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

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Review Article

Hydrogen gas sensing methods, materials, and approach to achieve parts per billion level detection: A review Pankaj Singh Chauhan a, Shantanu Bhattacharya a,b,* a

Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, Uttar Pradesh, India b Design Programme, Indian Institute of Technology Kanpur, Kanpur, 208016, Uttar Pradesh, India

highlights
Hydrogen gas detection at different utilization stages is essential due to explosive nature of the gas.
Above 4% concentration level of hydrogen is highly explosive and dangerous.
Small size of the H2 gas molecule leaks through small holes very easily.
The detection of H2 gas at ppb level is required to ensure the safety.
Improvement in sensing properties of some materials facilitates the ppb level H2 detection.

article info

abstract

Article history:

Being a clean source of energy, hydrogen gas is in high demand in various industrial and

Received 12 February 2019

commercial applications. However, the explosive nature of H2 gas above 4% concentration

Received in revised form

makes it highly dangerous to store, transport and use. Further, the small size gas molecules

1 August 2019

of H2 are prone to leak through the smallest possible holes and cracks. Hence, the detection

Accepted 8 August 2019

of H2 gas becomes essential even at trace levels. This article reviews various gas sensing

Available online 5 September 2019

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

Keywords:

approaches which are available in the literature to detect parts per billion (ppb) level of H2

Hydrogen

gas concentrations. This review article aims at explaining the different aspects of H2 gas

Energy

sensing technology in a simple yet exhaustive manner.

Sensing

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Sensors

* Corresponding author. Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, Uttar Pradesh, India. E-mail address: [email protected] (S. Bhattacharya). https://doi.org/10.1016/j.ijhydene.2019.08.052 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26077 H2 sensing technologies/methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26079 Thermal detection scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26080 Electrochemical detection scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26080 Electrical detection scheme through change in conductivity/resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26080 Work function based detection scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26081 Mechanical scheme of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26082 Optical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26082 Acoustic method of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26083 Catalytic method of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26083 Triboelectric method of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26085 Materials for hydrogen gas detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26085 Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26086 Metal oxide semiconducting materials (MOS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26087 Carbon based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26087 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26089 Effect of shape of nanostructures on gas sensing performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26089 Nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26089 Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26089 Nanosheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26090 Nanospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26090 Parts per billion (ppb) level hydrogen gas detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26090 Advances, current issues and future scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26095 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26095 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26096

Introduction As the world population is phenomenally increasing almost in a routine manner, the demand of various products and services is also increasing exponentially. The current scenario of industrial growth coupled with drastic growth of population warrant a very high demand for energy, which needs to be supplied through conventional and renewable sources of energy. Sustainable energy sources have definitely been the foundation of many recent scientific and technology development activities. The scientists have been trying rigorously to find out the alternate solutions to this crisis like situation through techniques, which are by and large green and do not add to the existing carbon footprint [1]. In this context, it is probably worth mentioning that Hydrogen based energy generation has been a globally accepted practise for obtaining clean energy. Hydrogen is also abundantly used in various industrial and space related applications and thus definitely has shown a way ahead for managing clean energy stringent requirements of the world. Hydrogen gas has high energy density and becomes explosive when its concentration increases beyond 4% [2]. Therefore, the production, storage and transportation of hydrogen gas becomes very risky. Hence, it is essential to monitor the concentration level of hydrogen gas to avoid any hazardous situation. Many convenient methods

and technologies have been developed by researchers to monitor the hydrogen gas concentration. Different type of analytical instruments such as infra-red (IR) spectroscope, chemiluminescence spectrometer, ultraviolet (UV) adsorption spectroscope and gas chromatography columns have been used abundantly for monitoring of trace hydrogen concentrations [3e6]. These instruments have shown good response and wide range of detection, however they suffer from shortcomings like large size and weight, high cost, time consuming process, requirement of trained personnel to operate them, maintenance and portability issues etc. The above mentioned limitations restrict the continuous operation of such instruments. During the current decade sensor development technologies based on metal oxide semiconductors (MOS) have emerged as a light weight, cost effective, fast, sensitive, and simple detection method [7,8]. The materials in the metal oxide semiconductor category which are widely used for hydrogen detection are: Zinc oxide (ZnO), Titanium dioxide (TiO2), Tin oxide (SnO2), Tungsten oxide (WO3), Vanadium pentoxide (V2O5), and Iron oxide (Fe2O3) etc. Some studies suggest the effective use of carbon based materials such as graphene oxide (GO) and carbon nanotubes for the sensitive detection of H2 gas [9e12]. The shape and size of the MOS plays a pivotal role in surface adsorption applications such as gas sensing and photocatalytic reactions [13,14]. The nanostructures such as

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nanosheets and hollow spheres offer a large surface area which facilitates greater interactions with the diffusive gas molecules [15]. Researchers have been focused to develop different techniques and strategies for sensitivity enhancement and changing efficacy of the sensing element in gas sensor design and development. Through their continuous effort in technological development of sensor performance by using different elements have seen drastic enhancements in recent times [16]. The sensor performance includes the sensitivity and selectivity towards different types of gases, low response and recovery times of the sensing element, low operating temperature at which sensing may be carried out, and low power utilization etc. [17]. The methods used for performance enhancement includes light assisted sensing, doping of metal nanoparticles with MOS, and making composite with carbon based materials [18e22]. In presence of visible light the electrons jump from valence band of the semiconductor material to the conduction band and increases the overall response by increasing the number of charge carriers. The loading of noble metals such as Gold (Au), Platinum (Pt), Silver (Ag), and Palladium (Pd) etc., causes the performance enhancement of the gas sensors by improving the charge transfer process [18,23e25]. Similarly, carbon based materials improve the charge transfer efficiency of the sensor by providing a conducting path for the excited electrons. Few studies suggest that the composite of metallic, carbon, and polymer based material as an effective sensing element for the detection of H2 gas at room temperature. The functional groups attached with graphene enable it to interact with polar molecules/ groups of polymers. This type of interaction is useful in making composite material between two materials. Also, the combination of the electronic, chemical, and physical properties of the two materials improves the H2 sensing performance. Addition of metallic nanoparticles help to adsorb H2 gas molecules through catalytic dissociation of molecular H2 into atomic H. The atomic H easily gets diffused to the interstitial sites of the metallic structure. The polymer based chemiresistive sensors are also found suitable for H2 gas sensing application. Srivastava et al. used TiO2 decorated polyaniline (PANI) thin films for the detection of H2 gas at room temperature [26]. Hydrogen gas molecules protonate the nitrogen atoms of PANI and forms a bridge between the two adjacent chains. This reaction provides the additional delocalized charge carriers and increases the conductivity of the thin film. Doping of TiO2 nanoparticles with PANI forms a heterojunction between the two materials which creates a positively charged depletion region on the surface of TiO2 nanoparticles. Hence the response of the sensor increases due to improved inter-particle electron transfer. The hydrogen gas can also be sensed by the solid electrolyte electrochemical cell system. Utilizing this type of system, Fadeyev et al. used Zn, Sn, Cr, and In based metal oxides as sensing electrodes with silver as reference electrode for high temperature (450e700  C) detection of H2 gas [27]. When a reducing gas such as H2 is passed in the system, the redox reactions take place at the respective electrode and generate a potential difference across the electrodes. The kinetics of the reactions depends upon the structure and type of electrode material. In this study, the electrode made of SnO2 showed highest response.

