3D non-precious metal-based electrocatalysts for the oxygen reduction reaction in acid media

international journal of hydrogen energy 35 (2010) 8295–8302

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3D non-precious metal-based electrocatalysts for the oxygen reduction reaction in acid media Hui-Juan Zhang a, Qi-Zhong Jiang a,*, Liangliang Sun b, Xianxia Yuan a, Zongping Shao b, Zi-Feng Ma a a b

Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China

article info

abstract

Article history:

A series of non-precious metal catalysts for oxygen reduction reaction (ORR) in PEMFCs are

Received 18 September 2009

prepared by pyrolysis of different precursors at 800  C under an Ar atmosphere. These

Received in revised form

precursors are 3d transition metal triethylenetetramine chelates supported on carbon

20 November 2009

(MTETA/C, M ¼ Fe, Co, Ni, Cu, Zn and Mn). Results by XRD and TEM indicate that the

Accepted 4 December 2009

catalyst transforms to a carbon structure embedded with metal or metallic carbon after

Available online 13 January 2010

pyrolysis. Electrochemical measurements are achieved by rotating disk electrode and cycle voltammograms in 0.5 M H2SO4 solution, as well as single cell tests. The catalytic activities

Keywords:

decreases as Fe > Co > Zn > Mn > metal-free >>Cu > > Ni. This suggests that the transition

3d transition metal

metal plays an important role in formation of the active sites. Single cell tests show that

Electrocatalysts

they have some potential applications as the cathode catalysts. Some potential catalytic

Triethylenetetramine

active sites are also propounded.

Oxygen reduction reaction

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

PEMFCs

1.

Introduction

Proton exchange membrane fuel cells (PEMFCs) are clean and efficient power sources for residential and transportation applications. Due to the sluggish kinetics of oxygen reduction reaction (ORR), catalysts are needed to improve the electrochemical reactions at the cathode of PEMFCs. So far, Pt/C catalyst remains the best cathode catalyst for PEMFCs. However, Pt is an expensive metal and of low abundance. Searching for efficient and cost-effective catalysts for the cathode oxygen reduction is a crucial issue from practical and theoretical points on fuel cell techniques [1–3]. Comprehensive reviews on the development of nonprecious metal catalysts for the ORR have been published recently [3]. Among these catalysts, MN4 macrocyclic compounds (e.g., cobalt or iron porphyrin), adsorbed on

carbon and heat treated under an inert atmosphere, are the most promising catalysts [4–7]. Although the exact nature of these active sites for the ORR is controversial, the choice of a transition metal, a source of nitrogen, a source of carbon and heat treatment under an inert atmosphere are all believed to influence the formation of the active sites [3,8–17]. Effect of the central metal for N4-phthalocyanines on the ORR activity was reported as Fe > Co > Ni > Cu z Mn [15]. Dodelet’s group reported that the catalytic activity for the ORR decreased according to the following order: Cr > Fe > Co > > V, where the catalysts were obtained from pyrolysis of transition metal hydroxides supported on carbon in acetonitrile vapor [16]. However, they also illustrated the increasing ORR activity as Fe > Co > Cr > Mn, when transition metal acetates, ammonia and perylenetetracarboxylic diahydride (PTCDA) were employed as the metal precursor, nitrogen source and carbon

* Corresponding author. Tel.: þ86 21 3420 6255; fax: þ86 21 5474 7717. E-mail address: [email protected] (Q.-Z. Jiang). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.015

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source, respectively [17]. Dahn obtained the catalytic activity orders of the TM–C–N (TM ¼ V, Cr, Mn, Fe, Co and Ni) catalysts toward the ORR as follows: Fe > Co > Cr > Ni > Mn > V in 0.1 M HClO4 solution, and Co > Ni > Mn > V > Cr in 0.1 M KOH solution [3]. All these results show that both transition metal and nitrogen source play important roles in the formation of the catalytic active site. The most interesting phenomenon is that the 3d transition metals have different catalytic activity orders when different nitrogen-containing precursor is selected. Recently, we have reported a novel N4-containing precursor, triethylenetetramine (H2NCH2CH2NHCH2CH2NHCH2CH2NH2, TETA), to prepare non-precious metal catalyst (CoTETA/C) for the ORR [18,19]. In this work, effects of the first row 3d transition metal on the catalytic activity in acid electrolyte are investigated, while TETA is chosen as the nitrogen source. Rotating disk electrode (RDE), cycle voltammograms (CVs) and single cell tests are performed to evaluate the activity of catalysts. Some potential catalytic active sites are also discussed based on various physical and chemical characterizations.

