Claudio ; Bianchini
email: claudio.bianchini@iccom.cnr.it

Pei Kang Shen

Francesco Vizza
email: francesco.vizza@iccom.cnr.it

Lianqin Q Wang

Valentina Bambagioni

Manuela Bevilacqua

Jonathan Filippi

Andrea Marchionni

Iccom Cnr

Sun Kang

Yat

Q Wang

Lianqin ; Sun

Ethylene Glycol Electrooxidation on Smooth and Nanostructured Pd Electrodes in Alkaline Media

Keywords: Ethylene glycol electro-oxidation, Electrocatalyst, Pd electrocatalysts, In situ spectroelectrochemical FTIR studies, Cyclic voltammetry Ethylene glycol electro-oxidation, Pd electrocatalysts, In situ spectroelectrochemical FTIR studies, Cyclic voltammetry

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The electrooxidation of ethylene glycol (EG) on different types of electrodes has been intensely studied over the last 25 years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16], largely due to the fact that this diol can be an effective fuel in direct fuel cells in both alkaline and acidic media [17][18][19][20][21]. The interest in EG as an electrochemical fuel has been recently enhanced, because it may be produced from abundant and renewable resources such as cellulose [22]. However, despite some promising achievements [17][18][19][20][21]23], direct ethylene glycol fuel cells (DEGFCs) are still a laboratory device that are not commercially available. This unavailability is largely due to a lack of active and stable anode catalysts, generally based on platinum and platinum alloys, and also to the scarcity of suitable electrolytes for the alkaline environment. Indeed, the alkaline environment offers a number of advantages over the acidic one for alcohol oxidation (see list below) [24,25] but traditional alkaline fuel cells (AFCs) with a liquid electrolyte, suffer some insurmountable problems for an effective application (for instance, membrane carbonation).

• Usability of both noble and non-noble metals to manufacture the electrocatalysts. • Improved kinetics at both cathode and anode, in particular low anodic over-voltages for alcohol oxidation.

• Alcohol cross-over from anode compartment to cathode compartment reduced by electroosmotic drag of hydrate hydroxyl ions.

• Easier water management as water is formed at the anode side where an aqueous solution already exists, while the electro-osmotic drag transports water away from the cathode preventing its flooding.

• Reduced risk of corrosion of the materials, including catalysts and carbons. • Reduced adsorption of spectator ions that might limit electrocatalysis.

The recent development of efficient and chemically stable anion-exchange membranes (AEMs) [27][28][29][30][31][32][33][34][35][36][37][38][39] can address the electrolyte issue in DEGFCs as well as contribute to design a new generation of alcohol oxidation electrocatalysts. For example, Pd may effectively replace Pt in alkaline media [25,[38][39][40] where also non-noble metals are sufficiently stable for electrochemical applications. Therefore, it is likely that Pd may be diluted with 2 non-noble metals to prepare smart catalytic architectures that are capable of functioning as anode electrodes for the rapid and stable oxidation of alcohols. This would decrease the cost of the membrane electrode assemblies (MEAs), thus favouring the commercial viability of DEGFC stacks.

Understanding the mechanism of EG electrooxidation on Pd-based electrocatalysts is therefore an important issue for the development of effective DEGFC anodes. Unlike Pt-based electrodes [1][2][3][4][7][8][9][10][11][12]14], a limited number of studies of the electrocatalysis of EG oxidation on Pd-based electrodes has been carried out so far [5,6,13,15,16,20] and no in situ experiment has been ever reported. Aimed at filling this gap, we report here a study of the electrooxidation of EG on different types of Pd electrodes. A smooth Pd metal electrode was used for an in situ FTIR electrospectroscopy analysis of EG oxidation at different pH values, while cyclic voltammetry (CV) studies have been performed using the nanostructured catalysts Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C. These electrocatalysts, obtained by a redox transmetalation process involving the spontaneous deposition of palladium onto Ni-Zn alloys supported on Vulcan XC-72 [40], have already shown remarkable activity for the oxidation of ethanol in both passive and active fuel cells [38,39]. For comparative purposes, we have investigated also the EG electrooxidation on a nanostructured Pd/C catalyst prepared by chemical reduction of H 2 PdCl 4 adsorbed onto Vulcan XC-72 [40].

Experimental

Materials and product analysis

All manipulations, except as stated otherwise, were routinely performed under argon or nitrogen atmosphere. Aqueous solutions were freshly prepared with doubly distilled-deionized water. The electrocatalysts Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C were prepared as previously described [40] (C = Vulcan XC-72). ICP-AES analysis (wt%) for Pd-(Ni-Zn)/C: Pd 6. The qualitative/quantitative analysis of the CV cell exhausts was obtained by 13 C{ 1 H} NMR spectroscopy using a Bruker Avance DRX-400 instrument with the chemical shifts relative to external TMS. The calibration curves for the quantitative analysis were obtained using authentic samples of the various products in the appropriate range of concentrations, using 1,4-dioxane as internal standard. Ionic chromatography (IC) was also used to identify and quantify the oxidation products (Metrohm 761 Compact instrument equipped with a Metrosep Organic Acids column).

Electrochemical measurements

Ink preparation for the CV study

A portion of each catalyst (about 45 mg) was introduced into a 5 mL high-density polyethylene container together with 1.01 g of water, 65 mg of KOH (99.99% Sigma-Aldrich), 0.50 g of absolute ethanol (99.8% Fluka) and 0.37 g of 5% Nafion ® ion-exchange resin in alcohol solution (Sigma-Aldrich). The resulting suspension was sonicated for 30 min with a Branson 3200 bath. Each suspension was freshly prepared just before carrying out the experiment scheduled. The metal loading on each glassy carbon electrode (vide infra) was determined by weighting the amount of ink deposited on the glassy carbon disk.

Apparatus for cyclic voltammetry studies

The cell used for the cyclic voltammetry (CV) experiments was a Kelef cylinder with a 7.2 mm inner diameter and a 50 mm outer diameter [40]. The inner volume of the cell was about 1 mL. The working electrode, Glassy Carbon (Sigradur  G) (0.867 cm 2 ), covered by the catalyst, was housed in a cavity at the top end of the cylinder, and the counter electrode was a gold disc placed at the bottom end. The solution contained in a Pyrex flask was previously purged by bubbling N 2 and then flushed into the cell by a pressure as low as 0.3 bar of N 2 . The miniaturized reference electrode, Ag/AgCl/KCl sat , was placed on the outlet tubing. This design allows one to avoid contamination and at the same time is sufficiently close to the working electrode to reduce the uncompensated resistance. All CV studies were carried out using a Parstat 2277 potentiostat-galvanostat (Princeton Applied Research).

In situ FTIR spectroelectrochemical study

A cell equipped with a CaF 2 window was used for the electrochemical and in situ FTIR measurements. The working electrode was a palladium electrode with a 6 mm diameter. A platinum foil and an Hg/HgO electrode were used as the counter and reference electrode, respectively. The electrochemical measurements were performed on a 263A potentialstat/galvanostrat (EG&G-PARC, USA), connected to the FTIR spectrometer 3 (Nicolet 5700 with DTGS detector, Thermo Electron Corporation, USA). The measurements were carried out at 25 °C. The EG concentration was fixed at 1 M. Electrolytes with different NaOH concentrations were used to investigate the pH effect on the EG electrooxidation. When necessary, NaClO 4 was added to the NaOH solutions to ensure a constant ionic strength. All solutions were purged with nitrogen prior to use. Prior to each experiment, the palladium electrode was polished with alumina powder, followed by a supersonic rinsing in pure water and a potential cycling in 0.1 M HClO 4 at 0.1 V s -1 between 0.242 V and 1.2 V vs. SCE with a reductive final potential. Afterwards, the palladium electrode was immersed into the electrochemical cell. During the in situ FTIR spectroelectrochemical experiments, the electrode was pressed against the CaF 2 window to form a thin layer solution. Each infrared spectrum was recorded from 128 interferograms at the resolution of 4 cm -1 . The reference spectrum (R ref ) was collected at 0 V vs. RHE. The electrode potentials were increased by 0.1 V intervals from the reference potential up to 1.2 V. The equilibrium at each potential was achieved in 5 s before the collection of the sample spectra (R s ). Each final spectrum was reported using Eq. ( 1). Accordingly, the negative bands represent the species produced and the positive bands the species consumed.

s ref ref R R R R R - ∆ = (1)
All the potentials in the figures were quoted with respect to the reversible hydrogen electrode (RHE).

