Mench M.M. Fuel Cell Engines

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340 Polymer Electrolyte Fuel Cells

(a)(b)

Figure 6.51 9-cm

Mobion

R

direct methanol fuel cell chips, which are used by Mechanical

Technology, Inc. (MTI) as the power source for consumer electronics applications. The Mobion

technology is based on a passively fed DMFC, with 100% methanol fuel. A power density of over

50 mW/cm

can be achieved. (Image courtesy of MTI.)

such as the coolant and humidiﬁcation subsystems can be eliminated. In the simplest case,

the system contains only fuel storage and delivery/recirculation systems, the fuel cell,

and microelectronics. By elimination of various subsystems, the portable fuel cell system

proﬁles can become extremely compact, even though the fuel cell stack (without ancillary

components) could be smaller if operating on pure hydrogen. Figure 6.51 shows 9-cm

DMFC chips used to power consumer electronics.

Direct alcohol fuel cells do suffer some drawbacks compared to hydrogen PEFCs:

1. Low operating efﬁciency

2. Reduced performance, due to more complex kinetics, and ﬂooding issues

3. Increased cost per kilowatt due to much higher catalyst loadings, on the order of

1–8 mg/cm

total (anode plus cathode) precious metal loading, compared to 0.2–

1.0 mg/cm

total loading for the H

PEFC

However, considering the portable market, efﬁciency and cost are less important than

system size, so these trade-offs are acceptable. The most developed DAFC is the direct

methanol fuel cell (DMFC). Many prototype and nearly commercial DMFC systems exist

for powering small electronics, laptop computers, cell phones, hand-held electronics, and

other devices. Direct alcohol fuel cells are expected to occupy a growing market for portable

power for years into the future.

The search for high-performance alternative fuels to hydrogen has led to many can-

didates. Table 6.4 provides a summary of safety, thermodynamic, and other data collected

on the potential candidates. Some fuels are potentially attractive alternatives due to ease

of handling or improved safety ratings. As with any fuel cell, there is no perfect solution,

and each potential fuel has its own limitations. Alternative fuels to hydrogen are an attempt

to eliminate one or more of the disadvantages of another fuel. One way to think about

alternative fuels is as hydrogen carriers. That is, they are high in hydrogen content but with

a greater hydrogen density than hydrogen gas because they are typically stored in liquid or

solid form. There are many potential fuels that can be used in portable applications where the

desire is for a compact size and low efﬁciency and higher cost per kilowatt can be absorbed.

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Table 6.4 Properties of Various Methanol Alternatives Being Considered for Portable Fuel Cell Use

Molecular Evaporation Rate Freezing/ Flash

Chemical Weight Density Solubility (relative to butyl Boiling Point Theoretical

Name Composition (g/mol)

(g/mL)

in Water

acetate = 1)

Point (

◦

(

◦

Capacity

Methanol CH

OH 32.0312 0.791 Completely Miscible 4.6 −98/64.6 12 5.03 Ah/g

Ethanol C

OH 46.0468 0.789 Miscible >= 10 g/

100 mL at 23

◦

N/A −114.1/78.3 12 N/A

Dimethoxy-methane

(DMM)

76.0624 0.086 33 g/100 mL 14.4 −105/42 −18 N/A

Trimethoxy-methane

(TMM)

106.078 0.97 N/A N/A −53/101–102 13 N/A

Trioxane C

90.0468 1.17 > 10 g/100 mL at

◦

N/A 64/114.5 45 N/A

Formic Acid CH

46.0156 1.21 Completely Miscible 2.1 8.3/100.7 69 N/A

Formaldehyde CH

O 30.0156 1.09 Very Soluble 55 g/mL N/A −118/−19.5 60 N/A

Dimethyl Ether (DME) (CH

)

O 46.0468 Soluble N/A −138.5/−22 −41 N/A

Ethylene Glycol (EG) C

62.0468 1.1155 Miscible >= 10 g/

100 mL at 17.5

◦

0.01 −13/195 111 4.32 Ah/g

Dimethyl Oxalate

(DMO)

118.0468 1.148 Limited N/A 52/163.4 75 3.18 Ah/g

Oxalic Acid C

94.0468 1 10 g/100 mL N/A 189/190 N/A 43 Ah/g

(Continued)

341

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Table 6.4 (Continued)

NFPA Hazard Global

Name Rating (H/F/R) Toxicity Reaction Efﬁciency Losses

Methanol 1/3/0

Skin/Eye irritant, Poisinous in large

quantities

O + O

→ CO

+ 2H

O Membrane Crossover

(causes mixed potential,

parasltic O

consumption)

Ethanol 0/3/0

Concentrations below 1000 ppm

usually produce no signs of

intoxication. It is a central nervous

system depressant in humans.

