Optical Fiber Grating Hydrogen Sensors: A Review

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.

In-Line Mach Zehnder Interferometer Based on Ytterbium Doped Fiber with Up-Taper Structure in Fiber Ring Laser and Its Application in Sensing

We report an ytterbium (Yb) doped fiber Mach Zehnder interferometer (MZI) based on the up-taper fiber structure in a fiber ring laser (FRL) cavity. Different from the traditional FRL sensing system, in which additional filters are required, the designed structure simultaneously acts as a filter, sensor and gain medium. Furthermore, thanks to the high thermal-optical coefficient of Yb doped fiber, the temperature sensitivity of 0.261 nm/°C can be achieved in the range of 10-50 °C. In addition, benefiting from the unique characteristics of the laser system itself, the designed structure has a narrower linewidth (-0.2 nm) and a higher signal-to-noise ratio (SNR) (-40 dB) than the sensor system based on a broadband light source (BBS). Meanwhile, the refractive index (RI) response and stability of the system are measured. The RI sensitivity is up to 151 nm/RIU, and the wavelength fluctuation range within two hours is less than 0.2 nm. Therefore, the designed structure is expected to play a significant role in human life safety monitoring, aircraft engine temperature monitoring, etc.

Palladium/Graphene Oxide Nanocomposite for Hydrogen Gas Sensing Applications Based on Tapered Optical Fiber

Gaseous pollutants such as hydrogen gas (H₂) are emitted in daily human activities. They have been massively studied owing to their high explosivity and widespread usage in many domains. The current research is designed to analyse optical fiber-based H₂ gas sensors by incorporating palladium/graphene oxide (Pd/GO) nanocomposite coating as sensing layers. The fabricated multimode silica fiber (MMF) sensors were used as a transducing platform. The tapering process is essential to improve the sensitivity to the environment through the interaction of the evanescent field over the area of the tapered surface area. Several characterization methods including FESEM, EDX, AFM, and XRD were adopted to examine the structure properties of the materials and achieve more understandable facts about their functional performance of the optical sensor. Characterisation results demonstrated structures with a higher surface for analyte gas reaction to the optical sensor performance. Results indicated an observed increment in the Pd/GO nanocomposite-based sensor responses subjected to the H₂ concentrations increased from 0.125% to 2.00%. The achieved sensitivities were 33.22/vol% with a response time of 48 s and recovery time of 7 min. The developed optical fiber sensors achieved excellent selectivity and stability toward H₂ gas upon exposure to other gases such as ammonia and methane.

Optical Fiber Probe Microcantilever Sensor Based on Fabry–Perot Interferometer

Optical fiber Fabry–Perot sensors have long been the focus of researchers in sensing applications because of their unique advantages, including highly effective, simple light path, low cost, compact size, and easy fabrication. Microcantilever-based devices have been extensively explored in chemical and biological fields while the interrogation methods are still a challenge. The optical fiber probe microcantilever sensor is constructed with a microcantilever beam on an optical fiber, which opens the door for highly sensitive, as well as convenient readout. In this review, we summarize a wide variety of optical fiber probe microcantilever sensors based on Fabry–Perot interferometer. The operation principle of the optical fiber probe microcantilever sensor is introduced. The fabrication methods, materials, and sensing applications of an optical fiber probe microcantilever sensor with different structures are discussed in detail. The performances of different kinds of fiber probe microcantilever sensors are compared. We also prospect the possible development direction of optical fiber microcantilever sensors.

Bragg grating sensor for refractive index based on a D-shaped circular photonic crystal fiber

In this paper, a silica-based D-shaped circular photonic crystal fiber Bragg grating sensor for refractive index sensing is proposed theoretically. D-shaped fiber construction can effectively enhance the coupling effect between the guiding mode and external liquid analyte, which then causes a distinct shift in the typical reflection spectrum as the refractive index of the analyte varies. This design exhibits highly improved sensitivity of 487 nm/RIU in a large refractive index range from 1.30 to 1.40 compared with the previous fiber grating sensors. Study of the dependence of sensing performance on the structure parameters suggests that the resonance peak shifts towards longer wavelengths with the increased air-hole diameter of fiber, while it is almost immobile as the hole spacing and the number of air-hole layers change in a certain range. For the influence of the Bragg grating structure, results show that the resonance peak is not sensitive to the grating length, but linearly increases as the grating period expands. The effects of polishing depth and fiber preparation error on the sensor are also discussed in detail. This high-sensitivity sensor based on a D-shaped photonic crystal fiber and Bragg grating has great potential in biochemical detection, environmental monitoring, and medical sensing.