Results and Discussion

In situ FTIR study

The influence of the pH on the EG electrooxidation on a smooth Pd electrode was investigated by CV using 1 M EG solutions with different NaOH concentrations but comparable ionic strengths (Figure 1). From a perusal of this figure one may readily realize that the catalytic activity of the Pd electrode decreases by decreasing the pH, until it almost disappears at pH 12. No activity was detected in neutral or acidic media. This finding has been already observed for the oxidation of ethanol on Pd electrodes and rationalized both experimentally and theoretically [41,42]. In particular, DFT calculations suggest that the dehydrogenation of ethanol to acetaldehyde requires a high OH ads coverage of the Pd surface [42].

Figure 1 here FTIR spectroscopy was used to gain insight into the mechanism of EG electrooxidation on Pd at different pH values (Figure 2). At NaOH concentrations ≥ 1 M, the stretching bands of the glycolate ion at 1580 cm -1 (C=O) and 1070 cm -1 (C-O) were clearly observed together with lower-intensity bands at 1410 and 1310 cm -1 assignable to carbonate and oxalate, respectively (Figures 2c-d) [4]. The stretch of the oxalate group expected at 1600 cm -1 is likely masked by the band of glycolate. Consistent with the CV study, increasing the pH decreases the onset potential of EG oxidation. At NaOH concentrations ≤ 1 M, the characteristic band of CO 2 at 2343 cm -1 was observed already at 0.7 V [3,4]. The detection of CO 2 suggests that the pH drops in the thin layer due to the consumption of OH -groups (vide infra). No band due to CO was detected at any pH or potential. In this respect, it is worth recalling that the IR detection of CO ads is particularly difficult in an alkaline electrolyte [43], especially at the very low concentrations expected in the present case. The occurrence of C-C bond breaking of an alcohol with formation of CO 2 at relatively low pH values is in line with previous reports according to which a high OH coverage of the Pd surface is essential for releasing carboxylate products by Pd-acyl/OH coupling [38][39][40][41].

Figure 3 shows the potential dependence of the integrated intensities of the glycolate C=O (A) and C-O (B) bands at 1580 and 1070 cm -1 , respectively, in the thin layer solution. The onset potential of EG oxidation apparently decreases by increasing the NaOH concentration and the intensity of either band reaches a maximum at ca. 0.9 V. Above this potential, however, the intensities of these two bands diverge, which can be attributed to an increase of the oxalate band at 1600 cm -1 enhancing the intensity of the glycolate band (see also the increase in intensity of the band at 1310 cm -1 ). formation at the expense of glycolate above 0.9 V. What really distinguishes Pd from Pt is the higher electrochemical activity of the former metal in alkaline media.

The lack of reports in the relevant literature dealing with in situ FTIR studies of EG electrooxidation on Pd does not allow one to make a direct comparison with the present results. However, it is worthwhile to discuss some studies of EG oxidation on Pt electrodes in alkaline media. In view of EMIR spectroscopy, Kardigan et al. have concluded that the adsorption of EG at a Pt electrode is dissociative at any pH with formation of CO ads [2], which contrasts with the behavior of Pd-based catalysts (vide infra). Besides confirming the presence of linearly bonded CO at high pH values, a SNIFTIR study by Christensen and Hamnett has shown the formation of glycolate, oxalate and carbonate upon oxidation of EG on Pt in alkaline media [4]. Based on the potential window at which these products were detected as well as the EG concentration, it was suggested that glycolate and carbonate are produced from the same intermediate (e. g. Pt-CH(OH)CH 2 OH), while oxalate is obtained by oxidation of desorbed glycolate. Likewise, a following SPAIR study of EG electrooxidation on Pt/C, PtPd/C, PtBi/C and PtBiPd/C electrodes has provided evidence that oxalate is not a primary product of EG oxidation but it comes from adsorption and oxidation of glycolate [20]. Traces of glyoxylic acid and formic acid were also detected by HPLC. Coutanceau et al. have found that the addition of Pd to Pt-base catalysts reduces the amount of strongly adsorbed carbonyl species at Pt, leading to higher current densities by virtue of the low propensity of Pd to cleave C-C bonds [20]. It was also proposed that Bi exerts a beneficial effect on the onset potential of EG electrooxidation as well as on the catalyst stability by virtue of the greater propensity of Bi to adsorb and activate hydroxyl groups as compared to Pd.

CV studies

The electrochemical activity of the nanostructured catalysts Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C for EG oxidation was investigated by CV at room temperature. A preliminary study was carried out to find suitable conditions in terms of catalyst loading on the electrode and of KOH concentration. For all catalysts, Pd loadings varying from 24 to 28 µg cm -2 , corresponding to 18-20 mg of ink, gave the highest current density values in 2 M KOH at a scan rate of 50 mV s -1. A series of cyclic voltammograms, recorded at KOH concentrations spanning from 0.01 to 4 M, showed the generation of the highest current densities in the range 1.5-2 M KOH. The following electrochemical studies in half cells were carried out using a 2 M KOH solution, a proper condition to keep a relatively high OH -concentration during the CV and chronoptentiometric experiments. Indeed, the oxidation of EG in alkaline media consumes OH -groups, irrespective of the oxidation level of the substrate (glycolate, oxalate or carbonate, Eqs. ( 1

The EG concentration was fixed at 5 wt% as this value has been found appropriate for DEGFCs with AEMs [44]. On the other hand, a previous CV study of EG electrooxidation on a smooth Pd electrode has demonstrated that the current densities are practically independent on the EG concentration but proportional to the OH - concentration with a reaction order of ca. 1 [5].

In conclusion, all the CV measurements reported below were performed at a sweep rate of 50 mV s -1 in deoxygenated 5 wt% EG, 2 M KOH solutions and electrodes containing a Pd loading between 24 and 28 µg cm - 2 .

Figure 4 shows the cyclic voltammograms of the EG oxidation reaction on Pd/C, Pd-(Ni-Zn-P)/C and Pd-(Ni-Zn)/C electrodes.

Figure 4 here Relevant electrochemical parameters such as the peak current density (J p ), the specific peak current density (Sa p ), the forward anodic peak potential (V p ), the onset oxidation potential (V onset ) and the Tafel slopes, together with the electronic transfer coefficient α, are given in Table 1. The three catalysts show high activity for the EG oxidation reaction with onset potentials at ca. 0.4 V vs. RHE and specific current densities higher than 3 mA µgPd -1 for Pd-(Ni-Zn)/C and Pd/C. Accordingly, Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C can be classified amongst the best performing electrocatalysts ever reported for EG oxidation. As expected, the oxidation of EG starts at the potential corresponding to the coverage of the electrode surface by OH groups [5,15,16,38,40]. A specific CV study of the Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C electrodes in 2 M KOH solution without EG has been previously reported [40] and the corresponding CVs are also shown in Figure 7.

