2 oxidizing aqueous solution, a process that absolutely requires an

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An Integrated Metal-Air Battery and Selective Electrolytic Leaching Cell for the Preparation of Nanoporous Metals Jintao Fu, John S Corsi, Zeyu Wang, Heng Wei, and Eric Detsi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00919 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Nano Materials

oxidizing aqueous solution, a process that absolutely requires an electrical energy to drive the removal of Ag. In our integrated cell configuration, no external electrical energy was used. This new concept can be extended to the fabrication of a broad range of nanoporous materials under mild conditions, including the use of pure water as electrolyte as demonstrated in this work.

1. Introduction Selective leaching has been widely used for the fabrication of various types of 3D microand mesoporous materials including porous metals,1–18 metalloids,19–22 metal oxides and their composites23 and tunable nanoporous carbon also known as carbide-derived carbon (CDC).24,25 These porous materials are attractive for applications in electrochemical energy conversion and storage,7,9,12,13,19,20,22 (electro)catalysis,8,18,26,27 actuators and sensors

28–32

as well as gas storage and

CO2 capture in the case of CDC.24,33 In general, during selective leaching in aqueous medium, a sacrificial element, usually the most electrochemically active element, is preferentially removed from a parent alloy using an acidic or alkaline aqueous corroding medium. Sometimes an external electrical bias voltage can be used to drive the dealloying process if the reactions involved are not spontaneous. In the former case where no external electrical field is required to assist the dissolution of the sacrificial component, the leaching process is referred to as free corrosion dealloying. In the latter case where there is an external energy supply, the process is referred to as electrolytic dealloying. Once the sacrificial component of an alloy is removed, the remaining metal adatoms coalesce in a diffusion limited manner that generally results in a very homogeneous 3D bicontinuous network of randomly interconnected metal nanowires commonly referred to as ligaments, as well as randomly interconnected nanochannels usually referred to as pores. Although selective leaching is usually a spontaneously process that does not require an electrical energy to drive the underlying reactions,1,2,11–14 in most cases highly aggressive corroding chemicals or high temperatures are required. A common example corresponds to the use of concentrated nitric acid (a highly oxidizing chemical), to fabricate nanoporous gold (NP-Au) through spontaneous selective 2 ACS Paragon Plus Environment

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removal of silver from gold-silver alloys at ambient conditions.1,34 Another example is the use of high temperatures (200-1000 ℃) during selective leaching of metallic elements from MAX phase materials and carbides to produce tunable nanoporous carbon.24,35 Instead of these extreme conditions (i.e. concentrated acids, strong oxidizing agents, and high temperatures), nanofabrication routes involving selective leaching under mild conditions are desirable. A very promising alternative nanofabrication route to porous materials under benign conditions corresponds to selective “electrolytic” leaching, during which electrical energy is used to drive the nonspontaneous reactions involved in the process. In particular, selective electrolytic leaching has been used to produce NP-Au from gold-silver alloys as mentioned above, but in neutral pH aqueous solutions or in diluted non-oxidizing acids, instead of using the strongly oxidizing nitric acid.36–38 Selective electrolytic leaching has also been used to produce CDC from MAX phase materials as discussed above, but in neutral aqueous solutions and at ambient temperature, instead of using high temperatures.25 Thus, through electrical energy supply, it is possible to overcome the use of highly aggressive chemicals and high temperatures during nanofabrication of porous materials by selective leaching. On the other hand, due to our heavy energy-dependence over the past century and the resulting environmental issues, the scalability of nanofabrication routes through selective electrolytic leaching is not promising. Innovative energy-efficient processing routes are needed to fill the gap. This requires thinking beyond conventional “energy conversion and storage” approaches by using renewable energy resources to do work on-demand, without any prior storage of these renewable energy resources. Here we demonstrate an integrated metal-air battery and selective electrolytic cell concept where no external electrical energy is required to do the leaching work. In the Result and discussions section under Background, we first present the fundamental mechanisms behind the proposed integrated metal-air battery and selective electrolytic leaching cell. Next, under Proof-of-concept we demonstrate the feasibility of this novel concept through the fabrication of nanoporous Au film from a Au-Ag parent alloy with composition Au35Ag65 at. % used as the anode in a wireless metal-air battery, in combination with commercially available 3 ACS Paragon Plus Environment

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activated carbon cloth (ACC) used as the cathode, and 2 M HCl solution used as the electrolyte. Following this step, the fabricated nanoporous Au was characterized using electron microscopy and x-ray diffraction techniques.

