The effect of alkalinity and temperature on the performance of lithium-air fuel cell with hybrid ele
Journal of Power Sources 196 (2011) 5611–5616
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Journal of Power
Sources
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j p o w s o u
r
The effect of alkalinity and temperature on the performance of lithium-air fuel cell with hybrid electrolytes
Ping He,Yonggang Wang,Haoshen Zhou ?
Energy Technology Research Institute,National Institute of Advanced Industrial Science and Technology (AIST),Umezono 1-1-1,Tsukuba,305-8568,Japan
a r t i c l e i n f o Article history:
Received 27October 2010
Received in revised form 21February 2011Accepted 22February 2011
Available online 26 February 2011Keywords:
Lithium-air fuel cell
Lithium super ionic conductor glass Electrochemical performance Galvanistatic measurement
Electrochemical impedance spectra
a b s t r a c t
A lithium-air fuel cell combined an air cathode in aqueous electrolyte with a metallic lithium anode in organic electrolyte can continuously reduce O 2to provide capacity.Herein,the performance of this hybrid electrolyte based lithium-air fuel cell under the mixed control of alkalinity and temperature have been investigated by means of galvanistatic measurement and the analysis of electrochemical impedance spectra.Electromotive force and inner resistance of the cell decrease with the increase of LiOH concentra-tion in aqueous electrolyte.The values ranged from 0.5to 1.0M could be the suitable parameters for the LiOH concentration of aqueous electrolyte.Environment temperature exhibited a signi?cant in?uence on the performance of lithium-air fuel cell.The lithium-air fuel cell can provide a larger power at elevated temperature due to the decrease of all resistance of elements.
? 2011 Elsevier B.V. All rights reserved.
1.Introduction
Metal-air batteries,such as zinc-air batteries,Mg-air batteries and Al-air batteries,have been extensively studied for a long period [1–4].The essential advantage of this system is to use inexhaustible oxygen in air as reagent,rather than carry the necessary chemicals around inside the battery,which results in a high energy density.Metallic lithium is the most negative metal while at the same time possessing an ultrahigh capacity of 3860mAh g ?1,thus facilitating the design for higher energy density.The ?rst lithium-air battery with organic electrolyte was introduced in 1996,it had attracted extensive interests due to its superior energy density,which is orig-inated from large free energy for the reaction of metallic lithium with oxygen [5].At a nominal potential of about 3V,the theoreti-cal speci?c energy of the Li-air battery is around 3500Wh kg ?1for the reaction forming Li 2O 2(2Li +O 2?Li 2O 2).However,this kind of lithium-air battery is suffered from the diffusion of oxygen,carbon dioxide and water through the electrolyte and their subsequent reaction with metallic lithium.Furthermore,the discharge prod-uct Li 2O 2is not soluble in organic electrolyte,and clogs porous air electrode gradually,which deteriorates the battery performance [5–10].
Recently,a hybrid electrolyte,by uniting air cathode in aque-ous electrolyte and a metallic lithium anode in organic electrolyte with a water-stable lithium super ionic conductor glass (LISICON)
?Corresponding author.Tel.:+81298615795;fax:+81298615799.E-mail address:hs.zhou@aist.go.jp (H.Zhou).plate,was proposed to circumvent the problems of the Li-air battery only using organic electrolyte [10,11].According to our previous study,the preferred direction of the lithium-air system should be regarded as a fuel cell rather than a rechargeable battery [10,12].Subsequently,to protect the LISICON plate from being destroyed in strong alkaline electrolyte,a recycle aqueous electrolyte system had also been developed [12].The O 2reduction in aqueous solution differs from that in organic solution.OH ?,the O 2reduction prod-uct,can be soluble in aqueous solution instead of being obstacle in porous air electrode in organic electrolyte.This lithium-air fuel cell can provide a high theoretical energy density of 5698W kg ?1,which is higher than that of the rechargeable Li-air battery with organic electrolyte (3500Wh kg ?1)[10].The electrodes reactions within this lithium-air fuel cell can be described as followed similar to our previous work:Cathodereaction :(1/4)O 2+(1/2)H 2O +e ?→OH ?(R1)Anodereaction :Li →Li ++e ?
(R2)Batteryreaction :
Li +(1/4)O 2+(1/2)H 2O →Li ++OH ?
