Self-standing 3D nanoporous Ag2Al with abundant surface oxygen species facilitating oxygen electroreduction for efficient hybrid Zn battery

Journal Pre-proofs Self-standing 3D nanoporous Ag2Al with abundant surface oxygen species fa‐ cilitating oxygen electroreduction for efficient hybrid Zn battery Ming Peng, Yang Zhao, Jiao Lan, Yijin Qiao, Yongwen Tan PII: DOI: Reference:

S2095-4956(20)30705-1 https://doi.org/10.1016/j.jechem.2020.10.015 JECHEM 1640

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Journal of Energy Chemistry

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15 July 2020 18 September 2020 10 October 2020

Please cite this article as: M. Peng, Y. Zhao, J. Lan, Y. Qiao, Y. Tan, Self-standing 3D nanoporous Ag2Al with abundant surface oxygen species facilitating oxygen electroreduction for efficient hybrid Zn battery, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.10.015

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Self-standing 3D nanoporous Ag2Al with abundant surface oxygen species facilitating oxygen electroreduction for efficient hybrid Zn battery Ming Penga, Yang Zhaoa, Jiao Lana, Yijin Qiaoa, Yongwen Tana, * a

College of Materials Science and Engineering, Hunan University, Changsha 410082, Hunan, China *Corresponding author. E-mail address: [email protected] (Y. Tan) ABSTRACT The hybrid battery integrating a typical Zn redox battery and a Zn-air battery is a promising green technology for energy storage, and the cathode integrating the redox reaction and electrocatalytic oxygen reduction is a key point for efficient electrochemical energy conversion. Herein, we report a scalable strategy to fabricate nanoporous Ag2Al intermetallic compound as a self-standing cathode for the hybrid Zn battery. The abundant surface oxygen species, the Ag-Al intermetallic interaction and the np-Ag2Al@AgAlOx interface cooperatively contributed to the catalytic ORR activity. The electrode endows efficient catalytic oxygen reduction (a Tafel slope of 38.0 mV/dec and an onset potential of 0.998 V) and regulated redox activity as compared with Ag. The nanoporous channels allow efficient ion transport, interface charge exchange and gas molecular diffusion. Significantly, the assembled hybrid Zn-Ag2Al/air battery delivers a high capacity of 3.23 mAh/cm2 as compared with recent reports. As far as we know, this is the first exploration for the electrochemical property of Ag2Al, and it would inspire more exploration in developing multifunctional materials and robust hybrid batteries for practical applications. Keywords: Nanoporous metal; Ag2Al; Oxygen reduction; Intermetallic interaction; Hybrid battery 1. Introduction

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As a green technology for energy storage and sustainable development, the hybrid Zn battery that integrates a Zn redox battery and a Zn-air battery is attracting increasing attention [1,2]. The hybrid device possesses a high power output and Faraday efficiency of the redox battery, and an extended energy density originated from the Zn-air battery, which is considered as a promising candidate for smart-grid energy storage and electric vehicle propulsion. The cathode is the key component for the hybrid Zn battery, which requires efficient redox property for charging/discharging reaction and effective oxygen reduction reaction (ORR) property after the redox reaction state [3–7]. In recent explorations, the hybrid battery based on NiO/Ni(OH)2 nanoflakes delivered a discharging voltage of 1.7 V in the ZnNi battery process and a capacity of over 800 mAh/g as the Zn-air battery [3]. Co3O4 nanosheets on a carbon cloth as the cathode contributed to the Zn-Co3O4/air battery, showing a working platform of ~1.85 V and a capacity of ~792 mAh/g [4]. NiCo2O4 [5], MnCo2O4 [6] and Ag/RuO2/carbon nanotubes [7] based cathodes have also contributed to hybrid batteries with robust performance. However, these metal oxides showed high redox response, but relatively limited catalytic activity for ORR. In this respect, suitable cathodic materials are still in great request for efficient hybrid devices. The gas-liquid-solid interface of the air electrode with sufficient ORR and redox activity is the key to high energy output, which remains a big challenge in the hybrid battery as analyzed above. Generally, scarce platinum is regarded as one of the most efficient catalysts toward ORR, by virtue of its preferred electronic structure and transition state [8,9]. Nevertheless, the utilization of platinum in various nanostructures with extremely large specific surface areas, typically commercially available Pt/C catalyst, not only costs too much for large-scale applications but also exhibits inferior ORR durability in basic environment. Attempts have been made to develop Pt-based alloys and non-precious metal based catalysts with electronic orbital hybrization to replace pure Pt for lower cost [10,11]. Recently, Al based intermetallic compound with well-defined crystal and electronic structures has been proved to be effective 2

precursors for new composite materials with attractive catalytic properties via interface selforganized transformation. In addition, the alloying effect combining Pt with the main group element Al introduces a strong covalency into the chemical bonding, and thereby increasing the chemical stability. PtAl2 with noticeable leaching of Al and formation of intermetallic Pt showed excellent oxygen evolution performance [12]. PtAl catalysts dealloyed from Pt8Al21 alloy consisting of atomic-layer-thick Pt skin and Pt-Al intermetallic compound skeletons effectively enhanced ORR performance [13]. Furthermore, the dealloying Al in the alloy results in abundant nanopore structure for the intermetallic compounds, which not only facilitates ion transport and interface redox reaction [14,15], but also endures a gas transport channel and highly active ORR surface [16]. Metallic Ag and its paragroup element alloys (such as AgCu, AgCo) with modified d-band center have shown excellent ORR activity under alkaline conditions, which completely reduce O2 via a four-electron approach to OH− with attractive stability [17]. Meanwhile, Ag is a reversible cathodic active material for Ag-Zn battery with high power output. We are wondering whether the Ag-Al metal system could contribute to new type of efficient ORR catalysts with nanoporous structure, and further construct a novel hybrid battery based on the electrochemical property as related with Ag. Especially, there is rare report about intermetallic compound for hybrid battery yet according to our knowledge. Thus, understanding the relationship of metal bonds and electrochemical property of intermetallic compounds would provide more inside of materials science. Here, we report a scalable approach for self-standing nanoporous Ag2Al (denoted as npAg2Al) intermetallic compound for the hybrid Zn-Ag2Al/air battery. The abundant surface oxygen species, the Ag-Al intermetallic interaction and the np-Ag2Al@AgAlOx interface cooperatively contribute to the catalytic ORR activity. As a whole, the np-Ag2Al cathode exhibits a Tafel slope of 38.0 mV/dec and an onset potential of 0.998 V for ORR. Based on the special electrochemical property, the hybrid battery integrating Zn-Ag2Al battery and Znair battery is successfully constructed, which delivers a capacity of 3.23 mA h/cm2 and an 3

