Hong Wang ,Xin Wei,Yujun Zhang ,Ronghua Ma ,Zhen Yin *,Jianxin Li,*

1 State Key Laboratory of Separation Membranes and Membrane Processes/National Center for International Joint Research on Separation Membranes,China

2 School of Materials Science and Engineering,Tianjin Polytechnic University,Tianjin 300387,China

3 Key Laboratory of Advanced Energy Materials Chemistry(Ministry of Education),Nankai University,Tianjin 300071,China

4 School of Environmental and Chemical Engineering,Tianjin Polytechnic University,Tianjin 300387,China

Keywords:Electrocatalytic membrane reactor(ECMR)MnO x/Ti membrane electrode Electro-oxidation of 2,2,3,3-tetra fluoro-1-propanol(TFP)Electrochemical reactor(ECR)Electrochemical measurement

ABSTRACT The different electrocatalytic reactors could be constructed for the electrocatalytic oxidation of 2,2,3,3-tetra fluoro-1-propanol(TFP)with two typical MnO x/Ti electrodes,i.e.the electrocatalytic membrane reactor(ECMR)with the Ti membrane electrode and the electrocatalytic reactor(ECR)with the traditional Ti plate electrode.Forthe electro-oxidation ofTFP,the conversion with membrane electrode(70.1%)in the ECMRwas 3.3 and 1.7 times higher than that of the membrane electrode without permeate flow(40.8%)in the ECMR and the plate electrode(21.5%)in the ECR,respectively.Obviously,the pore structure of membrane and convection-enhanced mass transfer in the ECMR dramatically improved the catalytic activity towards the electro-oxidation of TFP.The specific surface area of porous electrode was 2.22 m2·g-1.The surface area of plate electrode was 2.26 cm2(1.13 cm2×2).In addition,the electrochemicalresults showed that the mass diffusion coefficient ofthe MnO x/Ti membrane electrode(1.80 × 10-6 cm2·s-1)could be increased to 6.87 × 10-6 cm2·s-1 at the certain flow rate with pump,confirming the lower resistance ofmass transfer due to the convection-enhanced mass transfer during the operation of the ECMR.Hence,the porous structure and convection-enhanced mass transfer would improve the electrochemical property of the membrane electrode and the catalytic performance of the ECMR,which could give guideline for the design and application of the porous electrode and electrochemical reactor.

1.Introduction

The electrochemical technique of using electricity to drive organic reactions or organic degradation in water provides a green and sustainable pathway and has gained significant interest[1,2].However,the high energy consumption and low currentefficiency restricttheir applications.Hence,how to solve current efficiency and the energy consumption of the electrochemical reactor becomes a great challenge during the practical applications of the electrochemical technique.

Recently,the porous membrane electrode has attracted more and more attention due to their high specific surface area and excellentperformance[3–11].The porous membrane electrode can be used as the support to immobilize catalysts to realize the recovery and reuse of catalyst.Furthermore,the porous membrane electrode can be used as an anode to assemble the electrocatalytic membrane reactor(ECMR),which can not only enhance the contact between catalysts and reactants,but also remove the products from catalytic sites via the control of the permeate flow rate during the reactor operation.For example,Yang etal.[3,4]reported that TiO2nanoparticles were loaded on the porous carbon membrane to fabricate TiO2/carbon membrane electrode as filter and anode to assemble an ECMR,which was used for the oily wastewater.In ECMR,the TiO2/carbon membrane generated reactive intermediates under electric field that decomposed organic foulant into CO2and H2O or small biodegradable products,and thus resulted in its strong self-cleaning function.Simultaneously,the micro flows and permeate flow generated by ECMR could effectively remove the products from the active sites,leading to a high efficiency.The electricity consumption for this membrane reactor was 0.166 kW·h per ton of water.The removal rates of oil and chemical oxygen demand(COD)were up to 86%and 94%,respectively.Similarly,the dual tubular membrane electrode reactor was also reported to improve the tricyclazole degradation by enhanced mass transfer[5],in which a tubular membrane Ti/IrO2–Ta2O5was employed as anode and a carbon black–polytetra fluoroethylene modified graphite membrane as cathode.As a result,the removalrate oftricyclazole was 79%in 85 mg·L-1tricyclazole solution under the conditions:membrane flux of 103 L·m-2·h-1,current density of 10 A·m-2and pH value of 3.However,the current ef ficiency(CE)for the tricyclazole degradation in ECMR ranged from 9.9%to 50.86%and showed instability[5].