2. Result and discussions 2.1. Background Metal-air batteries are conventionally primary batteries (i.e. non-rechargeable) that can generate electricity through the oxidation of a metal or metalloid used as anode, in combination with the reduction of oxygen from air on a carbon-based cathode material.39,40 Theoretically, any metal or metalloid having a standard reduction potential below the oxygen reduction potential of 1.23 V can be used as anode in metal-air battery (unless otherwise mentioned, all potentials given in this article are vs. the standard hydrogen electrode). In fact, to date a broad range of materials have been investigated as anode in metal-air batteries including Li-air, Na-air, Al-air, Mg-air, Zn-air, Siair, Fe-air, Ge-air, Ca-air, K-air batteries.41–48 These electrochemical energy storage systems are primarily designed to generate and supply energy in the form of electricity or chemical bonds (i.e. H2 production) for external use.47,48 In this work, we demonstrate a radically different approach: the use of metal-air batteries to carry out specific works, namely the fabrication of nanostructured materials by selective leaching. For leaching processes which involve non-spontaneous reactions, instead of using an external battery or any other external electrical energy source to drive these reactions,10,25,37,38 we propose the integration of a metal-air battery in the leaching process, so that both “selective leaching” and “battery discharging” processes occur simultaneously. In this article we will focus on the selective electrolytic leaching of silver from gold-silver alloys discussed earlier as a proof of concept. However, this new concept can be applied to a broad range of materials systems since any material with standard reduction potential below 1.23 V can act as the anode in a metal-air battery. 4 ACS Paragon Plus Environment


Silver

metal

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oxidizes

through the reaction: Ag Ag+ + e− (Eq. 1), which requires at least 0.79 V. Therefore, silver metal

cannot

spontaneously

dissolve in non-oxidizing acids through Eq. 1 because the hydrogen

reduction

Figure 1. (a) A typical Ag-Air battery setup. Anode: Ag; Cathode: ACC; Electrolyte: 2M HCl. (b) Integrated, wireless Ag-Air battery for selective electrolytic etching.

counter

reaction associated with non-oxidizing acids only generates 0.0 V: 2H+ + 2e− H2 (Eq. 2). However, in a Ag-Air battery configuration as illustrated in Figure 1a, the counter reaction is the oxygen reduction reaction (ORR), O2 + 4 H+ + 4 e− 2 H2O (Eq.3), which can generate 1.23 V in non-oxidizing acids. This potential is high enough to oxidize Ag, eventually resulting in the full dissolution of the silver anode. If instead of a pure silver anode, silver with gold impurity is used as the anode (i.e. silver-rich gold-silver alloy), silver will still oxidize and dissolve in the acid solution, but gold is not expected to dissolve. This is because gold oxidation, Au Au3+ + 3e- (Eq. 4), requires at least 1.56 V, while the ORR counter reaction can only provide 1.23 V. Fundamentally, since selective leaching of silver from silver-rich gold-silver alloys results in the formation of NPAu,1 replacing a pure silver anode by a silver-rich gold-silver anode in our silver-air battery from Figure 1a should result in the formation of NP-Au under benign conditions (no concentrated nitric acid), and without any external electrical energy supply. More importantly, since the aim of such a silver-air battery is not to harvest electricity for external use, an external wire connection between the anode and the cathode as shown in Figure 1a is not necessary. This means the Ag-air battery can operate in a “wireless” configuration where the metal anode is in direct physical contact with the cathode as illustrated in Figure 1b.

