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Bi-functionality of samarium- and praseodymium-based perovskite catalysts for oxygen reduction and oxygen evolution reactions in alkaline medium
Journal of Power Sources 446 (2020) 227234
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Bi-functionality of samarium- and praseodymium-based perovskite catalysts for oxygen reduction and oxygen evolution reactions in alkaline medium Praveen Kolla a, b, Golibsho Nasymov a, Rudresh Rajappagowda a, Alevtina Smirnova a, b, * a b
Materials Engineering and Science Program, South Dakota School of Mines & Technology, Rapid City, SD, 57701, USA Chemistry and Applied Biological Sciences Department, South Dakota School of Mines & Technology, Rapid City, SD, 57701, USA
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Praseodymium and samarium-based perovskites were synthesized. � Catalytic activity towards OER and ORR is observed in alkaline media. � OER/ORR bi-functionality is described through graphene support-catalyst interaction.
A R T I C L E I N F O
A B S T R A C T
Keywords: Praseodymium Samarium Perovskites Oxygen evolution reaction Oxygen reduction reaction Metal-air batteries Regenerative fuel cells
The present study ellucidates bi-functional catalytic activity of the graphene-supported praseodymium (PrNixCo1-xO3-δ) and samarium (SmNixCo1-xO3-δ) perovskites towards oxygen evolution and oxygen reduction reactions in alkaline solutions. The perovskites with varied Ni and Co content (of x ¼ 0.1, 0.5, and 0.9) were synthesized via sol-gel glycine-nitrite combustion followed by heat-treated at 700, 900 and 1200 � C. The cubic perovskite phase of PrNixCo1-xO3-δ was stabilized at x ¼ 0.1, 0.5 in the entire temperature range (of 700 � C–1200 � C), while the perovskite phase of SmNixCo1-xO3-δ (x ¼ 0.1, 0.5) was observed only at lower tem peratures (of 700 and 900 � C). Consequently, changes in structural properties resulted in superior oxygen evo lution reaction (OER) electrochemical activity in comparison to the state-of-the-art IrO2. Among the Sm-based perovskites, SmNi0.1Co0.9O3-δ (700 � C) demonstrated OER mass activity of 495 mA/mg and the highest oxygen reduction reaction (ORR) activity of 95 mA/cm2⋅mg. For Pr-based perovskites, the highest electrocatalytic ac tivity toward OER and ORR was observed for PrNi0.1Co0.9O3-δ (900 � C) resulting in 60 mA/mg for ORR and 680 mA/mg for OER which is 50% higher than the mass activity of the state-of-the-art IrO2 catalyst. The overall performance of the composites is discussed in terms of graphene support interactions, Co and Ni redox processes, relative concentration of oxygen-vacancy sites and OER/ORR bi-functionality.
* Corresponding author. Chemistry and Applied Biological Sciences, South Dakota School of Mines & Technology, Rapid City, SD, 57701, USA. E-mail address: [email protected] (A. Smirnova). https://doi.org/10.1016/j.jpowsour.2019.227234 Received 30 January 2019; Received in revised form 29 September 2019; Accepted 30 September 2019 Available online 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
rate-limiting multiple reaction pathways. The B-site transition metal ions in an ABO3-δ perovskite perform an important role in binding the oxygen intermediates on the surface of the perovskite particle and thus affect the overall OER efficiency. This mechanism is based on the mo lecular orbital bonding framework formed as a result of the corre sponding interactions between the eg transition metal orbitals with negatively charged oxygen adsorbates and with metal-oxygen covalency as a descriptor for its catalytic activity. Samarium and praseodymium-based perovskites have been tested earlier as catalysts for different catalytic processes, among them carbon dioxide reforming of methane [36], hydrogen and oxygen evolution [37], and syngas production [38]. However, to the best of our knowl edge, they have not been tested as bifunctional catalysts in OER and ORR. In this regard, the goal of the proposed study was to investigate the ORR and OER electrocatalytic behavior of the doped SmNixCo1-xO3 and PrNixCo1-xO3 perovskites. Perovskites in its ideal state assumes stoi chiometric chemical form of ABO3. However, with increase in partial doping of B-site element (Co3þ) with B0 -element having lower oxidation state (Ni2þ) would change the relative concentration of mixed oxidation state of B (Co2þ/Co3þ) and/or B’ (Ni3þ/Ni2þ). In addition, partial doping with low-valent atoms may also cause increase in density of oxygen-vacancy sites. In this context, the effect of the B-site doping as a function of nickel to cobalt molar ratio (x ¼ 0.1, 0.5, and 0.9) vs. the heat-treatment temperature is discussed in terms of crystal structure and oxygen stoichiometry. The electrocatalytic activity of the SmNixCo1-xO3-δ and PrNixCo1-xO3-δ perovskites was further evaluated in comparison with the state-of-the-art platinum and IrO2 catalysts for ORR and OER, respectively.
Perovskites is a large group of materials that is well known in catalysis and electrocatalysis [1] for different types of reactions ranging from reduction of air pollutants to electric power generation and storage in sustainable energy applications. Among them, one of the most chal lenging research areas is focused on metal/air rechargeable batteries [2–7] that require inexpensive, stable, and bifunctional catalysts for both oxygen reduction reaction (ORR) and its reverse reaction of oxygen evolution (OER). Since the efficiency of lithium/air cells depends on differences in voltages and electrochemical reaction rates of the corre sponding charge and discharge processes, investigation of novel cata lysts in both ORR and OER is especially important. Perovskites with a chemical formula ABO3-δ, where A is a rare-earth metal atom occupy the eight corners of cuboidal with transition metal B in the center with octahedral 6-fold coordination of oxygen atoms. Pe rovskites have attracted considerable attention as alternatives to noble metal-based catalysts, such as Pt, Ru, and Ir [8,9] that perform either as oxygen reduction [10] or oxygen evolution catalysts [11], respectively. On contrary to noble metals, metal oxides and perovskites, especially in alkaline solutions [12], demonstrate bifunctionality in ORR/OER multistep two-electron kinetic routes resulting in different intermediates [13]. This capability highlights potential perovskite applications in fuel cells [14], electrolyzers [15], and aqueous metal-air batteries [16]. The bifunctional catalytic properties of perovskites towards ORR and OER is defined by a number of factors, such as surface and bulk elec tronic properties, synthesis conditions, surface area and particle size [17], catalyst surface hydroxylation, catalyst support, and the perov skite phase purity. Moreover, the catalytic properties of perovskites can be tuned by A-site [18] or B-site relative-doping [19]. Among different perovskites, single and double perovskitestructures, e.g. NdBa0.5Sr0.5Co1.5Fe0.5O5þδ [20] have been tested in re gard to their bifunctional catalytic behavior in OER and ORR. For example, the electrocatalytic activity of lanthanum-based perovskite LaCoO3 [21] synthesized by colloidal method onto nitrogen-doped carbon support has three times higher than the state-or-the-art IrO2 in OER [22]. Furthermore, it demonstrates strong OER and ORR bifunc tional character with mass activity reaching 24 mA/mg. Another perovskite, LaNiO3 on nitrogen-doped carbon [23] is also about three times more active toward OER in comparison to IrO2. In these and many other cases [24] carbon support played an important role as electrode component that compensates the lack of electronic conductivity and ensures maximum utilization of the perov skite catalysts in ORR and OER. Moreover, carbon phase catalyzes O2 reduction to H2O2 that is further decomposed and/or reduced by the LaCoO3 perovskite [25]. On the contrary, LaCoO3, La0.8Sr0.2MnO3 in perovskite–carbon composite also contributes to the H2O2 formation, suggesting that a surface non-specific process of outer-sphere charge transfers might be possible [26]. Besides recent significant achievements [27–30], the design of an effective bi-functional catalyst is still in progress since it is complicated by the multielectron electrochemical processes utilizing different reac tion pathways and mechanisms [31]. Perovskites are highly active for the ORR on different supports due to their ability to disproportionate peroxide [21]. There are two main mechanisms proposed for peroxide disproportionation on perovskites: a surface transition metal mediated route and a vacancy mediated route. Furthermore, the effect of the ox ygen deficiency and transition metal oxidation states created in B-site doped perovskites contributes to their oxygen exchange redox behavior at ambient conditions making this analysis even more challenging [32–34]. As alternatives to noble metals, perovskites were extensively studied for ORR and OER, especially in alkaline electrolytes. In this case, the structure, electron configuration, and the crystal field of perovskites, e. g. A- or B-site doping [35], is considered as the main factors that define reaction kinetics due to formation of various intermediates and
