Zinc Bromine Redox Flow Battery Research Articles

Modeling of Zinc Bromine redox flow battery with application to channel design

Here we present a 2-D combined mass transfer and electrochemical model of a zinc bromine redox flow battery (ZBFB). The model is successfully validated against experimental data. The model also includes a 3-D flow channel submodel, which is used to analyze the effects of flow conditions on battery performance. A comprehensive analysis of the effects of flow is included. We find that the zinc deposition decreases with flow distance during charging, and the opposite for the zinc de-plating process during discharging. The simulations also show that multiple curved flow channels can slightly improve the voltage efficiency of the ZBFB while the coulombic and energy efficiencies are slightly reduced. The model can be used for improving ZBFB design.

Activated Carbon‐Anchored 3D Carbon Network for Bromine Activity and its Enhanced Electrochemical Performance in Zn−Br2 Hybrid Redox Flow Battery

AbstractThere has been a huge demand on the usage of biomass‐derived activated carbon as an electrode and electrocatalyst for energy storage and conversion applications owing to its abundance, high surface area, good chemical stability, low cost, and eco‐friendliness. Herein, we report for the first time, activated carbon (AC) derived from Phyllanthus Emblica Leaves (PEL) (Indian gooseberry) biomass as an electrocatalyst for enhancing the bromine electro‐kinetics. The AC‐anchored graphite felt (AC−GF) showed excellent improvement in the bromine reversibility, which reinforces the overall performance of the zinc bromine redox flow battery (ZBRFB). Specifically, the AC−GF showed significant improvement in voltage efficiency. The flow cell employing AC in the positive electrode showed high Coulombic, voltage, and energy efficiency of 99, 83 and 82 %, respectively at a current density of 30 mA cm−2. Furthermore, the cell showed good cycle performance with 96.43 % Coulombic efficiency retention even after 100 cycles. Thus, the significant improvement in the cell performance unveils the good electrochemical/catalytic activity of the AC and therefore can be used as a potential low‐cost electrocatalyst in ZBRFB.

Realizing highly efficient energy retention of Zn–Br2 redox flow battery using rGO supported 3D carbon network as a superior electrode

Zinc–Bromine redox flow battery (Zn–Br2 RFB) is one of the most promising aqueous metal hybrid flow batteries used to store high energy in mega scale. This aqueous system provides high cell voltage with high practical energy density over the other redox flow systems. Conversely, poor reversibility of bromide/bromine redox couple strictly affects cell performances such as rate capability, cell efficiency and cycle life. In the present work, the rate capability is enhanced to 80 mA cm−2 by improving the bromine reversibility in the positive electrode. First time, reduced Graphene Oxide (rGO) is employed to improve the bromine kinetics in the Zn–Br2 redox flow battery. The electrochemical analysis showed highly renovated bromine kinetics with less peak separation potential (103 mV) and very closer onset potentials (anodic reaction: 0.92 V & cathodic reaction: 1.08 V). Moreover, the obtained anodic (Ia) and cathodic (Ic) peak current ratio (Ia/Ic) 0.83 indicates the improved reversibility of bromine reactions on rGO supported 3D carbon felt electrodes. Importantly, the cell showed increased energy efficiency of 80.75% for the rGO supported electrode based flow system over the studied cycle range. Thus, the rGO based electrode materials have fascinated more interest as an efficient electrocatalyst for Zn–Br2 RFB development.

Metal Oxide Coatings on Carbon Electrodes with Large Mesopores for Deeply Charged Zinc Bromine Redox Flow Batteries

Carbon nanofibers with various large mesopores (>10nm) were made and tested as active cathode materials for zinc bromine redox flow batteries. The largest mesopores demonstrated improvements over activated carbon in coulombic efficiency as well as rate capability up to 30mA/cm2. Afterwards, the mesoporous carbon nanofibers were coated with four amorphous metal oxides made from liquid precursors to form a protective coating, and their protective nature was examined through deep and shallow charge cycles. The effect of the coating on battery performance was then analyzed through galvanostatic testing, cyclic voltammetry, and electrochemical impedance spectroscopy. It was determined that the metal oxide coatings improved rate capability and voltaic efficiency by adsorbing the bromine complex more readily, while the large mesopores aided performance from a combination of absorption and more efficient diffusion.

