Kinetics Of Oxygen Electroreduction Research Articles

Effect of plasma-assisted electrochemical treatment of the boron-doped synthetic diamond compact electrodes on the oxygen electroreduction kinetics

The effect of plasma electrochemical activation of boron-doped diamond compact electrodes on the oxygen electroreduction reaction kinetics is studied. The conducting compact electrodes are prepared via graphite-to-diamond conversion in C–B growth system at high pressure and high temperature. The diamond compact electrode surface was modified in Na2SO4 aqueous solution by exposition to cathodic–anodic electrolytic plasma produced by the application of voltage pulses with amplitude up to 300 V. The cathodic–anodic plasma alone proved to produce but negligible catalytic effect with respect to the oxygen reduction reaction. However, on the cathodic reduction of thus treated electrode, it acquired significant electrocatalytical activity manifested itself in the predominant O2-to-H2O reduction by a four-electron pathway. At the same time, the cathodic treatment of the plasma-modified electrodes has no real impact on the rate constant of electron transfer in the “fast” [Ru(NH3)6]2+/3+ and [Fe(CN)6]4–/3– redox systems. The observed electrocatalytical effect with respect to the oxygen reduction reaction is assumed to be caused by the quinone groups formation at the boron-doped diamond electrode surface under the combined action of cathodic–anodic plasma and cathodic polarization, these groups being active centers for the four-electron oxygen reduction.

An adept approach to convert titanium carbide to titanium nitride and it’s composite with N-doped carbon nanotubes for efficient oxygen electroreduction kinetics

We report an in situ synthesis of titanium nitride (Ti3N2Tx) from its carbide (Ti3C2Tx) and concurrently composite formation with N-doped carbon nanotubes (N-CNTs). Unlike conventional wet-chemical method, a simple approach has been adopted for the synthesis of Ti3N2Tx and it’s composite with N-CNTs. The crystallographic study shows an obvious phase transformation from carbide to nitride. Importantly, an optimized composite electrocatalyst (Ti3N2@NCNT(20)-800) exhibits promising oxygen reduction ability under alkaline conditions with positive onset potential (Eonset) of 1.0 V versus reversible hydrogen electrode (RHE) and current density (JL) of 5.3 mA/cm2. The remarkable oxygen reduction reaction (ORR) properties mainly originate from the fascinating carbon-nitrogen exchange (Ti3C2Tx into Ti3N2Tx) in the MXene lattice along with N-doping in CNTs, which eventually lead to the formation of their composite and generating enough active centres for dioxygen adsorption followed by electroreduction. Moreover, the rotating ring disk electrode (RRDE) studies were carried to particularly detect peroxide (HO2−) formation and further to get an insight into ORR kinetics. Additionally, a robust performance of Ti3N2@NCNT(20)-800 electrocatalyst over Pt/C has been observed during the electrochemical cycling stability up to 10,000 (10 k) durable cycles. Thus, we believe that the present methodology provides an adept approach to convert carbide into nitride and further to extend it to synthesize the MXene based composite materials for energy conversion applications.

Efficient oxygen electroreduction kinetics by titanium carbide@nitrogen doped carbon nanocomposite

