•The density functional theory (DFT) calculation reveals that the doping of Ta is energetically favorable.•The doping defect acts as shallow donor to improve the conductivity of hematite by providing more electron carrier.•Mott-Schottky measurement confirms the increased shallow donor density and the energy level of hematite after doping with Ta.The improved photoactivity of Ta doped hematite, which was reported in our previous research, was studied by density functional theory (DFT) calculation and electrochemical measurement. The doping of Ta was calculated to produce slight changes in the local geometry of hematite crystal structure and have a low defect formation energy, indicating that the doping of Ta is energetically favorable and Ta impurity can be stably doped in hematite lattice to replace the Fe site (TaFe). The analysis of the electronic structure of Ta doped hematite indicates that the transition level of corresponding TaFe2+ defect (1.99 eV) lies below the conduction band minimum (CBM), meaning that the doping defect acts as shallow donor to provide more electron carrier, and thus improving the conductivity of hematite. The increased shallow donor density and the energy level induced by the doping of Ta were also confirmed by Mott-Schottky measurement.Download high-res image (80KB)Download full-size image

Carbon nanofibers (CNFs) with different content of surface functional groups which are carboxyl groups (CNF–OX), carbonyl groups (CNF–CO) and hydroxyl groups (CNF–OH) and nitrogen-containing groups (CNF–ON) are synthesized, and their electrocatalytic activities toward oxygen reduction reaction (ORR) in alkaline solution are investigated. The result of X-ray photoelectron spectroscopy (XPS) characterization indicates that a higher concentration of carboxyl groups, carbonyl groups and hydroxyl groups are imported onto the CNF–OX, CNF–CO and CNF–OH, respectively. Cyclic voltammetry shows that both the oxygen- and nitrogen-containing groups can improve the electrocatalytic activity of CNFs for ORR. The CNF–ON/GC electrode, which has nitrogen-containing groups, exhibits the highest current density of ORR. Rotating disk electrode (RDE) characterization shows that the oxygen reduction on CNF–ON/GC electrode proceeds almost entirely through the four-electron reduction pathway, the CNF–OX/GC, CNF–CO/GC and CNF–OH/GC electrodes proceed a two-electron reduction pathway at low potentials (−0.2 V to −0.6 V) followed by a gradual four-electron reduction pathway at more negative potentials, while the untreated carbon nanofiber (CNF–P/GC) electrode proceeds predominantly by a two-electron reduction pathway within the whole range of potential studied.Highlights► Different surface functional groups were successfully imported onto CNF surface. ► CNF–ON exhibited the highest ORR activity, followed by CNF–OX, CNF–CO and CNF–OH. ► CNF–ON could catalyze ORR through the 4e− pathway.

Pt nanoparticles supported on the acid-treated carbon nanofiber (CNF-OX) and LiAlH4-treated carbon nanofiber (CNF-OH) are synthesized via ethylene glycol reduction method. The nature of oxygen-containing surface groups on the CNF-OX and CNF-OH is investigated by potentiometric titration and XPS characterization. Titration of the support materials shows that LiAlH4 can effectively convert the carboxylic acid groups (from 0.21 mmol g−1 to 0.06 mmol g−1) to hydroxyl groups (from 0.09 mmol g−1 to 0.17 mmol g−1), which is agreed well with the results of XPS characterization. High resolution transmission electron microscopy (HRTEM) characterization shows that the Pt nanoparticles are highly dispersed on the two modified CNFs, and the Pt nanoparticles supported on the CNF-OH have a smaller particle size and a more uniform particle size distribution. Rotating disk electrode (RDE) analysis reveals that Pt/CNF-OH exhibits a better activity for ORR than Pt/CNF-OX, and this may be associated with the smaller particle size and better dispersion of Pt nanoparticles on the CNF-OH.Highlights► We studied the effect of oxygen-groups on the particle size and deposition of Pt particles. ► Pt/CNF-OH exhibits smaller Pt mean particle size compared with Pt/CNF-OX. ► Pt/CNF-OH/GC electrode exhibits a better ORR activity than Pt/CNF-OX/GC electrode.

