Design and fabrication of a MEMS-based metal hydride/air accumulator for energy harvesting

Microaccumulators are one of the key components that will decide on the success of energy harvesting based autonomous micro-electro-mechanical systems (MEMS). Little efforts however were made to integrate high-capacity accumulators on chip level. Since the film thickness and therefore the achievable capacity per unit area of integrated Li accumulators is rather low, we propose a new integrated planar metal hydride/air microaccumulator of very high capacity, suitable for autonomous microsystem applications. The alkaline microaccumulator consists of a microstructured thin film air-breathing cathode and a high-capacity low-pressure type metal hydride anode. Both electrodes are separated by an anionic conducting polymer electrolyte membrane. A MEMS-based process was developed to fabricate the prototype microaccumulators. The preliminary testing results showed that the microaccumulator is able to deliver an open circuit voltage of 0.88 V and a maximum power density of 0.559 mW/cm2. This result shows that the accumulator is competitive or even superior to today's thin film Li accumulators. The air-breathing electrode however was identified to mainly limit the cell performance and its fabrication process is therefore still under optimization for further improvement.