OH + 3O

→ 2CO

+ 3H

O Carbon-Carbon bonds

make CO

formation

very difﬁcult

Dimethoxymethane

(DMM)

1/3/2

Irritates Eyes, Skin, and Airways C

+ 2O

→ 3CO

+ 4H

O Hydrolyzes in the fuel cell

to produce methanol

Trimethoxymethane

(TMM)

N/A N/A C

+ 5O

→ 4CO

+ 5H

O Hydrolyzes in the fuel cell

to produce methanol

Trioxane 2/2/0

Can evolve formaldehyde when

heated strongly or in contact with

strong acids. Permitted as an

additive in food for human

consumption.

+ 3O

→ 3CO

+ 3H

ON/A

Formic Acid 3/2/0

Toxic. Proved very harmful to test

animals.

+ 1/2O

→ CO

+ H

ON/A

Formaldehyde 2/4/0

Cancerous CH

O + O

→ CO

+ H

ON/A

Dimethyl Ether (DME) 2/4/1

Mild anesthetic. Large quantites can

cause bloodthining.

(CH

)

O + 3H

O → 2CO

+ 12H

+ 12e

−

N/A

Ethylene Glycol (EG) 1/1/0

Mildly toxic by skin contact. A

suspected carcinogen.

+ 2H

O → 2CO

+ 2H

O Lower Energy Conversion

than Methanol

Dimethyl Oxalate

(DMO)

N/A N/A C

+ 1/2O

→ 4CO

+ 3H

O Lower Energy Conversion

than Methanol

Oxalic Acid 3/1/0

Oral irritant (poisonous if swallowed) C

+ 1/2O

→ 2CO

+ H

ON/A

E. Peled (A38 - A41)

http://www.orcbs.msu.edu/nfpa/nfpa.html

http://chemﬁnder.cambridgesoft.com/

Source: Adapted from Ref. [43].

342

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6.4 Direct Alcohol Polymer Electrolyte Cells 343

6.4.1 Direct Methanol Fuel Cell

The global electrochemical reactions for the DMFC are as follows:

Anode oxidation reaction:

OH + H

O → 6e

−

+ 6H

+ CO

(6.42)

Cathode reduction reaction:

+ 6e

−

→ 3H

O (6.43)

Overall cell reaction:

OH +

→ 2H

O + CO

(6.44)

Notice that at the anode some water is consumed in the global oxidation reaction, although

the overall fuel cell is still a net water generator. On the cathode of the DMFC, the main

reactions are the same as for the hydrogen fuel cell. Even though methanol and water are

used as the fuel, charge transfer at the anode catalyst and through the electrolyte is still

the same; hydrogen protons are generated by the methanol oxidation and travel though the

electrolyte.

The DMFC has similar design features as the hydrogen PEFC, except that a liquid

solution of methanol and water is used as the fuel, rather than hydrogen gas. A schematic

of the various transport phenomena in a DMFC is shown in Figure 6.52.

In the DMFC, a liquid solution of methanol and water is fed to the anode and air

is supplied to the cathode. At the anode side, a countercurrent ﬂux of carbon dioxide

bubbles formed via electrochemical reaction ﬁghts through the DM against the liquid-

phase transport of methanol from the ﬂow channel to the catalyst layer. Due to the potential

Figure 6.52 Schematic of direct methanol fuel cell and associated mass transfer processes. (Cour-

tesy of K. Sharp, Penn State University.)

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344 Polymer Electrolyte Fuel Cells

blockage effect and the need to efﬁciently transport methanol solution to the anode surface,

the anode-side DM material is typically highly hydrophilic porous carbon cloth or paper.