energy density of 3.11 mW h/cm2. The self-standing multi-channel of np-Ag2Al provides efficient ion transport, interface charge exchange and gas diffusion; and its electrochemical catalytic ORR activity and redox property contribute to the efficient hybrid power source. The work would inspire more explorations in functional intermetallic compounds and hybrid devices. 2. Experimental 2.1. Preparation of np-Ag2Al and np-Ag For the nanoporous materials, the AgAl (50:50) precursor was prepared by arc-melting of pure Ag (99.9%, Sinopharm group Co., Ltd.) and Al (99.996%, Trillion Metals Co., Ltd.) in high-purity Ar atmosphere. The AgAl mother ribbons with 20 μm in thinness and 2 mm in width were produced by melting-spinning the precursor with a rotation speed of 4 K rpm of the copper wheel. A three-electrode system with AgAl mother ribbon as the working electrode, Pt wire as the counter electrode and AgCl/Ag (3 M KCl) as the reference electrode was involved for selectively etching. During etching, the anodic reaction was the corrosion of Al (Al − 3e− → Al3+) and the cathode reaction was the reduction of H2O (2H2O + 2e− → H2↑ + 2OH−). The self-supporting np-Ag2Al was prepared by electrochemical dealloying the ribbons in 1 M NaCl solution with a bias of −0.4 V vs Ag/AgCl for 1600 sec. For comparison, the control groups were etching 800 and 2200 sec. And the self-supporting np-Ag was prepared by electrochemical dealloying of the AgAl mother ribbons in 1 M NaCl solution with a bias of 0.1 V vs. Ag/AgCl for 3600 sec. For the np-Ag2Al and np-Ag sample, the area density was measured as 6.41 and 4.88 mg/cm2, respectively. The as-prepared samples were rinsed in pure water (18.2 MΩ cm) to remove chemical residues, then rinsed with ethanol and naturally evaporated to dry. The ratio of Ag and Al of the sample is determined to be 2.0. For comparison, Ag2Al alloy without porous structure was prepared by arc-melting of pure Ag and Al with an atom ratio of 2:1 and melting-spinning the ingot. 4

2.2. Material characterizations The structure and morphology of the samples were characterized by Tescan MIRA3 SEM equipped with Oxford energy-dispersive spectroscopy (EDS). X-ray diffraction spectroscopic (XRD) was performed on Bruker D8 Advance X-ray diffraction with Cu Kα radiation (λ = 1.5418 Å). The X-ray absorption spectroscopy (Ag L1-edge) was performed at BL16A1 at National Synchrotron Radiation Research Center (NSRRC, Taiwan). X-ray photoelectron spectroscopic (XPS) measurement was performed on Thermo Scientific Escalab 250Xi with Al Kα monochromatic. The nitrogen adsorption/desorption data was recorded with a Micromeritics ASAP 2020 apparatus at the liquid nitrogen temperature (77 K), and the specific surface area was calculated according to the BET equation. The ratio of Ag and Al in the sample is determined by inductively coupled plasma-optic emission spectrometer (ICP-OES). 2.3. Electrochemical measurement Electrochemical measurements were conducted on a CHI 760E electrochemical workstation (CH Instruments) coupled with a Pine rotating disk electrode (RDE) system (Pine Instruments Co., Ltd. USA). All electrochemical reduction experiments were carried out in a three-electrode electrochemical cell with Pt wire as the counter electrode and Ag/AgCl (in 3 M KCl electrolyte) as the reference electrode and with oxygen flowing. The self-standing npAg2Al or np-Ag samples were directly fixed on pre-polished RDE via Cu tape and the sides of the strip were sealed with Teflon tape to minimize the parasitic currents. Linear sweep voltammetrys (LSVs) were measured in 0.1 M KOH saturated with oxygen at a scan rate of 2 mV/s under disk rotation rates of 400, 625, 900, 1600 and 2000 rpm. All the potentials were recorded in reference to the Ag/AgCl electrode. All the potentials were recorded in reference to the reference hydrogen electrode (RHE), the potentials were calculated according to the formula of E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 + 0.0591 × pH = E (vs. Ag/AgCl) + 0.9683 V under basic condition. 5

The electron transfer number (n) was determined based on the linear scan curves according to the Koutecky-Levich equation: 1 1 1 1 1     1/2 J J l J k B Jk

B  0.62nFC 0 D02 / 3 1 / 6

where J, Jl and Jk is the measured current density, the limiting current density and kinetic current density, respectively; ω is the angular velocity of RDE, n is the overall number of electrons transferred in catalytic ORR, F is the Faraday constant (96485 C/mol), C0 is the saturated concentration of O2 (1.2×10−6 mol/mL), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.93×10−5 cm2/s), and ν is the kinematic viscosity of the electrolyte (1.09×10−2 cm2/s) [18,19]. The battery was fabricated with np-Ag2Al as the cathode and commercial Zn sheet as the anode and a small piece of non-dust cloth as the separator, which could be tested as the Zn-air battery, Zn-Ag2Al battery and the hybrid Zn-Ag2Al/air battery. The electrolyte was a solution of 6 M NaOH. The cyclic voltammetry, chonoamperometry, chronopotentiometry and galvanostatic charging/discharging tests were recorded on the CHI 760E electrochemical workstation at room temperature in the air.

