The Oxygen Reduction Electrocatalytic Activity of Cobalt and Nitrogen Co-doped Carbon Nanocatalyst Synthesized by a Flat Template
- Chaozhong Guo†1Email author,
- Youcheng Wu†1,
- Zhongbin Li1,
- Wenli Liao1Email author,
- Lingtao Sun1,
- Chao Wang1,
- Bixia Wen1,
- Yanrong Li1 and
- Changguo Chen2Email author
© The Author(s). 2017
Received: 21 September 2016
Accepted: 18 December 2016
Published: 22 February 2017
The design of noble-metal-free catalysts for oxygen reduction reaction (ORR) is very important to the commercialization of fuel cells. Here, we use a Co-modified montmorillonite (Co-MMT) as a flat template to prepare Co- and N-doped nanocarbon ORR catalysts derived from carbonization of polyaniline at controlled temperatures. The use of flat template can hinder the agglomeration of polyaniline during pyrolysis process and optimize the N-rich active site density on the surface. The addition of transition metal Co in the flat MMT template can largely promote the formation of Co–N sites in prepared catalyst, facilitating the effective improvement of catalytic activity towards the ORR with a direct four-electron transfer pathway. The excellent ORR activity may be mainly attributed to high contents of graphitic N, pyridinic-N, and Co-N configurations. This study opens a new way to rationally design cheap and active ORR catalysts by using simple flat compound as a direct template.
With the reduction of the storage volume of fossil fuels and the increasing emphasis on green environmental protection, people are trying to explore sustainable and non-polluting power sources. The development of high-efficiency fuel cells (or metal-air batteries) as the promising clean energy power generation technology has become the key solution to solve the problem of energy shortage and environment pollution around the world. However, the oxygen reduction reaction (ORR) at the cathode of these power sources has exhibited disadvantages of slow kinetics and the diversity of ORR pathway, which brings lots of negative effects to the battery system and further decreases the general performance of the battery [1, 2]. Platinum and its alloys are currently the best catalyst with ultrahigh ORR electrocatalytic activity , but we cannot perform the large-scale application because of their high price, low stability, and inferior tolerance to fuel molecules. Therefore, the search for cheap, highly active, and stable alternatives to the Pt catalyst can facilitate the commercialization of fuel cells.
Over the past few decades, many types of ORR non-Pt catalysts , including non-precious metal catalyst (NPMCs) , transition metal oxides (TMOs) , heteroatom-doped carbon material (HDCMs) , have been developed. Especially, nitrogen-doped carbon materials (NDCMs) have attracted much attention owing to their unique inner structure and catalytic properties, which are considered to be a new ORR catalyst for fuel cells . In recent years, N-doped graphene (NG) , N-doped carbon nanotubes (N-CNTs) , N-doped carbon nanospheres (N-CNSs) , and other carbon nanomaterials have been effectively synthesized. Although they exhibit reasonable ORR catalytic activity and durability, the ORR catalytic mechanism and real active sites are unclear to this day. To be sure, heteroatoms (especially N atom) being introduced into the inner structure of carbon materials will improve the ORR catalytic activity in spite of doping methods (e.g., in situ and post-treatment doping). The enhancement of ORR activity in NDCMs is mainly due to the difference of bond length, valence electrons, and atomic size, which can result in the damage of electric neutrality of adjacent carbon atoms owing to the doping of nitrogen atoms [12, 13].
The main method of achieving higher ORR catalytic efficiency is to expose nitrogen-containing active sites for the ORR on the catalyst surface as soon as possible . We previously formed a new class of ORR catalysts by pyrolysis of protein-rich biomass (e.g., animal biomass, blood protein, and enoki mushroom) modified on carbon materials (e.g., CNTs and CNSs) under innert atmosphere at high temperature [14–17]. We can find that the active sites exposed on the surface of CNTs or CNSs can largely enhance the ORR activity in alkaline and acidic media. Moreover, the addition of carbon support can hinder further birdnesting of decomposed products and promote the surface density of active sites for the ORR [15, 17]. Therefore, we report a new strategy to design a cobalt-nitrogen-doped carbon (Co-NC) composite nanocatalyst for oxygen reduction by using polyaniline (PANI) as nitrogen/carbon sources and program-controlled pyrolysis process at high temperatures. The montmorillonite can be used as a flat template in the synthesis of Co-NC catalyst. This catalyst exhibits an ORR electrocatalytic activity with a four-electron transfer pathway in both acidic and alkaline solutions.
Synthesis of Co-NC-900 Catalyst
Structural and Electrochemical Characterizations
where n is electron transfer numbers per oxygen molecule involved in the ORR, CO is the O2 saturation concentration in the electrolyte, DO is the O2 diffusion coefficient in the electrolyte, ν is the kinetic viscosity of the electrolyte, ω is the electrode rotation rate, and 0.62 is a constant when the rotation rate is expressed in rounds per minute.
