The oxygen reduction reaction (ORR) is a crucial process in many energy conversion devices, such as fuel cells and metal/air batteries ^[1]. Although platinum-based materials have proven to be the most efficient catalysts for the ORR, the high cost and scarcity of this metal significantly hinder the large-scale applications of these devices ^{[2, 3]}. Recently, non-precious metal and metal-free ORR catalysts have attracted significant research interest as alternatives to platinum-based compounds ^{[4, 5]}. Nitrogen-doped carbon materials represent typical metal-free catalysts and exhibit excellent ORR activity as the result of nitrogen incorporation, as confirmed by experimental studies and quantum mechanical calculations ^[6-8]. Aniline derivatives have unique structures consisting of aromatic rings connected via imino groups, and so are frequently used as the nitrogen source in nitrogen-doped carbon materials ^[9-12]. However, a remaining challenge associated with these catalysts is their insufficient durability. The electrochemical stability and durability in air of nitrogen-doped carbon materials are substantially lower than those of platinum-based compounds, and this has hampered their applications ^{[4, 13]}. Many groups have reported on the electrochemical stability of nitrogen-doped carbon materials, and great progress has been made in this respect over the past several years ^{[4, 14]}. However, there have been few reports of the durability in air of nitrogen-doped carbon materials.

Platinum-based materials exhibit excellent ORR activity because of their appropriate oxygen binding and OH bonding energy values ^{[15, 16]}. During the ORR, oxygen is eventually reduced to water as electrons flow through an external circuit to the anode. However, when platinum-based catalysts are exposed to air, this electrochemical reaction does not proceed ^{[17, 18]}. Because the adsorption of atmospheric oxygen on the Pt surface generates an oxide coating ^[19], platinum-based catalysts must undergo an activation treatment to remove the surface oxidation layer before they exhibit optimal performance ^[20]. In the case of nitrogen-doped carbon materials, oxygen also adsorbs on the surface of the catalyst during the ORR ^{[8, 21]}. This raises the question of whether or not oxygen will react with the catalyst and, if so, how this impacts the catalytic activity.

Herein, we report a poly-p-phenylenediamine (PpPD)/ carbon black (CB) composite catalyst that shows high ORR activity, such that it outperforms a commercially available Pt/C catalyst (10% Pt,Johnson Matthey Corp) in KOH solution (0.1 mol/L). More importantly, we demonstrate that the exposure of the composite to air results in a reaction between oxygen and the catalyst, leading to a decrease in the ORR activity.

2 Experimental

2.1 Chemicals and apparatus

Both p-phenylenediamine (AR, 97%) and ammonium persulfate (AR,≥98%) were purchased from the Aladdin company (Shanghai,China) and were used without further purification. All the electrochemical experiments were carried out using a Parstat 2273 electrochemical station. Scanning electron microscopy (SEM) images were obtained with a Nova NanoSEM 230 and N₂ adsorption-desorption isotherms were acquired with a Micromeritics Tristar 3000 analyzer at -196 ℃. X-ray photoelectron spectroscopy (XPS,ESCALAB 250) was used to characterize the surface compositions of the samples.

2.2 Preparation of catalysts

To prepare composites consisting of PpPD and CB (Black Pearls 2000) (termed 0.44PpPD/CB, 0.88PpPD/CB and 1.32PpPD/CB),p-phenylenediamine (0.44, 0.88 or 1.32 g) was dissolved in 200 mL deionized water, following which 0.5 g CB was added. After stirring for 5 h, a solution of the oxidant (NH₄)₂S₂O₈ was added dropwise. The polymerization was allowed to proceed first with the container immersed in an ice bath (0 ℃) for 6 h, and then at room temperature for another 18 h. The resulting products were filtered and washed with deionized water, then dried at 60 ℃. Finally, the mixtures were ground into powder and heated under a N₂ flow at 1000 ℃. A sample of pure PpPD was prepared in the same manner, but without adding CB.

2.3 Characterization of catalysts

The electrochemical characterization of specimens was carried out in a standard three-electrode cell using a Pt plate counter electrode and a Ag/AgCl/KCl (3 mol/L) reference electrode (0.197 V vs. NHE) at 25 ℃. In brief, the catalyst was dispersed in an ethanol/Nafion (5%) mixture and ultra-sonicated for 30 min to form a uniform black ink. This well-dispersed catalyst ink was applied to a pre-polished glassy carbon disk (5 mm diameter, 0.19625 cm² area) and dried at room temperature. The ORR was performed in O₂-saturated KOH solution (0.1 mol/L) with a rotating-disk electrode (RDE) system. The accelerated durability trials were carried out by acquiring cyclic voltammograms (−0.3 V to 0.2 V) at 50 mV/s in O₂-saturated KOH solution (0.1 mol/L). All potentials were reported versus Ag/AgCl/KCl (3 mol/L).

