Longzhou Zhang

Defect Graphene as a Trifunctional Catalyst for Electrochemical Reactions

Defects derived by the removal of heteroatoms from graphene are demonstrated, both experimentally and theoretically, to be effective for all three basic electrochemical reactions, e.g., oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER). Density function theory calculations further reveal that the different types of defects are essential for the individual electrocatalytic activity for ORR, OER, and HER, respectively.

A Heterostructure Coupling of Exfoliated Ni–Fe Hydroxide Nanosheet and Defective Graphene as a Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the authors demonstrate a heterostructured NiFe LDH-NS@DG10 hybrid catalyst by coupling of exfoliated Ni-Fe layered double hydroxide (LDH) nanosheet (NS) and defective graphene (DG). The catalyst has exhibited extremely high electrocatalytic activity for oxygen evolution reaction (OER) in an alkaline solution with an overpotential of 0.21 V at a current density of 10 mA cm-2 , which is comparable to the current record (≈0.20 V in Fe-Co-Ni metal-oxide-film system) and superior to all other non-noble metal catalysts. Also, it possesses outstanding kinetics (Tafel slope of 52 mV dec-1 ) for the reaction. Interestingly, the NiFe LDH-NS@DG10 hybrid has also exhibited the high hydrogen evolution reaction (HER) performance in an alkaline solution (with an overpotential of 115 mV by 2 mg cm-2 loading at a current density of 20 mA cm-2 ) in contrast to barely HER activity for NiFe LDH-NS itself. As a result, the bifunctional catalyst the authors developed can achieve a current density of 20 mA cm-2 by a voltage of only 1.5 V, which is also a record for the overall water splitting. Density functional theory calculation reveals that the synergetic effects of highly exposed 3d transition metal atoms and carbon defects are essential for the bifunctional activity for OER and HER.

Seaweed biomass derived (Ni,Co)/CNT nanoaerogels: efficient bifunctional electrocatalysts for oxygen evolution and reduction reactions

Developing earth-abundant, active and stable electrocatalysts which operate in two-electrode rechargeable metal–air batteries, including both oxygen evolution and reduction reactions (OER and ORR), is vital for renewable energy conversion in real application. Here, we demonstrate a three-dimensional (3D) bifunctional nanoaerogel electrocatalyst that exhibits good electrocatalytic properties for both OER and ORR. This material was fabricated using a scalable and facile method involving the pyrolysis of (Ni,Co)/CNT alginate hydrogels derived from sustainable seaweed biomass after an ion exchange process. The bifunctionality for oxygen electrocatalysis as shown by the OER–ORR potential difference (ΔE, the OER and ORR potentials are taken at the current densities of 10 mA cm−2 and −3 mA cm−2 in 0.1 M KOH, respectively) could be reduced to as low as 0.87 V, comparable to the state-of-the-art non-noble bifunctional catalysts. The good performance was attributed to the ternary Ni/NiO/NiCo2O4 catalytic center for charge transfer and 3D hierarchical mesoporous hybrid framework for efficient mass transport. More importantly, the Zn–air battery fabricated with the hybrid nanoaerogel as a bifunctional electrocatalyst displays very high energy efficiency (58.5%) and long-term stability. Prospectively, our present work may pave a new way to develop earth-abundant and low cost high-performance bifunctional electrocatalysts for rechargeable metal–air batteries.

Platinum stabilized by defective activated carbon with excellent oxygen reduction performance in alkaline media

The exploration of highly active and durable cathodic oxygen reduction reaction (ORR) catalysts with economical production cost is still the bottleneck to realize the large-scale commercialization of fuel cells and metal-air batteries. Given that carbon support is crucial to the electrocatalysts, and Pt is the best-known ORR catalyst so far, in this work, we employed a simple impregnation method for synthesizing a kind of defective activated carbon (D-AC) supported low Pt content electrocatalysts for the ORR. The reduction conditions of the Pt-containing precursor were firstly optimized, and the influence of the Pt loading amount on the ORR was investigated as well. The results show that the obtained D-AC@5.0%Pt sample (contains 5 wt% Pt) has surpassed the commercial Pt/C with 20 wt% Pt for the ORR in an alkaline solution. In the meantime, it is more stable than the commercial Pt/C. The outstanding ORR performance of the D-AC@5.0%Pt confirms that both the unique defects in the D-AC and the introduced Pt particles are indispensable to the ORR. Particularly, the ORR activity of the synthesized catalysts is superior to most of the reported counterparts, but with much easier preparation methods and lower production cost, making them more advantageous in practical fuel cell applications.

Activity Origins in Nanocarbons for the Electrocatalytic Hydrogen Evolution Reaction

Sustainable hydrogen production is an essential prerequisite for realizing the future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the cornerstone of exploring the mechanism of water electrolysis, has attracted extensive interest in the past decades. Carbon-based materials with significant merits such as abundance, low cost, high conductivity, and tunable molecular structures, are considered as promising candidates for replacing the commercial noble metal electrocatalysts. To date, activity origins of these carbon-based electrocatalysts are mainly attributed to the dopants (e.g., N, B, P or S), whereas the contribution of intrinsic/induced carbon defects has recently been a hot research topic. In this Review, besides the development of heteroatoms doping strategies, the latest studies on defective carbon-based materials for HER electrocatalysis are summarized, especially for various approaches to prepare defective carbons and the detailed introduction regarding the defect catalysis mechanism. Finally, an outlook into the development of future defective carbon-based HER electrocatalysts is presented.

314 HRTEM Studies of Plastic Deformation in Silicon Wafers due to Micro Indentation

The plastic deformation of silicon wafers caused by micro-indentation was studied by means of HRTEM. It was shown that under a low maximum indentation load (P_ ) the transformation zone is amorphous but its boundary is rough and nano-crystals of diamond silicon appear near an amorphous / crystalline border. Increasing P_ gives rise to crystalline high-pressure R8/BC8 phases in the transformation zone. The size of high-pressure phase grains varies between 3 nm and 15 nm, suggesting a coherent nucleation and growth mechanism. The micro-deformation outside the transformation zone proceeds by initial distortion of crystalline planes concentrated in nano-zones accompanied by initiation of dislocations that are advanced to slip systems. Details of the microstructure and its effect on the nano-deformation in micro -indentation are also discussed.