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Oxidation of formate on hydrogen-loaded palladium
International Journal of Hydrogen Energy 27 (2002) 99–105
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Oxidation of formate on hydrogen-loaded palladium O. Y)epeza , B.R. Scharifkerb; ∗ a PDVSA-INTEVEP,
b Departamento
Sector El Tambor, Los Teques, Estado Miranda, Venezuela de Qu!mica, Universidad Simon Bol!var, Apartado 89000, Caracas 1080-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. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Carbon monoxide; Electrocatalysis; Palladium; Hydrogen
1. Introduction One of the major obstacles for the large-scale practical application of room temperature fuel cells is their need of pure hydrogen feed [1,2]. Due to present fuel availability, current technological e9orts are being directed to providing fuel cells with the purest hydrogen obtained from gasoline [3]. However, these e9orts are a9ected by a series of complications and energetic ine;ciencies, 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 [5,6]. 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 signi=cantly enhances CO tolerance in hydrogen–CO reformed fuels. This, combined with addition of hydrogen peroxide to the humidi=cation water of the cell, allows the use of H2 containing 100 ppm of CO [2]. Although this is a promising approach, it introduces further complication to cell design ∗ Corresponding author. Tel.: +58-212-906-3980; fax: +58-212906-3969. E-mail addresses: [email protected] (O. Y)epez), benjamin @usb.ve (B.R. Scharifker).
and requires H2 O2 , which is expensive. Another approach, involving CO removal by injection of oxygen into the fuel gas Gow, has been shown to reach cell performances comparable to CO-free hydrogen feeds [7]. 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 cell is of current interest, although it su9ers from limited fuel availability as well as CO poisoning [8]. This is avoided at high temperatures, and although working temperatures ◦ ◦ have decreased from 800 C to as low as 500 C with good performance recently [9], this technology still su9ers from poor material temperature resistance. Poisoning may be also avoided using mixtures of methanol and an oxidant as fuel, but then performance is severely limited by slow kinetics [10]. 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 1970 [11]. The spectroscopic establishment of the identity of the strong adsorbed intermediate formed during electrooxidation of formic acid on noble metals advanced considerably the understanding of the mechanism of electrooxidation of small organic molecules. The in situ identi=cation of the adsorbed species at the electrode surface, obtained with
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electrochemically modulated infra-red spectroscopy (EMIRS), as well as measurements of the charge required for its oxidation to CO2 , indicated that CO(ad) was the principal poison during formic acid electrooxidation [8,13]. Experiments in acid solutions with oxygen labelling of HCOOH revealed dissociation of the C–OH bond before formation of CO(ad) [14], leading to the following mechanism for the electrochemical oxidation of HCOOH, HCOOH → −HCO(ad) + OH(ad) ;
(1)
−HCO(ad) → CO(ad) + H+ + e− ;
(2)
CO(ad) + OH(ad) → −COOH(ad) ;
(3)
−COOH(ad) → CO2 + H+ + e− :
(4)
Recently, we have described a novel way to overcome the poisoning e9ect of strongly adsorbed intermediates on the palladium surface, particularly CO(ad) [15], eliminating them by chemical reaction with occluded hydrogen at the electrocatalyst surface. Hydrogen atoms di9using 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) [8,13], 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, 1 mm diameter (0:016 cm3 volume, 0:64 cm2 geometrical surface area) palladium (99.9%) wire, sealed to glass through a joint with platinum 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. 1 mm from the electrode surface. Potentials are reported with respect to SCE. All solutions were prepared from ultra-=ltered (Barnstead NanopureJ ) distilled water and analytical grade reagents, and Gushed with nitrogen (99.9%, GIV) before the experiments. An EG&G PAR model 173 potentiostat=galvanostat was used throughout the experiments. The e9ects of occluded hydrogen on the electrochemical oxidation of HCOO− were examined with the following sets of experiments.