A simple approach towards preparation of non-noble metal electrocatalyst for oxygen reduction reaction in low temperature fuel cells has been necessity for a sustainable green technology. Herein, a cost-effective and facile method of preparation of TiC@N-doped graphene like carbon nanocomposite (TiC@NC) has been discussed. The composite structure of as-prepared TiC@NC was confirmed using structural analysis and morphological studies. Interestingly, the optimized TiC@NC(0.2)-800 electrocatalyst shows remarkable oxygen reduction reaction (ORR) kinetics with better onset potential +1.08 vs RHE and significant current density of 4.8 mA/cm2 in alkaline medium. Further, obtained catalyst exhibits four electron transfer mechanism similar to Pt-based electrocatalysts. Additionally, TiC@NC(0.2)-800 shows better mass activity (∼410 mA/mg) as compared to other compositions. Moreover, the single step kinetics mechanism has been seen due to lower (<5%) peroxide yield. The relatively lower charge transfer resistance at electrode/electrolyte interface of TiC@NC (0.2)-800 electrode supports for higher catalytic activity. Additionally, electrochemical cycling reveals the better stability by TiC@NC(0.2)-800 even after 10,000 cycles (10 mV negative shift in E1/2) than that of state of art Pt/C catalyst (80 mV negative shift in E1/2). The presence of N-doped carbon around TiC crystals is responsible for better electrocatalytic activity (due to optimal doping synergy), though the support of TiC makes the electrocatalyst more stable in nature (thanks to strong TiC-NC interactions). Additionally, TiC@NC(0.2)-800 does not show any response towards methanol oxidation reaction, annulling the cross-over effects. Hence, TiC@NC(0.2)-800 could be hopeful substitute for conventional Pt/C electrocatalyst for energy conversion.

Influence of Pr6O11 on oxygen electroreduction kinetics and electrochemical performance of Sr2Fe1.5Mo0.5O6-δ based cathode

Oxygen electroreduction kinetics and electrochemical performance of the Pr6O11-impregnated Sr2Fe1.5Mo0.5O6-δ – Ce0.8Sm0.2O1.9 (SFM–SDC) cathode have been first studied. By means of distribution of relaxation times and non-linear least squares analysis of impedance spectra were shown that the addition of Pr6O11 into the cathode leads to the increase in the rate of the low-frequency step. It is suggested that the observed phenomenon is associated with the increase in the rate of surface oxygen exchange. It is shown that the introduction of praseodymium oxide into the cathode results in a decrease in the area specific polarization resistances of the cathode at equilibrium potentials from 0.23 to 0.06 Ω cm2 at 800 °C in air. The maximum power density of symmetrical solid oxide fuel cell (SOFC) with impregnated SFM–SDC electrodes and supporting 760 μm La0.85Sr0.15Ga0.85Mg0.15O3-δ electrolyte without buffer/barrier and collector layers under air/wet hydrogen (dry CH4) condition was about 0.5 (0.26) W cm−2 at 800 °С. The overvoltage of the cathode was higher than that of the anode under air/wet hydrogen and vice versa when methane was supplied to the anode. The obtained results elucidate that the impregnated SFM–SDC is a promising cathode for SOFC application.

Kinetics of Oxygen Electroreduction on Me–N–C (Me = Fe, Co, Cu) Catalysts in Acidic Medium: Insights on the Effect of the Transition Metal

The influence of three different transition metals (Me = Fe, Co, Cu) on the oxygen reduction reaction (ORR) kinetics in acidic medium of Me–N–C catalysts synthesized using Me(II)-phthalocyanine as precursors is investigated in this work. Through a detailed electrochemical characterization using cyclic voltammetry and rotating ring-disk electrode, several kinetic parameters such as Tafel slope, reaction order for oxygen and proton, apparent activation energy, selectivity toward hydrogen peroxide production, and kinetics of reduction of adsorbed oxygen were determined. The behavior of these three catalysts is analyzed in detail. A comparison between each other of the catalysts, and with a Pt-based catalyst is done. The results obtained provide clear evidence of the important role played by each transition metal in the formation of more or less effective active sites. The ORR kinetics behavior can be well interpreted according to the occurrence of a redox-mediated coverage of the active sites at low overpote...

