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Biomass-derived 3D hierarchical N-doped porous carbon anchoring cobalt-iron phosphide nanodots as bifunctional electrocatalysts for LiO2 batteries
Journal of Power Sources 412 (2019) 433–441
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Biomass-derived 3D hierarchical N-doped porous carbon anchoring cobaltiron phosphide nanodots as bifunctional electrocatalysts for LieO2 batteries
T
Kailing Suna, Jing Lia, Lulu Huanga, Shan Jib, Palanisamy Kannanb, Du Lia, Lina Liua, Shijun Liaoa,∗ a The Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510641, China b College of Biological, Chemical Science and Chemical Engineering, Jiaxing University, Jiaxing, 314001, China
H I GH L IG H T S
CoFeP nanodots anchored on biomass-derived N-doped porous carbon was fabricated. • The anchoring of CoFeP NDs accelerates the OER kinetics of the catalyst. • The catalyst CoFeP/EWC exhibited high capacity and long-term cyclability in LieO battery. • The • The synergetic effects of Co with Fe, and CoFeP NDs with EWC were investigated. 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Rechargeable lithium-oxygen batteries Cobalt-iron-phosphide nanodots Porous carbon Oxygen reduction and evolution reactions
Developing lithium-oxygen batteries with high reversibility and long cyclability requires an electrocatalyst with superior catalytic activity and excellent stability to achieve an efficient cathode. Herein, a three-dimensional hierarchically porous carbon framework embedded with cobalt-iron-phosphide nanodots nanocomposite is fabricated via a lyophilization-pyrolysis-phosphorization process. The synthetic catalyst displays excellent performances towards both the oxygen reduction reaction and the oxygen evolution reaction. With our optimal sample, the half-wave potential for the oxygen reduction reaction is up to 0.83 V versus reversible hydrogen electrode, and its potential for the oxygen evolution reaction at a current density of 10 mA cm−2 is as low as 1.53 V in 0.1 M KOH solution. The catalyst also exhibits improved electrochemical performances in a rechargeable lithium-oxygen battery, including a high specific capacity (11969 mAh g−1 at 100 mA g−1) and a long cycle life (141 cycles at a cut-off capacity of 1000 mAh g−1). All of these results make our catalyst a promising candidate for the development of highly efficient electrocatalysts for the rechargeable lithium-oxygen batteries.
1. Introduction Among the various renewable energy storage systems, non-aqueous lithium-oxygen batteries (LieO2 batteries) are regarded as the most promising candidates for future electric vehicles owing to their high theoretical specific energy (∼3600 Wh Kg−1) [1–8]. However, some problems are still existed for this type of battery, such as low energy efficiency, limited cycle life and poor rate capability [4,9,10], which are closely related with the sluggish cathode reactions, including the oxygen reduction reaction (ORR) during discharging and the oxygen evolution reaction (OER) while charging [2,3,6]. Thus, it is crucial to alleviate these problems by designing a bifunctional catalyst for the
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ORR and OER with high catalytic activity and excellent stability [2,8,11]. To date, substantial efforts have been made to develop efficient and durable bifunctional electrocatalysts for the LieO2 battery [2,3,12]. Among these, porous doped carbon materials significantly boost the performance of the LieO2 battery due to their large specific surface areas and sufficient porous structure to host the discharge products and facilitate the reactant transmission [13–16]. For instance, N-doped porous carbon materials display efficient ORR catalytic activity through N inducing atomic charge density redistribution, which promotes the adsorption of large amounts of O2 and enhance ORR activity [17–19]. However, doped carbon materials lack sufficient OER activity, resulting in poor battery cyclability. To overcome this
Corresponding author. E-mail address: [email protected] (S. Liao).
https://doi.org/10.1016/j.jpowsour.2018.11.079 Received 7 September 2018; Received in revised form 30 October 2018; Accepted 25 November 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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under an Ar atmosphere. For comparison analysis, Co2P/EWC and Fe2P/EWC composites were also synthesized using the same procedure as described above but without the addition of Fe or Co precursors, respectively.
problem, design and development of carbon-based composites through supporting transition metals or their compounds on doped carbon materials have emerged as an attractive pathway for preparing highperformance ORR/OER bifunctional electrocatalysts [20–26]. Recently, transition-metal phosphides (TMP) have emerged as remarkable candidates for OER catalysts in alkaline electrolytes [27–30]. These bimetallic phosphides generally exhibit higher OER activity than single-metal phosphides, possibly benefiting from electronic transmission between different metals, which facilitates the OER initialization process [31–36]. For instance, Mendoza-Garcia et al. reported that (Co0.54Fe0.46)2P showed excellent OER activity through synergistic interaction with Co/Fe, reaching a specific current density (SCD) of 10 mA cm−2 at an overpotential of 370 mV, which was more efficient than Co2P or Fe2P catalysts [34]. Tang et al. reported that Fe-doped CoP nanoarray on Ti foil (FeeCoP/Ti) required an overpotential of only 310 mV to reach an SCD of 100 mA cm−2, while RuO2/Ti and CoP/Ti required 430 and 450 mV, respectively [31]. Very recently, Liu et al. found that FeeCoeP alloy catalyst exhibited excellent OER activity, requiring an overpotential of 252 mV to reach an SCD of 10 mA cm−2, which was lower than FeeCo oxide catalysts [37]. Hence, cation doping plays a vital role in modulating OER activities in TMP-based catalysts [38]. However, the application of mixed metal phosphides in aprotic LieO2 batteries has rarely been reported but is expected to improve the performance of based LieO2 battery. Inspired by the above ideas, we designed and prepared a novel nanocomposite, three-dimensional (3D), hierarchically porous carbon framework embedded with cobalt-iron-phosphide nanodots (CoFeP/ EWC) for using in LieO2 batteries, through a lyophilization-pyrolysisphosphorization process. We chose egg white (EW) as the precursor of N-doped carbon materials due to that the high content of nitrogen element in EW can be intrinsically retained in the carbon framework and improves catalytic activity without the addition of extrinsic Ndoping sources [39,40]. By freeze-drying the egg white dispersion, then pyrolyzing it in an Ar atmosphere, we get a network-structured dopedcarbon material with a 3D honeycomb-like morphology and high specific surface area; these characteristics could provide the cathode with large storage space, an enriched solid-liquid-gas reaction interface, and a facile electron transport pathway. Further, after a phosphorization process, CoFeP nanoparticles are uniformly anchored on the conductive 3D EWC porous structure. As expected, LieO2 batteries with our CoFeP/EWC composite materials as the cathode exhibit an obvious reduction in charge overpotential, good rate performance, and longterm cyclability.
2.2. Characterization X-ray diffraction (XRD) measurements were recorded using a TD3500 powder diffractometer (Tongda, Cu-Kα radiation) at a scanning rate of 2° min−1 from 10 to 90°. Scanning electron microscopy (SEM) images were obtained on a Nova Nano 430 system (FEI, Netherlands) and transmission electron microscopy (TEM) images were obtained on a JEM-2100HR system (JEOL, Japan). Nitrogen adsorption–desorption measurements were performed on a Tristar II 3020 adsorption analyzer (Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 X-ray photoelectron spectrometer (Thermo-VG Scientific, USA). Raman spectra measurements were performed with a Lab RAM Aramis Raman spectrometer (HJY, France). 2.3. Electrochemical measurements Electrochemical measurements were performed using three electrode cells with glassy carbon (5 mm diameter), a graphite rod, and Hg/ HgO (1 M KOH solution) as the working, counter, and reference electrodes, respectively, using an Autolab PGSTAT30 (Metrohm Autolab) electrochemical analyzer. Polarization LSV curves for the ORR and OER catalytic reactions were obtained in 0.1 M aqueous KOH solution at a scan rate of 5 mV s−1. The catalyst ink-loaded working electrode was prepared by dropping 20 μL catalyst ink (5 mg catalyst in 1 ml 0.25 wt% Nafion/ethanol solution) on a glassy carbon electrode and allowed it to dry at room temperature. The LieO2 battery was assembled by the following procedure. The synthesized composites (80 wt%) were mixed with poly(tetrafluoroethylene) (PTFE, 20 wt%) as a binder in ethanol to form a slurry. Next, the slurry was sprayed onto carbon paper (the load ratio of catalyst was about 0.2 mg cm−2 and the area of the carbon paper was 1 cm2) and dried at 80 °C for 24 h. Then the cell was assembled in an Arfilled glove box with lithium foil as the anode, polypropylene membrane as the separator, and 1.0 M LiN(CF3SO2)2 in tetraethylene glycoldimethyl ether (TEGDME) as the electrolyte. Finally, the assembled cells (CR 2032) were rested for 8 h under high-purity oxygen. The galvanostatic charge–discharge measurements of the cells were tested on a Neware (Shenzhen, China) testing system in a 1 atm O2 atmosphere. CV analysis of the cells was conducted in the potential range of 2.0–4.5 V at a rate of 0.3 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were conducted for the nanocompositemodified electrodes in the frequency range of 100 kHz to 0.01 Hz. All specific capacity data were normalized by the weight of the active material (synthesized catalyst) loaded on the oxygen cathode.
