Electrostatic Spray Deposition-Based Manganese Oxide Films-From Pseudocapacitive Charge Storage Materials to Three-Dimensional Microelectrode Integrands

In this study, porous manganese oxide (MnO_x) thin films were synthesized via electrostatic spray deposition (ESD) and evaluated as pseudocapacitive electrode materials in neutral aqueous media. Very interestingly, the gravimetric specific capacitance of the ESD-based electrodes underwent a marked enhancement upon electrochemical cycling, from 72 F∙g^-1 to 225 F∙g^-1, with a concomitant improvement in kinetics and conductivity. The change in capacitance and resistivity is attributed to a partial electrochemical phase transformation from the spinel-type hausmannite Mn₃O₄ to the conducting layered birnessite MnO₂. Furthermore, the films were able to retain 88.4% of the maximal capacitance after 1000 cycles. Upon verifying the viability of the manganese oxide films for pseudocapacitive applications, the thin films were integrated onto carbon micro-pillars created via carbon microelectromechanical systems (C-MEMS) for examining their application as potential microelectrode candidates. In a symmetric two-electrode cell setup, the MnO_x/C-MEMS microelectrodes were able to deliver specific capacitances as high as 0.055 F∙cm^-2 and stack capacitances as high as 7.4 F·cm^-3, with maximal stack energy and power densities of 0.51 mWh·cm^-3 and 28.3 mW·cm^-3, respectively. The excellent areal capacitance of the MnO_x-MEs is attributed to the pseudocapacitive MnO_x as well as the three-dimensional architectural framework provided by the carbon micro-pillars.