Introduction

Hydrogen gas (H2) is expected to be a future clean-energy source to mitigate global warming and the exhaustion of fossil fuels. The use of H2 as an energy carrier requires safe materials to be used in the H2 infrastructure1,2,3. To satisfy this safety requirement, the embrittling effects of H2 exposure have been studied in steel, stainless steel, aluminium, and alloys4,5,6,7,8.

However, many kinds of rubber polymers are used as sealants in H2 environments: polytetrafluoroethylene (PTFE) is used as a sealant in mechanical compressors, and nitrile butadiene rubber (NBR), ethylene propylene diene monomer (EPDM), and fluoroelastomer (FKM) rubbers are used as sealants and gaskets in valves and pipelines9. Research on rubber polymers used for gas sealants and liners at the H2 station has focused on understanding the origin of explosive failure by decompression and the swelling behaviour of O-ring rubber under high-pressure H2 10,11,12. Research on how H2 affects the physical properties and morphology after pressurization and decompression has been conducted, but in situ measurements during pressurization and decomposition under high pressure have rarely been performed.

In this study, by using impedance spectroscopy (IS) to measure the capacitance and the dissipation factor (DF), we performed in situ, real-time monitoring of the dynamic macroscopic behaviour of the penetration of the H2 molecule into O-ring rubber and its desorption from the rubber during the processes of pressurization and decompression under high-pressure H2 gas. The results of the impedance measurement for NBR at various pressures and exposure times to H2 gas were analysed. The quantities considered include H2 diffusion, interfacial polarization, plasticization, permeation properties, and the physicochemical interaction between H2 and the polymer during the processes of pressurization and decompression. The results of scanning electron microscopy (SEM) and a COMSOL simulation support the IS observations.

Experimental

Sample preparation

NBR (Fig. 1, Table 1) is a synthetic rubber copolymer of acrylonitrile and butadiene. NBR is widely used as a sealing material due to its excellent gas resistance. The NBR used in this study was synthesized by a Korean domestic company, and 22% carbon black was included as a filler during the fabrication of the NBR specimen.

Figure 1
figure 1

Molecular structure of the NBR copolymer of acrylonitrile and butadiene; m: number of butadiene moieties; n: number of acrylonitrile moieties.

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Table 1 Chemical composition of the NBR specimen.
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Impedance spectroscopy system

The IS system (Fig. 2) for the in situ measurement under high pressure comprises a hydrogen gas supply, input and vent valves, a pressure gauge, a hydrogen vessel, and an impedance analyser with a general purpose interface bus (GPIB) interface to a PC. The hydrogen vessel contains the specimen, electrodes and a high-pressure feed-through to withstand the seal at high pressure. The NBR used for this measurement was disc-shaped, 49 mm in diameter and 2 mm in thickness. The specimen was inserted between two electrodes and later fixed in place by mechanical pressure. The variation in the impedance of the specimen was monitored in real time using an Agilent 4294 A impedance analyser and an automated measurement program at the frequencies of the applied voltage of 40 Hz ≤ frequency ≤ 10000 Hz. The in situ capacitance CNBR and DF DFNBR were simultaneously measured as a function of pressure and exposure time for several days.

Figure 2
figure 2

Configuration of the whole in situ impedance spectroscopy system under high-pressure H2.

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Thermal desorption analysis

Thermal desorption gas analysis (TDA) was performed using a 7890A analyser with a pulsed discharge detector from Agilent. To quantify the charged hydrogen content in the NBR, a specimen that had been exposed to H2 gas at 10 MPa for 24 h was mounted in the tube furnace of the analyser. The specimen was 20 mm in diameter and 2 mm in thickness. The release of H2 gas was measured by gas chromatography every 5 min for the first hour and then at intervals of 1 h for 16 h.

