Multiphase Nb–TiCo alloys: The significant impact of surface corrosion on the structural stability and hydrogen permeation behaviour

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Multiphase NbeTiCo alloys: The significant impact of surface corrosion on the structural stability and hydrogen permeation behaviour Erhu Yan a,b,*, Ruonan Min a, Haoran Huang a, Ping Zhao c, Pengru Huang a, Yongjin Zou a, Hailiang Chu a, Shuhui Sun b,***, Lixian Sun a,** a

Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, PR China b Department of Energy, Materials and Telecommunications, INRS-EMT, Quebec J3X 1S2, Canada c College of Materials Science and Engineering, Qingdao University of Science & Technology, Qingdao 266044, China

article info

abstract

Article history:

Multiphase NbxTi(100-x)/2Co(100-x)/2 (x ¼ 30e60) alloys are a promising material for hydrogen

Received 5 February 2019

separating membranes. These alloy membranes exhibit a rapid decline in hydrogen

Received in revised form

permeation flux within ~12 h when operated at 773 K. To address this issue, a dense oxide

24 April 2019

(e.g. Nb2O5, TiO2 and CoO) layer was prepared between a Pd coating layer and an NbeTiCo

Accepted 25 April 2019

substrate by surface corrosion for improving their thermal stability, and the corrosion

Available online 18 May 2019

resistance of NbeTiCo alloys was investigated. An increase in the Nb content (x) lowers the corrosion resistance of these alloys, but makes it easier to form the above oxide layer.

Keywords:

Substantial enhancement of hydrogen permeability and thermal stability at 773 K was

NbeTieCo alloys

observed for the alloys (x ¼ 30 and 40) after corrosion, which can be ascribed to an increase

Hydrogen separation

in hydrogen diffusivity. This improved permeability and stability are closely related to the

Surface corrosion

formation of the above surface oxide layer that impeded interdiffusion between the Pd film and NbeTiCo substrates. This study demonstrates that insertion of a diffusion barrier between the Pd and Nb-based substrates by surface corrosion is a viable approach to enhance the high-temperature stability of Pd-coated NbeTiCo alloys, an aspect not widely explored in Nb-based hydrogen separation and purification membranes. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Group 5 metals (V, Nb and Ta etc.) are considered some of the most promising materials for hydrogen permeable

membranes to substitute for the currently used Pd-based alloys because they process considerably higher hydrogen permeabilities and attractive prices than Pd [1e4]. Nevertheless, unlike Pd alloy, these metals have little or no catalytic activity for the dissociation/reassociation of H2; hence, it is common

* Corresponding author. Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, PR China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (E. Yan), [email protected] (S. Sun), [email protected] (L. Sun). https://doi.org/10.1016/j.ijhydene.2019.04.253 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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to prepare a catalytic Pd film (~200 nm) on each side of the membrane (see Fig. 1(a)) [1,3e6]. Another drawback of these metals is that hydrogen embrittlement usually occurs when they absorb hydrogen due to their more open body-centred cubic (BCC) lattice structure [7,8]. Therefore, there is a long way to go and more efforts is needed for developing Group 5 metals as H2 separating membranes to meet the U.S. Department of Energy (DOE) performance targets (e.g. high H2 flux of ~100 m3 h1 m2 and good durability of ~5 years) [7,9]. Generally, two different fracture modes exist in these BCC metal membranes according to their test temperature [1,3e8,10,11]. First, at temperatures below 673 K, hydrogen embrittlement readily occurs due to their relatively higher hydrogen solubility at low temperature [11e13]. To mitigate the above effects, a feasible approach is reducing the solubility of the membranes by alloying with another metal. Common alloying elements mainly include Ti, Ni, Co, W, Mo, Cu, Zr, Hf and Fe [3e5,7e9,11,13e20]. Correspondingly, some new hydrogen-permeable composite-metal membranes, such as NbeTieNi/Fe, V/TaeTieNi and NbeZreNi etc., have been developed by Aoki et al. [4,14,15], Dolan et al. [7,11,17], Park et al. [18], Nishimura et al. [19] and Yamaura et al. [20,21]. Through extensive studies of the above membranes, it has been confirmed that specific additions could lower the dissolved hydrogen concentration and suppress the formation of hydrides to some extent, thus suppressing hydrogen embrittlement. For example, Yukawa et al. [20,21] demonstrated that the hydrogen solubility decreased dramatically by adding 5 mol% Ru into NbeW alloys, thus improving their resistance to hydrogen embrittlement. A similar case was also found in the NbeTieNi alloy system by adding an appropriate amount of Mo or W [22,23]. Second, above 723 K, membrane fracture (or failure) can be ascribed to the loss of catalytic activity caused by the coating-metal layer (e.g. Pd and/or PdeAg etc.)

Fig. 1 e Schematic of the sandwiched Pd-coated NbeTiCo hydrogen separation membranes (a) without and (b) with the intermediate oxide layer, and the hydrogen permeation membrane module (c).

