Harry E. Hoster

Single-crystalline NiCo2O4 nanoneedle arrays grown on conductive substrates as binder-free electrodes for high-performance supercapacitors

In this work, we have successfully grown single-crystalline nanoneedle arrays of NiCo2O4 on conductive substrates such as Ni foam and Ti foil through a simple solution method together with a post-annealing treatment. Remarkably, the NiCo2O4–Ni foam binder-free electrode exhibits greatly improved electrochemical performance with very high capacitance and excellent cycling stability.

Formation of ZnMn2O4 Ball‐in‐Ball Hollow Microspheres as a High‐Performance Anode for Lithium‐Ion Batteries

Novel ZnMn(2)O(4) ball-in-ball hollow microspheres are fabricated by a facile two-step method involving the solution synthesis of ZnMn-glycolate hollow microspheres and subsequent thermal annealing in air. When evaluated as an anode material for lithium-ion batteries, these ZnMn(2)O(4) ball-in-ball hollow microspheres show significantly enhanced electrochemical performance with high capacity, excellent cycling stability and good rate capability.

Hierarchical hollow spheres composed of ultrathin Fe2O3 nanosheets for lithium storage and photocatalytic water oxidation

Hollow hierarchical spheres self-organized from the ultrathin nanosheets of α-Fe2O3 were prepared by a simple process. These ultrathin nanosheet subunits possess an average thickness of around 3.5 nm and show preferential exposure of (110) facets. Their Li ion storage and visible-light photocatalytic water oxidation performance are tested. Such hierarchical nanostructures show high Li storage properties with good cycling stability and excellent rate capabilities. The water oxidation catalytic activity is 70 μmol h−1 g−1 for O2 evolution under visible light irradiation and can be maintained for 15 hours. The structural features of these α-Fe2O3 nanocrystals are considered to be important to lead to the attractive properties in both Li storage and photocatalytic water oxidation, e.g. hollow interior, ultrathin thickness and largely exposed active facets.

Building 3D Structures of Vanadium Pentoxide Nanosheets and Application as Electrodes in Supercapacitors

Various two-dimensional (2D) materials have recently attracted great attention owing to their unique properties and wide application potential in electronics, catalysis, energy storage, and conversion. However, large-scale production of ultrathin sheets and functional nanosheets remains a scientific and engineering challenge. Here we demonstrate an efficient approach for large-scale production of V2O5 nanosheets having a thickness of 4 nm and utilization as building blocks for constructing 3D architectures via a freeze-drying process. The resulting highly flexible V2O5 structures possess a surface area of 133 m(2) g(-1), ultrathin walls, and multilevel pores. Such unique features are favorable for providing easy access of the electrolyte to the structure when they are used as a supercapacitor electrode, and they also provide a large electroactive surface that advantageous in energy storage applications. As a consequence, a high specific capacitance of 451 F g(-1) is achieved in a neutral aqueous Na2SO4 electrolyte as the 3D architectures are utilized for energy storage. Remarkably, the capacitance retention after 4000 cycles is more than 90%, and the energy density is up to 107 W·h·kg(-1) at a high power density of 9.4 kW kg(-1).

Strongly coupled carbon nanofiber–metal oxide coaxial nanocables with enhanced lithium storage properties

A facile two-step strategy involving a polyol method and subsequent thermal annealing treatment is successfully developed for the general synthesis of metal oxide/carbon coaxial nanocables. Benefitting from the strong coupling effect, these hybrid nanocables exhibit remarkable lithium storage properties with high capacity, long cycle life and excellent rate capability.

Pt–Ru model catalysts for anodic methanol oxidation: Influence of structure and composition on the reactivity

The activity of different types of PtRu surfaces towards anodic methanol oxidation has been investigated. As expected the activity of Pt(111) modified with Ru and analyzed in a UHV environment depends on the total number of Pt–Ru pair sites. Their population can be increased by artificially creating additional surface defects before or after ruthenium deposition. Ruthenium alloyed into smooth Pt(111) terraces in turn does not lead to comparable electrocatalytic activity, moreover the current density under potentiostatic conditions undergoes an exponential decline towards zero. Other model surfaces are also found to present a continuous loss in activity during chronoamperometric tests, which consists of a fast initial current decrease during the first 5–10 min followed by a slower one over several hours. The latter decay exhibits hyperbolic behavior which we can explain kinetically as being caused by a second-order process. The first current decay can be repeatedly observed by re-starting the experiment after setting the potential back to the initial value, thus indicating a certain degree of reversibility. The slow loss in activity cannot be recovered at low potentials. However, the original surface activity can be restored by applying a potential step to higher anodic values, e.g. up to 1.2 V for a few seconds. Structure optimized porous PtRu surfaces, on the other hand, do not show any current decrease during the chronoamperometric experiment.

