Durability of carbon-supported manganese oxide nanoparticles for the oxygen reduction reaction (ORR) in alkaline medium

MnO_x/C-based electrocatalysts, prepared by the chemical deposition of manganese oxide nanoparticles on carbon, were tested towards the Oxygen Reduction Reaction (ORR) in their as-synthesized state and after ageing, either in ambient air for a year (mild ageing) or in an O₂-saturated molar KOH solution at 80 °C for three weeks (premature ageing). For each electrocatalyst, the morphology and composition were characterised using TEM, XRD and chemical analysis. ORR kinetic parameters were evaluated using the Rotating Disk Electrode (RDE) and Rotating Ring Disk Electrode (RRDE) setups. Whilst the oxygen reduction activity of the electrocatalysts barely changes after mild ageing, it decreases after premature ageing following dramatic modifications to both the chemical and crystalline structures of the carbon-supported MnO_x nanoparticles. The peroxide yield also sharply increases after premature ageing. Doping MnO_x/C with nickel or magnesium divalent cations is beneficial since it improves both the catalytic activity and selectivity towards the 4-electron ORR pathway, even after ageing.     

Enhancement of electrocatalytic O<Subscript>2</Subscript> reduction on carbon nanotube-supported Pt alloys nanoparticles in gas diffusion electrodes

The use of Pt binary and ternary alloys prepared by alloying of Pt with transition metals, as catalysts for fabricating of gas diffusion electrodes (GDEs) is reported. Electrocatalytic properties of oxygen reduction reaction (ORR) were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, polarization experiments and chronoamperometry. The morphology of the GDEs and elemental compositions of the Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The results indicate that the introduction of Pd and Cd as transition metals in Pt alloys provides fast ORR kinetics. The performance of GDEs with Pt–Pd alloy surfaces for ORR was also studied as a function of overall composition and surface atomic distribution of Pt alloys. The results also show that alloying of Pt with transition metals and various amounts of Pt and Pd in the binary catalysts has a large effect on the performance of GDEs for ORR.     

Oxygen reduction reaction on carbon-supported CoSe2 nanoparticles in an acidic medium

We investigated the effect of CoSe₂/C nanoparticle loading rate on oxygen reduction reaction (ORR) activity and H₂O₂ production using the rotating disk electrode and the rotating ring-disk electrode techniques. We prepared carbon-supported CoSe₂ nanoparticles with different nominal loading rates and evaluated these samples by means of powder X-ray diffraction. All the catalysts had an OCP value of 0.81 V vs. RHE. H₂O₂ production during the ORR process decreased with an increase in catalytic layer thickness. This decrease was related to the CoSe₂ loading on the disk electrode. H₂O₂ production also decreased with increasing catalytic site density, a phenomenon related to the CoSe₂ loading rate on the carbon substrate. The cathodic current density significantly increased with increasing catalytic layer thickness, but decreased with increasing catalytic site density. In the case of 20 wt% CoSe₂/C nanoparticles at 22 μg cm⁻², we determined that the transfer process involves about 3.5 electrons.     

Electrochemical behavior of an Ag/TiO2 composite surfaces

Silver in a form of randomly distributed submicron-sized particles was electrodeposited, and simultaneously, titanium dioxide layer was formed, on the surface of mechanically polished titanium, by its potentiodynamic polarization in an aqueous AgNO₃ solution. In such obtained Ag/TiO₂ composite layer, the particle size distribution, and silver loading were determined by means of scanning electron microscopy. The electrochemical response of this composite layer was studied in both oxygen-free and oxygen saturated aqueous 0.1 M NaOH solution. The catalytic activity toward oxygen reduction reaction of this and other forms of silver- and platinum-based materials was compared.     

