Fuel Cell Defrosting Simulation with AVL CRUISE? M

Every Saturday, Thomas G. and Michael Bambula are excited to bring the developers on stage and provide you an extended read into various simulation topics.

by Christoph P?tsch

The crucial advantages of the electrochemical energy conversion process in PEM fuel cells are the high efficiency given by the two-step, low temperature reaction and water being the only reaction product. As it happens, it’s exactly those advantages, which conspire to one crucial Achilles' heel (we call it engineering challenge) for PEM fuel cells in real-life operation.

Before going into the specific technical challenge, let’s recap the basic structure of a fuel cell (experts, please skip this section). Hydrogen and oxygen are supplied at two separate electrodes (anode & cathode). The active reaction and diffusion layers which separate the reactants feature a sandwich design, as shown in Figure 1:

• In the core of the sandwich there is the solid membrane electrolyte which acts as separator for gas species and electron transfer while allowing the conduction of protons. In PEM fuel cells, this membrane also stores a part of the process water, i.e. it acts as a sponge. This water is crucial for the ionic conductivity of the membrane and the given storage capacity opens the possibility for active water management.
• On either side of the “sponge”, there are thin catalyst layers (CL) consisting of porous support structures coated with platinum to enable the two half-reactions. At the anode, that is the oxidation of hydrogen, H2 → 2?H+ + 2?e- and at the cathode the reduction of protons in the presence of oxygen, 2?H+ + 0.5?O2 + 2?e- → H2O We see, it is the cathode CL where the process water is generated.?

At the outside of the sandwich the gas diffusion layers (GDL) are located. As the name suggests, the reactants and the product water need to pass the diffusion resistance of this porous structure between the catalyst layers and the adjoined gas channels.

What’s the issue with fuel cell cold start?

When starting a fuel cell electric vehicle at sub-freezing conditions the water produced by the reaction may bring us in trouble. The gaseous process water from the cathode reaction would liquidize and further freeze inside the reaction catalyst and gas diffusion layers. Simultaneously, as we operate the fuel cell, the system is heated and the big question is, is it heating fast enough? If not, the catalyst layer (which is by far the thinnest layer in the sandwich) is covered by ice before the system temperature reaches the freezing point and the electrochemical reaction stops. A self-sustained system cannot recover from this state – it would require external heating or to move the vehicle to warm conditions. Needless to say, ice formation can cause serious structural damage in the fragile layers.

Key phenomena and model

In order to simulate cold-start, it is necessary to identify and take into account all relevant phenomena dealing with water management. For this purpose, AVL CRUISE??M’s real-time capable?stack compeont?was extended to consider the storage, transport and phase change of water as sketched in Figure 2. In each of our three layers, the model considers a liquid (dissolved) and a frozen water state. Further, it describes the mass transfers due to diffusive transport and capillary action between adjacent layers, as well as the phase change processes (freezing and melting). The water content states (dissolved, liquid, frozen) in the different layers of the fuel cell impair the physical properties i.e. conductivity, diffusion resistance and reactive surface area and consequently the overall fuel cell performance. In detail we have:

• The membrane humidification model calculates the water balance in the ionomer taking into account diffusion between the gas channels and the membrane, as well as electro-osmotic drag. At normal (warm) conditions, the membrane water is dissolved in the ionomer. At sub-freezing conditions, a part of the membrane water freezes. (Even at very low temperatures, the membrane water does not freeze entirely.) The dissolved water content determines the membrane ionic conductivity and therefore ohmic loss. Frozen membrane water does not contribute to the ionic conductivity and thus it enhances ohmic loss.
• When the membrane reaches saturation, due to cold and/or very humid conditions, liquid water forms and enters the (cathode) catalyst layer. Liquid and frozen water in the CL hinder the diffusive transport in the porous layer and block the active surface area for the electrochemical reaction. This results in higher activation and transport loss.

The model considers the transfer of liquid water in between the porous structures of the catalyst and gas diffusion layers due to capillary action. Liquid and frozen water in the GDL block the diffusive gas species transport. At high GDL water saturation, water is dragged to the gas channels.

What can our model do now?

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Enough of dry theory, let’s look at some results! First, we look into the principle behavior when starting a fuel cell stack at sub-freezing temperatures. For this purpose we compare two freeze start events at different start temperatures. Figure 3 (left) shows a startup from -10°C. Current is applied after 10?s, which is visible in the decay of cell voltage (a). The water formed in the electrochemical reaction accumulates in the membrane (c). When saturation is reached at ~17?s (red marker), liquid water forms and accumulates in the catalyst layer, where it almost entirely freezes (d). The heat generated by process losses warms up the system that is operated in adiabatic conditions (b). When the temperature approaches the melting point, there is a transition from ice to liquid water with a residence phase of the temperature around 0°C due to the latent heat of fusion. Due to the heterogeneous temperature distribution in the stack (1D), the freezing and melting phase overlap. At ~42?s, the stack has entirely passed the freezing point. During the process, the CL reaches a maximum frozen water saturation of ~ 0.4, which is sufficiently low to sustain proper stack operation.

When starting from -25°C (Figure 3, right), the membrane saturation and the rate of water formation in the CL are initially similar (c)/(d). However, icing continues as system temperature remains below the freezing point (b) for a longer period. From ~35?s onwards, when the CL water saturation exceeds ~0.6, a significant drop in the cell voltage is observed (a)/(d) due to the increased transport and activation losses. Fate takes its course until the cell voltage drops to zero. At that point, the stack cannot generate any further electric power or heat. Game over.

What can be done to facilitate a successful freeze start? Let’s look into one selected aspect. Every successful start-up has its roots in a prescient shut-down, which would leave the stack in a condition which steers the race between freezing and heat-up in favor of the latter. Here, the membrane humidification state offers one degree of freedom for optimization. A low initial water content would offer a higher storage capacity for the process water in the membrane sponge to be filled before liquid (frozen) water formation starts. At the same time, the membrane needs to retain a certain minimum water content to enable ionic conduction.

In order to prove this theory, a shut-down – rest – start-up sequence is investigated in two variants: with and without active drying of the membrane during shut-down. The results for the two cases are compared in Figure 4 below. The model enters the stage at warm conditions (b) in a constant current / voltage point (a). At ~25?s, the system is shut-down to zero current. In case of active drying (dashed lines), the cathode is flushed with warm and dry air for 10?s (35 – 45?s). This drains a large part of the dissolved membrane water. As a consequence, the water content drops from ~9 (full saturation) to ~2 (c). From 50 – 60?s, the system cools-down to the ambient temperature of -15°C (for demonstration purposes, this step is artificially accelerated in the simulation). At ~66?s, the system is started again and a constant current is applied. Process water forms and partially accumulates in the membrane. If the membrane is already saturated (no drying), water would immediately liquidize & freeze in the CL, as shown in the solid lines (d). Consequently, the start-up process terminates at ~92?s due to CL freezing (a). If the membrane provides water storage capacity (thanks to previous drying), it can dissolve a larger part of the process water, which establishes a time margin of ~10?s for heat-up, before liquid water formation starts (d). The system start-up is successful.

As much as we love to nerd out about simulation and read lengthy articles about it, we have to cut it short at this point.

We want to thank Christoph Poetsch for the insights and the impressive work that is performed day to day behind the scenes.

Real-world activities and their real-time limitations bring this Simulation Saturday to an end, but stay tuned for another one soon!

Cheers, Thomas and Michael

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