As shown by the traces in Figure 4, the three electrocatalysts are characterized by rather similar activities with Pd-(Ni-Zn)/C being slightly more active than Pd/C, especially in terms of forward and backward anodic Since we observed for all catalysts a slow but constant decrease of the peak back current with increasing the anodic limit (from 0.9 to 1.6 V), the backward scan peak can be associated to the oxidation of freshly chemisorbed EG. In fact, previous CV studies of ethanol oxidation on Pd have shown that the decrease in current at high voltages is mainly due to the formation of a PdO layer, which blocks the adsorption of the substrate [41]. The electrocatalytic activity is restored in the negative-going sweep once the potential reaches the value at which PdO is reduced to Pd. On the other hand, one cannot rule out that a contribution to the current decrease may be due to mass transfer phenomena as well as the instantaneous shortage of OH -groups in the thin layer. In fact, adsorbed OH species are required to release the carboxylic acid upon coupling with Pd-acyl species as shown in Scheme 1 [39]. Previous studies of ethanol oxidation on Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C [40] have unambiguously shown that the Ni-based supports, though able to oxidise ethanol in alkaline media, are not directly involved in the ethanol oxidation reaction occurring at the largely negative potentials of the Pdcontaining electrocatalysts. It has been proposed that Ni increases the amount of OH ads groups on the catalyst surface, which would favour the formation of the carboxylic acid by coupling with acyl ads species [25,40] Table 1 here Scheme 1 here Tafel plots for the EG oxidation reaction on the Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C electrodes were obtained at a scan rate of 5 mV s -1 in the potential interval from 0.3 to 0.5 V (2 M KOH, 5 wt% EG). The values for the Tafel slopes and of the α coefficients (Table 1) are comparable with each other as well as with those reported for nanostructured Pd electrocatalysts supported on the same carbon black, also promoted by metal oxides [13,15,38,40]. These values are higher than those reported for a smooth Pd electrode (120 mV dec -1 ) [5], most likely due to the porous high surface area of the present electrodes [38,15]. The Tafel slopes indicate a similar reaction mechanism for the three electrocatalysts, while the α parameter of ca. 0.3 is consistent with an electrochemical rate limiting step for the EG oxidation reaction.

For all electrodes, the anodic peak current density for EG oxidation was plotted against the square-root of the scan rate (Figure 5). A linear relationship, typical of an electrochemical reaction under diffusion control, was found for the Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C electrodes. In contrast, a slightly parabolic curve was observed for the Pd/C electrode, which suggests a reaction controlled by the activation polarization. As previously reported for the oxidation of ethanol on a similar Pd/C electrode [40], this effect can be related to the very low number of catalytically active sites on the electrode surface due to the low Pd dispersion: just by increasing the metal loading is the reaction controlled by substrate diffusion [40].

The performance stability of the three electrocatalysts for EG oxidation was investigated by chronopotentiometry. Steady state measurements, carried out with a constant current density polarization of 3.46 mA cm -2 showed a negligible potential oscillation for 2 hours, indicative of no strongly adsorbed species on the catalyst surface. Notably, the selective formation of glycolate with trace amounts of oxalate and carbonate (Eq. ( 1)) was determined by ionic chromatography/ 13 C NMR analysis of the chronopotentiometric exhausts produced by Pd/C as electrocatalyst. Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C gave a different product distribution. On both electrocatalysts EG was prevalently converted to glycolate (57-63%) but appreciable amounts of oxalate (35-30%) and carbonate (8-7%) were also formed, revealing a different chemoselectivity of EG oxidation of the nanostructured catalysts promoted by the Ni-Zn and Ni-Zn-P alloys as compared to plain Pd/C. In no case were glyoxaldehyde or its hemiacetal detected by NMR spectroscopy on the cell exhausts.

The different product distribution provided by Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C as compared to Pd/C seems to be inconsistent with a similar reaction mechanism for the three catalysts, as suggested by the Tafel slopes. A clear-cut explanation of this phenomenon is not available at this stage: the rate-determining step seems to be electrochemical in nature but it has not been identified among the several oxidation steps occurring for EG oxidation on the electrode surfaces. On the other hand, a precise quantification of the oxidation products may be inaccurate due to the little EG conversion in the chronopotentiometric experiments.

Figure 5 here Overall, the electrooxidation of EG on Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C resembles that of ethanol oxidation on the same electrodes that selectively yields potassium acetate [39,40]. Unlike ethanol, however, EG contains two hydroxyl groups so that in alkaline media at least three carboxylate products (glycolate, glyoxylate and oxalate) plus formate and carbonate may be obtained upon electrooxidation (Scheme 2). This scheme illustrates the overall electrochemical oxidation of EG in alkaline media as inferred from a perusal of the literature [3,4,7,9,11,12,14,20] as well as the present results. The scheme includes all the species that have been either isolated (boxes) or detected in situ (circles). In an attempt to gain insight into the EG electroxidation on the present nanostructured Pd catalysts, independent CV experiments have been carried out using solutions of potassium glycolate, potassium glyoxylate and potassium oxalate as anolytes.

The cyclic voltammograms of glycolic acid and glyoxylic acid oxidation on Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C or Pd/C in 2 M KOH are shown in Figure 6.

Figure 6 here

The glycolate ion is much more difficult to oxidize on Pd than EG as shown by the corresponding Sa p values which are one order of magnitude lower than those observed for EG (Figure 6A). Slightly higher current densities are produced by glyoxylate (Figure 6B) which can be an intermediate along the conversion of EG and glycolate to oxalate (Scheme 2). The aldehyde group is oxidized on Pd at a more negative potential than the alcohol group as shown also by the shoulder at ca. 0.85 V in Fig. 6 (right) corresponding to the oxidation of the glycolate formed upon disproportionation of the aldehyde in alkaline media. Notwithstanding the low current density observed, there is little doubt that glycolate can be oxidized to oxalate on Pd. On the other hand, just the very low current densities provided by glycolate oxidation on Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C and the fact that both electrocatalysts yield relatively large amounts of oxalate and carbonate indicate that the oxidation of EG on these catalysts proceeds by parallel routes with path c prevailing over paths a-b for oxalate production (Scheme 3).

Scheme 3 here

In an attempt to understand whether oxalate is oxidized to CO 2 on the present nanostructured Pd-based electrocatalysts, a CV study has been carried out on an alkaline solution of potassium oxalate. The corresponding cyclic voltammograms are given in Figure 7 that also shows those recorded on KOH solutions with no substrate. Upon addition of oxalic acid to the latter solutions, a significant decrease of the peak associated to PdO reduction in the reverse scan at ca. 0.65 V is observed together with an oxidation peak at 0.47 V featured by a very low specific current. Accordingly, one may conclude that the carbonate formed upon electrooxidation of EG on Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C is prevalently obtained from glycolate with a very minor contribution from oxalate. The electrooxidation of EG on Pd is dramatically influenced by the pH. Below pH 12, CO 2 is formed and detected in the thin layer by FTIR, while at higher pH values glycolate, carbonate and oxalate are formed almost simultaneously at a potential of ca. 0.4 V vs. RHE. Above 0.9 V glycolate is oxidized to oxalate and carbonate.

The electrodes with the nanostructured catalysts Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C are much more active than the smooth Pd electrode and give different distributions of the oxidation products. Pd/C is the most selective electrocatalyst yielding glycolate, while mixtures of glycolate (major > 60%), oxalate and carbonate are obtained with Pd-(Ni-Zn)/C or Pd-(Ni-Zn-P)/C. Carbonate is produced by oxidation of both glycolate (major contribution) and oxalate, while the major part of oxalate seems to be produced by the direct oxidation of EG (Scheme 3, route c). The Ni-Zn and Ni-Zn-P phases supporting the Pd clusters might favour the chelating adsorption of EG, leading to its direct oxidation to oxalate, by virtue of the oxophylic nature of both Ni and Zn [40].

In light of the results obtained in this work, current studies in our laboratories are aimed at developing MEAs with Pd-based anode electrodes and anion-exchange membranes for both passive and active DEGFCs.

F

Figure 2 here

 2 Figure 2 here

  Scheme 2 here

Figure 7

 7 Figure 7 here

Figure 1 . 1 Figure 2 .

 112 Figure 1. Cyclic voltammograms on a Pd electrode of the following solutions: (a) 0.01M NaOH + 0.99 M NaClO 4 + 1 M EG; (b) 0.1 M NaOH + 0.9 M NaClO 4 + 1 M EG; (c) 1 M NaOH + 1 M EG; (d) 2 M NaOH + 1 M EG. Scan rate: 50 mV s -1 Figure 2. In situ FTIR spectra obtained under potential step polarization of the following solutions: (a) 0.01 M NaOH + 0.99 M NaClO 4 + 1 M EG; (b) 0.1 M NaOH + 0.9 M NaClO 4 + 1 M EG; (c) 1 M NaOH + 1 M EG; (d) 2 M NaOH + 1 M EG. Scan number: 128.

