Use of carbon monoxide and cyanide to probe the active sites on nitrogen-doped carbon catalysts for oxygen reduction (original) (raw)
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Metal-free carbon electrodes with well-defined composition and smooth topography were prepared via sputter deposition followed by thermal treatment with inert and reactive gases. XPS and Raman spectroscopies show that three carbons of similar N/C content that differ in Nsite composition were thus prepared: an electrode consisting of almost exclusively graphitic-N (NG), an electrode with predominantly pyridinic-N (NP) and one with ca. 1:1 NG:NP composition. These materials were used as model systems to investigate activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity towards 4e-reduction of O2 is strongly influenced by the NG/NP site composition, with the material possessing nearly uniform NG/NP composition being the only one yielding a 4e-reduction. Computational studies on model graphene clusters were carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure NGdoping or NP-doping of model graphene clusters, adsorption of hydroperoxide and hydroperoxyl radical intermediates, respectively, is weak thus favoring desorption prior to complete 4e-reduction to hydroxide. Clusters with mixed NG/NP sites display synergistic effects, suggesting that co-presence of these sites improves activity and selectivity by achieving high theoretical reduction potentials while facilitating retention of intermediates.
Nature communications, 2017
Single-atom catalysts with full utilization of metal centers can bridge the gap between molecular and solid-state catalysis. Metal-nitrogen-carbon materials prepared via pyrolysis are promising single-atom catalysts but often also comprise metallic particles. Here, we pyrolytically synthesize a Co-N-C material only comprising atomically dispersed cobalt ions and identify with X-ray absorption spectroscopy, magnetic susceptibility measurements and density functional theory the structure and electronic state of three porphyrinic moieties, CoNC, CoNC and CoNC. The O electro-reduction and operando X-ray absorption response are measured in acidic medium on Co-N-C and compared to those of a Fe-N-C catalyst prepared similarly. We show that cobalt moieties are unmodified from 0.0 to 1.0 V versus a reversible hydrogen electrode, while Fe-based moieties experience structural and electronic-state changes. On the basis of density functional theory analysis and established relationships between ...
Carbon
Metal-free nitrogenated amorphous carbon electrodes were synthesised via dc plasma magnetron sputtering and post-deposition annealing at different temperatures. The electrocatalytic activity of the electrodes towards the oxygen reduction reaction (ORR) was studied as a function of pH using cyclic voltammetry with a rotating disk electrode. The trends in onset potential were correlated to the carbon nanostructure and chemical composition of the electrodes as determined via Raman spectroscopy and X-ray photoelectron spectroscopy analysis. Results suggest that: 1) the ORR activity in acidic conditions is strongly correlated to the concentration of pyridinic nitrogen sites. 2) At high pH, the presence of graphitic nitrogen sites and a graphitized carbon scaffold are the strongest predictors of high ORR onsets, while pyridinic nitrogen site density does not correlate to ORR activity. An inversion region where pyridine-mediated activity competes with graphitic-N mediated activity is identified in the pH region close to the value of pK a of the pyridinium cation. The onset of the ORR is therefore determined by the activity of different sites as a function of pH and evidence for distinct reduction reaction pathways emerges from these results.
Nature communications, 2017
Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M-N x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe-N-C and especially Ni-N-C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M-N x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity...
Electrochemical Science Advances, 2024
Oxygen reduction reaction (ORR) is key in many green energy conversion devices like fuel cells and metal-air batteries. Developing cheap and robust electrocatalysts is crucial to expedite the slow ORR kinetics at the cathode. Lately, transition metal (TM) and heteroatom-doped carbon catalysts have surfaced as promising cathode materials for ORR as they display admirable electrocatalytic activity and distinguished properties like tunable morphology, structure, composition and porosity. This review summarizes the recent breakthrough in TM (Fe, Co, Mn and Ni) and heteroatoms (N, S, B, P and F) doping in carbon materials. Moreover, their ORR activity and active sites are inspected for future augmentation in making ORR catalysts for electrochemical devices. The existing challenges and prospects in this field are ratiocinated in conclusion.
