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Etching a new future for electrolyser manufacturing

02 September 2022

As the world continues to adapt to powering its vehicles, towns and cities with zero-carbon energy, the demand for new technologies is gathering pace. Karl Hollis, Director of Engineering at Precision Micro, looks at how the current design and manufacturing process for bipolar plates, a component used in electrolysers that produce and store green hydrogen, could be made faster and more cost-effectively using chemical etching technology.

Electrolysers use electrical energy from wind, solar or hydroelectric sources to break water into hydrogen and oxygen using electrolysis. The green hydrogen produced can be used to power anything from buses and cars, to generators, heating systems and machinery.

Many modern low-temperature electrolyser systems are built with modules (‘stacks’) based on proton exchange membrane (PEM) electrolysis technology. And a crucial component of the PEM electrolyser is the bipolar plate, which has several important functions. 

Bipolar plates have precisely manufactured, often complex channels that evenly distribute water in the electrolyser stack. Their core functions are for cooling the electrolyser, supplying reactant gases to the anodic sides, and evacuating the hydrogen and gases produced during the reaction. 

Though bipolar plates are one of the most important electrolyser components, they are also one of the most costly. Photochemical etching provides a viable manufacturing alternative for mechanical design engineers looking to reduce these costs.

A material world

For PEM electrolysers, bipolar plates made of carbon or carbon composites have traditionally been used because of their chemical resistance. While carbon-carbon composites and carbon-polymer composites have advantageous properties, they have low mechanical strength and low electrical conductivity – not to mention high machining costs.

Using metals for bipolar plates is usually preferred due to their low cost, low resistance and good mechanical properties. However, they must stand up to the operating conditions required in the production of hydrogen. 

One of the easiest ways for engineers to increase margin on electrolysers is by revisiting the manufacturing processes of the components within them, such as the bipolar plate. 

Photochemical etching is probably the most versatile of all the sheet metal machining processes. Its subtractive nature means that virtually any metal can be etched and as such, specialist corrosion-resistant metals like titanium can be machined more cost-effectively than competing processes.

What is photochemical etching?

An alternative to traditional stamping and laser cutting, photochemical etching is a subtractive sheet metal machining process that uses chemical etchants to create complex and highly accurate precision components from almost any metal. 

The geometric complexity and tolerances offered by photochemical etching not only make it a desirable manufacturing process but, in some instances, the only technology suitable for mission or safety-critical metal components. 

Scaling up 

The photochemical etching process offers manufacturers significant advantages when producing complex fluidic components such as bipolar plates, reducing inefficiencies while maintaining precision and decreasing time to market. 

Tooling in stamping and hydroforming can be slow and uneconomical to produce and, in some instances, can take many months, increasing development timelines. In addition to extending project timescales, prototyping complex channel configurations using traditional methods can run into tens, if not hundreds of thousands of pounds.

The crucial differentiator between traditional machining processes and photochemical etching is that it requires no hard tooling and instead uses digital tooling which is inexpensive to produce and adapt, giving engineers the flexibility to optimise designs at minimal cost. 

Usually, photochemically etched prototypes can be created quickly and easily for hundreds, rather than thousands of pounds. Costs can also be reduced further by increasing channel feature density and manufacturing thinner bipolar plates. 

Channel complexity and accuracy

The geometry and design of flow channels in a bipolar plate have a huge effect on its performance. 

The complexity or depth of the channels is, for example, limited by stamping and hydroforming techniques that are so common in the industry. Hydroformed bipolar plates are prone to rupture because of the thinning of metal sheets during the forming process. As such, more complex designs can be challenging. 

Stamping, in addition to presenting design difficulties, can also be tough to replicate accurately due to wrinkles, surface roughness and spring back of the material. Smaller and more complex flow channels require greater stamping tonnage, which leads to the substantial increase of the machine capital cost and cycle time.

Photochemical etching, however, offers almost unlimited part complexity and crucially produces burr and stress-free, completely flat components with extreme consistency, which is especially important for bipolar plates where imperfections can compromise stack bonding. 

Unlike CNC machining, hydroforming and stamping the photochemical etching process does not apply any mechanical or thermal stress that may affect the metal’s properties and achieves channel accuracy to ±0.020 mm. 

Unlike traditional processes, chemical etching removes metal simultaneously, meaning complex channels or flow fields can be etched on both sides of the plate. This versatility enables designers to vary the size and shape of channels and incorporate headers, collectors and port features without additional cost.

A better path forward

There are hundreds of research papers which explore the efficiency, quality and financial limitations of producing bipolar plates, but they rarely consider photochemical etching as a viable process. If the industry is to keep up with the growing demand for hydrogen production, it must start to think outside the realms of costly machining processes, especially during prototyping.

Photochemical etching allows design engineers to produce bipolar plates with lead times measured in days, not months while offering the flexibility to create complex, high-performance bipolar plates.

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