2.

Experimental

3d transition metal catalysts were prepared through the procedure as described elsewhere [18,19]. Briefly, desired amounts of the first row 3d transition metal salts (MClx, M ¼ Fe, Co, Ni, Cu, Zn and Mn) were dissolved in ethanol absolute, respectively. TETA were added into the above solutions to form MN4-containing chelates under stirring conditions (Fig. 1), followed by the addition of carbon black (BP 2000). The solvents were removed in a rotary evaporator, and the resulting powders were heat treated at 800  C for 90 min under an Ar atmosphere. The nominal amount of metal loaded on the carbon support was kept 10 wt%. They were denoted as MTETA/C (M ¼ Fe, Co, Ni, Cu, Zn and Mn). The electrochemical measurements were performed at room temperature in a three-electrode single-compartment cell containing 0.5 M H2SO4 solution. A platinum mesh and a saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. All potentials in this work are referred to a normal hydrogen electrode (NHE). The catalyst ink was prepared by blending 5 mg finely ground catalyst powders with 0.5 ml distilled water and 50 ml 5 wt% Nafion solution in an ultrasonic bath. Then 10 ml of this ink was deposited onto the glassy carbon disk (d ¼ 4 mm) for air-drying. Catalytic activity for the ORR was evaluated in the oxygen-saturated acidic electrolyte. Firstly, the electrode was

scanned by the potential between 1.04 and 0.04 V at a sweep rate of 50 mV s1 to clean the surface of catalyst. Then CVs were recorded by scanning the potential between 1.04 and 0.04 V at a scan rate of 5 mV s1. One cycle was completed with the electrode stationary (0 rpm) and other cycles were completed with the electrode rotated at 100–900 rpm. The single cell performances with some MTETA/C cathode catalysts (M ¼ Co, Fe, Zn and Mn) were examined under actual PEMFCs conditions at 25  C. A catalyst coating membrane method was adopted to prepare the MEA [20]. 20 mg MTETA/C catalysts were ball-milled with 5 wt% Nafion solution and ethanol absolute for 1 h to prepare ink, respectively. Then they were sprayed onto Nafion 212 membranes. The anode catalysts were prepared with the commercial 20 wt% Pt/C (ETEK). The geometric area of the MEA was 2.6  2.6 cm2. Pure hydrogen gas and pure oxygen gas both at 200 ml min1 and without humidified were supplied to the anode and cathode compartments, respectively. X-ray diffraction (XRD) was performed on an automated Rigaku diffractometer equipped with a Cu Ka radiation. Data acquisition was carried out in the scanning angle range of 10– 80 at a scan rate of 5 min1. The JCPDS database was referred to for the peak identification. Transmission electron microscopy (TEM) was used to determine the distribution of metal particles in the catalysts.

3.

Results and discussion

Fig. 2 shows the powder XRD patterns of the MTETA/C catalysts (M ¼ Fe, Co, Ni, Cu, Zn and Mn). Series of standard characteristic diffraction patterns are also shown for comparison. All catalysts clearly exhibit a broad diffraction peak located at 24.5 , which is signed to C(002). The characteristic diffraction peaks of the first row 3d transition metals become intensive (except for Zn and Mn), which means that nanometallic Co, Fe, Cu and Ni might form after pyrolysis. Visual confirmations about the presence of metals are given in Fig. 3. The gray area is attributed to the basic macrostructure of amorphous carbon framework and the dark spots correspond to the metallic crystallites formation from each figure. From the XRD data in Fig. 2, it is known that the C(002) peak becomes weaker. These results confirm the deposition of metal particles onto the carbon support. Some similar instances were shown in other group [16,17,21]. Fig. 4 shows the CVs of the MTETA/C catalysts (M ¼ Fe, Co, Ni, Cu, Zn and Mn) in oxygen-saturated 0.5 M H2SO4 solution. It is a traditional way to represent the relative catalytic activity

Fig. 1 – Schematic illustration of synthesis of the MTETA chelates (M [ Fe, Co, Ni, Cu, Zn and Mn) in ethanol absolute at room temperature. Coordination of a single TETA chain as a multidentate ligand (N4) is also anticipated.

