Component Matters: Paving the Roadmap toward Enhanced Electrocatalytic Performance of Graphitic C3N4‑Based Catalysts via Atomic Tuning Zengxia Pei,†,⊥ Jingxing Gu,‡,⊥ Yukun Wang,† Zijie Tang,† Zhuoxin Liu,† Yan Huang,† Yang Huang,§ Jingxiang Zhao,‡ Zhongfang Chen,*,‡ and Chunyi Zhi*,†,∥ †
Department of Physics and Materials Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong 999077, China ‡ Department of Chemistry, The Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico 00931, United States § College of Materials Science and Engineering, Shenzhen University, Shenzhen 518000, China ∥ Shenzhen Research Institute, City University of Hong Kong, High-Tech Zone, Nanshan District, Shenzhen 518057, China S Supporting Information *
ABSTRACT: Atomically precise understanding of componential influences is crucial for looking into the reaction mechanism and controlled synthesis of efficient electrocatalysts. Herein, by means of comprehensive experimental and theoretical studies, we carefully examine the effects of component dopants on the catalytic performance of graphitic C3N4 (g-C3N4)-based electrocatalysts. The g-C3N4 monoliths with three types of dopant elements (B, P, and S) embedded in different sites (either C or N) of the C−N skeleton are rationally designed and synthesized. The kinetics, intrinsic activity, chargetransfer process, and intermediate adsorption/desorption free energy of the selected catalysts in oxygen reduction reaction and hydrogen evolution reaction are investigated both experimentally and theoretically. We demonstrate that the component aspect within the g-C3N4 motifs has distinct and substantial effects on the corresponding electroactivities, and proper component element engineering can be a viable yet efficient protocol to render the metal-free composites as competent catalysts rivaling the metallic counterparts. We hope that this study may shed light on the empirical trial-and-error exploration in design and development of g-C3N4-based materials as well as other metal-free catalysts for energy-related electrocatalytic reactions. KEYWORDS: electrocatalysis, graphitic carbon nitride, heteroatom, oxygen reduction, hydrogen evolution
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molecular structure, abundance, and robustness in corrosive (both acidic and alkaline) electrolytes.1,2,12 In spite of the great progresses, however, it is still urgent to further improve the activity of these metal-free catalysts to meet practical applications (approaching the present noble metal-based catalysts). In pursuing this advance, probably the most effective strategy is to investigate the coherent interplay between the component structure−property aspects both experimentally and theoretically.1,2,12,13 Graphitic C3N4 (g-C3N4) is a stacked two-dimensional (2D) π-conjugated polymer consisting of repeating in-plane heptazine moieties. Due to its unique electronic structure,
lectrocatalytic reactions play pivotal roles in a renewable energy economy and the sustainability of modern society.1−3 Electrochemical oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), for instance, lie at the heart of a series of most promising regenerative energy conversion and storage technologies, including fuel cells, metal-air batteries, and water-splitting devices.1,2,4−8 Presently, the core challenge of electrocatalysis remains on the development of highly active, cost-effective (with regard to the noble metal catalysts, like Pt/ C, IrO2, etc.) and durable catalysts and concurrently, on the indepth understanding of the underlying kinetics of different reactions together with the nature of active sites.1,2,9−11 The past decade has witnessed the flourish of various metal-free materials (dominantly carbon based) for catalyzing the aforementioned reactions, mainly by virtue of their tailorable © 2017 American Chemical Society
Received: March 20, 2017 Accepted: June 12, 2017 Published: June 12, 2017 6004
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Figure 1. Digital images of the as-prepared different g-C3N4 samples: (a) bare CN and (b) B-, (c) P-, and (d) S-doped CN. TEM images showing the morphology and microstructure of different g-C3N4 species: (e and i) bare CN and (f and j) B-, (g and k) P-, and (h and l) Sdoped CN. EELS element mapping images showing the distribution of (m) N in bare CN, (n) B in B-CN, (o) P in P-CN, and (p) S in S-CN sample.
whether and how it works in the electrocatalysis domain remain unclear. Only very sporadic works are available for this exploration, and the conclusions are quite elusive to date. Thus, it is imperative to further understand the effect of the altered component within the g-C3N4 motif over its electrocatalytic performances. Herein, by means of mutually corroborated experimental and theoretical studies, we comprehensively investigated the electrochemical activity of the g-C3N4 with different heteroatom dopants (B, P, S) supported on macroporous carbon (PC) for ORR and HER, targeting on understanding the general influence of different substituents and configurations (i.e., B/P substituting C, S substituting N) at different active reaction sites, specifically, ORR (O2, OOH−, etc.) on C sites, while HER (H+) on N sites. Our study demonstrates clearly that the component dopants can strikingly affect the overall electrocatalytic performance of g-C3N4, and proper doping protocol is indeed a valid approach to boost the composite hybrid as one of the most efficient and stable metal-free electrocatalysts. The in-depth understanding of dopant-induced performance differences is obtained by joint electrochemical measurements and theoretical calculations, which elucidate the kinetics, intrinsic activity, charge-transfer process, and inter-
excellent thermal and chemical stability as well as environmental-friendliness, g-C3N4 has risen to be one of the most appealing metal-free materials in photocatalysis, bioimaging, fuel cell, and other energy-related applications.14−18 In particular, as an analogue of the prototypical nitrogen-doped graphene, the intrinsic high N content (both pyridinic and graphitic configurations) and the abundant edge sites within its motifs also enable g-C3N4 to be capable versatile electrocatalysts for ORR, OER, and HER.15,16,19−22 Within the last years, it has been discovered that diverse conductive substrates (carbon black, carbon nanotube/fiber, graphene, and porous carbon) can be utilized validly to tackle the low conductivity of semiconductor-behaved g-C3N4,15,16,19 while nanodot, nanoribbon, and nanosheet structures have been fabricated to unravel the influence of nanoarchitecture of this polymer on its different electroactivities.21,22 Although inspiring advances have been achieved, these arduous prior endeavors focus exclusively on the exterior modification over the g-C3N4 species, while the underlying role of another significant aspect, component, toward the electrocatalytic performance has rarely been explored. The doping protocol has shown great success in modifying the electronic structure and catalytic behavior of gC3N4 in other fields (e.g., photocatalysis),14,18,23−26 however, 6005
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ACS Nano Scheme 1. Schematic Illustration for the Fabrication of the PC Supported B/P/S-Doped g-C3N4 Samplesa
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(I) The PC substrate was mildly oxidized to introduce hydrophilic functional groups on its pore shell. (II) The precursors were cross-linked together onto the PC pore shell driven by the acid−base interactions between − COOH on the PC shell, basic dicyandiamide, and acidic dopant species. (III) The assembled precursors were polymerized within the pore of the PC substrate, forming an intimately contacted X-CN/PC catalyst. For clarity, the separate B/P/S doping processes are incorporated into one framework.
