UiO-66-NO2 as an oxygen âpumpâ for enhancing oxygen reduction

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UiO-66-NO2 as an oxygen “pump” for enhancing oxygen reduction reaction performance Shanshan Zeng, Fucong Lyu, Ligang Sun, Yawen Zhan, Fei-Xiang Ma, Jian Lu, and Yang Yang Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04934 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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Chemistry of Materials

UiO-66-NO2 as an oxygen “pump” for enhancing oxygen reduction reaction performance Shanshan Zeng‡, Fucong Lyu‡, Ligang Sun, Yawen Zhan, Fei-Xiang Ma, Jian Lu *, Yang Yang Li * Dr. S. Zeng, F. Lyu, Dr. Y. Zhan, Dr. F. -X. Ma, Prof. Y. Y. Li Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Kowloon, Hong Kong, China Dr. S. Zeng, Dr. Y. Zhan, Dr. F. -X. Ma, Prof. Y. Y. Li Department of Material Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong, China F. Lyu, Dr. L. Sun, Prof. J. Lu Department of Mechanical Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong F. Lyu, Prof. J. Lu Hong Kong Branch of National Precious Metals Material Engineering Research Centre, City University of Hong Kong, Kowloon, Hong Kong F. Lyu, Prof. J. Lu Centre for Advanced Structural Materials, City University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park, Nanshan District, Shenzhen, China Prof. Y. Y. Li City University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road, Shenzhen Hi-Tech Industrial Park, Nanshan District, Shenzhen, China ABSTRACT: In this work, UiO-66-based metal−organic frameworks (MOFs) are investigated as an ORR catalyst for the first time. UiO-66-NO2 is solvothermally grown on the surface of cobalt phthalocyanine-anchored carbon nanotube (CoCNT) surface, serving as an oxygen ‘pump’ to accelerate the oxygen reduction reaction (ORR). The UiO-66-NO2 attached CoCNT (UiO-66NO2@CoCNT) exhibits superior electrochemical catalytic properties, exceeding the state-of-the-art commercial 20% Pt/C catalyst with more positive half-wave potential (15 mV difference, at 1,600 rpm), better stability (no significant degradation for UiO-66NO2@CoCNT vs. 19% degradation for 20% Pt/C after 25,000 s), and higher methanol tolerance. When assembled in flexible zincair battery, the UiO-66-NO2@CoCNT remains a competitive alternative to commercial 20% Pt/C catalyst with comparable power density and excellent flexibility, suggesting its potential in wearable electronic devices. The outstanding performance of UiO-66NO2@CoCNT composite is closely related to the synergetic effect among the three components: CNT as conductive backbone, CoPc as the oxygen reduction catalytic active site, and UiO-66-NO2 as an ideal oxygen adsorption “pump” (the oxygen diffusion rate is 4.8 times that of 20% Pt/C, and 17.7 times that of CoCNT). The synergy between the three components facilitates oxygen adsorption, transfer of adsorbed oxygen molecules, oxygen reduction, and electron conducting.

INTRODUCTION Fuel cells that directly convert the chemical energy of a fuel into electricity by electrochemical reactions are regarded as a promising class of energy storage device in realizing sustainable fossil-fuel-free economy.1 However, noble metal (e.g. Pt, Ir, or Ru)-based catalysts are typically required in achieving desirable oxygen reduction reaction performances, refraining them from large-scale cost-effective commercial applications.2-5 Thus, much research effort has been dedicated to the development of earth-abundant catalysts with competitive ORR performance.6-8 Encouraging progress has been achieved in improving ORR performance by engineering the active sites of the catalysts (e.g. through doping9-10 or enlarging surface area11). For instance, Guo et al.12 prepared highly efficient metal- and nitrogen- doped carbon-based ORR catalysts in metal organic framework (MOF) template, which ensured a high surface area and homogeneous distribution of

