Oxygen In Acidic Media Research Articles

Copper-doped ruthenium oxide as highly efficient electrocatalysts for the evolution of oxygen in acidic media

The development of highly active and acid-stable catalysts for oxygen evolution reaction (OER) is necessary and challenging. In this work, we report a highly efficient Cu-doped RuO2 OER electrocatalyst (Cu-RuO2) synthesized via one-step calcination of amorphous RuCu sample in air atmosphere. Results show that the catalytic performances of Cu-RuO2 catalysts toward OER are strongly dependent on the calcination temperature. Among the series of Cu-RuO2 catalysts, Cu-RuO2 prepared at the calcination temperature of 300 °C (Cu-RuO2-300) exhibits the highest activity for OER. At the current density of 10 mA cm−2, Cu-RuO2-300 shows an extremely low OER overpotential of 201 mV, which is much lower than the benchmark commercial RuO2 (233 mV) and most noble metal/non-noble metal catalysts reported in literature. The Tafel slope is as low as 55 mV dec−1, which is also lower than that on RuO2 catalyst (76 mV dec−1), suggesting a faster OER kinetics. In addition, Cu-RuO2-300 can maintain excellent durability in H2SO4 electrolyte. During the chronopotentiometric (CP) test, the potential of Cu-RuO2-300 catalyst shows very small attenuation (65 mV) at the time of 24 h whereas that of the reference RuO2 catalyst exhibits a 127 mV of change only within 4 h of test. Large electrochemical surface area and abundant exposed high-index facets are responsible for the high catalytic performances of the Cu-RuO2-300 catalyst. This work is instructive for the design and preparation of highly efficient and stable noble metal-based OER catalysts in acidic media.

High oxygen reduction reaction activity and durability of Pt catalyst photo-deposited on SnO2-coated and uncoated multi-walled carbon nanotubes

• Acid-treated CNTs were coated with SnO 2 by chemical method. • Platinum was photo-deposited on CNTs and SnO 2 -CNT supports. • Surface morphology was investigated by SEM and HAADF-STEM. • Pt/CNT and Pt/SnO 2 -CNT showed high oxygen reduction activity in acid media. Platinum nanoparticles were photo-deposited on acid-treated multi-walled carbon nanotubes (CNTs) and SnO 2 -coated CNTs. The SnO 2 -CNT nanocomposite was synthesised by a chemical method. Scanning and scanning transmission electron microscopy results indicated that the CNTs surface is partially covered by the SnO 2 layer. The surface morphology of the Pt catalyst depends on the nature of the support material. For instance, porous Pt aggregates of various sizes are formed at specific nucleation sites on the CNT surface, while a selective deposition of Pt was observed on the metal oxide part of the SnO 2 -CNT. Cyclic voltammetry and CO stripping investigations in 0.1 M HClO 4 solution confirmed different surface electrochemistry of the Pt catalysts. It is revealed that photo-deposition induces the formation of Pt-Sn nanoalloys due to increased interaction at the oxide interfaces. Both Pt/CNT and Pt/SnO 2 -CNT catalysts showed remarkable activity towards the electroreduction of oxygen in acid media. Durability measurement showed that photo-deposited Pt nanoparticles are more resistant to degradation. The corrosion-resistant nature of the SnO 2 -CNT support further improves the catalysts stability. Pt/CNT and Pt/SnO 2 -CNT catalysts retained 86% and 88% of the initial surface area respectively, after 10,000 potential cycles between 0.6 and 1.2 V RHE .

