Preparation and characterization of Cu–N–C electrocatalysts for oxygen reduction reaction in alkaline anion exchange membrane fuel cells

Accepted Manuscript Title: Preparation and Characterization of Cu-N-C Electrocatalysts for Oxygen Reduction Reaction in Alkaline Anion Exchange Membrane Fuel Cells Authors: Yun Sik Kang, Yoonhye Heo, Pil Kim, Sung Jong Yoo PII: DOI: Reference:

S1226-086X(17)30135-1 http://dx.doi.org/doi:10.1016/j.jiec.2017.03.019 JIEC 3332

To appear in: Received date: Revised date: Accepted date:

26-1-2017 12-3-2017 12-3-2017

Please cite this article as: Yun Sik Kang, Yoonhye Heo, Pil Kim, Sung Jong Yoo, Preparation and Characterization of Cu-N-C Electrocatalysts for Oxygen Reduction Reaction in Alkaline Anion Exchange Membrane Fuel Cells, Journal of Industrial and Engineering Chemistryhttp://dx.doi.org/10.1016/j.jiec.2017.03.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation

and

Characterization

of

Cu-N-C

Electrocatalysts for Oxygen Reduction Reaction in Alkaline Anion Exchange Membrane Fuel Cells

Yun Sik Kang a,1, Yoonhye Heo b,1, Pil Kim b*, Sung Jong Yoo a,c*

a

Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea

b

School of Chemical Engineering, Chonbuk National University, Jeonju 54896, Jeonbuk, Republic of

Korea c

Clean Energy and Chemical Engineering, Korea University of Science and Technology, Daejeon 34113,

Republic of Korea

1

These authors contributed equally to this work.

* Corresponding author. E-mail: [email protected], [email protected].

1

Graphical abstract

Highlights •

Cu-N-C catalysts were prepared by annealing metal-adsorbed polyaniline (PANI) or Vulcan carbon (VC).

•

Catalysts prepared by PANI showed higher oxygen reduction reaction (ORR) activity than those with VC.

•

Cu metal was used for the generation of active sites for oxygen reduction reaction (ORR).

•

NH3 heat-treated Cu-PANI catalyst exhibited the highest ORR performance among the catalysts.

Abstract In this study, Cu-N-C catalysts were prepared by annealing metal-adsorbed polyaniline (PANI) or Vulcan carbon (VC), and their electrocatalytic properties were investigated. The results showed that Cu is used for the generation of active sites as well as utilized as the active component for oxygen reduction reaction (ORR). The catalysts prepared using PANI delivered higher ORR activity than those prepared with VC. Furthermore, the ORR activities of the prepared catalysts can be tuned by the heat-treatment conditions. The PANI-derived catalyst obtained under an NH3 atmosphere showed the highest ORR performance among the prepared catalysts.

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Keywords: Oxygen reduction reaction; Anion exchange membrane fuel cells; Transition metal-nitrogencarbon catalyst; Polyaniline;

1. Introduction The oxygen reduction reaction (ORR) is widely regarded as an essential process in many electrochemical energy conversion devices, such as polymer electrolyte membrane fuel cells (PEMFCs), solid oxide fuel cells, and metal-air secondary batteries [1-5]. Owing to the sluggish kinetics of the ORR, however, high loading of precious metal-based catalysts such as Pt or Pt alloys is currently required to produce the desired power necessary for their application in many electronic devices. Consequently, the cost of these catalysts weakens the economic competitiveness of the corresponding devices on the market [6-9]. To cope with this technical issue, therefore, numerous studies on non-Pt electrocatalysts including non-noble metal alloys, transition metal oxides, chalcogenides, transition metal/nitrogen on carbon materials (M-NC), and conductive polymers have been conducted in order to lower manufacturing cost of the devices aforementioned while maintaining high catalytic activity [10-17]. In particular, among the various types of low-cost alternative catalysts, non-noble transition metal/nitrogen on carbon materials (M-N-C) are the most promising candidates in terms of catalytic activity and durability [13-15, 18]. Up to now, many researchers have focused on the development of Fe-based M-N-C catalysts because many Fe-N-C catalysts exhibit ORR activities superior to those of other non-noble metal catalysts. However, despite efforts in this direction, it is reported that Fe-N-C catalysts rapidly lose their catalytic activity when exposed to acidic conditions in PEMFCs which induce the oxidation and dissolution of Fe [12, 19-24]. To solve this problem, many studies have been conducted with the aim of improving the catalytic activity and stability of M-N-C catalysts by applying other transition metals, such as Co and Cu, or by optimizing the preparation conditions like the annealing temperature or duration [13, 25-29]. These previous researches have demonstrated that the active site in M-N-C catalysts for the ORR is the bonding sites between transition metal and N atoms and that these catalysts can be easily prepared by annealing a mixture of transition metal precursors, carbon materials, and N-containing precursors.

