Oxygen Reduction Current Density Research Articles

Effects of Oxygen Defects in Zr Oxide-Based Thin Films on Oxygen Reduction Reaction in Acidic Media

The use of carbon-supported platinum as cathode catalysts of polymer electrolyte fuel cells has some problems such as high cost and low amount of resources of platinum. Therefore, we have attempted to develop substitute of platinum based on group 4 metal oxides. In the case of Zr oxide-based powder catalysts, we found that zirconium oxide-based compounds with oxygen defects and deposited carbon derived from organometallic precursors had definite activity of oxygen reduction reaction (ORR) [1][2]. However, the ORR activities of these catalysts still remain low. In order to increase the ORR activity drastically, it is necessary to clarify the catalyst design. This requires an understanding of the active sites and the electron conduction path separately, which were not separated in powder catalysts. On the other hand, Sugino et al. investigated the effect of oxygen vacancies and nitrogen doping on the active site based on first-principles calculations [3]. Defects by oxygen vacancies and nitrogen dopants were introduced into the tetragonal ZrO2(101) slab. It was found that the energy barrier of the rate-determining step, or the removal of adsorbed OH, was comparable to (or only slightly lower than) that of the pristine surfaces, indicating that defects play only a minor role in enhancing the ORR activity. In other words, it was suggested that oxygen vacancies and nitrogen dopants contributed to the emergence of ORR activity by enhancing the electrical conductivity of ZrO2 rather than by forming high quality active sites. Simple models are suitable for experimentally confirming the results of theoretical calculations and for understanding the active sites and the electron conduction paths separately. Therefore, we attempted to prepare nitrogen doped ZrOx nanofilms by RF magnetron sputtering method with different targets and atmospheres and to investigate their potential as model electrodes.ZrOx thin films of ca. 3-5 nm thickness were prepared by RF magnetron sputtering method using glassy carbon (GC) plates as substrates and ZrN or Zr metal plates as targets under Ar, Ar+O2 and Ar+N2. The sputtering power, the deposition time, and the substrate heating temperature were 100 W, 10 min, and 300 ℃, respectively. The flow rates of Ar, O2, and N2 gas were controlled to keep the total chamber pressure at approximately 0.6 Pa. When the gases were mixed, the partial pressure ratio was set to 1:1. Electrode prepared with a ZrN target in Ar was denoted as "ZrN_Ar", and electrodes prepared with a Zr target in Ar:O2 and Ar:N2 were denoted as "Zr_Ar+N2" and "Zr_Ar+O2", respectively. A conventional three-electrode cell was used for electrochemical measurements with Reversible Hydrogen Electrode (RHE) as a reference and a GC plate as a counter electrode in 0.5 M sulfuric acid at 30 ℃. The oxygen reduction current density i ORR (geometric area standard) was calculated by subtracting the current in N2 from the current in O2.Figure 1 shows the effect of sputtering condition on potential-ORR current curves of the ZrOx thin film electrodes. For comparison, the result of the GC substrate only is also plotted as "GC only." The ORR activity of the ZrN_Ar was lower than that of the GC only. In addition, the current of the steady-state cyclic voltammogram of the ZrN_Ar was smaller than that of the GC only. This may be due to the electrochemical oxidation of ZrN surface and the formation of an insulating layer without active sites, presumably Zr hydroxide. On the other hand, the onset potential of the Zr_Ar+N2 and the Zr_Ar+O2 for the ORR were higher than that of the GC only and the Zr_Ar+N2 showed highest ORR activity. The steady-state cyclic voltammograms of the Zr_Ar+N2 and the Zr_Ar+O2 were larger than that of GC only, revealing that these electrodes might have high conductivities. In addition, XPS measurements suggest that the Zr_Ar+N2 contains nitrogen and the Zr_Ar+N2 and the Zr_Ar+O2 have oxygen vacancies. These results suggested that their high ORR activities were caused by nitrogen doping and oxygen defects. Therefore, the electrodes prepared by RF magnetron sputtering method may be able to separate the active sites from the electron conduction by changing the sputtering conditions such as film thickness.AcknowledgementThe authors thank the financial support of the New Energy and Industrial Technology Development Organization (NEDO).References(1) A. Ishihara et al., J. Phys. Chem. C, 123, 18150 (2019).(2) M. Chisaka et al., ACS Omega, 2, 678 (2017).(3) S. Muhammady et al., J. Phys. Chem. C, 126, 15662 (2022). Figure 1

Kinetics of Oxygen Reduction Reaction of Polymer-Coated MWCNT-Supported Pt-Based Electrocatalysts for High-Temperature PEM Fuel Cell

