Mixed valence manganese oxides (MnO_x) have attracted significant research interest in recent years due to the easily reversible redox reactions between manganese oxidation states (Mn⁺², Mn⁺³, and Mn⁺⁴)¹, which has enabled applications in catalysis², energy storage³, and gas sensing⁴. Of the manganese oxides manganese dioxide (MnO₂) has been of particular interest due to its wide variety of synthesized structural polymorphs ((β)1x1 tunnel⁵, (α)1x2 tunnel⁵ (γ) spinel⁵, (δ) layered⁵) which allow for enhanced control over the available surface area and reactive properties of MnO_2. Among these structural polymorphs, the α and δ phases exhibit mixed-valence character with Mn⁺³ defects being found throughout the crystal structure⁵. Mn⁺³ defects have been found to increase the catalytic activity of MnO₂ making its presence desirable⁶. Water stabilized hexagonal δ-MnO₂ (HMnO₂) contains a large number of Mn⁺³ ions as the interlayer contains Mn^+2/+3 ions to neutralize the layer charge from Mn⁺³ lattice defects⁷. However, the direct synthesis of HMnO₂ has not been reported extensively in the literature, as most synthesis routes rely on permanganate salts (e.g KMnO_4, NaMnO₄)¹, which leads to the synthesis of ion intercalated δ-MnO₂ and a reduction in Mn⁺³ defects in the lattice⁹. Additionally, these methods rely on the use of harsh chemicals¹, high temperatures¹, and long reaction times¹, which limits their sustainability. In this work, we present the green facile synthesis of high Mn⁺³ content HMnO₂ via electrodeposition on epitaxial graphene/silicon carbide (EG/SiC) substrates, we also demonstrate that these as-grown films can be intercalated with a variety of ions via cyclic voltammetry to form alternate birnessites. The electrodeposition was carried out in a three-electrode electrochemical cell with an Ag/AgCl reference electrode, a Pt counter electrode, and the substrate (EG/SiC) as the working electrode utilizing a 100 mM manganese acetate solution. The electrodeposition process contained three steps, an initial pulse at the oxidation potential to seed the surface with oxygen, a second pulse at the reduction potential to deposit manganese on the surface, and a third pulse at the oxidation potential to oxidize the deposited manganese, with the potentials determined by cyclic voltammetry. The resulting δ-MnO₂ thin films were then characterized through Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy(AFM) to determine their crystalline phase and surface morphology. Raman spectroscopy (Fig. 1) confirmed the formation of MnO₂ on both substrates due to the presence of Raman peaks between 650-640cm^-1,8 575-585cm^-1,8, and 490-510cm^-1,8, which are associated with the Mn-O symmetric stretching bond⁸, the Mn-O stretching bond in the basal plane⁸, and Mn-O stretching bond in the [MnO₆] octehedra⁸. The formation of δ phase is confirmed by the presence of a weak peak at 130cm^-1,2 consistent with the literature. The AFM images indicated that the surface was made up of nanofibrous nanoparticles (Fig 2.a) which SEM confirmed (Fig. 2.b-c) and indicated that the deposition on EG/SiC was made up of microplates larger than 40 μm x 40 μm in size. After this characterization, the resulting films were intercalated with alkali metal ions (Na⁺) via cyclic voltammetry. This was carried out in the same three-electrode cell as the deposition, with an Ag/AgCl reference electrode, a Pt counter electrode, and a δ-MnO₂/EG/SiC working electrode using a 2 M sodium nitrate solution. The cyclic voltammetry was run for 10 cycles with a window from 0.8V to -0.5V at a scan rate of 5 mv/s. Following this intercalation, the samples were analyzed with optical microscopy and Raman spectroscopy. Optical microscopy indicated two distinct regions formed in the film, large multicolored regions and small blue and black islands(Fig. 3.a). Raman analysis of these regions found that the islands were unintercalated MnO₂ but the colored regions were Na-type birnessite, with a redshift of the peak at 650cm^-1 to 638cm^-1 and a blueshift of the peak at 575cm^-1 to 581cm^-1 consistent with the literature⁹ (Fig. 3.b). This we have demonstrated the capability of forming H-type manganese birnessite, and partially converting it to alternate birnessites which makes this a good platform for advanced energy systems. However additional work needs to be done to improve the uniformity of the intercalation process.

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