Bonelli Stéphane
email: stephane.bonelli@cemagref.fr

Brivois Olivier

Lachouette Damien

The scaling law of piping erosion

Keywords: piping erosion, dams, dykes

Piping erosion is one of the main cause of serious hydraulic work failure. This phenomenon involves the progression of a continuous tunnel between the upstream and the downstream side. Starting from the bulk equations for diphasic flow with diffusion and from the jump equations with erosion, we propose a two parameters scaling law for piping erosion. The first parameter is the critical shear stress. The second parameter is the erosional characteristic time, which is a function of the initial hydraulic gradient and of the erosion coefficient. The comparison with experimental data has confirmed the validity of the scaling law. We deduce an estimation of the time for breaching in hydraulic works (dams, dykes).

Résumé :

L'érosion par renard hydraulique est l'une des causes de rupture des ouvrages hydrauliques. Elle est liée au développement d'un tunnel continu entre l'amont et l'aval. A partir des équations d'écoulement diphasique avec diffusion, et des équations de saut avec érosion, une loi d'échelle à deux paramètres pour l'érosion par renard est proposée. Le premier paramètre est la contrainte seuil d'érosion. Le second paramètre est le temps caractéristique d'érosion, fonction du gradient hydraulique initial et du coefficient d'érosion. La comparaison avec des résultats expérimentaux valide la loi d'échelle. On en déduit une estimation du temps de rupture d'un ouvrage hydraulique (barrage, digue).

Introduction

Erosion of soil resulting from piping, seepage or overtopping is the main cause of serious hydraulic work (dykes, dams) failure, in terms of the risk of flooding areas located downstream. The present study concerns the first process: the enlargement of a crack, which leads to an erosion process known as "piping" in soil mechanics. Piping erosion often occurs in hydraulics works, involving the formation and the progression of a continuous tunnel between the upstream and the downstream side. When this phenomenon is suspected of occurring or has already been detected in situ, the time to failure is difficult to predict. To be able to develop effective emergency action plans preventing heavy casualties and damage to property, it is necessary to have a characteristic time to use as a basis. The aim of this study was to draw up a useful model for estimating this time characteristic for breaching.

Two-phase flow equations with interface erosion

A large literature on soil erosion also exists in the field of hydraulics and river engineering [START_REF] Chanson | The Hydraulics of Open Channel Flows: An Introduction[END_REF], and in the field of poromechanics and petroleum engineering [START_REF] Vardoulakis | Hydromechanical aspects of sand production problem[END_REF]. In the field of geomechanics, several experimental methods have been developed for 2 simulating the piping erosion process experimentally, with particular attention focussed on the hole erosion test [START_REF] Wan | Investigation of internal erosion and piping of soils in embankment dams by the slot erosion test and the hole erosion test[END_REF]. The experience acquired on more than 200 tests on several soils has confirmed what an excellent tool this test can be for quantifying the rate of piping erosion in a soil. A model for interpreting the hole erosion test has been developed by [START_REF] Bonelli | On modelling of piping erosion[END_REF].

The soil is eroded by the flow, which then carries away the eroded particles. As long as the particles are small enough in comparison with the characteristic length scale of the flow, this two-phase flow can be said to be a continuum. We take to denote the volume of the twophase mixture and the fluid/soil interface. For the sake of simplification, sedimentation and deposition processes are neglected, and the soil is taken to be saturated. The mass conservation equations for the water/particles mixture and for the mass of the particles as well as the balance equation of momentum of the mixture within can be written as follows in a Eulerian framework:

t + ( u) = 0 , Y t + ( Y u) = J , u t + ( u u) = .
In these equations, is the density of the mixture, depending on the particles mass fraction Y , u is the mass-weighted average velocity, J is the mass diffusion of the flux of particles, and is the Cauchy stress tensor in the mixture. As there is a process of erosion, a mass flux crosses the interface . Let us take n to denote the normal unit vector of oriented outwards from the soil, and v to denote the normal velocity of . The jump equations over are

( v u) n = 0 , Y ( v u) n = J n , u( v u) n = n ,
where a = a s a w is the jump of any physical variable a across the interface, and a s and a w stands for the limiting value of a on the solid and fluid sides of the interface, respectively. The soil is assumed to be homogeneous, rigid and devoid of seepage. The co-ordinate system depends on the soil in question. The total flux of eroded material (both particles and water) crossing the interface is therefore m = s v n , where s is the density of the soil. Erosion laws dealing with soil surface erosion by a tangential flow are often written in the form of threshold laws such as (see Chansonfor a comprehensive review (1999)):

m = k er ( w c ) if b > c , 0 otherwise ,
where c is the critical shear stress involved in the erosion, k er is the coefficient of soil erosion, and w is the tangential shear stress at the interface defined as follows:

w = ( n) 2 ( n 2 w .
.

Application to piping erosion, the scaling law

This complete set of equations was previously used to study various situations involving boundary layer and free surface flows over an erodable soil [START_REF] Brivois | Contribution to strong slope erosion by a two-phase turbulent flow[END_REF]. The use of these 3 equations has been extended to the study of piping erosion [START_REF] Bonelli | On modelling of piping erosion[END_REF], by introducing a spatial integration over a cylinder with radius R (initial value R 0 ) (Fig. 1). Reference velocity is

V 0 = Q 0 / R 0 2
, where Q 0 is the initial input flow. The flow time is t 0 = R 0 /V 0 . The driving pressure is P = R(p in p out ) / (2L) (initial value P 0 ), where p in and p out are the input and output pressures, respectively ( p in > p out ), and L denotes the characteristic geometrical length of the system (it is not a wave lengthscale).

We take V er = k er P 0 / s to denote the reference erosion velocity. The erosion time is therefore t er = R 0 /V er . The total volume flow of eroded material is

Q er = 2 R 0 LV er .
The flow rate scale ratio is Q er / Q 0 = 2LV er / (R 0 V 0 ) . We take therefore c 0 to denote the reference volume fraction, defined by c 0 = (1 n)Q er / Q 0 where n is the soil porosity. Finally, we define k er = k er V 0 as the kinetics of erosion number. These are two key dimensionless numbers in our analysis.

When c 0 1 n , the amount of eroded material is much more lower than of the water (Q er Q 0 ). In this case, the fluid is a dilued suspension, and the volume fraction does not affect the system. This case arises when the pipe is not too long ( L R 0 k er 1 ). When k er 1 , the flow time scale is much more lower than of the erosion time scale ( t 0 t er ), and the erosion velocity is much more lower than of the flow velocity (V er V 0 ). Since surface erosion is a slow process, the erosion evolves at a very long scale compared to the flow scale. The flow can be considered as quasisteady, and inertial terms can be neglected.

To settle the equations, we introduce the dimensionless variables:

t = t t er , R = R R 0 , V = V V 0 , P = P P 0 , Q = Q Q 0 , c = c P 0
Under the dilued suspension situation ( L R 0 k er 1 ) and the low kinetics of erosion situation ( k er 1 ), obtained equations simplify considerably. The velocity and the flow are related to the pressure and the radius by R P = V 2 and R 5 P = Q 2 . We consider a unit step of driving pressure P( t ) = 0 if t < 0, 1 if t > 0 . The closed-form solution of the obtained system is entirely given by the following evolution of the radius:

R( t ) = 1 if t 0 or c 1 c + (1 c ) exp( t ) if t > 0 and c < 1
This is the scaling law of piping erosion with constant pressure drop. This important result has far-reaching practical consequences. It can be used to fit experimental data, expressing the scaled radius R c as a function of the scaling time t + ln(1 c ) , and making possible a unified description of piping erosion of differents soils, in pipe of various diameters, etc.

