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PEMFC Cathode Catalyst Contamination Evaluation with a RRDE- Acetylene
Electrochimica Acta 133 (2014) 65–72
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
PEMFC Cathode Catalyst Contamination Evaluation with a RRDE- Acetylene Junjie Ge ∗ , Jean St-Pierre, Yunfeng Zhai Hawaii Natural Energy Institute, University of Hawaii- Manoa, 1680 East-West Road, POST 109, Honolulu, HI 96822, USA
a r t i c l e
i n f o
Article history: Received 5 February 2014 Received in revised form 21 March 2014 Accepted 5 April 2014 Available online 16 April 2014 Keywords: Acetylene ORR RRDE Airborne Contaminant PEMFC
a b s t r a c t The effect of C2 H2 on the oxygen reduction reaction (ORR) for a commercial Pt/C catalyst was investigated using rotating ring disk electrode (RRDE) technique in acidic solution. This study was undertaken to provide insight into the mechanism of C2 H2 contamination of the cathodes in proton exchange membrane fuel cells. The cyclic voltammetry results show a high C2 H2 coverage on the Pt surface and an almost complete loss of the electrochemical surface area in the presence of 0.14 mM C2 H2 . The RRDE was used to measure the ORR polarization curves and H2 O2 production in air. The introduction of C2 H2 shifts the ORR onset potential in the negative direction by 330 mV, and no limiting current can be observed in the potential scan window. The significant retardation of the ORR is associated with the complete loss of the ECSA, as the adsorption of C2 H2 on the Pt sites results in the inhibition of both HUPD and O2 adsorption. Furthermore, it is proposed that C2 H2 adsorption also has an impact on the adsorption configuration of O2 molecules; the Pauling configuration prevails due to the spatial limitations imposed by the presence of adsorbed C2 H2 on Pt. As a result, both the ring-disk and Koutecky-Levich measurements show a shift in the reaction pathway from a 4- to a 2-electron reduction: the H2 O2 production increases and the charge transfer number decreases. The ORR rate determining step is observed to be shifted from the first electron transfer to other possible steps. This change is confirmed by the Tafel slope measurement, which increases significantly and is most likely due to the changes in the adsorption energy of O2 . Nearly complete recovery of the performance is attainable by stopping the C2 H2 exposure. The unrecovered performance is attributed to the remaining surface adsorbates. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Because the development of renewable energy and cleaner transportation are high on the world’s priority list, proton exchange membrane fuel cells (PEMFCs) have been drawing much attention due to their low operating temperatures, short start-up times, high efficiencies, and zero emissions [1–4]. However, PEMFC performance still suffers in the presence of contaminants. For example, PEMFCs use ambient air that contains oxygen as the oxidant and, therefore, the fuel cell exposure risk to a multitude of air contaminants is significant [5]. As the cathodic catalyst, Pt/C is highly active and has an affinity for a large range of inorganic and organic substances. Several inorganic species were investigated (SO2 , NO2 ,
∗ Corresponding author. Hawaii Natural Energy Institute, University of HawaiiManoa, 1680 East-West Road, POST 109, Honolulu, HI 96822, USA Tel.: +1 808 593 1714; fax: +1 808 593 1719. E-mail addresses: [email protected], [email protected] (J. Ge), [email protected] (J. St-Pierre), [email protected] (Y. Zhai). http://dx.doi.org/10.1016/j.electacta.2014.04.005 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
NH3 , and Cl− ), and the deleterious effects on the PEMFC performance were confirmed [3,6–8]. However, little attention has been paid to the poisoning effects of organic molecules on the Pt/C catalysts towards ORR [4,9]. In our research group, 7 organic airborne contaminants were down selected from 260 airborne contaminants using a two-tier selection [10]. The 7 contaminants covered several functional groups, including alcohol (isopropanol), N-containing compound (acetonitrile), alkene (propene), alkyne (acetylene), ester (methyl methacrylate), aromatic ring (naphthalene), and halide (bromomethane). According to the in-situ results, all seven compounds have detrimental effects on the performance of the PEMFC [10]. However, the mechanisms of these effects have not yet been explored. Therefore, understanding these contamination mechanisms is important for both fundamental studies and practical applications. Acetylene (C2 H2 ) is a contaminant of interest because it is widely used as a welding fuel and chemical intermediate; it is considered a potential fuel cell contaminant due to its reaction with Pt. It also allows the effect of the C-C triple bond on PEMFC performance to be studied. The chemical/electrochemical adsorption of
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C2 H2 on Pt and its subsequent reaction have been studied extensively as Pt-based catalysts have long been used for the removal of unburned hydrocarbons from exhaust streams [11,12]. The adsorption of C2 H2 on platinum is a nondissociative process that occurs through the partial break of the triple bond in the molecule and its direct interaction with the metal surface. The number of sites blocked by each adsorbed C2 H2 is found to be approximately 2.0 when the C-C axis is parallel to the surface and the C-C plane is tilted relative to the surface normal [13]. The C2 H2 -Metal bonding includes its donation to the surface and surface back-donation to acetylene *, giving rise to ethylenic like -bonding orbitals between acetylene and the surface. Acetylene displaces adsorbed water (2.1 water molecules per acetylene molecule) from the platinum electrode due to its stronger bonding, which results from the additional back-bonding [14,15]. For the electrochemical reaction on the Pt surface, it has been found that at potentials lower than 0.2 V (vs. the normal hydrogen electrode), C2 H2 is reduced to ethylene and ethane. At potentials greater than 0.35 V, it is oxidized to carbon dioxide with a current maximum at 1.18 V [13]. The rate determining step for C2 H2 oxidation was proposed to be the first deprotonation of the adsorbed acetylene with cooperation from adsorbed OH [12,16]. In addition, the C–C donation to the Pt(111) surface from acetylene increases the C–H bond strength, thus limiting oxydehydrogenation, which means that the C–H bond is less reactive in alkynes than in alkenes. The above literature results can provide insight into the probable influence of C2 H2 on the ORR, in which a retarding effect is expected due to the strong adsorption of C2 H2 on the Pt surface and its competition with O2 adsorption [11]. However, the electrochemical behavior of O2 in the presence of C2 H2 is still unknown. In this paper, we investigated the influence of C2 H2 on the ORR reaction using a RRDE to provide mechanistic information about the performance loss of PEMFC in the presence of C2 H2 . The potentiodynamic measurements from the disk allow us to evaluate the coverage and redox information about C2 H2 and its influence on ORR kinetic loss. The collected ring current and the Koutecky-Levich plot provide information about the change in the ORR pathway. Using this approach, we can determine if the adsorption of C2 H2 changes the mechanism of ORR from a 4-electron pathway to 2-electron pathway.
