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Zhiyi Lu, Ming Sun, Tianhao Xu, Yingjie Li, Wenwen Xu, Zheng Chang, Yi Ding, Xiaoming Sun,* and Lei Jiang Direct liquid-feed fuel cells (DLFCs), which enable the conversion of chemical energy directly into electrical energy by oxidizing liquid fuels (such as methanol and hydrazine) into gas products (CO2 and N2), have received considerable attention as low-cost and clean power sources for future mobile and stationary applications due to their high energy and power densities, which both are much superior to conventional batteries.[1] To pursue better performance of the DLFCs, catalysts on both electrodes must demonstrate low operating overpotentials, which are critical for enhancing the energy conversion efficiency.[2,3] In contrast to the tremendous progress in exploring various nanostructured catalysts, the management of the gas products on the microscopic electrode surfaces received very limited attention. This process is actually of substantial importance because at high reaction rates, the gas slugs accumulated on the electrode surface may not only hinder the liquid fuels transport to the catalyst sites, but also impose a higher pressure in the flow field, thus deteriorating the fuel crossover.[4–6] It was reported that adhesion behavior of gas bubbles (e.g., "superaerophilic bursting state”[7] and “superaerophobic pinning state”[8] could be flexibly tuned by the surface architecture construction. For example, we recently discovered that direct construction of electrochemically active materials into micro-/ nanostructured film can offer a “superaerophobic” effect to improve hydrogen evolution.[9] The discontinuous three phase (solid–liquid–gas) contact line (TPCL) afforded by creating the high surface porosity would significantly reduce the adhesion force toward the gas products, result in accelerated gas evolution behavior and much improved electrocatalytic performance at high reaction rates.[10] In this work, the severe bubble adhesion effect caused by the nitrogen bubble evolution in hydrazine oxidation reaction (HzOR) on the electrode surface is minimized by constructing Z. Lu, M. Sun, T. Xu, Y. Li, W. Xu, Dr. Z. Chang, Prof. X. Sun State Key Laboratory of Chemical Resource Engineering University of Chemical Technology PO Box 98, Beijing 100029, PR China E-mail: [email protected] Prof. Y. Ding, Prof. X. Sun Institute for New Energy Materials and Low-Carbon Technologies and School of Materials Science and Engineering Tianjin University of Technology Tianjin 300384, PR China Prof. L. Jiang Center of Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, PR China
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“superaerophobic” nanostructured films. The HzOR involves four electrons to oxidize one hydrazine molecule and form nitrogen gas product in basic environment (see Reaction 1). Cu, an inexpensive metal catalyst with high HzOR activity[11,12] comparable with precious metals such as Pt[13] and Pd,[14] was made into 3D nanostructured film to obtain a “superaerophobic” surface for diminishing the negative effects induced by the gas bubble adhesion. As expected, the highly porous nanostructured film showed a negligible adhesion force to the gas bubbles, resulted in small releasing size and fast removal of asformed nitrogen gas bubbles at high reaction rates, in contrast to the planar counterpart with continuous TPCL (Figure 1a). Consequently, the Cu-nanostructured film exhibited excellent HzOR performances with small overpotentials for achieving high current densities and long-term stability. Moreover, their application in actual direct hydrazine fuel cells (DHFCs) was also examined. With a redox pair of N2H4/O2, the DHFC based on superaerophobic anode could reach a peak power density of 160.8 mW cm−2, ≈3 times of the performance using the commercial Pt/C anode and over one order of magnitude higher to the Cu counterpart. N 2H4 + 4OH → N 2 + 4H2O − 4e
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Superaerophobic Electrodes for Direct Hydrazine Fuel Cells
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One-step electrodeposition at a negative potential prior to the hydrogen evolution reaction was employed to fabricate 3D nanostructured Cu film. After electrodeposition for half an hour, a yellow film was formed on the Cu foil. The top-view and cross-view scanning electron microscopy (SEM) images of the product (Figure 1b,c) demonstrated that a rough surface with vertically aligned nanoplates was formed directly on the Cu foil, and the high-magnification image (Figure 1d) further revealed an average size of ≈500 nm and a thin thickness (≈50 nm) of each nanoplate. The well separated morphology created wide open spaces surrounded by adjacent nanoplates, resulted in a highly porous structure. It should be noted that this electrodeposition process for fabricating 3D nanostructured Cu film is also applicable in Cu foam with zig-zag skeleton and high porosity, which is beneficial to increase the active surface area (Figure S1, Supporting Information). The high resolution transmission electron microscopy (TEM) image (Figure S2, Supporting Information) revealed a typical lattice spacing of 0.242 nm, which was much larger than that of Cu (111) plane (0.208 nm) but matched well with the Cu2O (111) plane, indicating that the thin nanoplates was oxidized once the nanoplate deposited on Cu substrate was exposed in air. This phenomena was further confirmed by the X-ray photoelectron spectroscopy result (XPS, Figure S3, Supporting Information), which indicated a Cu 2p3/2 binding energy of 932.3 eV, corresponding to the values of both Cu(0)[15] and Cu(Ι).[16]
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Figure 1. Schematic and structure information of the nanostructured Cu film. a) Schematic illustration of flat and nanostructured Cu films for HzOR, the nanostructured Cu film can offer a discontinuous TPCL, result in small releasing size of the N2 product; b) and c) top-view and cross-view SEM images of the nanostructured Cu film, demonstrating that the Cu nanostructures were directly aligned on the substrate to form a cross-linked 3D structure; d) high magnification SEM image of the nanostructured Cu film, revealing the nanosize of the Cu nanoplates.
