Search

The challenges associated to oxygen reduction reaction (ORR) electrocatalysis are undoubtedly the most intricate impediments to solve regarding fuel cell vehicle (FCV) commercialization, mainly due to the complexity of this reaction and the scarcity of the required electrocatalysts. The synthesis of non-precious electrocatalysts with high intrinsic selectivity is indeed crucial in this field, but the methodology utilized to come to selectivity conclusions is undeniably equally important. In this work, we demonstrate how the higher required electrode loadings of a Fe-N-MWCNT catalyst in a rotating ring disc electrode (RRDE) prevent access to all of the peroxide produced by the catalyst during ORR. Via a spraying technique, much smaller amounts of the same catalyst were deposited on a flat boron-doped diamond (BDD) and their loading was determined meticulously by atomic force microscopy (AFM). These ultra-low loadings were investigated using scanning electrochemical microscopy (SECM), which revealed higher peroxide production and lower selectivity. This finding may have crucial implications for future ORR investigations in the laboratory, considering the harmful effect peroxide has on the catalyst.

Ordered mesoporous carbon (OMC) is a promising material for oxygen reduction reaction (ORR) in fuel cells owing to its high surface area and more number of catalytic active sites. Herein, OMC was synthesized by hard template method, i. e., nanocasting of sacrificial silica template (SBA‐15) and then the sacrificial template was abolished using three different etching chemicals namely, hydrofluoric acid (HF), sodium hydroxide (NaOH) and polyvinylidene fluoride (PVDF) respectively, in order to study the etching effect. The etched OMCs derived from the SBA‐15 template was confirmed by various characterization techniques. In OMC, the porous nature, surface morphology and defect density changes with respect to etching chemicals were evidently determined. From the electrochemical studies, it has been concluded that the NaOH etched OMC showed higher electrocatalytic activity than others by exhibiting more positive onset (0.83 V), half‐wave potential (0.75 V), high current density (3.3 mA/cm2), Tafel slope (83 mV/dec) and n‐value (2.4). Eventually, the electrochemical impedance study (EIS) was performed to determine the charge transfer resistance (Rct) of various etched OMCs and has been found that NaOH etched OMC shows the lowest Rct value due to the presence of more defective sites and result in enhancing oxygen adsorption during ORR. Ordered mesoporous carbons (OMCs) is a promising electrocatalyst for ORR in fuel cells owing to its surface area and active sites. Here, OMCs were synthesized by using silica template (SBA‐15) then the template is removed by three different etching agents such as HF, NaOH and PVDF (polyvinylidene fluoride). The porosity, morphology and surface functional groups changes with respect to etching chemicals. Further, the electrochemical studies concluded that the NaOH etched OMC showed better activity than others. [ABSTRACT FROM AUTHOR]

The development of efficient and durable earth-abundant electrocatalysts for the oxygen reduction reaction (ORR) is an ongoing challenge. In this work, we directly anchored CoS nanocatalysts (20 wt%) on different supports, namely, carbon nanotubes (CNTs) and reduced graphene oxide by a single-pot hydrothermal route. The developed synthesis processes enabled us to alter the composite nature of CoS by changing the sulfur precursor used. The corresponding electrocatalytic performance of these composite catalysts toward the ORR was studied in alkaline medium. The increase in the effective electrode surface area that accompanies the CoS nanocomposite substantially boosts its ORR catalytic performance. The CoS-CNT-Cy hybrid material shows an onset potential of 0.85 V, which is 60–200 mV less negative than that of other catalysts tested. The catalytic current density achieved with the hybrid material at 0.2 mg cm −2 is comparable with that of commercial Pt/C (20%). The synergistic effect of CNTs and CoS enhances the overall performance of the hybrid catalyst. Extension of this nanocomposite strategy to other earth-abundant materials could similarly enable inexpensive electrocatalysts that lack the high intrinsic activity of the noble metals. [ABSTRACT FROM AUTHOR]

