Oxygen Reduction Reaction

Nine non-noble-metal catalysts (NNMCs) from five different laboratories were investigated for the catalysis of O 2 electroreduction in an acidic medium. The catalyst precursors were synthesized by wet impregnation, planetary ball milling, a foamingagent technique, or a templating method. All catalyst precursors were subjected to one or more heat treatments at 700-1050°C in an inert or reactive atmosphere. These catalysts underwent an identical set of electrochemical characterizations, including rotatingdisk-electrode and polymer-electrolyte membrane fuel cell (PEMFC) tests and voltammetry under N 2 . Ex situ characterization was comprised of X-ray photoelectron spectroscopy, neutron activation analysis, scanning electron microscopy, and N 2 adsorption and its analysis with an advanced model for carbonaceous powders. In PEMFC, several NNMCs display mass activities of 10-20 A g -1 at 0.8 V versus a reversible hydrogen electrode, and one shows 80 A g -1 . The latter value corresponds to a volumetric activity of 19 A cm -3 under reference conditions and represents one-seventh of the target defined by the U.S. Department of Energy for 2010 (130 A cm -3 ). The activity of all NNMCs is mainly governed by the microporous surface area, and active sites seem to be hosted in pore sizes of 5-15 Å. The nitrogen and metal (iron or cobalt) seem to be present in sufficient amounts in the NNMCs and do not limit activity. The paper discusses probable directions for synthesizing more active NNMCs. This could be achieved through multiple pyrolysis steps, ball-milling steps, and control of the powder morphology by the addition of foaming agents and/or sulfur.

The kinetics of oxygen reduction was investigated in acid solutions on Pt monolayers deposited on modified carbon-supported PdIr nanoparticles using the rotating diskelectrode technique. Iridium is introduced into the Pd substrate in order to fine-tune the Pt-Pd interactions and to improve Pd stability under operating conditions of the fuel cell. The kinetics of the oxygen reduction reaction shows enhancement with the Pt monolayer on the PdIr nanoparticle surfaces in comparison with the reaction on Pt/

Surface oxide films were grown on 99.99% copper and brass (copper-zinc alloy, Cu77Zn21Al2) in 0.1 mol L −1 borax solution at open circuit potential and were characterized using various experimental techniques. The composition of the passive films formed in situ on the different materials was studied using differential reflectance spectroscopy. The thickness of the oxide layers on copper and brass was compared by chronopotentiometric curves and potentiodynamic reductions. The electrical properties of each oxide were analyzed by means of electrochemical impedance spectroscopy. Their influence on the oxygen reduction reaction was also investigated using voltammetry hydrodynamic tools such as the rotating disk electrode. The results show that the incorporation of Zn to Cu in brass changes the composition and the thickness of the surface film. The films grown on brass tend to be thicker but less resistive and Zn compounds incorporate to the film. This is supported by results from reflectance and impedance spectroscopy. The kinetics of oxygen reduction is strongly inhibited on oxidized electrodes, particularly in the case of brass. The global number of exchanged electrons remains close to four and seems to be independent of the presence of surface oxides. .ar (S. Ceré). circuit potential and the relationship between the oxidized surface and the oxygen reduction reaction.

This paper reviews over 120 papers regarding the effect of heat treatment on the catalytic activity and stability of proton exchange membrane (PEM) fuel cell catalysts. These catalysts include primarily unsupported and carbon-supported platinum (Pt), Pt alloys, non-Pt alloys, and transition metal macrocycles. The heat treatment can induce changes in catalyst properties such as particle size, morphology, dispersion of the metal on the support, alloying degree, active site formation, catalytic activity, and catalytic stability. The optimum heat-treatment temperature and time period are strongly dependent on the individual catalyst. With respect to Pt-based catalysts, heat treatment can induce particle-size growth, better alloying degree, and changes in the catalyst surface morphology from amorphous to more ordered states, all of which have a remarkable effect on oxygen reduction reaction (ORR) activity and stability. However, heat treatment of the catalyst carbon supports can also significantly affect the ORR catalytic activity of the supported catalyst. Regarding non-noble catalysts, in particular transition metal macrocycles, heat treatment is also important in ORR activity and stability improvement. In fact, heat treatment is a necessary step for introducing more active catalytic sites. For metal chalcogenide catalysts, it seems that heat treatment may not be necessary for catalytic activity and stability improvement. More research is necessary to improve our fundamental understanding and to develop a new strategy that includes innovative heat-treatment processes for enhancing fuel cell catalyst activity and stability.

