Salem Ould-Amara

Eddy Petit

Dominique Granier

Pascal G Yot

Umit B Demirci
email: umit.demirci@umontpellier.fr

Alkaline aqueous solution of sodium decahydro-closodecaborate Na 2 B 10 H 10 as liquid anodic fuel

Keywords: Boron cluster, Na2B10H10, Oxidation, Polyborate anion, Sodium decahydro-

The potential of the decahydro-closo-decaborate anion B10H10 2-in alkaline aqueous solution as anodic fuel was investigated by using cyclic voltammetry and three different bulk metal electrodes (platinum, gold and silver). The sodium salt NaB10H10 was first synthesized, fully characterized and assessed for its relative stability in alkaline medium for 25 days. Then, oxidation of B10H10 2-in alkaline aqueous solution was studied. With platinum, the electrochemical activity is nil. With gold and silver, oxidation takes place at >0 V vs. SCE, suggesting direct oxidation of B10H10 2-. A current density of e.g. 15.1 mA cm -2 at 0.51 V vs. SCE is produced, supporting an electrocatalytically activity for both electrodes. There is even some reversibility of the process (i.e. reduction of intermediate species) with silver. The most important oxidation products were identified as being B7-based anions for both silver and gold. Such results suggest the occurrence of partial oxidative degradation of B10H10 2-at positive potential and may open new application prospects to polyborate anions.

Introduction

In the recent decades huge efforts have been dedicated to the development of a wide variety of materials for energy/hydrogen storage [START_REF] Liu | Advanced materials for energy storage[END_REF], including boron hydrides [START_REF] Demirci | Sodium borohydride for the near-future energy: a "rough diamond" for Turkey[END_REF]. A typical and early example is sodium borohydride NaBH4, much investigated for hydrogen storage [START_REF] Brack | Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH4) solutions[END_REF] and as anodic fuel for chemical energy storage [START_REF] Sison Escaño | Electrocatalysis of borohydride oxidation: a review of density functional theory approach combined with experimental validation[END_REF]. A more recent example is sodium dodecahydrocloso-dodecaborate Na2B12H12. It belongs to the family of the polynuclear borane anion-based salts [START_REF] Williams | The polyborane, carborane, carbocation continuum: architectural patterns[END_REF], which have been considered: (i) for hydrogen storage and production (by hydrolysis) [START_REF] Safronov | Chemical hydrogen storage using polynuclear borane anion salts[END_REF]; (ii) as anodic fuel for chemical energy storage [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]; and (iii) as solid electrolyte in electrochemical energy storage [START_REF] Verdala | Complex high-temperature phase transitions in Li2B12H12 and Na2B12H12[END_REF].

Both of the aforementioned boron hydrides have been considered as potential carbon-free anodic fuels owing to their solubility and relative stability in water. With respect to the borohydride anion BH4 -, it spontaneously hydrolyzes when put into contact with water, which results in hydrogen evolution [START_REF] Mirkin | Borohydride oxidation at a gold electrode[END_REF]:

BH4 -+ 4H2O → B(OH)4 -+ 4H2 (1)
The reaction can be drastically limited by increasing the solution pH up to 10-13 [START_REF] Kreevoy | H2BH3 as an intermediate in tetrahydridoborate hydrolysis[END_REF]. The asobtained alkaline aqueous solution of BH4 -is then an attractive anodic fuel (E° = -1.24 V) [START_REF] Indig | Sodium borohydride, an interesting anodic fuel[END_REF]:

