Gas diffusion layers (GDLs) play a critical role in fuel cell performance by facilitating reactant gas transport, managing water removal, and providing electrical conductivity. Conventional GDLs, typically made from carbon fiber paper or woven cloth, face limitations in pore structure uniformity, hydrophobicity control, and mass transport efficiency. Nanofiber-based GDLs, particularly those fabricated via electrospinning, offer a promising alternative due to their tunable morphology, high porosity, and enhanced mass transport properties. Among these, electrospun carbon nanofibers (ECNFs) stand out for their high surface area, interconnected pore networks, and mechanical stability, making them ideal candidates for advanced GDL applications.

Electrospinning enables precise control over nanofiber diameter, alignment, and mat porosity, which directly influence GDL performance. By adjusting polymer concentration, solvent composition, and processing parameters such as voltage and flow rate, researchers can tailor the pore size distribution of ECNF mats to optimize gas diffusion and water management. For instance, studies have shown that ECNFs with average fiber diameters below 500 nm exhibit pore sizes ranging from 1 to 10 micrometers, creating a hierarchical structure that enhances gas permeability while maintaining mechanical integrity. This contrasts with conventional carbon fiber GDLs, where pore sizes are less uniform and often larger, leading to inefficient water removal and flooding risks under high current densities.

Hydrophobicity is another critical factor in GDL design, as it determines water droplet behavior and prevents pore blockage. Electrospun nanofibers can be functionalized with hydrophobic agents such as polytetrafluoroethylene (PTFE) or fluorinated polymers during or after the spinning process. Incorporating PTFE into ECNFs has been shown to increase contact angles beyond 140 degrees, significantly improving water repellency compared to untreated carbon fiber substrates. Additionally, the nanoscale roughness of electrospun fibers enhances hydrophobicity through the Cassie-Baxter effect, where air pockets trapped between fibers reduce liquid-solid contact. This property is particularly advantageous in proton exchange membrane fuel cells (PEMFCs), where efficient water expulsion is crucial to avoid cathode flooding.

Mass transport optimization in nanofiber-based GDLs relies on balancing porosity and tortuosity. The interconnected networks of ECNFs reduce tortuosity compared to conventional GDLs, where fibers are randomly oriented and less continuous. Lower tortuosity translates to shorter diffusion paths for reactant gases, improving fuel cell efficiency at high power densities. Experimental data indicate that ECNF GDLs can achieve through-plane gas permeability values up to 30% higher than commercial carbon paper, with in-plane conductivity remaining competitive at approximately 200 S/cm. These properties are attributed to the alignment of graphitic domains within carbonized nanofibers, which enhances electron transfer while maintaining gas accessibility.

Performance comparisons between ECNF GDLs and conventional materials highlight several advantages. In PEMFC testing, ECNF-based GDLs demonstrate reduced mass transport losses at current densities above 1.5 A/cm², a regime where traditional GDLs often suffer from flooding. Polarization curves reveal that cells equipped with optimized ECNF GDLs maintain higher voltages under humidified conditions, with peak power densities exceeding 1 W/cm² in some configurations. Durability tests further show that ECNF mats exhibit less degradation over 1000-hour operation cycles, as their fibrous structure resists compression and cracking better than brittle carbon paper.

Modifications to ECNFs further enhance their suitability for GDL applications. Doping with nitrogen or sulfur during carbonization introduces catalytic sites for oxygen reduction, though this aspect verges on catalyst topics and will not be detailed here. Alternatively, creating gradient porosity through multilayer electrospinning allows for spatially controlled gas and water transport. For example, a dual-layer ECNF GDL with a dense top layer and porous bottom layer can effectively manage water while ensuring uniform gas distribution to the catalyst layer. Such designs mimic the microporous layer (MPL) in conventional GDLs but with superior interfacial contact due to the seamless integration of nanofibers.

Challenges remain in scaling up ECNF production for commercial GDLs. Batch-to-batch consistency in fiber diameter and mat thickness must be improved to meet industrial standards. Additionally, the carbonization process, typically conducted at temperatures above 800°C, adds energy costs compared to conventional GDL manufacturing. However, recent advances in continuous electrospinning and roll-to-roll processing show promise for large-scale fabrication. Hybrid approaches, where ECNFs are integrated with conventional carbon fibers as reinforcement, may also bridge the gap between performance and scalability.

In summary, electrospun carbon nanofiber-based GDLs offer significant improvements over conventional materials in pore structure control, hydrophobicity, and mass transport efficiency. Their tunable morphology and functionalization potential make them versatile for various fuel cell applications, particularly where high power density and durability are critical. While challenges in large-scale production persist, ongoing research into processing optimization and hybrid designs positions ECNF GDLs as a viable next-generation solution for advanced fuel cell systems. Future work should focus on standardizing fabrication protocols and further validating long-term performance under real-world operating conditions.