Review of Electrochemical Hydrogen Sensors

Ultra-Low-Power, Extremely Stable, Highly Linear-Response Thermal Conductivity Sensor Based on a Suspended Device with Single Bare Pt Nanowire

The continuous and stable monitoring by sensors is crucial for ensuring the safe utilization of hydrogen due to its inherent high explosiveness. Currently, catalyst aging and oxygen dependence often limit the lifetime of most sensors, which stems from the sensing materials and catalytic reaction in comparison to thermal conductivity sensors. Thermal conductivity sensors possess superior sensing characteristics such as lowpower consumption and exceptional stability attributed to their free-catalysts or free-oxygen nature. Herein, we present an ultralow-power hydrogen-thermal conductivity sensor based on suspended bare platinum nanowires. This sensor incorporates two suspended independent working elements (serpentine/bridge), each of which is thermally decoupled from the substrate. Also, the bridge element operates at significantly lower power levels (the lowest ∼3.32 μW) compared to existing direct-current hydrogen-thermal conductivity sensors. Furthermore, it demonstrates a 99.99% linearity between hydrogen concentration and response under various operating powers. Finally, our sensor shows remarkable stability through a repeatability test (>30,000 cycles). This developed platform provides an optimal structure scheme for integrated sensors with ultralow-power, extremely stable, highly linear-response sensing characteristics, which is expected to be widely used for hydrogen detection and leakage warning under various pipeline distribution systems.

2D Amino-Functionalized Black Phosphorus: A New Approach to Improve Hydrogen Gas Detection Performance

In recent years, hydrogen has gained attention as a potential solution to replace fossil fuels, thus reducing greenhouse gas emissions. The development of ever improving hydrogen sensors is a topic that is constantly under study due to concerns about the inherent risk of leaks of this gas and potential explosions. In this work, a new, long-term, stable phosphorene-based sensor was developed for hydrogen detection. A simple functionalization of phosphorene using urea was employed to synthesize an air-stable material, subsequently used to prepare films for gas sensing applications, via the drop casting method. The material was deeply characterized by different techniques (scanning electron microscopy, X-ray diffraction, X-ray photoelectron, and Raman spectroscopy), and the stability of the material in a noninert atmosphere was evaluated. The phosphorene-based sensor exhibited high sensitivity (up to 700 ppm) and selectivity toward hydrogen at room temperature, as well as long-term stability over five months under ambient conditions. To gain further insight into the gas sensing mechanism over the surface, we employed a dedicated apparatus, namely operando diffuse reflectance infrared Fourier transform, by exposing the chemoresistive sensor to hydrogen gas under dry air conditions.

Structure and Chemical Reactivity of Y-Stabilized ZrO2 Surfaces: Importance for the Water-Gas Shift Reaction

The surface structure and chemical properties of Y-stabilized zirconia (YSZ) have been subjects of intense debate over the past three decades. However, a thorough understanding of chemical processes occurring at YSZ powders faces significant challenges due to the absence of reliable reference data acquired for well-controlled model systems. Here, we present results from polarization-resolved infrared reflection absorption spectroscopy (IRRAS) obtained for differently oriented, Y-doped ZrO2 single-crystal surfaces after exposure to CO and D2O. The IRRAS data reveal that the polar YSZ(100) surface undergoes reconstruction, characterized by an unusual, red-shifted CO band at 2132 cm-1. Density functional theory calculations allowed to relate this unexpected observation to under-coordinated Zr4+ cations in the vicinity of doping-induced O vacancies. This reconstruction leads to a strongly increased chemical reactivity and water spontaneously dissociates on YSZ(100). The latter, which is an important requirement for catalysing the water-gas-shift (WGS) reaction, is absent for YSZ(111), where only associative adsorption was observed. Together with a novel analysis Scheme these reference data allowed for an operando characterisation of YSZ powders using DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). These findings facilitate rational design and tuning of YSZ-based powder materials for catalytic applications, in particular CO oxidation and the WGS reaction.

Influence of the Pd Oxidation State in PdNi Thin Films on Surface Acoustic Wave Hydrogen Sensing Performance

PdNi alloy thin films demonstrate exceptional hydrogen sensing performance and exhibit significant potential for application in surface acoustic wave (SAW) hydrogen sensors. However, the long-term stability of SAW H2 sensors utilizing PdNi films as catalysts experiences a substantial decrease during operation. In this paper, X-ray photoelectron spectroscopy (XPS) is employed to investigate the failure mechanisms of PdNi thin films under operational conditions. The XPS analysis reveals that the formation of PdO species on PdNi thin films plays a crucial role in the failure of hydrogen sensing. Additionally, density functional theory (DFT) calculations indicate that hydrogen atoms encounter a diffusion energy barrier during the penetration process from the PdNiOx surface to the subsurface region. The identification of PdNi film failure mechanisms through XPS and DFT offers valuable insights into the development of gas sensors with enhanced long-term stability. Guided by these mechanisms, we propose a method to restore the hydrogen sensing response time and magnitude to a certain extent by reducing the partially oxidized surface of the PdNi alloy under a hydrogen atmosphere at 70 °C, thereby restoring Pd to its metallic state with zero valence.

