1.Introduction

Fuel cells offer high efficiency,environmental protection,and unlimited sources of reactants.Therefore,the fuel cell is expected to have a widespread commercial application in the field of transportation[1-3].Oxygen reduction reaction(ORR)is a crucial reaction in fuel cells.For oxygen reduction reaction in fuel cells,noble Pt-based materials are generally used as active catalysts[4,5].However,the commercial largescale application of Pt-based materials still suffer from high cost,inferior electron-transfer kinetics and poor durability under operating conditions including dissolution,sintering,and agglomeration,which results in a loss of performance.Thus, finding cheap alternative catalysts is critical[6,7].

Heteroatom-doped carbon materials are excellent nonmetallic catalysts and have recently attracted significant attention[8].Compared to commercial Pt/C,heteroatomdoped carbon materials offer better electrocatalytic activity for oxygen reduction reactions in both alkaline and acidic media[9].In particular,pyridinic N offers remarkable ORR active sites[10].Sugar and urea[11],ammonium persulfate[12],cysteine[13],melamine and phytic acid[14],and sulfocarbamide[15]are generally selected as heteroatom resources.Furthermore,heteroatoms(e.g.,N,S,B,P,etc.)with dual and ternary-doped carbon materials are used as electrocatalysts for ORR due to their low cost,good tolerance toward fuel,and strong durability in acid/alkaline environments.The N and S co-doped carbon materials are able to afford high catalytic sites and exhibit excellent electrocatalytic activity for ORR,and they can be used for heteroatom doping[16].All of these features lead researchers to consider heteroatom codoped carbon materials as nonmetal catalysts for ORR.

Sustainable water manage mentin cluding wastewater treatment is a major challenge all over the world[17].Worldwide dye production from textile industries can reach over 1,500,000 tons annually.During dyeing,about 10-15%of the dye is lost.Among the several methods for dyecontaining wastewater treatment,porous carbon(PC)is generally employed as an absorbent due to its strong ability to absorb dyes[18].After adsorption,the PC(i.e.,sludge)can be treated in any of three main ways:(a)burning,(b)disposal in land fills,and(c)regeneration and reuse as absorbent[19,20].The method of regeneration reduces environmental and economic impact.Currently,there are thermal,chemical,microbiological,and vacuum methods for regenerating carbon[21].However,the absorption capacity of dye on regenerated PC can be significantly lower than fresh PC.A key focus on wastewater sustainability is the ability to reduce waste by utilization of by-products that can provide additional value.Due to the high amount of heteroatoms in azo dyes,they can be used as precursors to prepare heteroatom-doped carbon materials[22,23].

Herein,bananapeel-derived porouscarbon materials(BPPC)are employed as an absorbent in the treatment of methyl orange(MO)wastewater.The BPPC with MO was successfully transformed into N,S-doped banana peel-derived porous carbon(i.e.,BPPC-MO50)rather than being regenerated.BPPC itself contains some heteroatoms such as N and S.The surface chemistry of BPPC-MO50 demonstrates high electrocatalytic activity and raised N and S content could enhance the activity of BPPC as an electrochemical catalyst[24].Alternately,modified porous carbon can offer superior catalytic activity for ORR[25,26].The BPPC-MO50 developed here has greater activity and selectivity for the ORR in alkaline electrolytes than BPPC does.To the best of our knowledge,this is the first report to describe agriculture waste porous carbon that was activated and then used to adsorb azo dyes via enhanced heteroatom(N,S)content.These results contribute to the sustainable development of dye wastewater treatment by transforming used PC into an effective material.It also suggests potential applications in fuel cells.

2.Materials and methods

2.1.Synthesis of BPPC and BPPC-MO50 materials

The raw banana peel was air-dried,collected and put in a glass dryer prior to use.The as-received PC was grounded and passed through 80-100 mesh.The PC powder was sintered in a tube furnace and carbonized at 800°C for 5 h inflowing N2.The resulting powder was washed with deionized water and dried under air in an oven at 60°C for 12 h to produce banana peel-derived porous carbon(BPPC).The adsorption behavior of MO on BPPC adsorbents was tested at 25°C(50 mL total volume with an initial dye concentration of 100 mg/L and 0.100 g adsorbent).The BPPC-MO50 obtained after treatment of MO wastewater was ground and annealed at 800°C for 3 h in an Ar atmosphere.