BH4 -+ 8OH -→ BO2 -+ 6H2O + 8e - (2)
The resulting open circuit potential is as high as 1.64 V. A lot of efforts have been made to develop the related direct borohydride fuel cell technology but the progression objectives have been impeded by several critical issues [START_REF] Ma | Direct borohydride fuel cells -Current status, issues, and future directions[END_REF]. One of the main problems relates to the unwanted hydrolysis reaction. It very slowly takes place even in alkaline conditions [START_REF] Kreevoy | H2BH3 as an intermediate in tetrahydridoborate hydrolysis[END_REF], but more critically it catalytically takes place over the metal electrode [START_REF] Braesch | Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations[END_REF][START_REF] Olu | Influence of the concentration of borohydride towards hydrogen production and escape for borohydride oxidation reaction on Pt and Au electrodes -experimental and modelling insights[END_REF]. One of the main challenges ahead is to better understand the oxidation and hydrolysis mechanisms in order to search for novel anode electrocatalysts allowing depressed hydrogen evolution and making the electro-oxidation of BH4 - an eight-or near-eight-electron process [START_REF] Nurul | A comprehensive review of direct borohydride fuel cells[END_REF].

Alternatively, other boron hydrides, most being derivatives of NaBH4 [START_REF] Demirci | Sodium borohydride for the near-future energy: a "rough diamond" for Turkey[END_REF], have been considered.

For example, oxidations of alkaline aqueous solutions of ammonia borane NH3BH3, dimethylamine borane (CH3)2HNBH3, hydrazine borane N2H4BH3 and hydrazine bisborane N2H4(BH3)2 over carbon-supported platinum and palladium nanoparticles were investigated and the boranes were compared each other [START_REF] Zadick | Ubiquitous borane fuel electrooxidation on Pd/C and Pt/C electrocatalysts: toward promising direct hydrazine-borane fuel cells[END_REF]:

XBH3 + 6OH -→ XH + + BO2 -+ 4H2O + 6e - (3)
where X is equal to NH3, N2H4 and NH(CH3)2. For each of the boranes, oxidation is in competition with heterogeneous hydrolysis, the H2 evolution being more important over the platinum electrode. Another potential boron hydride is the octahydrotriborate anion B3H8 - [START_REF] Pylypko | A preliminary study of sodium octahydrotriborate NaB3H8 as potential anodic fuel of direct liquid fuel cell[END_REF]:

B3H8 -+ 20OH -→ 3BO2 -+ 14H2O + 18e - (4)
It oxidizes over a bulk gold electrode but the effective number of electrons is about 10 out of a theoretical total of 18 electrons (Eq. 4). Complete oxidation is not effective because heterogeneous hydrolysis. A last potential boron hydride was recently proposed. Oxidation of the dodecahydro-closo-dodecaborate anion B12H12 -was studied with platinum, gold and silver electrodes [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]. The anion was chosen owing to its high stability in aqueous solution at any pH (no hydrolysis). Partial oxidative degradation through a complex oxidation process was evidenced.

One of the products was identified as being B11H11O(OH) -and its formation was tentatively suggested to take place as follows:

B12H12 2-+ 8OH -→ B11H11O(OH) -+ B(OH)4 -+ 2H2O + 6e - (5)
The most attractive feature with B12H12 -is the absence of heterogeneous hydrolysis. The evolving electrons are all due to partial oxidation of the anion.

The recent achievements briefly described above created a window of opportunity for alkaline aqueous solutions of polynuclear borane anions as possible anodic fuels. In the present work we examined the potential of the decahydro-closo-decaborate anion B10H10 2-because, after the anion B12H12 2-, it is the second most stable species against hydrolysis [START_REF] Klanberg | Chemistry of boranes. XXVII. New polyhedral borane anions, B9H9 2and B11H11 2[END_REF]. The sodium salt, i.e.

Na2B10H10, was synthesized and the successful synthesis and the purity of the sample were verified. Its stability in aqueous solution was investigated so as to launch oxidation experiments.

Herein is reported, for the first time, the oxidation behavior of the anion B10H10 2-with platinum, gold and silver bulk electrodes.

Material and Methods

Na2B10H10 was prepared from the commercially available triethylammonium decahydro-closodecaborate of formulae ((C2H5)3NH)2B10H10 (Katchem, 98%) by cationic substitution in alkaline medium:

((C2H5)3NH)2B10H10 + 2NaOH → Na2B10H10 + 2((C2H5)3NH)OH (6)
The synthesis was performed under argon atmosphere. A 2-gram aliquot of ((C2H5)3NH)2B10H10

was dispersed in 100 mL of an alkaline aqueous (0.13 M NaOH) solution at room temperature. conditions identical to the TGA experiment).