Critical Sensing Modalities for Hydrogen: Technical Needs and Status of the Field to Support a Changing Energy Landscape

Decarbonization of the energy system is a key aspect of the energy transition. Energy storage in the form of chemical bonds has long been viewed as an optimal scheme for energy conversion. With advances in systems engineering, hydrogen has the potential to become a low cost, low emission, energy carrier. However, hydrogen is difficult to contain, it exhibits a low flammability limit (>40000 ppm or 4%), low ignition energy (0.02 mJ), and it is a short-lived climate forcer. Beyond commercially available sensors to ensure safety through spot checks in enclosed environments, new sensors are necessary to support the development of low emission infrastructure for production, transmission, storage, and end use. Efficient scalable broad area hydrogen monitoring motivates lowering the detection limit below that (10 ppm) of best in class commercial technologies. In this perspective, we evaluate recent advances in hydrogen gas sensing to highlight technologies that may find broad utility in the hydrogen sector. It is clear in the near term that a sensor technology suite is required to meet the variable constraints (e.g., power, size/weight, connectivity, cost) that characterize the breadth of the application space, ranging from industrial complexes to remote pipelines. This perspective is not intended to be another standard hydrogen sensor review, but rather provide a critical evaluation of technologies with detection limits preferably below 1 ppm and low power requirements. Given projections for rapid market growth, promising techniques will also be amenable to rapid development in technical readiness for commercial deployment. As such, methods that do not meet these requirements will not be considered in depth.

Hydrogen 4.0: A Cyber-Physical System for Renewable Hydrogen Energy Plants

The demand for green hydrogen as an energy carrier is projected to exceed 350 million tons per year by 2050, driven by the need for sustainable distribution and storage of energy generated from sources. Despite its potential, hydrogen production currently faces challenges related to cost efficiency, compliance, monitoring, and safety. This work proposes Hydrogen 4.0, a cyber-physical approach that leverages Industry 4.0 technologies-including smart sensing, analytics, and the Internet of Things (IoT)-to address these issues in hydrogen energy plants. Such an approach has the potential to enhance efficiency, safety, and compliance through real-time data analysis, predictive maintenance, and optimised resource allocation, ultimately facilitating the adoption of renewable green hydrogen. The following sections break down conventional hydrogen plants into functional blocks and discusses how Industry 4.0 technologies can be applied to each segment. The components, benefits, and application scenarios of Hydrogen 4.0 are discussed while how digitalisation technologies can contribute to the successful integration of sustainable energy solutions in the global energy sector is also addressed.

CMOS Compatible Hydrogen Sensor Using Platinum Gate and ALD-Aluminum Oxide

In this study, a p-Si/ALD-Al2O3/Ti/Pt MOS (metal oxide semiconductor) device has been fabricated and used as a hydrogen sensor. The use of such a stack enables a reliable, industry-compatible CMOS fabrication process. ALD-Al2O3 has been chosen as it can be integrated into the back end of the line (BEOL) or in CMOS, post processing. The device response and recovery are demonstrated with good correlation between the capacitance variation and the hydrogen concentration. Detection down to 20 ppm at 140 °C was obtained and a response time of 56 s for 500 ppm was recorded.

Enhanced response of WO3 thin film through Ag loading towards room temperature hydrogen gas sensor

This study investigates the enhancement of hydrogen gas-sensing performance by introducing silver (Ag) nanoparticles onto tungsten trioxide (WO3) thin films. Herein, the WO3 thin films are deposited onto SiO2/Si substrates using a sputtering technique and Ag nanoparticles are loaded onto the WO3 surface through a spin coating technique. To evaluate the sensing performance of a hydrogen gas, interdigitated titanium (Ti) electrodes are deposited onto the Ag:WO3 layer. Structural, chemical, and morphological analyses are conducted for both pristine WO3 and Ag:WO3 thin films, followed by the investigation of gas-sensing performance by varying hydrogen gas concentrations from 100 ppm to 300 ppm and operating temperatures between 30 °C and 300 °C. The obtained results demonstrate that Ag:WO3 thin films exhibit a notably enhanced response of 5.08% when exposed to a concentration of 100 ppm of hydrogen gas at room temperature, compared to the pristine WO3 of 3.40%. The fabricated Ag:WO3 sensor exhibits a response time of 3.0 s, a recovery time of 4.5 s, and also demonstrates excellent stability over 45 days period. Finally, with the superior sensitivity and fast response time, the fabricated Ti/Ag:WO3/Ti hydrogen gas sensor test-device can be a potential for improvement of safety from both industrial and environmental perspectives.

Minimum Detection Concentration of Hydrogen in Air Depending on Substrate Type and Design of the 3ω Sensor

Hydrogen has emerged as a promising carbon-neutral fuel source, spurring research and development efforts to facilitate its widespread adoption. However, the safe handling of hydrogen requires precise leak detection sensors due to its low activation energy and explosive potential. Various detection methods exist, with thermal conductivity measurement being a prominent technique for quantifying hydrogen concentrations. However, challenges remain in achieving high measurement sensitivity at low hydrogen concentrations below 1% for thermal-conductivity-based hydrogen sensors. Recent research explores the 3ω method's application for measuring hydrogen concentrations in ambient air, offering high spatial and temporal resolutions. This study aims to enhance hydrogen leak detection sensitivity using the 3ω method by conducting thermal analyses on sensor design variables. Factors including substrate material, type, and sensor geometry significantly impact the measurement sensitivity. Comparative evaluations consider the minimum detectable hydrogen concentration while accounting for the uncertainty of the 3ω signal. The proposed suspended-type 3ω sensor is capable of detecting hydrogen leaks in ambient air and provides real-time measurements that are ideal for monitoring hydrogen diffusion. This research serves to bridge the gap between precision and real-time monitoring of hydrogen leak detection, promising significant advancements in the related safety applications.

Resonant Gas Sensing in the Terahertz Spectral Range Using Two-Wire Phase-Shifted Waveguide Bragg Gratings

The development of low-cost sensing devices with high compactness, flexibility, and robustness is of significance for practical applications of optical gas sensing. In this work, we propose a waveguide-based resonant gas sensor operating in the terahertz frequency band. It features micro-encapsulated two-wire plasmonic waveguides and a phase-shifted waveguide Bragg grating (WBG). The modular semi-sealed structure ensures the controllable and efficient interaction between terahertz radiation and gaseous analytes of small quantities. WBG built by superimposing periodical features on one wire shows high reflection and a low transmission coefficient within the grating stopband. Phase-shifted grating is developed by inserting a Fabry-Perot cavity in the form of a straight waveguide section inside the uniform gratings. Its spectral response is optimized for sensing by tailoring the cavity length and the number of grating periods. Gas sensor operating around 140 GHz, featuring a sensitivity of 144 GHz/RIU to the variation in the gas refractive index, with resolution of 7 × 10-5 RIU, is developed. In proof-of-concept experiments, gas sensing was demonstrated by monitoring the real-time spectral response of the phase-shifted grating to glycerol vapor flowing through its sealed cavity. We believe that the phase-shifted grating-based terahertz resonant gas sensor can open new opportunities in the monitoring of gaseous analytes.

