Full-Cell Simulation of Cathodic Degradation in Polymer Electrolyte Fuel Cell System at Start-up, with and without Control Method to Reduce Degradation

Overview

Fuel-Cell vehicles (FCVs) powered by Polymer-Electrolyte Fuel Cells (PEFCs) are under development in several car manufacturers for commercial release in near future. Durability of PEFCs is one of most important issues regarding production cost of FCVs. Degradation of membrane-electrode-assembly (MEA) takes place during start-up process and limits life-span of PEFCs [1,2].

In this study, full-cell scale numerical investigation of degradation process of at start-up of PEFCs is carried out. Time-dependent behavior of degradation process is calculated, and distribution of amount of carbon corrosion can be predicted. Reduction of degradation by discharging corrosion current can also be simulated.

  Numerical Modeling

A Model for cathodic corrosion is implemented based on our simulation software for PEFCs. An important feature of this software is that macroscopic models are applied for microscopic phenomena in the MEA such as electrochemical reactions and proton/water transport. These models are coupled with multi-dimensional heat and mass transport equations, including effective parameters for gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow channel.

During start-up process, the fuel-cell is divided into power-source and load region by H₂/air front. Carbon corrosion reaction (COR) takes place in cathode at load region. A kinetic model for electrode reactions including both normal power-source reactions and corrosion reactions is implemented, coupled with multi-dimensional heat, mass and species transport equations. Simulation of control method for reduction of degradation can be done by applying time-dependent external current conditions.

  Results

As a performance test, numerical investigation of start-up process of 29.16cm²test cell with single serpentine flow anode/cathode channel was carried out. Anode inflow gas is assumed to be air with 25% humidity, then switched to hydrogen with 25% humidity at 0.05s. Back pressure is assumed to be 1atm, and operation temperature is 353.15K.

In order to simulate discharge control, following two external current conditions are considered.

(a)     Without control. Output current is fixed to be 0.0A.

(b)     With control. Output current is changed to 0.2A/cm²at 0.1s.

Time-dependent behavior of corrosion (bold) and power-source (thin) currents is shown in Fig.1. In case (a), two currents are exactly the same since output current is zero. The corrosion current increases at the beginning of start-up process, and decreases to zero when start-up is completed. Significant reduction of corrosion current can be seen in case (b). Distribution of corrosion current in case (a) at 0.1s and 0.2s is shown in Fig.2. As H₂/air front proceeds, the area of corrosion become narrower, while corrosion current become more concentrated. The same distribution in case (b) at 0.2s is shown in Fig.3, which indicates significant reduction of corrosion. Final distribution of the ratio of carbon corrosion (left) in cathode catalyst layer is shown in Fig.4. Here, initial carbon loading is assumed to be 2.0mg/cm². Degradation concentrates around the outlet of anode channel, because of long period of corrosion and concentration of corrosion current. In case (a), according to the middle of shown range, the estimated ratio of carbon corrosion after 10000 times of start-up is an order of 1-10%. This corrosion can be reduced by a factor of about 1/5 by discharge control.

  References

1. Y.Ishigami et.al., J.Power.Sources, 196 (2011), 3003-3008
2. J.P.Meyers and R.M.Darling, J.Electrochem.Soc., 153 (2006), A1432-A1442