2. Experiments A thin film electrode made from a Pt/C catalyst was used as the working electrode and prepared as follows: The commercially available 46.6 wt.% Pt/C (Ion Power) catalysts were used for all the electrochemical tests. A stock solution (100 mL) was prepared in advance by mixing 79.8 mL DI water, 20 mL IPA, and 0.2 mL of a 10.07 wt.% Nafion solution. Then, 15 mg of the Pt/C catalyst was placed in a 20 mL glass vial and 15 mL of the stock solution was added; the ink was mixed thoroughly using a bath sonicator (VWR, MODEL 50D) for 1 h, stirred overnight, and then sonicated again for 45-60 min before use. The bath temperature was kept at < 40 ◦ C during sonication. A ring-disk electrode with a glassy carbon inner disk (Ø = 4 mm, A = 0.126 cm2 ) and a polycrystalline Pt (ID = 5 mm, OD = 7 mm) outer ring was used for each test. The collection efficiency of the ring was measured in 2 mmol/L K3 [Fe(CN)6 ] in a 0.1 mol/L KNO3 solution, the collection efficiency was 0.428. The ring-disk electrode was polished for 4 min to a mirror finish using a 0.05 m alumina-particle suspension (ALS) on a moistened polishing cloth before each use. The polished electrodes were rinsed well with nanopure water and mounted to the electrode rotator (AlS RRDE-3A). The rotational drying method, developed by Garsany et al. [17], was used to prepare the thin film, acquire a uniform dispersion of the catalyst, and minimize the coffee-ring
structure. While the bare GC electrode was rotating at 800 rpm, a 5.41 L aliquot of the well-dispersed ink (Pt loading at 20 gPt cm−2 ) was pipetted onto the glassy carbon electrode. The electrode was held at 800 rpm for 20 min to ensure that the film was completely dry. The electrode was unloaded and rinsed gently with DI water to ensure the removal of gas bubbles on the electrode surface. The resulting thin film was used as the working electrode for the ORR in 0.1 M HClO4 solution. A spiral Pt wire and an Ag/AgCl/NaCl (3 M) were used as the counter and reference electrodes, respectively. The reference electrode was separated from the testing solution by two frits to avoid Cl− contamination. The potentials are reported versus the reversible hydrogen electrode (RHE). The electrochemical measurements were performed using Biologic VSP Bipotentiostat. An iR-drop correction of the measured potentials was performed by determining the solution resistance (R) using electrochemical impedance spectroscopy (EIS) at 0.57 V vs. RHE. The typical value of R is 28-31 ohm. An adjustment of 85% of the iR correction was made according to the manual from Biologic because a positive feedback was performed in the system and too much correction would result in system instability. During the tests, the largest current flow was approximately 0.25 mA; therefore, the uncompensated voltage had a maximum of approximately 1 mV. The testing temperature was maintained at 30 ◦ C using a water-jacket cell connected to a chiller. The chiller was operated for 20 min prior to the test. The RRDE was immersed in an electrochemical cell containing 65 mL of an HClO4 solution saturated with N2 (Ultra Zero N2 , 99.998%, Air Gas, purged for at least 20 min). The disk potential was cycled 50 times between 0.03 - 1.3 V vs RHE at 500 mV s−1 to remove any contaminants adsorbed on the surface, and the ring potential was cycled 100 times between 0.03-1.4 V at 500 mV s−1 . For the ORR tests, the measurements were conducted in an electrolyte saturated with air (prepurified, Air gas). The effect of C2 H2 on both the CV and ORR were tested. For C2 H2 , 4040 ppm in N2 and 4030 ppm in air were used for the measurements. These gases were previously used for in situ studies. Although the concentration of acetylene in air is ∼0.06 ppm, the local and transient concentration may be higher due to its use as a welding fuel and a chemical manufacture precursor. As this report constitutes the first study of the acetylene effect on the ORR activity using a RRDE, the present work employed a larger concentration than in ambient air to ensure the presence of an effect and facilitate the determination of the ORR activity loss mechanism, including the loss in the ECSA, MA due to the coverage effect, the shift in reaction pathway, the change in RDS, and the recovery effect of the catalyst. The solubility of the gases in the electrolyte is calculated using Henry’s law, shown in Equation 1: c(M) = kH ( kH =
M ) × p(atm) atm
ca = kH × exp pg
(1)
− H 1 sol R
T
−
1 T
(2)
The kH value for C2 H2 at 30 ◦ C is 0.0353 M/atm, which was taken from the CRC handbook and calculated using Equation 2. The impurity gas bubble rate was kept at 60 mL/min. The solubility of C2 H2 was calculated to be 0.14 mM at 30 ◦ C. For 65 mL of electrolyte, the estimated time needed for the saturation of the solution was 13 min for C2 H2 if all of the gas that passed through the solution was absorbed. Therefore, C2 H2 were bubbled for 0.5 h before the CV and ORR tests to ensure saturation of the solution and electrode surface. With the impurity gas bubbling at 60 mL/min, the C2 H2 consumption rate versus the replenishment rate is also calculated to determine if the electrolyte can be kept at saturation during the tests. The fastest consumption was determined to be during the CV collected at a fixed rate of 50 mV/s over the potential range from 0.03-1.5 V. Using Faraday’s law, the consumption rate
J. Ge et al. / Electrochimica Acta 133 (2014) 65–72
was calculated to be 2.94*10−9 mole/cycle. The replenishment rate for C2 H2 , however, was 8.6*10−6 mole/cycle, which was enough to maintain the saturation of the electrolyte during the test. For each test, including both CV and ORR, a new film was prepared. The ORR curves were corrected from a baseline CV where the CV scans were collected in N2 -saturated clean or contaminated cells, accordingly. 3. Results and discussions Fig. 1 shows the CV results collected in 0.1 M HClO4 with or without C2 H2 bubbling in the solution. Acetylene at 4040 ppm in N2 was tested and was equivalent to 0.14 mM dissolved in the solution. In the absence of C2 H2 , well-defined under potential deposition (UPD) of Hads is observed below 0.4 V. The electrochemical surface area of the catalyst can be calculated using Equation 3, after correcting for the double layer charging, and the hydrogen desorption area are evaluated from Fig. 1a. The correction is performed by deducing the current at 0.40 V from the total current and assuming that the capacitance is constant at different potentials. The charge of full coverage for clean polycrystalline Pt is 210 C cm−2 , and is used as the conversion factor. The Pt electrochemical surface area (ECSAPt ) is reported in m2 gPt −1 ; Q represents the HUPD charges over the range from 0.05 to 0.4 V, LPt is the working electrode Pt loading (mgPt cm−2 ) and Ag (cm2 ) is the geometric surface area of the glassy carbon electrode (i.e., 0.126 cm2 ) [18]. −1 ECSApt (m2 gPt )=[
QH−UPD (C) 210C cm−2 L (mgPt cm−2 )Ag (cm2 ) Pt Pt
]105
(3)
The positive scan was used for the measurement of the ECSA; the resulting value is 76.91 m2 gPt −1 . At potentials greater than 0.6 V, OH adsorption occurs, which is followed by the gradual formation of PtO as the potential increases. The behavior of the Pt/C electrode in the presence of 0.14 mM C2 H2 differs greatly from the clean 0.1 M HClO4 solution. From Fig. 1a, it can be observed that the hydrogen desorption peak is completely suppressed in the first positive scan, indicating the complete loss of the Pt surface area due to the C2 H2 adsorption. The onset of C2 H2 oxidation begins at ∼0.6 V and increases slowly as the potential increases to higher values. The adsorption of oxygen species is suppressed by the C2 H2 . In the negative scan, the lack of a Pt reduction peak indicates that there is a high C2 H2 coverage on the Pt surface. At lower potential values (E < 0.25 V) in the HUPD region, a sharp increase in the reduction current is observed, which is associated with the reduction of C2 H2 to ethylene and ethane, coupling with HUPD adsorption [13]. In the following cycles, the CV curves reach a steady state and the ECSA value increases to 15.55 m2 gPt −1 , 19.4% of the initial ECSA. The onset of C2 H2 oxidation moves negatively to 0.4 V due to the reduced coverage of C2 H2 and the increase in coverage of water, a promoter for the C2 H2 oxidation [12,16]. Fig. 1b shows the CV curves with/without C2 H2 scanned over an extended potential region, i.e., 0.03-1.5 V. The oxidation peak for C2 H2 is located at approximately 1.18 V, a typical oxidation peak for hydrocarbons on Pt [13], which suggests that the typical potential region where ORR is measured, i.e., the potential region shown in Fig. 1a, is not sufficient for the complete oxidation of C2 H2 . The lower oxidation peak values in the following cycles show that the readsorption of C2 H2 occurs during cycling, although not as fast as the potential cycling speed. As both the oxidation of C2 H2 and the readsorption occur during potential cycling, the effect of the electrode rotation on the measured ECSA needs to be addressed because the electrode rotation would exert an influence on the mass transfer of C2 H2 as well as O2 . Fig. 2a shows the CV results as a function of the rotation rates and cycling conditions are same as that in the following ORR measurements. Fig. 2b shows the enlarged area for the C2 H2
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reduction and the HUPD desorption. The reduction current increases with increases in the electrode rotation speed, suggesting faster C2 H2 transport and adsorption onto the catalyst surface. As a results, the coverage of the catalyst greatly increases as the ECSA decreases from 15.55 m2 gPt −1 in the static electrolyte to ∼0 at rotational rates >400 rpm. Therefore, the ECSA of the electrode during the ORR measurements should also be approximately 0, due to the facilitated readsorption of C2 H2 under rotation. Fig. 3a shows the impact of the 0.14 mM C2 H2 on the oxygen reduction activity of the Pt/C working electrode. The initial ORR polarization curve was recorded for the same working electrode in a clean air-saturated 0.10 M HClO4 electrolyte solution prior to the poisoning step. The initial ORR curve has a well-defined diffusion limiting current of 1.28 mA cm−2 (geometric) from 0.100.80 V, followed by a region with mixed kinetic-diffusion control from 0.80-1.00 V. In the presence of C2 H2 , the ORR onset potential shifts in the negative direction by 330 mV and no limiting current can be observed in the potential window, suggesting significant retardation of the ORR kinetics. The mass activity (MA) at 0.9 V is widely used as a comparison criterion for the activity of catalysts. From the polarization curve, the kinetic current can be extracted from the mixed kinetic-diffusion region through the mass transfer correction shown in Equation 4 [19]: 1/iK = 1/i − 1/il,c
(4)
where iK represents the mass transfer corrected current (kinetic current), i represents the current on the curve and il,c represents the limiting current. The MA value for the clean electrode is 0.114 A mgPt −1 , while no mass activity can be extracted from the C2 H2 adsorbed Pt surface and the ORR starts at 0.7 V. The complete loss of the MA at 0.9 V is consistent with the ECSA measurement because the entire surface is covered by the C2 H2 while the electrode is rotating. Fig. 3b shows the currents collected on the ring, which represent the amount of the ORR that occurs through the 2 electron reduction pathway with H2 O2 as the final product on the disk. On the clean Pt/C electrode, a minimal amount of current is observed at potentials greater than 0.4 V. At more negative potentials, the ring current increases to larger values due to the adsorption of Hads on the disk, which leads to an increase in the production of H2 O2 . Compared to the clean electrode, the H2 O2 oxidation currents on the C2 H2 contaminated Pt/C greatly increase for E < 0.7 V, and the peak current is located at ∼0.25 V, showing a significant contribution from the 2-electron reduction pathway in the contaminated cell. At E < 0.25 V, the currents start to decrease to lower values, indicating a decrease in production of H2 O2 on the disk. A decrease in the ring current can be observed with increasing cycle number, which can be attributed to the slight change in the surface properties during cycling. To confirm that the ring current is mainly due to H2 O2 , rather than the oxidation of C2 H2 , the ring current is also collected with the disk potential held at OCV. As shown in the inset of Fig. 3b, the ring current (<2 A) observed during the measurement can be attributed to the oxidation of C2 H2 , which is rather low compared to the values that can be observed during potential cycling of the disk. Due to the contribution from the oxidation of C2 H2 during potential cycling, an even lower ring current is expected to be collected because the solution reaches the disk first and subsequently spreads to the ring. As a result, it is confirmed that the collected ring current is mainly due to H2 O2 oxidation. For the shift in the reaction pathway of ORR, i.e., from a 4e reduction mechanism (forming water) to a 2e reduction (forming H2 O2 ), the percentage of the ORR that takes place through the H2 O2 pathway can be calculated using Equation 5 [4]: X H2 O2 [%] = 100 ×
IR N I ID + NR
2×
(5)
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Fig. 1. CV scans in the initial clean cell and the cell bubbled with 4040 ppm C2 H2 : a) over the range 0.05-1.03 V vs RHE at 20 mV/s for 10 cycles; and b) over the range 0.03-1.5 V vs RHE at 50 mV/s for 10 cycles. The 1st , 5th , and 10th cycles are shown in the figures.
