Novel nanowire-structured polypyrrole-cobalt composite as efficient catalyst for oxygen reduction reaction

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received: 09 November 2015

accepted: 22 December 2015

Published: 10 February 2016

# Novel nanowire-structured polypyrrole-cobalt composite as efficient catalyst for oxygen reduction reaction

Xianxia Yuan 1 , Lin Li 1 , Zhong Ma 1 , Xuebin Yu 2 , Xiufang Wen 3 , Zi-Feng Ma 1 , Lei Zhang 4 ,
David P. Wilkinson 5 & Jiujun Zhang 4

A novel nanowire-structured polypyrrole-cobalt composite, PPy-CTAB-Co, is successfully synthesized
with a surfactant of cetyltrimethylammounium bromide (CTAB). As an electro-catalyst towards
oxygen reduction reaction (ORR) in alkaline media, this PPy-CTAB-Co demonstrates a superior ORR
performance when compared to that of granular PPy-Co catalyst and also a much better durability than
the commercial 20 wt% Pt/C catalyst. Physiochemical characterization indicates that the enhanced ORR
performance of the nanowire PPy-CTAB-Co can be attributed to the high quantity of Co-pyridinic-N
groups as ORR active sites and its large specific surface area which allows to expose more active sites for
facilitating oxygen reduction reaction. It is expected this PPy-CTAB-Co would be a good candidate for
alkaline fuel cell cathode catalyst.

Polymer electrolyte membrane Fuel cells (PEMFCs) are a kind of prospective green power sources that can convert chemical energy in the fuel directly into electricity with high energy efficiency and low/zero emissions 1–4 .
Their performance including energy/power densities and life-time strongly depends on the electrocatalysts for
promoting the oxygen reduction reaction (ORR) at cathode. At current state of technology, platinum- and its
alloys-based materials are the most efficient and practical ORR catalysts which can give both high electrochemical
activity and acceptable durability. However, the scarcity of platinum and its high cost limit fuel cells’ wide and
sustainable commercialization.

Currently, there are two major types of PEMFCs, one is the proton exchange membrane fuel cells (also called
Acid-PEMFCs) where the membranes are proton conductive, and the other is the Alkaline-PEMFCs where the
membranes are hydroxide conductive. In normal, due to the acidic environment of Acid-PEMFCs, their cathode ORR is slower than that in Alkaline-PEMFCs. Thus, Alkaline-PEMFCs may present more opportunities for
non-Pt–based electrocatalysts for ORR than that Acid-PEMFCs do. In this sense, Alkaline-PEMFCs are considered to be the feasible and promising alternatives to Acid-PEMFCs in terms of their possible usage of non-noble
metal catalysts as well as their better stability of component materials than that in Acid-PEMFCs 5–7 .

Regarding non-noble metal catalysts for PEMFC applications, a wide range of catalyst materials have been
explored in the last several decades, including transition metal macrocycles 8–10, manganese oxides 11–13, spinel
AB2O4 complexes 14–16, heat-treated carbon supported transition metal-nitrogen complexes (M-N/C, M = Fe, Co,
Mn, etc.) 17–19, perovskite-type oxides 20–22 and metal free catalysts 23–25 . Among them, the M-N/C catalysts have
been paid great attentions owing to their higher activities and four-electron-transfer selectivity towards ORR. As
a representative member of M-N/C catalysts, polypyrrole (PPy)-based cobalt-contained catalyst has been investigated in recent years as ORR catalyst in alkaline media and demonstrated promising performance 26,27 . Even the
metal free pristine PPy and pyrolyzed PPy have also been investigated as ORR catalysts in acidic solutions 28,29 .
Depending on the preparation conditions, PPy could be formed into various morphologies 30, such as film 31,

1Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China. 2Department
of Materials Science, Fudan University, Shanghai, 200433, China. 3 The School of Chemistry and Chemical
Engineering, South China University of Technology, Guangzhou, 510640, China. 4 Energy, Mining & Environment,
National Research Council of Canada, Vancouver, BC V6T 1W5, Canada. 5 Department of Chemical and Biochemical
Engineering, University of British Columbia, Vancouver, Canada. Correspondence and requests for materials should
[be addressed to X. Y. (email: yuanxx@sjtu.edu.cn)](mailto:yuanxx@sjtu.edu.cn)

Figure 1. SEM images of PPy-CTAB ( a ), PPy-CTAB-Co ( b ), PPy ( c ), and PPy-Co ( d ).

nanoparticle 32, nanotube array 33, nanowire 34,35, microtube 36, hollow structure 37 and so on. The morphology of
PPy has been identified to influence its ORR performance as the metal free catalyst. Morozan _et al._ 38 reported that
pyrolyzed PPy with tubules-like morphology could exhibit a much better ORR catalytic performance in 0.1 M
KOH than that with granular-like morphology. However, the morphology effects of metal-contained PPy catalysts on their ORR performance in either alkaline or acidic media have seldom been seen in literature.