(R3)
During the discharge process,O 2from air continuously diffuses into the porous air catalytic electrode where electrocatalytic reduc-tion reaction takes place.Simultaneously,Li +,the oxidation product of lithium metal in organic electrolyte,diffuses across the LISICON plate to form the product LiOH.Considering the reaction (R3),the OH ?concentration can affect the energy and power of lithium-air fuel cell directly through following three aspects:(i)the thermo-dynamic potential of oxygen reduction;(ii)the catalytic activity of air catalytic electrode;(iii)solution conductivity.On the other
0378-7753/$–see front matter ? 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.jpowsour.2011.02.071
5612P.He et al./Journal of Power Sources
196 (2011) 5611–5616
Fig.1.A schematic representation of the lithium-air fuel cell with hybrid electrolyte. hand,environment temperature also has a signi?cant impact on the cell system.However,there have been few systematic studies and a lack of understanding on the performance of this lithium-air fuel cell in?uenced by the alkalinity and temperature.Herein, we systematically studied for the?rst time the effects of alkalinity and temperature on the performance of lithium-air fuel cell with a structure of Li|organic electrolyte|LISICON plate|LiOH aqueous electrolyte|Mn3O4/C catalytic electrode by means of electrochem-istry.
2.Experimental
The preparation of Mn3O4/activated carbon composite used as catalytic electrode is given in previous report[13].The catalyst material contains Mn3O4(25wt.%)and active carbon(75wt.%).The air catalytic electrode includes a catalyst layer and a gas diffusion layer.The prepared Mn3O4/carbon composite(85wt.%)and polyte-tra?uoroethylene(PTFE)(15wt.%)were well mixed,and then was roller-pressed into a sheet to form catalytic layer.The gas diffusion layer was prepared by mixing acetylene black(60wt.%)and PTFE emulsion(40wt.%)with isopropanol into paste and then rolling the paste into a?lm.The air catalytic electrode was?nished by pressing the catalyst layer and gas diffusion layer onto a nickel mesh.The area of the air electrode was1cm2,and the mass load of catalyst layer was4.5mg cm?2.
The structure of developed lithium-air fuel cell was shown in Fig.1and can be described as:(1)the organic electrolyte(1M LiClO4in ethylene carbonate/dimethyl carbonate)and LiOH aque-ous electrolyte are separated by a LISICON plate.(2)The organic electrolyte is just a thin liquid layer(or electrolyte adsorbed by porous membrane)that is used to separate wet metallic Li-anode and LISICON plate.(3)The air catalytic electrode combining catalyst layer with the gas diffusion layer is located between aqueous elec-trolyte solution and air atmosphere.The area of Li anode is about 2cm2.The distance between Li anode and air electrode is about 4cm and the cell contains5ml aqueous electrolytes.The catalyst layer contacts the aqueous electrolyte while the gas diffusion layer faces the air atmosphere.The used LISICON plate is provided by Ohara Inc.,Japan.The thickness of LISICON plate is150?m.The electrical conductivity of the LISICON plate is about4×10?4S cm?1 at room temperature.
Electrochemical tests were performed using a Solartron1287 Electrochemical Interface and a Solartron1255B Frequency Response Analyzer,and controlled by Corrware and Z-plot from a PC.For the electrochemical impedance spectra(EIS)test metallic lithium was used as the counter and reference electrode.The fre-quency limits were typically set between10kHz and0.01Hz.The AC oscillation was10mV.The obtained data of EIS were?tted from equivalent circuit using Zview2.70software.
3.Results and discussion
3.1.The performance of Li-air fuel cell in various alkalinity aqueous electrolytes
The Li-air fuel cell with acidic aqueous solution has a higher operating voltage comparing with that in neutral and basic elec-trolyte,which results in a larger energy density.However,only expensive noble metal such as Pt and Pd can be used as catalytic electrode in this acidic environment.So,the acidic electrolyte based Li-air fuel cell is hardly practical for industrial generalization until the substitution of noble metal has been developed.In our experi-ment,a Mn3O4/activated carbon composite material was selected as catalyst in LiOH alkaline electrolyte based Li-air fuel cell due to its low cost and high activity.
