Ag-Cu nanoalloyed film as a high-performance cathode electrocatalytic material for zinc-air battery
© Lei et al.; licensee Springer. 2015
Received: 17 March 2015
Accepted: 10 April 2015
Published: 23 April 2015
A novel Ag50Cu50 film electrocatalyst for oxygen reduction reaction (ORR) was prepared by pulsed laser deposition (PLD) method. The electrocatalyst actually is Ag-Cu alloyed nanoparticles embedded in amorphous Cu film, based on transmission electron microscopy (TEM) characterization. The rotating disk electrode (RDE) measurements provide evidence that the ORR proceed via a four-electron pathway on the electrocatalysts in alkaline solution. And it is much more efficient than pure Ag catalyst. The catalytic layer has maximum power density of 67 mW cm−2 and an acceptable cell voltage at 0.863 V when current densities increased up to 100 mA cm−2 in the Ag50Cu50-based primary zinc-air battery. The resulting rechargeable zinc-air battery exhibits low charge-discharge voltage polarization of 1.1 V at 20 mAcm−2 and high durability over 100 cycles in natural air.
Metal-air batteries have attracted a lot of attention as a possible energy storage solution for decades . Among various metal-air batteries, zinc-air battery holds the greatest promise for future energy applications , owing to its high power density, safety, and economic viability and the abundant zinc reserve in earth [3-5]. The key point for development of zinc-air battery is finding high-performance oxygen reduction reaction (ORR) catalysts. Precious metals, like Pt, are usually used as ORR catalysts [6,7]. In general, Pt-based catalysts can be prepared via various routes, such as electrochemical deposition , chemical vapor deposition [9,10], and facile hydrothermal method . Since the catalyst utilization in the fuel cell is determined mainly by the contact surface area of catalyst with electrolyte, the reduction of the thickness of catalytic layer can result in an improvement of the catalyst utilization and reduction of the fuel cell cost . Therefore, pulse laser deposition (PLD) is also used popularly in the field [13-15], because the approach is a feasible way to control the thickness of catalyst layer. Moreover, compared to aforementioned chemical preparations, PLD method also owns high repeatability and stability in process, making it to be a suitable route to obtain electrocatalyst with film state.
Although Pt-based film catalyst has already obtained progress via PLD method , it is still expected to further decrease the cost of ORR catalyst. Therefore, the 3d transition metal oxides [16-19], silver [20-27], and its related alloy with transition metals, such as Ag-Co and Ag-Mn [28-31], which consume less cost than Pt were investigated extensively in an alkaline environment. Beyond that, Ag-Cu alloyed materials also might be suitable electrocatalyst for reasons. Theoretical calculations indicate that Ag-Cu alloyed nanoparticles exhibit strong adsorption energies and low activation-energy barriers [32,33]. Meanwhile, Ag and Cu own the same facet-center cubic structure and similar cell parameters. Synthesis of Ag-Cu alloyed catalyst is supposed to be easier than other types of Ag alloy . It is reasonable to believe that Ag-Cu nanoalloy could become a new generation of catalysts. However, successful synthesis of real Ag-Cu alloyed catalysts in nanoscale has seldom been reported aside from some Ag-Cu heterostructures which do not have alloy state [35,36].
Based on the aforementioned background, herein, we expect PLD method can be applied to prepare Ag-Cu alloyed catalyst film, which not only does greatly reduce the catalytic cost but also can obtain effective ORR catalytic activity. In this work, we demonstrate a design of Ag-Cu alloyed film electrocatalyst synthesized via pulsed laser deposition (PLD). The ORR catalytic property of the as-prepared Ag-Cu electrocatalyst has been tested and compared to commercial Pt/C, Ir/C, and Ag film catalysts. Both of the resulting primary and rechargeable zinc-air battery show good performance in natural air.
Synthesis of Ag50Cu50 film electrocatalyst
Ag-Cu alloyed catalyst was prepared by PLD method in a vacuum chamber with a pressure of 2 × 10−4 Pa. The target of Ag-Cu alloy with atomic ratio of 50:50 was irradiated with a nanosecond Q-switched Nd:YAG laser beam (EKSPLA, Lithuania). The wavelength was set to be 266 nm, and the pulse duration was ranging from 3 to 6 ns. The laser beam diameter was around 1 mm, with an energy density of 200 mJ/pulse. Both target and substrate (nickel foam) rotated at a speed of 5 rpm during deposition, and target was irradiated for 2 min at 10 Hz to clear away the oxide on the surface before deposition. The laser was operated at the frequency of 10 Hz. The deposition time is set as 90 min. The as-prepared product is Ag50Cu50 catalyst.
