- Email: [email protected]
Fe3+ in iron phosphates for enhanced electrocatalytic activity in Li-O2 batteries
Chemical Engineering Journal 388 (2020) 124294
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Redox effect of Fe2+/Fe3+ in iron phosphates for enhanced electrocatalytic activity in Li-O2 batteries Gwang-Hee Lee, Yoon Seon Kim, Dong-Wan Kim
T
⁎
School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 02841, South Korea
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
phosphates laundry-ball-like na• Iron nostructures are prepared as electrocatalysts.
Porous Fe P O enables an excellent • cycle reversibility for Li-O batteries. coordination of phosphate • Flexible anions stabilizes the intermediate Fe 2 2
7
2
cations.
Superior performances of Fe P O • sult from the optimized Fe /Fe
2 2 7 re2+ 3+
redox effect.
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron phosphates Thermal reaction Fe2+/Fe3+ redox effect Electrocatalysts Li–O2 battery
FePO4 and porous Fe2P2O7 laundry-ball-like nanostructures (FePO4 LBs and p-Fe2P2O7 LBs, respectively) were prepared to investigate their functionalities as oxygen-electrode (O2-electrode) electrocatalysts in Li–O2 batteries. These structures were synthesized in two steps, via hydrothermal and thermal reactions. FePO4 LBs were synthesized through thermal dehydrogenation of as-prepared FePO4·2H2O precursors (FePO4·2H2O → FePO4 + 2H2O), and p-Fe2P2O7 LBs were synthesized through thermochemical reduction of same precursors under an H2 atmosphere (2FePO4·2H2O + H2 → Fe2P2O7 + 5H2O). As an O2-electrode electrocatalyst in Li–O2 cells, p-Fe2P2O7 LBs exhibited a higher discharge capacity (30,000 mA h gcatalyst–1 at a current density of 500 mA gcatalyst–1), higher reversibility (300 cycles at a current rate of 500 mA gcatalyst–1), and lower voltage gap, compared to FePO4 LBs. These superior performances of p-Fe2P2O7 LBs result from the Fe2+/Fe3+ redox effect and porous structure, which enhance the oxygen reduction or evolution reaction activities.
1. Introduction Immense research has been conducted on metal oxide-type catalysts, which are capable of catalyzing oxygen reduction/evolution reactions, in the field of Li-O2 batteries. The important factors that affect the catalytic activity of these metal oxide-type catalysts are as follows: (i) isolation of active sites, (ii) metal–oxygen bond strength, (iii) crystalline structure, (iv) phase cooperation, (v) the oxidation states of the
⁎
surface cations (redox properties), (vi) the nature of the surface oxygen species, and (vii) multi-functionality [1]. These factors are the basis for choosing single or mixed metal oxides of metal oxide-type catalysts for use as oxygen reduction/evolution reaction catalysts. Particularly, the attractive properties of mixed metal oxides (ABO3, ABO4, A2B2O7, etc.) are often discussed in connection with partially occupied transition metal (TM) d-orbitals and corresponding local atomic structures [2–10]. In TM oxides, the formation of oxygen vacancies (Ovac) results
Corresponding author. E-mail address: [email protected] (D.-W. Kim).
https://doi.org/10.1016/j.cej.2020.124294 Received 31 October 2019; Received in revised form 2 January 2020; Accepted 30 January 2020 Available online 31 January 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
in the change of coordination and the formal charge of the surrounding TM cations simultaneously [10]. In addition, the TM oxide surfaces are terminated by O2− anions because their sizes are much larger than those of TMn+ cations, and therefore, the symmetry and coordination of TMn+ cations can be lost at the surface. Furthermore, the oxide surfaces may contain different types of structural defects, such as terraces, steps, and kink, which play an important role in the catalytic activity [1]. However, the thermodynamically unsaturated surface is usually compensated by the reaction of gaseous O2 or the decomposition of solid Li2O2, resulting in the formation of O2− ions according to the following chemical equations: O2 (g) → 2O2− or Li2O2 (s) → 2Li+ + 2O2−. Nazar et al. developed pyrochlore oxide with a fraction of surface defects, which demonstrates lower overpotentials. So far, various electrocatalysts have been reported that contain oxygen vacancies and/ or defect sites [11]. For example, α-MnO2 nanotubes provide excellent electrocatalytic activity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) when more Mn3+ ions are exposed on the surface than Mn4+ [12]. In another case, C/Fe3O4 hollow granules show more oxygen vacancies when providing more reduced Fe2+/Fe3+ redox couples. C/Fe3O4 hollow granules show better ORR/OER performance than Fe2O3 hollow granules with less oxygen vacancies [13]. We focused on the flexible coordination of TMOs. TMOs with reduced TM are more stable in the oxidizing atmosphere, and oxidized defect sites can be used as excellent active sites. TM phosphates have strong structural stability and can appear in various crystalline and amorphous forms with different orientations of phosphate group. In addition, the flexible coordination of phosphate groups can stabilize the intermediate state of the TM cations by changing their local positions easily while ensuring effective redox changes of the TM cations [14–18].The divalent pyro-phosphates (M2P2O7; M = Cu, Fe, Zn, or Mg cations) are composed of P2O7 layers that are parallel to the (0 0 1) crystallographic plane and cations that have an irregular octahedral − coordination with oxygen atoms in the C1 space group (triclinic phase) [18,19]. For example, the triclinic Fe2P2O7 (β-phase), which is a distorted form of the triclinic FePO4 phase (2Fe(III)PO4 → Fe − (II)2P2O7 + O), belongs to the C1 space group. Therefore, in β-phase, the oxygen atoms of the P-O-P bridge can be assumed to be disoriented positionally; however, the flexible coordination of the phosphate anions ([PO4]3− and [P2O7]4−) can stabilize the intermediate state of the Fe cations by changing their local positions easily while ensuring effective redox changes (Fe3+ → Fe2+). For these reasons, we suggest tha the phosphate anions can serve as a solid support for Fe-P-O crystal structure. In our previous work, CuGeO3 nanowires had reducing Cu+/Cu2+ redox couples during the thermal reduction process. However, due to the unchanged [GeO3]2− anion in CuGeO3, the crystal structure was unchanged and many Ovac were generated on the crystal surface. Therefore, the variability and stability of the anions ensure generation of suitable redox couples and Ovac, which can exhibit excellent catalytic activity [4]. Herein, we explore the catalytic capabilities of Fe2P2O7 and FePO4 in terms of the Fe2+/Fe3+ redox change effect and the corresponding Ovac effect by using them as O2-electrode catalyst materials in Li-O2 batteries. These different redox couples and Ovac provide a platform to understand the catalytic activity of the iron phosphates in the oxygen reduction/evolution reactions better.
