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The Impact of BPP Coatings

Metal Bipolar Plates are the Future

In a fuel cell, the bipolar plates evenly distribute fuel and oxidant across the whole active area of the membrane electrode assembly, serve as the electrical connection between cells, assist with water (by-product) removal, enable humidification of the inlet gases and provide pathways for cooling of the cells. Bipolar plates should be electrically conductive for minimum resistive losses, chemically resistant against the corrosive environment within a fuel cell, and thermally conductive for optimum heat management across each cell.

Bipolar plates can be made of graphite or metal. While graphite plates have excellent corrosion resistance, low bulk resistivity and low contact resistance, they have several disadvantages including sizeable weight and volume, poor mechanical properties (brittle) and high processing costs. Metal plates, particularly those made with stainless steel, are inexpensive and light; compact and easy to fabricate; and possess excellent electrical and thermal conductivity. This allows fuel cells to be made much smaller and lighter, enabling a wider range of applications.

However, metal plates suffer from severe corrosion issues which limits their lifetime under the operating environment of a fuel cell.

Carbon is one of the most remarkable elements in the periodic table

Carbon compounds form the basis of all known life on Earth. It enables the formation of an extraordinary variety of compounds, most of which are comparatively stable under normal conditions.

Carbon has the largest number of allotropes and is well-used in more than 90% of all known chemical substances. Carbon-based solid materials can exist in various forms with unique properties, for instance:

In diamond form: super-high hardness, along with excellent electrical insulation and thermal conductivity;

In graphite form: adequate softness and high electrical conductivity

Carbon has also been the key element in the synthesis of thin-film coatings, including graphite, diamond and Diamond-Like Carbon (DLC).

For graphite coating, a low density (2.25 g/cm3) and a high electrical conductivity (resistivity 10-7 to 10-6 Ω.m) thin film can be achieved. This is mainly due to the vast electron delocalization between the sp2-hybridized carbon layers which allows the valence electrons to move freely.

This is in contrast to diamond coatings, which exhibits a dense structure (density of 3.515 g/cm³) as it constitutes mainly sp3-hybridized carbon atoms, each bonded to four nearest neighbors in a tetrahedral coordination. This atomic configuration leads to a high electrical resistivity (electric resistivity is 1011 to 1018 Ω.m) as there are no free electrons for charge conduction; while its rigid tetrahedral structure leads to its high hardness (100 GPa) and brittleness. The relatively small coating area, poor toughness and high synthesis temperature make it difficult to be used in many industrial applications.

Tetrahedral Amorphous Carbon (taC)

The Tetrahedral Amorphous Carbon (taC) film is a special type of advanced DLC material with a large percentage of sp3 bonding (up to 87%) and containing no hydrogen atoms. This tetrahedral network of high sp3 bonding results in unique diamond-like properties such as an extremely high density, excellent chemical stability, and high temperature resistance. Meanwhile, by adjusting the deposition process parameters or doping with selected elements, the taC film can concomitantly exhibit the excellent electrical properties of graphite.

Sydrogen has the key to unlock Metallic Bipolar Plates

Sydrogen’s advanced material solutions have demonstrated enhanced performance by applying an ultra-thin coating layer of taC on metallic bipolar plates, importantly, improving the corrosion resistance and interfacial contact resistance while retaining the inherent advantages of metal over graphite. Our coatings can be easily customized to your application to achieve the best performance out of your fuel cells and electrolysers and overcome existing design limitations. Contact us to find out more

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