Decorating g-C3N4 Nanosheets with Ti3C2 MXene Nanoparticles for

Subscriber access provided by UNIV OF BARCELONA

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Decorating g-C3N4 nanosheets with Ti3C2 Mxene nanoparticles for efficient oxygen reduction reaction Xuelian Yu, Wenchao Yin, Tao Wang, and Yihe Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03456 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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 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 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.

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

Langmuir

Decorating g-C3N4 nanosheets with Ti3C2 Mxene nanoparticles for efficient oxygen reduction reaction Xuelian Yu*, Wenchao Yin, Tao Wang, Yihe Zhang Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, 100083, Beijing, China. Corresponding Author *Xuelian Yu. E-mail address: [email protected] Abstract One of the major challenges associated with fuel cells is exploring highly efficient and low-cost electrocatalysts for the oxygen reduction reaction (ORR). Herein, the feasibility of using Ti3C2 MXene nanoparticles (NPs) to enhance the electrocatalytic activity of g-C3N4 for ORR was investigated. By varying the content of Ti3C2 NPs, series of g-C3N4/Ti3C2 heterostructures were obtained, displaying enhanced electrocatalytic activity, including positive shift in both onset and peak potentials towards ORR compared to the original g-C3N4 in basic solution. We attribute the improvement to the favorable electrical conductivity of well-dispersed Ti3C2 Mxene nanoparticles and also enhanced O2 adsorption due to the electronic coupling effect between g-C3N4 and Ti3C2 in the heterostrucutres. This work demonstrates the potential of earth-abundant MXene family materials to construct low-cost and high performance electrocatalysts.

1

ACS Paragon Plus Environment

Langmuir 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

Introduction Currently, the deteriorated global energy problem has largely stimulated the development of new energy. Among them, fuel cells have demonstrated large potential owing to their environmental friendliness and high reliability.[1-3] The oxygen reduction reaction (ORR) that occurs on the cathode of fuel cells is a limiting step and it severely depends on the low abundance and high cost platinum(Pt)-based materials as electrocatalysts.[4] In addition, Pt-based electrocatalysts suffer from the time-dependent drift and CO deactivation problems.[5] Therefore, looking for the inexpensive and highly active electrocatalyst to replace Pt is of paramount significance. Nowadays, a wide range of noble metal-free electrocatalysts have been explored for ORR, such as heteroatom (N, B, S, and P)-doped carbon materials, transition metal oxides, chalcogenides and nitrides.[6-9] Among them, the N-doped carbon nanotube was reported with a desirable ORR electrocatalytic activity for ORR in alkaline media due to the promoted oxygen adsorption and facilitated reduction induced by the heteroatoms.[10] In this regard, graphitic-carbon nitride (g-C3N4), the metal-free and high nitrogen-doped carbon, can be identified as one of the most potential substitutes for Pt as ORR electrocatalysts owing to its convenient preparation methods, nontoxicity and excellent stability.[11-12] However, the single-component g-C3N4 as an electrocatalyst for ORR usually exhibits low activity due to its poor electrical conductivity, which hinders the transformation of adsorbed oxygen toward reduction reaction.[13-14] Various studies have been conducted, such as incorporation of conductive supports, 2


Page 2 of 25

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

Langmuir

doping, structure engineering, and noble metal decoration, to facilitate the ORR with 4-electron reduction and improve the activity of the catalyst.[15-16] For example, Qu reported a one-step hydrothermal pathway to synthesize graphene quantum dots decorated sulfur-doped g-C3N4 and remarkably enhanced catalytic activity was achieved as compared to the original g-C3N4.[17] Zhi has examined the effects of component dopants on the catalytic performance of graphitic C3N4-based electrocatalysts to render the metal-free composites as competent catalysts rivaling the metallic counterparts.[18] MXene, a new family of two-dimensional (2D) metal carbides, nitrides and carbonitrides, has triggered great research interest because of its superior electrical conductivity and high surface area.[19] Among them, Ti3C2 are the most studied MXene, which can be readily fabricated by selectively etching Al from the Ti3AlC2 phase with HF.[20,21] Due to their excellent electrical conductivity, chemical stability and hydrophilicity, Ti3C2 Mxene is highly anticipated as a promising material for energy storage, electrocatalysis and photocatalysis.[22,23] Many applications of Ti3C2 have been focused on photoelectrocatalytic reduction of CO2, nitrogen photofixation and water purification, lithium-ion batteries and supercapacitors etc.[24-29] Considering the above outstanding properties of the MXene family, it is anticipated that coupling g-C3N4 nanosheets and Ti3C2 nanoparticles will be a promising material to be employed in electrocatalysis by taking advantage of high electronic conductivity of Ti3C2 nanoparticles and the interaction between them. In this work, Ti3C2 MXene nanoparticles were fabricated and used to construct 3


