The microporous structure of silver/silver chloride electrodes and the implications for Harned cell operation

The stability of silver/silver chloride reference electrodes when moved between different electrolyte solutions is limited by the small amount of solution transferred. The time required for the Ag/AgCl electrode potential to stabilize has been shown to increase with electrode size in an empirical fashion. The results presented suggest the presence of a microporous structure in the electrodes that limits the rate at which traces of any previous solutions are diluted by any new solution environment. Operational implications for the stability and accuracy of the Harned Cell used as the primary standard for pH measurements and the certification of primary reference buffers are discussed. Presented to the CCQM Electrochemical Analysis Working Group, Paris, April 2004.     

Behaviour of glass and silver/silver chloride electrodes in some non-aqueous media

The stability of glass electrodes and silver/silver chloride electrodes in isopropanol, methyl ethyl ketone and a mixture of equal volumes of these has been measured, and found to be of the order of 1 mV. Changes in the electrode potential due to addition of water and to addition of supporting electrolyte have been investigated. The glass electrode responded reversibly to hydrogen ion activity changes in buffers of picric acid-tetraethylammonium picrate, and perchloric acid-di-isopropylamine. The autoprotolysis constants at 25 degrees were calculated to be 2 x 10(-19) in isopropanol, 2 x 10(-26) in the ketone and 8 x 10(-19) in the mixture.     

CNx-modified Fe3O4 as Pt nanoparticle support for the oxygen reduction reaction

A novel electrocatalyst support material, nitrogen-doped carbon (CN_x)-modified Fe₃O₄ (Fe₃O₄-CN_x), was synthesized through carbonizing a polypyrrole-Fe₃O₄ hybridized precursor. Subsequently, Fe₃O₄-CN_x-supported Pt (Pt/Fe₃O₄-CN_x) nanocomposites were prepared by reducing Pt precursor in ethylene glycol solution and evaluated for the oxygen reduction reaction (ORR). The Pt/Fe₃O₄-CN_x catalysts were characterized by X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The electrocatalytic activity and stability of the as-prepared electrocatalysts toward ORR were studied by cyclic voltammetry and steady-state polarization measurements. The results showed that Pt/Fe₃O₄-CN_x catalysts exhibited superior catalytic performance for ORR to the conventional Pt/C and Pt/C-CN_x catalysts.     

Pt nanoparticle netlike-assembly as highly durable and highly active electrocatalyst for oxygen reduction reaction

A Pt nanoparticle netlike-assembly (Pt-NNA) synthesized through a facile hydrothermal method, with high specific surface area and large overall size, exhibits much higher durability and 2.9 times higher mass activity for oxygen reduction reaction than commercial Pt black catalyst.     

The role of anions in oxygen reduction in neutral and basic media on gold single-crystal electrodes

Oxygen reduction on well-defined single-crystal electrodes in different basic and neutral media has been studied using the impinging jet system. The results obtained with this system in 0.1 M NaOH are comparable to those reported in the literature for rotating disk electrodes, indicating that the impinging jet system behaves as a system in which the thickness of the diffusion layer is stationary. The activity of the Au(100) electrode is considerably higher than the rest of the basal planes in all media and yields water when E>–0.2 V and hydrogen peroxide for E<–0.2 V on the SHE scale. For Au(111) and Au(110) the activity of the electrode is much smaller and the final product is always hydrogen peroxide. The transition between both mechanisms for the Au(100) is, essentially, independent of the solution pH. In acid media the final product is always hydrogen peroxide for all the electrodes studied. The differences between the activities of the electrodes have been explained in the light of the different adsorption properties of the surfaces in relation to HO₂ ^–. In the case of the Au(100) electrode, the existence of a negative charge density on the metal inhibits further reduction of HO₂ ^–.Dedicated to Prof. Dr. Wolf Vielstich on the occasion of his 80th birthday in recognition of his numerous contributions to interfacial electrochemistry     

Superoxide anion is the intermediate in the oxygen reduction reaction on platinum electrodes

