A Highly Nanoporous Nitrogen-Doped Carbon Microfiber Derived from Bioresource as a New Kind of ORR Electrocatalyst
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Synthesis of metal-free carbon-based electrocatalysts for oxygen reduction reaction (ORR) to replace the conventional platinum-based catalysts has currently become a hot topic of research. This work proposes an activation-assisted carbonization strategy for the fabrication of nitrogen-doped nanoporous carbon microfibers (Me-CFZ-900) with a high BET surface area (~ 929.4 m2 g−1) via using melamine as a promoter/nitrogen source and bamboo-carbon biowastes as the carbon source with the help of a zinc chloride activator. Electrochemical tests showed that the Me-CFZ-900 material has exhibited excellent ORR electrocatalytic activity and long-term stability, and also displayed a quasi-four-electron ORR pathway in alkaline electrolyte. We also find that the graphitic-N may be the catalytically active site for the ORR, but the formation of planar-N can further help to promote the ORR activity for our catalysts. The results open a new space and provide a new idea to prepare valuable porous nanocarbon materials on the basis of carbonaceous solid wastes for catalysis of a wide range of electrochemical reactions in the future.
KeywordsNanoporous carbon Carbon microfiber Oxygen reduction reaction Electrocatalyst Bamboo-carbon biowaste
Accelerated aging test
Field-emission scanning electron microscopy
High-resolution transmission electron microscopy
Linear sweep voltammetry
Nitrogen-doped porous carbon microfibers
Oxygen reduction reaction
Rotation disk electrode
Reversible hydrogen electrode
Rotation ring-disk electrode
Saturated calomel electrode
X-ray photoelectron spectroscopy
Advanced electrochemical energy systems, such as fuel cells and metal-air batteries, are considered as promising alternatives for traditional fossil fuels [1, 2]. The oxygen reduction reaction (ORR) is an important reaction in those energy technologies, but it suffers from several shortcomings such as high over-potential, sluggish ORR kinetics, and pathway diversity, limiting the improvement of general performance and conversion efficiency [3, 4]. At present, the Pt-based catalysts have been widely employed to enhance the ORR in practical applications, but high cost and restricted resource of metal-Pt hamper the commercialization [5, 6, 7]. Thus, the exploration of cheap, active, and stable Pt-free ORR catalysts is significant to rapidly develop clean energy technologies.
To look for some valuable substitutes for metal-Pt catalysts, the doping of heteroatoms into carbon allotropes such as graphene , graphdiyne , and carbon nanotube  is popularly studied owing to their distinctive physical and electronic structures. Although an immense improvement has been carried out on controlled-fabrication of the doped-carbon catalysts, the origin of ORR catalytic activity is still unclear, which becomes a technical bottleneck in this field [11, 12]. Commonly, the enhancement of ORR activity of doped-carbon catalysts can be attributed to charge modulation and broken electroneutrality caused by the heteroatom doping in the carbon framework [13, 14]. Other researches also demonstrated that the ORR activity of the carbon-based catalysts originates from appropriate doping location and configuration [15, 16, 17]. Besides, the doping of heteroatoms such as nitrogen can induce carbon surface polarization, which helps to form new nitrogen-containing active sites, thereby favoring adsorption of atoms and ions . Thus, understanding the contribution of N-rich carbon structures is important for clarifying the ORR catalytically active sites, which can also pave a way to directional design ORR-active and stable doped-carbon catalysts.
The use of natural biomass (e.g., soybean , silk fibroin , kidney bean , and hemoglobin ) and animal biowastes (e.g., fish scale  and animal blood ) as a direct precursor or the nitrogen source of catalytically ORR-active sites was thought as an effective pathway to fabricate the doped-carbon catalysts. More recently, Li et al. also synthesized a doped carbon-based ORR catalyst with three-dimensional porous network via using hemin biomaterial as a single-source precursor and using self-assembled sodium chloride crystallines as the template . Jiang et al.  converted the biological enzyme of blood centers into Fe–Nx catalytically active sites for ORR electrocatalysis by the multi-step pyrolysis of blood biowaste. The resulting electrocatalyst shows superior ORR catalytic activity, indicating that the Fe–Nx structure of heme in blood cells is beneficial in the formation of ORR active centers and therefore can promote the performance of catalysts. These studies may be an aspiration that a new kind of high-performance doped-carbon catalysts can be prepared by appropriately controlling pyrolysis processes and choosing inexpensive biomass materials as the precursors.
Herein, inspired by cheap and easily available biowaste-derived heteroatom-doped carbon for superior ORR performance, we develop a strategy to synthesize a N-doped nanoporous carbon microfiber as a new kind of ORR electrocatalyst (Me-CFZ-900) by pyrolysis of wasted bamboo-carbon tissues with the activation of zinc chloride, combined with the use of melamine as a promoter/nitrogen source. To the best of our knowledge, there are no reports on the design of porous carbon microfibers as an ORR catalyst via facile conversion of bamboo-carbon biowastes until now. We find that the prepared Me-CFZ-900 catalyst has a large number of uniform mesopores with an average pore-diameter of 2.23 nm and a high surface area (~ 929.4 m2 g−1), which can be beneficial to the mass transportation of O2 electrocatalytic reduction. This study opens a new space and provides a new idea to prepare valuable porous nanocarbon materials, which can function as the promising ORR electrocatalysts by further improving pore characteristics and content of active N species.
