Study of the electrochemical performance of VO2+/VO2+ redox couple in sulfamic acid for vanadium redox flow battery
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- He, Z., He, Y., Chen, C. et al. Ionics (2014) 20: 949. doi:10.1007/s11581-013-1051-6
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The present work was performed in order to evaluate sulfamic acid as the supporting electrolyte for VO2+/VO2+ redox couple in vanadium redox flow battery. The oxidation process of VO2+ has similar electrochemical kinetics compared with the reduction process of VO2+. The exchange current density and standard rate constant of VO2+/VO2+ redox reaction on a graphite electrode in sulfamic acid are determined as 7.6 × 10−4 A cm−2 and 7.9 × 10−5 cm s−1, respectively. The energy efficiency of the cell employing sulfamic acid as supporting electrolyte in the positive side can reach 75.87 %, which is adequate for redox flow battery applied in energy storage. The addition of NH4+ to the positive electrolyte can enhance the electrochemical performance of the cell, with larger discharge capacity and energy efficiency. The preliminary exploration shows that the vanadium sulfamate electrolyte is promising for vanadium redox flow battery and is worthy of further study.
KeywordsVanadium redox flow batterySupporting electrolyteSulfamic acidElectrochemical kinetics
With a large scale of consumption of fossil fuels, environmental pollution and limited reserves have led to an increasing use of clean and renewable energy, such as solar energy, wind energy, etc. However, the intermittent varied nature of these clean and renewable resources makes it difficult to integrate these valuable energies into electrical supply grids . One solution to this problem would be to employ large-scale electrical energy storage. There are several energy-storage technologies which have been applied in electrical energy storage, such as flywheels, compressed air, superconducting magnetic energy storage, and flow battery [2–4]. Among these technologies, redox flow battery systems have attracted much attention, such as Br−/Br2 vs. Zn2+/Zn , Fe3+/Fe2+ vs. Cr2+/Cr3+ [6, 7], V5+/V4+ vs. V2+/V3+ , etc. [7–10].
Sulfuric acid is a common supporting electrolyte in the VRFB. It was mentioned that trifluoromethanesulfonic acid (CF3SO3H), methanesulfonic acid (CH3SO3H), perchloric acid (HClO4), nitric acid (HNO3), hydrochloric acid (HCl), and sulfamic acid (NH2SO3H) could be chosen as bifunctional acid electrolytes for the redox flow battery [1, 2, 15, 16]. In recent years, other supporting electrolytes such as methanesulfonic acid, hydrochloric acid, and mixed sulfate-chloride electrolyte have been investigated in VRFB [1, 17, 18]. Kim et al.  reported that >6 M hydrochloric acid supporting electrolyte for VRFB has better thermal stability than the sulfuric acid supporting electrolyte, capable of dissolving more than 2.3 M vanadium at varied valence states and remained stable at 0–50 °C. The improved thermal stability is attributed to the formation of a vanadium dinuclear [V2O3·4H2O]4+ or a dinuclear-chloro complex (V2O3Cl·3H2O)3+ in the solution over a wide temperature range. Moreover, VRFB with chloride electrolytes demonstrated excellent reversibility and fairly high efficiency. Meanwhile, the thermal stability of the vanadium electrolyte at various valence states and the electrochemical behavior of a VRFB using mixed sulfate-chloride electrolyte were studied by Li et al. . Peng et al.  reported the investigation of using CH3SO3H-H2SO4 mixed acid as a supporting electrolyte for VRFB. Compared to that in the sulfuric acid solution system, the redox reaction kinetics of VO2+/VO2+ couple in the CH3SO3H-H2SO4 mixed solution is increasing, with stable cycling performance and higher energy density. However, as a supporting electrolyte, hydrochloric acid has strong corrosivity, and chlorine evolution may occur at the electrode in the anode process due to the inconsistency of the cells. Nitrous oxide evolution also occurs at the surface of the electrode with nitric acid as supporting electrolyte. Trifluoromethanesulfonic acid is the strongest organic acid and has strong hydroscopicity, which may impose some safety problems and strict requirements to equipment.
Sulfamic acid has been widely used as an efficient and environmental friendly heterogeneous catalyst for acid-catalyzed reactions as an alternative to metal catalyst [24, 25]. It is also used in the electroplating field . Under consideration of the sustainable development, sulfamic acid as a new supporting electrolyte in VRFB is studied. This paper reports the kinetic characteristics of VO2+/VO2+ redox couple in sulfamic acid solution and the charge-discharge performance of a VRFB.
Preparation of vanadium sulfamate solution
Electrolyte of vanadium(IV) sulfamate was prepared by electrolytic dissolution of V2O5 (Jishou Huifeng Mining Industry Co., Ltd., China) in NH2SO3H (Shanghai Shanpu Chemical Industry Co., Ltd., China) solution in a two-compartment electrolysis cell . The NH2SO3H solution at an appropriate concentration was employed as the anolyte. The NH2SO3H solution with an appropriate weight of V2O5 was employed as the catholyte. NH2SO3H was employed as anion-matched V(IV) ions and supporting electrolyte. A graphite plate with an area of 49 cm2 was employed as the electrode. Electrolysis was performed with DC Power Supply System (Ming Shing Engineering Co., Hong Kong) providing constant current of 3 A. At the end of electrolysis, the final vanadium concentration was analyzed by redox titration. The vanadium electrolytes in other valence states were prepared by electrolysis with vanadium(IV) sulfamate as the original electrolyte and terminated by controlling the electrolysis time strictly. Vanadium(IV) sulfamate solutions containing NH4+ at different concentrations were prepared by adding NH4HCO3 and NH2SO3H into the vanadium(IV) sulfamate solution. All reagents in the experiment were of analytical reagent grade.