Mechanisms and Application of Gas-Based Anticancer Therapies

Cancer is still one of the major factors threatening public health, with morbidity and mortality rates at the forefront of the world. Clinical drawbacks, such as high toxicity and side effects of drug therapy, and easy recurrence after surgery affect its therapeutic effect. Gas signaling molecules are essential in maintaining biological homeostasis and physiological functions as specific chemical substances for biological information transfer. In recent years, the physiological regulatory functions of gas molecules in the cancer process have been gradually revealed and have shown broad application prospects in tumor therapy. In this paper, standard gas therapies are classified and introduced. Taking H2, CO2, NO, CO, H2S, and SO2 gases as examples, the research progress and application of gas therapies in malignant tumors are mainly introduced in terms of biological characteristics, anticancer mechanisms, and treatment strategies. Finally, the problems and prospects for developing gases as anticancer drugs are outlined.

A Review of the Carbon-Based Solid Transducing Layer for Ion-Selective Electrodes

As one of the key components of solid-contact ion-selective electrodes (SC-ISEs), the SC layer plays a crucial role in electrode performance. Carbon materials, known for their efficient ion-electron signal conversion, chemical stability, and low cost, are considered ideal materials for solid-state transducing layers. In this review, the application of different types of carbon materials in SC-ISEs (from 2007 to 2023) has been comprehensively summarized and discussed. Representative carbon-based materials for the fabrication of SC-ISEs have been systematically outlined, and the influence of the structural characteristics of carbon materials on achieving excellent performance has been emphasized. Finally, the persistent challenges and potential opportunities are also highlighted and discussed, aiming to inspire the design and fabrication of next-generation SC-ISEs with multifunctional composite carbon materials in the future.

Supraparticles for naked-eye H2 indication and monitoring: Improving performance by variation of the catalyst nanoparticles

The recent transition to H₂-based energy storage demands reliable H₂ sensors that allow for easy, fast, and reliable detection of leaks. Conventional H₂ detectors are based on the changes of physical properties of H₂ probes induced by subsurface H-atoms to a material such as electrical conductivity. Herein, we report on highly reactive gasochromic H₂ detectors based on the adsorption of H₂ on the material surface. We prepared supraparticles (SPs) containing different types of noble metal nanoparticles (NPs), silica NPs, and the dye resazurin by spray-drying and tested their performance for H₂ detection. The material undergoes a distinct color change due to the hydrogenation of the purple resazurin to pink resorufin and, finally, colorless hydroresorufin. The stepwise transition is fast and visible to the naked eye. To further improve the performance of the sensor, we tested the reactivity of SPs with different catalytically active NPs by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We show that the choice of the NP catalyst has a pronounced effect on the response of the H₂ indicator. In addition, we demonstrate that the performance depends on the size of the NPs. These effects are attributed to the availability of reactive H-atoms on the NP surface. Among the materials studied, Pt-containing SPs gave the best results for H₂ detection.

Metalorganic Chemical Vapor Deposition Approach to the Synthesis of Perovskite BaCeO3 and BaCe0.8Y0.2O3 Thin Films

In the present energetic scenario, the development of materials with high potentiality in the technological fields of energy conversion processes, production and storage of hydrogen, are of great interest in the scientific community. In particular, we report for the first time the fabrication of crystalline and homogeneous barium-cerate-based materials in the form of thin films on various substrates. Starting from the β-diketonate precursor sources Ce(hfa)₃diglyme, Ba(hfa)₂tetraglyme and Y(hfa)₃diglyme (Hhfa = 1,1,1,5,5,5-hexafluoroacetylacetone; diglyme = bis(2-methoxyethyl)ether; tetraglyme = 2,5,8,11,14-pentaoxapentadecane), a metalorganic chemical vapor deposition (MOCVD) approach has been successfully applied to the fabrication of BaCeO₃ and doped BaCe_0.8Y_0.2O₃ systems in the form of thin films. Structural, morphological and compositional analyses allowed for an accurate determination of the properties of deposited layers. The present approach represents a simple, easily scalable, and industrially appealing process for the production of compact and homogeneous barium cerate thin films.

On-chip ultrasensitive and rapid hydrogen sensing based on plasmon-induced hot electron-molecule interaction

Hydrogen energy is a zero-carbon replacement for fossil fuels. However, hydrogen is highly flammable and explosive hence timely sensitive leak detection is crucial. Existing optical sensing techniques rely on complex instruments, while electrical sensing techniques usually operate at high temperatures and biasing condition. In this paper an on-chip plasmonic-catalytic hydrogen sensing concept with a concentration detection limit down to 1 ppm is presented that is based on a metal-insulator-semiconductor (MIS) nanojunction operating at room temperature and zero bias. The sensing signal of the device was enhanced by three orders of magnitude at a one-order of magnitude higher response speed compared to alternative non-plasmonic devices. The excellent performance is attributed to the hydrogen induced interfacial dipole charge layer and the associated plasmonic hot electron modulated photoelectric response. Excellent agreements were achieved between experiment and theoretical calculations based on a quantum tunneling model. Such an on-chip combination of plasmonic optics, photoelectric detection and photocatalysis offers promising strategies for next-generation optical gas sensors that require high sensitivity, low time delay, low cost, high portability and flexibility.

