Hemin: A Highly Effective Electrocatalyst Mediating the Oxygen

ARTICLE pubs.acs.org/JPCC

Hemin: A Highly Effective Electrocatalyst Mediating the Oxygen Reduction Reaction Zhen-Xing Liang,* Hui-Yu Song, and Shi-Jun Liao School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, P.R. China

bS Supporting Information ABSTRACT: Carbon-supported hemin (hemin/C) was investigated as a nonpreci ous metal electrocatalyst mediating the oxygen reduction reaction (ORR). The effect of the heat treatment and metal content on the electrocatalytic activity was extensively studied. We found that the electrocatalytic activity was significantly improved after the heat treatment, and the best performance was achieved at the treatment temperature of 600 °C. Physico-chemical characterizations indicated that the heat treatment temperature yielded a coincident effect on the composition and microstructure of the catalyst. X-ray photoelectron spectroscopy (XPS) revealed that the atomic ratio of Fe to N was 1:4.7 for pristine hemin/C, which amounted to the highest value of 1:1.9 for hemin/C heat-treated at 600 °C. Nitrogen ad/desorption analysis showed that the pyrolysis at 600 °C resulted in the largest microporous surface area. On the basis of the above findings, we suggest that FeN2 be the active center in hemin/C and the micropore play an important role in catalyzing the ORR. Furthermore, we also found that the electrocatalytic activity of hemin/C was improved with increasing the metal content. More inspiringly, we report that the optimized hemin/C electrocatalyst could yield a comparable electrochemical performance with the commercial Pt catalyst in alkaline media.

1. INTRODUCTION Hemin, a natural porphyrinatoiron complex, shows wide potential applications as the heterocatalyst in addressing the environment and energy related issues, including the electrochemical reduction of uranium,1 nitrite,2,3 nitric oxide,4-6 hydrogen peroxide,7-9 carbon dioxide,10 and the oxidation of peroxynitrite.11 Also, hemin is found to be able to electrocatalyze the oxygen reduction reaction (ORR).12-17 For example, Antoniadou et al.12 reported that hemin was active to the ORR in both aqueous and methanolic solutions. Arifuku et al.13 studied the effect of pH on the electrocatalytic reduction of oxygen at a hemin/glassy carbon electrode. They found that oxygen was reduced via a one-step reduction through a four-electron way at pH < 11 and via two successive reductions at pH > 12. Platinum is the most widely used electrocatalyst in lowtemperature fuel cells; however, its scarcity and high cost has been the key issue hindering the development of this technology. To explore nonprecious metal electrocatalysts is one of the most challenging tasks in the fuel cell community.18-23 Metallomacrocyclics, for example, cobalt or iron porphyrin, has been regarded as the most promising alternative electrocatalyst, and enormous work has been devoted to improve the electrocatalytic activity.18,19 As mentioned above, hemin does yield electrocatalytic activity for the ORR, and this material is easily available and pretty cheap in cost. Therefore, we believe that hemin will be a potential nonprecious metal electrocatalyst to be used in fuel cells or sensors. r 2011 American Chemical Society

Previous work has been focused on the fundamental issues of electrocatalysis; however, little work has been done on improving the electrocatalytic activity of hemin to meet the fuel cell demand. In this work, we test the applicability of hemin as the fuel cell electrocatalyst for the ORR in either acid or alkaline media. It is found that pristine hemin shows high overpotentials, which however can be significantly decreased by the heat treatment at high temperatures. Also, the performance can be further improved by increasing the metal content in the catalyst. It is inspiring to note that the optimized hemin/C catalyst yields comparable activity to the Pt catalyst in alkaline media.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Hemin (ferriprotoporphyrin IX chloride, >95%) was purchased from Shanghai Boao Biochemistry Co. Ltd. (Shanghai, China) and used as received without further purification. Vulcan XC-72 carbon was supplied by ETEK Company and pretreated in the solution containing 10% HNO3 and 30% H2O2 at 80 °C for 5 h. All other chemicals were of reagent grade. 2.2. Catalyst Preparation. Hemin was first dissolved in N,Ndimethylformamide (DMF) to form the solution of 39.0 mM. The impregnation of hemin on carbon powders was then Received: November 25, 2010 Revised: December 14, 2010 Published: January 7, 2011 2604

