Understanding the Interplay Between GDL

Understanding the Interplay Between GDL Compression and Operating Conditions in Liquid-Feed DMFCs P. A. Garc´ıa-Salaberri, M. Vera

http://fluidosuc3m.es/

Dept. de Ingenier´ıa T´ermica y de Fluidos, Universidad Carlos III de Madrid, Legan´es, Madrid, Spain

Multiphysics Model

PEM

Figure 1: Numerical domains of the 2D

GDL

anode/cathode GDL models and the 1D local model comprising the CLs, MPLs and PEM.

Anode CL MOR : CH3 OH + H2 O → CO2 + 6 H+ + 6 e− HER : 6 H+ + 6 e− → 3 H2 Cathode CL ORR : 3/2 O2 + 6 H+ + 6 e− → 3 H2 O MOR : CH3 OH + H2 O → CO2 + 6 H+ + 6 e−

cathode

ch CO 2

70

50

I [mA/cm2 ]

Ip [mA/cm2 ] 3

ch CO 2

3.5 4 [mol/m3 ]

= 1 M (CR = 10%)

4.5

0

5

(40%)

2 M (10%)

2

2.5

(40%)

3

ch CO 2

3.5 4 [mol/m3 ]

3 M (10%)

4.5

5

(40%)

50

600

60 80

50

400

60 200

50 50

40

40 30 0.1

1

2

40

2

60 mW/cm

20

y [μm]

300

anode ηa

= 2 mol/m (Graphite/Metallic)

3 mol/m (G/M)

ch Cml

4 mol/m3 (G/M)

= 3M

3

ch [M] Cml

4

5

5 mA/cm2

30 0.1

6

1

0

10

20

CR [%]

30

40

for graphite/metallic bipolar plates and different operating conditions: (a) oxygen concentrach = 4 mol/m3, and various methanol concentrations, and (b) methanol concentration, tion, CO 2 ch = 3 M, and various oxygen concentrations; 1 − sch = 0.3; sch = 0.4; T = 70 ◦C. Cml a c

0.5

1

3 C ch O2 = 2 mol/m

CO2 [mol/m3 ] 5

anode

0

ηa

H2 evol.

1.2

ηc

0.6 0

-250 -550

cathode 0

0

150

3 mol/m3

4 mol/m3

0.5 x [mm]

1

2.5 0

P @ 0.2 V [mW/cm2 ]

120

60

30

5

6

90

sch c = 0.2 sch c = 0.4 sch c = 0.6

Low Influence Flooding High Influence Flooding

60

30

1

2

ch Cml [M]

3

4

0 0.1

1

2

ch Cml [M]

3

4

300 450 I [mA/cm2 ]

Future Work

5 mol/m3 600

750

Figure 4: Polarization curves for various oxy-

[V]

Figure 2: Variation of the current density, I, with GDL compression ratio, CR, at 0.2 V

0.6 0

cathode

300

50

0

1.2

ηc

-250

0

0.3

2 mol/m3 ηa

Condensation Dominant

Gas Blockage Dominant

0.1

anode

ch [M] Cml

4

Cathode Saturation Level (CR=10%) 150

= 0.1 = 0.3 = 0.6

3

ch , and cell temperature, T , at 0.2 V for CR = 10%. (down) Variamethanol concentration, Cml ch , at 0.2 V corresponding to various tion of the power density, P , with methanol concentration, Cml ch ch 3 anode, sch a , and cathode, sc , saturation levels for CR = 10%. Graphite BPP; CO2 = 6 mol/m ; other operating conditions as in Fig. 2.

0.2

1

3 C ch O2 = 3.5 mol/m

0

-550

0

0.5

1− 1− 1−

sch a sch a sch a

2

Figure 5: (up) Variation of the power density, P , and parasitic current density, Ip, with

0.4

CO2

cathode

300

350

0.6 0

-250 -550

3

1.2

ηc

90

0 0.1

0.5

current

0

ch 3 C ch ml = 3 M, varying C O2 [mol/m ]

0.6

3 C ch O2 = 5 mol/m

450

50

120

The impact of GDL compression is extended to the whole polarization curve as the availability of oxygen decreases. Oxygen starvation arises primarily under the rib, leading for low oxygen concentrations to a spontaneous hydrogen evolution at the anode that may accelerate catalyst degradation [5].

y [μm]

40

70 100

ch , corresponding to 10% and 40% compressed GDLs: (a) power methanol concentrations, Cml density, P ; and (b) parasitic current density, Ip, and methanol utilization, FU = (I + Ip)/I . Graphite BPP; other operating conditions as in Fig. 2.