Comparison with experimental data

The hole erosion test was designed to simulate piping flow erosion in a hole. An eroding fluid is driven through the soil sample to initiate erosion of the soil along a pre-formed hole. The results of the test are given in terms of the flow rate versus time curve with a constant 4 pressure drop. Therefore, the flow rate is used as an indirect measurement of the erosion rate.

For further details about this test, see [START_REF] Wan | Investigation of internal erosion and piping of soils in embankment dams by the slot erosion test and the hole erosion test[END_REF].

The scaling law is now compared with previously published data [START_REF] Wan | Investigation of internal erosion and piping of soils in embankment dams by the slot erosion test and the hole erosion test[END_REF]. Analysis were performed in 17 tests, using 9 different soils. The initial radius and the length of the pipe were R 0 =3 mm and L =117 mm. Table 1 contains particle size distribution, and critical stress and erosion coefficient. The comparison with experimental data confirms the validity of the scaling law (Fig. 2).

5

It is well known fact that different soils erode at different rate. However, the relationship between the erosion parameters ( c ,k er ) and geotechnical or chemical properties of the soils remain unknown [START_REF] Wan | Investigation of internal erosion and piping of soils in embankment dams by the slot erosion test and the hole erosion test[END_REF]. Therefore, it is recommended to use hole erosion tests in order to evaluate the erosion parameters on any sample of cohesive soil from a site.

5

Possible application to dams and dykes

The rate of erosion has a significant influence on the time for progression of piping and development of a breach in earthdams, dykes or levees. This provides an indication of the amount of warning time available to evacuate the population at risk downstream of the dam, and hence has important implications for the management of dam safety.

Given that erosion has initiated, and the filters are absent or unable to stop erosion, the hydraulics of flow in concentrated leaks are such that erosion will progress to form a continuous tunnel (the pipe, Fig. 3). The rate of which this pipe will enlarge is dependent on the hydraulic gradient and the erodibility of the soil as measured by the coefficient of erosion k er . Gross enlargement of the pipe may form a breach by collapse of the column of soil located above (the roof). The main issue adressed by testing and modelling is: what is the time for progression of piping and breaching ?

The time of piping can be re-written as t er = 2 s L / (k er g w H w ) , therefore a function of well known engineering quantities: the coefficient of erosion k er , the soil density s , the water density w , the head drop H w between upstream and downstream, the length of the pipe L , and the gravitational constant g . Let us assume that critical stress c and coefficient of erosion k er are known, with preliminary laboratory tests. Let us assume that we know the maximum radius R collapse of a hole in the dam before collapse of the roof, with a preliminary geotechnical analysis. A sketch of the evolution of the pipe as a function of time is represented in Fig. 4.

After initiation by backward erosion, phenomenon which remains far from known, the piping process begins with initial -and unknown -radius R 0 . A visual inspection can provide an estimation of the output flow, thus an estimation of the actual radius R d . The scaling law of piping erosion gives us the following estimation of the remaining time to breaching t u :

t u t er ln R collapse R min R d R min , R min = 2L c w H w .
Application to Teton dam case gives rough estimation of the remaining time between eyewitnesses observations and breaching. The dam failed during its first filling on June 5, 1976. It was the highest embankment dam that had ever failed catastrophically in the entire history of earth dam construction [START_REF] Penman | Teton Investigation-A review of Existing Findings[END_REF].

Head drop and pipe length can be roughly estimated: H w 30 m and L 100 m. Laboratory tests gave 20 Pa for the critical stress c and 10 -3 s/m for the coefficient of erosion k er (Wan and Fell, 2002). Therefore, R min 13 cm. The maximum diameter before collapse is unknown. We take 2R collapse = 2H / 3 where H 90 m is the relevant dam height.

Eyewitnesses noticed a leak flowing about Q 1 m/s by 9 :30 AM. We can estimate the corresponding diameter of the hole to be 2R d 50 cm. The estimation of the remaining time t u between eyewitnesses observations and breaching is therefore about 2:30 hours. This result agrees with reality: at 11h20 AM the eroded hole in the dam was so large that bulldozers sent to fill the hole sank into the flow, and at about 11h55 AM the dam crest was breached as a complete failure occurred.

6

The coefficient of erosion k er can therefore serve as an indicator of the remaining time to breaching: a value of 10 -4 s/m would give a remaining time of 20 hours ! FIG. 3 The hole erosion test appears to be an efficient and simple means of quantifying the rate of piping erosion at laboratory. The scaling law obtained for interpreting the results of this test makes possible a unified description of piping erosion of differents soils, in pipe of various diameters. This description provides an estimation of the remaining time to breaching of hydraulic works (earthdams, dykes) when piping erosion occurs. This is an indication of the amount of warning time available for evacuation. However, two conditions are required: early (visual) detection of piping, and preliminary laboratory (hole erosion) tests.

FIG. 1 -

 1 FIG. 1 -Axisymmetrical flow with soil erosion and transport of the particles.

  FIG.3-Sketch of the piping erosion in a dyke or a dam.

Acknowledgement

This research project is sponsored by the French National Research Agency under grant 0594C0115 (ERINOH). The authors wish to thank Professor Robin Fell and Dr. Chi Fai Wan for their valuable experimental data.


S Dessources

C Morais

C Canaff

T W Napporn
email: teko.napporn@univ-poitiers.fr

K B Kokoh

Ensemble Effects of Nickel in Surfactant-less Prepared Pt-Ni Materials on the Carbon Monoxide Oxidative Removal

Keywords: PtxNiy electrocatalyst, CO-to-CO2 mechanisms, CO-stripping, BAE synthesis method