Langmuir 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

g-C3N4/Ti3C2 heterostructured electrocatalyst by interface electrostatic interaction. At this process, both the N2 atmosphere protection and capping effect of g-C3N4 nanosheets can restrict the surface oxidation of Mxene. The generated layered g-C3N4 can not only act as matrix for the growth of Ti3C2 nanoparticles, but also ensure the good dispersibility and inhibit their aggregation. Due to the strong interfacial interactions and improved charge separation, g-C3N4/Ti3C2 heterostructures exhibited significantly improved electrocatalytic behavior for the ORR in 0.1 M KOH solution. Moreover, the prepared catalyst also demonstrated enhanced tolerance against methanol and a significant improvement in terms of stability in comparison to the stare-of-the-art Pt/C catalyst. Experimental Synthesis of g-C3N4 nanosheets: A certain amount of melamine was put in a crucible with a cover and heated at 550 °C for 4 h in a muffle furnace with a heating rate of 5 °C min-1. After cooled naturally to room temperature, the resultant yellow solid was ground into powder for further use. Synthesis of Ti3C2 NPs: Firstly, the Al species in Ti3AlC2 (Aladdin Reagent, China) were etched by 49 % HF at 60 ºC for 20 hours, and the obtained sample was washed with de-ionized water for six times to get the layered-structure Ti3C2. Then 100 mg layered-structure Ti3C2 was added into 30 mL of de-ionized water and subjected to ultrasonic cell pulverization for 7 hours under the protection of N2 atmosphere, followed by centrifugation at 8000 rpm for 5 minutes.[30] After removal of the precipitates, a homogeneous dispersion of Ti3C2 NPs in the supernatant was obtained. 4


Page 4 of 25

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

Langmuir

The concentration of the supernatant was about 1.0 mg mL–1 by drying and weighing the sediments in the centrifugal tube. Synthesis of g-C3N4/Ti3C2 heterostructures: In a typical synthetic process, 50 mg of g-C3N4 nanosheets were added to 30 mL de-ionized water and kept sonication for 1 h at room temperature. Then, a certain amount of Ti3C2 NPs in aqueous solution was added into the above solution under vigorous stirring. Following that, the mixture was continuously stirred for 6 h and heated to 70 ºC to gradually remove the solvent. Then, the obtained sample was dried at 60 °C in vacuum oven, and finally annealed at 120 °C for 1 h in N2 atmosphere for strengthening the interaction between g-C3N4 and Ti3C2 nanoparticles. The mass ratios of Ti3C2 to g-C3N4 were 0.5, 1, 2 and 3 wt%, and they were labeled as CT0.5, CT1, CT2 and CT3, respectively. Characterization The powder X-ray diffraction (XRD) analysis was carried out using a Bruker D8 Advanced diffractometer. The transmission electron microscopy (TEM) analysis was conducted on a Tecnai G2 F30 S-TWIN electron microscope equipped with an EDS detector at an acceleration voltage of 200 kV. The scanning electron microscopy (SEM) analysis was conducted on FEI 250 system. The X-ray photoelectron spectroscopy (XPS) spectra were obtained by using a PHI Quantera II ESCA System with Al Ka radiation at 1486.8 V. Electrochemical

measurements:

Cyclic

voltammetry

(CV),

linear

sweep

voltammetry (LSV), and chronoamperometric experiments were performed on an electrochemical work station (CHI 760E, Shanghai Chenhua, China) with a 5


Langmuir 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

three-electrode system. A glassy carbon rotating disk electrode (GC RDE, 4 mm in diameter) was used as the working electrode and a platinum wire was used as the counter electrode. An Ag/AgCl electrode calibrated with respect to the reversible hydrogen electrode (+0.949 V vs. RHE) was used as the reference electrode. All the given potentials were converted in terms of the RHE potential. The working electrode was prepared by dropping each of the catalyst solution onto a GC RDE. Typically, 1.5 mg of the as-synthesized sample was firstly loaded on 3.5 mg carbon black (Ketjen carbon, C) and then dispersed in the mixture of 0.25 mL isopropanol, 0.75 mL deionized water and 3 μL Nafion (0.5 wt%) and under ultrasonic for 1 h. Finally, 10 μL of solution, giving a loading density of 0.4 mg cm-2, was deposited on the GC RDE and dried in air overnight for electrochemical characterization. For comparison, a GC RDE coated with Pt/C (20 wt% Pt on Vulcan XC-72, purchased from Alfa Aesar) was also fabricated using the same procedure. Prior to each experiment, the electrolyte (0.1 M KOH) was saturated with oxygen by bubbling O2 for 30 min. A flow of O2 was maintained over the electrolyte during the measurements. For the rotating ring-disk electrode (RRDE) technique, the disk electrode was scanned cathodically with a rotation speed of 1600 rpm at a scan rate of 10 mV s-1, and the ring potential was constant at 0.5 V vs. Ag/AgCl. The % HO2- and n were calculated from the ratio of the ring current (Ir) and the disk current (Id). The Electrochemical Impedance Spectroscopy (EIS) was carried out in 0.1 M KOH electrolyte with continuous O2 flow at a rotating speed of 1600 rpm. Impedance data was recorded at 0.071 V vs Ag/AgCl (0.815 V vs RHE) for ORR. The frequency 6


