EXAFS analysis of the L3 edge of Ce in CeO2: effects of multi‐electron excitations and final‐state mixed valence

Cerium oxide (IV) (CeO2) is extensively employed in heterogeneous catalysis, particularly as a promoter of noble metal action in three-way catalysts. For this reason there is a great scientific and economical interest in the development of any possible chemical or structural analysis technique that could provide information on these systems. EXAFS spectroscopy has revealed itself as a powerful technique for structural characterization of such catalysts. Unfortunately, good quality K-edge spectra of cerium are not yet easily obtainable because of the high photon energy required (>40 keV). On the other hand, at lower energies it is easy to collect very good spectra of the L3 edge (5.5 keV), but L3-edge spectra of cerium (IV) are characterized by the presence of two undesired additional phenomena that interfere with EXAFS analysis: final-state mixed-valence behaviour and intense multi-electron excitations. Here, a comparative analysis of the K, L3, L2 and L1 edges of Ce in CeO2 has been made and a procedure for obtaining structural parameters from L3-edge EXAFS, even in the presence of these features, has been developed. This procedure could allow further studies of catalytic compounds containing tetravalent cerium surrounded by oxygen ligands

Effects of N-functional groups on the electron transfer kinetics of VO²⁺/VO₂⁺ at carbon:Decoupling morphology from chemical effects using model systems

Carbons and nanocarbons are important electrode materials for vanadium redox flow battery applications, however, the kinetics of vanadium species are often sluggish at these surfaces, thus prompting interest in functionalization strategies to improve performance. Herein, we investigate the effect of N-functionalities on the VO2+/VO2+ redox process at carbon electrodes. We fabricate thin film carbon disk electrodes that are metal-free, possess well-defined geometry and display smooth topography, while featuring different N-site distribution, thus enabling a mechanistic investigation of the intrinsic surface activity towards VO2+/VO2+. Voltammetry and electrochemical impedance spectroscopy show that N-functionalities improve performance, with pyridinic/pyrrolic-N imparting the most significant improvements in charge transfer rates and reversibility, compared to graphitic-N. This was further supported by voltammetry studies on nitrogen-free electrodes modified via aryldiazonium chemistry with molecular pyridyl adlayers. Computational modeling using an electrochemical-chemical mechanism indicates that introduction of surface pyridinic/pyrrolic-N can increase the heterogeneous rate constants by approximately two orders of magnitude relative to those observed at nitrogen-free carbon (k0 = 1.29 × 10−4 vs 9.34 × 10−7 cm/s). Simulations also suggest that these N-functionalities play a role in affecting reaction rates in the chemical step. Our results indicate that nitrogen incorporation via basic functional groups offers an interesting route to the design of advanced carbon electrodes for VRFB devices.</p

Bioinspired aryldiazonium carbohydrate coatings: reduced adhesion of foulants at polymer and stainless steel surfaces in a marine environment

Surface treatments that minimize biofouling in marine environments are of interest for a variety of applications such as environmental monitoring and aquaculture. We report on the effect of saccharide coatings on biomass accumulation at the surface of three materials that find applications in marine settings: stainless steel 316 (SS316), Nylon-6 (N-6), and poly(ether sulfone) (PES). Saccharides were immobilized via aryldiazonium chemistry; SS316 and N-6 samples were subjected to oxidative surface pretreatments prior to saccharide immobilization, whereas PES was modified via direct reaction of pristine surfaces with the aryldiazonium cations. Functionalization was confirmed by a combination of X-ray photoelectron spectroscopy, contact angle experiments, and fluorescence imaging of lectin–saccharide binding. Saccharide immobilization was found to increase surface hydrophilicity of all materials tested, while laboratory tests demonstrate that the saccharide coating results in reduced protein adsorption in the absence of specific protein–saccharide interactions. The performance of all three materials after modification with aryldiazonium saccharide films was tested in the field via immersion of modified coupons in coastal waters over a 20 day time period. Results from combined infrared spectroscopy, light microscopy, scanning electron and He-ion microscopy, and adenosine-triphosphate content assays show that the density of retained biomass at surfaces is significantly lower on carbohydrate modified samples with respect to unmodified controls. Therefore, functionalization and field test results suggest that carbohydrate aryldiazonium layers could find applications as fouling resistant coatings in marine environments

In Situ and Real Time Characterization of Spontaneous Grafting of Aryldiazonium Salts at Carbon Surfaces

Aryldiazonium cations are widely used to covalently functionalize carbon substrates that display a wide range of composition, from 100% sp² such as graphite or graphene to 100% sp³ such as diamond and nanodiamond. In this work we investigated the effect that changes in carbon composition have on aryldiazonium adsorption rates and surface reaction mechanism. Quartz crystal microbalance (QCM) was used to investigate the rates of adsorption in situ and in real time at two amorphous carbon substrates, one with high sp² content (a-C) and one with high sp³ content (a-C:H). A reversible Langmuir adsorption model was found to satisfactorily describe adsorption at a-C:H, yielding an adsorption rate coefficient <i>k</i>_a = 3.1 M^–1 s^–1 and a free energy of adsorption Δ<i>G</i>_a = −20.1 kJ mol^–1. This model, on the other hand, could not be applied for the interpretation of adsorption curves at a-C. Using electrochemical methods and X-ray photoelectron spectroscopy (XPS), we found that adlayers formed at a-C:H and a-C surfaces differ considerably in composition; in particular, a-C surfaces were found to display higher rates of dediazoniation with respect to a-C:H surfaces. Our findings are interpreted and discussed in the context of current proposed mechanisms for aryldiazonium reactions at surfaces that consist of an adsorption/desorption step followed by a chemisorption via dediazoniation step. Our observations are consistent with proposed mechanisms and strongly suggest that differences in carbon composition result in differences in the relative magnitude of adsorption and chemisorptions rate coefficients

