Jacques Grangeon

Philippe Lesage

A robust, low-cost and well-calibrated infrasound sensor for volcano monitoring

Keywords: Instrumentation, infrasound, sensor, volcano monitoring

Volcano acoustic signals contain valuable information on shallow magmatic and hydrothermal processes. In some cases, the detection of acoustic waves is the only clear evidence of the occurrence of volcanic explosions. Their study is thus complementary to that of seismic signals. For this reason, acoustic sensors are more and more frequently integrated in volcano monitoring systems as well as in temporary instrumental deployments. We have developed a broadband, robust and low-cost infrasound sensor designed for the detection and analysis of acoustic waves on volcanoes. It is based on a microelectromechanical differential pressure transducer (MEMS). The reference pressure is balanced with the atmospheric pressure through a pneumatic high-pass filter. Its low corner frequency, usually set to 60 mHz, can be adjusted from a few to tens of mHz. Its amplitude range is ± 240 Pa and its sensitivity is 20 mV Pa -1 , with a noise level less than 0.05 Pa RMS. The power consumption is 42 mW (3.5 mA with 12 V voltage). A direct output of the MEMS also provides a signal with sensitivity of about 500 µV Pa -1 and range ± 1245 Pa. This sensor is not sensitive to mechanical vibrations. The instrumental response of each infrasound sensor is carefully measured from 1 mHz to more than 100 Hz using two specially designed calibration systems which deliver sinusoidal pressure variations. The mechanical elements of the sensor are produced by 3D printer and filled with epoxy resin which guarantees high robustness in the aggressive environment of most volcanoes. The sensor dimensions (26x45x80 mm) and weight (100 g) makes it very easy to handle and install.

Introduction

A large variety of volcanic processes produce perturbations of the atmosphere that propagate as acoustic waves. These include sudden or continuous emissions of material, in solid, liquid or gaseous state, at the Earth's surface, mass transfers associated with dome collapse, pyroclastic density currents, or rockfalls, and seismic ground vibrations coupled with the air.

The frequency of these atmospheric perturbations can be in the audio range (> 20 Hz and < 20 000 Hz), but strong acoustic energy is often observed in the infrasound range, i.e. at frequency lower than 20 Hz [START_REF] Johnson | Volcano infrasound: a review[END_REF][START_REF] Fee | An overview of volcano infrasound: From hawaiian to plinian, local to global[END_REF]. Therefore, the spectrum of acoustic waves may have components at frequencies smaller and larger than the lower limit of the audible band for human ear. The detection, recording, and analysis of the infrasonic signals produced by volcanoes highlight the physical processes related with volcanic activity and provide important information such as mass flux and velocity of gas emission, over-pressure associated with explosions, and source location [START_REF] Johnson | Volcano infrasound: a review[END_REF][START_REF] Fee | An overview of volcano infrasound: From hawaiian to plinian, local to global[END_REF][START_REF] Garcés | Volcano acoustics[END_REF]. This information can be quantitative and of physical significance if the sensors used are calibrated. Since low frequency acoustic waves propagate in the atmosphere with little attenuation, they can be observed and analyzed at long distances from the source and even all around the Earth for large explosions [START_REF] Delclos | Processing and interpretation of microbarograph signals generated by the explosion of Mount St[END_REF]Le Pichon et al., 2010).

Volcano monitoring is primarily based on the observation of seismic activity with networks of seismometers deployed on or close to the edifices. An important task in volcano observatories is to detect and classify the events according to pre-defined classes such as volcano-tectonic event, long-period event, tremor, or explosion. However, in many cases, it is difficult to identify and separate LP events from explosion quakes, because both types of event share common temporal and spectral features. Thus, when there are no visual observations due to night or cloudy conditions, volcanic explosions may pass unnoticed when using only seismic methods. In this kind of situation, the use of infrasound sensors to complement the monitoring system may be of invaluable help for detecting and discriminating explosions [START_REF] De Angelis | Detecting hidden volcanic explosions from Mt. Cleveland Volcano, Alaska with infrasound and ground-coupled airwaves[END_REF]. Furthermore, the deployment of arrays of at least 3 or 4 infrasound sensors is very useful to improve the detection of small events, to determine back-azimuth to locate their source and to separate the signals according to their origin (Johnson et al., 2011). However, although their use tends to rapidly extend, there are still few infrasound sensors available with characteristics well-adapted for research and monitoring purposes in volcanology.

We have developed an infrasound sensor suited to the conditions found on volcanoes and easy to integrate in monitoring networks. The sensor has the following specifications: a low noise level and a dynamic range designed for detecting small to moderate explosions at a few kilometers from the source; a good resistance to harsh environmental conditions, including rain, humidity, temperature variations, corrosive gas, insolation, and insects; low power consumption and easy connection to most seismic digitizers; large bandwidth with adjustable low corner frequency (typically 60 mHz); precise calibration; low sensitivity to mechanical vibrations; easy installation and possible connection to acoustic noise reducers; small dimensions and weight; low cost. In this paper, we describe the principle of the IST-2018 sensor, the mechanical configuration, the electronic design, and its main features. In order to validate the sensor design, we compare signals of volcanic explosions recorded by our sensor and by a commercial high-precision and high-sensitivity sensor. We also present devices to easily calibrate infrasound sensors from one mHz to several hundred Hz and to test their sensitivity to mechanical vibrations.

