Structural Evolution in Photodeposited Nickel (oxy)hydroxide Oxygen Evolution Electrocatalysts

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 structureproperty 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 (NiO x ) thin films and anneal them at temperatures up to 1000 • C. 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 • C, followed by an amorphous-to-amorphous phase transition to a NiO-like structure by 300 • C, thus supplying evidence for different 1 amorphous polymorphs in this Ni-O system. Above 400 • C 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.

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“…27–29,40,41 Moreover, the XPS spectrum shows three characteristic peaks (852.1 eV, 856.4 eV, and 858.9 eV for HDDE 0.9 BGE 0.1 Ni 6-1) and one satellite peak (865.6 eV). According to previous literature reports, 42–46 it can be inferred that both hydroxyl and imidazole are involved in the coordination with Ni (Fig. S18 and S19†).…”

A Comparison of Photodeposited RuOx for Alkaline Water Electrolysis

“…This is consistent with the X-ray diffraction results, and an amorphous RuO 2 material, a -RuO 2 . This behavior has been observed in similar amorphous photodeposited metal (oxy)­hydroxides thin films. ,, The coordination numbers for outer shells bonding are all lower than the crystalline model compound; the Ru–Ru path for corner-sharing RuO 6 octahedra is shortened. This effect likely can be attributed to the displacement of surface RuO 6 octahedron from their ideal positions toward the surface of RuO 2 nanoparticles.…”

“…From the Scherrer estimation of the crystallite size slightly greater ordered domains are inferred for the Ru Cl NIR over the Ru 2eh NIR . On the other hand, the UV-deposited sample was found to be fully XRD-amorphous, as is typical of the UV photodeposition method. ,,,, …”

“…The enhanced catalytic performance of these amorphous materials however remains largely unexplained. X-ray absorption spectroscopy (XAS) has proven to be an invaluable tool in resolving mechanistic aspects of the OER mediated by these photodeposited materials; however such studies have to date focused on first-row transition-metal-based materials. − …”

Photodeposited Polyamorphous CuOx Hole-Transport Layers in Organic Photovoltaics

“…During this transition, we move from linear low-valence coordination modes in a predominantly Cu + material to a primarily octahedral coordination environment with much larger Cu 2+ presence. Amorphous-to-amorphous phase transitions are much rarer than their counterparts in crystalline systems but have been observed in semiconductor materials such as α-Si; we recently observed similar behavior in amorphous Ni and Fe oxyhydroxides used for electrocatalytic water splitting. , Given that we have observed changes in electrocatalytic behavior induced by amorphous phase transitions in other materials, we were immediately interested in whether these similar transitions would lead to changes in the electronic properties of our a-CuO x systems.…”