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Intrinsically Conductive Perovskite Oxides with Enhanced Stability and Electrocatalytic Activity for Oxygen Reduction Reactions Xiaoming Ge, Yonghua Du, Bing Li, T. S. Andy Hor, Melinda Sindoro, Yun Zong, Hua Zhang, and Zhaolin Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02493 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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ACS Catalysis

Intrinsically Conductive Perovskite Oxides with Enhanced Stability and Electrocatalytic Activity for Oxygen Reduction Reactions Xiaoming Ge,†,# Yonghua Du,‡,# Bing Li,† T. S. Andy Hor,†,⊥ Melinda Sindoro,§ Yun Zong,*, † Hua Zhang,*,§ and Zhaolin Liu*,† †

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science,

Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Republic of Singapore 138634 ‡

Institute of Chemical and Engineering Science (ICES), A*STAR (Agency for Science,

Technology and Research), 1 Pesek Road, Jurong Island, Republic of Singapore 627833 ⊥Department

of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR,

China §

Center for Programmable Materials, School of Materials Science and Engineering, Nanyang

Technological University, 50 Nanyang Avenue, Republic of Singapore 639798 #

These authors contributed equally.

Corresponding authors: * E-mail for Z. L. L.: [email protected].

ACS Paragon Plus Environment

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* E-mail for H. Z.: [email protected]. * E-mail for Y. Z.: [email protected].

ABSTRACT

The oxygen reduction reaction (ORR) is traditionally catalyzed by carbon-supported precious metals, heteroatom-doped carbons, and transition metal-carbon hybrids. Despite their good electric conductivity and high catalytic activities, these carbon-containing catalysts suffer from electrochemical carbon corrosion which limits their utilities in metal-air batteries and fuel cells. Here, we report a class of perovskite La0.5Sr0.5Mn1-xNixO3-δ nanocrystals that are intrinsically conductive with good electrocatalytic activity for ORR. Among these perovskites, La0.5Sr0.5Mn0.9Ni0.1O3-δ (δ=0.06, LSMN) exhibited the highest electrocatalytic activity for ORR with an onset potential of 1.02 V, half-wave potential of 0.80 V, and Tafel slope of ‒68 mV decade-1 in 0.1 molar potassium hydroxide aqueous solution. Negligible degradation of oxygen reduction currents was observed after 300 cyclic voltammetry scans from 1.08 V to 0.15 V. We demonstrated that the electrically conductive perovskites with transition metal redox couples and oxygen vacancies are essential. Our work demonstrates the possibility of carbon-free oxygen electrocatalysis with widely promising applications.

KEYWORDS: oxygen reduction reaction, carbon-free, conductive perovskite, transition metal oxide, reaction mechanism


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INTRODUCTION The harvest of renewable energy has driven increasing demands on energy storage and conversion devices,1,2 some of which rely on the oxygen electrocatalysis.3,4 Oxygen reduction reaction (ORR) is one of the most important electrode reactions in the field of electrocatalysis.5,6 In recent years, the ORR in alkaline media has been attracting attention due to the strong interest in metal-air batteries and alkaline fuel cells.7,8 The performance of these energy storage and conversion devices is constrained by the high overpotential of oxygen electrocatalysts. Albeit their good activity,9 state-of-the-art precious metal catalysts often suffer from insufficient stability due to the formation of oxide layer10 and metal dissolution during the ORR.11,12 Non-precious metal catalysts, including heteroatom-doped carbons13 and transition metal(TM)/transition metal oxide (TMO)‒carbon hybrids,14 have been applied in ORR systems. TMO is generally used together with carbonaceous materials which function as the conducting agents. In that, the incorporation of carbon additives helps to mitigate the poorly conducting nature of most TMO.15 Carbon, however, may suffer from the corrosion problem, leading to poor operation durability.16‒18 Carbon materials are poor catalysts for the electrochemical reduction of HO2‒

19,20

and chemical disproportionation of H2O2.21 As ORR proceeds via a two-electron

(2e‒) pathway on most of the carbon materials,22 HO2‒ ions tend to accumulate during the ORR process and oxidize the carbon materials.23 Such carbon corrosion becomes even more severe when the electrochemical devices are under polarized conditions.24 Carbon-free ORR catalysts are hence highly desirable for the development of efficient and low-cost catalysts despite of their serious technical challenges that still persist. Perovskite (ABO3) consists of corner-shared BO6 octahedra and A-site cations at corners of the unit cell. When used alone, most perovskites exhibit sluggish reaction kinetics and low