Electrochemical data were collected on a Zahner Zennium-E electrochemical workstation (Germany) with a convential three-electrode cell at room temperature. A glass-carbon rotation disk electrode (GC-RDE, 4 mm in diameter, Model 636, Princeton Applied Research), a saturated calomel electrode (SCE), and a Pt foil with geometric area of 1 cm2 were used as working electrode, reference electrode, and counter electrode, respectively. All potentials are quoted versus a reversible hydrogen electrode (RHE) in this study. The preparation of working electrode was performed by a coating method. Typically, the obtained carbon catalyst was well-dispersed in the 0.5 wt.% Nafion/isopropanol solution. Five microliters of 10 mg ml−1 dispersion was transferred onto the GC-RDE surface and then naturally dried. The mass loading was estimated to be around 0.40 mg cm−2. A commercial Pt/C catalyst (20 wt.% Pt, E-ETK) on the GC-RDE surface was prepared in the same way, but its mass loading was kept at about 0.32 mg cm−2.
Results and Discussion
Oxygen Reduction Electrocatalyic Activity and Stability
We further investigate the ORR electrocatalytic mechanism of Co-NC-900 using the RDE at different rotation rates (400–3600 rpm), as shown in Fig. 4c. As can be seen, an increase in the ORR current density with the rotation rate was observed at the Co-NC-900-catalyzed electrode. The good linearity of the Koutecky–Levich (K–L) plot (Fig. 4d) suggests the first-order dependence of the ORR kinetics at different potentials (0.3–0.5 V). The average ORR electron transfer number (n) is calculated to be about 3.9 at 0.3~0.5 V for Co-NC-900 and the average kinetic current density (j k) was calculated to be ~13.8 mA cm–2 for Co-NC-900, respectively, based on the slopes and intercepts of K–L plots obtained at 0.3–0.5 V (versus RHE). Our results indicate that the ORR on the Co-NC-900 catalyst proceeds mainly with four-electron reduction pathway (2 H2O + O2 + 4 e− → 4 OH−), very similar to the ORR catalyzed by a state-of-the-art Pt/C catalyst measured in KOH solution . More significantly, the ORR activity of Co-NC-900 is comparable to that of the best metal/nitrogen-doped carbon electrocatalysts and metal-free carbon-based electrocatalysts reported to date, in particular the catalysts derived from various nitrogen-containing organic molecules [14–17, 24–26]. Hence, we can reasonably conclude that the Co-NC-900 catalyst is a very promising candidate for the commercial Pt-based electrocatalyst in alkaline medium.
To explain the ORR catalytic mechanism of the Co-NC-900 catalyst in acidic electrolyte, we also measured the ORR polarization curves in 0.1 mol l–1 HClO4 at different rotation speeds (400−3600 rpm), as displayed in Fig. 5c. The ORR current densities measured on Co-NC-900 increase with the increasing of RDE rotation rates. The good linearity of Koutecky–Levich (K–L) plots (Fig. 5d) and near parallelism of fitting lines synergistically show the first-order dependence of the ORR kinetics and similar electron transfer numbers for ORR at different potentials. The average electron transfer number (n) was calculated to be ca. 4.1 for Co-NC-900 and the average kinetic current density (j k) was calculated to be ~7.32 mA cm–2 for Co-NC-900, respectively, based on the slopes and intercepts of K–L plots obtained at 0.2–0.4 V (versus RHE). Hence, the ORR on Co-NC-900 proceeds with a direct four-electron reduction pathway (O2 + 4 H+ + 4e– → 2H2O), very similar to the ORR catalyzed by the commercial Pt/C catalyst in 0.1 mol l–1 HClO4 solution .
Herein, we report a simple and new method to design a Co-NC catalyst by using a Co-modified montmorillonite as a flat template and using polyaniline as a single precursor of carbon and nitrogen, which can avoid the usage of complex chemicals substances in the synthetic process. The use of flat template can hinder the agglomeration of polyaniline during pyrolysis process and optimize the N-rich active site density on the surface owing to the intrinsic characteristics of Co-MMT template, resulting in the improvement of the ORR electrocatalytic activity in acidic and alkaline media. This study shows that under the condition of controlled temperatures, the Co-MMT template-assisted conversion of polyaniline is feasible to prepared a series of high-performance Co-NC catalysts for electrochemical reactions.
This work was supported by the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501118), the Basic and Frontier Research Program of Chongqing Municipality (cstc2015jcyjA50032 and cstc2014jcyjA50038), and the Talent Introduction Project of Chongqing University of Arts and Sciences (R2014CJ02). We gratefully thank Prof. Zhongli Luo and Yujun Si for helpful discussions.
CG and YW carried out the electrochemical experiments and wrote the manuscript. ZL, WL, LS, CW, BW, and YL prepared the samples and performed the characterizations. CG and CC checked and revised the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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