Electrochemical Reduction Of O2 Research Articles

New insights into the effects of alloying Pt with Ni on oxygen reduction reaction mechanisms in acid medium: a first-principles study.

The effects of alloying Pt with transition metal Ni on oxygen reduction reaction (ORR) mechanisms was investigated based on a systematic density functional theory (DFT) calculation explored in the present work. New insights into the ORR mechanisms were reported at the atomic level on Pt-segregated Pt3Ni(111). Only one molecular chemisorption state with the end-on OOH configuration was identified through geometry optimization and minimum energy path (MEP) analysis; top-bridge-top configuration as observed on pure Pt(111) was not identified on Pt-segregated Pt3Ni(111), indicating that alloying Pt with Ni influences the intermediates of ORR, and leads to only the dissociation mechanism of chemisorption state OOH species being involved in acid medium on Pt-segregated Pt3Ni(111). By contrast, the dissociation mechanisms of chemisorbed O2 molecule with top-bridge-top configuration and OOH species both were involved on pure Pt(111). The rds of the entire four-electron ORR was changed after Pt alloying with Ni. The rds of the entire ORR is the proton and electron transfer to O2 to form OOH on Pt-segregated Pt3Ni(111), whereas it is the reaction of O atom protonation to form OH species on pure Pt(111), indicating that sublayer Ni strongly influences the rds of ORR. The comparison of the ORR mechanisms explained that Pt3Ni alloy enhanced the ORR electrocatalytic activity more than pure Pt. The effect of electrode potential on ORR pathway on the pure Pt and Pt3Ni alloy was considered to obtain further insights into the electrochemical reduction of O2. Results showed that the proton and electron transfer becomes difficult at high potential. The ORR can easily proceed when the electrode potential lowers. For pure Pt- and Pt-based alloys, this phenomenon may imply the origin of the overpotential.

Low Temperature Electrochemical Reduction of NO and O2 on Pt Electrode Using YSZ, GDC and Lsgm Electrolyte

The pollutant NO mainly from diesel/gasoline engine exhausts without purification into environment can give rise to the acid rain and photochemical smog[1] which causes great potential safety hazard to human health and global environment. Accompanied by NO, there exists excess O2 in exhausts. Electrochemical reduction of NO and O2 can save the huge reducing agents storage system with the low operation temperatures simultaneously[2-3]. The exhaust temperature is low that high O2- conductivity is needed to guarantee high conversions of NO and O2 in the low temperature range. In this study, we fucused on the electrochemical reduction of NO and O2 in solid oxide electrolyte cell(SOEC) on symmetric Pt electrode with the temperature varying from 250℃ to 500℃ using three different electrolytes including YSZ, Ce0.9Gd0.1O1.95(GDC) and La0.9Sr0.1Ga0.8Mg0.2O2.85(LSGM). The GDC and LSGM electrolyte were prepared by tape-casting method and characterized by SEM after electrochemical tests. The symmetric cells were characterized by electrochemical measurements including linear sweep voltammetry(IV) and electrochemical impedance spectroscopy(EIS) at open circuit voltage(OCV). The polarization range was set from 2V to 0V containing 800ppm NO and 8% O2. The experiment testing temperatures and the concentrations of the NO and O2 are choosen according to the exhausts of the diesel or gasoline engine. The GDC and LSGM electrolyte showed higher electrochemical performance than YSZ by polarization measurements especially at low temperature of 250℃ to 350℃. The EIS experiment results were fitted well with equivalent circuit model R(C(RW)) containing the serial resistance Rs, electron transfer resistance Rt, double layer capacitance C and Warburg component W. It can be observed that the activation energy of the Rs and Rt were close to each other and the activation energy of the Rs were 0.581eV, 0.702eV and 0.719eV in GDC, LSGM and YSZ electrolytes. The EIS results yielded typical semi-infinite diffusive character in three electrolytes. The activation energy of the electron transfer process for the YSZ, GDC, and LSGM electrolytes were 1.057eV, 0.933eV and 1.031eV in 800ppm NO with 8% O2. And the YSZ electrolyte displayed the maxmium activation energy of electron transfer resistance Rt indicating the larger resistance of the charge transfer process. The gases in the Pt/YSZ/Pt cell showed the lowest diffusion coefficients obtained from the Warburg component in low frequency in the electrode resulted from largest charge transfer resistance that blocked the diffusion of the gases. This studies demonstrated that GDC and LSGM can be promising electrolytes for low temperature electrochemical removal of NO and provided theoretical guidance for the design of practical NOx elecrochemical application device at low temperatures.