3. Results and discussion 3.1. Preparation of np-Ag2Al Nanoporous Ag2Al was prepared by the selective etching Al phase from the AgAl mother alloy. In Fig. 1(a), the upper layer shows the evolution of nanoporous structure in the cross section and the lower layer shows the bicontinuous nanoporous structure in the 3D space. According to the Ag-Al phase diagram (Fig. S1), AgAl alloy with atomic ratio of 1:1 as the precursor contains Ag2Al phase and Al phase. Linear scan of AgAl alloy in 1 M NaCl solution shows three sections as the potential increasing (Fig. S2a), which corresponds to the 6

corrosion of metal Al phase, the corrosion of Al in Ag2Al together with Ag reconstruction, and the formation of AgCl from Ag oxidation, respectively. Electrochemical polarization with a bias of −0.4 V vs. Ag/AgCl was applied to prepare np-Ag2Al and 0.1 V vs. Ag/AgCl to prepare np-Ag, respectively (Fig. S2b). The AgAl alloy, np-Ag2Al and np-Ag appears as selfstanding strips with obvious metallic luster (Fig. S3) and the electrochemical etching method provides a scalable way for materials production. Their resistivity was determined to be 0.3, 0.2 and 0.1 mΩ cm, respectively. They could be self-supporting electrodes without adhesive and conductive additives as compared with traditional powder-based electrodes [20,21], which provides significant convenience for direct electrode characterization and device fabrication. X-ray diffraction (XRD) patterns (Fig. 1b) confirm that the AgAl ingot composed of Ag2Al phase and Al phase. Compared with the referenced PDF cards, the peaks of Ag2Al slightly up-shift and Al slightly down-shift, which are resulted from the interaction between the two phases. After dealloying of Al (PDF #04-0787), the peaks locate at 35.9°, 39.2°, 41.1°, 54.2°, 64.5°, 72.4°, 78.2° and 79.4° correspond to the (100), (002), (101), (102), (110), (103), (112) and (201) crystal plane of the Ag2Al phase (PDF #14-0647), respectively. Compared with the crystal planes in the mother alloy, the (101) plane rather than (002) becomes the dominant orientation which is likely resulted from the loss of interaction between the two phases and the atomic assembly and diffusion at the alloy/electrolyte interface during dealloying [22,23]. After totally dealloying of Al, the peaks locate at 38.7°, 44.3°, 64.4° and 77.4° correspond to the (111), (200), (220) and (311) crystal plane of the Ag phase (PDF #893722), respectively. No crystal oxides of Al and Ag are reflected in the XRD patterns, but we could not rule out the formation of some amorphous species during the electrochemical oxidation process. Ag L3-edge of near edge X-ray absorption spectroscopy (NEXAS) of npAg2Al shows higher white-line peak as compared with np-Ag (Fig. 1c), due to the

7

comprehensive effect of Ag-Al metallic interaction. The edge peak assigned to the transition from 2p orbital to 5s orbital is enhanced by the hybridization of 5s and 4d orbital.

Fig. 1. (a) Schematic diagram of the preparation process for np-Ag2Al and np-Ag. The upper layer shows the cross section and the lower layer shows the bicontinuous nanoporous structure in the 3D space. (b) XRD patterns of np-Ag2Al, np-Ag and AgAl alloy. The PDF 8

card: Ag2Al (red, PDF #14-0647); Ag (blue, PDF #89-3722); Ag2Al (green, PDF #04-0787). (c) Ag L3 XANES of np-Ag2Al and np-Ag. High resolution XPS of Ag 3d (d), Al 2p (e) and O 1s (f). (g) Diagram of AgAlOx layer on the np-Ag2Al matrix (np-Ag2Al@AgAlOx). In the structure cell of Ag2Al (Space group: P63/mmc), red and blue position present interchangeable Ag and Al. The high-resolution XPS of Al 2p (Fig. 1e) shows the peak at 72.8 eV corresponding to Al-Ag intermetallic interaction and the peak at 74.3 eV corresponding to Al3+ species due to partial surface oxidation (O-A1-Ag), while no Al signal is observed for np-Ag. The highresolution XPS of O 1s possesses three peaks (Fig. 1f): the bonding oxygen species (Ag-O-Al) at 529.3 eV, the surface oxygen species (Osur) at 530.5 eV and the adsorbed oxygen species (Oads) at 532.4 eV [25,26]. The bonding oxygen species based on the weak peak intensity confirms the partial oxidation to form a self-constructed oxidized layer (AgAlOx) with a low crystallinity on the np-Ag2Al (np-Ag2Al@AgAlOx, Fig. 1g), which could significantly improve the stability of the nanoporous material and provide sufficient active sites for both catalytic and electrochemical applications [27,28]. The abundant Osur and Oads species endow good surface affinity to oxygen species, laying good foundation for ORR process [29]. The formation of the oxidation layer could be related to both the nanoporous structure and the chemical environment. Firstly, the nanoporous structure possesses abundant and highly active under-coordinated surface atoms like step faces and edges on the curved ligaments with interfacial tension [30], thus self-grown oxy-hydroxide is easily formed [31,32]. Especially, since the oxidation potential of Ag is much higher than that of Al dissolution, the stress release of interfacial self-regulation plays an important role in the formation of monovalent Ag. Also, random metal−O bonds are formed for the same reason and result in amorphous layer. Secondly, the hydrogen evolution on the cathode make the solution alkaline gradually, which creates a preferable oxidation environment for the metal component during the anodic

