Supporting Online Material for

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1 Supporting Online Material for High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt Gang Wu, Karren L. More, Christina M. Johnston, Piotr Zelenay* *To whom correspondence should be addressed. Published 22 April 2011, Science 332, 443 (2011) DOI: /science This PDF file includes: Materials and Methods Table S1 Figs. S1 to S8 References

2 Content Text: 1. Materials and Characterization 2. Kinetic Analysis 3. FTIR Data 4. SEM Images as a Function of Heat-Treatment Temperature 5. Catalyst Stability Test by Potential and Voltage Cycling 6. SEM Images of PANI-FeCo-C(1) Catalyst 7. HR-TEM Characterization Figures S1-S8 Table S1 References S1-S14

3 1. Materials and Characterization Catalyst synthesis. Carbon support, typically a commercial carbon Ketjenblack EC 300J (AkzoNobel), with BET surface area of about 950 m 2 g -1 and good corrosion resistance (S1), was treated in HCl solution for 24 hours to remove metal impurities. In a typical approach, 2.0 ml aniline was first dispersed with 0.4 g acid-treated carbon black in 0.5 M HCl solution. The suspension was kept below 10 C while the oxidant (ammonium peroxydisulfate, APS) and transition metal precursors were added. FeCl 3 and Co(NO 3 ) 2 6H 2 O were used as precursors for PANI-Fe-C and PANI-Co-C catalysts, respectively. The molar ratio of precursors used to prepare the PANI-FeCo-C(1) catalyst was 3:1. After constant mixing for 24 hours to allow the polymerized PANI to uniformly mix and cover the carbon black particles, the suspension containing carbon, polymer and transition metal(s) was vacuum-dried using a rotary evaporator. The subsequent heat-treatment was performed at temperatures ranged from C in an inert atmosphere of a nitrogen gas for 1 hour. The heat-treated sample was then pre-leached in 0.5 M H 2 SO 4 at 80 C for 8 hours to remove unstable and inactive species from the catalyst, and thoroughly washed in de-ionized water. Finally, the catalyst was heat-treated again in nitrogen-gas atmosphere for 3 hours. PANI-FeCo-C(2) catalyst was obtained in a two-step process, in which EDA-Co-C synthesized in the first step served the role of support in the second synthesis step involving Fe precursor and PANI (as in the regular single-step synthesis of PANI-Fe-C). In this approach, cobalt precursor was complexed with EDA (ethylenediamine) to form an EDA-Co chelate. The complex was then used to thoroughly impregnate an HCl-treated carbon black (Ketjenblack EC-300J). After vacuum-drying, heat treatment at 900 C in N 2 atmosphere, and acid leach in 0.5 M H 2 SO 4 at 80 C, heat-treated EDA-Co-C material was used instead of carbon in the second step, analogous to the one-step synthesis of a PANI-Fe-C catalyst described above. The overall molar Fe-to-Co ratio in the two-step synthesis of PANI-FeCo-C(2) was 3:1, the same as in the one-step process used in the preparation of PANI-FeCo-C(1). Rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) measurements. ORR activity and four-electron selectivity of catalysts were evaluated using a rotating disk electrode (RDE) and a rotating ring disk electrode (RRDE), respectively. RDE/RRDE measurements were performed using CHI Electrochemical Station (Model 750b) in a conventional three-electrode electrochemical cell. To avoid any potential contamination of a non-precious metal catalyst by platinum, experiments were carried out using a graphite rod as the counter electrode. An Ag/AgCl electrode in 3.0 M NaCl (0.209 V vs. NHE) was used as a reference electrode. All potentials were later converted to the RHE scale. Non-precious catalyst data in this were 2

