Isotope Exchange Raman Spectroscopy (IERS): A Novel Technique to Probe Physicochemical Processes In Situ.

A novel in situ methodology for the direct study of mass-transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges, is developed. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, which is not accessible by conventional methods, enabling complementary insights for the study of ion-transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) is demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary-ion mass spectrometry (ToF-SIMS) characterization and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness, and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate the understanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research, and beyond.

Decoupling the Contributions of Different Instability Mechanisms to the PEMFC Performance Decay of Non-noble Metal O2-Reduction Catalysts.

Non-noble metal catalysts (NNMCs) hold the potential to replace the expensive Pt-based materials currently used to speed up the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes, but they feature poor durability that inhibits their implementation in commercial PEMFCs. This performance decay is commonly ascribed to the operative demetallation of their ORR-active sites, the electro-oxidation of the carbonaceous matrix that hosts these active centers, and/or the chemical degradation of the ionomer, active sites, and/or carbon support by radicals derived from the H2O2 produced as an ORR by-product. However, little is known regarding the relative contributions of these mechanisms to the overall PEMFC performance loss. With this motivation, in this study, we combined four degradation protocols entailing different cathode gas feeds (i.e., air vs N2), potential hold values, and durations to decouple the relative impact of the above deactivation mechanisms to the overall performance decay. Our results indicate that H2O2-related instability does not depend on the operative voltage but only on the ORR charge. Moreover, the electro-oxidation of the carbon matrix at high potentials (which for the catalyst tested herein triggers at 0.7 V) seems to be more detrimental to the NNMCs' activity than the demetallation occurring at low potentials.

On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts.

Fe-N-C catalysts containing atomic FeNx sites are promising candidates as precious-metal-free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The durability of Fe-N-C catalysts in fuel cells has been extensively studied using accelerated stress tests (AST). Herein we reveal stronger degradation of the Fe-N-C structure and four-times higher ORR activity loss when performing load cycling AST in O2 - vs. Ar-saturated pH 1 electrolyte. Raman spectroscopy results show carbon corrosion after AST in O2 , even when cycling at low potentials, while no corrosion occurred after any load cycling AST in Ar. The load-cycling AST in O2 leads to loss of a significant fraction of FeNx sites, as shown by energy dispersive X-ray spectroscopy analyses, and to the formation of Fe oxides. The results support that the unexpected carbon corrosion occurring at such low potential in the presence of O2 is due to reactive oxygen species produced between H2 O2 and Fe sites via Fenton reactions.

A membraneless air-breathing hydrogen biofuel cell based on direct wiring of thermostable enzymes on carbon nanotube electrodes.

A biocathode was designed by the modification of a carbon nanotube (CNT) gas-diffusion electrode with bilirubin oxidase from Bacillus pumilus, achieving high current densities up to 3 mA cm(-2) for the reduction of O2 from air. A membraneless air-breathing hydrogen biofuel cell was designed by combination of this cathode with a functionalized CNT-based hydrogenase anode.

Microfabrication, characterization and in vivo MRI compatibility of diamond microelectrodes array for neural interfacing.

Neural interfacing still requires highly stable and biocompatible materials, in particular for in vivo applications. Indeed, most of the currently used materials are degraded and/or encapsulated by the proximal tissue leading to a loss of efficiency. Here, we considered boron doped diamond microelectrodes to address this issue and we evaluated the performances of a diamond microelectrode array. We described the microfabrication process of the device and discuss its functionalities. We characterized its electrochemical performances by cyclic voltammetry and impedance spectroscopy in saline buffer and observed the typical diamond electrode electrochemical properties, wide potential window and low background current, allowing efficient electrochemical detection. The charge storage capacitance and the modulus of the electrochemical impedance were found to remain in the same range as platinum electrodes used for standard commercial devices. Finally we observed a reduced Magnetic Resonance Imaging artifact when the device was implanted on a rat cortex, suggesting that boron doped-diamond is a very promising electrode material allowing functional imaging.

Wiring laccase on covalently modified graphene: carbon nanotube assemblies for the direct bio-electrocatalytic reduction of oxygen.

Reduced graphene oxide (RGO) was covalently functionalized by the in situ generation and reduction of anthraquinone diazonium salt. Deposition on multi-wall carbon nanotube (MWCNT) electrodes prevents the aggregation of RGO nanosheets and allows the stable deposition of modified graphene, accompanied with excellent electron transfer properties. Laccases were immobilized on the nanostructured electrode by the interaction between the anthraquinone moiety and the laccase hydrophobic pocket located near the T1 copper center. The MWCNT/f-RGO electrode exhibits efficient bioelectrocatalytic oxygen reduction, with current densities of up to 0.9 mA cm(-2) .

Interfacial structure and photocatalytic activity of magnetron sputtered TiO2 on conducting metal substrates.