were added to 150 ml of aqueous solution. The reaction mixture was then vigorously stirred for 30 min. This transparent solution was then added to 0.3 M Na2SO4 with continuous stirring for 30 min. Subsequently, this solution was transferred to a Teflon-lined autoclave with a capacity of 200 ml, and was heated at 120 °C for 5 h. The resulting precipitate was then collected by centrifugation and was washed several times with distilled water and ethanol. Finally, the washed precipitate was dried overnight at 90 °C. 2.2. Thermal treatments of FePO4 and porous Fe2P2O7 laundry-ball-like nanostructures For synthesis of FePO4 laundry-ball-like nanostructures (FePO4 LBs), the thermal treatment was performed at 400 °C for 24 h under air atmosphere. For synthesis of porous Fe2P2O7 laundry-ball-like nanostructures (p-Fe2P2O7 LBs), the thermal treatment was performed at 400 °C for 24 h under flowing H2 (300 sccm). 2.3. Material characterization All samples were characterized by field-emission scanning electron microscopy (FESEM; SU-70, Hitachi, Japan), transmission electron microscopy (TEM; JEM-2100F, JEOL, USA), X-ray diffraction (XRD; Smartlab, Rigaku, Japan), Brunauer–Emmett–Teller analysis (BET; BELSORP mini II, BEL, Japan), and X-ray photo-electron spectroscopy (XPS; PHI X-tool, ULVAC-PHI, Japan). 2.4. Li–O2 cell studies The O2-electrodes were fabricated by mixing the powders (FePO4‧2H2O LBs, FePO4 LBs, and p-Fe2P2O7 LBs) with Super P carbon black and carboxymethyl cellulose (Aldrich, average Mw ~ 700000) in a mass ratio of 27:63:10, and subsequently pasting the mixture on Ni foam. The electrochemical tests were carried out by using Swageloktype Li-O2 cells with air holes on the O2-electrode side. The Li-O2 cells were assembled in an Ar-filled glove box by using a Li foil as the anode, glass fiber separator (Whatman), 0.5 M lithium bis(trifluoromethyl) sulfonylimide (Sigma Aldrich, 99.95%) in dimethyl sulfoxide as the electrolyte, the prepared porous O2-electrode as the cathode, and a carbon cloth (W0S1002, CeTech) as the gas diffusion layer. All measurements were conducted under 1.5 bar dry oxygen atmosphere (> 99.999%) without any humidity. The active masses of all O2-electrodes were calculated based on the electrocatalyst. The galvanostatic discharge–charge profiles and cyclic voltammetry were recorded by using an automatic battery cycler (WBCS 3000, WonaTech) after holding at open circuit potential for 6 h. 3. Results and discussion Fig. 1a schematically illustrates the synthesis process of p-Fe2P2O7 LBs and FePO4 LBs from FePO4‧2H2O LBs. The growth mechanism of FePO4‧2H2O LBs is as follow [20–23]. Based on the growth mechanism by hydrothermal reaction, the precipitation reaction is slowed to separate the nucleation and growth process. Therefore, FePO4‧2H2O nanoparticles were formed at a relatively slow rate in the initial stage of the reaction. In the second stage (aggregation stage), furthermore the hydrothermal reaction aggregates small nanoparticles into microsphere structures consisting of plates in order to reduce the surface energy through oriented growth. In the third stage (Ostwald ripening stage), the prolonged hydrothermal reaction leads to dissolution and recrystallization of particles. The sulfate ions of Na2SO4 etches out the surfaces and leads to the morphological reconstruction, which offer many nucleation sites for further growth. Further increase in the hydrothermal reaction leads to interweaving of nanoplates to form FePO4‧2H2O LBs. The thermal treatment process involves the following steps: (i) in the initial step, FePO4‧2H2O LBs were synthesized by the
2. Experimental 2.1. Synthesis of FePO4‧2H2O laundry-ball-like nanostructures FePO4‧2H2O laundry-ball-like nanostructures (FePO4‧2H2O LBs) were synthesized by using a hydrothermal method. The starting materials used to synthesize FePO4‧2H2O LBs were FeCl3 (Aldrich, purity 97%), H3PO4 (Aldrich, purity 99.0%), and Na2SO4 (Aldrich, purity 98%). As per the typical procedure, 0.12 M FeCl3 and 0.12 M H3PO4 2
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 1. (a) Schematic illustration of fabrication of p-Fe2P2O7 LBs and FePO4 LBs. FESEM images of (b, c) p-Fe2P2O7 LBs and (d, e) FePO4 LBs.
hydrothermal process; and (ii) in the two different final steps, pFe2P2O7 LBs were synthesized by the thermal treatment of FePO4‧2H2O LBs under a constant H2-flow (as shown in Eq. (1)) and FePO4 LBs were synthesized by the thermal treatment of FePO4‧2H2O LBs under air-atmosphere (as shown in Eq. (2)).
2FePO4 2H2 O + H2 FePO4 2H2 O
→
Thermal treatment
→
Thermal treatment
Fe2 P2 O7 + 5H2 O (in H2)
FePO4 + 2H2 O (in air)
we found that the porous structure of p-Fe2P2O7 LBs was developed by removing a large amount of crystalline water contained in FePO4‧2H2O as well as oxygen in Fe-P-O bonds. Further, as shown in Fig. 2a and b, both TEM and high-angle annular dark-field scanning TEM (HAADF STEM) images of p-Fe2P2O7 LBs indicate porous structure. The crystal structure of p-Fe2P2O7 LBs was further investigated in detail by high-resolution TEM (HRTEM) (Fig. 2c). The HRTEM image indicates that the interplanar distance for p-Fe2P2O7 LBs is 0.362 nm, which is consistent with the (1 1 1) plane of the triclinic Fe2P2O7 crystalline phase (PDF card No. 76-1762). The XRD pattern of pFe2P2O7 LBs was compared with that of the standard Fe2P2O7 crystalline phase (Fig. 3a). The good agreement between the XRD patterns indicates that Fe2P2O7 exists in the crystalline form. The XRD pattern of FePO4 LBs could be uniquely indexed to that of the triclinic FePO4, because of the absence of impurity (Fig. 3b) [24]. Table S1 lists the
(1) (2)
Following the thermal treatment, we observed the morphology of the obtained samples through FESEM images (Fig. 1b–e). As shown in Fig. 1b and c, p-Fe2P2O7 LBs have diameters in the range of approximately 1 μm and have porous surfaces. As shown in Fig. 1d and e, FePO4 LBs have similar particle size and morphology as those of pFe2P2O7 LBs; however, they do not have porous surfaces. In addition, 3
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 2. (a) TEM, (b) HAADF STEM, and (c) HRTEM images of p-Fe2P2O7 LBs. (d) TEM image of FePO4 LBs.
and Fe3+ oxidation states, respectively [16–28]. The results presented in Table 1 show that the Fe2+/Fe3+ redox couple is present in lower oxidation states, i.e., p-Fe2P2O7 LBs, because the Fe2+/Fe3+ molar ratio of p-Fe2P2O7 LBs is approximately unity (Fe2+/Fe3+ = 1.01). However, the Fe2+/Fe3+ molar ratio at the surface of FePO4 LBs decreases from 1.01 to 0.62. In the P 2p spectra, the P 2p peak of p-Fe2P2O7 LBs shifts to a higher binding energy when compared with that of FePO4 LBs, corresponding to the Fe2+/Fe3+ redox change. The binding energy shift was found to be ~0.6 eV, indicating that the heat treatment under H2 atmosphere induces a change in the chemical bonding of Fe-P-O groups. The O 1 s spectra present two peaks at binding energies of ~531.1 and ~532.7 eV, representing the O 1 s level in p-Fe2P2O7 LBs and FePO4 LBs [4,10,29]. The XPS survey spectra exhibit the oxygen contents of p-Fe2P2O7 LBs and FePO4 LBs. The oxygen content of pFe2P2O7 LBs is similar to the oxygen content of FePO4 LBs (Fig. S3 and Table S2). However, the concentration of Ovac in p-Fe2P2O7 LBs is higher than that in FePO4 LBs (Table 1). From this observation, it is evident that p-Fe2P2O7 LBs are more deficiently coordinated with O2 during annealing under reducing conditions than in the case of FePO4 LBs. Therefore, we expect that p-Fe2P2O7 LBs will offer an opportunity to explore the effect of the Fe2+/Fe3+ redox ratio and Ovac/Olattice ratio. We evaluated the electrochemical performance of O2-electrodes containing p-Fe2P2O7 LBs and FePO4 LBs. As shown in Fig. 5a, the initial galvanostatic full discharge tests of both p-Fe2P2O7 LB electrode and FePO4 LB electrode were performed at a current rate of 500 mA g−1 to 2.5 V. The p-Fe2P2O7 LB electrode exhibits a discharge capacity of ~30,000 mA h g−1, and the FePO4 LB electrode exhibits a discharge capacity of ~15,000 mA h g−1, which is approximately half the discharge capacity of the p-Fe2P2O7 LB electrode. We then compared the discharge–charge cycling results for the p-Fe2P2O7 LB electrode and
values of the BET surface area, pore volume, and pore diameter obtained from N2 adsorption–desorption isotherms and Barrett-JoynerHalenda (BJH) pore size distribution curves for all the samples. The BET surface areas of p-Fe2P2O7 LBs and FePO4 LBs were determined to be 10.44 and 4.31 m2 g−1, respectively; and their pore volumes were found to be 0.045 and 0.012 cm3 g−1, respectively. The average pore diameter increased from 133.39 nm for FePO4 LBs to 172.27 nm for pFe2P2O7 LBs because of the removal of a large amount of crystalline water as well as oxygen. When compared with the BJH pore size distribution curve of p-Fe2P2O7 LBs that had well-developed pores in the mesopore region (< 300 Å), the BJH pore size distribution curve of the FePO4 LBs had insignificant values, indicating no developed pores (Fig. S1). As shown in Fig. 2d, the TEM image of FePO4 LBs indicates the formation of dimples. Thus, the existence of only p-Fe2P2O7 LBs indicates mesopores (Fig. S2). Further, as indicated by the XPS profiles in Fig. 4, the difference in the surface oxidation states of p-Fe2P2O7 LBs and FePO4 LBs is quite significant. We performed peak separation of all XPS spectra based on Gaussian function. The binding energy peak position of specific element depends on the oxidation state and local chemical environment of that element (i.e., shifts in the core-level binding energies induced by the immediate chemical environment of an atom). The peak shift in XPS is most of time related to a change of oxidation state of the element. Here, higher binding energies mean also higher oxidation states. The element of a higher positive oxidation state exhibits a higher binding energy due to the extra coulombic interaction between the photoemitted electron and the ion core. In most metals there is a positive shift between the elemental form and mono-, di- or trivalent ions (M3+ → M2+ → M+ → M0) [25]. The Fe 2p3/2 spectra could be decomposed into two peaks at binding energies of ~713.5 and ~710.7 eV, corresponding to the Fe2+ 4
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Table 1 Comparison of Fe2+/Fe3+ redox couple and Ovac/Olattice ratios by XPS analysis.
