Borohydride Electrooxidation Research Articles

Insights into the potential dependence of the borohydride electrooxidation reaction mechanism on platinum nanoparticles supported on ordered carbon nanomaterials

Borohydride oxidation reaction (BOR) was studied on platinum nanoparticles deposited on vertically-aligned carbon nanofilaments (VACNF). Depending on the amount of Pt deposited on the VACNF support, Pt/VACNF electrodes with a wide range of Pt nanoparticle density on the VACNF substrate were obtained. Cyclic voltammetries of BOR on these model Pt/VACNF electrodes using the rotating disk electrode (RDE) setup demonstrated the existence of two different and potential-dependent pathways for the BOR on such nanostructured Pt surfaces. The low potential BOR pathway (E < 0.55V vs. RHE) does not occur (in near-steady-state conditions) when the Pt nanoparticle density of the Pt/VACNF electrode is too low, pointing out to the sensitivity of the low-potential BOR towards the density of free Pt sites. The complete dissociative adsorption of BH4−, a reaction strongly demanding for free Pt sites, was proposed as the first step of the BOR, in agreement with recent DFT and experimental studies. At higher potential values (E > 0.55V vs. RHE), this influence of the Pt site density levels off, mainly due to the dual role of hydroxyl species adsorbed on the Pt surface. Indeed, while reducing the fraction of free Pt sites, adsorbed hydroxyls also accelerate the removal of the stable BOR adsorbed intermediates that block the Pt surface (and are responsible for the sensitivity of the low-potential BOR towards the density of free Pt sites) by a Langmuir-Hinshelwood type electrooxidation. In the meantime, voltammograms plotted using an Au-ring rotating ring-disk electrode (RRDE) specifically enabled the detection of BH3OH− released in this potential region at the Pt/VACNF disk electrode. The first steps of this high-potential BOR pathway, including the Pt-oxides formation, were proposed. The whole tentative BOR mechanism has been discussed using a mean field microkinetic model, where experimental variation of Pt nanoparticle density has been modelled by a variation of Pt site density. The main features of the Au-ring RRDE experiments are qualitatively reproduced.

Investigation of Carbon Supported Nanostructured PtAu Alloy as Electrocatalyst for Direct Borohydride Fuel Cell

AbstractCarbon supported bimetallic PtAu electrocatalysts for sodium borohydride electrooxidation are prepared by a modified citrate stabilized NaBH4 reduction process at different pH and temperature values. The physical properties of the materials are characterized by X‐ray diffraction spectroscopy, energy dispersive spectrometry, X‐ray photoelectron spectroscopy and transmission electron microscopy. Nano sized electrocatalysts have narrow size distributions and are uniformly dispersed on the surface of carbon support. Electrochemical performances of catalysts for sodium borohydride electrooxidation are tested with 25 cm2 single fuel cell. The highest performance is obtained at a peak power density of 161 mW cm−2 with 20 wt. % PtAu/C catalyst of 7.03 nm. Impact of the fuel cell operation parameters including concentration of NaBH4, flow rates of oxidant and fuel, and fuel cell operation temperature are investigated. The best operation parameters are obtained at 1 M NaBH4 concentration, 3 cm3 min−1 NaBH4 flow rate, 0.2 dm3 min−1 oxygen flow rate and 65 °C fuel cell temperature.

Borohydride electro-oxidation in a molten alkali hydroxide eutectic mixture and a novel borohydride–periodate battery

The electrochemical oxidation of BH4− in a molten NaOH–KOH eutectic mixture (0.515:0.485 mole fractions), is investigated for the first time by cyclic voltammetry and electrochemical impedance spectroscopy. Anodically oxidized Ni is electrocatalytically more active than Pt for BH4− oxidation in the molten alkali electrolyte as shown by the more than three times higher exchange current density (i.e. 15.8 mA cm−2 vs. 4.6 mA cm−2 at 185 °C). Next the proof-of-concept for a novel BH4−/IO4− molten alkali electrolyte battery is presented. Using oxidized Ni mesh anode and Pt mesh cathode a maximum power density of 63 mW cm−2 is achieved at 185 °C.

Borohydride electro-oxidation on Pt single crystal electrodes

Abstract The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates.

