Effect of deactivation and reactivation of palladium anode catalyst on performance of direct formic acid fuel cell (DFAFC)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 7 1 9 e1 4 7 2 4

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Effect of deactivation and reactivation of palladium anode catalyst on performance of direct formic acid fuel cell (DFAFC) S.M. Baik a, Jonghee Han b, Jinsoo Kim a,*, Yongchai Kwon c,** a

Department of Chemical Engineering, Kyung Hee University, 1 Seocheon-dong Giheung-gu, Yongin, Gyeonggi-do 449-701, Republic of Korea Fuel Cell Research Center, KIST, 39-1 Hawolgok-dong, Seongbuk-gu, Seoul 130-650, Republic of Korea c Graduate School of Energy and Environment, Seoul National University of Science and Technology, 232 Gongneung ro, Nowon-gu, Seoul 139-743, Republic of Korea b

article info

abstract

Article history:

In the present study, degradation and recovery in cell performance of direct formic acid

Received 5 March 2011

fuel cells (DFAFCs) are investigated. For DFAFC tests, palladium (Pd) and platinum (Pt) are

Received in revised form

used as anode and cathode catalysts, respectively, and are applied to a Nafion membrane

16 April 2011

by catalyst-coated membrane (CCM) spraying. As multiple repeated DFAFC operations are

Accepted 21 April 2011

performed, the cell performance of DFAFC is steadily degraded. This behavior is ascribed to

Available online 25 September 2011

the electrooxidation of Pd into PdeOH, which occurs between 0.1 and 0.55 V. To investigate the dependency of the cell performance on the PdeOH and to evaluate how the cell

Keywords:

performance is regenerated, cyclic voltammetry (CV) tests are executed. In CV experiments

Direct formic acid fuel cell

where the voltages applied to the DFAFC single cell are lower than 0.7 V vs. DHE, the cell

Palladium deactivation

performance is further deactivated due to continuous production of PdeOH. Conversely, in

Palladium reactivation

CV experiments where the voltage is higher than 0.9 V vs. DHE, cell performance is reac-

Palladium electroredox reaction

tivated due to redox reactions of PdeOH into PdeO and PdeO into Pd. ATR-FTIR and XPS

ATR-FTIR

are used to confirm the transformations of Pd. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Recently, there has been significant demand for miniaturized fuel cell systems as battery replacements for consumer and military electronic devices. Direct formic acid fuel cells (DFAFCs), which use liquid formic acid to generate power, have great potential for use as fuel cell systems [1e3]. Indeed, formic acid has better edges than other fuel candidates such as methanol and hydrogen. For example, the electrochemical oxidation of formic acid is faster than that of methanol due to

a simple molecular structure of the formic acid with high open circuit voltage of formic acid. Also, the fuel crossover rate of formic acid is lower than that of methanol, making it possible to use highly concentrated formic acid solutions and thinner membranes. The toxicity of formic acid is also very low [2e7]. Compared to polymer electrolyte membrane fuel cells (PEMFCs), DFAFC can avoid potential danger of explosive hydrogen and save cost for additional expenditure such as hydrogen gas container. It has also higher gas-phase energy density than PEMFCs [8]. DFAFCs using formic acid fuel are

* Corresponding author. Tel.: þ82 31 201 2492; fax: þ82 31 204 8114. ** Corresponding author. Tel.: þ82 2 970 6805; fax: þ82 2 970 6011. E-mail addresses: [email protected] (J. Kim), [email protected] (Y. Kwon). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.181

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therefore being considered more proper baseline fuel cell system than direct methanol fuel cells (DMFCs) and PEMFCs. Although platinum (Pt) has been used as a main catalyst for DFAFCs [4,6,8,9], to date the cell performances of DFAFCs incorporating Pt do not satisfy market needs due to Pt poisoning by carbon monoxide (CO). The subsequent retardation of electrochemical reactions caused by the Pt poisoning of formic acid results in low efficiency and power density. Reaction (1) shows the main electrochemical reaction of formic acid under Pt. Pt þ HCOOH/PteCO þ H2 O/Pt þ CO2 þ 2Hþ þ 2e

(1)

