Field Trials Testing of a Mixed Potential Electrochemical Hydrogen Safety Sensor at a Commercial Hydrogen Filling Station
Authors: Brosha Eric L. , Romero Christopher J , Poppe Daniel , Strada Michael , Williamson Todd L , Mukundan Rangachary 
Published: February 2020

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.

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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.