The oxidation of the anion B10H10 2-(of Na2B10H10 in aqueous solution) was studied by cyclic voltammetry (with a µAutolab ® Type III potentiostat). A borosilicate cell of 100 mL was used (thermostated at 20°C). It was systematically, and before each experiment, cleaned with peroxymonosulfuric acid H2SO5 (Caro's Acid). Outgassed Milli-Q Water (18.2 MΩ cm, < 3 ppb total organic carbon) was used. The electrodes were as follows: saturated calomel electrode (SCE) as reference electrode; platinum wire as counter electrode; and a rotating disk (with bulk platinum ∅ 2 mm, or bulk gold ∅ 2 mm, or bulk silver ∅ 4 mm) as working electrode. We selected platinum, gold and silver because, historically, they have been among the most investigated metals for oxidation of the anion BH4 - [START_REF] Ma | Direct borohydride fuel cells -Current status, issues, and future directions[END_REF][START_REF] Braesch | Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations[END_REF][START_REF] Olu | Influence of the concentration of borohydride towards hydrogen production and escape for borohydride oxidation reaction on Pt and Au electrodes -experimental and modelling insights[END_REF][START_REF] Nurul | A comprehensive review of direct borohydride fuel cells[END_REF]; otherwise, we use to consider these metals only, for screening any new B-based anodic fuel, since we well know about their catalytic behavior towards the unwanted hydrolysis of BH4 -. The surface of the working electrodes was polished with diamond paste. The electrolyte solution was prepared from an aqueous solution of NaOH at the concentration of 0.1 M. The concentration of B10H10 2-was 0.001 M; it was kept so low to limit the interactions between the molecules. The cell voltage range was fixed as follows:

-1.05 to +0.6 V vs. SCE (in order to avoid water splitting).

Mass spectrometry (MS) experiments allowed qualitative analyses of the oxidation by-products.

A Quattro Micro mass spectrometer, with electrospray ionization in negative mode (from Waters Micromass; Wythenshawe, Manchester, UK) and operating at constant flow rate (0.25 mL min -1 ) was used. The electrolyte samples taken were analyzed by direct injection (Waters 2695 pumpautosampler, with 5 µl loop). A mixture of water and acetonitrile (50/50 in vol. %) was used as mobile phase. The nebulizer gas was N2. For detection, the conditions were: capillary potential of 3.5 kV; cone potential of 30 V; source temperature at 120°C; desolvation temperature at 450°C; cone gas flow of 50 L h -1 ; desolvation gas flow of 450 L h -1 . The recorded m/z values were compared to comparable molecular weights (of polyborate anions) and then the experimental spectra were compared to modeled/calculated spectra in order to confirm the identifications made.

Results and discussion

Characterizations of Na2B10H10

The 11 B NMR spectrum of the anion B10H10 2-(Figure 1) shows two doublets centered at -1 ppm (JBH = 140.1 Hz) and -29 ppm (JBH = 125.4 Hz), suggesting two kinds of boron environments in the molecule (2 equivalent apical boron atoms + 8 equivalent equatorial boron atoms) [START_REF] Gabel | The formation of boron-carbon bonds to closodecaborate(2-) and closo-dodecaborate(2-))[END_REF]. The relative area of these doublets is 4 to 1, which is in good agreement with the equatorial 8 boron atoms and the 2 apex boron atoms [START_REF] Lipscomb | Probable structure of the B10H10 2anion[END_REF][START_REF] Muetterties | Chemistry of boranes. VIII. Salts and acids of B10H10 2and B12H12 2[END_REF]. The purity of the sample is also confirmed.