Electrocatalysis of gold nanoparticles/layered double hydroxides nanocomposites toward methanol electro-oxidation in alkaline medium

A nanocomposite based on layered double hydroxides (LDHs) and gold nanoparticles (AuNPs) was prepared via hydrothermal treatment followed by a reduction procedure. The AuNPs were obtained in Mg-Al LDHs, and they maintained good stability. The electrocatalytic activities of AuNPs/LDH-modified glassy carbon electrodes for methanol oxidation in alkaline medium were investigated in detail. Under the same conditions, the modified electrode exhibited higher electrocatalytic activity than both the pure AuNPs-modified electrode and LDH-modified electrode. The role of the AuNPs and LDHs in this composite system was explored by cyclic voltammetry and chronoamperometry, respectively. Further studies demonstrated that the promoting effect of LDHs could be due to its strong adsorption and partly to the discharge of OH⁻ during methanol oxidation. This work indicates that LDHs is expected to be a good supporting material in the development of methanol anode catalysts.     

Electrochemical impedance spectroscopy investigation of spinel type cobalt oxide thin film electrodes in alkaline medium

Spinel type Co₃O₄ thin films, for the oxygen evolution reaction (OER) in 1 M KOH, have been prepared, on stainless steel supports, using the thermal decomposition method at 400 °C. The electrochemical behaviour of the oxide film/1 M KOH interface was investigated by cyclic voltammetry and impedance techniques. The impedance measurements were carried out at different positive potentials, from the open circuit potential to a potential in the OER region and the electrical equivalent circuit, L (R₁Q₁) (R₂Q₂) (R₃Q₃) was used to fit the experimental results. At each potential, a good correlation between experimental and simulated data is found, thereby validating the proposed equivalent circuit model. The roughness factor value determined in the potential region where the charge transfer reaction is negligible is similar to that obtained by cyclic voltammetry, with a value of 70 ± 2.     

The electrochemical reduction of oxygen on zinc corrosion films in alkaline solutions

Kinetics of the O₂ reduction has been characterized on Zn corrosion films by Pt/Zn rotating ring-disc electrode (RRDE) and EIS methods. On zinc-oxide films a two-step reduction was identified in various buffer solutions of pH 10.5, while a small quantity of H₂O₂ intermediate could be detected. On the basis of results obtained from Pt/Zn and Pt/Pt RRDE experiments in solutions containing H₂O₂, it was further confirmed that the HO₂⁻ was reduced to OH⁻ through the zinc-oxide corrosion layer. Capacitance data of the zinc-oxide/electrolyte interface calculated from steady-state impedance diagrams measured at various cathodic potentials indicate the presence of a space charge layer of the semi-metallic ZnO. The solid-state reaction mechanism of HO₂⁻ disproportion with participation of Zn_i⁺ interstitials, oxygen ion vacancies of the non-stoichiometric Zn-oxide, and chemisorbed HO₂⁻ is discussed.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

Influence of sputtering pressure on surface structure and oxygen reduction reaction catalytic activity of thin platinum films

The work presents a study on the influence of the sputtering pressure on the surface structure and morphology of low Pt loaded electrodes and their electrochemical behaviour toward oxygen reduction reaction (orr) in sulphuric acid solution and polymer electrolyte membrane (Nafion 117). Pt was deposited as thin film upon hydrophobic carbon paper substrates at sputtering pressure varied in the range 2-13 Pa. The test samples are analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The catalytic activity is assessed by applying the methods of linear sweep voltammetry (LSV) on rotating disc electrode (RDE) and cyclic voltammetry (CV). The results obtained show strong influence of the sputtering pressure on the surface structure and crystal orientation which in turn, affects the orr efficiency. The best electrode performance in both electrolytes used is obtained for the Pt film deposited at pressure of 9 Pa. The results obtained in Nafion 117 show that catalyst utilisation in this electrode exceeds significantly the one for a commercial ELAT electrode at the same operation conditions. The research demonstrated that by simple variations in the sputter regime it is possible to optimise the catalysts morphology in order to increase the catalytic activity toward the electrochemical reaction of interest at the same time controlling precisely the required precious metal loading.     

