Focus on H2 PEM Fuel Cells
1 H2 PEM Fuel Cells
•  The use of a pla-num catalyst makes spli5ng the hydrogen molecule rela-vely easy, easier that spli5ng the stronger oxygen molecule bonds •  This spli5ng of the oxygen causes significant ac-va-on losses (the main kine-c limita-on) •  Another significant source of performance loss is the resistance of the membrane to proton flow, which is minimized by making the membrane as thin as possible (around 50 µm) •  Overall a PEM fuel cells high power output opera-on depends on the performance of all the sub-­‐systems, catalyst, membrane as well as parameters such as temperature and humidity 2 GDLs
•  The GDL requirements are
o  allow uniform transport of reactant gases to the electrode
o  conduct electrons
o  remove product water from the electrode
o  transfer heat to maintain the cell temperature
o  provide mechanical support for the MEA.
•  To fulfill these functions, an ideal GDL material should have
small gas transfer resistance, good electron conductivity, and
good thermal conductivity.
•  The porosity of a GDL structure is the most important parameter
for reactant transfer
•  Water management is an important consideration
•  The ability to remove water is one of the key properties
3 GDLs
•  The cell performance can be improved by placing a fine
layer (such as a microporous layer) between the GDL and
the catalyst layer because facilitates the removal of water
•  Commonly used GDL materials are made of porous carbon
fibers, including carbon cloth and carbon paper.
•  Carbon cloth has better performance under high-humidity
conditions (low tortuosity) and rough textural surface
facilitate droplet detachment)
•  Under dry conditions, carbon paper GDL has shown better
performance than carbon cloth GDL because it is capable of
retaining the membrane hydration level 4 Catalyst layers
•  The catalyst layer on both sides (anode and cathode)
needs to be able to
o  transport H2 or O2 to the catalyst sites
o  catalyze the HOR/ORR process
o  carry protons away from/to the reaction sites to/from
the membrane electrolyte
o  remove electrons from the anode or transport them to
the cathode
o  prevent flooding by liquid water produced in the
reaction or condensed from the vapour in the gas
streams
o  transfer heat in or out of the reaction zone
5 ・・・
Catalyst layer
Carbon particles
Cel
•  Composed of ionomer and Pt/C sprayed or dropped onto the membrane or GDLs in a liquid (water/
Cel
glycerol) and dried at higher temperature. The GDLs z
and membrane are hot pressed to form the MEA x
•  The Nafion conducts protons; the C and Pt conduct Pt nano particle
electrons (a)
•  The Pt/C par-cles tend to form clusters called agglomerates, coated by Nafion ionomer Figure 2. (a) A constructed 3-D model
distributed Pt/C catalyst intermixed
•  Nafion and Pt/C content have drama-c effect on of PEMFC
performance •  A balance is required for ingress of reactants, 2.2. Tortuosity of gas in pore network
conduc-vity of the protons and conduc-vity of the electrons, as well as ac-ve surface area for reac-on The tortuosity, of a pore network
•  Too much Nafion or a low porosity can cause pores. These connected pores form continu
flooding ratio of the average path of gas flow in free s
•  Too liYle Nafion leads to low H+ conduc-vity Lc /
In computation of the mean
6 tortuo
computation procedure for the pore tortuosit
Anode
•  The hydrogen oxidation reaction (HOR) can be expressed in the Bulter-Volmer
+
form (equation (17) in the kinetics slides) for 2H2 → 4e + 4H
(
i = i0,a CH 2 CH 2 ,0
) (α )
2α
H+
4(1−α )
nFη
−(1−α )nFη
⎛ αRT
⎞
RT
e
−
e
⎜⎝
⎟⎠
(1)
•  Here we use activity for the oxidized form (H+) and take aH+=1.
•  The exchange currant density (i0) depends on the catalyst morphology, the
catalyst-electrolyte interface, the properties of the reaction media (pH,
electrolyte, temperature, concentration, etc.)
•  Reported value ranges from 10-5 to 10-2 A/cm2 for different Pt electrodes in
various acidic media.
•  Rate-limiting process is the ORR because i0 for the ORR is 10-6 ~10-11 A/cm2
•  Anode materials research has been centered mostly on CO-tolerant catalysts
operating with CO-contaminated fuels (such as reformate),
7 Cathode
•  The ORR is the main limitation in terms of kinetics (we say it is rate limiting)
-
+
•  We can write the current density as (for O2 + 4e + 4H → 2H2O)
(
i = i0,c CO2 CO2 ,0
) (α )
1−α
H 2O
2α
nFη
−(1−α )nFη
⎛ αRT
⎞
RT
−e
⎜⎝ e
⎟⎠
(2)
•  We use activity for the reduced form (H2O) and take α H 2O = 1
•  The slow ORR kinetics is responsible for the steep slope in the activation
polarization region (see later)
•  A small but noticeable H2 crossover current at the open circuit (OC) is largely to
blame for the open-circuit voltage (OCV) loss even for a state-of-the-art
membrane electrolyte.
