UC Berkeley

This study describes the generation of electrolytic hydrogen bubbles in a recently proposed artificial-photosynthetic device. Bubble growth and the resulting size distributions are important inputs for predictive tools that model a device's performance factors, such as light scatter, electrolyte resistance, and volumetric gas collection. Motivated by efforts to model the performance of a micropillar-based artificial-photosynthetic device, hydrogen bubbles are produced electrochemically on a single electrode pillar. High-speed visualization and bubble-tracking algorithms are used to track the growth and record the bubble-size distributions.

The study of bubble growth defines four different modes of growth after the onset of nucleation: 1) isolated growth, 2) proximity growth, 3) surrounded growth, and 4) trailing growth. As predicted by theory, bubble diameters grow proportionally with the square root of time. Experimental results suggest that this proportionality is dependent on the mode of growth. Bubbles undergoing surrounded growth show the fastest growth rates. And when present, bubbles in trailing growth show the slowest growth rate. The former is due primarily to local microconvection currents replenishing dissolved gas at the bubble interface, and the latter is due to bubbles competing for the same gas supply. Additionally, the growth rates are observed to increase proportionally with the square root of current density as well. A formal dimensional analysis is carried out to derive the rational dependence of bubble diameters on both time, current density, and growth mode.

The study of bubble-size distribution examines the final sizes of bubbles after detachment. The size distributions of these bubbles resemble log-normal distributions, which are consistent with observations cited in literature. Experimental results reveal that most of the distributions are bimodal, where a secondary distribution of bubbles results primarily from bubble coalescence prior to detachment. Correlations are developed between the salient characteristics of the bimodal distributions and the applied current density. These correlations predict the size distributions for bubbles generated within the target current-density range of 5 to 15 mA/cm^2.

Lastly, a collection efficiency is defined to quantify how efficient is gas collection in practice. At the highest tested current density of 15 mA/cm^2, the collection efficiency is 68%. The complementary losses are due primarily to unaccounted bubbles remaining on the electrode surfaces. A loose curve fit is used to describe the progressively decreasing slope between points. This correlation provides a target current density to achieve depending on the desired collection efficiency.

These results contribute to both the fundamental-discovery and prototype-development goals behind developing a working artificial-photosynthetic device. Current density is quantified as a fundamental factor in electrolytic bubble growth. And correlations for bubble size distributions and collection efficiencies serve as inputs for modeling device performance.