PEMFC Cathode Catalyst Contamination Evaluation with a RRDE- Propene and Naphthalene

Electrochimica Acta 138 (2014) 437–446

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PEMFC Cathode Catalyst Contamination Evaluation with a RRDE- Propene and Naphthalene 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 8 May 2014 Received in revised form 7 June 2014 Accepted 20 June 2014 Available online 11 July 2014 Keywords: Propene Naphthalene ORR Airborne Contaminant PEMFC

a b s t r a c t The effects of two unsaturated hydrocarbons, C3 H6 and C10 H8 , on the oxygen reduction reaction (ORR) at Pt/C electrodes were investigated using the rotating ring disk electrode (RRDE) method. Cyclic voltammetry (CV) measurements were performed to evaluate the electrochemical surface area losses. C3 H6 coverage is found to be mass transfer and potential dependent. C10 H8 is found to maintain high coverage across the whole potential range, independent of the mass transfer rates. The ORR kinetics is influenced by the hydrocarbon adsorption effect, and the mass activity decreases to lower values accordingly. The H2 O2 collected on the ring also increases greatly due to the influence of the two contaminants, and this increase corresponds to an increase in the 2-electron O2 reduction route. The increase in H2 O2 production and decrease in the charge transfer number is ascribed to spatial limitations arising from the adsorption of the contaminants because the rupture of the O-O bond requires two adjacent Pt active sites. The ring current collected on the C3 H6 -contaminated Pt surface differs greatly for the forward and reverse scans due to the interference of C3 H6 reduction and the change in electrode coverage. Increases in the Tafel slopes are observed to a different extent for the different contaminants, which indicates that electron transfer for ORR at the electrode surface is manipulated by contaminant adsorption. A nearly full recovery was achieved for the C3 H6 -contaminated surface, indicating that C3 H6 can be sufficiently removed by stop injection in in situ cell tests. Only partial recovery was obtained for C10 H8 -contaminated electrodes, with the unrecovered performance due to the high affinity of C10 H8 for the Pt/C electrode, which requires high potential for oxidative removal. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction The successful operation of proton exchange membrane fuel cells (PEMFCs) requires the maintenance of high cell performance for an acceptable lifetime. The polarization at the cathode (oxygen reduction reaction, ORR) is highly significant, which makes the cell performance largely dependent on the cathode performance [1]. Pt is the state-of-art catalyst in use for ORR, but it is vulnerable to poisoning by various contaminants due to the affinity of many species for the Pt/C catalyst [2]. One degradation mechanism for PEMFC is caused by the deterioration of the Pt/C catalyst by contaminants entering the cell with air [3,4]. Trace impurities, such as SO2 , NO2 , NH3 , and Cl- , have been

∗ Corresponding author. Address: Hawaii Natural Energy Institute, University of Hawaii- Manoa, 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.2014.06.147 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

confirmed to detrimentally affect PEMFC performance and sometimes cause permanent damage to the membrane electrode assemblies (MEAs)[1,2,4–7]. While sources of contamination are not limited to the above-mentioned inorganic molecules and ions, research into the effects of organic contaminants has been generally overlooked [3,8]. In our research group, 7 organic contaminants, which include 7 different functional groups, were selected from a list of more than 260 possible contaminants, for systematic evaluation [9,10]. The 7 organic contaminants are isopropanol (alcohol), acetonitrile (nitrogen compound), propene (alkene), acetylene (alkyne), methyl methacrylate (ester), naphthalene (polycyclic aromatic ring), and bromomethane (halide). The probability of their contamination of PEMFC was assessed in a recent paper published by our group [10]. According to the in situ test, the 7 compounds behaved differently in PEMFC with respect to their contamination and recovery. However, the mechanisms for these effects have not yet been explored, and in situ measurement has limited potential for mechanistic investigations. To investigate the mechanism of PEMFC cathode material contamination by the above airborne molecules, thin film rotating

