Mench M.M. Fuel Cell Engines

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400 Other Fuel Cells

Figure 7.16 PC25C 200-kWe PAFC power plant and accesories. (Reproduced with permission

from Ref. [38].)

advances in durability and lifetime were also achieved during operation of these units.

In 2002, operation of over 245 units all over the world had accumulated 5 million hours

of operation, with durability between major system overhauls exceeding 40,000 h [38].

Worldwide, over 500 PAFC systems had been installed by 2004 [39]. Shown in Figure

7.16, like many fuel cell power plants, the fuel cell itself is actually a smaller fraction of the

overall size and weight of the system compared to the ancillary components. In the case of

the 200-kWe PC25 PAFC, the stack constitutes only about 25% of the system volume and

cost [40], which is a reasonable estimate for most nonportable fuel cell systems. Ancillary

systems provide the following:

1. Fuel Management: The PC25 reforms natural gas to hydrogen fuel.

2. Heat Management: The fuel cell stack must be cooled to avoid electrolyte loss. To

increase the overall efﬁciency, the waste heat can be used as cogenerated power

through a hot-water heat exchanger system.

3. Water Management: The stack product water is rejected to the atmosphere and used

for steam reforming of the natural gas.

4. Flow and Power Management: The air and hydrogen ﬂow into the stack must be

controlled, and the direct current (DC) produced must be conditioned to produce

the voltage and current levels and phases desired.

5. Sensing and Control: The entire system is operated and controlled as a turnkey,

with no direct operator input in normal operation [41].

The total electrical efﬁciency of the PC25 200-kWe unit is shown in Figure 7.17.

A polarization curve from a cell in an early PC25 unit is given in Figure 7.18. Modern

PAFCs can be higher than this, as shown in Figure 7.19. For operation in the electric mode

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7.3 Phosphoric Acid Fuel Cells 401

Figure 7.17 Measured electrical and overall (combined heat and power) efﬁciency for United

Technologies Fuel Cell, PC25C 200 kWe PAFC power plant. (Reproduced with permission from Ref.

[38].)

only, around 40% chemical-to-electrical conversion is realized, which is better than many

combustion systems for submegawatt power application. When the waste heat recovery to

the hot water is included (the system puts out about 200–225 kW of recoverable heat),

the system efﬁciency reaches 80%, an outstanding ﬁgure that compares very well with

combustion-based technologies. Overall combined efﬁciency can now reach 87% in the

cogeneration mode [42].

Overall, the PC25 unit represents the ﬁrst real fuel cell product to be commercially

sold. The system size, weight, and reliability have been continuously improved, and

Figure 7.18 Polarization curve from 200-kWe PC25 PAFC system operating at atmospheric pres-

sure. (Adapted from Ref. [43].)

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402 Other Fuel Cells

Figure 7.19 Polarization curve from 200 kWe PC25 PAFC system operating at 180

◦

C and atmo-

spheric pressure. Pt-CO/C supported cathode catalyst, and SiC:ZrSiO

electrolyte matrix. (Adapted

from Ref. [43].)

operation in all environments around the world has been technically quite successful.

However, the overall system cost remains at $3000–4000 per kWe, which is about three

times higher than needed for deep stationary power market penetration [45]. The reliable

off-grid power and quiet, low emission output have found a niche in premium power

market venues such as banks, hospitals, or government buildings, however. In these

applications, a combustion-based generator may be undesired or comparably unattractive,

and the PAFC cost is acceptable. In the case of the Police Barracks in Central Park, New

York, the cost of bringing in additional electrical capacity to the barracks ($1.2 million)

was greater than the PC25 system ($800,000) [46]. A gas turbine generator would have

been less expensive but would have been much louder in the middle of the park. In special

applications such as these, the PAFC and its reliable power (>95% service reliability has

been observed) are ideal. An added beneﬁt is the heat cogeneration, which can be used

for a variety of purposes, including space heating, water heating, or even absorption cycle

air conditioning. Future research and development of these systems will revolve around

achieving a product more cost-competitive with other stationary power systems.

