An in-depth study on the thermodynamics and kinetics of disproportionation behaviors in ZrCo-H system

Hydrogen introduced disproportionation reactions are the main reason for capacity fading in ZrCo-H system during cycling, but its essential performances, including thermodynamics and kinetics, are still inconclusive so far. Herein, the thermodynamic and kinetic investigations of hydriding disproportionation (2ZrCo+ H2→ ZrCo2+ ZrH2) and dehydriding disproportionation (ZrCoH3→ 1/2ZrCo2+ 1/2ZrH2+ H2) as well as hydrogenation (2/3ZrCo+ H2→ 2/3ZrCoH3) and dehydrogenation (2/3ZrCoH3→ 2/3ZrCo+ H2)reactions were carried out systematically. Importantly, the thermodynamic equilibrium of hydriding disproportionation was resoundingly sought through a series of discontinuous reactions. Therefore, the hydrogen plateau pressure (0.220 bar at 600 oC) as well as the corresponding enthalpy change ΔH (-123.12 kJ/mol H2) and entropy change ΔS (-224.32 J/mol·K H2) were accurately measured. Moreover, by establishing thermodynamic relationships between the dehydrogenation and hydriding disproportionation reactions, the thermodynamic parameters of dehydriding disproportionation (ΔH= 69.36 kJ/mol H2, ΔS= 222.75 J/mol·K H2) were figured out dexterously. To our surprise, the thermodynamic conditions of de-/hydriding disproportionation are easier to be satisfied comparing with de-/hydrogenation reactions. Further kinetic studies reveal that both de-/hydriding disproportionation reactions require high temperature (>380 oC) to trigger, since the activation energy is much higher than that of the de-/hydrogenation reactions. Thus, we consider both disproportionation reactions are inevitable during cycling in terms of thermodynamics. However, the high energy barrier restricts the disproportionation reaction from terminating completely in one cycle, but controls it occurring gradually in each cycle. This research well explains the attenuation model of cycling capacity in ZrCo-H system and we suggest the future works, dedicated to improving the cycling stability of ZrCo-H system, should focus on changing the de-/hydrogenation paths to avoid thermodynamically disproportionation reactions regions.