4 obtained at room temperature (~25 C) with a catalyst loading of 0.6 mg cm -2 in 0.5 M H 2 SO 4 at a rotating disk speed of 900 rpm. The reference Pt data were recorded with a 20 wt% E-TEK Pt/C catalyst at two different loadings of 20 μg Pt cm -2 and 60 μg Pt cm -2 in 0.1 M HClO 4 solution (a non-adsorbing electrolyte for Pt catalysts). The catalysts were ultrasonically dispersed in an alcoholic solution containing suspended Nafion ionomer for one hour to form a catalyst ink, later applied to the glassy-carbon disk surface. ORR steady-state RDE polarization plots were recorded in O 2 -saturated electrolyte using potential step of 0.03 V and wait-period of 30 s between two subsequent potentials. The disk rotation rate was from 400 to 2500 rpm. Cyclic voltammetry (CV) characterization of the catalysts in the absence of oxygen was typically carried out in the potential range from 1.0 to 0.0 V at a scan rate of 10 mv s -1 in nitrogen-saturated 0.5 M H 2 SO 4 electrolyte (0.1 M HClO 4 in the experiments involving Pt reference). RDE cycling stability tests of the PANI-Fe-C catalyst were performed in N 2 -saturated 0.5 M H 2 SO 4 in the potential range from 0.6 to 1.0 V at room temperature. In RRDE experiments, the ring potential was set to 1.2 V. Before experiments, the Pt catalyst in the ring was activated by potential cycling in 0.5 M H 2 SO 4 from 0 to 1.4 V at a scan rate of 50 mv s -1 for 10 minutes. The four-electron selectivity of catalysts was evaluated based on the H 2 O 2 yield, calculated from the following equation (S2): I / N I N + I R H2O 2(%) = 200 ( R / ) D Here, I D and I R are the disk and ring currents, respectively, and N is the ring collection efficiency. The ring collection efficiency was independently determined using 10 mm ferricyanide (K 3 [Fe(CN) 6 ]) in 0.1 M KCl solution and PANI-Fe-C catalyst on the carbon disk at a typical loading of 0.6 mg cm -2. The measured N value was 36%, very close to the RDE manufacturer s value of 37%. Fuel cell testing. Catalysts were tested in the fuel cell cathode to evaluate their activity and durability under PEFC operating conditions. The catalyst ink was prepared by ultrasonically mixing the catalyst powder with Nafion suspension for four hours. The thus obtained ink was then applied to the gas diffusion layer (GDL, ELAT LT 1400W, E-TEK) by successive brush-painting until the cathode catalyst loading reached ca. 4 mg cm -2. The Nafion content in the dry catalyst layer was maintained at ca. 35 wt%. Commercial Pt-catalyzed carbon cloth GDL (E-TEK, 0.25 mg Pt cm -2 ) was used at the anode. The cathode and anode were hot-pressed onto two separate pieces of a Nafion 1135 membrane, later put together to form a two-layer membrane-electrode assembly (MEA). This approach 3

