•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

•The assessment of the effects of TAA+ cations on CO2 reduction is established.•The onset potential of CO2 reduction is dependent on the strength of ion pairing.•The peak current of CO2 reduction is determined by the quantity of the adsorbed TAA+ cation.The effect of tetraalkylammonium cation (TAA+) chain length on CO2 electroreduction is investigated on an Ag electrode in a DMF solution. Linear sweep voltammetric (LSV) studies show that the onset potentials of CO2 reduction move to increasingly negative potentials upon increasing the alkyl chain length of the tetraalkylammonium cation of the supporting electrolyte. Density functional theory (DFT) computations indicate that the onset potential of CO2 reduction is dependent on the strength of ion pairing between TAA+ cation and the electrogenerated CO2.−. CO2 disturbance experiments reveal that the peak current of CO2 reduction is determined by the quantity of TAA+ cation adsorbed on the Ag surface.

•The significant electrocatalytic effects of imidazolium salts are due to a strong ion-pairing.•DFT calculation predicts the ion pair [BMIM+][CO2.−] conformation.•The extent of ion-pairing decreases with increasing the size of the alkyl substituent.The roles of ion pairing on CO2 electroreduction involving imidazolium-based salts are explored on an Ag electrode in a DMF solution. The electroreductive behaviours of CO2 and the IR results establish the dominant role of the cation of imidazolium-based salts in CO2 electrocatalytic reduction process. Density functional theory (DFT) calculations predict that the ion pair formation is mainly driven by the attraction of the four nearby and positively charged hydrogen atoms which are at the C2, N1 and N3-positions of an imidazolium cation for the negatively charged oxygen atoms in the CO2.− species. The electrostatic interaction between an imidazolium cation and the CO2.− species decreases with increasing the size of the alkyl substituent at the N1-position of that imidazolium cation.

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.