Oxygen Reduction

Transition metal oxides are attractive noble metal-free catalysts of the oxygen reduction for application at the cathode of alkaline membrane fuel cells or metal-air batteries. However, despite of a rapidly increasing number of publications devoted to the oxygen electrocatalysis on transition metal oxides, a clear picture regarding the relations between their structure and composition on the one hand and electrocatalytic activity on the other hand is lacking. This short review discusses challenges facing researchers seeking to understand electrocatalysis of the oxygen reduction reaction on transition metal oxides.

Knowledge on the mechanisms of oxygen reduction reaction (ORR) and descriptors linking the catalytic activity to the structural and electronic properties of transition metal oxides enable rational design of more efficient catalysts. In this work ORR electrocatalysis was studied on a set of single and complex Mn(III) oxides with a rotating disc electrode method and cyclic voltammetry. We discovered an exponential increase of the specific electrocatalytic activity with the potential of the surface Mn(IV)/Mn(III) red-ox couple, suggesting the latter as a new descriptor for the ORR electrocatalysis. The observed dependence is rationalized using a simple mean-field kinetic model considering availability of the Mn(III) centers and adsorbate-adsorbate interactions. We demonstrate an unprecedented activity of Mn2O3, ca. 40 times exceeding that of MnOOH and correlate the catalytic activity of Mn oxides to their crystal structure. Keywords: manganese oxides; oxygen reduction reaction (ORR); red-ox transition; rotating disc electrode (RDE); meanfield kinetic modeling * Note that Suntivich et al. found significant effect of cations on the ORR at Pt, the activity increasing by ca. a factor of 2 from NaOH to KOH ** Potentials in the reference are given in Hg/HgO scale and recalculated into RHE scale via addition of + 0.93 V † Recalculated from the data presented in the article as mass activity divided by specific activity † † Recalculated from the data presented in the article considering the values of specific and mass activity at 0 V vs NHE and the value of the Tafel slope ‡ Oxide surface area estimated from cyclic voltammetry as 2.8 cm 2 (to be compared to ca. 24 cm 2 for Mn2O3_wet_3 in this work)

Perovskite oxides are promising materials for the ORR in alkaline media. However, catalytic layers prepared from perovskite powders suffer from high Ohmic losses and low catalyst utilization. An addition of carbon to the catalytic layers greatly improves the performance of the electrodes in the ORR. In this work composite thin film electrodes comprised of a perovskite oxide (either LaCoO 3 or La 0.8 Sr 0.2 MnO 3 ) and pyrolytic carbon of the Sibunit family were investigated in aqueous 1 M NaOH electrolyte using cyclic voltammetry and rotating disc electrode (RDE) method with the objective to unveil the influence of carbon on the catalyst utilization and on the ORR electrocatalysis. By systematically varying the oxide to carbon ratio we arrive to the conclusion on the dual role of carbon in composite electrodes. On the one hand, it is required to improve the electrical contact between perovskite particles and the current collector, and to ensure maximum utilization of the perovskite surface. On the other hand, carbon plays an active role in the ORR by catalyzing the O 2 reduction to H 2 O 2 . Composite electrodes catalyze the 4e - ORR in contrast to carbon which is only capable of catalyzing the 2e -reduction. For LaCoO 3 composite electrodes, carbon is responsible for the catalysis of the first steps of the ORR, the role of LaCoO 3 being largely limited to the hydrogen peroxide decomposition and/or reduction. For La 0.8 Sr 0.2 MnO 3 composite electrodes, along with the catalysis of the chemical decomposition and/or reduction of H 2 O 2 produced on carbon, the perovskite also significantly contributes to the first steps of the ORR. The results of this work suggest that the ORR on the carbon and the oxide components of composite cathodes must be considered as coupled reactions whose contributions cannot be always separated, and that neglecting the contribution of carbon to the ORR electrocatalysis may lead to erroneous values of the catalytic activity of perovskite materials.

The low durability of Pt/C electro-catalysts in polymer electrolyte membrane fuel cells (PEMFC), e.g., as a result of carbon oxidation to CO2 in an acid medium, has been recognized as one of the most important hindrances to long-term stability. In this work, Pt electrocatalysts supported on TiO2-carbon and Vulcan carbon were prepared by the chemical vapor deposition method. The physical and electrochemical properties of Pt/TiO2-C and Pt/C electrocatalysts were investigated by the characterization techniques of XRD, TEM, CO stripping and cyclic and linear voltammetry. The prepared materials were electrochemically evaluated in the oxygen reduction reaction (ORR) in an acid medium at room temperature. The XRD results show crystalline fcc platinum formation. The mean particle size is between 2 and 4 nm with a spherical morphology. Pt/TiO2-C electrocatalysts showed a higher electrochemical active surface area and better activity results for the ORR compared with the Pt/C samples. The add...