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interdiffusion with the BCC metal in the middle or the agglomeration of Pd atoms, as reported by Ohtsu et al. and Wolden et al. [3,24]. The traditional approach to solve this problem is to place an inter-layer (as a diffusion barrier) between the Pd layer and the substrate. Hatano et al. [25,26] demonstrated that a non-porous niobium subcarbide (Nb2C) layer prepared by heat treatment in a gas mixture (10% CH4 þ 90%H2) could suppress the interdiffusion between a thin Pd coating and Nb, thus improving the high-temperature durability. Similarly, niobium nitride (NbN) [27], hafnium nitride (HfN) [28], zirconium nitride (ZrN) [29] and oxides (e.g. Nb2O, Ti4Ni2O, TiO2 etc.) [30], as potential candidate materials for suppressing the interdiffusion, have been reported by Ohtsu et al., Hatano et al., Chotirach et al. and Ishikawa et al. Despite the issue of durability being resolved by the above experimental studies, another serious problem, the degradation of permeability at elevated temperatures, still remains unsolved. Durability improvement is always accompanied by a permeability decrease [7,9,27e30]. Therefore, it is necessary to search for new candidate materials for the diffusion barrier that can not only improve the stability by retarding the reaction between the Pd coating and the substrate but also enhance the permeability of the composite membranes. Recently, we reported that Pd-coated NbeTieCo alloy membranes exhibited attractive H2 permeabilities and higher durability compared with other Nb-based alloys at temperatures below 673 K [31]. In particular, the alloys located in the pseudo-binary NbeTiCo isopleth, i.e. NbxTi(100-x)/2Co(100-x)/2 (30  x  60), exhibit a good balance between permeability and durability at 673 K (see Fig. 5(c) in Ref. [31]). Nevertheless, a significant reduction in H2 flux through these NbeTiCo alloy membranes at 773 K was observed (described later). The deterioration of durability at high temperatures is one of the major problems remaining to be solved for these membranes, which limits their application in industrial conditions, such as reforming systems (operating temperature: ca. 773e873 K) [7,32]. If the oxide layer forms on the surface of the NbeTiCo substrate so that the interdiffusion between the Pd coating layer and base-metal layer is suppressed at the Pd/substrate interface, the enhancement of thermal stability is expected to be enhanced (see Fig. 1). Up to now, we have tried many experiments to prepare such an oxide layer. One way to achieve this is heat-treatment in air or an oxygen atmosphere [30], and we believe that surface corrosion is an alternative way. Dense oxide layers can be prepared rather easily in acidic conditions, for example, acidic solutions. However, there is limited information about the effects of a surface oxide layer on the hydrogen permeation behaviour of Pd-coated NbeTiCo alloy membranes, and it is not yet known whether or not the degradation in Pd coating effects at temperatures above 673 K can be avoided or substantially mitigated by the formation of an oxide layer on the surface of NbeTiCo substrates. With these considerations in mind, the present study first investigated the hydrogen permeation characteristics of NbeTiCo alloys at elevated temperature conditions (i.e. 773 K) and the corrosion resistance behaviour was also studied. Second, an intermediate oxide layer was prepared by surface corrosion of the substrate, and its effect on high-temperature stability and/or permeability of these composite membranes was elucidated. Finally, to clarify the mechanisms of the

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intermediate oxide layer for improving thermal stability and/ or permeability, the hydrogen solubility and diffusivity are evaluated and compared with that of the membranes without the oxide layer. In short, this work introduces, for the first time, a new practical method for preparing an intermediate oxide layer for improving the durability and permeability of NbeTieCo alloys above 673 K and helps to promote its practical application in reforming systems and polymer electrolyte fuel cells (PEFCs).

shown in Fig. 1(c). After evacuation to 5  103 Pa, it was heated to 573 K at 5 K/min and kept for 1 h. Subsequently, pure H2 gas (99.999% purity) was introduced onto both sides of the membrane. The feed-side (Pu) and permeation-side (Pd) hydrogen pressures applied in this work were 0.5 and 0.1 MPa, respectively. The H2 flux (J) permeating through the membranes was then measured using a calibrated mass flow meter, and the hydrogen permeability (F) was calculated from these values using Eq. (1). J¼

Experimental Membrane fabrication Four alloys in the NbeTiCo pseudo-binary isopleth (Nb60Ti20Co20, Nb50Ti25Co25, Nb40Ti30Co30 and Nb30Ti35Co35) were investigated and denoted as 1#, 2#, 3# and 4#, respectively (see Table 1). These alloys were formed from high-purity Nb, Ti and Co (99.99 wt% for all), purchased from CNM (P.R.C.). Three metals, Nb, Ti and Co, were arc-melted together in an argon atmosphere. Each ingot was remelted six times to ensure homogeneity. F16 mm  0.6 mm thick disk-type samples were cut from the ingot, and then each surface of the disk samples was polished using 1 mm diamond paste. The polished samples were then subjected to an electrochemical workstation (Zahner-IM6) for surface corrosion. The samples, saturated calomel and platinum were chosen as the working electrode, reference electrode and auxiliary electrode, respectively. The working electrode was sealed with epoxy resin, and its working surface area for corrosion was about 1.14 cm2. To obtain the oxide layers, these samples were corroded using a 1 mol/L HCl solution at 25  C for 1 h, and then the corresponding dynamic potential polarization curves were monitored and recorded. After the corrosion test was complete, all the samples were rinsed with de-ionized water and dried at 60  C in a vacuum oven for 1 h. Thereafter, a thin Pd coating (~200 nm) was deposited onto the NbeTiCo substrate using a sputtering technique, and these membranes were further used as specimens for hydrogen permeation tests.

H2 permeation measurements The disk samples were sealed with oxygen-free copper gaskets and placed into the hydrogen permeation module, as

D,KðDP0:5 Þ FðDP0:5 Þ FðPu 0:5  Pd 0:5 Þ ¼ ¼ L L L

(1)

Where D and K are the hydrogen diffusivity and solubility in the membrane, respectively. L represents the thickness of the composite membrane. The hydrogen permeation tests were repeated at 623, 673, 723 and 773 K. More detail about the experimental procedure is given in our previous papers [5,31].