In this work, a nanowire-structured PPy-Cobalt composite was synthesized using nanowire polypyrrole
and cobalt acetate by surface immobilization of cobalt ions followed by a pyrolysis in an inert atmosphere, its
ORR performance in alkaline solution was electrochemically evaluated and compared with that of the PPy-Co
granules. Experiment results showed that this novel catalyst had a superior ORR performance compared to a
granular PPy-Co catalyst and also a better durability than commercial 20% Pt/C catalyst. To understand the
enhancement effect of this catalyst for ORR, some physiochemical characterizations, including X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron
spectroscopy (XPS), N2 adsorption-desorption isotherm and Inductive Coupled Plasma Emission Spectrometer
(ICP), were employed. It is concluded that the ORR enhancement effect of such a catalyst is due to the resulted
Co-pyridinic-N active sites for ORR and also a larger specific surface area with more active site exposure.

Results
Synthesis and characterization of catalysts. SEM images were taken to observe morphologies of the
as-prepared polypyrrole and the cobalt-anchored catalysts. Figure 1a identifies nanowire morphology of the PPyCTAB prepared with CTAB as the surfactant, its nanowire structure can still be maintained after the addition
of Co metal and the pyrolysis process, as shown in Fig. 1b for the PPy-CTAB-Co catalyst. However, the average
nanowire size was slightly increased probably owing to the agglomeration of nanowire PPy during the pyrolysis. In contrast, a granular morphology can be observed for both PPy (Fig. 1c) which was prepared without
surfactant and the cobalt-contained catalyst of PPy-Co (Fig. 1d), but the average size of the PPy-Co catalyst is
slightly smaller, which could be ascribed to the decomposition of PPy and the structure re-configuration during
the pyrolysis. Both the nanowire and granular structures of PPy-CTAB-Co and PPy-Co catalysts could also be
observed with the TEM images (Fig. 2).

Electrochemical performance of the catalysts towards ORR. The catalytic ORR activity of the
nanowire-structured PPy-CTAB-Co was evaluated in O2-saturated 0.1 M KOH electrolyte with CV in a
three-electrode cell at 5 mV s −1, the obtained result is shown in Fig. 3a, where the CV curves of both PPy-Co
granules and 20 wt% Pt/C catalysts are also shown for comparison. Although the ORR activities of both nanowire PPy-CTAB-Co and granular PPy-Co catalysts are apparently lower than that of 20 wt% Pt/C catalyst as
seen from the ORR peak potentials, the activity of PPy-CTAB-Co is, however, significantly higher than that
of PPy-Co. Its ORR peak at about − 0.182 V is apparently more positive than that of − 0.213 V for the granular
PPy-Co. Moreover, the ORR peak current density of PPy-CTAB-Co is about twice higher than that for PPy-Co.
These results imply the higher ORR catalytic activity of nanowire PPy-CTAB-Co than granular PPy-Co catalyst.

Figure 2. TEM images of PPy-CTAB-Co ( a ) and PPy-Co ( b ) catalysts.

Figure 3. Cyclic voltammograms (CVs) ( a ), and I-V polarization curves at 1600 rpm ( b ) of PPy-CTAB-Co
and PPy-Co catalyzed glassy carbon electrodes in O2-saturated 0.1 M KOH solution. Potential scanning rates for
both CVs and I-V curves are 5 mV s −1 ; Catalyst loadings: 0.40 mg cm −2 .

Figure 3b presents the polarization curves of the PPy-CTAB-Co and PPy-Co catalysts measured at an electrode
rotating rate of 1600 rpm with a potential scanning rate of 5 mV s −1 . Both the onset and half wave potentials of
PPy-CTAB-Co catalyst are obviously higher than that of granular PPy-Co, indicating again the higher catalytic
activity of the nanowire PPy-CTAB-Co.