metal/nitrogen active sites. Nevertheless, another key step of ORR, the oxygen adsorption and transfer process, is largely overlooked by the researchers. In this study, we report that this step can be dramatically speeded up and thus the ORR activity greatly enhanced, through the novel design of equipping the catalyst framework with “oxygen pump”, i.e., to furnish cobalt phthalocyanines (CoPc)-adorned carbon nanotubes (CoCNTs) with nitro-substituted zirconium terephthalate UiO-66 (UiO66-NO2). The catalyst framework utilized in this study is cobalt phthalocyanines (CoPc)-anchored carbon nanotube (CoCNT). Anchoring metal phthalocyanines (MPc) on carbon materials such as Vulcan XC-7213 and carbon nanotubes14 is an effectively method for fabricating ORR catalyst. However, this method suffers serious problems including easy aggregation, demetalation and degradation, leading to low activity and poor durability.15 Heat-treatment of MPc/C

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composite (typically at 800-1000 °C) helps enhance its ORR activity and stability, but impairs integrity of the adsorbed macrocyclic compounds and leads to complexity in identifying the nature of the catalytic active site, offering very little insights on how further improving the MPc/C catalyst through rational designs.16 In the light of this, this work provides an alternative solution for better MPc/C catalyst by incorporating a strong oxygen adsorption component (“oxygen pump”), eliminating the need for high-temperature post-treatment. The “oxygen pump” design here is realized through the UiO-66-NO2 MOF. This design is based on the consideration that MOFs are well documented for their gas adsorption functionality, owning to their exceptionally high adsorption capacity, selectivity, and large surface area.17-18 As one of the few known water-stable MOFs, UiO-66-NO2 is a promising candidate for ORR application thanks to their exceptional stability, superior oxygen adsorption ability, and synthesis convenience. However, UiO-66 type MOFs has not been reportedly applied for ORR, possibly due to the inactive Zr metal centers and their insulating nature. In this study, UiO-66 type MOF is for the first time applied for ORR, innovatively utilized as an “oxygen pump”. The UiO-66-NO2-furnished CoCNT (UiO-66-NO2@CoCNT) displays remarkably improved ORR activity. RESULTS AND DISCUSSION The structure of UiO-66-NO2@CoCNT is illustrated in Figure 1. CoPc was first anchored on multi-walled CNTs by strong π–π interactions, which enhanced conductivity while refrained CoPc aggregation. UiO-66-NO2 was then solvothermally grown on the surface of the CoCNT. The UiO66-NO2, CoPc, and CNT serve respectively as oxygen adsorption “pump”, oxygen reduction catalytic site, and conductive backbone, providing synergy facilitating the adsorption, migration and reduction of oxygen molecules as well as electron transfer.

Figure 1. Schematic structure of UiO-66-NO2@CoCNT.

The XRD patterns of the pristine CNT, CoCNT, UiO-66NO2, and UiO-66-NO2@CoCNT composites was presented in Figure 2. CoCNT displayed the same diffraction peaks as the pristine CNT, suggesting the amorphous nature of the absorbed CoPc molecules. For UiO-66-NO2, the peaks appeared at the same locations as in previous studies,19 although slightly broadened due to the small grain size. For UiO-66-NO2@CoCNT, besides the peaks inherited from CoCNT, a strong peak at ~7.1o were observed, which can be ascribed to UiO-66-NO2, indicating the successful integration of UiO-66-NO2 with CoCNT.

Figure 2. XRD patterns of UiO-66-NO2@CoCNT, CoCNT, pristine CNT and UiO-66-NO2.

