Hsin-Cheng Hsu

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.

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.

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.

Directly Grown Carbon Nanotubes Applied in Direct Methanol Fuel Cell

Carbon nanotubes (CNTs) were directly grown on a carbon cloth (DGCNTs), which were employed as the electrocatalyst support in the direct methanol fuel cell (DMFC). The CNTs were prepared by an iron-assisted catalyst grown method using the microwave plasma-enhanced chemical vapor deposition. From the morphological analysis, the as-prepared DGCNT is a multi-walled and bamboo-like structure with a diameter of 10-20 nm. The Pt-Ru electrocatalysts were deposited on the DGCNTs using sputtering process (Pt-Ru/DGCNT), which were acted as an anode in the DMFC. The diameters of Pt-Ru nanoparticles deposited on the DGCNTs are ca. 3.54 nm, which are the high-degree alloy by X-ray diffraction analysis. For fabricating a membrane electrode assembly (MEA), 0.2 mg/cm^2 Pt-Ru/DGCNT and 4.0 mg/cm^2 Pt black deposited on carbon cloth were employed as the anode and cathode, respectively, which the Nafion 117 was sandwiched in between the electrodes by a hot-pressing process. The polarization curve shows that the maximum power density is ca. 1.52 W/Pt-mg feeding 1 M methanol and pure oxygen to the anode and cathode, respectively, operating at 60℃. The low amount of precious metal loaded on DGCNTs applied in a MEA shows a high performance, which is suitable for a DMFC.