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ring-disk electrodes (RRDEs) offer an effective approach to acquiring information on the contaminant coverage on Pt/C, the steady state ORR kinetics, the influence of O2 mass transfer, the ORR pathway, and the change in the ORR mechanism [11–13]. Among the 7 contaminants, propene (C3 H6 ) and naphthalene (C10 H8 ) are the ones for which C = C double bond interactions are expected, with the molecules bonded to the Pt surface through a C = C bridge configuration [14,15]. However, differences in behavior are also expected, due to the differences in molecule structure (e.g., the linear structure of C3 H6 vs. the rigid, aromatic structure of C10 H8 ) and adsorption energy on Pt surface (-89.7 kJ mol−1 for C3 H6 and -126 kJ mol−1 for C10 H8 ) [15,16]. According to our earlier paper [10], the maximum annual concentration on a 24 h average basis for propene and naphthalene are 0.1 and 0.05 ppm C, respectively. However, a higher concentration is used to study the effect of contaminants on PEMFC performance and clearly separate contamination effects from other degradation mechanisms. At the concentration of 20 ppm, the minimum accumulation times for propene and naphthalene to reach a steady state are 0.19-0.25 and 0.36-0.41 h, respectively. The decrease in the dimensionless, steady state performance of PEMFC differed significantly for C3 H6 (approximately 30%) and C10 H8 (80 ± 10%) and under the same testing conditions (45 ◦ C, 1 A cm−2 , 2/2 stoichiometry, 100/50% relative humidity, 10 kPag) [10]. In this paper, the contamination effects of C3 H6 and C10 H8 are investigated using RRDEs to provide mechanistic information about performance loss in PEMFC. 2. Experimental 2.1. Preparation of the Pt/C catalyst ink and the thin film electrode A thin film electrode made from a Pt/C catalyst was used as the working electrode; the detailed preparation method has been described elsewhere [17]. The commercially available 46.6 wt. % Pt/C (Ion Power) catalysts were used for all electrochemical tests. A ring-disk electrode with a glassy carbon inner disk (Ø = 4 mm, A = 0.126 cm2 ) and a polycrystalline Pt outer ring (ID = 5 mm, OD = 7 mm) was used for each test. A 1 mg mL−1 catalyst ink was prepared in solution containing 79.8% deionized water (18 M, Millipore, Super-Q water purification system), 20% isopropyl alcohol (Fisher Scientific, Certified ACS Plus), and 0.2% Nafion Solution (10.07 wt. % Nafion, Ion Power). The rotational drying method, developed by Garsany et al. [11], was used to prepare the thin film with a uniform dispersion of the catalyst and minimal coffee-ring structure. The Pt loading at the electrode was controlled at 20 ␮gPt cm−2 , with an appropriate amount of Pt/C ink cast onto the electrode. The collection efficiency of the ring, measured in a solution of 2 mmol L−1 K3 [Fe(CN)6 ] and 0.1 mol L−1 KNO3 , was 0.428. 2.2. Electrochemical equipment and operating conditions A spiral Pt wire and an Ag/AgCl/NaCl (3 M) electrode 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. Potentials are reported versus the reversible hydrogen electrode (RHE). The electrochemical measurements were performed using a Biologic VSP Bipotentiostat. An iR-drop correction of the measured potential was performed by determining the solution resistance (R) using electrochemical impedance spectroscopy (EIS) at 0.57 V vs. RHE. The typical value of R was found to be 28-31 . An adjustment of 85% of the calculated iR correction was made, in accordance with the Biologic manual, 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 waterjacket 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). 2.3. Cyclic voltammetry (CV) and ORR measurements The electrode was activated before use to remove any contaminants adsorbed on the surface. The disk potential was cycled 50 times between 0.03 V and 1.3 V vs RHE at 500 mV s−1 , and the ring potential was cycled 100 times between 0.03-1.4 V at 500 mV s−1 . CVs were collected between 0.05 V and 1.03 V at 20 mV s−1 for 3 cycles. The initial ORR measurements were collected between 0.05 V and 1.03 V at 20 mV s−1 for 3 cycles. The last cycle was used for analysis, and the typical rotation rate was 1600 rpm, unless otherwise noted. The effects of C3 H6 and C10 H8 on both the CV and ORR were tested. For C3 H6 , 1010 ppm in N2 /air was used for the measurements. The solubility of the gases in the electrolyte is calculated using Henry’s law, as shown in Equation 1: C (M) = kH kH =

 m  atm

× p (atm)

ca  = kH × exp pg

(1)

 − H  1 sol R

T

−

1 T

 (2)

The kH value for C3 H6 at 30 ◦ C is 0.004 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−1 . The solubility of C3 H6 was calculated to be 4.02 × 10−6 M at 30 ◦ C. For 65 mL of electrolyte, the estimated time needed for the saturation of the solution was 90 s for C3 H6 , assuming that all of the gas that passed through the solution was absorbed. In our tests, C3 H6 were bubbled for 0.5 h before the CV and ORR tests to ensure the saturation of the solution and electrode surface. For C10 H8 , 0.5 g of solid C10 H8 was added to the solution to ensure the saturation of the electrolyte, where the solubility of C10 H8 is 31.6 mg L−1 (from CRC handbook). The C10 H8 solid was kept in solution during the test to ensure the replenishment of C10 H8 . For each test, including both CV and ORR, a new film was prepared. The ORR curves were corrected from a baseline CV, with the CV scans collected in N2 -saturated, clean, or contaminated cells, accordingly. 3. Results and discussion 3.1. CV measurements Fig. 1 shows the CV results in 0.1 M HClO4 in the absence and presence of C3 H6 bubbling in the solution. In the absence of C3 H6 , the under potential deposition (UPD) of hydrogen (HUPD ) is well defined at E< 0.4 V, which is followed by a double layer region and then the formation of oxygen-containing species at E > 0.6 V. The electrochemical surface area (ECSA) of the electrocatalyst can be calculated by assuming a monolayer coverage of HUPD (210 ␮C cm−2 ) in the potential range of 0.05-0.4 V with the correction of double layer capacitance [18]. The hydrogen desorption peak is used for this calculation, and the result is 79.3 m2 gPt −1 for the clean electrode, which is a typical value for the Ion Power 46.6% Pt/C catalyst [19]. For those measurements run in the presence of C3 H6 , the potential was cycled ten times, and the 1st , 5th , and 10th cycles are shown to illustrate the changes with cycling. In the first cycle, the suppression of the HUPD is observed, due to the adsorption of C3 H6 , and the ECSA decreases to 58.3 m2 gPt −1 . At E > 0.5 V, the oxidation of C3 H6 is initiated, with water oxidation products at the Pt electrode/solution interface (i.e., Pt-OH and Pt-O species)

