Graphenated Carbon Nanotubes for Enhanced Nucleation of Manganese Oxide Electrodeposits in High Performance Composite Electrodes

Carbon nanotubes (CNTs) and graphenes are emerging electrode materials currently being investigated for supercapacitor applications due to their unique properties that offer advantages over traditional activated carbons including ultra-low resistivity (~ 10e^-6 Ω cm) (1), high specific surface area (2630 m²/g for single graphene sheet (2)), and tailorable mesopore size distributions.  In particular, the vertically oriented forms deposited by plasma enhanced chemical vapor deposition (PECVD) have received strong attention recently due to the preponderance of exposed graphitic edge-planes, the ability for direct growth of nanostructures perpendicular to metal current collectors, and increases in capacitance by fine-tuning the inter-tube or inter-sheet distance. These attributes are important since the graphitic edge-plane sites are known to have 20x greater double-layer capacitance (50-70 μF/cm²) than the basal plane sites (3 μF/cm²) (3), thus improving specific energy density.  Likewise, growth directly on metallic substrates can significantly reduce contact resistance with current collectors and negate inter-particle resistances in the electrode, thereby lowering equivalent series resistance (ESR) and improving power densities.  Furthermore, CNTs, graphenes, and related hybrid materials serve as highly conductive nanostructured platforms for battery materials or metal oxides that provide significant psuedocapacitance for use in asymmetric or hybrid supercapacitors.  Recently, manganese oxides (MnOx) have gained attention as a preferred battery material in aqueous electrolytes due to their high theoretical capacity (~ 1100 F/g) (4) and relatively lower cost. 

Recently, we have developed a single PECVD process (5) to grow vertically aligned carbon nanotubes with few-layered graphene protruding orthogonally from the sidewalls, referred to as graphenated carbon nanotubes (g-CNTs).  The fundamental advantage of g-CNTs is the 3D high volume-density framework of the CNT forest coupled with the high charge density of the 2D graphene edges.  We previously demonstrated that electrodes made from g-CNTs achieved a 7x improvement in specific capacitance relative to a CNT electrode without graphitic foliates when charged with high frequency pulses (6).

Here, we demonstrate that g-CNTs are an ideal platform for formation of composite electrodes with manganese oxides for energy storage applications due to their superior performance.  Preliminary results show that a g-CNT/MnOx electrode had up to 5x greater specific capacitance than a CNT/MnOx electrode (Figure 1) formed under an identical electrodeposition process.  The key factor in the improved performance is that the graphene foliates appear to serve as nucleation sites for nanostructured MnOx deposits that form along individual g-CNTs (Figure 2).  This is in contrast to results from REF (7) which indicate that MnOx nanoflowers only form at the junctions of overlapping or intersecting CNTs with no deposits on an isolated nanotube. Furthermore, the ability to vary the density of reactive graphene foliates on g-CNTs by increasing growth time (Figure 3) enables one to control or optimize the MnOx loading and utilization efficiency in the composite structure for maximum specific capacitance at a particular current density (Figure 4).  Overall these results which are yet to be optimized have revealed important benefits and promise for potential applications using this exciting new material.                

References:

1.         D. A. Areshkin, D. Gunlycke, C. T. White, Nano Lett. 7, 204 (2006).

2.         M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Nano Lett. 8, 3498 (2008).

3.         J. P. Randin, E. Yeager, J. Electrochem. Soc. 118, 711 (1971).

4.         S. L. Candelaria et al., Nano Energy 1, 195 (2012).

5.         C. B. Parker, A. S. Raut, B. Brown, B. R. Stoner, J. T. Glass, J. Mater. Res. 27, 1046 (2012).

6.         B. R. Stoner, A. S. Raut, B. Brown, C. B. Parker, J. T. Glass, Appl. Phys. Lett. 99, 183104 (2011).

7.         H. Zhang et al., Nano Lett. 8, 2664 (2008).