Choice and principle of infrasound sensor

Infrasounds are perturbations of the atmospheric pressure that can be detected by absolute and differential pressure sensors. The pressure at Earth's surface is about 1000 hPa at sea level and can vary in a range of up to 800 hPa, depending on the altitude and meteorological conditions [START_REF] Ponceau | Low-noise broadband microbarometers[END_REF]. On the other side, the smallest detectable perturbations above the background noise are of the order of one mPa, depending on the recording site and the frequency [START_REF] Bowman | Ambient infrasound noise[END_REF]Le Pichon et al., 2010). Thus the dynamic range of an absolute pressure sensor, or micro-barometer, able to measure the full scale of atmospheric pressure and to detect the smallest perturbations must be of the order of 10 8 . Such sensors, with high sensitivity and large dynamic range, are difficult and expensive to design and produce from the mechanical, electronic, and digital points of view. Differential sensors compare the instantaneous pressure to a reference value which is an average of the previous atmospheric pressures. They provide a high-pass filtered measure of the pressure, which requires much smaller dynamic range than absolute sensors [START_REF] Rocard | Demande de brevet pour procédé et dispositif de détection et d'enregistrement des variations de la pression atmosphérique[END_REF].

When selecting a type of infrasound sensor for carrying out measurements on volcanoes, high-quality microbarometers, such as the Martec MB2005 and the Chaparral Physics Model 50, that are used in the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization, appear expensive and poorly portable and require complex installation procedures. Some industrial pressure sensors, such as the SETRA270, are poorly suited for applications on volcanoes because of their high noise level, power consumption, and cost. Condenser microphones have interesting sensitivity characteristics but their low frequency response is limited. For example, a high-quality microphone, such the GRAS 40E, has a sensitivity of 50 mV Pa -1 and a low cut-off frequency of 3 Hz, while a basic microphone, such as SHURE PGA81, has a sensitivity of 3.8 mV Pa -1 and a low cut frequency of 40 Hz. On the other hand, these devices are sensitive to corrosion and cannot operate permanently in the field. Industrial pressure sensors and microphones also require signal conditioning electronics -filters and amplifiers -that are not always delivered by the manufacturers. Nevertheless, some infrasound sensors have already been designed for volcanological studies, such as the Chaparral 60 UHP (http://chaparralphysics.com/model60.html) or the ones developed at the New Mexico Institute of Mining and Technology and at the Boise State University, USA [START_REF] Marcillo | Implementation, Characterization, and Evaluation of an Inexpensive Low-Power Low-Noise Infrasound Sensor Based on a Micromachined Differential Pressure Transducer and a Mechanical Filter[END_REF][START_REF] Anderson | The Gem Infrasound Logger and Custom-Built Instrumentation[END_REF].

After exploring the technological solutions presented above, we decided to use a piezoresistivity differential pressure transducer base on microelectromechanical systems (MEMS) technology (Honeywell NSCDJJN005NDUNV). This transducer is designed for static measurements and thus can work at very low frequency, although it is still sensitive at frequency of several hundreds of Hz. The device measures the pressure difference between the atmosphere and a reference cavity which is connected to the external pressure through a pneumatic filter. We tested several technical solutions for this analog filter and we chose to insert a capillary tube between the reference cavity and the exterior. This tube acts as a small leak that progressively equilibrates the pressures between both sides. The principle of this differential pressure sensor was described by [START_REF] Rocard | Demande de brevet pour procédé et dispositif de détection et d'enregistrement des variations de la pression atmosphérique[END_REF] and [START_REF] Richiardone | The transfer function of a differential microbarometer[END_REF] and its physical behavior was studied by [START_REF] Mentink | Frequency response and design parameters for differential microbarometets[END_REF] and [START_REF] Marcillo | Implementation, Characterization, and Evaluation of an Inexpensive Low-Power Low-Noise Infrasound Sensor Based on a Micromachined Differential Pressure Transducer and a Mechanical Filter[END_REF]. Therefore, it is possible to change it in the field when necessary. The structure and packaging are made in ABS by using a 3D printer. The resulting case contains the MEMS, the backing volume, the capillary tube and the electronic board. The unused volume of this case is filled by epoxy resin before calibration of the sensor. The mechanical parts of the MEMS are also made in plastic. Thus, the design and the material used allow a very good resistance to corrosion and to the aggressive environments that can be found on volcanoes. The small sensor dimensions (26x45x80 mm 3 ) and weight (~100 g) make it very easy to handle and to install. The atmospheric pressure input and the end of the capillary tube are located in a unique ½" BSP connector which allows an easy link with any noise reducer [START_REF] Walker | A review of wind-noise redaction methodologies[END_REF]. This connector is equipped with a bug filter. The corner frequencies of these two filters are adjustable by setting the values of their resistors and capacitors when wiring the electronic boards. Together with the pneumatic filter, they allow to define the spectral range of interest. The micro-power voltage regulator delivers a stable 10 V power supply to the electronic components. The instrumentation amplifier excites the bridge and amplifies the electrical variations. All the electronic components were selected in order to lower the power consumption and to minimize the intrinsic noise level.

Mechanical configuration

Design of the electronic board

The board, designed with a minimum number of components, includes three outputs. 1) The '5 Vref' is the precision voltage reference output, accurate to +/-0.5 % with +/-35 ppm/°C drift. 2) The 'MEMS' output delivers an unfiltered signal with sensitivity of approximately 500 µV Pa -1 , depending on the MEMS characteristics. 3) The 'infrasound' (IS) output delivers a signal filtered by the 10 mHz high-pass filter, which compensates the offset of the sensor, and by the 40 Hz low-pass filter, which reduces the high-frequency noise. The sensitivity, fitted with the variable gain amplifier, is 20 mV Pa -1 (+/-2%). The sensor was designed to be connected to seismic recorders which have often free input channels. The 10 V micro-power voltage regulator powers the electronic sensor in a single mode. Consequently, the signal outputs are strictly positive compared to the sensor ground.

The output range is approximately 0 to 10 V; the signal is centered on the 5 V reference.

