What are Gas Diffusion Layers?

Gas diffusion layers (GDLs) are porous, electrically conductive materials located between the catalyst layer and gas channels in a fuel cell. They serve several critical functions for the fuel cell to operate efficiently. GDLs facilitate gas transport between gas flow channels and catalyst layers of the MEAs. They also remove generated water from the catalyst layer and provide mechanical support for the thin catalyst layer. Finally, GDLs aid in electrically conducting electrons between electrodes and current collectors.

Gas Transport through GDLs

One of the primary roles of GDLs is transporting reactant gases like hydrogen and oxygen from the supply channels to the catalyst layers. GDLs contain micropores and nanopores that allow for seamless gas diffusion. The GDL pores help disperse the reactant gases uniformly across the catalyst layer for faster electrochemical reactions. The hydrophobic nature of GDLs also helps remove water produced during the reactions. Any excess water accumulating in the catalyst layer can block gas transport pathways and limit cell performance.

Various types of carbon papers and carbon cloths are commonly used as GDL materials due to their gas permeability, electrical conductivity and hydrophobic properties. These carbon-based substrates provide high surface areas for gas adsorption and diffusion. Their microporous structure assists in managing water removal from catalyst layers while still enabling reactant gas access. The porosity of GDLs plays a crucial role in fuel cell efficiency by balancing reactant gas transport and water drainage.

Electrical Conductivity of Gas Diffusion Layer

In addition to transporting gases, GDLs must efficiently conduct electrons between the catalyst layer and current collectors. This is necessary to draw power externally from the fuel cell. The electrical resistance of GDL materials must be very low for smooth electron flow. Common GDL carbons like carbon paper and cloth have high electrical conductivity due to their graphitic nature. Additional conductive agents such as carbon black or graphite may also be added to the substrate to further enhance conductivity.

A low electrical resistivity is vital to minimize IRlosses and maximize power output from the cell. Proper contact between electrodes, Gas Diffusion Layer and current collectors is also important. Compressive loading during MEA assembly helps establish low contact resistance interfaces. Changes to GDL surface properties or addition of non-conductive contaminants can negatively impact electrical conductivity over time. This should be minimized through quality material selection and cell maintenance.

Supporting the Catalyst Layer

Since catalyst layers are very thin (~10-20 μm), they require structural backing from GDLs to prevent tearing or breakage. The GDL substrates provide flexible mechanical support to maintain the integrity of electrocatalyst coatings. This is vital especially during fuel cell operation involving gas pressure changes, temperature fluctuations and humidity gradients across the MEA. Along with support, GDL compresses the catalyst layer to ensure optimized electrolyte uptake and contact with electrode surfaces.

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