Reduction In Acid Solution Research Articles

Simultaneously Incorporating Atomically Dispersed Co‐Nx Sites with Graphitic Carbon Layer‐Wrapped Co9S8 Nanoparticles for Oxygen Reduction in Acidic Electrolyte

AbstractA facile yet robust synthesis is reported herein to simultaneously incorporate atomically dispersed Co‐Nx sites with graphitic layer‐protected Co9S8 nanoparticles (denoted as Co SACs+Co9S8) as an efficient electrocatalyst for oxygen reduction in acidic solution. The Co SACs+Co9S8 catalyst shows low H2O2 selectivity (∼5 %) with high half‐wave potential (E1/2) of ∼0.78 VRHE in 0.5 M H2SO4. The atomic sites of the catalyst were quantified by a nitrite stripping method and the corresponding site density of the catalyst is calculated to be 3.2×1018 sites g−1. Besides, we also found the presence of a reasonable amount of Co9S8 nanoparticles is beneficial for the oxygen electrocatalysis. Finally, the catalyst was assembled into a membrane electrode assembly (MEA) for evaluating its performance under more practical conditions in proton exchange membrane fuel cell (PEMFC) system.

Quantitative Understanding of Cation Effects on the Electrochemical Reduction of CO2 and H+ in Acidic Solution

Conducting electrochemical reduction of CO2 in acidic media can effectively increase the utilization efficiency of CO2. Alkali cations (M+) are indispensable for CO2 reduction in acidic media, while the high concentration of M+ results in bicarbonate precipitation in a gas diffusion electrode. To develop selective and sustainable CO2 reduction techniques in acidic media, quantitative understandings of cation effects on reduction rates of CO2 and H+ are demanded. Previous study shows that M+ in acidic media can modulate the electric field distribution in a double layer, which suppresses H+ migration and stabilizes the intermediate of CO2 reduction. In this work, we conducted a more quantitative study through the combination of electrochemical experiments and generalized modified Poisson–Nernst–Planck (GMPNP) simulations. When the concentration of M+ is higher than that of H+, the migration of H+ is substantially suppressed. The diffusion rate of H+ is also influenced by the concentration of M+. Furthermore, the concentration and identity of M+ affect the electric field within the Stern layer, which is the driving force of the electron transfer from the cathode to CO2. Only in M+-containing solutions, the electric field strength within the Stern layer increases as the potential moves negatively, and CO2 reduction can be accelerated by applying a larger overpotential. These aspects together determine the selectivity of CO2 reduction in acidic solution.

One-step in situ growth of high-density POMOFs films on carbon cloth for the electrochemical detection of bromate

