Search

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.

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.

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.

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.

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.

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).

Herein, we present a novel method to design nanoscale molecular wires by exploiting well-established electrocatalytic molecular platforms based on metallophthalocyanine blocks. Metallophthalocyanines exhibit high catalytic activity for a wide variety of electrochemical reactions of practical interests. To this aim, metallophthalocyanine molecules can be attached to an electrode surface via a conjugated mercaptopyridine axial ligand that provides (i) stable chemical binding to the metal surface through the thiol-anchoring group, and (ii) a good electrical communication between the metallophthalocyanine ring and the electrode surface. Our previous work demonstrates that long mercaptopyridinium blocks act as excellent linkers in such electrocatalytic platform, resulting in an optimal electrocatalytic activity of the metallophthalocyanine unit. Here we profit from this optimized electrocatalytic molecular platform to design new molecular wires that connect a metal nanoscale junction in a highly efficient and tunable way. To this aim, we use an STM break-junction approach to control the formation of a nanometric gap between two Au electrodes, both functionalized with mercaptopyridinium (bottom) and mercaptopyridine (top). When metallophthalocyanine is introduced into the functionalized metal nanojunction, stable molecular connections between the two electrodes are formed through axial coordination to the top and bottom pyridine moieties. We show that the highest conductance of the resulting nanoscale molecular wire corresponds to an Fe-phthalocyanine as compare to a Cu-phthalocyanine, which follows the electrocatalytic trend for such molecular systems. These results not only demonstrate a new strategy to design new families of highly conductive and tunable nanoscale molecular wires, but it also brings a new nanoscale electrical platform to help understanding some fundamental mechanistic aspects of molecular electrocatalysis.

Cyrhetrenyl crown ether complexes [(η5-C5H4CH N-M)Re(CO)3] (where M = 4-benzo-15-crown-5 (3a), 4-benzo-18-crown-6 (3b), 2-methyl-15-crown-5 (3c), or 2-methyl-18-crown-6 (3d)) were synthesised from cyrhetrenylcarboxaldehyde (1) and the corresponding crown ether amines 2a–d. All the complexes were characterised by IR spectroscopy, 1H and 13C NMR spectroscopies, and mass spectrometry. The stereochemistry for imine compounds 3a–d were determined using the 1H and 13C NMR spectroscopy data, which indicated that these complexes have the anti-(E) conformation. This was also confirmed by the X-ray crystal structures of 3b and 3c in the solid state. Additionally, the electrochemical behaviours of 1 and 3a–d were studied.

The recently introduced new electroanalytical protocol of stripping voltammetry microprobe (SPV) has been considerably improved with respect to analytical performance. The determination of the silver ions is shown to be possible by the precipitation of a solution of silver drops on an electrode surface, which is previously moistened with a solution of NaCl. The electrode thus modified, i.e., with the silver chloride precipitate on the surface, is introduced into a chloride electrolyte and is negatively polarized to reduce the silver salt to metallic silver. Whereas this reduction process produces only a broad signal, the following electrochemical oxidation of the silver metal back to AgCl provides a highly sensitive and narrow signal, which lends itself for analysis. The linear range for determination of silver ions following this protocol is 1.4 × 10 −7 to 7.4 × 10 −4 mol L −1 and the detection limit is 4.6 × 10 −8 mol L −1 .

In this work, we have studied the interaction between the hydrazine N 2 H 4 molecule with several FeN4 macrocyclic complexes (FePc's). In order to modulate the electron density located on the metal center using iron-phthalocyanine (FePc) as the reference, we used substituted iron-phthalocyanines with different types of substituents electron-donating groups such as iron-tetraamino-phthalocyanine (4β(NH 2 )FePc) and iron-octamethoxyphthalocyanine (8β(OCH 3 )FePc), and with electron-withdrawing groups such as iron-tetranitrophthalocyanine(4β(NO 2 )FePc) and iron-hexadecachlorophthalocyanine (16(Cl)FePc), respectively. We have found that the energy of interaction between hydrazine and the Fe center in the macrocycle increases as the electron-withdrawing power of the substituents increases. When rate constants instead of currents are compared in a semilog plot versus Δ ɛ D-A , a linear correlation is found where log k increases as the intermolecular hardness of the systems decreases.