Low-Overpotential High-Activity Mixed Manganese and Ruthenium Oxide Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media

Mixed Mn/Ru oxide thermally prepared electrodes using different compositions of Mn and Ru precursor salts have been fabricated on Ti supports via thermal decomposition at two annealing temperatures. Subsequently, the oxygen evolution reaction (OER) activities of these electrodes were determined. A majority of the mixed Mn/Ru catalysts are highly active for the OER, exhibiting lower overpotential values compared to those of the state-of-the-art RuO₂ and IrO₂ type materials, when measured at a current density of 10 mA cm^–2. These Mn/Ru oxide materials are also cheaper to produce than the aforementioned platinum group materials, therefore rendering the Mn/Ru materials more practical and economical. The Mn/Ru catalysts are also evaluated with respect to their Tafel slopes and turnover frequency numbers. Interestingly, scanning electron microscopy reveals that the morphologies of the electrodes change to a mud-cracked morphology, similar to that of the RuO₂, with minimal amounts of the Ru precursor salt added to the Mn salt. Fourier transform infrared spectroscopy and X-ray diffraction show that the Mn material fabricated in this study at the two annealing temperatures is largely Mn₃O₄, while the Ru material is predominately RuO₂. X-ray photoelectron spectroscopy was also used to investigate the Mn and Ru composition ratios in each of the films

Controlling the Carbon-Bio Interface via Glycan Functional Adlayers for Applications in Microbial Fuel Cell Bioanodes

International audienceSurface modification of electrodes with glycans was investigated as a strategy for modulating the development of electrocatalytic biofilms for microbial fuel cell applications. Covalent attachment of phenyl-mannoside and phenyl-lactoside adlayers on graphite rod electrodes was achieved via electrochemically assisted grafting of aryldiazonium cations from solution. To test the effects of the specific bio-functionalities, modified and unmodified graphite rods were used as anodes in two-chamber microbial fuel cell devices. Devices were set up with wastewater as inoculum and acetate as nutrient and their performance, in terms of output potential (open circuit and 1 kΩ load) and peak power output, was monitored over two months. The presence of glycans was found to lead to significant differences in startup times and peak power outputs. Lactosides were found to inhibit the development of biofilms when compared to bare graphite. Mannosides were found, instead, to promote exoelectrogenic biofilm adhesion and anode colonization, a finding that is supported by quartz crystal microbalance experiments in inoculum media. These differences were observed despite both adlayers possessing thickness in the nm range and similar hydrophilic character. This suggests that specific glycan-mediated bioaffinity interactions can be leveraged to direct the development of biotic electrocatalysts in bioelectrochemical systems and microbial fuel cell devices

Quantifying Graphitic Edge Exposure in Graphene-Based Materials and Its Role in Oxygen Reduction Reactions

Oxygen electrochemistry is at the core of several emerging energy conversion technologies. The role of carbon nanostructures in the electrocatalysis of the oxygen reduction reaction is not well understood. Herein we report an investigation of the role of graphitic edges in oxygen electrochemistry. A new synthetic method was used to create all-carbon model electrode materials with controlled morphology. Electron microscopy results show that synthesized materials possess a high density of graphitic edges. Electrochemical intercalation experiments, however, indicate that the density of electroactive edges does not correlate positively with microscopy results. The materials were then characterized as electrodes for the oxygen reduction reaction in alkaline media. Results suggest that electrochemical determinations of edge and defect density more accurately predict electrocatalytic activity, thus suggesting that in situ characterization techniques are needed to understand the carbon/electrolyte interface

Tailored glycosylated anode surfaces Addressing the exoelectrogen bacterial community via functional layers for microbial fuel cell applications

International audienceGrafting of aryldiazonium cations bearing a p-mannoside functionality over microbial fuel cell (MFC) anode materials was performed to investigate the ability of aryl-glycoside layers to regulate colonisation by biocatalytic biofilms. Covalent attachment was achieved via spontaneous reactions and via electrochemically-assisted grafting using potential step experiments. The effect of different functionalisation protocols on MFC performance is discussed in terms of changes in wettability, roughness and electrochemical response of modified electrodes. Water contact angle measurements (WCA) show that aryl-mannoside grafting yields a significant increase in hydrophilic character. Surface roughness determinations via atomic force microscopy (AFM) suggest a more disordered glycan adlayer when electrografting is used to facilitate chemisorption. MFCs were used as living sensors to successfully test the coated electrodes the response of the MFCs in terms of start-up time was accelerated when compared to that of MFC equipped with non-modified electrodes, this suggests a faster development of a mature biofilm community resulting from aryldiazonium modifications, as confirmed by cyclic voltammetry of MFC anodes. These results therefore indicate that modification with glycans offers a bioinspired route to accelerating biofilm colonisation without any adverse effects on final MFC outputs