The development of catalysts for electrochemical reduction of carbon dioxide (eCO 2 RR) with high activity and selectivity remains a grand challenge to render the technology useable. As promising candidates, metal−nitrogen−carbon (MNC) catalysts with metal atoms present as atomically dispersed metal−N x moieties (MN x , M = Mn, Fe, Co, Ni, and Cu) were investigated as model catalysts. The distinct activity for CO formation observed along the series of catalysts is attributed to the nature of the transition metal in MN x moieties because of otherwise similar composition, structure, and morphology of the carbon matrix. We identify a volcano trend between their activity toward CO formation and the nature of the transition metal in MN x sites, with Fe and/or Co at the top of the volcano, depending on the electrochemical potential. Regarding selectivity, FeNC, NiNC, and MnNC had Faradaic efficiency for CO >80%. To correctly model the active sites in operando conditions, experimental operando X-ray absorption near edge structure spectroscopy was performed to follow changes in the metal oxidation state with electrochemical potential. Co and Mn did not change the oxidation state with potential, whereas Fe and Ni were partially reduced and Cu largely reduced to Cu(0). Computational models then led to the identification of M 2+ N 4 −H 2 O as the most active centers in FeNC and CoNC, whereas Ni 1+ N 4 was predicted as the most active one in NiNC at the considered potentials of −0.5 and −0.6 V versus the reversible hydrogen electrode. The experimental activity and selectivity could be rationalized from our density functional theory results, identifying in particular the difference between the binding energies for CO 2 * − and H* as a descriptor of selectivity toward CO. This indepth understanding of the activity and selectivity based on the speciation of the metals for eCO 2 RR over atomically dispersed MN x sites provides guidelines for the rational design of MNC catalysts toward eCO 2 RR for their application in highperformance devices.
At present, the synthesis of ammonia through the Haber−Bosch (HB) process accounts for 1.2% of the global carbon emissions, representing roughly one-fourth of the global fossil consumption from the chemical industry, which creates a pressing need for alternative low-carbon synthesis routes. Analyzing seven essential planetary boundaries (PBs) for the safe operation of our planet, we find that the standard HB process is unsustainable as it vastly transgresses the climate change PB. In order to identify more responsible strategies from this integrated perspective, we assess the absolute sustainability level of 34 alternative routes where hydrogen (H 2 ) is supplied by steam methane reforming with carbon capture and storage, biomass gasification, or water electrolysis powered by various energy sources. We found that some of these scenarios could substantially reduce the global impact of fossil HB, yet alleviating the impact on climate change could critically exacerbate the impacts on other Earth-system processes. Furthermore, we identify that reducing the cost of electrolytic H 2 is the main avenue toward the economic appeal of the most sustainable routes. Our work highlights the need to embrace global impacts beyond climate change in the assessment of decarbonization routes of fossil chemicals. This approach enabled us to identify more suitable alternatives and associated challenges toward environmental and economically attractive ammonia synthesis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.