Mixed Acid Electrolyte Research Articles

Electrode Materials for the All Vanadium Redox Flow Battery Using Mixed Acid Electrolyte

Operation of vanadium redox flow batteries at low (below 5◦C) and high (above 40◦C) temperatures presents a significant challenge for commercialization due to issues with the solubility of vanadium under these conditions. One strategy that has been developed is the use of a mixed sulfuric and hydrochloric chloride acid electrolyte to stabilize the vanadium species. In this study, the electrochemical characteristics of the mixed acid system was explored using cyclic voltammetry at low and high temperature, with a range of electrode materials, including carbon paper, graphite felt and nanoporous carbon scaffold (NCS). Scanning electron microscopy (SEM) was used to characterize the morphology of the electrodes materials. The results show that the mixed acid system stabilized the vanadium redox system and enable operation at higher concentrations, leading to significant improvement in electrochemical characteristics at low and high temperature. Also, NCS electrode materials were found to have higher activity than graphite paper and graphite felt for the vanadium redox systems in the mixed acid electrolyte.

Optimization of mechanical properties for pulsed anodizing of aluminum

Mechanical properties such as thickness, hardness, and wear resistance of an anodic oxide layer on aluminum must be high enough for industrial purposes. Due to burning phenomenon, hardness and wear resistance of anodic oxide layer decrease as the thickness of the layer increases. These properties of oxide layer fabricated by regular anodizing on aluminum in mixed sulfuric-oxalic acid electrolyte were optimized using pulsed currents. For this purpose, four variable response surface design (duty cycle, frequency, maximum and minimum current densities) was utilized. Desirability function was used to maximize thickness, microhardness and wear resistance of the anodic oxide layer. Wear resistance of the layer was measured by means of ‘pin on disk’ method according to ASTM G89 standard. The determined optimal anodizing conditions were: duty cycle=65%, frequency=326Hz, imax=3.5 and imin=0.5A/dm2, while the corresponding estimated responses values were 170μm, 526Hv0.1 and 2.12×10−7g/N.m for thickness, microhardness, and wear rate, respectively. Frictional behavior of optimized pulsed, and direct current anodic oxide layers was discussed in terms of friction coefficient and examination of worn surfaces.

Sustainability of vanadium redox-flow batteries: Benchmarking electrolyte synthesis procedures

This work focuses on the assessment, in terms of effectiveness, feasibility and sustainability, of three different procedures for the synthesis of vanadium-based mixed-acid electrolytes to be used in vanadium redox-flow batteries (VRFBs). Procedures considered consisted in: a) the mere mixing of suitable vanadium precursors (Electrolyte A); b) the chemical reduction of V2O5 by oxalic acid (Electrolyte B); c) the electrochemical reduction of V2O5 using a home-made “H-shaped” electrolysis cell (Electrolyte C). VRFB properties such as energy efficiency, mean charge–discharge voltages, cycle duration as well as stability and conductivity of the electrolyte were analysed and compared with the state-of-the-art. Experimental tests carried out on a laboratory scale VRFB battery comprised: thermal stability test, cyclic voltammetry, electrochemical impedance spectroscopy measurements and charge–discharge tests. A Life Cycle Assessment of the three electrolytes is also presented for benchmarking purposes.

All Vanadium Redox Flow Battery Development for Stationary Energy Storage Applications

