Graduate School of Engineering Hokkaido University / Laboratory of Energy Conversion Systems

Control of the Balance between Vapor and Heat Transfer
for the Reduction of Oxygen Transport Resistancein High Current Density PEFC Operation

Yuki Kitami, Yutaka Tabe, Takemi Chikahisa
Division of Energy and Environmental Systems,

Graduate School of Engineering Hokkaido University / Laboratory of Energy Conversion
Systems
14/November/2004

Introduction

Slide 2 of 14

Background
Flooding (the blockage of the gas supply by the accumulation of water)⇒Oxygen transport resistance increases
𝑂2However, we confirmed the increase inoxygen transport resistance not by flooding 𝐻2 𝑂

Objective
To improve cell performance by reducing oxygen transport resistance1) To clarify the increase in oxygen transport resistance2) Control of the balance between Vapor and Heat Transfer toreduce oxygen transport resistance3) Effect of channel and flow rate

Experimental Methods and Conditions

Slide 3 of 14

IV curve and oxygen transport resistance measurements
𝑂2 ：1~24%

Limiting Current Method
Oxygen transport resistance :𝐶0
𝑅T = 4𝐹
𝐼Lim
F : Faraday’s constant
C0 ： Oxygen concentration
Ilim ： Limiting current density（V=0.1V）

Conditions

CathodeCell Temperature・Active area : 1.8cm² (2cm×0.9cm)
・Rib channel width : 0.3mm
・GDL with MPL : CB-MPL, 28BC, 38BC

Anode
35~80℃

Gas

Mixed Gas
(O2+N2)

H2

Flow Rate

4000sccm

100sccm

O2: 1~24%

H2: 100%

Pressure

100kPa100kPaHumidity

81%RH

81%RH

Causes of Increase in 𝑅𝑇 (CB-MPL)
35℃

Slide 4 of 1480℃

The RH of the gas increases

The RH of the gas increases

The increase in the 𝑅𝑇 shifts to lower 𝐼𝐿𝑖𝑚

The increase in the 𝑅𝑇 shifts to higher 𝐼𝐿𝑖𝑚

FloodingDrying

Causes of Increase in 𝑅𝑇 (CB-MPL)

Slide 5 of 14

80℃

81%RH

TheThecellresistancekept similar
RHof the gasisincreases
The voltage drops suddenly
The increasein thelower
𝐼𝐿𝑖𝑚𝑇 shifts toof
⇒Not bythe𝑅dry-out
PEM

The RH of the gas increases
The increase in the 𝑅𝑇 shifts to higher 𝐼𝐿𝑖𝑚Drying

Cryo-SEM Observation

Slide 6 of 14

Freezing Method
Observation method of the water distribution in the cell by freezing
and immobilizing the water in ice form
[1] Y. Aoyama, K. Suzuki, Y. Tabe, and T. Chikahisa, Electrochem. Commun., 41, 72 (2014).

Freezing method

OperationCooling(-40 ºC)

Making samples
in liquid nitrogen

Cryo-SEM observation(- 150 ºC)

Cryo-SEM Observation Results

Flooding condition(50℃, 81%RH, 220kPa)

A large amount of ice
in MPL and CL

Slide 7 of 14

High temperature condition(80℃,81%RH, 100kPa)

No ice in MPL and CL

One-dimensional Analysis

Slide 8 of 14

Vapor diffusion and Heat conduction model

𝑹𝑯𝑪𝑳 =
𝑇𝐶𝐿 =𝑷𝑾,𝑪𝑳𝑷𝑺,𝑪𝑳 (𝑻𝑪𝑳 )𝑍𝐻 𝐸 − 𝑉 𝑖ℎ𝐺𝐷𝐿 + ℎ𝑀𝑃𝐿 + 𝑇𝑠𝑘𝑃𝑠,𝐶𝐿

7.5𝑇𝐶𝐿
237.5+𝑇𝐶𝐿= 6.11 × 10𝑖𝑅
𝑍𝐻 𝐸 − 𝑉 𝑖 (ℎ𝐺𝐷𝐿 + ℎ𝑀𝑃𝐿 )2𝑃𝑊,𝐶𝐿 = 𝑍𝑊+ 𝑇𝑠 (ℎ𝐺𝐷𝐿 + ℎ𝑀𝑃𝐿 ) + 𝑃𝑊,𝐶ℎ2𝐹𝐷𝑒𝑓𝑓𝑘2

Estimation of RH in the CLSlide 9 of 14

Vapor diffusion and Heat conduction model
80℃

𝑅𝑇 increases at around 40%RH
300
250200
Cause : Drying in the CL
150
Decrease in water content of the ionomer100⇒Poor oxygen permeability
500

RT [s/m]

53%RH
66%RH
81%RH

[7] H. F. M. Mohamed, et al, Polymer., 40 (2008), 3091.

806040
RHCL [%RH]20

Estimation of RH in the CL

Slide 10 of 14

Vapor diffusion and Heat conduction model80℃

RT [s/m]

𝑅𝑇 increases at around 40%RH
300
250
20015010050
053%RH
66%RH
81%RH

806040
RHCL [%RH]20

Cause of Drying in the CL

Slide 11 of 14

RH in the CL decreases as 𝐼𝐿𝑖𝑚 increases
80℃

Low thermal conduction of the CB-MPL→Temrperature in the CL (𝑇𝐶𝐿 ) increases→Saturated vapor pressure (𝑃𝑆,𝐶𝐿 ) increases

CB-MPL

28BC

38BC

Thermal Conduction k [W/m･K]

0.116

0.500

0.350

Diffusivity D / Effective Diffusivity Deff

4.10

4.50

4.50

Vapor Diffusion and Heat Conduction

Slide 12 of 14

Controlling the balance between vapor and heat transfer(RH is constant)
To reduce the oxygen transport resistance
CB-MPL

28BC

38BC

CB-MPL

28BC38BC

Thermal Conduction k [W/m･K]

0.116

0.500

0.350

Diffusivity D / Effective Diffusivity Deff

4.10

4.50

4.50

Results (81%RH,100kPa)
CB-MPL

28BC

Slide 13 of 1438BC

Unrealistic Operating Conditions (2)• Cathode Gas Flow Rate : 4000sccm⇒ To prevent water from staying in the channel
0.3mm
80℃, 81%RH, 100kPa, 28BC
1.04000sccm

0.8Voltage[V]2000sccm

0.60.41.0mm
0.2

0.00.0 0.5 1.0 1.5 2.0 2.5 3.0
Current Density[A/cm2]

Slide 14 of 14

Unrealistic Operating Conditions (1)Slide 15 of 14• Rib channel width : 0.3mm → 1.0mm
80℃, 81%RH, 100kPa, 28BC
1.00.3mm

0.3mm
0.8

Voltage[V]

1.0mm
0.6
0.4
0.2
0.0
0.0

0.5

1.0

1.5

2.0

2.5

Current Density[A/cm2]

3.01.0mm

Conclusions

Slide 16 of 14• Under high temperature conditions where water is less likely tostay in the GDL and channel, the increase in oxygen transport
resistance can be due to the drying of the ionomer in the CL.
• It was confirmed that the increase in oxygen transport resistancecould be suppressed by controlling the balance between vapor
diffusion and heat conduction. This is considered to be usefulknowledge for designing GDL / MPL structure.
• In this research, the liquid water is less likely to stay inside thecell (narrow channel, high gas flow rate). It is necessary todesign the cell structure so that the increase in oxygen transport
resistance can be suppressed even under conditions closer toactual operation.