2. Experimental section 2.1. Materials preparation In a typical synthesis, 30 g egg white was diluted with 50 ml of deionized water and stirred for 10 min. Next, 10 g NaCl was gradually added into the solution, and stirring was continued to yield a homogeneous solution. Afterwards, 15 ml of 0.1 mol L−1 C12H22O14Fe·2H2O/ C12H22CoO14 mixed with 6 mmol EDTA was added dropwise to the homogeneous solution and stirred continuously for another 2 h. Then, lyophilization was performed by drying the mixture for about 15 h at −10 °C. Finally, the dried sample was transferred into a ceramic crucible and heat treated under an Ar atmosphere at 850 °C for 2 h. The heat-treated sample was cooled to room temperature, then washed with deionized water for 5 h. The obtained sample is denoted as CoFe/EWC. To obtain the CoFeP/EWC composite, CoFe/EWC was placed in a porcelain boat along with 0.5 g NaH2PO2 at the upstream side of the furnace. After being flushed with Ar, the center of the furnace was elevated to 450 °C at a ramping rate of 2 °C min−1 and kept at this temperature for 2 h, during which time PH3 was formed by the decomposition of sodium hypophosphite, and reacted with CoFe/EWC to form CoFeP/EWC, which was naturally cooled to ambient temperature
3. Results and discussion Scheme 1 shows a schematic diagram of the preparation process for our new catalyst. With the help of freeze-drying technique, uniform small cubic NaCl crystallites were formed and covered by a thin layer of dried egg white, playing a crucial templating role in the formation of the 3D honeycomb morphology. EDTA was chosen as a powerful chelating agent. The formation of EDTA complexes with Co and Fe ions ensured the homogenous distribution of Co and Fe ions in the EWC framework. After pyrolysis in Ar flow and washing with deionized water to remove the NaCl template, 3D honeycomb-like doped carbon was obtained, while the metal cations were completely reduced to CoFe alloy nanodots (Supporting Information Fig. S1). Finally, the CoFe alloy nanodots were transformed into CoFeP through phosphorization at 450 °C. The XRD patterns and Raman spectra shown in Fig. 1a and Fig. 1b 434
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its ultra-small crystallite size [44]. However, both the element maps and XPS results demonstrated the co-existence of cobalt and iron, indicating the formation of Fe doped CoP [31,32]. Fe ions may enter the lattice of CoP by replacing Co. Clearly, once Fe and Co were doped simultaneously, a synergetic effect arose between the doped Fe and Co, resulting in the formation of phosphides with different structures. Fig. 2a shows the N2 adsorption-desorption isotherms. All four samples have almost the same isotherms, indicating that EWC plays a key role in their N2 adsorption-desorption behaviors; it can be seen from the isotherms that the materials have fewer micro- and mesopores but many macropores. The calculated surface areas of EWC, Co2P/EWC, Fe2P/EWC and CoFeP/EWC are 610.3 m2 g−1, 450.2 m2 g−1, 398.2 m2 g−1, and 420.6 m2 g−1, respectively. Clearly, our egg whitederived carbon materials have very high surface areas, possibly due to their honeycomb structure. Furthermore, doping with Fe or Co caused a great decrease in the materials' surface area, which may be caused by the pores being blocked by iron or cobalt phosphide. In addition, the pore-size distribution diagram (Fig. 2b) based on the Barrett-Joyner-Halenda (BJH) method clearly indicates that the EWCs possess both mesopores (centered at 2 and 30 nm) and macropores (centered at 120 nm); as we predicted previously, the materials have more macropores and fewer micro- and mesopores. This pore distribution should benefit Li+ ion transportation, O2 diffusion, and discharge product storage, resulting in high discharge capacity and long-term cyclability for LieO2 battery [45]. As shown in the SEM and TEM images, the EWC and phosphideembedded materials (CoFeP/EWC) exhibited a honeycomb morphology (Fig. 3a and b). Co-doping with Co and Fe did not noticeably change the morphology. The formation of this honeycomb-like porous morphology/structure may be ascribed to many factors, but we believe the application of the NaCl template played a crucial role. TEM images show the ultra-thin pore walls of both EWC (Fig. 3c) and CoFeP/EWC (Fig. 3d). Fig. 3d shows the uniform dispersion of embedded phosphide nanoparticles (CoFeP) on the EWC substrate, with a uniform particle size of ∼2 nm. Generally, small nanoparticles expose more active sites, resulting in enhanced kinetics for catalytic reactions [44,46]. The elemental EDS mapping images clearly reveal the existence of Fe, Co, and P in the CoFeP/EWC material, and the uniform distribution of these elements further confirms the successful doping of Fe and Co and the formation of phosphides. It should be pointed out that Co2P/EWC (Fig. S2a) and Fe2P/EWC (Fig. S2b) show similar morphology/structure to CoFeP/EWC. Fig. 4 presents the XPS spectra of the Co2P/EWC, Fe2P/EWC, and CoFeP/EWC nanocomposites. Fig. 4a shows the survey spectra of three samples. Their surface compositions obtained from the survey spectra are listed in Table 1, where it can be seen that the co-doping/
Scheme 1. Schematic illustration showing the synthesis of a 3D hierarchical Ndoped porous carbon network embedded with cobalt-iron phosphide nanodots (CoFeP/EWC) via a lyophilization-pyrolysis-phosphorization process.
confirm the formation of egg white-derived doped carbon (EWC). All the samples show a diffraction peak at ∼24.4°, which corresponds to the (002) plane of graphitic carbon in the nanocomposite. The D-band to G-band density ratio (ID/IG) of CoFeP/EWC (0.91) was lower than that of the EWC (0.96), Co2P/EWC (0.94), and Fe2P/EWC (0.93) samples, revealing that the CoFeP/EWC nanocomposite has a higher degree of graphitization than the other three samples (Fig. 1b). In other words, co-doping with Co and Fe affected the pyrolysis of the freeze-dried egg white and the graphitization of the derived carbon materials. The XRD patterns shown in Fig. 1a and Fig. S1 also confirm the formation of phosphide, and the transformation of the alloy nanodots. It is interesting that the XRD pattern of our CoFeP/EWC is quite different from Co2P/EWC and Fe2P/EWC. For the Co2P/EWC sample, diffraction peaks at 40.7°, 43.2°, 44.1° and 52.1° good matches with the standard JCPDS File No. 32-0306 of Co2P [41], and Fe2P/EWC shows diffraction peaks at 31.6°, 40.1°, 44.0° and 54.3°, which can be assigned to Fe2P (standard JCPDS Card No. 33-0670) [42,43] (Fig. 1a). Notably, in CoFeP/EWC, the CoFe nanodots were converted into neither Fe-doped Co2P nor Co-doped Fe2P composite during the phosphorization process. CoFeP/EWC exhibits diffraction peaks at 31.5°, 36.3°, 46.2°, 48.2°, 52.0°, 56.1° and 56.7°, which seems to be attributable to the CoP phase (JCPDS No. 29-0497) [33,37]; the broadened peaks can be ascribed to
Fig. 1. X-ray diffraction pattern (a) and Raman spectra (b) of as-prepared EWC, Co2P/EWC, Fe2P/EWC, and CoFeP/EWC samples. 435
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Fig. 2. N2 adsorption-desorption isotherms (a) and corresponding pore-size distribution curves (b) of EWC, Co2P/EWC, Fe2P/EWC, and CoFeP/EWC.
