Scalable Self-Supported Graphene Foam for High-Performance

Subscriber access provided by READING UNIV

Article

Scalable Self-Supported Graphene Foam for High Performing Electrocatalytic Oxygen Evolution Yunpei Zhu, Jingrun Ran, and Shi-Zhang Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13836 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

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

ACS Applied Materials & Interfaces

Scalable Self-Supported Graphene Foam for High Performing Electrocatalytic Oxygen Evolution Yun-Pei Zhu,1 Jingrun Ran,1 and Shi-Zhang Qiao*,1,2 1

School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia.

2

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China.

KEYWORDS: Self-supporting, Electrochemical Expansion, Graphene, Chemical Doping, Electrocatalysis.

ABSTRACT. Developing efficient electrocatalysts consisting of earth-abundant elements for oxygen evolution reaction (OER) is crucial for energy devices and technologies. Herein, we report self-supported highly porous nitrogen-doped graphene foam synthesized through the electrochemical expansion of carbon-fiber paper and subsequent nitrogen plasma treatment. A thorough characterization, such as electron microscopy and synchrotron-based near edge X-ray absorption

fine

structure,

indicates

the

well-developed

porous

structures

featuring

homogeneously doped nitrogen heteroatoms. These merits ensure enriched active sites, enlarged active surface area and improved mass/electron transport within the continuous graphene framework, thus leading to an outstanding capability towards electrocatalyzing OER in alkaline media, even competitive with the state-of-the-art noble-/transition-metal and non-metal electrocatalysts reported to date, from the perspectives of the sharp onset potential, small Tafel slope and remarkable durability. Furthermore, a rechargeable Zn-air battery with this self-

ACS Paragon Plus Environment

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

Page 2 of 25

supported electrocatalyst directly used as the air cathode renders a low charge/discharge overpotential and considerable lifespan. The finding herein suggests that a rational methodology to synthesize graphene-based materials can significantly enhance the oxygen electrocatalysis, thereby promoting the overall performance of the energy-related system.

INTRODUCTION Featuring intriguing electrical conductivity and tunable physicochemical properties, graphene has attracted tremendous attention for the potential applications in the areas of nanocatalysis, nanoelectronics and advanced devices for energy storage.1-6 As a two-dimensional material, its role in aforementioned fields are crucially dependent on the structural uniformity, which actually dominates the charge transfer capability and mechanical properties.7,8 The well-established strategies for graphene preparation include bottom-up (e.g., chemical vapor deposition) and topdown (e.g., liquid-phase expansion and ball milling) strategies,9-13 though extensive treatments such as multistep transferring processes and pre-oxidation/post-reduction are indispensable for these approaches. These can not only exacerbate the resultant structural inhomogeneity and compromise the electrochemical performance, but also pose a challenge in cost increment and environmental risks. Although the self-assembly of graphene nanosheets on the basis of relatively weak intersheet interactions (for example, van der Waals force and hydrogen bonding) into macroscopic oriented films can render interconnected graphene networks,14,15 the unfavourable resistance of these films caused by an enormous amount of intersheet junctions and poor porosity induced by closely packed nanosheets prohibit further enhancement of the apparent performance in some multiphase reactions like oxygen electrochemisitry, as these two factors can respectively influence the electronic conductivity and provide smooth pathways for efficient


2

Page 3 of 25

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


mass transport.16-18 Preparing graphene-based materials with continuous network and wellstructured pores through an effective method, however, remains challenging. Oxygen evolution reaction (OER) is a key half reaction that occurs at the oxygen electrodes or cathodes of sustainable water electrolysis devices and rechargeable metal-air batteries.19-22 To accelerate the sluggish OER reaction kinetics involving multi-electron transfer, noble metals and transition-metal-based catalysts (generally supported on conductive substrates like graphene) are usually employed,23,24 but metal-containing electrocatalysts are prone to gradual deactivation under reaction conditions, thus greatly impairing the maintenance of longterm electrocatalytic activity and reproductivity. By contrast, non-metal materials with controllable compositions (e.g., C and N) and adjustable nanostrucutures have been widely recognized as promising alternatives towards active and durable electrochemical OER.25-27 Among them, heteroatom-modified carbonaceous materials have been developed as efficient OER electrocatalysts, which is stimulated by the fact that chemical doping can impart the variation of electronic structures and/or redistribution of spin density, and consequently facilitate the oxygen evolution process.28-31 Correspondingly, a diversity of methods haven been conducted to introduce heteroatoms into the carbon frameworks, such as post-modification using dopant precursors (e.g., thermally treated ammonia gas/air) and pyrolysis of organic components consisting of heteroatom-enrich motifs (e.g., polypyrrole, polyaniline).32-34 Nevertheless, energyconsuming thermal treatment is typically needed, which leads to the deterioration of preformed structures and the formation of powdery catalysts requiring complicated post-processing before their application in electrochemical devices. Noticeably, mild plasma technique represents a simple but efficient methodology for tuning the intrinsic properties of various materials without changing the initial macro-/nanostructures (e.g., metal oxides and graphene), which is indeed