The effect of urea hydrolysis was studied on the preparation and properties of Pt/C catalyst. The hydrolysis of urea provided hydroxyl ions in a controlled manner that prevented the localized pH change in the solution and only surface precipitation is resulted in a particle growth. From rotating ring disk electrode (RRDE) experiments, the Pt/C prepared in the presence of urea showed better exchange current density toward the oxygen reduction reaction. The rate of H 2 O 2 formation was reduced when urea was used in the alcohol reduction process and it was further decreased in the Pt/C catalyst prepared by the polyol process combined with urea.

Measurements of the pH dependence of the surface-charge density on oxide particles are correlated with the pH dependence of the cyclic voltammograms and of the chemical activity of the oxide for electrolysis/fuel cell reactions. Studies of the pyrochlores Pb2M2-xPb,07-y and the rutiles MOz (M = Ru or Ir) and of the perovskite Sr,,Nb0,4 show that (1) the oxygen-reduction reaction, found on the pyrochlores in alkaline solution, occurs by exchange of a surface OH-species at an 0' site with an adsorbed 02-solution species, (2) the oxygen-evolution and chlorine-evolution reactions occur at a surface 0-species made accessible by surface oxidation of a redox couple lying close to the top of the 02-:2p6 valence band, and (3) the hydrogen-evolution reaction occurs at the surface OH, species bonded to cations with a surface redox couple lying close to the H+/H2 level in solution.

Using the rotating ring-disk technique (RRDE) the oxygen reduction reaction (orr) was studied on the Pt(111) surface in the presence of Br − anions. We found that the orr at the Pt(111) Br ad interface is always accompanied quantitatively by H 2 O 2 oxidation currents on the ring electrode, implying that in the presence of Br − anions the orr does not proceed entirely through the 4e − reduction pathway, as in the Br − -free solution. We propose that strongly adsorbed Br − can simultaneously suppress both the adsorption of the O 2 molecule and the formation of pairs of platinum sites needed for the breaking of the O -O bond. Besides elucidating the effects of Br ad , these studies also shed some light on the role of OH ad on the kinetics of the orr on the Pt(111) surface in solution free of Br anions. We have developed a theoretical model and from simulations of I -E curves we propose two modes of action of the OH ad state on the kinetics of the orr on the Pt(111) surface: (i) OH ad can block the adsorption of O 2 on active platinum sites, i.e. they compete for the same sites, and (ii) OH ad can alter the adsorption energy of intermediates which are formed during the orr on the bare Pt sites adjacent to the OH ad .

A cobalt-polymer-MWCNT composite has been developed as an electrocatalyst for the oxygen reduction reaction (ORR) in PEMFC. The suitability of this electrocatalyst for the ORR in direct methanol fuel cells and direct ethanol fuel cells has been examined by taking Pt-Ru/ MWCNT and Pt-Sn/MWCNT, respectively as an anode electrocatalysts. The study results in improved power densities for hydrogen, methanol and ethanol based fuel cells compared to the previously reported non Pt based electrocatalysts, highlighting the use of this cobalt-polymer-MWCNT composite as a potential candidate for ORR in fuel cells.

The slow oxygen reduction reaction (ORR) kinetics on Pt catalysts is among the most limiting factors in the energy conversion efficiency of the state-of-the-art PEMFC. Also, Pt is scarcely available at a high price. Alternative materials are therefore highly sought for fuel cell applications. This review will concentrate on the electrochemical ORR on non-Pt catalysts, especially the experimental development during the past five years. Emphasis is placed on the understanding of the fundamental materials chemistry. This understanding seems pivotal for new designs of PEM cathode materials. Future outlooks are provided for researchers in engineering to select promising candidates for ORR.