Fig. 3. SEM and TEM images of EWC (a, c), and CoFeP/EWC (b, d) nanocomposites. The enlarged TEM image shows CoFeP nanodots distributed in the EWC framework (inset of d). STEM image (e) of the mapping area (red square) of CoFeP/EWC and the corresponding elemental mapping images (f) of C (cyan), N (blue), O (pink), Co (red), Fe (green), and P (yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 436
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Fig. 4. XPS survey spectra (a) and high-resolution XPS of N 1s (b), Co 2p (c), Fe 2p (d), and P 2p (e) for Co2P/EWC, Fe2P/EWC, and CoFeP/EWC materials.
Regarding phosphorus, its binding energies in the three samples are almost same, with only slight shifts observable, indicating that the coexistence/embedding of Co and Fe did not significantly change the electronic status of P. Fig. 5 shows the ORR/OER performance of the catalysts in 0.1 M KOH solution measured by the rotating disk electrode method. As we expected, CoFeP/EWC exhibited much better ORR and OER performance. Its ORR onset potential is up to 0.94 V, and its half-wave potential 0.83 V, much higher than those of EWC, Co2P/EWC, and Fe2P/ EWC, and 0.02 V (half-wave) higher than those of commercial Pt/C catalyst (Fig. 5a), indicating the excellent ORR performance of our CoFeP-embedded EWC catalyst. As shown in Fig. 5b, the OER overpotential of our CoFeP/EWC is 0.3 V@10 mA cm−1, almost half that of EWC (N-doped carbon) (0.54 V@10 mA cm−1) and 30% and 40% lower than those of Co2P/ EWC and Fe2P/EWC, confirming that the embedded CoFeP and the synergetic interaction between Fe and Co greatly improve the OER performance of the EWC materials. The potential difference (ΔE) at a current density of 10 mA cm−1 and the half-wave potential (ΔE = Ej=10 ‒ E1/2) are generally considered as critical parameters for assessing ORR/OER bifunctional performance [48,49]. The ΔE value of our CoFeP/EWC catalyst is 0.70 V (Fig. 5c), which is significantly lower than that of EWC (1.0 V), Co2P/EWC (0.83 V), and Fe2P/EWC (0.87 V), further confirming its superiority for the OER and ORR compared with the other three samples. It should be pointed out that phosphorization significantly enhanced the OER performance of the catalysts (Fig. S4). Fig. 5d shows the CV profiles of LieO2 batteries with EWC, Co2P/ EWC, Fe2P/EWC, and CoFeP/EWC as the cathode. The cathodic and anodic peak potentials (corresponding to ORR/OER) of the CoFeP/EWC battery are 2.47 and 3.22 V, which were positively/negatively shifted by 130/40, 20/5, and 140/50 mV compared with those obtained for
Table 1 Elemental composition analysis of Co2P/EWC, Fe2P/EWC, and CoFeP/EWC nanocomposites. Samples
C (wt%)
N (wt%)
O (wt%)
P (wt%)
Fe (wt%)
Co (wt%)
Fe2P/EWC Co2P/EWC CoFeP/EWC
88.9 85.6 83.7
3.0 3.6 2.5
4.8 6.3 6.8
1.0 2.1 2.9
2.3 – 2.1
– 2.4 2.0
embedding of Co and Fe caused the N content to decrease from 3.6 wt% (Co2P/EWC) to 2.5 wt% (CoFeP/EWC). The ratio of Co and Fe in CoFeP/EWC is 1.1:1, closing to the experimental recipe ratio of 1:1. Fig. 4b shows that the high-resolution spectra of N1s at 398.5, 400.1, 401.0, and 402.2 eV can be attributed to pyridinic, pyrrolic, graphitic, and oxidized N, respectively [40]; the compositions of these four types of N in each sample are presented in a column diagraph (Fig. S3). Significantly, the relative content of graphitic N was about 32% for Co2P/EWC, 39% for Fe2P/EWC, and 48% for CoFeP/EWC, indicating the high degree of graphitization in the CoFeP/EWC nanocomposite, which is consistent with the Raman results. According to previous reports [40,47], the high graphitic N content should aid in boosting catalyst activity. Fig. 4c presents the high-resolution XPS spectra of Co2P/EWC and CoFeP/EWC. The peaks at 798.5 eV and 781.7 eV for CoFeP/EWC correspond to Co2+ species, whereas the satellite peaks at 805.9 eV and 786.7 eV indicate the presence of a high quantity of Co3+ species [28,32]. Compared with Co2P/EWC, the binding energy of Co 2p1/2 in CoFeP/EWC shifted positively by 0.3 eV; meanwhile, the shifts of Co3+ (satellite peaks) are high up to 2.0 and 1.1 eV. Almost same results are obtained by comparing the Fe in Fe2P/EWC and CoFeP/EWC (Fig. 4d). All these results strongly point to the existence of interactions between doped Fe and Co. 437
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Fig. 5. ORR LSV profiles (a); OER LSV profiles (b) with overpotentials for the OER at 10 mA cm−2 in the inset; overall LSV curves (c) of the EWC, Co2P/EWC, Fe2P/ EWC, and CoFeP/EWC nanocomposites; the inset shows the value of ΔE (ΔE = Ej=10 ‒ E1/2) in 0.1 M KOH solution at 1600 rpm (scan rate: 5 mV s−1); CV curves (d) of LieO2 battery with CoFeP/EWC cathode in 1 M LiN(CF3SO2)2/TEGDME electrolyte.
initial discharge-charge terminal voltages (2.78/3.92 V). All these results indicate the outstanding cyclability of the battery with a CoFeP/ EWC cathode. The selected discharge-charge profiles are shown in Fig. 6c. The charge voltage plateau of the CoFeP/EWC electrode stabilized around 3.8 V during the subsequent 30 cycles, and the discharge voltage plateau stayed at 2.7 V for 60 cycles. On the other hand, the selected discharge/charge profiles of Co2P/EWC (Fig. S5b) and Fe2P/EWC (Fig. S5c) show increasing overpotential from the 10th cycle and afterwards. Furthermore, as the cut-off capacity increases to 2000 mAh g−1, the CoFeP/EWC battery shows a lifetime of 72 cycles (Fig. 6d), further indicating its superior cycling performance. We further investigated the rate performance of the battery with the CoFeP/EWC cathode by varying the current density range from 100 to 1000 mA g−1 for a discharge and charge range from 2.0 to 4.3 V (Fig. 6e). Notably, when the current density increased from 100 to 200 mA g−1, the CoFeP/EWC cathode exhibited decreasing dischargespecific capacity from 11969 mAh g−1 to 10306 mAh g−1; concomitantly, the discharge voltage decreased from 2.64 to 2.55 V and the charge voltage increased from 3.95 to 4.15 V. When higher current densities (800 and 1000 mA g−1) were applied, the charge potential sharply increased to over 4.0 V. For comparison, the Co2P/EWC composite cathode was also tested (Fig. S6). It clearly displayed larger discharge-charge overpotential and lower discharge capacity than the CoFeP/EWC cathode, indicating the latter's better rate performance. The morphology of the discharge products (Li2O2) has significant influence on the performance of the LieO2 battery [50,51], especially on cyclability. Fig. 7 shows ex situ SEM and TEM images of the CoFeP/ EWC cathode (Fig. 7a, b, and c) and the Co2P/EWC cathode (Fig. 7d, e, and f) at different discharge-charge stages. Fig. 7a shows SEM images of
EWC, Co2P/EWC, and Fe2P/EWC batteries, further demonstrating the superior ORR and OER catalytic activity of CoFeP/EWC in LieO2 battery conditions. Fig. 6a exhibits the first discharge-charge voltage curves of LieO2 batteries with EWC, Co2P/EWC, Fe2P/EWC, and CoFeP/EWC as the cathode at a rate of 100 mA g−1 from 2.0 to 4.3 V (vs. Li+/Li). We found that embedding the phosphides could effectively enhance the discharging capacity and coulombic efficiency. The capacities of the batteries follow the order EWC < Fe2P/EWC < Co2P/EWC < CoFeP/EWC. The iron and cobalt binary phosphide-embedded EWC exhibits the highest initial discharge capacity 11969 mAh g−1 (2.4 mAh cm−2), which is almost 40% and 20% higher than those of the EWC and Co2P/EWC cathodes. Furthermore, the initial discharge voltage plateaus of these batteries appear around 2.6 V, but substantial differences can be identified on the charge curves. The charging voltage plateau of the CoFeP/EWC battery was located at 3.95 V, while the plateaus of EWC, Co2P/EWC, and Fe2P/EWC appeared at 4.2, 4.0, and 4.07 V, respectively, indicating the higher energy efficiency achieved by the battery with a CoFeP/EWC cathode. Compared with CoFeP/EWC cathode, pure carbon paper as cathode exhibited negligible capacity contribution and much larger discharge-charge overpotential (Fig. S5a). It is exciting that the battery with a CoFeP/EWC cathode exhibited superior cyclability. Fig. 6b shows the cycling profiles of batteries with EWC, Fe2P/EWC, Co2P/EWC, and CoFeP/EWC as the cathode at a rate of 100 mA g−1 with a cut-off capacity of 1000 mAh g−1. The cycling lifetime of the CoFeP/EWC battery reached 141 cycles, compared with only 28 cycles for EWC, 54 cycles for Co2P/EWC, and 39 cycles Fe2P/ EWC. Notably, after 141 cycles, the discharge and charge terminal voltages of the battery with a CoFeP/EWC cathode were about 2.49 and 4.30 V, decreasing by only 10.4% and increasing about 9.7% from its 438
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Fig. 6. The initial discharge-charge profiles (a) of LieO2 battery with CoFeP/EWC cathode in the potential range from 2.0 to 4.3 V; cycling performances (b); the profiles of a battery with CoFeP/EWC cathode at a cut-off capacity of 1000 mAh g−1 (c) and 2000 mAh g−1 (d) at a charging-discharging rate of 100 mA g−1; the rate performance profile (e) of a battery with CoFeP/EWC cathode.
