pem fuel cell testing and diagnosis || the effects of temperature on pem fuel cell kinetics and...

Chapter 4

The Effects of Temperatureon PEM Fuel Cell Kineticsand Performance

PE

Cop

Chapter Outline

4.1. Introduction 12
1
4.2. Anode H2 Oxidation on Pt

Catalysts 122

4.3. Cathode O2 Reduction on Pt

Catalyst 122

4.4. Polarization Curve Analysis

Using EIS 124

4.5. Temperature Effects on PEM

Fuel Cell Kinetics 125

4.5.1. Temperature Effect on

PEM Fuel Cell

Thermodynamics and

OCV
125

the Kinetics of the

HOR in PEM Fuel

Cells
127
M Fuel Cell Testing and Diagnosis. http://dx.doi.org/10.101

yright � 2013 Elsevier B.V. All rights reserved.


the Kinetics of the

ORR in PEM Fuel

Cells 1
31

Membrane

Conductivity and

Hydrogen Crossover 1
32

Mass Transfer in PEM

Fuel Cells 1
35
4.6. The Effect of Temperature

on the Overall Performance

of a PEM Fuel Cell 136

4.7. Chapter Summary 138

References 139

4.1. INTRODUCTION

Temperature is one the most important operating conditions of proton exchangemembrane (PEM) fuel cells, and it can significantly influence cell performance.Generally, an increase in the temperature can improve performance. In light ofthis fact, high-temperature PEM (HT-PEM) fuel cells [1–5] operated above 95 �C(usually 95–200 �C) have recently been developed. This performance improve-ment at higher temperatures is mainly due to increased membrane proton

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122 PEM Fuel Cell Testing and Diagnosis

conductivity [6,7], enhanced electrode kinetics for the oxygen reduction reaction(ORR) and the hydrogen oxidation reaction (HOR) [1,2,8], and improved masstransfer of the reactants. In addition, increasing the temperature can also increasethe tolerance of electrocatalysts to contaminants [4,9–12]. However, higheroperating temperatures can lead to membrane dehydration [3], increasedhydrogen crossover rate [13], and the degradation of components such as elec-trocatalysts [14,15], gasket materials, and bipolar plates, resulting in a shortenedfuel cell lifetime. Chapter 10 presents a detailed discussion of HT-PEM fuel cells.This chapter will focus on conventional (i.e. low-temperature) PEM fuel cells thatuse perfluorosulfonic acid (PFSA)membranes (e.g. Nafion� membranes) and areusually operated below 95 �C (typically from room temperature to 80 �C).

In general, fuel cell performance can be affected by several operatingconditions, such as temperature, pressure, and relative humidity (RH). We willdiscuss the effects of RH and pressure in Chapters 8 and 9, respectively. In thischapter, only the temperature effects on the performance of PEM fuel cells willbe discussed in detail.

4.2. ANODE H2 OXIDATION ON PT CATALYSTS

Carbon-supported Pt nanoparticles (e.g. Pt/C) are currently the most efficientelectrocatalysts for the HOR in PEM fuel cells. If pure hydrogen is used as thefuel, the overpotential of the HOR at the Pt anode is small due to the fastelectrode kinetics of the reaction. Normally, the overpotential is <20 mV, evenat a current density of 1.0 A cm�2. The mechanism and kinetic behavior of theHOR on a Pt electrode or Pt/Nafion� interface has been studied extensively inboth acidic and alkaline media [9,16–20].

The anode electrochemical reaction of a H2/O2 PEM fuel cell can beexpressed using Reaction (1.I) in Chapter 1. The generally accepted mechanismof the HOR on a Pt catalyst includes three steps [9,17,21–23]: (1) theadsorption of H2 on the Pt surface, (2) the dissociated chemical adsorption ofthe adsorbed H2, which is considered the rate-determining step of the HOR, and(3) the fast electrochemical oxidation of adsorbed hydrogen atoms, producingprotons. These three steps are expressed in Reactions (1.IV), (1.V), and (1.VI),respectively, whereas the electrode kinetics of the HOR has been addressed inSection 1.3.1 of Chapter 1.

