Straightforward Synthesis of Carbon-Supported Ag Nanoparticles and Their Application for the Oxygen Reduction Reaction

In this paper we report a simple and environmentally friendly synthesis of silver nanoparticles (AgNps) and their activities towards the oxygen reduction reaction (ORR). Ultraviolet spectroscopy (UV-vis) and transmission electron microscopy confirmed the formation of poly(vinyl pyrrolidone)-protected colloidal AgNps through direct reduction of Ag+ by glycerol in alkaline medium at room temperature. For the ORR tests, the AgNps were directly produced onto carbon to yield the Ag/C catalyst. Levich plots revealed the process to occur via 2.7 electrons, suggesting that the carbon support contributes to the ORR. We discuss here possibilities of improving the catalytic properties of the Ag/C for ORR by optimizing the parameters of the synthesis.

Electrodeposition of PVA-protected PtCo electrocatalysts for the oxygen reduction reaction in H2SO4

In this paper we report the electrosynthesis of PVA-protected PtCo films (PVA = poly(vinylalcohol)) and their activities towards the oxygen reduction reaction (ORR). PtCo electrodeposits were potentiostatically obtained in the presence and absence of PVA at distinct potentials. The film morphology and composition were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), which revealed that the use of PVA in the electrodeposition of PtCo films was decisive to achieve better film composition control. Cyclic voltammetry for PVA-protected PtCo films showed that the electrochemical surface area is dependent on the electrodeposition potentials and suggested different adsorption strengths of oxygen-containing species. Films produced in the presence of PVA presented the following activity order towards ORR as a function of the electrodeposition potential (vs. Ag/AgCl): -0.9 V> -0.8 V> -1.0 V> -0.7 V. In contrast, PtCo films electrodeposited in the absence of PVA displayed very similar activities regardless of the electrodeposition potential. The simplicity of the electrodeposition method combined with its effectiveness enabled the production of "model electrodes" for investigating the fundamental aspects of the reactions taking place in the fuel cell cathodes. (C) 2011 Elsevier B.V. All rights reserved.

Oxygen reduction reaction on a Pt/carbon fuel cell catalyst in the presence of trace quantities of ammonium ions: An RRDE study

The effect of trace quantities of ammonia on oxygen reduction reaction (ORR) on carbon-supported platinum catalysts in perchloric acid solutions is assessed using rotating ring disk electrode (RRDE) technique. The study demonstrates that ammonia has detrimental effects on ORR. The most significant effect takes place in the potential region above 0.7 V vs RHE. The effect is explained by the electrochemical oxidation of ammonia, which blocks Pt active sites and increases the formation of H2O2. This leads to losses in the disk currents and increments in the ring currents. The apparent losses in ORR currents may occur in two ways, namely, through the blocking of the active sites for ORR as well as by generating a small anodic current, which is believed to have a lower contribution. In addition, a detrimental effect of sodium cations in the potential range below 0.75 V vs RHE was demonstrated. This effect is most likely due to the co-adsorption of sodium cations and perchlorate anions on the Pt surface. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Probing the Electrocatalytic Oxygen Reduction Reaction Reactivity of Immobilized Multicopper Oxidase CueO

The bioelectrocatalytic (oxygen reduction reaction, ORR) properties of the multicopper oxidase CueO immobilized on gold electrodes were investigated. Macroscopic electrochemical techniques were combined with in situ scanning tunneling microscopy (STM) and surface-enhanced Raman spectroscopy at the ensemble and at the single-molecule level. Self-assembled monolayer of mercaptopropionic acid, cysteamine, and p-aminothiophenol were chosen as redox mediators. The highest ORR activity was observed for the protein attached to amino-terminated adlayers. In situ STM experiments revealed that the presence of oxygen causes distinct structure and electronic changes in the metallic centers of the enzyme, which determine the rate of intramolecular electron transfer and, consequently, affect the rate of electron tunneling through the protein. Complementary Raman spectroscopy experiments provided access for monitoring structural changes in the redox state of the type 1 copper center of the immobilized enzyme during the CueO-catalyzed oxygen reduction cycle. These results unequivocally demonstrate the existence of a direct electronic communication between the electrode substrate and the type 1 copper center.

