Sponge Effect Boosting Oxygen Reduction Reaction at the Interfaces

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Energy, Environmental, and Catalysis Applications

Sponge Effect Boosting Oxygen Reduction Reaction at the Interfaces between Mullite SmMn2O5 and Nitrogen-Doped Reduced Graphene Oxide Meng Yu, Li Wang, Jieyu Liu, Hui Li, Xiuyao Lang, Chunning Zhao, Zhanglian Hong, and Weichao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04451 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 27, 2019

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ACS Applied Materials & Interfaces

Sponge Effect Boosting Oxygen Reduction Reaction at the Interfaces between Mullite SmMn2O5 and Nitrogen-Doped Reduced Graphene Oxide Meng Yu†, ‡, Li Wang†, ‡, Jieyu Liu†, Hui Li†, Xiuyao Lang†, Chunning Zhao†, Zhanglian Hong#, and Weichao Wang†, §, * † Department of Electronics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin, 300350, China # State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, No. 38 Zheda Road, Hangzhou, 310027, China § Renewable Energy Conversion and Storage Center, Nankai University, Tianjin, 300350, China ‡ These authors contributed equally.

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KEYWORDS: sponge effect, SmMn2O5, N-doped graphene, bidentate adsorptions, Zn-air batteries

ABSTRACT

Exploring the effect of interfacial structural properties on catalytic performance of hybrid materials is essential in rationally designing novel electrocatalysts with high stability and activity. Here, insitu growth of mullite SmMn2O5 on nitrogen-doped reduced graphene oxide (SMO@NrGO) is achieved for highly efficient oxygen reduction reaction (ORR). Combining X-ray photoelectron spectroscopy and density functional theory calculations, interfacial chemical interactions between Mn and substrates are verified. Interestingly, as revealed by charge density difference, the interfacial Mn-N(C) bonds display a sponge effect to store and compensate electrons to boost the ORR process. In addition, bidentate adsorptions of oxygen intermediates instead of monodentate ones are observed in hybrid materials, which facilitates the interactions between intermediates and active sites. Experimentally, the hybrid catalyst SMO@NrGO exhibits a half-wave potential as high as 0.84 V, being comparable to benchmark Pt/C and higher than that of the pure SMO (0.68 V). The Zn-air battery assembled with SMO@NrGO performs a high discharge peak power density of 244 mW cm-2 and superior cycling stability against noble metals.


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Introduction Exploration of earth-abundant and chemically stable cathode materials with high electrocatalytic activity for oxygen reduction reaction (ORR) to replace the noble metal catalysts is crucial to ensure the high performance of fuel cells and metal-air batteries.1-5 Among all the nonplatinum electrocatalytic materials, ternary ceramic oxides (TCOs) ABxOy with general properties such as high melting temperature and thermal stability like perovskites and spinels (where A denotes a rare-earth or alkaline earth cation and B a transition metal cation), have attracted great attention because of their cost efficiency, high intrinsic activities and superior stability compared to their binary metal oxide counterparts.6-8 The catalytic behavior of TCOs is highly dependent on the B site elements and their coordination, and the existence of A sites primarily increases the structure stability.4 As a result, the activity is open to optimization by tuning the electronic structures of B site elements.9 Nevertheless, most TCOs are insulators or semiconductors with poor electron conductivity, which largely limits the further enhancement of their performance in electrocatalysis. One of the effect schemes is to develop hybrid materials composed of TCOs and conductive carbonaceous substrate.10-12 In addition to facilitating the interfacial electron transfer, the performance enhancement of hybrid materials is generally attributed to the “synergic effect”. Even though previous research has shown that metal-nitrogen-carbon (M-N-C) bonds at interfaces are of vital importance for the high electrocatalytic activity, the fundamental boosting mechanism along with the effect of M-N-C bonds onto the interfacial electronic structures is still under debate.13-15 In order to reveal the interfacial boosting effect in hybrid electrocatalysts, a series of samples were developed by in-situ growth of Mn-based mullite SmMn2O5 on reduced graphene oxide with


