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Activating the bifunctionality of a perovskite oxide toward the oxygen reduction and oxygen evolution reactions Yuqi Lyu, and Francesco Ciucci ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10216 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

Activating the bifunctionality of a perovskite oxide toward the oxygen reduction and oxygen evolution reactions Yu-Qi Lyu,a and Francesco Ciucci a,b,* a

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science

and Technology, Hong Kong b

Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science

and Technology, Hong Kong KEYWORDS: Oxygen reduction reaction • Oxygen evolution reaction • electrocatalysis • Bifunctional catalyst • Perovskite

ABSTRACT

This article presents a facile and effective approach to activate the bifunctionality of calciummanganese perovskites towards the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). We substituted Nb into the Mn site of CaMnO3 (CMO) and treated the material with H2. The as-obtained CaMn0.75Nb0.25O3−δ (H2-CMNO) displays the same structure as CMO

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and, compared to CMO, H2-CMNO exhibits significantly improved OER performance, including a lower overpotential, a reduced Tafel slope, a higher mass activity, and enhanced stability. In addition, the ORR performance of H2-CMNO is also greatly enhanced, relative to CMO, with a higher ORR activity and a more efficient electron-transfer pathway. H2-CMNO shows even higher activity-per-catalyst-cost and superior stability than state-of-the-art materials such as IrO2 and Pt/C. This great enhancement in ORR and OER activity of H2-CMNO is attributed to several factors including phase stabilization, optimized eg filling, better OH- adsorption, and improved electrical conductivity.

Introduction Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are of crucial importance to many clean-energy technologies, including low temperature fuel cells, metal-air batteries, and water electrolyzers1-5. The most active catalysts for ORR and OER are based on noble metal based materials, such as Pt/C, a benchmark catalyst for ORR6-7, and IrO2, a benchmark catalyst for OER8-9. However, the scarcity, high cost, and low stability of these noble metals highly limit their commercialization3, 10-11. An additional challenge lies in the fact that few noble metal-based catalysts exhibit an effective bifunctional performance for both ORR and OER12. This, in turn, constrains the regenerative operation of these clean-energy devices1, 13-14. Therefore, to overcome these challenges, effort is required to develop bifunctional catalysts that can replace noble metals, with special focus on lowering the cost and improving the performance.


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Perovskite materials, with a nominal formula ABO3, where the A and B site are usually occupied by rare-earth ions and transition metal ions, respectively, have been recently considered as promising candidates as oxygen catalysts because of their good electrocatalytic activity and stability in alkaline solutions

2, 15-24

. CaMnO3 has been widely studied as one of the most active

perovskites for ORR because of its high activity and stability, low cost, and environmentally friendliness10, 25-36. Unfortunately, the OER activity of CaMnO3 is reported to be poor25, 30-31, impeding its use in regenerative fuel cells and metal-air batteries. Shon-Horn el al. pointed out that the OER activity can be effectively affected by the filling of eg orbitals of transition metal ions6, 8. Tuning the eg filling of a perovskite close to unity can lead to the highest OER activity8, 25-26, 30, 37-38

. Based on this principle, Chen el al. treated CaMnO3 with H2 in order to adjust its

oxygen stoichiometry and eg occupancy25. However, it is challenging to realize the controllable modulation of eg filling toward unity because of the phase instability of CaMnO325. Excessive H2-treatment can lead to the phase transformation from Pnma (CaMnO3) to Pbam (CaMnO2.5), which greatly reduces the electrical conductivity of the perovskite and inhibits the further control of the eg level25, 30. Conversely, Wu el al. opted for substituting Ca with trivalent rare-earth metals to adjust the eg level of CaMnO326. Yet, the maximum substitution level is 10% because of potential phase transformations26, 39-40. It is clear that in order to promote the electrocatalytic performance it is key to stabilize the Pnma phase of CaMnO3. In this work, we report a facile strategy to stabilize the crystal phase of CaMnO3 (CMO). This in turn enables a controllable adjustment of its OER activity and thereby activates its bifunctionality. First, we doped CaMnO3 with Nb, a 4d transition metal at the B-site, with special attention towards improving its phase stability. Nb is an ideal dopant for this purpose because it has higher electronegativity than Mn and may form more stable Nb-O bonds, which are expected


