6597
2007, 111, 6597-6600 Published on Web 04/13/2007
Two-Step Carbonization as a Method of Enhancing Catalytic Properties of Hemoglobin at the Fuel Cell Cathode Jun Maruyama,*,† Junji Okamura,‡ Kuninori Miyazaki,‡ and Ikuo Abe† EnVironmental Technology Department, Osaka Municipal Technical Research Institute, 1-6-50, Morinomiya, Joto-ku, Osaka 536-8553, Japan, and Catalyst Research Center, Nippon Shokubai Co., Ltd., 992-1, Aza Nishioki, Okihama, Aboshi-ku, Himeji, Hyogo 671-1292, Japan ReceiVed: February 20, 2007; In Final Form: March 21, 2007
Recently, there has been increased demand for a polymer electrolyte fuel cell (PEFC) that functions efficiently with far less or no Pt. In this study, the catalyst for the O2 reduction at the cathode was formed by carbonizing hemoglobin, which could be abundantly and inexpensively obtained. A substantial enhancement of catalytic properties was achieved by a change in the carbonization process from one step to two steps. The PEFC using the carbonized hemoglobin formed in the modified carbonization process generated a high power comparable to those formed using a Pt-free cathode as reported already. The power densities of 0.11 and 0.16 W cm-2 were attained using H2 and O2 gases at atmospheric pressure (H2 and O2 partial pressures, 54 kPa) and using pressurized gases (H2 and O2 partial pressures, 254 kPa), respectively. The carbonization modification reduced the Fe valence state from 3+ to 2+ in the carbonized hemoglobin, which was demonstrated by X-ray photoelectron spectroscopy. The catalysis enhancement was probably attributed to the increase in Fe(II), which was required as an O2 bonding site in the first step of the multistep cathodic O2 reduction process.
A polymer electrolyte fuel cell (PEFC), which consists of an anode, a cathode, and a polymer electrolyte sandwiched between them, is an electricity generator through anodic H2 oxidation and cathodic O2 reduction with only H2O being formed in principle. Because of the fast ionic conduction in a perfluorosulfonate ion-exchange membrane used as the polymer electrolyte, the PEFC is able to efficiently generate a high power around 80 °C when active heterogeneous catalysts are used in the electrodes. This temperature is within a reasonable temperature range for start-up of the PEFC from room temperature and makes the PEFC advantageous for use as a power source of electric vehicles and cogeneration systems for domestic electricity and heating.1 Nanoparticles of Pt or Pt alloys supported on electron-conductive carbon black have hitherto been used as the catalyst. However, the limitation of Pt reserves and supply will prohibit the widespread use of the PEFC. Because its widespread use is a prerequisite to realize an improvement of the environment,2 there is an increased demand for a catalyst that functions with far less or no Pt, especially for the cathode due to the slow O2 reduction, which requires a sufficient amount of Pt.3 Several PEFCs have been reported using a Pt-free cathode. The cathode catalyst prepared by pyrolyzing a Fe-porphyrin complex adsorbed on a carbon material, which is one of the most extensively studied types of Pt-free catalysts, generated 0.20 W cm-2 in the PEFC at the O2 partial pressure, p(O2), of * Corresponding author. E-mail:
[email protected]. Tel: +81-6-6963-8043. Fax: +81-6-6963-8049. † Osaka Municipal Technical Research Institute. ‡ Nippon Shokubai Co.
