Hydrogen Absorption and Evolution Capabilities of Porous Ti-15V-3Al-3Cr-3Sn Electrodes from Hot-Press Sintering

Abstract

Ti-15V-3Al-3Cr-3Sn alloy (Ti-153) plates were subjected to a hydriding process to obtain Ti-153 powder which was further hot-press sintered (HPS) into porous electrodes to see their hydrogen storage and evolution capabilities. The effects of processing parameters on the hydrogenation behavior were characterized qualitatively and quantitatively utilizing X-ray diffractometry (XRD: D2 PHASER, BRUKER, USA, 30 kV-10 mA, Cu-Kα λ = 1.541838 Å), scanning electron microscopy (SEM: Hitachi 4800), and elemental analysis (EA: Heraeus vario III-NCSH). XRD and EA analyses demonstrate that the hydrogen absorption and evolution capabilities of the powdered electrodes are superior to those of the original bulk sheet specimens. The hydrogen uptake of the specimens was evaluated by cyclic electrolytic hydrogenation at 50 mA/cm² for various periods in a 1N H₂SO₄ aqueous solution containing 0.1 g/L As₂O₃, followed by solution treatment in a muffle furnace at 300 °C for 1 h. The subsequent dehydrogenation process was conducted at 300 °C and 4.67 Pa for 2 h. The optimal processing conditions in this study are as follows: (1) first, the Ti-153 sheets are treated in a hydrogen furnace at 300 °C under a hydrogen pressure of 50 atm for 15 h to obtain the raw powder; (2) subsequently, the raw powder is processed in a hot-pressing sintering furnace at 600 °C under 25 MPa for 2 h to yield the sintered electrodes.

1. Introduction

Ti-15V-3Al-3Cr-3Sn (Ti-153) is a typical metastable β-titanium alloy widely used in the aviation and aerospace industries. Owing to its high content of β-stabilizing elements, this alloy suppresses α-phase precipitation even after accumulated heat treatment at 300 °C for 6 h. Furthermore, due to their open body-centered cubic (BCC) structure, β-titanium alloys exhibit higher hydrogen solubility than their α-counterparts [1,2,3]. Consequently, through a cyclic electrolytic hydrogenation process combined with subsequent solution treatment, Ti-153 can absorb a substantial amount of hydrogen without undergoing phase transformation or structural instability.

Although previous investigations and industrial applications of Ti-153 have rarely highlighted its hydrogen storage capability [4,5,6,7], this study explores this potential. Herein, Ti-153 sheet specimens were initially subjected to a thermal hydriding process at 300 °C to synthesize Ti-153 powder. This powder was then fabricated into porous electrodes via hot-press sintering (HPS). The hydrogen storage capabilities of these HPS electrodes were evaluated after cyclic electrolytic hydrogenation and subsequent solution heat treatment at 300 °C for 1 h. Additionally, their hydrogen evolution performance was investigated following a vacuum annealing process at 300 °C and 4.67 Pa for 2 h.

2. Methods and Materials

The starting material was received in the form of sheets that were 3 mm thick. Its chemical composition is summarized in Table 1. Specimens measuring 70 × 40 × 3 mm³ (designated as B0) were machined from the sheets, abraded with emery paper (from 220 to 600 grit), ultrasonically cleaned, and then dried. A subset of the B0 specimens was first thermo-hydrogenated into powder in an autoclave at 300 °C under 50 atm of gaseous H₂ for 15 h. The resulting powder, designated as BP, was subsequently hot-press sintered into circular electrodes (1.5 mm thick and 20 mm in diameter). Another subset of B0 specimens was then machined to be 1.5 mm thick and 20 mm in diameter to facilitate further electrochemical hydrogenation under the same conditions. Given that the variations in specimen volume are marginal, the weights of the different specimens vary as a function of their density (porosity), ranging approximately from 1.5 to 2.0 g. The complete process designations and fabrication conditions for the electrodes are summarized in Table 2. Figure 1a schematically illustrates the preparation processes for Ti-153 and the electrodes, while Figure 1b depicts the workflow of the cyclically electrolytic hydrogenation, hydrogen storage and evolution evaluations.