3


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

Page 4 of 25

achieved by easily controlling parameters like gas atmospheres (for instance, N2 for nitridation, O2 for oxidation, and H2 for reduction).35,36 Accordingly, the synthesis of chemically doped carbon with well-defined structures involving the plasma technique towards high-performance electrocatalysis is highly desirable. Herein self-supported N-doped graphene foam on carbon-fiber paper (denoted as NGFCFP) is designed as a high-performance oxygen electrode. The graphene foam with a continuous and porous framework prepared through electrochemical expansion of carbon fiber paper (CFP), followed by nitrogen plasma treatment to induce substantial N doping. The resultant electrocatalyst exhibits outstanding performance in catalyzing OER in terms of superior activity, favourable kinetics and excellent durability, even rivaling the precious Ir/C benchmark. EXPERIMENTAL SECTION Synthesis of Expanded Graphene Foam-Carbon Fiber Paper (GF-CFP). In a typical run, commercially available CFP (Fuel Cell store, USA) was first washed with 1 M HCl and distilled water for several times, followed by calcinating at 300°C in a muffle oven to introduce suitable defective sites (e.g., grain boundaries) on the fiber surface. The pretreated CFP adhered to a conductive carbon tape was used as the working electrode and a Pt wire was used as the counter electrode. These two electrodes were immersed in 0.1 M Na2SO4 aqueous solution, while the Na2SO4 electrolyte was bubbled with N2 for at least 30 min before use. A static low bias of 1.2 V was first applied to the CFP to wet the carbon fibers efficiently and possibly lead to the gentle intercalation of SO42– anions to the defective sites of the carbon fibers. Then the positive bias was increased to 10 V to the CFP electrode to initiate the expansion process. After completion, the treated CFP was collected and washed with distilled water and ethanol thoroughly, followed


4

Page 5 of 25

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


by drying overnight at 80°C and thermally treated at 300°C in a tube furnace under the protection of argon gas, and the final materials was denoted as GF-CFP. Synthesis of N-Doped Graphene Foam-Carbon Fiber Paper (NGF-CFP). Typically, the synthesized GF-CFP was placed in the vacuum chamber of a plasma equipment that was evacuated by rotary and roots pump. Nitrogen gas was introduced into the chamber through a mass flow controller at a pre-set flow rate of 60 sccm. Once the process pressure of the chamber was steady, capacitively coupled radio frequency discharge was created between the perforated disc shaped cathode and the grounded substrate holder inside the reactor chamber via supplying 45 W power. The GF-CFP was treated for a given period of 10 min at ambient temperature. Finally, the material was washed with distilled water and ethanol alternatively, dried overnight, and annealed at 300°C in a tube furnace under the protection of argon gas to remove residual functional groups on the surface. The resulting material was then marked as NGF-CFP. Physicochemical Characterization. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were recorded on the FEI Quanta 450 at an accelerating voltage of 10 kV and the Philips CM200 microscope at an acceleration voltage of 200 kV, respectively. X-ray diffraction (XRD) patterns were collected on a powder X-ray diffractometer at 40 kV and 15 mA using Co-Kα radiation (Miniflex, Rigaku). X-ray photoelectron spectra (XPS) were obtained using an Axis Ultra (Kratos Analytical, UK) XPS spectrometer equipped with an Al Kα source (1486.6 eV). The NEXAFS measurements were conducted in an ultrahighvacuum chamber (~10−10 mbar) of the undulator soft X-ray spectroscopy beamline at the Australian Synchrotron, and the XANES spectra were recorded in the surface sensitive total electron yield with use of specimen current. The WiTEC alpha300R Raman microscope equipped with a 532 nm solid laser was used to obtain the Raman spectra, while Fourier


5


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

Page 6 of 25

transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 spectrometer, and the detection range of spectrograms was 4000 to 400 cm‒1. The contact angle of the synthesized materials was determined on a Theta/Attension Optical Tensiometer. N2 adsorption-desorption isotherm were collected on a Quantachrome Autosorb-1 physical adsorption analyzer at 77 K. All samples were degassed at 453 K overnight prior to the adsorption-desorption tests. As such, pore size distribution curves were determined on the basis the adsorption branch of the isotherms by non-local density functional theory (NLDFT) method; according to the multi-point BrunauerEmmett-Teller (BET) model, the adsorption data in the isotherm at the relative pressure of P/P0 = 0.05‒0.30 were used to calculate the specific surface areas. Electrochemical Testing. All measurements were conducted in a classical three-electrode glass cell with an Ag/AgCl in 4 M KCl solution and a platinum wire utilized as the reference electrode and the counter electrode, respectively. The detected potentials versus the Ag/AgCl electrode were converted to potentials versus reversible hydrogen electrode (RHE) (ERHE = EAg/AgCl + 0.0591 × pH + 0.205). A flow of O2 was bubbled through the alkaline electrolyte (0.1 M KOH) during the course of electrochemical testing. The linear sweep voltammogram (LSV) measurements compensated by iR drop were conducted at a slow sweeping rate of 2 mV s‒1, and the cycling stability test was carried out through accelerated cyclic voltammogram (CV) scanning at a scan rate of 100 mV s‒1 for 5000 cycles and chronoamperometric response recorded in O2-saturated 0.1 M KOH solution was used to further investigate the long-term durability. Polarization curves before and after the accelerated CV testing were collected for comparison. Electrochemical impedance spectrum (EIS) tests were carried out using an AC voltage with an amplitude of 5 mV in a frequency range from 10‒1 to 105 Hz. In addition, the electrochemical active surface areas of the synthesized carbon electrocatalysts were evaluated by


