Charge Storage in Cation Incorporated α-MnO₂

Abstract

Electrochemical supercapacitors utilizing α-MnO 2 offer the possibility of both high power density and high energy density. Unfortunately, the mechanism of electrochemical charge storage in α-MnO 2 and the effect of operating conditions on the charge storage mechanism are generally not well understood. Here, we present the first detailed charge storage mechanism of α-MnO 2 and explain the capacity differences between α-and β-MnO 2 using a combined theoretical electrochemical and band structure analysis. We identify the importance of the band gap, work function, the point of zero charge, and the tunnel sizes of the electrode material, as well as the pH and stability window of the electrolyte in determining the viability of a given electrode material. The high capacity of α-MnO 2 results from cation induced charge-switching states in the band gap that overlap with the scanned potential allowed by the electrolyte. The charge-switching states originate from interstitial and substitutional cations (H + , Li + , Na + , and K + ) incorporated into the material. Interstitial cations are found to induce chargeswitching states by stabilizing Mn-O antibonding orbitals from the conduction band. Substitutional cations interact with O[2p] dangling bonds that are destabilized from the valence band by Mn vacancies to induce charge-switching states. We calculate the equilibrium electrochemical potentials at which these states are reduced and predict the effect of the electrochemical operating conditions on their contribution to charge storage. The mechanism and theoretical approach we report is general and can be used to computationally screen new materials for improved charge storage via ion incorporation.

The influence of large cations on the electrochemical properties of tunnel-structured metal oxides

“…The simulated density of states (shown in Fig. 4a) indicates that pure α-MnO 2 has a bandgap of ∼2.8 eV suggesting semiconductor behaviour, and agrees well with the reported value of Young et al 16. For K 0.25 MnO 2 , newly formed occupied states appear inside the original MnO 2 bandgap, indicating mixed Mn 4+ and Mn 3+ in K 0.25 MnO 2 , which is compatible with previous reports32.…”

Bandgap reduction of photocatalytic TiO2 nanotube by Cu doping

“…3), we evaluated the work functions, shown in Table 3, using a scheme given in the literature 53 . We ought to note that the calculated work functions are overestimated when compared with experimental values (∼4.90 eV 54 ) by 20%.…”

Sodium Charge Storage in Thin Films of MnO2Derived by Electrochemical Oxidation of MnO Atomic Layer Deposition Films

“…20,21 In addition, recent theoretical work predicted that charge storage in α-MnO 2 results from "charge-switching" interstitial and substitutional cations in α-MnO 2 . 27 In this work, we study charge storage in MnO 2 films. The MnO 2 films were prepared by electrochemically oxidizing MnO films with precise initial thicknesses that were grown using atomic layer deposition (ALD).…”