Table 1. Chemical composition of the as-received sheets (wt.%).

V	Al	Cr	Sn	Fe	H	C	N	O	Ti
15.3	3.31	3.02	2.91	0.25	0.015	0.05	0.05	0.13	balanced

Table 2. Sintering parameters and processing designations for various series.

Designations	Processing Parameters
B0	Ti-153 bulk specimen
BP	hydrogenated B0 at 300 °C and 50 atm (gaseous H2) for 15 h to obtain BP powder
Sa	HPS BP powder into Sa sinter at 600 °C-25 MPa for 2 h
Sa0	vacuum annealed Sa sinter at 760 °C-4.0 × 10−5 Pa for 4 h
Sb	HPS BP powder into Sb sinter at 700 °C-25 MPa for 2 h
Sb0	vacuum annealed Sb sinter at 760 °C-4.0 × 10−5 Pa for 4 h
Sc	HPS BP powder into Sc sinter at 800 °C-25 MPa for 2 h
Sc0	vacuum annealed Sc sinter at 760 °C-4.0 × 10−5 Pa for 4 h

Figure 1. Flowcharts for (a) preparing sintered electrodes and (b) evaluating hydrogen storage and evolution capabilities.

Hydrogen storage evaluation was conducted via electrolytic hydrogenation at 50 mA/cm² for various periods in a 1 N H₂SO₄ aqueous solution containing 0.1 g/L As₂O₃ (a hydrogenation promoter), followed by solution treatment in a muffle furnace at 300 °C for 1 h. The processing sequence is designated as ImKn based on the parameters of hydrogenation, solution treatment, and cycle timing. Specifically, I and K designate the treatment types: L refers to hydrogenation at 50 mA/cm² for 3 h; H refers to hydrogenation at 50 mA/cm² for 6 h; A refers to muffle furnace solution treatment at 300 °C for 1 h, and V refers to vacuum annealing at 300 °C and 4.67 Pa for 2 h. The subscripts m and n represent the respective cycle counts (1, 2, 3, …). For hydrogen evolution evaluation, a final vacuum annealing was performed at 300 °C and 4.67 Pa for 2 h. Table 3 outlines the detailed process designations and parameters for the cyclical hydrogenation-subsequent solution annealing and the final vacuum heat treatments. Table 4 categorizes the specimen designations and process parameters for evaluating hydrogen storage and evolution capabilities.

Table 3. Process designations and processing parameters for cyclically electrolytic hydrogenation.

Process Designations	Processing Parameters
L	hydrogenation at 50 mA/cm2 for 3 h in 1 N H2SO4 (aq) + 0.1 g/L As2O3
H	hydrogenation at 50 mA/cm2 for 6 h in 1 N H2SO4 (aq) + 0.1 g/L As2O3
L1A1	solution treated L specimens in a muffle furnace at 300 °C for 1 h
A1L2	hydrogenated L1A1 specimens again according to L process
L2A2	solution treated A1L2 specimens again according to L1A1 process
A2H3	hydrogenated L2A2 specimens according to H process
H3A3	solution treated A2H3 specimens again according to L1A1 process
A3H4	hydrogenated H3A3 specimens again according to H process
H4V	vacuum annealed A3H4 specimens at 300 °C and 4.67 Pa for 2 h

Table 4. Specimen designations and process parameters for evaluating hydrogen storage and evolution capabilities.