Characterization Microstructures of the membranes were observed by using a field emission scanning electron microscope (SEM) (FEI Quanta 600). The chemical compositions of the membranes before and after permeation testing were measured using an energy dispersive X-ray spectrometer (EDX). The chemical states of the substrate after corrosion were analysed by X-ray photoelectron spectroscopy (XPS). For the XPS measurement, the pass energy and the photoelectron take-off angle were set to 58.7 eV and 45 , respectively. The binding energy was first calibrated using a standard hydrocarbon peak at 284.8 eV, and the measured spectral order was as follows: Nb 3d, C 1s, Ti 2p, O 1s and Co 2p regions. The binding energy of these spectra in the present and other tests in Refs [3,27,30] show only minor differences, confirming a good reproducibility. The crystal structures of the specimens were measured by X-ray diffraction (XRD, Cu Ka radiation) at a step of 0.02 at room temperature.

Results and discussion As described previously, the hydrogen permeation flux as a function of time for the NbxTi(100-x)/2Co(100-x)/2 (x ¼ 30, 40, 50 and 60) alloys was investigated at temperatures below 673 K in our previous work [31]. These membranes exhibit a stable H2 flux and good durability at various temperatures. Nb30Ti35Co35

Table 1 e Hydrogen permeation permeability/durability along with the volume fraction of primary phase and eutectic for the multiphase NbeTiCo alloys. Number

1# 2# 3# 4# a

Alloys

Constituting phases

Nb60Ti20Co20 Nb50Ti25Co25 Nb40Ti30Co30 Nb30Ti35Co35

Bcc-Nb þ eutectic (bcc-Nb þ TiCo) Bcc-Nb þ eutectic (bcc-Nb þ TiCo) Bcc-Nb þ eutectic (bcc-Nb þ TiCo) Eutectic (bcc-Nb þ TiCo)

Volume fractions (Vol. %) Hydrogen permeability, Ф Фexp/Фest Stability (108 mol H2 m1s1Pa0.5) (%) timea, t (min) Primary bcc-Nb Eutectic Experimental Estimated Фest phase phase Фexp 51 38 23 0

49 62 77 100

6.81 5.68 4.93 4.16

The term “stability time” denotes the time when the H2 flux decreases to 10% of its maximum value.

8.10 6.79 5.91 4.99

84.1 83.7 83.4 83.3

195.9 273.7 429.8 682.1

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(4#), for instance, could continuously permeate hydrogen for about 40 h at 673 K, as shown in Fig. 2(a). In order to examine their hydrogen permeation characteristics at elevated temperatures, hydrogen permeation tests were performed at 723 K and 773 K. There are two main reasons for choosing 773 K as the maximum temperature in this study. First, it is known that the hydrogen permeability of Group 5 metals decreases sharply and they are prone to hydrogen embrittlement at T > 773 K [7,9]. Instead, 773 K allows membrane failure and/or permeability degradation to be easily documented within reasonable timescales. Second, this temperature is high enough to induce the interdiffusion between the substrate and Pd film during the hydrogen permeation process. Moreover, seal failure of copper gasket seal failure occurs easily at elevated temperatures (T > 773 K), which leads to hydrogen leakage and inevitable explosion [9].

Fig. 2 e Hydrogen permeation characteristics of the multiphase NbeTiCo alloys. (a) Hydrogen permeation flux of the alloys 1# (Nb60Ti20Co20), 2# (Nb50Ti25Co25), 3# (Nb40Ti30Co30), 4# (Nb30Ti35Co35) as a function of the permeation time at 773 K. (b) Temperature dependence of the hydrogen permeability for these alloys and pure Pd. Inset in (a): the appearance of the alloy 1# sandwiched between two copper seal gaskets after the hydrogen permeation test.