Two electroactive polyoxometalates (POMs)-based metal-organic frameworks (MOFs) films (POMOFs) in situ grown on carbon cloth (NENU-3/CC and NENU-5/CC) were synthesized by a facile one-step solvothermal method containing Cu(NO3)2, 1,3,5-benzentricarboxylate (H3BTC) and POMs (PMo12 and PW12), in which the POMs were wrapped into MOFs to increase stability. The analysis of electrochemical behavior of the NENU-3/CC and NENU-5/CC films reveals that they can serve as conductive electrode materials. Subsequently, two kinds of film electrodes exhibit enhanced electrocatalytic activity toward bromate reduction in acidic solution. Thus, they can act as electrochemical sensors to detect bromate. Amperometric detection results of NENU-3/CC to bromate show a high sensitivity of 45.11 μA cm −2 mM−1 and a low detection limit of 0.55 μM. Furthermore, NENU-5/CC can exhibit a detection limit of 1.18 μM, and a high sensitivity of 18.83 μA cm−2 mM−1. Experimental results show that two POMOFs films have high electrochemical stability as working electrode materials and good selectivity for detecting bromate as sensor materials. Novelty statementUp to now, electrochemical methods to detect bromated have attracted considerable attention due to their simplicity, rapidity, high sensitivity, excellent selectivity and environmentally friendliness with low-cost equipment compared with other techniques. Polyoxometalates (POMs) have been widely used for the electrocatalytic reduction of bromate because of their high redox properties. However, when POMs are used as conductive electrodes materials, they dissolve easily in the electrolyte, and the instability of POMs can increase performance degradation. For solving the above-mentioned problem, POMs are encapsulated into MOFs to generate POM-based MOFs (POMOFs), it combine the merits of POMs and MOFs and increase the stability of skeleton.However, POMOFs materials can increase the reversible electrochemical capacity, the leaching and dispersity of POMOFs in the electrocatalytic measurement can make them unstable, and it hinders the conductivity in electrochemical behaviors. Thus, Proper POMOFs films have been developed to overcome the aforementioned difficulties by packing POMOFs on the conductive substrates uniformly to increase the stability and performance in electrochemical catalysis.In this paper, two electroactive polyoxometalates (POMs)-based metal-organic frameworks (MOFs) films (NENU-3 and NENU-5) were chosen as target materials in the synthesis of POMOFs film. And they were located on the surface of the carbon cloth through a facile in situ one-step solvothermal method as conductive electrode materials. According to report, NENU-3 and NENU-5 were coated on the substrate using hot-pressing and electrochemical method, we firstly prepared NENU-3/CC and NENU-5/CC films by in situ one-step solvothermal method, and the as-synthesized POMOFs materials using this method are more uniform, facile, and stable than other methods. The results of amperometric detection of NENU-3/CC to bromate show a wide linear range of 5–560 μM with a high sensitivity of 45.11 μA cm−2 mM−1 and a low detection limit of 0.55 μM. Furthermore, NENU-5/CC can exhibit a good linearity range of 15–380 μM, a detection limit of 1.18 μM, and a high sensitivity of 18.83 μA cm−2 mM−1. As a result, the amperometric determination demonstrates that two as-synthesized POMOFs/CC films as working electrode materials exhibit good sensitivity, selectivity, and stability to detect bromate as sensor materials.

(Invited) Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) recombination. In order to circumvent these limitations considerable research efforts have been directed at creating more complex architectures involving e.g. tandem cells, nanostructures, and buried junctions to correctly position the bands and provide enough driving force to surmount overpotentials and the 1.23 eV energetic barrier of the water splitting reactions. Therefore, developing novel III-V photoabsorbers with adequate band energetics is key to conceiving simpler, yet efficient single semiconductor PEC cells for production of solar fuels at a competitive cost. First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb narrows the band gap in Ga(Sbx)N1-x and changes the electronic band gap from indirect to direct in GaSbP. Theoretical computations predict that, with band gaps in the order of 2 eV, these materials straddle the potential window for water oxidation and proton reduction in acidic solution. Single crystalline films of these two materials have been deposited by halide/hydride vapor phase epitaxy and metalorganic chemical vapor deposition systems, in a wide range of Sb incorporation without phase segregation. Experimental results corroborate the significant band gap reduction in GaSbN from 3.4 to 1.5 eV(Fig 1a) and suggest the conversion of excitonic transitions in the electronic band structure of GaSbP (Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data and low temperature photoluminescence spectroscopy. Electrodes comprising Ga(Sbx)N1-x and Ga(Sbx)P1-x have been benchmarked employing 2- and 3-electrode standard methods for assessment of their characteristic attributes, i.e. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situ fluorescence probing. This presentation will highlight recent advances in the understanding of the inter-relationship of processing/synthesis, material structure and photoelectrochemical properties of this new class of materials. Also, preliminary data on un-assisted photoactivity involving p-Si cathode and n-type GaSbP anode will be presented. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909). Collaborations with Dr. Todd Deutsch & Dr. James Young of NREL, and Dr. Madhu Menon of Univ. of Kentucky are acknowledged. Also acknowledge contributions of prior PhD students (Alejandro Garcia and Swathi Sunkara).

Electrochemical detection of chlorate on a novel nano-Au/TiO2NT electrode

The present work describes the synthesis, characterization and application of Au nanoparticles modified titanium nanotube electrode (Nano-Au/TiO2NT) as a novel and sensitive electrode material for electrochemical sensing of chlorate. For this purpose, highly ordered TiO2 nanotube (TiO2NT) electrode was synthesized by anodization of metallic titanium foil in ethylene glycol-water solution. Nano-Au/TiO2NT was obtained by drop casting a solution of homogeneous and fine structures of gold nanoparticles, which were prepared from HAuCl4 using a new simple photochemical process. Structural and elemental analysis characteristics of the resulting Nano-Au/TiO2NT electrode were investigated by scanning electron microscopy and X-ray diffraction analysis. The electrocchemical activity of resulted Nano-Au/TiO2NT electrode towards chlorate reduction in acidic solution was confirmed by cyclic voltammetry and chronoamperometry methods An enhancement of amperometric current in the presence of chlorate was afforded to chlorate detection with a low detection limit of 12μM and linear range of 50μM to 4.8mM.

Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) recombination. In order to circumvent these limitations considerable research efforts have been directed at creating more complex architectures involving e.g. tandem cells, nanostructures, and buried junctions to correctly position the bands and provide enough driving force to surmount overpotentials and the 1.23 eV energetic barrier of the water splitting reactions. Therefore, developing novel III-V photoabsorbers with adequate band energetics is key to conceiving simpler, yet efficient single semiconductor PEC cells for production of solar fuels at a competitive cost. First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb narrows the band gap in Ga(Sbx)N1-x and changes the electronic band gap from indirect to direct in GaSbP. Theoretical computations predict that, with band gaps in the order of 2 eV, these materials straddle the potential window for water oxidation and proton reduction in acidic solution. Single crystalline films of these two materials have been deposited by metal organic chemical vapor deposition and halide vapor phase epitaxy, in a wide range of Sb incorporation without phase segregation. Experimental results corroborate the significant band gap reduction in GaSbN from 3.4 to 1.5 eV(Fig 1a) and suggest the reduction of the indirect band gap of GaP when Sb is incorporated(Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data, low temperature photoluminescence spectroscopy, and photocurrent spectroscopy. Electrodes comprising Ga(Sbx)N1-x and Ga(Sbx)P1-x have been benchmarked employing 2- and 3-electrode standard methods for assessment of their characteristic attributes, i.e. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situ fluorescence probing. New insights into these materials regarding the donor concentration in GaP alloys, and the effect of incorporation help in fundamentally understanding the thermodynamics at a solid liquid electrolyte interface and work function of GaSbP to realize the full potential of these novel III-V alloys. This poster will highlight recent advances in the understanding of the inter-relationship of processing/synthesis, material structure and photoelectrochemical properties of this new class of materials. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).

(Invited) Novel Band-Gap Engineered III-V Alloys for Unassisted Water Photoelectrolysis

Photoelectrochemical water splitting cells comprising the alloys of III-V semiconductors hold record solar-to-hydrogen efficiencies. However, the performance of state-of-the-art III-V alloy-based single absorber devices, particularly of InGaP2, InGaAs, GaPN, and GaAsN, comes short of unassisted water splitting due to a) inadequate alignment of band edges with respect to the HER and OER redox potentials, b) insufficient photovoltage to favor charge separation at the solid electrolyte liquid junction, and/or iii) recombination. In order to circumvent these limitations considerable research efforts have been directed at creating more complex architectures involving e.g. tandem cells, nanostructures, and buried junctions to correctly position the bands and provide enough driving force to surmount overpotentials and the 1.23 eV energetic barrier of the water splitting reactions. Therefore, developing novel III-V photoabsorbers with adequate band energetics is key to conceiving simpler, yet efficient single semiconductor PEC cells for production of solar fuels at a competitive cost. First-principles DFT+U calculations incorporating the local density approximation and generalized gradient approximation have shown that incorporation of Sb narrows the band gap in Ga(Sbx)N1-xand changes the electronic band gap from indirect to direct in GaSbP. Theoretical computations predict that, with band gaps in the order of 2 eV, these materials straddle the potential window for water oxidation and proton reduction in acidic solution. Single crystalline films of these two materials have been deposited by metal organic chemical vapor deposition and halide vapor phase epitaxy, in a wide range of Sb incorporation without phase segregation. Experimental results corroborate the significant band gap reduction in GaSbN from 3.4 to 1.5 eV (Fig 1a) and suggest the conversion of excitonic transitions in the electronic band structure of GaSbP (Fig 1b) as determined by Tauc plot analysis of diffuse reflectance data and low temperature photoluminescence spectroscopy. Electrodes comprising Ga(Sbx)N1-x and Ga(Sbx)P1-x have been benchmarked employing 2- and 3-electrode standard methods for assessment of their characteristic attributes, i.e. flat-band and onset potentials, photovoltage, zero-bias photocurrent density, fill factors, carrier concentrations, and most importantly, their ability for gas evolution by in situfluorescence probing. New insights into these materials raise questions regarding the native conductivity type of GaP alloys and the effect of incorporation or defects in the extent of band bending at a solid liquid electrolyte interface and work function of GaSbN. This presentation will highlight recent advances in the understanding of the inter-relationship of processing/synthesis, material structure and photoelectrochemical properties of this new class of materials. Acknowledgements: Financial support from US Department of Energy (DE-FG02-07ER46375) and NSF (DMS1125909).

