Boosting the activation rate and capacitance retention of MnOx electrodes prepared using capping agents

Abstract

Manganese oxides are prepared by a hydrothermal method from MnCl2 solutions. The feasibility of tetrahydroxy-1,4-quinone (THQ) and 2-hydroxy-1,4-naphthoquinone (HNQ) application as capping agents (CAs) at hydrothermal conditions is demonstrated. The analysis of synthesis results provides an insight into the effect of chemical structure of THQ and HNQ on their adsorption on nanoparticles, phase content and dispersion of the nanoparticles. The use of CAs facilitates size reduction of the nanoparticles and offers multiple benefits for MnOx applications in cathodes of supercapacitors. An important finding is the ability to form MOF-like particle-based structures, which opens a door for the development of novel materials with advanced functionality. This finding expands the range of building blocks that can be used for producing MOF-like structures. Electrochemical testing of electrodes with mass of 39.8 ± 0.3 mg cm−2 is performed in 0.5 M Na2SO4 electrolyte. Testing of electrodes prepared using HNQ (Mn-HNQ) and THQ (Mn-THQ) shows highly accelerated electrode activation rate and the ability to avoid the long activation procedure, which are critically important for practical applications. Cyclic voltammetry shows high capacitances of 5.29 F and of 5.58 F cm−2 for Mn-HNQ and Mn-THQ, respectively at a sweep rate of 1 mV s−1, which are achieved at low electrode resistances (<1 Ohm for 1 cm2 electrode area). Another key advantage of the developed electrodes is their high cyclic stability and enhanced capacitance retention of 67.5 % and 68.6 % for Mn-HNQ and Mn-THQ, respectively at a sweep range of 1–100 mV s−1. As a result, the electrodes exhibit remarkably high capacitances at a sweep rate of 100 mV s−1. The approach of this investigation successfully addresses key challenges in the development of manganese oxide supercapacitors and opens an avenue for further advances in the fabrication of cathodes for supercapacitors.