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The changes of the H2 flux and hydrogen permeability during the measurement at 773 K are also shown in Fig. 2(a). All four samples displayed qualitatively similar behaviour. Initially the H2 flux rose rapidly, and shortly after ~1 h it reached a maximum and then began to decline at varying rates. This behaviour is obviously different from that of these membranes tested at 673 K (marked by an olive line). Moreover, the maximum flux of these membranes follows the order: J1# (5.86 cc H2 cm2min1)>J2# (4.89 cc H2 cm2min1)>J3# (4.24 cc H2 cm2min1)>J4# (3.58 cc H2 cm2min1). Nevertheless, Nb60Ti20Co20 (1#) exhibits the most rapid decline in H2 flux and brittle fracture occurs (see the inset in Fig. 2 (a)) after only 392 min of testing. Nb50Ti25Co25 (2#) and Nb40Ti30Co30 (3#) show somewhat improved stability, displaying stability times of 273.7 and 429.8 min, respectively. The H2 flows of these two membranes are too small to be detectable after 2400 min, which indicates that their permeability is almost lost. Nb30Ti35Co35 (4#) has a surprising result in that the H2 flux through this membrane not only shows a slower declining trend, but also exhibits a relatively higher value throughout the 2400 min of hydrogen permeation testing at 773 K. This result proves that the H2 fluxes through these NbeTiCo alloy membranes at 773 K are all markedly reduced in a manner that is closely related to their original composition and Nb30Ti35Co35 (4#) shows the highest thermal stability at temperature up to 773 K. From the above H2 flux values, the maximum hydrogen permeability (Фexp) of each membrane at 773 K was calculated, as shown in Fig. 2(b) and Table 1. The values for these samples at T < 773 K (723 K, 673 K, 623 K and 573 K) and pure Pd are also included for comparison. Below 723 K, F values increase with increasing temperature, implying that hydrogen permeation is controlled by the diffusion process rather than by surface reactions such as hydrogen dissociation/recombination (H2 4 2Hþ þ 2e) in these membranes. Additionally, the relationship between the above two parameters obeys the Arrhenius equation, i.e. F ¼ F0exp (-Ea/RT), where F0 and R are the vibration factor and the molar gas constant, respectively. The activation energies Ea for alloys 1#, 2#, 3# and 4# are 26.12, 24.86, 23.58 and 21.32 kJ mol1, respectively. These values are higher than that (~12.21 kJ mol1) for pure Pd [24,30,33], implying that the permeation of hydrogen through the above NbeTiCo alloys is somewhat inhibited compared to pure Pd. In addition, the linearity (i.e., the relationship of F vs. 1000/T) is not suitable for the samples tested at 773 K, and Fexp values are significantly lower than that (Fest) estimated from the Arrhenius plot as indicated by the grey area in Fig. 2(b) (marked as ‘I’), see Table 1. In contrast, F values of Pd increase almost linearly in the Arrhenius plot at all test temperatures. These results suggest that permeation through Pd-coated NbeTiCo membranes at 773 K is affected by surface reaction in addition to diffusion. This behaviour has also been found in Pd-coated NbeTiNi composite membranes as previously reported by Ishikawa et al. [30]. In their study, hydrogen permeability degradation occurred due to the destructive modifications of the coating composition and/or morphology caused by interfacial diffusion. From the results in Fig. 2, we inferred that a change in the composition of the Pd film or matrix affecting the above surface reaction may be the main

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cause of the degradation of hydrogen permeability and/or the decline of H2 flux. To further confirm the above results, EDX analysis was performed along the depth direction for the membranes after the permeation tests at 773 K, as shown in Fig. 3. The Pd/ substrate interface needs careful observation, so the positions of the tested points near the interface were set at 25 nm and other positions of the analysis were spaced ~50 nm apart. The

metallic element (e.g. Nb, Ti, Co) in the substrate diffuses into the Pd film with varying degrees, forming a thin reaction layer (the grey shaded area) near the Pd/substrate interface (ca. 200 nm from the upper surface of Pd). The thickness of this layer shows a decreasing tendency (4# < 3#<2# < 1#) accompanied by a reduction of Nb content in these alloys. The grey area observed in Fig. 3(a)e(d) mainly corresponds to the intermetallic compound Pd3Nb phase [34] (Pd/Nb ratio of cPd/

Fig. 3 e The EDX and XRD analysis of the specimens after the hydrogen permeation test. (a) 1# (Nb60Ti20Co20), (b) 2# (Nb50Ti25Co25), (c) 3# (Nb40Ti30Co30), (d) 4# (Nb30Ti35Co35) and (e) typical XRD patters of alloy 1#. The figures (a), (b), (c) and (d) correspond to the specimens (i.e., 1#, 2#, 3# and 4#) shown in Fig. 2(a) tested at 773 K, respectively.

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cNb ¼ 3:1), possibly with a small number of other phases such as Pd3Ti, Pd3Ti2 [35] and/or PdCo [36], because the formation of above intermetallic compounds were observed by XRD analysis after the hydrogen permeation test (see Fig. 3(e)). A similar case can be seen in Pd-coated V membranes, i.e., the formation of intermetallic compounds (e.g. PdV3, PdV2 and Pd3V) in the penetration layer [37]. All the above phases have very low permeabilities of ~1012 mol H2 m1s1Pa0.5 [34e37], which is four orders of magnitude less than the composite membranes, and seriously hinders hydrogen permeation through the membrane. This might explain why the values of permeability measured at 773 K, Фexp, are lower than estimated, Фest, (see Fig. 2 and Table 1). Moreover, the Pd concentration ratio was lower than 58% within ~100 nm away from the Pd/substrate interface in all cases, which may be also one of the main reason for the deterioration of the hydrogen permeability and the rapid decline of the H2 flux (see Fig. 2). In other words, if the interdiffusion of these metals into Pd can be mitigated and/or avoided during measurement at 773 K, the maximum permeability (Фexp) will increase gradually and tends toward the estimated value, Фest, for these Pd-coated composite membranes. To test this hypothesis, further investigation is needed to validate its feasibility. In this paper, we made a first attempt to use the oxide layer prepared by surface corrosion as the protective layer and achieved good results (described later). Fig. 4 shows the SEM micrographs of NbxTi(100-x)/2Co(100-x)/2 (x ¼ 30e60) alloys before and after surface corrosion. XRD patterns of these alloys are shown in Fig. 5. Before corrosion, primary a-Nb dendrites embedded in the eutectic phase (aNbþ TiCo) are observed in Nb60Ti20Co20 (1#), Fig. 4 (a). A similar