dx.doi.org/10.1021/jp1112334 | J. Phys. Chem. C 2011, 115, 2604–2610

The Journal of Physical Chemistry C performed as follows. A portion of 200.0 mg carbon powder was added into the hemin solution of specified volume, followed by being ultrasonicated for 2 h to form a homogeneous paste. Then, the paste was transferred into a vacuum oven at 70 °C to evaporate the solvent. Finally, the resultant powders were ball-milled and heat-treated under a constant flow of nitrogen at three different temperatures (400, 600, and 800 °C) for 2 h in a tube furnace. The pristine and treated samples were referred to as hemin/C and hemin/C(X), respectively. Here, X refers to the heat-treated temperature, namely, 400, 600, and 800 °C. 2.3. Physico-Chemical Characterization. Thermogravimetric analyses (TGA) were made using a TA Instrument SDT 2960. The experiment was performed at 20 °C min-1 from room temperature to 800 °C in either nitrogen (99.999%) or air at a flow rate of 20 mL min-1. The X-ray photoelectron spectroscopy (XPS) measurement was carried out with a Physical Electronics PHI 5600 multitechnique system using an Al monochromatic X-ray at a power of 350 W. Nitrogen adsorption/desorption isotherms were measured at 77 K using a Micromeritics TriStar II 3020 analyzer. Before adsorption measurements, each sample was outgassed under vacuum for at least 2 h at 200 °C. The total surface area was analyzed with the wellestablished BET method, and the microporous surface area was obtained with the MP method (t-plot method) provided by the software package. 2.4. Electrochemical Characterization. The electrochemical behavior of the catalyst was characterized by the cyclic voltammetry (CV) and linear sweeping votammetry (LSV) using a three electrode cell with an IVIUM electrochemical workstation at room temperature (25 °C). A platinum wire and an Ag/AgCl (saturated KCl solution) electrode were used as the counter and reference electrodes, respectively. The working electrode was a glassy carbon disk (5 mm in diameter) covered with a thin layer of Nafion-impregnated catalyst. Typically, the thin-film electrode was prepared as follows. A portion of 10 mg of the catalyst was dispersed in 1 mL of Nafion/ethanol (0.84 wt % Nafion) by sonication for 20 min. Then, 10 μL of the dispersion was transferred onto the glassy carbon disk by using a pipet. The metal loading of the catalyst on the electrode surface was 0.0086 mg. For comparison, we also measured the electrocatalytic activity for ORR of the commercial Tanaka 50 wt % Pt/C catalyst with the same metal loading on the electrode. The electrolyte solution, for example, 0.10 M HClO4 or 1.0 M KOH, was first bubbled with nitrogen for 40 min. Then, a CV test was conducted at 20 mV s-1 in the potential range between 0 and 1.2 V (vs RHE) for 20 cycles. LSV was collected by scanning the potential from 0 up to 1.2 V at 20 mV s-1 in the oxygen-saturated electrolyte solution under 1600 rpm, from which the ORR activity was extracted by subtracting the capacitive current (Ic).24 The upward scan in 20th cycle of CV in nitrogensaturated electrolyte was used as the capacitive current. The current, I, during the upward scan in oxygen-saturated electrolyte was corrected by subtracting Ic to yield the Faradaic current, IF.

