2000621 (1 of 22) © 2020 Wiley-VCH GmbH www.small-methods.com REVIEW A Review of Carbon-Supported Nonprecious Metals as Energy-Related Electrocatalysts Jian Wang,* Juwon Kim, Subin Choi, Hongsheng Wang, and Jongwoo Lim* Dr. J. Wang, J. Kim, S. Choi, Prof. J. Lim Department of Chemistry Seoul National University Seoul 08826, South Korea E-mail: jwangbx@connect.ust.hk; jwlim@snu.ac.kr Dr. H. Wang Department of Chemical System Engineering School of Engineering The University of Tokyo Tokyo 113-0032, Japan Prof. J. Lim Institute of Applied Physics Seoul National University Seoul 08826, South Korea The ORCID identifcation number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smtd.202000621. DOI: 10.1002/smtd.202000621 development of sustainable energy conver- sion and storage technologies. [1] Nowadays, the electrolysis of oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO 2 RR) is being extensively researched. [2] The con- version efciencies of these electrochemical reactions are critical to the performance of sustainable energy devices, including fuel cells, metal–air batteries, and electro- lyzers. [3] Particularly, ORR is the cathode reaction of fuel cells which are character- ized by a higher energy efciency than that of conventional combustion engines. [4] ORR and OER occur at the gas electrode of a rechargeable metal–air battery. [5] In the production of electro-fuels (e.g., H 2 or CO), HER and CO 2 RR serve as the cathode reac- tions in electrolyzers with H 2 O and CO 2 as the reactants, respectively. [6] The develop- ment of high-performance electrocatalysts is critical for the mass adoption of those sustainable energy applications. [7] Carbon-supported nonprecious metals (C@NPMs) have recently attracted signif- cant attention for the promotion of the above-mentioned reactions during electrolysis owing to their low cost and potentially high activity. [8] Carbon frameworks can be easily modifed and can take the form of various nanostructures, such as 1D carbon nanotubes (CNTs), [9] 2D graphene, [10] and 3D porous carbons. [11] The cou- pling of nonprecious metals with carbons can mitigate the cor- rosion issues of these metals under harsh electrolysis conditions (e.g., strong alkaline/acidic electrolytes and oxidative potentials), and reduce agglomeration by enhancing the dispersion of the metal moieties. [12] In addition, the coupling can promote charge transfer between the carbon and the metal components, thereby tuning the electronic structure for catalysis. [13] Thus, the carbon support not only works as a conductive substrate but also interacts electronically with the metal species, modifying the electronic/ electrochemical properties of the composite. [14] The catalytic prop- erties of C@NPMs can be further optimized by introducing het- eroatom dopants, engineering topological defects, modulating metal size, tuning the carbon shell, and coupling multimetals. [15] Owing to the tremendous eforts of the research community in this feld, C@NPMs have seen signifcant advancements. For instance, Fe or Co coupled with N-doped carbon, i.e., FeNC and CoNC are one of the most promising ORR electrocatalysts to replace precious Pt in both acidic and alkaline electrolytes. [16] The development of sustainable energy conversion and storage devices, such as fuel cells, metal–air batteries, and electrolyzers is highly dependent on the catalytic oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO 2 RR). Carbon-supported nonprecious metals (C@NPMs) with variable metal sizes (single atom, cluster, and nanoparticle) are attracting signifcant interest for catalyzing these reactions. Therefore, a systematic summary of the recent advances in this feld is necessary; however, C@NPMs have seldom been comprehensively reviewed. Herein, the most recent progress (in the years 2015–2020) of C@NPMs for ORR/OER/HER/CO 2 RR catalysis are reviewed. The review begins with the introduction of various synthesis methods for C@NPMs. Next, promising design strategies to modulate the electrochemical properties of C@NPMs are elaborated, including defect engineering, porosity fabrication, metal size modulation, carbon shell tuning, and multimetal coupling. Subsequently, the recent advances in C@ NPMs for ORR, OER, HER, and CO 2 RR catalysis are analyzed, covering the fundamentals (elemental steps, performance metrics, and catalytic mechanisms) of these reactions. This review not only analyzes the progress in the feld but also conveys methods and provides insights, which can guide the future development and practical applications of C@NPMs. 1. Introduction Industrial expansion, population explosion, and serious concerns regarding global warming have led to a surge in the demand for renewable energy. Electrolysis is playing increasingly impor- tant roles in the production of renewable energies and the Small Methods 2020, 2000621