H2 Sensor Research Articles

A sub-second response/recovery hydrogen sensor based on multifunctional palladium oxide modified heterojunctions

H2 possesses a wide range of applications. Due to the low ignition energy of H2 and the low explosion limit in air, it is necessary to establish rapid detecting systems. However, the sub-second response/recovery is still a serious issue for H2 sensors. In this work, a sub-second response/recovery H2 sensor based on multifunctional PdO modified SnO2/ZnO heterojunctions is successfully fabricated. The Pd-SnO2/ZnO sensors exhibit the response/recovery time of 0.8/0.8 s to 50 ppm H2, which are faster than the existing H2 sensors of analogous structures. The strong metal–support interaction between PdO and SnO2/ZnO enhances the presence of oxygen vacancy (VO), vacancy oxygen (Ov) and adsorbed oxygen (Oc) in the sensitive materials, which are crucial components for achieving rapid response/recovery times. Another function of PdO is that H2 is oxidized more easily when it was fixed on SnO2/ZnO. Multilevel heterojunction enriches the surface electrons of Pd-SnO2/ZnO, which is conducive to adsorb more oxygen molecules and target gas. Due to the multifunctional function of PdO and the modification of multilevel heterojunction, the response/recovery time of H2 sensors are shortened to sub-second. This study may provide a new approach for the design of fast response gas sensor, and Pd-SnO2/ZnO sensors have been shown to be potential candidates for real-time hydrogen monitoring.

High durability for PdNi nano-film hydrogen sensor

Stability is a critical parameter for hydrogen (H2) sensors. To enhance stability, we investigated the microstructure and morphology of palladium-nickel (PdNi) nano-film H2 sensors under various sputter pressures and annealing temperatures. Our findings indicate that an increase in the work sputtering pressure reduces the energy of the sputtering atom, resulting in the formation of small gaps between the PdNi nanoparticles at a pressure of 5 Pa. Furthermore, compared to the (111) peak position of pure Pd, the lattice constant of the prepared PdNi nano-film is significantly reduced. At 300 °C, the surface grains exhibit tip passivation. We conducted 20 cyclic tests for 1% H2 concentration of PdNi nano-film sensors, and our repeatability tests demonstrate that small cracks between two PdNi nanoparticles effectively alleviate particle expansion caused by H absorption. However, during the cycles of 1% H2 or N2, significant fluctuations were observed in the response resistance (Rs) and recovery resistance (Ra) during the stable phase. As the annealing temperature increases, Ra and Rs fluctuations decrease noticeably. Furthermore, the PdNi nano-film H2 sensor annealed at 300 °C exhibits high stability, relatively high Rs, short response time (trs), and short recovery time (trc).

Highly responsive hydrogen sensor based on Pd nanoparticle-decorated transfer-free 3D graphene

H2 sensors play a crucial role in the development of hydrogen (H2) energy and safety monitoring. Therefore, efficiently and rapidly detecting low concentrations of H2 is a fundamental challenge that needs to be addressed in the field of H2 sensors. In this study, we present a highly sensitive H2 gas sensor based on Pd-nanoparticles (NPs)-decorated transfer-free three-dimensional (3D) graphene. 3D graphene provides an increased surface area for enhanced gas adsorption and reaction kinetics. The integration of PdNPs improved the electron transport properties, resulting in a higher sensitivity. The PdNP-decorated 3D graphene sensor exhibited a remarkable gas response of 41.9% under 3% H2 exposure. The transfer-free synthesis process ensures the excellent conductivity of 3D graphene to further enhance its overall sensing performance. These discoveries propel current gas-sensing technologies forward and introduce fresh opportunities for the development of dependable sensors that can effectively enhance industrial safety and public security.