Pati et al. studied indium doped ZnO for the high operating temperature (200e300  C) detection of H2 gas with selectivity over NO2 at lower operating temperature [28]. The sensing material shows p to n-type carrier change at higher operating temperature resulting into change of resistive response (increasing to decreasing) towards H2 (reducing) gas. While, for NO2 (oxidizing gas) no such change in the resistive response was observed. The reason of such behaviour was attributed to the charge carrier scattering (similar for electron and hole) in presence of oxidizing gas NO2 [29]. Interestingly, some of the biological entities such as bacteria Rhodobacter capsulatus can also be used as H2 sensing medium through fluorescent response. Wecker et al. used this method to detect H2 production by the algae [30]. The Rhodobacter capsulatus was grown with the algae (C. Reinhardtii) for the in situ detection of H2 production. Liu et al. developed a Pd/ WO3/ZnO/Si thin film based schottky diodes for H2 gas detection [31]. When the diode is exposed to H2 gas, the dissociated H atoms react with the adsorbed oxygen molecules and form a dipole layer. The formation of dipole layer raises the fermi level of the material and lowers the barrier height and allows large number of electrons to flow through the thin films. The voltage shift of the diode was analysed as a response in presence of H2 gas. The H2 gas is also used in radioactive environment and requires very developed monitoring system to avoid any catastrophic disaster. Duy et al. studied the effect of gamma ray irradiation on the H2 gas sensing property of PdeSnO2 thin films [32]. The gamma irradiation induces oxygen defect in SnO2 thin films and increases the response of the sensor. Also the doping of Pd improves the performance of the sensor by enabling the material to sense the low concentrations of H2 gas and low radioactive environment working conditions. The increase of oxygen vacancies under high dose of gamma irradiation was confirmed by optical analysis of the samples. Carbon based materials such as graphene and carbon nanotube (CNT) are also used as composite material to improve the performance of metal oxide based gas sensing materials. Dhall et al. used graphene-Pd/SnO2 composites for the sensitive detection of H2 and ethanol [33]. Graphene has 2D structure with high surface area, high electron conductivity, and lower work function than Pd which leads the electron transfer from graphene to Pd. In presence of H2 gas molecules the formation of hydride (PdHx) takes place which also has the lower work function. Hence the addition of graphene becomes helpful in improving the gas sensing performance of a metal oxide/metal nanoparticles based material. Similarly, Reddeppa et al. added reduced graphene oxide (rGO) with GaN nanorods and used the nanocomposite material for the detection of H2 gas under UV light illumination [34]. The addition of rGO provides improved charge transfer from GaN in presence of H2 gas. Also the exposure of UV light enables the transfer of electrons from the valence band of semiconducting material to the conduction band. These photo-generated electrons also take part in the gas sensing reactions. To improve the performance of some of the metal oxides, annealing has been found an effective method. Yang et al. annealed MoO3 nanoribbons at 300  C and found a significant improvement in the H2 gas sensing property [35]. The thermal annealing at elevated temperature increases the concentration of Mo5þ defect sites in the nanostructure of MoO3. These

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Mo5þ defect sites act as host to the environmental oxygen gas molecules for chemisorption. Hence, annealing increases the adsorbed oxygen species which further improves the sensing performance due to more H2 gas molecules interacting with oxygen species. Chemisorbed oxygen molecules capture the free electrons from the nanostructure surface which increases the width of the depletion region and increases the resistance of the material. In presence of H2 gas, the formation of H2O molecule releases captured electrons and decreases the width of the depletion region and resistance of the material. Most of the metal oxide based semiconducting materials have n-type charge (electron) carriers which shows decrease in resistance in presence of H2 gas. However, there are some ptype semiconducting materials such as CuO which are sensitive towards H2 gas at high operating temperature (200  C) [36]. The p-type semiconducting material have holes as the majority charge carrier. In presence of H2 gas, the release of captured electrons results into recombination of hole and electron. Hence, in presence of H2 gas the resistance of the ptype material increases. Metal oxide based semiconducting materials mostly respond through resistive change in presence of H2 gas. However, following a different approach Shafieyan et al. used MoO3 colloidal nanoparticles for the detection of H2 gas by colour change method [37]. In presence of H2 gas the colour of the solution turns blue from transparent. While, the long term (5 min) exposure makes the solution brown. Exposure to H2 gas turns the non-plasmonic MoO3 into plasmonic MoO3-x due intercalation of hydrogen atoms. The increase in charge carrier concentration and reduction in band gap was observed after exposure of H2 gas. This phenomenon was observed by UVeVis analysis through the appearance of a localized surface plasmon resonance (LSPR) absorption band. Srivastava et al. irradiated tantalum (Ta)/Polyaniline (PANI) thin films with energetic Auþ12 ions and analysed its effect on H2 gas sensing property [38]. The irradiation of Ta melts the thin films and forms a rough, porous surface with high surface area. This method significantly improves the hydrogen gas sensing property of the material due to increased number of adsorption sites. Choi et al. studied the effect of calcination on the H2 gas sensing property of p-type CuO thin films [39]. The calcination at high temperature (up to 600  C) increases the hole concentration, crystallinity, and surface to volume ratio which ultimately improves the sensor performance. The hydrogen sensors are being redesigned continuously with various performance improvements over past several years. The present technological scenario has now geared to sensing of extremely trace (ppb) level leakage detection of hydrogen gas. Hydrogen gas molecules leak through very small cracks and holes with high flow rate due to its smaller size. The concentration of the H2 gas can reach to the explosive limit in no time. Hence, the H2 gas monitoring is essential in various industrial applications to avoid any loss of money and human life. The requirements for a reliable hydrogen gas sensor can be summarised as follows [40]: i. The sensor should indicate low to moderate level of gas concentrations (0.01e10%). ii. The sensor should detect the gases with precision and accuracy.

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iii. The interference of humidity and other gases should be minimum. iv. It should work properly even at unfavourable operating conditions such as: high pressure, temperature, gas flow rate etc. v. The signal given by sensors should have a low overall noise. vi. The response and recovery times should be less (<5 s). vii. Fabrication and maintenance cost should be less. viii. The sensor should be small and compact. This paper aims to describe and review the different materials used as sensing materials for hydrogen sensing. Also, different techniques and sensing methods will be summarised and compared. The advantages and disadvantages of these methods will also be pointed out wherever possible. The data presented in this paper is taken from recent findings published in literature and authors’ self-experience in hydrogen sensing application domains.

H2 sensing technologies/methods Generally, gas sensors are used to measure the leakages of a particular gas in the vicinity of its storage tank, transport pipes, and working area. However, the hydrogen gas sensors are required to trace very small leakages (~ppb level leakages) because of the explosive behaviour of hydrogen gas itself. The H2 gas sensing technology utilizes various methods where a specific property of a sensing element changes in presence of H2 gas. This specific property can be a thermal based, resistance based, work function based, optically assisted, acoustically assisted, mechanical or electrochemical changes based, or catalytic activity based, respectively. The various methods of detection are shown schematically in Fig. 1. The sensing system also requires a transducer to convert this specific change into an

Fig. 1 e Schematic diagram representing the methods of detection for hydrogen sensing.

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electrical signal which is further processed and analysed [41]. These methods will be reviewed and discussed in details in this section.

Thermal detection scheme Hydrogen gas detection based on thermal conductivity measurements is an effective method and was widely used in past. The hydrogen gas has higher thermal conductivity (0.18 W/mK at 300 K) in comparison with normal air (0.026 W/mK at 300 K) which makes it useful to measure the presence of H2 gas in air. The operating principle of this method is based on the heat loss of a body to the surrounding gas. The extent of heat loss depends upon the thermal conductivity of the surrounding gas. The inclusion of more and more H2 gas in air will increase the heat dissipation due to an increase in the thermal conductivity of the medium. This method utilizes two resistors, one for the reference gas (air) and the other one for the target gas (H2). These resistors are connected to a Wheatstone bridge as shown in Fig. 2. When there is no target gas and only air is present in both the cells, the resistors lose heat equally and give the same reading. When the target gas (H2) enters the respective cell, the heat loss of the target gas resistor defers from the air resistor depending upon the type of gas (heat loss increases in presence of H2 gas). The comparative reading is calibrated in the presence of the ambient Hydrogen concentration. This heat loss difference causes the imbalance in Wheatstone bridge and is measured in form of the overall resistance. Simon and Arndt used the thermal conductivity sensor for the detection of hydrogen gas generated from the automotive fuel cells [42]. In this device the heating element was placed in a dielectric membrane and its size was varied to correlate it with the sensing performance. The sensor was able to detect the hydrogen level up to 0.2%. Tardy et al. used SiC microplate with screen printed platinum resistance as a heating element (with low power consumption ~ 5 mW) for the detection of H2 gas in mixture of other gases such as He, N2, CO, and CH4 [43].

