PROD. TYPE: COM. Oxidation of formate on hydrogen-loaded palladium UNCORRECTED PROOF

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1 pp: - (col.fig.: Nil) PROD. TYPE: COM ED: Jolly PAGN: Vishalam -- SCAN: Bindu International Journal of Hydrogen Energy 000 (200) Oxidation of formate on hydrogen-loaded palladium O. Yepez a, B.R. Scharifker b; a PDVSA-INTEVEP, Sector El Tambor, Los Teques, Estado Miranda, Venezuela b Departamento de Qumica, Universidad Simon Bolvar, Apartado 8000, Caracas 080-A, Venezuela Abstract Hydrogen occluded in palladium assists in the electrooxidation of formate ions at its surface, by chemically reacting with strongly adsorbed poisoning species and contributing to their release from the surface. Reaction between emerging occluded hydrogen and adsorbed CO regenerates surface sites for the continuous electrochemical oxidation of formate ions.? 200 Published by Elsevier Science Ltd. on behalf of the International Association for Hydrogen Energy. Keywords: Carbon monoxide; Electrocatalysis; Palladium; Hydrogen. Introduction One of the major obstacles for the large-scale practical application of room temperature fuel cells is their need of pure hydrogen feed [,2]. Due to present fuel availability, current technological eorts are being directed to providing fuel cells with the purest hydrogen obtained from gasoline []. However, these eorts are aected by a series of complications and energetic ineciencies, mainly associated with the need of fuel reforming, thus opposing the green credentials of fuel cells. Most developments in this maturing eld are related to the design and operation of fuel cells [4] rather than their fundamental chemistry; poisoning of the anode remains a major problem without an appropriate solution [,6]. Fuel cell electrode poisoning thus generally prevents the direct use of carbon-based fuels and=or hydrogen CO mixtures as fuels. The use of Pt Ru alloys signicantly enhances CO tolerance in hydrogen CO reformed fuels. This, combined with addition of hydrogen peroxide to the humidication water of the cell, allows the use of H 2 containing 00 ppm of CO [2]. Although this is a promising approach, it introduces further complication to cell design Corresponding author. Tel.: ; fax: addresses: yepezoj@pdvsa.com (O. Yepez), (B.R. Scharifker). and requires H 2O 2, which is expensive. Another approach, involving CO removal by injection of oxygen into the fuel gas ow, has been shown to reach cell performances comparable to CO-free hydrogen feeds []. Yet these solutions would not be necessary if CO poisoning of the electrocatalyst could be avoided altogether, permitting also the direct electrochemical oxidation of carbon-containing fuels. Among direct organic fuel cells, the direct methanol fuel 4 cell is of current interest, although it suers from limited fuel availability as well as CO poisoning [8]. This is avoided 4 at high temperatures, and although working temperatures have decreased from 800 C to as low as 00 C withgood 4 performance recently [], this technology still suers from poor material temperature resistance. Poisoning may be also 4 avoided using mixtures of methanol and an oxidant as fuel, but then performance is severely limited by slow kinetics 4 [0]. The ideal situation would be that of a cheap catalyst not poisoned at room temperature either from the use of raw carbon-based fuel or as a result of fuel reactions [8]. The oxidation of formate in alkaline solution has been intensely studied for fuel cell applications using Pd and Pd=Pt electrocatalysts. Early work was reviewed in 0 []. The spectroscopic establishment of the identity of the strong adsorbed intermediate formed during electrooxida- tion of formic acid on noble metals advanced considerably the understanding of the mechanism of electrooxidation of small organic molecules. The in situ identication of the 6 adsorbed species at the electrode surface, obtained with 060-/0/$ 20.00? 200 Published by Elsevier Science Ltd. on behalf of the International Association for Hydrogen Energy. PII: S060-(0)