4.3. CATHODE O2 REDUCTION ON PT CATALYST

The ORR in PEM fuel cells can be expressed using Reaction (1.II) and isa multielectron transfer process. The generally accepted mechanism for theORR in acidic media is presented in Fig. 4.1 [24–28]. As shown, there aretwo pathways: one is a direct 4-electron reaction that produces H2O, and theother is a 2-electron reaction that produces H2O2, which is undesirable inPEM fuel cells. Which pathway the ORR will take strongly depends on the

FIGURE 4.1 Observed mechanism of

the ORR on a Pt catalyst [24–28].

123Chapter | 4 The Effects of Temperature on PEM Fuel Cell Kinetics

type and the properties of the electrocatalyst. Currently, Pt and Pt/C arerecognized as the most effective electrocatalysts for the ORR in PEM fuelcells, although other catalyst materials have also been developed andinvestigated; these include Pt-M alloy catalysts (where M is a transitionmetal, e.g. Fe, Co, Ni, Pd) [29–32]; other precious metal catalysts, such asAu, Ir, and Rh, [33]; transition metal macrocyclic complexes, such astransition metal phthalocyanine and porphyrin [34–41]; as well as transitionmetal chalcogenides, such as Mo4Ru2Se8 [42,43]. On a Pt catalyst surface,the ORR proceeds primarily via the direct 4-electron transfer pathway, withH2O as the main product.

The kinetics of the ORR has been addressed in Chapter 1. The exchangecurrent density and Tafel slope are two important factors that describe theelectrode kinetics. According to Eqns (1.56)–(1.59) and Figure 1.3, the Tafel

slope,2:303RT

ð1� a2OÞna2OF, can be obtained, and if ErO2=H2O

in Eqn (1.59) is known,

the exchange current density ðioO2=H2OÞ for the ORR can be calculated. In the

literature, two sets of Tafel slopes can be found [24,44–47]. In the low currentdensity range (corresponding to high cathode potential), the value of the slopeis approximately 60 mV decade�1, whereas in the high current density range(corresponding to low cathode potential), the value of the slope is approxi-mately 120 mV decade�1. This disparity results from the differences in the Ptsurface. When the cathode potential is high (e.g. >0.8 V vs. standard hydrogenelectrode (SHE)), the Pt surface will be partially covered by PtO due to thefollowing reaction:

Pt þ 1

2O2 4 PtO Eo ¼ 0:88V vs:RHE (4.I)

It has also been reported [48–50] that PtO coverage can be approximately 0.3 atcathode potentials >0.8 V. Only when the cathode potential is <0.8 V is the Ptsurface close to being pure. Several studies [24,51,52] have demonstrated thatthe kinetics of the ORR on a pure Pt surface is different from that on a PtOsurface.

At the cathode of a PEM fuel cell, a pure Pt surface is not easy to achievebecause oxygen is present, leading to a mixed surface of Pt and PtO. Thus, thefuel cell thermodynamic open circuit voltage (OCV) at 25 �C is always


<1.23 V. The value most often obtained is about 1.06 V, which is a mixture ofthe thermodynamic OCVof O2/H2O and of Pt/PtO due to the coexistence of theORR and Pt oxidation [53,54]. As discussed in Chapter 7, hydrogen crossoverfrom the anode to the cathode can also reduce the fuel cell OCV.

4.4. POLARIZATION CURVE ANALYSIS USING EIS

In Chapter 1, Figure 1.4 shows a typical polarization curve of a PEM fuel cell.The voltage loss of a cell is determined by its OCV, electrode kinetics, ohmicresistance (dominated by the membrane resistance), and mass transfer property.In experiments, the OCV can be measured directly. If the ohmic resistance(Rm), kinetic resistance (Rt, also known as charge transfer resistance), and masstransfer resistance (Rmt) are known, the fuel cell performance is easily simu-lated. As described in Chapter 3, electrochemical impedance spectroscopy(EIS) has been introduced as a powerful diagnostic technique to obtain theseresistances. By using the equivalent circuit shown in Figure 3.3, Rm, Rt, and Rmt

can be simulated based on EIS data.It is well known that the electrode kinetics of the HOR is much faster than that

of the ORR, and thus, the anodic overpotential is much smaller than the cathodicone. By assuming that the anodic polarization is negligible, one can describe thesteady-state polarization curve by using the semiempirical Eqn (4.1) [3,55–57]:

E ¼ Eo � blnðIÞ � IRm � mmtexpðnmtIÞ (4.1)

where E is the fuel cell voltage; Eo is the fuel cell OCV-related constant, whichis determined by many factors, such as fuel cell OCV, exchange current density,hydrogen crossover, and Pt surface composition (i.e. the ratio of Pt to PtO); b isthe Tafel slope; I is the current density; Rm is the membrane resistance; mmt isthe mass transfer coefficient; and nmt is the simulation parameter for curvefitting. In Eqn (4.1), bln(I) is the contribution of the fuel cell reaction kinetics(dominated by the ORR), IRm is the contribution of ohmic resistance (domi-nated by membrane resistance), and mmtexp(nmtI) is the mass transfer contri-bution to the fuel cell polarization (dominated by oxygen transfer in theelectrode). The internal alternating current (AC) resistance of the fuel cell(Rcell) can be obtained by differentiating Eqn (4.1) [3]:

Rcell ¼ � vE

vI¼ b

Iþ Rm þ mmtnmte

nmtI (4.2)

If b/I in Eqn (4.2) can be defined as the charge transfer resistance (Rt) andmmtnmte

nmtI as the mass transfer resistance (Rmt), the fuel cell internal ACimpedance can be expressed as Eqn (4.3) [3]:

Rcell ¼ Rm þ Rt þ Rmt (4.3)

In Eqn (4.3), Rm is a function of water content and temperature, and both Rmt

and Rt are functions of current density and temperature. Therefore, to


understand the effect of temperature on the performance of a PEM fuel cell, it isnecessary to study its effect on these resistances and on their associatedexchange current density, membrane water uptake, and mass transfer property.

Once the values of Rm, Rt, and Rmt in Eqn (4.3) are simulated based on theAC impedance spectra and the equivalent circuit in Figure 3.11, the decreasesin cell voltage (DVi) caused by the individual resistances can be calculated ata constant current density, and their contributions (DVi%) to the overall declinein cell voltage (DVcell) can be calculated according to Eqn (4.4):

DVi% ¼ DVi

DVcell� 100 (4.4)

This can also be expressed as follows:

DVi% ¼ Ri

Rcell� 100 (4.5)

As shown in Fig. 4.2, at both 80 �C and 120 �C, in the low current density range(<0.5 A cm�2), the cell polarization is mainly due to the charge transferprocess, whereas in the high current density range (>1.5 A cm�2), the cellpolarization is dominated by the mass transfer process [3].

4.5. TEMPERATURE EFFECTS ON PEM FUEL CELL KINETICS

The temperature can significantly affect the electrode kinetics in PEM fuelcells. This section will discuss the effects of temperature on (i) fuel cell ther-modynamics and OCV, (ii) the kinetics of both the HOR and the ORR, (iii) theproton conductivity and hydration of the membrane, and (iv) mass transfer.

4.5.1. Temperature Effect on PEM Fuel CellThermodynamics and OCV

In H2/air PEM fuel cells, the overall electrochemical reaction is expressed asReaction (4.II):

H2 þ 1

2O2/H2O (4.II)

For Reaction (4.II), the fuel cell’s theoretical OCVor thermodynamic OCV canbe expressed as VOCV

theory or VOCVcell , as used in Eqn (1.3) in Chapter 1.

VOCVtheory ¼ VOCV

cell ¼ ErO2=H2O

� ErH2=H

þ

¼ EoO2=H2O

� EoH2=H

þ þ 2:303RT

2Flog

0@aH2

a12O2

aH2O

1A (4.6)

FIGURE 4.2 Percentage of cell individual voltage drops caused by Rm, Rt, and Rmt. (a) at 80�C

and (b) at 120 �C. Nafion�-112 membrane-based membrane electrode assembly(MEA) with an

active area of 4.4 cm2; RH for both H2 and air: 100%; 3.0 atm. [3].