2. Experimental methods 2.1. Material synthesis Praseodymium (PrNixCo1-xO3-δ) and samarium (SmNixCo1-xO3-δ) perovskites with different B-site doping of nickel and cobalt (x ¼ 0.1, 0.5, and 0.9) were synthesized using glycine-nitrate sol-gel synthesis. Stoichiometric quantities of Pr(NO3)3⋅6H2O, Sm(NO3)3⋅6H2O, Ni (NO₃)₂∙6H₂O, and Co(NO₃)₂∙6H₂O salts were dissolved in 20 mL of deionized water forming initially an aqueous homogeneous solution followed by addition of excess glycine (3 x molar equivalent of metal salts). A clear glassy solid was obtained after treatment of the homog enous glycine-nitrate solution for ~24 h at 60 � C. Heat-treatment of the produced solid at 400 � C for 2 h on the hot-plate at ambient conditions resulted in its complete combustion. The resulting mixture of carbon and metal oxides was further heat-treated in a muffle furnace at three different temperatures of 700, 900 and 1200 � C in air for 3 h to form Smand Pr-based complex oxide phases with diverse physical, chemical, and catalytic properties. The 40 wt% Pt/C (stock #47308) and IrO2 (stock #17849) powders used as control samples were purchased from AlfaAesar. 2.2. Characterization of structure, composition and morphology The structure of the perovskite materials was characterized by Rigaku Ultima Plus theta-theta X-ray diffraction (XRD) instrument with Cu-kα radiation (of λ ¼ 1.54178 Å) by measuring the XRD spectrum of materials from 10� to 80� with scan rate of 1� /min. The crystallite size was calculated based on Scherrer equation - L ¼ λ/(β*cos θ), where λ is the wavelength of Cu Kα radiation and β is the full width half maximaFWHM of peaks corresponding to the crystalline phase and θ is diffraction angle. The crystallite sizes were obtained by averaging par ticle size associated with the peaks corresponding to the same phase. Note that these crystallite sizes may not always directly correlated with the particle size. The surface morphology and elemental composition of the materials was analyzed with Energy Dispersive X-ray (EDX) spectrometer from 2
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Journal of Power Sources 446 (2020) 227234
Oxford Instruments attached to Zeiss Supra 40VP variable-pressure SEM. The reduction behavior of the catalyst material was studied using Temperature Program Reduction (TPR) by ramping the tempera ture from room temperature to 900 � C at 5 � C/min in 5% H2/Ar atmo sphere. The Thermal Conductivity Detector (TCD) signal plotted along Y-axis is directly proportional to the amount of H2 consumed during perovskite oxide surface reduction to water.
where Id - disc-current, Ir - ring-current, and N ¼ 0.37 - current collection efficiency of Pt-ring electrode. The number of the transferred electrons (n). In this experiment, the Pt-ring electrode was biased at a higher potential (1.0 V RHE) that is required for electrooxidation of the inter mediate peroxide species which were instantaneously produced and swept-away from the disc-electrode of varying potential (LSV). 3. Results and discussion
2.3. Electrochemical methods
3.1. Crystal structure of the materials
The electrochemical studies of the nanocomposites with 90 wt% graphene and 10 wt% praseodymium (PrNixCo1-xO3-δ) or samarium (SmNixCo1-xO3-δ) perovskites towards OER and ORR reactions were investigated in a freshly prepared 0.1 M KOH electrolyte, using an analytical three-electrode cell from Princeton Applied Research® with Ag, AgCl/3.5 M KCl reference electrode. A bipotentiostat (AFCBP1) and a speed control unit (AFMSRCE) from Pine Research Instrumentation® were used to measure the electrochemical response and maintain diffusion controlled hydrodynamic conditions of the cell, respectively. The electrode potential vs. SHE was calculated by adding þ0.205 V to each of the experimental values obtained with Ag/AgCl, Cl electrode. Then, potential Nernstian shift (59 mV/decade) associated with proton activity was added to further convert SHE to RHE, as provided by the equation below: E(RHE) ¼ EAg/AgCl þ 0.059 pH þ EoAg/AgCl,
The X-ray diffraction patterns for two groups of perovskites, specif ically PrNixCo1-xO3-δ and SmNi0.1Co0.9O3-δ, are presented in Figs. 1, 2 and Tables 1, 2 below. The X-ray diffraction spectra of PrNixCo1-xO3-δ (Fig. 1) reveal that the structure evolutions and segregation into perovskite phase largely de pends on the extent of B-site transition metals doping and the heattreatment temperature. Formation of the perovskite cubic phase (Fig. 1 and Table 1) is favorable at lower temperatures and lower Ni/Co ratios (x ¼ 0.1 and 0.5). On the contrary, high Ni/Co ratio (x ¼ 0.9) have not resulted in perovskites-phase and yielded a mixture of its constituent oxides. The shift of the characteristic XRD peaks toward higher 2θ angle (Fig. 1) in the case of PrNixCo1-xO3-δ, results from sintering during the crystal lattice formation. As demonstrated in Fig. 1 and Table 1, at higher temperatures Pr-based oxides with high Ni content (x ¼ 0.9) are not thermodynamically stable to form perovskite phase. This could be explained by the fact that ABO3-δ requires the ratio of cuboidal A-ion volume to octahedral B-ion volume to be exactly five [39]. This ratio is changing at higher temperatures due to the modification of the unit cell parameters causing collapse of the perovskite crystal-lattice. In terms of PrNixCo1-xO3-δ particle size (Table 1), the nanoparticles produced by a sol-gel approach increase their crystallite size with tem perature from ~30 nm at 700 � C to ~35 nm at 900 � C and ~40 nm at 1200 � C. This observation is in correlation with many other publications focused on metal oxide ceramic sintering [40]. Investigation of the chemical stability and crystal structure formation of the second group of SmNi0.1Co0.9O3-δ perovskites (Fig. 2, Table 2) reveals that the trends are similar to that of PrNixCo1-xO3-δ. Pure SmNi0.1Co0.9O3-δ cubic perovskite phases are formed only at lower temperatures (700 and 900 � C) and lower Ni content (Ni/Co ratio ¼ 0.1, 0.5). Similar to that of PrNixCo1-xO3-δ, the shift of the characteristic XRD SmNi0.5Co0.5O3-δ peaks toward higher 2θ angles (Fig. 2) can be attributed to the sintering effects. The smallest particles size of ~23–25 nm (Table 2) was observed for SmNixCo1-xO3-δ at 700 � C sintering temperature and at low Ni con tent (x ¼ 0.1 and 0.5), respectively. At higher temperatures, formation of the new phases different from perovskite phases were also observed. These phases include oxides associated with A-site lanthanum elements and B-site associated mixed spinel or individual oxides of transition metals. These secondary phases are mostly formed at high nickel or low cobalt content (x ¼ 0.9). Furthermore, additional XRD peaks at higher temperatures (900 � C and 1200 � C are associated with formation of Ruddelsden-Popper mixed perovskite phases [41]. These results can be explained by the differences in oxidation-states of A- and B-site metals that are induced upon doping. Specifically, higher temperatures tend to increase the number of oxygen vacancies thereby decreasing the oxidation states of the B-site metals in order to maintain an overall electroneutrality. This eventually leads to the crystal lattice expansion as B-site cations with lower oxidation states have larger ionic radii causing 2θ to shift towards lower angles. A different effect is expected for the A-site metals at higher temperatures: lattice parameters of praseodymium and samarium decrease causing lattice to shrink and angles shift to higher 2θ. This effect results in eventual formation of the secondary oxide and spinel phases. As these secondary phases are formed, the lattice strain associated with oxygen vacancies relax, which may have contributed to a gradual shrinkage of
(1)
where EoAg/AgCl ¼ 0.205 V at 25 � C and E Ag/AgCl is working potential. The catalyst inks were prepared by sonication of the 0.4 mL of 5% 1100 Nafion™ solution in 20 mL of isopropanol and 79.6 mL of water with 0.6 g of the dry catalyst powder. The catalyst inks were dispersed using Ultra Turrax1 homogenizer. The ink (10 μL) was deposited on the glassy carbon electrode surface and dried to form a thin film. Then, ~1–2 μL of 5% Nafion™ solution was added on the surface of the catalyst to prevent the catalyst film from delamination during the ex periments. Two types of working electrodes; specifically, a Rotating Disc Electrode (RDE-glassy carbon disc, AFE3T050GE, 5 mm in diameter) and a Rotating Ring Disc Electrode (RRDE-glassy carbon disc with Ptring, E7R9-series AFMSRCE, 5.61 mm in diameter) from Pine Research Instrumentation® were used for OER and ORR studies, respectively. The mass loading of the catalysts on the RDE and RRDE was calculated based on the active electrode area and the catalyst loading. The active size of RDE and RRDE were 5.0 and 5.6 mm that corresponds to the active areas of 0.196 and 0.247 cm2, respectively. The mass loadings of the catalyst were estimated based on the amount of catalyst present in the 10 μL of the ink. The perovskite-catalyst composites comprised of 10 wt% cata lyst and 90 wt% graphene. The baseline Pt/C catalyst contained 40 wt% of Pt metal on carbon, while 10 wt% of commercial IrO2 was used in combination with 90% graphene. Prior to the measurements of electrochemical polarization, catalytic activity, and selectivity towards OER/ORR of the synthesized perov skites, electrode conditioning for 50 cycles at 50 mV/s scan rate was applied. The conditioning step was used to ensure the surface of the electrocatalyst is free of impurities. However, due to high acidity of Nafion ionomer used as a binder, a partial dissolution of the metal oxide catalyst was expected, which may have contibuted to the overall elec trochemical performance of the catalyst. Therefore, electrochemical stability of the catalyst has not been considered in this study as it may not justify the original perovskite durabilty properties. To compare the ORR selectivity towards peroxide pathway (n ¼ 2) and direct ORR reduction pathway (n ¼ 4), the effective number of the electrons (n) transferred per each oxygen molecule was evaluated using equation (2). n¼4
Id Id þ INr
(2)
3
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Journal of Power Sources 446 (2020) 227234
Fig. 1. X-ray diffraction pattrens of the PrNixCo1-xO3-δ phases formed after heat-treatment at different temperatures and B-site Ni/Co ratios: (A) x ¼ 0.1, (B) x ¼ 0.5, and (C) x ¼ 0.9.
