Standard electrochemical data for high-quality, boron-doped diamond thin-film electrodes are presented. Films from two different sources were compared (NRL and USU) and both were highly conductive, hydrogen-terminated, and polycrystalline. The films are acid washed and hydrogen plasma treated prior to use to remove nondiamond carbon impurity phases and to hydrogen terminate the surface. The boron-doping level of the NRL film was estimated to be in the mid 1019 B/cm3 range, and the boron-doping level of the USU films was approximately 5 x 10(20) B/cm(-3) based on boron nuclear reaction analysis. The electrochemical response was evaluated using Fe-(CN)6(3-/4-), Ru(NH3)6(3+/2+), IrCl6(2-/3-), methyl viologen, dopamine, ascorbic acid, Fe(3+/2+), and chlorpromazine. Comparisons are made between the apparent heterogeneous electron-transfer rate constants, k0(app), observed at these high-quality diamond films and the rate constants reported in the literature for freshly activated glassy carbon. Ru(NH3)6(3+/2+), IrCl6(2-/3-), methyl viologen, and chlorpromazine all involve electron transfer that is insensitive to the diamond surface microstructure and chemistry with k0(app) in the 10(-2)-10(-1) cm/s range. The rate constants are mainly influenced by the electronic properites of the films. Fe(CN)6(3-/4-) undergoes electron transfer that is extremely sensitive to the surface chemistry with k0(app) in the range of 10(-2)-10(-1) cm/s at the hydrogen-terminated surface. An oxygen surface termination severely inhibits the rate of electron transfer. Fe(3+/2+) undergoes slow electron transfer at the hydrogen-terminated surface with k0(app) near 10(-5) cm/s. The rate of electron transfer at sp2 carbon electrodes is known to be mediated by surface carbonyl functionalities; however, this inner-sphere, catalytic pathway is absent on diamond due to the hydrogen termination. Dopamine, like other catechol and catecholamines, undergoes sluggish electron transfer with k0(app) between 10(-4) and 10(-5) cm/s. Converting the surface to an oxygen termination has little effect on k0(app). The slow kinetics may be related to weak adsorption of these analytes on the diamond surface. Ascorbic acid oxidation is very sensitive to the surface termination with the most negative Ep(ox) observed at the hydrogen-terminated surface. An oxygen surface termination shifts Ep(ox) positive by some 250 mV or more. An interfacial energy diagram is proposed to explain the electron transfer whereby the midgap density of states results primarily from the boron doping level and the lattice hydrogen. The films were additionally characterized by scanning electron microscopy and micro-Raman imaging spectroscopy. The cyclic voltammetric and kinetic data presented can serve as a benchmark for research groups evaluating the electrochemical properties of semimetallic (i.e., conductive), hydrogen-terminated, polycrystalline diamond.

International audienceThe electrochemical oxidation of carbon is a pivotal problem for low-temperature electrochemical generators, among which are proton-exchange membrane fuel cells (PEMFCs), and (non)aqueous-electrolyte Li-air batteries. In this contribution, the structure-sensitivity of the electrochemical corrosion of high-surface area carbon (HSAC) used to support catalytic materials in PEMFC electrodes is investigated in model (liquid electrolyte, 96 h potentiostatic holds at different electrode potentials ranging from 0.40 to 1.40 V at T = 330 K) and real PEMFC operating conditions (solid polymer electrolyte, 12,860 h of operation at constant current). Characterizations from Raman spectroscopy demonstrate that the disordered domains of HSAC supports (amorphous carbon and defective graphite crystallites) are preferentially oxidized at voltages related to the PEMFC cathode (0.40 < E < 1.00 V). Excursions to high electrode potential E > 1.00 V, witnessed during start-up and shut-down of PEMFC systems, accelerate this phenomenon and propagate the electrochemical oxidation to the graphitic domains of the HSAC. Thanks to X-ray photoelectron spectroscopy, a better understanding of the relationships existing between structural changes and carbon surface oxides coverage is also emerging for the first time