For the challenging nature of the zirconium environment analysis, this study consists to analyze the electrochemical behavior of Zirconium in both aqueous and organic media. To that end first the electrolytic media was selected on the basis of the Pourbaix potential-pH diagram, which provides informations on the predominance of Zr (IV) ion and Zr in aqueous media. In aqueous media, analyzes were first carried out in acidic media then in basic media. Studies have thus revealed that the acidic environment is not favourable for the electrochemical analysis of zirconium. Voltammograms obtained in an acidic environment show no zirconium detection signal; this is due to the strong presence of H + ions in the solution. We have also observed in acidic media the phenomenon of passivation of the electrode surface. In aqueous alkaline media (pH = 13), we have drawn in reduction several Intensity-Potential curves by fixingsome technical parameterslike scanning speed, rotation speed of the electrode. The obtained voltammograms show cathodic waves, starting from-1.5 V/DHW and attributed to the reduction of Zr (IV) to Zr (0). The last phase of this study focused on the electrochemical analysis of zirconium in an organic media. In this media, several intensity-potential curves were plotted in reduction and in cyclic voltammetry with various parameters. Through several reduction analysis, the Zr (IV) was reduced to Zr (0) to the potential of-1.5 V/DHW. The electrochemical analysis of zirconium in organic media seems globally easier to achieve thanks to its large solvent window (i.e. dimethylformamide (DMF) solvent window > 6 V).

Corrosion tests of carbon steel metal in 5% H2SO4 solution at different operating conditions were performed for ranges of temperature (30 to 60 o C) and rotational velocity (0 to 600 rpm) using both weight loss method and electrochemical polarization technique (Tafel extrapolation). The results revealed different trends of carbon steel against the experimental parameters. The corrosion rate was found to increase considerably with increasing temperature for the majority of results. However, increasing rotational velocity clearly enhanced the corrosion rate for carbon steel. The corrosion potential shows sharp decrease with time in first few minutes and then increase reaching an asymptotic value beyond that. In weight loss experiments, increasing temperature shifted the Ecorr to more positive at stationary condition. Increasing velocity from 0 to 400 rpm shifted the Ecorr to more negative at 40 °C. Then trended to positive with increasing velocity to 600 rpm. In polarization experimen...

LaMnO3 has been identified as one of the most active systems towards the 4‐electron oxygen reduction reaction (ORR) under alkaline conditions, although the rationale for its high activity in comparison to other perovskites remains to be fully understood. LaMnO3 oxide nanoparticles are synthesised by an ionic‐liquid based method over a temperature range of 600 to 950 °C. This work describes a systematic study of the LaMnO3 properties, from bulk to the outermost surface layers, as a function of the synthesis temperature to relate them to the ORR activity. The bulk and surface composition of the particles are characterised by transmission electron microscopy, X‐ray diffraction, X‐ray absorption and X‐ray photoemission spectroscopy (XPS), as well as low‐energy ion scattering spectroscopy (LEIS). The particle size and surface composition are strongly affected by temperature, although the effect is non‐monotonic. The number density of redox active Mn sites is obtained from electrochemical...

In this work, Pd-Ni catalysts supported on carbon nanofibers were synthesized, with metal contents and Pd:Ni atomic ratios close to 25 wt. % and 1:2, respectively. Previously, the carbon nanofibers were chemically treated, in order to create surface oxygen and/or nitrogen groups. The synthesized catalysts displayed low crystallinity degree and high dispersion on carbon supports, especially in those with surface functional groups. Oxygen reduction reaction (ORR) was studied by rotating ring-disk electrode (RRDE) techniques. When the kinetic current is normalized by the mass of Pd present in the electrode, higher activities were obtained for the synthesized materials in comparison with the activity observed for a commercial Pd/C E-TEK catalyst. Some differences are reported for the different materials under study, mainly dependent on the presence of oxygen surface groups on the carbon support. In light of the results, we can propose the synthesized catalysts as possible candidates for cathodes in alkaline direct methanol fuel cells.