We identified the superoxide anion as the intermediate in the oxygen (O2) reduction reaction on a platinum (Pt) electrode in alkaline solution (pH = 11) using a surface-enhanced infrared spectroscopy technique with an attenuated total reflection mode. Spectral and voltammetry data, together with the vibrational frequencies calculated using the density functional theory, provide evidence for the formation of O2-. The supporting evidence includes similar spectra that we obtained for O2 reduction on Pt in acetonitrile solutions and a lack of spectra in the absence of O2 or its reduction. The appearance of O2- means that the series reaction pathway operates during O2 reduction on Pt electrodes in alkaline solutions and very likely also in acid solutions. This finding opens up the possibility of formulating a detailed reaction mechanism on surfaces supporting a four-electron reduction, which is critical in completely understanding the kinetics of O2 reduction, thus resolving dilemmas in the theoretical treatment of its kinetics and the design of new electrocatalysts.     

Nitrogen-doped graphene wrapped around silver nanowires for enhanced catalysis in oxygen reduction reaction

Journal of Solid State Electrochemistry - Oxygen reduction reaction is the key process that determines the performances of fuel cells and metal-air batteries. Replacing highly expensive noble metal...     

Electrochemical behaviour of oxygen reduction on polymer carbon electrodes in alkaline media

The preparation of polymer carbon electrocatalysts by the controlled pyrolysis of polyfurfuryl alcohol polymer is described. The potentiostatic method was used to study the electrochemical behaviour of the oxygen reduction reaction on the prepared catalyst electrodes in potassium hydroxide electrolyte. A pure polymer carbon electrode and a cobalt chloride doped polymer carbon electrode were shown to be active in oxygen reduction, but the electrode containing cobalt chloride seemed the most active. The main reaction product at the pure polymer electrode is hydrogen peroxide, involving two electrons, whereas at a poly(CoCl₂) electrode the reduction process reaches partly its ultimate state, and involves at most three electrons.     

Rapid precipitation-reduction synthesis of carbon-supported silver for efficient oxygen reduction reaction in alkaline solution

Journal of Solid State Electrochemistry - Silver has emerged as a promising electrocatalyst for oxygen reduction reaction (ORR) in alkaline membrane fuel cell for its high stability in alkaline...     

Behaviour of silver—silver malonate and silver—silver succinate electrodes in aqueous media

The silver—silver oxalate electrode has been employed by many workers^1–3 in aqueous media as the second order reference electrode, but no work seems to have been done so far on the study of the behaviour of silver—silver malonate and silver—silver succinate electrodes. The present work deals with the study of these electrodes in ionic equilibria of malonate and succinate ions in aqueous media. These electrodes, in conjunction with a saturated calomel electrode, have been employed in the poten- tiometric determination of malonate and succinate ions in aqueous media. In additon, the effect of the added salts, such as, potassium nitrate and sucrose on the behaviour of these electrodes has also been examined in this media.     

Barium carbonate nanoparticle as high temperature oxygen reduction catalyst for solid oxide fuel cell

BaCO₃ nanoparticles are demonstrated as outstanding electrocatalysts to enhance the high temperature oxygen reduction reaction (ORR) in solid oxide fuel cells (SOFCs). BaCO₃ nanoparticles are formed from thermal decomposition of barium acetate, Ba(Ac)₂ infiltrated to porous cathode skeleton and shows good chemical compatibility with cathode materials. BaCO₃ nanoparticles can greatly reduce the area specific resistance (ASR) of typical SOFC cathode materials, including La_0.8Sr_0.2FeO_3 -δ (LSF), La_0.6Sr_0.4Co_0.2Fe_0.8O_3 -δ (LSCF) and La_0.8Sr_0.2MnO_3 -δ (LSM). For example at 700 °C, ASR for LSF on yttria-stabilized zirconia (YSZ) electrolyte decreases from 2.95 Ω cm² to 0.77 Ω cm² when 12.9 wt.% BaCO₃ nanoparticles are deposited on the surface of the porous LSF electrode. Impedance spectra analysis shows that the decrease in ASR mainly comes from the reduction of the low frequency resistance. Furthermore, BaCO₃ nanoparticles are found to greatly enhance the oxygen chemical exchange coefficient. Most importantly, it has been found that the catalytic activity of BaCO₃ nanoparticles is even higher than those of the precious metals such as Pd, Rh, Pt and Ag, infiltrated into LSF, LSCF and LSM electrodes supported on YSZ electrolytes.     