Chemiresistive Hydrogen Sensing with Size-Limited Palladium Nanoparticles in Iptycene-Containing Poly(arylene ether)s

Metal nanoparticles have been widely employed in chemical sensing due to their high reactivity toward various gases. The size of the metal nanoparticles often dictates their reactivity and hence their performance as chemiresistive sensors. Herein, we report that iptycene-containing poly(arylene ether)s (PAEs) have been shown to limit the growth of palladium nanoparticles (Pd NPs) and stabilize the Pd NPs dispersion. These porous PAEs also facilitate the efficient transport of analytes. Single-walled carbon nanotube (SWCNT)-based chemiresistors and graphene field-effect transistors (GFETs) using these PAE-supported small Pd NPs are sensitive, selective, and robust sensory materials for hydrogen gas under ambient conditions. Generalizable strategies including presorting SWCNTs with pentiptycene-containing poly(p-phenylene ethynylene)s (PPEs) and thermal annealing demonstrated significant improvements in the chemiresistive performance. The polymer:NP colloids produced in this study are readily synthesized and solution processable, and these methods are of general utility.

Application of Optical Communication for an Enhanced Health and Safety System in Underground Mine

The continuous monitoring systems developed for health and safety provisions in underground mines have always been a deep concern in mining activity, since this is one of the most dangerous work activities in the world. All mining activities must follow strict regulations that pose an increased responsibility for the design and implementation of novel technologies aiming to enhance both the workers' and equipment's safety in underground mines. Coal mines are some of the most dangerous, with high risk of explosion due to pit gas; therefore, we propose the use of visible light communication for local wireless transmission and optical fiber for remote-cabled transmission. We address, in this article, a complete system comprising real-time personnel tracking and environmental methane measurement, as well as the transmission of data, with a detailed explanation of the complete system and technical description with practical solutions applicable for industrial use.

Brief Review on High-Temperature Electrochemical Hydrogen Sensors

Hydrogen sensors, especially those operating at high temperatures, are essential tools for the emerging hydrogen economy. Monitoring hydrogen under process conditions to control the reactions for detecting confined species is crucial to the safe, widespread use and public acceptance of hydrogen as fuel. Hydrogen sensors must have a sensitivity ranging from traces of hydrogen (parts per million (ppm)) up to levels near the lower explosive limit (LEL = 4% H2 in the air) for safety reasons. Furthermore, they need to operate in cryogenic, ambient, and high-temperature environments. Herein, emphasis is given to hydrogen sensors based on solid oxide electrolytes (operating at high temperatures), in particular oxygen ion and proton conductors. The review is devoted to potentiometric, amperometric, and combined amperometric-potentiometric hydrogen sensors. Experimental results already reported in the international literature are presented and analyzed to reveal the configuration, principle of operation, and the applied solid electrolytes and electrodes of the high-temperature hydrogen sensors. Additionally, an amperometric sensor able to detect hydrogen and steam in atmospheric air through a two-stage procedure is presented and thoroughly discussed. The discussion reveals that high-temperature hydrogen sensors face different challenges in terms of the electrodes and solid electrolytes to be used, depending on the operating principle of each sensor type.

Environmental Monitoring: A Comprehensive Review on Optical Waveguide and Fiber-Based Sensors

Globally, there is active development of photonic sensors incorporating multidisciplinary research. The ultimate objective is to develop small, low-cost, sensitive, selective, quick, durable, remote-controllable sensors that are resistant to electromagnetic interference. Different photonic sensor designs and advances in photonic frameworks have shown the possibility to realize these capabilities. In this review paper, the latest developments in the field of optical waveguide and fiber-based sensors which can serve for environmental monitoring are discussed. Several important topics such as toxic gas, water quality, indoor environment, and natural disaster monitoring are reviewed.

A Review of Applications, Prospects, and Challenges of Proton-Conducting Zirconates in Electrochemical Hydrogen Devices

In the future, when fossil fuels are exhausted, alternative energy sources will be essential for everyday needs. Hydrogen-based energy can play a vital role in this aspect. This energy is green, clean, and renewable. Electrochemical hydrogen devices have been used extensively in nuclear power plants to manage hydrogen-based renewable fuel. Doped zirconate materials are commonly used as an electrolyte in these electrochemical devices. These materials have excellent physical stability and high proton transport numbers, which make them suitable for multiple applications. Doping enhances the physical and electronic properties of zirconate materials and makes them ideal for practical applications. This review highlights the applications of zirconate-based proton-conducting materials in electrochemical cells, particularly in tritium monitors, tritium recovery, hydrogen sensors, and hydrogen pump systems. The central section of this review summarizes recent investigations and provides a comprehensive insight into the various doping schemes, experimental setup, instrumentation, optimum operating conditions, morphology, composition, and performance of zirconate electrolyte materials. In addition, different challenges that are hindering zirconate materials from achieving their full potential in electrochemical hydrogen devices are discussed. Finally, this paper lays out a few pathways for aspirants who wish to undertake research in this field.

Amperometric Hydrogen Sensor Based on Solid Polymer Electrolyte and Titanium Foam Electrode

Trace hydrogen detection plays an important role in the safety detection of lithium-ion batteries (LIBs) due to the generation and leakage of trace hydrogen in the early stage of LIBs damage. In this work, an amperometric hydrogen sensor based on solid polymer electrolyte was reported. The sandwich device structure was realized, which could directly diffuse the gas from both sides to the three-phase interface (gas/electrode/electrolyte) to participate in the reaction through the optimal design of the gas diffusion path. Then, platinum nanoparticles (Pt-NPs) were loaded on the metal foam by electroplating, and the porous electrode was filled with solid polymer electrolyte. A sensor with high specific surface area, high catalytic activity, and high sensitivity was obtained. Finally, the hydrogen oxidation reaction (HOR) mechanism of the platinum-loaded (Pt-loaded) titanium foam (Ti foam) electrode under both anaerobic and aerobic conditions was verified, and the properties of the sensor was evaluated. The hydrogen sensor with a "sandwich" structure has the advantages of high sensitivity, good stability, low detection limit and low cost, which provides a technical solution for the safety and real-time monitoring of LIBs.