Results and Discussion

COMSOL simulation

To quantify the relationship between the measured CNBR and the relative dielectric constant εr of the rubber, COMSOL software was used to model the hydrogen vessel and the electrodes (Fig. 3). The electric current conservation equations, J = σE + Je, E = −∇V, where J is the current density, σ is the conductivity, Je is the external current density, E is the electric field, and V is the electric potential, are solved with a boundary condition set by a ground domain at one electrode extending to the outer cylinder and a 1 V potential domain at the other electrode. A default mesh condition with a fine mesh size was used. The capacitance is equal to the total charge accumulated at the electrode, because we applied a unit potential of 1 V. The charge was calculated by surface integration of the surface charge over the front and side faces of the counter electrode, including the centre rod. In order to simulate the grounded vessel, the surface charge integration was carried over the counter electrode only excluding the vessel. The COMSOL simulations (Fig. 3, C-simul) show the capacitance C versus the electrode gap d. For d < 1 mm, the C-simul deviated from the capacitances (C-analytic) calculated using an ideal parallel-plate model, CNBR = ε0 · R2/d (where R is the radius of the parallel electrode face), because of the limits of the geometric precision in the mesh modelling. The deviation between C-simul and C-analytic for d > 10 mm is attributed to a stray field that primarily forms between the side faces of the electrodes. It should be noted that the stray field between electrode faces and the inner faces of the vessel does not make any effect in capacitance because the vessel is electrically grounded. This is the main reason why the deviation for d > 10 mm is not very large. For high sensitivity and to avoid the stray field effect, we chose d = 2 mm. Another simulation with the rubber inserted between the electrodes was performed by varying the relative dielectric constant of the rubber material for d = 2 mm. The result was a simple linear relationship: CNBR = 0.31 + 8.22εr [pF]. The linear relationship is expected to be maintained for other values of d.

Figure 3
figure 3

COMSOL software simulation results. Inset: the mesh structure of the modelled vessel. C_simul: COMSOL software simulation results; C_analytic: capacitance values predicted by an ideal parallel-plate model.

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H2 effect on the impedance spectra

The effect of H2 on the spectra of CNBR versus frequency for NBR in the sequential overall processes was quantified in three steps. (1) The CNBR spectra versus frequency were measured before H2 pressurization (HP) (Fig. 4a). (2) The pressure was increased to 10 MPa over 30 min, and the CNBR spectra versus frequency were subsequently measured after HP at 10 MPa for 25 h (Fig. 4b; several lines overlay). (3) H2 decompression (HD) was conducted for 20 min, and then the CNBR spectra versus frequency were measured after HD for 30 h (Fig. 4c; several lines overlay). The capacitance decreased by more than an order of magnitude after HP and then recovered to almost its original value after HD. The DFNBR spectra versus frequency showed behaviour similar to that of the CNBR spectra. The observed large change in both CNBR and DFNBR implies that IS responds very rapidly to pressurization and decompression and is therefore an appropriate tool for detecting the effects of H2.

Figure 4
figure 4

Effect of hydrogen on the capacitance spectra versus frequency for NBR: (a) before HP, (b) after HP, and (c) after HD. The logarithmic scale is the same in all figures.

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The duration of exposure to H2 affected CNBR (Fig. 5) at frequencies of 90, 1040, and 10,000 Hz during H2 exposure at a pressure of 10 MPa. Figure 5 is an enlargement of Fig. 4b and shows CNBR as a function of exposure time at the three frequencies. CNBR decreased exponentially with increasing exposure time to H2; this trend is caused by the diffusion of H2 into the NBR. In many polymers, such diffusion of high-pressure H2 induces plasticization13,14,15, which results in a reduction in the dielectric constant and capacitance. The decrease in the dielectric constant will be discussed later.

Figure 5
figure 5

Capacitance decay measured at three different frequencies as a function of the exposure time during in situ hydrogen exposure at 10 MPa. The three solid lines are fitted with the equation C0 · exp(−AD1t) at the corresponding frequency.

