Zhiqing Zou

Nanosized Mn–Ru binary oxides as effective bifunctional cathode electrocatalysts for rechargeable Li–O2 batteries

Mn–Ru binary oxides have been synthesized via a hydrothermal approach and their performance as bifunctional catalysts for the air electrode is evaluated in non-aqueous Li–O2 secondary batteries. Characterization of the catalysts by X-ray diffractometry and transmission electron microscopy confirms that the as-prepared oxides contain γ-MnO2 and hydrous RuO2 with the morphology of fusiform nanorods and nanoparticles, respectively. Linear scanning voltammetric measurements reveal that the binary oxides exhibit remarkable electrocatalytic activities towards both the oxygen reduction and oxygen evolution reactions. Li–O2 batteries with Mn–Ru oxides as cathode catalysts show a higher discharge voltage plateau and a higher specific capacity of 6500 mA h gcarbon−1 than those with Ketjen black (KB) alone, and the subsequent charge voltage is ca. 500 mV lower than that with KB. Moreover, much enhanced cyclability of Li–O2 batteries is achieved with a capacity retention ratio of 73.2% after 5 cycles. The cyclability is maintained for 50 cycles without sharp decay under a limited discharge depth of 1100 mA h gcarbon−1, suggesting that such a bifunctional electrocatalyst is a promising candidate for the air electrode in Li–air batteries.

Conversion of PtNi alloy from disordered to ordered for enhanced activity and durability in methanol-tolerant oxygen reduction reactions

The development of cost-effective oxygen reduction reaction (ORR) catalysts with a high methanol tolerance and enhanced durability is highly desirable for direct methanol fuel cells. This work focuses on the conversion of PtNi nanoparticles from a disordered solid solution to an ordered intermetallic compound. Here the effect of this conversion on ORR activity, durability, and methanol tolerance are characterized. X-ray diffraction and transmission electron microscopy results confirm the formation of ordered PtNi intermetallic nanoparticles with high dispersion and a mean particle size of about 7.6 nm. The PtNi intermetallic nanoparticles exhibited enhanced mass and specific activities toward the methanol-tolerant ORR in pure and methanol-containing electrolytes. The specific activity of the ORR at 0.85 V on the PtNi intermetallic nanoparticles is almost 6 times greater than on commercial Pt/C and 3 times greater than on disordered PtNi alloy. Durability tests indicated a minimal loss of ORR activity for PtNi intermetallic nanoparticles after 5,000 potential cycles, whereas the ORR activity decreased by 28% for disordered PtNi alloy. The enhanced methanoltolerant ORR activity and durability may be attributed to the structural and compositional stabilities of the ordered PtNi intermetallic nanoparticles compared relative to the stabilities of the disordered PtNi alloy, strongly suggesting that the PtNi intermetallic nanoparticles may serve as highly active and durable methanol-tolerant ORR electrocatalysts for practical applications.

Enhanced catalytic dehydrogenation of LiBH4 by carbon-supported Pd nanoparticles

The investigation of thermally induced dehydrogenation of LiBH(4) catalyzed by carbon-supported Pd nanoparticles (Pd/C) reveals that LiBH(4) doped with the Pd/C catalysts shows superior dehydrogenation performance, compared to dehydrogenation in the absence of catalyst, even at very low catalyst content. We have found that smaller Pd nanoparticles result in greater enhanced catalytic dehydrogenation of LiBH(4) than do larger Pd nanoparticles. For doping with 10 wt.% catalyst of an average Pd particle diameter of ca. 5.3 nm, thermal dehydrogenation of LiBH(4) is found to start at ca. 280 degrees C with a total weight loss of 15.8 wt.% for the full temperature range. With increased loading of catalyst within a LiBH(4) sample, the onset dehydrogenation temperature and the two main desorption peaks from LiBH(4) are found to decrease while the hydrogen release amount is found to increase. All the hydrogen of ca. 18.5 wt.% can be released from LiBH(4) below 580 degrees C at a catalyst doped level of 50 wt.%. Importantly, we observed a reversible hydrogenation/dehydrogenation with a capacity of approximately 4.3 wt.%. Pd nanoparticles are found to play an important role in the catalyzed dehydrogenation/rehydrogenation of LiBH(4).

