Abstract
The characteristics of a polymer electrolyte membrane (PEM) are crucial for the success of PEM based fuel cells and electrolyzers. Membranes play a significant role regarding ohmic losses, fuel permeability and mechanical stability. For the resulting catalyst coated membrane (CCM), it is important to create an ideal contact between membrane and electrode to avoid gaps and enhance interfacial surface area. Production of CCMs for fuel cells or electrolyzers is mainly based on two techniques:
Decal Transfer: Fabrication of an electrode onto a carrier substrate and subsequent transfer by hot-pressing it onto a membrane [1].
Direct deposition: Spray-deposition onto heated and vacuum fixed membranes [2] or doctor-blade coating onto pre-swollen membranes [3].
Both techniques bear several problems:
During hot-pressing membranes are exposed to high temperatures and pressure. This can lead to deformations or cracks within the membrane. At the same time, transfer of electrode material can be insufficient and membrane electrode interface can suffer from delamination which causes gaps in terms of proton conductivity.
Direct deposition of electrode material onto membranes entails difficult processing. Swelling and shrinking during coating and drying can result in inhomogeneous electrodes.
Instead of coating membranes with electrodes, ultrasonic spray-deposition was employed to produce self-standing membranes. Being able to produce membranes allows the consecutive fabrication of all layers in a CCM and makes commercial membranes obsolete. It also reduces problems when an electrode is spray-deposited because the membrane is usually produced just before the electrode that is supposed to be spray-deposited. Thus, the membrane is attached to either a carrier substrate (PTFE) or another electrode that was spray-deposited at the very beginning.
Fabricating membranes by spray-deposition gives freedom in terms of several processing aspects:
Modification of the ionomer solution composition. The following variations were used: Solutions based on water/alcohol and water/alcohol + high boiling point solvents such as ethylene glycol, dimethyl sulfoxide, dimethylformamide and dimethylacetamide.
Application of thermal treatments
Variation of the membrane thicknesses
This approach reveals possibilities that would not be available with commercial Nafion membranes with predefined properties. Self-standing Nafion membranes, based on different ionomer solution compositions and thermal treatments were produced (120-130 µm). All samples were employed for CCM fabrication (Anode: 3.0 mg/cm² PtRu; Cathode: 0.7 mg/cm² PtNi). Further, they were characterized, regarding hydrogen permeability, ohmic resistances and single-cell polarization curves in direct methanol fuel cell (DMFC) operation. All samples were compared to commercial Nafion 115 membranes.
With some samples the performances of CCMs using spray-deposited membranes could be matched with Nafion 115. At the same time they showed lower ohmic resistances and partly lower hydrogen crossover values. Additionally, thinner membranes equivalent to Nafion 212 (50 µm) and Nafion 211 (25 µm) were fabricated and tested. They showed even better performances. While 50 µm thick membranes had moderate permeability levels comparable to 127 µm thick membranes, 25 µm samples showed high permeabilities. To reach high performances with relatively low permeation values, thin composite membranes (25 µm) were produced. These consisted of Nafion and graphene oxide, where graphene oxide was supposed to work as a blocking layer against permeation [4] while supporting proton conductivity.
It is also possible to deposit Nafion solutions or Nafion (composite) dispersions directly onto electrodes instead of producing self-standing membranes [5]. This would allow the fabrication of a whole CCM consecutively [6]. Even though this technique was used for fabrication of CCMs for DMFCs, it can also be transferred to hydrogen-based fuel cells (PEMFC) [6] or water-electrolysis (PEMWE) [7].
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Figure 1