Electrochemical detection scheme Electrochemical sensors are based on the charge transfer phenomena associated with electrodes placed in an electrolyte medium. These type of sensors can be amperometric or

potentiometric type, depending upon the signal that comes out of these sensors. An amperometric sensor uses a constant applied voltage and measures the diffused current. In amperometric sensor the sensing and counter electrodes are used with a reference electrode. The electrodes are generally made of noble metals suitable for hydrogen gas oxidation. In between the electrodes a liquid electrolyte is filled to ease the charge transfer process. The hydrogen gas gets oxidized when being passed through the sensing electrode as in the equation below [44]: H2 / 2Hþ þ 2e

(1)

While at the counter electrode the reduction of oxygen gas takes place following the equation given below: 1 = 2O2 þ 2Hþ þ 2e /H2 O

(2)

Thus the flow of electrons takes place from anode to cathode thus generating a current value which is directly proportional to the hydrogen gas concentration according to the conventional Faraday’s law of electrolysis given below [45]: I ¼ z ,F, Q

(3)

where, F is faraday constant; z is number of electrons transferred per molecule; and Q is the conversion rate of hydrogen in mol/s. This sensor can work in a wide operating temperature range (20 to 80  C). At very high temperatures ceramic electrolytes can be used which may not melt easily. The application of this type of sensors is limited to low humidity levels and negligible interfering gases such as CO, CO2, or hydrocarbons. Similarly in a potentiometric type electrochemical sensor the potential difference or electromotive force between the sensing and reference electrodes are measured. The potential of electrode is a function of the concentration of hydrogen gas and given by the Nernst equation as [46]:  E ¼ E0 þ

   RT a ln zF a0

(4)

where, E represents electrode potential; E0 represents standard electrode potential, R is an universal gas constant; T is the temperature; F is Faraday constant; z is the number of electrons transferred; a is chemical activity of the analyte, which is proportional to H2 gas concentration; and a0 is the activity of the reference electrode. This method gives a sensitivity that ranges from 10 ppm onwards and some of the authors have also found very fast response time of these sensors (<2 s) [47]. Arora and Puri, electrophoretically deposited PdO on ITO substrates and used it as a working electrode for sensitive detection of H2 gas with the limit of detection (LOD) of 0.1% [48]. Yi et al. developed a potentiometric H2 gas sensor with SnO2 based scaffold type sensing electrode which gave fast response (~5 s) at 450  C and a limit of detection of 40 ppm [49].

Electrical detection scheme through change in conductivity/ resistivity

Fig. 2 e Schematic diagram for thermal conductivity measurement through Wheatstone bridge.

In resistive sensors, the measurement of resistance variation is carried out when a sensing material is exposed to a target gas. Metal oxide semiconducting materials (MOS) are mostly used in resistive sensors because of their responsiveness

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Fig. 3 e Schematic of metal oxides based resistive sensor.

towards reducing gases such as hydrogen [50]. The metal oxide based resistive sensors are generally fabricated over a substrate which can be an insulating material such as Al2O3, SiO2 etc. Several layers are coated over the substrate in such sensors as schematically presented in Fig. 3. Typically, interdigitated electrodes (made of noble metal Au, Ag, Pt etc.) are coated over the insulating layer followed by the metal oxide layer etc. In this scheme of sensors a few approaches also suggest the PCB (printed circuit board) based Cu electrodes for bio-sensing and gas detection technologies [14,51]. At the same time, flexible substrates with inkjet printing have also been heavily explored as a future of sensing technology [52,53]. The resistance is measured across the electrodes coated with the MOS sensing element in presence and absence of H2 gas. The difference in resistive load of the element is calibrated with the ambient gas concentration. Some metal oxides have been found to work at a relatively higher temperature with a need of a heating element [54]. There are various metal oxides which have been used in hydrogen sensing application such as: ZnO, V2O5, In2O3, TiO2, VO2, WO3 etc. [55e59] O2 þ 2e /2O

(5)

H2 þ O /H2 O þ e

(6)

The sensing mechanism of metal oxide based sensors is simple and well understood. The environmental oxygen gets adsorb on the surface of metal oxides and captures free electrons (equation (5)) which reduces the conductivity or increases the resistance. In presence of hydrogen gas the adsorbed oxygen reacts with adsorbed hydrogen molecules. This reaction makes a water molecule and releases the free electrons (equation (6)) which decreases the resistance in ntype material and increases the resistance in p-type material. Kim et al. demonstrated a resistive sensor for the H2 gas by using Pd coated ZnO nanorods and observed very high response 12,400% in presence of 2% H2 gas [60]. Similarly, Krivetskiy et al. showed a selectivity analysis of SnO2 based sensor for the detection of an individual gas in a mixture of gases (CO, H2, CH4, C3H8, NO, NO2, H2S, SO2) [61]. They used different artificial neural network based algorithms to identify the CO þ H2 gas with NO2 as background gas. The use of three sensor array with adaptive data processing was suggested to be more accurate in comparison with a single sensor system.

Sometimes the resistance variation of metallic nanoparticles (Pd etc.) in presence of H2 gas molecules is utilized in detection of H2 gas. The phenomenon takes place due to metal hydride formation in presence of hydrogen gas. The Pd nanoparticles crystallize into face centered cubic (fcc) structure [62]. The H2 gas molecules catalytically dissociates into H atom on the surface of Pd. Then, the dissociated H atom diffuses interstitially into the bulk to form palladium hydride PdHx. The composition (x) of hydride formed depends upon the concentration of H2 gas. The formation of non-metallic PdHx from metallic Pd drastically changes the electronic structure of the material due to the insertion of H atoms in the octahedral site of fcc crystal structure of Pd [63e67]. This change in electronic structure increases the resistance of the material which is considered as the detection signal of H2 gas leakages.

Work function based detection scheme The work function is the minimum energy required (in electron volts) to remove an electron from the surface of a metal to infinity. For hydrogen sensing application a metal sensitive to hydrogen gas is coated over an oxide layer. The hydrogen gas atoms diffuse through the metallic layer and get adsorbed at the interlayer of metal and oxide. The hydrogen atoms get polarized at this stage which changes the work function of the metal. This change can be measured as voltage change and the presence of hydrogen gas is confirmed. There are several devices which work on this mechanism such as: i. Metalesemiconductor (Schottky) diode ii. Metaleinsulatoresemiconductor transistor (MISFET or MOSFET) iii. Metaleinsulatoresemiconductor capacitor In Schottky type device the metal is coated over a semiconducting material which aligns the fermi level of both the materials. Chang et al. developed a Pd/HfO2/GaOx/GaN based Schottky diode for sensitive detection of H2 gas with high response at 5 ppm concentration level, and fast response and recovery times (36 s and 35 s respectively) at room temperature [63]. While in MISFET devices a field effect transistor (FET) is utilized to transform the signal from work function change to electrical in presence of hydrogen gas. The metal is called ‘gate’

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for these type of sensors. In metaleinsulatoresemiconductor capacitor method the change in capacitance is measured to detect the hydrogen gas. Podlepetsky et al. developed a MISFET sensor using PdeTa2O5eSiO2eSi structure for sensitive detection of hydrogen gas and analysed the effect of temperature variation from 25  C to 180  C on the sensitivity of the device [68]. It was observed that increase in temperature results into increase in the sensitivity of the device. At the same time, on increasing the temperature the power consumption of the device was also found to increase. Hence the optimum sensitivity in this work was reported at 130  C, while maximum sensitivity was found at 180  C.

Mechanical scheme of detection The mechanical sensors are based on the change of physical property of a metal in presence of hydrogen gas. Generally, a metal sensitive to the hydrogen gas (palladium etc.) is coated over a micro cantilever, which gets expanded in presence of hydrogen gas due to the adsorption of gas molecules into the interstitial sites of lattice. This expansion can be observed in form of deflection or bending of a thin film micro cantilever as shown schematically in Fig. 4 [69]. Palladium is further deposited and micro-machined as a porous element which sometimes enhances the gas diffusivity within the metal structure thus resulting in enhanced sensitivity and reduced response time. This micro cantilever based gas sensing technique has many disadvantages such as: complex fabrication process and delamination of metallic coating due to repeated contraction/ expansion. The sputtering technique for metallic coating has been found to be effective for strong coating over the cantilever. Some researchers used adhesive layer of Cr with Pd for proper bonding with the cantilever [70]. Also, few studies suggested the use of PdeNi and PdeAg instead of using pure Pd to avoid the issue of delamination [71]. In recent advances of microcantilever based detection methods the disadvantages of metallic coating has been eliminated and uncoated silicon microcantilever (USMC) are being used to measure the hydrogen gas detection. However this type of device is suited only for limited applications where selectivity of the gas is not an issue and only one type of gas molecules are expected to

leak out from the system such as hydrogen gas release in radioactive waste disposal facilities [72]. Boudjiet et al. developed USMC sensor and detected the hydrogen gas at low level concentrations (<2%) with 0.02% as a limit of detection [73]. In USMC sensors the change in resonance frequency of silicon microcantilever is measured as a signal of gas leakage. The hydrogen gas molecules increases the surrounding fluid mass density which increases the equivalent mass of the cantilever thereby decreasing the resonance frequency of the cantilevers.

Optical method The optical method of gas sensing is based on a change of optical properties of certain materials after adsorption of gas molecules. In this method a metal or a chemochromic oxide is coated along the length or at the tip of an optical fibre. When this metallic coating is exposed to hydrogen gas it gets expanded axially and radially. This expansion can be measured by using interferometry. These type of optical fibre based sensors are called optodes. There are various methods of detection of hydrogen gas by using optical technique such as: i. ii. iii. iv. v.

Interferometric measurements Reflectivity measurements using micro-mirrors Measurement of surface plasmon resonance (SPR) Evanescent field measurements Chemochromic materials, etc.

