Xiang Ao

Rational Design of Cobalt–Iron Selenides for Highly Efficient Electrochemical Water Oxidation

Exploring active, stable, earth-abundant, low-cost, and high-efficiency electrocatalysts is highly desired for large-scale industrial applications toward the low-carbon economy. In this study, we apply a versatile selenizing technology to synthesize Se-enriched Co1-xFexSe2 catalysts on nickel foams for oxygen evolution reactions (OERs) and disclose the relationship between the electronic structures of Co1-xFexSe2 (via regulating the atom ratio of Co/Fe) and their OER performance. Owing to the fact that the electron configuration of the Co1-xFexSe2 compounds can be tuned by the incorporated Fe species (electron transfer and lattice distortion), the catalytic activity can be adjusted according to the Co/Fe ratios in the catalyst. Moreover, the morphology of Co1-xFexSe2 is also verified to strongly depend on the Co/Fe ratios, and the thinner Co0.4Fe0.6Se2 nanosheets are obtained upon selenization treatment, in which it allows more active sites to be exposed to the electrolyte, in turn promoting the OER performance. The Co0.4Fe0.6Se2 nanosheets not only exhibit superior OER performance with a low overpotential of 217 mV at 10 mA cm-2 and a small Tafel slope of 41 mV dec-1 but also possess ultrahigh durability with a dinky degeneration of 4.4% even after 72 h fierce water oxidation test in alkaline solution, which outperforms the commercial RuO2 catalyst. As expected, the Co0.4Fe0.6Se2 nanosheets have shown great prospects for practical applications toward water oxidation.

A Universal Method to Engineer Metal Oxide–Metal–Carbon Interface for Highly Efficient Oxygen Reduction

Oxygen is the most abundant element in the Earths crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes and energy converting/storage systems. Developing techniques toward high-efficiency ORR remains highly desired and a challenge. Here, we report a N-doped carbon (NC) encapsulated CeO2/Co interfacial hollow structure (CeO2-Co-NC) via a generalized strategy for largely increased oxygen species adsorption and improved ORR activities. First, the metallic Co nanoparticles not only provide high conductivity but also serve as electron donors to largely create oxygen vacancies in CeO2. Second, the outer carbon layer can effectively protect cobalt from oxidation and dissociation in alkaline media and as well imparts its higher ORR activity. In the meanwhile, the electronic interactions between CeO2 and Co in the CeO2/Co interface are unveiled theoretically by density functional theory calculations to justify the increased oxygen absorption for ORR activity improvement. The reported CeO2-Co-NC hollow nanospheres not only exhibit decent ORR performance with a high onset potential (922 mV vs RHE), half-wave potential (797 mV vs RHE), and small Tafel slope (60 mV dec-1) comparable to those of the state-of-the-art Pt/C catalysts but also possess long-term stability with a negative shift of only 7 mV of the half-wave potential after 2000 cycles and strong tolerance against methanol. This work represents a solid step toward high-efficient oxygen reduction.

Construction of MoO2 Quantum Dot–Graphene and MoS2 Nanoparticle–Graphene Nanoarchitectures toward Ultrahigh Lithium Storage Capability

Herein, MoO2 quantum dots (QDs; <5 nm) are synthesized through a one-step solvothermal process. MoO2 QD-bonded graphene sheets (MoO2-QDs@RGO) are facilely produced and can be further converted through sulfidation into MoS2 nanoparticle-bonded graphene sheets (MoS2-NPs@RGO). The novel MoO2-QDs@RGO electrodes demonstrate exceptionally attractive lithium storage capability (e.g., 1257 mA h g-1 at 100 mA g-1, being close to the highest values ever reported for a MoO2-based lithium ion battery electrode), rate capability, and cycle stability. Moreover, the MoS2-NPs@RGO delivered a superior capacity (1497 mA h g-1 at 100 mA g-1) with outstanding rate retention and cycling stability. The superior lithium storage capabilities are ascribed to the synergetic effects of the high-surface-area graphene sheets, the well-dispersed MoS2 nanoparticles, and their strong bonding with each other, which effectively prevents aggregation of MoS2 while the composite architecture allows fast transport of electrons and ions.