Novel Hydrogel-Derived Bifunctional Oxygen Electrocatalyst for

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A Novel Hydrogel-derived Bifunctional Oxygen Electrocatalyst for Rechargeable Air Cathodes Gengtao Fu, Yifan Chen, Zhiming Cui, Yutao Li, Weidong Zhou, Sen Xin, Ya-wen Tang, and John B. Goodenough Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b03133 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 11, 2016

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A Novel Hydrogel-derived Bifunctional Oxygen Electrocatalyst for Rechargeable Air Cathodes Gengtao Fu,†,‡,# Yifan Chen,†,# Zhiming Cui,‡,* Yutao Li,‡ Weidong Zhou,‡ Sen Xin,‡ Yawen Tang,†,* and John B. Goodenough ‡,* †

Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Centre of

Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China ‡

Materials Science and Engineering Program & Texas Materials Institute, The University of

Texas at Austin, Austin, Texas 78712, United States ABSTRACT: The commercialization of Zn–air batteries has been impeded by the lack of lowcost, high active and durable catalysts that act independently for the oxygen electrochemical reduction and evolution. Here, we demonstrate excellent performance of NiCo nanoparticles anchored on porous fibrous carbon aerogels (NiCo/PFC aerogels) as bifunctional catalysts towards the Zn–air battery. This material is designed and synthesized by a novel K2Ni(CN)4/K3Co(CN)6-chitosan hydrogel-derived method. The outstanding performance of NiCo/PFC aerogels are confirmed as a superior air-cathode catalyst for rechargeable Zn–air battery. At a discharge–charge current density of 10 mA cm−2, the NiCo/PFC aerogels enable a Zn–air battery to cycle steadily up to 300 cycles for 600 h with only a small increase in the round-trip overpotential, notably outperforming the more costly Pt/C+IrO2 mixture catalysts (60

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cycles for 120 h). With the simplicity of the synthetic method and the outstanding electrocatalytic performance, the NiCo/PFC aerogels are promising electrocatalysts for Zn–air batteries. KEYWORDS: K2Ni(CN)4/K3Co(CN)6-chitosan hydrogel, fibrous carbon aerogels, NiCo alloy, bifunctional electrocatalyst, Zn-air battery Rechargeable Zn–air batteries are attractive for energy generation or storage, because of the high energy density, good safety and low cost.1-12 One of the major hurdles for the Zn-air batteries’ practical application is the lack of good electro-catalysts for the rechargeable reactions at an air cathode: on discharge, the oxygen reduction reaction (ORR) moves to the right and on charge, the oxygen evolution reaction (OER) moves to the left.1-2 Nowadays, the best electro-catalytic performances are obtained with Pt-based catalytic particles for the ORR and either IrO2 or RuO2 for the OER. However, the high cost and limited cycle life of these electrocatalysts have made these materials increasingly unattractive.1-5 To substitute the ultimately expensive noble catalysts, low cost alternatives such as the carbon-based catalysts and transition metal-oxides have been explored;13-18 each has initially looked promising, but none have advanced far enough to meet the requirements of practical application of the Zn-air batteries due to the low electrical conductivity of metal oxides15, 19 and the poor stability of carbon at high potentials.20-21 Recent studies show that transition-metal alloys possess good stability with good electrical conductivity; They are expected to provide more advantages as bifunctional catalysts for oxygen electrocatalysis.5, 22-23 The structural stability of the catalysts is critical to cycling performance of the Zn-air battery. Most nanostructured catalysts suffer from agglomeration during operation of the batteries,24-25 which induces the catalyst degradation. One of the effective approaches to address this problem


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is to disperse of catalyst particles on carbon supports.20, 24-27 However, the methods to deposit particles on carbon supports (carbon black, carbon tubes and graphene, etc) have failed to give well-anchored particles,24,