Page 6 of 25

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

Langmuir

range was 0.01-100000 Hz with a signal amplitude of 5 mV. Results and discussion The typical synthesis of g-C3N4/Ti3C2 heterostructures is as following. First, Ti3C2 NPs were synthesized by the HF etching of Ti3AlC2 and subsequent sonication-assisted exfoliation process. Meanwhile, carbon nitride (g-C3N4) was produced to act as host by thermal condensation of melamine. Zeta potential data were determined by zetasizer 3000 (Malvern, UK). The layered g-C3N4 and Ti2C3 show a positive zeta potential (+20.4 mV) and negative potential (-10.3 mV), respectively, which is similar to the literature reported.[31,32] This interface electrostatic interaction leads to the dispersion of Ti3C2 NPs on the g-C3N4 nanosheets. Figure 1 shows the XRD patterns of pristine g-C3N4 nanosheets, Ti3C2 NPs and g-C3N4/Ti3C2 heterostructures. The XRD pattern of g-C3N4 nanosheets shows two typical diffraction peaks at 12.2 º , corresponding to (100) plane with in-plane structural packing motif, and 27.8º, corresponding to (002) plane of interplanar stacking with conjugated aromatic systems, respectively.[33] The diffraction pattern for the Ti3C2 NPs shows diffraction peaks at 9.2, 18.4 and 27.6º, corresponding to (002), (004) and (008) reflections, respectively.[34] And the transformation of Ti3AlC2 to Ti3C2 NPs is further confirmed by the disappearance of the strongest diffraction peak at 39.2 of Ti3AlC2 and the lower shift of the (004) peaks (Figure s1). After combining with 2 wt% Ti3C2 NPs, g-C3N4/Ti3C2 heterostructures exhibit no obvious change in the XRD pattern compared to that of g-C3N4, due to the low loading content and also implying the good dispersion of the Ti3C2 NPs on the g-C3N4 nanosheets. 7


Langmuir 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

Figure 1. XRD patterns of g-C3N4 nanosheets, Ti3C2 NPs and g-C3N4/Ti3C2 heterostructures. The

morphologies

of

g-C3N4

nanosheets,

Ti3C2

NPs

and

g-C3N4/Ti3C2

heterostructures were examined by SEM and TEM. As shown in Figure s2a, the SEM image of the Ti3AlC2 powder shows a densely layer-stacked structure with micrometer size distribution. After the HF etching process, the Ti3AlC2 was exfoliated into multilayered Ti3C2 with the thick of several nanometers due to the extraction of the Al layer (Figure s2b). When ultrasonic treatment was used, the large multilayered Ti3C2 were cut into small pieces of Ti3C2 NPs. The successful formation of Ti3C2 NPs is supported by the TEM image shown in Figure 2a, with the average diameter about 50 nm. We expect the successful engineering of three-dimensional Ti3C2 into Ti3C2 NPs will increase their surface area and reactivity, thus favoring the formation of heterostructure. The TEM image of the g-C3N4 nanosheets is shown in Figure 2b, and 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

Langmuir

it exhibits a stacked lamellar structure with folded edges. Further observation suggests that the average size of the sheets is around micrometers. For the g-C3N4/Ti3C2 heterostructures, obvious nanoparticles can be seen on the surfaces of g-C3N4 nanosheets after the deposition of Ti3C2 NPs (Figure 2c and s3). In the HRTEM image (Figure 2d), the lattice spaces of 0.5 nm are consistent with the (001) facets of Ti3C2 MXene. EDS mapping was shown in Figure s4, and the results reveal that the C, N, Ti and O elements are well distributed on the surface of g-C3N4/Ti3C2. All of these results indicate that the proposed approach is efficient in homogenous dispersion of Ti3C2 NPs on g-C3N4 nanosheets.

Figure 2. TEM images of Ti3C2 nanoparticles (a), g-C3N4 nanosheets (b) and g-C3N4/Ti3C2

heterostructures

(c).

