Page 1

Porous Structured Graphene Paper with Enhanced  Performance for Energy Storage Application Kewei Shu, Caiyun Wang, Chen Zhao, Gordon G. Wallace Intelligent Polymer Research Institute, ARC Centre of Excellence for Electromaterials Science,  University of Wollongong Email: ks323@uowmail.edu.au

Introduction

Abstract A novel porous structured graphene paper was prepared from pressed graphene cryogel, which was formed by freeze‐drying a solution containing chemically reduced graphene and graphene oxide (CRG/GO). Compared to the conventional graphene papers, much higher discharge capacity and rate capability can be delivered from the graphene cryogel paper. At current density of 2000 mA g‐1, it exhibits a discharge capacity higher than 400 mAh g‐1, in sharp contrast to the 229 mAh g‐1 at 50 mA g‐1 delivered by the conventional graphene papers. The electrochemical properties were greatly improved probably due to the porous structure and the concomitant high surface area. The CRG/GO ratio in the precursor solution determines the mechanical properties of the cryogel paper. The cryogel paper has a Young’s modulus nearly 9 times greater than an equivalent paper made from pure GO when the CRG/GO mass ratio was 2:1. With a simple freeze‐drying method, graphene paper with continuous pores can also be formed from a wet graphene oxide gel paper obtained by filtration. This porous graphene paper shows high discharge capacity and excellent rate capability in lithium batteries. The discharge capacity can reach 451 mAh g‐1 at a current density of 2000 mA g‐1. A flexible supercapacitor fabricated with such porous graphene paper and polymer electrolyte (PVA‐ H2SO4) shows a specific capacitance of 157 F g‐1 at 0.5 A g‐1, which was higher than 112 F g‐1 from the conventional graphene paper .

Research on flexible energy conversion and storage devices is motivated by the increasing demand in portable electronics including roll‐up display, implantable and wearable medical devices. For such flexible energy system, electrodes which possess the superior electrochemical properties along with the mechanical flexibility is highly demanded. Graphene papers combine excellent electrochemical properties and mechanical flexibility together, and they are the promising materials for flexible energy storage system. However, suffered from its compact structure, the performance of conventional filtrated graphene paper is far from satisfactory. Here, we applied a new strategy, freeze drying, to obtain porous/uncompact graphene paper.

Experimental and Results  Graphene Cryogel Paper Loose‐packed graphene paper is made by pressing a graphene cryogel originated from graphene oxide (GO)/chemically reduced graphene (CRG) dispersion.

 Porous Graphene Paper

Figure 3  Schematic procedures  to fabricate  graphene cryogel paper.  Figure 1  Schematic  procedures  to prepare graphene cryogel paper. 

After pressing, the porous cryogel (Figure 2a) turns into graphene paper with wrinkled surface (Figure 2b inset) and loose‐packed cross section (Figure 2b). (a)

Porous graphene paper is prepared by direct lyophilization of a filtrated Flexible Supercapacitor wet graphene oxide gel.

Lithium Battery

(c)

(a) (b)

(b) Figure 4  Electrochemical performance of  porous graphene paper: (a) as lithium  battery anode (b) as  electrode for flexible supercapacitor. 

(d)

(e)

Figure 2  SEM images of (a) graphene cryogel (b) cross section of graphene cryogel paper (inset: surface morphology of  graphene cryogel paper). (c) stress ‐strain curves of graphene papers with different CRG/GO ratio. (d) capacity versus cycle number of rGO-CRG-0.5 paper at 2000 mA g-1.(e) Charge discharge curves of cryogel papers with different  GO/CRG ratio at different current densities. 

The mechanical properties of such graphene cryogel can be tuned by the GO/CRG ratio in the precursor solution for cryogel (Figure 3c). It was significantly enhanced with the increase of the CRG/GO ratio in the precursor solution. The graphene cryogel paper exhibit excellent performance when used as lithium battery anode. The capacity can reach 355 mAh g‐1 even at a high current density of 3 A g‐1. The introduction of CRG in the precursor solution resulted in a strong graphene paper without sacrificing electrochemical performance.

Acknowledgement The authors thank the Australian Research Council (ARC) for financial support under the ARC Centre of  Excellence for Electromaterials Science, and the ANFF Materials Node for their provision of research  facilities. Kewei Shu and Chen Zhao acknowledge the support of the CSC scholarship from the Ministry  of Education of P. R. China. The authors also acknowledge the use of facilities within UOW Electron  Microscopy Centre. 

After 100 cycles, all of the graphene paper electrodes still maintain a  specific capacity of 451mAh g‐1 at current density of 2000 mA g‐1. (Figure  4a inset). For flex supercapacitor, the single electrode specific capacitance  calculated from charge discharge curves is about 157 F g‐1 at 0.5 A g‐1. The  capacitance decreases to 105 F g‐1 at higher current density of 2 A g‐1. 

Conclusions  A flexible graphene paper with a porous structure has been fabricated  by mechanically pressing a graphene cryogel, prepared from a  precursor solution containing different CRG/GO ratios. Its mechanical  property can be significantly enhanced by increasing the CRG/GO ratio  in the precursor solution.   Such graphene cryogel paper exhibit excellent electrochemical  properties in lithium batteries, including high discharge capacity, good  rate capability and cycling stability.   A porous graphene paper can be easily made by lyophilization of wet  graphene oxide gel. Such paper with continuous porous structure can  be used as flexible electrode, shows good electrochemical  performance in lithium batteries  and  supercapacitors. 

Kewei shu  
Advertisement