The DOE Office of Electricity’s – Energy Storage program at Pacific Northwest National Laboratory (PNNL) has been focused on the development of cost effective energy storage technologies to improve the grid reliability, power quality, and adoption of the intermittent renewable energy. Among the most promising large-scale energy storage technologies are redox flow batteries (RFBs). RFB’s are electrochemical devices capable of storing up to multi-megawatt-hours (MWhs) of electrical energy via a reversible electrochemical energy conversion. Unlike traditional batteries in which the electrodes serve as the supply/sink of ionic charge, RFB’s store electricity in liquid electrolytes that flow on either side of a membrane. As such, the power and energy of a RFB can be separately designed, offering greater flexibility for a greater selection of stationary applications. All-vanadium RFB system enlists the same element, vanadium, in both the catholyte and anolyte. Researchers at PNNL have revisited the standard vanadium sulfate electrolyte chemistry used in this system and developed an novel mixed acid electrolyte (sulfate and chloride) that can stabilize all four vanadium cations at concentrations ≥ 2.5M within a temperature range of 5~50oC. Recently, PNNL recently demonstrated a kW/kWh scale prototype battery utilizing this electrolyte system in order to better understand the system benefits of the mixed acid electrolyte. The system consisted of ~ 20 cell stack with 780cm2 of active area/cell, ~ 30 liter electrolyte tanks, pumps and associated piping. The stack incorporated new interdigitated flow fields for reduced pressure drop and thinner Nafion membranes for improved performance. Results of stack testing under a wide range of current densities and temperature conditions will be reported along with resulting energy efficiencies.

Invited: High Efficiency V/V Redox Flow Battery Stack Utilizing Mixed Acid Electrolyte Chemistry

The demand for large-scale electrical energy storage (EES) devices has been growing for both improved efficiency and flexibility of the current grid infrastructure and to enable a higher penetration of stochastic renewable sources such like solar and wind onto the grid. Among the most promising technologies for the grid-scale EES are redox flow batteries (RFBs), which are capable of storing a large quantity of electricity (multi-MWs/MWhs) in a relatively simple and straightforward design. With support from the Department of Energy-Office of Electricity’s Energy Storage Program, PNNL developed a mixed-acid electrolyte (hydrochloric and sulfuric acid) for the all vanadium redox flow battery that offers significant improvements in thermal stability and solubility over the conventional, sulfate only system.PNNL has incorporated the mixed acid electrolyte into kW scale systems in order to determine the optimal system efficiency at various flow, temperature, and current density conditions. A 15-cell stack utilizing Nafion® 115 was successfully operated at more 160 mA/cm2 over a 10 % - 90% SOC range with a stack energy efficiency of 74 %. At a flow rate of 6 lpm for both the catholyte and anolyte, the system obtained an overall efficiency (stack - pumping power) of 65%. With no active cooling the system reached a maximum temperature of 47°C. Replacing the 5 mil Nafion® 115 membrane with a 2 mil Nafion® 212, the energy efficiency increased to 79% while the operating temperature decreased to 43°C for the same 6 lpm flowrate. Further development was done on the catholyte and anolyte flow field to reduce the pressure drop through the stack. A new interdigitated flow field was designed and incorporated into the 15-cell stack. With the interdigitated flow field and the Nafion® 212 membrane, the stack efficiency remained at 79% but the system efficiency improved to 72% due to the lower pressure drop in the system. At 6 lpm flow rate, the interdigitated design resulted in ~ 3X decrease in the pressure drop across the cell (from an average of 6.5 psi to 2.5 psi for the interdigitated design). The operating temperature under these conditions was 40°C. Further testing of the interdigitated flow field and the Nafion® 212 membrane at 240mA/cm2 and the same electrolyte flow rate netted a stack energy efficiency of 71%, a system efficiency of 67%, and an operating temperature of 46°C.

Effect of Mixed Acid Media on the Positive Side of the Hybrid Zinc-Cerium Redox Flow Battery

The electrochemical activity of the Ce3+/4+reaction in the presence of various mixed acid electrolytes containing methanesulfonic acid (MSA) as the base electrolyte along with hydrochloric, nitric and sulfuric acid was investigated as a function of acid concentration and reaction temperature on a Pt disk electrode. Cyclic voltammetry in 0.6molL−1 cerium and 4molL−1 MSA with 0.5 - 1molL−1hydrochloric acid showed that the reversibility and kinetics of the Ce3+/4+reaction were significantly enhanced compared to that containing MSA only. Higher diffusion coefficients (D) and exchange current densities (jo) were also obtained for the mixed acid media containing sulfuric and nitric acid, but in this case the reversibility of the reaction was not that improved. Higher concentrations of hydrochloric, nitric and sulfuric acid (>1molL−1) made the oxidation of Ce3+ very hard to delineate from the cyclic voltammograms because of the side reactions occurring at such high potentials. The exchange current density (jo), obtained through polarization resistance and electrochemical impedance spectroscopy measurements (EIS), increased with temperature over the range 25 to 55°C. The EIS results showed that the solution resistance of the mixed electrolytes is considerably lower than that of the unmixed electrolyte facilitating the charge transfer reaction. Nonetheless, solubility issues arose at high Ce4+ concentrations (state of charge>60%) and elevated temperatures (>40°C).

