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Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors
international journal of hydrogen energy xxx (xxxx) xxx
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Fast hydrogen diffusion induced by hydrogen presplit for gasochromic based optical hydrogen sensors Haoran Wang a, Guohua Gao a,*, Guangming Wu a, Huiyue Zhao a, Wanyu Qi a, Kaicong Chen a, Wan Zhang a, Yunyun Li a,b,** a
Shanghai Key Laboratory of Special Artificial Microstructure, Tongji University, Shanghai, 200092, China Center for Phononics and Thermal Energy Science, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
b
article info
abstract
Article history:
Gasochromic based optical hydrogen sensors have attracted much attention as normal
Received 27 December 2018
temperature sensors. The behaviors of the hydrogen diffusion largely affect the reaction
Received in revised form
process and the sensitivity. However, few researches focused on the influence of hydrogen
1 April 2019
diffusion in gasochromic films. Here, we report a method to pre-split the H2 molecule and
Accepted 4 April 2019
separate the hydrogen molecule and hydrogen atom diffusion process by deposition of
Available online xxx
porous PdCl2/SiO2 catalyst films on WO3. The response time of WO3 at 844 Pa H2 partial
Keywords:
more than 160 cycling lifetime. A validation experiment was designed and proved the
Gasochromic
diffusion of the hydrogen atom and the two-step model of H2 reaction on WO3 films, on the
Sol-gel
basis of which the diffusion coefficient of the hydrogen atom has been estimated. More-
pressure reduced from 800 s to 200 s. The stability of WO3 film was largely improved to be
Hydrogen sensor
over, the PdCl2 doping SiO2 catalyst exhibits substrate versatility, which can also coat on
Diffusion
the nano-structured WO3. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The energy carrier with negligible environmental impact is one of the most important technology challenges of the 21-st century [1]. Except the traditional energy storage technologies, such as supercapacitors [2] and batteries [3], hydrogen is also a potential clean and renewable energy carrier. Unfortunately, with a wide explosive limit (4%e74.4%) and small molecule size, hydrogen leaking has become a serious safety issue in the industry [4]. Thus, there is a strong demand for hydrogen
sensors. For now, resistive-type sensors are an important research field for hydrogen sensors [5e7]. However, resistivetype sensors mainly work at high temperature, leading to the high energy cost and explosion risks [8]. For certain materials, their optical properties change when they interact with hydrogen under room-temperature, which is exploited as an optical-type hydrogen sensor [8]. Based on this, various optical-type sensors has been developed, including Fiber Bragg Gratings sensor [9e11], interferometric sensor [12e14] and gasochromic based sensor [15e18], etc. Among them, gasochromic based sensor possess a simple structure and can
* Corresponding author. ** Corresponding author. Shanghai Key Laboratory of Special Artificial Microstructure, Tongji University, Shanghai, 200092, China. E-mail addresses: [email protected] (G. Gao), [email protected] (Y. Li). https://doi.org/10.1016/j.ijhydene.2019.04.026 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Fig. 1 e The design of the WO3/SiO2þPd film and WO3þPd film.
Fig. 2 e Transmittance spectrum of the PdCl2 water solution. be easily assembled into a self-powered gasochromic based sensor, as demonstrated by Chen et al. recently [17], which may be a more secure, convenient and commercial candidate for hydrogen detection. The gasochromic device mainly makes of two parts: the catalyst and gasochromic materials. Upon exposure to hydrogen, hydrogen molecules will split into hydrogen atoms
on the surface of the catalyst, which are essentially noble metal simple substances or salts, known as “hydrogen spillover” [19]. Then the hydrogen atoms can be injected into the gasochromic materials, such as WO3, resulting in a drop in the transmittance of the device. This phenomenon can be reversed by the oxygen or air. Actually, in the gasochromic film, sensitivity is mainly controlled by H2 molecule diffusion, which is based on Brownian movement that needs large pore size [20]. On top of that, the current studies on the sensitivity of the gasochromic optical sensors mainly focus on porous modification of the materials. WO3 nanofibers were prepared by Fatemeh Tavakoli Foroushani et al. [16] with sensitivity to 1% H2 in 6 min at room-temperature by electrospinning. Nano-columnar WO3-Pd films were prepared by Young-Ahn Lee et al. [21], which can detect hydrogen below 1% at roomtemperature in 30 s but with a cycling life less than 50 times. A 2D WO3 nanostructure was reported by Shankara S.Kalanur et al. [22] with sensitivity to 1% H2 in 30 s at room-temperature. However, porous modification increases the difficulty, instability and cost of the synthesis process of WO3 films. In contract, the hydrogen atom generated by hydrogen spillover is diffused by surface diffusion [23], which does not depend on the large pore sizes. Meanwhile, the small size of the H can also benefit to the diffusion which has been reported in research of hydrogen storage [24]. So if the diffusion of the
Fig. 3 e Dark-field TEM image and EDS mapping of the SiO2þPd sol. Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Fig. 4 e SEM images of the surface of the (a) PdCl2 drop cast on WO3 film, (b) WO3/SiO2þPd film and c) cross section of the WO3/SiO2þPd film.
hydrogen atom is applicable, the pre-split of the H2 molecule before diffused into the gasochromic materials is the key to increase the sensitivity and stability of gasochromic films. For now, most of the researchers directly added the catalyst to the precursor of the gasochromic materials during the synthetize process [22,25,26], which is easy to achieve a homogeneous doping of the catalyst but the hydrogen needs to diffused into the film and reacts with catalyst. Although there are some reports about coating the catalyst film onto the WO3 layer [27,28], which can split the hydrogen outside of the WO3, the diffusion of the hydrogen atom has not been fully discussed, and the high costs and low porosity properties of sputtered film and homogeneity problem of the direct drop-cast method limited the scope of application of these methods. Sol-gel method has gained more and more attention during the past decade. It can generate a very homogeneous distribution of components at nanoscale from commercial available precursor, and does not require complicated instrument. During the sol-gel process, a cross-linked network of the materials is assembled in the sol. After gelation and the solvent removal, porous structure can be easily prepared [29]. Thanks to this, the porous film which is suitable for hydrogen molecule diffusion can be directly synthetized even on a special-shaped surface [30], through sol-gel route combined with coating method, such as dip-coating and spin-coating [31]. Herein, a homogeneous porous catalyst film with PdCl2 doped SiO2 for WO3 gasochromic have been obtained by a facile sol-gel route. The sensitivity and stability of the system were studied. The mechanism has been discussed based on relationship between the film porosities and the control processes of H2 or H diffusion. The H diffusion was proved by isolating the catalyst and WO3 by another un-doped porous
SiO2 layer. Moreover, a two-step reaction model of H2 was proposed and a validation experiment was designed. In addition, the absence of the catalyst during the preparation of the gasochromic materials avoids the side effect to the morphology and structure, bringing more potential synthesis route for the materials.
