Who knew smoking was good for the environment! Well, maybe that’s not the right way to put this…rather, if people must smoke (which they undoubtedly will for the foreseeable future), can’t we find something useful to do with all the waste? Luckily, some creative scientists were sick of seeing cigarette butts littering sidewalks and parks. In a new paper in Nanotechnology, a group from Seoul has developed a method to use old cigarette filters as the basis for supercapacitor electrodes. Put this in the category of crazy but true – let’s take a closer look…
First off, what is a supercapacitor? It sounds like the stuff of comic books and superheroes, but the device itself is actually quite simple and related to batteries. The setup is similar – two electrodes separated by a liquid electrolyte with a membrane separator right in the middle. A potential is applied to the two electrodes, which builds up positive charge on one (left above) and negative charge on the other (right above). This leads to the attraction of the opposite charge in the electrolyte to the electrode surface, setting up what is known in the business as the electrical double layer. Electrical energy is stored in these double layers in a supercapacitor, making it different from a battery, which stores energy in chemically in bonds that form at the electrodes (e.g., with lithium). The term capacitor comes from the fact that capacitance in physics is the ratio of the amount of charge that can be stored per unit of potential applied. So a high capacitance means a lot of charge stored at these electrode surfaces for only a little potential, which is a good thing when we’re trying to store a lot of energy!
The fact that supercapacitors store their energy electrically means that they can be charged or discharged much more quickly than batteries. Batteries usually have higher energy density, meaning more total energy stored per unit mass, but this quick charge/discharge give supercapacitors a much higher power density. This is useful for electronics and crucial for next-generation energy storage when energy may be required on short time scales.
So why do cigarette filters end up being such a good material for these supercapacitors? The reason begins with carbon, a ubiquitous terrestrial element that has great potential in so many areas of next-generation electronics (graphene, nanotubes, etc.). For supercapacitors, carbon is an excellent material to include in the electrode due to its high porosity, which increases the surface area so more electrolyte can interact with the electrode surface. Check out all the holes in this carbon sponge:
Carbon is also cheap because it’s so abundant and has a high conductivity and thermal stability. It just so happens that cigarette filters are generally made of cellulose acetate, which can be used to produce porous carbon through pyrolysis, which involves heating something up to high temperatures in the absence of oxygen (to prevent the formation of oxygen-related products). So this cheap, simple, widely used heating process can be used on cigarette filters to make an optimal electrode material for supercapacitors?Lee et al did just this, heating the cigarette filters at 900 C for 2 hours in argon or ammonia environments, and then tested the resulting porous carbon material (NCF in the figure above) as an electrode in supercapacitors. They also nitrogen-doped the carbon, as this has been shown the capacitance of the double layer (more charge stored!).
Once formed, the authors completed extensive tests analyzing the pore structure of the resulting carbon materials as well as the performance and cycle life of the supercapacitor. Most important, the electrodes based on cigarette filters show comparable surface area to activated porous carbon materials produced by standard methods. There is much data that could be delved into more detail, but the important point is that this pyrolytic process described above leads to a highly porous, high-surface area electrode that performs well over thousands of cycles of charging and discharging. Indeed, one of the advantages of supercapacitors over batteries is their improved stability over many more cycles, increasing their lifetime. These cigarette filter-based electrodes show this same extended stability. As seen in the graph below, the capacitance (C) of the capacitor shows no decay over 6000 cycles, and instead actually increases slightly:The authors attribute this slight increase due to less pore clogging over many cycles, which will increase the surface area to allow more interactions between the electrolyte and electrode.
So, while I encourage everyone to stop smoking, here’s a great way to use the waste, which has been estimated to be 1.69 billion pounds of cigarette butts per year!
Lee, M., Kim, G., Don Song, H., Park, S., & Yi, J. (2014). Preparation of energy storage material derived from a used cigarette filter for a supercapacitor electrode Nanotechnology, 25 (34) DOI: 10.1088/0957-4484/25/34/345601