The construction of self-assembled iron phthalocyanine (FePc) systems on gold electrodes modified by self-assembled monolayers (SAMs) is becoming an interesting strategy for obtaining electrocatalytic molecular building blocks for the oxygen reduction reaction (ORR). In this work, we have measured the conductance of pyridiniums axial ligands at the single molecule level using the scanning tunneling microscope-based break-junction method (STM-Break Junction) to study the role of the axial ligand on the activity of the self-assembled FePc systems on a gold electrode surface. The electron-pulling effect of pyridinium axial ligands is known to increase the electrocatalytic activity of FePc for the oxygen reduction reaction (ORR). We have used these systems as a platform for carrying out a comparative study for understanding the real influence of the proximal axial ligands. Further, these ligands act as molecular wires between the gold electrode surface and the FePc molecule. The pyridinium molecules were synthesized following a series of structural variations using a basic molecular backbone. From conductance measurements obtained for each pyridinium molecule, it was possible to establish that electron transport through each pyridinium does not influence the activity of FePc for ORR in alkaline media. In addition, the DFT calculations shows that the axial ligand in FePc modifies its catalytic activity by decreasing the binding energy of O2 to the Fe site.

In classical volcano correlations in electrocatalysis, the shape of (log io) plots versus the binding energy of the reactant to active sites, the maximum activity is associated to a situation where the surface coverage of reactant species is equal to 0.5 corresponding to a free energy of adsorption of the reactant equal to zero. When studying electrochemical reactions catalysed by macrocyclic transition metal complexes adsorbed on graphite electrodes, volcano correlations are found when plotting the activity versus the formal potential of the catalyst for several reactions, assuming that the formal potential is somehow related to the free energy of the adsorption of the reacting molecule. In this work we offer an interpretation different from that given in the literature for these correlations for the electrooxidation of hydrazine. The highest catalytic activity is achieved when the formal potential of the catalyst approaches the N2H4/N2 reversible potential. In contrast, if log i (normalized for the actual surface concentrations of Fe(II) active sites at constant potential) is plotted versus the difference between the Fe(II)/(I) formal potential of the catalysts and the standard potential of the N2H4/N2 reaction, the correlation is linear with a slope close to 2RT/F. Keywords: Falling region, Tuning formal potential, Design best catalyst, Surface redox catalysis

The antimicrobial effect of incorporating silver nanoparticles (AgNps) into zirconia matrix–polyether glycol was studied. AgNps of 4–6 nm in size were synthesized using the inverse micelles method, and different doses of metallic nanoparticles were incorporated into zirconia–polyether glycol mixtures during the ageing procedure. Atomic force microscopy (AFM) of the modified hybrid film showed a homogenous distribution of 20–80 nm diameter AgNps, indicating agglomeration of these structures during film modification; such agglomerations were greater when increasing the dosage of the colloidal system. The AgNps-hybrid films showed higher antimicrobial activity against Gram-positive bacteria than for Gram-negative bacteria. Hybrid films prepared with dioctyl sodium sulfosuccinate (AOT) stabilized AgNps presented enhanced antibacterial activity compared to that obtained through the addition of a high AgNO3 concentration (0.3 wt%).

A new hybrid sol–gel type film, composed of tetraethylorthosilicate (TEOS) and tetraocthylorthosilicate (TEOCS), and modified with different nanoparticle systems, has been investigated as a coating for protection of AA-2024-T3 aluminium alloy. The nanoparticle systems considered were either ZrO2 or CeO2 or their combination. The zirconia nanoparticles were prepared from a Zr (IV) propoxide sol (TPOZ), using an organic stabilizer, and the CeO2 nanoparticles were developed spontaneously after adding cerium nitrate solution to the hybrid sol. The chemical composition and the structure of the hybrid sol–gel films were examined by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The corrosion resistance of the coated AA-2024 alloy was examined by potentiodynamic polarization. The results revealed that, for short exposure times in the electrolyte, incorporation of ZrO2 or CeO2 nanoparticles in the hybrid film does not provide an increase in the corrosion resistance of the coated AA-2024 alloy. Further, the resistance was significantly reduced by increasing the nanoparticle content. Conversely, by incorporating both nanoparticles (ZrO2 and CeO2), the corrosion resistance of the resulting hybrid films increased slightly. The behavior changed significantly when the coated alloy was exposed to the electrolyte for 5 days. The corrosion resistance of the coatings, unmodified and modified with CeO2 or ZrO2 nanoparticles, decreased by two or three orders of magnitude, while the film modified with both nanoparticles (CrO2 and ZrO2) showed a relatively high corrosion resistance and responsiveness to activation processes during anodic polarization.

