carbon surface. The layers alternate between anion-rich and cation-rich. Using AFM force spectroscopy, the team detected these layers with great accuracy. They measured the ion density of the layers as a function of distance from the electrode. They found that the AFM measurement is particularly sensitive to the negative ion layer structure, providing a near-perfect match between peaks in the AFM force-distance data and the heights of the computer-simulated ion layers above the graphite surface. Also, they found that the inner ionic layer compresses against the surface, potentially allowing for more ions to approach the surface, thereby adding additional charge storage capacity.
The next step in her research? Black responded, "the unmatched lateral resolution of scanning probe techniques also opens the pathway to mapping the structure of the electrical double layer in a 3D manner." This, she went on to say, would allow them to probe how defects found in the structure of the electrical double layer on the electrode surface can be used to further increase the energy density of supercapacitors and determine its rate performance.
The FIRST team combined expertise in computational modeling and AFM to improve understanding of the charge storage mechanism. They also presented clues as to how ionic liquids can be used to increase the energy density of electrochemical capacitors. Improvements would allow for increased energy storage for supercapacitors in applications requiring high power for short time intervals, such as merging safely onto I-75 toward Oak Ridge, Tennessee.
