Monitoring CO2 flux is critical, particularly in the ocean, which absorbs approximatly 30% of the yearly CO2 global carbon emissions. Despite its significance, a major knowledge gap remains regarding carbon transfer at the ocean surface due to the ocean’s vast scale, limiting researchers’ ability to identify optimal carbon sink locations for effective carbon sequestration. While satellite-based imaging has been employed to monitor CO2 flux over large oceanic areas, it suffers from low resolution and limited in accuracy. Additionally, carbon flux can vary by up to 177% on any given day due to dynamic environmental conditions, underscoring the need for real-time monitoring—something satellite imaging cannot reliably provide. To improve the spatial resolution and accuracy of CO2 flux measurements, deploying a large-scale, long-term sensor network is essential. These sensors must operate with ultra-low power consumption to enable hourly monitoring over extended periods—ideally at least one year—without frequent energy resupply, which is logistically challenging for widespread ocean deployment. However, current in-situ CO2 sensors, such as photoacoustic or non-dispersive infrared (NDIR) types, are unsuitable for long-term ocean use due to high power demands and poor performance in high-humidity environments.
To overcome these limitations, a bubble-based sensor offers a promising alternative by combining high accuracy with low power consumption for long-term, real-time monitoring. These sensors have demonstrated low detection limits for various gas analytes and provide rapid response times due to their small sample volumes. However, many traditional bubble-based gas sensors rely on active gas flow to generate bubbles, making them impractical for in-situ environmental sensing. However, this challenge can be addressed utilizing electrolytic-based bubble production, which produces hydrogen and oxygen bubbles by decomposing water molecules that bubbles serve as a reference in which target CO2 gas can diffuse into over time.
This reference bubble expands in size proportionally overtime due to the CO2 concentration within the water, which is attributed to CO2’s high solubility in water, leading to a greater concentration of dissolved CO2 compared to other gases as described by Henry’s law. Furthermore, CO2 possesses a relatively high diffusion coefficient, resulting in a faster mass transfer rate from liquid to bubble, as described by Epstein-Plesset’s equation. When analyzing the bubble growth for typical oceanic gases, it is evident that CO2 plays a dominant role, contributing approximately 93% of the total growth compared to other gases.
By harnessing the low-power electrolytic process into a miniaturized sensor, our uniquly developed bubble-based sensors enables a novel approach for in-situ, energy-efficient CO2 monitoring—ideal for tracking oceanic carbon flux over an extended periods.