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or without nitrogen doping (SMO@rGO or SMO@NrGO). Different from perovskites, two Mn crystal fields coexist in Mn-based mullite, i.e., the Mn-centered pyramid (Mnpyr3+) and octahedron (Mnoct4+).16-18 However, only Mnpyr3+ satisfies the unit occupancy of the dz2 orbital around Fermi level, leading to a potential catalytic activity, while the other half of Mn species (Mnoct4+) is catalytically inert.19, 20 This theoretical insight has been demonstrated by our previous work, in which the exposure of more Mnpyr3+ active sites through morphology control would effectively improve the ORR performance.21 However, the promotion was still limited, probably owing to the poor electron conductivity of SmMn2O5 relative to perovskites.22-24 Via interfacial engineering with graphene substrate, significant conductivity enhancement of the hybrid materials was achieved. Charge transfer between SMO and (N)rGO was verified by combining X-ray photoelectron spectroscopy (XPS) and charge density difference based on density functional theory (DFT) calculations. Partial Mn4+ was transformed to catalytically active Mn3+. Extra electrons, which were mainly contributed by the neighbouring C atoms, tended to accumulate and then be stored at the Mn-N(C) bonds. Interestingly, a “sponge effect” was observed during the ORR process. As the electrons were consumed by reaction intermediates (e.g. replacement of OH* by OO*), the interfacial Mn-N(C) “sponge” allowed the releasing of electrons. After that, the depleted charges could be complemented through the conductive graphene substrate. This specific interfacial sponge effect, which possibly existed in many hybrid systems, could provide a deep insight into understanding the synergic effect between metal ions and carbonaceous substrates. Besides, DFT calculations demonstrated the formation of bidentate adsorptions of oxygen intermediates rather than monodentate ones at the interfaces between SMO and NrGO, which optimized the interactions between reaction intermediates and hybrid catalysts. Instead of graphite or pyrrolic N, Mn-pyridinic N-C bonds were recognized as the primarily active sites for the high


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ORR activity. Experimentally, a half-wave potential as high as 0.84 V was delivered by SMO@NrGO, which was comparable to commercial Pt/C. The specific activity of chemically correlated sample SMO@NrGO at 0.85 V was more than 24 and 300 times of the physically mixed counterpart SMO/NrGO and pure SMO, respectively. In full cell test, the primary Zn-air battery (ZAB) made with SMO@NrGO showed superior discharge performance with a higher peak power density of 244 mW cm-2 than that of Pt/C (170 mW cm-2). Rechargeable ZABs based on SMO@NrGO performed excellent cycling stability with only 50 mV increase of charge-discharge platform after 100 cycles, outperforming that of a state-of-art noble metal-based battery. Results and Discussion Materials Characterizations The in-situ growth of mullite SmMn2O5 on reduced graphene oxide (SMO@rGO-x, where x denoted the feeding ratio of SmMn2O5 to GO) was achieved through one-step hydrothermal method (section 1.1 in the Supporting Information). The X-ray diffraction (XRD) spectra of asprepared samples were provided in Fig. S1. For all the samples with different feeding ratios, the patterns matched well with standard SmMn2O5 (PDF NO. 52-1096) and no obvious impurity peak was detected. A single intensive peak was observed for GO at 11.4°, which was related to the (001) reflection of GO layered structure.25-27 This reflection peak disappeared after combing with SMO, indicating the exfoliation of original GO stack via intercalation, which was a strong evidence of successful in-situ growth.28, 29 As expected, the peak intensity of SMO decreased with decreasing feeding ratio of SMO to GO. The actual mass loadings of SMO@rGO-x were examined with thermogravimetric analysis (Fig. S2), and the results were summarized in Table S1. For example, the mass loading of SMO@rGO-2 based on TGA was 73.9%, slightly higher than 66.7%


