AEM Water Electrolysis Catalyst Layer Preparation Process

The fabrication of catalyst layers (CL) covers a variety of technical means, such as catalyst spraying (whether in slurry form or other forms), electrodeposition (electroplating, abbreviated ED), thermal decomposition, and screen printing. An ideal catalyst layer should have a large pore surface area to effectively transport reactants and products between multiple active sites of the electrode. Similar to other parts of the membrane electrode assembly (MEA), CL also needs to find a delicate balance between low interfacial contact resistance (ICR) and efficient fluid transport.

From a macroscopic perspective, an ideal CL exhibits a flat and porous surface, which provides a low ICR at the interface between the gas diffusion layer (GDL) and the catalyst layer (CL), while ensuring smooth flow of reactants and products. The microscopic level reveals the tiny structural features that constitute the high electrochemically active surface area (ECSA), which allow reactants to penetrate into the fine details of the CL surface.

Further zooming in to the nanoscale reveals the active sites of the catalyst, where reactants (water molecules in the hydrogen evolution reaction, or HER, for example) can react via either the Volmer-Tafel mechanism or the Volmer-Heyrovsky mechanism.

Fabrication Methods:
The catalyst layer (CL) can be fabricated using a variety of strategies, the most common of which are the catalyst coating directly on the substrate (CCS) and the catalyst coating on the membrane (CCM). These two strategies involve applying the catalyst to the substrate material or the ion-conducting membrane, respectively. Although each has its own unique advantages and limitations, the current mainstream trend favors the CCM approach to produce efficient anion exchange membrane water electrolysis (AEMWE) devices. The CCM approach has a low internal resistance due to the close bonding between the CL and the membrane, which is extremely beneficial for the performance improvement of electrolyzers and fuel cells.
In addition, this approach can increase the specific surface area, effectively reduce the ohmic resistance of the CCS-type membrane electrode assembly (MEA) by 50%, and improve the utilization efficiency of the catalyst. These factors together explain why the CCM process is regarded as the preferred approach to build high-performance electrolyzers and fuel cells.

In contrast, the CCS method is known for its flexibility, allowing researchers to freely choose the substrate material and perform necessary pretreatment, thus avoiding the risk of possible damage to the membrane in the CCM configuration (especially the relatively fragile membrane in the AEM). While the CCM configuration requires some engineering design to properly place the reference electrode when performing three-electrode measurements in the AEMWE, the implementation of three-electrode measurements in the CCS configuration is relatively simple, and the same electrodes can be used directly in the AEMWE without repeated preparation. Therefore, the CCS method is widely used in studying new electrocatalytic materials CLs.

Given the respective characteristics of the anode and cathode, and the advantages of the CCS and CCM methods, researchers began to explore the possibility of combining these two methods in a single cell. Some researchers have optimized the configuration by using CCS technology for the anode and CCM technology for the cathode. The CCS anode was selected due to the relatively weak stability of the CCM anode, which is attributed to the possible adverse interactions between the oxidizing electrochemical environment and the CL, especially when anion exchange resins are used, which have low chemical and mechanical stability.

Whether using CCS or CCM methods, the morphological control of the CL layer is crucial to the performance of the MEA. Recent studies have changed the pore structure of the CL layer by adjusting the spray process system. Specifically, a multi-step spray strategy was adopted to increase the distance between the nozzle and the electrode while reducing the amount of catalyst used in each step, thereby forming a denser structure on the catalyst surface. By adjusting these parameters, the pore structure of the CL layer can be optimized and the surface density can be reduced.

The issue of catalyst loading is more complicated than expected, because higher loading does not always mean better performance. More catalyst means thicker CL, which increases the resistance to gas diffusion. Studies using different loadings of IrO2 and 40wt.% Pt/C for the anode and cathode, respectively, showed that the optimal loading was 2mgmetal/cm² for the anode and 0.4mgmetal/cm² for the cathode. The calculated thickness of the anode CL is 10.11μm, which meets the limit of no more than 20μm thickness of the microporous layer (MPL).

Although the thickness of the CL has an impact on the performance of the electrolyzer, recent studies have shown that the density of the CL and the distribution of its catalyst on the surface have a more significant impact on the battery performance. The best performance was achieved with a dense catalyst layer close to the surface, which is consistent with previous studies on porosity gradients in GDL/MPL. The density of the CL is related to the change in membrane transfer (MT) overpotential, while the position relative to the surface is related to the activation overpotential. However, electron transfer is only part of the charge transport in CL; the transport of ionic charge is equally important. In this regard, anion exchange ionomers (AEIs) play a key role.

Ultrasonic catalyst coating systems are uniquely suited for these challenging applications by creating highly uniform, repeatable, and durable coatings. From R&D to production, our non-clogging technology results in greater control of coating attributes, significant reduction in materials usage, and reduced maintenance and downtime.

Ultrasonic spraying is ideal for deposition of solar cells, fuel cells, silicon cell coatings, and is increasingly used in research and production of spraying processes. Ultrasonic Spraying Materials technology can be used to deposit uniform and extremely thin coatings on substrates of any width. Ultrasonic spraying technology enables very thin coatings with extremely high uniformity, resulting in improved electronic conversion rates and transport.

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