On a heterostructure field-effect transistor (HFET) based hydrogen sensing system

An interesting heterostructure field-effect transistor (HFET) based hydrogen sensing system is developed and demonstrated. Even at a low hydrogen concentration of 15 ppm H₂/air, the studied sensor still exhibits high sensitivity at room temperature. Furthermore, a simple sensing system is also designed and reported. In contrast to conventional hydrogen measurement, the detecting system consists of a hydrogen sensor and some sensing circuits. This proposed hydrogen detection system is based on the micro-controller with advantages of low cost, fast response, portable, and easy operation. From experimental results, the proposed system is shown to be good for use.     

A resonant photoacoustic cell for hydrogen gas detection

Hydrogen gas (H₂) detection plays an important role in many fields. With the continuous demand and development of clean energy, it is urgent to study new hydrogen gas sensors for stable and accurate H₂ detection. The purpose of this research is to develop a new H₂ sensor based on the resonant photoacoustic (PA) cell as the sensing element. The sensitivity of the resonant PA cell to the resonant frequency is sufficiently utilized. The optimization of its resonance frequency was investigated minutely for the H₂ sensor. Detection utilizes resonance frequency differences between H₂ and air as a sensing mechanism. The resonance frequency tracking is adopted and implemented by the field-programmable gate array (FPGA) device. The minimum detection limit of about 74 ppm for H₂ has been demonstrated by preliminary experiments. The response time of the sensor is about 5 s. This sensor detects concentrations ranging from 74 ppm to 100% in 1 atm. The preliminary test result shows that the H₂ sensor based on this structure has a larger application perspective.     

Comprehensive investigation on planar type of Pd–GaN hydrogen sensors

This paper reviews both static and dynamic characteristics of a planar-type Pd–GaN metal–semiconductor–metal (MSM) hydrogen sensor. The sensing mechanism of a metal–semiconductor (MS) hydrogen sensor was firstly reviewed to realize the sensing mechanism of the proposed sensor. Symmetrically bi-directional current–voltage characteristics associated with our sensor were indicative of easily integrating with other electrical/optical devices. In addition to the sensing current, the sensing voltage was also used as detecting signals in this work. With regard to sensing currents (sensing voltages), the proposed sensor was biased at a constant voltage (current) in a wide range of hydrogen concentration from 2.13 to 10,100 ppm H₂/N₂. Experimental results reveal that the proposed sensor exhibits effective barrier height variations (sensing responses) of 134 (173) and 20 mV (1) at 10,100 and 2.13 ppm H₂/N₂, respectively. A sensing voltage variation as large as 18 V was obtained at 10,100 ppm H₂/N₂, which is the highest value ever reported. If an accepted sensing voltage variation is larger than 3 (5) V, the detecting limit is 49.1 (98.9) ppm. Moreover, voltage transient response and current transient response to various hydrogen-containing gases were experimentally studied. The new finding is that the former response time is shorter than the latter one. Other dynamic measurements by switching voltage polarity and/or continuously changing hydrogen concentration were addressed, showing the proposed sensor is a good candidate for commonly used MS sensors.     

Characteristics of resistivity-type hydrogen sensing based on palladium-graphene nanocomposites

We describe the characteristics of resistivity-type hydrogen (H₂) sensors made of palladium (Pd)-graphene nanocomposites. The Pd-graphene composite was synthesized by a simple chemical route capable of large production. Synthesis of Pd nanoparticles (PdNPs) of various sizes decorated on graphene flakes were easily controlled by varying the concentration of Pd precursors. Resistivity H₂ sensors were fabricated from these Pd-graphene composites and evaluated with various concentrations of H₂ and interfering gases at different temperatures. Characteristics for sensitivity, selectivity, response time and operating life were studied. The results from testing the Pd-graphene indicated a potential for hydrogen sensing materials at low temperature with good sensitivity and selectivity. Specifically H₂ was measurable with concentrations ranging from 1 to 1000 ppm in laboratory air, with a very low detection limit of 0.2 ppm. The response of the sensors is almost linear. The resistivity of sensors changed approximately 7% in its resistance with 1000 ppm H₂ even at room temperature. The robust mechanical properties of graphene, which supported these PdNPs, exhibit structural stability and durability in H₂ sensors for at least six months. Moreover, the advantages in this work are experimental reproducibility in synthesis Pd-graphene composite and sensor fabrication process.     

