Quantifying the Stability–Energy trade-off in H2–CH4 mixed hydrates for hydrogen-enriched natural gas storage: A fugacity-corrected thermodynamic framework

Hydrogen storage remains a critical bottleneck in clean energy systems, with conventional approaches imposing significant trade-offs between energy density, safety, and cost. This study develops a dual-metric framework to quantify stability–energy trade-offs in mixed H 2 –CH 4 hydrates, integrating fugacity-corrected thermodynamic regression, a destabilization index (ΔT), and a net energy index (NEI) across compositions from 0 to 65.9 mol% H 2 and pressures from 3.0 to 20.7 MPa. Fugacity correction using the Peng–Robinson equation of state yielded apparent dissociation enthalpies ranging from 62.5 to 71.1 kJ/mol, revealing a non-monotonic profile with cooperative stabilization at moderate hydrogen fractions attributed to complementary cage occupancy. The destabilization index followed a power-law relationship (R 2 = 0.976), with penalties remaining below 5 K for compositions under 42 mol% H 2 but accelerating to approximately 10 K at 65.9 mol% H 2 . The destabilization index is referenced to the lowest-hydrogen composition in the dataset (4.55 mol% H 2 ), which spans the full experimental pressure range, rather than to pure methane hydrate, whose three-phase equilibrium data are limited to pressures below ∼8 MPa. Energy analysis demonstrated that volumetric energy density declined from 3750 to 2100 MJ m −3 with increasing hydrogen content, with hydrogen contributing less than 45% of stored energy even at maximum loading—indicating that mixed hydrates function as methane carriers with incidental hydrogen co-storage. The unified model reproduced the 45-point training set with R 2 = 0.997 (RMSE = 0.23 K); against 11 independent literature points, interpolation to 20 mol% H 2 achieved R 2 = 0.90 (RMSE = 1.15 K), while extrapolation to 80 mol% H 2 degraded to RMSE ≈ 2.5 K, comparable in magnitude to the destabilization penalties at moderate compositions and restricting reliable application to 0–66 mol% H 2 . These findings establish quantitative boundaries for mixed hydrate applications, suggesting favorable prospects for hydrogen-enriched natural gas systems below 20 mol% H 2 while demonstrating fundamental limitations for dedicated hydrogen storage.