pH-Controlled Dealloying Route to Hierarchical ... - ACS Publications

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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pH-Controlled Dealloying Route to Hierarchical Bulk Nanoporous Zn Derived from Metastable Alloy for Hydrogen Generation by Hydrolysis of Zn in Neutral Water Jintao Fu,† Ziling Deng,† Timothy Lee,† John S. Corsi,† Zeyu Wang,†,‡ Dongyang Zhang,† and Eric Detsi*,† †

Department of Materials Science & Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272, United States State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China

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‡

S Supporting Information *

ABSTRACT: Dealloyed nanoporous metals made of very-reactive elements have rarely been reported. Instead, reactive materials are used as sacrificial components in dealloying. The high chemical reactivity of nonprecious nanostructured metals makes them suitable for a broad range of applications such as splitting water into H2 gas and metal hydroxide. On the other hand, the same high chemical reactivity hinders the synthesis of nanostructured metals. Here we use a pHcontrolled dealloying strategy to fabricate bulk nanoporous Zn with bulk dimensions in the centimeter range via the selective removal of Al from metastable face-centered cubic bulk Zn20Al80 at. % parent alloys. The corresponding bulk nanoporous Zn exhibits a hierarchical ligament/pore architecture characterized by primary ligaments and pores with an average feature size in the submicrometer range. These primary structures are made of ultrafine secondary ligaments and pores with a characteristic feature size in the range of 10−20 nm. Our bulk nanoporous Zn can split water into H2 and Zn(OH)2 at ambient temperature and pressure and continuously produce H2 at a constant rate of 0.08 mL/min per gram of Zn over 8 h. We anticipate that in this hierarchical bulk architecture, the macropores facilitate the flow of water in the bulk of the material, while the mesopores and ultrafine ligaments provide a high surface area for the reaction of water with Zn. The bulk nanoporous Zn/ water system can be used for on-board or on-demand H2 applications, during which H2 is produced when needed, without prior storage of this gas compressed in cylinders as it is currently the case. KEYWORDS: energy, water splitting, metal hydrolysis, on-board hydrogen, dealloyed nanoporous metals

1. INTRODUCTION In recent years, monolithic nanoporous metals have gained increasing interest because of their broad range of potential applications.1−13 However, much attention has been paid to the synthesis and applications of less-reactive nanoporous metals including nanoporous Au, Pd, Pt, Ag, Cu, Sn, and Ni.3,4,9,14−23 Nanostructured metals made of more-reactive earth-abundant elements with standard reduction potentials (SRPs) far below the standard hydrogen electrode (SHE) are desirable in several applications.24−27 However, the synthesis of this class of nanostructured materials is hindered by their high chemical reactivity. The dealloying strategy commonly used to produce nanoporous metals by selective alloy corrosion in aqueous media is more suitable for the fabrication of the less-reactive nanoporous metals mentioned above.1,28 This is because the SRPs of less-reactive metals are much higher than, or close to, the SHE so that in a starting parent material, the more-reactive sacrificial metal with very negative SRP can be selectively dissolved using acidic or alkaline aqueous media, without attacking the more-noble component with a more-positive or less-negative SRP. In the case of a more-reactive parent material where the SRPs of all the alloy components are very negative, however, during dealloying, © XXXX American Chemical Society

both the more-noble alloy component and the sacrificial phase are susceptible to be attacked by the aqueous corroding medium, causing the parent alloy either to vanish by full dissolution or to undergo passivation.28,29 Alloy passivation will automatically block the selective corrosion process.30 These fundamental issues have hindered the fabrication of more-reactive nanoporous materials from aqueous corroding media. Among reactive earth-abundant metals, Zn with an SRP of −0.76 V vs SHE is susceptible to spontaneously react with aqueous acidic or alkaline corroding media, which makes the fabrication of nanoporous Zn (NP-Zn) difficult.31,32 However, in this work, we demonstrate that NP-Zn with bulk dimensions in the centimeter range can be synthesized in aqueous media from a properly designed parent material in combination with a controlled pH of the corroding media. Next, we demonstrate the high reactivity of the synthesized NP-Zn through Zn hydrolysis in neutral water, i.e., the ability to split distilled water into H2 and Zn(OH)2 at ambient temperature and Received: March 15, 2018 Accepted: June 12, 2018 Published: June 12, 2018 A

