Nitrogen-Doped Carbon Spheres-Decorated Graphite Felt as a High-Performance Electrode for Fe Based Redox Flow Batteries

Anarghya Dinesh Jyothy Institute of Technology Anantha Mylarapattana Shankaranarayana Jyothy Institute of Technology Santosh Mysore Sridhar Jyothy Institute of Technology Narendra Kumar Muniswamy Jyothy Institute of Technology Krishna Venkatesh Jyothy Institute of Technology H B Muraralidhara (  muralidhara.hb@ciirc.jyothyit.ac.in ) Centre for Incubation Innovation Research and Consultancy https://orcid.org/0000-0003-0184-9762


Introduction
Storage of energy harvested from renewable sources of energy such as wind, solar, hydro, etc is of major interest in the current scenario [1,2]. In recent days redox flow batteries are emerging as the sustainable technology to store the energy developed or harvested from renewable sources of energy [1][2][3][4][5][6]. Redox flow batteries are the one, which stores energy developed in the liquid electrolytes, thus they are power and capacity decoupled [3,4,[7][8][9][10][11].
The various kinds of redox flow batteries comprising aqueous, non-aqueous, organic, or inorganic electrolyte systems that are gaining attention because of their high efficiency and lifecycles. Iron redox flow batteries (IRFBs) are the unique type of flow batteries that use the earth-abundant, economical, and environmentally friendly iron as the source of the reaction [12,13]. IRFBs ( Fig.1) is the one that stores chemical energy and reversibly converts it into electrical energy. The Fe 3+ /Fe 2+ , chloride electrolyte is used as a positive electrolyte and Fe 2+ /Fe 0 electrolyte as a negative electrolyte [14,15].
Overall: 3 2+ ↔ 0 + 2 3+ 0 = 1.21 . (3) The ammonium chloride added to the electrolyte acts as the supporting electrolyte which shifts the plating potential of the electrolyte more negative. It has been mentioned in the literature that complexing the Fe 3+ ions with ligands increases the stability of the anolyte and prevents precipitation Fe 2+ as Fe(OH)3 [16]. According to the literature, glycine exhibits a positively charged state, negatively charged state as well as zwitterion state. Addition of glycine to the iron-electrolyte forms a bond to iron ions in its negatively charged form and restricts the precipitation of Fe 2+ as Fe(OH)3 [16].
According to the literature, carbon-based electrodes are the majorly used in the field of redox flow batteries because of their low cost, high stability, corrosion resistance, and high conductivity. But, these are poor in kinetic reversibility and electrochemical activity [17,18].
Hence it is essential to modify with electrochemically active substances such as metal-oxides, activated carbons, carbon nanotubes, graphene oxide or metal-carbon composites, etc. to increase the electrochemically active sites on the electrode surface [18,19].
Studies reported in the field of energy storage consider carbon spheres as a promising material/catalyst because of their high surface area, chemical stability, and cost-effectiveness [20][21][22][23]. It has explained in the literature that, N-doped carbon materials facilitate the formation of defects sites on the electrode surface which helps in the electrolyte absorption and hence increases the wettability of the electrode [17,18,[24][25][26]. Also, these are described to exhibit greater electrocatalytic property in many electrochemical devices. Various reports explain the hydrothermal synthesis of carbon spheres from glucose precursors [27][28][29][30].
However very few reports available on the single-step hydrothermal synthesis of nitrogendoped carbon spheres using dextrose and ammonia as the precursors [20,21].
The recent studies carried out for the electrode modification in IRFB using nitrogen-doped carbon particles and metal oxide composite reported good cycle life and kinetic reversibility towards Fe 3+ /Fe 2+ reaction using ascorbic acid and glycine as the ligand respectively [18,19].
The cell employed nitrogen-doped carbon particles resulted in a voltaic efficiency of 53.3% and with the maximum current uptake of 50 mA/cm 2 . In these studies, the major focus has been given for the enhancement of coulombic efficiency (CE) and cycle stability using a known electrolyte system. Yet there is a gap in the literature that leads to modification of electrode to enhance the overall efficiency of the IRFB which intern enhances the voltaic efficiency (VE) and energy efficiency (EE). To achieve higher voltaic efficiency of the battery, it is necessary to modify the electrode using an effective electrocatalyst which increases the electrocatalytic activity of the electrode by reducing ohmic resistance [31]. This is novel and potential work in the field of IRFBs to propose an effective catalyst for the modification of a positive electrode to increase the battery efficiencies.

Materials
All chemicals used in the experiments were procured from Sigma Aldrich, Bangalore. The

Synthesis
NDCS were synthesized by the hydrothermal carbonization process using dextrose as the precursor. 10 g of dextrose was taken in 40 mL of water and 10% of ammonia solution was added to the mixture. The solution was stirred at 80ºC for about 15 minutes and transferred to 50 mL Teflon-lined autoclave and treated at 180ºC for 12 hours. The Autoclaved samples were allowed to cool till room temperature. The resultant black precipitate of NDCS was filtered, washed with water until the neutral pH, and dried at 100ºC [30].

Positive electrode preparation
The graphite felt electrode used in the study was sonicated using distilled water for 30 minutes to remove the loosely bound particles and dried in a hot air oven at 100ºC overnight.
The spray method was used for the modification of graphite felt using ethanol as a solvent.
NDCS were well dispersed in ethanol by continuous stirring for about 24 hours. Nafion solution (5 wt%) was added as a binder to the above solution and sonicated for five minutes.