This kind of 3D nanoporous structure is demonstrated to be effective in reducing the adhesion force to gas bubbles due to the discontinuous TPCL.[9] In experiments, the adhesion force of the flat and smooth Cu foil to the gas bubble was as high
as ≈80 µN (Figure 2a), while the 3D nanostructured Cu film showed a negligible adhesion response (below the detection limit, Figure 2d) underwater, indicating the importance of constructing highly porous surface for alleviating the gas bubble
Figure 2. Characterization of the gas bubble interactions to the planar and nanostructured Cu films. a) and d) Adhesive forces measurements of the gas bubbles on planar and nanostructured films, demonstrating that the nanostructured Cu film affords an extremely small bubble adhesive force and underwater “superaerophobic” surface, the inset images are the corresponding underwater gas bubble contact angles of the films; b) and c) digital and schematic images showing the bubble generation behavior on planar Cu film, demonstrating that the as-formed gas bubbles adhere severely on the planar surface, the inset is the statistical releasing size of the gas bubbles (≈150 µm); e,f) digital and schematic images showing the bubble generation behavior on nanostructured Cu film, indicating the gas bubbles became much smaller and released faster, the inset is the statistical releasing size of the gas bubbles (≈20 µm).
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demonstrated that constructing highly porous nanostructured film was crucial to promote the gas evolution behavior in HzOR, and thereby might circumvent the negative effects caused by the bubble adhesion in DHFC. Promoted gas evolution behavior indeed showed a high impact on the electrocatalytic performance for HzOR, as we investigated in 3 M NaOH solution with different N2H4 concentrations. It should be noted that once the nanostructured Cu film with naturally oxidized surface was immersed in the high concentrated hydrazine solution, several gas bubbles were generated on the surface, indicating that the surface oxide was reduced by hydrazine (leading to the formation of N2 bubbles) and the electrocatalyst for HzOR was Cu(0). The polarization curve in the solution without hydrazine indicated that the copper started to be oxidized obviously at around 0.55 V versus saturated calomel electrode (SCE) (Figure S5, Supporting Information), thus the potential window for HzOR testing was limited at –1 to –0.55 V to avoid the side reaction. The catalytic performance comparison of Cu foil and nanostructured-Cu film was shown in Figure 3a (black line and red line), which indicated a boost current density enhancement for HzOR. At the same potential of –0.6 V, the Cu foil only afforded a small current (≈7.9 mA cm−2), while a ≈8.8 times higher current (≈70 mA cm−2) was observed on nanostructured Cu film. It should be noted that after normalization by measuring their electrochemical surface areas (ESAs), the HzOR current of the nanostructured Cu film was still much higher than that of the Cu substrate (Figure S6, Supporting Information), demonstrating the effectiveness of accelerated nitrogen gas bubble
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adhesion. The difference is majorly a result from the surface structures. It is observed that the flat surface showed an aerophobic property with a gas contact angle (CA) of 138.7 ± 2.8° (inset of Figure 2a), while the nanostructured film could afford a “superaerophobic” surface (a gas CA of 169.6 ± 1.3°, bubble repellence, the inset of Figure 2d). The different adhesive properties would lead to different gas releasing behaviors in gas evolution reaction. To get a quantitative comparison, a digitally recording process was performed for Cu foil and nanostructured Cu film at the same current density (≈20 mA cm−2) for hydrazine oxidation. It is observed that, for the Cu foil, the as-formed N2 bubbles became larger gradually and adhered severely to the surface strongly (Figure 2b and Movie S1, Supporting Information). The size of the gas bubbles should be big enough to overcome the high adhesion force and leave the surface. Statistics (inset figures in Figure 2b) showed that the average releasing size of the as-formed gas bubbles was around 150 µm in diameter, whereas the bubbles were generated and left quickly before they grew larger than 20 µm in diameter at the 3D nanostructured Cu film (Figure 2e and Movie S2, Supporting Information). It is also found that, in a certain given area, only one big gas bubble was found on the Cu foil, while more than 10 small gas bubbles can be observed on nanostructured Cu film, further demonstrating that the releasing size of gas bubbles was reduced (Figure 2c,f). It should be noted that the gas bubbles appeared densely and uniformly on this nanostructured surface (Figure S4, Supporting Information), indicating that more catalytically active sites were created and welldistributed by the nanoscale engineering. These results clearly
Figure 3. HzOR performance of the nanostructured Cu films. a) Polarization curves of several electrodes for HzOR, demonstrating that the nanostructured Cu films afford much faster current increases and outperformed the Cu substrate and commercial 60 wt% Pt/C catalyst; b) Nyquist plots of the Cu foil and nanostructured Cu film, the inset is the corresponding equivalent circuit, indicating the nanostructured Cu film possesses a much smaller charge-transfer resistance; c) performance optimization and average releasing sizes of the nanostructured Cu films, indicating the sample obtained at 30 min shows the optimal HzOR performance; d) stability testing of the nanostructured Cu films, demonstrating a good stability.
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Figure 4. DHFC performance of the nanostructured-Cu film: a) schematic illustration of a DHFC assembled by the “superaerophobic” Cu film as anode and commercial Pt/C film as cathode; b) the overall DHFC performances containing the anodes of nanostructured-Cu film, commercial Pt/C film and Cu foam, respectively. The DHFC assembled by the nanostructured-Cu film and Pt/C shows the best performance than the other two counterparts.
