Cyclic voltammetry investigation of borohydride oxidation at a gold electrode

Sodium borohydride (NaBH₄) is receiving increasing attention during the last decade regarding its possible application in energy systems. NaBH₄ has the dual potential of generating hydrogen on demand or being directly oxidised in a direct borohydride fuel cell (DBFC). Progress on DBFCs relies on the development of systematic studies to allow a more comprehensive characterisation of the borohydride (BH₄⁻) oxidation process. In this paper, cyclic voltammetry (CV) is applied to study systematically the BH₄⁻ electrooxidation on a gold (Au) disc macroelectrode in 2 mol l⁻¹ NaOH solutions. Voltammograms are obtained for various NaBH₄ concentrations [0.03-0.12 mol l⁻¹], working temperatures [25-65 °C], and potential scan rates [0.02-20 V s⁻¹], over a wide potential range [−1.0-0.8 V vs. SCE]. Modelling of CV data indicates that BH₄⁻ oxidation on Au electrode follows a first irreversible electrochemical pathway via the direct BH₄⁻ oxidation reaction, involving nearly 8 mol of exchanged electrons per mole of BH₄⁻. A second pathway, at higher potentials, concerns a yet undetermined oxidation mechanism in the partially oxidised Au surface which, in a third pathway, is reactivated, allowing an electrochemical-adsorption mechanism to take place. Relevant parameters such as transfer coefficient, kinetic rate constant, standard rate constant, charge transfer activation energy, and number of exchanged electrons are estimated. The BH₄⁻ oxidation reaction on Au is found to be first order with respect to BH₄⁻.     

A rotating disk electrode study on the ammonia oxidation

The anodic oxidation of ammonia in alkaline solutions was investigated by using rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. The ammonia oxidation current on the rotating platinum disk was confirmed not to increase but to decrease with increasing the rotation rate at 25-60 °C. The in situ ring electrode detector gave two kinds of signals for the electroactive species transported from the disk electrode engaged in ammonia oxidation; one can be again reduced and another can further be oxidized on Pt or Ni ring. These results suggested that at least some of the intermediates involved in ammonia oxidation on Pt may not be strongly adsorbed but be detached from the surface regions.     

The electrochemical oxidation of sulfite on gold: UV-Vis reflectance spectroscopy at a rotating disk electrode

Certain aspects of the electrochemical oxidation of sulfite in buffered, mildly acidic aqueous solutions (pH 5.23) have been examined using in situ near normal incidence UV-Vis reflectance spectroscopy (NNI-UVRS) at a Au rotating disk electrode (RDE). The dependence of the limiting current, i_lim, on the rotation rate of the RDE was found to display classical Levich behavior up to potentials well within the range in which Au forms a surface oxide in the neat (sulfite-free) supporting electrolyte. However, simultaneous in situ NNI-UVRS measurements performed at λ=500 nm during sulfite oxidation failed to show any evidence for the presence of oxide on the Au surface within that entire potential range. Polarization of the Au RDE at more positive potentials led to a sudden drop in i_lim, ca. an order of magnitude, which correlated with an abrupt decrease in the intensity of the reflected light, consistent with formation of (one or more forms of) Au oxide on the surface. On the basis of these and other observations a model has been proposed in which sulfite reacts chemically with adsorbed oxygen on the surface (oxygen atom transfer) in the region that precedes partial inhibition. As the potential is increased, adsorbed oxygen undergoes Au-O place exchange forming two-dimensional nuclei on the surface, which undergo rapid (autocatalytic) growth, covering an area large enough to block significantly sulfite oxidation.     

新型钛基银电极上硼氢化钠的电化学氧化

利用水热法,以聚乙二醇为还原剂将硝酸银还原, 产生的金属银颗粒直接沉积于钛表面,制备出具有三维网状结构的新型钛基银电极 (Ag/Ti)。利用循环伏安（CV）技术研究了碱性溶液中,Ag/Ti对硼氢化钠氧化的电催化活性。结果表明,硼氢化物在Ag/Ti电极上的氧化属直接电化学氧化过程,Ag/Ti电极对硼氢化物的电化学氧化表现出极高的电流密度,并且硼氢化物氧化的起始电位较低,约为-0.64 V vs SCE,说明Ag/Ti电极对硼氢化物氧化具有高度的电催化活性,有望作为硼氢化物燃料电池的阳极材料而得到应用。     

Kinetic analysis of the hydrogen oxidation reaction at Nafion film covered Pt-black rotating disk electrodes

The kinetics of the H₂ oxidation reaction at Nafion film covered Pt-black rotating disk electrodes (RDEs) in 0.5 M H₂SO₄ at 298 K was investigated by varying the Pt loading, Nafion film thickness, and rotating rate. The equation describing the H₂ oxidation kinetics at an RDE with a Nafion film covered porous Pt layer was derived, assuming a Tafel-Volmer mechanism and taking into account the mass transfer resistances in the aqueous electrolyte, Nafion film, and Pt layer. The H₂ oxidation reaction at the Pt layer was proved to be reversible and the measurable current density was determined entirely by the mass transfer of H₂ in the aqueous electrolyte and the Nafion film; the apparent kinetic current density measured was due to the experimental error. More accurate results of kinetic analysis were obtained in this work than our results reported previously.     

