1. Introduction
For Lithium-ion batteries (LIBs), a high internal temperature probably induces additional violent exothermic chemical reactions for further heat generation, resulting in thermal runaway of LIBs. Hence, room temperature H
2 sensors applied inside of LIB is necessary. Resistive H
2 sensors operating at room temperature mainly rely on the catalytic adsorption of H
2 molecule on noble metal (for example Pd, Au, Pt). Among of them, Pd-based H
2 sensors are the current state-of-the-art resistive H
2 sensors. Beyond that, synergistic effect of heterojunctions, metallization effect of special metal oxide
[1][2][39,40], and efficient H
2 sensing capacity of emerging materials (i.e., MXene, TMDs) at low temperatures have also been investigated to develop RT H
2 sensor.
2. Pd-Based H2 Sensors
Basic H2 Sensing Mechanism
Generally, the H
2 sensing mechanism of Pd-based H
2 sensors are catalytic adsorption of H
2 on Pd, which induces the dissociation of H
2 molecule into H atoms and the formation of PdH
x (Equations (1) and (2))
[3][4][31,41].
Numerous resistive Pd-based H2 sensors have been developed, including pure Pd nanostructures and Pd-based composites, such as Pd-hetero metal, Pd-MOX, Pd-carbon material, and Pd-TMD.
Pure Pd materials. According to Equations (1) and (2), PdH
x is formed when H
2 molecules adsorb on Pd surface. Conductivity of PdH
x is poorer than Pd, which generates H
2 response in resistance of Pd
[3][5][31,42]. Plenty of resistive H
2 sensors based on pure Pd materials have been reported. With the significant development of nanoscience, various Pd nanostructures with further H
2 sensing mechanisms were developed to enhance the H
2 sensing capacities
[6][7][8][9][43,44,45,46].
Kim et al.
[6][43] and Jung et al.
[7][44] investigated the nanograin effects and the α-to-β phase transition of Pd on H
2 sensing. When the Pd crystallites < 10 nm, abundant nanogaps were created between the nanograins (<2 nm). The nanogaps between Pd nanograins intrinsically stem the electrical conduction. Exposed to different concentration of H
2, different kinds of conducting pathways were formed in Pd, yielding switchable H
2 sensing behaviors
[7][44]. Similar phenomena were demonstrated in Ref.
[3][31]: (i) when exposed to low concentrations of H
2 (2.5 to 100 ppm), α-PdH
x was formed expanding Pd nanograins, which closed the narrow nanogaps and further producing a new conduction pathway. Obvious negative resistance response was observed. (ii) Upon exposed to moderate concentrations of H
2 (250 to 2500 ppm), most of the nanogaps were closed, resulting in saturation of the negative resistance response. (iii) Upon exposed to high concentrations of H
2 (0.5 to 3%), all nanogaps were closed and β-PdH
x were formed, while surface electron scattering generated by β-PdH
x dominantly the electrical conductions, leading to significant positive resistance response. Consequently, ultrasmall grain size of Pd are critical for H
2 detection at low concentrations, α-to-β phase transition of Pd could detect a wide range of H
2 concentrations from sub-ppm to 4%. In addition, negative effect of oxygen on Pd-based H
2 sensing has been demonstrated in earlier literatures
[10][11][47,48].
Pd-metal alloy. Recently, many researchers developed Pd-based alloys to improve the H
2 sensing properties, such as limit detection and response/recovery behavior
[12][13][14][15][35,49,50,51]. Jung et al.
[13][49] developed Pd/Pt and Pd/Au high surface-to-volume ratio nanopatterns. Compared to Pd nanopattern, Pd/Pt nanopattern showed 45.5-fold higher response to 1% H
2, Pd/Au nanopattern showed about 73-fold and 4.6-fold enhancement in the response and recovery speed, respectively. The dramatic improvements in response and recovery behaviors to H
2 are attributed to the ultrasmall size (<5 nm), ultrathin nanopattern (<15 nm), grain interfaces and the lower adsorption/dissociation energy of H
2 on Pd/Pt and Pd/Au surfaces. Kim et al.
