Bacterial cellulose derived reduction electrocatalyst for zinc-air battery

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RAPID COMMUNICATION

Bacterial cellulose derived nitrogen-doped

carbon nano?ber aerogel:An ef?cient

metal-free oxygen reduction electrocatalyst

for zinc-air battery

Hai-Wei Liang1,Zhen-Yu Wu1,Li-Feng Chen,

Chao Li,Shu-Hong Yu n

Division of Nanomaterials and Chemistry,Hefei National Laboratory for Physical Sciences at Microscale,

Collaborative Innovation Center of Suzhou Nano Science and Technology,Department of Chemistry,

University of Science and Technology of China,Hefei,Anhui230026,PR China

Received14October2014;received in revised form3November2014;accepted4November2014

Available online13November2014

KEYWORDS

Carbon nano?ber

aerogels;

Nitrogen-doped;

Electrocatalyst;

Zinc-air battery;

Oxygen reduction;

Bacterial cellulose

Abstract

The prohibitive cost and scarcity of the platinum-based eletrocatalysts for oxygen reduction

reaction(ORR)in fuel cells and metal-air batteries hamper dramatically the commercialization

of theses clean-energy technologies.Here,we develop a highly active nitrogen-doped carbon

nano?ber(N-CNF)aerogel metal-free ORR electrocatalyst,prepared by direct pyrolysis of a

cheap,green,and mass-producible biomass,i.e.,bacterial cellulose,followed by NH3

activation.The N-CNF aerogel inherits the three-dimensional nano?brous network of bacterial

cellulose and meanwhile possess high density of N-containing active sites(5.8at%)and high BET

surface area(916m2/g).Such N-CNF aerogel shows superior ORR activity(half-wave potential

of0.80V versus reversible hydrogen electrode)and high selectivity(electron-transfer number

of3.97at0.8V),and excellent electrochemical stability(only20mV negative shift of half-

wave potential after10,000potential cycles)in alkaline media.The ORR activity of N-CNF

aerogel exceeds that of NH3-treated carbon blacks,carbon nanotubes as well as reduced

graphene oxide aerogels,and that of most reported metal-free catalysts.Importantly,when

used as a cathode catalyst for constructing the air electrode of Zn-air battery,the N-CNF

aerogel exhibits high voltages of1.34and1.25V at the discharge current densities of1.0and

10mA cmà2,respectively,which are highly comparable with the state-of-the art Pt/C catalyst

https://www.wendangku.net/doc/8c7377742.html,/10.1016/j.nanoen.2014.11.008

2211-2855/&2014Elsevier Ltd.All rights

reserved.

n Corresponding author.

E-mail address:shyu@https://www.wendangku.net/doc/8c7377742.html,(S.-H.Yu).

1H.W.Liang and Z.Y.Wu have contributed equally to this work.

Nano Energy(2015)11,366–376

(20wt%Pt,BASF),indicating the great potential of this metal-free catalyst as a promising alternative to the Pt/C for alkaline fuel cells and metal-air batteries.

&2014Elsevier Ltd.All rights reserved.

Introduction

Electrochemical reduction of oxygen is an important process for many energy conversion and storage technologies, including fuel cells,metal-air batteries,and electrolyzers. Due to the high overpotential caused by the sluggish nature of oxygen reduction reaction(ORR),development of ef?-cient ORR electrocatalysts is crucial for practical applica-tions of these electrochemical devices.Although Pt and Pt-based alloys,up to now,are known as the most ef?cient catalysts for ORR,the high costs and scarce reserves of Pt signi?cantly hinder its large-scale applications[1,2].Addi-tional problem associated with Pt is its poor durability during long-term electrochemical process,as Pt-based catalysts suffer from nanoparticle migration,coalescence, and even detaching from support materials in both acidic and alkaline electrolytes[3–5].

Accordingly,substantial efforts have been dedicated to searching for alternative ORR catalysts with low cost,high activity,and long-term durability[6–14].In particular,recent experimental observations and theoretical calculations both revealed that heteroatoms(e.g.,nitrogen or/and phos-phorus,boron)-doped carbon materials could serve as ef?-cient metal-free electrocatalysts for ORR as the result of their unique electronic properties,which are derived from the heteroatom-induced charge transfer and delocalization [15–20].In order to achieve a high ORR performance,it is essential to fabricate carbon-based materials with rationally nanostructural design that would offer a desirable combina-tion of high internal reactive surface area and straightfor-ward transport path leading to such a surface.In this respect, heteroatoms-doped carbon nanotubes[15,21],mesoporous carbon[16–18],and grapheme[22–26]have been developed, some of which exhibited comparable ORR performance with that of commercial Pt/C catalysts in alkaline media[15,17]. Of particular interest,Dai et al.reported vertically aligned nitrogen-doped carbon nanotubes(VA-NCNT s)as ORR elec-trocatalysts[15].Besides the electronic effect induced by nitrogen doping,the authors believed that the well-de?ned geometric features of aligned CNT s provide additional struc-tural bene?ts in achieving high electrocatalytic performance [15].Although certainly promising,considering the complex fabrication methods,it remains a question to what extent these materials will affect a real cost reduction when compared to Pt-based catalysts.An inexpensive,green and scalable process is quite desirable to lower the fabrication cost of carbon-based metal-free catalysts for industrial applications.

