Independent testing and validation of prototype hydrogen sensors

In this article, the independent testing and validation of a packaged, electrochemical prototype hydrogen sensor at the National Renewable Energy Laboratory (NREL) is reported. Custom electronics were developed to be compatible with the data acquisition system at NREL. The specialized hydrogen sensor-testing laboratory at NREL used a variety of standardized test protocols to assess sensor performance. The system controlled and monitored humidity, pressure, and hydrogen gas concentration and introduced interference gases such as methane, carbon dioxide, carbon monoxide and ammonia.     

Assessment of commercial micro-machined hydrogen sensors performance metrics for safety sensing applications

Hydrogen sensors are increasingly recognized as safety enhancing components in applications where hydrogen is used as a clean energy carrier. The availability of low-cost, reliable, high performance hydrogen sensors is critical for facilitating the widespread and safe deployment of hydrogen systems. Accordingly, new sensing element designs based on advanced manufacturing techniques are being developed. Using micro-machining techniques, miniaturized versions of conventional hydrogen gas sensing elements have already been introduced in the market, with the promise of low-cost and high performance sensing metrics. An assessment of commercial micro-machined sensing elements relative to their conventional counterpart is presented in this paper. The results show that although some performance improvements were observed for commercial micro-machined sensors relative to their conventional counterparts, some models of micro-machined sensors were plagued with significant performance degradation. Furthermore, actual sensor performance, as determined by laboratory assessment often did not meet the manufacturer's published specifications. This work verifies the sensing metrics improvements brought by the micro-technology as well as its shortcomings for guiding the end-user safety applications.     

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.     

Simulation of dynamic response of mixed-potential hydrogen sensors

Knowledge about factors affecting the dynamic response process is crucial for development of mixed-potential gas sensor for various applications such as hydrogen detection. Based on a double layer capacitor model and the Butler-Volmer equation, the present work studied the dynamic hydrogen sensing process by numerical simulation in combination with experimental assessment. Simulation shows that both response and recovery times are jointly determined by the decrease rate of the net reaction current and the magnitude of response. Increase of standard reaction rate constants, transfer coefficients, standard equilibrium potential, or gas concentrations accelerates both the response and recovery processes, while double layer capacitance has a reverse effect. A power-law concentration dependence of the response/recovery time is obtained under Tafel kinetics. The simulated response curves and behavior agree quite well with the experimental results. These findings shed lights on the sensing kinetics of the mixed-potential hydrogen sensor, and may help guide the sensor design.     

Metal oxide hydrogen,oxygen, and carbon monoxide sensors for hydrogen setups and cells

Use of hydrogen, oxygen, and carbon oxide semiconductor sensors made of metal oxides allows controlling electronically the content of these gases in operation of many hydrogen setups, cells and devices. Present review-paper gives a general idea of achievements in this field.     

Investigation of PdCuSi metallic glass film for hysteresis-free and fast response capacitive MEMS hydrogen sensors

In this study, we investigated PdCuSi metallic glass (MG) as a sensing material for capacitive MEMS hydrogen sensors. We first confirmed by film analysis that the fabricated PdCuSi film was MG and that it had a trigonal prism cluster. The measured pressure-composition-temperature curve of PdCuSi MG exhibited no hysteresis during hydrogen absorption and desorption. The response time was found to become faster by two orders of magnitudes compared with that of Pd polycrystal. These properties were attributed to the trigonal prism clusters. Strain was evaluated in the low hydrogen concentration regime of 0.05 vol% to 4.0 vol%, and the strain of PdCuSi MG was found to follow Sieverts' law well, indicating that hydrogen is present in the MG in a diffuse state. The hydrogen-concentration dependence of a capacitive MEMS hydrogen sensor was measured and hysteresis-free characteristics were obtained, implying advantages in hydrogen leak detection applications.     

Room temperature hydrogen gas sensors of functionalized carbon nanotubes based hybrid nanostructure: Role of Pt sputtered nanoparticles

Fast detection of H₂ gas at room temperature has constantly remained a challenge. The metal-oxide based gas sensors have shown excellent sensing properties for gases like H₂, NO, CO and NH₃. In the present work, the H₂ gas sensing characteristics of multiwalled carbon nanotubes based hybrid sensor (F-MWCNTs/TiO₂/Pt) has been reported. The fabricated sensor shows 3.9% sensitivity for low concentration i.e. 0.05% of H₂ with good repeatability and stability at room temperature. The sensing response of F-MWCNTs/TiO₂/Pt is interrelated to change in their resistance on the introduction of H₂ gas and this phenomenon is required for deep understanding the effect of H₂ adsorption on their electronic conduction. The improvement in sensitivity of F-MWCNTs/TiO₂/Pt as compared to MWCNTs/TiO₂ towards H₂ is because of the catalytic role of dispersed Pt nanoparticles deposited by sputtering.     