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Recent non-precious-metal catalysts (NPMCs) show promise to replace in the future platinum-based catalysts currently needed for the electroreduction of oxygen (ORR) in proton-exchange membrane fuel cells (PEMFCs). Among NPMCs, the most mature subclass of materials is prepared via the pyrolysis of metal (Fe and Co), nitrogen, and carbon precursors (labeled as metal–NC). Such materials often comprise different types of nitrogen groups and metal species, from atomically dispersed metal ions coordinated to nitrogen to metallic or metal–carbide particles, partially or completely embedded in graphene shells. While disentangling the different contributions of these species to the initial ORR activity of metal–NC catalysts with multidunous active sites is complex, following the fate of these different active sites during electrochemical aging is even more difficult. To shed light onto this, herein, six metal–NC catalysts were synthesized and characterized before/after aging with two different accelerated stress tests (AST) simulating PEMFC cathode operating conditions either in steady-state or transient conditions. The samples differed from each other by the nature of the metal (Fe or Co), the metal content, and the heating mode applied during pyrolysis. Catalysts featuring either only atomically dispersed metal-ion sites (metal–N x C y ) or only metal nanoparticles encapsulated in the carbon matrix (metal@N–C) were obtained after pyrolysis of catalyst precursors containing 0.5 or 5.0 wt % of metal, respectively. All six catalysts showed high beginning-of-life ORR mass activity, but the ASTs revealed marked differences in their ORR activity at end-of-life. After the load-cycling AST (10000 cycles), metal–NC catalysts with metal–N x C y sites retained most of their initial activity at 0.8 V (60–100%), while those with metal@N–C particles retained only a small fraction of initial activity (10–20%). Metal–NC catalysts with metal–N x C y sites lost only 25% of their initial ORR activity after 30000 load cycles at 80 °C, thereby reaching the 2020 stability target defined by US Department of Energy. After 10000 start-up/shut-down cycles, no catalyst showed measurable ORR activity at 0.8 V. However, after 1000 start-up/shut-down cycles, most of the metal–NC catalysts initially comprising metal–N x C y sites showed measurable ORR activity at 0.8 V, while those initially comprising metal@N–C particles did not. Energy-dispersive X-ray spectroscopy and Raman spectroscopy measurements of the cycled rotating disk electrodes revealed that demetalation of the catalytic centers and corrosion of the carbon matrix are the main causes of ORR activity decay during load-cycling and start-up/shut-down cycling, respectively. In contrast to what could have been intuitively expected, the metal–N x C y sites are more robust to both demetalation and carbon corrosion than metal@N–C sites.

Although nanodiamonds (NDs) appear as one of the most promising nanocarbon materials available so far for biomedical applications, their risk for human health remains unknown. Our work was aimed at defining the cytotoxicity and genotoxicity of two sets of commercial carboxylated NDs with diameters below 20 and 100 nm, on six human cell lines chosen as representative of potential target organs: HepG2 and Hep3B (liver), Caki-1 and Hek-293 (kidney), HT29 (intestine) and A549 (lung). Cytotoxicity of NDs was assessed by measuring cell impedance (xCELLigence® system) and cell survival/death by flow cytometry while genotoxicity was assessed by γ-H2Ax foci detection, which is considered the most sensitive technique for studying DNA double-strand breaks. To validate and check the sensitivity of the techniques, aminated polystyrene nanobeads were used as positive control in all assays. Cell incorporation of NDs was also studied by flow cytometry and luminescent N-V center photoluminescence (confirmed by Raman microscopy), to ensure that nanoparticles entered the cells. Overall, we show that NDs effectively entered the cells but NDs do not induce any significant cytotoxic or genotoxic effects on the six cell lines up to an exposure dose of 250 µg/mL. Taken together these results strongly support the huge potential of NDs for human nanomedicine but also their potential as negative control in nanotoxicology studies.

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.

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Fe-N-C catalysts containing atomic FeN x sites are promising candidates as precious-metal-free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells.T he 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 O 2 -v s. Ar-saturated pH 1e lectrolyte.R aman spectroscopyr esults show carbon corrosion after AST in O 2 , even when cycling at lowp otentials,w hile no corrosion occurred after any load cycling AST in Ar.T he load-cycling AST in O 2 leads to loss of asignificant fraction of FeN x sites,as shown by energy dispersive X-rays pectroscopya nalyses,a nd to the formation of Fe oxides.T he results support that the unexpected carbon corrosion occurring at such low potential in the presence of O 2 is due to reactive oxygen species produced between H 2 O 2 and Fe sites via Fenton reactions.