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to stabilize the phase structure of CaMnO3. Additionally, Nb5+ also help reduce Mn4+ towards Mn3+, facilitating the optimization of eg filling. Then, we applied H2 treatment to the as-prepared CaMn0.75Nb0.25O3 (CMNO) to further engineer its electronic structure and electrochemical properties. The H2-treated CaMn0.75Nb0.25O3-δ (H2-CMNO) preserves an identical orthorhomibic (Pnma) phase as CMNO and pristine CMO. In comparison to CMO, H2-CMNO exhibits significantly enhanced OER performance, including a one-order-of-magnitude higher current density at the overpotential of 600 mV, a sevenfold higher mass activity at low overpotential region (1.55 V to 1.7 V), a reduced Tafel slope, and a clearly enhanced stability. In addition, the ORR activity of H2-CMNO is also improved compared to CMO, with more positive onset potential, higher diffusion-limited current density, lower Tafel slope, and higher mass activity. The performance of H2-CMNO is also comparable to several of state-of-the-art catalysts for OER and ORR, with even better stability and activity-per-catalyst-cost. This work widens the potential applications of cost-effective Mn-based perovskites as bifunctional oxygen catalysts for regenerative devices. Experimental Preparation of CaMnO3 (CMO) and CaMn1-xNbxO3 (CMNO): CaMnO3 and CaMn1-xNbxO3 (x= 0.1, 0.2, 0.25, 0.3 and 0.4) were prepared using a conventional solid-state method. Stoichiometric quantities of CaCO3, MnO2, and Nb2O5 were mixed, ball milled for 3 h, and then compressed into pellets. CaMn1-xNbxO3 was obtained after calcining at 1400 °C in air for 6 h, and CaMnO3 was prepared by heating at 1000 °C in air for 6 h. Preparation of H2-treated CaMn0.75Nb0.25O3-δ (H2-CMNO): As-prepared CaMn0.75Nb0.25O3 was heated in a tube furnace at 350 °C for 2 h under a reducing atmosphere of 30% H2/70% N2.


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Physical characterization: X-ray powder diffraction (XRD) was performed using a PANalytical X’Pert Pro diffractometer with Cu K-alpha radiation ( λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was performed on a PHI5600 X-ray photoelectron spectrometer with a monochromatic Al-source X-ray. High-resolution Transmission Electron Microscopy (TEM) was performed with a JEM-2100 field emission electron microscope. The microscope was operated at an acceleration voltage of 200 kV. The Brunauer-Emmett-Teller (BET) specific surface area was determined using a surface analyzer (Beckman Coulter, SA3100). Electrochemical measurements: Electrocatalytic performance was evaluated using a conventional three-electrode system on a Rotating Ring Disk Electrode (RRDE) apparatus (RRDE-3A, BAS Inc.) with a potentiostat (ALS2325E, BAS Inc.). Pt-glassy carbon electrode, Pt electrode, and Ag/AgCl electrode (saturated with 3.0 M NaCl) were used as the working, counter, and reference electrodes, respectively. The disk area of the glassy carbon electrode was 0.125 cm2 and the total catalyst loading was 0.4 mg·cm-2 with a mass ratio of 3:1 of oxide catalyst to conductive carbon. 0.1 mol/L KOH aqueous solution saturated with O2 was used as the electrolyte. The scan rate was set at 10 mV/s and the rotation speed was 1600 rpm for the RDE measurement. For the RRDE study, the working electrode was rotated at varying speeds from 400 rpm to 2500 rpm, and the potential offset on the ring electrode was adjusted to 0.4 V vs RHE. Pt/C (20 wt. % load, Sigma-Aldrich) was also used as a comparison using an identical measurement method. To test the electrical conductivity, the samples were pressed in a barshaped mold and sintered at 900 °C for 2 h to obtain dense bricks. Subsequently, four silver leads were attached to the samples in a 4-probe configuration and the DC conductivity was measured.