Therefore, the signal can be measured in single mode. It is also possible, and preferable for noise consideration, to use a differential mode in which the 5 Vref is the E-input. In this case, the range is +/-5 V and the common voltage can reach 10 V. With some seismic recorders, such as the 'Nanometrics Taurus', the recorder and sensor supplies must be isolated. For this case, we have developed an optional isolated power source that can supply up to five sensors, at the expense of higher power consumption and noise level. We have also designed a specific adaptor for DataCUBE recorders the input range of which is +/-2 V.

Characterization

If the infrasound signals are used to constrain source physical parameters and to quantify volcanic explosions and activity, they must be recorded by well-calibrated sensors [START_REF] Fee | An overview of volcano infrasound: From hawaiian to plinian, local to global[END_REF]. The response of the sensors must also be stable over time and their amplitude must be preferably flat in the working frequency range. Usually, the calibration is carried out by installing the sensor in an infrasonic testing chamber in which well-controlled pressure variations are produced [START_REF] Kromer | Infrasound sensor models and evaluation[END_REF][START_REF] Kromer | Test definitions for the evaluation of infrasound sensors[END_REF][START_REF] Marcillo | Implementation, Characterization, and Evaluation of an Inexpensive Low-Power Low-Noise Infrasound Sensor Based on a Micromachined Differential Pressure Transducer and a Mechanical Filter[END_REF]. Because we had no such equipment, we designed a simple calibration system which consists in generating sinusoidal pressure variations with frequency from 1 mHz to hundreds Hz. This system is directly connected to three sensors in parallel: 1) a high quality well-calibrated sensor used as a reference, 2) a calibrated and unfiltered IST-2018 sensor which is used to measure the phase shift, and 3) the sensor to calibrate. At low frequency, the reference sensor used is a Martec MB2005 microbarometer, the transfer function and sensitivity of which are well-determined. At frequency higher than 5 Hz, a GRAS 40AE microphone is used as a reference instead of the MB2005. For each selected frequency, the sinusoidal pressure variations are recorded by the three sensors. The complex transfer function is obtained point by point by comparing the amplitude and phase of the corresponding signals.

Appendix A describes more in detail the calibration system and procedure. Some types of sensor may be sensitive to mechanical oscillations. In this case, they must be carefully isolated from ground motions in order to avoid spurious output signals. We thus studied the sensitivity of our sensors to mechanical vibrations. We built a simple vibrating platform on which we set together the MB2005 and IST-2018 sensors and an accelerometer.

We observed that, while the MB2005 is strongly sensitive to mechanical oscillations, the IST-2018 is not perturbed by vibrations. This implies that the later sensor can be fixed directly on the ground or even on trees without increasing its background noise. Appendix B gives more details on this experiment.

Field experiment

A team from the University of Liverpool, UK, and the Observatorio Volcanológico del INGEMMET, Peru, carried out a field experiment at Sabancaya volcano in March 2018. In order to compare them, they deployed a Chaparral Model 60 UHP and a IST-2018 infrasound sensor. The sensors were installed close to each other at a site located on top of a small hill protected from the wind by some rocks; the site is ~6 km from the active crater, with direct view to the vent. Several explosions occurred during the experiment, producing ash plumes of up to 5000 m above the crater (BGVN, 2018). Figure 5 displays two examples of explosion recorded by the two instruments, and their respective spectra. A good agreement is generally obtained between the waveforms, the amplitudes and the spectra. However, for the explosion

Discussion

The characteristics of a sensor result from the balance between several constraints and from some technological choices. In our case, the main objectives were to produce a robust, low power, inexpensive, and well-calibrated infrasound sensor that can be easily connected to seismic acquisition devices. Thus, because the IST-2018 sensor is based on a microelectromechanical systems technology and uses a limited number of electronic components, some of its features are not as good as those of high quality instruments such as the Martec MB2005 and the Chaparral 60 UHP. For example, although it is well-adapted to detect most infrasound signals described in the literature, the smaller amplitude range of its 'IS' output may result in clipping records of strong explosions at short distance. In this case the 'MEMS' output of the IST-2018 can be useful. However, the sensitivities and frequency ranges of these three sensors have similar values. The IST-2018 can also be compared with the BSU and the Gem sensors. The lower sensitivity and noise level of the latter two probably result from the lack of amplification or the use of a small gain amplifier in these devices.

Nevertheless, the noise level of the IST-2018 could be reduced by using oversampling and averaging techniques. We plan to further reduce the input voltage, consumption and noise level by using new electronic components. The other characteristics of the three sensorsfrequency and amplitude ranges, power -do not differ much. On the other hand, a IST-2018

sensor was installed very close to the Pisciarelli fumarole, Italy. During one year, it measured the acoustic level of a vent without failure, which is a good test of robustness in harsh and corrosive volcanic environments.

This infrasound sensor is relatively easy to build. The electronic boards are produced by a cabling company. The structure and packaging are made by 3D printer. In the lab, we prepare the capillary tube, assemble the different parts, finish the cabling to the connector, and we fill the case with resin. There are no major difficulties for a person with good technical skill. The longest task is to calibrate the sensors. This lab-made sensor is available for any volcano observatories and research groups at low cost (a few hundred euros, depending of the accessory devices delivered -power, breaking box, cables). The user manual of the IST-2018 presents a detailed description of the sensor, including specifications, cabling, and electrical connections. It gives some recommendations for field installations, such as the need of protection from direct solar radiation and from obstruction of the pneumatic inputs.