As shown in Fig. 5, optical method of hydrogen gas sensing utilizes an interferometer compatible optical cable, and a hydrogen sensitive metallic coating. As the hydrogen gas molecules get adsorbed over the surface of the Pd coating, the reflectivity of the surface changes. When a light beam is passed through the optical cable and received back by the interferometer, there is a change in its reflectivity that can be observed and correlated to the concentration of hydrogen gas. Xu et al. fabricated a hydrogen sensing device with polymer filled hollow core fibre with Pt loaded WO3/SiO2 coating [74]. The sensor uses fibre Bragg grating to detect the hydrogen gas and gives a fast response and recovery time scale with high sensitivity for 0e4% of hydrogen gas concentration.

Fig. 4 e Schematic representation of micro cantilever based hydrogen sensors.

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Fig. 5 e Schematic configuration of optical gas sensors based on interferometer and Pd reflector.

Surface plasmon resonance (SPR), a phenomenon which involves electromagnetic surface waves generated at the interface of a metal and dielectric material is also used for sensing purposes. These waves are sensitive to the interface properties hence can detect the changes caused by adsorbed gas molecules. This phenomenon is used in optical sensing of hydrogen gas. Palladium is found to be a SPR active material and also sensitive to the hydrogen gas which makes it useful in SPR based optical detection of hydrogen. Karker and Carpenter used Au nanorods to obtain high order SPR for the detection of hydrogen gas at high temperature [75]. Some metal oxides (such as WO3) are found to change their colour on exposure to the hydrogen gas. These type of materials are called chemochromic materials and are very useful in detection of hydrogen gas. The titanium dioxide (TiO2) supported palladium oxide (PdO) pigments were developed by Hwang et al. and encapsulated within silicon to form a tape sensor for hydrogen gas [76]. The effect of particle size ranging from 100e5000 nm, and concentration from 0.2 wt% to 10 wt% was analysed and the reported optimum concentration was 3 wt%. Similarly, Kim et al. fabricated PdO/metal oxide (ZnO, MgO, TiO2, and SiO2) nanoparticles for colorimetric detection of hydrogen gas [77]. Among all the metal oxides ZnO was found to be the best one to be hybridized with PdO to give the largest colorimetric change from brown to black for 4% (by volume) concentration of hydrogen gas.

Acoustic method of detection In acoustic method of gas detection, change in surface properties of a piezoelectric material due to adsorption of gas molecules is measured in form of surface generated acoustic waves. There are several devices which can be used to detect surface acoustic waves. A quartz crystal microbalance (QCM) is a simple device to measure the acoustic waves [78]. QCM device has a small and thin quartz disc with electrodes printed on each side which are used to cause deformation in its structure which ultimately results into resonance in the disc. The resonance frequency of the disc is highly sensitive to the mask of the disk. When the ambient gas molecules get adsorb on the coated surface (with the material sensitive to the gas) of the disk, its mass changes which is detected by change in resonance frequency (as shown in Fig. 6). This phenomenon is used to measure the presence of hydrogen gas in the environment. The QCM device response is highly interfered by coexisting gases and temperature which is the major disadvantage of this scheme of measurement. Yang and He used

QCM with grapheme oxide (GO) functionalization for adsorption/desorption based detection of HCHO [79]. The adsorption/ desorption of HCHO takes place via hydrogen bonding with the functional groups present at the surface of GO. Similarly, surface acoustic waves (SAW) generated at the surface of a piezoelectric material are utilized as an effective means of detection and sensing of hydrogen gas. In SAW sensors two interdigitated electrodes are coated on the surface of piezoelectric material (Shown schematically in Fig. 7). One electrode generates surface acoustic waves from electrical signal, while the other converts this wave signal into an electrical signal. The surface acoustic waves are highly sensitive to the surface properties and show a change in output in presence of adsorbed gas molecules on the surface. To use this phenomenon for hydrogen gas sensing application, a hydrogen sensitive material is coated in between the two electrodes which adsorbs the gas molecules and changes its mass and electrical properties. This change is detectable due to the difference in input and output signal of the device which is a result of the change in SAW propagation on the surface of piezoelectric material. Yang et al. developed Pd doped SnO2 sputter coated thin films over LiNbO3 piezoelectric material and used it as a SAW sensor for sensitive detection of hydrogen gas [80]. The sensor showed highest frequency shift of 115.9 kHz at 175  C for 2000 ppm of hydrogen gas with very fast response and recovery time of 1 s and 512 s, respectively. Similarly, Ha et al. fabricated Pd doped graphene based SAW sensor and showed a frequency shift of 25 kHz at room temperature for 0.5% of H2 gas with less than 10 s of response and recovery time [81]. The velocity of sound in hydrogen gas (1314 m/s) is more than in air (346 m/s) at 298 K, which makes the sound velocity measurement a useful technique to detect hydrogen gas. In this method a sound generator (usually a pulse generator) is used with a receiver to detect the propagation time in different gas concentrations. The acoustic methods for hydrogen gas detection have been found to be fast and reliable with a large range of detection (ppm level). However, they always lack in terms of long term stability, temperature dependence and interfering gases.

Catalytic method of detection The catalytic sensors are based on the principle of detection of heat released from the oxidation reaction of combustible gases at the surface of a catalyst [82]. Being a combustible gas, Hydrogen gets oxidized exothermically and releases heat. The

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Fig. 6 e Schematic diagram of quartz crystal microbalance (QCM) based hydrogen sensing device.

Fig. 7 e Schematic representation of surface acoustic wave (SAW) based hydrogen sensor.

heat of combustion for hydrogen gas is 141.9 kJ/g [83]. Pellistor type sensors works on this principle and are widely used for gas detection. In this sensor, two platinum coils covered with ceramic beads are used. One of the bead (active bead) is coated with a hydrogen sensitive catalyst material (eg. Pd etc.), and the other one (inactive bead) is used as a reference. The beads are attached to a Wheatstone bridge to monitor the resistance imbalance within the materials. During gas sensing process, an electrical current is passed through the element which heats the ceramic beads. At high temperature the hydrogen gas molecules get adsorbed on the surface of catalyst. The adsorbed gas molecules get oxidized by the adsorbed oxygen and forms water molecules exothermically. The heat released during this reaction increases the temperature of the active bead and changes its resistance. This resistance change causes the resistance imbalance in the Wheatstone bridge which can be measured and considered as a signal of presence of the gas. Harley-Trochimczyk et al. used Pt nanoparticle loaded graphene aerogel as a catalytic hydrogen sensor [84]. The sensor showed 1.6% sensitivity at 10,000 ppm of hydrogen gas with fast response and recovery time (0.97 s and 0.72 s, respectively), low power consumption (2.2 mW), low limit of detection (65 ppm), and negligible cross sensitivity to interfering gases. The catalytic sensors are also fabricated on the principle of Seebeck effect and are called thermoelectric sensors. The Seebeck effect states that, when there is a change in

temperature of two points of a conducting or semiconducting material, a voltage difference can be observed between these two points (Illustrated schematically in Fig. 8). During hydrogen gas sensing application the oxidation of the gas molecules at the surface of catalyst causes the increase in temperature which is detected in form of voltage change as a signature of hydrogen gas [85]. The change in concentration of the hydrogen gas can be correlated to the change in voltage. The change in voltage signal can be described by the following equation [86]: E ¼ s:DT

(7)

where, s represents the Seebeck coefficient, and DT is the temperature difference created by the exothermic oxidation of the hydrogen gas between the active point and reference point. Brauns et al. used platinum nanoparticles as a catalyst layer for the detection of hydrogen gas [87]. The miniaturized design of this sensor showed high Seebeck coefficient, high response signal of 0.22 mV at 10 ppm concentration level of H2 gas with a fast response of less than 150 ms. Catalytic sensors have certain disadvantages which restricts there use as effective sensors. The catalyst used are sensitive to other combustible gases also, which is a major issue for the selectivity of hydrogen gas. These sensors require high amount of power for heating and cooling of the beads which is a disadvantage in modern sensor technology and supresses the field deployability of the sensing devices.

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Fig. 8 e Schematic representation of the Seebeck effect.