where ErH2=H

þ is the reversible anode potential (V) at temperature T; aH2and

aHþare the respective activities of H2 and Hþ; EoH2=H

þ is the electrode potentialof the H2/H

þ redox couple under standard conditions (1.0 atm, 25 �C), which isdefined as zero voltage; nH is the electron transfer number (with a value of 2 forthe H2/H

þ redox couple); R is the universal gas constant (8.314 J K�1 mol�1);F is Faraday’s constant (96,487 C mol�1); Er

O2=H2Ois the reversible cathode

potential (V) at temperature T; aO2and aHþ are the respective activities of O2

and Hþ; EoO2=H2O

is the electrode potential of the O2/H2O redox couple understandard conditions (1.0 atm, 25 �C), which is 1.229 V (vs. the SHE); nO is theelectron transfer number (with a value of 4 for the O2/H2O redox couple); andaH2O is the activity of H2O.

For an approximate evaluation of theoretical electrode potentials and fuelcell voltage, aO2

, aH2, and aH2O can be replaced by their partial pressures, PO2

,PH2

, and PH2O, in the anode and cathode feed streams, respectively. Thus, Eqn(4.6) can be expressed as Eqn (4.7):

VOCVcell ¼ Eo

O2=H2O� Eo

H2=Hþ þ 2:303

RT

2Flog

0@PH2

P12O2

PH2O

1A (4.7)

In Eqn (4.7), EoH2=H

þ equals zero at standard conditions, whereas EoO2=H2O

has

been reported to be a function of temperature and can be expressed as Eqn (4.8)[58–60]:

EoO2=H2O

¼ 1:229� 0:000486ðT � 298:15Þ (4.8)

Therefore, the thermodynamic fuel cell OCV can be expressed as follows:

VOCVcell ¼ 1:229� 0:000486ðT � 298:15Þ þ 2:303

RT

2Flog

0@PH2

P12O2

PH2O

1A (4.9)

Our recent study indicated that the fuel cell OCV decreased with increasingtemperature, as shown in Fig. 4.3 [58]. It can be seen that both the theoreticaland the measured OCV decreased when the temperature increased from 23 to120 �C. This was mainly because of the effect of temperature on the fuel cellthermodynamics and hydrogen crossover, which will be addressed in detail inChapters 6 and 7, respectively.

4.5.2. Temperature Effect on the Kinetics of the HORin PEM Fuel Cells

In Chapter 1, Eqns (1.30) and (1.31) describe the relationship between theoverpotential and the exchange current density of the HOR. In Eqn (1.31),

FIGURE 4.3 Fuel cell OCV at different temperatures. Nafion�-112-membrane-based MEAwith

an active area of 4.4 cm2, at 100% RH and 3.0 atm backpressure [58]. (For color version of this

figure, the reader is referred to the online version of this book.)


2:303RT

aHnaHFis the Tafel slope, which is a function of temperature; naH is the

electron transfer number for the HOR, whose value is widely reported to be 2.0;and aH is the charge transfer coefficient, whose value is recognized as 0.5. Inthis equation, io

H2=Hþ is the apparent exchange current density for the HOR and

is a function of temperature. The relationship between ioH2=H

þ and T can beexpressed using Eqn (1.34), where Io

H2=Hþ is the intrinsic exchange current

density, which is the exchange current density at infinite temperature. Therelationship between io

H2=Hþ and Io

H2=Hþ can also be expressed using the

following equation [2,8]:

ioH2=Hþ ¼ ðEPSAÞa � IoH2=H

þ

PH2

PoH2

!0:5

(4.10)

where (EPSA)a is the electrochemical surface area of Pt at the anode, and PH2

and PoH2

are, respectively, the operating hydrogen partial pressure in the feedstream at the anode and the hydrogen pressure at standard conditions.

Recently, we studied the temperature effect on the exchange currentdensity of the HOR between 23 and 120 �C using a Nafion�-112 membrane-based PEM fuel cell, and between 120 and 200 �C using a phosphoric-acid-doped polybenzimidazole (PBI) membrane-based PEM fuel cell. Asshown in Tables 4.1 and 4.2, Io

H2=Hþ increases as the temperature is increased

from 23 to 200 �C.

