Prussian Blue Analogues Derived CuO@Co3O4 Nanoparticles As Effective Oxygen Reduction Reaction Catalyst For Magnesium-Air Battery


 Developing efficient, durable and cost-effective non-noble metal catalysts for oxygen reduction reaction (ORR) is remarkably significant to promote the efficiency and performance of Mg-air battery. Herein, the CuO@Co3O4 nanoparticles are synthesized by a low-cost and simple approach using the CuCo-based prussian blue analogues (PBA) as precursor of pyrolysis at different calcination temperatures. It is found that the CuO@Co3O4 nanoparticles calcined at 600 ºC (CCO-600) have relatively small size and superior ORR performance. The onset potential is 0.889 V and the diffusion limiting current density achieves 6.746 mA·cm-2, as well as prominent stability in 0.1 M KOH electrolyte. The electron transfer number of the CCO-600 is 3.89 under alkaline medium, which manifests that the reaction mechanism of ORR is dominated by 4 e process. It is similar to the commercial Pt. Moreover, the primary Mg-air battery with the CCO-600 as the cathode catalyst has been assembled. The Mg-air battery of the CCO-600 as the cathode catalyst possesses better discharge performance than the CuCo-based PBA. The open circuit voltage of CCO-600 arrives 1.76 V and energy density of 1895.95 mWh/g. This work provides an effective strategy to develop non-noble metal ORR catalyst for the application of metal-air batteries.

oxides [31,32]. Transition metal composite oxides are one of the most promising candidate for non-noble metal ORR catalysts due to the wide potential window, good electrochemical redox activity, outstanding stability and low cost [19,33,34]. However, the poor electrical conductivity severely restricts the ORR activities [35]. The high conductivity carbonaceous materials are generally used as supports to improve their conductivity and enhance the ORR activity [33]. Furthermore, the mixed metal oxides can elevate the ORR activity compared with single metal oxides, which attributes to the presence of multi valence states, more active sites and synergistic effect between different metals [35][36][37][38]. It is a simple and effective strategy to synthesize mixed metal oxides using PBA as precursor. But there are few studies on the synthesis of mixed metal oxides as ORR catalysts using PBA precursor.
In this work, the CuO@Co 3 O 4 composite oxide was successfully synthesized by calcining the CuCo-based PBA precursor in the air. The CuO@Co 3 O 4 nanoparticles display better ORR activities in alkaline and neutral medium compared with the CuCo-based PBA. The experimental results suggest that the electrochemical performance of CCO-600 is competitive to the commercial Pt catalyst in alkaline electrolyte among these as-prepared catalysts in this study. The primary Mg-air battery was assembled with the CCO-600 as the cathode catalyst and showed excellent discharge performance. This work provides a facile and straightforward method to design transition metal composite oxides as e cient non-noble metal ORR catalyst, which is also worthy to explore the potential application of these catalysts in metal-air batteries.

Preparation of CuO@Co 3 O 4 nanoparticles
The CuCo-PBA samples were calcined at different temperatures to obtain a series of CuO@Co 3 O 4 nanoparticles. The CuCo-PBA nanocubes were pyrolyzed in a mu e furnace with a heating rate of 5 ºC·min −1 to 400 ºC, 600 ºC and 800 ºC for 2 h in the air to obtain CuO@Co 3 O 4 nanoparticles. The corresponding samples were denoted as CCO-400, CCO-600 and CCO-800, respectively.

Materials characterization
X-ray diffraction (XRD, Rigaku D/max 2500PC, Cu Kα radiation) was performed to characterize the crystalline phase and chemical composition of samples. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) spectra were conducted to explore the chemical elements and surface valence state of as-prepared catalyst. The XPS spectra of catalyst were corrected by the binding energy of C 1s at 284.8 eV. The morphology and structure of as-prepared catalysts were observed by eld emission scanning electron microscope (SEM, JSM-7800F) and transmission electron microscopy (TEM, FEI Talos F200S G2).

Electrochemical measurements
The electrochemical performance of as-prepared catalysts was determined with an electrochemical workstation (RST, 5100F) equipped with a rotating disk electrode (RDE) device under identical test condition. A classic three-electrode con guration was conducted to the electrochemical measurements, making up of Pt electrode, Ag/AgCl (saturated KCl solution) electrode and catalyst ink supported glassy carbon (GC) electrode (5 mm in diameter), as counter electrode, reference electrode and working electrode, respectively. . Besides, the electrochemical impedance spectroscopy (EIS) was conducted with the frequency from 0.01 to 1000 kHz at an amplitude of 5 mV. All the electrochemical test results were not corrected by IR compensation in this work.