The 1 H NMR spectrum of the anion B10H10 2-(Figure 2) is characterized by several signals, apart from those due to the solvent H2O and to CH3CN present in the deuterated solvent. There are two sets of 7 peaks located between 4 and 2 ppm and between 1 and -1 ppm. They are typical of 11 B-H and 10 B-H bonds [START_REF] Reed | The role of NMR in boron chemistry[END_REF][START_REF] Ruman | The synthesis, reactivity and 1 H NMR investigation of the hydroxyborohydride anion[END_REF], and are in good agreement with the presence of two B-H environments in the molecular anion [START_REF] Shubina | Dihydrogen bonding of decahydro-closo-decaborate(2-) and dodecahydro-closo-dodecaborate(2-) anions with proton donors: experimental and theoretical investigation[END_REF]. The relative area of these signals is 4 to 1, in good agreement with the number of hydrogen elements bound to the equatorial 8 boron atoms and to the 2 apex boron atoms. Based on the FTIR observations, the thermally stable Na2B10H10 was analyzed by TGA and DSC (Figure 4). Under heating, the sample suffers a first weight loss (1 wt%) up to 50°C and a second one, more important (9.6 wt%), between 100 and 160°C. The latter is featured by an endothermic process. The weight losses are much likely due to removal of adsorbed water as reported elsewhere [START_REF] Skachkova | Thermal and thermo-oxidative properties of the decahydro-closo-decaborate anion B10H10 2in a silicate matrix[END_REF]. The overall weight loss was considered to calculate the number of adsorbed water molecule per mole of Na2B10H10. It was found about to be 1, suggesting the empirical formulae Na2B10H10⋅H2O. Na2B10H10⋅H2O is a white solid. Following the results obtained by TGA and DSC, the powder was placed into a 1 mm diameter glass capillary into a glove box then heat-treated at 110°C overnight under primary vacuum (to remove the residual water molecules) and then sealed to protect the as-obtained anhydrous Na2B10H10 from hydration. The XRPD pattern of the sample was finally recorded in the 2θ range 5°-100° using the Debye-Scherrer method (Figure 5). The solid is polycrystalline and the pattern is comparable to previously reported results [START_REF] Hofmann | Crystal strcutures of M2[B10H10] (M = Na; K; Rb) via real-space simulated annealing powder techniques[END_REF][START_REF] Wu | The structure of monoclinic Na2B10H10: a combined diffraction, spectroscopy, and theoretical approach[END_REF]. In our case, an impurity supposed to be NaCl (a = 5.639(1) Å, s.g. Fm-3m (No. 225)) was fitted using Le Bail method. Figure 5 also presents the refinement of the pattern obtained using JANA2006 software [START_REF] Petricek | Crystallographic computing system JANA2006: general features[END_REF]. The crystal structure was successfully solved in the monoclinic phase (Z = 4), [26]. To sum up, Na2B10H10⋅H2O was successfully prepared in our conditions. The sample is monohydrated and contains a small amount of NaCl. This is not problematic for an anodic fuel that has to be dissolved in alkaline aqueous solution. The presence of NaCl had nevertheless to be kept in mind for the preparation of the 0.001 M Na2B10H10⋅H2O solution used as electrolyte.

using

Stability of B10H10 2-in alkaline solution

As a preliminary step, the stability of B10H10 2-(10 -5 M) in alkaline solution (0.1 M NaOH) was assessed. The fresh solution was analyzed by 11 B NMR and then was stored for 25 days before being analyzed once more by 11 B NMR (Figure 1). A singlet of small intensity appeared at positive chemical shits (1.7 ppm). It is due to the formation of B-O bonds (by oxidation and/or hydrolysis) like for borates. Despite this, it may be stated that the spectrum indicates a relatively good stability of the anion in the aforementioned conditions. Alkaline solution of B10H10 2-is stable enough to be studied as possible anodic fuel.

