(Mn,Al)3O4 Nanosheet Nanocomposites with

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Nanoporous Platinum/(Mn,Al)3O4 Nanosheet Nanocomposites with Synergistically Enhanced Ultrahigh Oxygen Reduction Activity and Excellent Methanol Tolerance Conghui Si, Jie Zhang, Ying Wang, Wensheng Ma, Hui Gao, Lanfen Lv, and Zhonghua Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13840 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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ACS Applied Materials & Interfaces

Nanoporous Platinum/(Mn,Al)3O4 Nanosheet Nanocomposites with Synergistically Enhanced Ultrahigh Oxygen Reduction Activity and Excellent Methanol Tolerance Conghui Si, Jie Zhang, Ying Wang, Wensheng Ma, Hui Gao, Lanfen Lv, and Zhonghua Zhang*

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan, 250061, P.R. China

ABSTRACT: At present, metal/metal oxide composites are considered as potential oxygen reduction reaction (ORR) catalysts for energy-related applications like fuel cells. Here, we fabricated a high-activity, low Pt loading ORR electrocatalyst comprised of nanoporous Pt (np-Pt) in intimate contact with lamellar (Mn,Al)3O4 nanosheet (NS). In comparison to Pt/C (Johnson Matthey), the np-Pt/(Mn,Al)3O4 NS catalyst shows a 11.5-fold enhancement in the mass-normalized ORR activity and much better methanol tolerance, because of the metal-support interactions between np-Pt and (Mn,Al)3O4. Furthermore, the combination of electrochemical experiments with theoretical calculations verifies that the ORR on the np-Pt/(Mn,Al)3O4 NS catalyst is a direct 4 e- pathway in the alkaline solution. In addition, the electrocatalytic mechanisms have also been rationalized by density functional theory (DFT) calculations. 1

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KEYWORDS: strong metal support interactions (SMSIs), nanoporous platinum, oxygen reduction reaction (ORR), methanol tolerance, density functional theory (DFT) calculations

1. INTRODUCTION Being a crucial process in electrochemical devices (for example, fuel cells), oxygen reduction reaction (ORR) has recently aroused tremendous interest.1-3 Hitherto, Pt-based catalysts have played a major role in catalyzing ORR in both acid and alkaline solutions.4-7 Yet, the scarcity and hence prohibitively high cost, as well sluggish dynamics of ORR on the cathode side inevitably hinder their large-scale commercialization.3,8,9 Additionally, methanol permeation in direct methanol fuel cells (DMFCs) seriously poisons Pt-based catalysts, thereby decreases the cathode potential and reduces fuel efficiency.2,10-14 Therefore, it is of great significance to seek non-Pt alternatives or low Pt loading electrocatalysts. One effort is to develop non-Pt materials

such

as

non-precious

metal

composites,15,16

spinel

oxides,17-19

perovskites,20-22 or doped carbonaceous materials23,24 for ORR. Among these non-Pt materials, spinel oxides have been extensively explored owing to good ORR activity and inexpensive nature.19,25,26 Moreover, spinel oxides reveal excellent electrical conductivity owing to the electron hopping effect and could provide numerous reaction sites in ORR.19,25-27 Besides, compared to Pt materials, the lower activity of non-Pt electrocatalysts towards methanol oxidation is an attractive property for DMFCs considering the methanol crossover.28,29 Nevertheless, non-Pt materials show 2


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lower activities towards ORR in comparison to Pt catalysts.18,25,30 Hence, it is still difficult to meet the requirements of DMFCs with relying only on non-Pt materials. Alternatively, lowering the amount of Pt in catalysts has triggered extensive research interests. Several strategies were developed to fabricate Pt-based alloys with Fe,31,32 Co,33,34 Ni35,36 and Cu37,38 or to deposit a Pt monolayer onto other nanoparticles.39,40 Recently, various theoretical studies have demonstrated that strong metal support interactions (SMSIs) could significantly improve the electrocatalytic activity in ORR.41,42 Meanwhile, some experimental works have correlated the ORR activity with the SMSI effect.43-45 However, few reports46 have illuminated the ORR activity-SMSIs relationship combining experimental results with theoretical calculations. Additionally, theoretical simulation has elucidated that ligand/strain effects could drive a charge transfer and further change surface chemistry of Pt on the substrate.47-49 Herein, we designed and prepared novel nanoporous Pt/(Mn,Al)3O4 nanosheet nanocomposites (np-Pt/(Mn,Al)3O4 NS) composed of small amounts of nanoporous Pt in intimate contact with spinel oxide (Mn,Al)3O4 nanosheets through a facile dealloying-annealing strategy. The np-Pt/(Mn,Al)3O4 NS nanocomposites exhibit excellent catalytic performance and elevated methanol tolerance in alkaline solutions for ORR, in comparison to those of Pt/C. Furthermore, the enhanced catalytic activity of np-Pt/(Mn,Al)3O4 has been rationalized by the electronic effect between Pt and (Mn,Al)3O4 through density functional theory (DFT) calculations. Meanwhile, electrochemical measurements and DFT calculations verify the 4 e- ORR pathway and 3



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methanol tolerance of np-Pt/(Mn,Al)3O4 NS.

