Periodic Reporting for period 4 - EnTER (Enhanced Mass Transport in Electrochemical Systems for Renewable Fuels and Clean Water)
Reporting period: 2024-08-01 to 2025-01-31
The current energy transition, moving away from fossil fuel sources to renewable energy and sustainable products, is a huge societal and technological challenge for the next decades. To accelerate this transition, the largest need is for developing technologies to convert renewable electricity into useful products, such as green hydrogen, hydrocarbons, chemicals and clean water. Electrochemical technologies can leverage the renewable electricity, and make useful products, but the production rate of these technologies is far too low, making them not competitive to fossil fuel routes. The overarching problem in these electrochemical technologies is that species (e.g. CO2, water, ions) move too slow to and from an electrode. To force a breakthrough, this project aims to enhance the rate of electrochemical conversions via improving the mass transport of species, i.e. increasing the exchange of species between the electrode and electrolyte. In this project, we have studied novel routes, such as playing with pressure waves, making fluidized electrodes, and studying the transport via optical microscopy techniques. This aims to provide design rules for doubling the process rate (i.e. current density) , which will unlock the potential of electrochemical conversions such as CO2 electrolysis, water electrolysis and electrochemical water desalination. The overall objective of this project was to eliminate barriers for transport of reactants and products to/from the electrode or membrane surface. This is translated in three research directions:
1) To what extent can gas bubbles improve convective mixing instead of causing detrimental surface coverage?
2) What is the effectiveness of novel electrode structures that induce convective mixing and enlarging electrode area in terms of energy efficiency and electrical current density?
And 3) What can we learn from microfluidic reactor and membrane engineering to enhance mixing in electrochemical cells?
1) To what extent can gas bubbles improve convective mixing instead of causing detrimental surface coverage?
2) What is the effectiveness of novel electrode structures that induce convective mixing and enlarging electrode area in terms of energy efficiency and electrical current density?
And 3) What can we learn from microfluidic reactor and membrane engineering to enhance mixing in electrochemical cells?
As a result of the project, we found that gas bubbles in porous electrodes, particularly in foams operated in a flow-by configuration, are detrimental to the energy efficiency. These stagnant bubbles can be removed via a pressure swing: when suddenly lowering the pressure for a short period of time (~1 sec), the gas bubbles expand and buoyancy forces increase. When returning to the high pressure operation, the few remaining gas bubbles shrink, and the remaining gas bubble fraction is mimized. This pressure swing should be operated periodically (every 5-100 seconds). More results can be read at https://doi.org/10.1016/j.ijhydene.2024.01.147(opens in new window). Furthermore, we also discovered that bubbles can be advantageous; fast pressure pulses (50 Hz) can triple the mass transport due to bubble vibrations. We discovered this by using a simple pump from an espresso coffee machine, which produces a 50 Hz pulsating flow. This work is under review, and available as preview via the PhD thesis of one of the researchers in the EnTER project, Jorrit Bleeker (https://research.tudelft.nl/en/publications/mass-transport-in-gas-evolving-electrolysers(opens in new window)). We conclude that stagnant bubbles are causing a problem, and might be removed via pressure swings, while moving bubbles (via vibrations or convection) can also be beneficial due to the additional mass transport.
As an alternative direction for enhanced mass transport, we studied the use of carbon slurries and 3D electrodes. This idea behind this is: if it's difficult to bring the reactant to the electrode, why not bring the electrode to the reactant? These carbon slurries, even though their surface area is massive, appeared to not work out well for CO2 electrolysis and neither for H2O2 production. This is partly due to the mismatch in ionic conductivity and carbon particle conductivity, and partly due to parasitic reactions as activated carbon. We have explained these findings in three works, available at https://doi.org/10.1039/D3YA00611E(opens in new window) https://doi.org/10.1021/acssuschemeng.4c03919(opens in new window) and one manuscript under review (available via the PhD thesis of one of the researchers in the EnTER project, Nathalie Ligthart, https://research.tudelft.nl/en/publications/volume-based-electrodes-for-enhancing-limiting-currents-in-electr(opens in new window)).