The electrochemical production of hydrogen peroxide (H2O2) by 2-electron oxygen reduction reaction (ORR) is an attractive alternative to the present complex anthraquinone process. The objective of this paper is to provide a state-of-the-arts review of the most important aspects of this process. First, recent advances in H2O2 production are reviewed and the advantages of H2O2 electrogeneration via 2-electron ORR are highlighted. Second, the selectivity of the ORR pathway towards H2O2 formation as well as the development process of H2O2 production are presented. The cathode characteristics are the decisive factors of H2O2 production. Thus the focus is shifted to the introduction of commonly used carbon cathodes and their modification methods, including the introduction of other active carbon materials, hetero-atoms doping (i.e., O, N, F, B, and P) and decoration with metal oxides. Cathode stability is evaluated due to its significance for long-term application. Effects of various operational parameters, such as electrode potential/current density, supporting electrolyte, electrolyte pH, temperature, dissolved oxygen, and current mode on H2O2 production are then discussed. Additionally, the environmental application of electrogenerated H2O2 on aqueous and gaseous contaminants removal, including dyes, pesticides, herbicides, phenolic compounds, drugs, VOCs, SO2, NO, and Hg0, are described. Finally, a brief conclusion about the recent progress achieved in H2O2 electrogeneration via 2-electron ORR and an outlook on future research challenges are proposed.

Design and development of highly active and durable oxygen reduction reaction (ORR) catalyst to replace Pt-and Pt-based materials are present challenges in fuel cell research including direct methanol fuel cells (DMFC). The methanol crossover and its subsequent oxidation at the cathode is another unwanted issue that reduces the efficiency of DMFC. Herein we report cobalt-doped ceria (Co-CeO 2) as a promising electrocatalyst with competent ORR kinetics mainly through a four-electron reduction pathway, and it surpasses Pt/C by a great margin in terms of stability and methanol tolerance. The Co-CeO 2 nanoparticles of diameter 4−7 nm were uniformly grown on reduced graphene oxide (rGO) by a facile single-step hydrothermal process. The as-synthesized Co-CeO 2 nanoparticles/rGO nanocomposites are further demonstrated as active energy storage materials in supercapacitors, underscoring the importance of the studied materials in renewable energy industries.

AA6060-T6, an AlMg0.5Si0.4 model alloy and a-Al(Fe,Mn)Si phase electrodes have been subjected to chromate treatment in a commercial chromate-fluoride based solution. The coated surfaces were subsequently examined by use of field emission SEM, TEM, AES and electrochemical measurements in 0.1 M NaCl solution in order to study the effect of substrate microstructure on coating formation and properties. Non-uniform growth of the chromate conversion coating (CCC) on AA6060-T6 resulted in a porous morphology, with cracks extending down to the base metal. Poor coverage was particularly observed at the grain boundaries. The thickness of the CCC after completed treatment was about 150-200 nm, while significantly thinner films were formed on the a-Al(Fe,Mn)Si particles. AlMg0.5Si0.4 in the artificially aged (T6) condition exhibited a coating morphology similar to AA6060-T6, while CCC formation on homogenised AlMg0.5Si0.4 was characterised by growth of localised oxide particles and filaments, resulting in poor coverage. These observations indicated that precipitation of Mg 2 Si particles due to heat treatment promoted nucleation of the CCC. Chromate treatment caused a significant reduction of cathodic activity on AA6060 during subsequent polarisation in chloride solution. This was attributed to rapid formation of a thin chromium 0010-938X/$ -see front matter Ó oxide film on the a-Al(Fe,Mn)Si particles during the chromate treatment, resulting in significant cathodic passivation of the phase. Inhibition of the oxygen reduction reaction at cathodic intermetallic particles is suggested as an important factor contributing to the high performance of chromate pre-treatments on aluminium.

