X-ray photoelectron spectroscopy study of anodically oxidized SIMFUEL surfaces

The surface composition of anodically oxidized SIMFUEL (doped uranium dioxide) has been determined as a function of applied potential over the range −500 to +500 mV (versus SCE). Cathodically cleaned UO₂ specimens were anodically oxidized for 1 h and subsequently analyzed by XPS. Using published binding energies, the U (4f_7/2) and O (1s) peaks were resolved into contributions from U^IV, U^V, U^VI, O^II, OH⁻ and H₂O. It was shown that over the potential range −500 to approximately +50 mV a thin surface layer of U^IV/U^V oxide (UO_2+x) formed. At more positive potentials, a U^VI hydrated oxide (UO₃·yH₂O) was deposited on the electrode surface. At very positive potentials (≥400 mV) the rapid anodic formation and hydrolysis of UO₂²⁺ led to local acidification in pores in the deposited UO₃·yH₂O layer and its subsequent re-dissolution.     

The influence of temperature on the anodic oxidation/dissolution of uranium dioxide

The anodic dissolution of U^IVO₂ has been studied at 60 °C in 0.1 mol dm⁻³ KCl using a range of electrochemical methods and X-ray photoelectron spectroscopy (XPS). The results were compared to previous results obtained at 22 °C. This comparison shows that the threshold for the onset of anodic dissolution (−400 mV versus SCE) is not noticeably changed by this increase in temperature. However, both the oxidation of the surface (to U^IV/VO_2+x) and the rate of anodic dissolution (as U^VIO₂²⁺) leading to the formation of a U^VIO₃·yH₂O deposit are accelerated at the higher temperature. The XPS analysis shows that the conversion of U^V-U^VI occurs at lower potentials at 60 °C. Consequently, once the surface becomes blocked by the presence of a U^VIO₃·yH₂O deposit, rapid dissolution coupled to uranyl ion hydrolysis causes the development of locally acidified sites within the fuel surface at lower potentials at the higher temperature.     

The anodic dissolution of SIMFUEL (UO2) in slightly alkaline sodium carbonate/bicarbonate solutions

The corrosion of nuclear fuel under waste disposal conditions is likely to be influenced by the bicarbonate/carbonate content of the groundwater since it increases the solubility of the U^VI corrosion product, [UO₂]²⁺. As one of the half reactions involved in the corrosion process, the anodic dissolution of SIMFUEL (UO₂) has been studied in bicarbonate/carbonate solutions (pH 9.8) using voltammetric and potentiostatic techniques and electrochemical impedance spectroscopy. The reaction proceeds by two consecutive one electron transfer reactions (U^IV → U^V → U^VI). At low potentials (≤250 mV (vs. SCE) the rate of the first electron transfer reaction is rate determining irrespective of the total carbonate concentration. At potentials >250 mV (vs. SCE) the formation of a U^VIO₂CO₃ surface layer begins to inhibit the dissolution rate and the current becomes independent of potential indicating rate control by the chemical dissolution of this layer.     

XPS study of Li ion intercalation in V2O5 thin films prepared by thermal oxidation of vanadium metal

The intercalation and deintercalation mechanisms of lithium into V₂O₅ thin films prepared by thermal oxidation of vanadium metal have been studied by X-ray photoelectron spectroscopy (XPS) using a direct anaerobic and anhydrous transfer from the glove box (O₂ and H₂O < 1ppm), where the samples were electrochemically treated, to the XPS analysis chamber. Vanadium in the as-prepared oxide films is mostly (from 93 to 96% depending on samples) in a pentavalent state (V⁵⁺) with a stoichiometric O/V concentration ratio fitting that of V₂O₅. Four to seven percent of VO₂ is also observed. After the 1st and the 2nd intercalation steps at E = 3.3 and 2.8 V versus Li/Li⁺, respectively, the V2p core level spectra evidence a partial reduction to V⁴⁺ states with a remaining concentration of 73 and 56% of V⁵⁺, in agreement with the intercalation of about 1/2 mol of Li per V₂O₅ mol at each intercalation step. Intercalated lithium was observed at a binding energy of 56.1 eV for Li1s. Changes of the electronic structure of the V₂O₅ thin film after intercalation are evidenced by the observation, at a binding energy of 1.3 eV, of occupied V3d states (V⁴⁺) originally empty in the pristine film (V⁵⁺). The V2p and Li1s core level spectra show that the process of Li intercalation is partially irreversible. In the first cycle, 34 and 14% of the vanadium ions remain in the V⁴⁺ state after deintercalation at E = 3.4 and 3.8 V versus Li/Li⁺, respectively, indicating a partially irreversible process already after the 1st deintercalation. The analyses of C1s and O1s XP spectra show the formation of a solid-electrolyte interface (SEI). The analyzed surface layer includes lithium carbonate and Li-alkoxides.     

