Solid solutions of HfB 2 -ZrB 2 mixtures were prepared by high-energy ball milling of diboride and additive powders followed by spark plasma sintering (SPS). A mixture of stoichiometric 1:1 HfB 2 -ZrB 2 borides was the base composition to which Hf, Zr, Ta, LaB 6 or Gd 2 O 3 was added. Hf, Zr and Ta were added in order to bring the boron-to-metal ratio down to 1.86, rendering the boride as MeB 1.86 . In the case of LaB 6 and Gd 2 O 3 , 1.8 mol% was added. Electroanalytical behavior of hydrogen evolution reactions was evaluated in 1 M H 2 SO 4 and 1 M NaOH solutions. The LaB 6 additive material showed Tafel slopes of 125 and 90 mV/decade in acidic and alkaline solutions respectively. The Hf and Zr rich samples showed Tafel slopes of about 120 mV/decade in both electrolytes. The overpotentials of hydrogen evolution reactions (at 10 mA/cm 2 ) in the alkaline solution were about 100 mV lower than those in acidic solution. The metal-rich diborides and addition of LaB 6 showed better hydrogen evolution reaction (HER) activities than the base 1:1 HfB 2 -ZrB 2 stoichiometric diboride solid solution. The higher activity of metal-rich borides could be attributed to the increased electron population at the d-orbitals of the metal shown by band structure modeling calculations using the Density Functional Theory approach. The electrochemical hydrogen evolution reaction (HER) pathways on a metal surface are given by Volmer-Heyrovsky-Tafel mechanisms as follows:The Volmer reactions are considered to be the discharge or adsorption reactions, Heyrovsky reactions are the charge or desorption reactions, and Tafel's is a recombination reaction with Tafel slopes of 118, 39, 29.5 mV/decade, respectively for the Volmer, Heyrovsky and Tafel equations.2 Since HER involves successive steps of adsorption and desorption of hydrogen atoms, the electrode material should have optimal binding energy for hydrogen for enhanced electrocatalysts. Platinum groups metals have the highest catalytic properties for HER. However, several earth abundant low cost alternative materials for HER are being investigated.3 Metal borides, such as MoB, 4,5 Co 2 B, 6 CoB, 7 and NiB 8,9 are reported as potential electrocatalysts for HER. The high catalytic activity of metal mono-borides is attributed to the electron transfer from boron to metal in the amorphous state that enriches the d-band electron density. 7 The stability of the monoborides is attributed to the ability of boron to prevent the metal from oxidizing by donating electrons to stabilize the valance state and reductively remove surface oxides. 4,9 Furthermore, the lattice strains involved in the process of boron incorporation in the metal lattice is believed to lower the energy barrier for catalytic reactions. Only limited investigations have been reported on metal diborides as electrocatalysts for HER. Layered titanium diborides with and without exfoliation were investigated 10 and a significantly high overpotential of >1 V RHE was reported for a HER current density of −10 mA/cm 2 . Metal-rich borides...

Spark plasma sintered transition metal diborides such as HfB 2 , ZrB 2 and their solid solutions were investigated as electrode materials for electrochemical hydrogen evolutions reactions (HER) in 1 M H 2 SO 4 and 1 M NaOH electrolytes. HfB 2 and ZrB 2 formed complete solid solutions when mixed in 1:1, 1:4, and 4:1 ratios and they were stable in both electrolytes. The HER kinetics of the diborides were slower in the basic solution than in the acidic solutions. The Tafel slopes in 1 M H 2 SO 4 were in the range of 0.15-0.18 V/decade except for pure HfB 2 which showed a Tafel slope of 0.38 V/decade. In 1 M NaOH the Tafel slopes were in the range of 0.12-0.27 V/decade. The composition of Hf x Zr 1-x B 2 solid solutions with x = 0.2-0.8, influenced the exchange current densities, overpotentials and Tafel slopes of the HER. The EIS data were fitted with a porous film equivalent circuit model in order to better understand the HER behavior. In addition, modeling calculations, using density functional theory approach, were carried out to estimate the density of states and band structure of the boride solid solutions.

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A model for the development of a novel self-ordering honeycomb morphology of TiO2 is presented using a unique anodization process.

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