AC impedance or electrochemical impedance spectroscopy (EIS) has been demonstrated to be a powerful technique for characterizing and evaluating fuel cells. In this work, as an extension of our previous study on the stack impedance of a 500 W PEM fuel cell, we report the AC impedance studies on individual cells of the same fuel cell stack. The EIS of the stack with an active area of 280 cm 2 was measured at currents from 10 to 210 A in steps of 20 A using the combination of a FuelCon test station, a TDI loadbank and a Solartron 1260 Frequency Response Analyzer. Measurement of the individual cell EIS was carried out with the help of a rotary switch unit made in our lab. Two methods (floating mode and grounded mode) were utilized for measuring the impedance spectroscopy of the individual cells. The results show that both methods are applicable to individual cells. The results also indicate a good agreement between the total Ohmic loss in the stack and the combined Ohmic losses of the individual cells.

PEMFC MEA HOR HER a b s t r a c t This work reports on the influence of the geometric area ratio between identical electrodes in MEA on their electrochemical performance. The difficulties in obtaining half-cell electrochemical data of MEAs motivated this research. The problem arises from the equality in geometric area of working electrode (WE) and counter electrode (CE), the thin distance between them (PEM thickness in the range 15e200 mm), the need of a reliable reference electrode, similar electrocatalyst loadings but different kinetics and electrocatalyst activities toward oxygen and hydrogen partial reactions. Thus, half-cell results obtained from MEAs may be influenced by the overloaded and, under certain conditions degrading CE.

Electrocatalyst Oxygen reduction Platinum a b s t r a c t Platinum nanoparticles supported on fishbone carbon nanofibers (CNFs) were synthesized and studied for the oxygen reduction reaction (ORR). The crystalline and textural properties of the CNFs were modified by synthesizing them at different temperatures, allowing the comparison of supports with either improved graphitization degree or improved porosity.

Feature Article 1794 Y. Feng and N. Alonso-Vante: Metal catalysts for molecular oxygen-reduction reaction Feature Article 1796 Y. Feng and N. Alonso-Vante: Metal catalysts for molecular oxygen-reduction reaction Feature Article 1800 Y. Feng and N. Alonso-Vante: Metal catalysts for molecular oxygen-reduction reaction

This work constitutes detailed EIS (Electrochemical Impedance Spectroscopy) measurements on a PBIbased HT-PEM unit cell. By means of EIS the fuel cell is characterized in several modes of operation by varying the current density, temperature and the stoichiometry of the reactant gases. Using Equivalent Circuit (EC) modeling key parameters, such as the membrane resistance, charge transfer resistance and gas transfer resistance are identified, however the physical interpretation of the parameters derived from EC's are doubtful as discussed in this paper. The EC model proposed, which is a modified Randles circuit, provides a reasonably good fit at all the conditions tested. The measurements reveal that the cell temperature is an important parameter, which influences the cell performance significantly, especially the charge transfer resistance proved to be very temperature dependent. The transport of oxygen to the Oxygen Reduction Reaction (ORR) likewise has a substantial effect on the impedance spectra, results showed that the gas transfer resistance has an exponential-like dependency on the air stoichiometry. Based on the present results and results found in recent publications it is still not clear what exactly causes the distinctive low frequency loop occurring at oxygen starvation. Contrary to the oxygen transport, the transport of hydrogen to the Hydrogen Oxidation Reaction (HOR), in the stoichiometry range investigated in this study, shows no measurable change in the impedance data. Generally, this work is expected to provide a basis for future development of impedance-based fuel cell diagnostic systems for HT-PEM fuel cell.

When measuring the current due to the Oxygen Reduction Reaction (ORR) and hydrogen oxidation reaction (HOR) on Pt and Pt alloys in aqueous electrolyte, it is important to take care of two major sources of error that are relatively easy to correct for. First, when measuring ORR voltammetry, adsorption processes are superimposed on the current. Second, the system resistance causes an Ohmic drop that may have a profound effect on the measured curves, especially at the higher currents close to the diffusion limiting current. More importantly, we show that it also influences the kinetic part of the potential curve in such a way that the Tafel slope may be determined incorrectly when failing to correct for Ohmic drop. Finally, because electrolyte resistance lowers with increasing temperature, failure to compensate for Ohmic drop may lead to erroneous conclusions about the temperature-dependent activity of a catalyst as well as the corresponding activation energies.