The doped elements and their corresponding chemical binding states were then examined by X-ray photoelectron spectroscopy (XPS) analyses. Survey spectra in Figure S1a confirm the embedment of desired dopant in different samples. Both the featuring C and N 1s spectra can be deconvoluted into three binding species (Figure S1b,c), denoting the g-C3N4 nature of the as-synthesized samples. Specifically, the B 1s peak locates at 191.5 eV (Figure S1d), corresponding to the B−N coordination,23 suggesting that B atoms are exclusively introduced into the carbon sites in the g-C3N4 matrix to form B−N bond. The peak of P 2p binding energy is centered around 133.3 eV (Figure S1e), which is typical of the P−N bonding, indicating that the P dopants also replace the carbon atoms in the polymer motifs.15 The S atoms, by contrast, give doublet 2p3/2 and 2p1/2 peaks at 163.8 and 165.0 eV, respectively (Figure S1f), corresponding to those in the S−C bonds formed by substituting lattice nitrogen atoms.18 These results therefore confirm that two forms of doping, replacing the alternative constitutional C (by B or P) or N (by S) atoms, were achieved by the present doping protocol in g-C3N4, which could basically ensure a comparative study of the dopant influence on the corresponding electroactivity. The dopant content was rationally controlled to a comparable level, with the B/N, P/N, and S/N ratios (evinced by the XPS element analyses) being 3.6 at%, 4.9 at%, and 2.7 at%, separately. The integration of X-CN and PC substrate was realized by an impregnation method followed by in situ heating to facilitate the polycondensation of doped g-C3N4 (Scheme 1). The PC template was first mildly oxidized by HNO3 to increase its hydrophilicity, then dicyandiamide and dopant precursor were cross-linked within the voids of the PC bulk, which were further subjected to calcination at 600 °C to induce the polymerization
mediate adsorption/desorption free energy in different composite catalysts. To the best of our knowledge, such a comprehensive investigation focusing on the influential component factor for g-C3N4 in electrocatalysis domain has not been reported. We hope that this study can inspire the rational design of g-C3N4-based materials as well as other metal-free catalysts for different electrocatalytic reactions.
RESULTS AND DISCUSSION As a preliminary foundation for evaluating their electrocatalytic activities, different heteroatom-doped g-C3N4 (X-CN) basal motifs were first prepared by a controllable “bottom-up” synthetic method. Boric acid and ethylene diphosphonic acid were adopted as B and P sources, respectively, and they were expected to cross-link with the dicyandiamide via the acid−base interaction,15,23 thus rendering a homogeneous element doping. Trithiocyanuric acid served as the S-doped CN monomer through in situ polycondensation, which could also enable a uniform element distribution. After polycondensation, the X-CN samples show apparent distinct colors, as the bare CN does (see Figure 1a−d), which indicates their substantial differences in the underlying physicochemical properties. Transmission electron microscope (TEM) images (Figure 1e−l) unveil the different textures caused by dopants. The gC3N4 sheets become denser by B or P doping even after thorough sonication treatment, while the sulfur-doped ones are much thinner due to the delamination effect caused by the Scontaining leaving groups, similar to previous work.14 Electron energy loss spectroscopy (EELS) mapping (Figure 1m−p) reveals that all the three heteroatoms locate homogeneously within the carbon nitride sheets as we anticipated, which could allow a more accurate sample testing of their electroactivities. 6006
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Figure 2. TEM images showing the microstructure of (a) pristine porous carbon substrate and (b) representative CN/PC composite; insets are their SAED patterns, respectively; scale bar: 200 nm. (c) HRTEM image of the CN/PC samples. The d-space of the stacked g-C3N4 nanosheets is indicated by dashed lines; scale bar: 5 nm. (d) High-resolution C 1s and N 1s (inset) XPS spectra from the CN/PC sample. (e) XRD patterns and (f) FTIR spectra of different X-CN/PC hybrid samples with bare g-C3N4 and PC substrate as reference.