Electrochemical Nitrate Reduction at Sn-Modified Single Crystalline Metal Electrodes

Sn-modified Pt, Pd and PtPd alloy electrodes have been known as the most active electrode for the electrochemical reduction of nitrate, which becomes abandant in ground water and industrial drain water. However, these electrodes were usually polycrystalline and the research at well-defined electrode surfaces were required to understand the reaction mechanism and the nature of reaction sites. In the present study, we prepared Sn-modified Pt(111) and Pd(111) electrodes to investigate the nitrate reduction in acidic solution. Pd-doped Pt(111) electrodes are also prepared. The coverage dependences of Sn at single crystalline electrodes on the activity toward nitrate reduction have been estimated by CV in detail. For Pt(111) and Pd(111), the oxidation induced alloying between Sn and the substrate, which was confirmed by XPS measurements, caused enhancement in the electrochemical activity toward nitrate reduction.

Nicotinamide-assisted fabrication of high-stability gold-palladium nanoparticles on carbon fiber cloth for hydrogen peroxide electroreduction

Bimetallic nanoparticles with excellent activity and stability have been widely studied as H2O2 electroreduction catalysts. Hereby, we have successfully found a green fabrication of high-stability Au-Pd nanoparticles directly supported on carbon fiber cloth (Au-Pd NPs/CFC) by using tobacco extract (nicotinamide) as a simple and ecofriendly reducing agent. Transmission electron microscopy, scanning electron microscopy, X-ray diffraction analyses and high angle annular dark field scanning TEM are performed to characterize the morphology, composition and structure of the as-prepared electrode. Results show that alloy nanoparticles prepared with different mole ratios of HAuCl4/PdCl2 mixtures in the wide range of 10–100nm have a uniform distribution on every carbon fiber, presenting a regular circle shape. The 3D electrode is directly applied as the electrocatalyst for H2O2 reduction in acid solution and does not introduce into any polymer binders. The catalytic performance is investigated by chronoamperometry and voltammetry, exhibiting desired catalytic performance and improved utilization than other representative nanomaterials recently reported. Furthermore, Au-Pd NPs do not aggregate after the whole test due to the effect of in-situ immobilization, showing a great significance for the application in fuel cells. A direct peroxide-peroxide fuel cell (DPPFC) using Au-Pd NPs/CFC cathode with Ni/Ni-foam anode acquires a maximum power density of 22.8mWcm−2. Electrochemical stability test through long-term potential cycles (>9h) further confirms the high durability of the electrode.

Carbon fiber cloth supported Au nano-textile fabrics as an efficient catalyst for hydrogen peroxide electroreduction in acid medium

The size-controlled hierarchical textile-like Au nanostructures supported carbon fiber cloth (Au NTs/CFC) is successfully fabricated through a simple low-cost electrochemical route. The electrodes are characterised by scanning electron microscopy equipped with an energy dispersive X-ray spectrometer, transmission electron microscopy and X-ray diffractometer. Without any conducting carbons and polymer binders, the 3D electrode with unique structure is directly used as the electrocatalyst for H2O2 reduction in acid solution and the catalytic performance is evaluated by voltammetry and chronoamperometry. The Au NTs/CFC electrode exhibits much higher catalytic activity and remarkably improved utilization of Au than Au nanoparticles (Au NPs/CFC) prepared by the same method owing to its unique structure. In the solution of 3.0 mol L−1 H2SO4 + 0.1 mol L−1 H2O2, with the reduction potential of 0 V, the current of −0.72 A cm−2 mg−1 can be obtained on Au NTs/CFC electrode and only a current of −0.09 A cm−2 mg−1 can be achieved on Au NPs/CFC electrode. All these results reveal that the novel Au NTs/CFC electrode exhibits excellent catalytic performance and superior stability for H2O2 electroreduction in acid medium, benefitting from the unique 3D structure which can ensure high utilization of catalyst.

