Enhancement of the room-temperature hydrogen sensing performance of MoO3 nanoribbons annealed in a reducing gas

Uniform-sized orthorhombic MoO₃ nanoribbons were synthesized by a simple hydrothermal method at 240 °C. The nanoribbons grew along the [001] orientation, with average length, width and thickness of approximately 20 μm, 270 nm and 90 nm, respectively. The obtained nanoribbons were further annealed in a hydrogen atmosphere at different temperatures to modify their surface states. The treatment of the nanoribbons at 300 °C significantly elevated the concentration of non-stoichiometric Mo⁵⁺ to 24.7%, much larger than the original concentration (∼14.8%). A positive relationship was found between the non-stoichiometric Mo⁵⁺, chemisorbed oxygen ion and sensor response. The sensor based on the MoO₃ nanoribbons treated at 300 °C exhibited a faster response time of approximately 10.9 s, and a higher sensor response of 17.3 towards 1000 ppm H₂, compared with the results of original tests (∼21 s and ∼5.7, respectively), indicating the significantly improved gas sensing performance of the treated MoO₃. Meanwhile, the sensor also exhibited excellent repeatability and selectivity toward hydrogen gas. The enhancement of the hydrogen gas sensing performance of treated MoO₃ nanoribbons was attributed to the more effective adjustment of the width of the depletion region on the nanoribbon surface and the height of the potential barrier at the junctions, induced by the interaction between hydrogen molecules and higher-concentration oxygen ions. Our research implied that the gas sensing performance of nanostructured metal oxides could be successfully enhanced through annealing in the reducing gas.     

Understanding on the hydrogen detection of plasma sprayed tin oxide/tungsten oxide (SnO2/WO3) sensor

The plasma spray technique was well proven in producing metal oxide based gas sensors in the last two decades using different powder feedstocks. However, limited research was made to fabricate hydrogen gas sensor from tin oxide layer coated over tungsten oxide layer. This paper attempts to interpret the hydrogen gas sensing performances of plasma sprayed coating derived by depositing tin oxide layer over tungsten oxide (SnO₂/WO₃) layer. Plasma sprayed SnO₂/WO₃ sensor showed maximum response of 90% at 150 °C in contrast to stand-alone WO₃ (89% at 350 °C) and stand-alone SnO₂ (89% at 250 °C). The lower operating temperature of SnO₂/WO₃ sensor without compromising gas response was attributed to the WO₃–SnO₂ hetero-junction. SnO₂/WO₃ sensor showed selective sensing towards hydrogen with respect to carbon monoxide and methane gases. This sensor also possessed repeatable characteristics after 39 days from the initial measurement. In a nut-shell, plasma spayed SnO₂/WO₃ sensor showed stability of base resistance, repeatability after successive response and recovery cycles, selective sensing towards 500 ppm H₂ with significant magnitude of gas response of 90%, response time of 35 s and recovery time of 269 s at a temperature of 150 °C.     

Hydrogen embrittlement and associated surface crack growth in fine-grained equiatomic CoCrFeMnNi high-entropy alloys with different annealing temperatures evaluated by tensile testing under in situ hydrogen charging

The effect of the annealing temperature after cold rolling on hydrogen embrittlement resistance was investigated with a face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy using tensile testing under electrochemical hydrogen charging. Decreasing annealing temperature from 800 °C to 750 °C decreased grain sizes from 3.2 to 2.1 μm, and resulted in the σ phase formation. Interestingly, the specimen annealed at 800 °C, which had coarser grains, showed a lower hydrogen embrittlement susceptibility than the specimen annealed at 750 °C, although hydrogen-assisted intergranular fracture was observed in both annealing conditions. Because the interface between the FCC matrix and σ was more susceptible to hydrogen than the grain boundary, the presence of the matrix/σ interface significantly assisted hydrogen-induced mechanical degradation. In terms of intergranular cracking, crack growth occurred via small crack initiation near a larger crack tip and subsequent crack coalescence, which has been observed in various steels and FCC alloys that contained hydrogen.     

