Surfactant-template Preparation of Polyaniline Semi-tubes for Oxygen Reduction

Nitrogen and metal doped nanocarbons derived from polyaniline (PANI) have been widely explored as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. In this work, we report surfactant-template synthesis of PANI nanostructures and the ORR electrocatalysts derived from them. By using cationic surfactant such as the cetyl trimethyl ammonium bromide (CTAB) as the template and the negatively charged persulfate ions as the oxidative agent to stimulate the aniline polymerization in the micelles of CTAB, PANI with a unique 1-D semi-tubular structure can be obtained. The semi-tubular structure can be maintained even after high-temperature treatment at 900 °C, which yields materials exhibiting promising ORR activity.


Introduction
Seeking the highly-active electrocatalysts for oxygen reduction reaction (ORR) has become the one of the urgent demands for fuel cells, which would take a key role in the "hydrogen energy economy" [1].In recent years, non-precious metal and/or metal-free materials based on nitrogen (N)-doped nanocarbons have shown great promise in substituting Pt and its alloys for catalyzing the ORR [2,3].
Up to now, doped carbon electrocatalysts of different morphologies, such as nanoparticles [20], nanowires [21], nanotubes [22], nanorods [23], hollow nanospheres [24] and amorphous carbons [25], have been constructed by using various methods.The soft-template synthesis through self-assembly processes is among the most straightforward methods for nanostructure formation.In this work, we use assembly architectures of a variety of surfactants as the soft-templates to synthesize PANI nanostructures.In particular, PANI semi-tubes with uniform diameters of ~80 nm are obtained by using cationic surfactant.The electrocatalysts derived from these PANI semi-tubes show good ORR catalytic activity in alkaline.

Results and Discussion
Figure 1 shows the morphologies of PANI materials obtained by using CTAB (80 mM) as the template and APS as the oxidative agent.It can be seen that uniform 1-D nanostructures with diameters of ~80 nm and lengths of a few micrometers were obtained under this condition.Careful inspection revealed that these 1-D nanostructures possessed semi-cannular structures.As seen from the TEM images (Figure 1b and its insert), the walls of the individual tubes were highly rugged and full of cone-shaped protuberances of ~10 nm lengths, exhibiting centipede-like morphologies.We denoted this sample as PANIs-tubes.In the case when the preparation was conducted using the same procedure as that giving PANIs-tubes but the CTAB was absent, highly agglomerated PANI particles were obtained (Figure 2a).When the FeCl3 was used to replace APS as the oxidative agent to stimulate the polymerization of aniline in

