Catalytic performance of hybrid Pt@ZnO NRs on carbon fibers for methanol electro-oxidation☆

Dongyan Li,Chen Gu ,Feng Han ,Zhaoxiang Zhong ,*,Weihong Xing ,*

1 Department of Chemical Engineering and Materials,Nanjing Polytechnic Institute,Nanjing 210048,China

2 State Key Laboratory of Materials-Oriented Chemical Engineering,National Engineering Research Center for Special Separation Membrane,Nanjing Tech University,Nanjing 210009,China

1.Introduction

The use of fossil fuels has caused alarming environmental problems,including global warming,greenhouse effect,and air pollution[1].To meet increasing energy demand without further impacting on the environment,the search of environmentally friendly alternative energy sources such as solar,nuclear,and biology are imperative.A fuel cell converts chemical energy into electricity through an electrochemical reaction with higher energy conversion efficiency than conventional fossil fuel combustion[2].Furthermore,fuel cell produces water as the only by-product,eliminating the pollution caused by fuel burning[3,4].Among different type of fuel cells,direct methanol fuel cell(DMFC)has attracted more and more attention because of its abundant fuel source,high energy efficiency,low cost,and ease to transportation and storage[5,6].

Pt catalysts own the highest catalytic activity for methanol electrooxidation.However,Pt catalysts have low CO tolerance[7,8].Numerous studies were conducted in search ofa solution to the low COtolerance of Pt catalysts,such as alloying with other metals,combined function with conductive substrates,and metal oxide coupling[9-11].The use of metal oxides such as ZnO and TiO2has shown promising results in controlling the poisoning of Pt[12,13].Furthermore,ZnO and TiO2also generate photocurrent under UV illumination using composite catalysts when coupled with Pt,further enhancing methanol electrooxidation ability.

Carbon fibers(CFs)have many advantages in carrier material due to its superior electro conductivity,good chemical resistance,and high mechanical strength[14,15].Kong et al.modified carbon fiber with grown CoSe2nano particles as three-dimensional electrodes,exhibiting excellent catalytic activity for hydrogen evolution reaction[16].Huang et al.employed knitted carbon fibers for superca pacitors with improved capacity,energy and power density.However,the low specific surface area of carbon fibers resulted in low catalyst loading[17].

In this study,a novel Pt@ZnO nanorod/carbon fibers(NR/CFs)with hierarchical structure with enhanced methanol electro-oxidation was fabricated.Although both ZnO and TiO2have wide band gap,ZnO processes two to three orders of magnitude higher electron migration rate(115-155 cm2·V-1·s-1)[18]and exhibits higher exciton binding energy of around 60 meV,which could generates higher photocurrent under UV illumination.Besides,the existence of ZnO with high specific surface area can efficiently increase the loading of catalytic composition and enlarge contact areas with electrolytes[19,20].Therefore,it is significant to synthesize these multifunctional materials.To prepare the novel structure,ZnO nanorods were hydrothermally grown on CF after the immobilization of ZnO seeds on CF via atomic layer deposition(ALD).Then,Pt were dispersed on the surface of ZnO nanorods via MS.The coverage,size,and chemical state of Pt can also be controlled precisely by MS.In the end,the crystal phase composition,morphology,and elements of Pt@ZnO NR/CF were characterized by XRD,SEM,and EDS.The prompt current during on/off switching of the UV lamp was recorded to investigate the photoelectric response of ZnO NR/CF.Chronoamperometry and cyclic voltammetry(CV)were conducted in a standard three-electrode cell at room temperature to evaluate the effect of thickness of sputtered Pt and ZnO secondary support on the activity of methanol oxidation and the stability of the catalyst.

2.Experimental

2.1.Raw materials

Carbon fibers(T300-3k,Toray)with a diameter of7μmand length of 5 μm.The source of zinc and oxygen for ALD were diethyzinc(Zn(C2H5)2)and deionized water.Zinc nitrate hydrate(Zn(NO3)2·6H2O)and hexamethylenetetramine(HTMA,C6H12N4)were provided by Shanghai Lingfeng Co.Pt catalysts were purchased from Beijing Gaodewei metal Co.All chemical reagents were analytical grade in the experiment.