State of Charge and Electrolyte Composition Effects on the Kinetics of Vanadium RFBs

The all vanadium redox flow battery (VRFB) shows significant potential for grid scale energy storage with load leveling capability.1VRFBs, especially in the low-current, higher-efficiency region of operation, are controlled by the kinetics of the redox reactions in each half-cell. We focus this effort on understanding the effects of state of charge and electrolyte composition on the kinetics of the battery.The kinetic behavior of each electrode reaction was studied independently in-situ via the inclusion of a dynamic hydrogen electrode in an operating VRFB. The exchange current density was measured at states of charge ranging from 0% SoC to 100% SoC in a VRFB operating at 25°C with a pure sulfuric acid or a mixed sulfuric-hydrochloric acid electrolyte. The kinetics of the positive electrode reaction (Eq. 1) shows a stronger absolute response to state of charge than does the kinetics of the negative electrode reaction (Eq. 2), but the relative variability is actually similar. At each state of charge, the exchange current density of the positive couple was greater than that of the negative couple. When corrected for concentration effects, the exchange current density is remarkably constant as a function of state of charge.The mixed acid composition has the advantages of significantly increased vanadium solubility and a wider range of battery operating temperatures.2 Preliminary results indicate enhanced exchange current densities for each electrode at 50% SoC as well. The kinetic response of the mixed acid system as a function of state of charge will be determined via in-situstudies using a DHE. Initial investigations of exchange current at higher states of charge for this system were complicated by the generation of chlorine gas during charging steps. Finally, a relatively wide range of operating temperature can be considered with the use of a mixed acid electrolyte. A comparison of the operational advantages of each electrolyte composition and how they impact VRFB kinetics will conclude this talk.Acknowledgement: We gratefully thank Dr. Imre Gyuk and US DOE Office of Electricity for support of this work.(1) Ponce de León, C.; Frías-Ferrer, A.; González-García, J.; Szánto, D. A.; Walsh, F. C. J. Power Sources 2006, 160, 716–732.(2) Li, L.; Kim, S.; Wang, W.; Vijayakumar, M.; Nie, Z.; Chen, B.; Zhang, J.; Xia, G.; Hu, J.; Graff, G.; Liu, J.; Yang, Z. Adv. Energy Mater. 2011, 1, 394–400.

1 kW/1 kWh advanced vanadium redox flow battery utilizing mixed acid electrolytes

This paper reports on the recent demonstration of an advanced vanadium redox flow battery (VRFB) using a newly developed mixed acid (sulfuric and hydrochloric acid) supporting electrolyte at a kW scale. The developed prototype VRFB system is capable of delivering more than 1.1 kW in the operation range of 15–85% state of charge (SOC) at 80 mA cm−2 with an energy efficiency of 82% and energy content of 1.4 kWh. The system operated stably without any precipitation at electrolyte temperatures >45 °C. At similar electrolyte temperatures, tests with a conventional sulfuric acid electrolyte suffered from precipitation after 80 cycles. By operating stably at elevated temperatures (>40 °C), the mixed acid system enables significant advantages over the conventional sulfate system, namely; 1) high stack energy efficiency due to better kinetics and lower electrolyte resistance, 2) lower viscosity resulting in reduced pumping losses, 3) lower capital cost by elimination of heat exchanger, 4) higher system efficiency and 5) simplified system design and operation. Demonstration of the prototype stack with the mixed acid electrolyte has been shown to lower the cost of conventional VRFB systems for large-scale energy storage applications.