Raman and FTIR spectra (Figure S1-S2) were further investigated to understand the chemical interactions between CNT, CoPc, and UiO-66-NO2. The UiO-66-NO2@CoCNT composites displayed characteristic Raman peaks of CoCNT and UiO-66-NO2, with the D band shifting from 1341 to 1331 cm-1 and the G band shifting from 1587 to 1584 cm-1, indicating the non-covalent interaction between CoCNT and UiO-66-NO2.20-22 As the relative intensity of G and D bands (IG/ID) is an indicator of the degree of ordering in CNT,23 the decreased IG/ID from 1.559 to 1.051 suggested an increased disorder level after compositing UiO-66-NO2 with CoCNT. Moreover, the spectroscopic features of CoCNT and UiO-66NO2 were discernable throughout the spectral region of UiO66-NO2@CoCNT. In particular, the peaks at 727 and 549 cm-1 shifted to lower frequencies (716 and 536 cm-1, respectively), and the intensity of the peaks centered at 2923, 2516, 2137, 1797, and 879 cm-1 significantly increased, all revealing the interaction between CoCNT and UiO-66-NO2. The elemental bonding configurations were further studied through XPS measurements. The survey spectrum of UiO-66NO2@CoCNT exhibited peaks corresponding to C, O, N, Zr, and Co atoms (Figure 3a). Fine scan of Co 2p of UiO-66NO2@CoCNT displayed weaker signals than CoCNT due to the coverage of UiO-66-NO2 on CoPc (Figure 3b). Besides, compared with CoPc, 1.9 eV down-shift in Co 2p3/2 signal (centered at 779.0 eV, cf. 780.9 eV for CoPc in literature21) was observed for both CoCNT and UiO-66-NO2@CoCNT, which can be ascribed to the π–π interaction between CoPc and CNT.24-25 The π–π interaction-induced shift indicates the electron transfer from CNT to the Co ions in CoCNT or UiO66-NO2@CoCNT, reducing the effective oxidation state of the Co ions, and therefore its XPS signal down-shifted toward Co(0) (778.1 eV for Co metal). The donation of electron from UiO-66-NO2 to CoCNT was verified by the N 1s spectra (Figure 3c), which showed a 0.51 eV up-shift (from 403.64 to 404.15 eV, oxidized N of –NO2) for UiO-66-NO2@CoCNT compared with pure UiO-66-NO2. The shift of the Zr 3d spectra revealed the interaction between CoCNT and UiO-66NO2 (0.36 eV up-shift for Zr 3d3/2 and 0.33 eV down-shift for Zr 3d5/2). The N 1s spectra of CoCNT, UiO-66-NO2, and UiO66-NO2@CoCNT were fit into four kinds of N atoms: pyridinic N (~397.27 eV), pyrrolic N (~396.87 eV, mainly


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derived from CoCNT), –C–NO2 (~398.5 eV), and –NO2 (404.1 eV, mainly derived from UiO-66-NO2).26-27 It should be noted that all of the N species observed in CoCNT and UiO66-NO2@CoCNT, except the uncertain contribution of the oxidized N (–NO2 and –C–NO2), have been reported to play a crucial role in the ORR process.28-29

coupled with STEM reveals the existence of C, Co, and Zr in UiO-66-NO2@CoCNT (Figure S8), confirming the successful synthesis of a heterogeneous composite of CNT, CoPc, and UiO-66-NO2. Brunauer–Emmett–Teller (BET) measurements were conducted to examine the porous nature of UiO-66NO2@CoCNT. The N2 adsorption/desorption isotherms (Figure 4c) of UiO-66-NO2@CoCNT and CoCNT both displayed type-II characteristics, indicating their mesoporous structures. The specific surface area of UiO-66-NO2@CoCNT is 360 m2 g-1, approximately 3 times of CoCNT (131 m2 g-1) (Figure 4c). The Barrett-Joyner-Halenda (BJH) pore-size distribution showed that UiO-66-NO2@CoCNT inherited the pore size distribution ranging 8-30 nm from CoCNT, and pore size of 2-4 nm form UiO-66-NO2 (Figure 4d). The large surface area and hierarchical pore structure of UiO-66NO2@CoCNT promise a high density of catalytic site and fast oxygen diffusion rate, favorable for rapid mass and oxygen transfer in ORR.

Figure 3. Survey scan (a), Co 2p (b), N1s (c) and Zr 3d (d) XPS spectra.

