Active sites for the oxygen reduction reaction in nitrogen-doped carbon nanofibers

Understanding the role of iron and the nature of the active sites in nitrogen-doped carbon nanomaterials is vital for their future application as oxygen reduction electrocatalysts in fuel cells. In this paper, porphyrin-like Fe-N4 sites have been identified in nitrogen-doped carbon nanofibers (N-CNFs) grown from iron nanoparticles by chemical vapor deposition (CVD). Acid treatment of the N-CNFs removed the iron growth particles and about 50% of the nitrogen groups from the pristine N-CNFs, without affecting the oxygen reduction performance. Performing electron energy loss spectroscopy (EELS) on the acid treated and annealed N-CNFs confirmed that the CVD synthesis method leads to iron being atomically incorporated into the N-CNF structure. Furthermore, X-ray absorption near-edge structure (XANES) analysis of the Fe K-edge indicates that the iron atoms are stabilized by four nitrogen atoms, reminiscent of the Fe-N4 structure found in porphyrins. An evolution of the XANES spectrum was observed when performing the measurements under mildly reducing conditions, which was explained by oxygen being adsorbed on the Fe-N4 sites at room temperature. The Fe-N4 moieties embedded in the N-CNFs were resistant to acid leaching and the results suggest that these Fe-N4 sites are active sites for the oxygen reduction in N-CNFs.publishedVersio

Active sites for the oxygen reduction reaction in nitrogen-doped carbon nanofibers

Understanding the role of iron and the nature of the active sites in nitrogen-doped carbon nanomaterials is vital for their future application as oxygen reduction electrocatalysts in fuel cells. In this paper, porphyrin-like Fe-N4 sites have been identified in nitrogen-doped carbon nanofibers (N-CNFs) grown from iron nanoparticles by chemical vapor deposition (CVD). Acid treatment of the N-CNFs removed the iron growth particles and about 50% of the nitrogen groups from the pristine N-CNFs, without affecting the oxygen reduction performance. Performing electron energy loss spectroscopy (EELS) on the acid treated and annealed N-CNFs confirmed that the CVD synthesis method leads to iron being atomically incorporated into the N-CNF structure. Furthermore, X-ray absorption near-edge structure (XANES) analysis of the Fe K-edge indicates that the iron atoms are stabilized by four nitrogen atoms, reminiscent of the Fe-N4 structure found in porphyrins. An evolution of the XANES spectrum was observed when performing the measurements under mildly reducing conditions, which was explained by oxygen being adsorbed on the Fe-N4 sites at room temperature. The Fe-N4 moieties embedded in the N-CNFs were resistant to acid leaching and the results suggest that these Fe-N4 sites are active sites for the oxygen reduction in N-CNFs.publishedVersio

Evaluation of ORR active sites in nitrogen-doped carbon nanofibers by KOH post treatment

Oxygen reduction on N-doped carbon nanomaterials is believed to take place at either N-centered active sites (C-Nx) or Fe-centered active sites (Fe-Nx). In this work the origin of the oxygen reduction on nitrogen-doped carbon nanofibers (N-CNFs) is investigated by removing nitrogen and iron from the N-CNF surface using high temperature KOH treatment. The activities for the oxygen reduction reaction (ORR) in 0.5 M H2SO4 are correlated with the XPS results and discussed with respect to the contribution from C-Nx and Fe-Nx active sites. Increasing the time and temperature of the KOH treatment decreased the iron and nitrogen content at the N-CNF surface. The contribution from Fe-Nx active sites was found to be minor compared to the C-Nx active sites as the KOH-treated N-CNFs with no iron in the surface still showed considerable ORR activity. Furthermore, the activity was maintained when the fraction of pyridinic-N was greatly reduced compared to quaternary-N. Finally, even when no iron or nitrogen could be detected by XPS, 50% of the initial oxygen reduction activity of the N-CNFs persisted. It is therefore suggested that there are active sites not originating from iron or nitrogen atoms, but rather from a distinct carbon environment

Nitrogen-doped carbon nanofibers on expanded graphite as oxygen reduction electrocatalysts

