Evaluation of selectivity of commercial hydrogen sensors

The development of reliable hydrogen sensors is crucial for the safe use of hydrogen. One of the main concerns of end users is sensor reliability in the presence of species other than the target gas, which can lead to false alarms or undetected harmful situations. To assess the selectivity of commercial-off-the-shelf hydrogen sensors, a number of sensors of different technology types were exposed to various interferent gas species. Cross-sensitivity tests were performed in accordance with the recommendations of ISO 26142:2010, using the hydrogen sensor testing facilities of the National Renewable Energy Laboratory and the Joint Research Centre – Institute for Energy and Transport. Most of the sensor platform tested are unaffected by the exposure to the interferents. The metal-oxide and the thermal conductivity platform show a remarkable sensitivity to CH₄. None of the platforms tested were permanently affected by the exposure to the cross-sensitive species.     

An assessment on the quantification of hydrogen releases through oxygen displacement using oxygen sensors

Gas sensors that respond directly to hydrogen are typically used to detect and quantify unintended hydrogen releases. However, alternative means to quantify or mitigate hydrogen releases are sometimes proposed. One recently explored approach has been to use oxygen sensors. This method is based on the assumption that a hydrogen release will displace oxygen, which can be quantified using oxygen sensors. The use of oxygen sensors to monitor ambient hydrogen concentration has drawbacks, which are explored in the current study. It was shown that this approach may not have adequate accuracy for safety applications and may give misleading results under certain conditions for other applications. Despite its shortcomings, the Global Technical Regulation (GTR) for Hydrogen and Fuel Cell Vehicles has explicitly endorsed this method to verify hydrogen vehicles' fuel system integrity. Experimental evaluations designed to impartially assess the ability of oxygen and hydrogen sensors to reliably measure hydrogen concentration changes are presented. Specific limitations on the use of oxygen sensors for hydrogen measurements are identified and alternative sensor technologies that meet the requirements for several applications, including those of the GTR, are proposed.     

Reliability of commercially available hydrogen sensors for detection of hydrogen at critical concentrations: Part I – Testing facility and methodologies

A facility for testing the performance of hydrogen safety sensors under a wide range of ambient conditions is described. A specific test protocol was developed to test sensors under conditions which could reasonably be expected during the sensors' service life. The tests were based on those described in IEC 61779 and were adapted following consultation with car manufacturers and after careful consideration of the sensors expected service environmental conditions. The protocol was evaluated by using it to test a large number of commercially available sensors. Observations made and experience gained during the testing campaign allowed the test protocol to be fine-tuned bearing in mind the sensor performance and behaviour during tests. The result of this work is an experimentally evaluated methodology which may be used as a guideline for testing the suitability of hydrogen sensors for automotive applications.     

Reliability of commercially available hydrogen sensors for detection of hydrogen at critical concentrations: Part II – selected sensor test results

Reliable hydrogen sensors are essential to detect accidental hydrogen releases when hydrogen will be used to fuel future vehicles. To assess the performance of hydrogen safety sensors under conditions typical of automotive applications a test protocol has been defined. It has been experimentally evaluated by performing tests on commercially available hydrogen sensors. Catalytic sensors measured hydrogen concentration accurately and sensor response was largely independent of ambient parameters. However they were significantly cross sensitive to carbon monoxide and the detection limit was high. Metal-oxide semiconductive sensors had a low detection limit and showed a low cross sensitivity to carbon monoxide however almost all of these samples showed poor accuracy and a strong dependence on ambient parameters. Electrochemical sensors also had a low detection limit however ambient parameters, cross sensitivity and accuracy tests showed a high variation in results. Tests on a limited number of thermal conductivity sensors highlighted their high detection limit and strong dependence on temperature.     