SEM and TEM were used to characterize the sample morphology (Figure S3-S4). Pristine CNTs possessed a high aspect ratio with several micrometers in length and ~10 nm in diameter (Figure S3a-b). No apparent morphological change was spotted after combined with CoPc, indicating its uniform adsorption (Figure S3c-d). The high-resolution TEM (HRTEM) image of CoCNT (Figure S4a) revealed amorphous regions covering the graphite wall of CNT. The corresponding SAED measurement (Figure S4b) discovered the amorphous nature of the CoPc absorbed on CNT, consistent with the XRD analysis (Figure 1). EDS mapping coupled with STEM (Figure S4c) of C (Figure S4d), Co (Figure S4e), and N (Figure S4f) of CoCNT revealed that C constituted the main framework while Co and N from CoPc evenly distributed throughout the framework. The synthesized pure UiO-66-NO2 appeared to be uniform nanoparticles (Figure S5a-b). The TEM image of UiO-66-NO2 (Figure S6) revealed a thin amorphous graphite carbon coating of the nanoparticles. The lattice spacing of the nanoparticle is 0.286 nm (Figure S6c), corresponding to (640) planes of UiO-66-NO2 (obtained from the calculated XRD of UiO-66-NO2 in FigureS7). The SAED pattern of UiO-66-NO2 (Figure S6d) showed polycrystalline diffraction spots along with amorphous diffraction rings, indicating its low crystallinity, possibly due to the low synthetic temperature (120 ºC).30 Ultrafine nanoparticles were observed uniformly distributed on the wall of the well-defined carbon nanotube in UiO-66-NO2@CoCNT (Figure 4a). As marked in the HRTEM image of UiO-66-NO2@CoCNT (Figure 4b), the amorphous regions (the orange dashed circles) may be derived from CoPc on the CNT walls. Close to the amorphous region, a small nanoparticle was anchored in the CNT wall with a lattice spacing of 0.286 nm, corresponding to (640) of UiO-66-NO2. The TEM study evidences the successful incorporation of CoPc molecules and UiO-66-NO2 with CNT. EDS mapping

Figure 4. High-magnification TEM image of UiO-66NO2@CoCNT (a) with the HRTEM image of the area marked in (a) with a red square shown in (b). N2 adsorption isotherms (c) and BJH method-based pore size distribution of CNT and UiO66-NO2@CoCNT (d).

The thermal stability of UiO-66-NO2@CoCNT was investigated by TGA in N2 from room temperature to 800 ºC (Figure S9). For UiO-66–NO2, significant weight loss (33.9%) due to ligand decomposition began at ~380 ºC, indicating that UiO-66–NO2 can remain stable up to 380 oC, in good agreement with the previous studies reporting that UiO-66– NO2 is one of the most stable MOFs.31 UiO-66-NO2@CoCNT displayed a similar weight loss trend to CoPc, with the slight weight loss below 300 ºC attributed to the departure of adsorbed water, between 320 and 480 ºC due to the dehydration of CoPc and partial ligand decomposition of UiO66-NO2 16, 32 and over 480 oC resulted from the decomposition and carbonization of CoPc and UiO-66-NO2. The ORR catalytic activity of UiO-66-NO2@CoCNT and commercial state-of-the-art 20% Pt/C catalyst were first investigated using CV in O2- or Ar-saturated alkaline solution (0.1 M KOH) (Figure 5a). Featureless slopes were observed for UiO-66-NO2@CoCNT in the Ar-saturated electrolyte. In sharp contrast, a dominant cathodic peak at 0.891 V (versus RHE) appeared for UiO-66-NO2@CoCNT in the O2-saturated