Direct rotating ring-disk measurement of the sodium borohydride diffusion coefficient in sodium hydroxide solutions

This paper presents the experimental determination of the diffusion coefficient of borohydride anion and solution kinematic viscosity for a large panel of NaOH + NaBH₄ electrolytic solutions relevant for use as anolyte in Direct Borohydride Fuel Cells (DBFC). The diffusion coefficients have been measured by the transit-time technique on gold rotating ring-disk electrodes, and verified using other classical techniques reported in the literature, namely the Levich method and Electrochemical Impedance Spectroscopy on a gold RDE, or chronoamperometry at a gold microdisk. The agreement between these methods is generally good. The diffusion coefficients measured from the RRDE technique are however ca. twice larger than those previously reported in the literature (e.g. ca. 3 × 10⁻⁵ cm² s⁻¹ in 1 M NaOH + 0.01 M NaBH₄ at 25 °C in the present study vs. ca. 1.6 × 10⁻⁵ cm² s⁻¹ in 1 M NaOH + 0.02 M NaBH₄ at 30 °C in the literature, as measured by chronoamperometry at a gold microsphere), which is thoroughly discussed. Our measurements using chronoamperometry at a gold microdisk showed that such technique can yield diffusion coefficient values below what expected. The origin of such finding is explained in the frame of the formation of both a film of boron-oxide(s) at the surface of the (static) gold microdisk and the generation of H₂ bubbles at the electrode surface (as a result of the heterogeneous hydrolysis at Au), which alter the access to the electrode surface and thus prevents efficient measurements. Such film formation and H₂ bubbles generation is not so much of an issue for rotating electrodes thanks to the convection of electrolyte which sweeps the electrode surface. In addition, should such film be present, the transit-time determination technique on a RRDE displays the advantage of not being very sensible to its presence: the parameter measured is the time taken by a perturbation generated the disk to reach the ring trough a distance several orders of magnitude bigger than the film thickness, thus minimizing its effect.     

Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells

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, KCl_std) 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.     

Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte: Part I. BH4 electro-oxidation on Au and Ag catalysts

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.     

Electrochemical oxidation of borohydride on platinum electrodes: The influence of thiourea in direct fuel cells

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⁻.     

Simulation studies on a rotating ring disk electrode: Effects of ionic migration with kinetic complication-disk results

In continuation to my previous work (Guha S. AIChE J. 2013;59(4):1390-1399), in this work, effects of ionic migration are evaluated for disk region of a rotating ring disk electrode system by numerically solving complex differential equations, developed for mass transfer along with kinetic complication in presence of ionic migration under limiting current condition. The system for simulation is 0.01 M Fe₂(SO₄)₃ solution with H₂SO₄ as supporting electrolyte. Simulation cases are presence and absence of ionic migration with kinetic complication (oxidation of Fe²⁺ to Fe³⁺ under O₂ pressure). Results show that concentration boundary layer thickness of reactant Fe³⁺ reduces appreciably and steady-state disk current reduces substantially in presence of migration. Simulated steady-state disk current in absence of migration case agrees well with published data. Results indicate higher Fe²⁺ concentration in presence of migration and thereby higher rate of oxidation of Fe²⁺ to Fe³⁺ at all rate constant values.     

Investigation of Ti mesh-supported anodes for direct borohydride fuel cells

The use of titanium mesh-supported gold and silver anodes in direct borohydride fuel cells (DBFCs) is reported. The anodes were prepared by either thermal decomposition or electrochemical deposition and were characterised by scanning electron microscopy and X-ray diffraction analyses. The performance of the mesh electrodes was compared with that for carbon-supported electrodes. The mesh anodes gave current densities, for borohydride oxidation, up to 50% greater and cell power densities up to 20% greater than those obtained with carbon-supported anodes. The effects of catalyst loading and fuel cell operating conditions are also reported. Electrode stability was examined over a prolonged period.     