Metallophthalocyanines confined on the surface of electrodes are active catalysts for a large variety of electrochemical reactions and electrode surfaces modified by these complexes can be obtained by simple adsorption on graphite and carbon. However, more stable electrodes can be achieved by coating their surfaces with electropolymerized layers of the complexes, that show similar activity than their monomer counterparts. In all cases, fundamental studies carried out with adsorbed layers of these complexes have shown that the redox potential is a very good reactivity index for predicting the catalytic activity of the complexes. Volcano-shaped correlations have been found between the electrocatalytic activity (as log I at constant E) versus the Co(II)/(I) formal potential (E°′) of Co-macrocyclics for the oxidation of several thiols, hydrazine and glucose. For the electroreduction of O2 only linear correlations between the electrocatalytic activity versus the M(III)/M(II) formal potential have been found using Cr, Mn, Fe and Co phthalocyanines but it is likely that these correlations are “incomplete volcano” correlations. The volcano correlations strongly suggest that E°′, the formal potential of the complex needs to be in a rather narrow potential window for achieving maximum activity, probably corresponding to surface coverages of an M-molecule adduct equal to 0.5 and to standard free energies of adsorption of the reacting molecule on the complex active site equal to zero. These results indicate that the catalytic activity of metallophthalocyanines for the oxidation of several molecules can be “tuned” by manipulating the E°′ formal potential, using proper groups on the macrocyclic ligand. This review emphasizes once more that metallophthalocyanines are extremely versatile materials with many applications in electrocatalysis, electroanalysis, just to mention a few, and they provide very good models for testing their catalytic activity for several reactions. Even though the earlier applications of these complexes were focused on providing active materials for electroreduction of O2, for making active cathodes for fuel cells, the main trend in the literature nowadays is to use these complexes for making active electrodes for electrochemical sensors.

Catalysts for the oxygen reduction reaction (ORR) were prepared on carbon black (C) using FeIIphthalocyanine (FePc) and Cl–FeIIItetramethoxyphenylporphyrin (ClFeTMPP), as Fe precursors with and without a pyrolysis step at 800 °C. CO poisoning of the ORR catalytic sites for all these Fe/N/C electrocatalysts was attempted at pH 1 and 13, but to no avail, even if an iron ion is known to occupy the center of the active sites in at least the unpyrolyzed FePc/C or ClFeTMPP/C. The exact nature of the active center of these Fe-based heat-treated catalysts may still be a subject of debate but, in light of the absence of CO poisoning for unpyrolyzed FePc/C and ClFeTMPP/C, resistance to CO poisoning by the heat-treated catalysts cannot be used as evidence that the active center of their catalytic site is devoid of iron. Keywords: Fuel cell, Non-noble metal catalyst, CO, Oxygen reduction

We have investigated the electrocatalytic activity of tetraphenyl porphyrin, of substituted tetraphenyl porphyrins and of vitamin B 12 for the electro-oxidation of 2-aminoethanethiol, with the catalysts confined on graphite electrodes. Several N4-macrocyclic were used in order to have a great variety of formal potentials of the catalysts and compare this parameter with the catalytic activity. The electrocatalytic activity, measured as current at constant potential, increases with the formal potential of the catalysts as follows: Co-pentafluorotetraphenylporphyrin 12 . Vitamin B 12 exhibits the maximum activity. A correlation of log I (at constant potential) vs. the formal potential E ′ of the catalysts gives a volcano correlation when data of Co-phthalocyanines studied in previous work is included in the comparison. This clearly shows that the search for better catalysts for this reaction point to those N 4 -macrocyclic complexes with Co(II)/(I) formal potentials close to −1.0 V vs. SCE which corresponds to an optimum situation for the interaction of the thiol with the active site. When comparing reactivity trends of similar catalysts for the oxidation of other thiols ( l -cysteine, mercaptoacetate and 2-mercaptoethanol) E ′ max correlates linearly with the p K a of the thiol.