Ensemble effects of nickel in surfactant-less prepared

Pt-Ni materials on the carbon monoxide oxidative removal

Introduction

In the Direct Alcohol Fuel Cells (DAFCs) investigations, carbon monoxide (CO) is pointed out as the main poisoning intermediate through its strong adsorption at the Pt-based electrode surface [START_REF] Park | Infrared Spectral Comparison of Electrochemical Carbon Monoxide Adlayers Formed by Direct Chemisorption and Methanol Dissociation on Carbon-Supported Platinum Nanoparticles[END_REF][START_REF] Park | Chemical and Electronic Effects of Ni in Pt/Ni and Pt/Ru/Ni Alloy Nanoparticles in Methanol Electrooxidation[END_REF][START_REF] Babu | Metallic Nature and Surface Diffusion of CO Adsorbed on Ru Nanoparticles in Aqueous Media: A 13C NMR Study[END_REF][START_REF] Shao | Electrooxidation of ethanol on a Pt electrode in acid solutions: in situ ATR-SEIRAS study[END_REF]. The development of such solid polymer electrolyte (SPE) systems involves the design of materials capable of releasing the electrode surface at lower potentials, which prolongs the durability of the catalytic process [START_REF] Arenz | Carbon-supported Pt-Sn electrocatalysts for the anodic oxidation of H2, CO, and H2/CO mixtures.: Part II: The structure-activity relationship[END_REF][START_REF] Arenz | The Effect of the Particle Size on the Kinetics of CO Electrooxidation on High Surface Area Pt Catalysts[END_REF]. Amongst the main alcohols commonly studied as promising fuels for sustainable applications such as SPE fuel cells, our research group contributed during these last decades to the possible development of DMFC (methanol) [START_REF] Colmati | Pt-Sn/C electrocatalysts for methanol oxidation synthesized by reduction with formic acid[END_REF][START_REF] Chen | Synthesis of hydrous ruthenium oxide supported platinum catalysts for direct methanol fuel cells[END_REF], DEFC (ethanol) [START_REF] Colmati | Ethanol Oxidation on Carbon Supported Pt-Sn Electrocatalysts Prepared by Reduction with Formic Acid[END_REF][START_REF] Song | Recent progress in direct ethanol proton exchange membrane fuel cells (DE-PEMFCs)[END_REF][START_REF] Simões | Electroactivity of tin modified platinum electrodes for ethanol electrooxidation[END_REF] and DGFC (glucose) [START_REF] Chen | A binary palladium-bismuth nanocatalyst with high activity and stability for alkaline glucose electrooxidation[END_REF][START_REF] Holade | Highly Selective Oxidation of Carbohydrates in an Efficient Electrochemical Energy Converter: Cogenerating Organic Electrosynthesis[END_REF]. As can be evidenced through this literature, the breakthrough consists in adding a co-catalyst (Sn, Co, Ru, Ni,…) in the electrode composition to remove from the anodic surface at low potential values the remaining adsorbed CO formed during the alcohol oxidative conversion at the Pt-based material. This activity promoting element which is often a transition metal (TM) is considered to modify chemically, geometrically or electronically the properties of platinum. Consequently, the reaction can be controlled by the so-called bifunctional mechanism. If Sn and Ru were considered to contribute, respectively, to electronic and geometric effects in the enhancement of Pt electroactivity towards alcohol oxidation, Ni is a thermodynamically stable TM in alkaline medium and can play a co-catalyst role in the Pt-M anode towards alcohol oxidation. We synthesized by a surfactant-less method, e.g a bromide anion exchange approach, Pt-based catalysts for which the precious metal loading was "diluted" by adding various nickel amounts in order to investigate the effect towards the removal of CO from the electrode surface. This synergistic contribution of Ni observed by combining XPS and electrochemical methods (CO stripping/voltammetry) results in "ensemble effects" [START_REF] Strmcnik | Unique Activity of Platinum Adislands in the CO Electrooxidation Reaction[END_REF][START_REF] Rudi | pH-Induced versus Oxygen-Induced Surface Enrichment and Segregation Effects in Pt-Ni Alloy Nanoparticle Fuel Cell Catalysts[END_REF], hard to discriminate and mainly composed of chemical (early formation of hydroxyl species at the electrode surface), electronic (shift in Pt4f) and geometric properties in the PtxNiy electrode materials.

Experimental

The "Bromide Anion Exchange" (BAE) method, recently reported in ref. [START_REF] Holade | Electrochemical and Physicochemical Characterizations of Gold-Based Nanomaterials: Correlation between Surface Composition and Electrocatalytic Activity[END_REF] was adapted to synthesize a large range of PtxNiy/C nanoparticles (x and y being the molar ratios of Pt and Ni, respectively) [START_REF] Dessources | Reversible Electrocatalytic Activity of Carbon-Supported PtxNi1-x in Hydrogen Reactions[END_REF]. The carbon substrate (Vulcan XC-72R, Cabot), used as matrix for preparing PtxNiy/C materials with 20 wt.%. metal loading, was previously heat-treated at 400 °C under N2 atmosphere for 4 h. XPS was also used to probe and characterize the surface and oxidation states of the obtained Pt-based materials. Analysis was performed in a high vacuum chamber on a Kratos Axis Ultra DLD spectrometer at pressure ≤ 9 10 -8 Pa, with a monochromatic radiation source (Alk: 1486.6 eV) operating at 150 W (15 kV and 10 mA). Survey spectra were recorded with a step of 1 eV (transition energy: 160 eV), which permits to check after the cleaning step whether the as-prepared materials do not contain any impurities (boron and bromide species from NaBH4 and KBr). Based on the collected basic information, highresolution XPS spectra were collected at a step of 0.1 eV (transition energy: 20 eV). XPS peaks were fitted using Gaussian-Lorentzian profile function and asymmetry determined from an etched bulk Pt metal. The cross-section Tougaard background has been chosen. It should be noted that a systematic subtraction of the background noise preceded the normalization of the spectra and the measurement of binding energies (BE) was corrected based on the energy of C1s at 284.4 eV (Vulcan XC 72R carbon). Quantifications were done from the corresponding XPS peak area after correction with suitable sensitivity factors and the CASA XPS software was used for peak fitting. The electrochemical measurements were conducted in a threeelectrode Pyrex cell using a potentiostat EG&G PARC Model 362 (Princeton Applied Research). The current densities were normalized with the constant metal loading (79.3 μg cm -2 ) deposited onto a glassy carbon disc (0.071 cm 2 geometric surface area). This avoids expressing the activity of the Pt-based catalysts as function of the electrochemical active surface area, dependent of the Pt content, which decreases with the Ni addition. The CO stripping measurement consists of adsorbing during 5 min CO at constant potential of 0.1 V vs. RHE until the current decreases down to the capacitive one. Subsequently and at the same applied potential, nitrogen was bubbled in the solution during 25 min in order to remove remaining free CO. Afterwards, cyclic voltammetry (CV) was performed for recording 2 consecutive cycles: the first one permitted to oxidize adsorbed CO, while the second CV permits to confirm the complete oxidation of adsorbed CO since the first run. The shift of the main oxidative peak position, known to be at ca. 0.7 V vs. RHE at the Pt surface, allows appreciating how active the PtxNiy catalyst as ensemble effects.

Results and discussion

The PtxNiy/C materials were successfully prepared by the BAE method [START_REF] Dessources | Reversible Electrocatalytic Activity of Carbon-Supported PtxNi1-x in Hydrogen Reactions[END_REF]. From our previous paper [START_REF] Dessources | Reversible Electrocatalytic Activity of Carbon-Supported PtxNi1-x in Hydrogen Reactions[END_REF], data from different physicochemical characterizations were summarized in Table 1.

XRD analysis revealed that the increase of Ni content in PtxNiy/C materials, leads to the formation of Ni(OH)2 species which presence was observed at 33.5 and 59.3° 2 values, respectively [START_REF] Dessources | Reversible Electrocatalytic Activity of Carbon-Supported PtxNi1-x in Hydrogen Reactions[END_REF]. Consequently, samples with a high Ni content (yNi > 0.5) lead to a wide range of particles size (2 to 30 nm) due to the presence of these species, which are amorphous-like and not well crystallized. Therefore particles and crystallite size were not assessed for pure Ni/C to avoid any error of estimation. To provide relevant additional insights on the ensemble effects responsible for the CO-to-CO2 oxidative removal at lower potentials, the material surface state was investigated through XPS analysis (Fig. 1). Therefore, observations are focused on the modifications of the Pt4f regions in the different prepared PtxNiy materials. As can be seen in Fig. 1a, the 4f7/2 and 4f5/2 peaks are centered respectively at 71.2 eV and 74.5 eV in Pt/C, as reported [START_REF] Corcoran | Application of XPS to study electrocatalysts for fuel cells[END_REF]. The decomposition of these peaks suggests that the Pt material is mainly composed of Pt 0 (peaks at 71.1 and 74.5 eV), but it also contains two Pt oxide/hydroxide including Pt 2+ (peaks at 72.1 and 75.5 eV) and Pt 4+

(peaks at 74.6 and 77.7 eV). This speciation analysis reveals that the increase of the nickel content enhances the oxide/hydroxide species in the PtxNiy/C samples, which reaches ca. 25%

