mV for 50 mA/cm2, and 7 mV for 100 mA/cm2, which are much more stable than Pt/C catalysts. Furthermore, the turnover frequency (TOF) has been calculated with the simplified assumption that all Mo atoms are involved in the reaction (Figure 4.2e). For mC-Mo-850, a TOF of 2.0 H2 per s at overpotentials of 275 mV are found, while the non-mesoporous C-Mo-850 shows only 0.08 H2 per s at the same overpotential.
Notably, the TOF values of mC-Mo-850 even exceed the ones for Pt/C when the overpotentials are higher than 239 mV. For direct comparison, the reported TOF values of several state-of-the-art non-precious metal HER catalysts published recently, including MOF derived MoCx,[46] γ-Mo2N,[155] and CoP on carbon cloth (CoP/CC)[156]
are introduced into this graph, it can be clearly observed that our catalyst, mC-Mo-850, shows the highest TOF value.
Chapter 4. Metal-Organic Precursor Derived Mesoporous Carbon Spheres Coupled with Mo2C/Mo2N
5 Summary
Chapter 2 delivers a new type of metal-organic precursor, by employing DA as the organic precursor, FeCl3·6H2O as the metal precursor as well as active salt-template. The synthetic method is very simple and efficient, and the obtained metal-organic precursor shows a novel layered structure. The structure of the precursor was studied, as well as the thermal stability. By pyrolysis of the metal-organic precursor, 2D carbon nanosheets can be obtained with abundant microporous on the surface. The synthesized 2D microporous carbon exhibits a surface area as high as 1752 m2 g-1. Eventually, the doped Fe and N species enable a remarkable ORR activity, which even outperforms the-state-of-art Pt/C catalyst in alkaline condition. When applied as air cathode in a home-made Zn-air battery, it also outperforms the Pt/C cathode with a high working voltage even at discharge current density of 100 mA cm-2.
With the promising results in Chapter 2, a follow-up project is described in Chapter 3 to extend the application of the new layered DA-Fe metal-organic precursor. In this Chapter, an additional silica template is assembled into the system to synthesize 2D meso/microporous carbons with FeCo-Nx doping for construction of high performance bifunctional ORR and OER catalysts. The successfully assembled silica nanoparticles within the metal-organic precursor introduces a large amount of mesopores into the resulting carbon material. The FeCo-Nx doped 2D meso/micro-porous carbon shows even higher performance in ORR than the material obtained in Chapter 2. Meanwhile, the OER activity is also highly improved with FeCo co-doping. The best catalyst is then utilized for a rechargeable Zn-air battery. The battery shows good cycle performance with stable long-term charge/discharge processes. This project opens a new pathway for synthesizing 2D meso/micro-porous materials, meanwhile, the novel strategy is also very simple and can be easily scaled-up for production.
Chapter 4 introduces a new type of metal-organic precursor for synthesizing
Chapter 5. Summary
highly active HER catalyst. DA was employed again as organic precursor, but this time with sodium molybdate as the inorganic part. Silica nanoparticles were again used as hard template and assembled well with the DA-Mo precursor. The obtained precursor is carbonized to obtain Mo2C/Mo2N heterojunctions on mesoporous carbon spheres.
Distinct from previous studies on conventional MOF precursors, our novel synthetic strategy shows obvious advantages for large-scale production. Furthermore, the in situ and confined growth of small Mo-based nanocrystallites and Mo2C/Mo2N heterojunctions are guaranteed by the strong and atomic DA-Mo coordination structures, which may provide a novel and simple strategy to synthesize heterojunction-based catalyst. The optimized Mo-based mesoporous electrocatalyst shows comparable hydrogen evolution activity and much superior stability to the state-of-art Pt/C catalyst in alkaline condition.
Overall, in this thesis, new types of metal-organic precursors and novel synthetic strategies have been developed, which create new pathways for the controllable synthesis of different kinds of micro-/nanostructured catalysts. Furthermore, these precursor routes are a first step towards a fast and scalable production of advanced mesoporous carbon electrodes for a broad range of applications.