In this work, a rotating disk electrode was used to measure the cathodic kinetics on stainless steel as a function of diffusion layer thickness (6 to 60 μm) and chloride concentration (0.6 to 5.3 M NaCl). It was found that, while the cathodic kinetics followed the Levich equation for large diffusion layer thicknesses, the Levich equation overpredicts the mass-transfer limited current density for diffusion layer thicknesses less than 20 μm. Also, an unusual transitory response between the activation and mass-transfer controlled regions was observed for small diffusion layer thicknesses that was more apparent in lower concentration solutions. The presence and reduction of an oxide film and a transition in the oxygen reduction mechanism were identified as possible reasons for this response. The implications of these results on atmospheric corrosion kinetics under thin electrolyte layers is discussed.

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Scanning Kelvin Probe (SKP) and FTIR microscopy were applied to study the atmospheric corrosion of galvanized steel coated by electrophoretic epoxy resin (ED) at a defect. The SKP was useful to determine the spatial separation of the electrochemical reactions at a defect and surrounding metal/paint interface and to evaluate the formation of the galvanic cells. FT-IR microscopy was helpful to identify the composition and distribution of the corrosion products in the galvanic cells. It was shown that the cathodic delamination of coating takes place after deposition of a thick water electrolyte film in the defect. The anodic undermining of the coating is favoured in case of atmospheric corrosion under thin electrolyte films. The anodic de-adhesion starting from defect reaching the zinc layer and from the non protected cut edge in case of exposure in the salt spray conditions was also determined. The role of the formation of confined volume underneath the delaminated paint on the rate of anodic undermining is discussed.

The oxygen exchange kinetics of platinum on yttria-stabilized zirconia (YSZ) was investigated by means of geometrically well-defined Pt microelectrodes. By variation of electrode size and temperature it was possible to separate two temperature regimes with different geometry dependencies of the polarization resistance. At higher temperatures (550-700 • C) an elementary step located close to the three phase boundary (TPB) with an activation energy of ∼1.6 eV was identified as rate limiting. At lower temperatures (300-400 • C) the rate limiting elementary step is related to the electrode area and exhibited a very low activation energy in the order of 0.2 eV. From these observations two parallel pathways for electrochemical oxygen exchange are concluded. The nature of these two elementary steps is discussed in terms of equivalent circuits. Two combinations of parallel rate limiting reaction steps are found to explain the observed geometry dependencies: (i) Diffusion through an impurity phase at the TPB in parallel to diffusion of oxygen through platinummost likely along Pt grain boundaries -as area-related process. (ii) Co-limitation of oxygen diffusion along the Pt|YSZ interface and charge transfer at the interface with a short decay length of the corresponding transmission line (as TPB-related process) in parallel to oxygen diffusion through platinum.

The considerable potential of model-type thin film electrodes for the investigation of oxygen exchange pathways is demonstrated for different electrode materials on yttria-stabilized zirconia (YSZ). In particular, a correlation of voltage-driven O tracer experiments and electrical ac and dc measurements has proven to be helpful when aiming at mechanistic conclusions. For Pt electrodes, two different parallel reaction pathways can be identified under equilibrium conditions. At lower temperatures, a diffusion limited path through the electrode is dominant, whereas at higher temperatures, an electrode surface path with oxygen incorporation at the three-phase boundary determines the electrochemical activity. In addition, for high cathodic polarization, an electrolyte surface path with electron transfer via YSZ outperforms both other pathways. The oxygen incorporation zones of the bulk path as well as the electrolyte surface path can be visualized by O tracer incorporation experiments in combination with time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis. A successful separation of surface and bulk path can also be obtained for La 0.8 Sr 0.2 MnO 3Àd (LSM) electrodes by means of O tracer incorporation at different cathodic overpotentials. Under lower polarization, a surface path with oxygen incorporation at the three-phase boundary is dominant, whereas at higher cathodic overpotential, the bulk path becomes significantly more pronounced. These changes are discussed in terms of polarization-induced changes of the ionic conductivity in the LSM electrode. Measurements on the acceptor-doped perovskite-type materials La 0.6 Sr 0.4 CoO 3Àd (LSC) and La 0.6 Sr 0.4 FeO 3Àd (LSF) illustrate the limitations of the tracer incorporation method. In the case of highly active LSC electrodes with low polarization resistances, the tracer distribution is determined by the electrolyte, and thus the active sites of the electrodes can no longer be visualized. The effect of polarization-induced changes of the electrode's electronic conductivity is demonstrated for LSF. Only a region close to the current collector remains electrochemically active owing to limited lateral electron transport.