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oxygen reduction currents for ORR.25 Combination of carbon materials and perovskites is hence essential to increase the electrical conductivity of catalysts and/or improve the ORR activity.26,27 However, incorporation of carbon materials might introduce the carbon corrosion problem. Therefore, it is highly desirable to develop carbon-free perovskite catalysts with considerable conductivity and good reaction kinetics for ORR. Herein we design a class of nickel-doped lanthanum strontium manganites (La0.5Sr0.5Mn1xNixO3-δ)

and tune the Ni/Mn ratio in order to maximize its ORR activity. The electrically

conductive La0.5Sr0.5Mn1-xNixO3-δ attained fairly good reaction kinetics for ORR without the incorporation of conducting additives. La0.5Sr0.5Mn0.9Ni0.1O3-δ (δ=0.06, LSMN), the optimised composition of La0.5Sr0.5Mn1-xNixO3-δ, achieved the onset potential (Eonset), half-wave potential (E½), and Tafel slope of 1.02 V, 0.80 V, and ‒68 mV decade-1 in O2-staturated 0.1 M KOH solution, respectively. These results place LSMN as a highly efficient ORR catalyst in alkaline media, on par with the state-of-the-art precious metal/carbon catalysts. Possible ORR mechanism and reaction schemes on LSMN have been proposed based on the rotating disk electrode (RDE), rotating ring-disc electrode (RRDE), and material characterization results.

RESULTS AND DISCUSSION The materials La0.5Sr0.5Mn1-xNixO3-δ can be treated as La0.5Sr0.5MnO3-δ (LSM) doped with Ni in the B-site. Manganite perovskites are stable for ORR in alkaline media,28 in contrast to nickelite perovskites that decompose when subject to prolonged operation.29 We designed perovskite of the formula La0.5Sr0.5Mn1-xNixO3-δ taking into considerations the chemical and electrochemical stability of manganite and electrocatalytic activity of Mn3+/4+ and Ni2+/3+ redox couples. Samples of La0.5Sr0.5Mn1-xNixO3-δ were synthesized by a citric route to obtain single phase perovskite


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La0.5Sr0.5Mn1-xNixO3-δ (0 ≤ x ≤ 0.2) with precise stoichiometry control (see Figure S1 to S4 and Table S1 in the Supporting Information). A preliminary examination on the onset potential (Eonset) and half-wave potential (E½) of the iR-compensated polarization curves reveals pronounced composition effect on the ORR activity. Interestingly, single phase perovskites, La0.5Sr0.5Mn1-xNixO3-δ (0 ≤ x ≤ 0.2), exhibit more positive Eonset and E1/2 than the other compositions (Figure S5). La0.5Sr0.5Mn0.9Ni0.1O3-δ (δ =0.06, LSMN) among different compositions is the most ORR active. FESEM morphology images show that the particle sizes of LSMN are comparable to those of LSM and La0.5Sr0.5Mn0.9Ni0.1O3δ

(Figure S6). The Brunauer‒Emmett‒Teller (BET) surface areas were obtained from nitrogen

adsorption/desorption isotherms (Figure S7). The BET surface area of LSMN is 1.98 m2 g-1, smaller than those of LSM (2.05 m2 g-1) and LSMN5582 (3.25 m2 g-1). Based on morphology and surface area analyses, it is inferred that the better electrocatalytic activity of LSMN must be due to its intrinsic ORR activity but not the particle size and surface area effect. Diffusionlimiting currents, which are characteristic behaviours of conducting catalysts, are observed in the oxygen reduction waves of La0.5Sr0.5Mn1-xNixO3-δ (0 ≤ x ≤ 0.2). [Fe(CN)6]3‒/[Fe(CN)6]4‒ is a facile redox couple that measures electrical conductivity. In this work, the La0.5Sr0.5Mn1-xNixO3-δ catalyst layers were cast on glassy carbon RDE and the [Fe(CN)6]3‒/4‒ redox analysis revealed that LSMN is the most optimised composition in La0.5Sr0.5Mn1-xNixO3-δ in terms of conductivity (Figure S8). The statement of LSMN as the most conductive composition in La0.5Sr0.5Mn1xNixO3-δ

is confirmed by 4-probe resistivity tests of the sintered La0.5Sr0.5Mn1-xNixO3-δ pellets

(Figure S9). The impedance spectroscopy showed that the intrinsic conductivity (σe) of LSMN is 141 mS cm-1 at r.t. (Figure S10).


