Mench M.M. Fuel Cell Engines

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320 Polymer Electrolyte Fuel Cells

ChannelGDLMiPLCL

MiPL

GDL

at SS

out

Figure 6.30 Typical water distribution in fuel cell porous media under steady state after sufﬁcient

time to allow equilibrium in porous media between stored and ﬂowing liquid. The discontinuities in

the saturation level are a result of the changing pore size and hydrophobicity between the different

layers. Hydrophobic layers are shown, and all water generated is assumed to leave through a single

channel.

It should be emphasized that the capillary motion will be induced only for droplets along

a connected ﬂuid path. That is, isolated droplets which have condensed on a surface will

feel no force other than that from the droplet interaction with the surface. Therefore, some

fraction of liquid can remain as isolated droplets despite capillary effects in the continuous

liquid phase. At equilibrium conditions, a pressure balance in the gas and liquid phases

across interfaces must exist. Therefore, we can predict the water distribution and ﬂooding

locations depending on the hydrophobicity of the surfaces and the pore size distribution.

Figure 6.30 shows the liquid water saturation distribution in the porous media assuming

an isothermal condition with no phase change and hydrophobic surfaces at steady state.

In steady state, there is a ﬂow of water from the catalyst to the channel and out of the

fuel cell as condensed slugs and vapor. Discontinuities in the liquid saturation due to the

pore size variations result in a saturation jump for the same capillary pressure. At each

interface, the liquid-phase capillary pressure is balanced (in steady state). The typical pore

size distribution in the fuel cell soft goods (membrane, electrode, and DM) is as follows:

catalyst layer (∼50 nm) < microporous layer (∼100 nm) < DM (∼100–500 µm).

For the same interfacial liquid pressure and hydrophobicity, the media with the smallest

hydrophobic pores (catalyst layer) will have the lowest liquid saturation. If there were

similar levels of hydrophobicity in each larger, the capillary pressure gradient will drive

the liquid ﬂow from the catalyst layer through the microporous layer and DM into the ﬂow

channel. Recall however that the microporous layer is typically much more hydrophobic.

Therefore, the MPL can act as a barrier for liquid ﬂow unless the break though pressure is

reached. Typical MPLs have a break through pressure around 5–10 kPa.

In hydrophilic pores of the CL and DM, liquid will accumulate until removed by

evaporative or convective forces and will not spontaneously drain into channels.

In our discussion so far, we have not included the possibility of phase change and

temperature gradients which play a large role as well. While isothermal conditions may be

true at low current density, higher current densities include phase change and temperature

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6.2 Water Balance in PEFC 321

gradients. Under current, the polarization losses will generate a majority of the heat dis-

sipated by the fuel cell. Since most of the entropy change and activation polarization are

generated at the cathode, this is typically the hottest location in the fuel cell, by up to 5

◦

or more under high current depending on the thermal properties of the DM. At the cathode

catalyst layer, vapor-phase water will be transported into the electrolyte and back to the

anode or outward to the cathode ﬂow channel. As the temperature cools, there will be a

saturation plane that develops where liquid water condensation occurs and ﬁlls the porous

media. The location of condensation will have a tremendous effect on the ﬂooding. For the

mixed wettability catalyst layer, the condensation would tend to take place in the predomi-

nantly hydrophilic pores ﬁrst, ﬂooding them, while the hydrophobic pores remained mostly

liquid free. If the condensation plane is beyond the catalyst layer and in the microporous

or diffusion layer, the highly hydrophobic nature of the MPL will prevent backﬂow into

the catalyst layer. Some have also suggested the MPL acts to force liquid water toward

the anode, preventing dryout [31, 32]. This can be true of ﬁlled hydrophilic connected

networks in the catalyst layer. When the CL is ﬂooded and cannot overcome the break-

through pressure into the cathode MPL, then back ﬂow to the anode by pressure-driven ﬂow

can occur.

Gas-Phase Transport in Catalyst, Microporous, and Diffusion Layers The role of the

different porous media in the fuel cell (catalyst layer, microporous layer, and diffusion

media) in liquid transport can be grasped by understanding capillary ﬂow behavior in mixed

wettability porous media. The role of these porous media in controlling gas-phase ﬂow can

be simply understood through gas-phase transport relationships. Recall from Chapter 5 that

the diffusivity in porous media must be modiﬁed to account for the tortuosity and porosity

of the porous media:

eff

= D

(6.33)

Thus, the higher the tortuosity and the lower the porosity of the porous media, the more

the diffusion will be restricted. Therefore, from a gas-phase transport perspective the

microporous layer in the fuel cell restricts transport from the DM to and from the catalyst

layer. Lands also play an important role in restricting gas-phase transport to the catalyst

layer. The proper landing width is a balance beween reactant ﬂow restriction and electrical

contact limitation.

