Electrodeposition of Metal Oxides on Spray-Coated Nanostructured Carbon Framework As High Performance Electrode Materials for Asymmetric Pseudocapacitors
Authors: Rogier Clémence , Pognon Grégory , Bondavalli Paolo , Galindo Christophe , Nguyen Tran-Minh-Giao , Vancaeyzeele Cédric , Aubert Pierre-Henri 
Published: May 2020

Combining transition metal oxides and nanostructured hierarchical carbon materials is considered as one of the best strategy to achieve high performance supercapacitors electrodes [1]. Here is reported a novel way to increase energy densities while maintaining high power densities exploiting pseudocapacitance and nanostructuration of electrode materials.
In the first part of this study, carbon nanotubes/graphene/manganese dioxide nanostructured films were designed as pseudocapacitors electrodes (Fig. A). While MnO2 is a well-known pseudocapacitive cathode material and will increase energy densities [2], nanostructured carbon materials provide high electronic conductivity and specific surface. The challenge is to assemble these three materials (CNTs/G/MnO2) into a nanostructured electrode with controlled homogeneity, morphology, and composition. First, carbon nanotubes and graphene foils are associated in a specific multi-layered organization onto a current collector by dynamic spray gun deposition. It creates a controlled porous network in the electrodes. This deposition method used for nanostructuration of thin films generates homogeneous mats with tunable thickness. This structuration of the carbon nanomaterials is reproducible, easily scalable and low-cost considering that it is compatible with large surfaces. The CNTs/G multilayered structure provides a good accessibility with an appropriate porosity for ion insertion (pores of 2 to 6 nm), a high specific surface area (~360 m2/g), and an electrically conductive network (~3600 S/m), all necessary to confer high power densities. Our approach consists in synthesizing MnO2 by anodic electrodeposition directly onto the conductive nanostructured carbon framework. Electrodeposition is very versatile [3] and simply. The morphology of the manganese dioxide (shape, particles size, and mass loading) can be controlled and the electrochemical performances can be tuned by adjusting the deposition conditions. The MnO2 nanoparticles are deposited through the thickness (seen in cross section Figure A) of the carbon mat with controlled sizes ranging from 50 to 200 nm.
Materials are characterized by SEM, XPS, microporosity analysis and electrochemistry. Results demonstrate that the capacitance can reach up to 220 F/g for binder free electrodes with a MnO2 mass loading of 55%. The pseudocapacitive oxide insures a faradaic contribution that enhances the specific capacitance by a factor 5 compared to the carbon nanomaterials structure only. The electrodes present an outstanding stability of 96% over 3 000 cycles in aqueous electrolytes (Na2SO4 1M). Then an asymmetric system with activated carbon as the negative electrode is developed to demonstrate the energy density increase compared to a symmetric system with activated carbon as electrodes. When adding MnO2 to the cathode the exploitable potential window can be widen up to 1.4V and the energy density increases from 1 Wh/kg to 6 Wh/kg for a power density of 0.7 kW/kg. Stability of 85% in capacitance was demonstrated for 20 000 cycles on this system.
The second part of the contribution is devoted to molybdenum oxide (MoO3) that has been recently introduced as an appropriate anode material for pseudocapacitors in view of its low cost nature and high electrochemical activity [4,5]. It has been reported to be suitable in terms of potential window as a negative counterpart to manganese oxide in asymmetric pseudocapacitors [6]. This study focuses on the development of a carbon nanotubes/molybdenum oxide composite by spray-coating and cathodic electrodeposition (Fig. B). The growth of this specific metal oxide is quite complex with several possible intermediaries presenting different oxidation states depending on the environment [7]. Studies on the impact of the chemical and electrochemical conditions on the morphology, crystallinity, and homogeneity of the oxide layer are performed and thermal treatments aiming to enhance these parameters and stabilize the electrochemical performances are explored. Solubility of the deposited MoOx has been found to be a major issue in aqueous electrolytes and in that regard tests have been performed in organic electrolytes with lithium salts to insure stability of the final system. First results obtained for a simple binary MoOx-CNTs electrode already present a good maximum specific capacitance of 110 F/g.

[1] National Science Review, 4, 1 (2017) 71–90

[2] Prog. Mater. Sci. 74 (2015) 51-124.

[3] Ceram. Int. 44 (2018) 10863-10870.

[4] Chem. Commun., 2011, 47, 10058–10060

5] Materials Letters 66 (2012) 102–105

[6] Adv. Funct. Mater. 2013, 23, 5074–5083

[7] Analytica Chimica Acta 496 (2003) 39–51

Figure 1