Conghui Si

Nickel oxide nanopetal-decorated 3D nickel network with enhanced pseudocapacitive properties

Metal oxides possess high theoretical specific capacitance, but their pseudocapacitive properties are restricted by the poor electronic conductivity. Here we present a strategy to synthesize a three-dimensional binder/conducting agent-free nickel oxide (NiO) electrode through the combination of anodization with calcination. The NiO electrode is composed of a 3D conductive nickel network decorated with nanopetal-like NiO arrays. The influence of calcination temperature has been investigated, with respect to the microstructure and pseudocapacitive properties of the NiO electrodes. The NiO electrode demonstrates great electrochemical properties, especially remarkable rate capability (82% retention of the highest value for the 25-fold enhanced current density) and cycling stability (good capacitance retention after 30000 cycles). Moreover, an asymmetric supercapacitor has been assembled using NiO as the positive electrode and activated carbon (AC) as the negative electrode. The NiO//AC supercapacitor presents excellent cycling stability (91.3% retention after 10000 cycles), and could power a mini fan as well as a commercial red LED for more than 270 min.

[001] preferentially-oriented 2D tungsten disulfide nanosheets as anode materials for superior lithium storage

Rechargeable lithium ion batteries (LIBs) have transformed portable electronics and will play a crucial role in transportation, such as electric vehicles. For higher energy storage in LIBs, two issues should be addressed, that is, the fundamental understanding of the chemistry taking place in LIBs and the discovery of new materials. Here we design and fabricate two-dimensional (2D) WS2 nanosheets with preferential [001] orientation and perfect single crystalline structures. Being used as an anode for LIBs, the WS2-nanosheet electrode exhibits a high specific capacity, good cycling performance and excellent rate capability. Considering the controversy in the lithium storage mechanism of WS2, ex-situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses clearly verify that the recharge product (3.0 V vs. Li+/Li) of the WS2 electrode after fully discharging to 0.01 V (vs. Li+/Li) tends to reverse to WS2. More remarkably, the [001] preferentially-oriented 2D WS2 nanosheets are also promising candidates for applications in photocatalysis, water splitting, and so forth.

Self-supporting nanoporous gold-palladium overlayer bifunctional catalysts toward oxygen reduction and evolution reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial processes for energy conversion/storage systems, such as fuel cells, metal–air batteries, and water splitting. However, both reactions are severely restricted by their sluggish kinetics, thus requiring highly active, cost-effective, and durable electrocatalysts. Herein, we develop novel bifunctional nanocatalysts through surface nanoengineering of dealloying-driven nanoporous gold (NPG). Pd overlayers were precisely deposited onto the NPG ligament surface by epitaxial layer-by-layer growth. More importantly, the obtained NPG-Pd overlayer nanocatalysts exhibit remarkably enhanced electrocatalytic activities toward both the ORR and OER in alkaline media, benchmarked against a stateof- the-art Pt/C catalyst. The improved electrocatalytic performance is rationalized by the unique three-dimensional nanoarchitecture of NPG, enhanced Pd utilization efficiency from precise control of the Pd overlayers, and change in electronic structure, as revealed by density functional theory calculations.

A nanoporous PtCuTi alloy with a low Pt content and greatly enhanced electrocatalytic performance towards methanol oxidation and oxygen reduction

Pt-based electrocatalysts play a crucial role in both the anode and cathode reactions of direct methanol fuel cells (DMFCs), but their activity/durability and cost are still the main issues to be addressed. Through the combination of mechanical alloying with dealloying, here we have fabricated a nanoporous PtCuTi (np-PtCuTi) alloy with a low Pt content from a Cu-based precursor. The np-PtCuTi alloy exhibits a three-dimensional bi-continuous interpenetrating ligament/channel structure with a ligament size of 3.1 ± 0.6 nm. Electrochemical measurements show that the np-PtCuTi alloy exhibits superior electrocatalytic activities (CO tolerance, specific and mass activity) towards methanol oxidation at the anode, compared to commercial PtC catalysts. Moreover, the np-PtCuTi catalyst shows an enhancement of 1.9 and 4.2 times in the mass and specific activity towards the oxygen reduction reaction (ORR) at the cathode compared to PtC, respectively. More importantly, the np-PtCuTi catalyst shows excellent catalytic durability for the ORR, and the mass activity retains 91.8% of the initial value after 20 000 cycles. In addition, the mechanisms for the activity enhancement of np-PtCuTi have been rationalized on the basis of the structural effect, alloying effect and electronic effect through experiments and density functional theory calculations.

Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction

Atomic layer-by-layer construction of Pd on nanoporous gold (NPG) has been investigated through the combination of underpotential deposition (UPD) with displacement reaction. It has been found that the UPD of Cu on NPG is sensitive to the applied potential and the deposition time. The optimum deposition potential and time were determined through potential- and time-sensitive stripping experiments. The NPG-Pd electrode shows a different voltammetric behavior in comparison to the bare NPG electrode, and the deposition potential was determined through the integrated charge control for the monolayer UPD of Cu on the NPG-Pd electrode. Five layers of Pd were constructed on NPG through the layer-by-layer deposition. In addition, the microstructure of the NPG-Pdx (x = 1, 2, 3, 4 and 5) films was probed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The microstructural observation demonstrates that the atomic layers of Pd form on the ligament surface of NPG through epitaxial growth, and have no effect on the nanoporous structure of NPG. In addition, the hydrogen storage properties of the NPG-Pdx electrodes have also been addressed.