2. EXPERIMENTAL SECTION 2.1. Sample fabrication. The np-Pt/(Mn,Al)3O4 NS nanocomposites were synthesized through a dealloying-annealing strategy. Firstly, a ternary Al94.8Mn5Pt0.2 alloy (nominal composition, at.%) was fabricated from metals (Al, Mn, Pt, 99.9 wt.% purity) through casting in an induction furnace (Figure S1a). Subsequently, the Al94.8Mn5Pt0.2 ingot was rapidly solidified into foils (Figure S1b). Then the Al94.8Mn5Pt0.2 foils were dealloyed in a 2 M NaOH solution at ambient temperature. After washing and drying, the as-dealloyed samples were annealed in air (450 oC, 4 h), and the np-Pt/(Mn,Al)3O4 NS powders were obtained (Figure S1c). For comparison, the (Mn,Al)3O4 samples were prepared through the above method, using a binary Al95Mn5 alloy (at.%) as the precursor. 2.2. Microstructural characterization. X-ray diffraction (XRD, XD-3) was performed to identify the phase constitution of the obtained samples. Scanning electron microscopy (SEM, FEI QUANTA FEG250) and transmission electron microscopy (TEM, FEI Tecnai G2) were utilized to characterize the microstructure of the as-prepared samples. Additionally, scanning transmission electron microscopy (STEM) imaging and elemental analysis were carried out under a high-angle annular dark field (HAADF) mode. The Pt content of the as-annealed samples was obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Electron IRIS Intrepid II XSP). Surface compositions of the as-annealed samples were 4


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analyzed

by

X-ray

photoelectron

spectra

(XPS,

ESCALAB

250).

N2

adsorption–desorption isotherms were measured using a Gold APP V-Sorb 2800P analyzer at a temperature of 77 K. 2.3. Electrochemical measurements. Electrochemical properties of all catalysts were evaluated using rotating disk electrode (RDE) measurements in a CHI 760E potentiostat at room temperature. A three-electrode cell was used, containing sample modified glassy carbon (GC) electrode with a diameter of 5 mm (geometric surface area of 0.196 cm2), Pt plate, and saturated calomel electrode (SCE). The GC electrode coated with a catalyst film of 0.05 mg cm-2 was prepared as follows. Briefly, the catalyst suspension was obtained by sonicating the sample (4.0 mg), conducting carbon (XC-72, 6.0 mg) and Nafion solution (0.5 wt.%, 0.5 mL) in isopropanol solvent (1.5 mL) to form a homogeneous dispersion for at least 30 min. Similarly, the Pt/C suspension was obtained by sonicating 10 mg of Pt/C (40 wt.% Pt, Johnson Matthey) in the above-mentioned solution. 5 µL of the catalyst ink was pipetted onto the polished GC electrode and then dried. Cyclic voltammetry (CV) measurements were carried out in a N2-purged 0.1 M HClO4 electrolyte at 50 mV s−1. The ORR measurements were conducted from -0.8 to 0.1 V vs. SCE in a 0.1 M KOH solution saturated with O2 at 10 mV s-1. Furthermore, the working electrodes were scanned at different speeds (400 - 2500 rpm) to investigate the kinetics of ORR. The ORR experiments were also performed in the 0.1 M KOH solution containing methanol (MeOH, 0.1 or 0.5 M) in order to assess the methanol tolerance. The polarization curves were corrected through deducting 5



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background currents recorded in the N2-purged 0.1 M KOH electrolyte. Rotating ring (polycrystalline Pt, 0.189 cm2) -disk (GC, 0.248 cm2) electrode (RRDE) experiments were conducted to investigate the transferred electron number and peroxide yield in ORR. The disk electrode was scanned at a rate of 10 mV s-1, and 0.3 V vs. SCE was used for the ring electrode. Specially, we benchmarked electrochemical properties of our samples with commercial catalyst in the same case. The measured potentials were transformed into reversible hydrogen electrode (RHE) and calibrated to be iR-corrected potentials. 2.4. DFT calculation. The structure of np-Pt/(Mn,Al)3O4 NS is shown in Figure S2 and a periodically repeated slab was adopted, representing the (Mn,Al)3O4 (110) as the support for the Pt monolayer. The slab was covered with a vacuum layer (12 Å in thickness) to prevent interactions between neighboring species. Different from the fixing of the bottom part, the top two layers (Pt layer + (Mn,Al)3O4 top layer) were allowed to be absolutely relaxed. And the geometric optimization of the system was obtained through minimizing its total energy. For comparison, we also established Pt (110) with similar parameter settings. The CASTEP software in Materials Studio (version 7.0) of Accelrys Inc was utilized to perform the DFT calculations. The detailed parameter setting could be found in the literature.50