We report here a fast procedure to modify glassy carbon (GC) electrode using commercially available unsubstituted cobalt phthalocyanine (CoPc) and tetrasulfonated substituted nickel phthalocyanine (NiTSPc) simply adsorbed on oxidized single walled carbon nanotubes SWCNT. The electrocatalytic activity of the resulting SWCNT-MPc nanocomposite materials was evaluated toward the oxidation of two biologically relevant molecules, namely 2-mercaptoethanol (2-ME) and nitric oxide (NO). The obtained electrodes are highly stable under hydrodynamic conditions and the tailored hybrid surfaces allow enhancing electron transfer for the electrocatalytic oxidation of 2-ME and NO. Keywords: Single walled carbon nanotube, Cobalt phthalocyanine, Nickel tetrasulfonated phthalocyanine, Electrocatalysis, 2-Mercaptoethanol, Nitric oxide

The aim of this study is to examine chitosan, which is a good film former, as a corrosion protective coating for AA-2024-T3 aluminium alloy. The aluminium samples were first anodized to increase adhesion of the subsequently dip coated film from a chitosan-acetic acid solution. To further increase the protective ability of the chitosan coating, the coated samples were immersed in a copper ion solution for 24 h. The copper salts used were sulphate and acetate. The chitosan membranes exposed to copper ion solutions revealed a reduced permeability in comparison with the unexposed samples, and an increase in stability in aqueous solutions, as revealed by a steady state and a high open circuit potential in borax solution. From UV-Vis spectroscopic measurement of the film on ITO glasses, the reduction in permeability of the chitosan film modified by copper ions appears to be associated with copper cross linking within the chitosan structure.

Catalytic activity of cobalt tetra ethoxythiophene and cobalt tetra phenoxypyrrole phthalocyanine complexes towards oxidation of 2-mercaptoethanol, l -cysteine and reduced glutathione is reported. It was found that the activity of the complexes depends on the substitution of the phthalocyanine ring, pH, film thickness and method of electrode modification. The high electrocatalytic activity obtained with adsorbed complexes in alkaline medium clearly demonstrates the necessity of modifying bare carbon electrodes to endow them with the desired behaviour.

This study shows that cobalt macrocyclics, namely Co-phthalocyanine (CoPc), Co-hexadecafluorophthalocyanine (CoF16Pc), Co-octaethylhexyloxyphthalocyanine (CoOEHPc), Co-tetraaminophthalocyanine (CoTAPc) and Co-tetrasulfophthalocyanine (CoTSPc), strongly adsorbed on a graphite electrode surface, exhibit true electrocatalytic activity for the oxidation of glucose in alkaline solution. The Tafel analysis of the electrochemical process occurring at these chemically modified electrodes, that become “molecular phthalocyanine electrodes”, suggests that a first-one electron step is rate controlling with the symmetry of the energy barrier depending on the type of substituents grafted on the macrocycle. The effect of substituents on the phthalocyanine ring on the catalytic activity was analyzed and a non-linear correlation is found. The volcano-shaped plot obtained when comparing catalytic activities versus the Co(II)/(I) formal potential indicates that a narrow window of Co(II)/(I) formal potentials exists for achieving maximum activity. In the particular case of the present work, we find that the most active phthalocyanine is the unsubstituted CoPc.