main peak at 284.5 eV is assigned to the C−C coordination, and the intermediate peak at 285.6 eV is from the C−OH residuals from the sucrose precursor.29 Both peaks in the composite are identical with those in bare PC substrate (Figure S4). The emergence of the third peak centered at 288.1 eV is originated from the C−N bonds, resembling that in the bare gC 3 N 4 sample (Figure S1b), which also indicates the incorporation of carbon nitride species.28 Similarly, the N element (inset in Figure 2d) in the composite features three typical coordinations of the integrated g-C3N4, including the typical pyridine-like (C−NC, 398.6 eV), graphite-like (N− C)3, 400.1 eV), and terminal amine (C−N−H, 401.3 eV) N species.29 The X-ray diffraction (XRD) patterns (Figure 2e) clearly identify different X-CN/PC hybrids synthesized by the in situ polycondensation method. The pristine g-C3N4 gives two distinct 2θ diffraction peaks at 27.5° and 13.3°, resulting from the interlayer stacking reflection of conjugated aromatic structures and the in-plane structural motif between nitride pores, respectively.23 The bare PC substrate, by contrast, is characterized by a broad peak around 23.5°, similar to other amorphous carbons.19 In the composite samples, however, the characteristic diffraction peak of X-CN is homogenized and overlapped by the PC template, leading to a shifted broad intermediate peak centered at ca. 24.9°, which is presumably due to the uniform distribution of the drafted X-CN species.30 The loaded carbon nitride was additionally evidenced by the Fourier transformed infrared (FTIR) spectra (Figure 2f). In contrast to the characterless PC, the multiple stretching bands in the 1100−1800 cm−1 range stem from the aromatic C−N heterocyclic triazine ring in the g-C3N4 moiety, while another featuring a sharp band at 800 cm−1 is assigned to the deformation of its tri-s-triazine ring modes.23,29 Despite the weakening by the carbon matrix, all of these bands are discernible in the X-CN/PC composites, denoting the successful incorporation of the X-CN portion. The exact
process (see Experimental Section for details). The macroporous carbon was used here for its merits such as high conductivity for charge-transfer, large surface area for highly exposed active sites, and large pore size for facilitated mass transport, all of which are anticipated to warrant good electroactivities.27 TEM image (Figure 2a) exhibits that the pristine PC substrate has hierarchical macroporous texture, while the selected area electron diffraction (SAED) (inset in Figure 2a) patterns disclose its amorphous nature. The N2 adsorption/ desorption analyses (Figure S2 and Table S1) indicate that the PC has a large specific surface area of 758.4 m2 g−1 and a hierarchical nanostructure of micro-, meso-, and macroporosity. With the incorporation of the representative g-C3N4, the PC matrix becomes hazy but still discernible since the porous structure is partially filled by the C−N compounds (Figures 2b and S3). Meanwhile, the decreased specific surface area and pore volume in conjunction with the narrowed pore diameters (particular in the macropore region, see Figure S2 and Table S1) clearly suggest the filling effect, and the homogeneous distribution of the in situ synthesized different X-CN species.19,28,29 Note that the dopant-induced texture differences affect the nanoarchitecture of the X-CN/PC samples, from which the B/P-doped hybrids have smaller parameters (surface area, pore diameter, and pore volume, Figure S2 and Table S1) compared with the other two, coinciding with the TEM morphology observation (Figure 1e−l). The composite remains poorly crystallized as a whole from the SAED patterns (inset in Figure 2b), but the high-resolution TEM (HRTEM) image (Figure 2c) still reveals the stacked nanosheet structure with a d-spacing of ca. 0.325 nm, corresponding to the stacking distance of two adjacent layers in g-C3N4,14 suggesting that the carbon nitride species was drafted onto the PC substrate. The chemical states of elements in the representative CN/PC composite were then unraveled by XPS analyses. The C 1s spectrum (Figure 2d) can be best fitted into three peaks. The 6007
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Figure 3. Electrochemical catalytic performance toward ORR in 0.1 M KOH solution: (a) CV curves of various samples in N2 (dashed line) and O2 (solid line) saturated electrolyte with a scan rate of 20 mV s−1. (b) LSV curves of different catalysts recorded from RDE tests at 1600 rpm in O2 saturated electrolyte at 5 mV s−1; the commercial Pt/C (20 wt %) was recorded for comparison. (c) Comparison of Jk obtained within the mixed kinetic-diffusion region (−0.2 V vs Ag/AgCl) upon normalization of the electrode area and the BET surface area. (d) K−L plots of different catalysts at varied rotation speeds from RDE tests; inset is the n and Jk of different samples obtained from the corresponding K−L plots in (d). (e) RRDE tests of the different samples; the current densities of the Pt ring were amplified 5 times for better differentiation. (f) Calculated electron-transfer number and HO2− yield of the samples from RRDE tests in (e).