3. RESULTS AND DISCUSSION 3.1. Microstructure and morphology. The XRD results indicate that the Al94.8Mn5Pt0.2 precursor foils are composed of Al and Al6Mn phases (Figure S3a), 6


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and the as-dealloyed sample consists of Mn4Al2(OH)12CO3·3H2O and Pt phases (Figure S3b). Figure 1a presents the XRD pattern of the as-annealed samples, confirming that the samples are comprised of f.c.c. Pt and (Mn,Al)3O4 (be transformed from Mn4Al2(OH)12CO3·3H2O during annealing) phases. Compared to the standard profile of Mn3O4 (JCPDS 24-0734), the diffraction peaks of (Mn,Al)3O4 shift positively owing to the substitution of Al for Mn (Figure S4). Since the ionic radius of Al (III) (0.054 nm)51 is smaller than that of Mn (III) (0.061 nm),52 lattice contraction might occur in (Mn,Al)3O4. Typical SEM image shows a nanosheet morphology of the (Mn,Al)3O4 substrate with homogeneous thickness of ~ 20 nm (Figure 1b). However, it is difficult to observe the morphology of Pt due to the restricted resolution of SEM and the small amount of Pt. The morphology and crystalline nature of the np-Pt/(Mn,Al)3O4 NS composites were further clarified by TEM observation and electron diffraction (Figure 1c). Two morphologies exist including gray nanosheets and dark nanoporous ligament-channel structures. The gray nanosheets should be (Mn,Al)3O4 and the dark ligament-channel structures ought to be Pt because elements with high atomic numbers normally have a sharp contrast.53 Moreover, the size distribution plot of ligaments in nanoporous Pt is shown in Figure S5 and the average ligament size is about 2.6 nm. The other TEM images also reveal the laminated structure of (Mn,Al)3O4 and nanoporous Pt dispersed on the (Mn,Al)3O4 substrate (Figure S6). The electron diffraction pattern consists of a series of diffraction rings and bright spots (inset of Figure 1c, Figure S7). Here the rings correspond to (111) and (200) planes of f.c.c. Pt, and the bright spots of [13ത 1] zone 7



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axis indicate the single crystal characteristics of (Mn,Al)3O4. Figure 1d shows the HRTEM image of np-Pt/(Mn,Al)3O4. The lattice distance for dark fringes is 0.227 nm, consistent with that (0.226 nm) of Pt (111). The gray lattice spacing of 0.493 nm is coincident with that (0.492 nm) of Mn3O4 (101). In addition, the HRTEM image of the np-Pt/(Mn,Al)3O4 NS sample also reveals the coherent relationship between np-Pt and (Mn,Al)3O4 (Figure S8). Figure 1e shows the HRTEM image of (Mn,Al)3O4-rich region for the np-Pt/(Mn,Al)3O4 NS sample and the fast Fourier transform (FFT) pattern verifies the single crystal characteristics of (Mn,Al)3O4 nanosheet (inset of Figure 1e). The HRTEM image of Pt-rich region is shown in Figure 1f and the FFT pattern (inset of Figure 1f) confirms the nanocrystal characteristics of np-Pt. Additionally, Figure S9 shows the SEM and TEM images of (Mn,Al)3O4 sample, clearly displaying the typical nanosheet structures. Figure 2a and 2b shows the STEM image and the nanobeam-energy dispersive X-ray (NB-EDX) spectra corresponding to different positions of the np-Pt/(Mn,Al)3O4 NS sample. Position 1 in Figure 2a is particularly bright and the EDX result of this region reveals a quite high Pt content (upper panel of Figure 2b), which confirms that this bright area is primarily composed of np-Pt. Conversely, the EDX result of position 2 indicates a high content of Mn/Al and negligible Pt, verifying that this region mainly consists of (Mn,Al)3O4 (bottom panel of Figure 2b). These results further demonstrate that np-Pt is supported on the (Mn,Al)3O4 nanosheet. Additionally, the Pt content in the np-Pt/(Mn,Al)3O4 NS catalyst was determined to be 5.13 wt.% through the ICP-AES analysis. And the SEM-EDX result (Figure S10) is 8