Triboelectric method of detection Triboelectric method of gas detection is based on triboelectric effect generated between the two surfaces. When two dielectric surfaces on metallic electrode are contacted and released cyclically there is a charge separation between the two surfaces. Depending on the electron affinity one surface gains electrons and becomes negatively charged while the other one lose electrons and becomes positively charged. These surfaces induce counter charges on the respective facing electrodes and creates a potential difference. When these electrodes are connected by an external load the flow of electrons takes place between them. This whole phenomenon is called triboelectric effect and the device is called triboelectric generator. Uddin et al. utilized the triboelectric effect and developed a self-powered hydrogen gas sensor [88]. In this study, micropyramids of polydimethylsiloxane (PDMS) were used to make and break the contact between Pd decorated ZnO nanorods. The lower work function of Pd allows the migration of surface electrons from ZnO to Pd. When the device is exposed to air, the environmental oxygen molecules capture the migrated electrons and form chemisorbed oxygen ions (O 2 ). Hence, these captured electrons does not take part in the triboelectric phenomenon in presence of air. When the sensor is exposed to the H2 gas, the H2 gas molecules react with chemisorbed oxygen ions (O 2 ) and make H2O molecules as per the conventional mechanism. Now the captured electrons are free to take part in the triboelectric phenomenon. Large number of free electrons increase surface charge density and screen the triboelectric field resulting into voltage drop. In a similar observation, Shin et al. utilized the triboelectric effect for the detection of H2 gas [89]. In this study the Pd coated indium tin oxide (ITO) and polyethylene terephthalate (PET) films were exposed to the H2 gas. The output voltage of

the device was varying proportionally with H2 concentration. In this method there was no requirement of external power source and the results obtained were reproducible with sensitive response. The above section gives detailed review of different methods used for hydrogen gas sensing. Depending upon the working principles these methods have different characteristics in terms of sensitivity, detectable range, operating parameters, and power requirement etc. Every method has its own advantages and disadvantages and none of them provide an optimum performance. Hence, the selection of a hydrogen sensor becomes application specific, for example the metal oxide semiconductor based sensors are used for low gas concentrations because of their better selectivity and thermal conductivity based sensors are used for high gas concentrations. The summary of hydrogen gas sensing methods is given in Table 1.

Materials for hydrogen gas detection A variety of materials and material systems have been used for hydrogen gas sensing. The selection criteria for material for gas sensing application is primarily the material specific physical, chemical, electrical, and optical properties. To simplify the understanding of these materials, we have classified them into following four different categories: 1. 2. 3. 4.

Metals Metal oxide semiconducting materials (MOS) Carbon based materials Polymers

In the following subsections we will discuss these materials and their usage as H2 gas sensing materials.

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Table 1 e Summary of different methods used for hydrogen gas sensing technology. Sensing Method

Working principle

Thermal

Calorimetric

Electrochemical

Amperometric Potentiometric

Resistive

Resistance variation in presence of H2 gas

Advantage










Work function

Schottky diode MOSFET MIS capacitor

Mechanical

Micro-cantilever bending

Optical

Optodes/ Interferometric






Fast response Low cost Stable Wide measuring range Simple construction Robust Sensitive Low power consumption Working at high temperature is possible Heating element is not required High sensitivity Wide range of operating temperature Low cost Easy fabrication Fast response Low power consumption Small size Fast response Low power requirement High sensitivity and selectivity Low cost Less influence of ambient conditions Small size Does not require oxygen Ability to work in explosive atmosphere Micromachining is possible Can work without explosion Fast response

Acoustic

QCM SAW Sound velocity measurement







High sensitivity, Low power consumption Wide range of detection Fast response

Catalytic

Pellistor Thermoelectric


Wider operating temperature
Stability

Triboelectric

Charge separation between two surfaces

Self-powered devices, fast response



















Metals A sensing material requires large surface area, fast adsorption/desorption kinetics, high diffusion rate, and significant change in material property to be monitored in presence of gas molecules. The use of metallic nanoparticles, metallic core shell structures, and metallic thin films facilitate these requirements in gas sensing applications. For example, S‚ennik et al. used sputter coated thin films of platinum (Pt) to detect the resistive changes in presence of H2 gas [90]. The sensing mechanism of Pt is quite different from Pd which is based on hydride phase formation on exposure to H2 gas. In case of Pt, the atmospheric oxygen adsorbed at the surface of metallic thin film gets replaced by hydrogen gas molecule, gradually. The hydrogen molecule removes chemisorbed oxygen ions by

Disadvantage

Physical change


Sensitive to interfering gases
Heating element reacts with gas
Lower detection limit is high

Thermal conductivity Resistance









Electric current Voltage


Sensitive to interfering sound waves and vibrations
Unable to operate at high temperature
Interference of other gases is possible
High power requirement
Interference with other gases
High response time Complex fabrication







Costly Low life Cross sensitivity to other gases Specific electrolyte requirement Requires regular calibration Cross sensitivity with interfering gases and humidity Poor selectivity High operating temperature Requires O2 to work Affected by gas pressure

Resistance


Existence of Hysteresis losses
Possibility of drift
Saturation occurs at modest concentrations

Voltage Capacitance Current


Hard to fabricate,
Cross-sensitivity with interfering gases
Slow response
Aging effect
Cross-sensitivity with interfering gases and ambient light

Bending Curvature

Reflectance Wavelength Colour SPR Frequency Time Wave velocity

Resistance Voltage Voltage

formation of water molecules. The resistance change is evident due to decrease in charge carrier scattering in presence of adsorbed hydrogen gas molecules. While, in presence of adsorbed oxygen the charge carrier scattering effect is notably high, which increases the resistance value of the sensor. In the similar manner, Sanger et al. sputter coated thin film of Pd/Mg over hydrophobic silicon substrate and used it for H2 detection [66]. In this case the resistance of the sensing material has been found to increase in the presence of H2 gas. Firstly, the hydrogen gas molecules get adsorbed by Pd which catalytically dissociates H2 molecule into H atom. Further, the H atom reacts with Mg and forms MgH2 which increases the resistance. The metal to insulator (hydride formation) transformation has been found to be the main reason of increased

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electron scattering and increase in the resistance values. The large surface area offered by porous silicon substrates and high adsorption capability of Pd provides a faster response (<1 s), however the recovery time has been found to be slow (~1 min), which is a major disadvantage of this material system. In a similar manner Pd nanoclusters were utilized to sense hydrogen gas by modulation of tunnelling gaps in presence of H2 gas molecules [91]. Adsorption of H2 gas molecules on Pd clusters expand the size of the cluster and reduces the cluster gap which further decreases the resistance between the measuring electrodes. Fig. 9 shows the graph of resistance variation with respect to temperature in presence of H2 gas. Rajoua et al. studied the electronic and mechanical antagonist effects of Pd@Au core shell structures in detection of hydrogen gas [62]. The electronic effect arises by the conversion of metallic Pd to non-metallic PdHx due to the insertion of H atom into the crystal structure of Pd. This metallic to non-metallic conversion increases the resistance of the material. While, the mechanical effect happens due to the swelling of the Pd shell by absorption of H2 gas. The swelling of the shell closes the interparticle gaps and provides a conduction path to the charge carriers. Hence, the mechanical effect decreases the resistance of material. The overall response of the sensor is balanced by both of these effects for all H2 gas concentrations and shell thickness. In an another interesting study, Koo et al. utilized Pd nanowires as hydrogen gas sensing material with Zn-based zeolite imidazole framework (ZIF-8) [92]. The ZIF-8 framework acts as a nanofiltration layer to reduce the O2 gas hindrance in H2 detection. The response of pure Pd nanowires was improved by molecular sieving effect of ZIF-8. The O2 gas molecule reacts with H2 gas molecule and also blocks the adsorption sites. Large size O2 gas molecules are filtered out by the sieving effect of ZIF-8. The recovery of H2 gas is improved by ZIF-8 by faster desorption of H2 gas molecules.

Fig. 9 e Shows the temperature dependence of sensor resistance due to Pd clusters. Schematic diagram showing the tunnelling current through Pd clusters is given in inset image (Reprinted from Lith et al. [91], with the permission of AIP publishing).

