Coordination polymer derived Fe-N-C electrocatalysts with high performance for the oxygen reduction reaction in Zn-air batteries.

Developing high performance noble-metal-free electrocatalysts as an alternative to Pt-based catalysts for the oxygen reduction reaction (ORR) in energy conversion devices is highly desirable. We report herein the preparation of a coordination-polymer (CP)-derived Fe/CP/C composite as an electrocatalyst for the ORR with excellent activity and stability both in solution and in Zn-air batteries. The Fe/CP/C catalyst was obtained from the pyrolysis of an iron porphyrin Fe(TPP)Cl (5,10,15,20-tetraphenyl-21H,23H-porphyrin iron(III) chloride) grafted Zn-coordination polymer with dangling functional groups 4,4'-oxybisbenzoic acid and 4,4'-bipyridine ligands. The Fe/CP/C catalyst showed much higher ORR activity with a half-wave potential (E1/2) of 0.90 V (vs. RHE) than the Fe/C catalyst (E1/2 = 0.85 V) derived from the carbon-black-supported Fe porphyrins in 0.1 M KOH solution. When Fe/CP/C was used as the cathode electrocatalyst in Zn-air batteries (ZABs), the ZABs achieved a significantly higher open circuit voltage (OCV = 1.43 V) and maximum power density (Pmax = 142.8 mW cm-2) compared with Fe/C (OCV = 1.38 V, Pmax = 104.5 mW cm-2) and commercial 20 wt% Pt/C (OCV = 1.41 V, Pmax = 117.6 mW cm-2). Using dangling functional groups in CP to increase the loading efficiency of iron porphyrins offered a facile method to prepare high-performance noble-metal-free electrocatalysts for the ORR, which may provide promising applications to energy conversion devices.

A High-Nuclearity Copper Sulfide Nanocluster [S-Cu50] Featuring a Double-Shell Structure Configuration with Cu(II)/Cu(I) Valences.

This work represents an important step in the quest for creating atomically precise binary semiconductor nanoclusters (BS-NCs). Compared with coinage metal NCs, the preparation of BS-NCs requires strict control of the reaction kinetics to guarantee the formation of an atomically precise single phase under mild conditions, which otherwise could lead to the generation of multiple phases. Herein, we developed an acid-assisted thiolate dissociation approach that employs suitable acid to induce cleavage of the S-C bonds in the Cu-S-R (R = alkyl) precursor, spontaneously fostering the formation of the [Cu-S-Cu] skeleton upon the addition of extra Cu sources. Through this method, a high-nuclearity copper sulfide nanocluster, Cu50S12(SC(CH3)3)20(CF3COO)12 (abbreviated as [S-Cu50] hereafter), has been successfully prepared in high yield, and its atomic structure was accurately modeled through single-crystal X-ray diffraction. It was revealed that [S-Cu50] exhibits a unique double-shell structural configuration of [Cu14S12]@[Cu36S20], and the innermost [Cu14] moiety displays a rhombic dodecahedron geometry, which has never been observed in previously synthesized Cu metal, hydride, or chalcogenide NCs. Importantly, [S-Cu50] represents the first example incorporating mixed Cu(II)/Cu(I) valences in reported atomically precise copper sulfide NCs, which was unambiguously confirmed by XPS, EPR, and XANES. In addition, the electronic structure of [S-Cu50] was established by a variety of optical investigations, including absorption, photoluminescence, and ultrafast transient absorption spectroscopies, as well as theoretical calculations. Moreover, [S-Cu50] is air-stable and demonstrates electrocatalytic activity in ORR with a four-electron pathway.

Contracted Fe-N5-C11 Sites in Single-Atom Catalysts Boosting Catalytic Performance for Oxygen Reduction Reaction.