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approximately consistent with the ICP-AES analysis. In addition, the N2 adsorption–desorption isotherms (type IV with hysteresis) of the np-Pt/(Mn,Al)3O4 NS composites indicate the existence of mesopores (Figure 2c). And the Brunauer–Emmett–Teller surface area of np-Pt/(Mn,Al)3O4 reaches up to 98.42 m2 g-1. Figure 2d shows the pore size distribution of np-Pt/(Mn,Al)3O4, and the mesopore size ranges from 2 to 6 nm. The mesopores can also be recognized in the TEM images (marked with arrows in Figure 1c and Figure S6d). The XPS spectra of np-Pt/(Mn,Al)3O4 are presented in Figure 3 and Figure S11. The peaks of Mn2p3/2 and Mn2p1/2 at ~ 640.9 and ~ 652.4 eV belong to Mn (III) species, and the others at ~ 642.5 and ~ 653.8 eV for Mn2p3/2 and Mn2p1/2 correspond to Mn (II) (Figure 3a).43 The XPS analysis of Al2p in Figure 3b indicates the existence of Al (III) with the peak at ~ 73.6 eV.54 These results indicate that Mn (II) occupies the tetrahedral cation sites, meanwhile, Mn (III) and Al (III) occupy the octahedral cation sites in (Mn,Al)3O4. The binding energy of Pt4f in np-Pt/(Mn,Al)3O4 NS (~ 70.7 eV) has a ∼ 0.4 eV decrease compared with the simplex Pt (~ 71.1 eV) (Figure 3c).11 This decrease could be caused by charge transfer from (Mn,Al)3O4 to Pt in np-Pt/(Mn,Al)3O4. According to the Schottky theory,55 the charge transfer will occur from (Mn,Al)3O4 (with low work function of 4.4 eV56,57) to Pt (with high work function of 5.65 eV58). The Pt4f peak shift of np-Pt/(Mn,Al)3O4 towards lower energies indicates that there exist SMSIs between np-Pt and (Mn,Al)3O4 NS. Figure 3d shows the O 1s XPS spectrum of np-Pt/(Mn,Al)3O4 NS, which were deconvolved to four peaks at ~ 529.6, ~ 530.1, ~ 530.7 and ~ 531.5 eV corresponding to lattice 9



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oxygen species (O2-), highly oxidative oxygen species (O22-/O-), oxygen species combined with carbon (C=O), and surface adsorbed oxygen-containing species (OHor H2O),59,60 respectively. The relative ratio of peak area of various oxygen species is listed in Figure 3d. 3.2. Electrochemical performance. Figure S12 shows cyclic voltammograms of the np-Pt/(Mn,Al)3O4 NS and commercial Pt/C catalysts in the N2-purged 0.1 M HClO4 electrolyte. For the np-Pt/(Mn,Al)3O4 NS catalyst, the peaks at 0.05 ~ 0.45 V (vs. RHE) are associated with the hydrogen adsorption/desorption and the redox peaks at 0.98/0.72 V (vs. RHE) correspond to the formation/reduction of Pt oxides. The electrochemical surface areas (ECSAs) of np-Pt/(Mn,Al)3O4 and Pt/C were thus determined (see supporting information). The ECSA of np-Pt/(Mn,Al)3O4 NS is 118 m2 gPt-1, much larger than that of Pt/C (47 m2 gPt-1, Table 1). Figure 4a presents ORR polarization curves of the np-Pt/(Mn,Al)3O4 NS, (Mn,Al)3O4 and commercial Pt/C catalysts in the 0.1 M KOH electrolyte. Clearly, the np-Pt/(Mn,Al)3O4 NS catalyst exhibits a positive shift of the onset/half-wave potential and higher limiting current compared to (Mn,Al)3O4, indicative of the greatly enhanced ORR activity of np-Pt/(Mn,Al)3O4 NS. Furthermore, the onset potential and half-wave potential of np-Pt/(Mn,Al)3O4 NS show a slightly negative shift in comparison to Pt/C, and the limiting current of np-Pt/(Mn,Al)3O4 NS (-5.45 mA cm-2geo) is even higher than Pt/C (-5.35 mA cm-2geo). Meanwhile, the E@J= -3mA cm-2 and J@E=0.8V of the np-Pt/(Mn,Al)3O4 NS are shown in Fig. S13, which are obviously larger than those of (Mn,Al)3O4 and close to the values of Pt/C. Furthermore, the mass activities of np-Pt/(Mn,Al)3O4 NS 10