Fig. 2. CV scans under different rotation speeds in the cell bubbled with 4040 ppm C2H2 over the range 0.05-1.03 V vs RHE at 20 mV/s. a) the complete CV plots, and b) the enlarged HUPD area for clear observation.
IR and ID are the ring and disk currents, respectively, and N is the collection efficiency of the RRDE (0.428). Fig. 4a shows the contribution of the 2-electron reduction pathway to the ORR. For the clean cell, less than 1% of the ORR occurs through the 2electron pathway at E > 0.3 V. Due to the interference of the HUPD
at lower potentials (E < 0.3 V), the amount of H2 O2 that is generated increases to 10% at 0.05 V. On the C2 H2 contaminated surface, however, a significant amount of H2 O2 is generated. At 0.25-0.55 V, approximately 80% of the ORR occurs through the 2-electron reduction pathway, with H2 O2 as the product; at potentials below 0.25 V,
Fig. 3. ORR scans in the initial clean cell and the cell with C2 H2 in air at 30 ◦ C: a) disk current measured at 20 mV/s for 10 cycles in the positive scan direction; b) ring current measured with potential fixed at 1.2 V during cycling. The inset in Fig. 3b shows the ring current measured with the potential fixed at 1.2 V in the presence of C2 H2 and the disk held at the OCV.
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Fig. 4. a) The percentage of the ORR that occurs through the H2 O2 pathway. The potential axis corresponds to the potential on the disk, and b) the mechanism that causes the change in the ORR reaction pathway.
the fraction of H2 O2 that is generated decreases as the potential decrease, from approximately 80% to 40%. The change in the reaction pathway can be explained by the following mechanism: The adsorption of O2 molecules on Pt occurs either through a dual adsorption (Griffiths model/bridge model) or an end-on adsorption (Pauling model), as shown in Fig. 4b [20]. Using tight-binding extended Huckel calculations, it was suggested that the chemisorption of O2 is more favorable at the two-fold bridge sites than the end-on sites [21]. This stabilization is due to the better overlap of the O2 2u orbital with the Pt surface. The dual adsorption sites are likely to be involved in the breaking of the O-O bond and, therefore, lead to the 4-electron reduction. The end-on adsorption model, however, is more likely to lead to the formation of H2 O2 as the product due to the single adsorption [20]. On the clean Pt surfaces, the reduction of O2 occurs exclusively through the 4-electron pathway before in the HUPD occurs, due to the energy preference. On the contaminated surface, the preferential adsorption of O2 in the end-on configuration is a possible scenario due to the high coverage of C2 H2 , which significantly reduces the availability of active and contiguous Pt sites. Consequently, more H2 O2 is produced in the 0.25-0.55 V range. At E < 0.25 V, however, the reduction of C2 H2 occurs, which leads to the availability of more Pt active sites for the adsorption of HUPD and dual adsorbed O2 . The effect of HUPD on the ORR pathway is significantly less than C2 H2 , as can be observed in Fig. 3b. As a result, more ORR occurs through 4-electron reduction, producing H2 O, rather than H2 O2 as the final product. To confirm that the ring current is mainly from H2 O2 oxidation and not C2 H2 or its residues, the charge transfer number is calculated from the Koutecky-Levich plot using Equation 6, and the results are compared with the ring disk current [22]. 1 1 1 + = 2 1 1 i ik 0.62nFAD 3 ω 2 v− 6 C ◦
(6)
n = 4ID /(ID + IR /N)
(7)
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The ORR polarization curves in the absence/presence of C2 H2 are shown in Fig. 5 a) and b) at various rotation speeds, respectively. From the polarization curves, plots of i−1 versus −1/2 at various potentials can be extracted, as shown in Fig. 5c, leading to straight lines with intercepts corresponding to the reciprocal of the true kinetic currents and the slope to the so called B-factor [22]. The B-factor allows one to assess the number of electrons involved in the oxygen reduction reaction. In Equation 6, i is the measured electrode current density, n represents the charge transfer number, F is the Faraday constant (96,485 C mol−1 ), A is the geometric surface area of the RDE, Cb is the bulk concentration of O2 (1.26 × 10−3 × 0.21 M)[22], D0 is the diffusion coefficient of O2 (1.93 × 10−5 cm2 S−1 ), v is the kinematic viscosity of the solution (0.01 cm2 S−1 ) and is the angular rotation rate of the electrode expressed as radian per second (=2f/60, where f is the rotation in rpm) [3]. Thus, the apparent number of electrons exchanged per O2 molecule in the overall cathodic reaction can be calculated. The calculated result for n in the clean cell is 3.93, as shown in Fig. 5d, in excellent agreement with the theoretical value for the 4-electron process. For the C2 H2 contaminated cell, the charge transfer number increases as the potential decreases, from 2.26 at 0.4 V to 3.09 at 0.2 V, indicating a change in the ORR reaction pathway from the 2-electron reaction to the 4- electron reduction at lower potentials due to the easier desorption of the reduced product of C2 H2 . On the other hand, the disk-ring current relationship can also be used for the calculation of charge transfer number (Equation 7) [4] and the results are shown in Fig. 5d. The Koutecky-Levich plot and the ring disk evaluation are in excellent agreement, which confirms the decrease in total charge transfer number due to the adsorption of C2 H2 and the shift in reaction pathway to the 2-elctron reduction. At lower potential values, the ORR shifts back to the 4-electron reduction due to the reduction of C2 H2 and the consequent increase in the availability of contiguous Pt active sites. From the inset in Fig. 5b, another interesting finding can be noticed from the change in the ring current at different rotation speeds. As the rotation speed increases, the ring current increases to higher values due to the faster transference of O2 to the disk electrode. At the same time, the peak potential on the ring shifts to the lower values, i.e., from 0.31 V at 400 rpm to 0.19 V at 3600 rpm. The shift in peak potential indicates that at higher electrode rotation speed, the adsorption of the C2 H2 occurs faster, which leads to a higher surface coverage of C2 H2 and increases the barrier to hydrogenation. As a result, the H2 O2 production peak potential shifts in the negative direction due to the insufficient removal of the C2 H2 from Pt surface at higher potential values. Shown in Fig. 6 is the mass-transport corrected Tafel plot of potentiodynamic measurements. The correction is made using Equation 4. On the original ORR polarization curve, the diffusion limiting current is taken directly from the polarization curve. For the ORR in C2 H2 , the diffusion limiting current is calculated from the Levich plot, and a charge transfer number of 2.5 (extracted from the ring-disk current measurement) is used. The il,c is calculated to be 0.76 mA cm−2 . The current range from 0.2 to 0.8 il,c is selected for the Tafel slope evaluation to ensure the accuracy of the RDE mass-transport corrections. A slope of 61 mV/dec is observed for the initial ORR, consistent with the results in the literature [22,23], and the rate determining step is confirmed as the first electron transfer step after O2 adsorption, as shown in Equation 8 and 9. O2 + Pt = Pt(O2 )ads −
Pt(O2 )ads + e =
PT (O2− )ads
(8) (9)
After the adsorption of C2 H2 onto the electrode, the Tafel curve has two distinct slopes with values of 179 mV/dec above 0.41 V and 274 mV/dec below 0.41 V. The Tafel curve shifted towards a much lower potential, from 0.93 V to 0.39 V at 1 mA/cm−2 , indicating a
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Fig. 5. a) ORR scans at different rotation speeds in air at 30 ◦ C and 20 mV/s; b) ORR scans in air with the presence of 4030 ppm C2 H2 at 30 ◦ C and 20 mV/s. The inset Fig. shows the corresponding ring currents collected at 1.2 V; c) the corresponding Koutecky-Levich plots from the polarization curves; and d) the calculated charge transfer number of ORR.