Cobalt-nitride (Co 4 N) nanoparticle-decorated nitrogen-doped graphene sheets were obtained via the nitrogen doping of a graphene-oxide precursor and simultaneous nitride formation. The non-precious metal catalyst formed in this one-step synthesis exhibits high electrocatalytic oxygen reduction activity and hence provides a promising alternative to conventional Pt/C alkaline fuel cell cathode catalysts. The reported composites were formed from the mixture of lyophilized graphene-oxide nanosheets and cobalt(II) acetate in ammonia atmosphere at 600 °C. The average Co 4 N particle size increased from 14 to 201 nm with the increase in cobalt content. The oxygen reduction activity of the new catalysts was comparable to that of non-noble metal systems described in the literature, and also to the widely-used carbon black supported platinum catalysts. The highest reduction current density under alkaline conditions was found to be as high as 4.1 mA cm -2 with the corresponding electron transfer number of 3.6. Moreover, the new system outperformed platinum-based composites in terms of methanol tolerance, thus eliminating one of the major drawbacks (besides high price and limited availability), of noble metal catalysts.

A carbon-supported, dealloyed platinum-copper (Pt-Cu) oxygen reduction catalyst was prepared using a multi-step synthetic procedure. Material produced at each step was characterized using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), electron energy loss spectroscopy (EELS) mapping, x-ray absorption spectroscopy (XAS), x-ray diffraction (XRD), and cyclic voltammetry (CV), and its oxygen reduction reaction (ORR) activity was measured by a thin-film rotating disk electrode (TF-RDE) technique. The initial synthetic step, a co-reduction of metal salts, produced a range of poorly crystalline Pt, Cu, and Pt-Cu alloy nanoparticles that nevertheless exhibited good ORR activity. Annealing this material alloyed the metals and increased particle size and crystallinity. TEM shows the annealed catalyst to include particles of various sizes, large (>25 nm), medium (12-25 nm), and small (<12 nm). Most of the small and medium-sized particles exhibited a partial or complete coreshell (Cu-rich core and Pt shell) structure with the smaller particles typically having more complete shells. The appearance of Pt shells after annealing indicates that they are formed by a thermal diffusion mechanism. Although the specific activity of the catalyst material was more than doubled by annealing, the concomitant decrease in Pt surface area resulted in a drop in its mass activity. Subsequent dealloying of the catalyst by acid treatment to partially remove the copper increased the Pt surface area by changing the morphology of the large and some medium particles to a "Swiss cheese" type structure having many voids. The smaller particles retained their core-shell structure. The specific activity of the catalyst material was little reduced by dealloying, but its mass activity was more than doubled due to the increase in surface area. The possible origins of these results are discussed in this report.

Electrocatalysts are mainly characterized by their intrinsic adsorption properties. However, the observed electrocatalytic activity ultimately results from the interplay between such properties and various additional interactions within the electrified solid-liquid interface. One of such phenomena is solvation, which can substantially affect the stability of adsorbates. The incorporation of solvation in computational electrocatalysis models can be fully implicit (inaccurate for H-bonded adsorbates), fully explicit (challenging computation of free energies), or embedded. Here we show that without any need for explicit or implicit media, a micro-solvation approach with just 3 water molecules captures the contribution of coadsorbed water to the adsorption energies of *OH and *OOH (two important adsorbates for oxygen reduction) on platinum nanoparticles of various sizes. The approach enables an accurate yet inexpensive explicit modeling of solvent-adsorbate interactions in nanoparticles, and the calculation of solvation corrections, estimated as 0.59 0.14 eV -± and 0.47 0.13 eV -± for *OH and *OOH adsorption on Pt.

This perspective article provides an overview of recent developments in the field of 3d transition metal (TM) catalysts for different reactions including oxygen-based reactions such as Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER). The spin moments of 3d TMs can be exploited to influence chemical reactions, and recent advances in this area, including the theory of chemisorption based on spin-dependent d-band centers and magnetic field effects, are discussed. The article also explores the use of scaling relationships and surface magnetic moments in catalyst design, as well as the effect of magnetism on chemisorption and vice versa. In addition, recent studies on the influence of a magnetic field on the ORR and OER are presented, demonstrating the potential of ferromagnetic catalysts to enhance these reactions through spin polarization.