On the cathode side, the same ﬂooding issues prevalent in the hydrogen PEFC are even

more severe, since the diffusion gradient from the anode to the cathode is always toward

the cathode and the electro osmotic drag coefﬁcient is higher (∼2–5) for liquid contact

with the membrane due to Schroeder’s paradox. To mitigate ﬂooding and achieve a more

favorable net water transport coefﬁcient, a highly hydrophobic microporous layer and DM

are typically used.

For the DMFC, both anode and cathode activation polarizations are signiﬁcant. How-

ever, reduced performance compared to the H

PEFC is tolerable in light of other advantages

of the DMFC, namely:

1. Because the anode ﬂow is mostly liquid (gaseous CO

is a product of methanol

oxidation), there is no need for a separate cooling or humidiﬁcation subsystem.

2. Liquid fuel used in the anode results in lower parasitic pumping requirements

compared to gas ﬂow. In fact, many passive DMFC designs operate without any

external parasitic losses, instead relying on natural forces such as capillary action,

buoyancy, and diffusion to deliver reactants.

3. The highly dense liquid fuel stored at ambient pressure eliminates problems with

fuel storage volume. With highly concentrated methanol as fuel (>10 M), passive

DMFC system power densities can compare favorably to advanced Li ion batteries.

4. No reformer system is needed.

5. Methanol is ubiquitous and transportable, and an infrastructure already exists.

The DMFC solves many problems associated with the hydrogen PEFC system; however,

there are of course limitations to the DMFC. The main technical issues affecting perfor-

mance and design are as follows:

1. Water Management Even though external humidiﬁcation is not needed in the

DMFC, prevention of cathode ﬂooding is critical to ensure adequate performance.

2. Methanol Crossover There is a high crossover rate of methanol from anode to

cathode because of the high concentration of methanol at the anode side. This

results in a mixed potential at the cathode from crossover methanol oxidation and

greatly reduces the open-circuit voltage of the DMFC from the theoretical value of

∼1.2 V to around 0.7–0.8 V.

3. Poor Anode Kinetics The kinetics are inherently slower because of the more

complex anode oxidation reaction. Tafel kinetics are appropriate at both electrodes,

and compared to the H

PEFC, an order-of-magnitude higher precious metal loading

is typically used.

4. Counterﬂow and Removal of Carbon Dioxide Carbon dioxide is produced at the

anode surface, resulting in countercurrent two-phase ﬂow in the anode DM that can

block access to the catalyst layer.

5. Methanol Safety Methanol is slightly toxic, spreads more easily into the ground

than gasoline, and is highly ﬂammable and miscible in water so that contamination

with reservoirs is very simple.

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6.4 Direct Alcohol Polymer Electrolyte Cells 345

Water Management in DMFC In DAFCs, some water is needed at the anode to provide

additional oxygen for the oxidation reaction. As a result, ﬂooding at the cathode can be

more severe because the diffusion gradient will favor ﬂow toward the cathode and electro-

osmotic drug is enhanced. From Eq. (6.42), 1 mol of water is needed per mole of methanol

for the overall oxidation reaction. One can deﬁne an anode methanol and an anode water

stoichiometry in this case:



(6.45)

Ideally, some of the water generated in the reaction at the cathode can be directed to

the anode, and concentrated methanol can be used as the feed fuel, reducing the fuel

storage volume requirements. This balance can be achieved passively if the net transport

coefﬁcient can be maintained to the proper level using capillary pressure management or

other techniques. For the DMFC

net,a−c

= α

(6.46)

From Eq. (6.45), if α

=−0.17, then operation on neat methanol with only product water

back-fed to the anode can be theoretically achieved. Each DAFC will have a different value

for this coefﬁcient, depending on the number of electrons exchanged in the fuel oxidation.

This condition must be achieved with a capillary or gas-phase pressure difference, since the

electro-osmotic drag and diffusion are always toward the cathode in the DMFC. Practically,

methanol crossover reduces performance, so that some methanol dilution is typical, as

described in the next section.

External humidiﬁcation is not needed in the DMFC, due to the liquid anode solution,

but prevention of cathode ﬂooding is critical to ensure adequate performance. To combat

ﬂooding and methanol crossover in the DMFC, several approaches have been used, as

schematically shown in Figure 6.53:

1. Increased Electrolyte Thickness This also limits the methanol crossover from

diffusion through Fick’s law:

x-over

= DA

− C

PEM

(6.47)

where δ

PEM

is the electrolyte thickness and D is the diffusion coefﬁcient of methanol

in the electrolyte. As δ

PEM

increases, the diffusion rate of water decreases. However,

this solution reduces performance since the path length of ion travel is increased. In

typical DMFCs, the electrolyte thickness is greater than 100 µm to limit methanol

and water crossover.

