Electromodified NiFe Alloys as Electrocatalysts for Water Oxidation: Mechanistic Implications of Time‐Resolved UV/Vis Tracking of Oxidation State Changes

Abstract

Facile electromodification of metallic NiFe alloys leads to a series of NiFe oxyhydroxide surface films with excellent electrocatalytic performance in alkaline water oxidation. During cyclic voltammetry and after sudden potential jumps between noncatalytic and catalytic potentials, Ni oxidation/reduction was tracked with millisecond time resolution by a UV/Vis reflectance signal. Optimal catalysis at intermediate Ni/Fe ratios is explained by two opposing trends for increasing Fe content: a) pronounced slowdown of the Ni2+/Ni3+ oxidation step and b) increased reactivity of the most oxidized catalyst state detectable at catalytic potentials. This state may involve an equilibrium between Ni4+ ions and Ni2+ ions with neighboring ligand holes, possibly in the form of bound peroxides.

Spectroelectrochemical study of water oxidation on nickel and iron oxyhydroxide electrocatalysts

“…NiOOH(++) in our nomenclature) in XAS and XANES experiments on this type of catalyst 27,28 . 18 O-labelling experiments by Hu and coworkers also indicate different active sites for Ni and NiFe oxides (Fe ≈ 25%) 52 , as do light scattering experiments on similar samples by Dau and coworkers, in-line with our conclusions herein 53 . However, the Fe concentration at which the favoured MOOH(++) species accumulated switches from Ni-centred to Fe-centred remains to be determined.…”

NaBH4 induces a high ratio of Ni3+/Ni2+ boosting OER activity of the NiFe LDH electrocatalyst

“…3A, the NiFe LDH, NaBH 4 -NiFe LDH (1 h, 2 h, 3 h, and 6 h) showed anodic and cathodic peaks at $1.36 V (vs. RHE) and 1.43 V (vs. RHE) which are corresponding to simultaneous oxidation and reduction of Ni 2+ to Ni 3+/4+ system of Ni(OH) 2 and NiOOH. [56][57][58][59][60] Apparently, aer NaBH 4 treatment for 2 h, the charge of higher positive peak at 1.43 V (vs. RHE) was most and that of lower peak at $1.36 V (vs. RHE) was least among these LDHs. The positive shi of the redox couples resulted from the triggered high valence nickel species aer NaBH 4 treatment (the mechanism was shown in Fig.…”

Carbon fibre paper coated by a layered manganese oxide: a nano-structured electrocatalyst for water-oxidation with high activity over a very wide pH range

“…This study shows that birnessite-coated carbon bre papers (MnO x /CFP) are efficient and stable anodes for electrocatalytic water oxidation over nearly the entire aqueous pH regime (pH [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Furthermore, we observe that MnO x /CFP is a volume catalyst where the electrocatalytic activity scales with the number of deposited Mn-ions.…”

“…[92][93][94][95] The thus formed negatively charged oxygen moieties are thought to be crucial for WOC, as a reaction of such species with other neighbouring OH-ligands or hydroxide ions from the electrolyte could lead to the more facile formation of reactive oxyl radical-(Mn-Oc) or peroxido-(Mn-O-O-H, Mn-O-O À ) intermediates and hence to a higher WOC activity. [96][97][98] The formation of oxygen ligands with a formal oxidation state of À1 at high catalytic potentials was recently reported not only for NiFe oxides 14,99 but also concluded from in situ XAS spectroscopy for birnessite MnO x . 100 From the results presented in this section, we can draw some general conclusions concerning WOC by MnO x /CFP-electrodes under different pH conditions: (1) from the Tafel data analysis it is obvious that a change of the mechanism from acidic/ intermediate pHs to strongly alkaline conditions takes place.…”

“…[9][10][11] As an example for the very good OER performance at high pH, some mixed Ni/Fe oxides can reach j ¼ 10 mA cm À2 for h < 300 mV in 1 M KOH. [12][13][14] Signicant progress has also been made in the development of water oxidation catalysis (WOC) at near neutral conditions. This eld has received a boost in 2008, when Kanan and Nocera reported on a self-healing, low-cost cobalt oxide OER catalyst with promising WOC activity at pH 7 (j $ 1 mA cm À2 for h < 450 mV).…”