Mench M.M. Fuel Cell Engines
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c09 JWPR067-Mench December 16, 2007 15:15 Char Count=
460 Experimental Diagnostics and Diagnosis
Figure 9.8 Cyclic voltammogram of polycrystalline platinum in 1 M KOH alkaline electrolyte
solution at 20
◦
C with voltage sweep rate of 100 mV/s. (Reproduced with permission from Ref. [12].)
best crystallographic surface structure for the lowest oxidation potential. In an alkaline
electrolyte, hydrogen oxidation follows:
Pt–H + OH
−
→ Pt + H
2
O + e
−
(9.1)
In the acid electrolyte, hydrogen oxidation follows:
Pt–H + H
2
O → Pt + H
3
O
+
+ e
−
(9.2)
Figure 9.9 CV of polycrystalline platinum in acid 0.5 M H
2
SO
4
electrolyte solution with voltage
sweep rate of 50 mV/s. (Reproduced with permission from Ref. [12].)
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9.1 Electrochemical Methods to Understand Polarization Curve Losses 461
At higher potential, ∼0.45–0.55 in an alkaline solution and ∼0.45–0.8 V in an acid solution,
there is a plateau region where the current is very low. This region represents the double-
layer charging region with no significant electrochemical reactions. At higher surface
potentials, the electrochemical chemisorption of oxygen and platinum oxidation is initiated
with the following reactions: In the alkaline solution
Pt +OH
−
→ Pt–OH + e
−
(9.3)
and above ∼800 mV, platinum oxide formation is observed:
Pt–OH + OH
−
→ Pt–O + H
2
O + e
−
(9.4)
In the acid solution, the reactions are very similar:
Pt + H
2
O → Pt–OH + e
−
+ H
+
(9.5)
Pt–OH + H
2
O → Pt–O + H
3
O
+
+ e
−
(9.6)
At very high potentials, the oxygen evolution reaction occurs in the alkaline electrolyte:
4OH
−
→ O
2
+ 2H
2
O + 4e
−
(9.7)
In the acid electrolyte
2H
2
O → O
2
+ 4H
+
+ 4e
−
(9.8)
As the voltage is then swept backward to zero potential, oxygen and Pt–O reduction
reactions occur, until hydrogen adsorption on the platinum occurs in the alkaline electrolyte
via
Pt + H
2
O + e
−
→ Pt–H + OH
−
(9.9)
In the acid electrolyte
Pt +H
3
O
+
+ e
−
→ Pt–H + H
2
O (9.10)
At very low surface potential, hydrogen evolution begins in the alkaline electrolyte via
2H
2
O + 2e
−
→ 2OH
−
+ H
2
(9.11)
In the acid electrolyte
2H
3
O
+
+ 2e
−
→ 2H
2
O + H
2
(9.12)
When the catalyst morphology, type, crystallographic structure, temperature, gaseous en-
vironment, or other factors change, the CV response will also change. Comparison of the
CV results can reveal information on the reaction kinetics and the potential required for
the reaction in a given environment. For example, a CV of the same Pt–H electrode in a
gaseous environment poisoned with carbon monoxide will show a shift in the hydrogen
oxidation to higher potentials and a reduction in the hydrogen adsorption peak area, since
some of the catalyst will be occupied with carbon monoxide.
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462 Experimental Diagnostics and Diagnosis
Electrochemical Active Surface Area The characterization of the ECSA is a measure of
the efficiency of the catalyst loading. The ECSA is an extremely useful tool to compare the
efficiency of one electrode to another and also to examine the degradation of the electrode
over time resulting from a variety of physical and chemical mechanisms. For the ECSA
measurement on a platinum electrode, the experimental configuration shown in Figure 9.7
is used and two assumptions are made [13]:
1. Each platinum site can adsorb only one hydrogen proton.
2. Every available platinum site will be occupied with hydrogen during the transition
from hydrogen adsorption to hydrogen evolution.
To calculate the ECSA, a plot of the area under the current versus time during the
hydrogen adsorption peak in the CV (see Figure 9.10) is used to determine the total
charge per superficial electrode area, since the charge is equivalent to the total integrated
adsorption current over time. The value of charge is divided by the total catalyst loading.