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The change in the capacitance is expected only depend on the penetrated H2 content by assuming a first-order approximation. Thus, the evolution of the capacitance was affected by the diffusion-controlled process of H2. The content c(t) of H2 and the change in capacitance, which are the solutions of the diffusion equation, are expressed as follows11.

$$\begin{array}{c}{\rm{c}}({\rm{t}})=\frac{32}{{\pi }^{2}}c(0)\cdot [\mathop{\sum }\limits_{n=0}^{\infty }\,\frac{\exp (-{(2n+1)}^{2}{\pi }^{2}Dt/{z}^{2})}{{(2n+1)}^{2}}]\cdot [\mathop{\sum }\limits_{n=1}^{\infty }\,\frac{\exp (-\,D{\beta }_{n}^{2}t/{r}^{2})}{{\beta }_{n}^{2}}]\\ \approx \frac{32c(0)}{5.8{\pi }^{2}}\cdot exp[-(\frac{{\pi }^{2}}{{z}^{2}}+\frac{5.8}{{r}^{2}})Dt]\end{array}$$
(1)

where c(t) is the hydrogen content at time t and c(0) is the equilibrium hydrogen content. D is the diffusion coefficient (diffusivity) with a unit of [m2/s]. z and r are the thickness and radius of the disc-shaped specimen, respectively. βn is the root of the zero-order Bessel function. We obtain the diffusion coefficient of H2 by fitting the data of the capacitance evolution to Eq. (1). The experimental data were well fitted to Eq. (1) by assuming a single-exponential decay of C = C0 ∙ exp(−AD1t) (Fig. 5, solid lines), where C0 is the capacitance at t = 0, and A is a constant corresponding to \((\frac{{\pi }^{2}}{{z}^{2}}+\frac{5.8}{{r}^{2}})\) with z = 2 mm and r = 24.5 mm. The obtained value of D1 is a diffusion coefficient that depends on frequency (Table 2) for the H2 penetration into the rubber.

Table 2 Diffusion coefficients D1 and D2 for pressurization (Fig. 5) and decompression (Fig. 6), respectively, at three frequencies.
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Meanwhile, the recovery of the capacitance over time at atmospheric pressure after the decompression of the H2 pressure of 10 MPa exhibited an exponential increase at all frequencies to maxima that decreased with increasing frequency (Fig. 6). Figure 6 is an enlargement of Fig. 4c and presents CNBR versus time after decompression. The exponential increase over time is a result of H2 desorption from the rubber by deplasticization after the release of pressure. The evolution of the capacitance was also controlled by the diffusion process of H2. An exponential function C = C∞ ∙ [1 − exp(−AD2t)], where C∞ is a saturated capacitance at infinite time, was also fitted to the data, and the diffusion coefficient D2 for the H2 desorption from the rubber was already obtained (Table 2). Single-exponential growth was also assumed for this fit.

Figure 6
figure 6

Capacitance recovery with time after decompression at a hydrogen pressure of 10 MPa. The three solid lines are the data fits to C = C∞ ∙ [1 − exp(−AD2t)] at each frequency.

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The diffusion of H2 into the rubber saturates at ~12 h during pressurization (Fig. 5), and the clearance from the rubber during decompression takes ~15 h (Fig. 6). D2 is slightly larger than D1; this difference implies that the H2 desorption is slower than the absorption; therefore, a hysteretic phenomenon occurs, possibly because H2 interacts with the polymer chains.

We have also measured the penetrated H2 content into NBR as a function of time using TDA11 and electronic balances. According to the results in two methods, it takes about 10 h for the H2 contents to reach the maximum. This is consistent with the capacitance measurement as shown in Fig. 5. Thus, we could say the long-term change in capacitance is due to slow diffusion of H2. The results for decompression of Fig. 6 also shows similar behavior. The temperature and relative humidity were maintained within (23 ± 2) °C and (50 ± 5) %, respectively, in national calibration laboratory. The long-term drift effect on impedance analyser 4294A for several hours with a fresh specimen without H2 penetration was found to be negligible, which was less than approximately 3% of measured value. Therefore, the additional effects except of H2 was not included in the measurement.

