EPFL team develops effective membrane-less electrolysis process for H2 production; potential to outperform conventional designs
Researchers at EPFL in Switzerland have developed a system for producing hydrogen through a simplified membrane-less water electrolysis process. By working with the balance between fluid mechanic forces, the researchers eliminated the expensive membrane that sits between the electrodes in conventional electrolysis systems.
The membrane-less approach demonstrates for the first time an electrolyzer capable of operating robustly and continuously with various catalysts and electrolytes across the pH scale, while at the same time generating hydrogen gas streams the oxygen content of which is well below the safety limit. An open access paper on their discovery is published in the RSC journal Energy and Environmental Science.
In a conventional system, two electrodes are submerged in water and separated by a polymer membrane. An electric current is sent through one of the electrodes (the cathode) and then travels to the other (the anode). The current, with the help of a catalyst, causes the water molecules to break apart into oxygen and hydrogen. To prevent the two gases from mixing together and making an explosive mixture, polymer membranes are implemented between the catalysts to keep the gases separated.
In both research and industry, membranes used for ionic conductivity are most commonly made of Nafion, due to its great stability and ion conductivity. However, they are expensive, have a limited lifetime and only work in highly acidic solutions, which limits the choice of catalysts.
To rid themselves of these constraints, the scientists placed the electrodes less than a few hundreds micrometres apart in a microfluidic device. When the liquid moves above a certain speed between the electrodes, the gases are pushed in opposite directions—due to the lift forces caused by an effect known as Segré-Silberberg effect—without the need for a membrane to guide them into separate outlets.
Our device has the potential to surpass the performance of a similar water-splitting apparatus that relies on an ion conductive membrane. This is due to the higher ion conduction in liquid electrolytes than in common solid membranes.—Mohammad Hashemi, first author
The devices presented in the paper split water at current densities above 300 mA cm−2, with more than 42% power conversion efficiency. Crossover of hydrogen gas into the oxidation side was as low as 0.4%, leading to a non-flammable and continuous hydrogen fuel stream.
Additionally, the ability to use buffered electrolytes allows for the incorporation of earth-abundant catalysts that can only operate at moderate to high pH values.
The membrane-less electrolyzer demonstrated here has the ability to produce non-flammable hydrogen streams, continuously and stably across the pH scale. Comparing the device ohmic resistance with that of Nafion-based devices, it is clear that this device has the potential to surpass the performance of a similar water splitting apparatus that rely on ion conductive membranes for separation. Although a single electrode pair, such as the one in this proof of concept study, can only produce a limited amount of fuel, scaling it out can be achieved on multi-stack panels for enhanced throughput49 or the implementation of large area electrodes.
As the only dimension to be kept small is the inter-electrode distance, follow up studies are underway to develop high throughput devices where high surface area planar electrodes are used as the side walls of narrow electrolyte channels. Additionally, it is worth noting that this electrolyzer platform may be used in reverse as a fuel cell with two streams of the electrolyte, each saturated with H2 or O2, allowing for the production of electricity. The design simplicity of this membrane-less electrolyzer can facilitate mass production, especially by employing high resolution 3D printers or injection moulding techniques.—Hashemi et al.
This design sets the stage for the production of devices that work with all types of liquid electrolytes (containing ions) or catalysts, since there is no longer the risk of damaging the components due to a highly acidic environment. This versatility is not possible in conventional systems, in which only catalysts made of noble metals such as platinum can work with the low pH values imposed by the membrane.
The same team is now working on scaling up the design for higher production rates. As the only dimension that needs to remain small is the inter-electrodes distance, it is possible to implement the same concept using high surface area electrodes as side walls of narrow electrolyte channels.
S. Mohammad H. Hashemi, Miguel A. Modestino and Demetri Psaltis (2015) “A membrane-less electrolyzer for hydrogen production across the pH scale” Energy Environ. Sci. doi: 10.1039/C5EE00083A
Segre G, Silberberg A (1961) “Radial particle displacements in Poiseuille ﬂow of suspensions”. Nature 189: 209–210 doi: 10.1038/189209a0