Enzymatic decolorization of reactive blue 221 (RB221) using laccase was investigated in a dual-chamber microbial fuel cell (MFC). Suspended laccase was used in the cathode chamber in the absence of any mediators in order to decolorize RB221 and also improve oxygen reduction reaction in the cathode. Molasses was utilized as low cost and high strength energy source in the anode chamber. The capability of MFC for simultaneous molasses and dye removal was investigated. A decolorization efficiency of 87% was achieved in the cathode chamber and 84% COD removal for molasses was observed in the anode chamber. Laccase could catalyze the removal of RB221 and had positive effect on MFC performance as well. Maximum power density increased about 30% when enzymatic decolorization was performed in the cathode chamber.

The electrochemical reduction of oxygen on thin Pd films with a nominal thickness of 0.25-10 nm was studied in alkaline and acidic solution using the rotating disk electrode (RDE) method. The Pd films on glassy carbon electrodes were prepared by electron beam evaporation and coated with a Nafion layer. The surface morphology of the films was examined by transmission electron microscopy (TEM). In 0.1 M KOH the electrocatalytic activity of Pd electrodes towards oxygen reduction was high and the specific activity (SA) did not depend on the film thickness. In 0.05 M H 2 SO 4 the activity was lower and a decrease of SA with decreasing film thickness was observed. The oxygen reduction reaction on thin Pd films predominantly followed a four-electron pathway in both solutions and the reduction mechanism was similar to that on bulk Pd, the first electron transfer being the rate-limiting step.

Pt-Co/C Membrane-electrode assembly PEM fuel cell a b s t r a c t Nano-sized Pt-Pd/C and Pt-Co/C electrocatalysts have been synthesized and characterized by an alcohol-reduction process using ethylene glycol as the solvent and Vulcan XC-72R as the supporting material. While the Pt-Pd/C electrodes were compared with Pt/C (20 wt.% E-TEK) in terms of electrocatalytic activity towards oxidation of H 2 , CO and H 2 -CO mixtures, the Pt-Co/C electrodes were evaluated towards oxygen reduction reaction (ORR) and compared with Pt/C (20 wt.% E-TEK) and Pt-Co/C (20 wt.% E-TEK) and Pt/C (46 wt.% TKK) in a single cell. In addition, the Pt-Pd/C and Pt-Co/C electrocatalyst samples were characterized by XRD, XPS, TEM and electroanalytical methods. The TEM images of the carbon supported platinum alloy electrocatalysts show homogenous catalyst distribution with a particle size of about 3-4 nm. It was found that while the Pt-Pd/C electrocatalyst has superior CO tolerance compared to commercial catalyst, Pt-Co/C synthesized by polyol method has shown better activity and stability up to 60 C compared to commercial catalysts. Single cell tests using the alloy catalysts coated on Nafion-212 membranes with H 2 and O 2 gases showed that the fuel cell performance in the activation and the ohmic regions are almost similar comparing conventional electrodes to Pt-Pd anode electrodes. However, conventional electrodes give a better performance in the ohmic region comparing to Pt-Co cathode. It is worth mentioning that these catalysts are less expensive compared to the commercial catalysts if only the platinum contents were considered.

In this study, we examined how aluminium corrosion in Al-Cu/PANI galvanic couples in a marine environment is influenced by deposition of polyaniline (PANI) on copper. Polarization curves and immersion assays in 0.1 M NaCl were performed. The morphologies of etched Al and corrosion products were observed by SEM, and the Al ions in solution were quantified by atomic absorption spectroscopy. A reduction in aluminium damage due to galvanic corrosion was observed as a result of decreased effective area for the oxygen reduction reaction on Cu/PANI electrode. Furthermore, an electrochemical reduction of PANI from leucoemeraldine to emeraldine base is proposed.

AC impedance or electrochemical impedance spectroscopy (EIS) has been demonstrated to be a powerful technique for characterizing and evaluating fuel cells. In this work, as an extension of our previous study on the stack impedance of a 500 W PEM fuel cell, we report the AC impedance studies on individual cells of the same fuel cell stack. The EIS of the stack with an active area of 280 cm 2 was measured at currents from 10 to 210 A in steps of 20 A using the combination of a FuelCon test station, a TDI loadbank and a Solartron 1260 Frequency Response Analyzer. Measurement of the individual cell EIS was carried out with the help of a rotary switch unit made in our lab. Two methods (floating mode and grounded mode) were utilized for measuring the impedance spectroscopy of the individual cells. The results show that both methods are applicable to individual cells. The results also indicate a good agreement between the total Ohmic loss in the stack and the combined Ohmic losses of the individual cells.