Palladium Nanosheet-Based Dual Gas Sensors for Sensitive Room-Temperature Hydrogen and Carbon Monoxide Detection

Palladium has long been explored for use in gas sensors because of its excellent catalytic properties and its unique property of forming hydrides in the presence of H₂. However, pure Pd-based sensors usually suffer from low response and a relatively high limit of detection. Palladium nanosheets (PdNS) are of particular interest for gas sensing applications due to their high surface area and excellent electrical conductivity. Here, we demonstrate the design and fabrication of low-cost PdNS-based dual gas sensors for room-temperature detection of H₂ and CO over a wide concentration range. We fabricated sensors using multiwalled carbon nanotube@PdNS (MWCNT@PdNS) composites and compared their performance against pure PdNS devices for hydrogen sensing based on electrical resistive response. Devices using PdNS alone had a response and response time of 0.4% and 50 s, respectively, to 1% H₂ in air. MWCNT@PdNS (1:5 mass ratio) showed enhanced performance at a lower hydrogen concentration with a limit of detection (LOD_H2) of 5 ppm. Nearly an order of magnitude increase in response was observed on increasing the amount of MWCNT to 50 mass % in the nanocomposite, but the response fell off at low H₂ concentration. Overall, these PdNS-based sensors were found to show good repeatability, stability, and performance under humid conditions. Their response was selective for H₂ versus CH₄, CO₂, and NH₃; the response to CO was comparable in magnitude but opposite in sign to the response to H₂. Upon simultaneous exposure to equal concentrations (10 ppm each) of H₂ and CO, the response to CO was dominant. The PdNS showed high sensitivity to CO, detecting as little as 1 ppm CO in air at room temperature. The sensitivity to CO could be used either in a stand-alone room-temperature CO detector, where H₂ is known not to be present, or in combination with CO and combustible gas detectors to distinguish H₂ from other combustible gases.

Review of Dissolved CO and H2 Measurement Methods for Syngas Fermentation

Syngas fermentation is a promising technique to produce biofuels using syngas obtained through gasified biomass and other carbonaceous materials or collected from industrial CO-rich off-gases. The primary components of syngas, carbon monoxide (CO) and hydrogen (H2), are converted to alcohols and other chemicals through an anaerobic fermentation process by acetogenic bacteria. Dissolved CO and H2 concentrations in fermentation media are among the most important parameters for successful and stable operation. However, the difficulties in timely and precise dissolved CO and H2 measurements hinder the industrial-scale commercialization of this technique. The purpose of this article is to provide a comprehensive review of available dissolved CO and H2 measurement methods, focusing on their detection mechanisms, CO and H2 cross interference and operations in syngas fermentation process. This paper further discusses potential novel methods by providing a critical review of gas phase CO and H2 detection methods with regard to their capability to be modified for measuring dissolved CO and H2 in syngas fermentation conditions.

Hypersensitive H2 sensor based on polymer planar Bragg gratings coated with Pt-loaded WO3-SiO2

This Letter demonstrates a novel, to the best of our knowledge, hydrogen sensor based on a polymer planar Bragg grating coated with Pt-loaded WO₃-SiO₂. The reflected Bragg signal shows a distinct peak splitting correlated to substrate anisotropies originating from the injection molding process. Especially at low H₂ concentrations, both sensing peaks exhibit an outstanding response to the heat generated by the exothermic reaction between hydrogen molecules and coating. Thereby, a hydrogen volume ratio of 50 ppm leads to a Bragg wavelength shift of -37pm, which yields an outstandingly low detection limit of only 5 ppm H₂ in air. Thus, functionalized polymer planar Bragg gratings are eminently suitable for H₂ leak detection applications.

Ruthenium Decorated Polypyrrole Nanoparticles for Highly Sensitive Hydrogen Gas Sensors Using Component Ratio and Protonation Control

Despite being highly flammable at lower concentrations and causing suffocation at higher concentrations, hydrogen gas continues to play an important role in various industrial processes. Therefore, an appropriate monitoring system is crucial for processes that use hydrogen. In this study, we found a nanocomposite comprising of ruthenium nanoclusters decorated on carboxyl polypyrrole nanoparticles (Ru_CPPy) to be successful in detecting hydrogen gas through a simple sonochemistry method. We found that the morphology and density control of the ruthenium component increased the active surface area to the target analyte (hydrogen molecule). Carboxyl polypyrrole (CPPy) in the nanocomposite was protonated to increase the charge transfer rate during gas detection. This material-based sensor electrode was highly sensitive (down to 0.5 ppm) toward hydrogen gas and had a fast response and recovery time under ambient conditions. The sensing ability of the electrode was maintained up to 15 days without structure deformations.

Review of Fiber Optic Diagnostic Techniques for Power Transformers

Diagnostic and condition monitoring of power transformers are key actions to guarantee their safe operation. The subsequent benefits include reduced service interruptions and economic losses associated with their unavailability. Conventional test methods developed for the condition assessment of power transformers have certain limitations. To overcome such problems, fiber optic-based sensors for monitoring the condition of transformers have been developed. Flawlessly built-up fiber optic-based sensors provide online and offline assessment of various parameters like temperature, moisture, partial discharges, gas analyses, vibration, winding deformation, and oil levels, which are based on different sensing principles. In this paper a variety and assessment of different fiber optic-based diagnostic techniques for monitoring power transformers are discussed. It includes significant tutorial elements as well as some analyses.