Temperature dependence of sensing properties of hydrogen-sensitive extended-base heterojunction bipolar transistors

A planar-type metal-semiconductor-metal (MSM) hydrogen sensor forming on the collector layer was employed as an extended base (EB) of the InGaP-GaAs heterojunction bipolar transistors (HBTs). Then, hydrogen sensing transistors integrated were proposed and studied. After introducing sensing properties of the EB-hydrogen sensor, various sensing current gains defined were addressed for our hydrogen sensing transistor. Instead of the base current, N₂ and/or hydrogen-containing gases were used as a parameter while measuring common-emitter characteristics of the hydrogen sensing transistor at various temperatures. Experimental results show that maximum sensing base current gains in 1% H₂/N₂ is 330 at 25 °C while it is enhanced to 1800 at 50 °C, then to 2300 at 80 °C, and finally to 2800 at 110 °C. In contrast, a peak sensing collector current gain is as high as 1.2 × 10⁵ (4.3 × 10⁴) in 1% (0.01%) at 110 °C. In addition, response times obtained from the sensing diode (base) and collector currents in 0.01% H₂/N₂ are 485 (490) and 745 s at 25 °C. Together with important features including one power supply and low-power consumption, the proposed hydrogen sensing transistor is very promising for applications in detecting hydrogen.     

Evolution of Ti2Ni precipitations during annealing and their effects on properties of Ti–Nb–Ni foil as PEMFC bipolar plates substrate

In this work, Ti–Nb–Ni foil was cold rolled and different annealing temperatures were conducted. For foils annealed at 600 °C, 650 °C and 700 °C, Ti₂Ni particles were evenly distributed in the matrix. Grain growth was obviously confined during annealing at these temperatures. While more (α+Ti₂Ni) eutectoids along grain boundaries (GBs) formed in the cases of higher temperatures. Plasticity of as-cold rolled foil was obviously improved by annealing and elongations gradually decreased with annealing temperature. Elongations of foil at 600 °C possessed the highest value, viz, 31.6% and 39.8% for RD and TD. For corrosion resistance, 850 °C annealed foil had the lowest i_corr compared with other counterparts. ICR of as-cold rolled and annealed specimens at 1.4 MPa were all lower than 10 mΩ cm⁻², which satisfied the DOE 2025 target.     

Enhanced response speed of SAW based hydrogen sensor employing a micro-heater

To feature fast hydrogen detection, a new design of surface acoustic wave (SAW) based hydrogen sensor integrated with a micro-heater is proposed in this paper. A 200 MHz delay-line patterned SAW sensing chip coated Pd/Ni alloy hydrogen sensitive film and a micro-heater are prepared on a Y35^oX quartz wafer. The hydrogen gas adsorption in Pd/Ni thin-film modulates the SAW propagation, and against the temperature interference, the corresponding changes in acoustic attenuation are collected as the sensor signal. The Micro-heater is designed to catalysis the gas-sensitive effect by regulating the working temperature, and the influence law of heating temperature towards response speed is revealed theoretically, allowing determination of the optimal working temperature. The experimental results verify the theoretical prediction. Fast response (t₉₀ < 2 s), lower power consumption (<1.35 W), and the low detection limit (<15 ppm) are achieved at low working temperature of 75 °C. Furthermore, very low crossed humidity sensitivity and excellent long-term stability are observed.     

Ultra-fast response and highly selectivity hydrogen gas sensor based on Pd/SnO2 nanoparticles

It is still a challenging task to achieve the rapid detection of hydrogen (H₂) with the rapid development of hydrogen energy sector. In this work, the H₂ sensing capabilities of pristine and Pd-modified SnO₂ nanoparticles with the size of ～7 nm were systematically evaluated. The SnO₂ nanoparticles were synthesized via hydrothermal method and Pd modification was performed using impregnation route. Pd modification remarkably upgraded the H₂ sensing performances compared with the pristine SnO₂ gas sensor. The working temperature of SnO₂ decreased from 300 °C to 125 °C after Pd loading. Among the prepared Pd/SnO₂ gas sensors, 0.50 at.% Pd/SnO₂ sensor exhibited the highest response magnitude of 254 toward 500 ppm H₂ and rapid response/recovery time of 1/22 s at 125 °C. The enhanced H₂ sensing capabilities by Pd modification may be related to the catalytic effect and the resistance modulation.     

Flexible hydrogen gas sensor based on a capacitor-like Pt/TiO2/Pt structure on polyimide foil

Metal oxide semiconductor gas sensors of hydrogen with a typical capacitor-like Pt/TiO₂/Pt electrode arrangement exhibit excellent sensitivity to hydrogen even at room temperature. At the same time, very similar Pt/TiO₂/Pt cells can also be used as memristive elements exhibiting resistive switching between two resistive states, which has been recently exploited to create a gas sensor with built-in memory. Merging of these two functionalities within a single device also opens new possibilities for smart gas sensor arrays. However, so far such sensors have been prepared only on rigid substrates. In this work, a flexible hydrogen gas sensor with such capacitor-like Pt/TiO₂/Pt electrode arrangement fabricated on polyimide foil is presented and characterized in terms of hydrogen gas sensing properties and bending endurance. The sensor exhibits high response (R_air/R_H2) of more than 10⁵ to 10 000 ppm H₂ at 150 °C with minor decline at elevated humidity and is capable of room temperature operation. The lowest detected concentration was 3 ppm at 150 °C and 300 ppm at room temperature in dry conditions. Bending the sensor 10⁵ times over diameter of 10 mm led to slight improvement of the sensing performance.     