Polymer supported catalysts prepared by the chemical deposition of Pt on chemically prepared poly(3,4-ethylenedioxythiophene)/poly(styrene-4-sulfonate) have been studied as thin films in a Nafion matrix on carbon electrodes. Cyclic voltammetry has revealed that the Pt utilization is lower than for a commercial carbon supported catalyst, and this has been attributed to electronic isolation and poisoning of some of the Pt. Rotating disc voltammetry of oxygen reduction is essentially the same (similar n values, Tafel slopes, and real exchange current densities) at electrodes coated with polymer and carbon supported catalysts, with the current at any potential being approximately proportional to the active Pt area determined by cyclic voltammetry.

Prussian blue analogues (PBAs) with inherent ordered structures and abundant metal ion sites are widely explored as precursors for various electrochemical applications, including oxygen evolution reaction (OER). Using a range of characterization techniques including Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), this work discloses the process of replacement of K+ by NH4+ in the interstitial spaces of the CoFe PBA by a hot aqueous urea solution, which influences the transformation of PBAs under further heat treatment and the OER performance of the derivatives. After heat treatment at 400 °C under Ar flow, high-resolution transmission electron microscopy (HRTEM) images reveal that CoFe alloy nanoparticles grew on the crystalline cubes of CoFe PBA with K+, while CoFe PBA cubes with NH4+ become amorphous. Besides, the derivative of CoFe PBA with NH4+ (Ar-U-CoFe PBA) performs better than the derivative of CoFe PBA with K+ (Ar-CoFe PBA) in OER, registering a lower overpotential of 305 mV at 10 mA cm−2, a smaller Tafel slope of 36.1 mV dec−1, and better stability over a testing course of 20 h in 1.0 M KOH. A single-cell alkaline electrolyzer, using Ar-U-CoFe PBA and Pt/C for the anodic and cathodic catalyst, respectively, requires an initial cell voltage of 1.66 V to achieve 100 mA cm−2 at 80 °C, with negligible degradation after 100 h.

We report calculated oxygen reduction reaction energy pathways on multi-metal-atom structures that have previously been shown to be thermodynamically favorable. We predict that such sites have the ability to spontaneously cleave the O2 bond and then will proceed to over-bind reaction intermediates. In particular, the *OH bound state has lower energy than the final 2 H2O state at positive potentials. Contrary to traditional surface catalysts, this *OH binding does not poison the multi-metal-atom site but acts as a modifying ligand that will spontaneously form in aqueous environments leading to new active sites that have higher catalytic activities. These *OH bound structures have the highest calculated activity to date.

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of paramount importance towards to regenerative fuel cells and rechargeable metal-air batteries. Many efforts have been devoted to developing high active ORR and OER bifunctional electrocatalysts to improve the efficiency of devices. However, the possibility of using free solar energy in ORR/OER electrocatalysts to get higher efficiency in the devices has not yet been recognized. Herein, we report a photo-responsive bifunctional ORR/OER electrocatalyst with a built-in p-n heterojunction based on Ni 12 P 5 nanoparticles (nps) coupled strongly with nitrogen-doped carbon nanotubes (NCNT). The Ni 12 P 5 @NCNT hybrid catalyst exhibited unusually high catalytic activities for both ORR (onset potential at 0.90 V vs. RHE) and OER (K = 360 mv @10 ma cm -2 ) in 0.1 M KOH solution with a ΔE (e oer j=10 -E ORR 1/2 ) = 0.82 V, comparable to that of its state-ofthe-art counterparts (e.g., Pt for ORR, iro 2 for OER). Upon light irradiation (300 W Xenon lamp equipped with an AM 1.5G filter, 5 cm to electrode), the ΔE further decreased to 0.80V. A rechargeable Zn-air battery devised from a Ni 12 P 5 @NCNT cathode in 6 M KOH/0.2 M Zn(Ac) 2 exhibited a low charge-discharge voltage gap (~0.75 V @ 10 ma cm -2 , charge potential = 1.94 V and discharge potential = 1.19 V) and a remarkable cycling stability over 500 cycles, which, upon the light irradiation, showed an even lower charge potential of 1.90 V and higher discharge potential of 1.22 V (i.e., a charge-discharge voltage gap: 0.68 V @ 10 ma cm -2 ). The irradiation caused also a concomitant increase in the round-trip efficiency from ~61.3% to ~64.2%. To demonstrate the potential applications in portable/wearable electronic devices, we have also developed all solid-state flexible photo-assisted rechargeable Zn-air batteries based on the Ni 12 P 5 @NCNT air electrode with high-performance in the both presence and absence of light irradiation. This conceptual demonstration of the first photo-responsive ORR/OER bifunctional catalyst is significant as it reveals a novel catalytic mechanism, from which a class of new bifunctional catalysts with further enhanced photo-responsive performance could be developed, using in other rechargeable metal-air batteries.