2.2.Hydrothermal synthesis of ZnO nanorods

Fig.1 shows the synthesis route of Pt@ZnO NR/CFs.Firstly,CFs were placed in the ALD reactor to deposit ZnO seeding layers,and then the chamber was preheated to 130°C with both sides exposing to N2.The ALD process will start when the vacuum in the chamber reached 1 Torr.Two precursor vapors of DEZ and water were heated to 40°C and 60°C,which were alternately delivered into the reaction chamber.To ensure the precursors enter into the samples thoroughly,exposure mode was set in the experimental section.Under the condition of N2flow rate of 20 sccm and temperature of 130°C,CFs were suffered over 300 cycles.Secondly,the hydrothermal synthesis reactor for growing ZnO NRs was maintained at 90°C for 3 h,which contained 5 mmol·L-1zinc nitrate.The prepared ZnO NRs/CFs were washed several times with water and dried at 70°C for 2 h.

2.3.Pt nanoparticles loading

Pt nanoparticles loading were conducted via magnetron sputtering(VTC-600-2HD,MTI,Shenyang)with a circular Pt target at room temperature.During the process of sputtering,the sample table was acted as the support for ZnO NRs/CFs substrates with rotation speed of 20 r·min-1to guarantee the homogeneous dispersion of Pt nanoparticles.The working pressure and power of the apparatus chamber were set as 3.0 Pa and 30 W,respectively.Besides,the layer thickness of Pt nanoparticles(10 nm,30 nm,and 50 nm)were controlled by the thickness detectors,which are defined as Pt@ZnO NR/CF-10,Pt@ZnO NR/CF-30,and Pt@ZnO NR/CF-50,respectively.

2.4.Characterizations

The phase composition was detected by X-ray diffractometer(XRD;D8-Advance,Bruker,Germany)with Cu Kαradiation(wavelength of 0.154 nm),operated at 15 mA,40 kV,and a step width of 0.02°with a scanning range of 10°-80°.The microstructure was observed by field emission scanning electron microscope(FESEM,HitachiS-4800,Japan).

2.5.Measurements of photoelectrochemical response

A three-electrode cell with CHI760E electrochemical system(Chenhua Instruments,Co.,Shanghai)in which a 24-W UV lamp(LTD,λ=365 nm)was used in photoelectrochemical measurements served as light source.The working electrode was 10 nm Pt@ZnO NRs/CFs.Linear sweep voltammetry(LSV)was used to record the currentvoltage curves.First,the photoresponse was conducted by switching light lamp in the process of total 5 cycles and each cycle was set in the absence of light illumination from 0 to 40 s and then next 40 to 90 s under the continuous illumination of light.Finally,the lamp was turned off at 90 s measuring at 0 V(vs.SCE).Besides,we also investigated the effect of the applied voltage(0,0.1,0.3 and 0.6 V vs.SCE)on response current according to the experimental procedures mentioned above.

2.6.Electrochemical and photoelectrochemical reactions of methanoloxidation

The electrochemical performance of the prepared samples was conducted by a traditional three-electrode cell in electrochemical workstation.Pt was acted as counter electrode and saturated calomel electrode(SCE)was functioned as the reference electrode.Pt@ZnO NRs/CFs with the thickness of 10 nm,30 nm,and 50 nm were used as working electrode.Cyclic voltammetry method was conducted to characterize electrochemical active surface area(ECSA)and catalytic capability.In the methanol electro-oxidation experiment,the condition of electrode was 0.5 mol·L-1H2SO4and the mixed solution of 1 mol·L-1CH3OH and 0.5 mol·L-1H2SO4,respectively.In addition,chronoamperometry experiments was applied to characterize the stability of as-prepared working electrode and the scan rate was set as 30 mV·s-1in all electrochemical experiments.A UV lamp(LTD,24 W,λ=365 nm)was used as light source to test the effect of light source on the methanol-oxidation reaction performance of samples.

3.Results and Discussions

3.1.Microstructure and morphology

Fig.1.Synthesis route of Pt@ZnO NRs/CFs composites.

Fig.2.The XRD spectrums of Pt@ZnO NRs/CFs with different stages and Pt thickness of 10,30 and 50 nm(a),and the EDS spectrum of Pt@ZnO NRs/CFs sample(b).

