- Researchers wonder why nanoporous carbon electrodes store more energy.
- Computer simulations will show how supercapacitors charge.
- Future supercapacitors could be a cost-effective, efficient method for water desalination.
Supercapacitors are energy storage devices that can be charged and discharged in a few seconds. They are used instead of batteries in many applications where high power densities are required — like the stop/start systems in hybrid cars, and the Energy Recovery System used in Formula 1 racing.
Although supercapacitors have been used commercially for over 50 years, only incremental performance advances have been made. However, a recent experimental study showed a new charge storage mechanism for nanoporous-carbon based supercapacitors, and now many simulations are attempting to reproduce and understand this mechanism.
Nanoporous carbon electrodes, which have a larger ability to store a charge than do simpler geometries, might be expected to show poorer power performances because of the longer times taken by the ions to access the electrode interior. Simulations show this to not be the case, however.
The dynamics aspects of this problem have not been addressed because this typically requires much longer trajectories to extract relevant quantities. Mathieu Salanne of the Pierre and Marie Curie University has been using PRACE resources to simulate the charging of supercapacitors in order to explain their fast charging times.
“We started our work in this area six years ago, when we were trying to understand why electrodes made of nanoporous carbon store more energy,” says Salanne. “Our first experiments showed that they were using quite a different storage mechanism compared to supercapacitors of the past. We then wanted to work out how it was possible that these structures could still charge within the space of a few seconds despite this confinement. You would expect the confinement to push charging times up to minutes of time, but that is not the case.”
Salanne and his team have thus been looking to calculate macroscopic charging times of this new type of supercapacitor. “We also complemented these calculations with microscopic quantities such as in-pore diffusion coefficients and adsorption lifetimes. Understanding the transport in supercapacitor electrodes is very important in order to avoid synthesizing new materials with excellent storage capabilities but which would have limited practical applications due to slow charging times.”
Looking at the response time of a system of supercapacitor electrodes is computationally demanding. The system consists of two supercapacitor electrodes submerged in a liquid electrolyte made up of tens of thousands of atoms. Constant potential must be simulated inside the electrodes, so a special algorithm is used for the molecular simulations.
“In normal molecular dynamics you just need to convey the force at each step to move the atoms. In our case, before calculating the force, we had to data mine all the charges on all the electrode atoms,” notes Salanne. “This takes a number of steps, and makes the whole process more expensive in terms of computing time.”
Two types of simulations were carried out on up to 1,024 cores. The first type involved applying a finite potential difference, starting from 0V, between two nanoporous carbon based electrodes. The system was then charged, typically taking 10-20 nanoseconds for the nanometer-scale structures being used.
“These simulations showed some regions responded very quickly whereas others were much slower,” says Salanne. The second series of simulations involved studying the systems at equilibrium, with the electrodes set at a fixed voltage. This allowed the researchers to determine the diffusion coefficients as well as characteristic times for various microscopic processes occurring in the liquid.
“We have shown that in realistically-modeled carbide-derived carbon electrodes the diffusion of the ions is one order of magnitude slower than in the bulk electrolyte,” says Salanne. “This means that the effect of confinement is not too detrimental, and that a charging time of a few seconds can be extrapolated for macroscopic supercapacitors. This means that it is possible to use nanoporous carbons for high power applications.”
The common belief that nanoporous systems perform poorly from the power density point of view therefore does not hold. Salanne’s results confirm that they are the most promising structures for the next generation of supercapacitors.
“What we have shown is in agreement with existing experimental data. It has never actually been shown that nanoporous carbon is slow at charging — it has been a common belief that they should be slow but this has never been shown to be true,” says Salanne. “Although it is difficult to get hold of recent commercial supercapacitors, from looking at their much larger energy storage capabilities we can be almost certain that they are now using nanoporous carbons.”
Further experimentation in this area will involve tweaking the system in various ways, for example using different types of electrolyte. But as well as making excellent supercapacitors, nanoporous carbons have other potential uses.
“We are now applying similar methods to a different problem: the desalination of water,” says Salanne. “At the moment this is done using complex membranes that utilize reverse osmosis, but one could also use a supercapacitor in a process called capacitive deionization. The problem with membrane-based desalination is the cost of the membrane, whereas porous carbons are very cheap, so in terms of cost it would be much better than what is currently used. We now have a new PRACE project that is investigating this topic.”
