Manufacturing of membranes and electrodes

We employ various techniques to produce membranes from ion exchange polymers and electrodes from catalyst nanoparticles, support, and binder. A key to efficient electrochemical energy conversion is the interface between the single layers, which is generated when they are sandwiched to form a membrane electrode assembly. Here, we provide an overview of the methods and devices that are used in our labs for manufacturing.

Spray coating

Manufacturing of membranes and electrodes
The spray head of an ultrasonic spray coater produces a fine mist from an ink, which is deposited and dries by solvent evaporation to form a thin layer.

Manufacturing of membranes and electrodes
Manufacturing of an electrode (catalyst layer) using spray coating with a Sonotek ExactaCoat.

In spray coating, a low viscosity fluid (e.g., a polymer dispersion or catalyst ink) is finely atomized via an ultrasonic spray head. The fluid is deposited as a fine mist in consecutive spray runs onto a substrate, where it forms a thin, continuous layer after solvent evaporation. In this layer-by-layer approach, a spray coater produces a thin film of adjustable size and thickness. We employ ExactaCoat devices from SonoTek in our labs.

The main advantages of spray-coating are that the thickness of the films can simply be adjusted by the number of runs and the flow rate, and the solvent evaporation rate can be controlled by adjusting the substrate temperature. Furthermore, not only homogenous solutions can be sprayed, but also dispersions, which are fluids that contain a solid fraction, like catalyst and binder in electrode inks.

Doctor blade and Mayer rod coating

Manufacturing of membranes and electrodes
The gap size (distance between doctor blade and substrate) defines the height of the manufactured layer in doctor blading.

Manufacturing of membranes and electrodes
Coating with a Mayer rod employs the same principle as a doctor blade but uses a grooved rod to form layers of defined thicknesses.

A doctor blade allows the fabrication of membranes and electrodes from polymer dispersions or catalyst inks. A line of the fluid is applied on a substrate, e.g., a Teflon sheet, a glass plate, or a gas diffusion layer. The doctor blade then moves at a user-defined gap height and speed along the substrate to coat it with the fluid.

One advantage of a doctor blade is that the layer production is typically faster than by spray coating because the thin films can be fabricated with a single run. A relevant difference to spray coating is that doctor blading processes fluids with a high viscosity, whereas inks with low viscosities are needed for spray coating.

The Mayer rod technique is used to manufacture electrodes (layers of catalyst materials). The working principle is similar to the doctor blade. The main difference is the grooved structure of the Mayer rod. The depth and the distance of the grooves determine the thickness of the wet film. Furthermore, the presence of the grooves supports a uniform coating.


Manufacturing of membranes and electrodes
Sketch of a needle-based solution electrospinning setup. A syringe pump dispenses the polymer solution through a needle, and an electric field is applied between the needle and the substrate, which enables the formation of nanofibers.

Electrospinning is used for the fabrication of nanofibers (polymer fibers with a diameter of less than 1 µm). In this technique, a polymer solution is dispensed via a needle onto a substrate where a fibrous mesh is formed. A strong electric field between the needle and the substrate (about 10 to 20 kV) accelerates the solution jet, which dries during the flight toward the substrate. We employ a needle-based solution electrospinning setup from KatoTech.

The nanofibrous meshes can be produced from single polymers, polymer blends, or polymer solutions with various additives. In the context of composite membranes, the resulting nanofiber meshes can be integrated into the membrane, for example, as a mechanical reinforcement or as chemically modified interlayers with specific functional groups.

Manufacturing of membranes and electrodes
Electron micrograph of electrospun nanofibers made from PVDF-HFP and decorated with CeOx nanoparticles (bright particles on the fibers). PVDF-HFP is a mechanically stable polymer. Embedding the fiber mesh into a membrane improves its mechanical properties. CeOx acts as a radical scavenger, which can mitigate the chemical degradation of a membrane and thereby increase its lifetime within an electrochemical energy conversion system.

Membrane electrode assemblies

Manufacturing of membranes and electrodes
Schematic layout of a full electrochemical cell. The cell consists of two electrodes (anode and cathode), which are separated by a membrane (solid polymer electrolyte) that is electrically insulating but ionically conductive. A sandwich of several layers is required to render this setup as efficient as possible. Flow field plates (also termed bipolar plates) provide access and removal of gases and liquids such as hydrogen as fuel gas and water as a product. Gas diffusion layers (or porous transport layers) enable a homogeneous distribution of the gases and liquids toward the catalyst layers. The catalyst layers are the electrochemical center of the cell: An oxidation reaction takes place at the anode, and a reduction reaction occurs at the cathode. The core of a cell, the membrane electrode assembly, is the sandwich of gas diffusion layers, catalyst layers, and membrane.

After the production of catalyst layers and membranes, the different layers need to be assembled to form a membrane electrode assembly. This step is typically performed by hot-pressing: The layers are pressed together while applying heat. As a result, close interfacial contact is formed between the layers, which now form a catalyst-coated membrane and are referred to as a membrane electrode assembly together with the gas diffusion layers. Good interfaces between the layers are the key to achieving high performance of the final electrochemical cell, like a fuel cell or water electrolyzer, while all layers need to maintain their integrity to avoid gas leaks and electrical short circuits.

Manufacturing of membranes and electrodes
The membrane electrode assembly of a proton exchange membrane fuel cell. The transparent polymer film is the membrane, which is covered by catalyst layers and gas diffusion layers in the active area (4 cm² in this lab scale cell).

Finally, the full cell is prepared by assembling the membrane electrode assembly in between two flow field plates. In water electrolyzers, the diffusion media are termed porous transport layers instead of gas diffusion layers, which is their designation in fuel cells. Additionally, a sealing element is required to ensure that the cell can be operated leak-free. This single cell can then be operated to investigate its performance, longevity, and durability versus certain stress factors.

Last Modified: 16.07.2022