samples, suggesting the efficient reduction of purged O2. The dopant-free CN/PC sample features a reduction peak located at −0.28 V (vs Ag/AgCl, similarly hereinafter). B-doping is regretfully found to deactivate the ORR process by reducing the reduction peak to −0.32 V, while P atoms in the g-C3N4 matrix can boost this value to −0.25 V, suggesting a facilitated reduction reaction. Encouragingly, the S dopant exhibits the most enhanced electrocatalytic performance with an ORR peak potential of −0.22 V. These component induced activity differences are also evidenced in the linear sweep voltammograms (LSVs) recorded from rotating disk electrode (RDE) tests at 1600 rpm (Figure 3b). The B-CN/PC sample can implement ORR with an onset potential of −0.17 V, while the pristine and P- and S-doped X-CN/PC ones can exert the identical reaction at −0.14, −0.10, and −0.07 V, respectively, approaching that of the benchmark Pt/C catalyst (−0.01 V) successively. The reaction current density (J) of these four samples also follows the above sequence (Figure 3b), and the J value for S-CN/PC sample surpasses that of the Pt/C one when applied potential is negative than ca. −0.63 V, demonstrating the superiority of proper doping under operational conditions. What is the origin of the distinct catalytic activity of the XCN/PC hybrid samples? To address this question, we first examined the ORR performance of bare PC substrate for comparison. The observed two separate reduction peaks in the gauged range of CV and an obvious plateau in its polarization curve (Figure S7a,b) indicate an inefficient oxygen reduction process via a two-electron (2e−) reaction path.19 The X-CN/ PC samples, by contrast, deliver distinct benchmark Pt/Cresembling catalytic behaviors (Figure S7c, d), and the carbon nitride species should thus be the main active sites within the
loading mass of the carbon nitride species was determined by thermal gravimetric (TG) measurements (Figure S5), which ranged in a comparative level between 22 and 28 wt %. Last but not the least, the interfacial structures of the hybrid catalysts were also reviewed by Raman spectrum (Figure S6). All samples exhibit two distinct bands around 1345 and 1590 cm−1, attributing to the well-known D band and G band, respectively. The D band is caused by the defective/disorder sites through the A1g vibrational mode of carbon, while the G band is attributed to the presence of crystallized graphitic sp2 carbon via the E2g vibrational mode.6,27 The featuring relative intensity ratios (ID/IG) of all the hybrid catalysts converged in a small range of 1.18−1.21, which approach that of the bare PC substrate (1.16), thereby ensuring comparable physicochemical properties of the carbon substrate. It is therefore plausible that the electroactivity differences discussed below primarily stem from the dopant-induced property distinctions. The successful heteroatom doping and the incorporation of the X-CN/PC composites allow a detailed study on the dopantinduced impact. The high electron affinity of nitrogen atoms within the g-C3N4 motif can render a high positive charge density on the adjacent carbon atoms, which makes the latter as potential active sites for O2 adsorption and subsequent reduction reactions.1,19 Here ORR was first used as a probe reaction to investigate the influence of different dopants on the electrocatalytic activity of the X-CN/PC samples. Cyclic voltammetry (CV) tests were first conducted in 0.1 M KOH solution. As shown in Figure 3a, all the catalysts display a quasi-rectangular CV curve without significant redox peaks in the N2-saturated electrolyte, indicating simply the capacitive behavior of the material and no redox reactions. When O2 is introduced, obvious reduction peaks can be observed in all the 6008
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Figure 4. Optimized structure of pristine g-C3N4 as (a) top view and (b) side view, in which the N atoms are numbered from 1 to 8, while C atoms are numbered from 9 to 14. Top views of the optimized structures of the energetically most favorable (c) B-CN, (d) P-CN, and (e) SCN, in which the B and P atoms substitute the bay carbon C13, while S atom replaces the pyridinic nitrogen N7. In each structure, the largest value of charge and spin densities on carbon atoms are indicated by black and red colors, respectively; additionally, the related carbon atoms are illustrated by green arrows.
architectures and/or integrating with other conductive substrates), the conspicuous performance differences presented here firmly suggest that component is indeed a significant parameter that can substantially govern the ORR activity of gC3N4 electrocatalyst. Actually, the performance of the S-CN/ PC catalyst well rivals many recently reported metal-free ORR catalysts (Table S3), demonstrating the feasibility of proper doping protocol in g-C3N4. Moreover, the hybrid catalysts also deliver excellent stability and reaction selectivity during the oxygen reduction process (Figure S12), showing their decent potential as alternatives to the Pt-based ORR catalysts. To get a further in-depth understanding of the dopantinduced ORR activity differences at the atomic level, we performed DFT computations. Figure 4 presents the optimized structures of the pristine g-C3N4 and the energetically most favorable X-CN monolayers (for details, see Figure S13). It is worth noting that although XPS analyses give the information about the coordination forms of the dopants, they cannot provide the exact substitution sites of the heteroatoms. Then we conducted the theoretical calculations to search the energetically most stable sites of the dopants. As shown in Figure S13, for the B and P-doped g-C3N4, we found the baycarbon is the most favorable site for the dopant atoms substitution, while the pyridinic nitrogen is more stable for the replacement by S atom, which are consistent with previous works.18,25 Therefore, all the calculations in the present work were based on the most stable substitution sites (Figure 4). On the other hand, previous DFT studies suggested that the active sites of N-doped graphene-based metal-free 2D catalysts are the carbon atoms next to the pyridinic nitrogen atom.31,32 Also, earlier studies showed that B doping plays a crucial role for ORR catalytic activity in the B-doped carbon nanotubes,33 and the B atom was confirmed as the active site for HO2− adsorption in the B-doped graphene toward ORR.34 Additionally, the S−C bond in S-doped graphene was thought as an important ORR active site,35 and the S atom was confirmed as the active site in the S-doped graphene.36 Thus, in this work, we evaluated the catalytic activities of all the dopants and the carbon atoms nearby. Since the oxygen-containing species, such as O, O2, OH, and OOH, tend to adsorb to the atoms with most positive charge or largest spin density,31 we computed the Hirshfeld charge and