The concentration of LiOH solution ranged from0.01to 2.0mol L?1.Fig.2gives a series of relations between operating voltage(power density)and applied current density in various con-centration of LiOH aqueous electrolyte.As seen in Fig.2,with the growth of applied current densities,linear decrease of operating voltage is clearly observed.For a battery system,the decrease in operating voltage with increasing current density is considered to be caused by a combination of kinetic,transport,and ohmic limi-tations in system.The drop from the kinetic limitation is relatively small compared to the other drops in the system and hence any error in this would be negligible[14].In our case,the electrochem-ical reactions((R1)and(R2))take place at the interface between solid electrode and liquid electrolyte instead of a intercalation reac-tion.Thus it is not necessary to consider the transport limitation in the solid phase.We are still left with analyzing the ohmic limita-tion for the decrease in the operating voltage.The ohmic limitation is mainly consisting of solution resistance(the resistance of liquid solution and LISICON plate),contact resistance(resistance between the current collector and the porous electrode)and matrix resis-tance(resistance between the current collector end of the porous electrode and electrode/electrolyte interface through the matrix phase)[14].As we know,the operating voltage(U)can be expressed as followed formula:
U=E?I×R(1) where E is the electromotive force which is related with the chem-ical component of active electrodes and electrolyte,I is the applied current,R is the inner resistance of the fuel cell and mainly con-sisting of the solution resistance,contact resistance and matrix resistance.As seen from this formula,the operating voltage lin-ear decrease with increase of applied current,which is consistent with the experimental phenomenon displayed in Fig.2.The inner resistance of fuel cell was estimated from the decline rate of linear curves.The electromotive force(E)obtained from the OCV of Li-air batter and inner resistance(R)with various LiOH concentration and environmental temperature were summarized in Table1.Accord-ing to the data displayed in Table1,E decreases with increase of concentration of LiOH in aqueous electrolyte.The OCV potential of this Li-air hybrid cell can be relative to the reversible poten-
P.He et al./Journal of Power Sources 196 (2011) 5611–5616
5613
Pow wer density (W k 0
2
4
6
8
10
2
46810
12
2
46810
12Discharge current (mA)
Discharge current (mA)
kg -1)
Discharge current (mA)
Power density (W W kg -1)
V o l t a g e (V )
V o l t a g e (V )
Discharge current (mA)
Discharge current (mA)Discharge current (mA)
Fig.2.The relations between operating voltage (power performance)and applied current density in various concentration of LiOH aqueous electrolyte.
Table 1
Electromotive force (E )and inner resistance (R ),with various LiOH concentration and environmental temperature.
Temperature 25?C
40?C 55?C LiOH concentration (M)0.010.10.20.51211E (V) 3.46 3.30 3.25 3.20 3.19 3.16 3.21 3.23R ( )
4390
525
278
204
200
185
165
148
tial of OH ?/O 2,which obeys the Nernst equation.So OCV potential decreases with increase of OH ?concentration in our experimental.The inner resistance (R )also decreases with increase of concentra-tion of LiOH in aqueous electrolyte.The larger inner resistance of cell with the LiOH concentration lower than 0.1M is mainly due to the low conductance of aqueous electrolyte.It is suggested that the LiOH concentration has the signi?cant impact on both the electro-motive force and inner resistance.This phenomenon is reasonable because that the OH ?ion take a role of not only the reaction product (see (R1))but also the supporting electrolyte.
As we know,the output power (P )of the cell can be expressed as followed:
P =I ×U =I ×(E ?I ×R )=?I 2R +I ×E (2)P max =
E 2R
(3)
where de?nitions of I ,r ,U and E are same as that of formula (1).Derived from formula (2),the output power (P )should have a max-imum value at the applied current of E /2R .In other words,with the increase of applied current densities the power can increase and then decrease.Apparently,the maximum power depends on the LiOH concentration in aqueous electrolyte.Fig.3gives the mea-sured maximum power density of Li-air fuel cell in various LiOH concentrations.This calculation of power density is based on the total mass of catalytic electrode in order to facilitate comparison,
which was adopted in all previous reports [10,12].Herein,the cal-culation method of power density does not affect the results of our discussion.As seen in Fig.3,the maximum power density increase little with the LiOH concentration larger than 0.5M.Thus increas-ing the LiOH concentration above 0.5M is of little effective
for
1500
k g
-1
1000
n s i t y /W 500
o w e r d e 0
M a x P [LiOH]
Fig.3.The variation of maximum power density with different LiOH concentrations.