150

30

800

Figure 3: Cell performance at 0.2 V as a function of oxygen concentration, COch2 , for various

y [μm]

3

150

I p @ 0.2 V (CR=10%)

80

T [◦ C]

2.5

ch Cml

250

CR [%]

2

3

550

250

20

80

25

[V]

I [mA/cm2 ]

= 1 M (Graphite/Metallic) = 4 mol/m

P @ 0.2 V (CR=10%)

75

40

(b)

650

350

10

100

[V]

(a)

0

0

55

Oxygen Starvation

450

50

90

150

There is an optimum GDL compression ratio (CR) that maximizes cell performance due to the trade-off between concentration and ohmic losses. The optimum CR is strongly dependent on the bipolar plate material [3]. In addition, some variations arise due to the particular operating conditions of the cell [4]. Lower CRs lead to higher power densities when the availability of reactants is severely limited.

3 M (G/M)

180

Anode Gas Coverage Level (CR=10%)

Optimum Compression Ratio

ch CO 2

270

P @ 0.2 V [mW/cm2 ]

2D Model

2 M (G/M)

70

10

ch ch CO s c 2

550

85

2D Model

MPL

ch Cml

Vcell = 0.2 V

25

T

CL

650

360

100

Vcell [V]

x

(b)

(a)

BPP

ch ch sa Cml

y

Beyond GDL compression, complex interrelated phenomena affect cell performance in different ways depending on the operating conditions. At low-to-intermediate methanol concentrations, DMFC performance increases with cell temperature due to the higher species diffusivity/amount of methanol vapor and lower activation/ohmic losses in the cell. However, the growth of the parasitic current with methanol concentration and cell temperature offsets such positive effects [6]. As a result, an optimum working temperature exists at high methanol concentrations. The anode gas coverage factor and, especially, the cathode saturation level have also a major impact on cell performance.

T [◦ C]

anode

There exists an optimum methanol concentration due to the trade-off between anode polarization losses and cathode mixed potential. An over-compression of the GDL hinders transport of methanol and oxygen, leading to a decrease of the cell performance despite the reduction of the parasitic current.

FU [%]

1D local model Computational domain

Cell Temperature

P [mW/cm2 ]

The coupled effects of GDL assembly compression and operating conditions on the performance of Direct Methanol Fuel Cells (DMFC) are explored by means of an isothermal 2D/1D across-the-channel model [1–3]. The formulation incorporates a comprehensive 2D description of the anode and cathode GDLs, including two-phase phenomena, non-uniform anisotropic transport properties, and GDL/BPP electrical contact resistances. The effective properties of the GDL and contact resistances are evaluated from empirical data corresponding to Toray carbon paper. Two-phase transport in the MPLs, and water/methanol crossover and proton transport in the PEM are described by a 1D local model, while the catalyst layers are treated as infinitely thin surfaces.

Reactant Concentration

ch , gen concentrations at the cathode channel, CO 2 and a fixed methanol concentration at the anch = 3 M. (left) Oxygen concenode channel, Cml tration field at the cathode GDL, electric current lines, and anode/cathode overpotentials at 0.2 V for various oxygen concentrations and a ch = 3 M; other fixed methanol concentration, Cml operating conditions as in Fig. 2.

Further extensions of the present model will be presented in future work, including the development of a fully 3D model or the incorporation of electrical contact resistances at the MPL/CL and GDL/MPL interfaces.

References [1] P.A. Garc´ıa-Salaberri, M. Vera, R. Zaera, Int. J. Hydrogen Energy, 36 (2011) 11856–11870. [2] P.A. Garc´ıa-Salaberri, M. Vera, I. Iglesias, J. Power Sources 246 (2014) 239–252. [3] P.A. Garc´ıa-Salaberri, M. Vera, J. Power Sources (2015) DOI 10.1016/j.jpowsour.2015.02.112 [4] Y. Zhu, C. Liu, J. Liang, L. Wang, J. Power Sources 196 (2011) 264–269. [5] T. Arlt, I. Manke, K. Wippermann, et al., J. Power Sources 221 (2013) 210–216. [6] M.K. Ravikumar, A.K. Shukla, J. Electrochem. Soc. 143 (1996) 2601–2606.