(for Pt 2+ ) and 22% (for Pt 4+ ) (Fig. 1a). Even if the Pt4f7/2 peak is partially in the PtNi-based materials, Fig. 1b shows a slight shift in binding energy which can be associated with an electronic change in the precious element when increasing the nickel content as the ligand effect associated with the nickel amount stops having effect. From the physicochemical characterizations, it can be pointed out that introduction of the Ni element in the Pt structure leads to the formation of nickel hydroxide and Pt oxide/hydroxide species in the prepared PtxNiy/C catalysts. In addition to this chemical effect, the ligand one was evidenced by XPS measurements. Indeed, we are wondering how the ensemble effects match for releasing the electrode surface in the CO-to-CO2 removal at lower electrode potentials. As shown in Fig. 2, the electrochemical characterization of the electrode materials was performed through the CO stripping technique described above in the experimental section. A voltammogram of a Pt/C electrode in the absence and presence of CO (Fig. 2a) was first recorded to optimize the operating conditions of the electrochemical investigations. Therefore, the three main regions of the signature of a Pt electrode (reversible hydrogen adsorption/desorption, Pt double layer and irreversible Pt hydroxide/oxide) in alkaline medium can be well observed in this voltammogram in the second cycle. It is worthy of noting herein that the upper potential limit was set at the common one in Pt (i.e, we do not consider a higher potential of the NiOOH formation (ca. 1.32 V vs. RHE), which is convenient for comparing all the as-prepared PtxNiy/C materials including Pt/C with the same upper potential limit). Actually, if CO is the main intermediate compound which prevents advanced oxidation of carbonaceous fuels, the strategy that consists in removing it by bifunctional mechanism deserves to be more investigated. CO-stripping was performed onto PtxNiy/C catalyst prepared by bromide anion exchange method. One of the various advantages of the surfactant-less BAE method is that no pre-treatment is required before electrochemical experiments [START_REF] Lai | Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes[END_REF][START_REF] Bergelin | Oxidation of CO adlayers on Pt(111) at low potentials: an impinging jet study in H2SO4 electrolyte with mathematical modeling of the current transients[END_REF]. This means that the electrocatalysts keep for sure the as-prepared aspects and structure at the beginning of the COstripping which is a sensitive electrochemical surface probing technique to evaluate the activity of an electrode. Before each CO-stripping the background cyclic voltammogram (CV) of the catalyst in the supporting electrolyte was recorded. One observes in Fig. 2a a main CO oxidation peak (A) at 0.67 V vs. RHE. It is commonly assigned to the CO-to-CO2 oxidative removal following the Langmuir-Hinshelwood (L-H) mechanism [START_REF] Maillard | CO monolayer oxidation on Pt nanoparticles: Further insights into the particle size effects[END_REF][START_REF] Spendelow | Mechanism of CO Oxidation on Pt(111) in Alkaline Media[END_REF], which proceeds by a concerted desorption of Pt-COads and Pt-OHads, the latter adsorbate being the result of the dissociative adsorption of H2O molecules (or HO -adsorption in alkaline medium).

Pt -CO ads + Pt -OH ads + 3HO -→ CO 3 2-+ 2H 2 O + e - Eq.

(L-H)

Peak A is preceded by two shoulders (B and C) that are located at 0.58 and 0.40 V vs. RHE, respectively. It was reported that CO oxidation at 0.58 V vs. RHE occurs on terrace sites with (110) orientation, while that at 0.40 V vs. RHE is taking place on kink-type defects [START_REF] Lai | Mechanisms of Carbon Monoxide and Methanol Oxidation at Single-crystal Electrodes[END_REF][START_REF] Farias | On the behavior of CO oxidation on shape-controlled Pt nanoparticles in alkaline medium[END_REF]. RHE, synonymous of the electronic transfer of element which downshifts the center of the dband with respect to the Fermi level of Pt 4f (Figure 1). This enhancement of the activity of the electrode at lower potentials is undoubtedly due to the increasing formation of Ni(OH)2 species up to a molar ratio of yNi = 0.8 (Figure 2b). As at this electrode potential it is considered theoretically that no oxygenated species are present at the Pt surface, the increase in the peak current densities is certainly due to either Eley-Rideal (E-R) mechanism (with the interaction between HO -species from electrolyte and Pt-COads) [START_REF] Spendelow | Mechanism of CO Oxidation on Pt(111) in Alkaline Media[END_REF][START_REF] Mukherjee | Effect of the Eley-Rideal step on catalytic oxidation of CO under periodic external pressure[END_REF][START_REF] Urchaga | Evidence of an Eley-Rideal Mechanism in the Stripping of a Saturation Layer of Chemisorbed CO on Platinum Nanoparticles[END_REF] or a bifunctional catalysis (BC) mechanism that consists in the interaction between Ni hydroxyl and Pt-COads [START_REF] Roca-Ayats | PtSn nanoparticles supported on titanium carbonitride for the ethanol oxidation reaction[END_REF], as assumed below:

Pt -CO ads + Ni -OH ads + 3HO -→ CO 3 2-+ 2H 2 O + e - Eq. 2 (BC)

As result, for yNi = 0.5 peak C has a current density value equal to that of peak A at Pt/C, i.e an impressive 270 mV shift towards the lower potentials. For yNi > 0.5, peak A is disappearing, while peak C starts decreasing. This double phenomenon can be explained by the conjugation of repulsive adsorption of CO towards the HO species and the lowering Pt content, which inhibits the surface structure, i.e prevents the L-H mechanism at the Pt surface. Table 2 gives the coulombic charges associated with the CO oxidation on the different catalysts. As the Ni content increases in the bimetallic composition up to 50%, the coulombic charge associated with the CO oxidation to carbonate increases from 0.69 to 0.79 mC. If on Pt/C this charge is only associated with the above-mentioned L-H mechanism, its increase to 0.79 mC could result from the co-existence of the two mechanisms (L-H and BC) on Pt0.5Ni0.5/C, as can be seen through the two oxidation peaks at 0.4 and 0.6-0.7 V/RHE (Figure 2b). Then the coulombic charge decreases to 0.54 mC when the Ni content is higher in the bimetallic anode composition (Pt0.2Ni0.8/C). This corresponds to the progressive setting up of the BC and E-R mechanisms to that of L-H as a result of the marked decrease in the Pt content, which is responsible for and where CO adsorption phenomena take place.

Conclusion

As CO is the main poison species that is responsible for the deactivation of the anode catalyst in the direct alcohol fuel cells, its oxidation reaction was investigated on PtxNiy/C synthesized from a surfactant-less simple reduction approach. XPS measurements revealed to be herein complementary techniques that provided evidences on the ensemble effects allowing to appreciate the excellent ability of PtxNiy/C anodes towards the CO oxidation reaction. Indeed, the physicochemical analyses of the materials correlated with the electrochemical characterization allowed to show that the increase of the Ni content in the Pt structure does modify the initial L-H mechanism towards E-R and finally BC ones. The 270 mV negative shift of the CO oxidative removal cannot be explained as the sole effect of the low Pt 4f7/2 peak shift, but also by the contribution of the Ni-OH species in the close environment of Pt-COads, which mainly leads to a bifunctional mechanism. This trend, emphasized by the increase of the Ni content up to yNi = 0.8, makes these bimetallics, either a CO tolerant cathode or promising active anode material in a direct alcohol fuel cell. Importantly, this work shows Ni as low-cost and earth-abundant element that allows to decrease the amount of the precious metal in a catalyst composition, not only without decreasing the electroactivity, but with improving its ability towards organics oxidation.