A single-step chemical vapor deposition method using simple gaseous precursors was employed to grow nitrogen-doped carbon nanofibers from Fe and Ni particles on the surface of expanded graphite (N-CNF/EG). Due to the high electronic conductivity of the expanded graphite the N-CNF/EG could be used as electrocatalysts without the need for harsh purification procedures. Electrochemical testing showed that the N-CNFs grown from Fe exhibited a notable activity for the oxygen reduction in both acidic and alkaline electrolyte, in addition to demonstrating a high durability with a well-preserved catalytic activity after 1600 cycles in O2-saturated 0.5 M H2SO4. Physicochemical characterization revealed the formation of N-CNFs with a bamboo-like structure, encapsulated Fe particles and high pyridinic nitrogen content. The combination of high ORR-activity, an easily scalable synthesis approach and a highly conductive support material makes N-CNF/EG a promising oxygen reduction catalyst for low temperature fuel cells

Nitrogen-doped Carbon Nanofibers for the Oxygen Reduction Reaction: Importance of the Iron Growth Catalyst Phase

A systematic evaluation of the oxygen reduction reaction (ORR) on nitrogen-doped carbon nanofibers (N-CNFs) has been performed by tuning the properties of the N-CNFs by using chemical vapor deposition. Analysis of the as-synthesized N-CNFs shows that the iron used as the growth catalyst consists of iron carbides, including Fe7C3, χ-Fe5C2, and θ-Fe3C, depending on the carbon activity of the synthesis feed. Furthermore, a relationship between the growth catalyst phase, the N-CNF properties, and the electrocatalytic activity for the oxygen reduction in acidic electrolyte is revealed. The best catalytic activity and selectivity was achieved if the N-CNFs were grown from Hägg carbide, χ-Fe5C2, suggesting that this carbide phase favors the incorporation of active sites into the N-CNFs. Controlling the phase of the iron particles used as growth catalysts is therefore essential for obtaining N-CNFs with a high active site density for the oxygen reduction reaction

Active sites for the oxygen reduction reaction in nitrogen-doped carbon nanofibers

Understanding the role of iron and the nature of the active sites in nitrogen-doped carbon nanomaterials is vital for their future application as oxygen reduction electrocatalysts in fuel cells. In this paper, porphyrin-like Fe-N4 sites have been identified in nitrogen-doped carbon nanofibers (N-CNFs) grown from iron nanoparticles by chemical vapor deposition (CVD). Acid treatment of the N-CNFs removed the iron growth particles and about 50% of the nitrogen groups from the pristine N-CNFs, without affecting the oxygen reduction performance. Performing electron energy loss spectroscopy (EELS) on the acid treated and annealed N-CNFs confirmed that the CVD synthesis method leads to iron being atomically incorporated into the N-CNF structure. Furthermore, X-ray absorption near-edge structure (XANES) analysis of the Fe K-edge indicates that the iron atoms are stabilized by four nitrogen atoms, reminiscent of the Fe-N4 structure found in porphyrins. An evolution of the XANES spectrum was observed when performing the measurements under mildly reducing conditions, which was explained by oxygen being adsorbed on the Fe-N4 sites at room temperature. The Fe-N4 moieties embedded in the N-CNFs were resistant to acid leaching and the results suggest that these Fe-N4 sites are active sites for the oxygen reduction in N-CNFs

Active sites for the oxygen reduction reaction in nitrogen-doped carbon nanofibers