Electroreduction of oxygen on Vulcan carbon supported Pd nanoparticles and Pd–M nanoalloys in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied using the rotating disk electrode (RDE) technique. Palladium nanoparticles and Pd–M (PdCo and PdFe) nanoalloys supported on Vulcan XC-72R were prepared using two different synthetic routes. The catalyst samples were examined by transmission electron microscopy (TEM) and the average size of metal nanoparticles was determined. Electrochemical measurements were performed in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions. The influence of different synthetic conditions on the values of specific activity and other kinetic parameters was investigated. These parameters were determined from the Tafel plots taking into account the real electroactive area for each electrode. Pd nanoparticles and Pd–M nanoalloys exhibit significantly high electrocatalytic activity for the four-electron reduction of oxygen to water.     

Electrocatalytic properties of new active ternary ferrite film anodes for O2 evolution in alkaline medium

Some ternary ferrites with molecular formula, CoFe_2−xCr_xO₄ (0≤x≤1.0) have been synthesized at 70 °C by a precipitation method and were transformed into the film form at the pretreated Ni support (1.5×1.0 cm²) using an oxide-slurry painting technique. The study showed that Cr-substitution from 0.2 to 1.0 mol increased the electrocatalytic activity of the oxide towards the oxygen evolution reaction (OER), the optimum improvement in apparent electrocatalytic activity being with 0.8 mol Cr. At E=600 mV versus Hg/HgO in 1 M KOH (25 °C), the apparent oxygen evolution current density (j_a) with the catalyst, CoFe_1.2Cr_0.8O₄, was ∼80 times greater than that observed with the base oxide (i.e. CoFe₂O₄). The OER on Cr-substituted oxides showed two Tafel slopes, one (b=42±1 mV per decade) at low overpotential and the other (b=66±6 mV per decade) at higher potential. The reaction order with respect to OH⁻ concentration was ∼1.3±0.1 for each electrocatalyst. The thermodynamic parameters for the OER, namely, standard apparent electrochemical enthalpy of activation (ΔH°_el^#), standard enthalpy of activation (ΔH°^#) and standard entropy of activation (ΔS°^#) have also been determined. It was observed that values of the ΔH°_el^# and ΔH°^# decreased with Cr-substitution in the CoFe₂O₄ lattice; the decrement, however, being the greatest with 0.8 mol Cr. The ΔS°^# values were largely negative varying between ∼−61 and −126 J deg⁻¹ mol⁻¹.     

Electrochemical behavior of Au nanoparticle deposited on as-grown and O-terminated diamond electrodes for oxygen reduction in alkaline solution

Au nanoparticle was electrochemically deposited on both as grown and oxygen-terminated (O-terminated) boron-doped diamond (BDD) films. The surface coverages of Au nanoparticle were 0.07 and 0.18 corresponding to the areas of Au 0.012 and 0.029 cm², respectively, as noted from linear sweep voltammetry. The SEM studies indicated different morphologies of Au deposition such as random distribution of small spherical particles at both the grain boundaries and the facets on the as grown diamond film and clusters principally on the cross edges of two facets on the O-terminated diamond. The electrochemical behavior for oxygen reduction was examined using differential pulse voltammetry (DPV), which confirmed the higher catalytic efficiencies of Au deposited as grown and O-terminated BDD electrodes when compared to a polycrystalline Au electrode. Moreover, the mechanism of Au nanoparticle deposited BDD films for the oxygen reduction was investigated by ac impedance and hydrodynamic voltammetric methods.     

Kinetics of oxygen reduction reaction at tin-adatoms-modified gold electrodes in acidic media

In the present report, oxygen reduction reaction (ORR) at polycrystalline gold (Au (poly)) electrode in situ modified by the underpotential deposition (upd) of Sn-adatoms is addressed. The ORR was investigated at the Sn-adatoms-modified Au (poly) electrode by the hydrodynamic voltammetric technique with a view to evaluating the various related kinetic parameters. The results demonstrated that the underpotential deposited Sn-adatoms on the Au (poly) electrode substantially promoted the activity of the electrode towards an exclusive one-step four-electron ORR forming H₂O as the final product.     

