Facile Synthesis of PdCu Echinus‐Like Nanocrystals as Robust Electrocatalysts for Methanol Oxidation Reaction

Exploiting high‐performance and inexpensive electrocatalysts for methanol electro‐oxidation is conductive to promoting the commercial application of direct methanol fuel cells. Here, we present a facile synthesis of echinus‐like PdCu nanocrystals (NCs) via a one‐step and template‐free method. The echinus‐like PdCu NCs possess numerous straight and long branches which can provide abundant catalytic active sites. Owing to the novel nanoarchitecture and electronic effect of the PdCu alloy, the echinus‐like PdCu NCs display high electrocatalytic performance toward methanol oxidation reaction in an alkaline medium. The mass activity of echinus‐like PdCu NCs is 1202.1 mA mg_Pd^?1, which is 3.7 times that of Pd/C catalysts. In addition, the echinus‐like structure, as a kind of three‐dimensional self‐supported nanoarchitecture, endows PdCu NCs with significantly enhanced stability and durability. Hence, the echinus‐like PdCu NCs hold prospect of being employed as electrocatalysts for direct alcohol fuel cells.     

The Effect of KOH Treatment on the Chemical Structure and Electrocatalytic Activity of Reduced Graphene Oxide Materials

Reduced graphene oxide (rG‐O)‐based materials have great potential as metal‐free electrocatalysts for the oxygen reduction reaction (ORR) owing to their electrical and electrochemical properties and large surface area. Long‐term durability and chemical stability of the catalysts in the presence of electrolytes such as aqueous KOH solution are important for their use in practical applications. In this study, three types of rG‐O and rG‐O‐K (rG‐O after reaction with KOH) materials were synthesized. The chemical structures, surface areas, and catalytic ORR performances of the rG‐O materials were compared with those of the corresponding rG‐O‐K materials. The onset potentials of the rG‐O materials for electrocatalytic reduction of oxygen are almost the same as those of the corresponding rG‐O‐K materials; however, the current density and the number of transferred electrons are significantly reduced. These data show that the catalytic ORR performance of rG‐O‐based materials can be altered by KOH.     

通过调控晶体结构提高氧气还原反应电催化活性:MPt金属间相纳米晶（英文）

燃料电池的正极主要发生氧还原反应(ORR),但是该反应的动力学速率较慢,需要催化剂来降低反应的过电势.目前商用的催化剂是碳载铂纳米粒子催化剂,但是铂高昂的价格严重阻碍了燃料电池的大规模商业化.近年来的理论和实验研究表明,过渡金属(M)与铂(Pt)形成的纳米晶合金(MPt)能够作为有效的ORR催化剂,同时由于引入价格低廉的过渡金属,催化剂成本有所降低.然而,即使合金化的催化剂具有良好的初始催化性能,但是在燃料电池的实际操作环境,即高电压、高温和酸性条件,长时间运行之后,过渡金属很容易被腐蚀流失,从而留下表面配位数较低的铂原子,而这些铂原子对ORR反应几乎没有催化作用,导致催化剂逐渐失活,燃料电池的输出功率逐渐降低.最近一些研究表明,铂基催化剂在一定条件下,例如加热,能够发生固态相变,形成结构有序的即金属间纳米晶(iNCs).与无序排列的合金相比,这种有序的MPt能够调控表面铂原子与含氧中间体的结合能,可以进一步提高ORR活性;同时,由于在金属间纳米晶中铂原子与过渡金属原子具有很强的相互作用,过渡金属在酸性溶液中也不容易被腐蚀,从而大大提高了催化剂的稳定性.本综述以FePt,CoPt和PbPt为例,总结了它们的相变规律和条件,同时关注它们的合成-结构-性能的构效关系,突出金属间结构在提高活性和稳定性方面的优势.最后,为了进一步提高MPt金属间纳米晶的活性,我们提出一些可能的方向和观点,包括:(1)在实现无序-相变的同时实现形貌调控来提高催化剂活性;(2)关注尺寸效应,尽可能减小MPt金属间纳米晶的尺寸,提高铂的利用率,从而提高催化剂活性;(3)关注材料的有序程度,尽可能提高材料的有序度,充分发挥金属间纳米晶对于氧还原反应的优势     

A Mesoporous Nanorattle‐Structured Pd@PtRu Electrocatalyst

Pt‐based nanomaterials play important roles in the catalytic process toward oxygen reduction reaction (ORR). Rationally regulating the composition and morphology of the catalysts could enhance the catalytic performance effectively. In this work, an effective method is presented to synthesize Pd@ mesoporous PtRu nanorattles (Pd@mPtRu NRs) containing a Pd core and a mesoporous PtRu shell. Owing to the unique structure and PtRu alloy composition, the prepared Pd@mPtRu NRs exhibit an enhanced catalytic performance and durability toward ORR relative to mesoporous PtRu hollow nanoparticles (mPtRu HNs) and commercial Pt/C. The proposed approach may provide a general way to synthesize Pt‐based yolk‐shell structures with different compositions.     