In our previous work[6],an ECMR constituted by MnOxloading the tubular porous Ti-based membrane electrode as anode and stainless steel as a cathode was employed to yield high value-added sodium 2,2,3,3-tetra fluoropropionate (STFP) from 2,2,3,3-tetra fluoro-1-propanol(TFP)into via electrochemical oxidation.The conversion of TFP was up to 92%and the selectivity to STFP was more than 99%.However,the CE for the oxidation of TFP to STFP was only 13.7%under the optimum condition[6].Hence,understanding the kinetic properties of membrane electrode and thus improving the current efficiency of the ECMR become the most critical problem in the application of the ECMR.

Actually,the electrochemical properties or kinetic parameters of the porous electrode in the reactor including the effect of mass transfer and synergistic effect of electrochemical oxidation and convectionenhanced diffusion play an important role in the current efficiency and the energy consumption of the electrochemical reactor.In order to evaluate the electrochemical performances and investigate the electrochemical mechanism of the reactor,the electrochemical measurements have been widely adopted[12–14].For example,Niu et al.[13]fabricated a novel electrode with a three-dimensional(3D)porous nickel nanostructure for glucose detection and investigated the electrochemical properties via cyclic voltammetry(CV),electrochemical impedance spectroscopy(EIS)and chronoamperometry(CA)measurements.Compared with the traditional Ni plate electrode,the 3D porous Ni electrode exhibited better electrocatalytic properties,i.e.higher current response in CA measurements and smaller arc in EIS for the glucose oxidation due to the porous and robust frameworks.Similarly,Huang et al.[14]investigated the electrochemical properties of modified/bulk carbon paste electrodes by cyclic voltammetry and EIS methods in[Fe(CN)6]4-/3-solution.In their work,the important electrochemical kinetic parameter—the electrochemical active area(A)could be obtained from the relationship between peak currents versus square root of scan rate through CVs with different scan rates.In addition,Liu and Vecitis[15]investigated the effects of hydrodynamically enhanced mass transfer on the current density in the electrochemical filtration system versus a conventionalbatch electrochemicalsystem via CA methods.Their results confirmed that the non-negligible convective mass transferto the electrode surface wasvery importantin the filtration system,which was also vitalforthe effective oxidation in electrochemical system.Moreover,as another important kinetic parameter,the diffusion coefficient could be obtained by CA measurement in electrochemical study.Li et al.[16]reported that the average diffusion coefficient(5.67× 10-5cm2·s-1)of 2-naphthol with the copper nanostructures–graphene oxide(Cu/GO)modified glassy carbon electrode could be obtained by the consequent plots of i vs.t-1/2derived from the CA curves.

The aim of this present study is to investigate the electrochemical performance of the porous membrane electrode and the synergistic effect of electrochemical oxidation and mass transfer of for TFP oxidation by electrochemical measurements in ECMR.Meanwhile,the key kinetic parameters such as electrochemical reaction-the diffusion coefficient and electrochemical surface site were also explored during the electrocatalytic oxidation of TFP to produce STFP.In addition,the conventional electrodes with the Ti plate were fabricated and used as a control anode to construct ECR for comparison.

2.Experimental

2.1.Materials

TFP and STFP were of chromatographic purity and purchased from Meryer Co.,Ltd.Acommercially available plate Tielectrode(with a simple 2-dimensional surface structure and a thickness of 1 mm)and the porous Ti membrane electrode(with an average pore size of 7.0 μm,a thickness of 2.6 mm,and a porosity of 23.8%)were purchased from Northwest Institute for Nonferrous Institute Metal Research,Xi'an,China.All other reagents were of analytical grade and purchased from Tianjin Kermel Chemical Reagent Co.,Ltd.,China and used without further purification.All solutions were prepared with doubly distilled water.