7.3.1 Operation and Conﬁgurations

The PAFC is an acid-based electrolyte technology that is in many ways quite similar to the

PEFC, and much of the technology learned during early PAFC development is now being

applied (or relearned!) to PEFC systems. The PAFC operates at an elevated temperature

compared to the PEFC, at 160–210

◦

C. The higher temperature PAFC was chosen for

development for terrestrial applications after the failure of the (AFC) to demonstrate suitable

performance with CO

. The operating pressure of PAFCs ranges from 1 to 9 atm, and the

ﬂow stoichiometries are similar to other fuel cells. Higher operating pressures require larger

parasitic losses and ancillary component costs, so most installed systems now operate at

near atmospheric pressure. The observed performance increase observed for an increase in

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7.3 Phosphoric Acid Fuel Cells 403

Hydrogen oxidation reaction (HOR)

+ 2e

–

Anode

cathode

matrix

Oxidizer gas

Fuel gas

+ O

+ 2e

–

—

Separator plate

Oxidizer reduction reaction (ORR)

Figure 7.20 Illustration of PAFC electrode and components.

pressure when the cell operates at 190

◦

C and 323 mA/cm

has been correlated as [47]:

V (mV) = 146 log

(7.9)

As discussed in Chapter 4, this is higher than the Nernst voltage increase due to increased

cathode exchange current density.

Both systems have an acid-based electrolyte (PEFC is sulfuric acid based), although the

PAFC is a liquid electrolyte solution system and the PEFC electrolyte exists as a partially

bound solution in a solid polymer matrix. Between the PEFC and PAFC, the anode HOR

and cathode ORR are the same. A schematic of the materials and electrochemical reactions

in the PAFC system is shown in Figure 7.20. Both systems use a noble metal catalyst or

alloy with noble metals on the electrodes, and both suffer from poor ORR kinetics relative

to alkaline-based systems. Ironically, since operation of the PEFC at 80

◦

C results in catalyst

poisoning from CO as well as water management issues that the PAFC avoids, developers

seek higher temperature PEFC membranes that can operate at 120–200

◦

C like the PAFC

but maintain the high power density advantage of the PEFC.

Similar to a MCFC, in the PAFC, the electrolyte is stored between the electrodes within

athin∼50% porosity, 50–200-µm-thick porous matrix. The purpose for the silicon carbide

(SiC) porous matrix used in PAFCs is to retain the acid electrolyte by capillary forces. If

there were no matrix, the acid could simply drain into the ﬂow channels or bulge according to

gravimetric forces. It is critical that the bubble pressure of the matrix exceed 35 kPa, or elec-

trolyte blow-through and excessive crossover from internal pressure differentials would oc-

cur. The maximum pressure differential between the anode and cathode is limited to around

20 kPa to avoid this problem.

Like all components, the electrode structure has evolved over time, and now a carbon-

supported heterogeneous catalyst of platinum or platinum alloys with other metals such

as chromium, vanadium, or cobalt is commonly used [40]. Similar to PEFC electrodes,

precious metal catalyst loading of around 0.25 on the anode to 0.5 mg/cm

are used.

Although platinum is the base catalyst material, it is not a major contributor to the cost

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404 Other Fuel Cells

of the system and no longer impedes market implementation, since other components

dominate PAFC system cost [40].

The electrode is sprayed or cast onto a porous electrode substrate which is similar in

some ways to the DM used in polymer electrolytes and AFCs. The purposes of this porous

substrate are multiple:

1. To enable transport of reactants, products, electrons, and heat to and from the

electrode,

2. to act as electrolyte reservoirs for phosphoric acid, and

3. to serve as the ﬂow channels.

In the PEFC, only the ﬁrst purpose is relevant. The eventual depletion of electrolyte from the

PAFC matrix by evaporation results in gas crossover losses and low bulk ionic conductivity

that must be avoided or the system will reach end of life. Therefore, a resevior of electrolyte

is needed to replace lost electrolyte in service.

It is difﬁcult to reﬁll the electrolyte in the PAFC system while in operation, so a

lifetime of acid should be available at the beginning of life. In older designs, the electrolyte

storage matrix would be thicker or include an internal reservoir to accommodate additional

electrolyte. However, this has the detrimental effect of increasing system bulk and ohmic

losses through the electrolyte. In order to reduce the size of the SiC matrix needed for

prolonged operation as well as simplify stack design, some of the storage of the electrolyte

has been moved out of the electrolyte matrix and into the porous and wettable carbon

paper substrate material. Electrolyte storage in the carbon paper substrate is accomplished

in hydrophilic, small pores. In order to promote gas ﬂux, however, there is typically

some wet prooﬁng of the overall substrate with PTFE. The catalyst also shares a dual

hydrophobic–hydrophilic nature. The dual hydrophobic–hydrophilic nature of the substrate

and catalyst layer is similar to that in PEFCs, except that in the PEFCs the hydrophilic pores

in the catalyst layer and DM are ﬁlled with water, not electrolyte, which is not desired. In

the PAFC, the storage of electrolyte, which is in liquid form at operating conditions in the

substrate greatly extends operation life.