Figure 3 .

 3 Figure 3. (A) Intensity of the band at 1580 cm -1 as a function of the polarization potential. (a) 0.01 M NaOH + 0.99 M NaClO 4 + 1 M EG; (b) 0.1 M NaOH + 0.9 M NaClO 4 + 1 M EG; (c) 1 M NaOH + 1 M EG; (d) 2 M NaOH + 1 M EG. (B) Intensity of the band at 1070 cm -1 as a function of the polarization potential. (a) 0.01 M NaOH + 0.99 M NaClO 4 + 1 M EG; (b) 0.1 M NaOH + 0.9 M NaClO 4 + 1 M EG; (c) 1 M NaOH + 1 M EG; (d) 2 M NaOH + 1 M EG.

Figure 4 .

 4 Figure 4. Cyclic voltammograms (at the tenth cycle) of EG oxidation on Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C electrodes in 2 M KOH and 5 wt% EG. Scan rate: 50 mV s -1 . Scheme 1. Proposed mechanism for the electrooxidation of EG to glycolate acid on Pd in alkaline media.

Figure 5 .

 5 Figure 5. Plots of the anodic peak specific current density against the square-root of the scan rate for the oxidation of EG (5 wt%) on Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C in 2 M KOH.Scheme 2. Overall reaction scheme of EG oxidation on metal electrocatalysyts in alkaline media. Products in boxes have been isolated; products in circles have been detected spectroscopically.

Figure 6 .Scheme 3 .

 63 Figure 6. Cyclic voltammograms of (A) glycolic acid (2 wt%) and (B) glyoxylic acid (2 wt%) oxidation on Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C in 2 M KOH. Scan rate: 50 mV s -1 . Scheme 3. Direct and sequential routes for the electrooxidation of EG to oxalic acid.

Figure 7 .Figure 1 .Figure 2 .Figure 2 .Figure 2 .Figure 2 .Figure 3 .Figure 3 .Figure 4 .Scheme 1 .Figure 5 .Scheme 2 .Figure 6 .Figure 6 .Scheme 3 .Figure 7 .

 7122223341526637 Figure 7. Cyclic voltammograns of Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C in 2 M KOH or in 2 M KOH +2 wt% oxalic acid. Scan rate: 50 mV s -1 .

  4, Ni 9.7, Zn 2.6; for Pd-(Ni-Zn-P)/C: Pd 4.73, Ni 7.45, Zn 0.43; for Pd/C: Pd 5.2.

Table 1 .

 1 Selected electrochemical parameters of EG oxidation on Pd-(Ni-Zn)/C, Pd-(Ni-Zn-P)/C and Pd/C electrodes in 2 M KOH.

	o r
	P
	e
	e r
	R
	e
	v i e
	w

Wiley-VCH Fuel Cells

Acknowledgments

We acknowledge the financial support from the European Commission (Network of Excellence IDECAT, contract n. NMP3-CT-2005-011730), Regione Toscana (Progetto CESARE) and the MIUR (Italy) for the PRIN 2007 project 200775CREC-004. Thanks are also due to the Guangdong Sci. & Tech. Key Projects (2007A010700001, 2007B090400032) and Guangzhou Sci. & Tech. Key Projects (2007Z1-D0051  SKT[2007]17-11) for financial support.


Jean-Paul Viricelle
email: viricelle@emse.fr

Sorina Udroiu

Geoffroy Gadacz

Michele Pijolat

Christophe Pijolat

J-P Viricelle

Development of Single Chamber Solid Oxide Fuel Cells (SCFC)

Keywords: Cathode, Conductivity, Fuel Cell, Low Temperature SOFCs, Screen Printing, Partial Oxidation, Performance Improvement BSCF, Fuel cell, GDC, LSM, Single chamber, SOFC, Solid oxide fuel cell, SSC Tabs: 141.75 pt, Left + 481.95 pt, Right

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Solid Oxide Fuel cells have been studied intensively over the past two decades. Main efforts deal with the reduction of working temperature from 1000°C to around 600°C in order to reduce the cost due to the requirement of robust materials with adequate electrical and electrochemical properties, thermal stability, and due to the technological problems, especially sealing between anodic and cathodic chambers. More recently, a new type of device named "Single Chamber Solid Oxide Fuel Cells" (SCFC) has been proposed. In such device, contrary to conventional SOFC, anode and cathode are exposed to a gas mixture of fuel (hydrocarbon) and oxidant (air) so that no more sealing with electrolyte is necessary. Their operating principle is based on the different catalytic activities of anode and cathode: ideally, the anode has to be active for the oxidation of fuel while the cathode should only present a strong electro-activity for oxygen reduction. Most significant and initial developments of SCFC devices were performed by T. Hibino and co-workers who proposed a review on SCFC in [1]. In the first developments of SCFC devices [2][3][4], materials used in conventional SOFC were used, namely YSZ for electrolyte, Ni-YSZ cermet anode and LSM (La 0.8 Sr 0.2 MnO 3 ) cathode. As for conventional SOFC, such materials are now more and more replaced in order to significantly reduce the operating temperature while increasing or, at least maintaining performances. YSZ electrolyte is substituted by doped ceria oxide either with Gadolinium (GDC) or samarium (SDC). Most of the anodes are constituted by a cermet of previous electrolyte with nickel. For cathode, the trend is to use doped perovskites with cobalt, either Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF, [5]) or Sm 0.5 Sr 0.5 CoO 3 (SSC, [6][7][8]), or La 0.5 Sr 0.5 CoO 3 (LSC, [9]). With such SCFC devices, depending on the cell microstructure, geometry and flow conditions, maximum power densities of 700mW/cm 2 at 600°C have been reached [7]. However, such high values are not so easily obtained: even for fixed materials, all experimental details such as flow geometry [8] or the way of the SCFC initialization [10] for example, have significant influence on the performances. The aim of this study was to investigate the influence of experimental parameters, especially flow composition, in our testing bench conditions. More comprehensive studies on SCFC working principle were reported by Hibino [1] and Haile [7]. In the present paper, we present our recent results concerning SCFC developments, with GDC electrolyte, Ni-GDC anode and LSM or SSC or BSCF as cathode. Materials characterizations and SCFC evaluations were performed under propane / oxygen mixtures. Among possible hydrocarbons, propane was chosen because it can be cracked at lower temperature than methane, and it can also produce more hydrogen by partial oxidation due to its higher carbon number (Eq. ( 1)). 2 Experimental

Materials, SCFC preparation

The electrolyte material was made using a starting commercial powder (®NexTech Materials Ltd) of gadolinium doped ceria (Ce 0.9 Gd 0.1 O 1.95 named GDC), having a specific area around 9 m 2 /g. The particle size is in the range 0.1-10µm with a mean size (d 50 ) around 0.5µm. The powder was pressed into pellets (100MPa, 2min) and sintered at 1350°C for 4 hours with an intermediate step at 1200°C during 6 hours. Temperature rates were 10°C/min for both heating and cooling. Final pellets have a diameter of 19mm and a thickness of 0.8 mm. Their relative density is around 92%. Anode cermet was prepared from a mixture of 60%wt NiO (Sigma Aldrich) and 40%wt GDC. The initial NiO powder having a specific area around 3-4 m 2 /g was strongly agglomerated and was ball-milled in ethanol. The solvent was then evaporated while stirring and the resulting powder was passed through a 100µm screen sieve. The NiO powder particles size was then comparable to the one of GDC powder. The two powders were then mixed and ground in a mortar and pestle. Then, commercial binder and solvent were added to obtain an ink for screen-printing. The ink was homogenised with an ®Ultra-Turrax dispersion tool.