DOI: 10.1021/acsaem.8b00419 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

it can be deduced from eq 4 that when the pH of the corroding medium is 12.88 (or higher), there is no driving overpotential for Zn dissolution (i.e., −0.76 V minus −0.76 V), meaning that NP-Zn can be synthesized in alkaline solution with pH higher than 12.88. Note that controlling the pH is very critical, not only in acids but also in alkaline solutions. Indeed, Zn will be converted to Zn oxide in alkaline corroding media with much higher concentration (e.g., 2 M NaOH), which restricts the upper concentration limit for the corrosion window.34 Next, since the sacrificial element in the parent alloy used to produce NP-Zn should be more reactive than Zn, Al is a good candidate sacrificial material with its SRP of −1.66 V vs SHE. Indeed, metallic Al can dissolve through the oxidation reaction eq 5

pressure. H2 was continually produced at the constant rate of 0.08 mL/min per gram of Zn over 8 h.

2. RESULTS AND DISCUSSION 2.1. Fundamental Considerations. Prior to presenting our experimental results, we will briefly clarify the basic chemistry mechanisms used for the fabrication of NP-Zn. Metallic Zn can dissolve in aqueous corroding media through the following oxidation reaction Zn(s) → Zn 2 +(aq) + 2e−

(1)

This reaction requires at least −0.76 V vs SHE and will only proceed if it is coupled to a reduction reaction that can generate the required potential accordingly to the mixed potential theory of corrosion.21 In nonoxidizing acidic and alkaline media, such a reduction reaction is associated with protons (H+) or water reduction into H2 gas through eqs 2 and 3, respectively33 2H+(aq) + 2e− → H 2(g)

(2)

2H 2O(l) + 2e− → H 2(g) + 2OH−(aq)

(3)

Al(s) → Al3 +(aq) + 3e−

(5)

The potential of −1.66 V required to drive this reaction can be provided in alkaline solution by the reaction in eq 3. While there is no driving overpotential to dissolve Zn through eq 1 in combination with eq 3 at the pH of 12.88 or higher as discussed above, under similar conditions (pH = 12.88), there is still a very high overpotential of ∼0.90 V available for Al dissolution (i.e., −0.76 V minus −1.66 V). Therefore, if Al is used as the sacrificial component, it will selectively dissolve, while Zn will not. On the basis of these considerations, we used Zn20Al80 at. % alloys as parent materials. 2.2. Metastable Zn20Al80 at. % Parent Materials. The Zn20Al80 at. % parent materials were fabricated by melting pure Zn and pure Al metals with the appropriate ratio (see Experimental Section). Attempts to dealloy as-prepared bulk Zn20Al80 at. % alloys do not result in the formation of NP-Zn. This is because of the coexistence of two phases at equilibrium, namely, the Al-rich α phase (FCC) and the Zn-rich η phase (HCP). The maximum Al content in this latter phase is far below 5 at. % at room temperature (see Figure 2a35). This difference in sacrificial Al content between the α and η phases results in a high difference in Al removal kinetics between these two phases, making it more difficult to dealloy the Zn20Al80 at. % parent material at room temperature. A similar behavior (i.e., coexistence of two phases in the parent materials with different dealloying kinetics) was reported in AgxAl1−x parent alloys used to make nanoporous Ag, so dealloying at high temperatures was necessary in order to activate the corrosion of the less-reactive Ag-rich phase.36 However, instead of using high temperatures to drive the dealloying process, Detsi et al. suggested in the case of AgxAl1−x alloys the use of nonequilibrium (i.e., metastable) parent materials to overcome this issue;2 this concept is also applicable to other materials systems as demonstrated by Hayes et al. in the case of CuxMn1−x alloys.3 For the present Zn20Al80 at. % materials system, since the underlying two phases do not give rise to NP-Zn during removal of Al as discussed above, the Zn20Al80 at. % parent alloy was reheated and homogenized at 400 °C (red dashed line, Figure 2a) for several hours and then quenched in an ice bath to suppress the formation of the Zn-rich HCP η phase (see Experimental Section).2,37 This quenching process induces the formation of two FCC phases, namely, the αFCC phase (with Al content above 80 at. %) and the β-FCC phase containing ∼50 at. % of Al (see phase diagram in Figure 2a). Note that the sample was annealing at 400 °C (i.e., a much higher temperature than that of the α + β regime) to account for heat loss due to various factors, including argon

