DS0205 - Stockage, gestion et intégration dans les réseaux des énergies

Composite membranes as low relative humidity and intermediate temperature electrolyte for PEMFC – COMEHTE

Better performance at lower cost for proton exchange membrane fuel cells (PEMFC)

Water management in proton exchange membrane fuel cells is a systemic constraint. Increasing the operating temperature would simplify the cooling device, particularly constraining in the case of an automotive application. Decreasing the relative humidity during operation would eliminate the need for a humidifier and thus reduce by approximately 1/3 the volume of the system, a definite advantage for any embedded application.

Alternative to Nafion® membranes

One of the limitations of proton exchange membrane fuel cells is their low operating temperature. Increasing this temperature beyond 80-90 ° C would simplify the cooling device, particularly constraining in the case of an automotive application. The type of polymer widely used as electrolyte (perfluorosulfonic acid, PFSA) also requires to be humidified to drive the protons from the anode to the cathode. The humidifiers used are bulky. Being able to dispense with it would reduce the volume of the system by approximately 1/3, a definite advantage for any embedded application.<br />COMEHTE's goal is to develop novel clay-based polymer electrolytes to reduce the required moisture content and increase the thermomechanical strength of membranes.

Two clays were selected for their hygroscopic character and fibrous morphology: sepiolite and halloysite. They also have a large number of condensable functions that will be used to functionalize the surface and improve the interaction with the host matrix, with the possibility of a regioselective functionalization for halloysite (inside vs. outside of the fibers). Several functionalizations will be carried out and their impact evaluated on the characteristics of the prepared composite membranes and their performance in fuel cell type operation (after integration into membrane electrode assembly). The clays will also be integrated with the reinforcements prepared by electrospinning. The impact of the clays will also be evaluated on the electrocatalytic activity of the inks formulated to prepare the electrodes.

Two types of modification were carried out, one for compatibilization with the polymer matrix, the other for adding an antioxidant function.
Two fibrous mats were prepared as reinforcement from modified halloysite.
In general, the introduction of clay into the polymer matrix (10 wt%) leads to an increase in water uptake while limiting the swelling with a relatively homogeneous distribution, depending on the production parameters and the type of functionalization. The ion exchange capacity is not affected by the presence of clay and the proton conductivity is better when the clay is well distributed in the thickness of the membrane. Residual iron in clays could impact the chemical stability of composite membranes that release more fluoride after H2O2/H2SO4 treatment than unloaded membranes. The effect remains to be measured over time. The introduction of cerium 3+ makes it possible to limit the degradation of the membrane during a Fenton test. The kinetics of release also remains to be studied. Protocols for measuring mechanical properties in a controlled atmosphere are being developed.
The presence of clay in the catalytic ink, even at very high concentration (50 and 77 wt%), has little impact on the catalytic activity measured on a rotating disk electrode (same shape of voltammograms in a support medium or CO stripping). The mechanisms involved for the O2 reduction reaction are identical with and without clay in the ink. The active platinum surface area nevertheless decreases in the presence of clay, perhaps due to pollution by metal cations that can be released. A slight depreciation of the catalytic activity was recorded, more noticeable in the case of sepiolite added at 77 wt%.
Two reference AMEs prepared with XL-100 membrane and Gore select M820 were characterized for comparison with the project membranes (single cell tests are planned).

The results of the project will bring new knowledge on the functionalization of clays with an impact going beyond the single field of fuel cells.
The development of new intermediate-temperature proton conductive membranes, mechanically resistant and less prone to drying, will benefit any other application where these characteristics are limiting.
For example, the dimensional stability provided by the addition of clay could be an asset in reversible Pile / Electrolyser systems.