Here, the CO intermediate is adsorbed strongly on Pt, obstructing its active sites and inhibiting further reactions, thereby limiting the activity of Pt [4e6]. To address this problem, it is necessary to develop catalysts capable of promoting the electrochemical reactions of formic acid, thereby enhancing fuel cell efficiency and power density. To this end, palladium (Pd) has been regarded as alternative catalyst [3,5,10]. Importantly, DFAFCs using Pd exhibited superior cell performance to their Pt counterparts [2,11]. The enhancement in performance of Pd-containing DFAFCs is due to the role Pd plays in expediting electrochemical reactions of formic acid. Specifically, formic acid is electrooxidated to produce carbon dioxide (CO2) through dehydrogenation under Pd (see reaction (2)). Pd þ HCOOH/Pd þ CO2 þ 2Hþ þ 2e

(2)

Since there is little chance of Pd deactivation by CO absorption, Pd typically exhibit better activity than Pt in DFAFCs [4,10e14]. Notwithstanding, it remains unclear what is really happening in between Pd and formic acid during DFAFC operation. Further, although cell performance of DFAFCs incorporating Pd is better than that of Pt, the former is prone to steady degradation in multiple-cycle polarization curve [11]. These results imply that the electrochemical reactions related to the Pd in DFAFCs may be controlled by other reactions, and not just the dehydrogenation of reaction (2). However, in spite of a critical role of the Pd on cell performance of DFAFCs, there has been little known about how anodic Pd catalyst affects the redox reaction of formic acid during operation of DFAFCs, although some research groups have been suggested the reaction models of formic acid occurring under the existence of Pd catalyst. Amid the reports, Zhou et al. [15] and Pan et al. [16] proposed that initial decomposition of formic acid (HCOOH) into formate (COOH) ion that was adsorbed on the surface of Pd. The COOH ion was then decomposed into CO2. Miyake et al. [17] reported that HCOOH and water molecule (H2O) included in the formic acid solution were adsorbed as COOH ion and hydroxyl (OH) ion on the surface of Pd, respectively. The Pd-adsorbed COOH and OH then reacted to produce CO2 and H2O. Zhou et al. [18] also published that HCOOH and H2O were adsorbed as CO and OH on the surface of Pd, respectively, producing CO2 as a result of subsequent reaction between the CO and OH. In the present study, we reveal a new Pd electrochemical reaction route that is different from the aforementioned reactions. The role of the proposed reactions was evaluated using surface analytical techniques such as attenuated total

reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) as well as electrochemical characterization like cyclic voltammetry (CV).

2.

Experimental

For cell testing, membrane electrode assemblies (MEAs) with active area of 9.0 cm2 were prepared by catalyst-coated membrane (CCM) spraying. Pd black catalyst (SigmaeAldrich, High Surface Area) and Pt black catalyst (Johnson Matthey, HiSPEC 1000) were used to prepare anode and cathode electrodes, respectively. Inks were made by dispersing the catalyst powders into a 5% recast Nafion solution (Solution Technology, Inc., 1100EW) in Millipore-purified water. The inks were then airsprayed onto both sides of Nafion 115 membrane and dried for 30 min at 70  C. The loadings of Pd and Pt were 8 and 7 mg cm2, respectively. A carbon cloth diffusion layer (GDL, E-Tek division, plane weave, 0.35 mm cloth thickness and 30% polytetrafluoroethylene (PTFE)) was added by hot-pressing performed for 1 min at 130  C with the pressure of 2500 kg in order to cover the catalyst layers coated on Nafion membrane [6]. The particle distribution and its group size of Pd and Pt catalysts coated on the Nafion membrane were measured by using sizer of z-potential and were visually inspected by using TEM. In case of the Pd, its particle distribution range was 60e160 nm and average particle group size was w96 nm, while the values of Pt were 70e200 nm and w100 nm, respectively. Fig. S.1 shows the sizer and TEM results of Pd and Pt. For the purpose of composing DFAFC single cells, the completed MEAs were then placed between two graphite plates, which have serpentine flow channels. After the DFAFC single cells were manufactured, they were installed at a fuel cell testing station (Fuel cell control system, CNL, Inc.). Prior to implementing the cell tests, all the DFAFC single cells undertook a pre-conditioning that was performed at a fixed voltage of 0.6 V for 2 h. During the pre-conditioning, excessive hydrogen (200 sccm) and air (800 sccm) gases were supplied to anode and cathode, respectively and the single cell current variation was continuously checked. It was confirmed that the full activation of each MEA was achieved within 2 h at the fixed voltage. For the operation, the cell temperature and humidified temperature were maintained as 70  C and 65  C, respectively. Cell polarization tests using formic acid and air were performed in the same fuel cell station used for pre-conditioning. 5 M formic acid (5 mL min1) was fed to anode, while air (800 sccm) was fed to cathode. For cyclic voltammogram (CV) tests, deionized (DI) water was fed to the anode (working electrode) at a constant feed rate (5 mL min1), while H2 (400 sccm) was fed to cathode. Here, the Pt/H2 combination in cathode of the cell acted as a DHE as well as a counter electrode. The tests were performed at 30  C at a scan rate of 5 mV s1. For scanning, the voltage range went from initial voltage of 0.3 V to predetermined maximum voltage. Voltage scans at voltage less than 0.3 V were not performed to avoid Pd catalyst that was broken due to hydrogen absorption [17]. For ATR-FTIR tests, an FTIR-6100 (Jasco, USA) was used. The wavelength range was 400e4000 cm1 and all spectra