Herein,we report a new type of metal-free ORR catalysts based on nitrogen-doped carbon nano?ber(N-CNF)aero-gels,which are fabricated from bacterial cellulose(BC),a cheap and green biomass that has long been known as the raw material of an indigenous dessert food(nata-de-coco) of the South-East Asia.Our recent studies revealed that BC was an excellent precursor for producing highly conductive carbon nano?ber(CNF)aerogels by direct pyrolysis of lyophilized BC under an inert atmosphere[27–29],and heteroatom-doped CNFs for energy storage and conversion [30–32].In the present study,we demonstrate that the BC-derived CNF aerogels,following nitrogen-doping by anneal-ing them in NH3,can act as ef?cient electrocatalysts for ORR.The as-prepared N-CNF aerogels possess a high level of nitrogen doping(N content,5.8at%)and high Brunauer–Emmett–Teller surface area(S BET up to916m2/g).It is believed that the three-dimensional(3D)nano?brous struc-tures of N-CNF aerogels would be favorable for easy molecular/ionic diffusion throughout the highly porous architecture to the reactive surface and facilitate the rapid transport of electrons along the interconnected carbon ?brous networks during electrocatalysis.As a consequence, the ORR activity of the N-CNF aerogels is much higher than that of NH3-treated carbon blacks,carbon nanotubes,and reduced graphene oxide aerogels.More importantly,as a cathode electrocatalyst for the Zn-air battery,the N-CNF aerogel exhibits highly comparable performance with the state-of-the-art Pt/C catalyst(20wt%,BASF). Experimental section

Synthesis of N-CNF aerogels electrocatalysts

Puri?ed BC pellicles with?ber content of$1%(vol/vol) were kindly provided by Ms CY Zhong(Hainan Yeguo Foods Co.,Ltd.,Hainan,China).They were produced in an industry-scale by the bacterial strain Acetobacter xylinum using a culture of coconut milk and sucrose[33].The wet BC pellicles were?rst cut into rectangular shapes with a sharp blade,frozen in liquid nitrogen(–1961C)and then freeze-dried in a bulk tray dryer(Labconco Corporation,Kansas City,MO,USA)at a sublimating temperature of–501C and a pressure of0.04mbar.The dried BC aerogels were then pyrolyzed under?owing N2at8001C to generate black CNF aerogels.Finally,N-CNF aerogel catalysts were obtained by a second heat-treatment of CNF aerogels under an NH3 atmosphere at700–9001C.NH3heat-treatment was per-formed by placing CNF aerogels in a quartz tube under ?owing NH3with a heating ramp rate of51C/min to desired temperatures and the temperature was kept for1h.The heating and cooling steps were performed in a N2atmo-sphere.For comparison,four reference carbon materials, including Vulcan XC-72R(S BET,ca.240m2/g),Ketjenblack EC-300J(S BET,ca.800m2/g),CNTs,and reduced graphene oxide(RGO)aerogels,were also doped with nitrogen by the same NH3treatment process.RGO aerogels were prepared by hydrothermal treatment of GO solution(2mg/mL)at 1801C for2h[34].

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Metal-free oxygen reduction electrocatalyst for zinc-air battery

Electrocatalytic activity measurements

All the electrochemical measurements were carried out in a conventional three-electrode cell using WaveDriver20bipo-tentiostat(Pine Instrument Company,USA)controlled at room temperature.Ag/AgCl(4M KCl)and platinum wire were used as reference and counter electrodes,respectively.All poten-tials in this study refer to that of reversible hydrogen electrode(RHE).The potential difference between Ag/AgCl and RHE is0.96570.002V,based on the calibration measure-ment in H2saturated0.1M KOH with polished Pt wires as the working and counter electrodes,and the Ag/AgCl as the reference electrode.A RDE with a glassy carbon disk(5.0mm diameter)and a RRDE electrode with a Pt ring(6.25mm inner-diameter and7.92mm outer-diameter)and a glassy carbon disk(5.61mm diameter)served as the substrate for the working electrode for evaluating the ORR activity and selectivity of various catalysts,respectively.Prior to use,the glassy carbon electrodes in RDE/RRDE were polished using aqueous alumina suspensions on felt polishing pads.

The catalyst ink was prepared by blending N-doped carbon materials(10mg)with100m L Na?on solution(0.5wt%)and 3.9mL ethanol in an ultrasonic bath.A certain volume of catalyst ink was then pipetted onto the glassy carbon surface to result in a desirable catalyst loading.The catalyst loading are0.1and0.4mg/cm2for CV and RDE/RRDE tests,respec-tively.RDE/RRDE tests were measured in O2-saturated0.1M KOH at1600rpm with a sweep rate of10mV/s.In order to estimate the double layer capacitance,the electrolyte was deaerated by bubbling with argon,and then the voltammo-gram was evaluated again in the deaerated electrolyte.The oxygen reduction current was taken as the difference between currents measured in the deaerated and oxygen-saturated electrolytes.For detecting peroxide species formed at the disc electrode,the potential of the Pt ring electrode in the RRDE system was set to1.4V.

A state-of-the-art Pt/C catalyst(20wt%platinum on Vulcan carbon black,BASF)was measured for comparison. The working electrode was prepared as follows:5mg Pt/C and50m L Na?on solution(0.5wt%)was dispersed in970m L of ethanol by sonication for1h to obtain a well-dispersed ink. 4μL of catalyst ink was then pipetted onto the GC surface, leading to a Pt loading of20μg/cm2.Other experimental conditions were the same as for the metal-free catalysts.

