Borohydride Oxidation Reaction Research Articles

Electrocatalytic performance of environmentally friendly synthesized gold nanoparticles towards the borohydride electro-oxidation reaction

In this paper we report a simple and environmentally friendly synthesis of gold nanoparticles (AuNps) and their electrocatalytic activity for borohydride oxidation reaction (BOR). Ultraviolet spectroscopy (UV–vis) and transmission electron microscopy (TEM) confirmed the formation of poly(vinyl pyrrolidone)-protected colloidal AuNps through direct reduction of Au3+ by glycerol in alkaline medium at room temperature. For the BOR tests the AuNps were directly produced onto carbon to yield the Au/C catalyst. Levich plots revealed that the process occured via 7.2 electrons, therefore near the theoretical value of 8 electrons. When compared to bulk Au, the gold nanoparticles presented enhanced catalytic properties since the onset potential for BOR was shifted 200 mV towards negative potentials.

Borohydride electrooxidation on Au and Pt electrodes

Direct borohydride fuel cells (DBFCs) are attractive energy generators for powering portable electronic devices, mainly due to their high energy density and number of electrons per borohydride ion. However, the lack of a highly efficient electrocatalyst for the borohydride oxidation reaction limits the performance of these devices. The most commonly studied electrocatalysts for this reaction are composed of gold and platinum. Nevertheless, for these metals, the borohydride electrooxidation reaction mechanism (BOR) is not completely understood, and the total oxidation reaction, involving eight electrons per BH4− species, competes with parallel reactions, with a lower number of exchanged electrons and/or with heterogeneous chemical hydrolysis. Considering the above-mentioned issues, this work presents recent advances in the knowledge of the BOR pathways on polycrystalline (bulk) Au and Pt electrocatalysts. It presents the studies of the BOR reaction on Au and Pt electrodes using in situ Fourier Transform Infrared Spectroscopy (FTIR), and on-line Differential Electrochemical Mass Spectrometry (DEMS). The spectroscopic and spectrometric data provided physical evidence of intermediate species and the formation of H2 in the course of the BOR as a function of the electrode potential. These results enabled to advance in the knowledge about the BOR pathways on Au and Pt electrocatalysts.

Direct Borohydride Oxidation at Carbon Supported Pt-Sn Binary Catalyst

Carbon supported binary catalysts are not often reported with respect to the borohydride oxidation reaction (BOR) in direct borohydride fuel cell applications. Specifically, the synergistic effect of the catalysts toward the BOR has not been addressed. The kinetics and mechanistic studies on the direct BOR on PtSn/C were presented by comparing the data on Pt/C in three solutions with different concentrations of NaOH. The results based on stationary cyclic voltammetry (CV), rotating disk electrode (RDE), and chronopotentiometry manifested the synergistic effect of PtSn/C toward the BOR.

Comparative Study on Electrochemical Oxidation of Sodium Borohydride on Carbon Supported PtSn and Pt

Pt/C and PtSn/C were investigated for electrochemical activity towards borohydride oxidation in direct borohydride fuel cell applications. Voltammetry on stationary electrode and rotating disk electrode, chronopotentiometry and chronoamperometry were conducted on a thin film electrode. The results indicate that the presence of Sn in the catalyst may facilitate the borohydride oxidation reaction. An adsorption-electrochemical step is proposed to explain the enhanced activity for the PtSn/C.

First-principles based microkinetic modeling of borohydride oxidation on a Au(1 1 1) electrode

Borohydride oxidation electrokinetics over the Au(1 1 1) surface are simulated using first-principles determined elementary rate constants and a microkinetic model. A method to approximate the potential dependent elementary step activation barriers based on density functional theory calculations is developed and applied to the minimum energy path for borohydride oxidation. Activation barriers of the equivalent non-electrochemical reactions are calculated and made potential dependent using the Butler–Volmer equation. The kinetic controlled region of the borohydride oxidation reaction linear sweep voltammogram over the Au(1 1 1) surface is simulated. The simulation results suggest that B–H bond containing species are stable surface intermediates at potentials where an oxidation current is observed. The predicted rate is most sensitive to the symmetry factor and the BH 2OH dissociation barrier. Surface-enhanced Raman spectroscopy confirms the presence of BH 3 as a stable intermediate.