To quantitatively investigate the ORR performance of the nanowire PPy-CTAB-Co catalyst, Fig. 4a gives the
RDE curves measured at various electrode rotating rates along with that of PPy-Co shown in Fig. 4c for comparison. Generally, the number of transferred electrons ( _n_ ), the so-called 4-electron pathway selectivity, for ORR can
be calculated with Koutecky-Levich (K-L) equation 39 :

_j_ −1 = _jk_ −1 + _jdl_ −1 = _jk_ −1 + ( _Bw_ 1/2)−1 (1)

_B_ = 0 62. _nFC_ 0 _D_ 0 2/3 _v_ 0−/1 6 (2)

where _j_ is the disk current density, _j_ k is the kinetic current density, _j_ dl is the diffusion current density, _F_ is the
Faraday constant, _C_ 0 is the concentration of O2 in the electrolyte, _D_ 0 is the diffusion coefficient of O2 in the
electrolyte, _v_ 0 is the kinetic viscosity of the electrolyte, and _w_ is the rotating rate of the disk electrode. The K-L
plots derived from the current densities at 0.65, 0.55, 0.45 and 0.35 V ( _vs_ . RHE) are shown in Fig. 4b,d giving the
electron-transfer numbers of 3.71 and 3.35 for PPy-CTAB-Co and PPy-Co, respectively. This indicates that both
the nanowire PPy-CTAB-Co and granular PPy-Co undergo 4-electron-transfer dominated ORR, but the former
has higher 4-electron pathway selectivity.

Figure 4. RDE I-V polarization curves recorded on glassy carbon electrodes coated by PPy-CTAB-Co
( a ) and PPy-Co ( c ) in O2-saturated 0.1 M KOH solution, respectively. Potential scannig rate is 5 mV s −1 ; Catalyst
loadings: 0.40 mg cm −2 . K-L plots ( b,d ) of data in ( a,b ).

Stability of the catalysts. Stability is a necessary property for fuel cell catalysts. In the present work, the
stability of nanowire PPy-CTAB-Co and granular PPy-Co catalysts were evaluated by comparing the CV curves
in O2-saturated 0.1 M KOH before and after 500 CV cycles in Ar-saturated 0.1 M KOH, and that of the 20 wt%
Pt/C was also measured for comparison, the results are shown in Fig. 5. Excellent stability can be observed for the
nanowire PPy-CTAB-Co catalyst, almost no decay could be found in its ORR peak potential, while the granular
PPy-Co catalyst shows a slight decrease in ORR activity. For the commercial 20wt%Pt/C catalyst, however, its
ORR peak potential is drastically decreased by 60 mV after 500 cycles. These results indicate the better stability of
both PPy-CTAB-Co and PPy-Co catalysts than the 20 wt% Pt/C catalyst and the nanowire PPy-CTAB-Co catalyst
can give the best stability.

Discussions
In order to understand the superior ORR performance of the nanowire PPy-CTAB-Co catalyst, diverse physiochemical characterizations were employed. Closely similar XRD patterns for PPy-CTAB-Co and PPy-Co catalysts were obtained (Fig. 6) with two broad diffraction peaks at 2θ of about 25° and 43°, which correspond to
(002) and (101) phase of carbon, respectively, but none characteristic peaks for metallic cobalt or cobalt oxide
could be observed in both catalysts, agreeing well with the SEM/TEM results (Figs 1 and 2) of none Co/CoO
particles, probably suggesting that the Co species are not in a crystalline state. Actually the ICP measurements
could detect a cobalt content of 4.356 wt% and 0.676 wt% in the catalysts of PPy-CTAB-Co and PPy-Co, respectively. XPS experiments (Fig. S1, Supporting Information) could also display surface cobalt content of 1.03 at%
(PPy-CTAB-Co) and 0.24 at% (PPy-Co) in the catalysts, respectively. Consulting with our previous research 40,
these results imply that the cobalt in the catalysts are bonded to nitrogen as Co-N structure, but not exist as
metallic cobalt or its oxide.

Fig. S2 in Supporting Information displays the N2 adsorption-desorption isotherm of PPy-CTAB, PPy and the
resulted PPy-CTAB-Co and PPy-Co catalysts. The calculated BET surface areas for PPy-CTAB and PPy are 120
and 12 m 2 g −1, respectively. This may explain why PPy-CTAB-Co catalyst has a higher cobalt content, as measured
with both ICP and XPS discussed above. It is believed that the higher specific surface area of nanowire PPy-CTAB
should be able to help absorbing more cobalt ions during cobalt acetate impregnation to form Co-N structure in
the final catalyst 41 .