2.2.Physico-chemical characterization of the synthesized PCs

油菜品种分别为三北98和油研11号，均为贵州省油菜研究所选育。钾肥为俄罗斯-白俄罗斯产，K2O含量为60%。

Next,XPS measurements were carried out to further determine the doping of the catalyst and the surface chemical compositions.The N and S levels in the BPPC were about 1.29 at.%and 0.12 at.%,and the BPPC-MO50 was about 2.84 at.%and 0.47 at.%,respectively(Table 2).The doped N and S atoms were in favor of being the as-synthesized catalysts.The origin of the N and S was suspected to come from the transformation of N-containing and S-containing functional groups in MO.High-resolution scans of the N content of the BPPC and BPPC-MO50 were performed in Fig.4a and b.There were three types of N-containing groups around at 398.2 eV(pyridinic-N),399.9 eV(pyrrolic-N)and 401.0 eV(quaternary-N).The corresponding values were about 39.1%,31.8%and 29.1%for BPPC-MO50,while 19.0%,45.5%and 35.5%for BPPC.The pyridinic-N was the most active site for ORR[27].The N 1s XPS spectrum of BPPC-MO50 had a value for the pyridinic-N that was 39.1 at.%higher than BPPC-this favored ORR[10].

2.3.Electrochemical measurement

A CHI760D instrument was used to evaluate the ORR catalytic activity of the sample BPPC and BPPC-MO50.The powder(5 mg)were mixed with Na fion solution(5%)and ultrasonically disrupted for 30 min with 10 mL loaded on a glassy carbon(GC)electrode.The GC electrode with a thin layer of powder was fixed on a rotating disk electrode(RDE)as the working electrode(WE).The counter electrode(CE)is Pt foil,and the reference electrode(RE)is Ag/AgCl.The electrolyte is 0.1 M KOH.The rotation speed varied from 400 to 2025 rpm.

3.Results and discussion

To understand the formation mechanism of N,S-doped BPPC,a schematic illustration is proposed in Fig.1a.It mainly consists of three steps.Firstly,the fresh banana peels as agriculture waste biomass resources were air dried and translated into BPPC via heat treatment.Secondly,the as prepared BPPC with abundant of pore structures was used as an adsorbent for methyl orange(MO)wastewater removal.In the latter process,rather than burning,disposal and regeneration,the BPPC adsorbed MO as the sludge was then decomposited and carbonized into N,S-doped BPPC.The carbon from the MO reduced the pore size of BPPC,processed high specific surface area(SSA)and provides many heteroatoms such as N and S.This synthetic strategy is favorable for green treatment of agriculture waste,dye wastewater,and even sludge.Moreover,the as-obtained N,S-doped BPPC with high surface area shows potential merit for using as energy materials.

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2.2.2 基因测序。基于PCR技术的基因分型是根据已知的DNA序列和突变位点，通过扩增来检测。但如果出现了未知的新基因和突变位点，就需要基因测序来解决。

Fig.3 shows XRD patterns of BPPC and BPPC-MO50.There was a broad peak at 2θ =23°,which con firmed the typical(002)plane reflection of graphite.For BPPC-MO50,this peak was more pronounced and was broader.This typically corresponds to smaller sized crystallites.Additionally,there was a small shoulder peak at 2θ =44° typical of the(100)plane reflection of graphite.Thus,the possible pseudo graphite nature and amorphous phase was possibly present in the carbonaceous BPPC-MO50.

P9= cψψ9/mb，P"= (cψψ- sψψ9 2)/mb，P= (cψψ- 3sψψ9ψ- cψψ93)/mb，

Fig.1.(a)Schematic illustration of preparation of N,S-doped BPPC.SEM images of(b)BPPC and(c)BPPC-MO50.TEM images of(d)BPPC and(e)BPPCMO50.

Fig.2.(a)Nitrogen adsorption-desorption isotherms and(b)pore size distribution of BPPC and BPPC-MO50.

Table 1 BET surface area and BJH pore diameter of BPPC and BPPC-MO50.

Samples SBET(m2/g) DBJH(nm) Pore volume(cm3/g)BPPC 508.5 2.18 0.33 BPPC-MO50 1774.3 2.13 1.25

Fig.3.XRD patterns of BPPC and BPPC-MO50.