In Situ Infrared (FTIR) Study of the Mechanism of the Borohydride Oxidation Reaction on Smooth Pt Electrode

Although Pt has been thoroughly studied regarding its activity for the borohydride oxidation reaction (BOR), the BOR mechanism at Pt remains unclear: Depending on the applied potential, spontaneous BH4– hydrolysis can compete with the direct BOR. The goal of the present work is to provide more insight into the behavior of smooth Pt electrodes toward the BOR, by coupling in situ infrared reflectance spectroscopy with electrochemistry. The measurements were performed on a Pt electrode in 1 M NaOH/1 M NaBH4, so as to detect the reaction intermediate species generated as a function of the applied potential. Several bands were monitored in the B—H (ν ≈ 1180, 1080, and 972 cm–1) and B—O (ν = 1325 and ∼1425 cm–1) bond regions upon increased electrode polarization. These absorption bands, which appear sequentially and were already detected for similar measurements on Au electrodes, are assigned to BH3, BH2, and BO2– species. In light of these experimental data and previous results obtained in our group for Pt- ...

Mass transport effects in the borohydride oxidation reaction—Influence of the residence time on the reaction onset and faradaic efficiency

The borohydride oxidation reaction (BOR) was studied on Pt and Au electrodes by cyclic voltammetry in dilute alkaline borohydride solutions (0.1 M NaOH + 10 −3 mol L −1 NaBH 4). More specifically, the electrodes were considered as either Vulcan XC72-supported Pt or Au (noted as Pt/C and Au/C, respectively) active layers or smooth Pt or Au surfaces, the latter possibly being covered by a layer of (non-metalized) Vulcan XC72 carbon powder. The BOR onset potential and the number of electrons ( n e−) exchanged per BH 4 − anion (faradaic efficiency) were investigated for these electrodes, to determine whether the residence time of reaction intermediates (at the electrode surface or inside the porous layer) does influence the overall reaction pathway/completion. For the carbon-supported platinum, n e− strongly depends on the thickness of the active layer. While thin (ca. 0.5 μm-thick) Pt/C active layers yield n e− < 4, thick layers (approximately 3 μm) yield n e− ≈ 8, which can be ascribed to the sufficient residence time of the molecules formed within the active layer (H 2, by heterogeneous hydrolysis, or BOR intermediates) enabling further (near-complete) oxidation. This puts into evidence that not only the nature of the electrocatalyst is important to reach high BOR efficiency, but also the structure/thickness of the active layer. The same trend applies for Au/C active layers and for smooth Pt or Au surfaces covered with a layer of (inactive) Vulcan XC72. In addition, the BOR onset usually shifts negative when the reaction intermediates are trapped, which suggests that some of the intermediates are more easily oxidized than BH 4 − itself; based on literature data, BH 3OH − species is a likely candidate.

Nanoporous gold anode catalyst for direct borohydride fuel cell

Nanoporous gold (NPG) electrodes were fabricated in film and wire array formats by selectively dealloying Ag from Au 0.18Ag 0.82. Borohydride oxidation reaction (BOR) was studied by cyclic voltammetry at the NPG electrodes. The onset potential for the oxidation at NPG in a wire array format shifted to more negative potentials that than observed at an Au disc and higher currents were realized. An onset potential of −1.07 V vs SCE which is 0.207 V lower than that at an Au disc was recorded. The oxidation current for 20 mM borohydride in 1 M NaOH increased to 73.6 mA cm −2 from 3.17 mA cm -2 at an Au disc. An n value of 7.49 was determined for the oxidation peak at high potential (−0.49 V) while a value of 4.26 was determined at low potential for the oxidation plateau centered at −0.05 V. NPG presents an attractive alternative to gold nanoparticle-based catalysts for use in direct borohydride fuel cells. NPG can establish intimate contact with an electrical substrate and eliminates the need for a carbon support.

Borohydride Oxidation on Pt-Based Electrodes: Evidence of Residence Time Effect on the Reaction Onset and Faradaic Efficiency

The borohydride oxidation reaction (BOR) was studied on Vulcan XC72-supported Pt active layers by cyclic voltammetry in 0.1 M NaOH / 10−3 mol L-1 NaBH4. The BOR onset potential and the number of electrons (ne-) exchanged per BH4- anion (faradaic efficiency) were investigated for active layers of different thickness and/or compositions, to determine whether the residence time of reaction intermediates does influence the reaction. The results indicate that the ne- value exchanged in the BOR strongly depends on the thickness of the active layer. While thin (ca. 0.5 µm-thick) Pt/C active layers yield ne- < 4, thick layers (approximately 3 µm) yield ne- ≈ 8, which can be ascribed to the sufficient residence time of the molecules formed within the active layer (H2 by heterogeneous hydrolysis or BOR intermediates). This puts into evidence that not only the nature of the electrocatalyst is important to reach high BOR efficiency, but also the structure/thickness of the active layer.

