Mn³⁺ e_g Configuration and Electron Transfer in Na-Incorporating α-MnO₂ to Improve Electrochemical Supercapacitor: An In Situ and Ex Situ X-ray Absorption Spectroscopic Investigation

Abstract

Cryptomelane-type α-MnO 2 has a high specific capacitance and cyclic stability because of its combination of 1 × 1 and 2 × 2 tunnel structures that are formed from the edge-and corner-sharing octahedra. It, therefore, has the potential to host various cations, which improve its electrochemical properties. The Na incorporation provides an opportunity for charge compensation by forming mixed-valence Mn 3+ and Mn 4+ states in α-MnO 2 . This study shows that the incorporation of Na results in superior electrochemical properties by enhancing the specific capacitance and capacity retention of α-MnO 2 to 234.4 F g −1 and ∼96%, respectively. The electrochemical performance is improved following Na incorporation by the induced phase competition between α-MnO 2 and the induced β-MnO 2 , while the charge neutrality between Jahn−Teller (JT) active Mn 3+ and Mn 4+ ions is maintained. The formed Mn 3+ /Mn 4+ mixed-valence species increase the strength of the redox reaction at the electrode−electrolyte interface, as observed from the in-situ Mn K-edge X-ray absorption nearedge structure (XANES) and the extended X-ray absorption fine structure. Ex-situ Mn L 3,2 -edge XANES analysis further reveals that the introduction of e g from the Mn 3+ 3d orbitals causes electron transfer via O 2p states by establishing the double-exchange (DE) Mn 3+ −O−Mn 4+ interaction. This study suggests that the electron exchange also involves the super-exchange (SE) Mn 4+ −O−Mn 4+ interaction, based on field-dependent magnetization (M−H) measurement, which accelerates the redox reaction in α-MnO 2 . This work demonstrates that JT active Mn 3+ e g configuration is responsible for the great improvement of electrochemical properties that are induced by the incorporation and insertion/extraction of Na + ions with the charge/discharge process, which causes electron hopping between Mn 3+ /Mn 4+ and Mn 4+ through oxygen ligand by DE/SE in the α-MnO 2 .

Investigation of the Role of 3d-4d Elements in a Disordered Double Perovskite toward Efficient Photocatalytic Energy Conversion and Electrochemical Energy-Storage Behaviors

“…Further, the conduction band edge minima value can be observed to be −4.37 eV . In addition, the spectral fitted peaks were assigned from the fitted XP-VBS plot as depicted in Figure S2e, where e g (Ru 4 d ) exhibited at 0.6 eV, t 2g (Ru 4 d ) at 1.52 eV, e g (Mn 3 d ) at 2.45 eV, t 2g (Mn 3 d ) at 3.56 eV, and Mn 3 d /Ru 4 d –O 2 p hybridization (∼5.03 and ∼7.98 eV) along with an intense O 2 p peak (∼6.57 eV). , These observed results clearly support the role of the current catalyst in the presence of a photoabsorber during the photolytic water oxidation process.…”

“…On the other hand, the distorted crystal structure of DPs generally plays a crucial role in controlling their electronic structure which eventually leads to the catalytic properties. ,,− ,,,,− Therefore, to gain a further understanding about the active sites and the specific role of 3 d -4 d metal ions in the present DP structures, we performed DFT calculation studies (Figure ). First, we optimized the pristine structure of Ca 2 MnRuO 6 as a base model and displayed it in Figure S13 (see Supporting Information).…”

Interrogation of 3d Transition Bimetallic Nanocrystal Nucleation and Growth Using In Situ Electron Microscope and Synchrotron X-ray Techniques

“…Excitingly, recent advancements in in situ techniques provide a remarkable opportunity to address the aforementioned issues. In situ transmission electron microscopy (TEM) enables a direct observation of NCs growth at the nanoscale or even atomic scale, − while in situ synchrotron X-ray spectroscopy has proven to be a powerful tool for tracing the electronic structure and local coordination environment of atoms. − Especially, in situ scanning transmission X-ray microscopy with ptychography (STXM-ptychography) provides direct insights into dynamically changing electronic structure at a specific nanoscale area. Hence, the integration of in situ TEM and X-ray synchrotron spectroscopy is essential for establishing robust quantitative and visual structural evolution of atoms and electrons during the nucleation and growth processes of 3d transition bimetallic NCs.…”