Material and methods SiO2 sol was synthesized by acid hydrolyzing tetraethyl orthosilicate (TEOS) in ethanol (EtOH) [32]. In a typical synthesis, the TEOS was mixed with half of EtOH to form solution A, and another half of EtOH was mixed with H2O and hydrochloric acid to form solution B. Then the solution B was added dropwise to the solution A. After stirring for 1 h, the SiO2 sol was prepared. The mole ratio of the precursor was TEOS:H2O:HCl:EtOH ¼ 1: 2.3: 0.245: 38. The catalyst doping sol was prepared by adding 0.1 g PdCl2 into 50 ml SiO2 sol, called SiO2þPd sol. For WO3, the synthesis procedure was reported by Kudo et al. [33]. For short, 5.4 g metal tungsten powder was mixed with 20 ml EtOH and then reacted with 20 ml H2O2 (30%). After the removal of impurities, the solution was reflux at 80 C for 2 h to form a yellow sol. The volume of the sol was adjusted to 50 ml by adding EtOH and the concentration of WO3 was 0.6 M. For comparing, the 0.1 g PdCl2 was added to the WO3 sol to form the catalyst doping WO3 sol, called WO3þPd sol. For sensor fabrication, the WO3 film was first prepared on glass slides by dip-coating in WO3 sol, after condensation at 80 C for 10e15 min, the SiO2þPd film was coated on top of the WO3 film by the same method, so called WO3/SiO2þPd film.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Fig. 7 e Diameter distribution of the WO3 (solid line) and WO3þPd (dash line) sol. Black line for original sol, red line for aging 1 day, green line for aging 2 days, blue line for aging 3 days. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
Fig. 5 e FTIR spectrum of the (a) SiO2 and SiO2þPd films and (b) WO3, WO3þPd and WO3/SiO2þPd films.
JEOL JEM-2100F) with energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR, Bruker Tensor 27), dynamic light scattering particle size analyzer (DLS, HORIBA LB-550) and the UVeviseNIR spectrophotometer (JASCO V-570). For researching the gasochromic effect under high H2 concentration, the films were directly exposed to the 10% H2/Ar or pure O2 and the spectra were measured. For testing the hydrogen sensing properties of the films, the film was placed in a 600 ml sealed vacuum bottle, then certain amount of gas at 100 kPa was injected into the bottle, the change of the transmittance at 1000 nm was recorded at the same time due to the main transmittance difference during the gasochromic is occurred around 1000 nm [34e36]. The partial pressure of the H2 was used to identify the
The WO3þPd film was directly prepared by dip-coating in the WO3þPd sol. The designs of the film were shown in Fig. 1. The sensor and sol were characterized through scanning electron microscopy (SEM, Philips-XL-30FEG and Hitachi S-4800), transmission electron microscopy (TEM, FEI Tecnai G2 F20 and
Fig. 6 e The filling of the neck during the growth of the particles [45].
Fig. 8 e The spectrum of the gasochromic films at bleached (transparent) state and colored (blue) state. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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concentration of the hydrogen. The hydrogen response measurements were performed at 23 C and the relative humidity was 40% unless otherwise specified.
Results and discussion Reaction of the PdCl2 under the H2 and O2 To further analyze the sensor, we first demonstrate the role of the catalyst PdCl2. The PdCl2 solution was prepared as M. Ranjbar et al. [37], and then the gas was pumped into the solution. Fig. 2 shows the transmittance spectra of the PdCl2 solution under the different conditions. The black line represents the transmittance curve of the PdCl2 solution before hydrogen was pumped. We can see that there is an absorption peak at 425 nm, corresponding to the characteristic peak of Pd2þ. The red line presents the transmittance curve of the PdCl2 solution after hydrogen was pumped. It is worth noting that the intensity of absorption peak at 425 nm is lowered. The dramatic difference between the as-prepared solution and others were attributed to the reduction of the Pd2þ by hydrogen, but the absorption peak of PdCl2 around 425 nm did not completely disappear, indicating the presence of Pd2þ residues. However, the intensity of peak is slightly recovered after oxygen was pumped, suggesting that there was a reversible reaction during hydrogen pumping. Considering the hydrogen spillover effect, it can be assigned to the adsorption/desorption process of the hydrogen on Pd.
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peaks of the PdCl2 didn't show in the spectra due to the low doping concentration. Fig. 5(b) shows the FTIR spectra of the WO3, WO3þPd and WO3/SiO2þPd film. The peaks at 1066 and 1151 cm1 are ascribed to mode and LO mode of the SieOeSi respectively [39]; the band around 650, 815, 963 cm1 are related to the corner-sharing W-O-W stretching, edge-sharing W-O-W stretching and terminal W]O stretching, respectively [40]. As mentioned above, the edge-sharing W-O-W and corner-sharing W-O-W is related to the gasochromic properties of the WO3 [41], but the edge-sharing W-O-W is much benefit to the gasochromic properties. By comparing their FTIR peaks, the WO3 film has two peaks at 650 and 815 cm1 related to the gasochromic properties, but has no gasochromic ability due to the absent of the catalyst; the peak intensity of corner-sharing W-O-W is much stronger than that of edgesharing W-O-W in WO3þPd film, suggesting that doping PdCl2 in WO3 can affect the structure of WO3 and result in edge-sharing W-O-W is transformed into corner-sharing W-O-W, which may be cause the poor gasochromic properties; for WO3/SiO2þPd film, without doping PdCl2 in WO3, the peak intensity of corner-sharing W-O-W and edge-sharing WO-W is similar with the that of WO3 film, which may obtain a better gasochromic properties. Furthermore, the refractive index of the WO3 film and WO3þPd film were fitted by Film Wizard software using the
Distribution of PdCl2 in SiO2 Usually PdCl2 do not show good solubility in the sol, especially in alcohol system [38]. In order to identify the distribution of the catalyst in the SiO2 support, Fig. 3 shows the dark-field TEM image and EDS mapping images of the SiO2þPd sol. No PdCl2 particles can be found in the sol. Instead, a same distribution area of Si and Pd can be observed, proving that the PdCl2 is well dispersed in the SiO2 support. To demonstrate the advantage of our method on preparing uniformly catalysts layer, PdCl2 solution was directly drop on the surface of WO3 film for comparing and the results were carried out by SEM shown in Fig. 4. Clearly, the film prepared by directly drop on the surface of WO3 film shows obvious aggregation as labeled by arrow in Fig. 4(a), in contrast, the film prepared by our method is uniform and flat (Fig. 4(b)). Fig. 4(c) shows the cross section of the WO3/SiO2þPd film, the thickness of the WO3/ SiO2þPd film is 194 nm, and the thickness of SiO2þPd layer is 54 nm. In a word, the PdCl2 was uniformly distributed in the SiO2 sol, which avoided the aggregation on the WO3 layer.
Structure changes of the films by PdCl2 doping Fig. 5(a) shows the FTIR spectra of the SiO2 and SiO2þPd films. The peaks at around 1066 and 1145 cm1 can be assigned to the transverse optic (TO) vibrational mode and longitudinal optic (LO) mode of the SieOeSi, respectively [39]. There is no significant difference between two samples, indicating the doping PdCl2 cannot change the structure of the SiO2. The
Fig. 9 e Gasochromic cycling of the WO3/SiO2þPd film.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Cauchy equation [42] and porosity factor were calculated from it by following equation [43]:
r¼
n2 n2p ðn2 1Þ
(1)
where r is porosity factor of the porous film, n is the refractive index of the WO3 bulk (2.18 is selected) [40] and np is the refractive index of the porous film at 632.8 nm. The fitted refractive index of WO3 and WO3þPd film at 632.8 nm were 1.819 and 1.901 and calculated porosity factor were 38% and 30%, respectively. It suggests that the doping of the PdCl2 may destroy the balance of double layers in the sol and leading to the growth of the particles [44]. During this period, the particles suffer from the dissolution and reprecipitation and the small pore, such as neck, were filled in (as shown in Fig. 6) [45], which is bad for the diffusion of the hydrogen. In a word, the SiO2-Pd film protects the WO3 from losing the edge-sharing structure and porosity, which is good for gasochromic properties.