Carbon nanofiber (CNF) supported PdAu nanoparticles are synthesized with sodium citrate as the stabilizing agent and sodium borohydride as the reducing agent. High resolution transmission electron microscopy (HRTEM) characterization indicates that the synthesized PdAu particles are well dispersed on the CNF surface and X-ray diffraction (XRD) characterization indicates that the alloying degree of the synthesized PdAu nanoparticles can be improved by adding tetrahydrofuran to the synthesis solution. The results of electrochemical characterization indicate that the addition of Au can promote the electrocatalytic activity of Pd/C catalyst for formic acid oxidation and the CNF supported high-alloying PdAu catalyst possesses better electrocatalytic activity and stability for formic acid oxidation than either the CNF supported low-alloying PdAu catalyst or the CNF supported Pd catalyst.Highlights► CNF supported high-alloying PdAu nanoparticles are successfully synthesized. ► The high-alloying PdAu/CNF exhibits a good activity for formic acid electrooxidation. ► The electronic effect of Au on Pd for formic acid oxidation is proposed.

Carbon nanofiber (CNF) supported Pd nanoparticles are synthesized with sodium citrate and sodium borohydride served as stabilizing agent and reducing agent, respectively. The size and distribution of the supported Pd nanoparticles are controlled by adjusting the pH value of the synthesis solution. Analyses of the obtained Pd/CNF catalysts indicate that the supported Pd nanoparticles become more uniform in size and the average particle size is decreased from 5.85 to 3.62 nm with pH value of the synthesis solution increasing from 3.2 to 6.0. However, the further increasing of the pH value to 6.5 leads to an increased particle size and the formation of PdO phase in the synthesized Pd/CNF catalyst. The Pd/CNF catalyst synthesized at the pH value of 6.0 exhibits superior catalytic activity and stability for formic acid electrooxidation due to its small particle size and uniform size distribution.Highlights► CNF supported Pd nanoparticles of varying particle size are synthesized. ► The Pd particle size varies with the pH value of the synthesis solution. ► Highly dispersed Pd/CNF catalyst exhibits good activity for formic acid oxidation.

Highly dispersed and active palladium/carbon nanofiber (Pd/CNF) catalyst is synthesized by NaBH4 reduction with trisodium citrate as the stabilizing agent. The obtained Pd/CNF catalyst is characterized by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The results show that the Pd nanoparticles with an average particle size of ca. 3.8 nm are highly dispersed on the CNF support even with a small ratio of citrate to Pd precursor, which is believed to be due to the pH adjustment of citrate stabilized colloidal Pd nanoparticles. The cyclic voltammetry and chronoamperometry techniques show that the obtained Pd/CNF catalyst exhibits good catalytic activity and stability for the electrooxidation of formic acid.Research highlights▶ Through simple pH adjustment of the trisodium citrate stabilized Pd colloidal, the Pd nanoparticles with an average particle size of ca. 3.8 nm can be highly dispersed on the CNF support although the trisodium citrate/PdCl2 ratio is one. ▶ The obtained Pd/CNF catalyst exhibits a good catalytic activity and stability for formic acid oxidation. ▶ The process developed in this work is demonstrated to be an attractive approach to synthesize highly dispersed noble metal nanoparticles onto support materials for fuel cell catalysts.

Oxygen- and nitrogen-containing groups are successfully introduced onto the carbon nanofiber (CNF) surfaces by sonochemical treatment in mixed acids (concentrated sulfuric acid and nitric acid) and ammonia, respectively. Pt electrocatalysts supported on the acid-treated CNF (CNF-O) and ammonia-treated CNF (CNF-ON) are prepared and the effect of CNF surface functional groups on the electrocatalytic activities of supported catalysts for oxygen reduction reaction (ORR) is investigated. High resolution transmission electron microscopy reveals that Pt particles are uniformly dispersed on the two CNF supports and the CNF-ON supported Pt nanoparticles have a smaller average particle size and a more uniform particle size distribution. Cyclic voltammetric analysis shows the Pt/CNF-ON has a larger electrochemically active surface area than Pt/CNF-O. Rotating disk electrode measurements show that the Pt/CNF-ON exhibits a considerably higher electrocatalytic activity toward ORR as compared with Pt/CNF-O. It is believed that the good electrocatalytic activity of Pt/CNF-ON can be attributed to the smaller Pt particle size and more uniform particle size distribution, to the synergistic effect and the enhanced Pt–CNF-ON interaction, and to the unique structural and electronic properties of CNF-ON.Highlights► We developed a simple post-doping method to synthesize nitrogen-doped CNFs using a sonochemical process. ► We studied the effect of oxygen- and nitrogen-containing functional groups on the particle size and deposition of Pt nanoparticles. ► Pt/CNF-ON exhibited smaller Pt average particle size and narrower particle size distribution compared with Pt/CNF-O. ► Pt/CNF-ON/GC electrode exhibited a better electrocatalytic activity toward ORR compared with Pt/CNF-O/GC electrode.