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calculated from the feeding ratio. This phenomenon was partially attributed to the loss of GO surface functional groups during hydrothermal reduction process. The corresponding Raman spectra of GO and SMO@rGO-2 were provided in Fig. S3a. Two broad peaks located at 1536 cm-1 and 1596 cm-1 were observed for GO, which were assigned to the structure imperfection (D band) and the vibration of ordered carbon with sp2 hybridization (G band), respectively.30, 31 The integral intensity ratio of D band and G band (ID/IG), calculated from the fitted first-order spectra, was commonly adopted to evaluate the disordered degree.32 The ID/IG of GO and SMO@rGO-2 were 1.38 and 2.09, respectively, indicating increased defects in rGO after combing with SMO. Noticing that a slight blue shift of the G-band of SMO@rGO-2 relative to GO was visualized, possibly elucidating the existence of strain caused by the interaction between SMO and rGO.33 Besides, a shoulder band D* was also recognized, which was probably related to the sp2-sp3 bonds in disordered graphitic lattices.34 The second-order spectrum of GO mainly composed of two peaks ascribed to 2D band (2650 cm-1) and D+G band (2910 cm-1).35 Since the 2D band is related to the number of layers of graphene sheets, its broad shape suggested that the GO used in the experiment was multi-layered. D+G band was also a defect-related feature in the Raman spectrum of GO, which was corresponding to the overtone of D band and G band.36, 37

The reduction of GO could be further realized from the Fourier transform infrared (FTIR)

spectroscopy (Fig. S3b). Clear absorption peaks of surface oxygen functional groups such as CO-C (1105 cm-1) and C=O (1710-1740 cm-1) were observed for GO.26, 38 Those peaks became weakened or even vanished in the spectrum of SMO@rGO-2, while the signals corresponding to C-C (1240-1260 cm-1) and C=C (1570-1590 cm-1) were retained, which was a direct evidence of hydrothermal reduction of GO to rGO.39


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Different from the smooth surface of GO as examined by the transmission electron microscopy (TEM) shown in Fig. S4a, rGO performed wrinkled structure because of the interplanar crosslinking.40 The micro/nanostructures of SMO@rGO-x were provided in Fig. S5. Irregular nanoparticles were distributed randomly throughout the exfoliated layers of rGO, consistent with the XRD spectra. At high loadings (> 85%, i.e. SMO@rGO-5 and SMO@rGO-4), severe aggregation of SMO nanoparticles occurred and part of the accreted nanoparticles were covered inside rGO as visualized from the corners of Fig. S5b. Better dispersion was obtained when reducing the loading to 70% - 80% (i.e. SMO@rGO-3 and SMO@rGO-2). When the mass loading became pretty low (40% for SMO@rGO-1), the visualized concentration of SMO nanoparticles significantly decreased compared to other samples. Furthermore, specific surface areas (SSAs) of the abovementioned products were extracted from nitrogen adsorption-desorption isotherms based on Brunauer-Emmer-Teller (BET) theory (Fig. S6). Interestingly, from pure SMO to SMO@rGO-4, SSA decreased with decreasing feeding ratio of SMO to GO. This phenomenon was probably attributed to the aggregation of SMO nanoparticles caused by the addition of GO, which was agreed with the TEM observations. By further decreasing of metal precursors, the SSA of products increased and finally, high SSAs of 79.4 m2 g-1 and 165.9 m2 g-1 were obtained for SMO@rGO-2 and SMO@rGO-1, respectively. To further optimize the interfacial connection between SMO and carbonaceous substrate, nitrogen was introduced into the composite catalysts as n-type dopant by adding 1 ml ammonium hydroxide during the synthesis of SMO@rGO-2. The as-prepared SMO@NrGO-2 also presented a single phase with impurity signal under detection limit, as shown in Fig. 1a. However, when more ammonium was added, other phases including Mn3O4 (Fig. S7, PDF No. 24-0734) started to emerge because of the strong interaction between Sm3+ and OH- under high OH- concentration as


