Fig. 3: The role of Fe in EA operation.
a CV scans of NF in an unpurified KOH solution (Fe: 76 ppb), a 99.99% KOH solution (Fe: 50 ppb), and a purified KOH solution (Fe: 20 ppb) with a scan rate of 20 mV s–1. The pH value of each electrolyte was 14. b Overpotential difference between before (η0h) and after (η50h) 50 h of EA operation with varying Fe concentrations in 1 M KOH solutions. c Comparison of the α-Ni(OH)2/γ-NiOOH oxidation peak shift before and after various EA operations for 50 h. A positive value indicates an anodic shift after 50 h compared to the initial state. XPS (d) O 1 s and (e) Ni 2p spectra of NF before and after various EA operations. f Ratios of OL/Ototal and Ni3+/Ni2+ before and after various EA operations, which were determined by comparing XPS peak areas. g The total Fe content in NF after various operation conditions. The EA operation with 0.6 VRHE was first carried out for 20 h in unpurified KOH containing a natural isotopic abundance of 56Fe and 57Fe. Subsequently, a second 20 h experiment was conducted in purified KOH with the addition of 100 ppb 57Fe, operating under either high (0.5 A cm–2) or low current density (30 mA cm–2) conditions with or without the EA protocol. Error bars indicate standard errors obtained from 3 independent replicates.
