Investigating Carbon Expansion in Electrochemical Double Layer Capacitors
Alexander J. McBride
A sustainable global energy future will require effective energy storage. Production from prominent clean energy sources, like solar and wind, are unreliable, varying with climate conditions. Storing excess energy, consequently, is necessary for a continuous supply of energy. Additionally, as energy resources for transportation applications move away from combustible fuels, portable devices for providing electric power are necessary. Existing energy storage technology relies predominantly on batteries, which have several shortcomings. Lithium-ion batteries are commonly used due to their high energy densities (up to 180 watt hours), but such devices have high fabrication costs, slow power uptake/delivery performances, and limited lifetimes. Alternatively, electrochemical capacitors (EC) have high power densities (10 kW kg-1), moderate costs, and longer lifespans, when compared to batteries. The main disadvantage of ECs, also called supercapacitors, or ultracapacitors, is low energy densities (5 Wh kg-1). Despite this drawback, ECs are still ideal for load leveling of electrical spikes in inconsistent energy sources, as previously described. However, any increases in EC energy density would significantly improve such devices, allowing them to complement battery technology, and perhaps eventually replace them altogether.
There are three main types of electrochemical capacitors: electrochemical double layer capacitors (EDLC), pseudo-capacitors, and hybrid capacitors. EDLCs are designed around two carbon-based electrodes, which have high surface areas capable of storing hundreds to thousands of times more charge than conventional electrolytic capacitors. Electrostatic charge is stored in EDLCs by absorption of ions from the nearby electrode, which are subsequently desorbed during discharging. Pseudo-capacitors introduce redox reactions on or near the surface of the electrodes for charge storage. Hybrid capacitors combine ECs with a battery, to take advantage of their dissimilar properties. Factors effecting capacitance within ECs, include the specific surface area (SSA) of the electrode, the electrical conductivity of the electrode, carbon pore dimensions, and cation/anion sizes. The larger the electrolyte stability voltage window, the higher the supercapacitor cell voltage. Problems with aqueous and organic electrolytes are related to solvent decomposition at high voltages (0.9V and 2.7V). As a result, ionic liquid (IL) electrolytes are often chosen for their higher voltage capacities, despite their low ionic conductivities.
In recent years, studies have demonstrated that, after consecutively charging/discharging the EDLC, volume fluctuations in the carbon based electrodes effect ion electrosorption (i.e. charge storage). Traditionally, researchers list incredible device lifetimes, as an advantage of ECs. Electrode expansion, however, may eventually limit device cycle lifetimes. Further research is therefore required to better understand the effects of expansion, and ultimately quench any expansion from harming the long-term device lifetimes. Many factors affect the extent of expansion, including the type of carbon electrode, procedure used to fabricate the electrode, and intercalation. Now that expansion is a proven operational property of EC carbon electrodes, this investigation will vary numerable factors to eliminate expansion, truly allowing for extended device lifetimes.
2.1. Device Fabrication.
While experimental work will commence next week, based on literature reports, the procedure will be as follows:
Carbon-derived carbon (CDC) obtained via chlorine treatment of boron carbide (B4C) was purchased from Y-Carbon In. (USA) and commercial grade YP50 activated carbon was obtained from Kuraray Co. Ltd (Japan). Electrode characterization will be completed with a N2 gas sorption around -200 oC using a Quantachrome (USA) Autosorb-6 system. The Bruanuer-Emmet-Teller Equation (BET) and quenched solid density functional theory (QSDFT), will be used in calculating electrode pore characteristics. Both the working and counter electrodes will use a 5 mass% polytetrafluorethylene (PTFE, 60 mass% dispersion in water from Sigma-Aldrich) solution as the binding agent, with porous CDC and activated carbon as the material for the working/counter electrodes, respectively. This will form a carbon paste, and a roll-press will be used to form solid films. Electrochemical cells will be assembled in an Argon filled glovebox (O2, H2O <1 ppm) and a [Emim]+ [BF4]- (Sigma Aldrich) will be used as the IL electrolyte. It is important to note, the above procedure depicts the control electrodes, as other features will be varied later on to quench expansion.
2.2 ELDC Characterization. Cycling voltammetry (CV) is useful for identifying some of the general qualitative properties of reactions at the electrolyte/electrode interface. By forcing the EC to selected potentials, the amount of consumed charge at each potential can be determined. Plotting said current output verses the electrode potential, is useful for assessing the performance of electrochemical devices. Conversely, galvanostatic cycling (GC) tests the cell at a constant applied current, and measures the potential response. This provides the most accurate quantitative evaluation of EC performance.
Both CV and GV electrochemical measurements will be preformed at 20oC with a Biologic VP-300 potentiostat. Dilatometric measurements will be performed within a temperature-controlled chamber also set to 20oC, with an Agilent 34972 A instrument to monitor carbon expansion.
To be continued….
Interesting, different, and quite come to mind. We arrived in Frankfurt, Germany, on June 16th, before taking a train to Saarbrücken. Everyone, myself included, was pleasantly surprised with the physical layout of our apartments. They are larger than Drexel's dorm rooms, include a personal restroom/shower, and the apartment building contains a fully stocked common kitchen. The first day at the INM (institute for new materials) was awesome! The laboratory facilities are incredible, and I have my own office! In practice, Germany isn’t altogether too different than the U.S., though, the accumulation of numerous little variations magnifies the overall dissimilarity. Like Philly, there are a lot of people, busses, cars, restraints, and shops. Unlike Philly, Germany is very quite, everything closes at 8pm, and failing to recycle results in the death penalty. Well...maybe not the last one, but Germany is certainly very proactive about recycling. As a quick side note, if you found my project description a bit technical, I apologize. In the hopes of publishing my research (at the end of the summer), I wrote the introduction/experimental section in a fairly technical style, which I later intend modify for publishing in a science journal. Overall, I am excited about what this summer will bring, and ready to begin my research!