2.2.Preparation of MnOx/Ti electrode

The nano-MnOxloading Ti electrode(MnOx/Ti electrode)was prepared via the sol–gel method and thermal decomposition of manganese(II)nitrate on the originalTiplate electrode and Timembrane electrode,respectively.The detailed procedure can be found in the previous work[6].The specific surface area of the porous electrode was calculated from the adsorption data using the Brunauer–Emmett–Teller(BET)method,and the value was 2.22 m2·g-1.The surface area of the plate electrode was 2.26 cm2(1.13 cm2×2).The obtained MnOx/Tielectrodes were used as an anode to assemble ECR or ECMR,respectively.The residence time(RT)and responding permeate rate were 5,10,15,and 20 min and 2.20,1.10,0.72,and 0.53 ml·min-1,respectively,during the operation of ECMR.

2.3.Characterization of electrodes

The specific surface area was obtained via an adsorption analyzer(Quantachrome Autosorb IQ-C).The electrochemical measurements,such as CV,EIS and CA,were used to characterize the electrochemical behavior of differentworking electrodes.The electrochemical measurements were conducted on an electrochemical workstation(Zahner-Zennium)instrument in a conventional three-electrode cell at room temperature.Three types of electrodes were used as working electrode in the test and the further reactions:(1)the MnOx/Ti plate electrode in ECR,(2)the MnOx/Ti membrane electrode without permeate flow in the ECMR and(3)the MnOx/Ti porous electrode at a certain flow rate in ECMR.

A saturated calomel electrode(SCE)was used as the reference electrode,and a large-area platinum foil(30 mm×30 mm)was used as the counter electrode,which is particularly important for impedance measurements(impedance of a counter electrode must be negligible in comparison with a working electrode)[17].Allmeasurements were carried out in 1 mmol·L-1[Fe(CN)6]4-/3-containing 500 mmol· L-1KCl solution.The CV measurements were conducted at a scan rate of 0.5 mV·s-1with potential windows ranging from 0.1 to 0.8 V(vs.SCE).Moreover,the effect of scan rate from 0.1 to 0.6 mV·s-1was also conducted under the same conditions.The EIS measurement was conducted at the amplitude of 5 mV and the frequency range from 0.1 to 105Hz.The CA measurement was conducted at the initial potential of 0.5 V.

2.4.Electrochemical oxidation of TFP

During the electrochemical oxidation reaction,the MnOx/Ti electrodes as the anode and the stainless steel wire mesh as the cathode were connected by a DC power to constitute the reactors,which was similar to our previous works.

The reaction rate r(mmol· L-1· min-1)was calculated by Eq.(1)[7].

where CF-TFPis the initial molar concentration of TFP in feed solution(mmol·L-1)and CP-TFPis the molar concentration of TFP in permeate solution(mmol·L-1).

The distance between the anode and cathode was 30 mm.The reactor operating conditions were the TFP concentration of 40 mmol·L-1,reaction temperature of 60 °C and current density of 2.4 mA·cm-2,and the feed solution volume per electrode surface area was 10 ml·cm-2.It was noted thatthe above parameters were derived from the prior work.The calculation formula of RT,current density,current efficiency(CE),conversion of reactant TFP(CTFP)and selectivity of product STFP(SSTFP)can be found in previous work[6].

