Metal‐Free Carbon Electrocatalysts: Recent Advances and Challenges Ahead

metal impurities to the ORR activities of C-MFCs.Of particular interest, a study on highly oriented pyrolytic graphite (HOPG) model catalysts concluded that the improved electrocatalytic performance of C-MFCs is due to the intrinsic activities of heteroatom-doped carbon structures coupled with doping-induced charge transfer, rather than any residual metal presence.Carbon materials possess multiple advantages over metal catalysts, including earth abundance, low-cost, structural tunability at the molecular and morphological levels, and multifunctional catalytic activities.While the catalytic activity of most metal catalysts strictly relies on the attributes of the metal element, the catalytic active sites of C-MFCs can be modulated by introducing different dopants (e.g., "codoping"), structural deformations/defects ("defect doping"), adsorbents ("chargetransfer doping"), and/or 3D carbon architectures, leading to a powerful means of creating a large variety of highly efficient, multifunctional catalysts for specific reactions from the same graphitic carbon materials.This makes C-MFCs fundamentally different from and even more versatile than conventional metal-based catalysts.These significant advantages enabled the design of new C-MFCs capable of catalyzing different reactions.Great progress has been made since the first C-MFC was discovered over 10 years ago, but many scientific challenges still remain in order to understand, control, and eventually direct a wide range of catalytic reactions.This progress and related challenges are highlighted in this Advanced Materials Special Issue focusing on C-MFCs for renewable energy technologies with contributions from world-leading experts in the field.The controlled chemical approach in the rational design of C-MFCs is a major contribution to advancements in the field of metal-free catalysis.Liming Dai, Jian-Feng Chen, Xinliang Feng, Klaus Müllen, Sheng Dai, and co-workers (article number 1804863) present a comprehensive and critical review on chemical approaches to C-MFCs by summarizing important advancements, current challenges, and future perspectives in this exciting field.These chemical approaches combined with other physicochemical approaches, such as heteroatom-doping (reviewed by Shuangyin Wang, Rodney S. Ruoff, Liangti Qu, and co-workers in article number 1805121) and defect engineering (reviewed by Josue Ortiz-Medina, Feng Wang, Xiangdong Yao, Mauricio Terrones, Morinobu Endo, and coworkers in article number 1805717), have been widely applied to the development of various well-defined C-MFCs from CNTs and graphene (reviewed by Junji Nakamura and co-workers in article number 1804297), carbon nitride (reviewed by C.N.R. Rao and Manjeet Chhetri in article number 1803668), graphdiyne (reviewed by Yuliang Li and co-workers in article number 1803762), biomass (reviewed by Feng Wang and co-workers in article number 1805718) and polymeric precursors (reviewed by Krzysztof Matyjaszewski, Tomasz Kowalewski, and Eric Gottlieb in article number 1804626), for catalyzing many