Stability of the WO3 and WO3þPd sol To clarify the influence of PdCl2 on the stability of the WO3 sol, the diameter distributions of sol under different aging stages were measured. Fig. 7 shows the diameter distribution of the WO3 and PdCl2 doping WO3 sol (WO3þPd sol) under different aging stages. The solid line represents the diameter distribution of the WO3 sol, and the dash line
refers to that of the WO3þPd sol. Clearly, the diameter of the WO3 sol is nearly the same, and the color of sol remains yellow after 3 days. For WO3þPd sol, the diameter and color show an obvious change during the aging period. After aging for 2 days, the color changed from orange to dark green, and further deepened after 3 days. At the original stage (0 day), the diameter of the WO3þPd sol is about 0.034 mm; the diameter gradually increased to 4.4 mm, and a precipitate forms in the sol after 3 days. It shows that the doping PdCl2 can affect the stability of the WO3 sol and shorten the lifespan of the sol.
Gasochromic properties of the film Fig. 8 presents the gasochromic phenomena of the films. The film was colored by 10% H2/Ar and bleached by pure O2. After colored, the transmittance spectra shows a dramatically decrease, especially in the near infrared region. The transmittance of the WO3/SiO2þPd films is higher than WO3þPd films due to the anti-reflection effect of the SiO2 layer [32]. To demonstrate the cycling lifespan of the WO3/SiO2þPd films, gasochromic cycle was recorded by the change of the transmittance at 1000 nm due to the main transmittance difference during the gasochromic is occurred around here. As shown in Fig. 9, after 160 cycles, there is no significant decay of the transmittance difference or the speed of gasochromic, exhibiting the favorable cycling lifetime of the WO3/SiO2þPd film. For testing the hydrogen sensing properties of the films,
Fig. 10 e Hydrogen sensing properties of the (a) WO3þPd and (b) WO3/SiO2þPd films without air, and (c) is the hydrogen sensing properties with different relative humidity of the air for WO3/SiO2þPd films.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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certain amount of 10% H2/Ar was injected into a 600 ml sealed vacuum bottle with film in it, the change of the transmittance at 1000 nm was recorded at the same time. The partial pressure of the H2 was used to identify the concentration of the hydrogen. Fig. 10(a) shows that the WO3þPd film has a long coloring time with 100 ml H2/Ar, where the hydrogen partial pressure in bottle was 1688 Pa and total pressure was 16887 Pa, the change of the transmittance is slight at the lower concentrate; for WO3/SiO2þPd film as shown in Fig. 10(b), the coloring speed is notably faster than WO3þPd film at 1688 Pa hydrogen partial pressure, and transmittance difference is 43%; 16% decrease of the transmittance still can be get with 50 ml H2/Ar, where the hydrogen partial pressure in bottle was 844 Pa and total pressure was 8444 Pa; for 20 ml H2/Ar, where the hydrogen partial pressure in bottle was 338 Pa and total pressure was 3378 Pa, the transmittance only changed for 10%, but is still better than WO3þPd films. The hydrogen sensing properties under different relative humidity (RH) in air were also measured as follow: 200 ml of air with different relative humidity was injected into the bottle mentioned above as base gas, except 0% relative humidity air was achieved by synthetic air (80% N2/20% O2), then injected 100 ml of 10% H2/Ar into the bottle to trigger the gasochromic effect, where the hydrogen partial pressure in bottle was 1688 Pa and total pressure was 50 kPa. The results were showed in Fig. 10(c). The transmittance difference drop to nearly 20% due to the competition of the oxygen but still get a fast respond. Under the air with 0%, 30%, 50%, 70% and 90% relative humidity, the sample show a similar gasochromic process,
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which indicated the relative humidity has minor influence on the WO3/SiO2þPd film. In conclusion, the WO3/SiO2þPd film shows fast and stable gasochromic properties, even under different relative humidity.
Differences between the gasochromic process of WO3þPd films and WO3/SiO2þPd films under low-pressure To further reveal the different gasochromic process of the samples, the color centers should be considered, which can be measured from optical density (OD) changes. The optical density is obtained from the following equation: OD ¼ log
T0 T
(2)
where T and T0 are the transmitted light transmittance at the colored and bleach states, respectively. The OD change of the WO3þPd film in Fig. 11(a) could be divided into two part: at the beginning of the gasochromic the curve can be fitted by a single exponential growth function (dark yellow line), which represents a gradually increasing but slow reaction process; after certain seconds, the curve can be fitted by a single exponential decay function (blue line). But for the WO3/SiO2þPd film in Fig. 11(b), the curves can be fitted by a single exponential decay function (blue line), no exponential growth process can be found. All the fitted parameters of the films are shown in Table 1. From our previous research [40], two types of reaction are involved in the gasochromic: one is the chemical reaction and another one is the electrochemical
Fig. 11 e The fitting of optical densities of WO3þPd (a) and WO3/SiO2þPd (b) film; (c) the relation between fitting parameter y2 and In P(H2). Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Table 1 e The fitting parameters of the WO3þPd and WO3/SiO2þPd film at different H2 concentration. Gasochromic Parameters Type of the film WO3þPd
WO3/SiO2þPd
Fitting parameters for single exponential growth function y ¼ Ax/t1 1e þy1 and single ðxx Þ=t exponential decay function y ¼ A2e 0 2 þy2
H2 partial pressure (Pa)
A1
t1
y1
R2
A2
t2
y2
x0
R2
1688 844 338 1688 844 338
0.09114 0.01881 0.10210 e e e
574.71264 640.69341 2512.05744 e e e
0.10202 0.02248 0.10928 e e e
0.99223 0.99795 0.99714 e e e
0.32765 0.34001 0.20430 0.31449 0.09459 0.06332
1161.97973 2325.41836 2810.47687 32.88695 39.39086 94.2156
0.46248 0.42441 0.33751 0.31971 0.09834 0.06362
559 1135 2225 e e e
0.99998 0.99994 0.99999 0.9915 0.99086 0.99968
injection reaction. In a typical gasochromic process, a slow chemical reaction, which can be fitted by a single exponential growth function, will occurs, after the accumulation of the HxWO3, the fast electrochemical injection reaction, which can be fitted by a single exponential decay function, shall start and till the end of the gasochromic. The electrochemical injection reaction process is similar with the resistorecapacitor charging circuit where the hydrogen (chemical potential of the hydrogen, m(H2)) is similar with the power supply and the
film (chemical potential of the HxWO3, m(HxWO3)) is similar with the capacitor. So, at the end of the gasochromic, the chemical potential of the hydrogen should be equal to the chemical potential of the HxWO3, which should be in proportion to OD [40]: mðH2 Þ ¼ mðHx WO3 ÞfOD
(3)
According to the ideal gas law, mðH2 Þfln PðH2 Þ
(4)
where P(H2) is the partial pressure of the hydrogen. Combined Eqs. (3) and (4): ODfln PðH2 Þ
(5)
Refer to the fitting function, the parameter y2 represents the OD at the end of the gasochromic (t/þ∞). So y2 fODfln PðH2 Þ
(6)
The gasochromic of WO3þPd films fit quite well with the theory above, and as shown in Fig. 11(c), parameters y2 are linear with In P(H2); but WO3/SiO2þPd films show no exponential growth period, which indicate a short chemical reaction process. Moreover, the y2 of WO3/SiO2þPd films are poorly linear with In P(H2). These might be assigned to the hydrogen atom diffusion: the hydrogen atom diffusion possess a larger diffusion coefficient. With a fast diffusion speed, the hydrogen
Fig. 12 e The schematic of the WO3/SiO2/SiO2þPd films (a), the gasochromic results of WO3/SiO2/SiO2þPd films with different intermediate SiO2 layer thickness (b) and the fitting of the optical density to the thickness of the intermediate SiO2 layer (c).
