Minimum 2000kj/kg stored when cycling between at T_cold and T_hot.
T_cold not less than 500k
T_hot greater than T_cold but less than 1000k
3atm or less in a closed container. Gas phase is allowed but total density should not be less than 0.5kg/l.
Must be able to cycle at least 10,000 times without losing more than 20% of capacity.
Full bounty will be awarded for identifying a mix of chemicals that satisfies all of these criteria and showing your work to demonstrate it satisfies the criteria. There is no partial credit.
1,000Not a bounty claim, but one screen I would apply to candidates here is the gas-volume constraint. The hard combination is not just 2 MJ/kg; it is 2 MJ/kg while staying below 3 atm in a dense closed container.
For example, MgH2 is an attractive-looking near miss: Zhou et al. report about 76.5 kJ/mol H2 for dehydrogenation (https://doi.org/10.1039/D1CP02498A), so the gravimetric heat is above 2 MJ/kg of MgH2. But 1 kg MgH2 corresponds to about 38 mol H2. If the system density must be at least 0.5 kg/L, that kilogram only gets at most about 2 L. Ideal-gas pressure for 38 mol at ~600 K in 2 L is hundreds of atm, not 3 atm. At 3 atm it would need hundreds of liters of gas volume per kg. So simple metal hydride de/hydrogenation seems ruled out by the pressure+density pair even before cycle-life questions.
The calcium hydroxide class is a useful comparison in the other direction: Ca(OH)2/CaO is a real thermochemical storage system, but RSC gives 104.4 kJ/mol and up to about 1400 kJ/kg theoretical, with real volumetric density lower (https://doi.org/10.1039/C7RA06639B). That misses 2000 kJ/kg. The same paper also notes carbonate contamination/decarbonation issues above 800 C, which is awkward against the <1000 K ceiling.
So my current filter would be: avoid candidates whose storage step liberates a full gas mole per small formula unit unless another solid phase captures that gas internally. The plausible solution space, if any, looks more like a dense solid-solid or solid-solution reaction with unusually high reversible enthalpy and proven cycle stability, not a hydrate/carbonate/hydride that relies on a large free gas inventory.
not saying this is impossible (it's definitely not! thermochemical energy storage is a well-established concept in literature with decades of precedent) but I'm genuinely rolling at your performance targets here
there are startups that have raised >$10mil Series A to evaluate if they can get to 10,000 cycles going between like 800 <=> 1500 °C at like 1.3~1.8 MJ/kg
"There is no partial credit." LMFAO if anyone reading this thinks they can hit these KPIs, DM me for $500,000 of non-dilutive seed funding
vaporization of phosphorous would be like 3000kj/kg at 553K at 1atm but to get the requisite density of 0.5kg/l you need about 16 mol/l which at that temperature would take about 0.5*22.4*16=179 atms of pressure. Such pressure vessels exist and aren’t that expensive but this violates one of the OP criteria