In situ synthesis by organometallic method of vulcan supported PdNi nanostructures for hydrogen evolution reaction in alkaline solution

Pd and Pd-based catalysts for hydrogen production remain the best alternative to Pt substitution because of similarities in their electronic structure and more abundant reserves. In this work, it was carried out the in-situ synthesis of Pd_xNi_1-x/C electrode materials (x = 0, 30, 50, 70 and 100 wt%) by the displacement of ligands from organometallic compounds followed by an annealing process at 300 °C in Ar atmosphere. The electrocatalytic performance of these materials was evaluated on the hydrogen evolution reaction in alkaline medium (1 M KOH). The results showed that annealing process, after the synthesis of stabilized nanostructures by organometallic method, did not affect the particle size (4.27 ± 1.14 to 4.62 ± 1.59 nm) and dispersion (25.75–27.07%) of the alloyed nanostructures. The modification of Pd electronic features with low Ni amount facilitates the adsorption of the hydrogen on the bimetallic active surface of the catalysts. The PdNi alloys, especially Pd₇₀Ni₃₀/C, tend to display the overpotential of HER to more positive values in comparison with Pd/C catalysts. This behavior is clearly correlated with an improvement by the synergistic effect between the components, which in turn enhance the electrochemical surface area (ECSA) and the real area. Either the hydrogen adsorption resistance and charge transfer resistance are dependent of the Ni amount, due to Ni influences ion/atom recombination or hydrogen desorption.     

Hydrogen sensing properties of ZnO nanorods: Effects of annealing,temperature and electrode structure

In this study, the hydrogen (H₂) sensing properties of vertically aligned zinc oxide (ZnO) nanorods were investigated depending on annealing, Pd coating, temperature and electrode structure. ZnO nanorods were fabricated by using hydrothermal method on a glass substrate and an indium tin oxide (ITO) coated glass substrate. In order to determine the effects of annealing on the H₂ sensor performance, the nanorods were heated at 500 °C in dry air. H₂ sensing measurements were done in the temperature range of 25–200 °C. It was found that, the sensor response of Pd coated ZnO nanorods were much higher than the un-coated nanorods due to the catalytic effect of Pd thin film. Moreover, the un-annealed samples showed better sensor response than the annealed samples due to the number of oxygen deficiency. In addition, the lateral electrode structure showed higher sensor response than the sandwich electrode structure.     

Evaluating hydrogen detection performance of an electrospun CuZnFe2O4 nanofiber sensor

CuZnFe₂O₄ nanoparticles (<50 nm) are successfully synthesized and incorporated in polyvinyl alcohol (PVA) to fabricate nanofibers via electrospinning technique followed by calcination process under various temperatures. Scanning electron microscopy (SEM) is used to observe the morphological characteristics of both nanoparticles and nanofibers. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermo-gravimetric (TG) analyses along with energy dispersive X-ray spectrometer (EDX) analysis are conducted to evaluate structure and composition of the nanofibers, respectively. The results exhibit that the calcination temperature is substantially effective on nanofiber morphology and sensing performance in the context of forming grains (beads) on the nanofibers. The highest response and recovery performance values (response and recovery time of about 6.5 s) are obtained at the calcination temperature of 500 °C and sensor working temperature of 250 °C at 500 ppm of H2 gas concentration which also corresponds to 30% increment in detection performance compared to 300 °C-calcined nanofiber sensor. The sensor selectivity against various gases is also analyzed to compare the detection performance in air.     