A proportionality constant for a given catalyst is needed to convert the charge per area to
effective catalyst active area, based on the charge per unit area of a planar electrode surface:
ECSA =
I
(
t
)
dt
SC
= m
2
/g
catalyst
(9.13)
The proportionality constant S used to relate the catalyst area to the charge is 210 µC/cm
2
for platinum [14] and C is the loading of the catalyst in grams. Although we have shown
the ECSA measurements using the hydrogen absorption area, the technique can also be
applied to the hydrogen oxidation area of the curve or, in theory, to any other portion of the
curve with repeatable faradaic processes. The same procedure can be used for nonplatinum
Figure 9.10 The area shown under the dashed line represents hydrogen adsorption and can be used
to determine electrochemically active catalyst area. CV done on a 5 cm
2
active area fuel cell, in-situ.
50 sccm hydrogen on anode, 200 sccm nitrogen on cathode. RH anode/cathode =100/100%, scan rate
20 mV/sec.
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9.1 Electrochemical Methods to Understand Polarization Curve Losses 463
electrodes as well, providing a new proportionality constant can be found for the partic-
ular catalyst. Typical values for the ECSA of a platinum catalyst in a PEFC are around
50 m
2
/g Pt [15].
Tafel Plots Electrode kinetic parameter characterization requires data on the electrode
kinetic polarization as a function of current density in the absence of (or corrected for)
concentration and ohmic effects. For fuel cell electrodes, this can be accomplished ex situ
using a variety of methods, most commonly a rotating disk electrode (RDE) with a reference
electrode. The RDE is a common electrochemical tool and has a small rotating electrode
disk in an electrolyte solution [1]. The rotation rate is used to eliminate concentration
polarization, and ohmic, nonuniformity, and crossover effects are all eliminated or corrected
for, so that only the pure kinetic response of the electrode of interest is examined. In situ fuel
cell testing is useful, too, since it is a direct measurement. This can easily be accomplished
in hydrogen fuel cells since the pure hydrogen oxidation reaction (HOR) in low or high
temperature is quite facile for fuel cells that are acid or alkaline based. In this case, the
anode is used as the dynamic hydrogen electrode, which is the pseudoreference electrode.
The cathode becomes the electrode of interest, and all losses are assumed to come from
ohmic, crossover (if applicable), and oxidizer reduction reaction (ORR), or
E
cell
= E
◦
(T , P) −
η
a,c
−η
r
−η
x
(9.14)
where the terms used have been discussed in Chapter 4. In Eq. (9.14), the concentration
polarizations are not explicitly broken out but are considered part of the kinetic expression
through the exchange current density. To measure the Tafel slope and extract kinetic
parameters, the cell corrected voltage versus effective current density is plotted on a semilog
scale, as shown in Figure 9.11 [15]. The cell voltage must be corrected to remove any ohmic
or other effects that reduce the voltage besides kinetic losses at a single electrode. In order
to correct for ohmic losses, an HFR or current-interrupt technique should be used at each
current density, as the value may change with current. Any crossover current density must
also be measured and corrected for by adding to the current density, that is,
I
eff
= I
cell
+ I
x
(9.15)
From Chapter 4, we recall that the Tafel kinetics expression, which is generally valid
for the ORR and many other reactions, can be written as
η =
R
u
T
α
j
F
ln
i
i
o
= b ln
i + i
x
i
o
(9.16)
where b is the Tafel slope. From a semilog plot like that shown in Figure 9.11, the exchange
current density and the Tafel slope can be determined for an electrode as long as the ohmic
and crossover losses are properly accounted for and the anode reaction has very small
overpotential, which may not be the case for a particular fuel cell, especially if operating
on diluted or reformed hydrogen.
Another way to present the data is normalized on a catalyst loading basis, as shown in
Figure 9.12. In this approach, the ohmic and crossover corrected current density is divided
by the catalyst loading per geometric area. From Figure 9.12, it can be seen that on a
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464 Experimental Diagnostics and Diagnosis
Figure 9.11 Tafel slope preparation from corrected polarization curve data. Data are taken for a
50-cm
2
fully humidified PEFC at 80
◦
C, 270-kPa anode and cathode in a hydrogen/oxygen environ-
ment, for different mass loadings of platinum catalyst. Data are corrected for measured ohmic losses
and crossover. (Reproduced with permission from Ref. [15].)
mass-normalized basis the Tafel slopes in an air and oxygen environment for different
catalyst loadings can be collapsed, with a Tafel slope of around 0.06 V/decade
In a direct alcohol or other nonhydrogen fuel cell, the anode or cathode can be examined
using the DHE concept for the opposing electrode. By switching the location of the DHE,
Figure 9.12 Tafel slopes for PEFC electrodes in 50-cm
2
hydrogen/air environment and hydrogen/
oxygen PEFC for different mass loadings of platinum catalyst on mass specific current–density basis
(A/mg Pt). Data are taken from fuel cell polarization curves of a fully humidified PEFC at 80
◦
C,
270-kPa anode and cathode. Data are corrected for measured ohmic losses and crossover, but not
concentration losses, which results in deviation from Tafel slope at high current density. (Reproduced
with permission from Ref. [15].)