solution, and the peak current density was much more prominent than that of the Pt/C catalyst, implying the superior ORR activity of UiO-66-NO2@CoCNT. The ORR performance of UiO-66-NO2@CoCNT was further tested by RDEs and RRDEs. Figure 5b compares the LSV plots of UiO66-NO2@CoCNT, CoCNT, UiO-66-NO2, and commercial 20% Pt/C catalyst at 1,600 rpm. The UiO-66-NO2@CoCNT catalyst exhibited extraordinary ORR performance with a halfwave potential of 0.865 V, better than commercial Pt/C catalyst (0.850V) and CoCNT (0.770 V). As expected, due to the inactivity of the zirconia center and the insulating nature of UiO-66-NO2, it displayed poor oxygen catalytic performance with a half-wave potential of 0.536 V and limited diffusion current densities as low as 0.77 mA cm-2 (5.9 mA cm-2 for UiO-66-NO2@CoCNT). To further prove that the synergetic effect between the catalytic sites (CoCNT) and oxygen adsorption and transfer motif (UiO-66-NO2) enables superior ORR performance of UiO-66-NO2@CoCNT, the LSV plots of physically blended mixture of CoCNT and UiO-66-NO2 (“CoCNT+UiO-66-NO2”) were measured at different rotating speed (400–2500 rpm, Figure S10), delivering a half wave potential of 0.712 V at 1600 rpm, significantly inferior to UiO66-NO2@CoCNT (0.865 V). Obviously, the insufficient catalytic activity of “CoCNT+UiO-66-NO2” is resulted from the weak interaction between CoCNT and UiO-66-NO2 in the mixture. Hence, the superior catalytic performance of UiO-66NO2@CoCNT can be attributed to the synergistic effect between the three components, which facilitated oxygen absorption, transfer of absorbed oxygen molecules, oxygen reduction, and electron conducting. RDE with various mass loading of UiO-66-NO2@CoCNT was shown in Figure S11, indicating the persistency of the four-electron oxygen pathway as evidenced by the little change in the half-wave potential at different mass loading.

Figure 5. CV curves of UiO-66-NO2@CoCNT and commercial 20% Pt/C catalyst in Ar-saturated (dashed line) and O2-saturated (solid line) solution (a). LSV curves of UiO-66-NO2@CoCNT, CoCNT, and 20% Pt/C catalyst at 5 mV s-1 at 1,600 rpm after background correction (b). RRDE curve (c), long-time duration curve at 0.632 V versus RHE (d) of UiO-66-NO2@CoCNT.

Figure 5c showed the RRDE polarization curves for UiO66-NO2@CoCNT and CoCNT. The HO2- yield was below 9.37% for UiO-66-NO2@CoCNT over a potential range of

0.15–0.75 V versus RHE, while CoCNT rendered 18.75% higher HO2- yield, suggesting that increased oxygen adsorption and transfer lowers the HO2- Production. CA measurements at 0.632 V versus RHE for UiO-66NO2@CoCNT (Figure 5d) showed no significant decay after 30,000 s, whereas the current of Pt/C gradually dropped by ~19% within the same time frame, indicating the high catalytic stability of UiO-66-NO2@CoCNT. The possible crossover effect caused by small organic molecules, such as methanol, was tested by CA (Figure S12). After the addition of methanol (final concentration of 3 M), UiO-66-NO2@CoCNT quickly recovered to the original current level with no obvious decay. By contrast, for the Pt/C catalyst, addition of methanol triggered a sharp surge in current density that was difficult to recover from, illustrating much weaker tolerance against chemical corrosion than UiO-66-NO2@CoCNT.

Figure 6. CVs of UiO-66-NO2@CoCNT (a) and commercial Pt/C (b) in static O2-saturated 0.1 M KOH solution at scan rate (ν) of 2, 4, 6, 8, and 10 mV s−1; anodic current of UiO-66-NO2@CoCNT at 1.08 V (I1.08V) versus scan rate (ν) (c); cathodic peak current (Ipeak) of UiO-66-NO2@CoCNT versus the root of scan rate (ν1/2) (d).

The CV performance was studied at different scan rate for both UiO-66-NO2@CoCNT and 20% Pt/C. A typical quasirectangle appeared within the ORR inactive potential range (0.95–1.13 V versus RHE) in the CV curves for both catalysts (Figure 6a-b), indicating the corresponding capacitive processes, which was confirmed by the linearity of their I1.08V−ν plots (Figure 6c). Thereafter, catalytic ORR occurred during a more negative potential range through a diffusionlimited process, as revealed by the linearity of their Ipeak−ν1/2 plots in Figure 6d. For diffusion-limited reactions, oxygen transport behaviors can be characterized by the equivalent diffusion coefficient (DE) obtained from Equation 1 which involves the diffusion-limited current (ID) and scan rate (ν):33 1/2 1/2 𝜈 𝐼𝐷 = 0.4958𝑛𝐹𝐴𝐶 × 𝐷1/2 (1) 𝐸 (𝑛𝛼 𝐹/𝑅𝑇) where DE is the equivalent diffusion coefficient, C is the bulk concentration of O2, T is temperature, n is electron transfer number, 𝛼 is electron transfer coefficient, F and R are Faraday constant and gas constant respectively, and A is the electrochemically active area of the catalyst electrode, which is proportional to the slope of its linear I1.08V−ν curve here. The ratio of AUiO-NO2@CoCNT to APt/C was calculated to be 0.41


