Hydrogen sensors based on noble metal doped metal-oxide semiconductor: A review

With the exhaustion of traditional energy, increasing attention has been paid on the new energy resources like H₂. Because of its inflammable and explosive properties, it is imperative and challenging to detect ppm-level H₂ during the transport and use process. This paper firstly introduces the working principles and sensing mechanism of the hydrogen sensors based on noble metal doped metal-oxide semiconductors. Then, this paper focuses on the advancement of noble metal doped metal oxide hydrogen sensors, especially the room temperature hydrogen sensors, in the recent years. At the end, we propose that fabricating semiconductors with special morphologies, using two different noble metals for bimetallic doping or composite semiconductors with 2D nanomaterials like graphene/MoS₂ to improve the room temperature sensing properties towards low H₂ concentration should be the emphasis for the future work.     

Is the H2 economy realizable in the foreseeable future? Part I: H2 production methods

The efforts on energy system decarbonization and improved sustainable energy efficiency in developed countries led energy enthusiasts to explore alternative highly effective pathways in accomplishing these goals. Specifically, the transition from hydrocarbon to H₂ economy using fuel cells and H₂ technologies is a sustainable and favorable approach forward in meeting stationary, transportation, industrial, residential, and commercial sectors. This review in three Parts brings out the capability of H₂ for enabling an energy revolution through much-needed flexibility in renewable energy resources. The review identifies the developments and challenges within the H₂ generation, storage, transportation, distribution, and usage - as well as applications along with national and international initiatives in the field, all of which suggest a pathway for a greener H₂ society. The review also highlights the opportunities and challenges in major energy sectors for H₂ technologies. Part I of the series highlights the importance of H₂ economy and initiatives from various agencies, and presents several H₂ generation methods.     

An overview on progress,advances, and future outlook for biohydrogen production technology

The hydrogen (H₂) economy has been recognized globally from a socio-economic, and environmental viewpoint. Hydrogen can be produced using technologies such as electrolysis, thermochemical, photoelectrochemical, and biological methods. Biological methods help in waste management and energy production simultaneously. To make the biohydrogen process economic and viable at a commercial scale there is a need to utilize renewable raw materials efficiently. Improved reactor designs and advanced genetic improvements using metabolic pathways developments are mandated to improve microbial adaptation to the harsh process conditions. Finally, this review aimed to fill the void among technical and applied research and review the current advances in genetic engineering and metabolic pathway developments to improve H₂ productivity.     

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

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

Exploration of waste-generated nanocomposites as energy-driven systems for various methods of hydrogen production; A review

Recently, waste-generated nanomaterials have received a surge in consideration for energy production due to their excellent properties and eco-friendly effects. A plethora of literature has reported on the growing interest for the production of hydrogen (H₂) by utilizing sunlight through water splitting and the recycling of bio-waste into organic catalysts. To the best of our knowledge, there has been no review on waste-derived nanocomposites for the production of H₂. Herein, potential methods, modes of fabrication, and the efficacy of synthesized nanocomposites for H₂ production have been highlighted. Distinct attention has been given to waste-generated nanocomposites, including a variety of graphene nanomaterials. Remarkable efforts have been made to fabricate biomass-derived nanomaterials and to optimize their reaction conditions and efficient use in energy production. Finally, future prospects and challenges in improving photocatalytic H₂ production are also summarized.     

An effective method to screen carbon (boron,nitrogen) based two-dimensional hydrogen storage materials

The current critical environmental pollution caused by the huge fossil fuel burning together with the increasingly scarce energy source has inspired much attention on the renewable clean hydrogen energy. Thus, the hydrogen storage materials are very vital for the hydrogen application and will be screened by the high-throughput computational screening procedure in this paper. Generally, metal-modified carbon (boron, nitrogen) nanomaterials can exhibit excellent hydrogen storage capacities. An effective procedure is designed to screen the potential metal decorated carbon (boron, nitrogen) hydrogen storage materials from the Materials Project database, which can be proved to be easily realized and reliable. Totally six ideal structures are obtained for hydrogen storage by considering the thermodynamic stability, the metal decorating, the theoretical hydrogen gravimetric density larger than 5.5 wt%, the PBE band gap smaller than 1.0 eV, and the two-dimensional structure restriction. Furthermore, the binding energy of the metal atom to the screened 2D materials, the average adsorption energy per H₂ adsorbed by the metal, the density of states, the difference charge densities are calculated by the density functional method. We believe that our screening procedure can effectively and accurately search for hydrogen storage materials, which should be the theoretical basis for experimental researches.     