Fig. 13 e The split and diffusion of the hydrogen in WO3/ SiO2þPd film.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Fig. 14 e (a) TEM image of nano-WO3,(b) TEM, HRTEM image and EDS mapping of the SiO2þPd doping nano-WO3 and (c) Gasochromic effect of nano-WO3.
atom could form and accumulate HxWO3 faster than hydrogen, which possess a smaller diffusion coefficient. With the absent of the slow chemical reaction, the gasochromic could directly run into the fast electrochemical injection period, and a fast coloring speed is achieved. However, the hydrogen atom might be unstable during the diffusion and recombine to the hydrogen again, which influence the final
OD of the gasochromic and break the liner relation between y2 and In P(H2). In conclusion, the WO3þPd film showed a typical two-step gasochromic process with slow speed but the WO3/ SiO2þPd film showed a one-step gasochromic process with high speed. In addition, the abnormal relation with the hydrogen pressure and gasochromic in WO3/SiO2þPd films need further discussion.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Diffusion of the hydrogen atom in the film
Mechanism of the improvements on gasochromic properties
To prove the hydrogen atom was diffused in the film and lead to the gasochromic, as well as the recombination of the hydrogen atoms, a series of WO3/SiO2/SiO2þPd films were fabricated as Fig. 12(a). WO3 and SiO2þPd layers were prepared as above but the intermediate un-doped porous SiO2 layer was prepared by alkali hydrolyzing TEOS [32] following a dipcoating method with different dip-coating speed, which brings a different thickness of the intermediate porous SiO2 layer. The thicknesses of the layers were fitted by Film Wizard software using the Cauchy equation [42]. After that, the different films, where the thickness of the intermediate porous SiO2 layer were 53 nm, 133 nm. 180 nm and 571 nm, were coloring in the chamber mentioned above with 1688 Pa hydrogen partial pressure (total pressure was 16887 Pa). The gasochromic results are shown in Fig. 12(b), all films showed a gasochromic effect but with different speed. With the intermediate layer, the WO3 was fully separated with PdCl2, the hydrogen molecule which diffused to the WO3 cannot react with it. Therefore, the hydrogen atom should diffuse to the WO3 and contribute to the gasochromic. Based on this structure, the diffusion coefficient of hydrogen atom in sol-gel film was estimated to further prove the diffusion of the hydrogen atom. Because at the beginning the gasochromic curve mainly shows the diffusion information but later the reaction speed will be involved due to the accumulation of the hydrogen atom, we select the data at 10 s, where the accumulation of the hydrogen atom has small effect to the reaction speed due to the large numbers of unreacted WO3 and long enough for steady diffusion undergo at average speed and accumulate HxWO3, as shown in Fig. 12(c). The diffusion equation with decay is expressed as [20]:
The WO3/SiO2þPd film shows a better gasochromic properties can be attributed to these factors as following: First, preserving of the edge-sharing W-O-W structure can improve the cycling lifespan of the sensor; more importantly, as shown in Fig. 13, considering the hydrogen spillover effect, porous SiO2 film allow the hydrogen easy to interact with catalyst and pre-split into hydrogen atoms, so it isn't hydrogen molecules but hydrogen atoms diffusing into the WO3. WO3 films possess a microporous structure [40]. Under this condition, the diffusion coefficient is decreased with a larger molecules weight [20]. So the diffusion of the hydrogen atoms should be faster than the hydrogen molecules. The fast diffusion, which is achieved by pre-split effect of the SiO2þPd film, could lead to a fast gasochromic process, thereby increasing the sensitivity of the sensor.
vC v2 C ¼ D 2 kC vt vx
(7)
where C is the concentration, k is the decay coefficient, D is the diffusion coefficient and x is the distance to the source. In this case, C, k and D all refer to the properties of the hydrogen atom in the intermediate SiO2 layer, and x is the distance to the top of the intermediate SiO2 layer. At the steady diffusion process, the solution of Eq. (7) (for x 0) is [20]: pffiffiffiffiffiffi C ¼ C0 e k=Dx
(8)
where C0 is a constant. Under a steady diffusion, the change of the OD should be in proportion to the concentration at the bottom of the intermediate SiO2 layer. In this situation the x could be set as the thickness of the intermediate SiO2 layer, so the OD-thickness curve could be well fitted with this function where the √ (k/D) ¼ 0.01936 (R2 ¼ 0.99996). As reported by M. Green et al. [46], the decay coefficient of the hydrogen atom in silica at the room temperature is about 104, so the D z 2.67 1015 cm2/s, which is fitted with previous research on the hydrogen atom diffusion [47]. The fitting of the diffusion with decay and proper diffusion coefficient prove the existence of the hydrogen atom diffusion and recombination in the intermediate SiO2 layer.
Substrate versatility of the SiO2þPd layer To demonstrate substrate versatility of our method, a nanoWO3 was prepared through hydrothermal synthesis and activated by SiO2þPd sol. Fig. 14(a) shows the nano-WO3 possesses a nanosheet structure; then the SiO2þPd sol was dropped on the nano-WO3 powder. Fig. 14(b) is the TEM images of the powder after dropping sol. The WO3 sheet was coated with the amorphous SiO2, indicating that the catalyst doped into the SiO2 could uniformly coated, and it's consistent with the EDS mapping results. After exposed to the hydrogen, the yellow powder can be changed to dark blue uniformly as shown in Fig. 14(c). This result suggests that the SiO2þPd sol can disperse the catalyst uniformly at nano-structure, which could be benefited for future development of nano-structure WO3 based gasochromic devices.
Conclusion In conclusion, we have demonstrated a facile and versatility method to prepare a uniform catalyst layer for pre-splits hydrogen in the gasochromic devices by a sol-gel SiO2 catalyst supporter film. Electron microscopes prove that the PdCl2 can be uniformity distributed on WO3; FTIR and diameter distribution results show the improved structure stability of the sol. The WO3/SiO2þPd film show a long gasochromic cycle lifetime and a short respond time at low concentration of H2 than WO3þPd film. In addition, the WO3/SiO2þPd film shows small influence by humidity. The improvement of gasochromic properties is benefited from the PdCl2 doping SiO2 film deposited on the WO3 film pre-splits hydrogen into smaller hydrogen atom to diffuse into the WO3, and the hydrogen atom processes a larger diffusion coefficient in WO3, which brings a fast diffusion speed.