Electrochemical Characterization of Bis‐Cobalt Hexaphyrin: A Selective Electrocatalyst for the Two‐Electron Reduction of Oxygen in Acid Media

AbstractA bis‐cobalt(II) hexaphyrin was synthesized and characterized as to its electrochemical and spectroelectrochemical properties in nonaqueous media. The examined compound is represented as HexaPyCo2 where HexaPy represents an expanded macrocycle containing six pyrroles and six meso‐2,6‐difluorophenyl substituents. Three or four reductions can be observed for the bis‐cobalt hexaphyrin depending upon the solvent. The first two reductions are reversible and well separated in PhCN (E1/2=−0.50 and −0.90 V) or CH2Cl2 (E1/2=−0.52 and −0.87 V) containing 0.1 M TBAP. This contrasts with what is observed in pyridine where the first two one‐electron additions are overlapped in potential and occur at an apparent E1/2=−0.73 V vs SCE. The third and fourth reductions of the bis‐cobalt(II) hexaphyrin are irreversible and coupled to chemical reactions in PhCN or CH2Cl2 but well‐defined one‐electron transfers are seen in pyridine where the 250 mV difference between reversible E1/2 values (−1.28 and −1.53 V) matches almost perfectly with a 240 mV separation in potentials between the third and fourth reductions of a related bis‐Cu(II) hexaphyrin under the same solution conditions. The oxygen reduction reaction (ORR) was also investigated using HexaPyCo2 as a catalyst in 1.0 M HClO4. The results of rotating‐disk and rotating‐ring‐disk voltammetry show that the bis‐cobalt hexaphyrin is a selective catalyst for the 2‐electron ORR and gives 95 % H2O2 as the product in acid media.

Low-Noble-Metal-Loading Hybrid Catalytic System for Oxygen Reduction Utilizing Reduced-Graphene-Oxide-Supported Platinum Aligned with Carbon-Nanotube-Supported Iridium

Hybrid systems composed of the reduced graphene oxide-supported platinum and multiwalled carbon nanotube-supported iridium (both noble metals utilized at low loadings on the level of 15 and ≤2 µg cm−2, respectively) were considered as catalytic materials for the reduction of oxygen in acid media (0.5-mol dm−3 H2SO4). The electrocatalytic activity toward reduction of oxygen and formation of hydrogen peroxide intermediate are tested using rotating ring–disk electrode (RRDE) voltammetric experiments. The efficiency of the proposed catalytic systems was also addressed by performing galvanodynamic measurements with gas diffusion electrode (GDE) half-cell at 80 °C. The role of carbon nanotubes is to improve charge distribution at the electrocatalytic interface and facilitate the transport of oxygen and electrolyte in the catalytic systems by lowering the extent of reduced graphene oxide restacking during solvent evaporation. The diagnostic electrochemical experiments revealed that—in iridium-containing systems—not only higher disk currents, but also somehow smaller ring currents are produced (when compared to the Ir-free reduced graphene oxide-supported platinum), clearly implying formation of lower amounts of the undesirable hydrogen peroxide intermediate. The enhancement effect originating from the addition of traces of iridium (supported onto carbon nanotubes) to platinum, utilized at low loading, may originate from high ability of iridium to induce decomposition of the undesirable hydrogen peroxide intermediate.

A comparative study of the kinetics of the oxidation of iron(II) by oxygen in acidic media - mechanistic and practical implications

The results of this comparative study have shown that in 1 M acid solutions, the differences in reactivity of iron(II) for reduction of oxygen are within about an order of magnitude of each other for the systems studied at 40 °C. This applies to both the uncatalysed and copper(II) catalysed systems. The exception is the chloride system in which the rate can be significantly increased by increasing the chloride concentration. The other main conclusion is that although catalysis by copper(II) ions can be significant there exist conditions (iron(II) and (III) concentrations) for which the effect is not that important.Rate equations for both the uncatalysed and catalysed reactions have been derived that apply equally to all the systems studied. Values for the various rate constants have been obtained by analysis of extensive kinetic data in 1 M acid solutions at 40 °C. A limited electrochemical study has been used to derive values for the equilibrium constant for the reaction involving the iron(III)/iron(II) and copper(II)/copper(I) couples. An attempt has been made to compare relevant constants with previously published data and to derive a mechanism that is consistent with the kinetic results.This study has shown that the use of methanesulfonic acid as an oxidative leaching medium does not offer any advantages and is the least reactive of the systems studied.