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7 Publication Reprints
7.1 2D Porous Carbons prepared from Layered Organic–
Inorganic Hybrids and their Use as Oxygen-Reduction Electrocatalysts
This section reprints the following paper with permission of John Wiley and Sons.
Copyright 2017.
S. Li, C. Cheng, H.‐W. Liang, X. L. Feng, A. Thomas, 2D Porous Carbons prepared from Layered Organic–Inorganic Hybrids and their Use as Oxygen Reduction Electrocatalysts, Adv. Mater. 2017, 29, 1700707.
https://doi.org/10.1002/adma.201700707
Published version
Chapter 7. Publication Reprints
CommuniCation
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2D Porous Carbons prepared from Layered Organic–
Inorganic Hybrids and their Use as Oxygen-Reduction Electrocatalysts
Shuang Li, Chong Cheng, Hai-Wei Liang, Xinliang Feng,* and Arne Thomas*
S. Li, Prof. A. Thomas Functional Materials Department of Chemistry Technische Universität Berlin
Hardenbergstr. 40, 10623 Berlin, Germany E-mail: arne.thomas@tu-berlin.de Dr. C. Cheng
Department of Chemistry and Biochemistry Freie Universität Berlin
Takustrasse 3, 14195 Berlin, Germany Prof. H.-W. Liang, Prof. X. L. Feng
Center for Advancing Electronics Dresden (CFAED) Department of Chemistry and Food Chemistry Technische Universität Dresden
01062 Dresden, Germany
E-mail: xinliang.feng@tu-dresden.de
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201700707.
DOI: 10.1002/adma.201700707
has triggered enormous research interests on 2D carbon materials. Such 2D car-bons show sub-nanometer to nanometer thickness, infinite length, and confined 2D structural features with unique elec-trical charge transport pathways. Impor-tantly for electrochemical applications, in such 2D nanomaterials, the entire carbon framework is exposed to the electrolyte and can take part in the electrochemical reactions.[6–8] Combining the benefits of 2D morphology with meso- or micro-porous structures, Ruoff and co-workers fabricated a 2D microporous graphene sheet for the application in supercapaci-tors by a direct activation of graphene with molten potassium hydroxide. The surface area of this activated graphene reached up to 3100 m2 g−1.[9] Feng and co-workers reported several 2D porous carbons by using graphene as template via different strategies.[10–16] Beside these examples, it has been reported that 2D inorganic com-pounds, such as montmorillonite or layered double hydroxides, can also be applied for synthesizing 2D porous carbon nano-materials.[17,18] All these synthetic methods undisputedly yield materials with promising properties for various fields of appli-cations. However, the synthetic processes for fabricating 2D porous carbons are either quite complex or enormous time/
energy consuming and a large scale synthesis of these materials seems nearly impossible. It is indeed still a major challenge to develop more facile and efficient protocols for the fabrication of 2D porous carbons with controllable morphology and large specific surface areas.
An inorganic molten salt-templating method to synthesize porous carbons with high surface area was recently proposed by Antonietti and co-workers.[19] In this method, inorganic molten salts, such as LiCl, NaCl, ZnCl2, and others, are used as solvents and templates for the generation of high surface area carbon materials. The salts are simply mixed with carbon precursors, thermally treated for carbonization and after the reaction is complete, they can be easily and completely removed by washing.[19,20] However, the currently reported salt-templates are usually not influencing the carbonization process and are mainly generating porosity but no further distinct morphology of the carbon materials, e.g., the genera-tion of 2D nanostructures from the salt-templating method has rarely been reported.[21] In this work, a facile and scalable 2D porous carbon nanomaterials have attracted tremendous attention in
different disciplines especially for electrochemical catalysis. The significant advantage of such 2D materials is that nearly all their surfaces are exposed to the electrolyte and can take part in the electrochemical reaction. Here, a versatile active-salt-templating strategy to efficiently synthesize 2D porous carbon nanosheets from layered organic–inorganic hybrids is presented. The resulting heteroatom-doped carbon nanosheets (NFe/CNs) exhibit excep-tional performance for the oxygen-reduction reaction and in Zn–air bat-tery electrodes. The activity of the best catalyst within a series of NFe/CNs exceeds the performance of conventional carbon-supported Pt catalysts in terms of onset potential (0.930 vs 0.915 V of Pt/C), half-wave potential (0.859 vs 0.816 V of Pt/C), long-time stability, and methanol tolerance. Also, when applied as a cathode catalyst in a zinc–air battery the NFe/CNs presented here outperform commercial Pt/C catalysts.