27-28

which induces leaching, coarsening and aggregation of the

particles during continuous electrochemical cycles. Moreover, the methods do not control well the catalyst-particle size and their uniform dispersion on carbon supports. Thus it is highly desirable to explore an effective, economical, and scalable method to fabricate a highperformance, bifunctional oxygen electrocatalyst. “Hydrogel and aerogel” are two kinds of gels, classified based on the different media that they encompass. “Hydrogel” is a kind of the cross-linked, three-dimensional (3D) hydrophilic polymeric networks derived by the physical or chemical action.29 “Aerogel” have a similar structure with that of the hydrogel. They can be obtained via replacing the liquid of hydrogel with air without collapsing the 3D porous networks.30-33 Concerning the high surface area, large pore volumes, and low density of the aerogels, they have important applications as catalyst supports for designing advanced catalytic materials.30-32 Although several materials, including graphene,16, 34-35 carbon nanotubes,36-37 and cellulose nanofibers38-39 have recently been served as building blocks and assembled into hybrid hydrogels, the accurate control of the porosity and the building blocks’ size was not obtained. In addition, these established methods for hydrogel preparation suffer from the complicated steps and/or rigorous reaction conditions such as hightemperature hydrothermal processing, thus a better and milder method is desirable. Herein, we report a novel K2Ni(CN)4/K3Co(CN)6-chitosan (CS) hybrid hydrogel derived method that enables facile and scalable synthesis of NiCo nanoparticles anchored within porous fibrouscarbon aerogels (NiCo/PFC aerogels) with high catalyst dispersion. Compared with the previous established methods, our preparation method of the K2Ni(CN)4/K3Co(CN)6-CS hybrid hydrogels


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is more facile and milder. The resulting NiCo/PFC aerogels display comparable activity and better stability for ORR and OER versus the Pt/C and IrO2 catalysts, respectively. Moreover, they are shown to provide a superior, low-cost air cathode towards Zn-air battery. At a discharge–charge current density of 10 mA cm−2, the NiCo/PFC aerogels enable a Zn–air battery to cycle steadily up to 600 h with only a small increase in the round-trip overpotential, outperforming the Pt/C+IrO2 mixture catalyst. Typically, the NiCo/PFC aerogels were obtained via sol–gel polymerization of K2Ni(CN)4, K3Co(CN)6 and CS, combined with freeze-drying and pyrolysis treatments, as illustrated in Figure 1a. First, K2Ni(CN)4/K3Co(CN)6-CS hydrogels were readily prepared by adding soluble cyanometalates to the CS solution. After gelation, the cyanometalates are trapped in the gel structure and will be chelated by the functional groups of the CS (Figure S1): (i) the hydroxyl of CS and cyanide groups of cyanometalates can be combined by the hydrogen bridges, which reduces the ρ-donor ability of cyanides and improves the stabilization of the complex; (ii) the amino groups of the CS are able to coordinate metal ions and provide covalent anchoring of the cyano-bridged metallic networks at the specific sites; finally resulting in a stable K2Ni(CN)4/K3Co(CN)6-CS hydrogel. To maintain the 3D porous framework, the as-prepared hydrogels

were

directly

dehydrated

via

a

freeze-drying

process,

resulting

in

K2Ni(CN)4/K3Co(CN)6-CS aerogels (Figure 1a). After carbonization of this aerogels, the NiCo particles were distributed throughout the porous carbon aerogels, and the 3D porous nature was well retained in the final carbon aerogels (Figure 1a). The experimental details are presented in the Supporting information. XRD was used to confirm the phase purity of the NiCo alloy, as shown in Figure 1b. Three characteristic peaks at 44.2, 51.3 and 75.6o are assigned to the (111), (200) and (220) reflections


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of the NiCo alloy phase (space group: Fm-3m (225), JCPDS No. 01-074-5694). A broad diffraction peak at 26.2o was also observed, corresponding to the (002) reflection of the graphitic carbon. XRD results proved that the NiCo alloy phase was introduced into the porous carbon aerogels successfully. The micro/nanostructure of the products was characterized with SEM and TEM technologies. SEM images (Figure 1c-d and Figure S2) reveal an interconnected, 3D porous frameworks consisting of the fibrous carbon with percolating pores in the nanometer-size range. The carbon nanofibers interweave with one another building up a whole network of carbon aerogels. It is also clearly found that the NiCo particles having a size of around 40 nm are anchored uniformly on the carbon nanofibers. TEM images further validate the 3D porous nature and NiCo particles uniformly distribute on the carbon nanofibers (Figure 1e-1f). The NiCo particles within the 3D porous carbon aerogels make a strong bond with the carbon nanofibers that suppresses the agglomeration of particles, thus promoting the electrochemical performance.16, 40 HRTEM image and the profile of the lattice fingers (Figure S3) of a single NiCo particle show the lattice fringes with an inter-fringe distance of 0.202 nm, which matches well with (111) facet of NiCo (0.204 nm, JCPDS No. 01-074-5694). The NiCo particles make electronic contact to the external circuit through the carbon nanofibers, ionic contact with the liquid electrolyte in the pores network of the carbon nanofibers, and contact with the external O2 by the oxygen in the aerogels. Figure S4 displays the scanning TEM image and elemental mappings of the NiCo/PFC aerogels. The C, Ni, and Co elemental signals are evenly distributed in the selected area, suggesting a uniform distribution of Ni and Co elements within the carbon framework. The NiCo content of the NiCo/PFC aerogels was determined by thermogravimetric analysis (TGA, Figure 2a) during which NiCo was completely oxidized to NiO and Co3O4, and carbon was oxidized to