X-ray diffraction patterns of the samples are shown in Fig.2a.A obvious peak at 25°represented the existence of integral graphite structure[21].This indicated that CFs deposited on ZnO seeds by ALD and ZnO NR/CF showed three main peaks appearing at angles 2θ=31.94°,34.60°,and 36.42°,which could be attributed to(100),(002),and(101)planes of hexagonal wurtzite(JCPDS36-1451)[22,23].The pattern of sample of Pt@ZnO NR/CFs with 10 nm Pt coating were no Pt peaks,which may be due to the small size and amount of Pt nanoparticles[24].However,the crystalline phase peaks of Pt gradually increased with thickness of Pt.At 50 nm thick,two diffraction peaks at 39.5°and 46.8°were detected,which corresponded to(111)and(200)planes of Pt,respectively[25].The XRD analysis results confirmed the loading of Ptnanoparticles on the surface ofZnONRs.Fig.2b showed the EDS spectrum of Pt@ZnO NRs/CFs sample.It was used to further prove the existence of Pt on the surface of ZnO NRs.From Fig.2b,it can be obviously seen that the peak of Pt elementexisted on the surface of Pt@ZnO NRs/CFs.Furthermore,it also had C,Zn and O element.

Fig.3 shows the SEM images of CFs,ZnONR/CFs and Pt@ZnO NR/CFs with different thickness of Pt.The initial morphology of pure CFs is shown in Fig.3(a).After ALD process,the CF substrates are covered with a uniform thin layer of ZnO seed[Fig.3(b)].As shown in Fig.3(cd),it can clearly be seen that ZnO NRs were on the surface of the CFs.From Fig.3(e-f),Pt@ZnO NR/CF-10 shows a similar morphology as that of ZnO NRs/CFs without observing the Pt nanoparticles deposited.MS has been demonstrated to be an effective and easy-controlled method to depositcoating layers on various substrates.With increasing loading of sputtered Pt,the surface of ZnO NRs becomes rougher,as shown in Fig.3(g-j).From Fig.3(i-j),when the deposited thickness of Pt reaching to 50 nm,the original surface of ZnO NR were obviously wrapped by smallPtnanoparticles.According to BET testdata published on my previous paper,ZnO NRs acted as bonding layer between carbon fiber substrate and Pt nanoparticles,which provided higher active sites and contact areas[20].

3.2.Photoelectrochemical response of ZnO NR/CF

Photoelectrochemical responses of ZnO NRs/CFs were investigated by using LSV method to record current-voltage(I-V)curves from 0 to 0.8 V with the scan rate of 10 mV·s-1(Fig.4).The response current gradually increased along with applied potentials from 0 and 0.8 V regardless of lightirradiation.However,the currentin the presence of light was consistently higher than thatin the dark at the same potential.This increment of current may be ascribed to the generation of photo current[26].

Fig.3.The micromorphology of the prepared samples(a is original CFs;b is CFs deposited with a ZnO seed layer;c and d are the sample of ZnO NRs/CFs at different magnification;e,f are 10 nm Pt@ZnO NRs/CFs;g,h are 30 nm Pt@ZnO NRs/CFs;i,j are 50 nm Pt@ZnO NRs/CFs).

Fig.4.The I-V curves of ZnO NRs/CFs in dark and light illumination.

Fig.5.The on-off I-t curves of ZnO NRs/CFs at 0 V potential.

ZnO is a photosensitive material.Experiment with periodic irradiation was performed to investigate the photosensitive characteristics of ZnO by recording the photocurrent-time(I-t)data.At 0 V vs.SCE,the response current remains at-0.020 mA in the dark during the first 40 s(Fig.5).When UV light illumination was switched on,the response photocurrent immediately changed from-0.020 to 0.30 mA.Besides,the photocurrent maintained about 0.30 mA under UV light illumination for 50 s.When the light was switched off,the current returned to-0.020 mAin the dark.The generation and disappearance of photocurrent is rapid in the periodic irradiation experiment,which suggests that ZnO have excellent response to photoelectricity.The consistent photocurrent response under periodic irradiation indicates the stability of ZnO NR/CF.

Fig.6 shows the effects of applied potentials on the photocurrent generation of ZnO NR/CF.As shown in Fig.6,the as-prepared sample of ZnO NR/CF showed increasing sensitivity to light irradiation with applied potential.However,ZnO NR/CF showed remarkable response regardless of the applied potentials.When the applied potential increased from 0 V to 0.6 V,the photocurrent generated under light irradiation increased from 0.235 to 0.725 mA.This is because the higher potential promotes more effective separation and transfer of photogenerated electron-hole pairs while suppressing electron recombination,leading to increasing photocurrent[27].