Mass transport properties of a pair of non-Platinum Group Metal (non-PGM) catalysts in proton exchange membrane fuel cells (PEMFCs) were evaluated through methods developed by Reshetenko et al., demonstrating that the use of different carrier gases can allow for the determination of the mass transport coefficient for oxygen in the gas phase and the electrolyte phase. The gas-phase and non-gas-phase resistances can be elucidated from the slope and intercept, respectively, of the total mass transport coefficient plotted as a function of molecular weight. It was determined through these experiments that the primary sources of mass transfer limitations of the non-PGMs when compared to the PGMs were the catalyst layer (non-gas-phase), rather than the flow fields (gas-phase, primarily Knudsen Diffusion effects), and the gas diffusion layer. This work was combined with a pseudo-2D, isothermal, steady state numerical model to estimate the gas-phase mass transfer coefficient and the fraction of hydrophobic, gas-phase pores in the catalyst layer. Sensitivity studies were also carried out, allowing for more information regarding the influence of several inherent factors on the mass transport limitations, and allow for additional validation of the model beyond simply the quality of the fit.

Metal-supported graphene nanocomposites with single atoms or small clusters are of interest for various catalytic processes, including applications in batteries, fuel cells, water electrolysis, and chemical synthesis. Typically, graphene oxide is reduced in the presence of metal salts to produce metal-graphene nanocomposites. However, graphene itself has reductive properties and can react with metal ions in higher oxidation states in a suitable solvent. While direct reactions (dip and coat or wet coating) with metal salts have been described several times, less is known about the kinetics. This study investigates the reaction of suspended graphene nanoplatelets (GNP) in aerated water with the chlorocomplexes of gold (III), iridium (IV), platinum (IV), and palladium (II) to form nanocomposites covered with single atoms and small clusters. The maximum metal loading ranges from 3.3 mass% for palladium to 44 mass% for gold, increasing with redox potential. At high redox potentials, such as those of Ir (IV) and Au (III), the reactions follow pseudo-first order kinetics. In contrast, at lower potentials, such as those of Pt (IV) and Pd (II), the reaction adhere to pseudo-second order. This data enable kinetically controlled metal coating of the GNP. In contrast to the use of a reducing agent, gold, platinum, and palladium are present on the GNP in different oxidation states, which can be specifically modified, as shown for platinum-coated GNP. Iridium (IV) has been deposited as anhydrous and hydrated iridium (IV) oxide. The nanocomposites have great potential as single-atom catalysts. The described process can be transferred to other transition metals and is sustainable because the reaction media can be recycled.

Fuel cells are foreseen to have an important role in sustainable energy systems, provided that catalysts with higher activity and stability are developed. In this study, highly active sputtered thin films of platinum alloyed with yttrium (Pt3Y) are deposited on commercial gas diffusion layers and their performance in a proton exchange membrane fuel cell is measured. After acid pretreatment, the alloy is found to have up to 2.5 times higher specific activity than pure platinum. The performance of Pt3Y is much higher than that of pure Pt, even if all of the alloying element was leached out from parts of the thin metal film on the porous support. This indicates that an even higher performance is expected if the structure of the Pt3Y catalyst or the support could be further improved. The results show that platinum alloyed with rare earth metals can be used as highly active cathode catalyst materials, and significantly reduce the amount of platinum needed, in real fuel cells.

A selective review on the materials and construction principles used for bifunctional oxygen/air electrodes is given. The discussion emphasizes the catalytically active materials used for the construction of these electrodes, which are a key component in electrically rechargeable air breathing electrochemical systems. Whereas, in acid electrolytes normally noble metal catalysts must be used, there is a possibility to use less expensive transition metal oxides in alkaline electrolytes. Typical transition metal oxides have the perovskite, pyrochlore and spinel structure.

