(289f) Atomic Layer Deposition of NiAlxOy Catalysts for Electrochemical Oxidation of Water

The electrolysis of water to form hydrogen gas and oxygen gas has been highlighted as a promising method to store electrical/solar energy in the form of chemical bonds. Of the two half-reactions involved in electrolysis, the oxygen evolution reaction (OER), 4 OH^- â?? 2 H₂O +O₂ + 4e^- (in alkaline conditions), suffers from slow kinetics leading to losses in efficiency. Of earth-abundant materials, nickel-iron oxide based catalysts have been found to be among the most active for OER. Recently, it has been shown that the activity of these nickel-iron oxide catalyst can be further improved through the addition of aluminum.¹

To study the effects of aluminum on the activity of nickel oxide based catalyst, atomic layer deposition (ALD) was used to synthesize ternary NiAl_xO_y films. ALD allows for the deposition of conformal thin films with angstrom-level control over thickness. Depositing ternary materials with ALD is shown to provide good compositional control by switching between the individual ALD chemistries. Electrochemical characterization of these synthesized thin films shows that aluminum improves the onset potential for OER and the Tafel slope when compared to nickel oxide films.

Studies of the intrinsic activity of nickel aluminum oxide catalyst are complicated by two factors: (1) nickel oxide films are prone to incidental incorporation of iron from trace impurities found in the electrolyte, (2) aluminum oxide leaches in alkaline conditions leading to changes in composition and porosity of the catalytic films. This work addresses the first issue through additional purification of the electrolyte to remove trace iron. With an iron free environment, nickel aluminum oxide catalyst is shown to outperform nickel oxide catalyst. When NiAl_xO_y films are allowed to age in electrolyte containing trace iron, their activity significantly improves, performing better than as-deposited NiFe_xO_y films. The second complicating factor was further explored by comparing thin and thick films to examine the effect of porosity and its effect on performance. Though the as-deposited NiAl_xO_y films are initially unstable under alkaline conditions, our studies show that the films retain their high activity after aluminum leaching equilibrates with the electrolyte.

1. James B. Gerken, Sarah E. Shaner, Robert C. Massé, Nicholas J. Porubsky, and Shannon S. Stahl. Energy Environ. Sci.,2014, 7, 2376