composite catalysts for ORR. The intimate contact of the CN species and PC substrate is vital for the efficient charge-transfer process,19 which is also attested by the control mechanically mixed sample (Figure S8). Considering that the texture distinction of different pure X-CN samples alters their specific surface area (Figure S2), we performed Koutecky−Levich (K− L) analyses to assess the kinetic current densities (Jk) of different composite samples upon normalization in the mixed kinetic-diffusion controlled region. As illustrated in Figure 3c, the gaps between different X-CN/PC samples are reduced when their Jk values are normalized. For example, the difference of Jk between the B-CN/PC and S-CN/PC decreases from approximately 20 to 8 times, yet both of their Jk values are still different from that of the pristine CN/PC. These data suggest that the surface area could contribute, but only partially, to the varied ORR performance of different composite catalysts, therefore heteroatom dopant is evidently another factor that matters. The electrochemical impedance spectroscopy (EIS) tests (Figure S9) also uncover that the S-doped hybrid sample shows the smallest charge-transfer resistance among all the counterparts, corroborating the ORR kinetics of the composite catalyst can be promoted by suitable atomic doping. The merit of optimized doping protocol is further validated by RDE voltammograms at various rotating speeds. As illustrated in Figure 3d, all the K−L plots of the X-CN/PC catalysts show good linearity under an operational potential of −0.5 V, while the S-doped composite presents the highest reduction current density at all rotation speeds, exceeding that of the control CN/PC samples as well as the P/B-doped ones. Based on the K−L model, the Jk and electron-transfer number (n) within the S-CN/PC sample are calculated to be 19.2 mA cm−2 and 4, respectively (inset in Figure 3d), denoting an efficient one-pot, 4e− reduction pathway with a high activity outperforming other analogues. The prominence of the suitable dopants is also evinced by K−L plots at other potentials (Figure S10) and the parallel RRDE tests (Figures 3e,f and S11). The HO2− yield of the S-CN/PC catalyst in the experimental system is determined to be 6−13% in a wide gauged range, which refers to a 4e− overwhelming ORR process and is dramatically better than other samples. Though improvement can be expected via further optimizations (e.g., structure tailoring the X-CN species into other nano6009
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Figure 5. Free energy diagram of (a) pristine and different atom doped C3N4 at the equilibrium potential U0 = 0.46 V. (b) Free energy diagram of S-doped C3N4 at different potentials.
Figure 6. Electrochemical catalytic performance toward HER in 0.5 M H2SO4 solution: (a) Polarization curves of different catalysts on RDE with a scan rate of 5 mV s−1 at 3000 rpm. (b) RRDE measurements of hydrogen evolution on different catalyst-modified electrodes; the disk electrode records the reduction current during H2 generation, while the ring electrode records the oxidation current of the evolved H2. (c) Corresponding Tafel plots of varied catalysts derived from curves in (a). (d) Capacitive current density as a function of scan rates for different samples at 0.2 V (vs RHE); the double layer capacitance values are also listed. (e) Comparison of J0 of different samples upon normalization of the electrode area and the BET surface area. (f) Calculated free-energy diagram of HER at equilibrium potential for different catalysts; the ΔG value of the bare carbon is from ref 39.
spin density of each doped structure to search the most favorable adsorption sites (Table S2). In addition, we also computed the band structure for each doped structure based on different functionals (Figures S14 and S15). Figure 4 presents the carbon atoms with the largest charges or spin densities and related data of the dopants. From the above, we proposed that besides the B, P, and S atoms, C9 and C14 of pristine C3N4, C9
and C11 of B-doped and P-doped C3N4, and C10 and C11 of Sdoped C3N4 could serve as potential active sites. Then, we computed the Gibbs free energy change of each reaction step that occurs on potential active sites. It turned out that C9 in pristine C3N4, B in B-doped C3N4, and C11 in P-doped C3N4 as well as C10 and C11 in S-doped C3N4 have the best catalytic performance for each system. For clarity, Figure 5 presents the 6010
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heteroatom embedment can promote the ORR activity, and the optimal performance is achieved by S doping. However, as shown in our computational model structure, the ORR process involves mainly the C sites, whether the dopants also work on the residual N species is still inconclusive. The N atoms are believed to serve as the main adsorption sites for H+ in acidic solution,16 thus we further evaluated the HER catalytic activity of doped C3N4 composite catalysts in 0.5 M H2SO4 electrolyte. Figure 6a depicts the polarization curves of different g-C3N4-based samples, N-doped porous carbon (NPC), and the state-of-the-art Pt/C catalysts were also listed for comparison. The N-PC delivers the poorest HER activity, with an onset potential as high as ca. −350 mV (vs RHE, similarly hereinafter) and an overpotential of −534 mV to achieve a metric HER current density of 10 mA cm−2, presumably due to the low concentration of active sites and intrinsically low adsorption capability for H+ ions as reported by previous studies.16,39,40 The CN/PC shows much better activity, yet the influence of the doped heteroatoms is also distinct: S acts as the best dopant, while the B and P elements deteriorate the HER activity. Specifically, in the S-doped hybrid, a small onset potential of −55 mV is required to efficiently implement the HER, and an operational current density of 10 mA cm−2 is measured at −186 mV; both are approaching those of the Pt/C catalyst and are much superior than those of the pristine CN/ PC sample (−167 mV, −300 mV) as well as the P- (−201 mV, −343 mV) and B- (−254 mV, −405 mV) doped ones, which also surpass most metal-free HER catalysts and many