Effects of Ba Adsorption Layer on NO Electrochemical Reduction Reaction Mechanism

This study investigated the electrochemical reduction of NO and O2 on Pt symmetric electrode by adding NOx adsorbents for the removal of NO from the exhausts of diesel engine or gasoline engine. O2 is excess when compared with NO. Thus, a large mount of external electric field should be consumed by electrochemical decomposition of O2 which gives rise to a low selectivity towards NO. Electrochemical reduction of NO has confronted with the difficult of improving NO selectivity in NO/O2 mixtures. Two cells were prepared and tested: single Pt electrode, Pt electrode with Ba(NO3)2 impregnation. Linear sweep voltammetry(0V~2V), cyclic voltammetry(CV:0.5V~-0.5V) and electrochemical impedance spectroscope(EIS) measurements were carried out between 350℃ and 550℃. The Pt electrode was covered with fine particles on the impregnated electrode surface by scanning electron microscope(SEM). It could be observed from the polarization curves that the Ba impregnated electrode showed higher electrochemical performance than the single Pt electrode especially at low temperatures and high voltages (1.25V to 2V). It can be obtained from the CV tests that the Ba impregnated electrode was superior to the Pt electrode in 800ppm NO. When scanned from 0.5V to -0.5V, the electrochemical performance of NO with O2 was far better than NO especially at high voltages and was close to the performance of O2. While changed the sweep direction backward, NO showed the best electrochemical performance than NO with O2 and O2 near open circuit voltage. EIS results at open circuit voltage revealed that the Ba impregnated electrode exhibited lower resistance than the single Pt electrode due to the decreased polarization resistance in the low-frequency region that dominated the impedance spectra in all atmospheres. The polarization resistances in 800ppm NO showed the smallest while in 8% O2 showed the largest in both electrodes possibly resulted from the generated NO2 formed by NO oxidation on Pt sites, the storage of NOx in the form of Ba(NO3)2 in active sites or the direct electrolysis of Ba(NO3)2 in external electric field that enhanced the reactivity and selectivity of NO reduction. From the impedance analysis using by equivalent circuit model R(CR)(CR)(CR) consisted of serial resistance Rs(related with the conductivity of the electrolyte) and 3 (CR) elements. Results showed that the activation energy of Rs for the two electrodes were nearly the same. The activation energy of the total polarization resistance Rp­ for the Ba impregnated electrode and Pt electrode were 0.874eV and 1.396eV respectively in 800ppm NO and 8% O2. The results were very close to the results obtained in the pure 8% O2 atmosphere. And the activation energy were 0.92eV and 1.50eV for the Ba impregnated electrode and Pt electrode. This was very close to the results of O2 reduction on Pt electrode conducted by Bauerle[1] with the activation energy between 1.43eV and 1.78eV demonstrated that the dissociative adsorption of O2 can be the controlling reaction step. The activation energy in the low frequency(0.01Hz-2Hz) for the three electrodes were close to the results showed by Bauerle resulted from the adsorption, surface diffusion, transfer of O2 and NOx intermediates near/at the triple phase boundary(TPB) and the dissociative adsorption of O2 [1]. The activation energy for the Ba impregnated electrode and Pt electrode in the high frequency were 0.608eV and 0.646eV respectively. The high frequency arc indicated the diffusion of oxide ions to the electrode/electrolyte interface and charge transfer of oxide ions from electrode/electrolyte interface to the electrolyte[2-3]. The activation energy in the middle frequency may be attributed to the charge transfer reactions in the electrode and the dissociative adsorption of O2. And the activation energy were 0.657eV and 1.06eV for the Ba impregnated electrode and Pt electrode. Results showed that the Ba impregnated electrode exhibited lower activation energy in middle and low frequency region indicating the improvement mechanism. Reference [1]J.E. Bauerle, J. Phys. Chem. Solids, 30, 2657 (1969). [2]M. J. Jørgensen, M. Mogensen, J. Electrochem. Soc., 148, A438 (2001). [3]M. L. Traulsenz, K. K. Hansen, J. Electrochem. Soc., 158, P156 (2011). Figure 1