Effective monitoring and classification of hydrogen and ammonia gases with a bilayer Pt/SnO2 thin film sensor

The monitoring and classification of different gases, such as H₂ and NH₃ using a low-cost resistive semiconductor sensor is preferred in practical applications in hydrogen energy, breath analysis, air pollution monitoring, industrial control, and etc. Herein, porous bi-layer Pt/SnO₂ thin film sensors were fabricated to enhance H₂ and NH₃ sensing performance for effective monitoring and classification. Different Pt film thicknesses of 2, 5, 10, and 20 nm were deposited on 150 nm SnO₂ film-based sensors by sputtering method to optimize the response to H₂ and NH₃ gases. Gas sensing results showed that the fabricated Pt/SnO₂ films significantly improved the sensor response to NH₃ and H₂ compared to pure SnO₂ thin film. The sensors based on 5 and 10 nm Pt catalyst layers presented the highest responses to H₂ and NH₃, respectively. The optimal working temperature for NH₃ was in the range from 250 °C to 350 °C, and that for H₂ gas is less than 200 °C. The response of Pt/SnO₂ sensors to CH₄, CO, H₂S, and liquefied petroleum gas was much lower than that to NH₃ and H₂ supporting the high selectivity. On the basis of sensing results at different working temperatures or Pt thicknesses, we applied a radar plot and linear discriminant analysis methods to distinguish NH₃ and H₂. The results showed that H₂ and NH₃ could be classified without any confusion with different Pt layer thicknesses at a working temperature of 250 °C.     

Unidirectional sensing characteristics of structured Au–GaN–Pt diodes for differential-pair hydrogen sensors

A new metal-semiconductor-metal (MSM) hydrogen sensor was proposed to avoid (or to reduce) false alarms due to temperature drift when it is used in differential-pair hydrogen-sensing systems. A GaN semiconductor layer together with Pt as catalytic metal and Au as Schottky metal was employed to structure an Au–GaN–Pt MSM sensor. In particular, the structured Au–GaN–Pt MSM sensor can function as an active sensor and a reference sensor, depending on the polarity of applied voltage, in a differential-pair sensing circuit. Possible sensing mechanisms associated with the Au–GaN–Pt MSM sensor were described first to include band diagrams and graphical analysis. Experimental results reveal that an active sensor by forward-biasing the Au–GaN–Pt MSM sensor responses well to hydrogen-containing gases (50, 500, and 5000 ppm H₂/N₂) at various temperatures (25 °C, 50 °C, 70 °C, and 90 °C). High sensing current gains over 10⁴ were obtained. Further, the Au–GaN–Pt MSM sensor can also be reverse-biased to act as a reference sensor which shows negligible responses to hydrogen-containing gases. The differential-pair sensing circuit with the proposed Au–GaN–Pt MSM sensor reduces false alarms due to ambient temperature variation while it provides a short detection time.     

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.     

Hydrogen production enhancement using hot gas cleaning system combined with prepared Ni-based catalyst in biomass gasification

This paper investigates the hot gas temperature effect on enhancing hydrogen generation and minimizing tar yield using zeolite and prepared Ni-based catalysts in rice straw gasification. Results obtained from this work have shown that increasing hot gas temperature and applying catalysts can enhance energy yield efficiency. When zeolite catalyst and hot gas temperature were adjusted from 250 °C to 400 °C, H₂ and CO increased slightly from 7.31% to 14.57%–8.03% and 17.34%, respectively. The tar removal efficiency varies in the 70%–90% range. When the zeolite was replaced with prepared Ni-based catalysts and hot gas cleaning (HGC) operated at 250 °C, H₂ contents were significantly increased from 6.63% to 12.24% resulting in decreasing the hydrocarbon (tar), and methane content. This implied that NiO could promote the water-gas shift reaction and CH₄ reforming reaction. Under other conditions in which the hot gas temperature was 400 °C, deactivated effects on prepared Ni-based catalyst were observed for inhibiting syngas and tar reduction in the HGC system. The prepared Ni-based catalyst worked at 250 °C demonstrate higher stability, catalyst activity, and less coke decomposition in dry reforming. In summary, the optimum catalytic performance in syngas production and tar elimination was achieved when the catalytic temperature was 250 °C in the presence of prepared Ni-based catalysts, producing 5.92 MJ/kg of lower heating value (LHV) and 73.9% tar removal efficiency.     

Simulation and experimental study of the NOx reduction by unburned H2 in TWC for a hydrogen engine

The unburned H₂ can be used to reduce NO emission in conventional TWC (three-way catalyst) for a hydrogen internal combustion engine when it works at equivalence ratio marginally higher than the stoichiometric ratio. To explore the effects and feasibility of this reaction, a Perfectly Stirred Reactor simulation model of TWC has been built with simplified mechanisms. Experiments on a 2.3 L turbocharged hydrogen engine are used to verify the conclusion. It shows that rising initial temperature accelerates the reduction of NO and the maximum reaction rate occurs at 400 °C temperature. The conversion efficiency of NO remains approximately 0 when temperatures below 300 °C. The efficiency reaches a peak value of approximately 98% with 400 °C and declines gradually. The unburned H₂ to NO mixing ratio greater than 1.5 in TWC guarantees 100% NO conversion efficiency. The experiments indicate that the NOx concentration decreases from 2056 ppm to 41 ppm at the stoichiometric ratio after the treatment of TWC and NOx reaches 0 ppm with a rich ratio. Results also demonstrate that the suitable reaction temperatures for TWC locate in the range of 400 °C–500 °C. Therefore, if the temperature and the mixing ratio are appropriate, it can achieve zero emissions with NOx reduction by unburned H₂ in conventional TWC for a hydrogen engine.     