High performance zinc-bromine redox flow batteries: Role of various carbon felts and cell configurations

Abstract Optimization of the cell configuration utilizing various carbon felts for obtaining better performance in zinc-bromine redox flow battery (ZBRFB) system is reported. It is clearly observed that the reaction kinetics of both Zn and Br are enhanced when used in different cell configurations with carbon felts. Redox flow cells having carbon felts on both half-cells showed improved performance thanthe other cell configurations tested in the present study. Further, Rayon based carbon felt is evaluated as an electrode on both sides. It is noted that the Rayon carbon felt based cell exhibited good performance in terms of coulombic (96.26%), voltaic (83%) and energy efficiency (79.4%) at a current density of 20 mA.cm−2. In addition to that, Rayon felt based system showed good performance even at 40 mA.cm−2 (3250 mW h. l−1) which indicates the significance of the flow cell configuration on the overall cell performance.

The Research Progress Of Zinc Bromine Flow Battery

Zinc bromine redox flow battery (ZBFB) has been paid attention since it has been considered as an important part of new energy storage technology. This paper introduces the working principle and main components of zinc bromine flow battery, makes analysis on their technical features and the development process of zinc bromine battery was reviewed, and emphasizes on the three main components of zinc bromine battery, and summarizes the materials and applications of electrolyte, membrane and electrode. At the same time, the solution to the technical problems of zinc bromine flow battery is also briefly analyzed. Finally, the future development of zinc bromine battery system is prospected.

Investigation of Graphite and SWCNT Composite Electrode for Zinc-Bromine Redox Flow Batteries

The development of zinc bromine redox flow battery technology is experiencing a critical challenge due to the slow kinetics of bromine/bromide redox reactions, which results in poor efficiency of the cell. Here, we demonstrate a comparative electrocatalytic effect of platinum, graphite and graphite/single-walled carbon nanotube (G/SWCNT) composite electrodes for the 2Br–/Br2 redox couple for zinc bromine redox flow battery application. The anodic peak current density of G/SWCNT electrode is found to be higher than that of pristine Pt and graphite electrodes, indicating the enhanced electrocatalytic effect of G/SWCNT perhaps due to the incorporated basal planes. The peak separation between the anodic and cathodic process of graphite and G/SWCNT electrodes is 225 and 160 mV, respectively, demonstrating the quasireversible nature of the 2Br–/Br2 redox reaction. Further characterizations of composites electrodes are carried out using FTIR, Raman spectroscopy and scanning electron microscopy (SEM) before and after cycling. The electrochemical performance and characteristic of the zinc bromine flow cell fabricated with graphite and G/SWCNT as electrodes is discussed in detail.

Optimization and Design of the Minimal Architecture Zinc-Bromine Battery Using Insight from a Levelized Cost of Storage Model

This work demonstrates how a levelized cost of storage (LCOS) model can be used to optimize the performance of the minimal architecture zinc bromine battery (MA-ZBB). Cycling data is collected at charge times ranging from 4 to 48 hours and capacities ranging from 320 to 4000 mAh using scaled-up versions of the MA-ZBB. An LCOS model for the entire MA-ZBB system is proposed and used to demonstrate how the energy efficiency/discharge energy trade-off within the system can be exploited to minimize LCOS. The present, unoptimized cell is shown to approach an LCOS of $0.08 kWh−1 as electricity purchase prices approach $0.02 kWh−1. At all purchase prices, greater than 60% of the LCOS comes from the capital cost, where the main contributors are the carbon foam electrode and zinc bromine electrolyte in the cell (both accounting for 20% of the total capital cost). In addition, two case studies are conducted which show how the LCOS model can be used to determine the optimal electrode spacing (0.4 cm) and electrolyte concentration (1.0 M) in the cell. Finally, a comparison with existing technologies is conducted, indicating the system-level cost of the MA-ZBB is competitive with lithium-ion, lead-acid, vanadium redox flow, and zinc bromine redox flow batteries.