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process. Thirdly, the oxidation of Al and Al is the easily activated in the electrochemical cell with Cl- electrolyte of high coordination activity [33,34]. For comparison, the Ag2Al alloy was prepared by arc-melting of pure Ag and Al with an atom ratio of 2:1. The XRD patterns show Ag2Al, Al and Ag phase in the alloy (Fig. S4), which reflects the deviation from the thermodynamic equilibrium phase. The XPS spectrum of Ag2Al alloy reflects an Ag/Al ratio lower than that of np-Ag2Al as compared in Fig. S5, further confirming the distinguished component. The high resolution XPS shows the different surface species of Ag2Al alloy and np-Ag2Al. Especially, Ag and Al metal could be identified in Ag2Al alloy, and Al2O3 is the main oxidation species on the surface. The formation of Al2O3 rather than AgAlOx could be resulted from the absence of interfacial tension originated from the curved ligaments. Since Al2O3 is catalytically inert toward ORR and would shield the active sites, it is reasonable to exhibit inferior ORR performance than np-Ag2Al.

Fig. 2. Morphology of np-Ag2Al (a and b) and np-Ag (c and d) from the top view (a and c) and cross section (b and d). Low magnification TEM (e) and high magnification TEM (f) 10

image of as-prepared np-Ag2Al. (g) HAADF-STEM image and the corresponding EDS elemental mapping of Ag (h), Al (i) and O (j). Scale bars: 200 nm (a, c and e), 2 µm (b and d), 2 nm (f), 20 nm (g–j). As far as we know, there is no report about the catalytic activity of Ag2Al toward oxygen reduction yet. An important reason is the difficulty to prepare a pure phase Ag2Al with a controlled micro/nano structure. Chemically etching Ag-Al alloys with high Al content (70% or more) as reported in literature achieved only np-Ag [35,36]. Even little amount of Ag2Al was observed in some trials, it could not form the single-phase self-standing electrode [37]. Thus, present work provides a scalable strategy for Ag2Al intermetallic compound with excellent electron transporting ability and three-dimensional nanopore structure, which provide multiple continuous channels for gas molecular diffusion, ion transport and interface charge exchange. The nanoporous structures of np-Ag2Al and np-Ag are shown in the images of scanning electron microscope (SEM) (Fig. 2a–d). From the top view and the cross section, np-Ag2Al and np-Ag show uniform bicontinuous pore structure, which provides sufficient active contact for interface reactions. In addition, np-Ag2Al has a pore size of ~50 nm, smaller than that of np-Ag (~100 nm). Due to the same volume percentage of Al phase, higher pore volume could be obtained by np-Ag. According to the calculation in Fig. S1, the porosity of np-Ag2Al (24.7%) is half that of np-Ag (49.3%) evaluated by the volume percentage of Al phase and total Al in the AgAl alloy, which was in good agreement with the SEM observation. Moreover, the surface of np-Ag2Al shows less nanopores than the cross section due to the presence of surface oxidation layer. Transmission electron microscope (TEM) further confirms the porous structure and the contentious ligament of Ag2Al (Fig. 2e). High magnification TEM image (Fig. 2f) of as-prepared np-Ag2Al shows a clear interface of amorphous and crystal phase. The amorphous layer with thickness of about 1 nm could be the surface oxidation shell of AgAlOx, and the crystal core with a lattice spacing of 0.23 nm 11

reflected the (002) direction of Ag2Al. The elemental mapping (Fig. 2g–j) shows the surface composed of Ag, Al and O, further confirming the above material composite.

3.2. Nanoporous Ag2Al for ORR The electrocatalytic activity of the as-prepared materials is subjected to ORR test under alkaline condition using a rotating disk electrode. CV curves (Fig. S6) show the ORR property of bulk Ag2Al alloy, np-Ag and np-Ag2Al in O2 saturated solution as compared with the N2 case, and np-Ag2Al has exhibited superior current response. The linear scan curves of Ag2Al alloy, np-Ag and np-Ag2Al for catalytic oxygen reduction are presented in Fig. 3(a). Apparently, higher onset potential and limiting current density are achieved by np-Ag2Al as compared with the other two. Though its onset potential is lower than commercial Pt/C, the limiting current density is at least comparable with it. The np-Ag2Al electrode shows significantly improved catalytic ORR activity than the bulk Ag2Al alloy, revealing the promising inherent catalytic activity of np-Ag2Al and the abundant active sites associated with spatial confinement effect of nanoporous structure [38,39]. To better compare the inherent catalytic performance of np-Ag2Al and np-Ag, the activity is specified to the specific surface area (Fig. S7a) as shown in Fig. S7b [40]. The np-Ag2Al catalyst exhibits superior ORR performance than np-Ag, which is mainly ascribed to the substantial increase of the intrinsic catalytic activity of Ag2Al electrode. The Tafel slop of the catalytic ORR by npAg2Al, np-Ag and bulk Ag2Al alloy is 38.0, 61.3, and 97.5 mV/dec, respectively (Fig. 3b). The Tafel slope of np-Ag (61.3 mV/dec), similar with Pt/C (64.6 mV/dec), approaches to 2303RT/αF mV/dec (R: molar gas constant; T: absolute room temperature; α: transfer coefficient; F: Faraday constant, ~60 mV/dec at 25 °C), indicating that the first electron transfer is the rate-determining step [41]. While the Tafel slope 38.0 mV/dec of np-Ag2Al approaches to 2303×(2RT/3F) mV/dec (~39.4 mV/dec at 25 °C), indicating O2− protonation on the catalytic sites is the rate determining step [42]. This could be originated from the 12