5 minimized the risk of a possible cross-contamination of the cathode with platinum from the anode during MEA preparation and also greatly facilitated post-mortem characterization of the individual fuel cell electrodes. The geometric MEA area was 5.0 cm 2. Fuel cell testing was carried out in a single cell with serpentine flow channels. Pure hydrogen and air/oxygen, humidified at 85 C, were supplied to the anode and cathode at a flow rate of 200 and 600/400 ml min -1, respectively. Both electrodes were maintained at the same absolute pressure of 2.8 bar. Fuel cell polarization plots were recorded using fuel cell test stations (Fuel Cell Technologies Inc.). Recorded voltage, obtained with the two-membrane sandwich MEA, was corrected for the resistance of one of the membranes; as a result, the fuel cell polarization data presented in this paper correspond to an MEA with a single Nafion 1135 membrane. The MEA voltage cycling tests were carried out in an externally driven cell with hydrogen supplied to the anode and nitrogen to the cathode in the voltage range of V. Physical characterization. Mid-infrared spectra were recorded on a Nicolet 670 FTIR spectrometer using KBr pellets. Catalyst morphology was characterized by scanning electron microscopy (SEM) on a Hitachi S-5400 instrument. High-resolution transmission electron microscopy (HR-TEM) images were taken on a JEOL 3000F microscope operating at 300 kv at Oak Ridge National Laboratory. 2. Kinetic Analysis While cyclic voltammetry (CV) of PANI-C and PANI-Co-C in N 2 -saturated H 2 SO 4 solution is virtually featureless, the CV of PANI-Fe-C reveals a pair of well-developed redox peaks at ca V (Fig. S1). The half-height width of these peaks is ca. 100 mv, very close to the theoretical value of 96 mv, expected for a reversible one-electron process involving surface species (S3). There are two surface processes that can possibly give rise to the observed redox behavior in this case: (i) one-electron reduction/oxidation of the surface quinone-hydroquinone groups (S4) and (ii) Fe 3+ /Fe 2+ reduction/oxidation. In support of the latter reaction, an in situ electrochemical X-ray absorption study of the PANI-Fe-C system experiment shows good correlation between the change in the oxidation state of Fe species in the catalysts and the potential of the reversible CV feature in the CV of the PANI-Fe-C catalyst in Fig. S1. Koutecky-Levich plots of electrode potential vs. mass transport-corrected (kinetic) current density were used to determine the Tafel slope (b) and exchange current density of the ORR (i 0 ) (Fig. S2). The average values of the Tafel slope in the rpm range from 400 to 2500 were found to be 67 and 87 mv dec -1 for PANI-Co-C and PANI-Fe-C catalysts, respectively. Both these values differ from the well-established dual Tafel slope for the ORR on Pt (60 mv dec -1 and 120 mv dec -1 at potential higher and lower than 0.8 V, respectively) (S5). The Tafel slope values of 4

6 Current density (ma/mg) mv dec-1 observed with the PANI-Co-C catalyst (Fig. S2a) suggests that ORR rate on that catalyst may be determined by migration of adsorbed oxygen intermediates (S6). In the case of PANI-Fe-C, the Tafel slope of 87 mv dec-1 is indicative of a more complicated ORR mechanism, with rate-determining step likely involving both the migration of intermediates and electron transfer. The difference in Tafel slope values for PANI-Co-C and PANI-Fe-C also implies a different nature of the active ORR site in these two cases. 0 PANI-C -3 PANI-Co-C 100 mv PANI-Fe-C 0.2 One Electrone Exchange: 96 mv Potential (V vs NHE) Fig. S1. Cyclic voltammetry of PANI-derived ORR catalysts in N2-saturated 0.5 M H2SO4. Scan rate: 10 mv s-1; cell temperature: 25 C. The values of the ORR onset potential, half-wave potential, Tafel slope, and exchange current density for the two PANI-derived catalysts are listed in Table S1. Notably, the i0 value for the PANI-Fe-C catalyst ( A cm-2) is by two orders of magnitude higher than that measured with the PANI-Co-C catalyst ( A cm-2), confirming the generally observed much higher activity of Fe-based catalysts. For example, the current density of A cm-2 was reported for a Co(II) TSP catalyst (S7), much lower than the value of 10-7 A cm-2 measured with a FePc non-precious metal catalyst (S8). The i0 value for Pt is primarily dependent on the catalyst morphology, increasing from A cm-2 on partially oxidized Pt surface at high potentials to approximately around 10 7 A cm-2 the reduced surface of metallic Pt at lower potentials (S9, S10). 5

7 Potential (V vs NHE) rpm, b=68 mv/dec 900 rpm, b=67 mv/dec 1600 rpm, b=66 mv/dec 2500 rpm, b=69 mv/dec Kinetic current density (ma/cm 2 ) (a) 0.90 (b) Potential (V vs NHE) rpm, b=87 mv/dec 900 rpm, b=87 mv/dec 1600 rpm, b=87 mv/dec 2500 rpm, b=87 mv/dec Kinetic current density (ma/cm 2 ) Fig. S2. Tafel ORR plots measured in O 2 -saturated 0.5 M H 2 SO 4 with (a) PANI-Co-C and (b) PANI-Fe-C at various rotating speeds of the RDE. Table S1. ORR kinetic data for PANI-derived catalysts measured on RDE at 900 rpm. Catalyst ORR onset potential * (V) E ½ (V) Tafel slope (mv dec -1 ) i 0 (A cm -2 ) PANI-Fe-C PANI-Co-C * In order to minimize the effect of residual currents on the potential value the onset potential in this research has been defined as a potential required for generating an ORR current density of 0.1 ma cm -2 in a steady-state RDE experiment. 6