10.1021/jp071451+ CCC: $37.00
4.0 × 102 kPa.4,5 The Pd-Co-Au alloy supported on carbon black generated 0.13 W cm-2 at the p(O2) of 2.2 × 102 kPa.6,7 The PEFC using pyrolyzed Fe(II) acetate on N-enriched carbon generated 0.22 W cm-2 at the p(O2) of 2.6 × 102 kPa.8 Quite recently, it was reported that the catalyst consisting of a Copolypyrrole-carbon black composite generated 0.15 W cm-2 at the p(O2) of 2.6 × 102 kPa with an excellent durability.7 We previously formed a Pt-free catalyst by carbonizing hemoglobin, generating 0.045 W cm-2 at the p(O2) of 54 kPa.9 Hemoglobin could be abundantly obtained (about 2 million tons per year), especially from the meat industry producing more than 200 million tons of meat per year around the world and discarding the blood containing hemoglobin as waste. To utilize the abundance and inexpensiveness of hemoglobin, which are advantageous for the widespread use of the PEFC, we attempted the enhancement of the catalytic properties of the carbonized hemoglobin. In our previous study, hemoglobin was carbonized by heat treatment in flowing Ar at 825 °C for 2 h (this carbonized hemoglobin is hereafter called CHb825). The active site for the cathodic O2 reduction in CHb825 was assumed to be Fe(III) coordinated by four pyrrolic N atoms (Fe-N4 moiety) embedded on the surface of the carbon material, which was derived from protoheme in the hemoglobin. This assumption was based on our previous study showing the presence of the Fe-N4 moiety in the carbonized catalase which also contains protoheme10 and other previous studies reporting that the porphyrin-like structures were retained in the carboneous compounds after the pyrolysis of their precursor containing metal porphyrins, based on an analysis using X-ray photoelectron spectroscopy,11 X-ray adsorption © 2007 American Chemical Society
6598 J. Phys. Chem. C, Vol. 111, No. 18, 2007
Letters
TABLE 1: Activity and Selectivity of Carbonized Hemoglobin as a Catalyst for Cathodic Oxygen Reductiona
CHb825 CHb350700 CHb350800 CHb350900 CHb3501000 CHb350900C
IK (A g-1)b
nc
1.17 3.21 × 10-2 0.161 1.81 1.89 4.56
3.3 2.0 2.1 3.1 2.8 3.3
specific Fe pore content surface area volume (m2 g-1)d (mm3 g-1)d (wt %) 0.47 0.86 0.64 0.59 0.43 0.26
919
477
38 139 240 700
25 99 174 484
a The Fe contents, specific surface areas, and pore volumes are also listed. b IK was determined from the O2 reduction current per mass of catalyst at 0.7 vs a reversible hydrogen electrode (RHE) used as a reference electrode. c n was determined at 0.1 V vs RHE. d Specific surface area was determined by the Brunauer-Emmet-Teller (BET) plot of the N2 adsorption isotherm at -196 °C and the pore volume by the amount of adsorbed N2 at the relative pressure of 0.931 (corresponding to the pore volume below pore diameter of 30 nm). The amount of N2 adsorption on CHb350700 was insufficient to determine the values.
near-edge structure spectroscopy,12 and Mo¨ssbauer spectroscopy.13 The valence state of Fe in the active site in CHb825 was based on the X-ray photoelectron spectrum (XPS) of Fe 2p.9 This active site was simultaneously formed with carbon matrix as a support of the site, in contrast to the catalysts mentioned previously, in which the carbon materials were formed in advance and used as supports of the catalyst. Hence, we expected that altering the carbonization conditions would lead to modification of the carbon matrix and the active site and enhance the catalytic properties of the carbonized hemoglobin. In this study, hemoglobin was first decomposed in flowing Ar at 350 °C for 10 h. This temperature was chosen in order to exceed the decomposition temperature of tyrosine (344 °C), the highest among the amino acids in the hemoglobin. The obtained precursor was then heat-treated in flowing Ar at 7001000 °C for 2 h. The soluble Fe species in the carbonized material were removed using 0.5 mol dm-3 H2SO4 to obtain a sample for use as the catalyst for the O2 reduction. The catalyst produced at 700 °C in the second heat treatment is hereafter called CHb350700 and the others in a similar manner. The catalysis was first examined using glassy carbon (GC) rotating disk electrodes, forming thin layers of the catalyst on the GC surface and immersing it in an aqueous solution of 0.1 mol dm-3 HClO4. The O2 reduction current without influence of the mass transfer in the solution, IK, and the number of electrons involved in the reduction per O2 molecule, n, were determined and used as indicators of the activity and selectivity of the catalyst layer. These values are listed in Table 1. An increase in n could occur when H2O2 generated by the twoelectron reduction (reaction 1) is further reduced to H2O (reaction 2), or decomposed according to reaction 3, or when the proportion of the four-electron reduction to H2O (reaction 4) increases.
