A Simple and Expeditious Route to Phosphate-Functionalized, Water-Processable Graphene for Capacitive Energy Storage

Abstract

Phosphate-functionalized carbon-based nanomaterials have attracted significant attention in recent years owing to their outstanding behavior in electrochemical energy-storage devices. In this work, we report a simple approach to obtain phosphate-functionalized graphene (PFG) via anodic exfoliation of graphite at room temperature with a high yield. The graphene nanosheets were obtained via anodic exfoliation of graphite foil using aqueous solutions of H 3 PO 4 or Na 3 PO 4 in the dual role of phosphate sources and electrolytes, and the underlying exfoliation/ functionalization mechanisms are proposed. The effect of electrolyte concentration was studied, as low concentrations do not lead to a favorable graphite exfoliation and high concentrations produce fast graphite expansion but poor layer-by-layer delamination. The optimal concentrations are 0.25 M H 3 PO 4 and 0.05 M Na 3 PO 4 , which also exhibited the highest phosphorus contents of 2.2 and 1.4 at. %, respectively. Furthermore, when PFG-acid at 0.25 M and PFG-salt at 0.05 M were tested as an electrode material for capacitive energy storage in a three-electrode cell, they achieved a competitive performance of ∼375 F/g (540 F/cm 3 ) and 356 F/g (500 F/ cm 3 ), respectively. Finally, devices made up of symmetric electrode cells obtained using PFG-acid at 0.25 M possess energy and power densities up to 17.6 Wh•kg −1 (25.3 Wh•L −1 ) and 10,200 W/kg; meanwhile, PFG-salt at 0.05 M achieved values of 14.9 Wh• kg −1 (21.3 Wh•L −1 ) and 9400 W/kg, with 98 and 99% of capacitance retention after 10,000 cycles, respectively. The methodology proposed here also promotes a circular-synthesis process to successfully achieve a more sustainable and greener energy-storage device.

Effect of electrolyte and carbon material on the electrochemical performance of high-voltage aqueous symmetric supercapacitors

“…5) on the surface of P-rGO (94.9°) is lower than that of AC (110.8°), suggesting that the hydrophilic phosphate and oxygen functional groups on the surface of P-rGO improve the overall hydrophilicity of the electrode active material [45]. The introduction of P-functional groups in graphene enhances wettability, thus ensuring a more efficient transport of the electrolyte ions at the electrode surface [46].…”

“…At the same current regime, P-rGO exhibits a capacitive performance that is different from that of electrodes composed of AC, which is convergent with the results obtained from the CV experiments. The shape of the GCD curve for P-rGO shows some degree of distortion, and this deformation can be caused by the pseudocapacitive effect of the active material during charging and discharging [52,53].…”

“…If the supercapacitor is charged between 0 and 2.0 V, the operating potential windows for the positive and negative electrodes are [-0.09; 0.84] and [-1.16; -0.09] V versus Ag/AgCl, respectively. The longer charging and discharging time observed for the ACbased supercapacitors compared to those for the P-rGO-based device corresponds to a higher available capacity [53], calculated using Eq. ( 2), which is presented in Fig.…”

Catalyzing sustainability: phytic acid as a green precursor for metal-free carbon electrocatalysts in ORR

“…However, in recent years, there has been increasing interest in using P as a heteroatom to produce high-performance carbons. 36 As a result, P has been considered the “next heteroatom to be explored” by the scientific community. 37…”

Effects of Heteroatom Doping and Physicochemical Character on the Electrochemical Properties of Graphene Sheets

“…In addition to the concerns about electrochemical durability, electrical conductivity loss caused by the deterioration of the π-conjugated structure in the presence of dense defects and functional groups should be also avoided. [7] For these reasons, methods such as controlling the oxidation level, [2] increasing the density of selected functional groups, [5,10,11] and common heteroatom doping methods [1,8,12,13,14,15] have been employed to maximize the yield from the reaction sites in the electrochemical applications of GRMs. In the literature, heteroatom doping of GRMs has been reported to improve the specific capacitance of GRMs as a result of a variety of effects including quantum capacitance modification, [16,17] increasing the active sites on the surface by creating more defects [6] enhancing the electronic conductivity, [18] providing ion transportation channels [19] and preventing the stacking of sheets.…”