2+
3+
Fe /Fe Ovac/Olattice
Fe2P2O7
FePO4
1.01 0.49
0.62 0.19
provides reversible catalytic activity. The overall overpotential of the pFe2P2O7 LB electrode is lower than that of the FePO4 LB electrode during 400 discharge–charge cycles. In the initial discharge–charge curves, the final voltage gap of the p-Fe2P2O7 LB electrode is similar to that of the p-Fe2P2O7 LB electrode. However, in the 200th and 300th discharge–charge curves, the final voltage gap of the p-Fe2P2O7 LB electrode (1.59 V at 200th cycle) is smaller than that of the FePO4 LB electrode (1.65 V at 200th cycle) (Fig. S5). In addition, the p-Fe2P2O7 LB electrode has a more stable cycle retention (400 cycles) when compared with the FePO4 LB electrode (250 cycles) (Fig. 5c). Furthermore, we carried out the discharge–charge cycling test for a large capacity at a current rate of 500 mA g−1 and a fixed capacity regime of 2000 mA h g−1, as shown in Fig. 5d–f. In the initial discharge–charge curves, the final voltage gap of the p-Fe2P2O7 LB electrode is similar to that of the p-Fe2P2O7 LB electrode. However, in the 10th and 20th discharge–charge curves, the final voltage gap of the pFe2P2O7 LB electrode (1.61 V at 10th and 20th cycle) is remarkably smaller than that of the FePO4 LB electrode (Fig. 5e). In addition, the pFe2P2O7 LB electrode has remarkably stable cycling performance when compared with the FePO4 LB electrode during 80 discharge–charge cycles (Fig. 5f). The role of disordered Fe3+ ions in Fe2P2O7 structure is important for catalytic activity. The catalytic activity of p-Fe2P2O7 LBs is strongly correlated with its surface oxidation state for disordered Fe3+ ions containing oxygen vacancies with respect to the electrochemical performance. Disordered Fe3+ ions with oxygen vacancies provides active sites during discharge–charge process [10,30]. As possible ORR/OER mechanism for p-Fe2P2O7 LBs, the exposed Fe3+ sites provide sites for formation of active oxygen species (i.e. absorbed O2– and O2*). Then, The discharge products (Li2−xO2) form on active Fe3+ site during the discharge process. The p-Fe2P2O7 LB electrode exhibits two-steps of the galvanostatic discharge or charge curves, respectively (Fig. 5e). This formation reaction corresponds to the first step (step I), which shows the higher discharge voltage. Subsequently, the disproportionation reaction of Li2−xO2 leads to the reaction to obtain
Fig. 3. XRD patterns of (a) p-Fe2P2O7 LBs and (b) FePO4 LBs.
FePO4 LB electrode at a current rate of 500 mA g−1 and a fixed capacity regime of 500 mA h g−1, as shown in Fig. 5b and c. We carried out the discharge–charge cycling test of the pure Super P electrode, as shown in Fig. S4. The overall overpotential of the pure Super P electrode is stable only 5 discharge–charge cycles. It can be seen that iron phosphate
Fig. 4. Fe 2p, P 2p, and O 1s XPS profiles of (a) p-Fe2P2O7 LBs and (b) FePO4 LBs. 5
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 5. Electrochemical performance of p-Fe2P2O7 LBs and FePO4 LBs: (a) Full discharge–charge curves of p-Fe2P2O7 LBs and FePO4 LBs at a current density of 500 mA g−1. (b) 1st discharge–charge curve of p-Fe2P2O7 LBs and FePO4 LBs at a current density of 500 mA g−1 under the fixed capacity regime of 500 mA h g−1. (c) Comparison of the discharge–charge specific capacity with the cycle number of p-Fe2P2O7 LBs and FePO4 LBs, corresponding to (b). (d) 1st, and (e) 10th and 20th discharge–charge curves of p-Fe2P2O7 LBs and FePO4 LBs at a current density of 500 mA g−1 under the fixed capacity regime of 2000 mA h g−1. (f) Comparison of the discharge–charge specific capacity with the cycle number of p-Fe2P2O7 LBs and FePO4 LBs, corresponding to (d) and (e).
the p-Fe2P2O7 LB electrode is more active in terms of the formation of Li2O2, which is a positive reaction, than in terms of the formation of Li2CO3, which is a parasitic reaction. As show in Fig. S6, the discharge products of the p-Fe2P2O7 LB electrode were formed in film-like morphology. This phenomenon shows mainly that the discharge products have a conformal film-like morphology by surface mediated growth at high currents. The film-like products may contain more structural defects, which is advantageous for charge transport and OER process. Overall, we concluded that the Li-O2 battery conditions contribute toward the excellent oxygen reduction/evolution activity of p-Fe2P2O7 LBs because of the corresponding optimized Fe2+/Fe3+ redox, a large amount of Ovac in Fe2P2O7 phase, and the porosity of p-Fe2P2O7 LBs. Also, we checked the stable Fe2+/Fe3+ redox couples of p-Fe2P2O7 LBs after the 5th discharge process. As a result, the surface composition of p-Fe2P2O7 LBs retained Fe2+/Fe3+ redox couples as shown in the XPS results (Fig. S7). Therefore, p-Fe2P2O7 LBs is considered very stable electrocatalyst.
stable Li2O2 discharge products according to the deepening of the discharge process (step II). In charge process, the Li2−xO2 during the discharge process could decompose Li2O2, corresponding to the step III, which shows the lower charge voltage. Then, the Li2−xO2 is decomposed at the step IV with higher charge voltage. This proposed electrochemical reaction mechanism is based on the previously reported result [30]. For further detail on catalytic activity of the p-Fe2P2O7 and FePO4 LBs, the dQ/dV plots of the p-Fe2P2O7 LB electrode and FePO4 LB electrode were obtained from those for the 100th cycle in Fig. 4c (Fig. 6a–c). The p-Fe2P2O7 LB electrode exhibits a larger O2 reduction peak at a high voltage (2.75 V) when compared with the FePO4 LB electrode (at 2.7 V) during the discharging process (Fig. 6a and b). This suggests that the p-Fe2P2O7 LB electrode shows excellent ORR activity when compared with the FePO4 LB electrode. During the charging process (Fig. 6a and c), the FePO4 LB electrode exhibits a large O2 evolution reaction peak, indicative of side reactions that can decompose the organic solvent-based electrolyte or lithium carbonate over 4.3 V, which are consistent with the high charge voltage (Fig. S5). Importantly, the galvanostatic cycling performance of the p-Fe2P2O7 LB electrode was found to be stable because the major O2 evolution reaction peak was obtained below 4.3 V, which avoided vigorous side reactions. To demonstrate an enhanced cycling performance, we employed ex-situ XPS for observing the formation of discharge products such as Li2O2 and Li2CO3 after the 5th discharge process of the pFe2P2O7 LB electrode and FePO4 LB electrode, corresponding to Fig. 5c (as shown in Fig. 6d and e). The Li 1 s spectra are decomposed into two peaks at binding energies of 54.8 and 55.7 eV corresponding to Li2O2 and Li2CO3, respectively [31–33]. The Li2O2/Li2CO3 formation ratio of the p-Fe2P2O7 LB electrode (3.61) was found to be higher than that of the FePO4 LB electrode (1.63) (Table S3). Therefore, we concluded that
4. Conclusion A phase-controlled synthesis of p-Fe2P2O7 LBs and FePO4 LBs was performed to evaluate the functionalities of the Fe2+/Fe3+ redox ratio and the porous structure and tailor them to the Li-O2 battery conditions. The initial step for synthesizing the two controlled structures involved synthesis of FePO4·2H2O LBs by a hydrothermal process. In the next step, p-Fe2P2O7 LBs and FePO4 LBs were synthesized by the thermal treatment of FePO4·2H2O LBs under a constant H2-flow and air-atmosphere, respectively. This thermal process aids the formation of porous structures under the control of Fe2+/Fe3+ redox couple and Ovac, consisting of Fe2P2O7 crystalline phase. It was observed that the pFe2P2O7 LBs exhibit an enhanced electrocatalytic activity because of the 6
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 6. (a–c) dQ/dV plots of p-Fe2P2O7 LBs and FePO4 LBs. Li 1s XPS profiles of (d) p-Fe2P2O7 LBs and (e) FePO4 LBs.
efficient active sites formed by the tailored Fe2+/Fe3+ redox ratio and Ovac on the surface of the porous nanostructures. Consequently, it was proved that the p-Fe2P2O7 LB electrode exhibits better catalytic performance when compared with the FePO4 LB electrode, because of its low overpotential, high specific capacity, and long cycle-life during discharge–charge cycles.
[6]
[7]
[8]
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.