Reactor geometries for CO2 conversion are studied via simulations. This revealed that concentration gradients along the flow direction are relevant, and do not scale linearly (https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.2c06129(opens in new window)). We also demonstrated that the heat produced in CO2 electrolysis is an underexposed bottleneck (https://pubs.rsc.org/en/content/articlehtml/2025/ey/d4ey00190g(opens in new window)) which led to a new research project and a perspective paper in Nature Energy beyond this project (https://www.nature.com/articles/s41560-025-01745-5(opens in new window)). We also experimentally tested membranes with microchannels (https://pubs.rsc.org/en/content/articlehtml/2022/se/d2se00858k(opens in new window)) and gas diffusion electrodes for CO2 reduction reactors (https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.2c00195 , https://pubs.acs.org/doi/abs/10.1021/acsaem.2c02783(opens in new window)).
Finally, for deeper understanding of the mass transport, we used the fluorescence lifetime image microscopy (FLIM), which shows the local concentrations via the concentration-dependent decay in fluoresence lifetime of a newly developed dye. This mapping of local concentrations near electrodes demonstrates the mass transport enhancement. We conclude that this FLIM tool gives a unique insight in the local conditions in electrochemical systems, and has generated knowledge in three papers (https://pubs.acs.org/doi/abs/10.1021/acssensors.3c00316 , https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.3c01773 , https://www.sciencedirect.com/science/article/pii/S1385894725012793 ).
As an alternative direction for enhanced mass transport, we studied the use of carbon slurries and 3D electrodes. This idea behind this is: if it's difficult to bring the reactant to the electrode, why not bring the electrode to the reactant? These carbon slurries, even though their surface area is massive, appeared to not work out well for CO2 electrolysis and neither for H2O2 production. This is partly due to the mismatch in ionic conductivity and carbon particle conductivity, and partly due to parasitic reactions as activated carbon. We have explained these findings in three works, available at https://doi.org/10.1039/D3YA00611E(opens in new window) https://doi.org/10.1021/acssuschemeng.4c03919(opens in new window) and one manuscript under review (available via the PhD thesis of one of the researchers in the EnTER project, Nathalie Ligthart, https://research.tudelft.nl/en/publications/volume-based-electrodes-for-enhancing-limiting-currents-in-electr(opens in new window)).
Reactor geometries for CO2 conversion are studied via simulations. This revealed that concentration gradients along the flow direction are relevant, and do not scale linearly (https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.2c06129(opens in new window)). We also demonstrated that the heat produced in CO2 electrolysis is an underexposed bottleneck (https://pubs.rsc.org/en/content/articlehtml/2025/ey/d4ey00190g(opens in new window)) which led to a new research project and a perspective paper in Nature Energy beyond this project (https://www.nature.com/articles/s41560-025-01745-5(opens in new window)). We also experimentally tested membranes with microchannels (https://pubs.rsc.org/en/content/articlehtml/2022/se/d2se00858k(opens in new window)) and gas diffusion electrodes for CO2 reduction reactors (https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.2c00195 , https://pubs.acs.org/doi/abs/10.1021/acsaem.2c02783(opens in new window)).
Finally, for deeper understanding of the mass transport, we used the fluorescence lifetime image microscopy (FLIM), which shows the local concentrations via the concentration-dependent decay in fluoresence lifetime of a newly developed dye. This mapping of local concentrations near electrodes demonstrates the mass transport enhancement. We conclude that this FLIM tool gives a unique insight in the local conditions in electrochemical systems, and has generated knowledge in three papers (https://pubs.acs.org/doi/abs/10.1021/acssensors.3c00316 , https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.3c01773 , https://www.sciencedirect.com/science/article/pii/S1385894725012793 ).
The use of dynamic pressures in electrochemical cells addresses the bubble removal in 2 ways: 1) a short pulse of low pressure makes the gas bubbles expand, thereby releasing from the electrode, and 2) when applying very fast pressure pulses (vibrations), small bubbles attached to the surface will induce mixing of reactants and products near the electrode. In this way, the bubbles are not only the root of the problem, but are also used to our benefit. This can be applied in multiphase electrochemical systems, such as electrolysis. The slurry electrodes or 3D electrodes can also alleviate mass transport limitations, either for multiphase systems or single-phase systems. We found that both CO2 electrolysis and H2O2 production benefit from this concept, as they can finally use the full volume of the electrochemical reactor, bringing the concept closer to homogeneous reactions, and thus avoiding a 1D-diffusion limiting. In principle, also systems such as fuel cells and flow batteries could benefit from this concept. This all has to go in parallel with designing reactor components at cell-scale and system-scale, which can benefit from fundamental process technology and fluid dynamics in the form of static mixers and staged feeding.