Rapid hydrogen sensing response and aging of α-MoO3 nanowires paper sensor

In order to improve the hydrogen sensing of transition metal oxide nanomaterials at room temperature, MoO₃ nanowire paper was prepared and used as a hydrogen sensing materials on substrate. In this paper, orthorhombic phase, ultra-long (～1 mm) MoO₃ nanowires were synthesized through conventional hydrothermal method at 260 °C for 96 h. A flexible nanowires paper with size of 200 mm × 300 mm was obtained by further self-assembly formation process of pure α-MoO₃ nanowires in aqueous solution on hydrophobic substrate, and the thickness of paper can be controlled depend on the concentration of disperse nanowires. A novel hydrogen sensor with sensing area of 10 mm × 10 mm was obtained after Pt interdigital electrodes (IDE) deposited on the surface of α-MoO₃ nanowires paper and transferred into ceramic circuit board (CCB) without any surface modification. The response and recovery time are about 3.0 and 2.7 s toward 1.5% H₂, respectively. The sensors also show good selectivity toward H₂ against other reduce gas, such as C₂H₅OH, CO and CH₄. Large amounts of the porous structures and high specific surface of nanowires paper is beneficial to the absorption of oxygen molecules, which would lead to the high sensitivity, fast response and recovery speed of sensor at room temperature. MoO₃ nanowires paper sensors have excellent stability and reliability, which could work for 5.56 years at room temperature.     

Is the H2 economy realizable in the foreseeable future? Part II: H2 storage,transportation, and distribution

The goal of the review series on the H₂ economy is to highlight the current status, major issues, and opportunities associated with H₂ production, storage, transportation, distribution and usage in various energy sectors. In particular, Part I discussed the various H₂ (grey and green) production methods including the futuristic ones such as photoelectrochemical for small, medium, and large-scale applications. Part II of the H₂ economy review identifies the developments and challenges in the areas of H₂ storage, transportation and distribution with national and international initiatives in the field, all of which suggest a pathway for establishing greener H₂ society in the near future. Currently, various methods, comprising physical and chemical routes are being explored with a focus on improving the H₂ storage density, capacity, and reducing the cost. H₂ transportation methods by road, through pipelines, and via ocean are pursued actively in expanding the market for large scale applications around the world. As of now, compressed H₂ and its transportation by road is the most realistic option for the transportation sector.     

Sensors for anaerobic hydrogen measurement: A comparative study between a resistive PdAu based sensor and a commercial thermal conductivity sensor

Hydrogen sensors able to perform measurements in real time in anaerobic environment such as natural gas (NG) will greatly help the development of power to gas technology. For now, thermal conductivity (TC) gas sensors and Pd thin film based sensors have demonstrated their capability to measure H₂ in air and N₂ but there is still lack of testing in natural gas environment. In this study, the sensing performances (response, hysteresis, response time and selectivity) of two sensors were assessed in three anaerobic environments: N₂, CH₄, and NG. The first one is a homemade resistive sensor based on a PdAu thin film and the second one is a commercial thermal conductivity sensor. While most performances are equivalent for both technologies, only the PdAu sensor is able to detect selectively H₂, without any interfering effect with NG components. Thus, Pd based thin film sensors are promising for H₂ detection in anaerobic environments.     

