Metal-Rich Transition Metal Diborides as Electrocatalysts for Hydrogen Evolution Reactions in a Wide Range of pH

Abstract

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...

Phosphorus Vacancies that Boost Electrocatalytic Hydrogen Evolution by Two Orders of Magnitude

“…Commercial acidic proton‐exchange‐membrane electrolyzers utilize Pt/C catalysts, but the scarcity of Pt greatly limits the large‐scale application; alkaline electrolyzers use low‐cost Ni catalysts, but suffer from low efficiency and corrosion . Consequently, active, cost‐effective, and robust electrocatalysts for hydrogen evolution reaction (HER) have been intensively pursued, such as metal‐free catalysts, single‐atom catalysts, transition‐metal chalcogenides, borides, carbides, nitrides, and phosphides (TMPs) . In particular, the easily adjustable electronic structure and surface properties of TMPs due to under‐coordinated P atoms, permit facile manipulation of HER performance .…”

“…Cation and anion vacancies, have been investigated in electrocatalytic HER, because their ability to modify electronic structures of catalytic sites would optimize the hydrogen adsorption free energy (Δ G H ). For example, boron vacancies (Bv) have been reported to change the lattice parameters of HfB 2 ‐ZrB 2 and possibly introduce more electrons at the Fermi level to facilitate HER . While nitrogen vacancies (Nv) created in Ni 3 N 1− x can increase the electron density of Ni atoms next to Nv and downshift the d‐band center to strengthen the water adsorption and tune the adsorption–desorption behavior of intermediate H* during HER .…”

Cobalt‐Tungsten‐Boron as an Active Electrocatalyst for Water Electrolysis

“…According to the open literatures, only a few examples of transition metal borides or transition metal-boron alloys have been investigated over the past decades in terms of electrocatalytic properties towards the HER. [20][21][22][23][24][25][26][27][28][29][30] It was found that the binary NiÀB and CoÀB typically showed high activities towards the HER, which compare favorably with other types of transition metal compounds. These findings inspired us to explore the full potential of transition metal borides or transition metal-boron alloys as non-noble electrocatalysts.…”

“…The potential of transition metal borides or transition metal‐boron alloys as HER catalyst is still largely underexplored. According to the open literatures, only a few examples of transition metal borides or transition metal‐boron alloys have been investigated over the past decades in terms of electrocatalytic properties towards the HER . It was found that the binary Ni−B and Co−B typically showed high activities towards the HER, which compare favorably with other types of transition metal compounds.…”

Phosphorus Vacancies that Boost Electrocatalytic Hydrogen Evolution by Two Orders of Magnitude

“…[1] Commercial acidic proton-exchange-membrane electrolysers utilize Pt/C catalysts, but the scarcity of Pt greatly limits the large-scale application; [2] alkaline electrolysers use low-cost Ni catalysts, but suffer from low efficiency and corrosion. [3] Consequently, active, costeffective and robust electrocatalysts for hydrogen evolution reaction (HER) have been intensively pursued, such as metalfree catalysts, [4] single atom catalysts, [5] transition metal chalcogenides, [6] borides, [7] carbides, [8] nitrides, [9] and phosphides (TMPs). [10] Especially, the easily adjustable electronic structure and surface properties of TMPs due to under-coordinated P atoms, permit facile manipulation of HER performance.…”