Automate electrolysis for hydrogen generation via humidity

Question

I have been having an idea regarding the methodology used for electrolysis.

In practice, for electrolysis to occur at all, there needs to be an electrolyte present. This electrolyte generally would have to be supplied manually over time - I was thinking, what if plain CaCl2 (Calcium Cloride) was placed into the chamber where the electrolyte is supposed to be?

A characteristic of CaCl2 is drawing in moisture thats naturally present in the air, thereby essentially mixing with it to form a liquid, a brine - I think this would facillitate a permanent electrolyte provider, or would it not?

In contemplating regarding that potential I was thinking about how CaCl2 would only split at a specific voltage present in the given built cell. Afaik it takes more voltage to split CaCl2 than it would H2O. If that is true, then that implies, that given the voltage is above the H2O splitting threshhold but below the CaCl2 splitting threshhold, that the CaCl2 would remain completly, consistently drawing in moisture while H2O would get "used up". Is this correct thinking?

The following point I was thinking of then, was how electropotentials of electrodes interplay with this idea. In theory, given two inert electrodes were chosen - like pt (platinum) and au (gold) - could this then facillitate a cell which - not forever - but would work for "quite a while"? (Months? Years? Only the electrodes could "deplete" if deplete is the right word here.. well oxidize)

So essentially, combining these thoughts and questions together, the summarized question would be:

"Can a system be built for the sake of performing electrolysis, where the supply of the electrolyte is static (that being, permanently present) whilst also ensuring that the electrodes practically (ideally completly) remain present / "unchanged" per taking use of their electropotentials (linking the cells in series and parallel where neccessary) - so that only H2O remains continually used?"

-> I think this would make an interesting hydrogen generator, provided a multitude of cells were connected (in series and in parallel) so that the innate electropotentials of the electrodes would sustain the needed voltage / amperage / wattage while the CaCl2 provided the electrolyte. Regardless of lacking efficiency in "production" of Hydrogen (gas), is this realistic? Does this work at all?

I am excited for every answer, please feel free to overload me with information as I wish to learn more.

If possible, do feel free to answer every question, inside of every paragraph. Thanks in advance!

Answer 1

Your overall idea is possible (if just barely feasible), but not using $\ce{CaCl2}$ for the electrolyte: $\ce{Cl2}$ would be evolved, not $\ce{O2}$, and the electrolyte would be rapidly depleted.

However, there are hygroscopic chemicals that might be used, such as $\ce{LiOH}$ or $\ce{H2SO4}$, that would not be decomposed, in preference to water.

That said, there are some complications, such as:

• How much water is in the atmosphere? You can use online calculators such as ProcessSensing's to find that.
• How would the air with water vapor be introduced into the apparatus without losing electrolyzed $\ce{H2}$ and $\ce{O2}$? Hydrogen, in particular, presents some difficulties -- it can quickly penetrate (permeate) most membranes.

On the other hand, it might be an interesting project, as an artistic endeavor, to make a "perpetual electrolyzer", if you can overcome difficulties.

Answer 2

Using some hygroscopic material to catch water vapour from air to perform electrolysis is an active area of research. This hygroscopic material can double as the electrolyte. Here's an article describing just that: "Guo, J., Zhang, Y., Zavabeti, A. et al. Hydrogen production from the air. Nat Commun 13, 5046 (2022). https://doi.org/10.1038/s41467-022-32652-y".

Your system would work but you would need some material to selectively oxidize water over chloride as others have pointed out. Otherwise you will lose your hygroscopic material. What others have not mentioned however is that such materials do exist and are also still being researched.

About the question on cell potential, when it comes to actually pulling current and producing hydrogen not only thermodynamics count. You're right in the sense that the standard reduction potential of water oxidation is lower than that of chlorine evolution. However first, the thermodynamic activity (~ concentration) of the species (chloride, water, protons, hydroxide, chlorine, oxygen, ...) will not be at standard conditions so take that into account when making the argument based on thermodynamics. Second, the oxidation of chloride is a two-electron process. Typically less electron transfers means less kinetic limitations and most materials will indeed evolve chloride over oxygen. However, some do show a preference in the opposite direction. For example check out this publication: "Johannes G. Vos, Tim A. Wezendonk, Adriaan W. Jeremiasse, Marc T. M. Koper, MnOx/IrOx as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution https://pubs.acs.org/doi/10.1021/jacs.8b05382"