Current-Time Behavior of Smooth and Porous PtRu Surfaces for Methanol Oxidation

Smooth ultrahigh vacuum-cleaned PtRu alloys used as model catalysts for methanol oxidation, present a continuous loss of activity under potentiostatic conditions. After a potential step, e.g., from 50 to 500 mV vs. reference hydrogen electrode, chronoamperometric curves first show a steep decrease over 5-10 min followed by a slower decrease over several hours The latter decay exhibits a time -1 behavior for different catalyst compositions, with higher slopes for the catalysts with less activity. The first current decay can be repeatedly observed by restarting the experiment after setting the potential back to the initial value, thus indicating a certain degree of reversibility. The slow decrease in activity, however, cannot be recovered by this means. But stepping the potential to higher anodic values, e.g., up to 1.2 V, the original surface activity can be obtained again. Optimized porous PtRu surfaces, on the other hand, do not show any comparable decrease after a respective potential step. The possible origin of the different behavior of smooth and porous surfaces is discussed.

Synthesis of cobalt phosphides and their application as anodes for lithium ion batteries

A facile thermal decomposing method has been developed for the fabrication of Co(x)P nanostructures with controlled size, phase, and shape (e.g., Co(2)P rod and spheres, CoP hollow and solid particles). An amorphous carbon layer could be introduced by the carbonization of organic surfactants from the precursors. The electrochemical performance of typical CoP and Co(2)P samples as anode materials has been investigated and the CoP hollow nanoparticle with carbon coating layer depicts good capacity retention and high rate capability (e.g., specific capacity of 630 mA h g(-1) at 0.2 C after 100 cycles, and a reversible capacity of 256 mA h g(-1) can be achieved at a high current rate of 5 C).

Bulk antimony sulfide with excellent cycle stability as next-generation anode for lithium-ion batteries

Nanomaterials as anode for lithium-ion batteries (LIB) have gained widespread interest in the research community. However, scaling up and processibility are bottlenecks to further commercialization of these materials. Here, we report that bulk antimony sulfide with a size of 10–20 μm exhibits a high capacity and stable cycling of 800 mAh g−1. Mechanical and chemical stabilities of the electrodes are ensured by an optimal electrode-electrolyte system design, with a polyimide-based binder together with fluoroethylene carbonate in the electrolyte. The polyimide binder accommodates the volume expansion during alloying process and fluoroethylene carbonate suppresses the increase in charge transfer resistance of the electrodes. We observed that particle size is not a major factor affecting the charge-discharge capacities, rate capability and stability of the material. Despite the large particle size, bulk antimony sulfide shows excellent rate performance with a capacity of 580 mAh g−1 at a rate of 2000 mA g−1.

PtxRu1−x/Ru(0001) surface alloys—formation and atom distribution

The formation of PtRu surface alloys by deposition of submonolayer Pt films on a Ru(0001) substrate and subsequent annealing to about 1350 K and the distribution of the Pt atoms in the surface layer were investigated by scanning tunneling microscopy. Quantitative statistical analysis reveals (i) negligible losses of Pt into subsurface regions up to coverages close below 1 monolayer, (ii) a homogeneous distribution of the Pt atoms over the surface, and (iii) the absence of a distinct long-range or short-range order in the surface layer. In addition, the density of specific adsorption ensembles is analyzed as a function of Pt surface content. Possible conclusions on the process for surface alloy formation are discussed. The results are compared with the properties of PtRu bulk alloys and the findings in previous adsorption studies on similar surface alloys (H. Rauscher, T. Hager, T. Diemant, H. Hoster, F. Bautier de Mongeot and R. J. Behm, Surf. Sci., 2007, 601, 4608; T. Diemant, H Rauscher and R. J. Behm, J. Phys. Chem. C, in press).