Simple Complexing-Reduction Synthesis of Pd-Pt/C Alloy Electrocatalysts for the Oxygen Reduction Reaction

PdPt bimetallic alloy nanoparticles with different atomic ratios of Pd and Pt were synthesized by a simple aqueous complexing-reduction method at room temperature. Ethylenediaminetetraacetic acid disodium salt was used as the complexing agent with Pd ions to enhance alloy formation for Pd and Pt atoms in nanoparticles at low temperatures. Both X-ray diffraction and transmission electron microscopy characterizations indicate that the sizes of the bimetallic alloy nanoparticles are ca. 2.6-3.8 nm, with a narrow size distribution and broad dispersion on a carbon support. Among the prepared catalysts, the PdPt/C alloy catalyst with an atomic ratio of 1:1 and heat-treatment at 150°C exhibited a large electrochemical active surface area and a greater electrocatalytic activity for the oxygen reduction reaction (ORR); both measured results are found to be superior to that of the Pt/C catalyst acquired commercially. Most importantly, the Pd-Pt/C catalysts exhibited substantially higher methanol tolerance during the ORR than the Pt/C catalyst, ensuring a higher ORR performance while diminishing Pt utilization. We find that the Pd-Pt/C bimetallic catalysts prepared by the approach developed herein may serve as methanol-tolerant cathode catalysts in a direct methanol fuel cell.

Shape-controlled porous heterogeneous PtRu/C/Nafion microspheres enabling high performance direct methanol fuel cells

An anode catalytic layer for direct methanol fuel cells (DMFCs) with decreased PtRu loading as low as 1.0 mg cm−2 has been prepared by an electrospray method. The morphology of the electrosprayed composite of PtRu/C/Nafion/polyethylene oxide (PEO) is altered from irregular particles to porous microspheres and to nanofibers by adjusting the PEO content. A hybrid structure is assembled using the porous microspheres as the anode catalytic layer for DMFCs, leading to a remarkable enhancement in the maximum power density of 35.4 mW cm−2, which is ∼50% higher than that of the conventional one at the same PtRu loading of 1.0 mg cm−2 and is even comparable to that (31.5 mW cm−2) of the conventional one at a higher PtRu loading of 2.0 mg cm−2. Further investigation reveals that the improved performance is mainly attributed to its hierarchical factual structure. In the primary structure, a single microsphere is with well-distributed PtRu/C and is fully rich in nano-pores and nano-channels, resulting in an increase in the electrochemical active surface area and higher catalyst utilization. In the secondary structure, micro-sized pathways are formed by the stereoscopic microspheres, resulting in enhanced mass transport, higher current density and power density.

Plasmonic Pd Nanoparticle- and Plasmonic Pd Nanorod-Decorated BiVO4 Electrodes with Enhanced Photoelectrochemical Water Splitting Efficiency Across Visible-NIR Region.

The photoelectrochemical (PEC) water splitting performance of BiVO4 is partially hindered by insufficient photoresponse in the spectral region with energy below the band gap. Here, we demonstrate that the PEC water splitting efficiency of BiVO4 electrodes can be effectively enhanced by decorating Pd nanoparticles (NPs) and nanorods (NRs). The results indicate that the Pd NPs and NRs with different surface plasmon resonance (SPR) features delivered an enhanced PEC water splitting performance in the visible and near-infrared (NIR) regions, respectively. Considering that there is barely no absorption overlap between Pd nanostructures and BiVO4 and the finite-difference time domain (FDTD) simulation indicating there are substantial energetic hot electrons in the vicinity of Pd nanostructures, the enhanced PEC performance of Pd NP-decorated BiVO4 and Pd NR-decorated BiVO4 could both benefit from the hot electron injection mechanism instead of the plasmon resonance energy transfer process. Moreover, a combination of Pd NPs and NRs decorated on the BiVO4 electrodes leads to a broad-band enhancement across visible-NIR region.