Fig. 2. X-ray diffraction patterns of the SmNixCo1-xO3-δ phases formed after heat-treatment at different temperatures and B-site Ni/Co ratios: (A) x ¼ 0.1; (B) x ¼ 0.5 and (C) x ¼ 0.9.
the crystal lattice observed at higher heat-treatment temperatures.
3.2. Analysis of chemical composition and oxygen-vacancy of the materials The SEM images for the PrNixCo1-xO3-δ and SmNixCo1-xO3-δ as well as their chemical composition from the EDS analysis are presented in Fig. 3 4
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Ni:Co:O and Sm:Ni:Co:O) are very close to their corresponding stoi chiometric values. The relative compositions of oxygen-deficient sites are qualitatively studied based on the thermal reduction behavior of the perovskites. Temperature Programmed Reduction (TPR) profiles of samarium and praseodymium-based perovskites (Fig. S1) follows a typical reduction behavior of complex perovskite oxides with initial peaks at low tem peratures (between 200 and 400 � C) that indicate formation of an in termediate oxygen deficient structures. The peaks at 400 � C or above correspond to the reduction of B-site element cobalt or the second B-site dopant nickel. The peaks at higher temperatures beyond 600 � C arise from reduction of transition metals in lower-oxidation states or reduc tion of the constituent lanthanide oxides. Increasing the amount of nickel-doping from x ¼ 0.1 to 0.5 results in lower-onset reduction tem peratures, which indicates high reducibility of both SmNi0.5Co0.5O3 and PrNi0.5Co0.5O3. On the other hand, increase in heat-treatment temper atures promote perovskites towards highly oxygen-vacant structures, which is evident from a gradual increase of the onsets of the reduction temperatures. This observation is in correlation with the shifts for the XRD peaks (Figs. 1–2) to higher diffraction angles when the heattreatment temperatures increases. This phenomenon is explained by the lattice shrinkage caused by increase in density of the oxygen-vacant sites.
Table 1 Crystal structure, crystalline size, and lattice parameters of PrNixCo1-xO3 as a function of its relative (Ni/Co) composition and heat-treatment (HT) temperature. Material
HT
Perovskite Yield
Structure
Crystallite Size (nm)
PrNi0.1Co0.9O3
700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C
Yes Yes Yes Yes Yes Yes No No No
Cubic Cubic Cubic Cubic Cubic Rhombohedral – – –
30.4 34.7 43.0 29.6 33.6 38.9 – – –
PrNi0.5Co0.5O3 PrNi0.9Co0.1O3
Table 2 Crystal structure, crystalline size, and lattice parameters of SmNixCo1-xO3-δ as a function of its relative (Ni/Co) composition and heat-treatment (HT) temperature. Material
HT
Perovskite Yield
Structure
Crystallite Size (nm)
SmNi0.1Co0.9O3
700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C
Yes Yes No Yes Yes Partial No No No
Cubic Rhombohedral – Cubic Cubic Cubic – – –
23.4 39.8 – 25.4 35.4 – – – –
SmNi0.5Co0.5O3 SmNi0.9Co0.1O3
3.3. ORR of the catalysts As depicted in the Fig. S2, the explicit role of graphene can be un derstood by comparing the electrochemical polarization data of various perovskite-graphene compositions. It is evident that both graphene and perovskite display high overpotentials and poor ORR activity in their pristine forms. However, as the concentration of graphene increases, the composites gradually start to exhibit lower overpotentials and higher current densities. Here, the inherent ability of atomically thin graphene in percolating perovskite moiety is experimentally demonstrated.
and Table 3, respectively. The SEM images of the PrNixCo1-xO3-δ and SmNixCo1-xO3-δ (Fig. 3) heat-treated at 900 � C indicate that both crystal structures with the lowest Ni content (x ¼ 0.1) have the smallest grain sizes in the range of about 50 nm, high porosity, and, thus, more surface available for electrocatalytic activity (Fig. 3a, d) associated with cubic crystal structure. On the other hand, the samples with equal ratio of Ni to Co (x ¼ 0.5) (Fig. 3b, e) demonstrate a shift toward less porosity and formation of larger grains. The PrNixCo1-xO3-δ sample with the highest Ni content (x ¼ 0.9) has different morphology (Fig. 3c) compared to that of samarium-based material (Fig. 3d) with larger grains. Energy dispersive spectroscopy (EDS) was used to determine the chemical composition of the PrNixCo1-xO3-δ and SmNixCo1-xO3-δ perov skites heat-treated at 900 � C. No elements other than Pr, Ni, Co and O are observed by EDS (Table 3). The ratios of the constituting elements (Pr:
Table 3 Elemental composition of PrNixCo1-xO3 and SmNixCo1-xO3 heat-treated at 900 � C in terms of weight (and atomic) percentages obtained from EDS analysis. Wt.% Material (at.%)
Pr or Sm
Ni
Co
O
PrNi0.1Co0.9O3 PrNi0.5Co0.5O3 PrNi0.9Co0.1O3 SmNi0.1Co0.9O3 SmNi0.5Co0.5O3 SmNi0.9Co0.1O3
61.4 (23.6) 62.4 (24.2) 61.3 (23.6) 64.3 (27.4) 62.1 (25.1) 63.1 (25.2)
2.3 (2.1) 10.8 (10.1) 20.0 (18.5) 1.6 (1.7) 12.3 (12.7) 2.7 (2.8)
19.9 (18.3) 10.4 (9.6) 2.1 (1.9) 22.6 (24.5) 12.8 (13.2) 21.4 (22.4)
16.5 (55.9) 16.4 (56.1) 16.5 (60.0) 11.6 (46.4) 12.9 (49.0) 12.9 (49.7)
Fig. 3. SEM images of PrNixCo1-xO3 (a-c) and SmNixCo1-xO3 (d-f) heat-treated at 900 � C with different Ni content (x ¼ 0.1, 0.5 and 0.9). 5
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Consequently, graphene at higher content reduces charge-transfer re sistances of the composite electrodes. These results highlight the importance of an optimized perovskite to graphene ratio for achieving the highest synergistic bi-functional electrochemical performance. In regard to the ORR activity of the Pr- and Sm-based catalysts, three factors have the most noticeable impact, such as Pr or Sm of A-site metal, the heat-treatment temperature, and the nickel B-site doping (Figs. 4, 5). In comparison to Pt (40 wt%) C, both Pr- and Sm-based catalysts demonstrate higher onset overpotentials and slightly lower power den sities (Figs. 4, 5). Considering that the amount of platinum in the com mercial Pt catalyst is 4 times higher than the amount of perovskite (10 wt.%) in the nanocomposite with graphene, the demonstrated ORR catalytic activity is an important step towards development of the nonnoble metal catalysts for the energy generation and storage. Specifically, the ring currents (Fig. 4a) for the Pr-based perovskites heat-treated at lower temperatures are the lowest at the lowest Ni content (x ¼ 0.1). A relatively low number of electrons (n ¼ 3.5) compared to Pt (n ¼ 4) suggest that an alternate ORR pathway through peroxide intermediate takes place as perovskites could mediate a surface non-specific outershell electron transfer reactions involving ORR in alkaline medium. Regarding polarization currents (Fig. 4c), the best performance is demonstrated by the PrNi0.1Co0.9O3-δ catalyst with the lowest Ni content (x ¼ 0.1) and intermediate (900 � C) heat-treatment temperature. This sample shows the highest mass activity in comparison to the other Prbased catalysts (Fig. 4d). In comparison to the Pr-based catalysts, Sm-based catalysts demon strated overall higher ORR activity. The ring currents (Fig. 5a) are the lowest and the number of electrons involved in ORR (Fig. 5b) is the highest for the Sm-based perovskites heat-treated at lower temperatures and with the lowest Ni content (x ¼ 0.1). Regarding polarization cur rents (Fig. 5c), the best performance is demonstrated by the perovskite with the lowest Ni content (SmNi0.1Co0.9O3-δ) and the lowest (700 � C) heat-treatment temperature. These results are in correlation with the
