N-doped Reduced Graphene Oxide Aerogel Research Articles

High-Performance Asymmetric Supercapacitors of MnCo2O4 Nanofibers and N-Doped Reduced Graphene Oxide Aerogel.

The working potential of symmetric supercapacitors is not so wide because one type of material used for the supercapacitor electrodes prefers either positive or negative charge to both charges. To address this problem, a novel asymmetrical supercapacitor (ASC) of battery-type MnCo2O4 nanofibers (NFs)//N-doped reduced graphene oxide aerogel (N-rGOAE) was fabricated in this work. The MnCo2O4 NFs at the positive electrode store the negative charges, i.e., solvated OH-, while the N-rGOAE at the negative electrode stores the positive charges, i.e., solvated K+. An as-fabricated aqueous-based MnCo2O4//N-rGOAE ASC device can provide a wide operating potential of 1.8 V and high energy density and power density at 54 W h kg-1 and 9851 W kg-1, respectively, with 85.2% capacity retention over 3000 cycles. To understand the charge storage reaction mechanism of the MnCo2O4, the synchrotron-based X-ray absorption spectroscopy (XAS) technique was also used to determine the oxidation states of Co and Mn at the MnCo2O4 electrode after being electrochemically tested. The oxidation number of Co is oxidized from +2.76 to +2.85 after charging and reduced back to +2.75 after discharging. On the other hand, the oxidation state of Mn is reduced from +3.62 to +3.44 after charging and oxidized to +3.58 after discharging. Understanding in the oxidation states of Co and Mn at the MnCo2O4 electrode here leads to the awareness of the uncertain charge storage mechanism of the spinel-type oxide materials. High-performance ASC here in this work may be practically used in high-power applications.

Charge storage mechanisms of manganese oxide nanosheets and N-doped reduced graphene oxide aerogel for high-performance asymmetric supercapacitors.

Although manganese oxide- and graphene-based supercapacitors have been widely studied, their charge storage mechanisms are not yet fully investigated. In this work, we have studied the charge storage mechanisms of K-birnassite MnO2 nanosheets and N-doped reduced graphene oxide aerogel (N-rGOae) using an in situ X-ray absorption spectroscopy (XAS) and an electrochemical quart crystal microbalance (EQCM). The oxidation number of Mn at the MnO2 electrode is +3.01 at 0 V vs. SCE for the charging process and gets oxidized to +3.12 at +0.8 V vs. SCE and then reduced back to +3.01 at 0 V vs. SCE for the discharging process. The mass change of solvated ions, inserted to the layers of MnO2 during the charging process is 7.4 μg cm−2. Whilst, the mass change of the solvated ions at the N-rGOae electrode is 8.4 μg cm−2. An asymmetric supercapacitor of MnO2//N-rGOae (CR2016) provides a maximum specific capacitance of ca. 467 F g−1 at 1 A g−1, a maximum specific power of 39 kW kg−1 and a specific energy of 40 Wh kg−1 with a wide working potential of 1.6 V and 93.2% capacity retention after 7,500 cycles. The MnO2//N-rGOae supercapacitor may be practically used in high power and energy applications.

Enhancing the charge-storage performance of N-doped reduced graphene oxide aerogel supercapacitors by adsorption of the cationic electrolytes with single-stand deoxyribonucleic acid

Single-stand deoxyribonucleic acid/N-doped reduced graphene oxide aerogel (ssDNA/N-doped rGO) composites were synthesized and used as the electrode materials for supercapacitors. The mass change of the electrolytes adsorbed on the ssDNA/N-doped rGO electrode during a charge/discharge process determined by an electrochemical quartz crystal microbalance (EQCM) at a cathodic potential range (−0.5 to −0.1 V vs. Ag/AgCl) is 10-fold higher than that at an anodic potential range (−0.1–0.3 V vs. Ag/AgCl). This is because the ssDNA having negatively charged phosphate and nitrogen-containing nucleobase groups prefers cationic electrolytes to anionic ones. An as-fabricated supercapacitor of the ssDNA/N-doped rGO with a coin-cell size (CR2016) provides 644 F/g at 0.5 A/g, which is 1.3-fold higher than that of the N-doped rGO device. The specific power and specific energy of the as-fabricated ssDNA/N-doped rGO device are ca. 30 Wh/kg and 4500 W/kg, respectively with a capacity retention of 94.1% over 2500 cycles. Two as-fabricated supercapacitor cells connected in series can supply electricity to the red LED over 10 min. The composite materials in this work may also be used in other applications where the functional groups of ssDNA can play an important role to the adsorption of the cationic electrolytes.

N-doped reduced graphene oxide aerogel coated on carboxyl-modified carbon fiber paper for high-performance ionic-liquid supercapacitors

Nitrogen-doped reduced graphene oxide aerogel (N-rGO aerogel) with high porosity and ionic conductivity were synthesized by a hydrothermal reduction of graphene oxide with hydrazine and following freezing-dry method. N-rGO aerogel was spray-coated on carboxyl-modified carbon fiber paper with a hydrophilic surface and used as the supercapacitor electrode. Not only can the N-rGO aerogel electrode accelerate the diffusion of the electrolyte but also it can store electronic charge via a surface redox reaction due to the N-containing groups. Among the electrolytes studied, the ionic liquid-based supercapacitor of the N-rGO aerogel provides a wide working potential of ca. 4.0 V and rather high specific capacitance of 764.53 F/g at 1 A/g with a capacity retention of 86% over 3000 charge–discharge cycles as well as maximum specific power and energy of 6525.56 W/kg and 245.00 Wh/kg, respectively. The device prototype fabricated in a single coin cell shape (CR2016) can supply electricity to red LED over 17 min.