N 1 s core level XPS spectra in PPy-CTAB, PPy, and the PPy-CTAB-Co and PPy-Co catalysts were captured
with XPS and demonstrated in Fig. 7. In both nanowire PPy-CTAB and granular PPy, the nitrogen is dominated
by a main peak of pyrrolic-N (N2) in the range from 400.1 to 400.9 eV, which can be assigned to nitrogen atoms
in PPy ring. In both the catalysts of PPy-CTAB-Co and PPy-Co, however, the form of nitrogen has changed drastically because of the decomposition of PPy and structure re-configuration during high temperature pyrolysis.

Figure 5. Stability of electrodes coated by PPy-CTAB-Co ( a ), PPy-Co ( b ) and 20wt% Pt/C ( c ), respectively,
measured in O2-saturated 0.1 M KOH solution. Potential scannig rate is 5 mV s −1 ; Catalyst loadings: 0.40 mg
cm −2 .

Figure 6. XRD patterns of PPy-CTAB-Co and PPy-Co catalysts.

Actually, the nitrogen in both of the catalysts could be deconvoluted into pyridinic-N (N1, 398.0–399.5 eV),
pyrrolic-N (N2, 400.1–400.9 eV), quaternary-N (N3, 401–402 eV) and oxidative-N (N4, 402–410 eV) 40, as shown
in Fig. 7. According to Higgins _et al._ 42, pyridinic-N could provide an edge plane exposure to readily facilitate the
adsorption of oxygen, leading to improved ORR performance. Sha _et al._ 40,43 also discussed that pyridinic nitrogen
could play a significant role in the ORR performance of pyrolyzed carbon supported polypyrrole-cobalt catalysts
with the active sites of Co-pyridinic-N group. In this work, the evaluated concentrations of various types of nitrogen in the studied PPy-CTAB-Co and PPy-Co catalysts are listed in Table 1. It can be seen that the concentration
of pyridinic-N in PPy-CTAB-Co is obviously higher than that in PPy-Co. Combined with the higher cobalt
content in PPy-CTAB-Co as discussed above and the larger nitrogen content (11.38 at% compared to 8.46 at% for
PPy-Co) acquired with XPS spectra (Fig. S1, Supporting Information), it is inferred that the more Co-pyridinic-N
structure as ORR active sites in the nanowire PPy-CTAB-Co catalyst is a main factor responsible for the enhanced
ORR performance. Moreover, the BET surface areas of PPy-CTAB-Co and PPy-Co catalysts can be calculated

Figure 7. N 1 s XPS spectra for PPy-CTAB ( a ), PPy ( b ), PPy-CTAB-Co ( c ), and PPy-Co ( d ).

PPy-CTAB-Co PPy-Co

Pyridinic-N (N1) 0.4333 0.3606

Pyrollic-N (N2) 0.0464 0.0892

Quaterary-N (N3) 0.4415 0.4305

Oxidative-N (N4) 0.0788 0.1197

Table 1. Concentrations of various nitrogen groups in the catalysts of PPy-CTAB-Co and PPy-Co.

using the data in Fig. S2 (Supporting Information), which are 72.2 and 24.4 m 2 g −1, respectively. It is another possible reason for the higher catalytic ORR activity of PPy-CTAB-Co than PPy-Co, because more Co-pyridinic-N
active sites exist on the larger surface of PPy-CTAB-Co catalyst to facilitate the oxygen reduction reaction.

Conclusions
In summary, PPy-CTAB-Co catalyst with a nanowire morphology is successfully synthesized with CTAB as
a surfactant and it is tested for catalyzing ORR in alkaline solution. Both much better catalytic ORR activity
and higher four-electron-transfer selectivity as well as higher stability than the granular PPy-Co catalyst are
observed. Although this catalyst’s ORR activity is slightly inferior to that of the commercial 20 wt% Pt/C catalyst,
a much better stability than the Pt/C catalyst in alkaline solution is achieved. With the help of physicochemical
techniques, the enhanced ORR performance of nanowire PPy-CTAB-Co can be attributed to large quantity of
Co-pyridinic-N groups as ORR active sites and its larger specific surface area which allows more active sites to
expose for facilitating oxygen reduction reaction.