mass activity values that are the highest for this catalyst (Fig. 5d, sample 1). 3.4. OER of the catalysts Numerous publications address the perovskite bi-functionality and provide a broad range of ORR/OER mass and specific activity values obtained at different operating conditions [42,43]. To confirm the SmNixCo1-xO3-δ bi-functionality, the catalytic activity evaluation was performed in 0.1 KOH solution saturated by oxygen. Since perovskites demonstrate the overpotential of up to 150–200 mV compared to the state-of-the-art Pt/C catalyst with onset potentials in the range of 0.6V–0.7 V, their mass activities were calcu lated between their onset potentials and the half-wave potentials, spe cifically within the 0.5V-0.6 V range. Furthermore, in order to be consistent with perovskite ORR mass activities and compare them with other available OER studies, the OER mass activities with a unit of mA/ mg are presented in Figs. 6–7. The results indicate (Figs. 6–7) that the best OER catalytic activity is observed for SmNi0.1Co0.9O3-δ perovskite with the lowest nickel content (x ¼ 0.1) and sintered at the lowest temperature (700 � C). In case of the Pr-based perovskite (PrNi0.5Co0.5O3-δ) with intermediate Ni content (x ¼ 0.5) and heat-treated at the lowest temperature (700 � C) the highest polarization currents have been observed (Fig. 6a). However, consid ering mass activity, the perovskite with the lowest Ni content (x ¼ 0.1) and heat-treated at intermediate temperature (900 � C) demonstrated the highest mass activity in comparison to other Pr-based perovskites (Fig. 6b) that could be related to the resultant concentration of Ni (II) and/or Co (II/III) and the overall metal-oxygen covalency created due to the oxygen-deficient-sites. The Sm-based perovskite (SmNi0.5Co0.5O3-δ) with intermediate Ni content (x ¼ 0.5) and heat-treated at intermediate temperature (900 � C) demonstrates the highest polarization currents (Fig. 7a). However, the
Fig. 4. ORR performance of PrNixCo1-xO3-graphene composite in terms of (a) ring-current, (b) number of electrons participated, (c) polarization currents and (d) mass activity, in comparison to state-of-the-art Pt/C catalyst. 6
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Fig. 5. ORR performance of SmNixCo1-xO3-graphene composite in terms of (a) ring-current, (b) number of electrons participated, (c) polarization currents and (d) mass activity, in comparison to state-of-the-art Pt/C catalyst.
Fig. 6. OER performance of PrNixCo1-xO3-graphene composite in terms of (a) polarization current densities and (b) mass activities, in comparison to state-of-the-art IrO2 catalyst.
catalyst SmNi0.1Co0.9O3-δ with the lowest Ni content (x ¼ 0.1) and heattreated at the lowest temperature (700 � C) shows the highest mass ac tivity in comparison to other Sm-based perovskites (Fig. 7b). Based on the demonstrated results, it can be concluded that in comparison to Pr-based catalysts (Fig. 6b), the OER mass activity for PrNi0.1Co0.9O3-δ (Fig. 7b) with the lowest Ni content and heat-treated at the lowest temperature has the highest oxygen evolution reaction activity.
synthesized using a sol-gel Pechini method resulting in relatively small crystallite size. Specifically, the size of the perovskite crystallites in cubic phase after sintering at elevated temperatures (700–1200 � C) was in the range of 23.4–39.8 nm. Electronic configuration of A-site lanthanum cations can signifi cantly affect oxygen deficiency in perovskites. Specifically, Pr with [Xe] 4f2 electron configuration tend to form higher III/IV oxidation states, while Sm possesing [Xe] 4f5 electron configuration exhibits lower oxidation states: II/III. Consequently, Sm would promote perovskite structures with high metal-oxygen covalency and high oxygen defi ciency, when compared to Pr-based perovskites. Therefore, oxygen may readily interact with samarium in Sm-based perovskite thereby enhancing the ORR activity. On the other hand, B-site cations of Ni and Co exhibit II and II/III oxidation states, respectively. Therefore, stable perovskite structures are readily produced with high Co-content (x ¼ 0.1 and 0.5). This trend is less important for Pr-rather than Sm-based
4. Conclusions The catalytic activity and electrochemical performance of the graphene-supported praseodymium (PrNixCo1-xO3-δ) and samarium (SmNixCo1-xO3-δ) perovskites towards oxygen evolution and oxygen reduction reactions in aqueous alkaline solutions has been investigated. The Pr/SmNixCo1-xO3-δ–graphene nanocomposites were successfully 7
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Fig. 7. OER performance of SmNixCo1-xO3-Graphene composite in terms of (a) polarization current densities and (b) mass activities, in comparison to state-of-the-art IrO2 catalyst.
perovskites due to flexible oxidation-states associated with both Pr and Co. The perovskite yield also follows similar trend with heat-treatment temperature. Specifically, the Pr-based perovskites tend to form stable perovskite structures when compared with Sm-based perovskites pos sessing low oxidation states. Subsequently, Pr-based perovskites with high Co-content exhibit superior OER performance as these catalysts could promote high-redox activity. The relatively high ORR catalytic activity combined with high OER activity, exceeding the OER catalytic activity for IrO2, makes these perovskites as good candidate for cathodes in aqueous metal-air batteries and regenerative fuel cells.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support from the ACS Petroleum Research Fund (Project number 53614-UR10) and the NSF EPSCoR projects (Awards IIA-1330840 and IIA-1330842). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227234. References [1] J. Hwang, R.R. Rao, L. Giordano, Y. Katayama, Y. Yu, Y. Shao-Horn, Science 358 (2017) 751–756. [2] Y. Gong, X. Zhang, Z. Li, Z. Wang, C. Sun, L. Chen, ChemNanoMat 3 (2017) 485–490. [3] X. Xu, W. Wang, W. Zhou, Z. Shao, Small Methods 2 (7) (2018) 1800071. [4] X. Zhang, Y. Gong, S. Li, C. Sun, ACS Catal. 7 (2017) 7737–7747. [5] J. Bian, Z. Li, N. Li, C. Sun, Inorg. Chem. 58 (2019) 8208–8214. [6] J. Bian, R. Su, Y. Yao, J. Wang, J. Zhou, F. Li, Z.L. Wang, C. Sun, ACS Appl. Energy Mater. 2 (2019) 923–931. [7] J. Wang, X. Cheng, Z. Li, M. Xu, Y. Lu, S. Liu, Y. Zhang, C. Sun, ACS Appl. Energy Mater. 1 (2018) 5557–5566. [8] E. Antolini, ACS Catal. 4 (2014) 1426–1440. [9] T. Reier, O. Mehtap, S. Peter, ACS Catal. 2 (2012) 1765–1772. [10] Y.C. Lu, H.A. Gasteiger, Y. Shao-Horn, J. Am. Chem. Soc. 133 (2011) 19048–19051. [11] C.C. McCrory, S. Jung, J.C. Peters, T.F. Jaramillo, J. Am. Chem. Soc. 135 (2013) 16977–16987. [12] J. Tulloch, W.D. Scott, J. Power Sources 188 (2009) 359–366.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Bi-functionality of samarium- and praseodymium-based perovskite catalysts for oxygen reduction and oxygen evolution reactions in alkaline medium Praveen Kolla a, b, Golibsho Nasymov a, Rudresh Rajappagowda a, Alevtina Smirnova a, b, * a b
Materials Engineering and Science Program, South Dakota School of Mines & Technology, Rapid City, SD, 57701, USA Chemistry and Applied Biological Sciences Department, South Dakota School of Mines & Technology, Rapid City, SD, 57701, USA
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Praseodymium and samarium-based perovskites were synthesized. � Catalytic activity towards OER and ORR is observed in alkaline media. � OER/ORR bi-functionality is described through graphene support-catalyst interaction.