Explosion mechanism of aluminum powder mixed with low-concentration hydrogen

This paper examines dust explosion characteristics and mechanisms for aluminum powder (Alp) in four different low-concentration hydrogen environments (0%, 1%, 2%, and 4 vol%). The aluminum powder's combustion characteristics were determined and the minimum ignition temperature and minimum ignition energy were found to be 690 °C and 45 mJ, respectively. A 20-L apparatus was used to evaluate how the explosive characteristics varied under different hydrogen concentrations. The explosion pressure generated by 4 vol% H₂ was ca. 8.3% stronger than an explosion in the air, and the explosion pressure rise rate showed an increase of 48%. In addition, the time to maximum rate of pressure rise was shortened in the mixed Alp/hydrogen experiment. The experimental residues were observed, and from this it was inferred that the explosion mechanism changed when a low concentration of H₂ was present.     

Hazard assessment of complex hydrides as hydrogen storage materials

Hydrogen storage materials containing NaAlH₄, LiNH₂, Mg(NH₂)₂, LiH and LiBH₄ were subjected to standardized safety tests in order to assess the potential hazards caused by the environmental exposure of these materials. All the materials were judged ‘flammable’, ‘pyrophoric’ and ‘water-reactive’, resulted in being classified as the United Nations Packing Group I, the most stringent category of container regulations in transporting these materials. A small spark energy (1.4 mJ) can trigger an intense dust cloud explosion of the Mg(NH₂)₂ + LiH system of which the minimum explosive concentration was determined to be 90 mg dm⁻³. Although this value is lower than those of the hydrogen storage alloys, the minimum explosive concentration of complex hydrides can be comparable to the alloys if expressed in terms of the amount of stored hydrogen in the material. Also examined was the eruption test, a non-standard test, in which the sample powder was pushed out of a container into the atmosphere by pressurized H₂. Despite the pyrophoricity, we observed only one explosion of the Ti-doped NaAlH₄ in dozens of trials using all the materials. A comparison with other materials points to the inevitability of more cautious measures than metal hydrides when handling these complex hydrides.     

Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor

Highly-ordered, vertically oriented TiO₂ nanotubes are synthesized, and their hydrogen sensing properties are investigated. Self-organized TiO₂ nanotube arrays are grown by anodic oxidation of a titanium foil in an aqueous solution that contains 1 wt% hydrofluoric acid at 20 °C. We use a potential ramp at a rate of 100 mV s⁻¹, increasing from the initial open-circuit potential (OCP) to 20 V, and this final potential of 20 V is then held constant during the anodization process. The fabricated TiO₂ nanotubes are approximately 1 μm in length and 90 nm in diameter. For the sensor measurements, two platinum pads are used as electrodes on the TiO₂ nanotube arrays. The hydrogen sensing characteristics of the sensor are analyzed by measuring the sensor responses ((I − I₀)/I₀) in the temperature interval of 20–150 °C. We find that the sensitivity of the sensor is approximately 20 for 1000 ppm H₂ exposure at room temperature, and increases with increasing temperature. The sensing mechanism of the TiO₂ nanotube sensor could be explained with chemisorption of H₂ on the highly active nanotube surface.     

An excellent room-temperature hydrogen sensor based on titania nanotube-arrays

Hydrogen sensors have been fabricated from highly ordered TiO₂ nanotube arrays through anodization of a Ti substrate in an ethylene glycol solution containing NH₄F. The vertically oriented TiO₂ nanotube arrays containing Pt electrodes exhibit an ability to detect a wide-range of hydrogen concentrations at room temperature. On exposure to 2000 ppm (parts per million) hydrogen, the sensors exhibit seven orders of magnitude change in resistance with a response time of 13 s at room temperature. The TiO₂ nanotube arrays sensor equipped with Pt electrodes exhibited a diode-type current–voltage (I–V) characteristic in air, but nearly ohmic behavior in hydrogen balanced with argon. A significant response to hydrogen was observed without the presence of oxygen in the base atmosphere. The response of two kinds of sensors with either Pt or Pt/Ti electrodes to 500 ppm hydrogen was measured and the results suggested that the excellent hydrogen sensing properties in air resulted primarily from the variation of the Schottky barrier height at the Pt/TiO₂ interface.     