Fig. 1 .

 1 Fig. 1. High resolution XPS spectra of the PtxNiy/C materials prepared from the BAE method.

Figure 2 .

 2 Figure 2. CO-stripping of different molar ratio PtxNiy/C catalysts in a CO-saturated solution containing 0.1 mol L -1 NaOH recorded at 20 mV s -1 and at 21 °C; a) 1 st cycle (red line, presence

Table 1 .

 1 Physical properties of PtxNiy materials obtained from TGA/DTA, XRD and TEM

	measurements
				PtxNiy
	Molar ratio	1:0	0.7:0.3	0.5:0.5	0.3:0.7	0.2:0.8	0:1
	Metal loading (%)	20.6	19.3	22.0	23.2	22.8	21.3
	Lv/nm (XRD)	4.9	4.5	3.5	3.2	3.5	-
	Dm/nm (TEM)	2 to 11	2 to 9.5	1.5 to 7	2 to 20	3 to 35	-
	Further to these results, we scrutinized herein XPS measurements and the correlation with
	releasing the electrode surface from CO poisoning.

Table 2 .

 2 Charge associated with the CO stripping on different PtxNiy /C nanocatalysts.

	Nanomaterials	Charge related to CO
		oxidation peak/ mC
	Pt/C	0.69
	Pt0.5Ni0.5/C	0.79
	Pt0.2Ni0.8/C	0.54

The excellent catalytic ability of the prepared PtxNiy/C materials towards the CO oxidation at lower potential values, as the common and major poisoning intermediate in the organics electrooxidation, provides sound evidences to the utilization of this type of catalyst either as CO tolerant cathode or anode in the alcohol fuel cells.

Acknowledgements

The authors thank the European Union (ERDF) and "Région Nouvelle-Aquitaine for their support.


Charly Lemoine

Lionel Dubois

Teko W Napporn

Karine Servat

Boniface Kokoh
email: boniface.kokoh@univ-poitiers.fr

Kouakou B Kokoh

Electrochemical Energy Conversion from Direct Oxidation of Glucose on Active Electrode Materials

Keywords: Glucose fuel cell, Carbon substrate Pt and Au electrodes, Alkaline medium

Introduction

The interest on glucose in the domain of energy production is increasing because this sugar, which is abundant in living being body, is a promising fuel for new implantable energy sources [1][2][3][4]. Enzymes, such as glucose oxidase, have been largely studied for enzymatic oxidation of glucose, showing good activity and selectivity, but their lack of stability has been reported as their principal weakness [5]. As a result, the lifetime of enzymatic fuel cells does not exceed several weeks, limiting their application to short-term implantable sensors. Thereby, research investigations have been focusing last decades on abiotic systems [6,7]. The use of such catalysts made of supported metal nanoparticles as glucose fuel cell anode presents great advantages such as long-time stability, resistance to temperature modification, and large pH window.

In aqueous solution glucose takes a hemiacetalic form, which confers it two anomeric positions,  and -glucose, the latter one being the more reactive at the electrode surface [8,9]. It is also reported that glucose dehydrogenation to glucono--lactone occurs better at low potentials at the platinum surface [10,11], which is very sensitive to poisoning in the presence of the degradation products, for instance CO species [8]. For these reasons and considering the affinity of the Au anode towards aldehydes/hemiacetals, the association of Au and Pt as bimetallic electrode may lead to reduce the poisoning effect and emphasize the stability of the catalyst for long-term applications [12][13][14][15].

As the electrocatalytic activity of metal nanoparticles results from surface states, the performance of the catalyst is directly dependent of its available electrochemical surface area (ECSA), so on the size and morphology of the nanoparticles. The surface cleanliness is also very important because the electrocatalytic activity can be drastically inhibited by the adsorption of organic species such as capping agent during the synthesis. Furthermore, the supporting electrode plays a key role in the enhancement of the catalyst activity and stability. Supported materials not only maximize the availability of nanoparticles surface for electron transfer, but also provide better mass transport of reactants to the electrocatalyst. In addition, the conductive support facilitates efficient collection and transfer of electrons to the electrode surface [16]. Carbon black has been extensively employed because of its high electrical conductivity, surface area and low cost. Despite all that progress, the major obstacle for sustainable performances is the aggregation process of nanoparticles in time [17,18]. A. Zadick et al. [19] observed a severe degradation of Pt deposited onto a Vulcan XC 72 substrate. Indeed, after 150 cycles in the potential range 0.1 -1.23 V vs RHE in 0.1 M NaOH and at 25 °C, they noted 60% ECSA loss due to the Pt particles agglomeration. Recently, graphene derivatives like graphene oxide (GO) and reduced graphene oxide (rGO) have been reported to be promising electrode supports for metal nanoparticles growth in electrocatalysis field [20][21][22][23][24][25].

Graphene is composed of a single sheet of sp 2 hybridized carbon with a huge surface area (theoretical value is 2630 m²/g) and shows high career mobility, high electrical conductivity and an improved resistance to corrosion. Indeed, the sp 2 hybridized carbon leads to an increased strength of π sites on the support that acts as anchoring centers for platinum nanoparticles, thus strengthening the nanoparticle-support interaction [26]. Through its properties, rGO is a hopeful supporting material for ensuring to nanoparticles improvement of their electroactivity and longterm stability [27]. Accordingly, Au and Pt nanocatalysts were prepared by Bromide Anion Exchange (BAE) method [28]. This surfactant-free synthesis is based on the anion exchange of potassium bromide with the metal precursor salts before reduction by sodium borohydride, and allowed to obtain well-dispersed and small-sized nanoparticles without organic pollutant.

Monometallic and bimetallic catalysts were synthetized on two different carbon supports: amorphous carbon Vulcan and reduced graphene oxide (rGO). The present investigation was done in alkaline medium, instead of physiological buffer, to remove the electrolyte easier than phosphate, which facilitates the identification of all reaction products and ensures the absence of any toxic intermediate or final products. Electrochemical behavior of the catalysts towards glucose oxidation and oxygen reduction reaction were investigated separately in alkaline medium before implementing the glucose/O2 fuel cell with the best anode and cathode catalysts.

Experimental

Before using, all the glassware was successively cleaned in a mixed sulfuric acid and potassium permanganate solution then in acidic hydrogen peroxide solution and finally rinsed with boiling ultrapure water (18.2 MΩ cm at 20 °C). KOH (90%, Sigma-Aldrich) solution was prepared with ultrapure water. HAuCl4, 3H2O (99.9%), H2PtCl6, 6H2O (99.9%), NaBH4 (99%) and D-(+)-glucose (99.5%) were purchased from Sigma-Aldrich. KBr (99%) was purchased from Fisher Scientific. Filters Durapore Membrane, hydrophilic PVDF, 0.22 µm, 47 mm were purchased from Merck. Amorphous carbon Vulcan XC-72R, purchased from Cabot was thermally treated at 400 °C under nitrogen atmosphere during 4 h, as described elsewhere [29].

The rGO substrate was prepared by dissolving 16 g of commercial graphene in sulfuric acid (400 mL). Graphene nanoplatelets aggregates (sub-micron particles, surface area 750 m² g -1 from STREM chemical), and sulfuric acid were purchased from Sigma-Aldrich. The mixture was stirred for 7 hours at 200 ºC. After cooling, an acidic solution was carefully added in 400 mL of water and the obtained suspension was transferred to two beakers for its decantation.