The influence of chemical pretreatment of carbon support for oxygen reduction on palladium nano-particles in acidic electrolyte was studied. Vulcan XC-72R carbon as catalyst support for palladium nano-particles was pretreated with 5% HNO 3 , 0.07 M H 3 PO 4 , 0.2 M KOH and 10% H 2 O 2 . The effect of treatment on the properties of the carbon support was studied by N 2 adsorption and X-ray photoelectron spectroscopy (XPS). It was found that chemical treatment significantly changed the surface chemical properties and surface area of the carbon support. The surface area and pore volume of 5% HNO 3 and 10% H 2 O 2 treated carbon supports were drastically decreased due to the oxidative nature of treatment. Ethylene glycol (EG) reduction method was used to synthesise 20% Pd on pr-treated and un-treated carbon supports. Differences in catalyst morphology were characterized using X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscope techniques. It was observed that by using a mild reducing agent, namely EG, well-dispersed and nano-size Pd particles could be achieved during catalyst synthesis. The electrocatalytic activity of different Pd/C catalysts towards the oxygen reduction reaction (ORR) was examined by cyclic voltammetry (CV) on a rotating ring-disc electrode (RRDE) and compared with E-Tek 20% Pd/C catalyst under identical experimental conditions. The kinetics of ORR on these electrocatalysts predominantly involved a four-electron step reduction with the first electron transfer being the rate-determining step. However, the observed specific activity, mass activity and amount of hydrogen peroxide produced during ORR were greatly influenced by the pretreatment employed for carbon support.

The perovskite La 0.75 Sr 0.25 Cr 0.5 Mn 0.3 Ni 0.2 O 3−δ (LSCMMn 0.30 Ni 0.20 ) was evaluated as potential electrode for solid oxide fuel cells. The electrochemical performances of LSCMMn 0.30 Ni 0.20 for hydrogen oxidation and oxygen reduction reactions were studied at 800°C. Symmetrical cells LSCMMn 0.30 Ni 0.20 /YSZ/LSCMMn 0.30 Ni 0.20 were studied by electrochemical impedance spectroscopy. The total conductivity of LSCMMn 0.30 Ni 0.20 is 22 S cm −1 in air and 0.8 S cm −1 under wet 5% H 2 /Ar at 800°C. The area specific resistance (ASR) at 800°C for hydrogen oxidation reaction is 1 Ω cm 2 . The ASR for oxygen reduction reaction is 1.6 Ω cm 2 . An ageing study during 24 h shows a relatively good stability of the electrochemical performances. LSCMMn 0.3 Ni 0.2 seems to be a promising dual electrode for a symmetrical SOFC.

Electrocatalysis: theory and experiment at the interface Marc T. M. Koper, Faraday Discuss.A true theory of electrocatalysis will not be available until activity can be calculated a priori from some known properties of the materials.'' Toward this goal, we developed intrinsic kinetic equations for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) using as the kinetic parameters the free energies of adsorption and activation for elementary reactions. Rigorous derivation retained the intrinsic connection between the intermediates' adsorption isotherms and the kinetic equations, affording us an integrated approach for establishing the reaction mechanisms based upon various experimental and theoretical results. Using experimentally deduced free energy diagrams and activity-and-barriers plot for the ORR on Pt , we explained why the Tafel slope in the large overpotential region is double that in the small overpotential region. For carbon-supported Pt nanoparticles (Pt/C), the polarization curves measured with thin-film rotating disk electrodes also exhibit the double Tafel slope, albeit Pt(111) is several times more active than the Pt nanoparticles when the current is normalized by real surface area. An analytic method was presented for the polarization curves measured with H 2 in proton exchange membrane fuel cells (PEMFCs). The fit to a typical iR-free polarization curve at 80 C revealed that the change of the Tafel slope occurs at about 0.77 V that is the reversible potential for the transition between adsorbed O and OH on Pt/C. This is significant because it predicts that the Butler-Volmer equation can only fit the data above this potential, regardless the current density. We also predicted a decrease of the Tafel slope from 70 to 65 mV dec À1 at 80 C with increasing oxygen partial pressure, which is consistent with the observation reported in literature.