8 3. FTIR Data FTIR spectra indicate that the benzene-type (1100 cm -1 ) and quinone-type (1420 cm -1 ) structures on the main polyaniline chain break into small species (such as C=N) groups at temperatures above from 600 C (Fig. S3). Fig. S3. Effect of the heat-temperature on FTIR spectra of PANI-Fe-C catalysts. 4. SEM Images as a Function of Heat-Treatment Temperature Fig. S4. SEM images of PANI-Fe-C catalyst as a function of heat-treatment temperature (scale bar for the all SEM images is 500 nm; scale bar for the HR-TEM inset in the before pyrolysis image is 20 nm; scale bar in the 900 C image is 5 nm). 7

9 5. Catalyst Stability Test by Potential and Voltage Cycling Stability of the PANI-Fe-C catalyst was studied in this work under potential (voltage) cycling conditions, following an approach widely used in the testing of Pt-based fuel cell catalysts (accelerated stress testing, AST). The results of a potential-cycling experiment using an RDE and a voltage-cycling experiment in a fuel cell are shown in Fig. S5. 10,000 cycles of the PANI-Fe-C catalyst in N 2 -saturated 0.5 M H 2 SO 4 in the potential range from 0.6 to 1.0 V resulted in 10 mv potential loss at the half-wave potential (Fig. S5a). The good stability of the catalyst was confirmed in the fuel cell; 30,000 voltage cycles in N 2 in the voltage range of V led to no more than 20% loss in the current density at a reference fuel cell voltage of 0.8 V (Fig. S5b). At voltages below ~0.6 V, the performance was found to improve with cycling, most likely due to the dissolution of non-reactive phase of the catalyst and ensuing improvement in the mass-transport properties within the electrode. The results of cycling experiments provide further evidence of a very good stability of PANI-derived catalysts, initially determined in long-term fuel cell testing at a constant voltage. Current density (ma/cm 2 ) Cell voltage (V) (a) K 5K 10 K (b) Cycle number Potential (V vs RHE) Cycle number 0.4 initial 1K 5K K 20K cycling increasing K Current density (A/cm 2 ) Fig. S5. Durability test of the PANI-Fe-C catalyst by cycling in nitrogen-gas in the potential (RDE) and voltage (fuel cell) range from 0.6 to 1.0 V: (a) RDE experiment in 0.5 M H 2 SO 4 (catalyst loading 0.6 mg cm -2 ); (b) fuel cell experiment (back pressure: H 2 -O 2 / bar; cathode catalyst loading 2.0 mg cm -2 ). 8

10 6. SEM Images of PANI-FeCo-C(1) Catalyst Fig. S6. SEM images of the PANI-FeCo-C(1) catalyst: (a) before and (b) after heat treatment at 900 C and acid leach. 7. HR-TEM Characterization HR-TEM images of well fuel-cell performing PANI-FeCo-C(1) catalysts at different synthesis stages are shown in Fig. S7. Polyaniline in situ polymerized and deposited onto carbon black particles to form a thin film is depicted in Fig. S7a. In that figure, carbon black particles with dark contrast are covered by PANI with a relative light contrast. After the heat treatment, the composite graphitic nanostructures are formed as a result of the carbonization of polyaniline-feco complex and mixing with the particles of carbon support (Fig. S7b). Metal aggregates encapsulated into an onion-like graphitic shell are also seen (Fig. S7c). The electron diffraction pattern shown in Fig. S7d indicates a crystalline nature of the metal aggregates. Hollow carbon nanostructures can be observed, the result of metal-particle dissolution from the graphitic carbon shells during the acid leach (Fig. S7e). The growth of well-defined, metal-encapsulating, nitrogen-doped, onion-like carbon structures during the heat treatment can be explained using the vapor-liquid-solid model (S11). Briefly, during the carbonization of PANI-M complexes, gaseous carbon, nitrogen, and metal species are simultaneously formed. Metal atoms congregate first to form metal nanoparticles, then carbon and nitrogen species are gradually captured by the metal nanoparticles, and immediately catalytically decomposed into carbon and nitrogen atoms, respectively. These atoms participate in the formation of small nitrogen-doped carbon fragments around the metal particles. Structural defects, especially dangling bonds at the edges of carbon fragments, are likely to act as centers for the further assembly and re-arrangement of carbon fragments to ultimately form layered structures on the surface of metal particles. It is possible that nitrogen inclusion facilitates the 9

