ITPD20090041A1 - CELL TO MEASURE ELECTRIC SPECTRUMS OF MATERIALS IN DEW POINT CONTROLLED CONDITIONS OF PRESSURE, TEMPERATURE AND TEMPERATURE (DPT) - Google Patents

Description

CELL FOR MEASURING ELECTRIC SPECTRA OF MATE LIA IN CONTROLLED CONDITIONS OF PRESSURE, TEMPERATURE AND TEMPERATUREA DEW POINT (DPT)

DESCRIPTION

Field of the invention

The field of the invention relates to instruments for measuring under controlled conditions of pressure, temperature and Dew Point temperature of the chemical-physical properties of materials with proton conductivity to be used in fuel cells.

State of the art

The electrical properties of fuel cells based on proton exchange polymeric membranes (PEMFC) strongly depend on the water content inside the membrane, which must be kept in humid conditions in order not to undergo a conductivity decay that would block the device. The central component in PEMFCs is the proton exchange membrane (PEM). Nowadays the most widely used polymeric membrane in PEMFCs is Nafion, produced by Dupont, which consists of a perfluorinated polymer containing sulphonic functional groups. From a microscopic point of view, the structure of Nafion consists of hydrophobic domains containing small hydrophilic clusters (40 - 100 nm), the latter are responsible for most of the hydrophilicity of the material and the proton conductivity of the membrane. A fully hydrated Nafion membrane can have a volume of 22% greater than the same membrane in anhydrous conditions. In these conditions, there are about 20 water molecules for each acid group (-S03H), bringing Nafion to conductivity values of the order of 0.1 S / cm. From an application point of view, there are problems due to the production, maintenance and elimination of water within the polymeric membrane in PEMFCs.

In fact, low levels of humidity cause dehydration of the membrane with a consequent collapse of conductivity, while humidification levels that are too high cause the phenomenon of flooding, which significantly reduces the performance of the PEMFC. It follows that the water content inside the PEM is a parameter of fundamental importance to know and control for the use of these materials in fuel cells. Commonly the water content inside the membrane is expressed as a function of the number of water molecules per acid group (parameter λ), while the proton conductivity is expressed as a function of λ, of the water vapor activity (aw) and relative humidity (RH) present in the environment and temperature. The study of the electrical properties of polymeric membranes for different values of temperature and relative humidity therefore becomes essential both for the understanding of the proton conduction mechanism and for the definition of the best conditions to be achieved for the use of such PEMFC systems. This patent describes the design and construction of a cell for electrical measurements on solid matrices, at constant pressure, for different temperature values and Dew Point temperatures.

Detailed description of the measuring cell

Figures 1 and 2 respectively show the top and side sectional views of the measuring cell. The cell consists of three concentric cylindrical steel chambers. The innermost chamber (cylinder 1) contains the sample compartment and the temperature, dew point and pressure sensors. The intermediate chamber (cylinder 2) is crossed by a serpentine duct inside which the liquid for thermostating flows. The temperature control liquid enters the coil from the duct located at the base of the cell and exits from the duct located on its top. The external chamber (cylinder 3) is maintained under dynamic vacuum ("empty" duct, shown in Fig.1-2) to thermally isolate the cell from the external environment. The wet gas enters the internal chamber (cylinder 1) through the appropriate duct ("gas inlet duct" in Fig. 1-2): it then comes into contact with the sample to be analyzed and generates at the inside the cell a fixed relative humidity value. This gas of established composition comes out of the "gas outlet pipe", located at the bottom of the measuring cell. The thermal stability of the internal cell is guaranteed by the heating liquid that circulates in the intermediate chamber and by the vacuum maintained in the outermost chamber. The pressure, temperature and relative humidity sensors are located near the sample, to increase the accuracy in determining the measurement parameters. An important criterion in the design of the cell consists in the compact packing of the sensors and the sample holder compartment in a particularly small volume. This configuration was developed with the intention of minimizing the possibility of condensation and temporal drifts in temperature and relative humidity. On the top of the measuring cell (Fig. 1) there are the holes for the insertion, inside the cylinder 1, of the sensors, of the sample and of the gas.

EXAMPLES

The following descriptions, together with the attached graphs, must be considered specific information of particular examples, reported only for illustrative and not limiting purposes of the invention.

The functionality of the measuring cell has been verified:

1) Measuring the ability of the cell to maintain temperature and Dew Point values inside it stable over time and equal to the set values (Example 1).

2) Using the cell to measure the electrical properties, at ambient pressure and for different values of DPT and temperature, of a reference polymeric membrane (Example 2).