Highly Efficient and Selective Two-Electron Reduction of Dioxygen Catalyzed By Cobalt Chlorin Complexes to Produce Hydrogen Peroxide

Hydrogen peroxide (H2O2) is an ideal energy carrier alternative to hydrogen, because H2O2 can be produced by photocatalytic oxidation of water by O2 using solar energy,1 and H2O2 can be used as a fuel in a one-compartment H2O2 fuel cell with low cost material.2-4 H2O2 can also be produced by electrochemical two-electron reduction of O2 using solar cells.5 Thus, it is highly desired to develop efficient catalysts for selective two-electron reduction of O2 to produce H2O2. We report herein that cobalt chlorin complexes in Chart 1 act as highly efficient and selective catalysts for two-electron reduction of O2 to H2O2. The detailed study based on the kinetics and thermodynamics have provided valuable insight into the development of the more efficient electrocatalyst for two-electron re-duction of O2 to H2O2, which is essential to realize H2O2-energy society. of O2 by 1,1’-dimethylferrocene (Me2Fc) efficiently and selectively to produce H2O2 in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K (eq 1). The rate of formation of Me2Fc+was 2Me2Fc + O2 + 2H+ --> 2Me2Fc+ + H2O2 (1) CoII(Ch n ) proportinal to concentrations of CoII(Ch n ) and O2, whereas the rate was independent of concentraton of Me2Fc. On the other hand, the catalytic rate increased with increasing concentraiton of HClO4 to reach a constant value. Such kinetic results indicate that the rate-determining step in the catalytic cycle (Scheme 1) is the proton-coupled electron transfer reduction of O2 by CoII(Ch n ), followed by fast electron transfer from Me2Fc to [CoII(Ch n )]+. The one-electron reduction potential of [CoIII(Ch 1 )]+ was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [CoIII(Ch 1 )]+.6 The introduction of electron-withdrawing groups on the chlorin ligand in CoII(Ch n ) (n = 2, 3) resulted in the positive shift of the redox potential for Co(III/II) from 0.37 V to 0.45 and 0.40 V, respectively. In the presence of HClO4, however, no positive shifts of the redox potential for [CoIII(Ch n )]+/CoII(Ch n ) (n = 2, 3) were observed. As a result, CoII(Ch 3 ), which has the lowest one-electron oxidation potential in the presence of HClO4 among the cobalt chlorin complexes in Chart 1, exhibited the highest catalytic reactivity for the two-electron recution of O2 by Me2Fc in the presence of HClO4in PhCN. The electrocatalytic two-electron reduction of O2 was also examined by using a CoIICh 1 -modified carbon paper electrode with multiwalled carbon nanotubes (MWCNTs) in an aqueous solution (pH = 1). A catalytic current of two-electron reduction of O2 was observed at a small overpotential. The electrocatalytic two-electron reduction of O2 was combined with the photocatalytic oxidation of water using semiconductor photocatalysts to produce H2O2 from H2O and O2using solar energy (eq 2). hν 2H2O + O2 --> 2H2O2 (2) catalysts References [1] Kato, S.; Jung, J.; Suenobu, T.; Fukuzumi, S. Energy Environ. Sci. 2013, 6, 3756. [2] Yamada, Y.; Yoshida, S.; Honda, T.; Fukuzumi, S. Energy Environ. Sci. 2011, 4, 2822. [3] Yamada, Y.; Yoneda, M.; Fukuzumi, S. Chem.–Eur. J. 2013, 19, 11733. [4] Yamada, Y.; Yoneda, M.; Fukuzumi, S. Inorg. Chem. 2014, 53, 1272. [5] Yamada, Y.; Fukunishi, Y.; Yamazaki, S.; Fukuzumi, S. Chem. Commun. 2010, 46, 7334. [6] Mase, K.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 2800. Figure 1