Test methodologies for hydrogen sensor performance assessment: Chamber vs. flow-through test apparatus

Certification of hydrogen sensors to meet standards often prescribes using large-volume test chambers [1,2]. However, feedback from stakeholders such as sensor manufacturers and end-users indicates that chamber test methods are often viewed as too slow and expensive for routine assessment. Flow-through test methods are potentially an efficient and cost-effective alternative for sensor performance assessment. A large number of sensors can be simultaneously tested, in series or in parallel, with an appropriate flow-through test fixture. The recent development of sensors with response times of less than 1s mandates improvements in equipment and methodology to properly capture the performance of this new generation of fast sensors; flow methods are a viable approach for accurate response and recovery time determinations, but there are potential drawbacks. According to ISO 26142 [1], flow-through test methods may not properly simulate ambient applications. In chamber test methods, gas transport to the sensor is dominated by diffusion which is viewed by some users as mimicking deployment in rooms and other confined spaces. Conversely, in flow-through methods, forced flow transports the gas to the sensing element. The advective flow dynamics may induce changes in the sensor behaviour relative to the quasi-quiescent condition that may prevail in chamber test methods. The aim of the current activity in the JRC and NREL sensor laboratories [3,4] is to develop a validated flow-through apparatus and methods for hydrogen sensor performance testing. In addition to minimizing the impact on sensor behaviour induced by differences in flow dynamics, challenges associated with flow-through methods include the ability to control environmental parameters (humidity, pressure and temperature) during the test and changes in the test gas composition induced by chemical reactions with upstream sensors. Guidelines on flow-through test apparatus design and protocols for the evaluation of hydrogen sensor performance have been developed. Various commercial sensor platforms (e.g., thermal conductivity, catalytic and metal semiconductor) were used to demonstrate the advantages and issues with the flow-through methodology.     

Developments in gas sensor technology for hydrogen safety

Gas sensors are applied for facilitating the safe use of hydrogen in, for example, fuel cell and hydrogen fuelled vehicles. New sensor developments, aimed at meeting the increasingly stringent performance requirements in emerging applications, are reviewed. The strategy of combining different detection principles, i.e. sensors based on electrochemical cells, semiconductors or field effects in combination with thermal conductivity sensing or catalytic combustion elements, in one new measuring system is reported. This extends the dynamic measuring range of the sensor while improving sensor reliability to achieve higher safety integrity through diverse redundancy. The application of new nanoscaled materials, nanowires, carbon tubes and graphene as well as the improvements in electronic components and optical elements are evaluated in view of key operating parameters such as measuring range, sensor response time and low working temperature.     

An overview of hydrogen safety sensors and requirements

Internationally, there is a commitment to increase the utilization of hydrogen as a clean and renewable alternative to carbon-based fuels. Hydrogen safety sensors are critical to assure the safe deployment of hydrogen systems; but, because there exists a broad range of sensor options, selecting an appropriate sensor technology can be complicated. Some sensor technologies might not be a good fit for a specific application. Facility engineers and other end-users, however, are expected to select the optimal sensor for their systems. Making informed decisions requires an understanding of the general analytical performance specifications that can be expected for a given sensor technology. Although there are many commercial sensors, most can be classified into relatively few specific sensor types. Each specific platform has characteristic analytical trends, advantages, and limitations. Knowledge of these trends can guide the selection of the optimal technology for a specific application.     

Packaging and testing of a hydrogen safety sensor prototype

In this article, testing of an electrochemical, potentiometric hydrogen safety sensor is reported within a proposed packaging scheme. Device packaging under static and flow testing conditions are presented. During the static volume sensor testing, the sensors response is compared against a calibrated Gas Chromatography (GC) measurement. Also, a commercial H₂ sensor is incorporated into the test chamber to act as a benchmark for the sensor prototype. In the testing phase, H₂ selectivity is demonstrated using pulsed discharge technique.     

