Industrial electrochemical processes that exploit electrodes to produce fuels and chemical products are hampered by the formation of bubbles that block parts of the electrode surface, reducing the area available for vigorous reaction. This blockage reduces the efficiency of the electrodes by 10 to 25 percent.
But novel research reveals decades-long misunderstandings about the extent of this interference. The findings show exactly how the blocking effect works and could lead to novel ways of designing electrode surfaces to minimize inefficiencies in these widely used electrochemical processes.
It has long been assumed that the entire area of the electrode shaded by each bubble would be effectively inactivated. However, it turns out that a much smaller area – roughly the area where the bubble actually contacts the surface – is blocked from its electrochemical activity. Up-to-date insights can directly lead to novel ways of modeling surfaces to minimize contact area and improve overall performance.
The arrangements are in place which was reported in the magazine today in a paper by recent MIT alumnus Jack Lake PhD ’23, graduate student Simon Rufer, mechanical engineering professor Kripa Varanasi, research scientist Ben Blaiszik, and six others from the University of Chicago and Argonne National Laboratory. The team has released an open-source AI-based software tool that engineers and scientists can now exploit to automatically recognize and quantify bubbles forming on a given surface, a first step toward controlling the properties of the electrode material.
Gas-evolving electrodes, often with catalytic surfaces that promote chemical reactions, are used in a wide variety of processes, including the production of “green” hydrogen without the exploit of fossil fuels, carbon capture processes that can reduce greenhouse gas emissions, aluminum production, and the chlorination process alkaline used in the production of widely used chemical products.
These processes are very common. The chlor-alkali process alone accounts for 2 percent of total U.S. electricity consumption; aluminum production accounts for 3 percent of global electricity; and both carbon capture and hydrogen production are likely to enhance rapidly in the coming years as the world strives to meet greenhouse gas reduction targets. So the novel discoveries could have real significance, says Varanasi.
“Our work shows that engineering the contact and growth of bubbles on electrodes can have a dramatic impact” on how bubbles form and how they leave the surface, he says. “The knowledge that the area beneath the bubbles can be significantly active introduces a new set of principles for designing high-performance electrodes to avoid the harmful effects of bubbles.”
“The broader literature produced over the last few decades suggests that it is not just this small contact area that is passivated, but the entire area under the bulb,” says Rufer. The novel study reveals “a significant difference between the two models because it changes the way the electrode is developed and designed to minimize these losses.”
To test and demonstrate the consequences of this effect, the team created different versions of electrode surfaces with dot patterns that nucleated and trapped bubbles of different sizes and spacing. They were able to show that surfaces with widely spaced dots produce large-sized bubbles, but only in diminutive areas of contact with the surface, which helped explain the difference between expected and actual bubble coverage effects.
For the team’s analysis, it was necessary to develop software to detect and quantify bubble formation, Rufer explains. “We wanted to collect a lot of data and look at a lot of different electrodes, different reactions, and different vesicles, and they all look slightly different,” he says. Creating a program that could handle different materials and lighting and reliably identify and track bubbles was a complex process, and machine learning was key to its success, he says.
Using this tool, he says, they were able to collect “a really significant amount of data about the bubbles on the surface, where they are, how big they are, how fast they’re growing, and so on.” The the tool is now freely available for anyone to exploit via the GitHub repository.
By using this tool to link visual measures of bubble formation and evolution with electrical measurements of electrode performance, the researchers were able to disprove the accepted theory and show that only the area of direct contact is affected. The videos further supported this idea by revealing that novel bubbles were actively evolving directly beneath parts of the larger bubble.
Scientists have developed a very general methodology that can be used to characterize and understand the effect of bubbles on any electrode or catalyst surface. They managed to quantify the effects of bubble passivation using a novel performance metric they called BECSA (bubble-induced electrochemically vigorous surface), as opposed to ECSA (electrochemically vigorous surface), which is used in the field. “We defined the concept of the BECSA metric in a previous study, but until this work, there was no effective method for estimating it,” says Varanasi.
The knowledge that the area beneath the bubbles can be significantly vigorous introduces a novel set of principles for high-performance electrode design. This means that electrode designers should strive to minimize the contact area with the bubbles, not just cover them, which can be achieved by controlling the morphology and chemical composition of the electrodes. Surfaces designed to control bubbles can not only improve overall process efficiency and therefore reduce energy consumption, but can also save on upfront material costs. Many of these gas-evolving electrodes are coated with catalysts made of steep metals such as platinum or iridium, and lessons learned from this work can be used to design electrodes to reduce material loss through reaction-blocking bubbles.
Varanasi says that “insights from this work could inspire new electrode architectures that will not only reduce the use of valuable materials, but also improve the overall efficiency of the electrolyzer,” which would have huge environmental benefits.
The research team included Jim James, Nathan Pruyne, Aristana Scourtas, Marcus Schwarting, Aadit Ambalkar, Ian Foster and Ben Blaiszik from the University of Chicago and Argonne National Laboratory. The work was supported by the US Department of Energy under the ARPA-E program.