Effects of Fe Electrolyte Impurities on Ni(OH)₂/NiOOH Structure and Oxygen Evolution Activity

Abstract

Ni-(oxy)hydroxide-based materials are promising earthabundant catalysts for electrochemical water oxidation in basic media. Recent findings demonstrate that incorporation of trace Fe impurities from commonly used KOH electrolytes significantly improves oxygen evolution reaction (OER) activity over NiOOH electrocatalysts. Because nearly all previous studies detailing structural differences between α-Ni(OH) 2 /γ-NiOOH and β-Ni(OH) 2 /β-NiOOH were completed in unpurified electrolytes, it is unclear whether these structural changes are unique to the aging phase transition in the Ni-(oxy)hydroxide matrix or if they arise fully or in part from inadvertent Fe incorporation. Here, we report an investigation of the effects of Fe incorporation on structure− activity relationships in Ni-(oxy)hydroxide. Electrochemical, in situ Raman, X-ray photoelectron spectroscopy, and electrochemical quartz crystal microbalance measurements were employed to investigate Ni(OH) 2 thin films aged in Fe-free and unpurified (reagent-grade) 1 M KOH (<1 ppm Fe). We find that 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 Ni 1−x Fe x OOH catalysts. Conversely, Fe-free Ni(OH) 2 films exhibit a substantially lower activity and higher Tafel slope for the OER. Films aged in Fe-free electrolyte are predominantly disordered β-Ni(OH) 2 /β-NiOOH if maintained below 0.7 V vs Hg/HgO in 1 M KOH and will "overcharge" to form a mixture of γ-and β-NiOOH above this potential. Fe-containing Ni(OH) 2 films evidence a lesser extent of β-Ni(OH) 2 formation and instead exhibit NiOOH structural changes in accordance with the formation of a NiFe-layered double hydroxide phase. Furthermore, turnover frequency calculations indicate that Fe is the active site within this phase, and above ∼11% Fe content, a separate, Fe-rich phase forms. These findings are the first to demonstrate the in situ changes in the catalyst structure resulting from the incorporation of Fe electrolyte impurities within Ni-(oxy)hydroxide, providing direct evidence that a Ni−Fe layered double (oxy)hydroxide (LDH) phase is critical for high OER activity.

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. 29, arise before the onset of Ni oxidation.…”

“…This is consistent with recent in situ Raman measurements within the same potential region, which revealed a spectral signature of NiOOH prior to the oxidation wave in the corresponding voltammogram. 29 Recently, Trześniewski et al…”

Zero-Gap Alkaline Water Electrolysis Using Ion-Solvating Polymer Electrolyte Membranes at Reduced KOH Concentrations

“…Polarization curves were recorded after certain durations of set-point operation as shown in Figure 6b. Due to electrode activation, perhaps from iron impurities as discussed above, 53 the onset potential was observed to improve as experiments ran for longer periods of time and more polarization sweeps were recorded. Currently, Fe-doped nickel hydroxides are even considered the most active alkaline oxygen evolution catalyst.…”

“…Iron is present as impurities in the electrolyte, but is also bleeding from external system components exposing steel to the heated electrolyte such as pumps and valves. It is well known that iron impurities significantly improve the oxygen evolution activity of nickel, 52,53 but other factors such as membrane uniformity, precision of electrode alignment, and contamination are expected to affect the appearance of the polarization curves as well.…”

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

“…The behavior of Fe-doped Ni (Fe:Ni) oxide films under basic conditions (1 M KOH) has been revisited (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) and in this recent body of work, the role of Fe in these films has come under debate. X-ray absorption spectra of Fe:Ni oxide films supported by computational studies have led to the contention that Fe 3+ species are the active sites for water oxidation (13).…”