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and Pt/C were obtained at 0.9 V vs. RHE on account of the mass of Pt in the catalysts (the Pt loading of 2.57 µg cm-2). As shown in Figure 4b, the mass activity for the np-Pt/(Mn,Al)3O4 NS catalyst reaches 0.46 mA mgPt-1, and the activity of Pt/C is only 0.04 mA mgPt-1. To evaluate the methanol tolerance, the ORR curves of np-Pt/(Mn,Al)3O4 NS and Pt/C were measured in the 0.1 M KOH solution with different concentrations of methanol (Fig. 4c). As shown in Figure S14, the potential (EJ=

-0.95mA cm

-2

) is

approximately 240 mV negative shift for Pt/C after the addition of 0.1 M MeOH compared with that without MeOH. In contrast, surprisingly, almost no change occurs for the np-Pt/(Mn,Al)3O4 NS catalyst in the same solution. For the 0.1 M KOH + 0.5 M MeOH electrolyte, the potential (EJ= -0.95mA cm-2) of Pt/C is nearly 310 mV negative shift compared with that without MeOH, but only 30 mV negative shift occurs for np-Pt/(Mn,Al)3O4 NS. Furthermore, the methanol oxidation peaks for Pt/C appear at around 0.80 V vs. RHE (0.1 M KOH + 0.1 M MeOH) and 0.95 V vs. RHE (0.1 M KOH + 0.5 M MeOH), respectively (Figure 4c). On the contrary, the methanol oxidation peaks for np-Pt/(Mn,Al)3O4 NS occur in a more negative potential region, indicating that the methanol oxidation on np-Pt/(Mn,Al)3O4 NS is significantly weakened (Figure 4c). In the 0.1 M KOH electrolytes containing 0, 0.1 and 0.5 M MeOH, the mass activity of np-Pt/(Mn,Al)3O4 NS and Pt/C was calculated according to the kinetic current @ 0.85 V vs. RHE (Figure 4d). At the presence of methanol, the mass activity of Pt/C is nearly zero. In comparison, the ORR activity of np-Pt/(Mn,Al)3O4 NS shows no significant change in the 0.1 M KOH + 0.1 M MeOH 11



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solution. Additionally, the np-Pt/(Mn,Al)3O4 NS catalyst still possesses a high activity of 0.44 mA mg Pt -1 in the 0.1 M KOH + 0.5 M MeOH solution at 0.85 V vs. RHE. The ORR performance of np-Pt/(Mn,Al)3O4 NS is quite prominent in the alkaline electrolyte with MeOH, indicating that the np-Pt/(Mn,Al)3O4 NS catalyst has a superior ORR selectivity in the presence of MeOH. To probe the ORR mechanism, RRDE data (Figure 5a and 5b) were obtained for np-Pt/(Mn,Al)3O4 NS and Pt/C. The ring current of np-Pt/(Mn,Al)3O4 NS is quite weak (Figure 5a), indicating that the peroxide formation (2e- pathway) is greatly reduced. The transferred electron number (n) on the np-Pt/(Mn,Al)3O4 NS is about 3.93 (Figure 5c), suggesting an apparent 4e- ORR pathway. Additionally, the transferred electron number for np-Pt/(Mn,Al)3O4 NS was calculated to be 3.94 via the Koutecky−Levich equations (see supporting information, Figure S15), which agrees well with the RRDE results. The yield of peroxide species (H2O2) is smaller than 4% for the np-Pt/(Mn,Al)3O4 NS catalyst (Figure 5d), which further confirms the 4e- pathway of ORR on np-Pt/(Mn,Al)3O4 NS. Clearly, the ORR process of np-Pt/(Mn,Al)3O4 NS is analogous to the scenario of Pt/C. The microstructural characterizations of np-Pt/(Mn,Al)3O4 NS reveal the SMSIs between np-Pt and (Mn,Al)3O4 nanosheet (Figures 1, S6 and S8), which are of crucial importance to promote the ORR activity. In the np-Pt/(Mn,Al)3O4 NS nanocomposites, the Pt particles show typical nanoporous ligament-channel structures and the (Mn,Al)3O4 nanosheets show mesoporous laminated structures (Figures 1c and S6). The nanoporous structure and the associated high specific surface area possess an 12


