In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity

Abstract

Ni-based oxygen evolution catalysts (OECs) are cost-effective and very active materials that can be potentially used for efficient solar-to-fuel conversion process toward sustainable energy generation. We present a systematic spectroelectrochemical characterization of two Fe-containing Ni-based OECs, namely nickel borate (Ni(Fe)-B(i)) and nickel oxyhydroxide (Ni(Fe)OOH). Our Raman and X-ray absorption spectroscopy results show that both OECs are chemically similar, and that the borate anions do not play an apparent role in the catalytic process at pH 13. Furthermore, we show spectroscopic evidence for the generation of negatively charged sites in both OECs (NiOO(-)), which can be described as adsorbed "active oxygen". Our data conclusively links the OER activity of the Ni-based OECs with the generation of those sites on the surface of the OECs. The OER activity of both OECs is strongly pH dependent, which can be attributed to a deprotonation process of the Ni-based OECs, leading to the formation of the negatively charged surface sites that act as OER precursors. This work emphasizes the relevance of the electrolyte effect to obtain catalytically active phases in Ni-based OECs, in addition to the key role of the Fe impurities. This effect should be carefully considered in the development of Ni-based compounds meant to catalyze the OER at moderate pHs. Complementarily, UV-vis spectroscopy measurements show strong darkening of those catalysts in the catalytically active state. This coloration effect is directly related to the oxidation of nickel and can be an important factor limiting the efficiency of solar-driven devices utilizing Ni-based OECs.

Ambient-Pressure XPS Study of a Ni–Fe Electrocatalyst for the Oxygen Evolution Reaction

“…30 What is different between the observations here and in ref. 29, and the report by Trześniewski et al is that the "active oxygen" species was observed along with the oxidation of Ni 2+ to Ni 3+ , as indicated by UV-vis and XAS data, whereas the additional oxide species in our study and, likewise, the Raman peak at 560 cm -1 in ref.…”

Towards Versatile and Sustainable Hydrogen Production through Electrocatalytic Water Splitting: Electrolyte Engineering

“…The oxide can sustain local positive/negative charges on the surface, as discussed in Section 2, which allows decoupled pathways (i.e., PT and ET) to occur. An example of the decoupled ET and PT path for the OER is reported by Koper, Smith, and co‐workers 152, 153. In situ surface‐enhanced Raman spectroscopy was employed in their study, the spectra of which indicate the presence of superoxo‐type species formed by the PT step 153.…”

Influence of iron doping on tetravalent nickel content in catalytic oxygen evolving films

“…In contradistinction to an Fe active site model for Fe:Ni oxides, although Mössbauer studies of Fe:Ni layered double hydroxides provide direct evidence for the formation of Fe 4+ in Fe:Ni oxide catalysts during OER, these Fe 4+ sites do not account for the observed catalytic activity (24). Moreover, the formed "active oxygen" species have been found to be adsorbed on nickel sites in Fe:Ni films in borate buffer (25)(26)(27), suggesting Ni centers as active sites for OER. These results together suggest that the presence of Fe active sites may not be the primary reason for the enhanced activity of Fe:Ni catalysts films and point to another chemical role for Fe in oxidic Ni films.…”