[15][51] developed hollow Pd-Sn alloy nanotubes with high surface area of 223.0 m
2/g for H
2 sensing and ultrafine grain size. Interestingly, the Pd-Sn alloy effectively prevented degradation of H
2 sensing performances caused by the α-β transition of Pd. As a result, the hollow Pd-Sn nanotubes exhibited outstanding sensing properties to a wide concentration range of H
2 (50 ppm to 3%), especially fast response/recovery rate to high concentration of H
2 (20 s/17 s to 2% H
2). The noteworthy H
2 sensing mechanism was proposed: (i) The abundant interface gaps between the grains owing to larger atom size of Sn (0.141 nm) and smaller atom size of Pd (0.138 nm) dominated low concentration of H
2 (0.5 ppm to 0.02%) sensing behavior, due to the nanogap effect. (ii) For high concentration of H
2 (0.05% to 3%), the formation of β-PdH
x governed the H
2 sensing properties. The H
2 sensing mechanism agreed with the previous reports
[6][7][43,44].
Pd-MOXs. MOXs are the classical gas sensing semiconductor materials. However, their H2 sensing properties are limited at room temperature. Previously, many researchers developed Pd-MOX composites to achieve good H2 sensing at room temperature.
Zhang et al.
[9][46] investigated the interconvertible effect on H
2 sensing of catalyst nanoparticles and semiconductor support in Pd-decorated PdO hollow shells. They prepared PdO hollow shells, which were subsequently treated by NaBH
4 to be partially reduced into Pd on the PdO surface. The Pd nanoparticles were discretely and physically inlaid on the surface of PdO with a ultrasmall size of ~ 2 nm. The catalytic effect of Pd, as well as the Schottky-junction between Pd and PdO, enhanced the H
2 sensing performances even at 1 ppm. Notably, inlaid Pd in PdO shells prevented the agglomeration of Pd nanoparticles, which generated long-term stability.
In addition, Zhang et al.
[16][36] designed an unique conduction model by inserting a high-conductive metallic core Au into less-conductive p-type PdO to boost the RT H
2 sensing performances. As a result, Pd decorating Au@PdO demonstrated an ~90 times larger in H
2 response than Pd decorating Pd@PdO. And the boosted H
2 response helped the Pd-Au@PdO sensor showed ultralow LOD of 0.1 ppm. Pd-ZnO nanoflowers has been demonstrated RT H
2 sensing in ppb level with experimental LOD of 300 ppb, due to the change in channel conductance of ZnO nanoflowers based on the incorporation of Pd
[17][37].
Pd-carbon materials. Carbon nanotubes, graphene, and their derivatives are very promising in the room temperature gas sensor fields, due to their good electrical conductivity at RT. However, H2 molecule is very difficult to adsorb on the surface of carbon materials, due to weak H2 adsorption capacity. In regard of this case, lots of Pd-carbon material composites have been previously investigated for RT H2 sensors. Even so, low LOD and quick recovery behavior remain big challenges. As is well known, Pd particles at ultrasmall size are very helpful for the RT H2 sensing. However, they are very easy to aggregate to further cause degradation of overall H2 response.
During the synthesis process of one-pot solution method, DNA suppressed the stacking of graphene layers due to π-π interaction between DNA and graphene, meanwhile in-situ anchored PdO
2 subnanoscale clusters on the exfoliated single layer graphene. The design showed mimic wrinkled morphology and sensing mechanism of natural olfactory neuroepithelium, being named BONe. The BONe boosted H
2 sensing performance (25 s/35 s at 5 ppm H
2 with experimental LOD of 50 ppb) at RT with yearlong durability. High surface area of the BONe, good conductivity of graphene, as well as the subnanoscale of PdO
2, generated the ultra-sensitivity to ppb-level H
2 at RT; subnanoscale of PdO
2 anchored on graphene yielded yearlong stability. Notably, it was calculated that PdO
2 exhibited a d-band downshift of −2.58 eV, which was a much further downshift than that of PdO (−2.16 eV) and Pd (−0.179 eV), governing the complete and fast recovery behavior during H
2 detection. A similar d-band theory was proposed previously, that tuning of d-band energy level balanced the adsorption and desorption capacities, and the lower the d-band level, the weaker the adsorption
[18][52].