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Fig. 1. CV scans in the initial, clean cell (without C3 H6 ) and the cell with bubbling of 1010 ppm C3 H6 at 20 mV/s for 10 cycles: a) with a stationary electrode over the range 0.05-1.03 V vs RHE; b) with an electrode rotaion rate of 1600 rpm over the range 0.05-1.03 V; c) with an electrode rotation rate of 1600 rpm over the range 0.27-1.03 V vs RHE.

acting as the oxygen donors in the reaction. In the reverse scan, the PtO reduction peak is suppressed, indicating the incomplete oxidation of C3 H6 at the potential limit of 1.03 V and/or the occurrence of C3 H6 re-adsorption during cycling. In the HUPD region, an apparent increase in the HUPD adsorption peak is noticed, which can be ascribed to the hydrogenation of the C = C bond [14] and the formation of propane. In the following cycles, the ECSA value increases back to 90% of its initial value, indicating a decrease in the coverage by C3 H6 , due to the oxidation/reduction process. While CV curves in Fig. 1a were measured in a quasistatic solution, the RRDE measurements for ORR are taken under different hydrodynamic circumstances. Considering that C3 H6 can be oxidized and reduced (the final product for oxidation is CO2 and the reduction product is C3 H8 [14]) during cycling, the effect of C3 H6 mass transfer on surface coverage must be considered to compare the surface area loss with the ORR kinetic loss. CV measurements with an electrode rotation rate of 1600 rpm are shown in Fig. 1b. Steady state CV results demonstrate a more pronounced C3 H6 effect in comparison with the measurements taken in quasistatic solution, both in the HUPD region and in C3 H6 oxidation. The surface area loss is compared for both the hydrogen desorption peak and the PtO reduction area, from which coverages of 0.29 and 0.53 were calculated, respectively. The difference in coverage at high and low potential region is associated with the difference removal rate of C3 H6 through oxidation and reduction. To separate the effect of oxidation and reduction of propene on the coverage at Pt/C surface, CV measurements were collected between 0.27V-1.03 V, where the lower potential limit was high enough to avoid C3 H6 reduction to form C3 H8 , as shown in Fig. 1c. With the lack of interference from C3 H6 reduction, the total C3 H6 oxidation current decreases to lower value with increase in cycling numbers, indicating the accumulation of C3 H6 oxidation residuals at potential up to 1.03 V. The incomplete oxidation of C3 H6 is associated with the difficulty in C-C bond rupture and

the complicated 18-electron transfer reaction, in which the peak potential of oxidation is located at approximately 1.1 V [14].The accumulation of the oxidative residuals limits the adsorption of oxygen-containing species and impedes the new C3 H6 molecules to be adsorbed and oxidized. The increase in C3 H6 coverage, due to the change in potential window, is in agreement with the CV measurements shown in Fig. 1b, where the coverage of C3 H6 is found to be potential dependent. The lower surface coverage in low potential region can be attributed to the easier removal of C3 H6 and its oxidative intermediate through hydrogenation process, which forms the volatile C3 H8 , a compound with much weaker affinity for the Pt surface and leads to a lower surface coverage [14]. The mass transfer effect of C3 H6 is further considered with CV measurements taken at different rotation rates, the results are shown in Fig. 2a. As can be observed, the C3 H6 oxidation currents increase with the increasing rotation rate, consistent with the faster mass transfer of C3 H6 to the electrode at higher rotation speeds. The PtO reduction current, experiences the opposite effect due to the incomplete oxidation of C3 H6 and the resulting higher coverage [14]. In the HUPD region, an increase in reductive peak current is observed while the hydrogen desorption peak is barely influenced, indicating a more efficient removal of C3 H6 through hydrogenation. The C3 H6 coverage calculated from the HUPD and PtO reduction peaks is given as a function of electrode rotation speed in Fig. 2b. A less than 10% deviation was observed between the coverage values shown in Fig. 2b at 1600 rpm and those presented in Fig. 1, which was due to the use of different thin film electrodes. The coverage by C3 H6 is found to be higher at higher rotation speed in high potential region, as discussed above. Therefore, it is concluded that the C3 H6 coverage is mass transfer and potential dependent. Fig. 3a shows the influence of C10 H8 contamination on the CV results. The adsorption of C10 H8 on the Pt surface blocks the Pt surface in both the HUPD region and the oxide formation region. A 90%

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Fig. 2. a) CV scans at different rotation rates with 1010 ppm bubbled C3 H6 , over the range 0.05-1.03 V vs RHE at 20 mV/s. b) C3 H6 coverage calculated from the HUPD and the PtO reduction area.