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Figure 1a and 1b show typical morphology images of LSMN. Particle sizes of LSMN are in the range of 40–70 nm. This moderate particle size distribution is desirable for the oxygen electrocatalysis because smaller oxide particles would be prone to agglomeration and corrosion upon the electrochemical operation. HRTEM analysis revealed the lattice fringes of two nanocrystals with d= 0.27 nm and d= 0.19 nm (Figure 1c), corresponding to the (104) planes and the (024) planes of the perovskite structure, respectively. LSMN has a trigonal/rhombohedral structure with lattice parameters of a = 5.444 Å and c = 13.378 Å and a space group of R‒3cH (Figure 1d, Figure S11 and Table S2). The near unity Goldschmidt tolerance factor implies a robust perovskite structure of LSMN (Table S3). Figure 1E shows the structural visualization of corner-sharing MO6 octahedra of the perovskite. Figure 2 shows the X-ray absorption near edge structure (XANES) of the LSMN, the LSM, and the reference samples. Compared to that of LSM, the Mn K-edge spectrum of LSMN is slightly shifted to the high energy end as a consequence of B-site doping of Ni. The Mn valence in LSMN and LSM was found to be of 3.52 and 3.39, respectively (Figure 2a). This indicates that the Mn3+/4+ redox couple of LSMN comprises of 48.0 at.% of Mn3+ and 52.0 at.% of Mn4+, in comparison with the Mn3+(60.8 at.%)/Mn4+(39.2 at.%) redox couple of LSM. Figure 2b suggests that the Ni2+/3+ redox couple consists of 88.0 at.% Ni2+ and 12.0 at.% Ni3+. The covalent bonding between TM and O is evident from the characteristic Mn‒O and Ni‒O peaks of O K-edge X-ray absorption spectra (Figure S12). Specifically for the case of LSMN, the Ni 3d‒ O 2p peak at 530.1 eV is prominent but the signal at 526.4 eV, which is characteristic for octahedral Ni3+,30,31 is virtually invisible. This observation is consistent with XANES results and confirms the dominance of Ni2+ in the Ni2+/3+ redox couple of LSMN.

The oxygen non-

stoichiometry is calculated based on the electroneutrality and the mixed valence states of Mn and


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Ni cations. The formula of LSMN and LSM are described as La0.5Sr0.5Mn0.9Ni0.1O3-δ (δ=0.06) and La0.5Sr0.5MnO3-δ (δ=0.036), respectively. A better ORR activity of LSMN than that of LSM, as indicated in Figure S5, is partly attributed to the enhanced oxygen deficiency. Figure 3 shows the fitted XPS spectra of La 3d5/2, Sr 3d, Mn 2p, Ni 2p3/2, and O 1s of LSMN. The corresponding binding energy (BE) and atomic ratio are listed in Table S4. La 3d has satellite and shake-up peaks adjacent to the core level peak (Figure 3a). The BE separation between the shake-up and the main peak of La 3d5/2 (∆BE3d5/2) is a finger-print of the La‒O bond. The BE and ∆BE3d5/2 of La 3d5/2 show the characteristics of trivalent La.32 The Sr 3d spectrum is fitted by two sets of the Sr 3d5/2 and 3d3/2 doublets with an energy separation of 1.80 eV (Figure 3b). The low BE doublet is assigned to near-surface regions and the high BE doublet is associated with the surface termination.33,34 The Mn 2p and Ni 2p3/2 spectra are deconvoluted into Mn3+/4+ and Ni2+/3+ sub-peaks, respectively (Figure 3c and 3d). The doublet separation between the fitted peaks of Mn4+ 2p3/2 and Mn3+ 2p3/2 (11.43 eV) is smaller than that of MnO2 (11.7 eV) but larger than that of metallic Mn (11.25 eV).35 XPS peak analysis indicates 30.1 % Mn3+ characteristic of the Mn3+/4+ redox couple and 30.8 % Ni3+ characteristic of the Ni2+/3+ redox couple. Figure 3e shows the O 1s spectrum consisting of sub-peaks from the lattice, the surface, and surface phases such as hydrates and hydroxyl species.33,36 The XPS quantification listed in Table S4 shows several aspects of the surface property. The atomic ratio of (Mn+Ni)/(La+Sr) at 0.75 reveals the A-site rich surface of LSMN, in agreement with other typical perovskites such as LaMnO3.37 A Sr/La ratio of 1.77 indicates that Sr is the preferred A-site cation. Similar Sr surface enrichment was also observed for other perovskites, as reported in the literature.33 The enrichment of A-site cations and especially the Sr enrichment are confirmed by the TOF-SIMS (Figure S13). The preference of Ni over Mn on the