(d)

HRTEM

image

of

g-C3N4/Ti3C2

heterostructures. XPS measurements were undertaken to investigate the chemical composition and oxidation states of g-C3N4/Ti3C2 heterostructures. The XPS survey spectrum is shown in Figure s5. The Ti 2p spectrum shown in Figure 3a are deconvoluted into six 9


Langmuir 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

components including the peaks at 461.3 and 455.0 eV ascribed to Ti-C 2p1/2 and Ti-C 2p3/2, and peaks at 464.7 and 459.1 eV ascribed to Ti-O 2p1/2 and Ti-O 2p3/2. The peaks at 462.5 and 456.5 eV are ascribed to Ti−X (Ti2+), which correspond to substoichiometric titanium carbide or titanium oxycarbides. The binding energy of 283.9 and 286.9 eV of C 1s (Figure 3b) are attributed to C-C and C=N, respectively, which are the feature of g-C3N4.[35] The high-resolution N 1s spectra in Figure 3c can be decomposed into three Gaussian-Lorenzian peaks at 397.8, 398.9 and 400.3 eV, corresponding to sp2 C-N-C, sp3 N-(C)3 and C-NHx, respectively.[36,37] The O 1s spectra shown in Figure 3d can be fitted to two kinds of Gaussian peaks. The line located at 529.0 eV corresponds to oxygen ions in the crystal lattice, while the main peak at 531.7 eV corresponds to the absorbed oxygen species, indicating the formation of numerous -OH terminations on Ti3C2 nanoparticles. It has been reported that OH-terminated Ti3C2 will show excellent metallic conductivity and exceptional hydrophilicity, which is crucial for highly efficient electrocatalysts.

10


Page 10 of 25

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

Langmuir

Figure 3. XPS spectra of (a) Ti 2p, (b) C 1s, (c) N 1s and (d) O 1s for g-C3N4/Ti3C2 heterostructures. Firstly, the electrocatalytic activity of g-C3N4/Ti3C2 heterostructures was assessed by cyclic voltammetry (CV) measurements. For comparison, the electrocatalytic performance of g-C3N4 and Ti3C2 was also examined. Figure 4a and 4d shows representative CV curves of g-C3N4 and CT2 (the weight ratio of Ti3C2 to g-C3N4 is 2.0 wt %) modified electrodes in Ar and O2 saturated KOH solution at a scan rate of 50 mV s-1. The linear sweep voltammograms of Ti3C2 modified GC electrode at different rotating rates were shown in Figure s6. The main characteristics that can be obtained from such curves include the faradaic current onset potential (Eonset), the potential of cathodic peak current (Epc) and the peak current density (ipc). Values relative to the g-C3N4 modified electrode are Eonset = 0.99 V vs. RHE; Epc = 0.76 V vs. RHE; and ipc = 0.55 mA cm-2. In contrast, upon the deposition of Ti3C2 NPs on the 11


Langmuir 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

g-C3N4 nanosheets, these values to the CT2 modified electrode show both the Eonset and Epc positively shift to 0.924 V vs. RHE and 0.81 V vs. RHE respectively, with an increase of ipc to 0.71 mA cm-2. This result indicates g-C3N4/Ti3C2 heterostructure will be a promising ORR elelctrocatalyst in alkaline media. RDE experiments were used to study the ORR kinetics. Figure 4b and 4e show the linear sweep voltammetry (LSV) curves for g-C3N4 and CT2 at various rotation rates. As expected, the limiting current density increases with the raising of rotation rate due to the promoted diffusion of oxygen on the electrode surface. And at a fixed rotation rate, the current density of CT2 electrode is higher than that of g-C3N4 electrode. The limiting current density (jd) of CT2 modified electrode at 0 V vs. RHE reaches 5.2 mA cm−2, much higher than the g-C3N4 modified electrode (3.5 mA cm−2). Furthermore, the CT2 modified electrode exhibits a half-wave potential (E1/2) of 0.79 V vs. RHE, which is also superior to the g-C3N4 modified electrode (0.65 V vs. RHE). Figure 4c and 4f are the Koutecky-Levich plots at various potentials, which show the linear relationship between the inverse of rotation (ɷ0.5) and inverse of current density (j-1). The good parallelism and linearity of the curves indicates of first-order reaction kinetics.[38] From the slopes of the Koutecky−Levich plots at different potentials, the number of electrons transferred for the ORR was estimated to be 2.64-3.56 for the g-C3N4 modified electrode. While for the CT2 modified electrode, the number is 4.05-3.95 from 0.2 to 0.6V vs. RHE. The increased electron transfer numbers indicate its higher selectivity toward oxygen reduction, and that for the heterostructure the reaction is mainly dominated by a one-step, four-electron pathway. Thus all of the 12


Page 12 of 25

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

Langmuir

above results prove Ti3C2 NPs on the surface of the g-C3N4 nanosheets can enhance the ORR catalytic activity.

