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Climate change has evidenced the need to reduce carbon dioxide emissions into the atmosphere, and so for transport applications, lighter weight alloys have been studied, such as magnesium alloys. However, they are susceptible to corrosion; therefore, surface treatments have been extensively studied. In this work, the influence of argon plasma pretreatment on the surface properties of an AZ31 magnesium alloy focus on the enhancement of the reactivity of the surface, which was examined by surface analysis techniques, electrochemical techniques, and gravimetric measurements. The samples were polished and exposed to argon plasma for two minutes in order to activate the surface. Contact angle measurements revealed higher surface energy after applying the pretreatment, and atomic force microscopy showed a roughness increase, while X-Ray photoelectron spectroscopy showed a chemical change on the surface, where after pretreatment the oxygen species increased. Electrochemical measurements showed that surface pretreatment does not affect the corrosion mechanism of the alloy, while electrochemical impedance spectroscopy reveals an increase in the original thickness of the surface film. This increase is likely associated with the high reactivity that the plasma pretreatment confers to the surface of the AZ31 alloy, affecting the extent of oxide formation and, consequently, the increase in its protection capacity. The weight loss measurements support the effect of the plasma pretreatment on the oxide thickness since the corrosion rate of the pretreated AZ31 specimens was lower than that of those that did not receive the surface pretreatment.

The general trends in the construction of highly active electrode devices are focused on the science of materials. These are useful for developing 2D nanostructured electrodes, with well-defined active sites, which are excellent approaches for understanding the fundamentals of electrocatalytic reactions. Here we present an overview of the experimental self-assembled molecular catalyst configurations to develop excellent electrode materials containing molecular catalysts for energy conversion device applications. First, by applying well-known reactivity descriptors for electrocatalysis, nanoarchitectonics, and the self-assembled concept, we summarize the main molecular building blocks to achieve a technology system for arranging by a rational design, nanoscale structural units configuration that promotes electrocatalytic reactions such as oxygen reaction reduction (ORR) and water-splitting reactions. We focus the discussion on the MN4 molecular catalyst linked to electrode surfaces with the help of the axial blocks, bio-inspired self-assembled approaches such as biomimetic models of metalloenzymes active sites, and molybdenum sulfide clusters for hydrogen evolution reaction (HER). We briefly discuss the advantages of developing host-guest self-assembled molecular catalyst systems based on cyclodextrins anchored to electrodes to get well-defined active sites with local environment control.

The alkaline anion exchange membrane fuel cell (AEMFC) is one of the green solutions for the growing need for energy conversion technologies. For the first time, we propose a natural shungite based non-precious metal catalyst (NPMC) as an alternative cathode catalyst to Pt-based materials for AEMFCs application. The Co and Fe phthalocyanine (Pc)-modified shungite materials were prepared via pyrolysis and used for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) studies. The most promising ORR performance was observed in alkaline media for FePc-modified and acid-leached shungite-based NPMC material. The catalysts were also evaluated as cathode materials in a single cell AEMFC and peak power densities of 232 and 234 mW cm−2 at 60 °C using H2 and O2 gases at 100% RH were observed for CoPc- and FePc-modified acid-treated materials, respectively.

This review summarizes recent advances in the development of pyrolyzed M–N–C catalysts for the oxygen reduction reaction, focusing on activity, stability, and the reactivity descriptors proposed for the rational design of pyrolyzed M–N–C catalysts. We discuss the last advances in achieving high catalytic activity and stability and the new insights into the characterization of FeN4 active sites by Mossbauer spectroscopy in combination with Density Functional Theory (DFT) calculations of the Fe–N–C catalysts. In addition, we present the different reactivity descriptors proposed in the literature for the rational design of Fe–N–C pyrolyzed materials: (i) structural descriptors determined by X-Ray Photoelectron Spectroscopy (XPS) and Mossbauer spectroscopy and (ii) the redox potential of the active center MNx.

The oxygen reduction reaction (ORR) is a highly important reaction in electrochemistry. The following short review details recent advances in novel non-precious metal catalysts containing transition metal macrocycles for use in the ORR. Unbound, many of these electrodes were found to generate high levels of side products such as O2− and H2O2 via 2-electron processes, and for this reason, it is aimed to create systems which would favor a 4-electron process which would completely convert oxygen to H2O. The 4-electron reduction of O2 releases the most energy in a fuel cell. Novel catalytic materials containing metal macrocycles were created mimicking the structure of enzyme metal centers, the metal in the macrocycle bound to a fifth axial ligand. These structures were observed to exhibit improved catalytic activity, and in the case of cobalt, phthalocyanine systems were observed to move away from the inefficient 2-electron process towards the more complete 4-electron process in alkaline media. Both experimental results (XPS, EPR, cyclic voltammetry and polarization curves) and theoretical models were gathered for various pentacoordinate systems and various electronic effects of the axial ligand on metal center were proposed. Penta-coordinate macrocycles are an important tool for further manipulating and tuning the electronic behavior of transition metal centers for catalysis of the ORR.