3

Furthermore, studies have extensively investigated the use of polyaniline (PANI), a well-known Ncontaining conducting polymer, as a support material [14, 30-32]. Recently, it is reported that the generation of bonding between the transition metal and N atoms during the annealing process is much easier when polymers containing a regular distribution of N atoms instead of common carbon-based materials are used as precursors for the support material. For this reason, the application of PANI as a support material precursor is an excellent strategy for the fabrication of highly active M-N-C catalysts, and offer a method to address the many practical issues associated with the commercialization of PEMFC. In this work, to propose a solution to the technical issues related to M-N-C catalysts, we prepared CuN-C catalysts by annealing metal-adsorbed polyaniline (PANI) or Vulcan carbon (VC) and investigated their electrocatalytic properties through physical and electrochemical measurements. The results showed that Cu metal is used for the generation of active sites, as well as constituting the active component for the ORR. Furthermore, the catalysts prepared using PANI deliver higher ORR activity than those prepared with VC. In particular, it was revealed that the ORR activities of prepared M-N-C catalysts can be tuned by the heat-treatment conditions. The catalyst obtained under an NH3 atmosphere (Cu-PANI-NH3) showed the highest ORR performance among the prepared catalysts.

2. Experimental section 2.1. Preparation of Cu-N-C catalysts Cu-N-C catalysts were synthesized through the excess solution impregnation method. Polyaniline (Emeraldine Base, Sigma-Aldrich) or carbon black (Vulcan carbon XC-72, Cabot) were dispersed in anhydrous ethanol (Sigma-Aldrich). Cu(NO3)2 (Shinyo) was added to the solution and the resulting solution was evaporated and dried by rotary evaporation at 70°C. The Cu loading of the catalysts was set at 3 wt.%. The resultant catalysts were subjected to heat treatment at 800 °C for 1 h under N2 or NH3 atmosphere. The gas flow was controlled to 100 ccm. Additional acid treatment was conducted in 1.5 M HCl solution for 6 h to remove Cu particles from the catalysts. The prepared Cu-N-C catalysts were

4

designated as Cu-X-Y, where X denotes the support material used and Y denotes the gas used during heat treatment.

2.2. Characterization of Electrocatalysts 2.2.1. Physical characterization The particle size and morphology of the Cu-N-C catalysts were confirmed by transmission electron microscopy (TEM, HITACHI H-7650). Samples for TEM were prepared by placing a drop of catalysts sample solution onto a carbon-coated Cu mesh grid that was subsequently dried in an oven. The crystalline structures of Cu-N-C catalysts were examined by X-ray diffraction (XRD) using a Rigaku D/MAX 2500 diffractometer with Cu Kα radiation (40 kV, 30 mA, 0.02° sampling width, and 6°/min scanning speed). Elemental analysis was carried out with a CHNS analyzer (Vario EL) to confirm the N contents of the catalysts and X-ray photoelectron spectroscopy (XPS, CJ109, operated with Al Kα radiation) was used to identify the chemical states and concentrations of N species in the catalysts. All spectra were calibrated using the C 1s peak at 284.6 eV and were deconvoluted using XPSPEAK 4.1 software. 2.2.2. Electrochemical characterization The prepared catalysts were analyzed by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using a potentiostat (CH Instruments CHI700C) with a conventional three-electrode electrochemical cell comprising a rotating ring-disk electrode (RRDE, PINE AFE7R9GCPT, 19.64 mm2 geometric surface area) working electrode, a Pt gauze counter electrode, and a Hg/HgO reference electrode. Catalyst inks were prepared by mixing the catalyst with DI water (18.2 MΩ cm; Millipore, 50 μL), 5 wt% Nafion ionomer solution (50 μL) as a binding material, and isopropyl alcohol (IPA, 99.5%, 2000 μL) (Sigma– Aldrich). A sample of the catalyst ink was dropped onto the RRDE by using a micropipette. LSV curves for the ORR were obtained using an RRDE at 1200 rpm, in the potential range from -0.8 to 0.1 V (or 0.3V) vs. Hg/HgO with a scan rate of 5 mV s-1 in O2-saturated 0.1 M KOH solution. CV curves of the prepared catalysts were also measured in the potential range from -0.8 to 0.3 V vs. Hg/HgO at a scan rate of 50 mV s-1 in N2-purged 0.1 M KOH under N2 purging. The electrochemical performances of the 5