3. RESULTS AND DISCUSSION Figure 1 presents the TG curves of hemin, pristine hemin/C, and the pyrolyzed hemin/C in nitrogen. It can be seen that all of the samples yield weight loss in the measured temperature range;

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Figure 1. TG curves of carbon, hemin, hemin/C, and the heat-treated hemin/C in nitrogen.

however, the shape of the curve varies greatly for the five samples. Pure hemin shows four stages of weight loss, and the temperature of each stage can be clearly recognized. The first three waves correspond to the relative weight loss of about 5%, 15%, and 15% at 150-300, 330-440, and 440-700 °C, respectively. In comparison, for the hemin/C samples, the weight loss does occur with increasing the temperature; however, the temperature range for each wave changes significantly. Hemin/C shows three waves of weight loss, which correspond to 120-300, 300-500, and >500 °C, respectively. For the pyrolyzed samples, the weight loss is only found at above 350 °C, which can be attributed to the decomposition of the residue hemin after the prepyrolysis. The change in the composition of hemin can be recognized based on the relative weight loss at each stage. As such, for pure hemin, the first stage of weight loss (150-330 °C) can be attributed to the removal of chloride. The second stage mainly corresponds to the decomposition of the organic groups attached on the macrocycles, such as ;CH3, ;CHdCH2, and ;CH2CH2COOH. The third wave should result from the weight loss in the carbonization process of hemin, including the dehydrogenation and decomposition of the macrocycles. Therefore, the composition of hemin can be tuned by controlling the pyrolysis temperature. After being impregnated onto carbon support, hemin shows different thermochemical behaviors. The beginning temperature of weight loss shows a negative shift to 120 °C, indicating that there exists specific interaction between hemin and carbon support. Such an interaction can be understood as follows. First, it is understandable that there exists electronic interaction between hemin and carbon support, as further evidenced by XPS. Second, the anionic groups, for example, -COO-, on the carbon surface may be exchanged with the chloride ion in hemin, thereby releasing and facilitating its removal. Figure 2 shows the XPS spectra of Fe 2p (Figure 2a) and N 1s (Figure 2b). From Figure 2a, it can be seen that, upon being supported on the carbon support, the binding energy of Fe 2p in hemin shifts from 712.5 (2p3/2) and 726.3 eV (2p1/2) to 711.5 (2p3/2) and 724.9 eV (2p1/2), respectively. Such a pronounced shift (ca. 1 eV) in binding energy suggests that there exists a strong electronic interaction between hemin and carbon support, which is consistent with the TGA. For hemin/C pyrolyzed at different temperatures, the position of Fe 2p remains almost 2605

dx.doi.org/10.1021/jp1112334 |J. Phys. Chem. C 2011, 115, 2604–2610

The Journal of Physical Chemistry C

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Figure 2. XPS curves of Fe 2p (a) and N 1s (b) of hemin, hemin/C, and the heat-treated hemin/C.

Table 1. Content of Fe, N, and Cl of Hemin, Hemin/C, and Heat-Treated Hemin/C Cl content

N content

Fe content

Fe:N

(at.%)

(at.%)

(at.%)

(1:x)

hemin

0.87

5.9

1.38

1:4.3

hemin/C

0.23

1.8

0.37

1:4.7

hemin/C(400)

0.04

0.94

0.35

1:2.7

hemin/C(600) hemin/C(800)

0.04 0.04

0.98 0.33

0.51 0.08

1:1.9 1:4.1

unchanged, indicating that the heat treatment yields little effect on the electronic structure of metal ion when the pyrolysis temperature is above 400 °C. From Figure 2b, it is seen that the curve N 1s shows significant difference both in shape and peak position for the five samples. The binding energy shifts negatively from 401.8 to 398.5 eV after hemin is supported on carbon. This change is similar to that found for Fe 2p. For the pyrolyzed hemin/C, the binding energy shows negligible change when the treatment temperature is below 600 °C. A further increase in pyrolysis temperature (800 °C) yields a pronounced positive shift, indicating that the chemical environment of N atoms is significantly changed during the carbonization at 800 °C.