Figure 1. Potential−pH diagram of an aqueous corroding medium. Zn will not corrode in the small window highlighted in green.

The reaction through eq 2 or 3 is pH-dependent as illustrated in Figure 1, and the generated pH-dependent voltage is given by eq 433 −59.2 mV × pH (4) In the specific situation where the pH is null, eq 2 will generate 0.0 V vs SHE, meaning there is a very high overpotential of +0.76 V (i.e., 0.0 V minus −0.76 V) available to drive the dissolution of Zn through eq 1 in combination with eq 2. Thus, NP-Zn cannot be fabricated in a corroding medium with pH = 0. In an alkaline solution, for instance, when the pH = 12, there is still an overpotential of ∼50 mV (i.e., −0.71 V minus −0.76 V) available to drive the dissolution of Zn through eq 1 in combination this time with eq 3. Thus, even in such an alkaline solution (pH = 12), NP-Zn cannot be fabricated. However, a careful analysis of the potential−pH diagram of a nonoxidizing aqueous corroding medium (see Figure 1) suggests that there is a small pH window where Zn does not corrode as highlighted in the green region in Figure 1. Indeed, B


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mixture of FCC-Al (Fm3̅m space group, matching JCPDS card number 00−004−0787) and HCP-Zn (P63/mmc space group, matching JCPDS card number 00−004−0831) phases, while the rapidly cooled alloy predominantly consists of the FCC phase, with a very small residual of HCP phase as depicted by the peak at 2θ ≈ 84° (compare the purple and blue patterns from Figure 2b). The splitting of the FCC peaks (purple pattern in Figure 2b) is associated with the presence of two different FCC phases (α and β as mentioned above). In particular, the diffraction peaks of the quenched Zn-rich βphase were found to match the Fm3̅m(225) space group (PDF card number is #72−1538). Because of the difference in sacrificial Al content in these two FCC phases, we anticipate that removal of Al from the α-FCC phase produces big pores because of the high content of Al in this phase (above 80 at. %), while removal of Al from the β-FCC phase produces small pores because of the relatively lower content of Al in this second phase (∼50 at. %).38,39 The presence of two phases in the starting parent materials was further confirmed by analyzing the surface of this starting alloy using scanning electron microscopy in backscattered electrons mode (see Supporting Figure S1). 2.3. Synthesis of Hierarchical Bulk Nanoporous Zn. Bulk NP-Zn was obtained through free corrosion dealloying, during which Al was selectively removed from metastable Zn20Al80 at. % precursors in alkaline solutions with pH ≈ 14 through eq 5 in combination with eq 3 as explained in Section 2.1 above (see Experimental Section). During dealloying, H2 gas bubbles were released in agreement with eq 3. After 3 days, no H2 gas bubbles were observed, the sample color changed from metallic silver to black/dark gray, and it was easily broken into small fragments, which together indicate the completion of the dealloying process. Note that the dealloying time of 3 days is reasonable because of the relatively big size of the starting bulk parent materials. Figure 3a,b shows the fabricated bulk NP-Zn. Large pieces of NP-Zn with bulk dimensions in the range of a few centimeters were made, rinsed in distilled water, dried under an inert environment, and kept in an argonfilled glovebox. Because of the high reactivity of Zn, it is important to confirm that the fabricated material is metallic Zn, instead of Zn oxide. This was verified using XRD and energy dispersive spectroscopy (EDS) as shown in Figures 2c and 4, respectively. The XRD pattern of dealloyed NP-Zn (see purple pattern in Figure 2c) matches perfectly with that of metallic Zn (HCP Zn, P63/mmc space group, matching JCPDS card number 00− 004−0831), which indicates that the synthesized material is not Zn oxide. It should be emphasized that the dealloyed NPZn was crushed into powder and sandwiched between Kapton tape prior to XRD characterization to minimize the exposure of the sample to air. The EDS (see Figure 4) spectrum shows that most of the Al has been removed, and the average residual Al content was below 10 at. %. In addition, ICP analysis of the dealloyed NP-Zn was conducted to determine the bulk chemical composition. The result suggests that our NP-Zn consists of ∼96 at. % of Zn and ∼4 at. % of Al. Both ICP and EDS results are summarized in Table 1. It should be noted that the EDS result also indicates the presence of a small oxygen signal. Materials exposure during SEM/EDS characterizations is unavoidable, and we speculate that the oxygen signal primarily comes from sample exposure during these characterizations, since a similar oxygen signal is observed on the starting Zn20Al80 at. % parent alloy (Figure 4).