Characterization of the Efficiency Losses in the Minimal Architecture Zinc-Bromine Battery

The major design criteria for electrochemical energy storage on the grid-scale are low cost and long lifetime. To accomplish this, research efforts have largely focused on the development of low-cost active materials which are incorporated into traditional battery designs. These designs typically contain a considerable amount of passive components that are necessary for extending the lifetime of the battery. For instance, zinc bromine redox flow batteries have been shown to perform with less that 20% capacity fade for 10,000 cycles [1]. However, the cost of this technology (>$200 kWh) is still prohibitive even though the active materials and carbon electrodes only account for ~$8 kWh. The majority of the cost comes from the system level, where complexing agents, separation membranes, and flow systems are used to prevent the corrosive bromine from reacting with the zinc electrode. In light of this fact, our lab has recently developed a minimal architecture zinc-bromine battery (MA-ZBB), which utilizes the same chemistry from commercial zinc-bromine flow batteries, but does not require the need of any passive components, significantly reducing the cost (projected ~$94/kWh) [2] The original MA-ZBB is composed of two carbon based electrodes, which are spatially separated in the vertical direction, in an aqueous electrolyte containing zinc-bromide salt. The battery does not contain any passive components for controlling bromine/zinc interaction. Instead, the electrodes are vertically separated to utilize the variations in density between bromine and the aqueous electrolyte as an advantage to minimize self-discharge. At the negative electrode on top of the cell, zinc ions are reduced (Zn) and oxidized (Zn2+) on a carbon cloth during charge and discharge, respectively. At the positive electrode, which is located on the bottom of the cell, bromide ions are oxidized (Br2) and reduced (Br-) within a carbon foam electrode. To date, a proof-of-concept cell on the lab-scale has demonstrated an energy efficiency of over 60% for over 1000 cycles at an energy density of ~10 Wh/L. In this work, we present recent efforts aimed at improving the efficiency and capacity of the system. Galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) are utilized to determine the relative importance of ohmic overpotential, charge transfer overpotential, and mass transport limitations on the performance of the cell. Based on these results, variations in cell and electrode design/materials selection are investigated, which minimize the main sources of overpotential in the system and maximize the cell capacity (Figure 1). [1] R. M. Darling, K. G. Gallagher, J. A. Kowalski, S. Ha, and F. R. Brushett, Energy Environ. Sci., 7 (2014) 3459. [2] Shaurjo Biswas, Aoi Senju, Robert Mohr, Thomas Hodson, Nivetha Karthikeyan, Kevin. W. Knehr, Andrew G. Hsieh, Xiaofang Yang, Bruce E. Koel, Daniel A. Steingart, Energy Environ. Sci., 10 (2017) 114. Figure 1

Synthesis and Characterisation of Novel Additives for Use in the Hybrid ZnBr2 Flow Battery

Zinc-bromine redox flow batteries (RFB) are energy storage devices capable of integrating with renewable generators and so improve energy management from these intermittent sources. However, this system is susceptible to self-discharge via the electrogenerated bromine transferring through the separator and reacting with the plated zinc. Here we examine the use of novel quaternary ammonium complexes to capture the electrogenerated bromine but keep it in the aqueous phase (as opposed to the immiscible phase formed with N-methyl-N-ethylpyrrolidinium, MEP). Electrochemical analysis indicates that these complexes do not impact on the electrode kinetics and exhibit similar physical properties as to the MEP. However, the latter still shows an enhanced ability to complex the bromine to higher polybromide states (i.e. Br5 −).

Estimation of State-of-Charge for Zinc-Bromine Flow Batteries by In Situ Raman Spectroscopy

A zinc–bromine redox flow battery (ZBB) has attracted increasing attention as a potential energy-storage system because of its cost-effectiveness and high energy density. However, its aqueous zinc bromide phase and non-aqueous polybromide phase are inhomogeneously mixed in the positive electrolyte. Furthermore, various equilibrium reactions, e.g., charge-transfer reactions, polybromide formation, and complexation, simultaneously occur in the battery. Because of these complex reactions, it is difficult to systematically analyze its electrolyte, which a component crucial for the stable operation of the battery. Especially, although the state-of-charge (SoC) of an electrolyte is crucial for preventing overcharging or discharging and side reactions, its accurate estimation is difficult. As a result, there have been few studies on estimation of the SoC in ZBBs. In this study, in situ Raman spectroscopy is employed for the real-time estimation of the SoC in 25 charge–discharge cycles. To exclude errors arising from the inhomogeneous dispersion of the non-aqueous phase, SoC is monitored on the negative electrolyte. External standard solutions are measured, and the calibration curve is constructed just before in situ measurements at every cycle to minimize instrumental errors, e.g., those caused by alignment. This in situ methodology exhibits high accuracy and reproducibility.