Surface electrochemistry of UO2 in dilute alkaline hydrogen peroxide solutions: Part II. Effects of carbonate ions

The electrochemical reduction of hydrogen peroxide has been studied on uranium dioxide electrodes. The reduction kinetics are found to be influenced by dissolved carbonate/bicarbonate ions. The formation of hydrated U^VI species on the electrode surface is avoided in carbonate solutions, allowing H₂O₂ reduction to proceed at less cathodic potentials than in carbonate-free solutions. At more cathodic potentials, the adsorption of carbonate ions on the active reduction sites inhibits the H₂O₂ reduction reaction. Over a narrow potential region, the reduction of peroxide is catalyzed by coadsoption of H₂O₂ and HCO₃⁻/CO₃²⁻. The pH dependence of the H₂O₂ reduction reaction appears to be stronger in carbonate solutions than in solutions that do not contain carbonate. This can be attributed to the displacement of inhibiting CO₃²⁻/HCO₃⁻ adsorbed ions by OH⁻.     

Electrochemical deposition of uranium oxide in highly concentrated calcium chloride

The coordination circumstances and redox reactions of UO₂ ²⁺ in the aqueous solution concentrated by calcium chloride, such as CaCl₂·6H₂O (6.9 M CaCl₂), were studied by Raman spectroscopy and electrochemical methods. The frequency of the O=U=O symmetrical stretching vibration suggested that the complex formation of UO₂ ²⁺ with Cl⁻ leads to the weakening of U=O bond. In the electrochemical measurements, two-step cathodic currents were observed at −0.090 and −0.4 V (vs. Ag|AgCl) corresponding to the reduction of UO₂ ²⁺ to UO₂ ⁺ and that of UO₂ ⁺ to UO₂, respectively. It was found that UO₂ ⁺ formed at first cathodic current was disproportionated to form UO₂ ²⁺ and UO₂. The UO₂ was identified by X-ray diffraction analysis. Electrolytic deposition of UO₂ was observed in 6.9–4.7 M CaCl₂ and in 14 M LiCl. When small amount of proton, i.e., 0.005 M was coexisted in 6.9 M CaCl₂, UO₂ ²⁺ was reduced to form U⁴⁺ instead of UO₂.     

The distribution of lithium intercalated in V2O5 thin films studied by XPS and ToF-SIMS

The distribution of lithium in V₂O₅/V lower oxide duplex thin films prepared by thermal oxidation of V metal was analysed by XPS and ToF-SIMS after intercalation at 2.8 V versus Li/Li⁺ and de-intercalation at 3.8 V following cycling between 3.8 and 2.8 V in 1 M LiClO₄-PC. XPS analysis of the intercalated thin film evidenced a partial reduction (43 at.% V⁴⁺) of the V₂O₅ surface, the modification of its electronic structure and the presence of Li, consistent with the formation of the δ-Li_xV₂O₅ (0.9 ≤ x ≤ 1) phase. The Li in-depth distribution measured by ToF-SIMS shows a maximum in the outer layer of V₂O₅, but Li is also found at the oxide film/metal substrate interface indicating its diffusion across the inner layer of V lower oxides. The analyses performed after de-intercalation on the samples cycled 12, 120 and 300 times reveal the effect of aging on the trapping of lithium. A significant reduction (17-22 at.% V⁴⁺) of the V₂O₅ surface was measured after 300 cycles. The Li in-depth distribution shows a maximum at the interface between the outer layer of V₂O₅ and the inner layer of lower oxides. Aging favours the accumulation of lithium at this interface with a resulting enlarged distribution enriching the sub-surface of the outer layer of V₂O₅ and the inner layer of lower oxides after 300 cycles. Lithium is also found, but in smaller quantities, at the oxide film/metal substrate interface. Measurements performed in the non-electrochemically treated surface areas of the de-intercalated samples revealed the same type of modifications, evidencing the diffusion of lithium along the interfaces where it is trapped.     

