Covalent Porphyrin Framework-Derived Fe2P@Fe4N-Coupled

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Covalent porphyrin frameworks-derived Fe2P@Fe4N coupled nanoparticles embedded in N-doped carbons as efficient trifunctional electrocatalysts Xiaohong Fan, Fantao Kong, Aiguo Kong, Aoling Chen, Ziqian Zhou, and Yongkui Shan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11229 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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ACS Applied Materials & Interfaces

Covalent

porphyrin

frameworks-derived

Fe2P@Fe4N

coupled nanoparticles embedded in N-doped carbons as efficient trifunctional electrocatalysts Xiaohong Fan, Fantao Kong, Aiguo Kong,*Aoling Chen, ZiqianZhou, and Yongkui Shan * School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, China KEYWORDS: Covalent porphyrin polymer, Triphenylphosphine, Iron nitride, Phosphate iron, Multifunctional electrocatalysts

ABSTRACT: New porous covalent porphyrin framework (CPF) filling with triphenylphosphine was designed and synthesized using the rigid tetrakis(p-bromophenyl) porphyrin (TBPP) and 1,3,5benzene-tri-boronic acid trivalent alcohol ester as building blocks. The carbonization of this special CPF has derived the coupled Fe2P and Fe4N nanoparticles embedding in N-doped carbons (Fe2P/Fe4N@N-doped carbons). This CPF served as the “all in one” precursors of Fe, N, P and C. The porous property and solid skeleton of the CPF endow Fe2P/Fe4N@N-doped carbons with the porous structures and higher graphitization degree. As a result, Fe2P/Fe4N@N-doped carbons exhibited highly efficient multifunctional electrocatalytic performance for water splitting and oxygen electroreduction. Typically, Fe2P/Fe4N@C-800 obtained at the heat-treatment temperature of 800 °C was found to show the ORR half-wave potential of 0.80 V in alkaline media and 0.68 V in acidic

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media, close to the commercial Pt/C catalysts. Fe2P/Fe4N@C-800 also displayed the efficient OER and HER activities, comparative to other phosphide and nitride electrocatalysts. The coupled Fe4N and Fe2P nanoparticles embedding in carbons exert their unique catalytic efficiency in water-splitting and fuel cells.

1. INTRODUCTION Converting renewable energy into hydrogen and oxygen by electrocatalytic water splitting, and recovering them in the form of proton exchange membrane fuel cells (PEMFCs) showed the great promise for the “green” energy storage and conversion1-6. Currently, the worldwide efforts are directing toward seeking for high-efficiency, stable and inexpensive alternative catalysts to replace the precious metal catalysts (Pt and RuO2 etc.) for cathodic oxygen reduction reaction (ORR) in PEMFCs as well as oxygen and hydrogen evolution reactions (OER and HER) in water-spitting electrolytic system7-9. Carbon-supported transition metal sulfides, selenides, nitrides, carbides, phosphides, borides, oxides, and hydro(oxy)oxides have been demonstrated as effective ORR, OER, or HER catalysts10-20. The efficient multifunctional catalysts for the electrocatalytic conversion or production of H2O would decrease the complexity of catalyst layers and thus reduce the cost of the whole electrocatalytic system of water-splitting and PEMFCs. However, most of these catalysts only catalyzed the specific reaction and showed the lower overall catalysis activities for water-splitting or fuel cells. Typically, carbon-supported metal nitrides such as FeN21, Fe2N22, and Fe3N23 have been reported to be active ORR electrocatalysts in alkaline medium, but often insufficient activity for HER and OER. Metal phosphides such as CoP24, 25, NiP26 and recently MnCoP27 often were reported to be efficient HER and OER electrocatalysts, but often low active for ORR. It should be a good choice to obtain the effective trifunctional catalysts for ORR, OER and HER by coupling metal nitrides and