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9.1 Electrochemical Methods to Understand Polarization Curve Losses 465
the performance of both electrodes under normal operating conditions can be examined.
One should make sure that the assumptions made in the analysis are always true, however,
to avoid erroneous results. For example, in PEFCs, high current can dry the anode and
result in erroneously high Tafel slopes if the ohmic losses are not accounted for.
9.1.2 Experimental Determination of Ohmic Parameters
and Polarization
Ohmic losses consist of contact, ionic, and electronic resistance losses. There are many
methods for quantifying the total ohmic losses in a cell (contact and current flow resistances).
The two main methods are the current-interrupt and HFR methods. The current-interrupt
method is the simplest and only requires an oscilloscope and fuel cell load, but can be
difficult to achieve consistent measurements. The HFR technique is precise but requires
an electrical impedance analyzer. To determine the contact resistance, several methods
are available. Perhaps the simplest is to measure the total ohmic resistance between the
two surfaces and compare to the resistance of the individual components. The contact
resistance in fuel cells is generally a strong function of compression pressure or surface
oxide formation, depending on the fuel cell type.
Ohmic losses between two points in a media can be determined using a two- or four-
point probe technique, as illustrated in Figure 9.13. In the two-probe technique, the voltage
drop between two points is measured as a known current is drawn through two points
and the resistance is determined through Ohm’s law. This technique has the disadvantage
of including the unknown contact resistance between the probes and the substrate. To
correct for this, a four-probe technique can be used where the two probes are used to
deliver current and two separate probes are used to measure the voltage drop between two
different points along the current path. If the distance between the voltage-sensing probes is
precisely known, the ohmic resistance of the material can be determined without knowing
the contact resistance between the probes and the substrate, since the voltage-sensing probes
have almost no current through them and the high-impedance measurement device.
V,I
i
i
Contact
resistance
(a) two probe
V
i
(b) four probe
I
i
i
Figure 9.13 Illustration of (a)two-and(b) four-probe techniques for ohmic resistance measure-
ment.
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466 Experimental Diagnostics and Diagnosis
V
t
OCV
∆V = IR
ohm
Cell voltage at current, I
Concentration
polarization
adjustment
Figure 9.14 Voltage–time behavior for fuel cell during current-interrupt measurement.
Current Interrupt To apply the current-interrupt method, a high-frequency response data
acquisition system such as a digital oscilloscope is used. While operating the fuel cell, the
oscilloscope is used to record the cell voltage. When the current is suddenly reduced to zero,
the ohmic (IR) losses will immediately disappear, and the fuel cell will return to open-circuit
conditions. However, the open-circuit voltage is a function of the reactant concentration at
the catalyst surface, and the time scale of mass transfer adjustment is sufficiently longer
than the nearly instantaneous electrochemical adjustment. Therefore, the voltage history
will have a nearly instantaneous jump following the current shutoff that is related to the
IR drop in the fuel cell. The voltage will then gradually rise to a steady open-circuit
potential, as illustrated in Figure 9.14. The secondary rise in the voltage to open circuit
can be used as an indication of the mass transfer resistance (concentration polarization).
While this approach is simple in theory, in practice, it is often difficult to define the precise
break-off point between the IR-related voltage rise and the mass-transport-related rise.
Additionally, some current charging/discharging in the electrolyte and catalyst layers will
occur due to the capacitive effects of the double layer and charge transfer processes, so that
there are several time scales involved in the voltage recovery. If a PEFC is being analyed,
membrane hydration can also impact results. Thus, this technique is more useful as a quick
qualitative sensor to determine the relative state of the ohmic losses than as a precision
tool.
High-Frequency Resistance A more sophisticated approach to measurement of the ohmic
losses is the HFR measurement, as discussed in the previous section. In this approach, an
AC signal is superposed on the DC from the fuel cell. At very high frequencies, the AC
will render the various electrochemical double-layer capacitances to zero, and only the
purely ohmic resistance will be measured. Typical frequencies high enough to successfully
use this technique are >1 kHz for PEFCs. This approach only measures the path of least
resistance however, and care should be taken to properly understand results. For example,
in the electrodes, the HFR will normally measure the electrical resistance only, since it is
generally much less than the ionic loss in the mixed conductivity structure. Therefore, HFR
could not generally be used to determine electrode ionomer performance.