Carbonate and sulphate green rusts—Mechanisms of oxidation and reduction

The oxidation and reduction of carbonate, GR(CO₃), and sulphate, GR(SO₄), green rusts (GR) have been studied through electrochemical techniques, electrochemical quartz crystal microbalance (EQCM), FTIR, XRD and SEM. The used samples were made of thin films electrodeposited on gold substrate. The results from the present work, from our previous studies and from literature were compiled in order to establish a general scheme for the formation and transformation pathways involving carbonate or sulphate green rusts. Depending on experimental conditions, two routes of redox transformations occur. The first one corresponds to reaction via solution and leads to the formation of ferric products such as goethite or lepidocrocite (oxidation) or to the release of Fe^II ions into the solution (reduction) with soluble Fe^II-Fe^III complexes acting as intermediate species. The second way is solid-state reaction that involve conversion of lattice Fe²⁺ into Fe³⁺ and deprotonation of OH groups in octahedra sheets (solid-state oxidation) or conversion of lattice Fe³⁺ into Fe²⁺ and protonation of OH groups (solid-state reduction). The solid-state oxidation implies the complete transformation of GR(CO₃) or GR(SO₄) to ferric oxyhydroxycarbonate exGRc-Fe(III) or ferric oxyhydroxysulphate exGRs-Fe(III), for which the following formulas can be proposed, Fe^III₆(OH)_(12−2y)(O)_(2+y)(H₂O)_(y)(CO₃) or Fe^III₆(OH)_(12−2z)(O)_(2+z)(H₂O)_(6+z)(SO₄) with 0 ≤ y or z ≤ 2. The solid-state reduction gives ferrous hydroxycarbonate exGRc-Fe(II) or ferrous hydroxysulphate exGRs-Fe(II), which may have the following chemical formulas, [Fe^II₆(OH)₁₀(H₂O)₂]·[CO₃, 2H₂O] or [Fe^II₆(OH)₁₀(H₂O)₂]·[SO₄, 8H₂O].     

Two-step hydrothermal synthesis of Ru-Sn oxide composites for electrochemical supercapacitors

A two-step hydrothermal process was developed to synthesize hydrous 30RuO₂-70SnO₂ composites with much better capacitive performances than those fabricated through the normal hydrothermal process, co-annealing method, or modified sol-gel procedure. A very high specific capacitance of RuO₂ (C_S,Ru), ca. 1150 F g⁻¹, was obtained when this composite was synthesized via this two-step hydrothermal process with annealing in air at 150 °C for 2 h. The voltammetric currents of this annealed composite were found to be quasi-linearly proportional to the scan rate of CV (up to 500 mV s⁻¹), demonstrating its excellent power property. From Raman, UV-vis spectroscopic and TEM analyses, the reduction in mean particulate size is clearly found for this two-step oxide composite, attributable to the co-precipitation of (Ru_δSn_1−δ)O₂·xH₂O onto partially dissolved SnO₂·xH₂O and the formation of (Ru_δSn_1−δ)O₂·xH₂O crystallites in the second step. This effect significantly promotes the utilization of RuO₂ (i.e., very high C_S,Ru). The excellent capacitive performances, very similar to that of RuO₂·xH₂O, suggest the deposition of RuO₂-enriched (Ru_δSn_1−δ)O₂·xH₂O onto SnO₂·xH₂O seeds as well as the individual formation of (Ru_δSn_1−δ)O₂·xH₂O crystallites in the second hydrothermal step.     

