Understanding enhanced charge storage of phosphorus-functionalized graphene in aqueous acidic electrolytes

• Published in: Electrochimica acta 2020-11, Vol.361, p.136985, Article 136985
• Main Authors: Moreno-Fernández, Gelines, Gómez-Urbano, Juan Luis, Enterría, Marina, Cid, Rosalia, López del Amo, Juan M., Mysyk, Roman, Carriazo, Daniel
• Format: Article
• Language: English
• Subjects: Annealing; Aqueous electrolytes; Benzoquinone; Capacitance; Charge materials; Charge storage; Chemical evolution; Electrochemical activation; Electrochemical analysis; Electrolytes; Graphene; Hydrogen storage; Hydroquinone; Interlayers; NMR; Nuclear magnetic resonance; Phosphonates; Phosphoric acid; Phosphorus; Phosphorus functionalized graphene oxide; Solid state nmr; Sulfuric acid; Supercapacitor; Surface chemistry; X ray photoelectron spectroscopy
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2020.136985

•Phosphorous functionalized graphene samples are developed•Solid state NMR proves two different phosphorous species related to the temperature treatment•Charge storage is improved by prior electrochemical activation in 1 M H2SO4 electrolyte•No capacitance loss after 60,000 charge discharge cycles at 1.5 V The mechanisms behind enhanced charge storage of P-functionalized carbons are unraveled for the first time using non-porous graphene oxide treated with phosphoric acid and annealed at either 400 or 800 °C. The electrochemical study in 1 M H2SO4 reveals that phosphorus groups boost charge storage and electrochemical stability, with more effect for the higher annealing temperature. Annealing at 800 °C also leads to the material withstanding 60,000 charge-discharge cycles with no capacitance loss at 1.5 V. The improvement in the electrochemical performance is shown to be mainly governed by the change in surface chemistry comprehensively studied with NMR, FTIR and XPS characterization techniques. The collective analysis of electrochemical response and surface chemistry demonstrates that enhanced charge storage by phosphorus-functionalized graphene materials is made possible due to the following synergistic mechanisms: i) non-Faradaic charging; ii) nascent hydrogen storage in the interlayer; iii) benzoquinone-to-hydroquinone redox processes; iv) phosphate-to-phosphonate like transformation. From the practical perspective, the stored charge can be boosted due to the higher capacitance upon prior electrochemical activation in the vicinity of oxygen evolution potential and the wider usable electrochemical window enabled by phosphorus-related groups.