Stability, Electronic and Magnetic Properties of In-Plane Defects in Graphene: A First-Principles Study

The electronic and magnetic properties of graphene can be modified through combined transition-metal and nitrogen decoration of vacancies. In this study, we used density functional theory to investigate the following defect motifs: nitrogen doping, nitrogen decoration of single and double vacancies (SVs and DVs), TM doping (TM = Co, Fe), TM adsorption on nitrogen-doped graphene, and combined TM–nitrogen chemistries in SV and DV (TM–N_<i>x</i>) configurations. The results show that the highest magnetic moments are supported in TM–N_<i>x</i> defect motifs. Among these defects, Co–N₃, Fe–N₃, and Fe–N₄ defects are predicted to show ferromagnetic spin structures with high magnetic moments and magnetic stabilization energies, as well as enhanced stability as expressed by favorable formation energies, and high TM binding energies

Multianalytical Study of the PTFE Content Local Variation of the PEMFC Gas Diffusion Layer

<p>A segmented cell system was used to study the impact of localized variations of GDL polytetrafluoroethylene (PTFE) on the spatial<br>and overall fuel cell performance. An artificial defect was created by exchanging the standard cathode GDL treated by 13 wt% PTFE<br>and located at segment 4 with a defective GDL containing 8, 17, 26, or 35 wt% PTFE. Operation under wet (100/50% RH) and dry<br>(32/32% RH) conditions demonstrated the same trend, in which an increase in the PTFE loading caused a local performance decrease<br>at high current density; however, a low humidity mitigated the impact of PTFE loading defects. Spatial impedance spectroscopy<br>data together with analysis of the polarization curves revealed that such a performance decrease is attributed to an increase in the<br>mass transfer limitations ascribed to the changes in textural (decrease in porosity, pore volume, and pores size), structural, and even<br>chemical properties of the GDLs with PTFE loading. The impacts of PTFE loading on the surface structure and the morphology<br>of the GDLs were studied by digital image processing of the SEM images, which were deconvoluted into low- and high-frequency<br>components. Higher hydrophilicity zones were also identified and ascribed to partially fluorinated species detected by XPS.</p> <p> </p

Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal–Nitrogen–Carbon Electrocatalysts

Development and optimization of non-platinum group metal (non-PGM) electrocatalysts for oxygen reduction reaction (ORR), consisting of transition metal–nitrogen–carbon (M–N–C) framework, is hindered by the partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. In this study, we have analyzed more than 45 M–N–C electrocatalysts synthesized from three different families of precursors, such as polymer-based, macrocycles, and small organic molecules. Catalysts were electrochemically tested and analyzed structurally using exactly the same protocol for deriving structure-to-property relationships. We have identified possible active sites participating in different ORR pathways: (1) metal-free electrocatalysts support partial reduction of O₂ to H₂O₂; (2) pyrrolic nitrogen acts as a site for partial O₂ reduction to H₂O₂; (3) pyridinic nitrogen displays catalytic activity in reducing H₂O₂ to H₂O; (4) Fe coordinated to N (Fe–N_<i>x</i>) serves as an active site for four-electron (4e^–) direct reduction of O₂ to H₂O. The ratio of the amount of pyridinic and Fe–N_<i>x</i> to the amount of pyrrolic nitrogen serves as a rational design metric of M–N–C electrocatalytic activity in oxygen reduction reaction occurring through the preferred 4e^– reduction to H₂O

Protein–Support Interactions for Rationally Designed Bilirubin Oxidase Based Cathode: A Computational Study

An example of biocathode based on bilirubin oxidase (BOx) was used to demonstrate how density functional theory can be combined with docking simulations in order to study the interface interactions between the enzyme and specifically designed electrode surface. The electrode surface was modified through the adsorption of bilirubin, the natural substrate for BOx, and the prepared electrode was electrochemically characterized using potentiostatic measurements. The experimentally determined current densities showed that the presence of bilirubin led to significant improvement of the cathode operation. On the basis of the computationally calculated binding energies of bilirubin to the graphene support and BOx and the analysis of the positioning of bilirubin relative to the support and T1 Cu atom of the enzyme, we hypothesize that the bilirubin serves as a geometric and electronic extension of the support. The computational results further confirm that the modification of the electrode surface with bilirubin provides an optimal orientation of BOx toward the support but also show that bilirubin facilitates the interfacial electron transfer by decreasing the distance between the electrode surface and the T1 Cu atom

Kinetic and Mechanistic Parameters of Laccase Catalyzed Direct Electrochemical Oxygen Reduction Reaction

This article presents the kinetic studies of oxygen reduction by one of the most important multicopper oxidases (fungal laccase) using the classic tool of electrochemistry: rotating ring-disk electrode (RRDE). Laccase was immobilized on a multiwalled carbon nanotube (MWNT) modified inert disk electrode using 1-pyrenebutanoic acid succinimidyl ester (PBSE), as a tethering agent. The conditions for laccase immobilization on MWNT were optimized to prepare a highly active composite electro-catalyst for O₂ reduction. The mechanistic as well as kinetic parameters such as Tafel slopes, number of electrons transferred, electrochemical rate constants (for heterogeneous charge transfer) and electron transfer rate constant were calculated from the RRDE experiment results. The Tafel slope obtained was close to the value of that of ideal four-electron reduction of O₂ to water indicating a highly active laccase in the tethered composite. The RRDE results also suggested the presence of intermediate steps in the oxygen reduction reaction. A model pathway for O₂ reduction reaction at the laccase composite modified electrode was postulated, and rate constants for individual reactions in the pathway were calculated. The rate constant for four-electron O₂ reduction was determined to be 1.46 × 10^–3 mol s^–1, indicating excellent electro-catalytic activity of the laccase-MWNT composite catalyst

