In proton exchange membrane water electrolyzers, porous transport layers (PTLs), which are similar to gas diffusion layers in PEM fuel cells facilitate the transport of feed materials and reaction products to and from the electrolyzer stack. They also conduct electrons and dissipate waste heat. This demonstrates just how critical PTLs are to the performance and efficiency of water electrolyzers.
As discussed in our previous blog article, Benchmark Porous Transport Layers for Water Electrolyzers, we established that due to the inherent high potential and acidity on the anode side of PEMWE, standard carbon-based materials, such as graphitized carbon fiber papers, and carbon cloths and fabrics, are not the best choice for PTLs. Instead, titanium has emerged as the material of choice.
Again, the efficiency of water electrolyzers heavily depends on their components, including the PTL, which has led studies to focus on optimizing the surface characteristics of titanium PTLs. This includes factors like pore size, porosity, and fiber diameter in sintered titanium fibers– but that’s a discussion I’ll reserve for a future blog. In this blog, we’ll focus on a simpler approach to ensure PTLs perform at their best: cleaning prior to use. We will specifically discuss how cleaning is done and what some cleaning steps do for titanium PTLs.
Importance of Cleaning Titanium PTLs before Use
Titanium porous transport layers (sintered titanium powders or fibers) used in water electrolyzers are usually cleaned before use. This pretreatment removes impurities and improves the surface properties of the PTL, making them work much better during electrolyzer operation. Pristine (uncleaned or untreated) titanium PTLs might contain dust and other impurities that they picked up during manufacturing or shipping. These impurities can negatively impact PTL performance in several ways:
Contaminants like oxides, oils, dust, or residues create insulating barriers on the PTL surface, which disrupts electron flow. This results in higher resistance, which, as you can imagine, isn’t great for water electrolyzer performance.
On top of that, impurities can affect the adhesion of the catalyst layer on the PTL (if the catalyst layer is deposited on the PTL). This causes a loss of contact between the active material and the PTL, potentially leading to delamination in the long run. In the short term, it can disrupt electrical conduction pathways, definitely not ideal. All of these lower electrolyzer efficiency and accelerate its degradation.
Solvent or Wet Cleaning of Titanium PTLs
Wet and dry cleaning are both well-established methods for removing surface contaminants in industrial applications, and each has its advantages. However, for cleaning titanium PTLs used in water electrolyzers, wet cleaning with solvents is the most commonly used method. Wet cleaning, also referred to as solvent cleaning, offers numerous advantages:
Targeted Cleaning and Solvent Flexibility: Solvent cleaning is highly effective at removing a wide range of organic and inorganic contaminants. This method offers the flexibility to target specific types of impurities by selecting the appropriate solvent or cleaning agent based on the nature of the contaminants. Remember, the concept, “like dissolves like?” It applies here: organic solvents, such as alcohols, are used to dissolve organic impurities like oils, while inorganic solvents, such as water or acids, are used to remove contaminants, such as oxide layers.
Simplicity: Wet cleaning is relatively easy to implement, requiring minimal specialized equipment. Tools like solvent baths, ultrasonic cleaners, and rinsing setups are commonly available in many industrial facilities, making it straightforward to incorporate into existing workflows without the need for sophisticated equipment.
Thorough Cleaning: Wet cleaning stands out because it can get into the pores and fibers of the titanium PTL, pulling out even those contaminants that are deeply embedded. While dry cleaning does a good job removing surface dust and larger particles, it often misses the smaller impurities trapped inside the material. Wet cleaning, on the other hand, can break down and wash away those deeper contaminants, giving the PTL a much more thorough clean.
How to Effectively Clean Titanium PTLs though Wet or Solvent Cleaning
In a nutshell, wet cleaning is performed in three easy steps:
Step 1: Solvent Application
The cleaning solvent is applied to the surface of the titanium PTL using methods such as immersion, spraying, rinsing, or wiping, depending on the preferred approach and the equipment available. A typical cleaning procedure, commonly employed in both laboratory settings and industrial-scale stack manufacturing, often starts with rinsing the PTL with deionized water, followed by an organic solvent, typically alcohols like ethanol or isopropyl alcohol. Once the solvent comes into contact with the surface, it works to break down or dissolve the contaminants, effectively removing them from the PTL surface.