Accelerated Durability Tests for Pt-Nanoclusters Modified Carbide Derived Carbons Catalysts in Acidic Solution

Pt-nanoparticles activated porous carbon composite catalysts are widely used in PEMFC applications. The instability of these catalysts in harsh fuel cell working conditions is the main problem hindering the large-scale application of the fuel cell technology in commercial and energy recuperation systems [1]. Hierarchical microporous-mesoporous carbon supports C(Mo2C) were prepared from molybdenum carbide using high temperature chlorination method at different synthesis temperatures from 600 °C to 1000 °C. The Pt-nanoparticles were deposited onto the carbon support using the NaHB4method [2,3]. The X-ray diffraction, low temperature nitrogen sorption, thermogravimetric analysis (TGA) and high-resolution scanning electron microscopy (HRSEM) methods were used to characterize the catalysts materials. The medium size of the Pt-nanoparticles deposited was 4.5±0.6 nm. Corrosion of the Pt-C(Mo2C) catalysts were evaluated using an accelerated durability test (ADT) method (in oxygen (99.999%, AGA) saturated 0.5 M H2SO4 aqueous solution, from 0.58 V to 0.98 V vs SHE, at potential scan rate 50 mV s−1) [4]. The oxygen electroreduction reaction kinetics and the value of electrochemically active surface area (ECA) of the catalysts were determined after 10, 100, 1000, 3000, 10 000, 20 000 and 30 000 cycles. Electrochemical impedance spectra were obtained at 0.9 V vs SHE within ac frequencies from 0.01 to 10 000 Hz to determine the changes of the electrolyte resistance and capacitance values during the ADT. The results show that only 20% decrease in electrode capacitance and 15% increases in series resistance takes place during 30,000 potential cycles. The values of ECA were estimated in the solution saturated with nitrogen (99.999%, AGA) using the cyclic voltammetry at different potential scan rates from 30 to 1000 mV s−1. The rotating disc electrode method was used to evaluate the oxygen electroreduction kinetics in solution saturated with oxygen at rotation rate 1600 rev min−1 and at potential scan rate 20 mV s−1within the region of potentials from 1.03 V to 0.03 V vs SHE. Based on the cyclic voltammetry and rotating disk electrode data the main decrease in ECA took place during the first 1000 cycles and thereafter the ECA value stabilized. The half wave potential values depend weakly on the number of cycles (up to 30,000) measured. The content of Pt in the catalyst was determined before and after the ADT using the electrochemical dissolution of platinum from the electrode in 6 M HCl solution. The average weight percentage of Pt in different catalyst materials was 71.8±1.7. The content of Pt in the catalysts before the ADT was in a good agreement with the TGA data.

The correlation of the properties of pyrrolidinium-based ionic liquid electrolytes with the discharge–charge performances of rechargeable Li–O2 batteries

Abstract Pyrrolidinium-based ionic liquids (ILs), such as PYR 13 TFSI, PYR 14 TFSI, and PYR 1(2O1) TFSI, exhibit high thermal and electrochemical stability with wide electrochemical windows as electrolytes for application to rechargeable Li–O 2 batteries. In this work, several fundamental properties of three ILs are measured: the ionic conductivity, oxygen solubility, and oxygen diffusion coefficient. The oxygen electro-reduction kinetics is characterized using cyclic voltammetry. The performances of Li–O 2 batteries with these IL electrolytes are also investigated using electrochemical impedance spectroscopy and galvanostatic discharge–charge tests. The results demonstrate that the PYR 1(2O1) TFSI electrolyte battery has a higher first-discharge voltage than the PYR 13 TFSI electrolyte and PYR 14 TFSI electrolyte batteries. Both PYR 13 TFSI- and PYR 1(2O1) TFSI-based batteries exhibit higher first-discharge capacities and better cycling stabilities than the PYR 14 TFSI-based battery for 30 cycles. A theoretical analysis of the experimental results shows that the diffusion coefficient and solubility of oxygen in the electrolyte remarkably affect the discharge capacity and cycling stability of the batteries. Particularly, the oxygen diffusion coefficient of the IL electrolyte can effectively facilitate the electrochemical oxygen electro-reduction reaction and oxygen concentration distribution in the catalyst layers of air electrodes. The oxygen diffusion coefficient and oxygen solubility improvements of IL electrolytes can enhance the discharge–charge performances of Li–O 2 batteries.

