A Direct Liquid Fuel Cell with High Power Density Using Reduced Phosphotungstic Acid as Redox Fuel

Yiyang Liu,Ting Feng,Shanfu Lu*,Haining Wang,and Yan Xiang

Direct liquid fuel cells(DLFCs)are proposed to address the problems of high cost and complex storage and transportation of hydrogen in traditional hydrogen-oxygen proton exchange membrane fuel cells.However,present fuels of organic small molecules used in DLFCs are restricted to problems of sluggish electrochemical kinetics and easily poisoning of precious metal catalysts.Herein,we demonstrate reduced phosphotungstic acid as a liquid fuel for DLFCs based on its advantages of high chemical and electrochemical stability,high electrochemical activity on common carbon material electrodes,and low permeability through proton exchange membranes.The application of phosphotungstic acid fuel effectively solves the problems of high cost of anode catalysts and serious fuel permeation loss in traditional DLFCs.A phosphotungstic acid fuel cell achieves a peak power density of 466 mW cm-2at a cell voltage of 0.42 V and good stability at current densities in the range from 20 to 200 mA cm-2.

Keywords

direct liquid fuel cell,heteropoly acid,phosphotungstic acid,power density,renewable fuel

1.Introduction

Proton electrolyte membrane fuel cells(PEMFCs)have become one of the best candidates for electric vehicles,mobile power sources,and household power supplies[1,2]because of their high power density and low pollution.[3]However,the high cost hinders the application of traditional hydrogen-oxygen PEMFCs.On the one hand,hydrogen energy is secondary energy,which requires considerable energy for the preparations of high purity hydrogen.On the other hand,the storage and transportation of hydrogen fuel need a variety of safety precautions.[4-8]Therefore,some cheap organic liquid fuels such as methanol,[9]ethanol,[10]and formic acid[11]are proposed to substitute for hydrogen by constructing conventional direct liquid fuel cells(DLFCs).[12]However,these organic molecules can easily permeate through a proton exchange membrane(e.g.,Nafion)because of the small molecular radii and strong polarities,which results in the loss of fuel utilization rates and energy efficiencies.[13,14]Also,DLFCs need high loading of anode catalysts due to the sluggish kinetics of these liquid fuels,which results in the high cost.Moreover,the CO produced via incomplete oxidation of some liquid fuels can poison the anode and cathode catalysts.[15]Therefore,the performance of conventional DLFCs is fairly low than that of hydrogen-oxygen PEMFCs as so far.

Soluble redox couples are a kind of energy storage materials used generally in redox flow batteries.These redox couples generally have a relatively higher homogeneous kinetics rate,which enables the electrode reactions to be independent of the precious metal catalyst.Furthermore,redox couples can be recycled between charging and discharging states.If the electric energy for producing high purity hydrogen is used to offline charge these redox couples,it is expected to both solve the problems of storage and transportation of hydrogen and the high demand for precious metal catalysts.The schematic diagram of the redox fuel cells is shown in Scheme 1.So far,inorganic redox couples of V2+/V3+[16-21]and aqueous organic redox couples of methyl viologen,[22]quinoxaline,[23]and anthraquinone[24]have been used as redox fuels.However,some of the inherent properties of these redox couples make it difficult for their fuel cells to achieve high output performance.For instance,vanadium ions,which have a small radius and positive charge,can easily pass through commonly used proton exchange membranes,which will result in fuel loss and adverse electrochemical reactions at cathodes.[16,17,20]Furthermore,vanadium ions have a low kinetics rate on common carbon material electrodes,which restricts the maximum output power density of the fuel cells to be 42 mW cm-2.[16]For aqueous organic redox couples,the chemical and electrochemical stability are important factors that impact the long-term performance of fuel cells,especially in highly acidic or strong oxidation environments.[25-27]There is also a risk that decomposition products of organic couples can permeate through the proton exchange membranes and poison the cathode catalysts.

Scheme 1.The working principle of redox fuel cells.

