Heteroatom-Doped and Oxygen-Functionalized Nanocarbons for High-Performance Supercapacitors

Electrochemical capacitors (best known as supercapacitors) are high-performance energy storage devices featuring higher capacity than conventional capacitors and higher power densities than batteries, and are among the key enabling technologies of the clean energy future. This review focuses on performance enhancement of carbon-based supercapacitors by doping other elements (heteroatoms) into the nanostructured carbon electrodes. The nanocarbon materials currently exist in all dimensionalities (from 0D quantum dots to 3D bulk materials) and show good stability and other properties in diverse electrode architectures. However, relatively low energy density and high manufacturing cost impede widespread commercial applications of nanocarbon-based supercapacitors. Heteroatom doping into the carbon matrix is one of the most promising and versatile ways to enhance the device performance, yet the mechanisms of the doping effects still remain poorly understood. Here the effects of heteroatom doping by boron, nitrogen, sulfur, phosphorus, fluorine, chlorine, silicon, and functionalizing with oxygen on the elemental composition, structure, property, and performance relationships of nanocarbon electrodes are critically examined. The limitations of doping approaches are further discussed and guidelines for reporting the performance of heteroatom doped nanocarbon electrode-based electrochemical capacitors are proposed. The current challenges and promising future directions for clean energy applications are discussed as well.