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

Nanostructured Fe-N-C pyrolyzed catalyst for the H2O2 electrochemical sensing

11 pags., 7 figs., 3 tabs.Fe-N-C pyrolyzed materials have been proposed as substitutes of the noble-based catalyst for energy conversion reactions. However, their use as electrochemical sensors has not been deeply explored. In the present work, different Fe-N-C pyrolyzed catalysts were synthesized for the amperometric sensing of the HO reduction in neutral media. The catalysts were characterized by BET, TEM, FESEM, XPS, Mössbauer spectroscopy, and cyclic voltammetry. The catalysts present an N-doped graphitic matrix with a macroporous structure and mesoporous contribution. Different amounts of N-pyridinic, N-pyrrolic, N-graphitic, N-oxides, and FeN4 sites have been detected on the catalysts. Among the different active sites present in the catalysts, the FeN4 structure is proposed as the most catalytic active site for the hydrogen peroxide reduction reaction (HPRR). Under optimal conditions (0.61 V vs. NHE, 0.00 V vs. Ag/AgCl), the materials show a lineal amperometric response in the range of 0.08 and 14 µM, with a sensitivity of 31.3 µA µM cm, and a detection and quantification limits of 0.25 µM and 0.75 µM respectively. The amperometric results indicate that the best performance is reached when increasing the amount of FeN4 active sites, and the redox potential of the FeN4 species becomes more positive. The Fe-N-C catalyst stands out for the more positive working potential than other materials proposed in the literature.We are grateful to FONDECYT Grant 1170352 & ANID- PFCHA/PhD fellowship No. 21160955 awarded to C. Candia-Onfray. F.J. Recio and N. Menendez are grateful to The Spanish Ministry of Economy and Competitiveness funded this research under project PGC2018-095642-B-I00

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