Miniaturization of spacecraft electrical power systems with solar-hydrogen power supply system

The concept of solar-hydrogen systems for spacecraft, orbital stations, lunar and Martian bases is currently receiving a new impetus. The supply of solar energy to energy receivers aboard space vehicles is limited. The number of everyday tasks and energy-intensive experiments on board space objects is growing with the development of astronautics. To perform energy-intensive work and experiments, energy consumption exceeds the incoming solar fluxes. The modern solar-hydrogen system ensures the reception, conversion and accumulation of excess incoming energy in the form of chemical energy - hydrogen. Cryogenics makes it possible to miniaturize the solar-hydrogen system. The safety of the cryogenic solar-hydrogen energy system is ensured by hydrogen concentration and leakage sensors.The article proposes a comprehensive solution for miniaturization of Power Conversion Unit (PCU) of Energy Power System (EPS) of spacecrafts (SC), which consists in a joint solution of four key problems of miniaturization of power devices: energy, structural, design and technological, system. Miniaturization of the solar-hydrogen energy system (SHES) is achieved by installing onboard hydrogen and oxygen microcryogenic refrigerators, as well as hydrogen and oxygen cryogenic tanks, water tank, electrolyzer and hydrogen fuel cells (FC). The accumulation of chemical hydrogen energy on board the orbiting spacecraft ensures reliable operation when entering the shadow. Storage of hydrogen in cryogenic form significantly reduces the volume required. A cryocooler based on the Stirling cycle provides the process of liquefying hydrogen after the electrolysis of distilled water. In addition, cryogenic temperatures of 20.2 K can be used to thermostat precision instruments placed on board the spacecraft, For example, to ensure the operation of the SQUID. A 3D - model of a voltage stabilization module (VSM), on the basis of which EEC can be produced with different output power and redundancy depth. We give an example of a complete structural scheme of the EEC, which allows implementing all the fulfillment of all the tasks, assigned to the EEC of the PCU SC.     

Characterization of a selective,zero power sensor for distributed sensing of hydrogen in energy applications

The use of hydrogen as a clean and renewable energy source is increasing rapidly for both vehicle and stationary applications. There are safety concerns for the locations in which hydrogen is made, used, and transported (i.e., pipelines and tanker trucks). Sensors are needed to comply with safety regulations and to enable a smooth and safe rollout of hydrogen as an alternative energy. However, hydrogen sensors do not yet exist that have the combined features of small size and low power for easy deployment coupled with high-volume manufacturability and low cost. This is necessary to accommodate the emerging fixed and mobile markets while retaining critical metrological metrics, including measurement range, detection limits, selectivity, fast response, stability, and long lifetime. An amperometric gas sensor for hydrogen (AGS) has been developed using an innovative manufacturing method. The sensor was designed using scalable fabrication strategies based on “Printed Electronics” (PE) methodology which are compatible with large-scale production. Prototype sensors were batch fabricated with multiple individual elements on a substrate compatible in size with a standard 8-inch wafer to enable high-volume, low-cost manufacturing, thereby leveraging PE and semiconductor fabrication infrastructure. This novel AGS was instrumented with control circuitry and evaluated for hydrogen detection in the range of 0–5000 ppm_v H₂ in air. Specific performance evaluations included assessment of the sensor measurement range, repeatability, and selectivity.     

The United States Department of Energy's high temperature,low relative humidity membrane program

The U.S. Department of Energy (USDOE) Hydrogen Program works with industry, academia, and National Laboratories through research and development to overcome technical barriers of fuel cell and hydrogen production, delivery, and storage technologies. Two of the major challenges in the advancement of fuel cell technology are cost and durability of the polymer electrolyte membranes used for proton conduction in the fuel cell. To address these challenges, DOE initiated new membrane research and development projects to design membranes that meet its 2010 technical targets and will lead to membranes that operate in a fuel cell system that performs as well and costs as little as internal combustion engines. Three strategies are employed in the program: implementation of phase segregation in the membrane to create proton conduction pathways, use of non-aqueous proton conductors for operation under dry conditions, and hydrophilic additives to retain water at low relative humidity.     

Identifying performance gaps in hydrogen safety sensor technology for automotive and stationary applications

A market survey has been performed of commercially available hydrogen safety sensors, resulting in a total sample size of 53 sensors from 21 manufacturers. The technical specifications, as provided by the manufacturer, have been collated and are displayed herein as a function of sensor working principle. These specifications comprise measuring range, response and recovery times, ambient temperature, pressure and relative humidity, power consumption and lifetime. These are then compared against known performance targets for both automotive and stationary applications in order to establish in how far current technology satisfies current requirements of sensor end users. Gaps in the performance of hydrogen sensing technologies are thus identified and areas recommended for future research and development.     

