Hacker News Reader

Comment by fernly | Hacker News Reader

Could someone ELI5 how the recharge cycle works? On charging it evolves H2 gas; how does that get put back on recharge?

This is very much NOT explained by the paragraph,

> The researchers did this by re-attaching their power source to the depleted prototype, this time with the goal of inducing the manganese dioxide particles clinging to the electrode to combine with water, replenishing the manganese sulfate salt. Once this salt was restored, incoming electrons became surplus, and excess power could bubble off as hydrogen gas, in a process that can be repeated again and again and again.

View on HN

In the modern sense of the word, this is not a battery. It is a device that cracks water into hydrogen, which can then be fed into a fuel cell. Presumably, it consumes distilled water, which is why it is targeting utility scale storage and not home installations.

The work sounds interesting, but the article is painfully written.

Lead acid batteries produce hydrogen too. I think the insight is that this can’t produce electricity directly, so the chemistry can be much more stable.

> ...not a battery.

The paper, linked below (thanks xelxebar) would beg to differ, it says

> we report a rechargeable manganese–hydrogen battery, where the cathode is cycled between soluble Mn2+ and solid MnO2 with a two-electron reaction, and the anode is cycled between H2 gas and H2O through well-known catalytic reactions of hydrogen evolution and oxidation.

They give an equation for the cell operation as,

    Mn2+ + 2H2O ↔ MnO2 + 2H+ + H2

Let me try that with _sub_ and ^sup^, does HN support that?

    Mn2^+^ + 2H_2_O ↔ MnO_2_ + 2H^+^ + H_2_

Anyway, that double-ended arrow says that you can read it from right to left, i.e. evolved H_2_ gets back into (or stays in?) the solution to react with the O_2_ from the magnesium oxide to make water. Quote,

> During discharge of the battery, the uniform layer of as-deposited MnO2 on the cathode is dissolved back to soluble Mn2+ electrolyte and H2 is oxidized on the anode.

So, I guess it stays in solution...?

HN formatting capabilities are not great, but you can use Unicode sub-/superscripts:

Mn₂⁺ + 2H₂O ⇌ MnO₂ + 2H⁺ + H₂

(Shameless plug: I generated the above with https://github.com/jwilk/chemiscripts .)

Just in case anyone's interested, I think this is the primary paper being discussed:

https://doi.org/10.1038/s41560-018-0147-7

which redirects to the Nature Energy page and so is behind a paywall. I'm not sure how Sci-Hub is viewed here, but this links directly to the pdf:

https://sci-hub.tw/10.1038/s41560-018-0147-7

This puzzled me also. Perhaps it's been kicked down the road as an engineering problem and we'd end up with a hydrogen extraction system which is responsible for that part. We'd probably need to feed protons back in though...

Reply on news.ycombinator.com