Understanding Hydrothermally Reduced Graphene Oxide Hydrogels: From Reaction Products to Hydrogel Properties

Abstract

We studied the chemical processes that take place during hydrothermal gelation of graphene oxide (GO), quantifying the reaction products generated during hydrothermal reduction. The gelation proceeds with disproportionation of GO yielding a large amount of CO2 (about a quarter of the original mass of GO), organic acidic fragments, and CO. The CO2 that is formed is trapped in the hydrogel creating macroscopic voids which can lead to cracking of the hydrogel during compression. We were able to quantify the amount of CO2 produced in situ by adding ammonia during the synthesis, and converting CO2 into ionic carbonate species that we could easily quantify by titration. We used titration to evaluate the formation of organic acidic fragments too and evaluated the amount of H2O and CO produced by thermogravimetric analysis and mass balance. The conversion of CO2 into ionic species allowed us to produce void-free hydrogels which remain structurally stable after extensive compression. However, such hydrogels on average showed lower mechanical strength and electrical conductivity than the hydrogels with voids. This is a result of the difference in chemistry and morphology between hydrogels reduced under acidic pH and basic pH. Our work provides for the first time a clear quantitative estimate of CO2 evolution and organic fragment formation during hydrothermal reduction of GO, an overall picture of the reaction products, and a deepened understanding of the conditions that can be used to prepare stronger and more conductive graphene hydrogels and aerogels.

Enhanced Electrocatalysis via 3D Graphene Aerogel Engineered with a Silver Nanowire Network for Ultrahigh‐Rate Zinc–Air Batteries

“…Formation of large channels in GA could be caused by gaseous products. As previously reported, nearly half of the mass of GO was lost and released in the form of carbon dioxide (CO 2 ) as well as a small amount of carbon monoxide (CO) during hydrothermal reduction . The reducing agent NaHSO 3 may also lead to the generation of sulfur dioxide (SO 2 ).…”

Achieving High Capacitance of Paper‐Like Graphene Films by Adsorbing Molecules from Hydrolyzed Polyimide

“…In previous work, researchers have used graphene hydrogels as supercapacitor electrodes to achieve high areal mass loading . Though the graphene mass loading of a compressed graphene hydrogel film can be several mg per square centimetre, the typical preparation methods are to condense GO sheets from aqueous dispersion into a black sponge with size much smaller than the volume of original solution, and then mechanically compress pieces of sliced sponge into thin films to act as supercapacitor electrodes . This approach limits the size of such films.…”

“…[19,20] Though the graphene mass loading of a compressed graphene hydrogel film can be several mg per square centimetre, the typical preparation methods are to condense GO sheets from aqueous dispersion into a black sponge with size much smaller than the volume of original solution, and then mechanically compress pieces of sliced sponge into thin films to act as supercapacitor electrodes. [19,21] This approach limits the size of such films. In this work, NG films were prepared with different mass loadings ranging from 0.5 to 4.8 mg cm −2 .…”

Ultrafast Delamination of Graphite into High‐Quality Graphene Using Alternating Currents

“…Thel ow value of the I d /I g ratio (0.16) reveals al ow level of disorders or defects at inplane region of EG sheets.T his value is much smaller than that of graphene materials based on GO (generally exceeding 1) [25] and anodic exfoliated graphene from aqueous electrolytes (0.25-0.95). [27] Epoxy groups (C-O-C) and carboxyl groups (-C(O)-O) are absent, which is most likely due to the electrochemical reduction in the exfoliation process [28] (the conversion reactions are proposed in the Supporting Information, Figure S8). Based on results from XPS,the oxygen content of the EG powder was evaluated to be 4.5 atom %, thus being higher than that of the graphite precursor (1.8 atom % oxygen).…”