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Results and Discussion The phase structure of Nb-doped CMO, CaMn1-xNbxO3 (x =0.1, 0.2, 0.25, 0.3, and 0.4), is analyzed by X-ray diffraction (XRD), with corresponding patterns displayed in Figure 1a. All diffraction patterns can be well indexed to an orthorhombic phase with a space group of Pnma, same with the pristine CMO (Figure S1), suggesting no phase transformation after Nb-doping. As the Nb content (x) increases, the diffraction peaks shift slightly to a lower angle direction, showing that the lattice expands. This is consistent with the calculated lattice parameters by Rietveld refinement analysis (Table S1) and can be linked to the presence of the large Nb ions. Figure 1b shows the crystal structure, taking CaMn0.75Nb0.25O3 as an example. The Mn/Nb cation is coordinated with six oxygen ions and forms a corner-sharing MnO6 octahedron. The electrochemical catalytic performance of the CaMn1-xNbxO3 series was evaluated by RDE measurements using a conventional three-electrode system in 0.1 M KOH solution saturated with O2 at rotation speed of 1600 rpm. Figure 1c and 1d show the OER and ORR kinetic activity of CaMn1-xNbxO3 series. With reference to the OER activity, all perovskites with Nb doping outperform the pristine CMO. At an overpotential of 370 mV (1.60 V vs RHE), the current density has the following descending trend: CaMn0.75Nb0.25O3 > CaMn0.8Nb0.2O3 > CaMn0.9Nb0.1O3 > CaMn0.7Nb0.3O3 > CaMn0.6Nb0.4O3 > CMO. As for ORR, CaMn1-xNbxO3 (x =0.1, 0.2, 0.25, and 0.3) also show improved activity in comparison to pristine CMO, while CaMn0.6Nb0.4O3 shows a decreased performance. At 0.78 V, the current density follows the descending trend of CaMn0.75Nb0.25O3 > CaMn0.8Nb0.2O3 > CaMn0.9Nb0.1O3 > CaMn0.7Nb0.3O3 > CMO > CaMn0.6Nb0.4O3. Consistent results are obtained from the LSV curves of all six catalysts, as shown in Figure S2, where CaMn0.75Nb0.25O3 displays the best OER and ORR performance.


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As more Nb doped into the perovskite lattice, more Mn4+ ions are reduced into Mn3+. The Mn4+ (3d3) and Mn3+ (3d4) in perovskites have electron configuration of and ,

respectively[8d,

15]

. While Nb substitution ratio increases from 10% to 40%, the Mn valence

changes from 3.82 to 3.26 (Table S2) and the eg electron filling approaches to 1. However, Nbdoping not only favorably affects the eg orbital occupancy, but also inevitably decreases the concentration of the Mn ions, which work as the active sites for ORR and OER. As shown in Figure 1e, both ORR and OER activity increases with the Nb substitution ratio x, when x is lower than 25%; and the activity diminishes by further increasing x. The balance, therefore, is achieve with CaMn0.75Nb0.25O3 (CMNO).


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Figure 1. (a) XRD patterns of CaMn1-xNbxO3 with x = 0.1, 0.2, 0.25, 0.3, 0.4, (b) Crystal structure of CaMn0.75Nb0.25O3. (c) OER and (d) ORR Tafel slope of CaMn1-xNbxO3 (x = 0.1, 0.2, 0.25, 0.3, 0.4) and CMO in O2-saturated 0.1 M KOH solution under rotation speed of 1600 rpm. (e) The combined effect of eg electron filling and Mn ion ratio at B-site on the current density at 0.78 V and 1.63 V. In order to further modify its electrochemical properties, CMNO was then treated with H2. As shown in Figure S3, the H2-treated CaMn0.75Nb0.25O3-δ (H2-CMNO) shows similar XRD diffraction pattern as the non-treated CMNO, suggesting that the Pnma phase is well preserved


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after H2 treatment. Figure 2a shows the XRD Rietveld refinement analysis of H2-CMNO, where the experimental data well matches the refined profile, suggesting the presence of little impurities. This phase stability is surprising and in stark contrast with the effects of the H2 treatment on CMO as reported in the literature25,

30

and in this work. Under the same H2

treatment conditions as those use for CMNO, the H2-treated CMO (H2-CMO) shows a distorted phase with space group of Pbam (Figure S4), which is clearly different to that of the starting CMO materials (Pnma). This further shows that Nb-doping can enhance the phase stability of CMO with respect to H2-treatment. Moreover, the crystal phase of H2-CMNO was further revealed by TEM and the corresponding Fast Fourier Transform (FFT) pattern. As shown in Figure 2b, the lattice fringes are well observed, and the d-spacing is measured as 0.37 nm and 0.44 nm for (020) and (011) planes, respectively, consistent with the XRD results.