oxygen species on np-Ag2Al. Electrochemical impedance spectroscopy in Fig. 3(c) show the charge transfer resistance of ORR as a semicircle at high frequency region and the diffusion of dissolved O2 in the electrolyte as an arc at low frequency region. The charge transfer resistance of np-Ag2Al is significantly lower than the other two, while the diffusion process is obviously higher, confirming that the bicontinuous nanoporous structure facilitates gas diffusion, ion transport and interface charge exchange. In addition, the linear scan curve of ORR on the free-standing np-Ag2Al (Fig. 3d) shows an onset of 0.998 V and a limiting current density ~6.3 mA/cm2 at 1600 revolutions per minute (rpm), better than recently reported Ag-based ORR catalysts (Table S1). Consequently, the np-Ag2Al catalyst presents high promise as a Pt-free catalyst for ORR. The Koutecky-Levich analysis of the diffusion control range demonstrates a four-electron process in a wide bias range (Figs. S8 and 3e). Thus, the oxygen reduction response of np-Ag2Al is well confirmed and keeps constant in the continuous run (Fig. 3f). In addition, the concentration of Ag and Al dissolved in the electrolyte during the ORR response is below the ppm level in 0.1 M KOH (Fig. S9), and a very small amount of Al was dissolved in the electrolyte and kept stable during the ORR preceding, which further reveals the stability of the electrocatalyst. Furthermore, the nanoporous topography and chemical composition show a negligible change after long term ORR operation (Fig. S10), indicating the stable ORR was originated from the chemical and structural stability of the self-standing 3D np-Ag2Al electrode. However, when the Al phase is not totally removed from the mother alloy, the ORR response is obviously inhibited as shown in Fig. S11. Because the mixture phase has less ORR activity than Ag2Al with abundant oxygen species, and the ORR response would tend to be stable after the residual Al removed to form the stable intermetallic compound Ag2Al under the biased condition, which could be considered as a self-optimized behavior [43]. Also, this is an evidence that the intermetallic Ag2Al could contribute to ORR activity. Furthermore, as-prepared AgAl alloys show an increased Ag/Al ratio with the etching time (Table S2). The oxygen reduction catalyzed by 13

AgAl alloy etching for 1600 sec exhibits the best performance (Fig. S12) as compared with the control groups, further confirming the intermetallic Ag2Al is the best component.

Fig. 3. (a) Linear scan curves of bulk Ag2Al alloy, np-Ag, np-Ag2Al and 20% Pt/C at 1600 rpm in 0.1 M KOH. (b) Tafel analysis of the linear scan curves. (c) Electrochemical 14

impedance spectroscopy of bulk Ag2Al alloy, np-Ag, np-Ag2Al at 1600 rpm. (d) Linear scan curves of np-Ag2Al at different rotating speeds. (e) Electron transfer number with different bias. (f) ORR response of np-Ag2Al in 0.1 M KOH at 1600 rpm. (g) Continuous discharging with cycled current pattern. (h) The enlarged discharging cycle as selected in (g). Since the exposure of AgAlOx on np-Ag2Al, the intrinsic ORR activity of the np-Ag2Al catalyst could be associated with the following factors. Firstly, the nanoporous structure comprised of nanosized ligaments rendered a stable matrix of catalytic active sites to enhance ORR kinetics. Particularly, the curved Ag2Al ligament surfaces, which are geometrically required by the bicontinuous porosity, lead to a high density of under-coordinated surface atoms and surface strains [44,45], and they contribute to significant electronic (ligand) and geometrical (strain) effects that would simultaneously play a key role in enhancing the ORR catalytic activity [46,47]. Especially, the Ag2Al intermetallic interaction could effectively modify the d-band electronic property of Ag, achieving intrinsic activity enhancements [48]. Due to the orbital hybridization of Ag 4d and Al 3p as described above, this electronic structure is similar to the Pt-based oxygen reduction catalysts [23], which could reduce the adsorption energy of transition states such as O*, HO* and HOO* during oxygen reduction, thereby reducing the reaction barrier [49,50]. The substitution of Al effectively tunes the crystal fields around Ag induce different electronic structures, band structures and catalytic activities [51,52]. From Ag to Ag2Al, the lattice structure not only retains the short-range order of Ag base, but also produces the longrange order of Ag-Al base, and the ordered Ag2Al intermetallic phase has a higher electrocatalytic ORR activity than Ag [53,54]. Moreover, the bicontinuous nanoporous structure facilitates charge and mass transport, and gives rise to fast charge transfer for ORR kinetics [14], and thus improves the onset potentials and the limiting current density. Secondly, the self-constructed oxidized surface along with abundant oxygen species on np-Ag2Al contribute to the efficient ORR performance. AgAlOx could be regarded as a typical 15

transition metal oxide catalyst. The 4-electron processed ORR occurs through a 2-electron plus 2-electron process (Eqs. 1 and 2) as the dominant mechanism [55]. O 2 + H 2 O + 2e − → HO 2 − + OH −

(1)

HO 2 − + H 2 O + 2e − → 3OH −

(2)

The oxygen species absorbed on the surface (like OHad) of catalyst may involve the following reaction. The associated electron transfer process firstly forms [O2− ·(H2O)n]aq, in which dissolved O2 interacts with the OHads via a hydrogen bond (Eq. 3). Then, after desolvation, O2,ads− is generated on the surface of the active sites of the ORR catalyst (Eq. 4) and further transferred to HO 2 − (Eqs. 5 and 6). M−OH + [O 2 ·(H 2 O) n ] aq + e − → M−OH + [O 2 − ·(H 2 O) n ] aq [O 2 − ·(H 2 O) n ] aq → O 2 , ads − + nH 2 O