9.1.3 Experimental Determination of Concentration Polarization
To measure the concentration polarization, several techniques can be applied. As discussed,
the current-interrupt technique can be used to gage the relative significance of the overall
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9.1 Electrochemical Methods to Understand Polarization Curve Losses 467
concentration polarization on the fuel cell performance. Another simple technique involves
comparison of performance for different reactant mole fractions in the fuel or oxidizer
mixture. For instance, pure oxygen can be used instead of air to delineate oxygen transport
limitations. A mixture of helium and oxygen instead of air can also be used. On the anode
side, the hydrogen or other fuel concentrations can be varied and the IR, crossover, and
kinetic loss corrected data can be compared to identify concentration losses. Due to con-
centration polarization, the Tafel slope will deviate from the constant value as the current
reaches higher values, as shown in Figure 9.12 for a PEFC air cathode. Once the kinet-
ics, crossover, and ohmic portions are well defined, the remainder must be concentration
polarization, which can be modeled in several ways, as discussed in Chapter 4.
In PEFCs, there is the additional concern of the highly nonlinear concentration po-
larization caused by electrode, diffusion media, or flow channel flooding. It is extremely
difficult to analytically correlate flooding behavior, since it is location dependent, and so
sensitive to current density, temperature, and other operating conditions. It is also very
difficult to separate the electrode flooding from gas-phase transport limitations, since,
ultimately, flooding is a local effect while fuel cell performance is a lumped measurement.
One technique that can be applied to delineate the flooding from nonflooding concentration
polarization losses is a rapid polarization curve, disussed in Chapter 6. In a rapid polar-
ization curve, the cell is held at OCV for several minutes after operating in a nonflooded
(dry) condition, then the cell voltage is rapidly decreased from OCV to low voltages to
obtain a polarization curve in a rapid time-scale. In this period of time, the accumulation
of liquid water is minimal. By comparison with a true steady-state polarization curve, the
liquid flooding effects can be delineated. It should be noted that a dry PEFC membrane
will tend to hydrate when going to higher current densities, on the timescale of around
10–20s. Therefore, this approach can be used if the timescale between voltage change is
long enough to allow membrane equilibrium (∼20 s) but short enough to preclude liquid
accumulation (∼1 min). The transient response of a fuel cell to a voltage change can also
be used to identify the hydration or flooded state of the system, because the characteristic
response is different between a dry, fully moist, and flooded fuel cell due to the presence
or absence of a rehydration or flooding response peaks.
9.1.4 Experimental Determination of Fuel Crossover
Fuel crossover is readily oxidized at the cathode due to the high local potential. The
magnitude of the voltage decay from normal levels of crossover becomes insignificant at
higher current, but it does cause a significant reduction in the OCV, since some overpotential
is required to oxidize the fuel at the cathode. Fuel crossover is also indicative of the quality of
the fuel cell build, compression, and sealing integrity and is also used as a beginning-of-life
quality control metric.
Fuel crossover generally affects PEFCs the most and is also used as a metric to
determine the durability of a membrane in service. That is, the fuel crossover increases
with time of service and is an indirect indication of membrane thinning and pinhole
formation. Fuel crossover can also diffuse into and across liquid electrolytes in PAFC,
AFC, and MCFC applications. In SOFCs, crossover is generally not a problem, but finite
electrical conductivity of the electrolyte results in the same effect of a reduced OCV. The
application where crossover causes the greatest performance loss is liquid-fueled DAFCs,
where methanol or other fuel crossover can result in a reduction of OCV of 0.5 V or more
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468 Experimental Diagnostics and Diagnosis
Anode
Cathode
FM
Flowing or
stopped
anode flow
Stopped
cathode inlet
Crossover flowmeter
Figure 9.15 Bubble gauge measurement in PEFC.
compared to the Nernst value. In order to measure the gas- or liquid-phase crossover, several
techniques are available, as described.
Gas-Phase Fuel Crossover In the case of gas-phase reactants, there are several methods
to determine reactant crossover. The most commonly used are the (1) bubble gauge, (2) the
OCV decay test, and (3) the limiting current test. The bubble gauge and OCV decay test
can be easily performed and are the most convenient approaches, while the limiting current
test requires some additional experimental complexity and can be a little unreliable.