Electrochemical Oxidation of Ce(III) to Ce(IV) in Mixed Acid (H<sub>2</sub>SO<sub>4</sub> and CH<sub>3</sub>SO<sub>3</sub>H)

Electrochemical behaviors of Ce3+/Ce4+ couple in mixed acid (H2SO4 and CH3SO3H) were investigated on Pt electrode. Polarization curves and electrochemical impedance spectroscopy (EIS) were used to optimize mixed acid composition, and the mixed acid consisting of 1.0 M methanesulfonic acid (MSA) and 0.8 M sulfuric acid is singled out as the optimum electrolyte. Subsequently, the effects of current density and electrolyte temperature on the electrochemical oxidation of Ce3+ to Ce4+ were researched. A 92.2% current efficiency was achieved at 40 mA•cm-2 in cerium (III) solution with the optimum mixed acid electrolyte above at 313 K. It can satisfy the application of mediated electrochemical oxidation process with Ce3+/Ce4+ mediator.

Designing and Constructing a Controlled-Flow Apparatus to Study the Effect of Surface Flow Velocity on the Quality of Electropolishing of Niobium

The focus of this project was to design and construct a model controlled-flow apparatus to study electropolishing of one cm2 niobium coupons at surface flow rates typical of cavity processing. A simulation of the apparatus was constructed using CFDesign, a flow and thermal simulation software, to ensure that the selected dimensions may be expected to provide steady-state, laminar flow across the surface of the niobium coupon. Based on these dimensions, a sample system and apparatus was produced to determine the correct reservoir elevation heights for the desired flow rates for fluid viscosities represented in the mixed acid electrolyte. From the CFDesign simulations, it was found that the flow channel supplied laminar flow rates when the center of the niobium coupon was located 40 mm downstream from the inlet stream. The corresponding system, based on the CFDesign simulations, showed that the reservoir elevation heights for flow rates of 0 cm/s to 5 cm/s ranged from 0 to 1.27 cm. The correlation between pressure heads and flow rates has been analyzed and an equation for flow rate was determined using experimental results. The detailed dimensions regarding the flow channel and information regarding the respective pressure heads serve as resources to finding the optimal flow rate for electropolishing the niobium cavities. Although previous research has found a correlation between the quality of electropolishing and internal surface flow rates, research facilities, including Jefferson Lab, did not have the equipment to pursue further analysis. Each nine-cell niobium cavity costs over $50,000, so it is cost prohibitive to use real cavities to conduct early stage research. The prototype built through this research work provides a cost effective alternative. It can be used to validate some of the theoretical results obtained through simulation. In addition, the device allows for easy variable measurements that are either difficult or impractical with an enclosed niobium cavity, as sensors can be embedded into the device in the construction stage. Moreover, the data collected through our experiment furthers superconducting radiofrequency (SRF) technology by allowing Jefferson Lab to design a more effective electropolishing process.

Study of the Ce3+/Ce4+ Redox Couple in Mixed-Acid Media (CH3SO3H and H2SO4) for Redox Flow Battery Application

The present paper first reports a kind of supporting electrolyte, mixed-acid media (CH3SO3H and H2SO4), used in redox flow battery (RFB) technology. Experimental work is performed with the aim of evaluating the Ce3+/Ce4+ redox couple in mixed-acid electrolyte for use in RFB technology. The mixed-acid media have their special advantages: (i) the Ce3+/Ce4+ electrode reaction appears to be more reversible in mixed-acid solution; the peak splittings in mixed acid are significantly less than in sulfuric acid; the diffusion coeffcient in mixed acid is larger than in single acid (CH3SO3H or H2SO4); the exchange current density and standard rate constant of the Ce3+/Ce4+ redox reaction in mixed-acid media on graphite electrode are 8.05 × 10–3 A cm–2 and 4.17 × 10–4 cm s–1; (ii) the solubility of cerium salt in mixed acid is larger than in sulfuric acid; a solution of 1 mol dm–3 cerium containing 2 mol dm–3 MSA and 0.5 mol dm–3 H2SO4 is sufficiently stable for more than 1 month at temperatures up to 313 K; (iii) Coulombic and energy efficiencies of the test cell using mixed-acid electrolyte are both higher than that using single CH3SO3H electrolyte; the average Coulombic and energy efficiencies of the cell using mixed acid are 87.1 and 73.5%, respectively, which are comparable to that of the all-vanadium RFB.