Electrochemical Response of Mixed Conducting Perovskite Enables Low-Cost High-Efficiency Hydrogen Sensing

High-performance noble metal-free gas sensors are crucial for widespread applications in various areas. Non-Nernstian electrochemical sensors have attracted tremendous attention, but are limited by the high cost and low efficiency of Pt electrode. Moreover, responses from different electrodes usually have the same polarity, degrading the sensor performance. Here we report a reverse response on a series of mixed ionic-electronic conductors (MIECs). Exemplary SrFe0.5Ti0.5O3-δ (SFT50) perovskite shows excellent H2 sensing properties, including high sensitivity and selectivity, humidity resistance, and long-term stability. Strikingly, the response is positive, as opposed to the usual one. Such an unusual response is ascribed to the change of the surface electrostatic potential due to the gas chemical reaction, which outcompetes traditional mechanisms, thereby reversing the response polarity. A conceptual noble-metal-free sensor with dual oxide electrodes of opposite polarity is designed by substituting SFT50 for the benchmark Pt, achieving a 1.5-2.0× increase in H2 response, sensitivity, and selectivity and a low limit of detection of 16 ppb. The ideal unity of excellent sensing and unusual polarity for MIECs can be used to optimize the performance of a variety of conventional sensors while reducing the cost. Our findings provide new insights into electrochemical gas sensing and offer a facile approach for developing low-cost high-performance gas sensors.

Sensing Hydrogen Seeps in the Subsurface for Natural Hydrogen Exploration

The recent detection of natural hydrogen seeps in sedimentary basin settings has triggered significant interest in the exploration of this promising resource. If large economical resources exist and can be extracted from the sub-surface, this would provide an opportunity for natural hydrogen to contribute to the non-carbon-based energy mix. The detection and exploration of hydrogen gas in the sub-surface is a significant challenge that requires costly drilling, sophisticated instrumentation, and reliable analytical/sampling methods. Here, we propose the application of a commercial-based sensor that can be used to detect and monitor low levels of hydrogen gas emissions from geological environments. The sensitivity, selectivity (K > 1000), and stability (<1 ppm/day) of the sensor was evaluated under various conditions to determine its suitability for geological field monitoring. Calibration tests showed that the hydrogen readings from the sensor were within ±20% of the expected values. We propose that chemical sensing is a simple and feasible method for understanding natural hydrogen seeps that emanate from geological systems and formations. However, we recommend using this sensor as part of a complete geological survey that incorporates an understanding of the geology along with complementary techniques that provide information on the rock properties.

Noble Metal-Decorated Nanostructured Zinc Oxide: Strategies to Advance Chemiresistive Hydrogen Gas Sensing

Hydrogen (H₂ ) is known as the key player in the alternative and renewable energy revolution and henceforth H₂ production, transportation, storage and usage have been a major interest of current research. However, due to severe safety concerns, strategies are indispensable to devise superior H₂ sensors, particularly selective and sensitive H₂ sensors. In this personal account, three specific gas sensing constructs; zinc oxide (ZnO) nanostructures-, noble metal nanoparticles-decorated ZnO- and noble metal nanoparticles-decorated ZnO nanostructures on reduced graphene oxide (rGO)-based H₂ sensors have been demonstrated. The dynamic response and H₂ sensing characteristics of ZnO nanostructures-based H₂ sensors were found to be improved compared to those of pristine ZnO. High-resolution field emission scanning electron microscopy (FESEM) confirmed the flower-like nanostructures that had higher surface area around the nanoscale petals. The mechanism behind the superior sensing characteristics of ZnO nanostructures-based H₂ sensor has been demonstrated. Decoration of ZnO nanostructures with noble metal nanoparticles, particularly platinum (Pt) and gold (Au) was observed to be useful in achieving better H₂ sensing performance compared to that of ZnO nanostructures. The Pt- and Au-decorated ZnO nanostructures followed the well-known "Spill-over" mechanism in enhancing the H₂ sensing characteristics. Abundant free electrons/holes generation and higher conductivity are two important parameters for designing selective and sensitive gas sensors. In this context, a hybrid nanocomposite, rGO-ZnO has been developed and decorated with noble metal nanoparticles, particularly Pt and Au. The ultimate sensing material has been characterized and compared to those of pristine ZnO, ZnO nanostructures and Pt- and Au-decorated ZnO for H₂ gas sensing applications. Such systemic and focus strategies is critical not only for developing efficient H₂ gas sensors but also for better understanding the mechanisms underlying such superior performance.

Highly Sensitive and Selective Detection of Hydrogen Using Pd-Coated SnO2 Nanorod Arrays for Breath-Analyzer Applications

We report a breath hydrogen analyzer based on Pd-coated SnO₂ nanorods (Pd-SnO₂ NRs) sensor integrated into a miniaturized gas chromatography (GC) column. The device can measure a wide range of hydrogen (1–100 ppm), within 100 s, using a small volume of human breath (1 mL) without pre-concentration. Especially, the mini-GC integrated with Pd-SnO₂ NRs can detect 1 ppm of H₂, as a lower detection limit, at a low operating temperature of 152 °C. Furthermore, when the breath hydrogen analyzer was exposed to a mixture of interfering gases, such as carbon dioxide, nitrogen, methane, and acetone, it was found to be capable of selectively detecting only H₂. We found that the Pd-SnO₂ NRs were superior to other semiconducting metal oxides that lack selectivity in H₂ detection. Our study reveals that the Pd-SnO₂ NRs integrated into the mini-GC device can be utilized in breath hydrogen analyzers to rapidly and accurately detect hydrogen due to its high selectivity and sensitivity.