2 2 O. Yepez, B.R. Scharifker / International Journal of Hydrogen Energy 000 (200) electrochemically modulated infra-red spectroscopy (EMIRS), as well as measurements of the charge required for its oxidation to CO 2, indicated that CO (ad) was the principal poison during formic acid electrooxidation [8,]. Experiments in acid solutions withoxygen labelling of HCOOH revealed dissociation of the C OH bond before formation of CO (ad) [4], leading to the following mechanism for the electrochemical oxidation of HCOOH, HCOOH HCO (ad) +OH (ad) ; () HCO (ad) CO (ad) +H + +e ; (2) CO (ad) +OH (ad) COOH (ad) ; () COOH (ad) CO 2 +H + +e : (4) Recently, we have described a novel way to overcome the poisoning eect of strongly adsorbed intermediates on the palladium surface, particularly CO (ad) [], eliminating them by chemical reaction with occluded hydrogen at the electrocatalyst surface. Hydrogen atoms diusing from the bulk of the electrocatalyst contribute to liberate the poisoned surface, thereby maintaining the electrocatalyst performance. Since the electrochemical oxidation of formic acid involves poisoning by CO (ad) [8,], the aim of the present work has been to verify whether this new method may be applied to maintain the rate of this electrochemical reaction. 2. Experimental The experiments were carried out in a three-compartment electrochemical cell. The working electrode was a 2 cm long, mm diameter (0:06 cm volume, 0:64 cm 2 geometrical surface area) palladium (.%) wire, sealed to glass througha joint withplatinum embedded in the glass seal, so that only Pd was exposed to the solution. The counter electrode was a platinum wire directly sealed to glass and placed in a separate compartment, separated from the working compartment by a porous glass membrane. A saturated calomel electrode (SCE) was used as reference. It was located in a third compartment connected to the working electrode section through a Luggin capillary, with its tip at ca. mm from the electrode surface. Potentials are reported withrespect to SCE. All solutions were prepared from ultra-ltered (Barnstead Nanopure J ) distilled water and analytical grade reagents, and ushed with nitrogen (.%, GIV) before the experiments. An EG&G PAR model potentiostat=galvanostat was used throughout the experiments. The eects of occluded hydrogen on the electrochemical oxidation of HCOO were examined withthe following sets of experiments. 2.. Cyclic voltammetry 4 Cyclic voltammetry (CV) was carried out at a potential scan rate of 0 mv s between 000 and +00 mv 4 (SCE) under dierent conditions: 2... CV of hydrogen-loaded palladium in NaOH 4 solution A constant cathodic current of ma was passed 4 through the palladium electrode in 0:00 M HCl solution during 00 s (0 C). The amount of hydrogen loaded by this process was equivalent to 0:0 H=Pd, corresponding to the region of coexistence of the and phases of hydrogen in palladium [6]. The average penetration depth of H may be estimated as (2Dt) =2, where D is the diusion coecient of H in Pd (ca. 0 cm 2 s [,8]), and t is the loading time. The average penetration depth is then ca. m, a small fraction of the electrode volume, i.e., the hydrogen load was not uniformly distributed throughout the palla- dium sample. Cyclic voltammetry of the hydrogen-loaded Pd electrode was carried out after transferring it to 0: M 6 NaOH solution CV of the hydrogen-free palladium electrode in 6 HCOO solution The Pd electrode was kept in 0: NaOH solution at 0: V 6 (SCE) during 0 min prior to eachmeasurement to ensure it was free of occluded hydrogen. Cyclic voltammetry was 6 carried out in 0:26 M NaHCOO + 0:2 M NaOH solution CV of the hydrogen-loaded palladium electrode in 6 HCOO solution The palladium electrode was loaded with hydrogen as described in Section 2.. and voltammetry in HCOO