Synthesis and characterization of P-doped TiO 2 nanotubes

Titanium dioxide (TiO 2) doped with phosphorus (P) was synthesized by anodization of Ti in the mixed acid electrolyte of H 3PO 4 and HF and characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis spectrum. The morphology greatly depends on the applied voltage. The as-formed nanotubes under the optimized condition, at 20 V for 2 h, are highly ordered with ~ 200 nm in length and the average tube diameter is about 100 nm. By annealing the initial samples at different temperatures, the importance of the crystalline nature is confirmed. Significantly, the peak positions of anatase in XRD patterns shifts to lower diffraction angles with an increase in the amount of H 3PO 4 ion. A remarkable red shift of the absorption edge has been observed for the sample formed in the electrolyte of HF and H 3PO 4, which is related to the introduction of P 5+ into TiO 2 crystallization and the possible impurity energy level formed in the TiO 2 band gap. The presence of P 2p state in XPS spectrum can further confirm the P 5+ which can replace a part of Ti 4+ has been introduced into TiO 2 crystallization. The present convenient synthesis technique can be considered to the composition of other doped oxide materials.

Fabrication of multi-non-metal-doped TiO 2 nanotubes by anodization in mixed acid electrolyte

Multi-non-metal-doped TiO 2 nanotubes were fabricated by electrochemical anodization of Ti in the mixed acid electrolyte of C 2H 2O 4·2H 2O and HIO 3 containing NH 4F. The samples were annealed in air and characterized by FE–SEM, XRD, XPS and DRS. The results indicate non-metals of N, F and I are successfully doped into TiO 2 nanotubes in aqueous solution by adjusting the electrolyte composition. The multi-non-metal-doped samples display a significant visible-light response. Additionally, the atomic concentration of non-metals is closely related with the electrolyte composition.

Preparation and characteristics of well-aligned macroporous films on aluminum by high voltage anodization in mixed acid

Well-aligned macroporous films on aluminum were prepared by high voltage anodization (HVA) in mixed acid electrolyte consisting of oxalic acid, sodium tungstate, phosphoric acid and hypophosphorous acid. The total concentration (about 10 g l − 1 ) of the electrolyte is much lower than that of normal anodization electrolyte. This anodizing process is characterized by a “saddle” shape change of the current density and short anodization period (around 15 min), comparing to conventional anodizing process. The aligned cell arrangement was markedly affected by the peak current density, which was optimized as 6–10 A dm − 2 . The effects of the concentrations of phosphoric acid, oxalic acid and hypophosphorous acid on the morphologies of the films were discussed in detail. The results showed that the pore diameter ranged from 250 to 500 nm, and increased with increasing of the concentration of phosphoric acid, oxalic acid and hypophosphorous acid. The pore geometry could change from round to hexagonal, which was related to the electrolyte components and their concentrations.

Electrochemical behavior of lead alloys in sulfuric and phosphoric acid electrolyte

The electrochemical behavior of lead, lead–calcium–tin–aluminum and lead–calcium–tin–aluminium–bismuth alloy electrode has been studied in the electrolyte of sulfuric, phosphoric and mixed acid electrolyte. The electrochemical measurements were performed using a cyclic voltammetry technique (CV) with three-electrode system. The results demonstrated that bismuth(III) doped in the lead electrode can increase electrical capacity. The oxidation peak potential of lead alloy electrode in phosphoric acid shifted to the positive compared to that in sulfuric acid. The formation of PbHPO 4 in phosphoric acid and its passivation are responsible for the positive shift of the oxidation.

622 高Cu含有アルミ合金ピストンリング溝への硬質陽極酸化(OS5 21世紀に挑むエンジン技術)

622 高Cu含有アルミ合金ピストンリング溝への硬質陽極酸化(OS5 21世紀に挑むエンジン技術)