Figure 4. CV curves of g-C3N4 (a) and CT2 (d) modified electrodes in an O2-saturated and Ar-saturated KOH solution. LSV curves of g-C3N4 (b) and CT2 (e) modified electrodes at various rotation rates in O2-saturated KOH solution. The corresponding Koutecky-Levich plots of g-C3N4 (c) and CT2 (f) modified electrodes at various potentials. To gain deep insight into the influence of Ti3C2 on the electrocatalytic activity of g-C3N4/Ti3C2 heterostructure, samples with different mass ratios of Ti3C2 to g-C3N4 were synthesized and named as CT0.5, CT1 and CT3 in the following (the weight ratio of Ti3C2 to g-C3N4 is 0.5 wt%, 1 wt% and 3 wt%, respectively). The LSV curves of these catalysts with different rotating speeds are presented in Figure 5a-c. Their electrocatalytic activities are summarized in Figure 5d by comparing the LSV curves at 1600 rpm. It can be seen that the onset potentials shift towards the positive direction and limiting current densities increase when increasing the amount of Ti3C2 13


Langmuir 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

NPs from g-C3N4 to CT2, which is close to that of the commercial Pt/C catalyst. However, further increasing the amount of Ti3C2 NPs to CT3 leads to a decrease of the limiting current density. The declining in their catalytic activity may be ascribed to the compromise of active sites and internal conductivity of the heterostructured catalysts. Nevertheless, CT3 still retains a much higher limiting current density than that of g-C3N4. Figure s7 shows their corresponding Koutecky-Levich plots and the electron transfer numbers at 0.3 V calculated from the plots for the CT0.5, CT1 and CT3 samples are 3.49, 3.36, and 3.28, respectively. Overall, the results suggest that Ti3C2 NPs play an important role for the efficient oxygen reduction and the highest ORR activity is obtained from CT2 with the optimal composition. Their performances are comparable to most of the advanced catalysts reported in the literature (Table s1).

Figure 5. LSV curves at various rpm of CT0.5 (a), CT1 (b) and CT3 (c) modified 14


Page 14 of 25

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

Langmuir

electrodes. (d) Comparative ORR activities of g-C3N4, CT0.5, CT1, CT2, CT3 and Pt/C at 1600 rpm. The best performing CT2 catalysts were further characterized through rotating ring-disk electrode (RRDE) technique to verify the ORR pathway by monitoring the formation of intermediate peroxide species (HO2-). As shown in Figure 6a and b, the CT2 yields lower than 12 % HO2- over the potential range from 0 to 0.6 V vs. RHE, with the electron transfer number (n) ranging from 3.7 to 3.94. These are consistent with the value calculated from Koutecky-Levich equation and close to the RRDE results of commercial Pt/C (Figure s8). The results indicate that the CT2 electrode exhibits a direct four-electron transfer process for ORR.

Figure 6. (a) RRDE curves of CT2 modified electrodes at a rotation rate of 1600 rpm and (b) Yield of the peroxide species and calculated electron transfer number on the CT2 modified electrodes. To comprehensively understand the basic principles involved in the enhanced ORR activity achieved by g-C3N4/Ti3C2 heterostructure, we present the following characterization. Firstly, the electrochemical impedance spectroscopy (EIS) was used to investigate the interfacial processes. The Nyquist plots of g-C3N4 and CT2 catalysts 15


Langmuir 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

are shown in Figure 7a. It is obvious that CT2 shows a smaller semicircle diameter than that of g-C3N4, which means a much lower interfacial charge-transfer resistance.[39] This result indicates the introduction of Ti3C2 NPs makes the g-C3N4/Ti3C2 possess better conductivity and faster electron transfer ability, which is desirable for highly efficient electrocatalysts. The other important feature is the electronic coupling between the g-C3N4 and Ti3C2. The XPS spectra of g-C3N4 nanosheets and Ti3C2 NPs were performed. And as shown in Figure 7b, compared with the Ti 2p3/2 binding energies of Ti3C2 NPs, an obvious shift to higher values is observed in the CT2 heterostructure (from 458.8 eV to 459.1 eV), which means that more electrons are transferred from Ti3C2 to g-C3N4.[40,41] This interaction will probably result in a substantial decrease in the local electron density around the Ti3C2 sites, which will improve the chemisorption of O2.[42] The electronic coupling effect is also proved by the shift of the valence band from XPS spectrometer shown in Figure s9, which indicates the significant electron transfer behavior in the heterostructure. This interaction is analogous to the enhanced catalytic activity of CuS-Pt composites for methanol oxidation.[43] Besides, the hydrophilic property of Ti3C2 NPs is also beneficial for O2 adsorption and reduction. The process is schematic shown in Figure s10.