•  Much of the cathode research has been directed at finding a cathode catalyst to
improve the slow ORR kinetics and to find a cheap replacement for Pt.
•  Pt and Pt alloys (Ru, Co, Ni) are the most active catalysts for the ORR
8 Cathode
•  Cathode durability presents a special challenge in
PEM fuel cells.
•  The major problems associated with the catalysts are
Pt agglomeration, dissolution, and redistribution.
•  The cathode environment is highly oxidative and
corrosive due to high voltage (e.g., 0.6 to 1.0 V), low
pH, elevated temperature, and the presence of water
and oxygen
•  Although Pt has low solubility at normal cell operating
voltages, its solubility increases significantly at round
1.1 V (near OCV)
9 entration of (spatially distributed) species i. For very flow rates
<1 ml s"1), a high degree of stratification in the reactant concentraons would develop between the inlet and outlet to the cell. For the
ow rates considered here, the stratification is expected to be low.
he proton activity has been approximated by the concentration (in
mol dm"3) of protons, ½Hþ ' in the positive electrode. Note that the
membrane
electrolyte
must
several requirements:
nal term A
in Eq.
4 could be written
in terms of the
pH of meet
the positive
lectrode solution, pH ¼ " log10 ½Hþ '. The formal potentials depend
•  high proton conductivity over a wide RH range
n temperature.22 The variations are, however, small and therefore
eglected.•  low electrical conductivity
The ohmic losses associated with the current collector, memrane and •
thelow
electrolyte
be modelled as follows
gascan
permeability,
particularly for H2 and O2 (to
wc
wm
we
minimize
crossover)
; ðIRÞmH
¼2j/O
;
ðIRÞ
¼
j
[5]
ðIRÞc ¼ jappl
appl 2
appl 3=2
e
rc
rm
e re
Membrane
•  good mechanical properties under cycles of humidity and
espectively, where, rc , rm and re are the conductivities and wc ,
the widths of the current collector, membrane and the
wm and we aretemperature
lectrolyte, respectively, as shown in Fig. 2. Note that a Bruggeman
cost
orrection• hasstable
been usedchemical
in the case ofpropertieslow
ðIRÞe to obtain the effective
onductivity e3=2 re . For a Nafion membrane, the following empiri23
al relationship can beProton employed
conduc-vity !
#
$"
1
1
λ = “[6]
water content” = (moles of water) "
rm ¼ ð0:5139k " 0:326Þ exp 1268
303 T
÷ (moles of SO3 groups) n S m"1, where k is the membrane water content (moles of H2O to
moles of SO"
3 ). Since the membrane is in constant contact with the
quid electrolyte on both sides, it is reasonable to assume that it is
ully saturated; that is,24 k ¼ 22.
10 Membrane
•  The life time of a membrane is related to its original
thickness, its mechanical integrity, and its chemical
stability
•  Thin membranes lower resistance but are much more
likely to fail, even when reinforced
•  Membranes can develop pinholes, tears, cracks, can
delaminate and can suffer degradation due to attack by
chemical species (.OH) generated on metal impurities
via Fenton reactions
11 Membrane
•  Many alterna-ves to Nafion (much less expensive) •  Issues are mainly proton conduc-vity and stability (durability) •  Nafion remains the op-mal although much progress has been made with composite and grafed membranes •  Durability (and cost) of the PEM is one of the most cri-cal degrada-on issues •  Need a breakthrough 12 Bipolar plates
•  The func-ons of bipolar plates are o  supply and separate reactant gas without introducing impuri-es o  conduct electrons o  remove the heat and control the fuel cell opera-ng temperatures o  remove the product water from the system •  To fulfill these func-ons, an ideal bipolar plate material should have high electrical conduc-vity, high thermal conduc-vity, and good chemical and electrochemical stability in the fuel cell environment •  From a manufacturing perspec-ve, the bipolar plate material should have low density, good mechanical strength, ease-­‐of-­‐manufacture (in bulk) and low cost. •  The most common material is graphite 13 H2 PEM Fuel Cells: Bipolar plates
•  Due to the brittle nature of graphite, graphite plates cannot be easily
fabricated, and the minimum thickness is limited to about 5 mm
•  This result in a large stack volume/weight and low efficiency in heat
transfer
•  The plate cost can be up to 40 % of the total stack cost
•  The brittleness of the graphite limits the methods of fabrication
(machining), which leads to a high manufacturing cost
•  Alternatives are stainless steel (SS) and composite materials
•  The issue is corrosion (particularly of bare SS) and high interfacial
contact resistance (ICR)
•  Can have low ICR and low corrosion rates with various treated SS and
composite plates but not both simultaneously
•  The is an active area of research 14 H2 PEM Fuel Cells: Real performance
In reality, fuel cells achieve their maximum output voltage at open circuit (no load) condi-ons and the voltage drops off with increasing current draw. This is known as polariza-on !