Seedless, one-step synthesis of porous Pt-Pd nanoflowers for electroreduction of oxygen in acidic medium

Flower-shaped Pt-Pd nanostructures supported on carbon (Pt-Pd/C) were successfully synthesized by simple wet-chemical processing by controlling the rate of reduction of a dissolved metal precursor. L-ascorbic acid (AA) was employed as a green and clean reductive agent for the coreduction of [PtCl6]2− and [PdCl4]2− in an aqueous solution. The electrocatalytic activity of nanoporous Pt-Pd/C on the oxygen reduction reaction (ORR) was investigated by rotating disk electrode voltammetry in a 0.1 M HClO4 solution. In particular, the Pt77-Pd23/C catalyst showed surpassing ORR activity and acceptable stability under acidic conditions compared to the commercial Pt electrocatalyst. The outstanding electrocatalytic activity was achieved by a combination of the high surface area of flower-like nanostructures and the bimetallic synergetic effect. The morphology, composition and structure of the Pt-Pd/C nanoflowers were confirmed by scanning transmission electron microscopy (STEM), TEM, X-ray photoelectron spectroscopy and X-ray diffraction. Overall, the Pt-Pd/C bimetallic nanostructures are expected to become promising cathode material in fuel cells to replace monometallic Pt ones.

Reduced-Graphene-Oxide with Transition Metal Hexacyanometallates As Active Support for Traces of Platinum Catalyst at Low Loading during Oxygen Electroreduction

Reduced graphene-oxide-supported the various metals analogue of polynuclear Prussian Blue are considered here together with traces of platinum as well as low loadings of platinum (10-15 mg cm-2) as catalytic materials for reduction of oxygen in acid media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of oxygen in acid solution (the highest currents and the most positive electroreduction potentials) using cyclic and rotating ring-disk electrode (RDE) voltammetric experiments. Among important issues are possible activating interactions between analogue of Prussian Blue, platinum nanoparticles and the support, as well as presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles with trace amount of Pt modified reduced graphene-oxide-supported is also characterized here by additional diagnostic experiments, namely galvanodynamic measurements with use of so called gas diffusion electrode (GDE) operating under conditions resembling those characteristic of the cathode in a real fuel cell.

Towards a fuel-flexible direct alcohol microfluidic fuel cell with flow-through porous electrodes: Assessment of methanol, ethylene glycol and glycerol fuels

A mixed media fuel-flexible direct alcohol microfluidic fuel cell (μDAFC) with flow-through configuration is introduced. The anode of the fuel-flexible μDAFC can interchangeably use methanol, ethylene glycol or glycerol in alkaline media, while saturated oxygen in acidic media feeds the cathode to generate ∼30–40 mW cm−2 of maximum power density. The performance in terms of output power density of the μDAFC is highly dependent on the fuel concentration in each case. The output power is proportional to the fuel concentration for ethylene glycol and methanol. Contrarily, glycerol shows higher power at low concentrations and vice-versa, which is rationalized as a consequence of the poisoning effect due to the limited glycerol catalysis on Pt at high concentration cases. The fuel-flexible μDAFC with consistently high power density is achieved by balancing the number of carbons participating in the ethylene glycol and methanol oxidation reactions, while glycerol must be administered in low concentrations. This concept therefore shows promise as a practical and cost-effective fuel cell solution that can operate on a range of organic, biomass-derived fuels without sacrificing power density at room temperature.

Activation of Reduced-Graphene-Oxide Supported Pt Nanoparticles by Aligning with WO3-Nanowires toward Oxygen Reduction in Acid Medium: Diagnosis with Rotating-Ring-Disk Voltammetry and Double-Potential-Step Chronocoulometry

The chemically-reduced graphene-oxide has been used as support to anchor catalytic Pt nanoparticles and, while subsequently modified with tungsten oxide nanowires, has produced a hybrid electrocatalytic system for electroreduction of oxygen in acid medium. Such characteristics of reduced graphene oxide as defective surface and low degree of organization of graphitic structures facilitate dispersion of platinum nanocenters. Further combination with WO3 nanonowires is expected to increase the system's interfacial hydrophilicity and porosity. Most likely WO3 nanostructures are attached mainly to the edges of graphene thus preventing its stacking and creating space for the flux of oxygen and the reaction products. The results of electrochemical diagnostic experiments are consistent with the view that the electrocatalytic activity of the system utilizing tungsten oxide nanowires toward the reduction of oxygen in acid medium has been enhanced even at the Pt loading as low as 30 μg cm−2. Furthermore, the rotating ring-disk electrode voltammetry data is consistent with decreased formation of the undesirable hydrogen peroxide intermediate in the presence of WO3. The conclusions are supported with mechanistic and kinetic studies performed with use of double-potential-step chronocoulometry as an alternative diagnostic tool to rotating ring-disk voltammetry.

