Atomic layer deposition for tuning the surface chemical composition of nickel iron phosphates for oxygen evolution reaction in alkaline electrolyzers.

Transition metal phosphates are promising catalysts for the oxygen evolution reaction (OER) in alkaline medium. Herein, Fe-doped Ni phosphates are deposited using plasma-enhanced atomic layer deposition (PE-ALD) at 300 °C. A sequence offFe phosphate PE-ALD cycles andnNi phosphate PE-ALD cycles is repeatedxtimes. The Fe to Ni ratio can be controlled by the cycle ratio (f/n), while the film thickness can be controlled by the number of cycles (xtimes (n+f)). 30 nm films with an Fe/Ni ratio of ∼10% and ∼37%, respectively, are evaluated in 1.0 M KOH solution. Remarkably, a significant difference in OER activity is found when the order of the Ni and Fe phosphate PE-ALD cycles in the deposition sequence is reversed. A 20%-45% larger current density is obtained for catalysts grown with an Fe phosphate PE-ALD cycle at the end compared to the Ni phosphate-terminated flavour. We attribute this to a higher concentration of Fe centers on the surface, as a consequence of the specific PE-ALD approach. Secondly, increasing the thickness of the catalyst films up to 160 nm results in an increase of the OER current density and active surface area, suggesting that the as-deposited smooth and continuous films are converted into electrolyte-permeable structures during catalyst activation and operation. This work demonstrates the ability of PE-ALD to control both the surface and bulk composition of thin film electrocatalysts, offering valuable opportunities to understand their impact on performance.

Plasma-enhanced atomic layer deposition of nickel and cobalt phosphate for lithium ion batteries.

A plasma-enhanced ALD process has been developed to deposit nickel phosphate. The process combines trimethylphosphate (TMP) plasma with oxygen plasma and nickelocene at a substrate temperature of 300 °C. Saturation at a growth per cycle of approximately 0.2 nm per cycle is observed for both the TMP plasma and nickelocene, while a continuous decrease in the growth per cycle is found for the oxygen plasma. From ERD, a stoichiometry of Ni3(P0.8O3.1)2 is measured, but by adding additional oxygen plasma after nickelocene, the composition of Ni3(P0.9O3.7)2 becomes even closer to stoichiometric Ni3(PO4)2. The as-deposited layer resulting from the process without the additional oxygen plasma is amorphous but can be crystallized into Ni2P or crystalline Ni3(PO4)2 by annealing under a hydrogen or helium atmosphere, respectively. The layer deposited with the additional oxygen plasma shows two X-ray diffraction peaks indicating the formation of crystalline Ni3(PO4)2 already during the deposition. The resulting PE-ALD deposited nickel phosphate layers were then electrochemically studied and compared to PE-ALD cobalt and iron phosphate. All phosphates need electrochemical activation at low potential first, after which reversible redox reactions are observed at a potential of approximately 2.5 V vs. Li+/Li. A relatively high capacity and good rate behavior are observed for both nickel and cobalt phosphate, which are thought to originate from either a conversion type reaction or an alloying reaction.