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Figure 2. (a) The XRD pattern and the Rietveld refinement plot of H2-CMNO (orthorhombic, pnma), (b) TEM image and the corresponding FFT pattern of H2-CMNO. (c) The Mn 2p XPS spectra and (d) the Mn 3s XPS spectra of CMNO and H2-CMNO. The Mn oxidation state is investigated by X-ray photoelectron spectroscopy (XPS) and iodometric titration. Figure 2c shows the Mn 2p XPS spectra of CMNO and H2-CMNO. The Mn 2p3/2 band of CMNO could be deconvoluted into two peaks at 641.6 eV and 642.8 eV, corresponding to Mn3+ and Mn4+, respectively. In contrast, the three peaks in the Mn 2p3/2 band of H2-CMNO denote the presence of trivalent Mn (Mn2+, Mn3+, and Mn4+). From the Mn 2p spectra, the nominal oxidation state of Mn is calculated to be 3.62 and 3.03 for CMNO and H2CMNO, respectively. The Mn 3s spectra support the hypothesis that the oxidation state of Mn changes after H2 treatment. As shown in Figure 2d, the separation energy (∆E) of CMNO and H2-CMNO is 5.15 eV and 5.54 eV, respectively. Owing to the monotone increasing relationship between ∆E of Mn 3s peaks and the Mn valence[16], the oxidation state of Mn of two materials correspond to the following order: 4 > CMNO > H2-CMNO ≈ 3. This is well matched with the Mn 2p spectra analysis and further confirmed by iodometric titration (Table S2). The RDE measurement was conducted to evaluate the electrocatalytic activity of H2-CMNO in O2-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm. Figure 3a shows the iRcorrected OER LSV curves for CMO, CMNO, and H2-CMNO. H2-CMNO has an onset potential of 1.51 V (at 0.5 mA·cm-2), a value similar to that of most-recently reported perovskite catalysts24,

38, 41-42

, and the state-of-the-art IrO2 (1.47 V)37, and obviously lower than that of

CMNO (1.53 V) and CMO (1.6 V). Moreover, at a current density of 10 mA·cm-2, H2-CMNO shows a small overpotential η (0.55 V), which is slightly more positive than BSCF and PBC15. It


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is worth noting that many calcium-manganese perovskites, such as CMNO, CMO, Ca2Mn2O530, and CaMnO3-x25, fail to reach 10 mA·cm-2 over the whole measured potential range, which means that H2 treatment resulted in an one-order-of-magnitude increase in the OER activity.

Figure 3. (a) The LSV curves, (b) Tafel plots, and (c) the mass activity of CMO, CMNO, and H2-CMNO in O2-saturated 0.1 M KOH solution at 1600 rpm. (d) O1s XPS spectra of CMO and


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H2-CMNO. (e) The chronoamperometric (CA) response of H2-CMNO and CMO at 1.65 V vs. RHE. (f) Activity per catalyst cost of H2-CMNO, BSCF, PBS and IrO2 collected at 0.4 V overpotential. The Tafel plots of H2-CMNO, CMNO, and CMO, which show the kinetic activity, are depicted in Figure 3b. The measured Tafel slopes are 98, 140, and 208 mV dec-1 for H2-CMNO, CMNO, and CMO, respectively. H2-CMNO shows the lowest Tafel slope, suggesting the highest kinetic activity. Moreover, H2-CMNO has the highest mass activity among the three with a current density 2 to 4 times higher than that of CMO. This excellent OER kinetic and intrinsic activity of H2-CMNO may be related to several factors. First, the Mn3+ in H2-CMNO gives

configuration, which favorably affects the binding of intermediate oxygen-species (e.g. OHgroups) on the oxygen surface, thus showing a better OER performance. The Mn4+ in CMO, on the other hand, has configuration, which is inferior to bind with OH-. This is also verified by

O 1s XPS spectra, as shown in Figure 3d, where the peak around 531.2-531.9 eV corresponds to OH- groups. Obviously, H2-CMNO has more OH- groups on its surface than CMO, suggesting a stronger OH- adsorption ability. Second, H2 treatment did not change the phase structure of CMNO, but simultaneously created more oxygen vacancies. This contributes to the enhancement of electronic transport. The electrical conductivity of H2-CMNO is measured to be 2.19×10-2 S/m, which 10 times higher than that of CMO (3.33 ×10-3 S/m). Stability is another key property of a catalyst. The chronoamperometry (CA) measurements carried out at 1.65 V vs. RHE clearly indicate that H2-CMNO is a more stable OER catalyst than CMO, see Figure 3e. At the end of a 7,000s continuous test, CMO lost as much as 90% of its initial activity, while H2-CMNO shows a much higher stability with an 84.5% retention. H2-