(3) (4)

M m+ −OH − + O 2 , ads − → M (m+1)+ –O–O 2− + OH −

(5)

M (m+1)+ –O–O 2− + H 2 O + e − → M m+ −OH − + HO 2 −

(6)

Since the protonation of O2− is the rate determining step, we believe that the oxygen species is an important aspect for efficient and continuous ORR. Previous studies have also showed that the partial oxidation of a metal/alloy surface to create abundant active centers and the activity arises from the oxides/metal interface [55], which could strengthen our point. Thus, the abundant surface oxygen species facilitates the ORR catalytic proceeding. Thirdly, the Ag2Al/AgAlOx interface could further cooperatively contribute to the ORR process. On the one hand, the Ag2Al base enhances the electrical conductivity of AgAlOx amorphous layer and provides fast electron transfer, which can accelerate the ORR kinetics. On the other hand, there is a synergy effect between Ag2Al and AgAlOx during the intermediate transition, which is cooperatively beneficial to the ORR activity. These reflect the necessity and importance of Ag-based intermetallic compounds and the construction of nanoporous structure for structure-function integrated materials. Significantly, since the np-Ag2Al material appears as self-standing ribbons with high electric conductivity, 16

they could be directly used as electrodes without any binders and additives, distinguished from conventional discrete particulate catalysts. To further confirm the ORR property, metal-air batteries were fabricated with the freestanding np-Ag2Al electrode. The as-prepared Zn-air device exhibited an open circuit voltage of 1.42 V (Fig. S13a) and it did not show significant change in cyclic voltammetry tests (540 cycles) in the range of 0–1.3 V (Fig. S13b), confirming the excellent stability of np-Ag2Al electrode. The self-standing nanoporous electrode as the cathode of a protype Zn-air and Alair battery exhibits moderate performance at different current densities (1–10 mA/cm2) (Fig. S14). Furthermore, the Zn-air battery could work with cycled current pattern (Fig. 3g and h). Thus, the self-standing np-Ag2Al could act as an efficient cathode for catalytic ORR process. 3.3. Nanoporous Ag2Al for hybrid battery The redox property of np-Ag2Al is confirmed by cyclic voltammetry in comparison with np-Ag (Fig. 4a). The two pairs of redox peaks corresponding to Ag2O/Ag and Ag2O/AgO are observed for np-Ag [7]. While only one pair of peaks is observed for np-Ag2Al, corresponding to the AgAlO2/Ag2Al redox couple due to the intermetallic interaction of AgAl. As shown in the XPS spectrum (Fig. 4b–d), the charged state shows the characteristic peak of Al 2p at 73.8 eV for Al3+, Ag 3d at 368.3 and 374.3 eV for Ag+, together with the O combined with Ag and Al, which confirms the formation of AgAlO2 [24,57]. In the discharged state, the peaks of metallic Ag-Al appears, together with reduced amount of combined O. Thus, np-Ag2Al could be a cathode with reversible redox reaction of the rechargeable Zn battery. It is worth noting that the surface oxygen species remains in both charged and discharged state, which lay good foundation for ORR and the hybrid battery. In addition, the nanoporous structure of np-Ag2Al remains in both charging and discharging states (Fig. S15). However, the Ag/Al ratio according the XPS decreases from 3.99 to 3.77 after a charging/discharging cycle, indicating the dissolution of Al under such concentrated alkali solution. For comparison of the electrochemical property, np-Ag2Al (1600s) exhibit a 17

slight better response than the control groups (Fig. S16), due to the comprehensive effect of nanoporous structure and surface Ag/Al ratio. Thus, np-Ag2Al is selected to fabricate the ZnAg2Al battery, which achieves a capacity of 0.6 mAh/cm2 at 6.25 mA /cm2 with an energy density of 0.96 mWh/cm2 and a power density of 9.88 mW/cm2 (Fig. S17). The discharging is available at a high current of up to 93.75 mA/cm2, because the intermetallic compound endows a good conductivity for current collection and the nanoporous structure provides the nanoscale multichannel for fast interface charge exchange.

Fig. 4. (a) Cyclic voltammetry of np-Ag2Al and np-Ag based Zn battery. High resolution XPS spectrum of Ag 3d (b), Al 2p (c) and O 1s (d) of np-Ag2Al electrode in the charged and discharged state.

18

Due to the success application of np-Ag2Al in ORR and rechargeable Zn-Ag2Al battery, the hybrid Zn-Ag2Al/air battery is designed as shown in Fig. 5(a). During typical charging and discharging process, the battery works as an conventional Zn-Ag2Al battery based on the conversion of Ag2Al/AgAlO2. After totally discharging, np-Ag2Al and Zn forms a Zn-air battery and the discharging state is maintained with long-term current output based on npAg2Al catalyzed oxygen reduction (Fig. 5b). Thus, the hybrid battery exhibits two discharging voltage plateaus with the first discharging plateau at 1.58 V due to chemical reduction and the second plateau at about 0.9 V due to ORR at the cathode. In the process, the hybrid battery delivers a capacity of 3.23 mAh/cm2 with an energy density of 3.11 mWh/cm2 and an average power density of 0.96 mW/cm2. The Zn-air battery contributes to 76.2% of the energy density, which could be improved by increasing the amount of Zn. Thus, the hybrid battery is successfully constructed with significantly improved the energy density than barely Zn-Ag2Al battery via integrating the batteries with two different discharging mechanisms. Especially, the present area energy density is higher than those of hybrid batteries based on Co3O4/carbon cloth electrode (0.25 mAh/cm2) [4], Ag/RuO2/carbon nanotubes (1 mAh/cm2) [7], Co3O4 nanowire-assembled clusters/Ni foam (1.67 mAh/cm2) [58], indicating the promising role of np-Ag2Al for hybrid Zn batteries. Compared with other cathodic materials for hybrid batteries, np-Ag2Al based material has a very low specific surface area, which might be the reason for the low tolerance of current density [59]. The hybrid battery would maintain stable charging/discharging cycles (Fig. 5c and d), achieving stepped discharging platforms at different current densities. Two hybrid batteries in series output a working platform of ~3.0 V during the Zn-Ag2Al battery period and ~1.8 V during the Zn-air battery period (Fig. 5e). Significantly, the module could light up the red and green LEDs in parallel at the high working platform and continuously power the red LED at the low working platform (Fig. 5f). This kind of platform could be applied to the case where high power was required for start and lower platform for long-term running, like electric 19

