Thermodynamic-kinetic compensation and non-dilute diffusivity correction in V–Ni–Mn hydrogen separation membranes

Pd-coated V-(15-x)Ni-xMn (x = 3, 6, 9, 12 at.%) bcc alloy membranes were prepared and assessed for hydrogen transport within the temperature range of 523–673 K. Pressure-Composition-Temperature (PCT) measurements reveal a non-monotonic dependence of the dilute solubility coefficient (S_dilute) on Mn content. By separating the enthalpic and entropic contributions via Van't Hoff analysis, it is found that the decline in solubility at high Mn content (≥9 at.%) is driven by a substantial reduction in the pre-exponential factor (S₀), which is consistent with the obstruction of interstitial sites. Significantly, the controversy regarding the anomalous temperature dependence of permeability in V-based alloys is resolved by strictly decoupling the thermodynamic and kinetic contributions. It is demonstrated that the near-zero apparent activation energy (E_Φ) is a quantitative outcome of thermodynamic–kinetic compensation, E_Φ = E_D+ ΔH_sol. Moreover, a non-dilute deviation factor (r) is introduced to quantify the errors in traditional Sieverts' analysis. It is shown that the conventional effective diffusivity (D_eff = Φ/S_dilute) overestimates the intrinsic chemical diffusivity (D_chem) by a factor of 1.1∼1.4 under realistic operating conditions. Notably, the hydrogen flux of the developed V–Ni–Mn alloys is approximately 2∼3 times higher than that of the industrial benchmark Pd₇₇Ag₂₃ membrane at 673 K. Finally, constant-pressure slow-cooling tests define the operational stability window, identifying V–9Ni–6Mn as an optimal composition that strikes a balance between high permeability and resistance to cooling-induced embrittlement.