3 Results and discussion

3.1 SEM and N₂ adsorption-desorption tests

The SEM image in Fig. 1(a) shows that the PpPD had an irregular sheet-like morphology with a smooth surface, and that the particle sizes of the PpPD were on the micrometer scale. The SEM image of the 0.88PpPD/CB (Fig. 1(b)) reveals that the CB particles aggregated on the surface of the PpPD, and that the size of the PpPD sheets was decreased ^[22]. The composite structure seen here is different from previously reported structures, in which the CB particles were covered with a PpPD shell ^{[12, 23]}. This difference is the result of variations in the process used to polymerize the pPD. In the present work, the polymerization was conducted without adding hydrochloric acid, thus the water solubility of the undoped PpPD was very low ^[24]. As a result, the PpPD sheets tended to agglomerate, thus increasing the particles size. Because the interaction between the hydrophobic surfaces of the CB and the PpPD is strong, the CB particles also aggregated on the PpPD surfaces and retarded the polymerization reaction. Thus, the PpPD particle sizes in the 0.88PpPD /CB were smaller than in the pure PpPD sample. To gain further insight into the structure of the PpPD/CB composites,N₂ adsorption-desorption measurements were carried out. Fig. 2(a) shows the adsorption-desorption isotherms, while the corresponding pore size distributions are presented in Fig. 2(b) and the results are summarized in Table 1. As expected, the Brunauer-Emmett-Teller (BET) surface area of the CB was very large, while the BET surface area of the PpPD was only 125 m²/g, which is in good agreement with the image in Fig. 1(a). Therefore, the BET surface area of the PpPD/CB composite decreased as the amount of PpPD increased. It is interesting to observe that the micropore area of the 0.88PpPD/CB was larger than that of the 0.44PpPD/CB. This occurred because both the PpPD and the CB contributed to the micropore area, thus the 0.88PpPD/CB had a superior composite structure because it contained the optimal ratio of PpPD to CB.

Fig. 1

Fig. 1. SEM images of PpPD (a) and 0.88PpPD/CB (b).

Fig. 2

Fig. 2. N2 adsorption-desorption isotherms (a) and the corresponding pore size distributions (b) of the prepared catalysts.

表 1 (Table 1)

Table 1 BET surface, micropore and external surface areas of the prepared catalysts. Catalyst BET surface area (m2/g) Micropore area (m2/g) External surface area (m2/g) PpPD 125 122 12.7 1.32PpPD/CB 286 127 158 0.88PpPD/CB 523 263 260 0.44PpPD/CB 590 247 342 CB 1480 618 862

Table 1 BET surface, micropore and external surface areas of the prepared catalysts.

3.2 Electrochemical characterization

We performed RDE measurements in KOH solution (0.1 mol/L) to evaluate the ORR activity of the prepared catalysts. As shown in Fig. 3(b), the onset potential of the PpPD was rather high, and its current density increased slowly as the potential decreased (Fig. 3(a)). This is attributed to the very low BET surface area of the PpPD (with an external surface area of only 12.7 m²/g), because surface area is vital to mass transport in this system. Thus, the current density obtained from the PpPD was 2.2 mA/cm² even at -1 V, and a plateau appeared at approximately -0.5 V, suggesting the production of HO₂^-. As a result of the addition of CB, the PpPD/CB composite exhibited a substantial improvement in ORR activity and the plateau disappeared. As shown in Fig. 3(a), the ORR activity of the 0.88PpPD/CB was far superior to those of the 0.44PpPD/CB and 1.32PpPD/CB. Because these catalysts were all prepared in the same manner, except for the amount of p-phenylenediamine, the differences in catalytic activity must have resulted primarily from variations in their physical properties, rather than their chemical characteristics. As noted above, the 0.88PpPD/CB had an advanced composite structure, in which the N-rich surface of the PpPD had an increased active center concentration and the high external surface area of the CB was conducive to the transport of oxygen and products. Thus the 0.88PpPD/CB outperformed the commercial Pt/C catalyst, as demonstrated by the higher current density (2.17 mA/cm² at -0. 1 V, compared to 1.11 mA/cm² for the Pt/C) and the more positive half-wave potential (E_1/2).