The effective carbon supported core–shell structure of Ni@Au catalysts for electro-oxidation of borohydride

Carbon-supported Ni–Au nanoparticles with a core–shell structure (Ni@Au/C) are prepared by a combination of reverse microemulsion and two-step reduction. The prepared Ni@Au/C nanoparticles are used as anode electrocatalysts in direct borohydride-hydrogen peroxide fuel cells (DBHFCs). The physical and electrochemical properties of the as-prepared catalysts are investigated using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical quartz crystal microbalance (EQCM), chronoamperometry (CA), chronopotentiometry (CP) and fuel cell test. The average size of the particles are approximately 10 nm. Among the resultant catalysts obtained in this study, the Ni1@Au1/C catalyst exhibits the highest catalytic activity for the direct electro-oxidation of borohydride. DBHFCs fabricated using Ni1@Au1/C as an anode catalyst and Pt mesh (1 cm × 1 cm) as a cathode electrode achieves a maximum power density of 74 mW cm−2 with a discharge current density of 130 mA cm−2 at 20 °C.

Nanoporous Ag–ZrO2 composites prepared by chemical dealloying for borohydride electro-oxidation

A novel two-step dealloying method is adopted to prepare nanoporous Ag–ZrO2 composite catalysts with the enhanced electrocatalytic performance by chemical dealloying of the melt-spun Al–Ag–Zr precursory alloys. The Zr atoms released from the precursory alloys are oxidized into ZrO2 during dealloying and are loaded on the inner surface of nanoporous Ag. The dealloyed ribbons exhibit an interpenetrating ligament-channel structure with nanometer length scales. The X-ray Photoelectron Spectroscopy (XPS) results reveal that the electron charge transfer takes place between ZrO2 film and Ag ligaments. The electrochemical tests demonstrate that the current density peak increases with calcination temperatures in a certain range and that the nanoporous composites with optimized ZrO2 content exhibit a higher catalytic activity, resulting in the oxidation current density increase of 91.3% compared with nanoporous Ag. The excellent catalytic activity can be attributed to the interfacial interaction and electron charge transfer between Ag and ZrO2.

Au hollow nanospheres on graphene support as catalyst for sodium borohydride electrooxidation

Abstract The Au hollow nanospheres on graphene support (HNAu/G) are prepared by galvanic replacement reaction between sacrificial cobalt nanoparticles and AuCl4− ions. The reaction is conducted at room temperature in aqueous solution. The prepared HNAu/G is used as the anode electrocatalyst for sodium borohydride electrooxidation. The physical and electrochemical properties of as-prepared electrocatalyst are investigated by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), chronoamperometry (CA), and chronopotentiometry (CP). The results show that Au hollow nanospheres on graphene support are coreless with the size of about 18 nm. The Au hollow nanospheres on graphene support exhibit an enhanced electrocatalytic performance for borohydride electrooxidation compared with the solid Au nanoparticles on graphene support (NAu/G).

High electrocatalytic activity of cobalt–multiwalled carbon nanotubes–cosmetic cotton nanostructures for sodium borohydride electrooxidation

Flexible and wearable cobalt electrode with a unique three-dimensional hierarchical-network structure is prepared by electrodeposition of spherical Co particles onto multiwalled carbon nanotubes (MWNTs) which are assembled on the skeleton of cosmetic cotton (CC). The morphology and phase structure of the cobalt–multiwalled carbon nanotubes–cosmetic cotton (Co–MWNTs–CC) electrode are characterized by scanning electron microscope, transmission electron microscope and X-ray diffraction spectrometer. The NaBH4 electrooxidation performance on the Co–MWNTs–CC electrode is investigated by means of cyclic voltammetry and chronoamperometry. Results show that the Co–MWNTs–CC electrode exhibits remarkably high catalytic activity and good stability for NaBH4 electrooxidation. The oxidation current density reaches as high as 170 mA cm−2 at −0.7 V in 1.0 mol dm−3 NaOH and 0.1 mol dm−3 NaBH4, which is higher than the most-related previous results.