releasing behavior on the HzOR performance. Electrochemical impedance spectroscopy is usually considered as a powerful tool to study the electrode kinetics in catalytic reaction.[17] As shown in the Nyquist plots measured under the same potential (Figure 3b), semicircles were observed for both electrodes, indicating this HzOR followed a similar mechanism and was kinetically controlled.[17] The different diameters of the semicircles indicated a different charge transfer resistance (Rct in the corresponding equivalent circuit), whereas the nanostructured Cu film showed a much smaller Rct value than that of Cu foil, demonstrating an accelerated kinetics for HzOR. To exclude the possible Ni doping (see Experimental Section) effect on the HzOR performance, we prepared a Cu film with relatively smooth surface (Figure S7, Supporting Information) as comparison. Although the Ni amount (≈3%) was a little higher than that in the nanostructured Cu film (≈1%), the HzOR performance was inferior to the nanostructured Cu film (Figure S8, Supporting Information) even after the ESA normalization, firmly demonstrating the effectiveness of alleviating the bubble adhesion force on the catalytic performance. It is found that the performance of the nanostructured Cu film on Cu foil was highly dependent on the electrodeposition time. By varying the depositing time from 5 to 40 min, the HzOR performance increased gradually until the time reached 30 min and no further increase was observed by prolonging the depositing time to 40 min (Figure 3c and Figure S9, Supporting Information). This phenomenon was consistent with the SEM results (Figure S10a–d, Supporting Information), which demonstrated that the size of the Cu nanostructure would become larger with the depositing time increasing (5–30 min), thus offering a higher active surface area for HzOR. However, further extending the depositing time might decrease the surface roughness, leading to a degraded HzOR performance. The gas evolution behaviors of the nanostructured Cu films obtained above 20 min were all promoted relative to the sample obtained below 10 min in terms of the releasing sizes (Figure S10e–h, Supporting Information), further confirming the essentiality of constructing highly porous nanostructures. Commercially, 60 wt% Pt/C film fabricated by drop-casting method was also tested for HzOR, as shown the blue line in Figure 3a. Although the onset potential of the Pt/C catalyst was more negative than that of the nanostructured Cu film, the current den-
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sity increased much slower and fluctuated severely, resulting in a small and unstable HzOR current at a high potential. The inferior performance of the Pt/C film was attributed to the high adhesion force to the gas bubble (≈60 µN) and large releasing size of the as-formed bubbles (≈300 µm), as shown in Figure S11 (Supporting Information). Growing the 3D nanostructure on the macroporous Cu foam could further improve the HzOR performance. As shown, the black and red dash lines in Figure 3a, the nanostructured-Cu film achieved ≈135 mA cm−2 at –0.6 V even in a much lower hydrazine concentration (≈0.2 M, the current would be over range of the testing instrument at higher concentrations), ≈1.8 times higher to that of the pure Cu foam. This enhancement was also partially ascribed to the accelerated gas evolution behavior, as shown in Movie S3 and S4 (Supporting Information). The stability testing of the nanostructured Cu films on both Cu foil and Cu foam demonstrated that the HzOR current densities could maintain above 80% of their initial current densities for around 5000s at a constant potential (Figure 3d), revealing the prominent stability of these nanostructured electrodes. The practical application was demonstrated by testing a DHFC assembled by the nanostructured Cu film on Cu foam and commercial Pt/C catalyst as the anode and cathode, respectively (Figure 4a). Pt/C and native Cu foam-based anodes were also tested as comparisons. The polarization curves of the “superaerophobic” electrode-based DHFC at different operating temperatures for the N2H4/O2 fuel cell were shown in Figure S12 (Supporting Information). The open-circuit potential of the cell was ≈1 V, and the maximum power densities of 29.1, 70.9, 121.9, and 160.8 mW cm−2 were achieved when the cell was operated at room temperature, 313, 333, and 353 K, respectively. These values are ≈2 times higher than that of the DHFCs assembled with the commercial Pt/C catalysts (58.5 mW cm−2) and one order of magnitude higher than native Cu foam anode (14.79 mW cm−2, the detailed results can be found in Figure S13, Supporting Information), as shown in Figure 4B. Moreover, the “superaerophobic” electrode-based DHFC also outperforms to the performances of previous reported DHFCs with Cu catalysts,[12,18] and the DHFCs containing novel catalysts,[3,19] as listed in Table S1 (Supporting Information).
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Supporting Information
Experimental Section Synthesis of Nanostructured-Cu Films: The nanostructured Cu films were fabricated by one-step electrodeposition process. First, the substrates (Cu foil and Cu foam) were precleaned in diluted H2SO4 solution to remove the surface oxides. After that, the electrodeposition process was performed on the precleaned substrates in an aqueous electrolyte containing CuSO4·5H2O (0.03 M), NiSO4·6H2O (0.0025 M), NaH2PO2·H2O (0.24 M), Na3C6H5O7·2H2O (0.05 M), H3BO3 (0.50 M), and polyethylene glycol (PEG, 5 ppm) at a constant potential of –0.95 V versus SCE. The pH value of the electrolyte was adjusted at 7–9 and the temperature should be kept at 65 oC in the whole electrodeposition process. The electrodeposition time was varied at 5–40 min to investigate the optimized condition for HzOR. The mass loading of the sample obtained at 30 min was ≈3 mg cm−2. The Cu film with relatively smooth surface was fabricated by a similar method but without PEG in the electrolyte and altering the depositing potential at –0.9 V. A dropcasting method was employed to synthesize the Pt/C catalyst film with the same mass loading. Specifically, the commercial 60 wt% Pt/C catalyst was dispersed in an ethanol solution with a concentration of 6 mg mL−1. Then we drop-cast the catalyst on Cu substrate in an oven heated up to 80 °C. Characterizations: The structural information of the samples were characterized using a field-emission SEM (Zeiss SUPRA 55) operating at 20 kV and a high-resolution TEM system (JEOL 2100) operating at 200 kV. XPS spectrum was carried out by using a model of ESCALAB 250. The nitrogen bubble releasing process was recorded by a highspeed CCD camera (X-Motion, AOS Technologies) mounted on a microscope (SZ-CTC, OLYMPUS). The interaction force between the gas bubbles and electrode interfaces can be assessed by a high-sensitivity microelectromechanical balance system (Dataphysics DCAT11, Germany), and the bubble CA was measured by the captive bubble method (Dataphysics OCA20).[9] The volume of the gas bubble was
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about 2 µL for each testing. All the experiments were repeated for more than five times. Electrochemical Measurements: The HzOR performance was carried out at room temperature in a three-electrode glass cell connected to an electrochemical workstation (CHI 660D, Chenghua, Shanghai). Cyclic voltammetry and linear sweep voltammetry with scan rate of 5 mV·s−1 were conducted in 3 M NaOH solution with different hydrazine concentrations using SCE as the reference electrode. Pt wire was used as the counter electrode. AC impedance measurements were carried out in the same configuration at –0.7 V from 105–0.1 Hz with an AC voltage of 5 mV. Movie S1 and S2 in the Supporting Information were recorded when using a Cu foil and a nanostructured Cu film on Cu foil at an HzOR current density of 20 mA cm−2 in 3 M NaOH solution containing 1 M hydrazine, and Movie S3 and S4 in the Supporting Information were recorded when using a Cu foam and a nanostructured Cu film on Cu foam at an HzOR current density of 60 mA cm−2 in a 3 M NaOH solution containing 0.2 M hydrazine. In all the measurements, we used SCE as the reference. The stability testing of the nanostructured Cu films were operated at constant overpotential (–0.55 V for the nanostructured Cu film on Cu foil and –0.6 V for the nanostructured Cu film on Cu foam) for achieving high initial current densities. The potential application of the nanostructured-Cu film in DHFC was demonstrated by assembling the nanostructured Cu film as the anode and the commercial 60 wt% Pt/C film as the cathode. Nafion 115 membrane was used as the solid electrolyte. The liquid feed in the anode side was the aqueous solution containing 20% N2H4 and 4 M NaOH with a flow rate of 10 mL min−1 by a peristaltic pump and silicone tubes, and pure O2 was purged in the cathode side. The cell temperature was controlled via a temperature controller and monitored by thermocouples buried in the graphite blocks. The steady state polarization curves were recorded by an automatic Electric Load (PLZ 70UA, Japan). The pure Cu foam and commercial Pt/C film were used as the anodes for comparison purposes.