Conclusion

Many advancements in sciences resulted from improvements of instrumentation. In volcano seismology for example, the use of broadband seismometers allowed the discovery of new phenomena such as the very-long-period (VLP) events which are associated with mass movements in the plumbing system [START_REF] Neuberg | Highlights from a seismic broadband array on Stromboli volcano[END_REF][START_REF] Ohminato | Broadband seismic observations at Satsuma-Iwojima volcano, Japan[END_REF][START_REF] Chouet | A multi-decadal view of seismic methods for detecting precursors of magma movement and eruption[END_REF]. The integration of acoustic sensors in permanent networks or in temporary deployments provides complementary observations to seismic records and improves the ability to understand some volcanic processes, such as Strombolian [START_REF] Braun | Interaction of seismic and air waves recorded at Stromboli volcano[END_REF]Vergniolle and Brandeis, 1994) and Vulcanian eruptions [START_REF] Johnson | Relative partitioning of acoustic and seismic energy during Strombolian eruptions[END_REF][START_REF] Valade | Explosion mechanisms at Arenal volcano, Costa Rica: An interpretation from integration of seismic and Doppler radar data[END_REF], and to reduce the related risks, for example by detecting pyroclastic density currents or rock fall [START_REF] Jones | Mapping complex vent eruptive activity at Santiaguito, Guatemala using network infrasound semblance[END_REF]. The infrasound sensor IST-2018 we have developed and presented in this paper is well suited to the measurements of acoustic waves produced by volcanoes. It is characterized by broad frequency band, high dynamic range, low noise level and low power consumption. It is designed to work in harsh environmental conditions after easy installation. The careful determination of its transfer function and sensitivity with the especially designed calibration system makes the IST-2018 sensor a true instrument for the measurement of acoustic phenomena. Its low cost and easy connection to most seismic digitizers may foster its use in many monitoring networks and acoustic arrays on volcanoes, as well as for a large variety of applications. In summary, the principle and design of these two calibration devices are simple and their cost is modest. They can be built easily in a research laboratory. They result easy to use and

Figure 1 .

 1 Figure 1. Left panel: block diagram of the infrasound sensor, showing the MEMS, the capillary tube, the reference cavity, the electronic board, and the connector. All these elements are inserted in the packaging displayed on the right side of the picture. Right panel: picture of the sensor.

Figure 1

 1 Figure 1 displays a picture of the sensor packaging and configuration. One side of the differential pressure transducer (MEMS) is directly open to the atmosphere, while the other side is connected through a backing volume and a capillary tube protected, on the atmospheric side, by a hygrophobe Teflon membrane with pore size of 0.45 µm. The length of the capillary tube determines the corner frequency of the pneumatic high-pass filter. For example, we obtain a corner frequency of 55 mHz with a 35 mm-long tube with internal diameter of 65 µm and a backing volume of approximately 230 mm 3 . The capillary tube is easily removable.

Figure 2 .

 2 Figure 2. Electronic diagram of the IST-2018 sensor.

Figure 3 .

 3 Figure 3. Amplitude (top) and phase (bottom) responses of the IST-2018 for its infrasound (IS -blue) and MEMS (red) outputs. Dots are values measured with the calibration system. Amplitude responses of the MB2005 and the GRAS 40AE microphone are also displayed in top panel for comparison. Double arrows indicate the frequency ranges used for each reference sensor in the calibration procedure.

Figure 3

 3 Figure3displays the amplitude and the phase of the transfer function of the two outputs of the IST-2018 sensor. For the IS output, the response is flat from 50 mHz to 40 Hz. The lower corner frequency is that of the pneumatic filter and can be easily modified. We carried out tests with corner frequency as low as 6 mHz. The 'MEMS' output is not low-pass filtered. Its response is also flat up to a few hundred Hz. At 1000 Hz, i.e. outside the normal working frequency band, a peak appears in the amplitude curve of both outputs. It is probably due to a resonance of the MEMS itself. The linearity is obtained by comparing the amplitudes of the pressure variations recorded by the MB2005 and the IST-2018 sensors (figure4). The mean relative deviation from linearity is 0.66 % between 1.2 and 220 Pa. This demonstrates that the response of the IST-2018 sensor to pressure variations is almost as linear as that of the high quality MB2005 sensor. The intrinsic background noise of our sensor is less than 0.05 Pa RMS on average in the range [0.06 -40] Hz on the 'IS' output and less than 1 Pa RMS on the

Figure 4 .

 4 Figure 4. Comparison of amplitude variations measured by the MB2005 and the IST-2018 sensors, as a test of relative linearity of the later one.

  at 21:05, the record obtained by the IST-2018 sensor contains larger variations at low frequency. This is mainly due to the difference in the low corner frequency of the high-pass filter of the instruments used in this experiment, which were 30 and 7 mHz for the Chaparral and IST-2018, respectively. More field tests will be necessary to confirm the good functioning and calibration of the IST-2018 sensor.

Figure 5 .

 5 Figure 5. Comparison of waveforms and spectra of 2 explosions of Sabancaya volcano, Peru, recorded by a Chaparral 60 UHP and a IST-2018 collocated infrasound sensors.

Figure A2 .

 A2 Figure A2. Picture of the 'low-frequency' calibration device displaying the pressure converter on the top of the Low-frequency Generator, the valves, and the three sensors.

Figure A3 .

 A3 Figure A3. Example of sinusoidal signals recorded by the two reference sensors MB2005 (green) and IST2018-1 (red, not filtered) and the IST-2018 to calibrate (black). Here the input pressure varies in the range +75 Pa/-55 Pa at a frequency of 1 Hz.

Figure A5 .

 A5 Figure A5. Picture of the 'high-frequency' calibration device displaying the loudspeaker chamber (top), the reference microphone, and the IST-2018 sensor to calibrate.

Table 1

 1

	summarizes the main

Table 1 .