A new approach to synthesize nitrogen-doped carbon nanotubes (NCNTs) as catalysts for oxygen reduction by treating oxidized CNTs with ammonia is presented. The surface properties and oxygen reduction activities were characterized by cyclic voltammetry, rotating disk electrode and X-ray photoelectron spectroscopy. NCNTs treated at 800°C show improved electrocatalytic activity for oxygen reduction as compared with commercially available Pt/C catalysts.

Xerogel carbon support Graphene Carbon nanotubes a b s t r a c t Non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton-exchangemembrane fuel cells (PEMFCs) were obtained by pyrolysis of iron citrate and polyacrylonitrile on mesoporous xerogel carbon support. Chemical-physical characterizations, electrochemical studies by the rotating disc electrode, and electrochemical tests in a PEMFC configuration demonstrated that the porosity of the pristine carbon promotes the formation of graphene and carbon nanotube structures featuring ORR catalytic activity. (M. Mastragostino).

Three cathode catalysts (60% Pt/C, 30% Pt/C and 60% Pt-Fe/C), with a particle size of about 2-3 nm, were prepared to investigate the effect of ethanol cross-over on cathode surfaces. All samples were studied in terms of structure and morphology by using X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. Their electrocatalytic behavior in terms of oxygen reduction reaction (ORR) was investigated and compared using a rotating disk electrode (RDE). The tolerance of cathode catalysts in the presence of ethanol was evaluated. The Pt-Fe/C catalyst showed both higher ORR activity and tolerance to ethanol cross-over than Pt/C catalysts. Moreover, the more promising catalysts were tested in 5 cm 2 DEFC single cells at 60 and 80°C. An improvement in single cell performance was observed in the presence of the Pt-Fe catalyst, due to an enhancement in the oxygen reduction kinetics. The maximum power density was 53 mW cm -2 at 2 bar rel. cathode pressure and 80°C.

Polyaniline (Pani), vanadium oxide (V 2 O 5 ), and Pani/V 2 O 5 nanocomposite were fabricated and applied as a cathode catalyst in Microbial Fuel Cell (MFC) as an alternative to Pt (Platinum), which is a commonly used expensive cathode catalyst. The cathode catalysts were characterized using Cyclic Voltammetry and Linear Sweep Voltammetry to determine their oxygen reduction activity; furthermore, their structures were observed by X-ray Diffraction, X-ray Photoelectron Spectroscopy, Brunauer-Emmett-Teller, and Field-Emission Scanning Electron Microscopy. The results showed that Pani/V 2 O 5 produced a power density of 79.26 mW/m 2 , which is higher than V 2 O 5 by 65.31 mW/m 2 and Pani by 42.4 mW/m 2 . Furthermore, the Coulombic Efficiency of the system using Pani/V 2 O 5 (16%) was higher than V 2 O 5 and Pani by 9.2 and 5.5%, respectively. Pani-V 2 O 5 also produced approximately 10% more power than Pt, the best and most common cathode catalyst. It declares that Pani-V 2 O 5 can be a suitable alternative for application in a MFC system.

The effect of ammonia on polymer electrolyte membrane fuel cells (PEMFC) was tested by adding 10 ppm NH 3 to the hydrogen feed to PEMFCs based on Gore™ PRIMEA ® membrane electrode assemblies (MEAs). A significant loss in performance was observed. The poisoning process was slow taking 24 h or more to reach a steady state. In some cases no steady state performance was reached during the experiment. The performance loss was reversible in most cases, but only after operation on neat hydrogen for several days. Additions of 1 ppm NH 3 for 1 week also resulted in significant performance loss. An MEA based on carbon supported Pt anode and cathode catalyst did not differ from the Gore™ MEA based on PtRu anode and Pt cathode catalyst. The performance losses were higher than could be explained by the observed increase in ohmic resistance in the cell. There was also a significant decay in performance in a H 2 |H 2 cell, especially at high current density where a reaction limiting current was observed showing that the Hydrogen Oxidation Reaction (HOR) was affected. The Oxygen Reduction Reaction (ORR) on the cathode was also significantly affected by ammonia.