Novel Pt catalyst on ruthenium doped TiO2 support for oxygen reduction reaction

Ruthenium doped titanium oxide support was synthesized. The support was characterized by BET (Brunauer, Emmett, Teller) and X-ray diffraction techniques (XRD). Determined specific surface area was 41m2g−1. XRD revealed presence mainly TiO2 anatase phase and some peaks belonging to rutile phase. No Ru compounds have been detected.Platinum based catalyst on this support was prepared by borohydride reduction method. The catalyst was characterized by scanning transmission electron microscopy (STEM, HAADF) and electron energy loss spectroscopy (EELS). Homogenous Pt particle distribution over the support, with average Pt nanoparticle diameter of 3nm was found. This novel catalyst was tested for oxygen reduction in acid solution. It exhibited remarkable higher catalytic activity in comparison with Pt/C, as well as with Pt nanocatalysts at titanium oxide based supports, reported in literature.

Effect of tungsten carbide in carbon Pt catalyst support on electrochemical oxygen reduction in acid solution

Tungsten carbide was synthesized by calcination of carbon cryogel with embedded tungsten in a form of metatungstate. This material was used as a support for Pt nanoparticles. XRD pattern of W-C support indicates the presence of WC, W2C, and unreacted W, as well as graphitized carbon. According to the previous TEM analysis of W-C support, it contains particles with core-shell structure, where W particle was covered with the shell of a mixture of WC and W2C. The average Pt grain size calculated from XRD pattern was about 6 nm. Cyclic voltammogram of W-C support was recorded within potential range relevant for its application as a catalyst support in fuel cells. Pair of anodic/cathodic peaks close to the negative potential limit could be ascribed to the intercalation of hydrogen within hydrous tungsten oxide, which is always present on the surface of WC in aqueous solutions. Cyclic voltammogram of Pt/W-C indicated that tungsten oxide species are present on tungsten carbide shell as well as on the surface of Pt nanoparticles. Pt surface is only partially covered by hydrous tungsten oxide. Hydrogen intercalation in hydrous tungsten oxide is enhanced in the presence of Pt nanoparticles. Also, the presence of hydrated tungsten oxide leads to the decrease of OH chemisorbed on Pt surface. Stripping of underpotentially deposited copper was used for the assessment of Pt surface area and the specific surface area of Pt was estimated to 41 m2 g-1. Electrochemical oxygen reduction reaction was examined on the synthesized Pt/W-C catalyst and compared with the results on the commercial Pt/C catalyst. It was found that the current densities at Pt/W-C are almost double as those on Pt/C. The Tafel plots for both catalysts are characterized with two Tafel slopes: -0.060 V dec-1 at low current densities, and -0.120 V dec-1 at high current densities. From the rotational dependence of the reaction rate, it was found that oxygen reduction on both Pt/W-C and Pt/C follows the first order kinetics with respect to O2 and that four electrons are transferred per O2 molecule. The results show that the presence of tungsten carbide in support material i.e. hydrous tungsten oxide on Pt surface, leads to promotion of oxygen reduction on the Pt/W-C catalyst. It was assumed that oxophilic hydrated tungsten oxide hinders OH adsorption on Pt surface, thus increasing Pt surface area available for O2 adsorption and its electrochemical reduction.

Electrochemical behaviour of carbon supported Pt electrocatalysts for H2O2 reduction

Carbon supported bimetallic Pt-alloys (Pt0.75M0.25/C, with M = Ni or Co) are investigated as novel electrode materials for H2O2 reduction in acid solution. The alloy electrocatalysts, Pt0.75Ni0.25/C and Pt0.75Co0.25/C, as well as carbon supported Pt (Pt/C) are characterised using cyclic voltammetry. The electrocatalytic activity of the materials is studied using a rotating disc electrode system with a combination of linear scan voltammetry and chronoamperometry. It is found that the activity of Pt0.75M0.25/C electrocatalysts for H2O2 reduction is comparable to the activity of Pt/C electrocatalyst, with Pt0.75Co0.25/C exhibiting the best performance.