solution was carried out as described in Section Potential steps The eect of occluded hydrogen on the electrooxidation of formate ions was further studied with potential pulse experiments: Current transients during oxidation of occluded hydrogen Hydrogen was loaded in 0:00 M HCl solution at ma during 60 s (2 C), for a hydrogen load of 0:02 H=Pd, 8 non-uniformly distributed throughout the volume of the electrode. The electrode was immediately transferred to 8 0: M NaOH, and current transients during hydrogen oxidation were recorded at dierent potentials Current transients during oxidation of formate ions on hydrogen-free palladium 8 Prior to eachmeasurement, the Pd electrode was kept in 0: M NaOH solution at 0: V (SCE) during 0 min to 8

3 O. Yepez, B.R. Scharifker / International Journal of Hydrogen Energy 000 (200) ensure the absence of occluded hydrogen. After transferring to 0:26 M NaHCOO + 0:2 M NaOH solution, current transients from the open circuit potential to 00; 0, and +00 mv (SCE) were obtained Current transients during oxidation of formate ions on hydrogen-loaded palladium The palladium electrode was loaded with hydrogen as described in Section Then transients in HCOO solution, as described in Section 2.2.2, were obtained pulsing to 00; 0, and +00 mv (SCE).. Results Cyclic voltammetry Fig. shows the cyclic voltammogram of palladium in 0: M NaOH. The voltammogram displays four signicant features: (a) palladium oxide formation during the positive sweep, starting at ca. 400 mv; (b) palladium oxide reduction during the negative sweep, between 200 and 400 mv; (c) hydrogen ad=absorption, starting at ca. 0 mv and extending to the lower potential limit, and (d) hydrogen oxidation=desorption, from 80 to 400 mv during the sweep in the positive direction. Fig. 2 shows the initial four voltammograms of the palladium electrode, obtained immediately after loading with 0 C of hydrogen, as described in Section 2... The anodic Fig.. Steady state cyclic voltammetry of palladium in 0: M NaOH aqueous solution at 0 mv s. Fig. 2. Initial cyclic voltammograms of palladium electrode after loading withhydrogen at 2 ma cm 2 during 00 s (: 0 Ccm, equivalent to non-uniformly distributed 0:0 H=Pd atomic ratio), in 0: M NaOH at 0 mv s. currents observed are dominated by the unimpeded oxidation of hydrogen diusing from the bulk of Pd, and succes- 2 sive cycles show lower currents as the hydrogen occluded in palladium during cathodic loading is consumed. Cycles 2 2, recorded after 0 min of continuous cycling, are shown in Fig.. As the hydrogen loading decreases, a peak in the anodic current is observed during the positive scan, as well as a depression of the anodic current during the neg- ative scan, at potentials coinciding withthe formation and reduction of palladium oxide on the electrode surface. Thus electrochemical formation of palladium oxide inhibits hydrogen oxidation as the hydrogen loading decreases. Com- parison of the responses shown in Figs. 2 and therefore indicates that suciently high concentrations of occluded hydrogen prevent the formation of palladium oxide at the electrode surface, due to chemical reaction at the interface, 4 which may be represented as follows: 2H + PdO Pd + H 2O; () where H represents occluded hydrogen. 4 Fig. 4 shows the cyclic voltammogram of palladium in NaHCOO 0:26 M + NaOH 0:24 M (cf. Section 2..2). In 4 this case there are two dominant features. The anodic current increases continuously during the positive potential sweep 4 and drops steeply at ca. 20 mv. This current is due to the electrochemical oxidation of formate, HCOO,toCO 2, 4 HCOO + OH CO 2 +2H 2O+2e : (6) The formate oxidation current is negligible within the region between 00 and +00 mv, indicating that (6) does not occur when the palladium electrode is covered by its oxide and=or poisons []. During the negative potential sweep,