15 pags., 9 figs., 2 tabs., Pyrolyzed non-precious metal catalysts have been proposed as an alternative to substitute the expensive and scarce noble metal catalysts in several conversion energy reactions. For the oxygen reduction reaction (ORR), the pyrolyzed catalyst M−N−C (M: Fe or Co) presents remarkable catalytic activity in acid and alkaline media. These pyrolyzed materials show a high heterogeneity of active sites being the most active in the MNx moieties. The activity and stability of these catalysts are also conditioned by other structural parameters such as the area, the N-doping, and by the presence of metal particles. In this study, we explore the use of FeO nanoparticles as templates and as iron sources to synthesize Fe−N−C. The best performance for the ORR in acidic media was reached with the catalysts using nanoparticles covered by PANI and iron salts as the precursor, with an onset potential of 0.85 vs. RHE and a direct 4-electrons mechanism. We corroborated the use of the catalysts’ redox potential as reactivity descriptors and discussed the detrimental role of the presence of FeC metallic particles in the mechanism. Based on the experimental results, we performed DFT calculations to explore the influence of N-doped species on the electronic density of the iron centers of FeN4 active sites, and we propose a theoretical model for increasing the activity based on the distance and ratio of N-doping to iron center., This work was supported by Fondecyt Regular Project 1161117, Conicyt Scholarship 21160212, Fondecyt Postdoctoral Projects 3180509 and 3170330, and Anillo Project ACT-192175. By MICIN grant PGC2018-095642-B-I00, MCIN/AEI/10.13039/501100011033 RTI2018-095303-B-C51, by ERDF A way to making Europe, and by CISC grant 2021AEP056.

The oxygen reduction reaction (ORR) is essential for energy conversion devices such as fuel cells and metal-air batteries. The use of expensive and scarce noble metal materials to catalyze the ORR is a limitation for the massification of these energy conversion technologies. Copper molecular catalysts that mimic the active sites of metalloenzymes such as laccase are under continuous development. In this minireview, we present the strategies to increase the activity and stability of the copper catalysts for the ORR. The flexibility, lability, and electronic character of the ligands are crucial to promote the ORR. In addition, the use of polymers as backbone for multicopper catalysts and the synthesis of copper carbon-based pyrolyzed catalysts present remarkable results with promissory applications, This work has been supported by FONDECYT 11221073, FONDECYT 1221798, FONDECYT Postdoctoral Project 3170330 and Anillo Project ACT-192175

In this work we have tested the Fe(III)/(II) redox potential of the catalysts as a reactivity descriptors of iron macrocyclic complexes (FeN4) adsorbed on multi-walled carbon nanotubes (MWCNTs) and deposited on ordinary pyrolytic graphite (OPG). The reaction examined is the oxidation of glutathione (GSH) a biologically important molecule. The experiments were conducted in 0.1 M NaOH and kinetic measurements were performed on MWCNT previously modified with FeN4 macrocycle complexes. This modified FeN4–MWCNTs were deposited on pristine OPG electrodes. From previous work it is known that for FeN4 complexes directly adsorbed on OPG, the activity as (log i)E plotted versus the Fe(II)/(I) redox potential follows a volcano correlation for the oxidation of glutathione. We wanted to test these correlations on hybrid electrodes containing MWCNTs and essentially the carbon nanotubes have no influence in these correlations and the redox potentials a are good reactivity descriptors, regardless of the way the FeN4 catalysts are attached to the electrode. Further, we find volcano correlations when using the Fe(II)/(I) and the Fe(III)/(II) redox potentials as reactivity descriptors. The volcano correlation when using the Fe(III)/(II) redox potential exhibits a maximum at E° = –0.26 V vs SCE which is close to the potential for comparing the different activities. This interesting result seems to indicate that the maximum cannot be explained only in terms of the Sabatier principle where θRS, the surface coverage of adsorbed intermediate is close to 0.5 but instead to a surface coverage of active sites θFe(II) equal to 0.5, which occurs at the Fe(III)/(II) formal potential.

We have systematically studied the catalytic activity of a series of substituted bisphenanthroline Cu complexes adsorbed on glassy carbon electrodes for the oxygen reduction reaction (ORR) in aqueo...