Influence of Temperature on the Oxygen Electroreduction Activity at Micro-Mesoporous Carbon Support

Influence of temperature on the kinetics of oxygen electroreduction on molybdenum carbide derived carbon based catalysts has been analyzed in O2 saturated 0.1M KOH solution using rotating disc electrode and cyclic voltammetry methods. The calculated kinetic current density value (|jkin| = 62.6 A m−2) at the half-wave potential (−0.05 V vs. Hg|HgO|0.1M KOH) of platinum nanoparticles modified catalyst was maximal at 60°C. Two different linear areas in the Tafel-like plot have been observed for unmodified and Pt nanoparticles modified catalysts within the studied temperature range from 10 to 80°C. The calculated experimental apparent activation energy values for the oxygen reduction were within a range from 19 to 27 kJ mol−1 for Pt nanoparticles modified carbon catalyst within potential range from 0.05 to −0.07 V vs. Hg|HgO|0.1M KOH. The oxygen reduction peak current density values decrease with increasing the temperature from 10 to 80°C.

Effect of Platinum Nanoparticle Loading on Oxygen Reduction at a Pt Nanocluster-Activated Microporous–Mesoporous Carbon Support

Very high surface area carbon-supported Pt nanoparticle catalysts have been applied to the oxygen reduction reaction in alkaline solution. The distribution effect of deposited Pt nanoparticles onto the carbon support on the oxygen reduction reaction kinetics has been established by comparing the various carbon-supported Pt (20 and 60 wt% metal) catalysts. The various Pt catalysts were prepared by using a sodium borohydride reduction method. All the catalysts showed a face-centered cubic crystal structure as determined by X-ray diffraction method; the average platinum particle sizes were ∼4.2, ∼4.8, ∼5.4, and ∼27.2 nm for 20 wt% Pt–C(Mo2C), 60 wt% Pt–C(Mo2C)-I, 60 wt% Pt–Vulcan XC72, and 60 wt% Pt–C(Mo2C)-II catalysts, respectively. The X-ray photoelectron spectra for all the catalysts indicated that most of the platinum nanoparticles have an oxidation state of 0. The low-temperature N2 sorption, time-of-flight mass spectrometry, scanning electron microscopy, and transmission electron microscopy experiments have been carried out to characterize the structure of prepared materials. The cyclic voltammetry and rotating disk electrode techniques were used to study the oxygen electroreduction kinetics.

USE OF PLATINUM MODIFIED TiÎ2 MICROELECTRODE FOR THE DETERMINATION OF OXYGEN CONCENTRATION IN BLOOD PLASMA

A microelectrode for the determination of oxygen dissolved in biologically active liquids by an electrochemical method has been developed, the sensitive material of which is nanodispersed titanium oxide, modified by Pt. The kinetics of oxygen electroreduction at TiO2 electrodes in blood plasma and sorbilactum solution have been investigated. It has been found that at cathodic potentials of -0.5 to -1.0 V (with respect to silver-chloride electrode) dependence of oxygen reduction current on concentration of oxygen in the rangeNI2 = (0,9÷4,6) ∙10-2 g∙l -1 was linear and passed through the beginning of coordinates. It has been shown that the number of electrons at the discharge stage in the oxygen reduction reaction is n=1, and that the total number of electrons in the reaction is ns ≈2. The electrodes investigated were distinguished by a high reproducibility of characteristics in longtime cycling and promise much for use in oxygen electrochemical sensors for biologically active liquids.

The Impact of Pt-Nanocluster Deposition and Nafion® Content on the Oxygen Electroreduction Kinetics on Molybdenum Carbide Derived Carbon Synthesized at 1000°C

Molybdenum carbide derived carbon was used as a porous support for platinum catalyst. Platinum was deposited onto carbon support in the form of nanoparticles using NaBH4 reduction method in ethylene glycol, so that the loading of platinum was 8.5 wt% and the average size of platinum nanoparticles was 3 nm. The high resolution scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction and the low temperature N2 sorption experiments were applied to characterize the structure of materials prepared. The kinetics of oxygen electroreduction was studied using cyclic voltammetry and rotating disc electrode methods in 0.5 M H2SO4 solution. The content of Nafion® was varied in the range from 5 to 30 wt%. As synthesized carbon exhibited high specific surface area and Nafion® binder disperses uniformly, the percentage of Nafion® has only slight influence on the electrochemical activity. Prepared catalyst materials demonstrated high activity towards oxygen electroreduction.