Acknowledgements This work was supported by Joint Funds of the National Natural Science Foundation of China (U1503292), National Key Research and Development Program of China
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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(2017YFA0204600) and National Natural Science Foundation of China (51872204, 51472182).
references
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Available online at www.sciencedirect.com
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Fast hydrogen diffusion induced by hydrogen presplit for gasochromic based optical hydrogen sensors Haoran Wang a, Guohua Gao a,*, Guangming Wu a, Huiyue Zhao a, Wanyu Qi a, Kaicong Chen a, Wan Zhang a, Yunyun Li a,b,** a
Shanghai Key Laboratory of Special Artificial Microstructure, Tongji University, Shanghai, 200092, China Center for Phononics and Thermal Energy Science, School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
b
article info
abstract
Article history:
Gasochromic based optical hydrogen sensors have attracted much attention as normal
Received 27 December 2018
temperature sensors. The behaviors of the hydrogen diffusion largely affect the reaction
Received in revised form
process and the sensitivity. However, few researches focused on the influence of hydrogen
1 April 2019
diffusion in gasochromic films. Here, we report a method to pre-split the H2 molecule and
Accepted 4 April 2019
separate the hydrogen molecule and hydrogen atom diffusion process by deposition of
Available online xxx
porous PdCl2/SiO2 catalyst films on WO3. The response time of WO3 at 844 Pa H2 partial
Keywords:
more than 160 cycling lifetime. A validation experiment was designed and proved the
Gasochromic
diffusion of the hydrogen atom and the two-step model of H2 reaction on WO3 films, on the
Sol-gel
basis of which the diffusion coefficient of the hydrogen atom has been estimated. More-
pressure reduced from 800 s to 200 s. The stability of WO3 film was largely improved to be
Hydrogen sensor
over, the PdCl2 doping SiO2 catalyst exhibits substrate versatility, which can also coat on
Diffusion
the nano-structured WO3. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The energy carrier with negligible environmental impact is one of the most important technology challenges of the 21-st century [1]. Except the traditional energy storage technologies, such as supercapacitors [2] and batteries [3], hydrogen is also a potential clean and renewable energy carrier. Unfortunately, with a wide explosive limit (4%e74.4%) and small molecule size, hydrogen leaking has become a serious safety issue in the industry [4]. Thus, there is a strong demand for hydrogen
sensors. For now, resistive-type sensors are an important research field for hydrogen sensors [5e7]. However, resistivetype sensors mainly work at high temperature, leading to the high energy cost and explosion risks [8]. For certain materials, their optical properties change when they interact with hydrogen under room-temperature, which is exploited as an optical-type hydrogen sensor [8]. Based on this, various optical-type sensors has been developed, including Fiber Bragg Gratings sensor [9e11], interferometric sensor [12e14] and gasochromic based sensor [15e18], etc. Among them, gasochromic based sensor possess a simple structure and can
* Corresponding author. ** Corresponding author. Shanghai Key Laboratory of Special Artificial Microstructure, Tongji University, Shanghai, 200092, China. E-mail addresses: [email protected] (G. Gao), [email protected] (Y. Li). https://doi.org/10.1016/j.ijhydene.2019.04.026 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Fig. 1 e The design of the WO3/SiO2þPd film and WO3þPd film.
Fig. 2 e Transmittance spectrum of the PdCl2 water solution. be easily assembled into a self-powered gasochromic based sensor, as demonstrated by Chen et al. recently [17], which may be a more secure, convenient and commercial candidate for hydrogen detection. The gasochromic device mainly makes of two parts: the catalyst and gasochromic materials. Upon exposure to hydrogen, hydrogen molecules will split into hydrogen atoms
on the surface of the catalyst, which are essentially noble metal simple substances or salts, known as “hydrogen spillover” [19]. Then the hydrogen atoms can be injected into the gasochromic materials, such as WO3, resulting in a drop in the transmittance of the device. This phenomenon can be reversed by the oxygen or air. Actually, in the gasochromic film, sensitivity is mainly controlled by H2 molecule diffusion, which is based on Brownian movement that needs large pore size [20]. On top of that, the current studies on the sensitivity of the gasochromic optical sensors mainly focus on porous modification of the materials. WO3 nanofibers were prepared by Fatemeh Tavakoli Foroushani et al. [16] with sensitivity to 1% H2 in 6 min at room-temperature by electrospinning. Nano-columnar WO3-Pd films were prepared by Young-Ahn Lee et al. [21], which can detect hydrogen below 1% at roomtemperature in 30 s but with a cycling life less than 50 times. A 2D WO3 nanostructure was reported by Shankara S.Kalanur et al. [22] with sensitivity to 1% H2 in 30 s at room-temperature. However, porous modification increases the difficulty, instability and cost of the synthesis process of WO3 films. In contract, the hydrogen atom generated by hydrogen spillover is diffused by surface diffusion [23], which does not depend on the large pore sizes. Meanwhile, the small size of the H can also benefit to the diffusion which has been reported in research of hydrogen storage [24]. So if the diffusion of the
Fig. 3 e Dark-field TEM image and EDS mapping of the SiO2þPd sol. Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Fig. 4 e SEM images of the surface of the (a) PdCl2 drop cast on WO3 film, (b) WO3/SiO2þPd film and c) cross section of the WO3/SiO2þPd film.
hydrogen atom is applicable, the pre-split of the H2 molecule before diffused into the gasochromic materials is the key to increase the sensitivity and stability of gasochromic films. For now, most of the researchers directly added the catalyst to the precursor of the gasochromic materials during the synthetize process [22,25,26], which is easy to achieve a homogeneous doping of the catalyst but the hydrogen needs to diffused into the film and reacts with catalyst. Although there are some reports about coating the catalyst film onto the WO3 layer [27,28], which can split the hydrogen outside of the WO3, the diffusion of the hydrogen atom has not been fully discussed, and the high costs and low porosity properties of sputtered film and homogeneity problem of the direct drop-cast method limited the scope of application of these methods. Sol-gel method has gained more and more attention during the past decade. It can generate a very homogeneous distribution of components at nanoscale from commercial available precursor, and does not require complicated instrument. During the sol-gel process, a cross-linked network of the materials is assembled in the sol. After gelation and the solvent removal, porous structure can be easily prepared [29]. Thanks to this, the porous film which is suitable for hydrogen molecule diffusion can be directly synthetized even on a special-shaped surface [30], through sol-gel route combined with coating method, such as dip-coating and spin-coating [31]. Herein, a homogeneous porous catalyst film with PdCl2 doped SiO2 for WO3 gasochromic have been obtained by a facile sol-gel route. The sensitivity and stability of the system were studied. The mechanism has been discussed based on relationship between the film porosities and the control processes of H2 or H diffusion. The H diffusion was proved by isolating the catalyst and WO3 by another un-doped porous
SiO2 layer. Moreover, a two-step reaction model of H2 was proposed and a validation experiment was designed. In addition, the absence of the catalyst during the preparation of the gasochromic materials avoids the side effect to the morphology and structure, bringing more potential synthesis route for the materials.