2. Capillary Pressure Management Use of a highly hydrophobic microporous layer

on the cathode can be used to maintain a capillary pressure difference between

the liquid-ﬁlled hydrophilic pores of the cathode and anode, and pump water (and

crossover methanol) back toward the anode by capillary pressure forces.

3. High Cathode Flow Rate The crossover water and generated water can be removed

by a high ﬂow rate of dry air. This has the drawback of increasing parasitic losses for

the cathode ﬂow. However, many compact fans provide adequately high ﬂow rate.

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346 Polymer Electrolyte Fuel Cells

(a)

DM CL PEM

Diffusion

barrier

DMCL

Air flow

Methanol

solution

Crossover

Sharp

drop

DM CL PEM DMCL

Air flow

Methanol

solution

(b)

(c)

DM CL

PEM

MPL

DMCL

Air flow

Methanol

solution

Capillary back-

flow of

water/CH

Figure 6.53 Schematic of various water and methanol management approaches used in DMFC: (a)

diffusion barrier approach, (b) thicker membrane approach, and (c) capillary back ﬂow management.

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6.4 Direct Alcohol Polymer Electrolyte Cells 347

To prevent ﬂooding by airﬂow, the cathode air ﬂow must be adequate to remove water at

the rate that it arrives and is produced at the cathode surface. The water arrival and production

at the cathode surface by diffusion, electro-osmotic drag, and hydraulic permeability can be

expressed by Eq. (6.46). For the case where the cathode is just ﬂooding, the diffusion to the

cathode will be minimal. Therefore, an estimate of the minimum amount of water that must

be removed by the cathode to avoid ﬂooding without capillary pressure management can

be derived by balancing the water generated and dragged by electro-osmosis to the cathode

with the drying capability of the cathode ﬂow. The maximum amount of water vapor that

can be removed at the exit for an air cathode can be shown as [44]

O,removed

(

− 1

)

0.84F



g,sat

(T )

− P

g,sat

(T )



(6.48)

where λ

is the cathode stoichiometry at operating conditions and P

g,sat

is the saturation

pressure. In this calculation, the assumed oxidizer is air, which results in the factor of 0.84

in the denominator. This does not allow for water removal in the liquid phase. For the case

of ﬂooding just at the exit, P

is equal to the total pressure of gas leaving the fuel cell. The

minimum stoichiometry required to prevent liquid water accumulation in the limit of zero

water diffusion through the membrane can be shown as

cathode, min











2.94

g,sat

/(P

− P

g,sat

)

+ 1inair

g,sat

/(P

− P

g,sat

)

+ 1inO

(6.49)

The factor of 1 in Eq. (6.49) is a result of the consumption of oxygen in the cathode by the

electrochemical oxygen reduction reaction. A plot of this minimum boundary as a function

of pressure over the relevant temperature range is shown in Figure 6.54. This boundary

serves as a baseline for discussion purposes. Depending on the use other methods to control

the net drag coefﬁcient of water, this boundary can shift considerably.

Figure 6.54 Plot of minimum stoichiometry requirements for ﬂooding avoidance as function of

operating temperature and pressure for DMFC. (Adapted from Ref. [44].)

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348 Polymer Electrolyte Fuel Cells

It can be seen that, without capillary pressure management, the minimum cathode

stoichiometry for a DMFC is determined by ﬂooding avoidance rather than by electro-

chemical requirements. Therefore, cathode stoichiometries in the DMFC are typically

much greater than unity. Also note from Figure 6.54 that the lowest curve is that of air at

3 atm pressure with no electro-osmotic drag. For this curve, Eq. (6.49) was modiﬁed to

eliminate the electro-osmotic drag term, which represents a result achievable by capillary

pressure management. This results in a vast reduction in the required cathode ﬂow rate

to avoid ﬂooding and shows the potential of capillary pressure management to improve

performance.

Methanol Crossover Another critical issue in the DMFC is methanol crossover from

the anode to the cathode from diffusion, electro-osmotic drag, and hydraulic permeation.

