Separating bulk and surface processes in NiO_x electrocatalysts for water oxidation

Abstract

Nickel oxide-based catalysts currently represent the state of the art in electrochemical water oxidation in alkaline pH. However, much of their functionality remains poorly understood, particularly regarding catalytically active sites...

Push‐Pull Electronic Effects in Surface‐Active Sites Enhance Electrocatalytic Oxygen Evolution on Transition Metal Oxides

“…However, the estimated capacitances associated to the spinels show a clear peak close to the onset of the catalysis that is followed by a decrease in the catalytic region. This capacitive peak can be attributed to the accumulation of electrogenerated holes in the spinels before driving the OER, as it has been previously reported for other Ni‐based OER electrocatalysts, where the size of the redox wave identified by cyclic voltammetry was directly associated to the density of catalytic sites [10,29] . The NiZnFeO n spinel shows a higher capacitive peak compared to NiFe 2 O n , which can be attributed to a higher density of states, and hence a higher activity towards OER.…”

“…This capacitive peak can be attributed to the accumulation of electrogenerated holes in the spinels before driving the OER, as it has been previously reported for other Ni-based OER electrocatalysts, where the size of the redox wave identified by cyclic voltammetry was directly associated to the density of catalytic sites. [10,29] The NiZnFeO n spinel shows a higher capacitive peak compared to NiFe 2 O n , which can be attributed to a higher density of states, and hence a higher activity towards OER. The difference in the capacitance is 2.27 × 10 À 5 (F cm À 2 ), while the difference in energy of these two peaks (x-direction) is 0.04 eV, similar to the changes in the vacancy formation energy for the Fe 2 Ni, FeNi 2 and FeNiZn environments, see Table S6.…”

Solution-Processed Ni-Based Nanocomposite Electrocatalysts: An Approach to Highly Efficient Electrochemical Water Splitting

“…63 This procedure has been successfully employed on different materials like TiO2, 62 Silicon, 64 conducting polymers, 65 etc. The obtained DOS, in good agreement with previous reports for highly defective Ni(OH)2/NiOOH electrocatalysts, 58 reflects an increase of the density of catalytic sites for films baked at lower temperatures, with a higher density of structural/electronic defects (oxygen vacancies) as confirmed by XRD, TEM, Raman, conductivity and PL. Furthermore, it has been recently reported that amorphous Ni-based catalysts can expose bulk catalytic active sites, leading to a higher OER activity, while their crystalline counterpart only exposes surface sites.…”

“…[60][61] Although VFB does not show a clear trend with baking temperature, NA increases with lower baking temperatures, consistently with a higher density of structural/electronic defects, as recently reported for sputtered NiOx films. 58 At more anodic potentials (1.4 -2 V vs. RHE), the capacitance increases with applied potential with a welldefined peak over the redox wave and a gradual decrease over the catalytic region, as shown in Supporting Information, Figure S9b for the NiOx nanocomposite film baked at 50 ºC. This behavior has been recently observed on NiOx electrocatalysts, 58 and holds at each baking temperature (Figure 5b).…”

Pencil graphite rods decorated with nickel and nickel–iron as low-cost oxygen evolution reaction electrodes

“…In fact, and according to the literature, the two peaks observed at low overpotentials are attributed to two different Ni 2+ /Ni 3+ redox transitions. 40,41 As commented before, the energy of these transitions depends on the Fe content but also to the phase segregation in the sample, which is a function of the deposition environment and the ame annealing treatment. 42 There are two possible phases for Ni(OH) 2 molecules present at the Ni surface: b-Ni(OH) 2 , which is the normal and stable phase and the a-phase, a hydrated form of the nickel hydroxide, 3Ni(OH) 2 $2H 2 O.…”