Electrochemical impedance spectroscopy study of borohydride oxidation reaction on gold—Towards a mechanism with two electrochemical steps

This paper focuses on the mechanism and kinetics of the borohydride oxidation reaction (BOR). This reaction was studied by quasi-steady-state hydrodynamic cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on a gold rotating disk electrode (RDE). The best operating conditions to obtain stable and reproducible experimental data were first surveyed: only (i) electrolyte solutions much more diluted than practical DBFC anolyte ( e.g. 0.01 M NaOH + 5 × 10 −5 M NaBH 4) and (ii) polarization of the gold electrode strictly below the region of gold oxide formation, are compatible with quasi-steady-state measurements. Both this diluted solution and a more classical one (0.1 M NaOH + 10 −3 M NaBH 4) were eventually investigated, the latter for comparison purposes with relevant literature data. From the modeling of the CV and EIS experimental data and consecutive parametric identification, a simplified reaction pathway was proposed, the theoretical behavior of which agrees with the experimental findings. Whatever the composition of electrolyte used, this pathway includes at least two electrochemical steps (EE) for the BH 4 − oxidation. Using the expression of the faradaic current density, concentration impedance and electrode impedance for the redox EE reaction, the theoretical steady-state hydrodynamic CV and EIS spectra can be calculated. The obtained simulated plots are representative to the corresponding experimental CV and EIS traces. However, the values of the kinetic parameters calculated from such modeling are weak, suggesting that the redox EE is still a simplification of the actual borohydride oxidation reaction pathway on gold in diluted sodium borohydride solutions.

Gold is not a Faradaic-Efficient Borohydride Oxidation Electrocatalyst: An Online Electrochemical Mass Spectrometry Study

Direct borohydride fuel cells are promising high energy density portable generators. However, their development remains limited by the complexity of the anodic reaction: The borohydride oxidation reaction (BOR) kinetics is slow and occurs at high overvoltages, while it may compete with the heterogeneous hydrolysis of . Nevertheless, one usually admits that gold is rather inactive toward the heterogeneous hydrolysis of and presents some activity regarding the BOR, therefore yielding to the complete eight-electron BOR. In the present paper, by coupling online mass spectrometry to electrochemistry, we in situ monitored the yield during BOR experiments on sputtered gold electrodes. Our results show non-negligible generation on Au on the whole BOR potential range (0–0.8 V vs reversible hydrogen electrode), thus revealing that gold cannot be considered as a faradaic-efficient BOR electrocatalyst. We further propose a relevant reaction pathway for the BOR on gold that accounts for these findings.

Study of the Borohydride Oxidation Reaction on Gold in Alkaline Medium Using On-Line Mass Spectrometry

The Direct Borohydride Fuel Cell, a potential high-energy-density portable generator, remains limited by the sluggish borohydride oxidation reaction (BOR) kinetics, the mechanism of which is not completely understood. Nevertheless, one usually admits that gold promotes the complete 8-electron BOR. In the present paper, by coupling on line Mass Spectrometry to electrochemistry (OLEMS), we in situ monitored the H2 yield during BOR experiments on sputtered gold electrodes. Our results show non-negligible H2 generation on Au on the whole BOR potential range (0 - 0.8 V vs. RHE), thus revealing that gold cannot be considered as a faradaic-efficient BOR electrocatalyst. From this fact and knowing gold yields slow BOR kinetics, one may conclude that Au is not an ideal electrocatalyst for DBFC anodes.

In situ infrared (FTIR) study of the mechanism of the borohydride oxidation reaction

Early reports stated that Au was a catalyst of choice for the BOR because it would yield a near complete faradaic efficiency. However, it has recently been suggested that gold could yield to some extent the heterogeneous hydrolysis of BH, therefore lowering the electron count per BH, especially at low potential. Actually, the blur will exist regarding the BOR mechanism on Au as long as no physical proof regarding the reaction intermediates is not put forward. In that frame, in situ physical techniques like FTIR exhibit some interest to study the BOR. Consequently, in situ infrared reflectance spectroscopy measurements (SPAIRS technique) have been performed in 1 M NaOH/1 M NaBH(4) on a gold electrode with the aim to detect the intermediate species. We monitored several bands in B-H (nu ∼ 1180, 1080 and 972 cm(-1)) and B-O bond regions (nu = 1325 and ∼1425 cm(-1)), which appear sequentially as a function of the electrode polarization. These absorption bands are assigned to BH(3), BH(2) and BO species. At the light of the experimental results, possible initial elementary steps of the BOR on gold electrode have been proposed and discussed according to the relevant literature data.