Sluggish oxygen reduction reaction (ORR) of electrodes is one of the main challenges in fuel cell systems. This study explored the kinetics of the ORR reaction mechanism, which enables us to understand clearly the electrochemical activity of the electrode. In this research, electrocatalysts were synthesized from platinum (Pt) catalyst with multi-walled carbon nanotubes (MWCNTs) coated by three polymers (polybenzimidazole (PBI), sulfonated tetrafluoroethylene (Nafion), and polytetrafluoroethylene (PTFE)) as the supporting materials by the polyol method while hexachloroplatinic acid (H2PtCl6) was used as a catalyst precursor. The oxygen reduction current of the synthesized electrocatalysts increased that endorsed by linear sweep voltammetry (LSV) curves while increasing the rotation rates of the disk electrode. Additionally, MWCNT-PBI-Pt was attributed to the maximum oxygen reduction current densities at −1.45 mA/cm2 while the minimum oxygen reduction current densities of MWCNT-Pt were obtained at −0.96 mAcm2. However, the ring current densities increased steadily from potential 0.6 V to 0.0 V due to their encounter with the hydrogen peroxide species generated by the oxygen reduction reactions. The kinetic limiting current densities (JK) increased gradually with the applied potential from 1.0 V to 0.0 V. It recommends that the ORR consists of a single step that refers to the first-order reaction. In addition, modified MWCNT-supported Pt electrocatalysts exhibited high electrochemically active surface areas (ECSA) at 24.31 m2/g of MWCNT-PBI-Pt, 22.48 m2/g of MWCNT-Nafion-Pt, and 20.85 m2/g of MWCNT-PTFE-Pt, compared to pristine MWCNT-Pt (17.66 m2/g). Therefore, it can be concluded that the additional ionomer phase conducting the ionic species to oxygen reduction in the catalyst layer could be favorable for the ORR reaction.

CVD-Deposited Oxygen-Selective Fluorinated Siloxane Copolymers as Gas Diffusion Layers

Copolymer thin films of 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4), 2-(perfluorohexylethylacrylate) (PFHEA), and 2-(perfluoroalkylethylmethacrylate) (PFEMA) were synthesized via initiated chemical vapor deposition (iCVD) as potential candidates for gas diffusion layers (GDLs) in gas diffusion electrodes (GDEs) for aqueous metal–air batteries. Thin-film GDLs exhibited an average water vapor transmission rate of 7.5 g m–2 day–1 and enhanced oxygen diffusion with oxygen permeabilities as high as 3.53 × 10–15 mol m m–2 s–1 Pa–1 (10.5 Barrer). The electrochemical performance of GDEs fabricated using commercial catalysts, current collectors, and synthesized GDLs was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and potentiodynamic polarization measurements. The fabricated GDEs exhibited higher oxygen reduction current densities (228.2 mA cm–2) compared to commercial GDEs (132.7 mA cm–2). Copolymer GLDs exhibited an order of magnitude higher oxygen diffusion (39.5 × 10–8 cm2 s–1) in GDEs compared to commercial counterparts (1.84 × 10–8 cm2 s–1). Due to the high oxygen solubility of V4D4 and excellent hydrophobic behavior of PFHEA and PFEMA, their copolymers can effectively promote the diffusion of oxygen and restrict moisture intake, making them ideal materials for GDLs. Combining well-balanced properties of siloxane and fluorinated polymer chemistries, the iCVD process is an excellent low-cost method for the fabrication of GDLs for metal–air battery applications.

Effect of Temperature and Cl- Concentration on the Catalytic Activity of a Stainless-Steel Oxide

Galvanic couples between high-strength, noble metal fasteners, such as those made out of titanium or stainless-steel, can cause extensive damage in aluminum alloy airframes. These galvanic couples can drive higher rates of oxidation than would be expected for general corrosion and the corrosion damage can become a stress concentration point for the nucleation of cracks. Understanding the factors that govern the catalytic activity for the oxygen reduction reaction on stainless-steel oxides is important for enabling predictions of the amount of corrosion that can occur for a given set of environmental conditions. We have focused on measurements of the diffusion limited current density for oxygen reduction on UNS S13800 stainless-steel in solutions of varying NaCl concentrations from 0.1 M to 4.6 M and air temperatures ranging from 10oC to 40oC. Estimates of the oxygen diffusion limited current were obtained from potentiodynamic polarization measurements under the various electrolyte concentrations and environmental conditions. The experimental data was then compared with calculated ilim values for the same conditions and assuming a constant diffusion layer thickness. Regression and physics-based models for the effect of environmental conditions on the experimental data have been developed and are used to estimate the effect of the electrolyte on the stainless-steel oxide.The U.S. Naval Research Laboratory sponsored this work under work unit 1L66. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy, or the U.S. government.