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surface is reflected from the Mn/Ni ratio of 3.45, compared to the stoichiometric value of 9. The ratio of O/(La+Sr+Mn+Ni) at 0.88, much lower than the theoretical value of 1.47 as calculated based on electroneutrality, is consistent with the proposal of cation-rich surface termination and the existence of oxygen vacancies. To summarize, the LSMN surfaces are nonstoichiometric with features of oxygen deficiency and cation surface termination in the order of A-sites (Sr > La) > B-sites (Ni > Mn). High-valency TM cations, i.e. Mn4+ and Ni3+, are populated on the surfaces (Table S5). This implies a relatively high oxidizing environment on the surface compared to the bulk, which could result in a more disordered oxygen lattice that is characterized by the presence of oxygen vacancies. Oxygen vacancies have been reported in other manganese-based oxides such as MnO2 and CaMnO3-δ.38,39 Oxygen vacancies would facilitate the adsorption and charge transfer reactions of oxygen-containing species such as hydroxyl and superoxide intermediates during the oxygen electrocatalysis.36

Surface B-site cations in form of truncated BO5 octahedra was

reported to be active for the ORR.40 It has been generally accepted that nickelate perovskites are highly active for oxygen electrocatalysis.41,42 Therefore, the surface exposure of oxygen vacancies and B-site cations of LSMN, especially Ni, would play a positive role for the ORR. The electrocatalytic behaviours for ORR were investigated by half-cell testing. The CV analysis in N2-saturated electrolyte gives no reduction currents except capacitive background currents. CV experiments in O2-staturated electrolyte revealed almost identical Eonset of LSMN and Pt/C (Figure 4a). The polarization resistance (Rp), defined by the difference between the lowfrequency intercept and the high-frequency intercept of an impedance spectrum, is an indicator of the reaction kinetics. Rp of LSMN is about 133 Ω cm2 at 0.58 V, which is almost an order of magnitude smaller than that of Pt/C (Figure 4b). The CV and impedance spectroscopy results


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demonstrate the salient ORR performance of LSMN. Koutecky‒Levich (K‒L) plots are generated from the capacitive-corrected and the iR compensated RDE voltammograms. The n values obtained from the K‒L plots are 3.97, 3.79, and 3.68 at potentials of 0.70 V, 0.60 V, and 0.40 V, respectively. Eonset defined as the potential required for an ORR current of -25 µA cm2 28,40,43

,

E1/2, and the Tafel slope of LSMN catalyst were found to be 1.02 V, 0.80 V, and ‒68 mV

decade-1, respectively (Figure 4c and 4d), compared with Eonset of 0.99 V, E1/2 of 0.82 V, and -76 mV decade-1 of Pt/C (Figure S14 and S15). In CV durability test between the potential ranging from 1.08 V to 0.15 V, LSMN underwent activation after 150 CV scans and showed negligible degradation after 300 CV scans (Figure S16). These impressive results put LSMN as a highly efficient ORR catalyst, on par with precious metals/carbon,44‒46 non-metal doped carbons,47‒49 carbon‒TM hybrids,24,50,51 and TMO/carbon composite materials (Table S6).27,43,52‒55 To better understand the reaction mechanism, the formation of peroxide ions (HO2‒) and n at various potentials were monitored by RRDE. The n values given by RRDE are generally in agreement with those given by RDE (Figure S17). The RRDE and RDE results inferred that ORR proceeds through a mixed four-electron (4e-) and 2e- pathway with the former being the dominant contributor but the latter gains some momentum at low potentials. The ORR is characterized by 92.5 % of the 4e- pathway and 7.5% of the 2e- pathway at a typical operating potential of 0.70 V. It is unclear whether the dominant 4e- pathway on LSMN is a direct 4epathway: O2 + 2H2O + 4e– ⇌ 4OH–

