Periodic Reporting for period 1 - PERMEABLE (Powder Metallurgy Approaches for Next-Generation Bipolar Plate Materials)
Periodo di rendicontazione: 2021-06-01 al 2023-05-31
The implementation of hydrogen technologies is moving at a fast pace while many aspects of the technology are still requiring further research. One of these are the bipolar plates in the dominant type of fuel cells, the Proton-Exchange Membrane (PEM). These plates are key for the performance of the system, because they are in charge of distribution of reactants and products, transport of electrons between cells and help with thermal management, while they are responsible for up to 40% of the cost of the cell.
While the conventional flow structure for the gases is made consisting of channels, that are either machined or formed, alternative designs using porous materials to create 3D flow fields have shown improvements in the efficiency of the cells. However, the features of this porosity is dependent on the technique, and most of the available evidence is gathered with techniques that are limited in terms of pore size and volume.
In a scenario where millions of fuel cells will need to be manufactured per year, technologies that are able to allow the fabrication of plates with specific flow geometries while also being able to reach the production needs are required.
The objective of the project is to evaluate the feasibility of powder metallurgy as a technology to manufacture porous structures based in titanium with a variety of features and the application of surface modification techniques that are suitable for these type of structures and that result in improvement of the properties of the material.
It is feasible to use powder metallurgy techniques to manufacture open pore flow field bipolar plates, thanks to the effective control on the pore size and amount of porosity and to the development of surface modifications that improve the corrosion resistance and improve the performance of the material in the conditions of the fuel cell.
While the conventional flow structure for the gases is made consisting of channels, that are either machined or formed, alternative designs using porous materials to create 3D flow fields have shown improvements in the efficiency of the cells. However, the features of this porosity is dependent on the technique, and most of the available evidence is gathered with techniques that are limited in terms of pore size and volume.
In a scenario where millions of fuel cells will need to be manufactured per year, technologies that are able to allow the fabrication of plates with specific flow geometries while also being able to reach the production needs are required.
The objective of the project is to evaluate the feasibility of powder metallurgy as a technology to manufacture porous structures based in titanium with a variety of features and the application of surface modification techniques that are suitable for these type of structures and that result in improvement of the properties of the material.
It is feasible to use powder metallurgy techniques to manufacture open pore flow field bipolar plates, thanks to the effective control on the pore size and amount of porosity and to the development of surface modifications that improve the corrosion resistance and improve the performance of the material in the conditions of the fuel cell.
The project has tackled the processing of porous materials based on titanium to manufacture PEM fuel cell bipolar plates. This included the fabrication of substrates with different porosity features and the surface modification of the material. Ti displays an excellent corrosion resistance in the environment of the fuel cell, however, this comes with a drawback: that there are voltage losses due to the contact of the titanium with other components.
The first part of the project involved the fabrication of porous substrates. These needed to have a large amount of porosity, between 40 and 70%, in order to allow the movement of gases while also keeping a structure with mechanical integrity and good thermal and electrical conductivity. Thanks to the powder metallurgy technology, different amounts of porosity and pore size distribution were able to be produced in a way that was cost-effective and with good control of the features. This was achieved using three different materials: pure titanium, a reference titanium alloy (titanium grade 5) and an advanced ceramic material with similar conductive properties to metals (Ti2AlC). The corrosion resistance of dense and porous titanium-based materials were almost on target with the requirements for use in bipolar plates, but the contact resistance was very high due to the protective oxide layer.
The next part was the surface modification of these materials, especially pure titanium and the alloy. This was done using two different technologies. In one of them, a conductive ceramic film was grown on the materials using a high temperature process in a controlled atmosphere. This resulted in films with an adequate thickness for the application using a process that can be incorporated into the powder metallurgy processing, reducing the post-processing costs drastically. This route resulted in materials that had the external and internal porous structure modified with the ceramic film. The corrosion resistance and the contact resistance were significantly improved thanks to these surface modifications, meeting the targets for the application. The second technology involved the deposition of graphene in composites or as the only compound using wet chemistry approaches, which are very cost-effective. Some of the formulations successfully improved both the corrosion and the contact resistance, but these were hard to apply to porous materials successfully. Most of these coatings displayed interesting corrosion protection properties, but the conductivity was poor.
In the end, a set of subtrate conditions and coatings was succesful in meeting the requirements for the production of open pore flow field bipolar plates.
This work has resulted in the filling of an international PCT patent, pending entering the different patent offices, on the desing and methods of manufacturing of a bipolar plate for PEM fuel cell. The results have been disseminated at four international conferences with experts in the field of metallurgy and hydrogen energy and one article is currently is under review.
The first part of the project involved the fabrication of porous substrates. These needed to have a large amount of porosity, between 40 and 70%, in order to allow the movement of gases while also keeping a structure with mechanical integrity and good thermal and electrical conductivity. Thanks to the powder metallurgy technology, different amounts of porosity and pore size distribution were able to be produced in a way that was cost-effective and with good control of the features. This was achieved using three different materials: pure titanium, a reference titanium alloy (titanium grade 5) and an advanced ceramic material with similar conductive properties to metals (Ti2AlC). The corrosion resistance of dense and porous titanium-based materials were almost on target with the requirements for use in bipolar plates, but the contact resistance was very high due to the protective oxide layer.
The next part was the surface modification of these materials, especially pure titanium and the alloy. This was done using two different technologies. In one of them, a conductive ceramic film was grown on the materials using a high temperature process in a controlled atmosphere. This resulted in films with an adequate thickness for the application using a process that can be incorporated into the powder metallurgy processing, reducing the post-processing costs drastically. This route resulted in materials that had the external and internal porous structure modified with the ceramic film. The corrosion resistance and the contact resistance were significantly improved thanks to these surface modifications, meeting the targets for the application. The second technology involved the deposition of graphene in composites or as the only compound using wet chemistry approaches, which are very cost-effective. Some of the formulations successfully improved both the corrosion and the contact resistance, but these were hard to apply to porous materials successfully. Most of these coatings displayed interesting corrosion protection properties, but the conductivity was poor.
In the end, a set of subtrate conditions and coatings was succesful in meeting the requirements for the production of open pore flow field bipolar plates.
This work has resulted in the filling of an international PCT patent, pending entering the different patent offices, on the desing and methods of manufacturing of a bipolar plate for PEM fuel cell. The results have been disseminated at four international conferences with experts in the field of metallurgy and hydrogen energy and one article is currently is under review.
The project has advanced the state of the art in the fabrication and surface modification of materials for open pore flow field bipolar plates. It is expected to foster further research into these type of bipolar plates considering the advances in the control of porosity and surface composition, which could lead to improvements in the fuel cell system without compromising costs.