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(Figure 6c). The DE ratio of UiO-NO2@CoCNT to commercial Pt/C was then further determined to be 4.8, from the slope ratio of the linear Ipeak−ν1/2 plots, which means the oxygen supply ability of UiO-66-NO2@CoCNT is 4.8 times of Pt/C. According to the Koutecky–Levich plots of Pt/C, it was speculated that oxygen on Pt/C transports nearly like molecular oxygen in liquid-phase with a diffusion coefficient (DL) of 1.9 × 10−5 cm2/s.34 Therefore, the DE of UiO-66NO2@CoCNT was 9.12×10−5 cm2/s. Moreover, the ratio of AUiO-NO2@CoCNT to ACoCNT was 0.37, and the DE ratio of UiO-66NO2@CoCNT to CoCNT was 17.7 (Figure S13), implying that the attachment of UiO-66-NO2 dramatically increased the oxygen diffusion rate by a factor of 17.7. To sum up, the oxygen supply ability of UiO-66-NO2@CoCNT is 4.8 and 17.7 times higher than Pt/C and CoCNT, respectively, due to the exceptional O2 adsorption ability of UiO-66-NO2, significantly enhancing its ORR performance.

Figure 7. Detailed atomistic structures of the relaxed adsorption configurations of UiO-66-NO2@CoCNT.

To further verify that the dramatically enhanced oxygen adsorption ability of UiO-66-NO2@CoCNT, theoretical simulations were conducted using the Adsorption Locator and Forcite program of Material Studio. The adsorption energy was calculated as the system’s free energy change after the free O2 molecule adsorbed onto [email protected] Hence, the absolute value of the negative adsorption energy indicated the extent of attraction between the catalytic active site and O2. The detailed atomistic structures of the relaxed adsorption configurations were presented in Figure 7. The calculated adsorption energy of O2 on UiO-66-NO2@CoCNT was -1907.36 kcal/mol, nearly 180 times lower than that on the surface of CoCNT (-10.64 kcal/mol), suggesting that the oxygen adsorption during ORR was tremendously enhanced with UiO-66-NO2 incorporated into CoCNT. The O2 adsorption test was carried out at 77K (Figure S14).37 The results show that CoCNT possesses rather low O2 adsorption which is dramatically enhanced after the incorporation of the oxygen ‘pump’, UiO-66-NO2, on its surface (Fig. S14). Flexible zinc-air batteries were assembled to investigate the performance of the catalysts under real battery operation conditions. Figure 8a displayed a schematic representation of the flexible zinc-air battery composed of a zinc foil anode,

PVA and KOH-based electrolyte, and the electrocatalystloaded air cathode. The polarization and power density curves of the UiO-66-NO2@CoCNT-based zinc-air battery (Figure 8b) revealed a peak power density of ∼26 mW cm−2, comparable to that of the Pt/C air cathode (∼28 mW cm-2). Compared with the not-bent status, when bent to 90°and 180°, the battery’s galvanostatic discharge potential remained unaffected (Figure 8c), indicating the high tolerance of mechanical stress of the fabricated zinc–air battery under extreme bending conditions. Furthermore, two UiO-66-NO2@CoCNT-based zinc–air batteries connected in series can generate a sufficiently high

Figure 8. Schematic diagram of the fabricated flexible zinc-air battery (a). Discharge curves, polarization curves, and the corresponding power density plots of UiO-66-NO2@CoCNT and commercial Pt/C electrodes in a dried PVA gel electrolyte containing 2 M KOH with a freshly polished Zn anode. The current density was normalized to the geometric area of the working electrode (b). Galvanostatic discharge curves at 1.0 mA cm−2 when bent at 0°, 90°, and 180° (c). Photograph of a 3 V LED light powered by two CoCNT@UiO-NO2-based Zn–air batteries (d).