Therefore,an appropriate redox couple combining high electrochemical activity,low ion permeability,and high stability is at the core to addressing the above dilemma.Heteropoly acids(HPA)are a type of efficient redox couples in electrochemical energy storage devices.[28-32]We propose that HPA is a safe,efficient,and sustainable fuel for DLFCs because of their favorable properties.Firstly,HPAs often contain a highly delocalized conjugated structures,[33-36]which facilitates a fast electron transfer[28]and enables high power density of fuel cells without anode noble metal catalysts.Secondly,HPAs often have a relatively large size and high negative charges.[37]Thus,it is difficult for HPAs to cross the cation exchange membranes because of Donnan’s effect,which reduces the loss of fuel and energy efficiency.Furthermore,some HPAs are chemically stable,and the reduction of HPAs is often coupled to protons transfer,[38]which avoids highly charged anions and results in high electrochemical stability.[35]The stability of HPAs is beneficial for safe storage and transport,as well as the recycling as a sustainable fuel.Finally,some HPAs can reversibly store and release multiple electrons,[39]which contribute to maintaining the energy density of the fuel cells.[40,41]Based on the above properties of HPA,keggin-type silicotungstic acid(H4[SiW12O40])was applied as early as the 1980s in PEMFCs as redox active materials.[42,43]In 2017,keggin-type phosphomolybdate(H3[PMo12O40])was used as redox active materials for biofuel cells.[44-47]However,there were huge gaps between the power density of these fuel cells and that of traditional PEMFCs,which limited their popularization and application.

Herein,12-phosphotungstic acid(PTA),which is a commercial keggin-type HPA,is selected as a redox fuel.PTA has been used as an active material in RFBs because its advantages of low ions permeation,high electrochemical activity,and high stability.[29]Herein,we demonstrate reduced phosphotungstic acid as a liquid fuel for DLFCs.Compared with previous DLFCs,PTA fuel cell exhibits slight fuel penetration loss and higher output performance without any anodic catalysts.It achieves a high peak power density of 466 mW cm-2and good stability at current densities in the range from 20 to 200 mA cm-2.

2.Results and Discussion

2.1.Characteristics of PTA

Cyclic voltammetry tests are performed on 5 mMPTA+1MHClO4solution at different scan rates under an argon atmosphere to investigate the electrochemical characteristics of PTA.As seen in Figure 1a,PTA exhibited three pairs of reversible redox peaks,and these correspond to one two-electron reaction and two one-electron reactions.[29]The equilibrium potentials that correspond to the three redox reactions are calculated to be-0.36 V,-0.03 V,and 0.21 V versus SHE,respectively.The peak potential differences for the three pairs of peaks are 43,63,and 66 mV,respectively.These do not show any significant changes when various scan rates are used,and values of the peak potential differences for the two one-electron reactions are close to the theoretical value of the reversible process(59 mV).Furthermore,the previous study proved that the kinetic reduction rate constant of PTA is high(0.015 cm s-1,Figure S1,Supporting Information),and this is much higher than the value of V2+/V3+(k0=2.2×10-5cm s-1).[48]The high redox reversibility eliminated the need to use a catalyst for PTA to undergo oxidation and reduction.Figure 1b showed the charging and discharging curves of PTA with different electrochemical processes.Oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)reaction occurred on two counter electrodes(IrO2modified carbon felt electrode and GDE),respectively.When the PTA only underwent two one-electrode reduce reactions,the charge and discharge processes are reversible.However,when trying to further reduce PTA,no corresponding discharge platform is found.This is because that the two-electron reduce reaction of PTA had a competitive relationship with a side reaction of hydrogen evolution at low potential.In a highly acidic environment,the two-electron redox reaction of PTA is almost completely replaced by the hydrogen evolution side reaction.This phenomenon is similar to our previous research on PTA RFB.[29]Therefore,in this work,the charging cutoff voltage of PTA fuel is limited to 2.0 V at 100 mA cm-2,only two one-electron reactions of PTA are used as the anode reaction of the PTA fuel cell,and the electrode reactions of PTA[35,49]and total electrochemical reaction of the cell can be written as follows:

Figure 1.a)CV of 5 mMPTA in 1MHClO4solution under various scan rates.b)Charging and discharging curves of PTA with different electrochemical processes.OER and ORR reaction occurred on the counter electrodes,respectively.The charging and discharging current density are 100 mA cm-2.c)The ion permeation of PTA,methanol and VO2+through Nafion?211 membrane.d)The ORR curve in 0.1M HClO4with different additives.The catalyst is 40% Pt/C(40 wt% Pt on XC-72).Addition amount is 5 mM.

