Gas Diffusion Layer (GDL)

Gas Diffusion Layers (GDLs) are crucial components that can affect entire fuel cell performance [1-7]. The GDL is placed between the bipolar plate and Catalyst Coated Membrane (CCM) on each Anode/Cathode side (figure 1). The GDLs are generally constructed from carbon-based materials with a thickness in the range of 100-300 μm [2]. Carbon-based materials are preferred due to their capability of providing good gas permeability, durability and electrical conductivity [3].    

 

 

 

Figure 1. The diagram shows a typical structure of Proton Exchange Membrane (PEM) Membrane Electrode Assembly (MEA) [1].

 

 

 

Functions of GDLs

 

The main functions of GDLs are [4-5]:  

• Ensuring a uniform distribution of reactive gases on the surface of the electrodes 
• Keeping the membrane in a hydrated state for high ionic conductivity
• Providing transport of electrons between an external circuit to CCM 
• Avoiding overheating by transferring heat during cell operation
• Providing mechanical strength against repeated membrane swelling and contracting 
• Removing by-produced water outside of the CCM and preventing flooding of the electrode (To prevent the water flooding, sometimes, PTFE is coated on the surface of the carbon fibres to make a suspension [6].)

 

 

 

Composition of GDLs

 

The GDL consists of two main layers, macroporous substrate (MPS) and microporous layer (MPL) (figure 2). 

 

MPS works by providing gas diffusion through carbon-based materials such as carbon paper or carbon cloth and collecting the current [3].

The purpose of MPL is to reduce the contact resistance between the CCM and GDL and also water management between the two layers [3]. 

  

 

Figure 2. The diagram shows the structure of GDL [3].

 

 

 

 

Design Consideration

 

In order to achieve the best performance of GDLs, there are many properties to consider such as porosity, pore size, gas permeability, structure, wettability, electrical/thermal conductivities, and surface morphology [3]. 

GDL thickness also plays a key role in fulfilling the optimum cell performance. The thickness of GDLs can affect mass transport within the cell and also the electrical/thermal conductivity of the GDLs [7].

Selecting the thickness of GDLs highly depends on cell operating conditions. For instance, in dry operating conditions, thicker GDLs are preferred as they can absorb more water, therefore the hydrated membrane condition can be achieved. On the other hand, thinner GDLs could be a better option for wet operating conditions due to their better water removability. Additionally, the thinner GDLs tend to have high current densities and perform better for parallel flow channels.   

 

 

 

 

 

 

 

References 

[1]"What is the Purpose of a Gas Diffusion Layer (GDL)?", Fuel Cells Etc, 2013. [Online]. Available: https://fuelcellsetc.com/2013/02/purpose-of-a-gas-diffusion-layer-gdl/. [Accessed: 22- May- 2022].

[2]B. Pollet, A. Franco, H. Su, H. Liang and S. Pasupathi, "Proton exchange membrane fuel cells", Compendium of Hydrogen Energy, pp. 3-56, 2016. Available: 10.1016/b978-1-78242-363-8.00001-3 [Accessed 23 May 2022].

[3]A. Öztürk, "Chapter 1 - Introduction to fuel cells", Direct Liquid Fuel Cells, pp. 1-47, 2021. Available: https://www.sciencedirect.com/science/article/pii/B9780128186244000017. [Accessed 22 May 2022].https://doi.org/10.1016/B978-0-12-818624-4.00001-7. 

[4]"Gas Diffusion Layers", Fuelcellstore.com, 2022. [Online]. Available: https://www.fuelcellstore.com/fuel-cell-components/gas-diffusion-layers. [Accessed: 22- May- 2022].

[5]J. Garche, "Chapter 11 - Regenerative fuel cells", Electrochemical Power Sources: Fundamentals, Systems, and Applications, pp. 365-406, 2022. Available: https://www.sciencedirect.com/science/article/pii/B9780128194249000070. [Accessed 22 May 2022].

[6]C. Hartnig, L. Jörissen, J. Scholta and W. Lehnert, "Gas diffusion media, flowfields and system aspects in low temperature fuel cells", Polymer Electrolyte Membrane and Direct Methanol Fuel Cell Technology, vol. 1, pp. 81-116, 2012. Available: 10.1533/9780857095473.1.81 [Accessed 23 May 2022].

[7]R. Omrani, "Gas diffusion layer for proton exchange membrane fuel cells", PEM Fuel Cells, pp. 91-122, 2022. Available: 10.1016/b978-0-12-823708-3.00017-1 [Accessed 23 May 2022].