
Redox-Flow Cell Stores Renewable Energy as Hydrogen - jonbaer
https://spectrum.ieee.org/energywise/energy/renewables/storing-renewable-energy-hydrogen-redoxflow-cell
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acidburnNSA
Good to see progress in large-scale energy sources. As always we ideally would
like to know current and prospective costs per MWh of storage, per MW of
capacity, round-trip efficiency, raw material mass flows and recyclability,
and space requirements to fully judge. I understand that these metrics are not
always reliably available in new issue tech. We should make a worksheet or
something to help navigate these complex issues.

At least more people are talking about hydrogen as an energy carrier these
days as opposed to a raw energy source, which was a huge misconception in the
hydrogen excitement bubble of ~2005.

And a nitpick: headline should be about storing intermittent energy, not
renewable energy. I personally hate the term renewable because not all
renewable energy is low-carbon (e.g. most biofuels) and not everything that's
sustainable for hundreds of millions of years is considered renewable (e.g.
nuclear with breeder reactors or seawater uranium extraction).

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twic
So how does this thing work?

> A redox-flow battery, in essence a reversible fuel cell, is typically made
> up of a positive and negative electrolyte stored in two separate tanks. When
> the liquids are pumped into the battery cell stack situated between the
> tanks, a redox reaction occurs, and generates electricity at the battery’s
> electrodes.

> By comparison, the new invention has only one electrolyte, comprised of an
> iron salt (rather than the more commonly used vanadium) dissolved in acid.
> When hydrogen ions react with the iron salt (Fe2+), hydrogen gas is produced
> at the platinum-coated carbon cathode in the battery stack.

> “We introduce iron as a middleman, so we can separate electrolysis into two
> reactions,” says Wang. Doing so allows one to control where and when to
> reverse the reaction to produce electrical energy to supply to the grid.
> “The system gives you flexibility... you could do the regeneration during
> evening time when electricity prices are at a peak,” he says.

> Regenerating Fe2+ in the reverse reaction also allows for the continuous
> production of hydrogen gas, he says. “And because the hydrogen-iron cell
> uses about half the voltage of a traditional electrolyzer, you can generate
> hydrogen at a much cheaper cost if you do everything right.”

It sounds to me like there are two things going on here.

Firstly, they have a redox cell which has one electrolyte, in which iron ions
move between the II and III redox states.

Secondly, they have a way to shuffle electrons between iron and hydrogen, so
they can take energy from the iron in the redox cell and use it to make
hydrogen.

So, over short timescales, the system just works like a redox battery. No
hydrogen is produced or consumed. But if the battery gets full, or they want
hydrogen for other purposes, they can produce hydrogen, and in doing so,
regenerate the iron.

Is that right? If so, what are the reactions here? What is the composition of
the electrolyte, and what stops the iron ions decaying to the lower-energy
state? How do they use iron ions to make hydrogen from water?

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throwaway154213
Redis, Redux, Redox, and now Redox-Flow. My brain is getting a little confused
now.

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AtlasBarfed
So electrolysis has a thermodynamic efficiency currently at best at 80%. Fuel
cells are 40-60% for the regeneration of the electricity.

I've heard that pumping water for gravitational potential energy is 90% end-
to-end though.

I don't see a thermodynamic efficiency metric, so that's not encouraging.