Borohydride electro-oxidation by Ag-doped lanthanum chromites

The electrocatalytic activity of Ag-doped lanthanum chromites electrode materials viz., LaCr0.4Ag0.6O3 and LaCr0.7Ag0.3O3 prepared by decomposing the precursor complex is studied. Pure LaCrO3 is synthesized by combustion route using oxalic acid as a fuel. The decomposition behaviour of the as-synthesized powder obtained in the latter method is characterized by TGA-DTA and XRD. Both the precursor complex and the as-synthesized powder are calcined at 900°C for 7 and 10 h, respectively. XRD of the final product after calcinations indicated the formation of perovskite phase with minor amounts of impurity phases of component oxides in the Ag-doped lanthanum chromites and pure perovskite phase in the undoped one. The surface morphology of the perovskites is studied by SEM. The electrocatalytic activity of the perovskite powders for borohydride oxidation is studied by using cyclic voltammetry (CV) at a catalyst loading of 0.7 mgcm−2 for both Ag-doped and undoped LaCrO3 coated on glassy carbon substrate. Calibration plots are obtained by plotting the anodic peak current versus concentration of borohydride in the range of 20–100 mM. The sensitivities of the three perovskites towards borohydride oxidation indicated that LaCr0.4Ag0.6O3 is the best among all the perovskites studied giving a value of 1.395 μA/mM. The undoped and Ag-doped lanthanum chromites (LaCr0.4Ag0.6O3 and LaCr0.7Ag0.3O3) synthesised by us are compared for activity towards borohydride electro-oxidation using cyclic voltammetry. Among these, LaCr0.4Ag0.6O3 shows highest sensitivity towards borohydride oxidation (1.395 µA/mM). The results are interpreted by comparing this work with those of Ag-based electrocatalysts.

New route for synthesis of electrocatalytic Ni(OH)2 modified electrodes—electrooxidation of borohydride as probe reaction

Immobilization of redox species like Ni(OH)2 onto the electrode surface is important in the application areas such as super capacitor, electrochromic displays and electrocatalysis. Nickel hexacyanoferrate (NiHCF) modified glassy carbon could be further derivatized with Ni(OH)2 by electrochemical cycling in alkali. The electrodeposition of Ni(OH)2 was usually carried out onto the electrode surface from nickel salt at high interfacial pH. This paper reports the preparation of Ni(OH)2 from insoluble nickel tetracyanonickelate supported on carbon (NTN/C). This insoluble precursor complex was decomposed by two methods. (1) By potential cycling of modified electrode with the above complex in alkali. (2) By thermal decomposition of the precursor complex (NTN/C) to form metallic nickel followed by cycling in alkali. Ni(OH)2 modified electrodes formed using both methods were characterized by cyclic voltammetry and also by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. Further, electrocatalytic properties of Ni(OH)2/C modified electrodes formed by the above two methods were studied and compared using borohydride oxidation as probe reaction.

Investigation of Borohydride Oxidation on Graphene Supported Gold-Copper Nanocomposites

Graphene as catalyst support has incurred intense interest in fuel cell applications due to its unique, outstanding physicochemical properties. Pure gold and gold-based catalysts show an enhanced electrocatalytic activity towards the direct oxidation of borohydride. Combination of graphene and gold-based catalysts allows reducing the amount of noble metal and fabricating novel materials with high efficiency towards borohydride electro-oxidation.In this study the graphene supported AuCu catalysts (denoted as AuCu/GR) with different Au:Cu molar ratio were prepared by microwave heating of Au(III) and Cu(II) salts in ethylene glycol (EG) solutions at 150 oC for 30 min. It has been determined that the composition of AuCu/GR catalysts depends on pH of reaction mixture. When the reaction mixture pH was 7.0, 10.1 and 12.2, the AuCu/GR catalysts having the Au:Cu molar ratio 37:1, 1:13 and 1:32, respectively, were synthesized. The morphology, structure and composition of the prepared catalysts were examined by transmission electron microscopy (TEM), X-ray diffraction (XRD) and inductively coupled optical emission spectroscopy (ICP-OES). The electrocatalytic activity of the synthesized catalysts was investigated towards the oxidation of borohydride by means of cyclic voltammetry and chronoamperometry.It was found that Au nanoparticles were successively deposited onto the surface of graphene nanosheets. Au nanoparticles were uniform and well dispersed on the surface of graphene. The analysis of composition of the prepared AuCu/GR catalysts by means of ICP-OES confirms the presence of Au and Cu.The Au loadings were 0.055 mg Au cm–2 in the Au/GR and 0.107, 0.091 and 0.088 mg Au cm–2 in the synthesized AuCu/GR catalysts having the Au:Cu molar ratio 37:7, 1:13 and 1:32, respectively. It has been determined that the graphene supported AuCu catalysts show an enhanced electrocatalytic activity towards the oxidation of BH4 – ions comparing with the graphene supported bare Au catalyst and graphene supported bare Cu catalyst. Furthermore, borohydride oxidation current densities at the graphene supported AuCu/GR catalysts having the Au:Cu molar ratio 1:12 and 1:32 are ~13 times higher than those obtained at the AuCu/GR catalyst with the Au:Cu molar ratio 37:1. Acknowledgement This research was funded by a Grant (No. ATE-08/2012) from the Research Council of Lithuania.

