Mench M.M. Fuel Cell Engines

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360 Polymer Electrolyte Fuel Cells

Irreversible Modes of Chemical Degradation

1. Electrolyte Loss Electrolyte material can be lost through a variety of physico-

chemical mechanisms. For PEFCs, the polymer itself can degrade physically and

chemically, particularly from peroxide radical attack [73]. This results in loss of

mass and conductivity in the electrolyte and possibly catastrophic pinhole forma-

tion. Mechanical stress and cycling have also been linked to accelerated polymer

degradation [74]. Over time, the electrolyte in a PEFC can thin considerably as the

ionomer is lost. This can result in a temporary increase in performance, since the

ohmic losses decrease with a thinner electrolyte. However, over time, the thinning

electrolyte is more susceptible to pinhole formation and excessive crossover, lead-

ing to failure. This mode of damage is especially ampliﬁed in extreme conditions

of high temperature and low humidity. In PEFCs, electrolyte degradation can be

monitored by measurement of the ﬂuorine content in efﬂuent liquid and vapor and

has been found to correlate with membrane degradation.

2. Platinum Dissolution and Migration Platinum migration from the cathode of a

PEFC occurs due to the following steps [75];

a. Platinum dissolves into Pt–O in the catalyst layer at high potentials. On the

cathode at high potential, a mobile Pt

species is created according to the

reaction

PtO + 2H

↔ Pt

+ H

O (6.52)

b. The PtO is mobile in Naﬁon and can diffuse on the surface of Pt/C, further

exposing underlying Pt in clusters.

c. Under the voltage gradient between the anode and cathode, PtO migrates away

from the catalyst layer through the electrolyte toward the anode.

The result is an irreversible loss with fewer active sites in the cathode for the

reaction, resulting in a gradually decreasing exchange current density (i

)atthe

cathode. Under load cycling to high cathode potential, the degradation rate is greatly

accelerated. This is because of the continuous formation and disruption of mobile

oxide ﬁlm leading to mobile Pt

species. The bottom line is that Pt is susceptible

to dissolution and migration toward the anode under high potential and load cycling

conditions, leading to loss of ECSA.

3. Ionic Impurity Contamination Ionic impurities from metals in the fuel cell system

will readily absorb into the fuel cell electrolyte, since it is an ionic conductor.

When the ionic impurity gets into the membrane, it can greatly reduce the ionic

conductivity and alter water transport in the membrane and catalyst layers, reducing

performance from very minute quantities of impurities [76]. Postmortem testing of

used membrane electrode assemblies (MEAs) has demonstrated the presence of

a surprising array of metallic impurities, such as calcium, iron oxides, copper,

magnesium, and various other metals, as shown in Figure 6.62. As a result, most

fuel cell systems are designed to avoid contact of the reactant ﬂow stream with any

metal connectors or couplings, and special plastics deemed compatible for fuel cell

service have been developed for fuel cell systems.

4. Degradation of Hoses and Gaskets Oxidation of components such as metallic

current collectors, gaskets, and hose components and ﬁttings can become a ma-

jor loss in fuel cells over time [77]. The oxidation and decomposition products

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6.5 PEFC Degradation 361

Figure 6.62 Bar chart with cationic impurities found in Naﬁon 112 electrolyte operated at

0.25 A/cm

at 75

◦

C after 9300 h of operation. (Reproduced with permission from Ref. [76].)

from these materials can permanently change the ionomer conductivity and wetta-

bility of components, changing the water management or accelerating membrane

degradation.

Carbon Monoxide Poisoning In the near term, it is likely that a majority of the hydrogen

produced will come from a hydrogen reformation process, in which a carbon-containing

fossil fuel (e.g., natural gas or methanol) is partially oxidized into a mixture of mostly

hydrogen, carbon dioxide, and water [78]. One of the undesired byproducts of the refor-

mation process is a small fraction of carbon monoxide (CO). Although the amount of CO

in the feed stream varies for different reformation processes, even small levels of CO can

adversely affect the performance of low-temperature noble metal catalyst fuel cells, such

as the PEFC. As a result, a large portion of the total fuel cell reformer system must be

devoted to CO removal. Even with bulky and complex CO scrubbing subsystems, nominal

CO levels can still be in the 10–50-ppm range, and transient operation during load changes

or startup can produce sporadic pulses above the 1000-ppm level. The complexity involved

in dynamic control of the reformation subsystem is a barrier in transient automotive and

military fuel cell systems, where it would be especially desirable to utilize a conventional

liquid fuel with onboard reformation to circumvent hydrogen infrastructure limitations.