Effect of high rates of mass transport on oxygen reduction at copper electrodes: Implications for aluminium corrosion

The reduction of oxygen at copper microelectrodes (25 μm diameter) in aqueous solution (pH 7) has been studied under conditions of high mass transport, similar to those experienced by μm sized copper-containing inclusions in aluminium alloys during corrosion. Contrary to previous studies at copper electrodes operating at lower mass transport rates, oxygen reduction limiting currents show an apparent number of electrons which decrease towards 2 as the mass transport rate increases (mass transfer coefficients up to ca. 0.55 cm s⁻¹), rather than a 4-electron process. These new data suggest that the treatment of oxygen reduction as a 4-electron transfer process at micron and smaller-sized copper intermetallics in aluminium alloys may require revision.     

Effect of graphene surface functionalization on the oxygen reduction reaction in alkaline media

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Investigations of the catalytic properties of manganese oxides for the oxygen reduction reaction in alkaline media

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on different manganese oxides, dispersed on a carbon powder (MnO_x/C). The oxides were prepared by different methods, for producing MnO, Mn₃O₄ and MnO₂ as major phases dispersed on the Vulcan XC-72 carbon. The oxides were characterized by XRD (X-ray diffraction) and in situ XANES (X-ray absorption near edge structure). The electrochemical measurements were made using cyclic voltammetry and steady state polarization curves carried out in an ultra-thin layer rotating ring/disk electrode. The results have shown lower activity for the ORR on the MnO_x/C species compared to that on Pt/C, but higher activity compared to that of pure Vulcan carbon. Formation of involving 2e⁻ per O₂ molecule is the main path of the ORR in the studied MnO_x/C catalysts but, at low overpotentials and rotation rates the number of electrons is raised to 4 due to the occurrence of a disproportionation reaction. Large differences of electrocatalytic activity were seen for the different oxide species, and these were related to the presence of a Mn(IV) phase and the occurrence of a mediation processes involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen.     

Performance of silver nanocubes based on electrochemical surface area for catalyzing oxygen reduction reaction

Ag nanocubes that are 45 nm in size are synthesized and successfully used as catalysts in oxygen electroreduction. Electrochemical surface areas (ESAs) are considered to determine the effect on HO₂⁻ production, which is found to be in the following order: nanocubes < nanoparticles. Comparative data generated using Tafel analyses in 0.1 M NaOH electrolyte without and with methanol show that unchanged slopes on the prepared cubic catalysts can indicate high resistance of Ag nanocubes for methanol oxidation during oxygen reduction reaction. Among these Ag catalysts, nanocubes exhibit 9.29 × 10^− 2 mA cm^− 2 (at − 0.15 V vs. Ag/AgCl), the better activity in the oxygen reduction reaction.     

An overview of the electrochemical reduction of oxygen at carbon-based modified electrodes

We present an overview of the electrochemical reduction of oxygen in water, focussing on carbon-based and modified carbon electrodes. This process is of importance for gas sensing, in fuel cells and in the electrosynthesis of hydrogen peroxide.     

Iron phthalocyanine coated on single-walled carbon nanotubes composite for the oxygen reduction reaction in alkaline media

Iron (II) phthalocyanine coated on single-walled carbon nanotubes was synthesized as a non-noble electrocatalyst for the oxygen reduction reaction (ORR). The composite exhibited higher activity than the commercial Pt/C catalyst, and excellent anti-crossover effect for methanol oxidation in the ORR.     

The in situ investigation of the polyaniline-derived N-doped carbon with the interdigitated array electrodes towards the oxygen reduction reaction

Journal of Solid State Electrochemistry - Nowadays, tremendous effort has been made in searching clean, high-efficient and sustainable energy, and related energy conversion methods. Oxygen...