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unusual combination of abundant adsorption sites, interconnected ligaments with high conductivity and large numbers of easily accessible nanopores,61-67 which are extraordinarily beneficial for electrocatalysis in contrast to nanoparticle catalysts.39,68 The specific ligament-channel structure of np-Pt could facilitate the transport of electrons (in the ligament network) and involved species (in the hollow channels) during the ORR process. Moreover, the XPS results (Figure 3) and the coherent relationship (Figure S8) signify the SMSIs between np-Pt and (Mn,Al)3O4 nanosheet. This interaction accelerates the charge transfer during the ORR process so as to improve the electrocatalytic performance of np-Pt/(Mn,Al)3O4 NS. Furthermore, the (Mn,Al)3O4 NS also plays a central role in improving the electrocatalytic activity of np-Pt/(Mn,Al)3O4. The O 1s XPS spectrum (Figure 3d) reveals that the area ratio of the O1s peak at 531.5 eV is obviously higher than that of the other peaks, indicating the strong O2 adsorption on the np-Pt/(Mn,Al)3O4 NS surface.69 3.3. Simulation analysis. We also did DFT calculations to address the correlation between ORR activity and electronic effects. So far, the electronic effects are mainly divided into two types: the ligand effect caused by the metal-support charge transfer, as well as the strain effect originating from the lattice strain.52 These strain/ligand effects are typical SMSI effects occurring in the metal/metal oxide composites.46 We first focus on the ligand effect, being regarded as the major origination for the enhancement of ORR activity.51 Figure 6 presents electronic density of states (DOS) of np-Pt/(Mn,Al)3O4 NS, (Mn,Al)3O4, and Pt obtained from the DFT calculations. The d-band centers of (Mn,Al)3O4, Pt and np-Pt/(Mn,Al)3O4 NS are located at -2.216, 13



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-2.411 and -2.408 eV, respectively. Among them, the d-band center of (Mn,Al)3O4 is the highest, which signifies the O−O bond breakage (Figure 6a). However, oxygenated intermediates are difficult to desorb owing to strong adsorption of O2. In contrast, Pt (Figure 6b) and np-Pt/(Mn,Al)3O4 NS (Figure 6c) with lower d-band centers have high ORR activities because of the low adsorption strength of O2 and easy desorption of oxygenated intermediates, despite the slow breakage of O−O bonds. The decrease in d-band center of np-Pt/(Mn,Al)3O4 NS is due to the reduction of Pt. In other words, np-Pt/(Mn,Al)3O4 NS has an electron-rich phase on the surface because electrons could transport from Mn,Al)3O4 to Pt in SMSIs. This is consistent with the tendency of the broadening d-band caused by the increase of d-band superposition. Accordingly, the d-band center will decrease with the donation of electrons from d-band.52 Moreover, the adsorption strength of O2 on the np-Pt/(Mn,Al)3O4 NS surface is slightly higher than Pt (Table 2, Figures S16 and S17), indicating that O2 is more easily adsorbed on the np-Pt/(Mn,Al)3O4 NS surface than Pt. On the other hand, the strain effect also plays an important role in enhancing the electrocatalytic activity. Mavrikakis et al. have revealed that the tensile or compressive strain induces positive or negative shift of d-band center in Pt-based systems, respectively.53 Compared with (Mn,Al)3O4, the d-band overlap increases and its shape broadens in np-Pt/(Mn,Al)3O4 NS (Figure 6), resulting in the downshift of d-band center associated with the compressive strain. The compressive strain induces more antibonding states and increases Pauli repulsion.70 This brings about increasing 14


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occupation and weakens the adsorption of oxygenated intermediates, leading to the enhancement of the electrocatalytic activity. DFT calculations were also performed to theoretically explore the direct 4 epathway of np-Pt/(Mn,Al)3O4 NS. Figure 7 shows the optimized structural configurations of HOOH molecules adsorbed on np-Pt/(Mn,Al)3O4 NS. The O-O bond length is only 1.465 Å in an isolated HOOH molecule (Figure 7a). However, the DFT simulation forecasts that the O-O bond length increases to 2.771Å in the HOOH molecule adsorbed on the np-Pt/(Mn,Al)3O4 NS (110) (Figure 7b). Therefore, the O-O bond breakage will occur during the process of H2O2 adsorption on the surface of np-Pt/(Mn,Al)3O4, which is a precondition for the 4 e- ORR. For comparison, the optimized structural configurations for HOOH adsorbed on Pt were also calculated (Figure S18). Similarly, the O-O bond length reaches up to 3.067 Å when adsorbing on Pt (110), indicating the 4 e- pathway on Pt. Finally, the DFT simulation was carried out to explain why np-Pt/(Mn,Al)3O4 NS has much better methanol tolerance than Pt/C. Figures S19 and S20 show the optimized

structural

configurations

for

methanol

molecules

adsorbed

on

np-Pt/(Mn,Al)3O4 NS (110) and Pt (110). The calculated adsorption energies are 0.24 and 0.33 eV for methanol molecules adsorbed on np-Pt/(Mn,Al)3O4 NS (110) and Pt (110), respectively. These results indicate that it is easier for methanol molecules adsorbed on Pt (110) than np-Pt/(Mn,Al)3O4 NS (110). The methanol molecules might occupy active adsorption centers and impede further adsorption of O2. Therefore, the np-Pt/(Mn,Al)3O4 NS has better methanol tolerance than Pt/C. 15