Also,PTA has good chemical and electrochemical stability and low permeation.[29]As seen in Figure 1c,the permeability of PTA through a Nafion211 membrane is only 1.96×10-10cm2min-1(Figure S2,Supporting Information),which is 3-4 orders of magnitude smaller than that of VO2+[50]and methanol.[51]The lower permeability contributed to a slower self-discharge rate and higher fuel efficiency for the fuel cell and decreased the risk of catalyst poisoning at the cathode.Moreover,given the sensitive nature of the ORR catalyst(40 wt% Pt on XC-72),the effects of how different additives influence ORR are also investigated.As seen in Figure 1d,using PTA had little impact on oxygen reduction behavior,whereas using CH3OH and VOSO4both resulted in dramatically reduced ORR potential.

2.2.Fuel Cell Performance

To verify the feasibility of using the PTA fuel cell and to optimize its performance,A PTA fuel cell is assembled and tested under different operating conditions.The polarization curves and power curves of the fuel cell with 0.2Mre-PTA at different operating temperatures are shown in Figure 2a.The fuel cell can operate from 30 to 60°C and the peak power density of the PTA fuel cell increased from 125to222 mW cm-2with the increasing temperature. Obviously, the increased operate temperature significantly accelerated the electrode reactions and decreased the internal resistance of the fuel cell.Figure 2b shows polarization plots and power curves of the PTA fuel cell with 0.2M re-PTA at different O2feed rates.When the current density is lower than 300 mA cm-2,the O2feed rate had little impact on the cell power density.Even when the current density is high,only low O2feed rates(0.01 and 0.05 L min-1)had a slight impact on the cell power density.This can be attributed to the high ORR rate.Also,the minimum O2feed rate of our instrument(0.01 L min-1)already met the theoretical demand for the electrode reaction.Figure 2c shows polarization plots and power curves of the PTA fuel cell with 0.2Mre-PTA at different fuel feed rates.With an increase in the fuel feed flow rate,the peak power density of the fuel cell gradually increased.The maximum peak power density reached 163 mW cm-2at a flow rate of 40 mL min-1.The higher flow rate intensified the convection at the reaction interface,and this decreased the thickness of the reaction diffusion layer and then decreased the mass transfer resistance of the anode reaction.Meanwhile,the higher fuel feed flow rate increased the re-PTA concentration at the reaction interface per unit time,and this helped alleviate the mass transfer problem that occurs at a high current density.Therefore,the performance of the PTA fuel cell gradually improved,particularly at a high current density.The performance of the fuel cells at different concentrations of re-PTA is shown in Figure 2d.The peak power density of the cell increases with an increase in the re-PTA concentration;specifically,it increased from 84 mW cm-2with 0.1Mre-PTA to 327 mW cm-2with 0.6Mre-PTA.On one hand,with an increase in the re-PTA concentration,the concentration of the anode reactant increased,and this decreased the mass transfer resistance of the anode reaction.On the other hand,with an increase in the proton concentration,the internal resistance of the fuel cell decreased.Furthermore,at the same fuel feed flow rate,when the concentration of re-PTA is higher,the feed rate of re-PTA is higher,and this helped alleviate the mass transfer problem that occurs at a high current density.

Figure 2.The optimization of PTA fuel cell performance.a)Cell polarization plots and power density curves versus current density at different operate temperature.Fuel feed flow rate is 20 mL min-1,and O2 feed flow rate is 0.1 L·min-1.b)Cell polarization plots and power density curves versus current density of different O2feed flow rates.Fuel feed flow rate is 20 mL min-1.c)Cell polarization plots and power density curves versus current density of different fuel feed flow rates.O2feed flow rate is 0.1 L min-1.d)Cell polarization plots and power density curves versus current density of different re-PTA concentrations.Fuel feed flow rate is 20 mL min-1,and O2feed flow rate is 0.1 L min-1.e)The optimized PTA fuel cell.Operating temperature is 60°C,fuel feed flow rate is 40 mL min-1,re-PTA concentration is 0.6M,and O2 feed flow rate is 0.2 L min-1.f)The polarization of re-PTA oxidizing reaction and ORR versus current density from(e).

