Solid electrochemical energy storage for aqueous redox flow batteries: The case of copper hexacyanoferrate

All redox flow batteries suffer from low energy storage density in comparison with conventional Li-ion batteries. However, this issue can be mitigated by utilization of solid energy storage materials to enhance the energy storage capacity. In this paper we demonstrate the utilization of copper hexacyanoferrate (CuHCF) Prussian blue analogue for this purpose, coupled with N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride (TEMPTMA) as a soluble redox mediator to target the redox transitions of the solid material. In this case, indirect charging and discharging of CuHCF suspended in the electrolyte by electrochemically oxidized/reduced TEMPTMA was observed by chronoamperometry. Secondly, electrochemistry of different CuHCF composites with carbon black and multi-walled carbon nanotubes were investigated, highlighting that the high conductivity of the solid energy storage materials is crucial to access the maximal charge storage capacity. Finally, a CuHCF-TEMPTMA/Zn aqueous redox flow battery achieved stable cycling performances with high coulombic efficiency of 95% and volumetric capacity of 350 C mL−1.

Redox flow battery and indirect water electrolysis

Véronique Amstutz

The progressive integration of renewable energy sources such as wind turbines and photovoltaic pannels in the current electrical network, and the rise of the electrical mobility, provoke a change of paradigm in the sector of energy management. The increasing variability of the electricity production and consumption profiles, together with the requirement for a reliable supply of energy, necessitates the implementation of energy storage means of different scales and for a wide range of applications. Redox flow batteries (RFBs) are well adapted for buffering the fluctuations of solar or wind energy production. They present a fast response time, can withstand a large number of charge-discharge cycles and their ouput power is independant of their energy capacity. The main limitation of RFBs resides in their low energy density. The objective of the present work was to test a mean to overcome this low energy density. A new concept was developed, which rests on the addition of a second pathway to discharge the RFB. This pathway allows to generate hydrogen and oxygen, without affecting the functioning of the RFB. This concept was called dual-circuit RFB and was patented. RFBs are based on two liquids electrolytes, each stored in a reservoir, and flowing through an electrochemical cell for their electrochemical conversion. Each electrolyte contains one redox couple and Ce(IV)/Ce(III) and V(III)/V(II) were selected as positive and negative redox couples in the RFB developed here. Charge-discharge curves of a VâCe RFB were measured for characterisation purposes and for the preparation of the charged electrolytes. The latter ones can be discharged electrochemically in the RFB to generate electricity (electrochemical discharge mode), or they can be directed in a secondary circuit where they are discharged for the production of hydrogen or oxygen (chemical discharge mode). The chemical discharge of the negative electrolyte consists of the reaction of V(II) with protons to produce hydrogen and V(III). Mo2C was selected as heterogenous catalyst for the characterisation of the reaction. A conversion close to 100% suggested no loss of current. A kinetic analysis provided some insights into the catalytic mechanism of this reaction. The chemical discharge of the positive electrolyte aimed at the conversion of Ce(IV) to Ce(III) by the oxidation of water to oxygen and also necessitates a catalyst. Iridium dioxide (IrO2) and ruthenium dioxide (RuO2) were evaluated in terms of conversion and kinetics. The composition of the positive electrolyte was shown to be of importance for this reaction. To show the feasibility of this concept, a larger-scale demonstrator system was designed based on a 10 kW (40kWh) commercially available vanadium RFB. A characterisation of this RFB was first performed. The design a suitable Mo2C catalyst and its corresponding catalytic bed is also discussed. As a conclusion, the concept developed and experimentally tested in the present work leads to a RFB system which is characterised by two discharge modes, increasing its energy storage capacity and energy density. The dual-circuit RFB also represents a crossing between electrical grid and hydrogen mobility as the hydrogen produced by surplus electricity could be delivered to fuel cell cars. The application of this system in a local distribution electrical network, close to a wind or solar source seems a promising approach.