3.Results and Discussion

3.1.Cyclic voltammetry and electroactive surface area of electrode

It can be seen from Fig.1 that a couple of redox peaks of each curve were observed,indicating the transformation between Fe(III)and Fe(IV)as the redox probe.The peak current density of the membrane electrode(Ipa)was 0.352 mA·cm2at Epa=0.580 V,which was more than three times of that of the plate electrode(0.105 mA·cm2)at Epa=0.468 V.At the same time,the peak potential separation(ΔEp)obtained from the membrane electrodes(130 mV)was higher than that of the plate electrode(119 mV)as shown in Fig.1.Furthermore,it also can be found that the CV curve of the membrane electrode exhibited a more rectangular shape without obvious redox peaks.It indicated that the membrane electrode had a short ion diffusion distance so as to provide fast ion transport pathways[18].This is because the electrical double layer of the membrane electrode can be reorganized rapidly at the switching potentials with short ion diffusion distance,and then the response current reached a steady state,resulting in the rectangular shaped curve[19].

According to the above CVresults,itis interesting to find thata linear correlation between Ipand the square root of scan rate(ν1/2)obtained from both of plate and membrane electrodes in the range of scan rate existed as shown in Fig.2.At the same time,the regression equation can be expressed as follows:

In particular,the electroactive surface area could be calculated by the slopes based on the Randles–Sevcik equation(Eq.(2))[14]:

Fig.1.Cyclic voltammograms of(a)plate electrode and(b)membrane electrode in the[Fe(CN)6]4-/3-and KCl solution with scan rate of 0.5 mV·s-1.

Fig.2.Plotof I pa versusν1/2 atplate electrode and membrane electrode in the[Fe(CN)6]4-/3-and KCl solution.

where n is the number of transfer electron,A is the electroactive surface area,D is the diffusion coefficient of the electroactive molecule at electrode surface,v is the scan rate,and c is the concentration of the probe molecule.For[Fe(CN)6]4-/3-,n=1,D=7.6 × 10-6cm2·s-1.The electroactive surface area of the membrane electrodes was 2.68 cm2,about 4 times higher than that of the plate electrode(0.79 cm2),confirming that the porous structure in the membrane electrode can increase the electroactive surface area of electrode.

3.2.Voltammetric charge of the electrodes

It is reported thatthe integrated charge(q*)ofthe voltammogramis not only related to the real surface area but is also affected by the electroactivity of the sites[20].Hence,q*can be used to measure the electrochemical activity sites of different electrodes coupled with the proton exchange between the oxide and the aqueous electrolyte[21].

According to the study of Trasatti et al.[22],q*decreased as the potential scan rate increases.This is because the diffusion limitation slows the accessibility of protons to the inner surface of the electrode.Thus,as scan rate(ν)approaches∞,the voltammetric charge(q*)tends to the outer voltammetric charge(q*O).Conversely,as the scan rate(ν)approaches 0,the voltammetric charge(q*)trends to the total voltammetric charge(q*T).Herein,the inner voltammetric charge(q*I)is calculated from q*Tand q*Oby subtraction,which is related to the less proton accessible surface sites(such as grain boundaries,crevices,and cracks,etc.).Fig.3 shows the CV of the plate electrode and porous electrode at various scan rates from 0.1 to 0.6 mV·s-1.Clearly,q*is linearly related to ν-1/2,and(q*)-1to ν1/2,respectively.Additionally,the value of q*Ocan be obtained from the extrapolation of q*to ν = ∞according to the plot of q*vs.ν-1/2as shown in Fig.3a.The total voltammetric charge(q*T)consists of q*Oand q*Iobtained from the extrapolation of q*to ν=0,according to the plot of q*-1vs.v1/2(Fig.3b),which is related to the whole active surface sites including the outerand inner active surface sites.Essentially,the value of q*Iis given by the difference between q*Tand q*O.The charge results of the plate electrode and membrane electrode obtained from Fig.3 were listed in Table 1.Clearly,the ratio of outer charge to total charge(q*O/q*T)of the membrane electrode(0.22)was much larger than that of the plate electrode(0.02).Itindicated thatthe porous membrane electrode has much more proton accessible surface sites,which would show the significant performance in electrochemical reaction[23].

Fig.3.Extrapolation of the total voltammetric charge(q*T)(a)and outer voltammetric charge(q*O)(b)for the plate and porous membrane electrodes.

