Scientific Achievement

Electrochemical characterization, in situ Raman spectroscopy, and quartz crystal microbalance measurements were used to differentiate the effects of structural changes vs Fe incorporation after aging Ni(OH)2/NiOOH films in KOH.

Significance & impact

We find that ppb-level Fe impurities significantly impact the Ni(OH)2/NiOOH structure and oxygen evolution activity.  Ni films aged in unpurified electrolyte can incorporate ≥20% Fe after 5 weeks of aging, and the maximum catalyst activity is comparable to that reported for optimized Ni1–xxFeOOH catalysts.  These findings are the first to demonstrate the changes in the catalyst structure resulting from the incorporation of Fe electrolyte impurities and provide direct evidence that a Ni–Fe layered double (oxy)hydroxide (LDH) phase is critical for high OER activity.

Top Left:  Cyclic voltammograms for Ni(OH)2 films deposited on polished Au RDEs in (a) Fe-free and (b) unpurified 1 M KOH after each day of aging.  Top Right:  In situ Raman spectra of NiOOH after aging in Fe-free and unpurified KOH for six days; the decrease of the 480/560 peak height ratio indicates the presence of a Ni-Fe LDH.  Bottom Left:  Oxygen evolution activity of electrodeposited NiOOH at 300 mV overpotential and 10 mA·cm−2 geometric current density in 1 M KOH as a function of Fe content (acquired solely from electrolyte impurity uptake).