As discussed in Chapter 5, the pore size and hydrophobicity control the capillary

pressure in the liquid. In a hydrophylic media, the smaller pore sizes have a liquid suction

pressure, drawing liquid in. In the PAFC, the SiC matrix has uniformly small pores and is

more hydrophilic than the catalyst layer or substrate reservoir, so that losses in electrolyte

are readily replaced by suction from stored electrolyte these locations. The pressure and

pore size distribution set up the electrolyte–catalyst–reactant interface area and thus are

critical factors to control the performance of the electrode. The uniformity and control of the

pore size in the matrix are extremely important to prevent local drainage spots with severe

crossover. Control of the pore size distribution and hydrophobicity of the catalyst layer,

substrate, and storage matrix are critical to ensure long life and maximized triple-phase

boundary area for reaction in the catalyst layer.

There have been many internal conﬁgurations of the PAFC stack plates, the most

recent involving the use of carbon paper substrate as an electrolyte reservoir and a ﬂow

distributor, as discussed. The ﬂow ﬁeld design in a PAFC is similar to a PEFC, but there is

no special provision for ﬂooding, since this is not an issue with the medium-temperature

PAFC. Typically, the ﬂow ﬁelds are aligned in plane and perpendicular to one another

(cross-ﬂow), and external manifolding is used, as shown in Figures 7.21 and 7.22. By

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7.3 Phosphoric Acid Fuel Cells 405

Figure 7.21 Illustration of PAFC stack ﬂow ﬁeld assembly with stored electrolyte. (Reproduced

with permission from Ref. [26].)

shaping the carbon paper in parallel channels, there is no need to separately machine a

bipolar plate with ﬂow ﬁelds, and system size and cost savings are realized.

The parallel and straight nature of the ﬂow ﬁelds cannot be easily fed fuel in a stack

using internal manifolding, and an external manifold system like that illustrated in Figure

7.22 is commonly used. This leads to sealing problems along the sides of the porous substrate

not used for gas ﬂow, where the edge pores must be closed by dipping or sealing the substrate

edges. The entire assembly is then completed with a ﬂat, thin separation plate that serves

to isolate the reactants and electrolyte and transport current between cells in series.

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Figure 7.22 PAFC stack external manifolding concept. (Reproduced with permission from Ref.

[26].)

The main advantage of the PAFC is that the higher temperature eliminates or reduces

two major problems with the PEFC, CO poisoning sensitivity and water management. The

PAFC cannot accomplish internal reformation like the MCFC or SOFC, but because of the

elevated temperature of 200

◦

C compared to 80

◦

C for the PEFC, the anode in the PAFC can

tolerate a 1–2% CO in the feed stream. This allows operation on reformed natural gas and

other fuel feedstocks with minimal CO ﬁltering, greatly reducing reformer size and control

requirements.

Due to the high-temperature operation and electrolyte behavior, water management in

the PAFC is not a major concern. The electrolyte is a highly concentrated acid solution

that is conductive without water and has a very low vapor pressure at high concentration.

The electrolyte concentration varies between 90 and 100% during operation depending

on the ﬂow rate, current density, and operating temperature. During operation, water is

generated at the cathode, which is readily evaporated into the ﬂow stream or absorbed into

the electrolyte. If the water is absorbed into the electrolyte, dilution increases the vapor

pressure and drives off the water at a faster rate. Therefore, water management in the

electrolyte is self-regulating. The generated water leaves the system as steam, which can

be used for the steam reformation process in the fuel-processing subsystem or to provide

thermal energy for cogeneration application.

Another major advantage of the electrolyte system is that control over freeze damage

can be accomplished by dilution of the electrolyte to lower concentrations, which can reduce

the freeze point of the electrolyte to below −40

◦

C [45] before shipping. Once in operation,

the PAFC will self-regulate the acid concentration when the operating temperature reaches

a high normal value through the vapor pressure dependency discussed. Compared to the

SOFC, the PAFC does not suffer the major material compatibility or manufacturing difﬁ-

culties. Neither the SOFC nor the PAFC is a rapid-startup system, but operation at 200

◦

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7.3 Phosphoric Acid Fuel Cells 407

does not present the major thermal stress and seal leakage issues the SOFC suffers from.