Ultrahigh sensitivity of hydrogen detection with a perforated Pd film on a miniature fiber tip

Hydrogen (H2) is a renewable energy gas and an important industrial raw material playing important roles in many fields. However, H2 is colorless, odorless and highly flammable within a wide concentration range. So H2 detection is very important for leakage monitoring especially at low concentration. Here we propose an ultrahigh sensitivity H2 sensor with a perforated palladium (Pd) film on the tip of an optical fiber. The suspended Pd nanohole film forms a Fabry-P é rot (FP) interferometer with a silica capillary fused onto the fiber. Upon H2 adsorption, the Pd nanohole film bends inwards leading to spectral shift of the FP cavity. With the film perforated, its Young’s modulus becomes effectively smaller giving rise to larger spectral shift and higher H2 sensitivity. We experimentally studied the effect of the structural parameters of the holey film on the H2 sensing performance and achieved a H2 detection sensitivity of 7.1 pm/ppm and a detection limit of 1.7 ppm. The sensor also shows good repeatability and gas selectivity and is expected to find applications as optical H2 sensors working at low concentrations.

Impact of gaseous interferents on palladium expansion for hydrogen optical sensing: A time stability study

Continuous monitoring of hydrogen (H2) concentration is critical for safer use, which can be done using optical sensors. Palladium (Pd) is the most commonly used transducer material for this monitoring. This material absorbs H2 leading to an isotropic expansion. This process is reversible but is affected by the interaction with interferents, and the lifetime of Pd thin films is a recurring issue. Fiber Bragg Grating (FBG) sensors are used to follow the strain induced by H2 on Pd thin films. In this work, it is studied the stability of Pd-coated FBGs, protected with a thin Polytetrafluoroethylene (PTFE) layer, 10 years after their deposition to assess their viability to be used as H2 sensors for long periods of time. It was found that Pd coatings that were PTFE-protected after deposition had a longer lifetime than unprotected films, with the same sensitivities that they had immediately after their deposition, namely 23 and 10 pm/vol% for the sensors with 150 and 100 nm of Pd, respectively, and a saturation point around 2 kPa. Furthermore, the Pd expansion was analyzed in the presence of H2, nitrogen (N2), carbon dioxide (CO2), methane (CH4) and water vapor (H2O), finding that H2O is the main interferent. Finally, an exhaustive test for 90 h is also done to analyze the long-term stability of Pd films in dry and humid environments, with only the protected sensor maintaining the long-term response. As a result, this study emphasizes the importance of using protective polymeric layers in Pd films to achieve the five-year lifetime required for a real H2 monitoring application.

Influence of the Cu doping on the physical and H2 gas sensing properties of TiO2

Due to poor selectivity, existing H2 sensors based on pure TiO2 nanostructures do not exhibit good efficiency at low gas concentrations. Therefore, many studies are carried out to improve the gas sensor properties of TiO2s. In this study, the structural and H2 gas sensing parameters of Cu doped TiO2 thin films obtained by using sputtering method were investigated as a function of Cu doping. In the co-sputtering system, direct current (DC) and radio frequency (RF) sources were used to simultaneously deposit of the Ti (fixed at 100 W in DC) and Cu (25, 35 and 45 W in RF). The structural, optical and morphological properties of the films were discussed in the light of XRD, XPS, UV-VIS, SEM and AFM results. XRD results revealed that the produced thin films have anatase TiO2 structure and the crystallite sizes in the most dominant A(110) plane are in the range of 14–18 nm. According to XPS results, copper ratios in Cu25, Cu35 and Cu45 samples were calculated as 0.433%, 1.598% and 2.233%, respectively. It was also found that CuO and Cu2O formations were formed in addition to the anatase phase only in Cu45 sample from these films. It was determined from the 2D and 3D AFM images that the surface roughness values of the films decreased from 63.92 nm to 12.99 nm with increasing Cu doping. In addition to the structural properties of the produced films, hydrogen gas detection performances at 1000 ppm gas concentration and 200 °C temperature were investigated with the current sensitive gas sensing system. The results reveal that Cu doping significantly improves the H2 gas responses and response times of TiO2s. Among the films produced, the highest gas response (2284) and shortest times (τres:128s, τrec:129s) was obtained in Cu45 thin film. This increase in gas responses was explained by the oxygen vacancy model and the results are presented by comparing pristine TiO2 and other 3d transition metal doped TiO2s in the literature.

Fabry-Pérot Cavities with Suspended Palladium Membranes on Optical Fibers for Highly Sensitive Hydrogen Sensing.