3.3.Role of ZnO secondary carrier layer in electro-oxidation of methanol

Fig.6.The on-off I-t curves of ZnO NRs/CFs at different potentials(0,0.1,0.3,0.6 V).

Cyclic voltammetry(CV)is a common method to calculate ECSA of Pt catalyst.The actual value of ECSA not only revealed the number of active sites of the catalyst,but also estimated the catalytic performance and the choice of suitable substrates.The ECSA were calculated by the area ratio of hydrogen adsorption/desorption peaks in the range of negative potential[28].CV diagram of Pt@CF-10 and Pt@ZnO NR/CF-10 catalyst were measured in 0.5 mol·L-1H2SO4solution as shown in Fig.7.Both catalysts had distinct hydrogen adsorption/desorption peaks between-0.2 and 0.1 V.According to the equation,ECSA=QH/（210 × Pt）[29],the ECSA(cm2·g-1)values of Pt@CF-10 and Pt@ZnO NR/CF-10 catalysts are 10.78 m2·g-1and 23.19 m2·g-1,respectively.The higher ECSA value of Pt@ZnO NR/CF-10 suggested that the existence of ZnO secondary carrier layer,which improved the distribution and the size of Pt nanoparticles on the surface of ZnO NR,and this was bene ficial to the improvement of catalytic efficiency.

Fig.7.The electrochemically active surface areas measured in 0.5 mol·L-1 H2SO4 of10 nm Pt@CFs and Pt@ZnO NRs/CFs with 30 mV·s-1 sweep rate.

Fig.8 showed the CV curves of the catalytic activity for electronoxidation methanol of ZnO NR/CFs,Pt@CFs-10,and Pt@ZnO NR/CFs-10.As shown in the Fig.8,it can be seen that CV curves of the three catalysts showed two similar current peaks located at the potentials of about 0.7 V in forward scan and 0.5 V in backward scan.The current peaks in 0.7 V and 0.5 V corresponded to the peaks of methanol oxidation(If)and CO oxidation peak(Ib),respectively.The peak current density at about 0.7 V was a crucial parameter to represent the catalytic ability for methanol oxidation[30].Otherwise,Fig.8 also showed that the peak of current density(If)of three catalysts resided in accordance with the following order:Pt@ZnO NR/CFs-10＞Pt@CFs-10＞ZnO NR/CFs.Pt@ZnO NR/CFs-10 possessed the highest current density for methanol oxidation because of the synergistic effect between the Pt nanoparticles and the ZnO NRs.Moreover,the ratio of Ifand Ibwas an indication of cataly sttolerance towards CO,where higherratio of If/Ibindicates higher CO tolerance of catalysts[31].The calculated If/Ibvalue of Pt@ZnONR/CFs-10 and Pt@CFs-10 were 1.72 and 1.41,respectively.The increased ratio of If/Ibrevealed that the existence of ZnO enhanced the CO tolerance by oxidizing CO in lower potential[12].In summary,the existence of ZnO secondary carrier layer effectively improved the ECSA,the methanol catalytic oxidation activity and the CO tolerance.

Fig.8.Cyclic voltammograms of ZnO NRs/CFs,10 nm Pt@CFs,10 nm Pt@ZnO NRs/CFs in 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol with 30 mV·s-1 sweep rate.

3.4.Methanolelectro-oxidation by Pt@ZnONRs/CFs electrode with different Pt loadings

Fig.9 showed the CVs curves of catalysts with different thickness of Pt(10,30,and 50 nm)in 0.5 mol·L-1H2SO4solution between-0.2 and+1.2 V.As shown in Fig.9,the Pt@ZnO NRs/CFs with 10,30,and 50 nm thickness all exhibited hydrogen adsorption/desorption peaks in scan range of-0.2-0.1 V.Calculated results indicated that ECSA of the three prepared samples of 10,30,and 50 nm Pt@ZnO NRs/CFs are 23.19 m2·g-1,29.73 m2·g-1,and 43.65 m2·g-1,respectively.The results further indicated that the ECSA values of catalysts increases with the increasing loading of Pt[32],thus improving the ability of methanol electro-oxidation.

Fig.9.The electrochemically active surface areas measured in 0.5 mol·L-1 H2SO4 of Pt@ZnO NRs/CFs with different thickness of Pt(10,30,and 50 nm)at a sweep rate of 30 mV·s-1.

