Inside most electrolyzers sits a thin, expensive, finicky component: a membrane that lets ions through while keeping the hydrogen and oxygen gases on opposite sides. It is essential and it is a cost-and-durability headache. A 2021 EPFL patent asks a provocative question — what if you skipped it? — and answers it not with chemistry but with fluid dynamics.

First, why the membrane exists. When you split water, hydrogen forms at one electrode and oxygen at the other, and you absolutely do not want them mixing — hydrogen and oxygen together is an explosive combination. The membrane is the referee that keeps them apart while still letting the charge-carrying ions cross to complete the circuit. It also tends to be the part that degrades and the part that costs the most.

“The present invention concerns a membrane-less electrolyzer comprising a fluidic channel for receiving an electrolyte fluid; a first electrode and a second electrode located inside the fluidic channel, the first and second electrode permitting to extract a first gas and a second gas inside the fluid…”— U.S. Patent No. 10,907,262 source

EPFL's grant US10907262B2, “Membrane-less electrolyzer,” replaces that physical barrier with a cleverer use of how fluids flow. The patent first names what removing the membrane buys you: “Membrane-less electrolysis allows for the operation of devices at any pH, reduces the complexity by the elimination of separation membranes and decreases the ionic resistance thanks to the higher ion-mobility in liquid electrolytes.” A membrane is not just a cost; it is also an obstacle that ions have to push through, so taking it out can actually lower the resistance of the cell.

The hard part is keeping the gases apart without the referee. The patent's answer is to manage the bubbles with flow. As it states, “to mitigate gas crossover in the membrane-less scheme… a fluid dynamic approach is implemented in order to control the position and trajectory of gas bubbles as they evolve from electrodes.” The two electrodes sit side by side in a single channel of flowing electrolyte; hydrogen bubbles peel off one, oxygen off the other, and the liquid's motion is engineered to sweep each stream toward its own wall before they can intermingle.

The mechanism that does the sweeping is described in physical terms. The electrolyte does not flow at uniform speed across the channel — it is fastest in the middle and slower near the walls, and that shape matters: “the hyperbolic profile of the electrolyte flow velocity creates an inertial force in the cross flow direction pointing towards the fluidic channel walls. This force keeps each stream of gas bubbles close to the corresponding wall and therefore, unmixed until they go into opposite directions to outlets, via for example, the T-junction branches and get separated.” The geometry in claim 1 backs this up: two elongated electrodes running side by side, two outlets facing in different directions, and a tightly specified electrode gap — a separation distance “in a range of 5 mm>d≥50 µm.” Close, but not touching.

The most important design knob, though, is speed, and the patent is explicit that crossover is a race the operator can win by pushing harder. “The predominant crossover mechanism is diffusion, whose magnitude can be controlled through minimization of the residence time of bubbles in the device via increasing flow rate of the electrolyte.” The device is built to exploit exactly that: it can “modify the flow rate of the electrolyte fluid to increase the flow rate and set it at a value that minimizes or eliminates crossover of the gases via diffusion.” In plain terms: pump the liquid fast enough and the bubbles are flushed out before they have time to wander across to the wrong side. Purity becomes a flow-rate setting rather than a property of a material.

The electrode geometry is doing quiet work here too. The claim requires “continuous elongated electrodes extending both separately and side-by-side” with a “continuously exposed electrolysis surface,” and it puts the two outlets “facing” one another so the gas-laden streams leave in different directions. That arrangement is what turns a single shared channel into two effectively separate exits without a wall down the middle: each electrode's bubbles ride its own wall to its own outlet. The tight inter-electrode gap — under five millimeters but at least fifty microns — keeps the ionic path short (which is part of why resistance drops) while still leaving room for the flow field to do its sorting. Every dimension in the claim is in service of the same goal: let hydrodynamics, not a membrane, keep hydrogen and oxygen on their own sides.

The appeal is straightforward cost and durability. Strip out the most expensive, most failure-prone component and an electrolyzer gets cheaper to build and potentially longer-lived, while gaining the pH flexibility and lower ionic resistance the patent describes. For green hydrogen to compete, every dollar per kilowatt of capacity matters, and the membrane is a fat target.

The honest counterweight is in the physics the patent itself lays out: the design trades a hard material problem for a hard engineering problem. Separation now depends on holding the right flow profile and a high enough flow rate across the whole device, at scale, over a range of conditions — and diffusion never goes fully to zero, it only shrinks as residence time shrinks. Run the pump too slow, or let the channel geometry drift, and the safety margin erodes. This 2021 grant is a claim on an approach and a specific bubble-steering mechanism, not evidence that it has displaced membrane electrolyzers in the field. But it is exactly the kind of from-first-principles rethink — replacing a material with a flow field — worth tracking as the industry hunts for cost-out.