Electrochemical measurements

The cathode was prepared by a slurry coating procedure and carbon cloth was used as the current collector. Typically, Se@C, Super-P carbon black (Alfa Aesar, 99%), and polyvinylidene fluoride (PVDF) binders (Alfa Aesar) were added in N-Methyl-2-pyrrolidone (NMP, Aladdin, 99.5%) with a weight ratio of 80:10:10 to form slurry (using a mortar, in air) was then uniformly deposited onto a carbon cloth. The cathode was obtained after dried under vacuum at 60 °C for 12 h. The electrochemical performance of the Se@C-based composite working electrodes was investigated in CR2032 coin cell configuration using Cu foil as counter electrode and about 200 μL of 0.5 mol L⁻¹ CuSO₄ aqueous electrolyte solution. The copper anode also acts as the reference electrode and the reference potential of the Cu²⁺/Cu is 0.33 V vs. standard hydrogen electrode (SHE). The electrochemical performance was tested on a Land cycler (Wuhan Kingnuo Electronic Co., China), all cells were first discharged. The gravimetric specific capacities were calculated based on the mass of Se. The typical areal Se loading is about 2 mg cm⁻², and the typical electrode thickness (Se@C on carbon cloth) is about 350 µm. Higher areal Se loading of about 4, 8, 12 mg cm⁻² were also fabricated and tested for higher areal capacity. The Galvanostatic intermittent titration technique (GITT) test is performed by a series of galvanostatic discharge or charge pulses of 300 s at 500 mA g⁻¹ followed by 1800 s rest. Electrochemical impedance spectroscopy (EIS) was tested under AC amplitude of 5 mV at the frequency from 100 kHz to 1 Hz under the open-circuit potential (constant potential). The recording number of data points was 12 (per decade). The Zn | |Se@C-48 full cells were tested in home-made sealed-cell³³, where Se@C-48 as the cathode, Zn foil as the anode, an aqueous electrolyte solution comprising of 1 mL of 0.5 mol L⁻¹ CuSO₄ and 1 mL of 0.5 mol L⁻¹ ZnSO₄, and an anion-exchange membrane (fumasep, FAB-PK-130, Suzhou Wingrise energy technology Co. Ltd.,) was employed for the compartmentalization of the Cu²⁺ and Zn²⁺ ions. All electrochemical energy storage tests are carried out in an environmental chamber with a temperature of 25±0.5 °C.