Fig. 3. CV scans in the initial, clean cell and in the C10 H8 -saturated cell, over the range 0.05-1.03 V vs RHE at 20 mV/s for 10 cycles, showing a) the 1st , 5th , and 10th cycles and b) different rotation speeds.

ECSA loss is calculated from the hydrogen desorption area of the HUPD region, corresponding to a C10 H8 surface coverage of 0.9. The adsorption of the oxygen-containing species (i.e., -OH and -O) is also seriously retarded, and the onset potential for C10 H8 oxidation and/or formation of PtOH and PtO initiates at 0.8 V, which is 200 mV higher than the onset potential of Pt oxidation. In the negative scan, the PtO reduction peak is almost lost entirely, which is consistent with the high coverage calculated from the hydrogen desorption peak. Similar to C3 H6 measurements, the mass transfer influence of C10 H8 on the CV behavior was also investigated and is shown in Fig. 3b. The results show the CV curves measured at different

rotation rates are nearly superimposed, meaning that a steady coverage had been reached at all rotation rates and the potential window was not sufficient for C10 H8 removal; the replenishment of C10 H8 from solution was thus not necessary. The oxidation behavior of C10 H8 in an extended potential window is shown in Fig. 4a. For the first cycle, the anodic current for the C10 H8 oxidation increases quickly at E > 1.2 V, and a peak at 1.48 V is observed with a shoulder at 1.37 V. The PtO reduction peak in the reverse scan indicates the occurrence of Pt oxidation at higher potential. The increased irreversibility of the Pt oxidation (positive shift of at least 200 mV from 0.6 to 0.8 V) and PtO reduction (negative shift of 30 mV from 0.7

Fig. 4. CV scans in the initial, clean cell and in the C10 H8 -saturated cell: a) over the range 0.03-1.5 V vs RHE at 50 mV/s for 10 cycles; b) over the range 0.05-1.03 V vs RHE at 20 mV/s for 10 cycles, following cycling in the range of 0.03-1.5 V. The 1st , 5th , and 10th cycles are shown in both figures.

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suggests that a high surface coverage by the contaminants, which is usually associated with more severe performance loss on ORR, does not necessarily cause high OCV loss with ORR. Therefore, the effect of different contaminants on the OCV in PEMFCs may not match that of the performance loss observed during the in situ measurements. 3.3. ORR measurements

Fig. 5. The influence of C3 H6 and C10 H8 on the OCV for ORR in Air.

to 0.67 V) is observed, which is attributed to the combined effect of naphthalene coverage on the catalyst surface and the induced alteration of the electronic structure (discussed in section 3.2). At 0.527 and 0.403 V, two small reductive peaks are observed, which can be ascribed to the reduction of products of reactions at higher oxidative potentials. In the following scans, two reversible redox peaks are noted at 0.439/0.403 mV (Ep = 36 mV) and 0.572/0.527 mV (Ep = 45 mV), which are noted as peaks 1/1 and 2/2 . At higher E, oxidative peaks 3 and 4 are associated with the oxidation of C10 H8 . For a potential window of 0.05-1.03 V, as shown in Fig. 4b, the two coupled redox peaks still exist, with no observable change in ECSA. This result indicates that the oxidative products formed at 1.03 V < E < 1.5 V have a high affinity for the electrode surface and cannot be removed efficiently through potential cycling in the 0.05-1.03 V potential range. 3.2. Contaminant impact on OCV Fig. 5 shows the influence of C3 H6 and C10 H8 on the OCV for ORR at the Pt/C electrode. In the initial, air saturated electrolyte, the OCV is approximately 1.03 V for both electrodes. The much lower OCV for the clean electrolyte, compared with the E␪ , is attributed to the occurrence of counter balance reactions, such as Pt oxidation and dissolution, which readily balances the sluggish ORR, with an exchange current density of approximately 10−8 A cm−2 [20]. For the case of the C3 H6 , it is assumed that partial oxidation occurs, serving as a counterpart reaction for ORR; therefore, the OCV decreases by 75 mV at 1000 s. Because only partial oxidation occurs at potentials lower than 1.03 V, a decrease in anodic current is expected with extended time, which leads to the slight increase in the OCV value. For C10 H8 , a slight increase in OCV by 5 mV can be observed immediately after contaminant injection. The effect may be ascribed to trace amounts of C10 H8 capable of altering the electronic structure of the electrode. Using density functional theory (DFT), Santaross et al. [15] found that C10 H8 preferentially adsorbs on the Pt surface at di-bridge sites. It was also found that an electron transfer occurs from occupied ␲ bands of C10 H8 to unoccupied d orbitals of the Pt surface and from the occupied d states of Pt to ␲* orbitals of C10 H8 . The Pt-to-C10 H8 electron transfer is found to be the main contribution, partially counteracted by the C10 H8 -to-Pt electron transfer, resulting in an increase of the Pt d orbital vacancies. However, the small amount of C10 H8 has a limited impact on the ORR current. With the increasing C10 H8 coverage at the electrode, the oxidative current (although quite slow due to the low potential) increases, resulting in a decrease in OCV. However, the more sluggish oxidation of C10 H8 , compared with that of C3 H6 , leads to a less significant effect on the OCV, such that the OCV only decreases by 19 mV. The comparison of OCV for C10 H8 and for C3 H6