15 H2 PEM Fuel Cells: Real performance
There are several major types of losses in a fuel cell •  Ac-va-on Losses – caused by the slowness of the reac-ons taking place on the surfaces of the electrodes. A propor-on of the voltage (free energy available) is lost in driving the electron transfer reac-ons •  Ohmic Losses – The voltage drop due to the resistance to the flow of electrons and ions through the material of the electrodes (some of the voltage available is required to transport the charge) •  Concentra-on Losses – Losses that result from resistance to mass transport of the fuel and oxidant 16 H2 PEM Fuel Cells: Real performance
•  The actual cell voltage can be expressed as follows: Ecell = Ecell,OCV
EOCV
X
k
Vohm,k
|⌘act,an |
|⌘act,ca |
|⌘con,an |
|⌘con,ca |
(1) •  The first term on the rhs is the OCV (from Nernst equa-on) •  The second term represents the ohmic losses in each component k of the cell (ionic and electronic) •  The 3rd and 4th terms represent the ac-va-on losses for the anode and cathode respec-vely •  The 5th and 6th terms represent the mass transport losses in the anode and cathode respec-vely •  The first term is usually measured or subs-tuted by E0cell 17 H2 PEM Fuel Cells: Ohmic losses
•  The ohmic loss of the fuel cell has to be determined component by component •  In the gas diffusion layers, only electrons are transported so the ohmic loss relates only to the GDL resistance •  In the catalyst layers, there is resistance to electron flow in the C and Pt, as well as H+ flow in the ionomer •  In the membrane, only H+ exist, so the ohmic loss related only to the resistance to proton migra-on •  We can see that an equal number of protons and electrons (n=4) are generated by the reac-on at the anode. Electroneutrality demands that all protons are transferred to the cathode. So the ionic (H+) current is equal to the electronic current 18 H2 PEM Fuel Cells: Ohmic losses
•  Usually, the ohmic losses are calculated for a certain applied current density iapp
•  The cell operates in galvanosta-c mode (constant current) as opposed to poten-osta-c (constant cell voltage) •  The ohmic loss is normally calculated using the conduc-vity, which is the reciprocal of the resis-vity (a length independent version of resistance) •  This is because the cross sec-on is constant (next slide) 19 H2 PEM Fuel Cells: Ohmic losses
•  Consider a flow of charge (current i) through some medium directed normal to a constant cross sec-on A
i A i l •  Ohm’s law states that the drop in poten-al (ΔV) along a path of length l for a current i passing though a constant cross sec-on of area A of the medium is equal to (Ai)R, where R is the resistance to the flow along the length l •  The resis-vity of the medium (cross sec-on A) is defined as ρ = RA/l so that ✓ ◆
✓ ◆
⇢l
l
V = AiR = A
i = (⇢l) i =
i
A
•  where =
1/⇢
is the conduc-vity of the medium 20 Ohmic losses
•  In the GDL we have to correct for the fact that the medium is porous with porosity ε. Therefore the volume frac-on of carbon is 1 -­‐ ε •  The means that the conduc-on path is not con-nuous so we need an effec-ve conduc-vity •  The conduc-vity is replaced with (1 -­‐ ε)3/2 × σ , which is called a Bruggeman correc-on (the factor (1 -­‐ ε)3/2 accounts for the tortuosity of the conduc-on path) •  Thus the Ohmic loss in the GDL is VGDL =
(1
lGDL
iapp
✏)3/2 carbon
•  Here σ is the conduc-vity of pure carbon and lGDL is the thickness of the GDL •  In the catalyst layers, the carbon has volume frac-on εC. We can ignore the contribu-on of Pt to electron transport since εPt is very small •  Then the electronic ohmic drop in the CLs (which have thicknesses lCL) is lCL
VCL,elec = 3/2
iapp
✏C carbon
(3) (4) 21 Ohmic losses
•  The ionomer in the CLs has conduc-vity σm and volume frac-on εm, so the effec-ve conduc-vity is εm3/2 σm lCL