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2.2. Proof-of-concept To verify the above hypothesis, we make

a

wireless

Ag-air

battery

consisting of ~100 nm-thick goldsilver alloy film with composition Au35Ag65 at. % used as the anode (the Au35Ag65 at. % alloy was purchased from Wehrung & Billmeier Co.), a

Figure 2. (a) Au35Ag65 in 2M HCl in air for several days. No Ag dissolution is observed. (b) Integrated wireless Ag-Air battery using the same Au35Ag65 as the anode, 2M HCl as electrolyte and ACC as the cathode. This time Ag dissolves.

piece of activated carbon cloth (ACC) used as the cathode (the ACC with specification SpectracarbTM 2225 – Type 900 was purchased from Engineered Fibers Technology, LLC), and 2 M hydrochloric acid solution (HCl) as the electrolyte (HCl was purchased from Fisher Chemical). Maintaining the starting gold-silver alloy anode in 2 M HCl in air at ambient temperature and pressure for several days does not result in selective dissolution of silver from this silver-rich alloy (see Figure 2a). This is because the counter reduction reaction in diluted HCl as given in Eq. 2 can only generate 0.0 V, which is far below the 0.79 V required at least to oxidize silver. However, when a piece of ACC is deposited on top of the gold-silver alloy, within two days, the silver appearance of the alloy is fully changed into a brown color, which is the typical color of dealloyed NP-Au (see Figure 2b). This indicates that silver dissolution from the starting gold-silver anode has taken place in the wireless Ag-air battery configuration, resulting in the formation of NP-Au. Note that although it can be expected that the removed Ag precipitates into AgCl passive film that blocks the dealloying process, we did not observe any AgCl blocking the sample surface to prevent the dealloying from proceeding. This can be confirmed by the fact that nearly all Ag was removed during dealloying, and no Cl peak was detected from EDS analysis. We speculate that AgCl does not precipitate because the working principle of our cell is based on “macrocell” corrosion, instead of “microcell” corrosion.49 Fundamentally, in microcell corrosion, both the oxidation half reaction 6 ACS Paragon Plus Environment


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and the reduction half reaction take place on the same piece of metal, while in macrocell corrosion the two half reactions take place on two different materials (i.e. Au35Ag65 for the oxidation raction and ACC for the reduction reaction).49 In a similar way, the reaction of positively charged silver ions with negatively charged chlorine ion to precipitate solid AgCl becomes unfavorable on one single electrode (i.e. in macrocell corrosion). During a subsequent experiment, the performance of our wireless Ag-air battery in the absence of oxygen was investigated by bringing the Au35Ag65 at. % in contact with ACC and in 2 M HCl under argon flow. After four days, no change in the silvery color of the alloy was observed, which indicates that oxygen is absolutely required for silver to be dissolved. In other words, the two half reactions in our wireless Ag-air battery are: Ag oxidation through Eq.1 and oxygen reduction through Eq. 3 as stated earlier in our hypothesis. In order to further verify that the proposed cell operates as a metal-air battery as suggested, a gold-silver alloy anode and the ACC cathode were connected to an ammeter to probe the discharge current during selective leaching. A discharge currents up to 7 µA was recorded. See supporting Figure S1. Thin NP-Au leaves with thickness in the range of ~50 nm up to ~1 µm have been widely fabricated for various applications including advanced electrochemical energy storage systems,13,14 (electro)catalysis, localized surface plasmon resonance effect and the corresponding application as substrates for sensors based on surface-enhanced Raman scattering (SERS).50,51 The fabrication of these thin NP-Au films by chemical leaching of silver from gold-silver alloys requires concentrated nitric acid, usually with molarity in the range of 15 M (or 70 wt%).1,34

This is because in

concentrated nitric acid, the reduction of nitrate ion (NO3−) through the following reaction: NO3− + 3H+ + 2e− HNO2 + H2O (Eq. 5) produces a potential of 0.93 V, which is high enough to drive the oxidation of silver through the reaction in Eq. 1.52 On the other hand, in diluted nitric acid solutions with concentration in the range of ~5 M (or less), due to the sluggish kinetics under room temperature, dissolution of silver will not be observed even after several days.53 Hence, “concentrated” nitric acid is absolutely required for the fabrication of NP-Au films from gold-silver alloys without the use of external electrical energy. Alternatively, thin NP-Au films can be 7 ACS Paragon Plus Environment

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fabricated in diluted non-oxidizing acid through electrolytic dealloying.32,36,50 However, the use of external electrical energy during conventional electrolytic leaching is not favorable for the scalability of the process as discussed earlier. Furthermore, from a practical point of view, making proper electrode contacts on “thin” films without damaging these films (usually sub-micrometer thicknesses) to supply the bias voltage or current needed for electrolytic leaching is not straightforward. Therefore, the wireless selective electrolytic leaching concept demonstrated in this work is expected to fill the processing gap in the field. The

fabricated

confirmed

through

characterizations diffractometer

NP-Au

was

materials

using (XRD),

X-ray scanning

electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and

transmission

microscope experimental

electron

(TEM). section

for

See more

details. The SEM images from Figure 3a and b show the typical

Figure 3 (a, b) SEM image of NP-Au obtained from selective electrolytic etching of Au35Ag65. (a) Under low magnification and (b) high magnification. (c, d) TEM image of NP-Au under low and high magnification, respectively. The light web phase from (c) represents the graphene oxide from the TEM sample substrate.

