[Paper Review] Coalescence induced late departure of bubbles improves water electrolysis efficiency

Tao Wu, Bo Liu+5 more|arXiv (Cornell University)|Feb 9, 2024

Minerals Flotation and Separation TechniquesEnvironmental Science3 citations

TL;DR

This study demonstrates that promoting bubble coalescence in water electrolysis enhances efficiency by over 30% by delaying bubble departure through continuous merging with microbubbles. The process reduces bubble size on the electrode to under 10 μm and induces intense, localized agitation (up to 1 m/s) lasting ~10 ms, significantly improving mass and heat transfer while freeing active catalytic sites.

ABSTRACT

In water electrolysis, bubbles form on the electrode and interact through processes such as collision and coalescence. However, the impact of bubble coalescence a fundamental process governing electrolytic bubble behaviour-on electrolysis efficiency remains unclear. Here, we show that enhancing bubble coalescence improves electrolysis efficiency by more than 30% compared to systems where coalescence is inhibited. One key feature is the continuous coalescence of a newly detached bubble with microbubbles on the electrode, which delays the former from departing. Experimental observations and numerical simulations reveal two key benefits of bubble coalescence for electrolysis efficiency: (1) it liberates surface bubbles from the electrode at much smaller sizes, reducing their diameter from approximately 60-80 um to less than 10 um, thus freeing the active sites of the electrode from bubble coverage; (2) it induces strong agitation, with velocities reaching 1m/s in a small region near the electrode (at a depth of 10-5 m), thereby significantly improving the heat/mass transfer locally. Importantly, the chaotic agitation effect lasts for approximately 10 ms, two orders of magnitude longer than the coalescence process, which occurs in around 0.2 ms. This work provides valuable insight into bubble management in water electrolysis and other gas-evolution electrochemical reactions.

Motivation & Objective

• To investigate the impact of bubble coalescence on electrolysis efficiency in water electrolysis.
• To understand how bubble dynamics, particularly coalescence, influence electrode surface coverage and mass transfer.
• To explore the role of bubble-induced agitation in enhancing local heat and mass transfer near the electrode surface.
• To develop a strategy for improving electrolysis efficiency by engineering bubble coalescence behavior.

Proposed method

• Conducted experiments using controlled electrolysis setups to observe bubble formation, coalescence, and departure dynamics on electrodes.
• Employed high-speed imaging to capture bubble behavior and quantify departure timing and size.
• Performed numerical simulations to model bubble coalescence and fluid flow patterns near the electrode surface.
• Analyzed fluid velocity fields to assess the intensity and duration of agitation induced by coalescence.
• Compared systems with enhanced coalescence versus inhibited coalescence to isolate its effects on efficiency.
• Used surface tension and hydrodynamic forces in simulations to predict bubble detachment behavior.

Experimental results

Research questions

• RQ1 How does bubble coalescence affect the size and timing of bubble departure from the electrode surface in water electrolysis?
• RQ2 What is the impact of coalescence-induced agitation on local heat and mass transfer in the electrolyte near the electrode?
• RQ3 How does coalescence influence the availability of active sites on the electrode by reducing bubble coverage?
• RQ4 What is the duration and intensity of the fluid agitation triggered by bubble coalescence?
• RQ5 To what extent does promoting coalescence improve overall electrolysis efficiency compared to systems where coalescence is suppressed?

Key findings

• Enhancing bubble coalescence increased electrolysis efficiency by more than 30% compared to systems where coalescence was inhibited.
• Bubble coalescence reduced the diameter of surface bubbles from approximately 60–80 μm to less than 10 μm, significantly decreasing electrode coverage.
• Coalescence-induced agitation reached velocities of up to 1 m/s in a region 10–5 m deep near the electrode surface.
• The chaotic agitation effect persisted for approximately 10 ms, which is two orders of magnitude longer than the coalescence process itself (~0.2 ms).
• The delayed departure of bubbles due to coalescence freed active catalytic sites on the electrode more effectively, improving reaction kinetics.
• Numerical simulations confirmed that coalescence enhances local mass transfer by generating transient, high-velocity fluid flows.