2.1. Cyclic voltammetry Cyclic voltammetry (CV) was carried out at a potential scan rate of 50 mV s−1 between −1000 and +300 mV (SCE) under di9erent conditions: 2.1.1. CV of hydrogen-loaded palladium in NaOH solution A constant cathodic current of −33 mA was passed through the palladium electrode in 0:003 M HCl solution during 1500 s (50 C). The amount of hydrogen loaded by this process was equivalent to 0:30 H=Pd, corresponding to the region of coexistence of the and phases of hydrogen in palladium [16]. The average penetration depth of H may be estimated as (2Dt)1=2 , where D is the di9usion coe;cient of H in Pd (ca. 10−7 cm2 s−1 [17,18]), and t is the loading time. The average penetration depth is then ca. 3 m, a small fraction of the electrode volume, i.e., the hydrogen load was not uniformly distributed throughout the palladium sample. Cyclic voltammetry of the hydrogen-loaded Pd electrode was carried out after transferring it to 0:5 M NaOH solution. 2.1.2. CV of the hydrogen-free palladium electrode in HCOO− solution The Pd electrode was kept in 0:5 NaOH solution at −0:3 V (SCE) during 30 min prior to each measurement to ensure it was free of occluded hydrogen. Cyclic voltammetry was carried out in 0:26 M NaHCOO + 0:23 M NaOH solution. 2.1.3. CV of the hydrogen-loaded palladium electrode in HCOO− solution The palladium electrode was loaded with hydrogen as described in Section 2.1.1 and voltammetry in HCOO− solution was carried out as described in Section 2.1.2. 2.2. Potential steps The e9ect of occluded hydrogen on the electrooxidation of formate ions was further studied with potential pulse experiments: 2.2.1. Current transients during oxidation of occluded hydrogen Hydrogen was loaded in 0:003 M HCl solution at −33 mA during 60 s (2 C), for a hydrogen load of 0:012 H=Pd, non-uniformly distributed throughout the volume of the electrode. The electrode was immediately transferred to 0:5 M NaOH, and current transients during hydrogen oxidation were recorded at di9erent potentials. 2.2.2. Current transients during oxidation of formate ions on hydrogen-free palladium Prior to each measurement, the Pd electrode was kept in 0:5 M NaOH solution at −0:3 V (SCE) during 30 min to
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ensure the absence of occluded hydrogen. After transferring to 0:26 M NaHCOO + 0:23 M NaOH solution, current transients from the open circuit potential to −300; 0, and +300 mV (SCE) were obtained. 2.2.3. Current transients during oxidation of formate ions on hydrogen-loaded palladium The palladium electrode was loaded with hydrogen as described in Section 2.2.1. Then transients in HCOO− solution, as described in Section 2.2.2, were obtained pulsing to −300; 0, and +300 mV (SCE). 3. Results 3.1. Cyclic voltammetry Fig. 1 shows the cyclic voltammogram of palladium in 0:5 M NaOH. The voltammogram displays four signi=cant 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. −750 mV and extending to the lower potential limit, and (d) hydrogen oxidation=desorption, from −850 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 50 C of hydrogen, as described in Section 2.1.1. The anodic
Fig. 2. Initial cyclic voltammograms of palladium electrode after loading with hydrogen at 52 mA cm−2 during 1500 s (3:1 × 103 C cm−3 , equivalent to non-uniformly distributed 0:30 H=Pd atomic ratio), in 0:5 M NaOH at 50 mV s−1 .
currents observed are dominated by the unimpeded oxidation of hydrogen di9using from the bulk of Pd, and successive cycles show lower currents as the hydrogen occluded in palladium during cathodic loading is consumed. Cycles 35 –52, recorded after 30 min of continuous cycling, are shown in Fig. 3. 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 negative scan, at potentials coinciding with the formation and reduction of palladium oxide on the electrode surface. Thus electrochemical formation of palladium oxide inhibits hydrogen oxidation as the hydrogen loading decreases. Comparison of the responses shown in Figs. 2 and 3 therefore indicates that su;ciently high concentrations of occluded hydrogen prevent the formation of palladium oxide at the electrode surface, due to chemical reaction at the interface, which may be represented as follows: 2H• + PdO → Pd + H2 O;
(5)
•
where H represents occluded hydrogen. Fig. 4 shows the cyclic voltammogram of palladium in NaHCOO 0:26 M + NaOH 0:24 M (cf. Section 2.1.2). In this case there are two dominant features. The anodic current increases continuously during the positive potential sweep and drops steeply at ca. −250 mV. This current is due to the electrochemical oxidation of formate, HCOO− , to CO2− 3 , − HCOO− + 3OH− → CO2− 3 + 2H2 O + 2e :
Fig. 1. Steady state cyclic voltammetry of palladium in 0:5 M NaOH aqueous solution at 50 mV s−1 .
