Hydrogen safety sensors must meet specific performance requirements, mandated by the U.S. Department of Energy, for hydrogen fueling station monitoring. Here, we describe the long-term performance of two zirconia-based mixed potential electrochemical hydrogen gas sensors, developed specifically with a high sensitivity to hydrogen, low cross-sensitivity, and fast response time. Over a two-year period, sensors with tin-doped indium oxide and strontium doped lanthanum chromite electrodes were deployed at two stations in four field trials tests conducted in Los Angeles. The sensors documented the existence of hydrogen plumes ranging in concentration from 100 to as high as 2700 ppm in the area surrounding the dispenser, consistent with depressurization from 700 bar following vehicle refueling. As expected, the hydrogen concentration reported by the mixed potential sensors was influenced by wind direction. Baseline stability testing at a Chino, CA station showed no measureable baseline drift throughout 206 days of uninterrupted data acquisition. The high baseline stability, excellent correlation with logged fueling/depressurization events, and absence of false alarms suggest that the zirconia-based mixed potential sensor platform is a good candidate for protecting hydrogen infrastructure where frequent calibrations or sensor replacement to reduce the false alarm frequency have been shown to be cost prohibitive.
Hydrogen refueling stations (HRSs) that dispense hydrogen to fuel cell vehicles need to ensure the quality of hydrogen to avoid contamination of the vehicle’s expensive fuel cell stacks. Currently, stations verify their fuel quality only periodically to ensure that they meet the strict fuel quality standards specified by either International Organization for Standards (ISO) or Society for Automotive Engineers (SAE). The development of hydrogen contaminant detectors (HCDs) that can provide low cost continuous monitoring at the HRS can be an invaluable asset in protecting fuel cell vehicles from any fuel contamination in-between infrequent expensive analysis of hydrogen fuel quality. An HCD capable of detecting < 200 ppb of CO in hydrogen is presented in this paper. The HCD is based on an electrochemical hydrogen pumping cell whose ultra-low loaded working electrode is poisoned by the contaminant, thus reducing its hydrogen oxidation reaction rate. The hydrogen pumping cell consists of a Nafion® membrane, a sputtered Pt working electrode, a Pt/Ru counter/pseudo-reference electrode and an internal water wicking system that provides humidification to the membrane and electrodes. When this HCD is operated in a pulsed voltammetry mode, it can provide stable CO response for thousands of hours in a HRS.
Hydrogen safety sensors must meet specific performance requirements, mandated by the U.S. Department of Energy, for hydrogen fueling station monitoring. Here, we describe the longterm performance of two zirconia-based mixed potential electrochemical hydrogen gas sensors, developed specifically with a high sensitivity to hydrogen, low cross-sensitivity, chemical stability across operating temperatures, and fast response time. Over a two-year period, sensors with two different working electrodes-tin-doped indium oxide and strontium doped lanthanum chromite-were deployed at hydrogen filling stations in the Los Angeles area. The sensors exhibit high baseline stability and excellent correlation with logged filling/release events, making these materials good candidates for applications requiring long-life, drift-free baseline operation, in which sensor calibrations and replacements are prohibitive.
Hydrogen sensors are recognized as an important to critical component in the safety design for any hydrogen system. In this role, sensors can perform several important functions including indication of unintended hydrogen releases, activation of mitigation strategies to preclude the development of dangerous situations, activation of alarm systems and communications to first responders, and may even be called upon to autonomously initiate a system shutdown. The hydrogen sensors may be used is such a manner that they operate separate from the system being monitored, thereby providing an independent safety component that is not affected by the system itself. The importance of hydrogen sensors has been recognized by the DOE Safety and Codes Standards sub-program within the Fuel Cell Technologies Office and has for the past several years supported hydrogen safety sensor research and development. Since 2008, Los Alamos National Laboratory (LANL) and Lawrence Livermore National Laboratory (LLNL) have been developing durable, high temperature electrochemical sensors based on zirconia oxygen ion-conducting solid electrolytes1 because, among the various modalities in hydrogen sensing, electrochemical solid-state sensors can be produced at low cost as evidenced by the widely used oxygen exhaust gas sensor. Also, solid-state electrochemical technology is well suited for meeting the most stringent requirements including operation over a wide temperature range, fast response times, and reliability under variations in humidity2-4. In 2013, several packaged prototype mixed-potential hydrogen safety sensors together with control electronics (designed and built by Custom Sensor Solutions, Inc. Oro Valley AZ) were tested in validation and verification experiments at the National Renewable Energy Laboratory (NREL) Hydrogen Sensor Test Laboratory5. This preliminary testing not only provided an independent evaluation and feedback for the performance of the sensor6, but also identified design requirements for the rest of sensing system that would be required to move forward with deployment in a field trials testing environment. Recently, we have started to test electrochemical, mixed-potential hydrogen sensor technology at a California commercial fuel cell vehicle hydrogen filling station. In the first field trials experiment, data were collected over a month time period during two modes of station operation: a) station hydrogen supplied by a hydrogen tube trailer and b) hydrogen generated on-site from a methane reformer. The sensor unit – comprised of a heater control board and commercial wireless transmitter inside of a NEMA-8 enclosure – was located inside the dispensing island at the City of Burbank Hydrogen Filling Station (Burbank CA). The inside of the dispensing island enclosure location was selected because it was in an area with least expectation of measuring a hydrogen exposure and because co-location with an existing commercial safety sensor was possible. The commercial sensor was one component of a larger safety system used to signal the authorities in case a customer ran into difficulties during refueling. Over the course of the testing, the mixed potential sensor was stable and showed no evidence of baseline drift and did not appear to be affected by over a month of unusually frequent and active weather systems that produced severe at times. The stable operation and lack of influence to temperature and humidity changes agreed well with earlier testing results from NREL. During the first field trials experiment, the mixed-potential sensor reported numerous hydrogen releases with some as high as 14% of LFL and these events correlated well to activities when customers resupplied their fuel cell vehicles. During periods that the station was supplied using hydrogen from the reformer, elevated levels of hydrogen (200-400 ppm) showed oscillations with regular periodicity over a 280hr experiment well within the 600hr reformer duty cycle. Releases up to 20% of LFL were reported from the mixed potential-based sensor system during this phase of station operation and these releases were attributed to the reformer and onsite compressor usage. In this presentation, we will present the first field trails results and discuss expansion of this work to include logging multiple mixed-potential sensors at distributed locations within the filling. References 1. Sekhar, PK et al., Sens Actuators B 148(2010) 469-77. 2. Korotcenkov G, Han SD, Stetter JR, Rev 109(2009), 1402-33. 3. Lange U, Hirsch T, Mirsky VM, Wolfbeis OS, Electrochem Acta 65(2011) 3707-12. 4. Hubert T, Boon-Brett L, Black G, Banach U, Sens Actuators B 157(2011) 329-52. 5. http://www.nrel.gov/hydrogen/facilities_hsl.htm 6. Sekhar, PK et al., Int. J. of Hydrogen Energy 39(2014) 4657-4663. Acknowledgements The authors would like to thank Charles (Will) James Jr. and the DOE Hydrogen Fuel Cell Technology Office and Hydrogen Safety Codes and Standards Sub-program provided funding for this work.
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