ZIF-8 Derived, Nitrogen-Doped Porous Electrodes of Carbon Polyhedron Particles for High-Performance Electrosorption of Salt Ions

Three-dimensional (3-D) ZIF-8 derived carbon polyhedrons with high nitrogen (N) content, (denoted as NC-800) are synthesized for their application as high-performance electrodes in electrosorption of salt ions. The results showed a high specific capacitance of 160.8 F·g−1 in 1 M NaCl at a scan rate of 5 mV·s−1. Notably, integration of 3-D mesopores and micropores in NC-800 achieves an excellent capacitive deionization (CDI) performance. The electrosorption of salt ions at the electrical double layer is enhanced by N-doping at the edges of a hexagonal lattice of NC-800. As evidenced, when the initial NaCl solution concentration is 1 mM, the resultant NC-800 exhibits a remarkable CDI potential with a promising salt electrosorption capacity of 8.52 mg·g−1.

the electrical conductivity of the carbon electrode. The data was collected in the frequency range from 10 mHz to 100 kHz with 5 mM a amplitude. The GC test was used to measure the reversibility and inner resistance (iR drop) of the carbon electrode.
Voltage profiles were obtained at a current density of 100 m A·g -1 in the potential range of −0.4 to 0.6 V. The CV measurement was executed to evaluate the capacitive performance of the carbon electrode. CV for assessing the EDL capacitance were measured in a potential window −0.4 to 0.6 V at various scan rate, ranging from 5 to 1000 mV·s −1 . The specific capacitance derived from the CV curves can be estimated according to the following equation: where C is the specific capacitance, v is the scan rate, m is the mass of the carbon material, and I is the current density. V c and V a represent the high and low potential limits of the cyclic voltammetry tests.

CDI Application of ZIF-8-derived NC-800:
The CDI electrodes were prepared by mixing a slurry of 90 wt% porous carbon powder and 10 wt% of polyvinylidene fluoride (PVDF, M.W. = 534,000, Sigma-Aldrich) binder in N, N-Dimethylacetamide (DMAc, 99%, Alfa Aesar) solution, followed by stirring for 12 h to ensure homogeneity. The slurry of the mixture was coated onto a titanium plate, dried in a 120 °C oven for 2h and in a 80 °C vacuum oven for 2h to remove organic solvents.
The CDI experiments of NCs were conducted in a batch-mode recycling system, in which the NaCl solution was continuously circulated through the CDI unit cell using a peristaltic pump (EYELA MP-1000) at a flow rate of 5 mL·min −1 . The CDI unit cell consisted of a couple of carbon electrodes and a pair of titanium plates as the current collectors, which were separated by a spacer by the distance of 2 mm for solution flow.
Prior to each experiment, the CDI cell was flushing by 18-ΩM deionized water until the solution conductivity decreased to value near zero. The electrical voltage of 0.8, 1.0 or 1.2 V was applied to the two carbon electrodes using a CHI 627D potentiostat. The regeneration was carried out by discharging the cell at 0 V. The change in solution conductivity was also continuously monitored at the outlet of the CDI cell by an online conductivity meter (SC-2300, Suntex). The concentration of the NaCl electrolyte was further determined by the linear relationship between the NaCl concentration and the conductivity in solution. The electrosorption capacity (Q) is calculated as the following equation: where C 0 and C e are the initial and equilibrium concentrations, respectively, and V NaCl is the solution volume.

Characterization:
SEM images of the samples were recorded by using a scanning electron microscope (Nova Nano SEM) operating at an acceleration voltage of 5.00 kV. Highresolution transmission electron microscopy (HR-TEM) images were obtained by using a JEOL JEM-2100F microscope operated at an accelerating voltage of 200 kV. Thermo gravimetric analyze (TGA) of samples were conducted by using a SDT Q600 thermo gravimetric analyzer in N 2 from room temperature to 900 °C at a heating rate of 10 °C min -1 . X-ray photoelectron spectroscopy was collected on a Thermo Scientific spectrometer of type "Sigma Probe" and XPS spectra of the samples were further deconvoluted into several narrow-scan spectra of the C 1s , O 1s and N 1s by using the      Figure S6. The corresponding deconvoluted spectra high-resolution N1S XPS spectra of NC-800.