Facile synthesis of novel graphene sponge for high performance capacitive deionization

Abstract

Capacitive deionization (CDI) is an effective desalination technique offering an appropriate route to obtain clean water. In order to obtain excellent CDI performance, a rationally designed structure of electrode materials has been an urgent need for CDI application. In this work, a novel graphene sponge (GS) was proposed as CDI electrode for the first time. The GS was fabricated via directly freeze-drying graphene oxide solution followed by annealing in nitrogen atmosphere. The morphology, structure and electrochemical performance of GS were characterized by scanning electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The electrosorption performance of GS in NaCl solution was studied and compared with pristine graphene (PG). The results show that due to the unique 3D interconnected porous structure, large accessible surface area and low charge transfer resistance, GS electrode exhibits an ultrahigh electrosorption capacity of 14.9 mg g(-1) when the initial NaCl concentration is ~500 mg L(-1), which is about 3.2 times of that of PG (4.64 mg g(-1)), and to our knowledge, it should be the highest value reported for graphene electrodes in similar experimental conditions by now. These results indicate that GS should be a promising candidate for CDI electrode.

Figure 1

(a) Schematic diagram of the CDI process. The basic principle of CDI involves the application of a voltage (<2.0 V) between two oppositely placed electrodes. Saline water flows through a spacer sandwiched between the oppositely placed electrodes. On the application of an electric potential to the CDI cell, the Na⁺ and Cl⁻ are attracted towards the oppositely charged porous carbon electrodes and subsequently stored inside them. The resulting freshwater thus contains a reduced amount of salt. This phenomenon is similar to energy storage in supercapacitors and batteries. (b) Schematic illustration of the procedure for the preparation of GS; high-resolution C1s XPS of (c) GOS and (d) GS.

Figure 2. Morphology and structure of GS and PG.

(a–d) SEM images of PG (a, b) and GS (c, d) at low and high magnification. (e, f) TEM images of PG (e) and GS (f). Insets of (e) and (f) are SAED patterns of PG and GS, respectively.

Figure 3. Nitrogen sorption isotherms of (a) GS and (b) PG.

Insets are the pore size distribution of GS and PG, respectively.

Figure 4. (a) CV curves of GS and PG measured at a scan rate of 5 mV s⁻¹ and (b) Nyquist plots of GS and PG electrodes in 1 M NaCl aqueous solution.

Inset of (b) is the corresponding expanded high-frequency region of the plots.

Figure 5. (a) Electrosorption capacity and (b) current transient for GS and PG electrodes over 30 minutes in NaCl solution with an initial concentration of ~50 mg L⁻¹ at an applied voltage of 1.5 V.

Figure 6. Linear fitting of the electrosorption of NaCl by GS and PG electrodes using (a) pseudo-first-order kinetic equation and (b) pseudo-second-order kinetic equation.

Figure 7. (a) Electrosorption capacities of GS and PG electrodes in NaCl solution; (b) electrosorption performance of GS electrode investigated in NaCl solution and simulative solution. Initial concentration: ~500 mg L⁻¹; applied voltage: 1.2 V.

Figure 8. (a) Experimental and fitting data by employing Langmuir and Freundlich isotherms for GS and PG; (b) electrosorption and regeneration cycles of GS in ~500 mg L⁻¹ NaCl solution at an applied voltage of 1.2 V.