Local Plant-Derived Carbon Sheets as Sustainable Catalysts for

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Local Plant-Derived Carbon Sheets as Sustainable Catalysts for Efficient Oxygen Reduction Reaction Guosheng Han, Yanyan Liu, Jie Gao, Lei Han, Huaqiang Cao, Xiangyu Wang, Xianli Wu, and Baojun Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04705 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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ACS Sustainable Chemistry & Engineering

Local Plant-Derived Carbon Sheets as Sustainable Catalysts for Efficient Oxygen Reduction Reaction Guosheng Han,† Yanyan Liu,†,‡ Jie Gao,§ Lei Han,† Huaqiang Cao,¥ Xiangyu Wang,*,† Xianli Wu,*,† and Baojun Li*,†,¥ †

College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Science Road, Zhengzhou

450001, P. R. China ‡

National Engineering Laboratory for Biomass Chemical Utilization, Institute of Chemical Industry of

Forest Products, CAF, 16 Suojinwucun, Nanjing 210042, P. R. China §

Integrated Analytical Laboratories, 273 Franklin Road, Randolph, New Jersey 07869, United States

¥

Department of Chemistry, Tsinghua University, 1 Tsinghua Park, Beijing 100084, P. R. China

*

Corresponding Author. E-mail: [email protected] (X.Y. Wang), [email protected] (X.L. Wu), and [email protected] (B.J. Li)

Abstract: Catalysts for oxygen reduction reaction as crucial factor occupy a key position for the practical application of oxygen-based energy systems. Developing robust materials with high catalytic performances remains a great and serious challenge for researchers. Herein, a novel type of porous nitrogen-containing carbon sheets transformed from a conveniently accessible and available plant biomass, Typha orientalis, are reported. The preparation involves H2SO4-assisted hydrothermal treatment in dilute acid medium, calcination, chemical activation and heteroatom-doping with melamine precursor. The final carbon has high surface area up to 1726 m2·g–1, abundant micropores, appropriate pore volume and 2.98% of N content with abundant carbon defects. Higher catalytic ability toward reduction of oxygen and excellent tolerance to methanol poisoning compared to that of 20% Pt/C are obtained in alkaline media for the carbon nanosheets. It also shows good catalytic performances for acidic oxygen reduction. This simple approach provides a low-cost, readily scalable and convenient route for the fabrication of electro catalytic materials from primary biomass. Broad applications may be expected in the fields of storage and conversion systems of renewable energy. Keywords: Biomass-derived carbon; Low-cost; Nitrogen-containing; Oxygen reduction reaction; Porous carbon

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Introduction Oxygen-related electrochemistry plays a key role in the next-generation power devices for energy-cyclic technologies, including oxygen-based fuel cells, microbial battery and metal–air batteries. The oxygen reduction reaction (ORR) as a critical step occurs at the cathode in discharging process of advanced energy output technologies.1-4 Effective electro catalysts are crucial for high efficient ORR, due to the sluggish reaction kinetics of ORR.5-8 So far, the high-performance platinum (Pt)-based catalytic materials have been adopted as a benchmark catalysts to promote the kinetic of ORR by facilitating the conversion from O2 into H2O via a four-electron pathway.9-10 However, the large-scale commercial application of Pt-based catalysts is severely discouraged by their very high cost, low natural abundance, low poisoning tolerance and poor stability.10,11 It still is of great significance to explore new substitutes

for

current

Pt-based

catalysts

toward

ORR.

The

emergence

of

heteroatom-containing porous carbon materials as effective electro catalysts has given a new choice for alternative of Pt/C, because of their high catalytic activity for ORR, large specific surface areas (SSA), good electrical conductivities, and high chemical stability.12-17 Most of those carbon materials suffer from either harmful and uncommon precursors or time-consuming complex synthetic processes, thus hindering their upscale employment.18,19 Carbon nanotubes (CNTs) and graphene are good choice for preparing heteroatom-containing porous carbon materials because of their electrochemical properties. Despite the promising catalytic performance can be obtained with those composite electrodes based on CNTs or graphene. Their widely commercial application still is presently limited by their complex preparation technique and high costs.20,21 Therefore, extreme abundant biomass resource have 2