prepared catalysts were compared with those of commercial Pt/C and Pd/C catalysts (20 wt.% Pt or Pd on Vulcan carbon; Premetek).

3. Results and discussion The XRD patterns of catalysts prepared under different conditions are exhibited in Fig. 1,. All the catalysts display a broad peak at 2θ = 26 °, which is assigned to the (002) diffraction of graphitic carbon. For the Cu containing catalysts, face-centered cubic (FCC) Cu and CuO peaks are observed (2-theta peaks at 43.317 °, 50.499 °, 74.126 ° and 36.3 °, which are assigned to Cu (111), (200), (220) and CuO (111) diffractions, respectively), confirming the existence of Cu in the prepared catalysts [33, 34]. In addition, NH3-treated catalysts present much narrower and sharper diffraction patterns compared to those of the N2-treated ones, indicating that the particle size of the NH3-treated catalysts is larger than those of theN2treated catalysts. The surface morphologies of the catalysts were investigated by TEM images and the images are presented in Fig. 2 (Fig.2 (a) and (b) for Cu-PANI-N2 and Cu-PANI-NH3 and (c) and (d) for Cu-VC-N2 and Cu-VC-NH3). From these images it is clear that bulky Cu particles are distributed inhomogeneously over the PANI and VC. To confirm the electrochemical characteristics of the prepared catalysts, CV measurements in N2saturated 0.1 M KOH solution were conducted (Fig. 3 (a)). The CV was repeated 20 times at a scan rate of 50 mV/s and the last scan is shown. When comparing the prepared catalysts in terms of the support materials used and the heat-treatment atmospheres, we found that the catalysts prepared from PANI and treated under an NH3 atmosphere showed larger and more rectanglular double layer capacitance curves than those prepared with VC and/or treated under a N2 atmosphere. This is due to an increased in the surface area of the catalysts, which occurs due to the etching effect of the heat-treatment under an NH3 atmosphere [35-37]. It is widely accepted that the double layer charging current is proportional to surface area of materials measured, 𝑖𝑐 = A𝐶𝑑 𝑣 6

where 𝑖𝑐 id the double layer charging current, A is the surface area of materials, 𝐶𝑑 is the capacitance of materials, and 𝑣 is the scan rate of potential sweep method. [35, 38, 39] In this study, we have conducted cyclic voltammetry with a fixed scan rate (50 mV/S) and it can be assumed that the Cu-N-C catalysts in this study have similar capacitance values. Therefore, we can believe that the double layer charging current of Cu-N-C catalysts is related to surface area of catalysts only. As mentioned above, due to the etching effect of the heat-treatment under an NH3 atmosphere, the roughness of NH3-treated Cu-NC catalysts surface increased and the surface area of the catalysts also enlarged. As a result, we can simply conclude that the NH3-treated catalysts exhibited increased double layer charging current caused by increased surface area. The LSV curves obtained in O2-saturated 0.1 M KOH are expressed in Fig. 3 (b). Among the prepared catalysts, the NH3-treated catalysts exhibit higher onset potential values than the N2-treated catalysts. Especially, Cu-PANI-NH3 catalyst shows the highest onset potential among the prepared catalysts. These results are attributed to the higher amount of N and the better coordination between Cu and N species in the micropores of the NH3-treated catalysts than those in the N2-treated catalysts, as the Cu-N bonds act as active sites for the ORR. (As shown in Fig. 3 (b), the onset potentials increased in the order: VC-NH3 < Cu-VC-N2 < PANI-NH3 < Cu-PANI-N2 < Cu-VC-NH3 < Cu-PANI-NH3 < Pd/C < Pt/C) For a clearer comparison of the ORR activities of the prepared catalysts, we calculated their ORR kinetic current densities using the Koutecky–Levich equation: 1 𝑖

