Chen-Hao Wang

Highly Efficient Visible Light Photocatalytic Reduction of CO2 to Hydrocarbon Fuels by Cu-Nanoparticle Decorated Graphene Oxide

The production of renewable solar fuel through CO2 photoreduction, namely artificial photosynthesis, has gained tremendous attention in recent times due to the limited availability of fossil-fuel resources and global climate change caused by rising anthropogenic CO2 in the atmosphere. In this study, graphene oxide (GO) decorated with copper nanoparticles (Cu-NPs), hereafter referred to as Cu/GO, has been used to enhance photocatalytic CO2 reduction under visible-light. A rapid one-pot microwave process was used to prepare the Cu/GO hybrids with various Cu contents. The attributes of metallic copper nanoparticles (∼4-5 nm in size) in the GO hybrid are shown to significantly enhance the photocatalytic activity of GO, primarily through the suppression of electron-hole pair recombination, further reduction of GOs bandgap, and modification of its work function. X-ray photoemission spectroscopy studies indicate a charge transfer from GO to Cu. A strong interaction is observed between the metal content of the Cu/GO hybrids and the rates of formation and selectivity of the products. A factor of greater than 60 times enhancement in CO2 to fuel catalytic efficiency has been demonstrated using Cu/GO-2 (10 wt % Cu) compared with that using pristine GO.

High performance of catalysts supported by directly grown PTFE-free micro-porous CNT layer in a proton exchange membrane fuel cell

A proton exchange membrane fuel cell (PEMFC) uses a solid polymer electrolyte, viz. Nafion®, sandwiched between the two electrodes. Nafion® not only plays the role as an electronic insulator and gas barrier but also allows rapid proton transport and supports high current densities. In order to maintain the high proton conductivity of Nafion®, humidified H2 and O2 are passed through the two electrodes. However, water gets easily condensed in the electrodes. This process, called water-flooding, degrades the performance of PEMFC. Hence, a hydrophobic agent, viz. polytetrafluoroethylene (PTFE), is normally incorporated into the electrodes to prevent this phenomenon. Since it is electrically insulating, the incorporation of PTFE increases the internal resistance of the fuel cell. In this study, we successfully demonstrate a PEMFC with catalyst layer comprising of low loading of platinum nanoparticles (0.05 mg cm−2) supported by a directly grown micro-porous carbon nanotube (CNT) layer without incorporation of PTFE, (Pt/MPL-CNT). This cell performs well without exhibiting water-flooding. A commercial electrode, the catalyst layer of which was supported by a conventional micro-porous layer of carbon black mixed with 30 w.t.% PTFE, was used as a reference (Pt/PTFE-MPL-CB). In the single cell tests, PEMFCs with 0.05 mg cm−2Pt/MPL-CNT and 0.25 mg cm−2Pt/PTFE-MPL-CB were used at the cathodes. These cells yielded maximum power densities of 902 mW cm−2 and 824 mW cm−2, respectively, at 70 °C when operated with H2/O2. Notably, the Pt-loading of Pt/MPL-CNT cell is one-fifth of that of Pt/PTFE-MPL-CB, but the former still outperforms the latter. It is shown that the directly grown micro-porous CNT layer has low electronic resistance and is intrinsically hydrophobic, which are the properties responsible for the high performance obtained here.

Electrocatalytic activity of Nb-doped hexagonal WO3 nanowire-modified graphite felt as a positive electrode for vanadium redox flow batteries

In this paper, we report a facile hydrothermal method to synthesize low-cost, high-catalytic-activity, and stable niobium-doped hexagonal tungsten trioxide nanowires (Nb-doped h-WO3 NWs); these NWs were employed as catalysts to improve the electrocatalytic activity of graphite felt (GF) electrodes for use as positive electrodes in an all-vanadium redox flow battery (VRFB). The effect of Nb doping and its composition on the electrochemical performance of GF electrodes for a VRFB was investigated. Cyclic voltammetry and electrochemical impedance spectroscopy results showed that Nb-doped h-WO3 NWs with a Nb/W atomic ratio of 0.03 exhibited the highest electrocatalytic activities for VO2+/VO2+ couples among all the tested electrodes. This observation was attributed to the optimal Nb-doping concentration producing moderate defect states, thereby creating structural disorders, such as oxygen vacancies, in WO3 and leading to the generation of more active sites for the VO2+/VO2+ redox reaction on the electrode. Moreover, in charge–discharge tests, a VRFB single cell using the Nb-doped h-WO3 NW (Nb/W = 0.03) catalyst demonstrated an excellent energy efficiency of 78.10% with a current density of 80 mA cm−2. This efficiency is much higher than that demonstrated by VRFB cells with untreated GF (67.12%) and heat-treated GF obtained through the conventional method (72.01%). Furthermore, in the stability test of a VRFB single cell with the Nb-doped h-WO3 NW (Nb/W = 0.03) catalyst, almost no decay of the cell was observed even after 30 cycles. This observation indicates the outstanding stability of the cell during the redox reaction of vanadium ions under highly acidic conditions.

