Dafeng Yan

Defect Chemistry of Nonprecious‐Metal Electrocatalysts for Oxygen Reactions

Oxygen electrocatalysis, including the oxygen-reduction reaction (ORR) and oxygen-evolution reaction (OER), is a critical process for metal-air batteries. Therefore, the development of electrocatalysts for the OER and the ORR is of essential importance. Indeed, various advanced electrocatalysts have been designed for the ORR or the OER; however, the origin of the advanced activity of oxygen electrocatalysts is still somewhat controversial. The enhanced activity is usually attributed to the high surface areas, the unique facet structures, the enhanced conductivities, or even to unclear synergistic effects, but the importance of the defects, especially the intrinsic defects, is often neglected. More recently, the important role of defects in oxygen electrocatalysis has been demonstrated by several groups. To make the defect effect clearer, the recent development of this concept is reviewed here and a novel principle for the design of oxygen electrocatalysts is proposed. An overview of the defects in carbon-based, metal-free electrocatalysts for ORR and various defects in metal oxides/selenides for OER is also provided. The types of defects and controllable strategies to generate defects in electrocatalysts are presented, along with techniques to identify the defects. The defect-activity relationship is also explored by theoretical methods.

Electropolymerized supermolecule derived N, P co-doped carbon nanofiber networks as a highly efficient metal-free electrocatalyst for the hydrogen evolution reaction

In this work, for the first time, we report the synthesis of N, P co-doped carbon derived from electrochemically polymerized supermolecules. The as-obtained N, P co-doped carbon fiber networks show ultra-efficient HER activity as a metal-free electrocatalyst with an overpotential of 151 mV to reach a current density of 10 mA cm−2.

Monolayer MoS2 with S vacancies from interlayer spacing expanded counterparts for highly efficient electrochemical hydrogen production

It is challenging to prepare monolayer MoS2 with activated basal planes in a simple and efficient way. In this study, an interlayer spacing expanded counterpart, ammonia-intercalated MoS2, was obtained by a simple hydrothermal reaction of ammonium molybdate and elemental sulfur in hydrazine monohydrate solution. Then, the ammonia-intercalated MoS2 could be easily exfoliated by ultrasonication to get monolayer MoS2. Importantly, this monolayer MoS2 possessed rich S vacancies. The produced MoS2 demonstrated a proliferated active site density as well as low-loss electrical transport for efficient electrochemical hydrogen production from water. As expected, the monolayer MoS2 with S vacancies exhibited an excellent electrocatalytic hydrogen evolution reaction performance with a low overpotential (at 10 mA cm−2) of 160 mV (V vs. RHE) in acid media and a small Tafel slope of 54.9 mV dec−1. Furthermore, the catalyst displayed a good long-term stability and chemical stability during the electrochemical hydrogen production process. Computational studies prove that the S vacancies enabled the inert basal planes by introducing localized donor states into the bandgap and lowered the hydrogen adsorption free energy. This study could open new opportunities for the rational design and a better understanding of structure–property relationships of MoS2-based catalysts for water splitting or other applications.

Polyaniline-Reduced Graphene Oxide Hybrid Nanosheets with Nearly Vertical Orientation Anchoring Palladium Nanoparticles for Highly Active and Stable Electrocatalysis.

We report a nearly vertical reduced graphene oxide (VrGO) nanosheet coupled with polyaniline (PANI) for supporting palladium (Pd) nanoparticles. The PANI-coupled VrGO (PANI@VrGO) nanosheet is prepared by a simple one-step electrodeposition technique ,and Pd nanoparticles are anchored on the support of PANI@VrGO through the spontaneous redox reaction of PANI with a palladium salt. The designed PANI@VrGO nanosheet efficiently exposes the surface of rGO sheets and stabilizes metal nanoparticles. Consequently, the Pd/PANI@VrGO electrocatalyst exhibits high catalytic activity and excellent durability for alcohol oxidation reaction. The proposed nanoarchitecture offers a new pathway to greatly promote the performances of rGO in various applications; moreover, this work provides a powerful and universal synthetic strategy for such an architecture.

Palladium Nanoparticles Supported on Vertically Oriented Reduced Graphene Oxide for Methanol Electro‐Oxidation

Reduced graphene oxide (rGO) is a promising support material for nanosized electrocatalysts. However, the conventional stacking arrangement of rGO sheets confines the electrocatalysts between rGO layers, which decreases the number of catalytic sites substantially. We report here a facile synthesis of vertically oriented reduced graphene oxide (VrGO) through cyclic voltammetric electrolysis of graphene oxide (GO) in the presence of Na2 PdCl4 . Experiments without Pd nanoparticles or with a low loading amount of Pd nanoparticles results in the deposition of rGO parallel to the electrodes. The vertical orientation of Pd/rGO nanoflakes causes a remarkable enhancement of the catalytic activity toward methanol electro-oxidation. The mass activity (620.1 A gPd (-1) ) of Pd/VrGO is 1.9 and 6.2 times that of Pd/flat-lying rGO (331.8 A gPd (-1) ) and commercial Pd/C (100.5 A gPd (-1) ), respectively. Furthermore, the Pd/VrGO catalyst shows excellent resistance to CO poisoning. This work provides a simple wet-chemical method for VrGO preparation.