Current trends in structural development and modification strategies for metal-organic frameworks (MOFs) towards photocatalytic H2 production: A review

Photocatalytic H₂ generation using semiconductor photocatalysts is considered as a cost-effective and eco-friendly technology for solar to energy conversion; however, the present photocatalysts have been recognized to depict low efficiency. Currently, porous coordination polymers known as metal-organic frameworks (MOFs) constituting flexible and modifiable porous structure and having excess active sites are considered to be appropriate for photocatalytic H₂ production. This review highlights current progress in structural development of MOF materials along with modification strategies for enhanced photoactivity. Initially, the review discusses the photocatalytic H₂ production mechanism with the concepts of thermodynamics and mass transfer with particular focus on MOFs. Elaboration of the structural categories of MOFs into Type I, Type II, Type III and classification of MOFs for H₂ generation into transition metal based, post-transition metal based, noble-metal based and hetero-metal based has been systematically discussed. The review also critically deliberate various modification approaches of band engineering, improvement of charge separation, efficient irradiation utilization and overall efficiency of MOFs including metal modification, heterojunction formation, Z-scheme formation, by introducing electron mediator, and dye based composites. Also, the MOF synthesized derivatives for photocatalytic H₂ generation are elaborated. Finally, future perspectives of MOFs for H₂ generation and approaches for efficiency improvement have been suggested.     

A critical review on improving hydrogen storage properties of metal hydride via nanostructuring and integrating carbonaceous materials

Hydrogen-based economy has a great potential for addressing the world's environmental concerns by using hydrogen as its future energy carrier. Hydrogen can be stored in gaseous, liquid and solid-state form, but among all solid-state hydrogen storage materials (metal hydrides) have the highest energy density. However, hydrogen accessibility is a challenging step in metal hydride-based materials. To improve the hydrogen storage kinetics, effects of functionalized catalysts/dopants on metal atoms have been extensively studied. The nanostructuring of metal hydrides is a new focus and has enhanced hydrogen storage properties by allowing higher surface area and thus reversibility, hydrogen storage density, faster and tunable kinetics, lower absorption and desorption temperatures, and durability. The effect of incorporating nanoparticles of carbon-based materials (graphene, C60, carbon nanotubes (CNTs), carbon black, and carbon aerogel) showed improved hydrogen storage characteristics of metal hydrides. In this critical review, the effects of various carbon-based materials, catalysts, and dopants are summarized in terms of hydrogen-storage capacity and kinetics. This review also highlights the effects of carbon nanomaterials on metal hydrides along with advanced synthesis routes, and analysis techniques to explore the effects of encapsulated metal hydrides and carbon particles. In addition, effects of carbon composites in polymeric composites for improved hydrogen storage properties in solid-state forms, and new characterization techniques are also discussed. As is known, the nanomaterials have extremely higher surface area (100–1000 time more surface area in m²/g) when compared to the bulk scale materials; thus, hydrogen absorption and desorption can be tuned in nanoscale structures for various industrial applications. The nanoscale tailoring of metal hydrides with carbon materials is a promising strategy for the next generation of solid-state hydrogen storage systems for different industries.     

Hydrogen gas sensing methods,materials, and approach to achieve parts per billion level detection: A review

Being a clean source of energy, hydrogen gas is in high demand in various industrial and commercial applications. However, the explosive nature of H₂ gas above 4% concentration makes it highly dangerous to store, transport and use. Further, the small size gas molecules of H₂ are prone to leak through the smallest possible holes and cracks. Hence, the detection of H₂ gas becomes essential even at trace levels. This article reviews various gas sensing strategies including methods, materials, and integrated systems available for the sensitive detection of H₂ gas for a bunch of different applications. The article also reviews some approaches which are available in the literature to detect parts per billion (ppb) level of H₂ gas concentrations. This review article aims at explaining the different aspects of H₂ gas sensing technology in a simple yet exhaustive manner.     