Factors Affecting the Rate of Electrocatalytic O2 Reduction By Iron Porphyrin Complexes

Electrochemical O2 reduction reaction (ORR) catalyzed by iron porphyrin complexes are investigated using surface enhanced resonance Raman spectroscopy coupled to rotating disc electrochemistry. The data suggests that the cleavage of O-O bond is the rate determining step (rds). Further the competitive hydrolysis of this rate determining FeIII-OOH species results in 2e/2H+ reduction of O2. Additional functionalities are included in the design of these iron porphyrin complexes to enhance the O-O cleavage rate and thus increase the rate of ORR. While inclusion of residues that allow facile PCET to the active site helps deter the 2e/2H+ pathway, introduction of the push effect of a thiolate axial ligand or the pull effect of protonated residues in the distal side is found to enhance the rate of ORR by 100-1000 times relative to an unfunctionalized porphyrin.

Concerted proton-electron transfer in electrocatalytic O2 reduction by iron porphyrin complexes: axial ligands tuning H/D isotope effect.

The electrochemical O2 reduction by thiolate- and imidazole-bound iron porphyrin complexes and H/D isotope effects on 4e(-) (determined by rotating disc electrochemistry) and 2e(-) (determined by rotating ring disc electrochemistry) O2 reduction rates are investigated. The results indicate that a thiolate axial ligand shows an H/D isotope effect greater than 18 and 47 for the 4e(-) and 2e(-) O2 reductions, respectively. Alternatively, an imidazole axial ligand results in H/D isotope effects of 1.04 and 4.7 for the 4e(-) and 2e(-) O2 reduction, respectively. The catalytic O2 reduction mechanism is investigated in situ with resonance Raman coupled with rotating disc electrochemistry. The data indicate that the rate-determining step changes from O-O bond heterolysis of Fe(III)-OOH species for a thiolate axial ligand to an O-O bond heterolysis of an Fe(II)-OOH for an imidazole axial ligand.

Enhanced photocatalytic activity of Cl-residual rutile TiO2 nanorods after targeted co-modification with phosphoric and boric acids.

The promotion of O2 adsorption on semiconductor surfaces for effectively capturing photogenerated electrons in the photocatalytic degradation of pollutants is highly desired. In this study, the targeted co-modification of residual chlorine rutile TiO2 nanorods with phosphoric and boric acids has been accomplished for the first time by simple wet chemical processes. The key to targeted co-modification is to connect -P-OH and -B-OH to the Cl-residual TiO2 surfaces by -Ti-OH and -Ti-Cl, respectively, consequently forming -Ti-O-P-OH and -Ti-Cl:B-OH ends. By means of the atmosphere-controlled surface photovoltage spectroscopy, the degrees for capturing photogenerated electrons by the adsorbed O2 as receptors on the resulting TiO2 nanorods are quantitatively analyzed. It is confirmed that the targeted co-modification could greatly promote the capture of the photogenerated electrons compared to the phosphate and borate modification alone. This is attributed to increased amounts of adsorbed O2 based on electrochemical O2 reduction and O2 temperature-programmed desorption measurements, further leading to the enhanced separation of photogenerated charges, characterized by an increase in the amount of produced hydroxyl radicals. This is responsible for the obviously enhanced photocatalytic activity of TiO2 nanorods towards the degradation of colorless gas-phase acetaldehyde and liquid-phase phenol. This work would provide us a feasible route for the co-modification with inorganic acids to synthesize efficient nanosized TiO2-based photocatalysts.