Electrocatalysts
Porous carbon nanomaterials are intriguing materials, which combine good electrical conductivity, low weight, and high sur-face areas with chemical/electrochemical activity and durability.
These properties have motivated numerous studies concerning the applications of porous carbon nanomaterials in various areas, for example, as electrodes in lithium-ion batteries or supercapacitors, for the storage of gases or as catalysts or cata-lyst supports.[1–5] On the other hand, the discovery of graphene
7.1. 2D Porous Carbons prepared from Layered Organic–Inorganic Hybrids and their Use as Oxygen Reduction Electrocatalysts
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active-salt-templating approach is presented in which a metal salt and an organic molecule form a layered metal–organic hybrid material at room temperature, which predefines the structure of the final carbon material, yielding highly porous and N-doped carbon nanosheets.
Iron(III) chloride hexahydrate (FeCl3·6H2O) possess a crystal structure composed of 2D layers[22] (see Scheme S1, Supporting Information) and is widely used in the intercalation of graphite or few layer graphene to increase the layer distance, which, e.g., yield an optimal performance for lithium-ion batteries.[22–24]
Inspired by this intercalation reactions and its chemical com-plexation ability,[24] FeCl3·6H2O is applied as an active salt tem-plate, which first interacts with the organic precursors, bearing hydroxyl, and/or amino-groups, to form a layered organic–
inorganic hybrid structure. This layered structure is preserved during the carbonization process and controls the final carbon morphology to yield 2D porous carbon nanosheets (CNs) with a few nanometer thicknesses after removal of the inorganic template. Doping of the carbon materials with heteroatoms can be achieved applying different carbon-rich precursors, such as dopamine hydrochloride (DA), catechol (Cat), or aniline (An).
As a representative model containing both catechol and amino groups, DA was investigated in detail in this work.
In a typical synthesis, FeCl3·6H2O powder was mixed directly with the carbon precursor, here first DA. After vigorous grinding, the mixture of dopamine–FeCl3·6H2O (DA–Fe) was aged for several days. Catechol type compounds can strongly bind to transition metals; thus, it is reasonable that during
grinding the catechol groups of dopamine coordinate to the Fe3+ ions to form a metal–organic coordination compound (Figure 1a). It should be noted that in this approach an excess of the iron salt is used (i.e., 1:7 molar ratio of DA:FeCl3·6H2O if not noted otherwise) to avoid molecular Fe(dopamine)3 com-plex formation so that the dopamine molecules coordinate most probably along the layers of the FeCl3·6H2O salt (Figure 1a).[25]
The strong interaction between the iron salt and the dopamine is proven by a notable color and constitution change during grinding. The mixture transfers into a viscous paste, which starts to solidify apparently into crystals after standing in air (Figure 1b; Figure S1 and S2, Supporting Information). X-ray diffraction (XRD) was measured directly after the grinding pro-cess and then after different aging times (Figure 1c). The rela-tively simple XRD patterns with just a few but sharp peaks indi-cate the formation of a periodic layered structure after 3–5 days seen in the occurrence of a peak at 2θ = 15.7°, which can be attributed to the interlayer distance of the DA–Fe complex of 5.64 Å. Changing the ratio between DA and FeCl3·6H2O, reveals similar patterns however with changing intensity of the peaks (Figure S2a, Supporting Information).