Catalytic combustion of methane on substituted strontium ferrites

Sr-hexaferrites prepared by co-precipitation method and calcined at 700-1000 °С have been characterized by thermogravimetric and differential thermal analysis (TG-DTA), Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H₂-TPR), and Ar adsorption techniques. It has been shown that hexaferrite phase formed after calcination at 700 °С is amorphous and its crystallization occurs at 800 °С. Specific surface area (S_BET) of the samples calcined at 700 °С is 30-60 m²/g. Reduction in hydrogen proceeds in several steps, Fe(III) in the hexaferrite structure being practically reduced to Fe⁰. Amount of hydrogen necessary for the reduction of the samples decrease in the order: SrMn₂Fe₁₀O₁₉ > SrFe₁₂O₁₉ > SrMn₆Fe₆O₁₉ > SrMn₂Al₁₀O₁₉. Surface composition of the ferrites differs from bulk. According to XPS data, the surface is enriched with strontium. Sr segregation is most probably explained by the formation of surface carbonates and hydroxocarbonates. The main components on the surface are in oxidized states: Mn³⁺ and Fe³⁺. Maximum activity in the methane oxidation is achieved for the SrMn_xFe_12−xO₁₉ (0 ? x ? 2) catalysts. These samples are characterized by highest amount of the hexaferrite phase, which promotes change of oxidation state Mn(Fe)³⁺ ↔ Mn(Fe)²⁺.     

Synthesis and characterization of ZnxMg1 − xGa2O4:Co fabricated by low-temperature burning method

A series of Zn_xMg_1 − xGa₂O₄:Co²⁺ spinels (x = 0, 0.25, 0.5, 0.75, and 1.0) was successfully produced through low-temperature burning method by using Mg(NO₃)₂·4H₂O, Zn(NO₃)₂·6H₂O, Ga(NO₃)₃·6H₂O, CO(NH₂)₂, NH₄NO₃, and Co(NO₃)₂·6H₂O as raw materials. The product was characterized by X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The product was not merely a simple mixture of MgGa₂O₄ and ZnGa₂O₄; rather, it formed a solid solution. The lattice constant of Zn_xMg_1 − xGa₂O₄:Co²⁺ (0 ≤ x ≤ 1.0) crystals has a good linear relationship with the doping density, x. The synthesized products have high crystallinities with neat arrays. Based on an analysis of the form and position of the emission spectrum, the strong emission peak around the visible region (670 nm) can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴A₂(⁴F)] of Co²⁺ in the tetrahedron. The weak emission peak in the near-infrared region can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴T₂(⁴F)] of Co²⁺ in the tetrahedron.     

Oxygen reduction on platinum electrodes in base: Theoretical study

Electrode-potential-dependent activation energies for electron transfer have been calculated using a local reaction center model and constrained variation theory for the oxygen reduction reaction on platinum in base. Results for four one-electron transfer steps are presented. For the first, O₂(ads) is predicted to be reduced to adsorbed superoxide, O₂⁻(ads), which dissociates with a low activation barrier to O(ads) + O⁻(ads). Then a proton transfer form H₂O(ads) to O⁻(ads) takes place, forming OH(ads) + OH⁻(aq). The second electron transfer reacts O(ads) with H₂O(aq) to form a second OH(ads) + OH⁻(aq). The third and fourth electron transfers react the two OH(ads) with two H₂O(aq) to form two H₂O(ads) + two OH⁻(aq). All three different surface reduction reactions are predicted to have reversible potentials in the −0.24 V(SHE) to −0.29 V(SHE) range for 0.1 M base and activation energies for the superoxide formation step are close to the experimentally observed range in 0.1 M base for the overall four-electron to water over the three low index (1 1 0) (1 0 0) and (1 1 1) surfaces: 0.38-0.49 eV at 0.35 eV respectively at 0.88 V(RHE). Predicted reversible potentials for forming O₂⁻(ads) are compared with estimates from the experimental literature. The difference between the acid mechanism, where the peroxyl radical, OOH(ads) is the first reduction intermediate, and the base mechanism, where superoxide, O₂⁻(ads) is the first reduction intermediate, is discussed.     

