Peter Bogdanoff

Cross-Laboratory Experimental Study of Non-Noble-Metal Electrocatalysts for the Oxygen Reduction Reaction

Nine non-noble-metal catalysts (NNMCs) from five different laboratories were investigated for the catalysis of O(2) electroreduction in an acidic medium. The catalyst precursors were synthesized by wet impregnation, planetary ball milling, a foaming-agent technique, or a templating method. All catalyst precursors were subjected to one or more heat treatments at 700-1050 degrees C in an inert or reactive atmosphere. These catalysts underwent an identical set of electrochemical characterizations, including rotating-disk-electrode and polymer-electrolyte membrane fuel cell (PEMFC) tests and voltammetry under N(2). Ex situ characterization was comprised of X-ray photoelectron spectroscopy, neutron activation analysis, scanning electron microscopy, and N(2) adsorption and its analysis with an advanced model for carbonaceous powders. In PEMFC, several NNMCs display mass activities of 10-20 A g(-1) at 0.8 V versus a reversible hydrogen electrode, and one shows 80 A g(-1). The latter value corresponds to a volumetric activity of 19 A cm(-3) under reference conditions and represents one-seventh of the target defined by the U.S. Department of Energy for 2010 (130 A cm(-3)). The activity of all NNMCs is mainly governed by the microporous surface area, and active sites seem to be hosted in pore sizes of 5-15 A. The nitrogen and metal (iron or cobalt) seem to be present in sufficient amounts in the NNMCs and do not limit activity. The paper discusses probable directions for synthesizing more active NNMCs. This could be achieved through multiple pyrolysis steps, ball-milling steps, and control of the powder morphology by the addition of foaming agents and/or sulfur.

Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells

Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by (57)Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH(3) at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN(4)-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (Fe(x)N, with x≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN(4)-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e(-) per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials.

EXAFS, XPS and electrochemical studies on oxygen reduction catalysts obtained by heat treatment of iron phenanthroline complexes supported on high surface area carbon black

Oxygen reduction catalysts have been prepared on the basis of heat-treated, carbon supported iron phenanthroline complexes. The activity of the catalyst towards oxygen reduction depends on the surface area of the carbon used in the synthesis. It is higher than the activity of other alternative oxygen reduction catalysts based on ruthenium. Related to the amount of metal, the activity is comparable to that of the state-of-the-art oxygen reduction catalyst, platinum supported onto carbon. EXAFS measurements indicate that the structure of the active centre of the catalyst consists of an iron ion, which is coordinated to four nitrogen atoms. No crystalline particles can be found in the catalyst using TEM.

Mott-Schottky Analysis of Nanoporous Semiconductor Electrodes in Dielectric State Deposited on SnO2 ( F ) Conducting Substrates

This paper analyzes the dark capacitance of nanostructured electrodes in the dielectric state, with particular emphasis on TiO 2 electrodes deposited over a transparent conducting substrate of SnO 2(F). It is shown that at those potentials where the TiO 2 nanostructure is in the dielectric state, the capacitance is controlled by the contact SnO 2(F)/~electrolyte, TiO2). The partial or total covering of the substrate by a dielectric medium causes a modification of the Mott-Schottky plot of the bare substrate. We provide a mapping of the various Mott-Schottky curves that will appear depending on the film characteristics. If the dielectric nanoparticles completely block part of the substrate surface, the slope of the Mott-Schottky plot increases ~with the same apparent flatband potential! as an effect of area reduction. The covering of a significant fraction of the surface by a thin dielectric layer shifts the apparent flatband negatively. Measurements on several TiO 2 nanostructured electrodes show that the capacitance contribution of the semiconductor network in the dielectric state is very low, indicating that the field lines penetrate little into the TiO2 network, not much further than the first particle. The different surface covering observed for rutile-anatase and pure anatase colloids is explained by lattice matching rules with the substrate. By comparing different electrodes, the Helmholtz capacitance at the SnO2(F)/solution interface was calculated and the apparent flatband potential was corrected for the effect of band unpinning.

Unveiling N-protonation and anion-binding effects on Fe/N/C-catalysts for O2 reduction in PEM fuel cells

The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinumbased catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction, but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C-catalysts prepared through a pyrolysis in NH3 is mostly imparted by acid-resistant FeN4-sites whose turnover frequency for the O2 reduction can be regulated by fine chemical changes of the catalyst surface. We show that surface N-groups protonate at pH 1 and subsequently bind anions. This results in decreased activity for the O2 reduction. The anions can be removed chemically or thermally, which restores the activity of acid-resistant FeN4-sites. These results are interpreted as an increased turnover frequency of FeN4-sites when specific surface N-groups protonate. These unprecedented findings provide new perspective for stabilizing the most active Fe/N/C-catalysts known to date.