=

1 𝑖𝑘

+

1 𝑖𝑑

where 𝑖 is the measured current, 𝑖𝑘 is the measured kinetic current, and 𝑖𝑑 is the diffusion limiting current [35, 40]. The mass-normalized activity (jk,mass) at -0.05 V vs. Hg/HgO were determined by dividing 𝑖𝑘 by the catalysts mass and the results are shown in Fig. 3(c). Among the prepared catalysts, Cu-PANINH3 exhibits the highest jk,mass value, which is in accordance with the onset potential value mentioned above. The mass-normalized activities (jk,mass) at -0.05 V vs. Hg/HgO of the catalysts increase in the order:

7

Cu-VC-N2 (0.01 mA/mg) < VC-NH3 (0.03 mA/mg) < Cu-PANI-N2 (0.08 mA/mg) < Cu-VC-NH3 (0.27 mA/mg) < PANI-NH3 (0.48 mA/mg) < Cu-PANI-NH3 (3.90 mA/mg). To identify whether the measured current density for Cu-PANI-NH3 is from ORR or the reduction current of Cu oxides in the catalyst, CV curves of Cu-PANI-NH3 were measured in N2- and O2-saturated 0.1 M KOH, and are shown in Fig 3 (d) [41]. The results revealed that the reduction peak appears at -0.1 V vs. Hg/HgO when the CV was measured in O2-saturated electrolyte, which is not shown in N2-saturated one. This indicates that the measured current density for Cu-PANI-NH3 only results from the ORR of the catalyst, not the reduction current of Cu oxides. In addition, the number of electrons transferred (𝑛) and the fraction of hydrogen peroxide produced (𝑋𝐻2 𝑂2 ) during ORR over the catalysts were calculated from the measured ring and disk currents according to the relationship, 𝑛=

4𝐼𝐷 , 𝐼 𝐼𝐷 + 𝑁𝑅

𝑋𝐻2 𝑂2 = 50(4 − 𝑛)

where IR and ID are the measured ring and disk currents of the catalysts, respectively, and N is the collection efficiency of the RRDE, which is 0.37, provided by the manufacturer [42]. The results are shown in Fig. 4. The fraction of hydrogen peroxide produced (𝑋𝐻2 𝑂2 ) of the catalysts was calculated from the oxidation current for hydrogen peroxide at a fixed ring potential of 0.3 V vs. Hg/HgO. The calculated 𝑛 for Cu-PANI-NH3 is approximately 4, and 𝑋𝐻2 𝑂2 of Cu-PANI-NH3 is only about 2%, which indicates that the ORR proceeds via the more favorable 4-electron pathway. (O2 + 4e- + 2H2O → 4OH-) We also conducted XPS measurement of the prepared catalysts to analyze the relationship between their ORR activities and the chemical state of Cu and/or electronic state of the N species. In Fig. S1, the N 1s photoelectron spectra for the prepared catalysts are exhibited and these are deconvoluted into contributions from four components of pyridinic (398.7 eV), pyrrolic (400.5 eV), quaternary (401.3 eV), and graphitic (402-405 eV) N species [32, 43, 44]. When comparing the N 1s photoelectron spectra of N2- and NH3-treated catalysts, the NH3-treated catalysts show higher peak intensities at 397-399 eV than those of the N2-treated catalysts, which indicates that the ratio of pyridinic N increases upon heat8