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 4 7 1 9 e1 4 7 2 4

were recorded at a resolution of 4 cm1 with the number of scans per sample set at 200. For testing all samples using attenuated total reflectance mode, a diamond ATR accessory (Pike, USA) was attached to the FTIR-6100. XPS analyses were performed using a VG Scientific photoelectron spectrometer operated at 10 kV and 250 W, a base pressure of 2  1010 torr, and with Al Ka radiation (1486.6 eV) as the X-ray source.

3.

Results and discussion

Fig. 1 shows the cell polarization curves of the DFAFC. To investigate how the electrochemical reactions of anode Pd catalyst affected cell performance of DFAFC, the polarization curve were measured four times in a consecutive manner. As the multiple-cycle of polarization curve was performed, the cell performance of DFAFCs steadily degraded [19]. Although the dehydrogenation (reaction (2)) that occurs between Pd and formic acid is well known, its mechanism does not explain why cell performance of the DFAFC steadily degrades with the multiple-cycle of polarization curve. One of the most plausible explanations is the presence of water molecules in the formic acid solution [17,20,21]. According to some reports [22e25], Pd loaded on the anode side can react with water molecules to oxidize Pd into PdeOH and/or PdeO via the following reaction pathways [22e25]. Pd þ H2 O/PdeOH þ Hþ þ e

(3)

Pd þ H2 O/PdeO þ 2Hþ þ 2e

(4)

Here, the reaction (3) occurs at relatively lower voltages between 0.35 and 0.7 V vs. DHE, while reaction (4) occurs at a high voltage of w0.9 V vs. DHE [22e25]. In addition, cathode voltage of DFAFC is 0.8e0.9 V vs. DHE [26,27] and anode voltage

Fig. 1 e Cell polarization curves of DFAFC incorporating a Pd catalyst-deposited anode and a Pt catalyst-deposited cathode. Cell polarization tests were performed four times and were conducted at 30  C. For the tests, 5 M formic acid was fed to anode at a feed rate (5 mL minL1), while air (800 sccm) was fed to cathode. All fuel cell tests were conducted at 30  C.

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of DFAFC cannot reach higher than w0.6 V vs. DHE [26,27]. Therefore, when a single cell voltage is defined as the difference between cathode voltage vs. DHE and anode voltage vs. DHE, voltage ranges of reaction (3) and reaction (4) are roughly equivalent to single cell voltages of 0.1e0.55 V and w0 V, respectively. Taken together, the above data demonstrate that available single cell voltage for DFAFCs is 0.2e0.9 V, which means that reaction (3) is likely to occur, whereas there is little chance for reaction (4) to occur within the given single cell voltage range. To determine whether Pd is oxidized during the multiplecycle polarization curve and to evaluate its correlation with DFAFC single cell performance, ATR-FTIR and XPS analysis were performed, the results of which were shown in Figs. 2 and 3. In Fig. 2, three kinds of samples were prepared; the first sample that underwent the multiple-cycle polarization curve, the second sample that did not undergo any polarization curves and the third sample that underwent both the multiple-cycle polarization curve and CV. We denote the first sample as “deactivated sample”, the second sample as “fresh sample” and the third sample as “reactivated sample”, respectively. In the ATR-FTIR spectra of Fig. 2, three distinct differences between “deactivated sample” and “fresh sample” were observed. First, “fresh sample” was flat during IR scanning without observation of any concrete Pd reaction peaks except for the CO2 peak of ambient appeared at w2300 cm1 [28,29], indicating there were no specific chemical reactions between Pd and formic acid. Conversely, “deactivated sample” showed two distinct peaks around 3500 cm1 and 1630 cm1, with the former large peak indicative of OH stretching mode and the latter small peak indicative of HOH bending mode [30]. Appearance of the OH peak means creation of PdeOH, while

Fig. 2 e ATR-FTIR spectra of Pd catalysts prepared from “fresh sample”, “deactivated sample”, and “reactivated sample”. The chemical properties of the three samples were examined by measuring reflectance of the samples over wavenumbers of 400e4000 cmL1. All spectra were recorded at a resolution of 4 cmL1 and a total of 200 scans per sample were executed.