Electrocatalytic oxygen reduction on single-walled carbon nanotubes supported Pt alloys nanoparticles in acidic and alkaline conditions

The objective of this study is to improve the catalytic activity of platinum by alloying with transition metal (Pd) in gas diffusion electrodes (GDEs) by oxygen reduction reaction (ORR) at cathode site and comparison of the acidic and alkaline electrolytes. The high porosity of single-walled carbon nanotubes (SWCNTs) facilitates diffusion of the reactant and facilitates interaction with the Pt surface. It is also evident that SWCNTs enhance the stability of the electrocatalyst. Functionalized SWCNTs are used as a means to facilitate the uniform deposition of Pt on the SWCNT surface. The structure of SWCNTs is nearly perfect, even after functionalization, while other types of CNTs contain a significant concentration of structural defects in their walls. So catalysts supported on SWCNTs are studied in this research. The electrocatalytic properties of ORR were evaluated by cyclic voltammetry, polarization experiments, and chronoamperometry. The morphology and elemental composition of Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The catalytic activities of the bimetallic catalysts in GDEs have been shown to be not only dependent on the composition, but also on the nature of the electrolytes. The GDEs have shown a transition from the slow ORR kinetics in alkaline electrolyte to the fast ORR kinetics in the acidic electrolyte. The results also show that introduction of Pd as transition metal in the Pt alloys provides fast ORR kinetics in both acidic and alkaline electrolytes. The performance of GDEs with Pt–Pd alloy surfaces towards the ORR as a function of the alloy’s overall composition and their behavior in acidic electrolyte was also studied. These results show that the alloy’s overall composition and also the nature of the electrolytes have a large effect on the performance of GDEs for ORR.     

On the role of the thermal treatment of sulfided Rh/CNT catalysts applied in the oxygen reduction reaction

Low loading sulfided rhodium catalysts supported on carbon nanotubes (CNTs) were prepared from RhCl₃ by deposition–precipitation using hydrogen peroxide, followed by an exposure to hydrogen sulfide and an additional thermal treatment in the range from 400 °C to 900 °C. Hydrogen sulfide was generated online from hydrogen and sulfur vapor over molybdenum disulfide as catalyst. By elemental analysis, the Rh loading of the prepared catalysts was found to be 1.4–1.8 wt%. Morphology and composition of the resulting catalysts were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), and X-ray photoelectron spectroscopy (XPS). Nanoparticles were found to be highly dispersed on the CNTs with an average diameter as small as 1.0 nm determined by TEM. Sintering occurred during heat treatments at 650 °C and 900 °C in helium, as evidenced by XRD, TEM, and XPS. The treatment with hydrogen sulfide significantly enhanced the activity of the supported rhodium catalysts for the oxygen reduction reaction (ORR) in hydrochloric acid, as determined by rotating disc electrode measurements. The sulfided catalyst annealed at 650 °C with a particle size of about 2.5 ± 1.0 nm showed the best performance for the ORR, which is discussed based on the presence of a more stable rhodium sulfide layer on the metallic rhodium particles.     

Effect of hydrothermal treatment on the performance of RuO2-Ta2O5/Ti electrodes for use in supercapacitors

Hydrothermal treatment (HTT) of RuO₂-Ta₂O₅/Ti electrode, as a method for improving their performance, for use in supercapacitors was investigated.The results show that HTT significantly enhances the stability of the electrodes. The specific capacitance of electrodes, subject to HTT in the temperature range 180-250 °C remains unchanged after 1000 CV cycles between −0.2 and 1.1 V vs. SCE; without HTT a decay to 97% of the initial is observed. The results also show that HTT decreases the activity of the electrodes for O₂ and H₂ evolution and increases the voltage window by 56-135 mV for supercapacitors, but with a specific capacitance decrease of 7-27%. XPS analyses show the existence of more hydroxides after the HTT, which leads to a little increase in the interplanar distance as indicated in the XDR results. Contact angle measurements show the presence of a more hydrophilic surface after HTT.     