Limiting Current Density of Oxygen Reduction under Ultrathin Electrolyte Layers: From the Micrometer Range to Monolayers

AbstractThe oxygen reduction reaction (ORR) under ultrathin electrolyte layers is a key reaction in many processes, from atmospheric corrosion to energy conversion in fuel cells. However, the ORR current under ultrathin electrolyte layers is difficult to measure using conventional electrochemical methods. Hence, reliable data are scarce for the micrometer range and totally missing for the sub‐micrometer range of the electrolyte layer thickness. Here, we report a novel hydrogen‐permeation‐based approach to measure the ORR current underneath thin and ultrathin electrolyte layers. By using a Kelvin‐probe‐based measurement of the potential, which results from dynamic equilibrium of oxygen reduction and hydrogen oxidation, and the corresponding hydrogen charging current density, the full current‐potential relationship can be constructed. The results shed a new light on the nature of the limiting current density of ORR underneath ultrathin layers of electrolyte.

Effect of Semiconducting Properties of Oxide-Based Compounds on Oxygen Reduction Activity in Acidic Media

Polymer electrolyte fuel cells (PEFCs) are attractive as energy conversion systems with high conversion efficiency. Although theoretical energy conversion efficiency is high, 83% at 295 K, the actual efficiency is very low because of a large overpotential of oxygen reduction reaction (ORR). In addition, the durability of the catalysts becomes important for commercialization of PEFCs. Pt supported carbon cathode is easy to degrade at high potential region and potential fluctuation. Therefore, we have focused on group 4 and 5 oxide-based compounds as oxides with active sites and electro-conductive supports. We successfully demonstrated that the surface of the oxides had high quality active sites [1] and superior durability against start/stop and load cycle tests [2].Recently, DFT calculation by Sugino’s group revealed that Pd or Rh doped TiO2 had possibility that these surfaces reached equilibrium potential for the ORR [3]. However, the DFT calculation was only based on the adsorption Gibbs energies of reaction intermediates. Considering the actual active sites, sufficient electron supply is necessary to proceed ORR fluently. Group 4 and 5 oxide-based compounds are essentially semi-conductors. Therefore, the semi-conducting physico-chemical properties such as carrier density and Flat-band potential affect the supply of electrons to the active sites. However, the effect of the semi-conducting properties of the oxide-based compounds on the oxygen reduction activity in acidic media has not been investigated yet. In this study, we attempted to reveal the effect of Flat-band potential and capacitance of space charge layer on the ORR activity. Thus, we focused on titanium oxides and prepared Nb, Ta, and Zr added titanium oxide to investigate the effect of foreign element doping.The oxide-coating film was prepared on a Ti plate (10 mm× 10 mm× 1 mm) substrate by the conventional dip-coating method using a 0.5 M butanolic solution of metal salt: titanium tetrabutoxide, zirconium tetrabutoxide, niobium pentaethoxide or tantalum pentaethoxide. The titanium substrate was etched with 10wt% oxalic acid at 80 °C for 1 h and then rinsed with deionized water before the dipping procedure. Calcination of the dip-coated salts was con-ducted in air at 350 °C. The dip-drying/calcination (alternating 10 min each) procedure was typically repeated 15 times.Electrochemical measurements were carried out using a three-electrode cell at 30 ± 1 °C in 0.1 M H2SO4. Cyclic voltammetry was performed with 5 mV sec-1 in the potential range from 0.2 to 1.2 V in an oxygen and nitrogen atmosphere to obtain oxygen reduction current. The oxygen reduction current density, i ORR, was normalized by the geometric area of the electrode. The ORR activity was evaluated by onset potential for the ORR, E onset, defined as the electrode potential at i ORR = 0.02 mA/cm2 and i ORR at 0.2 V, |i ORR@0.2V|.Electrochemical impedance measurements were performed with amplitude of 0.01 V and frequency of 1000 Hz to obtain capacitance of space charge layer, C scl, every 0.1 V below 1.1 V. In addition, the values of the (1/C scl)2 were plotted against the electrode potential, that is, Mott-Schottky plot, to obtain the flat-band potential, E fb.Figure 1 shows the relationship between E fb and E onset of Nb, Ta, or Zr doped TiO2 electrodes. The relation of E onset－E fb≒0.3 V was obtained and it was independent of the doped elements. The difference between the electrode potential and E fb corresponded to the barrier height of space charge layer. When the electrode potential is higher than the E onset, the electrons cannot pass through the barrier of the space charge layer, resulting that the ORR cannot proceed because the supply of electrons from bulk of semi-conductor to the active sites at the interface between oxide-electrolyte. When the electrode potential reaches at Eonset, the ORR starts to proceed due to the continuous supply of the electrons because the electrons can pass through the barrier. According to the relation of E onset－E fb≒0.3 V, the barrier height of the space charge layer, which the electrons can pass through, was estimated to be 0.3 V.AcknowledgementThe authors wish to thank the support of the New Energy and Industrial Technology Development Organization (NEDO).