The above results indicate that increases in the operating temperature,fuel feed flow rate,and re-PTA concentration all significantly improve the performance of the PTA fuel cell.The performance of optimized PTA fuel cell is shown in Figure 2e.The operating temperature is 60°C,fuel feed flow rate is 40 mL min-1,re-PTA concentration is 0.6M,and O2feed flow rate is 0.1 L min-1.The PTA fuel cell achieve a peak power density of 466 mW cm-2at a cell voltage of 0.42 V.To further investigate the impact condition of PTA fuel cell performance,the polarization of re-PTA oxidizing reaction and ORR are performed via H2GDE.The ohmic polarization caused by the internal resistance is also taken into consideration.The internal resistance of the fuel cell is 240 mΩ·cm2.The results show that the electrochemical polarization of ORR mainly affects the fuel cell voltage at low current density,while the fuel cell voltage is mainly limited by the ohmic polarization at high current density.It is worth noting that the concentration polarization of the re-PTA oxidation reaction at higher current density is not the major limiting factor of the fuel cell voltage after operating condition optimization.Therefore,the farther improvement of PTA fuel cell performance should start from reducing the internal resistance.For comparison,Table 1 shows the parameters and output performance of representative liquid fuel cells at a cell voltage of 0.4 V.The output power density of the PTA fuel cell stands out from the other recently reported liquid fuel cells.

Table 1.The parameters and output performance of representative liquid fuel cells.

2.3.Recycle of PTA Fuel

The stability and electrochemical stability are of most important for the renewability of redox fuel.Although in our previous work,we have already proved the electrochemical stability of PTA via a PTA-I2RFB during 700 cycles in 280 h,[29]to intuitively verify the renewability of the PTA fuel,the charge and discharge cycles of PTA fuel paired with O2/H2O couple is performed.The schematic of the device operation has been shown in above(Figure 3a).During the charging process,the PTA reduction reaction occurred on the carbon felt electrode of the cell on the left,and the OER occurred on the IrO2modified carbon felt electrode.During the discharge process,the re-PTA oxidation reaction occurred on the carbon felt electrode of the cell on the right,and the ORR occurred on GDE with Pt/C catalyst.The detail charge/discharge cycle performance is shown in Figure S4,Supporting Information.Figure 3b shows the typical charge/discharge curves of PTA coupled with OER and hydrogen oxidation reaction(HOR),respectively.The coulombic efficiency of PTA-OER is approximate to 95% ,while the coulombic efficiency of PTA-HOR is approach 100% .This is probably because the oxygen can permeate through the thin Nafion211 membrane,which oxidized the re-PTA.Furthermore,the high electrode potential and high electrochemical polarization of OER lead to high energy consumption of the PTA regeneration.When HOR is substituted for OER,the electricity consumption is greatly reduced.However,hydrogen also has a high cost.Therefore,biomass with suitable electrode potential should be sought for reduction of PTA in further work.To further investigate the stability of the PTA fuel,a PTA fuel cell with 0.2Mre-PTA is tested at various current densities for 240 min.Because of the limited amount of re-PTA,0.2Mre-PTA solution is used as fuel to keep the cell continuous discharging.The cell was stopped after each discharge test and then restarted after regenerating the re-PTA solution,which caused the periodic fluctuation of discharge curves.UV-vis spectrum of PTA before first charged and after stability test are shown in Figure 3c,the concentration of PTA solution changed slightly.IR spectrums are also performed to confirm the structural stability of PTA during charge and discharge.As shown in Figure S3,Supporting Information,four obvious characteristic peaks of α-Keggin-type heteropoly acid which corresponding νP-O, νW-Oand two νW-O-W,respectively,were achieved between 700 and 1100 cm-1.After stability test,none of the four characteristic peaks changed significantly.The discharge curves of PTA fuel cell are shown in Figure 3d,and the cell voltage first rapidly increase and then substantially stabilize at about 0.82V at a current density of 20 mA cm-2.This is because when the fuel cell is started,the ORR catalyst is gradually activated,and the cell performance increases substantially.When the current density steps to 200 mA cm-2,the cell voltage slowly decreases.The average discharge current density of the cell remained around 0.60 V,and the average output power density is 120 mW cm-2.This is because at large discharge current densities,re-PTA is quickly consumed because of the limited concentration and pump flow rate.Thus,the concentration of reactant at the reactive interface decrease gradually,and this caused a slow decrease in the cell voltage.Therefore,after the discharge current density is reduced to 150 mA cm-2,the discharge curve of the cell gradually stabilizes.Then,the discharge current density is gradually reduced to 50 mA cm-2and increased back to 200 mA cm-2for retesting.The results show that when the discharge current density is returned to 200 mA cm-2,the cell voltage remain stable above 0.63 V,and the performance is slightly higher than the initial level.Moreover,when the current density plummets again to 20 mA cm-2,the discharge voltage get back above 0.81 V and almost no attenuated during discharge.All the tests indicate that the PTA fuel cell have stable operation.

