http://pubs.acs.org/journal/aelccp
Active Sites on Heterogeneous Single-IronAtom Electrocatalysts in CO2 Reduction Reaction Xueping Qin,†,§ Shangqian Zhu,†,§ Fei Xiao,†,§ Lulu Zhang,† and Minhua Shao*,†,‡ Department of Chemical and Biological Engineering and ‡Energy Institute, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
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S Supporting Information *
ABSTRACT: Nitrogen-coordinated single-metal-atom catalysts (Me−N−C) are promising candidates for CO2-to-CO electrocatalytic conversion. The nature of real active sites in this type of electrocatalyst, however, is not clear. In this Letter, we study the specific interactions between the reaction intermediates and a model single-iron-atom catalyst (Fe−N−C) by combining in situ infrared absorption spectroscopy and density functional theory (DFT) calculations. For the first time, we confirm that the Fe centers in Fe−N4 moieties hosted by the complete graphitic layer are poisoned by strongly adsorbed CO and should not be the real active sites for gaseous CO production. Further DFT calculation results suggest that the high CO selectivity and reaction rate may originate from Fe−N4 moieties embedded in a defective graphitic layer that have balanced binding energies of adsorbed COOH and CO species. These findings add significant new insights into the mechanisms of CO2 reduction on carbon-based single-atom electrocatalysts.
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fundamental understanding is critical in developing strategies to further improve the performance of Me−N−C catalysts. Herein, a high-quality single Fe atom catalyst (Fe−N−C) was used as the model surface to study the site-specific reaction mechanisms of the CO2RR. The detailed synthesis protocols and characterizations of the Fe−N−C19 and N-doped carbon (N−C)20 materials can be found in our previous studies. In brief, a zeolitic imidazolate framework (ZIF-8) was employed as a self-template to obtain the Fe−N−C catalyst consisting of uniformly dispersed Fe single atoms. N−C was synthesized by the pyrolysis of polyaniline. The presence of abundant Fe single atoms in the Fe−N−C can be clearly identified using scanning transmission electron microscope high-angle annular dark-field imaging (STEM-HAADF, Figure S1).19 X-ray absorption spectroscopic characterizations confirmed that the Fe center was coordinated with four N atoms (Figure S2).19 The CO2RR product selectivity and current density of the catalysts were measured in CO2-saturated 0.5 M KHCO3 solutions. As shown in Figure 1 (solid lines), the cathodic current appeared at −0.2 V versus the reversible hydrogen electrode (RHE) and quickly increased at potentials lower than −0.3 V on Fe−N−C. In contrast, no obvious current can be
he electrochemical reduction of CO2 into various chemicals and fuels provides an attractive option for storing renewable energy and mitigating the increase in CO2 concentration.1−3 Among the CO2 reduction reaction (CO2RR) products, CO features relatively fast reaction kinetics, a high faradaic efficiency (FE), and compatibility with the traditional chemical industry4,5 and hence is worthy of efforts to scale up from laboratory-level demonstrations to practical applications. One key hurdle in this transition lies in the development of advanced electrocatalysts, which should effectively suppress the competing hydrogen evolution while sustaining the fast generation of CO. Single metal atoms embedded in the N-doped carbon matrix (Me−N−C) have been studied in various electrocatalytical reactions.6,7 In the past few years, it has also emerged as a promising candidate for CO2-toCO electrocatalytic conversion with a high CO selectivity.8−13 Theoretical calculations suggested that the metal centers were active sites for CO2 adsorption and reduction;14 however, the experimental validation of this hypothesis has not been achieved. As compared with Me−N moiety-containing homogeneous CO2RR counterparts (e.g., metal-complexed molecules15), the inhomogeneity of Me−N sites16,17 and the consequential complicated specific interactions between the reaction intermediates and the catalyst surfaces8,18 bring more significant challenges in directly probing the reaction mechanisms on heterogeneous Me−N−C catalysts. This © 2019 American Chemical Society
Received: May 10, 2019 Accepted: June 28, 2019 Published: June 28, 2019 1778
DOI: 10.1021/acsenergylett.9b01015 ACS Energy Lett. 2019, 4, 1778−1783
Letter
Cite This: ACS Energy Lett. 2019, 4, 1778−1783
Letter
ACS Energy Letters
detected for N−C until −0.7 V, indicating that the N−C catalyst was almost inactive for either the CO2RR or the hydrogen evolution reaction. The difference in the FE of CO was also significant on these catalysts (dashed lines in Figure 1). CO started to be detected at −0.2 V on Fe−N−C, that is, an overpotential of only 90 mV, with an FE of ∼20%. A high CO FE (>83%) can be maintained between −0.3 and −0.6 V, with a maximum value of 93.5% at −0.5 V. To the best of our knowledge, the overall performance (overpotential and CO FE) of this catalyst is among the best of all of the Fe−N−C materials reported.14,18,21−25 The presence of metal centers was found to be critical to the selective generation of CO because the CO FE was always