Provision for removal of the waste heat is accomplished through cooling pipes embedded

between every 5–10 cells in the stack.

Electrode and Electrolyte System Phosphoric acid, in high concentrations (90–100%),

was chosen as the electrolyte in PAFC systems because of its low vapor pressure in high

concentrations, which permits long-term operation with a ﬁxed electrolyte system. Below

160

◦

C, phosphoric acid is a poor ionic conductor, and problems of carbon monoxide

poisoning that plague PEFCs become problematic. Therefore, operation between 160 and

below 210

◦

C is typical. Above this temperature severe electrolyte loss becomes a limiting

factor. The electrolyte inventory in a PAFC is critical to maintain performance over time

and protect downstream piping from severe corrosion from high-acid efﬂuent. Therefore,

there must be an electrolyte reservoir to allow changes in the electrolyte volume with time.

Electrolyte volume can be retained if the cell temperature or cathode ﬂow stoichiometry is

decreased, and more water remains in the electrolyte, diluting the solution. The electrolyte

inventory can be lost over time due to three reasons:

1. Evaporation of Electrolyte The vapor pressure of phosphoric acid is extremely low

(a few ppm at 150–200

◦

C [45]) but ﬁnite. Over time, electrolyte loss is substantial.

This loss can be mitigated by reduced-temperature operation to reduce the vapor

pressure, a higher initial inventory of acid, or an acid condensation zone near the

exit. In this approach, an uncatalyzed cold zone near the fuel cell exit is used to

collect condensed phosphoric acid and return it to the matrix inventory [48].

2. Reaction with Poisons or Fuel Cell Components Obviously, the catalysts and other

materials used in the PAFC must be compatible with H

electrolyte. However,

the activity of a material is related to the surface area, and when the normally

inert SiC electrolyte matrix particles were reduced from 5 to 0.5 µm to decrease

electrolyte thickness and stack power density, acid consumption from an H

–Si

reaction is observed. The size of the SiC particles used in the electrolyte matrix is

now around 1 µm [48]. Consumption by poisons can be eliminated with ﬁltering of

the reactants.

3. Uptake by Fuel Cell Components The separator plates used in PAFC operation

have a closed porosity of around 15%. During operation, the pores become open,

drawing acid in by capillary action. By the end of lifetime, signiﬁcant electrolyte

can be stored in the separator plate. Additionally, electrochemical pumping occurs,

in which the acid anion accumulates at the anode during high-current operation.

This increases the pressure at the anode and can result in loss of performance due

to electrolyte excursion into the electrode or channel. Additionally, the electrolyte

pumping can result in cell-to-cell motion through the separator plate. Over time,

the cathode side of the stack can become depleted of electrolyte ﬁrst, resulting in

excessive crossover and performance decay.

Example 7.2 Loss of PAFC Electrolyte by Vaporization Over Time Given a 0.5-m

active-area PAFC operating at 150 mA/cm

and an anode stoichiometry of 1.4, determine

the amount of electrolyte lost to the anode ﬂow over 40,000 h operation. Assume the

equilibrium concentration of H

in vapor form is 3 ppm at the operating temperature

of 200

◦

C. The anode ﬂow is pure hydrogen.

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SOLUTION We must solve for the total vapor uptake in the ﬂow, assuming it reaches

equilibrium fully saturated conditions at the exit. The relationships are the same as for

moist air mixtures, only now it is the “relative humidity” of H

4,v

with which we are

concerned.

From Chapter 3, the deﬁnition of mole fraction is

4,v

total

others

⇒

≈ y

4,v

others

(small

)

where

others

is the molar ﬂow rate of everything but the acid vapor. Since there is no water

generated at the anode and the input is dry, this is equivalent to the total molar ﬂow rate of

hydrogen out of the fuel cell:

,out

= (λ

− 1)

= (0.4)

(0.150 A/cm

)(0.5m

)(10,000 cm

)

(2 e

−

eq/mol H

)(96,485 C/eq)

= 1.55 × 10

−3

mol H

The vapor equilibrium is 3 ppm, a mole fraction of 3.0 × 10

−6

. Therefore

≈ (3.0 ×10

−6

mol H

/mol H

)(1.55 × 10

−3

mol H

/s) (40,000 h) (3,600 s/h)

= 0.669 mol

Since the molecular weight of H

is 98 g/mol, this is equivalent to losing 65 g of acid

just from the hydrogen side during the 40,000-h lifetime of operation.

