Zhenghua Tang

Mesoporous N-Doped Carbons Prepared with Thermally Removable Nanoparticle Templates: An Efficient Electrocatalyst for Oxygen Reduction Reaction

Thermally removable nanoparticle templates were used for the fabrication of self-supported N-doped mesoporous carbons with a trace amount of Fe (Fe-N/C). Experimentally Fe-N/C was prepared by pyrolysis of poly(2-fluoroaniline) (P2FANI) containing a number of FeO(OH) nanorods that were prepared by a one-pot hydrothermal synthesis and homogeneously distributed within the polymer matrix. The FeO(OH) nanocrystals acted as rigid templates to prevent the collapse of P2FANI during the carbonization process, where a mesoporous skeleton was formed with a medium surface area of about 400 m(2)/g. Subsequent thermal treatments at elevated temperatures led to the decomposition and evaporation of the FeO(OH) nanocrystals and the formation of mesoporous carbons with the surface area markedly enhanced to 934.8 m(2)/g. Electrochemical measurements revealed that the resulting mesoporous carbons exhibited apparent electrocatalytic activity for oxygen reduction reactions (ORR), and the one prepared at 800 °C (Fe-N/C-800) was the best among the series, with a more positive onset potential (+0.98 V vs RHE), higher diffusion-limited current, higher selectivity (number of electron transfer n > 3.95 at +0.75 V vs RHE), much higher stability, and stronger tolerance against methanol crossover than commercial Pt/C catalysts in a 0.1 M KOH solution. The remarkable ORR performance was attributed to the high surface area and sufficient exposure of electrocatalytically active sites that arose primarily from N-doped carbons with minor contributions from Fe-containing species.

MoS2 nanosheet-coated CoS2 nanowire arrays on carbon cloth as three-dimensional electrodes for efficient electrocatalytic hydrogen evolution

The design and engineering of low-cost and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) has attracted increasing interest in renewable energy research. Herein, MoS2 nanosheet-coated CoS2 nanowire arrays supported on carbon cloth (MoS2/CoS2/CC) were prepared by a two-step procedure that entailed the hydrothermal growth of Co(OH)2 nanowire arrays on carbon cloth followed by reaction with (NH4)2MoS4 to grow an overlayer of MoS2 nanosheets. Electrochemical studies showed that the obtained 3D electrode exhibited excellent HER activity with an overpotential of −87 mV at 10 mA cm−2, a small Tafel slope of 73.4 mV dec−1 and prominent electrochemical durability. The results presented herein may offer a new methodology for the design and engineering of effective multilevel structured catalysts for the HER based on earth-abundant components.

Porous Carbon-Supported Gold Nanoparticles for Oxygen Reduction Reaction: Effects of Nanoparticle Size

Porous carbon-supported gold nanoparticles of varied sizes were prepared using thiolate-capped molecular Au25, Au38, and Au144 nanoclusters as precursors. The organic capping ligands were removed by pyrolysis at controlled temperatures, resulting in good dispersion of gold nanoparticles within the porous carbons, although the nanoparticle sizes were somewhat larger than those of the respective nanocluster precursors. The resulting nanocomposites displayed apparent activity in the electroreduction of oxygen in alkaline solutions, which increased with decreasing nanoparticle dimensions. Among the series of samples tested, the nanocomposite prepared with Au25 nanoclusters displayed the best activity, as manifested by the positive onset potential at +0.95 V vs RHE, remarkable sustainable stability, and high numbers of electron transfer at (3.60-3.92) at potentials from +0.50 to +0.80 V. The performance is comparable to that of commercial 20 wt % Pt/C. The results demonstrated the unique feasibility of porous carbon-supported gold nanoparticles as high-efficiency ORR catalysts.