Design and Fabrication Three-Dimensional Structure Electrode and Its Application in NaBH4 Electrooxidation and H2O2 Electroreduction

The direct liquid feed borohydride–hydrogen peroxide fuel cell (DBHPFC), comprised of borohydride oxidation at the anode and H2O2 reduction at the cathode, has attracted increasing attentions recently due to its high energy density, compactness, capable of operating in air-free environment (space and underwater), and the easy storage and distribution of both the fuel and the oxidant[ 1-2 ] . The main issues existing in DBHPFC are the hydrolysis and incomplete electrooxidation of borohydride at the anode resulting in evolution of H2 and the chemical decomposition of H2O2 at the cathode leading to the formation of O2. The releases of H2 and O2 not only reduce the energy density of DBHPFC, but also cause problems in the design and safety management of fuel cell systems[ 3-4 ] . Another important issue is how to reduce the use of noble metals in both borohydride oxidation and H2O2reduction. Nowadays, nano-materials have attracted much attention due to their unique physical and chemistry properties. The unique properties of nano-materials in facilitating the mass transport, ion diffusion and electron transfer, thus dramatically boosting the electrochemical performance, suggests that the main challenge of DBHPFC could be resolved or greatly ameliorated. In general, self-supported nano-materials growing directly on a current-collecting substrate represent an amazing architecture. Such structures have been demonstrated to possess larger electrochemical active surface area, higher utilization efficiency of the active materials, and superior mass transport property than conventional electrodes fabricated by mixing and pressing powder of active material with conducting materials (e.g., carbon black) and polymer binders (e.g., polytetrafluorethylene). State of the art, Solid template and structure-directing agents are commonly used to fabricate materials with hierarchical structures. However, some problems and challenges still remain, since complete template removal is needed, which means a much more complicated process including the selection of appropriate solvent or calcination at elevated temperature. Furthermore, impurities can be introduced from the templates or agents that affect the properties adversely. Therefore, it is necessary to find a simple and feasible method for the preparation of electrodes with three-dimensional structure .We reported a newly designed and fabricated electrode with three-dimensional structure[ 5 ]. The electrode consists of a conducting nanoarray substrate and Pd nanoparticles. The substrate is an array of TiO2 nanowires with a carbon coating layer prepared via a thermal evaporation method. Pd nanoparticles were electro-deposited on the substrate surfaces by the potentiostatic pulse method. The electrode exhibited excellent catalytic performance for NaBH4 electro-oxidation. The current density for NaBH4 electro-oxidation at the electrode (524 mA mg-1) is about 5 times of that at conventional Pd/C (100 mA mg-1). This enhanced performance is likely to be due to the improved mass transport of NaBH4, good electronic conductivity and high Pd utilization of the electrode.We alos reported a porous (Co, Mn)3O4 nanowires freely standing on Ni foam electrode, which are synthesized via a template-free growth method, followed by a thermal treatment in the air[ 6 ]. Results show that thermal treatment leads to the conversion of solid nanowires of MnCO3+CoCO3 to porous nanowires of (Co, Mn)3O4 via decomposition and reconfiguration, which is identified to be the catalytic active component for H2O2 electroreduction. The nanowires calcined at 300 °C exhibit the highest activity for the H2O2 reduction and a current density of 329 mA cm-2 is obtained in 3.0 mol dm-3 KOH + 0.6 mol dm-3 H2O2 at -0.4 V (vs. Ag/AgCl, KCl). The catalytic activity of (Co, Mn)3O4 nanowires is almost twice than that of Co3O4nanowires. The enhanced catalytic activity is explained by the decrease in the energy barrier of O-O bond cleavage caused by the introduction of Mn.In conclusion, compared to conventional fuel cell electrodes fabricated by mixing active materials with conducting agents and polymer binders, this three-dimensional structure electrode directly grown on substrate has superior mass transport property, which combining with its low-cost and facile preparation, make it a promising electrode for DBHPFC.