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The above results clearly manifest that constructing nanoporous architecture is of great importance for achieving high HzOR performance. The main reason is that the “superaerophobic” surface can effectively drive the gas product from the catalyst surface in a small critical size (≈20 µm), thus maximizing the real working area and keeping it constant. Besides, the uniform loading of the vertically aligned nanostructures is able to greatly enhance the porosity of the film while maintaining a high conductivity, thus resulting in a high electrochemically active surface area and utilization of the active materials. In summary, by simply electrodepositing a vertically aligned nanostructured Cu film, a “superaerophobic” electrode with low gas bubble adhesion force was achieved, resulting in a small releasing size of the gas product and fast evolution behavior. Benefit from this unique property, the highly porous nanostructured Cu film showed an extraordinary HzOR performance and potential promise in DHFC, along with other advantages of the porous nanostructures. This study exhibits the effectiveness of nanoscale engineering on HzOR performance, thus providing a facile and efficient way for solving the gas adhesion problem in DHFC. Coupling with the optimization of engineering technologies (e.g., channel construction[4] and operating parameters,[5] the performance of DHFC with “superaerophobic” electrode is believed to be maximized to serve as an efficient energy supply. Together with our previous demonstration of “superaerophobic” electrodes for hydrogen evolution, the promise of this concept to gas evolution reactions is firmly demonstrated, not only for electrochemical water splitting, but also for fuel cells.
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Z.L. and M.S. contributed equally to this work. The authors appreciate the useful discussion with Prof. Limin Liu. This work was supported by the NSFC, the 973 Program (Grant Nos. 2011CBA00503 and 2012CB93280), the 863 Program (Grant No. 2012AA03A609), the Program for Changjiang Scholars and Innovative Research Team in University. Note: The second affiliation was updated on 1 April 2015 after original online publication. Received: January 6, 2015 Revised: February 9, 2015 Published online: February 27, 2015
[1] a) A. S. Arico, S. Srinivasan, V. Antonucci, Fuel Cells 2001, 1, 133; b) H. S. Liu, C. J. Song, L. Zhang, J. J. Zhang, H. J. Wang, D. P. Wilkinson, J. Power Sources 2006, 155, 95; c) K. Asazawa, K. Yamada, H. Tanaka, A. Oka, M. Taniguchi, T. Kobayashi, Angew. Chem. Int. Ed. 2007, 46, 8024; d) A. Serov, C. Kwak, Appl. Catal. B: Environ. 2010, 98, 1; e) N. V. Rees, R. G. Compton, Energy Environ. Sci. 2011, 4, 1255; f) A. Serov, M. Padilla, A. J. Roy, P. Atanassov, T. Sakamoto, K. Asazawa, H. Tanaka, Angew. Chem. Int. Ed. 2014, 53, 10336. [2] a) Y. Wu, D. Wang, G. Zhou, R. Yu, C. Chen, Y. Li, J. Am. Chem. Soc. 2014, 136, 11594; b) Z. Niu, D. Wang, R. Yu, Q. Peng, Y. Li, Chem. Sci. 2012, 3, 1925; c) S. Guo, S. Zhang, X. Sun, S. Sun, J.
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Am. Chem. Soc. 2011, 133, 15354; d) X. Yan, F. Meng, Y. Xie, J. Liu, Y. Ding, Sci. Rep. 2012, 2, 941; e) J. Sanabria-Chinchilla, K. Asazawa, T. Sakamoto, K. Yamada, H. Tanaka, P. Strasser, J. Am. Chem. Soc. 2011, 133, 5425; f) Y. Meng, X. Zou, X. Huang, A. Goswami, Z. Liu, T. Asefa, Adv. Mater. 2014, 26, 6510. K. Yamada, K. Yasuda, H. Tanaka, Y. Miyazaki, T. Kobayashi, J. Power Sources 2003, 122, 132. P. Argyropoulos, K. Scott, W. M. Taama, Electrochim. Acta 1999, 44, 3575. H. Yang, T. S. Zhao, Q. Ye, J. Power Sources 2005, 139, 79. K. Scott, P. Argyropoulos, P. Yiannopoulos, W. M. Taama, J. Appl. Electrochem. 2001, 31, 823. a) X. Chen, Y. Wu, B. Su, J. Wang, Y. Song, L. Jiang, Adv. Mater. 2012, 24, 5884; b) J. Wang, Y. Zheng, F.-Q. Nie, J. Zhai, L. Jiang, Langmuir 2009, 25, 14129. a) J. Wang, Q. Yang, M. Wang, C. Wang, L. Jiang, Soft Matter. 2012, 8, 2261; b) W. Barthlott, T. Schimmel, S. Wiersch, K. Koch, M. Brede, M. Barczewski, S. Walheim, A. Weis, A. Kaltenmaier, A. Leder, H. F. Bohn, Adv. Mater. 2010, 22, 2325. Z. Lu, W. Zhu, X. Yu, H. Zhang, Y. Li, X. Sun, X. Wang, H. Wang, J. Wang, J. Luo, X. Lei, L. Jiang, Adv. Mater. 2014, 26, 2683.
