The effect of porous structures on the electrocatalytic activity of N-doped carbon is studied by using electrochemical analysis techniques and the result is applied to synthesize highly active and ...stable Fe–N–C catalyst for oxygen reduction reaction (ORR). We developed synthetic procedures to prepare three types of N-doped carbon model catalysts that are designed for systematic comparison of the porous structures. The difference in their catalytic activity is investigated in relation to the surface area and the electrochemical parameters. We found that macro- and mesoporous structures contribute to different stages of the reaction kinetics. The catalytic activity is further enhanced by loading the optimized amount of Fe to prepare Fe–N–C catalyst. In both N-doped carbon and Fe–N–C catalysts, the hierarchical porous structure improved electrocatalytic performance in acidic and alkaline media. The optimized catalyst exhibits one of the best ORR performance in alkaline medium with excellent long-term stability in anion exchange membrane fuel cell and accelerated durability test. Our study establishes a basis for rationale design of the porous carbon structure for electrocatalytic applications.
Demand on the practical synthetic approach to the high performance electrocatalyst is rapidly increasing for fuel cell commercialization. Here we present a synthesis of highly durable and active ...intermetallic ordered face-centered tetragonal (fct)-PtFe nanoparticles (NPs) coated with a “dual purpose” N-doped carbon shell. Ordered fct-PtFe NPs with the size of only a few nanometers are obtained by thermal annealing of polydopamine-coated PtFe NPs, and the N-doped carbon shell that is in situ formed from dopamine coating could effectively prevent the coalescence of NPs. This carbon shell also protects the NPs from detachment and agglomeration as well as dissolution throughout the harsh fuel cell operating conditions. By controlling the thickness of the shell below 1 nm, we achieved excellent protection of the NPs as well as high catalytic activity, as the thin carbon shell is highly permeable for the reactant molecules. Our ordered fct-PtFe/C nanocatalyst coated with an N-doped carbon shell shows 11.4 times-higher mass activity and 10.5 times-higher specific activity than commercial Pt/C catalyst. Moreover, we accomplished the long-term stability in membrane electrode assembly (MEA) for 100 h without significant activity loss. From in situ XANES, EDS, and first-principles calculations, we confirmed that an ordered fct-PtFe structure is critical for the long-term stability of our nanocatalyst. This strategy utilizing an N-doped carbon shell for obtaining a small ordered-fct PtFe nanocatalyst as well as protecting the catalyst during fuel cell cycling is expected to open a new simple and effective route for the commercialization of fuel cells.
Abstract
Utilization of carbon dioxide (CO
2
) molecules leads to increased interest in the sustainable synthesis of methane (CH
4
) or methanol (CH
3
OH). The representative reaction intermediate ...consisting of a carbonyl or formate group determines yields of the fuel source during catalytic reactions. However, their selective initial surface reaction processes have been assumed without a fundamental understanding at the molecular level. Here, we report direct observations of spontaneous CO
2
dissociation over the model rhodium (Rh) catalyst at 0.1 mbar CO
2
. The linear geometry of CO
2
gas molecules turns into a chemically active bent-structure at the interface, which allows non-uniform charge transfers between chemisorbed CO
2
and surface Rh atoms. By combining scanning tunneling microscopy, X-ray photoelectron spectroscopy at near-ambient pressure, and computational calculations, we reveal strong evidence for chemical bond cleavage of O‒CO* with ordered intermediates structure formation of (2 × 2)-CO on an atomically flat Rh(111) surface at room temperature.
In situ analysis for CO oxidation on Pd surfaces under near ambient pressure conditions with XPS and mass spectroscopy reveals catalytically active phases.
•Recent studies on CO oxidation on Pd ...surfaces with NAP-XPS were reviewed.•Catalytically active phases on the Pd surfaces were characterized.•All the catalytically active phases include surface-segregated high-density oxygen.•Reaction kinetics on the Pd surfaces under NAP and UHV conditions were compared.
X-ray photoelectron spectroscopy (XPS) is useful for in situ analysis of catalytically active surfaces under reaction conditions. We conducted in situ observations of CO oxidation on Pd single-crystal surfaces with different orientations under near ambient pressure (NAP) conditions with XPS. In this short review we compare catalytically active surfaces of Pd with the different orientations. Both surface oxides and atomic oxygen chemisorbed on metallic surfaces can be active for CO oxidation depending on surface geometric structure. We propose similarity between the surface oxides and the chemisorbed oxygen from a viewpoint of availability of active oxygen. Catalytic activity of the metallic surface under NAP condition is also compared with that under ultrahigh vacuum conditions.
The CO oxidation reaction on the Pd(111) model catalyst at various temperatures (200–400 °C) under hundreds mTorr pressure conditions has been monitored by in situ ambient pressure X-ray ...photoelectron spectroscopy and mass spectroscopy. In situ observation of the reaction revealed that the Pd(111) surface is covered by CO molecules at a lower temperature (200 °C), while at higher temperatures (300–400 °C) several oxide phases are formed on the surface. We found that the reactivity is enhanced in the presence of a surface oxide and significantly suppressed by formation of a cluster oxide and the PdO bulk oxide. CO titration experiments suggest that less coordinated oxygen atoms are more reactive for CO oxidation.
The slow rate of the oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cell (PEMFC) is the main limitation for automotive applications. We demonstrated that the Pt₃Ni(111) ...surface is 10-fold more active for the ORR than the corresponding Pt(111) surface and 90-fold more active than the current state-of-the-art Pt/C catalysts for PEMFC. The Pt₃Ni(111) surface has an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. Under operating conditions relevant to fuel cells, its near-surface layer exhibits a highly structured compositional oscillation in the outermost and third layers, which are Pt-rich, and in the second atomic layer, which is Ni-rich. The weak interaction between the Pt surface atoms and nonreactive oxygenated species increases the number of active sites for O₂ adsorption.
Abstract
Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO
2
) activation. Despite extensive reactivity benchmarks, a correlation between CO
2
...conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO
2
(g). The CO
2
dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO
2
conversion to renewable energy sources during C
1
chemical reactions.
Nitrogen–carbon (N–C) species is a potential electrocatalyst for oxygen reduction reaction (ORR) in electrochemical energy conversion cells, but its mechanistic origin of ORR on the N–C surface is ...still unclear. We show our facile approach to the synthesis of highly active Co-modified N–C catalyst and investigated the origin of ORR activity of electrospun N–C species by removing the metal with hydroxide carbon etching and acid metal leaching. Through the detailed investigation on the origin of ORR electrocatalysis for electrospun N–C nanofibers, we revealed that pyrrolic-N and highly graphitized carbon structure are mainly responsible for the enhanced ORR activity of metal-free N–C nanofiber and embedded Co metal got involved in the creation of the pyrrolic N site.
One of the key objectives in fuel-cell technology is to improve and reduce Pt loading as the oxygen-reduction catalyst. Here, we show a fundamental relationship in electrocatalytic trends on Pt(3)M ...(M=Ni, Co, Fe, Ti, V) surfaces between the experimentally determined surface electronic structure (the d-band centre) and activity for the oxygen-reduction reaction. This relationship exhibits 'volcano-type' behaviour, where the maximum catalytic activity is governed by a balance between adsorption energies of reactive intermediates and surface coverage by spectator (blocking) species. The electrocatalytic trends established for extended surfaces are used to explain the activity pattern of Pt(3)M nanocatalysts as well as to provide a fundamental basis for the catalytic enhancement of cathode catalysts. By combining simulations with experiments in the quest for surfaces with desired activity, an advanced concept in nanoscale catalyst engineering has been developed.
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