Electrochemical synthesis of hydrogen peroxide: Rotating disk electrode and fuel cell studies

The electrochemical reduction of oxygen on various catalysts was studied using the thin-layer rotating disk electrode (RDE) method. High-surface-area carbon was modified with an anthraquinone derivative and gold nanoparticles. Polytetrafluoroethylene (PTFE) and cationic polyelectrolyte (FAA) were used as binders in the preparation of thin-film electrodes. Our primary goal was to find a good electrocatalyst for the two-electron reduction of oxygen to hydrogen peroxide. All electrochemical measurements were carried out in 0.1 M KOH. Cyclic voltammetry was used in order to characterise the surface processes of the modified electrodes in O₂-free electrolyte. The RDE results revealed that the carbon-supported gold nanoparticles are active catalysts for the four-electron reduction of oxygen in alkaline solution. Anthraquinone-modified high-area carbon catalyses the two-electron reduction at low overpotentials, which is advantageous for hydrogen peroxide production.In addition, the polymer electrolyte fuel cell technology was used for the generation of hydrogen peroxide. The cell was equipped with a bipolar membrane which consisted of commercial Nafion 117 as a cation-exchange layer and FT-FAA as an anion-exchange layer. The bipolar membranes were prepared by a hot pressing method. Use of the FAA ionomer as a binder for the anthraquinone-modified carbon catalyst resulted in production of hydrogen peroxide.     

Determination of the kinetic parameters of oxygen reduction on copper using a rotating ring single crystal disk assembly (RRDCu(h k l)E)

The kinetics of the oxygen reduction reaction (orr) on Cu(h k l) surfaces are investigated in perchloric acid and sulfuric acid solutions using rotating ring disk electrode (RRD_Cu(h k l)E). Parameters, such as reaction order, kinetic current, rate constant, Tafel slopes as well as the number of electrons transferred are determined. The variation in the activity and reaction pathway with the crystal faces in different electrolytes is related to the surface characteristics of Cu(h k l) and the structure-sensitive inhibiting effect of the adsorbed anions on their surfaces. In 0.1 M HClO₄, the difference in activity is clearly observed on Cu(h k l) surfaces (Cu(1 0 0) > Cu(1 1 1) although it is relatively small). The higher activity of Cu(1 0 0) arises from its more open characteristics which may facilitate the co-adsorption of O₂. On the other hand, the adsorption of oxygenated species on Cu(1 1 1) at E > −0.35 V induces a 2 e⁻ pathway; while a 4 e⁻ reduction is observed on Cu(1 0 0) in the entire potential region (−0.70 V < E < −0.10 V). In 0.5 M H₂SO₄, the sequence in activity between Cu(1 1 1) and Cu(1 0 0) varies with the potentials, i.e., Cu(1 0 0) is initially more active than Cu(1 1 1) at −0.35 V < E < −0.15 V, however, the reversal in the activity between Cu(1 1 1) and Cu(1 0 0) is observed at more negative potentials (−0.45 V < E < −0.35 V). The desorption of strongly adsorbed (bi)sulfate anions on Cu(1 1 1) induces the 2 e⁻ reduction via peroxide formation, however, a 4 e⁻ reduction is dominant on the Cu(1 0 0) surfaces. The major effect of (bi)sulfate anions and oxygenated species on the orr kinetics and reaction pathway on Cu(h k l) surfaces is the blocking of active copper sites for the adsorption of O₂ molecules.     

Anodic oxidation of Co(II) complexes with amines on a rotating Au electrode: Ligand effects in relation to the application for autocatalytic metal deposition

For the comparison of the electrochemical activity of Co(II)-amine complexes, the electrochemical response of an Au rotating disk electrode in alkaline Co(II)-glycine solutions to six amines: ethylenediamine (en), propane-1,2-diamine (pn-1,2), propane-1,3-diamine (pn-1,3), cyclohexane-1,2-diamine (chn), butane-1,4-diamine (bn), diethylenetriamine (dien), was studied. Addition of amines tested (except for bn) in mM levels shifts the open-circuit potential to more negative values by up to 0.5 V and enhances dramatically the anodic Co(II) oxidation current, as a result of Co(II) complex transformation into more stable and electrochemically active Co(II)-amine species. The effect of amines on the open-circuit potential changes in the line: dien ∼ en > pn-1,2 ∼ chn > pn-1,3 ? bn, and on the anodic current in the sequence: dien ∼ en > pn-1,2 > chn > pn-1,3 ? bn. The procedure described helps to select ligands for Co(II) complexes used as reducing agents in electroless plating solutions. The amines of high electrochemical response: dien, en, pn-1,2, and possibly, chn, are suitable for electroless copper deposition, pn-1,3 (a lower response), for electroless silver deposition, and bn (no response), not suitable for electroless plating solutions.     