Hydrogen (H2) sensors are critical to various applications such as the situation where H2 is used as the clean energy for industry or the indicator for human disease diagnosis. Palladium (Pd) is widely used as the hydrogen sensing material in different types of sensors. Optical fiber H2 sensors are particularly promising due to their compactness and spark-free operation. Here, we report a Fabry-Pérot (FP)-cavity-based H2 sensor that is formed with a freestanding Pd membrane and integrated on a conventional single-mode optical fiber end. The freestanding Pd membrane acts both as the active hydrogen sensing material and as one of the reflective mirrors of the cavity. When the Pd film absorbs H2 to form PdHx, it will be stretched, resulting in a change of the cavity length and thus a shift of the interference spectrum. The H2 concentration can be derived from the amplitude of the wavelength shift. Experimental results showed that H2 sensors based on suspended Pd membranes can achieve a detection sensitivity of about 3.6 pm/ppm and a detection limit of about 3.3 ppm. This highly sensitive detection scheme is expected to find applications for sensing low-concentration H2.

Homojunction TiO2 thin film-based room-temperature working H2 sensors with non-noble metal electrodes

Metal oxide semiconductor heterojunctions and homojunctions are considered to have better performance than single phase material for room temperature hydrogen gas sensing. In this work, TiO2 facet homojunction (TiO2-FH) thin films were prepared by a two-step hydrothermal method. The TiO2-FH thin films were decorated with Pt nanoparticles using a green photodeposition method and then evaluated for H2 sensing. Non-noble metal Al was used as finger electrodes instead of noble metal Pt. Compared with the TiO2-FH sensor, the H2 detection limits of the platinum loaded TiO2-FH@Pt sensors are remarkably decreased from 8000 ppm to 1 ppm at room temperature. The optimized sensor achieves an excellent sensing performance of S = 1.21 and fast response / recovery times of about 42 s / 30 s when exposed to 1 ppm H2 concentration. The synergy between the Schottky barrier at the Pt/TiO2 interface and the barrier between the (110) and (001) facets of TiO2 is the key to the improvement of the sensing performance. Replacement of noble metal Pt with Al and C electrodes greatly help to clarify the mechanism of the junctions in the thin film sensors, especially the function of the Pt/TiO2 junction and the (110)/(001) TiO2 junction. The simple and green fabrication method and cheap Al interdigitated electrodes are advantageous for practical applications.

CeO2 nanorods decorated In2O3 nanoparticles for enhanced low temperature detection of hydrogen

Hydrogen (H2) detection sensors based on pure CeO2 nanorods (NRs) and In2O3-loaded CeO2 NRs with different In2O3 contents (5 wt%, 10 wt%, and 15 wt%) were investigated. The crystal phase evolution, morphology features, and response characteristics of the In2O3-loaded CeO2 NRs were analyzed in detail using characteristic techniques. Gas sensing results showed that the In2O3-loaded CeO2 gas sensors exhibited superior H2 sensing performances compared with pure CeO2 NRs under low operating temperature (100 °C). Notably, the gas sensors based on the 10 wt% In2O3-loaded CeO2 nanocomposite showed the most refined performance, with response values of 2.79 and 2.43 times higher than those of the pure CeO2 NRs sensors for 1000 ppm and 4000 ppm H2, respectively. The response values exhibited a linear relationship with the detection concentration. Moreover, the In2O3-loaded CeO2 gas sensor demonstrated enhanced H2 selectivity, good recyclability, and long-term stability. Finally, the improved gas sensing mechanism of the In2O3-loaded CeO2 gas sensor for H2 was also proposed.

Changes in the negative logarithm of end-tidal hydrogen partial pressure indicate the variation of electrode potential in healthy Japanese subjects