Organic–inorganic composites based on room temperature ionic liquid and 12-phosphotungstic acid salt with high assistant catalysis and proton conductivity

Proton-conducting composite material was synthesized from 1-butyl-3-methyl-imidazolium chloride (BMImCl) and 12-phosphotungstic acid (PWA). The structure, assistant catalytic effect and ionic conductivity of the composites for the as-synthesized, 200 and 400 °C annealed samples were studied, respectively. The as-synthesized salt was crystal and kept Keggin structure even being annealed at 400 °C, but the organic part was partly decomposed with increasing of the annealing temperature. The partly decomposed BMIm/PWA salt formed by annealing at 400 °C associated with Pt catalyst had excellent assistant catalytic effect on methanol electro-oxidation and displayed a high proton conductivity of 2 mS cm⁻¹ at 30 °C under 96% relative humidity condition.     

A small pilot reactor containing active MgTi–based hydrides at low operational temperatures

This study was investigated to utilize innovatively oil-free diaphragm pump to forcibly desorb the hydrogen from the small pilot MgH₂–TiH₂ based hydride reactor below the theoretical temperature of 278 °C. Active MgH₂-0.1TiH₂ composites were prepared using ball milling. Their hydrogenation performances at 25–300 °C were measured under a constant H₂ flow mode using a modified Sieverts apparatus. The dehydrogenation rates at 250–350 °C with or without diaphragm pump were investigated to examine whether the pilot reactors could be integrated with a proton exchange membrane fuel cell (PEMFC) for power generation. At a H₂ flow rate of 25 ml min⁻¹ g⁻¹, the reactors exhibited excellent hydrogenation, achieving gravimetric hydrogen storage capacities of 2.9–5.2 wt% (excluding the weight of the reactors) at 25–300 °C after 22 min. All hydrided MgTi–based reactors could be dehydrogenated at 250 °C at an average rate of 5 ml min⁻¹ g⁻¹ under vacuum. This is the first demonstration of Mg-based reactors that were hydrogenated at 100 °C and dehydrogenated at 250 °C to power a small PEMFC, yielding a measured conversion efficiency of 18%.     

Two operating modes of palladium film hydrogen sensor based on suspended micro hotplate

Palladium film hydrogen sensor based on suspended micro hotplate has been fabricated to operate at elevated temperature with low power consumption. Below 150 °C, the response of the sensor to H₂ is represented by an increase in resistance. At higher temperature, the phenomenon of resistance reduction appears when it comes into contact with H₂. We have researched the reasons for this phenomenon and proposed that the sensitive mechanism is the redox reaction of Pd film on the suspended structure. The suspended substrate can affect the temperature at which redox of the Pd film occurs, and be sensitive to the changes of the surrounding gas stream. When the working temperature is 400 °C, the magnitude of response (S) changes to −0.4% within 2 s for 200 ppm H₂, and S changes to −3% within 10 s for 4000 ppm H₂. This micro hotplate based hydrogen sensor can control the range of operating temperature according to the performance requirements.     

Cobalt oxide as photocatalyst for water splitting: Temperature-dependent phase structures

This study investigated the best phases of cobalt oxide for the photochemical and photoelectrochemical (PEC) water-splitting reaction. Cobalt oxide was produced via a hydrothermal process of cobalt nitrate hexahydrate and then annealed at different temperatures from 450 °C to 950 °C. The Co₃O₄ phase was produced during pre-annealing and annealing at 450 °C. The mixed phase of Co₃O₄ and CoO was produced during annealing at 550 °C and 650 °C, and pure CoO was produced during annealing from 750 °C to 950 °C. The Co₃O₄ phase produced the highest photocurrent density with a value of 1.15 mA cm⁻² at a −0.4 V potential bias vs. Ag/AgCl. This value two times higher than that reported by other researchers at the same potential bias. Furthermore, the highest rate of hydrogen collected by Co₃O₄ was ~272.6 μmol h⁻¹ g⁻¹ after 8 h photocatalytic process. The amount of collected hydrogen was stable until 12 h of the process.