 1 Main features of the IST-2018 infrasound sensor. (*) Adjustable value.

Acknowledgements

The development of the infrasound sensor has been supported by grants from Labex OSUG@2020 (Investissements d'avenir -ANR10 LABX56) and from AAP Université Savoie Mont Blanc. We would like to thank Silvio De Angelis, Alejandro Díaz Moreno and Roger Machacca for carrying out the field experiment at Ubinas volcano and for sharing their data. We are indebted to Jean Vandemeulebrouck for his useful and encouraging advices and for improving the manuscript. We also thank two anonymous reviewers for their useful comments.

Appendix A

To calibrate our sensors, we have developed two generators of sinusoidal pressure variations operating in the frequency ranges from 1 mHz to 10 Hz, and from 5 Hz to 10 kHz, respectively. Each generator consists of a variable volume chamber connected in a closed circuit to reference sensors and to the sensor to calibrate. At each measuring point, the chamber is equilibrated to the atmospheric pressure and then the pressure oscillations are generated at the desired frequency.

A1. Variable pressure generator from 1 mHz to 10 Hz

Appendix B

In order to study the sensitivity of the sensors to mechanical vibrations, we fixed together an accelerometer, an IST-2018, and a MB2005 sensors. We placed this equipment on a plastic canister that is used as a vibrating platform ("vibrator" in figure B1). Through the proportional solenoid valve, "prop. EV", we sent a variable pressure into the canister, which made it vibrate. These vibrations were thus transmitted synchronously to all sensors.


S Ould-Amara

J Dillet

Sophie Didierjean

Marian Chatenet

G Maranzana

Pr Gaël Maranzana
email: gael.maranzana@univ-lorraine.fr

Operating heterogeneities within a direct borohydride fuel cell

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Electrochemical energy converters (notably fuel cells) are a serious alternative to thermal machines. Their energy efficiency is in practice higher and they do neither generate fine particles nor nitrogen oxides when operating at low temperatures. The most mature fuel cell technology is the proton exchange membrane fuel cell (PEMFC), which is used in particular for mobility applications [START_REF] Corbo | Hydrogen Fuel Cells for Road Vehicles[END_REF][START_REF] Zohuri | Hydrogen-Powered Fuel Cell and Hybrid Automobiles of the Near Future[END_REF]. PEMFCs are supplied with hydrogen that can be obtained from renewable energies by electrolysis of water, and therefore emit (at least potentially) only a very small amount of greenhouse gases. In addition, the production of hydrogen by electrolysis will be a way to valorize the surplus production inherent in the variable nature of electricity production from the wind or the sun. However, the hydrogen carrier is very difficult to store in gaseous form due to its very low energy density [START_REF] Sdanghi | [END_REF][START_REF] Godula-Jopek | Hydrogen Storage Technologies: New Materials, Transport, and Infrastructure[END_REF]. One solution is to store it in liquid form by producing methanol, formic acid, or even hydrocarbons or ammonia by using an additional source of carbon dioxide or nitrogen. This is the channel called "power-to-liquids". The inverse transformation is carried out either by the preliminary reforming of the liquid fuel to obtain the stored hydrogen, or by the direct oxidation of the fuel in the fuel cell. Direct oxidation is preferable for reasons of simplicity of the system [START_REF] Wee | [END_REF] but, at low temperature, it is limited by the C-O bond or simply impossible to achieve with the available electrocatalysts for number of liquid fuels. There are some exceptions, though, one of which concerns sodium borohydride in alkaline media [6]. Concentrated and stable solutions can be obtained [7], leading to a gravimetric energy of 2.4 kWh/kg by considering an effective storage capacity of 7.51%w. The direct oxidation of the borohydride at pH of 14 is feasible, from the potential of -0.41 V vs RHE and ideally releases 8 electrons according to the reaction (1): BH4 -+ 8 OH -→ BO2 -+ 6 H2O + 8 e - E 0 = -0.41 V vs RHE (1)

However, depending on the electrocatalyst at stake, this electrochemical reaction is in competition with the catalytic hydrolysis of the borohydride (2) and the hydrogen evolution reaction (from water reduction below 0 V vs RHE) (3, backward) followed by the oxidation of the produced hydrogen (3, forward) above 0 V vs RHE, as is the case for Pt and Au electrocatalysts [8][9][10].

BH4 -+ 2 H2O → BO2 -+ 4 H2 (2) H2 + 2 OH -↔ 2 H2O + 2 e - E 0 = 0 V vs RHE (3)
Direct oxidation of borohydride is therefore possible, but also desirable to maximize the energy conversion efficiency of a direct borohydride fuel cell (DBFC). Research has been conducted by various teams to develop the good catalyst that allows the borohydride oxidation at low-potential without producing too much hydrogen [11,12]. Platinum seems to favour hydrolysis but good results have been obtained with palladium [10] or palladiumnickel-boron alloy on carbon nanotubes [9]. Encouraging performances as high as 420 mW/cm 2 have been obtained at the level of the cell [13], although the electrodes used are often derived from hydrogen fuel cells, including hydrophobic processing. A significant margin of progress is therefore to be expected. Anodes adapted to the liquid fuel without necessarily including ionomer have to be designed; they should be more open and thicker, made of hydrophilic materials, so as to ease mass-transfer phenomena, as anticipated by

Cheng and Scott [14,15] and Freitas et al. [16]. It is also desirable to determine the mass and charge transfer mechanisms so that optimization can be performed based on modelling results.

