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Subject of the thesis

Pillared-graphene materials for supercapacitors: from material development to electrical double-layer characterization

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Published on 25 November 2019
Summary
The goal of the PhD project is to develop pillared graphene matrixes with improved storage performances for supercapacitor (SC). These architectures will be obtained using pillar molecules selected to promote ions intercalation and transport (polarity modification or electrolyte nature). The electrochemical evaluation of the materials will be performed in order to qualify these new systems with respect to other existing carbon material not displaying 2D porosity. Ion transport inside these structures is a key aspect that will be studied notably by the mean of ex-situ ss-NMR. This in-depth investigation of the electrical-double layer formation mechanism inside these materials will allow to optimize further the materials and cells (electrolyte choice, pillar nature, conductivity enhancement…). Considering the model-like nature of these materials, these studies will also provide wider knowledge on the ion dynamics inside 2D porosity.

Detailed subject
Context:
Electrochemical double-layer capacitors (EDLC), also known as supercapacitors (SCs), are devices that store energy through charge separation from electrolytic ion sorption on charged electrode surfaces. Porous carbons such as activated carbons (ACs) are traditionally used as electrode materials due to their high surface areas and low costs. In parallel to ACs, various graphene derivatives have been proposed as potential materials for SCs owing to their high electrical conductivities, large surface areas and mechanical flexibilities. Reduced graphene oxide (RGO), readily prepared from graphene oxide (GO), is extensively studied as a model graphene-like material. RGO displays good power capability but suffers from low capacitances as the reduced graphene sheets partially restack through π- π interactions.
It has been shown that graphene sheets could be assembled to form structured graphene frameworks to limit this restacking but also translate the properties of individual sheets to functional materials and allow practical applications. The key features of these frameworks in terms of electrochemical storage applications are their graphitization level, their structural or textural disorder, and their porosity.
Exploring the layered structures of graphene derivatives for ion sorption is another approach followed to avoid graphene layers restacking. The graphitic stack with 3.3 Å inter-layer separation is too small for ion sorption but could be tuned with an intercalant to exhibit an expanded layer structure.

Group recent work:
In our group, we synthesized a class of pillared graphene materials with varied inter-layer separation using alkyl diamines as pillars speculating that such expanded layered structures could offer additional ion sorption sites and improve storage performances in supercapacitors (SCs). These pillared graphene materials have then been assembled into graphene hydrogel to optimize the ions transport inside electrode bulk porosity. The impressive storage performances (Cv = 200 F/cm 3) achieved – among the best reported to date for graphene-derived samples - demonstrated the success of this strategy.

Related publications:
Banda, H. et al. Ion Sieving Effects in Chemically Tuned Pillared Graphene Materials for Electrochemical Capacitors. Chemistry of Materials 2018, 30, 3040–3047.
Banda, H. et al. Sparsely Pillared Graphene Materials for High-Performance Supercapacitors: Improving Ion Transport and Storage Capacity ACS Nano 2019, 13, 1443-1453.

PhD project objectives:
The goal of the PhD project is to develop further these pillared graphene matrixes to obtain improved storage performances for supercapacitor (SC). These architectures will be obtained using pillar molecules selected to promote ions intercalation and transport (polarity modification or electrolyte nature). The electrochemical evaluation of the materials will be performed in order to qualify these new systems with respect to other existing carbon material not displaying 2D porosity. The ion transport inside these structures is a key aspect that will be studied notably by the mean of ex-situ ss-NMR. This in-depth investigation of the electrical-double layer formation mechanism inside these materials will allow to optimize further the materials and cells (electrolyte choice, pillar nature, conductivity enhancement…); and considering the model-like nature of these materials these studies will also provide new knowledge on ion dynamics inside 2D porosity.

Technical content of the PhD project:
The tasks of the PhD will be to design, synthesize and characterize the next generation pillared graphene materials. New pillar molecules chosen to facilitate and promote the ions transport inside the graphene galleries will be investigated. Graphene assemblies with varying bulk porosity will be prepared. Physico-chemical characterization (ss-NMR, XPS, TGA, SEM…) will be performed on all samples to allow a comprehensive comparison of the various materials properties. The most interesting graphene–based assemblies will be selected and tested electrochemically in supercapacitor cells using galvanostatic cycling, cycling voltammetry and electrochemical impedance spectroscopy. The formation of the electrochemical double-layer as well as the ionic charge transfer will investigated by the mean of ss-NMR and XPS ( in situ).

Outcome for the student:
This PhD project is dealing with a highly interesting societal issue, which is the electrochemical storage of energy. The student will hence be sensibilized to these issues. He will also gain extensive knowledge on electrochemical storage, supercapacitors and graphene-based materials as well as hands-on operating skill on a lot of characterization equipment as well as electrochemical techniques. In the course of this PhD work the student will have been trained to be an active member of the laboratory life reporting his work, taking part to manuscript writing, presenting his work to conferences...

Laboratories involved
IRIG/CAMPE:
L. Dubois – PhD director and F. Duclairoir – PhD advisor
Material design, preparation and characterization
Electrochemical assessment of the materials

IRIG/MEM:
G. De Paepe – D. Lee
ss-NMR characterization of the materials

Starting date
Beginning September-October 2020