Although there are alternative theories, the global reaction mechanism of CO

poisoning of hydrogen PEFCs with platinum catalyst has been modeled by Springer and

co-workers [79]:

CO + M ←−−−−−−−−−−−−−−→

1 f

(M–CO) (6.53)

+ 2M ←−−−−−−−−−−−−−−→

2 f

2(M–H) (6.54)

(M–H) −−−−−−−→

3 f

+ e

−

+ M(6.55)

O + (M–CO) −−−−−−−→

4 f

M + CO

+ 2H

+ 2e

−

(6.56)

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362 Polymer Electrolyte Fuel Cells

The forward reactions of Eqs. (6.53) and (6.54) dominate the reverse reactions and respec-

tively represent the parallel processes of CO adsorption and H

dissociative chemisorption

on an active and available platinum catalyst site, M. Equation (6.53) is the CO poisoning

step. Here, adsorbed CO takes up active catalyst surface sites, rendering them unavailable

for hydrogen dissociative chemisorption of Eq. (6.54) and the desired current-producing

hydrogen electrochemical oxidation reaction of Eq. (6.55). In steady state, surface

coverage of CO sites with gas streams containing as little as 100 ppm CO can reach 95%.

This blockage of available catalyst sites results in reduced fuel cell efﬁciency (lower cell

voltage for identical current). Equation (6.56) is the electrochemical oxidative desorption

of CO from the catalyst site and is the rate-limiting step in the overall mechanism.

Based on the mechanisms proposed in Eqs. (6.53)–(6.56), an expression relating the

time-dependent fractional surface coverage of the CO (θ

) species can be shown [79]:

dθ

= k

1 f

(

1 − θ

− θ

)

− K

− k

4 f

exp



Fη



(6.57)

where the ﬁrst term on the right-hand side represents the CO adsorption on the catalyst

surface (poisoning step), the second term represents the desorption of CO (reverse of

poisoning), and the third term represents the electrochemical oxidative desorption rate

equation (6.56). Due to the uncertainty in the various kinetic parameters, the fuel cell

dynamic performance is very difﬁcult to predict without use of arbitrary ﬁtting parameters.

Carbon monoxide poisoning remediation technology is fairly developed, including air

bleeding, alternate catalyst selection, and other methods. In air bleeding, a small percentage

(usually <5%) of air is mixed with the anode ﬂow to promote chemical and electrochemical

oxidation of CO and reduce poison coverage of the anode catalyst [80]. The drawback of

this technique is increased pumping requirements and electrochemical potential losses

associated with fuel/oxidizer mixing in the anode feed. Additionally, the mixing of moist

air and hydrogen poses safety concerns. The use of CO-tolerant catalysts such as Pt–Ru

can greatly reduce the performance loss for operation on moderate amounts of CO in the

fuel feed stream.

In summary, various modes of physical and chemical degradation exist, which limit

life times of operating DEFCs. In particular, membrane moisture variation or chemical

impurities can rapidly degrade performance and must be controlled.

6.6 MULTIDIMENSIONAL EFFECTS

In the study of fuel cells, there is a natural tendency to imagine the proﬁles of heat, mass,

current, and other parameters as uniform across the active area. There is also a natural

tendency to think that a perfectly homogeneous distribution of these operational parameters

is ideal. In fact, almost nothing is uniform in PEFC stack operation in the channelwise,

through-plane, or plate-to-plate directions. From an engineering design perspective, many

of these gradients can be intentionally exploited for performance optimization. An excellent

example already discussed is that of water management and temperature distribution. In

order to provide a fully moist condition throughout the fuel cell without excessive ﬂooding

and simultaneously reduce the load on the humidiﬁer system, the temperature distribution

can be adjusted through the coolant ﬂow rate to increase steadily from the inlet region to

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6.6 Multidimensional Effects 363

the exit region, so that the relative humidity of the ﬂow is approximately uniform and 100%

throughout and the water generated is absorbed as vapor reducing ﬂooding.

The computational or analytical prediction of the current, species, and temperature

distributions in a fuel cell can be generated by extension of the basic concepts of this

text into multidimensional models with various levels of complexity and solved using the

tools of computational ﬂuid dynamics (CFD). Models of various complexity abound in the

literature, and commercial packages now exist from several CFD software developers for

this purpose.