Effect of purification of carbon nanotubes on their electrocatalytic properties for oxygen reduction in acid solution

The oxygen reduction reaction has been investigated on acid-treated single-walled (SWCNT) and multi-walled carbon nanotubes (MWCNT) modified glassy carbon (GC) electrodes in acid media using the rotating disk electrode (RDE) method. Different acids were used for the carbon nanotube (CNT) purification. A systematic study was carried out to elucidate whether the metal catalyst impurities of CNTs play a role in the electroreduction of oxygen on the CNT modified GC electrodes. The surface morphology of the carbon nanotube samples was examined by transmission electron microscopy and the concentration of metal catalysts in the CNT materials was determined by energy dispersive X-ray spectroscopy. The acid-treated MWCNTs were also characterised by Raman and X-ray photoelectron spectroscopies. Aqueous suspensions of SWCNTs and MWCNTs used for GC surface modification were prepared in the presence of Nafion. The RDE results indicated that the acid-treated CNT modified GC electrodes are less active catalysts for oxygen reduction than as-received CNTs which could be explained by the absence of metal catalysts on the surface of purified CNTs.

Nitrogen-doped ordered mesoporous carbons as electrocatalysts for methanol-tolerant oxygen reduction in acid solution

A simple method is described for the production of Pt supported on nitrogen-doped ordered mesoporous carbons (Pt-OMCN x) by the pyrolysis of carbon and metal precursors in a N-containing organo-functionalized mesoporous silica as the primary N sources and hard templates. The resultant Pt-OMCN x samples were thoroughly characterized by a variety of different spectroscopic and analytical techniques. Results obtained from X-ray diffraction, N 2 adsorption/desorption isotherms and transmission electron microscopy show that Pt-OMCN x possessed well-dispersed Pt nanoparticles (2–3 nm) supported on high surface area (450 m 2 g −1) and regular pore channels (2.5 nm). Elemental analysis and X-ray photoelectron spectroscopy indicate that ca. 1.8 wt% of N species presented on the carbon rods of Pt-OMCN x were mostly quaternary-N (55%) and pyridinic-N (45%). The Pt-OMCN x was found to have superior electrocatalytic activity and the tolerance to methanol crossover during the oxygen reduction reaction (ORR), which can be attributed to the dispersion and unique nanostructure of Pt nanoparticles particularly in the presence of pyridinic-N atoms in the mesoporous carbon supports.

Effects of poly-1,5-diaminoanthraquinone morphology on oxygen reduction in acidic solution

The poly-1,5-diaminoanthraquinone (P15DAAQ) modified Pt electrodes show electrocatalytic activity for oxygen reduction reaction (ORR) with oxygen reduction peak at about 0.39 V in 0.1 M H 2SO 4. The P15DAAQ with different thickness has different morphology. The effects of morphologies on the electrocatalytic behaviors of P15DAAQ for oxygen reduction reaction are investigated using cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) measurements. We propose two different O 2-transport processes on electrodes modified with thin P15DAAQ and thick P15DAAQ. Together with the quantitative analysis with O 2-transport dynamics, electron-transfer resistance, and catalytic reaction rate during ORR, thin P15DAAQ electrode performs better electrocatalysis for ORR, although thick P15DAAQ provides higher real surface area and more reactive sites which is beneficial for ORR within a short time.

Soil Chemistry Effect on Feasibility of Cr-decontamination by Acid-Washing

Soil washing with sirnple acid has been proven to be effective for removal of cationic heavy metals from contaminated soils. Since the adsorption of anionic heavy metals is enhanced in acidic medium, the efrrciency of acid-washing may not be guaranteed for soils that are doubly contaminated with cationic and anionic heavy metals. To evaluate the efficiency of acid-washing, nine soils were artificially contaminated with chromate and chromium was extracted with hydrochloric acid of 0.5mrnol L-* to I mol L-'. A part of spiked chromate was reduced to trivalent chromium and the remaining chromate ions were almost quantitatively extracted wlth the hydrochloric acid. However, increasing hydrochloric acid concentration was not effective for the extraction. Possible mechanisms underlying the observed low efficiency were considered to be the enhanced reduction in acidic solution, complexation by humic substance and adsorption via cation exchange reactions. We concluded that acid-washing satisfactorily works for chromate removal in non-allophanic soils but inefficient for removal of trivalent chromium particularly from soils having high effective cation exchange capacity and organic matter content.