Fig. 6 depicts the impact of C3 H6 on the ORR of the Pt/C working electrode. The initial ORR polarization curves were recorded for the same working electrode in a clean, air-saturated, 0.10 M HClO4 electrolyte prior to poisoning. Fig. 6a shows both the forward and reverse polarization curves. The initial ORR curve has a well-defined diffusion limiting current of 1.32 mA cm−2 (geometric area) at 0.100.80 V, within the 10% margin of the theoretical diffusion limiting current (i.e., 1.27 mA cm−2 ), which is calculated using the Levich equation [21], and clearly indicates a negligible contribution from O2 diffusion through the Nafion film. A mixed kinetic-diffusion control region is located at 0.80 V < E < 1.00 V. The oxygen reduction rate is observed to be faster for the positive scan than it is for the negative scan in the kinetic region. The hysteresis in the polarization curves (the difference in half-wave potential (E1/2 ) is 26 mV) is attributed to the higher coverage by the structure-sensitive OH species, which is believed to be a site-blocking species for ORR due to the electrode history [21]. The ring current behaves similarly in both sweep directions, accounting for only a rather low fraction of current on the disk (0.6%) at E > 0.2 V, clearly indicating a 4-electron reduction pathway at the disk. Ring currents increase to larger values at E < 0.2 V, due to the adsorption of hydrogen on the disk and the generation of more H2 O2 . In the presence of C3 H6 , the polarization curve for both scan directions shifts negatively and is accompanied by a decrease in diffusion limiting current. The E1/2 decreases to 17 mV with C3 H6 , which is ascribed to the interference of C3 H6 adsorption in Pt oxidation. The kinetic current can be extracted from the mixed kinetic-diffusion region of the polarization curve through the mass transfer correction shown in Equation 3[12]: 1/iK = 1/i − 1/id

(3)

where iK represents the mass transfer-corrected current (kinetic current), i represents the current at a given point on the curve, and id represents the diffusion limiting current measured as 0.60 V. The corrected mass activity (MA) values are calculated and compared at 0.90 V. The positive scan is chosen for this calculation, and the MA value decreases from 0.144 A mgPt −1 for the initial conditions to 0.068 A mgPt −1 in the presence of C3 H6 , corresponding to a 53% loss. This value agrees well with the surface area loss calculated from the Pt reduction peak (53%, from Fig. 1b, and 49% from Fig. 2b) at 1600 rpm, clearly indicating a relationship between the decrease in Pt active surface area and the mass activity. An increase in ring current is observed in comparison to the initial ring current, indicating an increase in H2 O2 production at the disk. A much larger ring current can be observed for the negative scan than for the positive scan, corresponding to a smaller disk current in the range of 0.1-0.55 V for the negative scan. The ring current increases with decreasing potential for the negative scan, with a peak located at ∼0.26 V; at E < 0.26 V, a drop in the current can be observed, which is related to the reduction of C3 H6 to C3 H8 and the consequent decrease in surface coverage (as shown Fig. 1b and Fig. 2a). For the positive scan, a peak current is observed at ∼0.5 V, with the initiation of C3 H6 oxidation (as shown in Fig. 1), giving rise to more available active Pt sites. The lower production rate of H2 O2 during the positive scan is attributed to the above-mentioned reduction of C3 H6 to form the volatile C3 H8 in the HUPD region, which in turn leads to a higher ECSA value during the positive scan. A quantitative

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Fig. 6. a) ORR polarization curves measured on the disk (air, 30 ◦ C, 20 mV/s, 1600 rpm) and corresponding ring currents (potential holds at 1.2 V) measured in the initial, clean cell and the cell bubbled with 1010 ppm C3 H6 in air (the 10th cycle is shown for both disk and ring); b) The percentage of ORR occurring through the H2 O2 pathway. The x-axis potential refers to the disk potential.

representation of the H2 O2 can be calculated using Equation 4[3], and the result is shown in Fig. 6b. X · H2 O2 [%] = 100 ×

2 × IR /N ID + IR /N

(4)

In Equation 4, IR and ID are the ring and disk currents, respectively, and N is the collection efficiency of the RRDE (0.428). At 0.4 V, the H2 O2 production increases from 1%, under initial conditions, to 6% and 17%, with C3 H6 , for the positive and negative scans, indicating the large impact of C3 H6 adsorption on the reaction pathway of the ORR, where some of the ORR has been shifted from a 4-electron reduction mechanism (to form water) to a 2-electron reduction (to form H2 O2 ). The partial alteration in reaction pathway is attributed to the impediment of the parallel adsorption mode of O2 on Pt (necessary for O-O bond breaking) and the occurrence of end-on adsorption (Pauling model), due to the coverage by C3 H6 , which reduces the availability of active and contiguous Pt sites[3]. As a result, more O2 are adsorbed through the end-on configuration, and more H2 O2 are produced as the final product. In Fig. 7, the lower potential limit of the ORR measurement was altered to 0.27 V to avoid the effects from C3 H6 hydrogenation, as the actual operating potential of the PEMFC cathode is much higher than the potential for C3 H6 hydrogenation. The ORR polarization curve encounters increased retardation in the kinetic region for both scanning directions, corresponding to a decrease in mass activity of the Pt/C electrode. Increased current on the ring is also noted, with that of the positive scan surpassing that of the negative scan. The decrease in mass activity of the Pt/C electrode