Inorganic Chemistry, 2020
The rational design of electronically tuned transition-metal-doped conductive carbon nanostructures has emerged as a potential substitution of a platinum-group-metal (PGM)-free electrocatalyst for oxygen reduction reaction (ORR). We report here a universal strategy using a one-step thermal polymerization reaction for transition-metal-doped graphitic carbon nitride (g-C 3 N 4) without any conductive carbon support as a highly efficient ORR electrocatalyst. X-ray absorption spectroscopy evidences the presence of Fe−N x active sites with a possible three-coordinated Fe atom with N atoms. The as-prepared Fe-g-C 3 N 4 with improved surface area, graphitic nature, and conductive carbon framework exhibits a superior electrochemical performance toward ORR activity in an alkaline medium. Interestingly, it displays a 0.88 V (vs reversible hydrogen electrode, RHE) half-wave potential (E 1/2) with a four-electron-transfer pathway and excellent stability outperforming platinum/carbon (Pt/C) in an alkaline medium. More impressively, when the Fe-g-C 3 N 4 catalyst is used as a cathode material in a zinc−air battery, it presents a higher peak power density (148 mW cm −2) than Pt/C (133 mW cm −2), which further established the importance of the low-cost material synthesis approach toward the development of an earth-abundant PGM-free catalyst for fuel-cell and air battery fabrication.
Journal of Colloid and Interface Science, 2019
Nitrogen-doped hierarchical porous carbon (CN x-Co) samples embedded with cobalt nanoparticles are selectively prepared with polyethylenimine (PEI) as both the carbon and nitrogen sources. By processing at different temperature, CN x-Co-800 and CN x-Co-1000 are selectively prepared and the materials exhibit excellent electrocatalytic activity in the oxygen reduction reaction (ORR). The ORR measurements show that sample processed at the higher temperature delivers better performance due to the larger Co and graphitic nitrogen concentrations. CN x-Co-1000 also shows more tolerance against methanol crossover and outstanding durability towards ORR, making it a promising Pt-free electrocatalyst for ORR under alkaline conditions. The method demonstrated here is a general strategy to prepare other metal or metal alloy/porous carbon hybrid materials.
Carbon
Knowledge of the carbon active sites for Oxygen Reduction Reaction (ORR) remains confusing and controversial and thus the detailed mechanism is still not clarified. Considering the nature of the carbon-oxygen interaction in active sites during the gasification reaction, it could be inferred that the sites for this reaction are the same as those participating in the ORR. Herein, the relationship between carbon-oxygen gasification properties and ORR activities was elucidated. Carbon materials including different structures and porosities were selected and extensively characterized in terms of structural and electrochemical properties. Regarding gasification properties, active surface area (ASA) and reactivity for carbon-oxygen reaction were determined. A good linear correlation is found between ORR activity and carbon-oxygen gasification reactivity. Interestingly, similar correlation is found between ORR activity or gasification reactivity and ASA, although two different slopes are observed for the analyzed samples, being higher for the carbon nanotubes (CNT) based samples. The results suggest that the active sites participating in the gasification reaction can be catalytic sites for ORR although the specific catalytic activity is determined by the carbon structure. Thus, active sites in CNT show higher activities toward ORR and carbon gasification reactivities than others.
Metal-free doped carbon materials as electrocatalysts for the oxygen reduction reaction
J. Mater. Chem. A, 2014
Carbon materials such as graphite, graphene, carbon nanotubes and ordered mesoporous carbon have attracted a lot of attention for their use in fuel cells, due to beneficial properties like high conductivity, high mechanical and chemical stability and, for the latter, high surface area. Doping these materials with nitrogen or, less commonly, other elements alters their (electronic) properties, making them particularly suitable for application as electrocatalysts for the oxygen reduction reaction (ORR) in a fuel cell. This paper reviews the synthesis methods employed for the doping of these different types of carbon materials with various elements and the characterization techniques used to investigate their physicochemical properties such as degree of graphitization, dopant content, dopant configuration and surface area. Furthermore, their application as electrocatalysts for the oxygen reduction in a fuel cell is reviewed. Finally, the possible mechanisms for the ORR on N-doped carbon materials are critically discussed and compared to the mechanisms of commercial Pt/C electrocatalysts.