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demonstrated by our previous research.15 The SSA of SMO@NrGO-2 was measured to be 85.9 m2 g-1, quite similar to that of SMO@rGO-2 (Fig. S6). Fig. S3a also provided the Raman spectrum of SMO@NrGO-2, in which an enhanced ID/IG of 2.18 indicated that the defect ratio in SMO@NrGO-2 was slightly improved relative to SMO@rGO-2 caused by N doping.41 The successful reduction of GO was again realized by FTIR spectrum of SMO@NrGO-2 (Fig. S3b). Similar to the morphology of SMO@rGO-2, the micro/nanostructure of SMO@NrGO-2 revealed by TEM images in Fig. 1b suggested the random distribution of numerous anomalous-shaped nanoparticles all over the substrate. The polycrystalline nature of SMO@NrGO-2 was confirmed by the selected area electron diffraction (SAED) provided in Fig. 1b inset. Diffraction rings belonging to (001), (121) and (112) planes were recognized. The representative high-magnification TEM image in Fig. 1c presented a clear visualization of the carbonaceous substrate and several supported nanoparticles. Elemental mappings (Fig. 1d) demonstrated the uniform and random distribution of nitrogen element all over the NrGO substrate. Strong signals of Sm, Mn and O were detected in the position of nanoparticles, verifying the existence of SMO. In addition, highresolution TEM (HRTEM) image of a single nanoparticle marked in the white dash box in Fig. 1c was provided in Fig. 1e. Two sets of crystal planes with lattice spacing of 0.309 nm/0.252 nm and intersection angle of 32.1° were ascribed to (121) and (112) planes of SMO, respectively. To reveal the interaction between supported SMO and carbonaceous substrate, XPS was conducted for SMO@rGO-x and SMO@NrGO-2. The XPS spectra of physically mixed SMO and NrGO with ~73 wt% SMO (SMO/NrGO-2) were also recorded for reference. All the spectra were calibrated with C 1s peak located at 284.5 eV. High-resolution Sm 3d and Mn 2p peaks were adopted to identify the effect of substrate to SMO. According to Fig. S8a and Fig. S9, no obvious shift of Sm 3d peaks was detected for SMO/NrGO-2, SMO@NrGO-2 and all SMO@rGO-x


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samples (x = 1, 2, 3, 4, 5), implying no chemical bonding between Sm ions and rGO/NrGO substrate.42 On the contrary, clear peak deviation of Mn 2p for SMO@rGO-x (x = 1, 2, 3, 4, 5) offered a strong evidence of chemical interaction between Mn ions from metal oxides and rGO substrates for hybrid catalysts (Fig. S8b). Noticing that by decreasing the fraction of SMO, the Mn 2p1/2 peaks shifted towards negative binding energy, corresponding to a lower average valence state of Mn. In bulk SMO, two kinds of Mn crystal fields, pyramidal Mn3+ and octahedral Mn4+, coexist as illustrated in Fig. S10. Inspired by this, to develop further understanding over the hybrid interfaces, the Mn 2p spectra of SMO/NrGO-2, SMO@rGO-2 and SMO@NrGO-2 were fitted with three sub-peaks located near 641.5 eV, 642.7 eV and 644.9 eV, referring to the contribution of Mn3+, Mn4+ and a satellite peak, respectively, as shown in Fig. 2a.43-45 The atomic ratio of Mn3+ to Mn4+ should be proportional to the ratio of corresponding peak areas, and the average valence state of Mn was calculated from the weighted average of Mn3+ and Mn4+. Based on that, the ratio of Mn3+ to Mn4+ of physically mixture SMO/NrGO-2 approached 1, resulting in an average Mn valence of 3.47, close to that of pure SMO nanoparticles prepared via a similar hydrothermal method as reported previously.17, 21 For hybrid catalysts, more pronounced Mn3+ peaks relative to Mn4+ were observed and the average Mn valence decreased to 3.42 and 3.38 for SMO@rGO-2 and SMO@NrGO-2, respectively. On the other hand, the high-resolution N 1s spectra of SMO/NrGO2 and SMO@NrGO-2 provided in Fig. 2b further confirmed incorporation of N into the system and the formation of chemical bonds between N and Mn ions. Specifically, for each spectrum, three dominant components were obtained including pyridinic N (398.5-398.7 eV), pyrrolic N (399.8-400.0 eV) and graphitic N (401 eV).46, 47 Interestingly, the peaks attributed to pyridinic N and pyrrolic N of SMO@NrGO-2 obviously shifted to higher binding energy for about 0.2 eV compared to their counterparts of physically mixture SMO/NrGO-2. Considering the promoted


