WELTEMP: Water Electrolysis at Elevated Temperatures
The WELTEMP project funded by the European Union was a collaborative project under the Seventh Framework Programme (FP7). The project involved partners from five European countries (Denmark, Norway, Czech Republic, Switzerland and Italy), and was running from 1.1. 2008 to 30.04.2011.
Water electrolysis offers a practical way of hydrogen production in association with renewable energy sources. The barrier to its use is the economy, and new technologies must therefore be developed. Compared to the conventional alkaline electrolyte electrolyser, the polymer electrolyte membrane (PEM) electrolyser can operate at high current densities and pressure with compact design. The main challenges for PEM electrolysers are high capital cost of key materials, components and the overall system as well as insufficient long-term durability. The strategic development of the WELTEMP project is an elevated operating temperature of the PEM electrolyser. In this way the energy efficiency will be significantly improved because of the decreased thermodynamic energy requirement, enhanced electrode kinetics, and the possible integration of the heat recovery. The elevated temperature requires either that the water electrolysis is carried out on steam, or that the working pressure must be raised.
Key issues to achieve this strategic target are breakthroughs of fundamental materials developments, including catalysts, membranes, current collectors, bipolar plates and other construction materials. The WELTEMP activities include development of active and stable anodic catalysts based on mixed metal oxides, temperature-resistant PEM based on composite PFSA, sulfonated aromatic and/or acid-base cross-linked polymers, and highly conducting and corrosion-resistant tantalum thin surface coatings as current collectors and bipolar plates. As an alternative, the possibility to make an alkaline PEM electrolyser is also examined, i.e. to use an anion conducting polymeric electrolyte. In this case, the need for noble elements as electrocatalysts will be strongly diminished. On the other hand, the difficulty is to develop a polymeric membrane, which is sufficiently stable, mechanically as well as chemically, at elevated temperatures.
- Technical University of Denmark, Denmark
- Institute of Chemical Technology Prague, Czech Republic
- The Norwegian University of Science and Technology, Norway
- IHT Industrie Haute Technologie SA, Switzerland
- Acta S.p.a., Italy
- Tantalum Technologies A/S, Denmark
- Danish Power Systems ApS, Denmark
- Institute of Macromolecular Chemistry ASCR, Czech Republic
Activities during the project period
WELTEMP has surveyed materials and technologies within several different concepts of PEM electrolysers: 1) Steam electrolysis as well as electrolysis of pressurized water on the basis of proton conducting (acidic) membrane electrolytes. 2) Electrolysis on pressurized water using alkaline polymeric electrolytes.
The activities were organized in work packages related to the various fundamental components of which electrolyser cells are constructed.
Activities were carried out on materials active under acidic conditions as well as on materials active under alkaline conditions. For the acidic case the all the effort was put on the anode catalyst, i.e. the oxygen evolution reaction. Pt/carbon are used for the cathode (hydrogen evolution) in this case.
The development of the anode electrocatalyst for the acid - based system was initially focused on synthesis of a catalyst able to work in contact with phosphoric acid, later more generally to provide sufficient chemical resistance to acids and strongly anodic conditions at temperatures >100˚C. A row of powder materials with nanosized grains were prepared, including mixed oxides containing iridium oxide and other oxides. Various new candidates for catalyst support materials were also investigated. However, on the basis of the results it was decided to use pure iridium oxide, prepared by a new method, which produced powders showing very promising stability test results. Membrane electrode assemblies produced with such catalyst powders demonstrated polarization curves meeting the project goal. This preparation method was selected for preparation of the MEAs for the prototype electrolyser stack.
A series of non noble metal based electrocatalysts for alkaline water electrolysis at temperatures >100°C was developed for the oxygen evolution reaction as well as for the hydrogen evolution reaction. These catalyst have been fully characterized, and methods for production in upscaled quantities were developed. These non-noble metal system demonstrated performance comparable to noble metals based systems.
From the beginning of the project period the activities within proton conducting (acidic) membranes was based on modifications of two types of materials: 1) phosphoric acid doped polybenzimidazole (PBI) membranes, and 2) perfluorinated sulfonated (PFSA) membranes.
During the second year of the project a gradual shift of attention to the (PFSA) materials was apparent. This decision was evocated by two main reasons: (i) negative impact of the phosphoric acid on the anode catalyst activity and (ii) by the strongly limited stability of the PBI based membrane in the water electrolysis cell.
In the first instance commercial Nafion membranes were tested. They have proven sufficient conductivity under the condition of sufficient humidification. The main attention was paid to the research on enhancement of the mechanical and dimensional stability of PFSA materials at elevated temperature and pressure. The attention was also focused on enhancement of the proton conductivity at reduced relative humidity of the environment at elevated temperature. Most promising properties were obtained using membrane based on the polymer with shorter side chains and thus with lower flexibility of the functional groups. These materials are characterized by a higher softening temperature when compared to the classical Nafion type materials and thus by improved mechanical stability, while maintaining high chemical resistivity. Properties of the membrane were further improved by means of a porous PTFE support. This membrane has exhibited high stability and proton conductivity at the water electrolysis cell operational conditions (typical 120°C and 3-7 bar). The use of this PTFE support was considered more successful than the method of adding inorganic fillers for improvement of mechanical properties and conductivity.