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CMNO appears to stabilize after 500s, with negligible change in current density. In contrast, the OER activity of CMO continuously degrades until 2000s, when only 9% activity is preserved. The better stability of H2-CMNO may be ascribed to the enhanced phase stability resulted from the incorporation of Nb ions37, 43. In comparison to state-of-the-art catalysts, which contain Co or noble metals, additional advantage of H2-CMNO lies on the low-cost and negligible environmental impact of Mn. Figure 3f depicts the activity-per-catalyst-cost of H2-CMNO and other reported highly active OER catalysts8. (Honestly, it has to be mentioned that the elemental prices used here are of 5 years ago, therefore there might be small differences from that of today.) Even though the activity-percatalyst-cost of H2-CMNO is still lower than BSCF, which, however, has a poor stability44-45, it is 3 times higher than PBC and 50 times higher than IrO2. This is particularly significant because, with further work, low-cost Mn-based-perovskites could be viable bifunctional catalysts. Furthermore, the ORR catalytic activity of H2-CMNO was also evaluated and compared with CMO using RDE measurement in O2-saturated 0.1 M KOH solution. Figure 4a shows the LSV curves of H2-CMNO, CMNO, and CMO at 1600 rpm. H2-CMNO exhibits the highest ORR activity among the catalysts with the most positive onset potential (0.83 V), which is comparable with most-recently reported catalysts16, 41, 46. The diffusion-limited current density of H2-CMNO is measured to be 5.93 mA·cm-2. This value is 1.47 mA·cm-2 higher than that of CMO, 0.49 mA·cm-2 higher than that of CMNO, and even 0.41 mA·cm-2 higher than that of Pt/C. It is worth noting that a diffusion-limited current density of 5.93 mA·cm-2 at 1600 rpm is very close to the theoretical limit of the four-electron transfer[17], i.e., 6.0 mA·cm-2. The Tafel plots of four catalysts are depicted in Figure 4b. The kinetic current density (Jk) is calculated using Koutechy-


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Levich (K-L) equation47-48. A Tafel slope of 80 mV/dec was measured for H2-CMNO, while CMO and CMNO have much higher Tafel slopes, 123 and 136 mV/dec, further confirming the superior ORR performance of H2-CMNO. Furthermore, the mass activities of CMO, CMNO, and H2-CMNO are shown in Figure 4c at different potentials. In the potential range from 0.75 V to 0.6 V vs. RHE, H2-CMNO has the highest mass activity, which is 2 to 3 times greater than that of CMO. The ORR activity decay of H2-CMNO is also measured by CA study at 0.75 V vs RHE, as shown in Figure 4d. All three catalysts, CMO, CMNO, and H2-CMNO, exhibit excellent ORR stability, with remaining activity around 95% of their respective initial activity after 10,000s operation.


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Figure 4. (a) Linear sweep voltammetry (LSV) curves and (b) Tafel plots of CMO, CMNO, H2-CMNO, and Pt/C obtained in O2-saturated 0.1 M KOH solution at the rotation speed of 1600 rpm. (c) The mass activity at different potentials. (d) The chronoamperometric (CA) response of CMO, CMNO, and H2-CMNO at 0. 7 V. (e) LSV curves of H2-CMNO at rotation speeds from 400 rpm to 2500 rpm, and the corresponding Koutechy-Levich plots at different potentials. (f) HO2- yields and the electron transfer number (n) of CMO, CMNO, and H2-CMNO. We further studied the ORR electron transfer pathway of H2-CMNO via the Koutechy-Levich (K-L) method and compared it to CMO. Figure 4e and Figure S6 display the LSV curves of H2-


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CMNO and CMO at rotation speeds from 400 rpm to 2500 rpm together with the corresponding K-L plots. For both catalysts, as expected, higher rotation speeds lead to higher current densities, where all K-L plots show good linearity. The average electron transfer number (n) is calculated from the K-L plots as 3.86 and 3.58 for H2-CMNO and CMO, respectively. This means that H2CMNO is characterized by a more effective electron-transfer than CMO. Consistent results were obtained using the RRDE measurements at 1600 rpm. The current density of H2-CMNO, CMNO, and CMO collected on the disk electrode and the ring electrode are shown in Figure S7. H2-CMNO has the lowest current density on the ring electrode, even though its current density on the disk electrode is the highest among the three (Figure S7). This indicates that H2-CMNO has a small H2O2 yield and an efficient electron pathway. The actual H2O2 yield and the electron transfer number (n) can be calculated from the following equations10:

n=

⁄

H O % =

(1)

⁄

(2)