Oxidation of B10H10 2-over platinum

To our knowledge, oxidation of B10H10 2-in alkaline conditions over a metal electrode has not been investigated so far. An example of report available in the open literature deals with chemical oxidation in acidic conditions resulting in the formation of B20-based polynuclear borane anions [START_REF] Chamberland | Chemistry of boranes. XVIII. Oxidation of B10H10 2and its derivatives[END_REF]. Another example is about oxidation by MnO4 -at pH 7 and at room temperature, resulting in quantitative conversion to boric acid B(OH)3 [START_REF] Kaczmarczyk | Oxidative degradation of polyhedral boranes[END_REF]. Accordingly, the electro-oxidation results reported hereafter were analyzed with the help of the knowledge developed with the anion BH4 - [START_REF] Ma | Direct borohydride fuel cells -Current status, issues, and future directions[END_REF][START_REF] Braesch | Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations[END_REF][START_REF] Olu | Influence of the concentration of borohydride towards hydrogen production and escape for borohydride oxidation reaction on Pt and Au electrodes -experimental and modelling insights[END_REF][START_REF] Nurul | A comprehensive review of direct borohydride fuel cells[END_REF][START_REF] Rodriguez | Electrocatalysis on gold[END_REF] and in a lesser extent with the new boron hydride candidates [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF][START_REF] Zadick | Ubiquitous borane fuel electrooxidation on Pd/C and Pt/C electrocatalysts: toward promising direct hydrazine-borane fuel cells[END_REF][START_REF] Pylypko | A preliminary study of sodium octahydrotriborate NaB3H8 as potential anodic fuel of direct liquid fuel cell[END_REF].

The electro-oxidation of B10H10 2-was first investigated with the platinum electrode (Figure 6) at natural diffusion conditions (0 rpm). The voltammogram does not show apparent oxidation and/or reduction processes. Additional experiments were performed with different rotation rates (up to 2500 rpm) but no marked improvement was observed (voltammograms not reported). Such results do not support bulk platinum as suitable electrode for electro-oxidation of B10H10 2-. In comparison, the more-stable anion B12H12 2-was found to oxidize over the same electrode at <0 V vs. SCE, suggesting B12H12 2-dissociative adsorption and oxidation of Hads, together with some direct partial oxidation of the anion [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]. The lack of apparent oxidation of B10H10 2-could be explained by an absence of adsorption of the anion onto the electrode surface.

Oxidation of B10H10 2-over gold

Gold has been much investigated as effective electrode for oxidation of a boron hydride like NaBH4 [START_REF] Ma | Direct borohydride fuel cells -Current status, issues, and future directions[END_REF][START_REF] Nurul | A comprehensive review of direct borohydride fuel cells[END_REF][START_REF] Rodriguez | Electrocatalysis on gold[END_REF]. A bulk gold electrode was also considered in the present work (Figure 7).

Gold is unable to generate a current below -0.2 V vs. SCE. This suggests that it is neither able to partially dissociate B10H10 2-[14] nor to valorize adsorbed H species (Hads) [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]. The only oxidation peak (aAu) appearing during the forward sweep is in the metal-(hydr)oxide region [START_REF] Braesch | Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations[END_REF],

namely at +0.24 V vs. SCE for a current density of 3.5 mA cm -2 . It may be due to direct (at least partial) oxidation of adsorbed B10H10 2-[32]. A similar result was obtained with B12H12 2-in similar conditions [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]. For both anions, the process is not reversible. In contrast, in the present work, there is an oxidation peak during the backward sweep: cAu at 0 V vs. SCE (0.7 mA cm -2 ). It is reasonable to suggest oxidation of intermediates (as well as that of B10H10 2-) [START_REF] Smith | The mechanisms of electrolytic reduction for decaborane(14), B10H14, in an aprotic solvent. II. The second reduction step and the reduction of decaboran(13)ate(-1), B10H13 -1[END_REF] and/or oxidation of adsorbed H atoms [START_REF] Grimmer | The electrooxidation of borohydride: A mechanistic study on palladium (Pd/C) applying RRDE, 11 B-NMR and FTIR[END_REF]. Additional experiments were performed at disk rotation rates of 1000 and 2000 rpm (Figure 7). The current density for aAu increases up to 4 mA cm -2 at 2000 rpm (0.29 V vs. SCE). This is an indication of the low impact of poisoning, if it takes place, over the gold electrode [START_REF] Braesch | Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations[END_REF]. Because of too low concentrations, the oxidation products could not be identified by 11 B NMR [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]. Instead, they were detected and qualitatively analyzed by MS. The CV experiment was repeated while respecting the following procedure: the potential was scanned from -1.05 to +0.6 ). One of them (m/z = 99.3) was found to be predominant and it may be ascribed to B7H8O -. It is followed by the m/z value of 102.3, possibly due to B7H11O -. The others were in fact detected in much lesser amounts and their identification has been difficult: the m/z values of 100.4 and 116.7 suggest species containing 7 boron atoms (e.g.

