Open-pored, easily shaped, electrically conductive metal foam is providing new opportunities - whether used as a gas diffusion layer in metal-air batteries or fuel cells, or as a current collector in super capacitors. Doris Schulz reports.
When it comes to the development and optimisation of alternative drive concepts, the open-pored, easily shaped, electrically conductive metal foam produced by Alantum Europe GmbH is providing new opportunities. Whether used as a gas diffusion layer in metal-air batteries or fuel cells, or as a current collector in super capacitors, the innovative material can be adapted ideally to any application, thus facilitating increased energy densities or longer service lives.
The versatility of the metal foam is the result of a manufacturing process developed specifically by Alantum. Within this process, open porous nickel foam sheets with hollow struts are produced. The pore size is defined to 450, 580, 800 and 1200µm. Subsequent annealing keeps the material flexible and easily shaped. The electrically conductive metal foam is used in significant quantities as a cathode in nickel-metal hybrid (NiMH) batteries, which are state of the art in devices such as electric and hybrid vehicles. In this application, nickel-metal foam also offers optimisation potential for increasing energy density. Namely by applying a coating of graphite or other powdered materials suitable for energy storage to the open-pored structure in a special sintering process.[Page Break]
Improved power output
To achieve improvements in range and acceleration of vehicles, a great deal of research is being conducted around the world into battery technology. A promising approach is the use of metal-air batteries, whose performance can also be enhanced with the use of the porous material. As a gas diffusion layer, the foam optimises the flow rate of the metal electrode, such as zinc, with oxygen. This results in more even energy production and power output.
The material is available as pure nickel foam and as metal alloy foam for this purpose. Following production, the latter is coated with a high-alloy, application-optimised metal powder in a patented, stable process. This helps to promote resistance against aggressive atmospheres and corrosion as well as stability when exposed to temperatures of up to 1000°C.[Page Break]
Super capacitors, as well, are capable of delivering significant improvements to electromobility. This is thanks to their ability to compensate voltage peaks that occur during acceleration and braking, which in turn helps to lengthen the durability of batteries and fuel cells. As the so called 'supercaps' release and absorb energy more quickly than batteries, they do not have such a high energy density.
Whether or not this is an area that can be improved upon with the use of foam material was one of the themes of the 'Alternative Energy Technologies for Transportation (AETT)' research project carried out by the Fraunhofer Society in collaboration with the University of Michigan and the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) and Alantum Europe as the foam supplier. The metal alloy foam was used as a current collector. In contrast to conventional methods, whereby carbon is applied to a film, the foam was infiltrated with molybdenum- and vanadium-nitride as an active material.
In comparison to carbon, these materials have an energy density that is two to three times higher. Thanks to the special structure and large surface of the foam, it is possible to apply sufficient active material to achieve a two to three-fold increase in the energy density of the super capacitor.[Page Break]
Metal alloy foam can also offer a number of advantages when used as a gas diffusion layer in fuel cells. On the one hand it provides optimised equal distribution of the gas and the resulting increase in power density across the entire membrane surface. On the other hand, the material can be sintered to the bipolar plates in order to achieve a direct electrical connection. An additional advantage is the ability of the metal alloy foam to resist the very aggressive atmospheres that typically exist in fuel cells.
For use as a gas diffusion layer, the material can be coated with conventional alloys such as stainless steel alloy 316L and CroFer as well as with new, application-optimised metal powders.
Alantum currently produces around four million square metres of nickel foam in China annually. This is in addition to 500,000 square metres of metal alloy foam which are produced in Korea. In order to further optimise the material for applications in electromobility, the Munich-based company is working closely with IFAM in Dresden. Here, a research laboratory is also available in which tests may be conducted and samples can be produced.
Alantum Europe GmbH, Munich, Germany. www.alantum.com.