Water and Species Distribution Along the channel path from inlet to exit, one can

anticipate a reduction in the hydrogen and oxygen species concentrations that follow the

consumption by the electrochemical reactions. The mole fractions of the reactant species

may not be decreasing, however, since humidiﬁcation or dehumidiﬁcation of the ﬂow can

alter the water mole fraction. Consider a wet anode input with a dry cathode input and

cocurrent ﬂow, depicted in Figure 6.63. Conditions of dehumidifying ﬂow can actually

increase the reactant mole fraction, even with consumption, as shown in Figure 6.64. As

the ﬂow enters, the net drag will be heavily positive, favoring ﬂow of water from the

humidiﬁed anode inlet toward the dry cathode. The result will be a decreasing water mole

fraction, and increasing hydrogen mole fraction along the anode, despite the fact there is

hydrogen consumption along the channel. At the point where the water vapor concentration

on the anode and cathode are the same, there will be a diffusion ﬂux reversal point.

Temperature Distribution Along the fuel cell channel, the temperature distribution is

directly controlled by the heat transfer boundary conditions. For small fuel cell stacks, with

no active coolant ﬂow, the external boundary conditions control the temperature distribution

and at low current can be considered as uniform in temperature. For larger stacks with active

cooling, the temperature distribution can be engineered to match the desired humidity proﬁle

to control ﬂooding and promote longevity by elimination of dry- and hot-spot locations. In

the in-plane direction, temperature variation exists, with more water accumulation under

generally colder lands, as discussed. The temperature distribution in the stack can be fairly

Moist anode inlet

Dry cathode inlet

Diffusion reversal location

Net water drag

toward cathode

Electrolyte/electrode assembly

-net flux reversal may be different location

Figure 6.63 Schematic of dry cathode–wet anode inlet ﬂow that results in diffusion reversal process

in electrolyte. The arrows represent the direction of diffusion ﬂux, not the net mass ﬂux. (Adapted

from Ref. [11].)

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364 Polymer Electrolyte Fuel Cells

Figure 6.64 Water vapor and hydrogen mole fraction distributions for a concurrent ﬂowing ser-

pentine isothermal fuel cell with 100% RH anode inlet, 0% RH cathode inlet at 0.7 V opera-

tion. Even though current is being produced and hydrogen consumed, the hydrogen mole fraction

along-the-channel is increased near the inlet due to dehumidiﬁcation of the anode. (Adapted from

Ref. [11].)

well predicted using known thermal parameters and voltage current behavior to predict heat

generation.

Current Distribution The current distribution in a PEFC can be measured experimentally

with a variety of techniques, as discussed in Chapter 9. For larger fuel cell active areas,

gradients in temperature, reactant concentration, humidity, and liquid water can drastically

alter the local current density and should be considered in any advanced analysis. In order

to predict the current distribution in a cell on a qualitative basis, one has to keep in mind

the major driving forces that control the current at a given voltage:

1. Local relative humidity

2. Local reactant concentration

3. Local liquid water distribution

Since the temperature in the PEFC typically varies by at most ∼15

◦

C, the effect

of temperature distribution on the reaction kinetics will be relatively small compared to

the temperature interaction with the liquid water distribution and relative humidity that

controls membrane ionic conductivity. The local relative humidity in the anode phase

typically controls the local current density of an underhumidiﬁed fuel cell because electro-

osmotic drag exacerbates anode dryout, while water generation at the cathode diminishes

any electrolyte dryout in the cathode catalyst layer [11]. That is, if the other parameters

are constant, the local current distribution can be predicted with a knowledge of the anode

in an underhumidiﬁed cell [11]. Anode dryout can be the result of the loss of only a few

hundredths of a milligram of water per square centimeter active area in the catalyst layer

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6.6 Multidimensional Effects 365

Current

Location

(

x/L

)

Flooding/concentration

losses (may or may not

occur)

Dip due to

anode dry-out

at high i

Moist

anode/high

Dry anode

inlet

Water flux

reversal

to anode

Peak height increased

with current, net water

imbalance

Region II

Region I

Region III

Balanced

water

content

inlet

current inlet

Figure 6.65 Characteristic local current curve for undersaturated inlet conditions. (Adapted from

Ref. [11].)

[10]. Since the catalyst layer is only on the order of 15 µm thick and consists of about 30%

ionomer by weight, a very small amount of local drying can signiﬁcantly decrease the ionic

conductivity of the catalyst layer.