B7H9O -and B7H9O2 -); and the m/z values of 69 might be ascribed to a species with 4 boron atoms (e.g. B4H10O -). Though the identification of all of the species is difficult, the MS results are thus in good agreement with the CV results implying partial oxidation of B10H10 2-.

Oxidation of B10H10 2-over silver

Bulk silver was also considered as possible working electrode (Figure 8). The forward step is featured by two oxidation peaks: aAg1 at 0.22 V vs. SCE with 6.5 mA cm -2 , and aAg2 at 0.51 V vs.

SCE with 15.1 mA cm -2 . The first peak is comparable to that observed with the gold electrode and may be attributed to direct (at least partial) oxidation of B10H10 2- [START_REF] Chatenet | First insights into the borohydride oxidation reaction mechanism on gold by electrochemical impedance spectroscopy[END_REF]. With respect to the second oxidation wave, it may be also attributed to direct and possibly more complete oxidation of B10H10 2- [START_REF] Sanli | The oxidation of NaBH4 on electrochemicaly treated silver electrodes[END_REF] as well as to oxidation of adsorbed reaction intermediates [START_REF] Smith | The mechanisms of electrolytic reduction for decaborane(14), B10H14, in an aprotic solvent. II. The second reduction step and the reduction of decaboran(13)ate(-1), B10H13 -1[END_REF]. The electrooxidation ability of bulk silver is slightly better than that of bulk gold, making silver a more attractive candidate for further works on electrode materials. With BH4 -, silver oxide was reported to be the electroactive surface responsible of the direct oxidation of the adsorbed intermediate BH3OH -at similar potentials [START_REF] Chatenet | Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte: Part I. BH4electrooxidation on Au and Ag catalysts[END_REF]. Surface species such as Ag(OxHyBz) were reported elsewhere [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]. The backward step of the voltammogram shows a reduction (cAg at -0.15

V vs. SCE with -17.6 mA cm -2 ) of, much likely, an oxidation product. Such a reduction wave let us suggest that the electrochemical process involving B10H10 2-and silver is, at least partly, reversible: one (or more) of the species (see below for identified products) forming during the oxidation waves aAg1 and aAg2 is reduced at -0.15 V vs. SCE (wave cAg). The effect of the rotation rate of the disk electrode does not have a negative effect on the current density, which slightly increases along with the higher speed of the rate. Like for the gold electrode, this is an indication of the low impact of poisoning on the surface of bulk silver [START_REF] Braesch | Borohydride oxidation reaction mechanisms and poisoning effects on Au, Pt and Pd bulk electrodes: From model (low) to direct borohydride fuel cell operating (high) concentrations[END_REF]. Comparable results were obtained with B12H12 2-in identical operation conditions but the current densities were 4 times higher (at about 0.5 V vs. SCE) [START_REF] Pylypko | The highly stable aqueous solution of sodium dodecahydro-closo-dodecaborate Na2B12H12 as a potential liquid anodic fuel[END_REF]. Considering the above-reported cyclic voltammetry performances, the bulk electrodes can be discussed in terms of electrochemical activity. In our conditions, the highest current density was measured for bulk silver (followed by gold), making it the best candidate. Platinum is electrocatalytically inactive. Over silver and gold, the oxidation processes require high potentials (e.g. 0.51 V vs. SCE for aAg2 in Figure 8) and take place in the metal-oxide region, showing thus the importance of surface (hydr)oxides in oxidation of B10H10 2-. Though the present results are mixed, there are interesting and open opportunities for improvement. The potential of alkaline aqueous B10H10 2-as anodic fuel would depend on the development of multimetallic electrodes in order to achieve lower oxidation potentials (more realistic for a fuel cell application) and higher current densities (via complete oxidation). With the much-investigated BH4 -, such improvements were reported for e.g. Au-Cu [START_REF] Rostamikia | Direct borohydride oxidation: mechanism determination and design of alloy catalysts guided by density functional theory[END_REF][START_REF] Rostamikia | Electrocatalyst design for direct borohydride oxidation guided by first principles[END_REF], Ag-Cu [START_REF] Duan | Kinetics of sodium borohydride direct oxidation on carbon supported Cu-Ag bimetallic nanocatalysts[END_REF] and Ag-Pt alloys [START_REF] Molina Concha | Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt-Ag electrodes in basic media. Part I: Bulk electrodes[END_REF].

