Electrodeposition of manganese dioxide coatings onto graphene foam substrates for electrochemical capacitors
Highlights
- Graphene foams (Gii) with electrodeposited birnessite MnO2 via current pulse achieved a maximum capacitance of 530 mF/cm2 at 10 mA/cm2 compared to 385 mF/cm2 for galvanostatic deposition and 240 mF/cm2 for cyclic voltammetry deposition at the same current density.
- Current pulse MnO2 electrode demonstrated better rate capability (61% capacitance retention switching from 0.5 to 20 mA/cm2) and cycling retention (93% after 5000 cycles).
- Mild electrooxidation of the graphene foam was shown to increase the baseline capacitance from 2.5 mF/cm2 for the pristine electrode to 35 mF/cm2, due to pseudocapacitance from the functionalised graphene, identified via infrared spectroscopy as surface hydroxy (-OH) and C=O groups.
Abstract
Optimisation of electrodeposition routes of birnessite manganese dioxide (MnO2) coatings onto 3D graphene foam substrates enabled greater attainable capacitances. Current pulse deposition method resulted in highest achievable areal capacitance of 530 mF/cm2 under a 10 mA/cm2 current rate, cycling performance with 91% retention after 9000 cycles, as well as improved rate capability when compared to the cyclic voltammetry or galvanostatic deposition.
Introduction of oxygen functional groups to the graphene foam added initial pseudo capacitance and accelerated the rate for nucleation and growth of the MnO2 crystal grains, resulting in an areal capacitance of 410 mF/cm2 under a 10 mA/cm2 current rate. However, in this case the increase in specific capacitance was accompanied by sluggish kinetics for charge storage seen via impedance spectroscopy.
The charge storage mechanism of the deposited MnO2 films was investigated using in situ Raman microscopy and analysis of peak shifts revealed expansion and contraction of birnessite MnO2, relating to exchange of Na+ and H2O at the MnO2 interface.
Introduction
One of the main challenges for EC’s is to the increase energy density per foot-print area, but not at the expense of power density or cyclability. The energy density can be improved via enlarging the operational voltage window or by increasing the specific capacitance of the active material. For the latter, a common approach is to incorporate metal oxides that can add pseudocapacitance to the system increasing the energy content via fast reversible redox reactions [3].
Manganese oxides are often chosen as an electrode materials for such applications due to high theoretical capacitance, natural abundance, lower cost, non-toxicity, and suitability for applications in neutral electrolytes [4]. Unlike double-layer capacitors that utilise opposing charge separation in closely orientated layers of high surface area, the energy storage in these oxides (using the example of MnO2) occurs either by rapid surface intercalation/extraction of protons or alkali cations or via surface adsorption of electrolyte cations