A theoretical study of some aniline derivatives toward corrosion of copper in hydrochloric acid was carried out using cluster models for aniline–copper surface systems and ab initio Hartree Fock calculations. This study showed that the interaction of aniline molecules is highly favored in surface defects like the corner. The adsorption energy increases in several orders of magnitude rising from 30 to 67 kcal/mol. The interaction occurs between the surface defect and the amino group of the aniline molecule. The adsorption energy increases with anilines possesing electron donor substituents and decreases with anilines possessing electron acceptor substituents. These results showed that the corrosion inhibition by aniline molecules can be associated mainly to local properties acting on highly active sites, like defects, present in the copper surface. In addition, the possibility of ion pair formation between protonated aniline and chloride ion, and its influence on the general adsorption of aniline on copper is also analyzed.

Thin films of polyNi(II)tetrasulfophthalocyanine (polyNiTSPc) electrodeposited on a gold electrode by potential cycling in 0.1 M NaOH solutions containing 1 mM Ni(II) tetrasulfophthalocyanine were studied by cyclic voltammetry, electrochemical quartz crystal microbalance and impedance techniques. Both the mass increase due to the electrodeposited film, and the current of the Ni(II)/Ni(III) process in the film, increase monotonically with the number of electrodeposition cycles, a linear correlation existing between these two parameters. AFM micrographs show that the film is rugose and porous, this porosity being confirmed by the fact that the voltammetric features of gold oxide formation and reduction are unaffected by the film. The cyclic voltammogram of the film in a pH 11 carbonate–hydrogencarbonate buffer shows the characteristic anodic and cathodic peaks of the Ni ( II ) ⇔ Ni ( III ) process, with a large mass decrease upon Ni(II) electrooxidation and an equally large mass increase upon Ni(III) electroreduction. The sign of the mass change is compatible with a β-Ni(OH) 2 -character of the film. The large slopes of the plots of mass change vs. charge in the Ni(II)/Ni(III) region clearly show that large fluxes of water and/or ions accompany the electron transfer. The fairly low activation energy of Ni(II) electrooxidation, 38 kJ mol −1 , is in agreement with the moderately non-Nernstian behaviour of the Ni(II)/Ni(III) process. The super-Nernstian dependence on pH of the peak potentials of the Ni(II)/Ni(III) process indicates that the species involved are non-stoichiometric, similarly to the Ni 1 − x H x (OH) 2 species in nickel hydroxide.

This study reports on the role of both electron donor and electron acceptor substituents on the periphery of phthalocyanine cycle on the electrocatalytic activity of cobalt phthalocyanines for the oxidation of 2-aminoethanethiol (2-AET). By using these groups it is possible to modulate the Co(II)/(I) formal potential of the catalysts. The reported results show that the electrocatalytic activity of the adsorbed phthalocyanines containing electron-withdrawing groups decreases as the Co(II)/(I) formal potential of the catalyst becomes more positive, while electron-donating groups on the periphery of the ligand increase the catalytic activity.

We report here on the study of the electrocatalytic behavior of adsorbed and thin electropolymerized cobalt porphyrin and phthalocyanine films towards thiols oxidation with a special emphasize on the effect of the anion used for preparing the films and the nature of the macrocyclic ligand on the catalytic activity. Our results show that there are no effects related to the anions used during the preparation of the films on the electrocatalytic activity of the complex coatings, as evaluated by the cyclic voltammetry investigation of various thiols of different sizes. In contrast, the nature of the ligand around the cobalt centre of the macrocyclic complex has a strong effect on the catalytic activity for the oxidation of 2-mercaptoethanol (2-ME). This clearly illustrates the fact that the Co II /Co I formal redox potential of the macrocycle needs to be located in a rather narrow window to obtain maximum activity.

The reactivity of Co(II) phthalocyanine toward the oxidation of 2-mercaptoethanol was investigated using first principle theoretical methods. Calculations have been carried out using a hybrid Hartree–Fock/density functional approach (PBE0). Solvent effects have been included through a polarizable continuum model (C-PCM), while surface effects have been mimicked by a supramolecular (phthalocyanine + carbon cluster) approach. The influences of chemical environment and of the medium on reactivity were analyzed using two different reactivity descriptors, the donor–acceptor intermolecular hardness and the electrophilicity index. Our results show that solvent and surface have a cooperative effect, both increasing the reactivity of the adsorbed complex.