To examine if such layered structures can solely be formed from the DA–Fe complex, a catechol–FeCl3·6H2O (Cat–Fe) and an aniline–FeCl3·6H2O (An–Fe) mixture were also prepared, respectively, which again yields a paste which solidifies after several days (Figure S2c and S3, Supporting Information). XRD measurements show patterns that are comparable to those of the DA–Fe mixture, however, with a shift of the peaks to larger Figure 1. a) Schematic illustration of the overall synthetic procedure of 2D microporous carbon nanosheets (CNs). b) Photographs of freshly prepared DA–Fe paste and crystals after aging. c) XRD patterns of the paste after different aging time. d) SEM image of CNs obtained from the DA–Fe complex.
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angles (Figure S2a, Supporting Information), pointing to a decreased interlayer distance, which can be attributed to the smaller size of the catechol and aniline molecule.
Subsequently, the crystallized DA–Fe mixture was thermally treated under argon atmosphere. The 2D growth of porous carbon nanosheets is hypothesized to occur via thermal cross-linking of the DA ligands coordinated on the FeCl3·6H2O layered crystals. Indeed, DA–Fe derived materials prepared by heating the complex to lower temperatures (e.g., 200, 300, or 400 °C) already possess a layered or sheet-like structure (Figure S4 and S5, Supporting Information). However, for the envisaged application as advanced electrodes, i.e., as non-noble-metal catalysts for the oxygen-reduction reaction (ORR), a good elec-trical conductivity is needed. Therefore, the DA–Fe mixture was further heated to higher temperatures of 600–800 °C (named NFe/CNs-T, where “T” denotes the carbonization tempera-ture). After carbonization, the resulting materials were carefully washed with 1 m HCl to remove residual iron species. Trans-mission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements indicated that the obtained DA–Fe-based porous carbon prepared at 600 and 700 °C both showed 2D sheet-like morphologies with a size from hundreds of nanometers to several micrometers. When the carbonization temperature increased to 800 °C, the thin sheet-like morphology tends to transform into a vesicle-sheet-like structure (Figure S6, Supporting Information). High-resolution TEM images further indicate that the nanosheets NFe/CNs-600 and NFe/CNs-700 are highly microporous (Figure 2b,e). On the other hand, NFe/CNs-800 shows a distinct graphitic but less-porous structure (Figure 2h). Notably, no metal particles are observed within all the TEM images after washing with 1 m HCl solution. The thickness of the observed carbon nanosheets was investigated by AFM (Figure 2c,f,i), giving an average thick-ness of these nanosheets of about 30–40, 5–15, and 45–55 nm for NFe/CNs-600, NFe/CNs-700, and NFe/CNs-800, respec-tively. The increased thickness of NFe/CNs-800 can be attrib-uted to structural rearrangement of the first formed porous carbon sheets in the presence of the Fe catalysts. In addition, the influence of the DA/Fe salt ratio on the resulting carbon nanostructures was investigated. Different molar ratios of DA and FeCl3·6H2O (3:1, 1:1.5, 1:3.5, 1:7, and 1:14) were applied, and the chemical structures, morphologies, and surface areas of the resulting NFe/CNs after carbonization at 700 °C were studied (Figure S7 and S8, Supporting Information). It is noted that only when the DA–Fe molar ratio decreases to 1:3.5 or lower (1:7 and 1:14) the obtained CNs exhibit a uniform 2D morphology, proving the importance that the layered structure of FeCl3·6H2O is maintained after addition of the organic pre-cursors for the formation of 2D carbon structures.
Nitrogen-sorption measurements were carried out to analyze the porous characteristics of the NFe/CNs materials (Figure 3a).