Mg-air battery
In order to evaluate the application potential of as-synthesized catalysts in metal-air battery, a primary Mg-air battery was made up of self-assembly air cathode, electrolyte and metal anode. The air cathode was constituted of gas diffusion layer, conductive collector layer and catalyst layer. The hydrophobic carbon paper (YLS-30T) was used as gas diffusion and conductive collector layer. As for catalyst layer, the catalyst, Ketjen black, NaHCO 3 and PVDF were dispersed in N-N dimethylamide solution with a mass ratio of 6:1:1:2 to form a homogenous slurry. The slurry was uniformly coated on carbon paper and dried at 80 ºC 12 h to obtain the air cathode. 3.5 wt% NaCl solution was served as the electrolyte and AZ31 Mg alloy as anode.
The test mold of Mg-air battery is 6.0 cm high, 7.0 cm wide, the distance between the cathode and anode is 1.4 cm, the liquid capacity is 12 mL, and the effective exposure area is 1.0 cm 2 . All Mg-air battery tests were conducted on RST 5100F electrochemical workstation and Neware battery test systems at room temperature and without additional oxygen.

Structural and morphological characterization
The CuCo-PBA and CuO@Co 3 O 4 were synthesized with a simple chemical precipitation method and subsequent calcination. The crystalline characterizations of as-obtained samples were conducted by XRD analysis and the corresponding XRD patterns are displayed in Fig. 1. As can be seen from Fig The chemical constitution and elemental electronic state of CCO-600 were further employed by XPS measurement. The XPS spectra are displayed in Fig. 2. The full survey scan spectrum of CCO-600 is exhibited in Fig. 2(a) and con rmed the existence of Cu, Co, O and C elements without other elements. All the peaks of elements take C 1s peak (284.6 eV) as the reference. Fig. 2(b) represents the Cu 2p spectrum and shows two feature peaks at 933.4 and 953.4 eV, ascribing to the Cu 2p 3/2 and Cu 2p 1/2 . Besides, there are two obvious satellite peaks locating at 935 and 955 eV corresponding to the Cu 2+ , due to the existence of pure CuO [20,41]. As revealed in Fig. 2 To observe the morphology and structure of as-obtained samples, the SEM and TEM were further conducted. Fig. 3(a-c) display the CuCo-PBA nanocubes with an average dimension around 1 µm at different magni cations. Fig. 3(d-f) reveal the morphology of CCO-400, CCO-600 and CCO-800, respectively. It is obvious that the smaller CuO@Co 3 O 4 nanoparticles were obtained from the CuCo-PBA nanocube precursors calcining at different temperatures. The average size of CCO-400, CCO-600 and CCO-800 arrive at around 200, 100 and 500 nm, severally. Transformation of morphology from nanocube to nanoparticle caused by calcination could be explained by the evaporation of lattice water. It is known to all that the smaller the average size of the catalyst, the more active sites it can provide. The CCO-600 has the smallest average size among those catalysts, which can offer larger speci c surface area and increase the contact interface between electrode and electrolyte, boosting the ORR electronic and mass transfer process [30]. TEM image in Fig. 3(g) further illustrates that the average size of as-obtained CCO-600 nanoparticles are about 100 nm, which consist with the SEM results. The high-resolution TEM (HRTEM) and selected area elected diffraction (SAED) are shown in Fig. 3(h) [20,36,43], separately. The inset gure in Fig. 3(h) reveals the SAED pattern of the CCO-600, indicating a polycrystalline structure of catalyst. Furthermore, the TEM image and elemental mapping analysis are exhibited in Fig. 3(i) and con rmed the existence of Cu, Co and O elements and uniform distribution. The phenomenon is consist with the XPS survey spectrum consequence.