TABLE 4.1 Measured and Simulated Exchange Current Densities for the HOR and ORR in a Nafion�-112-Membrane-Based

PEM Fuel Cell at 3.0 atm (Absolute), 100% RH, and Different Temperatures, in a Low Current Density Range (ioO2=H2Ois the

Measured Apparent Exchange Current Density for the ORR, ioH2=H

þ is the Simulated Apparent Exchange Current Density for

the HOR, and IoO2=H2Oand Io

H2=Hþ are the Intrinsic Exchange Current Densities for the ORR and HOR, Respectively)[8]

Temperature (�C) 23 40 60 80 100 120

ioO2=H2OðA cm�2Þ 1.22�10�4 2.43�10�4 3.92�10�4 4.60�10�4 3.43�10�4 2.24�10�4

IoO2=H2OðA cm�2Þ 5.02�10�7 1.85�10�6 3.44�10�6 6.25�10�6 6.88�10�6 1.05�10�5

ioH2=H

þ ðA cm�2Þ 0.134 0.198 0.344 0.607 0.604 0.497

IoH2=H

þ ðA cm�2Þ 1.73�10�3 4.06�10�3 7.21�10�3 1.23�10�2 3.25�10�2 6.13�10�2

129

Chap

ter|4

TheEffects

ofTem

peratu

reonPEM

Fuel

Cell

Kinetics

TABLE 4.2 Apparent and Intrinsic Exchange Current Densities at Different Temperatures in the Low Current Density Range,

Measured at AMBIENT Pressure and Zero RH (ioO2=H2Ois the Measured Apparent Exchange Current Density for the ORR,

ioH2=H

þ is the Simulated Apparent Exchange Current Density for the HOR, and IoO2=H2Oand Io

H2=Hþ are the Intrinsic Exchange

Current Densities for the ORR and HOR, Respectively)[2]

Temperature (�C) 120 140 160 180 200

ioO2=H2OðA cm�2Þ 2.30�10�3 2.05�10�3 2.64�10�3 3.60�10�3 5.43�10�3

IoO2=H2OðA cm�2Þ 8.97�10�5 1.05�10�4 1.51�10�4 3.18�10�4 7.88�10�4

ioH2=H

þ ðA cm�2Þ 0.72 1.24 1.88 2.50 2.71

IoH2=H

þ ðA cm�2Þ 1.02�10�2 2.18�10�2 3.51�10�2 6.82�10�2 1.16�10�1

130

PEM

Fuel

Cell

Testingan

dDiagn

osis


4.5.3. Temperature Effect on the Kinetics of the ORRin PEM Fuel Cells

As discussed in Chapter 1, the Tafel slope of the ORR is2:303RT

ð1� a2OÞna2OF,which is a function of temperature. Here, a2O is the electron transfer coefficientin Reaction (1.XV) and is also dependent on temperature. On a Pt electrode, theelectron transfer coefficient for the ORR increases linearly with temperaturebetween 20 and 250 �C, in accordance with Eqn (4.11) [8,44]:

aO ¼ aoOT (4.11)

where aO is the electron transfer coefficient of the ORR on a Pt electrode, aoOequals 0.001678, and T is the temperature in Kelvin.

The relationship between the intrinsic exchange current density ðIoO2=H2OÞ

and the apparent exchange current density ðioO2=H2OÞ can be described using the

following equation [2,8]:

ioO2=H2O¼ ðEPSAÞc � IoO2=H2O

PO2

PoO2

!0:001678T

(4.12)

where (EPSA)c is the electrochemical surface area of Pt at the cathode, and PO2

and PoO2

are the operating oxygen partial pressure in the feed stream at thecathode and the oxygen pressure at standard conditions, respectively.