treatment under NH3 atmosphere. This is because heat-treatment under NH3 atmosphere easily generates pyridinic N, which acts as an active site for ORR, more easily than under N2 atmosphere. In Table 1, the N contents and XPS deconvolution results for prepared catalysts were given. The catalysts prepared with Vulcan carbon have showed much lower N contents than those prepared with PANI. The active site of M-N-C catalysts for the ORR has not yet been clearly identified until now. However, many researchers have argued that the pyridinic N in M-N-C catalysts acts as, or generates active sites for the ORR [11-13, 28, 29, 32, 40, 41, 43]. Accordingly, increasing the quantity of pyridinic N of M-N-C catalysts improves their catalytic activity owing to the increased number of active sites for the ORR. As shown in Fig. 5, NH3-treated catalysts exhibites higher pyridinic N contents than those of the N2-treated catalysts and the Cu-containing catalysts also shows higher pyridinic N ratio than others. Many researchers have reported that pyridinic N in catalyst micropores is active for the ORR [32, 40, 41, 43, 44]. In this study, the CV and XPS measurements prove that NH3-treated catalysts showed higher microporosities and pyridinic N content than N2-treated catalysts. Therefore, we can conclude that in catalysts with high pyridinic N contents, the coordination between Cu and pyridinic N generates more active sites for the ORR As a result, these catalysts show improved ORR activity. Finally, we investigated the effect of acid treatment on the ORR activity of the prepared catalysts. In common, PEMFCs utilize proton exchange membranes as solid electrolytes, which means that PEMFCs operate under acidic conditions [3, 5-8]. Consequently, the catalytic activity of the transition-metalcontaining catalysts is subjected to deterioration owing to the dissolution of the transition metal, especially in the ORR. Therefore, to find out the effect of acidic condition on the M-N-C catalysts, the Cu-PANINH3 catalyst, which showed the highest ORR activity among the prepared catalysts was subjected to acid treatment in 1.5 M HCl solution for 6 h. As shown by the XRD patterns of the Cu-PANI-NH3 catalyst before and after acid treatment in Fig. 6 (a), the Cu FCC peaks (Cu (111), (200), and (220)) of Cu-PANINH3 catalyst were disappeared after acid treatment due to the dissolution of Cu particles. In addition, the dissolution of Cu particles after the acid treatment is also confirmed by the TEM images and Cu 2p photoelectron spectra of the Cu-PANI-NH3 catalyst (Fig. 6 (b) and S2). To investigate the role of the Cu 9

particles of the prepared catalysts for in the ORR, we measured the LSV curves of Cu-PANI-NH3 and Cu-PANI-NH3+AT (where AT indicates acid treatment) in O2-saturated 0.1 M KOH using an RRDE (Fig. 7). The measured ring currents of the catalysts confirm that the ORR proceeds along the more favorable 4-electron pathway even after acid treatment. The number of electrons transferred (𝑛) and the fraction of hydrogen peroxide produced (𝑋𝐻2 𝑂2 ) during the ORR of Cu-PANI-NH3 and Cu-PANI-NH3+AT are shown in Fig. S4. In addition, Cu-PANI-NH3+AT showed decreased ORR activity (i.e., a much lower onset potential and a smaller limiting current density) compared to that of Cu-PANI-NH3. This is due to the dissolution of Cu particles upon acid treatment, resulting in active site loss. Additionally, as indicated by the elemental analysis data and N 1s XPS deconvolution results for Cu-PANI-NH3 and Cu-PANINH3+AT (Table 3 and Fig. S3), the total N contents and pyridinic N ratio decrease upon acid treatment. During acid treatment, the dissolution of Cu particles occurrs and the pyridinic N in Cu-PANI-NH3 is oxidized by oxygen species or chlorine ions. Consequently, the total N contents alsodecreases. As discussed above, the N contents and pyridinic N ratio of M-N-C catalysts are regarded as the main factors for ORR activity. As a result, Cu-PANI-NH3+AT exhibites lower ORR activity than that of Cu-PANINH3, which is in accordance with previous studies [11-13, 18].