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possible for Pd to be oxidized into PdeCOOH by the following reaction: Pd þ HCOOH/PdeCOOH þ Hþ þ e

Fig. 3 e XPS spectra of Pd catalysts of the fresh sample and the deactivated sample, with peak positions for Pd 3P3/2 and O 1s energy levels.

the HOH peak was taken to represent existence of the water squeezed from Pd [31]. Oxidation of Pd into PdeOH could be explained by the reaction (3), where such PdeOH impeded further electrochemical reaction of formic acid [8]; i.e., Pd was deactivated as it became oxidized to PdeOH. It is interesting to note that if HOH mode exists at high frequency region, the peak should be shown at 3638 cm1 and 3734 cm1 [31,32]. However, because the peak appearing at Fig. 2 is obviously less than 3638 cm1, it is reasonable to say that the peak around 3500 cm1 stems from OH, not from HOH. Neither “fresh sample” nor “deactivated sample” showed any PdeCO peaks, which is different from the results of similar experiments conducted with Pt [31]. According to a previous study [18,31], if CO is adsorbed on Pt and Pd, two IR peaks are supposed to appear at 1875 cm1 and 2073 cm1. Here, the former peak is for COs adsorbed at top-site of catalyst (linear COs), while the latter peak is assigned as bridge COs on catalyst. As neither of these two peaks was identified in Fig. 2, these results mean that Pd is only oxidized into PdeOH during the single cell operation of DFAFCs. Because CO is strongly adsorbed on noble metal, if PdeCO peaks existed, the peak should have been detected by ATR-FTIR. In addition, “deactivated sample” did not exhibit any peaks associated with PdeCOOH. As described previously [17], it is

(5)

If reaction (5) occurs during the repeated polarization sweeps, an IR peak corresponding to PdeCOOHs should appear at 1325 cm1 [17]; however, according to our ATR-FTIR data, no such peak was observed, confirming that there was little PdeCOOH production during the operation of DFAFCs. As shown by XPS spectra (Fig. 3), it is obvious that reaction (3) is the main reaction that occurred during the repeated polarization sweeps. In XPS spectra of both “fresh sample” and “deactivated sample”, a broad peak existed because O 1s peak of these samples was positioned closely to Pd 3P3/2 peak. However, the maximum peak of “deactivated sample” was shifted to lower binding energy relative to that of “fresh sample”, indicating the presence of PdeOHs [33]. This result proved that PdeOH was formed in “deactivated sample”, while little PdeOH was formed in “fresh sample”. It is noticeable that analyses of Figs. 2 and 3 were carried out under ex-situ condition, indicating that both “fresh sample” and “deactivated sample” were exposed to the ambient before and after the analysis. Accordingly, such an exposure may result in oxidation of Pd that is not related to reaction with formic acid occurring during the operation of DFAFC, especially, in “fresh sample”. However, in evaluation of Fig. 2, the “fresh sample” did not show any concrete Pd oxidation peaks, exhibiting there was no specific Pd oxidation reaction with the ambient. In summary, in spite of the exposure of Pd to the ambient, degree of Pd oxidation reaction by the ambient did not reach the level to be detected by ATR-FTIR. To further investigate the dependence of DFAFC performance degradation on the presence of PdeOH and to evaluate how cell performance is regenerated, CV and polarization curve tests were performed in four steps. In step 1, the same multiple-cycle polarization curve as Fig. 1 was carried out. In step 2, a CV test with a scan range starting at initial voltage of 0.3 V vs. DHE to maximum voltage of 0.6 V vs. DHE was performed. In step 3, the polarization curve test was repeated. Lastly, in step 4, steps 2 and 3 were iterated by gradual increase of CV maximum voltage until cell performance of the DFAFC was regenerated [17,19,34]. Fig. 4 shows the results of all the CV tests corresponding to steps 2 and 4, while Fig. 5a shows all of the polarization curves corresponding to steps 1, 3, and 4. Fig. 5b shows the DFAFC cell performances evaluated at “before CV” and “after CV” by voltages measured at 0.35 A cm2 of the polarization curves of Fig. 5a. It is noticeable that the maximum power densities of all the DFAFC single cells are obtained at near 0.35 A cm2. Thus, we can consider voltage measured at 0.35 A cm2 as a representative value for exhibiting DFAFC single cell performance. When the maximum voltage of CV was equivalent to 0.7 V or less than 0.7 V, the DFAFC cell performances measured after the CVs were far lower than that measured before CV. However, when the maximum voltage of CV was 0.8 V, the DFAFC cell performances measured after CV started to approach that measured before CV. Lastly, when the maximum voltages of CV were 0.9 V and 1.0 V, the DFAFC cell performances measured after CV reached the same level as that measured before CV, meaning that the DFAFC cell