As discussed previously, the Tafel slope at low current density (high potentialrange) is approximately 60 mV decade�1, while the Tafel slope at a high currentdensity (low potential range) is approximately 120 mV decade�1; thus, theexchange current densities should be different in these two current ranges. It isalso expected that the temperature dependencies of these two exchange currentdensities will differ. For example, Parthasarathy et al. [44] studied the tempera-ture dependence of theORRat a Pt/Nafion�-117 interface using amicroelectrodemethod and found that the exchange current density increased with increasingtemperature from 1.69 � 10�10 A cm�2 at 30 �C to 1.87� 10�8 A cm�2 at80 �C in the low current density range, whereas in the high current density range,it increased from 2.84� 10�7 A cm�2 at 30 �C to 1.39 � 10�6 A cm�2 at 80 �C.A recent study [8] of a Nafion�-112-membrane-based PEM fuel cell showed thatthe intrinsic exchange current densities of the ORR in the high and low potentialranges at 80 �C were 6.25� 10�6 A cm�2 and 3.87� 10�4 A cm�2, respec-tively. The temperature effects on the exchange current density of the ORRbetween 23 and 120 �Cusing aNafion�-112-membrane-based PEMfuel cell andin the temperature range of 120 –200 �C using a PBI-membrane-based PEM fuelcell have also been reported [2,8]. As shown in Tables 4.1 and 4.2, the IoO2=H2Oincreases with temperature from 23 to 200 �C.

The relationship between ioO2=H2Oand IoO2=H2O

can be expressed in anArrhenius form, as shown in Eqn (1.62). By plotting logðioO2=H2O

Þ vs. 1/T, the


reaction activation energy ðEoO2Þ of the ORR can be calculated based on the

slope of this plot. Using a Nafion�-112-membrane-based PEM fuel cell, EoO2

values of 28.3 kJ mol�1 and 57.3 kJ mol�1 were obtained in the low currentdensity range and the high current density range, respectively [8].

4.5.4. Temperature Effect on Membrane Conductivityand Hydrogen Crossover

It has been widely reported [61–64] that temperature can significantly affectthe proton conductivity (s) of a membrane. For a PFSA membrane (e.g.Nafion� membrane), the proton conductivity strongly depends on the watercontent of the membrane. Therefore, when studying the effect of temperatureon membrane conductivity, the RH or water content of the membrane mustbe considered. At a low RH, an increase in the temperature will causemembrane dehydration, resulting in decreased proton conductivity, whereaswith a well-hydrated membrane, the proton conductivity will increase withincreasing temperatures. For example, the conductivity of Nafion� 117 at100% RH increases from 0.1 to 0.2 S cm�1 when the temperature is raisedfrom 30 to 85 �C [65]. Generally, under well-hydrated conditions, thetemperature dependence of conductivity can be expressed in an Arrheniusform [2]:

logðsÞ ¼ logðsoÞ � 2:303Ema

R

�1

T

�(4.13)

where s, so, Ema , R, and T are the membrane conductivity (S cm�1), the pre-

exponential factor (S K�1 cm�1), the proton conducting activation energy(kJ mol�1), the ideal gas constant (J mol�1 K�1), and the temperature (K),respectively. Figure 4.4 shows the Arrhenius plots for the ionic conductivitiesof several X-form (X¼H, Li, Na) membranes, along with Nafion� 117 H-formin the fully hydrated state. It can be seen that in a fully hydrated state, the protonconductivity increases with increasing temperatures, regardless of whether themembrane is H-, Li-, or Na-form.

However, for a PFSA membrane operated at high temperatures, the situa-tion can be more complex due to the tradeoff between increased conductivityand membrane dehydration when the temperature is increased. As shown inFig. 4.5, the resistance of Nafion�-112 membrane decreases slightly when thetemperature is increased from 80 to 100 �C, which indicates an increase inproton conductivity, but the resistance increases when the temperature is raisedto 120 �C, suggesting a decrease in proton conductivity. This is because thenegative effect of dehydration is larger than the positive effect of temperatureon membrane conductivity.

Chapter 5 provides a more detailed discussion of the temperature effect onmembrane proton conductivity.

FIGURE 4.4 Arrhenius plots for ionic conduc-

tivities (s) of Flemion X-form (X¼H, Li, Na)

membranes, along with Nafion� 117 H-form

in the fully hydrated state [66].

FIGURE 4.5 Nafion� through-plane resistance as a function of current density at three fuel cell

operating temperatures: 100% RH for both the anode and the cathode; 3.0 atm backpressure with

hydrogen and air feeding; Nafion�-112-based baseline MEA with an active area of 4.4 cm2 [3].

(For color version of this figure, the reader is referred to the online version of this book.)


