Deutsche Version  ACT
Electrical Storage
Redox-Flow-Battery 12.4.2017

Simon Ressel conducts an experimental characterisation of the tubular redox flow batteries on the test rig at HAW Hamburg
© A. von Stryk

Energy from tubes

In the joint tubulAir project, scientists are developing a new redox flow battery type for stationary applications. This is intended to be cheaper to produce while having a considerably greater output and energy density. The new system works with just one electrolyte solution in combination with compressed ambient air. Also new is the geometry of the cells, which are constructed as micro-tubes.

Project status Investigation of basic principles
Project duration September 2012 until December 2017

The suitability of planar All Vanadium Redox Flow Batteries (VRB) for this purpose has been shown in various demonstrational projects. The comparatively low energy density (max. potential: 37.5 Wh/kg [1]) of the VRB as well as the cost intensive manufacturing of planar cell stacks require significant improvements as a prerequisite for a broad market entrance.

To achieve these goals, in this project the fluid electrolyte of the VRB on the cathode shall be replaced by an air/water steam electrode (where air resp. water is taken from the surrounding). Thus the energy density might basically be doubled compared to a VRB. To achieve a better cost effectiveness, a micro tubular cell structure will be developed.

  • Electrochemical investigations of electrolytes, electrode materials and membranes are conducted in the laboratory at the Dechema Research Institute (DFI). © DECHEMA-Forschungsinstitut
  • Cyclic voltammogram of a vanadium electrolyte © DECHEMA-Forschungsinstitut
  • The graphic shows a schematic depiction of the structure of a tubular vanadium/air redox flow battery © S. Ressel, C. Wiciok
  • Cell structure and reduction and oxidation reactions while charging and discharging a vanadium/air redox flow battery. © S. Ressel, N. Janßen
  • Group photo of the project partners in the tubulAir research project © A. von Stryk
  • Microscopic cross-sectional image of a porous sample: By means of atomic layer deposition, the walls of the parallel running, cylindrical nano-pores were furnished with a catalytically active platinum layer that is just a few nanometres thick. © Bachmann, Schumacher (FAU)
  • Microscopic image of galvanically grown platinum nano-tubes: The catalytically and electrocatalytically active tubes were generated in a porous matrix and are used for enlarging the electrode surface in batteries. © Bachmann, Licklederer (FAU)
  • The illustration shows a microscopic image of galvanically grown platinum nano-tubes in an inert, porous matrix. © Bachmann, Schumacher (FAU)
  • The illustration depicts a decavanadate tetrabutyl-phosphonium salt. © Universität Hamburg, P. Burger
  • The image shows a hexawolframate methyl-octylimidazolium salt and a cyclic voltammogram that shows the reversibility of the two redox processes. © Universität Hamburg, P. Burger
  • A researcher measures the prototypes for a tubular membrane in the material testing laboratory at Uniwell Rohrsysteme GmbH und Co.KG. © Uniwell Rohrsysteme GmbH und Co. KG, S. Fisch
  • Simon Ressel conducts an experimental characterisation of the tubular redox flow batteries on the test rig at HAW Hamburg © A. von Stryk

Principles of Design and Mass Transport

Within the scope of research and development carried out at HAW Hamburg, the interaction between the relevant process and material parameters for the operation of a VLRFB shall be examined. The influence of these parameters on the process control shall be explored and interdependencies shall be identified. Objective of this investigation is the detection of design principles for micro tubular VLRFB cells and the corresponding manufacturing and operation methods.

Computer based models for the examination of certain phenomena will assist the experimental research work in order to achieve a cost and time effective development of a micro tubular cell prototype.

One focus of the work is the examination and optimization of the mass transport in the porous structure of the bifunctional air/water steam electrode.

New Electrolytes with High Energy Density

Currently, the maximum energy densities of redox flow batteries are 70 Wh/L, which is substantially lower than those of Li ion batteries (200 Wh/kg, 400-500 Wh/L). There are three main targets to improve for the liquid systems:

  • high molar concentration of the electroactive compound
  • high number of electrons per formula unit of the electroactive component
  • large electrochemical potential(s) of the involved steps

The demand for high durability and facile maintainability relates to high chemical stability and high resistance toward water or oxygen. This led to the consideration of ionic liquids (ILs), which are readily available and highly chemical stable. In total, we anticipate that an increase by one order of magnitude is possible. Item i) we will be addressed by inclusion of the electroactive compound into the ionic liquid itself, either as the anionic or a cationic component.