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Metal oxide semiconducting materials (MOS) MOS materials are widely used for resistive sensing of H2 gas. The MOS possess significant properties such as semiconducting behaviour, surface defects, adsorbed oxygen, wide range of size and shape, and suitability for high temperature operation etc [93]. Motaung et al. have synthesized CeO2eSnO2 nanoparticles and used it for H2 gas detection [94]. The sensing material was tested at high (250e400  C) operating temperature and the optimum temperature for highest response was found as 300  C for the composite material. To understand the sensing mechanism, the formation of n-n type heterojunction by CeO2 and SnO2 needs to be analysed. CeO2 has lower work function value (4.69 eV) than SnO2 (4.9 eV) which allows an easy flow of electrons from CeO2 to SnO2 resulting into the band bending at the junction of the two materials. Also, the adsorption of environmental oxygen is promoted at the junction of the two materials, which improves the overall response behaviour of the sensing material. Fig. 10 shows the SEM images of Pd decorated VO2 nanowire used as sensing material for hydrogen gas. Pd decoration incorporates chemisorption and dissociation of the H2 gas molecules resulting into insulator to metal transition with significant decrease in resistance. In another observation Kim et al. have compared the effect of amorphous and crystalline coating of Pd over ZnO nanorods (NR) on the H2 sensing performance [96]. The amorphous and crystalline coating of Pd was achieved by using strong (NaBH4) and mild (Hydrazine) reducing agents during the synthesis process, respectively. The amorphous Pd coated ZnO nanorods have shown remarkable response of 12,400% at 2% H2 gas concentration in comparison with crystalline Pd coated ZnO nanorods. The crystalline Pd/ZnO is found to possess higher amount of chemisorbed oxygen species and follows the conventional resistive change mechanisms (discussed earlier) for the detection of H2 gas. The amorphous Pd/ZnO follows a different mechanism based on the work function reduction of Pd due to lesser chemisorbed oxygen species. In the presence of H2 gas, the Pd layer catalytically dissociates H2 molecule into H atoms which is further diffused into the Pd. The work function of Pd is found to decrease with increase in the interstitial H atoms [97]. The decrease of work function in presence of H2 gas causes the increase of electrical conductivity due to the ease of electron transfer between Pd layer and ZnO surface. Inyawilert et al. used PdOx-doped In2O3 nanoparticles for the detection of hydrogen gas [98]. The p-type PdOx and n-type In2O3 makes a heterojunction with additional charge carrier depletion region. The potential difference between the two materials induces the band bending effect. The doping of PdOx increases the number of adsorbed oxygen species on the surface of In2O3. In presence of H2 gas formation of H2O takes place with release of previously captured electrons by the adsorbed oxygen species. This phenomenon decreases the resistance of the sensing material.

Carbon based materials Carbon based materials possess some specific properties such as high charge transfer rate, large surface area, better

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Fig. 10 e (a) Pd decorated VO2 nanowire across Au electrodes for detection of H2 gas and (b) magnified image of Pd decorated VO2 nanowire (Reprinted with permission from Baik et al. [95]. Copyright (2009) American Chemical Society).

Fig. 11 e FESEM images of (a) CNT, (b) Pt decorated CNT, (c) Graphene, and (d) Pt decorated Graphene (republished with permission of Royal Society of Chemistry, from ‘Nanostructured Pt decorated graphene and multi walled carbon nanotube based room temperature hydrogen gas sensor’, Kaniyoor et al., 1, 2009) [101].

mechanical strength, and high flexibility which can be utilized very well in hydrogen gas sensing applications [99]. Yan et al. have fabricated sheets of stacked multi-walled carbon nanotubes (MWCNTs) with Pd functionalization and utilized them for hydrogen gas sensing application [100]. The stacking of CNT in form of sheets simplify the mass production

possibilities and the large scale commercialization of the sensor. MWCNT with three layers of sheets and 3 nm Pd thickness has shown the best results with high response (12.31%) and decent response timescale of 200 s at 4% H2 gas concentration. Similarly, Kaniyoor et al. used Pt decorated Graphene and carbon nanotubes (CNT) for the room

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temperature detection of H2 gas [101]. Pt/Graphene showed comparable response time but twice the sensitivity obtained by Pt/CNT. Fig. 11 shows the morphology of Pt decorated Graphene and CNT sensing material. The properties of carbon based materials have been very well utilized by Yaqoob et al. in their H2 sensing experiments [65]. The device was fabricated on foldable substrate (Nylon) and used Pd nanocube decorated multi-walled carbon nanotubes (MWCNT) and graphene oxide (GO) as sensing materials. MWCNT and rGO behave as a p-type semiconductor and the Pd has acted as a H2 adsorbing catalyst material dissociating the H2 molecule into H atom and formulation of PdHx state. The H atom diffuses through Pd and helps in reduction of its work function thus allows increased electron transfer from Pd to MWCNT-rGO. These electrons further recombine with holes present at p-type material MWCNT/rGO and decrease the number of majority charge carriers (holes), which ultimately leads to an increase in the resistance of the device. Jaidev et al. discussed the hydrogen gas sensing property of Pt decorated graphene-like carbon wrapped carbon nanotubes (GCNTs) [102]. The hydrogen gas sensing mechanism was based on the dissociation of molecular hydrogen into atomic hydrogen on Pt surface. This decreases the work function of Pt due to formation of Pt hydrides and results into accumulation of electrons at the GCNTs support. Hence the overall resistance of the material increases in presence of H2 gas. Han et al. studied the effect of functionalization of CNT with COOH and OH group on the sensing performance of H2 gas [103]. The functionalization and heat treatment of CNTs significantly improved the H2 sensing performance at room temperature. The p-type CNT has holes as majority charge carriers. At room temperature the detection of H2 gas molecules is difficult due to higher activation energy requirements for the charge transfer. The functional groups formed by heat treatment act as media for the charge transfer between the H2 gas molecule and CNTs. Thus the gas sensing performance of CNT is significantly increased by functionalization.

Polymers Organic polymers have certain properties such as low temperature operability, conductivity, selectivity, sensitivity, reproducibility, and stability which are all useful in hydrogen gas sensing applications. Utilizing these benefits, Sharma et al. have demonstrated polyaniline (PANI) in combination with SnO2 to detect H2 and CO gases, respectively [104]. PANI is a p-type semiconducting polymeric material which formulates a heterojunction with n-type SnO2. The hydrogen gas adsorbed at this junction layer modifies the material, electronically and results into conductivity variation through chemical reactions. The hydrogen gas molecule dissociates and bond with N atoms of imine group of PANI. This NeH bonding acts as a bridge between the two chains of the polymer. Therefore, the hopping conductivity of the material increases resulting into a decrease of the overall resistance. The synergistic effect of conducting polymer (PANI) and MOS (SnO2) composite material have shown improved H2 gas sensing performance and faster response and recovery time scale (<30 s) at 35  C temperature.

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Apart from conducting polymers (e.g. PANI), some polymers have been used as membranes in sensing devices to selectively detect the hydrogen gas among a variety of other gases. One such study was reported by Hong et al., where poly (methyl methacrylate) (PMMA) was used as a membrane for Pd coated single layer graphene to selectively detect H2 gas [105]. The free volume available on the PMMA membrane coating allows only H2 gas molecule to penetrate through because of its lower kinetic diameter (0.289 nm) in comparison with other interfering gases such as NO2 (0.4 nm), CO (0.33 nm), and CH4 (0.38 nm). Hence, the H2 selective property of PMMA membrane was successfully utilized in this study. Fig. 12 shows schematically, the effect/role of PMMA in H2 selective sensing and response of the sensor by PMMA/Pd/ Graphene composite material. The fabrication method included chemical vapour deposition (CVD) of graphene on a Cu substrate followed by etching of Cu and coating of Pd by galvanic transfer reaction using PdCl2 and finally the spin coating of PMMA. Jabor et al. electrodeposited Poly 4-bromoaniline thin films over a Si substrate and studied its behaviour for H2 gas sensing [106]. The polymer Poly 4-bromoaniline was observed as an efficient detecting material for the hydrogen gas at room temperature. The sensing properties were attributed to the excellent electron transfer between the amine group and benzene ring.

Effect of shape of nanostructures on gas sensing performance The hydrogen gas sensing performance depends upon the number of interactions between H2 gas molecules and the sensing material. The large surface area obtained from the porous nanostructures of the sensing material facilitates more number of interactions between the gas molecules and the active sites of adsorption of the adsorbent. The assembled nanoparticles form a certain shape such as: nanowire, nanotube, nanospheres, and nanoflowers etc. These shapes have different effect on the sensing performance which will be discussed in following section:

Nanowires Nanowire shape possess high aspect ratio which is beneficial in gas sensing application [55]. The ease of fabrication of nanowires with significant length is useful in forming a large semiconducting channel [92,95]. For smooth nanowires, only surface adsorption is possible, which is a disadvantage. Hence the nanowires are sometimes treated chemically or thermally to introduce surface defects or sharp edges. These defects improve the sensor performance by providing more number of adsorption sites.

Nanotubes The nanotubes have higher porosity and larger surface area due to hollow structure. Hence, nanotube is more suitable structure than nanowires for the gas sensing application. However, the fabrication of nanotube structure is difficult. Metal oxide based nanotubes are usually fabricated by

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Fig. 12 e Schematic representation of effect of PMMA membrane (a) and response of sensing material (PMMA/Pd/Graphene) with out (b) and with (c) PMMA membrane (Reprinted with permission from Hong et al. [105]. Copyright (2015) American Chemical Society).

hydrothermal synthesis method [107]. While the carbon nanotubes (CNTs) are fabricated by chemical vapour deposition (CVD) method [100]. The composite of 1-D (nanowires and nanotubes) and 2-D nanostructures (sheets) becomes a better performing sensing material.

using a one-step hydrothermal method [108]. The structure showed excellent H2 sensing property due to its large surface area, sharp edges which act as host for the chemisorption of atmospheric oxygen.