The use of electrochemical noise (EN) measurements for the investigation and monitoring of corrosion has allowed many interesting advances in the corrosion science in recent years. A special advantage of EN measurements includes the possibility to detect and study the early stages of localized corrosion. Nevertheless, the understanding of the electrochemical information included in the EN signal is actually very limited. The role of the cathodic process on the EN signals remains uncertain and has not been sufficiently investigated to date. Thus, an accurate understanding of the influence of the cathodic process on the EN signal is still lacking. On the basis of different kinetics of the oxygen reduction it was established that the anodic amplitude of transients arising from pitting corrosion on stainless steel can be decreased by the corresponding electron consumption of the cathodic process. Therefore, the stronger the electron consumption, the weaker the anodic amplitude of the EN signal becomes. EN signals arising from pitting corrosion on stainless steel can be measured because the cathodic process is inhibited by the passive layer. This was confirmed by means of EN measurements under cathodic polarisation. Since the cathodic process plays a decisive role on the form of transients arising from pitting corrosion, its influence must be considered in the evaluation and interpretation of the EN signals.

Oxygen reduction occurring at the passive layer is probably the most important cathodic reaction involved in corrosion processes on stainless steel. Furthermore, the influence of the surface state on the oxygen reduction reaction is a key point for the understanding of the mechanism of localized corrosion on stainless steel. In this study, electrochemical noise measurements under cathodic polarization were carried out to obtain new information about this influence. It has been confirmed that the surface state of stainless steel plays a very important role in the kinetic of this cathodic reaction. Oxygen reduction kinetics was significantly reduced on passivated surfaces and improved on pre-reduced and ground surfaces. In addition, electrochemical current noise measurements allowed to differentiate between the electrochemical activity produced by the oxygen reduction reaction and that due to the reduction of the passive layer, in direct dependence on the characteristics of the different surface states investigated.

The development of the superior Pt-free electrocatalyst for enhancing electrocatalytic activity and stability towards the EOR (ethanol oxidation reaction) and ORR (oxygen reduction reaction) are very important for the commercialization of fuel cells. In this work, the palladium nanoparticles (Pd NPs) deposited on titanium dioxide nanospheres (PdeTiO 2 ) were successfully prepared by the sonochemical reduction method. Hence, the TiO 2 nanospheres were directly used as the templates for nucleation and growth of Pd NPs over their surface using PdCl 4 À as the Pd precursor, PVP (Polyvinylpyrrolidone) as the stabilizer, ascorbic acid (AA) as the reducing agent, and water as the solvent. The size of Pd NPs is remarkably controlled using the sequential reduction steps by adding an excess of PdCl 4 . The excellent electrocatalytic activity towards EOR and ORR is ascribed by the synergistic effect of the Pd nanoparticles and TiO 2 nanospheres support. This unique nanocomposite should be of great benefit to the construction of commercial fuel cell systems.

 Synthesis of a new class of Au nanocrystals enclosed with high index surface supported on MoO3 nanorods by ultrasonic probe method  The role of supporting materials reduces the loading of Au and acts as a co-catalyst  The as prepared electrocatalyst exhibits enhanced catalytic activity and stability towards both EOR and ORR

Non-Precious Metal Catalysts (NPMC) are promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR), the cathode reaction in fuel cells. In this paper, we focus on an iron-nitrogen-carbon (Fe/N/C) catalyst in comparison to platinum and investigate how these different types of catalyst behave towards selective anion poisoning. The catalysts are studied with respect to their ORR activity using the rotating disk electrode (RDE) technique in aqueous HClO 4 , H 2 SO 4 , H 3 PO 4 and HCl electrolytes and the results are supported by density functional theory (DFT) calculations. We find that the ORR on the Fe/N/C catalyst is less affected by anion poisoning than platinum. Surprisingly, it is seen that phosphoric acid not only does not poison the Fe/N/C catalyst, but instead promotes the ORR; a finding in sharp contrast to the poisoning effect observed on platinum. This is a highly important finding as modern high-temperature proton exchange fuel

Non-Precious Metal Catalysts (NPMC) are promising alternatives to platinum-based catalysts for the oxygen reduction reaction (ORR), the cathode reaction in fuel cells. In this paper, we focus on an iron-nitrogen-carbon (Fe/N/C) catalyst in comparison to platinum and investigate how these different types of catalyst behave towards selective anion poisoning. The catalysts are studied with respect to their ORR activity using the rotating disk electrode (RDE) technique in aqueous HClO 4 , H 2 SO 4 , H 3 PO 4 and HCl electrolytes and the results are supported by density functional theory (DFT) calculations. We find that the ORR on the Fe/N/C catalyst is less affected by anion poisoning than platinum. Surprisingly, it is seen that phosphoric acid not only does not poison the Fe/N/C catalyst, but instead promotes the ORR; a finding in sharp contrast to the poisoning effect observed on platinum. This is a highly important finding as modern high-temperature proton exchange fuel