This may result in some effect on the electrocatalytic activity, as discussed later. On the basis of the XPS results, the content of Fe, N, and Cl can be quantified, as listed in Table 1. For pure hemin and untreated hemin/C, the measured element content agrees well with those calculated from the molecular formula. For pyrolyzed hemin/C, the element content changes with increasing the heat treatment temperatures. First, Cl cannot be detected at the pyrolysis temperature above 400 °C, indicating that Cl can be completely removed at 400 °C. Therefore, we confirm that, for pure hemin, the first stage of weight loss at 150-300 °C in the TG curve should be ascribed to the removal of chloride. Second, the N content basically decreases with increasing the pyrolysis temperature, which is due to the decomposition of the N-containing macrocycles of hemin. In comparison, with increasing the temperature, Fe content first increases until 600 °C and then drops at higher temperatures. The decrease in the surface Fe content can be rationalized by considering that Fe atoms may diffuse from the surface into the bulk carbon particle at higher temperatures. Table 1 also lists the ratio of Fe to N (1:x), which is helpful to understand the active center for catalyzing the ORR. The ratio is found to increase from 1:4.7 to 1:1.9 after hemin/C is heat-treated at 600 °C, which corresponds to FeN4.7 and FeN2 for pristine hemin/C and hemin/C(600). 2606


The Journal of Physical Chemistry C Figure 3 shows the change in the total surface area and the microporous surface area of hemin/C pyrolyzed at different temperatures. Both the surface area and the microporous surface area are found to increase after the heat treatment, amounting to the highest value at 400 and 600 °C, respectively. The increase in

Figure 3. Change in the total surface area and micropore surface area. (The point at 70 °C refers to the pristine hemin/C).

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the total surface area is easily understandable as a result of the decomposition of hemin at high temperatures. To explain the disagreement in the optimal temperature for the two surface areas, we here suppose that the decomposition of hemin on the outer surface of carbon support occurs prior to that in the micropores as the interaction between hemin and carbon is more pronounced in those micropores. Based on this understanding, the decomposition of hemin at higher temperatures (e.g., 600 °C) occurs more dominantly in the micropores, thereby yielding the largest microporous surface area. In summary, physicochemical characterizations reveal that the heat treatment yields a complicated effect on the composition and microstructure of hemin/C. As such, the redox behavior of hemin/C can thus be tailored by changing the heat treatment temperatures. Also, the interaction between hemin and carbon support can be enhanced by the pretreatment, further modulating redox potentials. Figure 4 presents the CV and the LSV curves of hemin/C and pyrolyzed hemin/C in 0.10 M HClO4 aqueous solution. From Figure 4a, it can be seen that for pristine hemin/C, multiple peaks are present in the cyclic voltammogram, which is however dominated by a pair of anodic and cathodic peaks centered at 0.70 and 0.44 V, respectively. The couple of peaks then show a positive shift after the pyrolysis at higher temperatures, and the largest positive shift is observed for hemin/C(600), of which the

Figure 4. CV curves (a), peak-to-peak potential splitting of each sample in CV curves (b), LSV curves at 1600 rpm (c), and the number of exchanged electrons in the ORR (d) for the pristine hemin/C and the hemin/C samples pyrolyzed at 400, 600, and 800 °C. (CV and LSV curves were collected in 0.10 M HClO4 solution saturated with nitrogen and oxygen, respectively.) 2607



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Table 2. Tafel Parameters of the LSV Curves at Low Overpotentials acid -1

hemin/C hemin/C(600)

alkaline

b (mV dec )

Rn

b (mV dec-1)