A R T I C L E I N F O
A B S T R A C T
Keywords: Praseodymium Samarium Perovskites Oxygen evolution reaction Oxygen reduction reaction Metal-air batteries Regenerative fuel cells
The present study ellucidates bi-functional catalytic activity of the graphene-supported praseodymium (PrNixCo1-xO3-δ) and samarium (SmNixCo1-xO3-δ) perovskites towards oxygen evolution and oxygen reduction reactions in alkaline solutions. The perovskites with varied Ni and Co content (of x ¼ 0.1, 0.5, and 0.9) were synthesized via sol-gel glycine-nitrite combustion followed by heat-treated at 700, 900 and 1200 � C. The cubic perovskite phase of PrNixCo1-xO3-δ was stabilized at x ¼ 0.1, 0.5 in the entire temperature range (of 700 � C–1200 � C), while the perovskite phase of SmNixCo1-xO3-δ (x ¼ 0.1, 0.5) was observed only at lower tem peratures (of 700 and 900 � C). Consequently, changes in structural properties resulted in superior oxygen evo lution reaction (OER) electrochemical activity in comparison to the state-of-the-art IrO2. Among the Sm-based perovskites, SmNi0.1Co0.9O3-δ (700 � C) demonstrated OER mass activity of 495 mA/mg and the highest oxygen reduction reaction (ORR) activity of 95 mA/cm2⋅mg. For Pr-based perovskites, the highest electrocatalytic ac tivity toward OER and ORR was observed for PrNi0.1Co0.9O3-δ (900 � C) resulting in 60 mA/mg for ORR and 680 mA/mg for OER which is 50% higher than the mass activity of the state-of-the-art IrO2 catalyst. The overall performance of the composites is discussed in terms of graphene support interactions, Co and Ni redox processes, relative concentration of oxygen-vacancy sites and OER/ORR bi-functionality.
* Corresponding author. Chemistry and Applied Biological Sciences, South Dakota School of Mines & Technology, Rapid City, SD, 57701, USA. E-mail address: [email protected] (A. Smirnova). https://doi.org/10.1016/j.jpowsour.2019.227234 Received 30 January 2019; Received in revised form 29 September 2019; Accepted 30 September 2019 Available online 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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1. Introduction
rate-limiting multiple reaction pathways. The B-site transition metal ions in an ABO3-δ perovskite perform an important role in binding the oxygen intermediates on the surface of the perovskite particle and thus affect the overall OER efficiency. This mechanism is based on the mo lecular orbital bonding framework formed as a result of the corre sponding interactions between the eg transition metal orbitals with negatively charged oxygen adsorbates and with metal-oxygen covalency as a descriptor for its catalytic activity. Samarium and praseodymium-based perovskites have been tested earlier as catalysts for different catalytic processes, among them carbon dioxide reforming of methane [36], hydrogen and oxygen evolution [37], and syngas production [38]. However, to the best of our knowl edge, they have not been tested as bifunctional catalysts in OER and ORR. In this regard, the goal of the proposed study was to investigate the ORR and OER electrocatalytic behavior of the doped SmNixCo1-xO3 and PrNixCo1-xO3 perovskites. Perovskites in its ideal state assumes stoi chiometric chemical form of ABO3. However, with increase in partial doping of B-site element (Co3þ) with B0 -element having lower oxidation state (Ni2þ) would change the relative concentration of mixed oxidation state of B (Co2þ/Co3þ) and/or B’ (Ni3þ/Ni2þ). In addition, partial doping with low-valent atoms may also cause increase in density of oxygen-vacancy sites. In this context, the effect of the B-site doping as a function of nickel to cobalt molar ratio (x ¼ 0.1, 0.5, and 0.9) vs. the heat-treatment temperature is discussed in terms of crystal structure and oxygen stoichiometry. The electrocatalytic activity of the SmNixCo1-xO3-δ and PrNixCo1-xO3-δ perovskites was further evaluated in comparison with the state-of-the-art platinum and IrO2 catalysts for ORR and OER, respectively.
Perovskites is a large group of materials that is well known in catalysis and electrocatalysis [1] for different types of reactions ranging from reduction of air pollutants to electric power generation and storage in sustainable energy applications. Among them, one of the most chal lenging research areas is focused on metal/air rechargeable batteries [2–7] that require inexpensive, stable, and bifunctional catalysts for both oxygen reduction reaction (ORR) and its reverse reaction of oxygen evolution (OER). Since the efficiency of lithium/air cells depends on differences in voltages and electrochemical reaction rates of the corre sponding charge and discharge processes, investigation of novel cata lysts in both ORR and OER is especially important. Perovskites with a chemical formula ABO3-δ, where A is a rare-earth metal atom occupy the eight corners of cuboidal with transition metal B in the center with octahedral 6-fold coordination of oxygen atoms. Pe rovskites have attracted considerable attention as alternatives to noble metal-based catalysts, such as Pt, Ru, and Ir [8,9] that perform either as oxygen reduction [10] or oxygen evolution catalysts [11], respectively. On contrary to noble metals, metal oxides and perovskites, especially in alkaline solutions [12], demonstrate bifunctionality in ORR/OER multistep two-electron kinetic routes resulting in different intermediates [13]. This capability highlights potential perovskite applications in fuel cells [14], electrolyzers [15], and aqueous metal-air batteries [16]. The bifunctional catalytic properties of perovskites towards ORR and OER is defined by a number of factors, such as surface and bulk elec tronic properties, synthesis conditions, surface area and particle size [17], catalyst surface hydroxylation, catalyst support, and the perov skite phase purity. Moreover, the catalytic properties of perovskites can be tuned by A-site [18] or B-site relative-doping [19]. Among different perovskites, single and double perovskitestructures, e.g. NdBa0.5Sr0.5Co1.5Fe0.5O5þδ [20] have been tested in re gard to their bifunctional catalytic behavior in OER and ORR. For example, the electrocatalytic activity of lanthanum-based perovskite LaCoO3 [21] synthesized by colloidal method onto nitrogen-doped carbon support has three times higher than the state-or-the-art IrO2 in OER [22]. Furthermore, it demonstrates strong OER and ORR bifunc tional character with mass activity reaching 24 mA/mg. Another perovskite, LaNiO3 on nitrogen-doped carbon [23] is also about three times more active toward OER in comparison to IrO2. In these and many other cases [24] carbon support played an important role as electrode component that compensates the lack of electronic conductivity and ensures maximum utilization of the perov skite catalysts in ORR and OER. Moreover, carbon phase catalyzes O2 reduction to H2O2 that is further decomposed and/or reduced by the LaCoO3 perovskite [25]. On the contrary, LaCoO3, La0.8Sr0.2MnO3 in perovskite–carbon composite also contributes to the H2O2 formation, suggesting that a surface non-specific process of outer-sphere charge transfers might be possible [26]. Besides recent significant achievements [27–30], the design of an effective bi-functional catalyst is still in progress since it is complicated by the multielectron electrochemical processes utilizing different reac tion pathways and mechanisms [31]. Perovskites are highly active for the ORR on different supports due to their ability to disproportionate peroxide [21]. There are two main mechanisms proposed for peroxide disproportionation on perovskites: a surface transition metal mediated route and a vacancy mediated route. Furthermore, the effect of the ox ygen deficiency and transition metal oxidation states created in B-site doped perovskites contributes to their oxygen exchange redox behavior at ambient conditions making this analysis even more challenging [32–34]. As alternatives to noble metals, perovskites were extensively studied for ORR and OER, especially in alkaline electrolytes. In this case, the structure, electron configuration, and the crystal field of perovskites, e. g. A- or B-site doping [35], is considered as the main factors that define reaction kinetics due to formation of various intermediates and
2. Experimental methods 2.1. Material synthesis Praseodymium (PrNixCo1-xO3-δ) and samarium (SmNixCo1-xO3-δ) perovskites with different B-site doping of nickel and cobalt (x ¼ 0.1, 0.5, and 0.9) were synthesized using glycine-nitrate sol-gel synthesis. Stoichiometric quantities of Pr(NO3)3⋅6H2O, Sm(NO3)3⋅6H2O, Ni (NO₃)₂∙6H₂O, and Co(NO₃)₂∙6H₂O salts were dissolved in 20 mL of deionized water forming initially an aqueous homogeneous solution followed by addition of excess glycine (3 x molar equivalent of metal salts). A clear glassy solid was obtained after treatment of the homog enous glycine-nitrate solution for ~24 h at 60 � C. Heat-treatment of the produced solid at 400 � C for 2 h on the hot-plate at ambient conditions resulted in its complete combustion. The resulting mixture of carbon and metal oxides was further heat-treated in a muffle furnace at three different temperatures of 700, 900 and 1200 � C in air for 3 h to form Smand Pr-based complex oxide phases with diverse physical, chemical, and catalytic properties. The 40 wt% Pt/C (stock #47308) and IrO2 (stock #17849) powders used as control samples were purchased from AlfaAesar. 2.2. Characterization of structure, composition and morphology The structure of the perovskite materials was characterized by Rigaku Ultima Plus theta-theta X-ray diffraction (XRD) instrument with Cu-kα radiation (of λ ¼ 1.54178 Å) by measuring the XRD spectrum of materials from 10� to 80� with scan rate of 1� /min. The crystallite size was calculated based on Scherrer equation - L ¼ λ/(β*cos θ), where λ is the wavelength of Cu Kα radiation and β is the full width half maximaFWHM of peaks corresponding to the crystalline phase and θ is diffraction angle. The crystallite sizes were obtained by averaging par ticle size associated with the peaks corresponding to the same phase. Note that these crystallite sizes may not always directly correlated with the particle size. The surface morphology and elemental composition of the materials was analyzed with Energy Dispersive X-ray (EDX) spectrometer from 2
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Oxford Instruments attached to Zeiss Supra 40VP variable-pressure SEM. The reduction behavior of the catalyst material was studied using Temperature Program Reduction (TPR) by ramping the tempera ture from room temperature to 900 � C at 5 � C/min in 5% H2/Ar atmo sphere. The Thermal Conductivity Detector (TCD) signal plotted along Y-axis is directly proportional to the amount of H2 consumed during perovskite oxide surface reduction to water.