Bioreduction of Precious Metals by Microorganism: Efficient Gold@N-Doped Carbon Electrocatalysts for the Hydrogen Evolution Reaction

The uptake of precious metals from electronic waste is of environmental significance and potential commercial value. A facile bioreductive synthesis is described for Au nanoparticles (ca. 20 nm) supported on N-doped carbon (Au@NC), which was derived from Au/Pycnoporus sanguineus cells. The interface and charge transport between Au and N-doped carbon were confirmed by HRTEM and XPS. Au@NC was employed as an electrocatalyst for the hydrogen evolution reaction (HER), exhibiting a small onset potential of -54.1 mV (vs. RHE), a Tafel slope of 76.8 mV dec(-1) , as well as robust stability in acidic medium. Au@NC is a multifunctional electrocatalyst, which demonstrates high catalytic activity in the oxygen reduction reaction (ORR), as evidenced by an onset potential of +0.97 V, excellent tolerance toward methanol, and long-term stability. This work exemplifies dual recovery of precious Au and fabrication of multifunctional electrocatalysts in an environmentally benign and application-oriented manner.

Graphene‐Supported Mesoporous Carbons Prepared with Thermally Removable Templates as Efficient Catalysts for Oxygen Electroreduction

Graphene-supported mesoporous carbons with rich nitrogen self-doped active sites (N-MC/rGO) are prepared by direct pyrolysis of a graphene-oxide-supported polymer composite embedded with massive, evenly distributed amorphous FeOOH that serve as efficient thermally removable templates. The resulting N-MC/rGO catalysts exhibit high surface areas and apparent electrocatalytic activity for oxygen reduction reaction in alkaline media. Among the series, the sample prepared at 800 °C displays the best performance with a more positive onset potential, higher limiting currents, much higher stability, and stronger poison resistance than commercial Pt/C. This is ascribed to the synergetic functions of the highly conductive graphene support and the mesoporous N-doped carbons that effectively impede the restacking of the graphene sheets and enhance the exposure of the rich nitrogen self-doped active sites.

Metal–Carbon Hybrid Electrocatalysts Derived from Ion‐Exchange Resin Containing Heavy Metals for Efficient Hydrogen Evolution Reaction

Transition metal-carbon hybrids have been proposed as efficient electrocatalysts for hydrogen evolution reaction (HER) in acidic media. Herein, effective HER electrocatalysts based on metal-carbon composites are prepared by controlled pyrolysis of resin containing a variety of heavy metals. For the first time, Cr2 O3 nanoparticles of 3-6 nm in diameter homogeneously dispersed in the resulting porous carbon framework (Cr-C hybrid) is synthesized as efficient HER electrocatalyst. Electrochemical measurements show that Cr-C hybrids display a high HER activity with an onset potential of -49 mV (vs reversible hydrogen electrode), a Tafel slope of 90 mV dec(-1) , a large catalytic current density of 10 mA cm(-2) at -123 mV, and the prominent electrochemical durability. X-ray photoelectron spectroscopic measurements confirm that electron transfer occurs from Cr2 O3 into carbon, which is consistent with the reported metal@carbon systems. The obtained correlation between metals and HER activities may be exploited as a rational guideline in the design and engineering of HER electrocatalysts.

Graphene Composites with Cobalt Sulfide: Efficient Trifunctional Electrocatalysts for Oxygen Reversible Catalysis and Hydrogen Production in the Same Electrolyte

Nitrogen and sulfur-codoped graphene composites with Co9 S8 (NS/rGO-Co) are synthesized by facile thermal annealing of graphene oxides with cobalt nitrate and thiourea in an ammonium atmosphere. Significantly, in 0.1 m KOH aqueous solution the best sample exhibits an oxygen evolution reaction (OER) activity that is superior to that of benchmark RuO2 catalysts, an oxygen reduction reaction (ORR) activity that is comparable to that of commercial Pt/C, and an overpotential of only -0.193 V to reach 10 mA cm-2 for hydrogen evolution reaction (HER). With this single catalyst for oxygen reversible electrocatalysis, a potential difference of only 0.700 V is observed in 0.1 m KOH solution between the half-wave potential in ORR and the potential to reach 10 mA cm-2 in OER; in addition, an overpotential of only 450 mV is needed to reach 10 mA cm-2 for full water splitting in the same electrolyte. The present trifunctional catalytic activities are markedly better than leading results reported in recent literature, where the remarkable trifunctional activity is attributed to the synergetic effects between N,S-codoped rGO, and Co9 S8 nanoparticles. These results highlight the significance of deliberate structural engineering in the preparation of multifunctional electrocatalysts for versatile electrochemical reactions.