VCL,ion = 3/2
iapp
(5) ✏m m
•  This leaves the ohmic losses in the membrane. The membrane and ionomer are usually the same material, so the membrane has conduc-vity εm, so Vmem =
lm
m
iapp
(6) •  where lm is the thickness of the membrane (there is only one phase in the membrane so we don’t need an effec-ve conduc-vity) 22 Activation losses
•  Recall the Bultler Volmer equa-on (1) (
i = i0,a C H 2 C H 2 ,0
)
−2 Fη
⎛ 2RTFη
⎞
⎛ 2Fη ⎞
RT
⎜⎝ e − e
⎟⎠ = 2i0,a C H 2 C H 2 ,0 sinh ⎜⎝ RT ⎟⎠ = −iapp
(
)
•  First of all we have assumed that the ac-vity of the H+ is 1. Second we assume α=1/2 (which is usually close to the actual value), which leads to the sinh •  If the applied current is constant then this equa-on can be inverted to obtain the overpoten-al, which is the ac-va-on overpoten-al •  Note that the conven-on for current is that it is posi-ve in the direc-on of the posi-ve to nega-ve terminal (cathode to anode) even though the electrons flow the other way! So iapp is nega-ve and the overpoten-al is nega-ve (this has to be the case because the second exponen-al is the anodic part) ηact,an
⎛ iappCH 2 ,0 ⎞
RT
=−
asinh ⎜
⎟
2F
⎝ 2i0,aCH 2 ⎠
(7) 23 Activation losses
•  By a similar argument (equa-on (2)) (
i = 2i0,c CO2 CO2 ,0
)
1/2
⎛ 2Fη ⎞
sinh ⎜
=i
⎝ RT ⎟⎠ app
•  First of all we have assumed that the ac-vity of water is 1. Second we again assume α=1/2, which again leads to the sinh •  If the applied current is constant then this equa-on can be inverted as before to obtain the overpoten-al, which is the ac-va-on overpoten-al •  This -me iapp is posi-ve and the overpoten-al is posi-ve ηact ,ca
⎛ iappCO1/22 ,0 ⎞
RT
=
asinh ⎜
1/2 ⎟
2F
2i
C
⎝ 0,a O2 ⎠
(8) 24 Mass transport losses
•  No-ce that the terms for the ac-va-on losses already include the concentra-ons of the reactants. •  Strictly speaking these concentra-ons are the surface concentra,ons •  PEMFCs u-lize porous electrodes at both the anode and cathode. Hydrogen fuel is fed to the cell at a constant flow rate. The fuel diffuses to the catalyst layers to react •  There are two resistances in series: 1.  Diffusional resistance (for H2) between the channel and the catalyst layer. The diffusion coefficient for H2 is D1 ( = D
H 2 ,eff
) and the diffusion length is r1 ( = lGDL). The hydrogen concentra-on in the channel = C
H 2 ,inlet
. The hydrogen concentra-on at the CL surface is Ci (i for intermediate). 25 Mass transport losses
2.  A further diffusional resistance in a boundary layer (BL) to get to the reac-on sites in the catalyst layer 3.  Physically, this represents absorp-on of H2 into the ionomer (electrolyte) films coa-ng the Pt/C and diffusion of the dissolved H2 through these films to reach the Pt 4.  The diffusion coefficient for H2 in the BL can be taken to be the diffusion coefficient in the ionomer film, D2 5.  The BL thickness is r1 (related to the volume frac-on of ionomer) 6.  The H2 concentra-on at the Pt surface is C H 2
26 Mass transport losses: Boundary layer model
H2 Pt r1 = lGDL Ionomer C Boundary layer of thickness r2 H2 Reac-on on this surface r1 = lGDL r2 27 Mass transport losses
•  At steady state, the flow rate through the film must balance the rate of consump-on at the Pt surfaces. So the flux of H2 through film = consump,on rate ν H 2 iapp iapp
D2
(Ci − C H 2 ) =
=
r2
nF
2F
•  At steady state, the H2 that flows through the film must be replenished by the H2 that flows in from the channel: flux of H2 through film equals flux of H2 through the GDL. D2
D1
(C
−
C
)
=
(CH 2 ,inlet − Ci )
i
H2
r2
r1
•  Combining these equa-ons to eliminate Ci gives: ⎛ r2
r1 ⎞ ⎛ iapp ⎞
(9) C =C
−
+
H2
H 2 ,inlet
⎜⎝ D
2
D1 ⎟⎠ ⎜⎝ 2F ⎟⎠
28 Mass transport losses