where Id - disc-current, Ir - ring-current, and N ¼ 0.37 - current collection efficiency of Pt-ring electrode. The number of the transferred electrons (n). In this experiment, the Pt-ring electrode was biased at a higher potential (1.0 V RHE) that is required for electrooxidation of the inter mediate peroxide species which were instantaneously produced and swept-away from the disc-electrode of varying potential (LSV). 3. Results and discussion
2.3. Electrochemical methods
3.1. Crystal structure of the materials
The electrochemical studies of the nanocomposites with 90 wt% graphene and 10 wt% praseodymium (PrNixCo1-xO3-δ) or samarium (SmNixCo1-xO3-δ) perovskites towards OER and ORR reactions were investigated in a freshly prepared 0.1 M KOH electrolyte, using an analytical three-electrode cell from Princeton Applied Research® with Ag, AgCl/3.5 M KCl reference electrode. A bipotentiostat (AFCBP1) and a speed control unit (AFMSRCE) from Pine Research Instrumentation® were used to measure the electrochemical response and maintain diffusion controlled hydrodynamic conditions of the cell, respectively. The electrode potential vs. SHE was calculated by adding þ0.205 V to each of the experimental values obtained with Ag/AgCl, Cl electrode. Then, potential Nernstian shift (59 mV/decade) associated with proton activity was added to further convert SHE to RHE, as provided by the equation below: E(RHE) ¼ EAg/AgCl þ 0.059 pH þ EoAg/AgCl,
The X-ray diffraction patterns for two groups of perovskites, specif ically PrNixCo1-xO3-δ and SmNi0.1Co0.9O3-δ, are presented in Figs. 1, 2 and Tables 1, 2 below. The X-ray diffraction spectra of PrNixCo1-xO3-δ (Fig. 1) reveal that the structure evolutions and segregation into perovskite phase largely de pends on the extent of B-site transition metals doping and the heattreatment temperature. Formation of the perovskite cubic phase (Fig. 1 and Table 1) is favorable at lower temperatures and lower Ni/Co ratios (x ¼ 0.1 and 0.5). On the contrary, high Ni/Co ratio (x ¼ 0.9) have not resulted in perovskites-phase and yielded a mixture of its constituent oxides. The shift of the characteristic XRD peaks toward higher 2θ angle (Fig. 1) in the case of PrNixCo1-xO3-δ, results from sintering during the crystal lattice formation. As demonstrated in Fig. 1 and Table 1, at higher temperatures Pr-based oxides with high Ni content (x ¼ 0.9) are not thermodynamically stable to form perovskite phase. This could be explained by the fact that ABO3-δ requires the ratio of cuboidal A-ion volume to octahedral B-ion volume to be exactly five [39]. This ratio is changing at higher temperatures due to the modification of the unit cell parameters causing collapse of the perovskite crystal-lattice. In terms of PrNixCo1-xO3-δ particle size (Table 1), the nanoparticles produced by a sol-gel approach increase their crystallite size with tem perature from ~30 nm at 700 � C to ~35 nm at 900 � C and ~40 nm at 1200 � C. This observation is in correlation with many other publications focused on metal oxide ceramic sintering [40]. Investigation of the chemical stability and crystal structure formation of the second group of SmNi0.1Co0.9O3-δ perovskites (Fig. 2, Table 2) reveals that the trends are similar to that of PrNixCo1-xO3-δ. Pure SmNi0.1Co0.9O3-δ cubic perovskite phases are formed only at lower temperatures (700 and 900 � C) and lower Ni content (Ni/Co ratio ¼ 0.1, 0.5). Similar to that of PrNixCo1-xO3-δ, the shift of the characteristic XRD SmNi0.5Co0.5O3-δ peaks toward higher 2θ angles (Fig. 2) can be attributed to the sintering effects. The smallest particles size of ~23–25 nm (Table 2) was observed for SmNixCo1-xO3-δ at 700 � C sintering temperature and at low Ni con tent (x ¼ 0.1 and 0.5), respectively. At higher temperatures, formation of the new phases different from perovskite phases were also observed. These phases include oxides associated with A-site lanthanum elements and B-site associated mixed spinel or individual oxides of transition metals. These secondary phases are mostly formed at high nickel or low cobalt content (x ¼ 0.9). Furthermore, additional XRD peaks at higher temperatures (900 � C and 1200 � C are associated with formation of Ruddelsden-Popper mixed perovskite phases [41]. These results can be explained by the differences in oxidation-states of A- and B-site metals that are induced upon doping. Specifically, higher temperatures tend to increase the number of oxygen vacancies thereby decreasing the oxidation states of the B-site metals in order to maintain an overall electroneutrality. This eventually leads to the crystal lattice expansion as B-site cations with lower oxidation states have larger ionic radii causing 2θ to shift towards lower angles. A different effect is expected for the A-site metals at higher temperatures: lattice parameters of praseodymium and samarium decrease causing lattice to shrink and angles shift to higher 2θ. This effect results in eventual formation of the secondary oxide and spinel phases. As these secondary phases are formed, the lattice strain associated with oxygen vacancies relax, which may have contributed to a gradual shrinkage of
(1)
where EoAg/AgCl ¼ 0.205 V at 25 � C and E Ag/AgCl is working potential. The catalyst inks were prepared by sonication of the 0.4 mL of 5% 1100 Nafion™ solution in 20 mL of isopropanol and 79.6 mL of water with 0.6 g of the dry catalyst powder. The catalyst inks were dispersed using Ultra Turrax1 homogenizer. The ink (10 μL) was deposited on the glassy carbon electrode surface and dried to form a thin film. Then, ~1–2 μL of 5% Nafion™ solution was added on the surface of the catalyst to prevent the catalyst film from delamination during the ex periments. Two types of working electrodes; specifically, a Rotating Disc Electrode (RDE-glassy carbon disc, AFE3T050GE, 5 mm in diameter) and a Rotating Ring Disc Electrode (RRDE-glassy carbon disc with Ptring, E7R9-series AFMSRCE, 5.61 mm in diameter) from Pine Research Instrumentation® were used for OER and ORR studies, respectively. The mass loading of the catalysts on the RDE and RRDE was calculated based on the active electrode area and the catalyst loading. The active size of RDE and RRDE were 5.0 and 5.6 mm that corresponds to the active areas of 0.196 and 0.247 cm2, respectively. The mass loadings of the catalyst were estimated based on the amount of catalyst present in the 10 μL of the ink. The perovskite-catalyst composites comprised of 10 wt% cata lyst and 90 wt% graphene. The baseline Pt/C catalyst contained 40 wt% of Pt metal on carbon, while 10 wt% of commercial IrO2 was used in combination with 90% graphene. Prior to the measurements of electrochemical polarization, catalytic activity, and selectivity towards OER/ORR of the synthesized perov skites, electrode conditioning for 50 cycles at 50 mV/s scan rate was applied. The conditioning step was used to ensure the surface of the electrocatalyst is free of impurities. However, due to high acidity of Nafion ionomer used as a binder, a partial dissolution of the metal oxide catalyst was expected, which may have contibuted to the overall elec trochemical performance of the catalyst. Therefore, electrochemical stability of the catalyst has not been considered in this study as it may not justify the original perovskite durabilty properties. To compare the ORR selectivity towards peroxide pathway (n ¼ 2) and direct ORR reduction pathway (n ¼ 4), the effective number of the electrons (n) transferred per each oxygen molecule was evaluated using equation (2). n¼4
Id Id þ INr
(2)
3
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Fig. 1. X-ray diffraction pattrens of the PrNixCo1-xO3-δ phases formed after heat-treatment at different temperatures and B-site Ni/Co ratios: (A) x ¼ 0.1, (B) x ¼ 0.5, and (C) x ¼ 0.9.