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Fig. 2 – XRD patterns of the MTETA/C catalysts (M [ Fe, Co, Ni, Cu, Zn and Mn). A series of standard characteristic diffraction peaks are also shown for comparison.

of catalysts with the ORR peak potential where the maximum oxygen reduction current occurs. The closest to the theoretical reversible potential of 1.23 V for the oxygen reduction at room temperature, the best catalytic activity the catalyst has. Fig. 4 indicates that the catalytic activity order of the first row 3d transition metal catalysts is Fe (725 mV) > Co (710 mV) > Zn (678 mV) > Mn (655 mV) >> Cu (583 mV) > > Ni (489 mV). Here, we can conclude that the 3d transition metals have different catalytic activity orders with TETA as the nitrogen source as compared to what others reported [3,15–17]. Apparently, Fe, Co, Zn and Mn are beneficial in formation of the active sites for the ORR in acidic solution. We may wonder that the catalytic activities of the catalysts are related to the outmost electron distributions of the 3d transition metals, which are: Fe (3d64s2), Co (3d74s2), Zn (3d104s2), Mn (3d54s2), Cu (3d104s1) and Ni (3d84s2), respectively. Fig. 5 shows the polarization curves of oxygen reduction on the MTETA/C catalysts (M ¼ Fe, Co, Ni, Cu, Zn and Mn). The measurements were performed in oxygen-saturated 0.5 M

H2SO4 solution at a rotation rate of 900 rpm. It illustrates that the catalytic activity for the ORR decreases as Fe > Co > Zn >> Mn >> Cu > > Ni. The CuTETA/C and NiTETA/ C catalysts exhibit lower activities for the ORR. It is observed that the half potential of oxygen reduction on the FeTETA/C catalyst shifts positively about 0.3 V as compared to the NiTETA/C catalyst. There are no well-expressed limiting current plateaus for these MTETA/C catalysts. A similar shape of polarization curves was reported for oxygen reduction on carbon-supported metal macrocycles [22]. In porous electrodes, the depth of oxygen penetration inside the electrode structure changes with the potential. When the catalyst is Pt/C, oxygen reduction is fast enough, the reaction is occurred on the outer part of the porous electrode, and a flat limiting plateau is observed. Here, these MTETA/C catalysts are poorer than Pt/C and some of the catalytic active sites inside the electrode might be in contact with O2 even at high overpotential. When distribution of the active sites is less uniform and reaction is slower, the plateau is more inclined [18,19,23,24].

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Fig. 3 – TEM images of the MTETA/C catalysts, (a) CoTETA/C (50 nm), (b) FeTETA/C (50 nm), (c) FeTETA/C (20 nm), (d) ZnTETA/ C (50 nm) and (e) MnTETA/C (50 nm).

For a film-coated electrode, Koutecky–Levich plots are determined by the following equation from the RDE data of the MTETA/C catalysts [22–24]. 1 1 1 1 1=2 I ¼ I1 k þ If þ Id ¼ jk þ If þ Bu


1

(1)

where I is the overall measured current, Ik is related to the kinetic current, Id is the boundary-layer diffusion-limited current, If represents the film diffusion-limited current, B is the Levich slope and u is the rotation rate of the working electrode. A series of Koutecky–Levich plots of MTETA/C catalysts at 0.2 V are illustrated in Fig. 6. All of these Koutecky–Levich

Fig. 4 – Cycle voltammograms of the MTETA/C catalysts (M [ Fe, Co, Ni, Cu, Zn and Mn) in oxygen-saturated 0.5 M H2SO4 solution.

plots are essentially parallel, which indicates that the number of electrons transferred per oxygen molecule and the active surface area for the reaction on the MTETA/C catalysts are not changed obviously (except for the NiTETA/ C catalyst) at that potential. A non-zero intercepts of I1 vs. u1/2 plots demonstrates the effect of the oxygen diffusion in the Nafion layer [23]. An uncertainty of the electrode surface area does not allow us to calculate precisely the number of electrons exchanged in the ORR, but the number of electrons can be gained by using rotating ring-disk experiments [22,23].

Fig. 5 – Current-potential polarization curves of the MTETA/C catalysts (M [ Fe, Co, Ni, Cu, Zn and Mn) in oxygen-saturated 0.5 M H2SO4 solution at a rotation rate of 900 rpm.

international journal of hydrogen energy 35 (2010) 8295–8302

Fig. 6 – Koutecky-Levich plots of the MTETA/C catalysts (M [ Fe, Co, Ni, Cu, Zn and Mn).