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been capturing increasing attention as fully sustainable alternative to fossil resources.22-23 Every year, the biomass as waste generated globally from agricultural and forest products is more than 140 billion metric tons.24 It has been gradually recognized that renewable waste biomass can be used as inexpensive source of high-value bulk chemicals and biomaterials.25 Hence, the effective production of novel materials from waste makes increasing sense if one considers the significant benefits related to waste elimination. In recent years, heteroatom-containing carbon materials derived from plant biomass have shown significant potential as catalysts toward clean energy conversion and storage.9,26-29 In their process, the carbon sources are cheap or free of charge. But it is difficult to largely collect most of them and there is little environmental benefit. Meanwhile, in previous researches, plant biomass-derived catalysts have shown relatively low surface area, small pore volume and relatively small limit current.26,30-34 The electro catalytic performances of carbon materials would be substantially improved through delicate controls or modification over their surface area, pore structure and surface properties.35,36 Therefore, it is still a seriously challenging but highly anticipated topic to rationally design catalysts derived from biomass with as large surface area, rich micropores and abundant catalytic active sites as possible. Besides environment and economy benefits from reuse of massive discarded biomass, many natural biomaterials are composed of unique and periodic structures.37 The hetero elements, such as S, N, P, etc. as the original ingredients of biomass, will be involved in the resulting carbon materials. The hetero elements contribute greatly to the high electrochemical active surface area, electron transport and surface wettability.38 Because the contents of heteroatom in biomass are often not enough for ideal catalytic activity, some artificial doping processes are required for effective active engineering in carbon-based catalytic materials. 3



Therefore, special interest should be casted onto the selection of activation agents to appropriately promote SSA, pore volume and hetero atom contents of carbon material derived from biomass. The famous wetland plant, Typha orientalis, also named as cattail, is concluding in the genus Typha. They are widely cultivated in the wider western Pacific (China, Philippines, Malaysia, Indonesia), Australia, and New Zealand. Cattail flocculus (CF), seeds of Typha orientalis, is commonly used as pillow interior or treated as waste, and are usually available free of charge.39,40 A solid foundation should be established for the development of low-cost catalysts. Biomass based carbon materials are usually obtained through pyrolysis with the aid of chemical or physical additives for effective activation. These strategies can introduce abundant pore and defective structures and enlarge the active surface area eventually.27 In this article, a low-cost and uncomplicated approach suitable for enlarging production is reported for the fabrication of porous N-containing carbon nanosheets from Typha orientalis. The synthesis process includes a hydrothermal process assisted by H2SO4, and then annealing with KHCO3 and N-doping with melamine in turn. This derived from CF material possesses high surface area, abundant micropores, 2.98% N-atom contents, and plentiful carbon defects. The raw materials are easily available and low cost, meanwhile the operation is ordinary. This carbon material shows high electro catalytic activity, ideal tolerance ability to methanol poisoning effect and highly robust stability in alkaline medium for ORR. The catalyst also exhibits good activity toward acidic ORR.

Experimental section Preparation of materials. Redistilled water (RW, 18 mL), H2SO4 (98%, 2 mL) and cattail 4


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flocculus (3.0 g) were loaded into a stainless-steel autoclave (Teflon-lined, 50 mL), and heated at 180 ºC for 18 h. After vacuum filtration, washing with RW, followed by drying at 90 ºC for 8 h, a solid was obtained. Sample (2.0 g) gained above and KHCO3 (8.0 g) were grinded and mixed, then the mixture was calcined at 900 ºC for 2 h under a N2 atmosphere at a heating rate of 6 ºC·min−1. The solid was soaked in aqueous HCl (1:1, 125 mL) under stirring for 6 h, followed by vacuum filtration, washed with RW. The filtered product was dried at 80 ºC for 12 h and a powder (0.500 g) was got and noted as CC. CC (0.251 g) was immersed in HNO3 (70%, 50 mL), and kept at room temperature for 40 min. A black powder was collected by filter and RW-washing, followed by drying at 100 ºC for 12 h. The product (0.205 g) was noted as ACC. Then a mixture of ACC (0.10 g) and melamine (4.00 g) went through a hydrothermal treatment at 100 ºC for 320 min in an autoclave. The sample, having been treated by hydrothermal method, was calcined at 400 ºC for 2 h, 550 ºC for 3 h and 650 ºC for 2 h in a N2 atmosphere at a heating rate of 5 ºC·min−1. A final product (0.213g) was collected and noted as NACC (N-containing ACC). As a contrast, water-NACC and NFBC were synthesized. For water-NACC, there is no H2SO4 addition at the first step of hydrothermal treatment. For NFBC, the biomass was pyrolyzed without hydrothermal treatment. Material characterization. The material morphologies were studied by using transmission electron microscope (TEM, FEI Tecnai G2 F20 S-TWIN, operating at 200 kV), combined with energy dispersive X-ray spectroscopy (EDS). The atom force micro analysis was conducted on a MFP-3D Infinity Asylum Research AFM (AFM, AIST-NT Instruments, USA). The phase structures of corresponding precursors and products were characterized with wide angle X diffraction (WAXD) and small angle X diffraction (SAXD) (XRD, Bruker D8 5