COMMENTS: The cathode would also absorb some electrolyte vapor as well and, since

the ﬂow rate and stoichiometry are higher on the cathode, would absorb more. On the

cathode, the solution is slightly more complicated since water vapor generated from reaction

will also be vaporized into the ﬂow. From this calculation, one can see how even a small

decrease in the vapor pressure can greatly extend the operational lifetime of the stack.

Durability Durability of the PAFC system has been demonstrated to be in excess of

40,000 hours between major overhauls of the system and stack components [38]. However,

several durability issues degrade performance with time and must be addressed with system

engineering design and material solutions. Many improvements were made in this regard

between the subsequent generations of PAFC development and have eliminated modes

of edge corrosion, damage during startup and shutdown cycles, or increased carbon and

catalyst support integrity. However, issues that shorten lifeimte in the PAFC system remain

and in many cases are remarkably similar to those observed in the PEFC system. Many of

the lessons learned in PAFC design and operation can be applied to PEFC system operation,

since many of the degradation modes are similar. Continuing durability issues for the PAFC

include the following [38]:

1. Electrolyte acid loss

2. Electrode stability and impurities

3. Carbon support corrosion

4. Anode starvation

5. Load cycling

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7.3 Phosphoric Acid Fuel Cells 409

Of these, acid loss is perhaps the most insidious problem, as discussed. Simply reducing

the operating temperature 25–30

◦

C can double the life of the stack by reducing electrolyte

vaporization and slowing other degradation modes [45]. This constant loss prevents the

potential elimination of the electrolyte reservoir storage in the substrate and prohibits

further major improvements in power density. Impurities in the fuel and oxidizer stream

can gradually degrade performance, as in all fuel cells [40]. Hydrogen sulﬁde (H

S) can

enter the anode stream through fuel impurities or be present in the air as impurity from

local industrial processes. At levels greater than 50 ppm, the stack-level damage to the

anode catalyst from H

S can be permanent. Chlorides, ammonia, and dust constitute other

potential poisons.

Carbon support corrosion and platinum dissolution at the cathode can occur at high

potentials, and as a result, operation or idling the fuel cell at voltages above 0.8 V is not

normally permitted. During startup, an auxiliary load cuts cell voltage below 0.8 V to prevent

damage. Heat treatment of the carbon support structure and use of graphitized carbon can

reduce this loss, which is an approach that also is successful in PEFCs. Carbon corrosion

is also observed in PEFCs during startup and shutdown and if there are locations of anode

starvation. This mode of irreversible loss is exacerbated in PAFCs, since the operating

temperature is higher. One fundamental disadvantage of higher temperature operation is

that all of the undesired reactions (e.g., corrosion of supports, piping, current collectors,

platinum dissolution) are accelerated by the increased temperature. Of course, the increased

temperature also hastens the kinetics of the desired HOR and ORR, too. This is a common

trade-off in fuel cell systems: At high temperatures, the kinetics are more facile, but the

undesired reactions are also accelerated. At low temperature, the degradation reactions are

slowed, but so are the kinetics of the desired reaction.

Fuel starvation can very rapidly degrade cell performance. If any portion of the anode

is starved of hydrogen, the electrode potential will rise to oxygen evolution, and irreversible

oxidation of the carbon support, substrate, and separator plate will ensue. To eliminate this,

the anode ﬂow ﬁeld often has several passes, so that the hydrogen concentration is not

reduced to low values in any locations [45].

7.3.2 Summary of Advantages and Disadvantages

The PAFC has some deﬁnite technical advantages. In many ways, the research thrusts in

PEFC technology are based on achieving the same midrange temperature of operation as

the PAFC while avoiding the drawbacks. The main advantages of the PAFC system are as

follows:

1. Ease of Water Management The water level in the electrolyte is self-regulating

by changing vapor pressure, and the temperature is high enough that liquid water

ﬂooding is not an issue.

2. Ability to Operate on 1–2% CO in Feed Stream The CO sticking coefﬁcient on

platinum is greatly reduced at 200

◦

C, so that the PAFC can operate on a wide variety

of fuel feeds with a simple steam reformer subsystem and minimal CO cleanup.

3. Demonstrated High Reliability and Developed System The PAFC system is the

ﬁrst fuel cell to reach the consumer production stage and has millions of operation

hours accumulated with hundreds of 200-kWe units. This system has demonstrated

high service reliability of over 95% as well as combined thermal efﬁciency of over

80% in the cogeneration mode.