Rn

130

0.45

81

0.73

75

0.79

66

0.90

cathodic peak resides at 0.60 V. The presence multiple anodic/ cathodic peaks of pristine hemin/C can be attributed to the complicated redox couples existing in the electrocatalyst. First, there exist two redox pairs of iron ions, namely, Fe(II)/Fe(I) and Fe(III)/Fe(II), yielding two sets of redox peaks. Second, hemin may exist in monomeric or dimeric form,12,25,26 generating different adsorption states and thereby at least two redox pairs in the CV curve. The redox peak of monomeric hemin is adjacent to but resides at higher potentials than that of the dimeric hemin.12 In the acid solution, the redox peaks at higher potentials (ca. 0.7 V), corresponding to the monomeric hemin, are found to be gradually eliminated with potential cycling (see Figure S1 of the Supporting Information). This should be due to the exchange of iron ion by protons in acid electrolyte and the consequent loss of metal ion. In comparison, the redox pair in the dimeric form remains rather stable during the CV scanning. Therefore, the redox peaks in the stable CV curve can be mainly attributed to the charge transfer of the redox pair Fe(III)/Fe(II) in the dimeric hemin, as represented in the following equation: P- Fe3þ 3 3 3 Fe3þ - P þ 2e - 1 f P- Fe2þ 3 3 3 Fe2þ - P in which P refers to the ligand of metal ion. The dimeric hemin can be formed in two ways. First, dimeric hemin was reported to exist in the concentrated solution,12 thereby deposited onto the carbon support in the impregnation step. Also, the pyrolysis process may induce the formation of the oxo-bridged hemin. The positive shift in the redox peaks after the pyrolysis should be related to the fact that the coordination environment is changed due to the decomposition of macrocycles, as mentioned in XPS. The peak-to-peak potential splitting (ΔEp) in CV curves can be used to characterize the Nernstian reversibility of the redox pair. On the basis of this understanding, we have plotted ΔEp versus the treatment temperature, as shown in Figure 4b. It can be seen that ΔEp decreases with increasing the treatment temperature, reaching the lowest value (70 mV) at 600 °C, indicating that the redox couple is the most reversible for hemin/C(600). This result will yield very unique electrocatalytic behavior for the ORR. Figure 4c shows the polarization curves in the oxygen-saturated acid solution. After the pyrolysis, the electrocatalytic activity is improved, and the optimal pyrolysis temperature is found at 600 °C. The onset potentials for hemin/C and hemin600/C are ca. 0.70 and 0.82 V, respectively. This is basically consistent with the potential shift in the redox pair after the pyrolysis, suggesting that the redox pair in hemin is the active center and mediates the ORR. This can be further confirmed by the Tafel analysis. The Tafel slope is calculated and listed in Table 2. It can be seen that the Tafel slope decreases from 140 to 75 mV dec-1 after the pyrolysis at 600 °C, and the Rn value is calculated to be 0.45 and 0.79, respectively. As mentioned above, the ORR is mediated by the redox pair, and the reaction can be recognized as an E-C type one. As the redox pair shows the highest reversibility (R ≈ 0.5) for hemin/C(600), the number of

Figure 5. CV (a) and LSV (b) curves of hemin/C pyrolyzed at 600 °C with different metal contents. (CV and LSV curves were collected in 0.10 M HClO4 solution saturated with nitrogen and oxygen, respectively.)

the electrons exchanged in the electrochemical step is 1.6 (approximate to 2) for hemin/C(600).26 This conclusion is in good agreement with the CV finding that the dimeric hemin is the main active center for hemin/C(600). We also measured the i-V curves at different rotating disk electrode rates, from which the number of the exchanged electrons in the reaction can be extracted. The results are plotted versus electrode potentials in Figure 4d. It can be seen that the number increases in the order of hemin/C(800) < hemin/ C < hemin/C(400) < hemin/C(600). The number of hemin/ C(600) is in the range of 3.0-3.3, suggesting a nearly complete four-electron pathway for the ORR. In comparison, the number is only ca. 2 for hemin/C and hemin/C(800), indicating that the ORR proceeds in two-electron pathway and the final product is hydrogen peroxide. In summary, the electrocatalytic activity is optimized at the heat treatment temperature of 600 °C. This finding coincides with the previous findings on the Fe:N ratio and microporous surface area. Accordingly, we suggest that FeN2 be the dominant active center for the ORR, and the micropore play an important role in catalyzing this reaction. We further studied the effect of the metal content on the electrochemical behavior of the hemin/C pyrolyzed at 600 °C. Figure 5 shows the CV and LSV curves of hemin/C(600) with the metal content of 1, 2, and 3 wt %. The samples are referred to as hemin-1/C(600), hemin-2/C(600), and hemin-3/C(600) in 2608