Nanostructured Electrodes

At the FAU Erlangen-Nürnberg, nanostructured electrodes with enhanced specific surface area will be prepared, investigated and applied. To this goal, the anodization of aluminium, an industrial process, for creating structures of accurately defined and tunable porosity will be exploited. The catalyst will then be deposited onto the porous structure either by atomic layer deposition (ALD) or by wet chemical methods (in particular galvanic plating). The increased surface area of the electrode will increase the current density of the vanadium-air redox flow battery, and thereby its power density.
First a fundamental investigation of the relationship between the surface morphology of the electrode and the current density that can be reached for electrochemical reactions will be carried out. This study will be carried out on planar samples, which can be used as model systems with very accurately tunable geometric parameters. The diameter and length of cylindrical pores arranged in parallel arrays will be varied systematically and the electrical current density will be measured as it depends on those parameters.
In the next step, a method for the controlled preparation of nanopores on substrates of technical relevance will be developed. The parameters of the anodization, the galvanic deposition, and the atomic layer deposition, will be modified for the application to non-planar, electrically conducting substrates such as mesh and felt. We will define the optimal parameters that enable one to reach the best possible control over the geometry (pore size and layer thickness) and the homogeneity for each sample. This optimization will be performed within the constraints determined at HAW that guarantee sufficient mass and energy transport.
Finally, the materials and procedures optimized by the other partners in order to create parts for the prototype will be combined. The best substrates obtained at the HAW Hamburg will structured with the methods developed at the FAU and coated with the best catalysts from the RWTH Aachen. Hereby, the necessary materials compatibility with the membranes (FumaTech), the extrusion procedure (Uniwell), as well as the electrolytes (University Hamburg) willl be bewared.

Bifunctional Catalysts

An efficient microtubular vanadium air redox flow battery (VARFB) relies heavily on high performance catalysts and tailor-made catalyst carriers. The DWI Aachen plays a leading role in the development of bifunctional catalysts and innovative membrane electrode assemblies within the BMBF lighthouse project tubulAir±.
DWI Aachen will cover the development of bifunctional catalysts for the application in microtubular VARFBs. The efficiency of state-of-the-art bifunctional catalysts can be increased with the addition of transition metals to the binary systems with the beneficial effect of a cost reduction. Aspects that influence the suitability of the catalyst are (a) the efficiency of reduction and oxidation of oxygen, (b) the long-term stability in operation, (c) cost and (d) the requirements concerning the application technology. In addition to the ternary bifunctional catalysts, suitable application technologies need to be developed.
An additional focus of DWI Aachen is the development of tailor-made tubular membrane electrode assemblies for the VARFB. High porosities for a high specific surface area and an architecture that reduces pore-blocking by water inclusion are crucial for the performance of the microtubular VARFB. The aim is to synthesize new tubular membrane electrode assemblies based on hollow fiber membrane spinning technology.

Membrane development and production

FuMA-Tech is developing and optimising polymers for manufacturing membranes, whereby both cationic and anionic basic polymers are being investigated. An important focus is on the production of these materials in the extrusion and spinning process. Here it is intended that all important properties of the polymers and membranes – such as the very small surface resistance and high selectivity – shall remain intact. In close collaboration with the cooperation partners, the membrane properties are being investigated, compared and tested and optimised under real conditions.

High Tech Tubings

Widespread knowledge in the field of extrusion of various polymers will function as a basis to develop, beside basic analysis of producibility of a tubular membrane with diameters of < 5 mm, an extrusion process for serial production of tubular membrane and membrane-electrode-assemblies in this subproject. Out of initially theoretical considerations regarding the production process (of tubular membrane) there will be a progress of constructional designing of machinery and tooling, followed by defining manufacturing parameters and realising an experimental production with suitable materials. Usage of promising, application fitted and more sophisticated polymeric materials from Fumatech shall be used for extrusion tests by Uniwell, to first form an optimized tubular membrane, later a prototype of a membrane-electrode-assembly.
In principle this subproject is based on optimisation of processing properties of Fumatech materials, to the end of a trouble-free, process-safe extrusion of a tubular membrane with defined low wall-thickness right up to 50 μm.

Process-safe production of tubular membrane initiates the next step in development – production techniques for a tubular membrane-electrode-assembly. Depending on that an extrusion line for production of tubular MEA will be build up for realizing Inline-assembly of membrane and electrode. On the one hand the subproject includes principles of trial, but also the aspect of a broad rollout of tubular systems with high economical efficiency.
On this basis later there will be the chance to demonstrate a competitive price segment.


The electrochemistry group of the DECHEMA-Forschungsinstitut covers a large spectrum of electrochemical methods and material characterization.
Within the frame of this project the electrochemistry is working as a connective link between manufacturers of the single components (electrolyte, electrode, catalyst, membrane) and users of the redox flow system. Analysis of the efficiency and stability of the components will be carried out by the electrochemistry group and the single components as well as their joint action will be characterized.
Special interest will be gained on the stability of the components under relevant conditions.

The scientific findings will be assembled to a catalogue of requirements for quality of redox-flow-batteries and their electrodes, membranes and electrolytes.

Website of the TubulAir Project

Further information can be obtained here.