Figure 2. (a) Equilibrium phase diagram of Zn−Al binary alloy. The blue dashed line indicates the alloy composition, and the red dashed line shows the annealing temperature for solid−solid transformation to induce the metastable phase after rapid cooling. (b) XRD diffraction patterns of the Zn20Al80 at. % parent alloy as-made (blue pattern) and after annealing followed by rapid cooling (purple pattern). (c) XRD diffraction patterns of the metastable Zn20Al80 at. % parent alloy before (blue pattern) and after Al removal to make hierarchical nanoporous Zn (purple pattern).

flow during the annealing step and rapid exposure of the sample to air during the quenching step. A key point here is that the latter β-FCC phase contains much more Al content than the 5 at. % Al in the previous HCP η phase. Thus, each of these two FCC phases contains enough sacrificial Al to generate a nanoporous Zn architecture after Al removal during dealloying. Figure 2b shows the X-ray diffraction (XRD) data of the phases present in the Zn20Al80 at. % parent alloy as-made (blue pattern) and after annealing followed by rapid cooling (purple pattern). It is seen that as-made Zn20Al80 at. % parent alloy is a C


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Table 1. ICP and EDS Data on the Chemical Composition of Dealloyed Bulk NP-Zn Zn (at. %) Al (at. %) EDS analysis ICP analysisa

percentage of Al extracted from the Zn−Al parent alloy

>90

97.2

95.8

4.2

98.9

a NP-Zn (with residual Al) was dissolved in a mixture of HNO3 and HCl for ICP analysis.

mentioning that when a dealloyed NP-Zn sample (initially kept in an argon-filled glovebox) was taken from the argon environment to air for materials characterization, the bulk NPZn was found to violently react with moisture from the air, generating enough heat to readily melt a polystyrene weighing boat (melting point 70 °C) as seen in Figure 3c. This also demonstrates the very “reactive” nature of this class of materials in the nanostructured form. Figure 3d shows the scanning electron microscopy (SEM) of the synthesized NP-Zn. It exhibits a hierarchical porous architecture. The formation of the hierarchical structure is further illustrated in Figure 5, where small ligament/pore structures with an average size in the range of 10−20 nm are observed, together with large ligament/pore structures with an average size in the submicrometer range, namely, 200−500 nm. TEM images in Figure 6 confirm that the big pores are in the submicrometer range, while the finer pore/ligament sizes are 10−20 nm. These TEM data also confirm that the finer pore/ ligament structures exist in the bulk of NP-Zn, as depicted from Figure 6b,c. As suggested above, this hierarchical architecture is attributed to the presence of two FCC phases with different sacrificial Al contents in the starting metastable Zn20Al80 at. % parent alloy. Removal of Al from the α FCC phase gives rise to the big ligament/pore structures due to the relatively high content of Al (more than 80 at. %) in this phase, while removal of Al from the β FCC phase produces small ligament/pore structures due to the relatively lower content of Al (∼50 at. %) in this second phase.38 2.4. Hydrogen Generation by Zn Hydrolysis in Neutral Water. Zn metal in the nanoscale format can spontaneously react with water to produce H2 according to the following reaction

Figure 3. (a) Bulk NP-Zn. (b) The bulk piece of NP-Zn is intentionally broken into small fragments to demonstrate its brittleness. (c) Broken pieces of NP-Zn violently react with water moisture from air, generating enough heat to readily melt a polystyrene weighing boat. (d) SEM image of dealloyed NP-Zn.