Fig.10 showed the CVs curves of samples with various deposited Pt thickness(10,30,and 50 nm)in mixed solution between 0 to 1 V vs.As shown in Fig.10,the forward anodic peak current(If)increased with the loading of Pt.The result was consistent with the ECSA experiment(Fig.9).The distribution,grain size,and morphology of Pt component had direct effect on the catalytic performance.From Fig.10,the sample of Pt@ZnO NR/CF-50 had the highest forward anodic peak current density(If),which may be due to the unique nanorod-bundle morphology of Pt caused by pressure and high temperature in the process of MS.The changed morphology of Pt could cause much great improvement in activity of methanol oxidation activity.For example,the peak current density ofPt@ZnONR/CFs-50 was approximately 3.75 times than thatof Pt@ZnO NR/CFs-30.However,the Ifvalue of Pt@ZnO NR/CFs-30 is only 2.5 times than Pt@ZnO NR/CFs-10.This enhancement may be due to the aggregation and growth of Pt cores resulting in the increase of contact area and active sites for methanol oxidation.However,the improved loadings of Pt bring about the decrement of CO tolerance of catalysts due to the easiness of Pt catalyst poisoning[10].

Fig.10.The CVdiagrams ofPt@ZnONRs/CFs with various thickness ofPt(10,30,50 nm)in 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol with 30 mV·s-1 sweep rate.

Chronoamperometric analytic approach is usually used to investigate the electric stability of catalysts.The current density was recorded for1800 s at0.6 V(vs.SCE)with 30 mV·s-1scan rate in electrolyte containing 0.5 mol·L-1H2SO4and 1 mol·L-1methanol.At 0.6 V constant potential,the current density of ZnO NR/CFs,Pt@CFs-10 and Pt@ZnO NR/CFs with different thickness of Pt(10,30,and 50 nm)decreased with time(Fig.11).This result revealed the formation of reaction intermediates and the oxide of Pt on the basis of proceeding in methanol oxidation reaction[33].Pt@ZnO NR/CFs-50 consistently showed the highest current density among all samples tested.In addition,Pt@ZnO NR/CFs-10 possessed higher current density than Pt@CFs-10,which indicated the existence of ZnO secondary support was bene ficial to improve the CO tolerance and activity of Pt.

3.5.Photoassisted methanol electro-oxidation

Fig.12 showed the CV curves of Pt@ZnO NR/CFs-30 for methanol electro-oxidation under light illumination.As shown in Fig.12,the peak current density in forward scan was 6.55 mA·cm-2under UV light,which was approximately 1.42 times than in the dark(4.6 mA·cm-2).This improvement of peak current density may be attributed to the occurrence of light-assisted methanol electro-oxidation.The increase of peak current density by 42%was attributed to the synergistic photocurrent produced on ZnO semiconductor[13].Under UV light,the electrons on ZnO semiconductor transferred from valance band to conduct band,which facilitate the formation of electron-hole pairs,thus generating additional photocurrent[26].

Fig.11.The recording of chronoamperometric curves at 0.6 V versus SCE of ZnO NRs/CFs,10 nm Pt@CFs,10 nm Pt@ZnO NRs/CFs,30 nm Pt@ZnO NRs/CFs,and 50 nm Pt@ZnO NRs/CFs.

Fig.12.CV diagrams of methanol electrochemical oxidation at the electrode of 30 nm Pt@ZnO NRs/CFs with or without UV light.

4.Conclusions

Firstly,a novel multilevel photo catalyst of Pt@ZnO NR/CFs was fabricated by ALD method united with hydrothermal synthesis and magnetron sputtering.With increasing loadings of Pt,the existing morphology of Pt changed from nanoparticles to nanorod bundles.ZnO NR/CFs is an excellent photosensitive material as demonstrated by the excellent and stable photoelectric response.The presence of ZnO can efficiently enlarge the ECSA of catalyst,promote the efficiency of methanol oxidation,and improve CO tolerance on methanol oxidation of catalysts.In addition,the changed morphology of Pt could cause much great improvement of methanol oxidation activity.The peak current density of methanol oxidation on Pt@ZnO NR/CFs-30 under UV illumination increased significantly compared with that in dark environment.The synergistic catalysis of lightand electricity resulted in the improvement of current for methanol oxidation.Most importantly,the novelcatalysts show good application promising due to its high catalytic efficiency and tenacious CO tolerance.

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