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

Defect Chemistry of Nonprecious-Metal Electrocatalysts for Oxygen Reactions

Proton exchange membrane fuel cells can use hydrogen and air to power clean electric vehicles. However, technical barriers including high cost, limited lifetime and insufficient power density limit their broad applications. Advanced cathode catalysts for the kinetically-sluggish oxygen reduction reaction (ORR) in acidic media are essential for overcoming these barriers. Here, we highlight breakthroughs, challenges and future directions for both platinum group metal (PGM) and PGM-free ORR cathode catalysts. Among PGM catalysts, highly-ordered PtM intermetallic nanostructures exhibit enhanced activity and stability relative to PtM random alloys. Carbon supports, with optimal balance between graphitization degree and porosity, play an important role in enhancing overall performance. Among PGM-free catalysts, transition metal and nitrogen co-doped carbons (M-N-C) perform best. However, degradation at practical voltages (>0.6 V) still prevents their practical application. For all catalysts, translating intrinsic activity and stability into device performance requires electrodes with robust three-phase interfaces for effective charge and mass transfer. Proton exchange membrane fuel cells can efficiently provide clean power for electric vehicles, although more efficient and economic cathode catalysts are still required. This Review highlights recent breakthroughs, challenges and future research directions for Pt group metal (PGM) and PGM-free oxygen reduction catalysts.

Achievements, challenges and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation

Electrocatalysis is crucial for the development of clean and renewable energy technologies, which may reduce our reliance on fossil fuels. Multimetallic nanomaterials serve as state-of-the-art electrocatalysts as a consequence of their unique physico-chemical properties. One method of enhancing the electrocatalytic performance of multimetallic nanomaterials is to tune or control the surface strain of the nanomaterials, and tremendous progress has been made in this area in the past decade. In this Review, we summarize advances in the introduction, tuning and quantification of strain in multimetallic nanocrystals to achieve more efficient energy conversion by electrocatalysis. First, we introduce the concept of strain and its correlation with other key physico-chemical properties. Then, using the electrocatalytic reduction of oxygen as a model reaction, we discuss the underlying mechanisms behind the strain–adsorption–reactivity relationship based on combined classical theories and models. We describe how this knowledge can be harnessed to design multimetallic nanocrystals with optimized strain to increase the efficiency of oxygen reduction. In particular, we highlight the unexpectedly beneficial (and previously overlooked) role of tensile strain from multimetallic nanocrystals in improving electrocatalysis. We conclude by outlining the challenges and offering our perspectives on the research directions in this burgeoning field. Tuning the surface strain in multimetallic nanomaterials represents an effective strategy to improve their electrocatalytic properties. In this Review, using the oxygen reduction reaction as a model, the underlying relationship between surface strain and catalytic activity is discussed, along with the introduction, tuning and quantification of strain in nanocatalysts.

Strain-controlled electrocatalysis on multimetallic nanomaterials

Recent Advances on Water-Splitting Electrocatalysis Mediated by Noble-Metal-Based Nanostructured Materials

Review of Metal Catalysts for Oxygen Reduction Reaction: From Nanoscale Engineering to Atomic Design

Despite intense research in past decades, the lack of high-performance catalysts for fuel cell reactions remains a challenge in realizing fuel cell technologies for transportation applications. Here we report a facile strategy for synthesizing hierarchical platinum-cobalt nanowires with high-index, platinum-rich facets and ordered intermetallic structure. These structural features enable unprecedented performance for the oxygen reduction and alcohol oxidation reactions. The specific/mass activities of the platinum-cobalt nanowires for oxygen reduction reaction are 39.6/33.7 times higher than commercial Pt/C catalyst, respectively. Density functional theory simulations reveal that the active threefold hollow sites on the platinum-rich high-index facets provide an additional factor in enhancing oxygen reduction reaction activities. The nanowires are stable in the electrochemical conditions and also thermally stable. This work may represent a key step towards scalable production of high-performance platinum-based nanowires for applications in catalysis and energy conversion.

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Surface engineering of hierarchical platinum-cobalt nanowires for efficient electrocatalysis

The development of superior non-platinum electrocatalysts for enhancing the electrocatalytic activity and stability for the oxygen-reduction reaction (ORR) and liquid fuel oxidation reaction is very important for the commercialization of fuel cells, but still a great challenge. Herein, we demonstrate a new colloidal chemistry technique for making structurally ordered PdCu-based nanoparticles (NPs) with composition control from PdCu to PdCuNi and PtCuCo. Under the dual tuning on the composition and intermetallic phase, the ordered PdCuCo NPs exhibit better activity and much enhanced stability for ORR and ethanol-oxidation reaction (EOR) than those of disordered PdCuM NPs, the commercial Pt/C and Pd/C catalysts. The density functional theory (DFT) calculations reveal that the improved ORR activity on the PdCuM NPs stems from the catalytically active hollow sites arising from the ligand effect and the compressive strain on the Pd surface owing to the smaller atomic size of Cu, Co, and Ni.

Ordered PdCu-Based Nanoparticles as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electrocatalysts.

We report a new 2D electron gas (2DEG) system at the interface between a Mott insulator, LaTiO3, and a band insulator, KTaO3. For LaTiO3/KTaO3 interfaces, we observe metallic conduction from 2 K to 300 K. One serious technological limitation of SrTiO3-based conducting oxide interfaces for electronics applications is the relatively low carrier mobility (0.5-10 cm2/V s) of SrTiO3 at room temperature. By using KTaO3, we achieve mobilities in LaTiO3/KTaO3 interfaces as high as 21 cm2/V s at room temperature, over a factor of 3 higher than observed in doped bulk SrTiO3. By density functional theory, we attribute the higher mobility in KTaO3 2DEGs to the smaller effective mass for electrons in KTaO3.

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LaTiO3/KTaO3 interfaces: A new two-dimensional electron gas system

Transport characteristics of ultrathin SrRuO3 films, deposited epitaxially on TiO2-terminated SrTiO3 (001) single-crystal substrates, were studied as a function of film thickness. Evolution from a metallic to an insulating behavior is observed as the film thickness decreases from 20 to 4 unit cells. In films thicker than 4 unit cells, the transport behavior obeys the Drude low temperature conductivity with quantum corrections, which can be attributed to weak localization. Fitting the data with 2-dimensional localization model indicates that electron-phonon collisions are the main inelastic relaxation mechanism. In the film of 4 unit cells in thickness, the transport behavior follows variable range hopping model, indicating a strongly localized state. Magnetoresistance measurements reveal a likely magnetic anisotropy with the magnetic easy axis along the out-of-plane direction.

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Xuan Shen

Papers

Surface engineering of hierarchical platinum-cobalt nanowires for efficient electrocatalysis

Ordered PdCu-Based Nanoparticles as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electrocatalysts.

LaTiO3/KTaO3 interfaces: A new two-dimensional electron gas system

Thickness-dependent metal-insulator transition in epitaxial SrRuO3 ultrathin films

Enhanced ferromagnetism at the rhombohedral–tetragonal phase boundary in Pr and Mn co-substituted BiFeO3 powders