Pt-NiO x /C, Pt-CoO x /C, and Pt-SnO 2 /C catalysts, prepared by thermal decomposition of polymeric precursors, are characterized for the oxygen reduction reaction in the presence of ethanol, and their tolerance to the presence of alcohol is determined. The electrocatalytic activities of Pt-NiO x /C, Pt-CoO x /C, and Pt-SnO 2 /C in the absence of ethanol are similar to those of the Pt/C catalysts or lower, depending on electrode composition. However, these catalysts present higher catalytic activity for the oxygen reduction reaction in the presence of ethanol, compared with the Pt/C catalyst. This makes Pt-NiO x /C, Pt-CoO x /C, and Pt-SnO 2 /C potentially applicable as cathodes in direct ethanol fuel cells, thereby reducing the effect of ethanol crossover.

Vulcan XC-72 carbon-supported Pt-Ni alloy nanoparticle catalysts with different Pt/Ni atomic composition were prepared via the carbonyl complex route and their structure was studied by X-ray diffraction spectroscopy at wide angles (WAXS) and Debye function analysis (DFA). The very good agreement between the WAXS pattern and DFA simulation revealed that all the as-prepared Pt-Ni alloy catalysts have a unique and highly disordered face-centered cubic structure (solid solution) and that the lattice parameter decreases with the increase of the Ni content in the alloys. Transmission electron microscopy (TEM) images indicated that the as-prepared Pt-Ni alloy nanoparticles were well dispersed on the surface of the carbon support with a narrow particle size distribution and that their mean particle size slightly decreased with the increase in Ni content. Energy-dispersive X-ray analysis (EDX) confirmed that the catalyst composition was nearly the same as that of the nominal value. Thus, a comparative study was made for the oxygen reduction reaction (ORR) using the thin-film rotating ring-disk electrode method to the behavior of Pt based catalysts on the same carbon support, having the same metal loading, the same disordered structure, and a similar particle size. As compared to the Pt/C catalyst, the bimetallic catalysts with different Pt/ Ni atomic ratios exhibited an enhancement factor of ca. 1.5 to 3 in the mass activity and of ca. 1.5 to 4 in the specific activity for the ORR and a lower production of hydrogen peroxide in pure perchloric acid solution. The maximum activity of the Pt-based catalysts was found with ca. 30 ∼ 40 at. % Ni content in the alloys, which could originate from the favorable Pt-Pt interatomic distance. The ring-current measurements on all the catalysts showed similar behavior for hydrogen peroxide production. The enhanced electrocatalytic activity of as-prepared Pt-Ni alloy catalysts for the ORR is attributed to the high dispersion of the alloy catalysts, to their disordered structure, and to the favorable Pt-Pt mean interatomic distance caused by alloying.

Titanium dioxide/zirconium dioxide nanocomposites were synthesized using a simple and modified sol-gel method. The synthesized particles were characterized through SEM, TEM, XRD, and DTA/TGA. SEM analysis shows micrographs with irregular and sharp edged particles. TEM analysis shows spherical particles of average diameter 10.5 nm. Tetragonal structure of TiO 2 /ZrO 2 nanocomposite particles with stabilized anatase phase was confirmed by XRD studies. The average crystallite size was calculated from XRD, using Scherrer's formula. Band gap was calculated from the DRS spectra using Kubelka-Munk function and Photoluminescence (PL) was done to study the recombination rate of charge carriers. Results showed a considerable increase in band gap on ZrO 2 addition and subsequent decrease in recombination rate. Impedance study showed a significant decrease in dielectric characteristics on ZrO 2 addition. Photocatalytic activity of the synthesized catalysts was studied by degradation of an azo-dye, Ponceau BS, using ultra-violet source of light. Optimum activity was observed on 6.0% ZrO 2 loading.