Fig. 2. X-ray diffraction patterns of the SmNixCo1-xO3-δ phases formed after heat-treatment at different temperatures and B-site Ni/Co ratios: (A) x ¼ 0.1; (B) x ¼ 0.5 and (C) x ¼ 0.9.
the crystal lattice observed at higher heat-treatment temperatures.
3.2. Analysis of chemical composition and oxygen-vacancy of the materials The SEM images for the PrNixCo1-xO3-δ and SmNixCo1-xO3-δ as well as their chemical composition from the EDS analysis are presented in Fig. 3 4
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Ni:Co:O and Sm:Ni:Co:O) are very close to their corresponding stoi chiometric values. The relative compositions of oxygen-deficient sites are qualitatively studied based on the thermal reduction behavior of the perovskites. Temperature Programmed Reduction (TPR) profiles of samarium and praseodymium-based perovskites (Fig. S1) follows a typical reduction behavior of complex perovskite oxides with initial peaks at low tem peratures (between 200 and 400 � C) that indicate formation of an in termediate oxygen deficient structures. The peaks at 400 � C or above correspond to the reduction of B-site element cobalt or the second B-site dopant nickel. The peaks at higher temperatures beyond 600 � C arise from reduction of transition metals in lower-oxidation states or reduc tion of the constituent lanthanide oxides. Increasing the amount of nickel-doping from x ¼ 0.1 to 0.5 results in lower-onset reduction tem peratures, which indicates high reducibility of both SmNi0.5Co0.5O3 and PrNi0.5Co0.5O3. On the other hand, increase in heat-treatment temper atures promote perovskites towards highly oxygen-vacant structures, which is evident from a gradual increase of the onsets of the reduction temperatures. This observation is in correlation with the shifts for the XRD peaks (Figs. 1–2) to higher diffraction angles when the heattreatment temperatures increases. This phenomenon is explained by the lattice shrinkage caused by increase in density of the oxygen-vacant sites.
Table 1 Crystal structure, crystalline size, and lattice parameters of PrNixCo1-xO3 as a function of its relative (Ni/Co) composition and heat-treatment (HT) temperature. Material
HT
Perovskite Yield
Structure
Crystallite Size (nm)
PrNi0.1Co0.9O3
700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C
Yes Yes Yes Yes Yes Yes No No No
Cubic Cubic Cubic Cubic Cubic Rhombohedral – – –
30.4 34.7 43.0 29.6 33.6 38.9 – – –
PrNi0.5Co0.5O3 PrNi0.9Co0.1O3
Table 2 Crystal structure, crystalline size, and lattice parameters of SmNixCo1-xO3-δ as a function of its relative (Ni/Co) composition and heat-treatment (HT) temperature. Material
HT
Perovskite Yield
Structure
Crystallite Size (nm)
SmNi0.1Co0.9O3
700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C 700 � C 900 � C 1200 � C
Yes Yes No Yes Yes Partial No No No
Cubic Rhombohedral – Cubic Cubic Cubic – – –
23.4 39.8 – 25.4 35.4 – – – –
SmNi0.5Co0.5O3 SmNi0.9Co0.1O3
3.3. ORR of the catalysts As depicted in the Fig. S2, the explicit role of graphene can be un derstood by comparing the electrochemical polarization data of various perovskite-graphene compositions. It is evident that both graphene and perovskite display high overpotentials and poor ORR activity in their pristine forms. However, as the concentration of graphene increases, the composites gradually start to exhibit lower overpotentials and higher current densities. Here, the inherent ability of atomically thin graphene in percolating perovskite moiety is experimentally demonstrated.
and Table 3, respectively. The SEM images of the PrNixCo1-xO3-δ and SmNixCo1-xO3-δ (Fig. 3) heat-treated at 900 � C indicate that both crystal structures with the lowest Ni content (x ¼ 0.1) have the smallest grain sizes in the range of about 50 nm, high porosity, and, thus, more surface available for electrocatalytic activity (Fig. 3a, d) associated with cubic crystal structure. On the other hand, the samples with equal ratio of Ni to Co (x ¼ 0.5) (Fig. 3b, e) demonstrate a shift toward less porosity and formation of larger grains. The PrNixCo1-xO3-δ sample with the highest Ni content (x ¼ 0.9) has different morphology (Fig. 3c) compared to that of samarium-based material (Fig. 3d) with larger grains. Energy dispersive spectroscopy (EDS) was used to determine the chemical composition of the PrNixCo1-xO3-δ and SmNixCo1-xO3-δ perov skites heat-treated at 900 � C. No elements other than Pr, Ni, Co and O are observed by EDS (Table 3). The ratios of the constituting elements (Pr:
Table 3 Elemental composition of PrNixCo1-xO3 and SmNixCo1-xO3 heat-treated at 900 � C in terms of weight (and atomic) percentages obtained from EDS analysis. Wt.% Material (at.%)
Pr or Sm
Ni
Co
O
PrNi0.1Co0.9O3 PrNi0.5Co0.5O3 PrNi0.9Co0.1O3 SmNi0.1Co0.9O3 SmNi0.5Co0.5O3 SmNi0.9Co0.1O3
61.4 (23.6) 62.4 (24.2) 61.3 (23.6) 64.3 (27.4) 62.1 (25.1) 63.1 (25.2)
2.3 (2.1) 10.8 (10.1) 20.0 (18.5) 1.6 (1.7) 12.3 (12.7) 2.7 (2.8)
19.9 (18.3) 10.4 (9.6) 2.1 (1.9) 22.6 (24.5) 12.8 (13.2) 21.4 (22.4)
16.5 (55.9) 16.4 (56.1) 16.5 (60.0) 11.6 (46.4) 12.9 (49.0) 12.9 (49.7)
Fig. 3. SEM images of PrNixCo1-xO3 (a-c) and SmNixCo1-xO3 (d-f) heat-treated at 900 � C with different Ni content (x ¼ 0.1, 0.5 and 0.9). 5
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Journal of Power Sources 446 (2020) 227234
Consequently, graphene at higher content reduces charge-transfer re sistances of the composite electrodes. These results highlight the importance of an optimized perovskite to graphene ratio for achieving the highest synergistic bi-functional electrochemical performance. In regard to the ORR activity of the Pr- and Sm-based catalysts, three factors have the most noticeable impact, such as Pr or Sm of A-site metal, the heat-treatment temperature, and the nickel B-site doping (Figs. 4, 5). In comparison to Pt (40 wt%) C, both Pr- and Sm-based catalysts demonstrate higher onset overpotentials and slightly lower power den sities (Figs. 4, 5). Considering that the amount of platinum in the com mercial Pt catalyst is 4 times higher than the amount of perovskite (10 wt.%) in the nanocomposite with graphene, the demonstrated ORR catalytic activity is an important step towards development of the nonnoble metal catalysts for the energy generation and storage. Specifically, the ring currents (Fig. 4a) for the Pr-based perovskites heat-treated at lower temperatures are the lowest at the lowest Ni content (x ¼ 0.1). A relatively low number of electrons (n ¼ 3.5) compared to Pt (n ¼ 4) suggest that an alternate ORR pathway through peroxide intermediate takes place as perovskites could mediate a surface non-specific outershell electron transfer reactions involving ORR in alkaline medium. Regarding polarization currents (Fig. 4c), the best performance is demonstrated by the PrNi0.1Co0.9O3-δ catalyst with the lowest Ni content (x ¼ 0.1) and intermediate (900 � C) heat-treatment temperature. This sample shows the highest mass activity in comparison to the other Prbased catalysts (Fig. 4d). In comparison to the Pr-based catalysts, Sm-based catalysts demon strated overall higher ORR activity. The ring currents (Fig. 5a) are the lowest and the number of electrons involved in ORR (Fig. 5b) is the highest for the Sm-based perovskites heat-treated at lower temperatures and with the lowest Ni content (x ¼ 0.1). Regarding polarization cur rents (Fig. 5c), the best performance is demonstrated by the perovskite with the lowest Ni content (SmNi0.1Co0.9O3-δ) and the lowest (700 � C) heat-treatment temperature. These results are in correlation with the