The anode layer of 6*6 mm 2 was then deposited by screen-printing technology on the previously described GDC pellet. To study the influence of electrode thickness, the number of deposited layer was varied, with an intermediate drying step at 130°C during 10 minutes between each deposit. The deposited films were fired at 1200°C for 2h, resulting in a porous anode layer with a thickness of either around 10µm (1 layer), 20µm (2 layer) or 40µm (4 layers). In order to activate the anode, reduction of nickel oxide was performed by thermal treatment in hydrogen. This part is detailed in paragraph 3.1. Cathode was prepared using a similar process as for the anode. The powders were passed though a 100 µm screen sieve, and then mixed with a binder and a solvent to obtain an ink. Cathodes layers were also deposited using screen-printing. Three materials were used: La 0.8 Sr 0.2 MnO 3 (named LSM) and Sm 0.5 Sr 0.5 CoO 3 (named SSC), commercial powders from ®NexTech Materials Ltd, and an in-house powder supplied by IRCELYON laboratory (France), Ba 0.5 Sr 0.5 Co 0.2 Fe 0.8 O 3 (named BSCF). Considering their thermal stability, these cathodes were fired at 1200°C, 1000°C and 950°C for LSM, BSCF and SSC respectively. Anode and cathode were successively deposited and fired as reported previously, on the same side of the electrolyte pellet. The space between the two square electrodes is around 1mm. For SCFC performances tests, a gold current collector was deposited on the cathode. This collectors consist in a grid (line space of 0.5mm) deposited by screen-printing, using a commercial gold paste. It was fired after deposition, at 900°C during 20 minutes. A SCFC prototype can be seen in Figure 1.

Material characterization, fuel cell performance test

Prior to fuel cell performances tests, materials were characterized in regards of their electrical conductivity, catalytic activity and thermal stability. Electrical conductivity of anode and cathode was measured according to Van Der Pauw method [11]. Particular samples supporting only the anode or the cathode on the GDC pellet were prepared, using the same elaboration process and parameters as described in previous part. Four thin platinum wires were stuck to the border of the studied layer with a platinum paste. These wires were connected to conventional electrical equipment to monitor current and voltage according to Van Der Pauw methodology. For cathode materials, measurements were conducted in air in the range 20-600°C. For anode, to avoid nickel reoxidation, air-propane mixture was set in the bench after the reduction process made in hydrogen. Catalytic activity of cathodes materials was investigated in a fixed bed reactor, towards propane conversion. The same quantity of cathode material in the form of powder dispersed in alumina was introduced to have comparative results. Propane conversion was measured thanks to a mass spectrometer coupled with a micro gas chromatograph (µGC-MS) placed at the bench outlet. Effectively, as the SCFC principle is based on the difference of catalytic activity of anode and cathode, cathode materials have to be the less active as possible with the hydrocarbon; they should only present a strong electrochemical activity towards oxygen reduction. Anode and cathode thermal stability was investigated in the SCFC running conditions, after exposure to airpropane mixtures, by X-Ray diffraction. For cathodes, same characterizations were performed after the hydrogen treatment of the anode in order to control if they can withstand it.

Deleted: In fact, generally speaking, propane can be cracked more easily than methane, and can also produce more hydrogen by partial oxidation (Eq. ( 1 SCFC performances were measured in a simple test bench consisting in a tubular furnace with a quartz tube. Electrical contacts are achieved thanks to gold wires in mechanical contact with gold collector on the cathode, and directly on the anode. In-situ activation of the anode was performed under a gas flow of 2% hydrogen balanced with argon. Then, after purging the system with nitrogen, the propane-air mixture was introduced. Three parameters were mainly investigated: total gas flow, C 3 H 8 /O 2 ratio and the temperature. The gas flow was varied in the range 15 l/h to 50l/h: it is constituted of air-propane mixture and, for safety reasons, of an additional nitrogen flow in order to decrease the oxygen content to 12% in the bench. For example, total gas flow of 45l/h was composed of 30l/h of air-propane mixture and 15l/h of nitrogen. Effectively, in air, the explosive domain of air-propane mixtures is in the range 2.2-10% of propane. The decrease of oxygen content reduces this area which disappears for oxygen content lower than 12%. Another important parameter is air-propane ratio which was investigated in the range 10-16% vol. in propane, corresponding respectively to C 3 H 8 /O 2 ratio in the range 0.53-0.95. It must be noted that the corresponding ratio for partial oxidation of propane (Eq. 1) is 0.66. SCFC were tested in the temperature range 580-650°C. These temperatures correspond to the regulated value of the furnace; depending on gas conditions, strong exothermic effect (overshoot of around 50°C) can occur during propane oxidation. If not specified, the reported temperatures are the furnace temperatures. A gas chromatograph and an infrared gas analyser performed gas analysis at the outlet of the test bench. The polarization curves of the SCFC devices were simultaneously measured in order to correlate performances with experimental conditions.

3 Results

Anode characterization

Anode reduction

The as-prepared NiO-CGO cermet anode has to be reduced before use in SCFC device. As in-situ reduction in air-propane mixture could not be easily achieved, a pre-treatment in H 2 (2% balanced in Ar) was conducted. In order to define the adequate temperature range, thermogravimetric experiments were conducted with NiO and GDC powders. These powders were previously fired in air at 1200°C during two hours, as done for the screen-printed layers. Results reported in Figure 2 show that NiO begins to reduce at around 400°C, with a total reaction reached around 800°C corresponding to the theoretical weight loss of 21.4%. For GDC, a significant weight loss attributed to partial reduction of cerium IV in cerium III is observed from 600°C.

Taking into account previous results, various reduction conditions of the anode in the same gas have been tested in the temperature range 400-600°C, with varying duration from 30 minutes to few hours. Optimum condition to obtain a total reduction of NiO in our test bench, as shown by X-ray diffraction analysis (Figure 3), was found for a treatment of 600°C during 30 minutes.

Anode conductivity

In order to check if the Ni-GDC anode has a satisfying electronic conductivity, conductivity measurements were performed as described in part 2.2. Results can be seen in Figure 4. Immediately after reduction at 600°C, the anode is stabilised under a nitrogen flow and the temperature is decreased to 520°C. During this period (domain (a) in figure 4), the conductivity is quite high around 900 S.cm -1 . However, as soon as nitrogen is replaced by the flow of air-propane (C 3 H 8 /O 2 ratio equal to 0.66), at 520°C, the conductivity begins to decrease (domain b). The temperature is then raised to 535°C, and step by step up to 570°C. At 535°C, the conductivity comes back to its initial value and keeps a nearly constant and high value in the range 535-570°C (domain c). Same experiment (not shown in figure 4) was done with decreasing temperature and a similar behaviour was observed. At temperature lower than 530°C, the conductivity begins to collapse. After cooling back to room temperature, a green colour of the anode was observed at the surface, indicating a re-oxidation of nickel. These results validate the preparation process of the anode, but indicate that, for SCFC operating in our experimental conditions, temperature must be kept higher than 520°C to avoid nickel re-oxidation. A thermodynamic study was performed to check these experimental observations. A good agreement was obtained: calculations of the free Gibbs energy indicate that, among possible reactions of nickel and nickel oxide in the studied system, the predominant reaction at temperature lower than 535°C is the re-oxidation of nickel (Eq. ( 2)). At higher temperature, reduction of nickel oxide by propane is favoured (Eq. ( 3)).

Ni + 1/2O 5 point out that SSC is the more conducted cathode with conductivity around 650 S.cm -1 at 600°C. The value for LSM and BSCF are respectively around 130 and 10 S.cm -1 at 600°C. The difference of electrical conductivity and especially the low value measured for BSCF cathode may be explained by the morphology of the layers: SEM observations (Figure 6) clearly point out that BSCF layer is quite more porous than SSC and LSM layers.

Cathode catalytic activity

As reported previously, the efficiency of a cathode in a SCFC device is based on its low catalytic activity in regards of hydrocarbon conversion. Figure 7 represents propane conversion measured by the µGC-MS equipment at the outlet of the catalytic test bench versus temperature, for the three studied cathode material.

Gas composition was set at a C 3 H 8 /O 2 ratio equal to 0.66, thus corresponding to stoechiometric conditions of partial oxidation (Eq. ( 1)).Results show a strong catalytic activity of LSM and SSC material in the range 450-600°C, quite higher than the one of BSCF. For this material, propane conversion increase slowly versus temperature but remains lower than 20% at 600°C, whereas it reaches respectively around 50% and 80% for SSC and LSM. Hence, considering catalytic activity criteria, BSCF is the more adequate cathode material for SCFC device.