5614P.He et al./Journal of Power Sources
196 (2011) 5611–5616
Powe V o l t a g e (V )
r (W/kg)
Discharge current (mA)
Power (W/kg)
V o l t a g e (V )
Discharge current (mA)
Discharge current (mA)
Fig.4.The variation of voltage and power density with applied current at different temperatures.
enhancing the power.Considering the cost and the weight of cell,the suitable LiOH concentration of aqueous electrolyte ranged from 0.5to 1.0M.
3.2.Effect of temperature on the performance of Li-air fuel cell Now,let us consider the effect of environment temperature on the performance of Li-air fuel cell.Fig.4illustrates the variation of voltage and power density with applied current at different tem-peratures.It was found that the Li-air fuel cell reached a maximum power density of 1166,1360and 1556W kg ?1at 25,40and 55?C,respectively.As the temperature elevated the maximum power density increases in value.The maximum power density varies with temperature due to the change of the electromotive force and the inner resistance,which can be derived from the formula (3).The electromotive force (E )of Li-air fuel cell that was measured from OCV shows little changes at various temperatures.There-fore,the increase of the maximum power density with elevating environmental temperature is mainly attributed to the decrease of inner resistance that can be estimated from the slope of linear rela-tion between voltages and applied current.The inner resistances at higher temperatures were also displayed in Table 1.It is sug-gested that the inner resistance of cell decreases with an increase of temperature.Thus the Li-air fuel cell can provide a larger power at higher temperature,although the operating temperature should not be too high to evaporate the electrolyte.
3.3.Electrochemical impedance spectroscopy of Li-air batteries As discussed above,it is con?rmed that the variation of power performance as the function of alkalinity and environmental tem-perature was mainly attributed to the inner resistance of the lithium-air fuel cell.Accordingly it seems important to analyze the detailed elements of inner resistance that are in?uenced by alkalin-ity and temperature.AC electrochemical impedance spectroscopy (EIS)technique has proven to be reliable and powerful tool for investigation of kinetic processes in cell system.Herein,impedance measurement was carried out with a structure of Li |organic electrolyte |LISICON plate |LiOH aqueous electrolyte |Mn 3O 4/C cat-alytic electrode.Nyquist plots of the Li-air fuel cell with air catalytic electrode in various concentrations LiOH are shown in Fig.5.The impedance spectra were analyzed using equivalent circuits with resistive (R i )/capacitive (C i )combination and the respective circuit elements were deduced by ?tting the experimental data points with the circuit shown in Fig.6.R e represents the resis-tance attributed to the electrolyte and cell components (including bulk resistance of LISICON plate and porous electrode etc.);the semicircle in high frequency can be ascribed to grain boundary resistance in the LISICON and air porous electrode (denoted as R gb );the depressed semicircle in high-intermediate frequency is attributed to the interfacial resistance between electrode and elec-trolyte (denoted as R int );the small semicircle in low frequency arises from charge-transfer resistance (denoted as R ct );W o is the Warburg diffusion contribution.CPE1,CEP2and CEP3are their associated capacitances.The non-homogeneity of the compos-ite electrode system re?ected as a depressed semicircle in the impedance response was accounted by considering a constant phase element (CPE)in place of a capacitor (C i )in the equivalent cir-cuit.Thus,from the impedance of the CPE,Z =1/B (j ω)n (j =(?1)1/2,ωis the angular frequency,B and n are constants),the degree of distor-tion of the impedance spectra can be obtained from the value of n ,and at n =1,B =C i ,the ideal capacitor.In Fig.5,the continuous lines
P.He et al./Journal of Power Sources 196 (2011) 5611–5616
5615
200
Z'/Ohm
200
250
100
150
100
15050
50Z'/Ohm
Z'/Ohm
-Z ''/O h m
-Z ''/O h m
-Z ''/O h m
Fig.5.Nyquist plots of the Li-air fuel cell with air catalytic electrode in various concentrations
LiOH.