6.2.3 Overall Role of Materials in Maintaining High-Performance

Electrode

Based on our understanding of the gas- and liquid-phase transport in the fuel cell, a uniﬁed

view of the role of the various porous media can be constructed and is shown in Figure

6.31. It should be noted that due to the lack of direct experimental observation of the very

small microporous and catalyst layers, there exist several different theories to resolve the

observed inﬂuences of the material properties on fuel cell performance.

Microporous Layer: Cathode Side A MPL on the cathode side has been experimentally

determined by many to enhance performance under high-current-density and high humidity

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Figure 6.31 Schematic of roles of different porous media in PEFC.

conditions, where catalyst layer and DM ﬂooding is prevalent. Since the performance in

this situation is generally limited by the ﬂooding, and not the gas-phase oxygen transport,

the additional diffusion resistance of the MPL does not signiﬁcantly reduce performance.

Based on the schematic shown in Figure 6.31, it is believed that the MPL serves as

a highly hydrophobic boundary to prevent water ﬂow back into the catalyst layer after

condensation and can also force water back to the anode by capillary pressure forces.

The capillary pressure gradient through the hydrophobic microporous and macroporous

layers also favors ﬂow toward the lower pressure channel, draining the DM and preventing

catalyst layer ﬂooding. The DM also serves as an intermediate transition between the very

small pore catalyst layer and the macroporous layer. With no MPL, small water droplets

emerging from the catalyst layer into a macroporous layer will see the equivalent of an open

channel, and capillary ﬂow will stop, pooling liquid in this location. The MPL therefore

provides a continuous, connected ﬂow of liquid emerging from the catalyst layer to ﬂow to

the macroporous DM and into the channel.

In low RH conditions, the cathode MPL can force ﬂow through the membrane into

the anode by capillary pressure forces, reducing dryout and increasing performance. The

backﬂow of water would occur primarily through the hydrophilic pore network in the

catalyst layer, since complete pore saturation in the catalyst layer would result in nearly

total performance loss, and high saturation in the hydrophobic pores of the CL would likely

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6.2 Water Balance in PEFC 323

overcome the breakthrough pressure of the MPL, resulting in sporadic liquid slug emission,

a phenomenon which has been observed to occur experimentally.

Microporous Layer: Anode Side The role of the MPL on the anode side is completely

different from that on the cathode side, although use of a MPL on the anode side has also

been observed to enhance performance in low-humidity environments where the water in

the anode ionomer can be easily lost to the dry hydrogen stream. The mass diffusivity of

water vapor into hydrogen is up to four times greater than into air, as shown in Chapter 5.

Thus, combined with electro-osmotic drag of water from the anode to the cathode,

local anode dryout is a common reason for low performance and reduced longevity in

low-humidity conditons. The use of an anode MPL limits the moisture removal from

the anode by acting as a diffusion barrier, reducing this dryout effect, as illustrated in

Figure 6.31. Under high-humidity conditions, the use of an anode MPL does not normally

have much of an impact on performance, which has been validated experimentally [33].

Mechanically, the MPL has also been suggested to serve to protect the electrolyte from

puncture from protruding ﬁbers from the macroporous layer.

Diffusion Media: Anode and Cathode Sides In terms of optimizing gas-phase trans-

port in the macro–diffusion layers or the MPLs, there is a key engineering trade-off to

consider. Obviously, high diffusivity of reactant to the catalyst is desired to promote re-

action and limit concentration polarization. However, high moisture in the electrolyte is

also desired. High-temperature and low-humidity conditions simplify system design but

can lead to anode dryout with accelerated degradation and poor performance. For high

reactant diffusivity, liquid saturation must be minimized, and there must be a high hy-

drophobic porosity. On the cathode side, oxygen transport is already limited by a low

initial mole fraction, high water saturation, and reduced diffusivity coefﬁcient, compared

to hydrogen. Here, the focus is generally on prevention of ﬂooding. On the anode, how-

ever, water vapor diffusivity loss into hydrogen can be severe, there is very little con-

centration polarization limitation at the anode, and hence a ﬂow-restricting structure is

preferred. The dominating role of the DM depends on the operating conditions. Under high

current conditions, the oxygen transfer to the electrode is limiting performance, and an

open hydrophobic structure promoting gas-phase transport with good liquid water removal

on the cathode is necessary. In low-humidity environments, the lack of moisture domi-

nates, and a closed-pore, less hydrophobic structure is needed to restrict vapor loss to the

ﬂow channel.