b)
CTAB solution, irregular PANI nanosheets were obtained (Figure 2b).These results indicated that the semi-cannular structured PANIs can be uniquely formed through the oxidative polymerization of anilines by APS in the assemblies of CTAB.We have also explored the effects of the surfactant types on the morphologies of the formed PANI materials.For anionic surfactants, e.g., SDBS, and non-ionic surfactants, e.g., X-100 and Span 40, mixtures of PANI nanoparticles and nanorods were obtained (Figure 3), which indicated the uniqueness of CTAB in producing the tubular structures of PANI.We believed that the formation of the PANI semi-tubes was related to the rod-like CTAB micelles and the opposite charges between the oxidative persulfate ions and the protonated aniline.Scheme 1 depicts the possible growth mechanism.The aniline molecules should be dissolved into the rod-like CTAB micelles.In the presence of HCl, the aniline molecules should be protonated.Therefore, they would be mainly located in the outer region of the micelles.The electrostatic attraction made the negatively charged persulfate ions approach the outer surface of micelles and oxidize the aniline molecules, which stimulate the polymerization in the outer region of cylindrical micelles.The formation of half instead of full PANI tubes was probably due to that there were only limited amounts of aniline molecules dissolved in the micelles.When the CTAB micelles were present or positively charged Fe(III) ions were used as oxidative agent, the mechanism shown in Scheme 1 could be altered.A deeper b) understanding of the effects of the oxidants and surfactants on the resultant PANI morphologies requires much more detailed investigation.Scheme 1.The possible growth process of 1-D PANIs-tubes by CTAB micelle as the soft-template.
The obtained PANI samples were converted into N-doped carbon materials by heat-treating at 900 °C under Ar atmosphere.As shown in Figure 4, semi-tubular morphologies were maintained after the heat-treatment.We investigated the physical properties and chemical composition of the PANIs-tubes materials before and after the heat-treatment.It was found that the heat-treatment resulted in materials exhibiting BET surface area (ca.351.8 m 2 /g) and pore volume (0.45 m 3 /g) which were much higher than that before heat-treatment (ca.54.8 m 2 /g and 0.29 m 3 /g respectively).This was probably due to the volatilizing release of some components during the heat-treatment, making the resulted materials more porous.XPS characterization results indicated that the contents of N and O decreased significantly after heat-treatment (Table 1), which confirmed the volatilization of some components.Figure 5 compares the ORR polarization curves of the materials obtained through heat-treating the PANI prepared without using surfactant (see Figure 2a), the PANIs-tubes prepared by using CTAB micelles as templates, and the PANIs-tubes/GS composite.For comparison, the ORR polarization curve for the pure GS is also given.It can be seen that the heat-treated PANIs-tubes exhibited significantly enhanced ORR activity as compared with the material derived from the PANI that was prepared without using surfactant template.This should be due to the open semi-tubular structure, which gave higher specific surface areas.As compared with the heat-treated PANIs-tubes, the GS exhibited slightly more positive ORR onset potential, but slower current rising rate and lower limiting current.The heat-treated PANIs-tubes/GS composite showed much higher ORR activity than that exhibited by the heat-treated PANIs-tubes and the GS alone.
Figure 5. ORR polarization curves for different catalyst samples in O2-saturated 0.1 M KOH at an electrode rotation speed of 1600 rpm.The PANI refers to the sample obtained by heat-treatment of PANI prepared without using surfactant; the PANIs-tubes/GS refers to the sample obtained by heat-treatment of PANIs-tubes and graphene sheets together.The catalyst loadings were 0.3 mg cm −2 for the non-precious metal catalysts and 0.1 mg cm −2 for Pt/C (20 μg cm −2 for Pt).
Since the ORR polarization curves in Figure 5 were obtained with same total mass loading of 0.3 mg cm −2 for the heat-treated PANIs-tubes, GS and PANIs-tubes/GS, one may expect that the ORR activity of the heat-treated PANIs-tubes/GS composite is between that of the heat-treated PANIs-tubes and GS.The actually higher ORR activity of the composite thus indicated that there were synergetic interaction between PANIs-tubes and GS.The introduction of GS may increase the electrical conductivity of the composite.On the other hand, the formation of composite between the 2-D GS and the 1-D PANIs-tubes would prevent the GS and PANIs-tubes from agglomerating and stacking during 0.2 0.4 0.6 0.8 the heat treatment and electrode preparation.There have been numerous studies showing that the electrochemical performance of nanomaterials can be enhanced by forming composites with GS, due to the good electric conductivity of GS and the capability of GS to improve the dispersion of the electroactive materials [26,27].
It can be seen that the limiting current of the composite was very similar to that of the Pt/C electrocatalyst.The value of the limited current is directly related to the electron transfer number of the reaction.It is known that the Pt-based electrocatalysts catalyze the ORR through a 4-electron pathway.Therefore, we have reason to believe that ORR proceeded on the heat-treated composite mainly through a 4-electron process.It is noted that the present PANIs-tubes materials were still less efficient for the ORR than the Pt/C catalyst.Further optimization on the material preparation is necessary to promote the electrocatalytic activity.

Materials Synthesis
In a typical synthesis, a desired amount of CTAB was mixed with 15 mL of ultrapure water under ultrasonication for more than 30 min.After fully dissolution of CTAB, 200 μL of aniline (AN) and 20 mL of 1 M HCl were successively added under ultrasonication for another 30 min and then the solution was allow to stay for 24 h. 5 mL of 0.4 M ammonium persulfate (APS) solution was then added into the solution to stimulate the polymerization of aniline in the CTAB assemblies.Different CTAB concentrations (10-80 mM) were explored and no substantial difference in the morphology was seen for the obtained PANI.During the reaction progress, the color gradually changed to blue and cloudy precipitates were formed, which were collected through ultrafiltration and then alternately washed by ethanol and water followed by freeze-drying.Finally, dark blue product was obtained.For comparison, the preparation was also conducted by replacing CTAB with other surfactants.
We also prepared the composite of PANIs with graphene nanosheets (GS) that were prepared by high temperature thermal reduction of graphene oxide [17,18].In this case, 30 mg of GS were added into the CTAB solution to prepare the PANI, which corresponded to a PANI/GS ratio of 1/8.Different PANI/GS ratios were explored and the 1/8 was found to be the most optimized value.To obtain electrocatalysts from the obtained nanostructured PANI, they were pyrolyzed under a flow of Ar at 900 °C for 1 h.

Characterization
Scanning electron microscopy (SEM) images were obtained by Hitachi S-4800 Scanning Electron Microsope (Tokyo, Japan).Transmission electron microscope (TEM) images were obtained at

Figure 2 .
Figure 2. Morphologies of PANI materials obtained when (a) the CTAB was absent, or (b) FeCl3 instead of APS was used as the oxidative reagent.The other conditions are the same as that for Figure 1.

Figure 3 .
Figure 3. Morphologies of PANI materials obtained when the CTAB was replaced by (a) sodium dodecyl benzene sulfonate (SDBS), (b) Triton X-100 or, (c) Span 40.The other conditions are the same as that for Figure 1.

Figure 4 .
Figure 4. Comparison between the morphologies of PANIs-tubes (a) before and (b) after heat-treatment at 900 °C.

Table 1 .
The content values of C, N and O estimated from XPS results for PANIs-tubes before and after heat-treatment.