The four-electron selectivity of catalysts was evaluated by using the RRDE techniques based on the H2O2yield, calculated from the following Eq.(1):

H2O2e%T?200?

I R=N

eI R=NTtI D

e1T

The electron transfer number can be calculated from the following Eq.(2):

n?4?

I D

eI R=NTtI D

e2T

Here,I D and I R are the disk and ring currents,respectively, and N=0.37is the ring collection ef?ciency.

Also,the number of electrons involved per O2in the ORR on N-CNF aerogel was further determined by the Koutecky–Levich equation:

1=j?1=j kt1=Bω1=2where j k is the kinetic current andωis the electrode rotating rate.B is determined from the slope of the K–L plots based on the Levich equation:

B?0:2nF D0

eT2=3ωà1=6C0

where n represents the overall number of electrons gained per O2,F is the Faraday constant(F=96,485C molà1),D0is the diffusion coef?cient of O2in0.1M KOH electrolyte (1.9?10à5cm2sà1),C0is the bulk concentration of O2 (1.2?10à6mol cmà3),ωis the kinetic viscosity of the electrolyte(0.01cm2sà1).

Zn-air batteries were tested in home-built electrochemi-cal cells,where catalysts loaded on the gas diffusion layer (Te?on-coated carbon?ber paper,1.0cm2,catalyst loading 1.0mg for all materials)as the air cathode and Zn foil as anode in6.0M KOH.Measurements were carried out on the as-fabricated cell at room temperature with a CHI760E(CH Instrument,USA)electrochemical workstation.

Characterization

Scanning electron microscopy(SEM)was performed with a ?eld emission scanning electron microanalyzer(Zeiss Supra 40)after coating the samples with Au layers using a Sputter Coater(Model BAL-TEC SCD005).Transmission electron microscopy(TEM)and energy-?ltered TEM(EFTEM)mapping were performed using JEM-ARM200F operating at an accelerating voltage of200kV.Elemental mapping were collected using a Gatan GIF Quantum965.X-ray photo-electron spectra(XPS)were determined by an X-ray photo-electron spectrometer(ESCALab MKII)with an excitation source of MgK a radiation(1253.6eV).N2sorption analysis was conducted on a TriStar3020accelerated surface area and porosimetry instrument,equipped with automated sur-face area,at77K using Barrett–Emmett–T eller(BET)calcula-tions for the surface area.Raman scattering spectra were recorded with a Renishaw System2000spectrometer using the514.5nm line of Ar+for excitation.Inductively coupled plasma atomic emission spectrometry(ICP-AES)measure-ments were conducted using an Atomscan Advantage Spec-trometer(Thermo Ash Jarrell Corporation,USA).

Results and discussion

Large quantity of wet BC pellicles with controllable thick-ness(3–20mm)were produced by an industry-scaled micro-bial fermentation process(step1in Figure1a;also see Supporting Information Figure S1).For preparing N-CNFs aerogels,the puri?ed BC pellicles were?rst cut into small pieces and lyophilized to form dry BC aerogels(step2in Figure1a).The dried BC aerogels were then subjected to pyrolysis under a N2atmosphere at8001C for2h to afford the CNF aerogels.Finally,N-CNF aerogel catalysts were obtained by a second heat-treatment of CNF aerogels under an NH3atmosphere(step3in Figure1a).For comparison, four other carbon materials,including Vulcan XC-72R(S BET, $240m2/g),Ketjenblack EC-300J(S BET,$800m2/g), CNTs,and reduced graphene oxide(RGO)aerogels,were also treated by the same NH3annealing process.

Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)observations indicated that the

H.-W.Liang et al.

368

mechanically robust nano ?brous network structures of original BC precursors maintained well even after high-temperature pyrolysis and NH 3annealing (Figure 1b and c),although both of the macroscopic dimensions of BC-derived aerogels (photos in Figure 1a)and the microscopic diameter of nano ?bers (Figure 1b,c and Figure S2)gradually shrank to about half of their original sizes after twice heat-treatment.High resolution TEM image revealed that the N-CNFs composed of randomly orientated graphene layers accompanied with amorphous and defective carbon struc-tures (Figure 1d).Based on the Raman spectroscopy ana-lyses (Figure S3),it was revealed that the D and G bands are positioned at 1345cm à1and 1595cm à1,respectively,and the intensity ratio of D to G band was about 0.92and 0.97for CNF aerogels and N-CNF aerogels,respectively.The increase of D/G ratio further con ?rmed that the defective structure of N-CNF aerogels was more pronounced than that of CNF aerogels due to the N-doping induced by NH 3treatment.The resulting doped-N defects at the graphene edges and in planes may serve as active sites for electro-catalysis of oxygen reduction.

