Metal-free platforms for molecular thin films as high-performance supercapacitors

Abstract

Controlling chemical functionalization and achieving stable electrode-molecule interfaces for high-performance electrochemical energy storage applications remain challenging tasks. Herein, we present a simple, controllable, scalable, and versatile electrochemical modification approach of graphite rods (GRs) extracted from low-cost Eveready cells that were covalently modified with anthracene oligomers. The anthracene oligomers with a total layer thickness of ∼24 nm on the GR electrode yield a remarkable specific capacitance of ∼670 F g^-1 with good galvanostatic charge-discharge cycling stability (10 000) recorded in 1 M H₂SO₄ electrolyte. Such a boost in capacitance is attributed mainly to two contributions: (i) an electrical double-layer at the anthracene oligomer/GR/electrolyte interfaces, and (ii) the proton-coupled electron transfer (PCET) reaction, which ensures a substantial faradaic contribution to the total capacitance. Due to the higher conductivity of the anthracene films, it possesses more azo groups (-N[double bond, length as m-dash]N-) during the electrochemical growth of the oligomer films compared to pyrene and naphthalene oligomers, which is key to PCET reactions. AC-based electrical studies unravel the in-depth charge interfacial electrical behavior of anthracene-grafted electrodes. Asymmetrical solid-state supercapacitor devices were made using anthracene-modified biomass-derived porous carbon, which showed improved performance with a specific capacitance of ∼155 F g^-1 at 2 A g^-1 with an energy density of 5.8 W h kg^-1 at a high-power density of 2010 W kg^-1 and powered LED lighting for a longer period. The present work provides a promising metal-free approach in developing organic thin-film hybrid capacitors.

Fig. 1. The formation of electrochemically grafted anthracene oligomeric films, plausible on-surface composition, and spectroscopic characterization. (a) Cyclic voltammograms of E-Chem grafting of anthracene diazonium salts of 5 mM in CH₃CN with 0.1 M TBAP recorded at 100 mV s⁻¹ for 20 CV scans. (b) Digital image of graphite rod (GR) on left; on right a schematic illustration of E-Chem-grafted anthracene (ANT) multilayers with different types of nitrogen site: pyridinic N (red), pyrolytic N (green), graphitic N (blue), N–N (sky blue) on a graphite rod (GR) electrode. (c) Raman spectra of bare GR and ANT/GR. (d) Deconvoluted and fitted XPS spectra of C1s of ANT/GR, and (e) N1s of ANT/GR.

Fig. 2. Computational studies on azo-functionalized anthracene grafted on GR/N-GR through chemisorption. DFT computed charge difference density plots for (a) 1@N-GR and (b) 1@GR (iso-value 0.007 a.u). The yellow region shows charge accumulation, whereas blue shows charge depletion. Colour code: C – brown, H – white, N – blue. ETS-NOCV computed electron deformation density for (c) 1@N-GR and (d) 1@GR (iso-value – 0.001 a.u). The red and blue contours correspond to the accumulation and depletion of electron density, respectively. Direction of the negative charge flow: red → blue.

Fig. 3. Electrochemical supercapacitor analysis of oligomeric films E-Chem grafted on a graphite rod. (a) Schematic showing E-Chem-grafted anthracene (ANT) multilayers with different types of nitrogen site on a graphite rod (GR) electrode in H₂SO₄ electrolyte. (b) Comparison of cyclic voltammograms of bare GR, ANT/GR in 1 M H₂SO₄ at 100 mV s⁻¹ and 500 mV s⁻¹ scan rates. (c) Bar diagram representing total areal capacitance of bare and modified electrodes calculated from CV at 500 mV s⁻¹ (with error bars in red). (d) Plots of log(i_p) vs. log(ν) from low to high scan rates for the ANT/GR electrode in the anodic regime. (e) Plot of j (mA cm⁻²)/ν^0.5 (mV s⁻¹)^0.5vs. (ν, mV s⁻¹)^0.5 at 0.29 V; inset shows CV at 30 mV s⁻¹ scan rate illustrating capacitive (green) and diffusion-controlled (white) charge storage process. (f) Pseudocapacitive contributions of ANT/GR at different scan rates.

Fig. 4. Galvanostatic charging/discharging analysis of anthracene oligomeric films at different current densities, and cycle numbers. Galvanostatic charging/discharging (GCD) curve of (a) bare GR and (b) ANT/GR at 30 μA cm⁻². (c) A single GCD cycle of ANT/GR at 30 μA cm⁻² showing negligible potential drop, V_drop. (d) GCD plot of ANT/GR at different applied current densities. (e) Corresponding capacitance values at different current densities. (f) Cycling performance of bare GR at an applied current density of 0.1 mA cm⁻² and ANT/GR at 1 mA cm⁻²; to ensure the experiment time was the same for bare GR and ANT/GR, 0.1 mA cm⁻² current density was applied.

Fig. 5. Electrochemical impedance responses and graphical analysis for the determination of CPE parameters on (a and b) the bare electrode, (c and d) ANT-modified GR, and (e and f) after 10⁴ GCD cycles at +0.3 V (vs. Ag/AgCl in 1 M H₂SO₄).

Fig. 6. Understanding the capacitance enhancement in the light of proton-coupled electron transfer mechanism. (a) DPV of the ANT/GR electrode recorded with varying pH of H₂SO₄ solution using 0.1 M KCl as electrolyte. (b) Reduction potential vs. pH plot (from DPV data). (c) Schematic illustration of interfacial PCET pathway to and from different nitrogens present in the ANT/GR electrode.