Complexing Additives to Reduce the Immiscible Phase Formed in the Hybrid ZnBr2 Flow Battery

The zinc-bromine redox flow battery (RFB) is one of a very few commercially viable RFB energy storage systems capable of integration with intermittent renewable energy sources to deliver improved energy management. However, due to the volatility of the electrogenerated bromine and potential for its crossover from positive to negative electrolytes, this system requires the use of quaternary ammonium complexes (N-methyl-N-ethylpyrrolidinium, (MEP)) to capture this bromine. This produces an immiscible phase with the Br2 which requires a complex network of pipes, pumps and automated controls to ensure access to the electroactive material during discharge. In this work, the use of novel quaternary ammonium complexes to capture the electrogenerated bromine but to keep it in the aqueous phase is examined. Three compounds, 1-(carboxymethyl) pyridine-1-ium, 1-(2-carboxymethyl)-1-methylmorpholin-1-ium and 1-(2-carboxymethyl)-1-methylpyrrolidin-1-ium, were found to successfully reduce the volume of the immiscible phase formed on complexing with the polybromide (Brx−) whilst displaying similar enthalpy of vaporization values as that of MEP. Electrochemical analysis also revealed that these compounds did not impact on the electrode kinetics of the Br−/Brx− reaction indicating that the resulting surface film formed with these compounds behaved as a chemically modified electrode, in contrast to the surface film formed with MEP.

Design and Optimization of Carbon Foam Electrode for Local Confinement of Bromine in Non-Flowing Single-Chamber Zinc Bromine Batteries

Zinc Bromine redox flow batteries are well studied, quite inexpensive, and have reasonably high power and energy densities. However, there has been limited commercial implementation of bromine bearing systems in general, and the Zn-Br2 systems in particular, due to the following premises: i) Br2(l), generated at the cathode during charging, can diffuse through the electrolyte and react with Zn deposited at the anode (crossover), leading to self-discharge; ii) Repeated electroplating and dissolution of zinc leads to dendrite formation, which can form a conductive bridge between the electrodes (shorting), and iii) Br2(l) has low miscibility in aqueous solutions (~2.8 vol%) and tends to stratify, resulting in non-uniform concentration distributions. Standard Zn-Br2 flow-cell designs alleviate these limitations with bromine-complexing agents to improve Br2(l) solubility, separation membranes to prevent crossover and shorting, and flowing electrolyte to remove bromine and to minimize dendrite formation. In the membrane-free, non-flowing single-chamber zinc-bromine (SC-Zn-Br2) battery design zinc dendrites are allowed to form freely and Br2(l) is allowed to stratify; expensive membranes, complexing agents, and pumps are eliminated; and yet overall energy efficiency is improved and cost is lowered. This is achieved by using a highly-porous carbon foam electrode (CFE) for local containment of Br2 generated during cycling, thus preventing crossover. Zinc dendrites are allowed to grow, but react with the highly corrosive Br2(l) on the surface of the CFE and dissolve back into the electrolyte as Zn2+ and Br-ions. This prevents shorting. Here, we discuss the design, composition, and fabrication of the CFE, and its effects on the performance of SC-Zn-Br2 battery. In particular, we demonstrate the optimization of CFE porosity through processing parameters to maximize the coulombic and energy efficiencies of the cell. We also introduce a color tracking and feedback monitoring scheme to actively control the active species (deep red Br2(l), yellow Br2(aq), and colorless ZnBr2 electrolyte) transport and prevent self-discharge. Finally, we demonstrate the effect of prolonged exposure to Br2(l) after multiple cycles on various CFE compositions using XPS analysis, and the potential degradation mechanisms. We show coulombic and energy efficiencies of 95% and 75%, respectively, for over 1000 cycles for SC-Zn-Br2 cells, with improvements possible with further optimization of the CFE.