(6)
The formate oxidation current is negligible within the region between −100 and +300 mV, indicating that (6) does not occur when the palladium electrode is covered by its oxide and=or poisons [13]. During the negative potential sweep,
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Fig. 4. Cyclic voltammogram of palladium in NaHCOO 0:26 M + NaOH 0:24 M at 50 mV s−1 .
Fig. 3. Cyclic voltammograms of palladium electrode loaded with hydrogen as in Fig. 2, recorded after 30 min of continuous cycling at 50 mV s−1 in 0:5 M NaOH.
after reduction of palladium oxide, an abrupt formate oxidation peak appears at −300 mV, indicating that (6) only occurs at potentials where the palladium electrode is free of surface oxide. Similar behaviour has been reported in acid H2 SO4 0:5 M + HCOOH 0:1 M media [12]. Fig. 5 shows =ve cyclic voltammograms obtained immediately after loading the palladium electrode with 50 C of hydrogen and transferring it to the NaHCOO 0:26 M + NaOH 0:24 M solution, as described in Section 2.1.3. 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 1 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 signi=cant and potential-dependent at more positive potentials, i.e. between 0 and +0:3, in the presence
Fig. 5. Initial cyclic voltammograms of palladium electrode loaded with hydrogen as in Fig. 2, in NaHCOO 0:26 M + NaOH 0:24 M solution, at 50 mV s−1 .
of hydrogen. After 3 12 h, the cyclic voltammogram corresponding to hydrogen-free palladium, cf. Fig. 4, was recovered. 3.2. Oxidation at constant potential Experiments at constant potential were carried out to corroborate the potentiodynamic results, and to otherwise
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Fig. 8. Current transients recorded at −300 mV, (a) in 0:5 M NaOH, after loading the palladium electrode with hydrogen at 52 mA cm−2 during 60 s; (b) hydrogen-free palladium, in NaHCOO 0:26 M + NaOH 0:23 M solution, and (c) in NaHCOO 0:26 M+NaOH 0:23 M solution, after loading the palladium electrode with hydrogen as in (a). Fig. 6. Cyclic voltammogram of palladium electrode loaded with hydrogen as in Fig. 2, recorded after 1 h of continuous cycling at 50 mV s−1 in NaHCOO 0:26 M + NaOH 0:24 M. The dotted line is the cyclic voltammogram of the electrode similarly loaded with hydrogen after continuous cycling during 1 h in 0:5 M NaOH solution.
Fig. 7. Current transients recorded at +300 mV, (a) in 0:5 M NaOH, after loading the palladium electrode with hydrogen at 52 mA cm−2 during 60 s, for a non-uniformly distributed 0:012 H=Pd atomic ratio; (b) hydrogen-free palladium, in NaHCOO 0:26 M + NaOH 0:23 M solution, and (c) in NaHCOO 0:26 M + NaOH 0:23 M solution, after loading the palladium electrode with hydrogen as in (a).
quantify the e9ects of occluded hydrogen on the oxidation of formate. Fig. 7 shows a comparison of current transients obtained at +300 mV under di9erent experimental
conditions. At this potential and in the absence of occluded hydrogen, the palladium surface is covered with oxide. Trace (a) in Fig. 7 shows the hydrogen oxidation current obtained in NaOH 0:5 M solution, after occluding hydrogen with a cathodic charge of 2:00 C. Integration of the current in 7(a) yields an oxidation charge of 1:64 C, indicating that hydrogen was loaded cathodically with ca. 82% e;ciency. The oxidation current of hydrogen-free palladium in HCOO− 0:26 M + OH− 0:23 M solution is shown as trace (b) in Fig. 7; its integration yields a much lower, almost negligible, oxidation charge. Trace 7(c) shows the oxidation current obtained in HCOO− 0:26 M+OH− 0:23 M solution after loading with the same amount of hydrogen as in trace (a). Integration of (c) yields an oxidation charge of 3:21 C, much larger than that used for loading with hydrogen. Fig. 8 shows the current transients obtained at −300 mV under otherwise similar conditions as those in Fig. 7. The currents are lower at this less positive potential. Also, the surface coverage of CO(ad) is lower [15], thus diminishing the surface poisoning, and the anodic currents for oxidation of HCOO− are sustained for longer. As obtained at +300 mV, the currents in HCOO− solution on H-loaded Pd were larger than those observed on H-free Pd. The integrated charge due to hydrogen oxidation in the absence of HCOO− was 1:64 C, whereas that of HCOO− oxidation on H-free Pd at −300 mV was 1:53 C, and that observed on H-loaded Pd in HCOO− solution was 2:44 C. 4. Discussion The results shown in Figs. 2 and 3 indicate that hydrogen oxidation is not impeded by the presence of an oxide