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 1160955,21160212

New experimental techniques and protocols are needed to study microbiologically influenced corrosion (MIC). In this work, we studied the capacity of microorganisms to modify their environment in vitro. The methodology involved cyclic voltammetry measurements using carbon electrodes modified with cobalt phthalocyanine, which is a known catalyst for the two-electron reduction of O2 to give peroxide. Mueller-Hinton (21 g/L) was used as an electrolyte in a sterilized aqueous medium. The open circuit potential (OCP) and oxygen concentration behaved similarly over time, with generally similar growth curves. However, there were peculiarities that indicated the presence of peroxide and catalase. Catalase activity was demonstrated by comparing the voltammetric responses of the culture medium in the absence and presence of bacterial strains of Escherichia coli (Gram +) and Staphylococcus aureus (Gram-), both facultative and catalase positive. With this system, which is capable of discriminating O2 and hydrogen peroxide, catalase activity is highly evident, and at its maximum at the end of the exponential stage of the growth curve.

Carbide-derived carbons (CDC) are porous carbon materials with tunable pore structure, very high specific surface area and of great potential for electrochemical applications. In this work CDC-supported transition metal macrocyclic electrocatalysts are prepared using two different type of CDC materials and iron phthalocyanine (FePc). Herein, we report the superior electrocatalytic properties of FePc/CDC catalysts fabricated via simple pyrolysis approach. The morphology, composition and structural features of these CDC-based non-noble metal catalysts are evaluated using scanning and transmission electron microscopies, X-ray photoelectron spectroscopy, N2 physisorption and X-ray diffraction analysis. The electrochemical oxygen reduction reaction (ORR) behavior of FePc/CDC catalysts is evaluated employing the rotating disk electrode (RDE) method. FePc modified CDC materials demonstrate a superior ORR electrocatalytic activity in alkaline conditions, showing onset potential of −0.05 V (vs. SCE) close to that of commercial Pt/C. Thus, the RDE results show great potential of these non-Pt catalysts as cathode materials in metal-air batteries and alkaline membrane fuel cells.

Platinum group metal–free electrocatalysts and in particular transition metal–nitrogen–carbon catalysts are becoming interesting candidates as cheap alternatives to Pt-based catalysts for the oxygen reduction reaction in polymer electrolyte fuel cells. Unified activity-stability correlations are needed to provide practical guidelines for a rational catalyst design. A discussion of different characterization techniques for studying possible activity descriptors is presented, with a specific focus on active site density and turnover frequency. These descriptors will be associated to the morphology of the various transition metal–nitrogen–carbon electrocatalysts investigated in the recent literature. The underlined correlation for this class of platinum group metal–free electrocatalysts offers important insights required for the development of the next generation of catalytic materials with enhanced stability that can solve the main activity and durability barriers needed for the replacement of Pt-based counterparts.

Iron(II) 2,3,9,10,16,17,23,24-octa(cyano)phthalocyanine (OCNFePc), was tested as a catalyst for the oxygen reduction reaction (ORR) adsorbed on carbon nanotubes. The composite was analyzed spectroscopically and electrochemically characterized at pH 13 and pH 1. The composite showed close to 4 electron processes at pH 13. Computational analysis indicates that the O2 molecule binds end-on to the metal center and that the dioxygen molecule is dissociated on both the Fe metal center and the corral ring. An Analysis of the molecular electrostatic potential confirms the behavior of cyano residues as electron-withdrawing moieties in OCNFePc. As a result of its catalytic behavior and theoretical analysis of its O2 binding energy, OCNFePc was placed in a high position on a volcano correlation of similar phthalocyanine composites.

We have been able to “tune” the electrocatalytic activity of iron phthalocyanine (FePc) and iron hexadodecachlorophthalocyanine (16(Cl)FePc) for the oxygen reduction reaction (ORR) by manipulating the “pull effect” of pyridinium molecules axially bounded to the phthalocyanine complexes (FePcs). These axial ligands play both the role of molecular anchors and also of molecular wires. The axial ligands also affect the reactivity of the Fe metal center in the phthalocyanine. The “pull effect” originates from the positive charge located on the pyridinium core. We have explored the influence of the core positions (Up or Down), in two structural pyridiniums isomers on the activity of FePc and 16(Cl)FePc for the ORR. Of all self-assembled catalysts tested, the highest catalytic activity was exhibited by the Au(111)/Up/FePc system. XPS measurements and DFT calculations showed that it is possible to tailor the FePc–N(pyridiniums) Fe–O2 binding energies, by changing the core positions and affecting the “pull effect”...

