Question

I am using a PEM cell to generate H2 (eventually D2). The PEM cell is fed water on the cathode (O2 side) only, but it transports about 3.5x as much water over to the cathode (H2 side) as it actually consumes in electrolysis.

Ultimately I want to have dry gas and want to minimize wastage since heavy water to get D2 is expensive. The equations for PEM electrolysis suggest that there is no liquid water transported, so I could dry it with desiccant. Actual case seems like I need to separate water from H2 output and recycle it. I only need a small volume so I am looking for simplicity.

Is this the nature of PEM cells or do I have the wrong kind of cell?

Answer 1

PEM electrolysis cells often transport more water to the cathode than consumed, due to osmotic effects. For drying hydrogen gas, especially when generated from heavy water, it is beneficial to separate excess water before drying with a desiccant to minimize wastage.

Step-by-step explanation:

Operation of PEM Electrolysis Cells

In a Proton Exchange Membrane (PEM) electrolysis cell, water electrolysis results in the production of hydrogen (H2) at the cathode and oxygen (O2) at the anode. Due to the nature of the PEM cell, water transport from anode to cathode is typical.

This is often due to osmotic drag and electro-osmotic drag within the membrane, which effectively means more water is transported than what is consumed in the electrolysis reaction itself, despite the theoretical predictions suggesting otherwise. Therefore, particularly when using expensive heavy water (D2O) to produce deuterium (D2), it is essential to separate the excess water from the hydrogen gas output and recycle it to minimize waste.

To achieve dry hydrogen gas, apart from mechanical separation, one might consider using a desiccant system to remove moisture from the hydrogen stream. However, due to the actual transport of liquid water across the PEM, this may not be sufficient for complete drying or for economic operation when using heavy water as the source. Hence, a combination of gas-liquid separation followed by drying with a desiccant would likely be the more effective approach.