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January 2012

Modeling the Invisible

How understanding nanoconfinement helps make better energy storage devices

Volker Presser

The capacitance of a nanoporous carbon electrode as a function of the pore size. Supercapacitor electrodes are usually based on porous carbon structures with a very large surface area and varying pore sizes. The simulation results agree well with experimental results; the latter, however, are limited to a narrow range of pore sizes. The orange line represents results from classical density functional theory; the red line with solid circles from molecular dynamics; the black line with solid squares from experiments.

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.”

Acknowledgments

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.

More Information

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”.

About the author(s):

  • Volker Presser is an Assistant Research Professor and Humboldt Research Fellow at Drexel University (Philadelphia). He is working on electric energy storage systems and the development of novel functional nanomaterials.

Fast Energy Storage with Supercapacitors

The capacitance of a nanoporous carbon electrode as a function of the pore size. Supercapacitor electrodes are usually based on porous carbon structures with a very large surface area and varying pore sizes. The simulation results agree well with experimental results; the latter, however, are limited to a narrow range of pore sizes. The orange line represents results from classical density functional theory; the red line with solid circles from molecular dynamics; the black line with solid squares from experiments.

Supercapacitors store energy as a static charge and not through electrochemical reactions, but suffer from low energy density compared to standard lithium-ion batteries. New research is tackling this problem. When scientists examined the energy density using laboratory experiments, they found an unexpected rise and fall in energy storage depending on the size of the supercapacitor’s pores. Because of the size and complexity of the material, scientists developed two computational models to examine the experimental findings. They showed that the oscillation results from a size-specific alignment of the ions which are confined in nanometer-sized pores. Understanding the relationship with these pores could lead to affordable, reliable and safe energy storage devices that perform the same after a million uses. This work was done by the Fluid Interface Reactions, Structures and Transport (FIRST) Energy Frontier Research Center, led by Oak Ridge National Laboratory.

Written by Volker Presser and Kristin Manke

More Information

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”.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.