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4. CONCLUSIONS The facile, scalable dealloying-annealing strategy has been explored to produce highly active, low Pt loading ORR catalyst (np-Pt/(Mn,Al)3O4 NS) composed of nanoporous Pt in intimate contact with lamellar (Mn,Al)3O4 nanosheets. The np-Pt/(Mn,Al)3O4 NS catalyst shows a significantly enhanced ORR activity as benchmarked with state-of-the-art Pt/C. The ECSA and mass activity (@ 0.9 V vs. RHE) of np-Pt/(Mn,Al)3O4 NS catalyst are approximately 2.5 and 11.5 times that of Pt/C, respectively. Furthermore, combining electrochemical measurements with DFT calculations, we confirm that the ORR on the np-Pt/(Mn,Al)3O4 NS catalyst is a direct 4 e- pathway in the alkaline solution. In addition, the np-Pt/(Mn,Al)3O4 NS catalyst has greatly enhanced methanol tolerance in comparison to Pt/C. Therefore, np-Pt/(Mn,Al)3O4 NS is a promising highly reactive electrocatalyst and suitable for energy-related applications like fuel cells, metal-air batteries, and so forth.

ASSOCIATED CONTENT Supporting Information Computational details, macrographs, atomic structure and XRD of the samples, the ligament size distribution, TEM, SAED, HRTEM, SEM-EDX and XPS images of np-Pt/(Mn,Al)3O4

NS,

SEM

and

TEM

images

of

(Mn,Al)3O4,

relevant

electrochemical curves for all of the samples analyzed, O2, HOOH and CH3OH adsorption models. (PDF)

16


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Zhonghua Zhang: 0000-0002-2883-4459 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51371106, 51671115), and Young Tip-top Talent Support Project (the Organization Department of the Central Committee of the CPC).

17



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Nordlund, D.; Ogasawara, H.; Toney, M. F.; Nilsson, A. Lattice-Strain Control of the Activity in Dealloyed Core–Shell Fuel Cell Catalysts. Nat. Chem. 2010, 2, 454-460.

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Figures and tables

Table 1. Comparison of electrochemical data for the np-Pt/(Mn,Al)3O4 NS and (Mn,Al)3O4 catalysts against the commercial Pt/C catalyst. np-Pt/(Mn,Al)3O4 NS

(Mn,Al)3O4

Pt/C

Onset potential (V vs. RHE)

0.96

0.75

1.03

Limiting current density (mA cm-2)geo

-5.45

-4.65

-5.35

118

—

47

0.46

—

0.04

3.94

—

3.97

3.93

—

3.98

ECSA (m2 gPt-1) Mass activity (mA mg-1Pt) at 0.9 V vs. RHE Transferred electron number determined by the Koutecky-Levich equations (n) Transferred electron number determined from the RRDE data (n)

26


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Table 2. Comparison of d-band center and molecular adsorption calculation results for np-Pt/(Mn,Al)3O4 NS (110), (Mn,Al)3O4 (110) and Pt (110) (corresponding to the optimized structural configurations in Figure 7 and Figures S16—S20). np-Pt/(Mn,Al)3O4 NS (110)

(Mn,Al)3O4 (110)

Pt (110)

-2.408

-2.216

-2.411

O2 adsorption energy (eV)

2.31

—

2.27

CH3OH adsorption energy

0.24

—

0.33

1.465→2.771

—

1.465→3.067

d-band center (eV)

(eV) O-O bond length change of HOOH (Å)

27



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Figure 1. (a) XRD pattern of the synthesized np-Pt/(Mn,Al)3O4 NS sample. (b) SEM image of the np-Pt/(Mn,Al)3O4 NS sample, inset: an enlarged image of the square region in (b). (c) TEM image of the np-Pt/(Mn,Al)3O4 NS sample, inset: corresponding SAED pattern. (d-f) HRTEM images of the np-Pt/(Mn,Al)3O4 NS sample, insets in (e,f) : corresponding FFT patterns. 28


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Figure 2. (a) STEM image of the np-Pt/(Mn,Al)3O4 NS sample. (b) NB-EDX spectra of the np-Pt/(Mn,Al)3O4 NS sample corresponding to the marked areas in (a) (the Cu signals are from the Cu mesh). (c) N2 adsorption–desorption isotherms of the np-Pt/(Mn,Al)3O4 NS sample. (d) Pore size distribution of the np-Pt/(Mn,Al)3O4 NS sample.

29



(b)

73.6 eV

660

655

Pt 4f

Intensity (a.u.)

