Study finds the wettability of porous electrode surfaces is key to making efficient water-splitting or carbon-capturing systems
As water-splitting technologies improve, often using porous electrode materials to provide greater surface areas for electrochemical reactions, their efficiency is often limited by the formation of bubbles that can block or clog the reactive surfaces. Now, a study at MIT has for the first time analyzed and quantified how bubbles form on these porous electrodes.
Bubble growth and departure are ubiquitous phenomena in gas-evolving reactions, which govern the overall energy and mass transport. However, an in-depth understanding of the relationship between bubble dynamics and the electrochemical processes, in particular, the wettability effect on a gas-evolving porous electrode remains elusive. Here, we report the bubble dynamics and overpotential observed during alkaline water splitting on a polytetrafluoroethylene (PTFE) deposited nickel porous electrode. A slight decrease in hydrophilicity induced a drastic transition of bubble dynamics and a significant increase of the transport overpotential.
We show that the porous electrode transitioned from a liquid-filled state to a gas-filled state when varying the wettability, which changed the bubble departure sizes and bubble coverage. As a result, there were substantial changes of the transport overpotential. Our work elucidates the fundamental relationship between wettability and water splitting characteristics, which provides a practical scenario for structuring the electrode for gas-evolving reactions.—Iwata et al.
The researchers found that there are three different ways bubbles can form on and depart from the surface, and that these can be precisely controlled by adjusting the composition and surface treatment of the electrodes. The work is described in the journal Joule, in a paper by MIT visiting scholar Ryuichi Iwata, graduate student Lenan Zhang, professors Evelyn Wang and Betar Gallant, and three others.
This image shows the interplay among electrode wettability, porous structure, and overpotential. With the decrease of wettability (moving left to right), the gas-evolving electrode transitions from an internal growth and departure mode to a gas-filled mode, associated with a drastic change of bubble behaviors and significant increase of overpotential. Credits: Courtesy of the researchers
Because the reaction constantly produces gas within a liquid medium, the gas forms bubbles that can temporarily block the active electrode surface. Control of the bubbles is a key to realizing a high system performance, says Iwata. Little study had been done on the kinds of porous electrodes that are increasingly being studied for use in such systems.
The team identified three different ways that bubbles can form and release from the surface:
In one, dubbed internal growth and departure, the bubbles are tiny relative to the size of the pores in the electrode. In that case, bubbles float away freely and the surface remains relatively clear, promoting the reaction process.
In another regime, the bubbles are larger than the pores, so they tend to get stuck and clog the openings, significantly curtailing the reaction.
In a third, intermediate regime, called wicking, the bubbles are of medium size and are still partly blocked, but manage to seep out through capillary action.
The team found that the crucial variable in determining which of these regimes takes place is the wettability of the porous surface. This quality, which determines whether water spreads out evenly across the surface or beads up into droplets, can be controlled by adjusting the coating applied to the surface. The team used a polymer called PTFE, and the more of it they sputtered onto the electrode surface, the more hydrophobic it became. It also became more resistant to blockage by larger bubbles.
New experiments showed that the wettability of the surface makes a big difference in the way bubbles form and leave the surface. On the left, a higher-wettability porous surface leads to small bubbles that leave quickly, while lower wettability, right, leads to bigger bubbles that clog the material's pores and reduce efficiency.
The transition is quite abrupt, Zhang says, so even a small change in wettability, brought about by a small change in the surface coating’s coverage, can significantly alter the system’s performance. Through this finding, he says, “we’ve added a new design parameter, which is the ratio of the bubble departure diameter [the size it reaches before separating from the surface] and the pore size. This is a new indicator for the effectiveness of a porous electrode.”
Pore size can be controlled through the way the porous electrodes are made, and the wettability can be controlled precisely through the added coating. Manipulating of these two effects will enable the precise control of these design parameters to ensure that the porous medium is operated under the optimal conditions, Zhang says. This will provide materials designers with a set of parameters to help guide their selection of chemical compounds, manufacturing methods and surface treatments or coatings in order to provide the best performance for a specific application.
While the group’s experiments focused on the water-splitting process, the results should be applicable to virtually any gas-evolving electrochemical reaction, the team says, including reactions used to electrochemically convert captured carbon dioxide, for example from power plant emissions.
What’s really exciting is that as the technology of water splitting continues to develop, the field’s focus is expanding beyond designing catalyst materials to engineering mass transport, to the point where this technology is poised to be able to scale. Now that we’re starting to really push the limits of gas evolution rates with good catalysts, we can’t ignore the bubbles that are being evolved anymore, which is a good sign.—Beta Gallant
The MIT team also included Kyle Wilke, Shuai Gong, and Mingfu He. The work was supported by Toyota Central R&D Labs, the Singapore-MIT Alliance for Research and Technology (SMART), the US-Egypt Science and Technology Joint Fund, and the Natural Science Foundation of China.
Article Ryuichi Iwata, Lenan Zhang, Kyle L. Wilke, Shuai Gong, Mingfu He, Betar M. Gallant, Evelyn N. Wang (2021) “Bubble growth and departure modes on wettable/non-wettable porous foams in alkaline water splitting” Joule doi: 10.1016/j.joule.2021.02.015