3D Micro-Supercapacitor Based on MnO2 Electrodes on Silicon Substrate
Authors: Eustache Etienne , Douard Camille , Tilmant Pascal , Yarekha Dmitri , Morgenroth Laurence , Lethien Christophe , Brousse Thierry 
Published: February 2020

Continuous progresses in micro/nanofabrication have led to the multiplication of MicroElectroMechanical Systems (MEMS). This trend meets the growing demand in miniaturized electronic devices with multiple on-board functionalities: sensors, actuators, computing and wireless communicating systems. Providing autonomy to these smart microsystems integrated in a few cubic centimeters or less is still a challenge. To fulfill the need of a high power at the microscale, electrochemical micro-supercapacitors seem to be good candidates. Although 3D-microbatteries (1) recently shows great power performances, the stability upon cycling is not demonstrated, the current trend to maximize the power on a small footprint area (<1 cm²) is to develop micro-supercapacitors with three-dimensional (3D) architectures (2, 3).
For supercapacitors, carbon-based materials (4, 5), conducting polymer (6) and metal oxides (7, 8) are the most widely studied materials. Among metal oxides, ruthenium oxide (RuO2) has high metallic conductivity and the highest specific capacitance (9), but its cost force to investigate other pseudocapacitive materials. Although manganese dioxide (MnO2) capacitance is not as high as RuO2, its cost effectiveness and its capacitance above 300 F/g reported for thin films produced by electrodeposition (10, 11) make it an attractive material in order to build a 3D microsupercapacitor : electrodeposition is a suitable technique to make a conformal deposit on a 3D substrate.
An attractive approach is to generate a 3D array of microstructures (walls, trenches, pillars) by a Deep Reactive-Ion Etching (DRIE) of a silicon wafer, thus having much more surface area per footprint than a classical 2D-substrate. In this study, we designed an original 3D array of high aspect ratio and dense silicon micropillars and microtubes (figure 1). This topology have several advantages: microstructures, are more robust than nanostructures, thus longer straight structures can be produced without sticking. The silicon substrate is firstly coated by atomic layer deposition of a platinum (Pt) layer which acts as a current collector, and then MnO2 conformal deposition is reached by pulsed electrodeposition means. In the present study, we will focus on one MnO2 electrode which will be investigated in standard 3 electrodes cell configuration (liquid electrolyte) in order to validate the use and advantages of the designed 3D topology: significant improvement of the MnO2 surface capacitance is really obtained as shown in figure 2. Nevertheless, the aim of our group is to produce by microfabrication processes a complete micro-supercapacitor with 3D interdigitated electrodes. A mixing between material science and microelectronics facilities should open the road to the fabrication of a 3D micro-supercapacitor based on MnO2 electrodes with interdigitated structures as shown in figure 3.

Acknowledgments: The authors want to thank the French network of the electrochemical energy storage (RS2E) for this support. This research is financially supported by the ANR and the DGA within the MECANANO project (ANR-12-ASTR-0032-01). The French RENATECH network and the CPER CIA are greatly acknowledged.
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