Gas diffusion layers (GDLs) play a crucial role in the performance and efficiency of fuel cells and water electrolyzers. These porous materials facilitate the transport of the reactants to the catalyst layer while providing electrical conductivity and mechanical support. Porosity affects GDL performance considerably. Hence, understanding its impact is important for optimizing the design and efficiency of fuel cells and electrolyzers.
What is porosity?
Porosity refers to the fraction of void spaces or pores within a material. The porosity of gas diffusion layers on the market typically ranges from 30 to 90%, with the most common range falling between 60% and 80%. In the case of LINQCELL titanium gas diffusion layers, our products boast a tailored porosity level ranging from 50% to 70%.
What is the impact of porosity on the performance of gas diffusion layers?
The total material flux or effective diffusivity increases in direct proportion to the porosity. So, what happens at high GDL porosities? With lower mass transport resistance, high porosities allow the transport of more reactants to the catalyst layer. In the case of fuel cells, more H2 and O2 gas become available at the anode and cathode sides, respectively. As a result, the electrochemical reactions generate high current, which consequently increases the cell performance. For fuel cells operating at such conditions, water management also becomes critical as more water is also expected to be produced.
The water retention and removal capability of GDLs need to be optimized. If the water retention capacity of the GDL is too low, the membrane suffers from dehydration, effectively reducing its ionic conductivity and the overall cell performance. Whereas, if the water removal capability of the GDL is too weak, the membrane may swell and cause flooding in the system, blocking the pores of the GDL and reducing its capability to deliver reactants to the electrochemically active sites. Going back to fuel cells with high water production rates or water elecltrolyzers with high water feed rates, fortunately, GDLs with high porosities exhibit high water removal efficiency, which shields the cell from suffering from flooding and consequently avoids the blockage of pores for reactant delivery.
Does this mean that GDLs with high porosity are effective for all fuel cell and water electrolyzer applications? Not always. While high porosity improves the O2 mass transport and water removal efficiency of the GDL, it also affects the electrical and thermal conductivities, as well as the mechanical properties, of the GDL. High porosities would mean that there is a high void fraction in the material, which tends to decrease the electrical and thermal conductivity of the GDL. Correspondingly, since there is less solid material to provide structural support, high porosity decreases the mechanical integrity of the GDL.
There is no “one size fits all” solution for all water electrolyzer and fuel cell designs.
Depending on the cell design and operating conditions, there exists an optimal porosity that balances the mass transport, electrical, thermal, and mechanical properties of the GDL. For example, for high current density applications and wet conditions, GDLs with high porosity are desired. However, when the device is operated at dry conditions, GDLs with relatively lower porosities are recommended to ensure that the membrane is hydrated during the operation.
How is the porosity of gas diffusion layers characterized?
Different techniques such as mercury intrusion porosimetry, gas permeability measurements, liquid displacement, and image analysis through scanning electron microscopy are used to characterize the porosity of gas diffusion layers. Each method comes with advantages and disadvantages, and the choice of particular method depends on the type of GDL material, desired accuracy and porosity metrics, and the equipment at hand.
Mercury Intrusion Porosimetry
Mercury intrusion porosimetry (MIP) is used to analyze the porosity and pore size distribution of porous materials. MIP can measure pores ranging from approximately 3 nm to 1000 μm, exceeding the limits of gas physisorption techniques at 500 nm. It provides insights into both hydrophobic and hydrophilic pore volumes, crucial for understanding the material’s characteristics. For reference, our LINQCELL graphitized carbon fiber paper GDLs have pores with internal widths more than 100 nm.
In MIP, the sample is subjected to high pressures, which forces mercury to infiltrate the material’s pores. The sample is placed in a vacuumed glass tube to make sure that only mercury is present. The initial volume of mercury is measured. Then, pressure is applied to allow mercury to penetrate the larger pores first, with the volume of intrusion recorded. This applied pressure is increased gradually, causing the mercury to fill smaller pores until a capillary pressure versus saturation curve is generated. This data helps determine total connected porosity, pore ares volume, pore size distribution, and apparent density. Although MIP is effective, accurate detection of total pore volume may be challenging, especially for samples with smaller pores, necessitating the use of different fluids like water or kerosene for more precise results.
Gas Permeability Measurements
Gas permeability in a porous medium refers to its capacity to allow gases to flow through its open pores. This property is crucial in various applications, particularly in fuel cells where efficient gas diffusion through the gas diffusion layer is important for optimum catalytic reactions.
The Gurley porosity technique is one of the most common methods to measure the gas permeability of gas diffusion layers. In this method, a Gurley apparatus, which assesses the time it takes for air to pass through a sample under controlled conditions, is used. This technique is suitable for materials with air permeance ranging from 0.1 to 100 µm/Pa⋅s but may not work well with rough surfaces that could cause leakage during testing.
During the test, the sample is placed in the Gurley apparatus, which is then filled with compressed air. The air is forced through the sample, and the time it takes for a set amount of air to pass through indicates the material’s porosity. Gas permeability is expressed in Gurley seconds or Gurley units. According ISO 5636-5:2003, Gurley permeability is the time required for 100 cm³ of air to pass through 1 in² of the material under standardized pressure conditions.
Imaging through Scanning Electron Microscopy
The pore characteristics of gas diffusion layers can also be studied through imaging techniques, including scanning electron microscopy (SEM). SEM works by scanning a focused beam of electrons across the surface of the sample. As the electrons interact with the atoms in the material, they generate signals such as secondary electrons, backscattered electrons, and characteristic X-rays. These signals are then detected and used to create high-resolution images of the sample’s surface.
Through SEM analysis, pore properties, such as pore size distribution, pore shape, pore connectivity, and pore interconnectivity are reported. This information is invaluable for understanding the structure–property relationships of gas diffusion layer and optimizing their performance for fuel cells and water electrolyzers.
Ready to improve your fuel cell or electrolyzer with optimized gas diffusion layers? CAPLINQ can help. Our team specializes in tailored solutions to boost performance and efficiency. Reach out today to improve your electrochemical devices!