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CO2 (Figure S5). According to the final content of metal oxide, the original content of NiCo is about 44.6 wt.%. The EDX mappings and line scanning profiles recorded through an individual NiCo particle (Figure S6) reveal that a homogeneous distribution of both Ni and Co in the NiCo phase, which is further indicative of NiCo alloy formation. EDX analysis demonstrated the molar ratio of Ni and Co is around 46.6: 53.4 (Figure 2b), in agreement with the stoichiometric ratio of Ni/Co (50: 50). In addition, XPS survey-scan spectrum confirmed the existence of N atoms in the NiCo/PFC aerogels; the content of N was calculated to be 1.60 wt.% (Figure S7). For a comparison, the Ni/PFC and Co/PFC aerogels were also prepared, respectively, by the same hydrogel-derived method. The detailed characterizations of the Ni/PFC and Co/PFC aerogels are presented in Figure S8 and Figure S9. As observed, the Ni/PFC and Co/PFC aerogels show the same structure as the NiCo/PFC aerogels. The surface area and pore size distribution of the three samples were determined from N2 adsorption–desorption isotherms. All samples exhibit type-IV isotherms with a distinctive hysteresis loop (Figure 2c), indicating the existence of the micro-/mesoporous structures. Such micro/mesoporous structures were also reflected by the pore-size distributions (Figure 2c, inset). The pore-sizes of the three samples are all centered mainly at 40.0 nm. The BET surface areas of the NiCo/PFC, Ni/PFC and Co/PFC aerogels are 568.9, 517.6 and 483.5 m2g−1, respectively. Clearly, the M/C composites (M=NiCo, Ni and Co) with the porous structure and the relatively high surface area could be synthesized by the hydrogel-derived method. The larger surface area is anticipated to provide more active surface sites, leading to a higher electrocatalytic activity. The Raman spectroscopy was employed to investigate the degree of graphitization of carbon phase in different samples (Figure 2d). The typical D band and G band were observed at around 1364 cm–1 and 1587 cm–1, respectively. The relative peak intensity ratio of the D band to G band calculated from the peak


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intensity can be considered as a measure of graphitization degree,41 which is 0.94 for NiCo/PFC aerogels, 0.97 for Ni/PFC aerogels and 1.01 for Co/PFC aerogels. Therefore, all the samples have the higher graphitization degree in the carbon phase than that of the XC-72r active carbon (1.10), which highly facilitates electrical conductivity in the electrocatalytic process. The ORR activities of three samples were investigated and compared in alkaline medium (Figure 3a). Remarkably, the NiCo/PFC aerogels exhibit a much more positive onset potential (0.92 V) than those of the Co/PFC (0.85 V) and Ni/PFC (0.72 V) aerogels. An enhancement of ORR activity on NiCo/PFC aerogels was also indicated by its half-wave potential (0.79 V) relative to Co/PFC (0.68 V) and Ni/PFC (0.49 V) aerogels, which proves that oxygen is more easily reduced on NiCo/PFC aerogels. XPS spectra (Figure S10) reveal that the binding energies of Ni and Co in the NiCo/PFC aerogels obviously shift compared with that in the Ni/PFC and Co/PFC aerogels, indicating the strong metallic interaction between Ni and Co. Thus, NiCo alloys are expected to provide additional synergistic effect during the oxygen catalysis reaction. The NiCo/PFC aerogels were further benchmarked with a Pt/C electrocatalyst. Although the half-wave potential of the NiCo/PFC aerogels (0.79 V) is more negative by 50 mV than that of the Pt/C (0.84 V), this value is comparable to many of the non-noble metal-based catalysts that have been reported (Table S1). Figure 3b displays a suit of ORR polarization curves of the NiCo/PFC aerogels recorded at different rotating speeds. The Koutechky–Levich (K-L) plots are provided in the inset of the Figure 3b, where the ORR electron transfer number (n) can be derived from the slopes.42-43 According to the average value calculated from different potentials, n is calculated to be 3.89, demonstrating that the NiCo/PFC aerogels favor a desirable 4-electron transfer pathway. The hydrogen peroxide (H2O2) yield and n can be calculated from the disk and ring currents (Figure S11). The ORR on NiCo/PFC aerogels yield about 12.7–13.3 % HO2– with


