Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal–air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure–property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

Electrochemical coupling of biomass valorization with carbon dioxide (CO₂) conversion provides a promising approach to generate value-added chemicals on both sides of the electrolyzer. Herein, oxygen-vacancy-rich indium oxyhydroxide (InOOH-O_V) is developed as a bifunctional catalyst for CO₂reduction to formate and 5-hydroxymethylfurfural electrooxidation to 2,5-furandicarboxylic acid with faradaic efficiencies for both over 90.0% at optimized potentials. Atomic-scale electron microscopy images and density functional theory calculations reveal that the introduction of oxygen vacancy sites causes lattice distortion and charge redistribution. Operando Raman spectra indicate oxygen vacancies could protect the InOOH-O_Vfrom being further reduced during CO₂conversion and increase the adsorption competitiveness for 5-hydroxymethylfurfural over hydroxide ions in alkaline electrolytes, making InOOH-O_Va main-group p-block metal oxide electrocatalyst with bifunctional activities. Based on the catalytic performance of InOOH-O_V, a pH-asymmetric integrated cell is fabricated by combining the CO₂reduction and 5-hydroxymethylfurfural oxidation together in a single electrochemical cell to produce 2,5-furandicarboxylic acid and formate with high yields (both around 90.0%), providing a promising approach to generate valuable commodity chemicals simultaneously on both electrodes.

Advances in research and development of carbon-based metal-free electrocatalysts (C-MFECs) have provided potential alternatives to precious metal catalysts for various reactions important to renewable energy and environmental remediation. This timely but critical review provides an overview of recent breakthroughs (within the past 5 years or so) on C-MFECs in all aspects, including the design and regulation of intrinsic catalytic active sites, design and synthesis of carbon composite and hybrid carbon catalysts, mechanism understanding, and potential applications in clean energy storage and energy/chemical conversion. Current challenges and future opportunities in the field of metal-free carbon electrocatalysis are also discussed to provide forward-looking opportunities for their potential applications in various catalytic processes of practical significance.

Recently, as electrocatalysts, high-entropy alloys (HEAs) have attracted extensive research interest. In this study, a PdCuMoNiCo HEA with a nano-hollow spherical structure on RGO₃-CNTs (PdCuMoNiCo NHSs/RGO₃-CNT) was synthesized via a facile one-pot solvothermal method as a superior bi-functional electrocatalyst for formic acid oxidation (FAO) and oxygen reduction reaction (ORR). The advantage of the developed catalyst, i.e., PdCuMoNiCo NHSs on carbon hybrid, was demonstrated by its comparable ORR activity, but much better long-term durability than that of the commercial 20% Pt/C and its highest FAO performance among the reported Pd-based catalysts. It was found that its catalytic activity and stability were mainly derived from the synergistic effect of its hollow structure and HEA effect, presenting a brilliant perspective for the fabrication of HEAs as efficient bi-functional electrocatalysts with high activity and good stability.