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layer on palladium. The anodic peak observed in Fig. 3 with low hydrogen loading at ca. −0:3 V originates from the presence of palladium oxide at more positive potentials, which is removed by chemical reaction with occluded hydrogen through (5), 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 [11]. 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 [19]. 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 [15] clear in situ FTIR evidence of CO adsorption on Pd in alkaline solution (pH 10). It is well known that adsorbed CO is the poisoning species during formic acid oxidation in acid medium [21], 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 su;ciently positive potentials adsorption of CO also occurs, HCOO− → CO(ad) + OH−
(7)
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. 5. 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 [15], CO(ad) + 2H• → H2 C = O(ad) ;
(8)
− H2 C = O(ad) + 6OH− → CO2− 3 + 4H2 O + 4e :
(9)
Comparison of Figs. 2 and 5 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 con=rmed the results found with cyclic voltammetry. Fig. 7 shows that no formate oxidation was observed at +300 mV on hydrogen-free palladium, whereas on H-loaded Pd the electric charge in the presence of formate in solution (3:2 C) doubles that transferred in its absence (1:6 C) and, furthermore, is signi=cantly higher than that used for hydrogen loading (2:0 C). The ine;ciencies
found between hydrogen charging and discharging cycles, ca. 20% in the experiments shown in Fig. 7, are due to molecular hydrogen production during galvanostatic charging, 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, with assistance from occluded hydrogen, can be proposed. In the absence of occluded hydrogen, CO(ad) formed through (7) progressively covers the surface, e9ectively poisoning the electrocatalyst for further oxidation. At low CO(ad) coverage, maintained at moderate positive potentials, adsorbed CO may oxidize to CO2 following the path depicted in (3) and (4), with overall anodic reaction in alkaline solution expressed as (6). In the presence of occluded hydrogen, H• reacts chemically with CO(ad) with formation of formaldehyde, with further electrooxidation according to (8) and (9), with overall anode reaction − HCOO− + 5OH− + 2H• → CO2− 3 + 4H2 O + 4e :
(10)
This mechanism is consistent with in situ spectroelectrochemical results reported elsewhere [15]. From (6) and (10), the ratio of molar charges involved in the electrochemical oxidation of formate in the presence of occluded hydrogen and in its absence is 2. Comparison of the currents measured during anodic oxidation of formate at −300 mV in the presence of occluded hydrogen with those in its absence, cf. Fig. 7, 2:44 C against 1:53 C respectively, yields a ratio of 1.6. This ratio is close to 2, even though not all the formaldehyde produced in (8) undergoes (9) [23].
5. Conclusion It has been shown that occluded hydrogen assists the electrooxidation of formate ions, by chemically reacting with, and therefore releasing the surface from, CO(ad) poison. The reaction between emerging occluded hydrogen and adsorbed CO regenerates surface sites for the further electrochemical oxidation of formate, and opens up new possibilities for overcoming the poisoning of electrode surfaces, and thus 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 Sim)on Bol)Qvar for discussions.
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References [1] Schmidt VM, Oetjen HF, Divisek J. J Electrochem Soc 1997;144:L237. [2] Divisek J, Oetjen H, Peinecke V, Schmidt VM, Stimming U. Electrochim Acta 1998;43:3811. [3] Vreeke MS, Mah DT, Doyle CM. J Electrochem Soc 1998;145:3668. [4] Chen CP, Vreeke MS. J Electrochem Soc 1997;144:3674. [5] Delime F, L)eger JM, Lamy C. J Appl Electrochem 1998;28:27. [6] del Valle MA, D)Qaz FR, Bodini ME, Pizarro T, C)ordova R, G)omez H, Schrebler R. J Appl Electrochem 1998;28:943. [7] Gottesfeld S. US Patent No. 4,910,099, 1990. [8] Parsons R, Vandernoot T. J Electroanal Chem 1988;257:9. [9] Doshi R, Richerds VL, Carter JD, Wang X, Krumpelt M. J Electrochem Soc 1999;146:1273. [10] Baxter SF, Battaglia VS, White RE. J Electrochem Soc 1999;146:437. [11] Vielstich W. Fuel cells. London: Wiley-Interscience, 1970.
