Aiming at a better understanding of the processes in future Mg-air batteries we have investigated the influence of different additives on the ORR/OER in magnesium-containing N-Butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) on a glassy carbon electrode. Specifically we focus on the complexing agent 18-crown-6, which can complex the magnesium ions and thus hinder the passivation caused by their reaction with ORR products such as superoxide and peroxide anions. Furthermore, to inhibit electrode passivation by reaction with trace impurities of water, we added borane dimethylamine complex (NBH) as potential water-removing agent. The electrochemical processes were characterized by differential electrochemical mass spectrometry (DEMS) to monitor the consumed and evolved O 2 in the ORR/OER and thus to determine the number of transferred electrons. Crown ether and also NBH were found to efficiently mask the Mg 2+ ions. When Crown ether was used over-stoichiometrically, a reduced formation of a passivation layer was observed, while too high concentration reduced the reversibility of the ORR/OER.

The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co3O4(1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co3O4(1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O-2) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co3O4(1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 V-RHE (RHE: reversible hydrogen electrode) at 0.1 mA cm(-2) was almost unchanged.

Amorphous and graphitized nitrogen-doped (N-doped) carbon spheres are investigated as structurally well-defined model systems to gain a deeper understanding of the relationship between synthesis, structure, and their activity in the oxygen reduction reaction (ORR). N-doped carbon spheres were synthesized by hydrothermal treatment of a glucose solution yielding carbon spheres with sizes of 330 ± 50 nm, followed by nitrogen doping via heat treatment in ammonia atmosphere. The influence of a) varying the nitrogen doping temperature (550–1000 °C) and b) of a catalytic graphitization prior to nitrogen doping on the carbon sphere morphology, structure, elemental composition, N bonding configuration as well as porosity is investigated in detail. For the N-doped carbon spheres, the maximum nitrogen content was found at a doping temperature of 700 °C, with a decrease of the N content for higher temperatures. The overall nitrogen content of the graphitized N-doped carbon spheres is lower than that of the amorphous carbon spheres, however, also the microporosity decreases strongly with graphitization. Comparison with the electrocatalytic behavior in the ORR shows that in addition to the N-doping, the microporosity of the materials is critical for an efficient ORR.

The influence of different additives on the oxygen reduction reaction/oxygen evolution reaction (ORR/OER) in magnesium-containing N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([BMP][TFSI]) on a glassy carbon electrode was investigated to gain a better understanding of the electrochemical processes in Mg-air batteries. 18-Crown-6 was used as a complexing agent for Mg ions to hinder the passivation caused by their reaction with ORR products such as superoxide and peroxide anions. Furthermore, borane dimethylamine complex (NBH) was used as a potential water-removing agent to inhibit electrode passivation by reacting with trace impurities of water. The electrochemical processes were characterized by differential electrochemical mass spectrometry to monitor the consumed and evolved O2 in the ORR/OER and determine the number of transferred electrons. Crown ether and NBH efficiently masked Mg2+ . A stochiometric excess of crown ether resulted in reduced formation of a passivation layer, whereas at too high concentrations the reversibility of the ORR/OER was diminished.

The performance of structurally and chemically well‐defined Ni‐free and Ni‐modified single‐crystalline Co$_{3}$O$_{4}$(1 1 1) thin‐film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni‐modified Co$_{3}$O$_{4}$(1 1 1) film electrodes were prepared and characterized under ultrahigh‐vacuum conditions by scanning tunneling microscopy and X‐ray photoelectron spectroscopy. Both Ni decoration (by post‐deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O$_{2}$) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co$_{3}$O$_{4}$ (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 V$_{RHE}$ (RHE: reversible hydrogen electrode) at 0.1 mA cm$^{-2}$ was almost unchanged.

Aiming at a better understanding of the molecular scale mechanism of the oxygen reduction reaction (ORR) on metal-free catalysts, we have systematically investigated this reaction in a combined experimental and theoretical approach on a set of catalysts consisting of nitrided carbon spheres. These catalysts, which were prepared similarly, but applying different carbonization/nitriding temperatures, were studied in acidic and alkaline electrolyte. The physical properties characterization of both, the bulk materials and the surface, was performed by transmission electron microscopy (TEM), N2 sorption, X-ray photoelectron spectroscopy (XPS), CHN analysis and energy dispersive X-ray spectroscopy (EDX) and Temperature Programmed Desorption (TPD) of CO2. Electrochemical and –catalytic properties were characterized by rotating ring disk electrode (RRDE) measurements. Mechanistic aspects were explored by kinetic analysis of the ORR and by evaluation of the kinetic isotope effect (H-D exchange), using deuterated electrolytes (KOD and D2SO4). In combination with density functional theory based calculations, these kinetic data provide detailed insights into the reaction mechanism and its dependence on pH effects. In acidic electrolyte, the first proton coupled electron transfer (PCET) is identified as rate determining step (RDS), while in alkaline electrolyte the first electron transfer (ET) to O2∗−ad is rate determining, followed by fast protonation. The potential of these highly active catalysts and the influence of structural effects are discussed.

