Photoelectrochemical Hydrogen Production

The possibilities for using CaNb2O6 as a photocatalyst in direct water splitting have been evaluated by investigating the electronic structure of the material. In addition the oxide was doped with nitrogen in order to modify the electronic structure and obtain visible light absorption. Experimental techniques such as electrochemical impedance spectroscopy (EIS), photocurrent, and diffuse reflectance spectroscopy (DRS) were combined with theoretical approaches to determine the bandgap, flatband potential and quasi-Fermi levels of the photocatalyst. CaNb2O6 was prepared by a sol-gel synthesis and doped with nitrogen by heat treatment of the oxide powder in an ammonia atmosphere. X-ray diffraction (XRD) confirmed phase pure orthorhombic CaNb2O6 for both pure and N-doped oxide and excluded a possible transformation of the oxide into an oxynitride. Upon illumination anodic photocurrents were observed implying that CaNb2O6 was an n-type semiconductor due to oxygen vacancies in the lattice. From the wavelength dependency of the photocurrent a direct bandgap of 3.7eV and an indirect bandgap of 3.4eV were determined for undoped CaNb2O6. Doping with nitrogen altered the optical properties of the oxide and shifted the absorption edge into the visible light region. Calculations using the density functional theory (DFT) attributed the change in absorption properties to the formation of narrow energy bands above the valence band of pure CaNb2O6. An alternative explanation could be a hybridization of N 2p and O 2p bands. Correspondingly a reduction of the bandgaps for N-doped CaNb2O6 with respect to the undoped oxide was identified. Impedance was applied to determine the flatband potential of CaNb2O6 from Mott-Schottky plots. However the obtained results seemed to be dominated by contributions from the electrode substrate. Theoretical investigations concluded that pinhole-free oxide layers creating an ohmic contact with the substrate are required in order to designate the observed impedance response to the space charge capacitance. Quasi-Fermi level measurements indicated a low photocatalytic activity of CaNb2O6 as no photocurrent could be detected. Further investigations are needed to identify the cause of the photocurrent limitations. Nevertheless probable explanations could be low conductivity in CaNb2O6, high concentrations of recombination centers or slow charge transfer kinetics. The latter was confirmed for porous oxide layers as the addition of a hole scavenger increased the measured photocurrent. Positive photocurrent transients were also observed for porous CaNb2O6 films and could be related to either the diffusion of electrons through the porous oxide layer or to a photoanodic decomposition of the photocatalyst

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 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

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

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

Enhancing capacitance of supercapacitor with both organic electrolyte and ionic liquid electrolyte on a biomass-derived carbon

Supercapacitor (SC) with organic electrolyte or ionic liquid (IL) electrolyte can generally store/release higher energy than that with an aqueous electrolyte, due to a larger operating voltage window of a non-aqueous electrolytes. A carbon is synthesized by a facile impregnate-activation method from renewable woody biomass, which has twice of the specific surface area and pore volume than the sample synthesized by conventional KOH activation. Biomass-derived carbons with high ion accessible surface area and highly integrated micropores and mesopores provide superior capacitance, excellent rate capability and good stability in both organic electrolyte and IL electrolytes. Significant enhancement in the capacitance and rate capability were obtained by the generation of micropores similar to the ion size and better pore network through removal of impurities in the biomass. High specific capacitances of 146 F g−1 in the organic electrolyte and 224 F g−1 in the IL electrolyte at current density of 0.1 A g−1 are achieved. Highly integrated micro- and mesoporous structure leads to a good rate capability of 100% capacitance retention at current density up to 10 A g−1 in the organic electrolyte and 67% capacitance retention at current density up to 7 A g−1 in the IL. With the large voltage offered by the non-aqueous electrolyte, the material can store/release high maximum energy of 26 W h kg−1 and 92 W h kg−1 in the organic electrolyte and IL electrolyte, respectively. It reveals that the biomass derived carbon is a promising and cost effective candidate for electrodes in high performance SCs