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hypo-eutectic microstructure can be found in the 2# and 3# alloys. Nb30Ti35Co35 (4#) alloy, is an exception and consists of a fully eutectic microstructure, as shown in Fig. 4 (d). The hydrogen permeability and the volume fraction of constituting phases for these alloys are summarized in Table 1. The volume fraction of eutectic phase increases with the decrease of Nb content (1#/2#/3#/4#), and reaches its highest value of ~100 vol% in Nb30Ti35Co35 (4#). As reported in the literature [31,33,38], the primary a-Nb phase contributes to permeability, while the eutectic phase mainly plays a role in embrittlement resistance. Thus, the changes in the volume fraction of constituting phases could explained the difference in hydrogen permeability and/or H2 flux for these alloys, as shown in Fig. 2, i.e., Nb30Ti35Co35 (4#) consisting of entirely of eutectic phase shows lower permeability than Nb60Ti20Co20 (1#) containing ~51 vol% primary a-Nb phase. Moreover, although the membrane surface before and after corrosion is largely unchanged and has a dual phase microstructure (see Fig. 4), there are some changes in the constituting phases, especially for Nb60Ti20Co20 (1#) and Nb50Ti25Co25 (2#), as shown in Fig. 5. After corrosion, no drastic change is observed for the original a-Nb and TiCo phases, whereas new Bragg peaks, such as Nb2O5, TiO2 and CoO, appeared in the XRD patterns of these alloys. Nb60Ti20Co20 (1#) additionally contains a tiny amount of Nb2C phase. The diffraction peak intensity of new phases increases with increasing Nb content (4#/1#), i.e. x value. To further confirm the above results, an XPS analysis was also performed on these samples, as shown in Fig. 6. All major peaks were assigned to Nb 3d, Nb 3p, Ti 2p, Co 2p, C 1s and O 1s (see Fig. 6 (a)). The high-resolution spectrum of Nb 3d consists of two

Fig. 4 e SEM images of the multiphase NbeTiCo alloys before and after surface corrosion. (a) 1# (Nb60Ti20Co20), (b) 2# (Nb50Ti25Co25), (c) 3# (Nb40Ti30Co30) and (d) 4# (Nb30Ti35Co35). The inset shows the morphology of each alloy after surface corrosion.

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Fig. 5 e XRD patterns of the multiphase alloys 1# (Nb60Ti20Co20), 2# (Nb50Ti25Co25), 3# (Nb40Ti30Co30) and 4# (Nb30Ti35Co35) after surface corrosion. The XRD diffractogram (pink line) of alloy 4# before surface corrosion is also included for comparison. (For interpretation of the references to in this figure legend, the reader is referred to the Web version of this article.)

main peaks of Nb3d 5/2 and 3d 3/2 centred at 206.7 eV and 209 eV, and the oxidation state of Nb, i.e. Nbþ5, can be observed clearly, as shown in Fig. 6 (b). These results are consistent with the results of the study conducted by Ishikawa et al. [30]. The oxidation state, such as TiO2 and CoO, is also confirmed by similar analysis in Fig. 6(c and d). The XPS results correspond to the results of the XRD analysis in Fig. 5. So, from these results, it is clear that a certain amount of oxide is formed on the membrane surface, and the type and amount of oxide are dependent on the initial composition, such as Nb content, in these alloys. In order to correlate the oxides on the surface with the composition and/or microstructure of the alloys, the corrosion resistance behaviour of the NbxTi(100-x)/2Co(100-x)/2 (x ¼ 30, 40, 50 and 60) alloys were examined by an electrochemical method. Fig. 7 shows the dynamic potential polarization curves for these alloys, and the corresponding corrosion parameters are listed in Table 2. One peak decrease of anode current density is observed in the polarization curve for each alloy. The reason for this change is the dissolution of metals (e.g. Nb, Ti, and Co) in the acidic medium with the formation of an oxide layer on the alloy surface (see the XPS results in Fig. 6). In general, the more positive the corrosion potential, the better the corrosion resistance performance of the test material. Similarly, the smaller the corrosion current, the slower the corrosion rate [39,40]. In the present study, Nb30Ti35Co35 (4#) has the highest corrosion potential (0.252 V) and

exhibits the smallest corrosion current (1.17  107 A cm2) (see the inset in Fig. 7 and Table 2), indicating that the corrosion resistance properties of this alloy are good. For the alloys with high Nb content, the corrosion resistance property changes in the following order: Nb60Ti20Co20 (1#) < Nb50Ti25Co25 (2#) < Nb40Ti30Co30 (3#). These results indicate that Nb30Ti35Co35 (4#) is less susceptible to corrosion, which leads to a relatively thin oxide layer on its surface under the same experimental conditions. This layer is too thin to be detected by SEM and XRD, as illustrated in Figs. 4 and 5. The oxide layer becomes dense and turns somewhat thick for Nb40Ti30Co30 (3#) and Nb50Ti25Co25 (2#); thus, a small oxidation peak appears in their XRD results, see Fig. 5. The thickest oxide layer was formed on the surface of Nb60Ti20Co20 (1#), which can be related to the fact that it has the lowest corrosion resistance property. Considering the results of the SEM, XRD, WPS and electrochemical test together, we find that the corrosion resistance properties of multiphase NbeTiCo alloys vary with their initial compositions and microstructures. Nb30Ti35Co35 (4#), consisting of a fully eutectic phase (see Fig. 4(d)), exhibits the best corrosion resistance. The corrosion resistance property is reduced with decreasing the volume fraction of eutectic phase (see Tables 1 and 2). Nb60Ti20Co20 (1#) has the highest volume fraction of the primary a-Nb phase but shows the lowest corrosion resistance. Consequently, the corrosion resistance property of NbeTiCo alloys is not realized by the primary a-Nb phase, but by the eutectic phases. In other words, TiCo in the eutectic phase and the eutectic structure itself contributes mainly to the corrosion resistance property of these alloys. In combination with previous studies [31,33,38], it can be found that the eutectic phase has dual roles in these NbeTiCo alloys, hydrogen embrittlement resistance and corrosion resistance. This result is particularly important for the application of these composite membranes in acidic atmospheres (e.g. CO2, H2S, SO2 etc.) because the interaction between the metal and environmental media made it possible for different degrees of corrosion to occur on the metal substrate during the application process. This implies that the corrosion resistance property is also an important factor to be considered in choosing a membrane for a given application, in addition to permeability and durability. On the basis of the analysis above, eutectic Nb30Ti35Co35 would be the superior material choice as a hydrogen permeable membrane in the above applications. To analyse the effect of the oxide layer on the hightemperature stability and/or permeability, the H2 flux of these membranes with oxide layer was measured at 773 K, and the corresponding results are shown in Fig. 8 and Table 3. For comparison, the results of Nb30Ti35Co35 (4#) without an oxide layer under the same experimental conditions are also included. Before surface corrosion, the H2 flux for this sample rapidly decreased and became less than 1/7th of its initial value within 2400 min. However, the decreasing rate of H2 flux vs. time was slowed down for this sample after surface corrosion, and the stability time is about 2 times larger than that for the membrane without corrosion (see Table 3), implying that its high-temperature stability has been improved. A similar result was obtained for Nb40Ti30Co30 (3#)