A perovskite-type KNbO3 nanoneedles based biosensor for direct electrochemistry of hydrogen peroxide

A KNbO₃ nanoneedles (KNs) based hydrogen peroxide (H₂O₂) biosensor was first proposed. Perovskite-type KNs can directly catalyze H₂O₂. The mechanism can be explained by Molecular Orbital Principles, with the formation of σ-bonding between the e_g orbital of surface niobium ions and surface adsorbed oxygen-related intermediate species. Direct electron transfer between the Horseradish peroxidase (HRP) and electrode surface was achieved. Co-catalyst system of both HRP and KNbO₃ was introduced to the oxidation of H₂O₂, thus the as-prepared biosensor exhibited high sensitivity (750 μA mM⁻¹ cm⁻²) and ultrafast response (1–2 s) to H₂O₂. Therefore, KNs provide a promising material for enzymes assembly and sensing application.     

Electrochemical quartz crystal microbalance studies of a palladium electrode oxidation in a basic electrolyte solution

Anodic oxidation of Pd in basic solutions (0.1 M KOH_aq and 0.1 M NaOH_aq) has been examined via cyclic voltammetry (CV) and an electrochemical quartz crystal microbalance (EQCM). Admittance tests show that Pd(II) layer behaves as a rigid one. The anodic vertex potential influences mass response during formation of the Pd(II) layer. For low anodic vertex potentials, obtained absolute mass per mole values suggest Pd(OH)₂ or PdO·H₂O to be oxidation products. At this stage of the oxidation process, contribution from adsorbed H₂O/OH⁻ in Pd(II) layer formation could explain the lower-than-expected mass gain, although the extent of H₂O/OH⁻ adsorption is unclear. The mass gain decreases with further increase in the anodic vertex potential, eventually reaching the value of ca. 8 g mol⁻¹ at about 700 mV vs. SCE. Comparing the influence of vertex potential in CV experiments on the mass and reduction potential of the Pd(II) species points to the formation of PdO at higher oxidation potentials. At this stage of the process, a fraction of the PdO species is generated during transformation of previously formed Pd(OH)₂/PdO·H₂O. A shift of the main Pd(II) reduction potential peak depends on both the anodic vertex potential and on the composition of the Pd(II) film. The order of the Pd(II) reduction process is the opposite of that observed for the oxidation process. The Pd(IV) species formed at E ≥ 500 mV vs. SCE and those reduced between 50 and 350 mV are hydrated or contain hydroxyl groups.     

Catalytic steam reforming of ethane and propane over CeO2-doped Ni/Al2O3 at SOFC temperature: Improvement of resistance toward carbon formation by the redox property of doping CeO2

Ni/Al₂O₃ with the doping of CeO₂ was found to have useful activity to reform ethane and propane with steam under Solid Oxide Fuel Cells (SOFCs) conditions, 700-900 °C. CeO₂-doped Ni/Al₂O₃ with 14% ceria doping content showed the best reforming activity among those with the ceria content between 0 and 20%. The amount of carbon formation decreased with increasing Ce content. However, Ni was easily oxidized when more than 16% of ceria was doped. Compared to conventional Ni/Al₂O₃, 14%CeO₂-doped Ni/Al₂O₃ provides significantly higher reforming reactivity and resistance toward carbon deposition. These enhancements are mainly due to the influence of the redox properties of doped ceria. Regarding the temperature programmed reduction experiments (TPR-1), the redox properties and the oxygen storage capacity (OSC) for the catalysts increased with increasing Ce doping content. In addition, it was also proven in the present work that the redox of these catalysts are reversible, according to the temperature programmed oxidation (TPO) and the second time temperature programmed reduction (TPR-2) results.During the reforming process, in addition to the reactions on Ni surface, the gas-solid reactions between the gaseous components presented in the system (C₂H₆, C₃H₈, C₂H₄, CH₄, CO₂, CO, H₂O, and H₂) and the lattice oxygen (O_x) on ceria surface also take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen (O_x) on ceria surface (C_nH_m+O_x→nCO+m/2(H₂)+O_x−n) can prevent the formation of carbon species on Ni surface from hydrocarbons decomposition reaction (C_nH_m⇔nC+m/2H₂). Moreover, the formation of carbon via Boudard reaction (2CO⇔CO₂+C) is also reduced by the gas-solid reaction of carbon monoxide (produced from steam reforming) with the lattice oxygen (CO+O_x⇔CO₂+O_x−1).     

