Modeling the Invisible
Imagine: you buy a new battery for your laptop computer, get a 25-year warranty with an affordable price and after charging the battery a million times, it still runs for the same number of hours as it did on the first day. Not only does it fully recharge in seconds, but it works at arctic- and desert-like temperatures, and has a zero chance of catching fire.
Supercapacitors, which store energy as a static charge and not through electrochemical reactions, could make this dream a reality. However, the major limitation of commercial supercapacitors is the low energy density compared to standard lithium-ion batteries. Scientists of the Fluid Interface Reactions, Structures and Transport or FIRST Energy Frontier Research Center are taking on this challenge by finding the best combination of porous carbon electrodes and liquid electrolytes, which contain ions of different sizes. But what is the “best” pore size? To make things more complicated: The scientists at FIRST found that the correlation between ion and pore sizes varies greatly in a non-linear and oscillating fashion.
A major challenge was that pore and ion sizes in the nanometer range, that is, a 100,000 times smaller than a human hair, called for sophisticated computer simulation techniques. Two different yet complementary computational models were used: classical density functional theory, which, from a computational standpoint, is a more efficient yet an overly simplistic approach; and molecular dynamics, which is a more realistic model from a chemistry perspective, but is computationally intensive. With safety concerns in mind, a novel class of electrolytes was chosen for study. These electrolytes show excellent chemical stability, are non-flammable and yield a very high energy density. Yet, our understanding of the behavior of ionic liquids inside nanopores remains limited.
The problem is that when confined to nanometer-sized pores, the properties of electrolytes are completely different from those that are unconfined or in the bulk state. The simulation studies were motivated by the observation of an anomalous increase in capacitance, stored charge per unit of surface area, when the pore size was matched to the ion size of a given electrolyte. However, no studies had investigated a wide range of pore sizes from sub-nanometer to tens of nanometer, which is the usual window of pores found in common porous carbons. Both computational modeling studies show that the observed variations of charge capacity per unit surface area are due to the interference of the layers of adsorbed ions inside nanopores, which is a new concept in the theory of supercapacitors with porous electrodes.
"I'm very excited about this work,” says De-en Jiang, R&D staff scientist at Oak Ridge National Laboratory and a key researcher on the work. “This is the first time one shows how the capacitance of an ionic liquid inside a nanopore should behave from micropore to mesopore range. It turns out that the experimental 'anomalous' increase of capacitance is only a small part of the whole picture.”
But how does this translate into real-world devices? “The porosity of real carbon materials shows always a distribution of pore sizes,” explains Guang Feng, postdoctoral research scientist at Vanderbilt University and a key researcher on the study.
The new simulations not only provide the tools needed to study commonly used carbons, but as Feng continues: “Based on our new models, we can now start to investigate novel and complex carbon pore architectures.”
Such architectures seek to combine fast ion mobility, which is improved for larger pores, with high energy density, which benefits from smaller pores, and must be based on cheap and abundantly available carbon materials—a challenging trifecta. But with the efforts of the FIRST scientists, don’t be surprised if your cell phone one day recharges faster than it takes you to check your emails and bears the label “warranty: forever.”
Feng G and PT Cummings. 2011. "Supercapacitor Capacitance Exhibits Oscillatory Behavior as a Function of Nanopore Size." The Journal of Physical Chemistry Letters. DOI: 10.1021/jz201312e.
“Jiang DE, Z Jin, and J Wu. 2011. "Oscillation of Capacitance inside Nanopores." Nano Letters. DOI: 10.1021/nl202952d”.
This work was supported by the FIRST EFRC Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. J Wu received funding from the National Science Foundation. This funding supported the density functional theory modeling work. This research used resources of the National Energy Research Scientific Computing Center and Clemson University.