CNBR (Fig. 6) was transformed to the imaginary part of the impedance, Zim = 1/(ωC) for a comparison with the TDA data. To find a correlation between IS and TDA, Zim normalized by C∞ was compared with the normalized H2 content obtained by TDA (Fig. 7). The two results were consistent overall, which implies again that the change in Zim is directly related to the desorbed H2 content. Such consistency is compatible with previous results obtained with TDA and NMR16. However, the IS data show a slower desorption of H2 than the TDA data (Fig. 7). Although there are unique measurement conditions with different dimensions in both of the specimens, this difference implies that the physics of the dependence of the impedance on the hydrogen content is different from that of the hydrogen diffusion process in the specimen. The TDA measurement only detects the hydrogen content released through the diffusion process. In addition to the effect of the released hydrogen content, IS is also affected by the chemical/physical interaction of hydrogen with the rubber polymer. Furthermore, the IS results may also due to the slow relaxation, i.e., the pressurization-induced state of the polymer matrix, or the fact that the filler distribution slowly relaxes towards another equilibrium and can continue to relax even after all the free H2 molecules are released during decompression. The relaxation may be irreversible; therefore, the original status cannot be recovered.

Figure 7
figure 7

Comparison between the hydrogen content profile with time after decompression at a H2 pressure of 10 MPa measured by IS and TDA. Inset: exponential fitting (Solid Line) for the log-transformed data of the normalized hydrogen content.

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The TDA experimental data (Fig. 7) could not be fitted well with a single exponential function; therefore, a two-exponential fit on the basis of Eq. (1) was employed (Fig. 7, inset), exp(−BD2,ft) + exp(−BD2,st), where B is a constant corresponding to \((\frac{{\pi }^{2}}{{z}^{2}}+\frac{5.8}{{r}^{2}})\) with z = 2 mm and r = 10 mm. D2,f and D2,s are fast and slow diffusion coefficients, respectively. The behaviour in the TDA data may arise from two kinds of diffusion processes for H2: a fast process (left side in the inset) for H2 in the polymer network matrix and a slow process (right side slope in the inset) from H2 in the filler (carbon black). D2,f and D2,s were found to be 3.9 × 10−10 m2/s and 4.2 × 10−11 m2/s, respectively.

The normalized capacitance Cnor was also obtained during cyclic pressurization and decompression under H2 and Ar gases (Fig. 8). The pressure dependences of Cnor were similar at all three frequencies. Cnor decreased exponentially as the pressure increased during pressurization and later increased as the pressure decreased during decompression. The behaviours are very similar to those of CNBR versus time (Figs 5 and 6).

Figure 8
figure 8

Change in the normalized capacitance with pressure in the pressurization and decompression processes under high-pressure hydrogen (ϒ) and argon (*) gases at three frequencies.

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The large change in the dielectric constant of the rubber at the low frequency of 40 Hz under high-pressure H2 (Fig. 8) can be explained by a change in the polarizability of the polymer chain matrix and the fillers, because the H2 in the rubber region contributes little to the total polarization. The strong frequency dependence of the dielectric constant of the rubber at the low frequency is explained by the polarization arising from the displacement or rotation of the fillers that have a net charge or a net dipole moment in the interface region around the fillers due to the interfacial polarization known as the Maxwell-Wagner-Sillars effect17,18.

The decrease in the dielectric constant with the exposure to high-pressure ambient H2 is expected, because the degrees of freedom of the polymer chain and the fillers decrease as the gas molecules occupy more space. A similar phenomenon was observed by pressurizing with Ar gas (Fig. 8).

According to the COMSOL simulation result CNBR = 0.31 + 8.22εr, the relative dielectric constant εr of the rubber sample at a pressure >5 MPa was found to be ~3, which has also been observed in silicon dioxide19 and many rigid plastic materials, such as epoxy glass20 and Bakelite21. A dielectric constant (~3) smaller than the value in a normal rubber polymer suggests that the decrease in the dielectric constant may occur because, due to the solvent, H2 causes the formation of a rigid plastic phase or a glass phase. This finding suggests that the penetration of high-pressure H2 with a low molecular mass in the process of pressurization, which generally induces plasticization for many polymers13,14,15, resulted in a reduction in the dielectric constant and capacitance of the polymers, whereas the increase in Cnor during decompression (Fig. 6) was caused by deplasticization after the release of pressure.