High‐Rate Oxygen Electroreduction over Graphitic‐N Species Exposed on 3D Hierarchically Porous Nitrogen‐Doped Carbons

Nitrogen‐doped species (NDs) are theoretically accepted as a determinant of the catalytic activity of metal‐free N‐doped carbon (NC) catalysts for oxygen reduction reaction (ORR). However, direct relationships between ND type and ORR activity have been difficult to extract because the complexity of carbon matrix impairs efforts to expose specific NDs. Herein, we demonstrate the fabrication of a 3D hierarchically porous NC catalyst with micro‐, meso‐, and macroporosity in one structure, in which sufficient exposure and availability of inner‐pore catalytic sites can be achieved due to its super‐high surface area (2191 cm² g^?1) and interconnected pore system. More importantly, in‐situ formation of graphitic‐N species (GNs) on the surface of NC stimulated by KOH activation enables us to experimentally reveal the catalytic nature of GNs for ORR, which is of great significance for the design and development of advanced metal‐free NC electrocatalysts.     

精确可控串联策略触发高效的氧还原活性

探索高效、经济的非金属氧还原(ORR)电催化剂已成为电化学能源体系的关键.科学界最具挑战性的目标之一是通过合理地验证和精确地调节活性位点来设计结构明确、性能优异的催化剂材料.本文提出一种精确和可控的串联协同作用的活性位点策略,以提高MFCOFs的ORR催化活性.以亚胺-N、噻吩-S和三嗪-N等作为结构单元,通过精确的串...     

Completely Green Synthesis of Colloid Adams’ Catalyst α‐PtO2 Nanocrystals and Derivative Pt Nanocrystals with High Activity and Stability for Oxygen Reduction

We report a first solution strategy for controlled synthesis of Adams’ catalyst (i.e., α‐PtO₂) by a facile and totally green approach using H₂PtCl₆ and water as reactants. The prepared α‐PtO₂ nanocrystals (NCs) are ultrasmall in size and have very “clean” surfaces, which can be reduced to Pt NCs easily in ethanol under ambient conditions. Such Adams’ catalysts have been applied as electrocatalysts beyond the field of heterogeneous catalysis. Noticeably, the water‐only synthesized α‐PtO₂ NCs and their derivative Pt NCs all exhibit much higher oxygen reduction reaction (ORR) activities and stabilities than that of the state‐of‐art Pt/C electrocatalysts. This study provides an example on the organics‐free synthesis of α‐PtO₂ and Pt NCs as promising cathode catalysts for fuel cell applications and, particularly, this simple, straightforward method may open a new way for the synthesis of other “clean” functional nanomaterials.     

Heterocyclization Strategy for Construction of Linear Conjugated Polymers: Efficient Metal‐Free Electrocatalysts for Oxygen Reduction

Exploring cost‐effective and efficient metal‐free electrocatalysts for the oxygen reduction reaction (ORR) is crucial for the development of energy conversion and storage technologies. Reported here is a novel heterocyclization strategy to construct efficient ORR catalysts based on linear conjugated polymers (LCPs), which are composed of N‐, S‐, or Se‐heterocycles. Among these polymers, the covalently linked pyridine and thiophene molecule ( P‐T ) with reduced graphene oxide (rGO) exhibits a remarkable half‐wave potential of 0.79 V (vs. RHE) and excellent electrochemical stability, which are among the highest values for metal‐free polymers as ORR catalysts. Density‐functional theory (DFT) calculations reveal that the molecule with a phenyl unit ( P‐Ph ) is catalytically inactive, and when a thiophene unit is introduced to replace the phenyl unit in the conjugated backbone it features highly efficient electrocatalytic active sites. More importantly, the well‐defined molecular structures and controllable active sites in the pyrolysis and metal‐free polymers highlight new opportunities for the catalytic metal‐free ORR.     