Figure 3.The recycle stability of 0.2MPTA fuel cell.a)Schematic of the PTA fuel cell(right)and the regeneration device of re-PTA(left).CF is short for carbon felt.b)The charge/discharge curves of PTA couple with H+/H2and O2/H2O.c)UV-vis spectrum of PTA before and after the recycle stability test.d)The discharge stability of the PTA fuel cell with 0.2 M PTA concentration at different current densities.O2feed flow rate is 0.1 L min-1.The periodic fluctuation of the discharge curve is due to the replacement of fresh electrolyte.

3.Conclusion

In this work,a highly active,redox active material with low-permeability,PTA,is selected as a fuel,which effectively solves the general problems associated with the high cost of an anode catalyst and the fuel permeation loss in traditional fuel cells.The constructed PTA fuel cell is safe,efficient,and sustainable.The result indicates that the re-PTA concentrations,fuel feed flow rate,and operating temperature all significantly affect the performance of the PTA fuel cell.By optimizing the operating conditions,the PTA fuel cell reaches a maximum peak power density of 466 mW cm-2at 0.42 V.Additionally,the discharge stability of the PTA fuel cell is tested with 0.2Mre-PTA,and it works steadily at a discharge current density ranging from 20 to 200 mA cm-2.In general,the PTA fuel cell proves that using HPAs as fuel cell fuel has research potential because of its excellent battery performance,and this provides a new idea for research regarding liquid fuel cells.

4.Experimental Section

Chemicals and Materials:The phosphotungstic acid(AR)is purchased from Macklin (China).All chemicals are put to use directly without purification.The Nafion?211 membrane(DuPont,USA)is used as a polymer electrolyte membrane.The carbon paper(TORAY,Japan)is used as the main part of cathode electrode.The carbon felt(Dalian Longtian Tech,China)is used as the anode electrode.All of the solutions are prepared with deionized water.

The Electrochemical Measurement of PTA:The electrochemical property of PTA is performed via cyclic voltammetry(CV)and linear sweep voltammetry(LSV)by a VersaSTAT4 Electrochemical Workstation(AMETEK,USA)in a threeelectrode system with a glassy carbon work electrode,a platinum wire counter electrode,and a saturated calomel reference electrode.All the electrode potentials tested with this system have been converted to the standard hydrogen electrode potential.The diffusion coefficient and kinetic reduction rate constants of PTA are measured via a rotating disk electrode system(RDE,5908Triangle Drive,USA Raleigh).All the electrochemical measurements are performed in the N2atmosphere at 25.0 °C ± 1.0 °C.