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electronegativity of pyridinic/pyrrolic N and decreased average valence state of Mn, it could be preliminarily concluded that partial electron transferred from N of graphene substrate to Mn of the interfacial SMO.48,

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Meanwhile, the fixed position of graphite N probably suggested that no

electron migration occurred between graphite N and metal ions. In order to verify the interaction between Mn and substrates, the exploration of charge density difference was then performed. According to the HRTEM image provided in Fig. 1e, the slab model of SMO along (121) direction with a thickness of 10 Å was built, as shown in Fig. S11.50 Besides, considering the fact that three kinds of nitrogen (graphitic N, pyrrolic N and pyridinic N) might coexist in NrGO (Fig. S12), the models of hybrid catalysts with different chemical bonds between Mn and carbonaceous substrates, i.e. SMO@rGO, SMO@GraphiteNrGO (SMO@Gra-NrGO), SMO@Pyrrolic-NrGO (SMO@Pyrr-NrGO) and SMO@PyridinicNrGO (SMO@Pyri-NrGO), were then constructed. The corresponding atomic structures were provided in Fig. S13. The length of Mn-C bonds between SMO and graphene was 2.79 Å, while the Mn-graphite N bond delivered a bond length of 2.85 Å, elucidating that the interaction between Mn and graphite N should be pretty weak. This phenomenon could be explained by the fact that n-type doping might be introduced with more valence electrons involved in N atom, compared with C atom. 51 The bond length of Mn-pyrrolic N and Mn-pyridinic N were 2.41 Å and 2.29 Å, indicating strong chemical interactions between Mn and pyrrolic/pyridinic N, which was in highly agreement with the XPS observations. Moreover, the schematic charge density differences of SMO@rGO, SMO@Pyrr-NrGO and SMO@Pyri-NrGO were illustrated in Fig. 2c. Charge migration with electrons accumulated at the Mn-N(C) bonds was clearly visualized for all the three structures. The accumulated electrons were mainly contributed by the graphene substrate nearby, and this phenomenon could be related to the intrinsic catalytic activity of hybrid materials.


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Specifically, whenever consumed in a catalytic reduction reaction, electrons could be replenished through the highly conductive graphene substrate. This process is here denoted as the “sponge effect”, and with the help of that, the charge transfer process could be effectively speeded up in hybrid materials. ORR Performance Evaluation The relationship between the structure and ORR performance of hybrid catalysts was then explored with rotating ring-disk electrode (RRDE) measurements. The electrolyte resistance was obtained to be ~47 Ω by electrochemical impendence spectroscopy (EIS, Fig. S14). For SMO@rGO-x, the linear sweep voltammograms (LSVs) in Fig. S15 showed that all the hybrid catalysts exhibited superior ORR performance than pure SMO and rGO in terms of onset potential and half-wave potential. Specifically, the half-wave potential increased from 0.73 V for SMO@rGO-5 to 0.79 for SMO@rGO-2, and enhancement of 0.11 V was achieved for SMO@rGO-2 comparing to pure SMO. The diffusion-limited current density of SMO@rGO-2 also reached -5.5 mA cm-2, larger than -4.8 mA cm-2 for pure SMO, confirming the ORR performance promotion contributed from effective interfacial engineering of hybrid catalysts. Surprisingly, SMO@rGO-1 with the largest SSA among all the samples showed relatively disappointing ORR activity, probably because of the reduced number of active sites due to the limited SMO loading. Another key factor that leaded to ORR performance enhancement was the improved conductivity of SMO@rGO-x. The resistivity of powder samples under different applied pressure was estimated with a home-made configuration (section 1.3 in the Supporting Information). As shown in Fig. S16, the powder resistivity of SMO@rGO-x samples drastically decreased with the feeding ratio of SMO to GO and reached to a level of 300-500 Ω cm-1 after x dropped to 3. For SMO@rGO-2, the powder resistivity was almost reduced by 4 orders of