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phosphides on the porous carbons. However, there are few reports about the coupled iron nitrides and phosphides for multifunctional electrocatalysis. Covalent porphyrin frameworks (CPFs) are the special organic polymers constructed by strong covalent bonds using porphyrin macrocycle molecules as building blocks. The rigidity and conjugation structures at the molecular level of CPFs endow them the inherent porous properties28. The tuning metal atoms in the form of coordinated metal-N4 and the location control of nonmetaldopant heteroatoms (for example, N and S) also served as the special functional regulators for the derived carbons29-31. As a result, the CPFs-derived carbon materials were explored as the effective and multifunctional “self-supported” electrocatalysts32,

33

. Two-dimensional (2D) covalent porphyrin

polymers with Mn, Fe, Co and Ni center atoms have successfully derived the efficient metal-N-C materials for ORR34-38. The location of metal distribution in carbons can be even controlled by carbonization of the synthesized heterometallic CPFs. However, although CPFs-derived catalysts are often active for ORR, the overall electrocatalytic performance for water splitting is insignificant. There are few reports about the CPFs-derived metal nitrides and especially phosphides electrocatalysts with better electrocatalytic performance for water splitting. Moreover, the reported CPFs often are 2D crystalline polymers owing to the strong π-π stacking effect of porphyrin rings, which may hinder the exposure of active metal species39, 40. The construction and conversion of new CPFs with open structures would be the promising strategy to obtain high-efficiency electrocatalysts for overall water splitting or fuel cells. In the present work, focusing on the low-cost and active iron-based trifunctional electrocatalysts, a new planar multi-directional porphyrin polymer (Fe-TPP-CPFs) was constructed by Suzuki-Miyaura cross-coupling reactions using the rigid iron(III)-5,10,15,20-tetrakis(p-bromophenyl) porphyrin (FeTBPP) and 1,3,5-benzene-tri-boronic acid trivalent alcohol ester (BTA) as building blocks (Figure 1).


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This new microporous 3D CPF featured a relatively large pore volume and narrow pore size distribution. In particular, triphenylphosphine (TPP) molecules may be coordinated to the Fe-N4 sites of porphyrin building blocks, in the form of octahedral Fe-N4-P2 groups. The thermal conversion of this special CPF induced the formation of coupled Fe4N (other than FeN, Fe2N and Fe3N) and Fe2P nanoparticles, which were together embedded into the nitrogen-doped carbon frameworks. The resultant materials by the conversion of such CPFs were demonstrated as the highly-active trifunctional electrocatalysts for HER, ORR and OER. 2. EXPERIMENTAL SECTION 2.1 Synthesis 2.1.1 Synthesis of 5,10,15,20-Tetrakis (4-bromophenyl) porphine (TBPP) The solution of 4-bromobenzaldehyde (18.5 g, 0.1 mmol) in acetic acid (500 mL) was heated up to 130 °C, and then freshly distilled pyrrole (6.7 g, 0.1 mmol) was added drop by drop. The reaction mixture was stirred at 130 °C for 10 h. After being filtered and dried in vacuo, a black solid was given. This solid was added in pyridine (150 mL) and refluxed for 5 h. The cooled mixtures was overnight at -4 °C to give TBPP as a bright purple powder. Fe-TBPP as the dark brown solid was prepared by refluxing TBPP and anhydrous FeCl3 in N, N-dimethylformamide (DMF) under nitrogen atmosphere using the reported synthesis method in literature41. 2.1.2 Synthesis of Fe-TPP-CPFs and Fe2P/Fe4N@C Fe-TBPP (0.893 g, 0.9 mmol), BTA (0.547 g, 1.2 mmol), and TPP (0.12 g, 0.46 mmol) were added in a three-necked flask with 100 mL DMF under flowing nitrogen. After that, an aqueous solution (30 mL) of Na2CO3 (2 g) and tetrakis(triphenyl phosphine) palladium (0) (0.1 g, 0.09 mmol, TPP-Pd)


