Graphene–carbon 2D heterostructures with hierarchically-porous P,N-doped layered architecture for capacitive deionization

Exploring a new-family of carbon-based desalinators to optimize their performances beyond the current commercial benchmark is of significance for the development of practically useful capacitive deionization (CDI) materials. Here, we have fabricated a hierarchically porous N,P-doped carbon–graphene 2D heterostructure (denoted NPC/rGO) by using metal–organic framework (MOF)-nanoparticle-driven assembly on graphene oxide (GO) nanosheets followed by stepwise pyrolysis and phosphorization procedures. The resulting NPC/rGO-based CDI desalinator exhibits ultrahigh deionization performance with a salt adsorption capacity of 39.34 mg g−1 in a 1000 mg L−1 NaCl solution at 1.2 V over 30 min with good cycling stability over 50 cycles. The excellent performance is attributed to the high specific surface area, high conductivity, favorable meso-/microporous structure together with nitrogen and phosphorus heteroatom co-doping, all of which are beneficial for the accommodation of ions and charge transport during the CDI process. More importantly, NPC/rGO exhibits a state-of-the-art CDI performance compared to the commercial benchmark and most of the previously reported carbon materials, highlighting the significance of the MOF nanoparticle-driven assembly strategy and graphene–carbon 2D heterostructures for CDI applications.


Preparation of NC/rGO
ZIF-8/GO nanosheets were converted to NC/rGO nanosheets by carbonization at 950 °C for 2 h under a N 2 atmosphere. The ramping rate was 1 °C min -1 in the first stage (below 350 °C) and increased to 5 °C min -1 in the second stage (350 to 950 °C). For comparison, ZIF-8 was obtained without adding the GO solution and employed as the precursor to NC by direct carbonization with the same process.

Preparation of porous NPC/rGO
NPC/rGO nanosheets were prepared by post-activation with PA. In a typical activation process, NC/rGO (30.0 mg) were dispersed in ethanol (1.5 mL) containing PA solution (170 μL). The suspension was then transferred to a quartz boat and dried at 60 °C. Subsequently, the quartz boat was placed in a quartz tube and heated under a N 2 atmosphere. The temperature was increased to 350 °C then 550 ℃ at a ramp rate of 5 °C min −1 . Each stage was held for 0.5 h. Temperature was then increased to 1000 °C and held for another 2 h.

Characterization.
Crystal identity of the samples was characterized by using powder X-ray diffraction (XRD, Ultima Rint 2000 X-ray diffractometer, RIGAKU, Japan) measurements using Cu Kα radiation (40 kV, 40 mA, 2° min −1 scan rate). Morphologies of the samples were observed by field-emission scanning electron microscopy (FESEM, HITACHI SU8000). Mesoporous structure of the samples was confirmed by transmission electron microscopy (TEM, JEOL JEM-2100). N 2 adsorption-desorption isotherms were obtained by using a Belsorp-max (BEL,Japan) instrument where the specific surface areas and pore size distributions were analyzed by the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) models, respectively. X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd) was used to investigate the chemical state of phosphorus and nitrogen in the samples. The Zeta potential measurements were done in aqueous solutions of the samples in function of pH values ranging from 3 to 11 by Zeta sizer Nano-ZS90 (Malvern, S-3 United Kingdom).

Electrochemical analyses
The electrochemical measurements were performed on CHI 660E electrochemical workstation in a threeelectrode system with 1 M NaCl aqueous electrolyte. A platinum (Pt) wire and Hg/HgO were selected as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) and gravimetric chargedischarge (GCD) measurements were carried out in the potential range of -0.5 to 0.5 V.
The specific capacitances (C, F g -1 ) were calculated from the CV curves by using the following equation: where i is the discharge current density (A), m is the mass of active materials (g), , and ΔV is the voltage window (V), v is the scan rate (V s -1 ).

Desalination analysis
Each individual CDI electrode was fabricated by depositing a mixture of the sample, Super-P and PVDF binder with a weight ratio of 8:1:1. The mixture was pressed onto graphite paper (2.5 × 2.5 cm 2 ) and dried under reduced pressure at 60 °C for 12 h. CDI tests were conducted using a batch-mode with continuous recycling system, which consists of a pair of anion-and cation-exchange membranes, a peristaltic pump, a power source, and a tank. The real-time saline concentration variation was monitored and measured by using an ion conductivity meter. The volume of the saline solution was fixed at 30 mL, the flow rate was 30 mL min -1 , and the operating voltage was 1.2 V.
The salt adsorption capacity (Γ, mg g -1 ) and mean salt adsorption rate (MSAR, mg g -1 min -1 ) at t min were calculated as follows: where C 0 and C t represent the concentrations of NaCl at the initial stage and t min, respectively (mg L -1 ); V is the volume of the NaCl solution (L); and m represents the total mass of the electrode materials (g).