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.     

The effect of ambient contamination on PEMFC performance

The effect of environmental contamination (NO_x, SO₂) on the performance of proton exchange membrane fuel cells (PEMFC) was studied. The performance of PEMFCs was tested for 100 h with different cathode reactants. According to the Ambient Air Quality Standard of PRC, three kinds of cathode gases were applied to operate the fuel cells, which were 1 ppm NO₂/air, 1 ppm SO₂/air and a mixture of contaminant gases. The gas mixture contained 0.8 ppm NO₂, 0.2 ppm NO and 1 ppm SO₂. Finally, the poisoning behavior and the mechanisms were analyzed by constant-current discharging and cycle voltammetry (CV). During the 100 h test, the potentials of the fuel cell degraded by 65%, 77% and 90% with 1 ppm SO₂/air, a gas mixture and 1 ppm NO₂/air, respectively.     

MEMS-microhotplate-based hydrogen gas sensor utilizing the nanostructured porous-anodic-alumina-supported WO3 active layer

Practical microsensors for fast, highly sensitive hydrogen gas detection were fabricated by combining silicon integral technology for MEMS microhotplate platform with newly developed technological, electrical, and electrolytic conditions for forming nanostructured porous-anodic-alumina-templated WO₃ layer as the sensing material. The morphology–structure–property relationship for the nanostructured sensing layer was determined by scanning electron microscopy, X-ray diffraction, and through systematically investigating the sensor performance at various H₂ concentrations (5–1000 ppm) and operating temperatures (20–350 °C). The sensors showed superior sensitivity to hydrogen gas, with the lowest detection limit ever reported for WO₃ semiconductors (5 ppm), the fast response and recovery times (2–3 min), and the best sensitivity at 150 °C, which was 100 times higher than that of a reference sensor having a smooth WO₃ active film. The technology developed enables high-volume, low-cost, and low-power sensor-on-a-chip solution for a hydrogen-based energy economy where the use of highly sensitive and low-power-consuming devices is encouraged.     

Pd doped WO3 films prepared by sol–gel process for hydrogen sensing

The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride salt was dissolved in the sol with different Pd:W molar ratios and coated on Al₂O₃ substrates by spin coating method. XRD and XPS techniques were used to analyze the crystal structure and chemical composition of the films before and after heat treatment at 500 °C. We observed that Pd can modify the growth kinetic of tungsten trioxide nanoparticles by reducing the crystallite size and as a result can improve hydrogen sensitivity. Resistance-sensing measurements indicated sensitivity of about 2.5 × 10⁴ at room temperature in hydrogen concentration of 0.1% in air. Considering all sensing parameters, an optimum working temperature of 100 °C was obtained.     

Novel hydrogen generator/storage based on metal hydrides

A novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and higher efficiency than a classical electrolyser. The prototype comprises a six-electrode cell assembly using an AB₅ type metal hydride and Ni plates as counter electrodes, in a KOH solution. Metal hydride electrodes with chemical composition LaNi₄.₃Co_0.4Al₀.₃ have been prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study of the alloy has been performed in a Sievert-type apparatus produced by Labtech Ltd. In the present prototype during charging, hydrogen is absorbed in the metal hydride and corresponding oxygen is conveyed out of the system. Conversely, in the case of discharging the hydrogen stored in the metal hydride it is released to an external H₂ storage. Released hydrogen is delivered into the hydrogen storage up to a pressure of 15 bar. It is anticipated that the device will be integrated as a combined hydrogen generator in a stand-alone system associated to a 1 kW fuel cell.     

