Supercapacitor carbons have been the focus of extensive research over the past couple of decades. Carbon–carbon supercapacitors offer higher power, better cycle life, and higher reliability than batteries, but have much lower energy density and higher self-discharge. While the currently available energy density is acceptable for applications such as emergency doors, memory backup, and energy recovery, limited energy density is popularly perceived as the main impediment to supercapacitor market growth. Much research and development has focused on increasing supercapacitor carbon energy density at a premium price. The supercapacitor carbon market, however, is much more sensitive to price than to performance, causing premium supercapacitor carbons to fail in the marketplace.
Today, virtually all supercapacitor manufacturers use coconut shell activated carbon as their active material. Activated carbon is made by charring a precursor, then oxidizing the charred body using an agent such as steam or carbon dioxide to create nanoscopic pores. Supercapacitor carbon is a premium activated carbon grade which is purified to reduce ash below 1% and to reduce halogen and iron impurities below 100 ppm to enable extended cycling. Kuraray supplies most of this product. Over time, this product has dropped from $150–$200 per kilogram to $15 per kilogram. Because activated carbon represents about half the total material cost of a supercapacitor, this low price point is a formidable barrier to entry for other carbons.
Many other porous carbons have been developed for supercapacitors. Carbon aerogels, which consist of nanoscale particles made by pyrolyzing polymeric aerogels, and resin derived carbons, which are activated carbons prepared from polymers, have previously been used in supercapacitors. They still attract some attention, but they have been largely supplanted by coconut shell carbon as its cost has decreased. Carbon nanotubes have been extensively studied as supercapacitor materials, but single-walled nanotubes remain prohibitively expensive, while multi-walled nanotubes offer comparable performance to activated carbon at a higher cost (over $50 per kilogram). Carbide-derived carbon, which is prepared by etching metal carbides with chlorine gas, has shown roughly double the energy density of activated carbon. However, its processing is expensive and difficult due to the corrosiveness of the reactants involved. Despite the variety of high energy density supercapacitor carbons developed, coconut shell activated carbon remains dominant.
In most applications, supercapacitors are small relative to what they power, giving little incentive to reduce supercapacitor size and weight while increasing cost. As described in Charged Electrical Vehicle Magazine, Maxwell’s main focus has been to reduce costs, and other manufacturers have a similar goal. In addition, WL Gore and Associates discontinued their supercapacitor electrode product. This may be due to slimming margins in the industry, which would encourage supercapacitor manufacturers to prepare electrodes in-house, rather than pay a third party to prepare them from porous carbon. In applications where reliability is less important and energy required is higher than current supercapacitors can provide, end-users are accustomed to batteries’ lifespan issues and are cost sensitive; Watt-hour for Watt-hour, batteries are much cheaper than supercapacitors. Supercapacitors also have much higher self-discharge than batteries, reducing their ability to displace batteries from many applications.
Cyclability is another issue affecting novel supercapacitor carbons. Most supercapacitor carbon research describes a few hundred or few thousand cycles of testing due to time constraints and the difficulties in obtaining well-sealed cells on a lab scale. Commercial supercapacitors, however, are rated for 100,000 cycles or more. Degradation often only becomes apparent following prolonged cycling. Additionally, lab-scale testing often uses ‘‘beaker tests’’, where the supercapacitor electrodes are immersed in a beaker of electrolyte during testing. This allows the impurities in the carbon to diffuse into a large volume of electrolyte during testing, reducing their impact on cycle life. By contrast, in practical devices the electrolyte is contained within the electrodes and a thin separator, so the impurities have little space to diffuse away and they can affect cycle life.
Today, much attention focuses on graphene as a promising supercapacitor electrode material. It consists of monolayers of graphite with a theoretical surface area of 2630 m²/g (as compared to ca. 2000 m²/g for coconut shell supercapacitor carbons), and impressive gravimetric energy densities have been demonstrated. A number of startups are working to scale-up graphene production; long-term cost projections range from roughly $5 per kilogram to $40 per kilogram. Graphene production processes are still being developed, so long-term numbers are uncertain, but the lower end of the price range is competitive with activated carbon. However, one issue which graphene in particular faces is poor volumetric energy density, which also affects other materials such as some highly activated carbons. Because it is easier to weigh electrodes than to measure their volumes, gravimetric energy numbers attract more academic attention than volumetric numbers. Graphene consists of long thin flakes resulting in low density when graphene particles are shaped in the form of electrode. When the low-density graphene electrode fills with electrolyte the electrolyte acts as ‘‘dead’’ volume and weight. Thus, the relative ‘‘fluffiness’’ of graphene and other low-density materials limit device energy density in practice on both a gravimetric and volumetric basis, and graphene based supercapacitors cannot be competitive unless a way is found to tightly pack graphene.
At this point, two potential opportunities exist in the supercapacitor carbon marketplace. If an electrode with an energy density well over double that of conventional activated carbon is discovered, it may find a market niche. There is also an opportunity for supercapacitor activated carbons with comparable performance to currently used coconut shell carbon at a lower cost. There is a substantial gap between the $4 per kg cost of commodity activated carbon and the $15 per kg cost of supercapacitor-grade carbon. While process control and purification requirements may increase the cost of producing a supercapacitor carbon to compete with coconut shell, the opportunity still exists; furthermore, both sugar derived carbons and high purity coal derived carbons may also be viable. However, attempts to develop a premium supercapacitor carbon in the performance range already demonstrated will fail in the future like they have in the past.