6

Page 7 of 25

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


measuring their electrochemical double layer capacitances (Cdl). The Cdl value is regarded to be of linearly proportional relationship to the electrochemically active surface area of the electrodes, wherein a potential range of 1.05–1.15 V featuring no apparent Faradic current in this region for each electrocatalyst was selected for the capacitance measurements. The detailed operation methods of rotating ring-disk electrode (RRDE) techniques to determine the reaction mechanisms and the assembly of Zn-air battery prototypes can be found in Supporting Information. RESULTS AND DISCUSSION The synthesis of NGF-CFP involves a two-step process, namely, electrochemical expansion and chemical doping, as depicted in Figure 1. Featuring a continuous graphitic network, remarkable electron conductivity and fascinating mechanical stability, commercial CFP was used as the working electrode for direct electrochemical expansion with the Na2SO4 aqueous solution employed as the electrolyte. The expansion process was initiated after applying a high bias voltage, together with the evolution of a large amount of bubbles on CFP. The high potential could result in the depolarization and expansion of the graphitic layers through the defect sites (for example, grain boundaries and edges sites) of CFP, thus facilitating the intercalation of guest species (e.g., H2O and SO42–) into the graphite interlayer spacings (Figure S1). Succedent reduction of sulphate anions and self-oxidation of water generated gaseous products like SO2 and O2,37,38 along with vigorous gas production during the electrochemical treatment. These gas products could impart suitable force to moderately swell and push the weakly bonded graphite layers apart15,39 and the escape of gaseous species along the exfoliated layers could create abundant porosity. The expansion of CFP can be first revealed by SEM image (Figure S2), presenting uniform pores on the exfoliated surface of the carbon fibers. After the electrochemical


7


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

Page 8 of 25

process, the exfoliated CFP was treated with nitrogen plasma to realize the incorporation of nitrogen dopants into the carbon backbone. After the completion of expansion and chemical modification, NGF-CFP can well retain the fibrous structure of the pristine CFP that features a 3D continuous network constructed by orderly interleaved carbon fibers (SEM images, Figure 2a, S3 and S4), as well as the high mechanical flexibility (inset in Figure 2a). Higher magnification SEM images (Figure 2b,c) demonstrate that the surface of NGF-CFP becomes rather rough with vesicle-like open pores uniformly distributed through the nearly transparent graphene layers. The diameters of these pores range from tens to hundreds of nanometers. TEM images further disclose the thin nature of the exfoliated surface of NGF-CFP (Figure 2d, S4) and its highly porosity (inset in Figure 2d). Indeed, the fascinating pore characteristics are expected to afford a large surface area. According to the nitrogen adsorption-desorption isotherm (Figure S5), GF-CFP shows a high surface area of 241 m2 g–1, which reveals the positive effect of electrochemical treatment in improving porosity. N2 sorption analysis reveals the high specific surface area (248 m2 g–1) of NGF-CFP (Figure 3a), together with a broad pore size distribution centered at approximately 10 nm. The similar surface area of NGF-CFP to that of GF-CFP suggests the negligible influence of the plasma treatment in the porous structures. The detected in good consistency with the enriched pores observed by electronic microscopy imaging. The successful incorporation of N into the graphene framework and the corresponding chemical state can be identified by synchrotronbased near-edge X-ray adsorption fine structure spectroscopy (NEXAFS) (Figure S6) and X-ray photoelectron spectroscopy (XPS) (Figure S7). The C K-edge NEXAFS spectrum of NGF-CFP (Figure 3b) displays typical sp2-hybridized carbon structures with the resonances of π* and σ* situated at 285.4 and 293.1 eV, related to transitions to specific unoccupied states like anti-