Pd-other materials. With the emerging of new materials (metal-organic frame MOF, Mxene, et al.), composites of Pd and kind of new materials were developed for H
2 sensing. The sensor based on flexible Ti
3C
2T
x MXene@Pd nanoclusters, reported in Ref.
[19][53] delivered a response/recovery time of 32/161 s and sensitivity of 23% to 4% H
2 at RT. The strong H
2 adsorption into lattice of Pd nanoclusters induced electrons doping in MXene, generating fast response behavior. However, the limit of detection (0.5%) needs to be improved. The Pd-decorated sodium titanate nanoribbons (Pd-NTO NRs) developed by Zhang et al.
[20][54] exhibited ultrafast response to 1% H
2 within 1.1 s at RT, and a wide detection range (0.8 ppm to 10% H
2). The excellent H
2 sensing capacity benefited from the laterally paralleled morphology and abundant oxygen vacancies on edge sites of nanoribbons, as well as monodispersed Pd nanoparticles in size of ~3.5 nm. Oxygen in air could block the reactive sites, leading to depress the H
2 sensitivity and retard the response/recovery rate
[10][11][47,48]. For that, Kim and Penner et al.
[21][55] designed patterned Pd nanowires covering ZIF-8 membrane (Pd NWs@ZIF-8) for H
2 sensing. Although the ZIF-8 membrane reduced the H
2 response slightly, 20-fold faster response/recovery rate (3.5% at 10/7 s to 1% H
2 versus 5.9% at 164/229 s of Pd nanowires) was achieved due to the molecular sieving and acceleration effects of ZIF-8 since the pore size of ZIF-8 is 0.34 nm, which is larger than the diffusion kinetic diameter of H
2 molecule (0.289 nm), but a little smaller than that of O
2 molecule (0.345 nm).
3. Other Noble Metal-Based H2 Sensors
Similar to H
2 adsorption property of Pd, other noble metals also exhibit H
2 adsorption behavior
[22][56]. Moreover, Au and Pt-based H
2 sensors has been demonstrated due to the formation of MH
x [23][24][25][57,58,59]. Guha et al.
[23][57] reported Pt-functionalized rGO for excellent H
2 sensing at RT, 65 s/230 s against 5000 ppm H
2 with LOD of 200 ppm in air ambience. Interestingly, for this Pt-rGO composite, H
2 response was larger in N
2 environment than that in air ambience; however, sensor recovered faster in air than in N
2. In N
2 and air environments, H
2 first physisorbed on Pt-rGO surface and then dissociated to from Pt-H (Equation (3)). But in air, H
2 also reacted with the adsorbed oxygen on the Pt-rGO surface to form Pt-H and H
2O (Equations (4) and (5)).
The H
2O competed with H
2 molecules to adsorb on the Pt-rGO surface, reducing the H
2 response. On the contrary, the adsorbed H
2 and the dissociated H-atoms reacted with adsorbed oxygen to convert to water vapor while purging air, exhibiting fast recovery in air ambience. Similar effect of oxygen on Pd- and Pd@Pt-based H
2 sensing has been demonstrated in earlier literatures
[10][11][47,48].
4. MOXs-Based H2 Sensors
Generally, most MOXs-based resistive gas sensors operated at high temperature (>150 °C). However, recent approaches were developed to realize RT H
2 sensing performances of MOXs, such as nanostructure design
[26][27][28][60,61,62], composites of MOXs
[29][30][63,64], and surface metallization
[17][37]. Huang et al.
[27][61] demonstrated the well-aligned MoO
3 nanoribbon arrays exhibited great H
2 response/recovery behavior at RT, with a response/recovery time of 3 s/16 s at 100 ppm H
2 which is much shorter than 59 s/151 s of randomly arranged MoO
3 nanoribbons. The accelerated H
2 response/recovery rate was due to the fact that the high alignment of nanoribbons could increase the surface activity of MoO
3 and suppress the nanojunction effect. On the contrary, in the randomly arranged MoO
3, the interface diffusion of adsorbed oxygen caused serious nanojunction effect, resulting in much slower H
2 response/recovery rate.
Yun et al.