The Mn +/M(n − 1)+ redox potential of MN4 macrocyclic molecular catalysts is a very good reactivity descriptor for several electrochemical reactions. One important feature about this reactivity descriptor is that it can be determined experimentally under the same conditions of the kinetic measurements in contrast to other descriptors like intermediate binding energies that are estimated from DFT calculations. However a linear correlation between both descriptors seems to exist. Plots of activity as (logj)E at constant E versus the Mn +/M(n − 1)+ redox potential gives volcano correlations. Another important aspect about this parameter is that it is possible to tune the Mn +/M(n − 1)+ redox potential of the MN4 catalyst by manipulating the structure of the macrocyclic complex and tailoring the electron-withdrawing power of the ligands to obtain the maximum activity. In this work we have probed the redox potential as a reactivity descriptor for the oxidation of cysteamine studying a series of substituted Fe phthalocyanines and Fe porphyrins adsorbed on glassy carbon and pyrolytic graphite in alkaline media. As expected the catalytic activity of these FeN4 species varies strongly with the Fe (II)/(I) redox potential of the different Fe phthalocyanines and a plot of activity as (logj)E versus E°Fe(II/I) gives a volcano-shaped correlation so a formal potential value exists for which the highest activity can be achieved demonstrating that the formal potential of the complexes seems to be an universal reactivity descriptor for electrochemical reactions.

We have studied the electro-oxidation of thioglycolic acid (TGA) catalyzed by iron phthalocyanines and iron porphyrins (FeN4 complexes) deposited on ordinary pyrolytic graphite and on multiwalled carbon nanotubes. The purpose of this work is to establish both experimental and theoretical reactivity descriptors of MN4 macrocyclic complexes for electrooxidation of thioglycolic acid (TGA) as an extension of previous studies involving other reactions using these types of catalysts. Essentially, the reactivity descriptors are all related to the ability of the metal center in the MN4 moiety to coordinate an extra planar ligand that corresponds to the reacting molecule. This coordinating ability, represented by the M-TGA binding energy can be modulated by tuning the electron-donation ability of the ligand and it is linearly correlated with the Fe(III)/(II) redox potential of the complex. Experimental plots of activity as (log j)E at constant potential versus the Fe(III)/(II) redox potential of the MN4 catalysts give volcano correlations. A semi-theoretical plot of catalytic activities (log j)E vs DFT calculated Fe-TGA binding energies (EbTGA) is consistent with the experimental volcano-type correlations describing both strong and weak binding linear correlations of those volcanos. On the other hand, the Hirshfeld population analysis shows a positive charge on the Fe center of the FeN4 complexes, indicating that electron transfer occurs from the TGA to the Fe center in the FeN4 complexes that act as electron acceptors. The donor (TGA)-acceptor (Fe) intermolecular hardness ΔηDA was also used as reactivity descriptor and the reactivity of the Fe centers as (log j)E increase linearly as ΔηDA increases. If activity is considered per active site, the trends is exactly the opposite, i.e. a plot of (logTOF)E increases linearly as ΔηDA decreases as expected form the Maximum Hardness-Principle. A plot of (logTOF)E versus E°’Fe(III)/(II) gives a linear correlation indicating that the activity per active site increases as the redox potential decreases.

Dilithium and zinc phthalocyanine derived catalysts supported on multiwalled carbon nanotubes (MWCNTs) have been studied as non-precious metal catalysts for oxygen electroreduction. The electrocatalysts were prepared by simple pyrolysis by varying the py

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.

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–O2 binding energy and M(n)+/M(n−1)+ 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 O2. Experimental and calculated O2–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.

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 160036

There has been a significant progress toward the development of highly active and stable platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells, promising a low-cost replacement for Pt group electrocatalysts. However, the success of such developments depends on the implementation of PGM-free electrocatalysts that are not only highly active but importantly, they also exhibit long-term durability under polymer electrolyte fuel cell operating conditions. This manuscript is an overview of the current status of a specific, most advanced class of PGM-free electrocatalysts: transition metal–nitrogen–carbon ORR catalysts. We present an overview for the understanding of catalysts’ performance descriptors for metal–nitrogen–carbon materials.