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Fig. 6 e XPS spectra of the multiphase alloys 1# (Nb60Ti20Co20), 2# (Nb50Ti25Co25), 3# (Nb40Ti30Co30) and 4# (Nb30Ti35Co35) after surface corrosion. (a) wide scan, (b) Nb 3d, (c) Ti 2p and (d) Co 2p.

with or without corrosion (see Figs. 2(a) and 8). An obvious improvement effect is clearly observed for Nb50Ti25Co25 (2#) and Nb60Ti20Co20 (1#), and there was no significant reduction in the H2 flux for these two membranes, although the latter was broken after testing for ~1205.4 min (see the inset in Fig. 8). In addition, the maximum hydrogen permeability F for each membrane after corrosion was also calculated (see Table 3). The F value for Nb60Ti20Co20 (1#) with an oxide layer is lower than that of the original specimen (pre-corrosion). Likewise, Nb50Ti25Co25 (2#) shows a similar tendency. However, permeability clearly increased for Nb40Ti30Co30 (3#) after

surface corrosion (4.93  108 / 5.01  108 mol H2 m1s1Pa0.5), although the rising trend is not particularly obvious. Surprisingly, Nb30Ti35Co35 (4#) exhibits a high hydrogen permeability of 4.56  108 mol H2 m1s1Pa0.5, which is obviously higher than that without corrosion (4.16  108 mol H2 m1s1Pa0.5). The reasons for these changes in permeability will be discussed later. Correlating the above results with the high-temperature stability (see Fig. 8), we find that the existence of an oxide layer not only improves the stability but is also beneficial for increasing the hydrogen permeability.

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Fig. 7 e Dynamic potential polarization curves for the multiphase alloys 1# (Nb60Ti20Co20), 2# (Nb50Ti25Co25), 3# (Nb40Ti30Co30) and 4# (Nb30Ti35Co35) at 298 K. Inset in figure: the magnified curves corresponding to local region (the pink square). (For interpretation of the references to in this figure legend, the reader is referred to the Web version of this article.)

In order to determine the element distribution in membranes after H2 permeation test at 773 K, an EDX analysis was performed and the results are depicted in Fig. 9. Here, we only focused on the distribution of Pd, Nb, Ti and Co in the specimen to compare with the results in Fig. 3. In fact, the thickness of the oxide layer is too thin and it is difficult to determine its precise position though EDX analysis for these membranes. As can be seen from Fig. 9, the atomic diffusion in the membranes was retarded, especially at the interface between the Pd and the substrate, compared with the result in Fig. 3. No plateau of the Pd concentration ratio was observed in any corroded sample. Additionally, the Pd concentration ratio on the top surface was about ~70%, which is higher than that for the samples (1#) without corrosion (~58%), see Fig. 3. A same case is also found for the specimens 2#, 3# and 4#. Taking into account the results in Figs. 7 and 8, it is indicative that the interdiffusion between the Pd protection film and NbeTiCo substrate is suppressed by the formation of the intermediate oxide layer, and this could be responsible for improving the high-temperature durability and/or hydrogen

Table 2 e Values of corrosion potential/curent obtained by analyzing the experimental results of Fig. 7. Number

1# 2# 3# 4#

Alloys

Corrosion potential, E_corr (V)

Corrosion current, I_corr (107 A cm2)

Nb60Ti20Co20 Nb50Ti25Co25 Nb40Ti30Co30 Nb30Ti35Co35

0.264 0.261 0.256 0.252

10.32 6.37 3.03 1.17

Fig. 8 e Hydrogen permeation flux of the multiphase NbeTiCo alloys (1#, 2#, 3# and 4#) after corrosion as a function of the permeation time at 773 K. Inset in figure: picture of the alloy (1#) membrane after testing at 773 K.