Material and methods SiO2 sol was synthesized by acid hydrolyzing tetraethyl orthosilicate (TEOS) in ethanol (EtOH) [32]. In a typical synthesis, the TEOS was mixed with half of EtOH to form solution A, and another half of EtOH was mixed with H2O and hydrochloric acid to form solution B. Then the solution B was added dropwise to the solution A. After stirring for 1 h, the SiO2 sol was prepared. The mole ratio of the precursor was TEOS:H2O:HCl:EtOH ¼ 1: 2.3: 0.245: 38. The catalyst doping sol was prepared by adding 0.1 g PdCl2 into 50 ml SiO2 sol, called SiO2þPd sol. For WO3, the synthesis procedure was reported by Kudo et al. [33]. For short, 5.4 g metal tungsten powder was mixed with 20 ml EtOH and then reacted with 20 ml H2O2 (30%). After the removal of impurities, the solution was reflux at 80 C for 2 h to form a yellow sol. The volume of the sol was adjusted to 50 ml by adding EtOH and the concentration of WO3 was 0.6 M. For comparing, the 0.1 g PdCl2 was added to the WO3 sol to form the catalyst doping WO3 sol, called WO3þPd sol. For sensor fabrication, the WO3 film was first prepared on glass slides by dip-coating in WO3 sol, after condensation at 80 C for 10e15 min, the SiO2þPd film was coated on top of the WO3 film by the same method, so called WO3/SiO2þPd film.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Fig. 7 e Diameter distribution of the WO3 (solid line) and WO3þPd (dash line) sol. Black line for original sol, red line for aging 1 day, green line for aging 2 days, blue line for aging 3 days. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
Fig. 5 e FTIR spectrum of the (a) SiO2 and SiO2þPd films and (b) WO3, WO3þPd and WO3/SiO2þPd films.
JEOL JEM-2100F) with energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR, Bruker Tensor 27), dynamic light scattering particle size analyzer (DLS, HORIBA LB-550) and the UVeviseNIR spectrophotometer (JASCO V-570). For researching the gasochromic effect under high H2 concentration, the films were directly exposed to the 10% H2/Ar or pure O2 and the spectra were measured. For testing the hydrogen sensing properties of the films, the film was placed in a 600 ml sealed vacuum bottle, then certain amount of gas at 100 kPa was injected into the bottle, the change of the transmittance at 1000 nm was recorded at the same time due to the main transmittance difference during the gasochromic is occurred around 1000 nm [34e36]. The partial pressure of the H2 was used to identify the
The WO3þPd film was directly prepared by dip-coating in the WO3þPd sol. The designs of the film were shown in Fig. 1. The sensor and sol were characterized through scanning electron microscopy (SEM, Philips-XL-30FEG and Hitachi S-4800), transmission electron microscopy (TEM, FEI Tecnai G2 F20 and
Fig. 6 e The filling of the neck during the growth of the particles [45].
Fig. 8 e The spectrum of the gasochromic films at bleached (transparent) state and colored (blue) state. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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concentration of the hydrogen. The hydrogen response measurements were performed at 23 C and the relative humidity was 40% unless otherwise specified.
Results and discussion Reaction of the PdCl2 under the H2 and O2 To further analyze the sensor, we first demonstrate the role of the catalyst PdCl2. The PdCl2 solution was prepared as M. Ranjbar et al. [37], and then the gas was pumped into the solution. Fig. 2 shows the transmittance spectra of the PdCl2 solution under the different conditions. The black line represents the transmittance curve of the PdCl2 solution before hydrogen was pumped. We can see that there is an absorption peak at 425 nm, corresponding to the characteristic peak of Pd2þ. The red line presents the transmittance curve of the PdCl2 solution after hydrogen was pumped. It is worth noting that the intensity of absorption peak at 425 nm is lowered. The dramatic difference between the as-prepared solution and others were attributed to the reduction of the Pd2þ by hydrogen, but the absorption peak of PdCl2 around 425 nm did not completely disappear, indicating the presence of Pd2þ residues. However, the intensity of peak is slightly recovered after oxygen was pumped, suggesting that there was a reversible reaction during hydrogen pumping. Considering the hydrogen spillover effect, it can be assigned to the adsorption/desorption process of the hydrogen on Pd.
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peaks of the PdCl2 didn't show in the spectra due to the low doping concentration. Fig. 5(b) shows the FTIR spectra of the WO3, WO3þPd and WO3/SiO2þPd film. The peaks at 1066 and 1151 cm1 are ascribed to mode and LO mode of the SieOeSi respectively [39]; the band around 650, 815, 963 cm1 are related to the corner-sharing W-O-W stretching, edge-sharing W-O-W stretching and terminal W]O stretching, respectively [40]. As mentioned above, the edge-sharing W-O-W and corner-sharing W-O-W is related to the gasochromic properties of the WO3 [41], but the edge-sharing W-O-W is much benefit to the gasochromic properties. By comparing their FTIR peaks, the WO3 film has two peaks at 650 and 815 cm1 related to the gasochromic properties, but has no gasochromic ability due to the absent of the catalyst; the peak intensity of corner-sharing W-O-W is much stronger than that of edgesharing W-O-W in WO3þPd film, suggesting that doping PdCl2 in WO3 can affect the structure of WO3 and result in edge-sharing W-O-W is transformed into corner-sharing W-O-W, which may be cause the poor gasochromic properties; for WO3/SiO2þPd film, without doping PdCl2 in WO3, the peak intensity of corner-sharing W-O-W and edge-sharing WO-W is similar with the that of WO3 film, which may obtain a better gasochromic properties. Furthermore, the refractive index of the WO3 film and WO3þPd film were fitted by Film Wizard software using the
Distribution of PdCl2 in SiO2 Usually PdCl2 do not show good solubility in the sol, especially in alcohol system [38]. In order to identify the distribution of the catalyst in the SiO2 support, Fig. 3 shows the dark-field TEM image and EDS mapping images of the SiO2þPd sol. No PdCl2 particles can be found in the sol. Instead, a same distribution area of Si and Pd can be observed, proving that the PdCl2 is well dispersed in the SiO2 support. To demonstrate the advantage of our method on preparing uniformly catalysts layer, PdCl2 solution was directly drop on the surface of WO3 film for comparing and the results were carried out by SEM shown in Fig. 4. Clearly, the film prepared by directly drop on the surface of WO3 film shows obvious aggregation as labeled by arrow in Fig. 4(a), in contrast, the film prepared by our method is uniform and flat (Fig. 4(b)). Fig. 4(c) shows the cross section of the WO3/SiO2þPd film, the thickness of the WO3/ SiO2þPd film is 194 nm, and the thickness of SiO2þPd layer is 54 nm. In a word, the PdCl2 was uniformly distributed in the SiO2 sol, which avoided the aggregation on the WO3 layer.
Structure changes of the films by PdCl2 doping Fig. 5(a) shows the FTIR spectra of the SiO2 and SiO2þPd films. The peaks at around 1066 and 1145 cm1 can be assigned to the transverse optic (TO) vibrational mode and longitudinal optic (LO) mode of the SieOeSi, respectively [39]. There is no significant difference between two samples, indicating the doping PdCl2 cannot change the structure of the SiO2. The
Fig. 9 e Gasochromic cycling of the WO3/SiO2þPd film.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Cauchy equation [42] and porosity factor were calculated from it by following equation [43]:
r¼
n2 n2p ðn2 1Þ
(1)
where r is porosity factor of the porous film, n is the refractive index of the WO3 bulk (2.18 is selected) [40] and np is the refractive index of the porous film at 632.8 nm. The fitted refractive index of WO3 and WO3þPd film at 632.8 nm were 1.819 and 1.901 and calculated porosity factor were 38% and 30%, respectively. It suggests that the doping of the PdCl2 may destroy the balance of double layers in the sol and leading to the growth of the particles [44]. During this period, the particles suffer from the dissolution and reprecipitation and the small pore, such as neck, were filled in (as shown in Fig. 6) [45], which is bad for the diffusion of the hydrogen. In a word, the SiO2-Pd film protects the WO3 from losing the edge-sharing structure and porosity, which is good for gasochromic properties.