The Electrocatalytic Oxidation of Sodium Borohydride at Palladium and Gold Electrodes for an Application to the Direct Borohydride Fuel Cell

Carbon supported palladium, gold and platinum nanoparticles were synthesized by microemulsion method in order to compare their catalytic activity for the electrooxidation of borohydride in alkaline medium. Two different pathways can be considered for this oxidation reaction: the direct oxidation of BH4- into BO2- and its indirect oxidation via hydrogen evolution. The electrochemical analysis of the polarisation curves recorded at different electrode rotation rates at Pd/C and Au/C in NaOH + NaBH4 electrolyte indicated that the electrooxidation process mainly occured following the direct pathway on both catalysts with 6 to 7 moles of exchanged electrons per mole of borohydride. On the other hand, on Pt/C catalyst, the oxidation reaction occurs with a strong hydrogen evolution. Furthermore, the kinetics parameters pointed out a ten times higher intrinsic catalytic activity of Pd/C compared to Au/C for borohydride oxidation reaction (bor).

Electrooxidation of Sodium Borohydride at Pd, Au, and PdxAu1−x Carbon-Supported Nanocatalysts

Carbon-supported palladium, gold, and bimetallic Pd−Au nanocatalysts with different compositions were synthesized by a “water-in-oil” microemulsion method. Their catalytic activity toward borohydride electrooxidation was evaluated in alkaline medium. Physical and electrochemical methods where applied to characterize the structure and surface of the synthesized catalysts. It was shown that PdxAu1−x/C catalysts were alloys, which present an increase of crystallite (X-ray diffraction) and particle (transmission electron microscopy) sizes with increasing Au atomic fraction. Their surfaces were palladium-rich whatever the Pd atomic ratios. The onset potential of NaBH4 oxidation was close to −0.2 V versus reversible hydrogen electrode (RHE) on Pd/C. PdxAu1−x/C catalysts presented lower onset potential for BH4− oxidation than Au and Pt (in the range from −0.2 to −0.1 V vs RHE against 0 and 0.3 V vs RHE for Au/C and Pt/C, respectively). The NaBH4 oxidation on Pd/C catalyst was found to be a first-order reaction with respect to borohydride concentration. From voltammetric measurements, rotating disk electrode experiments, and hydrogen production estimations it was proposed that NaBH4 oxidation on palladium-based nanocatalysts followed two pathways. The first one, at negative potentials, involved the formation of BH3OH− intermediate with H2 generation. The second one, at higher overpotentials, occurred mainly via the direct BH4− oxidation reaction, involving 6 mol of exchanged electrons per mole of borohydride. However, it was shown that addition of gold to palladium leads to increase significantly the hydrogen evolution rate. At last, comparison of the activity of the different catalysts toward the borohydride oxidation reaction showed that up to 50% of the palladium atoms can be replaced by a noncatalytic foreign metal like gold, while keeping identical catalytic activity than that of the monometallic Pd/C catalyst.

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media. Part I: Bulk electrodes

We studied the borohydride oxidation reaction (BOR) by voltammetry for BH 4 − concentrations between 10 −3 M and 0.1 M NaBH 4 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 NaBH 4 increases ( ∼ 3 e − in 1 mM and ∼ 6 e − in 10 mM BH 4 −/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 BH 4 − and subsequent oxidation of its by-products (e.g. BH 3OH − and H 2). BOR on Ag strongly depends on the pH: improved faradic efficiency is monitored for high pH ( ∼ 2 e − at pH 12.6 and ∼ 6 e − at pH 13.9 at 25 ° C). The BOR kinetics is faster for Pt than for Ag ( i Pt = 0.02 A cm −2, i Ag = 1.4 10 −7 A cm −2 at E = − 0.65 V vs. NHE in 1 mM NaBH 4/0.1 M NaOH, 25 ° C) both as a result from Pt high activity regarding the BH 4 − heterogeneous hydrolysis and subsequent HOR, above − 0.83 V vs. NHE and following direct oxidation of BH 4 − or BH 3OH − below − 0.83 V vs. NHE. Both Pt–Ag bulk alloys show unique behaviour: the number of electrons exchanged is rather high whatever the BH 4 − concentration and pH, while the kinetic parameters are quite similar to that of platinum, showing possible synergistic alloying effect.

