- Nano Express
- Open Access
A Nanopore-Structured Nitrogen-Doped Biocarbon Electrocatalyst for Oxygen Reduction from Two-Step Carbonization of Lemna minor Biomass
© The Author(s). 2016
- Received: 3 January 2016
- Accepted: 17 May 2016
- Published: 25 May 2016
So far, the development of highly active and stable carbon-based electrocatalysts for oxygen reduction reaction (ORR) to replace commercial Pt/C catalyst is a hot topic. In this study, a new nanoporous nitrogen-doped carbon material was facilely designed by two-step pyrolysis of the renewable Lemna minor enriched in crude protein under a nitrogen atmosphere. Electrochemical measurements show that the onset potential for ORR on this carbon material is around 0.93 V (versus reversible hydrogen electrode), slightly lower than that on the Pt/C catalyst, but its cycling stability is higher compared to the Pt/C catalyst in an alkaline medium. Besides, the ORR at this catalyst approaches to a four-electron transfer pathway. The obtained ORR performance can be basically attributed to the formation of high contents of pyridinic and graphitic nitrogen atoms inside this catalyst. Thus, this work opens up the path in the ORR catalysis for the design of nitrogen-doped carbon materials utilizing aquatic plants as starting precursors.
- Nitrogen-doped carbon
- Oxygen reduction
- Lemna minor
The increasing environmental pollution and depletion of fossil fuels have obliged peoples to develop green and clean energy sources. The fuel cell technology is considered as a very promising energy-conversion device all the time, because it possesses high-efficiency and environment-friendly characteristics . Currently, one of the main factors that hinder the commercialization of fuel cells is sluggish reaction kinetics of cathode oxygen reduction reaction (ORR) [2–4]. Up to date, noble-metal Pt-based materials are unanimously thought as the state-of-the-art ORR catalysts . Unfortunately, high cost and limited natural supply of platinum seriously restrict the development of conventional Pt/C catalysts and the sustainable application of fuel cells . Therefore, exploiting price-moderate and resource-rich catalysts for ORR to substitute for Pt-based catalysts has remained a great challenge.
In the past decades, numerous research studies were largely focused on the synthesis of various non-Pt catalysts such as metallic oxide catalysts , non-precious-metal (Me-N x /C) catalysts , heteroatom-doped carbon catalysts , and graphene-porphyrin MOF composite  in order to replace the precious Pt catalyst. Nitrogen-doped carbon materials with different morphologies, including N-doped graphene , N-doped carbon nanotube , N-doped carbon sphere , and N-doped carbon nanoweb [14, 15] were rapidly developed as effective ORR catalysts in various electrolytes. Besides, 2D sandwich-like zeolitic imidazolate framework (ZIF)-derived graphene-based nitrogen-doped porous carbon sheets (GNPCS ) were obtained by in situ growing ZIF on graphene oxide . Compared to the commercial Pt/C catalyst, the GNPCSs show comparable onset potential, higher current density, and especially, an excellent tolerance to methanol and superior durability in the ORR. Recently, efficient ORR catalysts derived from renewable and earth-abundant plant biomass are highly desirable. A large number of conveniently available biocarbon catalysts for ORR were developed from organic plants, e.g. Typha orientalis , monkey grass , Ipomoea aquatica , ginkgo leaf , soybean , and amaranthus waste , which can exhibit reasonable ORR catalytic performances. We previously synthesized a novel N-doped carbon nanomaterial with a superior ORR activity from thermal transformation of enoki mushroom biomass at high temperatures . Our results interestingly indicated that the plants rich in biological protein could largely benefit the formation of N-containing active centers for the ORR and help to enhance the electrocatalytic activity in both alkaline and acidic electrolytes.
Lemna minor is a species of Lemna with a subcosmopolitan distribution, native throughout most of Africa, Asia, Europe. and North America. It is an important food resource for many fish and birds (notably ducks), because it is rich in crude proteins. However, L. minor is structurally adapted to grow quickly and sometimes may be considered a pest or organic waste. Therefore, we here report a new strategy to design the nitrogen-doped biocarbon material by two-step carbonization of L. minor as a starting material under N2 protection. The electrocatalytic activity towards the ORR and long-term stability of this material were evaluated by the rotation disk electrode, and its structural characteristics were also examined by X-ray photoelectron spectroscopy, Raman spectroscopy, and nitrogen-sorption measurements.
