Electricity from air and magnesium
Researchers from Bonn, Ulm and Berlin develop the basis for rechargeable high energy batteries in the project MgLuft. This uses magnesium as a negative and an air electrode as the positive electrode. The magnesium-air battery is to be used in future as a novel electrochemical energy storage in power systems.
|Project duration||June 2013 until May 2017|
For a future sustainable supply with energy, novel high energy storage systems have to be developed. One of the most important possibilities are secondary batteries. In this collaborative research project we want to elucidate, whether batteries which use magnesium as a negative electrode and air gas diffusion electrode as a positive electrode (as known from fuel cells) are a feasible alternative to present systems. The advantage of such a battery is, that the available amount of magnesium is practically unlimited and that such a battery has a high theoretical energy density (in comparison to lithium ion batteries). The air electrode which serves for the reduction of oxygen from air (and for its evolution during charging) has the advantage, that heavy metals have only be used in small quantities as catalysts and not in stochiometric quantities as an active mass. This contributes to a higher energy density and better sustainability than in other battery systems.
In comparison with lithium batteries, a magnesium metal electrode has a lower specific capacity than lithium electrode (Mg: 2205 Ah/kg compared to Li: 3861 Ah/kg). Because of the higher density (Mg: 1.74 g/cm³ compared to Li: 0.53 g/cm³) the theoretical charge density (volume capacity) is higher (3837 Ah/L compared to 2046 Ah/L for Li). For a single cell, in combination with an air electrode and taking into account the mass of Mg and air, a theoretical specific energy (gravimetric energy density) of 2789 Wh/kg (compared to 5200 Wh/kg for the lithium O² cell) is calculated. Nevertheless this value is 4 times higher than that of up-to-date or state of the art lithium ion batteries (theoretically it was 600 Wh/kg related only to the active mass).
Cooperating with the University of Bonn are the IOK (University of Ulm), the ZSW (Centre for Solar Energy and Hydrogen Research in Baden Württemberg in Ulm) and the WIAS (Weierstraß-Institut) in Berlin.
The subject of this project is the establishment of a fundamental knowledge on the reactivity of magnesium electrodes on the one hand and on the oxygen reduction and evolution in non-aqueous electrolytes on the other hand. Although we can make use of the knowledge established in connection with lithium/air batteries and also on first literature results in the context of magnesium/ion batteries, this combination of magnesium electrodes with an air electrode creates new challenges: Grignard-based halogenide containing electrolytes which in principle are suited for magnesium batteries, cannot be used because halogens will probably be evolved at the oxygen electrode. Therefore new electrolyte compositions have to be developed. At the oxygen electrode peroxide will be found, which reacts with magnesium ions to probably insoluble Mg-peroxide. The reversibility of this process has to be established.
Charactersisation of electrode
TAs electrolytes we want to test ionic liquids on one hand, on the other hand also electrolytes shall be tested which are known from lithium/air system in combination with special magnesium salts. By adding special functional additives we also want to try to manipulate certain characteristics of the electrolyte, such as film formation and the characteristics of magnesium deposition. As an alternative for the magnesium electrode we also want to test conversion compounds.
In this collaborative research project partners with very different expertise have met: At the universities of Bonn and Ulm, several advanced methods for the characterization of electrodes and the examination of electrode reactions have been developed: with the help of electrochemical mass spectrometry products of decomposition for instance of the electrolyte can be detected with a high sensitivity, moreover this method also allows the determination of the amount of reduced or evolved oxygen; the characterization of the electrodes is done with the help of electron spectroscopy using special transfer systems to UHV. The ZSW in Ulm is well experienced in the context of lithium/ion batteries and their use of electrochemical methods for such batteries. The Theoretical Chemistry in Bonn is well renowned in the quantum chemical calculation of solid surfaces and interfaces, the WIAS Berlin developed methods for the calculation of diffusion and convection in complicated systems.
Electrochemistry group at the university of Bonn (Instutute of Physical and Theoretical Chemistry)
Microscopic characterisation of the structure and reactions at the anodeFor a successful approach of the above-mentioned goals the following fundamental tasks have to be carried out:
- Test of new electrolyte systems, characterization of electrochemical stability and reactions as well as film formation.
- How can a magnesium deposition be contolled - microscopic characterization of the structure is important as well as that of the reactions including products and intermediates at magnesium and the passive layers. Insertion and conversion compounds and alloys as alternate anode materials have to be tested.
- Microscopic characterization of insoluble magnesium oxides on the cathode.
Institute of Surface Chemistry and Catalysis, Ulm University
Microscopic characterization of reaction and transport processes at the cathode interface of magnesium-air batteries.
This part of the ‚Mg-air’ project aims at a detailed characterization of the interactions between electrode and electrolyte at the cathode interface (air-electrode) of Mg-air batteries as well as on the processes taking place at this interface caused by these interactions and by the O2 reduction reaction (ORR) or the O2 evolution reaction. The results of these investigations at defined model systems serve as mechanistic background and data base for the numeric simulations performed by the project partner WIAS and for the understanding of the studies at realistic electrochemical cells performed by the project partner ZSW. This will result in an improved understanding of the elementary processes and of their interrelations. Finally, these studies are the base for a systematic improvement of the cathodes in Mg-air batteries.
Practical approaches for electrodes and electrolytes in Mg-air batteries
- Identification and basic characterisation of suitable electrolytes for Mg-air systems
- Feasibility study on the use of alternative anode materials based on graphite ("insertion" or "intercalation materials") or metals / intermetallic compounds ("conversion materials")
- Investigation of reaction mechanisms of and passivation layer formation at the anode to gain insights on electrode-electrolyte interactions
- Synthesis and investigation of catalysts and additives for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of the air electrode
- Screening and selection of electrode substrates and preparation techniques for the gas diffusion electrode
- Design of real air electrodes with optimised four-phase interface
- Adaption and optimisation of electrode compositions with regard to power capability and degradation behaviour
Theoretical Chemistry, University of Bonn (Institute of Physical and Theoretical Chemistry)
This part of thee collaborative research project aims at the development of atomistic models for elementary chemical processes, which occur at the cathode and the anode of Mg-air batteries. The quantumchemical methods used are based on density functional theory and semiempirical level. Using simplified model systems, activation barriers and reaction enthalpies for the reaction of Mg with oxygen, water and solvens molecules shall be calculated. Intermediatees shall be characterised spectroscopically. The choice of the modelsystems will be done in cooperation with the partners. These theoretical calculations will lead to a deeper understanding of the relevant reaction mechanism during formation of the passivating interphases, and the role of the electrolyte and catalyst during oxygen reeduction. The thermodynamic and kinetic reaction parameters thus calculated will be used in the macroscopic models developped by the partners.
Macroscopic modeling of transport and reaction processes in magnesium air batteries
The aim of the sub-project is the development of macroscopic models of coupled transport and reaction processes in magnesium air batteries and in experimental electrochemical cells designed to for the investigation of particular components. Using these models, numerical simulation tools shall be implemented which allow to support the experimental investigations performed by the other project partners by improving the understandig of the interplay between the numerous physical processes in a magnesium air battery. Coupled to an inverse modeling approach, these simulations shall facilitate the extraction of characteristic data from experimental results.