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Fig. 4 e Cyclic voltammograms (CVs). To determine the maximum voltage, voltage sweep proceeded in positive direction until reaching the maximum voltage, after which the voltage sweep went in negative direction. The maximum voltage increased from 0.6 V vs. DHE to 1.0 V vs. DHE in increment of 0.1 V. For the CV tests, DI water was introduced to anode at a flow rate of 5 mL minL1; H2 was introduced to cathode at a flow rate of 400 sccm to form a DHE.

performances was completely regenerated (or recovered). Overall, the trend observed with CV experiments was well matched with that of the polarization curves. The close correlation between CV and polarization curves was attributed to the electrochemical redox reactions of Pd. As explained above, the initial polarization curve of DFAFC single cell led to reaction (3), producing PdeOH. Prior to reaching the maximum voltage of 0.7 V vs. DHE, Pd reacted with water molecules to generate more PdeOH during positive voltage sweep, resulting in further degeneration of performance. However, at a maximum voltage of 0.8 V vs. DHE, redox peaks began to appear in both positive and negative voltage sweeps, which were caused by the following reactions [22e25]. Oxidation : PdeOH/PdeO þ Hþ þ e

(6)

Reduction : PdeO þ 2Hþ þ 2e /Pd þ H2 O

(7)

According to the literatures, reaction (6) occurs at w0.9 V vs. DHE while reaction (7) occurs at w0.8 V vs. DHE [22e25]. A relationship between the CV experiments including electroredox reactions (reactions (6) and (7)) and the polarization curves can be explained as follows. When the maximum voltage is 0.8 V vs. DHE, reactions (6) and (7) begin to occur, partly regenerating activity of Pd in the DFAFC single cell. However, because reaction (6) is not completed, performance of the DFAFC is not fully recovered. Instead, at a maximum voltage of 0.9 V vs. DHE, reactions (6) and (7) occur vigorously, regenerating performance of DFAFC completely. Furthermore, even when the maximum voltage extends to 1.0 V vs. DHE, performance of DFAFC remains regenerated, indicating that reactions (6) and (7) still make an important role in regenerating activity of Pd and performance of DFAFC.

Fig. 5 e (a) DFAFC cell polarization curves. During the tests, the following four step process was executed: Step 1, the same polarization curves as Fig. 1. Step 2, initial CV of Fig. 4. Step 3, polarization curve. Step 4, repetition of step 2 and step 3. (b) Voltages measured at 0.35 A cmL2 of all the polarization curves.

To verify the effect of electrochemical reactions (reactions (6) and (7)) on DFAFC cell performance, ATR-FTIR analysis was performed for both “deactivated sample” and “reactivated sample”. In “reactivated sample”, no peak was observed, confirming that electroredox reactions occurred properly during CV and the deactivated Pd was well regenerated after performing CV.

4.

Conclusions

In this study, we investigate degradation and recovery in cell performance of DFAFCs when Pd and Pt are used as anode and cathode catalysts, respectively. As multiple-cycle DFAFC polarization curves are performed, the DFAFC cell performance is steadily degraded. That is attributed to the Pd deactivation by oxidation of Pd into PdeOH that takes place in between 0.1 and 0.55 V during operation of the polarization curve. In CV experiments performed to further evaluate the effect of DFAFC cell performance on the PdeOH, when the

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maximum voltages applied to the DFAFC single cell are 0.6 V vs. DHE and 0.7 V vs. DHE, the DFAFC cell performance is continuously deactivated due to continuous production of the PdeOH. On the contrary, when the maximum voltages are equivalent to and higher than 0.9 V vs. DHE, the DFAFC cell performance is reactivated because of revival in activity of the deactivated Pd by occurring of redox reactions - PdeOH into PdeO and PdeO into Pd. ATR-FTIR and XPS measurements are implemented to verify the transformations of Pd. The analyses validate the effects of redox reactions of Pd on DFAFC cell performance and catalytic activity of Pd.

Appendix. Supplementary material Supplementary data related to this article can be found online at doi:10.1016/j.ijhydene.2011.04.181.

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