permeability of Pd-coated NbeTiCo hydrogen permeation membranes. In general, the permeation of atomic hydrogen through BCC metal membranes follows the “solution-diffusion” mechanism [7,9,18e21,31,33], and the final permeability, F, for this process is controlled by both the hydrogen diffusivity, D, and the hydrogen solubility, K, in the membrane. Additionally, the latter has an important impact on the hydrogen embrittlement resistance and durability of the membrane. In order to better reveal the underlying causes of the changes in permeability and/or thermal stability for the samples before and after corrosion, the above two important parameters, i.e. D and K, were also evaluated and investigated. Fig. 10 shows the D values of the membranes with and without corrosion calculated using the time-lag method [41,42]. The hydrogen solubility coefficients, K, were also calculated from the definition of permeability, F ¼ D  K, in Eq. (1). All the D and K values are also summarized in Table 3. The values of K for these samples before and after corrosion are almost the same, whereas the values of D for the alloys after corrosion show a significant difference compared with their initial state. The variation trend of D before and after corrosion is similar to that of F for these alloys, which indicates that the variation of F for the specimens after corrosion is due to the change in D rather than K. The changes of D and/or F for the membranes before and after corrosion can be explained by the formation of the intermediate oxide layer. Although the oxide layer for Nb30Ti35Co35 (4#) after surface corrosion is very thin, it can effectively inhibit the interdiffusion between the Pd coating and the substrate, thus suppressing the formation of intermetallic compounds (e.g. Pd3Nb, Pd3Nb and PdCo), see Figs. 3, 5 and 9. This contributes to reducing the diffusion barrier for atomic H transport in the membranes. Therefore, the value of D is much higher than that of the composite membrane without corrosion. Nevertheless, the thickness of this oxide

1205.4 1339.6 565.6 1053.6 195.9 273.7 429.8 682.1 14.52 13.93 13.11 12.59 14.26 13.34 12.93 12.31 3.52 3.79 3.82 3.62 4.77 4.26 3.81 3.37 5.11 5.27 5.01 4.56 6.81 5.68 4.93 4.16 1# 2# 3# 4#

Post- corrosion Pre- corrosion Post- corrosion Pre- corrosion Post- corrosion Pre- corrosion Post- corrosion Pre- corrosion

Stability time, t (min) Hydrogen solubility, K (mol H2 m3Pa0.5) Hydrogen diffusion coefficient, D (109 m2s1) Hydrogen permeability, Ф (108 mol H2 m1s1Pa0.5) Number

Table 3 e Summary of hydrogen permeability (F), diffusivity (D), solubility (K) and stability time (t) for the multiphase NbeTiCo alloys before (Pre-) and after (Post-) surface corrosion.

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layer increases gradually with increasing Nb content in NbeTiCo multiphase alloys because of the excellent corrosion resistance property of eutectic phases in the microstructure (see Fig. 7). When the thickness of the oxide layer exceeds the critical value, this oxide layer will hinder the diffusion of atomic H, thus inevitably reducing the D values. The thickness of the oxide layer of Nb40Ti30Co30 (3#) is almost at the critical value because the values of D are almost the same for this specimen before and after corrosion. The thickness of the oxide layer is further increased for Nb50Ti25Co25 (2#) and Nb60Ti20Co20 (1#) due to their relatively lower corrosion resistance. In this case, the oxide layer, acting as a new diffusion barrier layer for hydrogen transport in the membranes, degrades the D values of the membranes, as shown in Fig. 10. In addition, it is noteworthy that the corroded samples have a higher Pd concentration ratio on the top membrane surface after the hydrogen permeation test, see Figs. 3 and 9. Generally, the higher surface Pd content should enhance the rate of H2 surface dissociation leading to the observed increase in the D value. This may help explain a tiny increase in the values of K for all the alloys after corrosion, compared with their initial state, see Fig. 10. Nevertheless, the corroded samples (1# and 2#) show significant lower D values which indicates that if the oxide layer is too thick, the diffusion and/ or transport of hydrogen in membrane is, to some extent, inhibited. Notwithstanding that, the oxide layer can effectively inhibit the intermetallic diffusion between the Pd coating and the substrate, thus improving the hightemperature stability of Pd-coated NbeTiCo alloys. Moreover, the durability of the corroded sample (4#) decreases with a higher rate after 1000 min, whereas 2# shows a notable durability improvement. The reason for this is currently unclear. The differences in the chemical compositions and phase constituents of the oxide layer between Pd and NbeTiCo substrates could be a possible cause, because new peaks of TiO2 (2q ¼ 28.1 and 54.2 ) and CoO (2q ¼ 41 ) appeared in the corroded sample (2#), and the diffraction peak intensity of these phases was increased obviously for Nb50Ti25Co25 (2#) (see red line in Fig. 5). For example, Nb40Ti30Ni30 alloy with various surface oxides exhibited different hydrogen permeation behavior, especially for long-term stability, which has already been confirmed by Ishikawa et al. [30]. Hatano et al. [25] and Edlund et al. [37] also demonstrated that the thickness of intermediate layer had important influence on the hydrogen permeability and the high temperature stability of PdeNb composite membranes for hydrogen separation. From these results, it is concluded that the type, phase constituents and thickness of the intermediate oxide layer has an important influence on the hydrogen diffusivity, permeability and/or durability of the composite membrane. The relationship between these parameters is worth further study. The morphology of the oxide layer will be further characterized by transmission electron microscopy. This work is underway. Finally, we compare the dense oxide layer prepared in this work to other protective layers between the substrate and the Pd coating in the literature [25e28,30,37,43e45]. All data are tabulated in Table 4. The thermal stability is improved for all the composite membranes with an intermediate layer, which

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Fig. 9 e Cross-sectional analysis of the Pd-coated corroded NbeTiCo alloys after the hydrogen permeation test. (a) 1# (Nb60Ti20Co20), (b) 2# (Nb50Ti25Co25), (c) 3# (Nb40Ti30Co30) and (d) 4# (Nb30Ti35Co35).