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n ranging from 3.72 to 3.75 (Figure S11b), which shows it is close to that of the Pt/C catalyst (HO2–: 5.4–8.6%; n: 3.82–3.89, Figure S11d). Figure 3c shows the OER polarization curves for the NiCo/PFC, Ni/PFC, Co/PFC and IrO2 catalysts in alkaline medium obtained with a RDE. As observed, the NiCo/PFC aerogels shows a low overpotential (0.40 V) compared with that of the Ni/PFC (0.57 V) and Co/PFC (0.61 V) aerogels at 10 mA cm−2. Particularly, the overpotential of the NiCo/PFC aerogels acquired at 10 mA cm−2 was comparable to that of the IrO2 (0.39 V) and those of the previous reported catalysts (Table S2). In addition, the NiCo/PFC aerogels exhibit the smallest Tafel slope (106 mV dec−1) relative to those of the Ni/PFC (180 mV dec−1), Co/PFC (199 mV dec−1) and IrO2 (110 mV dec−1) catalysts (Figure 3d), indicating the outstanding OER kinetics of the NiCo/PFC aerogels. The stability of the NiCo/PFC aerogels as a bifunctional ORR and OER catalyst was assessed through chronoamperometric measurement. As observed in Figure S12, the NiCo/PFC aerogels show the best stabilities during the OER and ORR among all the catalysts. The remarkable electrochemical activity and stability of the NiCo/PFC aerogels make them highly promising as a bifunctional oxygen catalyst. Overall oxygen electrode activities were evaluated by the potential difference between the OER potential obtained at 10 mA cm−2 and the ORR potential taken at half-wave (∆E =EJ10−E1/2). The lower the ∆E value, the better is the bifunctional activity of the catalyst.4 As shown in Figure 4, the NiCo/PFC aerogels exhibit the smallest ∆E among the studied catalysts with a value of 0.86 V, further confirming the high bifunctional activity of the NiCo/PFC aerogels for the ORR and OER. We also evaluated the overall oxygen electrode activities of the pure PFC aerogels. As shown in Figure S13, the PFC aerogels exhibit better bifunctional catalytic activity than that of the XC-72r active carbon. Based on the structural and compositional advantages of the


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NiCo/PFC aerogels, as shown in Scheme 1, we think that three important aspects are responsible for their superior ORR and OER activity and stability: (i) The large open space among the neighboring carbon nanofibers and nano-pores of the carbon nanofibers enable the active species to diffuse easily to/from the NiCo particles; The strong bond of the NiCo particles with the graphitized carbon aerogels provides excellent electronic contact to the external circuit; (iii) the uniform

distribution

of

covalently

anchored

catalytic

particles

suppress

the

agglomeration/dissolution of the particles, thus enhancing the electrocatalytic stability; (iv) Ndoping also may contribute to good oxygen redox catalysis, especially the pyridinic-N and graphitic-N, 44-45 in spite of the low content of N in the NiCo/PFC aerogels. The composition-dependent catalytic activities were investigated with overall oxygen polarization curves. The detailed characterizations of the Ni3Co/PFC and NiCo3/PFC aerogels are presented in Figure S14. Of the three kinds of catalysts, the NiCo/PFC and Ni3Co/PFC aerogels reveal the similar overall oxygen electrode activities with a lower ∆E value than that of the NiCo3/PFC aerogels (Figure S15). The result indicates that alloying Ni with moderate Co can enhance the bifunctional activity for the ORR and OER. The oxygen electrode activities of the NiCo/PFC aerogels also depends on the annealing temperature of the K2Ni(CN)4/K3Co(CN)6-CS aerogels. We found that increasing the NiCo/PFC aerogel annealing temperature (800 oC) caused the loss of catalytic activity (Figure S16d). The high annealing temperature may lead to large particles size with low electrochemical surface area, as verified by SEM images and the corresponding size distribution (Figure S16a-c). To examine the utility of the NiCo/PFC aerogels, a Zn-air battery was set up with NiCo/PFC aerogels as the air-cathode. The Zn-air battery configuration is shown in Figure S17. Figure S18 displays the polarization curves with NiCo/PFC aerogels and Pt/C+IrO2 mixture catalysts as the


