Energy Storage and Semiconductor Technology Research Group:
Dr Robert P. LYNCH
Mr John O’DONNELL
Mrs Maria AL HAJJI SAFI
Mr Niall Dalton
Dr Jennifer T. JOYCE
Dr Andrea BOURKE
Ms Mallory A. MILLER
Mr Marcus O’MAHONY
Mr Niall DALTON
Ms Pauline MARTIN
Ms Marion BUOT
Mr Kammal BENAKKI
Mr Rémi SUAU
Dr Xin GAO
Dr Mehrdad BALANDEH
Dr Joseph A Murphy
Mrs Salihah AL SHEHRI
Dr Nathan QUILL
Dr Catherine LENIHAN
Dr Daniela OBOROCEANU
Dr Cattleya PETCHSINGH
Dr Deirdre NÍ EIDHIN
Dr Sergiu ALBU
All-Vanadium Flow Battery (VFB) Research
Due to the intermittency of non-dispatchable power sources, such as solar and wind, their use is restricted to times of availability. Therefore, unless there is a means of storing the energy they produce in periods of high availability for utilization in periods of limited availability these sources of energy can cause significant reliability issues resulting in the burning of fossil fuels so as to ensure stability of electrical grids. Vanadium flow batteries (VFBs), also known as vanadium redox flow batteries (VRFBs or VRBs), are particularly attractive because, in addition to having long cycle life, they use the same chemicals in both halves of the battery (see Fig. 1). Therefore, they are essentially immune to cross-contamination problems due to mass transfer across the membrane that can limit the service life of the electrolyte in other systems.
We are currently performing research on several aspects of VFBs. This research includes research into increasing the energy density and power of VFBs, investigating the kinetics of reactions at carbon electrodes, investigation of production methods of electrolytes for VFBs, fundamental investigation of stability and equilibria of vanadium electrolytes, monitoring of the state of charge (SoC) and development of novel electrodes and designs for VFBs.
Currently, we have four PhD students working on this area of research. Three working on the kinetics of reactions at carbon electrodes and one working on the development of flow-past electrodes for VFBs. Furthermore, in collaboration with Prof. Noel Buckley, we have three post-doctoral researchers working on electrochemistry at carbon electrodes and state-of-charge monitoring of VFBs. The funding for these research projects comes from sponsored research funding from Renewable Energy Dynamics Technology Ltd. (RedT), PhD funding from the Irish Research Council and the Saudi Arabian Government and Commercialisation Funding from Enterprise Ireland (EI).
Electrolyte: The energy density of VFB systems can be increased by increasing the amount of vanadium that can be dissolved in the electrolyte. Recently, through an innovation partnership funded project that was cosponsored by REDT and EI, we have increased the energy density of VFBs from 18 Wh dm-3 to 25 Wh dm-3 (i.e. an increase of ~40%). Due to the relative low cost and long cycle-life of VFBs, achievements in this outcome will open opportunities for VFBs in municipal and factory transport while also making VFBs more attractive for their current market, e.g. stand-by power, grid stabilisation and isolated renewable energy systems.
Optical Monitoring: Both electrolytes in VFBs are highly coloured; VII, VIII, VIV, and VV have strong absorbance spectra in the visible region.1-5 Thus, ultraviolet-visible (UV-Vis) spectroscopy offers a precise method of independently measuring the state of charge (SoC) of both the positive and the negative half-cell electrolytes. In a fully discharged negative electrolyte all the vanadium is in the form of VIII and during charging this is converted to VII. As the battery charges the absorbance of the mixture estimated as a linear combination of the absorbance of its component VII and VIII is in excellent agreement with the measured absorbance. In a fully discharged positive electrolyte all of the vanadium is in the form of VIV and during charging this is converted to VV. However, it is observed that the absorbance varies with SoC (percentage of VV) in a very non-linear manner. In addition, other parameters, such as vanadium and sulphur concentration, have very significant effects on this degree of non-linearity. Therefore, since both water and vanadium can transfer across the cation exchange membrane in a VFB, there are many practical issues that must be addressed when determining SoC. Despite all these issues our research allows the SoC of both halves of a VFB to be determined with great accuracy. The research on SoC monitoring has resulted in a European patent being published and two further related patent applications are currently underway. This research should increase the fundamental understanding of VFB operation and increase the operational state-of-charge range of VFBs (i.e. further increase the useable energy density of these system).
Kinetics: The research on kinetics at VFB electrodes is fundamental research which is enhancing our understanding of the redox reactions and overpotential behaviour in VFBs. Such knowledge should eventually result in improved electrodes and guidelines for the operation of VFBs. Both of which should increase overall energy efficiency, reliability and cycle life of these systems.
Figure 1: Schematic of Vanadium Flow Cell (VFC) during discharge. A VFB is constructed from at least one pair of half-cells that are separated by an ion-selective membrane. The two half-cells are equivalent except a “negative” electrolyte (anolyte; i.e. purple and green in the diagram) flows through the negative half-cell and a “positive” electrolyte (catholyte; i.e. yellow and blue in the diagram) flows through the positive half-cell. In the negative electrolyte vanadium exists in the VII and VIII oxidation states, while in the positive electrolyte vanadium exists in the VIV (VO2+) and VV (VO2+) oxidation states. Each electrolyte is stored in a reservoir from which it is circulated through its respective half-cell by a pump. As the VFC is being discharged the vanadium changes oxidation states from VII (purple) to VIII (green) at the negative electrode and from VV (yellow) to VIV (blue) at the positive electrode, driving electrons from the negative to the positive electrode in the circuit connected to the battery. During charging the VIII reverts to VII and the VIV reverts to VV.