Mechanism of the effect of oxygen-modified carbon nanotubes on the kinetics of oxygen electroreduction on platinum

The relationship between the concentration of quinone groups in the electrode material containing carbon nanotubes (CNTs) and platinized carbon black and the efficiency of the use of the platinum surface in oxygen reduction was studied by cyclic voltammetry and rotating disc electrode methods. The effect of quinone groups on the oxygen coverage of the platinum surface and the density of the kinetic currents of molecular oxygen reduction on the platinum surface was investigated. A mechanism by which the oxygen-modified CNTs affect the kinetics of oxygen electroreduction on platinum was suggested.

The Impact of Pt-Nanocluster Deposition and Nafion® Content on ORR on Molybdenum Carbide Derived Carbon Synthesized at 1000°C

Carbide derived carbons (CDCs) are unique materials in which pore size, shape, uniformity and other parameters can be controlled in exact manner. Therefore these materials are interesting objects to be used as a catalyst support in polymer electrolyte membrane fuel cells (PEMFC) [1-3]. The structure of carbon material influences greatly electrochemical activity and therefore optimization of carbon material properties must be carried out in order to develop high power density low temperature fuel cells [4]. CDCs with incorporated platinum nanoparticles are highly active towards oxygen electroreduction (ORR) even at low Pt loadings. It has been suggested that microporous-mesoporous CDCs are viable fuel cell catalyst supports that are capable of realizing the full activity of Pt-nanoparticles with superior corrosion stability [1-3, 5].Carbon derived from Mo2C has been prepared using chlorination method at 1000°C. Pt was deposited onto carbon material using NaBH4 reduction method [1-3]. The structure of studied materials has been characterized using XRD, HRSEM, EDX and gas adsorption/desorption measurements. Electrochemical measurements were carried out in three-electrode cell. The kinetics of ORR has been analyzed using cyclic voltammetry, rotating disc electrode and electrochemical impedance spectroscopy methods in 0.5M H2SO4solution. The data of electrochemical and EDX analysis were in good agreement and demonstrated that the Pt content in the synthesized material was 10 wt%. It was shown that even the low Pt loading improves the electrochemical activity significantly by reducing the overpotential of ORR nearly by 0.8V. Number of electrons transferred per one oxygen molecule electroreduction calculated from Koutecky- Levich equation increases from two to four when activating the carbon material with Pt-nanoparticles.The content of Nafion®®was varied in the range 5-20 wt%. Results established show that the highest kinetic currents of ORR were observed with 15% Nafion®, but the percentage of Nafion® has only slight influence on the electrochemical activity. Thus, it was concluded that the Nafion® ionomer disperses very uniformly on the material studied and does not hinder the mass transport. This study demonstrates that investigated material is attractive for PEMFC applications. Figure 1. SEM image of Pt-activated carbon derived from Mo2C synthesized at 1000°C. Acknowledgements. This work was supported by Estonian target research project SF0180002s08, Estonian Centre of Excellence Project 3.2.1101.12-0019, Estonian Energy Technology Programme project SLOKT 10209T and ESF Grant ETF8267. References S. Sepp, E. Härk, P. Valk, K. Vaarmets, J. Nerut, R. Jäger, E. Lust, J. Solid State Electrochem. 2013, DOI 10.1007/s10008-013-2180-6.E. Härk, J. Nerut, K. Vaarmets, I. Tallo, H. Kurig, J. Eskusson, K. Kontturi, E. Lust, J. Electroanal. Chem. 2013, 689, 176-184.K. Vaarmets, S. Sepp, J. Nerut, E. Härk, I. Tallo, E. Lust, J. Solid State Electrochem 2013, 17(6), 1729-1741.B. Wang, J. of Power Sources 2005, 152, 1-15.L. Borchardt et al., Carbon 2012, 50, 1861-1870.