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air-cathodes. As observed, the NiCo/PFC exhibits a relative lower voltage gap between the charge and the discharge compared with Pt/C+IrO2 air-cathode, demonstrating a better chargedischarge ability.2, 4 Figure 5a and Figure S19a show the galvanostatic discharge–charge curve at a current density of 10 mA cm−2 and a cutoff time of 2 h per cycle with the NiCo/PFC aerogels as air-cathode. After a continuous 300 discharge–charge cycles (600 h), a slight increase of 0.20 V was observed for the charge-discharge voltage gap. The voltaic efficiency was obtained according to the discharge end voltage divided by charge end voltage to be 65.4 % at the beginning of operation stage and maintained at 56.7 % after 300 cycling. This is among the best cycle performances reported for Zn-air batteries so far (Table S3). A conventional Pt/C+IrO2 mixture catalyst demonstrates significant voltage losses after only 60 cycles (120 h) before the battery was stopped due to the high overpotentials (Figure 5b and Figure S19b). The voltage gap increases from 0.64 V at the 1st cycle to 1.10 V at the 60th cycle, while the voltaic efficiency decreased dramatically from 66.7% at the 1st cycle to 48.8 % at the 60th cycle. Clearly, the NiCo/PFC aerogels can enable Zn-air batteries with a long cycle life and a high efficiency. After 300 charge/discharge cycles, the overall morphology and structure of the NiCo/PFC cathode material were well preserved (Figure S20), further implying the excellent structural stability of the NiCo/PFC aerogels. In summary, we demonstrated a facile, economic and scalable approach to fabricate a NiCo/PFC catalyst, composed of NiCo nanoparticles and porous fibrous carbon aerogels with K2Ni(CN)4/K3Co(CN)6-CS hydrogel as precursor. The physiochemical characterization verifies that the high purity NiCo particles are well anchored on porous carbon aerogels, revealing the effectiveness of the synthesis method. This architecture synergistically combines the advantages of large open pores with high surface area and a uniform dispersion of NiCo particles. Benefiting


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from the structural advantages, the as-synthesized NiCo/PFC aerogels exhibit superior bifunctional activity and stability towards the ORR and OER. Moreover, the NiCo/PFC aerogels are highly durable and achieve higher cell efficiency and longer cycle life than that of the more costly Pt/C+IrO2 mixture catalyst in Zn-air batteries. Our present work provides a new route to synthesize cheap and highly durable bi-functional catalysts towards rechargeable Zn–air batteries.

ASSOCIATED CONTENT The characterization of samples (including TEM images, STEM-EDX mappings, XPS, TGA, Raman and XRD patterns), electrochemical test results and Zn-air batteries test are provided in Supporting Information. AUTHOR INFORMATION * E-mail: [email protected] (Zhiming Cui) [email protected] (Yawen Tang) [email protected] (John B. Goodenough) #

Gengtao Fu and Yifan Chen contributed equally.

ACKNOWLEDGMENT Physical measurements including TEM, Raman and BET were supported by the National Natural Science Foundation of China (21376122 and 21273116). The rest of the work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Science Engineering (DE-SC0005397).


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Figures and captions

Figure 1. (a) Fabrication process for the 3D NiCo/PFC aerogels. (b) XRD pattern, (c and d) SEM images and (e and f) TEM images of the NiCo/PFC aerogels.


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(b)

1.0

C

0.9 0.8

Intensity / a.u.