In the bubble gauge approach illustrated in Figure 9.15, a highly sensitive mass flowme-
ter (the term “bubble gage test” comes from a type of highly sensitivity mass flowmeter used
for this purpose) is used to measure the crossover flow. A gas-phase pressure difference
can be imposed across the anode and cathode to increase the crossover flow for ease of
measurement and comparison. The inlet on the nonpressurized side is also closed, and a
flowmeter on the nonpressurized side is used to measure the outgoing flow rate, which is
the crossover flow.
The bubble gauge approach is quite simple, but the mass flowmeter sensitivity to very
low levels of crossover is limited since the difference between the crossover and normally
expected flow rates is so large. A separate mass flowmeter can be installed prior to each
crossover test to eliminate this problem, but this is cumbersome.
Another approach to determine the crossover current with high sensitivity is the OCV
decay test. In this approach, the cathode and anode flows are shut off, the exits are closed,
and the cell is left at open-circuit conditions. The OCV will gradually decay, due to fuel
crossover through the membrane. The rate of decay can be correlated with a high level
of precision to the crossover flow rate. Variations of this approach can use a pressure
differential or flowing fuel to establish a faster or more repeatable decay correlation.
In the limiting current approach, the rate of fuel or oxidant crossover to the other
electrode can be estimated using cyclic voltammography by employing the same principles
discussed for the CV technique. Essentially, one electrode has nitrogen or other inert species
(or humidified nitrogen in the case of a PEFC) and the other electrode has hydrogen.
The inert electrode is polarized to oxidize any existing hydrogen until a limiting current
is reached where the current becomes invariant with additional polarization (until other
electrochemical reactions become significant). The rate of fuel or oxidant crossover to
the other electrode can be estimated using the maximum current measured during the
polarization. Either cyclic or linear sweep voltammetry can be used for this analysis.
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9.2 Physical Probes and Visualization 469
Liquid Fuel Crossover Many DAFCs suffer large performance losses from crossover. In
the case of liquid DAFCs, the crossover current can be determined from chemical species
measurement of the cathode effluent gas. Since the product of oxidation of methanol or
other fuel crossover is mostly carbon dioxide and carbon monoxide, measurement of these
gases along with any residual crossover fuel vapor and minor species can be used to perform
a carbon mass balance and deduce the crossover fuel current.
The liquid fuel crossover can also be measured using the limiting current approach
discussed above. Humidified nitrogen or other inert gas flows through the cathode as it is
polarized to oxidize any crossover fuel. The limiting current is the crossover current density
at the cell conditions.
9.2 PHYSICAL PROBES AND VISUALIZATION
9.2.1 Distributed Diagnostics
Besides online sensing and control, laboratory-based diagnostics have been advanced to
provide insight into competing phenomena and as benchmark data for fuel cell model
validation. The challenge is to gather more revealing distributed and transient data than
a normally instrumented fuel cell, where bulk current and voltage information can be
misleading. For example, in larger area fuel cells, performance varies between plates in a
stack, with location along the flow channels, and locally along the active area. At different
locations in a PEFC, the same cell can be simultaneously suffering flooding or drying
(PEFCs) or reactant/product transport limitations. This fact alone limits the quantitative
usefulness of bulk cell performance data. Multidimensional models are needed to capture
these variations, and diagnostic tools are needed to provide model validation data and
measure gradients in current, species, temperature, conductivity, and other parameters of
interest. Many useful tools have been developed for this purpose and some of the more
common approaches are summarized here as an introduction.
Current Distribution Measurement One of the key challenges to the advanced under-
standing of all fuel cell systems is the development of techniques to measure the current
distribution of an operating cell. Determination of current distribution is critical to design
optimization, material selection, and model validation and to fundamentally understand the
interrelated phenomena which determine the localized performance. Different fuel cells
have very different current distributions and influences. For example, a small active area
portable fuel cell may have a nearly uniform current distribution, while larger area PEFCs
have steep gradients that can vary with local humidity, flooding, oxygen concentration, or
other effects. In high-temperature fuel cells, electrolyte conductivity is almost solely con-
trolled by temperature, and current distribution generally follows the temperature profile,
so that distributed current measurement may not be necessary in solid oxide and molten car-
bonate fuel cells as long as electrolyte temperature is known. The methodologies to achieve
current distribution measurement in lower temperature PEFCs have been well developed
and can generally be applied to other fuel cell systems, although some are more suited to
a particular fuel cell. Methods for measurement of current distribution in PEFCs include
using magnetic loop arrays embedded in the current collector plate, masking different ar-
eas or partially catalyzing segments, isolating individual locations of catalyzed anode and