Oxygen Reduction Reaction on Pt Overlayers Deposited onto a Gold Film: Ligand, Strain, and Ensemble Effect

We study the oxygen reduction reaction (ORR), the catalytic process occurring at the cathode in fuel cells, on Pt layers prepared by electrodeposition onto an Au substrate. Using a nominal Pt layer by layer deposition method previously proposed, imperfect layers of Pt on Au are obtained. The ORR on deposited Pt layers decreases with increasing Pt thickness. In the submonolayer region, however, the ORR activity is superior to that of bulk Pt. Using density functional theory (DFT) calculations, we correlate the observed activity trend to strain, ligand, and ensemble effects. At submonolayer coverage certain atom configurations weaken the binding energies of reaction intermediates due to a ligand and ensemble effect, thus effectively increasing the ORR activity. At higher Pt coverage the activity is governed by a strain effect, which lowers the activity by decreasing the oxidation potential of water. This study is a nice example of how the influence of strain, ligand, and ensemble effects on the ORR can be deconvoluted.

Probing the Electrocatalytic Oxygen Reduction Reaction Reactivity of Immobilized Multicopper Oxidase CueO

The bioelectrocatalytic (oxygen reduction reaction, ORR) properties of the multicopper oxidase CueO immobilized on gold electrodes were investigated. Macroscopic electrochemical techniques were combined with in situ scanning tunneling microscopy (STM) and surface-enhanced Raman spectroscopy at the ensemble and at the single-molecule level. Self-assembled monolayer of mercaptopropionic acid, cysteamine, and p-aminothiophenol were chosen as redox mediators. The highest ORR activity was observed for the protein attached to amino-terminated adlayers. In situ STM experiments revealed that the presence of oxygen causes distinct structure and electronic changes in the metallic centers of the enzyme, which determine the rate of intramolecular electron transfer and, consequently, affect the rate of electron tunneling through the protein. Complementary Raman spectroscopy experiments provided access for monitoring structural changes in the redox state of the type 1 copper center of the immobilized enzyme during the CueO-catalyzed oxygen reduction cycle. These results unequivocally demonstrate the existence of a direct electronic communication between the electrode substrate and the type 1 copper center.

Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the oxygen reduction reaction

Functionalized carbon nanotubes (CNTs) using three aminobenzene acids with different functional groups (carboxylic, sulphonic, phosphonic) in para position have been synthesized through potentiodynamic treatment in acid media under oxidative conditions. A noticeable increase in the capacitance for the functionalized carbon nanotubes mainly due to redox processes points out the formation of an electroactive polymer thin film on the CNTs surface along with covalently bonded functionalities. The CNTs functionalized using aminobenzoic acid rendered the highest capacitance values and surface nitrogen content, while the presence of sulfur and/or phosphorus groups in the aminobenzene structure yielded a lower functionalization degree. The oxygen reduction reaction (ORR) activity of the functionalized samples was similar to that of the parent CNTs, independently of the functional group present in the aminobenzene acid. Interestingly, a heat treatment in N2 atmosphere with a very low O2 concentration (3125 ppm) at 800 °C of the CNTs functionalized with aminobenzoic acid produced a material with high amounts of surface oxygen and nitrogen groups (12 and 4% at., respectively), that seem to modulate the electron-donor properties of the resulting material. The onset potential and limiting current for ORR was enhanced for this material. These are promising results that validates the use of electrochemistry for the synthesis of novel N-doped electrocatalysts for ORR in combination with adequate heat treatments.