[9]
[10]
Acknowledgments [11]
This work was supported by a Korea University Grant, South Korea and, the National Research Foundation of Korea (NRF) Grant, South Korea funded by the Ministry of Science, ICT, and Future Planning [NRF-2017R1C1B2004869, 2019R1A2B5B02070203, and 2018M3D1A1058744], South Korea.
[12]
[13]
Appendix A. Supplementary data
[14]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2020.124294.
[15] [16]
References [17]
[1] J.C. Védrine, Heterogeneous Partial (amm)oxidation and oxidative dehydrogenation catalysis on mixed metal oxides, Catalysts 6 (2016) 22. [2] C. Yoan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488–1504. [3] K.H.L. Zhang, P.V. Sushko, R. Colby, Y. Du, M.E. Bowden, S.A. Chambers, Reversible nano-structuring of SrCrO3-δ through oxidation and reduction at low temperature, Nat. Comm. 5 (2014) 4669. [4] G.-H. Lee, M.-C. Sung, J.-C. Kim, H.J. Song, D.-W. Kim, Synergistic effect of CuGeO3/graphene composites for efficient oxygen–electrode electrocatalysts in Li–O2 batteries, Adv. Energy Mater. 8 (2018) 1801930. [5] J.-J. Xu, D. Xu, Z.-L. Wang, H.-G. Wang, L.-L. Zhang, X.-B. Zhang, Synthesis of
[18]
[19]
[20]
7
perovskite-based porous La0.75Sr0.25MnO3 nanotubes as a highly efficient electrocatalyst for rechargeable lithium–oxygen batteries, Angew. Chem. Int. Ed. 25 (2013) 3887–3890. Z. Ma, X. Yuan, L. Li, Z.-F. Ma, The double perovskite oxide Sr2CrMoO6–δ as an efficient electrocatalyst for rechargeable lithium air batteries, Chem. Commun. 50 (2014) 14855–14858. J. Cheng, M. Zhang, Y. Jiang, L. Zou, Y. Gong, B. Chi, J. Pu, L. Jian, Perovskite La0.6Sr0.4Co0.2Fe0.8O3 as an effective electrocatalyst for non-aqueous lithium air batteries, Electrochim. Acta 191 (2016) 106–115. Y. Wang, Z. Yang, F. Lu, C. Jin, J. Wu, M. Shen, R. Yang, F. Chen, Carbon-coating functionalized La0.6Sr1.4MnO4+δ layered perovskite oxide: enhanced catalytic activity for the oxygen reduction reaction, RSC Adv. 5 (2015) 974–980. Q. Xu, S. Song, Y. Zhang, Y. Wang, J. Zhang, Y. Ruan, M. Han, Ba0.9Co0.7Fe0.2Nb0.1O3-δ perovskite as oxygen electrode catalyst for rechargeable Lioxygen batteries, Electrochim. Acta 191 (2016) 577–585. G.-H. Lee, S. Lee, J.-C. Kim, D.W. Kim, Y. Kang, D.-W. Kim, MnMoO4 electrocatalysts for superior long-life and high-rate lithium-oxygen batteries, Adv. Energy Mater. 7 (2017) 1601741. Z. Chang, J. Xu, X. Zhang, Recent progress in electrocatalyst for Li-O2 batteries, Adv. Energy Mater. 7 (2017) 1700875. K. Song, J. Jung, Y.-U. Heo, Y.C. Lee, K. Cho, Y.-M. Kang, α-MnO2 nanowire catalysts with ultra-high capacity and extremely low overpotential in lithium–air batteries through tailored surface arrangement, Phys. Chem. Chem. Phys. 15 (2013) 20075. T.-S. Kim, G.-H. Lee, S. Lee, Y.-S. Choi, J.-C. Kim, H.J. Song, D.-W. Kim, Carbondecorated iron oxide hollow granules formed using a silk fibrous template: lithiumoxygen battery and wastewater treatment applications, NPG Asia Mater. 9 (2017) e450. H. Kim, J. Park, I. Park, K. Jin, S.E. Jerng, S.H. Kim, K.T. Nam, K. Kang, Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst, Nat. Comm. 6 (2015) 8253. K. Pogorzelec-Glaser, A. Pietraszko, B. Hilczer, M. Połomska, Structure and phase transitions in Cu2P2O7, Phase Transit. 79 (2006) 535–544. R. Lin, Y. Ding, L. Gong, W. Dong, J. Wang, T. Zhang, Efficient and stable silicasupported iron phosphate catalysts for oxidative bromination of methane, J. Catal. 272 (2010) 65–73. E. Muneyama, A. Kunishige, K. Ohdan, M. Ai, Reduction and reoxidation of iron phosphate and its catalytic activity for oxidative dehydrogenation of isobutyric acid, J. Catal. 158 (1996) 378–384. N.A. Blanc, Q. Williams, B.E. Bali, R. Essehli, A vibrational study of phase transitions in Fe2P2O7 and Cr2P2O7 under high-pressures, J. Am. Ceram. Soc. 101 (2018) 5257–5268. G.-H. Lee, S.-D. Seo, H.-W. Shim, K.-S. Park, D.-W. Kim, Synthesis and Li electroactivity of Fe2P2O7 microspheres composed of self-assembled nanorods, Ceram. Int. 38 (2012) 6927–6930. N.L.W. Septiani, Y.V. Kaneti, B. Yuliarto, H.K. Nugraha, T. Dipojono, J. Takei, Y. Yamauchi You, Hybrid nanoarchitecturing of hierarchical zinc oxide wool-balllike nanostructures with multi-walled carbon nanotubes for achieving sensitive and
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
5753. [28] K.O. Moura, R.J.S. Lima, A.A. Coelho, E.A. Souza-Junior, J.G.S. Duquec, C.T. Meneses, Tuning the surface anisotropy in Fe-doped NiO nanoparticles, Nanoscale 6 (2014) 352–357. [29] J. Zhang, R. Yin, Q. Shao, T. Zhu, X. Huang, Oxygen vacancies in amorphous InOx nanoribbons enhance CO2 adsorption and activation for CO2 electroreduction, Angew. Chem. Int. Ed. 58 (2019) 5609–5613. [30] S. Zhang, G. Wang, J. Jin, L. Zhang, Z. Wena, J. Yang, Self-catalyzed decomposition of discharge products on the oxygen vacancy sites of MoO3 nanosheets for lowoverpotential Li-O2 batteries, Nano Energy 36 (2017) 186–196. [31] S. Lee, G.-H. Lee, J.-C. Kim, D.-W. Kim, Magnéli-Phase Ti4O7 nanosphere electrocatalyst support for carbon-free oxygen electrodes in lithium−oxygen batteries, ACS Catal. 8 (2018) 2601–2610. [32] Y.-C. Lu, E.J. Crumlin, G.M. Veith, J.R. Harding, E. Mutoro, L. Baggetto, N.J. Dudney, Z. Liu, Y. Shao-Horn, In situ ambient pressure x-ray photoelectron spectroscopy studies of lithium-oxygen redox reactions, Sci. Rep. 2 (2012) 715. [33] R. Younesi, S. Urbonaite, K. Edström, M. Hahlin, The cathode surface composition of a cycled Li−O2 battery: a photoelectron spectroscopy study, J. Phys. Chem. C 116 (2012) 20673–20680.
selective detection of sulfur dioxide, Sens. Actuat. B Chem. 261 (2018) 241–251. [21] H. Lv, Y. Liu, J. Guang, Z. Ding, J. Wang, Shape-selective synthesis of Bi2WO6 hierarchical structures and their morphology-dependent photocatalytic activities, RSC Adv. 6 (2016) 80226. [22] T.S. Natarajan, H.C. Bajaj, R.J. Tayade, Synthesis of homogeneous sphere-like Bi2WO6 nanostructure by silica protected calcination with high visible-light-driven photocatalytic activity under direct sunlight, CrystEngComm 17 (2015) 1037. [23] M. Sakar, S. Balakumar, A mechanistic view into the morphology-reconstruction mediated facile synthesis of bismuth ferrite (BiFeO3) hierarchical nanostructures, Nano-Struct. Nano-Objects 12 (2017) 188–193. [24] D. Son, E. Kim, T.-G. Kim, M.G. Kim, J. Cho, Nanoparticle iron-phosphate anode material for Li-ion battery, Appl. Phys. Lett. 84 (2004) 5875. [25] J. Cazes, Ewing's Analytical Instrumentation Handbook, third ed., CRC Press, Florida, 2004, p. 416. [26] O. Pana, C. Leostean, M.L. Soran, M. Stefan, S. Macavei, S. Gutoiu, V. Pop, O. Chauvet, Synthesis and characterization of Fe–Pt based multishell magnetic nanoparticles, J. Alloys Compd. 574 (2013) 477–485. [27] Y. Yan, F. Xue, F. Muhammad, L. Yu, F. Xu, B. Jiao, Y. Shiau, D. Li, Application of iron-loaded activated carbon electrodes for electrokinetic remediation of chromium-contaminated soil in a three-dimensional electrode system, Sci. Rep. 8 (2018)
8
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Redox effect of Fe2+/Fe3+ in iron phosphates for enhanced electrocatalytic activity in Li-O2 batteries Gwang-Hee Lee, Yoon Seon Kim, Dong-Wan Kim
T
⁎
School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 02841, South Korea
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
phosphates laundry-ball-like na• Iron nostructures are prepared as electrocatalysts.