Promoting the catalyst performance for oxygen reduction reaction (ORR) in energy conversion devices through controlled manipulation of the structure of catalytic active sites has been a major challenge. In this work, we prepared Fe-N-C single-atom catalysts (SACs) with Fe-N5 active sites and found that the catalytic activity of the catalyst with shrinkable Fe-N5-C11 sites for ORR was significantly improved compared with the catalyst bearing normal Fe-N5-C12 sites. The catalyst C@PVI-(TPC)Fe-800, prepared by pyrolyzing an axial-imidazole-coordinated iron corrole precursor, exhibited positive shifted half-wave potential (E1/2 = 0.89 V vs RHE) and higher peak power density (Pmax = 129 mW/cm2) than the iron porphyrin-derived counterpart C@PVI-(TPP)Fe-800 (E1/2 = 0.81 V, Pmax = 110 mW/cm2) in 0.1 M KOH electrolyte and Zn-air batteries, respectively. X-ray absorption spectroscopy (XAS) analysis of C@PVI-(TPC)Fe-800 revealed a contracted Fe-N5-C11 structure with iron in a higher oxidation state than the porphyrin-derived Fe-N5-C12 counterpart. Density functional theory (DFT) calculations demonstrated that C@PVI-(TPC)Fe-800 possesses a higher HOMO energy level than C@PVI-(TPP)Fe-800, which can increase its electron-donating ability and thus help achieve enhanced O2 adsorption as well as O-O bond activation. This work provides a new approach to tune the active site structure of SACs with unique contracted Fe-N5-C11 sites that remarkably promote the catalyst performance, suggesting significant implications for catalyst design in energy conversion devices.

A P-doped PtNi alloy supported on N,C-doped TiO2 nanosheets as a stable electrocatalyst for the oxygen reduction reaction in an acidic electrolyte.

A P-doped PtNi alloy loaded on N,C-doped TiO2 nanosheets (P-PtNi@N,C-TiO2) exhibited excellent activity and durability for the oxygen reduction reaction (ORR) in 0.1 M HClO4 solution with mass (4×) and specific (6×) activity several times higher than those of commercial 20 wt% Pt/C, respectively. The P dopant mitigated the dissolution of Ni and strong interactions between the catalyst and the N,C-TiO2 support inhibited catalyst migration. This provides a new approach for the design of high-performance non-carbon-supported low-Pt catalysts to be used in harsh acidic environments.

Axial Ligand Coordination Tuning of the Electrocatalytic Activity of Iron Porphyrin Electrografted onto Carbon Nanotubes for the Oxygen Reduction Reaction.

The oxygen reduction reaction (ORR) is essential in many life processes and energy conversion systems. It is desirable to design transition metal molecular catalysts inspired by enzymatic oxygen activation/reduction processes as an alternative to noble-metal-Pt-based ORR electrocatalysts, especially in view point of fuel cell commercialization. We have fabricated bio-inspired molecular catalysts electrografted onto multiwalled carbon nanotubes (MWCNTs) in which 5,10,15,20-tetra(pentafluorophenyl) iron porphyrin (iron porphyrin FeF20 TPP) is coordinated with covalently electrografted axial ligands varying from thiophene to imidazole on the MWCNTs' surface. The catalysts' electrocatalytic activity varied with the axial coordination environment (i. e., S-thiophene, N-imidazole, and O-carboxylate); the imidazole-coordinated catalyst MWCNTs-Im-FeF20 TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNT-Im-FeF20 TPP was loaded onto the cathode of a zinc-air battery, an open-cell voltage (OCV) of 1.35 V and a maximum power density (Pmax ) of 110 mW cm-2 were achieved; this was higher than those of MWCNTs-Thi-FeF20 TPP (OCV=1.30 V, Pmax =100 mW cm-2 ) and MWCNTs-Ox-FeF20 TPP (OCV=1.28 V, Pmax =86 mW cm-2 ) and comparable with a commercial Pt/C catalyst (OCV=1.45 V, Pmax =120 mW cm-2 ) under similar experimental conditions. This study provides a time-saving method to prepare covalently immobilized molecular electrocatalysts on carbon-based materials with structure-performance correlation that is also applicable to the design of other electrografted catalysts for energy conversion.

Design and Preparation of Fe-N5 Catalytic Sites in Single-Atom Catalysts for Enhancing the Oxygen Reduction Reaction in Fuel Cells.