4 4 O. Yepez, B.R. Scharifker / International Journal of Hydrogen Energy 000 (200) Fig.. Cyclic voltammograms of palladium electrode loaded with hydrogen as in Fig. 2, recorded after 0 min of continuous cycling at 0 mv s in 0: M NaOH. after reduction of palladium oxide, an abrupt formate oxidation peak appears at 00 mv, indicating that (6) only occurs at potentials where the palladium electrode is free of surface oxide. Similar behaviour has been reported in acid H 2SO 4 0: M + HCOOH 0: M media [2]. Fig. shows ve cyclic voltammograms obtained immediately after loading the palladium electrode with 0 C of hydrogen and transferring it to the NaHCOO 0:26 M + NaOH 0:24 M solution, as described in Section 2... The sharp anodic peaks obtained in the absence of occluded hydrogen and shown in Fig. 4 do not appear in the presence of large amounts of occluded hydrogen, and formate continues oxidizing even at potentials where, in the absence of hydrogen, the surface is covered by oxide. Thus the processes responsible for the sharp drop in current after the peak during the positive sweep in Fig. 4 vanish in the presence of occluded hydrogen. Fig. 6 shows the voltammogram obtained after h of continuous cycling. As the amount of hydrogen loaded in Pd diminished during cycling, the intensity of the positive scan peak decreased, whereas the peak in the negative scan increased. Formate oxidation currents remained signicant and potential-dependent at more positive potentials, i.e. between 0 and +0:, in the presence Fig. 4. Cyclic voltammogram of palladium in NaHCOO 0:26 M + NaOH 0:24 M at 0 mv s. Fig.. Initial cyclic voltammograms of palladium electrode loaded withhydrogen as in Fig. 2, in NaHCOO 0:26 M + NaOH 0:24 M solution, at 0 mv s. of hydrogen. After h, the cyclic voltammogram cor- 2 2 responding to hydrogen-free palladium, cf. Fig. 4, was recovered Oxidation at constant potential Experiments at constant potential were carried out to 2 corroborate the potentiodynamic results, and to otherwise

5 O. Yepez, B.R. Scharifker / International Journal of Hydrogen Energy 000 (200) Fig. 6. Cyclic voltammogram of palladium electrode loaded with hydrogen as in Fig. 2, recorded after h of continuous cycling at 0 mv s in NaHCOO 0:26 M + NaOH 0:24 M. The dotted line is the cyclic voltammogram of the electrode similarly loaded withhydrogen after continuous cycling during hin 0: M NaOH solution. Fig.. Current transients recorded at +00 mv, (a) in 0: M NaOH, after loading the palladium electrode with hydrogen at 2 ma cm 2 during 60 s, for a non-uniformly distributed 0:02 H=Pd atomic ratio; (b) hydrogen-free palladium, in NaHCOO 0:26 M + NaOH 0:2 M solution, and (c) in NaHCOO 0:26 M + NaOH 0:2 M solution, after loading the palladium electrode withhydrogen as in (a). quantify the eects of occluded hydrogen on the oxidation of formate. Fig. shows a comparison of current transients obtained at +00 mv under dierent experimental Fig. 8. Current transients recorded at 00 mv, (a) in 0: M NaOH, after loading the palladium electrode with hydrogen at 2 ma cm 2 during 60 s; (b) hydrogen-free palladium, in NaHCOO 0:26 M + NaOH 0:2 M solution, and (c) in NaHCOO 0:26 M+NaOH 0:2 M solution, after loading the palladium electrode withhydrogen as in (a). conditions. At this potential and in the absence of occluded hydrogen, the palladium surface is covered with oxide. Trace (a) in Fig. shows the hydrogen oxidation current obtained in NaOH 0: M solution, after occluding hydrogen with a cathodic charge of 2:00 C. Integration of the current in (a) yields an oxidation charge of :64 C, indicating that hydrogen was loaded cathodically with ca. 82% eciency. The oxidation current of hydrogen-free palladium in HCOO 0:26 M + OH 0:2 M solution is shown as trace (b) in Fig. ; its integration yields a muchlower, almost negligible, oxidation charge. Trace (c) shows the oxidation current obtained in HCOO 0:26 M+OH 0:2 M solution after loading with the same amount of hydrogen as in trace (a). Integration