Stability of the WO3 and WO3þPd sol To clarify the influence of PdCl2 on the stability of the WO3 sol, the diameter distributions of sol under different aging stages were measured. Fig. 7 shows the diameter distribution of the WO3 and PdCl2 doping WO3 sol (WO3þPd sol) under different aging stages. The solid line represents the diameter distribution of the WO3 sol, and the dash line
refers to that of the WO3þPd sol. Clearly, the diameter of the WO3 sol is nearly the same, and the color of sol remains yellow after 3 days. For WO3þPd sol, the diameter and color show an obvious change during the aging period. After aging for 2 days, the color changed from orange to dark green, and further deepened after 3 days. At the original stage (0 day), the diameter of the WO3þPd sol is about 0.034 mm; the diameter gradually increased to 4.4 mm, and a precipitate forms in the sol after 3 days. It shows that the doping PdCl2 can affect the stability of the WO3 sol and shorten the lifespan of the sol.
Gasochromic properties of the film Fig. 8 presents the gasochromic phenomena of the films. The film was colored by 10% H2/Ar and bleached by pure O2. After colored, the transmittance spectra shows a dramatically decrease, especially in the near infrared region. The transmittance of the WO3/SiO2þPd films is higher than WO3þPd films due to the anti-reflection effect of the SiO2 layer [32]. To demonstrate the cycling lifespan of the WO3/SiO2þPd films, gasochromic cycle was recorded by the change of the transmittance at 1000 nm due to the main transmittance difference during the gasochromic is occurred around here. As shown in Fig. 9, after 160 cycles, there is no significant decay of the transmittance difference or the speed of gasochromic, exhibiting the favorable cycling lifetime of the WO3/SiO2þPd film. For testing the hydrogen sensing properties of the films,
Fig. 10 e Hydrogen sensing properties of the (a) WO3þPd and (b) WO3/SiO2þPd films without air, and (c) is the hydrogen sensing properties with different relative humidity of the air for WO3/SiO2þPd films.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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certain amount of 10% H2/Ar was injected into a 600 ml sealed vacuum bottle with film in it, the change of the transmittance at 1000 nm was recorded at the same time. The partial pressure of the H2 was used to identify the concentration of the hydrogen. Fig. 10(a) shows that the WO3þPd film has a long coloring time with 100 ml H2/Ar, where the hydrogen partial pressure in bottle was 1688 Pa and total pressure was 16887 Pa, the change of the transmittance is slight at the lower concentrate; for WO3/SiO2þPd film as shown in Fig. 10(b), the coloring speed is notably faster than WO3þPd film at 1688 Pa hydrogen partial pressure, and transmittance difference is 43%; 16% decrease of the transmittance still can be get with 50 ml H2/Ar, where the hydrogen partial pressure in bottle was 844 Pa and total pressure was 8444 Pa; for 20 ml H2/Ar, where the hydrogen partial pressure in bottle was 338 Pa and total pressure was 3378 Pa, the transmittance only changed for 10%, but is still better than WO3þPd films. The hydrogen sensing properties under different relative humidity (RH) in air were also measured as follow: 200 ml of air with different relative humidity was injected into the bottle mentioned above as base gas, except 0% relative humidity air was achieved by synthetic air (80% N2/20% O2), then injected 100 ml of 10% H2/Ar into the bottle to trigger the gasochromic effect, where the hydrogen partial pressure in bottle was 1688 Pa and total pressure was 50 kPa. The results were showed in Fig. 10(c). The transmittance difference drop to nearly 20% due to the competition of the oxygen but still get a fast respond. Under the air with 0%, 30%, 50%, 70% and 90% relative humidity, the sample show a similar gasochromic process,
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which indicated the relative humidity has minor influence on the WO3/SiO2þPd film. In conclusion, the WO3/SiO2þPd film shows fast and stable gasochromic properties, even under different relative humidity.
Differences between the gasochromic process of WO3þPd films and WO3/SiO2þPd films under low-pressure To further reveal the different gasochromic process of the samples, the color centers should be considered, which can be measured from optical density (OD) changes. The optical density is obtained from the following equation: OD ¼ log
T0 T
(2)
where T and T0 are the transmitted light transmittance at the colored and bleach states, respectively. The OD change of the WO3þPd film in Fig. 11(a) could be divided into two part: at the beginning of the gasochromic the curve can be fitted by a single exponential growth function (dark yellow line), which represents a gradually increasing but slow reaction process; after certain seconds, the curve can be fitted by a single exponential decay function (blue line). But for the WO3/SiO2þPd film in Fig. 11(b), the curves can be fitted by a single exponential decay function (blue line), no exponential growth process can be found. All the fitted parameters of the films are shown in Table 1. From our previous research [40], two types of reaction are involved in the gasochromic: one is the chemical reaction and another one is the electrochemical
Fig. 11 e The fitting of optical densities of WO3þPd (a) and WO3/SiO2þPd (b) film; (c) the relation between fitting parameter y2 and In P(H2). Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Table 1 e The fitting parameters of the WO3þPd and WO3/SiO2þPd film at different H2 concentration. Gasochromic Parameters Type of the film WO3þPd
WO3/SiO2þPd
Fitting parameters for single exponential growth function y ¼ Ax/t1 1e þy1 and single ðxx Þ=t exponential decay function y ¼ A2e 0 2 þy2
H2 partial pressure (Pa)
A1
t1
y1
R2
A2
t2
y2
x0
R2
1688 844 338 1688 844 338
0.09114 0.01881 0.10210 e e e
574.71264 640.69341 2512.05744 e e e
0.10202 0.02248 0.10928 e e e
0.99223 0.99795 0.99714 e e e
0.32765 0.34001 0.20430 0.31449 0.09459 0.06332
1161.97973 2325.41836 2810.47687 32.88695 39.39086 94.2156
0.46248 0.42441 0.33751 0.31971 0.09834 0.06362
559 1135 2225 e e e
0.99998 0.99994 0.99999 0.9915 0.99086 0.99968
injection reaction. In a typical gasochromic process, a slow chemical reaction, which can be fitted by a single exponential growth function, will occurs, after the accumulation of the HxWO3, the fast electrochemical injection reaction, which can be fitted by a single exponential decay function, shall start and till the end of the gasochromic. The electrochemical injection reaction process is similar with the resistorecapacitor charging circuit where the hydrogen (chemical potential of the hydrogen, m(H2)) is similar with the power supply and the
film (chemical potential of the HxWO3, m(HxWO3)) is similar with the capacitor. So, at the end of the gasochromic, the chemical potential of the hydrogen should be equal to the chemical potential of the HxWO3, which should be in proportion to OD [40]: mðH2 Þ ¼ mðHx WO3 ÞfOD
(3)
According to the ideal gas law, mðH2 Þfln PðH2 Þ
(4)
where P(H2) is the partial pressure of the hydrogen. Combined Eqs. (3) and (4): ODfln PðH2 Þ
(5)
Refer to the fitting function, the parameter y2 represents the OD at the end of the gasochromic (t/þ∞). So y2 fODfln PðH2 Þ
(6)
The gasochromic of WO3þPd films fit quite well with the theory above, and as shown in Fig. 11(c), parameters y2 are linear with In P(H2); but WO3/SiO2þPd films show no exponential growth period, which indicate a short chemical reaction process. Moreover, the y2 of WO3/SiO2þPd films are poorly linear with In P(H2). These might be assigned to the hydrogen atom diffusion: the hydrogen atom diffusion possess a larger diffusion coefficient. With a fast diffusion speed, the hydrogen
Fig. 12 e The schematic of the WO3/SiO2/SiO2þPd films (a), the gasochromic results of WO3/SiO2/SiO2þPd films with different intermediate SiO2 layer thickness (b) and the fitting of the optical density to the thickness of the intermediate SiO2 layer (c).