8

Page 9 of 25

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


bonding π* and σ* states, respectively,40 along with a resonance at 288.6 eV assignable to putative interlayer state.41 As for the N K-edge region (Figure 3c), two π* resonances situated at 398.6 and 401.5 eV can be assigned to the C–N species in the form of pyridinic and graphitic structures, respectively, while a shoulder resonance around 405.5 eV can be attributable to N–O species. The NEXAFS results coincide well with the N 1s XPS spectrum (Figure 3d), the deconvolution of which indicates the existence of pyridinic N (398.5 eV), graphitic N (401.1 eV), and N–O species (404.6 eV), and the corresponding N content is determined to be 1.56 at%.16,17 In the Raman spectra (Figure S8), a higher ratio of D band (1345 cm–1, imperfections in sp2-C structures) to G band (1588 cm–1, graphitic C atoms) for NGF-CFP than that for exfoliated CFP without plasma treatment (denoted as GF-CFP) suggests the formation of defects after plasma treatment.14,35 The slight shifting of G band for NGF-CFP renders additional evidence of nitrogen doping effect.14,17 The sharp signal of G band illustrates the well-defined graphitization, as further verified by XRD pattern (Figure S9) and FT-IR spectra (Figure S10). Flexibility and structural integrity of NGF-CFP make it feasible for the direct use as the working electrode without any binders or extra substrates in a three-electrode model (Figure S11). According to LSV polarizatioin curves (Figure 4a), the pristine CFP shows negligible electroactivity towards OER, whereas GF-CFP and N-CFP (N2-plasma-treated pristine CFP with expansion process, see the synthesis method in Supporting Information) renders enhanced performance in terms of onset potential and anodic current output, indicating that the electrochemical expansion and plasma treatment are beneficial for the enhancement of oxygen evolution activity. In contrast, the anodic current signal recorded on NGF-CFP delivers a sharp onset potential of 1.50 V with largely enhanced OER current after further scanning to positive potential, implying the positive role of N dopants in elevating the electrochemical activity. Noble


9


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

Page 10 of 25

Ir/C benchmark exhibits an earlier onset potential of 1.48 V than NGF-CFP, though the current density supplied by NGF-CFP surpasses that by Ir/C at potentials exceeding 1.61 V. Furthermore, NGF-CFP requires a potential of 1.61 V to afford a current density of 10.0 mA cm– 2

, nearly equal to that of Ir/C (1.61 V) and comparable or even superior to previously reported

state-of-the-art metallic OER electrocatalysts, such as RuO2 electrocatalysts (1.73 V),42 Ti3C2-g– C3N4 hybrid film (1.65 V),16 and Fe,N,S-decorated hierarchical carbon layers (1.60 V),43 and non-metallic electrocatalysts with the similar electrode configuration (for example, N-doped graphene-carbon nanotube film,44 1.70 V; P-g–C3N4 grown on CFP,45 1.63 V). A detailed comparison of diverse electroactive OER catalysts with different compositions and electrode structures is summarized in Table S1, revealing the superiority of NGF-CFP over the majority of efficient electrocatalysts developed so far. To evaluate the OER reaction kinetics, the linear fitting of the Tafel plot of NGF-CFP affords a Tafel slope of 64.8 mV dec–1 (Figure 4b), obviously smaller than that of Ir/C (71.2 mV dec–1), N-free GF-CFP counterpart (117.5 mV dec–1) and N-CFP (134.7 mV dec–1), as well as competing with previous high-efficiency OER electrocatalysts (Table S1), signifying that electrochemical expansion and plasma treatment play positive roles in promoting the reaction kinetics process occurred on NGF-CFP. Moreover, by conducting the RRDE to examine the reaction mechanism (Figure S12), the electrocatalytic oxygen evolution initiated on NGF-CFP is found to follow a four-electron (4e–) dominated pathway with insignificant generation of peroxide intermediates and a considerable Faradaic efficiency over 95% (Figure 4c). In addition to high reaction activity and selectivity, the chronoamperometric response displays the remarkable durability of NGF-CFP towards electrochemical OER (Figure 4d), demonstrating a slight current decay of 4.8% without morphology variation after a constant


10

Page 11 of 25

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


operation of 16 h (Figure S13), while an obvious activity attenuation of 28.9% can be observed on Ir/C. With regard to the chronopotentiometric response (Figure S14), NGF-CFP delivers a constant 10.0 mA cm–2 current density around 1.61 V, whereas the potential for the Ir/C electrode gradually increases to maintain the same current output. Also, after 5000 accelerated cyclic voltammogram cycles, there is seldom activity diminishment for NGF-CFP, in sharp contrast to the drastic shift of polarization curves recorded on Ir/C (Figure 4d inset). These stability measurements suggest the strong capability of NGF-CFP to withstand long-term and accelerated electrochemical processes, benefiting from superiority of the robust electrode configuration over the powdery electrocatalyts postcasted on conductive matrixes, as the latter suffer from detachment of electroactive components owing to the evolution of a great deal of oxygen gas.43,44 Further tests confirm that NGF-CFP can perform well in concentrated electrolytes (e.g., 1.0 M KOH solution) in the aspects of reaction activity and stability (Figure S15), ensuring the potential application in energy-related devices that need concentrated media. Indeed, the N doping as well as the open porous structure of NGF-CFP can assure accessible active centers and a large active surface area during electrocatalytic OER, which can be assessed by the electrochemical double-layer capacitance (Cdl). By calculating the slope of the plots of current density versus scan rate (Figure S16), Cdl of NGF-CFP is determined to be 15.8 mF cm–2, higher than that of dopant-free GF-CFP (9.5 mF cm–2). This illustrates the positive role of N heteroatoms in increasing electrocatalytically active surface area; thereby, the effective exposure and improved utilization of electroactive sites on the large active surface area of porous NGF-CFP can make a significant contribution to the impressive activity towards oxygen evolution. Chemical doping is efficient to modify the electronic structure of carbon framework, consequently causing an improved charge transfer ability.14,17,24 In the EIS spectrum, the