The textural properties of BC-derived aerogels were investigated by N 2sorption measurements and the results were summarized in Table https://www.wendangku.net/doc/8c7377742.html,pared to pristine BC aerogels,the CNF aerogels showed much improved BET surface area (from 83to 374m 2/g)and pore volume (from 0.19to 0.38cm 3/g),due to the micropores evolution caused by the evaporation of volatile species (such as CO,CO 2,methanol,and acetic acid)during pyrolysis of BC [27,35].NH 3treatment further increased the porosity signi ?cantly for N-CNF aerogels (S BET ,916m 2/g;pore volume,0.71cm 3/g).X-ray photoelectron spectroscopy (XPS)measurements were performed to analyze the content and chemical state of nitrogen atoms in CNF aerogels before and after NH 3treat-ment (Figure 2a,b).A week N1s signal was observed for the CNF aerogels (Figure S4)even before NH 3treatment.This small amount of “intrinsic ”nitrogen atoms (ca. 1.0at%)come from the residual nitrogen-containing compounds left by culture media and secretions [36].After NH 3treatment,about 5.8at%of N was detected for N-CNF aerogel.The high resolution N1s spectrum of N-CNFs aerogel could be ?tted well with three different signals having binding energies of 398.5,400.2,and 401.8eV ,corresponding to pyridinic N,pyrrolic N,and graphitic N,respectively (Figure 2b)[37–38].Elemental mapping with sub-nanoscale energy-?ltered TEM (EFTEM)imaging revealed that the N species were homo-geneously distributed throughout the individual nano ?ber as well as the whole nano ?brous network structure (Figure 2c and Figure S5).

Considering the possible trace amount of metal species in biomass precursors and its in ?uence on the electrocatalysis,[39]we used the inductively coupled plasma atomic emis-sion spectrometry (ICP-AES)technique to detect Fe and Co species for the N-CNFs aerogel.ICP-AES analyses suggested that the total metal content of N-CNFs aerogel is below 0.01wt%and much lower than the reported values of active metal-based catalysts with metal contents at least of 0.1–0.2wt%[8,40,41].Such results,therefore,exclude the possible contribution of metal species to the ORR perfor-mance of N-CNF aerogel.

The much improved porosity and nitrogen content for N-CNF aerogels can be explained well by the reactions between carbon and NH 3.At an elevated temperature,the radicals generated by the decomposition of NH 3could

etch

Figure 1Synthesis and morphologies of N-CNF aerogels.(a)Schematic diagram of the synthetic steps.(1)A large-sized BC pellicle (200?230?5mm 3,water content $99vol%)was produced by an industry-scaled microbial fermentation.(2)Photograph,scanning electron microscopy (SEM)image and schematic illustration of BC aerogels after cutting and freeze-drying of wet BC pellicles.(3)Black N-CNF aerogels were obtained ?nally by heat treatments of BC aerogels two times under a N 2and NH 3atmosphere,respectively.The ?brous networks survived after pyrolysis and various types of nitrogen were doped into carbon matrix successfully after NH 3treatment.(b,c)SEM and TEM images of N-CNF aerogels,respectively,showing the nano ?brous network structure.(d)HRTEM image of an individual N-CNF .

369

Metal-free oxygen reduction electrocatalyst for zinc-air battery

out carbon fragments,leaving behind the new generated micropores hosted with nitrogen-containing active sites [42–44].The replacement reaction of oxygen-bearing spe-cies on carbon surface with NH3also resulted in additional nitrogen-containing groups[41,45].Previous study has demonstrated that the nature of the pristine carbon mate-rials has a great in?uence on the capacity for capturing nitrogen during NH3treatment[44,46].For comparison, four other commercial and synthesized carbon materials, i.e.,Vulcan carbon,Ketjenblack carbon,CNTs,and RGO aerogel were also treated with NH3under the same condi-tions.RGO could capture only1.9at%N after NH3treat-ment,while the N contents of other three carbons were too low(o0.5at%)to be detected successfully by XPS(Table1

) Figure2Elemental composition of N-CNF aerogels.(a)XPS survey spectra of CNF and N-CNF aerogels.(b)High-resolution N1s spectrum of N-CNFs aerogels.(c)EFTEM elemental mapping of an individual N-doped CNF

.

H.-W.Liang et al. 370

Figure 3ORR performance of N-CNF aerogels and reference catalysts.(a)CV curves of N-CNF aerogels and Pt/C catalyst in O 2-saturated (solid line)and Ar-saturated 0.1M KOH (dash line).Catalyst loading was 0.1mg cm à2for both of the two catalysts.(b)ORR polarization plots of CNF aerogel,N-CNF aerogel,and Pt/C catalyst.(c)ORR polarization plots of N-CNF aerogel and other NH 3-treated carbon materials.(d)H 2O 2yield plots of N-CNF aerogel and reference catalysts.(e)ORR polarization plots for N-CNF aerogel in O 2-saturated 0.1M KOH with different speeds.(f)The corresponding Koutecky –Levich (K –L)plots at different potentials.For RDE and RRDE testing,the catalyst loading is 0.4mg cm à2for non-Pt catalysts and 20m g Pt cm à2for Pt/C catalyst.Electrode rotation speed,1600rpm;scan rate,10mV/s.

371

Metal-free oxygen reduction electrocatalyst for zinc-air battery

[47].Compared to the four reference carbon materials,the strong nitrogen-?xing ability of CNF aerogel was believed to be due to the numerous defective sites and oxygen-containing groups on surface of CNFs,as indicated by the Raman (Figure S3)and XPS (Figure 2a)analyses.Additionally ,the unique 3D nano ?brous network structure of CNF aerogel was more easily accessible to gaseous NH 3molecules than other carbons,[43]resulting in more ef ?cient activation processes with a homo-geneous distribution of functionalized sites.