A Nanoporous PdCo Alloy as a Highly Active Electrocatalyst for the Oxygen‐Reduction Reaction and Formic Acid Electrooxidation

A nanoporous (NP) PdCo alloy with uniform structure size and controllable bimetallic ratio was fabricated simply by one‐step mild dealloying of a PdCoAl precursor alloy. The as‐made alloy consists of a nanoscaled bicontinuous network skeleton with interconnected hollow channels that extend in all three dimensions. With a narrow ligament size distribution around 5 nm, the NP PdCo alloy exhibits much higher electrocatalytic activity towards the oxygen‐reduction reaction (ORR) with enhanced specific and mass activities relative to NP Pd and commercial Pt/C catalysts. A long‐term stability test demonstrated that NP PdCo has comparable catalytic durability with less loss of ORR activity and electrochemical surface area than Pt/C. The NP PdCo alloy also shows dramatically enhanced catalytic activity towards formic acid electrooxidation relative to NP Pd and Pd/C catalysts. The as‐made NP PdCo holds great application potential as a promising cathode as well as an anode electrocatalyst in fuel cells with the advantages of superior catalytic performance and easy preparation.     

Polyoxometalate‐Mediated One‐Pot Synthesis of Pd Nanocrystals with Controlled Morphologies for Efficient Chemical and Electrochemical Catalysis

Polyoxometalates (POMs), as inorganic ligands, can endow metal nanocrystals (NCs) with unique reactivities on account of their characteristic redox properties. In the present work, we present a facile POM‐mediated one‐pot aqueous synthesis method for the production of single‐crystalline Pd NCs with controlled shapes and sizes. The POMs could function as both reducing and stabilizing agents in the formation of NCs, and thus gave a fine control over the nucleation and growth kinetics of NCs. The prepared POM‐stabilized Pd NCs exhibited excellent catalytic activity and stability for electrocatalytic (formic acid oxidation) and catalytic (Suzuki coupling) reactions compared to Pd NCs prepared without the POMs. This shows that the POMs play a pivotal role in determining the catalytic performance, as well as the growth, of NCs. We envision that the present approach can offer a convenient way to develop efficient NC‐based catalyst systems.     

聚合物稳定的金纳米簇催化：理论计算与实验的相互影响

平均粒径为2–10 nm的聚合物稳定的Au纳米簇(NCs)表现出独特的催化性能。多个研究表明，影响聚合物稳定的Au NCs催化活性的主要因素为： Au NC尺寸的控制、聚合物的选择以及反应条件的优化。这是由于聚合物稳定的Au NCs在多个催化反应中表现出明显的尺寸效应，其催化活性也因所采用的聚合物和反应条件的不同而不同。为了阐明影响聚合物稳定的Au NCs催化活性的内在原因，众多研究者关注于聚合物稳定的Au NCs催化中的理论计算与实验的相互影响。本文主要总结了聚合物稳定的Au NCs中这种相互影响的研究进展。     

Formation of a Highly Reactive Cobalt Nanocluster Crystal within a Highly Negatively Charged Porous Coordination Cage

Earth‐abundant first‐row transition‐metal nanoclusters (NCs) have been extensively investigated as catalysts. However, their catalytic activity is relatively low compared with noble metal NCs. Enhanced catalytic activity of cobalt NCs can be achieved by encapsulating Co NCs in soluble porous coordination cages (PCCs). Two cages, PCC‐2a and 2b, possess almost identical cavity in shape and size, while PCC‐2a has five times more net charges than PCC‐2b. Co²⁺ cations were accumulated in PCC‐2a and reduced to ultra‐small Co NCs in situ, while for PCC‐2b, only bulky Co particles were formed. As a result, Co NCs@PCC‐2a accomplished the highest catalytic activity in the hydrolysis of ammonium borane among all the first‐row transition‐metals NCs. Based on these results, it is envisioned that confining in the charged porous coordination cage could be a novel route for the synthesis of ultra‐small NCs with extraordinary properties.     

“Metal‐Free” Catalytic Oxygen Reduction Reaction on Heteroatom‐Doped Graphene is Caused by Trace Metal Impurities

The oxygen reduction reaction (ORR) is of high industrial importance. There is a large body of literature showing that metal‐based catalytic nanoparticles (e.g. Co, Mn, Fe or hybrid Mn/Co‐based nanoparticles) supported on graphene act as efficient catalysts for the ORR. A significant research effort is also directed to the so‐called “metal‐free” oxygen reduction reaction on heteroatom‐doped graphene surfaces. While such studies of the ORR on nonmetallic heteroatom‐doped graphene are advertised as “metal‐free” there is typically no sufficient effort to characterize the doped materials to verify that they are indeed free of any trace metal. Here we argue that the claimed “metal‐free” electrocatalysis of the oxygen reduction reaction on heteroatom‐doped graphene is caused by metallic impurities present within the graphene materials.     