Figure 6.65 is a sketch of the generic relative current versus location along the ﬂow

channel for different combinations of inlet humidity on the anode and cathode with a

concurrent serpentine design and is key to understanding local performance for underhu-

midifed conditions. The generalized performance curve is qualitatively sinusoidal in shape

(although not necessarily symmetric or with equal amplitude or wavelength, as shown for

convenience). These results were obtained for a uniform coolant temperature boundary

condition, so any effects of a nonuniform temperature are not included. Region I begins at

the inlet of the fuel cell. If the anode is dryer than the cathode and the current is relatively

low, the net ﬂux of water is always neutral or toward the anode, resulting in an increasing

performance through region I. The steepness of the slope of this portion is exacerbated

with increased moisture imbalance between anode and cathode, increased current density,

and other thermodynamic and geometric parameters. In contrast, if the water vapor im-

balance initially favors the anode (dryer cathode), diffusion and electro-osmotic drag are

both initially toward the cathode, resulting in a drying condition at the anode catalyst layer

and electrolyte, reducing performance until a local minimum is reached, which terminates

region I. The minimum in performance is exacerbated by increased current density and

moisture difference between anode and cathode as well as ﬂow rates and other thermody-

namic and geometric parameters. Note that for closely hydrated anode and cathode at low

to moderate current density performance in region I can be homogeneous, as illustrated in

Figure 6.65. Note that the end of region I is shown as the same current for dry anode and dry

cathode conditions, when this is not necessarily the case. In fact, region II for a relatively

dry anode inlet is really a continuation of region I and no discrete boundary between the

two regions is deﬁned.

In region II, the case with relative dry anode feed continues increasing performance to

a maximum (if reached), which ends region II. The decrease from the maximum current is

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366 Polymer Electrolyte Fuel Cells

a result of decreasing reactant availability or ﬂooding and depends on operating conditions,

and therefore may not be severe, as shown in Figure 6.65. The dry relative cathode case

begins region II at the point of water mass ﬂux reversal from cathode to anode. In this

situation, this reversal may not occur with a thick electrolyte if back diffusion never

overcomes electro-osmotic drag to the cathode. This ﬂux reversal reduces anode-side

dryout performance loss and initiates an increase in performance. For both the dry relative

anode and cathode cases, region II concludes with a maximum in performance. Note that

the location of the maximum is dependent on the fuel cell and operating conditions.

In region III, for convenience, both dry anode and cathode cases are shown to peak at the

same location, although this depends on the individual conditions and is not necessarily the

case. Following the maximum local current, there is a downward trend resulting from local

ﬂooding or gas-phase mass transport losses at the electrode(s). This peak and downward

trend will only occur if a reactant starvation condition (via ﬂooding or high utilization) is

reached.

The generalized performance curve of Figure 6.65 closely follows anode gas channel

water vapor content through region II, indicating anode catalyst layer and electrolyte mois-

ture content plays a key role in controlling the distributed performance of an undersaturated

inlet PEFC. Cathode conditions are generally locally moist at the catalyst layer and control

the location of the maximum performance which initiates region III. The region I trend

depends on the side with greatest hydration and local water activity. The amplitude of the

peaks is increased with increasing current density and water imbalance between anode and

cathode at the inlet. The width of regions I–III depends on current density, ﬂow rate, water

imbalance, and membrane and electrode thicknesses as well.

In general, under high-humidity conditions with neat hydrogen ﬂow, the current proﬁle

follows oxygen distribution. The current distribution is reduced along locations of ﬂooding,

which typically occurs near the cathode exit or in cold spots in the fuel cell under landing

areas. In this case, excessive liquid water buildup restricts oxygen ﬂow to the catalyst layer,

and local current can be greatly reduced. In single-cell and stack situations, it is quite

possible that certain locations of the fuel cell are dry, and others are ﬂooded.

Example 6.7 Qualitative Prediction of Current Proﬁles in Simple Fuel Cell System For

an isothermal neat hydrogen fuel cell at moderate current density with adequate cathode

ﬂow stoichiometry, sketch the current distribution from inlet to exit for the following cases:

(a) Fully moist inlet anode and cathode, concurrent ﬂow

(b) Dry anode and cathode inlet, concurrent ﬂow

(d) Dry cathode and fully moist anode inlet ﬂow, concurrent ﬂow

SOLUTION

(a) In this case, the ﬂow is fully moist at both the inlets, so that the current should be

dominated by the oxygen mole fraction. The current should decrease from the inlet

to the exit in a decaying fashion, since the oxygen consumption will follow the

same qualitative shape as the current decays. At some point, the effects of ﬂooding

will likely induce severe performance loss, as sketched.

