Abstract
In this study, porous manganese oxide (MnOx) 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 MnOx/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 MnOx-MEs is attributed to the pseudocapacitive MnOx as well as the three-dimensional architectural framework provided by the carbon micro-pillars.
Highlights
The technological advancement toward small-scale and portable devices has resulted in an increased demand for micro-power systems
The broad peak centered at 18.64◦ is indexed as (002) plane of the birnessite MnO2 phase, whereas the two faint peaks located at 36.7◦ and 65.7◦ are identified as (006) and (119) planes, respectively (JCPDS Card Number: 00-018-0802) [35]
It should be noted that while the majority of the peaks signaling –OH bending and stretching are still visible, the peak at 602 cm−1 which signals the presence of the tetrahedral stretch of Mn-O disappears, indicating a loss of order in the crystal structure as compared to the as-synthesized manganese oxide (MnOx) films
Summary
The technological advancement toward small-scale and portable devices has resulted in an increased demand for micro-power systems. The majority of the micro-devices rely on batteries to provide the required energy and power. Electrochemical capacitors or supercapacitors, on the other hand, are electrochemical energy storage systems that possess higher power densities than batteries along with superior lifetime. Conventional supercapacitors, are too bulky for small-scale applications and their fabrication methods are not compatible with the currently existing Integrated Circuit (IC) technology. Of immediate need is downsizing supercapacitors with compatible microelectronic fabrication techniques, so that they can be placed directly on the chip. Such devices, referred to as micro-supercapacitors (MSCs), generally possess negligible active material masses and, their performance metrics are typically normalized by the area of the system. Volumetric/stack normalization is popular for reporting MSC performance, since it provides insight into intrinsic material properties, as well as device architecture
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