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[10] a) D. Kong, H. Wang, Z. Lu, Y. Cui, J. Am. Chem. Soc. 2014, 136, 4897; b) M. S. Faber, R. Dziedzic, M. A. Lukowski, N. S. Kaiser, Q. Ding, S. Jin, J. Am. Chem. Soc. 2014, 136, 10053. [11] a) F. Jia, J. Zhao, X. Yu, J. Power Sources 2013, 222, 135; b) H. Gao, Y. Wang, F. Xiao, C. B. Ching, H. Duan, J. Phys. Chem. C 2012, 116, 7719; c) C. Liu, H. Zhang, Y. Tang, S. Luo, J. Mater. Chem. A 2014, 2, 4580. [12] E. Granot, B. Filanovsky, I. Presman, I. Kuras, F. Patolsky, J. Power Sources 2012, 204, 116. [13] L. D. Burke, K. J. O’Dwyer, Electrochim. Acta 1989, 34, 1659. [14] Y. Fukumoto, T. Matsunaga, T. Hayashi, Electrochim. Acta 1981, 26, 631. [15] G. Schön, J. Electron Spectrosc. Relat. Phenom. 1972, 1, 377. [16] B. R. Strohmeiera, D. E. Levdena, R. S. Fielda, D. M. Herculesa, J. Catal. 1985, 94, 514. [17] D. Merki, H. Vrubel, L. Rovelli, S. Fierro, X. Hu, Chem. Sci. 2012, 3, 2515. [18] B. Filanovsky, E. Granot, I. Presman, I. Kuras, F. Patolsky, J. Power Sources 2014, 246, 423. [19] K. Yamada, K. Asazawa, K. Yasuda, T. Ioroi, H. Tanaka, Y. Miyazaki, T. Kobayashi, J. Power Sources 2003, 115, 236.
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Zhiyi Lu, Ming Sun, Tianhao Xu, Yingjie Li, Wenwen Xu, Zheng Chang, Yi Ding, Xiaoming Sun,* and Lei Jiang Direct liquid-feed fuel cells (DLFCs), which enable the conversion of chemical energy directly into electrical energy by oxidizing liquid fuels (such as methanol and hydrazine) into gas products (CO2 and N2), have received considerable attention as low-cost and clean power sources for future mobile and stationary applications due to their high energy and power densities, which both are much superior to conventional batteries.[1] To pursue better performance of the DLFCs, catalysts on both electrodes must demonstrate low operating overpotentials, which are critical for enhancing the energy conversion efficiency.[2,3] In contrast to the tremendous progress in exploring various nanostructured catalysts, the management of the gas products on the microscopic electrode surfaces received very limited attention. This process is actually of substantial importance because at high reaction rates, the gas slugs accumulated on the electrode surface may not only hinder the liquid fuels transport to the catalyst sites, but also impose a higher pressure in the flow field, thus deteriorating the fuel crossover.[4–6] It was reported that adhesion behavior of gas bubbles (e.g., "superaerophilic bursting state”[7] and “superaerophobic pinning state”[8] could be flexibly tuned by the surface architecture construction. For example, we recently discovered that direct construction of electrochemically active materials into micro-/ nanostructured film can offer a “superaerophobic” effect to improve hydrogen evolution.[9] The discontinuous three phase (solid–liquid–gas) contact line (TPCL) afforded by creating the high surface porosity would significantly reduce the adhesion force toward the gas products, result in accelerated gas evolution behavior and much improved electrocatalytic performance at high reaction rates.[10] In this work, the severe bubble adhesion effect caused by the nitrogen bubble evolution in hydrazine oxidation reaction (HzOR) on the electrode surface is minimized by constructing Z. Lu, M. Sun, T. Xu, Y. Li, W. Xu, Dr. Z. Chang, Prof. X. Sun State Key Laboratory of Chemical Resource Engineering University of Chemical Technology PO Box 98, Beijing 100029, PR China E-mail: [email protected] Prof. Y. Ding, Prof. X. Sun Institute for New Energy Materials and Low-Carbon Technologies and School of Materials Science and Engineering Tianjin University of Technology Tianjin 300384, PR China Prof. L. Jiang Center of Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, PR China
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“superaerophobic” nanostructured films. The HzOR involves four electrons to oxidize one hydrazine molecule and form nitrogen gas product in basic environment (see Reaction 1). Cu, an inexpensive metal catalyst with high HzOR activity[11,12] comparable with precious metals such as Pt[13] and Pd,[14] was made into 3D nanostructured film to obtain a “superaerophobic” surface for diminishing the negative effects induced by the gas bubble adhesion. As expected, the highly porous nanostructured film showed a negligible adhesion force to the gas bubbles, resulted in small releasing size and fast removal of asformed nitrogen gas bubbles at high reaction rates, in contrast to the planar counterpart with continuous TPCL (Figure 1a). Consequently, the Cu-nanostructured film exhibited excellent HzOR performances with small overpotentials for achieving high current densities and long-term stability. Moreover, their application in actual direct hydrazine fuel cells (DHFCs) was also examined. With a redox pair of N2H4/O2, the DHFC based on superaerophobic anode could reach a peak power density of 160.8 mW cm−2, ≈3 times of the performance using the commercial Pt/C anode and over one order of magnitude higher to the Cu counterpart. N 2H4 + 4OH → N 2 + 4H2O − 4e
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Superaerophobic Electrodes for Direct Hydrazine Fuel Cells
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One-step electrodeposition at a negative potential prior to the hydrogen evolution reaction was employed to fabricate 3D nanostructured Cu film. After electrodeposition for half an hour, a yellow film was formed on the Cu foil. The top-view and cross-view scanning electron microscopy (SEM) images of the product (Figure 1b,c) demonstrated that a rough surface with vertically aligned nanoplates was formed directly on the Cu foil, and the high-magnification image (Figure 1d) further revealed an average size of ≈500 nm and a thin thickness (≈50 nm) of each nanoplate. The well separated morphology created wide open spaces surrounded by adjacent nanoplates, resulted in a highly porous structure. It should be noted that this electrodeposition process for fabricating 3D nanostructured Cu film is also applicable in Cu foam with zig-zag skeleton and high porosity, which is beneficial to increase the active surface area (Figure S1, Supporting Information). The high resolution transmission electron microscopy (TEM) image (Figure S2, Supporting Information) revealed a typical lattice spacing of 0.242 nm, which was much larger than that of Cu (111) plane (0.208 nm) but matched well with the Cu2O (111) plane, indicating that the thin nanoplates was oxidized once the nanoplate deposited on Cu substrate was exposed in air. This phenomena was further confirmed by the X-ray photoelectron spectroscopy result (XPS, Figure S3, Supporting Information), which indicated a Cu 2p3/2 binding energy of 932.3 eV, corresponding to the values of both Cu(0)[15] and Cu(Ι).[16]