Electrochemical Characterization of Catalytic Activities of C60 Nanowhiskers to Oxygen Reduction in Aqueous Solution

We have investigated the catalytic function and activity of C60 nanowhiskers (C60NWs) on glassy carbon electrodes to the electrochemical O2 reduction in aqueous solution by cyclic voltammetry. It is found that C60NWs are active catalysts for O2 reduction in aqueous solution, although pristine C60 powders do not show any activities. The porous and conductive features of C60NWs play important roles in the electrochemical O2 reduction.

Core/shell Au/MnO nanoparticles prepared through controlled oxidation of AuMn as an electrocatalyst for sensitive H2O2 detection.

Monodisperse 5 nm AuMn nanoparticles were synthesized by hydride reduction of manganese acetylacetonate in the presence of Au nanoparticles. The alloy was formed through fast Mn diffusion into the Au structure. The AuMn nanoparticles were converted to Au-MnO composite particles through air annealing at 170 °C. These Au-MnO particles, especially the core/shell Au/MnO nanoparticles, were active for the electrochemical reduction of H2 O2 , with a detection limit reaching 8 nM. This highly sensitive electrochemical sensor based on the Au/MnO nanoparticles was used to monitor H2 O2 concentrations released from living cells, from which tumorigenic cells were discovered to release higher levels of H2 O2 than the non-tumorigenic cells.

Invited: the Aprotic Li-O2 Battery: O2 Reduction Mechanisms

While O2 reduction in aqueous environments has been studied extensively for many decades, O2 reduction in aprotic solvents has received much less attention. One spin-off of the recent interest in rechargeable Li-O2 batteries, Fig. 1,1,2 based on aprotic electrolytes is that it has highlighted the importance of understanding the fundamental mechanisms of O2 reduction in such a medium.3-10 Indeed understanding the mechanisms of O2 reduction are essential if progress is to be made towards realizing the potential of the rechargeable aprotic lithium-O2 battery.1,2 A number of authors have made important contributions to understanding O2 reduction in non-aqueous electrolytes.3-5,7 We have combined a range of electrochemical, spectroscopic and microscopy methods to investigate the mechanism of electrochemical O2 reduction in aprotic electrolytes. The results of these studies will be presented, along with the implications of the results for the future of rechargeable Li-O2 batteries.References(1) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Nat Materials 2012, 11, 19.(2) Li, F.; Zhang, T.; Zhou, H. Energy & Environmental Science 2013, 6, 1125.(3) Adams, B. D.; Radtke, C.; Black, R.; Trudeau, M. L.; Zaghib, K.; Nazar, L. F. Energy & Environmental Science 2013, 6, 1772.(4) Horstmann, B.; Gallant, B.; Mitchell, R.; Bessler, W. G.; Shao-Horn, Y.; Bazant, M. Z. The Journal of Physical Chemistry Letters 2013, 4, 4217.(5) McCloskey, B. D.; Scheffler, R.; Speidel, A.; Girishkumar, G.; Luntz, A. C. The Journal of Physical Chemistry C 2012, 116, 23897.(6) Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. The Journal of Physical Chemistry Letters 2013, 4, 1060.(7) Trahan, M. J.; Mukerjee, S.; Plichta, E. J.; Hendrickson, M. A.; Abraham, K. M. Journal of The Electrochemical Society 2013, 160, A259.(8) Peng, Z.; Freunberger, S. A.; Hardwick, L. J.; Chen, Y.; Giordani, V.; Barde, F.; Novak, P.; Graham, D.; Tarascon, J. M.; Bruce, P. G. Angewandte Chemie International Edition 2011, 50, 6351.(9) Sharon, D.; Etacheri, V.; Garsuch, A.; Afri, M.; Frimer, A. A.; Aurbach, D. The Journal of Physical Chemistry Letters 2012, 4, 127.(10) Zhai, D.; Wang, H.-H.; Yang, J.; Lau, K. C.; Li, K.; Curtiss, L. A.; Amine, K. Journal of the American Chemical Society 2013.