Porous Fe P O enables an excellent • cycle reversibility for Li-O batteries. coordination of phosphate • Flexible anions stabilizes the intermediate Fe 2 2
7
2
cations.
Superior performances of Fe P O • sult from the optimized Fe /Fe
2 2 7 re2+ 3+
redox effect.
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron phosphates Thermal reaction Fe2+/Fe3+ redox effect Electrocatalysts Li–O2 battery
FePO4 and porous Fe2P2O7 laundry-ball-like nanostructures (FePO4 LBs and p-Fe2P2O7 LBs, respectively) were prepared to investigate their functionalities as oxygen-electrode (O2-electrode) electrocatalysts in Li–O2 batteries. These structures were synthesized in two steps, via hydrothermal and thermal reactions. FePO4 LBs were synthesized through thermal dehydrogenation of as-prepared FePO4·2H2O precursors (FePO4·2H2O → FePO4 + 2H2O), and p-Fe2P2O7 LBs were synthesized through thermochemical reduction of same precursors under an H2 atmosphere (2FePO4·2H2O + H2 → Fe2P2O7 + 5H2O). As an O2-electrode electrocatalyst in Li–O2 cells, p-Fe2P2O7 LBs exhibited a higher discharge capacity (30,000 mA h gcatalyst–1 at a current density of 500 mA gcatalyst–1), higher reversibility (300 cycles at a current rate of 500 mA gcatalyst–1), and lower voltage gap, compared to FePO4 LBs. These superior performances of p-Fe2P2O7 LBs result from the Fe2+/Fe3+ redox effect and porous structure, which enhance the oxygen reduction or evolution reaction activities.
1. Introduction Immense research has been conducted on metal oxide-type catalysts, which are capable of catalyzing oxygen reduction/evolution reactions, in the field of Li-O2 batteries. The important factors that affect the catalytic activity of these metal oxide-type catalysts are as follows: (i) isolation of active sites, (ii) metal–oxygen bond strength, (iii) crystalline structure, (iv) phase cooperation, (v) the oxidation states of the
⁎
surface cations (redox properties), (vi) the nature of the surface oxygen species, and (vii) multi-functionality [1]. These factors are the basis for choosing single or mixed metal oxides of metal oxide-type catalysts for use as oxygen reduction/evolution reaction catalysts. Particularly, the attractive properties of mixed metal oxides (ABO3, ABO4, A2B2O7, etc.) are often discussed in connection with partially occupied transition metal (TM) d-orbitals and corresponding local atomic structures [2–10]. In TM oxides, the formation of oxygen vacancies (Ovac) results
Corresponding author. E-mail address: [email protected] (D.-W. Kim).
https://doi.org/10.1016/j.cej.2020.124294 Received 31 October 2019; Received in revised form 2 January 2020; Accepted 30 January 2020 Available online 31 January 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
in the change of coordination and the formal charge of the surrounding TM cations simultaneously [10]. In addition, the TM oxide surfaces are terminated by O2− anions because their sizes are much larger than those of TMn+ cations, and therefore, the symmetry and coordination of TMn+ cations can be lost at the surface. Furthermore, the oxide surfaces may contain different types of structural defects, such as terraces, steps, and kink, which play an important role in the catalytic activity [1]. However, the thermodynamically unsaturated surface is usually compensated by the reaction of gaseous O2 or the decomposition of solid Li2O2, resulting in the formation of O2− ions according to the following chemical equations: O2 (g) → 2O2− or Li2O2 (s) → 2Li+ + 2O2−. Nazar et al. developed pyrochlore oxide with a fraction of surface defects, which demonstrates lower overpotentials. So far, various electrocatalysts have been reported that contain oxygen vacancies and/ or defect sites [11]. For example, α-MnO2 nanotubes provide excellent electrocatalytic activity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) when more Mn3+ ions are exposed on the surface than Mn4+ [12]. In another case, C/Fe3O4 hollow granules show more oxygen vacancies when providing more reduced Fe2+/Fe3+ redox couples. C/Fe3O4 hollow granules show better ORR/OER performance than Fe2O3 hollow granules with less oxygen vacancies [13]. We focused on the flexible coordination of TMOs. TMOs with reduced TM are more stable in the oxidizing atmosphere, and oxidized defect sites can be used as excellent active sites. TM phosphates have strong structural stability and can appear in various crystalline and amorphous forms with different orientations of phosphate group. In addition, the flexible coordination of phosphate groups can stabilize the intermediate state of the TM cations by changing their local positions easily while ensuring effective redox changes of the TM cations [14–18].The divalent pyro-phosphates (M2P2O7; M = Cu, Fe, Zn, or Mg cations) are composed of P2O7 layers that are parallel to the (0 0 1) crystallographic plane and cations that have an irregular octahedral − coordination with oxygen atoms in the C1 space group (triclinic phase) [18,19]. For example, the triclinic Fe2P2O7 (β-phase), which is a distorted form of the triclinic FePO4 phase (2Fe(III)PO4 → Fe − (II)2P2O7 + O), belongs to the C1 space group. Therefore, in β-phase, the oxygen atoms of the P-O-P bridge can be assumed to be disoriented positionally; however, the flexible coordination of the phosphate anions ([PO4]3− and [P2O7]4−) can stabilize the intermediate state of the Fe cations by changing their local positions easily while ensuring effective redox changes (Fe3+ → Fe2+). For these reasons, we suggest tha the phosphate anions can serve as a solid support for Fe-P-O crystal structure. In our previous work, CuGeO3 nanowires had reducing Cu+/Cu2+ redox couples during the thermal reduction process. However, due to the unchanged [GeO3]2− anion in CuGeO3, the crystal structure was unchanged and many Ovac were generated on the crystal surface. Therefore, the variability and stability of the anions ensure generation of suitable redox couples and Ovac, which can exhibit excellent catalytic activity [4]. Herein, we explore the catalytic capabilities of Fe2P2O7 and FePO4 in terms of the Fe2+/Fe3+ redox change effect and the corresponding Ovac effect by using them as O2-electrode catalyst materials in Li-O2 batteries. These different redox couples and Ovac provide a platform to understand the catalytic activity of the iron phosphates in the oxygen reduction/evolution reactions better.
were added to 150 ml of aqueous solution. The reaction mixture was then vigorously stirred for 30 min. This transparent solution was then added to 0.3 M Na2SO4 with continuous stirring for 30 min. Subsequently, this solution was transferred to a Teflon-lined autoclave with a capacity of 200 ml, and was heated at 120 °C for 5 h. The resulting precipitate was then collected by centrifugation and was washed several times with distilled water and ethanol. Finally, the washed precipitate was dried overnight at 90 °C. 2.2. Thermal treatments of FePO4 and porous Fe2P2O7 laundry-ball-like nanostructures For synthesis of FePO4 laundry-ball-like nanostructures (FePO4 LBs), the thermal treatment was performed at 400 °C for 24 h under air atmosphere. For synthesis of porous Fe2P2O7 laundry-ball-like nanostructures (p-Fe2P2O7 LBs), the thermal treatment was performed at 400 °C for 24 h under flowing H2 (300 sccm). 2.3. Material characterization All samples were characterized by field-emission scanning electron microscopy (FESEM; SU-70, Hitachi, Japan), transmission electron microscopy (TEM; JEM-2100F, JEOL, USA), X-ray diffraction (XRD; Smartlab, Rigaku, Japan), Brunauer–Emmett–Teller analysis (BET; BELSORP mini II, BEL, Japan), and X-ray photo-electron spectroscopy (XPS; PHI X-tool, ULVAC-PHI, Japan). 2.4. Li–O2 cell studies The O2-electrodes were fabricated by mixing the powders (FePO4‧2H2O LBs, FePO4 LBs, and p-Fe2P2O7 LBs) with Super P carbon black and carboxymethyl cellulose (Aldrich, average Mw ~ 700000) in a mass ratio of 27:63:10, and subsequently pasting the mixture on Ni foam. The electrochemical tests were carried out by using Swageloktype Li-O2 cells with air holes on the O2-electrode side. The Li-O2 cells were assembled in an Ar-filled glove box by using a Li foil as the anode, glass fiber separator (Whatman), 0.5 M lithium bis(trifluoromethyl) sulfonylimide (Sigma Aldrich, 99.95%) in dimethyl sulfoxide as the electrolyte, the prepared porous O2-electrode as the cathode, and a carbon cloth (W0S1002, CeTech) as the gas diffusion layer. All measurements were conducted under 1.5 bar dry oxygen atmosphere (> 99.999%) without any humidity. The active masses of all O2-electrodes were calculated based on the electrocatalyst. The galvanostatic discharge–charge profiles and cyclic voltammetry were recorded by using an automatic battery cycler (WBCS 3000, WonaTech) after holding at open circuit potential for 6 h. 3. Results and discussion Fig. 1a schematically illustrates the synthesis process of p-Fe2P2O7 LBs and FePO4 LBs from FePO4‧2H2O LBs. The growth mechanism of FePO4‧2H2O LBs is as follow [20–23]. Based on the growth mechanism by hydrothermal reaction, the precipitation reaction is slowed to separate the nucleation and growth process. Therefore, FePO4‧2H2O nanoparticles were formed at a relatively slow rate in the initial stage of the reaction. In the second stage (aggregation stage), furthermore the hydrothermal reaction aggregates small nanoparticles into microsphere structures consisting of plates in order to reduce the surface energy through oriented growth. In the third stage (Ostwald ripening stage), the prolonged hydrothermal reaction leads to dissolution and recrystallization of particles. The sulfate ions of Na2SO4 etches out the surfaces and leads to the morphological reconstruction, which offer many nucleation sites for further growth. Further increase in the hydrothermal reaction leads to interweaving of nanoplates to form FePO4‧2H2O LBs. The thermal treatment process involves the following steps: (i) in the initial step, FePO4‧2H2O LBs were synthesized by the
2. Experimental 2.1. Synthesis of FePO4‧2H2O laundry-ball-like nanostructures FePO4‧2H2O laundry-ball-like nanostructures (FePO4‧2H2O LBs) were synthesized by using a hydrothermal method. The starting materials used to synthesize FePO4‧2H2O LBs were FeCl3 (Aldrich, purity 97%), H3PO4 (Aldrich, purity 99.0%), and Na2SO4 (Aldrich, purity 98%). As per the typical procedure, 0.12 M FeCl3 and 0.12 M H3PO4 2
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 1. (a) Schematic illustration of fabrication of p-Fe2P2O7 LBs and FePO4 LBs. FESEM images of (b, c) p-Fe2P2O7 LBs and (d, e) FePO4 LBs.