The oxygen reduction reaction (ORR) is a fundamental chemical reaction in industrial processes as well as in living systems, being O2, the final electron (e-) acceptor of many reactions. Since then its importance in the fuel cell technology where energy would be stored in the form of H2 to be oxidized in conjunction with the reduction of the cheap and abundant O2. However, the ORR is a complicated reaction which involves the transfer of 4 electrons and 4 protons and for its nature, it proceeds through the consequent formation of more than one intermediate accounting for the so-called “scaling correlations” [1]. The complexity of the reaction determines the slow kinetics and the difficulties into finding an adequate catalyst for it to proceed without energy losses. The “industrial standard” catalysts for this reaction are based on the very expensive and rare Pt metal. An uncountable number of non-precious metal catalysts (NPMC) have been studied with the purpose to replace Pt. In our research group, we are adopting various strategies to increase the activity and the stability of metal phthalocyanines like the substitution of planar neutral residues with more electron-negative ones, and the addition of axial back ligands [2-5]. In this research work we studied the electrocatalytic activity towards the ORR of a new compound i.e. the iron 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadeca(fluoro) phthalocyanine (16(F)FePc) in the absence and in the presence of a fifth pyridine axial ligand (FeN5). Interesting the 16(F)FePc appears to be the most active among all the FeN4 studied to the moment. The very high redox potential of the active Fe(III)/Fe(II) redox couple (-0.075 V and 0.44 V vs. SCE at pH 13 and at pH 1 respectively) confirms the correlation between the activity of the catalyst vs. redox potential of the active redox couple of the metal in the catalyst. 16(F)FePc was so active towards the ORR that the production of H2O2 measured at rotating ring electrodes was nil at pH 13 and very low at pH 1. In the presence of pyridine axial ligand, the redox potential increased of almost 60 mV and catalytic currents and stability of the catalyst also drastically increased. The catalysts were characterized with electrochemical techniques and by EPR and XPS spectroscopy in the presence and in the absence of O2. We could therefore experimentally evaluate the binding energy of O2 to the Fe metal centre. Ab initio calculations, confirm the experimental data and the importance of the pyridine axial ligand to lower the binding energy of O2 to the Fe metal centre because of decoupling of the Fe from the carbon support and changes in the geometrical and electronic structure. [1] Reactivity Descriptors for the Activity of Molecular MN4 Catalysts for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed., 55 (2016) 14510-14521. J.H. Zagal, M.T.M. Koper. [2] Adsorption of 4,4 ́-dithiodipyridine axially coordinated to Iron(II) phthalocyanine on Au(111) as a new strategy for oxygen reduction electrocatalysis. Chemphyschem, (2018), 19(13), 1599-1604.S. Herrera, F. J. Williams, E. J. Calvo, F. Tasca. [3] Biomimicking vitamin B12. A Co phthalocyanine pyridine axial ligand coordinated catalyst for the oxygen reduction reaction. Electrochimica Acta. (2018), 265, 547-555. J. Riquelme, K. Neira, W. Orellana, P. Hermosilla, J. F. Marco, J. H. Zagal, F. Tasca. [4] Comparison of the Catalytic Activity for the Reduction of O2 of Fe and Co MN4 Complexes Adsorbed on Edge Plane Graphite Electrodes and on Carbon Nanotubes. Physical Chemistry Chemical Physics. (2017), 19(31), 20441-20450. R. Venegas, F. J. Recio, J. Riquelme, K. Neira, J. F. Marco, I. Ponce, J. H. Zagal, F. Tasca. [5] Biomimetic reduction of O2 in acid medium on iron phthalocyanines axially coordinated to pyridine anchored on carbon nanotubes”. Journal of Material Chemistry A. (2017), 5(24), 12054-12059. R. Venegas, F. J. Recio, J. Riquelme, K. Neira, J. F. Marco, I. Ponce, J. H. Zagal, F. Tasca.

We have examined the electrocatalytic activity for the oxidation of glutathione of a wide variety of molecular catalysts: FeN4 complexes (MN4 = metal porphyrins and metal phthalocyanines) confined on the surface of graphite electrodes in order to establish reactivity descriptors for these catalysts for this reaction. We have conducted these studies mainly in alkaline media (pH = 13). The reaction order in OH− is 1 for pH values lower than 8. For higher pH values, the reaction becomes pH independent. The reaction order in glutathione is close to 1 in the concentration range examined (10−3–10−2 M). The activity of the surface-confined MN4 complexes is related to the Fe(II)/(I) and the Fe(III)/(II) redox transitions of the immobilized FeN4 complexes. The catalysts are active only in the potential range where the Fe(II) state predominates. The activity as (log j) E versus the Fe(II)/(I) formal potential varies in a non-linear fashion giving a volcano correlation as previously observed for the oxidation of L-cysteine and many other reactions catalyzed by MN4 complexes. A plot of (E)logj versus the Fe(II)/(I) formal potential gives also an asymmetrical volcano, with one of the branches with a slope close to unity. These volcano correlations clearly shows that the Fe(II)/(I) redox potential needs to be tuned to a certain potential to obtain a maximum activity for the oxidation of glutathione. Most Fe porphyrins show low activity because the metal center Fe(III) is in the wrong oxidation state in the potential range studied.