Cyclic voltammetry (CV)and rotating disk electrode (RDE)measurements were performed in 0.1M KOH at room temperature to assess the ORR activity of N-CNF aerogels.For comparison,four other N-doped carbon materials and a state-of-the-art Pt/C (20wt%Pt on Vulcan carbon black,BASF)catalyst were also tested under the same conditions.We ?rst examined the ORR activity of the N-CNF aerogels as a function of NH 3annealing temperature in the range of 700–9001C.It was found that the N-CNF aerogels obtained at 8001C showed the highest activity,as revealed by the onset and half-wave potentials in the ORR polarization plots (Figure S6),probably because a balance of surface area,

nitrogen content and type could be optimized at this temperature [23,43].Therefore,the N-CNF aerogels dis-cussed in the text were all produced at 8001C unless otherwise speci ?ed.

CV measurements in O 2statured electrolyte shown that the N-CNF aerogel afforded a peak potential of 0.85V versus reversible hydrogen electrode (RHE),which was comparable that of Pt/C catalyst (0.85V)and much higher than those of four other N-doped carbon materials (0.78–0.64V ,Figure 3a and Figure S7).The high ORR activity of N-CNF aerogel was then further con ?rmed by the RDE measurements.The pristine CNF aerogel could act as oxygen reduction catalyst at high electrode overpotential (Figure 3b),probably due to the defective structure and the “intrinsic ”nitrogen doping.After NH 3-treatment,the obtained N-CNF aerogel exhibited much enhanced catalytic activity with an ORR half-wave potential of 0.8070.01V ,which was only 50mV more negative than that of Pt/C catalyst (Figure 3b and T able 1)and higher than most of the values previously reported for metal-free catalysts (T able S2),indicating the very high electrocatalytic activity of N-CNF aerogel toward ORR in alkaline

media.

Figure 4Electrochemical durability and methanol tolerance of N-CNFs aerogel and Pt/C catalyst.(a,b)ORR polarization plots of N-CNF aerogel and Pt/C before and after 10,000potential cycles in oxygen-saturated 0.1M KOH,respectively.(c)Negative shift of the half-wave potential (E 1/2)of N-CNF aerogel and Pt/C catalyst with the number of potential cycles under O 2.(d)ORR polarization plots of N-CNF aerogel in 0.1M KOH with (black curve)and without (red curve)0.1M methanol.

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372

The four reference carbon materials,though obtained by the same NH3-activation process,showed much lower ORR activity than N-CNF aerogel in terms of half-wave potential and diffusion-limited current density(Figure3c).Compared with the reference carbon materials,the better electro-catalytic activity of the N-CNF aerogel can be attributed to its strong nitrogen-?xing ability during NH3treatment and the3D nano?brous network structure.The highly porous N-doped CNFs with large surface area(S BET,918m2/g)can exposure numerous nitrogen-containing active sites to elec-trochemical interface,meanwhile the3D nano?brous net-work offers“highways”for the rapid transport of ORR species,such as O2,OHà,and electrons[48].Note that the nano?brous network structure of N-CNFs survived well on RDE even after catalyst ink processing(Figure S8).It is believed that such reasonable combination of high reactive surface area and straightforward transport path results in the superior ORR performance of N-CNF aerogel.

Four-electron reduction of oxygen is highly preferred for energy conversion applications and high yield of H2O2 may damage the stability of catalyst.The selectivity of the four-electron reduction of oxygen was studied using the rotating ring-disk electrode(RRDE)technique and the potential of the Pt ring electrode was set to 1.4V for detecting peroxide species formed at the disc electrode. The H2O2yield of N-CNF aerogel remained below10%at all potentials and dropped to1.5%at0.8V,corresponding to a high electron-transfer number of3.97,which was higher than the reference carbon materials and comparable with Pt/C catalyst(Figure3d and Figure S9).The outstanding ORR selectivity of N-CNF aerogel was further con?rmed by the Koutechy–Levich analyses(Figure3e,f).The linear K–L plots at different potentials suggest the?rst-order reaction kinetics toward the concentration of O2on N-CNF aerogel from0.3V to0.6V.The numbers of electrons transferred per O2molecule(n)were calculated from the slope of the K–L plots to be between4.08and4.1,thereby indicating that the ORR is dominated by a4e process and O2is reduced to OHà,which is consistent with the RRDE results.

To investigate the durability of the catalysts,N-CNF aerogel and Pt/C were cycled from0.6to1.0V at a scan rate of50mV/s in0.1M KOH solution saturated with O2

. Figure5Zn-air battery performance of N-CNF aerogels and Pt/C catalysts.(a,b)Typic galvanostatic discharge curves of Zn-air batteries with N-CNF aerogel and Pt/C as cathode catalysts at different current densities:(a)1mA cmà2and(b)10mA cmà2.

(c)Long-term galvanostatic discharge curves of Zn-air batteries until complete consumption of Zn anode.The speci?c capacity was normalized to the mass of consumed Zn.

