Gas diffusion electrodes, used for example in electrochemical gas sensors or fuel cells, need a combination of high permeability to gases and controlled permeability to liquid electrolyte, in order to allow target gas access to a three-phase boundary between gas, catalyst and electrolyte, without allowing the electrolyte to leak out or flood the electrode. The desired combination of these properties is typically achieved by supporting the metal catalyst on a porous hydrophobic material (e.g. PTFE), and mixing additives such as PTFE in with the metal particles. (See, for example, “Liquid Electrolyte Fuel Cells”, B. S. Hobbs, A. D. S. Tantram & R. Chan-Henry, Ch.6 in “Techniques And Mechanisms In Gas Sensing”, Eds. P. T. Moseley, J. O. W. Norris & D. E. Williams, Pub. Adam Hilger, 1991, page 176).
Examples of conventional gas diffusion electrode fabrication techniques include: Puddling: for example, make a low viscosity (typically aqueous) suspension of catalyst and PTFE particles plus other additives such as surfactants, binder etc. spread over PTFE support tape or temporary support such as aluminum foil via pipette/syringe either manually or automatically.
Screen/stencil printing: make a high viscosity mixture of catalyst and PTFE with suitable vehicle (e.g. viscous organic, though aqueous based systems may also be used). Force through screen/stencil to pattern and deposit a defined thickness of material either directly onto PTFE support tape or temporary support.
Manual approach: mix catalyst and PTFE with suitable liquid to make a paste (like mixing cement), then spread over PTFE support tape or temporary support.
In all of the above approaches the electrode mixture may be applied directly to a porous material (e.g. PTFE tape) which will support the electrode when assembled into the sensor, or may be applied to a temporary supporting material such as aluminum foil, and subsequently transferred to the supporting tape. Typically the last step is achieved by pressing. Normally the material will be fired to drive off the carrier.
Disadvantages of this approach are that it is difficult to keep the mixture in suspension. PTFE tends to float to the top, and electrodes can crack under subsequent firing. Therefore constant agitation may be needed in use.
With conventional gas diffusion electrodes, it is very difficult to characterize and control the catalyst properties which affect the overall activity & wetting up. This results in widely varying performance of notionally identical batches. The reliable and repeatable production of gas diffusion electrodes of this type is therefore difficult.
It is desirable to avoid the need to mix a separate hydrophobic material with the catalyst. This can be achieved by making the catalyst itself suitably hydrophobic so that it does not simply saturate with the electrolyte but wets up to a controlled degree, thus maintaining the required mix of gas diffusion paths and wetted electrolyte.
The above results could potentially be achieved by chemically modifying the catalyst surface, but this approach has the undesirable effect of modifying the chemical and electrochemical properties of the material. It is therefore desirable to be able to change the hydrophobicity by a physical means that does not detrimentally affect its chemical properties.
Furthermore, it is also desirable to avoid the need to deposit the catalyst mixture onto a supporting porous tape. Such tapes are, by nature of their construction, fragile and prone to tearing or other damage with the result that electrolyte can potentially leak through them. This effect is worsened by the fact that the hydrophobicity of the catalyst mixture is typically not sufficient to prevent electrolyte coming into contact with the supporting tape. The supporting tape therefore needs to be a material which is chemically compatible with the electrolyte, PTFE is a common choice.
The above could potentially be achieved by utilizing a solid monolithic electrode material thereby removing the need for a support, however this would need to be in the form of a thin porous foil and would not be intrinsically hydrophobic.
It would thus be desirable to be able to manufacture electrodes of the type generally discussed above without the noted disadvantages.