advance with Cu Kα, λ = 1.5418 Å). Raman spectra were recorded on a HR Evoltion Raman Spectrometer (Horiba Scientific, France) with excitation by the 514 nm line from an Ar-ion laser (power of 5 Mw). X-ray photoelectron spectra (XPS) were recorded on a spectrometer (PHI quantera SXM) with an Al Kα = 1486.6 eV excitation source. The sample charge effect was reduced through calibrating all binding energies by referencing the C1s peak (284.8 eV). On surface area and pore size analyzer (ASAP2420-4MP, Micromeritics, USA), The N2 sorption isotherms were measured at 77 K. The specific SSA (SBET) of NACC was calculated by the multi-point Brunauer−Emmett−Teller (BET) method from the adsorption branch of isotherm curves in a P/P0 range of 0.05-0.35. The pore size/width distribution was evaluated by the non-localized density function theory (NLDFT). The total pore volume (Vtotal) was determined from the amount adsorbed at the relative pressure of about 0.999. Elemental analyzer (EA) was conducted on Flash EA 1112 (Thermo scientific, USA). Electrochemical measurements. All electrochemical measurements were performed with a three-electrode configuration, using a counter electrode (Pt wire) and a reference electrode (Ag/AgCl) on a CHI760E electrochemical workstation.41 The rotating ring-disk electrode (RRDE) of Pine electrochemical system consisted of a glassy carbon (GC) disk surrounded by a Pt ring (Pine Electrode Model: AFE7R9GCPT). The geometric area of the used rotating disk electrode (RDE) is 0.196 cm2 (Pine Electrode Model: AFE5T050GC, 5 mm OD disk). To prepare the working electrode, active materials (4 mg) were dispersed in ethanol (720 μL) and Nafion (80 μL) with ultrasonication for 40 min. Subsequently, the dispersion solution (10 μL) was added onto a glassy carbon of RRDE or RDE. The mass loading density was 0.255 mg·cm–2. KOH solution (0.1 M) and HClO4 solution (0.1 M) prepared with RW were used as the electrolyte. 6


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All potential values provided in the following text were converted to potential versus RHE according to the following equation (1): Evs.

RHE

 Evs.

Ag / AgCl

 E  vs.

Ag / AgCl

 0.059 pH

(1)

All catalysts were electrochemically cleaned by sweeping the potential in the range from 0.00 to 1.20 V (vs. RHE) at 50 mV·s–1 in an N2 or O2-saturated electrolyte until steady state cyclic voltammograms (CVs) were obtained. Linear sweep voltammetry (LSV) curves were measured in an O2-saturated electrode rotated at 10 mV·s–1 at 200, 400, 800, 1200, 1600, 2000, 2400 and 2800 rpm, respectively. The chronoamperometry (CP) were performed at 0.74 V for 22 h in O2-saterated KOH solution on a RDE at 200 rpm for NACC and at 0.92 V for 22 h for 20% Pt/C catalyst under the same conditions. The CP response NACC at 0.60 V and 20% Pt/C at 0.91 V were recorded with or without CH3OH at 800 rpm, 4 mL CH3OH for NACC and 0.5 mL CH3OH for 20% Pt/C being introduced into 100 mL of KOH solution at 400th second. Concentration of CH3OH is about 1 M and 0.125 M for NACC and 20% Pt/C, respectively. The electrolytic solution was bubbled over 30 min with either N2 or O2 at a flow rate of 40 sccm before and during measurement. To get the current density, the original Faradaic current was normalized by the electrode geometric area. The RRDE technology was conducted at 1600 rpm in KOH and the potential of ring is kept at 1.27 V (vs. RHE). The %H2O2 yield and electron transfer number (n) were calculated using the following equations (2) and (3):41

% H 2O2  200

n  4

IR / N I D  (I R / N )

(2)

ID ID  IR / N

(3)

Where IR is Faradaic current at ring electrode, and N (=0.41) is collection efficiency of ring 7



electrode, ID is Faradaic current at disk electrode. The value of N was obtained by testing K3[Fe(CN)6] on CHI760E electrochemical workstation.

Results and discussion

Figure 1. The synthesis processes of NACC.

The synthetic routes for NACC are shown in Figure 1. NACC is prepared by hydrothermal carbonization (HTC) of cattail flocculus in 18% H2SO4, then pyrolysis with KHCO3 and then N-doping with melamine. HTC converts original biomass into carbonated polymer through consequent dehydration, aromatization, condensation, and partial polymerization steps.27 HTC is usually conducted at relative moderate temperatures, below 300 °C, under self-generated pressures of water media.42 HTC is considered as not only a synthetic method, but also a highly effective and controllable approach for pore size, structure, and surface chemistry.21,42 In this research, HTC is carried out in 18% of H2SO4 at 180 °C to get better morphology and better catalytic performances. Generally, direct HTC-derived carbons display poor electron conductivity and low SSA (usually

Local Plant-Derived Carbon Sheets as Sustainable Catalysts for

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