A review of recent advances in hydrogen purification for selective removal of oxygen: Deoxo catalysts and reactor systems

As a response to climate change caused by a surge in greenhouse gas emissions and the associated transition to sustainable energy systems, hydrogen (H₂) is considered as an attracting alternative energy source. In that context, water electrolysis combined with renewable energy source, so called green H₂, has been recently receiving great attention from worldwide. However, for stable use of H₂ as an energy source in various type of fuel cells to produce electricity, the purity of H₂ must not only meet the required level but also be maintained stably. Among possible impurities in the produced H₂ steam, oxygen (O₂) must be controlled below certain level in any circumstances for safety, hence catalytic purification for intensive and selective removal of O₂ in H₂ steam is nowadays regarded as an essential technology for commercial application of green H₂ because of the intermittent nature of renewable energy. Nevertheless, the catalytic purification technology, especially the technology for catalyst and process development, is still in the basic stage, and the reported technologies have not been systemically organized yet. Therefore, this review 1) briefly summarizes the developmental trends and current available technologies of various H₂ purification technologies, such as membrane separation, pressure swing adsorption, metal hydride, and cryogenic separation 2) and introduces the developmental of deoxo catalysts and catalytic H₂ purification technologies with future research perspectives and suggestions.     

Resistive-type hydrogen gas sensor based on TiO2: A review

Owing to its high energy density and environmentally friendly nature, hydrogen has already been regarded as the ultimate energy of the 21st century and gained significant attention from the worldwide researchers. Meanwhile, there are increasing concerns about its safe use, storage and transport as, despite being colorless and odorless, after certain concentration level it becomes flammable and explosive in air. Therefore, it is imperative to develop H₂ sensors for real-time monitoring of the H₂ leakage for an early warning. This paper firstly introduces the general hydrogen gas sensing mechanism of TiO₂-based hydrogen sensors. Then we summarize and comment on the current hydrogen gas sensor based on various TiO₂ materials, which include pristine TiO₂, metal-assisted TiO₂, organic-TiO₂ composites, carbon-TiO₂ composites, MOX-TiO₂ composites and novel sensor concept with effective top-bottom electrode configuration. Finally, we briefly discuss the obstacles that TiO₂-based H₂ sensors have to overcome in the progress of the systematically practical application, possible solutions, and future research perspectives that can be focused in this area.     

Enhanced reversible hydrogen storage performance of light metal-decorated boron-doped siligene: A DFT study

The use of nanomaterials for hydrogen storage could play a very important role in the large-scale utilization of hydrogen as an energy source. However, nowadays several potential hydrogen storage nanomaterials do not have a large gravimetric density and stability at room temperature. In this work, we have investigated the hydrogen storage performances of Na-, K- and Ca-decorated B-doped siligene monolayer (BSiGeML) using density functional theory calculations. The results show that boron doping improves the interaction between the metal adatom and the siligene monolayer (SiGeML). The K- and Ca-decorated BSiGeMLs can bind up to seven H₂ molecules per metal adatom, whereas Na-decorated BSiGeML only adsorb four H₂ molecules per adsorption site. The effect of temperature and pressure on the hydrogen storage capacity of BSiGeMLs was also evaluated. At room temperature, all the H₂ molecules adsorbed on Na-, and Ca-decorated BSiGeML are stable at mild pressure. The metal decoration of both sides of BSiGeML may lead to hydrogen gravimetric densities exceeding the target of 5.5 wt% proposed by DOE for the year 2025. K- and Ca-decorated BSiGeML could be efficient hydrogen molecular storage media compared to undoped SiGeML and other 2D pristine materials.     

Pd (II) decorated conductive two-dimensional chromium-pyrazine metal-organic framework for rapid detection of hydrogen

The utilization of H₂ for versatile application has demanded highly selective, low cost and rapid hydrogen sensors that are proficient in sensing H₂ near flammability limit. In this report, Cr^IIICl₂(pyrazine)₂ MOF with negatively charged pyrazine linkers in its structure is used for the stabilization of Pd (II) via charge transfer interactions. This material design turned an innocent MOF into selective hydrogen sensor that can respond (through decrease in resistance under dynamic sensing setup) to H₂ in 5–7 s with a detection range of 0.25%–1% H₂ concentration. A correlation of H₂ sensing characteristics and the structure-property relationship is established using density functional theory (DFT) calculations. The calculations suggested that near fermi level in Pd^II@CrPy, the bandwidth increases upon interaction with H₂ thereby the phase space for electron delocalization increases leading to better carrier mobility. This new approach not only yields novel sensing properties but also enables limited usage of precious metal to develop cost-effective sensors.     