hydrothermal process; and (ii) in the two different final steps, pFe2P2O7 LBs were synthesized by the thermal treatment of FePO4‧2H2O LBs under a constant H2-flow (as shown in Eq. (1)) and FePO4 LBs were synthesized by the thermal treatment of FePO4‧2H2O LBs under air-atmosphere (as shown in Eq. (2)).
2FePO4 2H2 O + H2 FePO4 2H2 O
→
Thermal treatment
→
Thermal treatment
Fe2 P2 O7 + 5H2 O (in H2)
FePO4 + 2H2 O (in air)
we found that the porous structure of p-Fe2P2O7 LBs was developed by removing a large amount of crystalline water contained in FePO4‧2H2O as well as oxygen in Fe-P-O bonds. Further, as shown in Fig. 2a and b, both TEM and high-angle annular dark-field scanning TEM (HAADF STEM) images of p-Fe2P2O7 LBs indicate porous structure. The crystal structure of p-Fe2P2O7 LBs was further investigated in detail by high-resolution TEM (HRTEM) (Fig. 2c). The HRTEM image indicates that the interplanar distance for p-Fe2P2O7 LBs is 0.362 nm, which is consistent with the (1 1 1) plane of the triclinic Fe2P2O7 crystalline phase (PDF card No. 76-1762). The XRD pattern of pFe2P2O7 LBs was compared with that of the standard Fe2P2O7 crystalline phase (Fig. 3a). The good agreement between the XRD patterns indicates that Fe2P2O7 exists in the crystalline form. The XRD pattern of FePO4 LBs could be uniquely indexed to that of the triclinic FePO4, because of the absence of impurity (Fig. 3b) [24]. Table S1 lists the
(1) (2)
Following the thermal treatment, we observed the morphology of the obtained samples through FESEM images (Fig. 1b–e). As shown in Fig. 1b and c, p-Fe2P2O7 LBs have diameters in the range of approximately 1 μm and have porous surfaces. As shown in Fig. 1d and e, FePO4 LBs have similar particle size and morphology as those of pFe2P2O7 LBs; however, they do not have porous surfaces. In addition, 3
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 2. (a) TEM, (b) HAADF STEM, and (c) HRTEM images of p-Fe2P2O7 LBs. (d) TEM image of FePO4 LBs.
and Fe3+ oxidation states, respectively [16–28]. The results presented in Table 1 show that the Fe2+/Fe3+ redox couple is present in lower oxidation states, i.e., p-Fe2P2O7 LBs, because the Fe2+/Fe3+ molar ratio of p-Fe2P2O7 LBs is approximately unity (Fe2+/Fe3+ = 1.01). However, the Fe2+/Fe3+ molar ratio at the surface of FePO4 LBs decreases from 1.01 to 0.62. In the P 2p spectra, the P 2p peak of p-Fe2P2O7 LBs shifts to a higher binding energy when compared with that of FePO4 LBs, corresponding to the Fe2+/Fe3+ redox change. The binding energy shift was found to be ~0.6 eV, indicating that the heat treatment under H2 atmosphere induces a change in the chemical bonding of Fe-P-O groups. The O 1 s spectra present two peaks at binding energies of ~531.1 and ~532.7 eV, representing the O 1 s level in p-Fe2P2O7 LBs and FePO4 LBs [4,10,29]. The XPS survey spectra exhibit the oxygen contents of p-Fe2P2O7 LBs and FePO4 LBs. The oxygen content of pFe2P2O7 LBs is similar to the oxygen content of FePO4 LBs (Fig. S3 and Table S2). However, the concentration of Ovac in p-Fe2P2O7 LBs is higher than that in FePO4 LBs (Table 1). From this observation, it is evident that p-Fe2P2O7 LBs are more deficiently coordinated with O2 during annealing under reducing conditions than in the case of FePO4 LBs. Therefore, we expect that p-Fe2P2O7 LBs will offer an opportunity to explore the effect of the Fe2+/Fe3+ redox ratio and Ovac/Olattice ratio. We evaluated the electrochemical performance of O2-electrodes containing p-Fe2P2O7 LBs and FePO4 LBs. As shown in Fig. 5a, the initial galvanostatic full discharge tests of both p-Fe2P2O7 LB electrode and FePO4 LB electrode were performed at a current rate of 500 mA g−1 to 2.5 V. The p-Fe2P2O7 LB electrode exhibits a discharge capacity of ~30,000 mA h g−1, and the FePO4 LB electrode exhibits a discharge capacity of ~15,000 mA h g−1, which is approximately half the discharge capacity of the p-Fe2P2O7 LB electrode. We then compared the discharge–charge cycling results for the p-Fe2P2O7 LB electrode and
values of the BET surface area, pore volume, and pore diameter obtained from N2 adsorption–desorption isotherms and Barrett-JoynerHalenda (BJH) pore size distribution curves for all the samples. The BET surface areas of p-Fe2P2O7 LBs and FePO4 LBs were determined to be 10.44 and 4.31 m2 g−1, respectively; and their pore volumes were found to be 0.045 and 0.012 cm3 g−1, respectively. The average pore diameter increased from 133.39 nm for FePO4 LBs to 172.27 nm for pFe2P2O7 LBs because of the removal of a large amount of crystalline water as well as oxygen. When compared with the BJH pore size distribution curve of p-Fe2P2O7 LBs that had well-developed pores in the mesopore region (< 300 Å), the BJH pore size distribution curve of the FePO4 LBs had insignificant values, indicating no developed pores (Fig. S1). As shown in Fig. 2d, the TEM image of FePO4 LBs indicates the formation of dimples. Thus, the existence of only p-Fe2P2O7 LBs indicates mesopores (Fig. S2). Further, as indicated by the XPS profiles in Fig. 4, the difference in the surface oxidation states of p-Fe2P2O7 LBs and FePO4 LBs is quite significant. We performed peak separation of all XPS spectra based on Gaussian function. The binding energy peak position of specific element depends on the oxidation state and local chemical environment of that element (i.e., shifts in the core-level binding energies induced by the immediate chemical environment of an atom). The peak shift in XPS is most of time related to a change of oxidation state of the element. Here, higher binding energies mean also higher oxidation states. The element of a higher positive oxidation state exhibits a higher binding energy due to the extra coulombic interaction between the photoemitted electron and the ion core. In most metals there is a positive shift between the elemental form and mono-, di- or trivalent ions (M3+ → M2+ → M+ → M0) [25]. The Fe 2p3/2 spectra could be decomposed into two peaks at binding energies of ~713.5 and ~710.7 eV, corresponding to the Fe2+ 4
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Table 1 Comparison of Fe2+/Fe3+ redox couple and Ovac/Olattice ratios by XPS analysis.