and the increase in the ring current can be attributed to the increase in coverage of C3 H6 oxidative residuals, which is in agreement with the CV measurements in Fig. 1c. Fig. 8 shows the effect of C10 H8 on the ORR at the Pt/C working electrode. Similar to effect of C3 H6 on the disk, both the kinetic current and the diffusion limiting current are influenced by the presence of C10 H8 in solution (Fig. 8a). The mass activity loss for the positive scan is 66% at 0.9 V, and the retardation effect increases to 76% at 0.87 V, indicating a higher influence on the mass activity at lower potentials in the kinetic region. The mass activity losses for the contaminated Pt surface may be ascribed to the combined effect of C10 H8 on coverage and d orbital vacancy of Pt (a potential dependent parameter [22]). Unlike the initial ORR polarization curves, in which the diffusion limiting current is reached at 0.8 V, the C10 H8 -contaminated electrode only reached its diffusion limiting current at 0.6 V, indicating a serious retardation of ORR in the high current region. An increase in the amount of H2 O2 collected on the ring is observed, indicating the shift in reaction from the 4-electron to 2-electron pathway. Similar current behavior is observed for the forward and reverse scans, consistent with the CV measurement results, as no obvious redox behavior is observed in the potential scan region. Using Equation 4, the H2 O2 production (as shown in Fig. 8b) is found to increase from 1%, under initial conditions, to 16% and 17% for the positive and negative scans, showing that a large fraction of ORR has been shifted from a 4-electron reduction to a 2-electron reduction. 3.4. The change in charge transfer number The overall charge transfer number for the ORR is calculated to further analyze the steady-state voltammograms for the disk and the current at the ring using Equation 5 [3], the results are shown in Fig. 9.



n = 4ID / ID + IR /N

Fig. 7. The ORR measured in two potential ranges (0.05-1.03 V and 0.27-1.03 V), at 20 mV/s and 1600 rpm. The 10th cycles were used in the analysis of both disk and ring results for all potential ranges.



(5)

In the clean cell, a 4-electron reduction is observed for both measurements. A significant contribution of the 2-electron reduction pathway (i.e., hydrogen peroxide) during the course of the ORR leads to a decrease in the total charge transfer number in the presence of both C3 H6 (Fig. 9a) and C10 H8 (Fig. 9b). Again, the difference in n for the C3 H6 -adsorbed surface between the forward and reverse scans is caused by the difference in electrode coverage due to the surface oxidation/reduction reaction of C3 H6 . The n at 0.4 V is recorded to be 3.9 and 3.7 for the positive and negative scans, respectively. For the C10 H8 -adsorbed surface, n is 3.7 for both scan directions.

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Fig. 8. a) ORR polarization curves measured at the disk (air, 30 ◦ C, 20 mV/s, 1600 rpm) and ring (potential holds at 1.2 V), in both the initial, clean cell and C10 H8 -saturated cell, in air; b) The percentage of ORR occurring through the H2 O2 pathway. The x-axis potential refers to that of the disk.

Fig. 9. The influence of C3 H6 a) and C10 H8 b) on the charge transfer number of the ORR. The potential shown in the figures corresponds to the disk potential.

Fig. 10. The influence of rotation speed on the collection rate of the H2 O2 : ORR measured at different rotation speeds in the presence of C3 H6 (a) and C10 H8 (c); the H2 O2 production percentage, calculated from the ring current, for C3 H6 (b) and C10 H8 (d).

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Scheme 1. Schematic illustration of the possible reactions going on during ORR measurements. The subscript (diff) refers to diffusion of the H2 O2 into the bulk solution.

Fig. 12. Tafel plots for the ORR.

the disk potential held at OCV. Steady currents of 1.1 ␮A and 1.6 ␮A with C3 H6 and C10 H8 (results not shown) were observed and can be ascribed to their oxidation, accompanied by Pt oxidation. The lower current, compared to that observed during potential cycling of the disk, confirmed that the collected ring current is mainly due to H2 O2 oxidation. Fig. 11. Increase in the H2 O2 production rate at 0.4 V as a function of the rotation rate of the RRDE.