Fig.6.Equivalent circuit used to ?t the experimental data.R e represents the resis-tance attributed to the electrolyte and cell components;R gb is the grain boundary resistance in the LISICON and air porous electrode;R int is the interfacial resis-tance between electrode and electrolyte;R ct is the typical charge-transfer resistance (denoted as);W o is the Warburg diffusion contribution.CPE1,CEP3and CEP3are their associated capacitances.
represent the ?tting of the equivalent circuit whereas the symbols denote the experimental data points as a function of frequency at OCV.As can be seen the experimental points ?tted well with the values calculated from the equivalent circuit.The ?tting results of circuit elements in various LiOH concentrations were displayed in Table 2.As seen in Table 2,the resistances of the electrolyte and cell components ranged from 101.2to 2305.8 with the LiOH concen-tration decreased from 2.0to 0.01M.Simulations also show that no signi?cant changes of the grain boundary resistances in the LISICON and air porous electrode (R gb )were observed in the concentration range,thereby suggesting that alkalinity has little impact on the grain boundary resistance.R int and R ct slightly increase with the
decrease of alkalinity.Lithium ion concentration at solid–liquid interface affect the process of inter?cial charge transfer.Increasing LiOH concentration facilitate charge transfer at interface.Therefore charge transfer resistance increase when the LiOH concentration decreases.Note that W o has no obvious changes except for the LiOH concentration lower than 0.1M.All these results suggest that the LiOH concentration has the greatest impact on the resistance of R e .We speculate that R e varying with LiOH concentration arises from the change of conductivity of aqueous electrolyte.
In order to further extract the effect of temperature on the lithium-air fuel cell,the EIS tests were conducted at various tem-peratures with the LiOH concentration ?xed on 1M.Fig.7illustrates the Nyquist plot of the EIS measured experimentally at 25,40and 55?C.The EIS data were also ?tted using the equivalent circuit model shown in Fig.6.Similarly,solid lines represent the ?tting data while the symbols denote the experimental data points.Sim-ulated results of EIS data using the equivalent circuit mode at different temperatures were presented in Table 3.It can be seen from the ?tting results that all resistance of elements decrease when rising the temperature.It is easy to understand that ris-ing the temperature can enhance the conductivity of liquid and solid electrolyte,which leads to a decrease in R e .Particularly,LISI-
Table 2
Simulated results for the elements of equivalent circuit in various LiOH concentrations electrolyte.
R e /
R gb / R int / R ct / W o / 2M 101.244.336.113.650.21M 116.744.237.213.949.00.5M 124.443.940.914.149.30.2M 211.343.758.514.951.30.1M 835.744.581.221.8129.20.01M
2305.8
41.2
123.9
39.7
279.7
5616P.He et al./Journal of Power Sources 196 (2011) 5611–5616
Table 3
Simulated results for the elements of equivalent circuit at different temperatures.
R e /
R gb / R int / R ct / W o / 25?C 116.744.237.213.946.040?C 90.326.019.710.239.955?C
87.7
9.5
14.7
9.9
0.7
150
100
50
-Z ''/O h m
Z'/Ohm
Fig.7.The Nyquist plot of the EIS measured experimentally at various temperatures.
CON plate,the NASICON-type solid electrolyte,located between
organic and aqueous electrolyte plays a role of solid electrolyte.It has been reported that the resistances of grain and grain boundary in NASICON-type solid electrolyte decrease with rise of tempera-ture [15].The decrease of charge transfer resistance with rise of temperature is attributed to enhanced catalytic activity of cata-lyst layer at higher temperature.In addition,Warburg diffusion resistance and the interfacial resistance between electrode and electrolyte also have signi?cant decrease with an increase of tem-perature.Consequently,it is helpful to reduce the inner resistance of cell by elevating temperature,which leads to the enhancement of power performance of Li-air fuel cell.The variation of simulated results using equivalent circuit as a function of LiOH concentration and temperature is consistent with changes of maximum power described above.4.Conclusion
In this study,we have investigated the performance of hybrid electrolyte based lithium-air fuel cell under the mixed control of
alkalinity and temperature by means of galvanistatic measurement and the analysis of AC EIS.The operating voltage of the lithium-air fuel cell has a linear decrease with the growth of applied cur-rent densities,which is mainly due to the ohmic limitation.The electromotive force and inner resistance of the cell decrease with the increase of LiOH concentration in aqueous electrolyte.As for the power performance,increase of the LiOH concentration above 0.5M is of little effective for enhancing the power.Thereby con-centration ranged from 0.5to 1.0M is considered as the suitable parameters for the LiOH aqueous electrolyte.The environment temperature exhibited a signi?cant impact on the performance of lithium-air fuel cell.The Li-air fuel cell can provide a larger power at elevated temperature due to the decrease of all resistance of elements.This work promotes the practical use of lithium-air fuel cell.
Acknowledgement
We are thankful to the OHARA Inc.for providing ceramic LISI-CON plate.References
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