other metallic ones (see Table S4). The evolution of hydrogen gas was also studied by the RRDE technique, during which the electrode bearing the catalyst rotates at 1600 rpm to radically remove the evolved H2 bubbles, and the Pt ring potential was set at 0.7 V to oxidize the H2.40 As displayed in Figure 6b, all of these four g-C3N4-based catalysts deliver oxidation currents following the occurrence of their cathodic HER currents in defined potentials, confirming the true generation of H2 by all materials. The underlying H2 evolution kinetics was then unveiled by the Tafel plots (Figure 6c). The benchmark Pt/C features a small Tafel slope of 29 mV dec−1, corresponding to a well-known Volmer−Tafel mechanism, and the recombination of the adsorbed H* is the ratedetermining step at low overpotential.41−43 Regretfully, the detailed HER mechanism of most metal-free catalysts is still ambiguous. Unlike the Pt species, which has a very high H* coverage close to 1, the g-C3N4 is calculated to have a much smaller coverage of only around 1/3.16 The calculated range of Tafel slopes of g-C3N4-based samples is 84−100 mV dec−1 in the present system, suggesting that the hydrogen is generated most probably via the Volmer−Heyrovsky mechanism, in which the electrochemical desorption process acts as the ratedetermining step.41,42 The N-PC sample, by contrast, exhibits a large Tafel slope of 141 mV dec−1, attesting to its low HER kinetics, and the elementary proton adsorption (viz., the Volmer step) is the limiting step of its whole HER process. Taken together, these results suggest that carbon nitride, as a metal-free catalyst, could catalyze the HER with enhanced activity compared with N-doped carbon by affording facilitated proton adsorption sites, while its component heteroatoms can render strikingly different kinetics though subjecting to the same mechanism. We further evaluated the electrochemical active areas of the different carbon nitride incorporated composites by CV scan in a non-Faradaic potential range (0.15−0.25 V) at different rates
reaction pathway with the best performance for each doped C3N4 (for more details, see Figure S16). At the equilibrium potential U0 = 0.46 V, the S-doped C3N4 (dark cyan line) has the best catalytic performance, as indicated by the lowest overall reaction free energy change (Figure 5a). Moreover, for S-doped C3N4, only the third electron-transfer step is exothermic with ΔGeq 4c = −1.18 eV, while other three steps are endothermic with ΔGeq 4a = 0.25 eV, ΔGeq 4b = 0.31 eV, and ΔGeq 4d = 0.61 eV. Therefore, the last step, i.e., the uphill from *OH to OH− with a 0.61 eV endothermicity, is the most sluggish, or the rate-determining step. In comparison, the rate-determining step for pristine C3N4 is the first step whereby forming *OOH on the surface of catalysts. Instead, the ratedetermining step becomes the last electron-transfer step once doped by B, P, or S atoms. Taking P-CN (blue line) as an example, it is less active than S-CN, but more active than the pristine and B-CN. Similar to S-CN, the first and the last steps for P-CN are uphill, while the second and third steps are downhill. Although P-CN has more downhills than the S-doped analogue, the most sluggish step of P-doped C3N4 has a value of ΔGeq 4a = 0.66 eV. This value (0.66 eV) is larger than that of SCN (0.61 eV), but smaller than that of pristine C3N4 (0.91 eV for the first step of black line) and B-CN (1.39 eV for the last step of red line), indicating the catalytic activity trend: S-doped > P-doped > pristine > B-doped CN, which is in agreement with our experimental results. We further studied the reaction pathways of S-CN at different potentials (Figure 5b). Even at U = 0 V, there is still a 0.15 eV uphill to form hydroxide ion. Thus, a negative potential of −0.15 V is needed to make this step to be spontaneous, which corresponds to the theoretical onset potential of 0.62 V vs RHE, in reasonable agreement with our experimental value (ca. 0.90 V vs RHE). The above results manifest that the ORR activity of the pristine g-C3N4 can be altered by heteroatom embedment. According to our experiments and DFT computations, among the samples examined by this study, the pristine C3N4 has the least attraction to species containing O atoms (indicated by the positive free energies of black line in Figure 5a), which causes its relatively poor catalyzing capability. The least attraction may be caused by the lack of spin density and the large band gap of pristine C3N4, which is 3.16 eV (Figure S15).37 For B-CN, although the ORR pathway on B atom is the best among all the potential active sites according to the largest charge density of B atom, this material is the least active among all the tested CNbased material. This is because that the B atom is directly adjacent and bonded to the N atom forming a strong localized BN bond, thus can hardly improve the ORR catalytic property of CN, very similar to the case in the boron and nitrogen codoped carbon nanotubes.38 For P-CN, with the largest charge and spin density, the P atom is not the best active site, which is different from other doped systems in which the most active sites correspond to the atoms with largest charge or spin density. This phenomenon can be explained by the overwhelming strong interaction between the P atom and Ocontaining species (Figure S16c). For S-CN, there are two kinds of carbon atoms serving as active sites, one with the largest charge density (C10) and the other with the largest spin density (C11). Moreover, S-doping significantly reduces the band gap of the pristine C3N4 (3.16 eV) to 0.74 eV. The band gap reduction also holds true at lower concentrations (Figure S14d,e). Therefore, the appropriate redistribution of the spin and/or charge density and tuning of the band gap by 6011
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substantially distinct impacts over the corresponding electroactivities, among which the S-doped hybrid afforded the best reaction kinetics and intrinsic activities for boosting both ORR and HER. Theoretic calculations revealed that the dopant elements could render different charge and spin densities within the g-C3N4 motifs, which then alter the sorption free energies of different reaction intermediates and eventually lead to the enhanced or deteriorated activities. Our work highlights the underlying componential influences within g-C3N4 in electrocatalysis domain and also demonstrates that only proper component engineering could favor as a viable way for promoting electrocatalytic reactions.