the CoFeP/EWC cathode after its first discharge to 2.0 V. It can be seen that unique flake-shaped Li2O2 with relatively good crystallinity (Fig. 7a insert) was homogeneously deposited on the surface of the catalyst, noticeably altering the cathode's appearance; Fig. S7 shows the cathode before discharge. Flake-shaped Li2O2 particles with a uniform size (Fig. S8a) were vertically distributed on the CoFeP/EWC cathode, providing a sufficient Li2O2–electrolyte interface, which promoted the decomposition of Li2O2 during the charging process and thus enhanced
the LieO2 battery's capacity and cyclability. With the Co2P/EWC cathode, the discharge products (Fig. 7d) were partially agglomerated; this is further evidenced by the TEM image shown in Fig. S8b. The Li2O2 needed a high charge overpotential to make it decompose, which led to low energy efficiency. After the battery was recharged to 4.3 V, no obvious Li2O2 was observable on the surface of the CoFeP/EWC cathode (Fig. 7b), indicating the complete decomposition of the discharge products, or the
Fig. 7. SEM images of CoFeP/EWC and Co2P/EWC cathode first discharged to 2.0 V (a, d); charged to 4.3 V (b, e); after the 50th cycle at a cut-off capacity of 1000 mAh g−1 (c, f). 439
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Fig. 8. The impedance spectra of LieO2 batteries with CoFeP/EWC (a) and Co2P/EWC (b) as the cathode at various discharging-charging potentials.
superior catalyst performance was achieved by embedding binary CoFeP nanodots on the EWC materials with the synergetic effect between iron and cobalt.
good reversibility of the cathode; Further, no obvious discharge products were present on the surface of the CoFeP/EWC cathode after 50 fixed-capacity cycles (Fig. 7c), demonstrating that the CoFeP/EWC cathode efficiently catalyzed the reversible formation and decomposition of the Li2O2 during the cycling process, resulting in good cycling stability. However, the situation was quite different for the Co2P/EWC cathode. After the first recharge, undecomposed discharge products are rarely visible on the cathode (Fig. 7e), but after 50 fixed-capacity cycles, some agglomerated Li2O2 remained (Fig. 7f), demonstrating this cathode's poor rechargeability. This shows that the co-doping/embedding of Co and Fe plays a crucial role in the high performance of the CoFeP/EWC cathode. The XRD patterns of the CoFeP/EWC cathode during different discharging and charging stages also confirm these observations, as shown in Fig. S9. After discharge, three peaks related to Li2O2 were detected, which disappeared after the subsequent charge, suggesting that the deposited Li2O2 was reversibly decomposed on the CoFeP/EWC cathode. To explore the improvement mechanism of the CoFeP/EWC cathode, we conducted in situ electrochemical impedance spectra (EIS) measurements (Fig. 8a) at various potentials (see insets in Fig. 8) and compared these EIS results with the Co2P/EWC cathode (Fig. 8b). The CoFeP/EWC's ohmic resistance is 13 Ω close to that of Co2P/EWC, but the former's charge transfer resistances at each potential are much lower than the latter's. The initial Rct (before discharging) of the battery with the CoFeP/EWC cathode is 145 Ω, compared with up to 230 Ω for the Co2P/EWC; after discharging to 2.6 V, the Rct values for the two batteries are 182 and 353 Ω respectively; with further discharging to 2.0 V, the Rct increase to 223 and 472 Ω, respectively. Clearly, during the discharging process, the charge transfer resistance of the cathodes increases, likely due to insulated discharge products being formed/deposited on the cathode. The Rct of the LieO2 batteries decreased during the charging process. Interestingly, for CoFeP/EWC cathode recharged to 4.3 V, its Rct approximated its initial value, indicating that the insulated discharge products were almost completely decomposed during the charge process. However, for the Co2P/EWC cathode, the Rct value was still far higher than that initial value, which may indicate incomplete decomposition of the discharge products. After 50 cycles at a fixed capacity of 1000 mAh g−1, the Rct of the battery with the CoFeP/EWC cathode showed only a slight increase (see Fig. S10), indicating that fewer residual discharge products were aggregated on the cathode than Co2P/ EWC. The above results amply demonstrate the excellent rechargeability of the LieO2 battery with the CoFeP/EWC cathode, showing that
4. Conclusions In summary, we have successfully synthesized a high-performance ORR/OER bifunctional catalyst by embedding CoFeP nanodots on a honeycomb-like porous N-doped carbon material (EWC), which was fabricated by a lyophilization-pyrolysis-phosphorization approach using egg white as the precursor. We found that embedding iron and cobalt binary phosphide (or co-doping with iron and cobalt) significantly enhanced the catalytic activity towards the ORR and OER. Furthermore, due to the combination of the EWC's honeycomb-like porous structure and high surface area, when the material was used as the cathode catalyst in a non-aqueous LieO2 battery, superior capacity and excellent cycling stability were achieved. The capacity reached 11969 mAh g−1 (discharged at 100 mA g−1) with a charging potential of 3.95 V; with a fixed capacity of 1000 mA hg−1, it could be discharged and charged up to 141 cycles before the charge terminal voltage reached 4.3 V, which is almost 5.0 and 2.6 times of that achieved with EWC and Co2P/EWC, respectively. Based on the evaluation and characterization results, we ascribe the excellent performance of our catalyst to the following factors: 1) the 3D hierarchical porous structure and high surface area, which provide a large amount of space to host the discharge products, facilitating the access of oxygen molecules and Li ions to the catalyst's surface; 2) the embedding of phosphide nanodots on the EWC, which greatly enhances the ORR/OER catalytic activity, especially the OER, and significantly improves the LieO2 battery's cyclability; 3) the synergetic effect between Fe and Co in the phosphide, and the probable synergetic effect between the phosphide nanodots and the EWC, which led the CoFeP/EWC to exhibit much higher activity than Fe2P/EWC and Co2P/EWC, making the morphology of the discharge products tunable. All of these features of the CoFeP/EWC cathode make it promising for advanced LieO2 battery applications. Competing interests The authors declare no competing financial interests. Acknowledgements This work was supported by the National Key Research and Development Program of China (Project Nos. 2017YFB0102900 and 440
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2016YFB0101201), the National Natural Science Foundation of China (NSFC Project Nos. 21476088 and 21776105), the Guangdong Provincial Department of Science and Technology (Project No. 2015B010106012), and the Guangzhou Science Technology and Innovation Committee (Project Nos. 201504281614372 and 2016GJ006).
