Search

International audience; One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyse this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-Nitrogen-Carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for sufficiently long time in operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in liquid-electrolyte and PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

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.

Polymer electrolyte fuel cells (PEFC) convert chemicalenergy into electrical energy with higher efficiency thaninternal combustion engines. They are particularly suited fortransportation applications or portable devices owing to theirhigh power density and low operating temperature. The latter ishowever detrimental to the kinetics of electrochemicalreactions and in particular to the reduction of oxygen at thecathode. The latter reaction requires enhancing by the verybest catalyst, today platinum. Even so, the cathode isresponsible for the main loss of voltage in the cell. Moreover,the scarce and expensive nature of platinum craves theoptimisation of its use. The purpose of this thesis was to better understand thefunctioning of the porous cathode in the PEFC. This wasachieved by developing physical models to predict the responseof the cathode to steady-state polarisation, currentinterruption (CI) and electrochemical impedance spectroscopy(EIS), and by comparing these results to experimental ones. Themodels account for the kinetics of the oxygen reduction as wellas for the transport of the reactants throughout the cathode,i.e. diffusion of gases and proton migration. The agglomeratestructure was assumed for the description of the internalstructure of the cathode. The electrochemical experiments wereperformed on electrodes having a surface of 0.5 cm2 using alaboratory fuel cell. The response of the cathode to various electrodecompositions, thickness, oxygen pressure and relative humiditywas experimentally investigated with steady-state polarisation,EIS and CI techniques. It is shown that a content in thecathode of 35-43 wt % of Nafion, the polymer electrolyte, gavethe best performance. Such cathodes display a doubling of theapparent Tafel slope at high current density. In this region,the current is proportional to the cathode thickness and to theoxygen pressure, which, according to the agglomerate model,corresponds to limitation by oxygen diffusion in theagglomerates. The same analysis was made using EIS. Moreover,experimental results showed that the Tafel slope increases fordecreasing relative humidity. For Nafion contents lower than 35wt %, the cathode becomes limited by proton migration too. ForNafion contents larger than 40 wt %, the cathode performance athigh current density decreases again owing to an additionalmass transport. The latter is believed to be oxygen diffusionthroughout the cathode. The activity for oxygen reduction ofcatalysts based on iron acetate adsorbed on a carbon powder andpyrolysed at 900°C in ammonia atmosphere was alsoinvestigated. It was shown that the choice of carbon has atremendous effect. The best catalysts were, on a weight basis,as active as platinum. Keywords:polymer electrolyte fuel cell, cathode, masstransport, porous electrode, modelling, agglomerate model,electrochemical impedance spectroscopy, current interrupt,transient techniques, non-noble catalysts NR 20140805

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich., This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively., Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by 57Fe 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 NH3 at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN4-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 (FexN, 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 FeN4-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.