Cathodic Polarization Behavior of Rusted Carbon Steels Exposed to Atmospheric Environment for Different Duration

Carbon steel is most commonly employed as a structural material for infrastructures due to its superior mechanical strength as well as low cost. However, as carbon steel corrodes in atmospheric environment, improvement in corrosion resistance is required for steel infrastructures. It is well-known that during wet/dry cycle in atmospheric environment, the reduction reactions of rust on carbon steel as well as the oxygen reduction on rusted steel occur as cathodic reaction, affecting an anodic dissolution of underlying steel. Although the chemical composition and structure of rust layers on carbon steel have been extensively studied, the reduction reactions have not been focused. In this study, we have investigated reduction reactions on rusted carbon steels.Specimens were rusted carbon steels that had been exposed to a coastal region in Japan for different periods of 3 months, 1 year and 3 years. In the present work, polarization test was carried out to investigate reduction reactions on the specimens. The polarization was performed in a deaerated aqueous solution containing chlorides and bicarbonate for 500 s~30000 s at -0.8 V where the reduction of rusts on the specimens mainly occurs under the deaerated condition. After the defined periods of the rust reduction (td ), air was introduced into the aqueous solution.SEM observation of rusts on the rusted carbon steels revealed that the thickness of the rusts increased with prolonged periods of the exposure from 50 μm after 3 months to 130 μm after 3 years. In the polarization test, smaller reduction current density was observed on rusted steel with thicker rust layer during the potentiostatic polarization at -0.8 V under the deaerated condition. The subsequent introduction of air into the solution increased the reduction current density. The increase of the current density (ΔIc ) was attributed to oxygen reduction reaction on the specimens. Therefore, ΔIc was defined as oxygen reduction current density. The ΔIc varied depending on the periods of rust reduction (td ), that is, the ΔIc increased with increasing td and then decreased after maximum with td . The maximum of ΔIc was larger for specimen with thicker rust layer. Structures of rust layers were examined by XRD using synchrotron radiation before and after the polarization test. Obtained XRD patterns revealed that the main components of the rusts grown on the carbon steel were Fe3O4 and FeOOH. After the cathodic polarization, the fraction of Fe3O4 in the rust layers increased whereas that of FeOOH decreased with increasing td .According to the above results, the increased ΔIc with td in the initial stage of the polarization test can be attributed to the increased amount of electroactive Fe3O4, which facilitates electron transfer in the rust. However, the further increase of Fe3O4 may reduce the active surface area of Fe3O4, resulting in the decreased ΔIc in the later stage of the test. Furthermore, the larger maximum ΔIc observed on the specimen with thicker layer is also attributed to the amount of Fe3O4, that is, larger amount of Fe3O4 is formed in thicker rust layer. These results indicate that the reduction reactions on rusted steel are strongly related to the structure of rust layer, which will affect corrosion behavior of steel in atmospheric corrosion.

Factors Affecting ORR Activity of Nb-Added TiOx Catalyst Using Carbon Support for PEFC

