N, S, P-Codoped Graphene-Supported Ag-MnFe 2 O 4 Heterojunction Nanoparticles as Bifunctional Oxygen Electrocatalyst with High Efﬁciency

: Due to slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during discharging and charging processes, it is essential to rationally design and synthesize non-precious metal bifunctional electrocatalysts with good performance for metal-air batteries. Herein, Ag-MnFe 2 O 4 heterojunction nanoparticles supported on N, S, P-codoped graphene (NSPG) are developed with enhanced ORR and OER bifunctional electrocatalytic activities and stability. In contrast, S, P-doped graphene (SPG) and N, P-doped graphene (NPG) show less stabilization for the heterojunction particles. For example, under alkaline conditions, the ORR half-wave potential of Ag-MnFe 2 O 4 /NSPG can reach 0.831 V, and the over potential for OER is 0.56 V at the current density 10 mA · cm − 2 . Furthermore, Ag-MnFe 2 O 4 /NSPG shows better methanol resistance and durability than Pt/C catalysts.


Introduction
Design and synthesis of efficient bifunctional oxygen electrocatalysts are of critical importance but challenging for large-scale implementations of Zn-air batteries [1]. Recently, silver and metal oxides were reported as potential alternative candidates for Pt-based catalysts [2,3]. After loading onto carbon supports, these relatively inexpensive candidates can promote both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The combination of silver and metal oxide nanocrystals can improve their electrical conductivity and bifunctional activity due to direct or indirect coupling and electron transfer between them. For example, the combination of Mn-oxides with Fe, N-codoped carbon can scavenge hydrogen peroxide for ORR [4]. Different methods have been established to synthesize N-doped carbon materials from CO 2 [5] and biomass materials [6]. An increased surface nitrogen content was identified as one of the reasons for excellent activity [7]. However, the volume-specific activity and electronic conductivity of these candidates still need to be improved.
Graphene (G) has been explored as non-metal catalyst and support for electrochemical reactions. Pristine graphene has no catalytic activity [8], but chemical doping of heteroatoms into graphene lattices could transfer nearby carbon atoms into "active sites". Additionally, defects including single C atom vacancy, Stone-Wales defects, and grain boundaries can modify electronic structures [9]. For example, sole-doped carbons, dualdoped carbons (N/S [10,11], N/B [12], and N/P [13,14]), and triple-doped carbon materials (N/P/S [15]) demonstrated improved electrochemical properties. The introduction of N and/or S atoms could increase spin polarization and charge density of adjacent carbon atoms to enhance electrocatalytic activities [16,17]. P-doped graphene could provide active sites to facilitate the formation and dissociation of OOH, the rate-limiting step of ORR [18]. Furthermore, duel-doped and tri-doped graphene demonstrates even better performances as an electrocatalyst than single-doped graphene due to synergetic effects among different heteroatoms [9,19]. However, catalytic activities of doped graphene are still not comparable to Pt/C.
In addition, these dopped graphene can also be utilized to support and stabilize metalcontaining catalysts. According to the hard-soft-acid-base theory, the binding ability of hard base decreases along with its reducing electro-negativity [20]. Generally, silver is a soft acid [21], while sulfur is a soft base. The affinity interaction between silver and sulfur helps stabilize the heterostructure of catalysts. Similarly, the affinity interaction between iron and nitrogen in carbon materials improves the performance of oxygen electrocatalysts [22,23]. There are various bifunctional catalysts, including Co 3 O 4 @ graphene [24], Co 3 O 4 /N-doped graphene [25], and CoFe 2 O 4 /graphene aerogel [26]. Graphene supports are believed to promote electron transfer and facilitate mass transport of reactants to electrocatalysts. It is possible to boost the performances of metal heterojunction particle electrocatalysts by the supporting of multi-doped graphene.
Herein, S, P-co-doped graphene (SPG), N, P-co-doped graphene (NPG), and N, S, P-multi-doped graphene (NSPG) are synthesized and utilized as supports for Ag-MnFe 2 O 4 heterojunction nanoparticles. Compared with SPG and NPG, NSPG shows better stabilization for the heterojunction structure. As a result, Ag-MnFe 2 O 4 /NSPG demonstrates better electrocatalytic performance and higher catalytic activity for ORR and OER.