vehicle. Ag2Al with porous architecture not only facilitates ion transport and interface redox reaction but also endures a gas transport channel and highly active ORR surface. The electrochemical redox property and electrocatalytic oxygen reduction property of the selfstanding cathode are maintained during the repeated charge and discharge process, which provides a proof-of-concept demonstrating np-Ag2Al as a proper electrode for hybrid battery. 4. Conclusions In summary, we have developed a self-standing np-Ag2Al electrode by a facile and scalable alloying-etching strategy for efficient hybrid Zn battery. The electrochemical property of np-Ag2Al was explored for the first time. The abundant surface oxygen species, the Ag-Al intermetallic interaction and the np-Ag2Al@AgAlOx interface cooperatively contributed to the catalytic ORR activity. Also, the Ag-Al interaction could significantly regulate the reversible redox activity as compared with Ag. The unique architecture featuring a highly conductive and steady skeleton not only facilitates ion transport and interface redox reaction but also endures a gas transport channel for ORR. The np-Ag2Al electrode exhibited a remarkable ORR performance with a small Tafel slope of 38.0 mV/dec, a high onset potential of 0.998 V and electrochemical stability. Significantly, the assembled hybrid ZnAg2Al/air battery integrating Zn-Ag2Al battery and Zn-air battery delivered a capacity of 3.23 mA h/cm2 with an energy density of 3.11 mWh/cm2, showing genuine potential for efficient hybrid battery. This work provides a viable and effective strategy to afford high-performance bifunctional electrodes for hybrid batteries.

20

Fig. 5. (a) The schematic diagram of the hybrid Zn-Ag2Al/air battery. (b) A typical charging/discharging process of the hybrid battery. The charging current is 10 mA/cm2 and the discharge current is 1 mA/cm2. (c) Typical charging/discharging cycles of the hybrid battery. (d) The magnified view of the part enclosed in (c) is in the following order: charging the hybrid battery at 33.33 mA/cm2 (red zone), discharging as Zn-Ag2Al battery at 16.67 mA/cm2 (blue zone), discharging as Zn-Ag2Al battery at 0.83 mA/cm2 (green zone), and discharging as Zn-air battery at 0.83 mA/cm2 (yellow area). (e) Two hybrid batteries in series. 21

The charging and discharging process is in the following order: charging the hybrid battery at 3.33 mA/cm2 (red zone), discharging as Zn-Ag2Al battery at 3.33 mA/cm2 (green zone), discharging as Zn-Ag2Al battery at 0.50 mA/cm2 (purple zone), discharging as Zn-air battery at 0.50 mA/cm2 (blue zone). Insert: Two hybrid batteries in series achieved an open current voltage of 3.58 V and could power the blue LED (Eon = 3.2 V). (f) Two hybrid batteries in series could power the red (Eon = 1.8 V) and green (Eon = 2.8 V) LEDs in parallel. The module could light up the red and green LEDs in parallel at the high working platform (1 and 2) and continuously power the red LED at the low working platform (3 and 4). Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51771072 and No. 51901076), the Youth 1000 Talent Program of China, the Natural Science Foundation of Hunan Province (No. 2019JJ50051) and the Fundamental Research Funds for the Central Universities. The authors also thank to the project support from State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body in Hunan University (No. 71865007). Conflict of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] W. Shang, W. Yu, Y. Liu, R. Li, Y. Dai, C. Cheng, P. Tan, Meng Ni, Energy Storage Mater. 31 (2020) 44-57. [2] W. Shang, W. Yu, P. Tan, B. Chen, Z. Wu, H. Xu, Meng Ni, J. Mater. Chem. A 7 (2019) 15564-15574 . [3] D. Lee, J. Fu, M.G. Park, H. Liu, A.G. Kashkooli, Z. Chen, Nano Lett. 16 (2016) 17941802. 22

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Graphical Abstract Self-standing nanoporous Ag2Al with abundant surface oxygen species exhibits efficient electrochemical catalytic oxygen reduction and is used for hybrid Zn-Ag2Al/air battery.