Development and Investigation of a Hybrid Flywheel-Battery System Connected to the ESB Network
Through collaboration with Schwungrad Energie, Yokogowa, Hitachi, FreqCon, Beacon Power and Dr. Pican (at Cork Institute of Technology) we are working on a grid-scale project for the hybridisation of two well established power and energy storage technologies. The pilot of this system was officially opened near the end of 2015 (see Fig. 2). The synergy of flywheels and lead-acid batteries is intended to overcome shortfalls of either technology and addresses the need for additional system service provision, mitigating superfluous fossil fuel consumption: for example, currently up to 4% of fuel consumed by conventional energy generation is dedicated to system service provision. Integrating the hybrid system into the Irish grid – which as an island grid with high wind power penetration faces advanced frequency and voltage control issues – will test the suitability of lead-acid batteries and other battery technologies for such hybridisation and demonstrate the viability of such technology for the stabilisation of the grid. The potential market for this solution is estimated to be about 550 to 600 plants, each of 20 MW. Hence there is a huge growth opportunity and potential to provide employment.
Figure 2: Official opening of hybrid flywheel-battery pilot. Pictured from left to right are; Barry Cowen T.D. (Fianna Fail spokesperson on Environment & Local Government), Donal Bourke (Yokogawa), Ogata Hiroyuki (Yokogawa), Hiroyuki Kusunose (Hitachi Chemical), Yoshihiro Nomura (Vice CEO Hitachi Chemical), Doireann Barry (EirGrid), His Excellency Mr. Chihiro Atsumi (Ambassador of Japan to Ireland), Simon Rogers (Managing Director, Yokogawa UK and Ireland), Nigel Reams (Schwungrad Energie), Tom Kelly (Divisional Manager for Cleantech, Electronics and Lifesciences Enterprise Ireland), Marcella Corcoran Kennedy T.D. (Chair of Oireachtas Committee on Jobs, Enterprise & Innovation), Peter Duffy (Schwungrad Energie), John McCann (Programme Manager for Electricity and Wind, Sustainable Energy Authority of Ireland), Mr. Jing Ping (President of Liaoning Zhengbang Investment Group. Vice Chairman of Liaoning People’s Political Consultative Conference), Robert Lynch (University of Limerick), Colm Staunton (Schwungrad Energie), Liam Quinn (Local Councillor), Mervyn Keegan (Schwungrad Energie), Lea Collins (Schwungrad Energie), Frank Burke (Schwungrad Energie).
Micro- and Nano-Sized Porous Structures through Anodisation of Semiconductors and Metals
These projects are focus on nanoporosity in InP (indium phosphide) and the formation of titania nanotubes. The projects involve both lab-based research and modelling work. Through this research we have developed a fundamental understanding of these two systems facilitating much greater control of the growth parameters of such porous structures.
InP Anodisation: Our research on InP focuses on the formation of self-organised porous structures within n-type indium-phosphide electrodes when anodised in aqueous electrolytes. This research concentrates on the mechanisms that control the formation of these structures. The chemistry at n-InP-KOH interfaces is special since anodisation of the InP causes localised etching resulting in the formation of sub-surface pores in the semiconductor while little or no significant chemical etching occurs in parallel (see Fig. 3). These pores form as networks that connect back to pits in the surface of the semiconductor material. The majority of the surface remains virtually un-etched (i.e. the surface maintains its specular reflective properties) while the sub-surface etching creates a highly porous material beneath. The special chemistry of these interfaces was investigated through anodisation of the semiconductor using cyclic voltammetry, constant over-potential and galvanostatic experiments. Examination of the modification caused by the anodisation of this material has been performed in situusing a novel optical-microscopy technique and ex situ using electron microscopy techniques.
This research has been funded by the IFC and partly by Science Foundation Ireland (SFI) through National Access Programmes (NAPs) at Tyndall Institute, Cork for scanning electron microscopy (SEM) of samples.
Figure 3: A three-step mechanism of competitive kinetics results in <111>A pores:J12 (Step 1) hole generation at pore tips; (Step 2) hole diffusion at the surface; and (Step 3) electrochemical oxidation of the semiconductor to form etch products. Due to preferential etching such pores propagate along a characteristic set of directions, the <111>A directions.J11 Geometric models of <111>A pore propagation shows that truncated tetrahedron shaped domains of pores result (see TOP RIGHT). These domains have triangle and trapezium outlines in and cleavage planes, respectively, as shown in the respective SEM (LEFT) and TEM (RIGHT) images.
Titania Nanotubes: This research focuses on understanding and developing TiO2 nanotube layers (see Fig. 4) for dye-sensitized photoelectrochemical-cells and other applications. These layers of nanotubes are grown through electrochemical anodisation of titanium. The goal of the project is to discover applications for these porous structures due to their semiconductor nature, superior current-carrier diffusion-length (with respect to other TiO2 nanostructures) and high surface to volume ratio. In particular the research is aimed at applying these properties to solar-cell technology. Furthermore, we have developed a significant understanding of the mechanisms that govern the formation of these structures.
Figure 4: Similarities due to self-organisation can be seen in the images of (LEFT) TiO2 nanotubes, formed by anodisation of titanium at 100 V in ethylene glycol, and (RIGHT) the Giant’s Causeway, Antrim, formed by contraction during rapid cooling of molten basalt.
We also have an interest in scanning electron microscopy (SEM) and conducting in-situ electron, atomic-force and optical microscopy. Furthermore, we have significant research in the area of in-situ stress measurement during electrodeposition of metals used in the semiconductor industry.
Selected Patents and Publications