Reduced-Graphene-Oxide with Traces of Iridium or Gold as Active Support for Pt Catalyst at Low Loading during Oxygen Electroreduction

Chemically-reduced graphene-oxide-supported gold or iridium nanoparticles are considered here as active carriers for dispersed platinum with an ultimate goal of producing improved catalysts for electroreduction of oxygen in acid medium. Comparison is made to the analogous systems not utilizing reduced graphene oxide. High electrocatalytic activity of platinum (loading up to 30 µg cm-2) dispersed over the reduced-graphene oxide-supported Au (up to 30 µg cm-2) or Ir (up to 1.5 µg cm-2) nanoparticles toward reduction of oxygen has been demonstrated using cyclic and rotating ring-disk electrode (RRDE) voltammetric experiments. Among important issues are possible activating interactions between gold and the support, as well as presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide. The RRDE data are consistent with decreased formation of hydrogen peroxide.

Electronic metal-support interaction enhanced oxygen reduction activity and stability of boron carbide supported platinum

Catalysing the reduction of oxygen in acidic media is a standing challenge. Although activity of platinum, the most active metal, can be substantially improved by alloying, alloy stability remains a concern. Here we report that platinum nanoparticles supported on graphite-rich boron carbide show a 50–100% increase in activity in acidic media and improved cycle stability compared to commercial carbon supported platinum nanoparticles. Transmission electron microscopy and x-ray absorption fine structure analysis confirm similar platinum nanoparticle shapes, sizes, lattice parameters, and cluster packing on both supports, while x-ray photoelectron and absorption spectroscopy demonstrate a change in electronic structure. This shows that purely electronic metal-support interactions can significantly improve oxygen reduction activity without inducing shape, alloying or strain effects and without compromising stability. Optimizing the electronic interaction between the catalyst and support is, therefore, a promising approach for advanced electrocatalysts where optimizing the catalytic nanoparticles themselves is constrained by other concerns.

Copper Corrosion Under Non-uniform Magnetic Field in 0.5 M Hydrochloric Acid

The influence of a magnetic field on the electrochemical reactions taking place at the surface of a copper electrode immersed in a 0.5 M HCl solution at room temperature has been studied. The symmetry axis of the magnetic field was lined up in the same direction of the ion flow to minimize the Lorentz forces. Measurements of potentiodynamic polarization curves, electrochemical impedance spectroscopy and electrochemical noise allow concluding that the magnetic field significantly affects the cathodic reactions, with corrosion rates increasing under the presence of oxygen in acid media and decreasing when oxygen is eliminated.

Highly efficient degradation of hazardous dyes in aqueous phase by supported palladium nanocatalyst—A green approach

In this report, supported palladium nanocatalyst on silica coated Fe3O4 magnetic nanoparticles was employed as a novel catalyst towards the degradation of methyl red, methyl orange and methylene blue. The reaction between palladium nanoparticles and dissolved oxygen in acidic media produced palladium hydroperoxide, which has strong ability to oxidize the proposed dyes. The results showed that, the concentration of dyes changes exponentially with time and Langmuir–Hinshelwood model was used to describe the catalytic decolorization of these dyes in the presence of palladium nanoparticles. The efficiency of the catalyst and the effects of different variables such as initial pH, catalyst loading and temperature have been discussed in detail. Degradation sequence of dyes was further investigated by liquid chromatography–mass spectroscopy technique, which confirmed the degradation of dyes to non-toxic end products.