373 Metal-free oxygen reduction electrocatalyst for zinc-air battery

After10,000continuous potential cycles,the half-wave potential of N-CNF aerogel decreased$20mV(Figure4a), while there was$40mV loss of half-wave potential for the commercial Pt/C catalyst under the same cycling conditions (Figure4b),suggesting the superior electrochemical stabi-lity of N-CNF aerogel in alkaline medium.The high selectiv-ity for the four-electron reduction of O2probably is an important contributing factor to high stability of the N-CNF aerogel.And it is well known that Pt/C catalysts generally suffered from nanoparticle migration,coalescence,and even detaching from carbon supports during the accelerated durability tests[49].In addition to the high activity and excellent durability,as expected as other reported metal-free catalysts[15,16],the N-CNF aerogel developed here also exhibited better methanol tolerance than Pt/C catalyst (Figure4d and Figure S10),making it very attractive as cathode catalysts for alkaline direct methanol fuel cells.

Furthermore,to demonstrate the application potential of N-CNF aerogel for the ORR that typically proceeds in acidic media,such as proton-exchange membrane fuel cells,we also investigated the electrocatalytic properties of the N-CNF aerogel for oxygen reduction in an acid electrolyte. As shown by the polarization curves for the ORR in O2-saturated0.5M H2SO4(Figure S11),N-CNF aerogel exhibited high catalytic activity toward ORR,with an onset potential at0.83V(vs RHE)and a current density of2.10mA/cmà2at 0.6V,which were higher than that of the fabricated reference carbon materials and some reported metal-free catalysts,such as mesoporous N-doped carbon made from ionic liquid[17],dopamine-derived N-doped carbon spheres [50],and NH3-treated commercial carbon black[51].More-over,the calculated electron-transfer number of N-CNF aerogel based on the RRDE measurements was3.65–3.76 and higher than all reference carbon materials(Figure S12). Although the activity of N-CNF aerogel was still inferior to the state-of-the-art Pt/C catalyst(Figure S11),it is believed that incorporating simple metals(e.g.Fe and Co)into BC precursors before pyrolysis and NH3treatment might offer further potentialities in improving ORR performance in acidic media.

Finally,a laboratory Zn-air battery was constructed with N-CNF aerogel loaded on the gas diffusion layer(T e?on-coated carbon?ber paper)as the air cathode and with Zn foil as anode,to demonstrate the practical application potential of our catalyst.The Zn/air battery made of Pt/C catalyst with the same catalyst loading was also tested for comparison.An open-circuit voltage of$1.50V was observed(Figure S13), consisting with preciously reported values of such cells[52]. Galvanostatic discharge measurements(Figure5a,b)revealed that the N-CNF aerogel exhibited very high voltages of1.34 and1.25V at the discharge current densities1.0and10mA cmà2,respectively.The Zn-air battery performance of N-CNF aerogel was highly comparable to that of Pt/C catalyst (voltages of1.35and1.26V at1.0and10mA cmà2,respec-tively)and reported non-precious metal ORR catalysts (T able S3),and much higher than reported metal-free poly (3,4-ethylenedioxythiophene)electrode[52].Normalized to the mass of consumed Zn during the long-term galvanostatic discharge process,the speci?c capacity of the battery made with N-CNF aerogel was estimated to be$615mAh gà1 (Figure5c),corresponding to gravimetric energy density of $760Wh kgà1at the discharge density of10mA cmà2,which is comparable to the battery made of Pt/C catalyst(speci?c capacity630mAh gà1,gravimetric energy density$790Wh kgà1).These results clearly reveal that our developed N-CNF aerogel catalyst is a highly competitive alternative to commercial Pt/C for practical application in metal-air batteries.

Conclusions

In summary,we have developed a new N-CNF aerogel electrocatalyst for ORR by using a cheap,green,nanostruc-tured biomass,BC,as precursor,which were produced by an industry-scaled microbial fermentation process.The as-prepared N-CNF aerogels possessed high density of N-containing active sites(N content,5.8at%)and high BET surface area(916m2/g).Importantly,the3D nano?brous network structure was highly favorable for the rapid trans-port of ORR species.Such reasonable combination of high reactive surface area and fast transport path resulted in the superior ORR activity(half-wave potential0.80V),high selectivity(electron-transfer number 3.97at0.8V),and excellent electrochemical stability of N-CNF aerogel in alka-line media.The ORR performance of N-CNF aerogel exhibited a highly comparable ORR performance with the state-of-the-art Pt/C catalyst,as revealed by both half-cell(RDE techni-que)and full-cell(Zn-air battery)measurements.These results suggest that the N-CNF aerogel made from BC is a technologically and economically promising replacement of Pt-based ORR catalysts for practical applications in alkaline fuel cells and metal-air batteries.Extended works based on the BC-derived carbons in future will include preparation of other metal-free and even nonprecious metal nano?brous electrocatalysts with enhanced ORR performance. Acknowledgment

This work is supported by the Ministry of Science and T echnology of China(Grants2014CB931800,2013CB933900), the National Natural Science Foundation of China(Grants 21431006,91022032,91227103),the Chinese Academy of Sciences(Grant KJZD-EW-M01-1),and Hainan Province Science and T echnology Department(CXY20130046)for?nancial support.We thank Ms C.Y.Zhong for kindly providing pure bacterial cellulose pellicles.

Appendix A.Supporting information Supplementary data associated with this article can be found in the online version at https://www.wendangku.net/doc/8c7377742.html,/10.1016/ j.nanoen.2014.11.008.

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.

Hai-Wei Liang received his Ph.D.in inor-

ganic chemistry under the supervision of

Prof.Shu-Hong Yu in the Department

of Chemistry,University of Science and

Technology of China.He is currently a

Postdoctoral research associate under Prof.