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media: Part II. Carbon-supported nanoparticles

We studied the borohydride oxidation reaction (BOR) by voltammetry in 0.1 M NaOH/10 −3 M BH 4 − on carbon-supported Pt, Ag and alloyed PtAg nanoparticles (here-after denoted as Pt/C, Ag/C and Pt–Ag/C). In order to compare the different electrocatalysts, we measured the BOR kinetic parameters and the number of electrons exchanged per BH 4 − anion (faradaic efficiency). The BOR kinetics is much faster for Pt/C than for Ag/C ( i Pt = 0.15 , i Ag = 3.1 × 1 0 − 4 A cm −2 at E = − 0.65 V vs. NHE at 25 ° C), but both materials present similar Tafel slope values. The n value involved in the BOR depends on the thickness of the active layer of electrocatalysts. For a “thick layer” (approximately 3 μ m), n is nearly 8 on Pt/C and ∼ 4 on Ag/C, whereas n decreases for thinner Pt/C active layers ( n ∼ 2 for thickness < 1 μ m). These results are in favour of the sequential BH 4 − hydrolysis (yielding H 2) followed by hydrogen oxidation reaction (HOR), or direct sequential BOR on Pt/C, whereas Ag/C promotes direct but incomplete BOR (Ag has no activity regarding hydrogen evolution reaction, HER). The n value close to 8 for the thick Pt/C layer displays the sufficient residence time of the molecules formed (H 2 by heterogeneous hydrolysis or BOR intermediates) within the active layer, which favours the complete HOR and/or BOR. Two PtAg/C nanoparticles alloys have been tested (noted APVES-4C and APVES-E1). They show different behavior; the borohydride oxidation reaction kinetics is faster on APVES-E1 than on APVES-4C ( b = 0.15 , i − 0.65 V = 0.09 and b = 0.31 V dec −1, i − 0.65 V = 6.3 × 1 0 − 3 A cm −2, respectively, at 25 ° C), but the n values are higher on APVES-4C than APVES-E1 (nearly 8 vs. 3, respectively, at 25 ° C). These discrepancies probably originate from the heterogeneity of such bimetallic materials, as observed from physicochemical characterizations.

In situ infrared (FTIR) study of the borohydride oxidation reaction

The direct borohydride fuel cell (DBFC) is an interesting alternative for the electrochemical power generation at lower temperatures due to its high anode theoretical specific capacity ( 5 A h g - 1 ) . However, the borohydride oxidation reaction (BOR) is a very complex eight-electron reaction, influenced by the nature of the electrode material (catalytic or not with respect to BH 4 - hydrolysis), the [ BH 4 - ] / [ OH - ] ratio and the temperature. In order to understand the BOR mechanism, we performed in situ infrared reflectance spectroscopy measurements (SPAIRS technique) in 1 M NaOH/1 M NaBH 4 with the aim to study intermediate reactions occurring on a gold electrode (a poor BH 4 - hydrolysis catalyst). We monitored several bands in B–H (1184 cm −1) and B–O bond regions (1326 and 1415 cm −1), appearing sequentially with increasing electrode polarisation. Thanks to these experimental findings, we propose possible initial elementary steps for the BOR.

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 BH 4 − oxidation, which competes with the electrochemical adsorption reaction (EAR) of OH − anions at high potentials.

A Current−Decomposition Study of the Borohydride Oxidation Reaction at Ni Electrodes

On the basis of simultaneous measurements of the electric current and the hydrogen evolution rate, the reactions at Ni electrodes in alkaline borohydride solutions are successfully decomposed into two component reactions, i.e., borohydride oxidation and hydrogen electrode reaction. The former is a 4-electron anodic reaction with molecular hydrogen as a product, while the latter is either hydrogen evolution or hydrogen oxidation, depending on the borohydride concentration and electrode potential. Because of the change of the relative contributions of these two reactions, the apparent number of electrons in borohydride oxidation changes from 0 to 5 under the discharge conditions studied, much smaller than the theoretical electron number 8 for full oxidation of borohydride. The current decomposition result is able to interpret quantitatively the experimental linear relationship of open circuit potential to logarithmic borohydride concentration. The current decomposition also reveals a novel phenomenon, i.e.,...