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magnitude relative to pure SMO, which definitely would contribute to the electrocatalytic activity improvement. Moreover, further activity enhancement was observed for hybrid catalysts with nitrogen doping. With the assistance of strong Mn-N chemical bonds, the LSV curve of SMO@NrGO-2 in Fig. 3a exhibited a high half-wave potential of 0.84 V, which was comparable to the benchmark 20% Pt/C (0.87 V) and 0.16 V higher than that of pure SMO. The half-wave potentials and Tafel’s slopes of the samples in Fig. 3a were summarized in Fig. 3b. The ORR performance of SMO@NrGO-2 was significantly improved relative to its each constituent part and their physical mixture in terms of half-wave potential, Tafel’s slop and diffusion-limited current density. The XRD and LSV curves of SMO@NrGO-2 samples prepared with the different amount of ammonia (0.5 ml and 2 ml) were also recorded to elucidate the effect of N concentration (Fig. S7a and Fig. S17). The hybrid catalyst made with 0.5 ml ammonia showed limited ORR performance with a half-wave potential of 0.81 V, which could possibly be attributed to a reduced number of Mn-N active cites. Unexpectedly, increasing the amount of ammonia to 2 ml failed to further improve the ORR performance, either. Impurity phase of Mn3O4 (PDF No. 24-0734) was recognized from the XRD pattern. In order to dig out the correlation between activity degradation and the emergence of Mn3O4, the sample prepared without Sm precursor was synthesized as the controlled group. As expected, XRD pattern of the sample matched well with Mn3O4 (PDF No. 24-0734, Figure S7b), implying the sample to be N-doped reduced graphene oxide supported Mn3O4 (Mn3O4@NrGO). Considering the poor activity of Mn3O4@NrGO (Fig. S17), the formation of impurity phase Mn3O4 should be mainly responsible for the performance deterioration of SMO@NrGO-2 prepared with 2 ml ammonia.


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Furthermore, to shine a light on the structure-function relationship between the interfacial chemical bonds of hybrid catalysts and their kinetic activity, the kinetic current density of SMO/NrGO-2, SMO@rGO-2 and SMO@NrGO-2 at 0.85 V were extracted from LSV curves. As shown in Fig. 3c, the kinetic current density increased with reduced average Mn valence. SMO@NrGO-2 delivered more than 24 and 300 time specific activity than SMO/NrGO-2 and SMO, respectively. For the sample without effective interaction between metal oxides and carbonaceous substrate (SMO/NrGO-2), N doping or conductivity enhancement had only limited influence on catalytic activity improvement. Moreover, the hydrogen peroxide generated during the oxygen reduction process was dramatically inhibited from ~15% for SMO to ~5% for SMO@rGO-2 or SMO@NrGO-2, implying that the two hybrid catalysts preferred four electron kinetics. These facts suggested that the synergic effect of SMO and (N)rGO came from the strong Mn-N(C) chemical bonds, which effectively increased the number of active sites by changing catalytically inert Mn4+ into Mn3+ and caused the sponge effect to enable fast electron compensation, delivering an obvious advantage in boosting ORR activity. The stability of SMO@NrGO-2 was evaluated by both chronoamperometric measurement at 0.7 V and accelerated duration test (ADT) in O2-saturated 0.1 M KOH. As shown in Fig. 3e, after operating for 20000 s, more than 95 % of the original activity remained, much higher than 81% for Pt/C. After injecting 3 ml methanol into the electrolyte near the working electrode, only slight disturbance was observed for SMO@NrGO-2, suggesting the excellent methanol tolerance property. Fig. 3f presented the LSV curves of SMO@NrGO-2 after 30 and 1000 cycles between 0.6 V and 1.0 V in 0.1 M KOH. Only a slight drop of the half-wave potential (23 mV) was observed, indicating the excellent stability of the hybrid catalyst.