voltage to power a 3 V light-emitting diode (LED) (Figure 8d). The cycling performance displayed little change in the charging–discharging potentials, indicating the good cycling stability of the fabricated solid state zinc air battery (Fig. S15). These results demonstrate that UiO-66-NO2@CoCNT can achieve superior electrochemical catalytic performance for practical application in zinc–air batteries. CONCLUSION The first UiO-based ORR catalyst, UiO-66-NO2@CoCNT, has been fabricated and evaluated, exhibiting superior electrocatalytic properties outperforming the state-of-the-art commercial 20% Pt/C catalyst, with more positive half-wave potential (15 mV difference, 1,600 rpm), better stability, and higher methanol tolerance. When assembled as flexible zincair battery, the UiO-66-NO2@CoCNT remains a competitive alternative to commercial 20% Pt/C catalyst with comparable power density and high flexibility that is promising in wearable electronics. In UiO-66-NO2@CoCNT, CNT and CoPc serve as conductive backbone and catalytic active site for oxygen reduction and, respectively, while UiO-66-NO2 is perfect oxygen adsorption ‘pump’ (with an oxygen diffusion



rate 4.8 times of 20% Pt/C and 17.7 times of CoCNT. The synergistic effect between the three components facilitates oxygen absorption, transfer and reduction, and electron conducting. In theory, other molecular species capable of attracting oxygen, such as UiO-66-NH2, MIL-125-NH2, NTU9, and pyrrolizidines, may also potentially serve as oxygen pumps. This study opens an effective new route for dramatically improved ORR performance through rational design of the oxygen absorption and transfer process in the catalysts. EXPERIMENTAL SECTION Chemicals and materials Cobalt phthalocyanines, dimethyl formamide (DMF), ZrCl4, and nitroterephthalic acid were purchased from J&K Scientific Ltd., the carbon nanotubes from Shenzhen Nanotech Port Co. Ltd, Nafion (5%) from Sigma-Aldrich, and carbon paper from CeTech Co., Ltd. All the chemicals were used as received without any further purification. Material fabrication In a typical procedure, cobalt phthalocyanine (denoted as CoPc, 2.65 mg) and carbon nanotube (CNT, 20 mg) were dispersed in DMF (15 ml) and magnetically stirred overnight (Solution A). Then, ZrCl4 (9 mg) and nitroterephthalic acid (8.45 mg) (molar ratio of 1:1) were dissolved in DMF (15 ml) (Solution B). Solutions A and B were mixed together and transferred into a Teflon-lined autoclave (50 ml). The solvothermal reaction was performed in an electric oven at 120 °C for 22 h. After cooling to room temperature, the product was centrifuged and washed with DMF for three times and with ethanol twice. Finally, the precipitate was dried at 60 ºC to yield the final product. The final product was denoted as UiO-66-NO2@CoCNT. CoCNT and pure UiO-66-NO2 were fabricated using the same procedure of UiO-66-NO2@CoCNT, except no addition of ZrCl4 or nitroterephthalic acid for CoCNT, and no CoPc or CNT for UiO-66-NO2. The sample of “CoCNT+UiO-66-NO2” was fabricated through physical mixing of CoCNT and UiO-66-NO2. Characterization The sample morphologies were investigated with SEM (Philips XL-30 FESEM) and TEM (JEOL TEM 2100F FEG operated with an accelerating voltage of 200 kV). Raman spectrum was conducted with a Renishaw-200 visual Raman microscope (633 nm wavelength). XRD patterns were collected on an X-ray diffractometer (Rigaku SmartLab) using Cu Kα radiation. According to the BET theory, surface area was tested using a Micromeritics ASAP2020 gas sorption analyzer at 77 K. TGA (Mettler Toledo, TGA/SDTA851e) was performed in N2 at a scan rate of 10 °C min−1 from room temperature to 800 ºC. XPS measurements were carried out using a VG ESCALAB 220i-XL surface analysis system. The oxygen adsorption test was carried out at 77 K using a Quantachrome NOVA 1200e Gas Adsorption Analyzer. For ORR measurements, catalyst (4 mg) was ultrasonically dispersed in 0.5 wt% Nafion ethanol solution (400 µL) to form homogeneous slurry. The slurry was then transferred onto a glassy carbon electrode with a catalyst loading of 0.8 mg cm−2. Prior to use, the glassy carbon electrode (GCE, 0.19625 cm2) was polished with 0.05 μm alumina slurry and then washed