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Figure 1. Schematic and structure information of the nanostructured Cu film. a) Schematic illustration of flat and nanostructured Cu films for HzOR, the nanostructured Cu film can offer a discontinuous TPCL, result in small releasing size of the N2 product; b) and c) top-view and cross-view SEM images of the nanostructured Cu film, demonstrating that the Cu nanostructures were directly aligned on the substrate to form a cross-linked 3D structure; d) high magnification SEM image of the nanostructured Cu film, revealing the nanosize of the Cu nanoplates.
This kind of 3D nanoporous structure is demonstrated to be effective in reducing the adhesion force to gas bubbles due to the discontinuous TPCL.[9] In experiments, the adhesion force of the flat and smooth Cu foil to the gas bubble was as high
as ≈80 µN (Figure 2a), while the 3D nanostructured Cu film showed a negligible adhesion response (below the detection limit, Figure 2d) underwater, indicating the importance of constructing highly porous surface for alleviating the gas bubble
Figure 2. Characterization of the gas bubble interactions to the planar and nanostructured Cu films. a) and d) Adhesive forces measurements of the gas bubbles on planar and nanostructured films, demonstrating that the nanostructured Cu film affords an extremely small bubble adhesive force and underwater “superaerophobic” surface, the inset images are the corresponding underwater gas bubble contact angles of the films; b) and c) digital and schematic images showing the bubble generation behavior on planar Cu film, demonstrating that the as-formed gas bubbles adhere severely on the planar surface, the inset is the statistical releasing size of the gas bubbles (≈150 µm); e,f) digital and schematic images showing the bubble generation behavior on nanostructured Cu film, indicating the gas bubbles became much smaller and released faster, the inset is the statistical releasing size of the gas bubbles (≈20 µm).
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demonstrated that constructing highly porous nanostructured film was crucial to promote the gas evolution behavior in HzOR, and thereby might circumvent the negative effects caused by the bubble adhesion in DHFC. Promoted gas evolution behavior indeed showed a high impact on the electrocatalytic performance for HzOR, as we investigated in 3 M NaOH solution with different N2H4 concentrations. It should be noted that once the nanostructured Cu film with naturally oxidized surface was immersed in the high concentrated hydrazine solution, several gas bubbles were generated on the surface, indicating that the surface oxide was reduced by hydrazine (leading to the formation of N2 bubbles) and the electrocatalyst for HzOR was Cu(0). The polarization curve in the solution without hydrazine indicated that the copper started to be oxidized obviously at around 0.55 V versus saturated calomel electrode (SCE) (Figure S5, Supporting Information), thus the potential window for HzOR testing was limited at –1 to –0.55 V to avoid the side reaction. The catalytic performance comparison of Cu foil and nanostructured-Cu film was shown in Figure 3a (black line and red line), which indicated a boost current density enhancement for HzOR. At the same potential of –0.6 V, the Cu foil only afforded a small current (≈7.9 mA cm−2), while a ≈8.8 times higher current (≈70 mA cm−2) was observed on nanostructured Cu film. It should be noted that after normalization by measuring their electrochemical surface areas (ESAs), the HzOR current of the nanostructured Cu film was still much higher than that of the Cu substrate (Figure S6, Supporting Information), demonstrating the effectiveness of accelerated nitrogen gas bubble
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adhesion. The difference is majorly a result from the surface structures. It is observed that the flat surface showed an aerophobic property with a gas contact angle (CA) of 138.7 ± 2.8° (inset of Figure 2a), while the nanostructured film could afford a “superaerophobic” surface (a gas CA of 169.6 ± 1.3°, bubble repellence, the inset of Figure 2d). The different adhesive properties would lead to different gas releasing behaviors in gas evolution reaction. To get a quantitative comparison, a digitally recording process was performed for Cu foil and nanostructured Cu film at the same current density (≈20 mA cm−2) for hydrazine oxidation. It is observed that, for the Cu foil, the as-formed N2 bubbles became larger gradually and adhered severely to the surface strongly (Figure 2b and Movie S1, Supporting Information). The size of the gas bubbles should be big enough to overcome the high adhesion force and leave the surface. Statistics (inset figures in Figure 2b) showed that the average releasing size of the as-formed gas bubbles was around 150 µm in diameter, whereas the bubbles were generated and left quickly before they grew larger than 20 µm in diameter at the 3D nanostructured Cu film (Figure 2e and Movie S2, Supporting Information). It is also found that, in a certain given area, only one big gas bubble was found on the Cu foil, while more than 10 small gas bubbles can be observed on nanostructured Cu film, further demonstrating that the releasing size of gas bubbles was reduced (Figure 2c,f). It should be noted that the gas bubbles appeared densely and uniformly on this nanostructured surface (Figure S4, Supporting Information), indicating that more catalytically active sites were created and welldistributed by the nanoscale engineering. These results clearly
Figure 3. HzOR performance of the nanostructured Cu films. a) Polarization curves of several electrodes for HzOR, demonstrating that the nanostructured Cu films afford much faster current increases and outperformed the Cu substrate and commercial 60 wt% Pt/C catalyst; b) Nyquist plots of the Cu foil and nanostructured Cu film, the inset is the corresponding equivalent circuit, indicating the nanostructured Cu film possesses a much smaller charge-transfer resistance; c) performance optimization and average releasing sizes of the nanostructured Cu films, indicating the sample obtained at 30 min shows the optimal HzOR performance; d) stability testing of the nanostructured Cu films, demonstrating a good stability.