decrease of 8-9 orders (using the 61 mV/dec for the calculation) in the catalytic activity of the Pt/C catalysts towards to the ORR. The large change in the Tafel slope is attributed to a change in the rate determining step due to the high coverage of C2 H2. Fig. 7 shows the results of the attempts to recover the performance of Pt/C electrodes contaminated by C2 H2 . A simple DI water rinsing process was used to remove the adsorbed C2 H2 on the surface. During the electrode transfer and rinsing process, special care
Fig. 6. The Tafel plots for the ORR.
was taken to make sure the electrode was always covered with a drop of water, which prevented the electrode from direct contact with air. Fig. 7a shows the polarization curve of the recovered electrode in comparison to the initial ORR curve. A 91% recovery for mass activity was measured at 0.9 V vs RHE (0.121 A mg−1 Pt for the recovery electrode and 0.133 A mg−1 Pt for the initial electrode) for the recovered electrode, indicating the effectiveness of the water rinsing process in the removal of C2 H2 . A similar recovery effect was observed from our in-situ fuel cell test results [24], in which the cell performance can be recovered by simply stopping the C2 H2 injection. The almost fully recovered current from the ring also indicate the shift of the ORR back to the 4-electron reduction pathway. The inset of Fig. 7a shows the CV results of the initial electrode and the electrode after the recovery of the ORR. Both the ECSA and the Pt oxide formation/reduction results show the recovery of the active surface area through rinsing. From the calculation of the HUPD area, the ECSA recovers to 92% of the original value; this result is fairly consistent with the mass activity results from the ORR measurements. Therefore, it is safe to conclude that the loss of the mass activity value is caused by the loss of ECSA. Extended CV cycling was performed on the disk to look into the effect of the potential on the ECSA recovery. Fig. 7c shows the CV results from cycling over the range 0.03-1.2 V vs RHE. Through the continuous cycling, the ECSA value recoveries to 97% of its original value after the 10th cycle and comes to a steady state; the recovery effect can be ascribed to the removal of the remaining C2 H2 oxidation residues on the electrode surface. Over a wider potential cycling window, as shown in
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Fig. 7. a) The recovery effect of the ORR polarization curve on the disk at 30 ◦ C and 20 mV/s for 10 cycles in air. The inset in Fig. 7a shows the CV recovery effect after the ORR measurement in N2 ; b) the ring current with potential fixed at 1.2 V; c) The CV recovery effect after the ORR measurements from 0.03-1.2 V at 50 mV/s for 10 cycles in N2 ; and d) The CV recovery effect after the ORR measurements from 0.03-1.5 V at 50 mV/s for 10 cycles in N2 .
Fig. 7d, however, no further recovery is observed after cycling; the ECSA remained at 97% of its original value. The recovery of the surface area from 92% to 97% of its original value can be ascribed to the removal of the residuals of C2 H2 , where the rinsing process and potential cycling below 1.03 V is not sufficient for its removal. 4. Conclusion The influence of C2 H2 on the kinetics and reaction mechanisms of the ORR was investigated using a RRDE method. It was found that at a concentration of 14 mM C2 H2 in the electrolyte, the ECSA was completely lost due to the high C2 H2 coverage. As a result, the completely blocked Pt surface showed a significantly retarded ORR performance and the onset potential shifted in the negative direction by 330 mV. The C2 H2 not only blocks the active sites on Pt, thereby decreasing the kinetic of ORR, it also leads to a change in the ORR reaction pathway. Both the ring-disk and the KouteckyLevich measurements show a shift in reaction pathway from a 4to a 2-electron reduction; the H2 O2 production increases and the charge transfer number decreases. This situation is attributed to the presence of adsorbed C2 H2 , which significantly reduces the availability of active and contiguous Pt sites and promotes O2 adsorption in the end-on configuration. Due to the C2 H2 adsorption, which may change the adsorption energy of O2 , the ORR rate determining step shifted from the first electron transfer to a different step, as confirmed by the Tafel slope measurement. An almost complete performance recovery is obtained by removing the C2 H2 exposure,
and the performance that could not be recovered is ascribed to surface residues. Further efforts are needed for a complete mechanistic understanding of the C2 H2 influence on the PEMFC performance. First, the O2 adsorbate configuration in the presence of C2 H2 should be ascertained to validate the proposed mechanism. Second, the RDS of the ORR is left undetermined in the presence of C2 H2 . Third, the leftover residues, which need to be removed from the Pt surface to fully recover, require an oxidation potential greater than 1 V. Such a condition is rather challenging for an in-situ PEMFC cell, as the voltage is often kept lower than 1 V and a higher voltage is detrimental to the cell. Therefore, other recovery effects need to be considered. Fourth, the fact that more H2 O2 is produced exerts an undetermined influence from the C2 H2 on the durability of the MEA because H2 O2 is known to attack the Nafion ionomer and facilitate polymer decomposition. Therefore, the long term effects of C2 H2 on the PEMFC cathode are another factor to be considered in future work. Finally, it would be beneficial to relate ex situ RRDE results to in situ fuel cell test data for predictive purposes. Acknowledgements This work is supported by the Department of Energy (DEEE0000467). The equipment used for the tests are provided by the Office of Naval Research (N00014-11-1-0391). The authors are also grateful to the Hawaiian Electric Company for their ongoing support in the operation of the Hawaii Sustainable Energy Research Facility.
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References [1] D. van der Vliet, D.S. Strmcnik, C. Wang, V.R. Stamenkovic, N.M. Markovic, M.T.M. Koper, On the importance of correcting for the uncompensated Ohmic resistance in model experiments of the Oxygen Reduction Reaction, J. Electroanal. Chem. 647 (2010) 29–34. [2] Y. Zhai, G. Bender, S. Dorn, R. Rocheleau, The Multiprocess Degradation of PEMFC Performance Due to Sulfur Dioxide Contamination and Its Recovery, J. Electrochem. Soc. 157 (2010) B20–B26. [3] M.R. Rahman, M.I. Awad, F. Kitamura, T. Okajima, T. Ohsaka, A comparative study of ORR at the Pt electrode in ammonium ion-contaminated H2SO4 and HClO4 solutions, J. Power Sources 220 (2012) 65–73. [4] M.S. El-Deab, F. Kitamura, T. Ohsaka, Impact of acrylonitrile poisoning on oxygen reduction reaction at Pt/C catalysts, J. Power Sources 229 (2013) 65–71. [5] J. St-Pierre, M. Angelo, Y. Zhai, Focusing Research by Developing Performance Related Selection Criteria for PEMFC Contaminants ECS Transactions 41 (2011) 279–286. [6] O.A. Baturina, B.D. Gould, Y. Garsany, K.E. Swider-Lyons, Insights on the SO2 poisoning of Pt3Co/VC and Pt/VC fuel cell catalysts, Electrochimica Acta 55 (2010) 6676–6686. [7] B.D. Gould, O.A. Baturina, K.E. Swider-Lyons, Deactivation of Pt/VC proton exchange membrane fuel cell cathodes by SO2, H2S and CO, J. Power Sources 188 (2009) 89–95. [8] Y. Nagahara, S. Sugawara, K. Shinohara, The impact of air contaminants on PEMFC performance and durability, J. Power Sources 182 (2008) 422–428. [9] Y. Garsany, S. Dutta, K.E. Swider-Lyons, Effect of glycol-based coolants on the suppression and recovery of platinum fuel cell electrocatalysts, J. Power Sources 216 (2012) 515–525. [10] J. St-Pierre, Y.F. Zhai, M. Angelo, Quantitative ranking criteria for PEMFC contaminants, International Journal of Hydrogen Energy 37 (2012) 6784–6789. [11] D.J. Burnett, A.M. Gabelnick, D.A. Fischer, A.L. Marsh, J.L. Gland, Mechanism of acetylene oxidation on the Pt(111) surface using in situ fluorescence yield near-edge spectroscopy, Journal of Catalysis 230 (2005) 282–290. [12] X.Q. Lu, L. Liu, Y. Li, W.Y. Guo, L.M. Zhao, H.H. Shan, Theoretical analysis of the conversion mechanism of acetylene to ethylidyne on Pt(111), Phys. Chem. Chem. Phys. 14 (2012) 5642–5650.