In the past decades, the atmospheric CO2 emissions increased with the unrestrained combustion of fossil fuels to meet the growing energy demand, leading to serious environmental pollution and climate issues. The electrochemical CO2 reduction reaction (CO2RR) is a promising alternative to convert CO2 into value-added products, and has the potential to contribute to a carbon-neutral energy cycle by using surplus electricity generated from renewable sources (e.g., solar and wind). Among various materials, copper-based catalysts are most studied, given their unique capability to make hydrocarbons in considerable amounts and at reasonable overpotentials. In this thesis, in-depth understanding of the CO2 electroreduction process was firstly established by adjusting the local reaction environment of a CuOx nanoparticle (NP) model catalyst. A tunable product distribution during catalytic CO2 electroreduction could be achieved by varying areal particle densities and co-feeding CO2 reactant with CO. With higher areal density and lower mean interparticle distances, a shift in faradic efficiency towards C2H4 over CH4 was observed, which was attributed to enhanced CO(g) re-adsorption on catalyst surface sites in close proximity. Furthermore, the electroreduction of CO2 feed with CO-bleeding showed enhanced ethylene production over a broad potential range with various co-feed ratios. The origin of the carbon-atoms in C2H4 under co-feed conditions was traced and quantified by a custom-designed operando differential electrochemical mass spectrometry (DEMS) flow cell, giving unique mechanistic insight in the CO2-CO co-feed system. The co-feed mechanism was extended to novel tandem catalysts, in which a CO producer (Ag, or NiNC) works as local CO feeding source and combines with Cu-based catalysts, showing obvious enhancement in ethylene production compared to purely copper-based systems. Furthermore, a sheet-shaped CuOx catalyst was designed, developed and systematically investigated. In H-Cell measurements, a high activity for CO2RR could be observed, followed by tests in a micro flow cell, demonstrating a record C2H4 partial current density of 229 mA cm 2 and a C2+ partial current density of ~ 410 mA cm-2. Moreover, a combination of operando/(quasi) in situ XAS, WAXS, DEMS, S/TEM techniques were employed to discover structure-activity-selectivity relations under catalytic CO2RR operating conditions, delivering perspectives to design novel catalysts to produce hydrocarbons as the value-added products., CO2-Emissionen sind in den letzten Dekaden immer rasanter gestiegen, da der erhöhte Energiebedarf der wachsenden Weltwirtschaft vor allem durch fossile Energieträger gedeckt wurde und wird. Dies führt zu immer stärkerer Umweltverschmutzung und hat Auswirkungen auf das weltweite Klima. Die elektrochemische CO2-Reduktionsreaktion (CO2RR) bietet hierin eine vielversprechende Chance um CO2 aus der Atmosphäre oder als direkte Abgasverwertung zu entfernen und in höherwertige Produkte zu wandeln. Wird dazu überschüssige Elektrizität aus erneuerbaren Energien wie z.B. Solarenergie oder Windkraft genutzt, hat die CO2RR das Potenzial zu einer CO2-neutralen Energiewirtschaft beizutragen. Unter den bekannten Katalysatoren wurden kupferbasierte Materialien am häufigsten untersucht, da sie die einzigartige Eigenschaft besitzen zu großem Anteil Kohlenwasserstoffe zu produzieren ohne das Überpotential zu stark zu erhöhen. Diese Arbeit stellt zum ersten Mal ein tieferes Verständnis der CO2RR her, indem die lokale Umgebung eines CuOx-Nanopartikel (NP) Modellkatalysators systematisch variiert wurde. Durch Kontrolle der Partikelbeladungsdichte und der Zusammensetzung des Eduktstroms aus CO2 und CO kann die Produktverteilung gezielt eingestellt werden. Mit steigender Beladungsdichte, d.h. sinkendem Interpartikelabstand, wurde eine Verschiebung der Produkteffizienzen von CH4 zu C2H4 beobachtet, die hier auf verstärkte CO(g)-Readsorption auf nahe zusammenliegenden Katalysatorzentren zurückgeführt werden konnte. Des Weiteren ermöglicht der co-feed aus CO2/CO verstärkte Ethylenproduktion in einem breiten Potentialbereich indem das CO2/CO-Verhältnis angepasst wird. Zur Aufdeckung des Reaktionsmechanismus unter CO2/CO co-feed wurde die Herkunft der C-Atome im produzierten Ethylen über Isotopenmarkierung in einer eigens dafür designten Flusszelle für operando differentielle elektrochemische Massenspektrometrie (DEMS) untersucht und quantifiziert. Der vorgestellte co-feed Mechanismus konnte auf neuartige Tandemkatalysatoren übertragen werden. In diesem wird ein CO-produzierender Katalysator (z.B. Ag oder NiNC) mit einem kupferbasierten Katalysator kombiniert, was ebenfalls zu erhöhtere Ethylenproduktion gegenüber rein kupferbasierten Systemen führt. Außerdem, wurden in dieser Arbeit ein neuartiger, plattenförmiger CuOx Katalysator entworfen, synthetisiert und systematisch untersucht. Aufgrund herausragender CO2RR-Aktivität in H-Zellmessungen wurde der Katalysator in einer anwendungsnahen Mikroflusszelle getestet. Dort konnten für C2H4 und C2+ industrierelevante Rekordproduktstromdichten von 229 mA cm 2 bzw. ~410 mA cm 2 gezeigt werden. Weiterhin wurden durch Kombination von operando/(quasi) in situ XAS, WAXS, DEMS und S/TEM Techniken Struktur-Aktivitäts-Selektivitätsbeziehungen für die katalytische CO2RR entdeckt, die neue Wege zum Design von hochaktiven und –selektiven Katalysatoren aufzeigen, um gezielt höherwertige Kohlenwasserstoffe aus CO2 zu produzieren.