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.
Catalysts
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.
Acidic catalysts
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.
Alkaline catalysts
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.
Membrane materials
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.
Construction materials
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.
MEA (Membrane-Electrode-Assembly)
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.
Prototype stack
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.