[13] A.B. Delgado, A.M.C. Luna, W.E. Triaca, A.J. Arvia, The Adsorption and the Potentiodynamic Electrooxidation of Acetylene on Platinized Platinum, J. Electrochem. Soc. 129 (1982) 1493–1498. [14] S.P. Mehandru, A.B. Anderson, Dependence of carbon-carbon and carbonhydrogen bond activation on d band position: acetylene on platinum(111) and iron(100). An electrochemical model, J. Am. Chem. Soc. 107 (1985) 844– 849. [15] E.L. Muetterties, M.C. Tsai, S.R. Kelemen, Chemistry of Acetylene on Platinum (111) And (100) Surfaces, Proceedings of the National Academy of Sciences of the United States of America-Physical Sciences 78 (1981) 6571–6575. [16] J.W. Johnson, H. Wroblowa, J.O.M. Bockris, The Mechanism of the ElectroOxidation of Acetylene on Platinum, J. Electrochem. Soc. 111 (1964) 863– 870. [17] Y. Garsany, I.L. Singer, K.E. Swider-Lyons, Impact of film drying procedures on RDE characterization of Pt/VC electrocatalysts, Journal of Electroanalytical Chemistry 662 (2011) 396–406. [18] K.J.J. Mayrhofer, D. Strmcnik, B.B. Blizanac, V. Stamenkovic, M. Arenz, N.M. Markovic, Measurement of oxygen reduction activities via the rotating disc electrode method: From Pt model surfaces to carbon-supported high surface area catalysts, Electrochimica Acta 53 (2008) 3181–3188. [19] V. Stamenkovic, T.J. Schmidt, P.N. Ross, N.M. Markovic, Surface composition effects in electrocatalysis: Kinetics of oxygen reduction on well-defined Pt3Ni and Pt3Co alloy surfaces, J. Phys. Chem. B 106 (2002) 11970–11979. [20] J. Lipkowski, P.N., Ross, Electrocatalysis, Wiley, 1998. [21] A.W.E. Chan, R. Hoffmann, W. Ho, Theoretical aspects of photoinitiated chemisorption, dissociation, and desorption of oxygen on platinum(111), Langmuir 8 (1992) 1111–1119. [22] N.M. Markovic, H.A. Gasteiger, B.N. Grgur, P.N. Ross, Oxygen reduction reaction on Pt(111): effects of bromide, Journal of Electroanalytical Chemistry 467 (1999) 157–163. [23] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs, Appl. Catal. B-Environ 56 (2005) 9–35. [24] J. St-Pierre, Y. Zhai, M.S. Angelo, Effect of selected airborne contaminants on PEMFC performance J, Electrochem. Soc. 161 (2014) F280–F290.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
PEMFC Cathode Catalyst Contamination Evaluation with a RRDE- Acetylene Junjie Ge ∗ , Jean St-Pierre, Yunfeng Zhai Hawaii Natural Energy Institute, University of Hawaii- Manoa, 1680 East-West Road, POST 109, Honolulu, HI 96822, USA
a r t i c l e
i n f o
Article history: Received 5 February 2014 Received in revised form 21 March 2014 Accepted 5 April 2014 Available online 16 April 2014 Keywords: Acetylene ORR RRDE Airborne Contaminant PEMFC
a b s t r a c t The effect of C2 H2 on the oxygen reduction reaction (ORR) for a commercial Pt/C catalyst was investigated using rotating ring disk electrode (RRDE) technique in acidic solution. This study was undertaken to provide insight into the mechanism of C2 H2 contamination of the cathodes in proton exchange membrane fuel cells. The cyclic voltammetry results show a high C2 H2 coverage on the Pt surface and an almost complete loss of the electrochemical surface area in the presence of 0.14 mM C2 H2 . The RRDE was used to measure the ORR polarization curves and H2 O2 production in air. The introduction of C2 H2 shifts the ORR onset potential in the negative direction by 330 mV, and no limiting current can be observed in the potential scan window. The significant retardation of the ORR is associated with the complete loss of the ECSA, as the adsorption of C2 H2 on the Pt sites results in the inhibition of both HUPD and O2 adsorption. Furthermore, it is proposed that C2 H2 adsorption also has an impact on the adsorption configuration of O2 molecules; the Pauling configuration prevails due to the spatial limitations imposed by the presence of adsorbed C2 H2 on Pt. As a result, both the ring-disk and Koutecky-Levich measurements show a shift in the reaction pathway from a 4- to a 2-electron reduction: the H2 O2 production increases and the charge transfer number decreases. The ORR rate determining step is observed to be shifted from the first electron transfer to other possible steps. This change is confirmed by the Tafel slope measurement, which increases significantly and is most likely due to the changes in the adsorption energy of O2 . Nearly complete recovery of the performance is attainable by stopping the C2 H2 exposure. The unrecovered performance is attributed to the remaining surface adsorbates. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Because the development of renewable energy and cleaner transportation are high on the world’s priority list, proton exchange membrane fuel cells (PEMFCs) have been drawing much attention due to their low operating temperatures, short start-up times, high efficiencies, and zero emissions [1–4]. However, PEMFC performance still suffers in the presence of contaminants. For example, PEMFCs use ambient air that contains oxygen as the oxidant and, therefore, the fuel cell exposure risk to a multitude of air contaminants is significant [5]. As the cathodic catalyst, Pt/C is highly active and has an affinity for a large range of inorganic and organic substances. Several inorganic species were investigated (SO2 , NO2 ,
∗ Corresponding author. Hawaii Natural Energy Institute, University of HawaiiManoa, 1680 East-West Road, POST 109, Honolulu, HI 96822, USA Tel.: +1 808 593 1714; fax: +1 808 593 1719. E-mail addresses: [email protected], [email protected] (J. Ge), [email protected] (J. St-Pierre), [email protected] (Y. Zhai). http://dx.doi.org/10.1016/j.electacta.2014.04.005 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
NH3 , and Cl− ), and the deleterious effects on the PEMFC performance were confirmed [3,6–8]. However, little attention has been paid to the poisoning effects of organic molecules on the Pt/C catalysts towards ORR [4,9]. In our research group, 7 organic airborne contaminants were down selected from 260 airborne contaminants using a two-tier selection [10]. The 7 contaminants covered several functional groups, including alcohol (isopropanol), N-containing compound (acetonitrile), alkene (propene), alkyne (acetylene), ester (methyl methacrylate), aromatic ring (naphthalene), and halide (bromomethane). According to the in-situ results, all seven compounds have detrimental effects on the performance of the PEMFC [10]. However, the mechanisms of these effects have not yet been explored. Therefore, understanding these contamination mechanisms is important for both fundamental studies and practical applications. Acetylene (C2 H2 ) is a contaminant of interest because it is widely used as a welding fuel and chemical intermediate; it is considered a potential fuel cell contaminant due to its reaction with Pt. It also allows the effect of the C-C triple bond on PEMFC performance to be studied. The chemical/electrochemical adsorption of
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C2 H2 on Pt and its subsequent reaction have been studied extensively as Pt-based catalysts have long been used for the removal of unburned hydrocarbons from exhaust streams [11,12]. The adsorption of C2 H2 on platinum is a nondissociative process that occurs through the partial break of the triple bond in the molecule and its direct interaction with the metal surface. The number of sites blocked by each adsorbed C2 H2 is found to be approximately 2.0 when the C-C axis is parallel to the surface and the C-C plane is tilted relative to the surface normal [13]. The C2 H2 -Metal bonding includes its donation to the surface and surface back-donation to acetylene *, giving rise to ethylenic like -bonding orbitals between acetylene and the surface. Acetylene displaces adsorbed water (2.1 water molecules per acetylene molecule) from the platinum electrode due to its stronger bonding, which results from the additional back-bonding [14,15]. For the electrochemical reaction on the Pt surface, it has been found that at potentials lower than 0.2 V (vs. the normal hydrogen electrode), C2 H2 is reduced to ethylene and ethane. At potentials greater than 0.35 V, it is oxidized to carbon dioxide with a current maximum at 1.18 V [13]. The rate determining step for C2 H2 oxidation was proposed to be the first deprotonation of the adsorbed acetylene with cooperation from adsorbed OH [12,16]. In addition, the C–C donation to the Pt(111) surface from acetylene increases the C–H bond strength, thus limiting oxydehydrogenation, which means that the C–H bond is less reactive in alkynes than in alkenes. The above literature results can provide insight into the probable influence of C2 H2 on the ORR, in which a retarding effect is expected due to the strong adsorption of C2 H2 on the Pt surface and its competition with O2 adsorption [11]. However, the electrochemical behavior of O2 in the presence of C2 H2 is still unknown. In this paper, we investigated the influence of C2 H2 on the ORR reaction using a RRDE to provide mechanistic information about the performance loss of PEMFC in the presence of C2 H2 . The potentiodynamic measurements from the disk allow us to evaluate the coverage and redox information about C2 H2 and its influence on ORR kinetic loss. The collected ring current and the Koutecky-Levich plot provide information about the change in the ORR pathway. Using this approach, we can determine if the adsorption of C2 H2 changes the mechanism of ORR from a 4-electron pathway to 2-electron pathway.