2+
3+
Fe /Fe Ovac/Olattice
Fe2P2O7
FePO4
1.01 0.49
0.62 0.19
provides reversible catalytic activity. The overall overpotential of the pFe2P2O7 LB electrode is lower than that of the FePO4 LB electrode during 400 discharge–charge cycles. In the initial discharge–charge curves, the final voltage gap of the p-Fe2P2O7 LB electrode is similar to that of the p-Fe2P2O7 LB electrode. However, in the 200th and 300th discharge–charge curves, the final voltage gap of the p-Fe2P2O7 LB electrode (1.59 V at 200th cycle) is smaller than that of the FePO4 LB electrode (1.65 V at 200th cycle) (Fig. S5). In addition, the p-Fe2P2O7 LB electrode has a more stable cycle retention (400 cycles) when compared with the FePO4 LB electrode (250 cycles) (Fig. 5c). Furthermore, we carried out the discharge–charge cycling test for a large capacity at a current rate of 500 mA g−1 and a fixed capacity regime of 2000 mA h g−1, as shown in Fig. 5d–f. In the initial discharge–charge curves, the final voltage gap of the p-Fe2P2O7 LB electrode is similar to that of the p-Fe2P2O7 LB electrode. However, in the 10th and 20th discharge–charge curves, the final voltage gap of the pFe2P2O7 LB electrode (1.61 V at 10th and 20th cycle) is remarkably smaller than that of the FePO4 LB electrode (Fig. 5e). In addition, the pFe2P2O7 LB electrode has remarkably stable cycling performance when compared with the FePO4 LB electrode during 80 discharge–charge cycles (Fig. 5f). The role of disordered Fe3+ ions in Fe2P2O7 structure is important for catalytic activity. The catalytic activity of p-Fe2P2O7 LBs is strongly correlated with its surface oxidation state for disordered Fe3+ ions containing oxygen vacancies with respect to the electrochemical performance. Disordered Fe3+ ions with oxygen vacancies provides active sites during discharge–charge process [10,30]. As possible ORR/OER mechanism for p-Fe2P2O7 LBs, the exposed Fe3+ sites provide sites for formation of active oxygen species (i.e. absorbed O2– and O2*). Then, The discharge products (Li2−xO2) form on active Fe3+ site during the discharge process. The p-Fe2P2O7 LB electrode exhibits two-steps of the galvanostatic discharge or charge curves, respectively (Fig. 5e). This formation reaction corresponds to the first step (step I), which shows the higher discharge voltage. Subsequently, the disproportionation reaction of Li2−xO2 leads to the reaction to obtain
Fig. 3. XRD patterns of (a) p-Fe2P2O7 LBs and (b) FePO4 LBs.
FePO4 LB electrode at a current rate of 500 mA g−1 and a fixed capacity regime of 500 mA h g−1, as shown in Fig. 5b and c. We carried out the discharge–charge cycling test of the pure Super P electrode, as shown in Fig. S4. The overall overpotential of the pure Super P electrode is stable only 5 discharge–charge cycles. It can be seen that iron phosphate
Fig. 4. Fe 2p, P 2p, and O 1s XPS profiles of (a) p-Fe2P2O7 LBs and (b) FePO4 LBs. 5
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 5. Electrochemical performance of p-Fe2P2O7 LBs and FePO4 LBs: (a) Full discharge–charge curves of p-Fe2P2O7 LBs and FePO4 LBs at a current density of 500 mA g−1. (b) 1st discharge–charge curve of p-Fe2P2O7 LBs and FePO4 LBs at a current density of 500 mA g−1 under the fixed capacity regime of 500 mA h g−1. (c) Comparison of the discharge–charge specific capacity with the cycle number of p-Fe2P2O7 LBs and FePO4 LBs, corresponding to (b). (d) 1st, and (e) 10th and 20th discharge–charge curves of p-Fe2P2O7 LBs and FePO4 LBs at a current density of 500 mA g−1 under the fixed capacity regime of 2000 mA h g−1. (f) Comparison of the discharge–charge specific capacity with the cycle number of p-Fe2P2O7 LBs and FePO4 LBs, corresponding to (d) and (e).
the p-Fe2P2O7 LB electrode is more active in terms of the formation of Li2O2, which is a positive reaction, than in terms of the formation of Li2CO3, which is a parasitic reaction. As show in Fig. S6, the discharge products of the p-Fe2P2O7 LB electrode were formed in film-like morphology. This phenomenon shows mainly that the discharge products have a conformal film-like morphology by surface mediated growth at high currents. The film-like products may contain more structural defects, which is advantageous for charge transport and OER process. Overall, we concluded that the Li-O2 battery conditions contribute toward the excellent oxygen reduction/evolution activity of p-Fe2P2O7 LBs because of the corresponding optimized Fe2+/Fe3+ redox, a large amount of Ovac in Fe2P2O7 phase, and the porosity of p-Fe2P2O7 LBs. Also, we checked the stable Fe2+/Fe3+ redox couples of p-Fe2P2O7 LBs after the 5th discharge process. As a result, the surface composition of p-Fe2P2O7 LBs retained Fe2+/Fe3+ redox couples as shown in the XPS results (Fig. S7). Therefore, p-Fe2P2O7 LBs is considered very stable electrocatalyst.
stable Li2O2 discharge products according to the deepening of the discharge process (step II). In charge process, the Li2−xO2 during the discharge process could decompose Li2O2, corresponding to the step III, which shows the lower charge voltage. Then, the Li2−xO2 is decomposed at the step IV with higher charge voltage. This proposed electrochemical reaction mechanism is based on the previously reported result [30]. For further detail on catalytic activity of the p-Fe2P2O7 and FePO4 LBs, the dQ/dV plots of the p-Fe2P2O7 LB electrode and FePO4 LB electrode were obtained from those for the 100th cycle in Fig. 4c (Fig. 6a–c). The p-Fe2P2O7 LB electrode exhibits a larger O2 reduction peak at a high voltage (2.75 V) when compared with the FePO4 LB electrode (at 2.7 V) during the discharging process (Fig. 6a and b). This suggests that the p-Fe2P2O7 LB electrode shows excellent ORR activity when compared with the FePO4 LB electrode. During the charging process (Fig. 6a and c), the FePO4 LB electrode exhibits a large O2 evolution reaction peak, indicative of side reactions that can decompose the organic solvent-based electrolyte or lithium carbonate over 4.3 V, which are consistent with the high charge voltage (Fig. S5). Importantly, the galvanostatic cycling performance of the p-Fe2P2O7 LB electrode was found to be stable because the major O2 evolution reaction peak was obtained below 4.3 V, which avoided vigorous side reactions. To demonstrate an enhanced cycling performance, we employed ex-situ XPS for observing the formation of discharge products such as Li2O2 and Li2CO3 after the 5th discharge process of the pFe2P2O7 LB electrode and FePO4 LB electrode, corresponding to Fig. 5c (as shown in Fig. 6d and e). The Li 1 s spectra are decomposed into two peaks at binding energies of 54.8 and 55.7 eV corresponding to Li2O2 and Li2CO3, respectively [31–33]. The Li2O2/Li2CO3 formation ratio of the p-Fe2P2O7 LB electrode (3.61) was found to be higher than that of the FePO4 LB electrode (1.63) (Table S3). Therefore, we concluded that
4. Conclusion A phase-controlled synthesis of p-Fe2P2O7 LBs and FePO4 LBs was performed to evaluate the functionalities of the Fe2+/Fe3+ redox ratio and the porous structure and tailor them to the Li-O2 battery conditions. The initial step for synthesizing the two controlled structures involved synthesis of FePO4·2H2O LBs by a hydrothermal process. In the next step, p-Fe2P2O7 LBs and FePO4 LBs were synthesized by the thermal treatment of FePO4·2H2O LBs under a constant H2-flow and air-atmosphere, respectively. This thermal process aids the formation of porous structures under the control of Fe2+/Fe3+ redox couple and Ovac, consisting of Fe2P2O7 crystalline phase. It was observed that the pFe2P2O7 LBs exhibit an enhanced electrocatalytic activity because of the 6
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
Fig. 6. (a–c) dQ/dV plots of p-Fe2P2O7 LBs and FePO4 LBs. Li 1s XPS profiles of (d) p-Fe2P2O7 LBs and (e) FePO4 LBs.
efficient active sites formed by the tailored Fe2+/Fe3+ redox ratio and Ovac on the surface of the porous nanostructures. Consequently, it was proved that the p-Fe2P2O7 LB electrode exhibits better catalytic performance when compared with the FePO4 LB electrode, because of its low overpotential, high specific capacity, and long cycle-life during discharge–charge cycles.
[6]
[7]
[8]
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.
[9]
[10]
Acknowledgments [11]
This work was supported by a Korea University Grant, South Korea and, the National Research Foundation of Korea (NRF) Grant, South Korea funded by the Ministry of Science, ICT, and Future Planning [NRF-2017R1C1B2004869, 2019R1A2B5B02070203, and 2018M3D1A1058744], South Korea.