Cathode thermal stability

Although interesting performances have been obtained with the studied cathode materials used in SCFC, the problem of SSC and BSCF stability has been reported previously [9][10]. Hence, it is necessary to investigate the thermal stability of the three cathode materials both in hydrogen (conditions of anode reduction) and airpropane (SCFC test conditions) gas flow.

After a thermal treatment similar to the anode reduction one, 600°C during 30 minutes in H 2 (2% balanced in Ar), SSC and BSCF materials are degraded (Figure 8 for SSC). On the contrary, LSM material appears not affected, according to XRD analysis. Cathodes were also exposed to air-propane mixture at 630°C during 3 hours, without the reduction pre-treatment in H 2 . Effectively, other processes have been proposed in the literature [10] to avoid anode initialization by hydrogen pre-treatment, in order to prevent cathode degradation. However, in our conditions, SSC and BSCF materials still present degradation, even if it appears less drastic than in hydrogen. LSM did not present any degradation in the same conditions. In our actual preparation process of the SCFC device, both anode and cathode are deposited on the GDC support before the anode reduction treatment. Moreover, long term stability of BSCF and SCC materials is questionable in air-propane mixtures, at least in our experimental conditions. Hence, for the present study, it was only possible to use LSM material as a cathode, although we have confirmed that it is not the more adequate one, in regards of a too high catalytic activity for propane conversion. 9.

(Ni

After the in-situ reduction of the anode at 600°C, the sample temperature is set at 580°C and the air-propane mixture is introduced. At this temperature, the OCV is quite low, in the range 100-200mV. Then, the temperature is increased to find the optimum value corresponding to the highest OCV. Figure 9 shows that the OCV increases sharply from around 620°C with also increasing exothermic effect, but the OCV presents some strong oscillations. It tends to stabilize around 500mV at 700°C ( sample temperature; furnace is at around 650°C). If the temperature is more increased, a sudden reduction of OCV value is observed, which is reversible with temperature decrease. Hence, for this sample (20µm thick anode) and in the present experimental conditions, the optimum sample temperature is 700°C. In a previous paper [12], we have shown that the optimal conditions are strongly dependant on experimental parameters. Voltage-current characteristic measured at these optimum conditions for this sample is reported in Figure 10. The effective area considered for calculation of current and power densities, taking into account our planar geometry (Figure 1), is the section of the electrolyte between the two electrodes, equal to 4.8mm 2 (6*0.8mm 2 ). A maximum power density of around 10 mW/cm 2 is obtained. Characterisations of anode and cathode materials after the SCFC evaluation with various conditions were carried out by X-ray diffraction.

For LSM, no significant modification could be detected. For Ni-CGO anode, results are more various depending on the history of the SCFC device. A partial nickel re-oxidation is sometimes observed, but it is not clear if it occurs during the fuel cell test, or during cooling down to ambient after test, which would be in agreement with observations made during anode conductivity characterisation (part 3.1.2). In other conditions, especially higher C 3 H 8 /O 2 ratio (0.95), carbon deposition is observed on the anode.

Influence of experimental parameters on outlet gas composition

The influence of C 3 H 8 /O 2 ratio and of total gas flow was investigated in more detail thanks to gas analysis at the bench outlet. Thick anode sample (40µm) were used is this part of the study. For this sample, the optimum SCFC temperature was shifted to 620°C in conditions similar to previous ones (C 3 H 8 /O 2 ratio equal to 0.55 and a total flow of 45l/h). Hence, this sample temperature was fixed for following tests. Influence of C 3 H 8 /O 2 ratio with a fixed total flow of 45l/h is shown in Figure 11. The concentration (vol. %) of main constituents detected by the gas chromatograph are reported. Taking into account that the desired reaction is partial oxidation for CO and H 2 production (Eq. ( 1)), comments are focused on these gases. Figure 11 shows that, with increasing C 3 H 8 /O 2 ratio, CO and H 2 production decreases, contrary to concentrations of propane conversion sub products. SFC performances were also observed to decrease, both OCV and power densities with increasing C 3 H 8 /O 2 ratio, starting from a value of 0.53. The influence of total gas flow in the range 12-45 l/h was then investigated at 620°C and C 3 H 8 /O 2 ratio fixed at 0.53. Figure 12 indicates that CO and H 2 production increases with increasing flow higher than 25l/h. The same trend was also observed for SCFC performances. The polarization curve measured for our optimum conditions, 620°C, C 3 H 8 /O 2 ratio 0.53, and total gas flow of 45l/h can be seen in Figure 13. Main curve is obtained with a 40µm thick anode, while the insight shows measurements performed with a device having a 10µm thick anode. For the first one (40µm thick anode), OCV is around 780mV, and the maximum power density reaches 18mW/cm 2 . Hence performances are improved compared to previous tested device with a 20µm thick anode (Figure 10). Moreover, insight of Figure 13 shows that the 10µm thick anode device exhibits almost no more power density (0.035mW/cm 2 ). Observations of SCFC samples tested in our optimum conditions (Figure 14) reveals that the thin anode (10µm) is totally degraded with a black colour compared to grey colour of safe nickel anode. Strong exothermic effect due to high anode reactivity can explain this phenomenon. Hence, for fixed gas conditions (composition, flow), the thickness of the anode is of crucial importance for SCFC performances.

Conclusions

A SCFC device with a simple coplanar geometry of the electrodes was developed. The feasibility of operating in air-propane mixture was confirmed. However, with the actual configuration of SFC device (LSM-GDC-Ni) and test bench (furnace and gas feed geometry), limited performances were recorded. The maximum power density that we could obtain is around 20mW/cm 2 . Considering the too high catalytic activity of LSM towards hydrocarbon conversion, and the low studied temperature around 600°C, SSC and BSCF cathode materials should lead to higher performances compared to LSM. However, stability of SSC and BSCF materials in air-propane mixtures could be hardly achieved. For nickel anodes, in our test conditions, a minimum temperature of 535°C was required to avoid its re-oxidation. Moreover, a minimum thickness is also probably required, regarding strong exothermic effects during propane oxidation. In order to improve SCFC performances, investigations on anode behaviour in running conditions is still continued, and modelling of the reactor geometry and chemical reactions rates around cathode and anode is required to optimize the experimental conditions. Deleted: This temperature corresponds to the sample temperature, which is reported in Figure 7 and Figure 8.

Deleted: was

Deleted: of our SOFC device, 700°C for a C3H8/O2 ratio equal to 0.55. Effectively, these optimal conditions are strongly dependant on experimental parameters, in particular on fuel-air ratio, as shown in a previous paper [9]. The result Formatted: Font: Font color: Auto, (Asian) Chinese (PRC)

Formatted: English (U.K.)

Formatted: English (U.K.)

Formatted: English (U.K.)

Formatted: English (U.K.)

Formatted: Font: Italic, English (U.K.)

Formatted: Font: Italic

Formatted 7

 7 However, other processes may be proposed to overcome this inconvenient[8].Deleted: Ni-CGO / CGO / LSMDeleted:Deleted: , when the air-propane mixture is introduced Deleted: It must be noted that a strong exothermic effect occurs when the fuel mixture is introduced, which can induce an overshoot of 50°C.

Formatted:

  Font color: Auto Formatted: Font color: Auto Formatted: Font color: Blue Formatted: Subscript Formatted: Font color: Blue Formatted: Font color: Blue Formatted: Font color: Blue Formatted: Font color: Blue Formatted: Superscript Formatted: Font color: Blue Formatted: Font color: Blue Formatted: Font color: Blue Deleted: our Deleted: demonstrated

  has been presented at the Fundamental and Developments of Fuel Cells 2008 Conference (FDFC08) held in Nancy, France, on Dec. 10-12th, 2008.