We report on the electrocatalytic activity of immobilized coenzyme B12 and vitamin B12 (as aquocobalamin) for the electrooxidation of l-cysteine and their effects on the electrochemical reversibility of the l-cysteine/l-cystine redox couple, a crucial biological system. Cyclic voltammograms of coenzyme B12 adsorbed on a graphite electrode show that upon the reductive elimination of the 5′-deoxyadenosyl group from the cobalt center, at approximately −1.1 V, the electrochemical response of the modified electrode becomes similar to that of aquocobalamin. The electrochemically pretreated coenzyme B12 shows a high electrocatalytic activity for the electro-oxidation of l-cysteine at physiological pH that has never been observed before with the commonly used metallophthalocyanine catalysts. Also, its activity is slightly higher than that exhibited by aquocobalamin.

In this work we have examined the effect of the Co(II)/(I) formal potential of cobalt phthalocyanines (CoPcs) on their catalytic activity for the electro-oxidation of hydrazine. The activity of the different phthalocyanines was examined using these complexes adsorbed on graphite electrodes. When activities for the different metal chelates are compared as a plot of logI (at constant potential) versus the Co(II)/(I) formal potential an asymmetric volcano curve is obtained. For some complexes the rate of the reaction increases very sharply with the driving force of the catalyst (measured as its formal potential) and then it decreases for higher driving forces. The same plot is obtained when plotting log I versus the sum of the Hammett parameters of the substituents on the periphery of the phthalocyanine ring. Keywords: Cobalt phthalocyanines, Hydrazine oxidation, Electrocatalysis, Volcano plots, Hammett parameters

We have carried out a systematic study of the electrocatalytic activity of metallophthalocyanines (M(II)-Pcs) adsorbed on graphite electrodes for the electro-oxidation of thiols (R-SH) and for the electroreduction of molecular oxygen. We have found for both reactions that when comparing the activity of substituted phthalocyanines of the same metal, plots of log k at constant potential versus the redox potential of the metallophthalocyanine, the rate of the reaction decreases with increasing driving force of the catalyst. In this work we present evidence that this behavior can be rationalized in terms of the donor–acceptor intermolecular chemical hardness ( η DA ). The energies of frontier orbitals involved in the estimation of η DA were calculated using the PM3 semi-empirical theoretical method. For the donor–acceptor pairs the lower is the hardness, the higher is the reactivity.

We have investigated the electrocatalytic activity of cobalt tetra-aminophthalocyanine (CoTAPc) for the one-electron oxidation of 2-mercaptoethanol (2-ME) using adsorbed monomeric CoTAPc and electropolymerized poly-CoTAPc films of different thickness on vitreous carbon electrode. Our results show that the activity of poly-CoTAPc increases slightly with the thickness of the film, while the activity of the adsorbed monomeric CoTAPc is still greatest. The trend in activities, analysed by cyclic voltammetry, is the same for the electro-oxidation of 2-ME and for the electroreduction of the corresponding disulphide. Tafel plots obtained from rotating disk measurement for adsorbed CoTAPc and poly-CoTAPc film modified electrodes show slopes close to RT/F (ca. 70 mV) at low polarisation potentials which increase gradually with potential to values close to 2RT/F (120 mV). This suggests that at low polarisation the rate-determining step is a chemical step (formation of a metal complex–thiol adduct) preceded by a fast electron-transfer step involving the CoTAPc. At higher polarisations the one electron oxidation of the adduct to give the thiyl radical becomes rate controlling. Even though monomeric CoTAPc adsorbed on glassy carbon presents higher activity for the electro-oxidation of 2-ME, the high stability of the electropolymerized poly-CoTAPc makes them more attractive for applications in the activation and/or the amperometric detection of thiols.

