Activating the Basal Planes and Oxidized Oxygens in Layer‐Structured Na0.6CoO2 for Boosted OER Activity

Abstract With the CoO2 slabs consisting of Co4O4 cubane structure, layered Na x CoO2 are considered promising candidates for oxygen evolution reaction (OER) in alkaline media given their earth‐abundant and structural advantages. However, due to the strong adsorption of intermediates on the large basal planes, Na x CoO2 cannot meet the activity demands. Here, a novel one‐pot synthesis strategy is proposed to realize the high solubility of iron in Na x CoO2 in an air atmosphere. The optimist Na0.6Co0.9Fe0.1O2 exhibits enhanced OER activity compared to their pristine and other reported Fe‐doped Na x CoO2 counterparts. Such an enhancement is mainly ascribed to the abundant active sites on the activated basal planes and the participation of oxidized oxygen as active sites independently, which breaks the scaling relationship limit in the OER process. This work is expected to contribute to the understanding of the modification mechanism of Fe‐doped cobalt‐based oxides and the exploitation of layer‐structured oxides for energy application.

water / V isopropanol / V Nafion = 7.5/2.5/1),with a further ultrasonication for at least 1 h.After a homogenized dispersion of catalysts formed, 3 uL of the ink was taken and spread onto the surface of the glassy carbon electrode to yield a final mass loading of 0.386 mg cm −2 .The catalyst ink was dropped on a glassy carbon electrode with a diameter of 3 mm as the working electrode, and platinum wire and saturated Ag/AgCl electrode were used as the counter and reference electrodes, respectively.RuO 2 is purchased from Sigma-Aldrich, and the RuO 2 electrode preparation method is the same as that of NCFx.All the electrochemical tests were carried out by a typical three-electrode system on an electrochemical workstation (CHI 760E, Chenhua, Shanghai) in a 1.0 M KOH electrolyte solution.Linear sweep voltammetry (LSV) curves were collected at a scan rate of 5 mV s −1 .Electrochemical impedance spectroscopy (EIS) was performed at 1.6 V versus reversible hydrogen electrode (RHE) over frequencies from 0.1 Hz to 10 5 Hz.All the current was normalized to the glassy carbon electrode's geometric surface area.The potentials versus Ag/AgCl were converted to versus RHE according to the Nernst Equation (1): The E iR corrected is obtained following the equation: E iR corrected = E RHE -iR s (where i is the current, and R s is the uncompensated ohmic solution resistance resolved from Nyquist plots.)6][7] In this work, the TPD results are based on measurements in the high-temperature range, which corresponds to the desorption of lattice oxygen.As shown, each curve can be fitted to two regions, including the lower temperature region of Peaks 1 and 2 (named Region L), and the higher temperature
region containing Peaks 3 and 4 (named Region H).Obviously, region L accounts for the main part in NC and NCF0.1, while Region H plays the dominant role in 10% HCl-treated samples.Sergio et al.proved by DFT calculations that compared with (012), (110), and (104) surfaces, (001) terminations demonstrate the lowest energy and are more inclined to lose oxygen in Li x CoO 2 . 8Thus, we take Region L corresponding to the desorption of lattice oxygen with lower binding energy on the (001) basal planes, and Region H to the desorption of O on the edge areas.The fractions of edge areas significantly increase in the acid-treated samples, which is in line with our expectations.

Figure
Figure S8.(a-b) The linear fittings of the capacitive currents versus CV scans.(c)The derived Cdl values of the catalysts before and after acid-etching.

Figure
Figure S9.(a) Co 2p XPS spectra and (b) Fe 2p XPS spectra of the as-prepared catalysts.

Figure
Figure S10.(a) Co L-edge and (b) Fe L-edge soft XAS spectra of the as-prepared catalysts.

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Figure S12.(a) The modeled structure of Fe-doped NC, the blue atoms represent Co, the red atoms represent O, the orange atoms represent Na, and the green atom represents Fe.(b) Hirshfeld charge analysis based on the structure in Figure a, the number near the oxygen atom represents the calculated charge accumulation of that oxygen atom.

Table S1
. The inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of the prepared Na 0.6 Co 1-x Fe x O 2 .

Table S2 .
The calculated metal-oxygen bond length.