M E E T
The leading metal-air fuel cell company
Since 2001
Air diffusion electrodes
by MEET
Air diffusion electrode (ADE) is the core component for metal air electrochemical systems. ADE works as a cathode generating OH- ions from the reaction of air oxidation of water.
1/2O2(g) + H2O +2e = 2OH- 0.4V
As shown in this chemical reaction formula, the ADE is dependent on air (oxygen) introduction, electron transfer(electrical conduction), and water supply.
There are several types of ADEs with wide applications, and their performances vary in applications. MEET is producing the following types of ADEs, MnO2 catalyzed (ADE52) or Co3O4 catalyzed (ADE72) and PTFE porous film coated (ADE-F) or non coated (ADE-B). Table. 1 shows the specifications of the four types of ADEs. Also, Fig. 1 shows the pictures of ADE produced in MEET. They are fabricated from hot pressing machine and the sizes 20 x40 ㎠(the real sizes are slightly larger), thickness 0.5~ 0.75㎜.
B and F types according to PTFE film coating
ADE-B type has high power and easy assembling characteristics. The electrode has layers of Ni foam as current collector and catalytic layer in which the reaction occurs and composed of carbon powder, metal oxide powder and PTFE binder. The layer is hot pressed dense body and impermeable to electrolyte to a large extent, but do not stand long time. And it is recommended to use for Mg-air cells using unaggressive electrolyte like saltwater solution, or the other cells with a pressure controlled function keeping elevated air pressure than the pressure of the electrolyte, to push back the smeared electrolyte. This type exhibits high power from high rate of air inhalation and effective air cooling.
Nickel foam surface is contacting the external air and bondable with solder or glue. Both sides are electrically conducting.
ADE-F types have porous PTFE films to block electrolyte flooding. It stands longer time in metal-air cells using alkaline electrolyte. Air side has electrically insulating PTFE porous film. Soldering is not possible without removing PTFE film. Therefore, other mechanical processes like crimping, pressing, and riveting are required to attach terminals. Sometimes it needs removing or grinding off the PTFE layer for the better electrical contact.
How to test cathode performance?
There is a typical method of measuring overvoltage of electrodes, called Luggin probe method using Hg/HgO reference electrode. However, in this measurement, a Ni or Pt wire is used as a pseudo reference electrode (PRE) to compare the overvoltage changes with current densities. This PRE is easy to handle and manage, and gives reliable comparative results among ADEs, regardless of all conditions of electrolyte, while the normal redox electrode should be handled with great care from breaking, and baseline drifting from pH or concentration change of electrolyte, from intermixing or contamination of the standard solution filled in the electrode in case of long tests.
The effective electrode area is 7x7=49㎠. The electrolyte used in this experiment is 35% KOH solution. The size is large enough to neglect the screening by the Luggin probe.
The PRE in a Luggin probe made of PE tube is directly contacted on the surface of ADE in electrolyte. The electrolyte fills in the PE tube and a Ni-wire, then the overvoltage is measured by reading the potential difference between the ADE and the PRE. The open circuit voltages of PRE do not give correct overvoltage values as the metal wires exhibit their own electrochemical redox potentials. It is more resonable to read the values from at least 1 ㎃/㎠. From our test cells, the maximum current density was 100 ㎃/㎠, therefore the test range was from 1 to 100 ㎃/㎠, the data points were, as depicted in Fig. 5, numbered 13 and stayed for 30 seconds to reach stabilized potential at every points. All measurements were performed after the initial primal CC tests at 50㎃/㎠ for one hour in case of virgin samples, because they need enough wetting time for their hydrophobic surfaces.
As the discharge goes, zincate ions and other products partially fill the cell, and the passivation of the zinc surface occurs at high current density and high zincate concentration. Such conditions form inhomogeneous current flux, and decrease the measurable maximum current level. To have correct data in this experiment, there should be enough stirring of the electrolyte. The stirring breaks the high zincate regions and suppress the passivation.
The test cell is shown in Fig. 4, the frame is made of transparent PVC and has a opened top and a window with opening of 7×7 ㎠ on the front. A 9×9 or 9×12㎠ sized ADE specimen is glued on the peripheral of the opening.
A stainless steel plate with the same opening is attached after bonding the ADE on the frame, and then is clamped together. The upwardly exposed section of the ADE is clamped with terminals for current discharge and potential measurement.
The internal sizes of the cell are 7×7×5(㎝), the distance between the ADE and the Zn anode is about 5㎝ and the maximum current obtainable with the normal zinc plate anode with almost 100% density, fabricated from melt casting and rolling, is around 4.9A(100㎃/㎠). If you want to obtain more data at the higher current density, then replace with more active anodes, i.e., porous body or powder compact, Zn-Al alloy etc.
A direct single cell test
The test was conducted in a standard cell called “wattery®” having effective electrode area of 72.55㎠ (8.5㎝×8.5㎝). The ADE and a Zn plate anode are placed parallel with 20 degree inclination and with 2㎜ apart. A magnetic stirrer is used for minimize anodic passivation and inhomogeneous distribution of composition and temperature.
The constant current test was performed by stepwise increment of pulse current application. The current is set at certain value as in the Table and maintained for 1 minute and then read the voltage data, as the voltage is stabilized in a minute. After reading the data, the current is cut off and stayed another 1 minute to have time to be freed from passivation tendency. The next current is applied and repeat the procedure again, and the I-V curves results.