1. IntroductionCarbon-supported platinum catalysts used for the cathode of present polymer electrolyte fuel cells (PEFCs) have some problems such as a large amount of platinum use and low durability under operation condition. Therefore, we have attempted to develop substitute of platinum based on group 4 and 5 transition metal oxides. We found that stable oxygen vacancies on the surface and lattice distortion of these metal oxides could act as active sites of oxygen reduction reaction (ORR)1). On the other hand, cup-stacked carbon nanotubes (CSCNTs) coated with TiO2 have been investigated as durable supports for a platinum-based catalysts2). We already revealed that Nb-added titanium oxide with lattice distortion showed some ORR activity1). Then, we thought that CSCNTs coated with Nb-added titanium oxide might show definite ORR activity. In this study, we prepared CSCNTs coated with Nb-added TiOx and evaluated their behavior to investigate the factors which affect the ORR activity.2. ExperimentalThe CSCNTs (GSI Creos) without amorphous carbon on the surface was added to 2-propanol and dispersed ultrasonically, then a desired amount of Ti isopropoxide and Nb ethoxide were added with stirring. A sufficient amount of water mixed with 2-propanol were added to proceed hydrolysis and stirred overnight. Suction filtration was performed to obtain a precursor. In this experiment, Nb was added into TiO2 with an atomic% of 10 against Ti, and a loading of oxide was adjusted to 30 wt% of total catalyst. The precursor was heat-treated under an Ar gas containing 4% hydrogen at desired temperature and time to obtain Nb-added TiOx catalysts, designated as Nb-added TiOx/CSCNTs.The Nb-added TiOx/CSCNT was supported on a glassy carbon rod to make a working electrode. Electrochemical measurements were carried out using a three-electrode cell at 30 ± 1 °C in 0.1 M H2SO4. Cyclic voltammetry was performed with 5 mV sec-1 in the potential range from 0.2 to 1.2 V in an oxygen and nitrogen atmosphere to obtain oxygen reduction current. ORR current based on oxides was obtained by subtracting the reduction current of carbon from the reduction current of the entire catalyst. The oxygen reduction current density, iORR, was normalized by the mass of the oxides.3. Results and discussionFigure 1 shows the dependence of iORR at 0.6 V of Nb-added TiOX/CSCNTs on heat treatment temperature. The results of Nb-added TiOx powders prepared without carbon were also plotted. Because the electro-conductivities of the Nb-added TiOx powders were very low, Ketjenblack was mixed as an electro-conductive material to evaluate the ORR activity. The current value of Nb-added TiOx/CSCNT is significantly improved compared to oxide powder. This is because the electron supply increased due to the deposition of oxides on the CSCNT. The highest ORR current density was observed at 800 oC.Figure 2 shows the SEM images of Nb-added TiOX/CSCNTs. Amorphous TiO2 covers with the surface of CSCNTs in the unheated catalyst. At 700 ° C and 800 ° C, the oxide particles of around 10 nanometers were supported with high dispersion. However, at 900 ° C, the particles aggregated to become larger.Figure 3 shows the Ti 2p XPS spectra of Nb-added TiOx/CSCNTs heat-treated at 600-1000 °C. Ti2p peak shifted to the lower binding energy from 600 ° C to 800 ° C and to the higher energy from 800 ° C to 1000 ° C. The proportion of low-valence state Ti3+ increased as the temperature increased from 600 to 800 ° C. The reason why the shift to the higher binding energy occurred above 800 ° C was that the oxide surface prepared at higher heat-treatment temperature was oxidized by exposure of the air.Figure 4 shows the relationship between ratio of Ti3+ / (Ti3+ + Ti4+) and ORR current density at 0.6 V. The ratio of Ti3+ was calculated from the Ti 2p XPS spectra. The iORR at 0.6 V increases as ratio of Ti3+ increases. The ratio of Ti3+ indicates the presence of oxygen vacancies on the surface of the oxide. Therefore, it is considered that the increase of oxygen vacancies on the oxide surface strongly affected the ORR activity.AcknowledgementThe authors wish to thank the supply of the CSCNT from GSI Creos and the support of the New Energy and Industrial Technology Development Organization (NEDO), JSPS grants-in-aid for scientific research, Suzuki Foundation, and Tonen General Sekiyu Research / Development Encouragement & Scholarship FoundationReferences1) A. Ishihara, et. al., Electrochim. Acta, 283,1779 (2018).2) F. Ando, et. al., Electrochim. Acta, 404, 232 (2010) Figure 1

Porous Nitrogen‐doped Reduced Graphene Oxide Gels as Efficient Supercapacitor Electrodes and Oxygen Reduction Reaction Electrocatalysts

Summary of main observation and conclusionHeteroatom‐doped carbon materials have been widely used in energy storage and conversion such as supercapacitors and electrocatalysts. In this work, L‐asparagine (Asn), an amino acid derivative, has been used as a doping agent to prepare nitrogen‐ doped reduced graphene oxide gels (N‐GAs). The 3D interconnected structure gives rise to the superior electrochemical properties for supercapacitor and electrocatalytic oxygen reduction reaction (ORR). The N‐GA‐4 (the mass ratio of Asn to graphene oxide (GO) is 4 : 1 by hydrothermal method) electrode shows the capacitance of 291.6 F·g–1 at 0.5 A·g–1. Meanwhile, the assembled symmetric supercapacitor achieves a maximum energy density of 23.8 Wh· kg–1 when the power density is 451.2 W·kg–1, and demonstrates an ultralong cycling life that the retention of capacitance is 99.3% after 80000 cycles. What's more, the annealed aerogel N‐GA‐4‐900 exhibits an onset potential (Eonset) of 0.95 V, half wave potential (E1/2) of 0.84 V (vs. RHE) and the oxygen reduction current density of 5.5 mA·cm–2 at 0.1 V with nearly four‐electron transfer, which are superior to commercial Pt/C. This work offers a new insight into the synthesis and applications of N‐GAs materials towards high performance in supercapacitors and ORR.

Enhancing Oxygen Reduction Reaction Catalytic Activity Using a Sub‐Stoichiometric CaTiO3−δ Additive

AbstractPlatinum scarcity and its high cost have led to the requirement of alternative materials catalysing the oxygen reduction reaction (ORR), which is the main rate‐determining step occurring in electrochemical devices, including metal‐air batteries and fuel cells. We report a study on a sub‐stoichiometric calcium titanate (CaTiO3−δ, CTO) compound used as promoter for the ORR in order to reduce the Pt loading and improve its electrocatalytic activity. Composite catalysts based on Pt/C with different amounts of CTO were prepared and their activity was investigated by rotating disk electrode (RDE). The obtained results proved a higher catalytic activity for the composite electrode, with respect to pure Pt/C, in terms of electrochemically active surface area, oxygen reduction current density, onset potential and stability.

