Study identifies critical bubble size for green hydrogen production

A collaboration between researchers at Cornell and City University of Hong Kong has produced a new computational tool that could help overcome one surprisingly persistent obstacle to the efficient production of so-called green hydrogen: bubbles.

Green hydrogen, produced by splitting water using electricity from renewable sources, offers a promising alternative to fossil fuels, particularly for industries such as steelmaking and chemical manufacturing.

The researchers developed a computer model that reveals how bubbles forming during water electrolysis interfere with the process and increase the energy required to produce hydrogen. Their findings, published Nov. 9 in the journal EES Catalysis, identify a critical bubble size beyond which energy losses increase dramatically, providing a potential roadmap for designing more efficient hydrogen-producing equipment.

The study was led by Lenan Zhang, assistant professor in Cornell's Sibley School of Mechanical and Aerospace Engineering, and Weihong Li, associate professor of mechanical engineering at City University of Hong Kong.

Muhan Zhang of the City University of Hong Kong and Kaiyu Li of Cornell, who contributed equally to the research, are co-first authors.

One challenge to efficient production is that the water electrolysis process creates its own obstacles. As electricity passes through an electrolyzer, hydrogen and oxygen bubbles form on the electrodes where the chemical reactions occur. Those bubbles can cover active reaction sites, prevent liquid from reaching the electrode surface and interfere with the movement of electrically charged particles, forcing the system to consume additional energy.

The problem becomes especially consequential as electrolyzers grow larger and operate at higher current densities to increase hydrogen production.

Scientists have long recognized that bubbles reduce efficiency, and researchers have experimented with modifying electrode shapes, changing liquid flows and altering surface properties to encourage bubbles to detach. What has been missing, Zhang said, is a reliable way to quantify precisely how much energy bubbles waste and to determine when intervention becomes necessary.

That difficulty arises because the physical processes involved span enormous differences in scale, from electrochemical reactions occurring across distances smaller than a nanometer to the centimeter-scale dimensions of an electrolyzer.

"Experimentally it's really challenging to isolate the impact of bubbles on each individual scale," Zhang said.

To address that challenge, the researchers developed a three-dimensional mathematical modeling framework capable of simulating the interconnected processes across eight orders of magnitude in length. By introducing a new method for representing the smallest electrochemical interactions, they reduced the computational effort required by nearly 1,000 times without sacrificing numerical accuracy.

The result resembles a digital twin of an electrolyzer: a virtual device that can reproduce real-world electrode geometries, materials and operating conditions while revealing microscopic processes that conventional experiments cannot easily observe.

Using the model to examine individual hydrogen bubbles forming on an electrode in an alkaline electrolyzer, the researchers discovered that obstructing active electrode surface area was the largest source of bubble-induced energy loss.

More importantly, they identified a critical bubble radius. Once a bubble exceeds that size, the additional voltage required to sustain electrolysis increases exponentially. In certain modeled conditions, the total additional voltage can exceed its bubble-free value by more than 50%.

The critical radius depends on electrode geometry and how readily liquid wets the electrode surface, rather than on operating current density.

That finding suggests a practical design strategy: engineer electrode surfaces so bubbles detach before reaching the critical size. The researchers introduced a new design parameter comparing the radius at which bubbles detach with the critical radius. Their simulations indicate that keeping this ratio below one could limit the additional voltage required due to an individual bubble to within 3%.

The current study focuses on a single bubble, but Zhang and Li are extending the model to investigate interactions among multiple bubbles, which can grow, merge and influence one another. The researchers also envision combining their modeling framework with computational optimization and additive manufacturing to create electrodes designed specifically to minimize energy losses.

Ultimately, the researchers hope to develop their computational framework into broadly accessible software that engineers can use to evaluate materials, electrode structures and operating conditions.

Chris Dawson is communications coordinator for Duffield Engineering.