The work proposed here is positioned downstream from studies on materials. The aim is to evaluate the performance of a direct borohydride fuel cell with a local resolution between fuel inlet and outlet. The instrumented cell used makes it possible to locally measure the current density and the potential of the two electrodes with respect to reference electrodes located on the membrane along the active surface. With this setup, the operating heterogeneities that may be associated with the increase of hydrogen flow and decrease of BH4 -concentration down the channel direction are evaluated. The start / stop phases are also characterized and highlight the potential sustainability issues of the DBFC, as for regular H2-fed PEMFCs [17][18][19][20]. These observations at the level of the cell may help to guide the development of new electrodes. A first part is devoted to the description of the experimental device and a second to the presentation of the observations and analysis of the results. The segmented cell constitutes LEMTA's own know-how. Our first linear instrumented cell was developed in 2007 [21]. Then, it was improved many times in order to get better performances that are now representative of a non-segmented cell. The membrane electrode assemblies (MEAs) considered here are made of a cation-exchange membrane (CEM, Nafion® NR212), just positioned as received (without initial "cleaning" of exchange into the Na + form) between an anode and a cathode gas diffusion electrodes (GDE) without any hot pressing. The catalyst is made of regular 40% Pt/C (0.5 mgPt/cm 2 loading), and the active layers embed hydrophobic tetrafluoroethylene (PTFE) as a binder. As the alkaline anolyte solution impregnates the electrode and comes into contact with the membrane, sodium ion Na + can be transferred without any Nafion® ionomer in the electrode. This catalyst is coated on a gas-diffusion layer (GDL) possibly comprising a microporous layer (MPL). Different GDEs have been tested as anode and cathode by varying the PTFE loading in the electrode and in the GDL. Actually, the borohydride solution cannot cross a MPL. Only anodes without MPL can produce power and the best performances have been reached with highly hydrophobic cathode (40% PTFE loading with MPL) and less hydrophobic anode (10% PTFE loading without MPL). This is explained by the fact that (i) it is necessary to evacuate the soda formed at the cathode to allow the transport of oxygen and (ii) the solution has to be able to wet the anode catalyst layer.

PTFE gaskets were used to adjust the thickness of the electrodes to get 20% of compression of the initial thickness. Twenty reference electrodes spaced 15mm apart are positioned on the membrane along the active area (Figure 2). They are fed by humidified hydrogen thanks to a segmented channel placed on the side of the anodic channels. These twenty GDE discs are placed on the part of the membrane which is not submitted to the alkaline solution and therefore remains in H + form. These local reference electrodes can thus be considered as RHE electrodes (pH of 14).

Fig.2: 30 cm x 1 cm MEA with 20 reference electrodes (regular GDE) along the active area

The anolyte fuel was composed of 0.5 mol/L NaBH4 (Sigma aldrich, 98% purity) in 4 mol/L NaOH (VMR Analar ® Normapur ® ) dissolved in ultrapure water (18 mΩ cm). Pure oxygen (Messer, 99.999%) was used as oxidant. The borohydride solution was injected into the fuel cell by means of a pump (mzr-4665HNP) which regulates the flow of the solution.

Temperature regulation was achieved thanks to a circulating bath driven by a Pt100 probe inserted in the anodic plate. Before entering the cell, the thermo-regulated water passed through a heat exchanger, the function of which was to preheat the borohydride solution to the desired temperature. Without this heat exchanger, the performances were found to be very sensitive to the fuel flow rate, due to a progressive heating of the solution into the cell from the inlet to the outlet. As it will be presented in the next section, the performances of the cell are very sensitive to temperature. The measurement of the total amount of hydrogen released is performed using a phase separator and a H2/H2 fuel cell. A nitrogen flow rate is added to the borohydride-H2 mixture recovered at the cell outlet in order to increase the flow rate of gas and decrease the time response of the H2 flow rate measurement. The liquid returns to the anolyte reservoir and the gas is sent to the H2/H2

fuel cell operating at Ucell = 0.6 V. The current consumed by the hydrogen pump makes it possible to evaluate the flow of hydrogen escaping the DBFC anode.

III. Results and discussion

III.1. Start-up

The cell was first purged with distilled water to check for leaks. Then the borohydride solution was injected at a flow rate of 1 L/h. As shown in figure 3.b, before the borohydride injection the anode and cathode potentials with respect to the reference were equal to about E+ or -= 1 V vs SHE, which corresponds to the potential of an oxygen electrode in an acidic medium. Indeed the Nafion® membrane being used as received was therefore initially in its protonic form. Getting in the cell, the borohydride anolyte solution produces an overshoot of voltage up to 1.8 V, as shown in Figure 3.a. This overshoot is explained by the superposition of two phenomena:

-At the beginning of the filling process, the cell comprises a cathode in acidic medium (E+ ≈ + 1 V vs SHE) and an anode in basic medium with presence of borohydride (E-≈ -1.2 V vs SHE), what means potentially a 2.2 V cell voltage. During this transient phase, the protons in the membrane are gradually replaced by sodium ions from the anolyte, and the potential of the cathode gradually decreases to + 0.2 V vs RHE, which corresponds roughly to the potential of an oxygen electrode in alkaline medium (at that stage, the active area of the CEM is no longer acidic -all the H + have been replaced by Na + ). At the same time, the potential of the anode decreases from E-= 1 V vs RHE (initially, the CEM is still in H + form and the anode still under oxygen) to E-= -0.9 V vs. RHE (Figure 3b), i.e. a potential between that of hydrogen and borohydride at pH of 14. This transient lasts 10 s. Note that after a few minutes, due to the fuel crossover, these potentials stabilize to about E-= -1.12 V vs RHE for the anode and E+ = -0.1 V vs RHE for the cathode depending on the temperature, as will be discussed hereafter.