There is an urgent need for developing nonprecious metal catalysts to replace Pt-based electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Atomically dispersed M-Nx/C catalysts have shown promising ORR activity; however, enhancing their performance through modulating their active site structure is still a challenge. In this study, a simple approach was proposed for preparing atomically dispersed iron catalysts embedded in nitrogen- and fluorine-doped porous carbon materials with five-coordinated Fe-N5 sites. The C@PVI-(DFTPP)Fe-800 catalyst, obtained through pyrolysis of a bio-inspired iron porphyrin precursor coordinated with an axial imidazole from the surface of polyvinylimidazole-grafted carbon black at 800 °C under an Ar atmosphere, exhibited a high electrocatalytic activity with a half-wave potential of 0.88 V versus the reversible hydrogen electrode for ORR through a four-electron reduction pathway in alkaline media. In addition, an anion-exchange membrane electrode assembly (MEA) with C@PVI-(DFTPP)Fe-800 as the cathode electrocatalyst generated a maximum power density of 0.104 W cm-2 and a current density of 0.317 mA cm-2. X-ray absorption spectroscopy demonstrated that a single-atom catalyst (Fe-Nx/C) with an Fe-N5 active site can selectively be obtained; furthermore, the catalyst ORR activity can be tuned using fluorine atom doping through appropriate pre-assembling of the molecular catalyst on a carbon support followed by pyrolysis. This provides an effective strategy to prepare structure-performance-correlated electrocatalysts at the molecular level with a large number of M-Nx active sites for ORR. This method can also be utilized for designing other catalysts.

Bioinspired Transition-Metal Complexes as Electrocatalysts for the Oxygen Reduction Reaction.

The oxygen reduction reaction (ORR) is one of the most important reactions in life processes and energy conversion systems. To alleviate global warming and the energy crisis, the development of high-performance electrocatalysts for the ORR for application in energy conversion and storage devices such as metal-air batteries and fuel cells is highly desirable. Inspired by the biological oxygen activation/reduction process associated with heme- and multicopper-containing metalloenzymes, iron and copper-based transition-metal complexes have been extensively explored as ORR electrocatalysts. Herein, an outline into recent progress on non-precious-metal electrocatalysts for the ORR is provided; these electrocatalysts do not require pyrolysis treatment, which is regarded as desirable from the viewpoint of bioinspired molecular catalyst design, focusing on iron/cobalt macrocycles (porphyrins, phthalocyanines, and corroles) and copper complexes in which the ORR activity is tuned by ligand variation/substitution, the method of catalyst immobilization, and the underlying supporting materials. Current challenges and exciting imminent developments in bioinspired ORR electrocatalysts are summarized and proposed.

Efficient electrocatalytic O2 reduction at copper complexes grafted onto polyvinylimidazole coated carbon nanotubes.

A facile approach to prepare Cu complexes for an efficient oxygen reduction reaction (ORR) was developed. Copper complexes of 5-nitrophenanthroline were sandwiched between polyvinylimidazole layers wrapped on carbon nanotubes, which showed ORR activity comparable to a Pt/C catalyst in alkaline media.

Titanium Dioxide-Grafted Copper Complexes: High-Performance Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media.

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathodes of fuel cells significantly hampers fuel cell performance. Therefore, the development of high-performance, non-precious-metal catalysts as alternatives to noble metal Pt-based ORR electrocatalysts is highly desirable for the large-scale commercialization of fuel cells. TiO2 -grafted copper complexes deposited on multiwalled carbon nanotubes (CNTs) form stable and efficient electrocatalysts for the ORR. The optimized catalyst composite CNTs@TiO2 -ZA-[Cu(phen${{^{{\rm NO}{_{2}}}}}$)(BTC)] shows surprisingly high selectivity for the 4 e(-) reduction of O2 to water (approximately 97 %) in alkaline solution with an onset potential of 0.988 V vs. RHE, and demonstrates superior stability and excellent tolerance for the methanol crossover effect in comparison to a commercial Pt/C catalyst. The copper complexes were grafted onto the surface of TiO2 through coordination of an imidazole-containing ligand, zoledronic acid (ZA), which binds to TiO2 through its bis-phosphoric acid anchoring group. Rational optimization of the copper catalyst's ORR performance was achieved by using an electron-deficient ligand, 5-nitro-1,10-phenanthroline (phen${{^{{\rm NO}{_{2}}}}}$), and bridging benzene-1,3,5-tricarboxylate (BTC). This facile approach to the assembly of copper catalysts on TiO2 with rationally tuned ORR activity will have significant implications for the development of high-performance, non-precious-metal ORR catalysts.