Dr.Klaus Müllen and Prof.Dr.Xinliang Feng

at the Max Planck Institute of Polymer

Research in Mainz,Germany.His current research topics include non-precious metal and metal-free electro-catalysts for fuel cell and metal-air battery

.

Zhen-Yu Wu received his Bachelor Degree

at Nanjing Agricultural University in applied

chemistry in2011.He is currently a Ph.D.

candidate in chemistry under the supervi-

sion of Prof.Shu-Hong Yu in the Department

of Chemistry,University of Science and

Technology of China(USTC).His current

research focuses on designing carbon-

based materials for energy storage and

conversion,and environmental

sciences.

Li-Feng Chen received his B.E.degree in

2006from Anhui University of Technology

and Ph.D.in chemistry under the super-

vision of Prof.Shu-Hong Yu at University of

Science and Technology of China(USTC)in

2013.Then,he worked as a Postdoctoral

Research Fellow with Prof.Shu-Hong Yu

at USTC.His current research interests

are carbon-based nanomaterials/nanostruc-

tures for ef?cient energy storage and con-

version devices.

375

Metal-free oxygen reduction electrocatalyst for zinc-air battery

Chao Li received his B.S.degree in chem-istry in2012from University of Science and Technology of China(USTC).He is currently a Ph.D.candidate in Chemistry at USTC under the supervision of Prof.Shu-Hong Yu.His research interests mainly focus on preparation of new nanocarbon materi-als for capacitive deionization and super-

capacitors.

Shu-Hong Yu received his Ph.D.in Inorg.

Chem.in1998from the University of

Science and Technology of China(USTC).

From1999to2001,he joined in Tokyo

Institute of Technology as a Postdoctoral

Research Fellow.From2001to2002,he

was as an Alexander von Humboldt Research

Fellow in the Max Planck Institute of

Colloids and Interfaces,Potsdam,Germany.

He was appointed as a full professor in2002and the Cheung Kong

Professorship in2006by the Ministry of Education in the Depart-

ment of Chemistry,USTC.His current research interests include bio-

inspired synthesis and self-assembly of new nanostructured materi-

als and nanocomposites,and their related properties.He has

authored and co-authored more than360refereed journal publica-

tions,and17invited book chapters.He serves as an editorial

advisory board member of journals Accounts of Chemical Research,

Chemical Science,Materials Horizons,Chemistry of Materials,Nano

Research,CrystEngComm,Part.Part.Syst.Charact.and Current

Nanoscience.His recent awards include Chem.Soc.Rev.Emerging

Investigator Award(2010)and Roy-Somiya Medal of the Interna-

tional Solvothermal and Hydrothermal Association(ISHA)(2010).

H.-W.Liang et al. 376

四种再生纤维的概述

四种再生纤维的概述及鉴定方式 再生纤维具有优良的吸湿性、穿着舒适性，是纺织服装业最理想、最有开 发潜力的纺织原料。 再生纤维概述： 1.Tencel纤维 Tencel纤维是以针叶树为主的木浆、水和溶剂氧化胺混合，加热至完全溶解，在溶解过程中不会产生任何衍生物和化学作用，经除杂而直接纺丝，其分子结构是简单的碳水化合物。Tencel纤维在泥土中能完全分解，对环境无污染;另外，生产中所使用的氧化胺溶剂对人体完全无害，几乎完全能回收，可反复使用，生产中原料浆粕所含的纤维素分子不起化学变化，无副产物，无废弃物排出厂外，是环保或绿色纤维。该纤维织物具有良好的吸湿性、舒适性、悬垂性和硬挺度且染色性好，加之又能与棉、毛、麻、腈、涤等混纺，可以环锭纺、气流纺、包芯纺，纺成各种棉型和毛型纱、包芯纱等。 2.Modal纤维 Modal纤维是一种全新的纤维素纤维，Modal纤维的原料来自于大自然的木材，使用后可以自然降解。由于这类纤维是采用天然纤维素为原料，具有生物将解性，并且在纤维生产过程中不产生类似粘胶县委的严重污染环境问题，是21世纪的新型环保纤维。Modal纤维价格是Tencel纤维的一半，系第二代再生纤维素纤维。Modal纤维可与多种纤维混纺、交织，发挥各自纤维的特点，达 到更佳的服用效果。Modal纤维面料吸湿性能、透气性能优于纯棉织物，其手 感柔软，悬垂性好，穿着舒适，色泽光亮，是一种天然的丝光面料。 3.大豆蛋白纤维 大豆蛋白纤维是以出油后的大豆废粕为原料，运用生物工程技术，将豆粕中的球蛋白提纯，并通过助剂、生物酶的作用，使提纯的球蛋白改变空间结构，再添加羟基和氨基等高聚物，配制成一定浓度的蛋白纺丝液，用湿法纺丝工艺纺成。豆粕是油脂车间的副产品，在我国资源十分吩咐，属废物综合利用，资源取之不尽，用之不竭。大豆蛋白纤维可称为新世纪的“绿色纤维”。由于大豆蛋白纤维外层基本上是蛋白质，与人体皮肤亲和性好，且含有多种人体所必须的氨基酸，具有良好的保健作用。在大豆蛋白纤维纺丝工艺中加入定量的有杀菌消炎作用的中草药与蛋白质侧链以化学键相结合，药效显著且持