Influence of the surface morphology of smooth platinum electrodes for the sodium borohydride oxidation reaction

In this study, we show that the platinum electrode preparation procedure influences its behavior towards the borohydride electrooxidation reaction (BOR) mechanism. Cycling a smooth polycrystalline Pt electrode in alkaline electrolyte within the water stability domain prior to the BOR characterization radically changes the shape of the BOR voltammogram obtained in hydrodynamic conditions using the rotating disk electrode (RDE) setup, compared to the “classical” one measured on a smooth polycrystalline Pt electrode just polished before the BOR RDE study. This particular BOR voltammogram is reversibly brought back to the “classical” one after voltammetric cycling in borohydride alkaline media. These changes in the BOR voltammogram highlight the sensitivity of the BOR mechanism towards the Pt surface morphology. A first comparison of the Pt electrode surface before and after the voltammetric cycling in alkaline media using tapping mode atomic force microscopy (AFM) shows no morphological differences between the two surfaces within the AFM observation range, suggesting a very fine atomic structure disordering of the Pt surface. Such strong dependence of the BOR mechanism on Pt regarding the electrode atomic structuring opens the way to future studies focusing on the BOR on well-defined Pt single crystals.

Low-cost and binder-free, paper-based cobalt electrode for sodium borohydride electro-oxidation

A low-cost and binder-free cobalt electrode is prepared by the electrodeposition of Co nanoplates onto a flexible conductive substrate, which is prepared simply by scratching a piece of A4 paper with a common 8B pencil.

Mechanistic Insight into the Au-3d Metal Alloy-Catalyzed Borohydride Electro-Oxidation: From Electronic Properties to Thermodynamics

Recent theoretical and experimental findings have motivated the use of 3d transition metals as alloying elements to improve the performance of Au for the electro-oxidation of borohydride. In this paper, we provide mechanistic insights into the electrochemical oxidation of borohydride on pure Au and Au-3d alloys (Au3M with M = Cr, Mn, Fe, Co, Ni) using first principles calculations. We found that the initial oxidative adsorption of borohydride is the least exothermic among the elementary reactions considered for the complete eight-electron oxidation process. Interestingly, Au-3d metal alloy surfaces promote this oxidation step at a lower electrode potential compared to pure Au due the enhanced stability of borohydride on these alloy surfaces. The most negative borohydride oxidation potential is achieved by M = Co, followed by Fe, Ni, Mn, and Cr in order of increasing electrode potential. Subsequent to the initial oxidative adsorption of borohydride, the dehydrogenations of BH3*, BH2OH*, and BH(OH)2* are endothermic on pure Au at very low potentials. However, these activated and possibly limiting elementary reaction steps are more exothermic on Au-3d alloys than on pure Au. Following the adsorption of borohydride on the surface, all elementary reaction steps for the complete electro-oxidation process proceed downhill in energy at a lower electrode potential on Au-3d alloys than on pure Au.

Investigation of the porous nanostructured Cu/Ni/AuNi electrode for sodium borohydride electrooxidation

Abstract An electrochemical approach to nanostructured Cu/Ni/AuNi catalyst design using the electrodeposition process followed by galvanic replacement technique is presented. The procedure consisted of the electrodeposition of Ni–Zn on the Ni coating with subsequent replacement of the zinc by gold at open circuit potential in a gold containing alkaline solution. The surface morphologies and compositions of coatings were determined by energy dispersive X-ray and scanning electron microscopy techniques. The results showed that the Cu/Ni/AuNi coatings were porous composing of discrete Au nanoparticles. The electrocatalytic activity of Cu/Ni/AuNi electrodes for sodium borohydride electro-oxidation was studied using cyclic voltammetry, chronoamperometry, chronopotentiometry and electrochemical impedance spectroscopy techniques. The electro-oxidation current on Cu/Ni/AuNi catalyst is much higher than that on flat Au catalyst. The onset potential and peak potential on Cu/Ni/AuNi catalysts are more negative than that on flat Au catalyst for borohydride electrooxidation. The impedance behavior also shows different patterns, capacitive, and negative resistances and inductive loops at different applied anodic potentials. All results show that the Cu/Ni/AuNi catalysts can be applied as potential anode catalysts for the direct borohydride fuel cells.

Nickel particles supported on multi-walled carbon nanotubes modified sponge for sodium borohydride electrooxidation

A novel Ni electrode with a unique three-dimensional hierarchical macro-porous structure is prepared by electrodeposition of spherical Ni particles onto multi-walled carbon nanotubes (MWNTs) which are assembled on the skeleton of a sponge. The morphology and phase structure of the Ni@MWNTs/Sponge electrode are characterized by a scanning electron microscope, transmission electron microscope and X-ray diffraction spectrometer. The catalytic performance of the Ni@MWNTs/Sponge electrode for NaBH4 electrooxidation is investigated by means of cyclic voltammetry and chronoamperometry. Results show that the Ni@MWNTs/Sponge electrode exhibits a remarkably high catalytic activity and good stability for NaBH4 electrooxidation. The oxidation current density reaches as high as 300mAcm−2 at −0.7V in 2.0moldm−3 NaOH and 0.1moldm−3 NaBH4, which is significantly higher than that reported for other types of Ni electrodes.