(1)

, or a consecutive 2e- + 2e- pathway: O2 + 2e‒ + H2O ⇌ HO2‒ + OH‒

(2)

HO2‒ + 2e‒ + H2O ⇌ 3OH‒

(3)


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In the 2e- + 2e- pathway, HO2‒ is immediately reduced in the vicinity of catalysts electrochemically. To elucidate the electrochemical reduction of HO2‒, Figure S18 shows the RDE voltammograms of LSMN in N2-saturated 0.1 M KOH laden with 84 mM H2O2. The oxygen reduction current increases monotonically when polarized in the N2-saturated H2O2-laden 0.1 M KOH solution, while the diffusion-limiting current is observed at potentials more negative than 0.55 V when polarized in O2-saturated 0.1 M KOH solution. This result indicates that the LSMN catalyst is highly active for the electrochemical reduction of HO2‒ (Equation 3). Carbon-free perovskite oxides, such as La0.5Sr0.5CoO3,26 Ba0.5Sr0.5Co0.8Fe0.2O3-δ,27 La0.8Sr0.2MnO3,56 and LaNiO357 are good HO2‒ elimination catalysts but poor catalyst for direct O2 reduction. Perovskite oxides are mostly used together with carbon materials, where HO2‒ is generated by partial reduction of O2 on the carbon (Equation 2) and is reduced to OH‒ on the perovskite surfaces (Equation 3). The roles of carbon is more than an HO2‒ supplier and a conducting agent; the existence of electronic and covalent coupling between perovskite and carbon

materials

had

been

reported

in

perovskite/carbon

black

composite58

and

perovskite/graphene hybrid.53 The ORR reaction on perovskite/carbon composites has been known to proceed through a dual-site 2e- (on carbon) + 2e- (on perovskite) pathway, where OH‒ is reduced to HO2‒ by the carbon black and HO2‒ is further reduced to H2O by perovskite oxides. The ORR on perovskite/carbon catalysts involves the spill-over of reaction intermediates from carbon to the catalyst. Unlike those perovskite/carbon composite materials that catalyze ORR via a dual-site 2e- (on carbon) + 2e- (on perovskite) pathway, the ORR reaction on LSMN surfaces proceeds either by a direct 4e- pathway or the 2e- + 2e- pathway in the absence of carbon materials. This circumvents the carbon corrosion problem and the need for spill-over mechanism of composite catalysts. We therefore consider the reaction mechanism of ORR for LSMN as a


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“pseudo” 4e‒ pathway, which does not differentiate the 2e‒ + 2e‒ pathway from the direct 4e‒ pathway. Oxygen reduction on oxide surfaces proceeds through the formation of surface hydroxyls, adsorbed

oxygen,

surface

superoxides,

surface

hydroperoxides,

and

surface

oxide

regeneration:59,60 2TMm+‒O2‒ + 2H2O + 2e‒ → 2TM(m-1)+‒OH‒ + 2OH‒ O2 + e‒ → O2,ads‒

(4) (5)

TM(m-1)+‒OH‒ + O2,ads‒ → TMm+‒O‒O2‒ + OH‒ TMm+‒O‒O2‒ + H2O + e‒ → TM(m-1)+‒O‒OH‒ + OH‒ TM(m-1)+‒O‒OH‒ + e‒ → TMm+‒O2‒ + OH‒

(6) (7) (8)

