Direct-write planar microultracapacitors by laser engineering

We have successfully employed laser direct write and micromachining to fabricate high capacity hydrous ruthenium oxide (RuO_xH_y or RuO₂ · xH₂O) microultracapacitors. A laser direct-write process is used to deposit uniform pads of RuO₂ · 0.5H₂O in sulfuric acid under ambient temperature and atmospheric conditions. Ultraviolet laser micromachining is used to tailor the shape and size of the deposited material into planar electrodes. The specific capacitance of the laser-deposited materials is comparable to reported values of ∼720 F/g. The microultracapacitors demonstrate linear charge and discharge behavior at currents below 1 mA, as expected for an ideal capacitor. By studying the charge storage and power output as a function of discharge current, the power can be successfully modeled assuming only simple ohmic losses. Parallel and series combinations of these microultracapacitor cells provide the expected addition of capacitance. Maximum discharge currents of 50 mA are applied to two cells in parallel without damage to the microultracapacitor cells. The microultracapacitors exhibit high specific power and specific energy with over 1100 mW/g at approximately 9 mWhr/g for an 80 μg cell with a footprint of 2 mm² and a thickness of 15 μm.