Extending the detection range and response of TiO2 based hydrogen sensors by surface defect engineering

With the increasing usage of hydrogen energy, the requirements for hydrogen detection technology is increasingly crucial. In addition to bringing down the working temperature, further improvement in the response and broadening the detection range of hydrogen sensors in particular are still needed. TiO₂ based sensors show great promise due to their stable physical and chemical properties as well as low cost and easy fabrication, but their detection range and low concentration response requires further improvement for practical applications. Here (002) oriented rutile TiO₂ thin films are prepared by a hydrothermal method followed by annealing in either air, oxygen, vacuum or H₂ and the hydrogen sensing performance are evaluated. Raman results show that TiO₂ thin films annealed in vacuum and hydrogen have more oxygen vacancies, while those annealed in air and oxygen have a more stoichiometric surface. Annealing in an oxygen-rich atmosphere is shown to extend the detection range of the TiO₂ sensors while annealing in anaerobic atmospheres increases their response. At high hydrogen concentrations surface adsorbed O₂⁻ is the dominant factor, while at low concentrations the Schottky barrier between Pt and TiO₂ is key to achieving a high response. Here we show controlling the TiO₂ surface properties is essential for optimizing hydrogen detection over specific concentration ranges. We demonstrate that adjusting the annealing conditions and ambient provides a simple method for tuning the performance of room temperature operating TiO₂ based hydrogen sensors.     

Study of sensitivity and response kinetics changes for SnO2 thin-film hydrogen sensors

We present results of investigations devoted to searching of ways for sol–gel derived SnO₂ thin-film hydrogen sensors performance improvement. To that end we studied the as-prepared sensors parameters changes in real time during short- and long-term operation in gas–air mixture and known operating temperature conditions. It was established that in comparison with as-prepared sensor parameters such initial operation mode leads to the rise of the sensors sensitivity on more than two orders of magnitude, improvement of the sensor parameters stability in time. Obtained experimental data are in a good accordance with known theoretical results.     

A comparison of test methods for the measurement of hydrogen sensor response and recovery times

Response time is a critically important property of hydrogen safety sensors. Recovery times are less important from a safety perspective, but are often quoted as an indication of the speed of operation of a sensor. However, the measured values depend highly on the method used to evaluate them. The purpose of this work is to assess the suitability of different methods, both flow and diffusion-based, for the measurement of sensor response and recovery times. Four methods have been tested in terms of their repeatability and practicality of execution, as well as the accuracy of their results compared to the manufacturer’s specifications. It was found that each method has its own advantages and limitations, which are discussed herein. For the measurement of response times, a diffusion-based method was found to give the shortest and most precise values and is therefore recommended. However, the flow-based method was found to be the most convenient experimentally and is the only method that is suitable for the measurement of recovery times over a wide concentration range.     

Combined autothermal and sorption-enhanced reforming of olive mill wastewater for the production of hydrogen: Thermally neutral conditions analysis

A thermodynamic study and an energy analysis are performed on the autothermal reforming (ATR) and sorption-enhanced autothermal reforming (SE-ATR) of olive mill wastewater (OMW) to produce green hydrogen. For comparative purposes, the traditional reforming (TR) and sorption-enhanced reforming (SER) are also assessed. The thermodynamic equilibrium compositions are calculated by using the Gibbs free energy minimization method. The effect of temperature, pressure and steam-to-carbon molar ratio (S/C) is assessed for the different reactor configurations. The energy analysis is done by determining the level of oxygen needed to operate under thermally neutral conditions.The results show a reduced hydrogen yield for ATR and SE-ATR compared to their non-autothermal counterparts, but a decreased energy requirement per mole of hydrogen produced. For the ATR, the thermally neutral operation requires a high amount of oxygen, which reduces the hydrogen yield. However, for SE-ATR, the exothermal sorption reaction provides nearly enough energy for the process to be thermally neutral by itself.     