Methanol crossover from the anode to the cathode results in the following performance-

limiting effects:

1. A mixed potential on the cathode and oxidation of methanol at this location both

poison the cathode catalyst, consume oxygen, and greatly reduce the OCV, even

more so than hydrogen crossover in H

PEFC systems. Typical OCVs of DMFCs

are signiﬁcantly below 0.8 V.

2. The parasitic leakage of fuel through the membrane without current generation

represents an inefﬁciency. Without special design, or purging on shutdown, the

leakage will continue and consume methanol under idle conditions.

Methanol crossover to the cathode is quickly oxidized to CO

via Eq. (6.42). This consumes

, dilutes the cathode ﬂow, and creates a mixed potential parasitic current (I

) reaction at

the cathode:

= 6F

OH,cathode

(6.50)

The loss from this equivalent current can be modeled in the same way as hydrogen crossover

shown in Chapter 4.

Methanol crosses from anode to cathode by diffusion, hydraulic permeability, and

electro-osmotic drag:

=−DA

C

c−a

x



drag



−

P

c−a

(6.51)

Typically, the hydraulic permeability is either neutral or directed toward the anode by

capillary pressure management. Of the three modes of methanol crossover, diffusion

(estimated as 10

−5.4163-999.778/T

/s [45]) is dominant under normal conditions, espe-

cially at higher temperatures. Since the driving potential for oxidation is so high at the

cathode, the methanol that crosses over is almost completely oxidized to CO

, which

sets up a sustained maximum activity gradient in methanol concentration across the

electrolyte.

The electro-osmotic drag coefﬁcient of methanol is estimated to be 0.16 CH

OH/H

[46], or 2.5y, where y is the mole fraction of CH

OH in solution [47]. The electro-osmotic

drag coefﬁcient is relatively weak compared to water, which is a result of the nonpolar

nature of the molecule.

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6.4 Direct Alcohol Polymer Electrolyte Cells 349

In order to combat methanol crossover, several approaches have been used:

1. Use of Dilute Methanol Solutions Use of a low-molarity solution (0.5–2 M)of

methanol reduces the concentration of methanol at the anode, thus reducing the

crossover. However, this approach results in unacceptably large fuel storage tanks

and excessive water-pumping requirements.

2. Use of Thicker Electrolyte Similar to the water crossover, a thick electrolyte can

restrict crossover but also limits performance via increased ohmic losses through

the electrolyte.

3. Diffusion Barrier on Anode A diffusion barrier (shown in Fig. 6.53) is an al-

ternative to the use of a thicker electrolyte that places the diffusion restriction at

a location that will not affect ionic transport. With a diffusion barrier, there is a

steep concentration gradient from the DM through barrier, so that the methanol

concentration at the catalyst layer is minimized, reducing crossover [48].

4. Capillary Pressure Management This establishes a hydraulic pressure gradient

favoring liquid ﬂow toward the anode (Figure 6.53). The use of capillary pressure

management and an anode diffusion barrier permits simultaneous methanol and net

water crossover management, so that highly concentrated methanol solutions can

be used as the fuel source.

Of the various methods to restrict methanol crossover, typically a combination of thicker

electrolyte (∼100 µm) with diffusion barriers and capillary pressure management is used.

This approach is the most successful because it simultaneously enables water and methanol

management, with a high-concentration methanol solution at the anode, resulting in a

compact system design.

Anode Kinetics Since the cathode ORR is the same in DMFCs as it is in H

PEFCs and the

anode methanol oxidation is a complex series of intermediate reaction steps, Tafel kinetics

are generally valid for both the anode and the cathode. The theorized intermediate reaction

pathways are given in Figure 6.55 from [49]. There are believed to be two main routes to

methanol oxidation. The preferred pathway is to generate formaldehyde (CH

O), followed

by formic acid (CH

) oxidation to carbon dioxide (CO

). The nonpreferred route is also

initiated by generation of formaldehyde, which then oxidizes to carbon monoxide (CO),

ﬁnally oxidizing to produce carbon dioxide. The overall oxidation pathway (either preferred

OH CH

OH COH

COH

HCOOH COOH

Figure 6.55 Proposed methanol electrochemical oxidation pathway at DMFC anode. (Adapted

from Ref. [49].)