Nitridated carbon (NC) catalysts have attracted considerable interest as promising Pt-free alternatives to standard Pt/C catalysts in the oxygen reduction reaction (ORR). Aiming at a better understanding of the microscopic reaction mechanism and of the nature of the reaction-limiting step, we have investigated the ORR kinetics and in particular the kinetic isotope effects (KIEs) therein for three different NC catalysts with different nitrogen contents. The measurements were performed using ordinary and deuterated water electrolytes, under both alkaline and acidic solutions. From an analysis of the ORR kinetics on the most active NC-I catalyst, including the k(H)/k(D) ratio and the transfer coefficients, and from density functional theory (DFT) based computations, we derive that the initial proton-coupled electron transfer (PCET), whose process is to form OOH* from O-2*, acts as the potential-determining step (PDS) under acidic conditions and the initial electron transfer, whose process is to form (O-2(center dot-))* from O-2*, occurs under alkaline conditions. For the other NC catalysts with higher N concentrations we found significantly lower ORR activities. The DFT calculations support these conclusions and observations, showing that the formation of the OOH* intermediate as a result of the initial PCET to the metastable adsorbed O-2 is the key step for an efficient ORR on NC electrocatalysts under acidic conditions, acting as the PDS. Furthermore, they show that both the configuration of adsorbed O-2 and the N doping content sensitively affect the reaction pathway. This explains the experimental observation that only low nitrogen doping levels support an efficient ORR pathway. The work provides detailed insight into the microscopic mechanism of the ORR on the complex surfaces of NC catalysts and its dependence on the content and configuration of the N dopant atoms.

Aiming at Pt nanoparticle catalysts for application in polymer electrolyte membrane fuel cell (PEMFC) cathodes, which are highly active and more corrosion resistant under realistic mobile applications than common Pt/C catalysts, we have prepared and investigated Pt catalysts supported on highly stable, nanostructured composite materials containing carbon nanotubes (CNTs) and titania. TiO2@CNT composite materials are synthesized via sol-gel processing and subsequent Pt deposition. The physical and electrochemical properties as well as the stability of these catalysts, as determined by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), rotating ring disk electrode (RRDE) measurements and accelerated degradation tests (ADTs), were compared with those of commercial Pt/C, Pt/TiO2 and Pt/CNT. The measurements reveal a high activity of the composite catalyst, comparable to that of the Pt/C catalyst, but an almost complete loss of ORR activity upon an ADT procedure simulating start-stop behavior. In contrast to carbon supported catalysts, where degradation is mainly associated with corrosion at high potentials, we find the titania supported catalysts to mainly suffer from the reductive treatment in the ADTs. Consequences for the use of Pt catalysts supported on reducible oxides such as TiO2 as cathode catalysts in fuel cell applications are discussed.

As part of an ongoing effort to develop novel, highly active and stable Pt-free catalysts for the oxygen reduction reaction (ORR), we here report the synthesis, structural characteristics and electrochemical/electrocatalytic properties of novel core-shell composite materials, consisting of a spherical nitrided carbon core and a tantalum (oxy)nitride shell. The (nitrided) carbon core is supposed to improve the electrical conductivity of the material and the (oxy)nitride shell is intended to protect the core against electrochemical corrosion. Spherical core-shell TaOxNy@CmNn composite particles were synthesized by sol-gel deposition of tantalum oxide on preformed carbon spheres, which were prepared by hydrothermal carbonization of glucose and subsequent nitriding in ammonia vapor at different temperatures (700 degrees C - 1150 degrees C). The influence of the nitriding temperature on the structure and phase composition of the resulting composite particles was evaluated, employing a variety of techniques, including electron microscopy (SEM, TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), elemental analysis, thermogravimetric analysis (TGA), IR spectroscopy and N-2 sorption measurements, and correlated with changes in the electrochemical/electrocatalytic behavior. These core-shell composite materials show a significantly improved ORR activity compared to pure tantalum (oxy)nitrides, in particular upon nitriding at 1000 degrees C, while the selectivity for the 4-electron pathway to H2O still requires improvement. The physical origin of the high activity of these materials and contributions from different phases are discussed. (C) 2016 Elsevier Ltd. All rights reserved.