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1.
Electrochemical oxidation of borohydride on platinum electrodes: The influence of thiourea in direct fuel cells 总被引:2,自引:0,他引:2
The electrochemical behaviour of sodium borohydride on a platinum electrode in the absence and presence of thiourea (TU) was investigated by cyclic voltammetry. In the absence of thiourea, several overlapping peaks associated with the hydrolysis of BH4− appear in the domain of hydrogen oxidation, i.e., in the potential range of −1.25 to −0.50 V versus Ag/AgCl. As a consequence of secondary reactions, the borohydride oxidation in 3 M NaOH solution shows a four to six-electron process, according to its concentration, in direct fuel cells. A conveyable TU/NaBH4 concentration ratio of 0.6 inhibits the delivery of hydrogen simultaneously with catalytic hydrolysis of BH4−. Thus, the coulombic efficiency in direct fuel cell discharge was increased showing an about eight-electron process for the oxidation of BH4−. 相似文献
2.
Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells 总被引:2,自引:0,他引:2
El?d Gyenge 《Electrochimica acta》2004,49(6):965-978
The electrochemical oxidation of BH4− in 2 M NaOH on Pt and Au (i.e. catalytic and non-catalytic electrodes, respectively, for BH4− hydrolysis accompanied by H2 evolution) has been studied by cyclic voltammetry, chrono-techniques (i.e., potentiometry, amperometry, coulometry) and electrochemical impedance spectroscopy. In the case of Pt the cyclic voltammetry behaviour of BH4− is influenced by both, the catalytic hydrolysis of BH4− yielding H2 (followed by electrooxidation of the latter at peak potentials between −0.7 and −0.9 V versus Ag/AgCl, KClstd) and direct oxidation of BH4− at more positive potentials, i.e., between −0.15 and −0.05 V. Thiourea (TU, 1.5×10−3 M) was an effective inhibitor of the catalytic hydrolysis associated with BH4− electrooxidation on Pt. Therefore, in the presence of TU, only the direct oxidation of BH4− has been detected, with peak potentials between −0.2 and 0 V. It is proposed that TU could improve the BH4− utilization efficiency and the coulombic efficiency of direct borohydride fuel cells using catalytic anodes. The electrooxidation of BH4− on Pt/TU is an overall four-electron process, instead of the maximum eight electrons reported for Au, and it is affected by adsorbed species such as BH4− (fractional surface coverage ∼0.3), TU and possibly reaction intermediates. 相似文献
3.
Marian Chatenet 《Electrochimica acta》2006,51(25):5459-5467
The direct oxidation of sodium borohydride in concentrated sodium hydroxide medium has been studied by cyclic and linear voltammetry, chronoamperometry and chronopotentiometry for silver and gold electrocatalysts, either bulk and polycrystalline or nanodispersed over high area carbon blacks. Gold and silver yield rather complete utilisation of the reducer: around 7.5 electrons are delivered on these materials, versus 4 at the most for platinum as a result of the BH4− non-negligible hydrolysis taking place on this latter material. The kinetic parameters for the direct borohydride oxidation are better for gold than for silver. A strong influence of the ratio of sodium hydroxide versus sodium borohydride is found: whereas the theoretical stoichiometry does forecast that eight hydroxide ions are needed for each borohydride ion, our experimental results prove that a larger excess hydroxide ion is necessary in quasi-steady state conditions. When the above-mentioned ratio is unity (1 M NaOH and 1 M NaBH4), the tetrahydroborate ions direct oxidation is limited by the hydroxide concentration, and their hydrolysis is no longer negligible. The hydrolysis products are probably BH3OH− ions, for which gold displays a rather good oxidation activity. Additionally, silver, which is a weak BH4− oxidation electrocatalyst, exhibits the best activity of all the studied materials towards the BH3OH− direct oxidation.Finally, carbon-supported gold nanoparticles seem promising as anode material to be used in direct borohydride fuel cells. 相似文献
4.
Vankayala Kiran Suresh Babu Kalidindi Balaji Rao Jagirdar Srinivasan Sampath 《Electrochimica acta》2011,(28):10493
Electrochemical oxidation of sodium borohydride (NaBH4) and ammonia borane (NH3BH3) (AB) have been studied on titanium carbide electrode. The oxidation is followed by using cyclic voltammetry, chronoamperometry and polarization measurements. A fuel cell with TiC as anode and 40 wt% Pt/C as cathode is constructed and the polarization behaviour is studied with NaBH4 as anodic fuel and hydrogen peroxide as catholyte. A maximum power density of 65 mW cm−2 at a load current density of 83 mA cm−2 is obtained at 343 K in the case of borhydride-based fuel cell and a value of 85 mW cm−2 at 105 mA cm−2 is obtained in the case of AB-based fuel cell at 353 K. 相似文献
5.