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[12] Capon A, Parsons R. J Electroanal Chem 1973;44:239. [13] Corrigan D, Weaver M. J Electroanal Chem 1988;241:143. [14] Wolter O, Willsau J, Heitbaum J. J Electrochem Soc 1985;132:1635. [15] Y)epez O, Scharifker BR. J Appl Electrochem 1999;29:1185. [16] Lewis FA. The palladium hydrogen system. London: Academic Press, 1967. [17] Devanathan MAV, Stachurski Z. Proc Roy Soc 1962;270:90. [18] McBreen J. J Electroanal Chem 1990;287:279. [19] Beltowska-Brzezinska M, Heitbaum J. J Electroanal Chem 1985;183:167. [20] Kokkinidis G, Jannakoudakis D. J Electroanal Chem 1983;153:185. [21] Ross PN. In: Lipkowski J, Ross PN, editors. Electrocatalysis, Frontiers of Electrochemistry. New York: Wiley-VCH, 1998. p. 63– 66. [22] Beden B, Kadirgan F, Lamy C, Leger JM. J Electroanal Chem 1982;142:171. [23] Scharifker B, Y)epez O, De Jes)us JC, Ram)Qrez de Agudelo MM. US Patent No. 5,051,156, 1991.
www.elsevier.com/locate/ijhydene
Oxidation of formate on hydrogen-loaded palladium O. Y)epeza , B.R. Scharifkerb; ∗ a PDVSA-INTEVEP,
b Departamento
Sector El Tambor, Los Teques, Estado Miranda, Venezuela de Qu!mica, Universidad Simon Bol!var, Apartado 89000, Caracas 1080-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. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Carbon monoxide; Electrocatalysis; Palladium; Hydrogen
1. Introduction One of the major obstacles for the large-scale practical application of room temperature fuel cells is their need of pure hydrogen feed [1,2]. Due to present fuel availability, current technological e9orts are being directed to providing fuel cells with the purest hydrogen obtained from gasoline [3]. However, these e9orts are a9ected by a series of complications and energetic ine;ciencies, 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 [5,6]. 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 signi=cantly enhances CO tolerance in hydrogen–CO reformed fuels. This, combined with addition of hydrogen peroxide to the humidi=cation water of the cell, allows the use of H2 containing 100 ppm of CO [2]. Although this is a promising approach, it introduces further complication to cell design ∗ Corresponding author. Tel.: +58-212-906-3980; fax: +58-212906-3969. E-mail addresses: [email protected] (O. Y)epez), benjamin @usb.ve (B.R. Scharifker).
and requires H2 O2 , which is expensive. Another approach, involving CO removal by injection of oxygen into the fuel gas Gow, has been shown to reach cell performances comparable to CO-free hydrogen feeds [7]. 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 cell is of current interest, although it su9ers from limited fuel availability as well as CO poisoning [8]. This is avoided at high temperatures, and although working temperatures ◦ ◦ have decreased from 800 C to as low as 500 C with good performance recently [9], this technology still su9ers from poor material temperature resistance. Poisoning may be also avoided using mixtures of methanol and an oxidant as fuel, but then performance is severely limited by slow kinetics [10]. 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 1970 [11]. The spectroscopic establishment of the identity of the strong adsorbed intermediate formed during electrooxidation of formic acid on noble metals advanced considerably the understanding of the mechanism of electrooxidation of small organic molecules. The in situ identi=cation of the adsorbed species at the electrode surface, obtained with
0360-3199/02/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 8 6 - 6
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O. Yepez, B.R. Scharifker / International Journal of Hydrogen Energy 27 (2002) 99–105
electrochemically modulated infra-red spectroscopy (EMIRS), as well as measurements of the charge required for its oxidation to CO2 , indicated that CO(ad) was the principal poison during formic acid electrooxidation [8,13]. Experiments in acid solutions with oxygen labelling of HCOOH revealed dissociation of the C–OH bond before formation of CO(ad) [14], leading to the following mechanism for the electrochemical oxidation of HCOOH, HCOOH → −HCO(ad) + OH(ad) ;
(1)
−HCO(ad) → CO(ad) + H+ + e− ;
(2)
CO(ad) + OH(ad) → −COOH(ad) ;
(3)
−COOH(ad) → CO2 + H+ + e− :
(4)
Recently, we have described a novel way to overcome the poisoning e9ect of strongly adsorbed intermediates on the palladium surface, particularly CO(ad) [15], eliminating them by chemical reaction with occluded hydrogen at the electrocatalyst surface. Hydrogen atoms di9using 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) [8,13], 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, 1 mm diameter (0:016 cm3 volume, 0:64 cm2 geometrical surface area) palladium (99.9%) wire, sealed to glass through a joint with platinum 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. 1 mm from the electrode surface. Potentials are reported with respect to SCE. All solutions were prepared from ultra-=ltered (Barnstead NanopureJ ) distilled water and analytical grade reagents, and Gushed with nitrogen (99.9%, GIV) before the experiments. An EG&G PAR model 173 potentiostat=galvanostat was used throughout the experiments. The e9ects of occluded hydrogen on the electrochemical oxidation of HCOO− were examined with the following sets of experiments.