Designations	Process
B1	hydrogenated bulk B0 specimens to the designed A3H4 procedure to obtain B1
B2	evacuated B1 according to H4V
Sa1	hydrogenated Sa0 cathode to the designed A3H4 procedure to obtain Sa1
Sa2	evacuated Sa1 according to H4V
Sb1	hydrogenated Sb0 cathode to the designed A3H4 procedure to obtain Sb1
Sb2	evacuated Sb1 according to H4V
Sc1	hydrogenated Sc0 cathode to the designed A3H4 procedure to obtain Sc1
Sc2	evacuated Sc1 according to H4V

The microstructural variations and the hydrogen uptake were evaluated qualitatively and quantitatively by utilizing X-ray diffractometry (XRD: D2 PHASER, BRUKER, Billerica, MA, USA, 30 kV-10 mA, Cu-Kα λ = 1.541838 Å), metallographic optical microscopy (MOM), scanning electron microscopy (SEM: Hitachi 4800, Tokyo, Japan) and elemental analysis (EA: Heraeus vario III-NCSH, Hanau, Germany), respectively.

3. Results and Discussion

Previous research [3] demonstrated that bulk Ti-153 sheet specimens possess superior hydrogen absorption capacity, with a hydrogen mass payload 68 times higher than that of CP-Ti. Once fully hydrogenated, these Ti-153 sheet specimens can be readily crushed into powder and sintered via HPS to produce electrodes with customizable densities and porosities. Furthermore, these powder-sintered specimens exhibit higher hydrogen absorbency than corresponding bulk materials.

As shown in the cross-sectional views in Figure 2, the Sc specimen features the most dense and homogeneous powder and pore size distribution among the tested samples. The physical and chemical properties detailed in Table 5 and Figure 3 reveal that elevated processing temperatures resulted in sintered electrodes with lower porosity and much severe oxidation. Surface oxide layers impede hydrogen adsorption and absorption. Moreover, their relatively high density resulted in a deficiency in interconnected pores that facilitate hydrogen diffusion, as well as closed pores serving as hydrogen storage sinks. The Sc specimens were excluded from further investigation due to their high apparent density, which approached that of the corresponding bulk material, and low porosity.

Figure 2. Cross-sectional views of (a) Sa, (b) Sb and (c) Sc sinters.

Table 5. Density and porosity measurements.

Designations	Apparent Density	Porosity
Sa	3.131	41.2%
Sb	3.857	14.6%
Sc	4.230	4.5%

The measured density of Ti-153 sheet specimens is 4.42.

Figure 3. (a) XRD patterns of BP, Sa, Sb, Sc, B0, Sa0, Sb0 and Sc0 specimens respectively. (b) XRD patterns of B1, Sa1, Sb1, Sc1, B2, Sa2, Sb2 and Sc2 specimens respectively.

Figure 3 displays the XRD patterns for the BP, Sa, Sb, Sc, B0, Sa0, Sb0, Sc0, B1, Sa1, Sb1, Sc1, B2, Sa2, Sb2 and Sc2 specimens. The BP treatment clearly mineralized the originally bulk metallic Ti-153 specimens into a ceramic hydride powder, with its corresponding XRD pattern exhibiting exclusive δ-phase peaks. Because the processing temperature for Sb was 100 °C higher than that for Sa, the δ-hydrides decomposed during the sintering process of Sb. Consequently, the predominant phases in the hot-pressed sinters were α for Sa and α+β for Sb.

To accurately evaluate the hydrogen absorption capacity of the electrodes, the residual hydrogen present in sinters Sa and Sb must be first removed. Vacuum annealed sinters (Sa and Sb) at 760 °C and 4.0 × 10⁻⁵ Pa for 4 h led to the decomposition of δ hydrides and outgassing of solid solved hydrogen. The XRD patterns of as-received B0 specimens were all β peaks attributed to higher content of β-stabilizing elements. Vacuum anneal Sa specimens in the α-phase region to a hydrogen content as low as 0.029 wt% resulted in α phase as the major phase for Sa0. Hydrogen contents and the differences for typical specimens after various treatments were measured and listed in Table 6.

$$\mathrm{HAR}={\displaystyle \frac{\Delta \mathrm{H}(\mathrm{B}1-\mathrm{B}0)}{H\left(B1\right)}}={\displaystyle \frac{\left(2.165\right)}{\left(2.180\right)}}=0.993; \mathrm{HER}={\displaystyle \frac{\Delta \mathrm{H}(\mathrm{B}1-\mathrm{B}2)}{H\left(B1\right)}}={\displaystyle \frac{\left(0.911\right)}{\left(2.180\right)}}=0.418$$

Table 6. Hydrogen contents and the differences after various treatments.