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were added. The resulting mixture was stirred and refluxed at 120 °C for 12 h. The obtained solid mixture was filtered and washed with deionized water, tetrahydrofuran, ethanol and acetone successively. Finally, Fe-TPP-CPFs as brown powdery solid was obtained after dried in vacuum at 60 °C. Similarly, iron-free porphyrin frameworks (TBPP-CPFs) were also prepared by the abovementioned method using TBPP and BTA as substrates. The dried solid products of Fe-TPP-CPFs were heated at 600-900 °C for 4 h in a quartz tube furnace under nitrogen atmosphere, with a heating rate of 2 °C min-1. The prepared samples are assigned to Fe2P/Fe4N@C-n (n is the thermal-treatment temperature). For comparison, the TBPP-CPFs were also used as the precursors to yield N-C-n by the same heat-treatment procedures. For the removal of the main Fe2P and Fe4N particles, Fe2P/Fe4N@C-800 was grinded and leached in 5 wt% HF solution for 12 h to give Fe2P/Fe4N@C-800-HF. 2.2 Characterization X-ray diffraction patterns (XRD) for catalysts were obtained on a D8 Advance X-ray diffractometer (Bruker AXS, Germany) with CuKa radiation. Solid-state 13C NMR spectra were recorded at ambient temperature on a Bruker AVANCE III 600 MHz spectrometer. Transmission electron microscope (TEM) images were observed on JEM transmission electron microscope with the acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were measured on a Thermo ESCALAB 250 using Al Kα radiation (1486.6 eV). N2-sorption analysis measurements were carried out at 77 K on a QuadrasorbEvo surface area and porosity analyzer (Quanta chrome Instrument, USA). UV-visible diffuse reflectance spectra were carried out on a Shimadzu UV 2401 PC with an integrating sphere attachment. BaSO4 was used as background standard. Fourier transform infrared (FT-IR) spectra were recorded with an infrared spectrophotometer (Bruker Tensor 27) in KBr pellets.


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Raman spectroscopy of the samples was performed on a GX-PT-1500 (150) instrument with a 532 nm excitation laser (1 mW). 2.3 Electrochemical measurement The CHI-800C electrochemical analyzer with a three electrode system was used to investigate the electrochemical performance for the prepared samples. A glassy carbon RDE (5mm in diameter, Pine) was used as working electrode and the reference electrode was an Ag/AgCl, KCl (3 M) electrode. The counter electrode was a Pt wire in ORR and OER measurement or a graphite rod in HER. The typical working electrodes were prepared by pipetting the catalyst ink onto a glassy carbon electrode surface (0.196 cm2) and then drying under an infrared lamp. The catalyst ink was prepared by dispersing 10 mg catalysts in a solution containing 1.25 mL ethanol and 30 µL 5 wt% Nafion-isopropanol solutions. The ORR experiments of the prepared N-C-800 and Fe2P/Fe4N@C-n catalysts were carried out in 0.1 M KOH and 0.1 M HClO4 solution at a scan rate of 5 mV s-1 at room temperature. The reported potentials were corrected to the reversible hydrogen electrode (RHE) potentials (Figure S1). The general loading of carbon catalysts on the working electrodes was 0.30 mg cm-2 in 0.1 M KOH and 0.60 mg cm-2 in 0.1 M HClO4 solution. The catalyst loading of Pt/C was 0.1 mg cm-2 on the electrode in both electrolytes. The Koutecky-Levich (K-L) equations were used to calculate the kinetic parameters for ORR42: 1 1 1 1 1 = + = + ; B = 0.62nFC0 D02 / 3ν −1 / 6 ; J K = nFκC0 1/ 2 J J K J L Bω JK

For the OER performance of the prepared catalysts was estimated from linear scan voltammeter (LSV) plots in 1 M KOH solution at a scan rate of 5 mV s-1 at room temperature. The HER performance of the prepared catalysts was estimated from LSV plots in 0.5 M H2SO4 solution using the graphite rod


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as the counter electrode, instead of Pt electrode. The general loading of carbon catalysts in OER and HER on the working electrode was 1.5 mg cm-2. The LSV curves were replotted as overpotential (η) versus log current (log j) to get Tafel plots for quantification of the OER and HER performance of investigated catalysts. By fitting the linear portion of the Tafel plots to the Tafel equation (η=b log (j) + a), the Tafel slope (b) can be obtained. 3 RESULTS AND DISCUSSIONS 3.1 Synthesis of CPFs with the introduced TPP molecules The porphyrin-based covalent polymers with 3D network structure could be synthesized by a typical Suzuki-Miyaura cross-coupling reaction of TBPP and BTA (Figure 1). The structure of the porphyrinbased covalent polymer was confirmed by solid-state