L. minor (common duckweed, CDW) was completely dried at 120 °C in a drying oven, and then ground in an agate mortar for 1 h. 1.0 g of CDW dried powder was first carbonized at 300 °C for 2 h to promote thermal decomposition of bioproteins as far as possible, and the obtained carbonaceous material was marked for CDW-300. Subsequently, the CDW-300 was heat-treated in a tubular furnace at 700 °C for 2 h and then cooled to room temperature. The produced samples were further leached by 1.0 mol l−1 HCl solution, which was labeled as NPNC-700. As a control, the same method was applied to synthesize NPNC-600 and NPNC-800. All the heat treatment processes were carried out in nitrogen atmosphere with a heating rate of 10 °C min−1.
The surface and morphology were visualized using electron microscopy facility (JEOL FE-2010 high-resolution microscope operated at 200 kV). The Raman spectra were recorded with a Renishaw inVia unit using the Ar ion laser with an excitation wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) analysis was studied using a VG Scientific ESCALAB 220 iXL spectrometer with an Al Kα (hv = 1486.69 eV) X-ray source. Nitrogen adsorption and desorption isotherms were measured at 77 K using Micromeritics ASAP 2010 Analyzer (USA) to obtain the Brunauer-Emmett-Teller (BET) surface area and pore size distribution.
where j d and j k are the measured current density and kinetic limiting current density, respectively, F is the Faradaic constant (C mol−1), C O is the O2 saturation concentration in the aqueous solution (mol cm−3), D O is the O2 diffusion coefficient in the aqueous solution (cm2 s−1), v is the kinetic viscosity of the solution (cm2 s−1), ω is the electrode rotation rate (rpm), and 0.62 is a constant when the rotation rate is expressed in rpm.
Structural and Morphology Characterizations
Electrocatalytic Activity for Oxygen Reduction
To explain the ORR catalytic mechanism of the NPNC-700 catalyst in the alkaline electrolyte, we also measured the ORR polarization curves in 0.1 mol l−1 KOH at different rotation speeds (400–3600 rpm), as displayed in Fig. 5e. The ORR current densities measured on NPNC-700 increase with the increase of RDE rotation rates. The good linearity of Koutecky-Levich (K-L) plots (Fig. 5f) 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 ~3.9 for NPNC-700 and the average kinetic current density (j k) was calculated to be ~6.7 mA cm−2 for NPNC-700, respectively, based on the slopes and intercepts of K-L plots obtained at 0.5–0.7 V versus RHE. Hence, the ORR on NPNC-700 proceeds mainly with a four-electron reduction pathway, very similar to the ORR electro-catalyzed by the Pt/C catalyst . Results show that the NPNC-700 catalyst is a promising candidate for commercial Pt/C catalysts in the alkaline electrolytes.
In recent years, the exploration of active sites for the ORR in nitrogen-containing carbon-based catalysts was a hot topic all the time. Numerous studies were mainly focused on the geometry of the surface nitrogen-containing groups and their significant correlations with the electrocatalytic activity [13, 26, 27]. However, up to date, it is only confirmed that the doping of nitrogen atoms into the graphite lattice can promote the electrocatalytic activity thanks to the formation of chemically active and localized areas of higher electron density . In this study, we interestingly find that a high content of pyrrolic nitrogen in the catalyst has not played a key role in the ORR performance, but the formation of pyridinic and graphitic nitrogen during high-temperature pyrolysis can synergistically enhance the ORR electrocatalytic performance. Lai and his co-workers previously proposed that the activity of the doped carbon catalysts was found to be dependent on the graphitic nitrogen content, while the pyridinic nitrogen content improved the onset potential for ORR . In RDE experiments, the limiting current density of NPNC-700 was much higher than that of CDW-300 and the onset potential for ORR of NPNC-700 was more positive than that of CDW-300. These results may exactly correspond to the XPS analyses for N1s region of CDW-300 and NPNC-700. Thus, it can be reasonably concluded that the pyridinic and graphitic nitrogen atoms may be the catalytically active centers for the ORR in our catalysts.
In summary, a novel nitrogen-containing biocarbon material with nanoporous structures was synthesized by two-step carbonization of L. minor under the nitrogen atmosphere. This material exhibits the ORR electrocatalytic activity with an onset potential of around 0.93 V, and the excellent cycling stability in alkaline medium. The electron transfer number of the ORR on our catalyst was calculated to be around 3.9, almost identical to that on the commercial Pt/C catalyst. The pyridinic and graphitic nitrogen can be largely responsible for the enhancement of the ORR activity, which may be the catalytically active centers for the ORR. Our results provide a new idea for the design of renewable biocarbon materials from water plants, helping to construct ORR-active electrocatalysts.
This study was financially supported by the Basic and Frontier Research Program of Chongqing Municipality (cstc2015jcyjA50032, cstc2014jcyjA50038), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501118), and the Talent Introduction Project of Chongqing University of Arts and Sciences (R2014CJ02). We gratefully thank Dr. Qingshan Wu for the helpful discussions.
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