The kinetics of oxygen electroreduction: A long way from iron rust to lithium-air batteries

In this communication, we review the details of the oxygen electroreduction reaction on rust layers, passive iron and well defined iron oxides as cathodes for the reaction. In particular, we highlight the important electrocatalytic role of Fe(II) sites at the surface of the oxides in contact with the aqueous electrolyte. When the same reaction takes place at bifunctional cathodes with separated metal anodes which gives rise to metal–air batteries on which much attention has been paid recently in connection with electric vehicles.

Impact of the Pt catalyst on the oxygen electroreduction reaction kinetics on various carbon supports

Micro- and mesoporous carbide-derived carbons synthesized from molybdenum and tungsten carbides were used as porous supports for a platinum catalyst. Synthesized materials were compared with commercial Vulcan XC72R conducting furnace black. The scanning electron microscopy, X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, and low-temperature N2 adsorption methods were applied to characterize the structure of catalysts prepared. The kinetics of oxygen electroreduction in 0.5 M H2SO4 solution was studied using cyclic voltammetry and rotating disk electrode methods. The synthesized carbide-derived carbons exhibited high specific surface area and narrow pore size distribution. The platinum catalyst was deposited onto the surface of a carbon support in the form of nanoparticles or agglomerates of nanoparticles. Comparison of carbide-derived carbons and Vulcan XC72R as a support showed that the catalysts prepared using carbide-derived carbons are more active towards oxygen electroreduction. It was shown that the structure of the carbon support has a great influence on the activity of the catalyst towards oxygen electroreduction.

Electrochemical impedance characteristics and electroreduction of oxygen at tungsten carbide derived micromesoporous carbon electrodes

The electrical double layer characteristics and oxygen electroreduction kinetics in 0.5M H2SO4 aqueous solution has been studied at micromesoporous tungsten carbide derived carbon C(WC) electrodes. Carbon powders with various specific surface areas (1280–2116m2g−1) have been prepared from WC at chlorination temperatures 900°C, 1000°C and 1100°C. The porous structure of carbon substrate was characterised using nitrogen sorption, X-ray diffraction, high resolution TEM, electron energy loss spectroscopy, selected area electron diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy methods. Cyclic voltammograms at various potential scan rates from 2 to 70mVs−1, and rotating disc electrode data at rotation velocities from 0 to 3000revmin−1, were measured within the region of potentials from +0.4V to −0.6V vs. Hg|Hg2SO4|sat.K2SO4 in H2O (MSE). At E>−0.2V, the electroreduction of oxygen is mainly limited by the charge transfer step, and at −0.6V<E<−0.2V, by the mixed kinetics. The oxygen electroreduction mainly proceeds through the peroxide formation intermediate step on all electrodes studied. Despite of that the electrodes tested were very stable during the electrochemical experiment, indicating that the C(WC) is a suitable catalyst support material for polymer electrolyte membrane fuel cell. The electroreduction rate of oxygen depends strongly on the structure (graphitisation level) of carbide derived carbon used for preparation of an electrode and the oxygen reduction overvoltage decreases in the order C(WC) 1100°C>C(WC) 1000°C>C(WC) 900°C. Very high low-frequency capacitance values, independent of alternative current (ac) frequency at f<0.1Hz, have been established for C(WC) 1100°C, demonstrating that at ac f→0, mainly pseudocapacitive behaviour with adsorption limited step of reaction intermediates has been observed.