Type of Contaminant | Examples | Suitable Solvents |
| Organic Contaminants | Oils, grease, manufacturing organic residues | Alcohols (e.g., isopropanol, ethanol), hydrocarbon-based solvents |
| Inorganic Contaminants | Oxides, metal salts, particulate dust | Water, dilute acids (e.g., sulfuric acid, hydrochloric acid), alkaline cleaners |
If available, ultrasonic baths are used to effectively remove loosely bound particles and impurities from the PTL surface. During cleaning, sound waves create tiny bubbles that dislodge impurities through their movement and water flow. This process can also unblock pores, improving water and gas flow through the titanium PTL. However, since ultrasonic baths are not always accessible, stack manufacturers use them for PTL cleaning only when they are available, typically with deionized water.
Acid Etching for Oxide Layer Removal
Aside from loose impurities, titanium PTLs often develop oxide layers. When exposed to air at room temperature, a thin passive oxide film (5–10 nm) forms on the surface. In theory, this film has three layers:
- TiO (closest to the surface)
- Ti₂O₃ (intermediate layer)
- TiO₂ (outermost layer exposed to air)
In most practical cases, the oxide layer is mostly TiO₂. While this layer protects the titanium, it increases electrical resistance because TiO₂ is a semiconductor, not a conductor. Removing this layer is important to maximize the conductivity of the titanium PTL. As highlighted in Table 1, acid solutions (hydrochloric, nitric, phosphoric, or sulfuric) are commonly used to remove oxides through a process often referred to as acid etching. Etching the TiO2 layer exposes metallic Ti, lowering charge transfer and kinetic resistance and in turn, leading to better overall electrolyzer performance. Learn more about the electrochemical technique to probe conductivity changes in water electrolyzers (electrochemical impedance spectroscopy) in our previous blog, “How Pretreatment Affects Ion Exchange Membranes for Electrochemical Devices.”
Ultrasonic Bath + Acid Etching for Improved Performance
A study* showed that electrolyzer performance improves significantly when a titanium PTL undergoes both ultrasonic cleaning and acid etching. However, in industrial-scale stack manufacturing, incorporating more advanced cleaning techniques can impact cleaning time and, consequently, productivity, not to mention the availability of the required equipment. Still, if you’re exploring ways to further enhance PTL performance, these methods are worth considering.
*Qing Wang, Zheng Zhou, Kequan Ye, Mingruo Hu, Xiaoyu Hu, Sibo Wang, Chengyu Hu, The effect of pretreatment and surface modification of porous transport layer (PTL) on the performance of proton exchange membrane water electrolyzer, International Journal of Hydrogen Energy, Volume 53, 2024, Pages 163-172, ISSN 0360-3199, https://doi.org/10.1016/j.ijhydene.2023.12.057.
Step 2: Rinsing
After the solvent has done its job, the PTL is typically rinsed with deionized water to remove any remaining solvent and dissolved contaminants to make sure that no solvent residues remain on the PTL.
Step 3: Drying
The cleaned PTL is then dried, either at room temperature or with heat, depending on the process. Some methods use inert gases like argon or nitrogen to prevent oxidation. This step ensures all moisture is removed, leaving the PTL dry and ready for use in electrolyzers.
TL;DR: To clean titanium PTLs, start with a rinse or wash using deionized water, followed by alcohol. If you’ve got access to it, ultrasonic cleaning is a great option. For improved performance, consider acid etching, but honestly, rinsing with solvents works just fine. Finish with a rinse and drying step. And there you go, your titanium PTL is ready for your water electrolyzer.
Titanium Porous Transport Layers Available at CAPLINQ
We offer sintered titanium fiber or powder, with porosity ranging from 40% to 90% and thicknesses from 0.2 mm to 1.5 mm. Need it in circle or square? We can do both! The products listed below are our standard options, but we’re happy to customize to your specifications. Plus, we can apply a platinum coating with a thickness of 500 nm!
Ready to improve your water electrolyzer performance? Contact us today to discuss your specific needs or request a quote!