Zn(s) + 2H 2O(l) → H 2(g) + Zn(OH)2 (s)

(6)

Note that since Al is far more reactive than Zn, any residual Al near the surface of the dealloyed NP-Zn will exist in the form of aluminum oxide or hydroxide, which is not susceptible to react with water to produce hydrogen. Thus, it is reasonable to assume that only Zn contributes to hydrogen generation through eq 6. The theoretical gravimetric amount of H2 that can be produced from a stoichiometric amount of Zn and H2O is 1.97 wt %. This is the total mass of H2 over the stoichiometric mass of Zn and H2O. A fundamental challenge with this method to produce H2 is the fact that the metal oxide layer formed during the process will prevent water from coming into contact with pure metal to further react, blocking in that way the overall process. Thus, in general, hydrogen production by water hydrolysis will not proceed. 40−44 In this section, we demonstrate the use of our bulk NP-Zn to produce hydrogen through the reaction of Zn with distilled water (see Supporting

Figure 4. EDS spectra of Zn20Al80 at. % before (above) and after (below) dealloying. The strong Al signal in the starting parent alloy almost entirely vanishes after dealloying.

Furthermore, because the dealloying process was carried out in an aqueous alkaline medium containing oxygen species (OH−, O2−), one expects these oxygen species to be present at the surface of NP-Zn. However, the presence of such oxygen species on the surface of NP-Zn does not block the dealloying process by surface passivation, since we were able to dealloy the starting bulk materials. More importantly, the fabricated NP-Zn from aqueous solution was found to be highly reactive, which suggests that the surface coverage by oxygen species is not conformal. For the sake of illustration, it is worth D


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Figure 5. SEM images showing the typical hierarchical NP-Zn architecture at different magnifications. (a) Low magnification and (b) intermediate magnification where large ligament/pore structures with an average size in the range of 200−500 nm can be seen. (c) Under high magnifications, small ligament/pore structures with an average size in the range of 10−20 nm are observed.

Figure 6. TEM images showing the typical hierarchical NP-Zn architecture at different magnifications. (a) At low magnifications, large pores (of the order 100−300 nm) can be seen. (b,c) Under high magnifications, it is found that the “large” ligaments themselves are made of smaller ligament/pore structures with an average size in the range of 10−20 nm.

Video 1). The amount and rate of H2 produced was investigated using a customized Scion 456 gas chromatography (GC) system. The schematic setup for H2 measurement is shown in Figure 7a. Helium gas was used as carrier to transfer the produced H2 from the reaction chamber to the GC (see Experimental Section). The three curves in Figure 7b show typical raw GC data associated with the amount of H2 produced at random times of 65, 185, 260 min after injection of H2O: as time increases, the cumulative amount of H2 generated increases. Figure 7c shows the cumulative amount of hydrogen produced as a function of total reaction time (min). The H2 production rate was found to be constant over 8 h, as depicted by the linear fit (red line) in Figure 7c. From the slope of that linear fit, we derived the average hydrogen production rate of 7.26 μg/min per gram of Zn, which corresponds to 0.08 mL/min per gram of Zn So far, several water-reactive materials including Al, Zn, and Si have been proposed for H2 production through water hydrolysis.43−46 However, as mentioned above, the hydroxide layer (or “oxide” layer depending on the specific experimental conditions) formed on the metal surface at the beginning of the reaction prevents water from coming into direct contact

Figure 7. (a) Illustration of the setup used to measure the amount and rate of H2 generated from NP-Zn in contact with distilled H2O. Generated H2 is pushed into the GC using helium gas as the carrier. (b) Typical GC raw data showing H2 peak intensity versus reaction time. (c) Cumulative amount of H2 produced versus reaction time. The straight line is the corresponding linear fitting.