11


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

Page 12 of 25

semicircular diameter of NGF-CFP in the high-frequency region is apparently smaller than that of GF-CFP (Figure 5a), due to the lower contact and charge-transfer impedance within NGFCFP, corresponding to the favourable charge transfer kinetics. Importantly, the OER reaction kinetics is further boosted by the advanced electrode architecture. A smooth pathway for rapid penetration of the electrolyte can be achieved through the abundant porous channels, as well as the easy access of reactants (e.g., OH–) in the reaction media to the electroactive sites within the electrode and instant release of products (i.e., O2). Additionally, the current density in the water oxidation potential range shows little susceptibility to the scan rate (Figure S17), owing to the superior porosity that ensures accelerated mass/electron transfer within the porous N-doped graphene network and thus the high-rate performance. As such, NGF-CFP presents a Tafel slope even lower than that of precious Ir/C, indicating the more favourable reaction kinetics for NGF-CFP. The well-connected conductive network of NGF-CFP realized by the direct expansion of the CFP substrate can ensure the overall structural continuity during OER, as confirmed by the relatively poor performance of the control electrode that was prepared by postcoating NG onto CFP using polymeric binders. This control electrode affords an inferior OER performance relative to NGF-CFP from the activity and stability points of view (Figure 5b and S18). NGF-CFP is highly hydrophilic with a small contact angle of 18.5º (Figure 5c and S19), because of the existence of hydrophilic functional groups like surface hydroxyls and Ncontaining motifs. The hydrophilic nature can facilitate the infiltration and access of the electrolyte to the active sites of the self-supported electrode, holding the promise for the application in sustainable energy conversion/storage devices using aqueous medium. As a proofof-concept, a rechargeable Zn-air battery was constructed with NGF-CFP used as the air cathode


12

Page 13 of 25

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


to demonstrate its feasibility in real energy-related devices (Figure 5d). In the charge/discharge polarization profiles of the assembled Zn-air batteries (Figure S20), NGF-CFP can deliver a current density of 10 mA cm–2 at 2.16 V and 1.12 V for charging (OER) and discharging (oxygen reduction reaction, ORR), respectively. The performance of NGF-CFP greatly outperforms that of noble Ir/C cathode prepared by postcoating onto the CFP (2.18 V for charging and 1.07 V for discharging). This agrees well with the trend of the polarization curves derived by utilizing the three-electrode system, wherein NGF-CFP can not only efficiently electrocatalyze OER, but also reversibly catalyze ORR through a desirable 4e–-transfer mechanism (Figure 5e inset and S21). Remarkably enough, NGF-CFP exhibits no appreciable voltage fluctuation for over 20 charging/discharging pulse cycles, whereas a palpable overpotential increment for both charging and discharging processes can be seen after 30 cycles for Ir/C (Figure 5e and S22). The performance attenuation of the Ir/C benchmark electrode can be related to the instability of the noble metallic components and the deterioration of the organic binders employed for powder casting. CONCLUSIONS In summary, N-doped graphene foam is rationally prepared through electrochemical expansion of carbon-fiber paper, followed by a mild but efficient nitrogen plasma process to introduce N dopants. The resultant continuous network ensures the direct use as a superior binder-free oxygen electrode with high activity and robust durability, even competing noble metal benchmark. The reaction kinetics can be remarkably improved as revealed by the small Tafel slope, which is intimately associated with the outstanding structural advantages like 3D conductive framework, well-defined porosity and substantial chemical doping. Considering the easy feasibility of the present synthesis strategy, this work will provide a prospective way to


13


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

Page 14 of 25

synthesize other 3D materials like chemically modified carbon-metal composites with promising synergistic effect, creating new platforms to increase the efficiency of renewable energy systems even further. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional data as mentioned in the text; Figures S1−S12 and Table S1 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We would like to acknowledge the financial support from the Australian Research Council (ARC) through the Discovery Project and Linkage Project programs (DP140104062, DP160104866, DP170104464 and LP160100927) and the Natural Science Foundation of China (21576202). NEXAFS measurements were conducted on the soft X-ray beamline at Australian Synchrotron.