Platinum nanoparticles embedded in pyrolyzed nitrogen-containing cobalt complexes for high methanol-tolerant oxygen reduction activity

High oxygen reduction activity of methanol-tolerant catalysts was successfully reported using platinum nanoparticles embedded in cobalt-based nitrogen-containing complexes supported on carbon blacks (Pt–N-complex/C). The oxygen reduction reaction (ORR) of the Pt–N-complex/C was attributed to four-electron transfer pathway in which oxygen was directly reduced to water, yielding four electrons. In a methanol-containing solution, the platinum intrinsically favors the methanol oxidation reaction over the ORR, which is a major drawback for direct methanol fuel cells (DMFCs). In comparison, when the Pt–N-complex/C is introduced in a methanol-containing solution, not only is the methanol oxidation suppressed but also the four-electron-transfer in the ORR is maintained up to the diffusion-limiting region. Physicochemical characterization of the Pt–N-complex/C indicates that pyrrolic N-type poly-aromatic hydrocarbons were formed in a network structure around the catalysts and prevented them from the methanol oxidation reaction. In a DMFC test at elevated methanol concentrations, the one with the Pt–N-complex/C cathode showed superior stability over the one with the Pt-based cathode, which may offer a solution to the methanol crossover problem in DMFCs.

Deep eutectic solvent promoted one step sustainable conversion of fresh seaweed biomass to functionalized graphene as a potential electrocatalyst

Herein we report a facile method for the scalable production of Fe3O4/Fe doped graphene nanosheets (Fe3O4/Fe–GN) from a naturally abundant seaweed resource. The granules that remained after the recovery of liquid juice from a fresh brown seaweed, Sargassum tenerrimum, were utilized as a raw material and a deep eutectic solvent (DES) generated by the complexation of choline chloride and FeCl3 (ChoCl–FeCl3) was employed as a template as well as a catalyst for the production of graphene nanosheets. Pyrolysis of a mixture of seaweed granules and DES at 700–900 °C under a 95% N2 and 5% H2 atmosphere resulted in the formation of Fe3O4/Fe–GN with a high surface area (220 m2 g−1) and high electrical conductivity (2384.6 mS m−1). The synthesized nanosheets were then tested for their electrocatalytic activity in the oxygen reduction reaction (ORR) in an alkaline fuel cell. The electrocatalyst demonstrated a positive onset potential, high cathodic current density, low hydrogen peroxide formation ( 80% activity of the catalyst, making the functionalized graphene sheets derived from Sargassum tenerrimum a sustainable replacement for existing precious metal-based ORR catalysts.

Three-dimensional bifurcations of a two-phase Rayleigh-Benard problem in a cylinder

Three-dimensional (3D) bifurcations of a partially melted or solidified material in a cylinder heated from below are studied numerically. Through nonlinear calculations, bifurcation diagrams are constructed for a melt of a Prandtl number of one. As the interface is fixed, our calculated results agree reasonably well with previous calculations, but some discrepancies exist, which are further discussed through their dynamic evolutions and imperfect bifurcations of 5∞ tilt. As the interface is allowed to deform, the bifurcation behavior changes significantly, both for the onset of convection and its convection mode. For the initial melt aspect ratio of one, the primary bifurcation changes from supercritical to subcritical with the increasing solid amount, and the onset mode from an axisymmetric (m0) mode to a 3D (m1) mode. Although the free interface destabilizes the conductive mode and leads to an earlier onset of convection, it may stabilize some flow modes through its confinement. Imperfect bifurcations due to a 5∞ tilt are further illustrated. ” 2001 Elsevier Science Ltd. All rights reserved.