Zwischen allgemein bekannten Reaktivitatsdeskriptoren fur die Aktivitat von Metallelektroden bezuglich der Sauerstoffreduktion (ORR, oxygen reduction reaction) und der Reaktivitat von molekularen Katalysatoren, im Speziellen auf Elektrodenoberflachen haftenden makrocyclischen MN4-Metallkomplexen, lassen sich eindeutige Ahnlichkeiten 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-Ubergang von MnN4- und FeN4-Chelaten. Die Vulkankurve fur die Sauerstoffreduktion an MN4-Katalysatoren ahnelt 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-unabhangig auf der NHE-Skala ist (und damit pH-abhangig auf der RHE-Skala); Katalysatoren auf dem Abschnitt mit starker Bindung ergeben H2O als Produkt und zeigen die erwartete pH-Abhangigkeit auf der NHE-Skala. Die hier behandelten Deskriptoren treffen auch auf thermisch behandelte pyrolysierte MN4-Katalysatoren zu.

Nitrogen doped carbon nanotubes (N-CNTs) were synthesized by the chemical vapor deposition technique (CVD) using ferrocene and iron (II) phthalocyanine as carbon precursors and ethylendiamine as the carbon and nitrogen source. Fe2O3 nanoparticles, Co nanoparticles and MoO3 were supported on a sol–gel polymer and were used as catalyst and seed growth of the N-CNTs. The influence of the metal transition used as catalyst and the amount of ethylendiamine were studied in N-CNTs for oxygen reduction reaction (ORR) in alkaline media. The morphology of the N-CNTs was studied using scanning electron microscopy (SEM) revealing that de N-CNTs were multiwalled with averages of diameters ranging between 40 and 70 nm. The presence and type of nitrogen in the graphene grid was analyzed by means of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The obtained results indicated that N-CNTs catalyzed by MoO3 presented the highest content of pyridinic-type nitrogen (around 39%). The electrocatalytic activity of N-CNTs for the ORR was evaluated by rotating disk electrode (RDE) technique in 0.2 M NaOH. The electrochemical results showed that N-CNTs catalyzed with 0.2 wt.% MoO3 and synthesized with 19.5 wt.% of ethylendiamine exhibited higher activity for ORR than N-CNTs catalyzed with Co nanoparticles or Fe2O3 nanoparticles.

Gold–graphene interaction at the interface is evaluated through different polyaromatic hydrocarbons (PAH), accounted by C6H6, C24H12, C54H16, and C96H18, focusing into different energetic terms related to the overall interaction. Our results characterize the neutral gold–PAH interaction nature with 45% of dispersion character, 35% of electrostatic, and 20% of covalent character, suggesting that moderate van der Waals character is mostly involved in the interaction, which increases according to the size of the respective PAH. The resulting surface charge distribution in the graphene model is a relevant parameter to take into account, since the ability of the surface charge to be reorganized over the polycyclic structure in both contact and surrounding regions is important in order to evaluate interactions and different interacting conformations. Our results suggest that for a Au12 contact surface of radius 4.13 A, the covalent, electrostatic and dispersion character of the interaction are effectively accou...