⁄

where N is the collection efficiency of the electrode (which is 42.4% for RRDE-3A electrode), Iring is the current on the ring electrode, and Iring is the current on the disk electrode. Figure 4f shows the calculated n and H2O2 yield as a function of potential. The H2O2 yield of H2-CMNO is below 5% from 0.5 V to 0.7 V, which is much smaller than CMNO (9% to 12%) and CMO (17% to 27%). In the potential range from 0.2 V to 0.5 V, H2-CMNO also has a relatively lower H2O2 yield (below 10%) in comparison to CMNO and CMO. Furthermore, the calculated n for CMNO and CMO is 3.7-3.8 and 3.4-3.7, respectively. This suggests that electron-transfer is greatly enhanced by Nb substitution. Meanwhile, the n of r H2-CMNO is above 3.8 over the whole


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potential range, showing a dominant four-electron transfer pathway. All these results, in conjunction with the RDE measurements, clearly show that combining the reduction and Nbsubstitution can significantly improve the ORR activity and the electron transfer pathway. To evaluate the overall bifunctional performance of CMO, CMNO, and H2-CMNO, the difference between the OER potential at 10 mAcm-2 and ORR potential at -3 mAcm-2 was calculated (Table S5). In comparison to CMNO and CMO, H2-CMNO has the smallest potential (∆E) difference (i.e. 1.10 V), a value comparable to noble metal catalysts19, further supporting its promise as a bifunctional oxygen catalyst that is required by reversible energy devices. Conclusions We developed a novel bifunctional catalyst, CaMn0.75Nb0.25O3-δ (H2-CMNO), with remarkable OER and ORR activity in alkaline media. H2-CMNO shows one-order-of-magnitude higher OER activity and better stability than that of CaMnO3 (CMO). The activity-per-catalyst-cost of H2CMNO is 50 times higher than the benchmark catalyst IrO2. The ORR activity, as well as stability, of H2-CMNO also far surpass that of CMO and is comparable to commercial Pt/C. To the best of our knowledge, this is the first calcium-manganese perovskite that exhibits efficient performance towards both OER and ORR. This remarkable electrocatalytic performance may due to several beneficial factors, including enhanced phase stability, optimal eg filling level, good OH- adsorption ability, and improved electrical conductivity. Therefore, this work presents a new approach to activate the bifunctionality of low-cost Mn-perovskites as oxygen catalysts for reversible energy devices.


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ASSOCIATED CONTENT Supporting Information. Additional figures and tables showing crystal structure, chemical and electrocatalytic properties of CMO and CaMn1-xNbxO3, CMNO, and H2-CMNO using characterization techniques including XRD, SEM, BET, iodometric titration, and RRDE measurement. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

ACKNOWLEDGMENT The authors gratefully acknowledge HKUST for providing start-up funds, and the Research Grants Council of Hong Kong for support through the Project 16207615. REFERENCES

(1) Chen, D.; Chen, C.; Baiyee, Z. M.; Shao, Z.; Ciucci, F. Nonstoichiometric Oxides as LowCost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices. Chem. Rev, 2015, 115, 9869-9921. (2) Han, L.; Dong, S.; Wang, E. Transition‐Metal (Co, Ni, and Fe)‐Based Electrocatalysts for the Water Oxidation Reaction. Adv. Mater., 2016, 28, 9266-9291. (3) Kakati, N.; Maiti, J.; Lee, S. H.; Jee, S. H.; Viswanathan, B.; Yoon, Y. S. Anode Catalysts for Direct Methanol Fuel Cells in Acidic Media: Do We Have Any Alternative for Pt or Pt–Ru? Chem. Rev., 2014, 114, 12397-12429.


18

Page 19 of 25

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


(4) Lu, Z.; Wang, H.; Kong, D.; Yan, K.; Hsu, P.-C.; Zheng, G.; Yao, H.; Liang, Z.; Sun, X.; Cui, Y. Electrochemical Tuning of Layered Lithium Transition Metal Oxides for Improvement of Oxygen Evolution Reaction. Nat. Commun., 2014, 5, 4345. (5) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W. D.; Wang, X. A Metal–Organic Framework-Derived Bifunctional Oxygen Electrocatalyst. Nat. Energy, 2016, 1, 15006. (6) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Design Principles for Oxygen-Reduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal–Air Batteries. Nat. Chem., 2011, 3, 546-550. (7) Lim, B.; Jiang, M.; Camargo, P. H.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science, 2009, 324, 13021305. (8) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science, 2011, 334, 1383-1385. (9) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett., 2012, 3, 399-404. (10) Lyu, Y.-Q.; Chen, C.; Gao, Y.; Saccoccio, M.; Ciucci, F. In Situ Preparation of Ca0.5Mn0.5O/C as a Novel High-Activity Catalyst for the Oxygen Reduction Reaction. J. Mater. Chem. A, 2016, 4, 19147-19153. (11) Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res., 2013, 46, 1878-1889.