Borohydride oxidation has been investigated using a rotating disk electrode technique. The parameters, such as apparent rate constant, Tafel slope, Levich slope, number of electrons exchanged and reaction order, have been determined. The borohydride ion is oxidised on the gold electrode with an electrochemical rate constant of around 1 cm s−1 at intermediate potentials where side reactions had less effect. Influences of temperature, concentrations of borohydride and supporting electrolyte (NaOH) on the parameters were evaluated. 相似文献
6.
The stabilized aqueous solution of sodium borohydride NaBH4 is a promising hydrogen fuel but the stored hydrogen has to be released with the help of a catalyst through hydrolysis. In the present study, we developed Co- and clay-based supported catalysts. Three raw clays were taken from soil in Lebanon. Once purified and annealed, they were used as supports. Two of them, mainly composed of kaolinite and illite respectively, showed to be promising owing to their attractive specific surface areas (58.0 and 67.1 m2 g−1) as well as the high reactivity of the corresponding 15 wt.% Co catalysts (i.e. NaBH4 conversions of 100% and hydrogen generation rates up to ∼31 L(H2) min−1 g−1(Co)). A kinetic study was also carried out. The main results are reported and discussed herein. 相似文献
7.
Gholamreza Rostamikia 《Electrochimica acta》2010,55(3):1175-9237
The mechanism of borohydride oxidation and the competing hydrolysis reaction are examined over Pt(1 1 1) using density functional theory (DFT) methods. Adsorption of BH4− over Au(1 1 1) and Pt(1 1 1) is examined. Adsorption over Pt(1 1 1) is dissociative and extremely exothermic at potentials of interest, leading to a high surface coverage of H* for which gaseous hydrogen evolution is competitive with oxidation. Elementary surface reactions oxidizing B-containing intermediates are favorable over Pt(1 1 1) at −0.85 V (SHE), consistent with experimental voltammetry results in the literature. The energetics of the initial adsorption step dictate the activity limitation of gold anodes and the selectivity limitation of platinum electrodes. This adsorption energy can be rapidly calculated with DFT methods, enabling screening of pure metals, alloys, poisons, and promoters to optimize borohydride oxidation catalyst design. 相似文献
8.
Kinetic and mechanistic evaluation of tetrahydroborate ion electro-oxidation at polycrystalline gold
The anodic oxidation of tetrahydroborate ion is studied in NaOH at stationary and rotating polycrystalline Au disk electrodes. Linear sweep and cyclic voltammetry are applied varying the scan and rotation rate from 0.005 to 51.200 V s−1 and from 52.3 to 314.1 rad s−1, correspondingly. The effects of variation of BH4− and NaOH concentrations as well as of the potential limits of the ranges studied have been initially followed. Most of the experiments have been carried out with 10.9 mM NaBH4 in 1.04 M NaOH at 293 K in the potential range from −1.300 to 0.900 V (vs. Ag/AgCl). It is found that 6 electrons are exchanged in the overall oxidation transformation. The kinetic analysis of the processes determining the two anodic peaks recorded under static conditions at scan rates lower than 0.500 V s−1 shows that 1.4 electrons are exchanged in the potential range of the first one (at ca −0.5 V), while the rate of the second one (at ca +0.3 V) is determined by a quasi-reversible 1-electron transfer reaction. A kinetic evidence for the participation of surface bound intermediates in the electro-oxidation process is provided. Two additional well outlined anodic peaks are recorded in the aforementioned potential range under specific experimental conditions. A quasi-8 electron mechanism involving four oxidation and hydrolysis steps is advanced to explain the experimental results. It accounts for the involvement of borohydride oxidation species and the Au+/Au3+ mediator couple. 相似文献
9.