Negative electrode preparation
The graphite felt electrode was thermally treated at 400°C for 30 hours and cooled to room temperature. The thermally treated graphite felt (TTGF) electrode was used as a negative electrode for performance studies of the battery [34].

Electrochemical measurements
The electrochemical activity of the MGF electrodes decorated with different amounts of NDCS was evaluated using CV and potentiodynamic polarization studies to optimize the suitable electrode for the battery performance using the Electrochemical analyzer (CH608E).
The Electrochemical measurements were carried out using a three-electrode cell as shown in The results of the electrochemical measurements of the MGF electrodes with different weights of NDCS coated over the electrode surface were compared to each other to optimize the desired weight of NDCS to be coated over the electrode surface.

Electrodes characterization
The crystallography of the NDCS, UGF, and MGF electrodes was characterized by X-ray diffractometry (XRD) on a BRUKER eco-D8 ADVANCE system working with Cu-Kα radiation (λ=1.54 Å). The surface morphology and of TTGF and MGF electrodes were analyzed using scanning electron microscopy (SEM, HITACHI, SU3500) coupled to an energy dispersive analysis X-ray (EDAX) at an accelerating voltage of 10 kV. Raman spectra of electrodes were analyzed using (Horiba Jobin Yvon LabRAM) with 532 nm LASER at an exposure time of 5 seconds. The charge-coupled device (CCD) was used as a detector for the analysis with 1800 lines/mm grating.

Performance characterization using 132 cm 2 IRFB
The charge-discharge studies of the battery (132 cm 2 ) were studied using the Bitrode

Electrochemical measurements
From Fig. 3A, it is noticeable that the electrochemical activity of the reaction increased significantly over the graphite felt electrode as a consequence of modification with NDCS.
The oxidation and reduction peak current of the MGF electrode decorated with different weights of NDCS is given in Table 1 (The values are derived from the Electrochemical analyzer (CH608E)). The results indicate the shift in the oxidation peak potential of 2 mg/cm 2 MGF electrode towards a negative direction as shown in the figure.
The ratio of anodic and cathodic peak current (Ipa/Ipc) will describe the reversibility of the reaction. The ratio obtained for 2 mg/cm 2 MGF electrode is closer to 1 than that of the other electrodes. Based on these results, it can be concluded that 2 mg/cm 2 MGF electrode possesses greater electrocatalytic activity of Fe 3+ /Fe 2+ reaction and hence is favorable for improving the energy storage of IRFBs. From Fig. 3B it is also evident that the peak potentials for the Fe 3+ /Fe 2+ redox reaction on 2 mg/cm 2 MGF electrode is unchanged with increasing the potential scanning rate suggests that the concerned redox reaction is reversible on 2 mg/cm 2 MGF electrode [35].
The potentiodynamic polarization study revealed that 2 mg/cm 2 MGF electrode comprises fewer resistance values compared to other MGF electrodes. The Tafel plots obtained from the analysis has given in Fig. 4. The linear polarization value (derived using software) for 2 mg/cm 2 MGF electrode was found to be 2 Ω, which was least when compared to UGF (431 Ω), 1 mg/cm 2 MGF electrode (64 Ω) and 3 mg/cm 2

Morphological characterization of electrodes
The SEM images of the NDCS, UGF, and MGF electrodes are given in Fig. 5. The synthesized NDCS is in 3D -spherical shape with an average size of 5.80µm as shown in The diffractogram recorded for the NDCS, UGF, and MGF electrodes is given in Fig. 7.
The increase in the intensity of the (002) offset of 2 mg/cm 2 MGF electrode can be attributed to the decorated NDCS. The (100) and (110)

Charge-discharge studies using 132 cm 2 IRFB
The charge-discharge studies were carried out at different current densities using the MGF electrode as the positive electrode and TTGF electrode as the negative electrode. The results obtained from the analysis are given in Table 2 During charge-discharge cycling, the electrolyte was observed to lose its stability after the 5 th cycle [33]. This may be resulted due to the deposition of the active material of the electrolyte over the electrode or membrane surface. The experiment was continued by taking fresh electrolyte after every fifth cycle, the data obtained from the study resulted in the electrode was stable enough with the repeatability of the efficiency values up to 15 cycles. The chargedischarge cycling carried out at 30 mA/cm 2 charging has shown in Fig. 9C        A Charge-discharge curves for 2 mg/cm 2 MGF electrode at different current densities B Efficiencies of the UGF electrode at current density of 20 mA/cm 2 -10 mA/cm 2 and 2 mg/cm 2 MGF electrode at different current densities, and C Charge-discharge cycling for 2 mg/cm 2 MGF electrode at a current density of 30 mAcm 2 -15 mA/cm 2 ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Figure 1 Graphical representation of IRFBs The three-electrode cell assembly used in the electrochemical measurement studies Potentiodynamic polarization studies for UGF, TTGF, and MGF electrodes EDAX spectrum for NDCS and 2 mg/cm2 MGF electrode XRD studies for NDCS, UGF and 2 mg/cm2 MGF electrode Raman spectrum for UGF and 2 mg/cm2 MGF electrode A Charge-discharge curves for 2 mg/cm2 MGF electrode at different current densities B E ciencies of the UGF electrode at current density of 20 mA/cm2 -10 mA/cm2 and 2 mg/cm2 MGF electrode at different current densities, and C Charge-discharge cycling for 2 mg/cm2 MGF electrode at a current density of 30 mAcm2 -15 mA/cm2