The first results of the project were communicated at two conferences.
The first raised the question of the interest of integrating clays in membranes for PEMFC (19th International symposium on intercalation compounds, Assisi, Italy, 28 Mai - 1er Juin 2017, Halloysite nanotubes: friend or foe for PEMFC membranes ?, A. Akrout, A. Delrue, M. Zaton, B. Prelot, M. Taillades, S. Cavaliere,D. Jones, J. Rozière).
The second presented reinforcements incorporating clays made in the framework of the project (Congrès Electrospinning for Energy 2016 (ELEN 2016), 22-24 Juin 2016, Advanced electrospun Mat for reinforcement for PEM, A. Delrue, S. Cavaliere, M. Taillades, D. Jones, J. Roziere).

The necessary reduction of greenhouse gases emissions, as a worldwide concern, should more than ever motivate us to find alternatives to fossil resources as well as adapted energy conversion processes.
Among others, fuel cells will play a strategic role during the energy transition to come, and are likely to become a key player in the future energy mix. Proton Exchange Membrane Fuel Cells (PEMFC) are no doubt the most versatile (polyvalent) since they can meet many needs, across a wide range of power (from mW for portable applications to MW for stationary through dozens of kW for transport). In 2012, PEMFCs accounted for 88% of the shipments (457,000 units) and 41% of the megawatts shipped (166.7 MW).
A major concern in PEMFC development, especially in the transport sector, is to increase their operation temperature above 100°C while decreasing the relative humidity conditions. Today, the operating temperature is still limited by the electrolyte used in the membrane-electrodes assemblies (MEAs).
In COMEHTE, we propose to take benefit from clays hygroscopic properties to develop new composite membranes based on microfibrous sepiolite and tubular halloysite. These clays will be functionalized in order to direct the membrane morphology at the nanoscale, i) to make them proton conductive (addition of acid groups) and ii) to favor the interaction with Nafion®, the host matrix (addition of fluorinated groups). The particular elongated morphology of such clays will participate to improving the mechanical strength of the composite membrane. This approach will be coupled with the development of active nanofibre reinforcements to further improve the membrane mechanical resistance and with the incorporation of radical scavengers to limit the membrane degradation. The composite materials will also be used as the proton conductor in catalytic layers so as to optimize the membrane-electrode interface.
Chemical functionalization, plasma activation and thermal complexation will be studied as three complementary routes to modify raw materials. Silane based precursors will be used to efficiently react with the numerous hydroxyl groups covering both inner and outer surfaces of the selected clays. Neutral or reactive plasma will be applied either to create reactive radicals to favor the chemical post-functionalization or to directly functionalize the treated clays. Thermal complexation will notably help dispersing the loads in the polymer matrix. Dedicated characterizations will be performed so as to identify the proper treatment routes and to select the most promising composites. The selection will be realized on different criteria such as the ion exchange capacity, the proton conductivity, the swelling, the thermomechanical resistance or the thermochemical stability.
The selected materials will then be used to prepare membrane-electrodes assemblies (MEAs) from membranes and electrodes developed in the project. MEAs will be tested in severe conditions (intermediate temperature and low relative humidity, accelerated stress tests) in comparison to reference MEAs.
The most promising MEAs will finally be tested in short stack configuration, following automotive test cycles.

Project coordination

Christian BEAUGER (ARMINES, Centre Procédés Energies Renouvelables et Systèmes Energétiques de Mines ParisTech)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

SYMBIOFCELL SYMBIOFCELL
Symbio FCell Symbio FCell
PIGM'Azur PIGM'Azur
Paxitech Paxitech
ARMINES PERSEE ARMINES, Centre Procédés Energies Renouvelables et Systèmes Energétiques de Mines ParisTech
ARMINES (C2MA) ARMINES Centre des Matériaux des Mines d'Alès
ICGM Institut Charles Gerhardt, Equipe Agrégats, Interfaces et Matériaux pour l'Energie
LEPMI Laboratoire d'Electrochimie et Physico-chimie des Matériaux et Interfaces

Help of the ANR 754,147 euros
Beginning and duration of the scientific project: February 2016 - 42 Months

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