3.5. Change in H2 O2 production rate with different rotation speeds As we observed in Fig. 2a (Section 3.1), the coverage by C3 H6 is influenced by the electrode rotation rate, in which a higher rotation rate facilitates the mass transfer of both C3 H6 and O2 . It is expected that a higher percentage of H2 O2 can be produced with higher C3 H6 coverage because pairs of contiguous platinum sites for the rupture of the O-O bond are required [23]. Fig. 10a shows the RRDE measurement results for ORR at different rotation speeds in the presence of C3 H6 . The corresponding H2 O2 production percentage calculated using Equation 4 is shown in Fig. 10b. The H2 O2 production ratio increases with increasing rotation rates, as expected. Another source of the increase in H2 O2 production is illustrated in Scheme 1. The ORR occurs through either a direct 4-electron or a 2-electron reduction pathway [24]. The H2 O2 produced by the 2-electron reduction pathway can leave the surface through 3 different routes, i.e., continued reduction to water (k3 ), self-decomposition to O2 and H2 O (k4 ), or diffusion into the bulk solution (kdiff ). The ring only collects the part that diffuses into the bulk solution. As a result, an increased rotation rate leads to increased kdiff and an increase in the ratio between kdiff and k3 + k4 . The contribution from the change in the ratio of kdiff to k3 + k4 can be clearly observed from the C10 H8 measurements (Fig. 10c and Fig. 10d), where no C10 H8 coverage changes were observed with increasing rotation speed (Fig. 3b). Similar to the C3 H6 measurements, the H2 O2 collection ratio increases with an increasing rotation rate. To make a comparison between C3 H6 and C10 H8 , the H2 O2 production rate was plotted against the rotation rate and fitted linearly, as shown in Fig. 11. The slopes for C3 H6 and C10 H8 are 0.00289 and 0.00213, respectively. A steeper slope for C3 H6 can be interpreted as resulting from the change in its coverage, as discussed previously. Through the above discussion, it can be observed that an accurate evaluation of the RRDE results for ORR must be specific to the rotation rate, with measurements at several speeds considered to be superior to a single set of data. To confirm that the ring current measured by the contaminated electrodes is mainly resulting from the formation of H2 O2 , rather than the oxidation of C3 H6 or C10 H8 , the ring currents were also collected with

3.6. Tafel plots analysis The mass transport-corrected Tafel plots for the ORR measurements are shown in Fig. 12, with corrections made according to Equation 3. The current range of 0.2 to 0.8 id is selected for the Tafel slope evaluation to ensure the accuracy of the RDE mass transport corrections [12]. A slope of 67 mV/dec is observed for the initial ORR, which corresponds to 2.3RT/F and identifies the first charge transfer process as the rate determining step [12,22]. Both C3 H6 and C10 H8 exert influence on the Tafel plot of ORR, producing lower kinetic currents and higher slopes. The decrease in kinetic current shows the retardation effect of the contaminants on the ORR, with a more severe effect observed for the C10 H8 adsorbed Pt surface. Higher Tafel slope values are observed for both C3 H6 - and C10 H8 -contaminated electrodes, with the highest slopes observed at lower potentials. In the literature, a Tafel slope of (2.3RT/F)/(2 × 2.3RT/F) mV/dec identifies the first charge transfer reaction as the rate determining step for ORR [3,23,25]. In the present case, the increase in Tafel slope may suggest that the presence of the two contaminants has re-assigned the rate determining step. This alteration is likely due to the sluggish nature of the ORR and the specific adsorption of the two contaminants. Specifically, the adsorption of C3 H6 and C10 H8 at the inner Helmholtz plane (IHP) leads to a change in double layer structure at the electrode interface. Such a change in double layer structure has frequently been observed to influence charge transfer kinetics and the Tafel slope for different reactions [26,27]. Moreover, the Pt surface electronic structure is also influenced by the electron transfer between contaminant molecules and Pt [15], as reported in Section 3.2. Charge transfer effects differ for C3 H6 and C10 H8 , as indicated by different adsorption energies on Pt [15,16], therefore leading to a different Pt d-band vacancy. The Pt d-band vacancy, a potential dependent parameter [22] also influenced by the contaminants’ redox behavior, influences the Pt d-band center [28]. The strength of the coupling between the oxygen 2p states and the Pt d states, i.e., the adsorption energy of oxygen containing species, is affected by change in the position of the Pt d states (d-band center) relative to the Fermi level. As a result, the Pt catalyst performance [29] and ORR rate determining step (Tafel slope) [30,31] are influenced by changes in bonding energy of oxygen containing species to Pt in the presence of the two contaminants.

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Fig. 13. The recovery effect of the C3 H6 -contaminated electrode for pre-contamination, 1st , 5th , and 10th cycles: a) the recovery of the ORR on disk and ring; b) the recovery of the CV.

Fig. 14. The recovery effect of the C10 H8 -contaminated electrode for pre-contamination, 1st , 5th , and 10th cycles: a) the recovery of the ORR on disk and ring; b) the recovery of the CV.

