Molybdenum-based materials have been considered as alternative catalysts to noble metals, such as platinum, for the hydrogen evolution reaction (HER). We have synthesized four binary bulk molybdenum borides Mo B, α-MoB, β-MoB, and MoB by arc-melting. All four phases were tested for their electrocatalytic activity (linear sweep voltammetry) and stability (cyclic voltammetry) with respect to the HER in acidic conditions. Three of these phases were studied for their HER activity and by X-ray photoelectron spectroscopy (XPS) for the first time; MoB and β-MoB show excellent activity in the same range as the recently reported α-MoB and β-Mo C phases, while the molybdenum richest phase Mo B show significantly lower HER activity, indicating a strong boron-dependency of these borides for the HER. In addition, MoB and β-MoB show long-term cycle stability in acidic solution.

Non-noble metal nanomaterials (molybdenum sulfides, phosphides, carbides, and nitrides) have recently emerged as highly active electrocatalysts for the hydrogen evolution reaction (HER). Here we present experimental and theoretical studies of the first highly active molybdenum boride nanomaterial for the HER.

Two different boron layers, flat (graphene-like) and puckered (phosphorene-like), found in the crystal structure of MoB show drastically different activities for hydrogen evolution, according to Gibbs free energy calculations of H-adsorption on MoB. The graphene-like B layer is highly active, whereas the phosphorene-like B layer performs very poorly for hydrogen evolution. A new Sn-flux synthesis permits the rapid single-phase synthesis of MoB, and electrochemical analyses show that it is one of the best hydrogen evolution reaction active bulk materials with good long-term cycle stability under acidic conditions. MoB compensates its smaller density of active sites if compared with highly active bulk MoB (which contains only the more active graphene-like boron layers) by a 5-times increase of its surface area.

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Molybdenum-based materials have been considered as alternative catalysts to noble metals,s uch as platinum, for the hydrogen evolution reaction (HER). We have synthesized four binary bulk molybdenum borides Mo 2 B, a-MoB, b-MoB, and MoB 2 by arc-melting. All four phases were tested for their electrocatalytic activity (linear sweep voltammetry) and stability (cyclic voltammetry) with respect to the HER in acidic conditions.T hree of these phases were studied for their HER activity and by X-rayp hotoelectron spectroscopy( XPS) for the first time;M oB 2 and b-MoB show excellent activity in the same range as the recently reported a-MoB and b-Mo 2 C phases,w hile the molybdenum richest phase Mo 2 Bs how significantly lower HER activity,i ndicating as trong borondependency of these borides for the HER. In addition, MoB 2 and b-MoB show long-term cycle stability in acidic solution. Figure 5. Stability measurements (cyclic voltammetry) of b-MoB and MoB 2 for the first and the 1000th cycle in 0.5 m H 2 SO 4 .Scan rate was 100 mVs À1 .IR-drop was corrected. Angewandte Chemie Communications 5578 www.angewandte.org

Ultra-long Ag x Te y nanofibers were synthesized for the first time by galvanically displacing electrospun Ni nanofibers. Control over the nanofiber morphology, composition and crystal structure was obtained by tuning the Ag + concentrations in the electrolytes. While Te-rich branched p-type Ag x Te y nanofibers were synthesized at low Ag + concentrations, Ag-rich nodular Ag x Te y nanofibers were obtained at high Ag + concentrations. The Te-rich nanofibers consist of coexisting Te and Ag 7 Te 4 phases, and the Ag-rich fibers consist of coexisting Ag and Ag 2 Te phases. The energy barrier height at the phase interface is found to be a key factor affecting the thermoelectric power factor of the fibers. A high barrier height increases the Seebeck coefficient, S, but reduces the electrical conductivity, σ, due to the energy filter effect. The Ag 7 Te 4 /Te system was not competitive with Ag 2 Te/Ag system due to its high barrier height where the increase in S could not overcome the severely diminished electrical conductivity. The highest power factor was found in the Ag-rich nanofibers with an energy barrier height of 0.054 eV. INTRODUCTIONThe rising cost of compliance to laws and regulations (from the Clean Air Act to Geologic Sequestration) for consuming non-renewable energy resources is the key driver to improve the efficiency of environmentally-friendly and renewable energy generation. 1 Thermoelectric materials offer simple, silent and reliable solid-state energy conversion devices due to their unique ability to directly convert heat into electricity and viceversa without moving parts or bulk fluids. The efficiency of a thermoelectric device can be determined by the thermoelectric figure-of-merit (ZT), given by ܼܶ = ܵ ଶ ߪܶ/ߢ. Here S, σ, κ, T, and S 2 σ are the Seebeck coefficient, electrical conductivity, thermal conductivity, temperature, and power factor, respectively. Nonetheless, the interrelationships of these key parameters in a bulk material tend to offset one another making it difficult to improve ZT. 2 Recent research in low-dimensional, especially one-dimensional (1-D) thermoelectric nanostructures, has invigorated the field by identifying quantum confinement, the energy filtering effect, and stronger phonon scattering effects to enhance S 2 σ and reduce κ. 3-5 Quantum confinement shifts the Fermi level away from the conduction band, creating a greater energy difference between them thereby improving the power factor. 6 Energy filtering enhances the average carrier energy by filtering out low energy carriers at grain boundaries or interfaces, thus increasing the Seebeck coefficient and optimizing the power factor. 7 Stronger phonon scattering at the grain boundaries and interfaces is anticipated in 1-D nanostructures due to their larger surface-to-volume ratio, which can significantly decrease the thermal conductivity. Among 1-D thermoelectric materials, nanotubes benefit from their unique wall thickness, which provides an extra de-

Abundant transition metal borides are emerging as substitute electrochemical hydrogen evolution reaction (HER) catalysts for noble metals. In this study, we report on an unusual canonic-like behavior of the c lattice parameter in the AlB 2 -type solid solution Cr 1-x Mo x B 2 (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) and its direct correlation to the HER activity in 0.5 M H 2 SO 4 solution. The activity increases with increasing x, reaching its maximum at x = 0.6 before decreasing again. At high current densities, Cr 0.4 Mo 0.6 B 2 outperforms Pt/C, as it needs 180 mV less overpotential to drive an 800 mA cm -2 current density. Cr 0.4 Mo 0.6 B 2 has excellent long-term stability and durability showing no significant activity loss after 5000 cycles and 25 hours of operation in acid. First-principle calculations have correctly reproduced the nonlinear dependence of c-lattice parameter and have shown that the mixed metal/B layers, such as (110), promote hydrogen evolution more efficiently for x = 0.6, supporting experimental results.

Transition‐metal borides (TMBs) have recently attracted attention as excellent hydrogen evolution (HER) electrocatalysts in bulk crystalline materials. Herein, we show for the first time that VB and V3B4 have high electrocatalytic HER activity. Furthermore, we show that the HER activity (in 0.5 m H2SO4) increases with increasing boron chain condensation in vanadium borides: Using a −23 mV overpotential decrement derived from −0.296 mV (for VB at −10 mA cm−2 current density) and −0.273 mV (for V3B4) we accurately predict the overpotential of VB2 (−0.204 mV) as well as that of unstudied V2B3 (−0.250 mV) and hypothetical “V5B8” (−0.227 mV). We then derived an exponential equation that predicts the overpotentials of known and hypothetical VxBy phases containing at least a boron chain. These results provide a direct correlation between crystal structure and HER activity, thus paving the way for the design of even better electrocatalytic materials through structure–activity relationships.