Launched in 1965 in the United States, the HFIR (The High Flux Isotope Reactor) nuclear reactor at Oak Ridge National Laboratory is used not only for the production of isotopes in demand in medicine and industry, but also for carrying out a lot of scientific experiments. In particular, by observing the scattering of neutrons emitted by HFIR, scientists study new porous structures of supercapacitors to create powerful batteries in the future.
Historically, supercapacitors are intermediate between capacitors and batteries. The search for new porous materials for the electrodes of supercapacitors (in the Soviet and Russian literature – supercapacitors) promises to bring supercapacitors closer to batteries. At the same time, supercapacitors can deliver more power in a shorter time and have a very, very long resource. It is only necessary to find such materials, for which scientists need precise knowledge of the mechanisms of charge accumulation in supercapacitors.
Since it is better to see once than to speculate over and over again about the processes taking place in the plates of supercapacitors, a group of scientists from the Massachusetts Institute of Technology used the HFIR reactor as a source of neutrons. Neutrons have no charge and penetrate deeply into the porous material, where they interact with ions and paint the whole picture of the ongoing chemical processes in the supercapacitor.
In a specific experiment, MIT scientists studied the chemical processes in a new metal-organic framework (MOF) structure, which they proposed for the electrodes of a promising supercapacitor. The extremely porous structure of MOF allows the accumulation of powerful charges (many ions), which makes MOF materials promising for the creation of powerful traction batteries for electric vehicles.
A sodium triflate-based solution was created as an electrolyte for the experiment. Irradiation of MOF with electrolyte impregnation with a neutron flux from the HFIR reactor showed an interesting picture. Before energizing the electrodes, solvent molecules from the electrolyte freely penetrate the pores of the MOF, while sodium ions in the electrolyte form a thin layer on the MOF framework (see first image). Applying a voltage to the electrodes forced the ions to penetrate into the pores of the framework material, and the polarity depended on which ions penetrate into the pores: sodium ions or triflate ions.
The experiment not only clarified the mechanisms of chemical and physical processes in the supercapacitor, but also confirmed the high characteristics of the new MOF material as promising for future supercapacitors: this is good electrical conductivity, loss of only 10% of the capacity after 10 thousand charge / discharge cycles, and low internal resistance which hints at good durability for future commercial applications.
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