Additive manufacturing-derived 3D pyrolytic carbon electrodes for on-chip microsupercapacitors

Wearable and portable microelectronics coupled with the internet of things (IOT) are revolutionizing the world around us. They are playing a crucial role in advancing industries ranging from healthcare to all smart devices that are shaping our future. Micro energy storage systems are one of the fundamental components needed to power these autonomous microdevices. Traditional energy storage systems primarily rely on microbatteries because of their mature technology and high energy density. However, sluggish charge/discharge characteristics and short lifespan of microbatteries are major concerns of their application in microdevices. Moreover, the use of Li and other heavy metals in battery technology contributes to e-waste, which constitutes environmental and disposal challenges.

Microsupercapacitors (μSC) are alternative sustainable on-chip energy storage systems to power microdevices, due to their fast/discharge characteristics providing quick bursts of power and reducing downtime. Additionally, they have longer lifespan, which makes them more durable and reliable than microbatteries and suitable for powering microdevices that requires frequent cycling. Despite all these advantages, μSC are not commercialized yet because of several factors, which includes limited energy density and lack of an efficient, cost effective and streamlined fabrication process. Unlike macro-supercapacitors, the footprint area available for on-chip μSC is less, which limits the active material loading on the electrodes and hence the energy density. Fabricating hierarchical 3D electrodes is a promising solution to address this problem.

The main goal of this thesis was to fabricate 3D on-chip μSC based on hierarchical, high aspect ratio 3D carbon microelectrodes (3DCμE) for enhancing area specific capacitance and energy density. This was successfully achieved by developing a novel and viable fabrication method of combing high resolution on-chip additive manufacturing (AM) and pyrolysis. The high degree of freedom to fabricate 3D hierarchical structures by a high resolution AM method is an alternative for complex and time consuming processes of photolithography and etching.

One of the main challenges addressed in the research was ensuring the integrity and adhesion of the 3D printed polymer to the Si substrate during and after pyrolysis, which initially caused delamination and electrode shattering. The solution involved surface modifications of the Si substrate, including the etching of the Si surface to obtain nano-grass known as black silicon (BSi), the application of a bottom anti-reflective coating (BARC), and the use of silanization agents. Adhesion tests concluded that BSi provided the best long-term stability for fabricating reliable on-chip CμE.

The developed fabrication method not only allows for the fabrication of complex 3D carbon structures but also maintains high resolution without compromising the 3D printer capabilities. This innovation opens up new possibilities for microfabrication in various electrochemical applications, including energy storage.

The thesis specifically explored energy storage applications by developing two configurations of 3D on-chip μSC: Interdigitated microsupercapacitors (IDμSC) and sandwich microsupercapacitors (SWμSC). The IDμSC, with 3D CμE and polypyrrole deposition, exhibited the highest area specific capacitance of 135 ± 5 mF/cm² and energy density of 12.5 ± 1 μWh/cm²) at 0.5 mA/cm² in an aqueous electrolyte of 1M H2SO4. The same electrodes were also used for developing a quasi-solid state μSC with ultraviolet light curable polyethylene glycol diacrylate based gel polymer electrolyte. The quasi- solid state μSC exhibited an area specific capacitance of 125 ± 5 mF/cm2 and energy density of 11 ± 1 μWh/cm². The other configuration of SWμSC, using 3D CμE with reduced graphene oxide (rGO) and an organic electrolyte, achieved a lower area capacitance of 20 mF/cm2 but a higher energy density of 20.5 μWh/cm² and power density of 1.35 mW/cm² at 1 mA/cm².

Finally, the thesis also explored the possibility to tailor the graphitic and electrical properties of on-chip CμE. This was addressed by catalyzed pyrolysis of the 3D print polymer, using an optimized 0.8 wt.% Fe catalyst concentration in the 3D print resin. This approach enhanced the graphitic content and surface area, leading to superior supercapacitive properties. The optimized electrodes achieved the highest reported area-specific capacitances of 42.5 and 34 mF/cm² at 0.2 and 0.5 mA/cm² receptively for bare on-chip PyrC electrodes, and this value was further increased to 66.5 mF/cm² at 0.5 mA/cm² with 3D structures.