Results and Discussion
XRD analyses were conducted to identify crystallographic structures of carbon-based materials and transition metal oxide NPs. As shown in Figure 1a, there is a typical diffraction peak around 26 • , corresponding to the (002) plane of graphene. It indicates that various oxygen-containing functional groups on graphene oxide disappear to form graphene after calcination [27], and NPG, SPG, and NSPG have good structures. The      Table 1, the contents of N 1s and S 2p in Ag-MnFe 2 O 4 /NSPG are lower than N 1s in Ag-MnFe 2 O 4 /NPG and S 2p in Ag-MnFe 2 O 4 /SPG, respectively. Doping elements and metal nanoparticles can provide defect locations, and electron transfer between them can offer large number of ORR catalytically active sites to easily adsorb oxygen and greatly improve catalytic activity [29]. In addition, the combination of N and Fe and the effect of doping P on C and N have been shown to improve the ORR activity. Therefore, Ag-MnFe 2 O 4 /NSPG can have higher catalytic activities than other samples.   To evaluate ORR catalytic performances of obtained composites, cyclic voltammetry (CV), and linear sweep voltammetry (LSV) measurements were performed in O 2 -or N 2saturated 0.1 M KOH solution using a rotating disk electrode (RDE). Some important data of ORR catalytic performances are shown in Table 3. Figure 6a shows CVs of the three samples to explore their ORR catalytic activities. There are obvious reduction peaks appear for catalysts in O 2 -saturated electrolyte. The reduction peaks between 0.80 V and 1.10 V correspond to the reduction of AgO to Ag, while the peaks in the range of 0.60 V to 0.80 V are assigned to ORR catalytic activities. The ORR peak potentials of Ag-MnFe 2  indicating that Ag-MnFe 2 O 4 /NSPG has superior ORR activities. As reported, Ag-MnFe 2 O 4 nanoparticles supported on N, S-co-doped graphene was detected to be 0.824 V of halfwave potential in LSV [30]. This clearly shows that the doping of graphene with N, S, and P can significantly improve electrocatalytic activities of Ag-MnFe 2 O 4 toward ORR. Co-doping of NS, NP, SP, or NSP in graphene can produce asymmetric spin or charge density, resulting in more active sites and higher catalytic activity than single doping catalysts [31]. The results also show that NSPG can significantly improve electrocatalytic performances of Ag-MnFe 2 O 4 . The electrocatalytic performances of different catalysts are shown in Figure 7.  In order to clarify redox kinetic characteristics and catalytic pathway of Ag-MnFe 2 O 4 /NSPG, LSV measurements were conducted (Figure 6c). Limit current density rises with increasing electrode rotation rate, enabling more sufficient O 2 arrive on the surface of electrodes, which means higher conversion and faster diffusion kinetics for the catalyst. The corresponding K-L plots (Figure 6d) show an approximate linear relationship at different potentials, indicating the first-order reaction kinetics for ORR. The numbers of electron transfer for different samples during the ORR pathway are given in Figure 6e  To investigate the durability and methanol crossover effect of Ag-MnFe 2 O 4 /NSPG and Pt/C, chronoamperometric tests were carried out in O 2 -saturated 0.1 M KOH. As shown in Figure 8a, Ag-MnFe 2 O 4 /NSPG catalyst displays 13.2% decay after 15,000 s, while Pt/C shows 29.1% decay. Current versus time (I-t) plots along with methanol addition are shown in Figure 8b. As 5 mL methanol is added into electrolyte at 400 s, the ORR current of Ag-MnFe 2 O 4 /NSPG has no significant change, indicating that Ag-MnFe 2 O 4 /NSPG catalyst has high catalytic selectivity for methanol. In comparison, Pt/C shows sharp current change due to the oxidation of methanol. It demonstrates the better methanol tolerance of Ag-MnFe 2 O 4 /NSPG.

Material Synthesis
Synthesis of Ag-MnFe 2 O 4 NPs. Ag-MnFe 2 O 4 nanoparticles (NPs) were synthesized as reported [32]. Then, 1.0 mL oleic acid, 1.0 mL oleylamine, and 10 mL 1-dodecanol were mixed with stirring and heated to 220 • C. Then, 120 mg silver acetate was added and reacted for 1 h. The mixture was then cooled down to 200 • C. Next, 120 mg ferric acetylacetonate and 60 mg manganic acetylacetonate were subsequently added and reacted for another 1 h. The mixture was heated up to 220 • C and reacted 2 h. Finally, the mixture was cooled to room temperature. Products were collected and washed three times with ethanol.
Synthesis of NPG/SPG/NSPG. NPG, SPG, and NSPG were synthesized by similar route to NG [30]. Graphene oxide (GO) was synthesized by the Hummers' method [33]. Then, 100 mg GO was dispersed in 100 mL deionized water with ultra-sonication for 4 h to achieve a uniform solution. Then, 50 mg melamine and 50 mL phytic acid were slowly added into the dispersion. After 1 h ultra-sonication, the suspension was maintained under stirring at 90 • C for 2 h. After centrifugation with water, the precipitate was freeze-dried under vacuum. Finally, the powder was calcinated at 700 • C for 4 h at a heating rate of 5 • C min −1 with a flow of Ar gas, denoted as NPG. SPG and NSPG were also synthesized with similar routes except for the replacement of melamine with 50 mg dibenzyl disulfide for SPG, 25

Material Characterizations
Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopic (XPS), and Raman analyses of the samples were carried out as reported [30]. The content of nanoparticles in Ag-MnFe 2 O 4 /NSPG was determined by dissolving 0.25 g nanocomposite in 20 mL 5.0% HNO 3 solution for 2 h at room temperature and another 6 min at 80 • C, and then diluted to 100 mL with ultrapure water. Finally, elemental concentrations were measured by PerkinElmer Avio 200 ICP-OES.

Electrochemical Measurements
Ink of catalyst sample including 4.0 mg catalyst powder, 1.0 mg carbon black, 800 µL water, 200 µL isopropanol, and 20 µL Nafion (5%) was ultrasonicated for 1 h. Then, 3 µL ink was pipetted onto a glassy carbon disk. The ORR and OER performances were then studied with the rotating disk electrode (RDE) technique using the ALS RRDE-3A and CHI 760D system as reported [30].

Conclusions
In summary, heterojunction Ag-MnFe 2 O 4 nanoparticles have been synthesized and supported on various doped graphenes, including NPG, SPG, and NSPG. Among these composites, Ag-MnFe 2 O 4 /NSPG demonstrate superior ORR and OER electrocatalytic performances, excellent durability and methanol resistance. ORR half-wave potential of Ag-MnFe 2 O 4 /NSPG reaches 0.831 V, which exceeds that of commercial Pt/C (0.827 V). When the OER current density is 10 mA·cm −2 , the potential of Ag-MnFe 2 O 4 /NSPG is 1.79 V, which also exceeds that of Pt/C (1.97 V). The multi-doping also stabilizes the loading of metal particles, so that Ag-MnFe 2 O 4 /NSPG shows good methanol resistance and excellent durability. After 14,000 s, the current loss is only reduced by 13.2%, much lower than that of Pt/C (29.1%). This indicates that metal particles/doped graphene composites have great application potentials and good prospects in metal-air batteries.