Cooperative Fe sites on transition metal (oxy)hydroxides drive high oxygen evolution activity in base

Abstract

Fe-containing transition-metal (oxy)hydroxides are highly active oxygen-evolution reaction (OER) electrocatalysts in alkaline media and ubiquitously form across many materials systems. The complexity and dynamics of the Fe sites within the (oxy)hydroxide have slowed understanding of how and where the Fe-based active sites form-information critical for designing catalysts and electrolytes with higher activity and stability. We show that where/how Fe species in the electrolyte incorporate into host Ni or Co (oxy)hydroxides depends on the electrochemical history and structural properties of the host material. Substantially less Fe is incorporated from Fe-spiked electrolyte into Ni (oxy)hydroxide at anodic potentials, past the nominally Ni^2+/3+ redox wave, compared to during potential cycling. The Fe adsorbed under constant anodic potentials leads to impressively high per-Fe OER turn-over frequency (TOF_Fe) of ~40 s^-1 at 350 mV overpotential which we attribute to under-coordinated "surface" Fe. By systematically controlling the concentration of surface Fe, we find TOF_Fe increases linearly with the Fe concentration. This suggests a changing OER mechanism with increased Fe concentration, consistent with a mechanism involving cooperative Fe sites in FeO_x clusters.

Fig. 1. Foreign cations interaction with oxyhydroxides via redox signatures.

Cyclic voltammetry of Co-spiked NiO_xH_y (a, b) and of Ni-spiked CoO_xH_y (c, d) showing the evolution of redox features of the host metal hydroxide and phase formed by spiked ions under constant electrode potential. Inset numbers in (b) and (d) correspond to the cycle number, while the bottom cartoon illustrates schematically the process intentionally without atomic detail that is yet unknown. The data above was not iR_u-compensated.

Fig. 2. Surface-confined Fe sites via chronoamperometric (CA) metal-ion-spiking.

CA measurements of a NiOOH and c CoOOH at 1.55 V vs. RHE. After starting the measurement in purified Fe-free 1.0 M KOH electrolyte, aqueous Fe(NO₃)₃ was added to a concentration of 0.1 ppm. The first voltammetry cycle (red, 10 mV/s) after Fe-spiking CA measurements shows the dramatic effect of Fe incorporation on the OER activities of NiO_xH_y (b) and CoO_xH_y (d), but a minimal effect on the redox wave compared to the light green lines that show the initial voltammetry (cycle two) recorded in purified Fe-free 1.0 M KOH. The grey curves illustrate the large effect on the redox wave position for NiO_xH_y but not for CoO_xH_y after cycling, but that the OER activity does not further change much. The data is not iR_u compensated. The cartoons illustrate schematically the process intentionally without atomic detail that is yet unknown.

Fig. 3. Intrinsic OER activity measured by turnover frequency (TOF_Fe) at η = 300 mV.

The TOF_Fe is calculated based on the mass of all Fe sites determined by ICP-MS of each dissolved film. a Correlation between TOF_Fe and Fe/Ni atomic ratio for surface-confined Fe generated by CA, as well as mixed systems from cycling or co-deposition. b Correlation between TOF_Fe and Fe/Co atomic ratio. c Correlation between the TOF_Fe of surface-Fe sites on NiOOH (red) and CoOOH (grey) and the adsorbed Fe mass loading normalized by the electrochemical surface area of host oxyhydroxide. d Tafel slopes of Fe:NiO_xH_y (red) and Fe:CoO_xH_y (grey) as a function of Fe concentration (10, 40, 70, 100, and 200 ppb) in 1.0 M KOH electrolyte. Tafel analysis was performed using constant current steps from 0.18 to 3.2 mA·cm⁻², with each step held for 3 min. The steps were then repeated in reverse order. Before Tafel analysis, constant potential OER in Fe-spiked electrolyte was performed until the maximum OER current was reached. All current values used for TOF_Fe concentrations were iR_u-compensated where R_u was 15.1 ± 0.5 Ω. Error bars represent one standard deviation from the average of triplicate measurements.

Fig. 4. Plot (left panel) of the change in Fe loading by incorporation during CA and with CA plus 10 subsequent CV cycles versus the total Ni mass loading and film capacitance.

All elemental data was obtained with ICP-MS measurement of the dissolved films. The inset depicts how the mol or at % changed for each Fe incorporation technique from the lowest to highest Ni loading. While Fe incorporates at roughly constant mol % when the Ni and Fe species are allowed to mix by CV, the mol % of Fe incorporated by CA decreases substantially as the Ni loading is increased. The right panel TEM-EDX images of NiO_xH_y electrodeposited directly onto a gold TEM grid after Fe was adsorbed during chronoamperometry at 1.55 V vs RHE. While we observe localized Fe signal, it is not possible with this technique to distinguish between the surface-absorbed and internal/bulk sites nor to quantify accurately the size of the Fe clusters.

Fig. 5. Computational models and OER mechanisms for Fe sites on NiOOH.

a, b Schematic of adsorbed active sites on the NiOOH (0$\overline{1}$5) surface: a the isolated “Fe-O” case and b dimer “Fe-O-Fe” case, along with the mechanisms chosen for investigation. Values along the reaction pathways are the theoretical overpotential in units of eV for each step. c Summary of the total theoretical overpotentials for each depicted pathway including the effect on the overpotential of a single Ni substitution at an Fe in the isolated Fe-O monomer or Fe-O-Fe dimer. Models are shown at a larger scale in Supplementary Fig. 34.