2. Experiments A thin film electrode made from a Pt/C catalyst was used as the working electrode and prepared as follows: The commercially available 46.6 wt.% Pt/C (Ion Power) catalysts were used for all the electrochemical tests. A stock solution (100 mL) was prepared in advance by mixing 79.8 mL DI water, 20 mL IPA, and 0.2 mL of a 10.07 wt.% Nafion solution. Then, 15 mg of the Pt/C catalyst was placed in a 20 mL glass vial and 15 mL of the stock solution was added; the ink was mixed thoroughly using a bath sonicator (VWR, MODEL 50D) for 1 h, stirred overnight, and then sonicated again for 45-60 min before use. The bath temperature was kept at < 40 ◦ C during sonication. A ring-disk electrode with a glassy carbon inner disk (Ø = 4 mm, A = 0.126 cm2 ) and a polycrystalline Pt (ID = 5 mm, OD = 7 mm) outer ring was used for each test. The collection efficiency of the ring was measured in 2 mmol/L K3 [Fe(CN)6 ] in a 0.1 mol/L KNO3 solution, the collection efficiency was 0.428. The ring-disk electrode was polished for 4 min to a mirror finish using a 0.05 m alumina-particle suspension (ALS) on a moistened polishing cloth before each use. The polished electrodes were rinsed well with nanopure water and mounted to the electrode rotator (AlS RRDE-3A). The rotational drying method, developed by Garsany et al. [17], was used to prepare the thin film, acquire a uniform dispersion of the catalyst, and minimize the coffee-ring
structure. While the bare GC electrode was rotating at 800 rpm, a 5.41 L aliquot of the well-dispersed ink (Pt loading at 20 gPt cm−2 ) was pipetted onto the glassy carbon electrode. The electrode was held at 800 rpm for 20 min to ensure that the film was completely dry. The electrode was unloaded and rinsed gently with DI water to ensure the removal of gas bubbles on the electrode surface. The resulting thin film was used as the working electrode for the ORR in 0.1 M HClO4 solution. A spiral Pt wire and an Ag/AgCl/NaCl (3 M) were used as the counter and reference electrodes, respectively. The reference electrode was separated from the testing solution by two frits to avoid Cl− contamination. The potentials are reported versus the reversible hydrogen electrode (RHE). The electrochemical measurements were performed using Biologic VSP Bipotentiostat. An iR-drop correction of the measured potentials was performed by determining the solution resistance (R) using electrochemical impedance spectroscopy (EIS) at 0.57 V vs. RHE. The typical value of R is 28-31 ohm. An adjustment of 85% of the iR correction was made according to the manual from Biologic because a positive feedback was performed in the system and too much correction would result in system instability. During the tests, the largest current flow was approximately 0.25 mA; therefore, the uncompensated voltage had a maximum of approximately 1 mV. The testing temperature was maintained at 30 ◦ C using a water-jacket cell connected to a chiller. The chiller was operated for 20 min prior to the test. The RRDE was immersed in an electrochemical cell containing 65 mL of an HClO4 solution saturated with N2 (Ultra Zero N2 , 99.998%, Air Gas, purged for at least 20 min). The disk potential was cycled 50 times between 0.03 - 1.3 V vs RHE at 500 mV s−1 to remove any contaminants adsorbed on the surface, and the ring potential was cycled 100 times between 0.03-1.4 V at 500 mV s−1 . For the ORR tests, the measurements were conducted in an electrolyte saturated with air (prepurified, Air gas). The effect of C2 H2 on both the CV and ORR were tested. For C2 H2 , 4040 ppm in N2 and 4030 ppm in air were used for the measurements. These gases were previously used for in situ studies. Although the concentration of acetylene in air is ∼0.06 ppm, the local and transient concentration may be higher due to its use as a welding fuel and a chemical manufacture precursor. As this report constitutes the first study of the acetylene effect on the ORR activity using a RRDE, the present work employed a larger concentration than in ambient air to ensure the presence of an effect and facilitate the determination of the ORR activity loss mechanism, including the loss in the ECSA, MA due to the coverage effect, the shift in reaction pathway, the change in RDS, and the recovery effect of the catalyst. The solubility of the gases in the electrolyte is calculated using Henry’s law, shown in Equation 1: c(M) = kH ( kH =
M ) × p(atm) atm
ca = kH × exp pg
(1)
− H 1 sol R
T
−
1 T
(2)
The kH value for C2 H2 at 30 ◦ C is 0.0353 M/atm, which was taken from the CRC handbook and calculated using Equation 2. The impurity gas bubble rate was kept at 60 mL/min. The solubility of C2 H2 was calculated to be 0.14 mM at 30 ◦ C. For 65 mL of electrolyte, the estimated time needed for the saturation of the solution was 13 min for C2 H2 if all of the gas that passed through the solution was absorbed. Therefore, C2 H2 were bubbled for 0.5 h before the CV and ORR tests to ensure saturation of the solution and electrode surface. With the impurity gas bubbling at 60 mL/min, the C2 H2 consumption rate versus the replenishment rate is also calculated to determine if the electrolyte can be kept at saturation during the tests. The fastest consumption was determined to be during the CV collected at a fixed rate of 50 mV/s over the potential range from 0.03-1.5 V. Using Faraday’s law, the consumption rate
J. Ge et al. / Electrochimica Acta 133 (2014) 65–72
was calculated to be 2.94*10−9 mole/cycle. The replenishment rate for C2 H2 , however, was 8.6*10−6 mole/cycle, which was enough to maintain the saturation of the electrolyte during the test. For each test, including both CV and ORR, a new film was prepared. The ORR curves were corrected from a baseline CV where the CV scans were collected in N2 -saturated clean or contaminated cells, accordingly. 3. Results and discussions Fig. 1 shows the CV results collected in 0.1 M HClO4 with or without C2 H2 bubbling in the solution. Acetylene at 4040 ppm in N2 was tested and was equivalent to 0.14 mM dissolved in the solution. In the absence of C2 H2 , well-defined under potential deposition (UPD) of Hads is observed below 0.4 V. The electrochemical surface area of the catalyst can be calculated using Equation 3, after correcting for the double layer charging, and the hydrogen desorption area are evaluated from Fig. 1a. The correction is performed by deducing the current at 0.40 V from the total current and assuming that the capacitance is constant at different potentials. The charge of full coverage for clean polycrystalline Pt is 210 C cm−2 , and is used as the conversion factor. The Pt electrochemical surface area (ECSAPt ) is reported in m2 gPt −1 ; Q represents the HUPD charges over the range from 0.05 to 0.4 V, LPt is the working electrode Pt loading (mgPt cm−2 ) and Ag (cm2 ) is the geometric surface area of the glassy carbon electrode (i.e., 0.126 cm2 ) [18]. −1 ECSApt (m2 gPt )=[
QH−UPD (C) 210C cm−2 L (mgPt cm−2 )Ag (cm2 ) Pt Pt
]105
(3)
The positive scan was used for the measurement of the ECSA; the resulting value is 76.91 m2 gPt −1 . At potentials greater than 0.6 V, OH adsorption occurs, which is followed by the gradual formation of PtO as the potential increases. The behavior of the Pt/C electrode in the presence of 0.14 mM C2 H2 differs greatly from the clean 0.1 M HClO4 solution. From Fig. 1a, it can be observed that the hydrogen desorption peak is completely suppressed in the first positive scan, indicating the complete loss of the Pt surface area due to the C2 H2 adsorption. The onset of C2 H2 oxidation begins at ∼0.6 V and increases slowly as the potential increases to higher values. The adsorption of oxygen species is suppressed by the C2 H2 . In the negative scan, the lack of a Pt reduction peak indicates that there is a high C2 H2 coverage on the Pt surface. At lower potential values (E < 0.25 V) in the HUPD region, a sharp increase in the reduction current is observed, which is associated with the reduction of C2 H2 to ethylene and ethane, coupling with HUPD adsorption [13]. In the following cycles, the CV curves reach a steady state and the ECSA value increases to 15.55 m2 gPt −1 , 19.4% of the initial ECSA. The onset of C2 H2 oxidation moves negatively to 0.4 V due to the reduced coverage of C2 H2 and the increase in coverage of water, a promoter for the C2 H2 oxidation [12,16]. Fig. 1b shows the CV curves with/without C2 H2 scanned over an extended potential region, i.e., 0.03-1.5 V. The oxidation peak for C2 H2 is located at approximately 1.18 V, a typical oxidation peak for hydrocarbons on Pt [13], which suggests that the typical potential region where ORR is measured, i.e., the potential region shown in Fig. 1a, is not sufficient for the complete oxidation of C2 H2 . The lower oxidation peak values in the following cycles show that the readsorption of C2 H2 occurs during cycling, although not as fast as the potential cycling speed. As both the oxidation of C2 H2 and the readsorption occur during potential cycling, the effect of the electrode rotation on the measured ECSA needs to be addressed because the electrode rotation would exert an influence on the mass transfer of C2 H2 as well as O2 . Fig. 2a shows the CV results as a function of the rotation rates and cycling conditions are same as that in the following ORR measurements. Fig. 2b shows the enlarged area for the C2 H2