Fig. 7 shows the Tafel plots for the MTETA/C catalysts at low currents. Tafel slopes of the CoTETA/C, FeTETA/C, ZnTETA/C and MnTETA/C catalysts are all about 180 mV/ dec. As the Tafel slope of 58 mV/dec is the value for O2 reduction on Pt/C, and this was ascribed to the transfer of the first electron as a rate-determining step and Temkin conditions of intermediate adsorption [23,25]. According to the flooded-agglomerate, liquid-electrolyte model, (1) when the ORR is controlled by kinetics and the diffusion of dissolved O2, the data should show a doubled Tafel slope of Pt/C and be firstorder in O2 partial pressure; (2) when the ORR is controlled by kinetics and ionic transport, a doubled Tafel slope of Pt/C and half-order in O2 partial pressure should be expected; (3) in the case of kinetics, diffusion of dissolved O2 and ionic transport control, this model predicts a quadrupled Tafel slope of Pt/C and half-order in O2 partial pressure [23,25]. In this work, a series of triplication Tafel slopes (180 mV/dec of MTETA/C to 58 mV/dec of Pt/C) are obtained. We may guess that this flooded-agglomerate, liquid-electrolyte model could not explain the behaviors of MTETA/C. Other reports on nonprecious metal catalysts for the ORR also have the similar phenomenon [23,25]. For a more detailed analysis of the catalytic activities of the MTETA/C catalysts (M ¼ Fe, Co, Zn and Mn) developed in this work, these catalysts are tested in actual PEMFCs. Fig. 8

Fig. 7 – Tafel plots of the MTETA/C catalysts (M [ Fe, Co, Ni, Cu, Zn and Mn).

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displays the single cell performances based on the MTETA/C cathode catalysts. For comparison, a polarization curve obtained under the same operation conditions with 20 wt% Pt/ C cathode catalyst is also presented. The open circuit voltages of the above MTETA/C catalysts are all around 0.81 V, which is lower than that of the Pt/C (0.96 V). The current densities at 0.4 V of FeTETA/C, CoTETA/C, ZnTETA/C, MnTETA/C and Pt/C catalysts are 333, 285, 214, 150 and 1452 mA cm2, respectively. The performances of these 3d transition metal catalysts are much lower than the Pt/C counterpart. One clear reason is that their lower intrinsic activities of these non-precious metal catalysts in comparison with Pt/C. Another reason should be that these MTETA/C catalyst layers are thicker than Pt/C catalyst layer. Larger thickness of catalyst layer causes higher electrical resistance and mass transfer resistance. Although the performances of these MTETA/C catalysts are much lower than that of the commercial Pt/C, they still have some potential application as the ORR catalysts in PEMFCs systems. According to the above studies, it can be concluded that the MTETA/C catalysts are not stable at 800  C and these 3d transition metals display quite different effects on formation of the catalytic active sites. The native of the active sites obtained after pyrolysis still remains intricate. Based on our present results, as well as reports from others groups, we suggest the following candidates as the catalytic active sites.

3.1.

The MN4 moiety

Some researchers believe that the heat treatment does not destruct the macrocycle completely, but modify one ligand which preserves the central MN4 moiety and then the catalytic activity comes from this MN4 group [21,24]. Concerning the first row 3d transition metal phthalocyanines, the central metals Cr, Mn, Fe and Co show the redox process, while the redox processes of Ni, Cu and Zn phthalocyanines take place on the phthalocyanine ring [16]. In this work, ZnTETA/C catalyst has a better catalytic activity than MnTETA/C catalyst. On the other hand, the MTETA chelates are not stable at 800  C. Fig. 9 compares the catalytic activities of CoTETA/C

Fig. 8 – Polarization curves of PEMFCs based on the MTETA/ C cathode catalysts (M [ Fe, Co, Zn and Mn). The polarization curve based on the commercial Pt/C cathode catalyst is also shown for comparison.

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3.3. Nanometallic particles surrounded by carbon matrix Dodelet’s group reported that nanometallic Co with different size were synthesized in the range of increased activity, and XANES measurements at the Co and NK edges confirmed that the MN4 centers and nitrogen atoms were no longer detected after heat treatment [27]. Here, we demonstrate that the Co/C and Fe/C catalysts (obtained in the absence of TETA) show less catalytic activity, as compared to the CoTETA/C and FeTETA/C catalysts (more than 200 mV, Fig. 11), which seems that the nanometallic particles surrounded by carbon matrix are not the real catalytic active sites. Fig. 9 – Cycle voltammograms of the CoTETA/C precursor (unheated) and CoTETA/C catalysts in oxygen-saturated 0.5 M H2SO4 solution.

precursor (unheated), BP 2000 and CoTETA/C catalyst. The CoTETA/C catalyst shows high catalytic activity while the CoTETA/C precursor expresses no catalytic activity. This will mean that the catalytic active site of CoTETA/C catalyst is not the MN4 moiety. By the way, this model has been strongly opposed by our experimental findings on the CoTETA/C catalyst [18,19]. From the extended X-ray absorption fine structure (EXAFS), we have also found that the CoN4 moiety do not exist after pyrolysis at 800  C.