The measuring cell was connected to the Arbin Instruments “Dew Point Humidifier System 5SLPM 750 / 2200W” humidification system. This system is able to generate a humidified gas flow up to a maximum dew point of 99 ° C and to heat the outgoing gas flow up to a maximum temperature of 140 ° C. The gas used is nitrogen, humidified with double distilled water. The gas coming from the humidifier enters the measuring cell through the "gas inlet duct", shown in Fig. 1. A Julabo E06 circulation thermostat is used to heat the measuring cell, while as silicone oil (EN.CO.S.r.l.) was used, which has good thermal stability and can therefore be used in a temperature range between -30 and 180 ° C. The measuring cell (cylinder 3) was connected to the 803 CIT Alcatei High Vacuum Technology pump ("empty" duct, shown in Fig.1-2) and a pressure of IO was maintained inside the "cylinder 3" chamber <'6> bar, to ensure thermal insulation from the external environment. Electrical measurements were conducted on a Nafion <®> 117 (Dupont) membrane, using the Hewlett Packard (HP) Agilent 4294A impedance analyzer. For the control of the experimental variables and the collection, saving and visualization of the data, a software developed in the laboratory was used.

To measure the pressure inside the measuring cell, the microminiaturized XPCM10 transducer from Leane International s.r.l. was used. It has a full scale from 0 to 10 bar with an accuracy of ± 0.5% f.s. It can be used in a temperature range from -75 ° C to 220 ° C in both static and dynamic applications. The sensor is connected to a computer that acquires and stores the pressure data over time, thanks to a program developed in the LabVIEW (Laboratory Virtual Instrument Engineering Workbench) environment. The program allows you to view in real time the trends in pressure (bar) and voltage (Vdc) as a function of time (seconds). The measurement of the humidity contained in the nitrogen flow was performed through a cooled mirror Optidew Remote sensor, by Micheli Instruments. This sensor works from 20 ° C up to 115 ° C and can measure for each temperature the dew points corresponding to a relative humidity between 2% and 100%, with an accuracy of ± 0.2 ° C for the temperature of dew point and ± 0.1 ° C for that of the cell. The Optidew Remote sensor is connected to a computer via the RS232 connector. The Opti-software software, version 1.57 (April 2004), allows you to acquire data and display the measured parameters in real time on the screen. Inside the sampling chamber (cylinder I) there are two temperature sensors, P2 and 02. The P2 sensor, consisting of a PT100 thermocouple, is connected to the thermostat. Sensor 02, connected to the humidity sensor inside cylinder I, is a 100 Ohm 4-wire platinum thermometer.

Figure 3 shows a block diagram showing the assembly of the instrument object of the present invention, made for electrical measurements in controlled conditions of pressure, temperature and relative humidity. Block A includes the humidification system, block B describes the measuring cell, block C shows the sensors for controlling pressure, relative humidity and temperature. Blocks D and E indicate respectively the thermostat for heating the sample and the spectrometer for broadband dielectric measurements (BDS).

Example 1

The example describes the measurements made to verify the stability of the parameters inside the measuring cell, the cell temperatures (GAST) and the Dew Point (DPT) of the gas coming from the humidifier. The measurements were carried out by setting different combinations between the Dew Point temperature and the gas outlet temperature, at constant pressure (~ 1.35 Bar). In Figs. 4 and 5 it is shown that the system, inside the measuring cell, reaches the set Dew Point value for temperatures of 60 and 100 ° C, both in heating and in cooling.

Figure 6 clearly shows that the cell is able to stably maintain a DPT of 99 ° C inside it over time at a cell temperature of 105 ° C, with oscillations within ± 0.2 ° C. Parts a) and b) of Figure 7 respectively show the capacity of the cell to keep the dry gas stably inside it at temperatures of 75 and 90 ° C.

Example 2

This example shows the electrical measurements performed on a membrane of Nafion <®> 117 (Dupont), by means of the cell object of the following invention, operating for different combinations of GAS T = 50, 70, 90 ° C and DPT, (see Table 1). The membrane, properly washed and activated according to standard procedures [V. Di Noto, M. Piga, L. Piga, S. Polizzi and E. Negro, J. Power Sources, 178, 561-574 (2008)], was placed inside the sample holder cell and dried for 1 night in flow of nitrogen, so as to obtain the totally dry state of reference of the material. In this way it is possible to study the relationship between the electrical response of the membrane and the amount of water inside the material absorbed in known conditions of environmental humidity.

During the electrical measurements, the experimental conditions (pressure, DPT and GAS T) inside the cell object of the present invention were kept constant and monitored by computer through the software interfaced with the sensors, as shown in the diagram of the complete instrument in Fig. 3.