First insights into the borohydride oxidation reaction mechanism on gold by electrochemical impedance spectroscopy

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.     

A parametric study of a platinum ruthenium anode in a direct borohydride fuel cell

Data on the performance of a direct borohydride fuel cell (DBFC) equipped with an anion exchange membrane, a Pt–Ru/C anode and a Pt/C cathode are reported. The effect of oxidant (air or oxygen), borohydride and electrolyte concentrations, temperature and anode solution flow rate is described. The DBFC gives power densities of 200 and 145 mW cm⁻² using ambient oxygen and air cathodes respectively at medium temperatures (60 °C). The performance of the DBFC is very good at low temperatures (ca. 30 °C) using modest catalyst loadings of 1 mg cm⁻² for anode and cathode. Preliminary data indicate that the cell will be stable over significant operating times.     

Comments on the paper “Electrooxidation of borohydride on platinum and gold electrodes: Implications for direct borohydride fuel cell” by E. Gyenge, Electrochim. Acta 49 (2004) 965: Thiourea, a poison for the anode metallic electrocatalyst of the direct borohydride fuel cell?

The present discussion paper deals with the Gyenge's [E. Gyenge, Electrochim. Acta 49 (2004) 965] suggestion to add thiourea (H₂N-CS-NH₂) to the borohydride fuel of the direct borohydride fuel cell (DBFC). It is expected that thiourea inhibits the hydrogen evolution (stem from the borohydride hydrolysis, a side reaction) that occurs at the anode of the DBFC where in fact it is expected the direct oxidation of borohydride.However, thiourea is an organic sulphur compound and it is well known that the sulphur species are poisons for the metallic catalysts. Hence, the present discussion paper asks a question: may thiourea and the sulphur species stem from its decomposition act as poisons of metallic sites of catalysts used as DBFC anodes?     

A feasible kinetic model for the hydrogen oxidation on ruthenium electrodes

The hydrogen oxidation reaction (hor) was studied on a polycrystalline ruthenium electrode in H₂SO₄ solution at different rotation rates (ω). The experimental polarization curves recorded on steady state show the existence of a maximum current with a non-linear dependence of the current density on ω^1/2. On the basis of the Tafel-Heyrovsky-Volmer kinetic mechanism, coupled with a process of inhibition of active sites by the reversible electroadsorption of hydroxyl species, it was possible to appropriately describe the origin of the maximum current. The corresponding set of kinetic parameters was also calculated from the correlation of the experimental results with the proposed kinetic model.     

LaNi0.9Ru0.1O3 as a cathode catalyst for a direct borohydride fuel cell

LaNi_0.9Ru_0.1O₃ as cathode catalyst for a direct borohydride fuel cell (DBFC) was synthesized and investigated for the first time. The electrochemical experiments indicated that perovskite-type oxide LaNi_0.9Ru_0.1O₃ exhibited higher electrochemical performance compared with LaNiO₃, which suggested incorporation of element Ru into LaNiO₃ could further improve the catalytic ability for oxygen reduction reaction (ORR) in alkaline solution. LaNi_0.9Ru_0.1O₃ catalyst was found to have good tolerance of BH₄⁻. Meanwhile the maximum power density of 171 mW cm⁻² was obtained at 65 °C without using any precious ion exchange membrane. A life test indicated that the DBFC displayed no significant degradation for about 70 h testing. The electrochemical data suggested that LaNi_0.9Ru_0.1O₃, which provided a simple way to construct DBFCs without using any ion exchange membrane, might be promising cathode catalyst with high performance and low cost for DBFCs.     

Rotating disk electrode analysis of oxygen reduction at platinum particles under limiting diffusion conditions

An analysis is carried out of oxygen reduction under limiting diffusion conditions on a rotating disk electrode partially covered with platinum particles (‘particulate electrode’). First a model is developed for the current response at a rotating particulate electrode, because Levich equation, used for the classical continuous disk electrodes, is not applicable in this case. The model allows for the calculation of the limiting diffusion current by an iterative algorithm, as a function of the density and size of the particles, and the constants for the adsorption/desorption and diffusion of reactants over non-covered areas of the substrate. The model is valid when there is no overlapping of surface diffusion areas around the particles. In a second part, the oxygen reduction on platinum particles electrodeposited on a glassy carbon disk is studied. Platinum particulate electrodes with a variable density and size of particles are prepared by single pulse electrodeposition technique. Limiting diffusion currents for oxygen reduction are analysed on the light of the proposed model. Values for the oxygen surface diffusivity and the equilibrium adsorption/desorption constant on the glassy carbon substrate are obtained from the analysis.