Besides affecting the proton conductivity, the temperature can also influ-ence the hydrogen crossover of PEM fuel cell membranes. During hydrogencrossover, hydrogen diffuses across the membrane from the anode to thecathode, leading to a lower fuel cell efficiency and degradation of the


membrane. Obviously, hydrogen crossover is a diffusion-controlled process. Ithas been reported that hydrogen crossover can be affected by membranestructure [67] and by fuel cell operating conditions such as temperature,backpressure, and the RH of the reactant gases [13,68,69]. Hydrogen crossoverwill be dealt with in detail in Chapter 6.

Temperature affects hydrogen crossover mainly by affecting the hydrogenpermeability coefficient, which is temperature dependent and can be expressedin an Arrhenius form [13]:

lnJPEMH2

¼ lnJoH2

þ�� EPEM

H2

R

�1

T(4.14)

where JPEMH2

is the hydrogen permeability coefficient, JoH2

is the maximum

permeability coefficient (e.g. at infinite temperature), EPEMH2

is the activation

energy for hydrogen crossover, R is the gas constant, and T is the temperature inKelvin. As shown in Fig. 4.6, the hydrogen permeability coefficient increaseswith increasing temperature.

FIGURE 4.6 Arrhenius plots of the H2 permeability coefficient for both Nafion�-112- and

Nafion�-117-membrane-based MEAs at a 3.04 atm backpressure and a 100% RH in the

temperature range of 40–120 �C. Both MEAs have an active area of 4.4 cm2. Anode H2 stream

flow rate: 0.1 L min�1; cathode N2 stream flow rate: 0.5 L min�1; cathode potential: 0.5 V vs.

anode hydrogen electrode [13]. (For color version of this figure, the reader is referred to the online

version of this book.)

FIGURE 4.7 Effect of cell temperature and humidification on H2 crossover current density at

atmospheric pressure. H2 and Ar gases were humidified at the same temperature in each case.

Nafion�-112 membrane, H2/Ar¼ 300/300 mL min�1 [69].


Teranish et al. [69] showed that the H2 crossover current density increaseswith temperature and RH, as shown in Fig. 4.7; this indicates that the H2

crossover rate increases with temperature and RH.

4.5.5. Temperature Effect on Mass Transfer in PEM Fuel Cells

The temperature can also affect the gas mass transfer (or mass transport) inPEM fuel cells. However, when studying the temperature effect on gas masstransport, the RH, current density, backpressure, and flow rate (or stoichi-ometry) of the reactant gases should be considered. At low current densities(<0.5 A cm�2), the fuel cell polarization is under the control of activationand kinetics, and temperature has little effect on gas mass transport. At highcurrent densities (>1.5 A cm�2), the fuel cell polarization is under thecontrol of mass transport, and the effect of temperature can be significant,depending on other operating conditions. For example, at a low RH, anincrease in the temperature may cause membrane dehydration. At a high RH,a temperature increase will help to remove the water from inside the fuel cellelectrode and flow channels and will prevent “water flooding” and reduce themass transfer resistance. A high gas flow rate, or stoichiometry, will also helpto eliminate the “water flooding” phenomenon. Thus, to study the effect oftemperature on mass transport, a high current density is often used, and

FIGURE 4.8 Mass transfer resistance as a function of current density at 80 �C, 100 �C, and120 �C. Nafion�-112-membrane-based MEAwith an active area of 4.4 cm2, 3.0 atm backpressure,

and 100% RH; H2 flow rate: 0.3 L min�1, air flow rate: 1.0 L min�1 [3]. (For color version of this

figure, the reader is referred to the online version of this book.)


certain RHs and gas stoichiometries are applied for fuel cell operation.Figure 4.8 shows some results [3] obtained in a Nafion�-112-membrane-based PEM fuel cell. The fuel cell was operated at a 3.0 atm backpressureand 100% RH, and the flow rates of H2 and air were controlled at0.3 L min�1 and 1.0 L min�1, respectively. The Rmt values at differentcurrent densities and temperatures were simulated from the AC impedancespectra. As shown in Fig 4.8, the Rmt decreased when the temperature wasincreased from 80 to 100 �C and then to 120 �C, and the trend was valid inthe current density range of 0.8–1.7 A cm�2, which indicates the benefit ofhigh-temperature operation.