Understanding the role of iron and the nature of the active sites in nitrogen-doped carbon nanomaterials is vital for their future application as oxygen reduction electrocatalysts in fuel cells. In this paper, porphyrin-like Fe-N4 sites have been identified in nitrogen-doped carbon nanofibers (N-CNFs) grown from iron nanoparticles by chemical vapor deposition (CVD). Acid treatment of the N-CNFs removed the iron growth particles and about 50% of the nitrogen groups from the pristine N-CNFs, without affecting the oxygen reduction performance. Performing electron energy loss spectroscopy (EELS) on the acid treated and annealed N-CNFs confirmed that the CVD synthesis method leads to iron being atomically incorporated into the N-CNF structure. Furthermore, X-ray absorption near-edge structure (XANES) analysis of the Fe K-edge indicates that the iron atoms are stabilized by four nitrogen atoms, reminiscent of the Fe-N4 structure found in porphyrins. An evolution of the XANES spectrum was observed when performing the measurements under mildly reducing conditions, which was explained by oxygen being adsorbed on the Fe-N4 sites at room temperature. The Fe-N4 moieties embedded in the N-CNFs were resistant to acid leaching and the results suggest that these Fe-N4 sites are active sites for the oxygen reduction in N-CNFs

Nitrogen-doped carbon nanofiber catalyst for ORR in PEM fuel cell stack: Performance, durability and market application aspects

A noble metal-free catalyst based on N-doped carbon nanofibers supported on graphite (N-CNF1 ) was employed for the oxygen reduction at the cathode of a Nafion PEMFC with a commercial Pt/C anode. Obtained performance in pure H2 and O2 indicated the presence of significant mass-transport limitations when utilizing catalyst loadings between 1 and 10 mg cm-2 . Strategies to reduce the limitations were explored by optimization of the cathode ionomer content, catalyst loading and application technique. Pore-formers (Li2CO3, (NH4)2CO3 and polystyrene microspheres) were utilized to improve the mass-transport within the layer. A maximum of 72 mW cm-2 and 1400 A g-1 or 300 W g -1 at peak power was demonstrated. The catalyst was then applied to the cathode of a 10-cell fuel cell stack, and a 400-hour durability test was conducted. The average cell voltage decay amounted to 162 µV h -1 . Finally, a market application analysis was conducted by comparing the capital and operating costs of FC systems based on Pt/C and on N-CNF cathodes. While the cheap (3,32 € g-1 ) NCNF catalyst reduces the single MEA cost by almost a third, the total cost of ownership of an N-CNF based PEMFC system is still higher due to lower cell performance

Is the H2 economy realizable in the foreseeable future? Part II: H2 storage, transportation, and distribution

The goal of the review series on the H2 economy is to highlight the current status, major issues, and opportunities associated with H2 production, storage, transportation, distribution and usage in various energy sectors. In particular, Part I discussed the various H2 (grey and green) production methods including the futuristic ones such as photoelectrochemical for small, medium, and large-scale applications. Part II of the H2 economy review identifies the developments and challenges in the areas of H2 storage, transportation and distribution with national and international initiatives in the field, all of which suggest a pathway for establishing greener H2 society in the near future. Currently, various methods, comprising physical and chemical routes are being explored with a focus on improving the H2 storage density, capacity, and reducing the cost. H2 transportation methods by road, through pipelines, and via ocean are pursued actively in expanding the market for large scale applications around the world. As of now, compressed H2 and its transportation by road is the most realistic option for the transportation sector. Peer reviewe

Temperature dependent product distribution of electrochemical CO2 reduction on CoTPP/MWCNT Composite

| openaire: EC/H2020/722614/EU//ELCORELElectrochemical reduction of CO2 to valuable products on molecular catalysts draws attention due to their versatile structures allowing tuning of activity and selectivity. Here, we investigate temperature influence on CO2 conversion product selectivity over a Cobalt(II)-tetraphenyl porphyrin (CoTPP)/multiwalled carbon nanotube (MWCNT) composite in the range of 20-50℃. Faradaic efficiency of products changes with temperature and potential so that two-electron transfer product CO formation is enhanced at low potentials and temperatures while the competing hydrogen formation shows an opposite trend. Multi-electron transfer product methanol formation is more favorable at low temperatures and potentials whereas reverse applies for methane. Activity and selectivity are analyzed with DFT simulations identifying the key differences between the binding energies of CH2O and CHOH, the binding strength of CO, and the protonation of CHO intermediate. This novel experimental and theoretical understanding for CO2 reduction provides insight in the influence of the various conditions on the product distribution.Peer reviewe