The photocatalytic behavior of magnetron sputtered anatase TiO2 coatings on copper, nickel, and gold was investigated with the aim of understanding the effect of the metallic substrate and coating-substrate interface structure. Stoichiometry and nanoscale structure of the coating were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscope, and scanning and transmission electron microscopy. Photocatalytic behavior of the coating was explored by using optical spectrophotometry and electrochemical methods via photovoltage, photocurrent, and scanning kelvin probe microscopy measurements. The nature of the metal substrate and coating-substrate interface had profound influence on the photocatalytic behavior. Less photon energy was required for TiO2 excitation on a nickel substrate, whereas TiO2 coating on copper showed a higher band gap attributed to quantum confinement. However, the TiO2 coating on gold exhibited behavior typical of facile transfer of electrons to and from the CB, therefore requiring only a small amount of photon energy to make the TiO2 coating conductive.

Functionalization of glassy carbon with diazonium salts in ionic liquids.

The paper reports on the chemical functionalization of glassy carbon electrodes with 4-bromobenzene (4-BBDT) and 4-(4'-nitrophenylazo)benzene diazonium tetrafluoroborate (4-NAB) salts in ionic liquids. The reaction was carried out at room temperature in air without any external electrical bias in either hydrophobic (1-butyl-3-methylimidazolium hexafluorophosphate) or hydrophilic (1-butyl-3-methylimidazolium methyl sulfate) ionic liquids. The resulting surfaces were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electrochemical measurements. Electrochemical reduction of the terminal nitro groups allowed the determination of surface coverage and formation of an amine-terminated carbon surfaces. The results were compared to glassy carbon chemically modified in an aqueous solution in the presence of 1% sodium dodecyl sulfate (SDS) with the same diazonium salt. Furthermore, Raman spectroscopy coupled with electrochemical measurements allowed to distinguish between the reduction of -NO2 to -NH2 group and the -N=N- to -NH-NH- bond.

Raman imaging and Kelvin probe microscopy for the examination of the heterogeneity of doping in polycrystalline boron-doped diamond electrodes.

The issue of the heterogeneity of boron doping in microcrystalline diamond films was addressed by four different methods: micro-Raman spectroscopy and Raman imaging, Kelvin probe force microscopy, conducting atomic force microscopy, and scanning electrochemical microscopy. The samples were commercially available films from Windsor Scientific, with an average boron concentration of about 5 x 10(20) cm(-3). In agreement with previous works, all of the methods showed that the boron uptake was nonuniform across the surface of the electrode. Two different types of regions were evidenced, with metallic or semiconducting properties that were characterized with different types of Raman spectra. The line shape of these spectra was strongly dependent on the excitation wavelength. Local variations in electroactivity were evidenced by the SECM curves, which are related to the electronic properties of the individual grains, which, in turn, are governed by the boron content of the individual crystallites. In this study, two different micro-Raman imaging techniques were used that reveal the grain structure of the films: the images constructed from the diamond line intensity perfectly reproduced the optical image obtained by illuminating the sample in reflection. The method also allows detection of the presence of nondiamond carbon, especially in the metallic parts of the samples. Other spectral features (intensity of the boron-related broad lines, as well as the frequency and width of the diamond line) were used to construct images. In every case, the grain structure of the film was revealed, as well as twinning within individual crystallites. All approaches revealed that no enhanced doping or boron depletion occurred at the grain boundaries.

Nucleosides and ODN electrochemical detection onto boron doped diamond electrodes.

Boron doped diamond (BDD) is a promising material for electroanalytical chemistry due mainly to its chemical stability, its high electrical conductivity and to the large amplitude of its electroactive window in aqueous media. The latter feature allowed us to study the direct oxidation of the two electroactive nucleosides, guanosine and adenosine. The BDD electrode was first activated by applying high oxidizing potentials, allowing to increase anodically its working potential window through the oxidation of CH surface groups into hydroxyl and carbonyl terminations. Guanosine (1.2 V vs. Ag/AgCl) and adenosine (1.5 V vs. Ag/AgCl) could then be detected electrochemically with an acceptable signal to noise ratio. The electrochemical signature of each oxidizable base was assessed using differential pulse voltammetry (DPV), in solutions containing one or both nucleosides. These experiments pointed out the existence of adsorption phenomena of the oxidized products onto the diamond surface. Scanning electrochemical microscopy (SECM) was used to investigate these adsorption effects at the microscopic scale. The usefulness of BDD electrodes for the direct electrochemical detection of synthetic oligonucleotides is also evidenced.

Biotin grafting on boron-doped diamond.

Grafting of biotin on top of a polycrystalline boron-doped diamond layer was achieved by surface oxidation followed by an esterification reaction and revealed by fluorescently labelled streptavidin.