Fig. 13 e The split and diffusion of the hydrogen in WO3/ SiO2þPd film.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
international journal of hydrogen energy xxx (xxxx) xxx
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Fig. 14 e (a) TEM image of nano-WO3,(b) TEM, HRTEM image and EDS mapping of the SiO2þPd doping nano-WO3 and (c) Gasochromic effect of nano-WO3.
atom could form and accumulate HxWO3 faster than hydrogen, which possess a smaller diffusion coefficient. With the absent of the slow chemical reaction, the gasochromic could directly run into the fast electrochemical injection period, and a fast coloring speed is achieved. However, the hydrogen atom might be unstable during the diffusion and recombine to the hydrogen again, which influence the final
OD of the gasochromic and break the liner relation between y2 and In P(H2). In conclusion, the WO3þPd film showed a typical two-step gasochromic process with slow speed but the WO3/ SiO2þPd film showed a one-step gasochromic process with high speed. In addition, the abnormal relation with the hydrogen pressure and gasochromic in WO3/SiO2þPd films need further discussion.
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
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Diffusion of the hydrogen atom in the film
Mechanism of the improvements on gasochromic properties
To prove the hydrogen atom was diffused in the film and lead to the gasochromic, as well as the recombination of the hydrogen atoms, a series of WO3/SiO2/SiO2þPd films were fabricated as Fig. 12(a). WO3 and SiO2þPd layers were prepared as above but the intermediate un-doped porous SiO2 layer was prepared by alkali hydrolyzing TEOS [32] following a dipcoating method with different dip-coating speed, which brings a different thickness of the intermediate porous SiO2 layer. The thicknesses of the layers were fitted by Film Wizard software using the Cauchy equation [42]. After that, the different films, where the thickness of the intermediate porous SiO2 layer were 53 nm, 133 nm. 180 nm and 571 nm, were coloring in the chamber mentioned above with 1688 Pa hydrogen partial pressure (total pressure was 16887 Pa). The gasochromic results are shown in Fig. 12(b), all films showed a gasochromic effect but with different speed. With the intermediate layer, the WO3 was fully separated with PdCl2, the hydrogen molecule which diffused to the WO3 cannot react with it. Therefore, the hydrogen atom should diffuse to the WO3 and contribute to the gasochromic. Based on this structure, the diffusion coefficient of hydrogen atom in sol-gel film was estimated to further prove the diffusion of the hydrogen atom. Because at the beginning the gasochromic curve mainly shows the diffusion information but later the reaction speed will be involved due to the accumulation of the hydrogen atom, we select the data at 10 s, where the accumulation of the hydrogen atom has small effect to the reaction speed due to the large numbers of unreacted WO3 and long enough for steady diffusion undergo at average speed and accumulate HxWO3, as shown in Fig. 12(c). The diffusion equation with decay is expressed as [20]:
The WO3/SiO2þPd film shows a better gasochromic properties can be attributed to these factors as following: First, preserving of the edge-sharing W-O-W structure can improve the cycling lifespan of the sensor; more importantly, as shown in Fig. 13, considering the hydrogen spillover effect, porous SiO2 film allow the hydrogen easy to interact with catalyst and pre-split into hydrogen atoms, so it isn't hydrogen molecules but hydrogen atoms diffusing into the WO3. WO3 films possess a microporous structure [40]. Under this condition, the diffusion coefficient is decreased with a larger molecules weight [20]. So the diffusion of the hydrogen atoms should be faster than the hydrogen molecules. The fast diffusion, which is achieved by pre-split effect of the SiO2þPd film, could lead to a fast gasochromic process, thereby increasing the sensitivity of the sensor.
vC v2 C ¼ D 2 kC vt vx
(7)
where C is the concentration, k is the decay coefficient, D is the diffusion coefficient and x is the distance to the source. In this case, C, k and D all refer to the properties of the hydrogen atom in the intermediate SiO2 layer, and x is the distance to the top of the intermediate SiO2 layer. At the steady diffusion process, the solution of Eq. (7) (for x 0) is [20]: pffiffiffiffiffiffi C ¼ C0 e k=Dx
(8)
where C0 is a constant. Under a steady diffusion, the change of the OD should be in proportion to the concentration at the bottom of the intermediate SiO2 layer. In this situation the x could be set as the thickness of the intermediate SiO2 layer, so the OD-thickness curve could be well fitted with this function where the √ (k/D) ¼ 0.01936 (R2 ¼ 0.99996). As reported by M. Green et al. [46], the decay coefficient of the hydrogen atom in silica at the room temperature is about 104, so the D z 2.67 1015 cm2/s, which is fitted with previous research on the hydrogen atom diffusion [47]. The fitting of the diffusion with decay and proper diffusion coefficient prove the existence of the hydrogen atom diffusion and recombination in the intermediate SiO2 layer.
Substrate versatility of the SiO2þPd layer To demonstrate substrate versatility of our method, a nanoWO3 was prepared through hydrothermal synthesis and activated by SiO2þPd sol. Fig. 14(a) shows the nano-WO3 possesses a nanosheet structure; then the SiO2þPd sol was dropped on the nano-WO3 powder. Fig. 14(b) is the TEM images of the powder after dropping sol. The WO3 sheet was coated with the amorphous SiO2, indicating that the catalyst doped into the SiO2 could uniformly coated, and it's consistent with the EDS mapping results. After exposed to the hydrogen, the yellow powder can be changed to dark blue uniformly as shown in Fig. 14(c). This result suggests that the SiO2þPd sol can disperse the catalyst uniformly at nano-structure, which could be benefited for future development of nano-structure WO3 based gasochromic devices.
Conclusion In conclusion, we have demonstrated a facile and versatility method to prepare a uniform catalyst layer for pre-splits hydrogen in the gasochromic devices by a sol-gel SiO2 catalyst supporter film. Electron microscopes prove that the PdCl2 can be uniformity distributed on WO3; FTIR and diameter distribution results show the improved structure stability of the sol. The WO3/SiO2þPd film show a long gasochromic cycle lifetime and a short respond time at low concentration of H2 than WO3þPd film. In addition, the WO3/SiO2þPd film shows small influence by humidity. The improvement of gasochromic properties is benefited from the PdCl2 doping SiO2 film deposited on the WO3 film pre-splits hydrogen into smaller hydrogen atom to diffuse into the WO3, and the hydrogen atom processes a larger diffusion coefficient in WO3, which brings a fast diffusion speed.
Acknowledgements This work was supported by Joint Funds of the National Natural Science Foundation of China (U1503292), National Key Research and Development Program of China
Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026
international journal of hydrogen energy xxx (xxxx) xxx
(2017YFA0204600) and National Natural Science Foundation of China (51872204, 51472182).
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Please cite this article as: Wang H et al., Fast hydrogen diffusion induced by hydrogen pre-split for gasochromic based optical hydrogen sensors, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.04.026