From an overall porous media design perspective, the various porous media should be

tailored to achieve the desired liquid- and gas-phase transport behavior. In the examples

mentioned, and in most common materials, the properties are mostly uniform. However,

the potential for enhanced vapor and liquid ﬂow with hydrophobicity or pore size gradients

exists and has been exploited in some specialized materials. Overall, different membrane

electrode assembly conﬁgurations and materials are preferred by different manufacturers,

and the exact nature of transport in these regions is not yet perfectly understood. Complicat-

ing factors that must be considered include the tightly coupled heat transport phenomena,

nonisotropic material transport properties, and highly nonhomogeneous current density

along the electrode. Since phase-change plays a key role, the thermal conductivity is another

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Figure 6.32 SEM of PEFC catalyst layer with relatively large scale macro cracking. (Image by

Soowhan Kim, Penn State Fuel Cell Dynamics and Diagnostics Lab.)

design parameter of interest. Other factors include assembly sag into gas channels, which

has been linked to ﬂow maldistribution. Thus, a stiff DM is generally preferred for case of

assembly and prevention of interfacial gaps.

Mud Cracking, Interfacial and Morphological Effects Although it is easy to think of

pore size as a constant in the various porous media, there is a distribution of pores in the

DM and MPLs, and the catalyst layer can have signiﬁcantly large cracks and gaps between

interfaces in the catalyst surface, as shown in Figure 6.32. Catalyst layer surface cracking

is typically present from manufacture and is a result of the presence of volatile compounds

in the catalyst slurry. The viscous catalyst slurry is heated in an oven after application,

removing these volatiles and shrinking the catalyst layer, causing what some have termed

“mud cracks” to appear. From a multiphase ﬂow perspective, this situation is very different

than a continuous homogeneous phase, and some larger gaps and cracks may dominate ﬂow

physics in these regions. These cracks and gaps are orders of magnitude larger (they can be

upto ∼10 µm wide) than the normal pore size in the catalyst layer, and these cracks would

be regions of reduced capillary pressure, promoting liquid pooling. In terms of gas-phase

transport, these macrocracks may enhance reactant transport to catalyst regions by reducing

ﬂow resistance. These cracks also increase the effective catalyst layer porosity, enabling

high reactant species ﬂux to the catalyst surface. From a durability perspective, however,

these cracks have been linked to enhanced degration.

In PEFCs, the interface between the channel, land, and DM, and the interface between

the DM and catalyst layer can also affect transport of liquid. If there is a gap, this can

serve as a pooling location for water. At the interface between the land and the DM, water

stored in the DM can be drawn out with a hydrophilic surface or retained in the DM by a

hydrophobic surface (Figure 6.33).

Neutron imaging has also been used to conﬁrm the important role the land interface

has on the liquid water content stored at equilibrium. For fuel cells with otherwise identical

operating conditions, the greater the number of hydrophilic land interfaces, the less liquid

water content there is in the DM [34]. On the other hand, hydrophobic land surfaces have

been shown to retain water under the lands, and promote ﬂooding by restricting drainage

from the DM.

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6.3 PEFC Flow Field Conﬁgurations and Stack Design 325

Figure 6.33 Liquid water removal from under land location. With hydrophilic channel wall sur-

faces, there is a net suction from the DM to the channel. If the wall is hydrophobic, however, liquid

is retained in the diffusion media, causing earlier ﬂooding.

6.3 PEFC FLOW FIELD CONFIGURATIONS

AND STACK DESIGN

Design Considerations There are many different ﬂow ﬁeld conﬁgurations that have been

used for fuel cells in general and PEFCs in particular. The design of a ﬂow ﬁeld is a complex

balance between many coupled constraints, many with opposing functional dependencies

on the fuel cell performance, so that trade-offs must be made. The design constraints include

the following:

1. Gas-Phase Transport and Parasitic Pressure Loss The channel and stack manifold

design directly affects the ﬂow and mass transport to the catalyst surfaces as well

as the parasitic pressure drop. The ﬂow through the DM and to the catalyst surface

must be adequate to limit mass transport losses, and the pressure drop must be

minimized.