4. Conclusions In summary, we fabricated Cu-N-C catalysts by annealing metal-adsorbed PANI or VC and investigated their electrocatalytic properties and ORR activities by physical and electrochemical measurement techniques including XRD, TEM, XPS, and so on. The results showed that Cu particle are involved in the generation of active sites as well as constituting the active component for the ORR, proved by EA data, XPS data, and electrochemical analysis before and after acid treatment. The catalysts prepared using PANI delivered higher ORR activities than those prepared with VC. In particular, it was revealed that the ORR activities of M-N-C catalysts can be tuned by the heat-treatment conditions. Cu-PANI-NH3 showed the highest ORR performance among the prepared catalysts.

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Acknowledgement This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by NRF (2016M3A6A7945505) and the NRF grant funded by MSIP (2014R1A2A2A04003865, 2015R1A2A2A01007622). This work was also supported by the New & Renewable Energy Core Technology Program of KETEP, granted financial resource from MOTIE, Republic of Korea (20143030031340).

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Figures captions Fig. 1. XRD patterns of (a) PANI-NH3, (b) Cu-PANI-N2, (c) Cu-PANI-NH3, (d) VC-NH3, (e) Cu-VC-N2, and (f) Cu-VC-NH3. Fig. 2. TEM images of (a) Cu-PANI-N2, (b) Cu-PANI-NH3, (c) Cu-VC-N2, and (d) Cu-VC-NH3. Fig. 3. (a) CVs of prepared catalysts in N2 purged 0.1 M KOH solution and (b) LSVs obtained in O2saturated 0.1 M KOH. (c) Mass-normalized activities of prepared catalysts at -0.05 V vs. Hg/HgO. (d) Comparison of CVs Cu-PANI-NH3 in N2 and O2 purged 0.1 M KOH solution. Fig. 4. (a) Number of electron transferred and (b) the fraction of hydrogen peroxide produced during ORR of prepared catalysts. Fig. 5. Relative intensities of pyridinic and pyrrolic N of prepared catalysts. Fig. 6. XRD patterns of (a) Cu-PANI-NH3 and (b) Cu-PANI-NH3+AT catalysts. Fig. 7. LSVs obtained with Cu-PANI-NH3 and acid-treated Cu-PANI-NH3+AT catalysts in O2-saturated 0.1 M KOH. Fig. 8. Relative intensities of pyridinic and pyrrolic N of Cu-PANI-NH3 and Cu-PANI-NH3+AT catalysts.

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Fig. 1

CuO (111) Cu (111)

Cu (220)

Cu (200)

Intensity (a.u.)

(a)

(b) (c) (d) (e) (f) 20

30

40

50

60

70

80

2-Theta (degree)

15

Fig. 2

Cu-PANI-N2

(a)

Cu-PANI-NH3

(b)

Cu-VC-N2

(c)

Cu-VC-NH3

(d)

16

Fig. 3

(b)

-0.8

Current (A)

(a)

-0.6

-0.2

PANI-NH3

-50

Cu-PANI-N2 Cu-PANI-NH3

-40

VC-NH3

-30

Cu-VC-N2

-20

Cu-VC-NH3

-10

Pd/C Pt/C

0

0.0 0.0

Current (mA)

Current (mA)

-0.4

-60

0.2 PANI-NH3

0.4

Cu-PANI-N2 Cu-PANI-NH3 VC-NH3

0.6

Cu-VC-N2 Cu-VC-NH3

0.8 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.2 0.4 0.6 0.8 1.0 1.2 -0.6

0.4

-0.5

(c)

(d)

4.00

Cu-VC-N2 Cu-VC-NH3

0.6 0.48 0.4 0.27 0.2

0.0

0.1

0.2

0.4

Cu-PANI-NH3 (O2-saturated)

-0.2 0.0 0.2 0.4 0.6

0.08 0.0

-0.1

-0.4

Current (mA)

Mass activity (mA/mg)