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reduction and the HUPD desorption. The reduction current increases with increases in the electrode rotation speed, suggesting faster C2 H2 transport and adsorption onto the catalyst surface. As a results, the coverage of the catalyst greatly increases as the ECSA decreases from 15.55 m2 gPt −1 in the static electrolyte to ∼0 at rotational rates >400 rpm. Therefore, the ECSA of the electrode during the ORR measurements should also be approximately 0, due to the facilitated readsorption of C2 H2 under rotation. Fig. 3a shows the impact of the 0.14 mM C2 H2 on the oxygen reduction activity of the Pt/C working electrode. The initial ORR polarization curve was recorded for the same working electrode in a clean air-saturated 0.10 M HClO4 electrolyte solution prior to the poisoning step. The initial ORR curve has a well-defined diffusion limiting current of 1.28 mA cm−2 (geometric) from 0.100.80 V, followed by a region with mixed kinetic-diffusion control from 0.80-1.00 V. In the presence of C2 H2 , the ORR onset potential shifts in the negative direction by 330 mV and no limiting current can be observed in the potential window, suggesting significant retardation of the ORR kinetics. The mass activity (MA) at 0.9 V is widely used as a comparison criterion for the activity of catalysts. From the polarization curve, the kinetic current can be extracted from the mixed kinetic-diffusion region through the mass transfer correction shown in Equation 4 [19]: 1/iK = 1/i − 1/il,c
(4)
where iK represents the mass transfer corrected current (kinetic current), i represents the current on the curve and il,c represents the limiting current. The MA value for the clean electrode is 0.114 A mgPt −1 , while no mass activity can be extracted from the C2 H2 adsorbed Pt surface and the ORR starts at 0.7 V. The complete loss of the MA at 0.9 V is consistent with the ECSA measurement because the entire surface is covered by the C2 H2 while the electrode is rotating. Fig. 3b shows the currents collected on the ring, which represent the amount of the ORR that occurs through the 2 electron reduction pathway with H2 O2 as the final product on the disk. On the clean Pt/C electrode, a minimal amount of current is observed at potentials greater than 0.4 V. At more negative potentials, the ring current increases to larger values due to the adsorption of Hads on the disk, which leads to an increase in the production of H2 O2 . Compared to the clean electrode, the H2 O2 oxidation currents on the C2 H2 contaminated Pt/C greatly increase for E < 0.7 V, and the peak current is located at ∼0.25 V, showing a significant contribution from the 2-electron reduction pathway in the contaminated cell. At E < 0.25 V, the currents start to decrease to lower values, indicating a decrease in production of H2 O2 on the disk. A decrease in the ring current can be observed with increasing cycle number, which can be attributed to the slight change in the surface properties during cycling. To confirm that the ring current is mainly due to H2 O2 , rather than the oxidation of C2 H2 , the ring current is also collected with the disk potential held at OCV. As shown in the inset of Fig. 3b, the ring current (<2 A) observed during the measurement can be attributed to the oxidation of C2 H2 , which is rather low compared to the values that can be observed during potential cycling of the disk. Due to the contribution from the oxidation of C2 H2 during potential cycling, an even lower ring current is expected to be collected because the solution reaches the disk first and subsequently spreads to the ring. As a result, it is confirmed that the collected ring current is mainly due to H2 O2 oxidation. For the shift in the reaction pathway of ORR, i.e., from a 4e reduction mechanism (forming water) to a 2e reduction (forming H2 O2 ), the percentage of the ORR that takes place through the H2 O2 pathway can be calculated using Equation 5 [4]: X H2 O2 [%] = 100 ×
IR N I ID + NR
2×
(5)
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Fig. 1. CV scans in the initial clean cell and the cell bubbled with 4040 ppm C2 H2 : a) over the range 0.05-1.03 V vs RHE at 20 mV/s for 10 cycles; and b) over the range 0.03-1.5 V vs RHE at 50 mV/s for 10 cycles. The 1st , 5th , and 10th cycles are shown in the figures.
Fig. 2. CV scans under different rotation speeds in the cell bubbled with 4040 ppm C2H2 over the range 0.05-1.03 V vs RHE at 20 mV/s. a) the complete CV plots, and b) the enlarged HUPD area for clear observation.
IR and ID are the ring and disk currents, respectively, and N is the collection efficiency of the RRDE (0.428). Fig. 4a shows the contribution of the 2-electron reduction pathway to the ORR. For the clean cell, less than 1% of the ORR occurs through the 2electron pathway at E > 0.3 V. Due to the interference of the HUPD
at lower potentials (E < 0.3 V), the amount of H2 O2 that is generated increases to 10% at 0.05 V. On the C2 H2 contaminated surface, however, a significant amount of H2 O2 is generated. At 0.25-0.55 V, approximately 80% of the ORR occurs through the 2-electron reduction pathway, with H2 O2 as the product; at potentials below 0.25 V,
Fig. 3. ORR scans in the initial clean cell and the cell with C2 H2 in air at 30 ◦ C: a) disk current measured at 20 mV/s for 10 cycles in the positive scan direction; b) ring current measured with potential fixed at 1.2 V during cycling. The inset in Fig. 3b shows the ring current measured with the potential fixed at 1.2 V in the presence of C2 H2 and the disk held at the OCV.
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Fig. 4. a) The percentage of the ORR that occurs through the H2 O2 pathway. The potential axis corresponds to the potential on the disk, and b) the mechanism that causes the change in the ORR reaction pathway.
the fraction of H2 O2 that is generated decreases as the potential decrease, from approximately 80% to 40%. The change in the reaction pathway can be explained by the following mechanism: The adsorption of O2 molecules on Pt occurs either through a dual adsorption (Griffiths model/bridge model) or an end-on adsorption (Pauling model), as shown in Fig. 4b [20]. Using tight-binding extended Huckel calculations, it was suggested that the chemisorption of O2 is more favorable at the two-fold bridge sites than the end-on sites [21]. This stabilization is due to the better overlap of the O2 2u orbital with the Pt surface. The dual adsorption sites are likely to be involved in the breaking of the O-O bond and, therefore, lead to the 4-electron reduction. The end-on adsorption model, however, is more likely to lead to the formation of H2 O2 as the product due to the single adsorption [20]. On the clean Pt surfaces, the reduction of O2 occurs exclusively through the 4-electron pathway before in the HUPD occurs, due to the energy preference. On the contaminated surface, the preferential adsorption of O2 in the end-on configuration is a possible scenario due to the high coverage of C2 H2 , which significantly reduces the availability of active and contiguous Pt sites. Consequently, more H2 O2 is produced in the 0.25-0.55 V range. At E < 0.25 V, however, the reduction of C2 H2 occurs, which leads to the availability of more Pt active sites for the adsorption of HUPD and dual adsorbed O2 . The effect of HUPD on the ORR pathway is significantly less than C2 H2 , as can be observed in Fig. 3b. As a result, more ORR occurs through 4-electron reduction, producing H2 O, rather than H2 O2 as the final product. To confirm that the ring current is mainly from H2 O2 oxidation and not C2 H2 or its residues, the charge transfer number is calculated from the Koutecky-Levich plot using Equation 6, and the results are compared with the ring disk current [22]. 1 1 1 + = 2 1 1 i ik 0.62nFAD 3 ω 2 v− 6 C ◦
(6)
n = 4ID /(ID + IR /N)
(7)