The oxidation products forming over bulk silver were analyzed by MS by repeating the CV

Conclusion

The potential of the decahydro-closo-decaborate anion B10H10 2-(in alkaline aqueous solution) as anodic fuel (ultimately for direct liquid-fed fuel cell) was considered and assessed with the help of cyclic voltammetry and by using three different bulk metal electrodes such as platinum, gold and silver. As a first step the sodium salt was synthesized and the obtained white solid was characterized and identified by NMR, FTIR and XRPD. Rather stable in alkaline medium (oxidation in small extent with formation of B-O bonds after 25 days in a solution at 0.1 M NaOH), the anion B10H10 2-was found to be suitable for the cyclic voltammetry experiments.

With the bulk platinum electrode, the electrochemical activity is nil. In contrast, with bulk gold and silver, oxidation takes place at potentials higher than 0 V vs. SCE, suggesting direct oxidation of B10H10 2-. There is some reversibility of the oxidation process with the silver electrode. Both electrodes are electrocatalytically active and allow the production of current densities (e.g. 15.1 mA cm -2 at 0.51 V vs. SCE) that are however low for the targeted ultimate application. The most important oxidation products were identified by MS. The anion B7H8O - predominates for the gold electrode while B7H11O -is the preponderate species for bulk silver. In both cases, the results suggest the occurrence of partial oxidative degradation of B10H10 2-at positive potentials. From the present work there are thus three conclusions to be drawn. First, the well-known stability of the polynuclear borane anions is, to a certain extent, confirmed. In our conditions, B10H10 2-is stable towards oxidation over bulk platinum and it slightly oxidizes over gold and silver. Second, partial oxidative degradation of B10H10 2-over the last two electrodes could be valorized differently, namely by focusing on the synthesis of species like B7H8O -and B7H11O -provided novel and advanced electrodes are developed for improved conversion and selectivity. Third, silver appears to the best metal when alkaline aqueous B10H10 2-is considered as potential anodic fuel. However, the oxidation potentials are high and the current densities relatively low (in comparison to e.g. performance of the much-investigated BH4 -).

Notwithstanding, the anion B10H10 2-should not be discarded from the targeted application because of very attractive theoretical properties (cf. supplementary information): (i) a total oxidation of B10H10 2-could generate up to 22 electrons; (ii) the theoretical oxidation potential is as low as -3.3 V; and (iii) the theoretical specific energy of a fuel cell based on the couple B10H10 2-/O2 would be a high as ca. 18500 Wh kg -1 , which is much higher than the theoretical specific energy of a number of liquid fuels [START_REF] Demirci | Direct liquid-feed fuel cells: Thermodynamic and environmental concerns[END_REF]. However further works are necessary to make B10H10 2-useable and this calls for developments of silver-based multimetallic electrodes efficiently operating at low potential.

Figure 1 .