19


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 20 of 25

(12) Zhu, Y.; Zhou, W.; Yu, J.; Chen, Y.; Liu, M.; Shao, Z. Enhancing Electrocatalytic Activity of Perovskite Oxides by Tuning Cation Deficiency for Oxygen Reduction and Evolution Reactions. Chem. Mater., 2016, 28, 1691-1697. (13) Zeng, M.; Liu, Y.; Zhao, F.; Nie, K.; Han, N.; Wang, X.; Huang, W.; Song, X.; Zhong, J.; Li, Y. Metallic Cobalt Nanoparticles Encapsulated in Nitrogen‐Enriched Graphene Shells: Its Bifunctional Electrocatalysis and Application in Zinc–Air Batteries. Adv.Funct. Mater., 2016, 26, 4397-4404. (14) Cheng, Y.; Dou, S.; Veder, J.-P.; Wang, S.; Saunders, M.; Jiang, S. P. Efficient and Durable Bifunctional Oxygen Catalysts Based on Nifeo@ Mno X Core–Shell Structures for Rechargeable Zn–Air Batteries. ACS Appl. Mater. Interfaces, 2017, 9, 8121-8133. (15) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y.-L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun., 2013, 4, 2439. (16) Jung, J.-I.; Risch, M.; Park, S.; Kim, M. G.; Nam, G.; Jeong, H.-Y.; Shao-Horn, Y.; Cho, J. Optimizing Nanoparticle Perovskite for Bifunctional Oxygen Electrocatalysis. Energy Environ. Sci., 2016, 9, 176-183. (17) Jung, J. I.; Park, S.; Kim, M. G.; Cho, J. Tunable Internal and Surface Structures of the Bifunctional Oxygen Perovskite Catalysts. Adv. Energy Mater., 2015, 5, 1501560. (18) Stoerzinger, K. A.; Risch, M.; Han, B.; Shao-Horn, Y. Recent Insights into Manganese Oxides in Catalyzing Oxygen Reduction Kinetics. ACS Catal., 2015, 5, 6021-6031. (19) Gorlin, Y.; Jaramillo, T. F. A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. J. Am. Chem. Soc., 2010, 132, 13612-13614.


20

Page 21 of 25

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


(20) Hardin, W. G.; Mefford, J. T.; Slanac, D. A.; Patel, B. B.; Wang, X.; Dai, S.; Zhao, X.; Ruoff, R. S.; Johnston, K. P.; Stevenson, K. J. Tuning the Electrocatalytic Activity of Perovskites through Active Site Variation and Support Interactions. Chem. Mater., 2014, 26, 3368-3376. (21) Mueller, D. N.; Machala, M. L.; Bluhm, H.; Chueh, W. C. Redox Activity of Surface Oxygen Anions in Oxygen-Deficient Perovskite Oxides During Electrochemical Reactions. Nat. Commun., 2015, 6, 6097. (22) Mosa, I. M.; Biswas, S.; El-Sawy, A. M.; Botu, V.; Guild, C.; Song, W.; Ramprasad, R.; Rusling, J. F.; Suib, S. L. Tunable Mesoporous Manganese Oxide for High Performance Oxygen Reduction and Evolution Reactions. J. Mater. Chem. A, 2016, 4, 620-631. (23) Zhu, Y.; Zhou, W.; Shao, Z. Perovskite/Carbon Composites: Applications in Oxygen Electrocatalysis. small, 2017, 13, 1603793. (24) Zhu, Y.; Zhou, W.; Zhong, Y.; Bu, Y.; Chen, X.; Zhong, Q.; Liu, M.; Shao, Z. A Perovskite Nanorod as Bifunctional Electrocatalyst for Overall Water Splitting. Adv. Energy Mater., 2017, 7, 1602122. (25) Du, J.; Zhang, T.; Cheng, F.; Chu, W.; Wu, Z.; Chen, J. Nonstoichiometric Perovskite CaMnO3−δ for Oxygen Electrocatalysis with High Activity. Inorg. Chem., 2014, 53, 9106-9114. (26) Guo, Y.; Tong, Y.; Chen, P.; Xu, K.; Zhao, J.; Lin, Y.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y. Engineering the Electronic State of a Perovskite Electrocatalyst for Synergistically Enhanced Oxygen Evolution Reaction. Adv. Mater., 2015, 27, 5989-5994. (27) Han, X.; Cheng, F.; Zhang, T.; Yang, J.; Hu, Y.; Chen, J. Hydrogenated Uniform Pt Clusters Supported on Porous CaMnO3 as a Bifunctional Electrocatalyst for Enhanced Oxygen Reduction and Evolution. Adv. Mater., 2014, 26, 2047-2051.


