Fuel cells generate electricity through an electrochemical reaction, without combustion, and are touted as a promising clean energy technology. The key components of a fuel cell include two electrodes - an anode and a cathode, with an electrolyte in between. For the reactions to occur, gaseous reactants must be brought to the right electrode surfaces. This is where the gas diffusion layer (GDL) plays an important role.
What is a gas diffusion layer?
A Gas Diffusion Layer (GDL) is a porous and electrically conductive material placed between the catalyst layers (anode and cathode) and the bipolar plates (that serve as current collectors) in a fuel cell. GDLs help distribute reactants (i.e. fuel and oxidant gases) over the catalyst layers while allowing products (water, carbon dioxide etc.) to diffuse out. They also provide mechanical support, act as a conductivity path for electron transfer between the catalyst layer and plate, and prevent catalyst layer flooding or drying.
Materials used for Gas Diffusion Layer fabrication
The most commonly used material for GDL manufacture is carbon fiber paper or carbon cloth, due to its high electrical conductivity and low gas diffusion resistance. The porous carbon substrate provides pathways for reactant gases as well as products to pass through. Polytetrafluoroethylene (PTFE) is often applied to carbon paper as an hydrophobic agent by an impregnation process. This helps water management inside the fuel cell by facilitating drainage of liquid water.
Factors affecting GDL performance
Key parameters of the GDL that impact its performance include porosity, pore size distribution, thickness and hydrophobic-hydrophilic balance. Higher porosity and larger pore sizes allow for improved mass transport of gases. However, excessively large pores compromise the mechanical integrity. An optimal thickness of 200-400 μm yields low diffusion resistance. The right hydrophobic-hydrophilic balance ensures effective water removal without affecting gas diffusion. Fabrication methods that control these parameters precisely can enhance GDL functionality.
Design considerations
Specific design aspects are also important based on the fuel cell type. In proton exchange membrane fuel cells (PEMFCs), conventional carbon paper GDLs often fail at high humidity or temperature due to insufficient liquid water drainage. Newer designs incorporating microporous layers (MPLs) on the catalyst side address this issue. Similarly, for solid oxide fuel cells (SOFCs) operating at high temperatures (500-1000°C), graphite felts or woven fiber fabrics that can withstand such conditions are preferred as GDLs. Appropriate sealing and insulating designs are adopted as well to prevent reactant cross-over.
Gas Diffusion Layer testing and characterization
Proper testing and characterization methods help evaluate GDL performance. Gas permeability, electrical resistance, water transport properties are commonly assessed. Specifically, water uptake/management behavior under wet or dry conditions is studied using techniques like water droplet tests, water sorption analysis and impedance spectroscopy. Surface morphology examination via scanning electron microscopy provides insights into pore structure-property correlations. Compression tests under mechanical loading simulate long-term durability. Standardization of such characterization protocols facilitates material development and performance benchmarking.
Gas Diffusion Layer manufacturing processes
Besides materials selection, manufacturing techniques determine the microstructural properties of GDLs. Common processes involve paper/felt making followed by hydrophobic agent impregnation and drying. In papermaking, a slurry of carbon fibers, binders and solvents is prepared, poured onto a screen or belt and then pressed, dried and calendered to obtain porous sheets. Needlepunching and hydroentanglement are other routes. Newer approaches like 3D printing hold promise for tailored, graded porosity designs. Roll-to-roll coating of microporous layers on commercial gas diffusion media is also an industrially viable option. Quality control during these scalable processes is essential.
Latest developments and future scope
The roles of multifunctional GDL designs featuring micro/nano particles, additional carbon nanotube coatings and composite formulations are being explored actively. They aim to optimize parameters like wettability, electric/ionic conductivity, mechanical resilience and thermal stability. In-situ fabrication methodologies and advanced patterning are topics of ongoing research as well. With continued advancements, GDLs can be highly customized to support higher current densities from fuel cells operating under wider dynamic conditions. Their sustainable, affordable mass production would facilitate the commercialization of fuel cell vehicles and stationary power units worldwide.
Adequately designed gas diffusion layers addressing reactant/product transport, water and thermal management along with mechanical integrity form a mainstay of fuel cells for diverse applications. Concerted research focused on tailoring GDL microstructures, compositions and fabrication techniques will certainly facilitate improved fuel cell performance, longevity and competitiveness against established energy technologies. Harmonizing characterization standards with industrial-scale manufacturing capabilities holds the key to GDL commercialization and widespread large-scale fuel cell adoption.
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