640.9 eV 653.8 eV

642.5 eV 652.4 eV

650 645 Binding Energy (eV)

640

(c)

73.7 eV

76

74 72 Binding Energy (eV)

70

76

O 1s

70.7 eV

78

78

635

Intensity (a.u.)

Intensity (a.u.)

Al 2p

(a)

Mn 2p

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

68

Position 529.6 530.1 530.7 531.5

74 72 70 Binding Energy (eV)

(d)

531.5 eV area (%) 34.06 8.97 9.25 47.72

530.7 eV

536

68

534

529.6 eV

530.1 eV

532 530 528 Binding Energy (eV)

526

Figure 3. (a) Mn 2p, (b) Al 2p, (c) Pt 4f and (d) O 1s XPS spectra of the np-Pt/(Mn,Al)3O4 NS sample.

30


Jgeo/mA cm-2

-1

0.5

Pt/C np-Pt/(Mn,Al)3O4 NS

(a)

0.4

(Mn,Al)3O4

-2

(b)

Mass activity ECSA

100 80

0.3

-3

120

60

0.2

-4 -5

14 12 10 8 6 4 2 0

0.3

0.4

0.5

0.6 0.7 0.8 E/V vs.RHE

0.9

1.0

(0.1M MeOH) np-Pt/(Mn,Al)3O4 NS (0.5M MeOH) Pt/C(0M MeOH) Pt/C(0.1M MeOH) Pt/C(0.5M MeOH)

-4 0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

E/V vs.RHE

20

0.0

0 Pt/C

1.0

(c)

np-Pt/(Mn,Al)3O4 NS (0M MeOH) np-Pt/(Mn,Al)3O4 NS

-2

-6 0.2

1.1

-1 Mass activity (mA mg Pt)

-6 0.2

40

0.1

0.8

-1 2 ECSA (m gPt )

0

Jgeo/mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1 Mass activity (mA mgPt )

Page 31 of 36

Pt/(Mn,Al)3O4

Pt/C Pt/(Mn,Al)3O4

(d)

1.0 0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 0M MeOH

0.1M MeOH

0.5M MeOH

0.0

Figure 4. (a) Polarization curves for ORR on the Pt/C, np-Pt/(Mn,Al)3O4 NS and (Mn,Al)3O4 catalysts in the O2-saturated 0.1 M KOH electrolyte at the rotation rate of 1600 rpm and scan rate of 10 mV s-1. (b) Comparison of mass activity at 0.9 V vs. RHE and ECSA of the np-Pt/(Mn,Al)3O4 NS and commercial Pt/C catalysts. (c) Polarization curves for ORR on the Pt/C and np-Pt/(Mn,Al)3O4 NS catalysts in the O2-saturated 0.1 M KOH electrolyte containing 0, 0.1 and 0.5 M MeOH at 1600 rpm and 10 mV s-1. (d) Comparison of mass activity of the np-Pt/(Mn,Al)3O4 NS and commercial Pt/C catalysts at 0.85 V vs. RHE in the 0.1 M KOH electrolyte containing 0, 0.1 and 0.5 M MeOH.

31



(a)


n

Ring

0.1

(c)

4.0 3.9


3.8 0.0 0

(b)

-2 Disc -4 -6 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Peroxide Yield (%)

Jgeo/mA cm-2

0.2

Jgeo/mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

(d) 5.0 2.5 0.0 0.2

0.3

E/V vs. RHE

0.4 E/V vs. RHE

0.5

0.6

Figure 5. (a,b) RRDE voltammograms of the Pt/C and np-Pt/(Mn,Al)3O4 NS catalysts in the O2-saturated 0.1 M KOH at 1600 rpm and 10 mV s-1. (c) The transferred electron number (n) and (d) peroxide yield of Pt/C and np-Pt/(Mn,Al)3O4 NS at various potentials based on the corresponding RRDE data.

32


Page 33 of 36

(a)

(Mn,Al)3O4 − − − d-band center

-2.216eV

Density of state/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

Pt − − − d-band center

-2.411eV

(c)

np-Pt/(Mn,Al)3O4 NS − − − d-band center -2.408eV

-10

-8

-6

-4

-2

0

2

Energy/eV

Figure 6. The DOS of (a) (Mn,Al)3O4, (b) Pt and (c) np-Pt/(Mn,Al)3O4 NS. The d-band centers of (Mn,Al)3O4, Pt and np-Pt/(Mn,Al)3O4 NS are at -2.216, -2.411 and -2.408 eV, respectively. 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. The status of HOOH molecule adsorbed on the np-Pt/(Mn,Al)3O4 NS (110) (a) before and (b) after optimization.

34


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TOC

1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC

1


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(Mn,Al)3O4 Nanosheet Nanocomposites with

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