Vented confined explosions involving methane/hydrogen mixtures

The EC funded Naturalhy project is assessing the potential for using the existing gas infrastructure for conveying hydrogen as a mixture with natural gas (methane). The hydrogen could then be removed at a point of use or the natural gas/hydrogen mixture could be burned in gas-fired appliances thereby providing reduced carbon emissions compared to natural gas. As part of the project, the impact on the safety of the gas system resulting from the addition of hydrogen is being assessed. A release of a natural gas/hydrogen mixture within a vented enclosure (such as an industrial housing of plant and equipment) could result in a flammable mixture being formed and ignited. Due to the different properties of hydrogen, the resulting explosion may be more severe for natural gas/hydrogen mixtures compared to natural gas. Therefore, a series of large scale explosion experiments involving methane/hydrogen mixtures has been conducted in a 69.3 m³ enclosure in order to assess the effect of different hydrogen concentrations on the resulting explosion overpressures. The results showed that adding up to 20% by volume of hydrogen to the methane resulted in a small increase in explosion flame speeds and overpressures. However, a significant increase was observed when 50% hydrogen was added. For the vented confined explosions studied, it was also observed that the addition of obstacles within the enclosure, representing congestion caused by equipment and pipework, etc., increased flame speeds and overpressures above the levels measured in an empty enclosure. Predictions of the explosion overpressure and flame speed were also made using a modified version of the Shell Global Solutions model, SCOPE. The modifications included changes to the burning velocity and other physical properties of methane/hydrogen mixtures. Comparisons with the experimental data showed generally good agreement.     

A study of combined biomass gasification and electrolysis for hydrogen production

An integrated system for the production of hydrogen by gasification of biomass and electrolysis of water has been designed and cost estimated. The electrolyser provides part of the hydrogen product as well as the oxygen required for the oxygen blown gasifier. The production cost was estimated to 39 SEK/kg H₂ at an annual production rate of 15?000 ton, assuming 10% interest rate and an economic lifetime of 15 years. Employing gasification only to produce the same amount of hydrogen, leads to a cost figure of 37 SEK/kg H₂, and for an electrolyser only a production cost of 41 SEK/kg H₂. The distribution of capital and operating cost is quite different for the three options and a sensitivity analyses was performed for all of these. However, the lowest cost hydrogen produced with either method is at least twice as expensive as hydrogen from natural gas steam reforming.     

Development of a hydrogen dual sensor for fuel cell applications

The development and application of a hydrogen dual sensor (HDS) for the application in the fuel cell (FC) field, is reported. The dual sensing device is based on a ceramic platform head with a semiconducting metal oxide layer (MOx) printed on Pt interdigitated contacts on one side and a Pt serpentine resistance on the back side. MOx layer acts as a conductometric (resistive) gas sensor, allowing to detect low H₂ concentrations in air with high sensitivity and fast response, making it suitable as a leak hydrogen sensor. The proposed Co-doped SnO₂ layer shows high sensitivity to hydrogen (R₀/R = 90, for 2000 ppm of H₂) at 250 °C in air, and with fast response (<3 s). Pt resistance serves as a thermal conductivity sensor, and can used to monitor the whole range of hydrogen concentration (0–100%) in the fuel cell feed line with short response-recovery times, lower than 13 s and 14 s, respectively. The effect of the main functional parameters on the sensor response have been evaluated by bench tests. The results demonstrate that the dual sensor, in spite of its simplicity and cheapness, is promising for application in safety and efficiency control systems for FC power source.     

Microfibrous structured catalytic packings for miniature methanol fuel processor: Methanol steam reforming and CO preferential oxidation

The microfibrous structured catalytic packings for miniature fuel processor consisting of a methanol steam reformer and a subsequent CO cleanup train has been investigated experimentally. A highly void and tailorable sinter-locked microfibrous carrier consisting of 3.5 vol% 8 μm diameter Ni-fibers is used to entrap 35 vol% 150-250 μm catalyst particulates for both methanol steam reforming (MSR) and CO preferential oxidation (PROX). We demonstrate a microfibrous entrapped Pd-ZnO/Al₂O₃ catalyst packings for high efficiency hydrogen production by the MSR reaction. The use of microfibrous entrapment technology significantly enhances the catalyst utilization efficiency by a 4-fold improvement of the weight hourly space velocity (WHSV), compared to the single Pd-ZnO/Al₂O₃ particulates as keeping the methanol conversion at >98%. The microfibrous entrapped Pt-Co/Al₂O₃ catalyst packings can drive the CO from 2% down to <50 ppm at 150 °C with O₂/CO ratio of 1 using a gas hourly space velocity (GHSV) of 32,000 h⁻¹. Finally, a prototype fuel processor system integrating MSR reformer and CO PROX train is demonstrated as three reactors in series. Such test rig is capable of producing roughly 1700 standard cubic centimeter per minute (sccm) PEMFC-grade H₂ (equivalent to ∼163 W of electric power) in a longer-term test, in which the MSR reactor is operated at 300 °C using a methanol/water (1/1.1, mole) mixture WHSV of 9 h⁻¹ and CO PROX reactors at 150 °C using an O₂/CO molar ratio of 1.3, respectively. In the test of this prototype system, MSR reactor delivers >97% methanol conversion throughout the entire 1200-h test; the CO cleanup train placed in line after 800-h MSR illustrates the capability to decrease the CO concentration from ∼3.5% to ∼1% at PROX-1 and then to less than 20 ppm at PROX-2 until to the end of test.     