[12]
[13]
Appendix A. Supplementary data
[14]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2020.124294.
[15] [16]
References [17]
[1] J.C. Védrine, Heterogeneous Partial (amm)oxidation and oxidative dehydrogenation catalysis on mixed metal oxides, Catalysts 6 (2016) 22. [2] C. Yoan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488–1504. [3] K.H.L. Zhang, P.V. Sushko, R. Colby, Y. Du, M.E. Bowden, S.A. Chambers, Reversible nano-structuring of SrCrO3-δ through oxidation and reduction at low temperature, Nat. Comm. 5 (2014) 4669. [4] G.-H. Lee, M.-C. Sung, J.-C. Kim, H.J. Song, D.-W. Kim, Synergistic effect of CuGeO3/graphene composites for efficient oxygen–electrode electrocatalysts in Li–O2 batteries, Adv. Energy Mater. 8 (2018) 1801930. [5] J.-J. Xu, D. Xu, Z.-L. Wang, H.-G. Wang, L.-L. Zhang, X.-B. Zhang, Synthesis of
[18]
[19]
[20]
7
perovskite-based porous La0.75Sr0.25MnO3 nanotubes as a highly efficient electrocatalyst for rechargeable lithium–oxygen batteries, Angew. Chem. Int. Ed. 25 (2013) 3887–3890. Z. Ma, X. Yuan, L. Li, Z.-F. Ma, The double perovskite oxide Sr2CrMoO6–δ as an efficient electrocatalyst for rechargeable lithium air batteries, Chem. Commun. 50 (2014) 14855–14858. J. Cheng, M. Zhang, Y. Jiang, L. Zou, Y. Gong, B. Chi, J. Pu, L. Jian, Perovskite La0.6Sr0.4Co0.2Fe0.8O3 as an effective electrocatalyst for non-aqueous lithium air batteries, Electrochim. Acta 191 (2016) 106–115. Y. Wang, Z. Yang, F. Lu, C. Jin, J. Wu, M. Shen, R. Yang, F. Chen, Carbon-coating functionalized La0.6Sr1.4MnO4+δ layered perovskite oxide: enhanced catalytic activity for the oxygen reduction reaction, RSC Adv. 5 (2015) 974–980. Q. Xu, S. Song, Y. Zhang, Y. Wang, J. Zhang, Y. Ruan, M. Han, Ba0.9Co0.7Fe0.2Nb0.1O3-δ perovskite as oxygen electrode catalyst for rechargeable Lioxygen batteries, Electrochim. Acta 191 (2016) 577–585. G.-H. Lee, S. Lee, J.-C. Kim, D.W. Kim, Y. Kang, D.-W. Kim, MnMoO4 electrocatalysts for superior long-life and high-rate lithium-oxygen batteries, Adv. Energy Mater. 7 (2017) 1601741. Z. Chang, J. Xu, X. Zhang, Recent progress in electrocatalyst for Li-O2 batteries, Adv. Energy Mater. 7 (2017) 1700875. K. Song, J. Jung, Y.-U. Heo, Y.C. Lee, K. Cho, Y.-M. Kang, α-MnO2 nanowire catalysts with ultra-high capacity and extremely low overpotential in lithium–air batteries through tailored surface arrangement, Phys. Chem. Chem. Phys. 15 (2013) 20075. T.-S. Kim, G.-H. Lee, S. Lee, Y.-S. Choi, J.-C. Kim, H.J. Song, D.-W. Kim, Carbondecorated iron oxide hollow granules formed using a silk fibrous template: lithiumoxygen battery and wastewater treatment applications, NPG Asia Mater. 9 (2017) e450. H. Kim, J. Park, I. Park, K. Jin, S.E. Jerng, S.H. Kim, K.T. Nam, K. Kang, Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst, Nat. Comm. 6 (2015) 8253. K. Pogorzelec-Glaser, A. Pietraszko, B. Hilczer, M. Połomska, Structure and phase transitions in Cu2P2O7, Phase Transit. 79 (2006) 535–544. R. Lin, Y. Ding, L. Gong, W. Dong, J. Wang, T. Zhang, Efficient and stable silicasupported iron phosphate catalysts for oxidative bromination of methane, J. Catal. 272 (2010) 65–73. E. Muneyama, A. Kunishige, K. Ohdan, M. Ai, Reduction and reoxidation of iron phosphate and its catalytic activity for oxidative dehydrogenation of isobutyric acid, J. Catal. 158 (1996) 378–384. N.A. Blanc, Q. Williams, B.E. Bali, R. Essehli, A vibrational study of phase transitions in Fe2P2O7 and Cr2P2O7 under high-pressures, J. Am. Ceram. Soc. 101 (2018) 5257–5268. G.-H. Lee, S.-D. Seo, H.-W. Shim, K.-S. Park, D.-W. Kim, Synthesis and Li electroactivity of Fe2P2O7 microspheres composed of self-assembled nanorods, Ceram. Int. 38 (2012) 6927–6930. N.L.W. Septiani, Y.V. Kaneti, B. Yuliarto, H.K. Nugraha, T. Dipojono, J. Takei, Y. Yamauchi You, Hybrid nanoarchitecturing of hierarchical zinc oxide wool-balllike nanostructures with multi-walled carbon nanotubes for achieving sensitive and
Chemical Engineering Journal 388 (2020) 124294
G.-H. Lee, et al.
5753. [28] K.O. Moura, R.J.S. Lima, A.A. Coelho, E.A. Souza-Junior, J.G.S. Duquec, C.T. Meneses, Tuning the surface anisotropy in Fe-doped NiO nanoparticles, Nanoscale 6 (2014) 352–357. [29] J. Zhang, R. Yin, Q. Shao, T. Zhu, X. Huang, Oxygen vacancies in amorphous InOx nanoribbons enhance CO2 adsorption and activation for CO2 electroreduction, Angew. Chem. Int. Ed. 58 (2019) 5609–5613. [30] S. Zhang, G. Wang, J. Jin, L. Zhang, Z. Wena, J. Yang, Self-catalyzed decomposition of discharge products on the oxygen vacancy sites of MoO3 nanosheets for lowoverpotential Li-O2 batteries, Nano Energy 36 (2017) 186–196. [31] S. Lee, G.-H. Lee, J.-C. Kim, D.-W. Kim, Magnéli-Phase Ti4O7 nanosphere electrocatalyst support for carbon-free oxygen electrodes in lithium−oxygen batteries, ACS Catal. 8 (2018) 2601–2610. [32] Y.-C. Lu, E.J. Crumlin, G.M. Veith, J.R. Harding, E. Mutoro, L. Baggetto, N.J. Dudney, Z. Liu, Y. Shao-Horn, In situ ambient pressure x-ray photoelectron spectroscopy studies of lithium-oxygen redox reactions, Sci. Rep. 2 (2012) 715. [33] R. Younesi, S. Urbonaite, K. Edström, M. Hahlin, The cathode surface composition of a cycled Li−O2 battery: a photoelectron spectroscopy study, J. Phys. Chem. C 116 (2012) 20673–20680.
selective detection of sulfur dioxide, Sens. Actuat. B Chem. 261 (2018) 241–251. [21] H. Lv, Y. Liu, J. Guang, Z. Ding, J. Wang, Shape-selective synthesis of Bi2WO6 hierarchical structures and their morphology-dependent photocatalytic activities, RSC Adv. 6 (2016) 80226. [22] T.S. Natarajan, H.C. Bajaj, R.J. Tayade, Synthesis of homogeneous sphere-like Bi2WO6 nanostructure by silica protected calcination with high visible-light-driven photocatalytic activity under direct sunlight, CrystEngComm 17 (2015) 1037. [23] M. Sakar, S. Balakumar, A mechanistic view into the morphology-reconstruction mediated facile synthesis of bismuth ferrite (BiFeO3) hierarchical nanostructures, Nano-Struct. Nano-Objects 12 (2017) 188–193. [24] D. Son, E. Kim, T.-G. Kim, M.G. Kim, J. Cho, Nanoparticle iron-phosphate anode material for Li-ion battery, Appl. Phys. Lett. 84 (2004) 5875. [25] J. Cazes, Ewing's Analytical Instrumentation Handbook, third ed., CRC Press, Florida, 2004, p. 416. [26] O. Pana, C. Leostean, M.L. Soran, M. Stefan, S. Macavei, S. Gutoiu, V. Pop, O. Chauvet, Synthesis and characterization of Fe–Pt based multishell magnetic nanoparticles, J. Alloys Compd. 574 (2013) 477–485. [27] Y. Yan, F. Xue, F. Muhammad, L. Yu, F. Xu, B. Jiao, Y. Shiau, D. Li, Application of iron-loaded activated carbon electrodes for electrokinetic remediation of chromium-contaminated soil in a three-dimensional electrode system, Sci. Rep. 8 (2018)
8