Bio-inspired multinuclear copper complexes covalently immobilized on reduced graphene oxide as efficient electrocatalysts for the oxygen reduction reaction.

Inspired by the multicopper active site of laccase, which efficiently catalyzes the oxygen reduction reaction (ORR), herein we report a novel bio-inspired ORR catalyst composed of a multinuclear copper complex that was immobilized on the surface of reduced graphene oxide (rGO) via the covalently grafted triazole-dipyridine (TADPy) dinucleating ligand. This rGO-TADPyCu catalyst exhibited high ORR activity and superior long-term stability compared to Pt/C in alkaline media.

Covalent grafting of carbon nanotubes with a biomimetic heme model compound to enhance oxygen reduction reactions.

The oxygen reduction reaction (ORR) is one of the most important reactions in both life processes and energy conversion systems. The replacement of noble-metal Pt-based ORR electrocatalysts by nonprecious-metal catalysts is crucial for the large-scale commercialization of automotive fuel cells. Inspired by the mechanisms of dioxygen activation by metalloenzymes, herein we report a structurally well-defined, bio-inspired ORR catalyst that consists of a biomimetic model compound-an axial imidazole-coordinated porphyrin-covalently attached to multiwalled carbon nanotubes. Without pyrolysis, this bio-inspired electrocatalyst demonstrates superior ORR activity and stability compared to those of the state-of-the-art Pt/C catalyst in both acidic and alkaline solutions, thus making it a promising alternative as an ORR electrocatalyst for application in fuel-cell technology.

[Surface enhanced Raman spectroscopic studies on the coadsorption of N-methylimidazole and 2,2'-bipyridine at Cu electrode].

The adsorption behavior and coadsorption of N-methylimidazole (NMIM) and 2,2'-bipyridine(2,2'-bipy) at copper electrode were investigated by in situ electrochemical surface enhanced Raman spectroscopy (SERS) in the acetonitrile solution. In situ SERS studies revealed that NMIM can adsorb stably onto Cu electrode in a quite different potential range, but the potential range for adsorbing 2,2'-bipy is narrow. With the introduction of 2,2'-bipy into the solution, the SERS could be divided into three parts: (a) under -0.8 V, NMIM molecule adsorption, (b) near the open potential, 2,2'-bipy molecule adsorption with cis-conformation, (c) at positive potential region, both NMIM and 2,2'-bipy were coadsorbed at Cu surface, and the SERS data also suggested that the NMIM bound to copper surface with a tilted orientation, while the 2,2'-bipy was adsorbed through cis-conformation to the surface.

[Surface coordination chemistry of benzotriazole probed by electrochemical Raman spectroscopy].

The surface coordination chemistry of benzotriazole (BTAH) at silver electrode was investigated by in situ electrochemical surface enhanced Raman spectroscopy (SERS) and electrochemical synthesis in the acetonitrile solution. In situ SERS studies revealed that BTAH underwent three processes of chemical adsorption of BTAH, formation of compact layer of (AgBTA)n and the loss of SERS activity due to the oxidation of silver. The surface coordination processes of BTAH with Cu, Ag, Zn, Ni and Fe were investigated and the surface complexes prepared by direct electrochemical synthesis method were characterized by microanalysis and Raman spectroscopy. The influence of the neutral ligand of triphenylphosphine (pph3) on the coordination process was deduced. The introduction of pph3 was found to affect the surface processes of BTAH with Cu and Ag, and appeared in the final complex, while it had no influence on the coordination of BTAH with Zn, Ni and Fe.