4.6. THE EFFECT OF TEMPERATURE ON THE OVERALLPERFORMANCE OF A PEM FUEL CELL

As discussed above, the temperature can influence the OCVof a PEM fuel celland thermodynamics, electrode kinetics, membrane conductivity, hydrogencrossover, and mass transfer process, and this influence will be reflected in theoverall cell performance. However, the dependence of performance ontemperature can be complicated by the fact that other conditions, such as RH,backpressure, gas stochiometry, flow field design, and electrode structure, alsoaffect performance.


The literature contains several studies on how the operating temperatureaffects PEM fuel cell performance [2,3,70–75]. For example, Yan et al. [71]studied the effect of temperature on performance at different gas humidificationtemperatures, by using Gore PRIMEA� 57 MEA. Figure 4.9 presents the fuelcell performance at different temperatures with a humidification temperature of50 �C. As shown, the fuel cell performance improves when the operatingtemperature is increased from 30 to 50 �C. This can be explained by enhancedelectrode kinetics and increased membrane conductivity. However, the fuel cellperformance decreases if the temperature is further increased from 50 to 75 �C.This may occur because the additional increase can cause the water inside themembrane and the electrode to evaporate quickly. Any operating temperaturehigher than the humidification temperature (50 �C in their experiment) willlead to a fast water evaporation rate, causing membrane dehydration and lowermembrane conductivity. This negative effect is larger than the positive effect ofkinetics enhancement, which results in decreased fuel cell performance. Theyalso studied the temperature effect on fuel cell performance at humidificationtemperatures of 70 �C, 60 �C, and 30 �C, and achieved better performance at30 �C than at 60 �C and 70 �C, with a fixed humidification temperature.

Recently, the temperature effect on the performance of an HT-PEM fuel cellwith a PBI-membrane-based MEA (purchased from PEMEAS Fuel Cell Tech-nologies) in the temperature range of 120–200 �C was also reported [2]. As theproton conductivity of a PBImembrane does not rely on its water content, the fuelcell was operatedwithout extra humidification (0%RH) at ambient backpressure.

FIGURE 4.9 Effect of cell temperature on cell performance at a humidification temperature of

50 �C [71].

FIGURE 4.10 Polarization curves obtained at ambient backpressure, 0% RH, and different

temperatures (as marked). The stoichiometries of H2 and air are 1.5 and 2.0, respectively [2]. (For

color version of this figure, the reader is referred to the online version of this book.)


As shown in Fig. 4.10, the fuel cell performance increased with increasingtemperature. At a current density of 1.0 A cm�2, the cell voltages were 0.366 V(120 �C), 0.415 V (140 �C), 0.44 V (160 �C), 0.465 V (180 �C), and 0.485 V(200 �C). The maximum power density of the fuel cell increased linearly withtemperature, which indicates the benefit of high-temperature operation.

4.7. CHAPTER SUMMARY

In this chapter, the effects of operating temperature on PEM fuel cell kineticsand performance were addressed in detail. In general, an increase in thetemperature will enhance the kinetics of both the HOR and the ORR. Fuel cellthermodynamics and OCV can be significantly affected by the operatingtemperature; raising the temperature will lower the fuel cell OCV. Thetemperature also influences the proton conductivity and hydrogen crossoverof membranes. Because the water and proton diffusion coefficients increasewith increasing temperature, the proton conductivity will increase for a well-hydrated PEM. Thus, the temperature affects the fuel cell kinetics, fuel cellOCV, membrane conductivity, hydrogen crossover, and mass transfer process.These effects are reflected in the overall fuel cell performance, but the rela-tionship of fuel cell performance to temperature can be complicated by thefact that the flow field design, the fuel cell structure, and operating conditionssuch as RH, gas flow rate, and backpressure can also influence fuel cellperformance.


By using AC impedance spectroscopy, the membrane resistance (Rm),charge transfer resistance (Rt), and gas mass transfer resistance (Rmt) of anoperating PEM fuel cell, and the associated temperature dependencies of theseresistances, can be separated and simulated by means of a suitable equivalentcircuit.

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