morphology of the Ag-air battery anode after 45 hours in 2 M HCl at low and high magnification, respectively. It is seen that a three-dimensional bicontinuous nanoporous structure was formed. EDS analysis (see Figure 4a) suggests that the starting silver-rich gold-silver anode with composition Au35Ag65 at. % is converted into a gold-rich anode with residual silver content below 10 at. %. TEM was used to investigate the characteristic nanoscale size distribution in the NP-Au. The TEM images from Figure 3c and d show the typical morphology of the Ag-air battery anode after 45 hours in 2 M HCl also at low and high magnification, respectively. The average ligament size was estimated to be 50-70 nm from the TEM images. Note that the light web phase on the right 8 ACS Paragon Plus Environment


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side of Figure 3c represents the carbon support film from the TEM grid. Finally, XRD was employed to verify the crystal structure of the NPAu anode. Figure 4b shows the XRD patterns of the Agair battery anode before and

Figure 4 (a) EDS spectrum of Ag-Air battery anode before (Au35Ag65) and after (NP-Au) selective etching. (b) XRD result for Ag-Air battery anode before and after selective etching. Corresponding peaks are indicated at the bottom.

after 45 hours of immersion in 2 M HCl. The starting gold-silver alloy (black pattern Figure 4b), with composition Au35Ag65 at. %, exhibits a face-centered cubic (FCC) crystal structure (JCPDS file No. 00-004-0783 for Ag, and JCPDS file No. 00-004-0783 for Au), which agrees with the gold-silver phase diagram. Note that grains in the starting thin gold-silver film are mostly in the (200) plane as depicted by the black XRD pattern. This is because as-purchased starting alloy film was cold worked by the manufacturer through a metalworking process known as “goldbeating” to achieve the desired thickness of ~100 nm.

At the end of the beating, the grains are preferentially oriented along the (200) plane.

Regardless, after selective electrolytic leaching, the anode material preserves the FCC crystal structure of the crystalline gold and the (200) grains orientation of the starting alloy. These materials characterization techniques confirm the formation of NP-Au by selective electrolytic leaching of silver from gold-silver anode in dilute HCl, and without the use of any external electrical energy to drive the process.

The performance of our integrated Ag-air battery/electrolytic leaching cell was scrutinized in different HCl concentrations, and the leaching time was observed to increase with decreasing acid concentration. In a subsequent experiment, the ability of our integrated cell to produce “bulk” NPAu was investigated and silver was successfully leached out from a ~5 µm-thick “bulk” gold-silver 9 ACS Paragon Plus Environment

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anode in 2 M HCl. Future works include investigating effective ways to tailor the structure size in the fabricated nanoporous anode, and to control the selective leaching time. In the latter case, the rate limiting reaction in metal-air batteries is the ORR at the cathode. Although there is a large potential difference of 440 mV (i.e. 1.23 V minus 0.79 V), the maximum voltage (i.e. assuming no overpotentials) available to drive the overall cell reaction, the discharge current measured is only on the order of a few microamps (see supporting Figure S1), which suggests that the total overpotential (arising from activation, concentration and charge transfer overpotentials) is relatively large. We anticipate that such overpotential can be lowered by using better catalyst materials at the cathode. The rate of the ORR could be enhanced through nitrogen-doping of the ACC oxygen reduction catalyst.54 Enhancing the rate of the cathodic reduction reaction in our Ag-air battery will automatically boost the rate of Ag dissolution from the anode, as we have previously demonstrated55 in the case of selective removal of aluminum from silver-aluminum alloys to make nanoporous Ag (NP-Ag): Typically, a Pt catalyst was used to speed up the rate of the counter hydrogen reduction reaction associated with aluminum oxidation in non-oxidizing acids, which in turn resulted in strong enhancement of the rate of this aluminum dissolution, accordingly to the mixed potential theory of corrosion.55