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Figure 4. DHFC performance of the nanostructured-Cu film: a) schematic illustration of a DHFC assembled by the “superaerophobic” Cu film as anode and commercial Pt/C film as cathode; b) the overall DHFC performances containing the anodes of nanostructured-Cu film, commercial Pt/C film and Cu foam, respectively. The DHFC assembled by the nanostructured-Cu film and Pt/C shows the best performance than the other two counterparts.
releasing behavior on the HzOR performance. Electrochemical impedance spectroscopy is usually considered as a powerful tool to study the electrode kinetics in catalytic reaction.[17] As shown in the Nyquist plots measured under the same potential (Figure 3b), semicircles were observed for both electrodes, indicating this HzOR followed a similar mechanism and was kinetically controlled.[17] The different diameters of the semicircles indicated a different charge transfer resistance (Rct in the corresponding equivalent circuit), whereas the nanostructured Cu film showed a much smaller Rct value than that of Cu foil, demonstrating an accelerated kinetics for HzOR. To exclude the possible Ni doping (see Experimental Section) effect on the HzOR performance, we prepared a Cu film with relatively smooth surface (Figure S7, Supporting Information) as comparison. Although the Ni amount (≈3%) was a little higher than that in the nanostructured Cu film (≈1%), the HzOR performance was inferior to the nanostructured Cu film (Figure S8, Supporting Information) even after the ESA normalization, firmly demonstrating the effectiveness of alleviating the bubble adhesion force on the catalytic performance. It is found that the performance of the nanostructured Cu film on Cu foil was highly dependent on the electrodeposition time. By varying the depositing time from 5 to 40 min, the HzOR performance increased gradually until the time reached 30 min and no further increase was observed by prolonging the depositing time to 40 min (Figure 3c and Figure S9, Supporting Information). This phenomenon was consistent with the SEM results (Figure S10a–d, Supporting Information), which demonstrated that the size of the Cu nanostructure would become larger with the depositing time increasing (5–30 min), thus offering a higher active surface area for HzOR. However, further extending the depositing time might decrease the surface roughness, leading to a degraded HzOR performance. The gas evolution behaviors of the nanostructured Cu films obtained above 20 min were all promoted relative to the sample obtained below 10 min in terms of the releasing sizes (Figure S10e–h, Supporting Information), further confirming the essentiality of constructing highly porous nanostructures. Commercially, 60 wt% Pt/C film fabricated by drop-casting method was also tested for HzOR, as shown the blue line in Figure 3a. Although the onset potential of the Pt/C catalyst was more negative than that of the nanostructured Cu film, the current den-
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sity increased much slower and fluctuated severely, resulting in a small and unstable HzOR current at a high potential. The inferior performance of the Pt/C film was attributed to the high adhesion force to the gas bubble (≈60 µN) and large releasing size of the as-formed bubbles (≈300 µm), as shown in Figure S11 (Supporting Information). Growing the 3D nanostructure on the macroporous Cu foam could further improve the HzOR performance. As shown, the black and red dash lines in Figure 3a, the nanostructured-Cu film achieved ≈135 mA cm−2 at –0.6 V even in a much lower hydrazine concentration (≈0.2 M, the current would be over range of the testing instrument at higher concentrations), ≈1.8 times higher to that of the pure Cu foam. This enhancement was also partially ascribed to the accelerated gas evolution behavior, as shown in Movie S3 and S4 (Supporting Information). The stability testing of the nanostructured Cu films on both Cu foil and Cu foam demonstrated that the HzOR current densities could maintain above 80% of their initial current densities for around 5000s at a constant potential (Figure 3d), revealing the prominent stability of these nanostructured electrodes. The practical application was demonstrated by testing a DHFC assembled by the nanostructured Cu film on Cu foam and commercial Pt/C catalyst as the anode and cathode, respectively (Figure 4a). Pt/C and native Cu foam-based anodes were also tested as comparisons. The polarization curves of the “superaerophobic” electrode-based DHFC at different operating temperatures for the N2H4/O2 fuel cell were shown in Figure S12 (Supporting Information). The open-circuit potential of the cell was ≈1 V, and the maximum power densities of 29.1, 70.9, 121.9, and 160.8 mW cm−2 were achieved when the cell was operated at room temperature, 313, 333, and 353 K, respectively. These values are ≈2 times higher than that of the DHFCs assembled with the commercial Pt/C catalysts (58.5 mW cm−2) and one order of magnitude higher than native Cu foam anode (14.79 mW cm−2, the detailed results can be found in Figure S13, Supporting Information), as shown in Figure 4B. Moreover, the “superaerophobic” electrode-based DHFC also outperforms to the performances of previous reported DHFCs with Cu catalysts,[12,18] and the DHFCs containing novel catalysts,[3,19] as listed in Table S1 (Supporting Information).