Buckypaper Thickness Dependent Bilirubin Oxidase Electrochemistry

Among various electrode materials, freestanding buckypapers made from carbon nanotubes have gained significance as they do not require a solid support material and thus facilitate miniaturization. We present the effect of buckypaper (BP) thickness on the electrocatalytic properties of bilirubin oxidase (BOD) enzyme. In this study, we prepared BPs of varying thicknesses ranging from 87 micron, the minimum thickness for suitable handling with good stability in aqueous experiments, to 380 micron. BOD was adsorbed overnight onto the BPs. We determined that the lower range BP thickness (< 220 micron) exhibited better sigmoidal shaped electrocatalytic currents than the higher BP thickness based BOD biofilms with larger capacitive currents. Oxygen reduction current density of up to 3 mA cm–2 is achieved without use of any redox mediators or tedious electrode modifications. Using the 87 micron thick BP as the representative case, we were able to obtain distinguishable peaks for all Cu sites of BOD, and assigned their types, T1, T2, and T3, based on peak-width at half-maximum in anaerobic cyclic voltammograms. Our peak assignment is further supported by the appearance of dual electrocatalytic reduction waves at higher scan rate region (> 10 mV s-1) in oxygen-saturated buffer, which is identified to be driven by an ~ 3.5-times faster electron transfer rate from the buckypaper to the T2/T3 center than the T1 Cu site. Findings from this study are significant for designing new enzyme electrocatalytic systems and biosensors. In addition, accessing electrochemical and catalytic properties of often-buried inaccessible redox centers of many important metalloenzymes can be achieved by the freestanding BP design of optimal thickness.

Buckypaper-Bilirubin Oxidase Biointerface for Electrocatalytic Applications: Buckypaper Thickness.

Electrode materials play an important role on the electrocatalytic properties of immobilized biocatalysts. In this regard, achieving direct electronic communication between the electrode and redox sites of biocatalysts eliminates the need for additional electron transfer mediators for biocatalytic applications in fuel cells and other electrochemical energy devices. In order to increase electrocatalytic currents and power in fuel cells and metal-air batteries, conductive carbon-nanostructure-modified large surface area electrodes are quite useful. Among various electrode materials, freestanding buckypapers made from carbon nanotubes have gained significance as they do not require a solid support material and thus facilitate miniaturization. In this article, we present the effect of buckypaper (BP) thickness on the electrocatalytic properties of a bilirubin oxidase (BOD) enzyme. In this study, we prepared BPs of varying thicknesses ranging from 87 μm, the minimum thickness for suitable handling with a good stability in aqueous experiments, to 380 μm. BOD was adsorbed overnight onto the BPs, mostly via hydrophobic and π-π interactions since the nanotubes used were not chemically functionalized. Furthermore, intercalation of the BOD molecules onto the nanotubes' multicylindrical network is feasible. We determined that the lower range BP thickness (<220 μm) exhibited better sigmoidal shaped electrocatalytic currents than the higher BP-thickness-based BOD biofilms with larger capacitive currents. An oxygen reduction current density of up to 3 mA cm-2 is achieved without the use of any redox mediators or tedious electrode modifications. Using the 87 μm thick BP as the representative case, we were able to obtain distinguishable peaks for all Cu sites of BOD and assign their types, T1, T2, and T3, based on the peak-width at half-maximum in anaerobic cyclic voltammograms. Our peak assignment is further supported by the appearance of dual electrocatalytic oxygen reduction waves at a higher scan rate region (>10 mV s-1) in oxygen-saturated buffer, which is identified to be driven by an ∼3.5 times faster electron transfer rate from the buckypaper to the T2/T3 center than the T1 Cu site. Findings from this study are significant for designing enzyme electrocatalytic systems and biosensors in general and fuel cells and aerobic energy storage devices in particular, where the cathodic oxygen reduction current is often inadequate.

Vulcan XC-72 treated by nitric acid and its electrocatalytic activity for oxygen reduction reaction

Vulcan XC-72 treated by nitric acid is employed as an oxygen reduction reaction (ORR) catalyst. The carbon catalyst exhibits higher electrocatalytic activity and stability in 6[Formula: see text]mol[Formula: see text]L[Formula: see text] KOH electrolyte. The current density for oxygen reduction is 45[Formula: see text]mA[Formula: see text]cm[Formula: see text] at [Formula: see text][Formula: see text]V (vs. HgO/Hg). A zinc–air battery using XC-72 carbon electrode was assembled, the discharge voltage could reach up to 0.6[Formula: see text]V under a high current density of 80[Formula: see text]mA[Formula: see text]cm[Formula: see text].