We have studied the metal-centered redox chemistry of unsubstituted cobaltphthalocyanine (CoPc) and the cobalt complexes of the substituted ligands, cobaltoctaethylhexyloxyphthalocyanine (CoOEHPc), cobalttetramethoxyphthalocyanine (CoMeOPc), cobalttetraneopentoxyphthalocyanine (CoTNPPc), cobalttetraaminophthalocyanine (CoTAPc), cobalttetrasulfonatophthalocyanine (CoTSPc) and cobalthexadecafluorophthalocyanine (CoPcF16) adsorbed on ordinary pyrolytic graphite (OPG). These complexes exhibit two distinct metal-centered reversible redox processes attributed to the Co(II)/Co(I) and Co(III)/Co(II) couples. The redox potentials of these couples are shifted according to the electron withdrawing or donating character of the groups located at the periphery of the phthalocyanine ligand, as seen by comparison to molecular orbital theoretical calculations. Correlations between the redox potential and the Hammett parameters of the substituents provide good linear correlations. The slopes of the linear free energy correlations give values of 0.41 and 0.49 Vσ−1 for the Co(III)/Co(II) and Co(II)/Co(I) redox couples, respectively. We have also tested the electrocatalytic activity of these adsorbed phthalocyanines for the reduction of 2-hydroxyethyldisulfide. A plot of log i (current density at constant potential) versus the Co(II)/Co(I) redox potential gives a linear correlation where the rate of the reaction increases with the driving force of the catalyst, suggesting that that the reaction takes place via an outer-sphere mechanism.

We have studied the effect of the redox potential of metallophthalocyanines (M-Pcs) adsorbed on graphite on their electrocatalytic activity for the oxidation of 2-mercaptoethanol (ME). This was achieved by: (i) changing the metal in the phthalocyanine (M-Pc where M=Cr, Mn, Fe, Co, Ni and Cu) and (ii) using cobalt phthalocyanines with electron-donor and electron-withdrawing substituents on the macrocyclic ligand. For phthalocyanines of different metals a plot of log k versus the redox potential of the catalyst gives a straight line of slope 0.10 V decade−1 which is close to the value obtained (0.12 V decade−1) from Tafel plots for all M-Pcs investigated. In contrast, when different substituted cobalt phthalocyanines are compared, a plot of log k versus redox potential gives a straight line of negative slope (−0.240 V decade−1) and the rate decreases with driving force. Since ME electrooxidation in aqueous media most likely proceeds via an inner-sphere mechanism and the rate-determining step is the same for all cobalt phthalocyanines investigated, the decrease in rate constant with driving force may be due to a decrease in the electronic coupling between the cobalt center and the sulfur in ME. Preliminary PM3 semi-empirical theoretical calculations of the electronic coupling associated with the interaction of the metal in the phthalocyanine and the ME molecule support this explanation. Keywords: Phthalocyanines, Inner sphere, Outer sphere, Redox electrocatalysis, Electronic coupling, Mercaptoethanol electrooxidation

Cobalt and iron tetrasulfonated phthalocyanines (M-TSPc) have been studied by electroreflectance spectroscopy in the adsorbed state on the basal plane of HOPG in a borate buffer (pH 9). Since both complexes exhibit electrocatalytic activity for the electro-oxidation of cysteine, we looked at the changes of the visible spectra of the adsorbed complexes upon adding millimolar amounts of cysteine to the electrolyte. Under zero-current conditions (open circuit) the addition of cysteine causes large changes in the reflectance spectra of both Co-TSPc and Fe-TSPc. In the case of Co-TSPc the spectra resemble that of Co(I)TSPc whereas for Fe-TSPc bands at 424 and 626 nm are observed. The open circuit potential shifts to values close to the standard potential of the M(II)/M(I) couple. We attribute these changes, particularly in the case of Fe-TSPc, to the formation of a radical adduct between the adsorbed phthalocyanine and the cysteine molecule, with partial electron-transfer from the cysteine to the metal center. Evidences for these adducts is not found when polarizing the electrode, which shows that they behave as very unstable intermediates in the catalytic process.

Cobalt phthalocyanine (CoPc) and cobalt naphthalocyanine (CoNPc) exhibit different redox and electronic properties. Redox processes on CoPc are more reversible than on CoNPc. The pH dependence of those processes is also different for these complexes, which could indicate that in the case of CoNPc, the redox couples observed involve the macrocyclic ligand and not the metal as with CoPc. This seems to explain the different redox properties and kinetics for O2 reduction and N2H4 oxidation of these two macrocyclics. In this study we compare the electrocatalytic activity of these two phthalocyanines, adsorbed on glassy carbon, for the electrochemical reduction of O2 and for the oxidation of hydrazine.