The indispensable role of TM redox couples as reaction sites is depicted in Equations 4, 6‒8. These redox couples are Mn3+/4+ and Ni2+/3+ for the case of LSMN. Mn3+ (3d4) in the BO6 octahedra of perovskite tends to adopt high spin (H. S.) configuration t2g3eg1,61 cf. t2g3eg0 of Mn4+ (3d3). Ni3+ (3d7) in perovskite structure tends to adopt low spin (L. S.) configuration t2g6eg1,62 as compared to t2g6eg2 of Ni2+ (3d8). The singly filled eg orbitals of Mn3+ (H. S.) and Ni3+ (L. S.) gain electrons and catalyse surface hydroxylation (Equation 4). The empty eg orbital of Mn4+ (t2g3eg0) and Ni2+ (t2g6eg2) can accept an electron pair from the O 2p orbital of O2,ads‒, rendering the replacement of surface hydroxyl group by the superoxide group (Equation 6). The surface protonation of Mm+‒O‒O2‒ is accompanied by the electron gain on the singly filled eg orbitals of Mn3+ (H.S.) and Ni3+ (L. S.). The catalyst surface is regenerated by the cleavage of O‒OH‒ bond. Surface oxygen vacancies, as evident from the XPS results, would be the anchoring sites of


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oxygen-related intermediates.59,63,64 It is worth noting that aforementioned reaction pathways on LSMN surfaces are rather hypothetical and in situ spectroscopic studies should be carried out to better understand the reaction pathways on conductive oxide electrocatalysts. Another implication from Equations 4, 5, 7, and 8 is the requirement of electronic conduction pathways in the vicinity of TM reaction sites. The ORR on traditional composite catalysts occurs at the three-phase boundaries (TPB) which is defined by the common boundaries of catalysts, conducting agent and O2-laden electrolyte. The total TPB is the summation of discrete TPB lines (Figure 5a). The reaction sites on conductive catalysts are double-phase boundaries (DPB) which are defined by the catalyst surfaces in contact with the electrolyte (Figure 5b). In the current case, LSMN is sufficiently conductive so that the reaction sites are the continuous DPB. LSMN has relatively low surface area (1.98 m2 g-1) and moderate particle size (40-70 nm), because of the high temperature calcination. We believe that the low surface area and moderate particle size are beneficial to form a robust DPB and are some of the reasons for the long-term operating stability of LSMN. Note that high surface area nanoparticles are prone to agglomeration, which inevitably leads to poor durability.

CONCLUSIONS Our study of carbon-free oxygen electrocatalysis using nickel-doped lanthanum strontium manganite perovskites (La0.5Sr0.5Mn1-xNixO3-δ) revealed that single phase perovskite structure existed in La0.5Sr0.5Mn1-xNixO3-δ with Ni content below 20 at.%. La0.5Sr0.5Mn1-xNixO3-δ exhibits intrinsic activity for ORR in the absence of conducting agents such as carbon materials. The nonstoichiometric formula represented by La0.5Sr0.5Mn0.9Ni0.1O3-δ (δ=0.06, LSMN), with an intrinsic conductivity of 141 mS cm-1, represents the best performing composition of La0.5Sr0.5Mn1-


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xNixO3-δ.

LSMN achieved Eonset of 1.02 V, E1/2 of 0.80 V, and the Tafel slope of ‒68 mV decade-1

in 0.1 M KOH solution. These salient results put LSMN as a highly efficient ORR catalyst. XANES and XPS analyses revealed the well-balanced TM redox couples, considerable portions of TM cations with a single eg electron (Mn3+ and Ni3+), and oxygen deficiency both in the bulk and on the surface. Non-stoichiometric surfaces were rich in cations in the order of A-sites (Sr > La) > B-sites (Ni > Mn). The ORR proceeded through a pseudo 4e- pathway on the DPB of LSMN surfaces. We have successfully demonstrated carbon-free oxygen electrocatalysis on TMO, attributed largely to the oxygen-deficient perovskite structure, well-balanced Mn3+/4+ and Ni2+/3+ redox couples, surface oxygen vacancies, and the electrically conductive nature of LSMN.

EXPERIMENTAL SECTION Lanthanum strontium manganese nickel oxides La0.5Sr0.5Mn1-xNixO3-δ were synthesized by a solgel method using nitrate salts and citric acid. Briefly, stoichiometric amounts of raw materials were dissolved in water and heated to form a viscous gel. The gel was baked at 180 °C and consequently calcined at 950 °C for 4 hours. The electrical conductivity was evaluated by ferri/ferrocyanide redox couple. Intrinsic electrical conductivity was measured by impedance spectroscopy.65 XPS spectra were obtained with a Al Kα radiation (VG ESCALAB 200i-XL). Mn and Ni K edge XAS spectra were collected in transmission mode at XAFCA beamline, Singapore Synchrotron Light Source (SSLS).66 The sample preparation and characterization details can be found in Supporting Information.