16


Page 16 of 25

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

Langmuir

Figure 7. (a) EIS Nyquist plots of the g-C3N4 and CT2 modified electrodes over the frequency range 0.01Hz–100000 Hz. (b) XPS spectra of Ti 2p3/2 in Ti3C2 NPs and CT2 heterostrucutes. For practical application, the methanol-crossover effect and stability of the catalysts is also very important. Thus we investigated the electrocatalytic selectivity of the CT2 catalysts against methanol oxidation. As shown in Figure 8, with the addition of 1 mL methanol, the current shows almost no change for the CT2 catalysts, while for the Pt/C catalysts, there is a significant loss, suggesting that CT2 has a good tolerance for methanol. Moreover, the stability of the CT2 catalysts is also tested and the result reveals the catalytic activity shows no obvious attenuation during a 40000s test, demonstrating the highly stable CT2 heterostructures. These results reveal that CT2 has long-term durability and immunity to methanol crossover, which makes them highly promising as a Pt-substituted ORR electrocatalyst for practical application.

17


Langmuir 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 8. Stability and methanol-tolerance evaluation of CT2 in O2-saturated KOH solution. The commercial 20% Pt/C was used for comparison. Conclusion In summary, we have presented a facile approach for the decoration of g-C3N4 nanosheets with Ti3C2 NPs. The novel g-C3N4/Ti3C2 heterostructures exhibit significant enhancement for ORR compared to that of pure g-C3N4 nanosheets. And the electrocatalytic activity is strongly dependent on the loading amount of Ti3C2 NPs. The optimum loading amount of Ti3C2 to g-C3N4 is around 2 wt%. The improved catalytic activity is attributed to the increasing oxygen adsorption and efficient charge separation due to the electronic coupling between g-C3N4 and Ti3C2. Particularly, compared with the commercial Pt/C, the g-C3N4/Ti3C2 heterostructures show better methanol tolerance but with no consumption of precious metal. Our work will inspire ongoing interest in utilizing Ti3C2 Mxene for other energy and environmental applications in addition to their use in fuel cells. Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities (2652015086). 18


Page 18 of 25

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

Langmuir

References (1) M. Lefèvre, E. Proietti, F. Jaouen, J.P. Dodelet, Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells, Science 324 (2009) 71-74. (2) H. Zhu, S. Zhang, D. Su, G. Jiang, S. Sun, Surface profile control of FeNiPt/Pt core/shell nanowires for oxygen reduction reaction, Small 11 (2015) 3545-3549. (3) L. Yang, X. Zeng, W. Wang, and D. Cao, Recent progress in MOF-derived, heteroatom-doped porous carbons as highly efficient electrocatalysts for oxygen reduction reaction in fuel cells, Adv. Funct. Mater. 28 (2018) 1704537. (4) F. Jaouen, M. Lefèvre, J.P. Dodelet, M. Cai, Heat-treated Fe/N/C catalysts for O2 electroreduction: are active sites hosted in micropores? J. Phys. Chem. B 110 (2006) 5553-5558. (5) C. Liu, J. Wang, J. Li, R. Luo, X. Sun, J. Shen, W. Han, L. Wang, Fe/N decorated mulberry-like hollow mesoporous carbon fibers as efficient electrocatalysts for oxygen reduction reaction, Carbon 114 (2017) 706-716. (6) W. Jiang, L. Gu, L. Li, Y. Zhang, X. Zhang, L. Zhang, J. Wang, J. Hu, Z. Wei, L. Wan, Understanding the high activity of Fe-N-C electrocatalysts in oxygen reduction: Fe/Fe3C nanoparticles boost the activity of Fe-Nx, J. Am. Chem. Soc. 138 (2016) 3570-3578. (7) F. Jaouen, E. Proietti, M. Lefèvre, R. Chenitz, J.P. Dodelet, G. Wu, H.T. Chung, C.M. Johnston, P. Zelenay, Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells, Energy Environ. Sci. 4 19


Langmuir 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

(2010) 114-130. (8) X. Yu, D. Wang, J. Liu, Z. Luo, R. Du, L. Liu, G. Zhang, Y. Zhang, A. Cabot, Cu2ZnSnS4 nanocrystals as highly active and stable electrocatalysts for the oxygen reduction reaction, J. Phys. Chem. C 120 (2016) 24265-24270. (9) Q. Niu, J. Guo, B. Chen, J. Nie, X. Guo, G. Ma, Bimetal-organic frameworks/polymer