[Display omitted] • SECM is used to detected ORR catalytic activity and H 2 O 2 production successively. • AFM was used to assess the volume and morphology of the catalyst agglomerates. • Selectivity acquired with SECM at ultra-low loadings suggests n ∼ 2.6. • n ∼ 4 was detected with conventional RRDE. • Investigation of the impact of the morphology on the selectivity is illustrated. The challenges associated to oxygen reduction reaction (ORR) electrocatalysis are undoubtedly the most intricate impediments to solve regarding fuel cell vehicle (FCV) commercialization, mainly due to the complexity of this reaction and the scarcity of the required electrocatalysts. The synthesis of non-precious electrocatalysts with high intrinsic selectivity is indeed crucial in this field, but the methodology utilized to come to selectivity conclusions is undeniably equally important. In this work, we demonstrate how the higher required electrode loadings of a Fe-N-MWCNT catalyst in a rotating ring disc electrode (RRDE) prevent access to all of the peroxide produced by the catalyst during ORR. Via a spraying technique, much smaller amounts of the same catalyst were deposited on a flat boron-doped diamond (BDD) and their loading was determined meticulously by atomic force microscopy (AFM). These ultra-low loadings were investigated using scanning electrochemical microscopy (SECM), which revealed higher peroxide production and lower selectivity. This finding may have crucial implications for future ORR investigations in the laboratory, considering the harmful effect peroxide has on the catalyst. [ABSTRACT FROM AUTHOR]

Hydrogen evolution reaction (HER) activities of self-assembled monolayers (SAMs) of [Mo3S7(S2CNMe2)3] and several other MoSx molecular clusters are presented on planer Au electrode. Our study suggests that such Mo-S clusters are unstable under HER reaction conditions of a strongly acidic electrolyte. The [Mo3S7(S2CNEt2)3]I monolayer prepared from DMF showed greater stability among all the studied precursors. The X-ray photoelectron spectroscopy (XPS) analysis on a monolayer of [Mo3S7(S2CNMe2)3]I in THF assembled on Au/ITO suggested sulfur-rich composition with S:Mo ratio of 2.278. The Mo-S monolayer clusters resulting from [Mo3S7(S2CNMe2)3]I in THF showed a Tafel slope of 75.74 mV dec−1 and required a lower overpotential of 410 mV to reach a high HER catalytic current density of 100 mA cm−2 compared to the other studied precursors. Surface coverage of the Mo-S clusters on the Au surface was confirmed by cyclic voltammetry (CV) curves from K3Fe(CN)6 and anodization of Au surface. Further, the rotating ring-disk electrode (RRDE) measurements were performed for the monolayer of [Mo3S7(S2CNMe2)3]I prepared in THF to study its reaction kinetics. The HER catalytic activity of such monolayer Mo-S clusters can further be improved by controlling the sulfur vacancy.

The oxidation of Cu(II) complexes with tetra, penta and hexaglycine in borate buffer aqueous solution, by dissolved oxygen is strongly accelerated by sulfite. The formation of Cu(III) complexes with maximum absorbances at 250 nm (e = 9000 mol-1 L cm-1) and 365 nm (e = 7120 mol-1 L cm-1) was also characterized by using rotating ring-disk voltammetry, whose anodic and cathodic components were observed in voltammograms recorded in solutions containing Cu(II). Voltammograms, obtained at various rotation speeds, showed that the Cu(III) species electrochemically generated is not stable over the entire time window of the experiment and in solutions containing tetraglycine the overall limiting current is controlled by the kinetics of an equilibrium involving Cu(II) species.The calculated first order rate constant of the decomposition was 4.37x10-3 s-1. Electrochemical experiments carried out in Cu(II) solutions after the addition of relatively small amounts of sulfite demonstrated that the Cu(III) species formed in the chemical reaction is the same as the one collected at the ring electrode when Cu(II) is oxidized at the disk electrode in ring-disk voltammetry. The concentration of Cu(III) complexes is proportional to the amount of added sulfite and the results indicated that indirect analytical methods for sulfite may be developed by means of spectrophotometric or amperometric detection of the chemically generated product.