Pd–Pt alloy as a catalyst in gasochromic thin films for hydrogen sensors

We have used Pd–Pt alloy as the catalyst in the hydrogen sensor thin film. Palladium and platinum were co-sputtered on top of a tungsten oxide layer grown by reactive sputtering. Both the sensitivity and the durability were dramatically improved over the case of a palladium single-component catalyst. The fractional change in the optical absorption on exposure to 1% hydrogen gas was increased by a factor larger than 2, and the fractional change decreased only a little after more than 1000 cycles of repeated exposure to 1% hydrogen and air. Moreover, the sensor film exhibited good selectivity to other organic vapors.     

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.     

Efficient room temperature hydrogen gas sensing based on graphene oxide and decorated porous silicon

In this study, a triple system for a hydrogen gas sensor was fabricated using graphene oxide, palladium nanoparticles, and porous silicon as a substrate. The fabricated sample was investigated by energy dispersive X-ray spectroscopy, field emission scanning electron microscopy, and Raman spectroscopy. Field emission scanning electron microscopy images displayed a relatively uniform distribution of Palladium nanoparticles over porous silicon. In addition, it was observed that the graphene oxide nanosheets accumulated over the Palladium nanoparticles. Hydrogen-sensing measurements demonstrated that the fabricated system can even detect hydrogen at 200 ppm and 15 °C. The formation of palladium hydride was the main mechanism for detection. In fact, this structure caused a change in resistance through the creation of new electron pathways. Furthermore, the H₂ concentration showed a linear function to the reciprocal of the response time; this suggests that the sensing kinetics of the sample depends on the atomization of hydrogen molecules, which occurs via Pd nanoparticles. Moreover, the fabricated sample displayed significant selectivity for hydrogen gas compared to other examined gases.     

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.     

Inter-laboratory assessment of hydrogen safety sensors performance under anaerobic conditions

Sensors are important devices for alerting to the presence of leaked hydrogen in any application involving the production, storage, or use of hydrogen. Key missions for the sensor test laboratories in the U.S. Department of Energy, National Renewable Energy Laboratory and in the European Commission Joint Research Centre, Institute for Energy and Transport are to assure the availability and proper use of hydrogen safety sensors. As an integral element in a safety system, sensor performance should not be compromised by operational parameters. For example, safety sensors may be required to operate at reduced oxygen levels relative to air, such as that which would exist for nitrogen purges. Some sensor platforms are amenable for anaerobic operation, whereas other platforms will be deactivated and possible permanently altered with anaerobic operation. The NREL and JRC sensors laboratories assessed the ability of a number of sensor platforms to detect hydrogen under conditions of varying oxygen concentration. The performance of three common hydrogen sensor platforms, the thermal conductivity sensor, combustible gas sensor, and a palladium thin-film (metallic resistor) sensor, to operate under anaerobic conditions is presented.     

A novel yttria-doped ZrO2 based conductometric sensor for hydrogen leak monitoring

Pure and (8%)Y₂O₃-doped zirconium oxide commercial samples were investigated for developing a high performance conductometric hydrogen sensor. The morphological, microstructural, optical, and electrical characteristics of the samples were studied and compared. Conductometric sensors based on these samples were fabricated using a planar platform in alumina provided with interdigitated electrodes, and sensing tests were carried at different operating temperatures and hydrogen concentrations. Sensing tests revealed that the fabricated sensor based on the tetragonal ZrO₂–Y₂O₃ (8%) showed the best performances in terms of sensor response (R₀/R_H2 = 7.3@10000 ppm of hydrogen), response and recovery time (5 and 10 s, respectively), and low operating temperature (150 °C). These characteristics have been exploited for developing the first hydrogen leak conductometric sensor based on ZrO₂ so far reported.     

Evaluation of barrier walls for mitigation of unintended releases of hydrogen

Hydrogen jet flames resulting from ignition of unintended releases can be extensive in length and pose significant radiation and impingement hazards. Depending on the leak diameter and source pressure, the resulting consequence distances can be unacceptably large. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. While reducing the extent of unacceptable consequences, the walls may introduce other hazards if not properly configured. An experimental and modeling program has been performed at Sandia National Laboratories to better characterize the effectiveness of barrier walls to reduce hazards. This paper describes the experimental and modeling program and presents results obtained for various barrier configurations. The experimental measurements include flame deflection using standard and infrared video and high-speed movies (500 fps) to study initial flame propagation from the ignition source. Measurements of the ignition overpressure, wall deflection, radiative heat flux, and wall and gas temperature were also made at strategic locations. The modeling effort includes three-dimensional calculations of jet flame deflection by the barriers, computations of the thermal radiation field around barriers, predicted overpressure from ignition, and the computation of the concentration field from deflected unignited hydrogen releases. The various barrier designs are evaluated in terms of their mitigation effectiveness for the associated hazards present. The results show that barrier walls are effective at deflecting jet flames in a desired direction and can help attenuate the effects of ignition overpressure and flame radiative heat flux.     