Process Designations	H (wt%)	ΔH (wt%)	Hydrogen Absorption Ratio (HAR)	Hydrogen Evolution Ratio (HER)
B0	0.015	---
B1	2.180	2.165 (B0→B1)	99.3%
B2	1.269	−0.911 (B1→B2)		41.8%
Sa	0.329	---
Sa0	0.029	−0.300 (Sa→Sa0)		91.1%
Sa1	2.869	2.840 (Sa0→Sa1)	99.0%
Sa2	1.073	−1.796 (Sa1→Sa2)		62.6%
Sb	0.162	---
Sb0	0.016	−0.146 (Sb→Sb0)		90.1%
Sb1	2.852	2.836 (Sb0→Sb1)	99.4%
Sb2	1.224	−1.628 (Sb1→Sb2)		57.1%
Sc	0.152	---
Sc0	0.016	−0.136 (Sc→Sc0)		89.5%
Sc1	0.769	0.633 (Sc0→Sc1)	82.3%
Sc2	0.502	−0.267 (Sc1→Sc2)		34.7%

Although the processing temperature slightly increased by 100 °C, Sb0 remained within the α-phase region during the 4 h vacuum treatment. However, the XRD pattern of Sb0 exhibited relatively distinct β(110), β(200), and β(220) diffraction peaks, indicating that the processing parameters of Sb0 were more favorable for β-structure formation compared to those of Sa0. Unlike the Sa0 samples, the XRD pattern of the Sb0 specimens exhibited a mixture of α and β peaks. Cyclic hydrogenation of the specimens led to the reformation of the δ phase because the hydrogen uptake approached the saturated solubility, a trend further supported by the data in Table 6.

The base material of the Ti-153 alloy sheet is β phase (B0). A hydrogenation process is applied to convert it into the δ phase, facilitating the production of powder (BP) for subsequent sintering into electrodes (Sa and Sb). During the hot-pressing sintering process, the Sa and Sb electrodes do not undergo significant oxidation due to the hydrogen-rich nature of the powder (BP). The low residual hydrogen in the Sa and Sb electrodes is removed by treating them at 760 °C-4.0 × 10⁻⁵ Pa for 4 h. The dehydrogenated specimens are designated as Sa0 and Sb0. The hydrogen evolution ratio (HER) is defined as follows: HER = ${\displaystyle \frac{\Delta \mathrm{H}(\mathrm{Sa}-\mathrm{Sa}0)}{\mathrm{H}\left(\mathrm{Sa}\right)}}$ = ${\displaystyle \frac{\left(0.300\right)}{\left(0.329\right)}}$ = 91.1%. The calculated HERs of specimens Sa0 and Sb0 are 91.1% and 90.1% respectively. After cyclic electrolytic hydrogenation of the Sa0 and Sb0 specimens, the resulting Sa1 and Sb1 specimens exhibited hydrogen contents of 2.869 wt% and 2.852 wt%, respectively. The hydrogen absorption ratio (HAR) of Sa1 is defined as follows:

$$\mathrm{HAR} = {\displaystyle \frac{\Delta \mathrm{H}(\mathrm{Sa}1-\mathrm{Sa}0)}{\mathrm{H}\left(\mathrm{Sa}1\right)}}={\displaystyle \frac{\left(2.840\right)}{\left(2.869\right)}}=99.0\%.$$

Similarly, the HAR of Sb1 was calculated to be 99.4%.