Edge-selectively phosphorus-doped few-layer graphene as an efficient metal-free electrocatalyst for the oxygen evolution reaction

The development of efficient, stable and cost-effective electrocatalysts for the oxygen evolution reaction (OER) is the key for water splitting. Carbon-based nanomaterials have found significant applications as metal-free OER electrocatalysts. In this study, for the first time, edge-selectively phosphorus-doped graphene (G-P) was synthesised for OER electrocatalysts. The G-P catalyst reached a current density of 10 mA cm-2 at a small overpotential of 330 mV for the OER with a Tafel slope as low as 62 mV dec-1, which is superior to most of the carbon-based electrocatalysts reported to date.

Three‐Dimensional Nitrogen‐Doped Reduced Graphene Oxide–Carbon Nanotubes Architecture Supporting Ultrafine Palladium Nanoparticles for Highly Efficient Methanol Electrooxidation

A three-dimensional (3D) nitrogen-doped reduced graphene oxide (rGO)-carbon nanotubes (CNTs) architecture supporting ultrafine Pd nanoparticles is prepared and used as a highly efficient electrocatalyst. Graphene oxide (GO) is first used as a surfactant to disperse pristine CNTs for electrochemical preparation of 3D rGO@CNTs, and subsequently one-step electrodeposition of the stable colloidal GO-CNTs solution containing Na2 PdCl4 affords rGO@CNTs-supported Pd nanoparticles. Further thermal treatment of the Pd/rGO@CNTs hybrid with ammonia achieves not only in situ nitrogen-doping of the rGO@CNTs support but also extraordinary size decrease of the Pd nanoparticles to below 2.0 nm. The resulting catalyst is characterized by scanning and transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Catalyst performance for the methanol oxidation reaction is tested through cyclic voltammetry and chronoamperometry techniques, which shows exceedingly high mass activity and superior durability.

Rapidly engineering the electronic properties and morphological structure of NiSe nanowires for the oxygen evolution reaction

The oxygen evolution reaction (OER) is one of the most important reactions in a wide range of renewable energy technologies. It is important to develop highly efficient electrocatalysts for the OER due to its sluggish kinetics. The electronic properties and morphological structure of electrocatalysts can significantly affect their OER performance. Electrocatalysts with the morphology of nanosheets can expose more active sites which would enhance the OER activity. Here, we report an extremely simple and fast method to synthesize a NixFe1−xSe@Ni(Fe)OOH core–shell nanostructure with a nanosheet shell by a facile solvothermal selenization and ion exchange reaction. The NixFe1−xSe@Ni(Fe)OOH core–shell nanostructure gives an excellent catalytic activity toward the OER with an overpotential as low as 260 mV to reach a current density of 100 mA cm−2 and excellent electrochemical long-term stability in 1 M KOH solution. The enhanced OER activity can be attributed to the dual modulation of electronic properties and the morphological structure by Fe doping.

N, P-dual doped carbon with trace Co and rich edge sites as highly efficient electrocatalyst for oxygen reduction reaction

Oxygen reduction reaction (ORR) is key to fuel cells and metal-air batteries which are considered as the alternative clean energy. Various carbon materials have been widely researched as ORR electrocatalysts. It has been accepted that heteroatom doping and exposure of the edge sites can effectively improve the activity of carbon materials. In this work, we used a simple method to prepare a novel N, P-dual doped carbon-based catalyst with many holes on the surface. In addition, trace level Co doping in the carbon material forming Co–N–C active species can further enhance the ORR performance. On one hand, the doping can adjust the electronic structure of carbon atoms, which would induce more active sites for ORR. And on the other hand, the holes formed on the surface of carbon nanosheets would expose more edge sites and can improve the intrinsic activity of carbon. Due to the heteroatom doping and the exposed edge sites, the prepared carbon materials showed highly excellent ORR performance, close to that of commercial Pt/C.摘要本文使用有机分子配位聚合作用一步聚合、 碳化、 酸洗得到了一种N,P双掺杂碳材料. 其具有痕量掺杂的金属钴、 且具有更多活性边缘. X射线光电子能谱显示杂原子成功进入碳材料当中, 并且发现酸洗后钴的信号非常低, 证明酸洗后, 材料表面形成非常多的孔, 暴露出更多的边缘催化位点. 制备的碳材料具有大量催化活性位点, 因此表现出极其优异的电催化氧还原性能. 另外, 与Pt/C相比, 制备的多孔碳材料还具有较好的抗毒性与稳定性, 进一步显示了其在新能源电池领域的应用前景.

Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting

Abstract Layered double hydroxide (LDH)‐based materials have attracted widespread attention in various applications due to their unique layered structure with high specific surface area and unique electron distribution, resulting in a good electrocatalytic performance. Moreover, the existence of multiple metal cations invests a flexible tunability in the host layers; the unique intercalation characteristics lead to flexible ion exchange and exfoliation. Thus, their electrocatalytic performance can be tuned by regulating the morphology, composition, intercalation ion, and exfoliation. However, the poor conductivity limits their electrocatalytic performance, which therefore has motivated researchers to combine them with conductive materials to improve their electrocatalytic performance. Another factor hampering their electrocatalytic activity is their large lateral size and the bulk thickness of LDHs. Introducing defects and tuning electronic structure in LDH‐based materials are considered to be effective strategies to increase the number of active sites and enhance their intrinsic activity. Given the unique advantages of LDH‐based materials, their derivatives have been also used as advanced electrocatalysts for water splitting. Here, recent progress on LDHs and their derivatives as advanced electrocatalysts for water splitting is summarized, current strategies for their designing are proposed, and significant challenges and perspectives of LDHs are discussed.