To gain further insights into the electron transfer kinetics of the ORR over the 0.88PpPD/CB, we studied the reaction kinetics by rotating-disk voltammetry. The voltammetric profiles in O₂-saturated KOH solution (0.1 mol/L) show that the current density was enhanced as the rotation rate increased from 400 to 2000 r/min (Fig. 3(c) inset) ^[25], and the corresponding Koutecky-Levich plots (J⁻¹ vs. ω^−1/2) at various potentials exhibit good linearity. In alkaline solution, the ORR generally proceeds by one of two pathways. The first is a direct 4-electron process in which O₂ is reduced to OH^-, while the other is a 2-electron reduction pathway such that the O₂ is reduced to HO₂^-, followed by further reduction of the HO₂^{- [26]}. We supposed that the ORR over Pt/C proceeded via the 4-electron reaction at -0.4 V, and calculated the number of electrons transferred at various electrode potentials of Pt/C and 0.88PpPD/CB. In the case of the 0.88PpPD/CB, the number of electrons transferred at various electrode potentials was constant at approximately 3.6 (Fig. 3(c)), suggesting that O₂ is reduced almost completely to OH^- through the 4-electron reduction process.

Fig. 3

Fig. 3. RDE linear sweep voltammograms (a) of the prepared catalysts (catalyst loading: 820 μg/cm2) and Pt/C (catalyst loading: 250 μg/cm2); tafel plots (b) obtained from the RDE measurements; the dependence of the electron transfer number on the potential (c) for 0.88PpPD/CB and Pt/C at various potentials (Inset shows the RDE voltammograms of 0.88PpPD/CB at various rotation rates); accelerated durability test data (d) for 0.88PpPD/CB after 100 and 1000 cycles. RDE data were recorded in KOH solution (0.1 mol/L); electrode rotation speed 900 r/min; scan rate 10 mV/s.

3.3 Durability investigations

To evaluate the electrochemical durability of the 0.88PpPD/CB, accelerated durability tests using cyclic voltammograms ranging from -0.3 to 0.2 V at 50 mV/s in O₂-saturated KOH solution (0.1 mol/L) were carried out. It is evident from Fig. 3(d) that a slight change in the half-wave potential (~18 mV) is observed after 1000 cycles, indicating the good electrochemical stability of the 0.88PpPD/CB. To evaluate the durability of this material, it was exposed to air for 30 d. The results following this exposure show that the catalytic activity of the 0.88PpPD/CB decreased after air exposure; the current density at -0.1 V decreased by 1.02 mA/cm² and the diffusion-limiting current density was also reduced (Fig. 4(a)). As previously discussed, platinum-based materials tend to oxidize upon exposure to air, generating a surface oxide layer. We believe that oxidation also occurs upon exposure of the 0.88PpPD/CB to the atmosphere, decreasing the ORR activity. XPS assessments were performed to determine the chemical composition and structure of the catalyst. As shown in Fig. 4(b), the oxygen content of the 0.88PpPD/CB increased by 1% (atomic fraction) following air exposure, while the N content decreased slightly. The increase in the oxygen content indicates the occurrence of an oxidation reaction, although this reaction cannot be directly attributed to atmospheric oxygen. Thus, in order to eliminate other factors, such as water vapor and carbon dioxide, we ran another experiment. Following heat treatment and cooling of the sample to below 100 ℃, the nitrogen atmosphere was transitioned to oxygen, exposing the 0.88PpPD/CB to high purity oxygen for 1 h. XPS results reveal that the oxygen content of the 0.88PpPD/CB almost doubled after this oxygen exposure, and th e O 1s spectrum in Fig. 4(d) clearly shows increases in the C=O (531.2 eV) and C-O (533 eV) components ^{[8, 27]}. In subsequent RDE measurements, the ORR activity of the 0.88PpPD/CB was found to have greatly decreased (Fig. 4(a)) after this oxygen exposure; the current density at -0.1 V was only 0.29 mA/cm² and the diffusion-limiting current density was less than 4 mA/cm². These data demonstrate that exposing 0.88PpPD/CB to air allows oxygen to react with the catalyst, and this oxidation decreases the ORR activity. Therefore, the 0.88PpPD/CB should be protected from oxygen exposure to preserve its catalytic ability.

4 Conclusions

Fig. 4

Fig. 4. RDE voltammograms (a) (KOH solution (0.1 mol/L); electrode rotation speed 900 r/min; scan rate 10 mV/s); the nitrogen and oxygen contents (b); C 1s spectra (c); O 1s spectra (d) of the 0.88PpPD/CB, following exposure to air for 30 d or to pure oxygen for 1 h after heat treatment.

In summary, nitrogen-doped carbon materials with high catalytic activity in alkaline solution were obtained via the pyrolysis of PpPD/CB composite. It was found that the exposure of this composite to the ambient atmosphere resulted in oxidation of the catalyst that in turn decreased the ORR activity. Thus stability in air must be taken into consideration when assessing the application of nitrogen-doped carbon materials as alternatives to platinum-based catalysts.

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