3.3.Electrochemical impedance of the electrode

EIS was employed to further investigate the interface information between the electrode and electrolyte solution as shown in Fig.4.Normally,the NyquistplotsofMnOx/Tielectrodes include two components:a semicircle part at higher frequencies and a linear part at lower frequencies,which correspond to the charge transfer process and the electrolyte diffusion process,respectively[14,24].It can be seen from Fig.4 that a smaller arc was obtained from the membrane electrode.It implied that the membrane electrode has the low charge transfer resistance,leading to a faster electron transfer kinetics for electrocatalytic oxidation[25].

Table 1 Total,outer and inner charges of the plate electrode and membrane electrode

Fig.4.Electrochemicalimpedance spectra(Nyquistplots)and fitting curves(solid lines)of MnO x/Ti plate electrode(a)and membrane electrode(b)and the electric equivalent circuit.

In addition,the electrochemical impedance spectra of both electrodes can be fitted using the Zview software and the corresponding electric equivalent circuit was also presented in Fig.4.The values of the equivalentcircuitcomponents are listed in Table 2.In the equivalent circuit,Rsand Rctrepresent the bulk solution resistance and charge transfer resistance,respectively.Obviously,the equivalent series resistance[26](ESR=Rs+Rct)of the membrane electrode(20.9 Ω·cm2)was much lower than that of the plate electrode(142.5 Ω·cm2)(Table 2).

Further,constantphase element(CPE)describes the double layercapacitance and the passivation film capacitance is represented by CPE1-T(capacity element)and CPE1-P(CPE exponent)[27].The former CPE1-T is related to the surface fractaldimension structure[28].Apparently,the value of CPE obtained from the membrane electrode(8.6 mF·cm-2)was much higher than that obtained from plate electrode(0.1 mF·cm-2)(Table 2).Similarly,the Warburg diffusion resistance consists of W1-R(resistive part),W1-T(inductive part)and W1-P(the exponent).It is easy to find from Table 2 that W1-R(23 Ω·cm2)obtained from the porous electrode at lower frequencies was smaller than that of the plate electrode(147 Ω·cm2).It suggested that the ion diffusion resistance between the porous electrode and electrolyte was lower,which was consistent with the results obtained from voltammetric charge of the electrodes,i.e.,the porous electrode with much more proton accessible surface sites.

Therefore,the EIS analysis proved that the membrane electrode had lower charge transfer resistance and higher ion diffusion rate than the plate electrode,which is coincident with the observations from CV analysis.

3.4.Chronoamperometry and mass transfer behavior of electrode in the reactor

In general,the diffusion efficiency of the plate electrode in ECR or membrane electrode without fluid in ECMR based on the concentration gradient is normally quite low.An increase of the mass transfer and more uniform hydrodynamic distribution along the electrode under the role of pump contribute to the efficiency enhancement of the electrochemical reactor[29].For instance,the enhanced mass transfer allows all reactants effectively to flow through the porous membrane electrode in ECMR directly via the driving force from the pump,leading to a high mass transfer coefficient and good contact between reactants and active sites[7].In order to further gain an insight into the mass transfer behaviors of the electrodes in the electrochemical reactors,CA was carried out(Fig.5).

Table 2 Values of the equivalent circuit elements obtained by fitting the experimental results in the Nyquist diagrams

It can be seen thatthe currentdensity of the porous membrane electrode in ECMR gradually reached a plateau along with the extension of testing time after a rapid decline(Fig.5c–f).Moreover,the steadystate value of the current density obtained in ECMR increased from 0.0046 to 0.0684 mA·cm2with the decrease of RT from 20 to 5 min or increase ofthe flow rate afterthe testing time of1800 s(Fig.5c–f).However,for the plate electrode(Fig.5a),itcan be seen thatthe currentdensity of the plate electrode decreased sharply from approximately 0.1 to 0.01 mA·cm-2in the first 300 s,and then approached zero with an extension of testing time due to the expansion of diffusion layer[8].The similar trend could be found for the porous membrane electrode without any fluid(Fig.5b).