(Figure S17). The positive and negative current density differences are plotted against the scan rates in Figure 6d, in which the slopes are twice the electrochemical double-layer capacitances (Cdl).21 The Cdl values shrink successively in the sequence of the S-, pristine, P-, and B-CN/PC samples, which accords well with the HER performances observed above. The S-doped sample gives the largest Cdl of 18.3 mF cm−2, implying highly exposed active sites, which also hints at the advantage of a large surface area under operational conditions. To get an insight into the dopant-induced contribution, we compared their exchange current densities (J0s), a parameter that reveals the intrinsic activity of HER catalysts, and normalized them to unit surface area. The J0 values of the Band P-doped composites are smaller than the dopant-free CN/ PC before and after normalization (Figure 6e), showing the unfavorable roles of B/P doping in g-C3N4 for HER. Conversely, S-CN/PC sample distinguished itself with the highest (though compromised after normalization) J0, highlighting the benefit from S doping for HER. In this context, the S-doped catalyst delivers the smallest charge-transfer resistance among all the hybrids (Figure S18), representing the most favorable overall HER kinetics resulting from both the intrinsic high activity and abundant exposed catalytic sites. The stability of the catalysts was also assessed by accelerated CV scan for 3000 cycles. At the end of the cycling tests, all four samples afford similar i−V curves as before with negligible loss, demonstrating the good durability of all hybrid catalysts in HER (Figure S19). A further atomic insight into the dopant-induced HER activity differences was acquired from the computed adsorption Gibbs free energy of the bonded hydrogen atom (|ΔGH*|). In the most favorable HER configurations (Figure S20), the proton bonds to the pyridinic N site due to the electronegativity and the lone pair electrons of the nitrogen atom. Generally, a favored HER catalyzer should feature a |ΔGH*| value around zero, which allows optimal adsorption/desorption kinetics.44 The calculated free energies of the doped composite samples are listed in Figure 6f, with the N-PC and bare carbon samples as references. Compared with the pristine C3N4 sample (−0.19 eV), the B- and P-doped composites (−2.25 and −1.87 eV, respectively) feature more negative ΔGH* values, indicating a more difficult desorption process of the H* species.16 In the cases of the N-PC and bare C samples, the ΔGH* values are too positive, and H* species cannot adsorb onto the catalyst surface, therefore they are also unsuitable for HER.39 The Sembedded hybrid catalyst, most promisingly, exhibits a desired ΔGH* value of −0.03 eV, which is even better than the wellknown Pt-based catalyst (|ΔGH*| = 0.09 eV), showing its competent thermodynamic potential as metal-free HER catalyst.44 Note that S-doped C3N4 is a n-type semiconductor, the dopant introduces an impurity band near the conducting band minimum (CBM), and the electrons could easily jump to the CBM by thermal excitation, which is rather favorable for H+ to capture electrons.45 Our theoretical results coincide very well with the HER experiments above, reinforcing again that the component is a significant factor which accounts for the distinguished activities.
EXPERIMENTAL SECTION Materials Synthesis. Preparation of Macroporous Carbon. The macroporous carbon (PC) was synthesized using as a hard template method. Typically, 3 g of silica powder (fumed, particle size ca. 0.2− 0.3 μm, Sigma-Aldrich) was dispersed into 50 mL of DI water, followed by addition of 3 g of sucrose and 0.3 g of sulfuric acid (96−97 wt %). This mixture solution was then sonicated for 20 min before being heated at 100 °C to evaporate all the liquid under stirring. The resulted solid was then heated at 160 °C for 10 h for the polymerization of sucrose and was eventually calcined at 900 °C for 3 h under Ar atmosphere with a ramp rate of 3 °C/min for carbonization of sucrose on the silica spheres (denoted as SiO2@C). The silica template was removed by immersing the SiO2@C in excessive 20 wt % HF solution for 24 h with magnetic stirring, followed by thorough washing with DI water until pH ∼ 7. Synthesis of the X (B, P, S)-CN/PC Composite. The PC supported B/P/S-doped g-C3N4 catalyst was fabricated by in situ polycondensation of different precursors with the presence of PC. The PC was first immersed in 65 wt % HNO3 solution for 1 h at room temperature to gently introduce hydrophilic functional groups (e.g., −COOH) and then was washed and dried. Dicyandiamide was adopted for synthesizing the g-C3N4 motif, and H3BO3 and ethylene diphosphonic acid were used as the B and P precursors, respectively. Bare trithiocyanuric acid served as the S-doped g-C3N4 precursor. In a typical procedure, 200 mg of dicyandiamide and a proper amount of the above B/P precursor were added into 10 mL water−ethanol (v:v = 1:1) solution with 50 mg PC. The mixture was sonicated for 1 h and heated at 70 °C under stirring to volatilize the solvent, during which the dicyandiamide and the acidic precursors assembled together onto the PC pore shell driven by the acid−base interactions between −COOH on the PC shell, basic dicyandiamide and boric acid/ ethylene diphosphonic acid. The resulting powder was then grinded and calcined at 600 °C for 4 h under Ar protection with a ramp rate of 5 °C/min in a crucible (with cover). The dopant-free CN/PC catalyst was synthesized with an identical procedure except bare dicyandiamide was mixed with PC. For the synthesis of S-CN/PC, 300 mg of trithiocyanuric acid was mixed with 50 mg of PC and treated with the same procedure. Physicochemical Characterization. The crystal structure of the catalyst was identified by a Bruker D2 Phaser XRD with Cu Kα radiation (λ = 0.15418 nm) operating at 30 kV and 10 mA, respectively. FTIR spectra were recorded from Nicolet 6700 FT-IR instrument. The morphology and microstructure of the samples were revealed by a JEOL-2001F field-emission TEM, and the accessory EELS was used to determine to composite elements. XPS analyses were conducted on an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scienctific) at 1.2 × 10−9 mbar using Al Kα X-ray beam (1486.6 eV). The XPS spectra were charge corrected to the adventitious C 1s peak at 284.5 eV. TG analyses were carried out on a TA #SDT Q600 analyzer at 30−800 °C with an Ar flow of 100 mL/ min. The nitrogen adsorption and desorption isotherms were characterized using a Micrometrics ASAP 2020 analyzer. Pore size distribution and specific surface area were obtained via Barrett− Joyner−Halenda and Brunauer−Emmett−Teller (BET) methods from
CONCLUSIONS In summary, we systematically investigated the dopant influences on the electrocatalytic ORR and HER performances of the macroporous carbon supported g-C3N4 hybrid catalysts. The different site-constituted B, P, and S elements exhibited 6012
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out at 5 mV s−1 from 0.2 to −0.6 V (vs RHE), and CV curves were recorded in a nonfaradaic potential range between 0.15 and 0.25 V (vs RHE) at scan rates from 20 to 200 mV s−1 to investigate the effective surface area of the catalyst. In the RRDE test, the potential of the Pt ring was set at 0.7 V (vs RHE) to oxidize the evolved H2 and record the oxidation current. All polarization curves were automatically corrected for the iR contribution from the cell. The working electrode was rotated at a speed of 1600 rpm to alleviate the accumulation of evolved hydrogen bubbles on the GCE surface. The durability of the catalyst was tested by potential cycling between −0.4 and 0.2 V (vs RHE) at 100 mV s−1 for 3000 cycles, during which a graphite rod was used as the counter electrode to avoid the possible contamination of Pt species.