In this work, three kinds of α-MnO 2 nano shapes, namely, nano-wires, nano-tubes and nano-particles have been prepared with a fine control over α-crystallographic form by employing hydrothermal procedure. The materials have been thoroughly characterized by X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) spectrometry, field-emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques. The MnO 2 nano shapes are used as a model system for examining the shape-influenced bi-functional electrocatalytic activity towards oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline medium. The bi-functional role has been investigated by cyclic voltammetry and linear sweep voltammetry with rotating ring disc electrode (RRDE) techniques. It is found that α-MnO 2 nano-wires possess enhanced electrocatalytic activity compared to other two shapes namely nano-tubes and nano-particles despite the nano-tubes having a much higher specific surface area. The insight of bi-functional electrocatalytic activity is analysed in terms of catalyst surface with the help of first principles density functional theory (DFT) calculations based on the fact of surface energies and adsorption of water on the surface for a facile reaction. [ABSTRACT FROM AUTHOR]

Highlights: [•] The ease of electro-oxidation of alcohols follows the C–H bond dissociation energy. [•] The radicals formed primarily are too short lived for an electrochemical detection. [•] The second oxidation product, H+, was detected by a rotating ring disc-electrode. [•] The reactant molecules probably also act as a base for H+. [•] Oxidation of t-butanol leads to a long lived intermediate (τ ∼some seconds). [Copyright &y& Elsevier]

Abstract: The electrochemical reduction of oxygen has been investigated for a cobalt-amine complex pyrolysed at various temperatures. The activity of the corresponding cobalt-free materials has also been studied. The selectivity of the catalysts towards hydrogen peroxide production has been determined using Koutecky–Levich analysis and rotating ring disc electrochemical measurements. A combination of the rotating ring disc data and studies of the reduction of hydrogen peroxide have been used to investigate the reaction mechanism kinetics. The selectivity towards hydrogen peroxide passes through a minimum at a pyrolysis temperature of 700°C. The highest activity for oxygen reduction and yield for hydrogen peroxide formation was observed for the catalysts treated at higher temperatures. [Copyright &y& Elsevier]

Abstract: The effect of chemical pre-treatment of the carbon support used for deposition of Pt nano-particles is reported. Data on particle size, distribution and their electocatalytic activity toward hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) are reported. Vulcan XC-72R carbon was pre-treated with 5% HNO3, 0.07 M H3PO4, 0.2 M KOH and 10% H2O2. The properties of carbon supports were studied by N2 adsorption and X-ray photoelectron spectroscopy (XPS). Chemical reduction with ethylene glycol (EG) was used to synthesize Pt on carbon supports and the differences in catalyst morphology were characterized using CO chemisorption, X-ray diffraction, energy dispersive X-ray analysis and transmission electron microscope techniques. The electrocatalytic activity of Pt/C catalysts toward HOR and ORR was examined by cyclic voltammetry (CV) on a rotating ring-disk electrode (RRDE) and compared with E-Tek Pt/C. The ORR was predominantly involved via four-electron process with the first electron transfer being the rate-determining step. However, the specific activity and mass activity were greatly influenced by the pre-treatment employed. [Copyright &y& Elsevier]

The oxygen reduction reaction in sulphuric acid on commercial carbon supported platinum and ruthenium catalysts as well as on a home-made carbon supported ruthenium selenide catalysts (RuSe x /C) was investigated. The RuSe x /C catalysts were synthesised using similar procedures to those found in the literature. A dependency of H2O2 formation on the selenium content was found using the thin-film rotating ring disc electrode technique, namely that the H2O2 formation in the typical operation range of a Direct Methanol Fuel Cell (0.7–0.4 V) on Pt/C is below 1% and 1–4% on Ru/C and RuSe x /C catalysts. Finally for comparing the intrinsic activities of the catalysts the electrochemically active surface areas were determined in-situ by means of copper underpotential deposition. Our results indicate a comparable activity of the present RuSe x /C catalyst to commercial Pt/C if the activities are related to the electrochemical active areas. [ABSTRACT FROM AUTHOR]

