We are excited to share our progress in developing PFAS-free hydrophobic coatings for carbon gas diffusion layers (GDLs). This on-going development aims to address the environmental and health concerns associated with the use of traditional PFAS coatings by offering an effective and eco-friendly alternative. Our PFAS-free coatings are designed to match the performance of conventional products while also being environmentally friendly and safe.
Background
As discussed in our previous blog articles, gas diffusion layers (GDLs) are key components in fuel cells and electrolyzers, performing several key functions. They ensure efficient diffusion of feed gasses to the catalyst layer, which is essential for continuous electrochemical reactions and electricity generation. The porous structure of GDLs optimizes gas permeability, while their conductive properties facilitate electron transport.
GDLs also prevent excessive water buildup that could cause membrane swelling and degrade stack performance. Lastly, GDLs help maintain the mechanical integrity of the membrane electrode assembly during stack compression, crucial for sustaining performance and durability.
Several material types can be used as GDLs, with the most common being carbon-based GDLs, such as graphitized carbon fiber papers, carbon cloths and fabrics, and graphite/carbon felts. These carbon-based materials are versatile and can effectively function as both the anode and cathode GDLs in PEM fuel cells. In the case of water electrolyzers, these carbon-based materials are used as cathode porous transport layers.
| Application | Recommended LINQCELL Grade |
| Stationary PEMFC | GDP180 GDP210-MP, GDP210-MPS GDP240 |
| Automotive PEMFC | GDP180 GDP210-MP, GDP210-MPS GDP240 |
| High-temperature PEMC | GDP340 CF400-MP |
| Direct Methanol FC | GDP340 CF400-MP |
| Water Electrolyzers | GDL1500, GDL1500B GDL1850 GDL1900 GDL2000 GDL2200 GDL2900 |
Per- and polyfluoroalkyl substances (PFAS) in Carbon Gas Diffusion Layers
To perform the aforementioned roles in fuel cell and water electrolyzer stacks, traditional carbon GDLs are composed of a macroporous substrate (MPS) and sometimes a microporous layer (MPL). MPS, also referred to as gas diffusion backing (GDB), is the support layer. As water management is critical for fuel cell stacks, the MPS is often subjected to a hydrophobization treatment. The hydrophobic, polymer coating on the MPS improves mass transport but also tends to reduce the porosity and increase the electrical resistance of the GDL.
For this reason, a thin microporous layer (MPL) is applied over the hydrophobic coating. MPLs, made from carbon powders dispersed in a polymeric base, have a mean pore size around 100 nm. This small pore size and hydrophobicity make the MPL an effective barrier against liquid water transport. Additionally, the electrically conductive carbon in the MPL formulation reduces contact resistance between the GDL (or MPS) and the catalyst layer, addressing water flooding issues without compromising the GDL’s electrical conductivity.
Unfortunately, because of these hydrophobization treatments, carbon GDLs contain PFAS. In these layers, fluoropolymers like polytetrafluoroethylene (PTFE) are used. PTFE acts as the hydrophobic coating in the MPS and hydrophobic polymer in the MPL.
Side note: Most porous transport layers (PTLs) for water electrolyzers consist only of MPS, without an MPL or hydrophobic coating. Contrary to fuel cells, electrolyzers typically use thicker carbon materials (thickness > 1 mm). These thick carbon effectively prevent water from reaching the catalyst due to their higher porosity and thickness, which provide a strong barrier against water penetration.
Need for PFAS-free Solutions for Carbon Gas Diffusion Layers
From an environmental perspective, the use of PFAS raises concerns. The strong C–F bonds in PFAS make them highly resistant to degradation, leading to their accumulation in the environment. This persistence raises potential risks to human health, including links to cancer, reproductive issues, and hormonal imbalances.
As a result, there is a proposal to ban PFAS. Learn more about the PFAS regulations here.
CAPLINQ’s Sustainable Hydrophobic Coatings
In response to the potential ban on PFAS due to their environmental and health effects, CAPLINQ is actively working on developing alternative hydrophobic coatings that do not contain PFAS. These new coatings are designed for use in carbon gas diffusion layers, aiming to provide a sustainable solution without compromising performance.
Our development approach focuses on creating coatings that offer similar or greater hydrophobic properties as traditional PFAS-based products but with environmentally friendly materials. Initial results have shown that the PFAS-free coated GDL has an average water contact angle of 154.4°, which is higher to those of a commercially available GDL with PFAS coating (153.8°) and GDP240 (144.7°).
For the electrical properties, the high-frequency resistance (HFR) was measured through electrical impedance spectroscopy. HFR accounts for the sum of all the ohmic resistances present, including those of the electrolyte, contacts, electrodes, and wires. Assuming that the resistances of the electrolytes, contacts, and wires are the same due to similar testing setups, HFR would then indicate the ohmic resistance corresponding to the tested working electrodes, which, in this case, is the coated carbon GDL. The HFR of the PFAS-free coated carbon GDL (1.8 kΩ) was comparable to that of a commercially available GDL with a PFAS coating (1.5 kΩ). With these initial results, the PFAS-free coated carbon GDL matches the performance of traditional PFAS-coated GDLs, demonstrating its potential as a strong, sustainable alternative.
Outlook on PFAS-free Carbon GDL Coatings
As part of our ongoing development, we will publish relevant data as it becomes available, including stability testing and fuel cell performance data. We know that the electrical conductivity of the carbon GDL decreases inevitably after hydrophobization treatment, whether PFAs or PFAS-free coatings are used. To address this, CAPLINQ is also working on developing a conductive PFAS-free hydrophobic coating.
We encourage water electrolyzer and fuel cell manufacturers to reach out and collaborate with us as we advance our PFAS-free coated carbon GDL technology. Contact us to explore how our sustainable technologies can be integrated into your products and help drive the future of clean energy.