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Biomass-derived 3D hierarchical N-doped porous carbon anchoring cobaltiron phosphide nanodots as bifunctional electrocatalysts for LieO2 batteries
T
Kailing Suna, Jing Lia, Lulu Huanga, Shan Jib, Palanisamy Kannanb, Du Lia, Lina Liua, Shijun Liaoa,∗ a The Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510641, China b College of Biological, Chemical Science and Chemical Engineering, Jiaxing University, Jiaxing, 314001, China
H I GH L IG H T S
CoFeP nanodots anchored on biomass-derived N-doped porous carbon was fabricated. • The anchoring of CoFeP NDs accelerates the OER kinetics of the catalyst. • The catalyst CoFeP/EWC exhibited high capacity and long-term cyclability in LieO battery. • The • The synergetic effects of Co with Fe, and CoFeP NDs with EWC were investigated. 2
A R T I C LE I N FO
A B S T R A C T
Keywords: Rechargeable lithium-oxygen batteries Cobalt-iron-phosphide nanodots Porous carbon Oxygen reduction and evolution reactions
Developing lithium-oxygen batteries with high reversibility and long cyclability requires an electrocatalyst with superior catalytic activity and excellent stability to achieve an efficient cathode. Herein, a three-dimensional hierarchically porous carbon framework embedded with cobalt-iron-phosphide nanodots nanocomposite is fabricated via a lyophilization-pyrolysis-phosphorization process. The synthetic catalyst displays excellent performances towards both the oxygen reduction reaction and the oxygen evolution reaction. With our optimal sample, the half-wave potential for the oxygen reduction reaction is up to 0.83 V versus reversible hydrogen electrode, and its potential for the oxygen evolution reaction at a current density of 10 mA cm−2 is as low as 1.53 V in 0.1 M KOH solution. The catalyst also exhibits improved electrochemical performances in a rechargeable lithium-oxygen battery, including a high specific capacity (11969 mAh g−1 at 100 mA g−1) and a long cycle life (141 cycles at a cut-off capacity of 1000 mAh g−1). All of these results make our catalyst a promising candidate for the development of highly efficient electrocatalysts for the rechargeable lithium-oxygen batteries.
1. Introduction Among the various renewable energy storage systems, non-aqueous lithium-oxygen batteries (LieO2 batteries) are regarded as the most promising candidates for future electric vehicles owing to their high theoretical specific energy (∼3600 Wh Kg−1) [1–8]. However, some problems are still existed for this type of battery, such as low energy efficiency, limited cycle life and poor rate capability [4,9,10], which are closely related with the sluggish cathode reactions, including the oxygen reduction reaction (ORR) during discharging and the oxygen evolution reaction (OER) while charging [2,3,6]. Thus, it is crucial to alleviate these problems by designing a bifunctional catalyst for the
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ORR and OER with high catalytic activity and excellent stability [2,8,11]. To date, substantial efforts have been made to develop efficient and durable bifunctional electrocatalysts for the LieO2 battery [2,3,12]. Among these, porous doped carbon materials significantly boost the performance of the LieO2 battery due to their large specific surface areas and sufficient porous structure to host the discharge products and facilitate the reactant transmission [13–16]. For instance, N-doped porous carbon materials display efficient ORR catalytic activity through N inducing atomic charge density redistribution, which promotes the adsorption of large amounts of O2 and enhance ORR activity [17–19]. However, doped carbon materials lack sufficient OER activity, resulting in poor battery cyclability. To overcome this
Corresponding author. E-mail address: [email protected] (S. Liao).
https://doi.org/10.1016/j.jpowsour.2018.11.079 Received 7 September 2018; Received in revised form 30 October 2018; Accepted 25 November 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.
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under an Ar atmosphere. For comparison analysis, Co2P/EWC and Fe2P/EWC composites were also synthesized using the same procedure as described above but without the addition of Fe or Co precursors, respectively.
problem, design and development of carbon-based composites through supporting transition metals or their compounds on doped carbon materials have emerged as an attractive pathway for preparing highperformance ORR/OER bifunctional electrocatalysts [20–26]. Recently, transition-metal phosphides (TMP) have emerged as remarkable candidates for OER catalysts in alkaline electrolytes [27–30]. These bimetallic phosphides generally exhibit higher OER activity than single-metal phosphides, possibly benefiting from electronic transmission between different metals, which facilitates the OER initialization process [31–36]. For instance, Mendoza-Garcia et al. reported that (Co0.54Fe0.46)2P showed excellent OER activity through synergistic interaction with Co/Fe, reaching a specific current density (SCD) of 10 mA cm−2 at an overpotential of 370 mV, which was more efficient than Co2P or Fe2P catalysts [34]. Tang et al. reported that Fe-doped CoP nanoarray on Ti foil (FeeCoP/Ti) required an overpotential of only 310 mV to reach an SCD of 100 mA cm−2, while RuO2/Ti and CoP/Ti required 430 and 450 mV, respectively [31]. Very recently, Liu et al. found that FeeCoeP alloy catalyst exhibited excellent OER activity, requiring an overpotential of 252 mV to reach an SCD of 10 mA cm−2, which was lower than FeeCo oxide catalysts [37]. Hence, cation doping plays a vital role in modulating OER activities in TMP-based catalysts [38]. However, the application of mixed metal phosphides in aprotic LieO2 batteries has rarely been reported but is expected to improve the performance of based LieO2 battery. Inspired by the above ideas, we designed and prepared a novel nanocomposite, three-dimensional (3D), hierarchically porous carbon framework embedded with cobalt-iron-phosphide nanodots (CoFeP/ EWC) for using in LieO2 batteries, through a lyophilization-pyrolysisphosphorization process. We chose egg white (EW) as the precursor of N-doped carbon materials due to that the high content of nitrogen element in EW can be intrinsically retained in the carbon framework and improves catalytic activity without the addition of extrinsic Ndoping sources [39,40]. By freeze-drying the egg white dispersion, then pyrolyzing it in an Ar atmosphere, we get a network-structured dopedcarbon material with a 3D honeycomb-like morphology and high specific surface area; these characteristics could provide the cathode with large storage space, an enriched solid-liquid-gas reaction interface, and a facile electron transport pathway. Further, after a phosphorization process, CoFeP nanoparticles are uniformly anchored on the conductive 3D EWC porous structure. As expected, LieO2 batteries with our CoFeP/EWC composite materials as the cathode exhibit an obvious reduction in charge overpotential, good rate performance, and longterm cyclability.
2.2. Characterization X-ray diffraction (XRD) measurements were recorded using a TD3500 powder diffractometer (Tongda, Cu-Kα radiation) at a scanning rate of 2° min−1 from 10 to 90°. Scanning electron microscopy (SEM) images were obtained on a Nova Nano 430 system (FEI, Netherlands) and transmission electron microscopy (TEM) images were obtained on a JEM-2100HR system (JEOL, Japan). Nitrogen adsorption–desorption measurements were performed on a Tristar II 3020 adsorption analyzer (Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB 250 X-ray photoelectron spectrometer (Thermo-VG Scientific, USA). Raman spectra measurements were performed with a Lab RAM Aramis Raman spectrometer (HJY, France). 2.3. Electrochemical measurements Electrochemical measurements were performed using three electrode cells with glassy carbon (5 mm diameter), a graphite rod, and Hg/ HgO (1 M KOH solution) as the working, counter, and reference electrodes, respectively, using an Autolab PGSTAT30 (Metrohm Autolab) electrochemical analyzer. Polarization LSV curves for the ORR and OER catalytic reactions were obtained in 0.1 M aqueous KOH solution at a scan rate of 5 mV s−1. The catalyst ink-loaded working electrode was prepared by dropping 20 μL catalyst ink (5 mg catalyst in 1 ml 0.25 wt% Nafion/ethanol solution) on a glassy carbon electrode and allowed it to dry at room temperature. The LieO2 battery was assembled by the following procedure. The synthesized composites (80 wt%) were mixed with poly(tetrafluoroethylene) (PTFE, 20 wt%) as a binder in ethanol to form a slurry. Next, the slurry was sprayed onto carbon paper (the load ratio of catalyst was about 0.2 mg cm−2 and the area of the carbon paper was 1 cm2) and dried at 80 °C for 24 h. Then the cell was assembled in an Arfilled glove box with lithium foil as the anode, polypropylene membrane as the separator, and 1.0 M LiN(CF3SO2)2 in tetraethylene glycoldimethyl ether (TEGDME) as the electrolyte. Finally, the assembled cells (CR 2032) were rested for 8 h under high-purity oxygen. The galvanostatic charge–discharge measurements of the cells were tested on a Neware (Shenzhen, China) testing system in a 1 atm O2 atmosphere. CV analysis of the cells was conducted in the potential range of 2.0–4.5 V at a rate of 0.3 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were conducted for the nanocompositemodified electrodes in the frequency range of 100 kHz to 0.01 Hz. All specific capacity data were normalized by the weight of the active material (synthesized catalyst) loaded on the oxygen cathode.
