Improving the oxygen reduction reaction (ORR) activity of the Fe3C-based catalyst is greatly significant for energy conversion devices, such as fuel cells and Zn–air batteries (ZABs). Herein, biomass peanut shell is used to synthesize B,N-codoped defective graphitic carbon entangled Fe3C nanoparticles (D-BNGFe) via carbonizing and doping, followed by a defect creation process. In the synthesis, the usage of iron species could effectively prevent the formation of the B–N covalent bond, which maximizes the ORR activity. Moreover, a simple acid dissolution process was used to only reserve Fe3C nanoparticles encapsulated by carbon nanostructures, which can be stable during the electrocatalytic ORR as certified by operando X-ray absorption fine structure spectra and transmission electron microscopy characterization. Combining the ORR catalyst of D-BNGFe with the oxygen evolution reaction catalyst of NiFe-LDH@CF (NiFe-LDH grown on the copper foam substrate) as the air cathode, the assembled rechargeable ZABs feature a low-voltage gap, high power density, and excellent stability, which are superior to the state-of-the-art Pt/C + RuO2 and most of the nonprecious metal-based catalysts. This work provides an effective strategy to synthesize Pt-free catalysts derived from the natural plentiful biomass for satisfying the wide application of ZABs.
Developing non-precious-metal bifunctional oxygen reduction and evolution reaction (ORR/OER) catalysts is a major task for promoting the reaction efficiency of Zn-air batteries. Co-based catalysts have been regarded as promising ORR and OER catalysts owing to the multivalence characteristic of cobalt element. Herein, the synthesis of Co nanoislands rooted on Co-N-C nanosheets supported by carbon felts (Co/Co-N-C) is reported. Co nanosheets rooted on the carbon felt derived from electrodeposition are applied as the self-template and cobalt source. The synergistic effect of metal Co islands with OER activity and Co-N-C nanosheets with superior ORR performance leads to good bifuctional catalytic performances. Wavelet transform extended X-ray absorption fine spectroscopy and X-ray photoelectron spectroscopy certify the formation of Co (mainly Co0 ) and the Co-N-C (mainly Co2+ and Co3+ ) structure. As the air-cathode, the assembled aqueous Zn-air battery exhibits a small charge-discharge voltage gap (0.82 V@10 mA cm-2 ) and high power density of 132 mW cm-2 , outperforming the commercial Pt/C catalyst. Additionally, the cable flexible rechargeable Zn-air battery exhibits excellent bendable and durability. Density functional theory calculation is combined with operando X-ray absorption spectroscopy to further elucidate the active sites of oxygen reactions at the Co/Co-N-C cathode in Zn-air battery.
The development of nonprecious metal catalysts with both oxygen reduction and evolution reactions (ORR/OER) is very important for Zn-air batteries (ZABs). Herein, a Co5.47 N particles and Fe single atoms co-doped hollow carbon nanofiber self-supporting membrane (H-CoFe@NCNF) is synthesized by a coaxial electrospinning strategy combined with pyrolysis. X-ray absorption fine spectroscopy analyses confirm the state of the cobalt nitride and Fe single atoms. As a result, H-CoFe@NCNF exhibits a superior bifunctional performance of Eonset = 0.96 V for ORR, and Ej = 10 = 1.68 V for OER. Density functional theory calculations show that H-CoFe@NCNF has a moderate binding strength to oxygen due to the coexistence of nanoparticle and single atoms. Meanwhile, the Co site is more favorable to the OER, while the Fe site facilitates the ORR, and the proton and charge transfer between N and metal atoms further lower the reaction barriers. The liquid ZAB composed of H-CoFe@NCNF has a charge-discharge performance of ≈1100 h and a peak power density of 205 mW cm-2 . The quasi-solid-state ZAB assembled by the self-supporting membrane of H-CoFe@NCNF is proven to operate stably in any bending condition.
Abstract Dual‐metal single‐atom catalysts exhibit superior performance for oxygen reduction reaction (ORR), however, the synergistic catalytic mechanism is not deeply understood. Herein, we report a dual‐metal single‐atom catalyst consisted of Cu−N 4 and Zn−N 4 on the N‐doped carbon support (Cu/Zn−NC). It exhibits high‐efficiency ORR activity with an E onset of 0.98 V and an E 1/2 of 0.83 V, excellent stability (no degradation after 10 000 cycles), surpassing state‐of‐the‐art Pt/C and great mass of Pt‐free single atom catalysts. Operando XANES demonstrates that the Cu−N 4 as active center experiences the change from atomic dispersion to cluster with the cooperation of Zn−N 4 during ORR process, and then turns to single atom state again after reaction. DFT calculation further indicates that the adjustment effect of Zn on the d‐orbital electron distribution of Cu could benefit to the stretch and cleavage of O‐O on Cu active center, speeding up the process of rate determining step of OOH*.