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Supporting Information Self-standing 3D Nanoporous Ag2Al with Abundant Surface Oxygen Species Facilitating Oxygen Electroreduction for Efficient Hybrid Zn Battery Ming Penga, Yang Zhaoa, Jiao Lana, Yijin Qiaoa, Yongwen Tana, * a

College of Materials Science and Engineering, Hunan University, Changsha 410082, Hunan,

China. *Corresponding author, E-mail: [email protected]

Fig. S1. The Ag-Al phase diagram (Binary Alloy Phase Diagrams, II Ed., Ed. T. B. Massalski, 1990, 1, 8-9.) The porosity of np-Ag2Al and np-Ag can be evaluated by the volume percentage of Al phase and total Al in AgAl alloy, respectively. Based on the lever law of the phase diagram, the volume percentage of Al phase (V1) and total Al (V2) are approximately estimated by: V1 = (MAl/ρAl)/(MAl/ρAl + MAg2Al/ρAg2Al) × 100% = 24.7% MAl (10.0%) and MAg2Al (90.0%) is the weight percentage of Al phase and Ag2Al phase in AgAl alloy, respectively. ρAl (2.70 g/cm3) and ρAg2Al (7.97 g/cm3) is the density of Al and Ag2Al, respectively. 28

V2 = (MAl/ρAl)/(MAl/ρAl+ MAg/ρAg) × 100% = 49.3% MAl (20.0%) and MAg (80.0%) is the weight percentage of total Al and Ag in AgAl alloy, respectively. ρAl (2.70 g/cm3) and ρAg (10.49 g/cm3) is the density of Al and Ag, respectively.

Fig. S2. (a) Linear scan voltammogram curve of AgAl in 1 M NaCl solution at 0.02 V/S. The peaks as potential increasing corresponds to the corrosion of metal Al phase, the corrosion of Al in Ag2Al together with Ag reconstruction, and the formation of AgCl from Ag oxidation, respectively. (b) Polarization of AgAl alloy with a bias of -0.4 V (vs Ag/AgCl) np-Ag2Al and 0.1 V (vs Ag/AgCl) for np-Ag.

Fig. S3. Optical photograph of AgAl alloy, np-Ag2Al and np-Ag.

29

Fig. S4. XRD patterns of Ag2Al alloy, containing Ag2Al, Al and Ag phase. Inset is the enlarged peaks showing distinguished patterns of two components.

Fig. S5. XPS of np-Ag2Al (Ag:Al = 80.0:20.0) and Ag2Al (Ag:Al = 15.7:13.4). (a) The full 30

spectrum. High-resolution XPS of Ag 3d (b), Al 2p (c) and O 1s (d). For Ag 3d, the peaks at 368 eV and 374 eV of Ag2Al between the O-Ag-Al and Ag-Al peak of np-Ag2Al are assigned to Ag. For Al 2p, the main peak down shifted as compared with O-Al-Ag is assigned to Al-O of Al2O3. The metallic Al could contain both Ag-Al and Al due to the slight peak deviation as compared with that of np-Ag2Al. For O 1s, the lattice O with a higher binding energy than Ag-O-Al corresponds to Al-O of Al2O3. The surface oxygen (Osur) and adsorbed oxygen species (Oads) are similar for Ag2Al and np-Ag2Al.

Fig. S6. CV curves of bulk Ag2Al alloy, np-Ag and np-Ag2Al in 0.1 M KOH solution saturated with O2 and N2.

Fig. S7. (a) Nitrogen isotherm adsorption plots of np-Ag2Al and np-Ag. Here, np-Ag2Al and 31

np-Ag achieved a BET specific surface area of 4.67 and 8.21 m2/g with pore volume of 0.0062 and 0.043 cm3/g, respectively. (b) Linear scan curves of np-Ag2Al and np-Ag in specified to the BET surface area at 1600 rpm in 0.1 M KOH.

Fig. S8. Koutecky-Levich analysis of the linear scan curves.

Fig. S9. Concentration of Ag and Al (below ppm level) in the electrolyte during the ORR response with the bias of o.5 V vs RHE in 0.1 M KOH at 1600 rpm.

32

Fig. S10. SEM topography (a) and EDS (b) of np-Ag2Al after ORR. Scale bar: 2 µm.

Fig. S11. ORR response of np-Ag2Al with residual Al in 0.1 M KOH at 1600 rpm.

33

Fig. S12. Linear scan curves for ORR catalyzed by AgAl alloys etched with different time at 1600 rpm in 0.1 M KOH.

Fig. S13. Characterization of np-Ag2Al based Zn-air battery. (a) The open circuit voltage is 1.42 V. Inset is the as-prepared device with open circuit voltage of 1.418 V. (b) Cyclic voltammetry tests (540 cycles) of np-Ag2Al based Zn-air battery at 0.10 V/S.

34

Fig. S14. Voltage output of np-Ag2Al based Zn-air battery (red) and Al-air battery (blue) at different current densities (1-10 mA/cm2).

Fig. S15. Morphology of np-Ag2Al after charging (a) and discharging (b). Scale bar: 2 µm.

35

Fig. S16. Cyclic voltammetry tests of AgAl alloys etched with different time at 0.02 V/S.

Fig. S17. Typical charging/discharging curves of the Zn-Ag2Al battery.

36

Table S1. Performance of Ag based nanomaterials for ORR Jlimiting Onset (mA/cm2) Tafel slope Electrocatalyst potential @1600 (mV/dec) (V) rpm np-Ag2Al 6.3 38.0 0.998 Fe doped Ag 5.3 47 0.99 lotus

Reference This work Nanoscale 2018, 10, 7304 Nat. Chem. 2014, 6, 828. J. Power Sources 2017, 370, 1.

AgCo alloy

4.0

/

~0.87

Ag/Co-NGr

4.6

73

0.90

Ag/N-RGO

5.3

70

0.96

RSC Adv. 2016, 6, 99179.

Ag-Cu core shell nanoparticles

5.8

/

/

J. Mater. Chem. A. 2017, 5, 7043.

Ag Nanowire/BG

5.88

90

~0.87

np-Ag

~6

/

~1 V

Sci. Rep. 2016, 6, 37731. Adv. Energy Mater. 2015, 5, 1500149

Table S2 Elemental information from EDS Etching time (s)

800

1600

2200

Ag

24.93

52.40

60.30

Al

24.61

22.71

18.76

O

24.88

50.46

20.94

Ag/Al

1.1

2.3

3.2

37

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

38