The normalized DFNBR was also affected by the pressure during both pressurization and decompression under both H2 and Ar gases at all three frequencies (Fig. 9). The behaviour was similar to that of Cnor under H2 and Ar gases. The measurement time for Figs 8 and 9 is about 2 h in the process of the pressurization and decompression. In Fig. 9, normalized dissipation factor (DF) value at 40 Hz is −0.06, for the capacitance of 25 pF in Fig. 8. In the case with the value of lowest frequency of 40 Hz and small capacitance under the applied AC voltage of 0.5 V, the magnitude of the measured DF is regarded as very small, because the 40 Hz is the lowest limit of the impedance analyzer 4294 A used and the manufacturer’s specification22 say the uncertainty of DF at this frequency and capacitance level amounts to 10%. Thus, the measured DF is virtually regarded as zero and the negative sign is attributed to the measuring instrument having large uncertainty of DF at low frequency of 40 Hz. However, with increasing the frequency, the uncertainty decreases.

Figure 9
figure 9

Change in the normalized DF with pressure in the pressurization and decompression processes under high-pressure hydrogen (○) and argon (+) gases at three frequencies.

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The normalized CNBR and normalized DFNBR were affected more in H2 than in Ar during cyclic pressurization and decompression under the same pressure conditions (Figs 8 and 9). This difference implies that H2 has a greater permeation capability than Ar, possibly because of the molecular mass of H2. The most remarkable difference between H2 and Ar was that the recovery of the dielectric constant and the DF after decompression was considerably lower under H2 than under Ar (Figs 8 and 9). After 124 h of decompression in H2, the capacitance and the DF recovered to 40~60% of the initial values, whereas after 150 h of release in Ar, the rubber polymer recovered to 90~100% of the initial capacitance and DF. This failure to recover completely implies that H2 reacts chemically with the polymer and causes voids, defects, and a scission of the polymer chain, whereas inert Ar gas only causes a scission of the polymer. The SEM results partially support these findings, but further analysis is required.

SEM results

SEM images (Fig. 10) were obtained from the NBR without exposure to H2 and the NBR specimens exposed to H2 and Ar at 10 MPa. After exposure to H2, the morphology of the NBR (b, b’) changed from a random distribution to a uniaxial directed distribution. One possibility is that percolated channelling between low-density regions occurs during the permeation of H2 gas under 10 MPa of ambient pressure stress and causes a density modulation. In this instance, the distance between the valleys is ~10 μm. The NBR specimen exposed to Ar (c, c’) was modified to a circular type of modulation, which is reminiscent of a swollen balloon under gas injection instead of uniaxial channelling. The morphology of the Ar-exposed NBR reveals an Amoeba-like circle without any directional preference. These results demonstrate that different types of gases can result in different morphological responses at the same pressure. Thus we think it causes percolated channelling to an uniaxial directed distribution due to greater permeation than that exposed to Ar. Consequently, H2 effect on NBR are consistent with that already proposed by IS in views of greater permeation capability, stronger effects on physically and chemically in H2 more than Ar.

Figure 10
figure 10

SEM images of the fresh NBR (a,a’) without exposure to hydrogen, NBR (b,b’) exposed to H2 at 10 MPa and NBR (c,c’) exposed to Ar at 10 MPa. Left column: image sequences obtained from the same spot; scale bar = 50 μm. Right column: sequences obtained from the same spot at a higher magnification; scale bar = 100 μm.

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Conclusions

This paper presents the development and evaluation of an IS system to measure the effects of high-pressure H2 gas on rubber polymers. The in situ capacitance measurement enables the observation of the correlation between macroscopic and microscopic phenomena under high pressure. The developed IS system could be used as an in situ probe to gather information such as diffusivity in H2 diffusion and desorption processes in rubber polymers.

Although there is a difference in the time evolutions between IS and TDA, the results of this study indicate that IS may be a useful probe to observe real-time and in situ changes in the H2 content with time and pressure during pressurization and subsequent decomposition and could supplement the ex situ TDA method and other methods. The proposed IS system could also clarify in situ the effect of the permeation properties of a gas as a function of time and pressure in rubber during sequential pressurization and decompression processes.