Abstract Thermally‐stable, ordered mesoporous anatase TiO 2 with large pore size and high crystallinity has been successfully synthesized through an evaporation‐induced self‐assembly technique, combined with encircling ethylenediamine (EN) protectors to maintain the liquid crystal mesophase structure of TiO 2 primary particles, followed by calcination at higher temperature. The structures of the prepared mesoporous TiO 2 are characterized in detail by small‐angle and wide‐angle X‐ray diffraction, Raman spectra, N 2 adsorption/desorption isotherms, and transmission electron microscopy. Experimental results indicate that the well‐ordered mesoporous structure could be maintained up to 700 °C (M700) and also possesses large pore size (10 nm), high specific BET surface area (122 m 2 g −1 ), and high total pore volumes (0.20 cm 3 g −1 ), which is attributed to encircling EN protectors for maintaining the mesoporous framework against collapsing, inhibiting undesirable grain growth and phase transformation during the calcination process. A possible formation mechanism for the highly stable large‐pore mesoporous anatase TiO 2 is also proposed here, which could be further confirmed by TG/FT‐IR in site analysis and X‐ray photoelectron spectroscopy. The obtained mesoporous TiO 2 of M700 exhibit better photocatalytic activity than that of Degussa P25 TiO 2 for degradation of highly toxic 2,4‐dichlorophenol under UV irradiation. This enhancement is attributed to the well‐ordered large‐pore mesoporous structure, which facilitates mass transport, the large surface area offering more active sites, and high crystallinity that favors the separation of photogenerated electron‐hole pairs, confirmed by surface photovoltage spectra.
Understanding the dynamic process of interfacial charge transfer prior to chemisorption is crucial to the development of electrocatalysis. Recently, interfacial water has been highlighted in transferring protons through the electrode/electrolyte interface; however, the identification of the related structural configurations and their influences on the catalytic mechanism is largely complicated by the amorphous and mutable structure of the electrical double layer (EDL). To this end, sub-nanometric Pt electrocatalysts, potentially offering intriguing activity and featuring fully exposed atoms, are studied to uncover the elusive electrode/electrolyte interface via operando X-ray absorption spectroscopy during the hydrogen evolution reaction (HER). Our results show that the metallic Pt clusters derived from the reduction of sub-nanometric Pt clusters (SNM-Pt) exhibit excellent HER activity, with an only 18 mV overpotential at 10 mA/cm2 and one-magnitude-higher mass activity than commercial Pt/C. More importantly, a unique Pt-interfacial water configuration with a Pt (from Pt clusters)–O (from water) radial distance of approximately 2.5 Å is experimentally identified as the structural foundation for the interfacial proton transfer. Toward high overpotentials, the interfacial water that structurally evolves from "O-close" to "O-far" accelerates the proton transfer and is responsible for the improved reaction rate by increasing the hydrogen coverage.
Selective oxidation of methane to oxygenates with O2 under mild conditions remains a great challenge. Here we report a ZSM-5 (Z-5) supported PdCu bimetallic catalyst (PdCu/Z-5) for methane conversion to oxygenates by reacting with O2 in the presence of H2 at low temperature (120 °C). Benefiting from the co-existence of PdO nanoparticles and Cu single atoms via tandem catalysis, the PdCu/Z-5 catalyst exhibited a high oxygenates yield of 1178 mmol g-1Pd h-1 (mmol of oxygenates per gram Pd per hour) and at the same time high oxygenates selectivity of up to 95 %. Control experiments and mechanistic studies revealed that PdO nanoparticles promoted the in situ generation of H2 O2 from O2 and H2 , while Cu single atoms not only accelerated the activation of H2 O2 for the generation of abundant hydroxyl radicals (⋅OH) from H2 O2 decomposition, but also enabled the homolytic cleavage of CH4 by ⋅OH to methyl radicals (⋅CH3 ). Subsequently, the ⋅OH reacted quickly with the ⋅CH3 to form CH3 OH with high selectivity.
Abstract The electrocatalysts for high-energy consumed anodic oxygen evolution reaction (OER) especially in water splitting are generally prone to reconfiguration, so the dynamic structural evolution mechanisms should be deeply investigated. Herein, coral-like nanoarray assembled by nanosheets were synthesized via the layered effect of cobalt (Co) and the one-dimensional guiding effect of vanadium (V). The unique structure facilitates the full contact between active sites and electrolyte to enhance the electrocatalytic activity. The hydrogen evolution reaction (HER) and OER activity can be respectively promoted through modulating the electronic structure with nitrogen and phosphate anions. Thus, the assembled anion exchange membrane electrolyzer exhibits a direct current energy consumption of 4.31 kWh Nm–3@250 mA cm–2 at 70°C. It only required 1.88 V voltage to achieve a current density of 500 mA cm–2 with excellent stability over 200 h. Operando synchrotron radiation and Bode phase angle analyses reveal that the dissolution of vanadium species makes the distorted Co-O octahedral to regular octahedral structure during OER, accompanying by a decrease of band gap and a shortening of the Co-Co bond length. Such a structural evolution plays as the key active site for the formation of oxygen-containing intermediates, thereby accelerating the reaction kinetics.
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