The main idea behind the Open Fuel Cell is to free your creativity when it comes to modifying individual components—or completely redesigning them to suit specific experimental needs. So why not replace “real” flow fields, made from electronic circuit boards by milling, with flow fields made entirely of sheet metal using the deep drawing process? Sheet-metal flow fields represent the “state-of-the-art” approach to most commercial fuel cell systems.
When you think of steel, you also think of rust—and rightly so. That's why there is so much emphasis on protecting the first board's flow field plates with gold thickness (see “The Great Corrosion” blog post). To gain a deeper understanding of the decomposition processes inside a working fuel cell, the Fachhochschule Südwestfalen and the ZBT started the research project “CORROMAP.” One idea for obtaining detailed information sounds simple at first glance: place multiple reference electrodes inside the MEA and metal flow fields, and use the metal cell under all possible—and impossible—conditions.
This is where the challenges begin.
Accommodating all those test electrodes inside a PEMFC requires a lot of space and extensive modifications to the cell design, especially if commercially available cells are used as a starting point. A second challenge is the testing equipment required to use “perfect” commercial cells. Unless you work in a company or laboratory specializing in fuel cell technology, this (expensive) equipment is usually not available.
Since the Open Fuel Cell was designed to be easily modified and operated with a small experimental setup, it was a natural choice to adapt the OFC and turn it into a workhorse for future research projects. And we synced it up—just look at the pictures (figure: Full metal OFC design, Full metal OFC).
Important design changes
1) Deep metal flow fields
The most obvious change is the flow fields themselves. They are made from 0.1 mm sheet metal using a deep drawing mold. In keeping with the spirit of the OpenFuelCell project, the mold is simply made by 3D printing. The pressing of the mold is done using tools borrowed from the wood workshop-better after a good breakfast (illustration: 3D printed deep-draw mold). Despite the simple tools and process, the plates can be produced with reasonable quality.
2) Connection adapter between flow fields and end plates
Some modifications were needed to the interface between the flow field and the end plates. For circuit board flow starting points, this interface is straightforward: the plates have a flat, solid surface that provides even contact with the end plates. Not so with deep drawn steel plates. These three-dimensional plates provide only a limited contact area to the end plates, and their structure can become flat if more force is applied. Therefore, adapter plates made of elastomer had to be developed and integrated with the modified OpenFuelCell. This part is also 3D printed—of course.
3) Updated current collection
More modifications were needed for the current collector. The original soldered on flow-field plates cannot be mounted on stainless steel plates because soldering does not work in this situation. Additionally, the electrical conductivity of the base material is important: the in-plane conductivity of a 35 µm copper layer on a circuit board is much higher than that of a 100 µm stainless steel sheet. To keep ohmic losses as low as possible, current collectors are tied to the sides of the metal flow fields.
4) Closed cathode with controlled airflow
With the full metal design, the previously open cathode is no longer actually open or directly exposed to the surrounding air. This means that air must be pumped into the cathode flow area using a compressor. The air flow delivered by the compressor can be varied by using different voltages. Without this additional compressor, the complete test setup remains simple and therefore can be used almost anywhere.
Does it work?
Yes—it's definitely possible. See below a working diagram of a full metal OFC using different levels of air flow to the cathode by varying the compressor voltage.
The main characteristic regions of the polarization curve—activation overpotential, ohmic overpotential, and diffusion overpotential—can be clearly seen. The closed cathode design combined with an external compressor also means that the air flow in the cathode flow area is variable and can be precisely controlled by adjusting the voltage applied to the compressor.
The conclusion
Within a certain range, your imagination is the only real limit when it comes to designing your fuel cell experiment.