2.1. Cyclic voltammetry Cyclic voltammetry (CV) was carried out at a potential scan rate of 50 mV s−1 between −1000 and +300 mV (SCE) under di9erent conditions: 2.1.1. CV of hydrogen-loaded palladium in NaOH solution A constant cathodic current of −33 mA was passed through the palladium electrode in 0:003 M HCl solution during 1500 s (50 C). The amount of hydrogen loaded by this process was equivalent to 0:30 H=Pd, corresponding to the region of coexistence of the and phases of hydrogen in palladium [16]. The average penetration depth of H may be estimated as (2Dt)1=2 , where D is the di9usion coe;cient of H in Pd (ca. 10−7 cm2 s−1 [17,18]), and t is the loading time. The average penetration depth is then ca. 3 m, a small fraction of the electrode volume, i.e., the hydrogen load was not uniformly distributed throughout the palladium sample. Cyclic voltammetry of the hydrogen-loaded Pd electrode was carried out after transferring it to 0:5 M NaOH solution. 2.1.2. CV of the hydrogen-free palladium electrode in HCOO− solution The Pd electrode was kept in 0:5 NaOH solution at −0:3 V (SCE) during 30 min prior to each measurement to ensure it was free of occluded hydrogen. Cyclic voltammetry was carried out in 0:26 M NaHCOO + 0:23 M NaOH solution. 2.1.3. CV of the hydrogen-loaded palladium electrode in HCOO− solution The palladium electrode was loaded with hydrogen as described in Section 2.1.1 and voltammetry in HCOO− solution was carried out as described in Section 2.1.2. 2.2. Potential steps The e9ect of occluded hydrogen on the electrooxidation of formate ions was further studied with potential pulse experiments: 2.2.1. Current transients during oxidation of occluded hydrogen Hydrogen was loaded in 0:003 M HCl solution at −33 mA during 60 s (2 C), for a hydrogen load of 0:012 H=Pd, non-uniformly distributed throughout the volume of the electrode. The electrode was immediately transferred to 0:5 M NaOH, and current transients during hydrogen oxidation were recorded at di9erent potentials. 2.2.2. Current transients during oxidation of formate ions on hydrogen-free palladium Prior to each measurement, the Pd electrode was kept in 0:5 M NaOH solution at −0:3 V (SCE) during 30 min to
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ensure the absence of occluded hydrogen. After transferring to 0:26 M NaHCOO + 0:23 M NaOH solution, current transients from the open circuit potential to −300; 0, and +300 mV (SCE) were obtained. 2.2.3. Current transients during oxidation of formate ions on hydrogen-loaded palladium The palladium electrode was loaded with hydrogen as described in Section 2.2.1. Then transients in HCOO− solution, as described in Section 2.2.2, were obtained pulsing to −300; 0, and +300 mV (SCE). 3. Results 3.1. Cyclic voltammetry Fig. 1 shows the cyclic voltammogram of palladium in 0:5 M NaOH. The voltammogram displays four signi=cant 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. −750 mV and extending to the lower potential limit, and (d) hydrogen oxidation=desorption, from −850 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 50 C of hydrogen, as described in Section 2.1.1. The anodic
Fig. 2. Initial cyclic voltammograms of palladium electrode after loading with hydrogen at 52 mA cm−2 during 1500 s (3:1 × 103 C cm−3 , equivalent to non-uniformly distributed 0:30 H=Pd atomic ratio), in 0:5 M NaOH at 50 mV s−1 .