Electrochemical characterization for ORR
To evaluate the ORR performance of as-obtained catalysts, the cyclic voltammetry (CV) measurements were preliminarily conducted in 0.1 M KOH electrolyte saturated by O 2 and N 2 at a scan rate of 20 mV·s −1 [30]. The corresponding CV curves of catalysts are illustrated in Fig. 4 and Fig. 5. As can been seen from the Fig. 5, all CV curves display signi cant oxygen reduction peaks in the O 2 -saturated electrolyte, while no noticeable reduction peaks are found in the N 2 -saturated electrolyte solution. The results con rm that these catalysts possess oxygen reduction activity in O 2 -saturated 0.1 M KOH solution [27]. Fig. 4 summarizes the CV curves of all catalysts under O 2 saturation condition, and it is found that the oxygen reduction peaks of CCO-400, CCO-600 and CCO-800 are more positive than CuCo-PBA. In particular, the CV curve of CCO-600 demonstrates the most positive reduction peak potential (about 0.66 V), indicating the CCO-600 may possess best ORR activity among these catalysts [25].
The linear sweep voltammetry (LSV) tests were performed to further explore ORR activities of catalysts by RDE in O 2 -saturated 0.1 M KOH solution [27,30]. Fig. 6(a-d) reveals the LSV curves of CuCo-PBA, CCO-400, CCO-600 and CCO-800 at various rotating rates (400 to 2000 rpm) with a scan rate of 5·mV s −1 (0.2 to 1.2 V vs RHE). With the increase of rotating rate, the diffusion limiting current density of catalysts constantly enhances because of shortening of the diffusion distance and promoting mass transport rate at GC electrode surface, signifying the ORR process of catalysts follows a rst-order kinetic process [26,27]. The onset potential (E onset ) is denoted as the potential at the current density of 0.10 mA·cm −2 in this work. Fig. 6(e) displays the LSV curves of these catalysts and commercial Pt at 1600 rpm in O 2 -saturated electrolyte. The onset and half-wave potentials of as-prepared CCO-600 (0.889 V, 0.665 V) are visibly higher than those of the CuCo-PBA (0.759 V, 0.620 V), CCO-400 (0.826 V, 0.654 V) and CCO-800 (0.771 V, 0.618 V), evidencing the ORR activity of CCO-600 outperform other catalysts. Besides, the onset potential of CCO-600 is 0.889 V, which is merely 57 mV more negative than commercial Pt. Moreover, the CCO-600 also reveals an outstanding limit current density of 6.746 mA·cm −2 at 0.164 V compared with CuCo-PBA (4.311 mA·cm −2 ), CCO-400 (4.627 mA·cm −2 ) and CCO-800 (5.331 mA·cm −2 ). It deserves to note that the limit current density of CCO-600 (6.746 mA·cm −2 ) at 0.164 V exceeds that of the commercial Pt (5.824 mA·cm −2 ), which can be attributed to the synergistic effect between CuO with Co 3 O 4 and a uent active sites [32,[35][36][37]. The above results illustrates the CCO-600 is the most excellent ORR catalyst among those electrocatalysts and is accord with the CV results.
To further investigate the ORR reaction mechanism of as-obtained catalysts, the corresponding Koutecky-Levich (K-L) curves at various potentials were drew from the LSV plots at different rotate speeds [17,24,26]. Fig. 6(f) and (g) show the K-L curves of CuCo-PBA and CCO-600, severally. The K-L curves of CCO-600 exhibit good linearity and almost identical slopes from 0.2 to 0.5 V vs RHE compared with CuCo-PBA, suggesting a steady kinetic reaction process and rst-order reaction dynamic mechanism [37,44]. Furthermore, the transfer electron number (n) is a crucial index to estimate the ORR activity of asobtained catalysts and can be calculated by the K-L equations as follow: (1) B = 0.2nFD 2/3 υ -1/6 C (2) where j and j k refer to the observed current density and the kinetic current density severally. ω represents the rotation rate, n expresses the transfer electron number. four-electron reaction pathway and the catalytic mechanism is equivalent to the commercial Pt catalyst [27]. In addition to the catalytic activity of ORR, the cyclic stability of the catalyst is also an important indicator [32,45]. As revealed in Fig. 6(i), the CCO-600 demonstrates outstanding durability and stability in 0.1 M KOH solution. Its half-wave potential merely has a negative shift of 13 mV after 5000 cycles. The above results indicate that CCO-600 catalyst has tremendous application potential as a non-noble metal ORR catalyst.
The electrochemical impedance spectroscopy (EIS) measurements were also carried out at 0.60 V to demonstrate the different ORR kinetics of as-prepared catalysts. The Nyquist impedance spectra of catalysts were revealed in Fig. 7, which are all comprised of the semicircle and sloping line at high and low frequency region, severally. The diameter of the semicircle can represent the value of the interface charge transfer resistance (R ct ) and the slope of the line can represent the solution resistance (R s ) [32,45]. The R ct and R s of CCO-600 catalyst are smallest among those catalysts as seen from in Fig. 7, manifesting the highest electron transfer capacity and the fastest electrical kinetics for CCO-600 catalyst during ORR [36,46]. The main reasons are attributed to the synergistic effect between CuO with Co 3 O 4 and the speci c surface area of CCO-600 catalyst. The synergistic effect of catalyst can facilitate the electron transfer and the large speci c surface area can provide su cient contact between the catalyst and the electrolyte, leading to the smaller impedance.
In this work, the ORR activity under neutral condition (3.5 wt% NaCl electrolyte) was also measured to explore the potential application of CCO-600 catalyst as cathode catalyst for Mg-air batteries. Fig. 8 exhibits the CV curves in O 2 -saturated NaCl solution. The reduction peaks of CuCo-PBA, CCO-400, CCO-600 and CCO-800 locates at 0.352 V, 0.365 V, 0.370 V and 0.363 V severally, and the CCO-600 possesses the most positive potential which demonstrates the best ORR activity. Figure 9(a-d) represent the LSV curves of different catalysts in 3.5 wt% NaCl solution at various rotate speeds with a scan rate of 5 mV s −1 (0.0 to 1.0 V vs RHE). The diffusion limiting current density is constantly enhanced with the increasing of rotating speeds which also demonstrates the ORR pathway of catalyst is controlled by kinetic [27,44]. Figure 9(e) displays the LSV curves of as-obtained catalysts in NaCl solution at 1600 rpm. The onset and half-wave potentials of CCO-600 (0.560 V, 0.376 V) exceed CuCo-PBA (0.508 V, 0.347 V), CCO-400 (0.541 V, 0.364 V) and CCO-800 (0.534 V, 0.355 V), certifying the ORR performance of CCO-600 is better than other catalysts. Moreover, the stability and durability of CCO-600 were performed in Figure 9(f) and half-wave potential only has a minus shift of 20 mV after 5000 cycles. The above results illustrate the CCO-600 still has the best ORR catalytic activity in 3.5 wt% NaCl electrolyte and may be an outstanding cathode catalyst for Mg-air batteries.