The S 2p XPS spectrum of BPPC and BPPC-MO50 are shown in Fig.4c and d.The three S-containing groups had peaks at 163.9 eV,165.3 eV,168.2 eV.These were attributed to the thiophene-S and oxidized sulfur,respectively.Their content was 24.4%,61.3%and 14.3%for the BPPC-MO50.The two characteristic thiophene-S peaks in the BPPC-MO50 indicated that the BPPC carbon framework was successfully doped with S atoms[28].The BPPC-MO contained thiophene-S of 85.7 at.%which was much bigger than 43.5 at.%for BPPC.It is worth mentioning that thiophene-S plays an essential role in enhancing ORR catalytic activity rather than oxidized-S[23,29].

Fig.5a-e show elemental mapping of an individual BPPC,and panels(f-j)show elemental mapping of BPPC-MO50.The N and S atoms were clearly shown and were homogeneously distributed in BPPC and BPPC-MO50.It was reasonable to con firm that the N and S atoms were successfully introduced into the carbon framework because the MO precursors were completely decomposed at high temperatures.

As shown in Fig.6a,the BPPC-MO50 exhibited an onset potential in the O2-saturated 0.1 M KOH solution rather than N2-saturated 0.1 M KOH solution,indicating the pronounced ORR electrocatalytic activity of BPPC-MO50.Linear sweep voltammetry(LSV)measurements were performed on a rotating-disk electrode(Fig.6b).As expected,BPPC-MO50 displayed the highest ORR onset potential of 0.93 V(vs.RHE)among all the carbon-based electrocatalysts-even better than BPPC.The results are comparable to those reported in literatures(Table 3),such as 0.88 V for N,S-doped graphene[13],1.0 V for N,F-doped graphdiyne[26],0.94 V for BCN nanosheets[30],0.85 V for N,P-doped graphene frameworks[31].0.77 V for N,P,S-tridoped graphene[32].

This work was supported by the Doctor Foundation of Bingtuan(No.2014BB004),National Natural Science Foundation of China(U130329),the Program for Changjiang Scholars,Innovative Research Team in University(No.IRT_15R46)and the Program of Science and Technology Innovation Team in Bingtuan(No.2015BD003).

Table 2 Atomic content of BPPC and BPPC-MO50.

Sample Content(at.%) Content of N species(at.%) Content of S species(at.%)C O N S Pyridinic Pyrrolic Graphitic 2p2/3 2p1/3 Oxidized BPPC 83.60 14.99 1.29 0.12 19.0 45.5 35.5 5.8 37.7 56.5 BPPC-MO50 88.12 8.57 2.84 0.47 39.1 31.8 29.1 24.4 61.3 14.3

Fig.4.High-resolution N 1s XPS spectra of(a)BPPC and(b)BPPC-MO50.High-resolution S 2p XPS spectra of(c)BPPC and(d)BPPC-MO50.

Fig.5.(a)SEM image and(b-e)corresponding elemental mapping of BPPC and(f)SEM image and(g-j)corresponding elemental mapping of BPPC-MO50.

here,n represents the electron transfer number,F is 96,485 C(Faraday constant),Co*is 1.2×10-3mol/cm3(oxygen bulk concentration),Do is oxygen diffusion coefficient about 1.9×10-5cm2/s,ν=1.1× 10-2cm2/s(electrolyte kinematic viscosity),j denotes the measured current density,ω =2πN(N is the linear rotation speed),jLis the diffusion-limiting current densities,and jkstands for the kinetic current densities.Fig.6d shows that the calculated transfer numbers for the BPPCMO50 was 2.71 at approximately-0.4 V to-0.6 V.This suggested a four-electron(4e-)and two-electron(2e-)hybrid pathway from-0.4 V to-0.6 V dominates the ORR of BPPC-MO50.

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As shown in Fig.7a,the cathodic current of BPPC-MO50 retained 68.9%of its initial activity after 30,000 s,while the Pt/C catalyst activity declined to about 52.2%.The high catalytic stability of BPPC-MO50 catalysts was attributed to the high degree of graphitization of the carbon phase.Methanol crossover testing was also performed for both BPPC-MO50 and commercial Pt/C by adding 4 M methanol into 100 mL solutions of the catalyst after 1000 s(Fig.7b).The BPPC-MO50 catalyst showed a stable amperometric response after the introduction of methanol,the current of BPPC-MO50 retained 97.2%of its initial catalytic activity indicating good tolerance against the methanol crossover effect.In contrast,the Pt/C catalyst declined to about 70.1%of its initial current on the addition of methanol under the same experimental conditions.

4.Conclusions

Fig.6.(a)The CV curves of BPPC-MO50 at a scan of 50 mV/s.(b)LSV curves of BPPC and BPPC-MO50 with different adsorption values at a scan of 10 mV/s,electrode rotating rate:1600 rpm.(c)The LSVs of BPPC-MO50 with various rotation rates at a scan of 10 mV/s and(d)the corresponding Koutecky-Levich plots at different potentials.