[17][37] designed holey engineered 2D ZnO-nanosheets for supersensitive H
2 sensing. The sensor exhibited 115% response to 100 ppm H
2 with short response/recovery time of 9 s/6 s at room temperature. And the experimental LOD was 5 ppm. Upon exposure to H
2, surface of the ZnO nanosheets became metallic Zn. Metallization of Zn on the ZnO surface governed the gas sensing mechanism about high response and great selectivity to H
2 at room temperature. Moreover, the synergetic effect of 2D nanosheets and interconnected holey/porous network of ZnO generated the excellent H
2 sensing performances by offering abundant active sites for H
2 molecules.
5. MoS2-Based H2 Sensors
In recent year, MoS
2 has shown great H
2 sensing potential due to its 2D van der Waals structures, H
2 adsorption capacity and enable RT operation
[31][32][65,66]. Unfortunately, sluggish response/recovery limited the applications of MoS
2-based H
2 sensors. In regard of this case, some composites based on MoS
2 were investigated for H
2 sensing. Huang and Chen at al
[33][38] designed hybrid interlinked MoS
2-ZnO nanotubes for RT H
2 sensing of 51.1% to 500 ppm with 14/19 s response/recovery and LOD of 10 ppm, due to the increased oxygen vacancies and surface-active sites. Hollow MoS
2/Pt-based chemiresistors, designed by Kim et al.
[31][65], exhibited great H
2 sensing performance with fast response/recovery rate at RT (8.1/16 s for 1%, and 2.7/16 s for 4% H
2), due to the catalytic H
2 spillover of Pt, as well as sufficiently permeable pathways and maximized active sites for H
2 produced by the hollow MoS
2. Similarly, vertically aligned edge-oriented Pd/MoS
2 nanofilm was reported for great H
2 sensing properties at RT, response of 33.7% to 500 ppm with response/recovery of 16 s/38 s, and LOD of 50 ppm
[32][66]. Even so, selectivity of RT H
2 sensors based on MoS
2 has not been investigated widely and deeply.
6. Other-Based H2 Sensors
With the development of nanoscience and nanotechnology, new composites for RT H
2 sensors have been developed. Xu and Ou et al.
[34][67] explored ultrathin nickel oxysulfide which exhibited a selective and fully reversible response to H
2 at RT for a wide range from 0.25% to 1%, due to the physisorption of H
2 on the surface. Dash et al.
[35][68] developed RT H
2 sensors based on rGO-ZnFe
2O
4-Pd nanocomposite, showing high sensitivity and fast response/recovery rate (11.43% to 200 ppm with 18/29 s response/recovery) to a wide range of H
2 (50–1000 ppm), due to the synergistic effect of rGO, ZnFe
2O
4 and Pd nanoparticles.
Up to now, there are lots of efforts to develop room temperature resistive H2 sensors. Unfortunately, no H2 sensors integrated inside of LIB cells have been reported. Table 1 summarizes the recent and representative room temperature resistive H2 sensors possibly integrated inside of LIB cells. Although new gas sensing materials are emerging all of the time, Pd and Pd-based materials have been the current state-of-the-art resistive H2 sensing material operating at room temperature. In conclusion, Pd is still the most excellent catalyst for RT H2 sensing with great selectivity due to the formation of PdHx. But the LOD (ppm level) and response/recovery behavior (tens of seconds) of Pd-based H2 sensors should be further improved. Sensors based on Pd-carbon materials should be state-of-the-art RT H2 sensors displaying LOD of ppb-level (due to the catalytic H2 adsorption/dissociation on Pd surface, as well as high surface area and RT good conductivity of carbon materials). However, they still suffer from slow response/recovery rate (tens of seconds) to ppb-H2 at RT. According to previous investigations, atomically dispersed sub-nano clusters (perhaps single atom) of Pd are in favor of ppb-level H2 detection at RT; optimal d-band energy level of Pd could yield complete and fast recovery; construction of conduction channel in Pd-based sensing material could generate fast response and recovery rate. Although some composites of MOXs exhibited good RT H2 sensing property, they inherently showed cross-sensitivity to other gases (such as NO2, CO and ethanol) and susceptibility to humidity. Despite some new structures and materials (ternary composites, MoS2, and MXene) emerging for RT H2 sensing, fast response in 1 s to ppb-H2 with special selectivity is still a critical issue.