Application in Mg-air battery
The CCO-600 catalyst may have great potential as a cathode catalyst for Mg-air battery due to excellent ORR performance. A primary Mg-air battery was assembled with 1 mm thick AZ31 Mg alloy plate as anode, 3.5 wt% NaCl as electrolyte and self-assembled cathode. The CuCo-PBA catalyst was also applied to air cathode under the same condition for comparison. Fig. 10 The working voltage of batteries continuously decreases following the increase of current density which ascribes to ohmic polarization [14,46]. The average discharge voltages of CuCo-PBA arrive around 1.19 V, 1.07V and 0.96 V and the CCO-600 maintains around 1.25 V, 1.13 V and 1.02 V at the current densities of 1.0 mA·cm −2 , 5.0 mA·cm −2 and 10.0 mA·cm −2 , respectively. The power density, speci c capacity density and energy density of primary batteries assembled with two catalysts were calculated by the effective anode quality and summarized in Table 1. As can been seen, the discharge performance of the batteries assembled by CCO-600 is better than that of the batteries assembled by CuCo-PBA. It is also proved that the CCO-600 has a better ORR performance than the CuCo-PBA, which is consistent with the previous ORR performance test results. In addition, the stability and durability of cathode catalyst are very crucial indexes to in uence the service life of Mg-air batteries [13]. As the Fig. 10(b) depicted, the Mg-air batteries assembled with CCO-600 have extremely stable discharge plateau and higher operating voltage at different current densities compared with the CuCo-PBA, which reveals greatly outstanding discharge behavior, demonstrating its excellent catalytic stability and durability. The above results further suggest that the better ORR catalytic activity of CCO-600 can improve the Mg-air battery performance compared with CuCo-PBA. Hence, the CCO-600 has been proofed to be an effective non-noble metal catalyst in the practical application of Mg-air battery and provides a very simple method for the synthesis of the non-noble ORR electrocatalysts.

Conclusion
A facile and effective strategy has been applied to synthesize CuO@Co 3 O 4 nanoparticles by calcining CuCo-PBA nanocubes and used as highly active cathode catalyst for Mg-air battery. The electrochemical measurements manifest the CCO-600 catalyst possesses the most excellent catalytic activities toward ORR compared with others catalysts in this work whether in alkaline or neutral electrolyte. The onset potential of CCO-600 is 0.889 V which is merely 57 mV more negative than commercial Pt and the diffusion limiting current density of CCO-600 (6.746 mA·cm −2 ) even surpasses the commercial Pt (5.824 mA·cm −2 ) in alkaline medium. The half-wave potential has only 13 and 20 mV negative deviation after 5000 cycles in alkaline and neutral electrolytes, respectively. The CCO-600 catalyst reveals superior ORR activity and outstanding stability in both alkaline and neutral electrolytes which attributes to the synergistic effect between CuO with Co 3 O 4 and large speci c surface area exposing more active sites.
The Mg-air battery assembled by the CCO-600 catalyst as cathode catalyst exhibits excellent discharge performance such as high open circuit voltage, operating voltage and good energy density, demonstrating the potential for practical application. This work provides a promise strategy to synthesis non-noble metal ORR catalyst for the development of metal-air batteries. Figure 1 XRD patterns of the as-obtained catalysts.      The Nyquist plots of EIS at 0.6 V of as-prepared catalysts.

Declarations
Page 20/22 Figure 8 CV curves of the as-prepared catalysts in O2 saturated 3.5 wt% NaCl.