Critical rate of electrolyte circulation for preventing zinc dendrite formation in a zinc–bromine redox flow battery

Abstract In a zinc–bromine redox flow battery, a nonaqueous and dense polybromide phase formed because of bromide oxidation in the positive electrolyte during charging. This formation led to complicated two-phase flow on the electrode surface. The polybromide and aqueous phases led to different kinetics of the Br/Br − redox reaction; poor mixing of the two phases caused uneven redox kinetics on the electrode surface. As the Br/Br − redox reaction was coupled with the zinc deposition reaction, the uneven redox reaction on the positive electrode was accompanied by nonuniform zinc deposition and zinc dendrite formation, which degraded battery stability. A single-flow cell was operated at varying electrolyte circulation rates and current densities. Zinc dendrite formation was observed after cell disassembly following charge–discharge testing. In addition, the flow behavior in the positive compartment was observed by using a transparent version of the cell. At low rate of electrolyte circulation, the polybromide phase clearly separated from the aqueous phase and accumulated at the bottom of the flow frame. In the corresponding area on the negative electrode, a large amount of zinc dendrites was observed after charge–discharge testing. Therefore, a minimum circulation rate should be considered to avoid poor mixing of the positive electrolyte.

Single-Step Synthesis of Halogenated Graphene through Electrochemical Exfoliation and Its Utilization as Electrodes for Zinc Bromine Redox Flow Battery

We have demonstrated a single step synthesis of halogen functionalized graphenes (HGs) through electrochemical exfoliation of graphite in aqueous potassium halide solutions. Few layers of as-synthesized HGs are characterized using powder X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Electron paramagnetic spectroscopy (EPR), Brunauer-Emmett-Teller (BET) surface area analysis, Field emission scanning electron microscopy (FESEM), and Transmission electron microscopy (TEM). The degree of halogenation is found to vary between 2.32 and 0.26 atomic % in fluorinated graphene (FG) and iodinated graphene (IG) respectively, which could be attributed to the difference in reactivity of the halogen species generated during the exfoliation process. A suitable mechanism is proposed in accordance with the exfoliation phenomena and functionalization of halogens on the graphene sheets based on the experimental observation. As synthesized HGs are utilized as electrode materials for zinc bromine flow batteries (ZBB) and the electrocatalytic effect of HGs electrodes for the 2Br−/Br2 redox couple has been investigated in detail. Cyclic voltammetry (CV), and charge-discharge studies were carried out to study the electrochemical performance of the HGs. Among all HGs, FG has shown the superior electrocatalytic behavior for 2Br−/Br2 redox reaction. The anodic (11.2 mA cm−2) and cathodic (10.7 mA cm−2) peak current densities are higher for FG than that of other halogenated graphenes. ZBB flow cell fabricated with FG as bromine electrode exhibits enhanced electrochemical performance in terms of efficiency (81% of voltaic efficiency and 72% energy efficiency) and durability up to 350 cycles.

Electrochemical Performance of Three Novel Bromine-Sequestering Agents for Zinc/Bromine Flow Battery Electrolytes

Three different bromine-sequestering agents (BSAs) were proposed and assessed for their fitness-for-utilization in zinc–bromine redox flow batteries (Zn/Br RFBs). These BSAs included the bromide salts of the following cations: 1-ethyl-1-methyl-imidazolium ([C2MIm]+), 1-ethyl-1-hydroxymethyl-imidazolium ([C2OHMIm]+) and 1-ethyl-hydroxypiperidinium ([C2OHPy]+). Half-cell cyclic voltammetry and a.c. impedance spectroscopy as well as full-cell cyclic charge–discharge tests were performed to determine the influence of these agents on both on zinc- and bromine-side electrochemical performance. All three BSAs were found to increase peak currents in both zinc- and bromine-side half-cells. [C2MIm]+ and [C2OHPy]+ performed best for bromine- and zinc-sides, respectively, while [C2OHMIm]+ performed overall poorly compared to the other BSAs. Impedance spectra indicate bromine-side performance is strongly linked to Warburg diffusion limitations. On the other hand, voltammetry results indicate zinc-side half-cell performance improves with decreasing nucleation overpotentials during zinc electrodeposition. [C2MIm]+ gave the highest voltaic and coulombic efficiencies during full-cell cycling, while [C2OHMIm]+ again showed overall relatively poor performance. Scanning electron micrographs indicate a strong relationship between chemical structure of the BSA employed and the crystallinity of zinc electrodeposits. Both [C2MIm]+ and [C2OHMIm]+ were found to improve the quality of zinc plating, with the later producing nanocrystalline needle-like deposits. These findings warrant further investigation of BSAs made up of weakly-coordinating cations carrying diffuse positive charges. While these compounds have been proposed for application in Zn/Br systems, they are also potentially applicable to other types of RFBs which employ the Br2/Br– redox couple and require the use of electrolytes containing BSAs.