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The ORR polarization curves in the absence/presence of C2 H2 are shown in Fig. 5 a) and b) at various rotation speeds, respectively. From the polarization curves, plots of i−1 versus −1/2 at various potentials can be extracted, as shown in Fig. 5c, leading to straight lines with intercepts corresponding to the reciprocal of the true kinetic currents and the slope to the so called B-factor [22]. The B-factor allows one to assess the number of electrons involved in the oxygen reduction reaction. In Equation 6, i is the measured electrode current density, n represents the charge transfer number, F is the Faraday constant (96,485 C mol−1 ), A is the geometric surface area of the RDE, Cb is the bulk concentration of O2 (1.26 × 10−3 × 0.21 M)[22], D0 is the diffusion coefficient of O2 (1.93 × 10−5 cm2 S−1 ), v is the kinematic viscosity of the solution (0.01 cm2 S−1 ) and is the angular rotation rate of the electrode expressed as radian per second (=2f/60, where f is the rotation in rpm) [3]. Thus, the apparent number of electrons exchanged per O2 molecule in the overall cathodic reaction can be calculated. The calculated result for n in the clean cell is 3.93, as shown in Fig. 5d, in excellent agreement with the theoretical value for the 4-electron process. For the C2 H2 contaminated cell, the charge transfer number increases as the potential decreases, from 2.26 at 0.4 V to 3.09 at 0.2 V, indicating a change in the ORR reaction pathway from the 2-electron reaction to the 4- electron reduction at lower potentials due to the easier desorption of the reduced product of C2 H2 . On the other hand, the disk-ring current relationship can also be used for the calculation of charge transfer number (Equation 7) [4] and the results are shown in Fig. 5d. The Koutecky-Levich plot and the ring disk evaluation are in excellent agreement, which confirms the decrease in total charge transfer number due to the adsorption of C2 H2 and the shift in reaction pathway to the 2-elctron reduction. At lower potential values, the ORR shifts back to the 4-electron reduction due to the reduction of C2 H2 and the consequent increase in the availability of contiguous Pt active sites. From the inset in Fig. 5b, another interesting finding can be noticed from the change in the ring current at different rotation speeds. As the rotation speed increases, the ring current increases to higher values due to the faster transference of O2 to the disk electrode. At the same time, the peak potential on the ring shifts to the lower values, i.e., from 0.31 V at 400 rpm to 0.19 V at 3600 rpm. The shift in peak potential indicates that at higher electrode rotation speed, the adsorption of the C2 H2 occurs faster, which leads to a higher surface coverage of C2 H2 and increases the barrier to hydrogenation. As a result, the H2 O2 production peak potential shifts in the negative direction due to the insufficient removal of the C2 H2 from Pt surface at higher potential values. Shown in Fig. 6 is the mass-transport corrected Tafel plot of potentiodynamic measurements. The correction is made using Equation 4. On the original ORR polarization curve, the diffusion limiting current is taken directly from the polarization curve. For the ORR in C2 H2 , the diffusion limiting current is calculated from the Levich plot, and a charge transfer number of 2.5 (extracted from the ring-disk current measurement) is used. The il,c is calculated to be 0.76 mA cm−2 . The current range from 0.2 to 0.8 il,c is selected for the Tafel slope evaluation to ensure the accuracy of the RDE mass-transport corrections. A slope of 61 mV/dec is observed for the initial ORR, consistent with the results in the literature [22,23], and the rate determining step is confirmed as the first electron transfer step after O2 adsorption, as shown in Equation 8 and 9. O2 + Pt = Pt(O2 )ads −
Pt(O2 )ads + e =
PT (O2− )ads
(8) (9)
After the adsorption of C2 H2 onto the electrode, the Tafel curve has two distinct slopes with values of 179 mV/dec above 0.41 V and 274 mV/dec below 0.41 V. The Tafel curve shifted towards a much lower potential, from 0.93 V to 0.39 V at 1 mA/cm−2 , indicating a
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Fig. 5. a) ORR scans at different rotation speeds in air at 30 ◦ C and 20 mV/s; b) ORR scans in air with the presence of 4030 ppm C2 H2 at 30 ◦ C and 20 mV/s. The inset Fig. shows the corresponding ring currents collected at 1.2 V; c) the corresponding Koutecky-Levich plots from the polarization curves; and d) the calculated charge transfer number of ORR.
decrease of 8-9 orders (using the 61 mV/dec for the calculation) in the catalytic activity of the Pt/C catalysts towards to the ORR. The large change in the Tafel slope is attributed to a change in the rate determining step due to the high coverage of C2 H2. Fig. 7 shows the results of the attempts to recover the performance of Pt/C electrodes contaminated by C2 H2 . A simple DI water rinsing process was used to remove the adsorbed C2 H2 on the surface. During the electrode transfer and rinsing process, special care
Fig. 6. The Tafel plots for the ORR.
was taken to make sure the electrode was always covered with a drop of water, which prevented the electrode from direct contact with air. Fig. 7a shows the polarization curve of the recovered electrode in comparison to the initial ORR curve. A 91% recovery for mass activity was measured at 0.9 V vs RHE (0.121 A mg−1 Pt for the recovery electrode and 0.133 A mg−1 Pt for the initial electrode) for the recovered electrode, indicating the effectiveness of the water rinsing process in the removal of C2 H2 . A similar recovery effect was observed from our in-situ fuel cell test results [24], in which the cell performance can be recovered by simply stopping the C2 H2 injection. The almost fully recovered current from the ring also indicate the shift of the ORR back to the 4-electron reduction pathway. The inset of Fig. 7a shows the CV results of the initial electrode and the electrode after the recovery of the ORR. Both the ECSA and the Pt oxide formation/reduction results show the recovery of the active surface area through rinsing. From the calculation of the HUPD area, the ECSA recovers to 92% of the original value; this result is fairly consistent with the mass activity results from the ORR measurements. Therefore, it is safe to conclude that the loss of the mass activity value is caused by the loss of ECSA. Extended CV cycling was performed on the disk to look into the effect of the potential on the ECSA recovery. Fig. 7c shows the CV results from cycling over the range 0.03-1.2 V vs RHE. Through the continuous cycling, the ECSA value recoveries to 97% of its original value after the 10th cycle and comes to a steady state; the recovery effect can be ascribed to the removal of the remaining C2 H2 oxidation residues on the electrode surface. Over a wider potential cycling window, as shown in
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Fig. 7. a) The recovery effect of the ORR polarization curve on the disk at 30 ◦ C and 20 mV/s for 10 cycles in air. The inset in Fig. 7a shows the CV recovery effect after the ORR measurement in N2 ; b) the ring current with potential fixed at 1.2 V; c) The CV recovery effect after the ORR measurements from 0.03-1.2 V at 50 mV/s for 10 cycles in N2 ; and d) The CV recovery effect after the ORR measurements from 0.03-1.5 V at 50 mV/s for 10 cycles in N2 .
Fig. 7d, however, no further recovery is observed after cycling; the ECSA remained at 97% of its original value. The recovery of the surface area from 92% to 97% of its original value can be ascribed to the removal of the residuals of C2 H2 , where the rinsing process and potential cycling below 1.03 V is not sufficient for its removal. 4. Conclusion The influence of C2 H2 on the kinetics and reaction mechanisms of the ORR was investigated using a RRDE method. It was found that at a concentration of 14 mM C2 H2 in the electrolyte, the ECSA was completely lost due to the high C2 H2 coverage. As a result, the completely blocked Pt surface showed a significantly retarded ORR performance and the onset potential shifted in the negative direction by 330 mV. The C2 H2 not only blocks the active sites on Pt, thereby decreasing the kinetic of ORR, it also leads to a change in the ORR reaction pathway. Both the ring-disk and the KouteckyLevich measurements show a shift in reaction pathway from a 4to a 2-electron reduction; the H2 O2 production increases and the charge transfer number decreases. This situation is attributed to the presence of adsorbed C2 H2 , which significantly reduces the availability of active and contiguous Pt sites and promotes O2 adsorption in the end-on configuration. Due to the C2 H2 adsorption, which may change the adsorption energy of O2 , the ORR rate determining step shifted from the first electron transfer to a different step, as confirmed by the Tafel slope measurement. An almost complete performance recovery is obtained by removing the C2 H2 exposure,
and the performance that could not be recovered is ascribed to surface residues. Further efforts are needed for a complete mechanistic understanding of the C2 H2 influence on the PEMFC performance. First, the O2 adsorbate configuration in the presence of C2 H2 should be ascertained to validate the proposed mechanism. Second, the RDS of the ORR is left undetermined in the presence of C2 H2 . Third, the leftover residues, which need to be removed from the Pt surface to fully recover, require an oxidation potential greater than 1 V. Such a condition is rather challenging for an in-situ PEMFC cell, as the voltage is often kept lower than 1 V and a higher voltage is detrimental to the cell. Therefore, other recovery effects need to be considered. Fourth, the fact that more H2 O2 is produced exerts an undetermined influence from the C2 H2 on the durability of the MEA because H2 O2 is known to attack the Nafion ionomer and facilitate polymer decomposition. Therefore, the long term effects of C2 H2 on the PEMFC cathode are another factor to be considered in future work. Finally, it would be beneficial to relate ex situ RRDE results to in situ fuel cell test data for predictive purposes. Acknowledgements This work is supported by the Department of Energy (DEEE0000467). The equipment used for the tests are provided by the Office of Naval Research (N00014-11-1-0391). The authors are also grateful to the Hawaiian Electric Company for their ongoing support in the operation of the Hawaii Sustainable Energy Research Facility.
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