of (c) yields an oxidation charge of :2 C, much larger than that used for loading with hydrogen. Fig. 8 shows the current transients obtained at 00 mv un- der otherwise similar conditions as those in Fig.. The currents are lower at this less positive potential. Also, the 2 surface coverage of CO (ad) is lower [], thus diminishing the surface poisoning, and the anodic currents for oxida- 2 tion of HCOO are sustained for longer. As obtained at +00 mv, the currents in HCOO solution on H-loaded 2 Pd were larger than those observed on H-free Pd. The integrated charge due to hydrogen oxidation in the absence 2 of HCOO was :64 C, whereas that of HCOO oxidation on H-free Pd at 00 mv was : C, and that observed 2 on H-loaded Pd in HCOO solution was 2:44 C. 4. Discussion The results shown in Figs. 2 and indicate that hydrogen oxidation is not impeded by the presence of an oxide

6 6 O. Yepez, B.R. Scharifker / International Journal of Hydrogen Energy 000 (200) layer on palladium. The anodic peak observed in Fig. withlow hydrogen loading at ca. 0: V originates from the presence of palladium oxide at more positive potentials, which is removed by chemical reaction with occluded hydrogen through (), while being readily regenerated electrochemically. In spite of the potential imposed, then, chemical reaction between palladium oxide and occluded hydrogen emerging at the surface results in the reduction of the oxide. The electrochemical oxidation of formate ions, reaction (6), leads to formation of carbonate. It has been postulated that formate oxidation occurs without adsorbed intermediates, leading only to hydrogen and carbonate []. It has been found that oxidation of organics in alkaline solution is less sensitive to the surface structure than in acid solution, and that formation of the poisoning species is diminished in alkaline solution []. On the other hand, adatoms enhance the rate of oxidation of methanol in alkali [20], providing strong indication for the adsorption of poison CO. We have recently found [] clear in situ FTIR evidence of CO adsorption on Pd in alkaline solution (ph 0). It is well known that adsorbed CO is the poisoning species during formic acid oxidation in acid medium [2], and oxidation of adsorbed CO is usually regarded as the rate determining step for the oxidation of organics in basic solution too [22]. Thus experimental evidence indicates that at suciently positive potentials adsorption of CO also occurs, HCOO CO (ad) +OH () blocking the surface towards further oxidation and giving rise to the sharp peak observed during voltammetric positive sweeps, as shown in Fig. 4. In the reverse sweep and upon palladium oxide reduction, the electrode becomes again free of CO (ad), formate resumes oxidation through (6), and a further sharp peak is obtained. These sharp peaks are no longer observed in the presence of occluded hydrogen, as shown in Fig.. Emerging hydrogen contributes to the displacement of carbon monoxide from the interface, leading to its oxidation and formation of other products, as we have shown elsewhere [], CO (ad) +2H H 2C=O (ad) ; (8) H 2C=O (ad) + 6OH CO 2 +4H 2O+4e : () Comparison of Figs. 2 and shows that the currents on hydrogen-loaded palladium are lower in the presence of formate in solution, due to blockage of the surface by CO (ad). Displacement of CO (ad) through (8) thus restores surface sites for the continual oxidation of HCOO through (6). Potential step experiments conrmed the results found with cyclic voltammetry. Fig. shows that no formate oxidation was observed at +00 mv on hydrogen-free palladium, whereas on H-loaded Pd the electric charge in the presence of formate in solution (:2 C) doubles that transferred in its absence (:6 C) and, furthermore, is signicantly higher than that used for hydrogen loading (2:0 C). The ineciencies found between hydrogen charging and discharging cycles, ca. 20% in the experiments shown in Fig., are due to