core-shell

nanofibers

derived

heteroatom-doped

carbon

materials as electrocatalysts for oxygen reduction reaction, Carbon 114 (2017) 250-260. (10) L. Zhang, X. Wang, R. Wang, M. Hong, Structural evolution from metal-organic framework to hybrids of nitrogen-doped porous carbon and carbon nanotubes for enhanced oxygen reduction activity, Chem. Mater. 27 (2015) 7610-7618. (11) G. Algara-Siller, N. Severin, S.Y. Chong, T. Bjorkman, R.G. Palgrave, A. Laybourn, M. Antonietti, Y.Z. Khimyak, A.V. Krasheninnikov, J.P. Rabe, U. Kaiser, A.I. Cooper, A. Thomas, M.J. Bojdys, Triazine-based graphitic carbon nitride: a two-dimensional semiconductor, Angew. Chem. Int. Ed. 53 (2014) 7450-7455. (12) S.M. Lyth, Y. Nabae, S. Moriya, S. Kuroki, M. Kakimoto, J. Ozaki, S. Miyata, Carbon nitride as a nonprecious catalyst for electrochemical oxygen reduction, J. Phys. Chem. C 113 (2009) 20148-20151. (13) Z. Li, Y. Luan, Y. Qu, L. Jing, Modification strategies with inorganic acids for efficient photocatalysts by promoting the adsorption of O2, ACS Appl. Mater. Interfaces 7 (2015) 22727-22740. (14) J. Liu, S. Xie, Z. Geng, K. Huang, L. Fan, W. Zhou, L. Qiu, D. Gao, L. Ji, L. 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

Langmuir

Duan, L. Lu, W. Li, S. Bai, Z. Liu, W. Chen, S. Feng, Y. Zhang, Carbon nitride supramolecular hybrid material enabled high-efficiency photocatalytic water treatments, Nano Lett. 16 (2016) 6568-6575. (15) Y. Sun, C. Li, Y. Xu, H. Bai, Z. Yao, G. Shi, Chemically converted graphene as substrate for immobilizing and enhancing the activity of a polymeric catalyst, Chem. Commun. 46 (2010) 4740-4742. (16) G. Wang, J. Zhang, S. Kuang, W. Zhang, Enhanced electrocatalytic performance of a porous g-C3N4 /graphene composite as a counter electrode for dye-sensitized solar cells, Chem. Eur. J. 22 (2016) 11763-11769. (17) C. Xu, Q. Han, Y. Zhao, L. Wang, Y. Li, L. Qu, Sulfur-doped graphitic carbon nitride decorated with graphene quantum dots for an efficient metal-free electrocatalyst, J. Mater. Chem. A 3 (2015) 1841-1846. (18) Z. Pei, J. Gu, Y. Wang, Z. Tang, Z. Liu, Y. Huang, Y. Huang, J. Zhao, Z. Chen, C. Zhi, Component matters: paving the roadmap toward enhanced electrocatalytic performance of graphitic C3N4 ‑ based catalysts via atomic tuning, ACS Nano 11(2017) 6004-6014. (19) M. Khazaei, M. Arai, T. Sasaki, C.Y. Chung, N.S. Venkataramanan, M. Estili, Y. Sakka, Y. Kawazoe, Novel electronic and magnetic properties of two-dimensional transition metal carbides and nitrides, Adv. Funct. Mater. 23 (2013) 2185-2192. (20) P. Simon, Two-Dimensional MXene with controlled interlayer spacing for electrochemical energy storage, ACS Nano 11 (2017) 2393-2396. (21) Q. Peng, J. Guo, Q. Zhang, J. Xiang, B. Liu, A. Zhou, R. Liu, Y. Tian, Unique 21


Langmuir 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

lead adsorption behavior of activated hydroxyl group in two-dimensional titanium carbide, J. Am. Chem. Soc. 136 (2014) 4113-4116. (22) M. Zhu, Y. Huang, Q. Deng, J. Zhou, Z. Pei, Q. Xue, Y. Huang, Z. Wang, H. Li, Q. Huang, C. Zhi, Highly flexible, freestanding supercapacitor electrode with enhanced performance obtained by hybridizing polypyrrole chains with MXene, Adv. Energy Mater. 6 (2016) 1600969. (23) T. Ma, J. Cao, M. Jaroniec, S. Qiao, Interacting carbon nitride and titanium carbide nanosheets for high-performance oxygen evolution, Angew. Chem. Int. Ed. 55 (2016) 1138-1142. (24) M. Naguib, J. Halim, J. Lu, K.M. Cook, L. Hultman, Y. Gogotsi, M.W. Barsoum, New two-dimensional niobium and vanadium carbides as promising materials for Li-ion batteries, J. Am. Chem. Soc. 135 (2013) 15966-15969. (25) M.R. Lukatskaya, O. Mashtalir, C.E. Ren, Y. Dall'Agnese, P. Rozier, P.L. Taberna, M. Naguib, P. Simon, M.W. Barsoum, Y. Gogotsi, Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide, Science 341 (2013) 1502-1505. (26) Y. Xu, S. Wang, J. Yang, B. Han, R. Nie, J. Wang, Y. Dong, X. Yu, J. Wang, H. Jing, Highly efficient photoelectrocatalytic reduction of CO2 on the Ti3C2/g-C3N4 heterojunction with rich Ti3+ and pyri-N species, J. Mater. Chem. A 6 (2018) 15213-15220. (27) Q. Liu, L. Ai, J. Jiang, MXene-derived TiO2@C/g-C3N4 heterojunctions for highly efficient nitrogen photofixation, J. Mater. Chem. A 6 (2018) 4102-4110. 22