currents observed are dominated by the unimpeded oxidation of hydrogen di9using from the bulk of Pd, and successive cycles show lower currents as the hydrogen occluded in palladium during cathodic loading is consumed. Cycles 35 –52, recorded after 30 min of continuous cycling, are shown in Fig. 3. 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 negative scan, at potentials coinciding with the formation and reduction of palladium oxide on the electrode surface. Thus electrochemical formation of palladium oxide inhibits hydrogen oxidation as the hydrogen loading decreases. Comparison of the responses shown in Figs. 2 and 3 therefore indicates that su;ciently high concentrations of occluded hydrogen prevent the formation of palladium oxide at the electrode surface, due to chemical reaction at the interface, which may be represented as follows: 2H• + PdO → Pd + H2 O;
(5)
•
where H represents occluded hydrogen. Fig. 4 shows the cyclic voltammogram of palladium in NaHCOO 0:26 M + NaOH 0:24 M (cf. Section 2.1.2). In this case there are two dominant features. The anodic current increases continuously during the positive potential sweep and drops steeply at ca. −250 mV. This current is due to the electrochemical oxidation of formate, HCOO− , to CO2− 3 , − HCOO− + 3OH− → CO2− 3 + 2H2 O + 2e :
Fig. 1. Steady state cyclic voltammetry of palladium in 0:5 M NaOH aqueous solution at 50 mV s−1 .
(6)
The formate oxidation current is negligible within the region between −100 and +300 mV, indicating that (6) does not occur when the palladium electrode is covered by its oxide and=or poisons [13]. During the negative potential sweep,
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Fig. 4. Cyclic voltammogram of palladium in NaHCOO 0:26 M + NaOH 0:24 M at 50 mV s−1 .
Fig. 3. Cyclic voltammograms of palladium electrode loaded with hydrogen as in Fig. 2, recorded after 30 min of continuous cycling at 50 mV s−1 in 0:5 M NaOH.
after reduction of palladium oxide, an abrupt formate oxidation peak appears at −300 mV, indicating that (6) only occurs at potentials where the palladium electrode is free of surface oxide. Similar behaviour has been reported in acid H2 SO4 0:5 M + HCOOH 0:1 M media [12]. Fig. 5 shows =ve cyclic voltammograms obtained immediately after loading the palladium electrode with 50 C of hydrogen and transferring it to the NaHCOO 0:26 M + NaOH 0:24 M solution, as described in Section 2.1.3. 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 1 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 signi=cant and potential-dependent at more positive potentials, i.e. between 0 and +0:3, in the presence
Fig. 5. Initial cyclic voltammograms of palladium electrode loaded with hydrogen as in Fig. 2, in NaHCOO 0:26 M + NaOH 0:24 M solution, at 50 mV s−1 .
of hydrogen. After 3 12 h, the cyclic voltammogram corresponding to hydrogen-free palladium, cf. Fig. 4, was recovered. 3.2. Oxidation at constant potential Experiments at constant potential were carried out to corroborate the potentiodynamic results, and to otherwise
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Fig. 8. Current transients recorded at −300 mV, (a) in 0:5 M NaOH, after loading the palladium electrode with hydrogen at 52 mA cm−2 during 60 s; (b) hydrogen-free palladium, in NaHCOO 0:26 M + NaOH 0:23 M solution, and (c) in NaHCOO 0:26 M+NaOH 0:23 M solution, after loading the palladium electrode with hydrogen as in (a). Fig. 6. Cyclic voltammogram of palladium electrode loaded with hydrogen as in Fig. 2, recorded after 1 h of continuous cycling at 50 mV s−1 in NaHCOO 0:26 M + NaOH 0:24 M. The dotted line is the cyclic voltammogram of the electrode similarly loaded with hydrogen after continuous cycling during 1 h in 0:5 M NaOH solution.
Fig. 7. Current transients recorded at +300 mV, (a) in 0:5 M NaOH, after loading the palladium electrode with hydrogen at 52 mA cm−2 during 60 s, for a non-uniformly distributed 0:012 H=Pd atomic ratio; (b) hydrogen-free palladium, in NaHCOO 0:26 M + NaOH 0:23 M solution, and (c) in NaHCOO 0:26 M + NaOH 0:23 M solution, after loading the palladium electrode with hydrogen as in (a).