Roles of adsorbed OH and adsorbed H in the oxidation of hydrogen and the reduction of UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack> ions at Pt electrodes under non-conventional conditions

The roles of adsorbed hydroxyl radicals, OH, at a high temperature and adsorbed hydrogen atoms, H, in an acidic solution were investigated in the electrochemical reactions on Pt electrode by using potentiodynamic polarisation experiment, cyclic voltammetry and constant-potential electrolysis combined with UV/VIS analysis. From the analysis of the polarisation curves obtained from Pt electrode in a 0.185 M H₃BO₃ solution at 473 K, it was found that the reducing capability of dissolved hydrogen is significantly enhanced due to the increases of the mass transfer and the electron transfer rates. Especially, it is suggested that the stable Pt-OH_ad plays a significant role in the passivation reaction in the potential range from 0.60 to 0.75 V_SHE. From the analyses of the experimental results for the electrochemical reduction of UO₂²⁺ ions on Pt surface in a 1.0 M HClO₄ solution, it is recognised that the reduction reaction of UO₂²⁺ to U⁴⁺ ions is strongly dependent on the hydrogen atoms adsorbed on Pt electrode (indirect reduction of UO₂²⁺) as well as on the electrons transferred from Pt electrode (direct reduction of UO₂²⁺). In addition, the reduction mechanism of UO₂²⁺ ions involved in Pt-H_ad is also proposed.     

Synthesis and characterization of nano-crystalline strontium hexaferrite using the co-precipitation and microemulsion methods with nitrate precursors

Nano-crystalline strontium hexaferrite (SrFe₁₂O₁₉) powder was synthesized using the classical co-precipitation and microemulsion methods. The precursors were obtained by precipitating Sr²⁺ and Fe²⁺ ions using tetramethylammonium hydroxide and calcinating at different temperatures ranging from 400 °C to 1000 °C in air. The influence of the Sr²⁺/Fe³⁺ mol ratio and the calcination temperature on the product formation and magnetic properties were studied. The formation of nanosized particles of SrFe₁₂O₁₉ with a relatively high saturation magnetization M_s = 64 Am²/kg, remanent magnetization of M_r = 39 Am²/kg and a coercitivity of H_c = 5.5 kOe was achieved at a Sr²⁺/Fe³⁺ mol ratio of 1:8 calcined at 900 °C. The formation of the SrFe₁₂O₁₉ was inspected using XRD analysis, thermogravimetric analysis (TGA), differential thermal analysis (DTA), TEM, and magnetic measurements.     