Magneto-Electronic Hydrogen Gas Sensors: A Critical Review

Devices enabling early detection of low concentrations of leaking hydrogen and precision measurements in a wide range of hydrogen concentrations in hydrogen storage systems are essential for the mass-production of fuel-cell vehicles and, more broadly, for the transition to the hydrogen economy. Whereas several competing sensor technologies are potentially suitable for this role, ultra-low fire-hazard, contactless and technically simple magneto-electronic sensors stand apart because they have been able to detect the presence of hydrogen gas in a range of hydrogen concentrations from 0.06% to 100% at atmospheric pressure with the response time approaching the industry gold standard of one second. This new kind of hydrogen sensors is the subject of this review article, where we inform academic physics, chemistry, material science and engineering communities as well as industry researchers about the recent developments in the field of magneto-electronic hydrogen sensors, including those based on magneto-optical Kerr effect, anomalous Hall effect and Ferromagnetic Resonance with a special focus on Ferromagnetic Resonance (FMR)-based devices. In particular, we present the physical foundations of magneto-electronic hydrogen sensors and we critically overview their advantages and disadvantages for applications in the vital areas of the safety of hydrogen-powered cars and hydrogen fuelling stations as well as hydrogen concentration meters, including those operating directly inside hydrogen-fuelled fuel cells. We believe that this review will be of interest to a broad readership, also facilitating the translation of research results into policy and practice.

A Robust Miniaturized Gas Sensor for H2 and CO2 Detection Based on the 3ω Method

Gas concentration monitoring is essential in industrial or life science areas in order to address safety-relevant or process-related questions. Many of the sensors used in this context are based on the principle of thermal conductivity. The 3ω-method is a very accurate method to determine the thermal properties of materials. It has its origin in the thermal characterization of thin solid films. To date, there have been very few scientific investigations using this method to determine the thermal properties of gases and to apply it to gas measurement technology. In this article, we use two exemplary gases (H2 and CO2) for a systematical investigation of this method in the context of gas analysis. To perform our experiments, we use a robust, reliable sensing element that is already well established in vacuum measurement technology. This helix-shaped thin wire of tungsten exhibits high robustness against chemical and mechanical influences. Our setup features a compact measurement environment, where sensor operation and data acquisition are integrated into a single device. The experimental results show a good agreement with a simplified analytical model and FEM simulations. The sensor exhibits a lower detection limit of 0.62% in the case of CO2, and only 0.062% in case the of H2 at an excitation frequency of 1Hz. This is one of the lowest values reported in literature for thermal conductivity H2 sensors.

An efficient factor for fast screening of high-performance two-dimensional metal-organic frameworks towards catalyzing oxygen evolution reaction

Two-dimensional (2D) Metal-Organic frameworks (MOFs) are promising materials for catalyzing oxygen evolution reaction (OER) due to abundant exposed active sites and high specific surface area. However, how to fast screen...

Thin Film and Nanostructured Pd-Based Materials for Optical H2 Sensors: A Review

In this review paper, we provide an overview of state-of-the-art Pd-based materials for optical H2 sensors. The first part of the manuscript introduces the operating principles, providing background information on the thermodynamics and the primary mechanisms of optical detection. Optical H2 sensors using thin films (i.e., films without any nanostructuring) are discussed first, followed by those employing nanostructured materials based on aggregated or isolated nanoparticles (ANPs and INPs, respectively), as well as complex nanostructured (CN) architectures. The different material types are discussed on the basis of the properties they can attribute to the resulting sensors, including their limit of detection, sensitivity, and response time. Limitations induced by cracking and the hysteresis effect, which reduce the repeatability and reliability of the sensors, as well as by CO poisoning that deteriorates their performance in the long run, are also discussed together with an overview of manufacturing approaches (e.g., tailoring the composition and/or applying functionalizing coatings) for addressing these issues.

Defect and Doping Engineered Penta-graphene for Catalysis of Hydrogen Evolution Reaction

Water electrolysis is a sustainable and clean method to produce hydrogen fuel via hydrogen evolution reaction (HER). Using stable, effective and low-cost electrocatalysts for HER to substitute expensive noble metals is highly desired. In this paper, by using first-principles calculation, we designed a defect and N-, S-, P-doped penta-graphene (PG) as a two-dimensional (2D) electrocatalyst for HER, and its stability, electronic properties and catalytic performance were investigated. The Gibbs free energy (ΔG_H), which is the best descriptor for the HER, is calculated and optimized, the calculation results show that the ΔG_H can be 0 eV with C2 vacancies and P doping at C1 active sites, which should be the optimal performance for a HER catalyst. Moreover, we reveal that the larger charge transfer from PG to H, the closer ΔG_H is to zero according to the calculation of the electron charge density differences and Bader charges analysis. Ulteriorly, we demonstrated that the HER performance prefers the Volmer-Heyrovsky mechanism in this study.

A dual-frequency phase-difference method for ultrasonic hydrogen-concentration detection

Hydrogen has a wide range of energy applications, but hydrogen energy systems can suffer from high-concentration leaks that pose security risks, therefore making the measurement of high-concentration hydrogen very important. Traditional ultrasonic gas-detection methods are based mainly on ultrasonic time-of-flight measurements and can be divided into threshold-detection and phase-difference techniques. Threshold detection suffers from low resolution and a complex structure in gas detection, while the phase-difference technique has high resolution and a simple structure but can only measure the time of flight within one period of the ultrasonic signal. In this study, a dual-frequency phase-difference technique is proposed that solves the problem of multi-period phase detection with the phase-difference technique and can be used to detect high-concentration hydrogen. Simulation analysis and an experiment show that the proposed technique can measure the multi-period phase difference accurately. The maximum hydrogen concentration can reach 50% with an uncertainty of less than 5%, which meets commercial requirements.