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Origin of the Enhancement of ORR Based on DFT Calculations In order to provide insights into the mechanism of ORR activity enhancement in the hybrid catalysts, DFT calculations were carried out (section 1.6 in the Supporting Information). The adsorption states of oxygen-containing species (OH*, OO*, OOH* and O*) on different systems were shown in Fig. S18 - Fig. S22. Fig. 4a provided the Gibbs free energy diagrams for SMO, SMO@rGO and SMO@Pyri-NrGO at the equilibrium potential. Compared with SMO, the overpotential decreased obviously with the construction of hybrid catalysts (from 0.770V for SMO to 0.597V for SMO@rGO). And when Mn-N interfacial interactions were introduced into the composite systems, the overpotential further reduced to 0.501 V and 0.455V for SMO@PyrrNrGO and SMO@Pyri-NrGO, respectively. This result implied that Mn-pyridinic N bonding, instead of Mn-pyrrolic N bonding, predominantly contributed to promote the ORR electrocatalysis.14, 52 Noticing that here in hybrid catalysts, the adsorption mechanism of oxygen intermediates was different from that in SMO because of the formation Mn-N(C) bonds. Bidentate adsorptions of oxygen intermediates instead of monodentate ones were observed at the interfaces between SMO and (N)rGO.53 Taking SMO@Pyri-NrGO as an example (Fig. 4b), after the oxygen intermediates adsorbing on the active site Mn, its neighboring Sm synergistically promoted the process under the influence of spatial local effects resulting from Mn-N bonds. In other words, the assistance of neighboring Sm ions facilitated the absorbing of oxygen intermediates. Importantly, sponge effect was visualized at the interface between SMO and graphene substrates. Fig. 4c demonstrated how the sponge effect eventually boosted the ORR. Initially, there was significant charge accumulation on the Mn-N bond when OH* adsorbing, and then, the accumulated electrons were gradually consumed with the OH* replaced by OO*. Interestingly, the consumed charge at the Mn-N bond was replenished, when OOH* adsorbing, which is the rate-determining step for the


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SMO@Pyri-NrGO. After that, stable charge was kept at the interface to facilitate the next reaction step, i.e., the formation of O*. Clearly, the interfacial Mn-N(C) bonds acted as sponges to store and transfer electrons to boost the ORR activity. In fact, the sponge effect could be a general phenomenon existing in hybrid systems to promote the reduction reaction process. Thus, such kind of concept might be employed in the future electrode catalysts design. Battery Performance Evaluation The application of SMO@NrGO-2 was demonstrated by ZAB constructed with polished Zn plate as anode, catalyst supported gas diffusion layer as cathode and alkaline electrolyte (section 1.5 in the Supporting Information). The battery performed a high discharge peak power density of 244 mW cm-2 (Fig. 5a), much higher than that of the ZAB made with benchmark 20% Pt/C (170 mW cm-2). Rechargeable ZAB based on SMO@NrGO-2 hybrid catalyst was then constructed by integrating NiFe layer double hydroxide grown on Ni foam (NiFe LDH@Ni) as oxygen evolution reaction (OER) catalyst.54 The detailed synthesis method, XRD and the corresponding OER performance of NiFe LDH@Ni were provided in the Supplementary Texts (section 1.1) and Fig. S23 in the Supporting Information. As shown in Fig. 5b, a proof-of-concept ZAB device performed superior cycle stability under repeatedly charging and discharging at 5 mA cm-2 for 100 cycles (20 min discharge, followed by 20 min charge). The charge-discharge overpotential increased only 50 mV compared to the initial value. As controlled group, the rechargeable ZAB made with benchmark Pt/C and RuO2 had a far worse cycle stability, with charge-discharge overpotential increasing from 0.67 V to 0.97 V after only 36 cycles. The cycle profile of rechargeable ZAB made with NiFe LDH@Ni only was also provided in Fig. S24 as the reference.