Vitamin B12 (acquo-cobalamine) adsorbs irreversibly on ordinary pyrolytic graphite (OPG) electrodes from aqueous solutions. The adsorbed vitamin exhibits cyclic voltammetric reversible waves corresponding to Co(II)/Co(I) and Co(III)/(II) couples and its redox chemistry is similar to that found in solution. The OPG/vitamin B12(ads) electrodes show substantial activity for the reduction of oxygen. In alkaline pH, a Tafel slope of −60 mV/decade and a chemical order in protons of −1 suggest a redox catalysis type of kinetics, governed by the Co(III)/Co(II) couple in the cobalamine. AtpH

The electrochemical oxidation of 2-mercaptoethanol has been examined on cobalt phthalocyanine (CoPc) + poly(2-chloroaniline) (PANCl) modified electrodes. A simple and reproducible procedure is described to obtain the modified-polymer electrodes. PANCl electrodes are modified by dispersing three different percentages of CoPc (0.1, 1 and 20%). These modified electrodes show electrocatalytic activity for 2-mercaptoethanol electro-oxidation in an acid medium. The evaluation of the electrocatalytical response of PANCl + CoPc films suggests that the diffusion rate of electroactive species into the film is improved when the polymer reaches the oxidized state.

In this paper we report a kinetic study of the electro-oxidation of hydrazine catalyzed by vitamin B 12 pre-adsorbed on an ordinary pyrolytic graphite electrode. Kinetic parameters were determined by linear sweep voltammetry and rotating-disk electrode polarization curves. The order of the reaction is 1 in OH − ions and Tafel plots give slopes of 80 mV/decade. A possible redox-catalysis mechanistic scheme is proposed.

The influence of benzotriazole, at concentrations to 10 g l −1 , on the efficiency of anodizing of Al-3.5 wt% Cu alloy at constant current density in 0.1 M ammonium pentaborate electrolyte has been investigated for two conditions of the alloy, namely solution-treated and solution treated and artificially aged. For the solution-treated alloy, the addition of benzotriazole has negligible effect on the voltage-time response and, hence, also the efficiency of film growth, which was about 92% for film formed to 200 V. In contrast, benzotriazole increases significantly the efficiency of film growth on the solution-treated and aged alloy, from ~ 45 to ~ 70% for films formed to 200 V. The increased efficiency is attributed mainly to the inhibition of oxygen evolution on CuAl 2 particles by formation of a polymeric, Cu I -benzotriazole film on the copper-rich phase. The influence of benzotriazole, from comparisons of voltage-time responses for both heat treatments of the alloy, is greatest at the commencement of film growth and subsequently diminishes, with little, or no, effect following film formation to 150 V. The diminishing influence is associated with eventual growth of relatively thick anodic film, which limits oxygen evolution.

Reflectance measurements were obtained in situ of methylene blue films on a stress-annealed pyrolytic graphite electrode. The optical response in the charging region of the double layer was observed for p polarization in the form of a small bipolar band at 690 nm, red shifted 0.08 eV with respect to the monomer species in solution. The band intensity was found to depend linearly on the modulation amplitude. On the contrary, only tiny changes in band shape and frequency with the bias potential were detected. The average orientation of the adsorbed dye is discussed together with the role of the Stark effect as a likely band source.

We have investigated the electrocatalytic activity of vitamin B 12 adsorbed on ordinary pyrolytic graphite (OPG) for the oxidation of cysteine and glutathione and for the reduction of cystine, using cyclic voltammetry and rotating-disc techniques. The activity of the bare graphite substrate is very low for all reactions studied. The presence of monolayer levels of vitamin B 12 on OPG substantially decreases the overpotential of these reactions. Tafel plots of slope RT/F are obtained for the oxidation of cysteine, suggesting a mechanism controlled by the Co(II/III) couple in the vitamin. Similar slopes are obtained for the oxidation of glutathione, but they gradually change to 2 RT/F for acid pH. No linear Tafel regions are obtained for cysteine reduction and the process occurs on Co(I).