The ORR activities of Ag50Cu50 catalyst were studied at room temperature via measuring rotating disk electrode (RDE) polarization curves. The experiments were performed with a classic three-electrode cell containing a saturated calomel electrode (SCE) as reference electrode, a Pt counter electrode, and Ag-Cu catalyst supported on nickel foam as the working electrode in the CHI660C electrochemical workstation. The electrolyte was 0.1 M KOH aqueous solution. The experiments were performed over the potential range of 0 to −0.8 V at a scanning rate of 10 mV s−1. All potentials reported in this work were converted from the SCE to the RHE scale using E(RHE) = E(SCE) + 0.991 V in 0.1 M KOH. The rotation rates were controlled at 400, 800, 1,200, and 1,600 rpm. The working area for oxygen diffusion in the air electrode was 0.785 cm2.
Performance test of zinc-air battery
The catalyst layer was made by the direct deposition of Ag-Cu alloyed catalyst onto the nickel foam current collector. The air cathode was then assembled by the catalyst layer and the gas diffusion layer prepared by mixing acetylene black and 60% PTFE with a mass ratio of 1:2. A zinc-air battery was then fabricated by the prepared air cathode and pure zinc plate anode and 6 M KOH electrolytes with 0.2 M zinc acetate solution. The cell voltage polarization, power density curves, and specific capacity for the primary zinc-air battery have been tested. The charge and discharge characteristic and the cycle performance of the resulting zinc-air battery were tested by the BTS-9000 battery test system in a homemade metal-air battery model in air where air was introduced from the backside of the gas diffusion layer.
Material characterization on AgCu alloyed catalysts
Ag50Cu50 catalyst was investigated through comprehensive characterization with scanning electron microscope (SEM), transmission electron microscope (TEM), high-resolution TEM, and high-angle annular dark field (HAADF) carried out on a JEM-2200FS TEM (JEOL Ltd., Akishima-shi, Japan). An empty Ni grid was placed in PLD instrument with nickel foam to directly obtain TEM sample for plain viewing observation.
Results and discussion
Characterization on Ag50Cu50 catalyst
Fast Fourier transform (FFT) analysis has been performed on particle marked with red rectangle in Figure 1b. Based on the inset FFT image, it is believed that the particle has FCC structure with zone axis of . The corresponding IFFT image is shown in Figure 1d. The d-spacing of (111) and (002) planes are measured to be 0.218 and 0.184 nm, respectively, which show contract upon alloying compared to d-spacing of (111)Ag and (002)Ag. The contracted value is around 7.35% for them. Owing to smaller atomic radius of Cu atom (0.12 nm) than Ag atom (0.1445 nm), only when Cu atoms have solute into Ag cell and form Ag-Cu alloy the shrinkage of d-spacing is reasonable. Here, it is of note that TEM used in this work is well calculated, and the deviation is under 2%. Therefore, interference caused by TEM deviation to the conclusion has been excluded. In addition, HAADF result shown in Figure 1c displays that contrast of the particles is brighter than gap area between particles, demonstrating a higher atomic number Z for nanoparticles. The lower Z corresponding gap area then is attributed from Cu element. This is because Z of Cu (Z = 29) is smaller than Ag (Z = 47). Combining the amorphous state in gap area observed in Figure 1b, we can draw that Ag50Cu50 catalyst actually is Ag-Cu alloyed nanoparticles embedded in amorphous Cu film.
We also compare the polarization curve of Ag50Cu50 catalyst to commercial Pt/C (20 wt%) and Ag catalysts on a RDE in O2-saturated KOH solution at 1,600 rpm, as shown in Figure 2d. Here, Ag catalyst was obtained through the same PLD process with Ag50Cu50. The limiting current density of Ag catalyst for ORR is only −2 mA mg−1 at 0.2 V. For Ag50Cu50 catalyst, it displays a significant improvement. The onset potential and limiting current density for ORR are 0.81 V and −12.3 mA mg−1 at 0.2 V, respectively. Pt/C catalyst has also been tested and is shown with black curve. Although there is a difference between Pt/C catalyst and Ag50Cu50 catalyst, it is still believed that the catalyst has excellent catalytic activity for ORR. Moreover, the low cost makes it an excellent candidate to substitute precious Pt-based catalysts.