A Study of the Effect of Heat-Treatment on the Morphology of Nafion Ionomer Dispersion for Use in the Passive Direct Methanol Fuel Cell (DMFC)

Aggregation in heat-treated Nafion ionomer dispersion and 117 membrane are investigated by 1H and 19F Nuclear Magnetic Resonance (NMR) spectra, spin-lattice relaxation time, and self-diffusion coefficient measurements. Results demonstrate that heat-treatment affects the average Nafion particle size in aqueous dispersions. Measurements on heat-treated Nafion 117 membrane show changes in the 1H isotropic chemical shift and no significant changes in ionic conductivity. Scanning electron microscopy (SEM) analysis of prepared cathode catalyst layer containing the heat-treated dispersions reveals that the surface of the electrode with the catalyst ink that has been pretreated at ca. 80 °C exhibits a compact and uniform morphology. The decrease of Nafion ionomer’s size results in better contact between catalyst particles and electrolyte, higher electrochemically active surface area, as well as significant improvement in the DMFC’s performance, as verified by electrochemical analysis and single cell evaluation.

High performance MWCNT–Pt nanocomposite-based cathode for passive direct methanol fuel cells

Reduction of the Pt loading required in cathodes is crucial for the development of passive direct methanol fuel cells (DMFCs). Herein, a novel membrane electrode assembly (MEA) that utilizes a MWCNT–Pt nanocomposite cathodic catalyst layer (CCL) with a 3D network structure is shown to require significantly less Pt loading. With a CCL Pt loading of 0.5 mg cm−2, the maximum power density of the prepared DMFC is 19.2 ± 0.4 mW cm−2 using 2.0 M methanol solution at 25 ± 1 °C, which is higher than that of the power density by a conventional MEA with twice the Pt loading (1.0 mg cm−2). Electrochemical tests show that the structure of the CCL decreases the charge transfer resistance of the cathode reaction and greatly increases the cathode catalyst utilization in comparison with the conventional MEA. The enhanced MEA performance is attributed to the discontinuous distributions of the Pt MWCNT structures and the formation of a cross-twined network within the CCL. This study could provide a promising way to reduce the cost of future commercialized DMFCs.

Controllable fabrication of ordered Pt nanorod array as catalytic electrode for passive direct methanol fuel cells

The nanostructure of the catalytic electrode has a great effect on the performance of direct methanol fuel cells (DMFCs), including catalyst utilization, precious metal loading, water balance, and oxygen mass transfer. In this work, ordered arrays of platinum nanorods with different diameters were directly grown onto microporous layers by electrodeposition via a sacrificial template, and were used as the catalytic cathode for passive DMFCs. The use of these ordered electrodes led to a dramatic decrease in cathode polarization behavior. The maximum power density of passive DMFCs fabricated with catalytic electrodes of 200 and 100 nm Pt nanorod arrays were 17.3 and 12.0 mW/cm 2 , respectively. The obtained improvement in performance was ascribed to the fact that the ordered nanostructured electrode not only increased the electrochemically active surface area and the catalyst utilization, but also enhanced oxygen mass transfer and water balance in the system.

Carbon-supported Pd-Pt Alloy Nanoparticles for Methanol Tolerant Oxygen Reduction

We report one-step synthesis of Pd-rich carbon-supported Pd-Pt alloy electrocatalysts of different Pd/Pt atomic ratios with and without the addition of trisodium citrate as the complexing agent and stabilizer. An activity comparison of the oxygen reduction reaction (ORR) on the Pd-Pt/C catalysts with the same fcc structure and a similar particle size has been investigated. The Pd3Pt1/C catalyst exhibits the highest ORR activity among all the alloy catalysts prepared with the addition of trisodium citrate and a comparative ORR activity with the Pt/C. However, the Pd3Pt1/C catalyst prepared without the addition of trisodium citrate shows an enhanced ORR activity in comparison with the Pt/C. All the bimetallic catalysts exhibited much higher methanol tolerance during the ORR than the Pt/C catalyst. The high methanol tolerance of the Pd-Pt/C alloy catalysts during the ORR may be ascribed to a composition effect and to the formation of Pd-based alloys.