Abstract: This study provides experimental evidence for proton and hydrogen formation caused by the anodic electropolymerization of cyclohexyl benzene (CHB), which is a popular electrolyte additive for overcharge protection of lithium-ion batteries (LIBs). It is found that considerable H2 evolution is observed in overcharged LiCoO2/graphite cells, especially when CHB is included as an electrolyte additive. In order to confirm the proton generation during the CHB oxidation, Pt/Pt-rotating ring disc electrode (RRDE) measurements are performed in 1M Li(C2F5SO2)2N ethylene carbonate/ethyl methyl carbonate (1/2, v/v) solutions with and without CHB. The cathodic ring current is intimately correlated to the anodic disc current, and the cathodic reaction at the ring is determined to be the reduction of the proton. The proton generation efficiency during the CHB oxidation is as high as 90%. Proton liberation is also observed during the anodic decomposition of the electrolyte solvents, although it occurs in a much less stoichiometric way compared with that during the CHB oxidation. [Copyright &y& Elsevier]

Abstract: The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O2 was reduced to H2O2 in a 2-electron reaction with the rest being reduced to OH− in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O2 was almost exclusively reduced to OH− in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than −1.2V vs. Ag/AgCl and the second step reduction (E <−1.2V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around −1.2V. The O2 reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O2 reduced to OH− in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment. [Copyright &y& Elsevier]

Abstract: Three types of spinel-based cathode powders, namely, LiMn2O4, Li1.07Mn1.93O4, and Li1.06Al0.2Mn1.74O4, are examined with rotating ring-disc collection experiments to measure manganese dissolution and capacity losses in lithium-ion cells. The cyclic voltammograms are similar at high scan rates for all three electrodes. Initially, all electrodes rapidly lose capacity during cycling. After about 25 cycles, the electrodes start to exhibit capacity retention, and the performance matches that of the best spinels. The cation-substituted spinel electrodes experience a moderate capacity loss of 0.2% per cycle over 200 cycles, which is half that suffered by a typical spinel electrode (LiMnO4). Manganese (Mn) dissolution from all the spinel-based samples is monitored in situ under various conditions. The ring cathodic currents for the cation-substituted spinel electrodes are similar to those for LiMn2O4, which shows that the Mn dissolution behaviour is the same for all spinel-based cathode powders. The ring currents reach maximum values at the end-of charge (EOC) and end-of discharge (EOD), with the largest peak at EOC. The results suggest that the dissolution of Mn from all spinel-based samples occurs during charge–discharge cycling, especially in a charged state at >4.0V and in a discharged state at &#60;3.1V. In addition, lower ring cathodic currents are observed in the cation-substituted spinel cathodes and this indicates that the rate of dissolution of Mn has decreased in these materials. [Copyright &y& Elsevier]

The electrochemical behaviour of electrodeposited Co–Cu/Cu multilayers from citrate electrolytes was investigated using cyclic voltammetry and stripping techniques at a rotating ring disc electrode. Copper and cobalt–copper alloy sandwiches were deposited from an electrolyte containing 0.0125 M CuSO4, 0.250 M CoSO4 and 0.265 M trisodium citrate at two different pHs, 1.7 and 6.0. The Cu/Co–Cu/Cu sandwich is representative of a single layer in a Co–Cu/Cu multilayer deposit, which is known to exhibit unusual physical and magnetic properties. Results from cyclic voltammetry and detection of dissolving species at the ring showed that cobalt is stripped from a Cu/Co–Cu/Cu sandwich even when a copper layer as thick as 600 nm covers the Co–Cu alloy. Scanning electron microscopy showed that cobalt can dissolve from the deposit easily because the copper layer covering the Co–Cu alloy is porous. A separate series of experiments with Cu/Co–Ni–Cu/Cu sandwich showed that cobalt does not dissolve from these deposits because the addition of nickel stabilises cobalt in the Co–Ni–Cu alloy. [ABSTRACT FROM AUTHOR]