A room-temperature ultrasonic hydrogen sensor based on a sensitive layer of reduced graphene oxide

It is challenging to increase the sensitivity of a hydrogen sensor operating at room temperature due to weak sorption and tiny mass of hydrogen. In this work, an ultrasonic sensor is presented for detecting hydrogen, which is composed of a 128° YX-LiNbO₃ substrate and a reduced graphene oxide (RGO) sensitive layer with a platinum catalyzer. By optimizing the depositing parameters of RGO and platinum, a considerably high sensitivity is achieved at room temperature. A frequency shift of 308.9 kHz is obtained in 100 ppm hydrogen mixed with argon, and a frequency shift of 24.4 kHz is obtained in 1000 ppm hydrogen mixed in synthetic air. It is demonstrated that in addition to strong sorption of the sensitive layer, the coaction of mass load and conductivity variation is key to high sensitivity of the sensor. By establishing the original conductivity of the sensitive layer within the “conductivity window” for enhancing electrical response, we improve the sensitivity of the ultrasonic sensor, which is available for detecting hydrogen with an extremely low concentration of 5 ppm.

Fiber Optical Hydrogen Sensor Based on WO3-Pd2Pt-Pt Nanocomposite Films

In this paper, WO₃-Pd₂Pt-Pt nanocomposite films were deposited on a single mode fiber as the hydrogen sensing material, which changes its reflectivity under different hydrogen concentration. The reflectivity variation was probed and converted to an electric signal by a pair of balanced InGaAs photoelectric detectors. In addition, the performance of the WO₃-Pd₂Pt-Pt composite film was investigated under different optical powers, and the irrigating power was optimized at 5 mW. With the irrigation of this optical power, the hydrogen sensitive film exhibits quick response toward 100 ppm hydrogen in air atmosphere at a room temperature of 25 °C. The experimental results demonstrate a high resolution at 5 parts per million (ppm) within a wide range from 100 to 5000 ppm in air. This simple and compact sensing system can detect hydrogen concentrations far below the explosion limit and provide early alert for hydrogen leakage, showing great potential in hydrogen-related applications.

Chemiresistive Hydrogen Sensors: Fundamentals, Recent Advances, and Challenges

Hydrogen (H₂) is one of the next-generation energy sources because it is abundant in nature and has a high combustion efficiency that produces environmentally benign products (H₂O). However, H₂/air mixtures are explosive at H₂ concentrations above 4%, thus any leakage of H₂ must be rapidly and reliably detected at much lower concentrations to ensure safety. Among the various types of H₂ sensors, chemiresistive sensors are one of the most promising sensing systems due to their simplicity and low cost. This review highlights the advances in H₂ chemiresistors, including metal-, semiconducting metal oxide-, carbon-based materials, and other materials. The underlying sensing mechanisms for different types of materials are discussed, and the correlation of sensing performances with nanostructures, surface chemistry, and electronic properties is presented. In addition, the discussion of each material emphasizes key advances and strategies to develop superior H₂ sensors. Furthermore, recent key advances in other types of H₂ sensors are briefly discussed. Finally, the review concludes with a brief outlook, perspective, and future directions.

Quo Vadis Micro-Electro-Mechanical Systems for the Study of Heterogeneous Catalysts Inside the Electron Microscope?

During the last decade, modern micro-electro-mechanical systems (MEMS) technology has been used to create cells that can act as catalytic nanoreactors and fit into the sample holders of transmission electron microscopes. These nanoreactors can maintain atmospheric or higher pressures inside the cells as they seal gases or liquids from the vacuum of the TEM column and can reach temperatures exceeding 1000 °C. This has led to a paradigm shift in electron microscopy, which facilitates the local characterization of structural and morphological changes of solid catalysts under working conditions. In this review, we outline the development of state-of-the-art nanoreactor setups that are commercially available and are currently applied to study catalytic reactions in situ or operando in gaseous or liquid environments. We also discuss challenges that are associated with the use of environmental cells. In catalysis studies, one of the major challenge is the interpretation of the results while considering the discrepancies in kinetics between MEMS based gas cells and fixed bed reactors, the interactions of the electron beam with the sample, as well as support effects. Finally, we critically analyze the general role of MEMS based nanoreactors in electron microscopy and catalysis communities and present possible future directions.

Molecular Sieve Based on a PMMA/ZIF-8 Bilayer for a CO-Tolerable H2 Sensor with Superior Sensing Performance

Semiconductor sensors equipped with Pd catalysts are promising candidates as low-powered and miniaturized surveillance devices that are used to detect flammable hydrogen (H₂) gas. However, the following issues remain unresolved: (i) a sluggish sensing speed at room temperature and (ii) deterioration of sensing performance caused by interfering gases, particularly, carbon monoxide (CO). Herein, a bilayer comprising poly(methyl methacrylate) (PMMA) and zeolitic imidazolate framework-8 (ZIF-8) is utilized as a molecular sieve for diode-type H₂ sensors based on a Pd-decorated indium-gallium-zinc oxide film on a p-type silicon substrate. While the PMMA effectively blocks the penetration of CO gas molecules into the sensing entity, the ZIF-8 improves sensing performances by modifying the catalytic activity of Pd, which is preferable for splitting H₂ and O₂ molecules. Consequently, the bilayer-covered sensor achieves outstanding CO tolerance with superior sensing figures of merit (response/recovery times of <10 s and sensing response of >5000% at 1% H₂).