-The injection of the borohydride solution is not instantaneous. A concentration front is transported from the inlet (entrance) to the outlet (exit). During this filling time of the anode compartment, a potential heterogeneity appears between the inlet and the outlet. In the same way as when starting a hydrogen fuel cell, this results in an overshoot of cell voltage (Figure 3a), the duration of which is longer than for "equivalents" at PEMFC startups. Note that the local potential of the cathode in front of the anode outlet reached E+ = 1.6 V vs. RHE for a PEMFC, which is an issue for the cathode catalyst durability [17][18][19][20]. In this first startup, the local potential of the cathode varies in a complex manner (Figure 3c): at the fuel inlet, the potential firstly decreases towards 0.1 V vs RHE (at minimum), while at the outlet, it firstly increases to 1.5 V vs RHE (at maximum), the minimum in the fuel inlet region and maximum at fuel outlet region being concomitant. Then, from these extrema and for the segments towards the middle of the cell, the local cathode potentials evolve towards ca. 0.8

V vs RHE after t = 10 s (at that stage the CEM is still in H + form at the cathode interface); at the end of this transient (t > 10 s), then all segments reach similar local potential values, and progressively reach a "stabilized" potential of ca. 0.2 V vs RHE after ca. t = 50 s (at that stage the CEM is in Na + form at the cathode interface, and of course at the anode interface as well).

Very surprisingly, in the present DBFC configuration, no internal current could be measured during this first start up from a CEM in acidic (H + ) form. The protons initially in the membrane can either participate in the reduction of oxygen by leaving the membrane on the cathode side, or be neutralized by the OH -ions by leaving the membrane on the anode side after replacement with sodium ions. This last hypothesis, which does not lead to the production of current, seems to be verified.

Things are different for the second startup, when then CEM was already completely exchanged in Na + form (alkaline). Figure 4 shows the internal currents measured during such a startup, with a membrane initially in sodium form and an initial high pH at the cathode.

The internal currents and potentials are substantially identical to those observed for a startup with a hydrogen fuel cell. The amplitude of the overshoot of the cell voltage does not exceed 1.2 V and negative currents appear in the passive part not yet fed by the fuel. These locally-reversed currents are likely to lead to degradation of the DBFC electrode materials, which could be studied in a forthcoming contribution.

Fig.3. a) Cell voltage as a function of time for the first start-up with a membrane initially in its acidic form, b) Potential of the anodes with respect to the distributed reference electrodes, c)

Potential of the cathodes, all results are obtained at 30°C.

III.2. Operating performances and local heterogeneities

Cell performances were measured for four different temperatures of T = 30, 40, 50 and 60°C. Figure 5 shows the average performances (polarization plots and anode/cathode potentials in Figure 5a, power density in Figure 5b) for an electrode membrane assembly consisting of a GDE (0.5 mg/cm 2 Pt/C -10% PTFE -without MPL) at the anode and a GDE (0.5 mg/cm 2 Pt/C -40% PTFE -with MPL) at the cathode. Note the high sensitivity of the results with respect to temperature. An increase in temperature has complex and antagonistic effects; it reduces the resistance of the membrane to the transfer of sodium ions but on the contrary may lead to increased crossover of the anolyte fuel to the cathode; it improves the kinetics of the various competing reactions at stake (oxidation of borohydride and hydrogen, hydrolysis of borohydride to hydrogen at the anode; reduction of oxygen at the cathode). In Figure 5a it is shown that the potential of the cathode depends rather little on the temperature, unlike the potential of the anode. It is probable that the cathode performances are limited by the solubility of oxygen in the sodium hydroxide that wets the cathode, and that the limitation of the anode is of kinetic order. The maximum power density achieved here is 120 mW/cm 2 (Figure 5b). It is possible to improve this performance by further increasing the temperature but the corrosive nature of sodium hydroxide solution at temperatures above T = 60°C is an issue. Figure 5c shows that the quantity of hydrogen produced is more important as the temperature is raised. It also shows that the evolution of hydrogen is a decreasing function of the current density. The hydrogen production stops for the current density corresponding substantially to the maximum power; this means that the oxidation of hydrogen (HOR) produced by (heterogeneous) hydrolysis of NaBH4 is likely to play an important role in obtaining a good power density, at least with the platinum catalyst used in the present anode. However, it is possible to increase the current density beyond the density which cancels the hydrogen evolution, which would prove that the oxidation of hydrogen is not the only reaction at the origin of the anodic current, and that "direct" oxidation of BH4 -(BOR) is also possible at Pt anodes, even if the kinetics of the BOR is less favorable than that of HOR on platinum [8][9][10]. Figure 5a shows an increasing anode potential as a function of the current density and a singular evolution of the cathodic potential: it is first decreasing, i.e. the activation overvoltage increases with the current density. Then it stabilizes and even increases slightly for high current densities. This effect may be explained by the suppression of the hydrogen crossover, which had the effect of decreasing the cathodic potential. Since all the hydrogen produced is oxidized at high current densities, crossover can no longer occur.

Heterogeneities of operation between the inlet and outlet.

Figure 6a shows the current densities as a function of the position for several average currents. On the one hand, it is observed that the current density does not evolve in large proportions from the inlet to the outlet (25% variation). It is slightly decreasing between fuel inlet and outlet for low current densities and increasing for high current densities. On the other hand, the respective potentials of anode and cathode are quite heterogeneous (Figure 6b). The anode potential at the inlet "where there is little hydrogen" is below that at the outlet for low current densities: it is -1.13 V vs RHE, which is below the equilibrium potential of hydrogen and demonstrates that "direct" oxidation of NaBH4 proceeds in this region and in these experimental conditions. In this case of low current densities, it is therefore more advantageous to directly oxidize the borohydride, the associated activation overvoltage

being not yet too penalizing. Then, when the average current density increases, the anodic potential of the inlet increases rather strongly compared to the outlet, because the oxidation of the hydrogen present in large quantity at the outlet is much easier on platinum than the direct borohydride oxidation. This confirms that this is hydrogen oxidation that makes it possible to obtain a good power density, at least with the platinum catalyst used herein. In addition, the direct oxidation of borohydride is interesting at low current densities, to valorize the associated lower oxidation-reduction potential (and this must be strongly depending on the anode catalyst used [22], although it is beyond the scope of the present paper to study so). Finally, it is observed that the anodic potential increases a lot for the cell voltage of 0.3 > Ucell > 0.2 V, which corresponds to the end of hydrogen production (see figure 5c). For these high current densities, all the hydrogen produced is oxidized in the cell.

