Development of Residential Fuel Cell System of Panasonic and Approaches to Enhance Durability

   Panasonic has been developing residential fuel cells (micro CHP) since 1999 and launched the world's first system, the ENE-FARM, in May 2009 in Japan. Panasonic has now shipped a total of approx. 21,000 units throughout Japan as of the end of December 2012.

   Fig.1 shows the state-of-the-art model of the ENE-FARM which was launched in April 2013. Several points are improved from the previous model. First of all, the model achieved world’s highest overall efficiency 95 %(LHV) by reviewing the heat insulation of the heat recovery path and improved the heat recovery efficiency (Fig.2). Secondly, the price dropped approx. 25% by reducing 20% system components, 50% noble metal in the selective oxidation catalyst in fuel processor, and 50% platinum catalyst in stack. And finally, the durability enhanced by 20% from the previous model by improving the durability of the electrolyte membrane in the MEAs .

   MEAs are one of the most important components of the ENE-FARM in terms of cost, performance and durability.  We have developed many core technologies such as high durability membrane, high activity catalyst, and low cost base material-less GDL. Among these materials, we consider that durability improvement of membrane is essential and further elucidation of its degradation mechanism is necessary. Although there have been many approaches to enhance the chemical stability [1] of membrane, mechanical stability haven’t discussed enough. However, in our previous study, long-term operation including start-up and shut down results in serious mechanical degradation. Hence, it is important to design membrane not only from chemical but mechanical aspects.

   In this study, we focused on mechanical degradation mechanism of membrane. It was also discussed about mechanical property to enhance membrane durability with keeping performance and cost.

   Membranes which consist of different mechanical strength and dimensional change were prepared. After fabricating MEAs in each membrane, acceleration experiment of mechanical degradation was conducted. The experiment is a cycle test that comprises OCV, potential cycle and wet and drying cycle. Each cycle was conducted in a short time to accelerate the degradation.  After 5000 times cycle, stress-strain curves of each membrane were measured by tensile tester to investigate mechanical property. The results showed that membranes of low tensile strength and high dimensional change were degraded more than those of high strength and low dimensional change. It indicates that to reduce mechanical stress attributes to the dimensional change and to enhance mechanical strength to sustain mechanical stress result in high mechanical durability. By optimizing membrane strength and dimensional change, mechanical durability enhanced without any performance loss and increase in cost. It is also suggested that membrane which has higher mechanical durability tends to show higher chemical durability.

   Fig.3 shows the actual operation of the ENE-FARM and dissolved material elution from the membrane in the previous and current model. Due to the improvement of membrane, the amount of dissolved material elution reduced by approx. 20% and 60000hrs operation guarantee has achieved.

   Further progress for next generation ENE-FARM will be shown on the presentation.

Acknowledgements

   This project has been supported by the fund from New Energy and Industrial Technology Development Organizations (NEDO), in Japan

References

[1] W. Delanay et al., ECS Trans., 11 (2007) 1093