Formatted:

  Font: Bold Formatted: Font: (Asian) Chinese (PRC) Formatted: Bullets and Numbering Formatted: Font: Italic Formatted: Font: Bold Formatted: Font: Font color: Auto, (Asian) Chinese (PRC) Formatted: Portuguese (Brazil) Formatted: Portuguese (Brazil) Formatted: Font: (Asian) Chinese (PRC), (Other) Portuguese (Brazil) Formatted: French (France) Deleted: al., Deleted: L Deleted: <#>C. Zhang, Y. Zheng, R. Ran, Z. Shao, W. Jin, N. Xu, J. Ahn, J. Power Sources 2008, 179, 640. ¶

Fig. 1

 1 Fig. 1 SCFC prototype on GDC pellet with screen-printed electrodes. Fig. 2 Thermogravimetric analysis of NiO and GDC powders under hydrogen (2% in Ar), temperature rising rate 10°C/min. Fig. 3 XRD pattern of NiO-GDC anode after reduction 600°C-30min under hydrogen (2% in Ar) Fig. 4 Ni-GDC anode conductivity in air-propane mixture (C 3 H 8 /O 2 = 0.66), versus temperature Fig. 5 LSM, SSC and BSCF cathodes conductivity in air, versus temperature Fig. 6 SEM of BSCF, SSC and LSM cathodes (as prepared layers) Fig. 7 Catalytic activity measured as propane conversion rate with a µGC-MS,of BSCF, SSC and LSM cathodes (powders in a fixed bed reactor)) Fig. 8 XRD patterns of SSC cathode a) as prepared, b)after reduction 600°C-30min under H 2 (2% in Ar), c) after C 3 H 8 /O 2 exposure at 630°C during 3 hours Fig. 9 Open Circuit Voltage time dependence of a Ni-CGO / CGO / LSM SCFC under Air-propane (C 3 H 8 /O 2 =0.55), versus time, with varying sample temperature Fig. 10 Voltage-current characteristic of a Ni-CGO / CGO / LSM SCFC at 700°C (SCFC temperature) under Airpropane (C 3 H 8 /O 2 =0.55) Fig. 11 Influence of C 3 H 8 /O 2 ratio on outlet gas composition, during SCFC test, at 620°C with a total gas flow of 45l/h. Fig. 12 Influence of total gas flow on outlet gas composition, during SCFC test, at 620°C with a C 3 H 8 /O 2 ratio set at 0.53. Fig. 13 Voltage-current characteristic of a Ni-CGO / CGO / LSM SCFC at 620°C (SCFC temperature) under Airpropane (C 3 H 8 /O 2 =0.53), with a total gas flow of 45l/h. Influence of anode thickness: 40µm in main curve, 10µm in insight. Fig. 14 Photographs of SCFC devices with a 10µm (left) and 40µm (right) thick anode, after tests shown in figure 13.

Formatted:Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10 Fig. 11 Fig. 12

 123456789101112 Fig. 1 SCFC prototype on GDC pellet with screen-printed electrodes. 131x65mm (96 x 96 DPI)

  Cathode conductivityElectrical conductivity of the three studied cathode material, LSM, SSC and BSCF was measured under air in the range 20-600°C. Values reported in Figure

	1/3 C 3 H 8 + NiO → Ni + CO + 4/3 H 2 (3)	Deleted: the device
				Deleted: s
	3.2	Cathode characterization
	3.2.1			Deleted: initially
				Deleted: set at 3
				Deleted: of
				Deleted: mixture
				Deleted: in
				Deleted: 55
				Deleted: However, for safety reasons,
				an additional nitrogen flow of 15l/h was
				added in order to decrease the oxygen
				content to less than 12% in the bench.
				Effectively, in air, the explosive domain
				of air-propane mixtures is in the range
				2.2-10% of propane. The decrease of
				oxygen content reduces this area which
				disappears for oxygen content lower than
			F o r	12%. Deleted: 500 Deleted: 700
			P
			e
			e r
			R
			e
			v i e
			w	Deleted: , in our experimental conditions
				Formatted: Portuguese (Brazil)
				Formatted: Portuguese (Brazil)
		2 → NiO	(2)	Formatted: Portuguese (Brazil)

Wiley-VCH Fuel Cells


Katherine Reekie

Andres Metspalu

Stephen J Chanock

Edison T Liu

Elaine R Mardis

Stephen W Scherer

Pui-Yan Kwok

Anthony J Brookes

Elaine ; <none> Scherer

HGV2009 Meeting: Bigger and better studies provide more answers and more questions

Keywords: HGV2009, SNP, Variation, GWAS, CNV Genome-Wide Association Studies (GWAS), progress beyond GWAS, sequencing developments, and bioinformatics approaches

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The 11 th International Meeting on Human Genome Variation and Complex Genome Analysis was held in Tallinn, Estonia on 11 th -13 th September 2009 at the Sokos Hotel Viru Conference Centre. Over three days there were seven sessions and a total of 35 speakers, interspersed with poster sessions. The workshop format, involving over 120 academic and commercial attendees with diverse expertise, encouraged vibrant discussion and collaboration.

A core theme during the meeting concerned the "next step after GWAS", including issues such as identifying causal genes and their role in disease, managing large volumes of GWAS data, meta-analyses and combining resources, and approaches to data sharing. Another main topic was technology development and the emergence of sequencing as an increasingly powerful and universal tool for large-scale genetic variation research. The increasing number of complete genomes available continues to emphasize the degree of variation that exists between individuals and between populations. And the massive amounts of data generated by modern techniques stresses the need for lab scientists to have a good knowledge of bioinformatics. More generally, investigations focussed upon the human exome, comprising 1% of all human DNA, were also a main feature of the meeting. Compared to previous meetings in this series, there was less focus on animal models of disease but more emphasis upon understanding and predicting a broad range of human phenotypes (Box 1).

What Next After GWAS?

The power of GWAS was illustrated in the meeting's first session by Pui-Yan Kwok, who spoke about moving on from GWAS given that such studies have now yielded many positive genome regions with encompassed candidate genes that can be variously explored. The success of GWAS to date was illustrated by Stephen Chanock in the realm of identifying multiple novel cancer susceptibility regions, most having a moderate effect on risk (odds ratios 1.1 -1.3), and by Laura Zagato who described hypertension studies which started with GWAS and progressed to ultimately confirm disease association with a single gene. However, many regions identified by GWAS do not contain any, or any obvious, genes or regulatory elements, and this makes it very difficult to move forward. This was discussed by Rick Kittles, who described the 8q24 locus that contains few known genes but many SNPs which have been strongly and repeatedly associated with prostate cancer. GWAS research will often suggest many regions of association, each having relatively small effect and containing many genes that could be involved in the disease risk modification. To help prioritise these for analysis, Yves Moreau described ENDEAVOUR -a tool that ranks genes based on their similarity to genes already implicated in a particular trait or process. Alternatively, one might prefer to follow a pathway-based strategy, as exemplified by Edison Liu, who showed how well this has worked with regard to breast and endometrial cancer.

Ultimately, it is hoped that GWAS and related research will enable the prediction of phenotypes based on genetic data. Beyond obvious uses in medicine, Manfred Kayser showed how an understanding of the genetics behind human phenotypes such as eye colour, hair colour and height can be used to help identify individuals. For example, eye colour is predictable with high

Diagnostics and Pharmacogenetics

GWAS has great potential in the area of pharmacogenetics and several examples of this were presented at the conference, with one session focused entirely on this subject. Eileen Dolan reported the discovery of "genetic signatures" contributing to susceptibility to chemotherapeuticinduced cytotoxicity using cell-based models. Some of these SNPs translated into clinical outcomes, highlighting SNP alleles significantly associated with survival in ovarian cancer patients receiving carboplatin and paclitaxel.

Another example came from Lorena Citterio, who showed that SNPs identified from candidate gene and GWAS studies could predict patients most likely to be responsive to a novel antihypertensive drug. Such progress will steadily open the door to patient sub-classification by genotype, enabling smaller and more cost-effective clinical trials. To progress in this direction, Michael Cariaso described an analysis of five complete human genomes that involved mining public databases (e.g., dbSNP, PubMed, and SNPedia) via a tool called Promethease that outputs a report listing, for example, medical issues related to drug metabolism specific to each individual genome.