Bio-inspired carbon electro-catalysis for the oxygen reduction reaction

We report the synthesis, characterisation and catalytic performance of two nature-inspired biomass-derived electro-catalysts for the oxygen reduction reaction in fuel cells. The catalysts were prepared via pyrolysis of a real food waste (lobster shells) or by mimicking the composition of lobster shells using chitin and CaCO3 particles followed by acid washing. The simplified model of artificial lobster was prepared for better reproducibility. The calcium carbonate in both samples acts as a pore agent, creating increased surface area and pore volume, though considerably higher in artificial lobster samples due to the better homogeneity of the components. Various characterisation techniques revealed the presence of a considerable amount of hydroxyapatite left in the real lobster samples after acid washing and a low content of carbon (23%), nitrogen and sulphur (<1%), limiting the surface area to 23 m2/g, and consequently resulting in rather poor catalytic activity. However, artificial lobster samples, with a surface area of ≈200 m2/g and a nitrogen doping of 2%, showed a promising onset potential, very similar to a commercially available platinum catalyst, with better methanol tolerance, though with lower stability in long time testing over 10,000 s.

The electrochemical reduction reaction of dissolved oxygen on Q235 carbon steel in alkaline solution containing chloride ions

Cyclic voltammetry, electrochemical impedance spectroscopy, and rotating disk electrode voltammetry have been used to study the effect of chloride ions on the dissolved oxygen reduction reaction (ORR) on Q235 carbon steel electrode in a 0.02 M calcium hydroxide (Ca(OH)(2)) solutions imitating the liquid phase in concrete pores. The results indicate that the cathodic process on Q235 carbon steel electrode in oxygen-saturated 0.02 M Ca(OH)(2) with different concentrations of chloride ions contain three reactions except hydrogen evolution: dissolved oxygen reduction, the reduction of Fe(III) to Fe(II), and then the reduction of Fe(II) to Fe. The peak potential of ORR shifts to the positive direction as the chloride ion concentration increases. The oxygen molecule adsorption can be inhibited by the chloride ion adsorption, and the rate of ORR decreases as the concentration of chloride ions increases. The mechanism of ORR is changed from 2e(-) and 4e(-) reactions, occurring simultaneously, to quietly 4e(-) reaction with the increasing chloride ion concentration.

The morphology dependent electrocatalytic activity of Ir nanostructures towards oxygen reduction

Iridium nanostructures with different morphologies are synthesized by a simple, environmentally friendly approach in aqueous media under mild conditions. The morphology dependent electrocatalytic activity of Ir nanochains and nanoparticles towards oxygen reduction reaction (ORR) has been demonstrated in both acidic and alkaline media. Comparative electrochemical studies reveal that nanochains exhibit significantly enhanced ORR activities in both acidic and alkaline media as compared with nanoparticles, as a result of the continuous structure of interconnected particles. The mechanism of oxygen reduction on Ir nanostructures predominantly follows a four-electron pathway in alkaline and acidic solutions. Excellent stability and good selectivity towards methanol tolerance are reported.

Electronic structure origin of conductivity and oxygen reduction activity changes in low-level Cr-substituted (La, Sr)MnO3

The electronic structure of the (La0.8Sr0.2)(0.98)Mn1-xCrxO3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La, Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L-3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t(2g) up arrow state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t(2g) up arrow state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La, Sr)MnO3 is presented. (C) 2015 AIP Publishing LLC.

Nanoporous Graphitic-C3N4@Carbon Metal-Free Electrocatalysts for Highly Efficient Oxygen Reduction

Based on theoretical prediction, a g-C3N4@carbon metal-free oxygen reduction reaction (ORR) electrocatalyst was designed and synthesized by uniform incorporation of g-C3N4 into a mesoporous carbon to enhance the electron transfer efficiency of g-C3N4. The resulting g-C3N4@carbon composite exhibited competitive catalytic activity (11.3 mA cm–2 kinetic-limiting current density at −0.6 V) and superior methanol tolerance compared to a commercial Pt/C catalyst. Furthermore, it demonstrated significantly higher catalytic efficiency (nearly 100% of four-electron ORR process selectivity) than a Pt/C catalyst. The proposed synthesis route is facile and low-cost, providing a feasible method for the development of highly efficient electrocatalysts.