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ultrasonically in water and ethanol (30 s each). The clean electrode was dried with a high-purity nitrogen steam. The catalyst-coated glassy carbon electrode, Hg/HgO (KOH, 0.1 M), and Pt wire were used as the working, reference, and counter electrodes, respectively. Electrochemical measurements were conducted at room temperature on a CHI700A electrochemical station with a flowing gas threeelectrode cell system. For comparison, commercial Pt/C powder purchased from Alfa Aesar was tested with the loading of 0.4 mg cm−2. All electrochemical experiments were performed in O2saturated 0.1 M KOH electrolyte unless otherwise specified. CVs were performed between -0.8 and +0.2 V versus Hg/HgO in 0.1 M KOH at a scan rate of 10 mV s-1. The RDE (φ = 5 mm, 0.19625 cm2) was investigated at different rotating speeds (400–2500 rpm), and the RRDE was investigated at 1,600 rpm. All the experiments were carried out at room temperature. A loading of 0.8 mg cm-2 was chosen since it was close to the most active catalysts and contained a proper amount of catalyst for RDE. All experiments were carried out with this loading unless stated otherwise. All potentials were later converted to the RHE scale (for Hg/HgO reference in alkaline conditions, VRHE = VHg/HgO + VHg/HgOθ + 0.059pH = VHg/HgO + 0.932 V). Construction of flexible zinc-air battery The all solid-state Zn-air battery was constructed using a polished zinc foil as anode, the synthesized catalyst as cathode, gel polymer as solid electrolyte, and carbon cloth as charge collector. The solid electrolyte was prepared as follow by dissolving polyvinyl alcohol (PVA) powder (1 g, MW 85800, Aladdin) in deionized water (10 mL) at 95 oC under magnetic stirring for about 2 h, followed by adding aqueous solution of KOH (18 M, 1 mL) and zinc acetate (0.02 M). After continuous stirring at 95 oC for another 1 h, the gel electrolyte was produced, which was then poured onto a glass plate, frozen at -20 oC for 3 h, and thawed at room temperature. The cycling test of the all solid-state Zn-air battery was carried out at a current density of 3 mA cm-2, with each cycle consisting a 15 min discharging followed by a 15 min charging. Theoretical simulation Simulation was conducted using the Adsorption Locator and Forcite programs in Material Studio. The simulation systems were described with the universal force field combined with QEq charges.38 A bent graphene sheet terminated with hydrogen to reduce the edge effect was constructed to represent the outer surface of CNTs with a chirality of (50, 50). CoPc and UiO-66-NO2 were then attached in succession with geometry optimization in Forcite programs. The O2 adsorption energy on UiO-66-NO2@CoCNT and CoCNT were calculated using the Adsorption Locator program.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Raman spectra, FTIR spectrum, SEM images, TEM images, XRD, TGA, Oxygen sorption isotherms, electrochemical test data.

AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected], [email protected]

Author Contributions The manuscript was written with contributions from all authors. / All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was jointly supported by the Innovation and Technology Commission of HKSAR through Hong Kong Branch of National Precious Metals Material Engineering Research Center, the City University of Hong Kong (Projects 9667143 and 9667125), and the Science and Technology Innovation Commission of Shenzhen Municipality (Key laboratory for prestressed & surface engineering of aerospace materials, Ref: ZDSYS201602291653165).

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UiO-66-NO2 as an oxygen âpumpâ for enhancing oxygen reduction

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UiO-66-NO2 as an oxygen âpumpâ for enhancing oxygen reduction