Here, hydrogen is responsible for the anode potential of -1.08 V vs RHE which is rather constant as a function of the current density. As long as hydrogen is present, it is thus the cathode which is limiting.

III.3. Flow rate sensitivity

The influence of a variation of the borohydride solution flow rate was evaluated and is shown in Figure 7. There is an increase in performance up to the flow rate corresponding to a stoichiometry of about 10; then, the performances stabilize and even slightly decrease, which can be explained by an imperfect thermalization of the solution at high flow rate. The flow of hydrogen produced evolves at the same pace as the cell voltage but stabilizes at a slightly higher rate. This limit reached by the hydrogen flow rate means that the hydrolysis of the borohydride is limited by the transport in the through-plane direction. This electrode manufactured similarly to a hydrogen fuel cell electrode is confined to the direct vicinity of the membrane and not very accessible to the solution. Reagents must diffuse from channels to the electrode through the GDL. Even if this mass-transfer is favored by greater pressure losses, which create a Darcy flow in the GDL, it remains limited.

III.4. Purging the cathode with distilled water

Figure 5a showed that the cathode overvoltage is far from negligible, although pure oxygen is used; the cathode employs highly-efficient Pt/C based oxygen reduction reaction (ORR) catalyst and has a very hydrophobic character (hence should not be completely drown by anolyte). This overvoltage can firstly be explained by the very low solubility and diffusivity of oxygen in the sodium hydroxide produced at the cathode (by combination of the OH - products of the oxygen reduction reaction, and of the Na + ions that migrate in the CEM under current flow) [23,24]; the concentration of NaOH formed at the cathode might indeed be large (the cathode is hydrophobic), severely impacting the mass-transfer of oxygen from the gas compartment to the active sites (that are wetted by the electrolyte). Secondly, fuel crossover is also probably, favoring mixed reactions (needless to say that Pt is active for the BOR) and subsequent catalyst poisoning with BOR intermediates and products [25].

Although the cathode is very hydrophobic (this is positive to avoid drowning, but detrimental because it cannot "dilute" the NaOH formed at the cathode when a CEM is used), an effect of purging with distilled water could be observed. Figure 8 shows the current response for a cell voltage maintained at = 0.5 V following brief purging (3 mL injected) with liquid water. There is a very positive effect on the current produced, and this effect dissipates in a few tens of minutes. It is likely that the liquid water "pulse" enables to expel NaOH (and possibly BOR) products, thereby transiently improving the cathode performances. This further confirms that the cathode is non-negligibly limiting the present DBFC performances, and that there is room for optimization of the cathode and whole cell, as exemplified hereafter. (i) Selective catalysts insensitive to borohydride permeation would be less hindered by the anolyte crossover [26][27][28][29]; with such catalysts, (ii) using an alkaline membrane or no membrane (but instead a porous separator) could be viable [29]. (iii)

Another solution envisaged by [START_REF] Wang | Nature Energy[END_REF] is to use a bipolar membrane in order to solve both borohydride permeation and soda production, which seems mandatory to obtain high stability of the DBFC performances.

IV. Conclusion

This paper reports some observations of a DBFC with a spatial resolution between the inlet and the outlet of the reagents. On the regular PEM fuel cell platinum electrodes considered here, the direct oxidation of borohydride is the primary reaction at stake for low current densities. It leads to a larger cell voltage than the oxidation of hydrogen, because the BOR initiates at lower potential than the HOR. For higher current densities, the oxidation of hydrogen is best able to provide power, because the activation overvoltage is lower (the BOR kinetics is slower that the HOR kinetics). The evolution of hydrogen from the Pt-based anode is not negligible, and proceeds both from the hydrogen evolution reaction (cathodic reduction of water) at or below 0 V vs RHE and from the (heterogeneous) hydrolysis of BH4 -; it generates hydrogen escape (in other words, H2 can exit the cell if it is not completely valorized), but this escape decreases as a function of the current density. Indeed, Pt enables the oxidation of the produced H2 in the HOR, provided the potential is above 0 V vs RHE. It is possible to operate without release of hydrogen, if the current density is sufficient (which means the anode potential is sufficiently above 0 V vs RHE). Otherwise, at low current density, it seems necessary to consider a solution to efficiently valorize the hydrogen produced. In the present system, the cathode is responsible for a significant part of the irreversibilities, because the Pt catalyst is not selective for the ORR (and instead is also active for the BOR), and because the production of sodium hydroxide penalizes the transport of oxygen to the catalytic sites. One way to cope with these issues would be to use an anionic membrane and a selective catalyst that is insensitive to borohydride permeation or a bipolar membrane.

V.

Fig. 1 :

 1 Fig.1: test bench used for segmented DBFC studies. The system is composed of a

Fig. 4 .

 4 Fig.4. a) Cell voltage during the second start-up with a membrane in its sodium form as a

Fig. 5 .

 5 Fig.5. a) polarization curve and average anodic and cathodic potentials with respect to H2

Fig. 6 :

 6 Fig.6: performances of the segmented DBFC in terms of (a) local current density, (b) local

Fig. 7 :

 7 Fig.7: influence of the fuel flow rate sensitivity on (a) the overall current produced by the

Fig. 8 :

 8 Fig.8: effect of liquid water injection at the cathode during operation of the DBFC at Ucell =

Acknowledgement

The authors wish to thank the ANR for the funding received under the Mobidic project (ANR-16-CE05-0009).