The Influence of Hydration Energy on Alkali-Earth Intercalated Layered Manganese Oxides as Electrochemical Capacitors

Insight into the influence of hydration energy of structural cations within birnessite-type layered MnO₂ on charge storage mechanisms via redox reaction and intercalation/deintercalation processes with the ion-exchange process is demonstrated. The redox activity and Mn utilization observed from ex situ X-ray absorption spectroscopy are Li–MnO_x > Ca–MnO_x > Sr–MnO_x > Ba–MnO_x. Although Li–MnO_x shows higher redox activity than Ca–MnO_x, the Ca–MnO_x exhibits higher specific capacitance due to its higher hydration energy of Ca²⁺ (−500 kcal mol^–1) as compared to −465 and −436 kcal mol^–1 of Sr–MnO_x and Ba–MnO_x, respectively, which dominates the ion-exchange affinity within the birnessite structure. Therefore, the charge storage mechanism of the birnessite depends strongly on the hydration energy of structural cations which can further be probed by inductively coupled plasma-optical emission spectrometry technique. Additionally, Ca–MnO_x with the smallest number of the remaining structural Ca²⁺ as compared with other cations demonstrates the highest specific capacitance followed by Sr–MnO_x, Li–MnO_x, and Ba–MnO_x. Furthermore, all the as-prepared samples demonstrate the excellent cycling stability (above 96%) after 11 000 cycles at a current density of 5 A g^–1. This finding may be useful for further development of practical manganese oxide supercapacitors.