Physical Characteristics of New Potential Electrolytes for High Energy and Power Density Electrochemical Devices

Physical Characteristics of New Potential Electrolytes for High Energy and Power Density Electrochemical Devices

Enhanced Visible Activities of α-Fe2O3 by Coupling N-Doped Graphene and Mechanism Insight

In this work, we first have prepared graphene doped with different amounts of N through a one-pot ammonia-modified hydrothermal process and then successfully coupled them with nanocrystalline α-Fe2O3 by a common wet-chemical method. On the basis of the atmosphere-controlled surface photovoltage spectra, time-resolved surface photovoltage responses, and photoinduced hydroxyl radical amount measurements, it is confirmed that the photogenerated charge separation of α-Fe2O3 could be enhanced in N2 or in air atmosphere after coupling with a certain ratio of graphene. It is especially obvious with the graphene doped with a proper amount of nitrogen. This is responsible for the obviously improved visible activities of α-Fe2O3 for photoelectrochemical water oxidation to produce O2 and photocatalytic degradation of gas-phase acetaldehyde and liquid-phase phenol after coupling graphene doped with a proper amount of N species. It is suggested for the first time, mainly by means of N1s XPS data, electrochemical impedance spectra, O2 temperature-programmed desorption curves, surface acidity-related pyridine-adsorbed FT-IR spectra, and electrochemical O2 reduction measurements, that the increased amount of doped quaternary-type N would be quite favorable for photogenerated charge transfer and transportation and for O2 adsorption. As a result, photogenerated charge separation of the resulting N-doped graphene–Fe2O3 nanocomposite is greatly promoted. In addition, the enhanced O2 adsorption of α-Fe2O3 results mainly from the increased surface acidity after coupling with graphene, especially with quaternary-type N-doped graphene. This work would help us to better understand the important roles of doped N in graphene in the fabricated nanocomposites and also provide us with a feasible route to improve visible photocatalytic activities of α-Fe2O3 greatly.

Aprotic Li–O2 Battery: Influence of Complexing Agents on Oxygen Reduction in an Aprotic Solvent

Several problems arise at the O2 (positive) electrode in the Li-air battery, including solvent/electrode decomposition and electrode passivation by insulating Li2O2. Progress partially depends on exploring the basic electrochemistry of O2 reduction. Here we describe the effect of complexing-cations on the electrochemical reduction of O2 in DMSO in the presence and absence of a Li salt. The solubility of alkaline peroxides in DMSO is enhanced by the complexing-cations, consistent with their strong interaction with reduced O2. The complexing-cations also increase the rate of the 1-electron O2 reduction to O2*- by up to six-fold (k° = 2.4 ×10-3 to 1.5 × 10-2 cm s-1) whether or not Li+ ions are present. In the absence of Li+, the complexing-cations also promote the reduction of O2*- to O22-. In the presence of Li+ and complexing-cations, and despite the interaction of the reduced O2 with the latter, SERS confirms that the product is still Li2O2.

Solvent-Coupled Catalysis of the Oxygen Electrode Reactions in Lithium-Air Batteries

The electrocatalysis of oxygen reduction reactions (ORR) in non-aqueous electrolytes is coupled to the ability of the solvents to modulate the Lewis acidity of Li+. This is accomplished through chemical interactions of Li+ with the solvent to form acid-base complexes of the general formula, Li(solvent)n+, which determine the relative stability of the ORR intermediates and the final products formed. In high Donor Number solvents such as dimethyl sulfoxide (DMSO), the ORR proceeds via an outer Helmholtz plane (OHP) reaction pathway, conforming to a homogeneous catalysis of the reaction, irrespective of the presence of a catalyst in the cathode. In low Donor Number solvents exemplified by tetraethylene glycol dimethyl ether (TEGDME) and CH3CN, catalysts such as cobalt phthalocyanine (CoPC), Pt and Au promote heterogeneous electrocatalysis at the inner Helmholtz plane (IHP) of the electrical double layer on the electrode. The catalysis in this case involve the adsorption of O2 as well as the ORR intermediates on the catalyst surface leading to lower activation energy of the reactions and increases in the discharge voltages of Li-air cells compared to uncatalyzed cells. The heterogeneous catalysis at the IHP may promote the full electrochemical reduction of O2 to O2−.