Obviously,the current density of the porous membrane electrode in ECMR was much higher than that of the plate electrode during the testing time of1500 s.Hence,the electrochemicalperformance of the Tiporous electrode in ECMR would be improved effectively because of mass transfer enhancement resulting from the fluid through the membrane under the role of pump during the ECMR operation[15].

Meanwhile,the diffusion coefficient(D)as an important kinetic parameter could be estimated by the current–time relationship for diffusion-limited electrochemical systems.The consequent plots of i vs.t-1/2with the best fits for the different electrodes were derived from the CA curves,as shown in Fig.6.The slopes of fitting lines can be used to calculate D through the Cottrell equation,Eq.(3)[30]:

where n is the number of transferred electrons,D is the diffusion coef ficient(cm2·s-1),c is the redox concentration(mol·L-1),A is the geometric electrode area(cm2),and I is the current density at time t(A).The fitting equation can be expressed as follows:

Fig.5.Chronoamperometry ofMnO x/Ti plate electrode in ECR(a)and MnO x/Timembrane electrode without permeate(b)and the membrane electrode with the RT of 20 min(c),15 min(d),10 min(e)and 5 min(f)in ECMR.

According to the above equations,a series of fitting slopes of i vs.t-1/2and the diffusion coefficients obtained according to Eq.(3)were summarized in Table 3.It can be found that the diffusion coefficient obtained from the porous electrode in ECMR increased from 2.67× 10-6cm2·s-1to 6.89 × 10-6cm2·s-1with the decrease ofresidence time from 20 min to 5 min,indicating that the mass transfer can be improved in the porous membrane electrode in ECMR[31].One point should be noticed that the D value of the porous membrane electrode(1.80 × 10-6cm2·s-1)without any fluid was slightly lower than that of the plate electrode(1.83×10-6cm2·s-1).Actually,the porous electrode can exhibit a larger surface area and more reaction sites and/or shorter diffusion paths.However,it has to face much higher diffusion resistance[32].

3.5.Electrocatalytic oxidation of TFP by ECR/ECMR

In order to investigate further the catalytic performance of the electrocatalytic reactor,the MnOx/Ti electrodes were used as an anode for the TFP oxidation.Meanwhile,the porous membrane electrode constituted an ECMR with the flow rate of 2.2 ml·min-1(RT=5 min)by a peristaltic pump.In addition,the permeate was recycled into the feed tank in order to keep the same volume of reactant solution in the ECR and ECMR.The conversion of TFP and STFP selectivity were showed in Fig.7.The TFP conversion of the different MnOx/Ti electrodes increased rapidly at the initial time of 12.5 h and then gently increased with the extension of the operating time(Fig.7).Obviously,TFP conversion obtained from ECMR(70.1%)was much higher than that obtained from ECR with plate electrode(21.5%)and membrane electrode without fluid(40.8%)with 20 h of operating,demonstrating that the ECMR exhibited an extraordinary catalytic performance towards the electrooxidation of TFP.It can be explained from two aspects.Firstly,the porous electrode with higher electroactive surface area and more proton accessible sites provided its superior electrochemical properties during the ECMR operating.Secondly,the higher diffusion coefficient of ECMR(6.89×10-6cm2·s-1)not only promoted the transfer of reactant from the bulk solution to the electrode surface,butalso accelerated the products to permeate through or leave the porous electrode in ECMR,which would enhance the catalytic performance of ECMR[33].In addition,it was also found that the selectivity to STFP was consistently higher than 99.9%during the operation,indicating the superior selectivity of the MnOx/Ti electrodes towards the electro-oxidation of alcohols.More importantly,the CE of ECMR(87.3%)was much higher than that of without fluid(46.7%)and ECR with plate electrode(28.0%)at the initial time of operating(0.5 h),as shown in Fig.8.Of course,the CE of the reactors decreased with the operating time owning to the decrease of reactant concentration.Hence,the above results further confirmed that ECMR exhibits excellent performance on the electrochemical oxidation of TFP.