Nanosheets

Parts per billion (ppb) level hydrogen gas detection

Nanosheet type structure is also widely used in H2 gas sensing applications due to larger surface area in comparison with the bulk material [101]. 2-D nanosheets also possess sharp edges which acts as adsorption site for the gas molecules. Graphene oxide nanosheets are widely used in gas sensing applications due to its high charge transfer capability [12]. The functional groups (-COOH and eOH) attached to its surface also charge transfer throughout the surface.

Nanospheres

Hydrogen is an odourless, colourless, and highly flammable gas with very small size molecules which can defuse through very small areas which makes it very prone to leakages. Although, it is flammable at the concentration level greater than 4%, but its detection at trace level or ppb level is essential to avoid any catastrophic damage to the system and human life. H2-air mixture can ignite very easily with low energy input. The detection of H2 gas leakage at ppb level is very important due to following reasons:

Hollow and porous nanospheres type structure is also an efficient shape for the gas sensing application [41]. The porous structure allows the gas molecules to diffuse into the structure and gives high sensitivity. Sometimes, the petals like structure gets assembled and makes a spherical nanoflowers/ urchin. Zhou et al. fabricated Pd doped W18O49 urchins by

1. The small molecular size of H2 allows it to leak through the small holes and cracks. 2. Due to fast leakage, it will not take much time to reach the explosive concentration level. 3. Early detection of the H2 gas leakage at ppb level is important, so that safety measures can be taken on time.

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4. To avoid any damage to human and machinery the explosive gases should be detected as early as possible and at lowest possible concentrations. The scientists are finding new methods or improvising the existing methodology of sensing to obtain a ppb level detection of hydrogen gas. Tian et al. have shown that hydrogen gas can be detected even at low level of 150 ppb [109]. The birnessite type manganese oxide (d-MnO2) nanoflakes were used to selectively detect the H2 gas at 200  C and ppb level by oxidation of H2 into H2O by MnO2 and the formation of MnOOH and O2. The proton in MnOOH is weakely bonded to OeO which increases the electrical conductivity of d-MnO2. Hence, in presence of H2 gas the content of H2O and MnOOH increases which further increases the conductivity of d-MnO2. This phenomenon helps to detect the small concentrations (ppb level) of H2 gas. The lower temperature operating conditions were found to decrease the sensitivity and limit of detection of the material up to 750 ppb at 100  C and 7.5 ppm at room temperature. The sensed concentration (of hydrogen gas) and response (current variation) were found to fit accurately to the Langmuir adsorption isotherm model. Similarly, Yang et al. reported the detection of hydrogen gas with a low level concentration of 500 ppb by using hydrothermally synthesized a-MoO3 nanoribbons [110]. The study also analysed the effect of hydrothermal synthesis temperature from 120 to 200  C on the sensing performance of a-MoO3 nanoribbons. The material fabricated at 200  C was showed the fastest response of 14.1 s for 1000 ppm of H2. The sensing mechanism for this study was based on metal oxide based resistive sensing of combustible gases i.e. redox reactions between adsorbed oxygen and H2 gas molecules. The higher sensitivity (ppb level) of the sensing device was attributed to the presence of Mo5þ which acted as defect sites for the adsorption of O2 gas from environment. The hydrothermal synthesis at high temperatures increased the number of Mo5þ ion defects in the nanostructure of MoO3. Increasing the content of Mo5þ results into higher amount of chemisorbed oxygen and increased sensitivity of the device up to ppb level. Hu et al. used different loading weight percentages (0.5e6%) of CeO2 with In2O3 hollow nanospheres for ppb level detection of H2 gas at different operating temperatures [15]. It was found that at 2 wt% loading of CeO2 with In2O3 provided the optimum sensing characteristics such as a high response at 160  C, fast response and recovery times (1 s and 9 s), low detection limit (10 ppb), negligible cross sensitivity with interfering gases (such as H2S, NH3, CO, and CH4), long term stability etc. The gas sensing behaviour in such material systems have been attributed to the n-type semiconducting behaviour of the synthesized material and the heterojunction formulated between CeO2 and In2O3 which is found responsible for the generation of an oxygen adsorbing accumulation layer. The accumulation layer is formed as a result of the electron migration from CeO2 to In2O3 due to smaller work function. The adsorbed oxygen captures surface electrons and reacts with H2 gas, formulating water molecules and releasing the captured electrons. The coexistence of Ce3þ and Ce4þ ions in the samples provide excess oxygen vacancies and at the same time Ce4þ behaves as an electron scavenger and converts into Ce3þ according to the following equation [15]:

3þ 2Ce4þ þ O þ chemisorbed ¼ 2Ce

1 O2 2

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(8)

This phenomenon was utilized to detect the ppb level concentration of H2 gas. Roso et al. synthesized crystalline octahedral type structure (shown in Fig. 13 c and d) of In2O3 for the ppb level resistance based detection of NO2 and H2 gases [111]. The sensing characteristics are found to be promising due to high response, low temperature operability (room temperature to 130  C), no effect of humidity, and low level detectability (1000 ppb) of the H2 gas. The effect of noble metal (Pt and Pd) nanoparticles addition with high temperature (250  C) operation has also been examined and the increase in H2 gas sensitivity and decrease in NO2 response has been observed. The addition of these noble metal nanoparticles can be utilized to improve the selectivity of the sensing material for a particular gas. The metal nanoparticles provide the catalytic oxidation mechanism which improves the sensing behaviour. The H2 gas molecule gets adsorbed at the metal by dissociating into H atom which is further transported to oxide surface to react with adsorbed oxygen gas molecules. Fig. 13 (a) and (b) shows the sensor resistance variation in presence of different concentration level of hydrogen gas and the effect of temperature on sensor response. The sensing mechanism was based on the n-type resistive sensor method which has been already discussed in previous section. Utilizing a very unique technique of non-thermal plasma (NTP) method, Darabpour and Doroodmand developed a glow discharge plasma based ionization gas sensor for the ppb level detection of hydrogen gas [112]. They used a two electrode system with graphite electrode (diameter: 6.5 mm) as anode and an aluminium disc (diameter: 2.4 mm) as the cathode with 0.7 mm inter-electrode distance. All the experiments were conducted under vacuum level (0.01 Torr). The sensor showed excellent sensing characteristics such as low detection limit (3.3 ppb), reproducibility of results, no interfering effect of other gases (such as Ar, He, CO2, CO, C2H2, O2, and CH3OH), and high reliability. This technique utilizes a specific property of every gas that is specific breakdown electric field at constant temperature and pressure. At this point the ionization of the gas takes place which can be identified by current flow through the electrodes. The NTP method has distinct features of non-thermal and near atmospheric pressure operability which eliminates the disadvantage of high power requirements. The use of nanomaterials and glow discharge plasma technique enables the sensor to detect the trace level (ppb) concentration of hydrogen gas. Various sensing methods and materials were discussed in details in previous sections to describe the current scenario of H2 gas sensing technology. Also, the need of trace level (ppb) detection of H2 gas was discussed with some of the reports based on recent developments in H2 sensing technology. To summarize the topic and ease of understanding, various materials which have been used in recent studies for the detection of H2 gas, are tabulated with their sensing performance characteristics such as key features, optimum operating temperature, detectable concentration level, effect of humidity, and interfering gases (Table 2).

Material, Year Pd gate/SiO2/Si sensor (MOS), 2009

Key feature

Optimum operating Response/ temperature (oC) recovery time (s)

Effect of Humidity

Non-interfering gases

Ref.