High-performance pyrolyzed iron corrole as a potential non-precious metal catalyst for PEMFCs

This work demonstrates the performance of carbon black-supported pyrolyzed Fe–corrole (py-Fe–corrole/C) as a cathode catalyst for the oxygen reduction reaction (ORR) in PEMFCs. The ORR measurements reveal that the py-Fe–corrole/C exhibits good ORR activity, via the direct four-electron reduction pathway, in the reduction of O2 to H2O. The H2–O2 PEMFC produces high activity and good stability. The enhanced ORR activity is attributable to the network structure of poly-aromatic hydrocarbons, the quaternary (graphitic)-type nitrogen and the coordination structure of the py-Fe–corrole/C. Square wave voltammetry has been applied to the py-Fe–corrole/C to perform a redox reaction of Fe(II)/Fe(III) at 0.6 V. Finally, detailed in situ X-ray adsorption spectroscopy has been applied to determine the ORR mechanism of py-Fe–corrole/C.

A high performance polybenzimidazole–CNT hybrid electrode for high-temperature proton exchange membrane fuel cells

Nitrogen-doped CNTs directly grown on a carbon cloth (CNT/CC) hybrid with polybenzimidazole (PBI) are utilized as hybrid electrodes for high-temperature proton exchange membrane fuel cells (HT-PEMFCs). Pt nanoparticles (NPs) of typically 2.5 nm diameter have been deposited uniformly on the electrode composed of either CNT/CC or carbon black on carbon cloth (CB/CC). Comparative study of 0.1 mg cm−2 Pt NPs on a PBI–CNT (Pt/PBI–CNT) electrode and 0.8 mg cm−2 Pt NPs on PBI–carbon black (Pt/PBI–CB) for oxygen reduction in the cathode has been performed. In H2/O2 HT-PEMFC single cell tests, the low Pt-loaded (0.1 mg cm−2) Pt/PBI–CNT/CC electrode outperforms the high Pt-loaded (0.8 mg cm−2) Pt/PBI–CB/CC electrode with maximum power densities of 643 mW cm−2 and 468 mW cm−2, respectively, at 160 °C. Notably, the Pt-loading of a Pt/PBI–CNT/CC cell is one eighth of that of the Pt/PBI–CB/CC cell, while the former outperforms the latter by approximately 38%. The enhancement is attributed to good interfacial continuity between the PBI membrane and the carbon cloth and the uniform dispersion of the catalyst on the PBI–CNT surface, leading to the formation of three-phase contacts, which is essential for the activity of the catalyst.

Effect of a sulfur and nitrogen dual-doped Fe–N–S electrocatalyst for the oxygen reduction reaction

Herein, we report the synthesis of a non-precious metal dual-doped catalyst. Melamine and lipoic acid were used as precursors for the nitrogen and sulfur dopant atoms, respectively, which were mixed with iron precursors and carbon black and pyrolyzed at 700 °C to yield the catalyst Fe-M-LA/C. Fe-M-LA/C shows high oxygen reduction reaction (ORR) capability, with an almost-ideal electron transfer number of 3.99. It shows almost no degradation after potential cycling for 30 000 cycles, and the structural analysis indicates that a graphene-like structure of Fe-M-LA/C can improve its ORR activity, electrical conductivity, and corrosion resistance. X-ray photoelectron spectroscopy (XPS) results show that the catalyst has high contents of pyridinic- and quaternary N atoms and thiophene S atoms that can significantly enhance the ORR performance of Fe-M-LA/C. X-ray absorption spectroscopy (XAS) data show that the oxidation state of iron and the interatomic distance of the heteroatoms in Fe-M-LA/C play an important role in determining the ORR ability. These data confirm that the presence of a nitrogen and sulfur dual-doped –Fe–N–S– structure can enhance the ORR kinetics.

Pyrolysis of melamine-treated vitamin B12 as a non-precious metal catalyst for oxygen reduction reaction

Active, inexpensive non-precious metal substitutes for current Pt-based catalysis are needed to reduce the cost of proton exchange membrane fuel cells (PEMFC). In this work, pyrolysis of a carbon black-supported melamine-treated vitamin B12 (py-B12-M/C) catalyst for the oxygen reduction reaction (ORR) establishes that the surface nitrogen content and nitrogen–carbon ratio strongly affect ORR activity. Under optimal conditions, the number of transferred electrons in py-B12-M/C and the hydrogen peroxide yield of its ORR are 3.95 and 2.5%, respectively.