2. Experimental section 2.1. Materials preparation In a typical synthesis, 30 g egg white was diluted with 50 ml of deionized water and stirred for 10 min. Next, 10 g NaCl was gradually added into the solution, and stirring was continued to yield a homogeneous solution. Afterwards, 15 ml of 0.1 mol L−1 C12H22O14Fe·2H2O/ C12H22CoO14 mixed with 6 mmol EDTA was added dropwise to the homogeneous solution and stirred continuously for another 2 h. Then, lyophilization was performed by drying the mixture for about 15 h at −10 °C. Finally, the dried sample was transferred into a ceramic crucible and heat treated under an Ar atmosphere at 850 °C for 2 h. The heat-treated sample was cooled to room temperature, then washed with deionized water for 5 h. The obtained sample is denoted as CoFe/EWC. To obtain the CoFeP/EWC composite, CoFe/EWC was placed in a porcelain boat along with 0.5 g NaH2PO2 at the upstream side of the furnace. After being flushed with Ar, the center of the furnace was elevated to 450 °C at a ramping rate of 2 °C min−1 and kept at this temperature for 2 h, during which time PH3 was formed by the decomposition of sodium hypophosphite, and reacted with CoFe/EWC to form CoFeP/EWC, which was naturally cooled to ambient temperature
3. Results and discussion Scheme 1 shows a schematic diagram of the preparation process for our new catalyst. With the help of freeze-drying technique, uniform small cubic NaCl crystallites were formed and covered by a thin layer of dried egg white, playing a crucial templating role in the formation of the 3D honeycomb morphology. EDTA was chosen as a powerful chelating agent. The formation of EDTA complexes with Co and Fe ions ensured the homogenous distribution of Co and Fe ions in the EWC framework. After pyrolysis in Ar flow and washing with deionized water to remove the NaCl template, 3D honeycomb-like doped carbon was obtained, while the metal cations were completely reduced to CoFe alloy nanodots (Supporting Information Fig. S1). Finally, the CoFe alloy nanodots were transformed into CoFeP through phosphorization at 450 °C. The XRD patterns and Raman spectra shown in Fig. 1a and Fig. 1b 434
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its ultra-small crystallite size [44]. However, both the element maps and XPS results demonstrated the co-existence of cobalt and iron, indicating the formation of Fe doped CoP [31,32]. Fe ions may enter the lattice of CoP by replacing Co. Clearly, once Fe and Co were doped simultaneously, a synergetic effect arose between the doped Fe and Co, resulting in the formation of phosphides with different structures. Fig. 2a shows the N2 adsorption-desorption isotherms. All four samples have almost the same isotherms, indicating that EWC plays a key role in their N2 adsorption-desorption behaviors; it can be seen from the isotherms that the materials have fewer micro- and mesopores but many macropores. The calculated surface areas of EWC, Co2P/EWC, Fe2P/EWC and CoFeP/EWC are 610.3 m2 g−1, 450.2 m2 g−1, 398.2 m2 g−1, and 420.6 m2 g−1, respectively. Clearly, our egg whitederived carbon materials have very high surface areas, possibly due to their honeycomb structure. Furthermore, doping with Fe or Co caused a great decrease in the materials' surface area, which may be caused by the pores being blocked by iron or cobalt phosphide. In addition, the pore-size distribution diagram (Fig. 2b) based on the Barrett-Joyner-Halenda (BJH) method clearly indicates that the EWCs possess both mesopores (centered at 2 and 30 nm) and macropores (centered at 120 nm); as we predicted previously, the materials have more macropores and fewer micro- and mesopores. This pore distribution should benefit Li+ ion transportation, O2 diffusion, and discharge product storage, resulting in high discharge capacity and long-term cyclability for LieO2 battery [45]. As shown in the SEM and TEM images, the EWC and phosphideembedded materials (CoFeP/EWC) exhibited a honeycomb morphology (Fig. 3a and b). Co-doping with Co and Fe did not noticeably change the morphology. The formation of this honeycomb-like porous morphology/structure may be ascribed to many factors, but we believe the application of the NaCl template played a crucial role. TEM images show the ultra-thin pore walls of both EWC (Fig. 3c) and CoFeP/EWC (Fig. 3d). Fig. 3d shows the uniform dispersion of embedded phosphide nanoparticles (CoFeP) on the EWC substrate, with a uniform particle size of ∼2 nm. Generally, small nanoparticles expose more active sites, resulting in enhanced kinetics for catalytic reactions [44,46]. The elemental EDS mapping images clearly reveal the existence of Fe, Co, and P in the CoFeP/EWC material, and the uniform distribution of these elements further confirms the successful doping of Fe and Co and the formation of phosphides. It should be pointed out that Co2P/EWC (Fig. S2a) and Fe2P/EWC (Fig. S2b) show similar morphology/structure to CoFeP/EWC. Fig. 4 presents the XPS spectra of the Co2P/EWC, Fe2P/EWC, and CoFeP/EWC nanocomposites. Fig. 4a shows the survey spectra of three samples. Their surface compositions obtained from the survey spectra are listed in Table 1, where it can be seen that the co-doping/
Scheme 1. Schematic illustration showing the synthesis of a 3D hierarchical Ndoped porous carbon network embedded with cobalt-iron phosphide nanodots (CoFeP/EWC) via a lyophilization-pyrolysis-phosphorization process.
confirm the formation of egg white-derived doped carbon (EWC). All the samples show a diffraction peak at ∼24.4°, which corresponds to the (002) plane of graphitic carbon in the nanocomposite. The D-band to G-band density ratio (ID/IG) of CoFeP/EWC (0.91) was lower than that of the EWC (0.96), Co2P/EWC (0.94), and Fe2P/EWC (0.93) samples, revealing that the CoFeP/EWC nanocomposite has a higher degree of graphitization than the other three samples (Fig. 1b). In other words, co-doping with Co and Fe affected the pyrolysis of the freeze-dried egg white and the graphitization of the derived carbon materials. The XRD patterns shown in Fig. 1a and Fig. S1 also confirm the formation of phosphide, and the transformation of the alloy nanodots. It is interesting that the XRD pattern of our CoFeP/EWC is quite different from Co2P/EWC and Fe2P/EWC. For the Co2P/EWC sample, diffraction peaks at 40.7°, 43.2°, 44.1° and 52.1° good matches with the standard JCPDS File No. 32-0306 of Co2P [41], and Fe2P/EWC shows diffraction peaks at 31.6°, 40.1°, 44.0° and 54.3°, which can be assigned to Fe2P (standard JCPDS Card No. 33-0670) [42,43] (Fig. 1a). Notably, in CoFeP/EWC, the CoFe nanodots were converted into neither Fe-doped Co2P nor Co-doped Fe2P composite during the phosphorization process. CoFeP/EWC exhibits diffraction peaks at 31.5°, 36.3°, 46.2°, 48.2°, 52.0°, 56.1° and 56.7°, which seems to be attributable to the CoP phase (JCPDS No. 29-0497) [33,37]; the broadened peaks can be ascribed to
Fig. 1. X-ray diffraction pattern (a) and Raman spectra (b) of as-prepared EWC, Co2P/EWC, Fe2P/EWC, and CoFeP/EWC samples. 435
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Fig. 2. N2 adsorption-desorption isotherms (a) and corresponding pore-size distribution curves (b) of EWC, Co2P/EWC, Fe2P/EWC, and CoFeP/EWC.
