Asymmetric Microelectrodes for Nanoliter Bubble Generation via Electrolysis

Microfabricated devices for gas bubble generation via water electrolysis typically employ two electrodes of identical size ( $i$ . $e$ . interdigitated electrodes or microelectrode pairs). Contrary to this practice, asymmetrically sized electrode pairs offer distinct advantages in the formation of single, trapped nanoliter-volume gas bubbles in microfluidic devices. Although such principles may apply generally to MEMS or micro-scale water electrolysis devices, our model system consisted of a Parylene C microfluidic structure to contain the bubble and embedded thin-film Pt electrodes immersed in $1\times $ phosphate buffered saline solution. The geometric area of one microelectrode was constant (working electrode) while that of the second electrode (counter electrode) was varied over five orders of magnitude. Although increased counter electrode size increases overall device size, it provides improvements in various performance metrics. The largest tested counter electrode size resulted in 41% energy savings per bubble, improved bubble volume uniformity, enabled successful discrimination between O₂ versus H₂ gas generation, and achieved bubble stability exceeding 500 minutes (a $25\times $ improvement over previous work). The underlying theory motivating the use of asymmetric electrodes is detailed and followed by experimental demonstration. Finally, a discussion on tradeoffs to assist in the design of MEMS-based electrolytic bubble generation devices is provided. [2021-0172]