Spectroelectrochemica Study Of The Catalytic Roles Of Nickel?iron In Anodic Oxygen Evolution In Alkaline Solutions

NiFe-based oxygen evolution catalysts（OECs）are highly active and cost-effective and have proven to be among the most active materials that can be potentially used in solar-to-fuel conversion for hydrogen energy generation.The OECs have been widely studied t o understand their compositions,structures and the electrocatalytic mechanisms for oxygen evolution reaction（OER）.It has been generally accepted that iron incorporation is required to enhance the OER activity of OECs in alkaline electrolytes,but the details of how Fe affects catalysis remain under active investigation.First,the constant current method was used to electrodeposit NiFe alloys on a nickel sheet substrate.The deposition of NiFe alloys with different compositions was carried out in the mixed electrolytes of Ni SO₄ and Fe SO₄with a molar ratio of Fe to Ni in the range of 0-0.2.The electrodeposition was carried out with 5 different current densities,but the amount of charge consumed by each deposition was kept constant by controlling the deposition time.The catalytic activity of the deposited alloys to OER was tested in 0.1 M Na OH.The results of cyclic voltammetry showed that the OER catalytic activity of the prepared NiFe electrodes was enhanced with the increase of Fe content in the Ni-Fe mixed solutions.The thin layer UV-vis spectroelctrochemistry was used to in situ follow the dynamic absorption spectra of the electrolytes in the constant potential OER reaction process,and to record the multi-step potential step chronoabsorptograms（MPCAs）at a fixed absorption wavelength.The results showed that there were soluble ions stripping into the electrolyte from the electrode during the OER process,and the increase of iron content in the Ni-Fe mixed solutions led to a decrease in the amount of dissolved species during the OER process.It is preliminarily determined that Fe incorporation into Ni-（oxy）hydroxide can inhibit the corrosive dissolution of NiFe catalyst at an OER potential,thus maintaining the structural stability of active components.Further,we adopt a double thin-layer strategy to acquire an indisputable experimental verification for the role of Fe and Ni atoms in OER mech anism.The OER activity depends on the local structure of the OEC surface but not of the bulk.The double thin-layer strategy can avoid the OEC/electrolyte interface information being obscured by both phase bulks,and allow the OEC and electrolyte to be studied as a whole.Traces of Fe were in situ deposited on a Ni substrate from the 1 ppm Fe³⁺-spiked OER electrolyte（0.1 M Na OH）to form an OEC thin-layer in a few nanometers thickness.The Fe content in NiFe catalytic layer was regulated by controlling Fe deposition time.On the other side,a thin layer electrolyte with a thickness of only 0.1mm was constructed using a thin-layer long-light-path spectroelectrochemical cell,and the species dissoving into the thin-layer electrolyte were in situ monitored.The OER activity,the catalyst composition and the electrolyte species were investigated together as a function of Fe deposition time.The results show that trace Fe incorporation into Ni-（oxy）hydroxide favors the formation ofβ?Ni OOH in the thin layer catalyst,and thus effectively inhibit the dissolution of Ni OOH in electrolyte.The results of double-potential step chronoabsorptometry and cyclic voltabsorptometry demonstrate the potential-dependent formation of a Ni³⁺intermediate in the electrolyte and,more importantly,the dissolution suppression effect due to Fe incorporation.These findings link the role of Fe in the OER catalysis to the increased insolubility of Ni³⁺active sites and highlight the importance of paying close attention to the active-site stability of an electrocatalyst impaired by the electrolyte at the reaction potentials.