Facile Synthesis of Platinum–Cerium(IV) Oxide Hybrids Arched on Reduced Graphene Oxide Catalyst in Reverse Micelles with High Activity and Durability for Hydrolysis of Ammonia Borane

Highly dispersed Pt‐CeO₂ hybrids arched on reduced graphene oxide (Pt‐CeO₂/rGO) were facilely synthesized by a combination of the reverse micelle technique and a redox reaction without any additional reductant or surfactant. Under a N₂ atmosphere, the redox reaction between Ce³⁺ and Pt²⁺ occurs automatically in alkaline solution, which results in the formation of Pt‐CeO₂/rGO nanocomposites (NCs). The as‐synthesized Pt‐CeO₂/rGO NCs exhibit superior catalytic performance relative to that shown by the free Pt nanoparticles, Pt/rGO, Pt‐CeO₂ hybrid, and the physical mixture of Pt‐CeO₂ and rGO; furthermore, the nanocomposites show significantly better activity than the commercial Pt/C catalyst toward the hydrolysis of ammonia borane (NH₃BH₃) at room temperature. Moreover, the Pt‐CeO₂/rGO NCs have remarkable stability, and 92 % of their initial catalytic activity is preserved even after 10 runs. The excellent activity of the Pt‐CeO₂/rGO NCs can be attributed not only to the synergistic structure but also to the electronic effects of the Pt‐CeO₂/rGO NCs among Pt, CeO₂, and rGO.     

Electrochemical Synthesis of Tetrahexahedral Rhodium Nanocrystals with Extraordinarily High Surface Energy and High Electrocatalytic Activity

Noble metal nanocrystals (NCs) enclosed with high‐index facets hold a high catalytic activity thanks to the high density of low‐coordinated step atoms that they exposed on their surface. Shape‐control synthesis of the metal NCs with high‐index facets presents a big challenge owing to the high surface energy of the NCs, and the shape control for metal Rh is even more difficult because of its extraordinarily high surface energy in comparison with Pt, Pd, and Au. The successful synthesis is presented of tetrahexahedral Rh NCs (THH Rh NCs) enclosed by {830} high‐index facets through the dynamic oxygen adsorption/desorption mediated by square‐wave potential. The results demonstrate that the THH Rh NCs exhibit greatly enhanced catalytic activity over commercial Rh black catalyst for the electrooxidation of ethanol and CO.     

Nitrogen‐Doped Carbon Nanosheets with Size‐Defined Mesopores as Highly Efficient Metal‐Free Catalyst for the Oxygen Reduction Reaction

Nitrogen‐doped carbon nanosheets (NDCN) with size‐defined mesopores are reported as highly efficient metal‐free catalyst for the oxygen reduction reaction (ORR). A uniform and tunable mesoporous structure of NDCN is prepared using a templating approach. Such controlled mesoporous structure in the NDCN exerts an essential influence on the electrocatalytic performance in both alkaline and acidic media for the ORR. The NDCN catalyst with a pore diameter of 22 nm exhibits a more positive ORR onset potential than that of Pt/C (?0.01 V vs. ?0.02 V) and a high diffusion‐limited current approaching that of Pt/C (5.45 vs. 5.78 mA cm^?2) in alkaline medium. Moreover, the catalyst shows pronounced electrocatalytic activity and long‐term stability towards the ORR under acidic conditions. The unique planar mesoporous shells of the NDCN provide exposed highly electroactive and stable catalytic sites, which boost the electrocatalytic activity of metal‐free NDCN catalyst.     

Bamboo‐Like Nitrogen‐Doped Carbon Nanotubes with Co Nanoparticles Encapsulated at the Tips: Uniform and Large‐Scale Synthesis and High‐Performance Electrocatalysts for Oxygen Reduction

In recent years, various non‐precious metal electrocatalysts for the oxygen reduction reaction (ORR) have been extensively investigated. The development of an efficient and simple method to synthesize non‐precious metal catalysts with ORR activity superior to that of Pt is extremely significant for large‐scale applications of fuel cells. Here, we develop a facile, low‐cost, and large‐scale synthesis method for uniform nitrogen‐doped (N‐doped) bamboo‐like CNTs (NBCNT) with Co nanoparticles encapsulated at the tips by annealing a mixture of cobalt acetate and melamine. The uniform NBCNT shows better ORR catalytic activity and higher stability in alkaline solutions as compared with commercial Pt/C and comparable catalytic activity to Pt/C in acidic media. NBCNTs exhibit outstanding ORR catalytic activity due to high defect density, uniform bamboo‐like structure, and the synergistic effect between the Co nanoparticles and protective graphitic layers. This facile method to synthesize catalysts, which is amenable to the large‐scale commercialization of fuel cells, will open a new avenue for the development of low‐cost and high‐performance ORR catalysts to replace Pt‐based catalysts for applications in energy conversion.     