Mário Simões 《Electrochimica acta》2010,56(1):580-591
In the past few years, borohydrides have gathered a lot of attention as an energy carrier for fuel cell application. Numerous investigations on both hydrogen generation and direct oxidation of NaBH4 have been published. Nonetheless, in our knowledge, only a few catalysts are capable to completely perform the direct oxidation of NaBH4 at low potentials without hydrogen evolution.In this work, carbon supported Pd1−xBix/C and Pt1−xBix/C nanocatalysts were synthesized by a “water in oil” microemulsion method. The influence of surface modifications of Pt and Pd by Bi on the electrooxidation of sodium borohydride in alkaline media was evaluated. Physical and electrochemical methods were applied to characterize the structure and surface of the synthesized catalysts.It was verified that bismuth is present at the surface of the bimetallic catalysts and that hydrogen adsorption/desorption reactions are strongly limited on Pt and Pd surfaces with high bismuth coverage. Although the onset potential for NaBH4 oxidation on PdxBi1−x/C catalysts is ca. 0.2 V higher than that for Pd/C, the presence of bismuth on palladium surface influences the reaction mechanism, limiting hydrogen evolution and oxidation in the case of Pd0.8Bi0.2 catalyst. On Pt0.9Bi0.1 catalyst the onset potential remains unchanged comparing to Pt/C and negligible hydrogen evolution was observed in the whole potential range where the catalyst is active. The number of exchanged electrons was calculated using the Koutecky-Levich equation and it was found that for Pt0.9Bi0.1 catalyst, ca. 8 electrons are exchanged per BH4− ion at low potentials. The presented results are remarkable evidencing that NaBH4 can be directly oxidized at low potentials with high energy efficiency. 相似文献
10.
Min Ku Jeon 《Electrochimica acta》2009,54(10):2837-2842
The effect of reduction conditions on a Pt28Ni36Cr36/C catalyst was investigated by using two different reduction methods: hydrogen reduction and NaBH4 reduction. In hydrogen reduced catalysts, dissolution of metallic Ni and Cr was observed during cyclic voltammetry (CV) tests, and a larger amount of Ni and Cr was dissolved when reduced at higher temperatures. For methanol electro-oxidation, the highest specific current density of 1.70 A m−2 at 600 s of the chronoamperometry tests was observed in the catalyst reduced at 300 °C, which was ∼24 times that of a Pt/C catalyst (0.0685 A m−2). In NaBH4 reduced catalysts, formation of an amorphous phase and a more Pt-rich surface was observed in X-ray diffraction and CV results, respectively, with increasing amounts of NaBH4. When reduced by 50 times of the stoichiometric amount of NaBH4, the PtNiCr/C catalyst (PtNiCr-50t) showed a current density of 34.1 A gnoble metal−1, which was 81% higher than the 18.8 A gnoble metal−1 value of a PtRu/C catalyst at 600 s of the chronoamperometry tests. After 13 h of chronoamperometry testing, the activity of the PtNiCr-50t (15.0 A gnoble metal−1) was 110% higher than the PtRu/C catalyst (7.15 A gnoble metal−1). The PtNiCr/C catalyst shows promise as a Ru-free methanol oxidation catalyst. 相似文献
11.
In order to mimic the operation of the air-cathode in a direct borohydride alkaline fuel cell, we studied the oxygen reduction reaction (ORR) in sodium hydroxide solution containing traces of borohydride. The activity of several ORR electrocatalysts, namely carbon-supported platinum, gold, silver and manganese oxide, has been investigated using slow-scan linear voltammetry. Whereas platinum is one of the best electrocatalyst in pure sodium hydroxide, none of the classical electrocatalysts: gold, silver and platinum, exhibit sufficient selectivity towards the ORR. When BH4− is present in solution, the potential taken by electrodes using such materials is a mixed potential, following the competition between the ORR and the NaBH4 hydrolysis and/or oxidation. Conversely, manganese oxide-based electrocatalysts exhibit very interesting behaviour towards the ORR in alkaline medium; while their intrinsic ORR activity in pure sodium hydroxide is quite as good as that for platinum, they still display a remarkable selectivity for this reaction when the electrolyte contains traces of sodium borohydride.As a result, carbon-supported manganese oxide-based nanoparticles seem very interesting materials to be used in direct borohydride fuel cell. 相似文献
12.
First insights into the borohydride oxidation reaction mechanism on gold by electrochemical impedance spectroscopy 总被引:1,自引:0,他引:1
Direct borohydride fuel cells (DBFC) exhibit some potential regarding the powering of small portable electronic devices, thanks to their high energy density as well as the facile and safe storage of borohydride salts. However, DBFC are hindered because (i) the borohydride oxidation reaction (BOR) is complex, (ii) its mechanism imperfectly determined yet and (iii) no practical electrocatalyst exhibits both fast BOR kinetics and high faradaic efficiency. In this context, we characterized the BOR mechanism for polycrystalline bulk gold (a classical model BOR electrocatalyst) in the rotating disk electrode (RDE) setup. Modeling cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data, we propose a simplified reaction pathway, the theoretical behavior of which agrees with the experimental findings. This pathway includes at least a first irreversible electrochemical step (E) for BH4− oxidation, which competes with the electrochemical adsorption reaction (EAR) of OH− anions at high potentials. 相似文献
13.