14

Page 15 of 25

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


REFERENCES (1)

Parviz, D.; Irin, F.; Shah, S. S.; Das, S.; Sweeney, C. B.; Green, M. J. Challenges in Liquid-Phase Exfoliation, Processing, and Assembly of Pristine Graphene. Adv. Mater. 2016, 28, 8796−8818.

(2)

Georgakilas, V.; Tiwari, J. N.; Kemp, K. C.; Perman, J. A.; Bourlinos, A. B.; Kim, K. S.; Zboril, R. Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications. Chem. Rev. 2016, 116, 5464−5519.

(3)

Liu, W.; Song, M. S.; Kong, B.; Cui, Y. Flexible and Stretchable Energy Storage: Recent Advances and Future Perspectives. Adv. Mater. 2017, 29, 1603436.

(4)

Wan, J.; Lacey, S. D.; Dai, J.; Bao, W.; Fuhrer, M. S.; Hu, L. Tuning Two-Dimensional Nanomaterials by Intercalation: Materials, Properties and Applications. Chem. Soc. Rev. 2016, 45, 6742−6765.

(5)

Eigler, S.; Hirsch, A. Chemistry with Graphene and Graphene Oxide-Challenges for Synthetic Chemists. Angew. Chem. Int. Ed. 2014, 53, 7720−7738.

(6)

Cong, L.; Xie, H.; Li, J. Hierarchical Structures Based on Two-Dimensional Nanomaterials for Rechargeable Lithium Batteries. Adv. Energy Mater. 2017, 7, 1601906.

(7)

Buzaglo, M.; Bar, I. P.; Varenik, M.; Shunak, L.; Pevzner, S.; Regev, O. Graphite-toGraphene: Total Conversion. Adv. Mater. 2017, 29, 1603528.

(8)

Dong, L.; Chen, Z.; Lin, S.; Wang, K.; Ma, C.; Lu, H. High Mobility, Printable, and Solution-Processed Graphene Electronics. Chem. Mater. 2017, 29, 564−572.

(9)

Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of HighQuality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314.

(10)

Baláž, P.; Achimovičová, M.; Baláž, M.; Billik, P.; Cherkezova-Zheleva, Z.; Criado, J. M.; Delogu, F.; Dutková, E.; Gaffet, E.; Gotor, F. J.; Kumar, R.; Mitov, I.; Rojac, T.;


15


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

Page 16 of 25

Senna, M.; Streletskii A.; Wieczorek-Ciurowa, K. Hallmarks of Mechanochemistry: From Nanoparticles to Technology. Chem. Soc. Rev. 2013, 42, 7571−7637. (11)

Paton, K. R.; Varrla, E.; Backes, C.; Smith, R. J.; Khan, U.; O’Neill, A.; Boland, C.; Lotya, M.; Istrate, O. M. ; King, P.; Higgins, T.; Barwich, S.; May, P.; Puczkarski, P.; Ahmed, I.; Moebius, M.; Pettersson, H.; Long, E.; Coelho, J.; O’Brien, S. E.; McGuire, E. K.; Sanchez, B. M.; Duesberg, G. S.; McEvoy, N.; Pennycook, T. J.; Downing, C.; Crossley, A.; Nicolosi, V.; Coleman, J. N. Scalable Production of Large Quantities of Defect-Free Few-Layer Graphene by Shear Exfoliation in Liquids. Nat. Mater. 2014, 13, 624−630.

(12)

Garg, R.; Elmas, S.; Nann, T.; Andersson, M. R. Deposition Methods of Graphene as Electrode Material for Organic Solar Cells. Adv. Energy Mater. 2017, 7, 1601393.

(13)

Rogers, C.; Perkins, W. S.; Veber, G.; Williams, T. E.; Cloke, R. R.; Fischer, F. R. Synergistic Enhancement of Electrocatalytic CO2 Reduction with Gold Nanoparticles Embedded in Functional Graphene Nanoribbon Composite Electrodes. J. Am. Chem. Soc. 2017, 139, 4052–4061.

(14)

Zhu, Y. P.; Guo, C.; Zheng, Y.; Qiao, S. Z. Surface and Interface Engineering of NobleMetal-Free Electrocatalysts for Efficient Energy Conversion Processes. Acc. Chem. Res. 2017, 50, 915−923.

(15)

Parvez, K.; Wu, Z. S.; Li, R.; Liu, X.; Graf, R.; Feng, X.; Müllen, K. Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts. J. Am. Chem. Soc. 2014, 136, 6083−6091.

(16)

Ma, T. Y.; Cao, J. L.; Jaroniec, M.; Qiao, S. Z. Interacting Carbon Nitride and Titanium Carbide Nanosheets for High-Performance Oxygen Evolution. Angew. Chem. Int. Ed. 2016, 55, 1138−1142.

(17)

Pei, Z.; Li, H.; Huang, Y.; Xue, Q.; Huang, Y.; Zhu, M.; Wang, Z.; Zhi, C. Texturing in situ: N,S-Enriched Hierarchically Porous Carbon as A Highly Active Reversible Oxygen Electrocatalyst. Energy Environ. Sci. 2017, 10, 742−749.