2. Electron Transport For acceptable fuel cell performance, the electrical resistance

must be low. This is provided by adequate contact area and material choices between

the land and DM material.

3. Heat Transport Since a vast majority of the heat generated by fuel cell operation

must be removed by the fuel cell and not through the exhaust gas, the landing–DM

interface area carries much of the heat into the coolant. Local hot spots caused by

poor design can cause dryout and lead to premature membrane failure. The coolant

and reactant ﬂow channels should be designed in tandem to minimize ﬂooding and

hot spots.

4. Liquid Water Storage and Transport The particular cell design has an impact on

the amount of liquid water stored in the PEFC during normal operation and the ease

with which it is removed. The presence of liquid water has a critical impact on the

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operating performance, pressure loss, degradation via ionic contaminants, and time

to start and degradation from a frozen condition in a fuel cell stack.

5. Gas-Phase Vapor Transport For low humidity operation, dryout of the ionomer

into the channel ﬂow can severely limit performance. Large channel widths pro-

mote dryout, while limiting dead zones under thinner lands. A channel to land

ratio must be found to balance the oxygen restrictions with dryout for low RH

operation.

6. Degradation Mitigation There are many different modes of degradation in PEFC

performance, as discussed in more detail in Section 6.5. The fuel cell system must

be designed to avoid unacceptably high performance loss rate from irrecoverable

losses (e.g., a pinhole in the membrane) and be capable of sensing and mitigating

recoverable losses (e.g., a ﬂooded electrode).

7. Freeze–Thaw Damage Mitigation/Rapid Start-up If the fuel cell will be operated

in ambient environments, survivability at subfreezing temperatures and rapid power

availability at startup are required.

8. Other Constraints Many other factors can affect the ultimate performance of the

PEFC stack. In this context, the term “performance” is meant to be a general measure

of the overall effectiveness of the fuel cell design, including power density, cost,

and durability.

It should be emphasized that the constraints listed are not independent. In many cases,

design to satisfy one constraint will impact others. An excellent example is the channel to

land width ratio, discussed below.

Design Constraints: Using Extremes to Illustrate Trade-offs Often in engineering sys-

tems the basic trade-offs can be best illustrated by consideration of extreme cases. Consider

the generic fuel cell channel design with typical geometric parameters shown in Figure

6.34. The lands are an obstruction for mass transport that can potentially cause dead zones

where reactant is unavailable. In the extreme case, the entire ﬂow channel would be land,

and no reaction could take place.

At the other extreme, as shown in Figure 6.35, the entire fuel cell is open in design,

with no mass transfer obstruction from lands. However, the performance of this design is

poor, resulting from lack of electron conduction contacts provided by lands, and DM sag

into the channel volume is likely. The best case is a balance between the two constraints that

L = land width

W = channel width

= draft angle

d = depth

W:L

(typically 0.2–2.5 mm)

(typically 0.5–2.5 mm)

(typically 0–15

)

(typically 0.5–2.5 mm)

(typically 3:1–1:1)

Figure 6.34 Typical PEFC channel/land design features. The draft angle can be used to tailor the

channel cross-sectional area without affecting the depth (stack volume) or land contact area.

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6.3 PEFC Flow Field Conﬁgurations and Stack Design 327

Open

Current collector

Figure 6.35 Extreme-case design-maximum channel width current collector along boundary and

otherwise open. This design has the least obstructed mass transfer but the worst electrical contact and

would suffer severe DM tenting.

minimizes the presence of both dead zones and electrical contact resistance losses. Under

low humidity conditions, a balance between vaper loss and heat transfer is also required.

Another common trade-off is channel velocity versus parasitic pressure drop. As dis-

cussed in Chapter 5, ﬂow in channels is typically laminar, so that pressure drop, ignoring

entrance and turbulence effects, can be written as

P =

32µLV

(laminar ﬂow) (6.34)

Some turbulence and minor losses from channel switchbacks and entrance effects are

common. For a given cell design, an empirically derived relationship that combines the

main laminar losses and minor losses can be used as

P = av + bv

(6.35)

On one extreme, a high channel velocity (and stoichiometry) ensures nearly uniform reactant

concentration from inlet to outlet and helps to remove any liquid droplets by shear force.

