Micro fuel cell fabrication technologies

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Gianmario Scotti Name of the doctoral dissertation Micro fuel cell fabrication technologies Publisher Unit Department of Materials Science and Engineering Series Aalto University publication series DOCTORAL DISSERTATIONS 29/2014 Field of research Microelectromechanical Systems Manuscript submitted 6 February 2014 Date of the defence 9 May 2014 Permission to publish granted (date) 11 February 2014 Language English Monograph Article dissertation (summary + original articles) Abstract Fuel cells are established devices for high efficiency conversion of chemical into electrical energy. Microfabricated fuel cells (MFC) promise higher energy density compared to rechargeable batteries currently used in portable applications (mobile phones, tablets, laptops etc.). In this work new fabrication technologies have been developed to make MFCs more viable alternatives to batteries.Fuel cells are established devices for high efficiency conversion of chemical into electrical energy. Microfabricated fuel cells (MFC) promise higher energy density compared to rechargeable batteries currently used in portable applications (mobile phones, tablets, laptops etc.). In this work new fabrication technologies have been developed to make MFCs more viable alternatives to batteries. Like other microfluidic devices, MFCs can be fabricated using a number of different techniques, each with its specific advantage and drawback. In this doctoral dissertation, three microfabrication technologies have been used to produce MFCs: deep reactive ion etching (DRIE) of silicon, laser ablation of silicon and bulk aluminium wet etching. In all cases, the substrate acted as current collector, so good conductivity was important. The science produced is of value beyond the scope of fuel cells alone: integration of black silicon with microfluidic devices, rapid prototyping of microfluidic devices with laser ablation, and using aluminium as a cheap (and cheap to micromachine) but sturdy material for microfluidics. In the case of DRIE-fabricated micro fuel cells, black silicon was studied as both a simple integrated gas diffusion layer (GDL), and as promoter of galvanic contact between highly-doped silicon and carbon felt GDLs. Integrating a GDL into a MFC increases the cost of the device, but this increase is minimized using black silicon. Creating tens to hundreds of micrometer thick GDLs proved to be difficult; using black silicon to integrate a commercial carbon cloth GDL easily solves this problem. The work on laser-ablated micro fuel cells yielded useful results for microfabricators that want to use picosecond laser ablation in microfluidics and other MEMS fields; the technique of picosecond laser ablation of silicon developed in this study enables the creation of ~60 micrometer deep channels at an overall speed of 15 mm/s and very low induced stress. The method enables the microfabrication of channels and through-hole gas inlets in the same process step, without the need of lithography. The bulk-aluminium MFCs produced very high current density (1.1 A/cm2) and power density (228 mW/cm2), but the study of bulk-aluminium microfabrication also offers detailed guidelines for microfabrication of other aluminium microfluidic devices.