After removal of the acidic phase, the washing process (400 mL of water) was repeated three times. The obtained black paste was separated by filtration and washed with water until pH 5-6. The extra-exfoliated graphene was lyophilized and then dried in a vacuum oven for 24 h at 80 °C. Then, 5 g was put in a vitreous carbon boat located in a silica tube with a H2 10 % / Ar inlet (3 L h -1 ). The system was placed in a tubular oven and thermally treated during 4 h at 750 °C with a 5 °C min -1 rate. After cooling, 3.5 g of over reduced rGO was finally obtained.

Preparation of the electrode materials from the BAE synthetic method

Au and/or Pt salts were introduced in a 500 mL round bottom flask and dissolved in MilliQ water to have 1 mM metallic cation in a 100 mL solution. After stirring for 10 min at room temperature to homogenize the solution, KBr was introduced to have a ratio of n(KBr)/n(metal) equal to 1.46, [30] stirring for 10 min to allow the ionic exchange between the bromide and the chloride. Afterwards, a quantity of carbon was added, corresponding to a final hybrid material M/C of 20/80 wt. %. This solution was then ultrasonicated for 45 min without temperature control. 15 mL of 0.1 M NaBH4 was prepared and cooled down with ice during 10 min. The reducing solution was introduced dropwise into the flask under vigorous stirring at room temperature. Finally, the resulting solution was maintained under stirring in an oil bath at 40 °C during 2 h then filtered. The solid was dried in an oven at 70 °C overnight.

Physicochemical characterizations

X-Ray Diffractometry (XRD). XRD patterns were obtained with an Empyrean

(PANanalytical) diffractometer in Bragg-Brentano (θ-θ) configuration. X-rays are generated by a copper anode powered at 45 kV and 40 mA. Cu Kα1 (λ= 0.15406 nm) primary beam was isolated thanks to a nickel filter installed in secondary optical before the linear detector

XCelerator, which open window has been set on 5 °. Patterns were recorded between scattering angles 2θ = 15° and 130°, with a step of 0.05°/2θ and an acquisition time of 2 min by step.

Transmission Electron Microscopy (TEM).

Nanocatalysts were examined with a JEOL 2100 UHR TEM equipped with a LaB6 filament. Images were obtained in bright field using a 200 kV accelerating voltage and a maximum resolution of 0.19 nm.

Raman spectroscopy. The analyses were performed with a HORIBA JOBIN YVON Labram HR800UV confocal microscope equipped with a CCD detector 74 cooled by the Peltier effect. The primary radiation (power of 0.05 mW) was produced by an Ar+ laser (Melles Griot) with a wavelength of 514.5 nm. An optical density filter was used to limit the irradiation of the sample. A 600.mm -1 diffraction grating was used and the confocal hole opening was fixed at 400 μm. The spectral resolution was 1.5 cm -1 and the survey area was about 1 μm². The spectrometer was previously calibrated with a silicon sample. A Pseudo-Voigt function was used to decompose the spectra.

X-Ray Photoelectron Spectroscopy (XPS). The XPS analyses were performed with a Kratos

Axis Ultra DLD spectrometer equipped with a monochromatic Al Mono radiation source (Kα(Al) : 1486.6 eV) operating at 150 W (15 kV and 10 mA). The supported catalysts were placed on a copper support and introduced into the high vacuum chamber (10-12 bar) through an airlock. The irradiated surface was 0.3 x 0.7 mm². The high-resolution spectra were performed with a step of 0.1 eV. The background noise was subtracted and the KE values were systematically corrected with respect to the C 1s band energy which must be at 284.2 eV with this device. The energy bands were deconvoluted and modelled by combining a Gaussian function and a Lorentzian function, modulated by an asymmetry factor.

Analysis of the reaction products by ionic chromatography. Samples were characterized in a

Dionex ISC 5000 apparatus, equipped with a conductimetric detector, very sensitive and specific to carboxylic compounds (CD-5000) followed by an amperometric one (ED-5000), which is selective to carbohydrate analysis. Before the chromatographic characterization, the samples were passed through a cationic resin (DOWEX 50WX8-200 from Sigma-Aldrich) to remove the electrolyte (K + , HO -) and to convert carboxylate to their protonic form. Then the solution was frozen and lyophilized (Freeze PowerDry LL3000, Thermo scientific) to remove water. The resulting solid was diluted in 4 mL of water, and 20 µL of solution is injected by an AS50 autosampler into the eluent and flows through an IonPac AS15, 2 x 250 mm column. The eluent was 10 mM NaOH and a constant 0.3 mL min -1 flow rate was employed.

Catalytic inks formulation

Catalytic inks were prepared as follows:

A. Ink composition for voltammetry: 4 mg of M/C catalyst, 500 µL water, 100 µL isopropanol, 50 µL of Nafion ® emulsion solution 5 wt. % (1.23 mgmetal mL -1 ). B. Ink composition for fuel cell tests: 4 mg M/C catalyst, 300 µL water, 25 µL of Nafion® emulsion solution 5 wt. % (2.46 mgmetal mL -1 ). 23 µL of ink is deposited onto each face of a carbon paper sheet (2 x 0.5 cm²) to have a covered surface of 0.3 cm² on each face, corresponding to a mass loading of 0.19 mgmetal cm -2 .

Electrochemical characterizations.

Cyclic voltammetry (CV). CV measurements were performed at room temperature using an Autolab PGSTAT 302N potentiostat connected to a three electrodes glass cell. A glassy carbon slab and Hg/HgO electrode (MOR) purchased from ALS Co, were used as counter and reference electrodes, respectively. The last one was introduced in a glass extension with a platinum wire at the tip to ensure the electrical conduction with the solution in the cell. 3 µL of ink A was deposited onto 3 mm diameter glassy carbon disk inserted in a Teflon cylinder and dried under nitrogen flux while rotating at 600 rpm. Finally, the catalyst metal loading which composed the working electrode was estimated at 52 µgmetal cm -2 . The electroactivity of the prepared materials was measured in N2-saturated supporting electrolyte 0.1 M KOH between -0.88 and 0.27 V vs. MOR, corresponding to 0.05 and 1.20 V vs RHE. Potential cycling was done at 50 mV s -1 scan rate to stabilize the catalyst surface before recording two other ones at 20 mV s -1 , in the absence and the presence of 50 mM glucose.

Direct Glucose Fuel Cell testings (DGFC).

The DGFC consists of two compartments, anodic and cathodic, separated by a Fumatech anion membrane. Each compartment was filled with 20 mL of the support electrolyte (0.1 M KOH), and glucose was added in the anodic one to reach 50 mM. The anodic compartment was deaerated with an inert gas (nitrogen), while bubbling oxygen in the cathodic side for the ORR process. Electrodes were made of 2 x 0.5 cm² carbon Toray sheets, electrically connected with a Pt wire and covered on both sides with ink B to obtain 2 x 0.3 cm² of covered surface. Steady-state currents were applied to the two electrodes, and the resulting cell voltage was recorded with an Autolab PGSTAT 302N potentiostat, while tiny Ag/AgCl reference electrodes were added in each compartment to measure the electrode potentials at the anode and the cathode, separately, during the fuel cell operation.