Jacqueline Argüello 《Electrochimica acta》2008,54(2):560-565
This paper describes the preparation, characterization and application of a composite electrode based on methylene blue adsorption to phosphorylated zirconia-silica mixed oxide particles prepared by a sol-gel process. This electrode electrocatalytically oxidizes ascorbic acid (AA), dopamine (DA) and uric acid (UA), allowing their simultaneous voltammetric detection. Well-defined and -separated oxidation peaks were observed by differential pulse voltammetry in a 0.35 mol l−1 Tris-HCl buffer solution (pH 7.4) containing 0.5 mol l−1 KCl. The anodic peak currents observed at −74, 94 and 181 mV increased with increasing concentrations of AA, DA and UA, respectively. Linear calibration plots were obtained over the range of 100-1600 μmol l−1 for ascorbic acid, 6-100 μmol l−1 for dopamine and 22-350 μmol l−1 for uric acid with detection limits of 8.3 ± 0.1, 1.7 ± 0.1 and 3.7 ± 0.2 μmol l−1, respectively. DA and UA concentrations could also be determined under conditions of excess AA (1 mmol l−1). 相似文献
14.
A new SiO2/SnO2 carbon ceramic composite was prepared by the sol-gel method, and its potential application in electrochemistry as a novel electrode material has been studied. The prepared xerogel was structurally and electrochemically characterized by scanning electron microscopy coupled to energy dispersive spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and cyclic voltammetry. The composite was pressed in a rigid disk-shape and used as a conductive substrate to immobilize a water-soluble organic-inorganic hybrid polymer, 3-n-propyl-4-picolinium chloride silsesquioxane. The oxidation of nitrite was studied on this polymer film coated electrode in aqueous solution using cyclic voltammetry and differential pulse voltammetry. This modified electrode exhibited a better defined voltammetric peak shifted negatively about 60 mV. The linear detection limit found for nitrite was from 1.3 × 10−5 to 1.3 × 10−3 mol l−1 and the detection limit was 3.3 × 10−6 mol l−1. 相似文献
15.
M. Reffass 《Electrochimica acta》2009,54(18):4389-4396
Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO3 + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential Eb with the phosphate concentration was investigated by linear voltammetry. Eb increased from −15 ± 5 mV/SCE for [HPO42−] = 0 to 180 ± 40 mV/SCE for [HPO42−] = 0.02 mol L−1. During anodic polarisation (E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na2HPO4 solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO42−] = 0.004 mol L−1, siderite FeCO3 was detected first. It was oxidised later into carbonated green rust GR(CO32−) by dissolved O2. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO32−) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO42−] = 0.02 mol L−1, siderite was accompanied by an amorphous precursor of vivianite, Fe2(PO4)3·8H2O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel. 相似文献
16.
Algirdas Vaškelis Rima Tarozait? Aldona Jagminien? Loreta Tamašauskait? Tamašiūnait? Remigijus Jušk?nas Marija Kurtinaitien? 《Electrochimica acta》2007,53(2):407-416
Gold colloid solutions were prepared by reduction of Au(III) with Sn(II) in both acidic (HCl) and alkaline (carbonate) media, and characterized by electron microscopy, X-ray diffraction and light absorption spectra. Depending on the colloid preparation conditions, metal particles of 5-30 nm in size were obtained. Light absorption spectra of the Au colloid solutions contain an absorption band at 500-600 nm, typical of small gold particles. According to XRD data, colloid particles contain the metallic Au and SnO2 phases. The colloid formation rate and colloid solution stability depend on solution pH; the alkaline colloid solutions are formed more slowly and are more stable. The gold nanoparticles adsorbed on the dielectrics surface were found to initiate the electroless copper deposition process. The glassy carbon (GC) electrode modified by Au nanoparticles (200-900 ng cm−2 Au) was found to be an electrocatalyst for dimethylamine borane, borohydride, hydrazine, and formaldehyde oxidation in alkaline solutions. The activity of the reducers can be arranged as follows: DMAB > NaBH4 > N2H4 > CH2O. 相似文献
17.
