In a proton exchange membrane (PEM) fuel cell, product water must leave the cathode through the gas diffusion layer (GDL) — a PTFE-treated carbon-fiber paper often coated with a microporous layer (MPL) — either as vapor carried by the gas stream or as liquid driven by capillary pressure. Dai et al. (2008) introduced a simple method to measure the water transport rate across the GDL and, on a single GDL type, identified the operating conditions corresponding to membrane drying and flooding "for the particular GDL used"; their 2009 companion study performed the PTFE/MPL experiments empirically, still on one GDL family, finding that higher PTFE content and the presence of an MPL each reduce the measured transport rate. The open problem is to establish quantitative, mechanistic, validated relations — a predictive model or operating map — linking PTFE loading and MPL design to the total water transport rate and its vapor/liquid partition and to the capillary breakthrough barrier and the vapor–liquid removal-mode transition boundary, generalized beyond a single GDL microstructure. Ex-situ capillary characterization, pore-scale simulation, and operando imaging now supply the components, but no coupled, validated account exists, and primary studies through 2025–2026 continue to attack pieces of it.

Background

A proton exchange membrane (PEM) fuel cell converts hydrogen and oxygen to electricity across a polymer membrane that conducts protons only when hydrated. Water is produced at the cathode, so the cell must simultaneously keep the membrane wet (to preserve ionic conductivity) and remove excess water (to avoid flooding the porous transport media and blocking oxygen supply). Between the catalyst layer and the flow channels sits the gas diffusion layer (GDL) — a carbon-fiber paper or cloth, typically 150–300 micrometers thick, treated with polytetrafluoroethylene (PTFE, the hydrophobizing agent in Teflon) to resist liquid-water imbibition; many GDLs additionally carry a microporous layer (MPL), a thin (roughly 10–50 micrometer) coating of carbon black bonded with PTFE having sub-micrometer pores, applied on the catalyst-facing side. Water crosses this diffusion media in two coexisting modes: vapor-mode removal, in which water evaporates into the cathode gas stream — including phase-change-induced flow, i.e., evaporation on one side of the porous medium and condensation on the other driven by a through-plane temperature gradient — and leaves by gas-phase diffusion and convection; and liquid-mode removal, in which liquid water percolates through the pores driven by capillary pressure (the pressure difference across a water–air meniscus in a pore, which sets the threshold pressure for water to enter or be displaced from pores of a given size and wettability) until it breaks through into the flow channel. Which mode dominates, and how fast water is removed, jointly decide whether the cell dries out or floods.

The source measurements are due to Dai et al. (2008), who introduced a simple protocol for measuring the water transport ('rejection') rate across the GDL of an operating cell as a function of cathode air-flow rate, using a single GDL type (Toray TGP-H-060 carbon paper with 40 wt% PTFE on the cathode side and untreated GDL on the anode side). The measurements establish the transport-rate method and identify membrane drying and flooding operating conditions for the particular GDL used, but leave open a quantitative, mechanistic account of how PTFE content and the MPL control vapor- versus liquid-mode removal and of the capillary pressure barriers and the vapor–liquid removal-mode transition between vapor-dominated and liquid-dominated removal. Their companion study (Dai et al. 2009) performed systematic PTFE and MPL experiments on the same GDL family and found empirically that higher PTFE content and the presence of an MPL each reduce the measured water transport rate across the GDL, a behavior that can be beneficial at low relative humidity by slowing the drying of the ionomer, the proton-conducting polymer that constitutes the membrane and also permeates the catalyst layers.

Adjacent work supplies pieces of the mechanism but not the whole. Two-phase modeling by Pasaogullari and Wang (2004) showed that the MPL acts as a capillary barrier suppressing liquid water flux from the catalyst layer into the GDL, shifting removal toward the vapor phase and toward back-diffusion of water through the membrane to the anode. Gostick et al. (2008, 2009) measured water–air capillary pressure–saturation curves of GDLs directly (ex situ; saturation being the fraction of pore volume filled with liquid water), establishing permanent capillary hysteresis, nominally neutral wettability with PTFE shifting water toward the non-wetting phase, and appreciable thickness and compression effects — the raw material for a capillary-barrier description, but measured outside operating cells. Pore-network simulations (flow computed on an idealized network of pores and connecting throats) and lattice-Boltzmann simulations (a mesoscopic, particle-density-based fluid-dynamics method) (Qin et al. 2016; Sarkezi-Selsky et al. 2023) reproduce liquid and vapor transport in GDL/MPL microstructures, including the controlling role of through-plane thermal gradients in fixing where vapor condenses. Operando neutron radiography (Owejan et al. 2007) and soft X-ray radiography (Deevanhxay et al. 2013) visualize where liquid water accumulates in running cells and show water crossing the MPL mainly through cracks, with accumulation growing with current density. Reviews of water transport in PEM fuel cells and of GDL properties and water management (Jiao and Li 2011; Okonkwo and Otor 2020) present the GDL/MPL water-removal mechanism as an ongoing challenge, and primary studies through 2025–2026 (Zhang et al. 2025; Biswas et al. 2026) continue to attack parts of it. What no study or review supplies is a validated quantitative account that predicts, from GDL PTFE loading and MPL design, the water transport rate, the vapor/liquid partition, and the location of the transition between removal regimes in an operating cell, generalized beyond one GDL paper type.