Nanoconfinement effects on hydrogen storage properties of MgH2 and LiBH4

To find a solution to efficiently exploit renewable energy sources is a key step to achieve complete independence from fossil fuel energy sources. Hydrogen is considered by many as a suitable energy vector for efficiently exploiting intermittent and unevenly distributed renewable energy sources. However, although the production of hydrogen from renewable energy sources is technically feasible, the storage of large quantities of hydrogen is challenging. Comparing to conventional compressed and cryogenic hydrogen storage, the solid-state storage of hydrogen shows many advantages in terms of safety and volumetric energy density. Among the materials available to store hydrogen, metal hydrides and complex metal hydrides have been extensively investigated due to their appealing hydrogen storage properties. Among several potentials candidates, magnesium hydride (MgH₂) and lithium borohydride (LiBH₄) have been widely recognized as promising solid-state hydrogen storage materials. However, before considering these hydrides ready for real-scale applications, the issue of their high thermodynamic stability and of their poor hydrogenation/dehydrogenation kinetics must be solved. An approach to modify the hydrogen storage properties of these hydrides is nanoconfinement. This review summarizes and discusses recent findings on the use of porous scaffolds as nanostructured tools for improving the thermodynamics and kinetics of MgH₂ and LiBH₄.     

Surface and interface properties of monolayer graphene on hydrophobic and hydrophilic ultrananocrystalline diamond structures for hydrogen sensing applications

Here, for the first time, we synthesize hybrid hydrophilic and hydrophobic nanocarbon materials with reliable and stable gas sensing performance. The hybrid monolayer graphene (Gr)–nitrogen and argon (N₂ and Ar) gas incorporated ultra-nanocrystalline diamond (Gr/N₂@UNCD and Gr/Ar@UNCD) structures were synthesized using a microwave plasma enhanced chemical vapor deposition (MPECVD) method. The presented nanohybrid combinations have a unique morphology with diamond defects (sp³) covered by a graphene sheet (sp²). Sample sensors with metal electrodes were fabricated to study the H₂ gas sensing properties of the material. Thus, the as-fabricated Gr/N₂@UNCD exhibited higher sensor response (14.6%) than those of the as-fabricated Gr, N-UNCD and Gr/Ar@UNCD (3.6, 1.07 and 11.2%) based devices. The Gr/N₂@UNCD nanohybrid based sensor showed outstanding repeatability, selectivity and stability over ～56 days. The substantial improvement in the H₂ sensing performance of the as-fabricated Gr/N₂@UNCD nanohybrid based sensor was attributed to the modifications in surface morphology and resistance. The partial-hydrophobic surface of Gr/N₂@UNCD alters the beneficial resistivity and improved absorption, which assists in the efficient transport of electrons and H₂ gas molecules. The hybrid nanostructure of Gr-N₂@UNCD exhibits several unique properties that paves the way to future opportunities for advanced gas sensor fabrication.     

Metal hydride hydrogen storage and compression systems for energy storage technologies

Along with a brief overview of literature data on energy storage technologies utilising hydrogen and metal hydrides, this article presents results of the related R&D activities carried out by the authors. The focus is put on proper selection of metal hydride materials on the basis of AB₅- and AB₂-type intermetallic compounds for hydrogen storage and compression applications, based on the analysis of PCT properties of the materials in systems with H₂ gas. The article also presents features of integrated energy storage systems utilising metal hydride hydrogen storage and compression, as well as their metal hydride based components developed at IPCP and HySA Systems.