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.

Defects in planar and vertically oriented nanographitic structures (NGSs) synthesized by plasma enhanced chemical vapor deposition (PECVD) have been investigated using Raman and X-ray photoelectron spectroscopy.

Electrochemical energy storage technology has emerged as one of the most viable solutions to tackle the challenge of fossil-fuel-based technology and associated global pollution. Supercapacitors are widely used for high-power applications, and there is tremendous ongoing effort to make them useful for high-energy storage applications. While electrode materials of supercapacitors play a central role in charge storage performance, insights into the contribution from different charge storage mechanisms are crucial from both fundamental and applied aspects. In this context, apart from the electric double layer and fast redox reaction at/near the surface, another pronounced contribution from the electrode is quantum capacitance (C_Q). Here, the origin of C_Q, how it contributes to the total capacitance, the possible strategies to improve it, and the state-of-art C_Q of electrode materials, including carbon, two-dimensional materials, and their composites, are discussed. Although most of the studies on quantifying C_Q are theoretical, some case studies on experimental measurements using standard electrochemical techniques are summarized. With an overview and critical analysis of theoretical studies on quantum capacitance of electrode materials, this review critically examines the supercapacitor design strategies, including choosing the right materials and electrolytes. These insights are also relevant to other types of clean energy storage technologies, including metal-ion capacitors and batteries.