Platinum‐ and Membrane‐Free Swiss‐Roll Mixed‐Reactant Alkaline Fuel Cell

Eliminating the expensive and failure-prone proton exchange membrane (PEM) together with the platinum-based anode and cathode catalysts would significantly reduce the high capital and operating costs of low-temperature (<373 K) fuel cells. We recently introduced the Swiss-roll mixed-reactant fuel cell (SR-MRFC) concept for borohydride-oxygen alkaline fuel cells. We now present advances in anode electrocatalysis for borohydride electrooxidation through the development of osmium nanoparticulate catalysts supported on porous monolithic carbon fiber materials (referred to as an osmium 3D anode). The borohydride-oxygen SR-MRFC operates at 323 K and near atmospheric pressure, generating a peak power density of 1880 W m(-2) in a single-cell configuration by using an osmium-based anode (with an osmium loading of 0.32 mg cm(-2)) and a manganese dioxide gas-diffusion cathode. To the best of our knowledge, 1880 W m(-2) is the highest power density ever reported for a mixed-reactant fuel cell operating under similar conditions. Furthermore, the performance matches the highest reported power densities for conventional dual chamber PEM direct borohydride fuel cells.

Oscillatory Instabilities in the Electrooxidation of Borohydride on Platinum

O ion boroidreto tem se mostrado um promissor combustivel alternativo. Grande parte das pesquisas aborda o aspecto eletrocatalitico da eletrooxidacao deste combustivel sobre platina e ouro. Apesar das conhecidas limitacoes cineticas e do intrincado mecanismo, nosso Grupo relatou recentemente a ocorrencia de duas regioes de biestabilidade e autocatalise no potencial do eletrodo durante a interacao em potencial de circuito aberto entre este ion e a superficie de platina oxidada. Seguindo esta contribuicao, neste trabalho fenomenos ainda mais complicados sao apresentados: a presenca de oscilacoes eletroquimicas durante a eletrooxidacao do ion boroidreto em platina em meio alcalino. Oscilacoes de corrente foram associadas a duas janelas distintas de instabilidades e caracterizadas no plano de potencial-resistencia. As caracteristicas dinâmicas de tais oscilacoes sugerem a existencia de mecanismos distintos de acordo com a regiao de potencial. Resultados publicados anteriormente obtidos sob regime nao-oscilatorio foram usados para fornecer algumas dicas sobre as reacoes superficiais por tras da dinâmica observada. The borohydride ion has been pointed as a promising alternative fuel. Most of the investigation on its electrochemistry is devoted to the electrocatalytic aspects of its electrooxidation on platinum and gold surfaces. Besides the known kinetic limitations and intricate mechanism, our Group has recently found the occurrence of two regions of bi-stability and autocatalysis in the electrode potential during the open circuit interaction of borohydride and oxidized platinum surfaces. Following this previous contribution, the occurrence of more complicated phenomena is here presented: namely the presence of electrochemical oscillations during the electrooxidation of borohydride on platinum in alkaline media. Current oscillations were found to be associated to two distinct instability windows and characterized in the resistance-potential parameter plane. The dynamic features of such oscillations suggest the existence of distinct mechanisms according to the potential region. Previously published results obtained under non-oscillatory regime were used to give some hints on the surface chemistry behind the observed dynamics.

Promoting Effect of Au on the Nanoporous Ag/CeO2 Composites Prepared by Dealloying for Borohydride Electro-Oxidation

Nanoporous Ag/CeO2 composites modified by Au were successfully achieved through simply dealloying the melt-spun Al-Ag-Au-Ce ribbons in a 5 wt% NaOH aqueous solution and calcining at 873 K. The High-Resolution Transmission Electron Microscopy (HRTEM) and Energy Dispersive X-ray Spectroscopy (EDS) results revealed that CeO2 particles with an average size of about 5 nm dispersed on the fine Ag-Au grains in the nanoporous structure. The electrochemical measurements showed that the nanoporous Ag-Au/CeO2 composites as anode electrocatalyst observably enhanced the oxidation of sodium borohydride and decreased the reaction resistance. The promoted catalytic activity of the composites originated from the strong interaction between the uniform Ag-Au solid solution and CeO2 particles. The content and state of Au had a significant influence on the catalytic performance and thermal stability of the nanoporous composites.