Zehui Yang

Large-scale growth of Cu2ZnSnSe4 and Cu2ZnSnSe4/Cu2ZnSnS4 core/shell nanowires

We present a fast and simple protocol for large-scale preparation of quaternary Cu(2)ZnSnSe(4) (CZTSe), as well as CZTSe/Cu(2)ZnSnS(4) (CZTS) core/shell nanowires using CuSe nanowire bundles as self-sacrificial templates. CuSe nanowire bundles were synthesized by reacting Cu(2 - x)Se nanowire bundles with sodium citrate solution. CZTSe nanowires were prepared by reacting CuSe nanowire bundles with Zn(CH(3)COO)(2) and SnCl(2) in triethylene glycol. X-ray diffraction (XRD) and selected area electron diffraction studies show that stannite CZTSe is formed. The formed CZTSe nanowire bundles have diameters of 200-400 nm and lengths of up to hundreds of micrometers. CZTSe/CZTS nanocable bundles with similar morphologies were grown by the addition of some elemental sulfur to the reaction system for growth of CZTSe bundles. The stannite CZTSe/kesterite CZTS core/shell structure of the grown nanocables was confirmed by XRD and high-resolution transmission electron microscope investigation. The influence of S/Se molar ratio in the reaction system on the crystallographic structures and optical properties of CZTSe/CZTS nanocables was studied. The obtained CZTSe/CZTS core/shell nanocable bundles show broad and enhanced optical absorption over the visible and near-infrared region, which is promising for use in photovoltaic applications.

Salt-templated synthesis of defect-rich MoN nanosheets for boosted hydrogen evolution reaction

Two-dimensional defect-rich molybdenum nitride (dr-MoN) nanosheets were successfully prepared via a NaCl template-directed synthesis route followed by an incomplete ammoniation of MoO3 nanosheets. With additional edge defects arising from the etched MoO3 compared to those in the intact MoN nanosheets, dr-MoN was capable of efficiently electro-catalyzing the hydrogen evolution reaction (HER) under both acidic and alkaline conditions with impressive activity and durability. The dr-MoN-0 catalyst possessed an ultra-small onset overpotential of approximately 10 mV in 0.5 M H2SO4 electrolyte, which is competitive with that of the Pt/C catalyst. Overpotentials of only 125 mV and 139 mV are required to deliver a current density of 10 mA cm−2 for the dr-MoN-0 catalyst in 0.5 M H2SO4 and 1 M KOH, respectively. More importantly, the dr-MoN-0 maintained a prominent amperometric (I–t) durability during a 20 h test and also a superior cycling stability with negligible overpotential loss, all of which are among the best results for current MoN based HER catalysts. The exceptional performance was attributed to the defect-abundant structure which resulted in the formation of tiny cracks on the surface of the nanosheets and caused the additional exposure of active edge sites. These findings highlight the prospective potential of dr-MoN with additional active edge sites as highly efficient and stable platinum-free electrocatalysts towards the HER.

Facile enhancement of the durability and CO tolerance of CB/PtRu by poly(2,5-benzimidazole) coating via in situ polymerization

Here, we describe a facile method to enhance the stability of commercial CB/PtRu used in direct methanol fuel cells. In this work, 3,4-diaminobenzoic acid was in situ polymerized to form poly(2,5-benzimidazole) (ABPBI) on CB/PtRu particles. The experimental results indicated that the CO tolerance and stability of the CB/PtRu particles were enhanced when they were covered with ABPBI.

Bottom-up design of a stable CO-tolerant platinum electrocatalyst with enhanced fuel cell performance in direct methanol fuel cells

Sluggish methanol oxidation reaction (MOR) and CO poisoning of platinum electrocatalysts are critical problems in direct methanol fuel cells (DMFCs). Here, we design a stable CO tolerant platinum electrocatalyst via a bottom-up method, in which the platinum nanoparticles are deposited on carbon black after coating with polybenzimidazole (PBI) and poly(vinyl pyrrolidone) (PVP). By comparison with the PVP post-coated electrocatalyst (CB/PBI/Pt/PVP), the PVP pre-coated electrocatalyst (CB/PBI/PVP/Pt) exhibits comparable durability and CO tolerance due to the similar amount of PVP in the electrocatalyst, suggesting the PVP pre-coating method shows negligible effect on CO tolerance and durability, while the Pt utilization efficiency, methanol oxidation activity and power density of CB/PBI/PVP/Pt are 1.6 times higher than those of CB/PBI/Pt/PVP. Thus, the PVP pre-coated electrocatalyst has better activity due to the non-coated Pt nanoparticles. Meanwhile, CB/PBI/PVP/Pt exhibits highly stable CO tolerance during the durability test, while the CO tolerance of the commercial CB/PtRu seriously deteriorates during the durability test due to the dissolution of Ru nanoparticles. To the best of our knowledge, the maximum power density of CB/PBI/PVP/Pt (104 mW cm−2) is one of the highest values in recent publications.

Remarkably durable platinum cluster supported on multi-walled carbon nanotubes with high performance in an anhydrous polymer electrolyte fuel cell

Reducing platinum (Pt) usage in the polymer electrolyte fuel cells (PEFCs) has become one of the main issues in the global commercialization of PEFCs. In this work, we describe a facile and scalable method to deposit Pt clusters (1.2 nm) on multi-walled carbon nanotubes (MWNTs) by the aid of NaOH in a reduction process. The electrocatalyst loses 50% of the initial electrochemical surface area (ECSA) after 200 000 potential cycles from 1.0 to 1.5 V vs. RHE, which is 20 times higher compared to commercial CB/Pt. The mass power density of the Pt cluster electrocatalyst measured under 120 °C without any humidification reaches 1320 mW mgPt−1, which is 6.7 times higher compared to that of commercial CB/Pt. To the best of our knowledge, the mass power density of our electrocatalyst is one of the highest values measured in high-temperature PEFCs.

