Iris Herrmann

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.

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.

Electrocatalysts for Oxygen Reduction Prepared by Plasma Treatment of Carbon-Supported Cobalt Tetramethoxyphenylporphyrin

during the pyrolysis of CoTMPP. Additionally, solid decomposition products of the oxalate metal and oxides form a framework embedded within the pyrolysis product which is removed by a subsequent acid treatment. Finally, a highly porous carbon matrix with embedded centers is obtained. With this procedure materials with high electrochemical activities toward the oxygen reduction have been achieved in rotating disk electrode RDE measurements in 0.5 M H2SO4 at 0.7 V normal hydrogen electrode NHE close to that of commercial 20% Pt/C E-TEK. However, even with this advanced technique scanning electron microscopy SEM images reveal particles of several micrometer dimension composed of that highly porous material. In technical applications e.g., gas diffusion electrodes in fuel cells this presumably leads to a lower efficiency due to long diffusion pathways for protons and gas molecules. Therefore, alternative synthesis techniques are desired which result in smaller particle sizes 50‐100 nm as it is state of the art for carbon supports which are utilized for commercial platinum catalysts Vulcan, Black Pearls ca. 20 nm. In the last decades plasma treatment of organic material was intensively investigated and is considered as a promising nanotechnological approach. Our contribution shows that the transfer of CoTMPP into highly electrochemical active CoN4 centers embedded in a carbon matrix can be reached by plasma treatment instead of conservative heat-treatment. In order to find optimal operating parameters CoTMPP on Black Pearls was processed by plasma treatment as well as by classical heat-treatment. The obtained products were compared in terms of structure and catalytic activity.

On the Influence of Sulphur on the Pyrolysis Process of FeTMPP-Cl-based Electro- Catalysts with Respect to Oxygen Reduction Reaction (ORR) in Acidic Media

Pyrolysis of chloroiron-tetramethoxyphenyl-porphyrin (FeTMPP-Cl) in the presence of iron oxalate (± sulphur) leads to the formation of higly porous and active catalysts for the oxygen reduction reaction (ORR). In order to clarify the influence of sulphur the pyrolysis process is analyzed by thermogravimetry (TG) and by high-temperature X-ray diffraction (HT-XRD). In the absence of sulphur iron carbide (FexC) is formed which catalyses the proceeding graphitisation of the pyrolysis products. As a result catalytic active centres are decomposed by this reaction. This can be avoided by the addition of sulphur because iron monosulphide (FeS; troilite) is formed instead of FexC. Furthermore, FeS can easily be removed in a successive etching step so that nearly all inactive by-products can be removed. The results are in accordance with the higher electrochemical performance of the sulphur containing catalysts.

Influence of the Molecular and Mesoscopic Structure on the Electrocatalytic Activity of Pyrolysed CoTMPP in the Oxygen Reduction

Novel oxygen reduction electrocatalysts are obtained by thermolysis of chelates such as cobalt- tetramethoxyphenylporphyrin (CoTMPP) in presence of the foaming agent FeC2O4. EXAFS spectra point to the presence of molecular Fe-N4 and Co N4 centres embedded in a graphite-like matrix. The foaming agent provokes the generation of a porous material (800 m2/g) exhibiting a high fraction of mesopores (5 to 30 nm). Adding elemental sulphur to the precursors CoTMPP and FeC2O4, the catalytic activity in ORR of the final catalyst significantly improves due to an increase of mesopores and the formation of extended graphene layers in a glassy carbon-type material. The added sulphur not only facilitates the removal of inorganic by-products formed in the process of pyrolysis, but also provides an appropriate environment for catalytic centres so that the formation of undesired H2O2 is significantly decreased. In RDE measurements a catalytic activity in ORR equal to commercial platinum catalysts has been achieved.