Carbon-Based Metal-Free Electrocatalysts: Past, Present, and Future

Abstract

ConspectusDue to the overuse of fossil fuels, various detrimental effects along with the excess CO2 emissions have induced global warming and sea-level rising. To tackle climate change and provide a cleaner environment for the air we breathe and water we consume, the existing energy mix needs to be changed into fossil-free, clean, renewable energy with zero emission (e.g., fuel cells). While providing a promising and scalable strategy to the energy and environmental challenges, renewable energy processes often involve noble-metal-based catalysts (i.e., Pt, RuO2). However, the disadvantages of noble-metal-based catalysts, including their high cost and scarcity, have hampered the large-scale application of renewable energy technologies. In 2009, we discovered earth-abundant carbon materials functioning as efficient low-cost, carbon-based metal-free electrocatalysts (C-MFECs) attractive for renewable energy and environmental remediation. Since then, C-MFECs have become an emerging new research field over the world. They are demonstrated to be efficient multifunctional catalysts for various key reactions important to renewable energy and environmental technologies, including oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), CO2 reduction reaction (CO2RR), and N2 reduction reaction (NRR), to name a few. Charge transfer/redistribution induced by heteroatom (e.g., N) and/or defect doping was recognized as the driving force for the metal-free catalytic activities. This finding has been used as a guidance to design and develop various new and multifunctional C-MFECs for many reactions even beyond the renewable energy and environmental remediation. In this Account, we first summarize our previous work on the development and mechanistic understanding of C-MFECs for ORR, HER, and OER to promote renewable energy conversion and storage. Then, we present recent advances in C-MFECs for new important reactions for environment remediation (e.g., CO2RR, NRR), seawater splitting, and metal–CO2 batteries. However, different dopant locations for C-MFECs even with the same doping element and content can cause variable catalytic properties for heteroatom-doped carbon materials. Therefore, vast opportunities remain for further developing numerous innovative C-MFECs with defined structures to gain a better understanding of their structure-based properties. In this context, we finally conclude with the current challenges and future perspectives in this exciting field.