Blessing and Curse: How a Supercapacitor’s Large Capacitance Causes its Slow Charging

Carbon supercapacitors, which are energy storage devices that use ion adsorption on the surface of highly porous materials to store charge, have numerous advantages over other power-source technologies, but could realize further gains if their electrodes were properly optimized. Studying the effect of the pore size on capacitance could potentially improve performance by maximizing the electrode surface area accessible to electrolyte ions, but until recently, no studies had addressed the lower size limit of accessible pores. Using carbide-derived carbon, we generated pores with average sizes from 0.6 to 2.25 nanometer and studied double-layer capacitance in an organic electrolyte. The results challenge the long-held axiom that pores smaller than the size of solvated electrolyte ions are incapable of contributing to charge storage.

Securing our energy future is the most important problem that humanity faces in this century. Burning fossil fuels is not sustainable, and wide use of renewable energy sources will require a drastically increased ability to store electrical energy. In the move toward an electrical economy, chemical (batteries) and capacitive energy storage (electrochemical capacitors or supercapacitors) devices are expected to play an important role. This Account summarizes research in the field of electrochemical capacitors conducted over the past decade. Overall, the combination of the right electrode materials with a proper electrolyte can successfully increase both the energy stored by the device and its power, but no perfect active material exists and no electrolyte suits every material and every performance goal. However, today, many materials are available, including porous activated, carbide-derived, and templated carbons with high surface areas and porosities that range from subnanometer to just a few nanometers. If the pore size is matched with the electrolyte ion size, those materials can provide high energy density. Exohedral nanoparticles, such as carbon nanotubes and onion-like carbon, can provide high power due to fast ion sorption/desorption on their outer surfaces. Because of its higher charge-discharge rates compared with activated carbons, graphene has attracted increasing attention, but graphene had not yet shown a higher volumetric capacitance than porous carbons. Although aqueous electrolytes, such as sodium sulfate, are the safest and least expensive, they have a limited voltage window. Organic electrolytes, such as solutions of [N(C2H5)4]BF4 in acetonitrile or propylene carbonate, are the most common in commercial devices. Researchers are increasingly interested in nonflammable ionic liquids. These liquids have low vapor pressures, which allow them to be used safely over a temperature range from -50 °C to at least 100 °C and over a larger voltage window, which results in a higher energy density than other electrolytes. In situ characterization techniques, such as nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), and electrochemical quartz crystal microbalance (EQCM) have improved our understanding of the electrical double layer in confinement and desolvation of ions in narrow pores. Atomisitic and continuum modeling have verified and guided these experimental studies. The further development of materials and better understanding of charged solid-electrolyte interfaces should lead to wider use of capacitive energy storage at scales ranging from microelectronics to transportation and the electrical grid. Even with the many exciting results obtained using newer materials, such as graphene and nanotubes, the promising properties reported for new electrode materials do not directly extrapolate to improved device performance. Although thin films of nanoparticles may show a very high gravimetric power density and discharge rate, those characteristics will not scale up linearly with the thickness of the electrode.