Effect of Ni, Pd, and Pt Nanoparticle Dispersion on Thick Films of TiO2 Nanotubes for Hydrogen Sensing: TEM and XPS Studies

Crystal structure, morphological features, and hydrogen-sensing properties of thick film sensors of TiO₂ nanotubes (NTs) impregnated with nanoparticles of elements of Group 10, viz., nickel, palladium, and platinum, having average grain size of about 25, 20, and 20 nm, respectively, are presented. The sensitivity is observed to be higher for Pd/TiO₂ NTs than for Pt/TiO₂ NTs. Ni/TiO₂ NTs exhibited very poor sensitivity. X-ray photoelectron spectroscopy (XPS) studies confirm reduction of the oxide layer of palladium nanoparticles, which, in turn, is responsible for the generation of Ti³⁺ ion in TiO₂ NTs through hydrogen spillover. For Pt/TiO₂ NTs, only reduction of the oxide layer over Pt nanoparticles takes place without any spillover effect. For Ni/TiO₂ NTs, neither NiO nor TiO₂ undergoes any reduction. Changes in the Fermi level difference of PdO and TiO₂ along with Ti³⁺ generation synergistically operate for Pd/TiO₂ NTs, whereas the difference in Fermi levels of PtO and TiO₂ alone operates for Pt/TiO₂ NTs during sensing.

Synthesis of Pt-Loaded Y2WO6:Eu3+ Microspheres and Their Hydrogen-Sensitive Turn-Off Luminescence

Pt nanoparticles were loaded on Y₂WO₆:Eu³⁺ phosphor microspheres to enhance hydrogen sensitivity at low temperatures through turn-off luminescence. Mesoporous Y₂WO₆:Eu³⁺ microspheres were synthesized first by a hydrothermal method. Pt loading on the surface of the Y₂WO₆:Eu³⁺ microspheres was then carried out by a method of the self-regulated reduction of surfactants using aqueous K₂PtCl₄ solutions. Structural and morphological properties of unloaded and Pt-loaded Y₂WO₆:Eu³⁺ microspheres were investigated by various techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The relative photoluminescence intensity of the Pt-loaded Y₂WO₆:Eu³⁺ microspheres, due to ⁵D₀ → ⁷F _J (J = 0, 1, 2, 3, 4) electronic transitions of doped Eu³⁺ ions, was found to decrease as low as 22% after the microspheres were kept in the hydrogen gas atmosphere at a low temperature of 150 °C for 15 min. Meanwhile, mechanisms underlying this turn-off luminescence of the Pt/Y₂WO₆:Eu³⁺ microspheres in response to hydrogen gas are illustrated in detail.

UV Lithography-Assisted Fabrication of Low-Cost Copper Electrodes Modified with Gold Nanostructures for Improved Analyte Detection

An in-house UV lithography setup has been optimized to fabricate low-cost disposable electrochemical sensing Cu electrodes using a copper clad board. In view of the high oxidation probability of copper, the low-cost electrodes were modified using different gold nanostructures and a conducing polymer PEDOT:PSS to attain maximal signal output and improved shelf-life. Zero-dimensional (0D) gold nanoparticles (∼40 nm) and three-dimensional (3D) gold nanoflowers (∼38 nm) mixed with PEDOT:PSS were used as signal-enhancing conductors for the ultrasensitive detection of our model contaminant, methylene blue dye (MB). The bare copper electrode was sensitive to MB, linearly within the range of 4-100 μM, with a limit of detection of 3.49 μM. While for gold nanoparticle-PEDOT:PSS-modified electrode, the sensitivity of the electrode was found to increase linearly in the range of 0.01-0.1 μM, and for gold nanoflowers-PEDOT:PSS, the sensitivity achieved was 0.01-0.1 μM with the LOD as 0.0022 μM. For a PEDOT:PSS-modified Cu electrode, used as a comparative to study the contributing role of gold nanostructures towards improved sensitivity, the linearity was found to be in the range of 0.1-1.9 μM with the LOD as 0.0228 μM. A 6 times improvement in signal sensitivity for the nanoflower-PEDOT:PSS electrode compared to the nanoparticle-PEDOT:PSS-modified electrode indicates the influence of nanoparticle shape on the electrode efficiency. 3D gold nanoflowers with a large surface area-to-volume ratio and a high catalytic activity prove to be a superior choice for electrode modification.

A new type low-cost, flexible and wearable tertiary nanocomposite sensor for room temperature hydrogen gas sensing

This paper reports on reduced graphene oxide (rGO), tin oxide (SnO₂) and polyvinylidene fluoride (PVDF) tertiary nanocomposite thick film based flexible gas sensor. The nanocomposite of 0.90(PVDF) − 0.10[x(SnO₂) − (1 − x)rGO] with different weight percentages (x = 0, 0.15, 0.30, 0.45, 0.6, 0.75, 0.90 and 1) have been prepared by the hot press method. Chromium (Cr) has been deposited on the surface by using E-beam evaporation system, which is used as electrode of the device. Crystal structure, morphology, and electrical characteristics of the device have been explored for the technological application. A correlation between crystallinity, morphology, and electrical properties with these thick films has also been established. The device has been tested at different hydrogen (H₂) gas concentration as well as at different response times. A superior response of 0.90(PVDF) − 0.10[0.75(SnO₂) − 0.25 rGO] nanocomposite thick film has been observed. Hence, this composition is considered as optimized tertiary nanocomposite for the hydrogen gas sensor application. The sensor response of 49.2 and 71.4% with response time 34 sec and 52 sec for 100 PPM and 1000 PPM H₂ gas concentration respectively have been obtained. First time a new kind of low cost and flexible polymer based nanocomposite thick film gas sensor has been explored.

Recent highlights and future prospects on mixed-metal MOFs as emerging supercapacitor candidates

Mixed-metal metal-organic frameworks (M-MOFs) consist of at least two different metal ions as nodes in the same framework. The incorporation of a second or more metal ions provides structural/compositional diversity, multi-functionality and stability to the framework. Moreover, the periodical array of different metal ions in the framework may alter the physical/chemical properties of M-MOFs and result in fascinating applications. M-MOFs with exciting structural features offer superior supercapacitor performances compared to single metal MOFs due to the synergic effect of different metal ions. In this review, we summarize several synthetic methods to construct M-MOFs by employing various organic ligands or metalloligands. Further, we discuss the electrochemical performance of several M-MOFs and their derived composite materials for supercapacitor applications.