Exploring the Activity-Stability Landscape of Nickel Iron (Oxy)hydroxide for the Oxygen Evolution Reaction Under Industrially Relevant Alkaline Electrolysis Conditions

Electrolytic hydrogen is presumed to play a crucial part in the generation of green fuels and chemicals, while also helping to balance demand and supply in the future power grid, which will rely heavily on intermittent renewable sources such as wind and solar for energy generation. Among competing technologies for green hydrogen production, alkaline water electrolysis is the most mature technology but has the lowest efficiency and production rate due to sluggish kinetics of the hydrogen and oxygen evolution reactions. The key to improving its efficiency and production rate to a competitive level lies within optimization of both the cathodic and anodic electrocatalysts.

This Ph.D. project thus involved assessing high-performing nickel-iron oxyhydroxide (Ni_1-xFe_xOOH) for the oxygen evolution reaction in alkaline environment. Given that the origin of its excellent electrocatalytic activity and the long-term stability under industrially relevant alkaline conditions are still debatable, the electrocatalytic activity investigations were coupled with spectroelectrochemical Raman studies and stability tests were conducted under technologically relevant OER conditions.

The first study of this thesis focuses on exploring the activity-stability landscape of Ni_1-xFe_xOOH under industrially relevant alkaline conditions. More specifically, Ni_1-xFe_xOOH catalytic films with various Ni:Fe ratios (i.e., 1:0, 4:1, 3:1, and 2:1) were hydrothermally synthesized on Ni mesh with a mesh size of 10 μm. The electrochemical activity of the as-prepared catalytic films was recorded at room and elevated temperatures. To probe the origin of activity enhancement upon Fe introduction in NiOOH lattice, spectroelectrochemical Raman studies were conducted on Fe-free and Fe-containing films. Combining in-situ Raman spectroscopy with hydrogen-deuterium (H-D) and ¹⁸O-water isotope labelling experiments unraveled lattice oxygen activation upon Fe doping in Ni oxyhydroxide (NiOOH) films, hence increasing oxygen evolution activity. Ni_1-xFe_xOOH (x = 0-0.33) films were tested under industrially relevant OER conditions unfolding severe electrode deterioration. Using SEM, Raman spectroscopy, and ICP-OES, it is demonstrated that the anode deactivates due to a diminishing number of active sites and the formation of unwanted Fe moieties, with metal dissolution underpinning both of these deactivation mechanisms.

The second study focuses on the influence of alkali cations on the reaction kinetics of OER on Ni₃Fe₁OOH. Coupling in-situ Raman spectroscopy with hydrogen-deuterium (H-D) and ¹⁸O-water isotope labelling unfolded evidence of stronger stabilization of oxygenated intermediates in the presence of Li⁺ relative to Cs⁺, thus leading to a subgrade activity in LiOH. Furthermore, long-term stability testing of Ni₃Fe₁OOH under industrially relevant alkaline conditions in various alkali hydroxides (MOH, M = Na, K, and Cs) revealed severe deterioration of catalytic films. Employing SEM, Raman, and ICP-OES, we show that the anode deactivates due to the delamination of the catalytic coating, the loss of active sites, and the formation of undesired FeOOH moieties, irrespective of the type of alkali metal cation.