Results and discussion

rGO supported nanocatalysts were characterized by X-ray diffractometry (XRD). Part of the recorded diffractograms is displayed in Figure 1. A zoom in the range 2θ = 37° to 41° allows to appreciate the coexistence of the metallic particles thanks to the Au (111) and Pt (111) planes: the maximum of the Au (111) diffraction peak shifts from 38.209 ° (Au/rGO) to 38.271° (Au30Pt70/rGO). Also, the Pt (111) diffraction peaks shifts from 39.953 ° (Pt/rGO) to 39.878 ° (Au50Pt50/rGO). Moreover, the more Au is present, the more intense is its (111) reflection plane and the less intense is the Pt (111) one. According to the Vegard's law, most of the metallic phases are not alloyed. The crystallite sizes extracted on the (111) planes from Debye-Scherrer equation and Bragg's law comprise between 5-10 and 10-15 nm for Pt and Au, respectively. TEM images of the same catalysts depicted in Figure 2 show a good dispersion and a small particles size; the mean particle size resulted from counting more than 300 particles was 1.8 ± 0.2 nm for both Pt catalysts and 4.4 ± 0.5 nm for Au/rGO. That observations suggest a large available electroactive surface, suitable to optimize the electrocatalytic activity towards glucose oxidation.

The structural properties of the carbon supports were investigated using Raman spectroscopy.

The spectra recorded in the 800 to 2000 cm -1 range are displayed in Figure 3a for Vulcan and Figure 3b for rGO. Their simulated profile and deconvolution were done using Pseudo-Voigt function. Both spectra show a band centered at 1200 cm -1 , commonly associated with the presence of sp 3 rich-domains (polyene-like structures). The D band centered at 1340 cm -1 is characteristic of the disorder in the materials [31][32][33][34], and is thinner in the case of rGO. The third band centered at 1515 cm -1 and visible for both materials traduces the presence of amorphous sp 2 domains [35]. The G band, associated with the inter-plane vibration of sp 2 carbon network [36], is shifted from 1596 cm -1 for Vulcan to 1578 cm -1 for rGO, for which the G band is also thinner. Moreover, an extra-band centered at 1615 cm -1 only appears in rGO spectra. It corresponds to the D' mode and is related to the presence of intercalated graphite compounds [37]. From that fitting some structural parameters have been extracted: the size of the graphitic domains La and the graphitization degree corresponding to the ratio of D and G bands integrated intensity ID/IG. The La value in nm was calculated according to equation 1 [38]: Table 1 compares the calculated values and show that the difference is very small. Despite all the common bands traducing presence of sp 2 and sp 3 domains in both materials, and that calculated values of ID/IG and La are close, the width of D and G bands, the shift of the G band and the presence of the D' band show a more ordered structure of rGO compared to Vulcan [39]. Electronic interactions occurring between metals were investigated by XPS measurements. As can be observed on Figures 4a and4b, the peaks corresponding to Pt 4f5/2 and Pt 4f7/2 orbitals are located, respectively, at 74.6 eV and 71.3 eV in monometallic catalysts Pt/Vulcan and Pt/rGO. PtO2 and PtOx species are detected in both catalysts, at a proportion of Pt oxides calculated at respectively 19 wt. % and 28 wt. % for Pt/Vulcan and Pt/rGO. On the XPS spectra of the bimetallic catalysts (Figures 4c and4d), the position of Pt 4f peaks do not shift when compared to the monometallic ones; this explain the non-alloy nature of Au and Pt in the bimetallic Au70Pt30/rGO and Au50Pt50/rGO materials. [40] CO-stripping investigations were performed on Pt-containing catalysts in 0.1 M KOH electrolyte. This experiment consists in adsorbing a monolayer of CO at the electrode surface by successive saturation of solution with CO gas, then with N2 in order to remove from the electrolytic solution all free CO, i.e. except the monolayer under potential of 0.10 V vs RHE.

𝐿 𝑎 = 2.
Then a CV was recorded to oxidize CO during the first anodic scan, while the second cycle would exhibit a reference Pt, Au or AuPt electrode surface. Voltammograms of the prepared materials characterized by CO-stripping can be seen in Figure 5. The dispersion of Pt nanoparticles on the carbon substrate is more favorable to Vulcan than rGO when regarding the shift of the CO oxidation peak from 0.71 (rGO) to 0.68 V vs RHE (Vulcan). For the same bimetallic composition (Au70Pt30), a similar trend is observed (E = 28 mV) from Au70Pt30/rGO to Au70Pt30/Vulcan. Whatever the carbon substrate, the presence of Au leads to a small shift of the CO-to-CO2 oxidation peak toward lower potential values (-22 mV) on the Au70Pt30 bimetallic catalysts compared to Pt alone (Figure 5).

Calculations of specific electrochemical surface areas (SECSA) summarized in Table 2 were calculated according to equation 2 [41]:

𝑆𝐸𝐶𝑆𝐴 = 𝐸𝐶𝑆𝐴 𝑚 𝑃𝑡 = (∫ 𝛥𝐼(𝐸)𝑑𝐸 𝐸 𝑜𝑓𝑓𝑠𝑒𝑡 𝐸 𝑜𝑛𝑠𝑒𝑡 ) × 1 𝑣×𝑄 𝑚𝑙 ×𝑚 𝑃𝑡 (Eq. 2)
where Eonset and Eoffset are the onset and offset potentials of CO oxidation, ΔI(E) the difference of intensity between the first and the second anodic scan, v the scan rate (20 mV s -1 ), Qml the charge associated with the desorption of a CO monolayer (420 µC cm -2 ), and mPt the mass of platinum on the electrode. Calculations were normalized to the mass of Pt because Au is not active in CO-stripping [42]. The SECSA values obtained are higher when the nanoparticles are deposited onto the rGO substrate than the same compositions onto Vulcan (Table 2). Moreover, the addition of the Au content in the Pt based materials improves not only the onset potential of CO oxidation, but also the activity of the bimetallics that present higher SECSA than those found for monometallics. Although the monometallic materials deposited onto Vulcan and rGO are similar sizes, the better activity exhibited by those onto rGO may be due to the performance of the charge transfer between the catalysts and the rGO support.

The prepared materials were characterized in alkaline medium. First of all, their recording in a supporting electrolyte allows to reveal the presence of Pt in Pt/rGO and in the bimetallic compositions (Figure 6a).

Although the CV of Au/rGO does not reflect the standard shape, the CV in the presence of glucose (Figure 6b) shows the expected oxidation peaks in alkaline medium, as recently reported [43]. As the glucose oxidation occurs earlier at the Pt surface via dehydrogenation on the anomeric carbon position [8], one can observe that the Au70Pt30/rGO anode has a similar electrochemical behavior than Pt/rGO in the Pt-like hydrogen zone between 0.05 and 0.40 V vs RHE. When comparing the current densities at low potential of an anode in a fuel cell application, with 30% less Pt than Pt/rGO, the Au70Pt30/rGO catalyst is as active as Pt alone.

For the cathodic counterpart, oxygen reduction reaction (ORR) has been studied in alkaline media on Pt/Vulcan containing a wt. 20% metal loading. Linear sweep voltammograms, displayed in Figure 7, have been recorded at 5 mV s -1 scan rate in a O2-saturated 0.1 M KOH solution, using a rotating disk electrode at different rotation rates (from 400 to 2500 rpm). The onset potential of ORR and the half-wave potential at 1600 rpm were found at 1.06 and 0.78 V vs RHE, respectively, showing a good activity of this catalyst towards the ORR. The number of electrons involved in the O2 reduction reaction was estimated at 3.7 using Levich equation at 0.30 V vs RHE, which suggests a 4-electron pathway in the cathodic side [44].