Ion Permeability:Ion permeability is determined by measuring the diffusion of PTA in a device with two symmetric reservoirs,divided by Nafion211 membrane.The PTA solution is in one side reservoir,and the H3PO4is in the other side.An appropriate amount of sucrose is added in the H3PO4side to ensure the balance of the liquid level on both sides.Samples of the solution from the H3PO4side are taken out at a regular time interval and used to measure the concentration of PTA by UV-vis spectroscopy.The standard curve is shown in Figure S2,Supporting Information.

Fuel Cell Characterization:The fuel cell is assembled with the reduced state PTA(Re-PTA)used as the anode active material and O2as the cathode active material.Besides,the Nafion?211 membrane is used as polymer electrolyte membrane.Moreover,the carbon felt(2 cm×2 cm×0.3 cm,Dalian Longtian Tech,China)without pretreatment are used as the anode electrode of the cell,and the gas diffusion electrode(GDE)is the carbon paper coated with 0.5-0.6 mg cm-2Pt/C(40 wt% Pt on XC-72;Premetek Co.,USA).The concentration of PTA is measured by a Cintra10e UV spectroscopy(GBC,AUS)in the wavelength range 200-400 nm.

The Recycle Device of the PTA Fuel:A IrO2modified carbon felt is prepared to catalyze the oxygen evolution reaction(OER)via thermal decomposition method.The detailed method is as follows.The carbon felt(2 cm×2 cm)is activated in 400°C and air atmosphere for 18 h and is taken out for use.The activated carbon felt is immersed in 5.5 mL 43.69 mg(NH4)2IrCl6aqueous solution.Take out the carbon felt,dry it in a vacuum drying oven at 60°C for 30 min,and then calcine it at 450°C for 15 min.The soaking/drying/calcination procedure is repeated twice,and the third calcination time is changed to 1 h to fully convert(NH4)2IrCl6to IrO2.The reaction principles and SEM,EDS,and XRD spectrum of IrO2modified carbon felt is shown in Figure S3,Supporting Information.The fuel recycle device is assembled as a RFB.The nafion membrane is sandwiched between one blank carbon felt and one IrO2modified carbon felt,and the graphite felt electrodes are inserted into two square rigid plastics frames(2 mm thickness),and then,the carbon flowfields(Shanghai Hongjun,China)are assembled in both sides,the area of contact between the carbon flowfields,plastics frames,and the membrane are sealed by rubber gaskets as thickness as 0.2 mm.The reactor is then fastened by four double-screw bolts,and its active area is 4 cm2.PTA solutions are pumped through the half-batteries of blank carbon felt.The cell characterizations are measured using a Neware 5V3A battery test system(Neware,Shenzhen,China).

All the experiments are performed under constant temperature of 25°C unless otherwise indicated.

Acknowledgments

Y.L and T.F contributed equally to this work.This research is financially supported by the National Key R&D Program of China(No.2018YFB1502303),the National Natural Science Foundation of China(No.21722601,U19A2017),and China Postdoctoral Science Foundation(No.2019M660389).

Conflict of interest

The authors declare no conflict of interest.

Nomenclature

Section references k0 Standard heterogeneous rate constant Symbol&abbreviation Meaning Usual units cm·s-1 2.1 φ0 Standard electrode potential V 2.1 E0 Electromotive force of a reaction V 2.1 T Temperature °C 2.2 P Power density mW cm-2 2.2 OCV Open circuit voltage V 2.2 PEMFC Proton electrolyte membrane fuel cell-1 DLFC Direct liquid fuel cell - 1 HPA Heteropoly acid - 1,3 PTA 12-phosphotungstic acid - 1,2,3,4 OER Oxygen evolution reaction - 2.1,2.3,4 ORR Oxygen reduction reaction - 2 GDE Gas diffusion electrode - 2,4 HOR Hydrogen oxidation reaction - 2.3 Re-PTA Reduced state PTA - 2.2,2.3,3,4 CV Cyclic voltammetry - 2.1,4 LSV Linear sweep voltammetry - 4 SEM Scanning electron microscope - 4 EDS Energy dispersive X-ray analysis - 4 XRD X-Ray diffraction - 4 IR spectrum Infrared spectrum - 2.3 UV-vis spectrum Ultraviolet visible spectrum - 2.3,4

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.