Plasmonic fiber optic hydrogen sensor using oxygen defects in nanostructured molybdenum trioxide film

Hydrogen is one of the most promising candidates for fulfilling the next energy demands in transportation, aerospace, heating, and power generation. Due to its highly explosive nature, hydrogen leakage sensors are considered a critical industrial need. We propose a room-temperature, high-sensitivity hydrogen sensor using oxygen defect-induced plasmonic features. The proposed sensing probe utilizes nanostructured α-MoO₃ thin film as the sensing material in which free carriers and plasmonic properties are induced in response to hydrogen exposure. A notable blue spectral shift of 70.6 nm is observed in response to hydrogen gas exposure from 150 ppm to 2000 ppm, which confirms the sensor's capability for efficient detection of low hydrogen concentrations. The sensor's sensitivity, linearity, and reversibility are experimentally investigated through a simple optical setup.

High-sensitivity and fast-response fiber-tip Fabry-Pérot hydrogen sensor with suspended palladium-decorated graphene

The safe utilization of hydrogen as a clean fuel and industry material requires the reliable detection of a gas leakage; thus, hydrogen sensors with high sensitivity and fast response are in urgent demand. Among various hydrogen detection techniques, optical hydrogen sensors are especially attractive because of their intrinsic safety. Herein, by integrating an optical fiber with a suspended palladium (Pd)-decorated graphene, we demonstrate a fiber-optic hydrogen sensor with fast-response that is sensitive, compact and capable of remote detection. The suspended Pd-decorated graphene, attached to the optical fiber tip with an air cavity, forms a flexible Fabry-Pérot interferometer. Upon hydrogen absorption, the ultrathin Pd film facilitates a fast hydrogen dissociation and the ultrathin graphene enables an effective conversion of the Pd lattice expansion to Pd/graphene film displacement, which can be readily measured by fiber-optic interferometery. With a hybrid film of ∼5.6 nm-thick Pd and ∼3 nm-thick suspended graphene, a low detection limit of ∼20 parts per million (ppm) and a short response time of ∼18 s have been achieved. In addition to providing a compact and all-optical solution to sensitive and fast-response hydrogen detection, the rationally-designed sensor can be used for diverse chemical/gas sensing applications by replacing Pd with other functional materials.

Colorimetric hydrogen gas sensor based on PdO/metal oxides hybrid nanoparticles

We have synthesized new colorimetric hydrogen-sensing materials, PdO/metal oxide hybrid nanoparticles, in which palladium oxide was loaded upon surface of substrate materials via an acid-base reaction between a H₂PdCl₄ solution and substrate materials, ZnO, MgO, TiO₂, and SiO₂ respectively at 25 °C. The colorimetric hydrogen gas sensing properties of all the samples, PdO/ZnO, PdO/MgO, PdO/TiO₂ and PdO/SiO₂, were characterized and compared in order to investigate how hydrogen gas sensitivity would be affected by surface property of substrate materials. It was confirmed that the amount of the loaded PdO, which was thought to be closely related with the colorimetric hydrogen sensitivity, was quite different according to the substrate materials and was increased with increasing of the basicity of substrate materials (ZnO > MgO > TiO₂ > SiO₂). Consequently, among the PdO/metal oxide hybrid nanoparticles, the largest amount of PdO was observed to be loaded on ZnO substrate nanoparticles due to its highest basicity. The best colorimetric hydrogen gas sensing properties (color difference, ΔE = 71.57) was observed in PdO/ZnO hybrid nanoparticles, showing the most prominent color change from brown to black, when the sample was exposed to hydrogen gas of 4 vol% balanced with nitrogen for 2 min.

A New Hydrogen Sensor Based on SNS Fiber Interferometer with Pd/WO₃ Coating

This paper presents a new hydrogen sensor based on a single mode–no core–single mode (SNS) fiber interferometer structure. The surface of the no core fiber (NCF) was coated by Pd/WO₃ film to detect the variation of hydrogen concentration. If the hydrogen concentration changes, the refractive index of the Pd/WO₃ film as well as the boundary condition for light propagating in the NCF will all be changed, which will then cause a shift into the resonant wavelength of interferometer. Therefore, the hydrogen concentration can be deduced by measuring the shift of the resonant wavelength. Experimental results demonstrated that this proposed sensor had a high detection sensitivity of 1.26857 nm/%, with good linearity and high accuracy (maximum 0.0055% hydrogen volume error). Besides, it also possessed the advantages of simple structure, low cost, good stability, and repeatability.