is largely because the interdiffusion between the substrate and the coating is suppressed by such oxide layers. Nevertheless, some membranes exhibit a reduction in hydrogen permeability due to the formation of an intermediate layer. For example, the hydrogen permeability of Pd-coated Nb membranes was further decreased by employing NbN as the intermediate layer [27]. These comparisons suggest that appropriate coating materials on the alloys/metals and

Fig. 10 e Hydrogen diffusivity (D) and solubility (K) of the Pd-coated NbeTiCo alloys as a function of their composition.

preparation techniques are of great importance for improving the permeability of the composite membranes. Moreover, to obtain intermediate layers (e.g. HfN, NbN, NbC etc.) satisfying the requirement, as reported in literature [25e28,43,44], heat treatment under high temperature and in different atmospheres (e.g. carburization, plasma nitriding etc.) is needed, all of which are complicated and costly. In contrast, the intermediate oxide layer in the present work can be prepared using a novel surface corrosion process at room temperature, which has provided an effective approach for resolving the problems mentioned above. After proper corrosion, Pdcoated NbeTiCo composite membranes have superior comprehensive properties such as higher permeability and excellent thermal stability. Additionally, it is generally difficult to prepare thin Pd layers on porous materials such as Al2O3, as reported by Edlund et al. [37]. Hence, the dense oxide layer developed in the present study has an advantage in this point, although it slightly degrades the hydrogen permeability of Nb50Ti25Co25 and Nb60Ti20Co20 alloys at 773 K. In short, the newly developed oxide layer in the present work is a much better candidate for overlay materials on Nb-based permeable alloys compared with others reported in the literature [25e28,30,37,43e45]. This research also has extremely important practical value, especially for the practical utilization of multiphase NbeTiCo alloy membranes for hydrogen purification and separation at elevated temperatures.

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4.99  107 4  109 ** **

(0e3.6)  108 ** (1e2.7)  107 5.11  108 5.27  108 5.01  108 4.56  108

2.76  107 ~4.2  108 ** **

3.2  108 ** 4.6  108 6.81  108 5.68  108 4.93  108 4.16  108 Yes ** Yes Yes Yes Yes Yes

In this study, a dense intermediate oxide layer of Nb2O5, TiO2 and CoO was prepared between the Pd coating and NbxTi(100-x)/ 2Co(100-x)/2 (x ¼ 30e60) substrates by surface corrosion, and its effects on the hydrogen permeability and highetemperature stability at 773 K were examined. The thickness of the intermediate layer on the surface of the NbeTiCo alloys increases with increasing Nb content, which can be attributed to a decrease in the corrosion resistance property of these alloys. These changes are closely related to the fact that the eutectic phase (a-Nb þ TiCo) in these alloys plays a major role in the corrosion resistance property. In addition, a substantial enhancement of hydrogen permeability and thermal stability at 773 K was observed for the alloys (x ¼ 30 and 40) after corrosion, which can be ascribed to an increase in hydrogen diffusivity. Typically, Nb30Ti35Co35 with an oxide layer exhibits a high hydrogen permeability of 4.56  108 mol H2 m1s1Pa0.5 and excellent durability of ~1053 min, which are 1.1 and 1.55 times higher than without corrosion, respectively. Such improved permeability and stability are closely related to the formation of the oxide layer, which impeded interdiffusion between the Pd film and NbeTiCo substrates. The present work demonstrates that a dense oxide layer prepared by surface corrosion is a potential protective layer to improve the high-temperature stability and the permeability of Pdcoated NbeTiCo alloy membranes at elevated temperature conditions.

973 873 773 773 873 773 873 973 773 773 773 773

K K K K, K K K K K K K K

Yes

Yes Yes Yes Yes

Conclusions

(Note: the symbol “**” represents the missing data and/or information in the literature.).

Oxidation heat treatment Reactive sputtering Heat treatment at 723 K in pure H2 Electrochemical corrosion Electrochemical corrosion Electrochemical corrosion Electrochemical corrosion Nb6O, TiO2, Ti4Ni2O, Nb2O5 HfN ** Nb2O5, TiO2, CoO Nb2O5, TiO2, CoO Nb2O5, TiO2, CoO Nb2O5, TiO2, CoO, Nb2C Nb40Ti30Ni30 [30] Al-Si [44] Ti-Al [45] Nb60Ti20Co20 [This Nb50Ti25Co25 [This Nb40Ti30Co30 [This Nb30Ti35Co35 [This

study] study] study] study]

** Plasma nitriding Plasma nitriding Carburization Porous Al2O3 HfN NbN NbC, Nb2C

Data availability statement

V [37] Ta [28,43] Nb [27] Nb [25,26]

Without With intermediate layer intermediate layer

Hydrogen permeability, Ф (mol H2 m1s1Pa0.5) Test Suppressed the Improve the temperature interdiffusion thermal stability Fabrication technique Intermediate layer Substrate materials

Table 4 e Comparison of fabrication technique, substrate/intermediate layer material and the changes in the hydrogen permeability and thermal stability of metal composite membranes reported in the literature [25e28,30,37,43e45] and the ones presented in this work.

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The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Acknowledgements This study was founded by National Natural Science Foundation of China (Grants nos. 51761009, 51701048, 51801041 and 51671062), the Guangxi Key Laboratory of Information Laboratory (171023-Z), the Innovation Project of Guet Graduate Education (2019YCXS109), the China Postdoctoral Science Foundation (No. 2018M642625), Guangxi Scientific Technology Team (2017AD23029) and the scholarship from Guangxi Education Department (GED) of Guangxi Zhuang Autonomous Region.

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