3.7. Recovery effect Fig. 13 and Fig. 14 show the recovery of the Pt/C electrodes from C3 H6 and C10 H8 contamination, for which a simple deionized water rinsing process was used to remove the adsorbed contaminants from the surface after the contamination tests. The cleaned electrode was dipped into the clean cell with 0.1 M HClO4 electrolyte for the ORR and CV tests. Fig. 13a shows the polarization curve and the ring current of the recovered electrode in comparison to the initial ORR curve prior to contamination. The polarization curve of the recovered electrode is nearly superimposed on the initial curve, indicating the effectiveness of the rinsing process for performance recovery. A 93% recovery was found by carefully calculating and comparing the mass activity at 0.9 V. The ECSA, calculated from the HUPD area, recovers to 91% of its original value by the 10th cycle (Fig. 13b), with potential cycling between 0.05-1.03 V, which is consistent with the mass activity results from the ORR measurements. The recovery is very different for C10 H8 , as can be observed in Fig. 14. Only partial recovery was achieved by the rinsing process, as observed in the polarization curve and the corresponding ring current (Fig. 14a). The mass activity rebounds to only 49% of its initial value. The ring current also partially recovered, with the H2 O2 current settled between the initial value and the contaminated cell value. The partial recovery for both ring and disk indicates that the rinse process is not sufficient for the complete removal of C10 H8 from the Pt surface, most likely due to its higher adsorption energy and correspondingly higher affinity for Pt compared with that of C3 H6 . The oxidative removal of C10 H8 through potential scanning from 0.03-1.5 V is shown in Fig. 14b, in which a slow recovery is

observed, as indicated by the recovery of the HUPD and PtO reduction areas, accompanied by the diminishing oxidative current with continued cycling. The ECSA value increases back to 82% of its original value by the 10th cycle (HUPD ); however, such a high potential is detrimental to the electrode, leading to Pt oxidation and dissolution and to the corrosion of the carbon. Therefore, other recovery techniques should be considered. 4. Conclusions The effect of two unsaturated hydrocarbons, i.e., C3 H6 and C10 H8 , on the Pt/C catalyst performance for ORR was investigated using RRDEs. The electrode coverage by each contaminant was studied using CV as a function of the electrode rotation rate. The C3 H6 coverage is found to be both potential and mass transfer dependent, so that a higher coverage is observed at a higher electrode potential and rotation speed. For C10 H8 , a high coverage is observed due to its high adsorption energy on Pt, and its coverage is found to be mass transfer independent. The ORR mass activity significantly decreases in comparison with the non-contaminated case due to the adsorption of C3 H6 and C10 H8 on Pt. A greater influence on the ORR was observed for C10 H8 . The amount of H2 O2 collected at the ring also increases greatly with the two contaminants, corresponding to an increase in the 2-electron O2 reduction route. The increase in H2 O2 production and decrease in charge transfer number is ascribed to the spatial limitations resulting from the adsorption of the contaminants because the rupture of the OO bond requires two consecutive Pt active sites. The ring current for the C3 H6 -contaminated Pt surface differs greatly between the

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forward and reverse scans due to the interference of C3 H6 reduction and the change in coverage. Increases in the Tafel slopes are observed to a different extent for the different contaminants, indicating that the passage of electrons for ORR at the electrode surface is manipulated by the contaminants’ adsorption. A nearly full recovery was achieved for the C3 H6 -contaminated surface, indicating that C3 H6 can be sufficiently removed by stop injection in the in situ cell tests. However, the removal of C10 H8 is more difficult and requires a higher potential for oxidation. Additionally, further investigation needs to be carried out using techniques such as FTIR to identify the species generated by the oxidation of C10 H8 . The current and other studies provide the following insights into the contamination by C3 H6 and C10 H8 for the in situ PEMFCs study: i) The loss in PEMFC performance can be attributed to contaminant adsorption on the Pt surface, which results in a decrease in ORR kinetics; ii) The recovery from C10 H8 contamination deserves more concern because only partial recovery can be obtained through the water rinsing process due to the higher affinity of C10 H8 for the Pt surface. The oxidative removal of C10 H8 requires potentials of 1.5 V or higher at 30 ◦ C, which indicates that other recovery methods (rising temperature, prolonged recovery period) should be considered for PEMFC de-contamination; iii) The fact that more H2 O2 is produced suggests that the contamination might have a long term effect on cell performance because H2 O2 is known to attack the Nafion ionomer and facilitate polymer decomposition; iv) Propene and naphthalene do not affect the membrane because they are not solvents or cations. However, the oxygen mass transfer in a fuel cell is influenced [32]; v) Water management is not expected to have an effect on contamination for these species as they are insoluble in water. Therefore, they are not scavenged by the product water droplets [10,33]. Acknowledgements This work is supported by the Department of Energy (DEEE0000467). The test equipment is supported by the Office of Naval Research (N00014-11-1-0391). Authors are also grateful to the Hawaiian Electric Company for their ongoing support of the operations of the Hawaii Sustainable Energy Research Facility. References [1] 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. [2] Y. Nagahara, S. Sugawara, K. Shinohara, The impact of air contaminants on PEMFC performance and durability, J. Power Sources 182 (2008) 422–428. [3] 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. [4] B.D. Gould, O.A. Baturina, K.E. Swider-Lyons, Deactivation of Pt/VC proton exchange membrane fuel cell cathodes by SO2, H2S and COS, J. Power Sources 188 (2009) 89–95. [5] 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. [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] Y. Zhai, K. Bethune, G. Bender, R. Rocheleau, Analysis of the SO2 Contamination Effect on the Oxygen Reduction Reaction in PEMFCs by Electrochemical Impedance Spectroscopy, J. Electrochem. Soc. 159 (2012) B524–B530. [8] 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.

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