Nature of the Catalytic Centers of Porphyrin-Based Electrocatalysts for the ORR: A Correlation of Kinetic Current Density with the Site Density of Fe−N₄ Centers

Abstract

In this work, it has been shown that structural changes of an as-prepared catalyst enable the assignment of the catalytic centers responsible for the direct and indirect oxygen reduction reaction, respectively, of porphyrinbased electrocatalysts. An iron porphyrin (FeTMPPCl)-based catalyst as well as a catalyst based on H 2 TMPP were prepared using the so-called foaming agent technique (FAT). The obtained iron catalyst was used as a generic material for the post-treatments. Structural changes were analyzed by 57 Fe Mo ¨ssbauer spectroscopy. The catalytic activity toward the oxygen reduction reaction (ORR) was determined using rotating (ring) disc electrode (R(R)DE) experiments. The catalysts exhibit a variation in the iron content between 2.9 and 4.5 wt % caused by the post-treatments. It has been found that the Mo ¨ssbauer spectra of all catalysts can be fitted assuming two different ferrous Fe-N 4 centers, a CFeN 2 center (Fe 2+ , S ) 2) and an Fe 3 C center (Fe 0 ). After the intensities found in the Mo ¨ssbauer spectra were normalized relative to the iron content, a linear correlation between the kinetic current density related to the direct oxygen reduction and the amount of in-plane Fe-N 4 centers is found. Beside this, there is evidence for a correlation between the kinetic current density related to the hydrogen peroxide formation and CFeN 2 centers. Heat-treated carbon-supported iron porphyrin, prepared as reference material, exhibits the same behavior as our FAT catalysts. The correlation enables us to obtain the turnover frequencies for both the direct and the indirect oxygen reduction reaction and to determine the site densities, in which we reach a third of the target value published by Gasteiger et al. (Appl. Catal., B 2005, 56, 9).

Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

“…The Tafel slopes of the more-active catalysts show values of about 70 mV dec −1 . This value is in good agreement with the values that were observed for other catalysts prepared by the (oxalate supported) pyrolysis of porphyrins [15,24,29] as well as alternatively prepared catalysts (without the use of a reactive gas heat-treatment) [19]. When the fraction of oxalic acid goes down the Tafel slope increases, getting closer to 100 mV dec −1 .…”

“…In this approach oxalate works as a structure-forming agent (SFA) as the final catalyst resembles its morphology [13][14][15]. In addition to this, the use of sulfur strongly affected the morphology and performance of the catalysts [16,17].…”

Effect of the Transition Metal on Metal–Nitrogen–Carbon Catalysts for the Hydrogen Evolution Reaction

“…The presence of the bcc structures in Fe-, Co-and Ni-N-C in particular does however not preclude the parallel existence of single-metal-atom moieties MeN x C y . [52][53][54][55][56][57] In general, the metal-based structures and oxidation states resulting from the reaction between metal acetates and organic precursors at high temperature may be regarded as a red-ox reaction between the metal cation and the organic precursor, here Z8FA. Metal salts having a redox potential for M II /M sufficiently positive oxidize organic ligands and the metal cations are reduced during pyrolysis.…”

Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst

“…It is worth noting that the HOOFePc-Py-CNT system has a lower spin state than the HOOFePc-CNT system. It has been reported that only specific iron centres in low spin states in pyrolysed Fe-N-C catalysts are responsible for the ORR activity 23,27 . …”

“…Although the pyrolysed catalysts offer better performance toward ORR than those without pyrolysis, their chemical structures undergo a complicated and unpredictable transformation during the heattreatment 25 . Accordingly, the nature of the active sites in heattreated Fe-N-C catalyst is still unclear 26,27 . The unpredictable structural change and unclear catalytic mechanism make it difficult to achieve rational design of Fe-N-C catalyst for ORR.…”