3.2.

The CNx moiety

3.4.

The M–Nx–C moiety

This model means that the transition metal was adsorbs on carbon support and has the interactions with the residual nitrogen derived from heat treatment under an inert atmosphere. Our previous study told that the M–Nx–C moiety might be reasonless as the catalytic active site: the catalyst showed very little catalytic activity when Fe salt, urea and carbon support were mixed mechanically and heat treated at 800  C under an Ar atmosphere. Dodelet’s group also reported that very little catalytic activity was obtained when Fe were adsorbed on a nitrogen-containing carbon surface [30].

3.5.

The MxCy moiety

After heat treatment, nitrogen releases and transition metal combines with carbon to form MxCy phases. Dahn

In order to further study on the role of transition metal to form the active sites, a metal-free catalyst is prepared as follows: only TETA is adsorbed on the carbon support BP 2000, followed by pyrolysis at 800  C under an Ar atmosphere. CVs of the metalfree catalyst and FeTETA/C catalyst are compared in Fig. 10. There exists a peak potential difference of 118 mV between these two kinds of catalysts, which make us to conclude that the CNx may not be the native of the catalytic active site. However, this model seems to be reasonable as Popov’s reports [26,28,29].

Fig. 10 – Cycle voltammograms of the FeTETA/C catalyst and the metal-free catalyst in oxygen-saturated 0.5 M H2SO4 solution.

Fig. 11 – Current-potential polarization curves of (a) the Co/ C and CoTETA/C catalysts, (b) the Fe/C and FeTETA/C catalysts in oxygen-saturated 0.5 M H2SO4 solution.

international journal of hydrogen energy 35 (2010) 8295–8302

investigated the sputtered TM–C–N films (TM ¼ V, Cr, Mn, Fe, Co and Ni) for the ORR. After pyrolysis, these films transformed from an amorphous structure to a mixture of N-containing carbon with V8C7, Cr3C2 or Mn7C3, except for Ni, Co and Fe. These catalysts also showed some catalytic activity towards the ORR [3]. In this work, there is Mn4C1.06, but no MxCy moieties (M ¼ Fe, Co, Ni, Zn and Cu) are found in Fig. 2.

3.6.

Other catalytic active sites caused by pyrolysis

According to the above illumination, a notion maybe exists that the catalytic active sites do not contain the 3d transition metals, but these 3d transition metals function as midwifery substance to form the catalytic active sites and different 3d transition metal deliver different midwifery effect. It seems that this hypothesis is more reasonable. It is also possible that some novel structures containing the 3d transition metals are the catalytic active sites, but they are still undetected.

4.

Conclusions

From this study, some conclusions can be obtained as follows: (1) Non-precious metal catalysts for the ORR in PEMFCs can be prepared by pyrolysis of the first row 3d transition metal triethylenetetramine chelates supported on carbon (MTETA/C, M ¼ Fe, Co, Zn and Mn) under an Ar atmosphere. When the same procedure is applied to prepare the CuTETA/C and NiTETA/C catalysts, very little activities are obtained. (2) The catalytic activity of the MTETA/C catalysts decreases in the order of Fe > Co > Zn > Mn >> metalfree >> Cu > > Ni. This suggests that the transition metal plays an important role in the formation of the catalytic active site for the ORR. (3) PEMFCs tests show that the performances of the MTETA/C catalysts are much lower than the Pt/C counterpart due to their lower intrinsic activity. They still have some potential application as the cathode catalysts in PEMFCs. (4) XRD analysis shows that the carbon-supported 3d transition metal triethylenetetramine chelates (MTETA/C, M ¼ Fe, Co, Ni, Cu, Zn and Mn) have decomposed at 800  C and metallic species or metallic carbide (depending on the metal used) embedded in the carbon matrix are formed. (5) Some possible catalytic active sites are also propounded based on physical and chemical characterizations.

Acknowledgments The authors are grateful for the financial support of this work by the National 863 Program (2007AA05Z145), National Science Foundation of China (20776085), Science and Technology Commission of Shanghai Municipality (07JC14024, 09XD1402400) and the support from Instrumental Analysis Centre of Shanghai Jiao Tong University. The authors also appreciate Professor Shao of Nanjing University of Technology, for his help in the PEMFCs tests.

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