The development of carbon-based catalysts for selective electrochemical nitrogen-to-ammonia conversion

Abstract

<p indent=0mm>Ammonia (NH₃) has been widely utilized as fertilizer in agriculture and synthetic building blocks for a couple of industrial products. It also works as a caron-free fuel in internal combustion engines. Therefore, NH₃ is of considerable significance for modern human community with over 160 million tons synthesized every year. However, NH₃ synthesis was commonly carried out by the energy-intensive and fossil fuel-dependent Haber-Bosch process where the high-cost of clean and pure H₂ produced from fossil fuel resources and huge energy inputs are needed. In addition, only 10%−15% NH₃ conversion efficiency can be achieved by single reaction even at the pressures as high as <sc>40 MPa.</sc> The harsh reaction conditions and relatively low conversion efficiency as well as the valuable and fossil fuel-dependent reactant of H₂ cause the Haber-Bosch process to be currently one of the largest global energy consumers and greenhouse gas emitters, contributed to 1.2% of the global anthropogenic CO₂ emissions. In this regard, it is of great importance to develop alternative sustainable approaches for NH₃ synthesis with cheap and abundant H resources (e.g., H₂O) under mild conditions. Electrochemical nitrogen reduction reaction (NRR) to NH₃ under mild conditions powered by renewable electricity is a potential alternative to the energy-intensive and fossil fuel-dependent Haber-Bosch process. The intrinsic inertness of N₂ molecule and competition of hydrogen evolution reaction (HER) in aqueous solutions are the major challenges for electrochemical NRR. In order to overcome obstacles and facilitate this sluggish reaction, extensive attentions have been given to explore efficient and robust electrocatalysts. Although transition metal-based catalysts can solve the kinetic limitation of N≡N activation through the π-back donation process, the d-orbital electrons in transition metal atoms also favor the formation of metal-H bond and therefore boost the undesired HER side reaction. Carbon-based materials featuring the tunable electronic structure, controllable electronegativity of carbon frameworks, and easy-making defects have significant potentials in catalyzing NRR. In fact, the strategies including doping with heteroatoms, creating defects, and surface functionalization have been widely used to improve the electrocatalytic NRR activity of carbon-based electrocatalysts in the past few years. However, the development in this topic is limited to the following issues: (1) The current research on carbon-based NRR electrocatalysts is case-by-case and lacks comprehensive and systematical talking; (2) the structure/composition-performance relationships are yet well understood and regularities in catalyst design are still not established. Therefore, a review on state-of-the-art carbon-based materials for NRR is urgently needed to provide coverage of recent advances in both theoretical and experimental aspects. Here, we review the state-of-the-art research advancements in carbon-based materials for electrochemical NRR under ambient, in the perspective of different strategies for efficient catalyst design, including heteroatom doping, edge site and topological defects, metal single atoms coordinated with nitrogen or other heteroatoms in carbon matrix. More importantly, the intrinsic relationships between the structures/configurations of the defective carbons and the NRR performance are discussed in detail, both theoretically and experimentally. After that, a summary is finally provided together with personal perspectives on critical challenges and future possible trends in the field. This review will provide an overall and prompt understanding on the recent development of carbon-based materials for NRR, which will shed new light on the advancement of this prosperous field.