Effect of Environment Variables and Material Properties on the Limiting Current Density for Oxygen Reduction

For aircraft alloys that have received surface treatments, primers, and topcoats to control self-corrosion due to precipitate phases, high-strength fasteners made of stainless steel or titanium alloys are the most obvious point of attack due to galvanic mismatches between the materials. However, the electrolytes that form from atmospheric processes can differ widely in composition and thickness. Dissolved oxygen concentrations vary due to temperature and salinity changes in the electrolyte. Recent attempts to model atmospheric corrosion have suggested that, at high salt loadings on the surface, electrolytes can form that have water layer thicknesses that appear as bulk electrolytes in terms of oxygen transport1. Experiments using rotating disk electrode (RDE) measurements and polarization curve data,2 have suggested an upper bound of approximately 800 mm on the diffusion layer thickness for oxygen that can be expected to form in a thin film of water. At distances beyond this thickness, convective flow is expected to dominate oxygen transport. In this report, we compare the results of RDE experiments that simulate thin film behavior on different substrate materials in different electrolytes with polarization data obtained under actual electrolyte thin films equilibrated at different relative humidity values, and bulk polarization measurements, as shown in Figure 1, in order to determine the effects of ohmic drop on the measurements and isolate relevant kinetic parameters. Isolating these effects is important in order to accurately model the corrosion currents arising from galvanic couples in which the oxides at which the reduction reactions are occurring can be changing in response to the changes in the electrolyte as the environment changes. Figure 1. Bulk cathodic polarization curves in 0.6M NaCl at 25◦C for Pt, UNS S31600, and UNS R56400. This work was sponsored by the Office of Naval Research, ONR, under grant/contract number N0001418WX00826; the views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy or the U.S. government. References 1 Van den Steen, N., Simillion, H., Dolgikh, O., Terryn, H. & Deconinck, J. An integrated modeling approach for atmospheric corrosion in presence of a varying electrolyte film. Electrochimica Acta 187, 714-723, doi:10.1016/j.electacta.2015.11.010 (2016). 2 Liu, C., Srinivasan, J. & Kelly, R. G. Electrolyte Film Thickness Effects on the Cathodic Current Availability in a Galvanic Couple. J Electrochem Soc 164, C845-C855, doi:10.1149/2.1641713jes (2017). Figure 1

Quantitative analysis of the polarization behavior of iron in an aerated acidic solution using SECM

The apparent corrosion current of pure iron in aerated 5 mM HClO4 + 0.1 M NaClO4 solution was separated into Fe oxidation, proton reduction and oxygen reduction currents using SECM in a modified tip generation/substrate collection (TG/SC) mode. The oxidation current density of Fe was 1.11 × 10−3 A/cm2, equal to the sum of proton reduction and oxygen reduction current density, around 5.26 × 10−4 and 5.84 × 10−4 A/cm2, respectively, implying that oxygen reduction and proton reduction are equally important at open circuit potential (OCP). Moreover, oxygen reduction was suppressed on the Fe electrode in the potential range between −0.7 and −1.1 V vs Ag/AgCl because of the inhibitory effect of the proton reduction reaction and a potential-dependent surface effect, then the suppression is weakened and oxygen reduction is enhanced. Finally, the Fe corrosion current was dominated by oxygen reduction and Fe oxidation at potentials more positive than −0.6 V.

Systematic Control of Redox Properties and Oxygen Reduction Reactivity through Colloidal Ligand-Exchange Deposition of Pd on Au.

Core-shell nanoparticles of Au@Pd with precise submonolayer, monolayer, or multilayer structure were synthesized using ligand-exchange reactions of palladate ions onto colloidal Au nanocrystals. Decoupling the palladate adsorption step from the subsequent reduction enables excellent precision, uniformity, and tunability in the Pd shell thickness. The redox properties of the surface Pd are directly correlated to the thickness of the Pd shell with a >+200 mV shift in the PdO reduction potential for submonolayer Au@Pd nanoparticles compared to pure Pd. Using these precisely controlled core-shell materials, the oxygen reduction catalytic activity can be directly correlated to PdO reduction potential and Pd surface coverage on Au. When the Pd oxide reduction peak is shifted by +240 mV compared to pure Pd, a 50 mV reduction in overpotential and a 4-fold increase in kinetic current density for oxygen reduction are observed. Colloidal ligand-exchange synthesis may be particularly useful for noble metal core-shell catalysts as a strategy to subtly tune the electronic properties of surface atoms in order to lower overpotential and increase catalytic turnover.

Effects of Oxygen Pressure and Chloride Ion Concentration on Corrosion of Iron in Mortar Exposed to Pressurized Humid Oxygen Gas