Hydrogen sensing properties of a Pd/oxide/InAlAs metamorphic-based transistor

A Pd/oxide/InAlAs metal–oxide–semiconductor (MOS) type metamorphic high electron mobility transistor (MHEMT)-based hydrogen sensor is fabricated and investigated. In comparison with the conventional HEMT-based sensors, the MOS MHEMT-based sensor exhibits significantly high sensitivity to the hydrogen. The found hydrogen sensing response is as high as 300%. Using the thermodynamic analysis to estimate the enthalpy value of hydrogen adsorption, the value for the proposed sensor is much lower than that for the other reported HEMT-based sensors. The MHEMT-based sensors are demonstrated to have a relatively fast response as comparing to other HEMT-based ones. The response time of the device is approximately 10 s under exposure to a 1% H₂/air gas. Consequently, the performance of the studied sensors shows the promise characteristics for practical applications.     

A novel technique to fabricate horizontally aligned CNT nanostructure film for hydrogen gas sensing

A simple method using a combination of nanocomposite plating and firing techniques for the production of horizontally aligned carbon nanotube (HACNT)-based hydrogen gas sensors is presented. This low temperature (100 °C) firing process generates cracks in which HACNTs are formed. Hydrogen sensing characteristics are measured in various gas concentrations from 200 ppm to 16,000 ppm at room temperature. The HACNT-based hydrogen gas sensor performs with low noise, short response time, and fast recovery time. It is found that the HACNT-based sensors have a much better sensitivity response (approximately 5 times) than the original CNT/Ni film sensors which use a nanocomposite plating technique only. The Raman spectra of the HACNT-based sensors show that more defects and oxidation were generated on the HACNTs after the firing process. The firing process decreases the oxygen vacancies of CNTs to enhance the sensitivity response of HACNT-based sensors. In addition HACNT-based sensors are relatively simple, cost-effective and mass-producible.     

Artificial neural networks and neuro-fuzzy inference systems as virtual sensors for hydrogen safety prediction

Hydrogen is increasingly investigated as an alternative fuel to petroleum products in running internal combustion engines and as powering remote area power systems using generators. The safety issues related to hydrogen gas are further exasperated by expensive instrumentation required to measure the percentage of explosive limits, flow rates and production pressure. This paper investigates the use of model based virtual sensors (rather than expensive physical sensors) in connection with hydrogen production with a Hogen^®20 electrolyzer system. The virtual sensors are used to predict relevant hydrogen safety parameters, such as the percentage of lower explosive limit, hydrogen pressure and hydrogen flow rate as a function of different input conditions of power supplied (voltage and current), the feed of de-ionized water and Hogen^®20 electrolyzer system parameters. The virtual sensors are developed by means of the application of various Artificial Intelligent techniques. To train and appraise the neural network models as virtual sensors, the Hogen^®20 electrolyzer is instrumented with necessary sensors to gather experimental data which together with MATLAB neural networks toolbox and tailor made adaptive neuro-fuzzy inference systems (ANFIS) were used as predictive tools to estimate hydrogen safety parameters. It was shown that using the neural networks hydrogen safety parameters were predicted to less than 3% of percentage average root mean square error. The most accurate prediction was achieved by using ANFIS.     

Research on hydrogen dispersion by Raman measurement

To safely handle hydrogen as an energy carrier, it is important to understand the behaviour of hydrogen. There are many methods available for measuring hydrogen concentration using conventional sensors; however, it is difficult to detect hydrogen gas from a distance. Here, we observed hydrogen behaviour over a narrow region of space with a Raman scattered light sensor. Generally, there is some delay in conventional sensors; however, there was almost no delay with the use of our sensor. We used 1- and 6-mm diameter holes as spout nozzles to change the initial velocities. To confirm the effectiveness of our method, we used hydrogen visualization sheets, which became transparent when hydrogen was detected enabling the hydrogen movement to be visualized. Hence, we observed the behaviour of hydrogen gas in a small container and optimized the device to increase the measurement distance from 4.5 to 7.5 m.