The use of a rotating ring disc electrode (RRDE) is introduced in this paper to study the formation of chromate–phosphate layers on rolled aluminium surfaces. A RRDE was developed containing an Al disc, which was converted with a chromate phosphate film, and a carbon ring, where the Cr6+ reduction rate was measured in situ. The principle of the method is that Cr6+ is transported from the bulk of the solution towards the disc and the ring electrode. At the disc electrode, a fraction of Cr6+ is consumed during the conversion of the Al disc electrode. This leads to a decrease in ring current, which is directly related to the amount of Cr6+ being reduced at the disc electrode. For proper operation of the RDDE system, an electrode with optimised geometrical characteristics is used. In order to be sure that the measured current on the ring is in relation with the Cr6+ reduction rate occurring at the aluminium disc, a number of requirements need to be fulfilled. After showing that the RDDE system is indeed able to monitor in situ the Cr6+ content during conversion, the amount of Cr6+ reduced at the disc electrode is calculated from the recorded ring current time curves. The method allows (i) to follow the growth of a conversion layer quantitatively in situ and (ii) to study the influence of important process parameters such as fluoride content, chromate content of the conversion bath and influence of mass transport. [Copyright &y& Elsevier]

The electrocatalytic activity of several materials for the nitrate reduction reaction was studied by cyclic voltammetry on a rotating ring disc electrode in solutions with different concentrations of sodium bicarbonate. Copper exhibited highest catalytic activity among the materials studied. Nitrate reduction on copper was characterized by two cathodic shoulders on the polarization curve in the potential region of the commencement of hydrogen evolution. In this potential range an anodic current response was observed on the Pt ring electrode identified as nitrite to nitrate oxidation. This indicates that nitrite is an intermediate product during nitrate reduction. These conclusions were verified by batch electrolysis using a plate electrode electrochemical cell. Copper and nickel, materials representing the opposite ends of the electrocatalytic activity spectra, were used in batch electrolysis testing. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR) was studied on a silver polycrystalline electrode in different NaOH concentrations with a help of rotating disc and rotating ring disc electrodes. Soluble reaction intermediate was detected at all alkaline concentrations, but its concentration increased with an increase of the level of impurities. ORR is not NaOH concentration dependent at low concentrations (0.1 and 1 M). In 11 M NaOH ORR onsets at more positive potentials in the region where underpotential silver oxide formation shows less reversible behaviour. The nonlinear frequency response analysis spectra show significant qualitative difference with NaOH concentration indicating a high capability of this method for kinetic mechanism investigations. [ABSTRACT FROM AUTHOR]

Hydrogen evolution reaction (HER) activities of self-assembled monolayers (SAMs) of [Mo3S7(S2CNMe2)3] and several other MoSx molecular clusters are presented on planer Au electrode. Our study suggests that such Mo-S clusters are unstable under HER reaction conditions of a strongly acidic electrolyte. The [Mo3S7(S2CNEt2)3]I monolayer prepared from DMF showed greater stability among all the studied precursors. The X-ray photoelectron spectroscopy (XPS) analysis on a monolayer of [Mo3S7(S2CNMe2)3]I in THF assembled on Au/ITO suggested sulfur-rich composition with S:Mo ratio of 2.278. The Mo-S monolayer clusters resulting from [Mo3S7(S2CNMe2)3]I in THF showed a Tafel slope of 75.74 mV dec−1 and required a lower overpotential of 410 mV to reach a high HER catalytic current density of 100 mA cm−2 compared to the other studied precursors. Surface coverage of the Mo-S clusters on the Au surface was confirmed by cyclic voltammetry (CV) curves from K3Fe(CN)6 and anodization of Au surface. Further, the rotating ring-disk electrode (RRDE) measurements were performed for the monolayer of [Mo3S7(S2CNMe2)3]I prepared in THF to study its reaction kinetics. The HER catalytic activity of such monolayer Mo-S clusters can further be improved by controlling the sulfur vacancy. [ABSTRACT FROM AUTHOR]