Page 22 of 25

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

Langmuir

(28) X. An, W. Wang, J. Wang, H. Duan, J. Shi, X. Yu, The synergetic effects of Ti3C2 MXene and Pt as co-catalysts for highly efficient photocatalytic hydrogen evolution over g-C3N4, Phys. Chem. Chem. Phys. 20 (2018) 11405-11411. (29) Y. Sun, D. Jin, Y. Sun, X. Meng, Y. Gao, Y. Agnese, G. Chen, X. Wang, g-C3N4/Ti3C2Tx (MXenes) composite with oxidized surface groups for efficient photocatalytic hydrogen evolution, J. Mater. Chem. A 6 (2018) 9124-9131 (30) Q. Xue, Z. Pei, Y. Huang, M. Zhu, Z. Tang, H. Li, Y. Huang, N. Li, H. Zhang, C. Zhi, Mn3O4 nanoparticles on layer-structured Ti3C2 MXene towards the oxygen reduction reaction and zinc–air batteries, J. Mater. Chem. A 5 (2017) 20818-20823. (31) A.M. Jastrzebska, A. Szuplewska, Wojciechowski, M. Chudy, W. Ziemkowska, L. Chlubny, A. Rozmysłowska, A. Olszyna, In vitro studies on cytotoxicity of delaminated Ti3C2 MXene, J. Hazard. Mater. 339 (2017) 1–8. (32) Y. Wang, G. Tana, T. Liu, Y. Su, H. Ren, X. Zhang, A. Xia, L. Lv, Y. Liu, Photocatalytic properties of the g-C3N4/{010} facets BiVO4 interface Z-Scheme photocatalysts induced by BiVO4 surface heterojunction, App. Catal. B Environ. 234 (2018) 37-49. (33) Z. Zhou, Y. Shen, Y. Li, A. Liu, S. Liu, Y. Zhang, Chemical cleavage of layered carbon nitride with enhanced photoluminescent performances and photoconduction, ACS Nano 9 (2015) 12480-12487. (34) T. Ma, J. Cao, M. Jaroniec, S. Qiao, Interacting carbon nitride and titanium carbide nanosheets for high-performance oxygen evolution. Angew. Chem. Int. Ed. 55 (2016) 1138-1142. 23


Langmuir 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

(35) X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76-80. (36) B. Xia, Y. Yan, X. Wang, X. Lou, Recent progress on graphene-based hybrid electrocatalysts, Mater. Horiz. 1 (2014) 379-399. (37) Q. Liu, J. Zhang, Graphene Supported Co-g-C3N4 as a novel metal-macrocyclic electrocatalyst for the oxygen reduction reaction in fuel cells, Langmuir 29 (2013) 3821-3828. (38) L. Qu, Y. Liu, J.B. Baek, L. Dai, Nitrogen-Doped Graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells, ACS Nano 4 (2010) 1321-1326. (39) R. Liu, H. Liu, Y. Li, Y. Yi, X. Shang, S. Zhang, X. Yu, S. Zhang, H. Cao, G. Zhang, Nitrogen-doped graphdiyne as a metal-free catalyst for high-performance oxygen reduction reactions, Nanoscale 6 (2014) 11336-11343. (40) J. Yang, J. Ying, Nanocomposites of Ag2S and noble metals, Angew. Chem. Int. Ed. 50 (2011) 4637-4643. (41) X. Yu, R. Du, B. Li, L. Liu, Y. Zhang, Cu2ZnSnS4-AuAg heterodimers and their enhanced catalysis for oxygen reduction reaction, J. Phys. Chem. C 121 (2017) 6712-6720. (42) J. Yang, J. Ying, Nanocomposites of Ag2S and noble metals, Angew. Chem. Int. Ed. 50 (2011) 4637-4643.

24


Page 24 of 25

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

Langmuir

(43) W. Sang, T. Zheng, Y. Wang, X. Li, X. Zhao, J. Zeng, J. Hou, One-step synthesis of hybrid nanocrystals with rational tuning of the morphology, Nano Lett. 14 (2014) 6666-6671.

TOC

25


Decorating g-C3N4 Nanosheets with Ti3C2 MXene Nanoparticles for

Recommend Documents