UV-Vis spectrometry of the vanadium electrolyte were measured with a UNIC 3802 UV/Vis spectrophotometer (Shanghai, China) in the range of 400–900 nm using a 1.0-cm quartz cell. The measured electrolytes are 0.04 M VO2+ in 3 M H2SO4 and 1.0 M NH2SO3H, respectively. The reference solutions used were 3.0 M H2SO4 and 1.0 M NH2SO3H.
Viscosity and electrical conductivity tests
Cyclic voltammetry and linear sweep voltammetry
Cyclic voltammetry measurements of the electrolyte with and without NH4+ were carried out from 0.3 to 1.2 V vs. saturated calomel electrode (SCE) at a scan rate range of 5–200 mV s−1. Linear sweep voltammetry measurements of the electrolyte were performed at a scan rate of 1 mV s−1. The cyclic voltammetry and linear sweep voltammetry were performed using the CHI660C electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., China) with a three-electrode system. A 1-cm2 graphite plate and 4-cm2 platinum sheet were used as working electrode and counter electrode, respectively. SCE along with a double salt bridge full of saturated potassium chloride solution was used as reference electrode. Prior to each measurement, the working electrode was polished with 600- and 1,200-grit SiC paper and then washed with distilled water as described in the literature . The reference electrode was washed with distilled water, and the solution in the salt bridge was replaced before use. Unless otherwise specified, the electrode potential is the potential relative to the SCE electrode.
A single static cell was assembled, which consisted of two pieces of polyacrylonitrile (PAN)-based graphite felts with an area of 9 cm2 (Shenhe Carbon Fiber Materials Co., Ltd.), two current collectors, and a perfluorinated ion-exchange membrane (Best Industrial & Trade Co., Ltd., China). The membrane was treated in 3 % H2O2 solution for 1 h and in 1 M H2SO4 solution for 30 min at 80 °C afterward before use. The graphite felt was oxidized in air at 400 °C for 6 h to enhance electrochemical activity and hydrophilicity . Before the cell was assembled, graphite felts were soaked in the original electrolyte for 24 h at an ambient temperature. The original electrolyte used in the tests includes 0.6 M VO2+ with NH4+ at different concentrations in 1.0 M NH2SO3H as positive electrolyte and 0.6 M V3+ in 3.0 M H2SO4 as negative electrolyte. The galvanostatic charge-discharge test was performed between 0.7 and 1.7 V at a current density of 20 mA cm−2 using CT2001C-10V/2A (Wuhan Land Co., China).
Results and discussion
Viscosity and electrical conductivity of the electrolyte
The viscosity and electrical conductivity of vanadium(IV) sulfamate solutions containing NH4+ at different concentrations
Concentration of NH4+ (M)
ρ (g mL−1)
η (m Pas)
κ (S cm−1)
UV-Vis spectra of vanadium(IV) sulfamate
Linear sweep voltammetry
Figure 6 shows the linear relationship between the anodic peak current and the square root of scan rates for the electrolyte with and without NH4+. The diffusion coefficient of VO2+ in sulfamic acid with and without NH4+ can be obtained by the slope value in Fig. 6 and by Eq. 9. The diffusion coefficient of VO2+ in sulfamic acid without NH4+ was estimated as 2.71 × 10−6 cm2 s−1, higher than that with NH4+ (2.10 × 10−6 cm2 s−1), which is due to NH4+ that hinders the diffusion of VO2+. The diffusion coefficient of VO2+ in sulfamic acid is higher than that in sulfuric acid compared with the literature . The better diffusion performance of vanadium ions in sulfamic acid may be related to the low complexation ability of NH2SO3− with V ions. The differences in the diffusion performance may reflect differences in the state of complexation and hydration in each solution.
Environmental friendly and commercially available sulfamic acid as supporting electrolyte for vanadium redox flow battery has been investigated. Compared with the sulfuric acid system, the bonding style between V and O is not changed. The electrical conductivity of the vanadium(IV) sulfamate solution can be improved significantly by adding NH4+. The VO2+/VO2+ couple in sulfamic acid has better kinetics than that in the sulfuric acid system. The exchange current density and standard rate constant of the VO2+/VO2+ redox reaction on the graphite electrode in sulfamic acid are determined as 7.6 × 10−4 A cm−2 and 7.9 × 10−5 cm s−1, respectively. The diffusion coefficients of VO2+ in sulfamic acid with and without NH4+ are 2.10 × 10−6 and 2.71 × 10−6 cm2 s−1, respectively. Vanadium redox flow battery employing sulfamic acid as supporting electrolyte in the positive side was assembled and evaluated. The energy efficiency of the cell employing pure sulfamic acid in the positive electrolyte can reach 75.87 %, and when the concentration of the added NH4+ is 1.0 and 3.0 mol L−1, the energy efficiency increases by 3.66 and 8.69 % compared with that without NH4+, respectively. The preliminary exploration shows that the vanadium sulfamate electrolyte is promising for vanadium redox flow battery and is worthy of further study.
This work was financially supported by the Major State Basic Research Development Program of China (973 Program, No. 2010CB227201) and Innovation Project of College Students of Central South University (No. CL12136).