Methods
Synthesis of catalysts. In order to prepare nanowire PPy, cetyltrimethylammounium bromide (CTAB)
was used as the surfactant. The typical procedure 44 is as follows: 1.5 mmol CTAB was stirred in 150 ml deionized
water at 40 o C to form a homogeneous solution, then this solution was cooled down to 0–3 o C to form a CTAB
crystalline suspension followed by the addition of pre-cooled ammonium peroxydisulfate (APS) aqueous solution
(4.5 mmol APS dissolved in 25 ml deionized water). After 1 minute, 500 μ l freshly distilled pyrrole was injected

and the obtained mixture was stirred for another 24 hours at 0–3 o C. The resulted mixture was washed with water
and ethanol alternately for several times, and then dried under vacuum at room temperature to obtain the PPyCTAB with a nanowire morphology.

For comparison, granular PPy with small particles was also prepared: 2 ml freshly distilled pyrrole was dissolved in 100 ml deionized water, followed by an addition of APS aqueous solution (57.6 mmol APS dissolved
in 200 ml deionized water). After a stirring for 24 hours, the mixture was washed several times with water and
ethanol alternately, and then dried under vacuum at room temperature to obtain the granular PPy.

In two individual experiments, 0.1 g PPy-CTAB or PPy was added into 300 ml saturated cobalt acetate solution, separately. The obtained mixture was stirred and refluxed at 60 o C for 10 hours, and then washed with excessive deionized water and dried under vacuum at 60 o C. The resulted black powders were then pyrolyzed in argon
atmosphere at 800 o C for 2 hours to obtain the final catalysts of PPy-CTAB-Co and PPy-Co, respectively.

Commercial 20 wt% Pt/C catalyst from BASF company was used as the baseline for ORR performance
comparison.

Characterizations. Powder XRD patterns were obtained on a Shimadzu 6000 X-ray diffract meter
using Cu _K_ α radiation ( _λ_ = 1.5406 Å). SEM images were recorded on a NOVA NANOSEM 450 instrument.
TEM images were recorded on a JEOL JEM-2100 instrument operating at 200 kV and 30 mA. XPS measurements were performed on Kratos AXIS ULTRA DLD with Al _K_ α as excitation source ( _hv_ = 14 kV). The N2
adsorption-desorption isotherms were measured using Micromeritics ASAP 2010 M + C instrument at 77 K to
calculate the Brunauer-Emmett-Telley (BET) specific surface areas. Cobalt content in the catalysts was detected
using a Thermal iCAP 6000 ICP spectrometer by soaking the catalysts in aqua regia.

Electrochemical measurements. The electrochemical activities of the catalysts towards ORR were evaluated in a three-electrode cell controlled by a CHI 750 A potentiostat/galvanostat. A platinum wire and a saturated
calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. A rotating disk
electrode employing a glassy carbon disk (∅ = 4 mm) was used as the working electrode. 0.1 M KOH was the
electrolyte.

The catalyst layer on the working electrode was prepared by pipetting 10 μ l catalyst ink onto the glassy carbon
disk and dried at room temperature. For catalyst ink preparation, 5.0 mg catalyst sample was ultrasonically dispersed in a solution containing 50 μ l Nafion® solution (5 wt%, DuPont) and 950 μ l deionized water.

Cyclic voltammograms (CVs) of the catalysts were recorded between 0.19 and 1.19 V ( _vs._ RHE) in O2-saturated
0.1 M KOH at 25 °C with a potential scanning rate of 5 mV s −1 . The rotating disk electrode (RDE) experiments
were carried out in an alkali O2- and Ar-saturated solution (0.1 M KOH), respectively, with the same conditions
as CV at various electrode rotating rates.

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Acknowledgements
The authors are grateful for the financial support of this work by the National Natural Science Foundation of
China (21176155 and 21476138).

Author Contributions
L.L., X.Y., D.P.W. and J.Z. designed the experiments, L.L., X.Y., Z.M., X.Y., X.W., Z.F. M. and L.Z. carried out the
experiments and analysis, L.L., X.Y., D.P. W. and J.Z. wrote the paper. All authors reviewed the manuscript.

Additional Information
Supplementary information [accompanies this paper at http://www.nature.com/srep](http://www.nature.com/srep)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Yuan, X. _et al._ Novel nanowire-structured polypyrrole-cobalt composite as efficient
catalyst for oxygen reduction reaction. _Sci. Rep._ 6, 20005; doi: 10.1038/srep20005 (2016).

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