Cu-PANI-NH3 VC-NH3

3.50

-0.2

Cu-PANI-NH3 (N2-saturated) -0.6

Cu-PANI-N2

3.75

-0.3

-0.8

3.90

PANI-NH3

-0.4

Potential (V vs. Hg/HgO)

Potential ( V vs. Hg/HgO)

0.03

0.01

0.8 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

Potential (V vs. Hg/HgO)

17

Fig. 4

(a)

(b)

5

150 PANI-NH3 Cu-PANI-N2

120

Cu-PANI-NH3 VC-NH3 Cu-VC-N2

3

90

XH O (%)

Cu-VC-NH3 Pd/C Pt/C

2

PANI NH3

2

Cu-PANI N2

2

Number of electrons

4

Cu-PANI NH3

60

VC NH3

1

Cu-VC N2

30

Cu-VC NH3 Pd/C Pt/C

0 -0.7

-0.6

-0.5

-0.4

-0.3

-0.2

Potential( V vs. Hg/HgO)

-0.1

0 -0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

Potential (V vs. Hg/HgO)

18

Fig. 5

Relative intensities (%)

60

Pyridinic N Pyrrolic N

50

40

30

20

10

0 PANI-NH3

Cu-PANI-N2 Cu-PANI-NH3 VC-NH3

Cu-VC-N2 Cu-VC-NH3

19

Fig. 6

Cu (111) Cu (200)

Intensity(a.u.)

Cu (220)

(a)

(b) 20

30

40

50

60

70

80

2-Theta (degree) Cu-PANI-NH3

(c)

Cu-PANI-NH3+AT

(d)

Cu particle

20

Fig. 7

Current (A)

-60

Cu-PANI-NH3

-50

Cu-PANI-NH3+AT

-40 -30 -20 -10 0

Current (mA)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

Potential (V vs. Hg/HgO)

21

Fig. 8

Relative intensities (%)

60

50

Pyridinic N Pyrrolic N

40

30

20

10

0 Cu-PANI-NH3

Cu-PANI-NH3+AT

22

Table 1. Summary of N contents and XPS deconvolution for N 1s spectra of prepared catalysts.

Sample

PANI-NH3

Cu-PANI-N2

Cu-PANI-NH3

VC-NH3

Cu-VC-N2

Cu-VC-NH3

Nitrogen Contents (wt.%)

XPS (N 1s) Species

Binding energy (eV)

Relative intensities (%)

Pyridinic N

397.9

31.1

Pyrrolic N

400.5

9.9

Quaternary N

400.6

51

Graphitic N

404.0

8.0

Pyridinic N

398

29.6

Pyrrolic N

400.5

2.3

Quaternary N

400.7

43.8

Graphitic N

403.7

14.3

Pyridinic N

398

34.7

Pyrrolic N

400.5

10.4

Quaternary N

400.6

47.6

Graphitic N

403.4

7.3

Pyridinic N

398.8

32.0

Pyrrolic N

400.0

10.6

Quaternary N

400.9

31.0

Graphitic N

403.8

26.4

Pyridinic N

398.7

17.4

Pyrrolic N

400.3

52.7

Quaternary N

401.3

5.6

Graphitic N

403.8

24.3

Pyridinic N

398.3

22.2

Pyrrolic N

399.3

8.4

Quaternary N

400.4

62.5

Graphitic N

404.0

6.9

7.58

6.90

8.03

0.37

0.23

0.53

23

Table 2. Summary of N contents and XPS deconvolution for N 1s spectra of Cu-PANI-NH3 and acidtreated Cu-PANI-NH3 catalysts.

Sample

Cu-PANI-NH3

Cu-PANI-NH3 +AT

Nitrogen Contents

XPS (N 1s)

(wt.%)

Species

Binding (eV)

Pyridinic N

398

34.7

Pyrrolic N

400.5

10.4

Quaternary N

400.6

47.6

Graphitic N

403.4

7.3

Pyridinic N

398.1

21.5

Pyrrolic N

400.4

56

Quaternary N

400.8

13.7

Graphitic N

403.5

8.8

energy

Relative (%)

intensities

8.03

7.11

24