Table 3 Comparison of ORR performance of BPPC-MO50 with literatures.

Catalyst Electrolyte Onset potential(V vs.RHE) Current density(mA/cm2) Ref.N,S-doped graphene 0.1 M KOH 0.875 4.99 [13]N,F-doped graphdiyne 0.1 M KOH 1.000 4.5 [26]BCN nanosheets 0.1 M KOH 0.940 ＞5.0 [30]N,P-doped grapheme frameworks 0.1 M KOH ＞0.845 ＞5.0 [31]N,P,S-tridoped graphene 0.1 M KOH 0.7739 ＞3.0 [32]BPPC-MO50 0.1 M KOH 0.9259 3.94 This work

Fig.7.(a)The chronoampero metric curves of BPPC-MO50 and 20 wt.%Pt/C at 0.36 V in O2-saturated 0.1 M KOH media at a rotational speed of 400 rpm.(b)Methanol crossover tests of BPPC and Pt/C by injecting 4 M of methanol into KOH media after 1000 s.

This study showed that BPPC acts as a good adsorbent for the MO removal from dye wastewater.We can controlling the adsorption quantity by controlling the dosing quantity.The asobtained BPPC-MO50 exhibits high activity compared to BPPC and shows enhanced durability and better tolerance to methanol crossover reduction reaction in alkaline electrolyte compared to commercial Pt/C in ORR.The results may contribute to the development of dye wastewater treatment.Our present work suggests that BPPC-MO50 obtained from dye wastewater has an added value due to the potential applications in fuel cells.

Scanning electron microscopy(SEM)images of BPPC and BPPC-MO50 are shown in Fig.1b and c,respectively.Some thin holes were observed from the BPPC.The BPPC-MO50 had more holes and uniform pore distribution,which could be related to the calcination process to obtain the doped material.In Fig.1d and e,the microstructure of BPPC and BPPCMO50 was characterized by transmission electron microscopy(TEM)confirming that the dye-activated materials had dense channels.These results were further con firmed by the BET data obtained from the BPPC and the BPPC-MO50 with different adsorption.As shown in Fig.2a,the BPPC-MO50 exhibited high SSA of 1774.3 m2/g,which was higher than 508.5 m2/g seen from BPPC.It may be because MO was decomposited and carbonized into carbon covering the pore wall and reducing the pore size.The BPPC and BPPC-MO50 exhibited the average pore size of 2.18 nm and 2.13 nm(Fig.2b),where the corresponding pore volume was 0.33 cm3/g and 1.25 cm3/g(Table 1),respectively.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgments

Next,Koutecky-Levich(K-L)plots(Fig.6d)of BPPCMO50 were created from LSVs of BPPC-MO50 with various rotation rates at a scan of 10 mV/s(Fig.6c)to measure the electron transfer number of BPPC-MO50 with the catalyst.The electron transfer number per O2molecule in ORR can be collected according to the following equations[33].

Author contributions

Y.Q.Wang and F.Yu designed the experiments.F.Yu and X.H.Guo administered the experiments.Y.Q.Wang,X.Y.Xue and F.Yu performed experiments.M.Y.Zhu,Y.C.Li,Y.L.Shi and B.Dai gave technical support.Y.Q.Wang and M.J.Zhang collected data.F.Yu,B.Dai and X.H.Guo gave conceptual advice.All authors analyzed,discussed the data and wrote the manuscript.

A UV-vis spectrophotometer(UV-1100D,Mapada,China)at 464 nm was used to analyze the concentration of MO.A Hitachi SU8010 microscope was used to perform scanning electron microscopy(SEM)imaging for the BPPC and BPPCMO50.X-ray diffraction(XRD)was used on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation(λ =1.5406 Å).An AMICUS/ESCA 3400 electron spectrometer was used to collect X-ray photoelectron spectroscopy(XPS)data using Mg Kα(12 kV,20 mA)radiation.The C1s line at 284.8 eV was the reference.A Micromeritics ASAP 2020 BET apparatus was used to analyze pore structure by Barrett-Joyner-Halenda(BJH)and specific surface area(SSA)by Brunauer-Emmett-Teller(BET)at 77.35 K via a N2 adsorption-desorption method.Mapping was performed using a JEM-ARM 200F microscope operating at 200 kV.

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