quantify the e9ects of occluded hydrogen on the oxidation of formate. Fig. 7 shows a comparison of current transients obtained at +300 mV under di9erent experimental
conditions. At this potential and in the absence of occluded hydrogen, the palladium surface is covered with oxide. Trace (a) in Fig. 7 shows the hydrogen oxidation current obtained in NaOH 0:5 M solution, after occluding hydrogen with a cathodic charge of 2:00 C. Integration of the current in 7(a) yields an oxidation charge of 1:64 C, indicating that hydrogen was loaded cathodically with ca. 82% e;ciency. The oxidation current of hydrogen-free palladium in HCOO− 0:26 M + OH− 0:23 M solution is shown as trace (b) in Fig. 7; its integration yields a much lower, almost negligible, oxidation charge. Trace 7(c) shows the oxidation current obtained in HCOO− 0:26 M+OH− 0:23 M solution after loading with the same amount of hydrogen as in trace (a). Integration of (c) yields an oxidation charge of 3:21 C, much larger than that used for loading with hydrogen. Fig. 8 shows the current transients obtained at −300 mV under otherwise similar conditions as those in Fig. 7. The currents are lower at this less positive potential. Also, the surface coverage of CO(ad) is lower [15], thus diminishing the surface poisoning, and the anodic currents for oxidation of HCOO− are sustained for longer. As obtained at +300 mV, the currents in HCOO− solution on H-loaded Pd were larger than those observed on H-free Pd. The integrated charge due to hydrogen oxidation in the absence of HCOO− was 1:64 C, whereas that of HCOO− oxidation on H-free Pd at −300 mV was 1:53 C, and that observed on H-loaded Pd in HCOO− solution was 2:44 C. 4. Discussion The results shown in Figs. 2 and 3 indicate that hydrogen oxidation is not impeded by the presence of an oxide
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layer on palladium. The anodic peak observed in Fig. 3 with low hydrogen loading at ca. −0:3 V originates from the presence of palladium oxide at more positive potentials, which is removed by chemical reaction with occluded hydrogen through (5), 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 [11]. 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 [19]. 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 [15] clear in situ FTIR evidence of CO adsorption on Pd in alkaline solution (pH 10). It is well known that adsorbed CO is the poisoning species during formic acid oxidation in acid medium [21], 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 su;ciently positive potentials adsorption of CO also occurs, HCOO− → CO(ad) + OH−
(7)
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. 5. 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 [15], CO(ad) + 2H• → H2 C = O(ad) ;
(8)
− H2 C = O(ad) + 6OH− → CO2− 3 + 4H2 O + 4e :
(9)
Comparison of Figs. 2 and 5 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 con=rmed the results found with cyclic voltammetry. Fig. 7 shows that no formate oxidation was observed at +300 mV on hydrogen-free palladium, whereas on H-loaded Pd the electric charge in the presence of formate in solution (3:2 C) doubles that transferred in its absence (1:6 C) and, furthermore, is signi=cantly higher than that used for hydrogen loading (2:0 C). The ine;ciencies
found between hydrogen charging and discharging cycles, ca. 20% in the experiments shown in Fig. 7, are due to molecular hydrogen production during galvanostatic charging, 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, with assistance from occluded hydrogen, can be proposed. In the absence of occluded hydrogen, CO(ad) formed through (7) progressively covers the surface, e9ectively poisoning the electrocatalyst for further oxidation. At low CO(ad) coverage, maintained at moderate positive potentials, adsorbed CO may oxidize to CO2 following the path depicted in (3) and (4), with overall anodic reaction in alkaline solution expressed as (6). In the presence of occluded hydrogen, H• reacts chemically with CO(ad) with formation of formaldehyde, with further electrooxidation according to (8) and (9), with overall anode reaction − HCOO− + 5OH− + 2H• → CO2− 3 + 4H2 O + 4e :
(10)
This mechanism is consistent with in situ spectroelectrochemical results reported elsewhere [15]. From (6) and (10), the ratio of molar charges involved in the electrochemical oxidation of formate in the presence of occluded hydrogen and in its absence is 2. Comparison of the currents measured during anodic oxidation of formate at −300 mV in the presence of occluded hydrogen with those in its absence, cf. Fig. 7, 2:44 C against 1:53 C respectively, yields a ratio of 1.6. This ratio is close to 2, even though not all the formaldehyde produced in (8) undergoes (9) [23].
5. Conclusion It has been shown that occluded hydrogen assists the electrooxidation of formate ions, by chemically reacting with, and therefore releasing the surface from, CO(ad) poison. The reaction between emerging occluded hydrogen and adsorbed CO regenerates surface sites for the further electrochemical oxidation of formate, and opens up new possibilities for overcoming the poisoning of electrode surfaces, and thus 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 Sim)on Bol)Qvar for discussions.
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