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Electrochemical performances of catalysts were evaluated by RDE and RRDE in threeelectrode configuration. The Ag/AgCl reference electrode was converted to the reversible hydrogen electrode (RHE), Evs RHE= Evs Ag/AgCl + 0.976 V. The electrolyte was O2-saturated 0.1 M KOH aqueous solution. All potentials were compensated with the ohmic drop (R = 39.0 Ω) given by impedance spectroscopy. The collection efficiency of RRDE was calibrated by ferri/ferrocyanide redox couple (Figure S19). Detailed test procedures and analysis are illustrated in Supporting Information.

ASSOCIATED CONTENT Supporting Information. Experimental, TGA, XRD, EDX, RDE, SEM, BET, CV, 4-probe resistivity, impedance spectroscopy, XAS, TOF-SIMS, Tafel plot, and RRDE. Figure S1 to S19, Table S1 to S6. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This research was supported by Advanced Energy Storage Programme (grants IMRE/12-2P0503 and IMRE/12-2P0504), A*STAR. H. Z. thanks the financial support from MOE under AcRF Tier 2 (ARC 26/13, No. MOE2013-T2-1-034; ARC 19/15, No. MOE2014-T2-2-093; MOE2015T2-2-057) and AcRF Tier 1 (RG5/13), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. The authors are grateful to H. Jiang for his assistance in materials synthesis, Z. Zhang and L. T. J. Ong for XPS, G. J. Du for TEM, M. Y. D. Lai for TOF-SIMS, and H. J. Qian and J. O. Wang for soft X-ray absorption spectroscopy.


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Figure 1. (a) FESEM (scale bar, 100 nm), (b) TEM (scale bar, 100 nm), (c) HRTEM (scale bar, 5 nm), and (d) XRD of LSMN nanocrystals. (e) Visualization of trigonal/rhombohedral perovskite structure from the a direction.


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Figure 2. (a) Mn K-edge X-ray absorption spectra of LaMnO3, La0.5Sr0.5MnO3-δ, La0.5Sr0.5Mn0.9Ni0.1O3-δ, and CaMnO3. (b) Ni K-edge X-ray absorption spectra of Ni(NO3)2·6H2O, La0.5Sr0.5Mn0.9Ni0.1O3-δ, and LaNiO3.


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Figure 3. High-resolution XPS spectra of LSMN. (a) La 3d5/2, (b) Sr 3d, (c) Mn 2p, (d) Ni 2p3/2, and (e) O 1s. Note that in (d) the right shoulder of Ni 2p3/2 overlaps with the left shoulder of La 3d3/2.


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Figure 4. (a) Cyclic voltammograms of LSMN and Pt/C (O2-saturated, solid line; N2-saturated, dashed line). (b) Impedance spectra of LSMN and Pt/C collected under a potential of 0.58 V and a rotating rate of 1600 rpm. (c) iR-compensated and capacitive-corrected RDE oxygen reduction polarization curves of LSMN under different rotating rates in the unit of rpm. The inset shows the corresponding Koutecky-Levich plots. (d) Tafel plot of iR-compensated and capacitivecorrected RDE oxygen reduction polarization curve under a rotating rate of 1600 rpm. The electrolyte is O2-saturated 0.1 M KOH solution.


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OH‒

O2, ad

a

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b

HO2‒ O2,ad

‒

OH

OH‒ -

e e-

‒

e

-

e

-

e

-

OH

e- e- e- e- e- ee-

HO2‒

O2,ad

‒

OH

e- e- e- e- e- e- e-

Poorly conducting catalyst

Conducting catalyst

Conducting agent

Figure 5. Schematic diagrams of TPB and DPB. (a) Composite catalyst/electrolyte interface. The inset shows discrete TPB interface (red color) which is defined by poorly conducting catalysts, conducting agents, and the electrolyte. (b) Single phase catalyst/electrolyte interface. DPB (violet color) is defined by the interface between intrinsically conductive catalysts and the electrolyte.


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Table of Contents (TOC)

‒

OH

HO2‒

O2,ad

‒

OH

LSMN

DPB e- e- e- e- e- e- e-


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Intrinsically Conductive Perovskite Oxides with ... - ACS Publications

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