Batteries contain chemicals that store and release electrical energy relatively quickly compared to capacitors, which are often used in applications that require power to be delivered quickly.
Capacitors can charge and quickly release energy using an electric field to store charge on a negative and positive plate. The plates are separated by electrolytes, solids or liquids that carry ions. Applying positive or negative electrical potential to capacitors causes ions to move in one direction or another.
Newer capacitors, called supercapacitors, are made of advanced composite materials and nanomaterials that offer higher energy storage capacity and increased power, with almost unlimited cycle life. However, higher energy densities are needed to enable supercapacitors to one day become the only power source in high power applications such as electric vehicles.
Scientists from the Massachusetts Institute of Technology conducted a neutron study at the Oak Ridge National (ORNL) National Energy Laboratory (DOE) to study the new, porous nanomaterials that could serve as high energy supercapacitors. The results of the study are published in Angewandte Chemie International Edition,
“with recently developed a metal-organic framework material that has good electrical conductivity and energy storage capacity, “said Mircea Dincă, energy professor WM Keck at the Department of Chemistry at MIT.” we thought it could make it a rough supercapacitor material. “
Developing the next generation of electrode materials requires in-depth knowledge of its energy storage mechanisms. MOF is a crystalline material made up of metal ions and organic molecules, and it has a microphone, which makes them a good model for learning charge and discharge mechanisms.
To investigate the mechanism of ion adsorption on the MIT shaft, conductive MOF, the team made electrodes from the material and dissolved in a solvent containing sodium triflate electrolyte. This enables positive and negative charge ions to flow freely when the researcher turns on or off the voltage and switches to the negative or positive and then reverses.
Using a small-angle neutron scattering experiment in ORNL’s High Flux Isotope Reactor (HFIR), the researchers found that when the voltage is applied to zero, the sodium ions in the electrolyte form a thin layer in building blocks such as MOF rods while solvent molecules penetrate into the pores. Applying positive or negative voltages causes sodium ions or triflate ions, respectively, to enter the pores. Further reversing the polarity causes the ions inside the pores to shift position with those outside.
Neutron data indicate that the charge storage mechanism in the microphone depends heavily on the polarization of the electrode. These findings shed new insights on nanomaterial charge storage mechanisms.
“MOFs usually have a high porosity, but poor electrical conductivity, which limits their use in high power applications,” said Lilin He, a neutron scattering scientist at ORNL. The MOF that implements this is a very porous nanomaterial with an incredible surface area when you factor in all interior pores, gaps, and surfaces.
“Equally important for its conductivity is that the MOF indicates only a 10% loss of capacitance and no increase in internal electrical resistance even after 10,000 cycles, which can indicate good durability for future commercial applications,” he said.
Neutron scattering is an ideal device for observing ion activity inside the MOF, as neutrons can penetrate almost any material. They are also sensitive to the presence of light elements, such as deuterium (hydrogen isotope) that researchers add to electrolytes. The hydrogen that is deuterized in the electrolyte provides contrast to assist in the presence of ions — even in the millions of holes inside the MOF.
Scientists further plan to produce variations of MOF material and again use neutrons to study its energy capacity and judge whether it is more efficient and faster, and how it performs at higher voltages.
References: “Observations of Ion Electrosorption in Metal-Organic Frame Micropores and Small-Angle Neutron Operating Spreads” by Dr. Lilin He, Luming Yang, Prof Mircea Dincă, Dr. Rui Zhang and Dr. Jianlin Li, March 11, 2020, Angewandte Chemie International Edition,
DOI: 10.1002 / anie.201916201
Support for neutron research is provided by the DOE Science Office and the Laboratory-Directed Research and Development program at ORNL.
HFIR is a DoE Office of Science User Facility. ORNL is managed by UT-Battelle LLC for DOE’s Office of Science, which supports a single basic research on physics in the United States. The DOE Science Office is working to address some of the most pressing challenges of our time.