Electrochemical reduction of dioxygen by copper complexes with pyridylalkylamine ligands dissolved in aqueous buffer solution: the relationship between activity and redox potential.

The Cu(II)/Cu(I) redox properties and electrochemical O2 reduction activity of a series of Cu(II)-complexes with pyridylalkylamine ligands were investigated in a neutral buffer solution. The relationship between Cu(II)/Cu(I) redox properties and O2 reduction activity was clearly demonstrated by voltammetric analyses.

In situ ESR spectro-electrochemical investigation of the superoxide anion radical during the electrochemical O2 reduction reaction in aprotic electrolyte

For the first time, electrochemically generated superoxide ions on a porous carbon electrode are investigated by means of in situ electrochemical ESR technique at ambient conditions. Superoxide ions (O2·−) are detected as the product of the electrochemical O2 reduction reaction. The redox couple (O2/O2·−) is reversible in DMSO electrolyte. The superoxide ions are believed to be absorbed and de-mobilized on the carbon surface. Overlapping a symmetric carbon ESR signal, the anisotropic signal of the superoxide is observed. The g-factor of the carbon ESR spectra is found to be 2.0025, which is almost identical to that of a free electron; Anisotropic g-factors for the O2·− are g⊥=2.0031, g//=2.0750.

Chemical Stability of CuWO4 for Photoelectrochemical Water Oxidation

Pure-phase CuWO4 photoanodes with 200 nm thickness were produced by spin-casting sol–gel precursors to evaluate their performance as photoelectrodes for water oxidation. The stability of CuWO4 in potassium phosphate (KPi) and potassium borate (KBi) buffers was evaluated as a function of pH and irradiance. CuWO4 photoanodes demonstrate higher stability at pH 3 and 5 in a 0.1 M KPi buffer and are significantly more stable over a 12 h period of illumination in a 0.1 M KBi buffer at pH 7 (∼75 μA/cm2 photocurrent at 1.23 V vs RHE (reversible hydrogen electrode) and 1 sun illumination) than in a 0.1 M KPi buffer at pH 7. The onset of photoelectrochemical water oxidation and electrochemical O2 reduction is dictated by Cu(3dx2–y2) states that reside at 0.4 V vs RHE, determined by linear sweep voltammetry. The onset for water oxidation is hindered by a large charge-transfer resistance, as high as 4.6 kΩ at 1 V vs RHE. Nevertheless, CuWO4 photoanodes show nearly quantitative faradic efficiency for water oxidation, ...

Ordered alloy formation for Pt3Fe/C, PtFe/C and Pt5.75Fe5.75Cuy/CO2-reduction electro-catalysts