Solar Thermal Hydrogen Production from Water over Modified CeO2 Materials

A series of cerium oxide based materials for hydrogen production from water was studied by temperature-programmed reduction, X-ray diffraction and X-ray photoelectron spectroscopy (XPS) in addition to thermal reactions with water vapour. The addition of uranium ions into CeO₂, making mixed oxides Ce _x U_1?x O₂, resulted in noticeable modification of the reduction properties of CeO₂; with the main observations being the decrease in reduction temperature and the increase of hydrogen consumption when compared to CeO₂ alone. XPS U4f of the as prepared Ce_0.5U_0.5O₂ showed the presence of large amounts of U⁶⁺ cations at 380.9 eV in addition to the U⁴⁺ cations at 379.9 eV; the ratio U⁴⁺ to U⁶⁺ cations was found equal to 0.35. XPS Ce3d showed, on the contrary, considerable amount of Ce³⁺ cations with an estimated ratio of Ce³⁺ to Ce⁴⁺ = ca. 0.5. Ar-ions sputtering results in decreasing the U⁶⁺ contribution and a dramatic increase of the Ce³⁺ contribution. The decrease of U⁶⁺ cations was, however, not mirrored by the increase in Ce³⁺ cations. After five minutes of Ar ions sputtering (1 kV, 10 mA) the surface and near surface Ce3d line shapes looked closer to those of Ce₂O₃ with prominent Ce3d_5/2 and Ce3d_3/2 lines at 885.6 and 904.0 eV attributed to v′ and u′, respectively. The Ce _x U_1?x O₂ series was tested for hydrogen production from water (where x = 0, 0.25, 0.5, 0.75 and 1). All uranium containing oxides had higher activity than CeO₂ or UO₂ alone. Ce_0.75U_0.25O₂ was found to have the highest activity in the studied series; about one order of magnitude higher than that of CeO₂ alone at the same temperature. The reason for the enhanced activity is linked to the ease by which oxygen ions are removed from the oxide materials.     

Electrocatalyst materials for fuel cells based on the polyoxometalates—K7 or H7[(P2W17O61)Fe(H2O)] and Na12 or H12[(P2W15O56)2Fe4(H2O)2]

Free acids of the iron substituted heteropoly acids (HPA), H₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (HFe1) and H₁₈[(P₂W₁₅O₅₆)₂Fe^III₂(H₂O)₂] (HFe2) were prepared from the salts K₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (KFe1) and Na₁₂[(P₂W₁₅O₅₆)₂Fe^III₄(H₂O)₂] (NaFe4), respectively. The iron-substituted HPA were adsorbed on to XC-72 carbon based GDLs to form HPA doped GDEs after water washing with HPA loadings of ca. 1 μmol. The HPA was detected throughout the GDL by EDX. Solution electrochemistry of the free acids are reported for the first time in sulfate buffer, pH 1-3. The hydrogen oxidation reaction was catalyzed by KFe1 at 0.33 V, with an exchange current density of 38 mA/cm². Moderate activity for the oxygen reduction reaction was observed for the iron substituted HPA, which was dramatically improved by selectively removing oxygen atoms from the HPA by cycling the fuel cell cathode under N₂ followed by reoxidation to give a restructured oxide catalyst. The nanostructured oxide achieved an OCV of 0.7 V with a Tafel slope of 115 mV/decade. Cycling the same catalysts in oxygen resulted in an improved catalyst/ionomer/carbon configuration with a slightly higher Tafel slope, 128 mV/decade but a respectable current density of 100 mA/cm² at 0.2 V.     

In situ analysis of oxygen partial pressure at the cathode catalyst layer/membrane interface during PEFC operation

To understand the concentration overpotential in the polymer electrolyte fuel cell (PEFC), we have performed an in situ analysis of the oxygen partial pressure (p[O₂]_CL/PEM) at the interface between the cathode catalyst layer (CL) and the polymer electrolyte membrane (PEM). Diffusion-limited oxygen reduction current was measured, with Pt probes inserted into the PEM, during cell operation by supplying H₂ to the anode and O₂ + N₂ to the cathode at 80 °C. It was found that the p[O₂]_CL/PEM decreased by ca. 20% when the current density was stepped from 0 to 2.0 A cm⁻² at p[O₂]_gas = 54 kPa and 100% RH at the cathode inlet, irrespective of the oxygen utilization UO₂ (from 10% to 50%). Such a change in p[O₂]_CL/PEM might result in a concentration overpotential of ca. 10 mV, based on the Tafel slope of 120 mV decade⁻¹ in the high current density region. It was also found that ohmic losses in the ionomer phase of the CL increased with decreasing humidity, from 100% to 80% RH, and became a dominant factor in the increased total overpotential, while the corresponding concentration overpotential was unchanged. The present results provide new insight into the transport of oxygen and water at the CL/PEM interface, especially at the high current densities required for the electric vehicle application.