mass activity values that are the highest for this catalyst (Fig. 5d, sample 1). 3.4. OER of the catalysts Numerous publications address the perovskite bi-functionality and provide a broad range of ORR/OER mass and specific activity values obtained at different operating conditions [42,43]. To confirm the SmNixCo1-xO3-δ bi-functionality, the catalytic activity evaluation was performed in 0.1 KOH solution saturated by oxygen. Since perovskites demonstrate the overpotential of up to 150–200 mV compared to the state-of-the-art Pt/C catalyst with onset potentials in the range of 0.6V–0.7 V, their mass activities were calcu lated between their onset potentials and the half-wave potentials, spe cifically within the 0.5V-0.6 V range. Furthermore, in order to be consistent with perovskite ORR mass activities and compare them with other available OER studies, the OER mass activities with a unit of mA/ mg are presented in Figs. 6–7. The results indicate (Figs. 6–7) that the best OER catalytic activity is observed for SmNi0.1Co0.9O3-δ perovskite with the lowest nickel content (x ¼ 0.1) and sintered at the lowest temperature (700 � C). In case of the Pr-based perovskite (PrNi0.5Co0.5O3-δ) with intermediate Ni content (x ¼ 0.5) and heat-treated at the lowest temperature (700 � C) the highest polarization currents have been observed (Fig. 6a). However, consid ering mass activity, the perovskite with the lowest Ni content (x ¼ 0.1) and heat-treated at intermediate temperature (900 � C) demonstrated the highest mass activity in comparison to other Pr-based perovskites (Fig. 6b) that could be related to the resultant concentration of Ni (II) and/or Co (II/III) and the overall metal-oxygen covalency created due to the oxygen-deficient-sites. The Sm-based perovskite (SmNi0.5Co0.5O3-δ) with intermediate Ni content (x ¼ 0.5) and heat-treated at intermediate temperature (900 � C) demonstrates the highest polarization currents (Fig. 7a). However, the
Fig. 4. ORR performance of PrNixCo1-xO3-graphene composite in terms of (a) ring-current, (b) number of electrons participated, (c) polarization currents and (d) mass activity, in comparison to state-of-the-art Pt/C catalyst. 6
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Journal of Power Sources 446 (2020) 227234
Fig. 5. ORR performance of SmNixCo1-xO3-graphene composite in terms of (a) ring-current, (b) number of electrons participated, (c) polarization currents and (d) mass activity, in comparison to state-of-the-art Pt/C catalyst.
Fig. 6. OER performance of PrNixCo1-xO3-graphene composite in terms of (a) polarization current densities and (b) mass activities, in comparison to state-of-the-art IrO2 catalyst.
catalyst SmNi0.1Co0.9O3-δ with the lowest Ni content (x ¼ 0.1) and heattreated at the lowest temperature (700 � C) shows the highest mass ac tivity in comparison to other Sm-based perovskites (Fig. 7b). Based on the demonstrated results, it can be concluded that in comparison to Pr-based catalysts (Fig. 6b), the OER mass activity for PrNi0.1Co0.9O3-δ (Fig. 7b) with the lowest Ni content and heat-treated at the lowest temperature has the highest oxygen evolution reaction activity.
synthesized using a sol-gel Pechini method resulting in relatively small crystallite size. Specifically, the size of the perovskite crystallites in cubic phase after sintering at elevated temperatures (700–1200 � C) was in the range of 23.4–39.8 nm. Electronic configuration of A-site lanthanum cations can signifi cantly affect oxygen deficiency in perovskites. Specifically, Pr with [Xe] 4f2 electron configuration tend to form higher III/IV oxidation states, while Sm possesing [Xe] 4f5 electron configuration exhibits lower oxidation states: II/III. Consequently, Sm would promote perovskite structures with high metal-oxygen covalency and high oxygen defi ciency, when compared to Pr-based perovskites. Therefore, oxygen may readily interact with samarium in Sm-based perovskite thereby enhancing the ORR activity. On the other hand, B-site cations of Ni and Co exhibit II and II/III oxidation states, respectively. Therefore, stable perovskite structures are readily produced with high Co-content (x ¼ 0.1 and 0.5). This trend is less important for Pr-rather than Sm-based
4. Conclusions The catalytic activity and electrochemical performance of the graphene-supported praseodymium (PrNixCo1-xO3-δ) and samarium (SmNixCo1-xO3-δ) perovskites towards oxygen evolution and oxygen reduction reactions in aqueous alkaline solutions has been investigated. The Pr/SmNixCo1-xO3-δ–graphene nanocomposites were successfully 7
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Fig. 7. OER performance of SmNixCo1-xO3-Graphene composite in terms of (a) polarization current densities and (b) mass activities, in comparison to state-of-the-art IrO2 catalyst.
perovskites due to flexible oxidation-states associated with both Pr and Co. The perovskite yield also follows similar trend with heat-treatment temperature. Specifically, the Pr-based perovskites tend to form stable perovskite structures when compared with Sm-based perovskites pos sessing low oxidation states. Subsequently, Pr-based perovskites with high Co-content exhibit superior OER performance as these catalysts could promote high-redox activity. The relatively high ORR catalytic activity combined with high OER activity, exceeding the OER catalytic activity for IrO2, makes these perovskites as good candidate for cathodes in aqueous metal-air batteries and regenerative fuel cells.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support from the ACS Petroleum Research Fund (Project number 53614-UR10) and the NSF EPSCoR projects (Awards IIA-1330840 and IIA-1330842). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227234. References [1] J. Hwang, R.R. Rao, L. Giordano, Y. Katayama, Y. Yu, Y. Shao-Horn, Science 358 (2017) 751–756. [2] Y. Gong, X. Zhang, Z. Li, Z. Wang, C. Sun, L. Chen, ChemNanoMat 3 (2017) 485–490. [3] X. Xu, W. Wang, W. Zhou, Z. Shao, Small Methods 2 (7) (2018) 1800071. [4] X. Zhang, Y. Gong, S. Li, C. Sun, ACS Catal. 7 (2017) 7737–7747. [5] J. Bian, Z. Li, N. Li, C. Sun, Inorg. Chem. 58 (2019) 8208–8214. [6] J. Bian, R. Su, Y. Yao, J. Wang, J. Zhou, F. Li, Z.L. Wang, C. Sun, ACS Appl. Energy Mater. 2 (2019) 923–931. [7] J. Wang, X. Cheng, Z. Li, M. Xu, Y. Lu, S. Liu, Y. Zhang, C. Sun, ACS Appl. Energy Mater. 1 (2018) 5557–5566. [8] E. Antolini, ACS Catal. 4 (2014) 1426–1440. [9] T. Reier, O. Mehtap, S. Peter, ACS Catal. 2 (2012) 1765–1772. [10] Y.C. Lu, H.A. Gasteiger, Y. Shao-Horn, J. Am. Chem. Soc. 133 (2011) 19048–19051. [11] C.C. McCrory, S. Jung, J.C. Peters, T.F. Jaramillo, J. Am. Chem. Soc. 135 (2013) 16977–16987. [12] J. Tulloch, W.D. Scott, J. Power Sources 188 (2009) 359–366.
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