Ordered mesoporous carbons-supported gold nanoparticles as highly efficient electrocatalysts for oxygen reduction reaction

Unlike bulk gold, gold nanoparticles (AuNPs) have been found to exhibit apparent electrocatalytic activity for oxygen reduction reaction (ORR). In this work, glutathione-capped AuNPs of ca. 4 nm in diameter were prepared by a wet chemical method and embedded into a mesoporous carbon matrix synthesized by using an SBA-15 amorphous silica template. Pyrolysis at elevated temperatures led to removal of the organic capping ligands and the formation of ordered mesoporous carbons loaded with a selected amount of AuNPs which exhibited no apparent agglomeration. The obtained nanocomposites exhibited apparent ORR activity in alkaline media, and the sample with 20 wt% AuNPs stood out as the best among the series, within the context of onset potential, kinetic current density and stability, in comparison with AuNPs alone, ordered mesoporous carbons, and commercial Pt/C catalysts. The remarkable performance was ascribed to the intimate interactions between the AuNPs and mesoporous carbons that facilitated fast electron transfer and rapid mass transport. This is the first time that ordered mesoporous carbons were employed to support AuNPs as ORR catalysts. The strategy may be exploited to prepare a wide range of electrocatalysts based on mesoporous carbons-supported metal nanoparticles with extraordinary reactivity and enhanced stability for ORR.

Peptide FlgA3 Based AuPd Bimetallic Nanoparticles toward Oxygen Reduction Reaction in Alkaline Solution

Given the synergetic properties of different metal species, bimetallic nanoparticles are of immense scientific interest and technological importance in the field of catalysis. A peptide‐based method represents a new avenue to fabricate bimetallic nanocatalysts with a controllable size, shape, composition, and subtle surface microstructure. However, the electrocatalytic abilities of these peptide‐based bimetallic nanoparticles remain largely unexplored. Herein, we employed the peptide sequence FlgA3 to fabricate a series of AuPd alloys that were used as catalysts for the oxygen reduction reaction (ORR). Among the samples tested, Au33Pd67 exhibited the best activity, evidenced by the most positive onset potential and the largest diffusion‐limited current. Notably, the ORR activity of Au33Pd67 was comparable with that of commercial Pt/C, and the long‐term durability was significantly superior to that of Pt/C. A peptide‐enabled approach might pave the way for the fabrication of bimetallic nanomaterials with enhanced electrocatalytic properties under mild conditions.

Co@Pd core-shell nanoparticles embedded in nitrogen-doped porous carbon as dual functional electrocatalysts for both oxygen reduction and hydrogen evolution reactions

Developing efficient bi-functional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for producing hydrogen and utilizing hydrogen effectively to promote electrochemical energy storage in proton membrane exchange fuel cells (PEMFCs). Herein, we report Co@Pd core-shell nanoparticles encapsulated in porous carbon derived from zeolitic imidazolate framework 67 (ZIF-67) for both ORR and HER. The controlled pyrolysis of ZIF-67 can lead to the formation of Co nanoparticles encapsulated in nitrogen-doped porous carbon (Co NC), which subsequently underwent galvanic replacement with Na2PdCl4 to form Co@Pd core-shell nanoparticles embedded in nitrogen-doped porous carbon (Co@Pd NC). The Co@Pd NC exhibited outperformance in ORR and HER than commercial Pd/C, as manifested by more positive onset potential and larger diffusion-limited current density in ORR tests, as well as a small overpotential to drive a current density of 10 mA cm-2, and much lower Tafel slope in HER tests. It also demonstrated more robust long-term stability than commercial Pd/C for both ORR and HER. Multiple techniques inter-confirmed that the Pd loading in the sample was very low. The findings can pave a path for fabricating a core-shell structured nanocomposite with ultralow noble metal usage as a bifunctional catalyst for electrochemical energy storage and conversion with high-efficiency and remarkable longevity.