Nanosheets Co3O4 interleaved with graphene for highly efficient oxygen reduction

Efficient yet inexpensive electrocatalysts for oxygen reduction reaction (ORR) are an essential component of renewable energy devices, such as fuel cells and metal-air batteries. We herein interleaved novel Co3O4 nanosheets with graphene to develop a first ever sheet-on-sheet heterostructured electrocatalyst for ORR, whose electrocatalytic activity outperformed the state-of-the-art commercial Pt/C with exceptional durability in alkaline solution. The composite demonstrates the highest activity of all the nonprecious metal electrocatalysts, such as those derived from Co3O4 nanoparticle/nitrogen-doped graphene hybrids and carbon nanotube/nanoparticle composites. Density functional theory (DFT) calculations indicated that the outstanding performance originated from the significant charge transfer from graphene to Co3O4 nanosheets promoting the electron transport through the whole structure. Theoretical calculations revealed that the enhanced stability can be ascribed to the strong interaction generated between both types of sheets.

Carbon-Supported Pt-TiO2 as a Methanol-Tolerant Oxygen-Reduction Catalyst for DMFCs

Carbon-supported Pt-TiO2 (Pt-TiO2/C) catalysts with varying at. wt ratios of Pt to Ti, namely, 1:1, 2:1, and 3:1, are prepared by the sol-gel method. The electrocatalytic activity of the catalysts toward oxygen reduction reaction (ORR), both in the presence and absence of methanol, is evaluated for application in direct methanol fuel cells (DMFCs). The optimum at. wt ratio of Pt to Ti in Pt-TiO2/C is established by fuel cell polarization, linear sweep voltammetry, and cyclic voltammetry studies. Pt-TiO2/C heattreated at 750 degrees C with Pt and Ti in an at. wt ratio of 2:1 shows enhanced methanol tolerance, while maintaining high catalytic activity toward ORR. The DMFC with a Pt-TiO2/C cathode catalyst exhibits an enhanced peak power density of 180 mW/cm(2) in contrast to the 80 mW/cm(2) achieved from the DMFC with carbon-supported Pt catalyst while operating under identical conditions. Complementary data on the influence of TiO2 on the crystallinity of Pt, surface morphology, and particle size, surface oxidation states of individual constituents, and bulk and surface compositions are also obtained by powder X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, energy dispersive analysis by X-ray, and inductively coupled plasm optical emission spectrometry.

A PEFC With Pt-TiO2/C as Oxygen-Reduction Catalyst

Carbon-supported Pt-TiO2 (Pt-TiO2/C) catalyst with varying atomic ratio of Pt to Ti, namely, 1: 1, 2: 1, and 3: 1, is prepared by sol-gel method and its electrocatalytic activity toward oxygen-reduction reaction (ORR) is evaluated for the application in polymer electrolyte fuel cells (PEFCs). The optimum atomic ratio of Pt to Ti in Pt-TiO2/C and annealing temperature are established by cyclic voltammetry and fuel-cell-polarization studies. Pt-TiO2/C annealed at 750 degrees C with Pt and Ti in atomic ratio of 2: 1, namely, 750 Pt-TiO2/C (2: 1), shows enhanced electrocatalytic activity toward ORR. It is found that the incorporation of TiO2 with Pt ameliorates both electrocatalytic activity and stability of cathode in relation to pristine Pt cathode, currently being used in PEFCs. A power density of 0.75 W/cm(2) is achieved at 0.6 V for the PEFC with 750 Pt-TiO2/C (2: 1) as compared with 0.62 W/cm(2) at 0.6 V achieved with the PEFC comprising Pt/C as cathode catalyst while operating under identical conditions. Interestingly, carbon-supported Pt-TiO2 cathode exhibits only 6% loss in electrochemical surface area after 5000 potential cycles while it is as high as 25% for Pt/C. DOI: 10.1115/1.4002466]