Fully Synthetic Approach toward Transition Metal–Nitrogen–Carbon Oxygen Reduction Electrocatalysts

We report a nonpyrolytic chemical synthesis of model iron–nitrogen–carbon electrocatalysts for oxygen reduction reaction (ORR) to elucidate the role of Fe–N centers in the catalysis mechanism. The graphene-supported and unsupported catalysts were analyzed in detail by X-ray spectroscopy techniques. The electrochemical analysis was performed by linear sweep voltammetry and square wave voltammetry in 0.5 M H₂SO₄ and 0.1 M KOH electrolytes. In this article, with the use of model catalysts, we manifest and confirm the difference in the specific role of Fe–N active sites toward ORR in acidic and alkaline environments

Application of the Discrete Wavelet Transform to SEM and AFM Micrographs for Quantitative Analysis of Complex Surfaces

The discrete wavelet transform (DWT) has found significant utility in process monitoring, filtering, and feature isolation of SEM, AFM, and optical images. Current use of the DWT for surface analysis assumes initial knowledge of the sizes of the features of interest in order to effectively isolate and analyze surface components. Current methods do not adequately address complex, heterogeneous surfaces in which features across multiple size ranges are of interest. Further, in situations where structure-to-property relationships are desired, the identification of features relevant for the function of the material is necessary. In this work, the DWT is examined as a tool for quantitative, length-scale specific surface metrology without prior knowledge of relevant features or length-scales. A new method is explored for determination of the best wavelet basis to minimize variation in roughness and skewness measurements with respect to change in position and orientation of surface features. It is observed that the size of the wavelet does not directly correlate with the size of features on the surface, and a method to measure the true length-scale specific roughness of the surface is presented. This method is applied to SEM and AFM images of non-precious metal catalysts, yielding new length-scale specific structure-to-property relationships for chemical speciation and fuel cell performance. The relationship between SEM and AFM length-scale specific roughness is also explored. Evidence is presented that roughness distributions of SEM images, as measured by the DWT, is representative of the true surface roughness distribution obtained from AFM

Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal–Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems

Detailed understanding of the nature of the active centers in non-precious-metal-based electrocatalyst, and their role in oxygen reduction reaction (ORR) mechanistic pathways will have a profound effect on successful commercialization of emission-free energy devices such as fuel cells. Recently, using pyrolyzed model structures of iron porphyrins, we have demonstrated that a covalent integration of the Fe–N_<i>x</i> sites into π-conjugated carbon basal plane modifies electron donating/withdrawing capability of the carbonaceous ligand, consequently improving ORR activity. Here, we employ a combination of <i>in situ</i> X-ray spectroscopy and electrochemical methods to identify the various structural and functional forms of the active centers in non-heme Fe/N/C catalysts. Both methods corroboratively confirm the single site 2e^– × 2e^– mechanism in alkaline media on the primary Fe²⁺–N₄ centers and the dual-site 2e^– × 2e^– mechanism in acid media with the significant role of the surface bound coexisting Fe/Fe_<i>x</i>O_<i>y</i> nanoparticles (NPs) as the secondary active sites

Cascade Kinetics of an Artificial Metabolon by Molecular Dynamics and Kinetic Monte Carlo

Natural enzyme cascades are able to employ electrostatic channeling as an efficient mechanism for shuttling charged intermediates between sequential active sites. Application of channeling mechanisms to artificial cascades has drawn increasing attention for its potential to improve cascade design. We report a quantitative model of a two-step artificial metabolon that accounts for molecular-level complexity. Conversion of glucose to phospho-6-gluconolactone by hexokinase and glucose-6-phosphate dehydrogenase, covalently conjugated by a cationic oligopeptide bridge, is simulated and validated by comparison to stopped-flow lag time analysis. Specifically, molecular dynamics (MD) simulations enable the calculation of energy-determined surface equilibrium constants and surface diffusivity, and a kinetic Monte Carlo (KMC) model integrated all rate constants from MD (e.g., surface diffusion and desorption rate) and experiments (e.g., turnover frequency), to estimate the product evolution on an experimental time scale, starting from presteady state. Simulations, conducted as a function of ionic strength, compare well to experiment and indicate that bridge-enzyme leakage is a major limitation accounting for significant lag time increase. Reducing the energy barrier between the channeling pathway and binding pocket could further improve channeling efficiency. Bridge length is also found to have a significant effect on overall kinetics

Cascade Kinetics of an Artificial Metabolon by Molecular Dynamics and Kinetic Monte Carlo

Natural enzyme cascades are able to employ electrostatic channeling as an efficient mechanism for shuttling charged intermediates between sequential active sites. Application of channeling mechanisms to artificial cascades has drawn increasing attention for its potential to improve cascade design. We report a quantitative model of a two-step artificial metabolon that accounts for molecular-level complexity. Conversion of glucose to phospho-6-gluconolactone by hexokinase and glucose-6-phosphate dehydrogenase, covalently conjugated by a cationic oligopeptide bridge, is simulated and validated by comparison to stopped-flow lag time analysis. Specifically, molecular dynamics (MD) simulations enable the calculation of energy-determined surface equilibrium constants and surface diffusivity, and a kinetic Monte Carlo (KMC) model integrated all rate constants from MD (e.g., surface diffusion and desorption rate) and experiments (e.g., turnover frequency), to estimate the product evolution on an experimental time scale, starting from presteady state. Simulations, conducted as a function of ionic strength, compare well to experiment and indicate that bridge-enzyme leakage is a major limitation accounting for significant lag time increase. Reducing the energy barrier between the channeling pathway and binding pocket could further improve channeling efficiency. Bridge length is also found to have a significant effect on overall kinetics