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C NMR spectroscopy (Figure 2), which was

used to detect carbon linkage of TBPP-CPFs. Carbon signals from the benzene ring merged together to form an easily distinguished peaks around 129.7 ppm as well as two shoulder peaks at 136.9 and 140 ppm. Additionally, two small spikes at 119 and 160.7 ppm attributing to the carbon atoms in the pyrrole ring were also observed38, 41. The FT-IR spectra (Figure 3A) of TBPP-CPFs showed that the band at 638 cm-1 corresponding to C-Br vibrations of TBPP precursors disappeared in TBPP-CPFs. The C-Br bond in TBPP was broken and replaced by C-C bond after undergoing the coupling polymer reaction, indicating the successfully construction of the covalent framework28. Clear FT-IR bands at 1586 and 1474 cm-1 suggested the main backbone of the phenyl and pyrrole rings in TBPP-CPFs (Table S1). UV-visible diffuse reflectance spectra (Figure 3B) displayed absorption bands at 401 nm attributing to the Soret band of the CPF. The bands at 524, 560, 598 and 651 nm represented the Q bands of porphyrin units41. The entire Q-bands in 500-700 nm have significantly changed due to the successful synthesis of polymer backbone, which was consistent with the NMR observations and FT-


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IR results. These results demonstrated the formation of TBPP-CPFs frameworks by this typical coupling reaction. The high similarity in the FT-IR vibration bands between Fe-TPP-CPFs and TBPP-CPFs hinted their similar organic frameworks41. The formation of Fe-TPP-CPFs from Fe-TBPP and BTA were confirmed by the vanishing of N-H stretching vibration peaks at 3313 cm-1 in the FT-IR spectrum of Fe-TPP-CPFs, owing to the formation of Fe-N4 moieties (Table S1). The characteristic Fe-N vibration band at 998 cm-1 was also clearly detected41. These results fully demonstrate that iron(III) has replaced pyrrole protons in porphyrin rings to produce iron porphyrins. The XPS measurement (Figure 4A-a) confirmed the existence of C, N, Fe, and P in the Fe-TPP-CPFs. The deconvoluted Fe 2P

3/2

of Fe-TPP-CPFs at binding energy of 711.3 eV also testified the successful introduction of

Fe(III) ion43. Triphenylphosphine was initially added into the reaction system for protecting the TPP-Pd (0) catalyst. It is interesting that part of TPP molecules were remained in the final Fe-TPP-CPFs and difficult to be washed by DMF solvent. The peak at 696 cm-1 in the FT-IR spectrum (Figure 3A) of Fe-TPP-CPFs corresponded to the vibration absorption peak of the P-C bonds. The deconvolution of the P 2p XPS (Figure 4D) signals of the Fe-TPP-CPFs gives a peak with binding energies centred at of 132.3 and 129.7 eV, assigning to P-C and Fe-P coordinated species of Fe-TPP-CPFs44, 45. Moreover, there was a weak vibration peak of Fe-P in the FT-IR spectrum of Fe-TPP-CPFs at 580 cm-146. TPP molecules may be coordinated to Fe(III) atoms in the porphyrin plane in the form of the octahedral structures (Fe-N4-P2). It actually served as a phosphorus source for the derived Fe2P/Fe4N@C. The large-angle XRD of TBPP-CPFs and Fe-TPP-CPFs (Figure S2) confirmed their amorphous backbones due to the twisted 3D porous network structures. The sheet-like stack morphology of


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TBPP-CPFs and Fe-TPP-CPFs were also observed in the corresponding SEM images (Figure S3-A, B). Plenty of micropores in the sheets of Fe-TPP-CPFs could be found by HRTEM (Figure S3-D). The N2-sorption isotherms (Figure 5) of Fe-TPP-CPFs material exhibited a significant uptake in the low pressure region (P/P0

Covalent Porphyrin Framework-Derived Fe2P@Fe4N-Coupled

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