Carbon supported Pt3Fe, PtFe and Pt5.75Fe5.75Cu catalysts are prepared via the impregnation of Fe and/or Cu-nitrate precursor salts onto a preformed nano-sized Pt/C catalyst. They are referred to as Pt3Fe/C, PtFe/C and Pt5.75Fe5.75Cu/C catalysts and the subscripts refer to the nominal Pt, Fe and Cu atomic ratios. The final catalysts are obtained by annealing the impregnated samples in a H2 enriched atmosphere. Pt5.75Fe6.75/C, Pt5.75Cu6.75/C and Pt5.75Fe5.75Cu2/C catalysts were also made. The as-prepared catalysts are characterized using X-ray diffraction, transmission electron microscopy, standard electrochemical methods and probed for the electrochemical O2 reduction reaction (orr). Rietveld analyses reveal that [Fm-3m] Pt, cubic [Pm-3m] Pt3Fe and tetragonal [P4/mmm] PtFe structure types are made. In case of the Pt5.75Fe5.75Cu/C catalyst, a nano-sized carbon-supported single-phase catalyst of [P4/mmm] structure is found to be made at 800°C, with Pt and Fe ordered on separate lattice planes, while Cu is distributed randomly onto both Pt and Fe atomic planes. The PtFe/C equivalent catalyst contains in addition a separate Fe phase as well as a slightly disordered [P4/mmm] phase. Pt5.75Cu6.75/C forms a disordered cubic [Fm-3m] alloy structure. The activity toward the orr was measured after 70 potential cycling, which can cause beneficial changes to the catalysts. In case of the specific orr activity, a dependence on the catalyst series is observed as follows: Pt5.75Fe5.75Cu/C [P4/mmm]>PtFe/C [P4/mmm]>Pt3Fe/C [Pm-3m]≈Pt5.75Cu6.75/C [Fm-3m]>Pt/C [Fm-3m]. The highest orr mass activities are also found for the Pt5.75Fe5.75Cu/C and subsequently for the PtFe/C catalysts. Estimated orr activities are well within the range reported in the literature. In fact the specific orr activities compare to results reported for so called de-alloyed catalyst that are among the highest orr mass activities reported in the recent literature.

Synthesis of rod and beadlike Co3O4 and bi-functional properties as air/oxygen electrode materials

In this paper, micrometer Co3O4 materials are synthesized via a method of chemical synthesis in combination with calcinations. The effect of calcination temperature on the morphology, structure and catalytic performance as air/oxygen electrode materials are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) spectroscopy and electrochemical techniques. The SEM observation reveals that the morphology of the Co3O4 products changes to a beadlike shape from the rod shape after calcinated at temperature of 700°C. The XRD measurements show that the both morphology Co3O4 products exhibit a cubic phase of Co3O4 spinel with a space group of Fd3m. But the beadlike Co3O4 product has better crystallinity. The electrochemical measurements show that the beadlike Co3O4 material has better active properties for electrochemical O2 reduction and O2 generation reactions. Also it exhibits the bi-functional performance in the oxygen-diffusion electrodes and has potential applications in rechargeable metal–air batteries and regenerative fuel cells.

Study of Direct Synthesis of Hydrogen Peroxide Acid Solutions at a Heat-Treated MnCl–Porphyrin/Activated Carbon Cathode from H2 and O2

Heat-treated MnCl–porphyrin supported on activated carbon (AC) catalyzed electrochemical reduction of O2 to H2O2 using an H2/O2 fuel cell reactor. Suitable preparation conditions for an Mn–porphyrin electrocatalyst were studied for the H2O2 formation, such as types of porphyrin rings, heat-treatment temperatures in He, and Mn loadings. The effects of reaction conditions, H2SO4 concentration, P(O2), P(H2), and electrolyte flow on the H2O2 formation were studied. Then, 0.5 wt % MnCl–TPP/AC heat treated at 823 K and 0.3 wt % MnCl–OEP/AC heat treated at 723 K showed good electrocatalytic activities for the H2O2 formation (TPP: 5,10,15,20-tetrakis(phenyl)-21H,23H-porphyrin, OEP: 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin). A maximum H2O2 concentration of 1.0 mol dm–3 with 47% current efficiency (CE) was obtained under suitable conditions. The heat-treated MnCl–TPP/AC and MnCl–OEP/AC electrocatalysts were characterized using temperature-programmed desorption, X-ray absorption near-edge structure spectroscopy, elemental analysis, cyclic voltammetry, and rotating ring-disk electrode voltammetry. Elimination of Cl from MnCl–TPP and MnCl–OEP supported on AC was essential to generate an active site for the H2O2 formation. A model of the Mn active site was proposed.