Fig.6.Relationship between i and t-1/2 ofthe plate electrode in the ECR(a)and the porous membrane electrode without fluid(b)and the membrane electrode with the RT of20 min(c),15 min(d),10 min(e)and 5 min(f)in ECMR.

Table 3 Fit slope of i vs.t-1/2 and the diffusion coefficients of plate electrode,porous electrode and the membrane electrode in ECMR with different RTs

Fig.7.TFP conversion and STFP selectivity obtained from the reactors with plate electrode(a)and porous membrane electrode without fluid(b)and porous membrane electrode with the flow rate of 2.2 ml·min-1(RT=5 min)(c)at the initial TFP concentration of 40 mmol·L-1.

Fig.8.Currentefficiency obtained from the reactor with plate electrode(a)and membrane electrode(b)and porous electrode in ECMR with the flow rate of 2.2 ml·min-1(RT=5 min)(c)at the initial TFP concentration of 40 mmol·L-1.

In addition,the effect of C0,TFPon the reaction rate of TFP oxidation under different RT was also investigated(Fig.9).It could be found that all reaction rates increased with the increase of TFP initial concentration(C0,TFP)from 10 to 80 mmol·L-1until it reached a plateau.The reaction rate obtained at RT of 5 min increased rapidly from 0 to 2.23 mmol· L-1· min-1with the increase of C0,TFPfrom 0 to 40 mmol·L-1,and then remained stable when further increasing C0,TFPfrom 40 to 80 mmol·L-1,suggesting that the rate determining step of this reaction was mass transfer at the range of C0,TFPfrom 0 to 40 mmol· L-1[7].However,when C0,TFPwas over 40 mmol· L-1,the electrocatalytic reaction rate was no longer limited by the mass transfer but by the electron transfer in the electrochemical oxidation[8,15,33],indicating that the electrocatalytic reaction was controlled by the mass transfer when RT was over 20 min.Specifically,with C0,TFP=40 mmol·L-1,the reaction rates increased from 0.44 to 2.21 mmol· L-1· min-1with the RT decrease from 20 to 5 min,implying that the reaction can proceed in a short RT and a high diffusion coefficient is in favor of the increase of reaction rate.

4.Conclusions

Fig.9.Reaction rates ofTFP oxidation with differentinitialTFP concentrations in the ECMR at different RTs:(a)5 min;(b)10 min;(c)15 min and(d)20 min.

The electrochemical reactors can be constructed with two typical MnOx/Ti electrodes for the electrocatalytic oxidation of the TFP,i.e.ECMR with MnOx/Ti membrane electrode and traditional ECR with MnOx/Ti plate electrode.A series of electrochemical measurements were employed to investigate the electrochemicalperformance and catalytic results ofMnOx/Tielectrodes with Tiplate or porous Timembrane in the ECRand ECMR.The membrane electrode displayed better electrochemicalproperties than plate electrode due to higherelectroactive surface area,lower charge transfer resistance and higher ion diffusion rate.Atthe same time,the membrane electrode in ECMR can display a higher diffusion coefficient than the conventional porous electrode in ECR due to the enhanced mass transfer in ECMR.TFP conversion obtained from ECMR was up to 70.1%,which was much higher than that of plate electrode(21.5%)and porous membrane electrode without fluid(40.8%)with 20 h of operating time.The excellent performance of ECMR is mainly attributed to the synergistic effect of electrochemical oxidation and convection-enhanced mass transfer,which not only provides the superior electrochemical property but also promotes reactant and product transfer so as to enhance the catalytic efficiency.In addition,the reaction rate of TFP electrochemical oxidation can be controlled by mass transfer at the range of C0,TFPfrom 0 to 40 mmol·L-1.When C0,TFPis higher than 40 mmol·L-1,the reaction rate can be limited by the electron transfer in the electrochemical oxidation.Our results can shed more insight into the application of electrochemical reactor and design of porous electrode.