Reactivity Descriptors for the Activity of Molecular MN4 Catalysts for the Oxygen Reduction Reaction

Catalysis and Surface Chemistr

Probing the Fen+/Fe(n − 1)+redox potential of Fe phthalocyanines and Feporphyrins as a reactivity descriptor in the electrochemical oxidation of cysteamine

Catalysis and Surface Chemistr

Experimental reactivity descriptors of M-N-C catalysts for the oxygen reduction reaction

9 pags., 6 figs., 2 tabs.Pyrolyzed non-precious metal catalysts (NPMCs) are promising materials to replace platinum-based catalysts in the cathode of the fuel cells. These catalysts present high catalytic activity both in alkaline and acid media for the oxygen reduction reaction (ORR). These catalysts are essentially heterogeneous as they can present different types of active sites. MNx structures have been proposed as the most active for the ORR, similar to those of the MN4 structures of metal porphyrins and phthalocyanines. Several parameters have been proposed as reactivity descriptors to correlate the structure of these materials with their catalytic activity, such as the amount of MNx and of pyridinic nitrogens in the graphitic structure. In this study, we have explored the metal center redox potential of the catalyst as an overall reactivity descriptor. We have investigated this descriptor for pyrolyzed and intact catalysts for the ORR in acid and basic media. We have found that for all catalysts tested, there is a linear correlation between the redox potential of the catalyst and the catalytic activity expressed as (log i)). The activity increases as the redox potential becomes more positive. The correlation gives a straight line of slope close to +0.12 V/decade which agrees with the theoretical slope proposed in a previous publication assuming the adsorbed M − O follows a Langmuir isotherm and that the redox potential is directly linked to the M − O binding energy. The Tafel plots present two slopes, at low and high overpotentials. Based on these results, we proposed two different mechanisms. The low Tafel slopes of −60 mV appear at potentials where the surface concentration of M(II) active sites is potential dependent (close to the onset potential). At higher overpotentials the surface coverage of M(II) becomes constant and the slope changes to −0.120 V/decade.This work was supported by Fondecyt Regular Projects 1161117,1181037, Fondecyt Postdoctoral Projects 3170330 and 3180509, andConicyt Scholarship 21160955

In search of the most active MN4 catalyst for the oxygen reduction reaction. The case of perfluorinated Fe phthalocyanine

Iron macrocyclic complexes (MN4) are promising catalysts for replacing platinum (the industrial standard) in electrocatalysis. In particular, FeN4 complexes have shown lower overpotential than Pt for the oxygen reduction reaction (ORR) in alkaline media. To predict the electrochemical activity of metal electrodes and molecular catalysts towards the ORR, reactivity descriptors with typical volcano correlation have been demonstrated. The most important are M-O binding energy and M/M redox potentials for the complexes. We studied a new Fe complex, which possesses powerful electron-withdrawing fluorine residues at the periphery of the phthalocyanine ring. Fe hexadecafluorophthalocyanine (16(F)FePc) was characterized by electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) in the presence and in absence of O. Experimental and calculated O-Fe binding energies, as well as electrochemical characterization confirms the excellent activity of this complex for the ORR placing this complex at the top of the MN4 volcano correlation

Biomimetic reduction of O2 in an acid medium on iron phthalocyanines axially coordinated to pyridine anchored on carbon nanotubes

An efficient and inexpensive catalyst for the oxygen reduction reaction (ORR) is the key missing component for large-scale development of fuel cells. Bio-inspired tethered electrocatalysts could be the solution to this problematic reaction. Either unsubstituted Fe phthalocyanine (FePc) or Fe hexadecachloro-phthalocyanine (16(Cl)FePc) was anchored to carbon nanotubes (CNTs) via a pyridine axial ligand. The results show that the fifth coordination plays a major role in increasing the catalytic activity of FePc and 16(Cl)FePc for the ORR. The coordination also allows the decoupling of the metal centre from the carbon support, thus changing the geometrical and electronic structure and hindering the production of HO. The pentacoordinated catalysts were stable in acidic pH according to the rotating disk analysis, but the activity of the hexadecachloro compound was not higher than that of the unsubstituted phthalocyanine. Cl atoms reduced the coupling between O and Fe, mismatching the energy of the frontier orbitals and lowering the activity towards the reduction of O.Peer Reviewe

Modulation of the electrocatalytic activity of Fe phthalocyanine to carbon nanotubes: Electrochemistry of L-cysteine and L-cystine

12 pags., 12 figs., 2 tabs.We have evaluated the electrocatalytic activity of hybrid electrodes containing Fe(II) phthalocyanine and tested these electrodes for L-cysteine oxidation and L-cystine reduction. The hybrid electrodes consisted of pristine multi-walled carbon nanotubes and functionalized with [sbnd]COOH, [sbnd]NH groups (MWCNT-p, MWCNT-c and MWCNT-a). These MWCNTs were modified with iron (II) phthalocyanine (FePc). The characterization of the hybrid systems was performed using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and AFM with the purpose of elucidating the type of interaction between FePc and MWCNTs. The Fe(II)/(I) and Fe(III)/(II) redox potentials associated with the metal center where evaluated using cycling voltammetry. The redox processes are slightly affected by the presence of MWCNTs. The activity for both reactions increases substantially by the presence of modified MWCNTs essentially by an area effect. However, when the currents are normalized by the amount of active sites estimated from the surface coverages of FePc, the activities are still higher for FePc attached to MWCNTs. The data for L-cysteine oxidation fits well on a volcano correlation published previously for several metal phthalocyanines and metalporphyrins.The authors are grateful to Fondecyt Projects 1140199, 1181037,Nucleo Milenio RC 120001, Anillo ACT-1412, Fondecyt PostdoctoralProject 3150271 and Conicyt Scholarships M.P.O. 21130168. Fon-dequip EQM 130149&Fondequip EQM 16003

Elucidating the mechanism of the oxygen reduction reaction for pyrolyzed Fe-N-C catalysts in basic media

5 pags., 2 figs., 2 tabs.The study of non-precious metal catalysts (NPMCs) as alternatives to platinum for oxygen reduction is crucial if the use of fuel cells is to become more widespread. Among NPMCs, pyrolyzed catalysts (Fe-N-C) are particularly promising in both basic and acid media. The characterization of active sites and the understanding of the oxygen reduction reaction (ORR) mechanism are crucial for the design of active Fe-N-C catalysts. In this study, we have tested the involvement of the metal centre in the ORR process at pH 13 for two pyrolyzed iron porphyrins. The pyrolyzed catalysts present a FeN4 active site structure similar to that of the porphyrin precursors. Regarding the mechanism, we have found evidence for the crucial role of the Fe(II) centres. There is a direct relation between the Fe(III)/(II) redox transition of the catalysts and the onset potential of the ORR, showing that the electrogeneration of Fe(II) from Fe(III)OH– controls the catalysis. The poisoning of iron centres with CN− induces a decrease in the ORR activity. However, the onset potential for H2O2 generation remains unchanged. The Tafel plots show two different slopes at high and low overpotentials. Based on these results, we propose two different mechanisms, both dependent on the redox potential of the catalysts and the FeO2 binding energy.This work was supported by Fondecyt Regular Projects 1161117,1181037. Fondecyt Postdoctoral Projects 3170330, 3180509, and Conicyt Scholar ships 1160955,2116021