Oxygen reduction on Pd nanoparticle/multi-walled carbon nanotube composites

The electrochemical reduction of oxygen was studied on multi-walled carbon nanotube-supported palladium nanoparticle (PdNP/MWCNT) modified glassy carbon (GC) electrodes. The catalyst materials were prepared using two different methods and their electrocatalytic activity towards oxygen reduction was explored in both acid and alkaline media using the rotating disk electrode (RDE) method. Cyclic voltammetry was employed for CO stripping experiments and for the determination of real surface area of carbon nanotube–supported Pd nanoparticles. The surface morphology of PdNP/MWCNT nanocomposites was examined by transmission electron microscopy (TEM). Oxygen electroreduction kinetics were compared with those of bulk palladium electrodes. Enhanced electrocatalytic activity of PdNP/MWCNT modified GC electrodes was observed. The Tafel behaviour of oxygen reduction was similar for PdNP/MWCNT and bulk Pd electrodes. The RDE data analysis showed that the reduction of oxygen on the PdNP/MWCNT electrocatalysts studied followed a four-electron pathway.

Comparing factors that affect kinetics of oxygen electroreduction on platinum- and palladium-containing catalysts

The effect of blocking species, namely, oxygen-containing species adsorbed from water and blank electrolyte anions, on the kinetics of oxygen electroreduction on 40% Pt ETEK (denoted as 40Pt/C) and 20% Pd ETEK (20Pd/C) commercial catalysts, as well as on 20% Pd ETEK 6% Co (denoted as 20Pd6Co/C) and 20% Pd ETEK 5% Pt 2.9% Co (20Pd5Pt2.9Co/C) catalysts synthesized at the laboratory, is studied in sulfuric and perchloric acid solutions. It is shown that on a monopalladium catalyst, the first inhibiting factor prevails, whereas, on a monoplatinum catalyst, a substantial role in decelerating the reaction is played by the adsorption of HS anions. In the case of a trimetal catalyst, the adsorption curve of oxygen-containing species is noticeably shifted toward the curve typical of platinum. When perchloric acid is replaced with sulfuric acid, the potential of the kink point of the Tafel dependence of the oxygen electroreduction (on going from the low to high polarization) is shifted toward the less positive potentials. This shift of the kink potential is most clearly pronounced in the case of a monoplatinum catalyst.

Influence of phosphate anion adsorption on the kinetics of oxygen electroreduction on low index Pt(hkl) single crystals

The detrimental effects of phosphate anion adsorption on the oxygen reduction reactions (ORR) on low index Pt single crystal electrodes were studied in 0.1 M perchloric acid by using a hanging meniscus rotating disk electrode in the presence of varied concentrations of H(3)PO(4). The kinetic current for ORR decreased dramatically on Pt(100), Pt(110), Pt(111), and PtSn(111) even with the addition of a small amount (1 mM) of H(3)PO(4) into the perchloric acid solution, most probably due to the adsorption of phosphate anions onto the Pt active sites that impeded the electroreduction of O(2). Remarkably, the extent of decline was found to vary with the specific single crystal surface, following the order of Pt(111) > PtSn(111) > Pt(110) ∼ Pt(100). Consistent behaviors were also observed in Tafel analysis and in electrochemical impedance spectroscopic measurements. Within the present experimental context, Pt(110) was found to be the optimal crystal surface for ORR in phosphoric acid fuel cells with the smallest charge transfer resistance, whereas the poisoning effects of phosphate anion adsorption were the most pronounced on Pt(111), most likely because the phosphate anions primarily adsorbed on the 3-fold sites on the Pt(111) faces, as manifested in in situ X-ray absorption spectroscopic measurements.

Development of porous cathode powders for SOFC and influence of cathode structure on the oxygen electroreduction kinetics

Meso/macroporous La 1− x Sr x CoO 3−δ powder with the specific surface area higher than 140 m 2 g −1 has been synthesized from the corresponding nitrates, using solution thermal decomposition method. These nanopowders have been used for preparation of SOFC cathodes, demonstrating lower oxygen electroreduction activation energy than that for less porous cathodes prepared from the powders synthesized using traditional solid state reaction method. To increase macroporosity of the cathodes the special pore forming agent has been added into the raw cathode paste. The very low total polarization resistance and activation energy values have been obtained for oxygen electroreduction, depending on the cathode porosity and potential applied.