Molecular hydrogen (H2) is produced by human colon microbiomes and exhaled. End-tidal H2 sampling is a simple method of measuring alveolar H2. The logarithm of the hydrogen ion (H+)/H2 ratio suggests the electrode potential in the solution according to the Nernst equation. As pH is defined as the negative logarithm of the H+ concentration, pH2 is defined as the negative logarithm of the H2 effective pressure in this study. We investigated whether changes in pH2 indicated the variation of electrode potential in the solution and whether changes in end-tidal pH2 could be measured using a portable breath H2 sensor. Changes in the electrode potential were proportional to ({mathrm{pH}}_{2}-2times mathrm{pH}) in phosphate-buffered solution (pH = 7.1). End-tidal H2 was measured in the morning (baseline) and at noon (after daily activities) in 149 healthy Japanese subjects using a handheld H2 sensor. The median pH2 at the baseline was 4.89, and it increased by 0.15 after daily activities. The variation of electrode potential was obtained by multiplying the pH2 difference, which suggested approximately + 4.6 mV oxidation after daily activities. These data suggested that changes in end-tidal pH2 indicate the variation of electrode potential during daily activities in healthy human subjects.

Enhancing the performance of TiO2 nanotube-based hydrogen sensors through crystal structure and metal electrode

In this research, the effect of metal electrodes and crystalline phase on gas detection of titanium dioxide (TiO2) nanotube-based hydrogen (H2) sensors was investigated. TiO2 nanotubes were produced using glycerol-based electrolyte and annealed at 300 °C and 700 °C to change the anatase and rutile crystalline phases, respectively. TiO2 nanotubes were coated by platinum (Pt), palladium (Pd), gold (Au) and silver (Ag) electrodes to fabricate metal/TiO2 nanotubes Ti H2 sensor devices and then the current-voltage (I–V) characteristics were investigated at room temperature. The structural properties of TiO2 nanotubes were characterized by SEM, FE-SEM, XRD, and Raman techniques. The H2 detection properties of the sensors were examined at the 1000 ppm - 5% H2 concentration range. The crystal structure and metal electrodes are the main factors that affect the H2 sensing properties of TiO2 nanotube-based sensors. The effect of crystal forms on sensitivity was not the same as for metal electrodes. The underlying sensing mechanisms for different types of metal electrodes and crystal structures are discussed and the relevance of their sensing performance to nanotubes and electronic properties is investigated. In addition, discussion of each metal electrode and crystal structure will make important contributions to the development of H2 sensors. The Pd-coated device annealed at 700 °C showed the best detection performance.

High-performance H2 sensor based on Polyaniline-WO3 nanocomposite for portable batteries and breathomics-diagnosis of irritable bowel syndrome

Low-trace airborne hydrogen (H2) detection at room temperature is a prerequisite in designing sensors for portable batteries and breathomics-based diagnosis of human diseases/disorders. The state-of-the-art H2 sensor engineering is concerned with developing organic-inorganic nanocomposites with controlled morphology and enhanced physicochemical attributes. This unprecedented study reports a chemo-resistive-module H2 sensor based on polyaniline-tungsten oxide (PAni-WO3) nanocomposite fabricated through a facile and economic one-pot polymerization route. The morphological and structural analysis revealed the formation of nanocomposite consisting of WO3 nanorods (∼100 nm diameter) dispersed in the PAni matrix consists of particles with size around 200 nm. The PAni-WO3 chemiresistor exhibited a 5-fold enhanced performance towards one ppm of H2 at room temperature compared to pristine WO3 nanorods/PAni. It demonstrated high performance for room temperature H2 detection in terms of low detection limit (1 ppm), high sensitivity, prompt response, fast recovery, and high repeatability. Besides, the fabricated sensor is highly selective and stable for H2 monitoring in capricious environmental conditions (in varying humidity, temperature and interfering analytes). The well-engineered nanocomposite system possessing rational morphology has latent for real-time monitoring of low-trace human breath-borne H2 for non-invasive irritable bowel syndrome (IBS) diagnosis and for sensing leaks in portable batteries, hydrogen production and storage industries, and environmental monitoring.

