Electrochemical study of the promoting effect of Fe on oxygen evolution at thin 'NiFe-Bi' films and the inhibiting effect of Al in borate electrolyte

Abstract

In this study, we investigated the electrochemical effects of Fe co-deposition in thin Ni oxo/hydroxo films in borate, termed 'NiFe-Bi', electrodeposited from a Ni:Fe ratio of 9:1, 6:4, and 4:6 in solution. The NiFe-Bi films were investigated as-deposited and after anodic conditioning, and compared to Ni-Bi without intentional Fe doping at Ni(OH)2 loading from a submonolayer to 10 layers. Fe co-deposition enhanced the oxygen evolution reaction (OER) of as-deposited NiFe-Bi films relative to as-deposited Ni-Bi; however, anodic biasing was still required to maximize catalysis. The Ni(OH)2/NiOOH redox peaks of NiFe-Bi deposited from 6:4 and 4:6 Ni:Fe were more cathodic and more reversible than Ni-Bi peaks. Anodic conditioning caused the Ni(OH)2/NiOOH redox peaks of Ni-Bi and NiFe-Bi to shift anodically but without narrowing of peak separation, different from the effects of Fe co-deposition. After anodic conditioning, the turnover frequency (TOF) for OER per Ni center for Ni-Bi and NiFe-Bi was more proportional to the Ni content within the first linear Tafel region, with a promoting effect of Fe, and the films exhibited similar Tafel slopes of 37-42 mV dec-1. With increasing overpotential however, the TOF increased more significantly at high Fe:Ni ratio and was not proportional to the Ni content, and the Tafel slopes varied while notably decreasing for some NiFe-Bi films. Electrochemical results can support a Ni active site at low overpotential, but point to possibly different roles of Fe at low versus high potential. In addition, the effect of adding Fe3+ and Al3+ to the electrolyte was studied after deposition of Ni-Bi. Adding Fe resulted in reaching almost maximum activity in a single potential scan, confirming the promoting role of Fe in Ni-Bi and providing a quick alternative to applying anodic bias for hours to increase activity, while Al in the electrolyte poisoned OER catalysis at Ni-Bi and at NiFe-Bi. In the presence of both Fe and Al ions in solution however, with their opposite effects on the OER, the Ni(OH)2/NiOOH redox peaks shifted anodically with potential scanning.