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Abstract
Amorphous metal oxides expand the range of material parameters significantly compared to their crystalline counter parts. However, predictions of the exact nature of the amorphous phase and its effect on material properties are still elusive. Thorough structure-property investigations of well-known model systems are thus necessary before predictive control of useful material properties is obtained. In this work, we fabricate a series of photodeposited nickel (oxy)hydroxide (NiOx) thin films and anneal them at temperatures up to 1000 oC. EXAFS, XRD and XPS are used to determine the local structure, allowing us to correlate it to measured electrochemical properties. We find an amorphous Ni(OH)2-like local structure for annealing conducted below 250 oC, followed by an amorphous-to-amorphous phase transition to a NiO-like structure by 300 oC, thus supplying evidence for different amorphous polymorphs in this Ni-O system. Above 400 oC a cubic NiO XRD diffraction pattern is detected. Electrochemically, we find a stepwise increase of the onset overpotential at this transition, indicating a change in potential-determining step and possibly OER reaction mechanism. The Tafel slope decreases linearly with annealing temperature, which we attribute to a decrease in (Ni)OOH reaction intermediary coverage, supported by in-operando UV-Vis electrochromism. Furthermore, we find that the (Ni)OOH coordination is increasingly strained with annealing temperature, which manifests in higher electrochromic coloring rates and lower binding energies. We identify this as the root cause of the lowered intermediary coverage. Thus, nano-crystalline NiO should kinetically be a superior catalyst to amorphous Ni(OH)2. However, at our benchmarking value of 10 mA cm-2 the amorphous material exhibits lower overpotential, due to a combination of lower onset potential, large chemically active surface area and mass transport limitations under our conditions.