This study examines the oxygen reduction reaction (ORR) in a homogeneous catalyst system, comparing between the outer-sphere and inner-sphere electron-transfer mechanisms. The rate constants are measured using aqueous trifluoromethane sulfonic acid (TFMSA) and water-soluble M * meso-tetra (pyridyl) porphine chloride complexes [M * TMPyP, M * = Fe(III), Co(III), Mn(III) and Cu(II)] at given pH and molar ratio of metal complexes to oxygen. An outer-sphere model consistent with Marcus theory explains that an outer-sphere electron transfer mechanism occurs in the activation-control region. However, higher rate constants than predicted suggests that a possible reaction pathway is a quasi-redox mechanism associated with the formation of an intermediate bond between M ' TMPyP [M ' = Fe(II), Co(II), Mn(II) and Cu(I)] with O 2 followed by proton-activated decomposition. An increase in the catalyst turnover frequency was also observed upon addition of imidazole base, indicating the role of protonation is crucial to the ORR mechanism. The results are encouraging for replacement of platinum with non-noble metal-polymer complex systems for oxygen reduction in that the reorganization barrier for reaction pathway significantly decreases. The positive effect of proton activation on the catalytic activity of the homogeneous redox catalysts is of considerable interest for future studies. In a three-dimensional, molecular catalysis model, the predicted results using the measured reaction rate suggest that the non-noble metal catalysts can be used for practical electrochemical cell designs.

Recently we reported the preparation and electrochemical behaviour of porous electrodes based on the controlled combination of carbon nanotubes and capped platinum nanoparticles towards oxygen reduction. Due to the organic crown of the nanoparticles, the electrodes exhibited low hydrogen underpotential deposition (H upd) electroactive surface areas but significant activity towards oxygen reduction was recorded down to very low platinum loadings of few g/cm 2 . While the presence of organic stabilizing material, at the surface of the electrocatalyst synthesized by wet chemistry, may be considered as a potential drawback in fuel cell community, we present in this paper results showing that our capped electrocatalyst associated with carbon nanotubes can be used without any pre-treatment and exhibit high performances in fuel cell devices, in spite of low platinum loadings. Beyond the practical interest of such capped nanoparticles in fuel cell technology demonstrated here, fundamental question related to the high performances of the capped electrocatalyst are still opened and are currently under investigation.

Hydrogen peroxide is a promising substitute for fossil fuels because it produces non-hazardous by-products. In this work, a glassy carbon GC was anodized in sulphuric acid at +1.8 V to prepare the working electrode. It was utilized to investigate the oxygen reduction reaction (ORR) in a basic medium containing 0.1 M NaOH as a supporting electrolyte. The objective of this investigation was to synthesize hydrogen peroxide. X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), linear polarization, cyclic voltammetry (CV), and rotating disk electrode voltammetry (RDE) were performed for characterization and investigation of the catalytic properties. The RDE analysis confirmed that oxygen reduction reactions followed two electrons’ process at an activated GC electrode. Hence, the prepared electrode generated hydrogen peroxide from molecular oxygen at a potential of around −0.35 V vs. Ag/AgCl (sat. KCl), significantly lower than the pristine GC surface. The transfer coefficient, standard reduction potential, and standard rate constant were estimated to be 0.75, −0.27 V, and 9.5 × 10−3 cm s−1, respectively.

Corrosion in concrete prevents in-situ observation, necessitating models to provide insight into the local reaction currents. We present a computational method for predicting corrosion rates of reinforcements within concrete under natural conditions, i.e. requiring the corrosion current to be supported by equal cathodic currents. In contrast to typical corrosion models, where these two counteracting currents are required to be co-located, we allow these currents to be separated such that pitting corrosion can be supported by cathodic reactions over a much larger area. Pitting corrosion is investigated, elucidating the effects of the concrete porosity and water saturation, the presence of dissolved oxygen, and chlorine concentration within the pore solution. The presented model is capable of capturing the dynamic growth of acidic regions around corrosion pits, showing the limited region over which the hydrogen evolution reaction occurs and how this region evolves over time. The ability of oxygen to diffuse towards the metal surface due to increased porosity is seen to have a major effect on the corrosion rate, whereas changes in the chlorine concentration (and thus changes in the conductivity of the pore solution) play a secondary role. Furthermore, external oxygen is seen to enhance corrosion but is not required to initialise and sustain acidic corrosion pits.