Doping Engineering of M‐N‐C Electrocatalyst Based Membrane‐Electrode Assembly for High‐Performance Aqueous Polysulfides Redox Flow Batteries

Abstract Polysulfides aqueous redox flow batteries (PS‐ARFBs) with large theoretical capacity and low cost are one of the most promising solutions for large‐scale energy storage technology. However, sluggish electrochemical redox kinetics and nonnegligible crossover of aqueous polysulfides restrict the battery performances. Herein, it is found that the Co, Zn dual‐doped N‐C complex have enhanced electrochemical adsorption behaviors for Na2S2. It exhibits significantly electrochemical redox activity compared to the bare glassy carbon electrode. And the redox reversibility is also improved from ΔV = 210 mV on Zn‐doped N‐C complex to ΔV = 164 mV on Co, Zn‐doped N‐C complex. Furthermore, membrane‐electrode assembly (MEA) based on Co, Zn‐doped N‐C complex is firstly proposed to enhance the redox performances and relieve the crossover in PS‐ARFBs. Thus, an impressively high and reversible capacity of 157.5 Ah L−1 for Na2S2 with a high capacity utilization of 97.9% could be achieved. Moreover, a full cell PS‐ARFB with Na2S2 anolyte and Na4[Fe(CN)6] catholyte exhibits high energy efficiency ≈88.4% at 10 mA cm−2. A very low capacity decay rate of 0.0025% per cycle is also achieved at 60 mA cm−2 over 200 cycles.


Material characterizations:
The crystal phases of Zn-N-C and CoZn-N-C were identified using Powder X-ray diffraction (XRD) on a Rigaku Ultima IV diffractometer by Rigaku Japan operating at 40 kV voltage and 15 mA current with Cu Kα X-ray source. X-ray photoelectron spectroscopy (XPS) measurements were performed at Thermo Scientific ESCALAB Xi+ using Al Kα radiation as the X-ray source. Binding energies reported herein are with reference to C (1s) at 284.5 eV. The concentrations of Co element and Zn element in samples are determined by the ICP-OES (Thermofish icap 7400). The morphology of these three samples were detected with scanning electron microscopy (SEM) on a Histachi S4800 microscope at 15 kV. In addition, transmission electron microscopy TEM (HRTEM), high angle annular dark field scanning TEM (HAADF-STEM) and EDX mapping were performed on a Themis 60-300 kV transmission electron microscope (Thermofisher, America) equipped with a spherical aberration corrector of the condensor lens and Super-X detector. Co and Zn K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) experiments were carried out at t 1W1B station in Beijing Synchrotron Radiation Facility (BSRF).
The storage rings of BSRF were operated at 2.5 GeV with the maximum current of 450 mA. Si(111) double-crystal monochromator crystals were used to monochromatize the X-ray beam. Data reduction, data analysis were performed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. The quantitative curve-fittings were carried out using the module ARTEMIS of IFEFFIT. Standard procedures were used to normalize XAS data and to extract the EXAFS data from the measured absorption spectra.

Zeta potential measurement
The zeta potentials the Zn-N-C and CoZn-N-C were measured with Zetasizer Nano ZS system. 2 mg of Zn-N-C was added into 10 mL ethanol to obtain mixture A. 2 mg of CoZn-N-C was added into 10 mL ethanol to obtain mixture B. Before measurement, A and B were ultrasonic dispersed for 1 hour, respectively. For each sample, three paralle tests were conducted.

Cyclic voltammetry test
CV curves were tested with a BioLogic VMP3 potentiostat system. A glass carbon electrode with a surface area of 0.071 cm 2 was used as working electrode, a graphite rod electrode was used as counter electrode and Hg/HgO (1 M KOH) was used as reference electrode. All the solutions were degassed with Ar for 30 min before test to remove oxygen. CV of the mixed solution (0.1 M Na2S2 + 0.5 M Na2SO4) were conducted at scan rate of 5mV s -1 .            respectively. The VESTA software 9 was used to depict all the atomic models and differential charge density distribution.

Assembly of Redox Flow Battery
Two endplates were made stainless steel. Two titanium plates with a 2 * 2 cm 2 sinking plane were used as bipolar plates. The Nafion 115 membrane was pretreated with 5 wt% H2O2 under 80℃ for 30 min and followed by boiling in 1M H2SO4 at 80℃ for 30 min.       In-operando Raman measurement for the full cell PS-ARFB.
The Raman spectum was collected with a confocal Raman microscope (Alpha-300, WITec) with an excitation wavelength of 532 nm every 10 s during the dischargingcharging process of the RFB. A 50 μm multimode fiber was used for Raman signal collection and also as a confocal hole. The signal was then sent via an optical fiber port to a UHTS-300 spectrometers (Witec) equipped with an EMCCD detector (1600 × 200 pixels, Newton, Andor) to achieve the highest sensitivity. During the charging/discharging of the full cell PS-ARFB, the anolyte and catholyte were flowing through a quartz flow cell with an optical path of 1 mm under a microscope objective.
The microscope objective was of 20× magnification with a numerical aperture of 0.45.
An edge filter was equipped to filter the exciting line, and an 1800 g/mm grating was employed for the experiments. The background of denoised spectra is removed by the auto-adaptive background subtraction of the used quartz flow cell.