Similarly, structural variation profiles of cancer can provide a useful diagnostic and prognosis indicator. Bauke Ylstra reported upon the use of array-CGH to distinguish between cancers that are newly arising, and those resulting from metastasis of a previous tumour. Furthermore, some different types of cancer have been shown to have characteristic array-CGH profiles and As such possibilities grow for the emergence of personalised and predictive medicine based upon genome data, many ethical questions arise. The importance of good genetic testing practice and considerations involved with the handling of such information was discussed by Jean-Jacques Cassiman. He also noted the need for good genetic counselling facilities, and made a point of asking who would be responsible for providing such testing and advice services in the future.

Currently, commercial providers are forging ahead in this new arena.

Data Sharing and Availability

The issue of identifying individuals from aggregate level GWAS data was addressed head-on by Kevin Jacobs, who provided guidance on how much information could be safely made public without risking the privacy of research participants. The scope for individual identification was shown to very much depend on factors such as the size of the study, and the number and characteristics of the variants under investigation, with no identification being possible if any number of p-values were reported without allele frequency data or directionality. If rational guidelines can now be based upon such new insights, then researchers (and databases) will be able to make openly available much more summary-level GWAS data than is currently being shared, helping to accelerate biomedical research substantially.

Nevertheless, many data components that relate to individuals and their phenotypes should probably never be openly shared, due not least to privacy concerns, commercial considerations, and legal and financial issues. Therefore, increasingly standardised and unified methods for controlling data access that do not unduly burden research progress must be devised. To this end, Anthony Brookes reported on the EU funded GEN2PHEN project (http://www.gen2phen.org), which is working towards effective data publication and data sharing enabled by digital user identities and federated database solutions. Such developments will help move biomedical databasing into the 'web 2.0' era, as exemplified by the current status and ongoing development plans of the HGVbaseG2P genetic association database (http://www.hgvbaseg2p.org), as described by Gudmundur Thorrisson. Furthermore, related ideas could be used to great benefit in clinical settings, as discussed by Andrew Devereau, with particular focus on the experiences of the National Genetics Reference Laboratory in the UK.

Spotlight on Populations

In the last year, the number of complete and semi-complete human genome sequences has increased from two Caucasian genomes to several fold more individuals from a diverse range of populations including Korean, Chinese, and African. Similarly, most previous GWAS have been conducted upon Caucasian individuals, but Jianjun Liu described large-scale GWAS research now in progress in China using Chinese population samples. This initiative has already uncovered differences in psoriasis susceptibility between Chinese and Caucasian populations, and dramatically increased the amount of genetic information available for the Chinese population. One clear message from HGV2009 was the need for further GWAS projects in other populations, to examine how disease susceptibility varies between populations.

Much is already known about the main differences in patterns of genetic variation between populations, and even between different regions within countries. Given this, individual countries have begun to create their own biobanks to enable their own population's genetic aetiology to be optimally explored. A leading example of this is the Estonian Biobank, presented by Andres Metspalu. Such resources will not only provide population-specific disease materials, but also supply properly matched control samples for future GWAS investigations.

Latest Technologies

Many new technologies are being developed to increase the speed and reduce the cost of studying genome variation. Han Cao described a novel method for the analysis of structural variation at the single molecule level, based upon a nano-channel array. This technique uses fluorescent labelling of specific motifs to "barcode" genomic DNA fragments, with these motifs then being detected as they pass down a nano-tube. Andreas Gnirke described target selection technologies in support of next-generation sequencing, wherein DNA regions such as sets of exons, or even complete exomes, are selectively recovered by DNA hybridisation and enrichment using RNA "baits".

The direct application of advanced sequencing technologies was very much in evidence at the meeting. Deep sequencing was used by Allen Roses to identify novel TOMM40 gene variants, adjacent to APOE genotypes but able to distinguish two evolutionary groups of APOE3 strands which associate with age of onset for Alzheimer disease. Previous age of onset curves were only accurate for APOE4 which is associated with "long" TOMM40 fragments 98% of the time.

Martin Kerick also reported on 2nd generation sequencing studies, showing how complete exome analysis in colon cancer patients suggests a multi-hit strategy for tumour development.

Complete resequencing of whole genomes is also starting to be evaluated as a disease research tool, e.g., for the analysis of cancer tissue versus normal tissue comparisons as presented by Elaine Mardis. Other sequencing work reported by Kelly Frazer used high-throughput technologies to study the 9p21 interval which has been signalled by GWAS association data to be involved in both Coronary Artery Disease and type 2 diabetes. Second generation sequencing could also start to replace array-CGH and microarrays as a means for structural variation analysis. Snehit Prabhu compared sequencing and arrays in this context, concluding that limitations such as sequence fidelity and data storage costs may limit the degree to which sequencing will soon completely replace array-based methods. One factor here will be how effectively large-scale sequencing data can be analysed, and progress in this direction was reported by Gabor Marth. He explained algorithms that could be applied to next-generation sequencing data from hundreds of samples to extract genetic variation profiles that span from single base pair differences right through to large structural variants.

New Avenues for the Study of Genetic Variation

Iuliana Ionita-Laza raised the question of how many new variants of various frequencies might still be unknown at any stage of research progress, and described statistical methods designed to help answer this question. Findings for rare variants revealed a clear distinction between the Chinese and the Japanese populations. Michael Eberle reported on the 1000 genomes project, which aims to directly discover rare SNPs in several populations, using low-depth and highdepth sequencing of many DNAs.

Beyond SNPs, there is a need to study copy-number variants (CNVs), and how this class of variation impacts phenotypes. But this goal is very challenging, not least because sequence differences exist between CNV unit copies, with potential allelic variations at these and other positions within each unit copy in the population. This complexity is termed multi-site variation (MSV), and it is very difficult to experimentally decipher. Marco Groth, nevertheless, showed that CNVs and MSVs may act together to result in an end phenotype, for example expression levels of the defensin gene DEFB4. Furthermore, Jonathan Mudge reported that gene

Transmission and Inheritance

Mendelian disease, wherein phenotypes are directly passed down across generations, is generally taken to be the result of the inheritance of single pathogenic gene mutations. However, several presentations showed that this is not always (or even usually) truly the case. Pablo Sandro Carvalho Santos described transmission distortion in the human genome (unequal chance of passing on two diploid alleles), presenting confirmed examples at two loci using SNP genotyping of family trios. The underlying mechanism remains unknown. As well as being important in its own right, this finding has implications for family-based disease association studies that assume the absence of transmission distortion. Doron Behar discussed the advantages and disadvantages of using uniploid and diploid genomes for tracing genetic variation.

Naisha Shah described how there may be a parental origin bias for de novo CNVs, with males showing an increased frequency of deletions. Joris Vermeesch then further complicated the story for CNV inheritance; emphasizing that phenotypic effects entail many forms of genetic inheritance, including quantitative effects and the interplay of X-inactivation. He also raised the question of what should constitute a CNV reference map, since individual CNV profiles vary so dramatically. Jonathan Mudge, however, argued that the wealth of data describing large and small scale variation does not mean that the idea of a single human reference sequence is obsolete, but that this data can be used to improve the fidelity of the assembly -as is being done in the HAVANA project. Extrapolating from the totality of work presented at HGV2009 and looking to the future, the next few years are certain to bring rapid progress in understanding complex disease aetiologies, revealing roles for both single base variants and structural variation alleles. This will be enabled by continued technology development and the support of new and powerful databasing and bioinformatics strategies. Clinical utility will be increasingly emphasized, with ethics and data sharing challenges being navigated. These dimensions, and many more, will be thoroughly explored at the HGV2010 meeting, being planned for September in the USA.

  a small number of genetic variants, suggesting a role in forensics and the profiling of crime suspects.

  that can be prognostic, as in the case of childhood leukaemias, as explained by Lili Milani.

  between individuals, and reported on efforts to determine why this is so, and whether it contributes to phenotypes. Such examples emphasize the huge gaps remaining in our understanding of genetic variation and the mechanisms of gene expression and phenotype determination.

John Wiley & Sons, Inc.Human Mutation

John Wiley & Sons, Inc.

Human Mutation