It has been demonstrated that steam electrolysis (ambient pressure and 120-130°C) can be carried out using PFSA type membranes, but in this case it is necessary to dope them with phosphoric acid in order to maintain protonic conductivity. For Nafion material improved performance was obtained by modification by adding zirconium phosphate as an inorganic filler. However, the best performances for steam electrolysis were generally obtained using acid doped membranes based on the polymer with shorter side chains.
The membrane conductivity can deteriorate in time, unless precautions are taken to avoid liberation ions of nickel and other metals from the tubing etc. in the systems.
In the field of alkaline polymers the attention has focused on enhancement of the membrane thermal stability by variation on the matrix polymer enabled by the method of a water soluble additive developed within early stages of the project. But the most significant attention has been paid to the development of a novel homogeneous alkaline polymeric electrolyte and binder for the catalytic layer.
The only metallic material which had acceptable corrosion rate (<0.01 mm/yr) under the most severe conditions tested (concentrated phosphoric acid, 150°C, anodic polarization up to 2.5 V/ Ag/AgCl was tantalum. Other promising materials were useless for various reasons. The corrosion rate of various types of stainless steel, and nickel alloys, niobium and titanium is too high at the temperatures in question (110°C and above).
Stainless steel mesh and felt materials have been coated successfully with tantalum for current collectors.
Bipolar plates from tantalum coated stainless steel performed well in the laboratory test rigs at DTU, NTNU and ICTP. No problems with contact resistance between catalyst, current collector and bipolar plate due to tantalum passivation have been detected so far. This was confirmed by measurements of contact resistance properties of tantalum and other materials after anodisation.
For the acidic MEA development it was decided that the operating conditions should be feeding of liquid water, moderate overpressure (3-7 bars), and temperature 120°C-130°C for the upscaled MEAs for testing at IHT. For membrane material, the short side-chain perfluorinated sulphonated polymer material mentioned under WP2 was chosen for upscaling within the project period.
The method of preparation of particularly the anode catalyst layer was investigated. Various methods have been tested, including the “decal method”, and to spray the catalyst layer (i.e. IrO2 and the ionomer) onto the porous tantalum coated current collector. It appeared that the best performances were obtained by spraying on the current collectors.
On the other hand for cathodes standard Pt/C on carbon cloth have been used.
Hot pressing of the components of the MEA was found to be advantageous for the phosphoric acid doped membranes (for steam electrolysis), whereas it was less important for the non doped membranes.
Performances of 1.65 V at 1 A/cm2 were obtained at 130°C.
During the testing of MEAs under pressurized conditions in which liquid water is circulated through the system, it was observed that even small amounts of corrosion products from stainless steel tubing etc. in the systems were slowly decreasing the overall performance. This was explained by ion exchange of protons in the membrane by such ions (like nickel), and by deposition of metals on the platinum on the cathode, thereby decreasing its catalytic activity.
Passivating the stainless steel surfaces by treatment with nitric acid was attempted to avoid the phenomenon, with some success. Coating of tubing inside by PTFE is another possibility which is currently in progress.
Concerning alkaline MEAs, anode and cathode electrodes based on non noble metal catalysts developed by ACTA were prepared and optimised. Both electrode types were prepared by applying catalyst layers onto the current collector material. The performance of these electrodes mounted together with a commercial anion exchange membrane at low temperature (40°C) was investigated in a test cell by ACTA. Performance of this MEA with a circulating alkaline electrolyte was stable over a long period (> 6000 h). At a working current density of 475 mA/cm2 the stable cell voltage was around 2.2 V. Other similar electrolyser cells showed a perfomance at 80°C as good if not better than platinum group based cells (500 mA/cm2 at 1.7 V).
Although this performance is far from project objectives (Vcell of 1.5 V at 1 A/cm2), the performance of such MEAs at the desired temperature of 120°C are expected to be considerably higher. Such alkaline MEAs were selected for further testing in upscaled cells and stacks. Thus, they were prepared in larger dimensions fitting the stacks and testrig discussed below.
A prototype electrolyser stack and test rig was built for demonstration and evaluation of polymeric electrolyte membrane assemblies at temperatures in the range of 120-130°C.
On the basis of the results discussed above, it was decided to design the testrig as well as the stack for being fed by pressurized liquid water, rather than by steam.
Initially it is used for working pressures of 3-7 bars, i.e., but the hardware part is able to withstand up to 70 bars. The cells are circular, and the active part of the electrodes have a diameter of approx. 110 mm. The design provides the possibility to test one single cell, several stacked cells - which may be different from each other - using the same setup.
The alkaline PEM systems are considered quite important, as it was demonstrated that reasonable durability is within reach, and that at the same time non-noble element catalysts can provide performances at least as good as noble metal based catalysts.