Xuecheng Yan

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.

Atomically isolated nickel species anchored on graphitized carbon for efficient hydrogen evolution electrocatalysis

Hydrogen production through electrochemical process is at the heart of key renewable energy technologies including water splitting and hydrogen fuel cells. Despite tremendous efforts, exploring cheap, efficient and durable electrocatalysts for hydrogen evolution still remains as a great challenge. Here we synthesize a nickel–carbon-based catalyst, from carbonization of metal-organic frameworks, to replace currently best-known platinum-based materials for electrocatalytic hydrogen evolution. This nickel-carbon-based catalyst can be activated to obtain isolated nickel atoms on the graphitic carbon support when applying electrochemical potential, exhibiting highly efficient hydrogen evolution performance with high exchange current density of 1.2 mA cm−2 and impressive durability. This work may enable new opportunities for designing and tuning properties of electrocatalysts at atomic scale for large-scale water electrolysis.

Defective-activated-carbon-supported Mn–Co nanoparticles as a highly efficient electrocatalyst for oxygen reduction

A highly active and durable cathodic oxygen reduction reaction (ORR) catalyst is synthesized by introducing a small amount of Mn-Co spinel into a kind of defective activated-carbon (D-AC) support. It is assumed that the synergetic coupling effects between the unique defects in the D-AC and the loaded Mn-Co spinel facilitate the ORR and enhance its durability.

Carbon for the oxygen reduction reaction: a defect mechanism

We demonstrate a new defect mechanism of carbons for the oxygen reduction reaction (ORR). It is predicted by the first principles calculations that a type of 585 defect on graphene (G585) is more effective than N-doping for the ORR, and our experimental investigations show strong support to this theoretical prediction.

One-step synthesis of nitrogen-doped microporous carbon materials as metal-free electrocatalysts for oxygen reduction reaction

In our work, a convenient preparation route is reported for the synthesis of nitrogen-doped microporous carbon materials by direct carbonization of an amine functionalized aluminium-based metal–organic framework (MOF) compound, amino-MIL-53(Al). Through the pore distribution and pore volume calculations it was found that the carbon products possess narrow pore size, which is approximately 1 nm, and high microporous surface area and volume. The highest proportion of microporous contribution is 93% for surface area and 88% for pore volume. As metal-free electrocatalysts, these carbon products exhibit attractive capability for oxygen reduction reaction (ORR) in alkaline medium. The investigation shows that these nitrogen-doped microporous carbon materials have promising applications in fuel cells.

Nanosheets Co3O4 Interleaved with Graphene for Highly Efficient Oxygen Reduction

Efficient yet inexpensive electrocatalysts for oxygen reduction reaction (ORR) are an essential component of renewable energy devices, such as fuel cells and metal-air batteries. We herein interleaved novel Co3O4 nanosheets with graphene to develop a first ever sheet-on-sheet heterostructured electrocatalyst for ORR, whose electrocatalytic activity outperformed the state-of-the-art commercial Pt/C with exceptional durability in alkaline solution. The composite demonstrates the highest activity of all the nonprecious metal electrocatalysts, such as those derived from Co3O4 nanoparticle/nitrogen-doped graphene hybrids and carbon nanotube/nanoparticle composites. Density functional theory (DFT) calculations indicated that the outstanding performance originated from the significant charge transfer from graphene to Co3O4 nanosheets promoting the electron transport through the whole structure. Theoretical calculations revealed that the enhanced stability can be ascribed to the strong interaction generated between both types of sheets.

Activated carbon becomes active for oxygen reduction and hydrogen evolution reactions

We utilized a facile method for creating unique defects in the activated carbon (AC), which makes it highly active for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). The ORR activity of the defective AC (D-AC) is comparable to the commercial Pt/C in alkaline medium, and the D-AC also exhibits excellent HER activity in acidic solution.

Defect-driven oxygen reduction reaction (ORR) of carbon without any element doping

A porous carbon (PC) material, containing carbon and oxygen only, was synthesized via carbonisation of a Zn-MOF (IRMOF-8) at 950 °C. Interestingly, the derived materials of this reaction exhibit excellent electrocatalytic activity, molecular selectivity and long-term durability. The fact that this material, which is effectively a “pure” carbon, lacking any elemental doping, exhibits excellent oxygen reduction reaction (ORR) activity suggests that a mechanism not dependent on elemental doping is being utilised. We suggest the formation of defects arising from the removal of Zn atoms as a consequence of the calcination procedure play the critical role in this process.

Y2O3:Yb3+/Er3+ Hollow Spheres with Controlled Inner Structures and Enhanced Upconverted Photoluminescence

Multishell Y2 O3 :Yb(3+) /Er(3+) hollow spheres with uniform morphologies and controllable inner structures are prepared successfully by using a glucose-template hydrothermal process followed by temperature-programmed calcination. Much enhanced upconverted photoluminescence of these Y2 O3 :Yb(3+) /Er(3+) are observed, which are due to the multiple reflections and the enhanced light-harvesting efficiency of the NIR light resulting from the special features of the multishell structures.