We present electrospinning as a versatile technique to design and fabricate tailored polymer electrolyte membrane (PEM) fuel cell gas diffusion layers (GDLs) with a pore-size gradient (increasing from catalyst layer to flow field) to enhance the high current density performance and water management behavior of a PEM fuel cell. The novel graded electrospun GDL exhibits highly robust performance over a range of inlet gas relative humidities (RH). At relatively dry (50% RH) inlet conditions that exacerbate ohmic losses, the graded GDL lowers ohmic resistance and improves high current density performance compared to a uniform GDL with larger pores and fiber diameters. Specifically, the graded GDL facilitates a beneficial degree of liquid water retention at the catalyst layer/GDL interface due to the high capillary pressure inherent in its microstructure, thereby improving membrane hydration. Additionally, enhanced graphitization and connectivity of the graded electrospun fibers improves heat dissipation from the catalyst layer interface compared to the GDL with larger fiber diameters, thereby reducing membrane dehydration. When the inlet RH is raised to fully humid (100% RH) conditions, the graded GDL mitigates liquid water accumulation and lowers mass transport resistance. Specifically, the pore size gradient directs the removal of liquid water from the GDL, resulting in superior performance at high current densities.

We investigated the 3-D pore-scale liquid water distribution within the cathode GDL via in operando synchrotron X-ray tomography during low current density fuel cell operation to capture the early appearance of liquid water pathways. We found that the invasion of liquid water into the GDL only partially filled certain GDL pores. Liquid water preferentially flowed along some GDL fibers, which was attributed to the hydrophilic nature of carbon fiber and the presence of pore-scale mixed wettability within the GDLs.

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Electrospinning has been demonstrated to be a versatile technique for producing hydrophobic gas diffusion layers (GDLs) with customized pore structures for the enhanced performance of polymer electrolyte membrane (PEM) fuel cells. However, the degradation characteristics of custom hydrophobic electrospun GDLs (eGDLs) have not yet been explored. Here, for the first time, we investigate the degradation characteristics of custom hydrophobic eGDLs via an ex situ accelerated degradation protocol using H2O2. The surface contact angle of degraded eGDLs (44 ± 12°) was lower than that of pristine eGDLs (137 ± 6°). The loss of hydrophobicity was attributed to the degradation (via hydrolysis) of the fluorinated monolayers (formed via a direct fluorination treatment) on the electrospun carbon fiber surfaces as evidenced by the reduction in surface fluorine content. Degradation of the surface monolayers affected fuel cell performance under multiple operating conditions. At 100% relative humidity (RH), the loss of monolayers led to higher liquid water content and lower cell voltages compared to the pristine eGDL. At 50% RH, the degraded eGDL led to lower cell voltages due to the lower electrical conductivity of the degraded materials. The lower electrical conductivity was attributed to the oxidation of carbon fibers upon loss of the monolayers. Our results indicate the importance of designing robust hydrophobic surface treatments for the advancement of customized GDLs for effective long-term fuel cell operation.

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To implement successful water management strategies in polymer electrolyte membrane (PEM) fuel cells, we need to understand how the complex heterogenous nature of the gas diffusion layer (GDL) impacts the pore-scale transport of liquid water. Here, we investigated the 3-D pore-scale liquid water distribution within the cathode GDL via in operando synchrotron X-ray tomography during low current density fuel cell operation to capture the early appearance of liquid water pathways. We found that the invasion of liquid water into the GDL only partially filled certain GDL pores. Liquid water wicked along some GDL fibers, which was attributed to the hydrophilic nature of carbon fiber and the presence of pore-scale mixed wettability within the GDLs. Mixed wettability and partial pore filling should be considered in the pore-scale modeling and design of next-generation GDLs.

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