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Conclusions In conclusion, the electrocatalytic performance of pure SmMn2O5 was significantly enhanced by interfacial structure engineering via one-step in-situ growth on N-doped reduced graphene oxide. Chemical interactions between Mn and N(C) were verified by a combined evaluation of XPS and charge density difference study. Electron migration from carbonaceous substrate to Mn intrinsically caused the activation of catalytically inert Mn4+ sites with extra electron accumulated at the bonding regions. At the interfaces of SMO and NrGO, DFT calculations revealed the occurrence of the bidentate adsorptions rather than monodentate ones of oxygen intermediates. Furthermore, these Mn-N(C) bonds worked as electron sponges, which helped the electron transfer process during electrocatalytic reactions and thus resulted in the higher ORR performance. As a result, the ORR activity enhancement of the hybrid catalysts was achieved. Experimentally, SMO@NrGO-2 performed a half-wave potential of 0.84 V with 0.16 V improvement over pure mullite SMO, which is comparable to commercial Pt/C. The battery made with SMO@NrGO-2 delivered a high discharge peak power density of 244 mW cm-2 and superior cycle stability when integrated with NiFe LDH@Ni relative to the benchmark Pt/C and RuO2 catalysts


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Figure 1. (a) XRD spectra of GO, SMO and SMO@NrGO-2 compared with standard SmMn2O5 spectrum. (b-c) TEM images of SMO@NrGO-2 at different magnifications, and the inset in (b) shows the corresponding SEAD image. (d) Element mappings of the image in (c). (e) HRTEM of the region marked in the white box in (c), and the inset shows a representative (121) plane of SMO.


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Figure 2. XPS spectra of (a) Mn 2p orbital of SMO/NrGO-2, SMO@rGO-2, SMO@NrGO-2 and (b) N 1s orbital of SMO/NrGO-2, SMO@NrGO-2. (c) The charge density difference of SMO@rGO, SMO@Pyrr-NrGO and SMO@Pyri-NrGO. The accumulation and loss of charge are represented by yellow and blue regions (isosurface value: 0.02 e Å-3), respectively.


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Figure 3. (a) LSV curves of SMO, NrGO, SMO/NrGO-2, SMO@rGO-2, SMO@NrGO-2, Pt/C and (b) the corresponding half-wave potential and Tafel’s slope. (c) The relation of kinetic current at 0.85 V and Mn valence of SMO/NrGO-2, SMO@rGO-2 and SMO@NrGO-2. (d) Number of electron transferred and yield of hydrogen peroxide of the samples in (a). (e) Chronoamperometric curves of SMO@NrGO-2 and Pt/C, and the inset shows the influence of methanol injection. (f) The LSV curves of SMO@NrGO-2 in 0.1 M KOH at 1600 rpm after 30 and 1000 cycles between 0.6 V and 1.0 V at 100 mV s-1.


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Figure 4. (a) The free energy diagram of ORR on SMO, SMO@rGO and SMO@Pyri-NrGO at the equilibrium potential (U = 1.23V). (b) ORR mechanism over SMO on N-doped reduced


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graphene oxide, taking SMO@Pyri-NrGO as an example, which displays the strongest interfacial interactions among the four hybrid systems. (c) The schematic diagram of sponge effect based on the charge accumulation on the Mn-C or Mn-N bonds during ORR process.

Figure 5. (a) The discharge profiles and power plots of the Zn-air batteries made with SMO@NrGO-2 and Pt/C. (b) The cycling profiles of the Zn-air batteries made with SMO@NrGO2+NiFe LDH and Pt/C+RuO2.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental sections including detailed materials synthesis, characterization techniques, battery assembly, calculation method and results. (PDF)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Weichao Wang: 0000-0001-5931-212X Meng Yu: 0000-0002-1686-9128 Jieyu Liu: 0000-0002-1751-1152 Author Contributions ‡ These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Key Research and Development Program (Grant No. 2016YFB0901600), Tianjin City Distinguish Young Scholar Fund, National Natural Science Foundation of China (21573117 and 11674289), the 1000 Youth Talents Plan and the Fundamental Research Funds for the Central Universities (63185015).


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SYNOPSIS


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Sponge Effect Boosting Oxygen Reduction Reaction at the Interfaces

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