E


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produces small ligament/pore structures (10−20 nm) due to the relatively lower content of Al (∼50 at. %). Thanks to the small feature sizes in this hierarchical structure, our nanoporous Zn could split water into H 2 and Zn(OH) 2 continuously over 8 h at a constant rate of 0.08 mL/min per gram of Zn. We speculate that the macropores effectively facilitate the flow of water into the bulk of material, while the mesopores provide a high internal surface area for the H2O splitting reaction to effectively proceed. Generating H2 from water-reactive materials is attractive for a broad range of applications including on-board H2 production to power portable devices and next generation H2 fuel cell vehicles.

with the metal underneath. To prove this issue and highlight the performance of our bulk NP-Zn toward hydrogen generation, we have compared the reactivity of our bulk NPZn with other forms of metal Zn including commercial Zn powder and commercial Zn foil (see Supporting Video 2). We have found that only our bulk NP-Zn can react with neutral water to produce hydrogen. Other forms of Zn do not react with neutral water due to the presence of native Zn oxide. Even if the native oxide present on commercial bulk Zn is removed, the reaction of bulk Zn with water will not continually proceed because of the absence of nanoscale features. In fact, several approaches have been investigated to overcome this issue of the water-blocking hydroxide layer in water-reactive metals, including chemical removal of the hydroxide layer through additives in water;47−50 chemical removal of the hydroxide layer through additives/promoters in the metal or use of alloys instead of pure metal;40−44 and mechanical removal of the hydroxide layer through frictions and milling.51,52 However, these solutions will add “nonactive” mass to the systemthrough use of a catalyst or a nonreactive alloying metaland thus lower its theoretical gravimetric hydrogen storage capacity. Nevertheless, our hierarchical NP-Zn can generate hydrogen continuously over 8 h due to the ultrafine size of NP-Zn. Indeed, it is less likely that such small structure sizes (10−20 nm) with irregular shapes and random curvatures can be covered conformally with Zn hydroxide to block the reaction of Zn with water by preventing water to come into contact with fresh nanostructured Zn. In addition, while the small structure size (10−20 nm) provides a high surface area for the reaction with water, the big ligaments and pores (200− 500 nm) in the hierarchical NP-Zn structure facilitates the reaction kinetics by effectively facilitating mass transport of the reactant (H2O) and product (H2) in and out of the bulk material. H2 measurements were stopped after ∼8 h while the system was still producing H2 at constant rate (see Figure 7c). On the other hand, the amount of H2 collected after the reaction has fully completed reaches ∼16% of the theoretical gravimetric capacity of the Zn/H2O system. We speculate that the relatively low capacity partly originates from the presence of oxidized species at the surface of NP-Zn, which is unavoidable since an aqueous solution was used for the dealloying process. We anticipate that appropriate postdealloying treatments of nanoporous Zn (e.g., briefly washing in suitable acid), so as to get rid of inactive surface species, will increase the hydrogen generation capacity of this material system. The present work opens new research avenues in this field including optimizing the NP-Zn-water system to improve its hydrogen generation capacity as well as fabricating and exploring new water-reactive nanoporous metals such as nanoporous aluminum and magnesium.

