Microstructure and electrochemical properties of high performance graphene/manganese oxide hybrid electrodes

Hybrids consisting of 2D ultra-large reduced graphene oxide (RGO) sheets (∼30 μm long) and 1D α-phase manganese oxide (MnO₂) nanowires were fabricated through a versatile synthesis technique that results in electrostatic binding of the nanowires and sheets. Two different hybrid (RGO/MnO₂) compositions had remarkable features and performance: 3 : 1 MnO₂/RGO (75/25 wt%) denoted as 3H and 10 : 1 MnO₂/RGO (90/10 wt%) denoted as 10H. Characterization using spectroscopy, microscopy, and thermal analysis provided insights into the microstructure and behavior of the individual components and hybrids. Both hybrids exhibited higher specific capacitance than their individual components. 3H demonstrated excellent overall electrochemical performance with specific capacitance of 225 F g^-1, pseudocapacitive and electrochemical double-layer capacitance (EDLC) contributions, charge-transfer resistance <1 Ω, and 97.8% capacitive retention after 1000 cycles. These properties were better than those of 10H; this was attributed 3H's more uniform distribution of nanowires enabling more effective electronic transport. Thermal annealing was used to produce reduced graphene oxide (RGO) that exhibited significant removal of oxygen functionality with a resulting interlayer spacing of 0.391 nm, higher D/G ratio, higher specific capacitance, and electrochemical properties representing more ideal capacitive behavior than GO. Integrating ultra-large RGO with very high surface area and MnO₂ nanowires enables chemical interactions that may improve processability into complex architectures and electrochemical performance of electrodes for applications in electronics, sensors, catalysis, and deionization.