Crystalline MnO[sub 2] as Possible Alternatives to Amorphous Compounds in Electrochemical Supercapacitors

Abstract

Manganese dioxide compounds with various structures were synthesized and tested as "bulk" composite electrodes for electrochemical capacitors. The capacitance of the set of MnO 2 compounds having Brunauer-Emmett-Teller ͑BET͒ surface areas larger than 125 m 2 g −1 reached a maximum value of about 150 F g −1 . The capacitance of all amorphous compounds ͑except one͒ is due to faradaic processes localized at the surface and subsurface regions of the electrode. Further increasing the surface area does not provide additional capacitance. The capacitance of the crystallized materials is clearly dependent upon the crystalline structure, especially with the size of the tunnels able to provide limited cations intercalation. Thus, the 2D structure of birnessite materials gives an advantage to obtain relatively high capacitance values ͑110 F g −1 ͒ considering their moderate BET surface area ͑17 m 2 g −1 ͒. 1D tunnel structure such as ␥or ␤-MnO 2 is characterized by only a pseudofaradic surface capacitance and therefore relies on the BET surface area of the crystalline materials. 3D tunnel structure such as -MnO 2 shows some intermediate behavior between birnessite and 1D tunnel structures.

To Be or Not To Be Pseudocapacitive?

“…2,3,4,5 This definition can be found in Conway's influential book, "Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. 1 Conway stated that: "Regular double layer capacitance arises from the potential-dependence of the surface density of charges stored electrostatically (i.e., non-faradaically) at the interfaces of the capacitor electrodes.…”

Charge Storage in Cation Incorporated α-MnO2

“…16,22 In contrast, crystalline α-MnO 2 materials exhibit a bulk capacitance of only ~200 F/g, while the β-MnO 2 crystalline phase has a meager bulk capacitance of ~10 F/g. 23,24 One explanation given in the literature for the capacity differences between α-MnO 2 and β-MnO 2 is the tunnel (also called channel) sizes of their crystal structure; α-MnO 2 has larger tunnel sizes that have been suggested to enhance ion diffusion (e.g. proton conductivity) and provide additional adsorption sites to accept cations.…”

Nanostructured MnO2 for Electrochemical Capacitor

“…This behavior suggests that faradic phenomena occur during the charge-storage mechanism. The presence of redox waves during the charge/discharge process was already reported for several MnO 2 -based electrodes (Hu & Tsou, 2002;Brousse et al, 2006;Ghodbane & Favier 2009;Chang et al, 2009;Lee et al, 2010). The electrochemical experiments in their work demonstrate that the crystallographic form of MnO 2 influences the electrochemical performance, and the small tunnel of -MnO 2 was not suitable to store cations, while the large tunnel size of -MnO 2 favors the storage of cations Cheng et al, 2010).…”