Although the working principle of the self-powered selective electrolytic leaching cell presented here is demonstrated using a gold-silver alloy as anode, it should be emphasized that this new concept is not specific to gold-silver alloys. We anticipate that a broad range of nanostructured materials could be made from appropriate anode alloys using our leaching model, provided that the standard reduction potential of the sacrificial component in the anode is less than 1.23 V to ensure enough overpotential to drive the overall leaching process. In fact, silver with its relatively high standard reduction potential of 0.79 V is one of the most “difficult” sacrificial elements to dissolve by selective leaching. Common sacrificial elements (e.g. Mg, Al, Mn, Cu, etc…) have their standard reduction potentials far below that of Ag, meaning that these more reactive elements 10 ACS Paragon Plus Environment


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should be much easier to dissolve compared to silver. Finally, to further illustrate the wider applicability of the proposed concept, it should be clarified that while in this work we used an acid as electrolyte (low pH), pure water (neutral pH) and alkaline solution (high pH) could be used as electrolytes as well, depending on the materials system (i.e. thermodynamically possesses with large enough overpotentials as driving force), and provided that the oxidation and passivity of the sacrificial element under different pH conditions are not hindering the process. Indeed, the ORR in Eq. 3 that drives the dissolution of the sacrificial component in the metal-air battery anode is pHdependent, and the corresponding reduction potential is given by: 1.23 V ‒ 0.059 V x pH (Eq. 7). Based on Eq. 7, the ORR from Eq. 3 will generate 0.82 V in a neutral aqueous solution (vs. 1.23 in acidic medium), which corresponds to only ~30 mV overpotential for silver dissolution. Hence, our Ag-air battery works better in low pH solutions where up to 440

mV

overpotential

is

available for Ag dissolution. Instead of using the proposed Ag-air battery configuration operating in acid and with a Ag-rich gold-silver alloy as the anode to make NP-Au, a Cu-air battery with a Cu-rich Au-Cu alloy as the anode

Figure 5: Evidence of metal corrosion in DI water. (a) Wireless Cu-Air battery. Anode: Cu foil on glass slide; Cathode: ACC on top of the Cu foil; Electrolyte: Distilled water. (b) After one day the glass slide becomes nearly transparent because the Cu foil has been dissolved in distilled water.

could be used in pure water. Indeed, since Cu oxidation requires 0.34 V, the maximum voltage (i.e. assuming no overpotentials) available for Cu dissolution in neutral pH is 480 mV, which is even higher than the maximum potential without overpotentials available for Ag dissolution in acids. Further, in the case of pure water as the electrolyte, there is no need to worry about the low ionic conductivity of water,56 11 ACS Paragon Plus Environment

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because the anode and cathode are in direct contact and electrons transfer instead of ions transfer takes place at the anode/cathode interface. Evidence of Cu dissolution in distilled water using this new integrated metal-air battery concept is shown in Figure 5. Typically, a piece of Cu (anode) foil in direct contact with ACC (cathode) was nearly fully dissolved in distilled water after one day, which demonstrates that our approach could be attractive as an environmentally benign synthesis route to nanoporous metals.

3. Conclusion In summary, an integrated metal-air battery and selective electrolytic leaching cell is reported. This new concept is expected to be particularly attractive in nanofabrication by selective leaching, where the proposed self-powered cell could be used to drive non-spontaneous reactions without the need of external electrical energy supply. In this report, a proof-of-concept is demonstrated through the fabrication of NP-Au by selective removal of Ag from an Au/Ag alloy in a non-oxidizing acid and at ambient conditions. We anticipate that this concept could be used to produce a broad range of nanoporous materials under mild conditions, including the use of pure water as electrolyte.

4. Experimental Section Materials characterizations: Materials characterizations were carried out using an X-ray diffractometer (XRD) equipped with a graphite monochromator and using the Kα1 line of a Cu Xray tube, a JEOL 7500F scanning electron microscope (SEM) equipped with an Oxford Instruments energy dispersive X-ray spectroscopy (EDS) detector and a 200 kV JEOL 2010F transmission electron microscope (TEM).



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

ASSOCIATED CONTENT Supporting Information Available: discharge current profile during dealloying.

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2 oxidizing aqueous solution, a process that absolutely requires an

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