An optical hydrogen sensor based on a Pd-capped Mg thin film wedge

We report the proof of concept of a thin film device with a one-to-one relationship between the H₂ partial pressure and the lateral progression of a visible optical change along a thin film multilayer 70 mm long. The device basically consists of a sensing Mg layer with a thickness gradient. It exploits the thickness dependence of the hydrogenation thermodynamics of Pd-capped Mg thin films. The optical change of the Mg layer during the metal–hydride transition can be detected both in reflection and in transmission. This optical sensor allows a continuous measurement of hydrogen partial pressure in the range between 200 and 4000 Pa.     

Mathematical methodology and software package for investigation of nonstationary hydrogen permeability in membranes used in electrolysers and hydrogen fuel cells

As part of the methodology to control gas permeability of functional materials, mathematical approach is considered that takes into account a diffusion process through a plate with the boundary conditions of the 1st and 2nd types. The methodology under discussion provides methods for estimating parameters of the diffusion by using methods of exceptional points, functional scale and statistical moments. Computer software package HPRON has been developed to provide mathematical modeling, planning, processing and data interpretation of different variants of gas permeability method. The results are used to optimize the performance of the membrane electrode assembly (MEA) for the automotive fuel cell and electrolyzer applications with a perfluorinated proton-conducting membrane such as nafion.     

The effect of the change in the amount of Sb doping in ZnO nanorods for hydrogen gas sensors

In this study, pure and variable content Sb doped ZnO nanorods (NRs) were grown by a simple spray pyrolysis method successfully. Structural analysis has showed that all the films are indicating preferential dominant c-axis (002) plane from x-ray diffraction (XRD) measurements. It is observed that Sb doping does not result in changes in lattice parameter indicating no lattice distortion. Raman measurements has indicated Sb doping related modes in ZnO NRs especially defect related. SEM images has shown uniform hexagonal close packing NR structures uniformly distributed throughout the film. X-ray photoelectron spectroscopy (XPS) has displayed lower incorporation of the Sb from the precursor to the Sb doped NRs. Hydrogen gas sensor performances of these NRs has investigated. 5.0 wt% Sb doped ZnO NR sample has showed outstanding response with 23-fold response to 10 ppm hydrogen gas level at 250 °C.     

Solar hydrogen production by two-step thermochemical cycles: Evaluation of the activity of commercial ferrites

In this work, we report on the evaluation of the activity of commercially available ferrites with different compositions, NiFe₂O₄, Ni_0.5Zn_0.5Fe₂O₄, ZnFe₂O₄, Cu_0.5Zn_0.5Fe₂O₄ and CuFe₂O₄, for hydrogen production by two-step thermochemical cycles, as a preliminary study for solar energy driven water splitting processes. The samples were acquired from Sigma–Aldrich, and are mainly composed of a spinel crystalline phase. The net hydrogen production after the first reduction–oxidation cycle decreases in the order NiFe₂O₄ > Ni_0.5Zn_0.5Fe₂O₄ > ZnFe₂O₄ > Cu_0.5Zn_0.5Fe₂O₄ > CuFe₂O₄, and so does the H₂/O₂ molar ratio, which is regarded as an indicator of potential cyclability. Considering these results, the nickel ferrite has been selected for longer term studies of thermochemical cycles. The results of four cycles with this ferrite show that the H₂/O₂ molar ratio of every two steps increases with the number of cycles, being the total amount stoichiometric regarding the water splitting reaction. The possible use of this nickel ferrite as a standard material for the comparison of results is proposed.     

Review on hydrogen safety issues: Incident statistics,hydrogen diffusion,and detonation process

The development and application of hydrogen energy in power generation, automobiles, and energy storage industries are expected to effectively solve the problems of energy waste and pollution. However, because of the inherent characteristics of hydrogen, it is difficult to maintain high safety during production, transportation, storage, and utilization. Therefore, to ensure the safe and reliable utilization of hydrogen, its characteristics relevant to leakage and diffusion, ignition, and explosion must be analyzed. Through an analysis of literature, in combination with our practical survey analysis, this paper reviews the key issues concerning hydrogen safety, including hydrogen incident investigation, hydrogen leakage and diffusion, hydrogen ignition, and explosion.