Nitrite reduction in acidic solution at a GC/Eastman-AQ-Os(bpy) 32+-PVP composite modified electrode

The negatively charged polymer polyester sulfonic acid (Eastman-AQ, abbreviated to AQ), positively charged polymer polyvinyl pyridine (PVP) and mediator Os(bpy) 3 2+ were used to construct composite modified glassy carbon (GC) electrodes (abbreviated to GC/AQ-Os(bpy) 3 2+-PVP). The reduction of NO 2 − in acidic solution was taken as a model reaction to explore the properties of the modified electrode. On the steady state polarization curve of NO 2 − reduction there were two well-developed waves with much enhanced plateau current densities and positively shifted half-wave potentials compared with bare GC electrodes. In 0.05 mol/l H 2SO 4 + 5 mmol/l NO 2 − the modified electrode exhibited plateau current densities of 723 and 1153 μA cm −2 and half-wave potential shifts of 0.29 and 0.56 V for the first and second current wave, respectively, showing promising potential for nitrite detection. The catalytic activity for NO 2 − reduction did not decrease appreciably over 4 months. A number of relevant kinetic and thermodynamic parameters are estimated experimentally. NO 2 − reduction at the composite modified electrodes is suggested to follow the SR mechanism (pure kinetic conditions involving mutual compensation between a catalytic reaction and substrate diffusion in the film in addition to diffusion in the solution phase) according to the Savéant–Andrieux theory.

Electrochemical reduction of p-nitrothiophenol-self-assembled monolayer films on Au( [formula omitted]) surface and coadsorption of anions and water molecules

Electrochemical reduction of p-nitrothiophenol (PNTP)-self-assembled monolayer (SAM) films on Au(1 1 1) surface was studied using attenuated total reflection-IR spectroscopy. The reduction potential was evaluated from the spectral changes, which depended on pH in electrolyte solutions, e.g. +0.05 V (pH 0.45), −0.55 V (pH 5.5) or −0.55 V (pH 13) (vs. Ag/AgCl) in accordance with voltammetric results. We found that the IR absorption bands of anions and water increases upon reduction in acidic solution at pH<p K a of p-aminothiophenol (PATP), while those for hydroxide anions decreases in alkaline solution. The observations result from the electrostatic interaction of these anions with partial charge at the PNTP- and PATP-SAM films on Au(1 1 1) surface. It suggests that the pKa of the PNTP-SAM film on the Au electrode can be evaluated from the IR spectral changes with the potential. Pronounced IR background shift observed during the PNTP reduction was discussed based on the superimposed changes from molecular structure, orientation of the SAM film and coadsorption of the anions.

Electrochemical Behavior of Highly Conductive Boron‐Doped Diamond Electrodes for Oxygen Reduction in Acid Solution

The electrochemical behavior of highly conductive, boron‐doped polycrystalline diamond thin films for oxygen reduction was examined in 0.5 M using linear sweep voltammetry. When the potential sweep is confined to the region negative of 0.0 V vs. Ag/AgCl, oxygen reduction is highly inhibited with cathodic current being observed at ∼ −0.6 V vs. Ag/AgCl, as compared with the standard potential for the two‐electron reduction of oxygen vs. Ag/AgCl at pH 0). The extreme inhibition of oxygen reduction may be due to an absence of catalytic sites. When the potential is swept to potentials positive of + 1.4 V vs. Ag/AgCl, the subsequent sweep into the negative region shows a reduction peak due to oxygen reduction. In this case, catalytic oxygen‐containing functional groups can be formed on carbon impurities. Relatively mild conditions are required to deactivate the catalytic functional groups, but strong oxidative treatment in base appears to substantially remove the carbon impurities. The oxygen reduction behavior in acid solution could be useful in characterizing diamond electrodes, i.e., as a diagnostic for the presence of carbon on chemical‐vapor‐deposited diamond thin‐film electrodes. It is proposed that diamond electrode surfaces free of carbon are highly insensitive to oxygen, which could be a useful feature in electroanalysis. © 1999 The Electrochemical Society. All rights reserved.