I work at a company that is creating better electrolyzers for this process. Ammonia producers are clamoring for this technology. It works, and it is happening!
Could ammonia be used for stable seasonal storage of energy?
The abstract suggests that ammonia is much nicer to transport and store than hydrogen. If we can use it to store summer solar to run winter heat-pumps with a low roundtrip efficiency but even lower costs per kWH of capacity it could really help.
It could work theoretically. The only question is if it is economical to do so. And the answer to that is probably not. It ranks pretty low on the list of possible solutions in terms of cost and efficiencies.
Hydrogen and ammonia (you typically generate one to generate the other) are interesting as a fuel in some use cases (anywhere the weight of lithium ion batteries is a problem basically). Main use cases seem to be shipping, and maybe long haul aviation. Probably not for road transport (battery electric seems adequate there for most vehicle categories).
But as a battery/energy storage solution it makes less sense. The round trip from solar/wind energy to hydrogen to ammonia and back to electricity loses most of the energy in the process. It's doable but there are probably more efficient and cheaper ways to store the energy. You lose about half (at least, that's a super optimistic percentage) of the energy creating the hydrogen. Then some more creating the ammonia. And then some more converting that back to electricity. It's pretty easy to waste less energy than that.
For heat, simple thermal mass is very efficient, low tech, and has already been demonstrated to work for seasonal storage. Throwing away half the energy to create ammonia simply makes no sense. All you need for thermal mass is some basalt, sand, etc. with a lot of mass, a container to put it in, and some cheap way to insulate it (wool would do the job). Heat it up in the summer, extract heat in the winter. It scales. The raw materials are dirt cheap (because they are literally dirt), the complexity is low (pipes, plumbing, insulators, sand/rock).
Long term storage solutions are dominated by capital cost. You can use batteries to store electricity from day to night because you recoup part of your capital investment every day for ten years or so. But if you can only sell electricity once a year, then whatever profit you make it better be good, because you only get that 10 times in 10 years. The alternative is to have dirt cheap capital cost.
Pumped storage works where the lake and dam already exist, because the capital storage is essentially zero.
Everywhere else, nothing works. Batteries don't work, chemical storage (like ammonia or hydrogen) doesn't, compressed air, or molten salt, nothing works.
What will work is a way to "cheat". If you can mimic the day-night cycle of batteries and make a profit every day, you win. The way to do that is by long-distance transportation: you make ammonia in Australia, or Morocco, or Saudi Arabia and sell it in Japan, China or Europe. You may incur round trip losses of 80% or more, but if the price you pay for one kwh is one cent, and you sell for 10 cents, you can still make a profit.
Ammonia can indeed be used as a longish term energy storage. It's not as great as methane (that can be pumped into underground caverns), but it liquefies at room temperature at just around 9 atmospheres or at -33C at atmospheric pressure.
The abstract suggests that ammonia is much nicer to transport and store than hydrogen. If we can use it to store summer solar to run winter heat-pumps with a low roundtrip efficiency but even lower costs per kWH of capacity it could really help.