However, very high ﬂow rate results in higher parasitic pressure losses and can also lead

to unintended maldistribution of ﬂow throughout the stack. At the other extreme of very

low ﬂow rates, liquid water slugs will not be removed by channel shear forces, and reactant

concentration losses will be severe near the fuel cell exit. The normal range of balance

in H

PEFCs is laminar ﬂow with a stoichiometry of 1.2–1.5 in the anode and 2.0–2.5

in the cathode. Many more trade-offs exist and involve complex interactions and multiple

considerations, so that no true optimal channel design exists. However, it is clear that a

prespective involving understanding of the coupled capillary, phase-change, heat transfer,

and gas phase ﬂows must be used to arrive at an ideal design.

PEFC Flow Field Designs The ideal fuel cell ﬂow ﬁeld design results in the following

qualities:

1. Excellent mass transport of the reactants to and products from the catalyst layer

with a proper water balance to achieve a moist electrolyte with minimal ﬂooding of

the DM and channels under a wide range of operating conditions.

2. Excellent electron transport to and from the catalyst layers.

3. Adequate heat transport to the coolant channels.

4. Low pressure drop from inlet to exit.

5. Low cost of manufacture.

6. Compact design.

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Figure 6.36 Schematics of some common ﬂow ﬁeld design patterns. Many other basic design

features have also been tried as well, including fractal and spotted post designs. (Image by Soowhan

Kim.)

The particular fuel cell design has a great deal of inﬂuence on the transport of reactants

to the electrode. Some basic fuel cell ﬂow channel designs are illustrated in Figure 6.36, with

additional details on the inherent trade-offs in Table 6.3. Most standard fuel cell designs are

a combination of parallel and serpentine designs, like that shown in Figure 6.37. By tailoring

the channel size and parallelization, the channel velocity and pressure drop can be balanced.

Table 6.3 Channel Designs in PEFCs

Flow Field Design Advantages Disadvantages

Serpentine High channel velocity, good water

removal, high performance

High pressure drop, uneven species

concentration distribution

Parallel Low pressure drop, more even

concentration distribution

Low channel velocity, poor liquid

water removal

Parallel–serpentine

combination

Can balance advantages of

serpentine and parallel design

Combination of parallel and

serpentine disadvantages

Interdigitated High performance, excellent water

removal from under landings

High pressure drop for forced ﬂow

through DM, possible long-term

damage to structure

Porous plate Excellent water uptake capabilities Expensive design, coolant used

must be freezable water

Mesh Low pressure loss, controllable

total contact area

Dead zones and water accumulation

away from path of least resistance

Spiral Lower humidity load since exit

channels run by inlet channels

Unintentional reactant bypass

between low-pressure exit can

high-pressure inlet channels

Metal foam plate Uniform compression on DM

surface, ease of manufacture

Higher pressure drop

Radial Lowest diffusion path length for

passive designs

Only used in passive system

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6.3 PEFC Flow Field Conﬁgurations and Stack Design 329

Figure 6.37 Photographs of graphite 50 cm

active area parallel–serpentine ﬂow ﬁeld used for

laboratory studies. The laboratory ﬂow ﬁeld plates are over 1 cm thick to improve durability in a

laboratory environment. Actual fuel cell stack ﬂow ﬁeld plates are considerably thinner to improve

power density.

The interdigitated design is a unique design that has no continuous channels. Through

a series of dead-end channel inlet ﬁngers, as shown in Figure 6.36, ﬂow is forced into the

DM and under the lands into the outlet channel ﬁngers. A crosssection of the interdigitated

design is shown in Figure 6.38. In the process, liquid water is removed from the DM, and

the reactant transport to the surface is greatly facilitated by forced convection. Performance

with the interdigitated design is very good, but the parasitic pressure drop is also increased.

United Technologies has developed a unique porous plate ﬂow ﬁeld technology,

illustrated in Figure 6.39. The ﬂow channels have a typical parallel and serpentine de-

sign, but the landings have many small hydrophilic capillary columns, which allow the

cell to wick water from the DM surface by capillary action. When the DM becomes sat-

urated, the porous plate wicks water away and into the coolant channels, maintaining a

low saturation level and improving high-current-density performance. This design is a

passive method to manage water transport and ﬂooding using capillary forces. Naturally

occurring capillary forces are quite effective to transport water (consider that trees have no

pumps). Interestingly, natural forces are very rarely the ﬁrst possible solution considered

by engineers, although they can often be exploited, as in this unique design.

Figure 6.38 Schematic of cross section of interdigitated design to force ﬂow convectively to catalyst

surface under land locations.