adsorption branch of the isotherm, at a relative pressure range of P/P0 = 0.06−0.25. Electrochemical Measurements. ORR Tests. All the electrochemical measurements were carried out on a CHI 760D electrochemical workstation integrated with a RRDE-3A rotating ring disk electrode apparatus in a typical three-eletrode system, in which a glassy carbon electrode (GCE, 3 mm in diameter) loaded with different catalysts was used as working electrode, with a Ag/AgCl (in 3 M KCl) electrode and a Pt mesh (1 × 1 cm) as reference and counter electrode, respectively. 0.1 M KOH solution served as the electrolyte for all the measurements. For the fabrication of working electrode, 3 mg of catalyst was dispersed in a 1.5 mL H2O−isoproponal (v: v = 4:1) mixture together with 15 μL Nafion solution (5 wt %). The mixture was sonicated for 1 h, and 5 μL of the solution was then pipetted onto the mirror-polished GCE, which resulted in a loading mass of ca. 140 μg cm−2. The commercial Pt/C (Alfa Aesar, 20 wt %) of identical loading mass was used as reference. The CV curves were obtained at a scan rate of 20 mV s−1 in N2 or O2 saturated electrolyte in the potential window of −1.0 to 0.2 V (vs Ag/AgCl). Linear sweep voltammetry (LSV) curves within the same potential range were recorded in the O2 saturated electrolyte solution with a scan rate of 5 mV s−1 at various rotating speeds ranging from 400 to 3600 rpm. Each catalyst was repeated at least three times for the above measurements to exclude possible incidental errors. The transferred electron numbers (n) per O2 molecule and the kinetic current densities (JK) were determined from the Koutecky− Levich (K−L) equation expressed as follows:
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01908. Calculation details. Necessary characterizations mentioned in the main text (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
1 1 1 1 1 = + = + J JK JL JK Bω1/2
ORCID
Zhongfang Chen: 0000-0002-1445-9184 Chunyi Zhi: 0000-0001-6766-5953
where J is the measured current density, JK and JL are the kineticlimiting current density and the diffusion-limiting current density, respectively, ω is the rotation rate of the RDE, and B is the Levich slope given by
Author Contributions ⊥
Notes
B = 0.2nFC0D02/3v−1/6
The authors declare no competing financial interest.
in which n is the number of electrons transferred in the reduction of one O2 molecule, F is the Faraday constant (F = 96,485 C mol−1), C0 is the concentration of O2 in the solution (C0 = 1.2 × 10−6 mol cm−3), D0 is the diffusion coefficient of O2 in 0.1 M KOH (D0 = 1.9 × 10−5 cm2 s−1), and v is the kinematics viscosity of the electrolyte (v = 0.01 cm2 s−1). Constant 0.2 is adopted when rotating speed is in rpm. The RRDE tests were conducted with a Pt ring surrounded 4 mm diameter GCE (with loading mass of about 120 μg cm−2). The Pt ring electrode was set at 0.5 V to detect the generated HO2− species. The value of n was also calculated through RRDE tests:
n=
ACKNOWLEDGMENTS This work was supported by the NSFC/RGC Joint Research Scheme, under Project N_CityU 123/15, the Science Technology and Innovation Committee of Shenzhen Municipality (the grant no. JCYJ20130401145617276), and a Grant from the City University of Hong Kong. REFERENCES
4id id +
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ir N
the HO2− yield was calculated from equation: HO2−% =
200ir
(
N id +
ir N
These authors contributed equally to this work.
)
here id and ir are the disk current and ring current, respectively, and N is the current collection efficiency of the Pt ring and was determined to be 0.44 (details see Figure S11). Long-term stability tests were conducted by measuring the current changes of the GCE loaded catalysts at a fixed potential of −0.3 V (vs Ag/AgCl) and rotation speed of 1600 rpm in O2-saturated electrolyte. The crossover tolerance tests were performed by comparing the CV curves before and after the addition of 10 vol % methanol into the electrolyte. HER Tests. All the measurements were conducted in the same configuration as that in the ORR tests except that 0.5 M H2SO4 (purged with N2) served as the electrolyte solution. The potential was converted to a RHE by adding a value of (0.210 + 0.059 × pH). The loading mass for all samples was 280 μg cm−2. LSV curves were carried 6013
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DOI: 10.1021/acsnano.7b01908 ACS Nano 2017, 11, 6004−6014