The Sa1 and Sb1 specimens were treated under relatively poor dehydrogenation conditions (300 °C and 4.67 Pa for 2 h) to obtain the Sa2 and Sb2 specimens, with their calculated hydrogen evolution ratios being 62.6% and 57.1%, respectively. The purpose of treating the Sa1 and Sb1 specimens under these less ideal conditions was to evaluate the HER of the hydrogen storage materials under practical operating conditions. The HAR and HER values for different processes are summarized and listed in Table 6.

The B0 specimen features a pore-free, ingot metallurgical (I/M) β-Ti electrode, which precipitates the δ phase in B1 specimen after undergoing cyclic electrolytic hydrogenation, exhibiting a hydrogen absorption ratio (HAR) of 99.3%. In contrast, porous and powder metallurgical (P/M) electrodes, such as the Sa0 and Sb0 specimens, possess connecting pores that facilitate the transport and storage of hydrogen [6,7,8,9]. Consequently, following subsequent electrolytic hydrogenation treatments, the HAR values of both the Sa1 and Sb1 specimens reach as high as 99%, comparable to the value observed for the B1 specimen (99.3%).

Furthermore, after the final low-temperature and low-vacuum dehydrogenation process, the hydrogen evolution ratios (HERs) for the Sa2 and Sb2 specimens are 62.6% and 57.1%, respectively, both of which are higher than the 41.8% of the B2 specimen; as previously mentioned, this is attributed to the porous structure resulting from the P/M process. Regarding the slightly higher HER value of the Sa2 specimen compared to that of Sb2, it is primarily ascribed to the lower hydrogen solid solubility limit in the α-phase structure of Sa2 than in the β-phase structure of Sb2, as well as the higher porosity of the Sa2 specimen (Table 5) [10,11,12,13]. Sc2 exhibits the lowest HER efficiency (34.7%), which is primarily attributed to the oxide layer impeding hydrogen evolution. The XRD pattern of the Sc2 specimen in Figure 3b still clearly displays rutile diffraction peaks, as well as a distinctly shifted β_H diffraction peaks, which highlights the absorbed hydrogen remaining in the matrix due to insufficient hydrogen desorption.

Ideal hydrogen storage materials require not only a higher hydrogen absorption ratio (HAR) but also a higher hydrogen evolution ratio (HER) for easily extracting hydrogen during utilization. The initial structure of the material determines its chemical properties, while the porosity achieved through various manufacturing processes constitutes its physical properties. When utilizing titanium alloys as hydrogen storage electrodes, focus should not be placed solely on the high hydrogen storage capacity of β-Ti [14]; the efficiency of the hydrogen extraction process must be considered simultaneously. Both the initial chemical (composition and phase) and physical properties (interconnected pores) of the materials significantly affect the hydrogen absorption and desorption behaviors of the electrodes. This study reveals that the Sa0 specimen, which combines an initial α structure with optimal porosity, exhibits relative superior hydrogen absorption and evolution performance. The performance, in descending order, is as follows: Sa0, Sb0, B0 and Sc0 specimens.

4. Conclusions

A thermo-hydrogen process, 300 °C and 50 atm (gaseous H₂) for 15 h, followed by an HPS process, 600 °C and 700 °C at 25 MPa for 2 h, were adopted to obtain porous Sa0 and Sb0 electrodes. Qualitative and quantitative analyses proved that the hydrogen storage ability of powdered electrodes are superior to that of original bulk sheet specimens. The optimal processing conditions in this study are as follows: (1) first, the Ti-153 sheets are treated in a hydrogen furnace at 300 °C under a hydrogen pressure of 50 atm for 15 h to obtain the raw powder; (2) subsequently, the raw powder is processed in a hot-pressing sintering furnace at 600 °C under 25 MPa for 2 h to yield the sintered electrodes.