The effects of oxygen pressure on the corrosion of iron embedded in mortar with various Cl− contents were examined by using a novel hyperbaric-oxygen accelerated corrosion test (HOACT). Exposure and electrochemical tests of a pure iron sample embedded in mortar with a cover thickness of 5 mm were performed in HOACT condition with pressurized 100% oxygen gas and with relative humidity of 95%. For comparison, these tests were also performed in ambient air with the same relative humidity. Oxygen reduction current density of iron embedded in mortar increased with an increase in oxygen pressure, suggesting that the corrosion of the embedded iron is expected to be accelerated with oxygen gas pressure assuming an oxygen reduction limited corrosion current density. Iron corrosion in mortar without Cl− was minimal regardless of oxygen gas pressure. In contrast, the corrosion of iron in mortar with Cl− was accelerated with increasing the oxygen gas pressure up to a certain pressure as expected, and the thickness of the rust formed on the iron surface increased with Cl− concentration under each oxygen gas pressure, indicating that Cl− is necessary to initiate corrosion which is enhanced in the pressurized oxygen gas. However, when the oxygen pressure exceeded a certain level, the corrosion was suppressed. Electrochemical impedance spectroscopy measurement using a sensor consisting of a pair of carbon steel electrodes in mortar showed that the charge transfer resistance of the carbon steel under excessively high oxygen gas pressure became high in comparison to that at lower oxygen pressure, indicating the suppression of corrosion initiation is due to formation of a protective passive film under the excessive supply of oxygen. Consequently, the pressurized oxygen gas accelerates the corrosion of iron in mortar with Cl−; however, excess oxygen suppresses corrosion. The optimum condition to efficiently accelerate corrosion of iron in mortar was evaluated.

A novel PtRuIr nanoclusters synthesized by selectively electrodepositing Ir on PtRu as highly active bifunctional electrocatalysts for oxygen evolution and reduction

To promote development of the unitized regenerative fuel cell, a new energy conversion-storage device, it is essential to look for a bifunctional catalyst with high efficiency and catalytic properties for oxygen evolution and reduction reactions. Herein, a novel PtRuIr electrocatalyst has been synthesized by a pulse electrodeposition method with selectively electrodepositing Ir on PtRu nanoclusters. The prepared catalysts were characterized by electrochemical analysis, scanning electron microscopy and energy dispersive spectrometer. The experimental results show that the PtRuIr catalyst with 10 mol% of Ir exhibits significantly higher oxygen evolution and reduction currents compared to the PtRu and the PtRuIr catalysts with other contents of Ir, which is also supported by the electrochemical impedance data. In addition, a single water electrolysis cell and a H2/O2 fuel cell were performed simultaneously to evaluate the oxygen evolution and reduction performances, respectively, of the novel catalyst. The optimized catalyst with 90 mol% PtRu and 10 mol% Ir shows higher oxygen evolution reactivity current density and higher round-trip efficiency in the water electrolysis single cell, and in the meantime, it also exhibits higher oxygen reduction current density and higher power density in the fuel cell, compared to those of the PtRu catalyst.

Zirconium Oxide-Based Compounds as Non-Platinum Cathodes for Polymer Electrolyte Fuel Cells

In order to clarify the active sites of zirconium oxide-based compounds as non-platinum cathodes for PEFCs, we investigated the active sites of the zirconium oxide-based compounds prepared from oxy-zirconium phthalocyanine using MWCNT as a support (ZrCxNyOz/MWCNT) and the variation of the surface chemical state under electrochemical reactions by using in-situ X-ray absorption spectroscopy and electrochemical X-ray photoelectron spectroscopy. The oxygen vacancies of the zirconium oxide phase in the ZrCxNyOz/MWCNT could work as active sites for the electrochemical oxidation and the oxygen reduction reaction providing oxygen species adsorption sites. In addition, the zirconium oxide-based compounds supported by MWCNT (20wt% oxide) prepared by arc-plasma deposition method showed high oxygen reduction current density (220 mA g-1 @ 0.8 V), indicating that the zirconium oxide-based compounds which contained no iron and nitrogen-doped carbon had active sites for the oxygen reduction reaction.

Preparation and Characterization of Nano Structured Pt−MOx/C for Oxygen Reduction Reaction in Acidic Medium

Pt-MOx/C (M = Ti, Ce or Zr) electrocatalysts was prepared for the purpose of developing a cheap and efficient electrocatalysts for oxygen reduction reaction (ORR) using a mixture of ethylene glycol and sodium borohydride (EG + NaBH4) as a reducing agent. Pt/C was studied for estimation of metal oxides effect.Pt-CeO2/C-1 was prepared through single reducing agent ethylene glycol for studying the effect of changing the reducing agent on the activity of electrocatalysts toward ORR and for studying ORR kinetics. The electrocatalytic activity of the prepared electrocatalysts towards (ORR) was evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on a rotating disc electrode (RDE). Pt-ZrO2 /C shows the best activity towards ORR among the studied electrocatalysts; the oxygen reduction current density for it is about 5 times; (5.84 mAcm-2) as that of Pt/C (1.26 mAcm-2). Pt-CeO2/C showed better ORR activity (2.24 mAcm-2) than Pt-CeO2/C-1(1.58 mAcm-2) indicating the great effect of changing the reducing agent not only on the electrocatalyst behavior towards ORR but also its particle size and the metal content of the resultant electrocatalysts. Oxygen reduction mechanism for Pt-CeO2/C and Pt-CeO2/C-1 was evaluated using through Koutecky-Levich, they showed first-order kinetics with ORR not controlled solely by diffusion. XRD, EDX and TEM analyses were used to characterize the prepared electrocatalyst.