《Inorganic chemistry communications》2007,10(8):891-893
Reaction of [UO2Cl2(THF)2]2 with NaBH4 in THF led to the formation of [UO2Cl2−x(BH4) x(THF)n], and addition of hmpa induced the crystallization of [UO2(BH4)2(hmpa)2], whose structure exhibits the bidentate ligation mode of the borohydride ligand; this uranyl borohydride, which is thermally unstable with respect to elimination of borane, is a catalyst in the hydroboration of substituted alkenes by NaBH4. 相似文献
18.
Chun-Hua Zhen Chun-Jie Fan Sheng-Pei Chen Bing-Wei Mao You-Jun Fan 《Electrochimica acta》2004,49(8):1249-1255
Dissociative adsorption and oxidation of glycine on Au(1 1 1) single crystal electrodes in alkaline solutions were studied in the present paper using cyclic voltammetry (CV), in situ FTIR spectroscopy (FTIRS) and electrochemical quartz crystal microbalance (EQCM). In situ FTIRS results demonstrated that adsorbates derived from glycine dissociative adsorption are adsorbed cyanide anions (CNad−). The CNad− species are stable on Au(1 1 1) surface in the potential region from −0.8 to 0.0 V, and can be oxidized when electrode potential is increased above 0.1 V. The oxidation of CNad− releases surface active sites for further glycine oxidation. The products of CNad− oxidation were determined by in situ FTIRS as cyanate (OCN−), aurous cyanide (AuCN) and aurous di-cyanide (Au(CN)2−). The formation of Au(CN)2− may initiate a dissolution of Au(1 1 1) surface atoms, which has been confirmed by a loss of surface mass determined in EQCM studies. It has revealed also that at high electrode potential region glycine may be split on Au(1 1 1) surface to form AuCH2NH2 and AuCOO− adsorbates. Further oxidation of these species yielded CO2 and -NH2, and the AuCH2NH2 may be also combined with surface Au oxide to form methylamine. The CO2 species produced in glycine oxidation are all retained in alkaline solutions to generate carbonate (CO32−) and bicarbonate (HCO3−) species that were clearly determined by in situ FTIRS studies. 相似文献
19.
We studied the borohydride oxidation reaction (BOR) by voltammetry for BH4− concentrations between 10−3 M and 0.1 M NaBH4 in 0.1–1 M NaOH for bulk polycrystalline Pt, Ag and alloyed Pt–Ag electrocatalysts. In order to compare the different electrocatalysts, we measured the kinetic parameters and the number of electrons exchanged (faradic efficiency). BOR on bulk Pt is more efficient when the concentration of NaBH4 increases (3e− in 1 mM and 6e− in 10 mM BH4−/0.1 M NaOH). BOR on Pt can occur both in a direct pathway and in an indirect pathway including hydrogen generation via heterogeneous hydrolysis of BH4− and subsequent oxidation of its by-products (e.g. BH3OH− and H2). BOR on Ag strongly depends on the pH: improved faradic efficiency is monitored for high pH (2e− at pH 12.6 and 6e− at pH 13.9 at 25 °C). The BOR kinetics is faster for Pt than for Ag (iPt=0.02 A cm−2, iAg=1.4 10−7 A cm−2 at E=−0.65 V vs. NHE in 1 mM NaBH4/0.1 M NaOH, 25 °C) both as a result from Pt high activity regarding the BH4− heterogeneous hydrolysis and subsequent HOR, above −0.83 V vs. NHE and following direct oxidation of BH4− or BH3OH− below −0.83 V vs. NHE. Both Pt–Ag bulk alloys show unique behaviour: the number of electrons exchanged is rather high whatever the BH4− concentration and pH, while the kinetic parameters are quite similar to that of platinum, showing possible synergistic alloying effect. 相似文献
20.
High quality free-standing poly(9-fluorenone) (PFO) films were synthesized electrochemically by direct anodic oxidation of 9-fluorenone (FO) in pure boron trifluoride diethyl etherate (BFEE). The oxidation potential of FO in this medium was measured to be only 1.48 V versus SCE, which was greatly lower than that determined in CH2Cl2 + 0.1 mol l−1 Bu4NBF4 (2.21 V versus SCE). PFO films obtained from BFEE showed good electrochemical behavior, good thermal stability with electrical conductivity of 7.8 × 10−3 S cm−1, indicating that BFEE was a better medium than CH2Cl2 for the electrosyntheses of PFO films. Structural studies showed that the polymerization of FO ring occurred at 2,7-position. As-formed PFO films can be partly dissolved in acetone and tetrahydrofuran (THF). Fluorescent spectral studies indicate that PFO is a good blue light emitter. To the best of our knowledge, this is the first report on the electrosyntheses of free-standing PFO films. 相似文献