The calculated Brunauer–Emmett–Teller surface areas of NFe/
CNs-600, NFe/CNs-700, and NFe/CNs-800 are 1071, 1752, and 395 m2 g−1, respectively. The pore-size distribution (PSD) analysis shows that the pores of all materials are mostly smaller than 2 nm, that is, the high surface areas of the NFe/CNs can be mainly attributed to microporosity. For all NFe/CNs, especially NFe/CNs-700, several maxima are seen in the PSD (marked in Figure 3b), showing smaller and larger micropores and even
some mesopores around 4 nm. It has been reported that such hierarchical pore structures may not only promote the forma-tion of metal–N active cites but also enhance the mass transfer during the ORR.[16,18]
XRD (Figure 3c) and Raman spectroscopy (Figure 3d) meas-urements were taken to investigate the degree of graphitization obtained after different thermal treatments. The obtained NFe/
CNs shows increased graphitization along with the increased temperature from the XRD results. The peaks for NFe/CNs-800 are much more pronounced than for NFe/CNs-600 and NFe/CNs-700, which indicates that NFe/CNs-800 possesses a much higher graphitic ordering. The XRD measurements of the NFe/CNs before acid washing (Figure S5, Supporting Infor-mation) show that a considerable amount of Fe/Fe3C has been formed when the temperature reaches 800 °C, which can be the reason for increased graphitization. Raman spectra support the conclusions from XRD measurements. 600, NFe/CNs-700, and NFe/CNs-800 all display two intensive peaks around 1350 and 1580 cm−1, which can be attributed to the D and G band. The G band arises from the bond stretching of all sp2 -bonded pairs, while the D band is associated with the sp3 defec-tive sites. The G band of NFe/CNs-800 is distinctly sharper than the one of NFe/CNs-600 and NFe/CNs-700, and the intensity ratio of the G to the D band (IG/ID) significantly increases from 1.1 (both NFe/CNs-600 and NFe/CNs-700) to 4.2 (NFe/CNs-800).
Furthermore, a sharp 2D band peak is observed in the spectrum of NFe/CNs-800 (corresponding IG/I2D is 3.9) indicating the formation of multilayer high quality graphene.[26] In summary, the XRD and Raman spectra confirm the high crystallinity with pronounced graphitic stacking for NFe/CNs-800, which on the other hand results in a much lower specific surface area.
The elemental composition and nature of the heteroatom doping in the resultant 2D carbon nanosheets were explored by X-ray photoelectron spectroscopy (XPS). The binding situation of nitrogen within nitrogen-doped carbons has been frequently reported to be an essential factor for their catalytic activity. XPS revealed nitrogen contents of 4.7, 4.0, and 1.1 wt% in NFe/CNs-600, NFe/CNs-700, and NFe/CNs-800, respectively (Figure S6d, Supporting Information). The high-resolution N 1s core-level spectra of these materials can be deconvoluted into four peaks at 398.5, 399.5, 401.0, and 402.6 eV, which are attributed to pyridine-like, pyrrole-like, graphitic N, and pyridinic N+O−, respectively.[27,28] It should be noted that mainly pyridine-like and graphitic nitrogen have been described to be beneficial for the catalytic activity in ORR. For NFe/CNs-600, all four nitrogen species can be detected with a majority of pyrrole-like N.
Notably, heating the samples to 700 °C drastically changes this binding situation, showing a strong decrease in the amount of pyrrole-like nitrogen and also, to a lower extent, of pyridine-like nitrogen, making graphitic nitrogen the overwhelming species.
Considering that the overall nitrogen amount has just slightly decreased (from 4.7 to 4.0 wt%), it can be assumed that essen-tially just the nitrogen located on edges of the carbon sheets are removed due to the increased temperature treatment. This observation is certainly promising for the ORR activity since inactive N in the catalyst is removed or transferred into more active species at 700 °C, i.e., into pyridine-like or graphitic N.[15] For NFe/CNs-800, in which the overall N content has decreased significantly, the binding situation has changed again