Mechanistic Discussion of the Oxygen Reduction Reaction at Nitrogen-Doped Carbon Nanotubes

The oxygen reduction reaction (ORR) at undoped and nitrogen-doped carbon nanotubes (CNTs and N-CNTs, respectively) was studied by cyclic voltammety, rotating disk electrode voltammetry, and gasometric analysis in neutral and alkaline aqueous solutions. At undoped CNTs, the ORR proceeds by two successive two-electron processes with hydroperoxide (HO2–) as the intermediate. At N-CNTs, the ORR occurs through a “pseudo”-four-electron pathway involving a catalytic regenerative process in which hydroperoxide is chemically disproportionated to form hydroxide (OH–) and molecular oxygen (O2). The ORR mechanism at both undoped and N-doped varieties is supported by steady state polarization and gasometric measurements of hydroperoxide disproportionation rates. An enhancement of over 1000-fold for hydroperoxide disproportionation is observed for N-CNTs, with rates comparable to the best known peroxide decomposition catalysts. A positive correlation between nitrogen content and ORR activities is observed where the ORR potential shifts by up to 11.6 mV per at. % N incorporated into the N-CNTs and exhibits an oxygen reduction potential, Ep, of −0.23 V vs Hg/Hg2SO4 (+0.640 V vs NHE) in 1 M Na2HPO4 for N-CNTs containing 7.4 at. % N. A detailed mechanism is proposed that involves a dual site reduction in which O2 is initially reduced at a N–C type site in a 2-electron process to form HO2–, which then can undergo either further electrochemical reduction to form OH– species or chemical disproportionation to form OH– species and molecular O2 at a decorating FexOy/Fe surface phase

Indirect Electrocatalytic Degradation of Cyanide at Nitrogen-Doped Carbon Nanotube Electrodes

Nitrogen-doped carbon nanotube (N-CNT) mat electrodes exhibit high catalytic activity toward O2 reduction, which can be exploited for the remediation of free cyanide (CN−). During the electrochemical O2 reduction process, the hydroperoxide anion (HO2−) is formed and then reacts to chemically oxidize cyanide (CN−) to form cyanate (OCN−). The proposed electrochemical−chemical (EC) mechanism for CN− remediation at N-CNTs is supported by cyclic voltammetry and bulk electrolysis, and the formation of OCN− is confirmed via spectroscopic methods and electrochemical simulations. Our results indicate that by exploiting their catalytic behavior for O2 reduction, N-CNTs can efficiently convert toxic CN− to the nontoxic OCN−

Effect of Nitrogen Concentration on Capacitance, Density of States, Electronic Conductivity, and Morphology of N-Doped Carbon Nanotube Electrodes

Heteroatom doping (e.g., boron and nitrogen) of graphitic carbon lattices affects various physicochemical properties of sp2 carbon materials. The influence of nitrogen doping in carbon nanotubes (N-CNTs) on their electrochemical and electrical properties such as the differential capacitance, density of states at the Fermi level (D(EF)), bulk conductivity, and work function is presented. Studies were performed on free-standing N-CNTs electrode mats to understand the intrinsic physicochemical properties of the material without relying on the secondary influence of another conductive support. N-Doping levels ranging from 0 to 7.4 atom % N were examined, and electrochemical impedance spectroscopy (EIS) was used to evaluate the differential capacitance and to estimate the effective density of states, D(EF). X-ray photoelectron spectroscopy (XPS) and Raman microscopy were used to assess the compositional and structural properties as a function of nitrogen doping. XPS N1s spectra show three principle types of nitrogen coordination (pyridinic, pyrrolic, and quaternary). Raman was used as diagnostic tool for estimating the amount of disorder by comparing D and G bands. A linear increase in the ratio of integrated D and G band intensities with nitrogen doping indicates that the amount of disorder and number of edge plane sites increase. Furthermore, D(EF) also increases with N doping and the amount of disorder and number of edge plane sites. UV photoelectron spectroscopy (UPS) was used to probe the valence band of N-CNTs in order to estimate the work function of the mats. The work function increased linearly from 4.1 to 4.5 eV for increasing N-doping levels. The bulk electrical conductivity of the N-CNT electrode mats appears to be junction dominated as shown by the relationship between the bulk conductivity and average N-CNT length within the mats determined using high-resolution scanning transmission electron microscopy (STEM)

Highly Stable Pt/Ordered Graphitic Mesoporous Carbon Electrocatalysts for Oxygen Reduction

In this manuscript, we have synthesized a stable fuel cell catalyst composed of presynthesized Pt nanocrystals (<4 nm) on graphitic mesoporous carbon. The catalyst shows negligible loss in mass activity and active surface area after an accelerated durability test (1000 cycles, 0.5−1.2 V), whereas the commercial Pt on amorphous carbon loses ∼70% in activity and area. Strong Pt−graphite interactions, resulting from metal/support orbital overlap (π-backbonding) coupled with partial charge transfer, as shown by XPS, and a low coverage of weakly bound ligands on the Pt surface facilitated high dispersion and loadings up to 20 wt %. The high oxidation resistance of the graphitized carbon, along with the strong Pt−C interactions, helped to maintain electrical contact between the metal and carbon while mitigating Pt dissolution, ripening, and coalescence. The ability to disperse well-defined metal nanoparticles onto graphitic mesoporous carbon offers the potential for creating highly stable and active catalysts

Highly Stable and Active Pt−Cu Oxygen Reduction Electrocatalysts Based on Mesoporous Graphitic Carbon Supports

The activity of oxygen reduction catalysts for fuel cells often decreases markedly (30−70%) during potential cycling tests designed to accelerate catalyst degradation. Herein we achieved essentially no loss in electrochemical surface area and catalyst activity during potential cycling from 0.5 to 1.2 V for presynthesized Pt−Cu nanoparticles of controlled composition that were infused into highly graphitic disordered mesoporous carbons (DMC). The high stability is favored by the strong metal−support interactions and low tendency for carbon oxidation, which mitigates the mechanisms of degradation. Electrochemical dealloying transforms the composition from Pt20Cu80 to Pt85Cu15 with a strained Pt-rich shell, which exhibits an enhanced ORR activity of 0.46 A/mgPt, >4 fold that of pure Pt catalysts. The high uniformity in particle size and composition both before and after dealloying, as a consequence of the presynthesis/infusion technique, is beneficial for elucidating the mechanism of catalyst activity and, ultimately, for designing more active catalysts