4. EXPERIMENTAL SECTION Samples Preparation. A silver−aluminum master alloy with a composition of Zn20Al80 at. % was made by melting ∼2.2 g of pure Zn (99.999%, Alfa Asear) and ∼3.5 g of pure Al (99.999%, CERAC Inc.) at 800 °C (OTF-1200X, MTI Corporation) in a graphite boat using a quartz tube under argon flow for 20 min. The ingot was cooled down, flipped, and remelted twice to ensure a homogeneous mixture. Following this step, some master alloys were diced into thin pieces and kept at 400 °C for 5 h and quenched in an ice bath (∼3 °C). NP-Zn Fabrication. The as-fabricated Zn20Al80 at. % precursor was first polished with sandpaper (silicon carbide water proof paper, type 180 and type 600) to remove surface oxide and impurities. It was then kept in 1 M NaOH solution (NaOH white pellets purchased from Fisher dissolved in DI water) for 3 days until no H2 bubbles were observed. Note that the solution was changed every 12 h to maintain the concentration. The sample was then washed with DI water to remove surface ion species (i.e., Na+). Although this process may initiate an NP-Zn reaction with water, we suggest that most of the material remains as fresh NP-Zn, which is confirmed by EDS in Figure 4. Finally, the sample was dried under an inert environment and kept in the glovebox. Hydrogen Generation Characterization. A gas chromatograph (GC, Scion 456) with a pulsed discharge detector (Molecular Sieve 5A packed column) was used to quantitatively characterize the hydrogen generation from the NP-Zn reaction with water. Initially, NP-Zn (70 mg) was crushed into powder (with grains in the submillimeter range) inside an argon-filled glovebox and put in the reaction chamber also inside the glovebox (no water at this stage). The chamber was then sealed tightly using a stopper sleeve (Kimble) and flushed with helium (99.9999% pure He, Airgas) purge flow (25 mL/min) for 30 min. No peak associated with hydrogen was detected. Then, 5 mL of DI water (bubbled with argon for 30 min) was pushed into the cell using a syringe and brought into contact with NP-Zn. Bubbles could be observed indicating the formation of hydrogen. The reaction chamber was flushed with helium at various time intervals ranging from 60 to 120 min to detect the amount of hydrogen generated, but in between, it was kept sealed for hydrogen to accumulate inside. Finally, the area of detected hydrogen peak was calibrated through a standard gas (Scott-Marrin Inc.), which has a hydrogen concentration of 1% (mol/mol). Materials Characterizations. A JEOL 7500F Scanning Electron Microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) were used to investigate the microstructure and bulk elemental composition of the starting Zn20Al80 at. % parent alloy and the corresponding dealloyed nanoporous Zn. The X-ray diffraction (XRD) data of these materials was taken with a Rigaku D/Max-B X-ray diffractometer with Bragg−Brentano parafocusing geometry operating with a Cu Kα1 line (λ = 1.5405 Å) and a horizontal goniometer collecting at an angle range of 10−95° with a 0.05° step size and 2°/min scanning speed, with a conventional copper target Xray tube set to 40 kV and 30 mA.

3. CONCLUSION In this contribution, we report for the first time on the fabrication of nanoporous Zn, a reactive metal that is so far rather used as sacrificial component in dealloying. A pHcontrolled dealloying route was used to fabricate our nanoporous Zn from quenched Zn20Al80 at. % parent alloys containing two face-centered cubic phases (α and β) with different Al content so that Al removal from each of these phases results in a hierarchical nanoporous architecture. Specifically, the α-FCC phase gives rise to big ligament/pore structures (200−500 nm) due to the relatively high content of Al (more than 80 at. %) in this phase, while the β-FCC phase F


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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00419. SEM image of a quenched Zn20Al80 parent alloy collected using backscattered electrons (PDF) Supporting Video 1: Hydrogen generation by hydrolysis of bulk nanoporous Zn in neutral water (AVI) Supporting Video 2: Hydrogen generation by hydrolysis of bulk nanoporous Zn in neutral water: bulk nanoporous Zn vs commercial Zn (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.D.) ORCID

Eric Detsi: 0000-0002-4009-7260 Author Contributions

E.D. designed and supervised the experiments. J.F. performed the dealloying experiment, carried out all Zn hydrolysis and GC experiments. J.F and E.D. wrote the manuscript. Z.D. and Z.W. melted the alloy precursors, collected and analyzed XRD data. J.S.C. collected the TEM data and performed backscattered electron microscopy. T.L. and D.Z. assisted at various levels and helped to prepare the manuscript and figures. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Penn Engineering for the financial support through the PI startup. REFERENCES

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