Synthesis of Carbon-Based ORR Catalysts
High-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) tests were acquired by Hitachi UHR S4800 (Japan) and FEI Tecnai-G2 F30 instrument with an acceleration voltage of 300 kV, respectively. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos XSAM800 spectrometer. A Micromeritics Analyzer (ASAP 2010) was applied to test N2-adsorption/desorption isotherms at 77 K. X-ray diffraction (XRD) analysis was carried out by using a Shimadzu XRD-6000 X-ray diffractometer (Japan) with Cu Ka1 radiation (λ =1.54178 Ǻ) at 4° min− 1. Raman spectroscopy data were recorded with Horiba HR800 Raman system with a laser excitation wavelength of 514.5 nm. XR X-ray was done using a Shimadzu XRD-6000 (λ = X-ray diffractometer (Japan) with Cu Ka1 radiation Open image in new window ).
Results and Discussion
RRDE measurements were carried out to get insights into the ORR kinetics of carbon-based catalysts, as shown in Fig. 5c. Besides, based on the RRDE data, the corresponding electron number transferred (n) and peroxide species (H2O2%) produced during ORR are calculated via using Eqs. (1) and (2), respectively. The calculation results are displayed in Fig. 5d. The H2O2 yield (< 14.0%) and electron transfer number (3.45–3.95) on Me-CFZ-900 can be found in the potential range of 0.2–0.8 V versus RHE, which indicates a quasi-four-electron pathway for ORR process being similar to the ORR kinetics of commercial Pt/C catalyst (Additional file 1: Figure S3). Compared to the Me-CFZ-900, higher H2O2 yield and smaller electron transfer number can be observed on both CF-900 and CFZ-900 in the same potential range. However, the H2O2 yield on CFZ-900 is higher than that on Me-CFZ-900, but the electron transfer number on CFZ-900 is similar to that on Me-CFZ-900, also suggesting a quasi-four-electron pathway for ORR process. Unfortunately, CF-900 has exhibited the lowest electron transfer number (2.64–3.56) and highest H2O2 yield (22.2–68.2%), implying that the ORR catalyzed by CF-900 mainly follows a mixed pathway of two-electron and four-electron processes. These results prove that the carbon catalysts prepared by the zinc chloride activation have exhibited higher ORR catalytic efficiency and electrocatalytic performance with or without the addition of melamine. Combined with XPS analysis and ORR activity data, we find that only graphitic-N species can exist in CF-900 and CFZ-900 but exhibits ORR catalytic activity, which proves that the graphitic-N can be one of electrocatalytically active sites contributing to the ORR electrocatalysis. It is remarkable that the addition of melamine into the precursor can promote the formation of pyridinic-N and pyrrolic-N species with planar structures, which can be responsible for the ORR activity enhancement, supported by the previously reported results . Besides, the excellent ORR performance of Me-CFZ-900 can be also ascribed to other aspects: (1) high BET surface area and mesoporous structure of Me-CFZ-900 can facilitate the adsorption and transportation of oxygen molecule and the exposure of more active sites; (2) higher electrical conductivity of Me-CFZ-900 can effectively boost the electron transportation of ORR process; and (3) more N atoms are incorporated into the carbon structure of Me-CFZ-900, which can produce more nitrogen-rich defected structures and active sites. Therefore, controlled synthesis of high contents of planar and graphitic nitrogen species is essential to produce the active carbon-based catalysts for ORR, but further improvement of electrical conductivity, nitrogen-doping efficiency, and mesoporous characteristics is the key issue to enhance the ORR catalytic activity.
In summary, we develop a new method to prepare nanoporous N-doped carbon microfibers (Me-CFZ-900) derived from bamboo-carbon biowastes for the electrocatalysis of oxygen reduction reaction in alkaline media. The as-prepared Me-CFZ-900 catalyst exhibits the ORR electrocatalytic activity with a half-wave potential of ~ 0.86 V and a peak potential of ~ 0.91 V. The peroxide yield less than 14% and the average electron transfer number of 3.84 are obtained on Me-CFZ-900, further showing a quasi-four-electron reaction pathway. An only 21 mV negative shift in half-wave potential and 2.0% decline in the limited current density are observed on Me-CFZ-900 after doing the accelerated aging test. Furthermore, high BET surface area (929.4 m2 g−1) and mesoporous structure of Me-CFZ-900 can facilitate the adsorption and transportation of oxygen molecule. This work can help the researchers to build the high-performance carbon-based ORR electrocatalyst derived from biomass wastes and to understand the origin of the ORR electrocatalytic activity.
This study was financially supported by the National Natural Science Foundation of China (Project No: 21805024), the Basic Research and Frontier Exploration Project of Chongqing Municipality (Project No: cstc2018jcyjAX0461), the Research Program of Yongchuan Science and Technology Commission (Ycstc2016nc6001), the Open Project of Engineering Research Center of New Energy Storage Devices and Applications of Chongqing Municipality (KF20170201), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1711289), the Scientific Research Program (P2016XC07) and Talent Introduction Project (R2014CJ02) of Chongqing University of Arts and Sciences, and the Innovation Team Project of Chongqing Municipal Education Commission (CXTDX201601037). Zhongli Luo was supported by Nature Science Foundation Project of CQ (CSTC) (cstc2015jcyjBX0072), and the grant from the National Nature Science Foundation of China (NSFC 31771101).
CG, YL, and YX carried out the electrochemical experiments and wrote the manuscript. QX, LS, WZ, WL, YS, and ZL prepared the samples and performed the characterizations. CG and YS provided the idea for this work. ZL revised the manuscript. All authors read and approved the final manuscript.
Chaozhong Guo received his Ph.D. at Chongqing University of China in 2013. He is a distinguished professor and master supervisor of chemical engineering and tip-top academic backbone at Chongqing University of Arts and Sciences. His research mainly focuses on design and development of nanocarbon catalysts in energy conversion and storage. Currently, he has authored over 30 papers in peer-reviewed journals (e.g., J. Mater. Chem. A, Nanoscale, Carbon, J. Power Sources, etc.).
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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