The first investigations of the glucose/O2 fuel cell were undertaken with two series of experiments, one on Vulcan supported catalysts (Figure 8a), while the second operation was performed at the anodes deposited onto rGO (Figure 8b). The evolution of the cell voltage Considering that the cathode potential remains stable for each experiment, the cell voltage is only dependent of the anode behavior. The glucose/O2 fuel cell delivers a maximum power density of 900 and 1080 µW cm -2 on the Pt/Vulcan and Pt/rGO anodes, respectively. The Au70Pt30 catalyst displays an intermediary activity compared to Au and Pt. Finally, all the rGO supported catalysts show higher electrochemical performances than those analogous deposited onto Vulcan. On can conclude that the structure of the carbon substrate clearly influences the activity of the catalysts.

Different bimetallic AuPt compositions of rGO supported catalysts were also tested in the glucose/O2 fuel cell to find the best operating conditions and the most active anode (Figure 9).

The same OCV of 0.94 V was obtained for Pt/rGO and all bimetallic catalysts. Considering the relation observed in TEM (Figure 2) between agglomeration and Pt loading in bimetallic nanoparticles, that order of activity cannot be correlated to the particle size or the electrochemical active surface but directly to the Pt amount in the bimetallic composition.

Indeed CO-stripping results show that the Pt amount at the surface is higher for rGO supported materials (Table 2).

The electrochemical performances recorded herein are in the same order than those recently reported, [6,15,[45][46][47][48] even if the concentrations of glucose and the supporting electrolyte are 5 and 10 times lower in our case. Due to their simple preparation method from BAE and their proved efficiency, Pt/rGO and bimetallic AuPt/rGO anodes are expected to be largely used in alkaline glucose fuel cells.

To obtain enough products to analyze, the fuel cell was maintained in nearly optimized conditions during a long time; the current density was set to correspond to the value just before the highest power density, 1 mA cm -2 for Au70Pt30/rGO anode and 1.5 mA cm -2 for Au/rGO and Pt/rGO anodes. As can be seen in Figure 10, the cell voltage decreases as function of time because of the glucose consumption at the anode.

At the end of the testing, the anodic solution was extracted off the cell, neutralized on cation exchange resin and lyophilized for 24 h. The resulting solid was diluted in water and injected in ionic liquid chromatography equipped with a conductimetric detector to reveal the produced carboxylate species. These products were determined by comparing the retention times to standard analytes under similar elution conditions (Figure 11). Accordingly, gluconate (8.4 mM) and glucuronate (0.2 mM) were detected as the sole glucose oxidation products i.e without detectable compounds issued from C-C bond cleavage. If gluconate is already known to be the main glucose oxidation product in alkaline media when noble metal catalysts are employed [49], the presence of glucuronate which results from the oxidation of the primary alcohol at C6-position, reveals importantly that the electrocatalysts synthesized from BAE are selective [50].

Conclusion

In this paper, we demonstrated the excellent reactivity of glucose in alkaline fuel cell over catalysts made of Au and Pt nanoparticles deposited onto reduced graphene oxide (rGO). The BAE synthesis allowed to lead to small-sized and well-dispersed nanoparticles able to catalyze glucose oxidation in alkaline fuel cell with higher electrochemical performance. As result, these fuel cell delivered a power density of 1080 µW cm -2 with glucose concentration 5 times lower attributed to the strong interaction between the metal nanoparticles and the sp 2 carbon substrate, and the excellent electrical properties of rGO. By analysis of oxidation products from glucose/O2 fuel cell we have demonstrated that gluconate and glucuronate were selectively produce without any detectable chemical issued from the C-C bond cleavage. This point is of great importance for implantable devices in order to preclude toxic product release in living body.

Figures Caption

versus

  current density in the home-made fuel cell containing a Fumatech anion exchange membrane, reveals that i) the power density increases as function of Pt content in the anodic composition; ii) the open circuit voltage (OCV) also increases with the Pt content when the catalyst was dispersed on a Vulcan substrate; iii) for the rGO supported catalysts the OCV with the gold anode increases slightly, while the Pt-containing electrocatalysts permit to deliver the same OCV values of 0.94 V. As result, the highest values of cell voltage and power density are obtained with Pt monometallic anodes, and the lowest, on the Au monometallic ones.

:

  Figure 1: XRD patterns in the window 2θ = 37 to 41° of rGO supported catalysts.

Figure 2 :

 2 Figure 2: TEM images of (a) Pt/Vulcan, (b) Pt/rGO, (c) Au70Pt30/rGO and (d) Au/rGO.

Figure 3 :

 3 Figure 3: Deconvoluted Raman spectra of (a) Vulcan support and (b) rGO support.

Figure 4 :

 4 Figure 4: XPS spectra of (a) Pt/Vulcan, (b) Pt/rGO, (c) Au70Pt30/Vulcan and (d) Au70Pt30/rGO.

Figure 5 :

 5 Figure 5: First cycle anodic scans of CO-stripping recorded at 20 mV s -1 in 0.1 M KOH, after CO adsorption by polarization at 0.10 V vs RHE. Inset: cyclic voltammograms of CO-stripping for Pt at the surface of the two carbon substrates (rGO and Vulcan).

Figure 6 :

 6 Figure 6: Cyclic voltammograms recorded at 20 mV s -1 in 0.1 M KOH (a) in the absence and (b) in the presence of 50 mM glucose at room temperature of Au/rGO, Au70Pt30/rGO and Pt/rGO.

Figure 7 :

 7 Figure 7: ORR polarization curves recorded on a Pt/Vulcan catalyst at 5 mV s -1 with a rotating disk electrode working at various rotation rates in O2-saturated 0.1 M KOH.

Figure 8 :

 8 Figure 8: Polarization and power density curves of (a) Vulcan and (b) rGO supported catalysts as anodes and Pt/Vulcan as cathode in 0.1 M KOH. The concentration of glucose in the anodic compartment is 50 mM, and O2 is bubbled in the cathodic compartment. Electrode surface: 0.6 cm²; Metal loading: 0,19 mgmetal cm -2 .

Figure 9 :

 9 Figure 9: Polarization and power density curves of rGO supported catalysts as anodes and Pt/Vulcan as cathode in 0.1 M KOH. The concentration of glucose in the anodic compartment is 50 mM, and O2 is bubbled in the cathodic compartment. Electrode surface: 0.6 cm²; Metal loading: 0.19 mgmetal cm -2 .

Figure 10 :

 10 Figure 10: Evolution of GFC voltage at 1.5 mA cm -2 with Au/rGO and Pt/rGO as anodes and Pt/Vulcan as cathode, and at 1 mA cm -2 with Au70Pt30/rGO as anode and same cathode, in 0.1 M KOH + 50 mM glucose in anodic compartment and O2 bubbling in cathodic compartment.

Figure 11 :

 11 Figure 11: Ionic chromatograms of standards samples (a) gluconate 10 mM and (b) glucuronate 5 mM; and of 50 mM glucose alkaline solution oxidized in the fuel cell with Au70Pt30/rGO for 2 hours at 1 mA cm -2 .

Table 1 :

 1 Graphitization degree and size of the graphitic domains of Vulcan and rGO extracted from Raman spectra.

		Vulcan XC 72 R treated	rGO
	ID/IG	2.26	2.31
	La / nm	8.5	8.3

Table 2 :

 2 Calculated SECSA of the prepared electrode materials from CO-stripping measurements.

	Catalyst	Pt/Vulcan	Pt/rGO	Au70Pt30/Vulcan Au70Pt30/rGO
	SECSA / m² g -1 Pt 26	51	96	129
			Graphical Abstract

Acknowledgments

Authors acknowledge the financial support from the IMABIC project funded by the French National Research Agency (ANR), the European Union (ERDF) and "Région Nouvelle-Aquitaine".