Nitrogen and Phosphorus Dual‐Doped Hierarchical Porous Carbon Foams as Efficient Metal‐Free Electrocatalysts for Oxygen Reduction Reactions

Despite tremendous progress in developing doped carbocatalysts for the oxygen reduction reaction (ORR), the ORR activity of current metal‐free carbocatalysts is still inferior to that of conventional Pt/C catalysts, especially in acidic media and neutral solution. Moreover, it also remains a challenge to develop an effective and scalable method for the synthesis of metal‐free carbocatalysts. Herein, we have developed nitrogen and phosphorus dual‐doped hierarchical porous carbon foams (HP‐NPCs) as efficient metal‐free electrocatalysts for ORR. The HP‐NPCs were prepared for the first time by copyrolyzing nitrogen‐ and phosphorus‐containing precursors and poly(vinyl alcohol)/polystyrene (PVA/PS) hydrogel composites as in situ templates. Remarkably, the resulting HP‐NPCs possess controllable nitrogen and phosphorus content, high surface area, and a hierarchical interconnected macro‐/mesoporous structure. In studying the effects of the HP‐NPCs on the ORR, we found that the as‐prepared HP‐NPC materials exhibited not only excellent catalytic activity for ORR in basic, neutral, and acidic media, but also much better tolerance for methanol oxidation and much higher stability than the commercial, state‐of‐the‐art Pt/C catalysts. Because of all these outstanding features, it is expected that the HP‐NPC material will be a very suitable catalyst for next‐generation fuel cells and lithium–air batteries. In addition, the novel synthetic method described here might be extended to the preparation of many other kinds of hierarchical porous carbon materials or porous carbon that contains metal oxide for wide applications including energy storage, catalysis, and electrocatalysis.     

The First Study on the Reactivity of Water Vapor in Metal–Organic Frameworks with Platinum Nanocrystals

We first studied the reactivity of H₂O vapor in metal–organic frameworks (MOFs) with Pt nanocrystals (NCs) through the water–gas shift (WGS) reaction. A water‐stable MOF, UiO‐66, serves as a highly effective support material for the WGS reaction compared with ZrO₂. The origin of the high catalytic performance was investigated using in situ IR spectroscopy. In addition, from a comparison of the catalytic activities of Pt on UiO‐66, where Pt NCs are located on the surface of UiO‐66 and Pt@UiO‐66, where Pt NCs are coated with UiO‐66, we found that the competitive effects of H₂O condensation and diffusion in the UiO‐66 play important roles in the catalytic activity of Pt NCs. A thinner UiO‐66 coating further enhanced the WGS reaction activity of Pt NCs by minimizing the negative effect of slow H₂O diffusion in UiO‐66.     

Effects of Capping Agents on the Oxygen Reduction Reaction Activity and Shape Stability of Pt Nanocubes

We investigated the formation of Pt nanocubes (NCs) and their electrocatalytic oxygen reduction reaction (ORR) properties and structural stability using two different capping agents, namely, polyvinylpyrrolidone (PVP) and oleylamine (OAm). The mono-dispersity of the obtained Pt NCs and their interactions with PVP and OAm were analyzed by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The TEM data show a high mono-dispersity (82 %) and a large mean particle size (9-10 nm) for the Pt NCs obtained by the oleylamine-assisted method compared to those prepared via the PVP-assisted procedure (68 %, 6–7 nm). FTIR, XPS, and TGA data show that PVP and OAm still remain at the Pt surface, despite washing. Interestingly, the OAm-capped Pt NCs show significantly higher electrochemically active surface area (ECSA) and ORR activity than the PVP-capped ones. An accelerated stress protocol, however, reveals that the OAm-capped NCs possess a poor structural stability during electrochemical cycling. The loss of a defined surface arrangement in the NCs is connected with a transformation into a near-spherical particle shape. In contrast, the PVP-capped NCs mainly retain their particle shape due to their strong capping behavior. In addition, we have developed a degradation model for NCs as a function of electrochemical parameters such as upper potential and cycle number. Altogether, we provide fundamental insights into the electronic interactions between capping agent and Pt NCs and the role of the adsorption strength of the capping agent in improving the electrochemical ORR performance as well as the structural stability of shape-controlled nanoparticles.