Relative Humidity Distribution along the Gas Flow Channel in a Wide Temperature Range
Authors: Polat Beste Balci , Nakamichi Kimiyo , Ma Yulei , Kageyama Miho , Kawase Motoaki 
Published: December 2024

Introduction

This study focuses on analyzing the distribution of relative humidity within a parallel flow channel while correlating it with temperature, T, and relative humidity, RH. The gas composition profile inside the cell is determined by space time, defined as a ratio of the active area, A, to the volumetric gas flow rate, v
0, at the cell inlet. By increasing the gas flow rate and hence decreasing the space time, the profile is elongated and the gas composition inside the cell at a longer space time can be measured at the outlet at a shorter space time. Measuring the relative humidity at the outlet of the cell with varying the active area or the gas flow rate for varying the space time yields the relative humidity distribution inside the cell. The measured relative humidity distributions are then compared with theoretical distributions calculated with the assumptions of no water permeation and quick permeation (equal RH on both sides) through the membrane.

Experimental

A membrane electrode assembly (MEA) with an active area of 5.0×5.0 cm2 was constructed using a Nafion membrane (50.8 µm thick) with a 10.3 µm thick catalyst layer (CL) for anode and one for cathode. The platinum loading was 0.30 mg/cm2, employing 50 wt% Pt/carbon black catalyst particles with an ionomer. The ionomer/carbon weight ratio was 1.0.
RH measurements were facilitated by tracking water flow rates at both the anode and cathode outlets. RH at the cell inlet was measured at the outlet of the cell with a non-permeable polytetrafluoroethylene (PTFE) sheet utilized instead of an MEA under identical conditions for maintaining the total pressure in the humidifier equal. To assess various oxygen conversions, the oxygen molar flow rate at the inlet, F
O0, was adjusted at each temperature under atmospheric pressure conditions at 0.7 V.

Results and Discussion

The relationship between oxygen conversion and space time was investigated (Fig. 1). The space time is varied by varying the active area [1] and varying the gas flow rate. The oxygen conversion is higher when the space time is longer. The oxygen conversion is determined by the space time regardless of the active area or the gas flow rate. Varying the active area and the gas flow rate are equivalent. The oxygen conversion changed monotonically with change in the space time. The oxygen conversion can represent the space time and therefore the position in the cell.
RH on the anode and cathode side are plotted against the oxygen conversion, which represents the RH distribution in the cell (Fig. 2). The RH increases from the inlet to the outlet as water is generated in the electrochemical reaction (higher increase at cathode due to water generation and water not permeating quickly from the cathode to the anode side). RH distribution calculated by assuming no permeation (Eqs. 1 and 2) of water is shown by dotted lines. On the anode, hydrogen consumption raised the RH downstream; on the cathode side, oxygen consumption and water generation raised the RH downstream. Solid lines represent RH distributions calculated by assuming the quick permeation (Eq. 3) of water yielding equal RHs on both sides which is located between the two lines calculated with the no-permeation model. At higher temperatures, the two models result in similar RH distributions because the water permeation rate is sufficiently low compared with the water flow rate in the channels. The actual RH distribution is located between RH distributions calculated with the two models, though data on the anode side at 80 oC include some experimental errors.
RH[A]=(RH0
[A]/(1-2F
g0
[C]
y
O0
x
O/F
g0
[A])(P
[A]/P
0
[A]) (1)
RH[C]=(RH0
[C]+2p
O0
x
O/p
S
sat)/(1+y
O0
x
O)(P
[C]/P
0
[C]) (2)
RH=((RH0
[A]
P/P
0
[A])+(RH0
[C]
P/P
0
[C])(1+2y
O0
x
O/y
S0
[C])(F
g0
[C]/F
g0
[A]))/(1+(1-y
O0
x
O)(F
g0
[C]/F
g0
[A])) (3)
At higher temperatures, RH becomes low unless more humid gas is supplied. Therefore, performance of PEFC can be adjusted by varying inlet RH. When the average RH at inlet and outlet of the cell is equal, higher temperature showed higher performance (Fig. 3).

Conclusions

RH distribution in the cell was investigated by varying oxygen conversion. The space time which determines oxygen conversion was varied by varying the gas flow rate in this study. At higher temperatures, more water generation is required to attain an equal RH because the saturated water vapor strongly depends on temperature. When the average RH at inlet and outlet of the cell is equal, higher temperature showed higher performance.

Reference(s)

[1] Ma Y., Kageyama M., and Kawase M. ECS Transactions, (2021) 104(8).

Figure 1