A classical reactivity descriptor in electrocatalysis is the binding energy of key intermediates to the active sites. This is well documented for the catalytic activity of metal electrodes [1], alloys and metal oxides and less studied for molecular catalysts [2]. The activity at constant potential plotted versus these descriptors has the shape of a volcano. This is explained by the classical Sabatier principle, which states that there is an optimum “bond strength” (not too strong, not too weak) establishing the best catalyst for a given reaction. Essentially one of the key intermediates is the binding of the reacting molecule “R” to the active sites at the rate determining step as: [MN4]ad + R(aq) ↔ [RMN4+]ad + e- for oxidations and [MN4]ad + R(aq) + e- ↔ [RMN4-]ad for reductions where MN4 is surface confined macrocyclic complex. If the reaction involves the transfer of several electrons, several adsorbed intermediates can be involved. If the controlling step is the first step, it is expected that for the binding step when Gad = 0 a maximum activity should be observed and the partial coverage of adsorbed intermediate is  = 0.5. This implies an equilibrium constant equal to one for the first step and for the best catalyst. In this work we have tested this hypothesis by studying hydrazine electrooxidation and glutathione in alkaline media using several iron porphyrins and iron phthalocyanines as catalysts immobilized on graphite and carbon nanotubes. Figure A shows the trends in reactivity versus the Fe(III)/(II) redox potential for the oxidation of glutathione describing a typical volcano correlation. However if the currents are divided by the θFe(II), the fraction of active sites calculated at E=-0.3 V vs SCE using the Nernst equation, log(i/ θFe(II),)E vs. E° Fe(III)/(II) gives a straight line of slope -0.140V/decade. The maximum corresponds to θFe(II) = 0.5 and occurs at a potential equal to that used for comparing the activities. A similar behaviour is observed for the oxidation of hydrazine but the continuation of the straight line is observed at potentials more positive than that of the maximum. Again the maximum is observed at the potential chosen for comparison. Those catalysts having Eo’ Fe(III)/(II) >> θFe(II)) than -0.56V are in the Fe(III) state as predicted by the Nernst equation. That oxidation state Fe(III) is inactive for the reaction as OH- ions are strongly bound to Fe(III), especially in alkaline media. So the falling of the activities in the strong binding side of the volcano can be attributed preferentially to a gradual decline in the number of Fe(II) active sites and not to gradual decrease of the fraction (1-θ ) of empty or available active sites due to occupancy by intermediates. This phenomena has also been observed for ORR and for thiols oxidation and seems to be a unique feature of molecular catalysts compared to metal catalysts even though Schmickler and Santos [3] have shown that some volcano correlations for HER on metals that are in an oxidized form, and then inactive for this reason. Acknowledgements: This work was supported by Anillo Project ACT 192175, Fondecyt, Projects 1181037 & 1171408 C.G.C is grateful to a Posdoctoral Fellowship from Dicyt-USACH. References: [1] J.K. Norskov et al., J Catal.,328 (2015) 35 [2] J.H. Zagal & M.T.M.Koper, Angew. Chem., 128 (2016) 14726 [3] E. Santos, W. Schmickler et al. Chem. Phys. Chem., 12 (2011) 2274. Figure 1

Iron(II) 2,3,9,10,16,17,23,24-octa(cyano)phthalocyanine (OCNFePc), was tested as a catalyst for the oxygen reduction reaction (ORR) adsorbed on carbon nanotubes. The composite was analyzed spectroscopically and electrochemically characterized at pH 13 and pH 1. The composite showed close to 4 electron processes at pH 13. Computational analysis indicates that the O2 molecule binds end-on to the metal center and that the dioxygen molecule is dissociated on both the Fe metal center and the corral ring. An Analysis of the molecular electrostatic potential confirms the behavior of cyano residues as electron-withdrawing moieties in OCNFePc. As a result of its catalytic behavior and theoretical analysis of its O2 binding energy, OCNFePc was placed in a high position on a volcano correlation of similar phthalocyanine composites.

The design of metal complex catalysts for the oxygen reduction reaction (ORR) [1] can be assisted with the use of computational chemistry and Quantum Mechanics (QM) methods. In this work, we have implemented theoretical models and have used the computational chemistry for the rational design of MN4 catalysts with optimal catalytic efficiency for ORR. The application of ab initio and DFT-based methodologies were employed to assess binding energies on the interaction of O2 and the metal center of MN4 systems. [2] The energetic assessment on reactive paths has proved to be affected, in organic systems, by dynamical effects and strongly influenced by the electronic structure of substituents [3,4]. Hence, the electrocatalytic performance of porphyrins and phthalocyanines were assessed by the analysis of volcano correlations against computed binding energies. In these plots, substitution patterns on the catalysts include both in-plane and axial ligands. Besides the binding energies, other descriptors were assessed as well (e.g., the DFT-based hardness). The systematic study of volcano plots provided a deeply understanding on the the pivotal role of the ligands and the nature of electronic effects underlying the Sabatier principle for the electrocatalysis of the ORR by MN4 molecular systems. Acknowledgements The authors want to acknowledge the funding of Anillo Project ACT 192175 References [1] A. Kulkarni, S. Siahrostami, A. Patel, J. K. Nørskov, Chem. Rev. 2018, 118, 2302. [2] J.H. Zagal, M.T.M. Koper, Angew. Chem. Int. Ed. 201 6, 5 5, 14510. [3] R.A. Matute and K.N. Houk, Angew. Chem. Int. Ed., 2012, 51, 13097. [4] D.E. Ortega, R.A. Matute, J. Phys. Chem. A 2020, 124, 3573.