Fig. 3. SEM and TEM images of EWC (a, c), and CoFeP/EWC (b, d) nanocomposites. The enlarged TEM image shows CoFeP nanodots distributed in the EWC framework (inset of d). STEM image (e) of the mapping area (red square) of CoFeP/EWC and the corresponding elemental mapping images (f) of C (cyan), N (blue), O (pink), Co (red), Fe (green), and P (yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 436
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Fig. 4. XPS survey spectra (a) and high-resolution XPS of N 1s (b), Co 2p (c), Fe 2p (d), and P 2p (e) for Co2P/EWC, Fe2P/EWC, and CoFeP/EWC materials.
Regarding phosphorus, its binding energies in the three samples are almost same, with only slight shifts observable, indicating that the coexistence/embedding of Co and Fe did not significantly change the electronic status of P. Fig. 5 shows the ORR/OER performance of the catalysts in 0.1 M KOH solution measured by the rotating disk electrode method. As we expected, CoFeP/EWC exhibited much better ORR and OER performance. Its ORR onset potential is up to 0.94 V, and its half-wave potential 0.83 V, much higher than those of EWC, Co2P/EWC, and Fe2P/ EWC, and 0.02 V (half-wave) higher than those of commercial Pt/C catalyst (Fig. 5a), indicating the excellent ORR performance of our CoFeP-embedded EWC catalyst. As shown in Fig. 5b, the OER overpotential of our CoFeP/EWC is 0.3 V@10 mA cm−1, almost half that of EWC (N-doped carbon) (0.54 V@10 mA cm−1) and 30% and 40% lower than those of Co2P/ EWC and Fe2P/EWC, confirming that the embedded CoFeP and the synergetic interaction between Fe and Co greatly improve the OER performance of the EWC materials. The potential difference (ΔE) at a current density of 10 mA cm−1 and the half-wave potential (ΔE = Ej=10 ‒ E1/2) are generally considered as critical parameters for assessing ORR/OER bifunctional performance [48,49]. The ΔE value of our CoFeP/EWC catalyst is 0.70 V (Fig. 5c), which is significantly lower than that of EWC (1.0 V), Co2P/EWC (0.83 V), and Fe2P/EWC (0.87 V), further confirming its superiority for the OER and ORR compared with the other three samples. It should be pointed out that phosphorization significantly enhanced the OER performance of the catalysts (Fig. S4). Fig. 5d shows the CV profiles of LieO2 batteries with EWC, Co2P/ EWC, Fe2P/EWC, and CoFeP/EWC as the cathode. The cathodic and anodic peak potentials (corresponding to ORR/OER) of the CoFeP/EWC battery are 2.47 and 3.22 V, which were positively/negatively shifted by 130/40, 20/5, and 140/50 mV compared with those obtained for
Table 1 Elemental composition analysis of Co2P/EWC, Fe2P/EWC, and CoFeP/EWC nanocomposites. Samples
C (wt%)
N (wt%)
O (wt%)
P (wt%)
Fe (wt%)
Co (wt%)
Fe2P/EWC Co2P/EWC CoFeP/EWC
88.9 85.6 83.7
3.0 3.6 2.5
4.8 6.3 6.8
1.0 2.1 2.9
2.3 – 2.1
– 2.4 2.0
embedding of Co and Fe caused the N content to decrease from 3.6 wt% (Co2P/EWC) to 2.5 wt% (CoFeP/EWC). The ratio of Co and Fe in CoFeP/EWC is 1.1:1, closing to the experimental recipe ratio of 1:1. Fig. 4b shows that the high-resolution spectra of N1s at 398.5, 400.1, 401.0, and 402.2 eV can be attributed to pyridinic, pyrrolic, graphitic, and oxidized N, respectively [40]; the compositions of these four types of N in each sample are presented in a column diagraph (Fig. S3). Significantly, the relative content of graphitic N was about 32% for Co2P/EWC, 39% for Fe2P/EWC, and 48% for CoFeP/EWC, indicating the high degree of graphitization in the CoFeP/EWC nanocomposite, which is consistent with the Raman results. According to previous reports [40,47], the high graphitic N content should aid in boosting catalyst activity. Fig. 4c presents the high-resolution XPS spectra of Co2P/EWC and CoFeP/EWC. The peaks at 798.5 eV and 781.7 eV for CoFeP/EWC correspond to Co2+ species, whereas the satellite peaks at 805.9 eV and 786.7 eV indicate the presence of a high quantity of Co3+ species [28,32]. Compared with Co2P/EWC, the binding energy of Co 2p1/2 in CoFeP/EWC shifted positively by 0.3 eV; meanwhile, the shifts of Co3+ (satellite peaks) are high up to 2.0 and 1.1 eV. Almost same results are obtained by comparing the Fe in Fe2P/EWC and CoFeP/EWC (Fig. 4d). All these results strongly point to the existence of interactions between doped Fe and Co. 437
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Fig. 5. ORR LSV profiles (a); OER LSV profiles (b) with overpotentials for the OER at 10 mA cm−2 in the inset; overall LSV curves (c) of the EWC, Co2P/EWC, Fe2P/ EWC, and CoFeP/EWC nanocomposites; the inset shows the value of ΔE (ΔE = Ej=10 ‒ E1/2) in 0.1 M KOH solution at 1600 rpm (scan rate: 5 mV s−1); CV curves (d) of LieO2 battery with CoFeP/EWC cathode in 1 M LiN(CF3SO2)2/TEGDME electrolyte.
initial discharge-charge terminal voltages (2.78/3.92 V). All these results indicate the outstanding cyclability of the battery with a CoFeP/ EWC cathode. The selected discharge-charge profiles are shown in Fig. 6c. The charge voltage plateau of the CoFeP/EWC electrode stabilized around 3.8 V during the subsequent 30 cycles, and the discharge voltage plateau stayed at 2.7 V for 60 cycles. On the other hand, the selected discharge/charge profiles of Co2P/EWC (Fig. S5b) and Fe2P/EWC (Fig. S5c) show increasing overpotential from the 10th cycle and afterwards. Furthermore, as the cut-off capacity increases to 2000 mAh g−1, the CoFeP/EWC battery shows a lifetime of 72 cycles (Fig. 6d), further indicating its superior cycling performance. We further investigated the rate performance of the battery with the CoFeP/EWC cathode by varying the current density range from 100 to 1000 mA g−1 for a discharge and charge range from 2.0 to 4.3 V (Fig. 6e). Notably, when the current density increased from 100 to 200 mA g−1, the CoFeP/EWC cathode exhibited decreasing dischargespecific capacity from 11969 mAh g−1 to 10306 mAh g−1; concomitantly, the discharge voltage decreased from 2.64 to 2.55 V and the charge voltage increased from 3.95 to 4.15 V. When higher current densities (800 and 1000 mA g−1) were applied, the charge potential sharply increased to over 4.0 V. For comparison, the Co2P/EWC composite cathode was also tested (Fig. S6). It clearly displayed larger discharge-charge overpotential and lower discharge capacity than the CoFeP/EWC cathode, indicating the latter's better rate performance. The morphology of the discharge products (Li2O2) has significant influence on the performance of the LieO2 battery [50,51], especially on cyclability. Fig. 7 shows ex situ SEM and TEM images of the CoFeP/ EWC cathode (Fig. 7a, b, and c) and the Co2P/EWC cathode (Fig. 7d, e, and f) at different discharge-charge stages. Fig. 7a shows SEM images of
EWC, Co2P/EWC, and Fe2P/EWC batteries, further demonstrating the superior ORR and OER catalytic activity of CoFeP/EWC in LieO2 battery conditions. Fig. 6a exhibits the first discharge-charge voltage curves of LieO2 batteries with EWC, Co2P/EWC, Fe2P/EWC, and CoFeP/EWC as the cathode at a rate of 100 mA g−1 from 2.0 to 4.3 V (vs. Li+/Li). We found that embedding the phosphides could effectively enhance the discharging capacity and coulombic efficiency. The capacities of the batteries follow the order EWC < Fe2P/EWC < Co2P/EWC < CoFeP/EWC. The iron and cobalt binary phosphide-embedded EWC exhibits the highest initial discharge capacity 11969 mAh g−1 (2.4 mAh cm−2), which is almost 40% and 20% higher than those of the EWC and Co2P/EWC cathodes. Furthermore, the initial discharge voltage plateaus of these batteries appear around 2.6 V, but substantial differences can be identified on the charge curves. The charging voltage plateau of the CoFeP/EWC battery was located at 3.95 V, while the plateaus of EWC, Co2P/EWC, and Fe2P/EWC appeared at 4.2, 4.0, and 4.07 V, respectively, indicating the higher energy efficiency achieved by the battery with a CoFeP/EWC cathode. Compared with CoFeP/EWC cathode, pure carbon paper as cathode exhibited negligible capacity contribution and much larger discharge-charge overpotential (Fig. S5a). It is exciting that the battery with a CoFeP/EWC cathode exhibited superior cyclability. Fig. 6b shows the cycling profiles of batteries with EWC, Fe2P/EWC, Co2P/EWC, and CoFeP/EWC as the cathode at a rate of 100 mA g−1 with a cut-off capacity of 1000 mAh g−1. The cycling lifetime of the CoFeP/EWC battery reached 141 cycles, compared with only 28 cycles for EWC, 54 cycles for Co2P/EWC, and 39 cycles Fe2P/ EWC. Notably, after 141 cycles, the discharge and charge terminal voltages of the battery with a CoFeP/EWC cathode were about 2.49 and 4.30 V, decreasing by only 10.4% and increasing about 9.7% from its 438
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Fig. 6. The initial discharge-charge profiles (a) of LieO2 battery with CoFeP/EWC cathode in the potential range from 2.0 to 4.3 V; cycling performances (b); the profiles of a battery with CoFeP/EWC cathode at a cut-off capacity of 1000 mAh g−1 (c) and 2000 mAh g−1 (d) at a charging-discharging rate of 100 mA g−1; the rate performance profile (e) of a battery with CoFeP/EWC cathode.