Preparation of a cost-effective, scalable and energy efficient all-copper redox flow battery

A design for the all-copper redox flow battery based on readily available, cost-effective construction materials is presented. The design results in a stack cost of between 70 and 300 $/m2, with the electrolyte costing 60 $/kWh. The area specific resistance of the cell is 1.5 Ω cm2.A maximum round trip energy efficiency of 72% occurs at a current density of 40 mA cm−2, this is similar to values reported for zinc–bromine redox flow batteries. The two major loss processes are attributable to permeation of Cu2+ through the microporous separator at current densities below 50 mA cm−2 and excessive electrode overpotentials at current densities greater than 75 mA cm−2.Two methods to characterise the current efficiency of the electrochemical processes in the cell are reported. A round trip current efficiency of 98% is consistently found when cycling the all-copper battery at a current density of 150 mA cm−2.

Comparative Electrocatalytic Performance of Single-Walled and Multiwalled Carbon Nanotubes for Zinc Bromine Redox Flow Batteries

Carbon nanotubes (CNTs) have been employed as electrode materials in rechargeable zinc bromine redox flow batteries (ZBB) owing to their high electrocatalytic activity, remarkable electrical conductivity, and excellent mechanical strength with high Young’s modulus. The electrocatalytic effect of single-walled carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT) electrodes for the 2Br–/Br2 redox couple has been investigated for zinc bromine redox flow battery application. The anodic peak current density of SWCNT electrode is found to be about 16 mA cm–2, which is almost 50% higher than that of MWCNT, indicating the enhanced electrocatalytic effect of SWCNT perhaps due to a large amount of basal planes. The peak separation between the anodic and cathodic process at SWCNT and MWCNT electrodes is 201 and 126 mV, respectively, demonstrating the quasireversible nature of the 2Br–/Br2 redox reaction. Moreover, the peak separation for the MWCNT electrode is 37% less compared to that on the SWCNT electr...

A Quaternized Polysulfone Membrane for Zinc-Bromine Redox Flow Battery

A quaternized polysulfone (QNPSU) composite membrane is fabricated for zinc-bromine redox flow battery. The structure of the membrane is examined by FT-IR spectra and SEM. The conductivity of the membrane is tested by electrochemical analyzer. After a zinc-bromine battery with this composite membrane is operated at different voltage while charging and at different current while discharging to examine the performance of the membrane, it is found that the discharge voltage was 0.9672 V and the power density was 6 mW/cm2at a current of 0.1 A, which indicated that the novel composite membrane is a promising material for the flow battery.

High Performance Carbon Nanotube Based Electrodes for Zinc Bromine Redox Flow Batteries

Carbon nanotubes (CNTs) have been utilized as positive electrodes in rechargeable zinc bromine redox flow battery (ZBB) due to their high electrocatalytic activity, enhanced electrical conductivity and unusual mechanical strength with high Young's modulus. The electrochemical behavior of 2Br−/Br2 at pristine glassy carbon electrode (GCE) and CNT modified GCE studied by cyclic voltammetry (CV) in 0.05 M aqueous zinc bromide solution shows clear evidence for enhanced electrocatalytic activity for CNT based electrodes. The voltammetric peak current density at CNT is 16 mA cm−2 (2.5 times higher than that at GCE) presumably due to more active sites, demonstrating high electrocatalytic effect of CNT toward 2Br−/Br2 redox reaction. Composition and morphological changes have been further investigated using FTIR, Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Zinc bromine flow cell with CNT coated carbon felt (CF) as bromine electrode reveals improved the electrochemical performance in terms of voltage (VE), columbic (CE) and energy efficiencies (EE). The VE, CE and EE of a zinc bromine flow cell with CNT (90%) modified bromine electrode are 87%, 77% and 67% respectively.