Operando X-Ray Imaging of Gas-Electrolyte Behavior in the Zinc-Air Batteries

Zinc-air batteries have the potential to be widely used due to their high energy density, low cost, and safety. However, some problems with the electrolyte in the batteries have prevented their widespread use. For example, hydrogen evolution can occur in the zinc anode, reducing the battery's usable capacity. The oxygen reduction performance of the air electrodes is also a challenge, as the electrolyte, catalyst, and gas all interact with each other. Understanding the behavior of the electrolyte and gas is crucial to addressing these problems. Previous analyses, such as operando gas analysis, have provided some insights, but they have not been able to pinpoint the specific locations where these problems occur. More detailed, real-time observation of the cell, known as operando visualization, is needed to better understand these issues.In this study, x-ray imaging is conducted to visualize the electrolyte behavior inside a zinc-air battery. X-rays are highly permeable and nondestructive, allowing for real-time, operando observation of electrolytes in the battery. This allows for the simultaneous observation and comparison of the electrochemical performance of the cell and the behavior of the electrolyte during open circuit voltage (OCV), charging, and discharging.To obtain clear x-ray images, the x-ray permeable cell shown in fig.1 is designed with a small electrode size (2 x 10 mm) and an ABS polymer cell casing. This allowed for sufficient contrast in the acquired images, enabling the observation of the liquid electrolyte between the electrodes and inside the catalyst layer. The anode of the cell was made of zinc, and the cathode was a mixture of carbon black, PTFE binder, and LCCO (La0.6Ca0.4CoO3) catalyst that was coated onto carbon paper as gas diffusion layer (GDL) using a hot pressing process. A 6 mol/L KOH aqueous solution is used as the electrolyte. X-ray images were obtained using a Rigaku nano3DX and a molybdenum target. During the observation, the charge/discharge condition was controlled using a potentio-galvanostat. The hydrogen gas (H2) concentration is also monitored using an electrochemical sensor.Figure 2 shows x-ray images of the cell near the air electrode in four different conditions: (a) without electrolyte, (b) with electrolyte, (c) during the discharge phase, and (d) during the charging phase. Because liquid electrolyte absorbs more x-rays than air, the areas with liquid electrolyte appear darker in the images than the areas without electrolyte (Figure 2 (a) and (b)). During the discharge phase, some bright circular shapes appear, which are thought to be bubbles (Figure 2 (c)). These bubbles are also generated during the charging phase (Figure 2 (d)), and it appears that the formation of the bubbles is not dependent on the cell voltage or current density. The experimental data from the H2 sensor shows that H2 concentration increased during the discharge phase, indicating that the bubbles contain hydrogen gas.The hydrogen evolution reaction (HER) in zinc-air batteries is a natural reaction given the electrochemical potential of hydrogen and zinc. Until now, the HER problem has only been discussed in relation to the open circuit voltage (OCV) and the charging phase. It was previously thought that HER hardly occurs during the discharging phase because the cell voltage typically decreases at this time. However, the experimental data shows that HER does not stop during the discharging phase. In addition, the researchers observed that the bubbles formed during this phase disrupt ion transfer efficiency, causing a drop in cell voltage.This study demonstrates the successful visualization of electrolytes inside a zinc-air battery using x-ray imaging. In addition, the researchers observed unexpected hydrogen evolution during the discharging phase. Since HER has a significant impact on the lifetime and safety of the battery, the mechanism behind HER during the discharging phase is an important problem for zinc-air batteries to address. Figure 1

Review of the Status and Prospects of Fiber Optic Hydrogen Sensing Technology

With the unprecedented development of green and renewable energy sources, the proportion of clean hydrogen (H2) applications grows rapidly. Since H2 has physicochemical properties of being highly permeable and combustible, high-performance H2 sensors to detect and monitor hydrogen concentration are essential. This review discusses a variety of fiber-optic-based H2 sensor technologies since the year 1984, including: interferometer technology, fiber grating technology, surface plasma resonance (SPR) technology, micro lens technology, evanescent field technology, integrated optical waveguide technology, direct transmission/reflection detection technology, etc. These technologies have been evolving from simply pursuing high sensitivity and low detection limits (LDL) to focusing on multiple performance parameters to match various application demands, such as: high temperature resistance, fast response speed, fast recovery speed, large concentration range, low cross sensitivity, excellent long-term stability, etc. On the basis of palladium (Pd)-sensitive material, alloy metals, catalysts, or nanoparticles are proposed to improve the performance of fiber-optic-based H2 sensors, including gold (Au), silver (Ag), platinum (Pt), zinc oxide (ZnO), titanium oxide (TiO2), tungsten oxide (WO3), Mg70Ti30, polydimethylsiloxane (PDMS), graphene oxide (GO), etc. Various microstructure processes of the side and end of optical fiber H2 sensors are also discussed in this review.