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6.6 Multidimensional Effects 367

Distance from Inlet

Current Density

Cathode

Anode

(b) In this case, both the anode and cathode are dry, and thus the performance should be

quite low at the inlet and gradually increase as water is produced along the channel

and hydrates the electrolyte. If the ﬂow path is long enough, the performance will

peak when the membrane is fully humidiﬁed and can eventually decrease due to

ﬂooding effects.

Distance from Inlet

Current Density

Possible flooding

Cathode

Anode

This is a ﬂow conﬁguration often employed in portable applications, where no

humidiﬁer system is used. In this underhumidiﬁed case, the performance should

be dominated by the anode moisture content. Along the anode ﬂow channel, the

moisture content will gradually increase to a peak as moisture is transferred by

diffusion from the cathode, then it should decrease toward the exit as the moisture

is transferred back to the cathode. The current proﬁle will look follow the water

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368 Polymer Electrolyte Fuel Cells

uptake trend, although the peak current location will depend on the relative ﬂow

rates of the anode and cathode dry ﬂows.

Distance from Inlet

Current Density

Anode

Cathode

(d) In this case, the current distribution will be dominated by anode moisture content,

since the overall ﬂow is underhumidiﬁed. As the water is transferred from the

moist anode to the cathode, the current will decrease to a minimum then increase

again after ﬂux reversal. Given a long enough channel length, the performance will

eventually decline due to ﬂooding.

Distance from Inlet

Current Density

Possible flooding

Cathode

Anode

COMMENTS: The sketches drawn should be thought of as qualitative tools only, to help

in understanding the internal distributions of the various controlling parameters.

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6.7 Summary 369

6.7 SUMMARY

The goal of this chapter is to present the reader with a summary of the operational issues

and design constraints of the PEFC. Within the PEFC scope is the hydrogen-powered

PEFC and the alternative fuel PEFC, normally called a DAFC, since the vast majority of

the alternative fuels proposed are alcohol based. The DAFCs all have inherently reduced

fuel cell performance compared to the H

PEFC because the anode kinetics are more

complex. Additionally, issues of fuel and water crossover and anode catalyst poisoning

from intermediate reactions further reduce the performance relative to the hydrogen fuel

cells. The reduced performance of DAFCs is deemed tolerable in portable applications,

however, where system size and simplicity are valued more than efﬁciency. Portable DAFC

systems can be designed to eliminate the humidiﬁcation, cooling, and bulky hydrogen

storage needed for H

PEFC systems.

Despite the fact that the PEFC is a water generation reactor, some humidiﬁcation of

the reactants is usually necessary to enable high performance and longevity. A dry inlet

feed results in poor local performance, hot spots, and internal stresses that can lead to

short lifetimes. There are various active and passive humidiﬁcation methods used to ac-

complish humidiﬁcation, including membrane humidiﬁcation, direct injection, and internal

or external recirculation.

One of the most difﬁcult engineering challenges in the PEFC is achieving a net water

balance without ﬂooding. Like dryout, a ﬂooding condition with liquid water accumulated

in the pores of the various media or in the ﬂow channels can reduce performance and

exacerbate various degradation mechanisms. From a global perspective, we seek to remove

the net water produced from the fuel cell to maintain a steady-state condition in terms

of water storage in the fuel cell. This condition is accomplished via Eq. (6.16), derived

from a control volume balance on the entire fuel cell assumed at steady state and repeated

here:



,in,dry

− 1



out,c

1 − χ

out,c

−



,in,dry



in,c

1 − χ

in,c



,in,dry

− 1



out,a

1 − χ

out,a

−



,in,dry



in,a

1 − χ

in,a



slugs,out,a

slugs,out,c





= 1

whereχ = RH · P

sat

(T )/P.

The condition of a balance can be obtained in practice through tailoring the inlet

humidity, ﬂow rates, exit temperature (through coolant ﬂow manipulation), and pressure

drop though the fuel cell. Since a condition of exact balance is rarely achieved and an

overall balance may not satisfy the desire for maximum performance, the fuel cell is

typically operated in a slightly ﬂooded situation, and a periodic growth and rejection cycle

of liquid droplet slugs from the fuel cell achieves a quasi-steady balance condition.

Even when the fuel cell achieves a global water balance, there can be areas of dry-

out and ﬂooding inside the fuel cell itself. Water transport inside the fuel cell through

the electrolyte is governed by diffusion, electro-osmotic drag, hydraulic permeability (of

gas and liquid phases), and temperature gradient effects. Electro-osmotic drag is always

toward the cathode, but the diffusion, temperature, and hydraulic permeability gradients

can be engineered to achieve a favorable water balance. The liquid permeability direction