Fabrication of a polymer electrolyte membrane with uneven side chains for enhancing proton conductivity

Construction of effective proton transport channels in proton exchange membranes is the key to the design of high performance proton conductive materials. Enhancement of proton conductivity of polymer electrolyte membranes was achieved by broadening the proton transfer channels via attaching acid groups to both long and short side chains of polymer electrolytes simultaneously. To demonstrate the effectiveness of the uneven side chains on the conductive properties of polymer membranes, three types of polyamide based electrolyte membranes with long side chains, short side chains and long/short side chains were prepared. It was found that among the three types of membranes with the same ion exchange capacity (IEC) value, the one with uneven side chains exhibits the highest proton conductivity. An increase of the IEC value in the uneven side chain membrane leads to a significant increase of proton conductivity. The study provides useful insight into the structural design of polymer electrolyte materials with high conductivity for fuel cell applications.

Remarkably stable CO tolerance of a PtRu electrocatalyst stabilized by a nitrogen doped carbon layer

Here, we synthesized a PtRu electrocatalyst with remarkably stable CO tolerance, in which the PtRu nanoparticles were stabilized by a nitrogen doped carbon layer derived from the carbonization of poly(vinyl pyrrolidone).

Bottom-up Design of High-Performance Pt Electrocatalysts Supported on Carbon Nanotubes with Homogeneous Ionomer Distribution

Homogeneous ionomer networks are primarily important for the enhancement of fuel cell performance. Here, we report a bottom‐up design of platinum (Pt) electrocatalysts with homogeneous ionomer layers, in which Pt nanoparticles are deposited on carbon nanotubes (CNTs) after sequentially wrapping with polybenzimidazole (PBI) and end‐capped hyperbranched sulfonated macromolecules (E‐HBM) (CNT/PBI/E‐HBM/Pt). E‐HBM, which are proton conductive macromolecules, are homogeneously coated on CNTs owing to the base–acid interaction between PBI and E‐HBM. The durability and oxygen reduction reaction (ORR) results indicate that CNT/PBI/E‐HBM/Pt exhibits higher Pt stability and ORR activity compared with the electrocatalyst without E‐BHM. The Nafion ionomer‐free membrane electrode assembly (MEA) fabricated from CNT/PBI/E‐HBM/Pt (704 mW cm−2) shows higher power density than the MEA from CNT/PBI/Pt with Nafion ionomer (30 wt %, 603 mW cm−2) owing to the unique bottom‐up design resulting in high Pt utilization efficiency.

Highly methanol-tolerant platinum electrocatalyst derived from poly(vinylpoyrrolidone) coating.

The design and fabrication of a methanol-tolerant electrocatalyst is still one of the most important issues in direct methanol fuel cells (DMFCs). Here, we focus on the design of a cathodic electrocatalyst in DMFCs and describe a new methanol-tolerant electrocatalyst fabricated from poly(vinylpyrrolidone) (PVP) coating on platinum nanoparticles assisted by hydrogen bonding between PVP and polybenzimidazole (PBI). The PVP layer has a negligible effect on the oxygen reduction reaction (ORR) activity, while the methanol oxidation reaction is retarded by the PVP layer. The PVP-coated electrocatalyst shows higher ORR activity under various methanol concentrations in the electrolyte, suggesting that the PVP-coated electrocatalyst has a higher methanol tolerance. Also, the PVP-coated electrocatalyst loses only 14% of the electrochemical surface area after 5000 potential cycles from 0.6-1.0 V versus the reversible hydrogen electrode, indicating better Pt stability than non-coated (27%) and commercial (38%) electrocatalysts due to the unique sandwich structure formed by the PVP and PBI. The power density of the PVP-coated electrocatalyst is four to five times higher compared to non-coated and commercial electrocatalysts with 12 M methanol feeding to the anode side, respectively. PVP coating is important for the enhancement of Pt stability and methanol tolerance. This study offers a new method for preparing a low-cost and high-methanol-tolerant Pt electrocatalyst, and useful information for real DMFC application to eliminate the methanol crossover problem in the cathode side.

High Performance Palladium Supported on Nanoporous Carbon under Anhydrous Condition

Due to the high cost of polymer electrolyte fuel cells (PEFCs), replacing platinum (Pt) with some inexpensive metal was carried out. Here, we deposited palladium nanoparticles (Pd-NPs) on nanoporous carbon (NC) after wrapping by poly[2,2′-(2,6-pyridine)-5,5′-bibenzimidazole] (PyPBI) doped with phosphoric acid (PA) and the Pd-NPs size was successfully controlled by varying the weight ratio between Pd precursor and carbon support doped with PA. The membrane electrode assembly (MEA) fabricated from the optimized electrocatalyst with 0.05 mgPd cm−2 for both anode and cathode sides showed a power density of 76 mW cm−2 under 120 °C without any humidification, which was comparable to the commercial CB/Pt, 89 mW cm−2 with 0.45 mgPt cm−2 loaded in both anode and cathode. Meanwhile, the power density of hybrid MEA with 0.45 mgPt cm−2 in cathode and 0.05 mgPd cm−2 in anode reached 188 mW cm−2. The high performance of the Pt-free electrocatalyst was attributed to the porous structure enhancing the gas diffusion and the PyPBI-PA facilitating the proton conductivity in catalyst layer. Meanwhile, the durability of Pd electrocatalyst was enhanced by coating with acidic polymer. The newly fabricated Pt-free electrocatalyst is extremely promising for reducing the cost in the high-temperature PEFCs.