TBAOH intercalated WO3 for high-performance optical fiber hydrogen sensor

Considering the poor response performance of fiber bragg grating (FBG) sensor based on WO₃, in this paper, a polymer intercalated method is proposed to accomplish high-performance optical fiber hydrogen sensor. Tetrabutylammonium hydroxide (TBAOH) molecule intercalated WO₃ are synthesized successfully and the effect of the intercalation amount to sensing performance has been investigated. The experiment results show that, at the concentration of 10,000 ppm, the t₉₀ response time of the sensor based on TBAOH-Pt/WO₃ is improved by 308% compared with that of WO₃ FBG sensor. In addition, the FBG hydrogen sensor based on the novel materials can detect hydrogen concentration ranging from 300 ppm to 12,000 ppm in air, demonstrating excellent stability and repeatability, which show wide application prospect in the future.     

Changes in hydrogen storage properties of carbon nano-horns submitted to thermal oxidation

The effect of thermal oxidation on the hydrogen storage properties of carbon nano-horns was investigated by gravimetric and electrochemical methods. The pristine nano-horn sample was oxidised at 673 K in air for different periods (15, 30 and 60 min) and the resulting materials were characterised. The N₂ adsorption experiments reveal a marked increase in the surface area, from 267 m² g⁻¹, for the pristine sample, up to 1360 m² g⁻¹ for the sample oxidised for the 60 min period, and a reduction in the average pore diameter. The gravimetric investigation, conducted at low temperature (77 K) showed an increase in the hydrogen storage, from 0.75 wt% for the pristine sample up to 2.60 wt% for the oxidised material. Reproducible and stable hydrogen storage was found for all the samples examined apart from the sample oxidised for 60 min. For the latter, a decrease in the amount of hydrogen stored between the first and second cycles was found. Electrochemical loading of hydrogen in the samples was performed at room temperature (298 K) in alkaline solution by the galvanostatic charge/discharge technique. The results obtained here however show a much lower hydrogen storage level by the samples as compared to the gas storage method, with a maximum value of 0.124 wt% H₂ and with very little dependence on the thermal oxidation treatment.     

Hydrogen purification using compact pressure swing adsorption system for fuel cell

Adsorption of CO and CO₂ in mixtures of H₂/CO/CO₂ was achieved using compact pressure swing adsorption (CPSA) system to produce purified hydrogen for use in fuel cell. A CPSA system was designed by combining four adsorption beds that simultaneously operate at different processes in the pressure swing adsorption (PSA) process cycle. The overall diameter of the cylindrical shell of the CPSA is 35 cm and its height is 40 cm. Several suitable adsorbent materials for CO and CO₂ adsorption in a hydrogen stream were identified and their adsorption properties were tested. Activated carbon from Sigma–Aldrich was the adsorbent chosen. It has a surface area of 695.07 m²/g. CO adsorption capacity (STP) of 0.55 mmol/g and CO₂ at 2.05 mmol/g were obtained. The CPSA system has a rapid process cycle that can supply hydrogen continuously without disruption by the regeneration process of the adsorbent. The process cycle in each column of the CPSA consists of pressurization, adsorption, blowdown and purging processes. CPSA is capable of reducing the CO concentration in a H₂/CO/CO₂ mixture from 4000 ppm to 1.4 ppm and the CO₂ concentration from 5% to 7.0 ppm CO₂ in 60 cycles and 3600 s. Based on the mixture used in the experimental work, the H₂ purity obtained was 99.999%, product throughput of 0.04 kg H₂/kg adsorbent with purge/feed ratio was 0.001 and vent loss/feed ratio was 0.02. It is therefore concluded that the CPSA system met the required specifications of hydrogen purity for fuel cell applications.