The effect of the hydrothermal treatment with aqueous NaOH solution on the photocatalytic and photoelectrochemical properties of visible light-responsive TiO2 thin films

The effect of the hydrothermal treatment with aqueous NaOH solution on the photoelectrochemical and photocatalytic properties of visible light-responsive TiO₂ thin films prepared on Ti foil substrate (Vis-TiO₂/Ti) by a radio-frequency magnetron sputtering (RF-MS) deposition method has been investigated. The hydrothermally treated Vis-TiO₂/Ti electrodes exhibited a significant increase in their photocurrent under UV and visible light irradiation as compared to untreated Vis-TiO₂/Ti electrode. SEM investigations revealed that the surface morphology of Vis-TiO₂/Ti are drastically changed from the assembly of the TiO₂ crystallites to the stacking of nanowires with diameters of 30–50 nm with increasing hydrothermal treatment time (3–24 h), accompanying the increase in their surface area. The separate evolution of H₂ and O₂ from water under solar light irradiation was successfully achieved using the Vis-TiO₂/Ti/Pt which is hydrothermally treated for 5 h, while the H₂ evolution ratio was 15 μmol h⁻¹ in the early initial stage, corresponding to a solar energy conversion efficiency of 0.23%.     

Heat treatment effect of Pt-V/C and Pt/C on the kinetics of the oxygen reduction reaction in acid media

This work studies the heat treatment effect of carbon-dispersed platinum and platinum-vanadium alloys on the kinetics of the oxygen reduction reaction (ORR) in acid medium. The catalyst powders were subjected to heat treatments at three temperatures for 1 h. The electronic and structural features of the materials were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The XANES results for the oxidized state composites showed an increase of the Pt 5d band occupancy with increased heat treatment temperature for the Pt/C catalyst, while no changes were noted for Pt-V/C for the same treatments. The electrochemical characteristics for the ORR were investigated by cyclic voltammetry and state-state polarization measurements. The results showed that the ORR takes place by the multi-electronic charge transfer process, following a four electron mechanism. The kinetics of the ORR was evaluated using Tafel diagrams. It was observed that the ORR activity of the Pt/C and Pt-V/C is enhanced with the increase of the heat treatment temperature. The catalytic activity of the materials was analyzed in terms of the electronic and structural properties of Pt in the metallic particles.     

Direct formation of gold nanoparticles on substrates using a novel ZnO sacrificial templated-growth hydrothermal approach and their properties in organic memory device

This study describes a novel fabrication technique to grow gold nanoparticles (AuNPs) directly on seeded ZnO sacrificial template/polymethylsilsesquioxanes (PMSSQ)/Si using low-temperature hydrothermal reaction at 80°C for 4 h. The effect of non-annealing and various annealing temperatures, 200°C, 300°C, and 400°C, of the ZnO-seeded template on AuNP size and distribution was systematically studied. Another PMMSQ layer was spin-coated on AuNPs to study the memory properties of organic insulator-embedded AuNPs. Well-distributed and controllable AuNP sizes were successfully grown directly on the substrate, as observed using a field emission scanning electron microscope followed by an elemental analysis study. A phase analysis study confirmed that the ZnO sacrificial template was eliminated during the hydrothermal reaction. The AuNP formation mechanism using this hydrothermal reaction approach was proposed. In this study, the AuNPs were charge-trapped sites and showed excellent memory effects when embedded in PMSSQ. Optimum memory properties of PMMSQ-embedded AuNPs were obtained for AuNPs synthesized on a seeded ZnO template annealed at 300°C, with 54 electrons trapped per AuNP and excellent current–voltage response between an erased and programmed device.     

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 N₂ gas containing O₂ 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 Nb₂O₅ 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 H₂SO₄ 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.