11 formation of nanocrystallites and suppresses diffusion of the carbon on the surface of metallic particles during graphitization, thereby leading to an increase in the d-spacing of the (002) crystal planes in the graphitic structures (S12). Aside from the onion-like graphitic nanoshells, graphitized carbon nanofibers are also formed in PANI-derived catalysts (Fig. S7c). The disorder in the nanofiber structure (Fig. S7f) may be attributed to the doping of nitrogen atoms into graphitic carbon structures (S13, S14). The onion-like carbon and nanofibrous structures are likely to play an important role in the ORR. Fig. S7. HR-TEM images of a PANI-FeCo-C(1) catalysts synthesized (a) before and (b-f) after a heat treatment at 900 C. To gain an insight into the role of Co and Fe in the catalyst synthesis, especially during the decomposition of polyaniline, the nanostructures of different catalysts was studied using HR-TEM and SEM (Fig. S8). The multi-layered graphene-sheet structures are abundant in PANI-Co-C (after pyrolysis and acidic leaching), but not in PANI-C and PANI-Fe-C. This implies that Co is more effective in catalyzing the formation of graphene sheets at higher heat-treatment temperatures. 10

12 Fig. S8. Representative HR-TEM and SEM images of PANI-C, PANI-Co-C and PANI-Fe-C catalysts. References S1. G. A. Rimbu, C. L. Jackson, K. Scott, J. Optoelectron. Adv. Mater. 8, 611 (2006). S2. S. L. Gojkovic, S. Gupta, R. F. Savinell, Electrochim. Acta 45, 889 (1999). S3. E. Laviron, J.Electroanal.Chem. 52, 395 (1974). S4. K. Kinoshita, J. A. S. Bett, Carbon 11, 403 (1973). S5. N. Wakabayashi, M. Takeichi, M. Itagaki, H. Uchida, M. Watanabe, J. Electroanal. Chem. 574, 339 (2005). S6. C. Coutanceau, M. J. Croissant, T. Napporn, C. Lamy, Electrochim. Acta 46, 579 (2000). S7. J. Zagal, R. K. Sen, E. Yeager, J. Electroanal. Chem. 83, 207 (1977). S8. M. Savy, P. Andro, C. Bernard, G. Magner, Electrochim. Acta 18, 191 (1973). S9. C. Song et al., Electrochim. Acta 52, 2552 (2007). S10. H. Meng, P. K. Shen, J. Phys. Chem. B 109, (2005). S11. B. S. Xu, New Carbon Mater. 23, 289 (2008). S12. G. M. Fuge, C. J. Rennick, S. R. J. Pearce, P. W. May, M. N. R. Ashfold, Diamond Relat. Mater. 12, 1049 (2003). S13. P. H. Matter, E. Wang, M. Arias, E. J. Biddinger, U. S. Ozkan, J. Phys. Chem. B 110, (2006). S14. J. W. Jang, C. E. Lee, S. C. Lyu, T. J. Lee, C. J. Lee, Appl. Phys. Lett. 84, 2877 (2004). 11