Problem Statement

Determine, quantitatively and mechanistically, how the water-removal function of the cathode gas diffusion media of a PEM fuel cell depends on GDL hydrophobization (PTFE loading) and on the presence and design of a microporous layer. Specifically, establish relations that predict, for a cell under given operating conditions (current density, cathode gas flow rate, inlet humidity, temperature): (i) the total water transport rate across the GDL/MPL and its partition between vapor-phase and liquid-phase pathways; (ii) the magnitude and location of the capillary pressure barrier controlling liquid water breakthrough from the catalyst layer through the MPL and GDL; and (iii) the criterion, expressible as an operating map, separating vapor-dominated from liquid-dominated water removal. These three deliverables form one linked objective — a validated predictive description of the water-removal mode of the gas diffusion media — rather than three separate problems. Because the source's identified operating conditions hold only "for the particular GDL used" (Dai et al. 2008), this statement sets generalization beyond the single Toray TGP-H-060 configuration as its own goal.

References

  1. Dai, Wei; Wang, Haijiang; Yuan, Xiao-Zi; Martin, Jonathan J.; Luo, Zhiping; Pan, Mu (2008). Measurement of the water transport rate in a proton exchange membrane fuel cell and the influence of the gas diffusion layer. Journal of Power Sources 185, 1267–1271. DOI: 10.1016/j.jpowsour.2008.07.033. https://doi.org/10.1016/j.jpowsour.2008.07.033
  2. Dai, Wei; Wang, Haijiang; Yuan, Xiao-Zi; Martin, Jonathan; Shen, Jun; Pan, Mu; Luo, Zhiping (2009). Measurement of water transport rates across the gas diffusion layer in a proton exchange membrane fuel cell, and the influence of polytetrafluoroethylene content and micro-porous layer. Journal of Power Sources 188, 122–126. DOI: 10.1016/j.jpowsour.2008.11.079. https://doi.org/10.1016/j.jpowsour.2008.11.079
  3. Pasaogullari, Ugur; Wang, Chao-Yang (2004). Two-phase transport and the role of micro-porous layer in polymer electrolyte fuel cells. Electrochimica Acta 49, 4359–4369. DOI: 10.1016/j.electacta.2004.04.027. https://doi.org/10.1016/j.electacta.2004.04.027
  4. Gostick, Jeff T.; Ioannidis, Marios A.; Fowler, Michael W.; Pritzker, Mark D. (2008). Direct measurement of the capillary pressure characteristics of water–air–gas diffusion layer systems for PEM fuel cells. Electrochemistry Communications 10, 1520–1523. DOI: 10.1016/j.elecom.2008.08.008. https://doi.org/10.1016/j.elecom.2008.08.008
  5. Gostick, Jeff T.; Ioannidis, Marios A.; Fowler, Michael W.; Pritzker, Mark D. (2009). Wettability and capillary behavior of fibrous gas diffusion media for polymer electrolyte membrane fuel cells. Journal of Power Sources 194, 433–444. DOI: 10.1016/j.jpowsour.2009.04.052. https://doi.org/10.1016/j.jpowsour.2009.04.052
  6. Owejan, Jon P.; Trabold, Thomas A.; Jacobson, D. L.; Arif, M.; Kandlikar, Satish G. (2007). Effects of flow field and diffusion layer properties on water accumulation in a PEM fuel cell. International Journal of Hydrogen Energy 32, 4489–4502. DOI: 10.1016/j.ijhydene.2007.05.044. https://doi.org/10.1016/j.ijhydene.2007.05.044
  7. Deevanhxay, Phengxay; Sasabe, Takashi; Tsushima, Shohji; Hirai, Shuichiro (2013). Observation of dynamic liquid water transport in the microporous layer and gas diffusion layer of an operating PEM fuel cell by high-resolution soft X-ray radiography. Journal of Power Sources 230, 38–43. DOI: 10.1016/j.jpowsour.2012.11.140. https://doi.org/10.1016/j.jpowsour.2012.11.140
  8. Jiao, Kui; Li, Xianguo (2011). Water transport in polymer electrolyte membrane fuel cells. Progress in Energy and Combustion Science 37, 221–291. DOI: 10.1016/j.pecs.2010.06.002. https://doi.org/10.1016/j.pecs.2010.06.002
  9. Okonkwo, Paul C.; Otor, Clement (2020). A review of gas diffusion layer properties and water management in proton exchange membrane fuel cell system. International Journal of Energy Research 45, 3780–3800. DOI: 10.1002/er.6227. https://doi.org/10.1002/er.6227
  10. Qin, Chao-Zhong; Hassanizadeh, S.; Van Oosterhout, Lucas (2016). Pore-Network Modeling of Water and Vapor Transport in the Micro Porous Layer and Gas Diffusion Layer of a Polymer Electrolyte Fuel Cell. Computation 4, 21. DOI: 10.3390/computation4020021. https://doi.org/10.3390/computation4020021
  11. Sarkezi-Selsky, Patrick; Schmies, Henrike; Latz, Arnulf; Jahnke, Thomas (2023). Lattice Boltzmann simulation of liquid water transport in gas diffusion layers of proton exchange membrane fuel cells: Impact of gas diffusion layer and microporous layer degradation on effective transport properties. Journal of Power Sources 556, 232415. DOI: 10.1016/j.jpowsour.2022.232415. https://doi.org/10.1016/j.jpowsour.2022.232415
  12. Zhang, Ning; Chen, Wenshang; Zhou, Haoran; Deng, Qihao; Chen, Ben (2025). Liquid water transport behavior in gas diffusion layer/microporous layer of proton exchange membrane fuel cells: A pore-scale investigation. Chemical Engineering Journal 516, 164227. DOI: 10.1016/j.cej.2025.164227. https://doi.org/10.1016/j.cej.2025.164227
  13. Biswas, Diptesh; Mondal, Nilanjan; Mukherjee, Rabibrata; Bakli, Chirodeep (2026). Effective removal of water from a fuel cell: Engineering the gas-diffusion layer. International Journal of Hydrogen Energy 203, 153145. DOI: 10.1016/j.ijhydene.2025.153145. https://doi.org/10.1016/j.ijhydene.2025.153145