[epistasis]

"Cycles" don't really make any sense here, but you can get some estimates of costs and efficiencies here:

https://en.wikipedia.org/wiki/Power_to_gas

Lithium ion batteries for grid storage typically use an NMC chemistry, and standard warranties are for 10 years of daily cycling.

[dnautics]

Duty cycles does make sense as the catalytic efficiency decreases per given use period (in this case, day is the most relevant) due to irreversible chemical reactions with the catalyst itself, in this case probably quite a bit of the metaphosphate going to plain rust.

[logicallee]

Thank you, but could you translate this to practical terms? I would like you to imagine some totally off-the-wall usage, I don't know, (this is completely illustrative example), imagine a tiny pacemaker that is self-powered through blood circulation and must charge and discharge itself essentially continuously - so, something like tens of thousands of duty cycles per day (low estimate, this is one cycle every few seconds), or 7m cycles per year.

By the end of one year, would it be totally depleted?

Please note that I did not want to name the application, so don't worry about the specifics of my illustrative example, it's totally made-up and I see the problems with it - just about the timing.

Additionally I am interested in the size and weight as compared with other rechargeable battery types.

I am sorry that I seem to be asking really strange questions. If they are not well-defined could you reply with your questions about my question?

Thank you so much for taking the time to understand my question and and reply. Basically, I'm asking if the effect you describe applies to hundreds of thousands, millions, tens of millions, hundreds of millions, or billions of charge/discharge cycles. Is it a tiny little effect or rather more significant than that?

I want you to really think out of the box, please, so just to expand your mind (as an "anchor"), even though it is not relevant to my application, consider:

>Modern automobile engines are typically operated around 2000–3000 rpm (33–50 Hz) when cruising, with a minimum (idle) speed around 750–900 rpm (12.5–15 Hz), and an upper limit anywhere from 4500 to 10,000 rpm (75–166 Hz) for a road car or nearly 20,000 rpm for racing engines such as those in Formula 1 cars (currently limited to 15,000 rpm)

So if the ctypical cruising speed is 2000 RPM, then in an hour of cruising, a piston would go through 120,000 cycles. If an hour of driving gets you, say, 100 miles (obviously higher than average) then 100,000 miles is 1,000 hours (likely actually more), so that we are at 120 million cycles per 100,000 miles. Obviously this is pretty crazy and I don't envision anything that does anything like that, plus it would need an electric generator, but I just wanted to expand your mind as to why in certain, as-yet unspecified, applications, hundreds of millions, billions, or tens of billions of duty cycles might well be completely reasonable to talk about, and therefore whether the answer to my questions is "tens of thousands" or "infinite" is by no means an equivalent answer!

Please note that both the heart-valve driven pacemaker, and the hybrid engine with per-cycle electric recovery, are not the domains I'm talking about. So I'm interested in a more pure or abstract description of the possible specifications here.

By the way if you happen to know off-hand of other kinds of energy storage (example: graphene supercapacitor; flywheel) that you happen to know are good for the mentioned hundreds of millions of cycles, then you could mention this as well. However, in this thread I was primarily asking about the number of duty cycles (and, to a lesser extent, the possible weight) of a reversible fuel cell - whether via water splitting or some other mechanism.

This is kind of out-there, but it also sounds like you are saying the catalyst may be thought of as an additional "fuel" to be added over time. Like, for the engine made-up example, someone might think of topping up their engine with both petrol and catalyst-fuel, every time they top up. So in this case it would matter how much in quantity we are talking about - if the catalyst is thought of as a consumable that gets used over time and adding some more is part of operations. If it gets used exceedingly slowly, this is no greater a burden than engine oil changes, which are already used regularly.

Thank you so much for your time. I hope I've been clear about my questions.

If all this is pretty crazy, then please just name the number of duty cycles until the catalyst is 50% depleted. (And please name a definition for what 50% depleted means.) Or, any other alternative measure you can think of, as long as it's well-defined. I would just like to know within 1-2 orders of magnitude! Thank you!

[logicallee]

(you might want to mention you're not OP in cases like this, as I addressed my comment specifically to them and almost thought I was still talking to them.)

of course, I appreciate your reply too. epistasis, why do you say "cycles don't make sense" -- is it because it's "infinite" or because it can't be run as a fuel cell immediately in the same place? Like you can't go back and forth immediately?

I read most of the article you linked (which had a couple of occurrences of the term 'fuel cell' - not many - therefore leading me to be confused as to why you said what you just said).

I am talking about using it as a fuel cell, same as a battery. If daily cycling means 1 cycle per day, then 10 years means 3650 cycles. How does "power to gas" compare? Infinite cycles?

[epistasis]

Apologies for the confusion. You were asking for an awful lot, in way that didn't make much sense, so I thought I would chip in a bit.

OP was talking about burning the hydrogen, as was I. Burning is not typically the term used with what goes on in fuel cells.

A battery has a fixed capacity attached to it, it has a fixed power to energy ratio. The component that stores the energy is directly attached to the part that discharges the energy. This is what makes "cycles" make sense.

With power to gas, or using fuel cells to consume energy from the process described in the article, the component that stores energy is not directly connected to the part that consumes energy. It's not necessary that they even have a fixed fuel stores attached to them, they could just be hooked up to a pipeline. The rate at which parts wear out would better be described in terms of total energy stored or consumed.

That's why I mentioned the cost estimates (€0.10/kWh for hydrogen in that article), because it's perhaps the best way to compare. The rapid drop of lithium ion grid storage has put it at $250/kWh capacity with 3650 cycles, which is ~$0.07/kWh before accounting for taxes, maintenance, siting, etc., which may or may not be included in that above estimate for hydrogen.

Both of these numbers blow me away by how small they are. We're in for a wild ride on tech changes over the next few years...

[logicallee]

Thanks. So after the fuel is burned for energy, it can't be unburned again, all in the same closed loop? (Reversible fuel cell?). That is not possible/practical?