O2 + 2H+ + 2e- f H2O2
(1)
H2O2 + 2H+ + 2e- f 2H2O
(2)
2H2O2 f 2H2O + O2
(3)
O2 + 4H + + 4e- f 2H2O
(4)
The occurrence of these reactions is closely related to the number of active sites, and also the pore development of the
carbon material, because the number of active sites would be increased by exposure to the pore surface inside the material due to the pore development. Another factor for the n increase due to the pore development is the possible occurrence of reaction 1 deep inside the pores, which increases the opportunity for reactions 2 and 3 to occur during the H2O2 transfer to the outside of the pores. IK and n increased with an increase in the temperature of the second heat treatment up to 900 °C; however, IK only slightly increased and n decreased when using CHb3501000, indicating that 900 °C was appropriate for the second heat-treatment step. A 55% increase in IK was attained compared to CHb825. The decrease in n might be attributed to the decrease in the active site in CHb3501000, which was associated with the Fe content decrease. The increase in the temperature of the second heat-treatment step probably increased the formation of the aggregated Fe species, such as Fe2O3 and metallic Fe,9 which were then removed during the catalyst production procedure. We further attempted the catalysis enhancement by developing micropores inside the carbon material to expose the active site, which was possibly hidden inside the carbon matrix of CHb350900, and found that using 25% CO2 + 75% Ar gases instead of pure Ar during the second heat-treatment step formed a carbonized material (CHb350900C) with a specific surface area of 700 m2 g-1. This result indicated that a simultaneous carbonization and pore development occurred during the second heat-treatment step. The latter phenomenon was attributed to CO2, which is a pore-forming agent in carbon materials and often used for the production of activated carbon. The increase in IK and n was attained compared to CHb350900 because of this pore development. The Fe content decrease in CHb350900C compared to CHb350900 might also be caused by the pore development, leading to the improved removal of the aggregated Fe species formed inside the catalyst particles. The PEFC was formed using CHb350900C in the cathode. The relationships between the cell voltage, the power density, and the current density were shown in Figure 1. The relationships for the PEFC formed using CHb825 were also shown, demonstrating the substantial performance improvement at the PEFC formed using CHb350900C, which attributed to the catalysis enhancement of the carbonized hemoglobin. The power density of 0.11 W cm-2 was attained for the PEFC formed using CHb350900C using humidified H2 and O2 gases at atmospheric pressure (H2 and O2 partial pressures, 54 kPa). The PEFC generated 0.16 Wcm-2 at p(O2) of 254 kPa, higher than the PEFC formed using the Pd-Co-Au alloy and Co-polypyrrole-carbon composite, but lower than the PEFC formed using the pyrolyzed Fe(II) acetate on N-enriched carbon. In addition, the power density was almost unchanged at the higher p(O2) (454 kPa) compared to that at 254 kPa. The reason for the lack of a power density increase is still unclear at present, but a possible reason might be the low dependency of the activity of the carbonized material on p(O2). Nevertheless, the substantial enhancement of the activity for O2 reduction leading to the much-improved power density of the PEFC was achieved, showing the potential of hemoglobin as a raw material for the PEFC catalysts. The much higher activity at CHb350900C than CHb825 is most probably attributed to the change in the Fe valence state, which was demonstrated by the XPS shown in Figure 2 because the other factors for the catalysis were unfavorable for the total enhancement at CHb350900C; the lower Fe content, the lower specific surface area, and nearly the same pore volume (Table 1). The negative peak shift in the XPS was observed for
Letters
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6599
Figure 1. Relationships between (a) cell voltage and current density, (b) power density and current density generated by PEFCs formed using CHb825 (O), and CHb350900C (4) under atmospheric pressure. Cell temperature: 80 °C. H2 and O2 were supplied to the cell at 100 cm3 min-1 after humidification to contain 47 kPa of water vapor. The partial pressures of H2 and O2 were 54 kPa. The cathodes contained the following: the carbon materials, 10 mg cm-2 (Fe content: CHb350900C, 26 µg cm-2; CHb825, 47 µg cm-2); carbon black, 1 mg cm-2; polymer electrolyte (Nafion), 10 mg cm-2. The anode was formed using carbon black loaded with Pt (10 wt %), 1 mg cm-2 and Nafion, 0.5 mg cm-2. The electrode area was 5 cm2. Relationships for PEFC formed using CHb350900C under backpressure of H2 and O2 at 200 kPa are also shown (0). The partial pressures of H2 and O2 were 254 kPa.
Figure 2. X-ray photoelectron spectra of Fe 2p in CHb825, CHb350900, and CHb350900C. Each spectrum was arbitrary shifted in the y-axis direction for easier comparison.
