Densification and Co-Doping of Laser-Induced Graphene for Boosting Electrochemical Performance of Flexible Supercapacitors

Laser-induced graphene (LIG) has been one of promising materials that can substitute for conventional graphene or graphene-like electrode materials due to its simple fabrication, mechanical flexibility, and electrochemical properties tantamount to three-dimensional graphene materials. LIG is produced by irradiating polymer, being represented by polyimide (PI), with a laser ranging from UV to near-IR lasers. While numerous studies demonstrated the promises of typical LIG for electrodes of energy storage devices such as supercapacitors, this as-obtained LIG suffered from inherently limited surface area. The small surface area of LIG is critically related to limited energy storage performance, which should be overcome for practically applying LIG in energy storage devices.<br/>In this study, we present three strategies for boosting electrochemical capacitance of LIG: densification, doping, and the incorporation of pseudocapacitive metal oxide. The duplicate laser-induced pyrolysis method has been developed to simultaneously implement these goals. Unlike typical methods for LIG fabrication, as implied by “duplicate”, laser irradiation is conducted twice in this method. First, LIG is produced by irradiating a PI sheet with a CO₂ laser beam, which corresponds to typical LIG. Then, the LIG is coated with a polyamic acid (PAA) layer that contains precursors for dopants and (or) pseudocapacitive metal oxides. The PAA layer is treated with heat for imidization, and the additional PAA layer is converted into PI. Finally, this sample is irradiated with the laser again, and a high-performance LIG electrodes is obtained. Because of the additional loading of PI and its pyrolysis into LIG, densification of LIG is achieved, and interestingly doping of LIG and (or) decoration of LIG with metal oxide nanoparticles are observed.<br/>Based on the duplicate laser pyrolysis of PI, LIG could be doped with nitrogen only, simultaneously with nitrogen and boron, or simultaneously with nitrogen and phosphorus. These LIG electrodes are denoted by N-LIG, NB-LIG, and NP-LIG, respectively. While the electrochemical properties of these doped LIG varied with laser processing parameters and the concentration of doping solutions, the optimally doped LIG electrodes all exhibited remarkably improved electrochemical capacitance superior to those of typical LIG. For instance, the specific capacitances of N-LIG, NB-LIG, and NP-LIG were 49.0, 104.3, and 163.6 mF/cm² at 0.2 mA/cm²in 1 M H₂SO₄ electrolyte, whereas that of undoped LIG was only ~9 mF/cm². These doped LIG maintained the excellent flexibility of LIG, showing negligible capacitance change by bending. Moreover, when a metal precursor was additionally loaded in the doping solution, metal oxide nanoparticles was generated on the surface of doped LIG, which potentially further boost the capacitance of the doped LIG because of pseudocapacitance of metal oxides.<br/>In summary, the duplicate laser-induced pyrolysis enabled the facile fabrication of heteroatom-doped LIG. While this method requires additional coating process and laser irradiation, densification and (co)doping of LIG were simultaneously achieved, and the resulting LIG electrodes exhibited starkly increased capacitance. We envision the use of this method for doping of other energy-storage carbon nanomaterials.