Ultrafast response/recovery and high sensitivity of a hydrogen gas sensor at room temperature based on electrochemically deposited Sb2Te3/polystyrene composite film

A novel thermochemical hydrogen (TCH) sensor was fabricated from a thermoelectric (TE) film and platinum nanoparticle (Pt-NP)-decorated graphene catalyst applied onto the TE film, and its hydrogen (H2) sensing performance was systemically investigated. Two types of films comprising stoichiometric antimony telluride (Sb2Te3) and a Sb2Te3/polystyrene (PS) composite were synthesized by cost-effective electrodeposition on a silicon (Si) substrate. The resulting Sb2Te3/PS composite film played an important role in improving the H2 sensing signal. Specifically, the sensing signal of the optimized TCH sensor based on the Sb2Te3/PS composite film was 29.3 times higher than that of the Sb2Te3-film-based TCH sensor under 2.5 vol% H2/air at room temperature (RT). This phenomenon can be explained by the lower thermal conductivity of the Sb2Te3/PS composite film, increasing the temperature difference relative to that of the Sb2Te3 film. The R-squared correlation coefficient (R2) of the composite-film-based TCH sensor with a range of 400 ppm to 7 vol% H2/air was 0.9916. The best response time and recovery time of the composite-film-based sensor were 4 s and 3 s at 3 vol% H2/air, respectively. Our results prove that this sensor provides a wide detection range, short response/recovery time, and high sensitivity at RT, which shows its potential for use as a commercial H2 sensor for infrastructure applications.

The distinguished hydrogen sensibility by selecting diameters of single-walled carbon nanotubes and adding sensitive materials

Hydrogen sensors based on palladium (Pd)/semiconducting single-walled carbon nanotube(s-SWNTs) have showed great performance in sensitivity, limit of detection (LOD), and response time. With the development of research, it is proved that diameters and chirality of s-SWNTs strongly influence the detection. In this paper, we fabricated two kinds of sensors with different diameters, and that results showed response of sensor with 0.7–1.2 nm s-SWNTs was almost 6 times higher than that of the one with 1.2–1.6 nm s-SWNTs. The former could achieve the response of change ≈6.09 for 1 ppm H2 in N2, and the latter was ≈1.07. Furthermore, with Pd/HKUST-1 as functionalization for capturing more hydrogen (H2), the system formed a synergistic effect to enhance the change of resistance. The fabricated Pd/HKUST-1/s-SWNTs film (0.7–1.2 nm) H2 sensor presents a very fast recovery time of 8 s at 10 ppm and a detection limit of 1 ppm H2 in nitrogen(N2). In addition, in the air the sensor can also work. Its response time of the sensor to 0.05% H2 in air are 0.183 s. By choosing the diameter of s-SWNTs and adding H2-sensitive materials, the sensors are expected to achieve the fast response and recovery with high sensitivity and low LOD.

Atomic layer deposited SnO2/ZnO composite thin film sensors with enhanced hydrogen sensing performance

Hydrogen gas (H2) has long been regarded as a clean and renewable energy carrier alternative to the fossil fuel, therefore the development of highly sensitive and reliable hydrogen sensor is imperative for the large-scale implementation of green energy hydrogen. In this work, SnO2, ZnO thin film and SnO2/ZnO composite thin film H2 sensors were fabricated on SiO2/Si substrates by atomic layer deposition (ALD). The effect of ALD cycles and heat treatment temperatures on the sensing properties of the thin film sensors was investigated deeply. It is found that the 100-cycle SnO2/150-cycle ZnO composite film shows the best H2-sensing performance such as large response of 93 to 30 ppm H2 at 200 ℃, fast response/recovery time of 50 s/29 s, and low detection limit of 250 ppb. Moreover, the response of SnO2/ZnO composite film sensor is 10 times and 2 times higher than that of the pure ZnO and SnO2 thin film ones at working temperature of 200℃, respectively. A possible mechanism for performance improvement of the SnO2/ZnO composite film H2 sensor is proposed, attributed to the synergistic effect of the SnO2-ZnO heterojunction and the oxygen vacancies in the SnO2 film. ALD may be a feasible strategy to construct Si-based highly sensitive hydrogen sensors for micro-electromechanical system application.