CHb350900C compared to CHb825, suggesting a decrease in the Fe valence state from 3+ to 2+ based on the peak position of the Fe 2p3/2 binding energy: Fe(III), 710.8-711.8 eV; Fe(II), 707.1-708.7 eV.14,15 It has been reported that a similar negative peak shift was related to the activity increase for the O2 reduction at the catalyst formed by pyrolyzing Fe(III) tetra(p-chlorophenyl)porphyrin chloride adsorbed on carbon black.16 The activity increase with an increase in Fe(II) could be explained by applying the O2 reduction mechanism proposed by Bouwkamp-Wijnoltz et al. for the pyrolyzed Fe porphyrin adsorbed on carbon, in which Fe(II) on the catalyst surface is required as an O2 bonding site during the first step of the multistep reaction,17 assuming that the mechanism is also valid for the carbon materials produced in this study containing Fe on the surface. This result indicates that the Fe valence state change and the catalysis enhancement were achieved by the carbonization modification. The almost equal binding energy of Fe 2p in the XPS for CHb350900 and CHb350900C was observed in Figure 2, suggesting the crucial effect of the introduction of first heat-treatment step on the Fe valence state in the carbonized hemoglobin and the independence of the flowing gas during the second heat treatment. An elemental analysis was carried out for hemoglobin and the precursor obtained by the first heat treatment. A signif-
icant change occurred in the oxygen content: hemoglobin, 24.0 wt %; the precursor, 6.5%. This lower oxygen content in the precursor than in hemoglobin might generate a more reducing atmosphere during the second heat treatment that produces the carbonized material with Fe in the lower valence state. In conclusion, the modification of the carbonization conditions caused an enhancement of catalytic properties for the cathodic O2 reduction of carbonized hemoglobin, an enormous amount of which is currently discarded as waste, which led to the formation of a PEFC generating a power density comparable to the highest level of the Pt-free cathode-type PEFCs reported previously. However, the power is still lower than conventional Ptbased PEFCs that can generate over 0.5 W cm -2 at atmospheric pressure using H2 and O2.18 In addition, in contrast to the excellent durability reported for the Co-polypyrrole-carbon black composite catalyst,7 a 100-h continuous operation of the PEFC formed from CHb350900C at 0.5 V showed a 53% current decrease, although the durability was improved compared to CHb825 that showed a 70% decrease. Further studies are continuing in order to enhance the catalysis and improve the durability using hemoglobin because its possibility as a raw material for Pt-free PEFC cathode catalysts was raised in this study. Acknowledgment. We thank Dr. T. Ioroi for his help with the fuel cell tests, Mr. H. Kawano for his help with the ICPAES measurements, and Dr. T. Hasegawa for discussions on the measurement of the N2 adsorption isotherm. This study was partly supported by a Grant-in-Aid for Scientific Research (project no. 18750183) given to J.M. from the Ministry of Education, Culture, Sports, Science and Technology, Japan, for which the we are grateful. The hemoglobin structure in table of contents image is cited from The Protein Data Bank (DOI: 10.2210/pdb2dn2/pdb). References and Notes (1) Handbook of Fuel Cells; Vielstich, W., Lamm, A., Gasteiger, H. A., Eds.; John Wiley & Sons: Chichester, England, 2003. (2) Jacobson, M. Z.; Colella, W. G.; Golden, D. M. Science 2005, 308, 1901. (3) Gasteiger, H. A.; Panels, J. E.; Yan, S. G. J. Power Sources 2004, 127, 162. (4) Me´dard, C.; Lefe`vre, M.; Dodelet, J. P.; Jaouen, F.; Lindbergh, G. Electrochim. Acta 2006, 51, 3202. (5) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, T. F. Appl. Catal., B 2005, 56, 9. (6) Ferna´ndez, J. L.; Raghuveer, V.; Manthiram, A.; Bard, A. J. J. Am. Chem. Soc. 2005, 127, 13100. (7) Bashyam, R.; Zelenay, P. Nature 2006, 443, 63.
6600 J. Phys. Chem. C, Vol. 111, No. 18, 2007 (8) Jaouen, F.; Charreteur, F.; Dodelet, J. P. J. Electrochem. Soc. 2006, 153, A689. (9) Maruyama, J.; Abe, I. Chem. Mater. 2006, 18, 1303. (10) Maruyama, J.; Abe, I. Chem. Mater. 2005, 17, 4660. (11) Widelo¨v, A.; Larsson, R. Electrochim. Acta 1992, 37, 187. (12) Jones, J. M.; Zhu, Q.; Thomas, K. M. Carbon 1999, 37, 1123. (13) Herod, A. J.; Gibb, T. C.; Herod, A. A.; Xu, B.; Zhang, S.; Kandiyoti, R. Fuel 1996, 75, 437.
Letters (14) Johansson, L. Y.; Larsson, R. Chem. Phys. Lett. 1974, 24, 508. (15) Choudhury, T.; Saied, S. O.; Sullivan, J. L.; Abbot, A. M. J. Phys. D: Appl. Phys. 1989, 22, 1185. (16) Widelo¨v, A. Electrochim. Acta 1993, 38, 2493. (17) Bouwkamp-Wijnoltz, A. L.; Visscher, W.; van Veen, J. A. R. Electrochim. Acta 1998, 43, 3141. (18) Passos, R. R.; Paganin, V. A.; Ticianelli, E. A. Electrochim. Acta 2006, 51, 5239.
