Electrochemical polymerization of pyrene derivatives on functionalized carbon nanotubes for pseudocapacitive electrodes

Electrochemical energy-storage devices have the potential to be clean and efficient, but their current cost and performance limit their use in numerous transportation and stationary applications. Many organic molecules are abundant, economical and electrochemically active; if selected correctly and rationally designed, these organic molecules offer a promising route to expand the applications of these energy-storage devices. In this study, polycyclic aromatic hydrocarbons are introduced within a functionalized few-walled carbon nanotube matrix to develop high-energy, high-power positive electrodes for pseudocapacitor applications. The reduction potential and capacity of various polycyclic aromatic hydrocarbons are correlated with their interaction with the functionalized few-walled carbon nanotube matrix, chemical configuration and electronic structure. These findings provide rational design criteria for nanostructured organic electrodes. When combined with lithium negative electrodes, these nanostructured organic electrodes exhibit energy densities of ∼350 Wh kg−1electrode at power densities of ∼10 kW kg−1electrode for over 10,000 cycles.

electrode as a function of cycle number, which were measured at a current density of 0.1 A g -1 once every 100 cycles until 1000 total cycles and then once every 499 cycles. The voltage was held before slow-rate charge and discharge for 30 min. Within each 100 (for the first 1000 cycles) or 499 cycles (for the subsequent cycles), these cells were cycled under an accelerated rate of 10 A g -1 . We speculate the initial capacity increase comes from the electrolyte having additional time to diffuse into the pores of the electrode, allowing access to additional active material. Additionally, the capacity increase can be attributed to the continued polymerization of monomers remaining on the surface of the electrode and in the electrolyte. (b) The specific capacity measured during the accelerated cycles at 10 A g -1 . One can see that there is a similar decrease in capacity at high rates as there is at low rates. Additionally, the high-rate-specific capacities show hysteresis from the slow discharge cycles.

Electrochemical measurements and calculations
All experiments on glassy carbon (GCE, CH Instrument Inc) and indium tin oxide working electrodes (Rs = 4-8Ω, Delta Technologies, LTD) were carried out in a threeelectrode cell at room temperature using a Biologic (SP300 or VSP300) potentiostat, coating has been found to vary from ~21-63% of total weight of the electrode. Cycling tests consisted of galvanostatic cycling at 10 A g -1 for 99 cycles, followed by a slower charge and discharge cycle at 0.1 A g -1 every 100 cycles up to 1000 cycles, with a 30 min voltage hold at 1.5 V or 4.5 V versus Li/Li + prior to low-rate charge or discharge, respectively. For cycle numbers between 1000 and 11,000, 499 cycles were performed for every slower charge and discharge cycle.
All performance metrics were normalized to the total weight of the positive electrode, including both the FWNT and polymer coating. Energy densities are calculated based on the integration of the discharge profiles of the potential as a function of specific capacity: Where e is the specific energy, V is the voltage of the cell, and q is the specific discharge charge. The average power density during discharge is reported on all Ragone plots and is calculated as the ratio of the energy density to the total time for discharge. The average specific capacitance is determined from the discharge profile as: Where ̅ is the average specific capacitance, q is the specifc discharge capacity (note one must multiply a value in mAh g -1 by 3.6 to convert to C g -1 for calculations of F g -1 ), and V is the difference in potential from the start to the end of discharge (3 V in the case of this research).
Binder-free electrodes were synthesized through vacuum filtration on Celgard 2500 membranes. Electrodes were further dried at 70 °C under vacuum and polymerized in a two-electrode cell typically through 5 CV cycles at 1 mV s -1 and subsequent 5 CV cycles at 5 mV s -1 between 1.5 -4.5 V versus Li/Li + .

Characterization
The microstructure of the polymer electrodes were investigated using a scanning helium-  For the neutral and oxidized form of each pyrene derivative, geometries were optimized in both the solution and gas phases. The output energies were plugged into the thermodynamic cycle (Equation 3 and Supplementary Fig. 24), which is commonly used in the DFT calculations of reduction potentials. 7 The energies were converted from hartrees to eV at T = 295 K. Due to our method of allowing the relaxed geometries to optimize in all phases and not just the gas phase, calculating the gas phase energies proved unnecessary, as the energy difference between the species in solution was identical to the energy difference calculated from the thermodynamic cycle in all cases.
Therefore, in the cases of perylene, pyrene, phenanthrene, and napthalene only the solution optimized structures and energies were calculated to determine the free energy of oxidation. ( The difference in energy between the neutral and oxidized species in solution was converted into a potential, all potentials and energy differences were referenced to those of pyrene.