Multicomponent platinum-free nanoporous Pd-based alloy as an active and methanol-tolerant electrocatalyst for the oxygen reduction reaction

Development of high-performance oxygen reduction reaction (ORR) catalysts is crucial to improve proton exchange membrane fuel cells. Herein, a multicomponent nanoporous PdCuTiAl (np-PdCuTiAl) electrocatalyst has been synthesized by a facile one-step dealloying strategy. The np-PdCuTiAl catalyst exhibits a three-dimensional bicontinuous interpenetrating ligament/channel structure with an ultrafine length scale of ~3.7 nm. The half-wave potential of np PdCuTiAl is 0.873 V vs. RHE, more positive than those of PdC (0.756 V vs. RHE) and PtC (0.864 V vs. RHE) catalysts. The np-PdCuTiAl alloy shows a 4-electron reaction pathway with similar Tafel slopes to PtC. Remarkably, the half-wave potential shows a negative shift of only 12 mV for np-PdCuTiAl in the presence of methanol, and this negative shift is much lower than those of the PdC (50 mV) and PtC (165 mV) catalysts. The enhanced ORR activity of np-PdCuTiAl has been further rationalized through density functional theory calculations.

Large-scale synthesis and catalytic activity of nanoporous Cu–O system towards CO oxidation

Nanoporous Cu–O system catalysts with different oxidation states of Cu have been fabricated through a combination of dealloying as-milled Al66.7Cu33.3 alloy powders and subsequent thermal annealing. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the microstructure and surface chemical states of Cu–O catalysts. The peculiar nanoporous structure can be retained in Cu–O catalysts after thermal treatment. Catalytic experiments reveal that all the Cu–O samples exhibit complete CO conversion below 170 °C. The optimal catalytic performance could be achieved through the combination of annealing in air with hydrogen treatment for the Cu–O catalyst, which shows a near complete conversion temperature (T90%) of 132 °C and an activation energy of 91.3 KJ mol−1. In addition, the present strategy (ball milling, dealloying and subsequent thermal treatment) could be scaled up to fabricate high-performance Cu–O catalysts towards CO oxidation.

Tungsten diselenide nanoplates as advanced lithium/sodium ion electrode materials with different storage mechanisms

Transition-metal dichalcogenides (TMDs) exhibit immense potential as lithium/ sodium-ion electrode materials owing to their sandwich-like layered structures. To optimize their lithium/sodium-storage performance, two issues should be addressed: fundamentally understanding the chemical reaction occurring in TMD electrodes and developing novel TMDs. In this study, WSe2 hexagonal nanoplates were synthesized as lithium/sodium-ion battery (LIB/SIB) electrode materials. For LIBs, the WSe2-nanoplate electrodes achieved a stable reversible capacity and a high rate capability, as well as an ultralong cycle life of up to 1,500 cycles at 1,000 mA·g–1. Most importantly, in situ Raman spectroscopy, ex situ X-ray diffraction (XRD), transmission electron microscopy, and electrochemical impedance spectroscopy measurements performed during the discharge–charge process clearly verified the reversible conversion mechanism, which can be summarized as follows: WSe2 + 4Li+ + 4e– ↔ W + 2Li2Se. The WSe2 nanoplates also exhibited excellent cycling performance and a high rate capability as SIB electrodes. Ex situ XRD and Raman spectroscopy results demonstrate that WSe2 reacted with Na+ more easily and thoroughly than with Li+ and converted to Na2Se and tungsten in the 1st sodiated state. The subsequent charging reaction can be expressed as Na2Se → Se + 2Na+ + 2e–, which differs from the traditional conversion mechanism for LIBs. To our knowledge, this is the first systematic exploration of the lithium/sodium-storage performance of WSe2 and the mechanism involved.

Nanoporous Platinum/(Mn,Al)3O4 Nanosheet Nanocomposites with Synergistically Enhanced Ultrahigh Oxygen Reduction Activity and Excellent Methanol Tolerance

At present, metal/metal oxide composites are considered as potential oxygen reduction reaction (ORR) catalysts for energy-related applications like fuel cells. Here, we fabricated a high-activity, low Pt loading ORR electrocatalyst composed of nanoporous Pt (np-Pt) in intimate contact with lamellar (Mn,Al)3O4 nanosheet (NS). In comparison to Pt/C (Johnson Matthey), the np-Pt/(Mn,Al)3O4 NS catalyst shows a 11.5-fold enhancement in the mass-normalized ORR activity and much better methanol tolerance because of the metal-support interactions between np-Pt and (Mn,Al)3O4. Furthermore, the combination of electrochemical experiments with theoretical calculations verifies that the ORR on the np-Pt/(Mn,Al)3O4 NS catalyst is a direct 4e- pathway in the alkaline solution. In addition, the electrocatalytic mechanisms have also been rationalized by density functional theory (DFT) calculations.

Transforming bulk alloys into nanoporous lanthanum-based perovskite oxides with high specific surface areas and enhanced electrocatalytic activities

The limited surface area of a perovskite oxide obtained through a conventional synthesis route has often limited its catalytic performance. Creating nanoporous perovskite oxides with abundant amounts of mesopores is one of the key challenges to realize perovskite potential for energy conversion and storage applications. Here, we report a novel, facile synthesis route that exploits the de-alloying process to transform bulk alloys into nanoporous lanthanum (La)-based perovskite oxides. The synthesized La-based perovskites display ultrahigh specific surface areas in the range of 8.10–20.13 m2 g−1 and a unique ligament-like porous framework. Our systematic ORR and OER tests revealed their high electrocatalytic activity and durability in an alkaline solution, which are consistent with the density functional theory (DFT) calculations. The synthesis route reported here opens up a new avenue to create a new generation of functional perovskite oxides that are applicable to other catalytic and electrochemical applications.