(18)

Wang, H.; Min, S.; Ma, C.; Liu, Z.; Zhang, W.; Wang, Q.; Li, D.; Li, Y.; Turner, S.; Han, Y.; Zhu, H.; Abou-hamad, E.; Hedhili, M. N.; Pan, J.; Yu, W.; Huang, K. W.; Li, L. J.;


16

Page 17 of 25

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


Yuan, J.; Antonietti, M.; Wu, T. Synthesis of Single-Crystal-Like nanoporous carbon membranes and their application in overall water splitting. Nat. Commun. 2017, 8, 13592. (19)

Shinde, S. S.; Lee, C. H.; Sami, A.; Kim, D. H.; Lee, S. U.; Lee, J. H. Scalable 3-D Carbon Nitride Sponge as an Efficient Metal-Free Bifunctional Oxygen Electrocatalyst for Rechargeable Zn–Air Batteries. ACS Nano 2017, 11, 347−357.

(20)

Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I. B.; Norskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, 146.

(21)

Tahir, M.; Pan, L.; Idrees, F.; Zhang, X.; Wang, L.; Zou, J. J.; Wang, Z. L. Electrocatalytic Oxygen Evolution Reaction for Energy Conversion and Storage: A Comprehensive Review. Nano Energy 2017, 37, 136−157.

(22)

Spöri, C.; Kwan, J. T. H.; Bonakdarpour, A.; Wilkinson, D. P.; Strasser, P. The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation. Angew. Chem. Int. Ed. 2017, 56, 5994–6021.

(23)

Reier, T.; Nong, H. N.; Teschner, D.; Schlogl, R.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction in Acidic Environments – Reaction Mechanisms and Catalysts. Adv. Energy Mater. 2017, 7, 1601275.

(24)

Wang, Z. L.; Xu, D.; Xu, J. J.; Zhang, X. B. Oxygen Electrocatalysts in Metal–Air Batteries: from Aqueous to Nonaqueous Electrolytes. Chem. Soc. Rev. 2014, 43, 7746– 7786.

(25)

Kale, V. S.; Sim, U.; Yang, J.; Jin, K.; Chae, S. I.; Chang, W. J.; Sinha, A. K.; Ha, H.; Hwang, C. C.; An, J. ; Hong, H. K.; Lee, Z.; Nam, K. T.; Hyeon, T. Sulfur-Modified Graphitic Carbon Nitride Nanostructures as an Efficient Electrocatalyst for Water Oxidation. Small 2017, 13, 1603893.

(26)

Li, J. C.; Hou, P. X.; Zhao, S. Y.; Liu, C.; Tang, D. M.; Cheng, M.; Zhang, F.; Cheng, T. A 3D Bi-Functional Porous N-Doped Carbon Microtube Sponge Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Energy Environ. Sci. 2016, 9, 3079−3084.


17


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

(27)

Page 18 of 25

Zhao, Z.; Li, M.; Zhang, L.; Dai, L.; Xia, Z. Design Principles for Heteroatom-Doped Carbon Nanomaterials as Highly Efficient Catalysts for Fuel Cells and Metal–Air Batteries. Adv. Mater. 2015, 27, 6834−6840.

(28)

Zhu, Y. P.; Jing, Y.; Vasileff, A.; Heine, T.; Qiao, S. Z. 3D Synergistically Active Carbon Nanofibers for Improved Oxygen Evolution. Adv. Energy Mater. 2017, 7, 1602928.

(29)

Hou, Y.; Qiu, M.; Zhang, T.; Ma, J.; Liu, S.; Zhuang, X.; Yuan, C.; Feng, X. Efficient Electrochemical and Photoelectrochemical Water Splitting by a 3D Nanostructured Carbon Supported on Flexible Exfoliated Graphene Foil. Adv. Mater. 2017, 29, 1604480.

(30)

Tang, C.; Zhang, Q. Nanocarbon for Oxygen Reduction Electrocatalysis: Dopants, Edges, and Defects Adv. Mater. 2017, 29, 1604103.

(31)

Wu, J.; Rodrigues, M. T. F.; Vajtai, R.; Ajayan, P. M. Tuning the Electrochemical Reactivity of Boron- and Nitrogen-Substituted Graphene. Adv. Mater. 2016, 28, 6239– 6246.

(32)

Xu, Y.; Kraft, M.; Xu, R. Metal-free Carbonaceous Electrocatalysts and Photocatalysts for Water Splitting. Chem. Soc. Rev. 2016, 45, 3039−3052.

(33)

Hu, C.; Dai, L. Carbon-Based Metal-Free Catalysts for Electrocatalysis beyond the ORR. Angew. Chem. Int. Ed. 2016, 55, 11736−11758.