On an Easy Way To Prepare Metal–Nitrogen Doped Carbon with Exclusive Presence of MeN4-type Sites Active for the ORR

Today, most metal and nitrogen doped carbon catalysts for ORR reveal a heterogeneous composition. This can be reasoned by a nonoptimized precursor composition and various steps in the preparation process to get the required active material. The significant presence of inorganic metal species interferes with the assignment of descriptors related to the ORR activity and stability. In this work we present a simple and feasible way to reduce the contribution of inorganic metal species in some cases even down to zero. Such catalysts reveal the desired homogeneous composition of MeN4 (Me = metal) sites in the carbon that is accompanied by a significant enhancement in ORR activity. Among the work of other international groups, our iron-based catalyst comprises the highest density of FeN4 sites ever reported without interference of inorganic metal sites.

Influence of selenium on the catalytic properties of ruthenium-based cluster catalysts for oxygen reduction

The favourable influence of selenium on the catalytic properties of Ru-based catalysts for the oxygen reduction reaction in acid electrolytes has been investigated by rotating disk electrode measurements. Compared to the oxygen reduction of selenium-free Ru-based catalysts, the overpotential at low current densities (ca. 10 μA cm−2) is not affected by the presence of selenium whereas selenium-containing catalysts show higher current densities under fuel cell relevant conditions. The kinetically controlled current density at 0.6 V versus SHE increases 4–5 fold with increasing selenium content. A maximum value is obtained at about 15 mol% Se. This effect is tentatively explained by a modification of the catalytic active centre, which is assumed to consist of RuCCO complexes. IR spectroscopic investigations indicate a reaction of selenium with these complexes. This model is also supported by the study of the electrooxidation of CO. In contrast to the selenium-free catalyst, no CO oxidation is observed on the selenium-containing catalyst. Additional effects of selenium are an enhanced stability towards electrochemical oxidation and a lower amount of Ru oxides formed during synthesis, as evidenced from XRD investigations. Direct four electron oxygen reduction to water is efficient and H2O2 production of these catalysts is small (about 5% at potentials <0.3 V vs. SHE ).

Influence of Sulfur on the Pyrolysis of CoTMPP as Electrocatalyst for the Oxygen Reduction Reaction

This work presents the preparation and investigation of pyrolyzed cobalt-tetramethoxyphenylporphyrin (CoTMPP) supported by iron oxalate with and without sulfur as electrocatalysts for the oxygen reduction reaction (ORR) in acid media. A preparation method which needs no addition of carbon supports allows the structural investigation of the pyrolysis products by X-ray photoemission spectroscopy, Raman spectroscopy, and X-ray diffractometry without any interferences of a carbon support. Already with low metal loading, rotating ring disk electrode measurements reveal the high ORR activity and enhanced selectivity which are apparently caused by an increased number of catalytic centers and higher efficient ones due to a well developed porosity and a suitable molecular structure of the formed carbon. A thermogravimetric investigation of the pyrolysis process shows that the addition of sulfur to the precursor influences the carbonization of the porphyrin in a favorable way. It has been found that extended graphene layers present a particularly suitable matrix for highly active catalytic centers.

Water oxidation by amorphous cobalt-based oxides: in situ tracking of redox transitions and mode of catalysis

Water oxidation by amorphous oxides is of high interest in artificial photosynthesis and other routes towards non-fossil fuels, but the mode of catalysis in these materials is insufficiently understood. We tracked mechanistically relevant oxidation-state and structural changes of an amorphous Co-based catalyst film by in situ experiments combining directly synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis. Unlike a classical solid-state material, the bulk material is found to undergo chemical changes. Two redox transitions at midpoint potentials of about 1.0 V (CoII0.4CoIII0.6 ↔ all-CoIII) and 1.2 V (all-CoIII ↔ CoIII0.8CoIV0.2) vs. NHE at pH 7 are coupled to structural changes. These redox transitions can be induced by variation of either electric potential or pH; they are broader than predicted by a simple Nernstian model, suggesting interacting bridged cobalt ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass spectroscopy reveals that accumulated oxidizing equivalents facilitate dioxygen formation. On these grounds, a new framework model of catalysis in an amorphous, hydrated and volume-active oxide is proposed: Within the oxide film, cobalt ions at the margins of Co-oxo fragments undergo CoII ↔ CoIII ↔ CoIV oxidation-state changes coupled to structural modification and deprotonation of Co-oxo bridges. By the encounter of two (or more) CoIV ions, an active site is formed at which the O–O bond-formation step can take place. The Tafel slope is determined by both the interaction between cobalt ions (width of the redox transition) and their encounter probability. Our results represent a first step toward the development of new concepts that address the solid-molecular Janus nature of the amorphous oxide. Insights and concepts described herein for the Co-based catalyst film may be of general relevance also for other amorphous oxides with water-oxidation activity.

Catalysts for oxygen reduction from heat-treated carbon-supported iron phenantroline complexes

Oxygen reduction catalysts were prepared by heat treatment of carbon supported iron phenantroline complexes in Ar or NH3. The optimum carbon black loading with iron was found to be 2%, the optimum heat treatment temperature was about 800 °C. X-ray diffractogramms and TEM showed the occurrence of crystalline species at higher catalyst loadings; however, these species seem not to contribute significantly to the catalytic activity. From the slope of the Koutecky–Levich plot, an average number of 3.7 electrons transferred per oxygen molecule was calculated, which is consistent with RRDE data. A Tafel slope of about 120 mV (decade)−1 indicates that the first electron transfer is rate determining.