Electrodeposited gold nanostructures at Nafion–poly(o-phenylenediamine) modified electrode and its electrocatalytic application

A facile method to prepare gold (Au) nanostructures at the Nafion–poly(o-phenylenediamine) (Nf–PPD) composite film modified electrode (ITO or GC/Nf–PPD/Au) and its application toward the electrocatalytic reduction of oxygen in acidic medium are reported. The formation of Au nanostructures at the Nf–PPD composite film modified electrode was monitored by in-situ spectroelectrochemical method. The oPD monomer was oxidatively polymerized during the electrodeposition of Au nanostructures. The Nf–PPD/Au modified electrode was characterized by cyclic voltammetry, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The composite film formed on the electrode surface was dissolved in ethanol and the Au nanostructures were characterized by high resolution transmission electron microscopy (HRTEM) analysis. The HRTEM images showed the formation of ginger shaped Au nanostructures by a fusion growth process at the Nf–PPD film. The GC/Nf–PPD/Au modified electrode displayed a better electrocatalytic activity in terms of peak current density, mass activity and stability toward oxygen reduction reaction (ORR) in acidic medium than the other controlled modified electrodes investigated in this work. The presence of PPD and Au nanostructures in the Nf matrix at the modified electrode showed a synergistic catalytic activity in the ORR.

Amine acceleration of oxidation of styrene epoxide with oxygen in acidic media

Additives of aromatic amines (aniline, α-naphtylamine, o-, p-, and m-aminophenols) accelerate the low-temperature (343 K) molecular oxygen uptake by styrene epoxides in acetonitrile solution of toluenesulfonic acid. Amines decrease the output of benzaldehyde (oxidation product) and nearly suppress the formation of phenylacetaldehyde, the product of acid-catalyzed conversion of styrene epoxide.

Cobalt triarylcorroles containing one, two or three nitro groups. Effect of NO2 substitution on electrochemical properties and catalytic activity for reduction of molecular oxygen in acid media

Cobalt(III) triarylcorroles containing 0–3 nitro groups on the para-position of the three meso-phenyl rings of the macrocycle were synthesized and characterized by electrochemistry, mass spectrometry, (UV–vis) and 1H NMR spectroscopy. The examined compounds are represented as (NO2Ph)nPh3−nCorCo(PPh3), where n varies from 0 to 3 and Cor represents the core of the corrole. Each compound can undergo two metal-centered one-electron reductions leading to formation of Co(II) and Co(I) derivatives in CH2Cl2 or pyridine containing 0.1M tetra-n-butylammonium perchlorate (TBAP). A stepwise two electron reduction of each NO2Ph group of the compound is also observed. The first is reversible and occurs in a single overlapping step at the same potential which involves an overall one-, two- or three-electron transfer process for compounds 2–4, respectively. This indicates the lack of an interaction between these redox active sites on the corroles. The second reduction of the NO2Ph groups is irreversible and located at a potential which overlaps the Co(II)/Co(I) process of the compounds. Thin-layer UV–visible spectroelectrochemical measurements in CH2Cl2, 0.1M TBAP demonstrate the occurrence of an equilibrium between a Co(III) π-anion radical and a Co(II) derivative with an uncharged macrocycle after the first controlled potential reduction of the nitro-substituted corroles. All four cobalt corroles were also examined as catalysts for the electroreduction of O2 when coated on an edge-plane pyrrolytic graphite electrode in 1.0M HClO4. This study indicates that the larger the number of nitro-substituents on the cobalt corrole, the better the compound acts as a catalyst.

Synthesis and electrochemistry of β-pyrrole nitro-substituted cobalt(II) porphyrins. The effect of the NO₂ group on redox potentials, the electron transfer mechanism and catalytic reduction of molecular oxygen in acidic media.