(34)

Shao, M. H.; Chang, Q. W.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction Chem. Rev. 2016, 116, 3594–3657.

(35)

Jeong, H. M.; Lee, J. W.; Shin, W. H.; Choi, Y. J.; Shin, H. J.; Kang, J. K.; Choi, J. W. Nitrogen-Doped Graphene for High-Performance Ultracapacitors and the Importance of Nitrogen-Doped Sites at Basal Planes. Nano Lett. 2011, 11, 2472−2477.

(36)

Wang, Y.; Shao, Y.; Matson, D. W.; Li, J.; Lin, Y. Nitrogen-doped graphene and its application in electrochemical biosensing. ACS Nano 2010, 4, 1790−1798.

(37)

Beck, F.; Jiang, J.; Krohn, H. Potential Oscillations During Galvanostatic Overoxidation of Graphite in Aqueous Sulphuric Acids. J. Electroanal. Chem. 1995, 389, 161−165.


18

Page 19 of 25

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


(38)

Beck, F.; Junge, H.; Krohn, H. Graphite Intercalation Compounds as Positive Electrodes in Galvanic Cells. Electrochim. Acta 1981, 26, 799−809.

(39)

Goss, C. A.; Brumfield, J. C.; Irene, E. A.; Murray, R. W. Imaging the Incipient Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite. Anal. Chem. 1993, 65, 1378−1389.

(40)

Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Jaroniec, M.; Qiao, S. Z. High Electrocatalytic Hydrogen Evolution Activity of an Anomalous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 16174−16181.

(41)

Schultz, B. J.; Patridge1, C. J.; Lee, V.; Jaye, C.; Lysaght, P. S.; Smith, C.; Barnett, J.; Fischer, D. A.; Prendergast, D.; Banerjee, S. Imaging Local Electronic Corrugations and Doped Regions in Graphene. Nat. Commun. 2011, 2, 372.

(42)

Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399−404.

(43)

Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y. Atomically Dispersed Iron–Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions. Angew. Chem. Int. Ed. 2017, 56, 610−614.

(44)

Chen, S.; Duan, J. J.; Jaroniec, M.; Qiao, S. Z. Nitrogen and Oxygen Dual-Doped Carbon Hydrogel Film as a Substrate-Free Electrode for Highly Efficient Oxygen Evolution Reaction. Adv. Mater. 2014, 26, 2925−2930.

(45)

Ma, T. Y.; Ran, J.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Phosphorus-Doped Graphitic Carbon Nitrides Grown In Situ on Carbon-Fiber Paper: Flexible and Reversible Oxygen Electrodes. Angew. Chem. Int. Ed. 2015, 54, 4646−4650.


19


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

Page 20 of 25

Figure 1. Schematic presentation for synthesizing NGF-CFP through a facile two-step route, i.e., electrochemical expansion and nitrogen plasma treatment.


20

Page 21 of 25

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


Figure 2. (a) Low- and (b,c) high-resolution SEM images of NGF-CFP. Inset of panel (a) displays the photograph of flexible NGF-CFP. (d) TEM image of N-doped graphene collected from NGF-CFP (inset: low magnification TEM image shows the enriched porosity).


21


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

Page 22 of 25

Figure 3. (a) N2 adsorption-desorption isotherm of NGF-CFP (inset: the corresponding pore size distribution curve). (b) C K-edge and (c) N K-edge NEXAFS spectra of NGF-CFP. (d) Highresolution XPS spectra of N 1s core level in NGF-CFP.


22

Page 23 of 25

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


Figure 4. (a) LSV polarization curves and (b) Tafel plots of NGF-CFP, Ir/C, GF-CFP and CFP in the O2-saturated 0.1 M KOH. Inset of panel (a) manifests the optical image of NGF-CFP directly employed as the working electrode at 1.70 V with oxygen bubbles accumulated on the electrode surface. (c) Ring current recorded using an RRDE setup with the ring potential set at 0.4 V. Inset of panel (c) shows the corresponding Faradaic efficiency profiles. (d) Chronoamperometric curves of NGF-CFP and Ir/C at a constant potential of 1.61 V (inset: OER LSV polarization curves before and after cycling testing).


23


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

Page 24 of 25

Figure 5. (a) EIS spectrum of NGF-CFP, GF-CFP, CFP and Ir/C. (b) LSV curves and inset in (b) shows the Tafel plots of NGF-CFP and the control electrode of scratched N-doped graphene postcoated on CFP using organic binders. (c) Contact angle of the NGF-CFP materials. (d) Configuration illustration of a rechargeable Zn-air battery. (e) Charge-discharge cycling curves of the Zn-air batteries using NGF-CFP and Ir/C as the cathodes, respectively. Inset of panel (e) shows the polarization curves in the whole ORR and OER region using the three-electrode model (O2-saturated 0.1 M KOH).


24

Page 25 of 25

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


TOC Graphics


25

Scalable Self-Supported Graphene Foam for High-Performance

Recommend Documents