Reaktivitätsdeskriptoren für die Aktivität von molekularen MN4‐Katalysatoren zur Sauerstoffreduktion

Abstract

Zwischen allgemein bekannten Reaktivitätsdeskriptoren für die Aktivität von Metallelektroden bezüglich der Sauerstoffreduktion (ORR, oxygen reduction reaction) und der Reaktivität von molekularen Katalysatoren, im Speziellen auf Elektrodenoberflächen haftenden makrocyclischen MN4‐Metallkomplexen, lassen sich eindeutige Ähnlichkeiten herstellen. Wir zeigen auf, dass ein Zusammenhang zwischen dem MIII/MII‐Redoxpotential von MN4‐Chelaten und ihren M‐O2‐Bindungsenergien besteht. Insbesondere die Bindungsenergie von O2 (und anderen O‐Spezies) folgen dem MIII‐OH/MII‐Übergang von MnN4‐ und FeN4‐Chelaten. Die Vulkankurve für die Sauerstoffreduktion an MN4‐Katalysatoren ähnelt der Vulkankurve von Metallkatalysatoren: Katalysatoren auf dem Abschnitt mit schwacher Bindung (vor allem CoN4‐Chelate) ergeben H2O2 als Hauptprodukt, wobei der Beginn der Sauerstoffreduktion pH‐unabhängig auf der NHE‐Skala ist (und damit pH‐abhängig auf der RHE‐Skala); Katalysatoren auf dem Abschnitt mit starker Bindung ergeben H2O als Produkt und zeigen die erwartete pH‐Abhängigkeit auf der NHE‐Skala. Die hier behandelten Deskriptoren treffen auch auf thermisch behandelte pyrolysierte MN4‐Katalysatoren zu.

P-functionalized carbon nanotubes promote highly stable electrocatalysts based on Fe-phthalocyanines for oxygen reduction: Experimental and computational studies

“…Table 2 shows the Fe III /Fe II cathodic peak potential for FePc/SWCNT, FePc/SWCNT-f, FePc/hCNT and FePc/hCNT-f, which can be used as a reactivity descriptor of FeN4 electrocatalysts for the ORR [21]. Comparing the two types of carbon nanotubes, responses for hCNT-related materials show more positive Fe III /Fe II redox potential, which indicates higher activity, especially for such catalysts prepared from functionalized carbon supports.…”

“…Moreover, MN4 macrocycles aroused much attention because of their low price, excellent thermal and chemical stabilities and tailorable catalytic activity [20]. Concerning the ORR, diverse factors are important for describing the activity of MN4 electrocatalysts, including the d electrons of the metal, the binding energy to dioxygen (donor-acceptor intermolecular interaction of N4-M-O2) or reacting molecule and intermediates, the catalytic site density and accessibility towards the active sites and the formal M (n)+ /M (n-1)+ redox potential of the catalysts, among others [21,22]. Interestingly, many of these parameters are strongly influenced by the central metal ion and the environment of the MN4 complex.…”

“…Interestingly, many of these parameters are strongly influenced by the central metal ion and the environment of the MN4 complex. Among all the metallic complexes, numerous contributions have highlighted the particular use of iron (II) phthalocyanines (FePc) supported by a wide variety of nanostructured carbon materials [16,23] as extraordinary candidates in terms of high ORR activity, resistance to CO deactivation and tolerance towards methanol crossover [21,23,24]. The catalytic behaviour of FePc is largely governed by the central Fe II atom in Fe-Nx configuration as the active sites for ORR, which is mainly due to the partially filled d orbitals in Fe complexes [21,25,26].…”

“…Among all the metallic complexes, numerous contributions have highlighted the particular use of iron (II) phthalocyanines (FePc) supported by a wide variety of nanostructured carbon materials [16,23] as extraordinary candidates in terms of high ORR activity, resistance to CO deactivation and tolerance towards methanol crossover [21,23,24]. The catalytic behaviour of FePc is largely governed by the central Fe II atom in Fe-Nx configuration as the active sites for ORR, which is mainly due to the partially filled d orbitals in Fe complexes [21,25,26]. However, further progress in the long-term durability and selectivity is still required, since Fe-N-C catalysts are known to suffer from degradation in fuel cell conditions [24,26].…”

“…Furthermore, it is essential to consider the large influence of the nature of the support on the electrocatalytic performance [20,23], sometimes even higher than the metal identity of these macrocycles or their functionalization/substitution. Therefore, a key factor is to find the most convenient support that favors the correct distribution of FePc molecules with sufficiently strong interaction, in order to facilitate the accessibility of oxygen to the active sites and the subsequent electron transfer through the inner-sphere mechanism during the electrochemical reduction of O2 [20,21,27]. Particularly, carbon nanotubes (CNTs) have been placed into the focus of recent studies for the immobilization of catalysts because of their extraordinary fast electron delivery to the ORR active sites, together with a high specific surface area, mechanical strength and chemical and electrochemical stabilities [28].…”