Weight loss / %

(a)

40.5 %

0.7

Co Co Ni

Ni

0.6

Wt. % (NiCo) = 44.6 % 0.5 0

100

200

300

400

500 o

600

700

0

1

2

3

Temperature / C

200 160

D

0.08 0.04 0.00 20 30 40 50 60 70 80 90 100 Pore Diameter / nm

120

NiCo/PFC Ni/PFC Co/PFC

80 0.0

5

(d)

0.12

0.2

0.4

0.6

Relative Pressure /

0.8

PP0-1

1.0

Intensity / a. u.

240

4

6

7

8

9

10

Energy / KeV

280 dV/dlog(D) /cm3g-1

(c) Volume Adsorbed / cm3g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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G

ID/IG=0.94

NiCo/PFC Ni/PFC Co/PFC XC-72r C

ID/IG=0.97 ID/IG=1.01 ID/IG=1.10 500

1000

1500

2000

2500

Raman Shift / cm-1

Figure 2. (a) TGA curve and (b) EDX spectrum of the NiCo/PFC aerogels. (c) N2 adsorption– desorption isotherms and corresponding pore distribution curves of the NiCo/PFC, Ni/PFC and Co/PFC aerogels. (d) Raman spectra of the NiCo/PFC, Ni/PFC, Co/PFC aerogels and XC-72r active carbon.


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0

(b) 32

NiCo/PFC Ni/PFC Co/PFC Pt/C

-1 -2

Current / mA cm-2

Current / mA cm-2

(a)

-3 -4 -5 -6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.35

0.57 0.61

40 35

(d) 0.60

NiCo/PFC Ni/PFC Co/PFC IrO2

0.40

45

0.39

30 0.1 0.2 0.3 0.4 0.5 0.6 25 Overpotentail / V

0.55

20 15 10

10 mA cm-2

0.50 0.45

NiCo/PFC -1 Ni/PFC ec d V -1 Co/PFC 199 m ec d IrO2 mV 180

0.40 0.35 0.30

dec V m 106

1

1

dec V m 110

0.25

5 0 1.1

n=3.89

Potential / V versus RHE

Overpotentail / V

(c)

50

0.30

1 0.25 0.2 V 0.20 0.3 V 0 0.4 V 0.15 -1 0.5 V 0.10 0.04 0.06 0.08 0.10 0.12 0.14 0.16 -2 -1/2 -1/2 1/2 ω / rad s -3 400 rpm -4 900 rpm -5 -6 1600 rpm -7 2500 rpm -8 3600 rpm -9 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2


Current / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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J -1 / m A-1 cm 2

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1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

0.20 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0


Log (J) / mA cm-2

Figure 3. ORR and OER performances of the catalysts in O2-saturated 0.1 M KOH (Rotation rate: 1600 rmp; Sweep rate: 5 mV s–1): (a) ORR polarization curves. (b) ORR polarization curves of the NiCo/PFC aerogels at different rotation speeds, Inset shows the corresponding K–L plots at different potentials. (c) OER polarization curves, inset shows the overpotentials at a chosen current density of 10 mA cm–2. (d) Tafel plots derived from (c).


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Figure 4. (a) The overall polarization curves of the catalysts within the ORR and OER potential window (Rotation rate: 1600 rmp; Sweep rate: 5 mV s–1). (b) The value of ∆E for five catalysts (∆E = Ej10-E1/2).


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Scheme 1. Schematic illustration of the advantages of the NiCo/PFC aerogels as electrocatalysts.


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Voltage / V

(a) 2.4

NiCo/PFC aerogels

2.0 1.6 1.2 0.8 0

20

Voltage / V

2.0 1.6

40

580

600

65.4 % 56.7 % 300th discharge

1.2 0.8

590

Time / h 300th charge 1st charge

2.4

1st discharge 0

1

2

599

600

601

Time / h

Voltage / V

(b) 2.4

Pt/C + IrO2 mixture catalyst

2.0 1.6 1.2 0.8 0

20

40

110

66.7 %

48.8 %

1.2 0.8

120

60th charge

1st charge

2.0 1.6

100

Time / h

2.4

Voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1st discharge 0

1

60th discharge 2

119

120

121

Time / h

Figure 5. The discharge and charge voltage profiles of Zn-air batteries with (a) NiCo/PFC aerogels catalysts, and (b) conventional Pt/C+IrO2 mixture catalyst at a current density of 10 mV cm–2 at room temperature.


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Table of Contents Graphic


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Novel Hydrogel-Derived Bifunctional Oxygen Electrocatalyst for

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