the CoFeP/EWC cathode after its first discharge to 2.0 V. It can be seen that unique flake-shaped Li2O2 with relatively good crystallinity (Fig. 7a insert) was homogeneously deposited on the surface of the catalyst, noticeably altering the cathode's appearance; Fig. S7 shows the cathode before discharge. Flake-shaped Li2O2 particles with a uniform size (Fig. S8a) were vertically distributed on the CoFeP/EWC cathode, providing a sufficient Li2O2–electrolyte interface, which promoted the decomposition of Li2O2 during the charging process and thus enhanced
the LieO2 battery's capacity and cyclability. With the Co2P/EWC cathode, the discharge products (Fig. 7d) were partially agglomerated; this is further evidenced by the TEM image shown in Fig. S8b. The Li2O2 needed a high charge overpotential to make it decompose, which led to low energy efficiency. After the battery was recharged to 4.3 V, no obvious Li2O2 was observable on the surface of the CoFeP/EWC cathode (Fig. 7b), indicating the complete decomposition of the discharge products, or the
Fig. 7. SEM images of CoFeP/EWC and Co2P/EWC cathode first discharged to 2.0 V (a, d); charged to 4.3 V (b, e); after the 50th cycle at a cut-off capacity of 1000 mAh g−1 (c, f). 439
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Fig. 8. The impedance spectra of LieO2 batteries with CoFeP/EWC (a) and Co2P/EWC (b) as the cathode at various discharging-charging potentials.
superior catalyst performance was achieved by embedding binary CoFeP nanodots on the EWC materials with the synergetic effect between iron and cobalt.
good reversibility of the cathode; Further, no obvious discharge products were present on the surface of the CoFeP/EWC cathode after 50 fixed-capacity cycles (Fig. 7c), demonstrating that the CoFeP/EWC cathode efficiently catalyzed the reversible formation and decomposition of the Li2O2 during the cycling process, resulting in good cycling stability. However, the situation was quite different for the Co2P/EWC cathode. After the first recharge, undecomposed discharge products are rarely visible on the cathode (Fig. 7e), but after 50 fixed-capacity cycles, some agglomerated Li2O2 remained (Fig. 7f), demonstrating this cathode's poor rechargeability. This shows that the co-doping/embedding of Co and Fe plays a crucial role in the high performance of the CoFeP/EWC cathode. The XRD patterns of the CoFeP/EWC cathode during different discharging and charging stages also confirm these observations, as shown in Fig. S9. After discharge, three peaks related to Li2O2 were detected, which disappeared after the subsequent charge, suggesting that the deposited Li2O2 was reversibly decomposed on the CoFeP/EWC cathode. To explore the improvement mechanism of the CoFeP/EWC cathode, we conducted in situ electrochemical impedance spectra (EIS) measurements (Fig. 8a) at various potentials (see insets in Fig. 8) and compared these EIS results with the Co2P/EWC cathode (Fig. 8b). The CoFeP/EWC's ohmic resistance is 13 Ω close to that of Co2P/EWC, but the former's charge transfer resistances at each potential are much lower than the latter's. The initial Rct (before discharging) of the battery with the CoFeP/EWC cathode is 145 Ω, compared with up to 230 Ω for the Co2P/EWC; after discharging to 2.6 V, the Rct values for the two batteries are 182 and 353 Ω respectively; with further discharging to 2.0 V, the Rct increase to 223 and 472 Ω, respectively. Clearly, during the discharging process, the charge transfer resistance of the cathodes increases, likely due to insulated discharge products being formed/deposited on the cathode. The Rct of the LieO2 batteries decreased during the charging process. Interestingly, for CoFeP/EWC cathode recharged to 4.3 V, its Rct approximated its initial value, indicating that the insulated discharge products were almost completely decomposed during the charge process. However, for the Co2P/EWC cathode, the Rct value was still far higher than that initial value, which may indicate incomplete decomposition of the discharge products. After 50 cycles at a fixed capacity of 1000 mAh g−1, the Rct of the battery with the CoFeP/EWC cathode showed only a slight increase (see Fig. S10), indicating that fewer residual discharge products were aggregated on the cathode than Co2P/ EWC. The above results amply demonstrate the excellent rechargeability of the LieO2 battery with the CoFeP/EWC cathode, showing that
4. Conclusions In summary, we have successfully synthesized a high-performance ORR/OER bifunctional catalyst by embedding CoFeP nanodots on a honeycomb-like porous N-doped carbon material (EWC), which was fabricated by a lyophilization-pyrolysis-phosphorization approach using egg white as the precursor. We found that embedding iron and cobalt binary phosphide (or co-doping with iron and cobalt) significantly enhanced the catalytic activity towards the ORR and OER. Furthermore, due to the combination of the EWC's honeycomb-like porous structure and high surface area, when the material was used as the cathode catalyst in a non-aqueous LieO2 battery, superior capacity and excellent cycling stability were achieved. The capacity reached 11969 mAh g−1 (discharged at 100 mA g−1) with a charging potential of 3.95 V; with a fixed capacity of 1000 mA hg−1, it could be discharged and charged up to 141 cycles before the charge terminal voltage reached 4.3 V, which is almost 5.0 and 2.6 times of that achieved with EWC and Co2P/EWC, respectively. Based on the evaluation and characterization results, we ascribe the excellent performance of our catalyst to the following factors: 1) the 3D hierarchical porous structure and high surface area, which provide a large amount of space to host the discharge products, facilitating the access of oxygen molecules and Li ions to the catalyst's surface; 2) the embedding of phosphide nanodots on the EWC, which greatly enhances the ORR/OER catalytic activity, especially the OER, and significantly improves the LieO2 battery's cyclability; 3) the synergetic effect between Fe and Co in the phosphide, and the probable synergetic effect between the phosphide nanodots and the EWC, which led the CoFeP/EWC to exhibit much higher activity than Fe2P/EWC and Co2P/EWC, making the morphology of the discharge products tunable. All of these features of the CoFeP/EWC cathode make it promising for advanced LieO2 battery applications. Competing interests The authors declare no competing financial interests. Acknowledgements This work was supported by the National Key Research and Development Program of China (Project Nos. 2017YFB0102900 and 440
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2016YFB0101201), the National Natural Science Foundation of China (NSFC Project Nos. 21476088 and 21776105), the Guangdong Provincial Department of Science and Technology (Project No. 2015B010106012), and the Guangzhou Science Technology and Innovation Committee (Project Nos. 201504281614372 and 2016GJ006).
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