Following previous studies where the metal-centered redox potentials of MN4 complexes are proposed as a reactivity descriptor for different electrochemical reactions, in the present work we expand this idea to a series of substituted Cu(I)-phenanthrolines adsorbed on glassy carbon electrodes as catalysts for the hydrogen peroxide reduction reaction (HPRR) in aqueous media. As the foot of the wave for HPRR on Cu-based modified electrodes is closely related to the Cu(II)/Cu(I) formal potential, we have found a linear correlation between the catalytic activity expressed as (log i)E and the E°'Cu(II)/Cu(I) redox potential of the complexes with a slope (dE°/dlogi)E close to +0.120 V dec−1, showing that the catalytic activity increases with the shift of E°'Cu(II)/Cu(I) to more positive potentials as a result of the electron-withdrawing nature of the substituents on the ligands. Moreover, the theoretical differences in the calculated chemical potentials (Δμ) of the reactive species follow a similar trend with the E°'Cu(II)/Cu(I) where a positive shift of this parameter is related with a higher Δμ and in consequence, with high catalytic activity. Furthermore, this strategy can be used for the smart design of biosensors as it was shown by the electroanalytical results.

We have investigated the electrocatalytic activity of pyrolysed iron phthalocyanine (FePc) modified carbide-derived carbon/multi-walled carbon nanotube (CDC/CNT) composite materials for the electroreduction of O2. For comparison, FePc supported on Ketjenblack (KB/FePc) and heat-treated at 800 °C was also investigated. All materials containing a higher weight ratio of the CNTs exhibit higher electrocatalytic activity for oxygen reduction reaction (ORR) in both alkaline and acidic media. The surface morphology, structure, elemental composition and porosity of the electrocatalysts were studied with scanning electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, microwave plasma atomic emission spectroscopy and N2 adsorption-desorption analysis. Anion exchange membrane fuel cell (AEMFC) tests were performed with the most active FePc modified CDC/CNT and KB/FePc catalysts. Excellent AEMFC performance was observed using these FePc derived cathode catalysts (maximum power density of 186 mW cm−2 for KB/FePc).

An experimental protocol was studied to improve the adhesion of a polymeric poly(methyl methacrylate) coating that was modified with silver nanoparticles to an aluminum alloy, AA2024. The nanoparticles were incorporated into the polymeric matrix to add the property of inhibiting biofilm formation to the anticorrosive characteristics of the film, thus also making the coating antibiocorrosive. The protocol consists of functionalizing the surface through a pseudotransesterification treatment using a methyl methacrylate monomer that bonds covalently to the surface and leaves a terminal double bond that promotes and directs the polymerization reaction that takes place in the process that follows immediately after. This results in more compact and thicker poly(methyl methacrylate) (PMMA) coatings than those obtained without pseudotransesterification. The poly(methyl methacrylate) matrix modified with nanoparticles was obtained by incorporating both the nanoparticles and the methyl methacrylate in the reactor. The in situ polymerization involved combining the pretreated AA2024 specimens combined with the methyl methacrylate monomer and AgNps. The antibiofilm capacity of the coating was evaluated against P. aeruginosa, with an excellent response. Not only did the presence of bacteria decrease, but the formation of the exopolymer subunits was 99.99% lower than on the uncoated aluminum alloy or the alloy coated with unmodified poly(methyl methacrylate). As well and significantly, the potentiodynamic polarization measurements indicate that the PMMA-Ag coating has a good anticorrosive property in a 0.1-M NaCl medium.

MN 4 macrocyclic metal complexes have been studied for many years as potential catalysts for the O 2 reduction reaction (ORR) in order to replace costly Pt and Pt group metals from the cathode of fuel cells. These macrocyclic complexes lack the necessary stability over long periods of time required for fuel cell operation especially in acidic electrolytes. However, they have provided very good models for establishing reactivity descriptors. The activity plotted with the M–O binding energy describes a volcano correlation. For Mn and Fe complexes, the onset for ORR occurs at potentials very close to the M(III)/(II) redox potential of the complexes in contrast to Co complexes. In general, the more positive the M(III)/(II) redox potential, the highest the activity. This is also true for MN 4 catalysts dispersed on carbon nanotubes using different strategies. MN x catalysts obtained by pyrolysis of MN 4 complexes show higher activity and stability and are more promising for fuel cell applications. The M(III)/(II) transition is a reactivity descriptor for these materials as observed for intact MN 4 complexes.

Summary This review discusses the electrocatalytic activity of MN4 macrocyclic complexes for the O2 reduction reaction. The main reactivity descriptors for these complexes, essentially the M(III)/(II) redox potential and the M O2 binding energies are considered. MNx pyrolysed catalysts are only discussed briefly as this will be covered in other articles of this issue. The performance of these catalysts in anion exchange membrane fuel cell (AEMFC) is thoroughly reviewed.

AISI 304L is used in Sudoscan(TM) technology (Impeto Medical Inc.) for the early diagnosis of small fiber neuropathy caused by type-2 diabetes or cystic fibrosis. In a recent paper, several substitute electrodes were analyzed, among which the biocompatib