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Page 22 of 25

(28) Han, X.; Zhang, T.; Du, J.; Cheng, F.; Chen, J. Porous Calcium–Manganese Oxide Microspheres for Electrocatalytic Oxygen Reduction with High Activity. Chem. Sci., 2013, 4, 368-376. (29) Hancock, C. A.; Ong, A. L.; Slater, P. R.; Varcoe, J. R. Development of CaMn1−XRuXO3−Y (X= 0 and 0.15) Oxygen Reduction Catalysts for Use in Low Temperature Electrochemical Devices Containing Alkaline Electrolytes: Ex Situ Testing Using the Rotating Ring-Disk Electrode Voltammetry Method. J. Mater. Chem. A, 2014, 2, 3047-3056. (30) Kim, J.; Yin, X.; Tsao, K.-C.; Fang, S.; Yang, H. Ca2Mn2O5 as Oxygen-Deficient Perovskite Electrocatalyst for Oxygen Evolution Reaction. J. Am. Chem. Soc., 2014, 136, 1464614649. (31) Yamada, I.; Fujii, H.; Takamatsu, A.; Ikeno, H.; Wada, K.; Tsukasaki, H.; Kawaguchi, S.; Mori, S.; Yagi, S. Bifunctional Oxygen Reaction Catalysis of Quadruple Manganese Perovskites. Adv. Mater., 2016. (32) Vesborg, P. C.; Jaramillo, T. F. Addressing the Terawatt Challenge: Scalability in the Supply of Chemical Elements for Renewable Energy. RSC Adv., 2012, 2, 7933-7947. (33) Wiechen, M.; Zaharieva, I.; Dau, H.; Kurz, P. Layered Manganese Oxides for WaterOxidation: Alkaline Earth Cations Influence Catalytic Activity in a Photosystem II-Like Fashion. Chem. Sci., 2012, 3, 2330-2339. (34) Najafpour, M. M.; Ehrenberg, T.; Wiechen, M.; Kurz, P. Calcium Manganese (III) Oxides (CaMn2O4⋅ xH2O) as Biomimetic Oxygen‐Evolving Catalysts. Angew. Chem., Int. Ed., 2010, 49, 2233-2237.


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Page 23 of 25

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


(35) Feng, J.; Liang, Y.; Wang, H.; Li, Y.; Zhang, B.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Engineering Manganese Oxide/Nanocarbon Hybrid Materials for Oxygen Reduction Electrocatalysis. Nano Res., 2012, 5, 718-725. (36) Xu, J. J.; Xu, D.; Wang, Z. L.; Wang, H. G.; Zhang, L. L.; Zhang, X. B. Synthesis of Perovskite‐Based Porous La0.75Sr0.25MnO3 Nanotubes as a Highly Efficient Electrocatalyst for Rechargeable Lithium–Oxygen Batteries. Angew. Chem., Int. Ed., 2013, 52, 3887-3890. (37) Zhu, Y.; Zhou, W.; Chen, Z. G.; Chen, Y.; Su, C.; Tadé, M. O.; Shao, Z. SrNb0.1Co0.7Fe0.2O3−∆ Perovskite as a Next‐Generation Electrocatalyst for Oxygen Evolution in Alkaline Solution. Angew. Chem., Int. Ed., 2015, 54, 3897-3901. (38) Petrie, J. R.; Cooper, V. R.; Freeland, J. W.; Meyer, T. L.; Zhang, Z.; Lutterman, D. A.; Lee, H. N. Enhanced Bifunctional Oxygen Catalysis in Strained Lanio3 Perovskites. J. Am. Chem. Soc., 2016, 138, 2488-2491. (39) Cohn, J.; Neumeier, J. Heat Conduction and Magnetic Phase Behavior in Electron-Doped Ca1−XLaXMnO3 (0

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