Half-disturbed spin desalination: Asymmetric high-spin states in MnO2 accelerate charge transfer in capacitive deionization

Abstract

Manganese dioxide (MnO2) has been recognized for its high theoretical capacitance. Recent advancements highlight the potential of modulating the spin state of Mn to elevate charge transfer kinetics. Nonetheless, the specific relationship between Mn spin distribution and desalination efficacy, alongside the underlying mechanism, remains elusive. This study introduces the novel concept of half-disturbed spin desalination, elucidating its complex mechanism via the strategic induction of oxygen vacancies on the MnO2 surface. Theoretical analyses reveal that electron transfer from the O py (up) and pz (up) to the Mn dz2 (up) orbitals alters MnO2's spin state from a symmetric low spin to an asymmetric high spin configuration. This transition disrupts spin up symmetry, bolstering dynamic processes and diminishing the ion diffusion energy barrier. Consequently, oxygen vacancy-enriched MnO2 (VO-MnO2) demonstrates an enhanced specific capacitance of 289 F g−1 at 1 A g−1. Remarkably, in the hybrid capacitive deionization (HCDI) process, it achieves a superior salt adsorption capacity (SAC) of 56.5 mg g−1 and a salt adsorption rate (SAR) of 2.5 mg g−1 min−1. This research pioneers the correlation between spin desalination mechanisms and spin state applications, paving the way for novel applications across diverse fields.