1e8%

RT

NA

NA

NA

[113]

100-40,000 ppm

RT

7/1500

NA

NA

[9]

0-40,000 ppm

RT

170/325

NA

NA

[114]

0.5e3%

RT

33/66

NA

[115]

300 ppbe150 ppm

145  C

660/NA

NA

Ethanol, Acetone, Isopropanol NA

[116]

0.5e4 bar

100

1/60

No effect (up to 80% RH)

NA

[66]

2 bar

RT*

3/3

NA

NA

[117]

500 ppbe1000 ppm

RT

14/75

NA

CO, Ethanol, Acetone

[110]

0.025e2%

RT

108/331

NA

CH4, CO, NO2

[105]

0-20,000 ppm

320

0.97/0.72

NA

Diethyl ether, n-Pentane

[84]

400 ppm-45 vol%

RT

1.3/NA

NA

NA

[85]

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Decrease of flat band voltage due to decrease of Pd work function Schottky type Pd/porous Si, 2010 The Hydrogen absorption at interstitial sites increases Pd resistance Pd/BeC/n-Si, 2012 The fermi level shifts due to H2 gas adsorption on Pd and the heterojunction of C and Si Pt catalytically dissociates Pt/In2O3, 2014 H2 gas molecule into H atom NiO:Pd, 2014 Increased electron transfer between adsorbed oxygen and Hydrogen gas in presence of Pd Pd/Mg thin films, 2015 Increased electron scattering due to metalhydride formation Pd/MgPd alloy, 2015 Induced Stress and strain due to H diffusion results low binding energy and faster response High synthesizing a-MoO3, 2015 temperature increases Mo5þ content and chemisorbed Oxygen PMMA/Pd/Graphene, 2015 Selective filtration of H2 gas molecules by PMMA membrane due to its small size improves selectivity Pt/Graphene aerogel, 2015 The Pt/Graphene material shows large surface area and high thermal conductivity for catalytic detection of H2 gas Chalcogenides (Bi2Te3 and Sb2Te3), 2015 Heat released due to exothermic formation of H2O from H2 and O2 molecules is transferred by high thermal conductive chalcogenides

Detectable range

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Table 2 e The summary of sensing performance and features of various materials (Year wise) used in H2 gas sensing application.

MWCNT, 2015

Pd nanoflower/Graphene, 2015

Pt/Pd bimetallic thin films, 2016

Pd/V2O5, 2016

SnO2/Polyaniline (PANI), 2016

Pd/MWCNT/Graphene, 2016

Pd-PANI-rGO, 2016

Pd/In2O3, 2016

Pd/SnO2 thin films, 2017

Pd/Graphene, 2017

40e86 ppb

RT

75/NA

At Vacuum

Ar, He, CO2, CO, C2H2, O2, CH3OH, Acetone

[112]

0.1e100 ppm

RT

<12/NA

At 30% RH

NO2, NH3

[118]

150 ppb- 500 ppm

200

<10/780

At 47% RH

toluene, methanol, ethanol, [109] acetone etc.

10-40,000 ppm

150

4/5

NO (up to 80% RH) N2, CO2, CO, NO2, O2

[67]

2e500 ppm

100

5/2

No effect

NH3, CO, H2S

[64]

1000e5000 ppm

35

<30/<30

NA

CO

[104]

10e10,000 ppm

RT

600/300

No effect (up to 80% RH)

CO2, N2, C2H2, O2, NO2

[65]

0.01e2%

200

10/32

At 42% RH

CO2, H2S, CH3OH

[119]

50 ppb- 20 ppm

250

200/300

At 50% RH

CO, Ethyl alcohol, H2S, H2, NO2,

[111]

100e2000 ppm

175

1/512

NA

NA

[80]

0.25e1%

RT

1/9

At 30% RH

NA

[81]

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(continued on next page)

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d-MnO2, 2015

MWCNT offers large surface area/aspect ratio which helps to detect the trace level detection by gas ionization method Lower work function phase transformed PdHx in presence of H2 increases electron transfer The release of weakly covalent bonded proton from MnOOH (from oxidation of MnO2) increases the electrical conductivity Increased electron scattering due to metalhydride formation Formation of Vanadium bronze in presence of adsorbed H atom Bonding of H atom and N atom (from PANI) forms bridge between the polymer chains and increases conductivity The composite material shows high sensitivity with foldable and flexible sensor Formation of PdHx increases electron scattering and p-type PANIrGO increases the resistance by charge neutrality effect Formation of Pd from PdO in presence of H2 gas releases electrons and increases conductivity Increased conductivity of SnO2 in presence of H2 slows down the SAW The electron transfer to from Pd to graphene interacts with the electric field accompanying the SAW which results frequency shift

Material, Year TiO2/PdO pigment, 2018 Amorphous Pd/ZnO, 2018

Pd/HfO2/GaOx/GaN MOS type schottky diode, 2018

CeO2eIn2O3, 2018

Pd/W18O49, 2018

MWCNT layered sheets, 2018

Change of colour due to PdO to metallic Pd conversion Catalytic dissociation of O2 and H2 by Pd causes fast adsorption and diffusion into ZnO Reduction of PdO into Pd and formation of Hþ at the working electrode in amperometric detection Treatment with H2O2 causes a rough GaOx dielectric layer formation over GaN which improves sensitivity Band bending effect due to heterojunction formation between the two materials The nanourchin structure of W18O49 offers high amount of adsorbed Oxygen which offers larger resistance Stacking of MWCNT offers more number of adsorption sites

RT ¼ Room temperature, NA ¼ Not available, RH ¼ Relative humidity.

Detectable range

Optimum operating Response/ temperature (oC) recovery time (s)

Effect of Humidity

Non-interfering gases

Ref.

NA

RT

NA

NA

NA

[76]

0.5e6%

80

156/61

Yes

CO2, Ethanol, Methanol, Acetone

[60]

10e70%

RT

15/NA

NA

NA

[120]

5-10,000 ppm

RT

36/35

Yes (Ineffective at higher temperature)

NA

[63]

10 ppbe1000 ppm

160

1 s/9 s

NA

NH3, CH4, CO, H2S

[15]

0.025e0.5% (by Volume)

100

60/4

NA

Propane, NH3, CH4, Ethanol, [108] Acetone

0.5e10%

RT

<200/NA

NA

C2H4, CO2

[100]

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PdO, 2018

Key feature

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Table 2 e (continued )

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Fig. 13 e Resistance variation with respect to time in different gas concentration levels (at 130  C) (a), and response of the sensor with respect to temperature variation (b). (c) SEM and (d) TEM images of In2O3 octahedral type structures at different magnification (Reproduced from Ref. [111] with permission from the Royal Society of Chemistry).

Advances, current issues and future scope With the technological development in the sensors area, the advancement of H2 gas sensors has been the major concern of the researchers. The advance sensing system demands online, fast, sensitive, and low cost detection methods/devices. Also, the performance of sensing device should not be affected by the environmental conditions such as temperature, humidity, and other interfering gases. Recently, Pt thin films have been found to work as an excellent sensing material for H2 gas at high humidity (RH 90%) level [121]. Sensor performance characteristics such as sensitivity, response/recovery time, and selectivity are also important for high performing devices. In a recent article, Yi et al. suggested porous hollow SnO2 nanofibers as sensing electrodes for the potentiometric detection of H2 gas [49]. The device shows high performance characteristics such as high response value (289.1 mV), low response time (5 s), and high temperature (450  C) operation. With the recent advancements in quantum dots and pigment research, the colour change based detection technique has become the most suitable sensing method. Recently, Hwang et al. demonstrated a colour changing tape sensor based on TiO2 pigments [76]. The current scenario of sensor application also focuses on wearable sensors based on flexible substrates. Inkjet-printed flexible electrodes are now being widely used in sensing devices [53]. The flexible sensing

devices are very helpful in detection of H2 gas due to ease of application in systems with complex shape. Although, the sensing technology has been improved very well in recent times but there are some issues which need to be taken care of such as: (i) repeatability of sensing results for large number of cycles, (ii) performance deterioration after long term exposure to humidity, (iii) complex fabrication method for electrodes and sensing material, (iv) effect of gas flow rate on sensor performance, (v) inaccurate prediction of concentration of the gas on the basis of sensor response, and (vi) complex analysis of results. The future aspect of H2 gas sensing technology is based on making the sensing methods more reliable and sensitive for large number of cycles. The complex analysis based techniques should be replaced by direct methods such as colour change based techniques with accurate concentration measurement. The online monitoring system of H2 gas should be integrated with smart-devices such as smartphones and tablets for ease of applicability. Paper based sensing method is also very useful for low cost, scalable, and efficient detection of H2 gas and can be utilized for high performance devices in future.

Conclusion The hydrogen gas sensing technology is used extensively due to vast use of hydrogen gas as a clean energy source. The

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explosive behaviour of H2 gas compels to detect it at a very low level of concentration. Hence, ppb level detection of H2 gas leakages becomes important to avoid hazardous situations. There are different techniques to detect H2 gas and all of them are well explored. This article has successfully reviewed the methods of detection, materials used for detection, performance enhancing techniques, and ppb level detection studies for H2 gas. Each method has its own merit and demerit, which allows user to choose the most suitable technique according to its application. The performance enhancement methods enable the sensing devices to operate at large range of temperatures, humid conditions, and different concentration levels. Selectivity has been an issue for all type of sensing devices, but with the advancement of nanomaterials the selectivity for hydrogen gas has been reported by many researchers. This article provides an overall insight of the hydrogen gas sensing technology with the idea of trace level detection.

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