As an example, the profiles of the electrical spectra of the Nylon membrane were collected in a frequency range between 40 and 10 <6> Hz using different combinations of cell temperature (GAS T) and dew point (DPT), as schematically reported in Tab.l.

Sections a) b) and c) of Figure 8 show the spectra of real conductivity σ 'and imaginary permittivity ε ", measured as a function of frequency and DPT at GAS T values equal to 50, 70 and 90 ° C respectively .

It is clearly evident that the electrical quantities measured for the Nylon 117 membrane significantly depend on the water content present inside the cell (and therefore adsorbed by the sample under examination). In particular, the values of the σDC conductivity increase as both the water content (DPT) inside the membrane and the cell temperature (GAS T) increase, as shown in Fig. 9.

Brief description of the figures

Fig ·! · Top view of the measuring cell object of the present invention: a) section A-A; b) section B-B. In particular, the inlet holes of the Dew Point, temperature, pressure and sample cell sensors are shown. The figure also shows the ducts for the gas inlet and the connection with the vacuum pump.

Fig. 2. Sectional view of the measuring cell object of the present invention: a) section A-A; b) section B-B. The cell consists of three concentric cylindrical chambers. The innermost chamber (cylinder 1) contains the sample holder cell and the temperature, humidity and pressure sensors; the intermediate chamber (cylinder 2) is crossed by a recirculation of thermostatic fluid while the external chamber (cylinder 3) is placed under vacuum in order to ensure thermal insulation from the external environment.

Fig.3. Block diagram of the instrument object of the present invention for carrying out electrical measurements on materials at controlled temperature, pressure and relative humidity (RH%). Block A includes the humidification system, block B describes the measuring cell, block C shows the sensors for controlling pressure, relative humidity and temperature. Blocks D and E indicate respectively the thermostat for heating the sample and the spectrometer for broadband dielectric measurements (BDS).

Fig. 4. Dependence on the DPT time (set point of 24.5 and 50 ° C), for a cabinet temperature equal to GAS T = 60 ° C.

Fig. 5. Dependence on the DPT time (set point of 75 and 90 ° C), for a cabinet temperature equal to GAS T = 100 ° C.

Fig. 6. Time dependence of the temperature of Dew Point DPT (set point of 99 ° C) and GAS T 105 ° C. Box b), in stable conditions, shows an oscillation within ± 0.2 ° C of DPT.

Fig. 7. Time dependence of the cabinet temperature (GAS T) at DPT = 0 ° C, for a set point of 75 ° C a) and 90 ° C b).

Fig. 8. Spectra of σ ’and ε" of a sample of Nafion 117 measured at different values of DPT and GAS T: a) 50 ° C; b) 70 ° C; c) 90 ° C.

Fig.9. Dependence of log [σDC] on the dew point temperature, DPT, for a membrane of Nafion 117 at a cell temperature equal to GAS T = 50, 70 and 90 ° C.

Claims (9)

CELL FOR MEASURING ELECTRIC SPECTRA OF MATELIA IN CONTROLLED CONDITIONS OF PRESSURE, TEMPERATURE AND TEMPERATURE AT DEW POINT (DPT) Claims 1. Three-chamber cell (measurement, thermostating and insulation) for the automatic measurement of chemical-physical properties of materials under controlled conditions of cell temperature, Dew Point and pressure. 2. The cell as in point 1, which allows the use of different types of gases and liquids (volatile solvents) for the controlled saturation of the gas and therefore of the DPT. 3. The cell as in points 1 and 2 to be connected to different instruments and equipment, such as: humidification systems; sensors for controlling pressure, relative humidity and temperature; thermostats and instruments for measuring different chemical-physical properties of the sample under analysis. 4. The cell as in points 1-3 consists of three concentric cylindrical steel chambers. Where the outermost chamber is kept under vacuum to ensure thermal insulation from the external environment, the intermediate one is crossed by a liquid for heating and maintaining the temperature and the innermost cell serves to house the sample measuring cell and the temperature, pressure and dew point sensors. 5. Cell as in points 1-4 capable of housing systems for carrying out electrical, sensor, optical, electrochemical and mass measurements using conventional or quartz microbalances. 6. Use of the cell of points 1-4 to make instrumental combinations, as schematized in Fig.l and in order to carry out all the measurements provided for in point 5. 7. Cell as in points 1-4 where heating of the innermost cell occurs in any way. 8. Cell as in points 1 -4 where the thermal stability of the internal cell is guaranteed by the heating liquid present in the intermediate chamber and by the vacuum (10 <'6> bar) created in the outermost chamber in any way. 9. Cell as in points 1-4 equipped in the innermost compartment with pressure, temperature and relative humidity sensors placed near the sample in order to increase the accuracy in determining the measurement parameters.