 1 Figure 1. (a) 11 B NMR spectrum of the anion B10H10 2-(from Na2B10H10). (b) 11 B NMR spectrum of the anion B10H10 2-in an alkaline solution (1 M NaOH) after 25 days.

Figure 2 .

 2 Figure 2. 11 H NMR spectrum of the anion B10H10 2-(from Na2B10H10).

Figure 3 .

 3 Figure 3. FTIR spectrum of Na2B10H10.

Figure 4 .

 4 Figure 4. TGA and DSC results for Na2B10H10.

  the space group P21/c (No. 14). The cell parameters were determined as: a = 6.7228(6) Å; b = 13.1435(11) Å; c = 11.9520(9) Å; β = 120.616(5)°; V = 908.88(13) Å 3 (GoF = 2.08; Rp = 8.62; wRp = 11.47; R(obs)/R(all): 10.06/11.11; wR(obs)/wR(all): 7.66/7.79). The result obtained for the anhydrous phase Na2B10H10 is in good agreement with the work previously reported in ref.

Figure 5 .

 5 Figure 5. Result of the refinement carried out on the XRPD pattern of anhydrous Na2B10H10 (λ = 1.54059 Å). The observed (in black) and calculated (in red) XRPD profiles for the Rietveld refinement of Na2B10H10 and Le Bail refinement of NaCl are shown.The bottom curve (in blue) is the difference plot on the same scale intensity. The tic marks (in green) are the calculated angles for the Bragg peaks for the two phases in 2θ.

Figure 6 .

 6 Figure 6. Cyclic voltammogram obtained on bulk platinum at 100 mV s -1 and at natural diffusion conditions (0 rpm). The voltammogram of 0.1 M NaOH (dashed lines) is superimposed to that of 0.1 M NaOH + 0.001 M Na2B10H10 (solid lines). The blue line indicates the forward scan and the red line the backward scan, which are besides shown by two arrows.

Figure 7 .

 7 Figure 7. (a) Cyclic voltammogram obtained on bulk gold at 100 mV s -1 and at natural diffusion conditions (0 rpm). The voltammogram of 0.1 M NaOH (dashed lines) is superimposed to that of 0.1 M NaOH + 0.001 M Na2B10H10 (solid lines). The blue line indicates the forward scan and the red line the backward scan, which are besides shown by two arrows. (b) Cyclic voltammogram obtained on bulk platinum at 100 mV s -1 and at different rotation rates of the disk electrode.

Figure 8 .

 8 Figure 8. (a) Cyclic voltammogram obtained on bulk silver at 100 mV s -1 and at natural diffusion conditions (0 rpm). The voltammogram of 0.1 M NaOH (dashed lines) is superimposed to that of 0.1 M NaOH + 0.001 M Na2B10H10 (solid lines). The blue line indicates the forward scan and the red line the backward scan, which are besides shown by two arrows. (b) Cyclic voltammogram obtained on bulk platinum at 100 mV s -1 and at different rotation rates of the disk electrode.

  experiment and following the conditions described for the gold electrode. Several m/z signals were detected. Only one signal (m/z = 102.3, ascribed to B7H11O -) predominated. This species could be that reducing during the backward step of the CV experiment (Figure8). The other m/z signals could be considered as minor (m/z = 145, suggested to belong to e.g. B11H10O -) or very minor products (m/z = 100.4 possibly for B7H9O -, 116.7 probably due to B7H9O2 -, and 142.6 for an unidentified compound). These results clearly indicate the partial oxidative degradation of B10H10 2-over the silver electrode and support the CV results discussed above.

Acknowledgements

This work was supported by the ANR Agence Nationale de la Recherche [program "Investissements d'Avenir", LabEx CheMISyst, grant number ANR-10-LABX-05-01]; the Région Languedoc-Roussillon [program "Chercheur(se)s d'Avenir 2013", project C3, grant number 2013 008555]. We thank Prof. M. Cretin (IEM, Univ Montpellier) for accessibility to cyclic voltammetry.