Electroreduction of Carbon Dioxide Driven by the Intrinsic Defects in the Carbon Plane of a Single Fe–N₄ Site

Abstract

various noble metal catalysts, such as Au, Ag, and Pd have demonstrated impressive potential for yielding CO through CO 2 RR. [2] Furthermore, Ir/Au [1a] and 3D Pd nanosheets [3] offer sufficient performances for Zn-CO 2 batteries. These electrocatalysts are, however, expensive and often require high overpotentials to generate desired products. In addition, the Faradaic efficiency (FE) for CO 2 RR in Zn-CO 2 batteries, particularly for CO 2-to-CO conversion, is typically below 50% at relatively high discharge currents (e.g., 5 mA cm −2). [4] As an alternative to noble metal catalysts, nitrogen-coordinated single-metal active sites anchored within porous carbon (M-N-C) have been recently identified as a new class of efficient CO 2 RR catalysts to enable CO 2-to-CO conversion, because of their abundance, high electrical conductivity, and good durability. [5] Among various M-N-C catalysts, Fe-N-C presents a low onset potential for CO production, whereas its CO Faradaic efficiency (FE CO) and partial current density (J CO) are not high, particularly for concentrated electrolytes, [6] owing to the strong binding of *CO on the single Fe-N x site. [7] Strategies such as modifying the morphology and pore structure of the carbon substrate, [6a,8] and adjusting the coordination structure as well as the local environment of the metal center, [4b,5f,9] were employed to enhance CO 2 RR performance. In addition to the single Fe-N x sites, nonmetal moieties in the carbon plane of M-N-C, such as N-doped sites and intrinsic defects, may additionally contribute to CO 2 RR (Figure S1, Supporting Information). Pyridinic and hydrogenated nitrogen species in Fe-N-C was demonstrated to exhibit preferential adsorption for CO 2 and acted as active sites for CO 2 RR. [5h,10] Moreover, the intrinsic carbon defects could also act as sole metal-free active sites for CO 2 RR, stemming from the electron redistribution around the defects, and consequently, form partially positive C atoms, as demonstrated very recently in certain studies. [11] For example, defectrich and metal-free mesoporous carbon materials exhibited enhanced CO generation from CO 2 RR, whereas the competing HER was suppressed. When an overpotential of 490 mV was applied, the FE CO was ≈80%, with a J CO of −2.9 mA cm −2. [11b] A positive correlation was also observed between the content of intrinsic carbon defects and the CO 2 RR performance of carbon-based catalysts. [11a] Despite these achievements, the J CO for the reported carbon catalysts containing intrinsic defects remained below 7 mA cm −2. [11] Considering that such catalysts Manipulating the in-plane defects of metal-nitrogen-carbon catalysts to regulate the electroreduction reaction of CO 2 (CO 2 RR) remains a challenging task. Here, it is demonstrated that the activity of the intrinsic carbon defects can be dramatically improved through coupling with single-atom Fe-N 4 sites. The resulting catalyst delivers a maximum CO Faradaic efficiency of 90% and a CO partial current density of 33 mA cm −2 in 0.1 m KHCO 3. The ...

Atomically Structural Regulations of Carbon‐Based Single‐Atom Catalysts for Electrochemical CO2Reduction

“…defects (DNG-Fe-N 4 ) for dramatically improving CO 2 RR activity (Figure 17). [231] They further experimentally verified the existence of intrinsic carbon defects by EPR and Raman. In contrast to the above results, they found that the CO 2 RR activity originates from carbon defects rather than traditional Fe-N 4 sites.…”

“…Reproduced with permission. [231] Copyright 2020, Wiley-VCH. could form the mixing-coordinated Ni-N 3 O structure within 2D carbon nanosheets at 500 °C, and further heating to 800 °C generated abundance carbon vacancies with Ni-N 3 .…”

“…f) Difference between the calculated limiting potentials for CO 2 RR and HER. Reproduced with permission [231]. Copyright 2020, Wiley-VCH.…”

From CO2 to Value-Added Products: A Review about Carbon-Based Materials for Electro-Chemical CO2 Conversion

“…Through different synthesis methodologies, graphitic carbon nitride (g-N 3 C 4 ) has been transformed as a raw material for obtaining carbonaceous matrices [129][130][131][132][133]. Li and co-workers [129] used topochemical transformation by carbon layer as a synthesis methodology, which allowed them to ensure the preservation in the structure of Ni-N 4 sites with a homogeneous distribution on the carbon sheets and avoided the agglomeration of Ni atoms to particles, thus allowing to obtain abundant active sites and, consequently, a maximum FE (CO) of 99% at −0.81 V, with a current density of 28.6 mA cm −2 .…”

“…The main achievement of this synthesis methodology is that it allows controlling the content of Ni single atoms, obtaining an optimum metal content that is crucial for high electrochemical performances in CO 2 electrochemical reduction [132]. Ni and co-authors [133] investigated the influence of intrinsic defects in the carbon plane on catalytic activity, however intrinsic defects in the carbon plane are formed during the pyrolysis process together with other actives sites such as M-Nx and doped nitrogen species, making it a challenge to exclusively study their effect. To do this, they designed two catalysts with two different pyrolysis temperatures (800 • C and 900 • C), using carbon-rich graphitic carbon nitride (C-g-C 3 N 4 ) that served as template and carbon resource during the pyrolysis process.…”