In our previous blog article, we highlighted that gas diffusion layers (GDLs) are commonly available in various thicknesses. Interestingly, electrolyzers tend to utilize thicker GDLs, while fuel cells typically require thinner ones. Case in point, customers building fuel cells prefer the 180-um thick LINQCELL GDP 180, whereas those manufacturing electrolysis stacks choose the thicker LINQCELL GDL 1500 (thickness = 1.5 mm). This brings us to the question, “Does the GDL thickness matter?”
Apparently, it does.
Thickness, which can affect the overall performance of the cell stack, is another critical parameter to consider in GDL selection and design. While thickness does not affect some of the structural properties of GDLs, including pore size distribution, it influences some transport and electrical properties.
First, increasing the thickness increases the breakthrough pressure of the GDLs. Breakthrough pressure is the maximum pressure required so that a fluid from a reservoir can pass through the pores of the GDL and start flowing out on the other side. Simply put, if the pressure from the reservoir is high enough to reach the breakthrough pressure, the fluid will flow through the GDL.
Similar to breakthrough pressure, water saturation, which is the degree at which the GDL is soaked or saturated with water, also increases with increasing thickness. Aside from the thickness, hydrophobization treatments also affect the water saturation of GDLs. As discussed in our previous article about the manufacturing process for GDLs, PTFE coatings and/or microporous layers are applied on top of the graphitized carbon paper substrate to improve its wettability and make it more hydrophobic. Hydrophobization treatments on thin GDLs lower the overall saturation. However, the effect of the hydrophobization treatment becomes insignificant as the thickness increases, which could have implications in the water management performance of the GDL.
Gas permeability, through-plane transport of water, and through-plane electrical conductivity all decrease with increasing GDL thickness. This is particularly detrimental for high current density and wet operations, which require efficient electrical conductance and water removal from the system.
Increasing the thickness resulted to a drastic drop in the contact angle of PTFE-treated GDLs (5 wt%), implying that the effect of the hydrophibization treatment became less significant.
The gas permeability, characterized through the Darcy coefficient, decreased at increasing thickness. This could have severe implications to reactant delivery during fuel cell operation.
Having all these discussed, the question, “Why do fuel cells use thin GDLs and electrolyzers use thicker ones?” remains.
To answer this question, we have to understand the fundamental reactions involved in fuel cells and electrolyzers.
Fuel cells produce water and generate electricity using input H2 and O2 gas. Conversely, water electrolyzers split water into H2 and O2 gas using electricity. These definitions may sound simple. However, in reality, much more complex processes are at play. Carbon-based GDLs are commonly used in the cathode side of fuel cells and water electrolyzers. In a polymer electrolyte membrane fuel cell, the oxygen reduction reaction (ORR) occurs (Equation 2) at the cathode in which oxygen gas reacts with hydrogen ions and electrons to form water. ORR occurs at electrode regions in which three different phases, namely, the solid catalyst at the catalyst layer, gaseous reactants, and ion-conducting phase of the electrolyte, meet. The interface between these phases are commonly known as the triple-phase boundary (TPB). How does this relate to the GDL thickness?
As previously discussed, as the GDL thickness increases, the effect of the hydrophobization treatment becomes insignificant, and the gas permeability and through-plane transport of water decreases. All of these things can affect the reactant delivery and water management in the stack. Water management is very crucial in fuel cell operations because water is produced during the cathodic reaction. Accumulation of water causes flooding, which decreases the available gas diffusion pathways, the concentration of TPBs at which the reaction can occur, and oxygen concentration on the catalyst layer. Poor water management decreases the efficiency of the fuel cell. As such, thinner GDLs with higher through-plane water transport, gas permeability, and hydrophobicity are preferred in fuel cells.
In contrast, water electrolyzer operation (Equation 3 and 4) only involves two phases: the solid catalyst and the liquid phase reactants and electrolyte ions. Looking at cathode in which the GDL is used, hydrogen gas is produced through the hydrogen evolution reaction during electrolysis (Equation 3). The absence of a gaseous reactant, which is used in fuel cells, simplifies the water management aspect in water electrolyzers to a certain extent. Therefore, compared to fuel cells, water electrolyzers can use thicker GDLs, which have good mechanical properties, uniform current distribution, and low pressure drop across the cell.
Ultimately, the optimal GDL thickness required to strike a balance between water management, electrical conductance, and reactant delivery depends on the operating conditions and cell design. Contact us, and our application engineers and in-house water electrolyzer and fuel cell experts will assist you in selecting the most suitable products tailored to your application.