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pyrolysis, and hemin-600/C only shows one cathodic peak at 0.840 V. The redox pair at ca. 0.28 V may be attributed to the single charge transfer between Fe(II)/Fe(I), and the cathodic peak at higher potentials can be attributed to the reduction of Fe(III) to Fe(II). So far, it is still unclear why the redox pair peaks disappear after pyrolysis, but this should be related to the change in the composition of hemin at high temperatures. The positive shift in the cathodic peak at higher potentials can be attributed to the decomposition-induced modulation on the ligand, which is consistent with the findings in acid media. Such a shift is beneficial for achieving higher electrocatalytic activity for the ORR, as seen in Figure 6b. The onset potential for hemin-1/C is ca. 0.92 V, which increases to be 1.0 V for hemin-1/C(600). Figure 6b also includes the ORR performance of hemin-2/ C(600) and the Tanaka Pt/C catalyst. Similar to that in acid solution, the metal content has a positive effect on the polarization performance. More inspiringly, it is found that, when the metal content is 2 wt %, the pyrolyzed electrocatalyst yields a similar ORR performance to the commercial Pt catalyst. Considering the facile availability and low cost, we suggest that hemin is a very promising candidate for the nonprecious metal electrocatalyst in fuel cells.

Figure 6. CV (a) and LSV (b) curves of hemin/C pyrolyzed at 600 °C with different metal contents. (CV and LSV curves were collected in 1.0 M KOH solution saturated with nitrogen and oxygen, respectively.)

the figures and following discussion. From Figure 5a, it is observed that the curve shape is similar for all three catalysts except that the pseudocapacitance current increases when the hemin content is doubled. A further increase in hemin content (3 wt %) then lowers the pseudocapacitance current. The increase in pseudocapacitance current is due to the enriched functional groups on the surface of the catalyst when the hemin content increases from 1 to 2 wt %. When the content further increases, more functional groups are generated on the surface; however, the total surface area of the catalyst is reduced as well, consequently resulting in the decrease in the pseudocapacitance current. On the basis of the above findings, it is unsurprising that this catalyst yields improved ORR performance with increasing the metal content from 1 to 2 wt %, as seen in Figure 5b. A further increase in metal content (3 wt %) yields a negligible effect on the ORR performance. This result further confirms that both the metal ion and the surface area are of paramount importance in determining the electrocatalytic activity. Figure 6 presents the CV and LSV curves of the hemin/C catalysts in 1.0 M KOH aqueous solution. As shown in Figure 6a, hemin/C and hemin/C(600) exhibit different electrochemical characteristics in alkaline media. Hemin/C shows three welldefined peaks consisting of one pair of reversible redox peaks at 0.288 and 0.270 V and another cathodic peak at 0.780 V. In comparison, the redox peaks at low potentials disappear after the

4. CONCLUSION In this work, hemin was investigated as the electrocatalyst for the ORR. We found that heat treatment yielded a pronounced effect on the composition and microstructure of hemin, which roughly agreed well with the change in the electrocatalytic activity. On the basis of the findings, we suggest that FeN2 is the active center and the micropore plays an important role in catalyzing this reaction. Finally, we found that the optimized hemin/C catalyst yielded comparable electrochemical performance to the commercial Pt catalyst in alkaline media, which makes it rather promising to be used as an alternative nonprecious metal catalyst in fuel cells or sensors. ’ ASSOCIATED CONTENT

bS

Supporting Information. Successive CV curves of the electrocatalysts and their TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: (86) 020-87113586. E-mail: [email protected] (Z.-X. L.).

’ ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (Grant No. 21003052), the Training Project for the Outstanding and Innovative Talents in Guangdong Colleges and Universities (Yumiao Project) 2009 and the Fundamental Research Funds for the Central Universities, SCUT. ’ REFERENCES (1) Lojou, E.; Bianco, P. J. Electroanal. Chem. 1999, 471, 96–104. (2) Bedioui, F.; Trevin, S.; Albin, V.; Villegas, M. G. G.; Devynck, J. Anal. Chim. Acta 1997, 341, 177–185. (3) Younathan, J. N.; Wood, K. S.; Meyer, T. J. Inorg. Chem. 1992, 31, 3280–3285. 2609



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Hemin: A Highly Effective Electrocatalyst Mediating the Oxygen

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