•  A typical D2 is O(10−10) m2/s, while the film thickness is on the order of 100 nm. The term r2/D2 is therefore much larger than r1/D1 and dominates the mass transfer resistance •  The ra-o r2/D2 will not be known and will therefore need to be fiYed in any model or es-mated from experiment 29 Mass transport losses
•  A similar analysis for the oxygen leads to: ⎛ r
r ⎞⎛i ⎞
CO2 = CO2 ,inlet − ⎜ 2 + 1 ⎟ ⎜ app ⎟
⎝ D2 D1 ⎠ ⎝ 4F ⎠
(10) •  r1 can be taken to be the same as before while D1 and D2 are now for O2 •  The ra-o r2/D2 will be expected to be larger than in the anode. It is in the cathode that the mass transport and ac-va-on losses dominate •  Expressions (9) and (10) replace the concentra-ons of H2 and O2 in (7) and (8) respec-vely •  The resul-ng expressions incorporate both the mass transport and ac-va-on losses 30 Ideal Potential
Region of Activation Polarization
(Reaction Rate Loss)
Cell Voltage (V)
1.  The figure to the right shows a H2/O2 fuel cell polariza-on curve. •  Given that the losses are mainly incurred at the cathode, try to sketch the magnitudes of the cathode ac-va-on loss and the combined ohmic losses. The anode ac-va-on loss is sketched in red. (Note that the 3 losses should sum to the total loss) •  Looking at expressions (3) to (6), what are the main ohmic losses in a H2/O2 fuel cell and why? Quiz
1.0
1.0
Total Loss
Region of
Concentration Polarization
(Gas Transport Loss)
Region of Ohmic Polarization
(Resistance Loss)
0.5
0.5
Cathode L
oss
Internal
Membrane
0.0
0
5
10
15
Loss
Resistance
Anode Loss
20
25
2
Current Density (mA/cm )
0.0
30
35
Anode ac-va-on loss 31 Quiz
2.  What allows the SOFC to operate without the need for a catalyst? 3.  Why are SOFCs suited to sta-onary rather than portable applica-ons? 4.  Consider a 10 cell DMFC stack in series with a total ac-ve area of 200 cm2. If the stack operates at a current of 60 A, use the polariza-on curve below to determine the stack power in W and rates of methane and O2 consump-on in g s-­‐1 (molar masses of methane and O2 are both 32 g mol-­‐1) 0.8
0.10
0.09
0.7
0.5
0.06
0.4
0.05
0.04
0.3
2
Power (Watts/cm )
0.07
Voltage (V)
Cell voltage V 0.08
0.6
0.03
0.2
60 C
0.02
70 C
0.1
0.01
80 C
0.0
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
2
Current density A cm−2 Current density (A/cm )
FIGURE 9 Several DMFC polarization curves taken at the Penn State Electrochemical Engine
Center, showing typical performance. Active area: 50 cm2, fuel solution molarity: 1.0 M,
cathode pressure: 0.205 MPa, anode pressure: 0.101 MPa, cathode flow rate: 2100 mL/min,
2
anode flow rate: 14 mL/min, catalyst loading: 4 mg/cm .
32 Quiz
5.  Sketch the power density curve in the previous example 6.  With reference to the polariza-on curve on slide 23 what is the equivalent power for a H2 PEM fuel cell stack of 10 cell opera-ng at the same current with the same ac-ve area? What is the rate of H2 consump-on in this case. Compare it to the rate of methane consump-on in Quest 4. 7.  The magnitude of the anode current density for a H2 PEM fuel cell has the following form (the anodic part on slide 89 dominates) −2 Fη
(a) RT
| i |= i0,a CH 2 CH 2 ,0 e
Recall from slide that at steady state −2 Fη
D2
| i | i0,a
RT
(Ci − CH 2 ) =
=
CH 2 CH 2 ,0 e
(b) r2
2F 2F
D2
D1
and
(Ci − CH 2 ) = (CH 2 ,inlet − Ci )
(c) r2
r1
Here we replace iapp with any general i. We also need the magnitude for the rela-onships to make sense
(
)
(
)
33 Quiz
•  Show that the current density can be wriYen in the form −2 Fη
i0,a
ξ CH 2 ,inlet CH2 ,0 e RT
D2 / r2 ) ( D1 / r1 )
(
|i | =
where ξ =
−2 Fη
i
D2 / r2 + D1 / r1
ξ + ( 21F ) CH0,a2 ,0 e RT
[Use (b) to write C
H 2 in terms of C
i , then use (c) to eliminate C
i from the resul-ng expression. Finally subs-tute the expression for C
H 2 back in to (a) •  What is the limi-ng value of |i| as η
→
∞
•  Think of the factors that will limit the current density ( )
( )
34