Confinement dependence of electro-catalysts for hydrogen evolution from water splitting

Abstract

Density functional theory is utilized to articulate a particular generic deconstruction of the electrode/electro-catalyst assembly for the cathode process during water splitting. A computational model was designed to determine how alloying elements control the fraction of H2 released during zirconium oxidation by water relative to the amount of hydrogen picked up by the corroding alloy. This model is utilized to determine the efficiencies of transition metals decorated with hydroxide interfaces in facilitating the electro-catalytic hydrogen evolution reaction. A computational strategy is developed to select an electro-catalyst for hydrogen evolution (HE), where the choice of a transition metal catalyst is guided by the confining environment. The latter may be recast into a nominal pressure experienced by the evolving H2 molecule. We arrived at a novel perspective on the uniqueness of oxide supported atomic Pt as a HE catalyst under ambient conditions.

Keywords: DFT; confinement; corrosion; electro-catalysis; hydrogen evolution.

Figure 1

(a) Representative structure for a model of a hydroxylated inter-grain interface comprising ZrO(OH)₂ on ZrO₂. Here, this interface is decorated with Fe ions with the oxidation state +2. Oxygen is represented as red, zirconium as light blue, iron as purple, and hydrogen as white. (b) One hydride ion and one hydroxide moiety prior to the hydride-proton recombination to form H₂ is displayed, reactant in Equation 6. (c) The product in Equation 6 is displayed, including M^X coordination to the additional oxygen ion replacing the hydride ion and the released grain boundary H₂. (d) Hydride-proton recombination energies for H₂ release into said interface (dashed black line at 1.1 eV), enthalpy change for H₂ release at ambient pressure (dashed black line at 0 eV), and corresponding Gibbs energy change (dashed black line at −0.2 eV). TM²⁺ blue, TM³⁺ red, weighted average green. (e) Comparison of theoretical data (green) and experimental HPUF data (black); * from [7] and ^o from [8]. The theoretical data is a weighted average between TM²⁺ and TM³⁺. The black dashed line is HPUF in pure ZrO₂ from [7]. The blue dashed line corresponds to HE from Zr⁴⁺ hydride at GB with Na⁺, Ca²⁺ and Sc³⁺ spectator. Sc³⁺ corresponds to the top line, Na⁺ to the middle line and Ca²⁺ to the bottom line.

Figure 2

The diagram on the left is identical to Figure 1d. The enlarged region exposes the overpotentials for the elements in the Pt and Au groups. Note that Pt⁺ associated hydride displays a negative overpotential implying that it is more stable than the H₂(g) asymptote (lower dashed line).

Figure 3

HER at electro-catalyst/electrode assembly. (A) Coalescence of proton and electrons to form the metal catalyst (MC) associated hydride, (B) Hydride-proton recombination to form H₂ at the interface. (C) The step between panel B and panel C comprises the HER following the hydride-proton recombination step.