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Supporting Information
Experimental Section Synthesis of Nanostructured-Cu Films: The nanostructured Cu films were fabricated by one-step electrodeposition process. First, the substrates (Cu foil and Cu foam) were precleaned in diluted H2SO4 solution to remove the surface oxides. After that, the electrodeposition process was performed on the precleaned substrates in an aqueous electrolyte containing CuSO4·5H2O (0.03 M), NiSO4·6H2O (0.0025 M), NaH2PO2·H2O (0.24 M), Na3C6H5O7·2H2O (0.05 M), H3BO3 (0.50 M), and polyethylene glycol (PEG, 5 ppm) at a constant potential of –0.95 V versus SCE. The pH value of the electrolyte was adjusted at 7–9 and the temperature should be kept at 65 oC in the whole electrodeposition process. The electrodeposition time was varied at 5–40 min to investigate the optimized condition for HzOR. The mass loading of the sample obtained at 30 min was ≈3 mg cm−2. The Cu film with relatively smooth surface was fabricated by a similar method but without PEG in the electrolyte and altering the depositing potential at –0.9 V. A dropcasting method was employed to synthesize the Pt/C catalyst film with the same mass loading. Specifically, the commercial 60 wt% Pt/C catalyst was dispersed in an ethanol solution with a concentration of 6 mg mL−1. Then we drop-cast the catalyst on Cu substrate in an oven heated up to 80 °C. Characterizations: The structural information of the samples were characterized using a field-emission SEM (Zeiss SUPRA 55) operating at 20 kV and a high-resolution TEM system (JEOL 2100) operating at 200 kV. XPS spectrum was carried out by using a model of ESCALAB 250. The nitrogen bubble releasing process was recorded by a highspeed CCD camera (X-Motion, AOS Technologies) mounted on a microscope (SZ-CTC, OLYMPUS). The interaction force between the gas bubbles and electrode interfaces can be assessed by a high-sensitivity microelectromechanical balance system (Dataphysics DCAT11, Germany), and the bubble CA was measured by the captive bubble method (Dataphysics OCA20).[9] The volume of the gas bubble was
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about 2 µL for each testing. All the experiments were repeated for more than five times. Electrochemical Measurements: The HzOR performance was carried out at room temperature in a three-electrode glass cell connected to an electrochemical workstation (CHI 660D, Chenghua, Shanghai). Cyclic voltammetry and linear sweep voltammetry with scan rate of 5 mV·s−1 were conducted in 3 M NaOH solution with different hydrazine concentrations using SCE as the reference electrode. Pt wire was used as the counter electrode. AC impedance measurements were carried out in the same configuration at –0.7 V from 105–0.1 Hz with an AC voltage of 5 mV. Movie S1 and S2 in the Supporting Information were recorded when using a Cu foil and a nanostructured Cu film on Cu foil at an HzOR current density of 20 mA cm−2 in 3 M NaOH solution containing 1 M hydrazine, and Movie S3 and S4 in the Supporting Information were recorded when using a Cu foam and a nanostructured Cu film on Cu foam at an HzOR current density of 60 mA cm−2 in a 3 M NaOH solution containing 0.2 M hydrazine. In all the measurements, we used SCE as the reference. The stability testing of the nanostructured Cu films were operated at constant overpotential (–0.55 V for the nanostructured Cu film on Cu foil and –0.6 V for the nanostructured Cu film on Cu foam) for achieving high initial current densities. The potential application of the nanostructured-Cu film in DHFC was demonstrated by assembling the nanostructured Cu film as the anode and the commercial 60 wt% Pt/C film as the cathode. Nafion 115 membrane was used as the solid electrolyte. The liquid feed in the anode side was the aqueous solution containing 20% N2H4 and 4 M NaOH with a flow rate of 10 mL min−1 by a peristaltic pump and silicone tubes, and pure O2 was purged in the cathode side. The cell temperature was controlled via a temperature controller and monitored by thermocouples buried in the graphite blocks. The steady state polarization curves were recorded by an automatic Electric Load (PLZ 70UA, Japan). The pure Cu foam and commercial Pt/C film were used as the anodes for comparison purposes.
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The above results clearly manifest that constructing nanoporous architecture is of great importance for achieving high HzOR performance. The main reason is that the “superaerophobic” surface can effectively drive the gas product from the catalyst surface in a small critical size (≈20 µm), thus maximizing the real working area and keeping it constant. Besides, the uniform loading of the vertically aligned nanostructures is able to greatly enhance the porosity of the film while maintaining a high conductivity, thus resulting in a high electrochemically active surface area and utilization of the active materials. In summary, by simply electrodepositing a vertically aligned nanostructured Cu film, a “superaerophobic” electrode with low gas bubble adhesion force was achieved, resulting in a small releasing size of the gas product and fast evolution behavior. Benefit from this unique property, the highly porous nanostructured Cu film showed an extraordinary HzOR performance and potential promise in DHFC, along with other advantages of the porous nanostructures. This study exhibits the effectiveness of nanoscale engineering on HzOR performance, thus providing a facile and efficient way for solving the gas adhesion problem in DHFC. Coupling with the optimization of engineering technologies (e.g., channel construction[4] and operating parameters,[5] the performance of DHFC with “superaerophobic” electrode is believed to be maximized to serve as an efficient energy supply. Together with our previous demonstration of “superaerophobic” electrodes for hydrogen evolution, the promise of this concept to gas evolution reactions is firmly demonstrated, not only for electrochemical water splitting, but also for fuel cells.
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements Z.L. and M.S. contributed equally to this work. The authors appreciate the useful discussion with Prof. Limin Liu. This work was supported by the NSFC, the 973 Program (Grant Nos. 2011CBA00503 and 2012CB93280), the 863 Program (Grant No. 2012AA03A609), the Program for Changjiang Scholars and Innovative Research Team in University. Note: The second affiliation was updated on 1 April 2015 after original online publication. Received: January 6, 2015 Revised: February 9, 2015 Published online: February 27, 2015
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