Four cobalt(II) porphyrins, two of which contain a β-pyrrole nitro substituent, were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds are represented as (TRPP)Co and (NO2TRPP)Co, where TRPP is the dianion of a substituted tetraphenylporphyrin and R is a CH3 or OCH3 substituent on the four phenyl rings of the macrocycle. Two reductions and three oxidations are observed for each compound in CH2Cl2 containing 0.10 M tetra-n-butylammonium perchlorate. The first reduction of the compounds without a nitro substituent is metal-centered and leads to formation of a Co(I) porphyrin which then reacts with the CH2Cl2 solvent to generate a carbon σ-bonded Co(III)-R complex. A further reduction then occurs at more negative potentials to generate an unstable Co(II) σ-bonded compound. In contrast to these reactions, the first reduction of the nitro-substituted porphyrins is macrocycle-centered under the same solution conditions and gives a Co(II) porphyrin π-anion radical product. This reversible electron transfer is then followed at more negative potentials by a second reversible one-electron addition to give a Co(II) dianion. Three reversible one-electron oxidations are also seen for each compound. The first is metal-centered and the next two involve the conjugated π-system of the macrocycle. Each neutral Co(II) porphyrin was also examined as to its catalytic activity for electroreduction of molecular oxygen when coated on an edge-plane pyrolytic graphite electrode in 1.0 M HClO4. The β-pyrrole nitro-substituted derivatives were shown to be better catalysts than the non-nitro substituted compounds under the utilized experimental conditions.

Enhanced O2 electrocatalysis by a highly conjugated cobalt(II) porphyrin

Cobalt(II) tetraphenyltetranaphthoporphyrin, with extended conjugation at the pyrrole units, has been synthesized and its spectroscopic and electrocatalytic behavior toward the reduction of oxygen has been investigated. The new complex shows a red shift for the Soret band of 75 nm compared to its cobalt(II) tetraphenylporphyrin analog. In addition, the new complex shows enhanced electrocatalytic ability for the reduction of oxygen in acidic media converting over 50% of the oxygen directly to water, a four electron process at relatively high potentials.

Structure of the catalytic sites in Fe/N/C-catalysts for O2-reduction in PEM fuel cells

Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by (57)Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH(3) at 950 °C. Four different Fe-species were detected at all iron concentrations: three doublets assigned to molecular FeN(4)-like sites with their ferrous ions in a low (D1), intermediate (D2) or high (D3) spin state, and two other doublets assigned to a single Fe-species (D4 and D5) consisting of surface oxidized nitride nanoparticles (Fe(x)N, with x≤ 2.1). A fifth Fe-species appears only in those catalysts with Fe-contents ≥0.27 wt%. It is characterized by a very broad singlet, which has been assigned to incomplete FeN(4)-like sites that quickly dissolve in contact with an acid. Among the five Fe-species identified in these catalysts, only D1 and D3 display catalytic activity for the oxygen reduction reaction (ORR) in the acid medium, with D3 featuring a composite structure with a protonated neighbour basic nitrogen and being by far the most active species, with an estimated turn over frequency for the ORR of 11.4 e(-) per site per s at 0.8 V vs. RHE. Moreover, all D1 sites and between 1/2 and 2/3 of the D3 sites are acid-resistant. A scheme for the mechanism of site formation upon heat-treatment is also proposed. This identification of the ORR-active sites in these catalysts is of crucial importance to design strategies to improve the catalytic activity and stability of these materials.

Partially oxidized niobium carbonitride as a non-platinum catalyst for the reduction of oxygen in acidic medium

Partially oxidized NbC 0.5N 0.5 has been evaluated as a non-platinum catalyst for the reduction of oxygen in acidic medium. NbC 0.5N 0.5 powder was partially oxidized in N 2 gas containing O 2 of 10 −4 atm at the temperature range of 700–1000 °C. Partially oxidized NbC 0.5N 0.5 had a definite oxygen reduction reaction (ORR) activity, while as-prepared NbC 0.5N 0.5 and completely oxidized Nb 2O 5 had a poor catalytic activity for ORR. The onset potential of the partially oxidized NbC 0.5N 0.5 for the ORR achieved 0.92 V vs. RHE in 0.1 M H 2SO 4 at 30 °C. The results of X-ray absorption spectroscopy and ionization potential measurements suggested that oxygen-vacancy defects might be responsible for the oxygen reduction capability by creating electronically favorable oxygen adsorption sites.