再生纤维概述

再生纤维具有优良的吸湿性、穿着舒适性，是纺织服装业最理想、最有开发潜力的纺织原料。 再生纤维概述： 1.Tencel纤维 Tencel纤维是以针叶树为主的木浆、水和溶剂氧化胺混合，加热至完全溶解，在溶解过程中不会产生任何衍生物和化学作用，经除杂而直接纺丝，其分子结构是简单的碳水化合物。Tencel纤维在泥土中能完全分解，对环境无污染；另外，生产中所使用的氧化胺溶剂对人体完全无害，几乎完全能回收，可反复使用，生产中原料浆粕所含的纤维素分子不起化学变化，无副产物，无废弃物排出厂外，是环保或绿色纤维。该纤维织物具有良好的吸湿性、舒适性、悬垂性和硬挺度且染色性好，加之又能与棉、毛、麻、腈、涤等混纺，可以环锭纺、气流纺、包芯纺，纺成各种棉型和毛型纱、包芯纱等。 2.Modal纤维 Modal纤维是一种全新的纤维素纤维，Modal纤维的原料来自于大自然的木材，使用后可以自然降解。由于这类纤维是采用天然纤维素为原料，具有生物将解性，并且在纤维生产过程中不产生类似粘胶县委的严重污染环境问题，是21世纪的新型环保纤维。Modal纤维价格是Tencel纤维的一半，系第二代再生纤维素纤维。Modal纤维可与多种纤维混纺、交织，发挥各自纤维的特点，达到更佳的服用效果。Modal纤维面料吸湿性能、透气性能优于纯棉织物，其手感柔软，悬垂性好，穿着舒适，色泽光亮，是一种天然的丝光面料。 3.大豆蛋白纤维 大豆蛋白纤维是以出油后的大豆废粕为原料，运用生物工程技术，将豆粕中的球蛋白提纯，并通过助剂、生物酶的作用，使提纯的球蛋白改变空间结构，再添加羟基和氨基等高聚物，配制成一定浓度的蛋白纺丝液，用湿法纺丝工艺纺成。豆粕是油脂车间的副产品，在我国资源十分吩咐，属废物综合利用，资源取之不尽，用之不竭。大豆蛋白纤维可称为新世纪的“绿色纤维”。由于大豆蛋白纤维外层基本上是蛋白质，与人体皮肤亲和性好，且含有多种人体所必须的氨基酸，具有良好的保健作用。在大豆蛋白纤维纺丝工艺中加入定量的有杀菌消炎作用的中草药与蛋白质侧链以化学键相结合，药效显著且持久，避免了棉制品用后整理方法开发的功能性产品，其药效难以持续的缺点。大豆蛋白纤维织物手感柔软、光滑，具有良好的吸湿透气性，有真丝般的光泽，抗皱性优于真丝，尺寸稳定性好。 4.竹纤维 竹纤维是继大豆蛋白纤维之后我国自行开发研制并产业化的新型再生纤维素纤维，竹纤维分竹素纤维和竹原纤维。竹素纤维是以毛竹为原料，在竹浆中加入功能性助剂，经湿法纺丝加工而成。竹原纤维是将毛竹经天然生物制剂处理后所制取的纤维。作为纺丝原料的竹浆粕，来源于速成的鲜竹，资源十分丰富。其废弃物土埋、焚烧不会造成环境污染，属于环保型纤维，满足绿色消费的需求。竹纤维是性能与粘胶纤维相类似，竹纤维织物具有良好的吸湿、透气性，其悬垂性和染色性能也比较好，有蚕丝般的光泽和手感，且具有抗菌、防臭、防紫外线功能

常见食品纤维素含量

常见食品纤维素含量常见食品的纤维素含量

麦麸:31% 谷物:4-10%，从多到少排列为小麦粒、大麦、玉米、荞麦面、薏米面、高粱米、黑米。

麦片:8-9%; 燕麦片:5-6% 马铃薯、白薯等薯类的纤维素含量大约为3%。 豆类:6-15%，从多到少排列为黄豆、青豆、蚕豆、芸豆、豌豆、黑豆、红小豆、绿豆。（无论谷类、薯类还是豆类，一般来说，加工得越精细，纤维素含量越少。 蔬菜类:笋类的含量最高，笋干的纤维素含量达到30-40%，辣椒超过40%。其余含纤维素较多的有:蕨菜、菜花、菠菜、南瓜、白菜、油菜。 菌类(干):纤维素含量最高，其中松蘑的纤维素含量接近50%，30%以上的按照从多到少的排列为:香菇、银耳、木耳。此外，紫菜的纤维。20%素含量也较高，达到． 黑芝麻、松子、10%以上的有:。坚果:3-14%以下的有白芝麻、核桃、榛子、胡桃、;10%杏仁

葵瓜子、西瓜子、花生仁。含量最多的是红果干，纤维素含量接近:水果大枣、酸枣、黑枣、其次有桑椹干、50%，樱桃、小枣、石榴、苹果、鸭梨。各种肉类、蛋类、奶制

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还可降低血压缓解头晕失眠治疗风湿性关炎等作用。其所含的膳食纤维能促进胃肠蠕动具有下气通便清肠排毒的作用经常食用还降低血压缓解头晕失眠治疗风湿性关节

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