A network-like carbon nanofiber (CNF) film with an open porous structure formed by the open space between entangled CNFs is fabricated by electrophoretic deposition. The performance of the CNF film as an electrocatalyst in the presence of electrodeposited Pd nanoparticles for ethanol oxidation in alkaline media is investigated. Cyclic voltammetric analyses show the electrocatalyst has a good electrocatalytic activity toward ethanol oxidation in KOH solution. This is believed to be due to the high dispersion of Pd on the CNF film with a three-dimensional network structure which can provide a large number of available Pd active sites for ethanol oxidation, and to the structural and electrical properties of the film.

Pd electrocatalysts supported on three types of carbon nanofibers (CNFs), viz. platelet CNFs (p-CNFs), fish-bone CNFs (f-CNFs) and tubular CNFs (t-CNFs) are prepared and the effect of CNFs microstructure on the activities of the electrocatalysts for ethanol oxidation reaction (EOR) is investigated. The information on structural characteristics is obtained by high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Electrochemical techniques are employed to characterize the microstructure effect of CNFs on the catalytic activities of catalysts. HRTEM images indicate the microstructure of CNFs has a powerful influence on the distribution of Pd particles. The results of the electrochemical characterization also indicate that the structure of CNFs significantly influences the catalytic activities of the electrocatalysts and p-CNFs supported Pd electrocatalyst has the best performance for ethanol oxidation in an alkaline medium because p-CNFs has the highest ratio of edge atoms to basal atoms and correspondingly the fastest electrode kinetics and strongest Pd–CNFs interaction.

A platinum/carbon nanofiber (Pt/CNF) nanocomposite with a platinum loading of 15 wt% is prepared by a modified electrophoretic deposition (EPD) method, and the as-grown nanocomposite is used as the electrocatalyst for oxygen reduction reaction (ORR). For comparison, a Pt/CNF composite with 40 wt% platinum loading is prepared by chemical reduction. High resolution transmission electron microscope (HRTEM) images show that the size of platinum nanoparticles formed by EPD is about 1 nm, much smaller than those by chemical reduction (about 3–5 nm). Cyclic voltammetric analysis in a nitrogen saturated electrolyte shows that the electrochemical surface area of electrocatalyst by EPD is larger than that by chemical reduction. Moreover, although the electrocatalyst prepared by chemical reduction has a higher electrochemical capacity, it is less active than that prepared by EPD. Analysis of the electrode kinetics using Tafel plot suggests that the electrocatalyst prepared by EPD provides a strong ORR activity. Cyclic voltammetric measurements at different scan rates confirm that the ORR on the nanocomposites prepared by EPD is a diffusion-controlled process. This work demonstrates that the Pt/CNF composites synthesized by EPD are effective for ORR.

Carbon nanofibers (CNFs) are grown on metal catalysts and electrochemical treatment is used to remove the metal catalyst residuals from the as-grown CNFs. For comparison, the CNFs are also purified by a chemical method and a thermal method. The oxygen reduction reaction (ORR) properties of CNFs purified by these three methods are examined by cyclic voltammetry. CNFs treated by the electrochemical method have a more positive ORR onset reduction potential and peak potential compared with those treated by chemical and thermal methods, and this is because the microstructures of CNFs are less changed by electrochemical method. However, they have a lower electrochemical capacity and ORR peak current than those treated by the chemical method. Cyclic voltammetric measurements at different scan rates confirm that the oxygen reductions on CNFs treated by electrochemical and chemical methods are controlled by diffusion, while on CNFs treated by thermal method is partially influenced by diffusion.

Carbon nanofibers (CNFs) with controlled microstructures, i.e. platelet CNF (p-CNF), fish-bone CNF (f-CNF) and tube CNF (t-CNF), are synthesized, and their behaviors in electrocatalytic oxygen reduction reaction (ORR) in acid media are investigated in this paper. The physico-chemical properties of the CNFs are characterized by high resolution transmission electron microscope (HRTEM), N2 adsorption–desorption and Raman spectrum. Cyclic voltammetry experiments show that the CNFs have higher ORR activities than graphite. The p-CNF, which has the highest ratio of edge atoms to basal atoms, demonstrates the most positive ORR onset potential and ORR peak potential. The f-CNF, which has the largest amounts of ORR active sites, exhibits the highest ORR peak current. The t-CNF demonstrates the most negative ORR onset potential, negative ORR peak potential, and the least ORR peak current, which is a result of the fewest catalytic active sites. Furthermore, the microstructures of CNFs can impact the reaction process. The ORR on p-CNF or f-CNF is controlled by diffusion, while the ORR on t-CNF is jointly controlled by surface reaction and diffusion.