Large-Area Printed Oxide Film Sensors Enabling Ultrasensitive and Dual Electrical/Colorimetric Detection of Hydrogen at Room Temperature.

Commercial hydrogen (H2) sensors operate at high temperatures, which increases power consumption and poses a safety risk owing to the flammable nature of H2. Here, a polymer-noble metal-metal oxide film is fabricated using the spin-coating and printing methods to realize a highly sensitive, low-voltage operation, wide-operating-concentration, and near-monoselective H2 sensor at room temperature. The H2 sensors with an optimized thickness of Pd nanoparticles and SnO2 showed an extremely high response of 16,623 with a response time of 6 s and a recovery time of 5 s at room temperature and 2% H2. At the same time, printed flexible sensors demonstrate excellent sensitivity, with a response of 2300 at 2% H2. The excellent sensing performance at room temperature is due to the optimal SnO2 thickness, corresponding to the Debye length and the oxygen and H2 spillover caused by the optimized coverage of the Pd catalyst. Furthermore, multistructures of WO3 and SnO2 films are used to fabricate a new type of dual-signal sensor, which demonstrated simultaneous conductance and transmittance, i.e., color change. This work provides an effective strategy to develop robust, flexible, transparent, and long-lasting H2 sensors through large-area printing processes based on polymer-metal-metal oxide nanostructures.

Highly stable and reversible hydrogen sensors using Pd-coated SnO2 nanorods and an electrode–substrate interface as a parallel conduction channel

We demonstrated highly stable and reversible H2 sensors using a dual electron-conduction channel comprising Pd-coated SnO2 nanorods (NRs) and a parallel conduction channel containing Cr/native oxide/p-type Si interface. The dual-channel sensor exhibited highly reversible resistance changes during H2 in-and-out cycles at operating temperatures of 25 °C− 100 °C, while typical single-channel sensors with SnO2 NRs fabricated on SiO2 (300 nm)/Si substrates showed irreversible response because of insufficient recovery from Pd-Hx to Pd-Ox. In particular, the sensor showed significantly high repeatability, long-term stability, and selectivity for H2 sensing at 80 °C. Additionally, the time taken to detect H2 above 1% was less than 2 s and the response to H2 above 500 ppm was unaffected by high humidity. These results indicate that the dual-channel Pd-coated SnO2 NR sensor is suitable for applications such as hydrogen leak detectors that require high speed and precision. Moreover, it was found that the sensor could detect H2 below 0.5 ppm even at low temperatures. This work provides a pathway for improving gas-sensing performance by interface engineering, enabling various practical hydrogen sensor applications.

Hydrogen online monitoring based on thermal conductivity for anaerobic microorganisms.

H2 -producing microorganisms are a promising source of sustainable biohydrogen. However, most H2 -producing microorganisms are anaerobes, which are difficult to cultivate and characterize. While several methods for measuring H2 exist, common H2 sensors often require oxygen, making them unsuitable for anaerobic processes. Other sensors can often not be operated at high gas humidity. Thus, we applied thermal conductivity(TC) sensors and developed a parallelized, online H2 monitoring for time-efficient characterization of H2 production by anaerobes. Since TC sensors are nonspecific for H2 , the cross-sensitivity of the sensors was evaluated regarding temperature, gas humidity, and CO2 concentrations. The systems' measurement range was validated with two anaerobes: a high H2 -producer (Clostridium pasteurianum) and a low H2 -producer (Phocaeicola vulgatus). Online monitoring of H2 production in shake flask cultivations was demonstrated, and H2 transfer rates were derived. Combined with online CO2 and pressure measurements, molar gas balances of the cultivations were closed, and an anaerobic respiration quotient was calculated. Thus, insight into the effect of medium components and inhibitory cultivation conditions on H2 production with the model anaerobes was gained. The presented online H2 monitoring method can accelerate the characterization of anaerobes for biohydrogen production and reveal metabolic changes without expensive equipment and offline analysis.