EPFL2015

Redox Solid Energy Boosters for Flow Batteries: Polyaniline as a Case Study

Véronique Amstutz, Alberto Battistel, Christopher Raymond Dennison, Hubert Girault, Pekka Eero Peljo, Heron Vrubel, Elena Zanzola

In this work, the viability of polyaniline as a solid charge storage material in an aqueous redox-mediated flow battery is investigated. Fe(III/II) and V(IV/III) were identified as potential redox mediators to target the emeraldine-pernigraniline and leucoemeraldine-emeraldine redox transitions of the polymer. An indirect chemical cycling method was developed and used to investigate the charging/discharging of the polymer by the selected redox mediators. With Fe(III/II) as a redox mediator, respectable specific capacity and cycling stability were demonstrated over 25 cycles. V(IV/III) was deemed unsuitable as a redox mediator, due to rather poor kinetics. When added to the electrolyte tanks of a complete flow battery, a conductive composite of polyaniline and carbon black provided a significant improvement in capacity, exhibiting a specific capacity of 64.8 mA h g−1 at a current density of 38.5 mA cm−2. This represents a three-fold improvement in volumetric capacity, compared with the electrolyte alone. Moreover, the addition of the polyaniline composite was observed to lower the average potential at the positive electrode, providing a considerable improvement in voltage efficiency. This work demonstrates the potential of utilizing redox mediators to enable bulk solid-phase charge storage in the tanks of aqueous redox flow batteries

Elsevier2017

Novel concepts of compressed air energy storage and thermo-electric energy storage

Young Min Kim

The interest in energy storage is currently increasing, especially from the perspectives of matching intermittent sources of renewable energy with customer demand and storing excess nuclear or thermal power during the daily cycle. Technologies to be considered for load leveling for large-scale energy systems, typically in the range of hours to days of discharge time, include pumped-storage hydroelectricity, compressed air energy storage (CAES), sodium sulfur (NaS) batteries, advanced absorbent glass mat lead acid batteries, and flow batteries. CAES is a promising technology because of significant advantages such as its high reliability, economic feasibility, and low environmental impact. Thermo-electric energy storage (TEES), which was recently proposed as a method for large-scale energy storage, is another mechanical storage method based on thermodynamic cycles. This thesis presents a guide to precisely understand each system along with energy and exergy analyses to characterize the key parameters for achieving high efficiency for each of the systems. In addition, some novel concepts for the systems are proposed in order to address some of the current drawbacks and to widen the scope of their applications. Conventional CAES systems are most commonly operated under constant volume conditions with a fixed, rigid reservoir and compressors and turbines that can operate over an appropriate pressure range. These varying pressure ratios can degrade the efficiencies of compression and power generation owing to deviations from design points. Although it is possible to increase the storage volume to reduce the operating pressure range, doing so results in reduced energy density and high construction costs. Therefore, in order to resolve such problems, a novel constant-pressure CAES system combined with pumped hydro storage is proposed. An energy and exergy analysis of the novel CAES system was performed in order to understand the operating characteristics of the system according to several different compression and expansion processes. The effects of the height of the storage cavern and heat transfer between two media (air and water) and the cavern on the performance of the novel CAES system were also examined. Although the large-scale CAES systems are dependent on the right combination of site-dependent geological factors for air storage, a micro-CAES system with man-made air vessels can be a very effective system for distributed power networks, because it provides energy storage, generates electric power using various heat sources, and incorporates an air cycle heating and cooling system. This thesis presents the results of energy and exergy analyses of different types of micro-CAES systems, as well as some innovative ideas for achieving high efficiency of these systems. Recently, another type of mechanical storage, namely, thermo-electric energy storage (TEES) systems, which use heat pumps and heat engines with thermal storage, have been proposed. The advantages of TEES systems are their higher energy densities and independence from geological formations. In particular, a TEES system with transcritical CO2 cycles is considered to be a promising method for large-scale energy storage. This thesis reviews current TEES systems and proposes a novel isothermal TEES system with transcritical CO2 cycles. It is shown that in the case of TEES systems with transcritical CO2 cycles, the roundtrip efficiency and energy density can be increased by isothermal compression/expansion. In addition, novel transcritical or supercritical CO2 cycles using both low-temperature (LT) and high-temperature (HT) heat sources are proposed to maximize the power output of the CO2 power cycle with the given HT heat sources for use in applications such as nuclear power, concentrated solar power, and combustion. Moreover, the previous TEES system with transcritical CO2 cycles can be combined with the proposed transcritical CO2 cycle using both LT and HT heat sources.