Zinc-air battery performance
According to all the above electrochemical characterization and the battery performance, the excellent ORR catalytic activity is believed. It is speculated that these are resulting from the alloying between Ag and Cu. Before we verify the speculation, it is necessary to analyze the formation of AgCu alloyed film firstly. In general, whether a metal forms a continuous film or grows into discrete clusters (or nanoparticles) depends on the relative magnitudes of the adhesion energy (E adh) and the metal/vacuum interfacial free energy (γ v/m). If E adh < 2γ v/m, three-dimensional growth is expected to occur via the Volmer-Weber growth mode [43,44]. As Ag satisfies the criterion , Ag nanoparticles have formed at the very beginning of PLD process, and they are showing as tiny amorphous particles at the beginning. In terms of Cu, it is speculated to form as amorphous Cu film using the criterion .
There are two possible ways for Cu atom solute into Ag cell. One is some Cu atoms may be surrounded inside the Ag nanoparticles during the PLD ejecting process. The second way is some Cu atoms included in the amorphous Cu film contacting surface of Ag nanoparticles. At this stage, Ag and Cu atoms mixed randomly within the tiny nanoparticles, and they have not formed the alloyed state. In Figure 4b, we can see that the amorphous particles are smaller than the crystallized ones in size. It indicates that they are in this stage.
After the above stage, the following are then the crystallization and growth for the Ag-rich amorphous nanoparticles. The substrate used in PLD is difficult to be definitely clear and pure, which consequently creates preferred sites promoting the nanoparticles to crystallize. In terms of growth, it may occur through coalescence, because of the relatively short diffusion distances . Eventually, Cu atoms have solute into Ag cell via substituting some of Ag atoms and form into silver-rich AgCu solid solution nanoparticles, that is, AgCu alloyed nanoparticles. With increment of deposit time, the alloying degree between Ag and Cu elevates. However, if the deposit time is too long, AgCu alloyed nanoparticles may grow bigger and bigger, like the huge nanoparticles shown in Figure 4a. The ORR activity will get deteriorated, because the bigger the nanoparticles, the smaller the specific surface area.
According to the formation mechanism of AgCu film, the alloying between Ag and Cu is verified from the aspect of PLD process. In our previous work, it demonstrates that pure Cu material catalyzed ORR barely . Therefore, the enhancement of ORR catalytic activity of Ag50Cu50 film actually is from AgCu alloyed nanoparticles. The synergistic effect between Ag and Cu within the alloyed nanoparticles calculated by simulation is verified through the experiments [47,33]. It is also consistent with the d-bond center theory [48,49]. The theory aims to design a highly active catalyst for ORR via synthesizing a binary or multi-alloy with various metals which have different absorption and desorption energy for oxygen . Ag has the strong oxygen desorption energy, yet the Cu has forceful absorption energy. As a result, when Ag and Cu are formed into AgCu alloy or bimetallic nanomaterials, the product will improve the ORR activity to a great extent owing to the synergistic effect between Ag and Cu atoms.
In addition, the enhancement in ORR activity of Ag50Cu50 over Ag catalyst also can be explained by the ligand effect and strain effect. Regarding the ligand effect, it actually originates from the random Cu atoms solute into (near) surface of structure of various AgCu catalysts. In terms of the strain effect, it could be caused by the shorter Ag-Ag bond length in Ag50Cu50 catalyst than Ag catalyst, given the fact that d-spacing of Ag50Cu50 planes were smaller than the corresponding Ag planes. The two effects both improve the catalytic activity of the catalyst, which can weaken Ag-O binding energies and strengthen the O2 molecular adsorption to the surface of AgCu solid solution .
We demonstrate a design of Ag-Cu film catalyst via PLD. The obtained Ag50Cu50 film shows excellent ORR catalytic activity. TEM results verify the catalyst actually is AgCu alloyed nanoparticles embedded in amorphous Cu film. The alloying between Ag and Cu from the embedded AgCu nanoparticles synergistically improves the ORR catalytic activity. Using the catalyst to assemble a zinc-air battery, the maximum power density is 67 mW cm−2 in primary zinc-air battery. The resulting rechargeable zinc-air battery shows low charge-discharge voltage polarization of 1.1 V at 20 mA cm−2 and high stability over 100 charge and discharge cycles in natural air.
This study was supported by the National Natural Science Foundation of China (grant nos. 51271148 and 50971100), the Research Fund of State Key Laboratory of Solidification Processing in China (grant no. 30-TP-2009), the Aeronautic Science Foundation Program of China (grant no. 2012ZF53073), and the Doctoral Fund of Ministry of Education of China (grant no. 20136102110013)
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