Every Step Counts: Importance of Final ALD Sequences for Oxygen Evolution Reaction Catalysts

Abstract

In achieving the global dream of a sustainable future, the development of new and alternative methods to generate electricity are of high importance. Current renewable energy sources are mainly dependent on sun and wind, and are thus very unpredictable. To obtain an overall reliable energy supply, we need to be able to store clean energy for extended periods of time to compensate for unfavorable weather conditions or sudden surges in energy demand.One way to store energy is by generating H2-gas, which has a set of interesting advantages as an energy carrier molecule. It has a very high energy density, is multifunctional as feedstock or energy carrier and it is not a greenhouse gas in itself. The production efficiency of green H2, however, still needs to be optimized before it can be adapted on a global scale. Electrochemical water splitting is mainly hampered by the sluggish kinetics of the oxygen evolution half-reaction (OER) of the process. This involves a transfer of four electrons, each with its own intermediary product(s) requiring catalysis, resulting in a significant overpotential[1].Atomic layer deposition (ALD) could be an interesting technique in the research, development and possibly production of such OER catalytic materials. This technique is suited to deposit thin films in a controlled layer-by-layer way with dimension control up to the Angström level. In this development, transition metals could be interesting catalyst candidates due to their availability and pricing over tradition OER catalysts based on noble metals [2]. Both oxides and phosphate variants have been explored; the latter occasionally outperforming the former[3][4].Nickel has shown to be one of the more OER-active transition metals, and doping or combining it with iron seems to further increase its performance [5]. Inspired by these results, ALD was used in this work to deposit and finetune the properties of a Ni-Fe ternary phosphate for its use as an OER-catalyst material. This material was deposited by alternating ALD cycles of Ni- and Fe-phosphate (denoted here as NiPO and FePO) based on previous works of Rongé et al.[6] and Henderick et al.[7].Ni-Fe phosphates with varying composition and thickness were deposited on a solid nickel disc and tested in a rotating-disc electrode (RDE) set-up to quantify their electrochemical performance. Applying a coating with a selected composition of 4 NiPO to 1 FePO cycles improves the performance over a bare Ni-PVD substrate (Fig 1). A current density of 91 mA/cm2 at 0.685 V vs Hg/HgO is obtained for this material which is 75% higher than the uncoated nickel disk (51 mA/cm2) which indicates that the material has catalytic properties for the OER increasing the production of H2 at an identical overpotential. An additional observation was that the pulsing order of the two sub-processes within an ALD super-cycle has an important impact on the resulting layer’s performance. If the FePO sub-cycle is pulsed last, the current density increases further to approx. 150 mA/cm2 for a total increase of 200% compared to the uncoated performance.The results of this explorative work in the use of ALD for the development of OER catalytic materials indicate that it is a promising technique due to the precise control that can be exerted over both the thickness and composition of the material. Specifically the option to exert high control over the surface of the catalyst can be a big asset in researching catalytic materials.