molecular hydrogen production during galvanostatic charg- ing, in addition to losses during transfer of the electrode to the test solutions. In spite of the hydrogen loss, the results clearly demonstrate the continuous anodic oxidation of formate on H-loaded Pd. From the results obtained, a mechanism for the displacement of CO (ad) produced during oxidation of formate on Pd, withassistance from occluded hydrogen, can be proposed. In the absence of occluded hydrogen, CO (ad) formed through 6 () progressively covers the surface, eectively poisoning the electrocatalyst for further oxidation. At low CO (ad) cov- 6 erage, maintained at moderate positive potentials, adsorbed CO may oxidize to CO 2 following the path depicted in () 6 and (4), withoverall anodic reaction in alkaline solution expressed as (6). In the presence of occluded hydrogen, H 6 reacts chemically with CO (ad) withformation of formaldehyde, with further electrooxidation according to (8) and (), 6 withoverall anode reaction HCOO + OH +2H CO 2 +4H 2O+4e : (0) This mechanism is consistent with in situ spectroelec- trochemical results reported elsewhere []. From (6) and (0), the ratio of molar charges involved in the electrochemical oxidation of formate in the presence of occluded hydrogen and in its absence is 2. Com- parison of the currents measured during anodic oxidation of formate at 00 mv in the presence of oc- cluded hydrogen with those in its absence, cf. Fig., 2:44 C against : C respectively, yields a ratio of.6. This ratio is close to 2, even though not all the formaldehyde produced in (8) undergoes () [2]. 8. Conclusion It has been shown that occluded hydrogen assists the elec- 8 trooxidation of formate ions, by chemically reacting with, and therefore releasing the surface from, CO (ad) poison. The 8 reaction between emerging occluded hydrogen and adsorbed CO regenerates surface sites for the further electrochemi- 8 cal oxidation of formate, and opens up new possibilities for overcoming the poisoning of electrode surfaces, and thus 8 for direct feeding of organic fuels to room temperature fuel cells. Acknowledgements We gratefully acknowledge PDVSA-INTEVEP for nancial support. We are also grateful to Mr. Michele Milo for technical assistance and the members of the electrochemistry group at Universidad Simon Bolvar for discussions.

7 O. Yepez, B.R. Scharifker / International Journal of Hydrogen Energy 000 (200) References [] Schmidt VM, Oetjen HF, Divisek J. J Electrochem Soc ;44:L2. [2] Divisek J, Oetjen H, Peinecke V, Schmidt VM, Stimming U. Electrochim Acta 8;4:8. [] Vreeke MS, MahDT, Doyle CM. J Electrochem Soc 8;4:668. [4] Chen CP, Vreeke MS. J Electrochem Soc ;44:64. [] Delime F, Leger JM, Lamy C. J Appl Electrochem 8;28:2. [6] del Valle MA, Daz FR, Bodini ME, Pizarro T, Cordova R, Gomez H, Schrebler R. J Appl Electrochem 8;28:4. [] Gottesfeld S. US Patent No. 4,0,0, 0. [8] Parsons R, Vandernoot T. J Electroanal Chem 88;2:. [] Doshi R, Richerds VL, Carter JD, Wang X, Krumpelt M. J Electrochem Soc ;46:2. [0] Baxter SF, Battaglia VS, White RE. J Electrochem Soc ;46:4. [] VielstichW. Fuel cells. London: Wiley-Interscience, 0. [2] Capon A, Parsons R. J Electroanal Chem ;44:2. [] Corrigan D, Weaver M. J Electroanal Chem 88;24:4. 2 [4] Wolter O, Willsau J, Heitbaum J. J Electrochem Soc 8;2:6. 2 [] Yepez O, Scharifker BR. J Appl Electrochem ;2:8. [6] Lewis FA. The palladium hydrogen system. London: 2 Academic Press, 6. [] Devanathan MAV, Stachurski Z. Proc Roy Soc 62;20:0. 2 [8] McBreen J. J Electroanal Chem 0;28:2. [] Beltowska-Brzezinska M, Heitbaum J. J Electroanal Chem 2 8;8:6. [20] Kokkinidis G, Jannakoudakis D. J Electroanal Chem 8;:8. [2] Ross PN. In: Lipkowski J, Ross PN, editors. Electrocatalysis, Frontiers of Electrochemistry. New York: Wiley-VCH, 8. p [22] Beden B, Kadirgan F, Lamy C, Leger JM. J Electroanal Chem 82;42:. [2] Scharifker B, Yepez O, De Jesus JC, Ramrez de Agudelo MM. US Patent No.,0,6,.